Green light inhibits GnRH‐I expression by stimulating the melatonin‐GnIH pathway in the chick brain

To study the mechanism by which monochromatic light affects gonadotrophin‐releasing hormone (GnRH) expression in chicken hypothalamus, a total of 192 newly‐hatched chicks were divided into intact, sham‐operated and pinealectomy groups and exposed to white (WL), red (RL), green (GL) and blue (BL) lights using a light‐emitting diode system for 2 weeks. In the GL intact group, the mRNA and protein levels of GnRH‐I in the hypothalamus, the mean cell area and mean cell optical density (OD) of GnRH‐I‐immunoreactive (‐ir) cells of the nucleus commissurae pallii were decreased by 13.2%‐34.5%, 5.7%‐39.1% and 9.9%‐17.3% compared to those in the chicks exposed to the WL, RL and BL, respectively. GL decreased these factors related to GnRH‐I expression and the effect of GL was not observed in pinealectomised birds. However, the mRNA and protein levels of hypothalamic gonadotrophin‐inhibitory hormone (GnIH) and GnIH receptor (GnIHR), the mean cell area and mean cell OD of the GnIH‐ir cells of the paraventricularis magnocellularis, and the plasma melatonin concentration in the chicks exposed to GL were increased by 18.6%‐49.2%, 21.1%‐60.0% and 8.6%‐30.6% compared to the WL, RL and BL intact groups, respectively. The plasma melatonin concentration showed a negative correlation with GnRH‐I protein and a positive correlation with GnIH and GnIHR proteins. Protein expression of both GnRH‐I and GnIHR showed a negative correlation in the hypothalamus. After pinealectomy, GnRH‐I expression increased, whereas plasma melatonin concentration, GnIH and GnIHR expression decreased, and there were no significant differences among the WL, RL, GL and BL groups. Double‐labelled immunofluorescence showed that GnIH axon terminals were near GnRH‐I neurones, some GnRH‐I neurones coexpressed with GnIHR and GnIH neurones coexpressed with melatonin receptor subtype quinone reductase 2. These results demonstrate that green light inhibits GnRH‐I expression by increasing melatonin secretion and stimulating melatonin receptor‐GnIH‐GnIH receptor pathway in the chick brain.