Melatonin modulates monochromatic light-induced GHRH expression in the hypothalamus and GH secretion in chicks

• GHRH-ir neurons were found in the infundibular nucleus of chick hypothalamus.
• Green light illumination enhanced GHRH expression of hypothalamus and plasma GH secretion.
• Melatonin played a critical role in green light-induced expression of GHRH in the hypothalamus.

To study the mechanism by which monochromatic lights affect the growth of broilers, a total of 192 newly hatched broilers, including the intact, sham-operated and pinealectomy groups, were exposed to white light (WL), red light (RL), green light (GL) and blue light (BL) using a light-emitting diode (LED) system for 2 weeks. The results showed that the GHRH-ir neurons were distributed in the infundibular nucleus (IN) of the chick hypothalamus. The mRNA and protein levels of GHRH in the hypothalamus and the plasma GH concentrations in the chicks exposed to GL were increased by 6.83–31.36%, 8.71–34.52% and 6.76–9.19% compared to those in the chicks exposed to WL (P = 0.022–0.001), RL (P = 0.002–0.000) and BL (P = 0.290–0.017) in the intact group, respectively. The plasma melatonin concentrations showed a positive correlation with the expression of GHRH (r = 0.960) and the plasma GH concentrations (r = 0.993) after the various monochromatic light treatments. After pinealectomy, however, these parameters decreased and there were no significant differences between GL and the other monochromatic light treatments. These findings suggest that melatonin plays a critical role in GL illumination-enhanced GHRH expression in the hypothalamus and plasma GH concentrations in young broilers.