Gamma-L-glutamyl-L-aspartate, interacting with NMDA receptors, affects appetitive visual discrimination tasks in mice

gamma-L-glutamyl-L-aspartate (gamma-LGLA), which interacts with NMDA receptors, has been shown to impair retention of an active avoidance task in mice. Here, we specified the behavioral effects of gamma-LGLA on acquisition and retention of appetitive nondelayed visual discrimination tasks. Three experiments were conducted: the peptide (0.25 and 2.5 microM/kg/25 ml. ip) was administered 3 min after each of the first six sessions of either original learning, reversal 1 or reversal 3. gamma-LGLA affected acquisition of the original task and of the first reversal, as revealed by an absence of improvement on initial sessions and an increased number of sessions to reach criterion fixed at 7 of 10 correct choices on three consecutive sessions. This deficit did not result from an action of the peptide on position habits (repetition of spatial choices) nor on motivational processes, suggesting a specific interference of gamma-LGLA with acquisition and memorization of the visual rule. In contrast, gamma-LGLA had no effect on acquisition of the third reversal, in which the positively reinforced visual stimulus was identical to that used on the first reversal. These results show that the behavioral deficits of gamma-LGLA, which had previously been demonstrated in an aversive task, can be generalized to appetitive tasks based on acquisition of a new rule.