Either of two peptide hormones secreted by the neurohypophysis, or posterior lobe of the pituitary gland, in humans. These hormones, oxytocin and vasopressin, each comprise nine amino acid residues. Vasopressin is responsible for arterial vasoconstriction (pressor action) and inhibition of water excretion through the kidneys (antidiuretic action), and has a weak effect on contraction of smooth muscle including that of the uterus. The principal action of oxytocin is stimulation of smooth muscle contraction, specifically that of the uterine muscle, and milk ejection from the mammary gland.
Oxytocin and vasopressin are synthesized in neurons in the hypothalamus and subsequently packaged into neurosecretory granules, which migrate down the axon of the neuron and are stored in the posterior lobe of the pituitary gland, from where they are secreted into the systemic circulation. These hormones are also secreted directly from the hypothalamus into the third ventricle and into the hypothalamo-hypophysial portal circulation of the anterior pituitary gland. See Neurosecretion
Major stimuli controlling the release of vasopressin include changes in osmolality of the blood, alterations in blood volume, and psychogenic stimuli such as pain, fear, and apprehension. Stimuli evoking release of oxytocin include nipple stimulation or suckling, and stretching of the cervix and vagina (Ferguson reflex).
Oxytocin probably plays an important role in the onset of labor and delivery (parturition) in primates. During lactation, significant amounts of oxytocin are released by the mother during suckling. When there is total destruction of the pituitary or the neurohypophysis, diabetes insipidus may occur. See Hormone, Pituitary gland