Toll like receptor (TLR)-4 as a regulator of peripheral endogenous opioid-mediated analgesia in inflammation.
BACKGROUND
Leukocytes containing opioid peptides locally control inflammatory pain. In the early phase of complete Freund's adjuvant (CFA)-induced hind paw inflammation, formyl peptides (derived e.g. from Mycobacterium butyricum) trigger the release of opioid peptides from neutrophils contributing to tonic basal antinociception. In the later phase we hypothesized that toll-like-receptor-(TLR)-4 activation of monocytes/macrophages triggers opioid peptide release and thereby stimulates peripheral opioid-dependent antinociception.


RESULTS
In Wistar rats with CFA hind paw inflammation in the later inflammatory phase (48-96 h) systemic leukocyte depletion by cyclophosphamide (CTX) or locally injected naloxone (NLX) further decreased mechanical and thermal nociceptive thresholds. In vitro β-endorphin (β-END) content increased during human monocyte differentiation as well as in anti-inflammatory CD14+CD16- or non-classical M2 macrophages. Monocytes expressing TLR4 dose-dependently released β-END after stimulation with lipopolysaccharide (LPS) dependent on intracellular calcium. Despite TLR4 expression proinflammatory M1 and anti-inflammatory M2 macrophages only secreted opioid peptides in response to ionomycin, a calcium ionophore. Intraplantar injection of LPS as a TLR4 agonist into the inflamed paw elicited an immediate opioid- and dose-dependent antinociception, which was blocked by TAK-242, a small-molecule inhibitor of TLR4, or by peripheral applied NLX. In the later phase LPS lowered mechanical and thermal nociceptive thresholds. Furthermore, local peripheral TLR4 blockade worsened thermal and mechanical nociceptive pain thresholds in CFA inflammation.


CONCLUSIONS
Endogenous opioids from monocytes/macrophages mediate endogenous antinociception in the late phase of inflammation. Peripheral TLR4 stimulation acts as a transient counter-regulatory mechanism for inflammatory pain in vivo, and increases the release of opioid peptides from monocytes in vitro. TLR4 antagonists as new treatments for sepsis and neuropathic pain might unexpectedly transiently enhance pain by impairing peripheral opioid analgesia.
Reine-Solange
RS
Sauer
Dagmar
D
Hackel
Laura
L
Morschel
Henrike
H
Sahlbach
Ying
Y
Wang
Shaaban A
SA
Mousa
Norbert
N
Roewer
Alexander
A
Brack
Department of Anesthesiology, University Hospital of Wuerzburg, Oberdürrbacher Strasse 6, D-97080 Würzburg, Germany. rittner_h@ukw.de.
Heike L
HL
Rittner
D000698:Analgesia; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002454:Cell Differentiation / Q000187:drug effects; D005620:Freund's Adjuvant / Q000008:administration & dosage / Q000494:pharmacology; D006801:Humans; D006930:Hyperalgesia / Q000150:complications / Q000378:metabolism / Q000473:pathology; D007249:Inflammation / Q000150:complications / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology; D018950:Lipopolysaccharide Receptors / Q000378:metabolism; D008070:Lipopolysaccharides / Q000494:pharmacology; D008264:Macrophages / Q000187:drug effects / Q000378:metabolism; D008297:Male; D009000:Monocytes / Q000187:drug effects / Q000378:metabolism; D059225:Nociception / Q000187:drug effects; D018847:Opioid Peptides / Q000494:pharmacology / Q000627:therapeutic use*; D051381:Rats; D017208:Rats, Wistar; D017452:Receptors, IgG / Q000378:metabolism; D011957:Receptors, Opioid / Q000378:metabolism; D051195:Toll-Like Receptor 2 / Q000378:metabolism; D051197:Toll-Like Receptor 4 / Q000378:metabolism*; D001615:beta-Endorphin / Q000378:metabolism
D018950:Lipopolysaccharide Receptors; D008070:Lipopolysaccharides; D018847:Opioid Peptides; D017452:Receptors, IgG; D011957:Receptors, Opioid; C495343:Tlr2 protein, rat; C495347:Tlr4 protein, rat; D051195:Toll-Like Receptor 2; D051197:Toll-Like Receptor 4; D001615:beta-Endorphin; D005620:Freund's Adjuvant; D002118:Calcium