title: Leukocyte opioid receptors mediate analgesia via Ca(2+)-regulated release of opioid peptides.
abstract: Opioids are the most powerful analgesics. As pain is driven by sensory transmission and opioid receptors couple to inhibitory G proteins, according to the classical concept, opioids alleviate pain by activating receptors on neurons and blocking the release of excitatory mediators (e.g., substance P). Here we show that analgesia can be mediated by opioid receptors in immune cells. We propose that activation of leukocyte opioid receptors leads to the secretion of opioid peptides Met-enkephalin, β-endorphin and dynorphin A (1-17), which subsequently act at local neuronal receptors, to relieve pain. In a mouse model of neuropathic pain induced by a chronic constriction injury of the sciatic nerve, exogenous agonists of δ-, μ- and κ-opioid receptors injected at the damaged nerve infiltrated by opioid peptide- and receptor-expressing leukocytes, produced analgesia, as assessed with von Frey filaments. The analgesia was attenuated by pharmacological or genetic inactivation of opioid peptides, and by leukocyte depletion. This decrease in analgesia was restored by the transfer of wild-type, but not opioid receptor-lacking leukocytes. Ex vivo, exogenous opioids triggered secretion of opioid peptides from wild-type immune cells isolated from damaged nerves, which was diminished by blockade of Gαi/o or Gβγ (but not Gαs) proteins, by chelator of intracellular (but not extracellular) Ca(2+), by blockers of phospholipase C (PLC) and inositol 1,4,5-trisphosphate (IP3) receptors, and was partially attenuated by protein kinase C inhibitor. Similarly, the leukocyte depletion-induced decrease in exogenous opioid analgesia was re-established by transfer of immune cells ex vivo pretreated with extracellular Ca(2+) chelator, but was unaltered by leukocytes pretreated with intracellular Ca(2+) chelator or blockers of Gαi/o and Gβγ proteins. Thus, both ex vivo opioid peptide release and in vivo analgesia were mediated by leukocyte opioid receptors coupled to the Gαi/o-Gβγ protein-PLC-IP3 receptors-intracellular Ca(2+) pathway. Our findings suggest that opioid receptors in immune cells are important targets for the control of pathological pain.
authors.affiliation: Department of Anesthesiology and Critical Care Medicine, Charité-Universitätsmedizin Berlin, Campus Benjamin Franklin, 12203 Berlin, Germany. Electronic address: ozgur.celik@charite.de.
authors.forename: Melih Ö
authors.initials: MÖ
authors.lastname: Celik
authors.affiliation: Department of Anesthesiology and Critical Care Medicine, Charité-Universitätsmedizin Berlin, Campus Benjamin Franklin, 12203 Berlin, Germany. Electronic address: dominika.labuz@charite.de.
authors.forename: Dominika
authors.initials: D
authors.lastname: Labuz
authors.affiliation: Department of Anesthesiology and Critical Care Medicine, Charité-Universitätsmedizin Berlin, Campus Benjamin Franklin, 12203 Berlin, Germany. Electronic address: henning.karen@gmx.de.
authors.forename: Karen
authors.initials: K
authors.lastname: Henning
authors.affiliation: Department of Anesthesiology and Critical Care Medicine, Charité-Universitätsmedizin Berlin, Campus Benjamin Franklin, 12203 Berlin, Germany. Electronic address: mellibusch@freenet.de.
authors.forename: Melanie
authors.initials: M
authors.lastname: Busch-Dienstfertig
authors.affiliation: Institut de Génétique et de Biologie Moléculaire et Cellulaire, Illkirch, France; Université de Strasbourg, Illkirch, France; Centre National de la Recherche Scientifique, UMR7104, Illkirch, France; Institut National de la Santé et de la Recherche Médicale, U964, Illkirch, France. Electronic address: gaveriau@igbmc.fr.
authors.forename: Claire
authors.initials: C
authors.lastname: Gaveriaux-Ruff
authors.affiliation: Institut de Génétique et de Biologie Moléculaire et Cellulaire, Illkirch, France; Université de Strasbourg, Illkirch, France; Centre National de la Recherche Scientifique, UMR7104, Illkirch, France; Institut National de la Santé et de la Recherche Médicale, U964, Illkirch, France; Douglas Hospital Research Center, McGill University, Montreal H4H 1R3, Canada. Electronic address: Brigitte.Kieffer@douglas.mcgill.ca.
authors.forename: Brigitte L
authors.initials: BL
authors.lastname: Kieffer
authors.affiliation: Institute of Molecular Psychiatry, University of Bonn, 53105 Bonn, Germany. Electronic address: a.zimmer@uni-bonn.de.
authors.forename: Andreas
authors.initials: A
authors.lastname: Zimmer
authors.affiliation: Department of Anesthesiology and Critical Care Medicine, Charité-Universitätsmedizin Berlin, Campus Benjamin Franklin, 12203 Berlin, Germany. Electronic address: halina.machelska@charite.de.
authors.forename: Halina
authors.initials: H
authors.lastname: Machelska
mesh_terms: D000698:Analgesia; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004195:Disease Models, Animal; D007962:Leukocytes / Q000378:metabolism*; D051379:Mice; D009437:Neuralgia / Q000188:drug therapy / Q000378:metabolism*; D018847:Opioid Peptides / Q000378:metabolism*; D011957:Receptors, Opioid / Q000819:agonists / Q000378:metabolism*; D015398:Signal Transduction; D016037:Single-Blind Method
chemical_list: D018847:Opioid Peptides; D011957:Receptors, Opioid; D002118:Calcium
keywords: Calcium; G-protein; IP(3) receptors; Immune cells; Neuropathic pain; Opioids; PLC; Pain