Oxytocin

Oxytocin is a peptide hormone (a small molecule made up of a chain of amino acids) produced in the hypothalamus, a part of the brain. It is stored and released into the bloodstream by the posterior pituitary gland (neurohypophysis). It also functions as both a hormone and a neurotransmitter, meaning it affects nerve cells within the brain.

Its best-known function is related to reproduction: it stimulates the muscles of the birth canal during labour, and causes milk to be released from the breast when an infant suckles. It has also been studied for its effects on social bonding, trust, and attachment between mother and infant.

Structure and synthesis

Oxytocin consists of nine amino acids (a nonapeptide) and contains a disulfide bond that forms a ring. Its structure is very similar to that of vasopressin, another hormone that is produced in the same part of the brain; the two differ by only two amino acids. For this reason, they occasionally cross-react with each other's receptors.

Physiological function

During labour, oxytocin stimulates the smooth muscles of the birth canal to contract. This operates through positive feedback: the pressure of the infant on the birth canal increases the release of the hormone, which in turn increases contractions. In milk letdown, the sensation of suckling in the breast causes the release of a hormone that stimulates myoepithelial cells, allowing milk to be expressed.

Medical use

A synthetic form of this hormone is used in hospitals to stimulate or augment labour, and also to control bleeding following delivery while the uterus contracts. It is administered by intravenous drip with careful monitoring. Improper dosing can cause excessive contractions, so medical supervision is necessary.

Research on social behaviour

Many studies have examined the relationship between oxytocin and social behaviours, including facial recognition, trust, and bonding between individuals. The results are not consistent and depend on the experimental conditions and method of administration. Terms such as "the love hormone" are media simplifications that do not fully represent the science.