Recently it has been suggested that MIF-I can act as an opiate antagonist for analgesia. Therefore, rats kept at 4°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.
Bibliographical noteFunding Information:
Shlomo Yehuda is a visiting professor on sabbatical leave from the Psychopharmacological Lab, Bar Ilan University, Ramat Gan, Israel. Reprint requests to: Dr. A. J. Kastin, V.A. Medical Center, 1601 Perdido Street, New Orleans, Louisiana 701 46. Supported in part by the Medical Research Service of the Veterans Administration, NIH (NS 07664), NIDA (DA 01806), and The American Parkinson Disease Foundation. The authors thank the Endo Company for supplying naloxone.