Pharmacological characterization, localization and quantification of expression of gonadotropin receptors in Atlantic salmon (Salmo salar L.) ovaries

The gonadotropins Fsh and Lh interact with their receptors (Fshr and Lhr, respectively) in a highly specific manner in mammals with little overlap in biological activities. In fish, the biological activities seem less clearly separated considering, for example, the steroidogenic potency of both Fsh and Lh. Important determinants of the biological activity are the specificity of hormone–receptor interaction and the cellular site of receptor expression. Here, we report the pharmacological characterization of Atlantic salmon Fshr and Lhr, identify receptor-expressing cells in the ovary, and validate receptor mRNA quantification systems. For the pharmacological studies, we used highly purified coho salmon gonadotropins and found that the Fshr preferentially responded to Fsh, but was also activated by ∼6-fold higher levels of Lh. The Lhr was specific for Lh and did not respond to Fsh. Photoperiod manipulation was used to generate ovarian tissue samples with largely differing stages of maturation. Specific real-time, quantitative (rtq) PCR assays revealed up to 40-fold (fshr) and up to 350-fold (lhr) changes in ovarian expression levels, which correlated well with the differences in ovarian weight, histology, and circulating oestrogen levels recorded in January and June, respectively. Vitellogenic ovaries were used to localise receptor-expressing cells by in situ hybridization. Granulosa cells of small and large vitellogenic follicles were positive for both receptors. Also theca cells of small and large vitellogenic follicles expressed fshr mRNA, while only in large vitellogenic follicles theca cells were (weakly) positive for lhr mRNA. While only ovulatory Lh levels seem high enough to cross-activate the Fshr, expression by both receptors by granulosa and theca cells suggests that homologous ligand receptor interaction will prevail.