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

The subunit composition and pharmacology of α-Conotoxin MII-binding (α-CtxMII) nicotinic acetylcholine receptors (nAChR) was studied by an improved [125I]-α-CtxMII membrane binding method. This binding method facilitates pharmacological studies that have been difficult to accomplish with [125I]-α-CtxMII autoradiography or α-CtxMII inhibition of [125I]-epibatidine binding. Binding densities and Kd-values obtained by this [125I]-α-CtxMII membrane binding were similar to the values obtained by autoradiography or α-CtxMII inhibition of [125I]-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: [125I]-α-CtxMII binding measures two sets of α6β2* nAChR (α4α6β2β3 or α6β2β3). Most nicotinic agonists and antagonists show monophasic inhibition of [125I]-α-CtxMII binding, indicating that α4α6β2β3 and α6β2β3 have similar binding properties. Comparison of the binding and activation profiles of α6β2* nAChR to those of other nAChR subtypes (α4β2* and β4*) indicates that these receptors have distinctly different pharmacology indicating that it may be possible to target α6β2* nAChR selectively to develop compounds that might be therapeutically useful.