SHORT-TERM AND LONG-TERM EXCITABILITY CHANGES OF THE INSULAR CORTICAL-NEURONS AFTER THE ACQUISITION OF TASTE-AVERSION LEARNING IN BEHAVING RATS

Conditioned taste aversion, a long-lasting type of learning establishe d after a single pairing of a novel taste and subsequent internal mala ise, (2,17) is an adaptive behavior to prevent animals from repeated i ntakes of poisonous substances, The present study was designed to iden tify the time-dependent excitability changes of cortical neurons to gu statory stimuli after the acquisition of conditioned taste aversion in freely behaving rats. Conditioned taste aversion to saccharin was est ablished by an intraperitoneal injection of lithium chloride, a sickne ss-inducing agent, soon after an intraoral infusion of saccharin, Twen ty minutes after the pairing, 25 (29%) of 86 rats showed aversive tast e reactivities to saccharin, and 30 min after the pairing, all of the rats showed aversive behaviors to saccharin; these behavioral changes lasted throughout the test session (over 360 min), When unit activitie s mere recorded from the insular cortex simultaneously with the behavi oral test, 14 (11%) of 122 neurons showed a significant enhancement of excitability in response to saccharin, but not to other taste stimuli , after the acquisition of taste aversion, Eight of these 14 neurons s howed a short-term enhancement: significant effects were detected only 30 min after the pairing. The remaining six neurons exhibited a long- term enhancement: the effects lasted over 360 min after the pairing. T he existence of such short-term and long-term excitability changes sug gests that the gustatory insular cortex is involved in different aspec ts of taste aversion learning. (C) 1998 IBRO. Published by Elsevier Sc ience Ltd.