Mechanisms of Drug Action on the Nervous System (Cambridge Texts in Physiological Sciences)

Mechanisms of drug action on the nervous system

Mechanisms of drug action on the nervous system SECOND EDITION Ronald W. Ryall Lecturer in Pharmacology, University of Cambridge and Fellow of Churchill College, Cambridge

The right of the University of Cambridge to print and sell all manner of books was grunted by Henry VIII in 1534. The University has printed and published continuously since 1584.

CAMBRIDGE UNIVERSITY PRESS Cambridge New York New Rochelle Melbourne Sydney

Published by the Press Syndicate of the University of Cambridge The Pitt Building, Trumpington Street, Cambridge CB2 1RP 32 East 57th Street New York NY 10022, USA 10 Stamford Road, Oakleigh, Melbourne 3166, Australia © Cambridge University Press 1979, 1989 First published 1979 Second edition 1989 British Library cataloguing in publication data Ryall, Ronald W. Mechanism of drug action on the nervous system.—2nd ed. 1. Drugs affecting nervous system. Action. Mechanisms I. Title 615\78 Library of Congress cataloguing in publication data Ryall, Ronald W. Mechanisms of drug action on the nervous system / Ronald W. Ryall.—2nd ed. p. cm.—(Cambridge texts in the physiological sciences : 1) Bibliography: p. Includes index. ISBN 0-521-25424-8. ISBN 0-521-27437-0 (pbk.) 1. Neuropharmacology. I. Title. II. Series. [DNLM: 1. Nervous System—drug effects. QV 76.5 R988m] RM315.R9 1989 615'.78—dcl9 DNLM/DLC 88-20353 ISBN 0 521 25424 8 hard covers ISBN 0 521 27437 0 paperback (first edition ISBN 0 521 22125 0 hard covers ISBN 0 521 29364 2 paperback) Transferred to digital printing 2004

BO

To Audrey

CONTENTS

Preface to the second edition Preface to thefirstedition List of abbreviations

xiii xvii xix

1 Introduction

1

2 Techniques Routes of drug administration Systemic administration Local administration Electrophysiological methods Biochemical and histochemical techniques

7

PERIPHERAL NERVOUS SYSTEM 3 Neuromuscularjunction Techniques Synaptic transmission The acetylcholine receptor Activation of the receptor Sites of drug action at the neuromuscularjunction Prejunctional drug action Postjunctional drug action Pharmacological characterisation of neuromuscular blocking agents Myaesthenia gravis Denervation supersensitivity 4 Autonomic nervous system Neurotransmitters Drug action in the autonomic nervous system Ganglionic sites of action

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Contents

The structure and function of sympathetic nerves The metabolism of catecholamines The uptake and storage of catecholamines Receptors for noradrenaline Membrane and intracellular consequences of adrenoceptor activation Directly and indirectly acting sympathomimetic amines Inhibition of uptake mechanisms Miscellaneous drug actions The importance of uptake mechanisms in the actions of some adrenergic neurone blocking drugs Other antihypertensive drugs Denervation supersensitivity Cholinergic transmission at autonomic postganglionic nerve endings Muscarinic receptors Cholinesterase inhibitors

CENTRAL NERVOUS SYSTEM

5 Central neurotransmitters and neuromodulators Acetylcholine Amino acids Catecholamines and 5-hydroxytryptamine Polypeptides

80

6 The blood-brain barrier The nature of the blood-brain barrier Factors affecting rate of transfer of substances to and from the brain Developmental aspects Neurotoxicity Summary

93

7 General anaesthetics Types of general anaesthetic Gaseous anaesthetics

101

Contents

ix

Volatile anaesthetics Soluble (intravenous) anaesthetics

Mechanisms of anaesthesia Physico-chemical theories Difficulties with physico-chemical theories

Localisation of the effects of anaesthetics on neurones Pre- and postsynaptic effects Differential effects on excitatory neurotransmitters Effects on presynaptic inhibition Selective effects upon different areas of the brain and on spinal reflexes

Conclusions Tolerance to anaesthetics 8 Pain and analgesia Peripheral pain mechanisms Peripheral nerve fibres Activation of pain receptors and mediators The action of aspirin The action of capsiacin Central pain pathways Processing in the spinal cord Morphine-like analgesics Structure of morphine-like drugs Actions of morphine-like drugs The opiate receptor Localisation of the receptor Endogenous ligands for opiate receptors Analgesia and opioid peptides Multiple receptors for opioid peptides Involvement of opioid peptides in pain Sites of opiate action Descending control and analgesia Cellular actions of opiates Tolerance to opiates

118

9 Drug interactions with inhibitory amino acids Convulsants Anxiety-reduction and sedative-hypnotics

144

Contents

Benzodiazepines Pharmacokinetics Pharmacological actions

Benzodiazepine receptors Other anxiety-reducing, sedative-hypnotic drugs Anti-epileptic drugs Characterisation of epileptic seizures The use of drugs in epilepsy Pharmacological mechanisms General conclusions 10

Drugs used in schizophrenia

171

Theories of schizophrenia Drugs used in schizophrenia The dopamine receptor Multiple receptors for dopamine

Extrapyramidal side-effects of antischizophrenic drugs Mechanisms in drug-induced dyskinesias

Summary 11 Affective and manic depression Endogenous depression

193

Monoamine oxidase inhibitors Tricyclic antidepressants Other classes of antidepressants

Mechanisms of antidepressant action Long-term effects of antidepressants

Conclusions Manic depression 12 Disorders associated with defined brain lesions Spasticity Wilson's disease Parkinson's disease Drug treatment

Huntington's disease Biochemical and structural changes

203

Contents

xi

Treatment Alzheimer's disease Selected reading Index

217 225

PREFACE TO THE SECOND EDITION

Since the first edition of this book was published in 1979 there have been some major advances in all areas of knowledge concerning the physiology and pharmacology of the nervous system, although some advances are much greater than others. Each of these advances can on the whole be attributed to advances in technology. New technology for the measurement of receptor binding and immunological techniques, combined with the production of monoclonal antibodies, have been responsible for the greatly increased understanding and apparent complexity of receptors and their ligands. Only recently have some notes of caution been raised concerning the interpretation of ligand binding data as necessarily reflecting the properties of receptors. Advances in electrophysiological techniques now permit not only noise analysis of single channel events but also direct recording of the electrical currents flowing through those channels: I refer of course to single channel current recording with 'patch-clamp' techniques and the use of modern, high frequency, voltage-clamp amplifiers. Considerations of space in a book of this size and scope does not allow more than a brief mention of the technology. Finally, the age of the computer has brought with it considerable benefits, together with some consequential difficulties. Among the benefits is the ability to analyse complex events or to build up complex pictures of three dimensional objects which was not possible in an earlier generation. Complex experiments are now easier to perform than ever before. Among the potential hazards is the proliferation of trivial data which are impediments rather than aids to understanding. There has been an explosion of information concerning the presence of polypeptides in the peripheral and central nervous

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Preface to the second edition

systems, but there is still a deficit in our understanding of their functions at specific locations. Despite immense efforts, a therapeutic advance from such studies has yet to appear. All too often excessive enthusiasm can lead to false conclusions: the mere presence of a particular substance in a nerve or that beautiful coloured picture of its distribution in nerve networks that stirs one's artistic imagination or the fact that binding sites exist, even when they are high affinity sites, does not preclude the possibility that the substance may normally function neither as a neurotransmitter nor as a neuromodulator. The next few decades will hopefully tell us whether the present has been guilty of the scientific crime of stretching conclusions beyond those justified by the data available. Two areas in particular have 'blossomed' since the first edition: these are the discovery of the wealth of endogenous opioid and other peptides and their receptors and the benzodiazepines and their receptors. In the case of the opioids both receptors and endogenous peptide ligands for them have been identified. As I write, I am conscious that fresh interest is currently being evoked in the possibility that morphine itself might be an endogenous ligand for the receptor! In contrast, the benzodiazepine receptors are probably still looking' for their endogenous ligands, although plenty of the synthetic variety are available and enthusiastic suggestions for candidates abound. In both of these areas there are drugs available which have a therapeutic use. I am intrigued that, despite the enormous amount of basic science that has been carried out, the increasing sophistication of technology and the tremendous increase in understanding of the actions of known drugs, there have been few major new conceptual advances in new drugs for the treatment of diseases of the peripheral or central nervous system over the last decade. One begins to wonder whether the 'sight of the wood is getting lost by the obscurity of the trees'. Put in another way, is there now too much emphasis on molecular mechanisms and too little on function and its disturbance in disease states? If the answer is yes then we shall continue to see an expansion in knowledge of the action of currently popular drugs but will see few new drugs developed.

Preface to the second edition

xv

However, there are some grounds for optimism. In the field of transmitters the excitatory amino acids are currently in vogue and are beginning to eclipse the 'conventional1 neurotransmitters and even the peptides. This renewed interest (attention was first drawn to them in the 1950s) is largely attributable to the production of interesting new compounds, rather than the study of old ones. There is a good involvement in the function of endogenous excitatory amino acids and their receptors and the current speculation on the possible role of NMDA receptors in memory and even Alzheimer's disease and stroke gives grounds for enthusiasm and hope for the possible emergence of radically new therapies. It is disappointing that no radically new therapies have been developed for the treatment of psychotic mental illness although in this case it is perhaps attributable to the poor understanding of the underlying neuropathology. There is hope, but little evidence on which to base it, for the development of new anti-schizophrenic compounds which interact with 5HT3 receptors. It is surprising that we can say little more than was possible ten years ago about the action of general anaesthetics. The difficulty may still be our lack of knowledge about the nature of the synaptic transmitters at specific synaptic locations, despite the extensive knowledge about how they work on isolated bits of membrane. The approach to this book and its objectives have changed little, if at all. Essentially, the objective has been to present a 'story' about each class of drugs which will give undergraduate students in science and medicine some broad insight into basic disorders of the nervous system and the way in which the drugs work to alleviate symptoms. If others find the book of value then this will be an added bonus since this was not the objective. A narrative style has been adopted and there is a deliberate avoidance of references in the text to help attain this objective. No attempt has been made to mention all drugs which are used: only those which have attributes making them worth special mention are presented. Nevertheless, the average student will not find that there are too many drug names to remember. He should take hope in the thought that before too long, especially if he is a medical student, each of these drugs, or perhaps a 'better' version, will soon become a 'household word'. The student is not spared con-

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Preface to the second edition

troversy, where this is appropriate, although I would hope that I have not included too much. Most of the book has been rewritten and reorganised, although a few chapters are little changed. The section on the autonomic nervous system has been expanded considerably since this was not given adequate treatment in the first edition. The book now gives a fairly balanced perspective of the action of most classes of drugs which act on the nervous system. Stimulant drugs, such as amphetamine, ritalin, lysergic acid diethylamide, cannabis (is it a stimulant?) and cocaine are not particularly useful therapeutically, even though many of them are of importance as drugs of abuse. Too little is known about their mechanisms to give a coherent picture and too little space was available to do justice to the problem of abuse. However, references to some of them will be found sporadically through the book. Perhaps I can be excused for this deliberate omission.

PREFACE TO THE FIRST EDITION

In recent years there have been many important advances in knowledge concerning the mechanisms of chemical synaptic transmission, the identification of the neurotransmitters and the mechanisms by which drugs act on the nervous system. These advances have necessitated a change in approach to the teaching of the pharmacology of the nervous system to undergraduate science and preclinical medical students from a basically therapeutic orientation to one which is more mechanistically minded. In giving such courses to students in Cambridge, the author has become painfully aware of the need for an undergraduate text which could fulfil the needs of students in this respect. There are of course many excellent textbooks of therapeutics available but few of them attempt to cope in detail with mechanisms of drug action, especially on the central nervous system, except from rather specialised viewpoints. It was therefore considered to be unnecessary to discuss therapeutic applications in detail in this book, although an attempt has been made to give a fairly balanced account of the physiological basis, applications and mechanisms of action of each class of drugs, within the limitations imposed by the objective of producing a concise account of drug actions. Advances are occurring at such a rate that some of the concepts which are current today may be superseded tomorrow: this is probably true for any subject that is 'alive' and progressing. However, this does create problems in deciding what to omit and what to include. As far as possible, the basic approach adopted in this volume is to present a coherent 'story' which will enable the student to develop concepts and, perhaps, ideas of his own. Only in this way is it likely that a continuation of progress can be

xviii

Preface to the first edition

assured and that future medical graduates will not see drugs simply as liquids in bottles to be administered in an empirical manner without understanding to patients with diseases of the nervous system: a reasonable concept, compatible with contemporary information, even if subsequently found to be incorrect in detail, is surely better than no concept at all. Nevertheless, where concepts are relatively insecure, or mechanisms completely unknown, no attempt has been made to disguise this fact in order to present a 'story': such an approach could lead to unjustified complacency.

ABBREVIATIONS

ACh ACTH Adr AMP ANS ATP Bmax CAT (scan) CIO P-CCE CCK CNS COMT CSF DA DOPA d-Tc EC50/ED50 ECF ECT EEG EMG epp EPSP GABA GAD GDP GMP GTP

Acetylcholine Adrenocorticotropic hormone Adrenaline Adenosine monophosphate Autonomic nervous system Adenosine triphosphate Concentration at which all receptors are saturated Computer aided axial tomography Decamethonium P-carboline-3-carboxylic acid ethyl ester Cholecystokinin Central nervous system Catechol-O-methyl transferase Cerebrospinal fluid Dopamine Dihydroxyphenylalanine d-Tubocurarine Concentration/dose to cause 50% effect Extracellular fluid Electroconvulsive therapy Electroencephalogram Electromyogram End plate potential Excitatory postsynaptic potential y-Amino-butyric acid Glutamic acid decarboxylase Guanosine diphosphate Guanosine monophosphate Guanosine triphosphate

xx

Abbreviations

HC-3 5-HT 5-HTP IC50/ID50

KA KD K, LHRH LSD MAO (-A or -B) mepp MPTP NA Ni

Ns NMDA NMR 6-OHDA PET PG PTMA PTP SIF SPECT TEA TOH TRF VIP

Hemicholinium 5-Hydroxytryptamine 5-Hydroxytryptophan Concentration/dose to cause 50% inhibition Affinity constant = \/KD Dissociation constant Dissociation constant for inhibition Luteinising hormone releasing hormone Lysergic acid diethylamide Mono amine oxidase (A & B forms) Miniature end plate potential N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine Noradrenaline Inhibitory regulatory protein Stimulatory regulatory protein N-methyl-D-aspartic acid Nuclear magnetic resonance 6-Hydroxydopamine Positron emission tomography Prostaglandin Phenyltrimethyl ammonium Post-tetanic potentiation Small intensely fluorescent neurone Single photon emission tomography Tetraethyl ammonium Tyrosine hydroxylase Thyrotrophin releasing factor Vasoactive intestinal polypeptide

Introduction

The junctions between nerve cells and either other nerve cells or the organs which they innervate are called synapses and form major sites for the targetting of new drugs with therapeutic effects and for the actions of toxic substances, including those produced by living systems. The synapses act as transducers, transforming the fairly constant energy of the action potential, which conducts nerve impulses from one end of the nerve to the other, into a variety of events upon the next cell in the chain. It is the variety of transmitters and modulators and the effects which they produce which enables drugs to act in relatively selective ways, either directly on the transmission process or the events which follow it. Side-effects are often due to unwanted actions upon some other system. However, it is perhaps less obvious that even if the drugs are highly specific in their actions then they may still have serious side-effects arising from the fact that individual processes are common to many nervous pathways, not all of which are equally affected by disease. Ideally, the development and design of new drugs for specific applications would arise from a thorough understanding of the basic physiology of the system and the pathology of the process which has led to dysfunction, together with a thorough understanding of the action of drugs and transmitters which affect the system. One would also know how modification to the structures of existing drugs would bring about changes in their pharmacological actions: the modern techniques of structure-activity relationships, as resolved by sophisticated computer simulations and calculations, can be considered to be in their infancy and have yet to lead to therapeutically useful drugs. Only rarely has a rational approach actually yielded important therapeutic

2

Introduction

advances, and it is significant that many of the drugs in current use, or their prototypes, are of plant origin and their major effects were known and exploited long before the advent of contemporary physiology, pharmacology or pathology. Serendipity has contributed a great deal to the contents of the modern pharmacopoeia and many of the drugs now used for one purpose were originally introduced for quite another. Despite these observations, the search for new drugs has undoubtedly yielded great improvements upon the activity of the prototypes and has occasionally, e.g. with the benzodiazepines, led to a completely different pharmacological approach to a therapeutic problem. Most of the drugs used to treat disorders of the nervous system produce an alleviation of symptoms, without curing the underlying disease. However, the alleviation of distressing symptoms is a worthwhile therapeutic objective and the curing of the disease and the reversal of the pathological changes which have already occurred is a tantalising goal to strive for. In some instances, the underlying pathology is fairly well established, as in the case of Parkinson's disease, and drug therapy rests upon a rational basis. In other illnesses, such as schizophrenia, little or nothing is known of the underlying pathological process although the pharmacological action of therapeutically effective drugs has been soundly evaluated. In such cases there is a tendency to develop a plethora of theories, most of them resting upon very insecure foundations, and the temptation to accept those theories without question must be strongly resisted. Uncritical acceptance can only slow the discovery of the real causes and produce complacency and a false sense of security. Although the sites of drug action will be evaluated in more detail in subsequent chapters, it is appropriate first to consider the question more generally. Most junctions between nerves and postjunctional structures, particularly in mammals, operate by the release of a chemical transmitter substance from the nerve termination acting upon specific sites or receptors upon the postjunctional membrane. Most of this book will be concerned with drug action at such sites. In addition, local anaesthetics and some neurotoxins impair impulse conduction in nerves by altering specific membrane conductances to particular ions and thereby

Introduction

3

will have effects on synaptic transmission, but they will not be considered in detail. Chemically transmitting junctions between a nerve and its postjunctional element are of two types. There are those in which the postjunctional element is a muscle or a gland and activation of the synapse results in either secretion or its inhibition, or contraction or its inhibition. There are of course also those synapses in which both the pre- and postjunctional elements are neuronal. In this instance the effect of the transmitter will be to modify the electrical excitability of the postsynaptic neurone so as either to excite it or to make it either more or less susceptible to excitation. Generally, the term inhibition is applied to a reduction in excitability of the postsynaptic neurone. However, the advent of the peptides as possible neurotranmitters has created some additional terminology in which the endogenous substance is said to modulate the excitability of the postsynaptic membrane. Such modulation may either increase or decrease the excitability of the cell. There are many potential targets upon which a substance could act at synapses. These targets are illustrated in Fig. 1.1, which forms the basis of much that will be described in this text. The sites of action may be broadly divided into prejunctional and postjunctional sites. At prejunctional sites there are susceptible processes involved in the conduction of the nerve impulses into the nerve terminal or with the entry of calcium ions into the terminal, which are both essential for the release of the neurotransmitter. Axonal transport may be involved not only in transporting transmitter-related substances, such as precursors, synthesising or degrading enzymes or even polypeptides manufactured in the cell body, but may also be involved in the transport of trophic factors responsible for maintaining the structural integrity of the junction. Such trophic factors may also control the efficacy of synaptic transmission in a dynamic fashion, perhaps by controlling, among other possibilities, the density of receptors upon the post- or prejunctional membranes. Other prejunctional processes include: uptake of precursors and the re-uptake of released transmitters; synthesis or degradation of transmitters or their precursors in cell membranes,

4

Introduction

organelles or cytoplasm; uptake into organelles in which there are storage mechanisms for the transmitter and subsequently the storage mechanism itself; the release mechanism, whether this be calcium-dependent, vesicular release or, as some believe, non-vesicular release; prejunctional receptors. When drugs act prejunctionally they must do so so efficiently that transmitter release is significantly decreased or increased. If the process upon which the drug acts is not rate limiting then the process of transmission will not be significantly altered unless Fig. 1.1. Cellular processes susceptible to drug action at chemically transmitting junctions. Axonal transport

Na + -

Synthesis and degradation Uptake into storage organelles Prejunctional receptors

Precursors Uptake"

Neuronal uptake Prejunctional re-uptake

oo Exocytosis and release 0 of transmitter Diffusion Postjunctional receptors

Postjunctional uptake

u

/ Intracellular 'messengers'

Postjunctional effect

Extrajunctional receptors

Introduction

5

the process is affected to such an extent that it does become rate limiting. Examples of such situations may be found in Chapter 4, which considers the biosynthetic pathway for catecholamines, in which the only rate limiting step is tyrosine hydroxylase. Postjunctionally, there are also many sites at which drugs could act. Again the process affected by the substance should either be rate limiting or should be so severely affected that it becomes so after drug treatment. The action of d-tubocurarine, acting upon skeletal muscle (Chapter 3), falls into this category. Here, the reduction of the end plate potential does not become rate limiting for the production of muscle action potentials until about 90% of the postjunctional acetylcholine receptors are occupied by the antagonist. The most common and best understood mechanism for postsynaptic action is upon specific subjunctional receptor sites. These sites may be considered to be chemically selective binding sites which modify the properties of the postjunctional cell. This may be effected either by opening or closing adjacent or integral ion channels, thereby affecting ion currents across the cell membrane, or by indirectly affecting function via a second messenger such as one of the cyclic nucleotides. Other postjunctional sites are also available. These include the enzymes which degrade the transmitter into inactive products; the enzymes may be located in the cell membrane or free within the cytoplasm or contained within or bound to the surface of cytoplasmic organelles. There may also be receptor sites at a distance from the junctional regions where the transmitter is liberated and the characteristics of the extrajunctional receptors are not necessarily identical to those located subjunctionally. Drug action, for obvious reasons, will usually involve reversible effects. Where the effects are not reversible the action of the drug is likely to be prolonged, limited by the de novo reconstitution of the inhibited process, or even toxic and possibly fatal if the process is essential, severely affected or perhaps completely eliminated. One can find examples of many such irreversible actions among the toxins and venoms produced by living organisms to their own peculiar advantage. Irreversible toxic effects could ensue from selective effects of

6

Introduction

the toxic substance upon particular junctions, e.g. 6hydroxydopamine selectively destroys adrenergic nerve terminals because it is selectively taken up by the adrenergic nerve terminal, or the toxicity may be more or less non-selective. Tetrodotoxin, by blocking sodium channels, will reduce the excitability of all electrically excitable tissues, whereas botulinus toxin, which prevents the release of acetylcholine at all cholinergic junctions, will selectively block the process of neurotransmission at all such junctions, both peripherally and in the central nervous system, but will leave unaffected adrenergic transmission or transmission mediated by other substances. Formation of free radicals could be the basis of some irreversible and toxic actions. The kinetics of drug action depends upon many processes. At the periphery, it may be limited by absorption from the site of administration, by binding to plasma proteins, by blood flow to the target organ, by rate of elimination from the plasma, either by excretion, metabolism or uptake by tissues, and by the kinetics of the drug-receptor interactions. For actions on the central nervous system, there is an additional barrier to access. This is the blood-brain barrier which for the most part behaves like a lipid membrane. The blood-brain barrier will be considered in detail in Chapter 6. If the substance is highly lipid soluble it will rapidly pass through the barrier at a rate dependent upon the concentration difference across it. If the lipid solubility is low then the rate of access will be determined not only by the lipid solubility but also by the blood flow. In summary, there are many possible sites of drug action at synapses. In practice, the most useful therapeutic substances will be those which affect processes peculiar to a limited range of junctions. Often the target will be the interaction between the endogenous transmitter or neuromodulator and its receptor sites, but other selective processes such as transmitter release or degradation may also yield the required degree of selectivity in drug action. Unfortunately, few drugs are absolutely specific in their effects. Examples of specific and less specific drug actions will be found throughout this book.

