The Posterior Pituitary Gland and Disorders of Vasopressin Metabolism

The posterior pituitary gland, also known as the neurohypophysis, shares a name with the anterior pituitary gland but is not related in structure; the neurohypophysis has indirect connections to the brain as the posterior pituitary gland contains the nerve endings of the vasopressin and oxytocin neurons which originate in the periventricular and supraoptic nuclei. Vasopressin is secreted from the posterior pituitary gland, as is oxytocin. Disorders of vasopressin metabolism may cause increased or decreased urine formation and subsequent abnormalities in body water, serum osmolality, and electrolytes. Rather than being controlled by secretions of the median eminence of the hypothalamus through the hypophysiotrophic portal system as found in the case of the anterior pituitary gland, these nerve endings in the posterior pituitary gland directly secrete their products into the general circulation. These neurons may be interrupted by trauma or surgery, and the level of the interruption determines the outcome. If the pituitary stalk section is low, some intact vasopressin neurons will remain as some terminate at the top of the pituitary stalk, and diabetes insipidus may be mild or not occur. If the stalk section is high, it is more likely that all vasopressin neurons will be damaged and that diabetes insipidus will be permanent.