Several reports have indicated that various stress stimuli modulate learning and memory processes. In the present study, the effects of adrenocortical suppression with the 11beta-hydroxylase inhibitor metyrapone on the psychological stress-induced changes in memory storage in inhibitory avoidance training and in serotonin turnover in various brain regions were investigated in rats. Retention of one-trial inhibitory avoidance and the plasma corticosterone level were significantly enhanced by post-training exposure to psychological stress for 1 h. Pretreatment with metyrapone (12.5 or 25 mg/kg, s.c.) 90 min beforehand dose-dependently blocked the enhancement of memory storage and of the plasma corticosterone level produced by psychological stress. These results suggest that the adrenocortical system may contribute to the memory-enhancing effect of psychological stress. In a neurochemical study, a significant increase in serotonin turnover in the hippocampus and limbic forebrain, including the nucleus accumbens, were observed in rats that were exposed to psychological stress. In contrast to the behavioral experiments, these changes in serotonin turnover produced by exposure to psychological stress were not antagonized by pretreatment with metyrapone; instead, a further increase in serotonin turnover was observed only in the hippocampus. These results suggest that the serotonergic system in the hippocampus might be selectively regulated by adrenal steroids in response to stress, and imply the existence of negative feedback mechanisms via a hippocampal serotonergic system in the memory enhancement associated with corticosterone and psychological stress.
Copyright 1999 Elsevier Science B.V.