Effects of sustained administration of testosterone in pre-pubertal sea bass (Dicentrarchus labrax L)

This study investigated the effects of testosterone (T) on the levels of immunoassayable salmon GnRH (sGnRH) in the brain and the pituitary in relation to blood plasma steroid levels and gonadal activity in juvenile male sea bass (Dicentrarchus labrax L, < 1-year of age). Gonad-intact male fishes were intraperitoneally implanted for 76 days with Silastic pellets containing no steroid (control) or testosterone (T; 100 microgram/g body weight). The serum T response, the sGnRH content in whole brain and pituitary and the testicular development at 0, 15, 30 and 76 days following T implantation were assessed. Treatment with T did not significantly affect somatic growth or weight of gonads. However, gonad maturation was histologically more advanced in T-treated fishes compared to control fish. Histological examination showed that 15 days after implantation, 100% of steroid-treated fishes exhibited differentiated gonads, compared to only 50% of the controls. In addition, discrete spermiation was observed in 10% of the steroid-treated fishes 45 days after implantation while spermiation did not occur in the controls. Steroid treatment did not affect plasma oestradiol (E2) levels and all animals exhibited similar basal levels (< 0.6 ng E2/ml). Fifteen days after T-implantation, the sGnRH content of brain and pituitary and the plasma levels of T were elevated. Over the following weeks a progressive decrease in all these parameters was observed. A second implant of T provoked a new and significant (p < 0.05) rise of both plasma T and whole brain sGnRH content, indicating that the GnRH system was still responsive to the additional steroid administration. These results provide evidence of a positive feedback of steroids at the brain and/or pituitary level, indicating a possible participation of sex steroids in the acceleration of gonadal differentiation and stimulation of spermatogenesis in pre-pubertal sea bass.