Reduced α4 subunit expression in α4+- and α4+-/β2+- nicotinic acetylcholine receptors alters α4β2 subtype up-regulation following chronic nicotine treatment

Background and PurposeGenomic 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 42-nAChRs, in humans and laboratory animals. Here, we have evaluated whether the initial level of receptor expression affects the increase in expression. Experimental ApproachMice differing in expression of 4 and/or 2 nAChR subunits were chronically treated with saline, 0.25, 1.0 or 4.0 mgkg(-1)h(-1)nicotine. Brain preparations were analysed autoradiographically by [I-125]-epibatidine binding, immunoprecipitation and Western blotting. Key ResultsImmunochemical studies confirmed that most of the [H-3]-epibatidine binding corresponds to 42*-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, 425-nAChR did not increase in response to nicotine. Conclusions and ImplicationsThese 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.