Taste receptor cell responses to the bitter stimulus denatonium involve Ca2+ influx via store-operated channels

Responses to di-sodium guanosine 5'-monophosphate and monosodium L-glutamate in taste receptor cells of rat fungiform papillae

The 5'-ribonucleotide guanosine 5'-monophosphate (GMP) is used widely as an umami taste stimulus and a potent flavor enhancer as it synergistically increases the umami taste elicited by monosodium glutamate. Transduction mechanisms for GMP and its synergy with glutamate are largely unknown. Using whole-cell patch-clamp and Ca(2+) imaging, we examined responses to GMP, glutamate, and a mixture of GMP and glutamate in taste-receptor cells of rat fungiform papillae. Our electrophysiological results showed that GMP induces responses that are similar to those of glutamate, e.g., an outward current, an inward current, or a biphasic response. Our Ca(2+) imaging results showed that applications of GMP, glutamate, and the mixture increased intracellular Ca(2+) levels. Interestingly, both patch-clamp and Ca(2+) imaging showed that some taste cells can respond to GMP and glutamate independently, indicating that glutamate and GMP likely activate different receptors. Simultaneous application of GMP and glutamate resulted in synergistic responses in a subset of cells; both response intensity and number of responding cells were increased. Most responses to GMP, as well as the synergy between GMP and glutamate, were suppressed by 8-bromo-adenosine 3',5'-cyclic monophosphate (8-bromo-cAMP) in patch-clamp recordings. Together, our results suggest that intracellular cAMP- and Ca(2+)-mediated pathways are involved in umami taste transduction for GMP and its synergistic responses with glutamate.

Capsaicin modulates K+ currents from dissociated rat taste receptor cells

Chili pepper is one of most widely used spices. The main active component of chili pepper is the capsaicin. The effects of capsaicin on sensory nerve endings are well known; however, little is known regarding the direct effect of capsaicin on taste receptor cells (TRCs). In this study, patch clamp methods were used to study the effects of capsaicin on the K(+) currents in TRCs isolated from the rat circumvallate papilla. Fura-2 microspectrofluorimetry was also used to determine the effects of capsaicin on the intracellular Ca(2+) concentration ([Ca(2+)](i)). In the resting state, whole-cell experiments identified outward-rectifying K(+) currents, which were inhibited by 5 mM tetraethylammonium (TEA(+)) chloride. Voltage-dependent K(+) channels with a conductance of 55+/-4 pS (mean+/-S.E.M.; n=3), were observed in cell-attached patches. Capsaicin (500 nM) completely inhibited the outward-rectifying K(+) current in the whole-cell recordings. In cell-attached patches 500 nM capsaicin significantly reduced the open probability (P(o)) of the K(+) channels from 0.401+/-0.052 (n=3) in the resting state, to 0.018+/-0.002 (n=3, P<0.05 by unpaired t-test). In the fura-2-loaded TRCs, micromolar concentrations of capsaicin increased [Ca(2+)](i) in a dose-dependent manner, e.g., 100 microM capsaicin consistently increased the 340:380 fluorescence ratio from 1.04+/-0.05 in the resting state to 1.40+/-0.05 (n=28). These results suggest that capsaicin can enhance or modify the gustatory sensation by inhibiting the K(+) currents of the TRCs directly.

A transient receptor potential channel expressed in taste receptor cells

We used differential screening of cDNAs from individual taste receptor cells to identify candidate taste transduction elements in mice. Among the differentially expressed clones, one encoded Trpm5, a member of the mammalian family of transient receptor potential (TRP) channels. We found Trpm5 to be expressed in a restricted manner, with particularly high levels in taste tissue. In taste cells, Trpm5 was coexpressed with taste-signaling molecules such as alpha-gustducin, Ggamma13, phospholipase C-beta2 (PLC-beta2) and inositol 1,4,5-trisphosphate receptor type III (IP3R3). Our heterologous expression studies of Trpm5 indicate that it functions as a cationic channel that is gated when internal calcium stores are depleted. Trpm5 may be responsible for capacitative calcium entry in taste receptor cells that respond to bitter and/or sweet compounds.

Acetylcholine increases intracellular Ca2+ in taste cells via activation of muscarinic receptors

Previous studies suggest that acetylcholine (ACh) is a transmitter released from taste cells as well as a transmitter in cholinergic efferent neurons innervating taste buds. However, the physiological effects on taste cells have not been established. I examined effects of ACh on taste-receptor cells by monitoring [Ca2+]i. ACh increased [Ca2+]i in both rat and mudpuppy taste cells. Atropine blocked the ACh response, but D-tubocurarine did not. U73122, a phospholipase C inhibitor, and thapsigargin, a Ca2+-ATPase inhibitor that depletes intracellular Ca2+ stores, blocked the ACh response. These results suggest that ACh binds to M1/M3/M5-like subtypes of muscarinic ACh receptors, causing an increase in inositol 1,4,5-trisphosphate and subsequent release of Ca2+ from the intracellular stores. A long incubation with ACh induced a transient response followed by a sustained phase of [Ca2+]i increase. In Ca2+-free solution, the sustained phases disappeared, suggesting that Ca2+ influx is involved in the sustained phase. Depletion of Ca2+ stores by thapsigargin alone induced Ca2+ influx. These findings suggest that Ca2+ store-operated channels may be present in taste cells and that they may participate in the sustained phase of [Ca2+]i increase. Immunocytochemical experiments indicated that the M1 subtype of muscarinic receptors is present in both rat and mudpuppy taste cells.