Daily rhythm in serum melatonin and leptin levels in the Syrian hamster (Mesocricetus auratus)

Melatonin is produced and secreted by the pineal gland in a rhythmic manner; circulating levels are high at night and low in the day. Leptin is a hormone secreted by adipocytes as a product of the obese gene and plays an important role in regulating body energy homeostasis and reproductive function in rodents and humans. The present study was conducted to examine daily fluctuations in serum levels of melatonin and leptin in Syrian hamster. We measured serum leptin and melatonin levels by ELISA in (a) intact and pinealectomized (pinx) male hamsters kept under long daylight conditions [14 h of light (14L)]; (b) intact and pinx hamsters under short daylight (10L); and (c) intact hamsters in constant light (24L). Blood samples were obtained every 2 h throughout a 24-h period. Statistically significant circadian variations were found in both melatonin and leptin profiles. Their relationship was inverse, i.e. when melatonin was high in the serum, leptin was comparably low. These results suggest that there is a rhythm in leptin levels in the adult male Syrian hamster and this rhythm is pineal gland (melatonin) and/or photoperiod dependent.