Seasonal Changes in mRNA Levels of Gonadotropin and Thyrotropin Subunits in the Goldfish,Carassius auratus

Abstract

Seasonal changes in the mRNA levels of glycoprotein α, gonadotropin (GTH) Iβ and IIβ, and thyrotropin (thyroid-stimulating hormone (TSH)) β subunits in the pituitary of goldfish were quantified by Northern blot analysis and laser densitometry. Reproductive development and thyroid activity were monitored by measuring gonadosomatic index, plasma GTH II, testosterone (T), estradiol, 11-ketotestosterone, and thyroid hormones (T4and T3). Plasma GTH II and steroids showed characteristic increases, while plasma thyroid hormones levels, in general, decreased in association with the reproductive period. In females, the mRNA levels of the α, GTH Iβ, and GTH IIβ subunits increased synchronously during early spawning period (April) and then decreased during ovarian regression (August). In males, the levels of the α and GTH IIβ subunits showed changes similar to those in females, but the GTH Iβ mRNA levels showed only a small increase during the late spawning period (May). In both sexes, TSHβ mRNA levels were high during winter to early spring (February and April) and low during late spring to summer (May and August). These results suggest that in goldfish the gonadotropins may be synthesized synchronously in order for asynchronous gametogenesis to take place. Additionally, the data suggest a negative feedback relationship between synthesis of the TSHβ subunit and the thyroid hormones.