Diet-induced plasticity in the taste system of an insect: localization to a single transduction pathway in an identified taste cell.

We studied exposure-induced sensitivity changes in an identified taste cell from Manduca sexta, a herbivorous caterpillar. This taste cell occurs within the lateral styloconic sensillum and responds selectively to compounds that humans characterize as bitter (e.g. caffeine, salicin and aristolochic acid). We made extracellular recordings from several classes of identified taste cell within the lateral sensillum, both before and after dietary exposure (for 48 h) to a suprathreshold concentration of caffeine, salicin or aristolochic acid. Our results revealed (1) that dietary exposure to caffeine desensitized the bitter-sensitive taste cell to caffeine, whereas dietary exposure to salicin or aristolochic acid did not desensitize the same taste cell to salicin or to aristolochic acid; (2) that dietary exposure to caffeine failed to alter the responsiveness of the sugar-, salt- or inositol-sensitive taste cells within the same sensillum; (3) that the caffeine-induced desensitization phenomenon generalized to salicin, a compound that stimulates the same transduction pathway as caffeine, but not to aristolochic acid, a compound that stimulates a different pathway; and (4) that chronically stimulating the lateral sensillum with caffeine, in the absence of ingestion, was sufficient to induce desensitization. We conclude that caffeine causes desensitization through a direct effect on a single transduction pathway within the bitter-sensitive taste cell.