Differential effects of Physical, Psychological, and Social Stress on Spatial Learning and Memory.

It has been postulated that stressful events may trigger mental dysfunction such as depression and posttraumatic stress disorder. The role of stress in affective or anxiety disorders has been partly linked to its influences on memory. However, studies have shown that different types of stress, such as physical, psychological or social ones, precipitate disorder symptoms with different likelihood. Numerous reports have indicated that various stressful stimuli modulate learning and memory processes. Yet evidence summarized by Lupien and McEwen (1997) indicates that stress has multiple and often conflicting effects on cognition. To elucidate the effects of stress on cognition, it is critical to evaluate the independent effects of different kinds of stress on different phases of learning and memory. In the present study, we evaluated the effects of physical, psychological, and social stress on different phases of spatial memory processing that engages the hippocampus: Spatial navigation in a Morris water maze. Groups of male Sprague-Dawley rats were subjected to stress induced by footshock, stimuli paired with footshock, or obseration of conspecifics being shocked, and received multiple training sessions at the Morris water maze. Pre-training application of physical stress (footshock paired with light) impaired acquisition of Morris water maze. However, pre-training application of psychological stress enhanced spatial learning. Pre-test manipulation of either physical stress or psychological stress did not influence the retrieval of well-formed spatial memory traces. Nevertheless, if rats went through only a single session of training, pre-test application of physical stress caused a mild retrieval deficit, and pre-test present of psychological stress produced significant retrieval enhancement. Both effects were restricted to the first trial of a three-trial test session. Finally, pre-training presentation of social stress induced a deficit in the second training session; this finding suggests that social stress impaired acquisition of spatial memory. Furthermore, posttraining present of social stress did not influence consolidation of spatial memory, but pre-test application of social stress produced retrieval enhancement. In conclusion, physical, psychological and social stress produces distinct effects on acquisition, formation or expression of spatial memory. The present results established an animal model in studying the differential influences of stress in various kinds on memory processing and may eventually lead to understanding of why traumatic experience of different kinds had different effects in precipitating psychological symptoms.