P2X7-related modulation of pathological nociception in rats

Growing evidence supports a role for the immune system in the induction and maintenance of chronic pain. ATP is a key neurotransmitter in this process. Recent studies demonstrate that the glial ATP receptor, P2X7, contributes to the modulation of pathological pain. To further delineate the endogenous mechanisms that are involved in P2X7-related antinociception, we utilized a selective P2X7 receptor antagonist, A-438079, in a series of in vivo and in vitro experiments. Injection of A-438079 (10-300 micromol/kg, i.p.) was anti-allodynic in three different rat models of neuropathic pain and it attenuated formalin-induced nocifensive behaviors. Using in vivo electrophysiology, A-438079 (80 micromol/kg, i.v.) reduced noxious and innocuous evoked activity of different classes of spinal neurons (low threshold, nociceptive specific, wide dynamic range) in neuropathic rats. The effects of A-438079 on evoked firing were diminished or absent in sham rats. Spontaneous activity of all classes of spinal neurons was also significantly reduced by A-438079 in neuropathic but not sham rats. In vitro, A-438079 (1 microM) blocked agonist-induced (2,3-O-(4-benzoylbenzoyl)-ATP, 30 microM) current in non-neuronal cells taken from the vicinity of the dorsal root ganglia. Furthermore, A-438079 dose-dependently (0.3-3 microM) decreased the quantity of the cytokine, interleukin-1beta, released from peripheral macrophages. Thus, ATP, acting through the P2X7 receptor, exerts a wide-ranging influence on spinal neuronal activity following a chronic injury. Antagonism of the P2X7 receptor can in turn modulate central sensitization and produce antinociception in animal models of pathological pain. These effects are likely mediated through immuno-neural interactions that affect the release of endogenous cytokines.

Discovery of P2X7 receptor-selective antagonists offers new insights into P2X7 receptor function and indicates a role in chronic pain states

ATP-sensitive P2X(7) receptors are localized on cells of immunological origin including peripheral macrophages and glial cells in the CNS. Activation of P2X(7) receptors leads to rapid changes in intracellular calcium concentrations, release of the proinflammatory cytokine interleukin-1beta and following prolonged agonist exposure, the formation of cytolytic pores in plasma membranes. Both the localization and functional consequences of P2X(7) receptor activation indicate a role in inflammatory processes. The phenotype of P2X(7) receptor gene-disrupted mice also indicates that P2X(7) receptor activation contributes to ongoing inflammation. More recently, P2X(7) receptor knockout data has also suggested a specific role in inflammatory and neuropathic pain states. The recent discovery of potent and highly selective antagonists for P2X(7) receptors has helped to further clarify P2X receptor pharmacology, expanded understanding of P2X(7) receptor signaling, and offers new evidence that P2X(7) receptors play a specific role in nociceptive signaling in chronic pain states. In this review, we incorporate the recent discoveries of novel P2X(7) receptor-selective antagonists with a brief update on P2X(7) receptor pharmacology and its therapeutic potential.

Pharmacological characterization of P2X7 receptors in rat peritoneal cells

OBJECTIVE AND DESIGN

P2X(7) receptor activation by ATP results in the release of IL-1beta and IL-18. Prolonged stimulation can lead to pore formation and cell death. In this study we pharmacologically characterized P2X(7) receptors on rat peritoneal cells (RPC) and on 1321N1 cells transfected with rat P2X(7) receptor (1321rP2X(7)-11).

MATERIALS AND METHODS

RPC were isolated from rats by lavage. P2X(7) agonist induced pore formation in RPC was measured by EtBr uptake. P2X(7)-stimulated pore formation and Ca(++) influx in 1321rP2X(7)-11 cells were measured by a fluorometric imaging plate reader. The effects of pyridoxal phosphate-6-azo phenyl -2'-4'-disulfonic acid (PPADS) on pore formation and Ca(++) influx were examined in both RPC and 1321rP2X(7)-11. P2X(7)-mediated IL-1beta release in RPC and the effect of PPADS were determined.

