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

A distinct family of membrane progestin receptors (mPRalpha, mPRbeta, and mPRgamma) 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 mPRalpha and mPRbeta in zebrafish was examined by RT-PCR, Western blotting and immunohistochemistry using mPR specific primers and antibodies. The proteins and mRNAs of mPRalpha and mPRbeta were co-localized in the major reproductive organs, including the ovary, testis, and pituitary. Both mPRalpha and mPRbeta were found in scattered cells in the pituitary for the first time. In the testis, immunostaining of mPRalpha was restricted to the sperm, while mPRbeta was found in spermatocytes and spermatogonia. In the ovary, both mPRalpha and mPRbeta were detected in denude oocytes and follicular layer cells. Furthermore, mPRalpha and mPRbeta 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.