Differential production and regulation of two gonadotropins (GTH I and GTH II) in teleost pituitary gonadotrophs during reproductive development.

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Two chemically distinct gonadotropins (GTH I and GTH II) have been purified from chum salmon pituitary. Chemical and biological characterizations have revealed that GTH II is the gonadotropin concerning gonadal maturation and ovulation, however, GTH I is a new hormone and its physiological function still remains unknown. Immunocytochemistry, Lising specific antisera to beta-subunit of GTH I and GTH II, have suggested that two distinct gonadotropins (GTH I and GTH II) are synthesized mainly in two distinct gonadotrophs in salmoned pituitary. In addition, GTH I content in blood plasma is relatively high in vitellogenic phase as compared with in ovulating phase in salmoneds.This study examined differential production and regulation of gonadotropins sub-units in rainbow trout (*) pituitary gonadotrophs during reproductive development, using cDNA probes of their alpha - and beta-subunits by in situ hybridization histochemistry. (1) The mRNA of alpha -subunit of GTH I and GTH II were detected prior to any signals of beta- subunits of GTH I or GTH II in immature rainbow trouts. (2) The mRNA of beta-subunit of GTH I were observed in the early phase of vitellogenic phase, always prior to that of GTH II, and increased during vitellogenic phase and then declined before maturation. (3) The mRNA of beta-subunit of GTH II began to increase extremely during post vitellogenic and maturating phases of gonadal development. These results suggest that GTH I-synthesis occurs from early vitellogenic phase prior to that of GTH II, and may play functional role during vitellogenesis in the ovary. However, functional relationship between GTH I and GTH II, just like FSH and LH in mammalian pituitary, still remains unsolved.