[The role of microglia and ATP receptors in a mechanism of neuropathic pain]

P2Y receptors in neuropathic pain

This review summarizes and evaluates the relationship between neuropathic pain and P2Y receptors from inception to 2019. Purinergic receptors have been well studied in recent years using various molecular biological methods. The main research objective of this review is to determine the association of P2Y1, P2Y2, P2Y6, P2Y12 and P2Y13 receptors with neuropathic pain. This review includes the most comprehensive subtypes of P2Y that related to neuropathic pain and the current therapeutic method of neuropathic pain. G protein-coupled P2Y receptors are located on neurons, astrocytes, oligodendrocytes and microglial cells and regulate neurotransmission. Nerve injury is the prime reason for abnormal regulation of P2Y receptor mRNA expression, subsequently, inducing neuropathic pain. Neuropathic pain is a type of chronic pain that is divided into peripheral, central and mixed. Numerous studies demonstrated a positive correlation between the expression level of P2Y receptors and neuropathic pain generation. Also, several reports showed that P2Y short hairpin RNA (shRNA) and P2Y antagonist can be used as an analgesic to relieve neuropathic pain via decreasing P2Y receptor expression level and neural cell activation. However, the transformation process from basic experiments to clinical applications is a long process. Current deficiencies and future research directions are discussed at the end of this review.

Topical moringin-cream relieves neuropathic pain by suppression of inflammatory pathway and voltage-gated ion channels in murine model of multiple sclerosis

Background

Neuropathic pain represents the major public health burden with a strong impact on quality life in multiple sclerosis patients. Although some advances have been obtained in the last years, the conventional therapies remain poorly effective. Thus, the discovery of innovative approaches to improve the outcomes for multiple sclerosis patients is a goal of primary importance. With this aim, we investigated the efficacy of the 4-(α−L-rhamnopyranosyloxy)benzyl isothiocyanate (moringin), purified from Moringa oleifera seeds and ready-to-use as topical treatment in experimental autoimmune encephalomyelitis, murine model of multiple sclerosis. Female C57BL/6 mice immunized with myelin oligodendrocyte glycoprotein (MOG_35–55) were topically treated with 2% moringin cream twice daily from the onset of the symptoms until the sacrifice occurred about 21 days after experimental autoimmune encephalomyelitis induction.

Results

Our observations showed the efficacy of 2% moringin cream treatment in reducing clinical and histological disease score, as well as in alleviating neuropathic pain with consequent recovering of the hind limbs and response to mechanical stimuli. In particular, Western blot analysis and immunohistochemical evaluations revealed that 2% moringin cream was able to counteract the inflammatory cascade by reducing the production of pro-inflammatory cytokines (interleukin-17 and interferon-γ) and in parallel by increasing the expression of anti-inflammatory cytokine (interleukin-10). Interestingly, 2% moringin cream treatment was found to modulate the expression of voltage-gated ion channels (results focused on P2X7, Nav 1.7, Nav 1.8 KV4.2, and α2δ-1) as well as metabotropic glutamate receptors (mGluR5 and xCT) involved in neuropathic pain initiation and maintenance.

Conclusions

Finally, our evidences suggest 2% moringin cream as a new pharmacological trend in the management of multiple sclerosis-induced neuropathic pain.

Basic/Translational Development of Forthcoming Opioid- and Nonopioid-Targeted Pain Therapeutics

