Failure of melatonin to increase serum prolactin levels in ovariectomized rats subjected to superior cervical ganglionectomy or pinelaectomy.

The effects of melatonin on serum prolactin levels were examined in ovariectomized rats primed with oestradiol and progesterone, and subjected to bilateral superior cervical ganglionectomy or pinealectomy. Ganglionectomy resulted in a significant depression of the serum prolactin concentrations, as well as in impairment of the prolactin release evoked by administration of steroid. Treatment with melatonin increased serum prolactin in control but not in ganglionectomized rats. Injection of melatonin protentiated the steroid-induced release of prolactin in control rats; this effect of melatonin was not detected in ganglionectomized rats. Pinealectomy did not affect basal prolactin levels, nor impair the release of prolactin evoked by steroid treatment; however, it was effective in blocking the melatonin-induced release of prolactin in vehicle-treated rats, as well as the potentiation of steroid-induced prolactin release by melatonin. Intracranial surgery by itself increased prolactin release. These results suggest that systemically administered melatonin needs an intact pineal gland to augment serum prolactin levels.