Increase in Inositol 1,4,5-Trisphosphate Levels of the Fungiform Papilla in Response to Saccharin and Bitter Substances in Mice

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Abstract
Inositol 1,4,5-trisphosphate (IP3) levels of the mouse fungiform papilla and nonsensory epithelial tissue in response to various taste stimuli were measured by use of a competitive protein binding assay. Stimulation of the tongue with 20 mM saccharin induced a significant increase in IP3 levels of the fungiform papillae without affecting those of the nonsensory epithelial tissue, whereas stimulations with 20 mM quinine and denatonium raised IP3 levels in not only the fungiform papilla but slightly also nonsensory epithelial tissue. No such increases in IP3 levels were observed to lower concentrations of saccharin, denatonium and quinine, and other basic taste stimuli, such as 0.5 M sucrose, 0.1 M NaCl and 10 mM HCl. Pretreatment of the tongue with pronase, a proteolytic enzyme which inhibits sweet responses, completely eliminated the increase in IP3 levels induced by saccharin, but not those induced by quinine or denatonium. These results suggest that an IP3 pathway in taste cells of the mouse fungiform taste bud is involved in the transduction not only for denatonium and quinine, but also for 20 mM saccharin. The increase of IP3 induced by saccharin, but not by denatonium and quinine, may occur through pronase-sensitive membrane components.

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