Role of the area postrema in vomiting and related functions.

Before 1949, the vomiting center was said to be located in the dorsal vagal nuclei of the medulla, but it was uncertain whether two centers existed separately for the control of direct and reflex actions of emetic agents. Borison and Wang then used a stereotaxic technique with electrical stimulation to localize the vomiting center in the reticular formation at a measurable distance from the dorsal vagal nuclei. They also formulated the concept of a separate emetic chemoreceptor trigger zone (CTZ) so that the vomiting center itself is not sensitive to emetic agents and serves solely to coordinate the reflex process. The CTZ was soon identified with the area postrema (AP), but the question remains unanswered whether the CTZ constitutes part or all of tht circumventricular organ. Furthermore, different chemosensory functions, as for defecation and certain forms of autonomic expression, may be represented regionally within the AP. Species that are unable to vomit, e.g., rodents, show other postrema-mediated effects as radiation-induced gastric stasis and drug-induced conditioned taste aversion. In sheep, digitalis-induced arrest of rumination is prevented by postremectomy. It is suggested that these behavioral end points in nonvomiting species may serve for biological assay of antinauseant drugs. Finally, a tabular summary is given of known causes of vomiting in which the AP has been implicated.