neurotransmitters, drugs and brain function

Section A BASIC ASPECTS OF NEUROTRANSMITTER FUNCTION 1 Neurotransmitter systems and function: overview 2 Control of neuronal activity Fast and slow events.. Appropriate methodology Secti

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Neurotransmitters, Drugs

and

Brain Function

Neurotransmitters, Drugs and Brain Function.

Edited by Roy Webster Copyright & 2001 John Wiley & Sons Ltd ISBN: Hardback 0-471-97819-1 Paperback 0-471-98586-4 Electronic 0-470-84657-7

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Department of Pharmacology, University College London, UK

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Copyright & 2001 John Wiley & Sons Ltd ISBN: Hardback 0-471-97819-1 Paperback 0-471-98586-4 Electronic 0-470-84657-7

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Copyright # 2001 by John Wiley & Sons Ltd.

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Library of Congress Cataloging-in-Publication Data

Neurotransmitters, drugs and brain function / edited by R A Webster

p , cm.

Includes bibliographical references and index.

ISBN 0-471-97819-1

1 Neurotransmitters 2 Neurotransmitter receptors 3 Brain±Pathophysiology.

4 Psychopharmacology I Webster, R A., Ph.D.

[DNLM: 1 Neurotransmitters±physiology 2 Brain±drug eects 3 Brain

Chemistry±drug eects 4 Synaptic Transmission±drug eects QV 126 N4955 2001]

QP364.7 N479 2001

612.8 0 042±dc21

2001024354

British Library Cataloguing in Publication Data

A catalogue record for this book is available from the British Library

ISBN 0 471 97819 1

Typeset in 10/12pt Times from authors' disks by Dobbie Typesetting Limited, Tavistock, Devon

Printed and bound in Great Britain by Biddles Ltd, Guildford and King's Lynn

This book is printed on acid-free paper responsibly manufactured from sustainable forestry,

in which at least two trees are planted for each one used for paper production.

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Section A BASIC ASPECTS OF NEUROTRANSMITTER FUNCTION

1 Neurotransmitter systems and function: overview

2 Control of neuronal activity

Fast and slow events Ion channels and 2nd messengers

Channel events Appropriate methodology

Section B NEUROTRANSMITTERS AND SYNAPTIC

TRANSMISSION Neurotransmitters;their location,

pathways, chemistry, receptors, effects (synaptic

and functional) and drug modification

5 Basic pharmacology and drug effects on neurotransmitter function

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11 Amino acids: inhibitory

13 Other transmitters and mediators

i Histamine, ATP, steroids, prostaglandins, trace amines ii Nitric oxide

R A Webster (with a section on nitric oxide by A H Dickenson) 265

Section C NEUROTRANSMITTERS IN DRUG ACTION

AND DISEASE STATES The possible roles of different

neurotransmitters in the aetiology of disease states and the

mechanism of action of clinically effective drugs Symptoms, therapy and animal models are covered

14 Study and manipulation of neurotransmitter function in humans

Section D NEUROTRANSMITTERS AND BEHAVIOUR

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List of Contributors

David A Brown, Alasdair J Gibb, S Clare Stanford, Anthony H Dickenson, MarkFarrant, and Roy Webster, all contributors are from the Department of Pharmacology,University College London, Gower Street, London WC1E6BT

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This book is about neurotransmitters, the substances released from neurons to act onneurons It covers what they do, how they do it and how their activity is involved inbrain function and affected by drugs and disease

After an overview of neurotransmitter systems and function and a consideration ofwhich substances can be classified as neurotransmitters, section A deals with theirrelease, effects on neuronal excitability and receptor interaction The synapticphysiology and pharmacology and possible brain function of each neurotransmitter

is then covered in some detail (section B) Special attention is given to acetylcholine,glutamate, GABA, noradrenaline, dopamine, 5-hydroxytryptamine and the peptidesbut the purines, histamine, steroids and nitric oxide are not forgotten and there is abrief overview of appropriate basic pharmacology

How the different neurotransmitters may be involved in the initiation andmaintenance of some brain disorders, such as Parkinson's disease, epilepsy,schizophrenia, depression, anxiety and dementia, as well as in the sensation of pain,

is then evaluated and an attempt made to see how the drugs which are used in theseconditions produce their effect by modifying appropriate neurotransmitter function(section C) The final section (D) deals with how neurotransmitters are involved in sleepand consciousness and in the social problems of drug use and abuse

The contents are based on lectures given by the contributors, all of whom areexperienced in research and teaching, in a neuropharmacology course for final-year BScstudents of pharmacology, physiology, psychology and neuroscience at UniversityCollege London The text should be of value to all BSc students and postgraduates inthose and related disciplines Those studying medicine may also find it useful especially

if working in neurology or psychiatry

We have tried to make the book readable rather than just factual and so referenceshave been kept to a minimum, especially in the early chapters on basic neuro-pharmacology and although more are given in the applied sections, they are selectiverather than comprehensive

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Acid-sensing ion channels (ASICS) 457

Action potential discharges, ion channels

synthesis and breakdown 269

Adenosine triphosphate (ATP) 265±8

a1Adrenoceptors 178±9

a2Adrenoceptors 98, 99, 173, 411, 471antagonists 483

calcium currents and 40ligands 177

subtypes 170

b (beta) Adrenoceptors 173, 182, 480, 489densensitised by antidepressants 444electrophysiology 188

mechanisms 179±80subtypes 180Affective disorders 425±50Afferent nerve fibres, types of 454±6Agranulocytosis 363

Akinesia 363b-Alanine 247Alcohol 504Allodynia 464Allylglycine 230Alphaxalone 238, 275Alzheimer's disease (AzD) 45, 128, 222,375±92

acetylcholine in 380±1aetiology 378±80aluminium in 379assessment scales 386±7attenuation of degeneration 389±91augmenting cholinergic function in 385±8b-amyloid formation in 377±8

chromosomes 14, 21 378±9genetic mutations in 378±9glutamate function in 388head injuries in 379inflammation in 379

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Alzheimer's disease (cont.)

