Melatonin secretion from organ-cultured pineal glands of rats: modulation by gonadectomy and gonadotropin-releasing hormone agonist administration

The objective of the study was to evaluate the effect of pretreatments such as gonadectomy in male and female rats, and gonadotropin-releasing hormone agonist (GnRHa) administration in female rats, on levels of secretion of m elatonin. using an organ culture of pineal glands. Gonadectomy 2 weeks befo re the animal was killed increased the amount of melatonin secreted into th e medium by the pineal glands of female rats but not of male rats. The incr ease in in vitro melatonin secretion after ovariectomy in female rats was p revented by estrogen replacement. Ovariectomy 3 and 4 weeks before death al so significantly increased the amount of melatonin secretion. Administratio n of GnRHa 2 weeks before decapitation significantly decreased serum estrad iol concentrations and significantly increased melatonin secretion by the p ineal glands of female rat. GnRHa administration 3 or 4 weeks before decapi tation also significantly decreased serum estradiol concentrations, but did not increase pineal secretion of melatonin. The results indicate that ovar iectomy increases melatonin secretion from organ-cultured pineal glands and that this increase is suppressed by estrogen in adult female rats. In cont rast, orchiectomy in male rats does not influence in vitro secretion of mel atonin. These results suggest that the GnRH-gonadotropin system may partici pate in the regulation of pineal melatonin secretion in adult female rats.