Effect of Sex Steroids on the mRNA Levels of Gonadotropin I and II Subunits in the Goldfish Carassius auratus

Abstract

In order to clarify the role of gonadal sex steroids in the expressions of gonadotropin (GTH) subunit genes in goldfish, we examined the effect of testosterone (T), estradiol-17β (E₂), and 11-ketotestoster-one (11-KT) on the pituitary mRNA levels of GTH Iβ, GTH IIβ and α subunits by Northern blot analysis in male and female fish at sexually early recrudescent and mature stages. Fish were implanted with a blank Silastic capsule, or capsules containing T, E₂, or 11-KT (200 μg), and kept at 20°C for 2 weeks. In early recrudescent fish (GSI: male, 2.6±0.2; female, 2.6±0.6), the mRNA of GTH Iβ in T, E₂, or 11-KT implanted fish showed significantly lower levels than those in the blank implanted fish in both sexes. The mRNA levels of GTH IIβ in T and E₂ implanted fish showed slightly increased levels whereas decreased levels were observed in 11-KT implanted fish in both sexes. The mRNA levels of the α subunit in 11-KT and E₂ implanted females were lower than those in the blank implanted females, but no differences were observed in males. In sexually mature fish (GSI: male, 3.5±0.3; female, 7.5±1.5), no clear effects of T, E₂, and 11-KT were observed on the mRNA levels of the all GTH subunits except for an increased level of GTH IIβ in E₂ implanted males. Thyrotropin β subunit mRNA levels were also measured in these fish but showed no clear changes by steroid hormone treatment except for a decrease by 11-KT and E₂ treatments in sexually early recrudescent females. These results indicate that the expressions of GTH Iβ and IIβ subunit genes are differentially regulated by sex steroids in the goldfish at the early recrudescent stage. The results also suggest that the responsiveness of GTH gene expression to sex steroids differs depending on gonadal maturity.