Effect of exogenously administered thyroid hormones on gonadotropin, thyrotropin and deiodinases encoding genes in the catfish, Heteropneustes fossilis (Bloch)

Abstract

Thyroid hormones are known to regulate the basal metabolism rate of an organism. They are also known to regulate the seasonal reproduction of long-day breeding vertebrates in response to thyrotropin induced deiodinase enzymes switching in the brain. The current study attempted to investigate the effect of intraperitoneal administration of thyroxine (T4) and tri-iodothyronine (T3) hormones at various doses on gonadal recrudescence, plasma estradiol-17β and quantitative expression analysis of genes encoding for gonadotropin, thyrotropin, and deiodinases. The estradiol-17β levels were not affected by either thyroid hormone; however, the gonado-somatic index (GSI) and ovarian histology were varying. The gonadotropin releasing hormone 2 (gnrh2) and follicle stimulating hormone-β subunit (fsh-b) gene expressions correspond to the fish GSI and ovarian histology. The gene expressions show that T4 inhibits the expression of thyroid stimulating hormone-β subunit (tsh-b) and type 3 deiodinase (dio3), though it enhances the expression of type 2 deiodinase (dio2). T3, on the other hand, inhibits tsh-b and dio2 expression while increasing dio3 expression. In summary, the T4 appears to regulate gonadal recrudescence in Heteropneustes fossilis in a dose-dependent manner, whereas the T3 appears to have no effect on gonadal activity.