Neuropathic Pain Phenotype Does Not Involve the NLRP3 Inflammasome and Its End Product Interleukin-1β in the Mice Spared Nerve Injury Model

Intrathecal gastrodin alleviates allodynia in a rat spinal nerve ligation model through NLRP3 inflammasome inhibition

BACKGROUND

Gastrodin (GAS), a main bioactive component of the herbal plant, Gastrodia elata Blume, has shown to have beneficial effects on neuroinflammatory diseases such as Alzheimer's disease in animal studies and migraine in clinical studies. Inflammasome is a multimeric protein complex having a core of pattern recognition receptor and has been implicated in the development of neuroinflammatory diseases. Gastrodin has shown to modulate the activation of nucleotide-binding oligomerization domain (NOD)-like receptor protein 3 (NLRP3) inflammasome. This study investigated the effects of GAS on the intensity of mechanical allodynia and associated changes in NLRP3 inflammasome expression at the spinal level using L5/6 spinal nerve ligation model (SNL) in rats.

METHODS

Intrathecal (IT) catheter implantation and SNL were used for drug administration and pain model in male Sprague-Dawley rats. The effect of gastrodin or MCC950 (NLRP3 inflammasome inhibitor) on mechanical allodynia was measured by von Frey test. Changes in NLRP3 inflammasome components and interleukin-1β (IL-1β) and cellular expression were examined in the spinal cord and dorsal root ganglion.

RESULTS

The expression of NLRP3 inflammasome components was found mostly in the neurons in the spinal cord and dorsal root ganglion. The protein and mRNA levels of NLRP3, apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC), caspase-1, and IL-1β were upregulated in SNL animals compared to Sham animals. IT administration of GAS significantly attenuated the expression of NLRP3 inflammasome and the intensity of SNL-induced mechanical allodynia. NLRP3 inflammasome inhibitor, MCC950, also attenuated the intensity of allodynia, but the effect is less strong and shorter than that of GAS.

CONCLUSIONS

Expression of NLRP3 inflammasome and IL-1β is greatly increased and mostly found in the neurons at the spinal level in SNL model, and IT gastrodin exerts a significant anti-allodynic effect in SNL model partly through suppressing the expression of NLRP3 inflammasome.

NLR family pyrin domain containing 3 (NLRP3) inflammasomes and peripheral neuropathic pain - Emphasis on microRNAs (miRNAs) as important regulators

Neuropathic pain is caused by the lesion or disease of the somatosensory system and can be initiated and/or maintained by both central and peripheral mechanisms. Nerve injury leads to neuronal damage and apoptosis associated with the release of an array of pathogen- or damage-associated molecular patterns to activate inflammasomes. The activation of the NLR family pyrin domain containing 3 (NLRP3) inflammasome contributes to neuropathic pain and may represent a novel target for pain therapeutic development. In the current review, we provide an up-to-date summary of the recent findings on the involvement of NLRP3 inflammasome in modulating neuropathic pain development and maintenance, focusing on peripheral neuropathic conditions. Here we provide a detailed review of the mechanisms whereby NLRP3 inflammasomes contribute to neuropathic pain via (1) neuroinflammation, (2) apoptosis, (3) pyroptosis, (4) proinflammatory cytokine release, (5) mitochondrial dysfunction, and (6) oxidative stress. We then present the current research literature reporting on the antinociceptive effects of several natural products and pharmacological interventions that target activation, expression, and/or regulation of NLRP3 inflammasome. Furthermore, we emphasize the effects of microRNAs as another regulator of NLRP3 inflammasome. In conclusion, we summarize the possible caveats and future perspectives that might provide successful therapeutic approaches against NLRP3 inflammasome for treating or preventing neuropathic pain conditions.

Involvement of Oxidative Stress and Nutrition in the Anatomy of Orofacial Pain

Pain is a very important problem of our existence, and the attempt to understand it is one the oldest challenges in the history of medicine. In this review, we summarize what has been known about pain, its pathophysiology, and neuronal transmission. We focus on orofacial pain and its classification and features, knowing that is sometimes purely subjective and not well defined. We consider the physiology of orofacial pain, evaluating the findings on the main neurotransmitters; in particular, we describe the roles of glutamate as approximately 30-80% of total peripheric neurons associated with the trigeminal ganglia are glutamatergic. Moreover, we describe the important role of oxidative stress and its association with inflammation in the etiogenesis and modulation of pain in orofacial regions. We also explore the warning and protective function of orofacial pain and the possible action of antioxidant molecules, such as melatonin, and the potential influence of nutrition and diet on its pathophysiology. Hopefully, this will provide a solid background for future studies that would allow better treatment of noxious stimuli and for opening new avenues in the management of pain.

