Effects of the aromatase inhibitor Fadrozole on reproductive steroids and spermiation in male coho salmon ( Oncorhynchuskisutch) during sexual maturation

Abstract

In male teleosts, plasma oestrogen levels are undetectable, or when detectable, are present in very low amounts. Several studies have shown that the testes are incapable of secreting oestrogens. In contrast, other studies have demonstrated that oestrogen can be formed in the male brain, and that oestrogens are involved with neuroendocrine and behavioral responses. Therefore, this study evaluated the effects of inhibiting oestrogen production in male coho salmon ( Oncorhynchus kisutch) during sexual maturation using the aromatase inhibitor Fadrozole (AI). Groups of fish, which were approximately 1.5 months before natural spawning, received injections of 1.0 or 10.0 mg AI/kg or five weekly injections of 10 mg AI/kg. Plasma sex steroid hormones (17β-oestradiol, testosterone, 17,20β-dihydroxy-4-pregnen-3-one (17,20β-P) and 11-ketotestosterone) were determined before and during 32 days after injection. Injection with AI inhibited 17β-oestradiol secretion by the brain. Fish injected with 10 mg AI/kg or 10 mg AI/kg weekly presented significantly higher plasma 17,20β-P levels (12.11±1.75 and 11.98±2.85 ng/ml, respectively) at 6 h after injection than before injection (0.19±0.19 and 1.00±0.42 ng/ml, respectively). At 16 days after injection, the groups injected with 10 mg AI/kg or 10 mg AI/kg weekly were already spermiating at significantly higher rates than the vehicle group. Gonadosomatic index (GSI) and sperm motility were not different among the groups. This study demonstrated that Fadrozole inhibited 17β-oestradiol in the brain, and prematurely increased plasma 17,20β-P levels.