Pathological consequences of stress-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis may be related to the duration rather than to the intensity of HPA axis activation after exposure to the stressor. Consequently a fine analysis of post-stress events is of importance. The present experiments were designed to study the importance of three key factors in HPA recovery: intensity of the stressor (experiment 1), duration of exposure to the stressor (experiment 2) and previous experience of the animals with the situation (experiments 3 and 4). In experiment 1, analysis of both the response to the stressor and the poststress period showed that the stronger the stressor, the greater the area under the curve of HPA activation. In experiment 2, different groups of rats were exposed to different periods of immobilization (IMO) (20 min, 1 h and 2 h) and sampled before, during and after exposure to IMO. The speed of recovery of plasma corticotropin (ACTH) levels was not related to the duration of exposure to the stressor. In experiments 3 and 4, the influence of previous experience with the stressor was studied in rats daily exposed to 20 min IMO or daily injected with hypertonic saline (HS) for 8 days and sampled on days 1, 2, 5 and 8. Whereas a significant decline in plasma ACTH levels was not observed immediately after IMO until day 8, a single previous exposure to IMO was enough to enhance recovery 90 min after the end of exposure to IMO. Corticosterone levels were related to the number of previous experiences with the stressor only in the post-IMO period. In response to a novel stressor (forced swimming), chronic IMO rats showed a slightly impaired recovery as compared to stress-naive rats, suggesting that enhanced recovery of the HPA axis was specific for the homotypic stressor. After daily HS injections, a pattern similar to that after IMO was observed, the delayed, but not the early response of the HPA axis being reduced as a function of the number of previous experiences with the situation. Taken together, the present results suggest that the speed of recovery of the HPA axis after its activation by stressors is sensitive to the intensity of the stressors but not to their duration, and that adaptation to a repeated stressor is more apparent during the delayed HPA response.

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