A cannabinoid receptor antagonist attenuates conditioned place preference but not behavioural sensitization to morphine.

The present study compared the effects of the cannabinoid receptor antagonist SR 141716 on morphine-induced locomotor sensitization (Experiment 1) and conditioned place preference (CPP, Experiment 2) in male albino Wistar rats. In Experiment 1, rats received seven consecutive daily treatments with morphine (10 mg/kg, SC) in combination with either SR 141716 (0, 0.1, 0.5 or 3.0 mg/kg, IP), or naloxone (10 mg/kg, IP). Three days later, all rats were challenged with a lower dose of morphine (5 mg/kg, SC). Rats pre-treated with morphine showed significantly elevated locomotor activity during the challenge session compared to vehicle-pre-treated animals indicating behavioural sensitization. Prior naloxone, but not SR 141716, co-administration with morphine, significantly attenuated the locomotor sensitization observed. In Experiment 2A, SR 141716 (0.1 mg/kg, IP), co-administered during conditioning, significantly attenuated the place preference produced by morphine (4 mg/kg, SC) in a standard unbiased two compartment place conditioning task. In Experiment 2B, the timing of drug administration and drug doses used were altered to be similar to Experiment 1, such that a comparison between the sensitization and CPP paradigms could be made. Thus, rats were conditioned with morphine (10 mg/kg, SC) combined with SR 141716 (0, 0.1, 0.5 or 3.0 mg/kg, IP) and tested for place preference under the influence of morphine (5 mg/kg, SC). SR 141716 attenuated morphine place preference at a dose (3.0 mg/kg) that did not itself affect place conditioning. Morphine also induced locomotor sensitization in the drug-paired compartment in Experiment 2B which was not blocked by any dose of SR 141716. We conclude that CB1 receptor antagonism modulates the rewarding value of opioids, but not the behavioural sensitization induced by chronic opioid administration.