REPEATED STRESS PERSISTENTLY ELEVATES MORNING, BUT NOT EVENING, PLASMA-CORTICOSTERONE LEVELS IN MALE-RATS

Repeated exposures to a stressor in our rat model of a chronic stress state cause elevated plasma corticosterone levels in the morning for s everal days after the last stressor. However, plasma corticosterone le vels are normally characterized by a circadian rhythm with low levels for much of the morning and higher levels near the onset of darkness. The current experiment examined the question of whether the elevated m orning levels after stressor exposures were accompanied by other chang es in this circadian rhythm. Male rats were given restraint-shock stre ssor sessions for 0, 1, or 3 days, after which plasma samples were col lected for 3 days at 0900 h and at three other times around the circad ian peak (1400, 1800, and 2200 h). Plasma corticosterone levels at 090 0 h were elevated for the first 2 days after three stressor exposures and for 1 day after a single stressor exposure compared to those in no nstressed controls. However, levels at 1400, 1800, and 2200 h were not different in stressed and control rats on the first 2 days after stre ssor exposures. In addition, the amplitude of the corticosterone rhyth m was suppressed after three stressor exposures, but not after one. Th is decease in amplitude was mostly due to increased morning levels, in asmuch as the evening levels were similar in stressed rats and control s. Because the hypothalamic-pituitary-adrenal axis is more sensitive t o glucocorticoid feedback in the morning, our data suggest that the me chanisms mediating feedback at this time of day may be disrupted by re peated stressor exposures. However, when feedback sensitivity is lower in the evening, repeated stressor exposures had little or no effect o n plasma corticosterone levels.