2. Taste sensation
It is the taste cells located in the taste buds of oral epithelium that are responsible for the perception of taste. There are three types of taste cells, including glial-like cells (Type I), sweet/bitter/umami detectors (Type II), and sour receptor cells (Type III) (Figure 1a), which are mainly distributed in circumvallate (CV) papillae, foliate (FL) papillae, and fungiform (FF) papillae[34,35]. All three kinds of cells are derived from K5+/K14+ cells, which form the precursors, Shh+ basal cells, of all three taste cell types in low level of β-catenin[34] (Figure 1b) and an average taste cell lifespan of 8-12 days in mammalians [36] and 10-14 in rodents, although different types of taste cells showed different lifespan[37]. Some taste cells transmit taste stimuli through taste receptors and then generate second messengers while others allow the stimulus to be transported into the cytoplasm to directly activate downstream reactions[38]. The most studied is the type II taste receptors, which means that bitter, sweet, and umami tastes use GPCRs and second messenger signaling mechanisms to transmit taste signals. The sour and salty stimulus use ion channels for transduction[39,40].
The information transmission pathway of each taste is different. The sense of salty taste is mainly completed by the specific sodium salt taste receptor epithelial sodium channel (epithelial sodium channel, ENaC) and the non-specific sodium salt taste receptor capsaicin receptor 1 (vanilloid receptor-1 nonselective cation channel 1, TRPV1). It is recognized on the front of the tongue. Sour taste is the taste sensation produced by H+ entering the oral cavity to stimulate the tongue mucosa. Transient receptor potential (TRP) channel members polycystin kidney dominant 1 like 3 (PKD1L3) and PKD2L1 are participate in sour sensation at the back of the tongue. The sweet and umami taste receptors (T1R2/T1R3 and T1R1/T1R3 receptors) distributed in the tongue epithelium are coupled to the taste-specific G protein α-gustducin present on the taste buds, releasing Gβγ subunits, and activating phospholipase C-β2 (phospholipase C-β2), PLC-β2) and adenylyl cyclase (AC). This will activate the downstream inositol triphosphate (IP3) and cAMP pathways, and the intracellular Ca2+concentration increases, which in turn activates the transient receptor potential channel M5 (transient receptor potential channel M5, TRPM5) depolarizing the cell membrane and releasing neurotransmitters, thereby producing sweetness. There are differences in the intracellular signal transduction of sweet receptors at different sites. The bitter taste receptor proteins (T2Rs) are distributed in the root of the tongue and combines with the bitter substance dissolved in the liquid phase to activate the taste cells and depolarize the cell membrane. Then it causes the nerve cells to post-synaptic excitement and is transmitted to the bitter taste center of the cerebral cortex[38].