Resumo

Negative anticipatory contrast (NAC) corresponds to the suppression in consumption of a first rewarding substance (e.g., saccharin 0.15%) when it is followed daily by a second preferred substance (e.g., sucrose 32%). The NAC has been interpreted as resulting from anticipation of the impending preferred reward and its comparison with the currently available first reward. In this context, one should expect that devaluation of the preferred substance after the establishment of the NAC would either reduce or abolish the contrast effect. In order to evaluate this prediction, Wistar rats were initially submitted to the NAC training, which consisted of exposition to 0.15% saccharin followed by 0.15% saccharin (.15-.15 condition) in a day and to 0.15% saccharin followed by 32% sucrose (.15-32 condition) in the following day, along 28 days. Distinct visual cues signalized the different conditions. The rats were then exposed to eight additional training sessions, when the experimental conditions were presented in a quasi-random schedule. This arrangement allowed us to evaluate the impact of an unpredictable sequence of conditions on the NAC effect. This was followed by the sucrose devaluation phase: either 0.15% saccharin or 32% sucrose solution was offered every other day for the course of six days. Immediately after each of the 15-min sucrose exposures, half of the subjects received an i.p. injection of LiCl (0.3 M; 0.4 ml/100 g body weight); the remaining subjects received an i.p. injection of saline (0.4 ml/100 g body weight). The 15-min saccharin exposures were never paired with any injection. Twenty-four hours after the last devaluation session, the subjects were exposed to the .15-.15 condition (Post-devaluation Day 1) and to the .15-32 condition in the following day (Post-devaluation Day 2). The corresponding visual discriminative cues used in association with these experimental conditions along the NAC training were also used in these post-devaluation testing days. Contrary to the initial prediction, the results of the present study show that the NAC effect was maintained despite the sucrose devaluation induced by its pairing with LiCl. This allows one to question the idea that the NAC results from anticipation of the impending preferred reward and its comparison with the currently available first reward. The results reported in this study support the view that the NAC effect is controlled by anticipation of the relative value of the first solution, which is updated daily by means of both a post-ingestive comparison of the first and second solutions and memory of past pairings.