Daytime melatonin treatment influences food-carrying (hoarding) behavior in rats.

Objective. Three experiments were performed to verify whether melatonin (MEL) may influence hoarding behavior in rats. This hypothesis was supported by the consideration that leptin treatment decreases food hoarding in hamsters and that an inverse relation exists between plasma leptin concentration and MEL treatment. Methods. Male Sprague-Dawley rats housed individually and kept on 12h:12h light-dark cycle were used. First experiment was performed to check whether a bimodal distribution of food hoarding scores exists in rats, and to select two groups of high (HH-rats) or low (LH-rats) hoarding. Second experiment was designed to verify whether MEL treatment modifies food-hoarding, while the third one was performed to investigate whether MEL treatment was able to modify the reciprocal relation between leptin and MEL plasma concentration. Results. In rats the hoarding tendency fell into a bimodal distribution and the plasma leptin concentration was significantly higher in HH-rats than LH-rats. When MEL was injected, circulating concentration of leptin was decreased in both HH-rats and LH-rats and such MEL treatment significantly increased the number of pellets hoarded by LH-rats but not that hoarded by HH-rats. Conclusions. MEL influences the food-hoarding in rats either directly, or indirectly by the MEL and leptin reciprocal interaction. Our results support the hypothesis that the endocrine system either directly, by the action of one or more combined hormones (MEL, leptin), or indirectly via its actions on neural substrates determines, at least in part, food-hoarding of rats.