Differential tolerance to antinociceptive effects of mu opioids during repeated treatment with etonitazene, morphine, or buprenorphine in rats.
BACKGROUND
Repeated treatment experiments with high and low efficacy agonists provide critical insight into possible mechanisms underlying development of opioid tolerance.


OBJECTIVE
Experiments in a tail-withdrawal assay tested the hypothesis that magnitude of tolerance to antinociceptive effects is inversely related to agonist relative efficacy in rats intermittently treated with etonitazene. morphine, or buprenorphine.


METHODS
The antinociceptive effects of five mu opioid agonists were tested in male, Sprague-Dawley rats in a warm-water tail-withdrawal assay. To induce tolerance, escalating doses of the higher efficacy agonist etonitazene, the high efficacy agonist morphine, or the lower efficacy agonist buprenorphine were administered twice daily for 2-8 weeks.


RESULTS
Etonitazene, etorphine, morphine, buprenorphine, and GPA 1657 [(1)-beta-2'-hydroxy-2,9-dimethyl-5-phenyl-6,7-benzomorphan] produced dose-dependent increases in tail-withdrawal latency until 100% maximum possible effect (%MPE) was obtained. Treatment with escalating doses of etonitazene, morphine, or buprenorphine produced greater tolerance to the lower efficacy agonists buprenorphine and GPA 1657 than to the higher efficacy agonists etonitazene, etorphine, and morphine. Treatment with buprenorphine, a lower efficacy agonist, produced greater tolerance than did treatment with equivalent doses of the higher efficacy agonists morphine or etonitazene.


CONCLUSIONS
Taken together, these data suggest that magnitude of antinociceptive tolerance is inversely related to relative efficacy of mu agonists, with lower efficacy agonists being more susceptible to tolerance than are higher efficacy agonists under these intermittent dosing conditions.
Walker
E A
EA
Research and Technology Development/Psychology, Albert Einstein Healthcare Network/La Salle University, Philadelphia, PA 19141, USA. walkere@aehn2.einstein.edu
Young
A M
AM
D000700:Analgesics / Q000494:pharmacology*; D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D001562:Benzimidazoles / Q000494:pharmacology*; D002047:Buprenorphine / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004361:Drug Tolerance / Q000502:physiology; D008297:Male; D009020:Morphine / Q000494:pharmacology; D009294:Narcotics / Q000494:pharmacology; D010147:Pain Measurement / Q000187:drug effects*; D051381:Rats; D017207:Rats, Sprague-Dawley; D017450:Receptors, Opioid, mu / Q000819:agonists* / Q000502:physiology
D000700:Analgesics; D000701:Analgesics, Opioid; D001562:Benzimidazoles; D009294:Narcotics; D017450:Receptors, Opioid, mu; D002047:Buprenorphine; D009020:Morphine; C084835:etonitazene