Full-length membrane-bound tumor necrosis factor-α acts through tumor necrosis factor receptor 2 to modify phenotype of sensory neurons

Neuroimmune Mechanisms Underlying Neuropathic Pain: The Potential Role of TNF-α-Necroptosis Pathway

The neuroimmune mechanism underlying neuropathic pain has been extensively studied. Tumor necrosis factor-alpha (TNF-α), a key pro-inflammatory cytokine that drives cytokine storm and stimulates a cascade of other cytokines in pain-related pathways, induces and modulates neuropathic pain by facilitating peripheral (primary afferents) and central (spinal cord) sensitization. Functionally, TNF-α controls the balance between cell survival and death by inducing an inflammatory response and two programmed cell death mechanisms (apoptosis and necroptosis). Necroptosis, a novel form of programmed cell death, is receiving increasing attraction and may trigger neuroinflammation to promote neuropathic pain. Chronic pain is often accompanied by adverse pain-associated emotional reactions and cognitive disorders. Overproduction of TNF-α in supraspinal structures such as the anterior cingulate cortex (ACC) and hippocampus plays an important role in pain-associated emotional disorders and memory deficits and also participates in the modulation of pain transduction. At present, studies reporting on the role of the TNF-α–necroptosis pathway in pain-related disorders are lacking. This review indicates the important research prospects of this pathway in pain modulation based on its role in anxiety, depression and memory deficits associated with other neurodegenerative diseases. In addition, we have summarized studies related to the underlying mechanisms of neuropathic pain mediated by TNF-α and discussed the role of the TNF-α–necroptosis pathway in detail, which may represent an avenue for future therapeutic intervention.

Association between increased inflammatory cytokine expression in the lateral habenular nucleus and depressive-like behavior induced by unpredictable chronic stress in rats

Depression induced by unpredictable chronic stress (UCS) has been widely studied using animal models. However, its underlying pathological mechanisms remain unclear. Increased inflammatory cytokines (ICs) in the central nervous system (CNS) are closely related to depressive disorder. UCS was used as an animal model in this study to investigate how UCS-induced changes in cytokine signaling lead to depression. We found that UCS could increase ICs in the CNS, especially in the habenular nucleus (Hb). UCS resulted in decreased expression of Menin in Hb and increased the activation of the NF-κB signaling pathway. Local administration of tumor necrosis factor-α in the lateral Hb (LHb) could induce depressive-like behavior in rats. The anti-inflammatory drug aspirin and the NF-κB inhibitor pyrrolidine dithiocarbamate could alleviate depressive-like behavior. This phenomenon was not observed for local administration in the dorsal raphe nucleus and paraventricular nucleus. These results indicate that LHb is the main central target for ICs to regulate depressive-like behaviors. We also found that LHb lesions could improve the inflammatory response in the hippocampus, reduce the activation of the NF-κB signaling pathway and the expression of ICs, and increase the expression of brain-derived neurotrophic factor and its receptor tropomyosin receptor kinase B, collectively improving the neuroinflammation caused by UCS. Moreover, LHb lesions improve not only hippocampal neurogenesis damage caused by UCS by activating the PI3K/mTOR signaling pathway but also hippocampal function by reducing the expression of apoptosis-related proteins, including phosphorylated p53, Bax, Bcl2, and cleaved-caspase3. In conclusion, our study sheds light on the pathogenesis of ICs-induced depression. Anti-inflammation in the CNS could be a new strategy in the treatment of depression.

Homeostatic regulation of neuronal excitability by probiotics in male germ-free mice

Emerging evidence indicates that probiotics can influence the gut-brain axis to ameliorate somatic and behavioral symptoms associated with brain disorders. However, whether probiotics have effects on the electrophysiological activities of individual neurons in the brain has not been evaluated at a single-neuron resolution, and whether the neuronal effects of probiotics depend on the gut microbiome status have yet to be tested. Thus, we conducted whole-cell patch-clamp recording-assisted electrophysiological characterizations of the neuronal effects of probiotics in male germ-free (GF) mice with and without gut microbiome colonization. Two weeks of treatment with probiotics (Lactobacillus rhamnosus and Bifidobacterium animalis) significantly and selectively increased the intrinsic excitability of hippocampal CA1 pyramidal neurons, whereas reconstituting gut microbiota in GF mice reversed the effects of the probiotics leading to a decreased intrinsic excitability in hippocampal neurons. This bidirectional modulation of neuronal excitability by probiotics was observed in hippocampal neurons with corresponding basal membrane property and action potential waveform changes. However, unlike the hippocampus, the amygdala excitatory neurons did not show any electrophysiological changes to the probiotic treatment in either GF or conventionalized GF mice. Our findings demonstrate for the first time how probiotic treatment can have a significant influence on the electrophysiological properties of neurons, bidirectionally modulating their intrinsic excitability in a gut microbiota and brain area-specific manner.

