ONDANSETRON DOES NOT INHIBIT THE ANALGESIC EFFECT OF ALFENTANIL

5-Hydroxytryptamine (5-HT) causes antinociception via presynaptic 5-HT 3 (5-HT subtype 3) receptors on primary afferent nociceptive neurones in the spinal cord dorsa I horn. Therefore, ondansetron (a 5-HT3 recep tor antagonist) may increase the perception of a noxious stimulus or d ecrease the effects of concurrently administered antinociceptive drugs . Using a randomized, double blind, crossover study design, we have te sted this hypothesis in eight healthy volunteers who, on three differe nt days, received either ondansetron and placebo, ondansetron and alfe ntanil or placebo and alfentanil. Experimental pain was induced with h eat, cold, mechanical pressure and electrical stimulation. Ondansetron alone did not change the response to any of the experimental tests, b ut alfentanil and the combination ondansetron-alfentanil significantly changed the response compared with ondansetron alone. There was no di fference between alfentanil alone and the combination ondansetuon-alfe ntanil. We conclude that ondansetron does not change the response to p ressure, heat, cold or electrical nociceptive stimuli or antagonize th e analgesic effect of alfentanil.