Opioids represent an efficacious therapeutic modality for some, but not all pain states. Singular reliance on opioid therapy for pain management has limitations, and abuse potential has deleterious consequences for patient and society. Our understanding of pain biology has yielded insights and opportunities for alternatives to conventional opioid agonists. The aim is to have efficacious therapies, with acceptable side effect profiles and minimal abuse potential, which is to say an absence of reinforcing activity in the absence of a pain state. The present work provides a nonexclusive overview of current drug targets and potential future directions of research and development. We discuss channel activators and blockers, including sodium channel blockers, potassium channel activators, and calcium channel blockers; glutamate receptor-targeted agents, including N-methyl-D-aspartate, α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid, and metabotropic receptors. Furthermore, we discuss therapeutics targeted at γ-aminobutyric acid, α2-adrenergic, and opioid receptors. We also considered antagonists of angiotensin 2 and Toll receptors and agonists/antagonists of adenosine, purine receptors, and cannabinoids. Novel targets considered are those focusing on lipid mediators and anti-inflammatory cytokines. Of interest is development of novel targeting strategies, which produce long-term alterations in pain signaling, including viral transfection and toxins. We consider issues in the development of druggable molecules, including preclinical screening. While there are examples of successful translation, mechanistically promising preclinical candidates may unexpectedly fail during clinical trials because the preclinical models may not recapitulate the particular human pain condition being addressed. Molecular target characterization can diminish the disconnect between preclinical and humans' targets, which should assist in developing nonaddictive analgesics.

Current and Future Issues in the Development of Spinal Agents for the Management of Pain

Targeting analgesic drugs for spinal delivery reflects the fact that while the conscious experience of pain is mediated supraspinally, input initiated by high intensity stimuli, tissue injury and/or nerve injury is encoded at the level of the spinal dorsal horn and this output informs the brain as to the peripheral environment. This encoding process is subject to strong upregulation resulting in hyperesthetic states and downregulation reducing the ongoing processing of nociceptive stimuli reversing the hyperesthesia and pain processing. The present review addresses the biology of spinal nociceptive processing as relevant to the effects of intrathecally-delivered drugs in altering pain processing following acute stimulation, tissue inflammation/injury and nerve injury. The review covers i) the major classes of spinal agents currently employed as intrathecal analgesics (opioid agonists, alpha 2 agonists; sodium channel blockers; calcium channel blockers; NMDA blockers; GABA A/B agonists; COX inhibitors; ii) ongoing developments in the pharmacology of spinal therapeutics focusing on less studied agents/targets (cholinesterase inhibition; Adenosine agonists; iii) novel intrathecal targeting methodologies including gene-based approaches (viral vectors, plasmids, interfering RNAs); antisense, and toxins (botulinum toxins; resniferatoxin, substance P Saporin); and iv) issues relevant to intrathecal drug delivery (neuraxial drug distribution), infusate delivery profile, drug dosing, formulation and principals involved in the preclinical evaluation of intrathecal drug safety.

Sex Differences in Pain

Females greatly outnumber males as sufferers of chronic pain. Although social and psychological factors certainly play a role in the differences in prevalence and incidence, biological differences in the functioning of the immune system likely underlie these observed effects. This Review examines the current literature on biological sex differences in the functioning of the innate and adaptive immune systems as they relate to pain experience. With rodent models, we and others have observed that male mice utilize microglia in the spinal cord to mediate pain, whereas females preferentially use T cells in a similar manner. The difference can be traced to differences in cell populations, differences in suppression by hormones, and disparate cellular responses in males and females. These sex differences also translate into human cellular responses and may be the mechanism by which the disproportionate chronic pain experience is based. Recognition of the evidence underlying sex differences in pain will guide development of treatments and provide better options for patients that are tailored to their physiology. © 2016 Wiley Periodicals, Inc.

Brain-derived neurotrophic factor from microglia: a molecular substrate for neuropathic pain

