Voluntary exercise increases resilience to social defeat stress in Syrian hamsters.

Exposure to social stressors can cause profound changes in an individual's well-being and can be an underlying factor in the etiology of a variety of psychopathologies, such as post-traumatic stress disorder (PTSD). In Syrian hamsters, a single social defeat experience results in behavioral changes collectively known as conditioned defeat (CD), and includes an abolishment of territorial aggression and the emergence of high levels of defensive behaviors. In contrast, voluntary exercise has been shown to promote stress resilience and can also have anxiolytic-like effects. Although several studies have investigated the resilience-inducing effects of voluntary exercise after exposure to physical stressors, such as restraint and electric shock, few studies have examined whether exercise can impart resilience in response to ethologically-based stressors, such as social defeat. In Experiment 1, we tested the hypothesis that voluntary exercise can have anxiolytic-like effects in socially defeated hamsters. In the elevated plus maze, the exercise group exhibited a significant reduction in risk assessment, a commonly used index of anxiety, compared to the no-exercise group. In the open-field test, animals in the exercise group exhibited a significant reduction in locomotor behavior and rearing, also an indication of an anxiolytic-like effect of exercise. In Experiment 2, we examined whether exercise can reverse the defeat-induced potentiation of defensive behaviors using the CD model. Socially defeated hamsters in the exercise group exhibited significantly lower levels of defensive/submissive behaviors compared to the no-exercise group upon exposure to the resident aggressor. Taken together, these results are among the first to suggest that voluntary exercise may promote resilience to social defeat stress in Syrian hamsters.