Structure of the high-affinity binding site for noncompetitive blockers of the acetylcholine receptor: serine-262 of the delta subunit is labeled by [3H]chlorpromazine

Radioligand binding and photoaffinity labelling studies show a direct interaction of phenothiazines at 5-HT3 receptors

The effects of a range of phenothiazines were examined on 5-hydroxytryptamine3 (5-HT3) receptors in membranes from NIE-115 neuroblastoma cells using radioligand binding. Chlorpromazine, fluphenazine, perphenazine, trifluoperazine and prochlorperazine inhibited specific binding of both the 5-HT3 receptor antagonist [3H]GR65630 and agonist [3H]meta-chlorophenylbiguanide (mCPBG), with Ki values ranging from 0.4 to 3.9 microM. The mode of action of chlorpromazine was further examined using photoaffinity labelling in the presence and absence of 5-HT. Saturation radioligand binding data with both [3H]GR65630 and [3H]mCPBG showed that photoaffinity labelling with chlorpromazine (1 microM) caused a decrease in the maximum number of binding sites observed (35% and 28% for agonist and antagonist, respectively). This decrease was not observed when the membranes were incubated in the presence of 5-HT. The results demonstrate a direct interaction of a range of phenothiazines at the 5-HT3 receptor binding site.

Determinants of nicotinic receptor gating in natural and unnatural side chain structures at the M2 9' position

A nonsense suppression method was employed to incorporate a total of four natural and six unnatural residues at the 9' position of the M2 region in the beta, gamma, and delta subunits of muscle nicotinic receptors. In 33 pairwise comparisons of functional properties as influenced by structural features including side chain length, branching, and substitution of oxygen for methylene carbons, it is concluded that increased polarity in the side chains at the 9' position consistently increases the sensitivity to acetylcholine. In addition, the stereochemistry of the side chain can have marked influences on the EC50, primarily because of changes in the single-channel open time. For the case of isoleucine versus allo-isoleucine in the delta subunit, these changes are themselves modified by mutations at the 9' position in other subunits. The data suggest an especially strong interaction between the beta and delta subunits in the pore region, leading in turn to a suggested arrangement of subunits within the pentamer.

The emerging three-dimensional structure of a receptor. The nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor is the neurotransmitter receptor with the most-characterized protein structure. The amino acid sequences of its five subunits have been elucidated by cDNA cloning and sequencing. Its shape and dimensions (approximately 12.5 nm x 8 nm) were deduced from electron-microscopy studies. Its subunits are arranged around a five-fold axis of pseudosymmetry in the order (clockwise) alpha H gamma alpha L delta beta. Its two agonist/competitive-antagonist-binding sites have been localized by photolabelling studies to a deep gorge between the subunits near the membrane surface. Its ion channel is formed by five membrane-spanning (M2) helices that are contributed by the five subunits. This finding has been generalized as the Helix M2 model for the superfamily of ligand-gated ion channels. The binding site for regulatory non-competitive antagonists has been localized by photolabelling and site-directed-mutagenesis studies within this ion channel. Therefore a three-dimensional image of the nicotinic acetylcholine receptor is emerging, the most prominent feature of which is an active site that combines the agonist/ competitive-antagonist-binding sites, the regulatory site and the ion channel within a relatively narrow space close to and within the bilayer membrane.

Serotonergic modulation of muscle acetylcholine receptors of different subunit composition

Modulation of muscle acetylcholine (AcCho) receptors (AcChoRs) by serotonin [5-hydroxytryptamine (5HT)] and other serotonergic compounds was studied in Xenopus laevis oocytes. Various combinations of alpha, beta, gamma, and delta subunit RNAs were injected into oocytes, and membrane currents elicited by AcCho were recorded under voltage clamp. Judging by the amplitudes of AcCho currents generated, the levels of functional receptor expression were: alpha beta gamma delta > alpha beta delta > alpha beta gamma > alpha gamma delta. The alpha beta gamma delta and alpha beta delta AcChoR Subtypes were strongly blocked by 5HT, whereas the alpha beta gamma receptor was blocked only slightly. The order of blocking potency of AcChoRs by 5HT was: alpha beta delta > alpha beta gamma delta > alpha beta gamma. 5HT receptor antagonists, such as methysergide and spiperone, were even more potent blockers of AcChoRs than 5HT but did not show much subunit selectivity. Blockage of alpha beta gamma delta and alpha beta delta receptors by 5HT was voltage-dependent, and the voltage dependence was abolished when the delta subunit was omitted. These findings may need to be taken into consideration when trying to elucidate the mode of action of many clinically important serotonergic compounds.

