REGULATION OF THE TYPE-II GONADOTROPIN ALPHA-SUBUNIT AND BETA-SUBUNIT MESSENGER-RNAS BY ESTRADIOL AND TESTOSTERONE IN THE EUROPEAN EEL

The gonadotrophic function of the European eel (Anguilla anguilla L.) at the silver stage is very weak: gonadotrophin-releasing hormone (GnRH) secretion is deficient and, moreover, dopamine inhibition overrides GnRH action. At the silver stage, oestradiol stimulates the biosynthesis of the type-II gonadotrophin (GTH-II). To study the molecular mechanism of this activation further, we examined the effect of testosterone and oestradiol administration on pituitary levels of mRNA encoding GTH-II-alpha and beta-subunits. Corresponding eel cDNA probes and Northern blot analysis were used. After 2 weeks, testosterone and oestradiol implantation resulted in a strong increase in mRNA encoding the GTH-II-beta-subunit (7-fold and 25-fold, respectively) and in a slight, but non-significant, rise in the a subunit mRNA level (1.8-fold and 1.5-fold, respectively). Co-implantation of these two steroids suggested a potentiation of their effects on the beta-subunit (104-fold) while an additive effect was indicated on the alpha-mRNA level (2.9-fold). Effects were detectable within 4 days and were maximal 4 weeks after implantation. These results indicate that in the European eel at the silver stage, gonadal steroids stimulate differentially the expression of GTH-II subunit genes at a pretranslational level.