Immunohistochemical localization of inhibin and activin-like proteins in the brain, pituitary gland, and the ovary of thin-lipped grey mullet, Liza ramada (Risso)

Abstract

Inhibin and activin, dimeric protein hormones originally isolated from mammalian gonads, are involved in the regulation of vertebrate reproduction. In the present study, we investigated the localization of inhibin and activin subunits (α, β A, and β B) immunoreactivity in the brain, pituitary, and ovary of female Liza ramada using immunohistochemistry. We demonstrate that activin β A subunit immunoreactivity is found in most of the brain regions: telencephalon (Tel), midbrain tegmentum (MT), and cerebellum (C). In the pituitary gland, activin β A subunit immunoreactivity is observed in nerve fibers that are in close contact with somatotrophs and gonadotrophs. Strong immunostaining of activin β B subunit is detected in melanotropin (MSH)-producing cells in pars intermedia (PI), which are in close contact with gonadotrophs. In the ovary, the cytoplasm as well as follicle cells of previtellogenic oocytes exhibited strong immunoreactivity of activin β A. However, the activin β A and β B immunoreactivity is decreased when the oocytes entered the vitellogenic stage (recrudescence). The progress in the accumulation of yolk within oocytoplasm is accompanied by a decrease in the activin β A and β B immunoreactivity and eventually the immunoreactivity disappears completely when the oocytes become fully mature. Interestingly, the vitellogenic oocytes of female L. ramada obtained from fresh water (captivity) contain only weak immunoreactivity of activin β A compared to that of saline water (natural habitat). Moreover, immunoreactivity of activin β B is observed in the phagocytic granulosa cells during resorption of oocytes (atresia) within the ovary of female reared in captivity. The immunostaining of inhibin α in the ovary is similar to that of activin β B but the reaction intensity is weaker. However, the mature oocytes show a positive reaction for inhibin α. These results provide evidence for the presence of inhibin and activin-like molecules within L. ramada and reveal the distribution of these molecules in L. ramada brain–hypophysial–gonadal system. Our observations add some new informations to our current understanding of activin system in fish and suggest that inhibin and activin may play an important role for L. ramada reproduction in the event of oocyte growth and maturation.