RESULTS

RPC express functional P2X(7) receptors that were activated by ATP analogs with a rank order of potency of 2'- 3'-O-(4-Benzoylbenzoyl) adenosine 5'-triphosphate (BzATP) > ATP > alpha,beta-methylene ATP. Activation of P2X(7) receptors by BzATP was inhibited by PPADS. Similar results were also obtained in 1321rP2X(7)-11 cells. Activation of P2X(7) receptors on RPC resulted in IL-1 beta secretion, which was inhibited by PPADS.

CONCLUSIONS

RPC express functional P2X(7) receptors that form pores and mediate the release of IL-1beta.

Reduced nicotinic receptor-mediated antinociception following in vivo antisense knock-down in rat

Pharmacological activation of neuronal nicotinic acetylcholine receptors can produce non-opioid antinociception in rodents. However, multiple nAChR subtypes exist, the most abundant of which contain alpha4 and beta2 subunits. The purpose of the present study was to investigate the role of alpha4-containing nAChRs in mediating nicotinic antinociception using an in vivo antisense strategy. Both i.c.v. infusion and repeated bolus injections into the cerebral aqueduct of an antisense oligonucleotide against the alpha4 subunit significantly attenuated the antinociceptive effects of the nAChR agonist A-85380 in the paw withdrawal test of acute thermal pain. Rats treated with a scrambled oligonucleotide displayed a full antinociceptive response to A-85380, while discontinuing antisense treatment restored the antinociceptive effects of the nicotinic agonist. Double immunohistochemical labeling revealed near-complete overlap of expression of the serotonin marker tryptophan hydroxylase and the alpha4 nAChR subunit in the dorsal raphe nucleus. The expression of alpha4-containing nAChRs by serotonergic neurons in the dorsal raphe offered a means to address nonspecific alpha4 knock-down, i.e., oligonucleotide-induced neurotoxicity. Immunohistochemical detection of alpha4 expression was reduced by nearly 50% in the dorsal raphe of antisense-treated rats as compared to either saline or missense-treated controls. In contrast, the expression of tryptophan hydroxylase, as well as, the alpha7 nAChR subunit in antisense-infused rats was similar to that observed in saline- and missense-treated controls. The results of these studies suggest that alpha4-containing nAChRs, possibly expressed by serotonergic neurons, are involved in nicotinic-mediated analgesia. However, these data do not eliminate the possibility that other nicotinic subunit combinations may also play a role in antinociception produced by nAChR activation.

Structure-activity studies related to ABT-594, a potent nonopioid analgesic agent: effect of pyridine and azetidine ring substitutions on nicotinic acetylcholine receptor binding affinity and analgesic activity in mice

Analogs of A-98593 (1) and its enantiomer ABT-594 (2) with diverse substituents on the pyridine ring were prepared and tested for affinity to nicotinic acetylcholine receptor binding sites in rat brain and for analgesic activity in the mouse hot plate assay. Numerous types of modifications were consistent with high affinity for [3H]cytisine binding sites. By contrast, only selected modifications resulted in retention of analgesic potency in the same range as 1 and 2. Analogs of 2 with one or two methyl substituents at the 3-position of the azetidine ring also were prepared and found to be substantially less active in both assays.

The role of neuronal nicotinic acetylcholine receptors in antinociception: effects of ABT-594

ABT-594, a nicotinic acetylcholine receptor agonist, has antinociceptive effects in rat models of acute thermal, persistent chemical, and neuropathic pain. Direct injection of ABT-594 into the nucleus raphe magnus (NRM) is antinociceptive in a thermal threshold test and destruction of serotonergic neurons in the NRM attenuates the effect of systemic ABT-594. However, lidocaine-inactivation of the NRM prevents the antinociceptive effect of systemic (-)-nicotine but not that of systemic ABT-594.

