Muscarinic acetylcholine receptors operative in mouse taste cells

Abstract

A heterogeneous population of taste bud cells includes cells of three different types (type I to type III) that serve distinctive physiological functions. Evidence implicates several signaling systems in mediating cell-to-cell communications in the taste bud. In particular, a variety of receptors and their ligands known as classical neurotransmitters/neuromodulators have been identified in the taste tissue. Although potentially, they all can be involved in extracellular signaling, their significance for taste bud physiology is not well understood. Here we focused on the cholinergic system to unveil whether one operates in the mouse taste bud, and if so, to elucidate taste cells functionally expressing ACh receptors. Taste cells identified electrophysiologically were stimulated by the non-hydrolizable ACh analog carbachol and their responsiveness was evaluated by Ca2+ transients elicited by this agonist. Carbachol stimulated cellular responses in a wide range of concentrations, exhibiting EC50 = 2.5 μM on average. Carbachol-elicited Ca2+ transients were observed in the taste cell cytoplasm even with 100 nM Ca2+ in the bath, while 1 μM atropine, a muscarinic receptor antagonist, completely suppressed cellular responses. Together, these observations and negligible responses to nicotine pointed out that muscarinic receptors predominantly mediated the carabachol-dependent Ca2+ mobilization in taste cells. Notably, telenzipine, a subtype-specific antagonist of M1-receptors, inhibited carbachol responses only partially. Consistently, the M1-agonist McN-A-343 was much less effective than carbachol. These findings indicate that although M1-receptors are functional in mouse taste cells, muscarinic receptors of other types, most likely M3 and/or M5, are largely responsible for Ca2+ transients elicited by carbachol. The analysis of expression of muscarinic receptor genes with RT-PCR revealed transcripts for M1-, M3-, and M5-isoforms in preparations of the taste tissue. It thus appears that solely type I cells are responsive to cholinergic agonists and recognize those by employing multiple muscarinic receptors coupled to the phosphoinositide cascade.