Role of monochromatic light on daily variation of clock gene expression in the pineal gland of chick

- Green light enhanced positive clock gene expression and decreased negative ones.
- Red light increased negative clock genes expression and decreased positive ones.
- Green light lifted melatonin level, but red light suppressed that in plasma.
- Green and red light affected melatonin level by modulating clock gene expression.
- Blue light advanced acrophases of negative clock genes and delayed positive ones.

The avian pineal gland is a master clock that can receive external photic cues and translate them into output rhythms. To clarify whether a shift in light wavelength can influence the circadian expression in chick pineal gland, a total of 240 Arbor Acre male broilers were exposed to white light (WL), red light (RL), green light (GL) or blue light (BL). After 2 weeks light illumination, circadian expressions of seven core clock genes in pineal gland and the level of melatonin in plasma were examined. The results showed after illumination with monochromatic light, 24 h profiles of all clock gene mRNAs retained circadian oscillation, except that RL tended to disrupt the rhythm of cCry2. Compared to WL, BL advanced the acrophases of the negative elements (cCry1, cCry2, cPer2 and cPer3) by 0.1-1.5 h and delayed those of positive elements (cClock, cBmal1 and cBmal2) by 0.2-0.8 h. And, RL advanced all clock genes except cClock and cPer2 by 0.3-2.1 h, while GL delayed all clock genes by 0.5-1.5 h except cBmal2. Meanwhile, GL increased the amplitude and mesor of positive and reduced both parameters of negative clock genes, but RL showed the opposite pattern. Although the acrophase of plasma melatonin was advanced by both GL and RL, the melatonin level was significantly increased in GL and decreased in RL. This tendency was consistent with the variations in the positive clock gene mRNA levels under monochromatic light and contrasted with those of negative clock genes. Therefore, we speculate that GL may enhance positive clock genes expression, leading to melatonin synthesis, whereas RL may enhance negative genes expression, suppressing melatonin synthesis.