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
melatonin, using an organ culture of pineal glands.
Gonadectomy 2 weeks before the animal was killed increased the amount of
melatonin secreted into the medium by the pineal glands of female rats but not of male rats. The increase in in vitro
melatonin secretion after
ovariectomy in female rats was prevented by
estrogen replacement.
Ovariectomy 3 and 4 weeks before death also significantly increased the amount of
melatonin secretion. Administration of GnRHa 2 weeks before
decapitation significantly decreased serum
estradiol concentrations and significantly increased
melatonin secretion by the pineal glands of female rat. GnRHa administration 3 or 4 weeks before
decapitation also significantly decreased serum
estradiol concentrations, but did not increase pineal secretion of
melatonin. The results indicate that
ovariectomy increases
melatonin secretion from organ-cultured pineal glands and that this increase is suppressed by
estrogen in adult female rats. In contrast,
orchiectomy in male rats does not influence in vitro secretion of
melatonin. These results suggest that the
GnRH-
gonadotropin system may participate in the regulation of pineal
melatonin secretion in adult female rats.