Neuropathic pain in animal models of nervous system autoimmune diseases

Combining fecal 16 S rRNA sequencing and spinal cord metabolomics analysis to explain the modulatory effect of PPARα on neuropathic pain

BACKGROUND

Existing evidence suggests that the composition of the gut microbiota is associated with neuropathic pain (NP), but the mechanistic link is elusive. Peroxisome proliferator-activated receptor α (PPARα) has been shown to be a pharmacological target for the treatment of metabolic disorders, and its expression is also involved in inflammatory regulation. The aim of this study was to investigate the important modulatory effects of PPARα on gut microbiota and spinal cord metabolites in mice subjected to chronic constriction injury.

METHODS

We analyzed fecal microbiota and spinal cord metabolic alterations in mice from the sham, CCI, GW7647 (PPARα agonist) and GW6471 (PPARα antagonist) groups by 16 S rRNA amplicon sequencing and untargeted metabolomics analysis. On this basis, the intestinal microbiota and metabolites that were significantly altered between treatment groups were analyzed in a combined multiomics analysis. We also investigated the effect of PPARα on the polarization fractionation of spinal microglia.

RESULTS

PPARα agonist significantly reduce paw withdrawal threshold and paw withdrawal thermal latency, while PPARα antagonist significantly increase paw withdrawal threshold and paw withdrawal thermal latency. 16 S rRNA gene sequencing showed that intraperitoneal injection of GW7647 or GW6471 significantly altered the abundance, homogeneity and composition of the gut microbiome. Analysis of the spinal cord metabolome showed that the levels of spinal cord metabolites were shifted after exposure to GW7647 or GW6471. Alterations in the composition of gut microbiota were significantly associated with the abundance of various spinal cord metabolites. The abundance of Licheniformes showed a significant positive correlation with nicotinamide, benzimidazole, eicosanoids, and pyridine abundance. Immunofluorescence results showed that intraperitoneal injection of GW7647 or GW6471 altered microglial activation and polarization levels.

CONCLUSION

Our study shows that PPARα can promote M2-type microglia polarization, as well as alter gut microbiota and metabolites in CCI mice. This study enhances our understanding of the mechanism of PPARα in the treatment of neuropathic pain.

Microglia-independent peripheral neuropathic pain in male and female mice

ABSTRACT

The dominant view in the field of pain is that peripheral neuropathic pain is driven by microglia in the somatosensory processing region of the spinal dorsal horn. Here, to the contrary, we discovered a form of neuropathic pain that is independent of microglia. Mice in which the nucleus pulposus (NP) of the intervertebral disc was apposed to the sciatic nerve developed a constellation of neuropathic pain behaviours: hypersensitivity to mechanical, cold, and heat stimuli. However, NP application caused no activation of spinal microglia nor was pain hypersensitivity reversed by microglial inhibition. Rather, NP-induced pain hypersensitivity was dependent on cells within the NP which recruited macrophages to the adjacent nerve. Eliminating macrophages systemically or locally prevented NP-induced pain hypersensitivity. Pain hypersensitivity was also prevented by genetically disrupting the neurotrophin brain-derived neurotrophic factor selectively in macrophages. Moreover, the behavioural phenotypes as well as the molecular mechanisms of NP-induced pain hypersensitivity were not different between males and females. Our findings reveal a previously unappreciated mechanism for by which a discrete peripheral nerve lesion may produce pain hypersensitivity, which may help to explain the limited success of microglial inhibitors on neuropathic pain in human clinical trials.

TNFα in MS and Its Animal Models: Implications for Chronic Pain in the Disease

Multiple Sclerosis (MS) is a debilitating autoimmune disease often accompanied by severe chronic pain. The most common type of pain in MS, called neuropathic pain, arises from disease processes affecting the peripheral and central nervous systems. It is incredibly difficult to study these processes in patients, so animal models such as experimental autoimmune encephalomyelitis (EAE) mice are used to dissect the complex mechanisms of neuropathic pain in MS. The pleiotropic cytokine tumor necrosis factor α (TNFα) is a critical factor mediating neuropathic pain identified by these animal studies. The TNF signaling pathway is complex, and can lead to cell death, inflammation, or survival. In complex diseases such as MS, signaling through the TNFR1 receptor tends to be pro-inflammation and death, whereas signaling through the TNFR2 receptor is pro-homeostatic. However, most TNFα-targeted therapies indiscriminately block both arms of the pathway, and thus are not therapeutic in MS. This review explores pain in MS, inflammatory TNF signaling, the link between the two, and how it could be exploited to develop more effective TNFα-targeting pain therapies.