430, 431±43, 491, 493adaptive changes in 5-HT1A receptors

to 444±6chronic administration, neurochemicalchanges in 446±7

extraneuronal monoamine levels 432latency of effect 443±6

Antihistamines 487Antimuscarinic drugs 383

in PD 315±17Anxiety 395±4205-HT in 413±19animal models 396±9drug effects in 397±9diagnostic criteria 395±6drug treatments 397, 401ethological models 397induction in humans 399integrated theories of 416±19monoamines in 410±19peptides in 419±20states 396

symptoms 396Anxiogenic drugs 411stimuli 397AP5 217, 340AP7 340Apolipoprotein E (ApoE) 378Apomorphine 151±2, 311, 490Arachidonic acid 281, 456, 510Arachidonylethanolamide 510Arachidonyl-glycerol (2AG) 510Arcuate nucleus (A12) 138Arecoline 486

Arginine vasopressin (AVP) 478Arousal 484±5

Artificial cerebrospinal fluid (aCSF) 83Ascending reticular activating system(ARAS) 484±5

Aspartate (L-Aspartic acid) 211Aspirin 281

ATP 33Atropine 5, 125, 315, 486Autoinhibition 16, 38Autoreceptors 98, 173Axoplasmic transport 171Azapirones 414

Baclofen 242, 340, 466Barbiturates 343±5, 401, 504

in epilepsy 340

on GABAAreceptors 237Befloxaton 436

Behavioural despair 431Benserazide 141, 306±7

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b-Carboline 237, 239, 389inverse agonist DMCM 389Carbolines b-CCE (ethyl-b-carboline-3-carboxylate) 404, 406

Carbon monoxide 281Catalepsy 352Catatonia 351Catechol-O-methyl transferase (COMT) 141,175±8

inhibitors in PD 308±10CCK 254, 256, 314and morphine 261and schizophrenia 264

in feeding 261Cerebrospinal fluid (CSF) 10, 202C-fos 255

Cheese reaction 433Chemoreceptor trigger zone 471p-Chloramphetamine (pCPA) 195, 413Chlordiazepoxide (Librium) 401Chloride equilibrium potential (ECl) 233m-Chlorophenylpiperazine (mCPP) 443Chlorpromazine 152, 353, 436±7Cholecystokinin (CCK) 260±1, 473, 495agonists 419±20

co-existence with dopamine 260±1receptors 260±1, 419

Cholesterol 406

in neurosteroid synthesis 273Choline 120, 385±6

Choline acetyltransferase (ChAT) 120,

380, 383cholinergic neuron marker 117Cholinergic agonists

muscarinic 128, 130

in AzD 388Cholinergic antagonistsmuscarinic 130, 486

in PD 315nicotinic 129Cholinergic neurons

in cortex 132±5

in spinal cord 131±2

in striatum 132vesicles in 120Cholinergic pathways (and function) inAlzheimer's disease 133

cognition and reward 134±5sleep and arousal 134Cholinergic receptors 123±130classification and structure 123distribution 125

function 126±8

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Cholinergic receptors (cont.)

selective serotonin reuptake inhibitors(SSRIs) in 439±42

serotonin and noradrenaline reuptakeinhibitors (SNRIs) in 441±3serotonin reuptake and receptor inhibitors

in 443symptoms of 426TCAs in 436±8Desipramine 434, 436±7Desmethyldiazepam 409Desmethylimipramine 144Dexamethasone

in depressed patients 447suppression test 448Dexamphetamine 512Dexedrine see dexamphetamineDextromethorphan 463, 471Diazepam (Valium) 345, 401Diazepam binding inhibitor 409Dibenzamines 353

5,7-Dihydroxytryptamine 414Diphenylhydantoin, see phenytoinDissociation equilibrium constant 78Dizocilpine 420

Domperidone 153, 311Donepezil 386±7Dopa decarboxylase 141DOPAC 141

Dopamine (DA) 137±61b-hydroxylase 167±8, 306agonists 152

in PD 310±13antagonists 153

in schizophrenia 354augmentation in PD 302±13brain concentrations 138central functions 153±8motor activity 155±6psychoses 156reward and reinforcement 156±8electrophysiological effects 150±1fluorescence 137

in drug dependence 518

in sleep 490malfunction in PD 299±300metabolism 141±2

neuronsneurochemistry of 138±44neurotoxins for 143±4O-methylation 305

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pathways 137±8

release and turnover 143

long term control 143

short term control 143

Drug withdrawal syndrome 401Drugs of dependence, classification 500±2DYFLOS122

Dynorphins 314, 468Dyskinesias 156, 355, 362±3Ecstasy (MDMA) Methylenedioxymeth-amphetamine 193, 195, 435, 510±12EEG 327, 494

in epilepsy 330

in sleep 481±4Eicosanoids 456Electroconvulsive shock 444Electroencephalogram (EEG) 56Electromyogram (EMG) 482Electroocculogram (EOG) 482Eletriptan 458

Elevated plus-maze 398Emesis, drugs in 202Encephalitis lethargica 320Endogenous opioid transmitters 468Endomorphins 258, 468

Endopeptidase 254b-Endorphin 468Endothelin 254Endothelium-derived relaxing factor(EDRF) 281

