Changes of melatonin and its receptors in synchronizing turbot (Scophthalmus maximus) seasonal reproduction and maturation rhythm

Abstract

In most fish, reproduction is seasonal or periodic under the suitable conditions. In turbot (Scophthalmus maximus) farms, one of the most economically important marine flatfish species, changes in daylength could cause changes in the spawning time. In this study, to characterize the regulation of reproductive physiology following light signals, three melatonin receptors (Mtnr) investigated in turbot were named smMtnr1, smMtnr2, and smMtnr1c. Distinct expression profiles demonstrated that Mtnr mRNAs were concentrated in the brain (as detected in the hypothalamus (Hy) and mesencephalon (Me)), gonad and eye. The most abundant Mtnr1 and Mtnr2 mRNA expression levels were detected in the central nervous system at the beginning of the breeding season, suggesting that Mtnr1 and Mtnr2 may play vital roles in the regulation of turbot gonadal development. In addition, the melatonin profiles gradually increased and reached to the highest level at the spawning stage, indicating that melatonin is a potent hormone in the regulation of fish oocyte growth and maturation. The results of this study suggested that melatonin is the primary factor that transduces the light signal and regulates the physiological functions of turbot seasonal reproduction. Moreover, the results of this study may establish a foundation for further research seeking to identify fish melatonin receptors involved in the gonadal development and gamete maturation.

Melatonin Improves Turbot Oocyte Meiotic Maturation and Antioxidant Capacity, Inhibits Apoptosis-Related Genes mRNAs In Vitro

“…In the present study, mtnr1 demonstrated higher mRNA levels than mtnr2 and mtnr3 in follicles and denuded oocytes in turbot. Zhao et al [77] found high mRNA levels of mtnr1 and mtnr2 during turbot ovarian development. The mtnr1 mRNA is significantly upregulated in oocytes and follicles after melatonin treatment, whereas those of mtnr2 and mtnr3 remained unchanged in the current study.…”

New Evidence for the Existence of Two Kiss/Kissr Systems in a Flatfish Species, the Turbot (Scophthalmus maximus), and Stimulatory Effects on Gonadotropin Gene Expression

“…The developmental stages of gonads were confirmed based on ovarian and testicular histology. All of the collected turbots were classified into II, III, IV, V, and VI stages according to the dominant gamete type and characteristics ( 56 , 57 ).…”