 Bitter taste is detected by about 30 different types of T2R chemosensory receptors. Each receptor represents a single transmembrane protein coupled to its own G protein. After receptor activation by the tastant, the G protein stimulates the enzyme phospholipase C, leading to increased intracellular production of inositol 1,4,5-trisphosphate (IP3), a second messenger molecule. IP3 in turn activates taste-specifc Na channels causing influx of Na ions, thus depolarizing the neuroepithelial cell. Depolarization of the plasma membrane causes voltage-gated Ca2 channels in neuroepithelial cells to open. Increasing the concentration of intracellular Ca2 levels either by influx of extracellular Ca2 into the cell (the effect of depolarization) or by its release from intracellular stores (direct IP3 stimulation) results in the release of neurotransmitter molecules,
FIGURE 16.6 • Diagram of taste receptors and their signaling mechanism. a. This diagram shows the signaling mechanism of bitter, sweet and umami receptors in the neuroepithelial cells. These cells selectively express only one class of receptor proteins; for simplicity all three taste receptors are depicted in the apical cell membrane. See text for details. PLC – phospholipase C, IP2–inositol-1,4-diphosphate, IP3 – inositol 1,4,5-trisphosphate (IP3). b. Signaling mechanism in sour sensation is generated by H protons that primary blocks K channels. The H protons enter the cell via amiloride-sensitive Na channels and through taste-specific H channels (PKD1L3 and PKD2L1) exclusively expressed in cells involved in sour taste transduction. c. Salty sensation derives from Na ions that enter the neuroepithelial cells through the amiloride-sensitive Na channels. Intracellular Na causes a depolarization of membrane and activation of additional voltage-sensitive Na and Ca2 channels. Calcium mediated release of neurotransmitters from synaptic vesicles results in stimulating gustatory nerve fiber.
which generate nerve impulses along the gustatory afferent nerve fiber (Fig 16.6a).
 Sweet taste receptors are also G protein–coupled receptors. In contrast to the bitter taste receptors, they have two protein subunits, T1R2 and T1R3. The sweet tastants bound to these receptors activate the same second messanger system cascade of reactions that the bitter receptors do (see Fig 16.6a).
 Umami taste is linked to certain amino acids (e.g., L-glutamate, aspartate, and related compounds) and is common to asparagus, tomatoes, cheese, and meat. Umami taste receptors are very similar to sweet receptors; they are also comprised of two subunits. One subunit, T1R3, is identical to that in the sweet receptor, but the second subunit formed by the T1R1 protein is unique for umami receptors (see Fig 16.6a). The transduction process is identical to that described previously for bitter taste pathways. Monosodium glutamate, added to many foods to enhance their taste (and the main ingredient of soy sauce), stimulates the umami receptors.
The mechanism of transtuction can be similar to several tastes (i.e., bitter or sweet); however, it is important to remember that neuroepithelial cells selectively express only one class of receptor proteins. Therefore, the messages about bitterness or sweetness from eating food are transferred to the CNS along different nerve fibers.
Sodium ions and hydrogen protons, which are responsible for salty and sour taste respectively, act directly on ion channels.
Signaling mechanisms, in the case of sour and salty tastes, are similar to other signaling mechanisms found in synapses and neuromuscular junctions.