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 ...

Recognition of sweet, bitter and umami tastes requires the non-vesicular release from taste bud cells of adenosine 5′-triphosphate (ATP), which acts as a neurotransmitter to activate afferent neural gustatory pathways1. However, how ATP is released to fulfill this function is not fully understood. Here we show that calcium homeostasis modulator 1 (CALHM1), a voltage-gated ion channel2,3, is indispensable for taste stimuli-evoked ATP release from sweet-, bitter- and umami-sensing taste bud cells. Calhm1 knockout mice have severely impaired perceptions of sweet, bitter and umami compounds, whereas sour and salty taste recognition remains mostly normal. Calhm1 deficiency affects taste perception without interfering with taste cell development or integrity. CALHM1 is expressed specifically in sweet/bitter/umami-sensing type II taste bud cells. Its heterologous expression induces a novel ATP permeability that releases ATP from cells in response to manipulations that activate the CALHM1 ion channel. Knockout of Calhm1 strongly reduces voltage-gated currents in type II cells and taste-evoked ATP release from taste buds without affecting the excitability of taste cells to taste stimuli. Thus, CALHM1 is a voltage-gated ATP release channel required for sweet, bitter and umami taste perception.

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Gustatory stimuli are detected by taste buds and transmitted to the hindbrain via sensory afferent neurons. Whether each taste quality (sweet, bitter etc.) is encoded by separate neurons (“labeled lines”) remains controversial. We used mice expressing GCaMP3 in geniculate ganglion sensory neurons to investigate taste-evoked activity. Using confocal calcium imaging, we recorded responses to oral stimulation with prototypic taste stimuli. Up to 69% of neurons respond to multiple tastants. Moreover, neurons tuned to a single taste quality at low concentration become more broadly tuned when stimuli are presented at higher concentration. Responses to sucrose and monosodium glutamate are most related. Although mice prefer dilute NaCl solutions and avoid concentrated NaCl, we found no evidence for two separate populations of sensory neurons that encode this distinction. Together, our data suggest that taste is encoded by activity in patterns of peripheral sensory neurons and challenge the notion of strict labeled line coding.

Taste buds are innervated by neurons whose cell bodies reside in cranial sensory ganglia. Studies on the functional properties and connectivity of these neurons are hindered by the lack of markers to define their molecular identities and classes. The mouse geniculate ganglion contains chemosensory neurons innervating lingual and palatal taste buds and somatosensory neurons innervating the pinna. Here, we report single cell RNA sequencing of geniculate ganglion neurons. Using unbiased transcriptome analyses, we show a pronounced separation between two major clusters which, by anterograde labeling, correspond to gustatory and somatosensory neurons. Among the gustatory neurons, three subclusters are present, each with its own complement of transcription factors and neurotransmitter response profiles. The smallest subcluster expresses both gustatory- and mechanosensory-related genes, suggesting a novel type of sensory neuron. We identify several markers to help dissect the functional distinctions among gustatory neurons and address questions regarding target interactions and taste coding.

Highlights d ipRGCs survive and regenerate axons well after injury d RNA-seq reveals unique sets of genes enriched in injured ipRGCs, including Thbs1 and Sdc1 d Neural THBS1 promotes axon regeneration in various RGC types d Neural THBS1 promotes axon regeneration in a syndecan1dependent manner

. Acid-sensitive two-pore domain potassium (K 2 P) channels in mouse taste buds. J Neurophysiol 92: 1928 -1936. First published May 12, 2004 10.1152 10. / jn.00273.2004 taste is postulated to result from intracellular acidification that modulates one or more acid-sensitive ion channels in taste receptor cells. The identity of such channel(s) remains uncertain. Potassium channels, by regulating the excitability of taste cells, are candidates for acid transducers. Several 2-pore domain potassium leak conductance channels (K 2 P family) are sensitive to intracellular acidification. We examined their expression in mouse vallate and foliate taste buds using RT-PCR, and detected TWIK-1 and -2, TREK-1 and -2, and TASK-1. Of these, TWIK-1 and TASK-1 were preferentially expressed in taste cells relative to surrounding nonsensory epithelium. The related TRESK channel was not detected, whereas the acid-insensitive TASK-2 was. Using confocal imaging with pH-, Ca 2ϩ -, and voltage-sensitive dyes, we tested pharmacological agents that are diagnostic for these channels. Riluzole (500 M), selective for TREK-1 and -2 channels, enhanced acid taste responses. In contrast, halothane (Յ ϳ17 mM), which acts on TREK-1 and TASK-1 channels, blocked acid taste responses. Agents diagnostic for other 2-pore domain and voltage-gated potassium channels (anandamide, 10 M; Gd 3ϩ , 1 mM; arachidonic acid, 100 M; quinidine, 200 M; quinine, 100 mM; 4-AP, 10 mM; and TEA, 1 mM) did not affect acid responses. The expression of 2-pore domain channels and our pharmacological characterization suggest that a matrix of ion channels, including one or more acid-sensitive 2-pore domain K channels, could play a role in sour taste transduction. However, our results do not unambiguously identify any one channel as the acid taste transducer.

Although adenosine triphosphate (ATP) is known to be an afferent transmitter in the peripheral taste system, serotonin (5-HT) and norepinephrine (NE) have also been proposed as candidate neurotransmitters and have been detected immunocytochemically in mammalian taste cells. To understand the significance of biogenic amines in taste, we evaluated the ability of taste cells to synthesize, transport, and package 5-HT and NE. We show by reverse transcriptase-polymerase chain reaction and immunofluorescence microscopy that the enzymes for 5-HT synthesis, tryptophan hydroxylase (TPH) and aromatic amino acid decarboxylase (AADC) are expressed in taste cells. In contrast, enzymes necessary for NE synthesis, tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) are absent. Both TH and DBH are expressed in nerve fibers that penetrate taste buds. Taste buds also robustly express plasma membrane transporters for 5-HT and NE. Within the taste bud NET, a specific NE transporter, is expressed in some presynaptic (type III) and some glial-like (type I) cells but not in receptor (type II) cells. By using enzyme immunoassay, we show uptake of NE, probably through NET in taste epithelium. Proteins involved in inactivating and packaging NE, including catechol-O-methyltransferase (COMT), monoamine oxidase-A (MAO-A), vesicular monoamine transporter (VMAT1,2) and chromogranin A (ChrgA), are also expressed in taste buds. Within the taste bud, ChrgA is found only in presynaptic cells and may account for dense-cored vesicles previously seen in some taste cells. In summary, we postulate that aminergic presynaptic taste cells synthesize only 5-HT, whereas NE (perhaps secreted by sympathetic fibers) may be concentrated and repackaged for secretion.