The present study examined the memory modulatory effect of epinephrine on latent learning of an inhibitory avoidance task. Male Sprague-Dawley rats on the first day were subjected to one of three conditions (no, short or long) in pre-exposure to the task apparatus. One day or several days later, they received the typical inhibitory avoidance training with a 0.5 mA/0.5 s foot shock. Memory of the inhibitory avoidance response was tested one day after the foot-shock training. The long pre-exposure group showed better memory than the no or short pre-exposure group, and this latent memory could last for 6 days: Retention scores of the long pre-exposure group were significantly better than those of the no pre-exposure group if the shock training was given 3 or 6 days, but not 12 or 21 days, after the pre-exposure. Epinephrine injected after the pre-exposure training modulated the latent memory in a dose- and time-dependent manner: 0.01 mg/kg given shortly after the short pre-exposure enhanced the memory, but 0.5 mg/kg given shortly after the long pre-exposure impaired it. Epinephrine injected 4 h after the pre-exposure had no effect, neither did that given to rats pre-exposed to a different context. Epinephrine (0.01 mg/kg) also made the latent memory lasting longer as the rats treated with it showed significant avoidance behavior when they had the shock training at 12 or 21 days after the pre-exposure. These findings suggest that epinephrine could modulate memory formed in the latent learning.
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