Circadian rhythm of melatonin receptors under monochromatic light and their regulatory effect on clock genes in the chick liver

Avians have a complex system of light sensation and circadian organization. We previously found that light wavelength could affect the circadian expression of clock genes in chick liver, and that melatonin participated in this process. Melatonin functions through membrane and nuclear receptors, but their circadian rhythm and the effect of monochromatic light remain unclear. The effects of different receptors on clock gene expression are also unknown. In this study, 144 newly hatched chicks were divided into sham-operation and pinealectomy groups. They were raised under white (WL), red (RL), green (GL) and blue (BL) light for two weeks, and their livers were sampled at 6 time points with 4 h intervals. Using qPCR and western blot, the mRNA and protein expression of three membrane receptors (Mel1a, Mel1b, Mel1c) and one nuclear receptor (RORα) were measured for circadian rhythm analysis. The results showed that melatonin receptors, especially Mel1a and Mel1b, displayed a significant circadian rhythm. The rhythm mesor and amplitude of the membrane receptors were enhanced by GL and reduced by RL; further, pinealectomy dampened their rhythm and diminished the effect of light wavelength. However, the rhythm of melatonin nuclear receptor contrasted that of the membrane receptors. In vitro experiments involving the addition of different agonists and antagonists into primary chick hepatocytes showed that Mel1a and RORα activation induced the expression of seven clock genes (cClock, cBmal1, cBmal2, cCry1, cCry2, cPer2, cPer3). Similarly, activation of Mel1b and Mel1c induced the expression of five clock genes (except cBmal1 and cCry1). Moreover, melatonin entrained the rhythm of cBmal1 and cPer2 in long-term cultured hepatocytes. Overall, this research elucidates the role of melatonin receptors in regulating clock gene expression, and enhances our understanding regarding the effect of light wavelength on chick circadian rhythm.