| Oxytocin is a nine amino acid peptide hormone synthesized in magnocellular neurosecretory cells in the hypothalamus and released by the posterior lobe of the pituitary gland. The sequence is cysteine - tyrosine - isoleucine - glutamine - asparagine - cysteine - proline - leucine - glycine). The leucine is the only difference from vasopressin, which has arginine or lysine at this
position. The cysteine residues form a sulfur bridge. Oxytocin is often bound to neurophysin (a carrier protein) as shown in the inset of the figure. Oxytocin is also secreted within the brain and from a few other tissues, including the ovaries and testes. Oxytocin differs from antidiuretic hormone in two of the nine amino acids. Both hormones are packaged into granules and secreted along with carrier proteins called neurophysins.
Oxytocin was discovered in 1909 when Sir Henry H. Dale found that an extract from the human posterior pituitary gland contracted the uterus of a pregnant cat. Oxytocin is synthesized in the soma (cell body),and then transported (using energy) all the way down the axon to the nerve terminals, where it can be released into the spinal cord and bloodstream. The production of the neuropeptide oxytocin differs from that of simple neurotransmitters because the process is more complicated and energetically demanding. Oxytocin is not only a hormone that circulates in the bloodstream, it is also a neurotransmitter that travels along nerve cells in the brain and elsewhere. The cells that make oxytocin in the hypothalamus and send it to the pituitary gland also send oxytocin to different regions of the brain.
Natural oxytocin is secreted by the posterior pituitary gland, which holds and secretes oxytocin produced by the hypothalamus. Oxytocin causes milk to be ejected from the breasts during lactation; the amount of oxytocin produced naturally, however, has little effect on uterine contractions and does not stimulate labour. Oxytocin's effect on uterine smooth muscle is dependent on the presence of estrogen, and for that reason oxytocin has little effect on the uterus during the early stages of pregnancy. In lactating (breastfeeding) mothers, oxytocin stimulates myoepithelial cells, causing milk to be ejected into the ducts of the mammary glands. Sucking by the infant at the nipple stimulates oxytocin release. Oxytocin is necessary for cervical dilation prior to birth and causes the contractions all mothers are familiar with. Oxytocin release during breastfeeding causes mild but often painful uterine contractions during the first few weeks of lactation. This also serves to assist the uterus in clotting the placental attachment point postpartum.
Oxytocin is used to induce labor, strengthen labor contractions during childbirth, control bleeding after childbirth, or to induce an abortion. During labor, oxytocin stimulates the uterine muscle to contract. Blood and brain levels of oxytocin rise in instances outside of pregnancy and nursing. A long-pregnant woman's uterus responds so strongly and readily to both the natural and synthetic versions because she has more oxytocin receptors at childbirth than at any other time in her life. By the time she gives birth, oxytocin receptors in her uterus have climbed to 300-fold their normal levels. |
