The Diversity of Neuronal Nicotinic Acetylcholine ...

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1993. 16:403---43 1993 by Annual Reviews Inc.

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NEURONAL NICOTINIC ACETYLCHOLINE RECEPTORS Peter B. Sargent Departments of Stomatology and Physiology and the Neuroscience Graduate Program, University of California, San Francisco, California 941 43-05 1 2 KEY WORDS:

calcium, presynaptic, heterologous expression, transsynaptic regu lation, channel gating

The transmitter acetylcholine (ACh) acts on two different classes of recep tors: nicotinic and muscarinic. Nicotinic receptors (nAChRs) are directly gated or ionotropic receptors and are members of a supergene family that also includes glycine, GABAA, and 5-HT 3 receptors. Muscarinic receptors are metabotropic receptors and belong to a different supergene family. In this review, I concentrate on nAChRs expressed by neurons, which are distinct from skeletal muscle nAChRs. Several recent reviews have treated both neuronal and muscle nAChRs (Claudio 1 989; Galzi et a1 1 99 1 ; Lukas & Bencherif 1 992; Schmidt 1 988; Schuetze & Role 1 987), whereas others have focused on neuronal nAChRs (Deneris et al 1 99 1 ; Lindstrom et al 1 987; Luetje et a1 1 990a; Role 1 992). Over the past several years, the application of molecular, immunological, and physiological techniques has greatly expanded our knowledge of neuronal nAChRs. By applying tools and insight gained from the study of muscle nAChRs, neuronal nAChRs have been purified and their subunits cloned. In addition, whole cell and patch clamp recordings have given us precise information about the function of nAChR ion channels. These studies have demonstrated that neuronal nAChRs are highly diverse: there are several different nAChR subunits, multiple forms of purified nAChRs, and many pliysiologically distinct classes of nAChR channels. The prin cipal challenge over the next several years will be to bring together molec403 0 1 47-006Xj9 3 j030 1 -0 403 $02.00

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ular, biochemical, and physiological observations to define the molecular basis of this diversity and to understand its functional significance. PHYSIOLOGICAL PROPERTIES OF NEURONAL

Annu. Rev. Neurosci. 1993.16:403-443. Downloaded from www.annualreviews.org by University of California - San Francisco UCSF on 07/29/14. For personal use only.

nAChRS

Nicotinic AChRs are present on autonomic neurons and adrenal chro maffin cells in the peripheral nervous system and on many neurons in the central nervous system (CNS). In nearly all instances, these nAChRs mediate "fast" inward currents, as do their skeletal muscle counterparts. Many of the properties of these neuronal nAChR channels, such as their ion selectivity and gating properties, resemble those of muscle nAChRs. However, neuronal AChRs are clearly distinct from muscle nAChRs and are themselves diverse.

Ion Selectivity Most neuronal nAChR channels, like muscle nAChR channels, are cation specific, but do not distinguish readily among cations (e.g. Fieber & Adams 199 1; Mulle & Changeux 1990; Nutter & Adams 1991). Neuronal nAChRs have a significant permeability to Ca2+; for example, PCa: PNa for rat parasympathetic cardiac neurons is 0.7 (Adams & Nutter 1992), and PCa: PNa for rat PC l 2 cells is approximately 2.5 (Sands & Barish 1 99 1 ; see also Vernino et aI 1 992). By contrast, Decker & Dani ( 1 990) calculated a PCa: PNa of 0.2 in skeletal muscle. Neuronal nAChR channels expressed heterologously in oocytes also have calcium permeabilities greater than muscle and, in some instances, greater than N-methyl-D-aspartate (NMDA) channels (Seguela et al 1 992; Vernino et al 1 992). Calcium entry through neuronal nAChR channels is sufficient to activate calcium-dependent chloride (Mulle et al 1 992a; Seguela et al 1992; Vernino et al 1 992) and potassium (Fuchs & Murrow 1 992b) conductances and may activate second messenger systems.

Channel Properties Open-channel conductances of neuronal nAChRs vary from less than 1 0 picosiemens (pS) to more lhan 5 0 pS. Comparison o f channel conductance measurements for different preparations is problematic, because they vary as a function of recording conditions, especially the concentrations of divalent cations (lfune & Steinbach 1 99 1 ; Mathie et al 1987; Neuhaus & Cachelin 1 990). However, under similar recording conditions, clear differences do exist among different neuronal nAChRs (e.g. Mulle el a1 199 1). The best evidence for functional heterogeneity of neuronal nAChR channels arises from instances in which multiple classes of open-channel