One of the most significant advances in pain research is the realization that neurons are not the only cell type involved in the etiology of chronic pain. This realization has caused a radical shift from the previous dogma that neuronal dysfunction alone accounts for pain pathologies to the current framework of thinking that takes into account all cell types within the central nervous system (CNS). This shift in thinking stems from growing evidence that glia can modulate the function and directly shape the cellular architecture of nociceptive networks in the CNS. Microglia, in particular, are increasingly recognized as active principal players that respond to changes in physiological homeostasis by extending their processes toward the site of neural damage, and by releasing specific factors that have profound consequences on neuronal function and that contribute to CNS pathologies caused by disease or injury. A key molecule that modulates microglia activity is ATP, an endogenous ligand of the P2 receptor family. Microglia expresses several P2 receptor subtypes, and of these the P2X4 receptor subtype has emerged as a core microglia-neuron signaling pathway: activation of this receptor drives the release of brain-derived neurotrophic factor (BDNF), a cellular substrate that causes disinhibition of pain-transmitting spinal lamina I neurons. Converging evidence points to BDNF from spinal microglia as being a critical microglia-neuron signaling molecule that gates aberrant nociceptive processing in the spinal cord. The present review highlights recent advances in our understanding of P2X4 receptor-mediated signaling and regulation of BDNF in microglia, as well as the implications for microglia-neuron interactions in the pathobiology of neuropathic pain.

P2X4 purinoceptor signaling in chronic pain

ATP, acting via P2 purinergic receptors, is a known mediator of inflammatory and neuropathic pain. There is increasing evidence that the ATP-gated P2X4 receptor (P2X4R) subtype is a locus through which activity of spinal microglia and peripheral macrophages instigate pain hypersensitivity caused by inflammation or by injury to a peripheral nerve. The present article highlights the recent advances in our understanding of microglia-neuron interactions in neuropathic pain by focusing on the signaling and regulation of the P2X4R. We will also develop a framework for understanding converging lines of evidence for involvement of P2X4Rs expressed on macrophages in peripheral inflammatory pain.

ATP receptors gate microglia signaling in neuropathic pain

Microglia were described by Pio del Rio-Hortega (1932) as being the 'third element' distinct from neurons and astrocytes. Decades after this observation, the function and even the very existence of microglia as a distinct cell type were topics of intense debate and conjecture. However, considerable advances have been made towards understanding the neurobiology of microglia resulting in a radical shift in our view of them as being passive bystanders that have solely immune and supportive roles, to being active principal players that contribute to central nervous system pathologies caused by disease or following injury. Converging lines of evidence implicate microglia as being essential in the pathogenesis of neuropathic pain, a debilitating chronic pain condition that can occur after peripheral nerve damage caused by disease, infection, or physical injury. A key molecule that modulates microglial activity is ATP, an endogenous ligand of the P2-purinoceptor family consisting of P2X ionotropic and P2Y metabotropic receptors. Microglia express several P2 receptor subtypes, and of these the P2X4, P2X7, and P2Y12 receptor subtypes have been implicated in neuropathic pain. The P2X4 receptor has emerged as the core microglia-neuron signaling pathway: activation of this receptor causes release of brain-derived neurotrophic factor (BDNF) which causes disinhibition of pain-transmission neurons in spinal lamina I. The present review highlights recent advances in understanding the signaling and regulation of P2 receptors expressed in microglia and the implications for microglia-neuron interactions for the management of neuropathic pain.

The reactions of glial cells and endoneurial macrophages in the dorsal root ganglion and their contribution to pain-related behavior after application of nucleus pulposus onto the nerve root in rats

STUDY DESIGN

Controlled, interventional, animal study.

OBJECTIVE

To observe the reaction of glial cells and endoneurial macrophages in the dorsal root ganglion (DRG) after application of nucleus pulposus (NP) and investigate whether activated DRG glial cells play a role in the pathogenesis of neuropathic pain.

SUMMARY OF BACKGROUND DATA

Peripheral nerve injury activated DRG and spinal cord glial cells and several cytokines and neurotrophins released from these activated glial cells might induce pain hypersensitivity.

METHODS

Adult male Sprague-Dawley rats were used. NP harvested from the tail was applied to the left L5 DRG. Behavioral testing was performed to investigate the mechanical withdrawal threshold. The numbers of activated satellite glial cells and endoneurial macrophages were counted, and the expressions of tumor necrosis factor-alpha (TNF-alpha) and glial cell-line derived neurotrophic factor (GDNF) were examined by double-labeled immunohistochemistry and immunoblotting.

