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Xylazine is an effective sedative analgesic that induces vomiting in the cat. A consistently effective intramuscular emetic dose of xylazine was established in normal cats in this laboratory. Animals in which the area postrema of the medulla oblongata had been chronically destroyed did not exhibit emesis in response to the standard test dose of xylazine but continued to show sedation. By contrast, sham-operated cats responded normally. Refractoriness to deslanoside, a known specific emetic drug, was used as functional proof for successful ablation of area postrema, and the lesions were also validated histologically. We conclude that xylazine exerts its emetic action on the chemoreceptor trigger zone of the area postrema. It is suggested that this action may be mediated by an opiate type of molecular receptors.

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A volume-distribution analysis of the water soluble contrast medium, meglumine iothalamate, injected into various ventricular and subarachnoid sites was accomplished radiographically in the cat with the aid of a newly designed screw-type cannula having a deadspace of 6 to 8 pl. The cannula is positioned stereotaxically and mounts directly and permanently in the cranium by a single self-tapping insertion maneuver. As little as 100 p1 of solution injected into the anterior horn of the lateral ventricle, or 50 and 20 pl into the third and fourth ventricles, respectively, was visualized immediately in the lateral apertures of the fourth ventricle connecting with the subarachnoid spaces. Injection of 50 pl into the posterior horn of the lateral ventricle delineated the entire ventricle as well as the interventricular foramen. A volume of 12 pl deposited in the base of the third ventricle served to define the hypothalamic cleft and infundihular recess. If rate of injection was not excessive, the solution could be introduced into the third and fourth ventricles without penetrating into the more anterior regions. Radiopaque medium injected into subarachnoid spaces (cisternae ambiens and cerebellomedullaris) did not enter the ventricular system.