nAChRs form a family of subtypes formed by five subunits (Gotti and Clementi 2004; Gotti et al. 2006). The subunits are encoded by nine (α2–α10) and three (β2–β4) subunit genes and the two major subfamilies are classified as either αBgtx-sensitive or αBgtx-insensitive. αBgtx-sensitive nAChRs are mostly homopentameric and the αBgtx-insensitive nAChRs are only heteropentameric, consisting of both α and β subunits. αBgtx-insensitive nAChRs are mostly comprised of the α4β2* subtype and may include a primary α6 or accessory α5 or β3 subunits. α5 and β3 do not form functional channels alone, but combined with other α-β combinations, they contribute to receptor targeting and localization in neuronal plasma membrane domains, and modify other receptor properties (Gotti et al. 2006, 2007).
