Effects of sex steroids on the mRNA levels of gonadotropin subunits in juvenile and ovariectomized goldfish Carassius auratus

SUMMARY: In order to elucidate the regulation of gonadotropin (GTH) gene expression by gonadal sex steroids, the effects of ovariectomy and sex steroid administration on pituitary mRNA levels of GTH subunits were examined in goldfish. Ovariectomy caused a marked increase in GTH Iβ mRNA levels, and steroid hormone replacement (testosterone, T; estradiol-17β, E2; and 11-ketotestosterone, KT) suppressed the elevated levels in sexually early recrudescent and mature females. Pituitary mRNA levels of α and GTH IIβ were not affected by ovariectomy and steroid hormone treatment. When sex steroids (T, E2, and KT) were administered to sexually immature juvenile fish, GTH Iβ mRNA levels were suppressed as observed in adult fish. However, GTH IIβ mRNA levels were observed to increase by sex steroids (T and E2 but not KT) unlike that observed with adult fish. These results indicate that sex steroids exert an inhibitory effect on GTH Iβ mRNA levels from the time goldfish are immature to the time they are mature. However, sex steroids (T and E2) have a stimulatory effect on GTH IIβ mRNA levels in juvenile goldfish and the effect becomes less pronounced when they become adult.