Dose-responses of male silver eels, Anguilla australis, to human chorionic gonadotropin and 11-ketotestosterone in vivo

• Human chorionic gonadotropin (hCG), but not 11-ketotestosterone (11KT), effectively induced spermatogenesis in eel in vivo
• Intramuscular injections of hCG were more effective than intra-peritoneal injections
• A deduced dose of 0. 47 IU/g BW or 0.85 IU/g BW yielded a half-maximal 11KT or GSI response, respectively, after 4 weeks
• 11KT treatment decreased the testicular expression of androgen receptors in male eels in vivo.

Human chorionic gonadotropin (hCG) is widely used to induce spermatogenesis in male anguillids, but neither the efficacy of intramuscular (IM) versus intraperitoneal (IP) injection, nor the dose leading to a half-maximum response has been determined. We therefore conducted in vivo dose-response studies with single administrations of hCG in silver-staged male shortfinned eels and evaluated the gonadosomatic index (GSI), testicular histology and plasma 11-ketotestosterone (11KT) levels. Half-maximum effects of hCG on GSI and 11KT levels were deduced to require 0.47 and 0.85 IU hCG/g body weight, respectively. Gonadal responses were significantly stronger for IM than for IP injections. Subsequently, the effects of IP implants containing 11KT, a major mediator of gonadotropin action in the eel testis, were determined; our prediction that 11KT would mimic hCG effects was largely rejected - although testes responded dose-dependently within the 11-KT range tested, testicular growth was modest and germ cell development appeared to be associated primarily with limited spermatogonial proliferation. Expression of the luteinizing hormone receptor and both androgen receptors in the testes was significantly lower in 11KT-treated eels than in controls. We conclude that i) IM hCG injections into male eels may be preferable over IP injections, ii) that 11KT implants can only partly mimic the effects of hCG in male eels in vivo, and iii) that hCG dose-response designs hold promise to evaluate treatment effects and compare between studies from different research groups.Statement of relevance/impactInduction of spermatogenesis in captive male eels using hCG.