A Crosstalk Between Pineal and Major Extra-Pineal Sources of Melatonin and its Role in Ovarian Growth and Maturation in Fish

Pinealocytes of the pineal gland in vertebrates mainly synthesize melatonin (5-methoxy-N-acetyl-tryptamine). Moreover, melatonin is synthesized in several extra-pineal cells, including the photoreceptor cells of the retina, the cells of the gut, and the hepatocytes of the liver in different vertebrates, including fish species. One of the remarkable features of pineal and retinal melatonin is that it is produced rhythmically in synchronization with the environmental Light-Dark (LD) cycle, with a daily nighttime peak. However, the melatonin synthesis in tissue/cells from the extra-pineal and extra-retinal origin(s) may not always undergo photoperiod-regulated daily variations but is also dependent on the environmental food entrainment factors (in the gut), acting as the most reliable synchronizer(s) in its daily rhythm features. Moreover, the regulation of the liver and ovary (important for fish reproduction) is unclear. In this review, we attempt a comparative account of the nature and regulation of endogenous melatonin synthesis between a source like the pineal gland and many other nonpineal origins, which have gained serious attention in the last ten years. We also review the functions of melatonin in regulating fish ovarian growth and maturation. The physiological melatonin levels, manipulated either endogenously (by photoperiodic modulations) or exogenously (by injections or by feeds), have tremendous effects on reproductive events in fish at the age of its first maturity, as revealed in recent findings. Characterization and identification of the importance of pineal gland melatonin in the growth of the oocytes via the hypothalamic-pituitary-gonadal axis have been explored several years back. The identification of melatonin receptors about fourteen years back on the wall of developing oocyte spurt the breakthrough, which introduced the concept of direct control of melatonin on developing oocytes. Thus, this review gains uniqueness by addressing the latest developments recorded in the field of melatonin and fish reproduction, particularly in improving oocyte maturation. Nonetheless, an attempt has been made to underline approaches that need to be developed to apply the molecule in large-scale aquaculture.