Functional expression of M3, a muscarinic acetylcholine receptor subtype, in taste bud cells of mouse fungiform papillae.

Abstract

Mammalian taste bud cells (TBCs) express M1, one of five muscarinic acetylcholine receptor (mAChR) subtypes, M1-M5. However, the expression of the other four subtypes has not been examined. Also, it remained unknown if the expression of the subtype depends on the TBC type. In this study, we examined the expression of mAChR subtypes in mouse TBCs in fungiform papillae innervated by the chorda tympani nerve and those in circumvallate papillae innervated by the glossopharyngeal nerve. Here we show that acetylcholine (ACh) applied to the basolateral membrane of fungiform TBCs elevates the intracellular Ca²⁺ level in a concentration-dependent manner. The Ca²⁺ responses occur in the absence of extracellular Ca²⁺, and are inhibited by 100 nM atropine, a specific antagonist against mAChR subtypes, and 10 nM 4-DAMP, a selective antagonist against M3. RT-PCR studies suggest the expression of M3, but not the expression of the other subtypes of mAChR in fungiform TBCs. In fungiform papillae, immunohistochemical studies showed that TBCs show immunoreactivity for M3, but not for M1, and that M3 occurs on type II and type III TBCs. The expression of vesicular ACh transporter (vAChT), a transporter that packs ACh into synaptic vesicle was undetectable in fungiform TBCs. In circumvallate taste buds, RT-PCR and immunohistochemical studies showed the expression of M1, M3 and VAChT. The role of ACh on taste transductions and the innervation-dependent expression of mAChR subtypes are discussed. [J Physiol Sci. 2007;57 Suppl:S101]