Techniques

In this chapter only some of the techniques which can be employed to investigate the action of drugs upon the nervous system and the process of synaptic transmission will be examined. There is no space in a book of this size to describe the many techniques in detail and no attempt is made to discuss behavioural techniques which are frequently employed in the evaluation of drugs acting on the central nervous system. The emphasis will be on the more recent developments in technology. Many of the techniques are common to studies of the peripheral or central nervous system. However, special techniques are required to overcome the problems associated with the access of drugs to the brain and the determination of the site of action. Since the first edition of this book was written, there have been a number of advances in the development of in vitro techniques for examining the properties of slices of brain tissue (brain slice techniques) and for maintaining cultures of neuronal tissue. These have enabled us to bypass the problems associated with the access of drugs to the brain although they have introduced some additional problems of their own. There have also been advances in electrophysiological techniques for examining the properties of single ion channels in isolated segments of cell membranes in vitro. It is likely that future major progress will accrue from the development of the techniques to the point when they can be applied to intact, functioning tissues. In other disciplines there have been advances in techniques for visualising receptors and enzymes and in synthetic methods for determining the structure of complex molecules.

8

Techniques

Routes of drug administration The route of drug administration is particularly important in studies of the actions of drugs on the brain since access of drugs to this structure may be limited. The access via the bloodbrain barrier is discussed in more detail in Chapter 6. Systemic administration The easiest way to give a drug is into the general circulation. An intravenous injection will give a rapidly equilibrating concentration in the blood. The blood levels attained during experiments can be compared with the therapeutic levels attained in man and in this way the relevance of experimental data to the therapeutic situation can be assessed. Oral administration, or other routes such as subcutaneous or intra-peritoneal injections may also be employed when appropriate but absorption via these routes is likely to be slower and more variable. Many of these routes may be adequate for study of peripheral mechanisms but may not be appropriate for the study of the actions of some drugs upon the central nervous system to which they may not have easy access via the blood stream. Clinically useful drugs will usually have access since clearly there is no other way in which they can be administered except occasionally under very supervised conditions, e.g. by direct injection into the cerebro-spinal fluid. Other substances, such as most neurotransmitters, have a very limited ability to cross the blood-brain barrier and so other means need to be found to administer them in order to study their effects. There are disadvantages with systemic administration when attempting to define the locus and mechanism of drug effects with any precision, because the drug may be exerting its effect indirectly on that part of the system under study. It may even be producing an effect on CNS function by changing the inflow of sensory information. Local administration A more restricted locus of drug action may be obtained by a variety of techniques. These include injection into or perfusion of the cerebro-spinal fluid (CSF), local irrigation of superficial parts of the brain with solutions of known composition, local

Routes of drug administration

9

perfusion of deeper areas by perfusion through 'push-puir cannulae, microinjection of small amounts into defined areas and microelectrophoresis, microelectro-osmosis or pressure microejection from micropipettes. The most restricted site of drug action is probably attained by those techniques in which the substance is ejected from micropipettes. All of these techniques suffer the disadvantage that the concentrations attained in the tissue are unknown, although they can often be estimated, but the rate of administration or the amount injected can be regulated and monitored. The more localised the administration the more uncertain is the concentration which is effective. This is because a concentration gradient is set up with the highest concentration at the focus of the injection and concentrations falling off exponentially with distance at a rate dependent upon the diffusion of the substance in the medium and factors affecting its rate of removal. It has commonly been assumed that the microinjection of small quantities, when small is usually defined as a few micrograms, produces low concentrations. It can readily be calculated that in fact within millimetres of the injection site the concentration can be in the millimolar range. In pharmacological terms such millimolar concentrations are usually considered to be high. This is the penalty that one perhaps has to pay for localised administration. In contrast, low, known concentrations can be established by in vitro techniques in which parts of tissues are bathed in solutions containing known drug concentrations. However, such techniques have their own but quite distinct limitations. Even when slices are superfused with solutions of known concentrations, the concentration at the receptor sites within the tissue will be lower than that in solution. It is still uncertain how far one may extrapolate data obtained from the highly artificial environments of slices and cultures of tissue, particularly the latter in which the cells may have changed their properties during the culturing process, to the functioning nervous system in vivo. Furthermore, there is the major drawback that much but not all of the functional integrity of the nervous system, upon which the nature of the system depends, is lost in these isolated systems. Nevertheless, the techniques have permitted answers to be obtained to

10

Techniques

some very fundamental questions. Provided that the assumptions made in extrapolating from data obtained with in vitro techniques are always kept in mind, then the in vitro techniques have undoubtedly led to some important advances and will continue to do so for some time yet. It may be concluded that no one technique for drug administration is ideal or will provide all of the answers that are needed. Advances will continue to be made by laboratories working with a variety of techniques. Electrophysiological methods The electrical activity of the nervous system can be observed in many ways. It is a relatively simple matter to record action potentials in peripheral nerves: in recent years it has been possible to record from or stimulate single nerve fibres in man. It is also relatively easy, even in man, to record spinal reflexes with electrodes placed upon the peripheral nerves. However, such recordings will generally tell us little about how a drug can produce a change in the record. The recording of the electrical activity of large areas of the brain, expressed as electroencephalograms (EEGs) or as evoked potentials has also been widely used and is of some value in the diagnosis of disorders or in the predictions of possible clinical applications of new drugs. However, EEGs and evoked potentials tell us little about the precise localisation or mechanism of drug actions. Computer technology has been applied to EEG recording, and computer-generated maps of activity in the brain may now be obtained. However, it must be remembered that an EEG represents the summed activity of many neurones in a relatively large part of the brain: asynchronous neuronal activity is therefore largely unobserved. At the next level of resolution, electrolyte-filled microelectrodes have been used for several decades to record the activity of single nerve cells in situ. These recordings may be obtained from the just extracellular environment and consist of recordings of action potentials. These recordings cause the least change to the neurones under observation, but are limited to recordings of the changes in the frequency or latency of firing of the cells. Since the action potential is the only line of fast com-

Electrophysiological methods

11

munication in the nervous system, this limitation may not be so important. Nevertheless, it does not allow a deeper understanding of the intracellular and membrane processes involved in drug action. Major advances in this area have been made with studies in which the pipette is stationed inside the cell membrane (intracellular recording). The technique can be employed in vivo as well as in vitro. It is possible to record either the voltage changes across the membrane or the current fluxes when the membrane voltage is 'clamped' to known potentials. Voltage clamp techniques have considerably advanced in recent years to the state at which the electronic circuitry will now stabilise voltage changes occurring at frequencies up to about 30 kHz, a frequency which is at the upper end of the biological frequency range. In principle, the voltage clamp technique is quite simple. The potential is monitored with a micropipette stationed inside the cell. Any tendency to change is counterbalanced by ejecting a current through either the same micropipette, or an independent pipette, of such a magnitude and polarity as to just counterbalance the change: this is the technique of negative feedback. The amplitude of the feedback current is equal and opposite to the change in membrane current induced by changes in membrane properties. The monitoring of membrane currents and voltage enables us to estimate the changes in electrical resistance and conductance of the cell membrane to ions. The great advantage of monitoring changes in membrane currents, rather than membrane voltage, is that it is now known that certain membrane ion conductances are sensitive to changes in membrane voltage. Voltage clamping enables studies to be made without needing to consider this complicating factor. Such techniques have given insight into the manner in which neurotransmitter substances may control neuronal activity. The final stage of resolution has been achieved in recent years by attaching a very small piece of cell membrane to a micropipette orifice with a very tight seal. This is the technique of fc patch-clamping\ The small area of membrane ensures that only one, or at most a few, ion channels in the membrane will be conducting (open) at any one instant. The tightness of the seal between membrane and micropipette ensures that the small current

12

Techniques

flowing through the single ion channel can be measured. Although most studies have been carried out on pieces of membrane which have been detached from the cell, it is also possible to make whole cell patch clamps. It has not so far been possible to make useful studies in vivo. It is possible to change the composition of the medium on either side of the membrane. The technique has given much insight into the changes in single ion channel conductances which accompany neurotransmitter action. Biochemical and histochemical techniques One of the most widely used techniques developed in the last decades is the ligand binding technique. Specific, saturable binding of specially prepared ligands to binding sites in nervous tissue has yielded a great deal of information on drug receptor interactions. However, there is still a tendency to equate ligand binding with receptor binding and this has led to a certain confusion and in some cases to an unjustified multiplicity of postulated receptor sites. Awareness of the problem should ultimately lead to a solution. The important thing to remember is that a binding site cannot be assumed to be the same as a receptor site until it has been unequivocally shown that the occupation of that binding site correlates with changes in tissue response which are identical to those caused by receptor occupation. Histologists have been most active in devising methods for visualising binding sites in nervous tissue. In this they have been greatly aided by the development of immunohistochemical methods for labelling binding sites with immunoreactive substances. The specificity of the antibodies used has in turn been dependent upon advances in cloning techniques for preparing monoclonal antibodies. In general, the results have corroborated those obtained with more conventional techniques but they have allowed extensive studies to be made of the distribution of binding sites for polypeptides for which no other techniques were available. Biochemical techniques now being used to study the nervous system include the countless studies on the role of cyclic nucleotides and the more recent studies on the phosphoinositol system,

Biochemical and histochemical techniques

13

which may be involved in the action of various neurotransmitters, especially in the mediation of the muscarinic actions of acetylcholine. Among the most recent developments are the techniques of CAT (computer aided tomography) and PET (positron emission tomography) scanning. These techniques are improving our knowledge of brain structure and changes in disease. Both techniques rely heavily upon sophisticated computer technology to build up complex three-dimensional images of some aspects of brain structure or function. PET scanning has the potential to tell us about changes in function in selected neuronal systems in living subjects. Although in their infancy, high hopes are held by some that these techniques will provide important answers to questions which are at present incapable of being answered by other techniques. Nevertheless, some caution must be expressed since the technology is exceedingly expensive and its use requires careful justification. Of course, there are many other techniques being used by biochemists to study the function of the nervous system and the action of drugs on it, but these are too numerous to mention here: reference to them will be found at appropriate places in the text.

Neuromuscular junction

The neuromuscular junction between motor nerve fibres and skeletal muscles is a good place at which to begin a study of the ways in which drugs can influence the processes of synaptic transmission. Other synapses may be considerably more complex. At the neuromuscular junction the system is both anatomically and functionally relatively simple and a functional system can be isolated from the whole organism. The majority of mammalian skeletal (striated) muscle fibres are focally innervated by motor axon terminals, each one forming a single junction on any one muscle fibre at the motor end plate. A motoneurone in the ventral horn of the spinal cord gives rise to a single axon which branches within the muscle to innervate from one to six muscle fibres, these groups of muscle fibres together comprising a single motor unit. This motor unit therefore behaves as a single functional entity (Fig. 3.1). The large, extrafusal muscle fibres which generate the major component of the force developed in muscle contraction are innervated by large a-motoneurones through myelinated A-fibres. The small intrafusal fibres of the muscle spindles are innervated by the small y-fibres of motor nerves. Some of the muscle fibres in amphibia and in birds, e.g. the rectus abdominis muscle in the frog and the biventer cervicis muscle in the chick respectively, have multiple endings upon them and this results in important differences in their physiological responses to nerve stimulation and in their pharmacological responses to drugs.

15

Neuromuscularjunction

Motor end plate Axon

Synaptic vesicles containing ACh Mitochondrion

Myelin sheath

Myofibrils

.. Spinal motoneurone Peripheral motor axon Single end plates Motor unit (focal innervation) Terminal branches Individual muscle fibres

Multiple innervation Separate motor axons

Multiple end plates

- Muscle fibre

Fig. 3.1. Innervation of skeletal muscle. After Couteaux, R. (1958). Exp. Cell Res., Supply 5, 294.

16

Neurom uscularjunction

Techniques There are many ways in which the actions of drugs may be studied upon neuromuscular transmission. These range from the simplest of techniques, in which recordings are made of the contractions of the muscle to stimulation of the peripheral end of the sectioned motor nerve and of the responses to administered drugs, either in vitro or in vivo, to the most sophisticated modern techniques of intracellular or patch clamp recording. The electromyogram (EMG) can be recorded in vivo via a metal electrode inserted into the muscle. Depending upon the size of the exposed electrode tip, recordings of either gross muscle action potentials or of the action potentials generated by a single motor unit can be obtained. When the muscle is maintained in isolation, perfused with physiological salt solution, then intracellular recording of end plate, miniature end plate and muscle action potentials becomes possible with fine capillary microelectrodes filled with potassium chloride solution. In order to record the miniature end plate potentials (mepps) or end plate potentials (epps), the micropipette must be located close to the end plate region. Modern voltage clamp amplifiers are able to clamp the fast voltage transients of these intracellular potentials (see Chapter 2) thus enabling the currents flowing through the membrane to be determined. The techniques of noise analysis and of patch clamp recording have been described in more detail in the previous chapter but were first applied in studies of transmission at the neuromuscular junction, and have greatly increased our understanding of the way in which transmission is effected and of the ways in which it is modified by drugs. Synaptic transmission The transmitter at the neuromuscular junction is acetylcholine (ACh) which is packaged in vesicles in the presynaptic terminals. The arrival of an action potential in the terminal causes a transient increase in the permeability of the terminal membrane to Ca2+, which in turn precipitates the simultaneous liberation of ACh from a number of vesicles. The ACh content of each vesicle produces a single miniature end plate po-

Synaptic transmission

17

tential (mepp), after diffusing across a narrow synaptic cleft of about 150 A in width and combining with specific receptors located at the end plate region on the postsynaptic membrane. The simultaneous release of many vesicles, of quanta, of ACh results in the production of the much larger end plate potential (epp). In normal muscles the receptors for ACh are only found at the junctional regions of the muscle fibres, but after denervation or before innervation during development, the receptors are far more diffusely located over the muscle fibre membrane. The activation of the receptors by ACh gives rise to local changes in membrane permeability and to local changes in membrane potential which do not propagate along the muscle fibres but only decay electrotonically, i.e. the miniature and end plate potentials decrease exponentially with distance along the fibres, the rate of decrease in amplitude being dependent upon the properties of the muscle fibres. In muscle fibres which are focally innervated, the epp gives rise to a propagated muscle action potential which in turn invades the transverse tubular system, to produce a translocation of Ca from the sarcoplasmic reticulum, so activating the contractile mechanism. At low frequencies of stimulation there is evoked a single, all or nothing twitch, but at higher frequencies of 30 to 100 Hz the twitches fuse to produce a tetanic contraction, many times the amplitude of the all or nothing twitch. The tetanic contraction at high frequencies of stimulation is due to the incomplete relaxation of the sliding filaments between each arriving nerve impulse. The transmitter at multiple innervated junctions is released from many sites on each fibre and at each site there is a local non-propagated epp which does not give rise to a propagated action potential. Instead the epps summate to produce slow and graded depolarisations, evoking in turn slow and graded contractures of the muscle fibres. After dissociation of the ACh-receptor complex, the ACh is hydrolysed by the enzyme acetylcholinesterase which is located predominantly in the postjunctional folds of the muscle fibre membrane. The by-product, choline, is either removed in the circulation or else taken up again by the nerve terminals by an active transport process: choline is the precursor of ACh synthesised by

18

Neuromuscularjunction

acetylcholinesterase. In the absence of electrical activity in the motoneuronal axon, individual vesicles release their content of ACh in a random fashion, and each unit amount of ACh produces a quantal change in the membrane potential. The amplitudes of the spontaneous miniature potentials range in size from about 0.1 to 0.7 mV, due to random liberation of single or multiple quanta of transmitter, and there are slight variations between different species. Each quantum probably represents the ACh released from a single vesicle. When a nerve impulse arrives in the terminal there is a sudden release of ACh from about 100 to 200 vesicles, causing a large epp to occur after a delay of about 0.5 ms. The synaptic delay is made up of a short period for the ACh molecules to diffuse across the cleft (about 0.15 ms) and a somewhat longer period required for the release process to be completed. The acetylcholine receptor Peripheral receptors for ACh fall into two types, characterised by their biochemical and pharmacological properties. Muscarinic receptors are activated by ACh itself, by some other choline esters such as acetyl-p-methylcholine and by the cholinomimetic substance muscarine. These muscarinic receptors are blocked by the antagonist atropine. Muscarinic receptors are found in effector organs innervated by postganglionic parasympathetic autonomic fibres and at a few Fig. 3.2. The acetylcholine receptor complex. Glycoprotein subunits a

65 000 /

NA_/

/

\ 40 000

50000 Hydrophilic pore

The acetylcholine receptor

19

other locations, including the central nervous system, as discussed elsewhere in this book. Nicotinic receptors are found at the neuromuscular junction and in autonomic ganglia and at a few sites in the central nervous system. At the periphery, the nicotinic receptors are activated by ACh and by nicotine, but not by acetyl-p-methylcholine, and are blocked by curare-like substances at the neuromuscular junction or by hexamethoniumlike substances in autonomic ganglia. It is therefore evident that there are differences between those nicotinic receptors found at the neuromuscular junction and those located in ganglia. This is further exemplified by the fact that the receptors at the neuromuscular junction are selectively activated by phenyltrimethylammonium, whereas those in ganglia are selectively activated by dimethylphenylpiperazinium. The nicotinic ACh receptor has been biochemically isolated in relatively abundant quantities from the electroplaque organ of the Torpedo; this organ is composed of stacks of motor end plates, arranged as a series of batteries, and capable of generating upon command from the central nervous system rather unpleasantly high voltages to shock unwary intruders. Immunological studies indicate that the receptor isolated from Torpedo is virtually identical to those found elsewhere. The nicotinic ACh receptor is a complex of four different glycoprotein subunits (a,p,y,5), arranged in a rosette of five subunits in which the a-unit is represented twice. The overall molecular weight of the complex is about 250 000 daltons and the molecular weight of each subunit is about 40 000 to 65 000 daltons. The subunits are each translated from a separate messenger RNA on ribosomes of the rough endoplasmic reticulum. The five subunits in the receptor are arranged around a central hydrophilic ion channel in the muscle membrane (Fig. 3.2). The toxin abungarotoxin, about which we shall hear more later, and other cholinergic receptor ligands bind with high affinity to the asubunit but the role of the other subunits is unknown. There is a very high density of receptors at the end plate region. The subsynaptic density is about 104 receptor complexes per square micrometre and the receptors extend, at reduced densities, for about 400 juim from the end plate.

20

Neurom uscularjunction

Activation of the receptor The interaction of ACh, or of any other suitable agonist, with the nicotinic receptor causes an increase in the permeability of the postjunctional membrane to cations, especially to Na + and K\ with a size limit of not more than twice the size of the hydrated K+ ion. The influx of Na+ ions causes a depolarisation of the postsynaptic membrane. The amplitude of the depolarisation depends upon a number of factors, including the concentration of ACh in the region of the receptors, the receptor density and the duration of the lifetime of the ionic channels opened by the transmitter-receptor interaction. The smallest mepp observed at the end plate has an amplitude of about 0.3 mV and corresponds to the net effect resulting from the opening of many channels activated by the release of the transmitter content of one vesicle, i.e. about 10-50 000 molecules of ACh. There have been two major advances in the last decade which have greatly increased our understanding of the molecular events accompanying receptor activation at the neuromuscular junction. These advances have utilised the two new techniques of noise analysis and patch clamping described in Chapter 2. Katz & Miledi were the first to approach this problem with noise analysis, which is essentially a statistical analysis of the small variations in membrane potential or current observed with the now relatively gross technique of intracellular recording at the end plate region. If each interaction between ACh molecules and receptors causes a constant and equal channel opening, so permitting a fixed charge to cross the membrane, the depolarisation produced by this unitary event (shot effect) is given by the expression: a = 2EVV mV where 'a' is the elementary depolarisation, E2 is the statistical variance of the membrane potential about the mean value of the depolarisation (V) caused by the external application of ACh. It will be noticed that V is independent of the concentration of ACh employed and this was confirmed experimentally. The amplitude was about 0.3 jwV in frog muscle and 0.7 jaV in rat muscle. The current flowing through an open channel is of the order of 2

Activation of the receptor

21

Table 3.1. Membrane noise and depolarisers of the neuromuscular junction (frog sartorius) a ACh Carbachol Suberyldicholine Acetylthiocholine Decamethonium

JJV

0.3 0.1 0.4 0.08 0.05

rms 1.0 0.3-0.4 1.65 0.12 0.1

After Katz & Miledi (1973). a is the elemental shot-effect; r is the duration of opening of the ionic gate.

pA and is independent of the cholinomimetic employed. However, different agonists evoke different amplitudes of elementary depolarisations (see Table 3.1). This is due to the fact that the duration of the channel opening produced by different agonists is not the same, as is also shown in the table: the duration of the channel opening was measured from the spectral density of the current noise recorded with a focal electrode . For ACh the value of Y, the channel open time, was 1 ms. The passive electrical properties of the cell membrane slow down the processes of depolarisation and of repolarisation so that the time constant for the decay of the depolarisation produced by ACh is about 10 ms, i.e. it is much longer than the channel open time (Fig. 3.3). The high potency of carbachol in depolarising the membrane, despite the small magnitude and brief time course of the elemental depolarisation, is due to its resistance to acetylcholinesterase, possibly allowing repeated interactions of a single agonist molecule with the receptor. Recording of membrane current noise under voltage clamp conditions, rather than of membrane depolarisation, gave direct measurements of the open channel lifetimes which agreed with the earlier results of Katz & Miledi in their pioneering studies. Investigations of the molecular events at the neuromuscular junction have now reached a new level of sophistication with the

22

Neurom uscular junction

recent introduction by Neher & Sakmann of the elegant technique of patch clamping. With this technique the amplitude and duration of the molecular events initiated by a single ACh-receptor interaction can be observed and measured directly, rather than by statistical inference. In general, the main conclusions reached by Katz & Miledi have been confirmed. However, the technique has also revealed other interesting phenomena for which there is at present no certain explanation. Among the more unexpected findings there are some which seem to show that there are multiple conductance states of a single ion channel and that there are 'flickering' transitions between open and closed states. Clearly this new technique is bringing a wealth of new information which may in the future bring new insights into the molecular events accompanying drug receptor interactions. Already these studies at the neuromuscular junction have spurred on efforts to study similar phenomena at other synaptic junctions, particularly those in the central nervous system. Sites of drug action at the neuromuscular junction Drugs affecting neuromuscular transmission may act at one or more of three possible sites and these are summarised,

Fig. 3.3. Diagram showing single channel currents flowing when recordings are made from one (upper record) or two (lower record) channels simultaneously. The large current steps, labelled c, are the algebraic sums of the two individual steps, a and b.

