The so-called love hormone is really a Greek word for swift birth
Oxytocin gets called the cuddle or love hormone, but its name comes from Greek words meaning swift childbirth. It was discovered as a chemical that makes the womb contract. Only later did researchers link it to bonding, and a 2023 study hints that even zebrafish use it to register another fish's fear.
Oxytocin is a small peptide, just nine amino acids long, made in the hypothalamus and released into the blood from the back lobe of the pituitary gland. Its best-known effects involve reproduction: it drives contractions during labour and the let-down of milk during breastfeeding. Both work by positive feedback. A contraction triggers more oxytocin, which triggers stronger and more frequent contractions, and the loop keeps building until the trigger stops. Levels climb three to fourfold during pregnancy, the same again during labour, and about fivefold while nursing.
Its history runs through several landmarks. The British pharmacologist Henry Hallett Dale found the womb-contracting effect in 1906, and in 1909 William Blair-Bell first used it clinically to bring on difficult births. In the early 1950s the American biochemist Vincent du Vigneaud worked out its amino acid sequence, the first polypeptide hormone ever sequenced, and in 1953 he synthesised it, another first, earning the 1955 Nobel Prize in Chemistry. The final step in making it inside the body depends on an enzyme that needs vitamin C, and many tissues rich in that enzyme also store high levels of the vitamin.
Oxytocin is not confined to childbirth or to women. It occurs in the brain at concentrations up to a thousand times higher than in the blood, reaching regions such as the nucleus accumbens, and it is also made in the placenta, the testes, the retina, the thymus and the pancreas, where its roles remain an open question. Levels roughly double in new romantic relationships and are also raised by exercise and stress. Estrogens boost both the hormone and its receptors in the brain, while evidence on testosterone is mixed.
The molecule is ancient. Nearly all vertebrates carry an oxytocin-like hormone for reproduction and a vasopressin-like one for water balance, encoded by neighbouring genes thought to have arisen from a duplication around 500 million years ago. Even the tiny worm C. elegans uses a version in mating. When gene editing removed oxytocin from zebrafish, they stopped reacting to other fish's fear, and restoring it brought the response back.
Source: Oxytocin
