title: Enhancement mu opioid antinociception by oral delta9-tetrahydrocannabinol: dose-response analysis and receptor identification.
abstract: The antinociceptive effects of various mu opioids given p.o. alone and in combination with Delta-9-tetrahydrocannabinol (Delta9-THC) were evaluated using the tail-flick test. Morphine preceded by Delta9-THC treatment (20 mg/kg) was significantly more potent than morphine alone, with an ED50 shift from 28.8 to 13.1 mg/kg. Codeine showed the greatest shift in ED50 value when administered after Delta9-THC (139.9 to 5.9 mg/kg). The dose-response curves for oxymorphone and hydromorphone were shifted 5- and 12.6-fold, respectively. Methadone was enhanced 4-fold, whereas its derivative, l-alpha-acetylmethadol, was enhanced 3-fold. The potency ratios after pretreatment with Delta9-THC for heroin and meperidine indicated significant enhancement (4.1 and 8.9, respectively). Pentazocine did not show a parallel shift in its dose-response curve with Delta9-THC. Naloxone administration (1 mg/kg s.c.) completely blocked the antinociceptive effects of morphine p.o. and codeine p.o. The Delta9-THC-induced enhancement of morphine and codeine was also significantly decreased by naloxone administration. Naltrindole (2 mg/kg s.c.) did not affect morphine or codeine antinociception but did block the enhancement of these two opioids by Delta9-THC. No effect was seen when nor-binaltorphimine was administered 2 mg/kg s.c. before morphine or codeine. Furthermore, the enhancements of morphine and codeine were not blocked by nor-binaltorphimine. We find that many mu opioids are enhanced by an inactive dose of Delta9-THC p.o. The exact nature of this enhancement is unknown. We show evidence of involvement of mu and possibly delta opioid receptors as a portion of this signaling pathway that leads to a decrease in pain perception.
authors.affiliation: Department of Pharmacology and Toxicology, Medical College of Virginia/Virginia Commonwealth University, Richmond, Virginia, USA.
authors.forename: D L
authors.initials: DL
authors.lastname: Cichewicz
authors.forename: Z L
authors.initials: ZL
authors.lastname: Martin
authors.forename: F L
authors.initials: FL
authors.lastname: Smith
authors.forename: S P
authors.initials: SP
authors.lastname: Welch
mesh_terms: D018712:Analgesics, Non-Narcotic / Q000494:pharmacology*; D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D013759:Dronabinol / Q000494:pharmacology*; D004357:Drug Synergism; D008297:Male; D051379:Mice; D008813:Mice, Inbred ICR; D009020:Morphine / Q000494:pharmacology; D009270:Naloxone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D010147:Pain Measurement / Q000187:drug effects; D011930:Reaction Time / Q000187:drug effects; D017465:Receptors, Opioid, delta / Q000037:antagonists & inhibitors / Q000187:drug effects; D017450:Receptors, Opioid, mu / Q000037:antagonists & inhibitors / Q000187:drug effects*; D015398:Signal Transduction / Q000187:drug effects
chemical_list: D018712:Analgesics, Non-Narcotic; D000701:Analgesics, Opioid; D009292:Narcotic Antagonists; D017465:Receptors, Opioid, delta; D017450:Receptors, Opioid, mu; D009270:Naloxone; D009020:Morphine; D013759:Dronabinol