oxytocin
n. a peptide produced in the hypothalamus and released by the posterior pituitary gland into the blood, where it acts as a hormone, or into the central nervous system, where it acts as a neurotransmitter and binds to oxytocin receptors to influence behavior and physiology. Although perhaps best known for its role in stimulating contractions of smooth muscle in the wall of the uterus to facilitate labor and in the mammary glands to facilitate expression of milk—the so-called milk letdown reflex—oxytocin is present and serves important functions in both sexes. It has earned a reputation as a facilitator of social affiliation, and the tend-and-befriend response in particular, and it has been shown to influence sexual pleasure, reproductive functions, and parental behavior (especially maternal behavior). Additionally, research with nonhuman animals suggests that oxytocin—and the structurally similar compound
vasopressin—is important for pair-bond formation, mate guarding, and recognition of social stimuli. A possible reason for the varied roles that oxytocin appears to play is that it has dual pathways and purposes. It has been theorized that when operating during times of low stress, oxytocin physiologically rewards, with feelings of well-being, those who maintain good social bonds. When operating during times of high stress or pain, however, it may produce physiological changes that encourage individuals to seek contact with others.