ABT-594 [(R)-5-(2-azetidinylmethoxy)-2-chloropyridine]: a novel, orally effective analgesic acting via neuronal nicotinic acetylcholine receptors: I. In vitro characterization

The discovery of (+/-)-epibatidine, a naturally occurring neuronal nicotinic acetylcholine receptor (nAChR) agonist with antinociceptive activity 200-fold more potent than that of morphine, has renewed interest in the potential role of nAChRs in pain processing. However, (+/-)-epibatidine has significant side-effect liabilities associated with potent activity at the ganglionic and neuromuscular junction nAChR subtypes which limit its potential as a clinical entity. ABT-594 [(R)-5-(2-azetidinylmethoxy)-2-chloropyridine] is a novel, potent cholinergic nAChR ligand with analgesic properties (see accompanying paper by Bannon et al., 1998b) that shows preferential selectivity for neuronal nAChRs and a consequently improved in vivo side-effect profile compared with (+/-)-epibatidine. ABT-594 is a potent inhibitor of the binding of [3H](-)-cytisine to alpha 4 beta 2 neuronal nAChRs (Ki = 37 pM, rat brain; Ki = 55 pM, transfected human receptor). At the alpha 1 beta 1 delta gamma neuromuscular nAChR labeled by [125I] alpha-bungarotoxin (alpha-Btx), ABT-594 has a Ki value of 10,000 nM resulting in a greater than 180,000-fold selectivity of the compound for the neuronal alpha 4 beta 2 nAChR. In contrast, (+/-)-epibatidine has Ki values of 70 pM and 2.7 nM at the alpha 4 beta 2 and alpha 1 beta 1 delta gamma nAChRs, respectively, giving a selectivity of only 38-fold. The S-enantiomer of ABT-594, A-98593 has activity at the neuronal alpha 4 beta 2 nAChR identical with ABT-594 (Ki = 34-39 pM), which demonstrates a lack of stereospecific binding similar to that reported previously for (+/-)-epibatidine. A similar lack of stereoselectivity is seen at the human alpha 7 receptor. However, A-98593 is 3-fold more potent at the neuromuscular nAChR (Ki = 3420 nM) and the brain alpha-Btx-sensitive nAChR (Ki = 4620 nM) than ABT-594. ABT-594 has weak affinity in binding assays for adrenoreceptor subtypes alpha-1B (Ki = 890 nM), alpha-2B (Ki = 597 nM) and alpha-2C (Ki = 342 nM), and it has negligible affinity (Ki > 1000 nM) for approximately 70 other receptors, enzyme and transporter binding sites. Functionally, ABT-594 is an agonist. At the transfected human alpha 4 beta 2 neuronal nAChR (K177 cells), with increased 86Rb+ efflux as a measure of cation efflux, ABT-594 had an EC50 value of 140 nM with an intrinsic activity (IA) compared with (-)-nicotine of 130%; at the nAChR subtype expressed in IMR-32 cells (sympathetic ganglion-like), an EC50 of 340 nM (IA = 126%); at the F11 dorsal root ganglion cell line (sensory ganglion-like), an EC50 of 1220 nM (IA = 71%); and via direct measurement of ion currents, an EC50 value of 56,000 nM (IA = 83%) at the human alpha 7 homooligimeric nAChR produced in oocytes. A-98593 is 2- to 3-fold more potent and displays approximately 50% greater intrinsic activity than ABT-594 in all four functional assays. In terms of potency, ABT-594 is 8- to 64-fold less active than (+/-)-epibatidine and also has less IA in these functional assays. ABT-594 (30 microM) inhibits the release of calcitonin gene-related peptide from C-fibers terminating in the dorsal horn of the spinal cord, an effect mediated via nAChRs. Pharmacologically, ABT-594 has an in vitro profile distinct from that of the prototypic nicotinic analgesic (+/-)-epibatidine, with the potential for substantially reduced side-effect liability and, as such, represents a potentially novel therapeutic approach to pain management.