Agonist self-inhibitory binding site of the nicotinic acetylcholine receptor

A major focus of current research on the nicotinic acetylcholine receptor (AChR) has been to understand the molecular mechanism of ion channel inhibition. In particular, we put special emphasis on the description of the localization of the agonist self-inhibitory binding site. Binding of agonist in the millimolar concentration range to this particular site produces inhibition of the ion flux activity previously elicited by the same agonist at micromolar concentrations. Due to the similitude in the pharmacological and electrophysiological behavior in inhibiting the ion channel of both high agonist concentrations and noncompetitive antagonists, we first describe the localization of noncompetitive inhibitor binding sites on the AChR. There is a great body of experimental evidence for the existence and location of luminal high-affinity noncompetitive inhibitor binding sites. In this regard, the most simple mechanism to describe the action of noncompetitive inhibitors which bind to luminal sites and, by its semblance, the agonist self-inhibition itself, is based on the assumption that these compounds enter the open channel, bind to different rings within the M2 transmembrane domain of the receptor, and block cation flux by occluding the receptor pore. However, the existence of high-affinity nonluminal noncompetitive inhibitor binding sites is not consistent with the open-channel-blocking mechanism. Instead, the presence of the quinacrine locus at the lipid-protein (alpha M1) interface approximately 7 A from the lipid-water interface and the ethidium domain located approximately 46 A from the membrane surface in the wall of the vestibule open the possibility for the regulation of cation permeation by an allosteric process. Additionally, the observed (at least partially) overlapping between the quinacrine and the agonist self-inhibitory binding site also suggests an allosteric process for agonist self-inhibition. For this alternative mechanism, cholinergic agonist molecules first need to be partitioned into (or to be adsorbed onto) the lipid membrane to further interact with its binding site located at the lipid-protein interface.

Luminal and non-luminal non-competitive inhibitor binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of the alpha subunit exist the binding sites for agonists such as the neurotransmitter acetylcholine, which upon binding trigger the channel opening, and for competitive antagonists such as d-tubocurarine, which compete for the former inhibiting its pharmacological action. For non-competitive inhibitors, a population of low-affinity binding sites have been found at the lipid-protein interface of the nicotinic acetylcholine receptor. In addition, at the M2 transmembrane domain, several high-affinity binding sites have been found for non-competitive inhibitors such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222 and the hydrophobic probe trifluoromethyl-iodophenyldiazirine. They are known as luminal binding sites. Although the local anaesthetic meproadifen seems to be located between the hydrophobic domains M2-M3, this locus is considered to form part of the channel mouth, thus this site can also be called a luminal binding site. In contraposition, experimental evidences support the hypothesis of the existence of other high-affinity binding sites for non-competitive inhibitors located not at the channel lumen, but at non-luminal binding domains. Among them, we can quote the binding site for quinacrine, which is located at the lipid-protein interface of the alpha M1 domain, and the binding site for ethidium, which is believed to interact with the wall of the vestibule very far away from both the lumen channel and the lipid membrane surface. The aim of this review is to discuss these recent findings relative to both structurally and functionally relevant aspects of non-competitive inhibitors of the nicotinic acetylcholine receptor. We will put special emphasis on the description of the localization of molecules with non-competitive antagonist properties that bind with high-affinity to luminal and non-luminal domains. The information described herein was principally obtained by means of methods such as photolabelling and site-directed mutagenesis in combination with patch-clamp. Our laboratory has contributed with data obtained by using biophysical approaches such as paramagnetic electron spin resonance and quantitative fluorescence spectroscopy.

Agonist-induced displacement of quinacrine from its binding site on the nicotinic acetylcholine receptor: plausible agonist membrane partitioning mechanism

It was previously demonstrated that high concentrations of cholinergic agonists such as acetylcholine (ACh), carbamylcholine (CCh), suberyldicholine (SubCh) and spin-labelled acetylcholine (SL-ACh) displaced quinacrine from its high-affinity binding site located at the lipid-protein interface of the nicotinic acetylcholine receptor (AChR) (Anas, H. R. and Johnson, D. A. (1995) Biochemistry, 34, 1589-1595). In order to account for the agonist self-inhibitory binding site which overlaps, at least partially, with the quinacrine binding site, we determined the partition coefficient (Kp) of these agonists relative to the local anaesthetic tetracaine in AChR native membranes from Torpedo californica electric organ by examining (1) the ability of tetracaine and SL-ACh to quench membrane-partitioned 1-pyrenedecanoic acid (C10-Py) monomer fluorescence, and (2) the ability of ACh, CCh and SubCh to induce an increase in the excimer/monomer ratio of C10-Py-labelled AChR membrane fluorescence. To further assess the differences in agonist accessibility to the quinacrine binding site, we calculated the agonist concentration in the lipid membrane (CM) at an external agonist concentration high enough to inhibit 50% of quinacrine binding (IC50), which in turn was obtained by agonist back titration of AChR-bound quinacrine. Initial experiments established that high agonist concentrations do not affect either transmembrane proton concentration equilibria (pH) of AChR membrane suspension or AChR-bound quinacrine fluorescence spectra. The agonist membrane partitioning experiments indicated relatively small (< or = 20) Kp values relative to tetracaine. These values follow the order: SL-ACh>SubCh>>CCh-ACh. A direct correlation was observed between Kp and the apparent inhibition constant (Ki) for agonists to displace AChR-bound quinacrine. Particularly, agonist with high KpS such as SL-ACh and SubCh showed low Ki values, and this relationship was opposite for CCh and ACh. The calculated CM values indicated significant (between 7 and 54 mM) agonist accessibility to lipid membrane. By themselves, these results support the conjecture that agonist self-inhibition seems to be mediated by the quinacrine binding site via a membrane approach mechanism. The existence of an agonist self-inhibitory binding site, not located in the channel lumen would indicate an allosteric mechanism of ion channel inhibition; however, we can not discard that the process of agonist self-inhibition can also be mediated by a steric blockage of the ion channel.