Targeting Neuroinflammation via Purinergic P2 Receptors for Disease Modification in Drug-Refractory Epilepsy

Treatment of epilepsy remains a clinical challenge, with >30% of patients not responding to current antiseizure drugs (ASDs). Moreover, currently available ASDs are merely symptomatic without altering significantly the progression of the disease. Inflammation is increasingly recognized as playing an important role during the generation of hyperexcitable networks in the brain. Accordingly, the suppression of chronic inflammation has been suggested as a promising therapeutic strategy to prevent epileptogenesis and to treat drug-refractory epilepsy. As a consequence, a strong focus of ongoing research is identification of the mechanisms that contribute to sustained inflammation in the brain during epilepsy and whether these can be targeted. ATP is released in response to several pathological stimuli, including increased neuronal activity within the central nervous system, where it functions as a neuro- and gliotransmitter. Once released, ATP activates purinergic P2 receptors, which are divided into metabotropic P2Y and ionotropic P2X receptors, driving inflammatory processes. Evidence from experimental models and patients demonstrates widespread expression changes of both P2Y and P2X receptors during epilepsy, and critically, drugs targeting both receptor subtypes, in particular the P2Y₁ and P2X₇ subtypes, have been shown to possess both anticonvulsive and antiepileptic potential. This review provides a detailed summary of the current evidence suggesting ATP-gated receptors as novel drug targets for epilepsy and discusses how P2 receptor–driven inflammation may contribute to the generation of seizures and the development of epilepsy.

Ion channels and neuronal hyperexcitability in chemotherapy-induced peripheral neuropathy; cause and effect?

Abstract

Cancer is the second leading cause of death worldwide and is a major global health burden. Significant improvements in survival have been achieved, due in part to advances in adjuvant antineoplastic chemotherapy. The most commonly used antineoplastics belong to the taxane, platinum, and vinca alkaloid families. While beneficial, these agents are frequently accompanied by severe side effects, including chemotherapy-induced peripheral neuropathy (CPIN). While CPIN affects both motor and sensory systems, the majority of symptoms are sensory, with pain, tingling, and numbness being the predominant complaints. CPIN not only decreases the quality of life of cancer survivors but also can lead to discontinuation of treatment, thereby adversely affecting survival. Consequently, minimizing the incidence or severity of CPIN is highly desirable, but strategies to prevent and/or treat CIPN have proven elusive. One difficulty in achieving this goal arises from the fact that the molecular and cellular mechanisms that produce CPIN are not fully known; however, one common mechanism appears to be changes in ion channel expression in primary afferent sensory neurons. The processes that underlie chemotherapy-induced changes in ion channel expression and function are poorly understood. Not all antineoplastic agents directly affect ion channel function, suggesting additional pathways may contribute to the development of CPIN Indeed, there are indications that these drugs may mediate their effects through cellular signaling pathways including second messengers and inflammatory cytokines. Here, we focus on ion channelopathies as causal mechanisms for CPIN and review the data from both pre-clinical animal models and from human studies with the aim of facilitating the development of appropriate strategies to prevent and/or treat CPIN.

Adrenomedullin mediates tumor necrosis factor-α-induced responses in dorsal root ganglia in rats

Adrenomedullin (AM), a member of the calcitonin gene-related peptide (CGRP) family, has been demonstrated to be a pain peptide. This study investigated the possible involvement of AM in tumor necrosis factor-alpha (TNF-α)-induced responses contributing to neuronal plasticity in the dorsal root ganglia (DRG). Exposure of the DRG explant cultures to TNF-α (5nM) for 48h upregulated the expression of AM mRNA. The treatment with TNF-α also increased the level of CGRP, CCL-2 and MMP-9 mRNA in the cultured DRG. This increase was attenuated by the co-treatment with the selective AM receptor antagonist AM22-52 (2μM). The blockade of AM receptors inhibited TNF-α-induced increase of the glial fibrillary acidic protein (GFAP), interleukin-1β (IL-1β), phosphorylated cAMP response element binding protein (pCREB) and nuclear factor kappa B (pNF-κB) proteins. On the other hand, the treatment with the AM receptor agonist AM1-50 (10nM) for 96h induced an increase in the level of GFAP, IL-1β, pCREB and pNF-κB proteins. The inhibition of AM activity did not change TNF-α-induced phosphorylation of extracellular signal-related kinase (pERK) while the treatment with AM1-50 still increased the level of pERK in the cultured DRG. Immunofluorescence assay showed the colocalization of AM-like immunoreactivity (IR) with TNF-α-IR in DRG neurons. The present study suggests that the increased AM receptor signaling mediated the many, but not all, TNF-α-induced activities, contributing to peripheral sensitization in neuropathic pain.

