Acetylcholine. chapter. Palmer Taylor Joan Heller Brown

chapter 11 Acetylcholine Palmer Taylor Joan Heller Brown CHEMITY F ACETYCHINE 186 GANIZATIN F THE CHINEGIC NEVU YTEM 186 Acetylcholine receptors have been classified into subtypes based on early studies of pharmacologic selectivity 186 The intrinsic complexity and the multiplicity of cholinergic receptors became evident upon elucidation of their primary structures 189 FUNCTINA APECT F CHINEGIC NEUTANMIIN 189 Both muscarinic and nicotinic responses are elicited in brain and spinal cord 189 Neurotransmission in autonomic ganglia is more complex than depolarization mediated by a single transmitter 190 Muscarinic receptors are widely distributed at postsynaptic parasympathetic effector sites 190 timulation of the motoneuron releases acetylcholine onto the muscle endplate and results in contraction of the muscle fiber 191 Competitive blocking agents cause muscle paralysis by preventing access of acetylcholine to its binding site on the receptor 191 YNTHEI, TAGE AND EEAE F ACETYCHINE 192 Acetylcholine formation is limited by the intracellular concentration of choline, which is determined by active transport of choline into the nerve ending 192 A second transport system concentrates acetylcholine in the synaptic vesicle 193 Choline is supplied to the neuron either from plasma or by metabolism of choline-containing compounds 193 A slow release of acetylcholine from neurons at rest probably occurs at all cholinergic synapses 194 The relationship between acetylcholine content in a vesicle and the quanta of acetylcholine released can only be estimated 194 Depolarization of the nerve terminal by an action potential increases the number of quanta released per unit time 194 All the acetylcholine contained within the cholinergic neuron does not behave as if in a single compartment 194 ACETYCHINETEAE AND THE TEMINATIN F ACETYCHINE ACTIN 195 Cholinesterases are widely distributed throughout the body in both neuronal and non-neuronal tissues 195 The primary and tertiary structures of the cholinesterases are known 195 Acetylcholinesterases exist in several molecular forms governed by alternative splicing 195 Cholinesterase catalysis and inhibition mechanisms involve formation of reversible complexes and covalent conjugates 197 Consequences of acetylcholinesterase inhibition differ with effector site 197 NICTINIC ECEPT 197 The nicotinic acetylcholine receptor (nach) was the first characterized neurotransmitter receptor 197 The nach consists of five subunits arranged around a pseudoaxis of symmetry 198 Analysis of the opening and closing events of individual channels has provided information about ligand binding and activation of the receptor 201 Continued exposure of nicotinic receptors to agonist leads to desensitization of the receptors 201 Nicotinic receptor subunits are part of a large superfamily of ligand-gated channels 202 Both nicotinic receptors and acetylcholinesterase are regulated tightly during differentiation and synapse formation 202 MUCAINIC ECEPT 203 Muscarinic receptor activation causes inhibition of adenylyl cyclase, stimulation of phospholipase C and regulation of ion channels 203 adioligand-binding studies have been used to characterize muscarinic receptors 205 The binding properties of the antagonist pirenzepine led to the initial classification of muscarinic receptors 205 Transgenic mice are being generated to assess the functions of receptor subtypes in vivo 207 Acetylcholine (ACh) and its targets of interaction have played a long-standing and critical role in the basic concepts of neurochemistry. At the turn of the last century, ACh emerged as a pivotal mediator in chemical neurotransmission and as a ligand for defining receptors. The natural alkaloids physostigmine (by inhibiting acetylcholinesterase) and nicotine and muscarine (for distinguishing Basic Neurochemistry: Molecular, Cellular and Medical Aspects 0-12-088397-X 185 2006, American ociety for Neurochemistry. All rights reserved. Published by Elsevier, Inc.

186 PAT II Intercellular ignaling receptor subtype) were invaluable tools for identifying ACh as a transmitter and delineating its sites of action. The unusual spatial localization of nicotinic acetylcholine receptors (nach) at the neuromuscular junction endplates enabled the discoveries of quantal release of neurotransmitter, receptor desensitization and channel gating properties of individual receptor molecules through single channel and noise analysis. ACh is widely distributed in the nervous system. It subserves all motor transmission in vertebrates, is the primary transmitter for peripheral ganglia, mediates parasympathetic actions of the autonomic nervous system and is a dominant transmitter in the central nervous system. Toxins from various predatory species have evolved to block motor activity of prey; others from coral and plants use paralysis induced by cholinergic agents for protection from predation. The high affinity and selectivity of these toxins enabled the nach to emerge as the first chemically characterized neurotransmitter receptor. ther toxins from snakes and snails have proved useful to identify receptor subtypes in various tissues. In Alzheimer s dementias, ACh-containing nerve terminals are preferentially affected and therapy is directed to extending the surface area and duration of action of released ACh (Ch. 47). Cholinergic nerve terminals are also the site of action in the treatment of certain myasthenias and disorders involving compromised or excessive smooth muscle activity and exocrine secretion in the periphery. Carbamate and organophosphate pesticides act to delay the termination of action of acetylcholine by inhibiting acetylcholinesterase, and volatile organophosphates that have been used in terrorism incidents also act in this manner. ACh arrived within the evolutionary scheme long before the design of the nervous system and functional synapses. Bacteria, fungi, protozoa and plants store ACh and possess biosynthetic and degradative capacities for turnover of the molecule. Even in higher organisms, ACh distribution is far wider than the nervous system. For example, ACh is found in the cornea, certain ciliated epithelia, the spleen of ungulates and the human placenta [1]. Although definitive evidence is lacking, ACh has been proposed to play a role in development and tissue differentiation. ACh was first proposed as a mediator of cellular function by Hunt in 1907, and in 1914 Dale [2] pointed out that its action closely mimicked the response of parasympathetic nerve stimulation (see Ch. 10). oewi, in 1921, provided clear evidence for ACh release by nerve stimulation. eparate receptors that explained the variety of actions of ACh became apparent in Dale s early experiments [2]. The nicotinic ACh receptor was the first transmitter receptor to be purified and to have its primary structure determined [3, 4]. The primary structures of most subtypes of both nicotinic and muscarinic receptors, the cholin esterases (ChE), choline acetyltransferase (ChAT), the choline and ACh transporters have been ascertained. Threedimensional structures for several of these proteins or surrogates within the same protein family are also known. B A C 3 H H H + C CH 2 CH 2 N C 3 H N + N + C H H H H CHEMITY F ACETYCHINE Torsional rotation in the ACh molecule can occur around bonds τ 1, τ 2 and τ 3 (Fig. 11-1). ince the methyl groups are disposed symmetrically around τ 3 and constraints are placed on τ 1 by the planar acetoxy group, the most important torsion angle determining ACh conformation in solution is τ 2. A view from the β-methylene carbon of the molecule (Fig. 11-1B,C) shows the energy configurations around τ 2. Nuclear magnetic resonance (NM) studies indicate that the gauche conformation is predominant in solution [5, 6]. tudies of the activities of rigid analogs of ACh suggest that the trans conformation may be the active conformation at muscarinic receptors [7], while results of NM and X-ray crystallography studies suggest a change in configuration when ACh is bound to the nicotinic receptor [6, 8]. Hence the bound conformations of this flexible molecule appear to differ substantially with receptor subtype. This finding should not be a great surprise, since structural modifications of ACh that enhance or diminish activity on muscarinic receptors are very different from those modifications that influence activity on nicotinic receptors. GANIZATIN F THE CHINEGIC NEVU YTEM Acetylcholine receptors have been classified into subtypes based on early studies of pharmacologic selectivity. ong before structures were known, two crude alkaloid fractions, containing nicotine and muscarine (Fig. 11-2), were used to subclassify receptors in the cholinergic nervous system (Fig. 11-3). The greatly different C H H C FIGUE 11-1 tructure of acetylcholine. (A) The three torsion angles, τ 1, τ 2 and τ 3. (B) Newman projection of the gauche conformation. (C) Newman projection of the trans conformation. The molecule is viewed in the plane of the paper from the left side, and the bond angles around τ 2 are compared.

CHAPTE 11 Acetylcholine 187 Cholinergic tage 3 tage 2 tage 1 Nicotinic muscle Agonist: Phenyltrimethylammonium Antagonist: Elapid α-toxins, d-tubocurarine Nicotinic Junctional [(α1) 2 βεδ] Nicotinic Embryonic [(α1) 2 βγδ] Nicotinic Agonist: Nicotine Antagonist: d-tubocurarine Nicotinic neuronal β2 Nicotinic neuronal β3 Nicotinic neuronal β4 Nicotinic neuronal β5 Agonist: Acetylcholine Nicotinic neuronal (α+β) Agonist: Dimethylphenylpiperazinium, Cytisine Antagonist: Trimethaphan, Neuronal bungarotoxin Nicotinic neuronal α2 (α-toxin insensitive) Nicotinic neuronal α3 (α-toxin insensitive) Nicotinic neuronal α4 (α-toxin insensitive) Nicotinic neuronal α5 (non-functional without other α subunits) Nicotinic neuronal α6 (α-toxin insensitive) Nicotinic neuronal α7 (α-toxin sensitive) Muscarinic Agonist: Muscarine, Pilocarpine Antagonist: Atropine Agonist: xotremorine Antagonist: Propylbenzilylcholine mustard, Quinuclidinyl benzilate Muscarinic m 1 M 1 Antagonist: Pirenzepine Muscarinic m 2 M 2 Antagonist: Methoctramine Muscarinic m 3 M 3 Antagonist: Hexahydrosiladifenidol Muscarinic m 4 M 4 Antagonist: Himbacine Muscarinic m 5 M 5 Nicotinic neuronal α8 (α-toxin sensitive) Nicotinic neuronal α9 Nicotinic neuronal α10 FIGUE 11-2 Classification of cholinergic receptors. The diagram shows a historical classification of receptors analyzed on the basis of distinct responses with crude alkaloids (stage 1), the partial resolution of receptor subtypes with chemically synthesized agonists and antagonists (stage 2) and the distinction of primary structures of the receptors principally through cloning by recombinant DNA techniques (stage 3). activities of the antagonists atropine on muscarinic receptors and d-tubocurarine on nicotinic receptors further supported the argument that multiple classes of receptors existed for ACh. ubsequently, it was found that all nicotinic receptors are not identical. Nicotinic receptors in the neuromuscular junction, sometimes denoted as N 1 receptors, show selectivity for phenyltrimethylammonium as an agonist; elicit membrane depolarization in response to bisquaternary amines, with decamethonium being the most potent; are preferentially blocked by the competitive antagonist d-tubocurarine and are blocked irreversibly by the snake α-toxins. Nicotinic receptors in ganglia, N 2 receptors, are stimulated preferentially by 1,1-dimethyl-4-phenylpiperazinium, blocked competitively by trimethaphan, blocked noncompetitively by bisquaternary amines, with hexamethonium being the

188 PAT II Intercellular ignaling Nicotinic and muscarinic agonists CH H + 3 CH + 3 C CH 2 CH 2 N N CH H 3 3 C Acetylcholine (N + M) Phenyltrimethylammonium (N 1 ) CH 2 N Muscarine (M) + + N N 1,1-Dimethyl-4- phenylpiperazinium (N 2 ) C + N CH 2 C C CH 2 N xotremorine-m (M) N N Nicotine (N 1 + N 2 ) Nicotinic antagonists C CH 2 N N CH 2 ( ) 2 N + CH 2 H HC CH H 2 C HC CH + 2 H 2 C H 2 C Trimethaphan (N 2 ) H 3 C CH 2 + H N H d-tubocurarine (N 1 ) H 3 C + N (CH2)6 + N H 3 C + N (CH2)10 + N Hexamethonium (N 2 ) Decamethonium (N 1 ) Muscarinic antagonists H N C H i CH 2 CH 2 CH 2 N N N C N CH 2 N N Hexahydrosiladifenidol (M 3 ) Pirenzepine (M 1 ) Himbacine (M 4 ) N C Atropine (M) CH 2 H CH NH (CH 2 ) 6 NH (CH 2 ) 8 NH (CH 2 ) 6 NH Methoctramine (M 2 ) FIGUE 11-3 tructure of compounds important to the classification of receptor subtypes at cholinergic synapses. Compounds are subdivided as nicotinic (N) and muscarinic (M). The compounds interacting with nicotinic receptors are subdivided further according to whether they are neuromuscular (N 1 ) or ganglionic (N 2 ). Compounds with muscarinic subtype selectivity (M 1, M 2, M 3, M 4 ) are also noted.

