Current Knowledge of the Nature and Identity of Progestin and Estrogen Membrane Receptors in Fish Gonads

Abstract

Specific binding sites for progestins, estrogens and androgens have been identified on plasma membrane fractions of spotted seatrout (Cynoscion nebulosus) and Atlantic croaker (Micropogonias undulatus) gonadal tissues and gametes. Moreover comprehensive biochemical studies have shown that the characteristics of all of these binding moieties fulfil the criteria for their designation as steroid membrane receptors (Patino and Thomas, 1990; Loomis and Thomas, 2000). The most comprehensive information on the nature of these receptors and their physiological roles has been obtained to date for the membrane progestin receptors mPR, (Thomas, 2000). The mPRs are localized on the plasma membranes of fully-grown oocytes and sperm in seatrout and act as intermediaries in progestin induction of final gamete maturation by a rapid, cell surface-mediated nongenomic action (Thomas et al., 2002). The membrane estrogen receptor (mER) is present in testicular tissues and is likely involved in the nongenomic action of estrogen to down regulate androgen production in this tissue (Loomis and Thomas, 2000). A membrane androgen receptor (mAR) has also recently been characterized in croaker ovarian tissues and may be involved in a nongenomic action of androgens to down-regulate ovarian estrogen production.