DIFFERENTIAL PRODUCTION AND REGULATION OF GONADOTROPINS (GTH-I AND GTH-II) IN THE PITUITARY-GLAND OF RAINBOW-TROUT, ONCORHYNCHUS-MYKISS, DURING OVARIAN DEVELOPMENT

Biosynthesis of salmon gonadotropins, GTH I and GTH II, during ovarian development, were examined by means of in situ hybridization histochemistry and indirect immunocytochemistry. In rainbow trout pituitary glands, expression of GTH I beta- and II beta-subunit genes appeared separately in distinct cells (GTH I- and GTH II-cells), whereas the GTH alpha-subunit gene was expressed in both cell-types. In the GTH I-cells, coordinated increases in GTH alpha and I beta-messenger ribonucleic acids (mRNAs) occurred coincident with the onset of vitellogenesis, indicating active synthesis of GTH I during vitellogenesis. In contrast, in the GTH II-cells, both GTH alpha-and II beta-mRNA signals markedly increased from a later stage of vitellogenesis and persisted throughout oocyte maturation and ovulation, supporting the idea that GTH II is actively synthesized as a maturational GTH. GTH alpha-mRNA levels in the GTH I-cells selectively decreased prior to final oocyte maturation, although I beta-mRNA levels remained elevated, thus suggesting a decline of biosynthesis of GTH I after vitellogenesis. These findings clarify how the synthesis of GTH I and GTH II are coordinated in the piscine pituitary, and indicate that the expression of GTH subunit genes during gametogenesis is regulated differentially in a cell-specific manner, both temporally and spatially.