| Sumario: | The follicle stimulating hormone (Fsh) and luteinizing hormone (Lh) are central endocrine regulators of gametogenesis in vertebrates, and gonadotropin-releasing hormones (Gnrh) have been postulated as the main regulators of their synthesis and secretion. Gonadal sex steroids have a feedback effect modulating the availability of gonadotropins. All these effects at the level of pituitary have a direct impact in gametogenesis progression. Previous in vivo studies in sea bass, during the sexual resting period, showed that Gnrh injections stimulated Lh synthesis and release, but had no effect on the expression of the Fsh beta-subunit gene. At the same time, different steroid implants repressed fsh-b expression, but activated the expression of lh-b in the pituitary. To elucidate how this system is organized in the pituitary of male sea bass, we analysed the annual expression profile of the three nuclear estrogen receptors (Esrs) in male pituitary and the circulating levels of estradiol in relation with the different stages of spermatogenesis. Also, immunohistochemistry studies have been performed to identify the pituitary cells containing steroid receptors and their relationship with gonadotrophs and GnRh1 fibers. Moreover, we have used an in vitro pituitary primary cell cultures stimulated with estradiol-17β (E2) to study its direct action on gonadotrophs, and E2 implants to study its effect in vivo on the pituitary and the gonad. We have concluded that E2 has an inhibitory effect on the gonadotropins, although with different specific actions, which is also reflected in the localization of the Esrs in the gonadotrophs, while Gnrh differentially regulates Lh and Fsh cells in male seabass.
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