Objective: 

Leptin, a hormone produced mainly by adipose tissue, regulates food intake and energy expenditure. It functions also as a metabolic and neuroendocrine hormone. In mammals, leptin shows a diurnal rhythm. Circadian rhythm governs cycles in rest-activity, food intake and metabolism. Such rhythm are mediated and imposed on the organism through the circadian system consisting of the hypothalamus and pineal gland. The main signal from this complex is hormone melatonin. The present study examined daily fluctuations in serum levels of melatonin and leptin.

Methods: 

We investigated a daily pattern of melatonin and leptin levels in serum of Japanese quail kept in light:dark cycle 16:8 h. Blood samples were taken from the wind jugular vein at 12:00, 16:00, 20:00, 00:00, 04:00 or 08:00 h throughout a 24-h period. Blood in dark period was taken under dim red light. Serum aliquots were aspirated and frozen at -20 0C. Hormones were measured by commercial ELISA kits.

Results: 

Significant daily patterns in concentration of melatonin and leptin were recorded. As in other species melatonin levels were low during the day (below 60 pg/ml) and high at night (280 pg/ml). Leptin showed a clear daily rhythm as being low during the day (below 7 ng/ml) and high at night (28 ng/ml)

Conclusions: 

The results show that in addition to melatonin, there is also a rhythmic change in leptin hormone, but melatonin/photoperiod may not contribute directly to the difference of leptin levels. Leptin might be controlled by different circadian oscillators or of the same set of circadian oscillators, but they are not closely coupled at least, in Japanese quail.