Sites of drug action at the neuromuscularjunction

23

together with examples in Fig. 3.4: i) On axonal conduction of the impulse into the nerve terminal, ii) On presynaptic terminals, affecting transmitter storage, release or synthesis. iii) On the postjunctional cell, either at the end plate or on the contractile mechanism at a step beyond the end plate. Therapeutically useful drugs which cause muscle relaxation are to be found among those acting on the motor end plates, producing either non-depolarising blockade of transmission or depolarising, desensitising block. A therapeutically useful facilitation of transmission is produced by neostigmine or germine, acting either as inhibitors of acetylcholinesterase or by facilitating transmitter release (see below). Actions at other sites are produced by a variety of toxins and venoms and sometimes as a side-effect during therapy with certain antibiotics. Prejunctional drug action Conduction. Although local anaesthetics block nerve conduction, they non-selectively block fast, voltage-dependent, sodium channels in all excitable tissues and so will not be considered here. Anticholinesterase agents and, in particular, neostigmine, are used in the treatment of myaesthenia gravis. The beneficial facilitation of transmission produce is most likely due to a number of factors. The inhibition of acetylcholinesterase reduces the breakdown of released ACh and so facilitates transmission when the concentration of the transmitter is rate limiting. However, neostigmine also causes a repetitive firing of action potentials both in the nerve terminals and in the muscle fibre in response to a single motor nerve volley. The depolarisation of the terminal is sufficient to cause impulses to propagate antidromically up the axon. Although the details of the mechanism are not certain, it does not seem to be related to inhibition of cholinesterase. Similar repetitive firing of action potentials is known to be caused by germine, which is not a cholinesterase inhibitor but is also useful in myaesthenia. Thus both inhibition of acetylcholinesterase and repetitive firing of action po-

24

Neuromuscularjunction

tentials may contribute to the mechanisms by which these drugs are effective in myaesthenia gravis. Germine, used as the monoacetate, is a veratrum alkaloid. It does not change the sensitivity of the muscle to ACh nor does it change the frequency of mepps or the amplitude of epps in doses which cause repetitive firing in frog muscle, but there may be a slight increase in mepp frequency in the diaphragm muscle of the rat. The repetitive firing is probably due to a delayed closure of Na+ channels, although other mechanisms have not been excluded. Tetrodotoxin blocks neuromuscular transmission by blocking the voltage-dependent fast Na+ channels in all excitable membranes, including the presynaptic terminals and muscle fibres, so preventing the initiation of action potentials. Fig. 3.4. Sites and structures of drugs acting at the neuromuscular junction. Presynaptic Repetitive activation:

neostigmine

germine CH 3

OCN(CH 3)2

S

JL.CH3

(CH3)3

^J^^^ O H

CH 3 OH

X

OH

OH Synthesis: diphenylbutyl acetate, hexamethylene-l,4-(l-naphthylvinyl)pyridinium-6-trimethylammonium hemicholinium (HC-3)

Uptake:

CH 3 CH 3 Storage: black widow spider venom, l-bungarotoxin Mobilisation: neomycin, streptomycin Release: botulinus toxin

CH3 CH 3

Sites of drug action at the neuromuscular junction Postsynaptic Cholinestcrase inhibitors: edrophonium

neostigmine O II OCN(CH 3 ) 2

^V/^N + (CH 3 ) 3

OH

^X^N-C2H CH3

Competitive blocking agents (reversible): d-tubocurarine (d-TC) OH

CH 3 O

X

Q

OCH3

CH3 CH3

pancuronium

O-CH 2 CH 2 N + (C2H5)3 -O-CH 2 CH 2 N + (C 2 H 5 ) 3 ^O-CH 2 CH 2 N + (C 2 H 5 ) 3

Competitive blocking agents (non-reversible): a-bungarotoxin Desensitising blocking agents: decamethonium (CH3)3N+(CH2)1ON+(CH3)3

succinylcholine

(CH3)3N+CH2CH2OCCH2

I

(CH3)3N+CH2CH2OCCH2 O Metaphilic antagonists (reversible): dinaphthyl decamethonium (DNC-10). Metaphilic antagonists (non-reversible): DNC-10 mustard

25

26

Neuromuscularjunction

Synthesis of transmitter. Choline acetyltransferase is blocked nonselectively by a number of substances such as diphenylbutyl acetate and some styrylpyridine analogues. The most potent of the latter substances is hexamethylene-1, 4-(l-naphthylvinyl)pyridinium-6-trimethylammonium, but this substance blocks acetylcholinesterase in vitro to about the same degree. The block of neuromuscular transmission which occurs in vivo is probably related to a direct effect on the contractile mechanism rather than to an inhibition of choline acetyltransferase. Inhibition of choline uptake. Hemicholinium (HC-3) is a quaternary ammonium substance which prevents the synthesis of ACh by blocking the uptake of the precursor, choline, into the terminal. In early experiments with HC-3 it was noted that intravenous injection in rabbits caused muscle relaxation and respiratory paralysis which was aggravated by exercise. It was erroneously thought that the effect was due to an action on the central nervous system, even though such a quaternary compound, being fully ionised, would be unlikely to cross the blood-brain barrier. Since synthesis of ACh is only rate limiting for transmission in normal muscles at high rates of stimulation, HC-3 has little effect on contractions of a muscle excited through its nerve at low frequencies of stimulation but there is a marked decline in the twitches if the rate of stimulation is increased to about 1 Hz and the reduction can be effectively counteracted by the intravenous administration of choline. This indicates that HC-3 blocks transmission by competing with choline at uptake sites on the terminal membrane. The conclusion is supported by the observation that the sizes of mepps as well as epps are reduced by HC-3, that the measured output of ACh on stimulation is diminished, and by biochemical determinations of choline uptake in the presence and absence of HC-3 in vitro. At high doses, HC-3 may also have a postjunctional, curare-like effect. Triethylcholine is transported almost as readily as choline and blocks transmission in a manner similar to that of HC-3. An earlier suggestion that triethylcholine might be acetylated, stored and then subsequently released as a false transmitter is only of historical interest as the first reported suggestion that substances

Sites of drug action at the neuromuscularjunction

27

taken into nerve terminals could act in this way. In fact, attempts to isolate the acetylated derivative have been unsuccessful. Tetraethylammonium (TEA) also has an action like that of triethylcholine but is better known for its ability to block some potassium channels. It may also have some postsynaptic blocking action, especially pronounced in autonomic ganglia. Storage of ACh. A number of venoms are known to prevent the storage of ACh in the presynaptic vesicles. In frog muscle, black widow spider venom causes at first a great increase in the number of mepps recorded with an intracellular electrode and this is followed by a block of transmitter output and a block of neuromuscular transmission. Electron microscopy has shown that at this stage the vesicles have disappeared. In cats, similar effects are followed by a complete disintegration of the terminals. (3-Bungarotoxin, obtained from the venom of a Formosan snake, and not to be confused with a-bungarotoxin which has a postsynaptic site of action, has a presynaptic action akin to that of the venom of the black widow spider. Mobilisation of transmitter. Not all of the ACh in the terminal is immediately available for release, much of it needing first to be 'mobilised'. Mobilisation, which may involve the movement of vesicles up to the release site on the presynaptic membrane, is dependent on the frequency of stimulation, is increased by an increase in extracellular Ca2+ or by depolarisation of the terminals by potassium ions. Some antibiotics, notably neomycin, streptomycin and kanamycin, occasionally cause muscle weakness during antibiotic therapy; there is some evidence to suggest that the weakness is caused by a reduction in transmitter mobilisation leading to a decrease in the quantal content of the epp. Release of ACh. A highly toxic substance with a molecular weight of about 60 000 is produced by the anaerobic bacterium Clostridium botulinum. Botulinus toxin produces neuromuscular paralysis by virtue of the fact that it very effectively prevents the liberation of ACh from the terminals without destroying the

28

Neuromuscular junction

vesicles. It therefore acts upon the release process. The fatal dose for a mouse is about 2 X 10"10 g kg"1. Making the assumption that a mouse has about 106 muscle fibres, Burgen has estimated that the fatal dose of the toxin would correspond to an availability of about 40 molecules of the toxin at each end plate! Even though the toxin is poorly absorbed from the gastrointestinal tract, the high potency is sufficient to ensure that is is easy to ingest toxic quantities. The toxic effects of botulinus toxin are attributable to a paralysis of cholinergic systems throughout the body, including not only those at the neuromuscular junction but also those of the autonomic nervous system. The symptoms include paralysis of eye muscles, causing diplopia, ptosis, dilated pupils and an inability to accommodate. Later there is muscle weakness, respiratory distress, difficulty in swallowing or speaking, constipation and urinary retention. The toxin does not affect the sensitivity of the membrane to ACh. There is no effect on the arrival of the impulse in the terminal nor is there any effect on the quantal size of the mepps. However, the frequency of the spontaneous mepps is greatly reduced and stimulation of the motor nerve elicits only a small or no epp and the assayed output of ACh declines. This is all consistent with the proposed mechanism of action. Simpson has suggested that the action of the toxin is dependent on ACh release. The block of transmission is prevented by maintaining a low concentration of Ca2+ in the medium to block ACh release, or by a maintained depolarisation of the terminals. However, it is more likely that the toxin blocks Ca2+ entry to the terminal, upon which the ACh release is dependent, or else changes the coupling between calcium entry and release from the vesicles: the block may be alleviated by the use of calcium ionophores, which promote the entry of Ca2+ or by TEA which prolongs action potentials by blocking repolarising potassium currents, so increasing the voltage-dependent Ca2+ entry. Guanidine sometimes alleviates muscle weakness in myaesthenia. It increases epp amplitude but there is no change in the size or frequency of the mepps. There is no effect on the

Sites of drug action at the neuromuscularjunction

29

postjunctional sensitivity of the membrane to ACh or on resting membrane potential or resistance. As the effect is calcium dependent it seems likely that guanidine increases the ability of nerve impulses to release ACh from the nerve terminals. Postjunctional drug action Drugs causing a block of neuromuscular transmission are used as adjuvants in surgery to achieve muscle relaxation, especially in abdominal surgery, or to prevent severe convulsions in tetanus or those caused by electroconvulsive therapy (ECT) in the treatment of psychiatric disorders. The useful drugs (Fig. 3.4) are either non-depolarising (competitive) or depolarising (desensitising) blocking agents: the metaphilic antagonists are only mentioned here as an example of a possibly different type of action but these drugs are not clinically useful. d-Tubocurarine (d-Tc) is a synthetic drug which is the active principle of the South American Indian arrow poison. In addition to blocking the nicotinic receptors at the neuromuscular junction, it also blocks those in autonomic ganglia, resulting in a fall in blood pressure, decreased tone and motility of the gastointestinal tract and a block of vagal actions. Histamine release can be an important problem which can lead to hypotension, bronchospasm and an increase in bronchial and salivary secretion in man. These effects may be reduced by the administration of antihistamines. The search for alternative drugs has concentrated on a reduction of these undesirable side-effects, together with different durations of action. Gallamine is a short-acting, purely synthetic drug with a minimal effect on sympathetic ganglia but it may nevertheless cause tachycardia by reducing vagal effects by a mechanism which is not certain. Pancuronium, a synthetic steroidal neuromuscular blocking drug, is more potent than d-tubocurarine and has a similar duration of action but does not block ganglia or release histamine. Fazadinium, (AH 8165), belonging to the azo-bisaryl-imidazopyridinium series has an action which is brief in du-

30

Neuromuscularjunction

ration and rapid in onset with minimal effects on the cardiovascular system: the brief duration is due to rapid metabolism in the liver. The differential ability of some of these drugs to block nicotinic receptors at the neuromuscular junction but not those in autonomic ganglia re-emphasise the conclusion that the nicotinic receptors at these two sites are different. All of the drugs so far mentioned act as competitive antagonists to ACh at the receptor sites. Another group of drugs, typified by decamethonium (CIO) and succinylcholine, act differently and are classed as depolarising, desensitising agents. A characteristic of the depolarising agents is that they produce an initial muscle fasciculation before the onset of neuromuscular blockade. This uncoordinated contraction of many muscle fibres may cause physical damage and result in muscle pain postoperatively. CIO has minimal effects on ganglia or on the release of histamine and therefore has minimal effects upon the circulation. Succinylcholine has a very brief duration of action because it is an excellent substrate for plasma cholinesterase and so is rapidly degraded. However, in some patients with very low levels of the esterase, there may be a very prolonged action if the drug is used in the normal dosages. These agents depolarise not only skeletal muscle but also autonomic ganglia. On vagal ganglia the depolarisation may be sufficient to cause bradycardia and hypotension. Hexafluorenium has a mild neuromuscular blocking action and in addition inactivates plasma cholinesterase. Benzoquinonium has characterisitics of both the competitive and the depolarising blocking drugs and additionally inhibits cholinesterase. It is therefore used to prolong the action of succinylcholine. a-Bungarotoxin is a toxic rather than therapeutic drug but it has been of great importance experimentally for the isolation of the nicotinic receptor with which it combines irreversibly to form a very stable drug-receptor complex. Mechanisms of neuromuscular blockade by competitive receptor an-

tagonists. The theory of drug-receptor interaction based upon the

Sites of drug action at the neuromuscularjunction

31

law of mass action predicts that in the presence of a reversible, competitive antagonist the log dose-response curve for an agonist will be shifted to the right, the curve will remain parallel to the control curve obtained in the absence of the antagonist and the maximum response of the tissue will be unchanged. Curarelike drugs should therefore be expected to conform to these criteria since they are believed to be competitive, reversible antagonists of ACh at the nicotinic receptor at the motor end plate. Dose-response curves for ACh are easily obtained upon the frog rectus muscle preparation. At low concentrations of d-Tc producing dose ratios for the agonist (ACh) of about 100 the curves remain reasonably parallel and the maximum response does not change appreciably. Thus, at these concentrations of antagonist there seems to be a competitive interaction with ACh at the receptor. At higher concentrations there is a progressive departure from parallelism and the maximum response declines. On the frog sartorius muscle in which Jenkinson compared the depolarisation of the end plate produced by carbachol in the presence and absence of d-Tc, the departure from parallelism and the reduction of the maximum response to ACh was even more marked at relatively low concentrations of the antagonist. In the tibialis muscle of the cat, Paton & Waud showed that gallamine caused very marked discrepancies from the anticipated curves. The situation in such experiments is complex and difficult to explain simply in terms of competitive antagonism but it was suggested by Paton & Waud that the response to agonists may not be equilibrium responses and that the antagonists in some way alter the equilibria. Thus, shifts in dose response curves in this instance does not give unequivocal evidence for or against the competitive nature of the anticholinergic action of d-Tc or gallamine. Evidence showing that the interaction of d-Tc with ACh at the motor end plate is competitive in nature was first obtained by Katz & Miledi in their experiments on membrane noise. They predicted that if the antagonism was competitive then the amplitude of the elemental shot effect produced by the interaction of one molecule of ACh with the receptor should be

32

Neuromuscularjunction

unaffected by the presence of the antagonist. They clearly showed that neither d-Tc, which is reversible, nor abungarotoxin, which is not, changed the amplitude of the ACh shot effect, fca\ The reduction of the depolarisation produced by a given concentration of ACh is therefore due to a lower frequency of activation of the receptors. At low frequencies of activation of the neuromuscular junction it is certain that competitive antagonism of ACh at the receptor site is the major mechanism involved and this is supported by the failure to show any reduction in ACh output by direct assay. However, the results obtained from direct assays of the amount of ACh released in the presence of a cholinesterase inhibitor, required to prevent hydrolysis by acetlcholinesterase, must be subject to a note of caution. Furthermore, there is a large component of the assayed ACh which is released in a non-quantal fashion, perhaps from non-vesicular sites, and this may mask any small change in quantal release produced by the antagonist. There is electrophysiological evidence that d-Tc causes a reduction in quantal release when the motor nerve is stimulated at higher frequencies. This prejunctional effect may therefore also contribute to neuromuscular paralysis under physiological conditions. If so, then it seems likely that there are prejunctional nicotinic receptors which facilitates the release of ACh at relatively high frequencies of stimulation. The interaction of ACh with its receptor and the reduction of the effect by a competitive antagonist can be represented as the molecular model shown below, which may later be compared with other mechanisms. ACh + R ^ AChR ^ ACI12R ^ ACI12R* dTc + R ^ dTcR In this model the interaction of ACh with the receptor is shown as a bimolecular interaction, in accord with the experimental evidence. R' represents the closed and R*' represents the open channel configuration of the receptor. Mechanism of neuromuscular block by depolarising agents. It will

be evident from some of the previous discussion that

33

Sites of drug action at the neuromuscular junction

decamethonium (CIO) and succinylcholine may act as agonists at the nicotinic receptor and that this fact must be considered in relation to the mechanism by which they block neuromuscular transmission. The discussion of the mechanism is complicated by the fact that these substances cause depolarisation, desensitisation and neuromuscular blockade and by the fact that there are differences in the effects obtained in different species or even between different muscles in the same animal. The systemic administration of CIO or succinylcholine in mammals causes a muscle fasciculation which is relatively shortlived and may be seen in the experiment illustrated in Fig. 3.5 as a slight elevation of the baseline tension immediately following the intravenous injection of 2 mg of succinylcholine. During this phase of the effect, the muscle twitches elicited by nerve Fig. 3.5. Contractions of the tibialis muscle of the cat to electrical stimulation of the peripheral stump of the sciatic nerve every 5 s. An intravenous injection of 1 mg of succinylcholine caused a facilitation of the contractions; a larger dose caused only a transient facilitation followed by a block of transmission during which tetanic stimulation at 30 Hz caused a small but maintained contraction. D-tubocurarine (0.5 and 1 mg) caused only a block of transmission during which a tetanus (100 Hz) was not maintained but gave rise to post-tetanic potentiation. Neostigmine (0.2 mg) reversed the blockade of neuromuscular transmission.

A

AT

ti 1 mg

2 mg 30 Hz

0.5 mg 1 mg 30 Hz 100 Hz

0.2 mg

34

Neuromuscularjunction

stimulation are at first enhanced but are later reduced and ultimately completely blocked. The fasciculation is a reasonably In 1951, Burns & Paton carried out experiments on the gracilis muscle of the cat with extracellular recording which suggested that the time course of the depolarisation by CIO in this muscle ran parallel to the time course of the neuromuscular blockade. This supported the idea that blockade of neuromuscular transmission was the result of excessive depolarisation of the end plate leading to an inactivation of the voltage dependent Na + channels in the surrounding, extrajunctional membrane, so preventing the initiation of propagated action potentials upon which the contraction of focally innervated muscle depends. However, the close correspondence between depolarisation and block is not seen on frog muscle (Fig. 3.6). Even in the early work on mammalian muscles it was evident that different muscles in the cat showed different pharmacological responses to decamethonium and that there were species differences. In all muscles of the monkey, dog, rabbit and rat and in the soleus muscle of the cat it was thought that decamethonium and succinylcholine produced a 'dual block', changing with time from a depolarising to a non-depolarising type. However, in the tibialis muscle and gracilis muscles of the cat and probably in some muscles in man the block was thought to be solely due to the prolonged depolarisation of the end plate. In 'Phase IV block in man, which occurs late during prolonged administration of depolarising agents, the block of neuromuscular transmission is relieved by anticholinesterase agents. Neostigmine does not relieve depolarisation or desensitisation block and the mechanism of Phase II block is unclear. One suggestion is that it is the result of the entry of the blocking agent into the interior of the muscle through open ion channels. In those muscles, for example frog muscle, in which depolarisation is clearly not the explanation of neuromuscular blockade Katz & Thesleff introduced the concept of receptor desensitisation. Desensitising block of the responses of frog muscle to ACh by a small, sustained application of ACh is illustrated in Fig. 3.6a. Here, the depolarisation produced by brief iontophoretic pulses of ACh are depressed by the sustained

Sites of drug action at the neuromuscularjunction

35

iontophoretic administration of a small quantity of ACh which produced only a transient depolarisation of the end plate. A possible molecular mechanism for such desensitisation could be: ACh + R - ACI12R* - AChRd Fig. 3.6. Desensitisation, depolarisation and neuromuscular block, (a) Iontophoresis of ACh on frog muscle. A prolonged (15 s) administration of ACh caused a depolarisation which was not sustained and the effect of repeated small administrations of ACh was reduced by the prolonged administration, even when the depolarisation produced initially was minute. After Katz, B. & Thesleff, S. (1967). J. PhysioL, 138, 63. (b) The intra-arterial injection of CIO in a cat (gracilis muscle) caused a block of neuromuscular transmission and a depolarisation of the muscle with only a slightly different time-course. After Burns, B.D. & Paton, W.D. (1951). J. PhysioL, 115, 41. (c) Bath application of CIO to frog muscle. The addition of decamethonium to the bath caused a sustained block of neuromuscular transmission but only a transient depolarisation. After Thesleff, S. (1955). Acta PhysioL Scand, 34, 218. In (a) and (c) the neuromuscular block is due to desensitisation but in (b) it is due to depolarisation. 5 m

ACh

ACh

ACh

ACh (c) Depolarisation Block

(b)

Depolarisation

Intra-arterial CIO

Block

Time

CIO in bath

3 Time

36

Neuromuscularjunction

in which Rd represents a desensitised configuration of the receptor associated with closed ionic channels, open ionic channels again being represented by R*. The second, desensitising phase of the reaction is considered to be a slow process compared with the first activation of the channel. Similar reactions could occur with the blocking agents. Changeux has characterised three affinity states of the ACh receptor isolated from Torpedo electroplaques. The highest affinity state, with a dissociation constant for ACh of 3 X 10~9 M, represented about 20% of the receptor population and corresponds to Rd. The lowest affinity state with a constant of about 10'4 M corresponds to R, the resting state of the receptor. Binding to the high affinity state, Rd, is facilitated by raising the Ca2+ concentration, which also increases desensitisation. It seems likely that binding to Rd blocks the sodium channel, rather than maintaining it in the resting state, but this type of channel blocking may be different from that produced by other channel blockers which bind to a different site in the channel. Although CIO enters the muscle fibre at the end plate region, the uptake, in contrast to the depolarisation, is not antagonised by d-Tc. It is unlikely that CIO acts intracellularly to block transmission, even though it is taken up. Using modern techniques, Wray (1981) has produced convincing evidence that, at least in the tenuissimus muscle of the cat, the blockade of transmission by CIO, succinylcholine or by high concentrations of ACh itself is due to depolarisation and not to desensitisation in this tissue. In rat muscles, which are less sensitive to the blocking agents, the mechanism is not due to depolarisation but to desensitisation. In man the mechanism seems to be depolarisation as in the tenuissimus muscles of the cat. A diagrammatic representation of the data obtained by Wray is shown in Fig. 3.7. At low concentrations of ACh (1 to 2 X 10"6 M) there was a maintained depolarisation of the end plate but a gradual decline in twitch tension attributable to a blockade of transmission. From his noise analysis records it was evident that there was a maintained increase in the frequency of opening of the ion channels by the agonist. At higher concentrations (5 to 10 X 10"6 M)

Sites of drug action at the neuromuscular junction

37

the frequency of channel opening at first increased and then rapidly decreased. This was accompanied, as might be expected, by a decrease in the mean level of depolarisation. Thus desensitisation was revealed in these experiments by a decreased frequency of opening of ion channels in the presence of high but not low concentrations of ACh, although block of transmission occurred at all concentrations in this range (2 to 10 X 10"6 M). The desensitisation had a relatively slow time course. Fig. 3.7. Diagram of depolarisation, channel opening and neuromuscular block in the tenuissimus muscle of the cat and rat in response to bath application of acetylcholine or succinylcholine. After Wray, 1981. /. Physiol, 310, 37. CAT Tension to nerve stimulation

Depolarisation of muscle fibres

Frequency of channel opening

RAT Tension to nerve stimulation

Depolarisation of muscle fibres

Frequency of channel opening

Low concentration

High concentration

38

Neurom uscularjunction

At concentrations which block transmission, CIO and succinylcholine depolarise by about 20 mV. Although the channel open time is shorter than for ACh (Table 3.1), the frequency of channel opening is greater so that the overall level of depolarisation is similar. Wray found that at concentrations which just blocked transmission there was no evidence of desensitisation, although it did occur, as with ACh, at higher concentrations. In similar experiments on rat muscles, which are less sensitive to the blocking agents, he has shown that blockade of transmission only occurred at concentrations which caused a decline in the frequency of channel opening: the mechanism in rat muscle is therefore desensitisation and not depolarisation. In conclusion, it will be appreciated that the mechanism of blockade of neuromuscular transmission by CIO and succinylcholine is complex. Both desensitising and depolarising block may occur, but the relative importance of these two effects depends both upon the particular muscle and the animal species. Metaphilic antagonism. Rang & Ritter found that the dinaphthyl derivative of decamethonium (DNC-10) only blocked the action of cholinergic agonists on frog and chick muscle in the presence of an agonist (Fig. 3.8). The rate of onset of antagonism was faster Fig. 3.8. (a) Lack of apparent effect of metaphilic antagonists after washout when desensitising agent is not administered during exposure to the antagonist, contrasted with the marked effect when both are present together (b). (a)

(b)

I Desensitising agonist Time -

Characterisation of blocking agents

39

if the agonist was given at frequent intervals and absent if no agonist was present during exposure to the antagonist. The effect of the metaphilic antagonist was greater with some agonists than with others. For example, the blocking action of DNC10 was greater when decamethylene-bis-trimethylammonium (C10TMA) was used as the agonist than it was with either CIO or carbachol. C10TMA itself caused more desensitisation than either carbachol or CIO. This type of antagonism was called metaphilic antagonism. It probably represents s special case of desensitisation in which the ligand binds only to the desensitised configuration of the receptor (Rd). Metaphilic antagonists have not yet provided any clinically useful compounds and have not been as extensively studied as other neuromuscular blocking agents. Pharmacological characterisation of neuromuscular blocking agents Neuromuscular blocking agents which act competitively, e.g. d-tubocurarine, can be distinguished from those which cause depolarisation, e.g. decamethonium, by a number of tests in whole animals or on isolated tissues. (Some of these have already been referred to in the preceding discussion and are illustrated in Fig. 3.5). i) A muscle depolarisation is not caused by agents which act competitively. They therefore do not cause an initial fasciculation of the muscle when injected intravenously in mammals nor do they cause a contracture of multiply innervated muscle fibres in frogs or birds. In birds, which have both focally innervated and multiply innervated fibres, even in the same muscle, the competitive blockers cause a flaccid paralysis in contrast to the spastic paralysis, characterised by opisthotonus and rigid limb extension, produced by depolarising blocking agents in this species. ii) Cholinesterase inhibitors, e.g. neostigmine, reverse block by d-Tc but not by CIO or succinylcholine, due to an increase in the safety factor for transmission.

