Anhedonia and motivational deficits in rats: impact of chronic social stress.

Stress, especially chronic stress, is one of the most important factors responsible for precipitation of affective disorders in humans. The animal models commonly used in the investigation of stress effects are based mainly on powerful physical stressors. In the majority of cases, these models are not relevant to situations that human beings encounter in everyday life. In our study, an animal model for chronic social stress has been developed for rats using a resident-intruder paradigm. This paradigm is considered a model of social defeat or subordination, and therefore may mimic situations occurring in humans. Rats were subjected daily to subordination stress for a period of five weeks and, in parallel, tested with a battery of behavioural tests. Chronically stressed rats showed behavioural changes, including decreased motility and exploratory activity, increased immobility in a forced swim test, and reduced preference for sweet sucrose solution (anhedonia). Reduced locomotor and exploratory activity represents a loss of interest in new stimulating situations, implying a deficit in motivation. Increased immobility in the forced swim test indicates behavioural despair, a characteristic of depressive disorders. Decreased sucrose preference may indicate desensitisation of the brain reward mechanism. Since anhedonia is one of the core symptoms of depression in humans, our findings suggest that the rat chronic social stress model may be an appropriate model for depressive disorders.