Oxytocin is a nine amino acid peptide hormone synthesized in magnocellular neurosecretory cells in the hypothalamus and released by the posterior lobe of the pituitary gland. Oxytocin is often bound to neurophysin (a carrier protein) as shown in the inset of the figure.

Structure

Oxytocin is a peptide of nine amino acids (a nonapeptide). The sequence is cysteine - tyrosine - isoleucine - glutamine - asparagine - cysteine - proline - leucine - glycine. The leucine is the only difference from vasopressin, which has arginine or lysine at this position. The cysteine residues form a sulfur bridge.

Functions

Functions of oxytocin:

• Letdown reflex – in lactating (breastfeeding) mothers, oxytocin stimulates myoepithelial cells , causing milk to be ejected into the ducts of the mammary glands. Sucking by the infant at the nipple stimulates oxytocin release.
• Uterine contraction – necessary for cervical dilation prior to birth and causes the contractions all mothers are familiar with. Oxytocin release during breastfeeding causes mild but often painful uterine contractions during the first few weeks of lactation. This also serves to assist the uterus in clotting the placental attachment point postpartum.
• Inducing maternal behaviour – Sheep and rat females given oxytocin antagonists after giving birth do not exhibit typical maternal behavior such as grooming towards newborns. Conversely, virgin sheep females show maternal behavior towards foreign lambs upon cerebrospinal fluid infusion of oxytocin, which they would not do otherwise.
• Plays some role in orgasms – for both males and females. In males, oxytocin is said to facilitate sperm transport in ejaculation.
• Thought to induce pair bonding in people. Levels of oxytocin are found to be higher amongst people who claim to be falling in love. It is also thought to mediate other forms of pair bonding such as friendship and family relationships as given above. Reduction of sociophobic behavior was shown after treatment with oxytocin.

Therapeutic use

Oxytocin analogues (pitocin, Syntocinon®) are used to induce labour and support labour in case of non-progression of the parturition. It has largely replaced ergotamine as the principal agent to increase uterine tone in acute postpartum haemorrhage.

An oxytocin nose spray may be a useful adjunct to stimulate breastfeeding.

Inhibition of oxytocin action is the mode of action of the new tocolytic agent atosiban (Tractocile®), which is registered in many countries to suppress premature labour between 24 and 33 weeks of gestation. It has fewer side-effects than drugs previously used for this purpose (ritodrine , salbutamol and terbutaline).