Role of the chemoreceptor trigger zone in histamine-induced emesis.

1. In unanaesthetized dogs, the emetic action of histamine was studied after its injection into the cerebral venricles through chronically implanted cannulae in order to elucidate the role of the chemoreceptor trigger zone (CT-zone), situated in the area postrema, for this emesis.2. On injection into the lateral cerebral ventricle, about 10 times larger doses of histamine (3 mg) were required regularly to produce emesis, and it occurred after a longer latency than on injection into the fourth ventricle. This is in accord with an action of histamine on the CT-zone.3. After bilateral ablation of the CT-zone, intraventricular injections of histamine no longer produced emesis even when injected in doses which were three to four times greater than those which regularly elicited vomiting in dogs with intact CT-zone. The emesis produced in dogs by intraventricular injections of histamine is thus fully accounted for by an action on the CT-zone.4. Injections of chlorpromazine intramuscularly or of the two antihistamines cyclizine and mepyramine, either intramuscularly or into the lateral ventricle, prevented the emesis caused by histamine injected into the lateral ventricle. This protective action of the antihistamines-which did not extend to the emesis produced by oral copper sulphate-suggests the presence of histaminergic receptors in the CT-zone.