Acute radiation-induced vomiting in area postrema-ablated cats

A dose-response relationship was established in normal unanesthetized cats for emetic incidence, latency to onset of vomiting, and duration of emetic activity over a period of 24 h after whole-body exposure to 60Co radiation at selected doses between 7.5 and 90 Gy. Each episode of vomiting, i.e., retching and expulsion, was recorded oscillographically as its characteristic intrathoracic pressure waveform by means of a catheter inserted some days before into the superior vena cava. The gamma-radiation dose of 45 Gy evoked vomiting optimally with an incidence of 92% and an average onset time of 98 min. When administered to animals prepared chronically with surgical ablation of the area postrema, the same dose of radiation evoked vomiting in four of five test cases and with an average time to onset that was not by either measure significantly different from normal. Vomiting was also elicited in all of six normal cats exposed to an intestinal dose of 45 Gy X radiation with the head shielded. The same form of irradiation evoked vomiting in two of three chronically postremectomized cats. Successful ablation of the area postrema was determined functionally by emetic refractoriness to an injection of digitalis and confirmed for completeness by histological examination. It is concluded that the area postrema is not an essential element in the reflex mechanism of radiation-induced vomiting and, therefore, no physiological basis exists for dependence on a centrally acting chemogenic factor in radioemesis.