RESULTS

The mechanical withdrawal threshold was significantly decreased for 28 days and then gradually recovered (P < 0.05). Long-term activation of endoneurial macrophages and satellite glial cells in the DRG was observed, and the reactions of these cells correlated well with pain-related behavior. TNF-alpha was expressed in both endoneurial macrophages and activated satellite glial cells, and TNF-alpha expression was significantly increased in the early stage (P < 0.05). Activated satellite glial cells also expressed GDNF, and its expression was significantly increased and persisted for 28 days (P < 0.05).

CONCLUSION

Activation of DRG glial cells and endoneurial macrophages plays an important role in the pathogenesis of the neuropathic pain state. TNF-alpha actively released from activated glial cells and endoneurial macrophages in the DRG might initiate and maintain the neuropathic pain together with TNF-alpha derived from the applied NP. In the recovery phase, persistent expression of GDNF from activated satellite glial cells might play an important role to restore the function of damaged neurons and recover from neuropathic pain.

The reactions of glial cells and endoneurial macrophages in the dorsal root ganglion and their contribution to pain-related behavior after application of nucleus pulposus onto the nerve root in rats

STUDY DESIGN

Controlled, interventional, animal study.

OBJECTIVE

To observe the reaction of glial cells and endoneurial macrophages in the dorsal root ganglion (DRG) after application of nucleus pulposus (NP) and investigate whether activated DRG glial cells play a role in the pathogenesis of neuropathic pain.

SUMMARY OF BACKGROUND DATA

Peripheral nerve injury activated DRG and spinal cord glial cells and several cytokines and neurotrophins released from these activated glial cells might induce pain hypersensitivity.

METHODS

Adult male Sprague-Dawley rats were used. NP harvested from the tail was applied to the left L5 DRG. Behavioral testing was performed to investigate the mechanical withdrawal threshold. The numbers of activated satellite glial cells and endoneurial macrophages were counted, and the expressions of tumor necrosis factor-alpha (TNF-alpha) and glial cell-line derived neurotrophic factor (GDNF) were examined by double-labeled immunohistochemistry and immunoblotting.

RESULTS

The mechanical withdrawal threshold was significantly decreased for 28 days and then gradually recovered (P < 0.05). Long-term activation of endoneurial macrophages and satellite glial cells in the DRG was observed, and the reactions of these cells correlated well with pain-related behavior. TNF-alpha was expressed in both endoneurial macrophages and activated satellite glial cells, and TNF-alpha expression was significantly increased in the early stage (P < 0.05). Activated satellite glial cells also expressed GDNF, and its expression was significantly increased and persisted for 28 days (P < 0.05).

CONCLUSION

Activation of DRG glial cells and endoneurial macrophages plays an important role in the pathogenesis of the neuropathic pain state. TNF-alpha actively released from activated glial cells and endoneurial macrophages in the DRG might initiate and maintain the neuropathic pain together with TNF-alpha derived from the applied NP. In the recovery phase, persistent expression of GDNF from activated satellite glial cells might play an important role to restore the function of damaged neurons and recover from neuropathic pain.

Purinoceptors in microglia and neuropathic pain

Emerging evidence indicates that microglia play a critical role in the pathogenesis of neuropathic pain, a debilitating chronic pain condition that can occur after peripheral nerve damage caused by disease, infection, or physical injury. Microglia are immunocompetent cells of the central nervous system and express various ionotropic P2X and metabotropic P2Y purinoceptors. After injury to a peripheral nerve, microglia in the spinal cord become activated and upregulate expression of the P2X4 receptor. Recent findings suggest that activation of P2X4 receptors evokes release of brain-derived neurotrophic factor from microglia and that this mediates microglia-neuron signaling leading to pain hypersensitivity. Thus, P2X4 receptors and the intracellular signaling mediators in microglia are promising therapeutic targets for the development of novel pharmacological agents in the management of neuropathic pain.