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

In male teleosts, plasma oestrogen levels are undetectable, or when detecta ble, are present in very low amounts. Several studies have shown that the t estes are incapable of secreting oestrogens. In contrast, other studies hav e demonstrated that oestrogen can be formed in the male brain, and that oes trogens are involved with neuroendocrine and behavioral responses. Therefor e, 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 approximatel y 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 hormone s (17 beta-oestradiol, testosterone, 17,20 beta-dihydroxy-4-pregnen-3-one ( 17,20 beta-P) and 11-ketotestosterone) were determined before and during 32 days after injection. Injection with AI inhibited 17 beta-oestradiol secre tion by the brain. Fish injected with 10 mg AI/kg or 10 mg AI/kg weekly pre sented significantly higher plasma 17,20 beta-P levels (12.11 +/- 1.75 and 11.98 +/- 12.85 ng/ml, respectively) at 6 h after injection than before inj ection (0.19 +/- 0.19 and 1.00 +/- 0.42 ng/ml, respectively). At 16 days af ter injection, the groups injected with 10 mg AI/kg or 10 mg AI/kg weekly w ere already spermiating at significantly higher rates than the vehicle grou p. Gonadosomatic index (GSI) and sperm motility were not different among th e groups. This study demonstrated that Fadrozole inhibited 17 beta-oestradi ol in the brain, and prematurely increased plasma 17,20 beta-P levels. (C) 2000 Elsevier Science B.V. All rights reserved.