Enkephalins 254±8, 461, 468, 472agonists in PD 315

Entacapone 142, 308Epilepsies 325±6Epilepsy (epilepsies) 325±40amino acids in 336±40animal models of 326±9predictive value 328±9cause 329±30

drugs in 342±9EEG patterns in 327GABA in 332GABA receptors in 335gliosis for 349

neurotransmitters in 335±41ACh 341

adenosine 341monoamines 341pathology 329±30surgery in 349Epileptic activity, approaches to thecontrol of 341±2

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distribution 242isoforms 244mechanisms of action 242pharmacology 242structure 242±3GABAC 244±8expression in Xenopus oocytes 244pharmacology 244

presynaptic 245structure 244±5Gabaculine 233Gabapentin 348, 464GABA-shunt 226GABA transaminase (GABA-T) 226, 347inhibitors in epilepsy 337±8

Galactorrhoea 359Galanin 263Gastrin-releasing peptide (GRP) 478Geller-Seifter test 399

Generalised anxiety disorder 395Generalised seizures (GM) 326Geniculohypothalamic tract (GHT) 478Globus pallidus 301±3

Glucocorticoid receptors 448Glutamate (L-Glutamic acid) 211±27exitotoxicity 221±2

metabotropic 74, 214, 218nomenclature 214non-NMDA (see also AMPA andkainate) 214±16

structure 66±8synapse 213Glutamic acid decarboxylase (GAD)226±30

inhibitors of 230isoforms of 229regulation of 227±9Glutamine 211±12synthetase 212Glutathione 320Glycine 246±8, 461neurochemistry 246±7NMDA receptors and 217

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G-protein coupled receptors 69±75

see also neurosteroids

Succinic semialdehyde dehydrogenase 226

Sucrose density-gradient centrifugation 94,

Tamsulosim 179Tardive dyskinesias 363Taurine 247

Temazepam 497Temporal lobe epilepsy 325Tetrabenazine 142, 172Tetraethylamonium 37Tetrahydrocannabinols (THC) 507±8Theophylline 269, 317

Thioperamine 488Thioridizine 353, 363Thiosemicarbazide 230Thioxanthenes 353THIP 234

Thromboxanes 456Tiagabine 340, 348, 406Tissue damage, chemical mediators of 456±7Tocopherol 320

Tolcapone 142, 310Toloxatone 436Tonic-clonic seizures 326Topiramate 348

Trace amines 277±80Tramadol 472Transgenic mice, receptor expression in 68Transplants in PD 318, 322

Tranylcypromine 430, 434±6, 444Trazodone 415, 434, 443

Triazolopyridazines 237, 239Triazolopyridine 443Tricarboxylic acid (TCA) cycle 226Tricyclic antidepressant drugs (TCAs) 433,436±8

side effects 438Triethylcholine 120Triptans 458Tryptamine 277±9body temperature and 277Tryptophan 191±2, 429Tryptophan hydroxylase (TH) 190±3Tuberomammillary nucleus 269, 487Tyramine 177, 433, 279±80

Tyrosine 139a-methyl-p-tyrosine 429Tyrosine hydroxylase (TH) 141, 143, 167±8phosphorylation of 169

regulation of 168±71Tyrosine kinase 458Unipolar depression 425

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Valproate (Sodium valproate, valproic

Ventrolateral preoptic area (VLPO) 487

Vesicle membrane associated transporters

VTA Ventral tegmental area See A10Waking 477±98

Wind-up 467

in nociception 463±4neuronal 284Xenopus oocytes 239Yohimbine 179, 411±12Zacopride 385, 415Zolantidine 270Zolmitriptan 458Zolpidem 405, 496Zonisamide 348Zopiclone 405, 496

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1 Neurotransmitter Systems and

Function: Overview

R A WEBSTER

INTRODUCTION

Analysis of Biological Function generally presumes that function at one level

arises from the interactions of lower-level elements It is often relatively

straightforward to identify elements that may be involved and their individual

interactions Modern cell and molecular biology, in particular, is very efficient

at identifying new molecules, and establishing which molecules `talk to'one

another However, as the accumulation of such studies gradually reveals a

complex network of interactions, its output Ð the biological function Ð

becomes ever harder to understand and predict The system is reduced to its

elements, but it is not clear how to integrate it again Yet this is the ultimate

functional goal (Brezina and Weiss 1997)

The molecules referred to are the neurotransmitters (NTs) and their receptors, found inthe brain; the biological function is the activity of the brain itself Our understanding ofthat must be the ultimate goal

We have no such pretensions in this book but we do hope to help you to understandhow neurotransmitters may be involved in brain function and more particularly howtheir activity is modified by disease and drugs As the above quotation implies, thiswill mean considering the synaptic characteristics of each neurotransmitter, but before

we do so, it is important to consider some more general and basic aspects of transmitter function Thus:

neuro- What is a neurotransmitter and how did the concept of chemical transmission arise? Which substances are neurotransmitters? Can they be sensibly classified and how do

we know they are transmitters?

Which neurons and pathways use which neurotransmitters and how are theyorganised?

How do neurotransmitters work? What effects do they have on neuronal activity? What is known about the receptors to which they bind?

How are neurotransmitters released and how is this controlled?

How can neurotransmitter function be modified?

