Receptors and signaling for sour and salty: the ionic taste qualities

This review outlines the specific ion channels, taste cell types, downstream signaling, and neural transmission mechanisms involved in the perception of sour and salty tastes. Both of these taste qualities arise from ionic stimuli and so can be detected via ion channels rather than G protein-coupled receptors. Sour taste, triggered by protons in acidic substances such as lemon juice and vinegar, is mediated within taste buds exclusively by type III taste cells. Protons enter the receptor cell through the apically located proton-selective channel OTOP1, thereby directly depolarizing the taste cell and reducing intracellular pH, which may block K+ channels to further amplify the response. Type III cells then release serotonin (5-HT) via conventional chemical synapses to activate nerve fibers. Salt taste involves both type II and type III cells. In a subset of type II cells, low NaCl concentrations, which are appetitive, pass through the apically located, sodium-selective, and amiloride-sensitive epithelial sodium channel. In response, these cells generate action potentials and release ATP through the CALHM1/3 "channel synapse" to stimulate gustatory afferents. High concentrations of NaCl, KCl, and NH4Cl, which are aversive, are detected by a different subset of type II cells and type III cells. Although the depolarizing mechanism for high concentrations of NaCl and KCl remains unidentified, NH4CL is detected by OTOP1 in type III taste receptor cells, and the chloride channel TMC4 may contribute to repolarization of the receptor cells, to enhance their responsivity to the salt stimulus.