Neurotransmitter receptors on taste bud cells (TBCs) and taste nerve fibres are likely to contribute to taste transduction by mediating the interaction among TBCs and that between TBCs and taste nerve fibres. 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. Ca 2+ imaging showed that similarly applied 1 μM ATP, 30 μM BzATP (a P2X 7 agonist), or 1 μM 2MeSATP (a P2Y 1 and P2Y 11 agonist) increased intracellular Ca 2+ concentration, but 100 μM UTP (a P2Y 2 and P2Y 4 agonist) and α,β-meATP (a P2X agonist except for P2X 2 , P2X 4 and P2X 7 ) did not. RT-PCR suggested the expression of P2X 2 , P2X 4 , P2X 7 , P2Y 1 , P2Y 13 and P2Y 14 among the seven P2X subtypes and seven P2Y subtypes examined. Immunohistostaining confirmed the expression of P2X 2 . 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 P2X 7 blocker). These results showed for the first time the functional expression of P2X 2 and P2X 7 on TBCs. The roles of P2 receptor subtypes in the taste transduction, and the renewal of TBCs, are discussed.

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Taste receptor cells fire action potentials in response to taste substances to trigger non-exocytotic neurotransmitter release in type II cells and exocytotic release in type III cells. We investigated possible differences between these action potentials fired by mouse taste receptor cells using in situ whole-cell recordings, and subsequently we identified their cell types immunologically with cell-type markers, an IP3 receptor (IP3 R3) for type II cells and a SNARE protein (SNAP-25) for type III cells. Cells not immunoreactive to these antibodies were examined as non-IRCs. Here, we show that type II cells and type III cells fire action potentials using different ionic mechanisms, and that non-IRCs also fire action potentials with either of the ionic mechanisms. The width of action potentials was significantly narrower and their afterhyperpolarization was deeper in type III cells than in type II cells. Na(+) current density was similar in type II cells and type III cells, but it was significantly smaller in non-IRCs than in the others. Although outwardly rectifying current density was similar between type II cells and type III cells, tetraethylammonium (TEA) preferentially suppressed the density in type III cells and the majority of non-IRCs. Our mathematical model revealed that the shape of action potentials depended on the ratio of TEA-sensitive current density and TEA-insensitive current one. The action potentials of type II cells and type III cells under physiological conditions are discussed.

Single taste buds in mouse fungiform papillae consist of ≈50 elongated cells (TBCs), where fewer than three TBCs have synaptic contacts with taste nerves. We investigated whether the non‐innervated TBCs were chemosensitive using a voltage‐sensitive dye, tetramethylrhodamine methyl ester (TMRM), under in situ optical recording conditions. Prior to the optical recordings, we investigated the magnitude and polarity of receptor potentials under in situ whole‐cell clamp conditions. In response to 10 mM HCl, several TBCs were depolarized by ≈25 mV and elicited action potentials, while other TBCs were hyperpolarized by ≈12 mV. The TBCs eliciting hyperpolarizing receptor potentials also generated action potentials on electrical stimulation. A mixture of 100 mM NaCl, 10 mM HCl and 500 mM sucrose depolarized six TBCs and hyperpolarized another three TBCs out of 13 identified TBCs in a taste bud viewed by optical section. In an optical section of another taste bud, 1 M NaCl depolarized five TBCs and hyperpolarized another two TBCs out of 11 identified TBCs. The number of chemosensitive TBCs was much larger than the number of innervated TBCs in a taste bud, indicating the existence of chemosensitivity in non‐innervated TBCs. There was a tendency for TBCs eliciting the same polarity of receptor potential to occur together in taste buds. We discuss the role of non‐innervated TBCs in taste information processing.

Lucifer Yellow CH (LY) is a membrane-impermeant, fluorescent dye. Once introduced into a cell, LY diffuses throughout the entire intracellular space and allows fluorescent visualization of the outline of the cell (Fig. 1). Therefore, electrophysiologists have added LY to electrode solutions to mark recorded cells as well as to study their morphology. Since early electrophysiological studies involved the use of high-resistant microelectrodes that released only small amounts of LY, little attention was paid to the pharmacology and effects of this dye. However, in whole-cell patch-clamp experiments, where the electrode solution readily dialyses the cell (Hamill et al. 1981), the effect of LY could not be negated.