Effects of luteinizing hormone-releasing hormone analog and pimozide on the release of luteinizing hormone and ovulation in artificially matured Japanese eel Anguilla japonica

Pituitary gonadotropins, follicle-stimulating hormone (Fsh) and luteinizing hormone (Lh), control oogenesis in all vertebrates. In particular, Lh plays a key role in stimulating the final oocyte maturation and subsequent ovulation. The biosynthesis and secretion of Lh are regulated by several neurohormones, including gonadotropin-releasing hormone (GnRH) and dopamine. GnRH analogs, also known as Lh releasing hormone analogs (LHRHa), and dopamine antagonists are commonly used to induce sexual maturation in teleosts. However, the effects of these reagents differ among fish species. Therefore, in the present study, the effects of LHRHa and pimozide (a dopamine antagonist) on Lh release and ovulation induction were investigated in female eels whose ovarian development was artificially induced by recombinant Fsh with ovaries containing oocytes at the migratory nucleus stage in vitro and/or in vivo. Both LHRHa and pimozide stimulated the release of Lh from pituitary cells in a dose-dependent manner in vitro. Furthermore, the synergistic effects of LHRHa and pimozide were observed in the release of Lh from the pituitary gland. In vivo experiments demonstrated that the administration of pimozide alone or in combination with LHRHa induced the release of Lh and the fusion of oil droplets in oocytes. In addition, 17 alpha-hydroxyprogesterone (OHP) combined with LHRHa and pimozide resulted in complete ovulation at a high rate (87.5% complete ovulation and 12.5% partial ovulation), whereas LHRHa and pimozide without OHP induced partial ovulation at a low rate (0% complete ovulation and 14.3% partial ovulation). These results suggest that combined treatment with LHRHa, pimozide, and OHP could induce ovulation in artificially matured female Japanese eels. However, future studies are needed to evaluate the quality of embryos produced by females induced with LHRHa, pimozide and OHP. Additionally, the pituitary gland obtained from female eels with artificially induced sexual maturation is suitable for investigating the mechanisms of Lh release.