Oxytocin is also a nonapeptide hormone produced by the hypothalamus. It is produced by both sexes, however the effects in women have been well documented for a number of years. It is secreted by the neurohypophysis in response to stimulation of the nipple, in particular the suckling reflex, visual or auditory stimulation from the baby, during intercourse and as a result of expansion of the uterus and cervix during birth. The presence of estrogen increases the uterus's response to oxytocin. Fear, anxiety, pain and alcohol inhibit the release of oxytocin. The presence of progesterone decreases the uterus's sensitivity to oxytocin.
The major effects of oxytocin are the milk “letdown" response, contraction of the uterine wall during birth and, possibly, the formation of maternal behaviors. The last effect has come under debate as experiments have shown conflicting conclusions. In the “letdown" response, milk has been formed and filled the alveoli of the mammary glands within the breast tissue. Oxytocin stimulates the ejection of the milk from the alveoli through the nipple in response to suckling. During birth, oxytocin causes the smooth muscle in the uterine wall to contract in order to aid in the birth process.
Several secondary effects of oxytocin have been posited. As the function of the hormone in men is not clearly elucidated, there is a hypothesis that it aids in the transport of sperm post-coitus, to increase the chances of reproduction. Likewise, it is suggested that it aids in egg transport in women. It has also been postulated that oxytocin plays a role in sexual response, particularly in men and non-pregnant women. The high levels of oxytocin in breast milk have led to the association of it with the mutual bonding of the offspring to the mother. Also, it may be somewhat responsible for the release of prolactin from the adenohypophysis; prolactin causes growth in the alveoli of the mammary glands and milk production.
Research has suggested a strong link between oxytocin and social behaviors, such as cooperation, generosity and, in men, empathy. This link has fostered several compelling research studies into the use of oxytocin as a possible treatment for patients with autism and Asperger's syndrome. Experimentally, oxytocin has shown social improvement in the condition, but is not currently a medically-recognized treatment.