Outside the uterus, oxytocin has long been known to be essential for milk secretion. In recent years, however, there has also been a growing body of evidence that oxytocin plays a role in the brain in modulation of maternal, sexual and social behaviour.
Oxytocin is required for lactation. Milk is initially secreted into small sacs (alveoli) within the mammary gland. These alveoli are surrounded by smooth muscle, which contract to eject the milk. The oxytocin receptor is the key receptor that mediates this ejection of the milk.
The central role of oxytocin has led to it being described as the 'love hormone', and recently Insel et al.  postulated that oxytocin is the molecular basis of monogamy. In their laboratory, they examined the behaviour of prairie voles, which form strong pair bonds after mating with prolonged, repeated bouts of copulation. It was found that central administration of an oxytocin antagonist inhibited the pair bond formation, whereas central infusion of oxytocin stimulated pair bond formation in the absence of mating. Since oxytocin is released during sexual arousal and orgasm in both men and women , it is possible that a similar role may play a part in the formation of pair bonds in humans.
Similarly to the formation of pair bonds between adult animals, it has been demonstrated that oxytocin is also required for the onset of maternal behaviour in rodents .
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