Effects of stress and of amphetamine on passive avoidance conditioning in rats.

This study examined the effects of immobilization stress combined with water immersion (ICS) and/or amphetamine (AM) on different memory phases in the passive avoidance task in rats. The performance of rats was evaluated in the retention tests 24 and 48 h after a single acquisition trial. ICS exposure lasting 1 h impaired retention of the learned avoidance response if applied 2 to 4 h before or immediately after training. The stressor did not affect retrieval if presented 5 or 2 h before the retention test. AM was used i.p. at the dose of 8 or 1 mg/kg. Neither 8 mg AM administered 4 h before nor 8 or 1 mg doses given after training did not impair the retention performance in unstressed rats. The 1 mg AM prevented the impairment of retention in animals exposed to the stressor 3 or 4 h before training but had no effect when the stronger impairment was induced by ICS 2 h before training. However, when given 1 h before retention testing, 1 mg AM attenuated even the severe impairment induced by the pre-training stressor exposure. Our results suggest that ICS impairs primarily the early phase of memory consolidation and a low dose of AM can prevent this effect.