40

Neuromuscular junction

iii)

The tension developed during tetanic stimulation of motor nerves at frequencies of 30 to 50 Hz is not maintained during partial blockade by a competitive agent but it is maintained during partial block by depolarising agents. The decline in tetanic tension after the administration of drugs such as d-Tc is attributed to two factors, a reduction in safety factor for transmission, caused by the reduced number of receptors available to ACh, and to the progressive reduction in ACh release with each stimulus in the tetanic train. This combined effect reduces the amplitude of the epps until they are no longer able to elicit action potentials. In untreated muscles, even though transmitter depletion still occurs during the tetanus, the end plate potentials remain above threshold for the initiation of muscle action potentials. Post-tetanic potentiation (PTP) is readily observed after partial neuromuscular block by the competitive but not the depolarising agents. PTP is attributed to an increase in the intracellular concentration of free Ca2+ in the terminals during the tetanus which only slowly declines over a period of minutes after cessation of the stimulus. This causes an increased liberation of ACh with each stimulus at low frequencies of stimulation and thereby an augmentation of the amplitude of the epps, partially reduced to near threshold levels by d-Tc. The effect is observed to only a small degree in untreated muscles where the safety factor for transmission is high. Potentiation is not evident during the period of high frequency of stimulation because under these conditions there is transmitter depletion from the terminals.

iv)

Myaesthenia gravis About one in every 6000 persons is afflicted with a crippling disease of the neuromuscular junction called myaesthenia gravis. The disease strikes either suddenly or slowly and may appear at any age between 10 and 40 years. The incidence of the disease is greater in women than in men but the

Myaesthen ia gravis

41

distribution of the disease is unaffected by geography, race or climate. There are marked changes in the structure of the end plate region including a reduction in the number and size of the nerve terminals and an increase in the diameter of the synaptic cleft. There is a reduced amplitude of the mepps recorded electrophysiologically. The symptoms are similar to those produced by neuromuscular blocking agents like d-Tc and there is a marked increase in the sensitivity to this agent. The muscle weakness increases during repetitive muscle activity and decreases with rest. It tends to increase throughout the day. In about 10% of patients there is a thymoma and in about 80% there are other abnormalities in the thymus, including mild thymitis. Possibly a viral infection is the causative factor. Thymectomy may give marked improvement but there is complete remission in only about one third of the patients. About 90% of patients have circulating antibodies which react with the ACh receptors on skeletal muscle and immunosuppressive agents produce some improvement. There is a correlation between the severity of the symptoms and the antibody titre. Symptoms may improve dramatically with plasmaphoresis to remove the circulating antibodies. Similar, although not necessarily identical, antibodies have been induced experimentally by the administration of purified ACh receptor proteins. It is therefore almost certain that the disease is of the autoimmune type. There is a correlation between the incidence of myaesthenia gravis and other autoimmune diseases and with the occurrence of neoplasms. Myaesthenic symptoms are reduced by the administration of the anticholinesterase, neostigmine; this is used in preference to physostigmine because it does not pass the blood-brain barrier. Germine monoacetate has also been found to produce beneficial results. A very short acting anticholinesterase, edrophonium, is employed as a diagnostic agent for the disease; if the symptoms improve, the test is diagnostic for myaesthenia. Related conditions, such a myaesthenic syndrome, are not improved by cholinesterase inhibitors and may in fact be made worse. In myaesthenic syndrome there may be a different defect

42

Neuromuscularjunction

in transmission in which there is a deficiency in the release mechanism. Denervation supersensitivity Muscles which have been denervated by cutting the motor nerve supply and allowing sufficient time for the fibres distal to the cut to degenerate become hypersensitive to ACh. The hypersensitivity is due to a spread of receptors along the muscle fibres, in contrast to the restricted distribution about the end plate in normal muscle. Similar effects may be produced by irreversibly blocking the receptors with a-bungarotoxin. Several factors may account for the spread of the receptors, or perhaps more strictly their restriction to the end plate region in normal fibres, including muscle activity and 'trophic' factors liberated from the nerve. The characteristics of the new receptors formed after degeneration of the nerve are similar, but not identical, to those of the normal end plate receptors. Although the new receptors remain nicotinic in type, the channel open times are longer than those at the normal end plate and the action potentials are no longer blocked by tetrodotoxin. In addition the supersensitivity to d-Tc is not as great as that to ACh indicating some change in the receptor affinity. There is some increase in the levels of cyclic adenosine monophosphate, possibly indicating that cyclic nucleotides play a role in the development of the supersensitivity.

Autonomic nervous system

The autonomic nervous system (ANS) is an efferent system conveying impulses from the central nervous system to the periphery. It controls the activity of most bodily functions except those of the skeletal muscles which are controlled by the somatic motor system discussed in the preceding chapter. It therefore affects such diverse physiological activities as salivary secretion, sweating, movement and secretions of the gastrointestinal tract, heart rate, calibre of the blood vessels, secretions of the pineal gland, contraction of the urinary bladder and its internal sphincter, penile erection, adrenaline secretion by the adrenal glands, accommodation in the eye and control of pupil diameter. Pharmacological interference with the ANS will therefore lead to widespread effects. A major application of drugs which selectively reduce vasomotor tone is in the treatment of cardiovascular disease. There are two major divisions of the ANS. The sympathetic division originates from preganglionic nerve cell bodies lying predominantly in the intermediolateral part of the thoracic and upper lumbar regions of the spinal cord. The parasympathetic division originates from nerve cells located in the brain stem and in the sacral spinal cord. Both systems may be distinguished from the skeleto-motor system by the interposition of a peripheral ganglionic synapse between the preganglionic neurone in the CNS and the peripheral, innervated effector organ (Fig. 4.1). The ganglia of the sympathetic division are situated either in the paravertebral chain ganglia or in one of the more distal sympathetic ganglia, such as the cervical sympathetic, stellate, coeliac, inferior mesenteric, or hypogastric ganglia. The parasympathetic ganglia are usually located within

44

Autonomic nervous system

or very close to the effector organ and so, unlike the sympathetic nerves, give rise to only very short postganglionic fibres, for example, those in the heart or on the wall of the urinary bladder. The parasympathetic nerve to the orbit, which synapses in the ciliary ganglion, is an exception to this generalisation. The postganglionic nerve fibres in both systems are mostly nonmyelinated C-fibres in contrast to the myelinated B-fibres of the preganglionic nerves. Neurotransmitters The most important transmitters in the autonomic nervous system are ACh and noradrenaline (NA). However, recent studies have shown that, particularly in the intestine, there is an abundance of neuronally located bioactive peptides such as substance P, enkephalins, vasoactive intestinal polypeptide (VIP), cholecystokinin and a number of other peptides. Sometimes the peptides co-exist with the more conventional transmitters. For example, ACh and VIP have been shown to be Fig. 4.1. Comparison of structure and transmitters in skeleto-motor and autonomic nervous system. Skeleto-motor

Central nervous system

Sympathetic

Parasympathetic t

A fibre

B fibre

B fibre Myelin

Myelin ^ ^ ^ Peripheral nerve ACh-^

^ ^ Sympathetic * ganglion C fibre

Effector organ

ACh^^ ACh^, HUH TTTff

Skeletal muscle

NA-^,

^ \)

Parasympathetic ganglion

t ^ C fibre

Smooth muscle or glands

Neurotransmitters

45

present in the same nerve fibres, but more than one peptide is usually not present simultaneously in the same neurone. This may not always be the case since there is some evidence that somatostatin and substance P may co-exist in some primary afferent fibres. The major part of the substance P at the periphery is associated with non-myelinated, sensory afferent fibres. There is good evidence for a role of substance P and of luteinising hormone-releasing factor in autonomic ganglia but the evidence for a physiological role of other peptides is as yet lacking. In addition to the peptides, there is growing evidence that purines may be involved in transmission at some sites in the viscera and on blood vessels. ACh is the transmitter liberated from preganglionic terminals in both the sympathetic and parasympathetic systems and excites the postganglionic neurones. It also functions as the transmitter at the postganglionic parasympathetic junctions, where it may either be excitatory, e.g. in the alimentary tract and the urinary bladder, or inhibitory as in the heart, or it may evoke secretions as in the salivary glands. The first conclusive evidence that ACh was the transmitter from the vagus nerve to the heart was obtained by Loewi in the 1920s, who demonstrated that the substance released on stimulation of the vagus in isolated frog hearts, slowed a second perfused heart in the same way as ACh: using the same technique he also showed that an adrenaline-like substance was liberated on stimulation of the sympathetic supply. It was not until the 1930s that it was shown that ACh was also the transmitter in sympathetic ganglia and at the neuromuscular junction. NA is liberated from postganglionic sympathetic nerve fibres and may be either excitatory as in the heart or inhibitory as in the intestine. The chromaffin cells of the adrenal medulla may be considered to be modified sympathetic neurones which are able to synthesise adrenaline from noradrenaline by N-methylation. In this case the amine is liberated into the circulation, where it exerts effects similar to those of NA. Dopamine may also be a neurotransmitter at some sites, e.g. in some ganglia and in the kidney, although its precise function at these sites remains uncertain. Some sympathetic nerves contain ACh, as in the sympathet-

46

Autonomic nervous system

ic supply to the sweat glands and to some blood vessels in sketetal muscle, but these fibres are probably of minor significance. Drug action in the autonomic nervous system Drugs may act at peripheral sites such as ganglia, terminals, postjunctional receptors or on postreceptor mechanisms of the autonomically innervated cell or they may act on central mechanisms in autonomic reflex pathways. Occasionally, they may also exert effects upon the afferent limb of the pathway. An example of this last action is the marked fall in blood pressure and heart rate which accompanies an intravenous injection of a large quantity of 5-hydroxytryptamine and other agents which stimulate vagal afferent fibres in the right atrium: this is the Bezold-Jarische reflex for which there is little evidence of a physiological function. Ganglionic sites of action At the turn of the century Langley noted that the local application of nicotine to autonomic nerves only caused effects when the agent was painted on at discrete spots, which we now call ganglia. He used this simple technique to map out most of what we know of the distribution of autonomic nerves to their effector organs. He observed that at low concentrations applied to sympathetic ganglia there was an excitant effect, similar to that produced by electrical stimulation of the nerve. At higher concentrations, there was also at first an excitant effect but this waned and at this time stimulation of the nerve was ineffective. This dual action of nicotine is due to an initial excitation of the ACh receptors followed by a blocking action due to desensitisation, probably similar in mechanism to that at the neuromuscular junction. In addition to the cholinergic synapses in ganglia, which are probably the most important to consider, there are also monoaminergic synapses which utilise either NA or dopamine (DA) as the neurotransmitter and, at least in some species, peptides may also be involved.

47

Ganglionic sites of action

Nicotinic receptors. The most obvious effects in ganglia resulting from the electrical stimulation of preganglionic nerves are due to the activation of the cholinergic receptors. The largest postsynaptic potential with the shortest latency is caused by the Fig. 4.2. Ganglionic mechanisms, (a) Diagrammatic representation of sequence of ganglionic potentials evoked by a stimulus (S) to the preganglionic nerve. N, surface-negative wave of depolarisation; P, surface-positive wave of hyperpolaristion; LN, late surface-negative wave of depolarisation. (b) postulated ganglionic synapses causing the recorded wave form. N, Nicotinic receptors (blocked by hexamethonium or D-tubocurarine); M, muscarinic receptors (blocked by atropine); A, adrenoceptor (blocked by dibenamine), SIF, small intensely fluorescent neurone. Note: there is evidence that in some situations a cholinergic step directly mediates the inhibition without the intervention of a SIF neurone.

DAorNA SIF neurone

48

Autonomic nervous system

interaction of ACh with nicotinic receptors on the postganglionic neurones. This is the equivalent of the N-wave, first recorded extracellularly from isolated ganglia of the rabbit by Eccles & Libet and shown diagrammatically in Fig. 4.2. The nicotinic receptors in ganglia are similar to but not identical to the nicotinic receptors at the neuromuscular junction. They can be distinguished by the actions of agonists and antagonists at the receptors. At both sites ACh, nicotine and carbachol are stimulants and d-tubocurarine is a competitive antagonist. However, the nicotinic receptors in ganglia but not at the neuromuscular junction are activated by dimethylphenylpiperazinium (DMPP) or tetramethylammonium and blocked competitively by tetraethylammonium or hexamethonium (C6). Phenyltrimethylammonium, which activates the nicotinic receptors at the neuromuscular junction, and decamethonium, which causes block, are not similarly effective in ganglia (Table 4.1). Studies with the bisquaternary ammonium series showed that peak ganglion blocking activity was present with a chain length of 5-6 carbon atoms (pentamethonium and hexamethonium) whereas 10 carbon atoms (decamethonium) are required at the neuromuscular junction. Alpha bungarotoxin binds to specific binding sites in ganglia as it does at the neuromuscular junction. However, it does not block transmission in ganglia, possibly because it has two opposite effects. Besides blocking the receptors, it also increases the mean channel lifetime by a factor of two, and this effect may counteract the block of transmission. Muscarinic receptors. Electrical stimulation of the preganglionic fibres in the cervical sympathetic nerve of the cat causes a contraction of the nictitating membrane which is blocked by hexamethonium and is due to the activation of nicotinic receptors in the superior cervical sympathetic ganglion. When the stimulation is terminated, the nictitating membrane relaxes but there follows a small second contraction which is resistant to hexamethonium but blocked by atropine, a known selective antagonist at muscarinic receptors. Electrical stimulation of the hypothalamus causes a contraction of the membrane which is only

Ganglionic sites of action

49

Table 4.1. Comparison of effects of agonists and antagonists at the neuromuscular junction and in autonomic ganglia

Muscarinic receptors a) Agonists: ACh Carbachol Muscarine Acetyl-beta-methylcholine McNeil A-343 (Mi) b) Antagonists (competitive): Atropine (Mi) and (M2) Pirenzapine (Mi)

Neuromuscular junction

Autonomic ganglia

+ + 0 0 0

+ + + 4+

0 0

44-

Nicotinic receptors c) Agonists: ACh 4Carbachol + Nicotine + Dimethylphenylpiperazinium (DMPP) 0 Phenyltrimethylammonium (PTMA) + d) Antagonists (competitive): D-tubocurarine 4Hexamethonium 0 Block release of ACh Botulinus toxin

4-

+ 44+ 0 + + +

0 = no effect 4- = effective; Mi muscarinic receptors are found in ganglia and in the brain and are activated by McNeil A-343 and blocked by pirenzapine; ACh and carbachol are effective on all muscarinic and nicotinic receptors. partially blocked by either hexamethonium or atropine administered separately but is completely blocked when both agents are administered simultaneously under conditions in which there is no effect on the frequency of impulses in the preganglionic nerve. It is therefore clear that this response involves both nicotinic and muscarinic receptors. The intra-arterial administration to the ganglion of acetyl-(3methylcholine, muscarine or McNeil A-343 (Table 4.1), all of which activate muscarinic receptors but not nicotinic receptors,

50

Autonomic nervous system

causes contractions of the membrane which are blocked by atropine but not hexamethonium: McNeil A-343 actually causes a rise in blood pressure, unlike other agonists at muscarinic receptors, because it selectively activates ganglionic receptors and not those on the blood vessels which cause vasodilatation and a fall in blood pressure. There is now some evidence to support the notion that there may be different subtypes of muscarinic receptors. The electrophysiological observations of Eccles & Libet in 1961 on transmission in the isolated rabbit superior cervical ganglion demonstrated the importance of both nicotinic receptors and muscarinic receptors in ganglionic transmission (Fig. 4.2). There was an early depolarising N-wave which is blocked by hexamethonium or d-tubocurarine and is attributable to activation of the nicotinic receptors by neuronally released ACh. There follows a hyperpolarising P-wave, which is blocked either by atropine acting on muscarinic sites or by dibenamine, an alphaadrenergic receptor antagonist. The P-wave was followed by another late depolarising negative wave called the LN-wave which is completely blocked by atropine. It was postulated that the P-wave was caused by muscarinic activation of a monoaminergic interneurone which synapsed with the postganglionic neurone: this explanation accounted for the block by dibenamine (see Fig. 4.2). Since all waves were blocked by botulinus toxin which prevents the release of ACh in ganglia, just as it does at the neuromuscular junction, all effects are clearly mediated via cholinergic preganglionic nerve fibres. The postulate of Eccles & Libet has received ample support from histochemical studies demonstrating the presence in the ganglia of small, intensely fluorescent (SIF) neurones containing either dopamine or noradrenaline according to the species. However, more recently the hypothesis has been questioned on several grounds. The most compelling is that muscarinic receptor agonists induce a hyperpolarisation of the ganglion cell membrane, by an increase in the potassium ion conductance, which occurs at low Ca2+ concentrations in the medium. This indicates that activation of the muscarinic receptor can directly hyperpolarise the membrane, without the intervention of

Ganglion ic sites of action

51

synaptic transmission. The slow inhibitory synaptic potential produced by stimulation of the preganglionic nerve has also been shown to be due to an increase in potassium ion conductance, in which no second messenger is involved, and which is monosynaptically mediated via a cholinergic synapse in the bullfrog, mudpuppy, rabbit, rat and cat. Nevertheless, there is evidence that dopamine receptor activation does facilitate the responses due to activation of muscarinic receptors and these effects of dopamine are blocked by the dopamine Dl receptor antagonist, butaclamol, but not by antagonists which are selective for the D2 receptor. Potentiation of these effects by theophylline, an inhibitor of phosphodiesterase, indicates that cylcic AMP could be involved as a second messenger. In this context it is of interest that dopamine and nerve stimulation both increase the level of cyclic AMP in the ganglion. Other evidence indicates that the receptors in ganglia are not dopamine but a-2 adrenoceptors and that the SIF cells have no processes and are too sparse to carry their postulated role. Clearly, there are still some controversial issues to be resolved in the mechanism of the inhibition in ganglia. Peptide involvement in ganglionic transmission. In addition to the

late negative wave demonstrated in mammalian ganglia, in the frog there is also a late slow excitatory postsynaptic potential (EPSP) which is probably due to luteinising hormone releasing hormone (LHRH), co-released with ACh from preganglionic fibres. In this species, stimulation of cholinergic, myelinated B-fibres gives rise only to cholinergically mediated responses. However, stimulation of the preganglionic, non-myelinated C-fibres (Fig. 4.3) causes activation of cells via the release of both ACh and LHRH. There is also an indirect activation of the cells due to diffusion of the LHRH from the C-fibres. The effects of exogenous LHRH are similar to those of the endogenously released substance and both are blocked by LHRH analogues which are antagonists of LHRH in the pituitary. There is also evidence for the involvement of the polypeptide substance P in ganglionic transmission (see Otsuka & Konishi,

52

Autonomic nervous system Fig. 4.3. Diagrammatic representation of cholinergic and peptide mediated effects in the inferior mesenteric ganglion of the guinea pig and in frog sympathetic chain ganglia. A. Inferior mesenteric ganglion in guinea pig

SP

Post

Inf. mes. g. Diagram of synaptic potentials

seconds minutes Stimulate preganglionic nerve B. Frog sympathetic ganglion

B-cell

B-fibres (myelinated) A

N^

\ ^ SCH3

CH 2 CH 2 NT I CH 3

Butyrophenones

Haloperidol

O II CCH 2 CH 2 CH,-N

OH

Diphenylbutylpiperidines

Pimozide

CHCH^CH.CH^-N

O

Fig. 10.1. Examples of antipsychotic drugs.

179

The dopamine receptor Thioxanthenes

Flupenthixol CHCH2CH2 — N

N—CH2CH2OH

Dibenzazepines

o

Clozapine

Cl

CH,

Benzamides Sulpiride

NH2SO2 /

\ - CONHCH2 OCH3

—L NNJ I

C2H5

dermatological problems of contact dermatitis and photosensitivity are sometimes seen. Blood dyscrasias, including agranulocytosis and leucopenia, are encountered rarely. Chlorpromazine causes a marked hypothermia and the drug has been used to produce 'artificial hibernation' which reduces metabolic demands for oxygen in major heart surgery. This action was known and used clinically before its use as an antipsychotic drug. Chlorpromazine markedly potentiates the action of other depressants of the central nervous system. The simultaneous intake of alcohol, for example, could have unexpectedly dramatic results. This action is mainly due to an action on the CNS itself, but is also partly the consequence of inhibition of the cytochrome P-450 system in the liver, reducing the enzymic degradation of drugs.

