Oxytocin is a peptide of nine amino acids (a nonapeptide). Its systematic name is cysteine-tyrosine-isoleucine-glutamine-asparagine-cysteine-proline-leucine-glycine-amine. The cysteine residues form a disulfide bond. Oxytocin has a molecular mass of 1007 daltons. One international unit (IU) of Oxytocin is the equivalent of about 2 micrograms of pure peptide. The trust-inducing property of oxytocin might help those who suffer from social anxieties and mood disorders, but with the potential for abuse with confidence tricks and military applications.
The structure of oxytocin is very similar to that of vasopressin, also a nonapeptide with a sulfur bridge, whose sequence differs from Oxytocin by two amino acids. Oxytocin and vasopressin were isolated and synthesized by Vincent du Vigneaud in 1953, work for which he received the Nobel Prize in Chemistry in 1955. Oxytocin and vasopressin are the only known hormones released by the human posterior pituitary gland to act at a distance. However, Oxytocin neurons make other peptides, including corticotropin-releasing hormone and dynorphin, for example, that act locally. Oxytocin has peripheral (hormonal) actions, and also has actions in the brain. Its actions are mediated by specific, high-affinity oxytocin receptors. The oxytocin receptor is a G-protein-coupled receptor that requires Mg2+ and cholesterol. Oxytocin is destroyed in the gastrointestinal tract, so must be administered by injection or as nasal spray. It has a half-life of typically about three minutes in the blood, and given intravenously does not enter the brain in significant quantities - it is excluded from the brain by the blood-brain barrier. Evidence in rhesus macaques indicates Oxytocin by nasal spray does enter the brain. Oxytocin nasal sprays have been used to stimulate breastfeeding, but the efficacy of this approach is doubtful.
In the hypothalamus, Oxytocin is made in magnocellular neurosecretory cells of the supraoptic and paraventricular nuclei and is stored in Herring bodies at the axon terminals in the posterior pituitary. Oxytocin is also made by some neurons in the paraventricular nucleus that project to other parts of the brain and to the spinal cord. Depending on the species, Oxytocin receptor-expressing cells are located in other areas, including the amygdala and bed nucleus of the stria terminalis. In the pituitary gland, Oxytocin is packaged in large, dense-core vesicles, where it is bound to neurophysin I. Secretion of Oxytocin from the neurosecretory nerve endings is regulated by the electrical activity of the oxytocin cells in the hypothalamus.
Outside the brain, Oxytocin containing cells have been identified in several diverse tissues, including the corpus luteum, the interstitial cells of Leydig, the retina, the adrenal medulla, the placenta, the thymus and the pancreas. The finding of significant amounts of this classically "neurophysiology" hormone outside the central nervous system raises many questions regarding its possible importance in these different tissues. The Leydig cells in some species have also been shown to possess the bio-synthetic machinery to manufacture testicular Oxytocin de novo, to be specific, in rats (which can synthesize vitamin C endogenously) and in guinea pigs, which, like humans, require an exogenous source of vitamin C (ascorbic).