title: Glial TLR4 signaling does not contribute to opioid-induced depression of respiration.
abstract: Opioids activate glia in the central nervous system in part by activating the toll-like receptor 4 (TLR4)/myeloid differentiation 2 (MD2) complex. TLR4/MD2-mediated activation of glia by opioids compromises their analgesic actions. Glial activation is also hypothesized as pivotal in opioid-mediated reward and tolerance and as a contributor to opioid-mediated respiratory depression. We tested the contribution of TLR4 to opioid-induced respiratory depression using rhythmically active medullary slices that contain the pre-Bötzinger Complex (preBötC, an important site of respiratory rhythm generation) and adult rats in vivo. Injection with DAMGO (μ-opioid receptor agonist; 50 μM) or bath application of DAMGO (500 nM) or fentanyl (1 μM) slowed frequency recorded from XII nerves to 40%, 40%, or 50% of control, respectively. This DAMGO-mediated frequency inhibition was unaffected by preapplication of lipopolysaccharides from Rhodobacter sphaeroides (a TLR4 antagonist, 2,000 ng/ml) or (+)naloxone (1-10 μM, a TLR4-antagonist). Bath application of (-)naloxone (500 nM; a TLR4 and μ-opioid antagonist), however, rapidly reversed the opioid-mediated frequency decrease. We also compared the opioid-induced respiratory depression in slices in vitro in the absence and presence of bath-applied minocycline (an inhibitor of microglial activation) and in slices prepared from mice injected (ip) 18 h earlier with minocycline or saline. Minocycline had no effect on respiratory depression in vitro. Finally, the respiratory depression evoked in anesthetized rats by tail vein infusion of fentanyl was unaffected by subsequent injection of (+)naloxone, but completely reversed by (-)naloxone. These data indicate that neither activation of microglia in preBötC nor TLR4/MD2-activation contribute to opioid-induced respiratory depression.
authors.affiliation: Department of Physiology, Neuroscience and Mental Health Institute, Women and Children's Health Research Institute, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, Canada;
authors.forename: Jennifer D
authors.initials: JD
authors.lastname: Zwicker
authors.affiliation: Department of Physiology, Neuroscience and Mental Health Institute, Women and Children's Health Research Institute, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, Canada;
authors.forename: Yong
authors.initials: Y
authors.lastname: Zhang
authors.affiliation: Department of Physiology, Neuroscience and Mental Health Institute, Women and Children's Health Research Institute, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, Canada;
authors.forename: Jun
authors.initials: J
authors.lastname: Ren
authors.affiliation: Discipline of Physiology, School of Medical Sciences, University of Adelaide, Adelaide, South Australia, Australia;
authors.forename: Mark R
authors.initials: MR
authors.lastname: Hutchinson
authors.affiliation: Chemical Biology Research Branch, National Institute on Drug Abuse and National Institute on Alcohol Abuse and Alcoholism, Rockville, Maryland; and.
authors.forename: Kenner C
authors.initials: KC
authors.lastname: Rice
authors.affiliation: Department of Psychology and The Center for Neuroscience, University of Colorado at Boulder, Boulder, Colorado.
authors.forename: Linda R
authors.initials: LR
authors.lastname: Watkins
authors.affiliation: Department of Physiology, Neuroscience and Mental Health Institute, Women and Children's Health Research Institute, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, Canada;
authors.forename: John J
authors.initials: JJ
authors.lastname: Greer
authors.affiliation: Department of Physiology, Neuroscience and Mental Health Institute, Women and Children's Health Research Institute, Faculty of Medicine and Dentistry, University of Alberta, Edmonton, Alberta, Canada; gf@ualberta.ca.
authors.forename: Gregory D
authors.initials: GD
authors.lastname: Funk
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D005260:Female; D005283:Fentanyl / Q000494:pharmacology; D008297:Male; D008526:Medulla Oblongata / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D008911:Minocycline / Q000494:pharmacology; D009270:Naloxone / Q000494:pharmacology; D009457:Neuroglia / Q000378:metabolism* / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D017450:Receptors, Opioid, mu / Q000819:agonists; D012119:Respiration / Q000187:drug effects; D012131:Respiratory Insufficiency / Q000139:chemically induced / Q000378:metabolism* / Q000473:pathology; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*; D051197:Toll-Like Receptor 4 / Q000378:metabolism*
chemical_list: D000701:Analgesics, Opioid; D017450:Receptors, Opioid, mu; C495347:Tlr4 protein, rat; D051197:Toll-Like Receptor 4; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D009270:Naloxone; D008911:Minocycline; D005283:Fentanyl
keywords: opioid; pre-Bötzinger complex; rat; respiratory depression; toll-like receptor
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