Combined restraint and cold stress in rats: effects on memory processing in passive avoidance task and on plasma levels of ACTH and corticosterone.

The effect of restraint stress combined with water immersion (IMO+C), applied at various intervals before and after the acquisition of a passive avoidance task, was studied in rats. The procedure started with two pre-training trials. On the single training trial the rats received a footshock (0.3 mA, 3s) after they entered the preferred dark compartment. The exposure to IMO+C lasting 1 h terminated 4 or 1 h before application of the footshock or started immediately or 3 h after this aversive stimulus. Retention tests were performed 1 and 2 days after the acquisition trial. In an attempt to relate the behavioural responses to the stressor with plasma levels of two stress hormones we measured ACTH and corticosterone under similar conditions as were used in the behavioural experiments. IMO+C exposure terminating 1 h before the training resulted in very short avoidance latencies during retention testing. A similar impairment of retention test performance was found in animals exposed to the stressor immediately after training. When IMO+C exposure terminated 4 h before training the stressed rats exhibited comparably long avoidance latencies as shown by the controls. IMO+C presented 3 h after acquisition trial also did not influence retention of avoidance learning. The hormones were estimated 1 and 4 h after IMO+C, both in the absence and presence of footshock. Both ACTH and corticosterone were significantly increased 1 h after IMO+C termination, and their plasma levels returned to control values within 4 h. Footshock alone increased plasma corticosterone, however, the hormone levels were significantly lower than those estimated after IMO+C terminating 1 h before blood collection. Footshock substantially increased ACTH levels in rats exposed to IMO+C 1 h before footshock, but not in stressed rats with already high levels of corticosterone. In conclusion, IMO+C represents a strong stress stimulus exerting amnesic effect when applied shortly before or after the acquisition trial. Further, the findings indicate the restraint and cold stressor to interfere with consolidation of passive avoidance response. We suggest that the moderate circulating levels of corticosterone found after footshock may be positively related to the memory consolidation, while the exceedingly high levels have an opposite effect.