180

Drugs used in sch izophrenia

Table 10.1. Severity of major side-effects of antischizophrenic drugs at effective antipsychotic doses Extrapyramidal symptoms

Drug

Sedative effect

Hypotensive effect

Chlorpromazine Thioridazine Triflupromazine Prochlorperazine Haloperidol Clozapine Pimozide

Fig. 10.2. Metabolism of dopamine. OH

OCH3 \

XV

COMT -CH,CH : NH : — • -

HO

CH2CH2NH2

Dopamine

OCH3 CHXTOOH Homovanillic acid

The dopamine receptor Our present knowledge of the dopamine receptor has derived almost entirely from the interest in the mode of action of the antischizophrenic drugs. Effects of antischizophrenic drugs on terminals One of the earliest pieces of evidence is that the administration of drugs which are effective in schizophrenia increase the turnover of dopamine in the CNS, as revealed by an increased production of the metabolite, homovanillic acid (Fig. 10.2). Sub-

The dopamine receptor

181

stances with a similar chemical structure but which are not effective in schizophrenia lack this effect. Although some of the neuroleptics also increase the turnover of noradenaline, not all do so and this effect may be more related to the incidence or severity of sedation. Most of the studies have been carried out in the caudate nucleus which derives its dopaminergic innervation from the substantia nigra. Two pieces of evidence indicate that the increase in turnover is due to an effect directly on the terminals and is not due to a change in a feedback loop as originally proposed (Fig. 10.4). First, the injection of kainic acid into the caudate nucleus, which destroys nerve cell bodies but not terminals, does not reduce the ability of antischizophrenic drugs to increase dopamine turnover. Secondly, the increased turnover is not abolished by acute transection of the nigro-striatal pathway. Antagonism of behavioural action of dopamine or dopamine agonists injected directly into the brain There have been many studies showing that apomorphine or amphetamine, both of which probably act upon the dopamine system, or dopamine itself causes turning to one side or stereotypy when injected into the cerebral ventricles of rats with unilateral nigro-striatal lesions. These effects are all readily blocked by systemic administration of antischizophrenic drugs. Antagonism of dopamine agonist effects upon single neurons While the acute effect of antischizophrenic drugs is clearly to reduce the inhibition of dopamine-sensitive neurones by the agonist, Bunney (1984) has emphasised his view that the chronic administration of antischizophrenic drugs causes a depolarisation blockade of dopamine sensitive cells. He claims that it is this effect which is correlated with the slow onset of the therapeutic action. Increased receptor density Prolonged administration of antischizophrenic agents causes an increase in dopamine receptor density (Bmax) in brain.

182

Drugs used in schizophrenia Negative feedback

o A

- > apomorphine = dopamine > bromocryptine > pergolide >> quinpirole

Antagonists:

Selective Dl to selective D2: SCH-23390 > > a-flupenthixol = fluphenazine = chlorpromazine > haloperidol > pimozide > spiperone > domperidone > (-)sulpiride

Adapted from Trends in Neurosci., 6, centrefold.

Extrapyramidal side-effects of antischizophrenic drugs

187

caused by 6-OHDA, even though the former does not destroy the nerve terminals. Furthermore, after destruction of the dopamine terminals with 6-OHDA, the administration of dopamine restored the 'missing' receptors. It became clear that the loss of receptors after 6-OHDA could not be used as evidence that the receptors were presynaptic, as assumed before. These receptors were thought to be high affinity Dl receptors located on the striatal neurones. It seems that the expression of dopamine receptors on the postsynaptic membrane is controlled by the amount of dopamine released in transmission. If this is reduced by lesions, produced surgically or by 6-OHDA, or by depletion of transmitter with reserpine then the postsynaptic receptor density is seen to decline. Elevating the level of dopamine by administration restores the prelesioned condition. Extrapyramidal side-effects of antischizophrenic drugs Disorders of movement known as dyskinesias are frequently observed in patients treated with antischizophrenic drugs. The dyskinesias are of several types. Least troublesome are akathisia, a need for continual movement as if agitated and dystonia, characterised by grimacing and torticollis. More frequent are Parkinsonian symptoms of akinesia and rigidity which tend to be most noticeable during the early stages of treatment and can be well controlled by drugs, especially the anticholinergic drugs, which are used in Parkinson's disease. The tremors are more troublesome and not so readily controlled. All Parkinsonian symptoms diminish rapidly if the antischizophrenic drug is either withdrawn or the dosage is reduced. More disturbing for the patient is tardive dyskinesia, observed in about 20% of patients being treated with drugs over long periods. Another, more descriptive, name for this side-effect is orofacial dyskinesia in which there are excessive and highly distressing movements of the lips, tongue and jaw. The symptoms seem to appear only after prolonged drug treatment and do not always disappear as soon as the drug is withdrawn. The occurrence of dyskinesias in schizophrenic patients was observed before the

188

Drugs used in schizophrenia

introduction of the modern drugs and it is possible that there is a particular tendency to tardive dyskinesia in such patients. Unfortunately, these symptoms do not respond favourably to the anticholinergic drugs and may be made worse by them. Thus the effective treatment of the Parkinsonian side-effects may worsen the tardive dyskinesia. The symptoms may subside when the drug is withdrawn. Clearly, this will not always be possible. Mechanisms in drug-induced dyskinesias If tardive dyskinesia is exacerbated by the antischizophrenic drugs, as seems probable, then the mechanism is obscure. Since the symptoms only occur after prolonged treatment the mechanism is presumed to be something which also follows a long time course. Although not quite on the right time scale, the proliferation of dopamine receptors after chronic treatment with neuropleptic drugs offers a possible but by no means certain explanation. There is a convincing explanation for the Parkinsonian-like dyskinesias. The cause of Parkinson's disease is well known to be the loss of dopamine neurones and of their terminals in the basal ganglia. The dopamine pathway involved is the nigro-striatal pathway originating in the substantia nigra. It is the loss of function in the basal ganglia which gives rise to the term extrapyramidal effects, meaning utilising those nervous pathways outside the pyramidal system. Agents which block dopamine receptors in the basal ganglia have the same ultimate effect as the destruction of the dopamine neurones in basal ganglia disease. Although all useful antischizophrenic drugs bind to the Dl receptor to some degree, we have noted that there is not a good correlation with the incidence of extrapyramidal effects. Nevertheless, a reasonably satisfying explanation has been adduced from the relative ability of different antischizophrenic drugs to block both dopamine receptors (stimulation of adenylate cyclase) and muscarinic receptors for acetylcholine, so effectively combining both cause of the problem and its cure within the same molecule. Table 10.5 summarises some data on the affinity of antischizo-

Extrapyramidal side-effects of antischizophrenic drugs

189

Table 10.5. Dissociation constants and relative affinities of some antipsychotic drugs for binding to muscarinic and dopamine receptors in brain homogenates. (After Miller & Hiley 1974) Muscarinic (M) Dopaminergic (D) Relative affinity K D nM KD nM 25 Thioridazine 55 Clozapine 160 Pimozide 350 Chlorpromazine 12000 Spiroperidol 4000 Trifluoperazine 2200 a-flupenthixol

130 170 140 48 95 19 1

5.2 3.1

0.87 0.14 0.008 0.005 0.0005

Note: Binding to dopamine receptors was measured by activation of adenylate cyclase. Relative affinity is the ratio of the affinity (l/KD) for muscarinic receptors compared with the affinity for dopamine receptors.

phrenic drugs to the Dl receptor, measured by inhibition of dopamine-activated adenylate cyclase and for muscarinic receptors. The relative affinity is the ratio of the affinity (l/^ o ) for muscarinic receptors compared with the affinity for dopamine receptors. The drugs in the table are arranged in descending order of relative affinity for muscarinic/dopamine receptors. It was observed that this order correlates well with the relative incidence of extrapyramidal effects with this series of drugs. Those drugs near the top of the table, such as thioridazine, clozapine and pimozide have a much lower incidence of extrapyramidal effects than have those at the bottom of the table. Dopamine turnover, measured by the production of homovanillic acid has confirmed that the extrapyramidal effects correlate with the anticholinergic properties of the molecule. However, the significance of the anticholinergic action depends upon the region of the brain under study. Table 10.6 shows in summary some data obtained in the caudate nucleus and nucleus accumbens, a part of the limbic forebrain. The number of arrows indicate the relative effects of each of the three compounds studied. Thioridazine, which causes

190

Drugs used in schizophrenia

Table 10.6. Effect of three antipsychotic drugs on dopamine turnover in two regions of the CNS in relation to their abilities to cause extrapyramidal symptoms. Motor side effects

Dopamine turnover (HVA production)

Thioridazine Chlorpromazine Fluphenazine

No anticholinergic present

Anticholinergic present

Caudate Ace

Caudate Ace

0 + +•

0 0 0

+++ +++ +++

Slight Moderate Severe

-I- = increase in DA turnover, Caudate = caudate N., Ace = N. Accumbens Based on data from Crow et aU 1976. The Lancet, 11 Sept, 563.

the least extrapyramidal action and has the highest ratio of anticholinergic to antidopaminergic effect, had the least effect on dopamine turnover in the caudate nucleus. By contrast, fluphenazine had the greatest effect on dopamine turnover in caudate, had the smallest ratio of affinities and had the highest incidence of motor effects. Chlorpromazine came in the middle. In nucleus accumbens all three agents had about the same effect on dopamine turnover. Of particular interest is the finding that an anticholinergic drug abolished all effects on dopamine turnover in the caudate but had no similar effect in the nucleus accumbens. It may be concluded from this study that the cholinergic system is less important in counteracting the dopamine system in the nucleus accumbens than in the basal ganglia. A hypothesis collating these various studies is presented in Fig. 10.5. The cholinergic system may act as a physiological balance to the dopamine system originating from the substantia nigra in the basal ganglia but not in the limbic system. When the control of the basal ganglia is disturbed by blocking the dopamine recep-

Summary

191

Fig. 10.5. Hypothesis relating the action of drugs in the basal ganglia and limbic system to antischizophrenic action and the production of motor disorders.

Block of DA receptors and reduction of DA release is responsible for antipsychotic effect

Block of DA receptors causes extrapyramidal effects

Cholinergic pathway

Block of ACh receptors balances block of DA receptors and reduces extrapyramidal effects

tors, then motor dyskinesias become evident. If there is a counterbalancing block of the cholinergic system, then the dyskinesia are less evident. In the limbic system it is hypothesised that the unopposed block of dopamine receptors is necessary for the antipsychotic action to be in evidence. The penalty for requiring substances to be antagonists at muscarinic receptors and at dopamine receptors is that such compounds may be prone to cause peripheral symptoms attributable to block of muscarinic receptors. Such common symptoms include dryness of the mouth, blurring of vision and constipation. (i)

(ii)

Summary There is increasing evidence that schizophrenia is associated with observable organic brain changes, including structural changes and an increase in the density of D2 receptors for dopamine. Drugs which are effective in treating the symptoms of schizophrenia are most effective on the positive signs

192

(iii)

(iv)

(v)

(vi)

(vii)

Drugs used in schizophrenia

and less effective on negative signs such as withdrawal. There is a good correlation between clinical efficacy and the affinity of the drugs for dopamine D2 receptors where they are antagonists to the transmitter, dopamine: the role of the Dl receptors is uncertain. The presumed target of action is in the limbic system but their is little direct evidence in support of this concept. There is a slowly developing increase in dopamine receptor density. This may explain the discrepancy between the time course of the onset of the therapeutic effect, which is slow, and the pharmacological action which is immediate. Alternatively, it is also possible that in part the slow onset of therapeutic action is due to slowly developing psychological readjustement, although this seems less likely. The Parkinsonian-like side-effects of antischizophrenic drugs are also related to the ability to block dopamine receptors in the basal ganglia, mitigated by their ability to also block muscarinic receptors at the same site. Anticholinergic drugs are therefore an effective countermeasure. Tardive dyskinesia is of slow onset. The mechanism can only be speculated upon and their is no effective countermeasure, except a reduction in dose, which is not always 100% effective. New compounds may be developed in which the primary action is not on dopamine receptors but on some other type of receptor (e.g. the 5-HT receptor) but the clinical efficacy of such drugs has yet to be established.

11 Affective and manic depression

The affective disorders include a variety of conditions characterised by mood changes unrelated to life events, i.e. they are not reactive. There is major depression, sometimes referred to as psychotic or endogenous depression, mania and bipolar or manic depression. Any extreme of mood may be associated with psychosis in which thinking may become irrational and delusional. Since drug therapy should be based upon symptomatology and not on diagnosis this should not cause problems. However, it has sometimes been rather difficult to differentiate clearly manic depression from schizophrenia and the boundaries may merge. In typical cases the distinction is clear. There may also be a tendency for the nature of the illness to change over the years and the diagnosis may change accordingly. Until the major mental illnesses can be characterised completely in terms of specific disorders in structure or function, diagnosis will need to remain linked to symptoms and treatment. A recent promising beginning is the association of a genetic abnormality with the illness.

Endogenous depression Drugs used (Fig. 11.1) in treating major depression include tricyclic compounds like imipramine, tetracyclics like mianserin and monoamine oxidase inhibitors, such as nialamide, which are no longer used to any great degree. Prior to the use of these substances the only available procedures included leptazol or insulin shock therapy and electroconvulsive therapy (ECT). Of these only ECT survives today, although the use of ECT varies greatly from one centre to another. All major theories assert that effective procedures modulate

194

Affective and manic depression

aminergic mechanisms in some way. However, the diversity of action of therapeutic measures defeats most attempts at creating a universal theory to explain the actions of all antidepressant drugs. If it is difficult to explain satisfactorily the mechanism in antidepressant drug action, it is even more difficult to define the aberrant mechanisms in the illness itself. In many studies there have been changes reported in levels of noradrenaline, dopamine or 5-hydroxytryptamine, or their metabolites, in urine, blood or cerebro-spinal fluid. Such changes have been seen in the manic and depressive stages of manic depression, for example. While such alterations may indeed reflect a changes in a monoaminergic system in depression, they do not tell us whether the changes cause the illness or are simply a reflection of its presence. Monoamine oxidase inhibitors The mood-elevating effect of iproniazid was first noted in 1951 when it was introduced for the treatment of tuberculosis. In 1952 it was shown to inhibit monoamine oxidase (MAO) but it was not until five years later that it was tried as an antidepressant. Many drugs with similar actions have now been investigated but severe toxicity, principally on the liver, has caused them to be withdrawn from regular use, although they may still be occasionally employed when all other measures fail. The inhibition of MAO-A prevents amines such as tyramine from being deaminated and thereby detoxified. An agent such as Deprenyl, which affects mainly MAO-B may be more free from such sideeffects but its value in depression remains to be established. The elevation of mood in depressed patients may take some weeks to become apparent. It is tempting to attribute the therapeutic action to inhibition of MAO, which controls the levels of monoamines in the cytoplasm. However, the correlation between ability to inhibit MAO and clinical efficacy is not good and the inhibition of MAO is immediate whereas the clinical improvement may take weeks.

Endogenous depression

195

Fig. 11.1. Structures of antidepressant drugs. Monoamine oxidase inhibitors O C-NHNHCH(CH3)2 CH — C H N H 2

"cH2 Tranylcypromine

Iproniazid (prototype)

CH 2 CH 2 NHNH 2

II O

II O

Nialamide

Phenelzine

Tricyclic

CH2NHCCH2NHNHC

compounds

CH 2 CH 2 CH 2 N(CH 3 ) 2 Imipramine

CH 2 CH 2 CH 2 NHCH 3 Desipramine

CHCH 2 CH 2 N(CH 3 ) 2 Amitriptyline Tetracyclic compounds

CH3 CH 2 CH 2 CH 2 N;

CH3 Mianserin

Iprindole

196

Affective and manic depression

Tricyclic antidepressants The prototype of these drugs is imipramine, which was originally developed as a possible histamine antagonist where its sedative action was noted. In 1958 it was first tested clinically as a sedative in agitated psychotic patients but was not very impressive. Quite coincidentally, it was found to be an antidepressant. The major metabolite of imipramine is desipramine, which was thought to be more active than the parent compound: this seems not to be the case but both are similarly active. Although the MAO inhibitors cause an elevation of mood in normal subjects, the tricyclic antidepressants tend to cause sedation which is accompanied by unpleasant subjective sensations. In depressed patients, imipramine causes less outright euphoria than MAO inhibitors but causes a greater attenuation of the depressive ideas. It is also interesting that the sedative action in normal individuals is rapid in onset with imipramine but the antidepressant action is slow to develop. Other classes of antidepressants There are a variety of other agents which seem to have some use in treating depression. This miscellany includes tetracyclic drugs like mianserin (Fig. 11.1) and other substances such as iprindole and trazodone. Mechanisms of antidepressant action All effective agents seem to interfere in some way or other with monoaminergic systems in the central nervous system. However, the passage of time and more numerous investigations has shown just how complex and varied these actions are. Instead of simplifying the picture it is perhaps becoming more obscure. The difficulties are compounded by the fact that most actions which are observed by pharmacologists are rapid in onset whereas the clinical improvement is very slow over weeks from the commencement of treatment. It would seem rational to concentrate upon only the long-term effects of the drugs if it were not for the fact that long-term effects may only be the long term, and

Mechanisms of antidepressant action

197

Table 11.1. Effects of some antidepressants on uptake ofnoradrenaline and 5-hydroxytryptamine into nerve terminals. Block of uptake (Relative potencies, imipramine = 1)

Desmethylimipramine Amitriptyline Iprindole Mianserin Imipramine KD (M)

NAdr

5-HT

20 18 0 9 1 X 10"6

0.2 0.6 0.3 0.0 2.8 X 10"7

secondary, changes produced by the immediate but maintained short-term action. Inevitably, this means that all actions must at this stage be considered as possibly important mechanisms. In the early days, when the only compounds available were MAO inhibitors and tricyclic antidepressants it seemed reasonable to propose that there was too little monoamine transmitter in depression. This was based upon the fact that either blocking MAO or decreasing amine uptake with tricyclic compounds was beneficial. At that time, although the tricyclic compounds blocked the uptake of both noradrenaline and 5-hydroxytryptamine, the alleviation of clinical depression correlated better with the ability to block the uptake of 5-hydroxytryptamine than with the block of noradrenaline uptake. Other antidepressant compounds with different chemical structures did not fit into this picture at all well. Thus mianserin and iprindole have little effect on uptake mechanisms for 5hydroxytryptamine or noradrenaline respectively (Table 11.2) and yet are effective antidepressants. The picture became even more blurred when it was seen that some antidepressants had a strong affinity for the histamine H-2 receptor. For example, amitriptyline is about seven times as potent as imipramine in this

198

Affective and manic depression

Table 11.2. Effects of some antidepressants as antagonists at various receptors. Receptor block

Desmethylimipramine Amitriptyline Imipramine Iprindole Mianserin Imipramine KD (M)

H2

ACh (Muse)

7 1.0 0.07

0.33 6 1.0 0.04

2.4 X 10"7

2 X 10"7

Pre-a-2

Active Inactive Inactive Active Inactive

Note: blank entries signify unknown result

respect and has a KD\n the nM range. Amitriptyline is also a more effective antagonist than imipramine at muscarinic receptors for acetylcholine. Again these actions may be purely coincidental because substances like iprindole have low activity. The ability of some antidepressants, this time including mianserin, to block presynaptic a-2 receptors for catecholamines at first sight seems promising. On closer investigation this seems less likely because iprindole and imipramine are not effective. Furthermore, the block of the alpha receptor was deduced from experiments in which cocaine was used to block the confusing effect of uptake 1. There is now evidence that cocaine blocks a new 5-HT receptor (5-HT-3) which is located on neurones and mianserin blocks yet another variety of 5-HT receptor (5-HT-2), also found on neurones. Nevertheless, several antidepressants displace the binding of clonidine, an a-2 agonist (Table 11.2), indicating that there is an affinity for these receptors. Suffice it to say that this complex picture is difficult to resolve. Long-term effects of antidepressants There are several long term-effects noted with antidepressant or ECT treatment, including an up-regulation of a-2

Mechanisms

of antidepressant

T a b l e 11.3. Effects of antidepressants cerebral cortex (3-1 Imipramine Clomipramine Desipramine Nortriptyline Mianserin Nialamide Nisoxetine

— — 0 - 0

action

199

on P-7 and a-2 receptor density in

a-2 +++ + ++ + +++ ++ 0

Notes: 1. + = Increase, - = Decrease, 0 = No effect on receptor density 2. p-receptors assessed by dihydroalprenolol binding 3. a-receptors assessed by clonidine binding 4. in limbic forebrain desipramine increased a- but did not decrease Preceptor density 5. nisoxetine decreased P-activated adenylate cyclase without an effect on receptor density

receptors and a down-regulation of P-l-receptors (Table 11.3). At the present time such changes are generally considered to be of vital importance in the generation of the antidepressant action (Tyrer & Marsden, 1985). A variety of effects have been observed with chronic antidepressant treatment. (i) The clinical efficacy of the tricyclic antidepressants correlates well with their ability to displace radio-labelled imipramine from a high affinity (KD = 4 nM) binding sites (Langer & Briley, 1981). Such sites have been identified in postmortem human brain, as well as in the brains of experimental animals. The high affinity binding of imipramine is depressed by about 50% in the platelets of depressed patients. The amount of binding does not correlate with the severity of the symptoms, nor does it change when the patients are treated with antidepressants. The binding site is probably some part of the 5-hydroxytryptamine uptake site on nerve terminals. ECT also decreases imipramine binding but the

200

(ii)

(iii) (iv)

(v)

(vi)

Affective and man ic depression

atypical antidepressants such as iprindole have no effect. It is possible that there is a defect in these sites in the brain of depressed patients, although this has yet to be established. Repeated injections of tricyclic antidepressants over two to three weeks decreases the density of a-1 noradrenergic receptors in the brain, with no change in their affinity, i.e. there is a reduction in Bmax with no change in KD. ECT has a similar effect. This effect is common to many but not all of the newer antidepressants (Table 11.3). One exception is mianserin, which has no significant action on the beta receptors. Repeated administration of antidepressants, including mianserin and nialamide, an MAO inhibitor, causes an up-regulation of a-2 receptors in brain. Although most of the antidepressants decrease the beta receptors and increase the number of a-2 receptors in cerebral cortex, in limbic forebrain desmethylimipramine decreases the beta receptors without affecting alpha receptors. This indicates that the regulation of the two receptor populations is not identical. There is also evidence that chronic treatment with tricyclic compounds, iprindole or pargyline, an MAO inhibitor, decreases the density of receptors for 5hydroxytryptamine in the brain. Chronic treatment with tricyclic antidepressants, iprindole or ECT causes a subsensitivity of a (3-1 activated adenylate cyclase in rat limbic forebrain. While this is often associated with a decreased density of adrenergic receptors, this is not invariably so: nixoxetine decreases the sensitivity without a change in receptor density.

