To investigate the effects of previous experience on stress response, rats were exposed to a stressor repeatedly and changes in plasma concentrations of stress hormones were measured. Rats were exposed to electric footshocks for 1 day (the first exposure (1S) group) or consecutive 3 days (the repetitive exposure (3S) group). A retardation of the rate of decrease from the peak corticosterone concentration, and an acceleration of the rate of decrease from the peak epinephrine concentration were observed in the repetitive exposure group as compared with the first exposure group. Moreover, a higher peak concentration of norepinephrine was observed in the repetitive exposure group than in the first exposure group. Immunohistochemical studies showed that the ratio of c-Fos and nitric oxide synthase (NOS) double stained cells/ c-Fos-positive cells increased in the paraventricular nucleus of the hypothalamus. The results suggest that previous experience of the same stressor results in the rapid termination of the energy consuming response, and the persistence of the energy- saving response. Therefore, it is speculated that memory of a previous stressful experience contributes to the development of a successful strategy for coping with the facing stressor.

Keywords: stress response, stressor, coping strategy, footshock