Reversal of scopolamine-induced amnesia of passive avoidance by pre- and post-training naloxone

In a series of five experiments, the modulating role of naloxone on a scopolamine-induced retention deficit in a passive avoidance paradigm was investigated in mice. Scopolamine, but not methyl scopolamine (1 and 3 mg/kg), induced an amnesia as measured by latency and duration parameters. Naloxone (0.3, 1, 3, and 10 mg/kg) injected prior to training attenuated the retention deficit with a peak of activity at 3 mg/kg. The effect of naloxone could be antagonized with morphine (1, 3, and 10 mg/kg), demonstrating the opioid specificity of the naloxone effect. Post-training administration of naloxone (3 mg/kg) as a single or as a split dose also attenuated the scopolamine-induced amnesia. Control experiments indicated that neither an increase in pain sensitivity (pre-training naloxone) nor an induced aversive state (post-training naloxone) appear to be responsible for the influence of naloxone on the scopolamine-induced retention deficit. These results extend previous findings implicating a cholinergic-opioid interaction in memory processes. A possible mechanism for this interaction involving the septo-hippocampal cholinergic pathway is discussed.