Changes in Melatonin and Dopamine Secretion During Ovarian Development in Japanese Eel

In the present study, we investigated the intricate mechanisms governing the reproductive physiology of female Japanese eels. By investigating the effects of weekly intraperitoneal injections of salmon pituitary extract on the gonadosomatic index and expression levels of reproduction-associated genes, including gnrh1, fshβ, lhβ, and aromatase (cyp19a), we elucidated the key processes underlying ovarian development and steroidogenesis. Our results demonstrated induced ovarian development at stages 1–4, accompanied by distinct transcriptional profiles in the brain, pituitary gland, and ovary. Notably, gnrh1 exhibited increased expression toward stage 3 in the brain, suggesting a potential role in regulating vitellogenesis, whereas fshβ and lhβ mRNA levels peaked during stages 2 and 3, respectively, highlighting their significance in ovarian growth. Furthermore, we observed a significant increase in estradiol (E₂) levels from stages 2 to 3 of vitellogenesis, which stabilized at stage 4. This trend in E₂ levels underscores the pivotal role of this hormone in supporting oocyte development during mid-vitellogenesis and indicates that a carefully regulated hormonal environment is necessary for successful ovarian maturation. A notable surge in cyp19a expression during mid-vitellogenesis underscores the crucial role of cyp19a in estrogen synthesis. Additionally, we observed significant variations in melatonin and dopamine levels in blood plasma, suggesting potential regulatory roles in eel reproduction. These findings deepen our understanding of the complex interplay between endocrine factors and environmental cues in shaping the reproductive strategies of Japanese eel and lay the groundwork for further research.