Effects of the aromatase inhibitor fadrozole on plasma sex steroid secretion and ovulation rate in female coho salmon, Oncorhynchus kisutch, close to final maturation

Plasma levels of 17 beta-estradiol, 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (17 alpha,20 beta-P), and testosterone were measured in adult female coho salmon in late vitellogenesis, approximately 1.5 months before spawning and just before and following intraperitoneal injection with the aromatase inhibitor (AI) Fadrozole. Injection at dosages of 0.1, 1.0, and 10.0 mg AI/kg body wt caused a significant drop in plasma 17 beta-estradiol levels relative to preinjection values within 3 or 6 h. Injection of 10 mg AI/kg body wt caused a significant increase in plasma 17 alpha-20 beta-P levels within 3 h. Ten days after injection 67% of the fish treated with 10 mg AI/kg body wt had ovulated in contrast with 0% in the group injected with 0.1 mg AI/kg body wt. The fertilization rate of the eggs varied between 96% in the control group and 85% in the groups injected with AI. We conclude that the shift from 17-beta-estradiol to 17 alpha,20 beta-P biosynthesis, which is characteristic of maturing Oncorhynchus sp., was advanced significantly by treatment with AI and that Fadrozole can be used as a tool to investigate periovulatory endocrine changes in salmon. (C) 1999 Academic Press.