CHAPTE 11 Acetylcholine 189 most potent, and show resistance to the snake α-toxins [9]. A large number of distinct neuronal nicotinic receptors are found in the CN; they are closer relatives of the nicotinic receptors in ganglia than of those in muscle. Muscarinic receptors also exhibit distinct subtypes. The antagonist pirenzepine (PZ) has the highest affinity for one subtype, M 1, which is found mainly in neuronal tissues. Another antagonist, methoctramine, has a higher affinity for M 2 receptors, which are the predominant muscarinic receptor subtype in mammalian heart. Hexahydrosiladifenidol is relatively selective for the M 3 receptors present in smooth muscle and glands, whereas himbacine exhibits high affinity for M 4 receptors. With this level of multiplicity of receptor subtypes, limitations on specificity preclude a single antagonist defining a distinct subtype. The intrinsic complexity and the multiplicity of cholinergic receptors became evident upon elucidation of their primary structures. In the CN, at least nine different sequences of α subunits and three different sequences of β subunits of the nicotinic receptor have been identified [10, 11]. Expression of the cloned genes encoding certain subunit combinations yields functional receptors with different sensitivities toward various toxins and agonists. At least five distinct muscarinic receptor genes have been cloned and sequenced. The genes are called m 1 to m 5. They correlate with the M 1 to M 5 receptors identified pharmacologically. The subtypes differ in their ability to couple to different G proteins and, hence, to elicit cellular signaling events. Thus, cholinergic receptor classification can be considered in terms of three stages of development. Initially, Dale [2] distinguished nicotinic and muscarinic receptor subtypes with crude alkaloids. Then, chemical synthesis and structure-activity relationships clearly revealed that nicotinic and muscarinic receptors were heterogeneous, but chemical selectivity could not come close to uncovering the true diversity of receptor subtypes. astly, analysis of subtypes came from molecular cloning, making possible the classification of receptors on the basis of primary structure (Fig. 11-2). FUNCTINA APECT F CHINEGIC NEUTANMIIN The individual subtypes of receptors often show discrete anatomical locations in the peripheral nervous system, and this has facilitated their classification. Nicotinic receptors are found in peripheral ganglia and skeletal muscle. Upon innervation of skeletal muscle, receptors congregate in the junctional or postsynaptic endplate area. Upon denervation or in noninnervated embryonic muscle, the receptors are distributed across the surface of the muscle, and these extrajunctional receptors are synthesized and degraded rapidly. Junctional receptors exhibit far slower rates of turnover and are distinguished by an ε subunit replacing a γ subunit in the assembled pentameric receptor. Ganglionic nicotinic receptors are found on postsynaptic neurons in both parasympathetic and sympathetic ganglia and in the adrenal gland. Ganglionic nicotinic receptors appear in tissues of neural crest embryonic origin and exhibit identical properties in sympathetic and parasympathetic ganglia. Muscarinic receptors are responsible for postganglionic parasympathetic neurotransmission and thus for control of a wide range of smooth muscle, cardiac muscle and secretory responses. ome responses originating in the sympathetic division of the autonomic nervous system, such as sweating and piloerection, also are mediated through muscarinic receptors. Both muscarinic and nicotinic responses are elicited in brain and spinal cord. A few specific central cholinergic pathways have been characterized. In higher brain centers, ACh is concentrated in interneurons and long projection neurons. In the striatum, cholinergic interneurons provide critical intercircuitry for extrapyramidal motor control and implicit memory. Four predominant nuclei in the basal forebrain send projecton neurons to the hippocampus, olfactory bulb and cerebral cortex. These include the nucleus basalis of Meynert, a region of primary degeneration in Alzheimer s disease. Four areas of the brain stem send cholinergic projections to the thalamus, interpeduncular nucleus and the superior colliculus. enshaw cells in the spinal cord play a role in modulating motoneuron activity by a feedback mechanism. timulation of enshaw cells occurs through branches of the motoneuron, and the transmitter is ACh acting on nicotinic receptors. Both nicotinic and muscarinic receptors are widespread in the CN. Muscarinic receptors with a high affinity for pirenzepine (PZ), M 1 receptors, predominate in the hippocampus and cerebral cortex, whereas M 2 receptors predominate in the cerebellum and brainstem, and M 4 receptors are most abundant in the striatum. Central muscarinic and nicotinic receptors are targets of intense pharmacological interest for their potential roles in regulating abnormal neurological signaling in Alzheimer s disease, Parkinson s disease and certain seizure disorders. Nicotinic receptors are largely localized at prejunctional sites and control the release of neuro transmitters [10, 11]. Histochemical studies using antibodies selective for receptor subtype, ChAT and presynaptic transport proteins, along with receptor autoradiography with labeled ligands, have produced detailed maps of the CN. In addition, the nerve cell bodies containing the mna encoding these proteins have been defined through in situ hybridization with a cdna or antisense mna. tudies involving iontophoretic application of transmitter, local stimulation and intracellular or cellsurface measurements of responses establish appropriate

190 PAT II Intercellular ignaling functional correlates. ecently, knockout mice strains, harboring deletions of M1 M5 machs and various nach subunits, have been generated, providing a direct means of determining the localization of function of these receptors [12, 13]. Neurotransmission in autonomic ganglia is more complex than depolarization mediated by a single transmitter. In autonomic ganglia, the primary electrophysiological event following preganglionic nerve stimulation is the rapid depolarization of postsynaptic sites by released ACh acting on nicotinic receptors. This activation gives rise to an initial excitatory postsynaptic potential (EPP), which is due to an inward current through a cation channel (see Chs 6 and 10). This mechanism is virtually identical to that in the neuromuscular junction, with an immediate onset of the depolarization and decay within a few milliseconds. Nicotinic antagonists such as trimethaphan competitively block ganglionic transmission, whereas agents such as hexamethonium produce blockade by occluding the channel. An action potential is generated in the postganglionic nerve when the initial EPP attains a threshold amplitude. everal secondary events amplify or suppress this signal. These include the slow EPP, the late, slow EPP and an inhibitory postsynaptic potential (IPP). The slow EPP is generated by ACh acting on muscarinic receptors and is blocked by atropine. It has a latency of approximately 1 sec and a duration of 30 60 seconds. The late, slow EPP can last for several minutes and is mediated by peptides found in ganglia, including substance P, angiotensin, luteinizing-hormone-releasing hormone (HH) and the enkephalins. The slow EPP and late, slow EPP result from decreased K + conductance and are believed to regulate the sensitivity of the postsynaptic neuron to repetitive depolarization [14]. The IPP seems to be mediated by the catecholamines dopamine and/or norepinephrine and is blocked by α-adrenergic antagonists as well as by atropine. ACh released from presynaptic terminals may act on a catecholamine-containing interneuron to release dopamine and perhaps norepinephrine. imilar to the slow EPP, the IPP has a prolonged latency and duration of action. These secondary events vary with the individual ganglia and are believed to modulate the sensitivity to the primary event. Hence, drugs that selectively block the slow EPP, such as atropine, will diminish the efficiency of ganglionic transmission rather than eliminate it. imilarly, agonists such as muscarine and the ganglion-selective muscarinic agent, McN-A-343, are not thought of as primary ganglionic stimulants. ather, they enhance the initial EPP under conditions of repetitive stimulation. ince parasympathetic and sympathetic ganglia exhibit comparable sensitivities to nicotinic agonists such as nicotine and ACh in producing the initial EPP, the pharmacological effect of ganglionic stimulation depends on the profile of innervation to particular organs or tissues (Table 11-1). For example, blood vessels are innervated only by the sympathetic nervous system; thus, ganglionic stimulation should produce only vasoconstriction. imilarly, the pharmacological effects of ganglionic blockade will depend on which component of the autonomic nervous system is exerting the predominant tone at the effector organ. Muscarinic receptors are widely distributed at postsynaptic parasympathetic effector sites. The response to systemically administered ACh is characteristic of stimulation of postganglionic effector sites rather than of ganglia. This is a consequence of the greater abundance of muscarinic receptors at effector sites in innervated tissues and the limited blood flow to ganglia. Muscarinic receptors are found in visceral smooth muscle, cardiac muscle, secretory glands and endothelial cells of the vasculature. Except for endothelial cells, each of these sites receives cholinergic innervation. esponses can be excitatory or inhibitory, depending on the tissue. Even within a single tissue the responses may vary. For example, muscarinic stimulation causes gastrointestinal smooth muscle to depolarize and contract, except at sphincters, where hyperpolarization and relaxation are seen (Table 11-2). In most tissues innervated by the cholinergic nervous system, smooth muscle exhibits intrinsic electrical and/or mechanical activity. This activity is modulated rather than initiated by cholinergic nerve stimulation. Cardiac muscle and smooth muscle exhibit spikes of electrical activity that are propagated between cells. These spikes are initiated by rhythmic fluctuations in resting membrane potential. In intestinal smooth muscle, cholinergic stimulation TABE 11-1 Predominance of sympathetic or parasympathetic tone at effector sites: effects of autonomic ganglionic blockade ite Predominant tone Primary effects of ganglionic blockade Arterioles ympathetic (adrenergic) Vasodilation, increased peripheral blood flow, hypotension Veins ympathetic (adrenergic) Dilation, pooling of blood, decreased venous return, decreased cardiac output Heart Parasympathetic (cholinergic) Tachycardia Iris Parasympathetic (cholinergic) Mydriasis Ciliary muscle Parasympathetic (cholinergic) Cycloplegia (focus to far vision) Gastrointestinal tract Parasympathetic (cholinergic) educed tone and motility of smooth muscle, constipation, decreased gastric and pancreatic secretions Urinary bladder Parasympathetic (cholinergic) Urinary retention alivary glands Parasympathetic (cholinergic) Xerostomia weat glands ympathetic (cholinergic) Anhidrosis

