Akihiro Takemura

Japanese eel ( Anguilla japonica ) is a commercially important fish species in Asia. Understanding factors like photoperiod, temperature, and lunar cycles is crucial for successful aquaculture and managing its reproduction. Melatonin and dopamine (DA) are essential for regulating reproduction in vertebrates, including fish. This study investigated the effects of melatonin and DA on the reproductive system of mature male Japanese eels to better understand reproductive regulation in fish. To clarify the effects of these hormones on sexual maturation in eels, a critical stage in the reproductive process, sexual maturation was induced by injecting human chorionic gonadotropin, which stimulates the production of sex hormones. To check the effect of melatonin and DA on sexual maturation, DA, melatonin, and DA + domperidone were intraperitoneally injected into fish from each group (six per treatment) at a dose of 1 mg/kg body weight. The fish were then examined using quantitative RT‐PCR by comparing the messenger RNA level of reproduction‐related genes (gonadotropin releasing hormone 1; gnrh1 , gonadotropin releasing hormone 2; gnrh2 , follicle stimulating hormone; fshβ , luteinizing hormone; lhβ and DA receptor 2b; d2b ), involved in the gonadotropic axis in eels, to those that received a control injection. The results indicate significant differences in the expression levels of gnrh1 , gnrh2 and d2b in the brain and d2b , fshβ , lhβ in the pituitary at different stages of sexual maturation. Melatonin appears to enhance the production of sex gonadotropins, whereas DA inhibits them. These findings suggest an interaction between melatonin and DA in regulating reproduction in Japanese eels.