Effects of pinealectomy and melatonin on gonadotropin-releasing hormone (GnRH) gene expression in the male rat brain

Melatonin, a pineal hormone, is known to be an important neurohormonal factor involved in the timing of reproductive events which occur seasonally in various mammalian species. In order to evaluate the influence of melatonin on neurons which are producing gonadotropin-releasing hormone (GnRH), we studied the effect of light-dark cycle as well as pinealectomy and melatonin administration on GnRH gene expression in the adult male rat medial preoptic area (MPOA) using quantitativein situ hybridization. The animals were kept under artificial light (light on 6:00 h–20:00 h). In animals which were sacrificed at 24:00 h (when endogenous melatonin levels are high), the hybridization signal was higher than that detected in animals sacrificed at 20:00 h (before the onset of darkness). Administration of melatonin during the light period (16:00 h) induced a 15% increase in the amount of GnRH mRNA after 4 h. Three weeks after pinealectomy mRNA levels were decreased by 35%. Injection of melatonin to pinealectomized rats 4 h before sacrifice increase the amount of GnRH mRNA, completely reversing the decrease in mRNA induced by pinealectomy. These results strongly suggest that melatonin produced by the pineal gland exerts a positive influence on GnRH neuronal activity in the male rat.