NLRP3-Mediated Piezo1 Upregulation in ACC Inhibitory Parvalbumin-Expressing Interneurons Is Involved in Pain Processing after Peripheral Nerve Injury

The anterior cingulate cortex (ACC) is particularly critical for pain information processing. Peripheral nerve injury triggers neuronal hyper-excitability in the ACC and mediates descending facilitation to the spinal dorsal horn. The mechanically gated ion channel Piezo1 is involved in the transmission of pain information in the peripheral nervous system. However, the pain-processing role of Piezo1 in the brain is unknown. In this work, we found that spared (sciatic) nerve injury (SNI) increased Piezo1 protein levels in inhibitory parvalbumin (PV)-expressing interneurons (PV-INs) but not in glutaminergic CaMKⅡ+ neurons, in the bilateral ACC. A reduction in the number of PV-INs but not in the number of CaMKⅡ+ neurons and a significant reduction in inhibitory synaptic terminals was observed in the SNI chronic pain model. Further, observation of morphological changes in the microglia in the ACC showed their activated amoeba-like transformation, with a reduction in process length and an increase in cell body area. Combined with the encapsulation of Piezo1-positive neurons by Iba1+ microglia, the loss of PV-INs after SNI might result from phagocytosis by the microglia. In cellular experiments, administration of recombinant rat TNF-α (rrTNF) to the BV2 cell culture or ACC neuron primary culture elevated the protein levels of Piezo1 and NOD-like receptor (NLR) family pyrin domain containing 3 (NLRP3). The administration of the NLRP3 inhibitor MCC950 in these cells blocked the rrTNF-induced expression of caspase-1 and interleukin-1β (key downstream factors of the activated NLRP3 inflammasome) in vitro and reversed the SNI-induced Piezo1 overexpression in the ACC and alleviated SNI-induced allodynia in vivo. These results suggest that NLRP3 may be the key factor in causing Piezo1 upregulation in SNI, promoting an imbalance between ACC excitation and inhibition by inducing the microglial phagocytosis of PV-INs and, thereby, facilitating spinal pain transmission.

Anti-Inflammatory and Analgesic Effects of Curcumin Nanoparticles Associated with Diclofenac Sodium in Experimental Acute Inflammation

The present study evaluated the anti-inflammatory and analgesic effects of conventional curcumin (cC) and curcumin nanoparticles (nC) associated with diclofenac sodium (D) in experimental acute inflammation (AI) induced by carrageenan administration. Seven groups of eight randomly selected Wistar-Bratislava white rats were evaluated. One group was the control (C), and AI was induced in the other six groups. The AI group was treated with saline solution, the AID group was treated with D, the AIcC200 and AInC200 groups were treated with cC and nC, respectively, while AIcC200D and AInC200D were treated with cC and nC, respectively, both associated with D. Conventional curcumin, nC, and D were administered in a single dose of 200 mg/kg b.w. for cC and nC and 5 mg/kg b.w. for D. Association of cC or nC to D resulted in significant antinociceptive activity, and improved mechanical pressure stimulation and heat thresholds at 3, 5, 7 and 24 h (p < 0.03). The association of cC and nC with D (AIcC200D and AInC200D groups) showed significantly lower plasma and tissue levels of tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), and interleukin-1β (IL-1β) up to 2.5 times, with the best results in the group who received nC. Moreover, AInC200D presented the least severe histopathological changes with a reduced level of inflammation in the dermis and hypodermis. The combination of nC to D showed efficiency in reducing pain, inflammatory cytokines, and histological changes in acute inflammation.

Hyperactivity of Innate Immunity Triggers Pain via TLR2-IL-33-Mediated Neuroimmune Crosstalk

The innate immune system responds to infections that give rise to pain. How the innate immune system interacts with the sensory nervous system and contributes to pain is poorly understood. Here we report that hyperactivity of innate immunity primes and initiates pain states via the TLR2-interleukin-33 (IL-33) axis. Toll-like receptors (TLRs) are upregulated in the complete Freund's adjuvant (CFA) pain model, and knockout of TLR2 abolishes CFA-induced pain. Selective activation of TLR2/6 triggers acute pain via upregulation of IL-33 in the hindpaw, dorsal root ganglia (DRG), and spinal cord in an NLRP3-dependent manner. The IL-33 increase further initiates priming of nociceptive neurons and pain states. Finally, blocking IL-33 receptors at the spinal level mediates analgesia during acute and chronic inflammatory pain, underscoring an important function of IL-33 in pain signaling. Collectively, our data reveal a critical role of the TLR2-IL-33 axis in innate immune activation for pain initiation and maintenance.

