Effects of Extreme Light Cycle and Density on Melatonin, Appetite, and Energy Metabolism of the Soft-Shelled Turtle ().

Constant darkness and constant light exposure often disturb the circadian rhythm in the behavior and energy metabolism of vertebrates. Melatonin is known as the hormonal mediator of photoperiodic information to the central nervous system and plays a key role in food intake and energy balance regulation in vertebrates. The popularly cultured soft-shelled turtle has been reported to grow better under constant darkness; however, the underlying physiological mechanism by which darkness benefits turtle growth is not clear yet. We hypothesized that increased melatonin levels induced by darkness would increase appetite and energy metabolism and thus promote growth in . In addition, in order to elucidate the interaction of photoperiod and density, juvenile turtles were treated under three photoperiods (light/dark cycle: 24L:0D, 12L:12D, 0L:24D, light density 900 lux) and two stocking densities (high density: 38.10 ind./m, low density: 6.35 ind./m) for 4 weeks, and then the blood and brain tissues of turtles were collected during the day (11:00-13:00) and at night (23:00-1:00) after 2 days of fasting. We examined changes in plasma melatonin levels, food intake (FI), and appetite-related hormones (plasma ghrelin and leptin), as well as growth and energy metabolism parameters such as specific growth rate (SGR), standard metabolic rate (SMR), plasma growth hormone (GH), and thyroid hormone/enzyme activity (plasma triiodothyronine T, thyroxine T, and T5'-deiodinase activity). Moreover, we also assessed the responses of mRNA expression levels of food intake-related genes (kisspeptin 1 (); cocaine amphetamine-regulated transcript (); neuropeptide Y ()) in the brain. The results showed that under high density, SGR was the lowest in 24L:0D and the highest in 0L:24D. FI was the highest in 0L:24D regardless of density. Plasma melatonin was the highest in 0L:24D under high density at night. SMR increased with decreasing light time regardless of density. Most expressions of the measured appetite-related genes (, , and ) were not affected by photoperiod, nor were the related hormone levels, such as plasma leptin, ghrelin, and GH. However, thyroid hormones were clearly affected by photoperiod. T level in 0L:24D under high density during the day was the highest among all treatment groups. T in 24L:0D under high density during the day and T5'-deiodinase activity in 24L:0D under low density at night were significantly reduced compared with the control. Furthermore, the energy metabolism-related hormone levels were higher under higher density, especially during the day. Together, melatonin secretion is not only modulated by light but also likely to be regulated by unknown endogenous factors and density. Altered plasma melatonin induced by constant darkness and density seems to be involved in the modulation of energy metabolism rather than appetite in the soft-shelled turtle.