Antagonistic actions of MIF-I on the hypothermia and hypomotility induced by beta-endorphin or morphine.

Recently it has been suggested that MIF-I can act as an opiate antagonist for analgesia. Therefore, rats kept at 4 degrees C were pretreated with MIF-I in an attempt to extend the observation to a nonanalgesic opiate effect by determining any blockade of the thermal response to beta-endorphin and morphine. MIF-I, at an ip dose of 1.0 mg/kg, was found to block the thermal responses to beta-endorphin injected ip at doses of 0.1 and 1.0 mg/kg. A lower dose (0.1 mg/kg, ip) of MIF-I, or naloxone (10 mg/kg), was also able to block the thermal effects of 30 and 60 mg/kg doses of morphine. However, an ip dose of 1.0 mg/kg MIF-I potentiated the hypothermic effects of morphine but, like naloxone, reduced the magnitude of the decrease in the level of motor activity induced by beta-endorphin or by morphine. The results of this study demonstrate a nonanalgesic situation in which MIF-I can act as an antagonist of opiate effects after peripheral injection.