Annett Häussler

Low-dose methotrexate reduces peripheral nerve injury-evoked spinal microglial activation and neuropathic pain behavior in rats.

&NA; Peripheral nerve injuries that provoke neuropathic pain are associated with microglial activation in the spinal cord. We have investigated the characteristics of spinal microglial activation in three distinct models of peripheral neuropathic pain in the rat: spared nerve injury (SNI), chronic constriction injury, and spinal nerve ligation. In all models, dense clusters of cells immunoreactive for the microglial marker CD11b formed in the ipsilateral dorsal horn 7 days after injury. Microglial expression of ionised calcium binding adapter molecule 1 (Iba1) increased by up to 40% and phosphorylation of p38 mitogen‐activated protein kinase, a marker of microglial activity, by 45%. Expression of the lysosomal ED1‐antigen indicated phagocytic activity of the cells. Unlike the peripheral nerve lesions, rhizotomy produced only a weak microglial reaction within the spinal gray matter but a strong activation of microglia and phagocytes in the dorsal funiculus at lumbar and thoracic spinal cord levels. This suggests that although degeneration of central terminals is sufficient to elicit microglial activation, it does not account for the inflammatory response in the dorsal horn after peripheral nerve injury. Early intrathecal treatment with low‐dose methotrexate, beginning at the time of injury, decreased microglial activation, reduced p38 phosphorylation, and attenuated pain‐like behavior after SNI. In contrast, systemic or intrathecal delivery of the glucocorticoid dexamethasone did not inhibit the activation of microglia or reduce pain‐like behavior. We confirm that microglial activation is crucial for the development of pain after nerve injury, and demonstrates that suppression of this cellular immune response is a promising approach for preventing neuropathic pain.

R-Flurbiprofen Reduces Neuropathic Pain in Rodents by Restoring Endogenous Cannabinoids

Background R-flurbiprofen, one of the enantiomers of flurbiprofen racemate, is inactive with respect to cyclooxygenase inhibition, but shows analgesic properties without relevant toxicity. Its mode of action is still unclear. Methodology/Principal Findings We show that R-flurbiprofen reduces glutamate release in the dorsal horn of the spinal cord evoked by sciatic nerve injury and thereby alleviates pain in sciatic nerve injury models of neuropathic pain in rats and mice. This is mediated by restoring the balance of endocannabinoids (eCB), which is disturbed following peripheral nerve injury in the DRGs, spinal cord and forebrain. The imbalance results from transcriptional adaptations of fatty acid amide hydrolase (FAAH) and NAPE-phospholipase D, i.e. the major enzymes involved in anandamide metabolism and synthesis, respectively. R-flurbiprofen inhibits FAAH activity and normalizes NAPE-PLD expression. As a consequence, R-Flurbiprofen improves endogenous cannabinoid mediated effects, indicated by the reduction of glutamate release, increased activity of the anti-inflammatory transcription factor PPARγ and attenuation of microglia activation. Antinociceptive effects are lost by combined inhibition of CB1 and CB2 receptors and partially abolished in CB1 receptor deficient mice. R-flurbiprofen does however not cause changes of core body temperature which is a typical indicator of central effects of cannabinoid-1 receptor agonists. Conclusion Our results suggest that R-flurbiprofen improves the endogenous mechanisms to regain stability after axonal injury and to fend off chronic neuropathic pain by modulating the endocannabinoid system and thus constitutes an attractive, novel therapeutic agent in the treatment of chronic, intractable pain.

