Vasopressin is a cyclic nonapeptide that is synthesized centrally in the hypothalamus. Vasopressin participates in the hypothalamic-pituitary-adrenal axis, and regulates pituitary corticotropin secretion by potentiating the stimulatory effects of corticotropin releasing factor. Vasopressin also can act as a neurotransmitter, exerting its action by binding to specific G protein-coupled receptors.