Modulatory effects of Gs-coupled excitatory opioid receptor functions on opioid analgesia, tolerance, and dependence.

Electrophysiologic studies of opioid effects on nociceptive types of dorsal root ganglion (DRG) neurons in organotypic cultures have shown that morphine and most mu, delta, and kappa opioid agonists can elicit bimodal excitatory as well as inhibitory modulation of the action potential duration (APD) of these cells. Excitatory opioid effects have been shown to be mediated by opioid receptors that are coupled via Gs to cyclic AMP-dependent ionic conductances that prolong the APD, whereas inhibitory opioid effects are mediated by opioid receptors coupled via Gi/Go to ionic conductances that shorten the APD. Selective blockade of excitatory opioid receptor functions by low (ca. pM) concentrations of naloxone, naltrexone, etorphine and other specific agents markedly increases the inhibitory potency of morphine or other bimodally acting agonists and attenuates development of tolerance/dependence. These in vitro studies have been confirmed by tail-flick assays showing that acute co-treatment of mice with morphine plus ultra-low-dose naltrexone or etorphine remarkably enhances the antinociceptive potency of morphine whereas chronic co-treatment attenuates development of tolerance and naloxone-precipitated withdrawal-jumping symptoms.