Oxytocin

Oxytocin acts as an agonist at the oxytocin receptor (OXTR), a G-protein-coupled receptor. Peripherally it triggers contraction of the smooth muscle of the uterus (labour-promoting) and mediates the milk-ejection reflex (milk let-down) by contracting the myoepithelial cells of the mammary gland. In the central nervous system, oxytocin is attributed a role in social and bonding-related (affiliative) behaviour; these central-nervous functions are predominantly the subject of research and have not been conclusively clarified in humans. Whether and to what extent intranasally administered oxytocin allows functionally relevant amounts to reach the brain via the nose-to-brain route is scientifically disputed.

Central-nervous uptake and efficacy after intranasal administration (nose-to-brain transport) is controversially discussed in the literature. Methodological problems — variable application, uncertain CNS penetration, low statistical power, publication bias — complicate the interpretation of the behavioural studies.

The chemical structure and synthesis of oxytocin was elucidated in 1953 by Vincent du Vigneaud; it was one of the first peptide hormones to be synthesised, for which du Vigneaud received the Nobel Prize in Chemistry in 1955. Synthetic oxytocin has been used in obstetrics for decades and is among the established medicines. The hypothesis of the "bonding" or "cuddle hormone" arose chiefly from animal-experimental and human behavioural research of recent decades and was later heavily marketed in popular-science form.

• Obstetrics: induction and augmentation of labour as well as control of bleeding after birth (established, approved use under clinical monitoring).
• Autism spectrum / social cognition: intranasal oxytocin is investigated as a possible modulator of social skills; meta-analyses show at most small effects on social behaviour and no clear effect on routinized/repetitive behaviours (evidence mixed, possible publication bias).
• Anxiety, stress reactivity and social trust: the subject of experimental behavioural research with inconsistent results — no robust basis for broad clinical application.
• Breastfeeding/lactation: physiological role in the milk-ejection reflex (research and physiological context).

• Obstetric effects: triggering or augmentation of uterine contractions; milk let-down.
• Uterine overstimulation: strong (hypertonic) or sustained (tetanic) contractions, which can lead to a precipitous course of labour and, in extreme cases, to uterine rupture.
• Water intoxication/hyponatraemia: through its antidiuretic intrinsic action, fluid retention, seizures and, in severe cases, coma can occur — particularly with prolonged infusion.
• Cardiovascular reactions: among others cardiac arrhythmias (e.g. ventricular extrasystoles), blood-pressure changes.