Monochromatic green light enhances the expression of positive-regulating clock proteins and the synthesis of melatonin in both pinealectomized and sham-operated chicks

Birds have a complex system of interacting central clocks, including SCN, pineal gland and retina. The pineal gland and retina possess the functions of sensing light, generating circadian oscillation, and secreting melatonin, making them crucial components for regulating circadian rhythms in birds. BMAL1 and CLOCK proteins form a heterodimer, playing a central role in regulating melatonin secretion and other downstream genes. However, the effect of monochromatic light on the expression of BMAL1 and CLOCK in the chick retina and melatonin, and whether the effect of monochromatic light on the retinal circadian clock depends on the regulation of the pineal gland have yet to be clarified. The present study raised newly hatched chicks under monochromatic white, red, green, and blue light for 14 days, with LD12:12 photoperiod, while the pinealectomy and sham operation were performed on the fourth day. ELISA, Western blot, immunohistochemistry, and immunofluorescence experiments were performed. The results showed that under different monochromatic lights, pinealectomy did not have any significant effect on the expression levels of BMAL1 and CLOCK proteins in the chick retina, neither altered the presence or absence of circadian rhythms of plasma melatonin, but significantly reduced the plasma melatonin levels. Monochromatic green light promotes the circadian expression of BMAL1 and the daily average expression level of CLOCK, as well as the secretion of melatonin. The BMAL1 and CLOCK proteins are mainly expressed in the photoreceptors of the retina. Therefore, chick retina is a circadian pacemaker that is relatively independent from the pineal gland, and monochromatic green light promotes the synthesis and secretion of melatonin by promoting the expression of the retinal BMAL1 and CLOCK in chicks.