Oxytocin, a natural mammalian hormone that was first synthesized in 1953, is a peptide  that primarily acts as a neurotransmitter in the brain.  Oxytocin plays an extensive role in the female portion of the reproductive and birthing process: during and after labor, facilitating the birthing process; and also during breastfeeding when the nipple is stimulated. Though oxytocin is best-known for the aforementioned roles, recent studies have looked for and found links to more complex social behavior: bonding, anxiety, trust, love social recognition, and more.

Oxytocin receptors are specific to oxytocin and are high-affinity rhodopsin-type G-protein-coupled receptors. They are found in the uterus, mammary gland, and various parts of the central nervous system. Oxytocin receptors are very chemically similar to vasopressin receptors which has created difficulty in prospective development of selective oxytocin receptor agonists.

In the so-called “Letdown reflex”, in lactating (breastfeeding) mothers, oxytocin activates receptors in the mammary glands, causing milk to be released into subareolar sinuses, and then excreted from the nipple.The mechanism of action begins by the infant sucking at the nipple; this information is relayed by spinal nerves to the hypothalamus. Neurons fire in intermittent bursts; these bursts result in the secretion of pulses of oxytocin from the neurosecretory nerve terminals of the pituitary gland.

Oxytocin also causes uterine contraction, which helps pre-birth cervical dilation and also causes contractions during the later stages of labor. Oxytocin released from breastfeeding sometimes causes mild (but painful) uterine contractions during the initial weeks of lactation. In knockout mice lacking the oxytocin receptor, reproductive behavior and parturition is normal.

Two studies have found increases in plasma oxytocin at orgasm in both men and women. Levels are notably increased around masturbatory orgasm and are still higher than baseline when measured 5 minutes after self arousal. Oxytocin’s effects on muscle contractions may facilitate sperm and egg transport. Oxytocin serves an important role in sexual arousal according to a study done on women. Genital orifice stimulation resulted in increased oxytocin immediately after orgasm. Another study reported increases of oxytocin during sexual arousal states which may be due in part to response to nipple, areola, genital, and/or genital orifice stimulation, which has been confirmed in other mammals.Murphy found while studying men that oxytocin levels were elevated throughout sexual arousal with no acute increase at orgasm. Another study on men found an increases in plasma oxytocin acutely after orgasm. The authors did note, however, that these changes “may simply reflect contractile properties on reproductive tissue.