Expression of nigrostriatal α6-containing nicotinic acetylcholine receptors is selectively reduced, but not eliminated, by β3 subunit gene deletion

mRNAs for the neuronal nicotinic acetylcholine receptor (nAChR) alpha 6 and beta 3 subunits are abundantly expressed and colocalized in dopaminergic cells of the substantia nigra and ventral tegmental area. Studies using subunit-null mutant mice have shown that alpha 6- or beta 3-dependent nAChRs bind alpha-conotoxin MII (alpha-CtxMII) with high affinity and modulate striatal dopamine release. This study explores the effects of beta 3 subunit-null mutation on striatal and midbrain nAChR expression, composition, and pharmacology. Ligand binding and immunoprecipitation experiments using subunit-specific antibodies indicated that beta 3-null mutation selectively reduced striatal alpha 6(*) nAChR expression by 76% versus beta 3(+/+) control. Parallel experiments showed a smaller reduction in both midbrain alpha 3(*) and alpha 6(*) nAChRs (34 and 42% versus beta 3(+/+) control, respectively). Sedimentation coefficient determinations indicated that residual alpha 6(*) nAChRs in beta 3(-/-) striatum were pentameric, like their wild-type counterparts. Immunoprecipitation experiments on immunopurified beta 3(*) nAChRs demonstrated that almost all wild-type striatal beta 3(*) nAChRs also contain alpha 4, alpha 6, and beta 2 subunits, although a small population of non-beta 3 alpha 6(*) nAChRs is also expressed. beta 3 subunit incorporation seemed to increase alpha 4 participation in alpha 6 beta 2(*) complexes. I-125-Epibatidine competition binding studies showed that the alpha-CtxMII affinity of alpha 6(*) nAChRs from the striata of beta 3(-/-) mice was similar to those isolated from beta 3(-/-) animals. Together, the results of these experiments show that the beta 3 subunit is important for the correct assembly, stability and/or transport of alpha 6(*) nAChRs in dopaminergic neurons and influences their subunit composition. However, beta 3 subunit expression is not essential for the expression of alpha 6(*), high-affinity alpha-CtxMII binding nAChRs.