THE EFFECT OF CONTINUOUS MORPHINE ANALGESIA ON CHRONIC THERMAL HYPERALGESIA DUE TO SCIATIC CONSTRICTION INJURY IN RATS

We employed hindfoot withdrawal latencies to radiant heat to assess th e analgesic effect of prolonged morphine infusion on thermal hyperalge sia induced by chronic constriction injury (CCI) of the rat sciatic ne rve. All CCI rats developed thermal hyperalgesia while sham-operated a nimals did not. Continuous systemic infusion of morphine dose-dependen tly reversed the thermal hyperalgesia in the CCI rats, In contrast, th ermal hyperalgesia persisted in saline-treated CCI rats. Tolerance to morphine's analgesic effect did not develop over a period of seven day s of morphine infusion, which is considered long-term for animal model s. These data suggest that morphine acts rapidly and effectively to re duce behavioral signs of hyperalgesia in rats with sciatic CCI, withou t the concomitant development of tolerance, Scheduled administration o f morphine might be an appropriate treatment regimen for relief of neu ropathic pain, and the infrequent use of opioids in equivalent human c linical pain syndromes due to fear of opioid unresponsiveness and tole rance might need to be re-evaluated.