Oxytocin

Oxytocin is a peptide hormone and neuropeptide produced by magnocellular neurons in the paraventricular and supraoptic nuclei of the hypothalamus and released by the posterior pituitary into the bloodstream. It exists as a nine-amino-acid cyclic peptide (nonapeptide) with the sequence CYIQNCPLG and a disulfide bond between the first and sixth cysteines. Oxytocin is synthesized as part of a larger precursor, prepro-oxytocin, which is processed and transported along axons with its carrier protein neurophysin I.

In the periphery, oxytocin stimulates uterine contractions during labor and the milk ejection reflex during breastfeeding. In the brain, it acts as a neurotransmitter and neuromodulator, influencing social behavior, bonding, maternal care, trust, and empathy. Its effects are mediated through the oxytocin receptor, a G protein-coupled receptor expressed in the uterus, mammary tissue, and several brain regions including the amygdala and nucleus accumbens. Effects occur via Gq/PLC/IP3/Ca2+ signaling and can modulate neural circuits involved in reward and stress.

Medical use and pharmacology: synthetic oxytocin is used to induce or augment labor and to prevent postpartum hemorrhage following delivery. It is administered intravenously or intramuscularly (and in some settings intranasally for research). Side effects include uterine tachysystole, fetal distress, hypotension, and hyponatremia when used with excessive fluid intake.

Research and social effects: intranasal oxytocin has been investigated for effects on social cognition and disorders such as autism, though results are variable and context-dependent. Overall, the role of oxytocin in human social behavior is complex and not fully understood, with effects highly dependent on environment, baseline social functioning, and receptor distribution.