Photolabeling reveals the proximity of the alpha-neurotoxin binding site to the M2 helix of the ion channel in the nicotinic acetylcholine receptor

A photoactivatable derivative of neurotoxin II from Naja naja oxiana containing a 125I-labeled p-azidosalicylamidoethyl-1,3'-dithiopropyl label at Lys-25 forms a photo-induced cross-link with the delta subunit of the membrane-bound Torpedo californica nicotinic acetylcholine receptor (AChR). The cross-linked radioactive receptor peptide was isolated by reverse-phase HPLC after tryptic digestion of the labeled delta subunit. The sequence of this peptide, delta-(260-277), and the position of the label at Ala-268 were established by matrix-assisted laser-desorption-ionization mass spectrometry based on the molecular mass and on post-source decay fragment analysis. With the known dimensions of the AChR molecule, of the photolabel, and of alpha-neurotoxin, finding the cross-link at delta Ala-268 (located in the upper part of the channel-forming transmembrane helix M2) means that the center of the alpha-neurotoxin binding site is situated at least approximately 40 A from the extracellular surface of the AChR, proximal to the channel axis.

Acetylcholine receptor channel imaged in the open state

The structure of the open-channel form of the acetylcholine receptor has been determined from electron images of Torpedo ray postsynaptic membranes activated by brief (< 5 ms) mixing with droplets containing acetylcholine. Comparison with the closed-channel form shows that acetylcholine initiates small rotations of the subunits in the extracellular domain, which trigger a change in configuration of alpha-helices lining the membrane-spanning pore. The open pore tapers towards the intracellular membrane face, where it is shaped by a 'barrel' of alpha-helices having a pronounced right-handed twist.

The nicotinic acetylcholine receptor: structure and autoimmune pathology

The nicotinic acetylcholine receptors (AChR) are presently the best-characterized neurotransmitter receptors. They are pentamers of homologous or identical subunits, symmetrically arranged to form a transmembrane cation channel. The AChR subunits form a family of homologous proteins, derived from a common ancestor. An autoimmune response to muscle AChR causes the disease myasthenia gravis. This review summarizes recent developments in the understanding of the AChR structure and its molecular recognition by the immune system in myasthenia.

Chimaeric nicotinic-serotonergic receptor combines distinct ligand binding and channel specificities

The neuronal nicotinic alpha 7 (nAChR) and 5-hydroxytryptamine (5HT3) receptors are ligand-gated ion channels with a homologous topological organization and have activation and desensitization reactions in common. Yet these homo-oligomeric receptors differ in the pharmacology of their binding sites for agonists and competitive antagonists, and in their sensitivity to Ca2+ ions. The alpha 7 channel is highly permeable to Ca2+ ions and external Ca2+ ions potentiate, in an allosteric manner, the permeability response to acetylcholine, as shown for other neuronal nAChRs. The 5HT3 channel, in contrast, is not permeable to Ca2+ ions, but blocked by them. To assign these properties to delimited domains of the primary structure, we constructed several recombinant chimaeric alpha 7-5HT3 receptors. We report here that one of the constructs expresses a functional receptor that contains the serotonergic channel still blocked by Ca2+ ions, but is activated by nicotinic ligands and potentiated by external Ca2+ ions.

