Forced swimming behavior is not related to the corticosterone levels achieved in the test: a study with four inbred rat strains.

The behavior of four inbred strains of rats in the holeboard and the forced swimming tests, and its relationship with a physiological index of stress (serum corticosterone) were studied in adult male rats. The strains were: Fisher 344 (FIS), Lewis (LEW), Spontaneously hypertensive (SHR), and Wistar-Kyoto (WKY). In the holeboard, SHR rats were the most active and WKY the less active, the other strains showing intermediate levels of activity. During the first exposure to forced swimming WKY were far more passive than the other three strains and the same was observed during the second exposure. When corticosterone levels after this second exposure to water was determined, LEW rats showed lower values than the other three strains. Therefore, no apparent relationship between behavior and stress-induced corticosterone secretion exists. Although a single point measurement of only on physiological index of stress has important limitations, the present data do not give support to a strong relationship between the behavior of the animals in the forced swimming test and emotional reactivity to stress. It is therefore possible that forced swimming behavior might not be mainly a panic-like reaction, but the result of the tendency of the animals to adopt passive strategies in inescapable situations. Although more studies are needed to firmly establish this assumption, WKY rats might be, at least potentially, a useful model of depressive-like behavior.