A mature taste bud contains multiple types of taste cells conventionally divided into three types. Type I cells constitute the majority of cells within each bud. They enwrap other cells with flattened processes, express proteins associated with neurotransmitter reuptake or catabolism, and form no apparent specialized contacts with the sensory nerve fibers. Type II cells are the receptor cells for the taste qualities of sweet, bitter, and umami (savory) mediated by the taste receptor (TR) family of G-protein coupled taste receptors and the related phospholipase C (PLC)-mediated downstream cascade. The points of contact between the type II cells and nerve fibers often exhibit a non-conventional specialization involving subsurface cisternae and atypical mitochondria. Type III cells do not express the TR family taste receptor proteins or downstream cascade, but do form conventional synapses with the afferent nerves, and are required for sour taste transduction but not for transmission of taste information from type II cells to the afferent nerves.
T1Rs mediate sweet and umami taste. They are functional only as heterodimers, with T1R3 serving as an obligate partner for both the umami receptor (T1R1 + T1R3) and the sweet receptor (T1R2 + T1R3). The sweet taste of mono- and oligosaccharides in the GPCR T1R2/T1R3 dimer responds to glucose and a number of artificial sweeteners by activating a specialized G-protein, gustducin, which is a member of the G alpha i family. Gustducin stimulates phosphodiesterase (PDE) and results in cyclic nucleotide degradation, but in parallel the umami receptor T1R1/T1R3 activates the Gbeta gama subunits which activate PLCbeta2, leading to IP3-mediated Ca2+ release. The consequent elevation of cytoplasmic Ca2+ activates the Ca2+-sensitive transient receptor potential channel M5 (TRPM5) triggering membrane depolarization and activation of voltage-gated Ca2+ channels. Activated gustducin stimulates PDE to hydrolyze cAMP; the decreased cAMP may disinhibit cyclic nucleotide-inhibited channels to elevate intracellular Ca2+. The balance between the activities of adenylyl cyclases and phosphodiesterases (PDEs) determines the level of cAMP in cells.
Bitter taste is mediated by the T2R GPCRs, products of the Tas2R family. T2Rs are classical 'A' type receptors similar in structure to opsins and olfactory receptors. They have short N-terminal domains, with ligand binding in the extracellular loops and transmembrane domains. The family consists of about 30 members in mammals, each of which binds structurally similar bitter compounds. These receptors have been considered to function as monomers, but recent data suggest that they can also form functional oligomers. T2R GPCRs that respond to bitter compounds activate gustation which colocalizes with Ggamma13 and Gbeta3 stimulating PDE.
PKD2L1 is expressed in a subset of taste cells different from those expressing either T1R or T2R receptors. The genetic ablation of PKD2L1-expressing cells in taste buds has been interpreted as strong support for the role of this channel in sour transduction. Yet the possibility exists that other channels and receptors may contribute to detection of acids by taste buds. The functional candidate sour taste receptor comprises two subunits: PKD2L1 and PKD1L3.
In the taste system, ATP is necessary for transmission of information from the sensory cells to the afferent nerve fibers. Genetic elimination of the P2X receptors on the sensory nerve fibers (P2X2 and P2X3) totally eliminates transmission of the signal from taste receptor cells to nerve fibers. Taste compounds induce the release of ATP through pannexin1 (Panx1) hemichannels which activate P2X and P2Y receptors on sensory nerve fibers. In addition to activation of afferent nerves, the released ATP (and its breakdown product ADP) activate purinergic receptors P2X and P2Y on the type II cells themselves to potentiate further ATP release.