Most of these points are considered in detail in subsequent chapters but some will betouched on collectively here

Neurotransmitters, Drugs and Brain Function Edited by R A Webster

&2001 John Wiley & Sons Ltd

Edited by Roy Webster Copyright & 2001John Wiley & Sons Ltd ISBN: Hardback 0-471-97819-1 Paperback 0-471-98586-4 Electronic 0-470-84657-7

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CHEMICAL TRANSMISSION

We might start by considering what we understand by the term `neurotransmitter'.According to the Oxford English Dictionary (2nd edition) it is:

A substance which is released at the end of a nerve fibre by the arrival of a

nerve impulse and by diffusing across the synapse or junction effects the

transfer of the impulse to another nerve fibre (or muscle fibre or some

receptor)

Other dictionaries carry similar definitions

Based on this definition a neurotransmitter could be exemplified by actylcholine(ACh) released from motor nerves to excite and contract the fibres of our skeletalmuscles Indeed the synapses there, i.e the junctions between nerve and muscle fibres,are anatomically and chemically geared to act as a fast relay station Acetylcholinereleased rapidly from vesicles in the nerve terminal, on arrival of the nerve impulse,binds quickly with postsynaptic sites (receptors) When activated these open channelsfor sodium ions which pass through into the muscle fibre to depolarise its membraneand cause contraction The whole process takes less than one millisecond and theACh is rapidly removed through metabolism by local cholinesterase so that con-traction does not persist and the way is cleared for fresh ACh to act Anatomicallythere is a precise and very close relationship between the nerve ending and the musclefibre at histologically distinct end-plates, where the receptors to ACh are confined It

is better than having the nerve directly linked to the muscle since the time lostthrough imposing a chemical at the synapse between nerve and muscle is insignificantand the use of a chemical not only facilitates control over the degree of muscle tonedeveloped, but fortuitously makes it possible for humans to modify such tonechemically

Blocking the destruction of ACh potentiates its effects while blocking the receptors

on which it acts produces paralysis (neuromuscular blockade) Indeed it was the curareimpregnated into the darts used by native South American hunters, so that they couldparalyse and then easily kill their prey, that motivated Claude Bernand to investigate itsactions at the end of the nineteenth century and so demonstrate the chemical sensitivity

of excitable tissue that led to the concept of chemical transmission He did a very simpleexperiment He took a sciatic nerve gastrocnemious muscle preparation from a frog(not the actual quest of the hunters), placed the muscle in one dish of appropriate saltsolution and extended the nerve into another Not surprisingly, simple wire electrodesconnected to an activated induction coil induced contractions of the muscle whetherplaced directly on the muscle or on the nerve to it When, however, curare was added tothe dish containing the muscle, direct stimulation of the muscle still induced acontraction, but activation of the nerve was ineffective This was not due to any effect

of curare on the nerve because when curare was added to the nerve rather than themuscle dish, stimulation of the nerve was still effective Thus there had to be achemically sensitive site on the muscle, where it was linked with the nerve, which wasaffected by the curare This did not prove that a chemical had been released from thenerve but some years later (1916) Otto Loewi found that if he cannulated the ventricle

of a frog's heart, isolated with its vagus nerve intact, then when this was stimulated notonly did the heart slow, as expected, but if fluid withdrawn from the ventricle wassubsequently reintroduced the heart slowed again This suggested the release of a

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chemical from the vagus, which was made even clearer by allowing the fluid perfusedthrough one frog heart to drip onto a second one and establishing that when the firstheart was slowed by stimulating its vagus the fluid from it also slowed the second heartwhen that was reached Loewi did not identify the chemical, which he called vagustoff,but it was later shown to be acetylcholine (ACh), the first identified neurotransmitter(and it was also found to transmit the neural stimulation of skeletal muscle, which hadbeen blocked by curare in the experiments of Bernard).

Now this brings us to the first problem with the dictionary definition of aneurotransmitter because in the heart ACh is not transmitting an excitatory impulsebetween nerve and muscle, it is causing inhibition There are also other differences Itscardiac effect, change in rate, occurs much more slowly, has nothing to do with thedirect opening of any ion channel and is not blocked by curare Thus the sites oncardiac muscle that are chemically sensitive to ACh, its so-called receptors, are differentfrom those for ACh on skeletal muscle In fact they are blocked by a different poison,namely atropine (from Atropa belladonna, Deadly Nightshade) These observationsraise two important issues First, it is the receptor which ultimately determines theeffects of a neurotransmitter and second, since only the excitatory effects of ACh at theneuromuscular junction fulfil the original definition of a neurotransmitter in trans-mitting excitation, either acetylcholine cannot be considered to be a neurotransmitter inthe heart, despite its effects, or the definition of a neurotransmitter needs modifying

We will return to this topic at the end of the chapter

Thus a neurotransmitter can clearly have more than one effect and a moment'sconsideration of what is involved for your nervous system in effecting the processes thatenable you to turn the pages of this book and read and remember some of its contentswill make you realise just how much the nervous system has to achieve and how manydifferent parts of it have to be involved and functionally integrated This is withoutconsidering whether you feel content, anxious, or depressed and how that can affectyour concentration and ability to read and learn or even turn over the pages Clearlysuch processes must involve many different neural pathways and types of neuronproducing different effects and presumably requiring a number of different chemicals(neurotransmitters) The importance and variety of such chemicals is also emphasisedfrom a look at drug usage and the study of how they work

There are many drugs that affect the nervous system for good (antidepressants,analgesics, anticonvulsants) and bad (toxins, poisons, drugs of abuse) and although itwould be naive to think that any drug has only one effect, i.e that an anti-epilepticdrug will never cause any sedation, their demonstrably different primary effects,coupled with the diversity of their chemical structures, suggest that not only are drugsaffecting different areas of the brain but as they are likely to do this at chemicalsynapses there must be a number of different chemical synapses and chemicals, i.e.neurotransmitters

NEUROTRANSMITTER CLASSIFICATION

The following substances, listed alphabetically, have been widely implicated andgenerally accepted as neurotransmitters in the central nervous system (CNS), althoughsome, such as glutamate, are much more important than others, e.g adrenaline Some

