title: Evidence that tricyclic small molecules may possess toll-like receptor and myeloid differentiation protein 2 activity.
abstract: Opioids have been discovered to have Toll-like receptor (TLR) activity, beyond actions at classical opioid receptors. This raises the question whether other pharmacotherapies for pain control may also possess TLR activity, contributing to or opposing their clinical effects. We document that tricyclics can alter TLR4 and TLR2 signaling. In silico simulations revealed that several tricyclics docked to the same binding pocket on the TLR accessory protein, myeloid differentiation protein 2 (MD-2), as do opioids. Eight tricyclics were tested for effects on TLR4 signaling in HEK293 cells over-expressing human TLR4. Six exhibited mild (desipramine), moderate (mianserin, cyclobenzaprine, imiprimine, ketotifen) or strong (amitriptyline) TLR4 inhibition, and no TLR4 activation. In contrast, carbamazepine and oxcarbazepine exhibited mild and strong TLR4 activation, respectively, and no TLR4 inhibition. Amitriptyline but not carbamazepine also significantly inhibited TLR2 signaling in a comparable cell line. Live imaging of TLR4 activation in RAW264.7 cells and TLR4-dependent interleukin-1 release from BV-2 microglia revealed that amitriptyline blocked TLR4 signaling. Lastly, tricyclics with no (carbamazepine), moderate (cyclobenzeprine), and strong (amitriptyline) TLR4 inhibition were tested intrathecally (rats) and amitriptyline tested systemically in wildtype and knockout mice (TLR4 or MyD88). While tricyclics had no effect on basal pain responsivity, they potentiated morphine analgesia in rank-order with their potency as TLR4 inhibitors. This occurred in a TLR4/MyD88-dependent manner as no potentiation of morphine analgesia by amitriptyline occurred in these knockout mice. This suggests that TLR2 and TLR4 inhibition, possibly by interactions with MD2, contributes to effects of tricyclics in vivo. These studies provide converging lines of evidence that several tricyclics or their active metabolites may exert their biological actions, in part, via modulation of TLR4 and TLR2 signaling and suggest that inhibition of TLR4 and TLR2 signaling may potentially contribute to the efficacy of tricyclics in treating chronic pain and enhancing the analgesic efficacy of opioids.
authors.affiliation: Department of Psychology and The Center for Neuroscience, University of Colorado at Boulder, Boulder, CO, USA.
authors.forename: M R
authors.initials: MR
authors.lastname: Hutchinson
authors.forename: L C
authors.initials: LC
authors.lastname: Loram
authors.forename: Y
authors.initials: Y
authors.lastname: Zhang
authors.forename: M
authors.initials: M
authors.lastname: Shridhar
authors.forename: N
authors.initials: N
authors.lastname: Rezvani
authors.forename: D
authors.initials: D
authors.lastname: Berkelhammer
authors.forename: S
authors.initials: S
authors.lastname: Phipps
authors.forename: P S
authors.initials: PS
authors.lastname: Foster
authors.forename: K
authors.initials: K
authors.lastname: Landgraf
authors.forename: J J
authors.initials: JJ
authors.lastname: Falke
authors.forename: K C
authors.initials: KC
authors.lastname: Rice
authors.forename: S F
authors.initials: SF
authors.lastname: Maier
authors.forename: H
authors.initials: H
authors.lastname: Yin
authors.forename: L R
authors.initials: LR
authors.lastname: Watkins
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology; D000818:Animals; D002478:Cells, Cultured; D006575:Heterocyclic Compounds, 3-Ring / Q000494:pharmacology*; D006801:Humans; D007375:Interleukin-1 / Q000378:metabolism; D008024:Ligands; D051218:Lymphocyte Antigen 96 / Q000737:chemistry / Q000502:physiology*; D008297:Male; D051379:Mice; D008807:Mice, Inbred BALB C; D018345:Mice, Knockout; D017628:Microglia / Q000187:drug effects / Q000378:metabolism; D008958:Models, Molecular; D009020:Morphine / Q000494:pharmacology; D053594:Myeloid Differentiation Factor 88 / Q000235:genetics; D010147:Pain Measurement; D011485:Protein Binding; D051381:Rats; D015398:Signal Transduction; D051195:Toll-Like Receptor 2 / Q000819:agonists / Q000037:antagonists & inhibitors / Q000502:physiology; D051197:Toll-Like Receptor 4 / Q000819:agonists / Q000037:antagonists & inhibitors / Q000235:genetics / Q000502:physiology; D051193:Toll-Like Receptors / Q000819:agonists / Q000037:antagonists & inhibitors / Q000502:physiology*; D014409:Tumor Necrosis Factor-alpha / Q000096:biosynthesis
chemical_list: D000701:Analgesics, Opioid; D006575:Heterocyclic Compounds, 3-Ring; D007375:Interleukin-1; C495377:LY96 protein, human; D008024:Ligands; D051218:Lymphocyte Antigen 96; C507780:Myd88 protein, mouse; D053594:Myeloid Differentiation Factor 88; D051195:Toll-Like Receptor 2; D051197:Toll-Like Receptor 4; D051193:Toll-Like Receptors; D014409:Tumor Necrosis Factor-alpha; D009020:Morphine
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