CONTINUOUS INTRAVENOUS-INFUSION OF NALOXONE DOES NOT CHANGE BEHAVIORAL, CARDIOVASCULAR, OR INFLAMMATORY RESPONSES TO SUBCUTANEOUS FORMALIN IN THE RAT

The opioid antagonist, naloxone, produces equivocal effects on the mag nitude of nociceptive responses in several animal models of persistent pain, including the formalin test. Hindpaw injection of dilute formal in produces not only inflammation but also phasic (Phase 1) and persis tent (Phase 2) behavioral and cardiovascular nociceptive responses in the rat. To test the hypothesis that endogenous opioid systems contrib ute to the magnitude of responses to intraplantar formalin injection, we evaluated the effects of continuous naloxone administration (0.01-1 00 mg/kg per h, i.v.) on formalin-evoked hindpaw inflammation, on beha vioral indices of pain, flinching and licking pain behavior, and on ch anges in mean arterial pressure and heart rate. We report that naloxon e, at doses less than 100 mg/kg per h, did not change any formalin-evo ked response. Although the 100 mg/kg per h dose significantly decrease d these responses, it also produced muscle rigidity and profound brady cardia. We conclude that endogenous opioids do not significantly modul ate the nociceptive processing induced by subcutaneous formalin.