Regulation of pituitary gonadotropin II and growth hormone content by sex steroids and pituitary extract in the aquacultured Japanese eel, Anguilla japonica

The objectives were to investigate the regulation of brain mammalian gonadotropin-releasing hormone (mGnRH), pituitary gonadotropin II (GTH II), growth hormone (GH), and gonadosomatic index (GSI) using sex steroids and salmon pituitary extract in aquacultured Japanese eels, Anguilla japonica. Yellow Japanese eels were collected from culture ponds and treated at 2-week intervals with various doses of estradiol-17beta (E-2), testosterone (T), and 5alpha-dihydrotestosterone (DHT), or weekly for varying periods with salmon pituitary extract. GSI was not significantly stimulated in females after three steroid injections over a 6-week period, nor in males after six injections over a 12-week period (E-2, T, and DHT; for each steroid with two doses, 0.75 and 3.75 mg/kg body weight). Salmon pituitary extract (12 mg/kg body weight) induced a progressive increase in GSI after 3-, 6-, 9- and 14-injections over a 14-week period. E-2 and pituitary extract significantly increased pituitary GTH II content in a dose-dependent manner but did not affect pituitary GH content. Androgens (T and especially DHT) inhibited pituitary GH content. Androgens also stimulated pituitary GTH H content but the effects were much weaker than E-2. Sex steroids did not stimulate brain mGnRH content. Brain and pituitary mGnRH content, and pituitary GTH HIIcontent were significantly increased in maturing eels (GSI>20) injected with salmon pituitary extract. Pituitary and plasma GH levels were not different between pituitary extract-treated eels and the control group. Factors other than mGnRH are suggested to mediate the in vivo estrogen effects on pituitary GTH II in cultured yellow eels. (C) 2002 Elsevier Science B.V. All rights reserved.