oxytocin

oxytocin is one of the two major hormones secreted from the posterior lobe of the pituitary gland, the other being vasopressin, the antidiuretic hormone. The posterior pituitary itself largely comprises endings of nerves whose cell bodies lie in the brain in the hypothalamus. Its hormones are extruded from the nerve endings directly into blood capillaries and thence into the general circulation. Oxytocin is a relatively small peptide hormone, composed of only 9 amino acids. It is synthesized in nerve cells in the hypothalamus in the form of a larger, precursor molecule, which is transported down the nerve fibres through the pituitary stalk to the posterior lobe. The active hormone is cleaved from the precursor during this process.

Oxytocin was the first hormone to have its structure identified and to be synthesized in the laboratory. This was achieved in 1953 by two groups, one working in France and the other in the United States. The hormone plays an important role in birth and in feeding the infant. It has two major actions, the first being to promote contractions of the uterus, an action which is used in obstetric practice when the hormone is infused if labour is protracted. The second action is to cause contraction of the muscular elements surrounding the alveoli and milk ducts in the breasts, thereby helping to expel the milk (milk let-down). Oxytocin is also found in men, but its role is not clear, although it has been suggested that it aids sperm transport during mating. The stimuli for release reflect the actions of the hormone and its actions are rapid because they are evoked by sensory nerve impulses which communicate with the hypothalamic nerve cells. Thus it is released during vaginal stimulation, so that during the process of birth, as the infant enters the birth canal, oxytocin is released and in turn causes increased uterine activity, allowing the process to proceed more rapidly. The oxytocin released at this time may also cause milk ejection, a response known as Ferguson's reflex. This reflex was first described in 1942, although herdsmen had been aware of it for centuries and had used it as an aid to milking; in the second century ad, Galen, the Greek physician and prolific writer, described how herdsmen would blow into the vagina of mares to improve milk yield. Oxytocin is also released in response to suckling and many other stimuli associated with breast feeding, such as the sight and sound of the infant. If the mother is not relaxed, this may act via the hypothalamus to inhibit oxytocin release so that problems with breast feeding may arise.

Interestingly, there have been no descriptions of clinical conditions associated with over- or underproduction of this hormone. Recent work with genetically modified mice which are unable to produce oxytocin indicates that the hormone is essential for lactation, but not for parturition.

Oxytocin exemplifies interplay between hormonal and neural systems: unlike most hormones, it is made in nerve rather than glandular cells. Sensory nerves stimulate the nerve cells to activate its extrusion into the blood so that — like other hormones — it reaches the ‘target’ site via the circulation. This is an example of a ‘neuroendocrine’ secretion.

Mary L. Forsling

See also birth; breast; hormones; infant feeding; pituitary.