Devaluation of ethanol reinforcement

Abstract

Postingestive CNS pharmacologic effects of ethanol are often assumed to provide the major stimuli for development and maintenance of ethanol self-administration in rats. However, there is little direct evidence to support this assumption. In all procedures that have been used to initiate ethanol intake in rats, some type of taste adaptation or taste conditioning could account for the increased and maintained ethanol intake. Thus, it remains critical to demonstrate that increased ethanol intake is related to postingestive CNS actions of ethanol, and not to a positive shift in the hedonic taste value of the solution. Two experiments were performed to examine this question. In both studies, rats were trained to self-administer 20% ethanol by using a sucrose-substitution initiation procedure. The rats were required to press a lever 25 or 30 times to gain access to 20% ethanol for 20 min from a sipper tube. Once initiated, extinction sessions were used to determine the strength of ethanol seeking by measuring the number of lever presses that occurred in 20 min with no presentation of the ethanol solution. After initial training, the rats were split into two groups: one that received pairings of a gavage of ethanol (1 g/kg), followed after 10 min by a lithium chloride (LiCl) injection (paired group), and one that also received ethanol gavage and LiCl injections, but separated by 24 h (unpaired group). This pairing of postingestive effects with the illness induced by LiCl injection has been shown to devalue other food and fluid reinforcers. In Experiment 1, the rats received four pairings, one after the other with no behavioral testing between. In Experiment 2, the rats received three pairings and were tested for devaluation after each pairing. Results from both experiments showed significant decreases in seeking behavior in both groups, but seeking behavior was decreased significantly greater in the paired group, even though neither group had access to ethanol during the extinction testing periods. In Experiment 1, when ethanol became available after the devaluation procedure, the pattern of intake in the paired group was unchanged early in the sipper tube availability period, supporting the suggestion that the devaluation effect was not mediated by taste stimuli. These findings support the assumption that postingestive effects contribute to the reinforcement produced by self-administered ethanol in rats.