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 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.

Melatonin Receptor Mel1b‐ and Mel1c‐mediated Green Light Induced the Secretion of Growth Hormone in Anterior Pituitary of Chicks

“…The three membrane receptors play different roles in different tissues and organs. For example, melatonin inhibited GnRH secretion by Mel1c in the chick hypothalamus (29). Melatonin mediated the proliferation of T lymphocytes in GL by Mel1b and Mel1c in the chick spleen (30).…”

Effects of Light-at-Night on the Rat Liver – A Role for the Autonomic Nervous System

“…Most of these pathways are also well-known for their glucoregulatory effects. However, besides the well-described effects of LAN on melatonin and corticosterone only very few studies, in chickens and related to growth responses, have reported associations between light exposure and gonadotropin-releasing hormone (Zhang et al, 2017) and thyroid hormone levels (Yang et al, 2016). Future studies should investigate whether light indeed also affects the hypothalamic-pituitary-gonadal- and hypothalamic-pituitary-thyroid-axis in rodents.…”

Role of Melatonin in Monochromatic Light Affecting the Expression of Somatostatin in Hypothalamus and Pituitary of Chicks

“…Mel1c mediates monochromatic light-induced the secretion of IGF-1 in chick liver [4]. Meanwhile, Mel1c inhibited GnRH expression via GnIH neurons in chick hypothalamus [32]. Whether MEL receptors mediated the secretion of SST is the same as those of MEL receptors mediated GH secretion in adenohypophysis under monochromatic green light?…”