Modulation of Voltage-Gated Sodium Channels by Activation of Tumor Necrosis Factor Receptor-1 and Receptor-2 in Small DRG Neurons of Rats

Tumor necrosis factor- (TNF-) α is a proinflammatory cytokine involved in the development and maintenance of inflammatory and neuropathic pain. Its effects are mediated by two receptors, TNF receptor-1 (TNFR-1) and TNF receptor-2 (TNFR-2). These receptors play a crucial role in the sensitization of voltage-gated sodium channels (VGSCs), a key mechanism in the pathogenesis of chronic pain. Using the whole-cell patch-clamp technique, we examined the influence of TNFR-1 and TNFR-2 on VGSCs and TTX-resistant NaV1.8 channels in isolated rat dorsal root ganglion neurons by using selective TNFR agonists. The TNFR-1 agonist R32W (10 pg/mL) caused an increase in the VGSC current (I(Na(V))) by 27.2 ± 5.1%, while the TNFR-2 agonist D145 (10 pg/mL) increased the current by 44.9 ± 2.6%. This effect was dose dependent. Treating isolated NaV1.8 with R32W (100 pg/mL) resulted in an increase in I(NaV(1.8)) by 18.9 ± 1.6%, while treatment with D145 (100 pg/mL) increased the current by 14.5 ± 3.7%. Based on the current-voltage relationship, 10 pg of R32W or D145 led to an increase in I(Na(V)) in a bell-shaped, voltage-dependent manner with a maximum effect at -30 mV. The effects of TNFR activation on VGSCs promote excitation in primary afferent neurons and this might explain the sensitization mechanisms associated with neuropathic and inflammatory pain.

Neuromodulatory properties of inflammatory cytokines and their impact on neuronal excitability

Increasing evidence underlines that prototypical inflammatory cytokines (IL-1β, TNF-α and IL-6) either synthesized in the central (CNS) or peripheral nervous system (PNS) by resident cells, or imported by immune blood cells, are involved in several pathophysiological functions, including an unexpected impact on synaptic transmission and neuronal excitability. This review describes these unconventional neuromodulatory properties of cytokines, that are distinct from their classical action as effector molecules of the immune system. In addition to the role of cytokines in brain physiology, we report evidence that dysregulation of their biosynthesis and cellular release, or alterations in receptor-mediated intracellular pathways in target cells, leads to neuronal cell dysfunction and modifications in neuronal network excitability. As a consequence, targeting of these cytokines, and related signalling molecules, is considered a novel option for the development of therapies in various CNS or PNS disorders associated with an inflammatory component. This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.

Effect of intermittent normobaric hyperoxia for treatment of neuropathic pain in Chinese patients with spinal cord injury

STUDY DESIGN

Prospective, randomized and controlled study.

OBJECTIVES

The aim of the study was to investigate the effect of intermittent normobaric hyperoxia (InHO) for treatment of neuropathic pain in patients with spinal cord injury (SCI).

SETTING

The First Affiliated Hospital of Nanhua University, Hengyang, Hunan Province, China.

METHODS

Patients with SCI from Hunan Province were recruited from the First Affiliated Hospital of Nanhua University. History, duration, localization and characteristics of pain were recorded. Visual analog scale (VAS), the Patient Global Impression of Change (PGIC) and Short Form-36 walk-wheel (SF-36ww) was used to investigate the effect of InHO. Patients were randomly assigned to study and control groups. In study group, patients were exposed to pure oxygen via non-rebreathing reservoir mask, which increased the provided oxygen at a rate of 7 l min^-1 for 1 or 4 h daily in 2 weeks. While in control group, patients breathed air via non-rebreathing reservoir mask at the same rate.

RESULTS

A total of 62 SCI patients with neuropathic pain were included in the study. The mean age of the patients was 36.85±10.71 years. Out of 62 patients, 21 were tetraplegic and 41 were paraplegic. Overall, 14 patients had complete SCI while 48 patients had incomplete injuries. Three groups were similar with respect to age, gender, duration, smoker or not, level and severity of injury. In the 4 h per day InHO groups, a statistically significant reduction of the VAS values was observed (P<0.05). Significant difference was also found in SF-36ww pain scores and PGIC (P<0.05). However, such an effect was not evident in the control group.

CONCLUSION

This study revealed that in treatment of neuropathic pain of SCI patients, InHO may be effective.

PERSPECTIVE

This article presents InHO may effectively complement pharmacological treatment in patients with SCI and neuropathic pain.