CHAPTE 11 Acetylcholine 191 TABE 11-2 Effects of acetylcholine (ACh) stimulation on peripheral tissues Tissue Effects of ACh Vasculature (endothelial cells) elease of endothelium-derived relaxing factor (nitric oxide) and vasodilation Eye iris (pupillae sphincter muscle) Contraction and miosis Ciliary muscle Contraction and accommodation of lens to near vision alivary glands and lacrimal glands ecretion thin and watery Bronchi Constriction, increased secretions Heart Bradycardia, decreased conduction (atrioventricular block at high doses), small negative inotropic action Gastrointestinal tract Increased tone, increased gastrointestinal secretions, relaxation at sphincters Urinary bladder Contraction of detrusor muscle, relaxation of the sphincter weat glands Diaphoresis eproductive tract, male Erection Uterus Variable, dependent on hormonal influence will cause a partial depolarization and increase the frequency of spike production. In contrast, cholinergic stimulation of atria will decrease the generation of spikes through hyperpolarization of the membrane. Membrane depolarization typically results from an increase in Na + conductance. In addition, mobilization of intracellular Ca 2+ from the endoplasmic or sarcoplasmic reticulum and the influx of extracellular Ca 2+ appear to be elicited by ACh acting on muscarinic receptors (see Ch. 22). The resulting increase in intracellular free Ca 2+ is involved in activation of contractile, metabolic and secretory events. timulation of muscarinic receptors has been linked to changes in cyclic nucleotide concentrations. eductions in camp concentrations and increases in cgmp concentrations are typical responses (see Ch. 21). These cyclic nucleotides may facilitate contraction or relaxation, depending on the particular tissue. Inhibitory responses also are associated with membrane hyperpolarization, and this is a consequence of an increased K + conductance. Increases in K + conductance may be mediated by a direct receptor linkage to a K + channel or by increases in intracellular Ca 2+, which in turn activate K + channels. Mechanisms by which muscarinic receptors couple to multiple cellular responses are considered later. timulation of the motoneuron releases acetylcholine onto the muscle endplate and results in contraction of the muscle fiber. Contraction and associated electrical events can be produced by intra-arterial injection of ACh close to the muscle. ince skeletal muscle does not possess inherent myogenic tone, the tone of apparently resting muscle is maintained by spontaneous and intermittent release of ACh. The consequences of spontaneous release at the motor endplate of skeletal muscle are small depolarizations from the quantized release of ACh, termed miniature endplate potentials (MEPPs) [15] (see Ch. 10). Decay times for the MEPPs range between 1 and 2 ms, a duration similar to the mean channel open time seen with ACh stimulation of individual receptor molecules. timulation of the motoneuron results in the release of several hundred quanta of ACh. The summation of MEPPs gives rise to a postsynaptic excitatory potential (PEP), also termed motor endplate potential. A sufficiently large and abrupt potential change at the endplate will elicit an action potential by activating voltage-sensitive Na + channels. The action potential propagates in two-dimensional space across the surface of the muscle to release Ca 2+ and elicit contraction. Therefore, the PEP may be thought of as a generator potential, found only in junctional regions and arising from the opening of multiple receptor channels. Normal resting potentials in endplates are about 70 mv. The PEP causes the endplate to depolarize partially to about 55 mv. It is the rapid and transient changes from 70 to 55 mv in localized areas of the endplate that trigger action potential generation [9, 15]. Competitive blocking agents cause muscle paralysis by preventing access of acetylcholine to its binding site on the receptor. Competitive blockade with agents such as d-tubocurarine result in maintenance of the endplate potential at 70 mv. Without frequent PEPs, action potentials are not triggered and flaccid paralysis of the muscle results. The actions of competitive blocking agents can be surmounted by excess ACh. Depolarizing neuromuscular blocking agents, such as decamethonium and succinylcholine, produce depolarization of the endplate such that the endplate potential is 55 mv. The high concentrations of depolarizing agent that are maintained in this synapse do not allow regions of the endplate to repolarize, as would occur with a labile transmitter such as ACh. ince it is the transition in membrane potential between 70 and 55 mv that triggers the action potential, flaccid paralysis also will occur with a depolarizing block [9]. Excess ACh will not reverse the paralysis by depolarizing blocking agents. As might be expected if depolarization occurs in a non-uniform manner in microscopic areas within individual endplates and in individual motor units, the onset of depolarization blockade is characterized by muscle twitching and fasciculations that are not evident in competitive block. nce paralysis occurs, the overall pharmacological actions of competitive and depolarizing blocking agents are similar, yet intracellular measurements of endplate potential can distinguish these two classes of agent.

192 PAT II Intercellular ignaling YNTHEI, TAGE AND EEAE F ACETYCHINE The biosynthesis and storage of ACh can be divided into three processes that allow for recovery of hydrolyzed transmitter by choline transport back into the nerve ending, conversion by acetylation to active transmitter and then storage in a vesicle for subsequent release [16 21] (Fig. 11-4). The synthesis reaction is a single step catalyzed by the enzyme choline acetyl transferase (ChAT): Choline + Acetyl coenzyme A a Acetylcholine + Coenzyme A. ChAT, first assayed in a cell-free preparation in 1943, has subsequently been purified and cloned from several sources [16]. The purification of ChAT has allowed ACh ACh ACh AcCoA ChAT Choline = VAChT = ChT AChE Choline + Ac ACh FIGUE 11-4 Transport, synthesis and degradative processes in a cholinergic presynaptic nerve terminal and synapse. The choline transport protein (ChT) functions at the nerve ending membrane to transport choline into the cytoplasm, where its acetylation by acetyl CoA is catalyzed by choline acetyltransferase (ChAT) to generate acetylcholine (ACh) in the vicinity of the synaptic vesicle. The vesicular acetylcholine transporter (VAChT) concentrates acetylcholine in the vesicle. ChT is also found on the vesicle but in a functionally inactive state. Upon nerve stimulation, depolarization and Ca 2+ entry, ACh-containing vesicles fuse with the membrane and release their contents. The fusion of the membrane results in more ChT being exposed to the synaptic gap, where it becomes active. ACh is hydrolyzed to acetate and choline catalyzed by acetylcholinesterase (AChE), allowing for recapture of much of the choline by ChT. Because of the differing ionic compositions in the extracellular milieu and within the cell, ChT is thought to be active only when situated on the nerve cell membrane. imilarly, the VAChT may only be active when encapsulated in the synaptic vesicle [19]. production of specific antibodies. Whereas acetylcholinesterase (AChE), the enzyme responsible for degradation of ACh, is produced by cells containing cholinoreceptive sites as well as in cholinergic neurons, ChAT is found in the nervous system specifically at sites where ACh synthesis takes place. Within cholinergic neurons, ChAT is concentrated in nerve terminals, although it is also present in axons, where it is transported from its site of synthesis in the soma. When subcellular fractionation studies are carried out, ChAT is recovered in the synaptosomal fraction, and within synaptosomes it is primarily cytoplasmic. It has been suggested that ChAT also binds to the outside of the storage vesicle under physiological conditions and that ACh synthesized in that location may be situated favorably to enter the vesicle. Brain ChAT has a K D for choline of approximately 1 mmol/l and for acetyl coenzyme A (CoA) of approximately 10 µmol/l. The activity of the isolated enzyme, assayed in the presence of optimal concentrations of cofactors and substrates, appears far greater than the rate at which choline is converted to ACh in vivo. This suggests that the activity of ChAT is repressed in vivo. urprisingly, inhibitors of ChAT do not decrease ACh synthesis when used in vivo; this may reflect a failure to achieve a sufficient local concentration of inhibitor, but also suggests that this step is not rate-limiting in the synthesis of ACh [18 20]. The acetyl CoA used for ACh synthesis in mammalian brain comes from pyruvate formed from glucose. It is unclear how the acetyl CoA, generally thought to be formed at the inner membrane of the mitochondria, accesses the cytoplasmic ChAT. This may also be a regulated, rate-limiting step. Acetylcholine formation is limited by the intracellular concentration of choline, which is determined by active transport of choline into the nerve ending. Choline is present in the plasma at a concentration of about 10 µmol/l. A low-affinity choline uptake system with a K m of 10 100 µmol/l is present in all tissues, but cholinergic neurons also have a Na + -dependent high-affinity choline uptake system, with a K m for choline of 1 5 µmol/l [17 20]. The high-affinity uptake mechanisms should be saturated at 10 µmol/l choline, so the plasma choline concentration is probably adequate for sustained ACh synthesis even under conditions of high demand, as observed in ganglia. ince the plasma concentration of choline is above the K m of the high-affinity choline-transport system, it is not expected that choline concentrations in the nerve ending would be increased by increasing the plasma concentration of choline or by changing the K m of the uptake system. However, neuronal choline content can be influenced by altering the capacity of the high-affinity choline-uptake mechanism, i.e. by changing the maximum velocity (V max ) for transport. The cloning of the high affinity choline transporter has provided considerable insight into the mechanisms by which events associated with neuronal

CHAPTE 11 Acetylcholine 193 + N H N H Vesamicol H H 3 C H 3 C Hemicholinium (HC-3) FIGUE 11-5 tructures of hemicholinium (HC-3) and vesamicol. activity enhance choline entry into neurons [17, 19]. If the K m of ChAT for choline in vivo is as high as that seen with the purified enzyme, one would expect ACh synthesis to increase in proportion to the greater availability of choline. Conversely, ACh synthesis should be diminished when high-affinity choline uptake is blocked. Hemicholinium-3 is a potent inhibitor of the highaffinity choline-uptake system, with a K i in the submicromolar range (Fig. 11-5). Indeed, treatment with this drug decreases ACh synthesis and leads to a reduction in ACh release during prolonged stimulation; these findings lend support to the notion that choline uptake is the ratelimiting factor in the biosynthesis of ACh [18]. Characterization and regulation of the high affinity choline transporter. The successful cloning of the high affinity choline transporter [17], now referred to as the CHT, was spurred by the discovery of a Caenorhabditis elegans transporter exhibiting cholinergic neuron specificity. The homologous rat CHT cdna conferred high affinity Na + and Cl dependent choline transport when expressed in Xenopus oocytes, and subsequently homologous cdnas were cloned from numerous species including mice and humans [19]. The gene structure is highly conserved amongst the species, and the encoded protein is a member of a family of Na + -dependent transporters. The structure reveals 13 transmembrane spans, glycosylation sites on the extracellular surface and several potential sites for phosphorylation on intracellular loops and at the C- terminus. The development of antibodies to CHT revealed the anticipated localization of CHT at presynaptic sites and exclusively in cholinergic neurons. urprisingly, however, much of the CHT immunoreactivity was found associated with intracellular vesicles rather than at the presynaptic plasma membrane. These vesicles appear to be those involved in ACh synthesis (and its subsequent + N storage and release) as CHT is co-localized with the vesicular ACh transporter (VAChT) described below. Interestingly, blocking endocytosis leads to CHT accumulation at the plasma membrane and an enhanced capacity for choline uptake [19]. Thus, it appears that the transporter is normally removed from the plasma membrane and recycled. Conversely, stimuli that trigger ACh release have been shown to increase CHT density at the plasma membrane, providing a mechanism by which uptake can be tightly coupled to the rate of ACh release. It is likely that post-translational modification, e.g. phosphorylation, also serves to regulate the transporter in response to altered synaptic activity. Whether this occurs via activation of a resident plasma membrane pool of CHT or alterations in activity or recycling of the vesicular CHT pool remains to be determined. The transporter precursor progresses down the axons by axoplasmic transport and is localized to the vesicular environment by poorly understood mechanisms. A second transport system concentrates acetylcholine in the synaptic vesicle. The vesicular ACh transporter (VAChT) also has been cloned and expressed (see also Ch. 5). Its sequence places it in the 12-membranespanning family characteristic of the other biogenic amine transporters found in adrenergic nerve endings [18, 20, 21]. Interestingly, the gene encoding the transporter is located within an intron of the ChAT gene, suggesting a mechanism for coregulation of gene expression for ChAT and VAChT [20]. ACh uptake into the vesicle is driven by a proton-pumping ATPase. Coupled countertransport of H + and ACh allows the vesicle to remain isoosmotic and electroneutral [18]. A selective inhibitor of ACh transport, vesamicol (Fig. 11-5), inhibits vesicular ACh uptake with an IC 50 of 40 nnol/l [16 18]. Inhibition appears noncompetitive, suggesting that this inhibitor acts on a site other than the ACh-binding site on the transporter. Vesamicol blocks the evoked release of newly synthesized ACh without significantly affecting high-affinity choline uptake, ACh synthesis or Ca 2+ influx. The fact that ACh release is lost secondary to the blockade of uptake by the vesicle supports the concept that the vesicle is the site of ACh release. The activity of the recombinant transporter also is inhibited by vesamicol [19, 20]. Choline is supplied to the neuron either from plasma or by metabolism of choline-containing compounds. The majority of the choline used in ACh synthesis is thought to come directly from recycling of released ACh, hydrolyzed to choline by acetylcholinesterase (Fig. 11-4). Presumably, uptake of this neurotransmitter-derived choline occurs rapidly, before the choline diffuses away from the synaptic cleft. Another source of choline is the breakdown of phosphatidylcholine, which may be stimulated by locally released ACh. Choline derived from these two sources is then subject to high-affinity uptake into the

