INVOLVEMENT OF OPIOID MU-1 RECEPTORS IN MORPHINE-INDUCED CONDITIONED PLACE PREFERENCE IN RATS

The main purpose of this study was to evaluate the role of mu 1-opioid receptors in morphine reward. Therefore, we studied the ability of a mu 1-selective antagonist, naloxonazine [15 mg/kg intraperitoneally (I P)], to antagonize the conditioned place preference (CPP) induced by m orphine [3 mg/kg subcutaneously (SC)]. In addition, effects of naloxon azine on morphine-induced catalepsy (15 mg/kg), analgesia (3 mg/kg), a nd hyperthermia (3 mg/kg) were studied. For comparison, the effects of a nonselective opioid receptor antagonist, naltrexone (2.5 mg/kg SC), and a selective delta-opioid receptor antagonist, naltrindole (2 mg/k g IF), on CPP induced by morphine were investigated. Morphine-induced CPP was clearly antagonized by pretreatment with naloxonazine and nalt rexone (12 h and 20 min prior to morphine, respectively) but not by na ltrindole (15 min before morphine). Naloxonazine also antagonized morp hine-induced catalepsy and analgesia but not morphine-induced hyperthe rmia. Naltrindore did not modify morphine-induced catalepsy. These res ults suggest an active role for mu 1-opioid receptors in morphine rewa rd, whereas morphine-induced hyperthermia does not appear to be mediat ed by mu 1-opioid receptors. Furthermore, delta-opioid receptors seem to be without significance in morphine-induced reward. (C) 1997 Elsevi er Science Inc.