Testosterone, 11-ketotestosterone, and estradiol-17 beta modify baseline 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 the levels of these hormones in the blood of the treated fish and had significant effects on plasma ACTH and cortisol levels in both unstressed and stressed rainbow trout and in stressed brown trout. E treatment significantly elevated resting levels of ACTH and cortisol and KT significantly suppressed resting ACTH levels in rainbow trout, although no effect of KT was noted on baseline cortisol levels. One hour of confinement 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 stress-induced plasma cortisol levels. In brown trout subjected to confinement stress for 96 hr, within 1 hr of the onset of confinement the stress-induced increase in plasma ACTH and plasma cortisol was significantly lower in T- and KT-implanted fish than in sham-implanted controls. 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 significantly 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 responsiveness observed in E-treated immature fish was not observed during confinement stress in untreated mature female trout, with naturally high plasma E levels. However, untreated mature male trout displayed a significantly reduced cortisol response to confinement. It is suggested that gonadal steroids are involved in the regulation of both baseline and stress-induced activity of the pituitary-interrenal axis in salmonid fish.