Repeated social defeat stress induces chronic hyperthermia in rats

Psychological stressors are known to increase core body temperature (T_c) in laboratory animals. Such single stress-induced hyperthermic responses are typically monophasic, as T_c returns to baseline within several hours. However, studies on the effects of repeated psychological stress on T_c are limited. Therefore, we measured T_c changes in male Wistar rats after they were subjected to 4 social defeat periods (each period consisting of 7 daily 1 h stress exposures during the light cycle followed by a stress-free day). We also assessed affective-like behavioral changes by elevated plus maze and forced swim tests.In the stressed rats, the first social defeat experience induced a robust increase in T_c (+ 1.3 °C). However, the T_c of these rats was not different from control animals during the subsequent dark period. In comparison, after 4 periods of social defeat, stressed rats showed a small but significantly higher (+ 0.2-0.3 °C) T_c versus control rats during both light and dark periods. Stressed rats did not show increased anxiety-like behavior versus control rats as assessed by the elevated plus maze test. However, in the forced swim test, the immobility time of stressed rats was significantly longer versus control rats, suggesting an increase in depression-like behavior. Furthermore, hyperthermia and depression-like behavior were still observed 8 days after cessation of the final social defeat session. These results suggest that repeated social defeat stress induces a chronic hyperthermia in rats that is associated with behavior resembling depression but not anxiety.