The area postrema and vomiting.

The area postrema (AP) has been implicated as a chemoreceptor trigger zone for vomiting (emesis) for over 40 years. The AP is located on the dorsal surface of the medulla oblongata at the caudal end of the fourth ventricle. It is one of the so-called circumventricular organs that serve as an interface between the brain parenchyma and the cerebrospinal fluid (CSF)-containing ventricles. The AP lacks a specific blood-brain diffusion barrier to large polar molecules (i.e., a "blood-brain barrier") and is thus anatomically positioned to detect emetic toxins in the blood as well as in the CSF. The AP along with the nucleus of the solitary tract (NTS) and the dorsal motor nucleus of the vagus makes up the so-called dorsal vagal complex, which is the major termination site of vagal afferent nerve fibers. Lesions of the AP prevent vomiting in response to most, but not all, emetic drugs. However, the AP is not essential for vomiting induced by motion or by activation of vagal nerve afferents. The role of the AP in radiation-induced vomiting remains controversial. Electrophysiological studies have reported that neurons in the AP increase their firing in response to emetic drugs. Similarly, studies using the 2-deoxyglucose uptake and c-fos expression techniques have shown that the AP is excited by systemic administration of emetic drugs. Activation of the AP probably leads to nausea and vomiting through its projection to the neighboring NTS. The NTS may serve as the beginning of a final common pathway by which different emetic inputs trigger vomiting.