The oxytocin receptor is a receptor for oxytocin (vasopressin). The activity of the oxytocin receptor is mediated by G proteins which activate a phosphatidylinositol-calcium second messenger system. Oxytocin is a mammalian hormone that acts primarily as a neuromodulator in the brain. Also known as alpha-hypophamine (αhypophamine), oxytocin has the distinction of being the very first polypeptide hormone to be sequenced and synthesized biochemically, by Vincent du Vigneaud et al. in 1953. Oxytocin is best known for roles in female reproduction: 1) it is released in large amounts after distension of the cervix and uterus during labor, and 2) after stimulation of the nipples, facilitating birth and breastfeeding.