Identification and initial structure-activity relationships of (R)-5-(2-azetidinylmethoxy)-2-chloropyridine (ABT-594), a potent, orally active, non-opiate analgesic agent acting via neuronal nicotinic acetylcholine receptors

New members of a previously reported series of 3-pyridyl ether compounds are disclosed as novel, potent analgesic agents acting through neuronal nicotinic acetylcholine receptors. Both (R)-2-chloro-5-(2-azetidinylmethoxy)pyridine (ABT-594, 5) and its S-enantiomer (4) show potent analgesic activity in the mouse hot-plate assay following either intraperitoneal (i.p.) or oral (p.o.) administration, as well as activity in the mouse abdominal constriction (writhing) assay, a model of persistent pain. Compared to the S-enantiomer and to the prototypical potent nicotinic analgesic agent (+/-)-epibatidine, 5 shows diminished activity in models of peripheral side effects. Structure-activity studies of analogues related to 4 and 5 suggest that the N-unsubstituted azetidine moiety and the 2-chloro substituent on the pyridine ring are important contributors to potent analgesic activity.

Synthesis and structure-activity relationships of pyridine-modified analogs of 3-[2-((S)-pyrrolidinyl)methoxy]pyridine, A-84543, a potent nicotinic acetylcholine receptor agonist

Analogs of 3-[2-((S)-pyrrolidinyl)methoxy]pyridine, (A-84543, 1) with 2-, 4-, 5-, and 6-substituents on the pyridine ring were synthesized. These analogs exhibited Ki values ranging from 0.15 to > 9,000 nM when tested in vitro for neuronal nicotinic acetylcholine receptor binding activity. Assessment of functional activity at subtypes of neuronal nicotinic acetylcholine receptors indicates that pyridine substitution can have a profound effect on efficacy at these subtypes, and several subtype-selective agonists and antagonists have been identified.

Evidence for nicotinic receptors potentially modulating nociceptive transmission at the level of the primary sensory neuron: studies with F11 cells

F11 cells are a dorsal root ganglion (DRG) cell line used to model the function of authentic type C, peptidergic, nociceptive neurons. The cellular events underlying the antinociceptive effects of (+/-)-epibatidine, a nicotinic acetylcholine receptor (nAChR) ligand that is 200-fold more potent than morphine, is unknown. The present study investigated the ability of cholinergic channel activators (ChCAs) to effect nAChR-gated ion flux and modulate the release of substance P (SP), a neuropeptide identified to play a critical role in nociception. The prototypical agonists (-)-nicotine and (-)-cytisine, the ganglionic stimulant 1,1-dimethyl-4-phenylpiperazinium, the novel ChCA ABT-418 [(S)-3-methyl-5-(-1-methyl-2-pyrrolidinyl)isoxazole], and (+/-)-epibatidine evoked a concentration-dependent stimulation of rubidium (86Rb+) efflux with EC50 values of 14.2 +/- 1.6, 63.4 +/- 24, 3.8 +/- 2.0, 29.8 +/- 2.6, and 0.019 +/- 0.001 microM as well as maximal intrinsic activities of 100, 97, 69, 75, and 102%, respectively. The noncompetitive nAChR antagonist mecamylamine potently antagonized (-)-nicotine-evoked ion flux, whereas the competitive antagonist dihydro-beta-erythroidine was a weak antagonist, giving support to an alpha3beta4 nAChR subtype. In addition, concentrations of (+/-)-epibatidine, similar to those necessary to induce maximal 86Rb+ efflux, evoked spontaneous release of SP from these cells, which was blocked by mecamylamine. Furthermore, prolonged exposure to (+/-)-epibatidine desensitized the functional response of the nAChR in this cell line (IC50 = 12 +/- 9 nM). These findings in F11 cells provide a model to investigate the role nAChRs play in modulating DRG cell function, and may lead to insights into the role these receptors have in modulating nociceptive transmission.

