SummaryAfferent output in type II taste cells is mediated by ATP liberated through ion channels. It is widely accepted that pannexin 1 (Panx1) channels are responsible for ATP release in diverse cell types, including taste cells. While biophysical evidence implicates slow deactivation of ion channels following ATP release in taste cells, recombinant Panx1 activates and deactivates rapidly. This inconsistency could indicate that the cellular context specifies Panx1 functioning. We cloned Panx1 from murine taste tissue, and heterologously expressed it in three different cell lines: HEK-293, CHO and neuroblastoma SK-N-SH cells. In all three cell lines, Panx1 transfection yielded outwardly rectifying anion channels that exhibited fast gating and negligible permeability to anions exceeding 250 Da. Despite expression of Panx1, the host cells did not liberate ATP upon stimulation, making it unclear whether Panx1 is involved in taste-related ATP secretion. This issue was addressed using mice with genetic ablation of the Panx1 gene. The ATP-biosensor assay revealed that, in taste cells devoid of Panx1, ATP secretion was robust and apparently unchanged compared with the control. Our data suggest that Panx1 alone forms a channel that has insufficient permeability to ATP. Perhaps, a distinct subunit and/or a regulatory circuit that is absent in taste cells is required to enable a high ATP-permeability mode of a native Panx1-based channel.

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TRPM5 are ion channels belonging to the TRP family, which demonstrate a nonselective permeability for monovalent cations and are activated by an increase in the intracellular calcium level. TRPM5 are present in taste receptor cells of type II responsible for reception of bitter, sweet, and umami taste sensations. Knockout of the trpm5 gene in mice results in a nearly complete loss of sensitivity to taste stimuli of the above-mentioned modalities (taste blindness). The physiological activity of TRPM5 in taste receptive cells has practically not been studied. Using a patch-clamp technique, we carried out a comparative analysis of the properties of recombinant TRPM5 and Ca 2+ -activated membrane channels in type-II taste cells in mice. Dialysis of the studied cells with a high-Ca 2+ solution and application of a calcium ionophore, ionomycin, caused activation of outward-rectification ion channels permeable for Na + , Cs + , and K + in CHO-strain cells with exogenous TRPM5. These channels were blocked by 100 µM triphenylphosphine oxide (TPPO). Calcium-activated channels in type-II taste cells also possessed analogous properties. Application of the calcium ionophore ionomycin or a stepwise increase in the intracellular Ca 2+ level using photolysis (uncaging) caused activation of channels nonselective with respect to Na + and Cs + and impermeable for N-methyl-D-glucamine (NMDG + ). These channels had the current-voltage characteristics of outward rectification and a high thermosensitivity (Q 10 = 6.7 ± 0.5); they could be blocked by TPPO. It should be emphasized that TRPM5 were specific with respect to type-II cells. An increase in the intracellular calcium level induced the appearance of Cl -current in type-I cells and did not influence the basic current in type-III cells.

It was shown that physiological processes in taste buds (peripheral sensory gustatory organs in vertebrates) are realized with the involvement of several signal systems. In these structures, a number of "classical" neurotransmitters, including glutamate, serotonin, GABA, ATP, noradrenaline, and others, as well as receptors to these agents, were identified. The physiological roles of the above systems (separate ones and all as a whole) remain, however, far from final elucidation. We studied purinergic and cholinergic systems in the taste buds. Based on the data obtained in behavioral experiments using knockout animals, which indicated that ATP is an afferent neurotransmitter, we found stimulation-induced secretion of ATP by type-II cells. The release of ATP does not require the entry of external calcium and is mediated by ion channels permeable for ATP. The obtained data allowed us to explain the fact that classical synaptic structures are absent in the type-II cells. The type-I cells coat other elements including type-II cells; they provide formation of compartments in the intercellular space of the taste buds (this limits ATP diffusion). We showed that taste cells of just type I mostly generate calcium signals in response to the action of ATP and acetylcholine. These cell responses are generated with the involvement of metabotropic purine receptors (isoforms P2Y1, P2Y2, and P2Y4) and muscarinic receptors (isoforms M1, M3, and M5), respectively. Functioning of these receptors is combined with a phosphoinositide cascade, mobilization of intracellular Са 2+ , and subsequent activation of calciumactivated Cl -channels. It seems probable that purinergic and cholinergic signal systems in type-I cells are elements of negative feedback in the taste buds, which promote the process of adaptation to the action of gustatory stimuli.

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