NEURONAL NICOTINIC AChRS

405

conductances are observed in single cells. Two populations of open-chan nel conductances are present in rat sympathetic neurons (Derkach et al 1 987) and chicken ciliary ganglion neurons (Margiotta & Gurantz 1 989), and three to four populations arc present in rat PC 1 2 cells (Bormann & Matthaei 1 983; Ifune & Steinbach 1990), rat cardiac parasympathetic neurons (Adams & Nutter 1992), bovine chromaffin cells (Cull-Candy et al 1 988), and chicken sympathetic neurons (Moss et al 1 989). Whether distinct conductance states result from nAChRs having different com binations of subunits and/or from nAChR subunits having different post translational modifications is not known. Neuronal nAChR channels, like muscle nAChR channels, display burst ing behavior. Burst durations of neuronal nAChRs vary from preparation to preparation, and even among different channel types on individual cells. Several nicotinic channels on autonomic neurons show burst lengths with time constants in the range of 5-1 0 ms (e.g. Kuba et al 1989; Mathie et al 1 987; Moss et al 1 989; Schofield et al 1 985). These times are longer than those recorded from muscle fibers. The functional consequence of longer burst durations is that more current flows through each activated nAChR channel during synaptic transmission.

A gonists and A ntagonists Neuronal nAChRs are distinct from muscle nAChRs in agonist potency. For example, suberyldicholine is generally more effective than ACh on muscle nAChRs, whereas ACh is more effective than suberyldicholine on neuronal nAChRs (e.g. Lukas 1 989). Neuronal nAChRs themselves are quite distinct with regard to agonist potency. Mulle et al (1991) found that the order of potency for neurons from the rat interpeduncular nucleus was cytisine > ACh> nicotine; for nAChRs on neurons from the medial habenula, it was nicotine> cytisine> ACh. These differences may be explained by differences in nAChR subunit composition (Luetje & Patrick 1991; discussed below). Antagonists have also been useful in distinguishing muscle nAChRs from neuronal ones and in distinguishing among different neuronal nAChRs. Paton & Zaimis ( 1 949) demonstrated that decamethonium (ClO) was more effective than hexamethonium (C6) in blocking muscle nAChRs, whereas C6 was more effective in autonomic ganglia. This led to the terms "C10 receptor" (muscle) and "C6 receptor" (neuronal). Currently, several antag onists distinguish between neuronal and muscle nAChRs (Lukas 1990). Snake toxins have proven useful for differentiating between neuronal and muscle nAChRs (reviewed in Chiappinelli 1 99 1 ; Loring & Zigmond 1 988). The first snake toxin to be used widely in studies on nAChRs was Ct-bungarotoxin (Ct-Bgt), which generally binds with high affinity and


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specificity to muscle nAChRs, but not to nAChRs that underlie synaptic transmission in autonomic ganglia. Early reports that synaptic trans mission in autonomic ganglia is blocked by oc-Bgt were due to the presence of contaminating toxins, one of which has been purified and found to block ganglionic nAChRs at nanomolar concentrations (ChiappineIIi 1 983; Loring et al 1 984 ; Ravdin & Berg 1 9 79). This toxin is referred to as bungarotoxin 3 . 1 (Ravdin & Berg 1 9 79), K-bungarotoxin (ChiappineIIi 1 983), or toxin F (Loring et aI 1 984); in this review, I adopt Lindstrom et aI's ( 1 98 7) suggestion that the toxin be named neuronal-bungarotoxin (n-Bgt). I In chicken and frog autonomic ganglia, n-Bgt at 5-1 00 nM blocks nicotinic synaptic transmission and responses of autonomic neurons to nicotinic agonists (ChiappineIIi 1 983; Loring et al 1 984; Ravdin & Berg 1 9 79; Sargent et al 1 99 1 ). Rat sympathetic ganglion neurons and bovine chromaffin cells show lower affinity for n-Bgt than those in chicken or frog (Higgins & Berg 1 98 7; Nooney et al 1 992; Sah et aI 1 98 7) . For all ganglionic nAChRs, however, n-Bgt is more potent than oc-Bgt, which is generally ineffective at blocking ganglionic nAChRs at concentration as high as 1 J-lM (but see Marshall 1 98 1 ). Muscle nAChRs, by contrast, have considerably greater affinity for oc-Bgt than for n-Bgt. Many central nAChRs resemble ganglionic nAChRs in their sensitivity to n-Bgt (Calabresi et a1 1 989; Lipton et a1 1 98 7; Loring et al 1 989; Schulz & Zigmond 1 989; Vidal & Changeux 1 989 ; Wong & Gallagher 1 99 1 ; Zhang & Feltz 1 990). However, nicotinic responses of neurons in the chicken lateral spiriform nucleus are apparently insensitive to 0.5-1 .0 J-lM n-Bgt (Sorenson & Chiappinelli 1 990), as are those of neurons in the inter peduncular nucleus and medial habenular nucleus of rats (Mulle et al 1 99 1 ). The nAChRs underlying the responses of these neurons are pharma cologically distinct from ganglionic nAChRs. In a few instances, Cl-Bgt does block central nicotinic responses. Tn the rat hippocampus, for example, nicotinic responses are virtually eliminated by 20 nM n-Bgt or 300 nM oc-Bgt (Alkondon & Albuquerque 1 99 1 ). These nAChRs thus represent a novel class of neuronal nAChRs, because they are blocked nearly irreversibly by relatively low concentrations of both oc Bgt and n-Bgt. nAChRs with similar properties have been characterized

