Steroid activation of the brain-pituitary complex gonadotropic function in the triploid rainbow trout Oncorhynchus mykiss.

Testosterone and estradiol were implanted into triploid rainbow trout, and their effects on brain and pituitary salmon gonadotropin-releasing hormone (sGnRH) content and on gonadotropin (GtH2) levels in the pituitary and blood were studied. In the hypothalamic areas, neither steroid altered sGnRH content. In the brain tissues (ventral telencephalon and preoptic areas), testosterone (0.5 mg/kg BW) significantly increased sGnRH contents and larger doses induced a more rapid increase. Estradiol induced a similar increase which occurred later than the response to the same dose of testosterone. Both steroids also stimulated pituitary sGnRH, with estradiol being less potent. Only testosterone induced long-term increases in plasma GtH2 content. Both steroids may act on the synthesis and release of GnRH, with testosterone having a greater influence on the control of gonadotropin secretion, especially at the end of the cycle.