Brain-pituitary-gonadal axis during early development and sexual differentiation in the rainbow trout, Oncorhynchus mykiss

Profiles of testosterone, 11-ketotestosterone, androstenedione, and estradiol were determined by RIA, and immunocytochemical techniques were employed to identify gonadotropin (GTH) I and II and gonadotropin releasing 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 differences in steroid content between males and females became evident around the 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 all 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 differences were seen between males and females in the timing of appearance, 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 for sexual differentiation of rainbow trout. (C) 1996 Academic Press, Inc.