Impact of gonadal steroids on pituitary gonadotropins in male European sea bass (Dicentrarchus labrax)

Abstract

The follicle stimulating hormone (Fsh) and luteinizing hormone (Lh) are central endocrine regulators of gametogenesis in vertebrates. Gonadotropin-releasing hormones (Gnrh) have been postulated as the main regulators of the synthesis and secretion of Fsh and Lh since long. In addition, sex steroids produced by the gonads 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 (August), showed that Gnrh injections stimulated Lh synthesis and release, but had no effect on the expression of the Fsh beta-subunit gene (fshb). At the same time, different steroid implants almost suppressed basal fshb expression, while activated the expression of lhb in the pituitary. To elucidate how this system is organized in the pituitary of sea bass, we analysed in a first step the annual profile of male pituitary expression of the estrogen, androgen and progesterone receptors, in relation with the different stages of spermatogenesis, the pituitary gonadotropin content and the circulating levels of sex steroids. In addition, immunohistochemistry studies have been performed to identify the pituitary cells containing steroid receptors. Next, we have used an in vitro pituitary primary cell culture stimulated with Estradiol, Testosterone Dihydroxyprogesterone and a Luteinizing Hormone-Releasing Hormone analogue (LHRHa) to analyse their impact on gonadotropin synthesis and release. It has been confirmed the strong effect of LHRHa on sea bass Lh release, while this effect is more weak on Fsh release and decreases with the advance of spermatogenesis. On the other hand, the main evidence of repression for both gonadotropins is due to Estradiol, with major intensity at the beginning of spermatogenesis.