Antagonism of oxytocin prevents suckling- and estradiol-induced, but not progesterone-induced, secretion of prolactin

Abstract

In female rats, estradiol (E 2) and suckling induce prolactin (PRL) secretion. This involves inhibition of hypothalamic dopaminergic tone and stimulation by a PRL-releasing hormone, possibly oxytocin (OT). Infusing an OT antagonist (OTA) iv, we evaluated the role of OT on suckling- and E 2-induced PRL secretion. Three days after parturition at 0900 h, lactating dams were fitted with 24-h osmotic minipumps filled with saline or OTA. Ond5of lactation, pups were separated from their dams for 6 h. Immediately or 20 min afterthe resumption of suckling, dam trunk blood was collected. Also, ovariectomized (OVX) rats were treated with E 2 (OVE) and OTA at 1000 h on d 1. Blood samples were obtained from 1300 to 2100 h on d 2 for PRL measurements. Additionally, OVX rats were evaluated on d 2 after receiving progesterone (P 4). OTA blocked suckling and E 2-induced release of PRL but not that induced by E 2 + P 4. Pups from treated dams failed to gain weight when allowed to nurse for 20 min on d 5 but gained more than 7 g when nursed ond7of lactation, indicating that the OTA was active 48 h later. Western blot analysis showed that E 2 treatment increased OT receptors in the anterior pituitary when compared with OVX animals. No further increase was observed in response to the P 4, suggesting that the enhancing effect of P 4on E 2-induced PRL release may act through mechanisms independent of OT. These data demonstrate the role of OT in the control of suckling and steroid-induced PRL secretion. Copyright © 2009 by The Endocrine Society.