The opiate agonist, levorphanol, injected into the amygdala complex of rats following passive avoidance conditioning produced time-dependent and dose-dependent decreases in retention. This effect obtained with levorphanol was observed to be stereospecific. In addition, post-training administration of the opiate antagonist, naloxone, into the amygdala significantly increased retention of passive avoidance conditioning in a time-dependent and dose-dependent manner. Finally, these opposing effects of opiate agonist and antagonist administration were blocked by combined administration of levorphanol and naloxone. These data support a possible role for amygdala opioid peptides in time-dependent memory processes.
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