Green Light Inhibits GnRH-I Expression by Stimulating the Melatonin-GnIH Pathway in the Chick Brain

Abstract

To study the mechanism by which monochromatic light affects gonadotropin-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 (LED) system for 2 wk. 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 the GnRH-I-ir cells of the nucleus commissurae pallii (nCPa) 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 pinealectomized (PINX) birds. However, the mRNA and protein levels of hypothalamic gonadotropin-inhibitory hormone (GnIH) and GnIH receptor (GnIHR), the mean cell area and mean cell OD of the GnIH-ir cells of the paraventricularis magnocellularis (PVN), 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. Both protein expressions of GnRH-I and GnIHR showed a negative correlation in the hypothalamus. After pinealectomy, GnRH-I expression increased, whereas plasma melatonin concentration, GnIH and GnIHR expressions decreased, and there were no significant differences among the WL, RL, GL and BL groups. Double-labeled immunofluorescence showed that GnIH axon terminals were near GnRH-I neurons, some GnRH-I neurons coexpressed with GnIHR, and GnIH neurons coexpressed with melatonin receptor subtype QR2. These results demonstrated that green light inhibits GnRH-I expression by increasing melatonin secretion and stimulating melatonin receptor-GnIH-GnIH receptor pathway in the chick brain.

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