ANTERIOR-PITUITARY RESPONSE TO STRESS - TIME-RELATED CHANGES AND ADAPTATION

A wide array of physical and psychological stressors alter the secreti on of anterior pituitary hormones. However, both the qualitative and t he quantitative features of the stressors as well as its duration mark edly influence the final endocrine response. In addition, among all an terior pituitary hormones, only ACTH and prolactin levels appear to re flect the intensity of the stress experienced by the animals. Although physical stressors show a somewhat specific neuroendocrine profile, t he response of the pituitary-adrenal (PA) and sympathomedulloadrenal a xes are common to almost all stressors. After an initial stimulatory e ffect of stress, an inhibition of all anterior pituitary hormones, exc ept ACTH, can be found provided the stressor is intense enough. The me chanisms responsible for this biphasic response to stress are likely t o be located at sites above the pituitary. When the animals are repeat edly exposed to the same stressor, some behavioural and physiological consequences of stress exposure are reduced, suggesting that the anima ls become adapted to the stimulus. This process has been also termed h abituation. Among all the pituitary hormones, only ACTH and prolactin levels are reduced as a consequence of repeated exposure to the same ( homotypic) stressor, although some negative results have been reported . However, it has been recently reported that subtle changes in the ch aracteristics of the stressors or in their regularity can greatly infl uence adaptation, and these factors might explain failure to find adap tation of ACTH and prolactin in some works. Habituation of ACTH and pr olactin, when observed, appears to be specific for the chronically app lied stressor so that the potentiality of the PA axis and prolactin to respond to a novel (heterotypic) stressor can be preserved. In the ca se of the PA axis, an intact or potentiated response to a novel stress or is observed in spite of presumably negative feedback exerted by dai ly stress-induced glucocorticoid release and the high resting levels o f glucocorticoids. This phenomenon has been termed as facilitation and can be unmasked alternating stress. Although with the exception of th e PA axis, developmental aspects of anterior pituitary response to str ess have been poorly studied, available data suggest that dramatic cha nges occur in some hormones during weaning, with some, but less profou nd, changes thereafter. Responsiveness to stressors appears to mature with age, but developmental patterns differ among the various anterior pituitary hormones. (C) 1998 ISDN. Published by Elsevier Science Ltd.