Desflurane and nitrous oxide, but not nonimmobilizers, affect nociceptive responses.

UNLABELLED: Nonimmobilizers (previously called nonanesthetics) do not prevent movement in response to a noxious stimulus, even at doses predicted to produce anesthesia. We hypothesized they would also lack antinociceptive effects. We tested this prediction using the tail-flick latency (TFL) test. As predicted, the two nonimmobilizers tested (1,2-dichlorohexafluorocyclobutane and perfluoropentane) did not alter TFL, whereas desflurane and nitrous oxide both lengthened TFL (nitrous oxide at a lower minimum alveolar anesthetic concentration [MAC]-multiple than desflurane). In addition, we found that 0.1 MAC desflurane had a hyperalgesic effect (shortened TFL). IMPLICATIONS: We studied the response of animals inhaling anesthetics or nonimmobilizers (compounds predicted to be anesthetics from the Meyer-Overton relation) to painful stimuli. Nonimmobilizers had no effect on these responses; at a low partial pressure, desflurane was hyperalgesic; nitrous oxide and, at higher partial pressures, desflurane were antinociceptive.