Structure-activity studies on 2-methyl-3-(2(S)-pyrrolidinylmethoxy) pyridine (ABT-089): an orally bioavailable 3-pyridyl ether nicotinic acetylcholine receptor ligand with cognition-enhancing properties

2-Methyl-3-(2(S)-pyrrolidinylmethoxy)pyridine, ABT-089 (S-4), a member of the 3-pyridyl ether class of nicotinic acetylcholine receptor (nAChR) ligands, shows positive effects in rodent and primate models of cognitive enhancement and a rodent model of anxiolytic activity and possesses a reduced propensity to activate peripheral ganglionic type receptors. The profiles of S-4, its N-methyl analogue, and the corresponding enantiomers across several measures of cholinergic channel function in vitro and in vivo are presented, together with in vitro metabolism and in vivo bioavailability data. On the basis of its biological activities and favorable oral bioavailability, S-4 is an attractive candidate for further evaluation as a treatment for cognitive disorders.

In vitro neuroprotective properties of the novel cholinergic channel activator (ChCA), ABT-418

Recent literature has shown that compounds interacting with neuronal nicotinic acetylcholine receptors (nAChRs) have the potential to be neuroprotective both in vitro and in vivo. ABT-418 is a novel ChCA that selectively stimulates discrete subtypes of the nAChRs and exhibits cognitive enhancing activity. In the present study, the neuroprotective effects of ABT-418 and (-)-nicotine, as measured by the release of lactate dehydrogenase (LDH) into the media, were investigated in a glutamate (Glu)-induced cytotoxicity assay using either primary rat cortical neurons or a human differentiated cell line, IMR 32. The neuroprotection elicited by ABT-418 and (-)-nicotine is both time and concentration dependent with an optimal concentration of 10 microM and an optimal pretreatment time of 2 h. ABT-418 remained neuroprotective and not cytotoxic to rat cortical cells following subacute exposure for 7 days. Protection appears to be mediated via an interaction with nAChRs, possibly the alpha 7 subtype, since the neuroprotection was prevented by alpha-bungaratoxin (alpha-Bgt) and methyllycaconitine (MLA), both selective alpha 7 antagonists. Removal of extracellular Ca2+ prevented the neuroprotective effects of ABT-418 and (-)-nicotine, consistent with the known ability of alpha 7 nAChRs to modulate calcium dynamics. These data support the idea that ABT-418 not only enhances cognition, but may possibly slow the progression of the neurodegenerative process.

Functional modulation of human "ganglionic-like" neuronal nicotinic acetylcholine receptors (nAChRs) by L-type calcium channel antagonists

Recent studies suggest that the neuronal nicotinic acetylcholine receptors present on chromaffin cells contain a 1,4-dihydropyridine-sensitive site whose occupation blocks membrane depolarization (1). In the present study, several L-type Ca2+ channel blockers inhibited the activation of the nAChRs present in the human neuroblastoma cell line, IMR 32, in a dose-dependent manner with IC50 values ranging from 0.8-3 microM. In contrast, omega-Conotoxin GVIA and omega-Agatoxin IVA had no effect up to 100 microM. Furthermore, the inorganic channel blocker, cadmium, had no effect either alone or on the modulatory role of the L-type antagonists, suggesting that the effects of these agents on nAChRs are not mediated via an interaction with calcium channels but possibly via a direct interaction with the nAChR ionophore.

Erysodine, a competitive antagonist at neuronal nicotinic acetylcholine receptors

