Rationale: Previous observations revealed that defeated and subsequently individually housed rats do not display behavioral anticipatory responses to the conditioned presentation of a bell/light stimulus associated with free access to a 5% sucrose solution reward. The absence of the appetitive responses suggests a decreased sensitivity to reward. This might be homologous to anhedonia, a symptom of human depression.

Objectives: To further validate the inability to anticipate as indicative for a depressionlike state we investigated whether antidepressant treatment restores the impaired anticipatory responses in defeated and subsequently individually housed animals.

Methods: Male rats were defeated and subsequently individually housed or subjected to a control treatment. In the 19-20th weeks after the exposure to defeat rats were either injected daily with imipramine (20 mg/kg per os, dissolved in water) or water. Anticipatory behavior was measured both before and after 3-5 weeks of chronic treatment with imipramine.

Results: The long-term impairment of anticipatory behavior in defeated and subsequently individually housed rats was restored by chronic imipramine treatment. Impaired appetitive behavior in socially stressed rats was not accompanied by a decreased consumption of the 5% sucrose solution in the anticipatory tests. The recovery of the appetitive responses was independent of open field activity, body weight, and 5% sucrose preference in the home cage.

Conclusions: Chronic imipramine administration restores the anhedonialike absence of anticipatory behavior in socially stressed rats. Predictive validity of the social stress model of human depression is suggested by the similar action of imipramine on the modeled behavior and on the anhedonia symptoms in depressive human patients.