The Effect of Repeated Exposure to Forced Swimming on Extracellular Levels of 5-Hydroxytryptamine in the Rat

The effects of repeated exposure to forced swimming was examined on extracellular concentrations of 5-hydroxytryptamine (5-HT), behavioral, and endocrine responses in rats. Animals were exposed to a 15-min swimming session on two consecutive days. On the first day, the swimming session increased extracellular 5-HT by 80 % over baseline in the striatum and reduced 5-HT to 40 % below baseline in the lateral septum. On the second day, however, the swimming session produced no effect on 5-HT in either brain region. Ratings of behavior showed that rats demonstrated climbing and swimming behaviors and developed immobility during the initial swimming session and that ratings of immobility increased and swimming decreased during the second swimming session. Immobility was positively correlated and swimming was negatively correlated with changes in extracellular 5-HT in the lateral septum but not in the striatum. Plasma corticosterone was equally elevated by 950 % after either I or 2 days of swimming exposure. These results show that there is rapid adaptation to the effects of repeated forced swimming on the regionally-specific, bi-directional response of extracellular 5-HT. In addition, changes in extracellular 5-HT in the lateral septum may he related to the behaviors produced during the forced swimming test that underlie its utility as an animal model of depression.