Photoaffinity labeling of Torpedo acetylcholine receptor by physostigmine

The plant alkaloid physostigmine, an established anti-cholinesterase agent of the carbamate type, has recently been shown to bind to the nicotinic acetylcholine receptor from Torpedo marmorata electrocytes [Okonjo, K. O., Kuhlmann, J. & Maelicke, A. (1991) Eur. J. Biochem. 200, 671-677]. Pharmacological studies of physostigmine-induced ion flux into nicotinic-acetylcholine-receptor-rich membrane vesicles, indicated distinct binding sites for physostigmine and acetylcholine. As shown in this study by photoaffinity labeling with [phenyl-(n)-3H](-)physostigmine, the physostigmine-binding site is located within the same subunit (alpha polypeptide) of the receptor as the acetylcholine-binding site. Using a variety of proteolytic cleavage conditions for the purified alpha polypeptide, several [3H]physostigmine-labeled peptides were isolated and sequenced. From the radioactivity released in the course of the Edman degradations of the labeled peptides, it was found that the label was associated in all cases with Lys125. These results identify a novel ligand-binding site for the Torpedo nicotinic acetylcholine receptor that is different in location from binding sites identified previously for acetylcholine, its established agonists and antagonists, and direct channel blockers.

Mutations at two distinct sites within the channel domain M2 alter calcium permeability of neuronal alpha 7 nicotinic receptor

The relative permeability for sodium, potassium, and calcium of chicken alpha 7 neuronal nicotinic receptor was investigated by mutagenesis of the channel domain M2. Mutations in the "intermediate ring" of negatively charged residues, located at the cytoplasmic end of M2 (site 1), reduce calcium permeability without significantly modifying other functional properties (activation and desensitization) of the receptor; a similar change of ion selectivity is also noticed when mutations at site 1 are done in the context of a receptor mutant that conducts ions in a desensitized state. Moreover, mutations of two adjacent rings of leucines at the synaptic end of M2 (site 2) have multiple effects. They abolish calcium permeability, increase the apparent affinity for acetylcholine by 10- to 100-fold, augment Hill numbers (up to 4.6-5.0) of acetylcholine dose-response relationships, slow rates of ionic response onset, and lower the extent of desensitization. Mutations at these two topographically distinct sites within M2 selectively alter calcium transport without affecting the relative permeabilities for sodium and potassium.

Channel blocking properties of a series of nicotinic cholinergic agonists

Inhibition of the nicotinic acetylcholine receptor (nAChR) by channel blockade has been demonstrated with a variety of large organic cations, including several nicotinic agonists. We have studied the kinetics of channel blocking of a series of agonists which vary systematically in size and hydrophobicity due to a hydrocarbon chain from one to six carbons in length, as well as one agonist with a tertiary isomer of one hydrocarbon chain. Single-channel recording was used in combination with three different analysis techniques for determining the kinetic and equilibrium parameters of channel blockade. With an increasing number of methylenes, the blocking rates were essentially constant and the unblocking rates decreased exponentially. This is consistent with studies of the blocking properties of alcohols at the nAChR channel. Also, a linear decrease in the depth to which the larger agonists penetrate the membrane spanning region of the channel was observed. The three smaller agonists, however, all traverse approximately 75% of the membrane field, in agreement with previous measurements of the location of the narrowest region of the channel, the selectivity filter.

Design, synthesis and functional characterization of a pentameric channel protein that mimics the presumed pore structure of the nicotinic cholinergic receptor

Nicotinic cholinergic receptors are membrane proteins composed of five subunits organized around a central aqueous pore. A pentameric channel protein, T5M2 delta, that emulates the presumed pore-forming structure of this receptor was generated by assembling five helix-forming peptide modules at the lysine epsilon-amino groups of the 11-residue template [K*AK*KK*PGK*EK*G], where * indicates attachment sites. Helical modules represent the sequence of the M2 segment of the Torpedo californica acetylcholine receptor (AChR) delta subunit; M2 segments are considered involved in pore-lining. Purified T5M2 delta migrates in SDS-PAGE with an apparent M(r) approximately 14,000, concordant with a protein of 126 residues. T5M2 delta forms cation-selective channels when reconstituted in planar lipid bilayers. The single channel conductance in symmetric 0.5 M KCl is 40 pS. This value approximates the 45 pS single channel conductance characteristic of authentic purified Torpedo AChR, recorded under otherwise identical conditions. These results, together with conformational energy calculations, support the notion that a bundle of five amphipathic alpha-helices is a plausible structural motif underlying the inner bundle that forms the pore of the pentameric AChR channel.

Acetylcholine receptor/channel molecules of insects

Acetylcholine-gated ion channels of the nicotinic type are abundant in the nervous system of insects. The channels are permeable to Na+, K+ and probably Ca(2+), and unlike most vertebrate neuronal nicotinic acetylcholine receptors the receptor/channel molecule is blocked by alpha-bungarotoxin (alpha-Bgt). Such alpha-Bgt-sensitive receptors are present at synapses and on cell bodies of insect neurones. Single channel recordings have shown the existence of multiple conductances of nAChRs. Studies on several different insect preparations have provided evidence for more than one open state and several closed states of insect nAChRs. Functional insect nAChR channels have now been investigated in situ, following reconstitution of a purified protein in bilayers, and as a result of expressing in Xenopus oocytes messenger RNA encoding receptor subunits.

