Increase in brain and pituitary radioimmunoassayable gonadotropin releasing hormone (GnRH) in the European silver eel treated with sexual steroid or human chorionic gonadotropin.

In the female silver eel, a single estradiol 17 beta (E2) injection significantly increased radioimmunoassayable GnRH (IRGnRH) in the di- and mesencephalon and also in the telencephalon and olfactory lobes, during the first following days; after a chronic estradiol treatment, the pituitary IRGnRH was doubled. In the male silver eel, a single injection of human chorionic gonadotropin (hCG), which is able to induce a progressive testicular development and a durable increase in androgens production, produced a long-term effect on IRGnRH: IRGnRH was significantly increased in the same brain areas as in E2-treated females; a more important rise (10-fold) was observed for pituitary IRGnRH, probably reflecting the accumulation of GnRH in the axonal endings which directly innervate the pituitary in teleosts. These results suggest a positive effect of sexual steroids on GnRH synthesis but not release in the silver eel.