NEUROTRANSMITTER SYSTEMS AND FUNCTION: OVERVIEW 5

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classification is appropriate and the simplest and most commonly used is that based onchemical structure with the substances grouped as follows:

substance P(Many others have been implicated)

In addition to the above it is now clear that the following substances may have animportant central action but whether they can be classified as true neurotransmitters isuncertain:

GNitric oxide (A gas but it is always in solution in the brain)

A glance at the structure of the classical neurotransmitters (Fig 1.1) shows that apartfrom the peptides (D) (and purines, E), most of them are fairly simple chemicals Someauthors therefore divide them into small (e.g A, B, C) and large (peptides, D) molecularNTs Although we will see that peptides certainly have some properties different fromother NTs, in that they rarely have a primary neurotransmitter function and usuallyjust complement the actions of those NTs in groups A±C, to put them in a class oftheir own and group all the others together simply on the basis of molecular size isinappropriate and misleading since it elevates the peptides to a status that is neitherproven nor warranted

NEURONS: STRUCTURE AND ENVIRONMENT

The neurons from which NTs are released number more than 7 billion in the humanbrain Each (Fig 1.2) consists of a cell body, the soma or perikaryon, with one majorcytoplasmic process termed the axon, which projects variable distances to otherneurons, e.g from a cortical pyramidal cell to adjacent cortical neurons, or to striatalneurons or to spinal cord motoneurons Thus by giving off a number of branches fromits axon one neuron can influence a number of others All neurons, except primarysensory neurons with cell bodies in the spinal dorsal root ganglia, have a number ofother, generally shorter, projections running much shorter distances among neigh-bouring neurons like the branches of a tree These processes are the dendrites Their

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absence from sensory, i.e initiating, neurons immediately suggests that their function isassociated with the reception of signals (inputs) from other neurons Neuron cell bodiesvary in diameter from 5 mm to 100 mm and axons from 0.1 mm to 10 mm, although theseare enlarged at their terminal endings Axons are generally surrounded by an insulatingmyelin sheath which is important for the propagation of action potentials generated inthe neurons and gives the axons and the pathways they form a white colour whichcontrasts with the grey appearance of those areas of the CNS dominated by thepresence of neuron cell bodies and their dendrites.

The axon terminals of one neuron synapse with other neurons either on the dendrites(axo-dendritic synapse) or soma (axo-somatic synapse) Synapses on another axon

Figure 1.1 The chemical structures of the main neurotransmitters The relatively simplestructure of acetylcholine, the monoamines and the amino acids contrasts with that of thepeptides, the simplest of which are the enkephalins which consists of five amino acids; substance Phas eleven

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pro-NEUROTRANSMITTER SYSTEMS AND FUNCTION: OVERVIEW 9

(b)

Figure 1.2 (b) Schematic representation of a neuron The main features of a neuron are showntogether with different synaptic arrangements (A) axo-dendritic, (B) axo-somatic, (C) axo-axonicand (D) dendro-dendritic For more detail see section on `Morphological correlates of synapticfunction'and Fig 1.7

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terminal are also found (axo-axonal) and occasionally even between dendrites dendritic) (see Fig 1.2(b)) The morphology of synapses is considered later.

(dendro-Like other cells, a neuron has a nucleus with genetic DNA, although nerve cellscannot divide (replicate) after maturity, and a prominent nucleolus for ribosomesynthesis There are also mitochondria for energy supply as well as a smooth and arough endoplasmic reticulum for lipid and protein synthesis, and a Golgi apparatus.These are all in a fluid cytosol (cytoplasm), containing enzymes for cell metabolismand NT synthesis and which is surrounded by a phospholipid plasma membrane,impermeable to ions and water-soluble substances In order to cross the membrane,substances either have to be very lipid soluble or transported by special carrier proteins

It is also the site for NT receptors and the various ion channels important in the control

of neuronal excitability

Microtubules (about 20 nm in diameter) and solid neurofilaments (10 nm) extendfrom the cell body into the axon and are found along its length, although notcontinuous They give structure to the axon but are not involved in the transport ofmaterial and vesicles to the terminal, which despite its high level of activity does nothave the facility for molecular synthesis possessed by the cell body Such transport isconsidered to be fast (200±400 mm per day), compared with a slower transport (1 mmper day) of structural and metabolic proteins Although axonal flow is mainly towardsthe terminal (ortho or anterograde) there is some movement (fast) of waste material andpossibly information on synaptic activity back to the cell body (retrograde) The neuron

is obviously very active throughout the whole of its length

In addition to neurons the CNS contains various neuroglia (often just called glia).These can outnumber neurons by up to 10:1 in some areas and include star-like astro-cytes with their long cellular processess which not only enable them to providestructural support for the nerve cells but also facilitate NT degradation and the removal

of metabolites Oligodendrites are glial cells which are involved in myelin formation andalthough they also have long processes, these are spirally bound rather than extendingout as in the astocytes

Neurons and glia are bathed in an ion-containing protein-free extracellular fluidwhich occupies less of the tissue volume (20%) in the brain than in other organs because

of the tight packing of neurons and glia In fact the whole brain is really suspended influid within its bony casing The brain and spinal cord are covered by a thin close-fittingmembrane, the pia mater and a thicker loose outer membrane, the dura mater In thespace between them, the subarachnord space, is the cerebrospinal fluid (CSF) This alsoflows into a series of ventricular spaces within the brain as well as a central canal in thecord and arises mainly as a secretion (ultra filtrate) of blood from tufts of specialisedcapillaries (the choroid plexus), which invaginate the walls of the ventricles While theCSF is contiguous with the extracellular fluid within the brain and contributes to it,much of this fluid comes directly from the copious network of capillaries foundthroughout the brain In fact neurons are never far from a capillary and their highmetabolic rate means that despite contributing only 2% towards body weight, thenervous system receives 15% of cardiac output

In most parts of the body, substances, other than large molecular ones like proteins,are filtered from the blood into the extracellular space through gaps betweenendothelial cells in the capillary wall Such gaps are much narrower, almost non-existent, in brain capillaries and it is likely that any filtering is further reduced by themanner in which astrocytes pack around the capillaries This constraint is known as the

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blood±brain barrier (BBB) It protects the brain from inappropriate substances,including all NTs and many drugs To enter the brain as a whole is therefore almost asdifficult for a substance as entering a neuron and again it has to be either very lipidsoluble, when it can dissolve in and so pass through the capillary wall, or be transportedacross it.

