Etorphine elicits anomalous excitatory opioid effects on sensory neurons treated with GM1 ganglioside or pertussis toxin in contrast to its potent inhibitory effects on naive or chronic morphine-treated cells

Dihydroetorphine: physical dependence and stereotypy after continuous infusion in the rat

In a previous study in this laboratory, exposure of rhesus monkeys to intermittent, high doses of dihydroetorphine for 42 days did not evoke behavioral signs of physical dependence on this opioid either after it was abruptly withdrawn or after challenge with a high dose of naloxone. To investigate further the physical dependence capacity of this opioid, it was given by infusion to rats thereby exposing receptors chronically and continuously to this opioid. Abstinence expressed as body weight loss, irritability, and wet-dog shakes was observed after abrupt withdrawal of the low-dose regimen (5,10, 40 and 40 microg/kg per day for 4 days, respectively). The high-dose regimen (10, 20 and 80 microg/kg per day for 3 days, respectively) produced stereotypy and physical dependence. Although many reported molecular events and dependence studies suggest otherwise, dihydroetorphine's propensity to produce physical dependence, an important determinant of opioid abuse, is real.

Modulation of opioid analgesia, tolerance and dependence by Gs-coupled, GM1 ganglioside-regulated opioid receptor functions

Studies of direct excitatory effects elicited by opioid agonists on various types of neurone have been confirmed and expanded in numerous laboratories following the initial findings reviewed previously by Stanley Crain and Ke-Fei Shen. However, the critical role of the endogenous glycolipid GM1 ganglioside in regulating Gs-coupled, excitatory opioid receptor functions has not been addressed in any of the recent reviews of opioid stimulatory mechanisms. This article by Stanley Crain and Ke-Fei Shen focuses on crucial evidence that the concentration of GM1 in neurones might, indeed, play a significant role in the modulation of opioid receptor-mediated analgesia, tolerance and dependence.

Etorphines: mu-opioid receptor-selective antinociception and low physical dependence capacity

Comparative analgesic studies revealed that dihydroetorphine was more potent than etorphine in the tail-flick and hot-plate tests, respectively and nearly equipotent in the phenylquinone assay. Both compounds were short acting. Studies with selective opioid receptor antagonists beta-funaltrexamine, nor-binaltorphimine and naltrindole revealed that both etorphines were mu-selective agonists. Presumptive evidence for competitive antagonism of these compounds with naloxone was provided by Schild regressions with slopes of near unity. In a suppression test in rhesus monkeys maximally dependent on morphine, dihydroetorphine and etorphine dose-dependently replaced morphine. Drug-naive simians chronically exposed to frequent, intermittent and escalating doses of dihydroetorphine for 42 days showed few withdrawal signs when challenged with large doses of naloxone or were abruptly withdrawn from this drug. The results suggest that these atypical opioids may be useful in the clinical treatment of pain and opiate drug abuse.

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.