Conclusions It is indeed difficult to draw firm conclusions from these observations. Antidepressant activity seems to be associated with many different types of acute interaction with monoaminergic

Mechanisms of antidepressant action

201

systems coupled with a down regulation of beta receptors and an up regulation of alpha receptors with chronic treatment or ECT. It is possible that the long-term effects on receptor density are regulated by the short-term effects on a variety of amine transmitter systems. As for the biochemical disorder in depression, we seem to be little further down the road than we were 15 years ago. The emerging re-emphasis on multiple receptor types for 5hydroxytryptamine perhaps will shed some new lights on this picture of confusion. However, with about four different types proposed, there may first need to be some rationalisation, as with other systems such as the dopamine system. In the meantime there is no single pharmacological action which will reliably predict the success of a new compound as an antidepressant and reliance still needs to be placed upon the clinical trial. Manic depression The pathology of manic depression is no more established than it is for depression. It is an interesting fact that one of the commonest elements in the earth's crust, lithium, has therapeutic value in the treatment of manic depression. Its first use in modern psychiatry was in 1950 but serious interest did not develop until about 1965. Lithium did not come into general use until 1971, possibly because its low cost did not really make it a very attractive commercial proposition. Lithium has no indication in the treatment of depression but does come into its own in the prophylactic therapy of manic depression. There is very little effect in the first 7-10 days of treatment. The plasma level must be carefully monitored to ensure that it does not rise above about 1.0 mEq/1, or 1.5 mEq/1 in extreme cases. If the plasma level rises higher then toxic effects are observed. Lithium is a simple monovalent cation, slightly larger than sodium ions in the hydrated state. It is not handled effectively by the sodium pump but can diffuse slowly across membranes. The resulting slow accumulation of sodium in cells leads to a progressive depolarisation. Because lithium is so slow to act, and does not cause sedation, it is not effective in initial management, especially in the manic

202

Affective and manic depression

phase of the illness. It is therefore frequently combined with relative large doses of phenothiazines to quieten the patient. The reported actions of lithium include: (i) stimulation of noradrenaline turnover (ii) inhibition of 5-hydroxytryptamine turnover (iii) stimulation of 5-hydroxytryptamine synthesis (iv) inhibition of stimulus induced amine release (v) inhibition or stimulation of amine uptake (vi) changes in the concentration of monoamine precursors and metabolites in CSF (vii) block of choline uptake in erythrocytes (viii) excitation of spinal Renshaw neurones by release of acetylcholine (ix) inhibition of the sodium pump (x) facilitation of synaptic transmission in the hippocampus (xi) increase in evoked potentials in animals and man There seems to be general consensus that the primary action of lithium may be the inhibition of the sodium pump and that all other effects are the secondary effect of this, with the apparent selectivity on different cells being the consequence of differing levels of dependence on the pump. Interesting though these effects may be, they do not tell us a great deal about the way in which lithium exerts its undoubted effect in manic depression.

12 Disorders associated with brain lesions If the cause of an illness is known then it should be possible to devise a therapy to combat that illness. However, if the cause is an anatomical lesion of the brain then it may be possible to devise a rational therapy to treat the symptoms but at first sight it would seem to be impossible to devise a cure because the neuronal loss is irreversible. There are two ways in which the effects of losing neurones might be overcome. One would be to train parallel neuronal circuits to become more efficient: this is the basis of some types of physiotherapy. Alternatively, it may in the future become possible to transplant nerve cells: already this seems to be possible, at least experimentally, with dopaminesecreting cells. Although the major neuronal deficit in Parkinson's disease and effective therapy have been established for many years, attention has more recently shifted first to another basal ganglia disorder: Huntington's chorea, and to the debilitating illness of Alzheimer's disease (senile dementia). In each of these illnesses the major impact has come from studies of changes in neurotransmitter systems, carried out in the hope that specific changes which might be amenable to drug therapy could be devised on a rational basis. Very recently there has also been a new interest in the cell loss which results from the hypoxia and ischaemia of stroke. It will be convenient to group together five disorders of the basal ganglia since these are all characterised by dyskinesias (disorders of movement) of various types.

204

Disorders associated with defined brain lesions

Spasticity Spasticity is muscle rigidity of central origin, as compared with various muscle spasms of peripheral origin such as those occurring in arthritis, migraine or trauma where treatment would be quite different. Spasticity of CNS origin, unlike spasms of peripheral origin, is rarely accompanied by pain. The brain lesions accompanying spasticity are varied and generally not discreetly localised. The extrapyramidal symptoms displayed are correspondingly varied. Typically, stretch reflexes are exaggerated, e.g. in the Babinski sign. There seems in general to be a loss of descending control mechanisms mediated by extrapyramidal pathways with some involvement of pyramidal tracts. Spasticity is often aggravated by anxiety and some of the drugs used with moderate success are anxiety-reducing agents such as diazepam. However, it is emphasised that there is as yet no really effective treatment. Baclofen is another drug for which some success has been claimed, especially in conditions associated with spinal cord lesions such as multiple sclerosis. The chemical name of this substance is p-chlorophenyl-GABA. It is a derivative of the inhibitory amino acid neurotransmitter, GABA. Like GABA, baclofen has inhibitory effects on neurones. However, unlike GABA, baclofen does not increase the conductance of the membrane to chloride ions, nor does it depolarise primary afferent terminals in the spinal cord. Nevertheless, there are specific binding sites to which both baclofen and GABA bind with high affinity. In sympathetic ganglia, in which it is well known that the transmitter is acetylcholine, it has been shown that baclofen and GABA inhibit transmission by decreasing the release of acetylcholine. This effect is not blocked by the competitive GABA-antagonist, bicuculline: it therefore does not interact with what is now termed the GABA-A receptor. The GABA-B receptor for which both GABA and baclofen have affinity is probably associated with a reduction in the permeability of the membrane to calcium ions which are necessary for transmitter release. However, it is also possible that an increase in the conductance of the terminal membrane to potassium ions decreases the probability

Parkinson fs disease

205

of invasion of the nerve terminals by an incoming impulse down the axon. Other drugs which have been used in spasticity include mephanesin, meprobamate and carisoprodal but baclofen and diazepam have made the use of these substances obsolete. Dantrolene is also used but acts peripherally by interfering with the release of calcium from the endoplasmic reticulum of skeletal muscle. Wilson's disease Wilson's disease is also known as hepato-lenticular degeneration because it is accompanied by definable lesions in the lenticular nucleus (globus pallidus and putamen) and in the liver, often presenting as chronic or viral hepatitis, sometimes with cirrhosis. It is a fairly rare familial disorder in which there is deficiency of ceruloplasmin, a copper-carrying protein, in the plasma. Copper is deposited in the basal ganglia and liver, and if untreated leads to permanent brain damage. Restriction of copper in the diet has no benefit but the administration of penicillamine, a copper chelating agent, descended from the original useful drug dimercaprol (British Anti-Lewisite): dimercaprol was developed to counteract arsenical poisoning and was found to be effective but toxic in Wilson's disease. If treatment with penicillamine is instituted before major lesions have been induced then there is hope that further deterioration can be prevented and physiotherapy may produce some recovery. Wilson's disease usually presents between the first and third decades of life. If it occurs early the main symptom is rigidity. Later, tremors, athetosis (swinging of the limbs between flexion and extension) or chorea (sudden, random, coordinated but involuntary flinging movements of the extremities). Parkinson's disease The modern approaches to pharmacological treatment of Parkinson's disease represent one of the few major advances made by a rational development of drugs designed to produce specific effects to correct a known disorder of function based

206

Disorders associated with defined brain lesions

upon an identified pathological lesion. Prior to the advent of Ldihydroxyphenylalanine (L-DOPA), the drugs used were based upon serendipity as in most other areas of therapy. The disease was first described by James Parkinson in 1817, who described patients suffering from what he called the 'shaking palsy' or paralysis agitans, what we now call Parkinson's disease. The treatment which he recommended was letting of blood from the neck followed by the application of vesicants and liniments to cause a purulent discharge. The illness can be considered to be of three types: (i) symptomatic; (ii) postencephalitic; (iii) idiopathic. The symptoms are similar but not necessarily identical in all three types. They consist of tremors, which tend to disappear during volitional movement, unlike the intentional tremors of cerebellar disease. There is also poverty or absence of movement (brady- or akinesia), including difficulties in initiating or arresting any one or a sequence of movements, mask-like facial expression, impairment of handwriting (micrographia), stooped, stiff postures, impaired speech and therefore difficulties in communication and 'cogwheel' rigidity. Mental faculties are generally not impaired. 'Charcot's triad' is the combination of akinesia, rigidity and tremor which are diagnostic. The tremor is relatively fast with a frequency of 3-4 per second with alternating contractions of flexor and extensor muscles, indicating a disturbance of fine motor control. (i) Symptomatic Parkinson's disease may follow injury to the CNS caused by trauma, senile arteriosclerosis, carbon monoxide poisoning, or manganese or other metallic poisoning. The lesions produced by these means may be irreversible. In contrast, Parkinsonian symptoms produced by antipsychotic drugs like the phenothiazines or butyrophenones are reversible upon withdrawal of the drug or reducing the dose. (ii) Postencephalitic Parkinson's disease was first observed as a consequence of an epidemic in 1916-17 of a disease of unknown etiology called encephalitis lethargica (sleeping sickness, Von Economo's disease) and presumed to be a virus. The first case appeared in Vienna in the winter of 1916-17, had spread to England by 1918 and to the USA, the following year. There have been

Parkinson's disease

207

no later resurgences of this illness. The neurological signs of the basal ganglia disorder occurred some years later, although they were seen, atypically, in some young children. (iii) Ideopathic Parkinson's disease is of unknown origin, rarely occurs before the age of 40 and reaches a peak between the ages of 50 and 60 years. The estimated incidence varies from about 1 : 40 to 1 : 500 and is currently at a plateau. Genetic factors are not clearly defined although males are slightly more susceptible than females. There is usually a progressive deterioration over a 10-15 year time span from the onset of symptoms and life expectancy is not greatly increased by drug treatment, even though it is often very effective in relieving the most distressing symptoms. In 1982, classical symptoms of Parkinson's disease suddenly appeared in a group of young drug addicts in California. This was traced to N-methyl-4-phenyl-1,2,3,6,-tetrahydropyridine (MPTP), a toxic substance accidentally produced as a 'designer drug' during the illegal preparation of'synthetic heroin'. It is possible, but not yet proven, that MPTP or a substance like it is present in the environment in small quantities and could be the causal agent of spontaneously occurring Parkinson's disease. The drug is absorbed by inhalation or skin contact. It is probable that MPTP is metabolised by MAO-B in brain to l-methyl-4phenylpyridinium, which may be the final toxic agent since inhibition to MAO-B reduces the toxicity. If such treatment is effective in preventing or reducing degeneration in Parkinsonian patients, then it will be a major step forward in the prevention of progressive deterioration, rather than just reducing symptoms with no effect on the underlying pathology, there is an increased susceptibility to MPTP with age, which may be related to increased activity of MAO in the aging process. MPTP is probably selectively taken up into dopaminergic neurones since these are selectively destroyed, just as in Parkinson's disease. MPTP has a number of pharmacological actions including depletion of dopamine, which may not be the cause of cell death, a block of the uptake of dopamine and of 5-HT and a reduction of dopamine release.

208

Disorders associated with defined brain lesions Drug

treatment

A number of drugs are available for treatment of Parkinson's disease (Fig. 12.1) but L-DOPA reigns supreme with bromocriptine coming a close second. The mechanisms can be separated into several different types: Antagonism at muscarinic receptors for acetylcholine. Belladonna

alkaloids (atropine) were first used by Charcot in 1892. Synthetic anticholinergic compounds include benztropine, ethopropazine, trihexyphenidyl and procyclidine. These are sometimes used in conjunction with L-DOPA but may have some adverse effect on the absorption of the latter. The anticholinergic compounds are the major drugs used in treating the Parkinsonian-like sideeffects of the antischizophrenic drugs. The action of the anticholinergic compounds is usually attributed to restoring the balance of cholinergic and dopaminergic transmission in the basal ganglia. However, these substances also block the uptake of dopamine in synaptosomes and benztropine, at least, causes the release of newly synthesised dopamine and the stimulant action in mice is blocked by a-methyl-/?-tyrosine, which blocks dopamine synthesis, but not by an acute lesion of the nigro-striatal tract (i.e. terminals are still intact). 'Replacement therapy': L-DOPA. The history of the introduction of L-DOPA is one of the few examples of the rational development of a new drug. In 1957 it was known that treatment of experimental animals with reserpine, which depletes nerve terminals of their catecholamine content, replicated some of the symptoms of Parkinson's disease. Especially noticeable in mice or rats treated with reserpine is the tremor, rigidity and akinesia. In 1957 it was shown that this action was reduced by the administration of DOPA. Two years later it was shown that dopamine is highly concentrated in the basal ganglia in normal human postmortem brain. It was in 1961 that dopamine was found to be low in the basal ganglia of patients dying with Parkinson's disease. Over the next five or six years DL-DOPA was tried in moderate doses with only slight improvement and numerous sideeffects. In 1967, large doses of L-DOPA (3-8 g by mouth) were

Parkinson's disease

209

employed. This gave a remarkable improvement in the akinesia and rigidity with lesser effect on the tremor. In 1971 it was shown that the entire dopaminergic projection to the basal ganglia arose from the substantia nigra. It is thought that the effect of administering large quantities of L-DOPA is to increase the amount of dopamine synthesised by those terminals which have not degenerated. Nausea was experienced in about 80% of the patients treated with L-DOPA, with vomiting and anorexia in a smaller proportion. However, these symptoms diminished with continuation of treatment. Involuntary movements of athetosis and chorea were the most disabling of all symptoms and occurred in about 60% of patients. The commonest reason for withdrawal of L-DOPA was the occurrence of psychic side-effects in about 15% of patients. It has been suggested (Paalzow & Paalzow, 1986) that under some circumstances treatment with L-DOPA may exacerbate the Parkinsonian symptoms and that fcon-off effects of L-DOPA during the course of the day may, in part, be related to pharmacokinetic considerations. Prevention of breakdown of L-DOPA. Carbidopa is N-(DL-seryl)-N-

(2,3,4,trihydroxybenzyl)hydrazine. It is an inhibitor of DOPA decarboxylase peripherally, but it is unable to enter the central nervous system. This prolongs the action of L-DOPA by reducing its inactivation. It is ineffective by itself but it enables smaller doses of L-DOPA to be used, with resulting lower blood levels and therefore fewer gastro-intestinal side-effects. However, the unwanted effects on the central nervous system, especially the involuntary movements, are unchanged. Deprenyl is a selective inhibitor of monoamine oxidase-B (MAO-B). MAO-B is present in parts of the CNS and it is thought that the effectiveness of deprenyl in Parkinson's disease is due to the prolongation of the action of endogenous dopamine. It is useful because it does not inhibit the peripheral MAO-A to any great extent so that breakdown of indirectly acting sympathomimetic amines such as tyramine may proceed as normally, with little risk of the hypertensive crises that can ensue when this enzyme is inactivated. However, deprenyl increases the

210

Disorders associated with defined brain lesions HO 3, 4-dihydroxyphenyl-L-alanine (L-DOPA)

HO

"A

V C H 2 'CHNH2

I

CH3

Benztropine

Trihexyphenidyl

Diethazine CH2CH2N(C2H5)2

Ethopropazine CH 2 CHN(CH 5 ) 2 I CH3

severity of hon-off effects with L-DOPA, particularly after prolonged treatment when the effect of L-DOPA is declining. If Parkinson's disease is due to a substance such as MPTP in the environment which needs to be broken down by MAO-B into a more toxic metabolite then treatment with deprenyl may reduce further structural deterioration. The answer to this will not be available until long-term studies are completed.

211

Parkinson's disease

Amantadine

Apomorphine

COOH

I

.CH2-C-CH3 NH-NH2 Carbidopa 1 -methyl-4-pheny 1tetrahydropyridine (MPTP)

Deprenyl

N—CH 3

cv

CH, CH 2 CH—N—CH 2 C=CH CH 3

Fig. 12.1. Drugs and Parkinson's disease.

Release of dopamine. Amantadine was introduced as an antiviral agent in 1964. Amantadine causes the release of dopamine from nerve terminals. The release can be prevented in vivo by a lesion of the nigro-striatal tract. Amantadine also blocks the re-uptake of dopamine into terminals, so increasing the effect of any which is released.

212

Disorders associated with defined brain lesions

Agonist binding to dopamine receptors. Apomorphine binds to

dopamine, Dl type, receptors and activates adenylate cyclase. It is effective in Parkinson's disease but hepato- and nephrotoxicity has prevented its widespread use. The binding of 3H-apomorphine in postmortem brain from Parkinsonian patients is reduced by about 60% compared with normal brain. This indicates that the binding sites are largely located presynaptically on the dopaminergic neurons. In contrast, the binding of 3H-haloperidol, a reasonably selective antagonist acting on D2 receptors for dopamine is increased by about 60%. This indicates that the postsynaptic D2 receptors proliferate. However, treatment with L-DOPA prevents the change in the haloperidol binding sites. Bromocriptine is an ergot derivative which has some selectivity as an agonist at D2 receptors for dopamine. It has found considerable use in Parkinson's disease, particularly when used in combination with L-DOPA. It has a longer duration of effect than LDOPA and causes fewer involuntary movements. It has been suggested that the action of bromocriptine is to potentiate the action of dopamine itself, since it atypically does not convert the high to the low affinity state of the receptor (Goldstein et dl. 1985). It was therefore predicted that it would be most effective if combined with low doses of L-DOPA. A disadvantage of bromocriptine is that hallucinations are more frequent with bromocriptine than with L-DOPA. Lisuride and pergolide act in a similar way to bromocriptine. Peripheral side-effects of nausea and vomiting are reduced by the administration of domperidone, a dopamine receptor antagonist which does not cross the blood-brain barrier. A combination of lisuride with L-DOPA has given promising results in causing less 'on-off effects. Lithium has also been found to reduce these unexplained effects, but the mechanism of action of lithium in this respect is unknown. The mechanisms of the effective anti-Parkinsonian agents is summarised in Fig. 12.2

Fig. 12.2. Sites of drug action in Parkinson's disease and Huntington's chorea. Huntington's disease

Parkinson's disease

Caudate Putamen Globus pallidus

Caudate Putamen Globus pallidus

Globus pallidus 5-HT normal GAB A decreased GAD decreased DA increased Ch. Ac. Tr. decreased ACh receptor decreased

Z. compacta Subs tan tia nigra

T-OH increased GABA receptors increased

5-HT ACh normal GABA DA decreased Apomorphine binding decreased Haloperidol binding increased

DA'

DA neurones degenerated

Z. reticulata Drugs used Tetrabenazine Chlorpromazine Haloperidol Also effective; ? Choline; lecithin; physostigmine

Drugs used L-DOPA (with decarboxylase inhibitor) Anticholinergics Amantadine Bromo crip tine ? Also effective but nephrotoxic, apomorphine

I

214

Disorders associated with defined brain lesions

Huntington's disease Huntington's disease, also known as Huntington's chorea or senile chorea is a hereditary disease, with autosomal dominance, of the basal ganglia and the cerebral cortex. A specific antineuronal antibody (Igg) has been found in 50% of patients, in 23% of close relatives and in 3-6% of the normal population. The disease occurs in both sexes of all races and only rarely does it miss a generation. The ancestry of Huntington's chorea in the USA has been traced to three immigrants from England in 1630. In Venezuela, the illness has been traced to a single German sailor who settled in 1860. Choreiform movements and mental deterioration occur in adult life, usually appearing between the ages of 30 and 60 years, and are accompanied by widespread lesions in the brain leading to death in about 15 years. The movements may be increased by emotional disturbance but tend to disappear during sleep. Chorea is increased by treatment with L-DOPA, which may even precipitate symptoms in siblings in whom there was no other evidence of the disease. Biochemical and structural changes Histologically, there is a considerable atrophy of parts of the brain, especially of the basal ganglia. The levels of tyrosine hydroxylase are increased in the striatum and substantia nigra in the brains of patients dying with Huntington's chorea. Dopamine in the caudate nucleus and nucleus accumbens is increased. It is likely that this apparent increase is only relative, due to the substantial loss of other neurones. There has been a similar increase in 5-hydroxytryptamine levels, which can probably also be attributed to the neuronal loss. In contrast, there is marked loss of substance P in the medial pallidum and substantia nigra, indicating the degeneration of a substance P projection from globus pallidus to substantia nigra. Similarly, there is a substantial loss of angiotensin converting enzyme in the striatum and substantia nigra. There is also some evidence for degeneration of a striato-nigral met-enkephalincontaining pathway.

Alzheimer's disease

215

More interest has been shown in the loss of gabergic neurons projecting from the caudate to the substantia nigra. The greatest loss of glutamic acid decarboxylase, the GABA synthesising enzyme, occurs in the zona reticulata, where the striato-nigral gabergic fibres terminate. There is an increase in the affinity for GABA binding, with no change in flmax in the substantia nigra. In some, but not all, patients dying with Huntington's chorea there is a loss of cholinergic receptors and of choline acetyltransferase from the putamen and caudate nucleus. This enzyme synthesises acetylcholine for choline and is a good marker for cholinergic neurones. Since the reduction of receptor is not always accompanied by a decrease in the synthesising enzyme, it has been postulated that postsynaptic lesions may precede presynaptic changes. In summary (Fig. 12.2) there is good evidence for a loss of gabergic neurones projecting from striatum to substantia nigra in Huntington's chorea. There is also a degeneration of some peptidergic pathways and in some, but not all individuals there is a loss of intra-striatal cholinergic neurones. Treatment It is curious that the most used treatment is with agents such as haloperidol or chlorpromazine: it will be recalled that these agents block receptors for dopamine and that the dopamine system is one which is relatively unchanged in Huntington's chorea. This may be seen as an attempt to restore the balance caused by the loss of other systems. Other agents which have been claimed to have been of some benefit include substances acting to increase the activity of the cholinergic system. These include physostigmine, which blocks acetylcholinesterase, choline itself and lecithin, which is a choline precursor. These agents cannot be considered to be established treatments, but are of interest in that they illustrate the attempts to compensate for selective neuronal loss by either increasing activity in a deficient system, or by blocking the activity in another system, the dopamine system, which may be relatively overactive, due to the loss of systems with which it normally acts in harmony.

216

Disorders associated with defined brain lesions

Alzheimer's disease Alzheimer's disease, or dementia of the Alzheimer type, occurs in about one in six persons over the age of sixty. The personality changes and loss of memory are catastrophic and accompanied by characteristic lesions in the brain in cerebral cortex, amygdala and hippocampus. These changes include the formation of tangles and plaques accompanied by neurochemical changes in a number of neurotransmitters. There is no known cause of the disease but there are many hypotheses. There some observations which indicate that the lesions could be produced by an increased sensitivity to an endogenous excitatory amino acid. First, there is a decreased population of receptors for glutamic acid of the N-methyl-D-aspartate (NMDA) type. A number of toxic actions have been demonstrated with toxic derivatives of the endogenous excitatory amino acids. In particular it has been shown that N-methyl-D-aspartic acid applied to the cerebral cortex produces biochemical changes similar to those of Alzheimer's disease and causes lesions of cholinergic neurones in the nucleus basalis. Uniquely, so far at least, glutamate induces the formation of structures in human spinal cord neurons in tissue culture which are similar to the neurofibrillary tangles characteristic of Alzheimer's disease. These findings suggest that substances which block glutaminergic transmission by acting as antagonists at NMDA receptors may prove to be of value in treating Alzheimer's disease. A number of such substances are now known, including the dissociative anaesthetic phencyclidine, but have yet to be evaluated. In the meantime, there is suggestive evidence that the use of cholinesterase inhibitors may improve memory in patients with Alzheimer's disease, even if the effect is only temporary. There are also pointers to the involvement of NMDA receptors in longterm potentiation and memory which may also lead to therapeutic advances in the future. There is also evidence that antagonists at NMDA receptors may be of prophylactic value in preventing cell death due to hypoxia or ischaemia. Such therapy would be of enormous value in stroke victims.

