Expression of membrane progestin receptors in zebrafish (Danio rerio) oocytes, testis and pituitary

A distinct family of membrane progestin receptors (mPRα, mPRβ, and mPRγ) that mediate rapid, nongenomic actions of progestins has been identified and characterized in several fish species as well as in frogs, rats, pigs, and humans. However, few studies to date have thoroughly examined tissue specific expression of mPR protein and transcripts in any model species. In the present study, the expression of both mPRα and mPRβ in zebrafish was examined by RT-PCR, Western blotting and immunohistochemistry using mPR specific primers and antibodies. The proteins and mRNAs of mPRα and mPRβ were co-localized in the major reproductive organs, including the ovary, testis, and pituitary. Both mPRα and mPRβ were found in scattered cells in the pituitary for the first time. In the testis, immunostaining of mPRα was restricted to the sperm, while mPRβ was found in spermatocytes and spermatogonia. In the ovary, both mPRα and mPRβ were detected in denude oocytes and follicular layer cells. Furthermore, mPRα and mPRβ proteins were localized at or near the oocyte membrane of maturationally competent stage IV oocytes in a probable location for mediating progestin-induced nongenomic signaling in oocytes.