Activation of salmon gonadotropin-releasing hormone synthesis by 17 alpha-methyltestosterone administration in yearling masu salmon, Oncorhynchus masou.

Juvenile salmonid pituitary gonadotropin (GTH) contents are elevated after steroid hormone treatment, but the involvement of gonadotropin-releasing hormone (GnRH) is unclear. Activation of salmon GnRH (sGnRH) synthesis by 17 alpha-methyltestosterone (MT) administration has been examined in the brain of yearling masu salmon (future precocious males and immature females) using an in situ hybridization technique combined with radioimmunoassay. Oral MT application markedly increased pituitary GTH II beta, but not GTH I beta, contents in both sexes. In future precocious males, MT treatment increased the number of cells expressing sGnRH mRNA in the preoptic area about threefold, whereas there were no significant differences in the olfactory bulbs and the ventral telencephalon. No significant changes were observed in cell sizes nor the numbers of silver grain per 100 microns2 cell in any of the brain regions. Thus, in future precocious males, preoptic sGnRH neurons may be activated by sex steroids. In contrast, no significant changes were observed in sGnRH mRNA levels of immature females after MT treatment. These differences in responses to sex steroids of sGnRH cells in the preoptic area between future precocious males and immature females suggest that MT has indirect actions via sGnRH and/or direct actions on the pituitary in the former, and that MT acts directly on the pituitary in the latter.