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The effects on inhibiting estrogen production on sexual development in coho salmon (Oncorhynchus kisutch) Afonso, Luis Orlando Bertolla


The effects of inhibiting estrogen production on sexual development in coho salmon (Oncorhynchus kisutch) were investigated in this study through in vivo and in vitro experiments, using an aromatase inhibitor. In the in vivo studies female were injected with the aromatase inhibitor Fadrozole during vitellogenesis and close to final maturation, and males during spermatogenesis. In both females and males the effects of Fadrozole were investigated by determining the changes in plasma sex steroid levels (17β-estradiol, 17α-20p-P, testosterone and 11-ketotestosterone) before and several times after intraperitonial injection with different doses of Fadrozole, and by evaluating the reproductive development. In the in vitro studies the effects of Fadrozole, gonadotropins (GTH I and GTH II), and steroid hormone precursors (testosterone and 17α-hydroxyprogesterone) on steroid hormone secretion (17β-estradiol, 17α-20β-P, and testosterone) by coho salmon ovarian follicles were determined during vitellogenesis (immature and central germinal vesicle) and close to final maturation (peripheral germinal vesicle). The in vivo experiments in females close to final maturation showed that injection of the aromatase inhibitor Fadrozole was effective in lowering plasma 17β-estradiol levels, and an associated increase in plasma 17α-20β-P levels occurred, indicating that the shift between plasma 17β-estradiol biosynthesis and 17α-20β-P biosynthesis, which is characteristic of maturating fish, was advanced by treatment with Fadrozole. It was also shown that Fadrozole can be used as a tool to induce ovulation in coho salmon. In vitellogenic females Fadrozole reduced plasma 17β-estradiol levels in a dose and time dependent manner, and increased plasma 17α-20β levels prematurely also in a dose and time dependent manner. The switch from 17β-estradiol to 17α-20β-P biosynthesis was transient in the groups that received only one injection. The study also showed that multiple injections with Fadrozole arrested ovarian development, indicating Fadrozole may have a potential as a tool to regulate sexual development in salmon. In males, injection with Fadrozole inhibited 17β-estradiol secretion in the brain and caused a premature and transient increase in plasma 17α-20β-P levels. Multiple injections caused further increases in plasma 17α-20β-P, and 16 days after the beginning of the experiment the treatment groups which received the highest doses of Fadrozole (10 mg/kg and 5 x 10 mg/kg) started to spermiate, suggesting that the administration of aromatase inhibitors such as Fadrozole may provide a means of accelerating spermiation in salmonids and potentially other species of fish. The series of in vitro experiments demonstrated that Fadrozole is capable of inhibiting 17β-estradiol secretion throughout the periovulatory period, and that inhibition of 17β-estradiol per se does not elicit 17α-20β-P secretion, showing that gonadotropin is necessary for its secretion. It was also evident that 17p-estradiol secretion is dependent on availability of the substrate testosterone, which in turn is secreted in response to the gonadotropins. On the other hand 17α-20β-P secretion is directly dependent on stimulation of 20β-hydroxysteroid dehydrogenase by the gonadotropins, especially GTH II. These experiments demonstrated that the aromatase inhibitor Fadrozole can be used as tool to study mechanisms involved in the regulation of sexual development in fish. Also an aromatase inhibitor such as Fadrozole could be used close to final maturation to advance ovulation in females and spermiation and males and during vitellogenesis to inhibit reproductive development in females.

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