Sites of emetic action of sodium salicylate in the cat

Abstract

Intravenous infusion of sodium salicylate at the rate of 20 mg/min until vomiting occurred was used to determine the threshold emetic dose in 45 cats. The ED50 was 275 mg/kg, and the mean with 95% confidence limits was 293 (260–327) mg/kg. The threshold values obtained by this procedure were relatively stable over time if infusions were spaced at least 10 days apart to avoid salicylate accumulation. Supradiaphragmatic vagotomy raised the mean emetic threshold of 8 cats from 314 mg/kg before surgery to over 575 mg/kg, while ablation of the medullary emetic chemoreceptor trigger zone (CTZ) raised the mean threshold in 4 cats from 277 mg/kg before surgery to over 790 mg/kg. Increased thresholds after CTZ ablation were also found using spaced rapid iv injections. Lateral cerebral ventricular injection of sodium salicylate eleicited vomiting in 5 of 19 cats while fourth ventricular injection caused vomiting in 1 of 7 cats. All but 1 of these sensitive cats responded on 2–5 separate occasions. It is concluded that salicylates can induce emesis by stimulation of receptors which are accessible from the cerebrospinal fluid (probably in the CTZ) and that simultaneous afferent input to the vomiting center from both these central receptors and from peripheral receptors is necessary for a normal emetic threshold to sodium salicylate administered iv.