The hypothalamic pituitary axis and vasopressin. Provasopressin is synthesized in neurohypophyseal neurons of the paraventricular nuclei (PVN) and supraoptic nuclei (SON) of the hypothalamus. Provasopressin is transported along the supraoptic neurohypophyseal tract in neurosecretory granules to the posterior pituitary gland. Provasopressin is cleaved by enzymes to vasopressin during transport to the posterior pituitary gland. Hypotension and hyperosmolality stimulate release of stored vasopressin (immediate) and synthesis (delayed) of vasopressin. In response to hypotension, vasopressin is released into the systemic circulation and also into the pituitary portal capillaries, flows to the anterior pituitary, binds to the AVPR1b receptor of corticotrophs, and stimulates release of adrenocorticotropic hormone. MNPO, median pre-optic nucleus.