194 PAT II Intercellular ignaling nerve ending. In the CN, these metabolic sources of choline appear to be particularly important because choline in the plasma cannot readily pass the blood brain barrier. Thus, the high-affinity uptake of choline into cholinergic neurons in the CN might not always be saturated, and ACh synthesis could be limited by the supply of choline, at least during sustained activity. This would be consistent with the finding that ACh stores in the brain are subject to variation, whereas ACh stores in ganglia and muscles remain relatively constant. A slow release of acetylcholine from neurons at rest probably occurs at all cholinergic synapses. Quantized ACh release was described first by Fatt and Katz, who recorded small, spontaneous depolarizations at frog neuromuscular junctions that were subthreshold for triggering action potentials. These MEPPs were shown to be due to the release of ACh. When the nerve was stimulated and endplate potentials recorded and analyzed, the magnitude of these potentials always was found to be some multiple of the magnitude of the individual MEPPs. It was suggested that each MEPP resulted from a finite quantity or quantum of released ACh and that the endplate potentials resulted from release of greater numbers of quanta during nerve stimulation (see Ch. 10). A possible structural basis for these discrete units of transmitter was discovered shortly thereafter when independent electron microscopic and subcellular fractionation studies by de obertis and Whittaker revealed the presence of vesicles in cholinergic nerve endings. These investigators defined procedures for subcellular fractionation of mammalian brain and Torpedo electric organs that yield resealed nerve endings, or synaptosomes, that can be lysed to release a fraction enriched in vesicles. More than half of the ACh in the synaptosome is associated with particles that look like the vesicles seen under an electron microscope. Therefore, it is clear that ACh is associated with a vesicle fraction, and it is likely that it is contained within the vesicle. The origin of the free ACh within the synaptosome is less clear. It may be ACh that is normally in the cytosol of the nerve ending, or it may be an artifact of release from the vesicles during their preparation (see Ch. 10). The relationship between acetylcholine content in a vesicle and the quanta of acetylcholine released can only be estimated. Estimates of the amount of ACh contained within cholinergic vesicles vary considerably [22]. Whittaker estimated that there are about 2,000 molecules of ACh in a cholinergic vesicle from the CN. A similar estimate of about 1,600 molecules of ACh per vesicle was made using sympathetic ganglia. The most abundant source of cholinergic synaptic vesicles is the electric organ of Torpedo. Vesicles from Torpedo are far larger than those from mammalian species and are estimated to contain up to 100 times more ACh per vesicle. The Torpedo vesicle also contains ATP and, in its core, a proteoglycan of the heparin sulfate type. Both of these constituents may serve as counter-ions for ACh, which otherwise would be at a hyperosmotic concentration. The amount of ACh in a quantum has been estimated by comparing the potential changes associated with MEPPs to those obtained by iontophoresis of known quantities of ACh. Based on such analysis, the amount of ACh per quantum at the snake neuromuscular junction was estimated to be something less than 10,000 molecules [22]. ince this compares with estimates of vesicle content, quanta are likely defined by the amount of releasable ACh in the vesicle. An alternative favored by some investigators is that ACh is released directly from the cytoplasm. In this model, definable quanta are evident because channels in the membrane are open for finite periods of time when Ca 2+ is elevated. A presynaptic membrane protein suggested to mediate Ca 2+ -dependent translocation of ACh has been isolated [21]. Although some arguments support this model, most investigators favor the notion that the vesicle serves not only as a unit of storage but also as a unit of release. The vesicle hypothesis and release of neurotransmitters are discussed also in Chapter 10. Depolarization of the nerve terminal by an action potential increases the number of quanta released per unit time. elease of ACh requires the presence of extracellular Ca 2+, which enters the neuron when it is depolarized. Most investigators are of the opinion that a voltage-dependent Ca 2+ current is the initial event responsible for transmitter release, which occurs about 200 µs later. The mechanism through which elevated Ca 2+ increases the probability of ACh release is not yet known; presumably activation of proteins in the synaptic vesicle membrane fusion complex causes the vesicle to fuse with the neuronal membrane. Dependence on Ca 2+ is a common feature of all exocytotic release mechanisms and it is likely that exocytosis is a conserved mechanism for transmitter release. There is good evidence that adrenergic vesicles empty their contents into the synaptic cleft because norepinephrine and epinephrine are released along with other contents of the storage vesicle. Although less rigorous data are available for cholinergic systems, cholinergic vesicles contain ATP, and release of ATP has been shown to accompany ACh secretion from these vesicles. Furthermore, Heuser and eese demonstrated, in electron microscopic studies at frog nerve terminals, that vesicles fuse with the nerve membrane and that vesicular contents appear to be released by exocytosis; it has been difficult to ascertain, however, whether the fusions are sufficiently frequent to account for release on stimulation. The nerve ending also appears to endocytose the outer vesicle membrane to form vesicles that subsequently are refilled with ACh [19, 23]. All the acetylcholine contained within the cholinergic neuron does not behave as if in a single compartment. esults of a variety of neurophysiological and biochemical experiments suggest the presence of two distinguishable

CHAPTE 11 Acetylcholine 195 pools of ACh, only one of which is readily available for release. These have been referred to as the readily available or depot pool and the reserve or stationary pool. The reserve pool refills the readily available pool as it is utilized. Unless the rate of mobilization of ACh into the readily available pool is adequate, the amount of ACh that can be released may be limited. It is also likely that newly synthesized ACh is used to fill the readily available pool of ACh because it is the newly synthesized ACh that is released preferentially during nerve stimulation. The precise relationship between these functionally defined pools and ACh storage vesicles is not known. It is possible that the readily available pool resides in vesicles poised for release near the nerve ending membrane, whereas the reserve pool is in more distant vesicles. ACETYCHINETEAE AND THE TEMINATIN F ACETYCHINE ACTIN Cholinesterases are widely distributed throughout the body in both neuronal and non-neuronal tissues. Based largely on substrate specificity, the cholinesterases are subdivided into the acetylcholinesterases (AChEs) (EC 3.1.1.7) and the butyrylcholinesterase (BuChE) (EC 3.1.1.8) [24 26]. Choline esters with acyl groups of the size of butyric acid or larger are hydrolyzed very slowly by AChE; selective inhibitors for each enzyme have been identified. AChE and BuChE are encoded by single but distinct genes. BuChE is synthesized primarily in the liver and appears in plasma; however, it is highly unlikely that appreciable concentrations of ACh diffuse from the locality of the synapse and elicit a systemic response. That the population distribution of BuChE mutations correlates with resistance of the enzyme to naturally occurring inhibitors suggests that this enzyme hydrolyzes dietary esters of potential toxicity. Although BuChE is localized in the nervous system during development, the existence of nonexpressing mutations in the BuChE gene within the human population also demonstrates that this enzyme is not essential for nervous system function. In general, AChE distribution correlates with innervation and development in the nervous system. The AChEs also exhibit synaptic localization upon synapse formation. Total knock out of the AChE gene in mice gives rise to an animal with continuous tremors, stunted growth and deficits in neurologic function [27]. The primary and tertiary structures of the cholinesterases are known. The primary structures of the cholinesterases initially defined a large and functionally eclectic superfamily of proteins, the α,β hydrolase fold family, that function not only catalytically as hydrolases but also as surface adhesion molecules forming heterologous cell contacts, as seen in the structurally related proteins neuroligin and the tactins. A sequence homologous to the cholinesterases and a presumed common structural matrix are found in thyroglobulin, in which tyrosine residues become iodinated and conjugated to form thyroid hormone [24, 25]. The initial solution of the crystal structure of the Torpedo enzyme [28], followed by the mammalian AChE structure [29], revealed that the active center serine lies at the base of a rather narrow gorge that is lined heavily with aromatic residues (Fig. 11-6). The enzyme carries a net negative charge, and an electrostatic dipole is oriented on the enzyme to facilitate diffusional entry of cationic ligands. Crystal structures of several inhibitors in a complex with AChE also have been elucidated [25]. Acetylcholinesterases exist in several molecular forms governed by alternative splicing. The open reading frame in mammalian AChE genes is encoded by three invariant exons (exons 2, 3 and 4) followed by three splicing alternatives. Continuation through exon 4 gives rise to a monomeric species. plicing to exon 5 gives rise to the carboxyl-terminal sequence signal for addition of glycophospholipid, while splicing to exon 6 encodes a sequence containing a cysteine that links to other catalytic or structural subunits. These species of AChE differ only in the last 40 residues in their C-termini. FIGUE 11-6 View of the active center gorge of mammalian acetylcholinesterase looking into the gorge cavity. The gorge is 18 20 Å in depth in a molecule of 40 Å diameter and is heavily lined with aromatic amino acid side chains. ide chains from several sets of critical residues are shown emanating from the α carbon of the α carbon-amide backbone: (a) A catalytic triad between Glu 334, His 447 and er 203 is shown by dotted lines to denote the hydrogen-bonding pattern. This renders er 203 more nucleophilic to attack the carbon of acetylcholine (shown in white with the van der Waals surface). This leads to formation of an acetyl enzyme, which is deacetylated rapidly. (b) The acyl pocket outline by Phe 295 and 297 is of restricted size in acetylcholinesterase. In butyrylcholinesterase, these side chains are aliphatic, increasing the size and flexibility in the acyl pocket. (c) The choline subsite lined by the aromatic residues Trp 86, Tyr 337 and Tyr 449 and the anionic residue Glu 202. (d) A peripheral site which resides at the rim of the gorge encompasses Trp 286, Tyr 72, Tyr 124 and Asp 74. This site modulates catalysis by binding inhibitors or, at high concentrations, a second substrate molecule.