NEUROTRANSMITTER FUNCTION

BASIC CIRCUITRY

In a classical neural pathway, such as that depicted in Fig 1.3, neuron A must exciteneuron B and at the same time inhibit neuron C in order to optimise the excitation of B

It could achieve this with one NT able to activate receptors linked to different events on

B and C Of course, neuron C would have other inputs, some of which would beexcitatory and if the same NT was used it could activate the inhibitory mechanism on C

as well Also, the NT released from A might be able to stimulate as well as inhibitneuron C (Fig 1.3(a)) Even the provision of separate receptors linked to excitation andinhibition would not overcome these problems since both would be accessible to the

NT One possible solution, used in the CNS, is to restrict the NT to the synapse atwhich it is released by structural barriers or rapid degradation Also the inputs andreceptors linked to excitation could be separated anatomically from those linked toinhibition and, in fact, there is electrophysiological and morphological evidence thatexcitatory synapses are mainly on dendrites and inhibitory ones on the soma of largeneurons (Fig 1.3(b)) Nevertheless, the problem of overlap would be eased if two NTswere released, one to activate only those receptors linked to excitation and another toevoke just inhibition, i.e place the determinant of function partly back on the NT (Fig.1.3(c)) This raises a different problem which has received much consideration Can aneuron release more than one NT?

It was generally assumed that it cannot and this became known as Dale's Law.During his studies on antidromic vasodilation he wrote (1935) `When we are dealingwith two different endings of the same sensory neuron, the one peripheral and con-cerned with vasodilation and the other at a central synapse, can we suppose that thediscovery and identification of a chemical transmitter at axon reflex dilation wouldfurnish a hint as to the nature of the transmission process at a central synapse Thepossibility has at least some value as a stimulus to further experiments'

This it certainly has been and in the last few years much evidence has been presented

to show that more than one substance (but not necessarily more than one conventionalNT) can co-exist in one nerve terminal This does not disprove Dale's Law (so called),since he was referring to `a'not `the'NT and to different endings of one neuron In fact

he was simply saying that if a neuron uses a particular transmitter at one of its terminals

it will use it at another, although he did not add, irrespective of whether or not it usesmore than one NT This makes good sense especially since it is difficult to conceive how

a neuron could achieve, let alone control, the release of different NTs from differentterminals, unless the NTs were synthesised solely at the terminals independently of thecell body In that way different substances might be released from different terminals of

a neuron by arriving action potentials without the neuron having to do anything special

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Figure 1.3 Some possible basic neurotransmitter±synaptic arrangements for the excitation andinhibition of different neurons (a) The single NT activates neuron B and inhibits neuron C bybeing able to activate both excitatory and inhibitory receptors or, more probably, acting onone receptor linked to both events There is potential, however, for the NT to activate anyinhibitory receptors that may be on B or excitatory receptors on C (b) The same NT is used

as in (a) but the excitatory receptors are now only on dendrites and separated from theinhibitory receptors only on the soma There is less chance of unwanted mixed effects (c)Neuron A releases distinct excitatory and inhibitory NTs from its two terminals each acting

on specific and morphologically separated receptors But this depends on a neuron being able

to release two NTs (d) Neuron A releases the same NT from both terminals It directlyexcites B but inhibits C through activating an inhibitory interneuron (I) which releases aninhibitory NT onto specific receptors on C This last scheme (d) is clearly more functional and

is widely used

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to achieve it Thus neuron A (Fig 1.3) could then conceivably always release one NT at

B and another at C or even two NTs at both but probably could not vary their releaseindependently at different (or the same) synapses

Fortunately there is another way in which one neuron can excite and inhibit differentneurons using just one NT Neuron A could excite B and inhibit C by the introduction

of an inhibitory interneuron the activation of which by A, using the same excitatory NT

as at B, automatically inhibits C (Fig 1.3(d)) This form of inhibition is quite common

in the CNS and in fact much inhibition is mediated by these so-called short-axoninterneurons and a neuron may inhibit itself through feedback via an axon collateralsynapsing onto an adjacent inhibitory short-axon interneuron (Fig 1.2)

It might therefore be possible to set up a CNS with two NTs exerting fast excitatoryand inhibitory effects through different receptors, situated on different parts of theneuron provided those were the only effects wanted But this is not so One neuron canreceive hundreds of inputs and its activity and responsiveness is in fact balanced by suchinputs producing different effects at differing speeds by using different NTs So whatare these different effects and how are they produced?

