In a rat model of localized hyperalgesic
inflammation induced by intraplantar injection of
carrageenin, the effect of a relatively high dose of
naloxone (1 mg/kg i.v.) was investigated using the measure of the vocalization threshold as a nociceptive test, on both the inflamed and non-inflamed paws. The effects of the
drug were determined at two different periods after the intraplantar injection of
carrageenin, in the same group of rats. We showed that 4 h after
carrageenin (a few hours after the onset of the inflammatory process),
naloxone induced a significant further decrease in the vocalization threshold induced by pressure on either paw, suggesting that
naloxone had reduced a tonically active inhibitory system involving endogenous
opioid peptides. Twenty-four hours after
carrageenin, a consistent hyperalgesic effect of
naloxone was observable only in rats which had recovered from their
carrageenin-induced
hyperalgesia. A significant negative correlation between the behavioral effect of
naloxone and the degree of
hyperalgesia determined for each animal was observed. This suggests that the tonic inhibition exerted by the endogenous
opioids was particularly effective in rats which recovered from their initial
hyperalgesia. By contrast, these
opioid controls could have been weaker in those rats which remained hyperalgesic.