Remimazolam alleviates neuropathic pain via regulating bradykinin receptor B1 and autophagy

OBJECTIVES

Neuropathic pain (NP) represents a broad scope of various pathological ramifications of the nervous system. Remimazolam is a proved sedative in treating neuropathic pain. Considering the Bradykinin receptor's vital role and the potentials of Bradykinin receptor B1 (BDKRB1) in the neuropathic pain-signalling pathway, we nominated them as a primary target for remimazolam.

METHODS

In this study, rats were injected with complete freund's adjuvant (CFA) to construct NP models in vivo. BV2 microglia cells were treated with LPS to establish NP model in vitro. qRT-PCR, ELISA, western blot and immunofluorescence were applied to determine gene expression.

KEY FINDINGS

Our findings revealed that BDKRB1 was overexpressed in NP models in vivo, while R715 (an antagonist of BDKRB1) suppressed the levels of BDKRB1 and inhibited the hyperpathia induced by spinal nerve litigation surgery. Moreover, remimazolam inactivated BDKRB1 signalling via suppressing NF-κB translocation and decreased the release of pro-inflammatory cytokines. Additionally, remimazolam suppressed the translocation of NF-κB, and inhibited autophagic lysosome formation in vivo and in vitro. However, R838 (an agonist of BDKRB1) reversed the effects of remimazolam.

CONCLUSIONS

Remimazolam downregulated BDKRB1, inhibited BDKRB1/RAS/MEK signalling pathway and regulated the autophagic lysosome induction, exhibiting a better outcome in the NP.

Upregulation of Mlxipl induced by cJun in the spinal dorsal horn after peripheral nerve injury counteracts mechanical allodynia by inhibiting neuroinflammation

Mlxipl regulates glucose metabolism, lipogenesis and tumorigenesis and has a wide-ranging impact on human health and disease. However, the role of Mlxipl in neuropathic pain remains unknown. In this study, we found that Mlxipl was increased in the ipsilateral L4-L6 spinal dorsal horn after Spared Nerve Injury surgery. Knockdown of Mlxipl in the ipsilateral L4-L6 spinal dorsal horn by intraspinal microinjection aggravated Spared Nerve Injury-induced mechanical allodynia and inflammation in the spinal dorsal horn, on the contrary, overexpression of Mlxipl inhibited mechanical allodynia and inflammation. Subsequently, the rat Mlxipl promoter was analyzed using bioinformatics methods to predict the upstream transcription factor cJun. Luciferase assays and ChIP-qPCR confirmed that cJun bound to the promoter of Mlxipl and enhanced its expression. Finally, we demonstrated that Mlxipl inhibited the inflammatory responses of lipopolysaccharide-induced microglia and that Mlxipl was regulated by the transcription factor cJun. These findings suggested that cJun-induced Mlxipl upregulation in the spinal dorsal horn after peripheral nerve injury provided a protective mechanism for the development and progression of neuropathic pain by inhibiting microglial-derived neuroinflammation. Targeting Mlxipl in the spinal dorsal horn might represent an effective strategy for the treatment of neuropathic pain.

Silencing of the RNA-binding protein HuR attenuates hyperalgesia and motor disability in experimental autoimmune encephalomyelitis

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system associated with progressive neuronal loss and axonal degeneration. Neuronal lesions and dysfunction lead often to neuropathic pain, the most prevalent and difficult to treat pain syndrome observed in MS patients. Despite its widespread occurrence, the underlying neural mechanisms for MS pain are not fully understood. For a better clarification of the pathophysiology of MS-associated pain, we investigated the role of HuR, an RNA-binding protein that positively regulates the stability of many target mRNAs, including several cytokines. The influence of HuR in the generation of the hypernociceptive response in a mouse model of relapsing-remitting experimental autoimmune encephalomyelitis (RR-EAE), an experimental model of MS, was investigated. HuR silencing, obtained through the repeated intrathecal administration of an antisense oligonucleotide (aODN) anti-HuR, completely attenuated hindpaw mechanical allodynia and thermal hyperalgesia developed by RR-EAE mice. Anti-HuR aODN also reduced severity of motor deficits as reflected by a reduction of clinical EAE score and improvement of rotarod performance. RR-EAE mice showed demyelination in spinal cord sections that was significantly reduced by HuR silencing. Double-staining immunofluorescence studies showed a neuronal localization of HuR within dorsal horn spinal cord, consistent with a neuronal mechanism of action. Our findings suggest the involvement of HuR in the hypernociceptive behaviour of RR-EAE mice providing the first pharmacological assessment of an antiallodynic and antihyperalgesic effect of HuR silencing. These data may provide support for HuR modulation as a therapeutic perspective for the management of MS-related neuropathic pain.