NEURONAL EXCITABILITY

POSTSYNAPTIC EVENTS

The neuronal membrane normally has a resting membrane potential around 770 mV(inside negative in respect of outside) with Na and Cl7 concentrated on the outsideand K on the inside prepared to move down their concentration gradients when theappropriate ion channels are opened (Fig 1.4) On arrival of an excitatory impulse the

Nachannels are opened and there is an increased influx of Na so that the restingpotential moves towards the so-called equilibrium potential for Na (50 mV) when

Nainflux equals Naoutflux but at 760 to 765 mV, the threshold potential, there is

a sudden increase in Na influx This depolarisation leads to the generation of apropagated action potential The initial subthreshold change in membrane potentialparallels the action of the excitatory transmitter and is graded in size according to theamount of NT released It is known as the excitatory postsynaptic potential (EPSP) andlasts about 5 ms

An inhibitory input increases the influx of Cl to make the inside of the neuron morenegative This hyperpolarisation, the inhibitory postsynaptic potential (IPSP), takes themembrane potential further away from threshold and firing It is the mirror-image ofthe EPSP and will reduce the chance of an EPSP reaching threshold voltage

Such clear postsynaptic potentials can be recorded intracellularly with trodes in large quiescent neurons after appropriate activation but may be somewhatartificial In practice a neuron receives a large number of excitatory and inhibitoryinputs and its bombardment by mixed inputs means that its potential is continuouslychanging and may only move towards the threshold for depolarisation if inhibition fails

microelec-or is overcome by a sudden increase in excitatmicroelec-ory input

Not all influences on, or potentials recorded from, a neuron have the same time-course

as the EPSP and IPSP, which follow the rapid opening of Na and Cl7 ion channelsdirectly linked to NT receptors There are also slowly developing, longer lasting andsmaller non-propagated (conditioning) changes in potential most of which appear to have

a biochemical intermediary in the form of G-proteins linked to the activation (Gs) or

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inhibition (Gi) of adenylate cyclase and cyclic AMP production or IP3 breakdown (seeChapter 2) They can be excitatory (depolarising) or inhibitory (hyperpolarising) generallyinvolving the opening or closing of Kchannels This can be achieved directly by the G-protein or second messenger but more commonly by the latter causing membranephosphorylation through initiating appropriate kinase activity.

Thus the activity of a neuron can be controlled in a number of ways by NTsactivating appropriate receptors (Fig 1.5) Two basic receptor mechanisms areinvolved:

(1) Ionotropic Those linked directly to ion channels such as those for Na(e.g AChnicotinic or some glutamate receptors) or Cl7(GABA), involving fast events withincreased membrane conductance and ion flux

(2) Metabotropic Those not directly linked to ion channels but initiating biochemicalprocesses that mediate more long-term effects and modify the responsiveness of theneuron With these the first messenger of synaptic transmission, the NT, activates asecond messenger to effect the change in neuron excitability They are normallyassociated with reduced membrance conductance and ion flux (unless secondary to

Figure 1.4 Ionic basis for excitatory postsynaptic potentials (EPSPs) and inhibitory postsynapticpotentials (IPSPs) Resting membrane potential (770 mV) is maintained by Nainflux and K

efflux Varying degrees of depolarisation, shown by different sized EPSPs (a and b), are caused

by increasing influx of Na When the membrane potential moves towards threshold potential(60±65 mV) an action potential is initiated (c) The IPSPs (a'b') are produced by an influx of C17.Coincidence of an EPSP (b) and IPSP (a') reduces the size of the EPSP (d)

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an increased Ca2conductance) and may involve decreased Nainflux (inhibitory)

or Kefflux (excitatory) Some amines (e.g noradrenaline) may increase Kefflux(inhibitory)

These two basic mechanisms could provide a further classification for NTs, namelyfast and slow acting, although one NT can work through both mechanisms usingdifferent receptors The slow effects can also range from many milliseconds toseconds, minutes, hours or even to include longer trophic influences What willbecome clear is that while one NT can modify a number of different membrane ioncurrents through different mechanisms and receptors, one current can also be affected

by a number of different NTs The control of neuronal excitability is discussed inmore detail in Chapter 2

Figure 1.5 The degree of ion channel opening can be controlled (gated) either directly(ionotropic effect) or indirectly (metabotropic effect) In the former the neurotransmittercombines with a receptor that is directly linked to the opening of an ion channel (normally NaorC17) while in the latter the receptor activates a G-protein that can directly interact with the ionchannel (most probably Kor Ca2) but is more likely to stimulate (Gs) or inhibit (Gi) enzymescontrolling the levels of a second messenger (e.g cAMP, GMP, IP3) These in turn may alsodirectly gate the ion channel but generally control its opening through stimulating a specificprotein kinase that causes phosphorylation of membrane proteins and a change in state of the ionchannel The latter (metabotropic) effects may either open or close an ion channel (often K) andare much slower (100s ms to min) than the ionotropic ones (1±10 ms) A variety of neuro-transmitters, receptors, second messengers and ion channels permits remarkably diverse andcomplex neuronal effects

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PRESYNAPTIC EVENTS

So far we have assumed that a NT can only modify neuronal activity by a postsynapticaction Recently, interest has also turned to presynaptic events It has been knownfor many years that stimulation of muscle or cutaneous afferents to one segment ofthe spinal cord produces a prolonged inhibition of motoneuron activity without anyaccompanying change in conductance of the motoneuron membrane, i.e no IPSP.Such inhibition is probably, therefore, of presynaptic origin and is, in fact,associated with a depolarisation of the afferent nerve terminals and a reduction inrelease of the excitatory NT If it is assumed that the amount of NT released from anerve terminal depends on the amplitude of the potential change induced in it, then ifthat terminal is already partly depolarised when the impulse arrives there will be asmaller change in potential and it will release less transmitter (Fig 1.6) There is nodirect evidence for this concept from studies of NT release but electrophysiologicalexperiments at the crustacean neuromuscular junction, which has separate excitatoryand inhibitory inputs, show that stimulation of the inhibitory nerve, which releasedGABA, reduced the EPSP evoked postsynaptically by an excitatory input withoutdirectly hyperpolarising (inhibiting) the muscle fibre Certainly when GABA is applied

to various in vivo and in vitro preparations (spinal cord, cuneate nucleus, olfactorycortex) it will produce a depolarisation of afferent nerve terminals that spreadssufficiently to be recorded in their distal axons