SELECTED READING

Chapter 1 Introduction Feldman, R.S. & Quenzer, L.F. (1984). Fundamentals of Neuropsychopharmacology. Sinauer. New York. Goodman Gilman, A., Goodman, L.S., Rail, T.W. & Murad, F. (1985). The Pharmacological Basis of Therapeutics. Seventh Edition. MacMillan. New York. Lamble, J.W., Ed. (1980) More about Receptors. Elsevier. Amsterdam. Lamble, J.W., & Abbott, A.C. Eds. (1984). Receptors Again. Elsevier, Amsterdam. Rang, H.P. & Dale, M.M. (1987). Pharmacology. Churchill-Livingstone. London. Snyder, S. (1986). Drugs and the Brain. Scientific American Books. New York. Chapter 2 Techniques Dingledine, R. (1983). Brain Slices. Plenum. New York. McBurney, R.N. (1983). New approaches to the study of rapid events underlying neurotransmitter action. Trends in Neurosci., 6, 297-302. Chapter 3 Neuromuscular junction Adams, P.R. (1978). Molecular aspects of synaptic transmission. Trends in NeuroscL, 1, 141-3. Behan, P.O. (1979). The immunology of myaesthenia gravis. Trends in Neurosci., 1, 31-3. Drachman, D.B. (1983). Myaesthenia gravis: immunobiology of a receptor disorder. Trends in Neurosci., 6, 446-51. Giraudat, J. & Changeux, J-P. (1980). The acetylcholine receptor. Trends in Pharmacol. ScL 1, 198-202. Grob, D. (Ed). (1976). Myasthenia Gravis. Ann. NYAcad ScL 274, 1682. Katz, B. (1966). Nerve Muscle and Synapse. New York: McGraw-Hill. Katz, B. & Miledi, R. (1972). The statistical nature of the acetylcholine potential and its molecular components. /. Physiol, 230, 665-99. Katz, B. & Miledi, R. (1973). The characteristics of'endplate noise' produced by different depolarising drugs. /. Physiol, 230, 707-17. Neher, E. & Sakmann, B. (1976). Single channel currents recorded from

218

Selected reading

membrane of denervated frog muscle fibres. Nature (Lond), 260, 799-802. Neher, E. & Stevens, C.F. (1977). Conductance fluctuations and ionic pores in membranes. Ann. Rev. Biophys. Bioengng., 6, 345. Rang, H.P. & Ritter, J.M. (1970). On the mechanism of desensitization at cholinergic receptors. Mol Pharmacol., 6, 383-90. Rang, H.P. & Ritter, J.M. (1970). The relationship between desensitisation and the metaphilic effect at cholinergic receptors. Mol Pharmacol, 6, 383-90. Thesleff, S. (1955). The mode of neuromuscular block caused by acetylcholine, nicotine, decamethonium and succinylcholine. Acta Physiol Scand, 34, 218-31. Thesleff, S. & Sellin, L.C. (1980). Denervation supersensitivity. Trends in Neurosci., 3, 122-4. Wray, D. (1981). Prolonged exposure to acetylcholine: Noise analysis and channel inactivation in cat tenuissimus muscle./ Physiol., 310, 37-56. Chapter 4 Autonomic nervous system Brown, D. (1982). Peptidergic transmission in ganglia. Trends in NeuroscL, 5, 34-5. Eccles, R.M. & Libet, B. (1961). Origin and blockade of the synaptic responses of curarized sympathetic ganglia. 7. Physiol, 157, 484-503. Jan, Y.N. & Jan, L.Y. (1983). A LHRH-like peptidergic neurotransmitter capable of 'action at a distance' in autonomic ganglia. Trends in Neurosci., 6, 320-5. Langer, S.Z. (1977). Presynaptic receptors and their role in the regulation of transmitter release. Br. J. Pharmac, 60, 481-97. Langer, S.Z. (1980). Presynaptic receptors and modulation of neurotransmission: pharmacological implications and therapeutic relevance. Trends in Neuroscl, 3, 110-12. Laverty, R. (1973). The mechanisms of action of some anti-hypertensive drugs. Br. Med. Bull, 29, 152-7. Libet, B. (1977). The role SIF cells play in ganglionic transmission. Ann. Rev. Pharmacol, 9, 135-47. Otsuka, M. & Konishi, S. (1983). Substance P - the first peptide neurotransmitter. Trends in Neuroscl, 5, 317-20. Trendelenburg, U. (1979). The extraneuronal uptake of catecholamines: is it an experimental oddity or a physiological mechanism? Trends in Pharmac. ScL, 1, 4-6. Chapter 5 Central neurotransmitters and neuromodulators Artola, A. & Singer, W. (1987). Longterm potentiation and NMD A receptors in rat visual cortex. Nature (Lond), 330, 649-52. Beart, P.M. (1982). Multiple dopamine receptors-new vistas. Trends in Pharmacol ScL, 2, 100-2.

Selected reading

219

Bradley, P.B. (1987). Pharmacology: 5-HT3 receptors in the brain? Nature (Lond), 330, 696. Collingridge, G. (1987). Synaptic plasticity: The role of NMDA receptors in learning and memory. Nature (Lond), 330, 604. Creese, I. (1982). Dopamine receptors explained. Trends in Neurosci 5 40-3. Creese, I. (1985). Dopamine receptor subtypes. Trends in Pharmacol. ScL 6, centrefold. Davidoff, R.A. (ed.) (1983). Handbook of the Spinal Cord. Marcel Dekker, New York. Hanley, M.R. & Jackson, T. (1987). Substance K receptor: return of the magnificent seven. Nature (Lond), 329, 766-7. Henry, H.L., Couture, R., Cuello, A.C., Pelletier, G., Querion, R. & Regoli, D. (eds.) (1987). Substance P and Neurokinins. SpringerVerlag, New York. Kilpatrick, G.J., Jones, B.J. & Tyers, M.B. (1987). Identification and distribution of 5-HT3 receptors in rat brain using radioligand binding. Nature (Lond), 330, 746-9. Leff, S.E. & Creese, I. (1983). Dopamine receptors re-explained. Trends in Pharmacol ScL, 4, 463-7. Masu, Y., Nakayama, K., Tamaki, H., Harada, Y., Kuno, M. & Nakanishi, S. (1987). cDNA cloning of bovine substance-K receptor through oocyte expression system. Nature (Lond), 329, 836-8. Porter, R. & O'Connor, M. eds. (1982). Substance P in the nervous system. Ciba Foundation Symposium No. 91. Pitman. London. Richardson, B. & Engel, G. (1986). The pharmacology and function of 5-HT3 receptors. Trends in Neurosci., 9, 424-8. Rogawski, M.A. & Barker, J.L. (1985). Neurotransmitter Actions. Plenum. New York. Chapter 6 The blood-brain barrier Bowman, W.C. & Rand, M. (1980). Textbook of Pharmacology. 2nd Edn. Bradbury, M. (1979). The Concept of a Blood-Brain Barrier, John Wiley, Chichester. Bradbury, M. (1979). Why a blood-brain barrier? Trends in Neurosci, 2, 36-8. Davson, H. (1978). The environment of the neurone. Trends in Neurosci., 2, 39-41. Lund-Anoksen, H. (1979). Transport of glucose from blood to brain. Physiol. Rev., 59, 305-52. Chapter 7 General anaesthetics Dundee, J.W. (1971). Comparative analysis of intravenous anaesthetics. Anesthesiology, 35, 137-48. Nicholl, R.A. (1979). Differential postsynaptic effects of barbiturates on chemical transmission. In, Neurobiology of Chemical Transmission.

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Selected reading

Otsuka, M. & Hall, Z.W. (eds.). John Wiley & Sons. New York. pp. 267-78. Pender, J.W. (1971). Dissociative anaesthesia,/ Am. Med. Ass., 215, 1126-30. Richards, CD. (1980). The mechanism of anaesthesia. In Topical Reviews in Anaesthesia. Norman, J. & Whitwam, J. (eds.). Vol. 1. Wright. Bristol. Richards, CD. & Hesketh, T.R. (1975). Implications for theories of anaesthesia of antagonism between anaesthetic and non-anaesthetic steroids. Nature (Lond), 256, 179-82. Seeman, P. (1972). The membrane actions of anesthetics and tranquillizers. Pharmacol. Rev., 24, 583-655. Study, R.E. & Barker, J.L. (1981). Diazepam and (-)-pentobarbital: Fluctuation analysis reveals different mechanisms for potentiation of GABA responses in cultured central neurons. Proc. Natl. Acad. Sci. USA., 78, 7180-4. Halsey, et al. (1974). Molecular Mechanisms in General Anaesthesia.

Churchill-Livingstone. Edinburgh. Weakly, J.N. (1969). Effect of barbiturates on 'quanta!' synaptic transmission in spinal motoneurones. J. Physiol, 204, 63-77. Chapter 8 Pain and analgesia Chung, Shin-Ho & Dickenson, A. (1980). Pain, enkephalin and acupuncture. Nature (Lond), 283, 243-4. Duggan, A.W. & North, R.A. (1983). Electrophysiology of opioids. Pharmacol. Rev., 35, 219-82.

Basbaum, A.I. & Fields, H.L. (1984). Endogenous pain control systems: brainstem spinal pathways and endorphin circuitry. Ann. Rev. NeuroscL, 7, 309-38. Hughes, J., Ed. (1983). Opioid peptides. Brit. Med Bull, 39, 1-100. Jesell, T.M. & Iversen, L.L. (1977). Opiate analgesics inhibit substance P release form rat trigeminal slices. Nature (Lond), 268, 549-551. Khatchaturian, H., Lewis, M.E., Schafer, M.K.-H. & Watson, S.J. (1985). Anatomy of the CNS opioid systems. Trends in NeuroscL 8, 111-19. Kosterlitz, H.W. (ed.) (1976). Opiates and Endogenous Opioid Peptides.

North Holland. Amsterdam. Kosterlitz, H.W. (1987). Biosynthesis of morphine in the animal kingdom. Nature (Lond), 330, 606. Kosterlitz, H.W. & Paterson, S.J. (1980). Characterization of opioid receptors in nervous tissue. Proc. R. Soc. B., 210, 113-22. Lord, J.A.H., Waterfield, A.A., Hughes, J. & Kosterlitz, H.W. (1977). Endogenous opioid peptides: multiple agonists and receptors. Nature (Lond), 267, 495-9. Mayer, D.J., Price, D.D. & Rafii, A. (1977). Antagonism of acupuncture analgesia in man by the narcotic antagonist naloxone. Brain Res., 121, 368-72. Miller, R. (1978). Enkephalin: a peptide with morphine-like properties. Trends in NeuroscL, 1, 29-31.

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Milton, A.S. (1976). Modern views on the pathogenesis of fever and the mode of action of antipyretic drugs. /. Pharm. Pharmacol, 28, 393-9. North, R.A. & Williams, J.T. (1983). How do opiates inhibit transmitter release? Trends in Neurosci., 6, 337-9. Pomeranz, B. (1977). Brain opiates at work in acupuncture. New Scientist, 6th June, 12-13. Roemar, D., Buescher, H.H., Hill, R.C., Pless, J., Bauer, W., Cardinaux, F., Closse, A., Hauser, D. & Huguenin, R. (1977). A synthetic enkephalin analogue with prolonged parenteral and oral analgesic activity. Nature (Lond), 268, 547-9. Schwartz, J-C. (1979). Opiate receptors on catecholaminergic neurones in brain. Trends in Neurosci., 2, 137-9. Wall, P.D. & Melzack, R. (1984). Textbook of Pain. ChurchillLivingstone, Edinburgh. Weber, E., Evans, C.J. & Barchas, J.D. (1983). Multiple endogenous ligands for opioid receptors. Trends in Neurosci., 6, 333-6. Chapter 9 Drug interactions with inhibitory amino acids Barnard, E.A., Darlison, M.G. & Seburg, P. (1987). Molecular biology of the GABA-A receptor/channel superfamily. Trends in Neurosci., 10, 502-9. Barker, J.L. and Matthers, D.A. (1980). GAB A receptors and the depressant action of pentobarbital. Trends in Neurosci., 1, 257. Braestrup, C. and Nielsen, M. (1980). Multiple benzodiazepine receptors. Trends in Neurosci., 3, 301-3. Burnham, W.M., Spero, L., Okazaki, M.M. and Madras, B.K. (1981). Saturable binding of 3H-phenytoin to rat brain membrane fraction. Can. J. Physiol. Pharmacol., 59, 402-7. Chiu, T.H. and Rosenberg, H.C. (1983). Multiple conformational states of benzodiazepine receptors. Trends in Pharmacol. Sci., 4., 34850. Fujimoto, M., Hirai, K. & Okabayashi, T. (1982). Comparison of the effects of GABA and chloride ion on the affinities of ligands for the benzodiazepine receptor. Life Sciences, 30, 51-7. Haefely et al. (1981). General pharmacology and neuropharmacology of the benzodiazepines. Handbook Exptl. Pharmacol, 55 (II), 13-262. Karobath, M. (1979). Molecular basis of benzodiazepine actions. Trends in Neurosci, 2, 166-8. Macdonald, R.L. and McLean, M.J. (1982). Cellular bases of barbiturate and phenytoin anticonvulsant drug action. Epilepsia, 23, Suppl. 1, S7-S18. Martin, I.L. (1980). Endogenous ligands for benzodiazepine receptors. Trends in Neurosci., 3, 299-310. Martin, I.L. (1986). The benzodiazepine receptor. In Neuromethods\ eds. G.B. Baker and A.A. Boulton. 4, 12.1-12.31. Humana Press. Turner, A.J. and Whittle, S.R. (1980). Sodium valproate, GABA and epilepsy. Trends in Pharmacol Sci., 2, 257-60. Willow, M. (1986). Pharmacology of diphenylhydantoin and

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Selected reading

carbamazepine action on voltage-sensitive sodium channels. Trends in Neuroscl, 9, 147-9. Chapter 10 Drugs used in schizophrenia Bissette, G., Nemeroff, C.B. & MacKay, A.V. (1986). Neuropeptides and schizophrenia. Progr. Brain Res., 66, 161-74. Bunney, B.S. (1984). Antipsychotic effects on the electrical activity of dopaminergic neurons. Trends in Neuroscl, 5, 212-15. Crawley, J.C.W., Crow, T.J., Johnstone, E.C., Oldland, S.R.D., Owen, F., Owens, D.G.C., Smith, T., Veall, N. & Zanelli, G.D. (1986). Uptake of 77Br-spiperone in the striata of schizophrenic patients and controls. Nucl Med. Commun., 7, 599-607. Crow, T.J. (1979). What is wrong with dopaminergic transmission in schizophrenia? Trends in Neuroscl, 2, 52-6. Crow, TJ. (1984). A re-evaluation of the viral hypothesis: is psychosis the result of retroviral integration at a site close to the cerebral dominance gene? Br. J. Psychiat., 145, 243-53. Crow, TJ. & Johnstone, E.C. (1978). ECT - Does it work? Trends in Neuroscl, 1, 51-3. Crow, T.J., Johnston, E.C, Deakin, J.W.F. & Longden, A. (1976). Dopamine and schizophrenia. Lancet, Sept. 11, 563. Farde, L., Weisel, F-A, Hall, H., Halldin, C, Stone-Elander, S. & Sedvall, G. (1987). No D2 receptor increase in PET study of schizophrenia. Arch. Gen. Psych., 44, 671-2. Hirsch, S.R. (1979). Do parents cause schizophrenia? Trends in neuroscl, 2, 49-52. Hornykiewicz, O. (1977)vPsychopharmacological implications of dopamine and dopamine antagonists: a critical evaluation of current evidence. Ann. Rev. Pharmacol, Toxicol, 17, 545-59. Iversen, L.L., Iversen, S.D. & Snyder, S.H. (1975). Handbook of Psychopharmacology. Plenum. New York. Kebabian, J.W., Agui, J.C., van Oene, J.C., Shigematsu, K. & Saavedra, J.M. (1986). The DI dopamine receptor: new perspectives. Trends in Pharmacol Scl, 7, 96-9. Mackay, A.V.P. (1984). High dopamine in the left amygdala. Trends in Neuroscl, 5, 107-8. McCann, S.M., Lumpkin, M.D., Mizunuma, H., Khorram, O., Ottlecz, A. & Samson, W.K. (1984). Peptidergic and dopaminergic control of prolactin release. Trends in Neuroscl, 5, 127-31. Miller, RJ. & Hiley, R. (1974). Antimuscarinic properties of neuroleptics and drug-induced Parkinsonism. Nature (Lond), 248, 596-7. Nemeroff, C.B. & Cain, S.T. (1985). Neurotensin-dopamine interactions in the CNS. Trends in Pharmacol. Scl, 6, 201-5. Phillips, A.G., Lane, R.F. & Blaha, CD. (1986). Inhibition of dopamine release by cholecystokinin: relevance to schizophrenia. Trends in Pharmacol. Scl, 7, 126-8. Reynolds, G.P. (1979). Phenylethylamine - a role in mental illness? Trends in Neuroscl, 2, 265-8.

Selected reading

223

Smythies, J.R. (1984). The transmethylation hypothesis of schizophreniz re-evaluated. Trends in NeuroscL, 5, 45-7. Stevens, J.R. (1979). Schizophrenia and dopamine regulation in the mesolimbic system. Trends in NeuroscL, 2, 102-5. Tyrer, P. & Mackay, A. (1986). Schizophrenia: no longer a functional psychosis. Trends in NeuroscL 9, 537-8. Wang, R.Y., White, F.J. & Voigt, M.M. (1984). Cholecystokinin, dopamine and schizophrenia. Trends in Pharmacol Sci., 5, 436-8. Weissman, M.M. (1986). Psychiatric diagnoses. Science, 235, 522. de Wied, D. (1979). Schizophrenia as an inborn error in the degradation of beta-endorphin - a hypothesis. Trends in Neuwsci., 2, 79-82. Wong, D.F., Wagner, H.N., Tune, L.E., Dannals, R.F., Pearlson, G.D., Links, J.M., Taminga, C.A., Broussolle, E.P., Ravert, H.T., Wilson, A.A., Toung, J.K.T., Malat, J., Williams, J.A., OTauma, L.A., Snyder, S.H., Kuhar, MJ. & Gjedde, A. (1986). Positron emission tomography reveals D2 dopamine receptors in drug-naive schizophrenics. Science, 234, 1558-63. Chapter 11 Affective and manic depression Akasura, M. Tsukamoto, T. & Hasegawa, K. (1982). Modulation of rat brain ai and p-adrenergic receptor sensitivity following long term treatment with antidepressants. Brain Res., 235, 192-7. Emrich, H.M., Aldenhoff, J.B. & Lux, H.D. (eds.) (1982). Basic Mechanisms in the Action of Lithium. Excerpta Med. Int. Congr. Ser, 572, 71-

9. Elsevier. Green, R.A. (1978). ECT-How does it work? Trends in Neuwsci., 1, 534. Green, R.A. & Maayani, M. (1977). Tricyclic antidepressant drugs block histamine H2-receptors in brain. Nature (Lond), 269, 163-5. Harper, B. & Hughes, I.E. (1979). Presynaptic a-adrenoceptor blocking properties of tri- and tetracyclic antidepressant drugs. Br. J. Pharmac, 67,511-17. Hodgkinson, S., Sherrington, R. Gurling, H.M.D. et al. (1987). Molecular genetic evidence for heterogeneity in manic depression. Nature (Lond), 325, 805-6. Kimmelberg, H.K. (1986). New antidepressant drugs: is there anything new they can tell us about depression? Trends in Neuwsci., 9, 314. Langer, S.Z. & Briley, M. (1981). High-affinity 3H-imipramine binding: a new biological tool for studies in depression. Trends in Neuwsci., 4, 28-31. Maas, J.W. (1979). Neurotransmitters and depression. Too much, too little or too unstable? Trends in Neuwsci., 2, 306-8. Maj, J. (1981). Antidepressant drugs: will new findings change the present theories of their action? Trends in Pharmacol Sci., 1, 80-3. Sulzer, F. (1979). Newperspectives on the mode of action of antidepressant drugs. Trends in Pharmacol Sci., 1, 92-4. Tyrer, P. & Marsden, C. (1985). New antidepressant drugs: is there anything new they can tell us about depression? Trends in Neuwsci., 8,427-31.

224

Selected reading

Chapter 12 Disorders associated with defined brain lesions Bartolini, G. (1980). Interactions of striatal dopaminergic, cholinergic and GABA-ergic neurons: relation to extrapyramidal function. Trends in Pharmacol ScL 1, 138-40. Bird, E.D. (1978). Huntington's disease (chorea). Trends in Neurosci., 1, 57-9. Bird, E.D. (1980). Chemical pathology of Huntington's disease. Ann. Rev. Pharmacol. ToxicoL 20, 533-51. Goldstein, M., Liberman, A. & Meller, E. (1985). A possible molecular mechanism for the antiparkinsonian action of bromocryptine in combination with levodopa. Trends in Pharmacol Sci., 6, 436-7. Hefti, F. & Melamed, E. (1980). LDOPA's mechanism of action in Parkinson's disease. Trends in NeuroscL, 3, 229-31. Hornykiewicz, O. (1973). Parkinson's disease from brain homogenate to treatment. Fed. Proc, 32, 183-90. Kindt, M.V., -Nicklas, W.J., Sonsalla, P.K. & Heikkila, R.E. (1986). Mitochondria and the neurotoxicity of MPTP. Trends in Pharmacol Sci., 7, 473-5. Langston, J.W. (1985). MPTP and Parkinson's disease. Trends in NeuroscL 8, 79-83. Langston, J.W. (1987). Closer to Parkinson's disease: MTP and the ageing nervous system. Trends in Pharmacol Sci., 8, 7-8. Larsen, T.A. & Calne, D.B. (1982). Recent advances in the study of Parkinson's disease. Trends in Neurosci., 5, 10-12. Lee, T., Seeman, P., Rajput, A., Farley, I.J. & Hornykiewicz, O. (1978). Receptor basis for dopaminergic supersensitivity in Parkinson's disease. Nature (Lond), 273, 59-61. Lloyd, K.G. & Davidson, L. (1979). 3H-GABA binding in brains from Huntington's chorea patients: altered regulation by phospholipids. Science, 205, 1147-9. Maragos, W.F., Greenamyre, J.T., Penney, J.B. & Young, A.B. (1987). Glutamate dysfunction in Alzheimer's disease: an hypothesis. Trends in Neurosci., 10, 65-8. Paalzow, G.H.M. & Paalzow, L.K. (1986). L-DOPA: how it may exacerbate Parkinsonian symptoms. Trends in Pharmacol ScL 7, 15-19. Von Voigtlander, P.F. & Moore, K.E. (1971). Dopamine: Release from the brain in vivo by amantadine. Science, 174, 408-10.