I

Neuronal-bungarotoxin does not block some neuronal nAChRs (Mulle et al 1991) and

it does block muscle-like nAChRs (Luetje et al 1990b), albeit at considerably higher con centrations than needed for neuronal nACh Rs. Moreover, still other toxins are present in the venom of Bungarus multicinctus that block ganglionic nAChRs (reviewed in Chiappinelli 1991). Thus, the term "neuronal-bungarotoxin" should not be interpreted to mean that there is only one such neuronal-specific toxin or even that the toxin is specific for and effective on all neuronal nAChRs.

NEURONAL NICOTINIC AChRS

407

in hair cells of the chicken cochlea (Fuchs & Murrow 1 992a) and in insects (e.g. Pinnock et al 1988).


Rectification Nicotinic responses of central and peripheral neurons are distinct from those of muscle in displaying pronounced inward rectification. Inward rectification of whole cell currents occurs independently of current polarity and depends primarily upon membrane potential (Mathic ct a1 1 990; Yawo 1 989). Rectification can, in principle, result from a voltage-dependent change in gating and/or ion permeation through single channels. In neurons, as in muscle, ion permeation through single channels displays some inward rectification, and this behavior is dependent upon internal Mg2+ (Ifune & Steinbach 1 990; Neuhaus & Cachelin 1 990; Sands & Barish 1 992). With no Mg2+ (or Ca2+) on either side of the membrane, single channel I-V curves are linear, but whole cell currents continuc to show inward rectification (Ifune & Steinbach 1 990; Mathie et a1 1 990; Neuhaus & Cachelin 1 990). This rectification is due only in small part to a voltage dependence of burst duration, which is weak for neuronal nAChRs (Mathie et aI 1 990). In rat sympathetic neurons, rectification of whole cell currents results primarily from a reduced probability of channel opening; at +50 mV, channels open less than one-tenth as often as at -50 mV (Mathie et al 1 990). In rat PC 1 2 cells, rectification is primarily caused by channels closing more rapidly at positive potentials (Ifune & Steinbach 1992; see also Sands & Barish 1992). STRUCTURE OF NEURONAL nAChRS

Patrick & Stallcup ( 1 977a,b) reported that nicotinic responses elicited from rat PC12 cells were blocked by an antiserum to electric organ nAChR and were thus homologous to muscle nAChRs. This homology suggested to several investigators that cDNA probes for muscle nAChR genes might be useful, under conditions of low stringency, in identifying genes that encode neuronal nAChR subunits.

Neuronal nA ChRs A re Encoded by a Family of Related Genes Recombinant DNA technology has resulted in the identification of nine to ten genes in rat and chicken neural tissue that are homologous to muscle nAChR genes (Table I; putative nAChR genes from Drosophila, cockroach, goldfish, and human species are also listed). These genes encode peptides with four hydrophobic, putative transmembrane domains (M l M4) in the approximate positions of their muscle counterparts (the struc-


Table 1

Species b Rat

Properties of cloned and sequenced neuronal nAChR genes

Probe

Subunit

a2

a2

chicken

a3

mouse al

a4-1c

rated, mouselXl

as

rat {33

a6

PCR

Consensus

Homology

N-linked

(subunit/%)

.a::. 0 00

Mature peptide Mr

glycosylation

# amino acids

sitesa

Cysteinesa

[conspecific, unless

55,500 484 54,800 474 67,100 600 48,800 424 53,300

29,79,185

133,147, 197,198

al/48

Wada et al

24,141

128,142, 192,193

ai/52; a2/58

Boulter et al

24,141

128,142, 192,193

ai/53; a2/68; a3/59

112,140, 186

127,141,191,192

a1--04/44--55

1987; Goldman et al 1987 Boulter et al 1990

24,141

66,128, 142,192,193

463

otherwise noted]

ai, a2,'4,,,5/41, 49, 49,45; a3/59; P2-[J4/43-47

References

""

1988

> � Cl

1986

t'rl

Boulter et al

Lamar et al

1990;

J. Patrick et ai, unpublished results

PCR (primers

a7

based on chicken ,,7,

fJ2

rat cd

fJ3

rat cd

fJ4

rat

non-a: d

2

a2

P2

universal

chicken

a 1, Y

1