Reduced alpha4 subunit expression in alpha4+- and alpha4+-/beta2+- nicotinic acetylcholine receptors alters alpha4beta2 subtype up-regulation following chronic nicotine treatmenpf

Background and Purpose: Genomic analysis has shown many variants in both CHRNA4 and CHRNB2, genes which encode the ?4 and ?2 subunits of nicotinic ACh receptors (nAChR) respectively. Some variants influence receptor expression, raising the possibility that CHRNA4 variants may affect response to tobacco use in humans. Chronic exposure to nicotine increases expression of nAChRs, particularly ?4?2-nAChRs, in humans and laboratory animals. Here, we have evaluated whether the initial level of receptor expression affects the increase in expression. Experimental Approach: Mice differing in expression of ?4 and/or ?2 nAChR subunits were chronically treated with saline, 0.25, 1.0 or 4.0 mg·kg-1·h-1nicotine. Brain preparations were analysed autoradiographically by [125I]-epibatidine binding, immunoprecipitation and Western blotting. Key Results: Immunochemical studies confirmed that most of the [3H]-epibatidine binding corresponds to ?4?2*-nAChR and that increases in binding correspond to increases in ?4 and ?2 proteins. Consistent with previous reports, the dose-dependent increase in nAChR in wild-type mice following chronic nicotine treatment, measured with any of the methods, reached a maximum. Although receptor expression was reduced by approximately 50% in ?2+- mice, the pattern of response to chronic treatment resembled that of wild-type mice. In contrast, both ?4+- and ?4+-/?2+- exhibited relatively greater up-regulation. Consistent with previous reports, ?4?2?5-nAChR did not increase in response to nicotine. Conclusions and Implications: These results indicate that mice with reduced expression of the ?4 nAChR subunit have a more robust response to chronic nicotine than mice with normal expression of this subunit