196 PAT II Intercellular ignaling 1α Cap 82 22 1070 1723 TGA 1 2 3 4 pa 6 pa Cap ATG TGA pa pa 5 TGA pa pa Transcriptional regulation Constant regioncatalytic core Membrane attachment mna stability Heteromeric Exon 6 Homomeric Exon 6 Exon 5 C G 1 (exon 4) ( H ( n G 1 C C ( ( n ( ( n G 2 ipid-linked (G 4 ) C C G 2 Asymmetric A 12 C C G 4 Hydrophilic (G 1,2,4 ) Glycophospholipidlinked (G 1,2,4 ) FIGUE 11-7 Gene structure of AChE. Alternative cap sites in the 5 end of the gene allow for alternative promoter usage in different tissues. keletal-muscle-specific regulation is controlled by the intron region between Exons 1 and 2. Exons 2, 3 and 4 encode an invariant core of the molecule that contains the essential catalytic residues. Just prior to the stop codon, three splicing alternatives are evident: 1, a continuation of exon 4; 2, the 4 5 splice; and 3, the 4 6 splice. The catalytic subunits produced differ only in their carboxy-termini and are shown in the lower panel. (Modified with permission from reference [24].) These AChE forms differ in solubility and mode of membrane attachment rather than in catalytic activity. ne class of molecular forms exists as homomeric assemblies of catalytic subunits that appear as monomers, dimers or tetramers (Fig. 11-7). These forms also differ in hydrophobicity, and their amphiphilic character arises from either exposure of an amphipathic helix or posttranslational addition of a glycophospholipid on the carboxyl-terminal amino acid. The glycophospholipid allows the enzyme to be tethered on the external surface of the cell membrane. The second class of AChEs exists as heteromeric assemblies of catalytic and structural subunits. ne form consists of up to 12 catalytic subunits linked by disulfide bonds to filamentous, collagen-containing structural subunits. These forms are often termed asymmetric, since the tail unit imparts substantial dimensional asymmetry to the molecule. The collagenous tail unit links by disulfide bonding at its proline rich N-terminus through a coiled coil arrangement to the C-terminus of two of the catalytic subunits [30]. The tail unit associates with the basal lamina of the synapse rather than the plasma membrane.

CHAPTE 11 Acetylcholine 197 Asymmetric forms are present in high density at the neuromuscular junction. A second type of structural subunit, found primarily in the CN, has a similar proline-rich attachment domain but contains covalently attached lipid, enabling this form of the enzyme to associate with cell membranes. The different C-termini and attachment modes lead to distinctive extracellular localizations of AChE but do not affect the intrinsic catalytic activities of the individual forms. Cholinesterase catalysis and inhibition mechanisms involve formation of reversible complexes and covalent conjugates. In ester catalysis, an initial acylation step proceeds through the formation of a tetrahedral transition state; acylation is rapidly followed by deacylation. Alkylphosphate inhibitors are tetrahedral in configuration and their geometric resemblance to the transition state in part accounts for their effectiveness as inhibitors of AChE. Acylation occurs on the active-site er 202, which is rendered nucleophilic by proton withdrawal by Glu 334 through His 447. The acetyl enzyme that is formed is short-lived, lasting approximately 10 µs; this accounts for the high catalytic efficiency of the enzyme (Fig. 11-6). The availability of a crystal structure of AChE has enabled investigators to assign residues and domains in the cholinesterase responsible for catalysis and inhibitor specificity [26, 28, 29]. everal AChE inhibitors are used therapeutically in glaucoma, myasthenia gravis and smooth muscle atony, whereas others have proved useful as insecticides. till others have been produced for more insidious uses in terrorism and chemical warfare. Inhibitors such as edrophonium bind reversibly to the active center of AChE at the base of the gorge (Fig. 11-6). ther reversible inhibitors, such as gallamine, propidium and the three-fingered peptide from snake venom, fasciculin, bind to a peripheral site located at the gorge rim. The carbamoylating agents, such as neostigmine and physostigmine, form a carbamoyl enzyme by reacting with the active-site serine. The carbamoyl enzymes are more stable than the acetyl enzyme; their decarbamoylation occurs over several minutes. ince the carbamoyl enzyme will not hydrolyze ACh, the carbamoylating agents are alternative substrates that are effective inhibitors of ACh hydrolysis. The alkylphosphates, such as diisopropylfluorophosphate or echothiophate, act in a similar manner; however, the alkyl phosphorates and alkylphosphonates form extremely stable bonds with the active-site serine on the enzyme. The time required for their hydrolysis often exceeds that for biosynthesis and turnover of the enzyme. Accordingly, inhibition with the alkylphosphates is effectively irreversible. Consequences of acetylcholinesterase inhibition differ with effector site. At postganglionic parasympathetic effector sites, AChE inhibition enhances or potentiates the action of administered ACh or ACh released by nerve activity. In part, this is a consequence of diffusion of transmitter and stimulation of receptors extending over a larger area from the point of transmitter release. imilarly, ganglionic transmission is enhanced by cholinesterase inhibitors. ince atropine and other muscarinic antagonists are effective antidotes of AChE inhibitor toxicity, some CN manifestations result largely from excessive muscarinic stimulation. By prolonging the residence time of ACh in the synapse, AChE inhibition in the neuromuscular junction promotes a persistent depolarization of the motor endplate. The decay of endplate currents or potentials resulting from spontaneous release of ACh is prolonged from 1 2 ms to 5 30 ms. This indicates that the transmitter activates multiple receptors before diffusing from the synapse. Excessive depolarization of the endplate, resulting from slower rates of decay of endplate currents, leads to a diminished capacity to initiate coordinated action potentials. In a fashion similar to depolarizing blocking agents, fasciculations and muscle twitching are observed initially with AChE inhibition, followed by flaccid paralysis. NICTINIC ECEPT The nicotinic acetylcholine receptor (nach) was the first characterized neurotransmitter receptor. The nach was purified a decade earlier than other neurotransmitter receptors. The electric organ of Torpedo, consisting of stacks of electrocytes, tissue of common embryonic origin to skeletal muscle, is a rich source of nicotinic receptors. Upon differentiation, the electrogenic bud in the electrocyte proliferates but the contractile elements are not expressed. The excitable membrane encompasses the entire ventral surface of the electrocyte rather than being localized to focal junctional areas covering 0.1% of the cell surface as found in skeletal muscle. The electrical discharge in Torpedo relies solely on a PEP resulting from depolarization of the postsynaptic membrane, rather than propagation from an action potential. The density of receptors in the Torpedo electric organ approaches 100 pmol/mg protein, which may be compared with 0.1 pmol/mg protein in skeletal muscle. In the early 1960s, it was established that snake α-toxins, such as α-bungarotoxin, irreversibly inactivate receptor function in intact skeletal muscle, and this finding led directly to the identification and subsequent isolation of the nicotinic ACh receptor from Torpedo [3]. By virtue of their high affinity and very slow rates of dissociation, labeled α-toxins serve as markers of the receptor during solubilization and purification. ufficient amino acid sequence of the receptor was obtained to permit the cloning and sequencing of the genes encoding the individual subunits of the receptor, first in Torpedo and then in other species [4]. As a consequence of the high density of nicotinic ACh receptors in the postsynaptic membranes of Torpedo, sufficient order of the receptor molecules is achieved in isolated membrane fragments such that image

198 PAT II Intercellular ignaling reconstructions from electron microscopy have allowed a more detailed analysis of structure [31]. abeling of functional sites, determination of subunit composition and structure modification through mutagenesis contributed to our understanding of the structure of nicotinic receptors [32]. ecently, a soluble acetylcholine binding protein (AChBP) from mollusks was purified and its three dimensional structure determined (Fig. 11-8) [33]). Not only are these pentameric proteins structural surrogates of the extracellular domain of the nach [33] but, when appropriately linked to the transmembrane spans of nach family, they gate ions in response to agonists and are blocked by antagonists [34]. The nach consists of five subunits arranged around a pseudoaxis of symmetry. The subunits display homologous amino acid sequences with 30 40% identity of amino acid residues [4, 11, 32]. In muscle, one subunit, designated α, is expressed in two copies; the other three, β, γ and δ, are present as single copies (Fig. 11-9). Thus, the receptor is a pentamer of molecular mass of approximately 280 kda. tructural studies show the subunits to be arranged around a central cavity, with the largest portion of the protein exposed toward the extracellular surface. The central cavity is believed to lead to the ion channel, which in the resting state is impermeable to ions; upon activation, however, it opens to a diameter of 6.5 Å. The open channel is selective for cations. The two α subunits and the opposing face of the γ and δ subunits form the two sites for binding of agonists and competitive antagonists and provide the primary surfaces with which the larger snake α-toxins associate. A similar pentameric ring of subunits is found for AChBP [8, 33, 34] with the binding sites located at the five interfaces between the identical subunits of this homomeric pentamer (Fig. 11-8). The sites for ligand binding are localized toward the external perimeter of each of the ligand binding subunit interfaces; occupation of multiple sites on the pentamer is necessary for receptor activation. Electrophysiological and ligand-binding measurements together with analysis of the functional states of the receptor indicate positive cooperativity in the association of agonists; Hill coefficients greater than unity have been described for agonist-elicited channel opening, agonist binding and agonist-induced desensitization of the receptor [3, 32]. Noncompetitive inhibitor sites within various depths of the internal channel also have been defined and are the sites of local anesthetic inhibition of receptor function. equence identity among the subunits appears to be greatest in the hydrophobic regions. Various models for the disposition of the peptide chains and subunit assembly have been proposed on the basis of hydropathy plots, electron microscopic reconstruction analysis of Torpedo receptor membranes and the AChBP crystal structure [30, 33] (Figs 11-8, 11-9). Homology among the four subunits strongly suggests that the same folding pattern is found in all subunits. A disulfide loop between Cys 128 and 142 in the α subunit is conserved in the entire receptor channel family (Fig. 11-8). A second disulfide is found in the α subunits between vicinal Cys 192 and 193, and this structural feature is also found in AChBP. Early studies showed that reduction of the Cys 192 193 bond allowed for labeling by the site-directed sulfhydryl-reactive agonist and antagonist, respectively, bromoacetylcholine and m-maleimidobenzyl trimethylammonium [32]. ubsequent studies involving photolytic labeling, labeling by the natural coral toxin, lophotoxin, and site-specific mutagenesis identified the region between residues 185 and 200 in the α subunit as being important for forming part of the agonist- and FIGUE 11-8 Features of the sequence of the acetylcholine receptor. (A) chematic drawing of the sequence showing candidate regions for spanning the membrane. The region M 2 is believed to be an α helical segment and lines the internal pore of the receptor. M 1, M 3 and M 4 contain hydrophobic sequences but it is not certain whether they traverse the membrane as full α helices. The nicotinic β, γ and δ subunits contain homologous M 1 through M 4 hydrophobic domains at similar positions in the linear sequence. Two disulfide loops, 128 142 and 192 193, in the α subunits are shown. While the other subunits contain the larger disulfide loop, they lack cysteines 192 and 193 and tyrosine 190. equence features of other homologous subunits in ligand-gated channel (5-hydroxytryptamine [5-HT 3 ], GABA and glycine) receptors are shown. The amino-terminal portion is found on the extracellular (synaptic) surface. The soluble ACh binding proteins characterized from ymnaea and Aplysia have truncated sequences ending at residue 202 204 and an absence of hydrophobic domains designed to interact with sequences that span the membrane or interact with membrane lipids. (B) tructure of the acetylcholine binding protein from ymnaea as a model of the extracellular domain of the nicotinic acetylcholine receptor. tructures are developed from the crystallographic coordinates of ixma and colleagues [33]. B-1. View of the pentameric protein from the apical surface. The protein is a pentamer of five identical subunits surrounding a central cavity, which in the receptor is the vestibule to the ion channel pore. Although the subunits are identical, they are distinguished by color. B-2. View of subunit interfaces from the outer perimeter. The C loop, containing the second disulfide bond with vicinal cysteines, overlaps the complementary face on the adjacent subunit. The subunit interfaces (shown in gray, orange and blue and located apical to the C loop) contain binding locations of nicotine and α-neurotoxin (ribbon diagram, shown in red and largely on the membrane side of the C loop) [33]. B-3. Interface of the binding protein to the transmembrane spans of the ion channel. Modification of selected residues allowed the acetylcholine binding protein, when linked to the transmembrane spans of the channel, to become a functional ligand-gated ion channel with the expected ligand specificity and channel gating kinetics. Top (a): chematic diagram of a chimaeric subunit composed of AChBP and 5-HT 3A sequences. The amino terminus contains AChBP (white bars and black font) and 5-HT 3A sequences (loop β1-β2, orange; Cys loop, green; β8-β9 loop, magenta). The 5-HT 3A pore domain follows, with transmembrane helices M1-M4 (red) and the M2-M3 linker (blue). ed font indicates the start of the M1 domain. Middle (b): Homology model of the chimaeric receptor with key regions of one subunit colour-coded as in Top figure. Bottom (c): View of the coupling zone from the pore showing the network of binding and pore domain loops. (With permission from reference [34].)

