title: Peripheral antinociceptive effects of exogenous and immune cell-derived endomorphins in prolonged inflammatory pain.
abstract: Endomorphins (EMs) are endogenous selective mu-opioid receptor agonists. Their role in inflammatory pain has not been fully elucidated. Here we examine peripheral antinociception elicited by exogenously applied EM-1 and EM-2 and the contribution of EM-containing leukocytes to stress- and corticotropin-releasing factor (CRF)-induced antinociception. To this end, we applied behavioral (paw pressure) testing, radioligand binding, immunohistochemistry, and flow cytometry in rats with unilateral hindpaw inflammation induced with Freund's adjuvant. EMs injected directly into both hindpaws produced antinociception exclusively in inflamed paws. This was blocked by locally applied mu-receptor-selective (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2) but not kappa-receptor-selective (nor-binaltorphimine) antagonists. Delta-receptor antagonists (naltrindole and N,N-diallyl-Tyr-Aib-Aib-Phe-Leu) did not influence EM-1-induced but dose-dependently decreased EM-2-induced antinociception. Antibodies against beta-endorphin, methionine-enkephalin, or leucine-enkephalin did not significantly change EM-2-induced antinociception. Both EMs displaced binding of [3H]-[D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin to mu-receptors in dorsal root ganglia (DRG). Using [3H]-naltrindole or [(125)I]-[D-Pen2,5]-enkephalin, no detectable delta-binding was found in DRG of inflamed hindlimbs. Numerous beta-endorphin-containing and fewer EM-1- and EM-2-containing leukocytes were detected in subcutaneous tissue of inflamed paws. Leukocyte-depleting serum decreased the number of immigrating opioid-containing immune cells and attenuated swim stress- and CRF-induced antinociception in inflamed paws. Both forms of antinociception were strongly attenuated by anti-beta-endorphin and to a lesser degree by anti-EM-1 and anti-EM-2 antibodies injected into inflamed paws. Together, exogenously applied and immune cell-derived EMs alleviate prolonged inflammatory pain through selective activation of peripheral opioid receptors. Exogenous EM-2 in addition to mu-receptors also activates peripheral delta-receptors, which does not involve actions via other opioid peptides.
authors.affiliation: Klinik für Anaesthesiologie und Operative Intensivmedizin, Charité-Universitätsmedizin Berlin, Campus Benjamin Franklin, D-12200 Berlin, Germany.
authors.forename: Dominika
authors.initials: D
authors.lastname: Labuz
authors.forename: Stephan
authors.initials: S
authors.lastname: Berger
authors.forename: Shaaban A
authors.initials: SA
authors.lastname: Mousa
authors.forename: Christian
authors.initials: C
authors.lastname: Zöllner
authors.forename: Heike L
authors.initials: HL
authors.lastname: Rittner
authors.forename: Mohammed A
authors.initials: MA
authors.lastname: Shaqura
authors.forename: Toni
authors.initials: T
authors.lastname: Segovia-Silvestre
authors.forename: Barbara
authors.initials: B
authors.lastname: Przewlocka
authors.forename: Christoph
authors.initials: C
authors.lastname: Stein
authors.forename: Halina
authors.initials: H
authors.lastname: Machelska
mesh_terms: D000700:Analgesics / Q000378:metabolism* / Q000494:pharmacology; D000818:Animals; D004305:Dose-Response Relationship, Drug; D007249:Inflammation / Q000188:drug therapy / Q000378:metabolism / Q000473:pathology; D008297:Male; D009504:Neutrophils / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology; D009842:Oligopeptides / Q000378:metabolism* / Q000494:pharmacology / Q000627:therapeutic use; D010146:Pain / Q000188:drug therapy / Q000276:immunology / Q000378:metabolism*; D010147:Pain Measurement / Q000187:drug effects / Q000379:methods; D051381:Rats; D017208:Rats, Wistar; D013997:Time Factors
chemical_list: D000700:Analgesics; D009842:Oligopeptides; C105293:endomorphin 1; C105294:endomorphin 2