Effects of diazepam and desipramine in the forced swimming test: influence of previous experience with the situation.

The effects of desipramine and diazepam on the behavior of rats in the forced swimming test were studied. Desipramine significantly enhanced struggling behavior, the intensity of its effect being greater in rats previously exposed to the forced swimming test for 1 or 4 days before drug administration. The effect of desipramine on immobility was significantly only in rats previously exposed to the test. Acute diazepam administration did not modify the behavior of rats in the test. Chronic administration (6 days) of two different doses of diazepam did not alter the behavior of rats in the forced swimming test, but significantly decreased the defecation rate, suggesting that the drug was effective as an anxiolytic. The present results suggest that the behavior of rats in the forced swimming test might be related to mood states rather than to fear/anxiety and that efficacy of the test to detect antidepressant activity might be enhanced by repeated exposure of the rats to the situation.