EFFECTS OF STEROIDS ON GTH-I AND GTH-II SECRETION AND PITUITARY CONCENTRATION IN THE IMMATURE RAINBOW-TROUT ONCORHYNCHUS-MYKISS

Using specific radio-immunoassays for rainbow trout GTH I and GTH II, the effects of testosterone and estradiol 17 beta have been studied or reinvestigated on the regulation of the secretion and the synthesis o f the these two pituitary gonadotropins in the immature rainbow trout. After steroid implantation, the GTH II pituitary concentration is sti mulated by testosterone and estradiol 17 beta for the entire period du ring which the plasma levels of these hormones are maintained to value s comparable to those measured in the adult vitellogenic female rainbo w trout On the other hand only testosterone induced a transient increa se in the GTH I pituitary content 15 days after implantation, and estr adiol provoked a decrease at day 30. The secretion of both GTH I and G TH II is stimulated by testosterone but not by estradiol 17 beta. Alto gether, these results show that in the immature rainbow trout testoste rone preferentially modifies GTH I secretion, but not that of GTH II. They confirm that the stimulation of GTH II accumulation after testost erone or estradiol treatment would correspond to a stimulation of horm one synthesis. They evidence a differential action of both steroids on the synthesis of the two gonadotropins, especially a possible inhibit ion of GTH I synthesis by estradiol. They let suppose that the regulat ion of GTH I synthesis would involve factors other than steroids.