Differential effects of chronic stress on memory processes in the tree shrew

Unpredictable and uncontrollable stressful events have been shown to affect cognitive processes. Interestingly, only hippocampus-mediated memory proce sses are thought to be sensitive to the effects of chronic stress. In contr ast, the hippocampus-independent memory processes have been shown to be res istant to chronic stressful experiences. A central feature of the stress re sponse is the activation of the hypothalamus-pituitary-adrenocortical (HPA) -axis, resulting in increased plasma levels of glucocorticoids, and several studies suggested that the performance of hippocampus-mediated memory proc esses might be directly modulated by these adrenal steroids. We investigate d the impact of chronic psychosocial stress on hippocampus-mediated and hip pocampus-independent memory processes in male tree shrews. By using a modif ied holeboard we followed memory performance during 23 weeks of alternating stress-free and stressful conditions. This schedule was designed to mimic a more realistic situation with stress-free conditions being sequentially i nterrupted by challenging events. The results indic ate that chronic stress differentially affects hippocampus-mediated and hippocampus-independent me mory processes in tree shrews. While hippocampus-independent memory process es remained unimpaired throughout the study, hippocampus-mediated memory wa s persistently impaired, not only during stress periods but also during rec overy periods. This persistent impairment seems not to be exclusively trigg ered by glucocorticoids because urinary free cortisol concentration returne d to normal during recovery periods. The present study is the first to eval uate the consequences of sequential stress exposure on memory performance i n animals. Apparently, the mechanisms modulating cognitive processes are fa r from being understood and need a very systematic analysis in animal model s with a high face and predictive validity to human stress-related memory d isorders. (C) 1999 Elsevier Science B.V. All rights reserved.