Anterior pituitary response to stress : time‐related changes and adaptation

Abstract

A wide array of physical and psychological stressors alter the secretion of anterior pituitary hormones. However, both the qualitative and the quantitative features of the stressors as well as its duration markedly influence the final endocrine response. In addition, among all anterior pituitary hormones, only ACTH and prolactin levels appear to reflect the intensity of the stress experienced by the animals. Although physical stressors show a somewhat specific neuroendocrine profile, the response of the pituitary-adrenal (PA) and sympathomedulloadrenal axes are common to almost all stressors. After an initial stimulatory effect of stress, an inhibition of all anterior pituitary hormones, except ACTH, can be found provided the stressor is intense enough. The mechanisms responsible for this biphasic response to stress are likely to be located at sites above the pituitary. When the animals are repeatedly exposed to the same stressor, some behavioural and physiological consequences of stress exposure are reduced, suggesting that the animals become adapted to the stimulus. This process has been also termed habituation. 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 characteristics of the stressors or in their regularity can greatly influence adaptation, and these factors might explain failure to find adaptation of ACTH and prolactin in some works. Habituation of ACTH and prolactin, when observed, appears to be specific for the chronically applied stressor so that the potentiality of the PA axis and prolactin to respond to a novel (heterotypic) stressor can be preserved. In the case of the PA axis, an intact or potentiated response to a novel stressor is observed in spite of presumably negative feedback exerted by daily stress-induced glucocorticoid release and the high resting levels of glucocorticoids. This phenomenon has been termed as facilitation and can be unmasked alternating stress. Although with the exception of the PA axis, developmental aspects of anterior pituitary response to stress have been poorly studied, available data suggest that dramatic changes occur in some hormones during weaning, with some, but less profound, change thereafter. Responsiveness to stressors appears to mature with age, but developmental patterns differ among the various anterior pituitary hormones.

Effects of nalbuphine on anterior pituitary and adrenal hormones and subjective responses in male cocaine abusers

“…The lower dose of nalbuphine was significantly correlated with "Sick". These subjective ratings are consistent with the classification of PRL as a stress-reactive hormone (Marti and Armario, 1998). In our previous studies, no change from baseline in subjective effects was observed after vehicle administration alone (Mendelson et al, 2003).…”

Behavioral and neuroendocrine consequences of juvenile stress combined with adult immobilization in male rats

“…Because stress-induced increases in plasma glucose are an index of the activation of the adrenal medulla and particularly of plasma adrenaline (Martí and Armario, 1998), the present finding argues against an effect of juvenile stress on the activation of the sympathetic-medulloadrenal axis. Despite this severity, IMO by itself had no long-term (7 days post-stress) anxiogenic effects in the EPM.…”

c- fos mRNA Induction in Acute and Chronic Audiogenic Stress: Possible Role of the Orbitofrontal Cortex in Habituation

“…Under certain conditions, the impact of repeated challenges is reduced through adaptive mechanisms, generally termed habituation (Christoffersen, 1997), that act to minimize further physical and psychological insult. In general, habituation to glucocorticoid and catecholamine release, some of the most widely studied stress responses, often develop following the relatively predictable exposures of experimental animals to the same (homotypic) stressor (McCarty et al, 1992;Marti and Armario, 1998). Importantly, these responses are either normal or facilitated in habituated animals exposed to a different (heterotypic), not previously experienced stressor (Marti and Armario, 1998).…”

“…In general, habituation to glucocorticoid and catecholamine release, some of the most widely studied stress responses, often develop following the relatively predictable exposures of experimental animals to the same (homotypic) stressor (McCarty et al, 1992;Marti and Armario, 1998). Importantly, these responses are either normal or facilitated in habituated animals exposed to a different (heterotypic), not previously experienced stressor (Marti and Armario, 1998). These findings have led to the suggestion that habituation occurs in the central limb of stress-responsive circuits (Akana and Dallman, 1997), thus allowing the hypothalamo-pituitary-adrenocortical (HPA) axis to respond normally, or even become sensitized, to new challenges.…”