BRAIN-PITUITARY-GONADAL AXIS DURING EARLY DEVELOPMENT AND SEXUAL-DIFFERENTIATION IN THE RAINBOW-TROUT, ONCORHYNCHUS-MYKISS

Profiles of testosterone, 11-ketotestosterone, androstenedione, and es tradiol were determined by RIA, and immunocytochemical techniques were employed to identify gonadotropin (GTH) I and II and gonadotropin rel easing hormone (GnRH) in monosex and mixed sex populations of rainbow trout from 1 to 126 days postfertilization (dpf). Steroid levels were relatively high at 1 dpf and declined until 25 dpf. At 30 and 48 dpf ( hatching) steroid levels increased slightly before they fell by 78 dpf and remained relatively constant thereafter. Trends toward difference s in steroid content between males and females became evident around t he time gonadal differentiation was histologically discernible (78 and 90 dpf). GTH I was present in the proximal pars distalis at all dates (48-126 dpf), whereas GTH II was not detectable. GnRH was found at al l dates (48-126 dpf) and was distributed in several areas of the brain including the nucleus preopticus periventricularis, nucleus lateralis tuberis, and the pituitary in the region where GTH I was found. No di fferences were seen between males and females in the timing of appeara nce, localization, or intensity of staining of these peptide hormones. Given that the brain-pituitary-gonadal axis seems to be intact during the process of sexual differentiation and the fluctuations of steroid levels during this process, sex steroids may play the driving role fo r sexual differentiation of rainbow trout. (C) 1996 Academic Press, In c.