It is well documented that the pituitary in teleosts produces two gonadotropins, namely gonadotropin-I (GTH-I) and gonadotropin-II (GTH-II), which may regulate different phases of the reproductive cycle. However, unlike in mammals, very little is known about the differential regulation of the two GTHs 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 in the differential regulation of mammalian FSH and LH, has opposite effects on GTH-Ibeta and GTH-IIbeta mRNA expression. Recombinant goldfish activin B stimulates GTH-Ibeta but significantly suppresses GTH-IIbeta mRNA levels in a dose-dependent manner in cultured goldfish pituitary cells. Administration of recombinant human follistatin completely abolished the effects of activin, thus demonstrating the specificity of the activin activities. The 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 temporal expression patterns of the two GTHs during the fish reproductive cycle, but also provides important information on the evolution of gonadotropin regulation in vertebrates.
Copyright 1999 Academic Press.