ATP has been shown to be a taste bud afferent transmitter, but the cells responsible for, and the mechanism of, its release have not been identified. Using CHO cells expressing high-affinity neurotransmitter receptors as biosensors, we show that gustatory stimuli cause receptor cells to secrete ATP through pannexin 1 hemichannels in mouse taste buds. ATP further stimulates other taste cells to release a second transmitter, serotonin. These results provide a mechanism to link intracellular Ca 2؉ release during taste transduction to secretion of afferent transmitter, ATP, from receptor cells. They also indicate a route for cell-cell communication and signal processing within the taste bud.afferent ͉ gustation ͉ serotonin ͉ synapses G ustatory receptor cells within taste buds detect sweet, bitter, and umami tastants via G protein-coupled taste receptors. Although detailed transduction mechanisms downstream of such receptors have been elucidated (1), our understanding of the signaling from taste cells to the afferent nerve is still limited. ATP has emerged as a key afferent neurotransmitter for taste buds (2). Gustatory stimulation of taste buds also results in release of serotonin (5-HT) (3). Yet, which cells release each neurotransmitter and the mechanisms of such release are unknown. These problems are particularly enigmatic, because in taste buds, the cells that express taste receptors (i.e., ''receptor cells'') comprise a separate population from the cells that possess synapses, express synaptic proteins, and exhibit depolarizationdependent calcium influx (''presynaptic cells'') (4-6). We have used cellular biosensors (3) to measure taste-evoked transmitter release and, particularly, to identify which cells secrete ATP and 5-HT. Our results show that only receptor cells release ATP and only presynaptic cells release 5-HT. Further, we demonstrate an unexpected mechanism for nonexocytotic ATP secretion and present evidence for cell-cell signaling between receptor and presynaptic cells upon taste stimulation. ResultsWe isolated taste cells from mouse circumvallate papillae, loaded them with the Ca 2ϩ indicator Fura2-AM, and measured responses to taste stimulation and to KCl depolarization. Concurrently, we also measured transmitter release from individual taste cells using cellular biosensors (see below). Taste cells were unambiguously identified either as receptor cells or presynaptic cells by whether they responded to taste stimulation (receptor cells) or to KCl depolarization (presynaptic cells) (4). Isolated receptor and presynaptic cells were present in sufficiently low density in the recording chamber that there were no interactions (e.g., diffusible signals) between individual taste cells. The only interactions measured were between an isolated taste cell and its apposed biosensor. Taste Receptor Cells Secrete ATP via Gap Junction Hemichannels.When a Fura2-loaded CHO cell stably expressing P2ϫ2/ P2ϫ3 receptors (hereafter, ''ATP biosensor'') was positioned in close proximity to a receptor cell (Fig. 1A), we ...

Synapses between gustatory receptor cells and primary sensory afferent fibers transmit the output signal from taste buds to the CNS. Several transmitter candidates have been proposed for these synapses, including serotonin (5-HT), glutamate, acetylcholine, ATP, peptides, and others, but, to date, none has been unambiguously identified. We used Chinese hamster ovary cells stably expressing 5-HT 2C receptors as biodetectors to monitor 5-HT release from taste buds. When taste buds were depolarized with KCl or stimulated with bitter, sweet, or sour (acid) tastants, serotonin was released. KCl-and acid-induced 5-HT release, but not release attributable to sweet or bitter stimulation, required Ca 2ϩ influx. In contrast, 5-HT release evoked by sweet and bitter stimulation seemed to be triggered by intracellular Ca 2ϩ release. These experiments strongly implicate serotonin as a taste bud neurotransmitter and reveal unexpected transmitter release mechanisms.

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