Structure, diversity, and ionic permeability of neuronal and muscle acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) form a family of ligand-gated, cation-selective channels that are concentrated at cholinergic synapses on vertebrate neurons and muscle cells. At the neuromuscular endplate, muscle nAChRs bind acetylcholine released by the presynaptic motor neuron. The receptors then undergo a conformational change that opens their ion channels. Cations move passively through the water-filled pores down their electrochemical gradients, completing synaptic transmission by depolarizing the postsynaptic muscle. The channel only weakly discriminates among permeant cations, which include all monovalent and divalent cations that are small enough to fit through the narrowest cross section. The membrane-spanning region of the pore is lined by uncharged domains that are bracketed by residues with net negative charge. The pore has large entrance vestibules, especially facing extracellularly. The narrowest cross-section is located near the cytoplasmic end of the membrane-spanning region, and this short narrow region probably provides the main cation binding site that is directly in the permeation pathway. Neuronal nAChRs share many of the properties of muscle nAChRs, but the neuronal receptor subtypes are more heterogenous genetically, pharmacologically, and functionally. There are especially important functional differences between muscle and neuronal nAChRs. For example, neuronal nAChRs are more highly permeable to Ca2+ and physiological levels of Ca2+ very potently modulate neuronal nicotinic currents. This variety of nAChRs suggests that these receptor/channels serve many roles in the excitable tissues of vertebrates.

Biochemical characterization of a novel channel-activating site on nicotinic acetylcholine receptors

We have studied the interaction of the reversible acetylcholine esterase inhibitor (-)physostigmine and several structurally related compounds with the nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata electric tissue by means of ligand-induced ion flux into nAChR-rich membrane vesicles, direct binding studies and photoaffinity labeling. (-)Physostigmine acts as a channel-activating ligand at low concentrations and as a direct channel blocker at elevated concentrations. Channel activation is not inhibited by desensitizing concentrations of ACh or ACh-competitive ligands (including alpha-bungarotoxin and D-tubocurarine) but is inhibited by antibody FK1 and several other compounds. From photoaffinity labeling using tritiated physostigmine and mapping of the epitope for the Phy-competitive antibody FK1, the binding site for physostigmine is located within the alpha-subunit of the Torpedo nAChR and is distinct from the acetylcholine binding site. Our data suggest a second pathway of nAChR channel activation that may function physiologically as an allosteric control of receptor activity.

Structure and function of inhibitory glycine receptors

Ion transport by peptide channels has been the major theme in the work of the late P. Läuger. His theoretical and experimental approaches provided the basis for a deeper understanding of pore-mediated ion permeation through biological membranes. This review on a ligand–gated ion channel protein from the mammalian brain is dedicated to the memory of this outstanding scientist.

The functional architecture of the acetylcholine nicotinic receptor explored by affinity labelling and site-directed mutagenesis

The scientific community will remember Peter Läuger as an exceptional man combining a generous personality and a sharp and skilful mind. He was able to attract by his views the interest of a large spectrum of biologists concerned by the mechanism of ion translocation through membranes. Yet, he was not a man with a single technique or theory. Using an authentically multidisciplinary approach, his ambition was to ‘understand transmembrane transport at the microscopic level, to capture its dynamics in the course of defined physiological processes’ (1987). According to him, ‘new concepts in the molecular physics of proteins’ had to be imagined, and ‘the traditional static picture of proteins has been replaced by the notions that proteins represent dynamic structures, subjected to conformational fluctuations covering a very wide time-range’ (1987).

Acetylcholine receptor channel structure probed in cysteine-substitution mutants

In order to understand the structural bases of ion conduction, ion selectivity, and gating in the nicotinic acetylcholine receptor, mutagenesis and covalent modification were combined to identify the amino acid residues that line the channel. The side chains of alternate residues--Ser248, Leu250, Ser252, and Thr254--in M2, a membrane-spanning segment of the alpha subunit, are exposed in the closed channel. Thus alpha 248-254 probably forms a beta strand, and the gate is closer to the cytoplasmic end of the channel than any of these residues. On channel opening, Leu251 is also exposed. These results lead to a revised view of the closed and open channel structures.

Mutations in the channel domain of a neuronal nicotinic receptor convert ion selectivity from cationic to anionic

Introduction by site-directed mutagenesis of three amino acids from the MII segment of glycine or gamma-aminobutyric acid (GABAA) receptors into the MII segment of alpha 7 nicotinic receptor was sufficient to convert a cation-selective channel into an anion-selective channel gated by acetylcholine. A critical mutation was the insertion of an uncharged residue at the amino-terminal end of MII, stressing the importance of protein geometrical constraints on ion selectivity.

