VOLATILE ANESTHETICS ANTAGONIZE NITROUS-OXIDE AND MORPHINE-INDUCED ANALGESIA IN THE RAT

We reported previously that nitrous oxide induces pre-emptive analgesi a that is partially antagonized by naloxone and totally antagonized by halothane. The aims of this study were to determine if halothane and isoflurane are similar in this respect and to examine if volatile anae sthetics antagonize the analgesic effect of exogenous opioids. We foun d that 75% nitrous oxide prolonged tail-flick latency by 37% and this analgesia was dose-dependently inhibited by halothane and, less effect ively, by isoflurane. In contrast, morphine 1.25 mg kg(-1) i.v. also p rolonged tail-flick latency by 35% but, unlike nitrous oxide-induced a nalgesia, this effect was attenuated only by high doses of halothane a nd was unaffected by isoflurane. Neither halothane nor isoflurane alon e altered the tail-flick response. We conclude that both halothane and isoflurane dose-dependently antagonized nitrous oxide analgesia but a ntagonized morphine-induced analgesia to a lesser extent.