Oxytocin System
The hormonal system centred on oxytocin, a 9 amino acid peptide involved in labour, lactation, and social bonding. Synthetic oxytocin is one of the most widely used peptide drugs globally, primarily for labour induction and prevention of postpartum haemorrhage. It appears on the WHO Essential Medicines List.
Technical Context
Oxytocin (Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2, differing from vasopressin at positions 3 and 8) acts through the oxytocin receptor (OXTR), a Gαq-coupled GPCR. In the uterus, OXTR expression increases dramatically near term (up to 200-fold) due to oestrogen priming. Oxytocin stimulates rhythmic uterine contractions through IP3-mediated calcium release in myometrial smooth muscle. A positive feedback loop (Ferguson reflex) amplifies the signal during labour: cervical stretch → afferent nerve signals to hypothalamus → more oxytocin release → stronger contractions → more cervical stretch. In the mammary gland, oxytocin causes myoepithelial cell contraction for milk ejection. Synthetic oxytocin (Pitocin/Syntocinon) is administered IV for labour induction/augmentation and IM for postpartum haemorrhage prevention.