Effects of melatonin on the release and synthesis of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) at the hypothalamus and pituitary levels have been explored in some species, but a similar study in the corpora lutea (CL) has not yet been conducted. In this study, the immunostaining for GnRH and LH was observed in luteal cells of porcine CL during pregnancy, and a significant effect of pregnant stage on the level of GnRH and LH was found; higher values for GnRH and LH immunostaining and mRNA were detected in the early and mid-stages CL than in the later-stage CL (P < 0.01). Furthermore, the patterns of melatonin membrane receptors (MT1 and MT2) expression were consistent with those of GnRH and LH expression in the CL of pregnant sows; the relative levels of MT1 and MT2 in the early and mid-stages were significantly higher than those in the later-stage (P < 0.01). In luteal cells, melatonin dose-dependently increased in GnRH and LH secretion and mRNA expression. Melatonin also increased the GnRH-induced accumulation of LH and the LH-induced secretion of P4 in luteal cells. Additionally, the effects of melatonin on luteal GnRH and LH production were blocked by luzindole, a non-selective MT1 and MT2 receptor antagonist. Our results demonstrate the stimulatory effects of melatonin on GnRH and LH production in luteal cells of pregnant sows, suggesting a potential role for melatonin in luteal function through regulating the release and synthesis of GnRH and LH in luteal cells.
Keywords: GnRH; LH; corpus luteum; melatonin; melatonin membrane receptors; pig.