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

The effects of gonadal steroids on brain serotonin (5-HT) and aromatase act ivity during the critical period of sexual differentiation were investigate d in tilapia, Oreochromis mossambicus. Treatment of tilapia with 17 beta-oe stradiol (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 signifi cant 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 ma y influence sexual differentiation, we examined the effects of para-chlorop henylalanine (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 signif icant increase in the female:male ratio. Again, treatment between days 10 a nd 20, or between days 20 and 30, had no significant effect. Both p-CPA and E-2 significantly depressed brain aromatase activity when administrated be tween 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 eff ect of E-2 on sexual differentiation may be mediated by the 5-HT system. Tr eatment of tilapia with 17 alpha-methyltestosterone (MT), by contrast, resu lted in a reduction in the female:male ratio, and treatment was most effect ive when given between days 10 and 20. The period of maximal effect of MT u pon sex ratio appears to coincide with the ability of MT to induce an incre ase in brain aromatase activity.