Tris+/Na+ permeability ratios of nicotinic acetylcholine receptors are reduced by mutations near the intracellular end of the M2 region

Tris+/Na+ permeability ratios were measured from shifts in the biionic reversal potentials of the macroscopic ACh-induced currents for 3 wild-type (WT), 1 hybrid, 2 subunit-deficient, and 25 mutant nicotinic receptors expressed in Xenopus oocytes. At two positions near the putative intracellular end of M2, 2' (alpha Thr244, beta Gly255, gamma Thr253, delta Ser258) and -1', point mutations reduced the relative Tris+ permeability of the mouse receptor as much as threefold. Comparable mutations at several other positions had no effects on relative Tris+ permeability. Mutations in delta had a greater effect on relative Tris+ permeability than did comparable mutations in gamma; omission of the mouse delta subunit (delta 0 receptor) or replacement of mouse delta with Xenopus delta dramatically reduced relative Tris+ permeability. The WT mouse muscle receptor (alpha beta gamma delta) had a higher relative permeability to Tris+ than the wild-type Torpedo receptor. Analysis of the data show that (a) changes in the Tris+/Na+ permeability ratio produced by mutations correlate better with the hydrophobicity of the amino acid residues in M2 than with their volume; and (b) the mole-fraction dependence of the reversal potential in mixed Na+/Tris+ solutions is approximately consistent with the Goldman-Hodgkin-Katz voltage equation. The results suggest that the main ion selectivity filter for large monovalent cations in the ACh receptor channel is the region delimited by positions -1' and 2' near the intracellular end of the M2 helix.

Alpha 4-2 beta 2 and other nicotinic acetylcholine receptor subtypes as targets of psychoactive and addictive drugs

1. Xenopus oocytes were injected with various muscle and neuronal nicotinic acetylcholine receptor (ACh receptor, cholinoceptor) subunit RNA combinations and their pharmacological properties studied using two-electrode voltage clamp. The functional expression of one of these combinations, rat alpha 4-2 beta 2, has not been previously described. The alpha 4-2 mRNA is a splicing variant transcribed from the alpha 4 gene. In the experiments reported here, the alpha 4-2 beta 2 subtype was functionally indistinguishable from the alpha 4-1 beta 2 subtype. 2. For each subtype, the relative potency of nicotine compared with acetylcholine was obtained by estimating the relative concentration of nicotine which would elicit the same current response as 0.1 microM Ach. The ratios of these concentrations (nicotine: ACh) for the mouse muscle ACh receptor-(alpha 1 beta 1 gamma delta) was 96.1:1. In contrast, the ratios for the rat neuronal subtypes were: alpha 2 beta 2, 1.01:1; alpha 3 beta 2, 2.01:1; alpha 4 beta 2, 0.76:1 and alpha 4-2 beta 2, 0.76:1. The much greater relative nicotine sensitivity of the neuronal subtypes as compared with muscle receptors illustrates their potential to mediate the psychoactive and addictive effects of nicotine. However, it does not appear that the differences in relative nicotinic sensitivity among the neuronal receptors themselves can be used as a simple discriminative tool in neuronal tissue. 3. The slopes of the log dose-log response curves at low ACh concentrations were all greater than 1 but less than 2, suggesting that at least two agonist binding sites mediate the functional response of each hetero-oligomer. 4. The response of all the neuronal subtypes to ACh could be inhibited by the psychoactive drugs mecamylamine, amitriptyline, phencyclidine, trifluoperazine and promethazine. With the exception of the very potent antagonist, mecamylamine, the degree of block of the peak current to ACh produced by 10 microM concentrations of these drugs was remarkably similar (around 50%). 5. The degree of inhibition produced when the antipsychotic drug, trifluoperazine, was co-applied with ACh increased as the duration of application increased. Such an effect was not observed with promethazine, a related phenothiazine derivative which does not have antipsychotic actions.

Unconventional pharmacology of a neuronal nicotinic receptor mutated in the channel domain

The putative channel-forming MII domains of the nicotinic, gamma-aminobutyric acid type A, and glycine receptors contain a highly conserved leucine residue. Mutation of this hydrophobic amino acid in the neuronal nicotinic receptor alpha 7 (Leu-247), reconstituted in Xenopus oocytes, modifies the ionic response to acetylcholine and alters desensitization. Furthermore, the Leu----Thr (L247T) mutant has two conducting states (46 pS and 80 pS), in contrast with the wild-type (WT) receptor, which has only one (45 pS). We now show that this mutant possesses a rather paradoxical pharmacology: antagonists of the WT receptor such as dihydro-beta-erythroidin, hexamethonium, or (+)-tubocurarine elicit ionic currents when applied to the L247T alpha 7 mutant and these responses are blocked by alpha-bungarotoxin. Furthermore, prolonged application of acetylcholine causes desensitization in the WT but leads to a potentiation of the responses to acetylcholine or dihydro-beta-erythroidin in the mutant. These data are consistent with a scheme in which mutation of Leu-247 renders a desensitized state in the WT channel a conducting state. They also strengthen the proposal that, in the WT, some competitive antagonists may stabilize desensitized states. Finally, these observations may shed light on properties of other ion channels, in particular the glutamate receptors, which display multiple conductance levels associated with various pharmacological agents.

