Naloxone reversal of morphine analgesia but failure to alter reactivity to pain in the formalin test.

Rats treated with naloxone in doses sufficient (10 mg/kg) to completely antagonize morphine analgesia showed no decrease in their thresholds of reactivity to formalin-induced chronic pain. These findings fail to support the hypothesis that tonic release of endogenous opioids acting at naloxone-sensitive receptors decreases sensitivity or reactivity to pain stimuli.