Targeting the pro-inflammatory factor CCL2 (MCP-1) with Bindarit for influenza A (H7N9) treatment

Influenza A viruses are important human and animal pathogens. Seasonal influenza viruses cause infections every year, and occasionally zoonotic viruses emerge to cause pandemics with significantly higher morbidity and mortality rates. Three cases of laboratory confirmed human infection with avian influenza A (H7N9) virus were reported in 2013, and there have been several cases reported across South East Asia, and recently in North America. Most patients experience severe respiratory illness, with mortality rates approaching 40%. No vaccine is currently available and the use of antivirals is complicated due to the emergence of drug resistant strains. Thus, there is a need to identify new drugs for therapeutic intervention and disease control. In humans, following H7N9 infection, there is excessive expression of pro-inflammatory factors CCL2, IL-6, IL-8, IFNα, interferon-γ, IP-10, MIG and macrophage inflammatory protein-1β, which has been shown to contribute to fatal disease outcomes in mouse models of infection. In the current study, the potent inhibitor of CCL2 synthesis, Bindarit, was examined as a countermeasure for H7N9-induced inflammation in a mouse model. Bindarit treatment of mice did not have any substantial therapeutic efficacy in H7N9 infection. Consequently, the results suggest that Bindarit may be ill-advised in the treatment of influenza H7N9 infection.

Peripheral and central neuronal ATF3 precedes CD4+ T-cell infiltration in EAE

Experimental allergic encephalomyelitis (EAE), an animal model of multiple sclerosis produced by immunization with myelin oligodendrocyte glycoprotein (MOG) and adjuvants, results from profound T-cell mediated CNS demyelination. EAE is characterized by progressive, ascending motor dysfunction and symptoms of ongoing pain and hypersensitivity, in some cases preceding or concomitant with the motor deficits. In this regard, the EAE model mimics major features of multiple sclerosis, where a central neuropathic pain state is common. Although the latter condition is presumed to arise from a CNS loss of inhibitory controls secondary to the demyelination, dysfunction of sensory neurons may also contribute. Based on our previous studies that demonstrated the utility of monitoring expression of activating transcription factor 3 (ATF3), a sensitive marker of injured sensory neurons, here we followed both ATF3 and CD4+ T cells invasion of sensory ganglia (as well as the CNS) at different stages of the EAE model. We found that ATF3 is induced in peripheral sensory ganglia and brainstem well before the appearance of motor deficits. Unexpectedly, the ATF3 induction always preceded T cell infiltration, typically in adjacent, but non-overlapping regions. Surprisingly, control administration of the pertussis toxin and/or Complete Freund's adjuvants, without MOG, induced ATF3 in sensory neurons. In contrast, T cell infiltration only occurred with MOG. Taken together, our results suggest that the clinical manifestations in the EAE result not only from central demyelination but also from neuronal stress and subsequent pathophysiology of sensory neurons.

The nitroxyl donor, Angeli's salt, reduces chronic constriction injury-induced neuropathic pain

