DISORDERS OF THE POSTERIOR PITUITARY



The posterior lobe of the pituitary gland (neu­rohypophysis) is an anatomical extension of the hypothalamus, containing the axons and axon ter­minals of neurons whose nuclei originate in the supraoptic and paraventricular areas of the hy­pothalamus. Two peptide hormones, arginine va­sopressin and oxytocin, and their carrier proteins, the neurophysins, are synthesized in the cell bod­ies of these neurons. These peptides and their car­rier proteins travel down the axons to the nerve terminals from which they are released in re­sponse to a variety of physiological stimuli.

Oxytocin causes release of breast milk and may promote uterine contractions during labor. There is no known function for this peptide hormone in men. Arginine vasopressin (also known as anti­diuretic hormone [ADH]) is the principal hor­monal factor regulating water metabolism. Defi­ciency of ADH or impaired action in its major target organ, the kidney, results in a polyuric state known as diabetes insipidus. Excessive and phys­iologically inappropriate secretion of ADH results in hyponatremia (the syndrome of inappropriate ADH secretion), as discussed elsewhere in this volume . Arginine vasopressin, as the name implies, also has pressor activity, the phys­iological relevance of which is unknown.