Voltage-dependent block by magnesium of neuronal nicotinic acetylcholine receptor channels in rat phaeochromocytoma cells

1. The effects of Mg2+ on the single-channel conductance of neuronal nicotinic acetylcholine receptors were examined using receptors expressed by the rat phaeochromocytoma cell line, PC12. PC12 cells express at least three conductance classes of channels that are activated by acetylcholine, the largest conductance class being the most prevalent. This receptor channel is blocked by intracellular and extracellular Mg2+. 2. The effects of Mg2+ are asymmetrical; at a given concentration, internal Mg2+ is more effective at blocking outward currents than external Mg2+ is at blocking inward currents. Receptor channels are blocked at concentrations of Mg2+ that are low compared to the concentration of the main permeant cation, Na+, and the block is voltage dependent. 3. The block by Mg2+ is not complete as Mg2+ can permeate the channel. With 80 mM-extracellular Mg2+ (no extracellular Na+), the channel has an inward slope conductance of 2.9 pS. 4. The block by extracellular Mg2+ can be described by a one site, two barrier model for the channel which includes a negative surface charge on the external surface of the membrane. The parameters of the model place the binding site for Mg2+ at 52% of the membrane field from the outside with an apparent dissociation constant of 14 mM. However, the same parameters cannot describe the block by intracellular Mg2+. The deviations from the model suggest that the receptor channel may have more than one binding site for Mg2+.

Mutations in the channel domain alter desensitization of a neuronal nicotinic receptor

A variety of ligand-gated ion channels undergo a fast activation process after the rapid application of agonist and also a slower transition towards desensitized or inactivated closed channel states when exposure to agonist is prolonged. Desensitization involves at least two distinct closed states in the acetylcholine receptor, each with an affinity for agonists higher than those of the resting or active conformations. Here we investigate how structural elements could be involved in the desensitization of the acetylcholine-gated ion channel from the chick brain alpha-bungarotoxin sensitive homo-oligomeric alpha 7 receptor, using site-directed mutagenesis and expression in Xenopus oocytes. Mutations of the highly conserved leucine 247 residue from the uncharged MII segment of alpha 7 suppress inhibition by the open-channel blocker QX-222, indicating that this residue, like others from MII, faces the lumen of the channel. But, unexpectedly, the same mutations decrease the rate of desensitization of the response, increase the apparent affinity for acetylcholine and abolish current rectification. Moreover, unlike wild-type alpha 7, which has channels with a single conductance level, the leucine-to-threonine mutant has an additional conducting state active at low acetylcholine concentrations. It is possible that mutation of Leu 247 renders conductive one of the high-affinity desensitized states of the receptor.

A second pathway of activation of the Torpedo acetylcholine receptor channel

We have studied the interaction of the reversible acetylcholine esterase inhibitor (-)physostigmine (D-eserine) with the nicotinic acetylcholine receptor (nAChR) from Torpedo marmorata electric tissue by means of ligand-induced ion flux into nAChR-rich membrane vesicles and of equilibrium binding. We find that (-) physostigmine induces cation flux (and also binds to the receptor) even in the presence of saturating concentrations of antagonists of acetylcholine, such as D-tubocurarine, alpha-bungarotoxin or antibody WF6. The direct action on the acetylcholine receptor is not affected by removal of the methylcarbamate function from the drug and thus is not due to carbamylation of the receptor. Antibodies FK1 and benzoquinonium antagonize channel activation (and binding) of eserine, suggesting that the eserine binding site(s) is separate from, but adjacent to, the acetylcholine binding site at the receptor. In addition to the channel activating site(s) with an affinity of binding in the 50 microM range, there exists a further class of low-affinity (Kd approximately mM) sites from which eserine acts as a direct blocker of the acetylcholine-activated channel. Our results suggest the existence of a second pathway of activation of the nAChR channel.

