Pharmacology of α-Conotoxin MII-Sensitive Subtypes of Nicotinic Acetylcholine Receptors Isolated by Breeding of Null Mutant Mice

Subtypes of nicotinic acetylcholine receptors (nAChR) containing α6 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 α-conotoxin MII (α-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 α6 subclass of nAChRs. Selected-null mutant mice were bred to generate isolated subtypes of α6β2* nAChRs expressed in vivo for assessing pharmacology of α6β2* nAChRs. Binding to striatal membranes and function in synaptosomes from (α4–/–)(β3+/+) and (α4–/–)(β3–/–) mice were measured and compared with wild-type (α4+/+)(β3+/+) mice. Gene deletions (α4 and β3) decreased binding of ¹²⁵I-α-CtxMII without affecting affinity for α-CtxMII or inhibition of α-CtxMII binding by epibatidine or nicotine. Deletion of the α4 subunit substantially increased EC₅₀ values for both nicotine- and cytisine-stimulated α-CtxMII-sensitive dopamine release from striatal synaptosomes. A further increase in EC₅₀ values was seen upon the additional deletion of the β3 subunit. The data indicate that one α-CtxMII-sensitive nAChR subtype, prevalent on wild-type dopaminergic terminals, has the lowest EC₅₀ for a nicotine-mediated function so far measured in mice. In conclusion, the gene deletion strategy enabled isolation of α6* subtypes, and these nAChR subtypes exhibited differential activation by nicotine and cytisine.