Modification of nociception in a model of localized inflammatory pain by long-term administration of naloxone.

Inoculation of the right hind paw with Mycobacterium butyricum led to an inflammation reflected in a reduction in the threshold to respond to noxious pressure. Chronic perfusion of naloxone at a 'high' dose of 3.0 mg/kg/hr blocked the antinociceptive action of the mu-agonist, morphine, and the kappa-agonist, U69 593. In contrast, a 'low' dose of 0.16 mg/kg/hr antagonized the action only of morphine. Two days postimplantation, the hyperalgesia to pressure was potentiated in rats receiving the high, but not the low dose of naloxone. At 6 days, this difference was no longer seen. At 7 days, pumps were removed; one day later, in rats which had been receiving the high, (but not low) dose of naloxone, thresholds on the inflamed paw no longer differed from those on the uninflamed paw. This may reflect supersensitivity to an endogenous opioid. The data suggest that kappa-receptors may contribute to control of nociception in inflammatory pain. However, this role does not appear to be essential.