Regulation of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) gene expression by gonadotropin-releasing hormone (GnRH) and sexual steroids in the Mediterranean Sea bass

Trabajo presentado en la 20th Conference of European Comparative Endocrinologists, celebrado en Faro (Portugal), del 5 al 9 de septiembre de 2000The secretion of gonadotropins, the key reproductive hormones in vertebrates, is controlled from the brain by the gonadotropin-releasing hormone (GnRH), but also by complex steroid feedback mechanisms. In this study, after the recent cloning of the three gonadotropin subunits of sea bass (Dicentrarchus labrax), we aimed at investigating the effects of GnRH and sexual steroids on pituitary gonadotropin mRNA levels, in this valuable aquaculture fish species. Implantation of sea bass, in the period of sexual resting, for 12 days with estradiol (E2), testosterone (T) or the non-aromatizable androgen dihydrotestosterone (DHT), almost suppressed basal expression of FSHß (four to 15-fold inhibition from control levels), while slightly increasing that of ¿ (1.5-fold) and LHß (approx. twofold) subunits. Further injection with a GnRH analogue (15 ¿g/kg BW; [d-Ala6, Pro9-Net]-mGnRH), had no effect on FSHß mRNA levels, but stimulated (twofold) pituitary ¿ and LHß mRNA levels in sham- and T-implanted fish, and slightly in E2- and DHT-implanted fish (approx. 1.5-fold). The GnRHa injection, as expected, elevated plasma LH levels with a parallel decrease on LH pituitary content, with no differences between implanted fish. In conclusion, high circulating steroid levels seems to exert different action on gonadotropin secretion, inhibiting FSH while stimulating LH synthesis. In these experimental conditions, the GnRHa stimulate LH synthesis and release, but have no effect on FSH synthesis.J. Mateos received a fellowship from the Spanish Ministry of Education and Science. E. Mañanós was a recipient of a European Marie-Curie post-doctoral grant (FAIR GT95-6462). This work was financially supported by the European Community (FAIR CT97-3785) and the CICYT (MARI1998-1542-CE)