The subunit composition and pharmacology of alpha-Conotoxin MII-binding nicotinic acetylcholine receptors studied by a novel membrane-binding assay.

The subunit composition and pharmacology of alpha-Conotoxin MII-binding (alpha-CtxMII) nicotinic acetylcholine receptors (nAChR) was studied by an improved [(125)I]-alpha-CtxMII membrane binding method. This binding method facilitates pharmacological studies that have been difficult to accomplish with [(125)I]-alpha-CtxMII autoradiography or alpha-CtxMII inhibition of [(125)I]-epibatidine binding. Binding densities and K(d)-values obtained by this [(125)I]-alpha-CtxMII membrane binding were similar to the values obtained by autoradiography or alpha-CtxMII inhibition of [(125)I]-epibatidine binding, verifying that each of these approaches measures the same nAChR population. Binding results with nAChR subunit-null mutant mice confirm and extend observations from earlier studies: [(125)I]-alpha-CtxMII binding measures two sets of alpha6beta2* nAChR (alpha4alpha6beta2beta3 or alpha6beta2beta3). Most nicotinic agonists and antagonists show monophasic inhibition of [(125)I]-alpha-CtxMII binding, indicating that alpha4alpha6beta2beta3 and alpha6beta2beta3 have similar binding properties. Comparison of the binding and activation profiles of alpha6beta2* nAChR to those of other nAChR subtypes (alpha4beta2* and beta4*) indicates that these receptors have distinctly different pharmacology indicating that it may be possible to target alpha6beta2* nAChR selectively to develop compounds that might be therapeutically useful.