Chronic pain is a major health problem worldwide. We have recently demonstrated the analgesic effect of the nitroxyl donor, Angeli's salt (AS) in models of inflammatory pain. In the present study, the acute and chronic analgesic effects of AS was investigated in chronic constriction injury of the sciatic nerve (CCI)-induced neuropathic pain in mice. Acute (7th day after CCI) AS treatment (1 and 3 mg/kg; s.c.) reduced CCI-induced mechanical, but not thermal hyperalgesia. The acute analgesic effect of AS was prevented by treatment with 1H-[1,2, 4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ, a soluble guanylate cyclase inhibitor), KT5823 (an inhibitor of protein kinase G [PKG]) or glibenclamide (GLB, an ATP-sensitive potassium channel blocker). Chronic (7-14 days after CCI) treatment with AS (3 mg/kg, s.c.) promoted a sustained reduction of CCI-induced mechanical and thermal hyperalgesia. Acute AS treatment reduced CCI-induced spinal cord allograft inflammatory factor 1 (known as Iba-1), interleukin-1β (IL-1β), and ST2 receptor mRNA expression. Chronic AS treatment reduced CCI-induced spinal cord glial fibrillary acidic protein (GFAP), Iba-1, IL-1β, tumor necrosis factor-α (TNF-α), interleukin-33 (IL-33) and ST2 mRNA expression. Chronic treatment with AS (3 mg/kg, s.c.) did not alter aspartate aminotransferase, alanine aminotransferase, urea or creatinine plasma levels. Together, these results suggest that the acute analgesic effect of AS depends on activating the cGMP/PKG/ATP-sensitive potassium channel signaling pathway. Moreover, chronic AS diminishes CCI-induced mechanical and thermal hyperalgesia by reducing the activation of spinal cord microglia and astrocytes, decreasing TNF-α, IL-1β and IL-33 cytokines expression. This spinal cord immune modulation was more prominent in the chronic treatment with AS. Thus, nitroxyl limits CCI-induced neuropathic pain by reducing spinal cord glial cells activation.

Rotarod motor performance and advanced spinal cord lesion image analysis refine assessment of neurodegeneration in experimental autoimmune encephalomyelitis

BACKGROUND

Experimental autoimmune encephalomyelitis (EAE) is a commonly used experimental model for multiple sclerosis (MS). Experience with this model mainly comes from the field of immunology, while data on its use in studying the neurodegenerative aspects of MS is scarce.

NEW METHOD

The aim of this study is to improve and refine methods to assess neurodegeneration and function in EAE. Using the rotarod, a tool used in neuroscience to monitor motor performance, we evaluated the correlation between motor performance, disease severity as measured using a clinical scale and area covered by inflammatory lesions.

RESULTS

The included parameters are highly correlated in a non-linear manner, with motor performance rapidly decreasing in the intermediate values of the clinical scale. The relation between motor performance and histopathological damage is exclusively determined by lesions in the ventral and lateral columns, based on a new method of analysis of the entire spinal cord. Using a set of definitions for distinct disease milestones, we quantified disease duration as well as severity.

COMPARISON WITH EXISTING METHODS

The rotarod measures motor performance in a more objective and quantitative manner compared to using a clinical score. The outcome shows a strong correlation to the surface area of inflammatory lesions in the motor systems of the spinal cord.

CONCLUSIONS

These results provide an improved workflow for interpreting the outcome of EAE from a neurological point of view, with the eventual goal of dissecting neurodegeneration and evaluating neuroprotective drugs in EAE for application in MS.

Effects of active immunisation with myelin basic protein and myelin-derived altered peptide ligand on pain hypersensitivity and neuroinflammation

Neuropathic pain is a debilitating condition in multiple sclerosis and experimental autoimmune encephalomyelitis (EAE). Specific myelin basic protein (MBP) peptides are encephalitogenic, and myelin-derived altered peptide ligands (APLs) are capable of preventing and ameliorating EAE. We investigated the effects of active immunisation with a weakly encephalitogenic epitope of MBP (MBP87-99) and its mutant APL (Cyclo-87-99[A(91),A(96)]MBP87-99) on pain hypersensitivity and neuroinflammation in Lewis rats. MBP-treated rats exhibited significant mechanical and thermal pain hypersensitivity associated with infiltration of T cells, MHC class II expression and microglia activation in the spinal cord, without developing clinical signs of paralysis. Co-immunisation with APL significantly decreased pain hypersensitivity and neuroinflammation emphasising the important role of neuroimmune crosstalk in neuropathic pain.

Pain management in Guillain-Barre syndrome: a systematic review

INTRODUCTION

Pain is a common symptom in patients with Guillain-Barre syndrome. Intensity is moderate to severe in most cases and pain may persist after resolution of the disease.

OBJECTIVE

Identify the most appropriate analgesic therapy for pain management in patients with Guillain-Barre syndrome.

MATERIAL AND METHODS

Systematic review and selection of scientific articles on treatment of pain in Guillain-Barre syndrome patients, published between January 1985 and December 2012. We included only randomised, double-blind, controlled trials assessing the effectiveness of drugs for pain management in these patients.

