The effects of estrogen and testosterone on the noradrenergic activity in the early developing tilapia brain were investigated. Seven-day-old (7 days post-hatch) tilapia, Oreochromis mossambicus, were continuously treated with 17 beta-estradiol (E-2) and methyltestosterone (T) until 60-day-old. The brain NE content of tilapia, before 30 days old, increased with age. During this period, a developing period of the central noradrenergic system of tilapia, continuous treatment with T (100 mg/kg diet) significantly increased the NE content in the brains of 10-day-old tilapia, but had no effect on the 20- and 30-day-olds. As while, continous treatment with E-2 (100 mg/kg diet) significantly increased the NE content in the brains of 20-day-old tilapia but had no influence on the 10- and 30-day-olds. After this developing period of central noradrenergic system, T increased the brain NE content of 40- and 60-day-olds, but had no effect on 50-day-olds. Whereas, E-2 had no effect during the experiments. These results indicate that T has a facilitative effect on the central noradrenergic activity during the periods from 0 to 10, 30 to 40, and 50 to 60 days old; while E-2 is from 10 to 20 days old. It is suggested that T and E-2 have a facilitative effect on the noradrenergic activity in the early developing tilapia brain with a specific effective period.
