Hydrocodone, but Neither Morphine nor Oxycodone, Is Effective in Suppressing Burn-Induced Mechanical Allodynia in the Uninjured Foot Contralateral to the Burn.

Opioids are commonly used to treat severe, burn-induced pain. However, there is a lack of rodent studies that examine the differential effects of various opioids on burn pain. We recently demonstrated that hydrocodone was superior to other opioids in suppressing the development of burn-induced mechanical allodynia in the burned limb. This study monitored the development of mechanical allodynia and compared the abilities of morphine, oxycodone, and hydrocodone to reduce burn-induced mechanical allodynia in the limb contralateral to the burn. Mice were examined for their baseline pain sensitivity thresholds using the von Frey filaments test. Then, they were subjected to burn or sham injury and treated orally with morphine, oxycodone, hydrocodone (20 or 40 mg/kg), or saline twice daily throughout the study. They were retested on days 4, 7, 11, 14, 21, and 28 postburn. Hyperalgesia was developed in the contralateral, uninjured foot beginning 21 days after the burn injury. Hydrocodone was effective in suppressing the development of burn-induced mechanical allodynia. In contrast, morphine and oxycodone had only minimal effects on the development of burn-induced mechanical allodynia. The abnormal pain sensitivities that develop as a result of burn injuries are very difficult to treat and remain a significant public health problem. More rodent studies are required to improve our understanding of the differences among the currently available opioid analgesics in order to optimize the care provided to burn victims as well as those suffering from other pain modalities.