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Naloxone impairs concurrent but not sequential flavor aversion: Resorting to a flexible/explicit learning.

The role of opiate systems has been extensively studied in relation to learning and memory. Naloxone (Nx), an opiate antagonist, was administrated in concurrent (Experiment 1) and sequential (Experiment 2) flavor aversion learning (FAL) tasks. The outcomes demonstrate that Nx impairs the acquisition of concurrent but not sequential FAL. In the concurrent learning (7 trials), both control (vehicle) and Nx2 (treated with Nx only on the first 2 days) groups learned the task. Furthermore, these 2 groups retained the learning in a discrimination test without drug administration (Day 8) but failed a reversal test (Day 9). In contrast, the Nx group (7 trials with Nx) showed no concurrent learning but correctly performed the discrimination test (Day 8) and, critically, the reversal test. These results suggest that Nx blocks concurrent (implicit) learning in these experiments but induces animals to resort to new strategies that are flexible, a characteristic of explicit learning.