Comparison of nociceptive behavior in prostaglandin E, F, D, prostacyclin and thromboxane receptor knockout mice

Antagonist at specific prostaglandin receptors might provide analgesia with a more favourable toxicity profile compared with cyclooxygenase inhibitors. We analyzed nociceptive responses in prostaglandin D, E, F, prostacyclin and thromboxane receptor knockout mice and mice deficient of cyclooxygenase 1 or 2 to evaluate the contribution of individual prostaglandin receptors for heat, mechanical and formalin‐evoked pain. None of the knockouts was uniformly protected from all of these pain stimuli but COX‐1 and EP4 receptor knockouts presented with reduced heat pain and EP3 receptor and COX‐2 knockout mice had reduced licking responses in the 2nd phase of the formalin assay. This was accompanied with reduced c‐Fos immunoreactivity in the spinal cord dorsal horn in EP3 knockouts. Oppositely, heat pain sensitivity was increased in FP, EP1 and EP1+3 double mutant mice possibly due to a loss of FP or EP1 receptor mediated central control of thermal pain sensitivity. Deficiency of either EP2 or DP1 was associated with increased formalin‐evoked flinching responses and c‐Fos IR in dorsal horn neurons suggesting facilitated spinal cord pain reflex circuity. Thromboxane and prostacyclin receptor knockout mice showed normal pain behavior in all tests. The results suggest a differential, pain‐stimulus and site‐specific contribution of specific PG‐receptors for the processing of the nociceptive stimuli, a differential modulation of nociceptive responses by COX‐1 and COX‐2 derived prostaglandins and compensatory and/or developmental adaptations in mice lacking specific PG receptors.

Inhibition of GTP cyclohydrolase attenuates tumor growth by reducing angiogenesis and M2-like polarization of tumor associated macrophages.

GTP cyclohydrolase (GCH1) is the key‐enzyme to produce the essential enzyme cofactor, tetrahydrobiopterin. The byproduct, neopterin is increased in advanced human cancer and used as cancer‐biomarker, suggesting that pathologically increased GCH1 activity may promote tumor growth. We found that inhibition or silencing of GCH1 reduced tumor cell proliferation and survival and the tube formation of human umbilical vein endothelial cells, which upon hypoxia increased GCH1 and endothelial NOS expression, the latter prevented by inhibition of GCH1. In nude mice xenografted with HT29‐Luc colon cancer cells GCH1 inhibition reduced tumor growth and angiogenesis, determined by in vivo luciferase and near‐infrared imaging of newly formed blood vessels. The treatment with the GCH1 inhibitor shifted the phenotype of tumor associated macrophages from the proangiogenic M2 towards M1, accompanied with a shift of plasma chemokine profiles towards tumor‐attacking chemokines including CXCL10 and RANTES. GCH1 expression was increased in mouse AOM/DSS‐induced colon tumors and in high grade human colon and skin cancer and oppositely, the growth of GCH1‐deficient HT29‐Luc tumor cells in mice was strongly reduced. The data suggest that GCH1 inhibition reduces tumor growth by (i) direct killing of tumor cells, (ii) by inhibiting angiogenesis, and (iii) by enhancing the antitumoral immune response.

Inhibitor κB Kinase β Deficiency in Primary Nociceptive Neurons Increases TRP Channel Sensitivity

Inhibitor κB kinase (IKK) regulates the activity of the transcription factor nuclear factor-κ B that normally protects neurons against excitotoxicity. Constitutively active IKK is enriched at axon initial segments and nodes of Ranvier (NR). We used mice with a Cre–loxP-mediated specific deletion of IKKβ in sensory neurons of the dorsal root ganglion (SNS–IKKβ−/−) to evaluate whether IKK plays a role in sensory neuron excitability and nociception. We observed increased sensitivity to mechanical, cold, noxious heat and chemical stimulation in SNS–IKKβ−/− mice, with normal proprioceptive and motor functions as revealed by gait analysis. This was associated with increased calcium influx and increased inward currents in small- and medium-sized primary sensory neurons of SNS–IKKβ−/− mice during stimulation with capsaicin or Formalin, specific activators of transient receptor potentials TRPV1 and TRPA1 calcium channels, respectively. In vitro stimulation of saphenous nerve preparations of SNS–IKKβ−/− mice showed increased neuronal excitability of A- and C-fibers but unchanged A- and C-fiber conduction velocities, normal voltage-gated sodium channel currents, and normal accumulation of ankyrin G and the sodium channels Nav1.6 at NR. The results suggest that IKKβ functions as a negative modulator of sensory neuron excitability, mediated at least in part by modulation of TRP channel sensitivity.

