Abstract
Nicotine evokes chorda tympani (CT) taste nerve responses and an aversive behavior in Trpm5 knockout (KO) mice. The agonists and antagonists of nicotinic acetylcholine receptors (nAChRs) modulate neural and behavioral responses to nicotine in wildtype (WT) mice, Trpm5 KO mice and rats. This indicates that nicotine evokes bitter taste by activating a Trpm5-dependent pathway and a Trpm5-independent but nAChR-dependent pathway. Rat CT responses to ethanol are also partially inhibited by nAChR blockers, mecamylamine and dihydro-β-erythroidine. This indicates that a component of the bitter taste of ethanol is also nAChR-dependent. However, at present the expression and localization of nAChR subunits has not been investigated in detail in taste receptor cells (TRCs). To this end, in situ hybridization, immunohistochemistry and q-RT-PCR techniques were utilized to localize nAChR subunits in fungiform and circumvallate TRCs in WT mice, Trpm5-GFP transgenic mice, nAChR KO mice, and rats. The expression of mRNAs for α7, β2 and β4 nAChR subunits was observed in a subset of rat and WT mouse circumvallate and fungiform TRCs. Specific α3, α4, α7, β2, and β4 antibodies localized to a subset of WT mouse circumvallate and fungiform TRCs. In Trpm5-GFP mice α3, α4, α7, and β4 antibody binding was observed in a subset of Trpm5-positive circumvallate TRCs. Giving nicotine (100 μg/ml) in drinking water to WT mice for 3 weeks differentially increased the expression of α3, α4, α5, α6, α7, β2 and β4 mRNAs in circumvallate TRCs to varying degrees. Giving ethanol (5%) in drinking water to WT mice induced an increase in the expression of α5 and β4 mRNAs in circumvallate TRCs with a significant decrease in the expression of α3, α6 and β2 mRNAs. We conclude that nAChR subunits are expressed in Trpm5-positive TRCs and their expression levels are differentially altered by chronic oral exposure to nicotine and ethanol.