Morphine is a potent opioid analgesic used to alleviate moderate or severe pain， but the development of drug tolerance and dependence limits its use in pain management. Previous studies showed that cannabinoid type 2 (CB) receptor ligands may modulate opioid effects. However， there is no report of the effect of CB receptor agonist on acute morphine tolerance and physical dependence. We therefore investigated the effect of a CB receptor agonist (AM1241) on morphine-induced morphine tolerance and physical dependence in mice. Repeated coadministration of AM1241 (1 or 3mg/kg intraperitoneally) and morphine (10mg/kg subcutaneously) for 7days increased the mechanical paw withdrawal threshold in mice as measured b... More
Morphine is a potent opioid analgesic used to alleviate moderate or severe pain， but the development of drug tolerance and dependence limits its use in pain management. Previous studies showed that cannabinoid type 2 (CB) receptor ligands may modulate opioid effects. However， there is no report of the effect of CB receptor agonist on acute morphine tolerance and physical dependence. We therefore investigated the effect of a CB receptor agonist (AM1241) on morphine-induced morphine tolerance and physical dependence in mice. Repeated coadministration of AM1241 (1 or 3mg/kg intraperitoneally) and morphine (10mg/kg subcutaneously) for 7days increased the mechanical paw withdrawal threshold in mice as measured by the von Frey filament test， and 3mg/kg AM1241 in combination with morphine increased the thermal paw withdrawal latency as measured by the hot-plate test. Combination with 3mg/kg AM1241 and morphine increased acute morphine antinociception. Coadministration of 1 or 3mg/kg AM1241 and morphine reduced acute morphine tolerance， and 3mg/kg AM1241 reduced chronic morphine tolerance. Coadministration of 1 or 3mg/kg AM1241 and morphine reduced naloxone-precipitated withdrawal jumping， but not diarrhea. Coadministration of AM1241 and morphine did not inhibit spontaneous locomotor activity. Pretreatment with 3mg/kg AM1241 decreased the chronic morphine-induced Iba1 expression in spinal cord. Coadministration of AM1241 (3 mg/kg) reduced the production of interleukin-1β， tumor necrosis factor-α， and interleukin-6 induced by long-term and acute morphine treatment. Our findings suggest that the coadministration of the CB receptor agonist and morphine could increase morphine antinociception and reduce morphine tolerance and physical dependence in mice.
