Definition: four small areas in or near the base of the brain that have fenestrated capillaries and are outside the blood-brain barrier. They are the neurohypophysis, the area postrema [TA], the organum vasculosum of the lamina terminalis [TA], and the subfornical organ [TA]. The neurohypophysis is a neurohemal organ. The other three are chemoreceptors: the area postrema triggers vomiting in response to chemical changes in plasma, the organum vasculosum of the lamina terminalis senses osmolality and alters vasopressin secretion, and the subfornal organ initiates drinking in response to angiotensin II.
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Examples: glitazone, GI cocktail, etc.