Regulation of gonadotropin β subunit gene expression by testosterone and gonadotropin-releasing hormones in the goldfish, Carassius auratus

Abstract

Sex steroids differentially regulate gonadotropin (GTH) β subunits (FSHβ and LHβ) gene expression in the pituitary of goldfish: a strong in vivo inhibitory effect on FSHβ mRNA production, but a weak stimulatory effect on LHβ in sexually immature and recrudescent fish. In the present study, to examine a direct effect of testosterone (T) and gonadotropin-releasing hormone (GnRH) on the mRNA levels of FSHβ and LHβ subunits in the pituitary, in vitro experiments were performed using dispersed pituitary cells of sexually immature, recrudescent, mature and regressed goldfish. T treatment in vitro did not significantly decrease FSHβ mRNA levels, but increased that of LHβ only in the cells of immature fish. Salmon-type GnRH increased FSHβ mRNA levels in cells of mature fish, but decreased the levels in cells of sexually regressed fish. From these results, it was suggested that: (1) in vivo effect of sex steroids on gene expression of GTH β subunits is not always exerted on the pituitary; and (2) the different responses of GTH β subunits by sex steroids between in vivo and in vitro are partly due to a complex pathway through hypothalamic factors, such as GnRH, in the case of in vivo.