TESTOSTERONE, 11-KETOTESTOSTERONE, AND ESTRADIOL-17-BETA MODIFY BASE-LINE AND STRESS-INDUCED INTERRENAL AND CORTICOTROPIC ACTIVITY IN TROUT

Estradiol-17 beta (E), 11-ketotestosterone (KT), and testosterone (T) were administered to immature rainbow and brown trout by implantation of steroid-containing cocoa butter pellets. This procedure elevated th e levels of these hormones in the blood of the treated fish and had si gnificant effects on plasma ACTH and cortisol levels in both unstresse d and stressed rainbow trout and in stressed brown trout. E treatment significantly elevated resting levels of ACTH and cortisol and KT sign ificantly suppressed resting ACTH levels in rainbow trout, although no effect of KT was noted on baseline cortisol levels. One hour of confi nement stress increased ACTH levels in rainbow trout, but less so in T - and KT-implanted fish than in sham-implanted fish. A similar pattern was observed in stress-induced plasma cortisol levels where T and KT treatment of rainbow trout resulted in a more than 50% attenuation of plasma cortisol levels while E implantation significantly increased st ress-induced plasma cortisol levels. In brown trout subjected to confi nement stress for 96 hr, within 1 hr of the onset of confinement the s tress-induced increase in plasma ACTH and plasma cortisol was signific antly lower in T- and KT-implanted fish than in sham-implanted control s. However, these differences were not sustained at subsequent sample points during the 96-hr period of continuous confinement. Nonetheless, overall mean ACTH levels for the entire confinement period were signi ficantly enhanced in E-implanted brown trout and significantly reduced in KT-implanted fish. Overall mean cortisol levels were significantly lower in T- and KT-implanted fish. The enhancement of stress responsi veness observed in E-treated immature fish was not observed during con finement stress in untreated mature female trout, with naturally high plasma E levels. However, untreated mature male trout displayed a sign ificantly reduced cortisol response to confinement. It is suggested th at gonadal steroids are involved in the regulation of both baseline an d stress-induced activity of the pituitary-interrenal axis in salmonid fish. (C) 1996 Academic Press, Inc.