Acute thermal hyperalgesia elicited by low-dose morphine in normal mice is blocked by ultra-low-dose naltrexone, unmasking potent opioid analgesia.

Our previous electrophysiologic studies on nociceptive types of dorsal root ganglion (DRG) neurons in culture demonstrated that extremely low fM-nM concentrations of morphine and many other bimodally-acting mu, delta and kappa opioid agonists can elicit direct excitatory opioid receptor-mediated effects, whereas higher (microM) opioid concentrations evoked inhibitory effects. Cotreatment with pM naloxone or naltrexone (NTX) plus fM-nM morphine blocked the excitatory effects and unmasked potent inhibitory effects of these low opioid concentrations. In the present study, hot-water-immersion tail-flick antinociception assays at 52 degrees C on mice showed that extremely low doses of morphine (ca. 0.1 microg/kg) can, in fact, elicit acute hyperalgesic effects, manifested by rapid onset of decreases in tail-flick latency for periods >3 h after drug administration. Cotreatment with ultra-low-dose NTX (ca. 1-100 pg/kg) blocks this opioid-induced hyperalgesia and unmasks potent opioid analgesia. The consonance of our in vitro and in vivo evidence indicates that doses of morphine far below those currently required for clinical treatment of pain may become effective when opioid hyperalgesic effects are blocked by coadministration of appropriately low doses of opioid antagonists. This low-dose-morphine cotreatment procedure should markedly attenuate morphine tolerance, dependence and other aversive side-effects.