Low concentrations of halothane increase response to a noxious thermal stimulus and attenuate the antinociceptive effect of intraventricular but not intrathecal morphine.

BACKGROUND Classically, the first plane of anesthesia is known as the stage of analgesia. Nonetheless, clinical evidence suggests that low doses of inhaled agents might enhance pain perception. The present experiments test the hypothesis that low concentrations of halothane increase response to a noxious thermal stimulus and attenuate the antinociceptive effect of intraventricular morphine via disruption of descending inhibition. METHODS In the first experiment, the temperature at which rats withdraw their tails from a heat source was measured in animals breathing various concentrations of halothane. In the second experiment, the effect of intraventricular or intrathecal morphine on tail-flick latency was assessed in rats breathing either oxygen or 0.23% halothane. RESULTS Low concentrations of halothane decreased the temperature threshold for tail-flick with a maximum effect at 0.06% atmospheres. Halothane attenuated the antinociceptive potency of intraventricular morphine but enhanced the efficacy of intrathecal morphine. CONCLUSIONS Subanesthetic concentrations of halothane may enhance response to a noxious stimulus. The differential effect on intraventricular and intrathecal morphine suggests that this enhancement results from disruption of descending inhibition.