Effects of gonadal steroids on brain serotonergic and aromatase activity during the critical period of sexual differentiation in tilapia, Oreochromis mossambicus

The effects of gonadal steroids on brain serotonin (5-HT) and aromatase activity during the critical period of sexual differentiation were investigated in tilapia, Oreochromis mossambicus. Treatment of tilapia with 17 beta-oestradiol (E-2) between days 7 and 10 posthatching resulted in a significant increase in the female : male ratio as determined at day 90, and a significant reduction in brain 5-HT content. Treatment between days 10 and 20, or between days 20 and 30, had no significant effect. Since the 5-HT system may influence sexual differentiation, we examined the effects of para-chlorophenylalanine (p-CPA), a 5-HT synthesis inhibitor. As with E-2, treatment of tilapia with p-CPA between days 7 and 10 posthatching resulted in a significant increase in the female:male ratio. Again, treatment between days 10 and 20, or between days 20 and 30, had no significant effect. Both p-CPA and E-2 significantly depressed brain aromatase activity when administrated between days 7 and 10, but not subsequently. In tilapia treated between days 7 and 10, the brain 5-HT content was lowered by E-2 to an extent similar to that seen with p-CPA, which is consistent with the suggestion that the effect of E-2 on sexual differentiation may be mediated by the 5-HT system. Treatment of tilapia with 17 alpha-methyltestosterone (MT), by contrast, resulted in a reduction in the female:male ratio, and treatment was most effective when given between days 10 and 20. The period of maximal effect of MT upon sex ratio appears to coincide with the ability of MT to induce an increase in brain aromatase activity.