GONADOTROPIN-RELEASING-HORMONE AND GONADOTROPIN IN GOLDFISH AND MASU SALMON

Reproductive activities in vertebrates are regulated by an endocrine s ystem, consisting of the brain-pituitary-gonad axis. In teleosts, gona dotropin-releasing hormone (GnRH) in the brain stimulates gonadotropin (GTH) release in the pituitary gland, but because of lack of the port al vessel, it is not known when and how much GnRH is released for the regulation of GTH release. There are multiple molecular types of GnRH in teleosts and several distinct populations of GnRH neurons in the br ain. However, we do not know which types and populations of GnRH neuro ns regulate reproductive activities. Here we summarize our recent stud ies on GnRH and GTH in masu salmon Oncorhynchus masou and goldfish Car assius auratus. Immunocytochemistry showed the location and molecular types of GnRH neurons. Salmon (sGnRH) and chicken-II GnRH (cGnRH-II) n euronal fibers were widely distributed in the brain of both masu salmo n and goldfish. Only sGnRH fibers were observed in the pituitary of ma su salmon, whereas both sGnRH and cGnRH-II fibers were observed in the goldfish pituitary, indicating that species specific GnRH profiles ar e involved in the regulation of pituitary function in teleosts. A seri es of experiments in masu salmon and goldfish suggest that among GnRH neuron populations GnRH neurons in the ventral telencephalon and the h ypothalamus regulate GTH release, and that GnRH of the terminal nerve origin is not essential to gonadal maturation and ovulation. The biolo gical function of other GnRH neurons remains unkown. Two GTHs appear t o be characteristic of teleost; however, regulation of reproduction by these GTHs is a question that remains to be elucidated. In salmonid s pecies, it is proposed that GTH I stimulates early gonadal development , whereas GTH II acts in later stages. When GTH expression was examine d in goldfish, both GTH I beta and II beta mRNA levels in the pituitar y gland showed increases in accordance with gonadal development, unlik e the sequential expression of GTH subunits in salmonids. The expressi on of these GTH subunit mRNAs were affected by water temperature, star vation, and steroid hormones in goldfish, but in what manner these two GTHs regulate gonadal development remains to be clarified.