Recombinant goldfish activin B stimulates gonadotropin-I beta but inhibitsgonadotropin-II beta expression in the goldfish, Carassius auratus

It is well documented that the pituitary in teleosts produces two gonadotro pins, namely gonadotropin-I (GTH-I) and gonadotropin-II (GTH-II), which may regulate different phases of the reproductive cycle. However, unlike in ma mmals, very little is known about the differential regulation of the two GT Hs in fish. Using goldfish as a model, the present study demonstrates, for the first time, that activin, a protein factor that plays a critical role i n the differential regulation of mammalian FSH and LH, has opposite effects on GTH-I beta and GTH-II beta mRNA expression. Recombinant goldfish activi n B stimulates GTH-I beta but significantly suppresses GTH-II beta mRNA lev els in a dose-dependent manner in cultured goldfish pituitary cells. Admini stration of recombinant human follistatin completely abolished the effects of activin, thus demonstrating the specificity of the activin activities. T he novel opposite effects of activin on the two goldfish GTHs make goldfish a very unique vertebrate model for activin studies. The present study not only contributes to our understanding of the mechanisms that control the te mporal expression patterns of the two GTHs during the fish reproductive cyc le, but also provides important information on the evolution of gonadotropi n regulation in vertebrates. (C) 1999 Academic Press.