Erysodine, an erythrina alkaloid related to dihydro-beta-erythroidine, was found to be a more potent inhibitor of [3H]cytisine binding at neuronal nicotinic acetylcholine receptors but a less potent inhibitor of [125I]alpha-bungarotoxin binding at muscle-type nicotinic acetylcholine receptors than dihydro-beta-erythroidine. Erysodine was a competitive, reversible antagonist of (-)-nicotine-induced dopamine release from striatal slices and inhibited (-)-nicotine-induced 86Rb+ efflux from IMR-32 cells. Erysodine was equipotent with dihydro-beta-erythroidine in the dopamine release assay but 10-fold more potent in the 86Rb+ efflux assay, suggesting differential subtype selectivity for these two antagonists. Erysodine, systemically administered to mice, entered the brain and significantly attentuated nicotine's hypothermic effects and its anxiolytic-like effects in the elevated plus-maze test. There was greater separation between antagonist and toxic doses for erysodine than for dihydro-beta-erythroidine, perhaps because of erysodine's greater selectivity for neuronal receptors. In rats, erysodine prevented both the early developing decrease and the late-developing increase in locomotor activity produced by (-)-nicotine. The potent and competitive nature of erysodine's antagonism together with its ability to enter the brain after systemic administration suggest that erysodine may be a useful tool in characterizing neuronal nicotinic acetylcholine receptors.

Effects of mutations of Torpedo acetylcholine receptor alpha 1 subunit residues 184-200 on alpha-bungarotoxin binding in a recombinant fusion protein

Residues between positions 184 and 200 of the Torpedo acetylcholine receptor alpha 1 subunit were changed by oligonucleotide-directed mutagenesis in a recombinant fusion protein containing residues 166-211. Amino acids were substituted with residues present in the snake alpha subunit, with an alanine, or with a functionally dissimilar residue. The competitive antagonist alpha-bungarotoxin bound to the fusion protein with high apparent affinity (IC50 = 3.2 x 10(-8) M), and binding was competed by agonists and antagonists. Mutation of His-186, Tyr-189, Tyr-190, Cys-192, Cys-193, Pro-194, and Asp-195 greatly reduced or abolished alpha-bungarotoxin binding, while mutation of Tyr-198 reduced binding, indicating these residues play an important role in binding either through functional interaction with neurotoxin residues or by stabilizing the conformation of the binding site. Molecular modeling of acetylcholine receptor residues 184-200 and knowledge of both neurotoxin and receptor residues essential for binding allow analysis of possible structure-function relationships of the interaction of alpha-bungarotoxin with this region of the receptor.

Sodium dodecyl sulfate- and carbamylcholine-induced changes in circular dichroism spectra of acetylcholine receptor synthetic peptides

The effect of sodium dodecyl sulfate (SDS) on the conformation of acetylcholine receptor alpha-subunit synthetic peptides was investigated by circular dichroism. In the presence of SDS (0.01-0.02%), the affinity of a 173-204 32 residue peptide and a 172-227 56 residue peptide for the competitive antagonist alpha-bungarotoxin increases about 10-fold to the nanomolar range. Circular dichroism spectroscopy of these peptides revealed significant changes in the secondary structure of the peptides in the presence of SDS at concentrations below the critical micelle concentration. It is concluded that SDS induces a conformation of the peptides that is conductive to high affinity binding. Carbamylcholine, an acetylcholine analog, produced small but significant changes in the spectrum of the 173-204 peptide. This change could be the result of agonist-induced conformational changes in this region of the acetylcholine receptor alpha-subunit or to changes in the asymmetric environments of aromatic chromophores in the binding site. These studies demonstrate that synthetic peptides alone are capable of retaining significant functional activity and contain significant secondary structure.

Substitution of Torpedo acetylcholine receptor alpha 1-subunit residues with snake alpha 1- and rat nerve alpha 3-subunit residues in recombinant fusion proteins: effect on alpha-bungarotoxin binding

