Bilateral lesions of the gustatory thalamus disrupt morphine- but not LiCl-induced intake suppression in rats: evidence against the conditioned tasteaversion hypothesis

Rats decrease intake of a saccharin conditioned stimulus (CS) when followed by: (1) the administration of an aversive agent such as Lithium chloride ( referred to as a conditioned taste aversion, CTA); (2) access to a very pal atable concentration of sucrose (referred to as an anticipatory contrast ef fect, ACE); or (3) the administration of a drug of abuse. It is not clear, however, whether the suppressive effects of drugs of abuse are mediated by their aversive or rewarding properties. The present set of experiments addr essed this issue by examining the suppressive effects of morphine in rats w ith a lesion thought to dissociate the two phenomena (i.e., CTA and ACE). T he results show that bilateral ibotenic acid lesions of the gustatory thala mus eliminate the suppressive effects of morphine, but fail to disrupt the suppressive effects of the aversive agent, lithium chloride. This pattern o f results argues against the CTA account in favor of the reward comparison hypothesis. Specifically, the data suggest that rats suppress intake of a s accharin CS in anticipation of the availability of a preferred drug of abus e and that the gustatory thalamus is essential for this type of reward comp arison process. (C) 2000 Elsevier Science B.V. All rights reserved.