Potential roles of the melatonin system in the promotion of ovarian estradiol secretion in the sea cucumber, Apostichopus japonicus.

Melatonin (MT), synthesized from tryptophan, modulates reproduction through its interaction with melatonin receptors (MTNRs). While its functions in vertebrates are well established, the MT system remains poorly understood in echinoderms. To elucidate MT synthesis regulation, we examined AjAsmt, a pivotal enzyme in MT biosynthesis, to determine its tissue-specific distribution in Apostichopus japonicus. AjAsmt was predominantly expressed in the nerve ring, polian vesicle, intestine, and muscle, with significantly reduced intestinal expression post-spawning, suggesting a role in ovarian development and physiological regulation. Building on this, we characterized the molecular and functional properties of AjMTNR using computational analyses and in vitro assays. MT treatment triggered AjMTNR internalization and elevated intracellular cAMP and Ca2+ levels. Notably, AjMTNR was highly expressed in the gonads, prompting an investigation into MT's role in estradiol (E2) secretion. In vitro experiments confirmed that MT stimulates E2 secretion in ovarian tissue in a concentration-dependent manner. These findings enhance our understanding of the MT system's physiological functions and provide valuable insights for the reproductive management and aquaculture of A. japonicus.