R‐flurbiprofen attenuates experimental autoimmune encephalomyelitis in mice

R‐flurbiprofen is the non‐cyclooxygenase inhibiting R‐enantiomer of the non‐steroidal anti‐inflammatory drug flurbiprofen, which was assessed as a remedy for Alzheimers disease. Because of its anti‐inflammatory, endocannabinoid‐modulating and antioxidative properties, combined with low toxicity, the present study assessed R‐flurbiprofen in experimental autoimmune encephalomyelitis (EAE) models of multiple sclerosis in mice. Oral R‐flurbiprofen prevented and attenuated primary progressive EAE in C57BL6/J mice and relapsing‐remitting EAE in SJL mice, even if the treatment was initiated on or after the first flare of the disease. R‐flurbiprofen reduced immune cell infiltration and microglia activation and inflammation in the spinal cord, brain and optic nerve and attenuated myelin destruction and EAE‐evoked hyperalgesia. R‐flurbiprofen treatment increased CD4+CD25+FoxP3+ regulatory T cells, CTLA4+ inhibitory T cells and interleukin‐10, whereas the EAE‐evoked upregulation of pro‐inflammatory genes in the spinal cord was strongly reduced. The effects were associated with an increase of plasma and cortical endocannabinoids but decreased spinal prostaglandins, the latter likely due to R to S inversion. The promising results suggest potential efficacy of R‐flurbiprofen in human MS, and its low toxicity may justify a clinical trial.

Inter-strain differences of serotonergic inhibitory pain control in inbred mice

BackgroundDescending inhibitory pain control contributes to the endogenous defense against chronic pain and involves noradrenergic and serotonergic systems. The clinical efficacy of antidepressants suggests that serotonin may be particularly relevant for neuropathic pain conditions. Serotonergic signaling is regulated by synthesis, metabolisms, reuptake and receptors.ResultsTo address the complexity, we used inbred mouse strains, C57BL/6J, 129 Sv, DBA/2J and Balb/c, which differ in brain serotonin levels. Serotonin analysis after nerve injury revealed inter-strain differences in the adaptation of descending serotonergic fibers. Upregulation of spinal cord and midbrain serotonin was apparent only in 129 Sv mice and was associated with attenuated nerve injury evoked hyperalgesia and allodynia in this strain. The increase of dorsal horn serotonin was blocked by hemisectioning of descending fibers but not by rhizotomy of primary afferents indicating a midbrain source. Para-chlorophenylalanine-mediated serotonin depletion in spinal cord and midbrain intensified pain hypersensitivity in the nerve injury model. In contrast, chronic inflammation of the hindpaw did not evoke equivalent changes in serotonin levels in the spinal cord and midbrain and nociceptive thresholds dropped in a parallel manner in all strains.ConclusionThe results suggest that chronic nerve injury evoked hypernociception may be contributed by genetic differences of descending serotonergic inhibitory control.

Anandamide deficiency and heightened neuropathic pain in aged mice.

Damaging of peripheral nerves may result in chronic neuropathic pain for which the likelihood is increased in the elderly. We assessed in mice if age-dependent alterations of endocannabinoids contributed to the heightened vulnerability to neuropathic pain at old age. We assessed nociception, endocannabinoids and the therapeutic efficacy of R-flurbiprofen in young and aged mice in the spared nerve injury model of neuropathic pain. R-flurbiprofen was used because it is able to reduce neuropathic pain in young mice in part by increasing anandamide. Aged mice developed stronger nociceptive hypersensitivity after sciatic nerve injury than young mice. This was associated with low anandamide levels in the dorsal root ganglia, spinal cord, thalamus and cortex, which further decreased after nerve injury. In aged mice, R-flurbiprofen had only weak antinociceptive efficacy and it failed to restore normal anandamide levels after nerve injury. In terms of the mechanisms, we found that fatty acid amide hydrolase (FAAH) which degrades anandamide, was upregulated after nerve injury at both ages, so that this upregulation likely did not account for the age-dependent differences. However, enzymes contributing to oxidative metabolism of anandamide, namely cyclooxygenase-1 and Cyp2D6, were increased in the brain of aged mice, possibly enhancing the oxidative breakdown of anandamide. This may overwhelm the capacity of R-flurbiprofen to restore anandamide homeostasis and may contribute to the heightened risk for neuropathic pain at old age.

