Chemical Senses Advance Access published online on September 13, 2007
Chemical Senses, doi:10.1093/chemse/bjm065
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Functional Expression of M3, a Muscarinic Acetylcholine Receptor Subtype, in Taste Bud Cells of Mouse Fungiform Papillae
Department of Brain Science and Engineering, Hibikino 2-4, Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu 808-0196, Japan
Correspondence to be sent to: Kiyonori Yoshii, Hibikino 2-4, Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu 808-0196, Japan. e-mail: yoshii{at}brain.kyutech.ac.jp
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Abstract |
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Taste bud cells (TBCs) express various neurotransmitter receptors assumed to facilitate or modify taste information processing within taste buds. We investigated the functional expression of muscarinic acetylcholine receptor (mAChR) subtypes, M1–M5, in mouse fungiform TBCs. ACh applied to the basolateral membrane of TBCs elevates the intracellular Ca2+ level in a concentration-dependent manner with the 50% effective concentration of 0.6 µM. The Ca2+ responses occur in the absence of extracellular Ca2+ and are inhibited by atropine, a selective antagonist against mAChRs. The order of 50% inhibitory concentration examined with a series of antagonists selective to mAChR subtypes shows the expression of M3 on TBCs. Perforated whole-cell voltage clamp studies show that 1 µM ACh blocks an outwardly rectifying current and that 100 nM atropine reverses the block. Reverse transcriptase–mediated polymerase chain reaction studies suggest the expression of M3 but not the other mAChR subtypes. Immunohistochemical studies show that phospholipase Cß-immunoreactive TBCs and synaptosome-associated protein of 25 kDa–immunoreactive nerve endings are immunoreactive to a transporter that packs ACh molecules into synaptic vesicles (vesicular acetylcholine transporter). These results show that M3 occurs on a few fungiform TBCs and suggest that a few nerve endings, and probably a few TBCs, release ACh by exocytosis. The role of ACh in taste responses is discussed.
Key words: Ca2+ imaging, immunohistochemistry, pharmacology, RT–PCR, vesicular acetylcholine transporter, whole-cell clamp
Accepted 8 August 2007