CHAPTE 11 Acetylcholine 199 A eceptor Nicotinic (α 1 -subunit) Nicotinic (non α 1 -subunits) 5-HT 3 GABA A (α 1 -subunit) Glycine (α 1 -subunit) Acetylcholine binding protein H 2 N H 2 N H 2 N H 2 N H 2 N H 2 N CH CH CH CH CH CH M1 M2 M3 M4 B-1 B-3 (a) (b) B-2 (c)

200 PAT II Intercellular ignaling α β δ α ynaptic face Cytoplasmic face A B C FIGUE 11-9 (A) ongitudinal view of the muscle nicotinic acetylcholine receptor with the γ subunit removed. The remaining subunits, two copies of α, one of β and one of δ, surround an internal channel with outer vestibule and its constriction or gating locus deep within the membrane bilayer region. pans of α helices with bowed structures from the M 2 region of the sequence form the perimeter of the channel (see C). Acetylcholinebinding sites, denoted by arrows, are found at the αγ- and αδ (not visible)-subunit interfaces. (C shows the data on which this structure is based.) (Adapted with permission from references [25, 59, 60].) (B) Image reconstruction of electron micrographs yielding a structure at 9 Å resolution. hown are side and synaptic views. (Adapted with permission from reference [25, 59, 60].) (C) ongitudinal view of the electron density of the receptor. The transmembrane area is shown between the dots. The visible transmembrane-spanning helixes are shown by the V-shaped solid lines. This helix is believed to be the M 2 region, the sequence of which is shown. The area inside the rectangle is the transmembrane-spanning region. The X denotes the conserved leucine. The additional density in the cytoplasmic region arises from an associated 43 kda protein, rapsyn. The shaded area to the right indicates the zone of narrowest constriction.

CHAPTE 11 Acetylcholine 201 antagonist-binding surface. The loop containing the vicinal cysteines overlaps with the neighboring subunit at the outer perimeter of the receptor, and internal to this loop is nested the ligand-binding site. Two other segments of sequence in the α subunit and four discrete segments on the opposing face of the γ and δ subunits also have been identified as forming regions that contribute to the binding surfaces at the αγ and αδ interfaces [34], findings that have now been confirmed from the AChBP ligand complexes. Four candidate membrane-spanning regions are found after residue 210 with a large cytoplasmic loop between membrane spans 3 and 4 (Figs 11-8, 11-9). Based on labeling experiments and site-specific mutagenesis, membrane span 2 was found to be proximal to the ion channel. This span, when constructed as an α helix, is amphipathic, with an abundance of serine and threonine residues pointed toward the channel lumen. Positions corresponding to α-thr 244, α-eu 251, α-val 255 and α-glu 262 in this transmembrane span have been labeled with the noncompetitive, channel-blocking inhibitors chlorpromazine and tetraphenyl phosphonium [3, 32]. Mutation of several of the hydroxyl groups on residues at these positions affects channel kinetics. The channel gate, or constriction, is thought to lie deep within the channel or even close to the cytoplasmic side. The ion selectivity of the channels appears to be controlled in part by rings of charges formed by all five subunits at the extracellular surface of the channel corresponding to α-glu 262 and at the cytoplasmic surface corresponding to α-glu241. Exposed amide backbone hydrogens and carbonyl groups and a ring of hydroxylated amino acids corresponding to α-thr 244 also contribute to ion selectivity and permeation [11, 32, 35]. Analysis of the opening and closing events of individual channels has provided information about ligand binding and activation of the receptor. Electrophysiological studies use high-resistance patch electrodes of 1 2 µm diameter, which form tight seals on the membrane surface [36]. They have the capacity to record conductance changes of individual channels within the lumen of the electrode (see Ch. 10). The patch of membrane affixed to the electrode may be excised, inverted or studied on the intact cell. The individual opening events for ACh achieve a conductance of 25 p across the membrane and have an opening duration that is distributed exponentially around a value of about 1 ms. The duration of channel opening is dependent on the particular agonist, whereas the conductance of the open-channel state is usually agonistindependent. Analyses of the frequencies of opening events have permitted an estimation of the kinetic constants for channel opening and ligand binding, and these numbers are in reasonable agreement with estimates of ligand binding and activation from rapid kinetic, or stopped-flow, studies. verall, activation events for the muscle receptor may be described by scheme 1 [3, 11, 36]: 2k+1 k+1 2 + k-1 2k-1 2 k-2 2 * (Closed) (Closed) (Closed) (pen) cheme 1 Two ligands () associate with the receptor () prior to the isomerization step to form the open-channel state 2 *. For ACh, the forward rate constant for binding, k +1, is 1 2 10 8 mol 1 l 1 s 1 ; k +2 and k 2, forward and reverse rate constants for isomerization, yield rates of isomerization consistent with opening events in the millisecond time frame. ince k +2 and k 2 are greater than k 1, the rate constant for ligand dissociation, several opening and closing events with the fully liganded receptor occur prior to dissociation of the first ligand. Binding of the first and second ligands appears not to be identical, even allowing for the statistical differences arising from the two sites. uch a conclusion is consistent with receptor structure since different subunits, such as the γ and δ subunits in muscle, are adjacent to the same face of the α subunits in the muscle receptor pentamer. Continued exposure of nicotinic receptors to agonist leads to desensitization of the receptors. This diminution of the response occurs even though the concentration of agonist available to the receptor has not changed. Katz and Thesleff examined the kinetics of desensitization with microelectrodes and found that a cyclic scheme in which the receptor existed in two states, and, prior to exposure to the ligand best described the process. To achieve receptor desensitization and activation by ligand, multiple conformational states of the receptor are required. The binding steps represented in horizontal equilibria are rapid; vertical steps reflect the slow, unimolecular isomerizations involved in desensitization (scheme 2). apid isomerization to the open channel state (scheme 1) should be added. To accommodate the additional complexities of the observed fast and slow steps of desensitization, additional states have to be included. A simplified scheme, in which only one desensitized and one open-channel state of the receptor exist, is represented in scheme 2, where is the resting (activatable) state, * the active (open channel) state and the desensitized state of the receptor; M is an allosteric constant defined by /, and K and K are equilibrium dissociation constants for the ligand. 2 + M 2 + k/2 k /2 2k 2k cheme 2 2 2 k-2 2 *

202 PAT II Intercellular ignaling In this scheme, M <1 and K <K. Addition of ligand eventually will result in an increased fraction of species because of the values dictated by the equilibrium constants. Direct binding experiments have confirmed the generality of this scheme for nicotinic receptors. Thus, distinct conformational states govern the different temporal responses that ensue upon addition of a ligand to the nicotinic receptor. No direct energy input or covalent modification of the receptor channel is required. Nicotinic receptor subunits are part of a large superfamily of ligand-gated channels. Nicotinic receptors on neurons, such as those originating in the CN or neural crest, show ligand specificities distinct from the nicotinic receptor in the neuromuscular junction. ne of the most remarkable differences is the resistance of most nicotinic neuronal receptors containing α2 α6 subunits to α- bungarotoxin and related snake α-toxins. This fact and the lack of an abundant source of neuronal CN receptors limited initial progress in their isolation and characterization. However, low-stringency hybridization with cdnas encoding the subunits of electric organ and muscle receptors provided a means to clone neuronal nicotinic receptor genes. Isolation of the candidate cdna clones, their expression in cell systems to yield functional receptors and the discrete regional localizations of the endogenous mnas encoding these receptor subunits revealed that the nicotinic receptor subunits are part of a large and widely distributed gene family. They are related in structure and sequence to receptors for inhibitory amino acids (GABA and glycine), to 5-hydroxytryptamine type 3 (5HT 3 ) receptors and, somewhat more distantly, to glutamate receptors (32). At least 12 distinct genes encoding neuronal nicotinic receptor subunits α2 α10 and β2 β4 have been identified in the central and peripheral nervous systems (Fig. 11-2). The α subunits are similar in sequence to the muscle α1 subunit and contribute to the ligand-binding interface. The β subunits fulfill the role of β1, γ and δ subunits in the muscle receptor. When certain pairs or triplets of cdnas encoding neuronal α and β subunits are cotransfected into cells or their corresponding mnas are injected into oocytes, characteristic ACh-gated channel function can be achieved. The α3 subunit is prevalent in peripheral ganglia, usually with β2, β4, and α5 subunits. The α4β2 subunit combination predominates in the CN, yet its elimination in knockout studies causes surprisingly limited changes in function. The α5 subunit appears unique in that it will not contribute to function in the absence of other α subunits; its global sequence features are more similar to those of the β subunits. Four permutations of subunits containing α4, α5, α6, β2 and β3 subunits are found in the striatum that mediate dopamine release [36]. The α7 and α8 subunits display function as homologous pentamers. eceptors containing α7 subunits have a high Ca 2+ permeability, and Ca 2+ entry may be integral to their function in vivo. These receptors also show characteristically rapid rates of desensitization and a high sensitivity to choline as antagonist. α9 and α10 subtypes function in vestibular and cochlear mechanosensory hair cells [37]. While not all combinations of α and β mnas lead to the expression of functional receptors on the cell surface, the number of permutations is large [11]. A future challenge is the assignment of pharmacological and biophysical signatures to all of the subunit combinations found in vivo. ubstantial evidence points to nicotinic receptors in the CN functioning at presynaptic locations to regulate release of several CN transmitters [10]. Electrophysiological and microdialysis studies provide evidence that glutamatergic, dopaminergic, serotonergic, peptidergic and cholinergic pathways are under the control of presynaptic nicotinic receptors. Hence, nicotinic receptors appear to play an important amplification and modulatory role in the CN. A prime example is the influence of presynaptic nicotinic receptors influencing the release of dopamine that appears to underlie the addictive actions of nicotine [10, 40]. ee also Ch. 56. Both nicotinic receptors and acetylcholinesterase are regulated tightly during differentiation and synapse formation. At present, we understand more about tissuespecific gene expression in muscle than in nerve [41 45]. Both the above proteins show enhanced expression during myogenesis upon differentiating from a mononucleated myoblast to a multinucleated myotube. Curiously, enhanced receptor expression occurs largely by transcriptional activation, while the increase in cholinesterase expression arises to a large extent from stabilization of the mna. The receptor appears to cluster spontaneously, which involves a protein on the cytoplasmic side of the membrane, termed 43K or rapsyn (see in Ch. 43) [43, 44]. This protein links the receptor to cytoskeletal elements and restricts its diffusional mobility. Following innervation and synaptic activity, expression of the receptor and AChE persists in endplate, or junctional, regions and disappears in extrajunctional regions. The collagen-tailcontaining species of AChE is localized to the basal lamina in the neuromuscular synapse; it links at its distal end to the proteoglycan perlecan [45]. With innervation and the development of electrically excitable synapses, the γ subunit of the receptor is replaced by an ε subunit; small changes in the biophysical properties of the receptor occur concomitantly. Upon denervation, many of the developmental changes associated with innervation are reversed and there is again an increase in expression of extrajunctional receptors containing the γ subunit. In multinucleated muscle cells, particular subsynaptic nuclei drive the expression of these synapsespecific proteins. The factors controlling these regulatory events are incompletely understood, but calcitonin gene-related peptide (CGP) and the protein ACh receptor-inducing activity (AIA) may be extracellular mediators of expression. In addition, intracellular Ca 2+,