A ring of uncharged polar amino acids as a component of channel constriction in the nicotinic acetylcholine receptor

The channel pore of the nicotinic acetylcholine receptor (AChR) has been investigated by analysing single-channel conductances of systematically mutated Torpedo receptors expressed in Xenopus oocytes. The mutations mainly alter the size and polarity of uncharged polar amino acid residues of the acetylcholine receptor subunits positioned between the cytoplasmic ring and the extracellular ring. From the results obtained, we conclude that a ring of uncharged polar residues comprising threonine 244 of the alpha-subunit (alpha T244), beta S250, gamma T253 and delta S258 (referred to as the central ring) and the anionic intermediate ring, which are adjacent to each other in the assumed alpha-helical configuration of the M2-containing transmembrane segment, together form a narrow channel constriction of short length, located close to the cytoplasmic side of the membrane. Our results also suggest that individual subunits, particularly the gamma-subunit, are asymmetrically positioned at the channel constriction.

Binding sites for alpha-bungarotoxin and the noncompetitive inhibitor phencyclidine on a synthetic peptide comprising residues 172-227 of the alpha-subunit of the nicotinic acetylcholine receptor

The binding of the competitive antagonist alpha-bungarotoxin (alpha-Btx) and the noncompetitive inhibitor phencyclidine (PCP) to a synthetic peptide comprising residues 172-227 of the alpha-subunit of the Torpedo acetylcholine receptor has been characterized. 125I-alpha-Btx bound to the 172-227 peptide in a solid-phase assay and was competed by alpha-Btx (IC50 = 5.0 x 10(-8) M), d-tubocurarine (IC50 = 5.9 X 10(-5)M), and NaCl (IC50 = 7.9 x 10(-2)M). In the presence of 0.02% sodium dodecyl sulfate, 125I-alpha-Btx bound to the 56-residue peptide with a KD of 3.5 nM, as determined by equilibrium saturation binding studies. Because alpha-Btx binds to a peptide comprising residues 173-204 with the same affinity and does not bind to a peptide comprising residues 205-227, the competitive antagonist and hence agonist binding site lies between residues 173 and 204. After photoaffinity labeling, [3H]PCP was bound to the 172-227 peptide. [3H]PCP binding was inhibited by chlorpromazine (IC50 = 6.3 x 10(-5)M), tetracaine (IC50 = 4.2 x 10(-6)M), and dibucaine (IC50 = 2.7 x 10(-4)M). Equilibrium saturation binding studies in the presence of 0.02% sodium dodecyl sulfate showed that [3H]PCP bound at two sites, a major site of high affinity with an apparent KD of 0.4 microM and a minor low-affinity site with an apparent KD of 4.6 microM. High -affinity binding occurred at a single site on peptide 205-227 (KD = 0.27 microM) and was competed by chlorpromazine but not by alpha-Btx.(ABSTRACT TRUNCATED AT 250 WORDS)

Distinctions in ligand binding sites on the nicotinic acetylcholine receptor

Ligand-gated ion channels possess intrinsic binding sites for noncompetitive inhibitors that differ substantially in ligand specificity and structural characteristics from most binding sites found on globular proteins. We have used the nicotinic acetylcholine receptor to examine the characteristics of such diverse sites because the high-affinity binding site in the proximity of the ion channel has unusual binding interactions and ligand specificity, whereas the site of agonist activation exhibits classical structure-activity characteristics. Noncompetitive inhibitors that bind to the former site show a wide degree of structural variation and appear to associate at separate loci and in distinct orientations in the vicinity of the channel. The receptor structure appears to provide a large domain with multiple hydrophobic crevices that can bind noncompetitive inhibitors, yet binding of these inhibitors is mutually exclusive. The mutually exclusive behavior suggests that association of a single ligand is sufficient to prevent access of additional ligands to distinct sites. This could occur either by physical occlusion to the site of binding or by formation of a conformational state that will not allow entry of additional noncompetitive inhibitors.

Reverse pharmacology of the nicotinic acetylcholine receptor. Mapping the local anesthetic binding site

We have been examining the interaction of a local anesthetic derivative, QX-222, with the ion channel pore of the muscle AChR, using a combination of mutagenesis, oocyte expression, and electrophysiology. Single channel recording, together with macroscopic voltage-jump relaxations, provides a measure of the residence time of the open channel blocker within the pore. We have found systematic changes in the apparent affinity of the open channel for QX-222 following amino acid substitutions in the proposed M2 transmembrane helix of each of the four subunits of the AChR. Assigning the number 1' to the residue at the cytoplasmic end of the M2 helix, positions 2',6',10',14', and 18' are modeled as forming the lining of the pore. Polar to nonpolar substitutions at 6' decrease QX-222 residence time, while the opposite effect is seen at position 10'. Nonpolar to polar substitutions have the converse effect. The distance between the aromatic and quaternary amine moieties of QX-222 corresponds almost exactly to the repeat distance of an alpha helix. This structural feature is common to many local anesthetic drugs. We propose a model for the binding of QX-222 within the ion channel of the AChR that is consistent with these observations.