Effects of melatonin and natural and synthetic analogues of arginine vasotocin on plasma prolactin levels in adult male rats.

A significant elevation in plasma prolactin was observed 10 min following the intravenous injection of 100 µg of melatonin into either estrogen-progesterone (EP) primed or into nonsteroid-treated male rats. 60 min postinjection in the EP-primed rat, the groups treated with 100 µg or 10 mg of melatonin had significantly elevated plasma prolactin levels while no effect was observed with these same doses in the nonsteroid-treated rats. Compared to diluent-treated controls, a significant elevation in plasma prolactin was observed at 10, 20 and 60 min following the intravenous injection of either 1 µg arginine vasotocin (AVT) or 1 mg melatonin into EP-primed male rats. A consistent rise in plasma prolactin was also evident after the injection of 1 µg of either arginine vasopressin, lysine vasopressin or AVT. Oxytocin had no effect on plasma prolactin values. The intravenous administration of 1 µg of (deamino-1,6 dicarba, 8-arginine)-vasotocin caused a significant elevation of plasma prolactin 10 and 20 min after injection. However, the injection of another analogue of AVT, (4-leucine, 8-arginine)-vasotocin, had no effect on prolactin release at the time points measured.