Effects of letrozole, non-steroidal aromatase inhibitor, on serum sex steroid levels, oocyte diameter and gonadosomatic index (GSI) in rainbow trout (Oncorhynchus mykiss)

The objective of this study was to contribute to investigate the efficacy of letrozole, a potent non-steroidal aromatase inhibitor, on serum sex steroid levels and oocyte growth in rainbow trout females. Serum steroid levels were measured using an indirect enzyme-linked immunosorbent assay (ELISA) before and ۲۲ days after an initial injection with ۱, ۲.۵ mg kg-۱ and subsequent weekly injection with ۲.۵ mg kg-۱letrozole. After the last blood collection, gonad tissues were removed and oocyte diameter and GSI were determined. In all groups treated with letrozole, serum ۱۷β- esteradiol levels (E۲) decreased significantly and inversely serum ۱۷α-۲۰ß dihydroxy-۴ pregnen- ۳- one (۱۷α-۲۰ß-P), testosterone(T) levels increased significantly from ۶ to ۹۶ h after injection (p<۰.۰۵). The lowest E۲ levels, oocyte diameter and GSI were measured in the group treated weekly with ۲.۵ mg kg-۱ letrozole (p<۰.۰۵). This study demonstrates that letrozole has the potential to interfere in the synthesis of endogenous estrogens from androgens and delay gonadal development.

Keywords:

Aromatase inhibitor , Sex steroid , GSI , Oncorhynchus mykiss