The opioid antagonist naloxone does not alter discharges of nociceptive afferents from the acutely inflamed knee joint of the cat.

Recent studies have shown peripheral antinociceptive effects of opiates in inflamed tissue. To test whether the afferent activity during an acute inflammation may also be suppressed by endogenous opioids, we studied whether the application of the opioid antagonist naloxone would alter the afferent discharges from the cat knee joint inflamed by kaolin and carrageenin. After i.a. bolus administration of naloxone (3 micrograms/kg and 1 mg/kg) close to the joint, neither the ongoing activity nor the responses to noxious and innocuous movements significantly changed in group III or group IV units. Since naloxone did not unmask opioidergic activity under these conditions, we conclude that the development of increased activity in joint afferents during an acute kaolin/carrageenin-induced inflammation is not tonically suppressed by endogenous opioids.