Dichotomy of CCL21 and CXCR3 in nerve injury-evoked and autoimmunity-evoked hyperalgesia

The chemokine CCL21 is released from injured neurons and acts as a ligand of the chemokine receptor, CXCR3, which likely contributes to pro-inflammatory adaptations and secondary neuronal damage. CCL21-CXCR3 signalling may therefore impact on the development of neuropathic pain. By using the respective knockout mice we show that deficiency of CCL19/21 in plt/plt mice attenuates nerve injury evoked pain but not the hyperalgesia evoked by autoimmune encephalomyelitis (EAE). Oppositely, CXCR3-deficiency had no protective effect after traumatic nerve injury but reduced EAE-evoked hyperalgesia and was associated with reduced clinical EAE scores, a reduction of the pro-inflammatory cell infiltration and reduced upregulation of interferon gamma and interleukin-17 in the spinal cord. In contrast, microglia activation in the spinal cord after traumatic sciatic nerve injury was neither attenuated in CXCR3(-/-) nor plt/plt mice, nor in double knockouts. However, the severity of EAE, but not the hyperalgesia, was also reduced in plt/plt mice, which was associated with reduced infiltration of the spinal cord with CCR7+ T-cells, an increase of CD25+ T-cells and reduced upregulation of CXCL9 and 10, CCL11 and 12. The data show that CCL21 and CXCR3 have dichotomous functions in traumatic and EAE-evoked neuropathic pain suggesting diverse mechanisms likely requiring diverse treatments although both types of neuropathic pain are mediated in part through the immune activation.

Inhibitor Kappa B Kinase Beta Dependent Cytokine Upregulation in Nociceptive Neurons Contributes to Nociceptive Hypersensitivity After Sciatic Nerve Injury

UNLABELLED Inhibitor kappa B kinase (IKK)-mediated nuclear factor-kappa B (NF-κB) activation is a major pathway for transcriptional control of various pro-inflammatory factors. We here assessed whether activation of this pathway specifically in primary nociceptive neurons of the dorsal root ganglia (DRG) contributes to the development of nociceptive hypersensitivity. Mice carrying a cre-loxP-mediated deletion of inhibitor kappa B kinase beta (IKKβ) in DRG neurons were protected from nerve injury-evoked allodynia and hyperalgesia. This effect was mimicked by systemic treatment with an IKKβ inhibitor but was not observed upon specific inhibition of IKKβ in the spinal cord, suggesting a specific role of IKKβ in the peripheral neurons. The deletion of IKKβ in DRG neurons did not affect constitutive neuronal NF-κB activity, but reduced nerve injury-evoked NF-κB stimulation in the DRG and was associated with reduced upregulation of interleukin-16, monocyte chemoattractant protein-1/chemokine (CC motif) ligand 2 (MCP-1/CCL2), and tumor necrosis factor alpha (TNFα) in the DRG. These cytokines evoked a rapid rise of intracellular calcium in subsets of primary DRG neurons. The results suggest that IKKβ-mediated NF-κB stimulation in injured primary sensory neurons promotes cytokine and chemokine production and contributes thereby to the development of chronic pain. PERSPECTIVE Inhibitors of IKK that do not pass the blood-brain barrier and act only in the periphery might be useful for reduction of the pro-inflammatory response in peripheral DRG neurons and reduce thereby nerve injury-evoked pain without affecting neuroprotective effects of NF-κB in the central nervous system.