title: Naloxone fails to produce conditioned place aversion in mu-opioid receptor knock-out mice.
abstract: There is growing evidence that tonic activity of the opioid system may be important in the modulation of affective state. Naloxone produces a conditioned place aversion in rodents, an effect that is centrally mediated. Previous pharmacological data using antagonists with preferential actions at mu-, delta-, and kappa-opioid receptors indicate the importance of the mu-opioid receptor in mediating this effect. We sought to test the mu-opioid receptor selectivity of naloxone aversion using mu-opioid receptor knock-out mice. mu-Opioid receptor knock-out and wild-type mice were tested for naloxone (10 mg/kg, s.c.) aversion using a place conditioning paradigm. As a positive control for associative learning, knock-out mice were tested for conditioned place aversion to a kappa agonist, U50,488H (2 mg/kg, s.c.). Naloxone produced a significant place aversion in wild-type mice, but failed to have any effect in mu-opioid receptor knock-out mice. On the other hand, both knock-out and wild-type mice treated with U50,488H spent significantly less time in the drug-paired chamber compared to their respective vehicle controls. We conclude that the mu-opioid receptor is crucial for the acquisition of naloxone-induced conditioned place aversion. Furthermore, in a separate experiment using C57BL/6 mice, the delta-selective antagonist naltrindole (10 or 30 mg/kg, s.c.) failed to produce conditioned place aversion.Taken together, these data further support the notion that naloxone produces aversion by antagonizing tonic opioid activity at the mu-opioid receptor.
authors.affiliation: University of California at Los Angeles, 20024, USA.
authors.forename: P D
authors.initials: PD
authors.lastname: Skoubis
authors.forename: H W
authors.initials: HW
authors.lastname: Matthes
authors.forename: W M
authors.initials: WM
authors.lastname: Walwyn
authors.forename: B L
authors.initials: BL
authors.lastname: Kieffer
authors.forename: N T
authors.initials: NT
authors.lastname: Maidment
mesh_terms: D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer / Q000494:pharmacology; D018712:Analgesics, Non-Narcotic / Q000494:pharmacology; D000818:Animals; D001362:Avoidance Learning / Q000187:drug effects / Q000502:physiology*; D001921:Brain / Q000187:drug effects / Q000378:metabolism*; D003213:Conditioning, Psychological / Q000187:drug effects / Q000502:physiology*; D004305:Dose-Response Relationship, Drug; D006185:Habituation, Psychophysiologic / Q000187:drug effects / Q000502:physiology; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009043:Motor Activity / Q000187:drug effects / Q000502:physiology; D009270:Naloxone / Q000494:pharmacology*; D009271:Naltrexone / Q000031:analogs & derivatives* / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology*; D009474:Neurons / Q000187:drug effects / Q000378:metabolism; D018847:Opioid Peptides / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010641:Phenotype; D017465:Receptors, Opioid, delta / Q000187:drug effects / Q000378:metabolism; D017473:Receptors, Opioid, kappa / Q000187:drug effects / Q000378:metabolism; D017450:Receptors, Opioid, mu / Q000172:deficiency* / Q000235:genetics; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology
chemical_list: D018712:Analgesics, Non-Narcotic; D009292:Narcotic Antagonists; D018847:Opioid Peptides; D017465:Receptors, Opioid, delta; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu; D009270:Naloxone; D009271:Naltrexone; D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; C055382:naltrindole