Transduction for Sweet Taste of Saccharin May Involve Both Inositol 1,4,5-Trisphosphate and cAMP Pathways in the Fungiform Taste Buds in C57BL Mice
The transduction pathways for sweet and bitter tastes were investigated with assays of inositol 1,4,5-trisphosphate (IP3) and cyclic adenosine monophosphate (cAMP) levels in mouse fungiform taste buds. Recordings of taste responses were also made in the chorda tympani nerve. Stimulation of the tongue with saccharin elicited a significant increase in IP3 levels in the fungiform papilla only at 20 mM but in cAMP levels at 3 and 20 mM, without affecting those of the nonsensory epithelial tissue. Formation of both IP3 and cAMP induced by 20 mM saccharin was suppressed by pretreatment of the tongue with pronase, a proteolytic enzyme which specifically inhibits sweet responses. Quinine and denatonium elicited both significant increases in IP3 levels at a concentration of 20 mM and slight decreases in cAMP levels at concentrations of 1–20 mM in the fungiform papilla. Recording of the chorda tympani nerve showed good responses by saccharin, quinine, and denatonium at concentrations of 1 mM and higher. These results suggest that the fungiform taste cells in C57BL mice have pronase-sensitive receptors for saccharin, coupled to both the IP3 and the cAMP pathways; the former participates only at high concentration, while the latter acts from low to high concentrations. The results also do not rule out the possibility that a phosphodiesterase-mediated cAMP decrease may be involved in bitter transduction for quinine and denatonium.