Pharmacology of alpha-conotoxin MII-sensitive subtypes of nicotinic acetylcholine receptors isolated by breeding of null mutant mice.

Subtypes of nicotinic acetylcholine receptors (nAChR) containing alpha6 subunits comprise 25 to 30% of the presynaptic nAChRs expressed in striatal dopaminergic terminals in rodents and 70% in monkeys. This class of receptors, potentially important in nicotine addiction, binds alpha-conotoxin MII (alpha-CtxMII) with high affinity and is heterogeneous, consisting of several subtypes in mice, possibly an important consideration for the design of compounds that selectively activate or antagonize the alpha6 subclass of nAChRs. Selected-null mutant mice were bred to generate isolated subtypes of alpha6beta2* nAChRs expressed in vivo for assessing pharmacology of alpha6beta2* nAChRs. Binding to striatal membranes and function in synaptosomes from (alpha4-/-)(beta3+/+) and (alpha4-/-)(beta3-/-) mice were measured and compared with wild-type (alpha4+/+)(beta3+/+) mice. Gene deletions (alpha4 and beta3) decreased binding of (125)I-alpha-CtxMII without affecting affinity for alpha-CtxMII or inhibition of alpha-CtxMII binding by epibatidine or nicotine. Deletion of the alpha4 subunit substantially increased EC(50) values for both nicotine- and cytisine-stimulated alpha-CtxMII-sensitive dopamine release from striatal synaptosomes. A further increase in EC(50) values was seen upon the additional deletion of the beta3 subunit. The data indicate that one alpha-CtxMII-sensitive nAChR subtype, prevalent on wild-type dopaminergic terminals, has the lowest EC(50) for a nicotine-mediated function so far measured in mice. In conclusion, the gene deletion strategy enabled isolation of alpha6* subtypes, and these nAChR subtypes exhibited differential activation by nicotine and cytisine.