Effects of antidepressants in the rat forced swimming test.

Effects of antidepressants and other drugs on the behaviour of rats in the forced swimming test were examined. Acute and chronic administration of antidepressants reduced the duration of immobility during the first 5 or 10 min of a 30 min test by prolonging escape-directed behaviour which appeared only during this period. However, the drugs did not affect the duration of immobility during the last 20 or 25 min, when the rats were in a state of almost complete immobility. In contrast, methamphetamine, caffeine and scopolamine reduced the duration of immobility not only during the first 5 min but also the next 15 or 25 min without prolonging the escape-directed behaviour but by increasing general motor activity. The effect of antidepressants was potentiated by chronic treatment as compared to acute administration. In the chronic experiments, a significant reduction in the duration of immobility was first observed on the 6th day of the treatment. Although a single injection of diphenhydramine caused an effect similar to antidepressants, this effect disappeared after chronic treatment. These results indicate that immobility itself is not affected by antidepressants. However, it is suggested that the reduction in the duration of immobility only during the first 5 min of the test, which was caused by the prolongation of the escape-directed behaviour, and the potentiation of the effect after chronic treatment are an action specific to antidepressants.