Dexmedetomidine relieved neuropathic pain and inflammation response induced by CCI through HMGB1/TLR4/NF-κB signal pathway

Neuropathic pain is one of the most intractable diseases. The lack of effective therapy measures remains a critical problem due to the poor understanding of the cause of neuropathic pain. The aim of this study was to investigate the effect of dexmedetomidine (Dex) in trigeminal neuropathic pain and the underlying molecular mechanism in order to identify possible therapeutic targets. We used a chronic constriction injury (CCI) model of mice to investigate whether Dex prevents neuropathic pain and the inflammation response. The α 2-adrenoceptors (α2AR) inhibitor BRL44408 and adenovirus for knocking down High mobility group box 1 (HMGB1) was administrated to confirm whether Dex exert its effect through targeting α2AR and HMGB1. The results indicated that Dex significantly inhibited CCI induced neuropathic pain through targeting α2AR and HMGB1. Dex inhibited the inflammatory response through decreasing the release and the mRNA expression of IL-1β, IL-6, and TNF-ɑ while increasing that of IL-10. Moreover, Dex participates in the regulation of HMGB1, Toll-like receptor 4 (TLR4), NFκb (p-65) expression and the phosphorylation of IκB-ɑ. In conclusion, Dex could relieve neuropathic pain through α2AR and HMGB1 and attenuate inflammation response.

CY-09 Inhibits NLRP3 Inflammasome Activation to Relieve Pain via TRPA1

Peripheral tissue damage leads to inflammatory pain, and inflammatory cytokine releasing is the key factor for inducing the sensitization of nociceptors. As a calcium ion channel, TRPA1 plays an important role in pain and inflammation, thus becoming a new type of anti-inflammatory and analgesic target. However, there is no consensus on the role of this channel in mechanical hyperalgesia caused by inflammation. Here, we aim to explore the role and underlying mechanism of the inflammasome inhibitor CY-09 in two classic inflammatory pain models. We evaluated pain behavior on animal models, cytokine levels, intracellular Ca2+ levels, transient TRPA1 expression, NF-κB transcription, and NLPR3 inflammasome activation. Consistently, CY-09 reduced the production of inflammatory cytokines, intracellular Ca2+ levels, and the activation of TRPA1 by inhibiting the activation of inflammasomes, thereby reducing the proinflammatory polarization of macrophages and alleviating animal pain and injury. Importantly, AITC (TRPA1 agonist) significantly reversed the analgesic effect of CY-09, indicating that TRPA1 was involved in the analgesic effect of CY-09. Our findings indicate that CY-09 relieves inflammation and pain via inhibiting TRPA1-mediated activation of NLRP3 inflammasomes. Thus, NLRP3 inflammasome may be a potential therapeutic target for pain treatment and CY-09 may be a pharmacological agent to relieve inflammatory pain, which needs further research.

Vincristine-induced peripheral neuropathy is driven by canonical NLRP3 activation and IL-1β release

Vincristine is an important component of many regimens used for pediatric and adult malignancies, but it causes a dose-limiting sensorimotor neuropathy for which there is no effective treatment. This study aimed to delineate the neuro-inflammatory mechanisms contributing to the development of mechanical allodynia and gait disturbances in a murine model of vincristine-induced neuropathy, as well as to identify novel treatment approaches. Here, we show that vincristine-induced peripheral neuropathy is driven by activation of the NLRP3 inflammasome and subsequent release of interleukin-1β from macrophages, with mechanical allodynia and gait disturbances significantly reduced in knockout mice lacking NLRP3 signaling pathway components, or after treatment with the NLRP3 inhibitor MCC950. Moreover, treatment with the IL-1 receptor antagonist anakinra prevented the development of vincristine-induced neuropathy without adversely affecting chemotherapy efficacy or tumor progression in patient-derived medulloblastoma xenograph models. These results detail the neuro-inflammatory mechanisms leading to vincristine-induced peripheral neuropathy and suggest that repurposing anakinra may be an effective co-treatment strategy to prevent vincristine-induced peripheral neuropathy.

The NLRP3-related inflammasome modulates pain behavior in a rat model of trigeminal neuropathic pain

AIMS

Nod-like receptor family pyrin domain containing 3 (NLRP3) may play an important role in neuropathic pain. Treatment for trigeminal neuropathic pain remains a challenge, as common drugs either do not demonstrate beneficial therapeutic effects or induce intolerance in patients.

MAIN METHODS

In a rat model of trigeminal neuropathic pain, pain caused by the malpositioning of dental implants is similar to that experienced by humans. We used masculine Sprague-Dawley rats with inferior alveolar nerve damage as a model to investigate the differential regulation of NLRP3. First, we confirmed the level of NLRP3 in the medullary dorsal horn and variation of pain response behavior after silencing the expression of NLRP3 inflammasome bodies in rats with trigeminal neuropathic pain. Second, under localized anesthesia, we extracted the lower left second molar, implanted a micro-dental implant, and deliberately injured the inferior alveolar nerve.

