Oxytocin is a relatively short polypeptide hormone composed of nine amino acids. It is produced primarily in the hypothalamus and acts as a neurotransmitter in the brain. Best known for its role in female reproduction, it is actually found in both male and female mammals.
Unlike most hormones, oxytocin is produced in nerve cells rather than glandular cells. It is primarily produced in the hypothalamus, where sensory nerves stimulate the nerve cells to promote its secretion. This electrical activity causes the hormone to bind to a substance in the pituitary gland known as neurophysin I. It is then released from the posterior lobe of the pituitary gland into the bloodstream, where it then reaches its destination through a process known as neuroendocrine secretion. Alternately, it may leave the hypothalamus to then be distributed to other parts of the brain and spinal cord, and attach to receptors to influence behavior and physiology.
This hormone plays a major role in female reproduction. First, it is released into the bloodstream as a result of increased uterine, cervical, and vaginal stimulation during childbirth. It then serves to promote contractions of the smooth muscle tissue of the uterus during and after labor, promoting a more rapid childbirth. Medical professionals also sometimes administer it intravenously to induce labor or strengthen contractions. After childbirth, the hormone continues to promote contraction of the uterus, helping to prevent hemorrhaging as well as helping to return the uterus to its pre-gravid state. During childbirth, oxytocin may even cause the involuntary ejection of milk, a reflex known as Ferguson's reflex.
In response to the sight, sound, or suckling of the infant, the hormone is released in new mothers. It causes the contraction of muscles surrounding the alveoli and milk ducts, which helps expel milk through a process known as the letdown reflex. If the mother is not relaxed, however, the hormone may not be easily released, resulting in problems with breastfeeding.
Oxytocin is also the first hormone to have its structure identified and created in a laboratory setting. This was achieved in 1953 by two research groups, one in France and one in the United States.
Although the hormone is present in male mammals, its role is not clearly known. It has been suggested that it may aid in the transportation of sperm during sexual intercourse.