Halothane antagonizes effect of morphine on the motor reaction threshold in rats.

The ability of halothane (used in "sub-MAC" concentrations) to modify the effect of morphine on motor response threshold to pressure was studied and compared with pentobarbital in 241 rat experiments. It was found that halothane (0.5-0.7%, insp.) decreased the reaction threshold to pressure, as did pentobarbital. Halothane (0.5%) increased morphine ED50 for the reaction threshold to pressure from 0.21 mg . kg-1 (95% fiducial limits: 0.13-0.29 mg . kg-1) to 0.52 mg . kg-1 (0.28-0.73 mg . kg-1, P less than 0.0001). Pentobarbital in a dose of 3 mg . kg-1 demonstrated a similar antanalgesic effect. Neither halothane nor pentobarbital antagonized the effect of morphine with motor response to the tail clamp. On the contrary, both agents strengthened this effect. It has been suggested that the effect of morphine on the motor response threshold to pressure results primarily from activation of inhibitory control mechanisms concerned with this response; halothane in a subanesthetic concentration depresses the inhibitory control mechanisms and, therefore, weakens the effect of morphine.