Chronic stress and plasma catecholamine and corticosterone levels in male rats.

Rats were exposed repeatedly to an immobilization, light and noise stressor for 3 weeks and re-exposed to the same stressor after a rest period of 3 weeks. At the beginning of each stress period, blood was obtained by a tail vein cut. Norepinephrine (NE), epinephrine (E) and corticosterone (COR) were determined. The stress response of NE or E remained relatively constant throughout 3 weeks of chronic stress. The stress response of COR remained constant for the first 2 weeks but increased markedly thereafter. After a 3-week recovery, repeat stress values of E were the same while those of NE and COR were significantly lower as compared with the last chronic stress value. Marked individual differences among rats were observed. Thus, the initial stress response during chronic stress showed no adaptation but actual sensitization for COR.