Evidence for prolactin-releasing and release-inhibiting effects of melatonin, serotonin and arginine vasotocin.

Adult female Wistar rats (in 12 hr light/12 hr dark) were pinealectomized (PX) or sham-operated (SO) either 21 days after ovariectomy or on the 15-17th day of pregnancy. Ovariectomized (OVX) rats were injected with estrogen and progesterone (EP) 48 hr before decapitation. Melatonin, serotonin or arginine vasotocin (AVT; 50, 100 or 200 micrograms) were administered intravenously into OVX-EP rats 9 days after pineal removal. In PX and SO groups, the same study was done 3 days after delivery. Sera and pituitaries were collected 30 min after injection in order to determine prolactin (PRL) levels. Fifty micrograms melatonin significantly suppressed serum PRL levels in PX-OVX-EP rats and PX postpartum rats, but had not significant effect in SO-OVX-EP or PX postpartum rats. After administration of AVT, serum PRL levels markedly rose in PX and SO rats. These results suggest that melatonin may act not only to stimulate but also to inhibit rat PRL secretion and that the stimulatory function would be superior to its inhibitory function when the pineal gland is intact.