Pituitary proopiomelanocortin-derived peptides and hypothalamus-pituitary-interrenal axis activity in gilthead sea bream (Sparus aurata) during prolonged crowding stress: Differential regulation of adrenocorticotropin hormone and alpha-melanocyte-stimulating hormone release by corticotropin-releasing hormone and thyrotropin-releasing hormone

Plasma levels of cortisol, growth hormone (GH), adrenocorticotropin hormone (ACTH), alpha-melanocyte-stimulating hormone (alpha-MSH), N-acetyl-beta-en dorphin, in vitro ACTH-stimulated cortisol secretion, and in vitro corticot ropin-releasing hormone (CRH)- and thyrotropin-releasing hormone (TRH)-stim ulated ACTH and alpha-MSH secretion were investigated in gilthead sea bream exposed to high stocking density (30 kg m(-3)) for 23 days. Within 3 days after the onset of crowding, plasma levels of cortisol, ACTH, alpha-MSH, an d N-acetyl-beta-endorphin were above control values. After 7 days, plasma p arameters had returned to control levels, but at 23 days, cortisol, cr-MSH, and N-acetyl-beta-endorphin levels were again elevated over controls, indi cating a long-term activation of the melanotrope cells. In contrast, crowdi ng stress elicited a prolonged reduction in plasma GH levels concomitant wi th the increased hypothalamus-pituitary-interrenal axis (HPI) activation. C rowding stress enhanced cortisol secretory activity of the unstimulated int errenal cells. However, interrenal tissue from crowded fish in vitro displa yed an attenuated response to ACTH stimulation compared with tissue from co ntrol fish, indicating a desensitization of these cells to ACTH during crow ding. The involvement of pituitary proopiomelanocortin-derived peptides in the HPI axis of sea bream is indicated by the observed modulation of the CR H and TRH responsiveness of the corticotropes and melanotropes in crowded f ish. At day 1, when there were crowding-induced plasma increases in ACTH an d alpha-MSH, there was an attenuated CRH-stimulated but not TRII-stimulated , ACTH release. However, at that time, CRH- and TRH-induced responses of al pha-MSH secretion, and the unstimulated secretory activity of the MSW cells , were enhanced in crowded sea bream. These data provide evidence for stimu latory roles of multiple hypothalamic (CRH and TRH) and pituitary (ACTH and alpha-MSH) peptides in the activation of the hypothalamus-pituitary-interr enal axis under crowding conditions in sea bream, (C) 2000 Academic Press.