Elevated plasma leptin levels of fasted rainbow trout decrease rapidly in response to feed intake

•Plasma leptin levels were after fasting and during refeeding of rainbow trout.•Anorexic fish were found in both the fasted and the ad lib fed groups.•Non-feeding and fasted fish had elevated plasma leptin levels.•Plasma leptin levels decrease rapidly when food enters the gastrointestinal tract.

Leptin has an anorexigenic effect in fish, indicating a role in regulation of growth and energy homeostasis. The study aimed to further clarify the physiological role of leptin in rainbow trout, specifically its short-term response to feed intake after a period of fasting. Utilizing a salmonid leptin radioimmunoassay, the study demonstrates differences in plasma leptin levels in fishes with different nutritional status and at the onset of feeding. Some of the fasted fish were clearly in a state of anorexia, and did not initiate feeding during the 72 h refeeding period. For those fish that did initiate feeding, both previously fed and fasted, plasma leptin levels rapidly decreased during the first 24 h in correlation with increased amount of food reaching the gastrointestinal tract, while non-feeding individuals retained a high plasma leptin levels. The data indicate that the leptin-induced anorexic state is broken after onset of feeding and that the regulatory mechanisms leading to decreased plasma leptin levels are linked to nutrient levels.