Estradiol and para-Chlorophenylalanine Downregulate the Expression of Brain Aromatase and Estrogen Receptor-α mRNA during the Critical Period of Feminization in Tilapia (Oreochromis mossambicus)

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Abstract
The period of maximal feminizing action of 17β-estradiol (E2) upon sex ratio is before 10 days posthatching in tilapia (Oreochromis mossambicus). The effect of E2 at this time is mimicked by para-chlorophenylalanine (p-CPA), a serotonin (5-hydroxytryptamine; 5-HT) synthesis inhibitor. The effect of E2 on sexual differentiation may be mediated by the 5-HT system, which is consistent with the suggestion in mammals. The masculinizing actions of 17α-methyltestosterone (MT) are most potent later at up to day 20 of age, and may depend on MT induction of aromatase activity. In the present study, the effects of gonadal steroids and p-CPA on brain aromatase and estrogen receptor (ER) mRNA expression during the critical period of sexual differentiation were investigated. Treatment of tilapia with E2 resulted in a significant decrease in the expression of brain aromatase and ERα between days 0 and 10, but not subsequently. The effect of E2 at this time can be mimicked by p-CPA. Treatment of tilapia with MT, by contrast, resulted in a significant increase in brain aromatase, ERα and ERβ mRNA expression when given between days 10 and 20. The downregulation of brain aromatase and ERα mRNA expression by E2 before 10 days of age and, in turn, the upregulation of brain aromatase and ERα and ERβ mRNA expression by MT at up to day 20 of age coincide with the period in which E2 and MT have the maximal effect on gonadal feminization and masculinization, respectively.

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