Serotonergic changes produced by repeated exposure to forced swimming: correlation with behavior.

Repeated forced swim resulted in a decrease in the concentrations of serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid in the hypothalamus and amygdala 24 h after the second swim session. This stressor also increased the mRNA levels for tryptophan hydroxylase-2, the rate-limiting enzyme in neuronal 5-HT synthesis, and 5-HT transporter in the midbrain as well as 5-HT1A receptor in the frontal cortex. Some of these serotonergic changes may be involved in the mechanisms of a depressive-like behavior induced by a stress of repeated swim in these animals.