Such presynaptic inhibition can last much longer (50±100 ms) than the postsynapticform (5 ms) and can be a very effective means of cutting off one particular excitatoryinput without directly reducing the overall response of the neuron How GABA canproduce both presynaptic depolarisation and conventional postsynaptic hyperpolar-isation by the same receptor, since both effects are blocked by the same antagonist,bicuculline, is uncertain (see Chapter 2) although an increased chloride flux appears

to be involved in both cases If nerve terminals are depolarised, rather than polarised by increased chloride flux, then their resting membrane potential must bedifferent from (greater than) that of the cell body so that when chloride enters and thepotential moves towards its equilibrium potential there is a depolarisation instead of ahyperpolarisation Alternatively, chloride efflux must be achieved in some way.This form of presynaptic inhibition must be distinguished from another means

hyper-of attenuating NT release, i.e autoinhibition This was first shown at peripheralnoradrenergic synapses where the amount of noradrenaline released from nerveterminals is reduced by applied exogenous noradrenaline and increased by appropriate(alpha) adrenoceptor antagonists Thus through presynaptic (alpha) adrenoreceptors,which can be distinguished from classical postsynaptic (alpha) adrenoreceptors byrelatively specific agonists and antagonists, neuronal-released noradrenaline is able toinhibit its own further (excessive) release It is a mechanism for controlling the synapticconcentration of noradrenaline This inhibition does not necessarily involve any change

in membrane potential but the receptors are believed to be linked to and inhibitadenylate cyclase Whether autoinhibition occurs with all NTs is uncertain but there isstrong evidence for it at GABA, dopamine and 5-HT terminals

There is also the interesting possibility that presynaptic inhibition of this form, with

or without potential changes, need not be restricted to the effect of the NT on theterminal from which it is released Numerous studies in which brain slices have beenloaded with a labelled NT and its release evoked by high Kor direct stimulation show

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Figure 1.6 Presynaptic inhibition of the form seen in the dorsal horn of the spinal cord (a) Theaxon terminal (i) of a local neuron is shown making an axo-axonal contact with a primaryafferent excitatory input (ii) (b) A schematic enlargement of the synapse (c) Depolarisation ofthe afferent terminal (ii) at its normal resting potential by an arriving action potential leads to theoptimal release of neurotransmitter (d) When the afferent terminal (ii) is already partiallydepolarised by the neurotransmitter released onto it by (i) the arriving acting potential releasesless transmitter and so the input is less effective

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that such release can be inhibited by a variety of other NTs A noradrenergic terminalhas been shown to possess receptors for a wide range of substances, so-calledheteroceptors (see Langer 1981, 1997) and although this may be useful for developingdrugs to manipulate noradrenergic transmission it seems unlikely that in vivo all of thereceptors could be innervated by appropriate specific synapses or reachable by their

NT They may be pharmacologically responsive but not always physiologically active(see Chapter 4)

CONTROL OF SYNAPTIC NT CONCENTRATION

Having briefly discussed the presynaptic control of NT release it is necessary toconsider how the concentration of a NT is controlled at a synapse so that it remainslocalised to its site of release (assuming that to be necessary) without its effect becomingtoo excessive or persistent

Although one neuron can receive hundreds of inputs releasing a number of differentNTs, the correct and precise functioning of the nervous system presumably requiresthat a NT should only be able to act on appropriate receptors at the site of its release.This control is, of course, facilitated to some extent by having different NTs withspecific receptors so that even if a NT did wander it could only work where it finds itsreceptors and was still present in sufficient concentration to meet their affinityrequirements Normally the majority of receptors are also restricted to the immediatesynapse

Nevertheless, from release (collection) studies we know that enough NT must diffuse(overflow) to the collecting system, be that a fine probe in vivo or the medium of aperfusion chamber in vitro, to be detected Thus one must assume that either theconcentration gradient from the collecting site back to the active synaptic release site is

so steep that the NT can only reach an effective concentration at the latter, or it is notunphysiological for a NT to have an effect distal from its site of release

Released NT, if free to do so, would diffuse away from its site of release at thesynapse down its concentration gradient The structure of the synapse and the narrowgap between pre- and postsynaptic elements reduces this possibility but this means thatthere must be other mechanisms for removing or destroying the NT so that it, and itseffects, do not persist unduly at the synapse but are only obtained by regulated impulsecontrolled release In some cases, e.g ACh, this is achieved by localised metabolisingenzymes but most nerve terminals, especially those for the amino acids andmonoamines, possess very high-affinity NT uptake systems for the rapid removal ofreleased NT In fact these are all Na- and Cl7-dependent, substrate-specific, high-affinity transporters and in many cases their amino-acid structure is known and theyhave been well studied Transport can also occur into glia as well as neurons and thismay be important for the amino acids Of course, a further safeguard against anexcessive synaptic concentration of the NT is the presence of autoreceptors to controlits release

Thus there are mechanisms to ensure that NTs neither persist uncontrollably at thesynapse nor produce dramatic effects distal from it Studies of glutamate release alwaysshow a measurable basal level (1±3 mM), although this may not all be of NT origin, andyet it is very difficult to increase that level even by quite intense stimulation Whetherthis is a safeguard against the neurotoxicity caused by the persistent intense activation

Định dạng
Số trang	532
Dung lượng	6,47 MB

Tiêu đề	Neurotransmitters, Drugs and Brain Function
Tác giả	Roy Webster
Trường học	University College London
Chuyên ngành	Pharmacology
Thể loại	book
Năm xuất bản	2001
Thành phố	London