In vertebrates, gonadal production of steroid hormones is regulated by follicle-stimulating hormone (FSH) and luteinizing hormone (LH) via their receptors designated FSHR and LHR, respectively. We have shown recently that steroid hormones are synthesized in the differentiating gonad of tadpoles during sex determination in the frog Rana rugosa. To elucidate the role of gonadotropins (GTHs) and their receptors in the production of gonadal steroid hormones during sex determination, we isolated the full-length FSHβ, LHβ, FSHR and LHR cDNAs from R. rugosa and determined gonadal expression of FSHR (FSH receptor) and LHR (LH receptor) as well as brain expression of FSHβ and LHβ during sex determination in this species. The molecular structures of these four glycoproteins are conserved among different classes of vertebrates. FSHβ expression was observed at similar levels in the whole brain (including the pituitary) of tadpoles, but it showed no sexual dimorphism during gonadal sex determination. By contrast, LHβ mRNA was undetectable in the whole brain of tadpoles. FSHβ-immunopositive cells were observed in the pituitary of female tadpoles with a differentiating gonad. Furthermore, FSHR expression was significantly higher in the gonad of female tadpoles during sex determination than in that of males, whereas LHR was expressed at similar levels in males and females. The results collectively suggest that FSHR, probably in conjunction with FSH, is involved in the steroid-hormone production during female-sex determination in R. rugosa.
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