A peripheral mechanism for behavioral adaptation to specific "bitter" taste stimuli in an insect

Animals have evolved several chemosensory systems for detecting potentially dangerous foods in the environment. Activation of specific sensory cells w ithin these chemosensory systems usually elicits an aversive behavioral res ponse, leading to avoidance of the noxious foods. Although this aversive be havioral response can be adaptive, there are many instances in which it gen erates "false alarms," causing animals to reject harmless foods. To minimiz e the number of false alarms, animals have evolved a variety of physiologic al mechanisms for selectively adapting their aversive behavioral response t o harmless noxious compounds. We examined the mechanisms underlying exposur e- induced adaptation to specific "bitter" compounds in Manduca sexta cater pillars. M. sexta exhibits an aversive behavioral response to many plant- d erived compounds that taste bitter to humans, including caffeine and aristo lochic acid. This aversive behavioral response is mediated by three pairs o f bitter- sensitive taste cells: one responds vigorously to aristolochic ac id alone, and the other two respond vigorously to both caffeine and aristol ochic acid. We found that 24 hr of exposure to a caffeinated diet desensiti zed all of the caffeine- responsive taste cells to caffeine but not to aris tolochic acid. In addition, we found that dietary exposure to caffeine adap ted the aversive behavioral response of the caterpillar to caffeine, but no t to aristolochic acid. We propose that the adapted aversive response to ca ffeine was mediated directly by the desensitized taste cells and that the a dapted aversive response did not generalize to aristolochic acid because th e signaling pathway for this compound was insulated from that for caffeine.