RESULTS

Four articles met the inclusion criteria. One evaluated the use of gabapentin, another evaluated carbamazepine, a third compared gabapentin to carbamazepine, and the last evaluated use of methylprednisolone. Both carbamazepine and gabapentin were useful for pain management. Patients experienced lower-intensity pain with gabapentin treatment in the study comparing that drug to carbamazepine. Methylprednisolone was not shown to be effective for reducing pain. The published data did not permit completion of a meta-analysis.

CONCLUSIONS

There is no robust evidence at present that would point to a single treatment option for this disorder. Further clinical studies of larger patient samples and with a longer duration are needed to characterise types of pain for each patient and measure pain intensity in an objective way.

Sex differences in a mouse model of multiple sclerosis: neuropathic pain behavior in females but not males and protection from neurological deficits during proestrus

BACKGROUND

Multiple sclerosis (MS), a demyelinating disease of the central nervous system, is one of the most prevalent neurological disorders in the industrialized world. This disease afflicts more than two million people worldwide, over two thirds of which are women. MS is typically diagnosed between the ages of 20-40 and can produce debilitating neurological impairments including muscle spasticity, muscle paralysis, and chronic pain. Despite the large sex disparity in MS prevalence, clinical and basic research investigations of how sex and estrous cycle impact development, duration, and severity of neurological impairments and pain symptoms are limited. To help address these questions, we evaluated behavioral signs of sensory and motor functions in one of the most widely characterized animal models of MS, the experimental autoimmune encephalomyelitis (EAE) model.

METHODS

C57BL/6 male and female mice received flank injection of complete Freund's adjuvant (CFA) or CFA plus myelin oligodendrocyte glycoprotein 35-55 (MOG35-55) to induce EAE. Experiment 1 evaluated sex differences of EAE-induced neurological motor deficits and neuropathic pain-like behavior over 3 weeks, while experiment 2 evaluated the effect of estrous phase in female mice on the same behavioral measures for 3 months. EAE-induced neurological motor deficits including gait analysis and forelimb grip strength were assessed. Neuropathic pain-like behaviors evaluated included sensitivity to mechanical, cold, and heat stimulations. Estrous cycle was determined daily via vaginal lavage.

RESULTS

MOG35-55-induced EAE produced neurological impairments (i.e., motor dysfunction) including mild paralysis and decreases in grip strength in both females and males. MOG35-55 produced behavioral signs of neuropathic pain-mechanical and cold hypersensitivity-in females, but not males. MOG35-55 did not change cutaneous heat sensitivity in either sex. Administration of CFA or CFA + MOG35-55 prolonged the time spent in diestrus for 2 weeks, after which normal cycling returned. MOG35-55 produced fewer neurological motor deficits when mice were in proestrus relative to non-proestrus phases.

CONCLUSIONS

We conclude that female mice are superior to males for the study of neuropathic pain-like behaviors associated with MOG35-55-induced EAE. Further, proestrus may be protective against EAE-induced neurological deficits, thus necessitating further investigation into the impact that estrous cycle exerts on MS symptoms.

Effects of vaccination with altered Peptide ligand on chronic pain in experimental autoimmune encephalomyelitis, an animal model of multiple sclerosis

Neuropathic pain is a chronic symptom of multiple sclerosis (MS) and affects nearly half of all MS sufferers. A key instigator of this pain is the pro-inflammatory response in MS. We investigated the behavioral effects of immunization with a mutant peptide of myelin basic protein (MBP), termed altered peptide ligand (APL), known to initiate immune deviation from a pro-inflammatory state to an anti-inflammatory response in experimental autoimmune encephalomyelitis (EAE), an animal model of MS. Male and female Lewis rats were injected with vehicle control or with varying doses of 50 or 100 μg guinea pig MBP in combination with or without APL. APL-treated animals established significantly lower disease severity compared to encephalitogenic MBP-treated animals. Animals with EAE developed mechanical, but not thermal pain hypersensitivity. Mechanical pain sensitivities were either improved or normalized during periods of clinical disease in male and female APL-treated animals as compared to the encephalitogenic group. No significant changes to thermal latency were observed upon co-immunization with APL. Together these data indicate that APL ameliorates disease states and selectively mediates an analgesic effect on EAE animals.