Chronic stress and sympathetic-adrenal medullary responsiveness.

Acute exposure of animals to stressful stimulation is attended by a significant activation of the sympathetic-adrenal medullary system. If animals are exposed to the same stressful stimulus each day for several weeks, a number of adaptive changes occur in the sympathetic-adrenal medullary system, including increased synthesis and storage of catecholamines, increased basal levels of circulating catecholamines, and decreased release of catecholamines into the circulation following exposure to the identical (homotypic) stressful stimulus. If chronically stressed animals are exposed to a novel (heterotypic) stressful stimulus, there is an exaggerated response of the sympathetic-adrenal medullary system compared to animals exposed to the same stressful stimulus for the first time. Other neuroendocrine systems share some characteristics with the sympathetic-adrenal medullary system in its pattern of adaptation to chronic stress. Several variables appear to influence this pattern of neuroendocrine adaptation to stressful stimulation, including predictability of the stressor, the intensity and duration of the stressor, the interval between each episode of stress, and the number of presentations of the stressor. The pattern of neuroendocrine adaptation to chronic intermittent stressful stimulation resembles in some respects the processes of habituation and sensitization.