Abstract
ATP, an intracellular high energy phosphate compound, extracellularly functions as a neurotransmitter or neuromodulator. Receptors for ATP, P2 receptors, consist of seven ionotropic (P2X) and eight G-protein-coupled (P2Y) receptor subtypes. Although the involvement of two P2X and four P2Y subtypes were shown in taste responses, that of other subtypes remained to be investigated. We investigated the functional expression of P2 receptor subtypes on TBCs of mouse fungiform papillae. Electrophysiological studies showed that 100 μM ATP applied to their basolateral membranes either depolarized or hyperpolarized a few cells per taste bud. Ca2+-imaging showed that similarly applied 1 μM ATP, 30 μM BzATP (a P2X7 agonist), or 1 μM 2MeSATP (a P2Y1 and P2Y11 agonist) increased intracellular Ca2+ concentration, but 100 μM UTP (a P2Y2 and P2Y4 agonist) and α,β-meATP (a P2X agonist except for P2X2, P2X4, and P2X7) did not. RT-PCR suggested the expression of P2X2, P2X4, P2X7, P2Y1, P2Y13, and P2Y14 among the seven P2X subtypes and seven P2Y subtypes examined. Immunohistostaining confirmed the expression of P2X2 and P2X7. The exposure of the basolateral membranes to 3 mM ATP for 30 min caused the uptake of Lucifer Yellow CH in a few TBCs per taste bud. This was antagonized by 100 μM PPADS (a non-selective P2 blocker) and 1 μM KN-62 (a P2X7 blocker). These results showed for the first time the functional expression of P2X2 and P2X7 on TBCs. The roles of P2 receptor subtypes in the taste transduction, and the renewal of TBCs, are discussed. [J Physiol Sci. 2007;57 Suppl:S39]