CHAPTE 11 Acetylcholine 203 membrane depolarization and protein kinase C play distinct roles in maintaining junctional expression of synapse-localized proteins [42 44]. A neurally derived signaling protein, agrin, acts through a receptor tyrosine kinase, MuK, in the formation of the specialized postsynaptic endplate by interaction with rapsyn. Thus, MuK rapsyn interactions are critical in forming the local scaffold for postsynaptic components in the motor endplate [43, 44]. An extensive variety of neuromuscular diseases have been uncovered as congenital conditions involving not only the nach and AChE but also proteins that control their expression and synaptic localization. These are discussed in Ch. 43. tudy of the underlying genotype or sequence differences has also proved helpful in unraveling the involvement of various amino acid residue determinants in function [41]. MUCAINIC ECEPT Muscarinic and nicotinic receptors are related more closely to other receptors in their respective families than to one another, both structurally and functionally. The nicotinic receptor is far more similar to other ligandgated ion channels than to the muscarinic receptor. The muscarinic receptor in turn belongs to a group of seven transmembrane-spanning receptors [46], which transduce their signals across membranes by interacting with GTP-binding proteins (see Ch. 19). everal macromolecular interactions are involved in the responses triggered by activation of the muscarinic receptor. These associations contribute to the 100 250 ms latency characteristic of muscarinic responses, which are slow compared with those mediated by nicotinic receptors. Muscarinic receptor activation causes inhibition of adenylyl cyclase, stimulation of phospholipase C and regulation of ion channels. Many types of neuron and effector cell respond to muscarinic receptor stimulation. Despite the diversity of responses that ensue, the initial event that follows ligand binding to the muscarinic receptor is, in all cases, the interaction of the receptor with a G protein. Depending on the nature of the G protein and the available effectors, the receptor G-protein interaction can initiate any of several early biochemical events. Common responses elicited by muscarinic receptor occupation are inhibition of adenylyl cyclase, stimulation of phosphoinositide hydrolysis and regulation of potassium or other ion channels [47] (Fig. 11-10). The particular receptor subtypes eliciting those responses are discussed below. (ee also Chs 20 and 21.) Inhibition of adenylyl cyclase by mach activation results in decreased camp formation. A decrease in camp is most apparent when adenylyl cyclase is stimulated, for example, by activation of adrenergic receptors with catecholamines or by forskolin. imultaneous addition of cholinergic agonists decreases the amount of camp formed in response to the catecholamine, in some tissues almost completely. The result is diminished activation of camp-dependent protein kinase (PKA) and decreased substrate phosphorylation catalyzed by this kinase. The mechanism by which the muscarinic receptor inhibits adenylyl cyclase is through activation of an inhibitory GTP-binding protein, G i. The α subunit of G i competes with the α subunit of the G protein activated by stimulatory agonists (G ) for regulation of adenylyl cyclase (see Chs 19, 21). Although muscarinic receptors do not interact with G s, increases in camp formation are seen in response to mach stimulation under some circumstances. These may result from stimulatory effects of βγ subunits released from G i proteins or from elevated intracellular Ca 2+ on specific isoforms of adenylyl cyclase or phosphodiesterase. Activation of phosphoinositide-specific phospholipase C by muscarinic agonists elicits phosphatidylinositol 4,5- bisphosphate stimulates phosphoinositide hydrolysis. The β 1 isoform of phosphoinositide-specific phospholipase C (PI-PC) is activated by its interaction with the α subunit of a GTP-binding protein, G q/11 [48]. This is the primary mechanism by which muscarinic receptors regulate this enzyme. However, some PC isoforms, most clearly β 2, also are activated by βγ subunits. This probably accounts for the pertussis-toxin-sensitive, G i /G o -mediated activation of PI-PC seen when high levels of mach subtypes that normally couple to G i (see below) are expressed in heterologous cell lines. The hydrolysis of phosphatidylinositol 4,5-bisphosphate yields two potential second messengers, inositol 1,4,5-trisphosphate (IP 3 ) and diacylglycerol (DAG) (see Ch. 20). DAG increases the activity of the Ca 2+ and phospholipid-dependent protein kinase (PKC). IP 3 mobilizes Ca 2+ from intracellular stores in the endoplasmic reticulum and thereby elevates cytosolic free Ca 2+. ubsequent responses are triggered by direct effects of Ca 2+ on Ca 2+ -regulated proteins and by phosphorylation mediated through Ca 2+ /calmodulindependent kinases and PKC. timulation of a phospholipase D, which hydrolyzes phosphatidylcholine, also occurs in response to muscarinic receptor activation. This appears to be secondary to activation of PKC and contributes to a secondary rise in DAG. egulation of K + channels. Muscarinic agonists cause rapid activation of G-protein-coupled, inwardly rectifying potassium channels (GIKs). This muscarinic effect can be mimicked by GTP analogs in whole-cell clamp experiments, and the response is sensitive to pertussis toxin, which ribosylates and inactivates G i and a related protein, G o (see Ch. 19). It is now generally agreed that GIK1 and GIK2 are activated directly by binding βγ subunits released from G i or G o. This is a primary mechanism by which muscarinic agonists cause hyperpolarization of

204 PAT II Intercellular ignaling Primary biochemical responses mediated by muscarinic acetylcholine receptors NE ACh Inhibition of adenylyl cyclase β-adr AC M 2 /M 4 mach γ γ β α αi β GTP ATP GTP camp ACh timulation of phospholipase C Protein kinase C γ M (1,3,5) mach PI-PC β PIP 2 PIP PI β α q/11 GTP Ca 2 + Diacylglycerol IP 3 ACh egulation of K + channels M 2 /M 4 mach α i/o β γ GTP K + FIGUE 11-10 Acetylcholine (ACh) interacts with a muscarinic receptor of the subtypes indicated to induce various responses. The M 2 and M 4 muscarinic acetylcholine receptors (mach) interact with the α subunit of GTP-binding protein, G i. When ACh binds, Gα i dissociates from βγ and inhibits adenylyl cyclase (AC). The M 1, M 3 and M 5 machs interact with GTP-binding proteins in the G q and G 11 family. The Gα q and α 11 subunits activate phosphoinositide-specific phospholipase C (PI-PC). The M 2 and M 4 machs regulate inwardly rectifying K + channels through the βγ subunit of G i or G o. Diffusible second messengers formed within the cell include camp, inositol trisphosphate (IP 3 ) and diacylglycerol (DAG). IP 3 is generated from phosphatidylinositol bisphosphate (PIP 2 ). NE, norepinephrine; β-adr, β-adrenergic receptor; PI, phosphatidylinositol; PIP, phosphatidylinositol-4-phosphate; PIP 2, phosphatidylinositol-4,5-bisphosphate.

CHAPTE 11 Acetylcholine 205 cardiac atrial cells, as well as of neurons [49]. This pathway contrasts with muscarinic inhibition of the M-current in sympathetic ganglia; suppression of this K + channel is mediated indirectly through muscarinic formation of a diffusible second messenger. Intracellular mediators of muscarinic receptor action. The three events described above, inhibition of adenylyl cyclase, stimulation of PC and regulation of K + channels, occur within the plasma membrane. They can be triggered directly by muscarinic receptor occupation independent of changes in cytosolic mediators. However, these primary events in turn affect the generation of diffusible second messengers such as camp, DAG, IP 3 and Ca 2+, which generate other metabolic sequelae. For example, an increase in cytosolic free Ca 2+ probably contributes to activation of phospholipase A 2, generating arachidonic acid, prostaglandins and related eicosanoids (see Ch. 33). These products in turn can stimulate cgmp formation and can regulate ion channel activity. Increased Ca 2+ also can activate Ca 2+ -dependent ion channels (K +, Cl ), regulate camp phosphodiesterase and activate Ca 2+ /calmodulin kinase-dependent protein phosphorylation. PKC is activated by DAG, generally in concert with Ca 2+ for conventional PKC isoforms, and has effects on ion channel activity, as well as on cholinergic secretory and contractile responses. Given the obviously complex set of possible interactions between these intracellular mediators, it is easy to explain how diverse cellular responses can be mediated through a single receptor activating relatively few primary responses (see Ch. 10). adioligand-binding studies have been used to characterize muscarinic receptors. In membranes or homogenates from heart, brain and other tissues, muscarinic agonists compete for antagonist-binding sites with Hill slopes of less than unity, suggesting that these agonists interact with more than a single population of muscarinic receptors [46]. Direct binding experiments with radiolabeled agonists also show multiple binding sites for agonists. Competition curves are best fitted by a model in which there are sites with low, high and, in some cases, super-high affinity for agonists. Addition of GTP to the binding assay can have a dramatic effect on the agonist competition curve or on direct agonist binding. The effect of GTP is to decrease the apparent affinity of the receptor for agonists. This results from a change in the interaction of the receptor with the GTP-binding protein that transduces its effects. Agonists vary in their binding properties. ome, like ACh, carbamylcholine and methacholine, bind with high affinity to a large percentage of the total sites and with low affinity to the remainder. In contrast, agonists such as oxotremorine and pilocarpine appear to bind to a single class of sites and may show relatively little highaffinity binding. The capacity of an agonist to induce high-affinity binding correlates with the efficacy of that agonist for eliciting responses such as contraction or phosphoinositide breakdown. It therefore appears that interaction of the receptor and G protein, as reflected in high-affinity agonist binding, is critical to production of the cellular response. Unlike agonists, most muscarinic antagonists, such as quinuclidinylbenzilate, N-methylscopolamine and atropine, bind to the receptor with Hill slopes of unity, as expected for a mass-action interaction with a single receptor type. There is little difference in affinity for these ligands in various tissues. imilar findings with other antagonists initially suggested that all muscarinic receptors were the same. However, a number of functional studies suggested that muscarinic receptors were heterogeneous, and putative subtype-selective antagonists have been described throughout the years. The binding properties of the antagonist pirenzepine led to the initial classification of muscarinic receptors. Pirenzepine (PZ), in contrast to other known mach antagonists, binds to muscarinic receptors in cortex, hippocampus and ganglia with relatively high affinity; these sites have been termed M 1. Heart, gland and smooth muscle mach muscarinic receptors, as well as those in brainstem, cerebellum and thalamus, show 30 50-fold lower affinity for PZ [46, 47]. The affinity for classic antagonists like N-methylscopolamine is the same in all of these regions, emphasizing the unique selectivity of PZ. Direct binding studies using [ 3 H]-PZ confirmed that only certain tissues and brain regions had receptors with high affinity for this antagonist. esults of pharmacological studies also demonstrated that PZ blocks muscarinic responses in ganglia better than responses in heart. Brain and heart were subsequently used to purify distinct receptors, termed M 1 and M 2 muscarinic receptors, and cdna clones corresponding to these receptors were isolated from rat brain and heart libraries. The cdnas for the muscarinic receptors encode apparent glycoproteins of 55 70 kda, which contain seven predicted transmembrane-spanning regions, similar to what is seen for the β-adrenergic receptor and other receptors that couple to G proteins (Fig. 11-11). There is only 38% amino acid identity between the proteins cloned from porcine brain and heart. The cdna encoding the receptor initially cloned from the brain has been termed m 1, whereas that cloned from the heart has been termed m 2, in keeping with the names (M 1 and M 2 ) applied to the purified recombinant receptors. The human and rat homologs of these receptor genes, as well as three additional subtypes termed m 3, m 4 and m 5, were subsequently cloned and expressed. Comparison of the amino acid sequences of the five muscarinic receptor subtypes suggests that they are members of a highly conserved gene family. The greatest sequence identity is in the transmembrane-spanning regions, whereas the long cytoplasmic loop (i 3 ) between transmembrane domains V and VI varies among the receptor subtypes [46, 50]. The

