Effects of nutritional status on plasma leptin levels and in vitro regulation of adipocyte leptin expression and secretion in rainbow trout

• Plasma leptin levels negatively correlate with mesenteric fat index in rainbow trout.
• Leptin is demonstrated to be expressed and secreted by freshly isolated adipocytes.
• Insulin and ghrelin stimulate dose-dependently adipocyte leptin secretion in trout.
• Fish nutritional condition influences leptin production in isolated adipocytes.
• Adipocytes from feed restricted fish respond to nutrients decreasing leptin secretion.

As leptin has a key role on appetite, knowledge about leptin regulation is important in order to understand the control of energy balance. We aimed to explore the modulatory effects of adiposity on plasma leptin levels in vivo and the role of potential regulators on leptin expression and secretion in rainbow trout adipocytes in vitro. Fish were fed a regular diet twice daily ad libitum or a high-energy diet once daily at two ration levels; satiation (SA group) or restricted (RE group) to 25% of satiation, for 8 weeks. RE fish had significantly reduced growth (p < 0.001) and adipose tissue weight (p < 0.001), and higher plasma leptin levels (p = 0.022) compared with SA fish. Moreover, plasma leptin levels negatively correlated with mesenteric fat index (p = 0.009). Adipocytes isolated from the different fish were treated with insulin, ghrelin, leucine, eicosapentaenoic acid or left untreated (control). In adipocytes from fish fed regular diet, insulin and ghrelin increased leptin secretion dose-dependently (p = 0.002; p = 0.033, respectively). Leptin secretion in control adipocytes was significantly higher in RE than in SA fish (p = 0.022) in agreement with the in vivo findings, indicating that adipose tissue may contribute to the circulating leptin levels. No treatment effects were observed in adipocytes from the high-energy diet groups, neither in leptin expression nor secretion, except that leptin secretion was significantly reduced by leucine in RE fish adipocytes (p = 0.025). Overall, these data show that the regulation of leptin in rainbow trout adipocytes by hormones and nutrients seems to be on secretion, rather than at the transcriptional level.