Volatile anaesthetics antagonize nitrous oxide and morphine-induced analgesia in the rat

We reported previously that nitrous oxide induces pre-emptive analgesia 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 anaesthetics antagonize the analgesic effect of exogenous opioids. We found that 75% nitrous oxide prolonged tail-flick latency by 37% and this analgesia was dose-dependently inhibited by halothane and, less effectively, by isoflurane. In contrast, morphine 1.25 mg kg-1 i.v. also prolonged tail-flick latency by 35% but, unlike nitrous oxide-induced analgesia, this effect was attenuated only by high doses of halothane and was unaffected by isoflurane. Neither halothane nor isoflurane alone altered the tail-flick response. We conclude that both halothane and isoflurane dose-dependently antagonized nitrous oxide analgesia but antagonized morphine-induced analgesia to a lesser extent.