206 PAT II Intercellular ignaling Extracellular space T N M A T I N P V V P A P A P G o 2 K G T P A A W Q H V A M G G W I T T T Y D F Y A A T V T D I F I II III IV V VI VII T E K i 1 Cytoplasm o 1 NH 3 + V i 2 C E M A G Q Y o 3 T I Y I M Q P T FIGUE 11-11 Predicted amino acid sequence and transmembrane domain structure of the human M 1 muscarinic receptor. Amino acids that are identical among the m 1, m 2, m 3 and m 4 receptors are dark orange. The shaded cloud represents the approximate region that determines receptor G-protein coupling. Arrows denote amino acids important for specifying G protein coupling. Amino acids predicted to be involved in agonist or antagonist binding are denoted by white letters [50]. V I W T Y G T V A T C W A A P I Y K A K E T N E A E A A A Q G E T P G K G G G E G G E A P Q E P C P T G P C C A P Q A Y W K E E E E E E G E D E M T I V E G E E P G E V K V F T K E K o 4 K D W Y K A V E E Y V I M W D D K i 4 W K I P K P G P K Q G K G T K K G D A P P K P K V M T N P P M V D P E A Q A P T K Q i 3 C Q P T G V H C cloned receptors, expressed in mammalian cells, show differences in antagonist affinity similar to the differences of the pharmacologically defined receptors. Thus, the expressed M 1 receptor is blocked selectively by PZ, the M 2 receptor is blocked by AFDX-116 and methoctramine and the M 3 receptor is blocked by hexahydrosiladifenidol [47]. The regions in the receptor responsible for differences in antagonist affinity have not yet been identified clearly. igands are believed to bind to the receptor at sites facing the extracellular space but located in a central cavity deep within the bundle formed by transmembrane domains III VII. The binding site for the covalent antagonist propylbenzilylcholine mustard has been mapped to a particular aspartic acid residue in the third transmembrane region. This amino acid is conserved in all biogenic amine G-protein-coupled receptors. Mutagenesis of this amino acid profoundly affects both agonist and antagonist binding to muscarinic receptors. It is hypothesized that this residue participates in ionic bonding with the ammonium headgroup of the cholinergic ligand [46]. Expression of the cloned receptors in Chinese hamster ovary cells, other mammalian cells and Xenopus oocytes has demonstrated differential coupling of these receptors to cellular responses. In general the M 1, M 3 and M 5 receptors regulate phosphoinositide hydrolysis by stimulating PC. This occurs through selective coupling of the receptor to the pertussis-toxin-insensitive G protein G q/11, which can activate the β isoform of PC [48]. Calciumdependent K + and Cl channels are activated secondarily to the PC-mediated increase in intracellular Ca 2+. In contrast, the M 2 and M 4 receptors couple through a pertussis-toxin-sensitive G protein (G i ) to inhibition of adenylyl cyclase. egulation of K + channels also is mediated through M 2 or M 4 receptor interaction with specific pertussis-toxin-sensitive G proteins. Chimeric receptors have been used to determine the regions critical for specifying coupling to particular responses. These studies demonstrate that it is the third intracellular (i 3 ) loop that defines functional specificity [47, 50]. A series of amino acids proximal to the transmembrane domain, that is, at the N- and C- termini of the i 3 loop, carry most of this information. These particular regions are similar in the M 1, M 3 and M 5 receptors and in the M 2 and M 4 receptors but distinguish these two groups from one another. Both site-directed and random mutagenesis studies have identified specific amino acids at the amino-terminus of the i 3 loop which are required for G protein recognition and activation [50, 51]. These critical noncharged amino acids are predicted to reside on the hydrophobic face of an α-helical extension of transmembrane domain V. Conserved amino acids in the carboxyterminus of the i 3 loop and adjacent transmembrane domain VI also have been demonstrated to specify coupling to G i - versus G q -mediated responses. The hydrophobic

CHAPTE 11 Acetylcholine 207 regions at the two ends of the i 3 loop thus are suggested to form a surface that binds to and discriminates between different classes of G protein [50]. ther regions, including a portion of the second intracellular loop, also contribute to specifying correct G protein coupling. The early and detailed characterization of distinct regions involved in selective G-protein coupling of mach provides a paradigm for analysis of other GPCs. The selectivity in muscarinic receptor coupling is not, however, absolute. verexpression of receptors or of particular G proteins supports interactions that may differ from those described above. For example, M 2 receptors expressed in Chinese hamster ovary cells not only inhibit adenylyl cyclase but also can stimulate phosphoinositide hydrolysis through a pertussis-toxin-sensitive G protein [52]; this is not seen, however, when M 2 receptors are expressed in Y1 cells. These findings indicate that caution must be exercised in interpreting data obtained when receptors are expressed, often at high levels, in cells in which they normally do not function. Muscarinic receptors of the M 1, M 3 and M 5 subclasses induce transformation or cell proliferation, a feature not shared by M 2 and M 4 receptors [53]. This property has been exploited to develop a high-throughput assay for screening effects of receptor mutations [51]. Mitogenactivated protein kinases (MAP kinases) also are activated by muscarinic receptors but, unlike transformation, this response occurs with receptors of the M 2 /M 4 subtype as well as of the M 1 /M 3 subtype. Notably, induction of cell growth by muscarinic receptor stimulation is cell-typespecific, however, and is seen only at high levels of receptor expression [53]. Thus, it is questionable whether there is a physiological role for ACh in growth regulation. Transgenic mice are being generated to assess the functions of receptor subtypes in vivo. Knowledge of the anatomical distribution and coupling properties of receptor subtypes can indicate which physiological responses they mediate in vivo. However, the lack of good subtypeselective antagonists limits the use of pharmacological approaches to address this question. Generation of transgenic mice in which muscarinic receptors are overexpressed or knockout mice in which receptor genes are disrupted by homologous recombination provides a powerful and necessary approach for evaluation of muscarinic receptor function. The m 1 gene was the first to have been targeted selectively, eliminating M 1 -receptor expression in the forebrain where it is abundantly expressed [54]. Homozygous M 1 -receptor-deficient mice are completely resistant to seizures produced by pilocarpine, implicating the M 1 receptor in this model of epilepsy. Furthermore, inhibition of the M-current in sympathetic ganglia, suggested by previous pharmacological experiments to be M 1 - receptor-coupled, is ablated in knockout mice. Interestingly, despite the abundance of M 1 receptors in the forebrain, most learning and memory tasks are not impaired in mice lacking M 1 receptors. With regard to the biochemical signaling pathways delineated above, mach-stimulated phosphoinositide hydrolysis is attenuated and MAP kinase activation lost in hippocampus and pyramidal neurons. The conclusion that brain M 1 mach selectively regulate these responses is supported by the observation that these responses are unaffected in the M 2 -, M 3 - and M 4 -receptor knockout mice described below. Both the M 2 and M 4 receptors are, as indicated above, coupled to G i pathways and appear to mediate similar responses. The M 2 receptor is widely expressed in the CN but also present in heart and smooth muscle, while M 4 is preferentially expressed in the CN, especially in forebrain. Ablation of the M 2 receptor leads to complete loss of muscarinic-agonist-stimulated bradycardia [55]. In the CN, deletion of the M 2 receptor abolishes oxotremorine induced akinesia and tremors [56]. Memory and learning tasks including passive avoidance and working memory are impaired in M 2 -receptor knockout mice, and there is decreased TP in hippocampal slices [12]. M 2 receptors are also predominant in the mouse spinal cord and here this receptor subtype appears, from knockout studies, to play a major role in signaling muscarinicagonist-induced analgesia (although M 4 receptors can also mediate this response) [56]. A major role for M 2 (and M 4 ) receptors is as presynaptic auto- (or hetero-)receptors. Thus studies using M 2 -receptor knockout mice reveal that the M 2 receptor plays a prominent role in inhibiting NE release from sympathetic nerves in atria and other peripheral tissues and that M 2 and M 4 autoreceptors regulate ACh release from atria, urinary bladder and cholinergic neurons in the hippocampus and striatum [12, 57]. Muscarinic and dopaminergic pathways in the CN interact in control of numerous pathways implicated in diseases, especially those controlling involuntary motor systems. Muscarinic effects on dopamine release are mediated in several ways via different mach subtypes. Thus mach facilitation of DA release appears to involve M 4 receptors on GABA projection neurons to the striatum, while M 3 receptors on striatal DA neurons are predicted to inhibit striatal DA release [12]. The role of the M 3 mach in the CN remains unclear. The phenotype of the M 3 receptor knockout is hypophagic and lean. This appears to result from altered hypothalamic appetite regulation, as M 3 receptors are relatively highly expressed in this region [12]. The most striking changes in the M 3 -receptor knockout animal are peripheral, specifically loss of contractile responses of tracheal, urinary bladder and pupillary smooth muscle to muscarinic agonists. The M 5 receptor also has an uncertain role in the CN, where it is present at low levels. ecent studies using M 5 -receptor knockout mice suggest that these receptors may reside primarily on endothelial cells of brain arteries and arterioles, mediating cholinergic relaxation of these vessels [58]. The information gleaned from studies with mach knockout mice is encouraging in demonstrating striking phenotypes that indicate that mach agonists and

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