Peripheral Morphine Administration Blocks the Development of Hyperalgesia and Allodynia after Bone Damage in the Rat

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BackgroundThe current study aimed to assess whether local administration of morphine could block the development of hyperalgesia and allodynia in a rat model of osteotomy or bone damage.MethodsWithdrawal responses to mechanical and thermal stimuli applied to the plantar surface of the hind paw were measured before and after bone damage. The bone was injured by drilling a 1-mm hole through the tibia during short-lasting general anesthesia. In separate groups of rats, the effects of administering morphine (20–80 [micro sign]g), either into the marrow cavity or systemically, on the development of hyperalgesia and allodynia after bone damage were assessed. In an additional group of rats, a selective [micro sign]-opioid receptor antagonist, clocinnamox (0.15 mg), was administered into the marrow cavity before the administration of morphine (40 [micro sign]g).ResultsIn animals that received no drug treatment, hyperalgesia and allodynia peaked 2 h after injury. Injection of morphine (40 and 80 [micro sign]g) into the marrow cavity immediately after bone injury prevented the development of hyperalgesia and allodynia. Clocinnamox (0.15 mg) injected into the marrow cavity before administration of morphine blocked the antihyperalgesic effect of morphine.ConclusionThis study shows that local application of a low dose of morphine effectively blocks the development of hyperalgesia and allodynia in a rat model of bone damage through [micro sign]-opioid receptor action. These findings provide further evidence that local application of morphine at the time of orthopedic surgery, bone graft, or bone marrow harvesting may reduce the amount of postoperative pain.

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