INDEX

A-fibres, 119-20, 123, 126 AH 8165 (fazadinium), 29-30 Acetazolamide, 96 Acetylcholine (ACh), 53, 77 action, 85 in autonomic nervous system, 44, 45 benzodiazepine effect on, 151 blocking agents: depolarising (desensitising), 25, 30, 39-40; mechanisms, 32-8; metaphilic antagonists, 38; non-depolarising (competitive), 25, 29-30, 30-2, 39-40 mobilisation, 27 at neuromuscular junction, 16-17, 18-22 receptor complex, 17, 18 receptors, 82 fig.; activation, 20-2; and antidepressant action, 198; autoantibodies, 41; muscarinic, 13, 18-19, 77; nicotinic, 19, 30 storage and release of, 27-9 structure, 83 Acetylcholinesterase, 17-18, 23-4, 77 Action potentials, 1, 10-11, 16 Acupuncture, 136 Adenylate cyclase, 140, 183 Adrenaline, 64, 76 cardiovascular effects, 64-5 in CNS, 91 mechanism of action, 70 structure, 83 uptake, 73 Affective disorders, 171 Akathisia, drug-induced, 187 Alpha-methyl DOPA, 55 fig., 75, 98 Alpha-receptors, 64, 68-70 blocking agents, 75-6 Alphaxalone, 106, 111fig.,112 Alzheimer's disease (senile dementia), 203, 216

Amantadine, 211fig.,211 Amino acids in CNS,. 85-9; receptors, 82 fig., 87-8; see also y-Ainino-butyric acid, Glycine, Glutamic acid inhibitory, drug reactions with, 144-70

y-Amino-butyric acid (GABA), 85, 88. 115 and benzodiazepines, 152-3, 155-6, 160 compared with baclofen, 204-5 drug interactions with, 144-70 passim

receptors, 82 fig., 88, 89 structure, 83 Amino-oxyacetic acid, 146 Amitriptyline, 195 fig., 197-8 Amphetamines, 71, 172, 180 Amylobarbitone, 107 fig., 11 fig. Analgesia aspirin-like drugs, 122-3, 124 local, 122-3, 124 morphine-like drugs, 128 opiates, 137-9 stress-induced, 137 Angiotensin, 76, 82 fig. Angiotensin-converting enzyme, 76 Antibiotics, 27 Anticholinesterase agents, 23-4, 97 Antidepressants, 193-4 long-term effects, 198-200 mechanisms of action, 196-8 monoamine oxidase inhibitors, 194, 195 fig. other, 196 tricyclics, 196-7 see also individual drugs

Anti-epileptics, 162, 164 clinical applications, 163, 165-6 mechanisms, 166-9 Antihypertensives, 74-6

225

226

Index

Antineuronal antibody (Igg), 214 Antipsychotic drugs, 178-9 fig. Anxiety, 147-8 Anxiety reducing agents, 147-8, 161-2 Apomorphine, 180, 210fig.,212 Aspartic acid, 83fig.,85 Aspirin, 122-3, 124 fig. Athetosis, 205 Atropine, 77, 208 Autonomic nervous system cholinergic transmission, 77 drug action, 46 ganglionic sites of action, 46-54 neurotransmitters, 44-6 skeletomotor nervous system compared, 44 fig. sympathetic nerves, structure and function, 54-6 Autoradiography, 89 Baclofen ((3-chlorophenyl-GABA), 89, 146, 204-5 Barbitone, 96 Barbiturates, 105-10, 108-9 fig., 110, 112, 116-17, 148 brain penetration, 96, 166 Basal ganglia, disorders of, 203-15 Benzodiazepines dependency, 148-9 long-term effects, 149-50 pharmaco kinetics, 150-1 pharmacological actions, 151-3, 166 receptors: binding affinity, 153-4; BZ1 compared to BZ2, 158-9; cellular localisation, 157; endogenous ligands, 157-8; and GABA receptors, 155-6; regional variation, 154-5, 158, 159; substances binding to, 159-60 structures, 149 Benzoquinonium, 30 Benztropine, 208, 210 fig. Benzylamine, 58 Beta-carbolines (p-CCE), 157 Beta-endorphin, 131, 132, 133 fig., 175 Beta-receptors, 64, 69-70 blocking agents, 75-6 Bethanecol, 77 Bethanidine, 74 Bezold-Jarische reflex, 46 Bicuculline, 88-9, 144-5, 146 Biochemical techniques, 12-13 Black widow spider venom, 24, 27 Blood-brain barrier

breakdown of, 97-8 developmental aspects, 98-9 nature of, 8, 93-5 permeability, factors affecting, 95-8 Bombesin, 82 fig. Botulinus toxin, 24, 27-8, 85 Bradykinin, 76, 120-1 Brain, electrical activity, recording, 10 Brain lesions, associated disorders, 203-16

Brain slice techniques, 7, 9 Brainstem, pain control, 124-5 Bretylium, 55fig.,74 Bromocriptine, 208, 211-12 a-Bungarotoxin, 19, 25, 30, 32, 42 p-Bungarotoxin, 27 Buprenorphine, 127 fig., 128 C-fibres, 119-20, 123, 126 Catechol-o-methyltransferase (COMT), 57 fig., 61, 63, 73 Catecholamines, 89-91, 198 cardiovascular effects, 64-5 metabolism, 56-9 receptors, 68-70 uptake and storage, 59-63; neuronal (uptake-1), 59, 60, 61 fig., 71; non-neuronal (uptake-2), 59, 60-2, 63, 72 CL218872 compound, 158, 159, 160 figCapsiacin, 123 Captopril, 76 Carbachol, 21, 31, 39, 77 Carbamazepine, 166 Carbidopa, 210 Cerebrospinal fluid (CSF), 8, 93-4, 96, 136, 137 Ceruloplasmin, 205 Charcofs triad, 206 Chloral hydrate, 162 Chloramphenicol, 97 Chlordiazepoxide (Librium), 148, 149 Chlorgyline, 59 Chloroform, 102 fig., 103-5 Chlorpromazine, 96, 129, 176-9, 178 figand dopamine turnover, 190 fig. in Huntington's chorea, 213 fig., 214 Cholecystokinin (CCK), 44, 82 fig., 175 Choline, 17,26,202,215 Choline acetyltransferase, 26, 84 Cholinergic pathways, in CNS, 84

227

Index Cholinergic system, autoregulation, 70 Cholinergic transmission, in autonomic nervous system, 77 Cholinesterase inhibitors, 78, 216 Chorea, 205 Choroid plexus, 93-4, 96 Chromaffin cells, 45, 76 Clathrates, 110 Clonazepam, 166 Clonidine, 55fig.,67, 70-1, 75

Clostridium botulinum, 27 Clostridiwn tetanic 146

Clozapine, 179 fig., 188-9 Cocaine, 60, 62 fig., 63, 71 Codeine, 126, 127 fig., 129-30 Computer aided tomography (CAT), 13, 175 Convulsants, 144-6 Cyclohexylamine (Ketamine), 106-7 Cyclopropane, 102 fig., 103, 105 Dl-4 (dopamine) receptors, 182 fig., 183-7, 188 Dantrolene, 205 Decamethonium (C10), 25, 30, 33-8, 39 Decamethylene-bistrimethylammonium (C10TMA), 39 Delta (8) receptors, 133-6 Dementia, senile see Alzheimer's disease Denervation supersensitivity, 43, 76 Depolarising (desensitising) blocking agents, 25, 30 mechanisms, 32-8 pharmacological characteristics, 39-40 Deprenyl, 59, 211fig.,211 Depression bipolar (manic), 193, 201-2 major (endogenous, psychotic), 193, 193-201 Desipramine, 195 fig., 196 Desmethylimipramine, 197 fig., 198 fig. Diazepam (Valium), 148, 149 fig., 154 P-CCE displacement of, 157 and benzodiazepine receptors, 158, 159 pharmacological profile, 161 in spasticity, 204, 205 for status epilepticus, 166 Diethazine, 210 fig.

Dihydro-p-erythroidine, 84, 85, 87, 138 Diisopropyl phosphofluoridate (DFP), 78, 97 Dimercaprol, 205 Dimethylphenylpiperazinium, 19 Dinaphthyl decamethonium (DNC10), 25, 38-9 Diphenylbutyl acetate, 24, 26 Diuretics, 76 Dopa decarboxylase, 56, 57fig.,58 Dopamine, 45 amantadine effect on, 212 anticholinergic drugs effect on, 208 level in depression, 194 level in Huntington's chorea, 214 metabolism of, 180 in Parkinson's disease, 212 receptors, 82 fig., 180-7; classification, 185-6; effect of antischizophrenic drugs, 180-7; location, 182 fig., 185-6 figs role in schizophrenia, 173-5 structure, 83 Dopamine-p-hydroxylase, 57fig.,58 Dopaminergic pathways, 89, 90 fig.

Drugs, 1-3, 8-10 see also individual drugs

Dynorphins, 120, 131, 133 fig., 134-5 Dyskinesias, 203, 204, 205, 206, 212 drug-induced, 187-8; mechanisms, 188-91 Dystonia, drug-induced, 187 Ecothiophate, 78 Edrophonium, 25, 41, 78 Electroconvulsive therapy (ECT), 177, 192, 198 Electroencephalogram (EEG), 10 Electromyogram (EMG), 16 Electrophysiological methods, 7, 10-12 Encephalitis lethargica (sleeping sickness, Von Economo's disease), 206 End plate potentials (epps), 16, 17, 18, 26,28 Enkephalins, 44, 82 fig., 131-2, 134-5 Ephedrine, 71 Epilepsy, 162-5, see also Anti-epileptic drugs Ether, 102 fig., 103-5 Ethopropazine, 208 Ethosuximide, 166 Etorphine, 129-30, 135

228

Index

Eugenols, 106 Evoked potentials, 10 Excitatory postsynaptic potential (EPSP), 51, 53, 56 Extracellular fluid (ECF), 94 FK33-824 compound, 132-3, 135 Fazadinium (AH 8165), 29-30 Flunitrazepam, 154, 158, 159 Flupenazine, 190 fig., 190 Flupenthixol, 177, 179 fig., 189 fig. Flurazepam, 148 GABA see y-Amino-butyric acid GABA-A receptor, 82 fig., 88, 89 GABA-B receptor, 82 fig., 88, 89 GABA modulin, 156 GABA transaminase inhibition, 146 Gallamine, 25, 29, 31 Ganglionic sites of action, 43-4, 46-54 blocking agents, 53-4, 55 ionic mechanisms, 53 muscarinic receptors, 48-51 nicotinic receptors, 46-8 peptide involvement in, 51-3 General anaesthetics, 102-17 convulsants related to, 110, 111 fig., 112 effects, 112-16 gaseous, 102-3 mechanisms, 107-12 soluble (intravenous), 105-7 structures of, 101, 102 tolerance to, 116-17 types of, 101-7 volatile, 103-5 see also individual anaesthetics

Germine, 24 Glaucoma, 77, 78 Glutamic acid, 83, 85 receptors, 87-8 Glutamic acid decarboxylase (GAD), 89, 146 Glycine, 82 fig., 83 fig., 85, 88 drug interactions with, 144-70 passim

Guanethidine, 55 fig., 74, 75 Guanidine, 28 Guanosine monophosphate (GMP), 153 Haloperidol, 178, 180fig.,213fig.,215 Halothane, 95-6, 102 fig., 103-5, 112, 113

Heart muscle activation, 68, 70 Hemicholinium (HC-3), 24, 26 Hexafluorenium, 30 Hexamethonium, 55fig.,97 Hexobarbitone, 109 fig. Histamine, 82 fig., 120 Histochemical techniques, 12 Homovanillic acid, 180 fig. Huntington's disease (Huntington's or senile chorea), 202, 214-15 Hydrochlorothiazide, 55 fig. 6-Hydroxydopamine (6-OHDA), 187 5-Hydroxytryptamine (5-HT), 46, 89 antidepressant effect on, 197-8 in CNS, 90fig.,91 in depression, 194 in Huntington's disease, 214 lithium effect on, 202 in nociception, 139 receptors, 82 fig., 176, 198, 201 in schizophrenia, 176 structure, 83 fig. 5-Hydroxytryptophan (5-HTP), 98 Hyoscine, 77 Hyperglycinaemia, non-ketotic, 145-6 Hypertension, 98, see also Antihypertensives Hypnotics, 147-8, 161-2 Hypoxanthine, 157 Imipramine, 192, 195 fig., 196, 197-8 Immunohistochemical methods, 12 Indomethacin, 122, 123, 124 fig. Inosine, 157 Insulin shock therapy, 177, 192 Intra-peritoneal injections, 8 Intracellular recording, 11, 16 Intravenous injections, 8 Ion channels conductance: measurement, 7, 11-12; states, 21-2 Iprindole, 195 fig., 196, 197-8 Iproniazid, 58, 194, 195 fig. Isoprenaline, 62, 63, 64-5, 70-1 Kainate receptor for glutamic acid, 82 fig., 87 Kainic acid, 157, 181 Kanamycin, 27 Kappa (K) receptors, 133-6 Kernicterus, 99 Ketamine (cyclohexylamine), 106, 107 Lathyrism, 99-100 L-dihydroxyphenylalanine

229

Index (L-DOPA), 98, 206, 213 fig. development of, 208-9 effect in Huntington's disease, 212 for Parkinson's disease, 208-12 Labetalol, 76 Lecithin, 215 Leptazol, 192 Leu-enkephalin, 131, 132, 133 fig., 134-5 Lisuride, 212 Lithium, 201-2 Local anaesthetics, 2-3, 120 Luteinising hormone releasing hormone (LHRH), 45, 51 M-current, 53 Malathion, 78 Mania, 193 MAO-A -B, see Monoamine oxidase McNeil A-343 (muscarine agonist), 77 Mu (u) receptors, 132, 133-6 Mecamylamine, 54, 55 fig., 97 Membrane noise, 7, 11-12, 20-1 Meningitis, 97, 98 Meperidine, 127 fig. 3-Mercaptoproprionic acid, 146 Merprobamate, structure, 149 Met-enkephalin, 131, 132, 133 fig., 134-5, 175 Meta-tyramine, 71 Metaphilic antagonists, 29, 38, 38-9 Methacholine (acetyl-pmethylcholine) 18, 19, 77 Methadone, 127 fig., 129 Methohexitone, 106, 109 fig. Methoxyfluorane, 102 fig., 105, 112 Methylatropine, 97 N-methyl-D-aspartic acid (NMDA), 87-8, 215-16 ct-Methylnoradrenaline, 75 Mianserin, 192, 195 fig., 96, 197-8 Microejection, 9 Microelectro-osmosis, 9 Microelectrophoresis, 9, 84 Microinjection, 9 Miniature end plate potentials (mepps), 16-18, 26, 28 Monoamine oxidase (MAO) in catecholamine metabolism, 58 receptors: MAO-A, 5, 210; MAO-B, 59,207,210-11 substrates, 61 Monoamine oxidase inhibitors (MAOIs), 58-9, 61, 73, 192, 194-5 mechanisms of action, 196-7

Monoclonal antibodies, 12, 157 Morphine, 96, 126, 127 fig. binding affinity, 134-5 characteristics, 129-30 endogenous, 132 Morphine-like drugs, 126-8 action, 128-9 addiction, 126, 128, 129 Motor end plate, 15, 16 Motor unit, 15 MPTP (N-methyl-4-phenyl-l, 2, 3, 6, • tetrahydropyridine), 207. 211 fig., 212 Muscarine, 13, 18, 77 Muscarinic receptors, 18-19, 77 Muscle relaxants, 23, 29, 106, 151 Myasthenia gravis, 23-4, 40-2, 97 Myasthenic syndrome, 41-2 N-wave, 46, 47 fig. Nalorphine, 127 fig., 128 Naloxone, 127 fig., 128, 136-7 Neomycin, 24, 27 Neostigmine, 23, 24-5 figs, 41, 78, 97 Nerve fibres, peripheral, 119-20 Nerve gases, 78 Neural transplants, 203 Neurokinins, 82 fig. Neuromuscular junction acetylcholine, receptors: activation, 20-2; sites, 17; types of, 18-19 synaptic transmission, 16-19 drug action: investigations, 16; sites, 22-39: postjunctional, 29-39; prejunctional, 23-9 synaptic transmission, 16-19 Neurones activity, recording, 10-12 general anaesthetic: effects on, 112-16; tolerance to, 116-17 nociceptive, central control of, 124-5 uptake of catecholamines, 71, 59, 60, 61 fig. Neurotensin, 82 fig. Neurotoxicity, 99-100 Neurotoxins, 2-3 Neurotransmitters central: identification of, 81-3; receptors and their significance, 82 k false\ 71-2 see also individual neurotransmitters

Nialamide, 58, 193, 195 fig. Nicotinamide, 157

230

Index

Nicotine, 19 Nicotinic receptors, 19, 30 at ganglia, 46-7 at neuromuscular junction, 19, 30, 47 Nitrazepam (Mogadon), 149 Nitrous oxide, 102-3 Nociception, 118, 118-19, 139 spinal and supraspinal mechanisms, 124-6 Nociceptors, peripheral, 119, 120-2 Non-depolarising (competitive) blocking agents, 25, 29-32, 39-40 Noradrenaline (NA), 44, 45-6, 76, 89, 180 antidepressant effect on, 197-8 cardiovascular effects, 64-5 in the CNS, 89-91 in depression, 194 feedback facilitation, 68 feedback inhibition, 67-8 lithium effect on, 202 mechanism of action, 70-1 metabolism, 56, 57 receptors, 64-8, 82 fig.; characteristics, 65; classification, 65 structure, 83 uptake and storage, 60, 61-3, 73 Noradrenergic pathways, 89, 90 Octopamine, 58 Opiates, 127-9 action sites, 137-9 cellular effects, 139-41 receptors, 129-36; endogenous ligands, 131-2; evidence for, 133-6; localisation, 130-1 tolerance, 141-3 Opioid peptides, 131-7 analgesic effects, 132-3 endogenous involvement in pain, 136-7 multiple receptors, 133-6 Opium, 126 Organophosphates, 78 Oxotremorine, 77 Pain central pathways, 123-6 measurement of, 118, 118-19 peripheral mechanisms, 119-23 Pancuronium, 25, 29 Parathion, 78 Paracetamol, 122, 123, 124 fig.

Parasympathetic nervous system cholinergic transmission, 77 effect of cholinesterase inhibitors on, 78-9 neurotransmitters, 44, 45 Parkinsonian symptoms, druginduced, 188, 189, 206 Parkinson's disease, 2, 203, 205-12 idiopathic, 207 postencephalitic, 206-7 symptomatic, 206 treatment, 98, 208-12, 213 fig. Patch clamping, 11-12, 16, 20, 22 Pempidine, 54, 55 fig. Penicillamine, 205 Penicillin, 97 Pentazocine, 127 fig., 128 Pentobarbitone, 111 fig., 113, 114, 117 Peptide hypothesis of schizophrenia, 175 Pergolide, 212 Personality disorders. 111, see also Affective disorders; Schizophrenia Phaeochromocytoma, 58, 76 Phenacetin, 122, 123, 124 fig. Phencyclidine, 106, 216 Phenelzine, 58, 195 fig. Phenobarbitone, 105, 108 fig., 117, 166 Phenothiazines, 177, 202 Phenoxybenzamine, 63, 64, 66, 67 Phentobarbitone, 107 fig. Phentolamine, 76 Phenylbutazone, 122, 123, 124, fig. Phenylethylamine, 58, 70-1 Phenyltrimethylammonium, 19 Phenytoin, 96, 166, 167-9 Phosphoinositol system, 12 Photoaffinity labelling, 154 Physostigmine (eserine), 41, 78, 97, 215 Picrotoxin, 89, 144-5 Pilocarpine, 77 Pimozide, 177, 178 fig., 180 fig., 188-9 Pirenzepine, 77 PK9084 compound, 160, 161 Positron emission tomography (PET), 13, 174 Postsynaptic membrane, neuromuscular, depolarisation, 17, 20-1 Pressure microejection, 9 Primidone, 166 Pro-dynorphin, 131, 133 fig. Pro-enkephalin-A, 131, 133 fig.

231

Index Pro-opiomelanocortin, 131, 133 fig. Prochlorperazine, 180 fig. Procyclidine, 208 Propanidid, 106 Propranolol, 55fig.,75 Prostaglandins (PG), aspirin inhibition of, 82 fig., 121, 122, 129 Psychoneurotic illness, 111, see also Anxiety Psychotic illness, 171 Pyridostigmine, 78 Quinolines, 160, 161 Quisqualate receptor for glutamic acid, 82fig.,87 Renin-angiotensin system, 75-6 Renshaw cells, 84, 85, 86-7 fig., 138 Reserpine, 55 fig., 60, 62 fig., 71, 73, 186 RO-15-1788 compound, 160, 161 Salbutamol, 64, 68, 70-1 Schizophrenia, 2, 171, 172-3 dopamine receptors: classification, 185-6; comparison of different, 180 fig., 183-7; effect of antischizophrenic drugs on, 180-7; location, 182 fig., 185-6 figs dopamine theory of, 173-5 drug treatment, 176-92; side-effects, 177, 179, 181, 187-91 non-drug treatment, 176-7 summary, 191-2 theories of, 173-6 Sedatives, 147-8, 161-2 Senile chorea see Huntington's disease Senile dementia see Alzheimer's disease Skeletal muscle contraction of, 17 electrical activity, measurement, 16 innervation of, 14-15 see also Neuromuscular junction Skeletomotor nervous system, 44 Sleep, 91 Smooth muscle activation, 64, 68 innervation, 56 stimulation, 57 Sodium barbitone, 105, 108 fig. Somatostatin, 45, 82 fig., 120, 175

Spasticity, 204-5 Spinal cord lesions, 204 Spiroperidol, 189 fig. Steroidal anaesthetics, 106, 111 fig., 112 Sterotypy, 130, 180 fig. Streptomycin, 24, 27 Stroke, 88, 203, 216 Subcutaneous injections, 8 Substance P, 44, 45, 82 fig. capsiacin effect on, 123 in ganglionic transmission, 51-2 in Huntington's chorea, 214 in pain fibres, 120, 121 in schizophrenics, 175 structure, 83 Succinylcholine, 25, 30, 33-8 Sympathetic nerves, structure and function, 54-6 Sympathetic nervous system cholinergic transmission, 77 neurotransmitters, 44-6 Sympathomimetic amines, 70-1, see also individual agents

Synapses, cholinergic, on Renshaw cells, 84, 85, 86-7 fig. Synaptic delay, 18 Synaptic transmission, neuromuscular, 14, 16-18 Taurine, 85 Tetanus toxin, 144-5, 146 Tetracyclics, 192 Tetracycline, 97 Tetraethyl pyrophosphate, 78 Tetraethylammonium (TEA), 27 Tetrodotoxin, 24 Thiohexitone, 106, 109 fig. Thiopentone, 95, 96, 106, 109 fig., 117 Thioridazine, 178 fig., 180 fig., 188-9 Tissue culture, 7 Torpedo, electroplaque organ of, 19 Tranylcypromine, 195 fig. Trazodone, 196 Tremorine, 97 Triazolopyridazines, 158, 159, 160 fig. Trichloroethylene, 112 Tricyclics, 96, 192, 195 fig., 197 Triethylcholine, 26-7 Trifluoperazine, 189 fig. Triflupromazine, 180 fig. Trihexyphenidyl, 208, 210 fig. Trimethadione, 166 Trizolopyridazines, 161 Trypan blue, 93. 96

232

Index

d-Tubocurarine (d-Tc), 25, 29, 31, 32 Tyramine, 58, 71, 73 Tyrosine hydroxylase (TOH), 56-8 Valproate, 166, 166-7 Vasoactive intestinal polypeptide

(VIP), 44, 82 fig., 175 Voltage clamping, 11, 16, 21 Wilson's disease (hepato-lenticular degeneration), 205