A fusion protein consisting of the TrpE protein and residues 166-211 of the Torpedo acetylcholine receptor alpha 1 subunit was produced in Escherichia coli using a pATH10 expression vector. Residues in the Torpedo sequence were changed by means of oligonucleotide-directed mutagenesis to residues present in snake alpha 1 subunit and rat nerve alpha 3 subunit which do not bind alpha-bungarotoxin. The fusion protein of the Torpedo sequence bound 125I-alpha-bungarotoxin with high affinity (IC50 = 2.5 x 10(-8) M from competition with unlabeled toxin, KD = 2.3 x 10(-8) M from equilibrium saturation binding data). Mutation of three Torpedo residues to snake residues, W184F, K185W, and W187S, had no effect on binding. Conversion of two additional Torpedo residues to snake, T191S and P194L, reduced alpha-bungarotoxin binding to undetectable levels. The P194L mutation alone abolished toxin binding. Mutation of three Torpedo alpha 1 residues to neuronal alpha 3-subunit residues, W187E, Y189K, and T191N, also abolished detectable alpha-bungarotoxin binding. Conversion of Try-189 to Asn which is present in the snake sequence (Y189N) abolished toxin binding. It is concluded that in the sequence of the alpha subunit of Torpedo encompassing Cys-192 and Cys-193, Try-189 and Pro-194 are important determinants of alpha-bungarotoxin binding. Tyr-189 may interact directly with cationic groups or participate in aromatic-aromatic interactions while Pro-194 may be necessary to maintain a conformation conductive to neurotoxin binding.

Structural and conformational similarity between synthetic peptides of curaremimetic neurotoxins and rabies virus glycoprotein

Antibodies were raised in rabbits against synthetic peptides corresponding to loop 2, the 'toxic' loop reacting with the acetylcholine-binding site on the nicotinic acetylcholine receptor, of curaremimetic neurotoxins and the structurally similar segment of the rabies virus glycoprotein. Some of the antibodies cross-reacted with the corresponding peptides confirming the structural similarity between the neurotoxin and glycoprotein peptides. A polyclonal antibody raised against a 29 residue glycoprotein peptide (175-203) in the presence of 0.1% sodium dodecyl sulfate reacted with native alpha-bungarotoxin and rabies virus. Circular dichroism spectroscopy of the 29 residue glycoprotein peptide and a 20 residue king cobra loop 2 peptide (25-44) revealed these peptides to be conformationally similar and composed predominantly of beta sheet structure. These results show the rabies glycoprotein segment is structurally and conformationally similar to neurotoxin loop 2. This similarity may confer on the glycoprotein the capability of interacting with the neurotoxin-binding site on the acetylcholine receptor.

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)

Synthetic peptides of neurotoxins and rabies virus glycoprotein behave as antagonists in a functional assay for the acetylcholine receptor

Peptides of portions of loop 2 (the "toxic" loop) of snake venom curare-mimetic neurotoxins (alpha-bungarotoxin and king cobra toxin b) and of a structurally similar region of the rabies virus glycoprotein were synthesized. The effect of the peptides on carbachol-induced 22Na+ flux into BC3H-1 cells, which contain nicotinic acetylcholine receptors on their surfaces, was measured. Both the neurotoxin and glycoprotein peptides inhibited ion transport with IC50 values of 10(-4) M to 7 x 10(-7) M. The most effective peptides correspond to neurotoxin loop 2 and inhibited 22Na+ flux in the micromolar range comparable to the competitive antagonist d-tubocurarine. These findings show that neurotoxin loop 2 and the corresponding rabies virus glycoprotein segment interact with the agonist binding site of teh acetylcholine receptor and that short synthetic peptides representing portions of larger molecules by themselves can exert a biological effect on a large macromolecular complex like the acetylcholine receptor.

Antibodies against an alpha-bungarotoxin-binding peptide of the alpha-subunit of the acetylcholine receptor

Polyclonal and monoclonal antibodies were raised against a peptide comprising residues 173-204 of the alpha-subunit of the acetylcholine receptor. The polyclonal and pooled monoclonal antibodies inhibited up to 50% of 125I-alpha-bungarotoxin binding to peptide 173-204. Some of the antibodies recognized native receptor but did not significantly affect alpha-bungarotoxin binding. Epitope mapping revealed that the antibodies are directed against residues 183-194 indicating this region is a major determinant of toxin binding. This region is most likely conformationally constrained in the native receptor.