Reception and Transmission of Taste Information in Type II and Type III Taste Bud Cells

Gustatory information processing begins with taste bud cells, which are activated by sapid molecules via specific taste receptors and transmit their signals to gustatory afferent fibers. Taste bud cells are morphologically classified into 4 groups (Type I – IV cells), two of which are involved in gustatory signaling. Type II cells express sweet, bitter, and umami taste receptors and transduction components and respond best to sweet, bitter, or umami stimuli, suggesting that sweet, bitter, and umami tastes are detected by different sets of Type II cells. Type III cells express putative sour taste receptors and respond to sour or multiple taste stimuli, indicating that sour tastes are mediated by Type III cells. These data suggest that each taste quality could be discriminated among taste bud cells. Type II cells do not possess a conventional synaptic structure but they release ATP in response to taste stimuli. Type III cells have a synaptic structure and they release serotonin and norepinephrine but not ATP. Therefore, each taste cell may use distinct mechanisms and transmitters for signal transmission to gustatory nerve fibers.