title: Anti-nociception mediated by a κ opioid receptor agonist is blocked by a δ receptor agonist.
abstract: OBJECTIVE
  The opioid receptor family comprises four structurally homologous but functionally distinct sub-groups, the μ (MOP), δ (DOP), κ (KOP) and nociceptin (NOP) receptors. As most opioid agonists are selective but not specific, a broad spectrum of behaviours due to activation of different opioid receptors is expected. In this study, we examine whether other opioid receptor systems influenced KOP-mediated antinociception.
  
  
  METHODS
  We used a tail withdrawal assay in C57Bl/6 mice to assay the antinociceptive effect of systemically administered opioid agonists with varying selectivity at KOP receptors. Pharmacological and genetic approaches were used to analyse the interactions of the other opioid receptors in modulating KOP-mediated antinociception.
  
  
  RESULTS
  Etorphine, a potent agonist at all four opioid receptors, was not anti-nociceptive in MOP knockout (KO) mice, although etorphine is an efficacious KOP receptor agonist and specific KOP receptor agonists remain analgesic in MOP KO mice. As KOP receptor agonists are aversive, we considered KOP-mediated antinociception might be a form of stress-induced analgesia that is blocked by the anxiolytic effects of DOP receptor agonists. In support of this hypothesis, pretreatment with the DOP antagonist, naltrindole (10 mg·kg(-1) ), unmasked etorphine (3 mg·kg(-1) ) antinociception in MOP KO mice. Further, in wild-type mice, KOP-mediated antinociception by systemic U50,488H (10 mg·kg(-1) ) was blocked by pretreatment with the DOP agonist SNC80 (5 mg·kg(-1) ) and diazepam (1 mg·kg(-1) ).
  
  
  CONCLUSIONS
  Systemic DOP receptor agonists blocked systemic KOP antinociception, and these results identify DOP receptor agonists as potential agents for reversing stress-driven addictive and depressive behaviours mediated through KOP receptor activation.
  
  
  BACKGROUND
  This article is part of a themed section on Opioids: New Pathways to Functional Selectivity. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2015.172.issue-2.
authors.affiliation: University of California, Los Angeles, CA, USA; University of California Irvine, Irvine, CA, USA.
authors.forename: A M W
authors.initials: AM
authors.lastname: Taylor
authors.forename: K W
authors.initials: KW
authors.lastname: Roberts
authors.forename: A A
authors.initials: AA
authors.lastname: Pradhan
authors.forename: H A
authors.initials: HA
authors.lastname: Akbari
authors.forename: W
authors.initials: W
authors.lastname: Walwyn
authors.forename: K
authors.initials: K
authors.lastname: Lutfy
authors.forename: F I
authors.initials: FI
authors.lastname: Carroll
authors.forename: C M
authors.initials: CM
authors.lastname: Cahill
authors.forename: C J
authors.initials: CJ
authors.lastname: Evans
mesh_terms: D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer / Q000494:pharmacology / Q000627:therapeutic use; D000698:Analgesia; D018712:Analgesics, Non-Narcotic / Q000494:pharmacology / Q000627:therapeutic use; D000701:Analgesics, Opioid / Q000494:pharmacology* / Q000627:therapeutic use; D000818:Animals; D001522:Behavior, Animal / Q000187:drug effects; D001549:Benzamides / Q000494:pharmacology; D003975:Diazepam / Q000494:pharmacology; D005048:Etorphine / Q000494:pharmacology / Q000627:therapeutic use; D005260:Female; D006358:Hot Temperature; D008297:Male; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D009271:Naltrexone / Q000031:analogs & derivatives / Q000494:pharmacology; D059225:Nociception / Q000502:physiology*; D010879:Piperazines / Q000494:pharmacology; D011957:Receptors, Opioid / Q000235:genetics; D017465:Receptors, Opioid, delta / Q000819:agonists* / Q000037:antagonists & inhibitors; D017473:Receptors, Opioid, kappa / Q000819:agonists*; D017450:Receptors, Opioid, mu / Q000235:genetics; D013315:Stress, Psychological; D000094942:Nociceptin Receptor
chemical_list: D018712:Analgesics, Non-Narcotic; D000701:Analgesics, Opioid; D001549:Benzamides; D010879:Piperazines; D011957:Receptors, Opioid; D017465:Receptors, Opioid, delta; D017473:Receptors, Opioid, kappa; D017450:Receptors, Opioid, mu; C088464:4-(alpha-(4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl)-N,N-diethylbenzamide; D005048:Etorphine; D009271:Naltrexone; D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; C055382:naltrindole; D003975:Diazepam; D000094942:Nociceptin Receptor; C000722287:Oprl1 protein, mouse