KEY FINDINGS

After nerve damage, the level of NLRP3-related inflammasomes was upregulated in microglia and the expression of a component of the inflammasome gradually increased during postoperative days 3-21. The suppression of adenovirus-shRNA-NLRP3 on postoperative day 1 markedly inhibited the expression of pro-inflammatory cytokines and the activation of the inflammasome and mechanical allodynia. Furthermore, it attenuated cell death in microglia, as evidenced by increased Bcl-2, Bcl-xL, Bax, and Bik expression.

SIGNIFICANCE

The level of NLRP3 in the dorsal horn is a pivotal factor in trigeminal neuropathic pain, and inhibition of the early expression of NLRP3 might serve as a potential therapeutic approach.

Palmitoylethanolamide and hemp oil extract exert synergistic anti-nociceptive effects in mouse models of acute and chronic pain

The use of products derived from hemp - i.e., cannabis varieties with low Δ⁹-tetrahydrocannabinol (Δ⁹-THC) content - as self-medication for pain and other health conditions is gaining in popularity but preclinical and clinical evidence for their effectiveness remains very limited. In the present study, we assessed the efficacy of a full-spectrum hemp oil extract (HOE; 10, 50 and 100 mg-kg^-1; oral route), alone or in combination with the anti-inflammatory and analgesic agent palmitoylethanolamide (PEA; 10, 30, 100 and 300 mg-kg^-1; oral route), in the formalin and chronic constriction injury (CCI) tests. We found that HOE exerts modest antinociceptive effects when administered alone, whereas the combination of sub-effective oral doses of HOE and PEA produces a substantial greater-than-additive alleviation of pain-related behaviors. Transcription of interleukin (IL)-6 and IL-10 increased significantly in lumbar spinal cord tissue on day 7 after CCI surgery, an effect that was attenuated to the same extent by HOE alone or by the HOE/PEA combination. Pharmacokinetic experiments show that co-administration of HOE enhances and prolongs systemic exposure to PEA. Collectively, our studies lend support to possible beneficial effects of using HOE in combination with PEA to treat acute and chronic pain.

Neuroinflammation, oxidative stress and their interplay in neuropathic pain: Focus on specialized pro-resolving mediators and NADPH oxidase inhibitors as potential therapeutic strategies

Neuropathic pain (NP) is a chronic condition that results from a lesion or disease of the nervous system, greatly impacting patients' quality of life. Current pharmacotherapy options deliver inadequate and/or insufficient responses and thus a significant unmet clinical need remains for alternative treatments in NP. Neuroinflammation, oxidative stress and their reciprocal relationship are critically involved in NP pathophysiology. In this context, new pharmacological approaches, aiming at enhancing the resolution phase of inflammation and/or restoring redox balance by targeting specific reactive oxygen species (ROS) sources, are emerging as potential therapeutic strategies for NP, with improved efficacy and safety profiles. Several reports have demonstrated that administration of exogenous specialized pro-resolving mediators (SPMs) ameliorates NP pathophysiology. Likewise, deletion or inhibition of the ROS-generating enzyme NADPH oxidase (NOX), particularly its isoforms 2 and 4, results in beneficial effects in NP models. Notably, SPMs also modulate oxidative stress and NOX also regulates neuroinflammation. By targeting neuroinflammatory and oxidative pathways, both SPMs analogues and isoform-specific NOX inhibitors are promising therapeutic strategies for NP.

The NLRP3 Inflammasome: Role and Therapeutic Potential in Pain Treatment

Pain is a fundamental feature of inflammation. The immune system plays a critical role in the activation of sensory neurons and there is increasing evidence of neuro-inflammatory mechanisms contributing to painful pathologies. The inflammasomes are signaling multiprotein complexes that are key components of the innate immune system. They are intimately involved in inflammatory responses and their activation leads to production of inflammatory cytokines that in turn can affect sensory neuron function. Accordingly, the contribution of inflammasome activation to pain signaling has attracted considerable attention in recent years. NLRP3 is the best characterized inflammasome and there is emerging evidence of its role in a variety of inflammatory pain conditions. In vitro and in vivo studies have reported the activation and upregulation of NLRP3 in painful conditions including gout and rheumatoid arthritis, while inhibition of NLRP3 function or expression can mediate analgesia. In this review, we discuss painful conditions in which NLRP3 inflammasome signaling has been pathophysiologically implicated, as well as NLRP3 inflammasome-mediated mechanisms and signaling pathways that may lead to the activation of sensory neurons.

NLRP2 inflammasome in dorsal root ganglion as a novel molecular platform that produces inflammatory pain hypersensitivity

Inflammatory pain hypersensitivity is associated with activation of primary afferent neurons. This study investigated the existence of the inflammasome in dorsal root ganglion (DRG) and the functional significance in the development of inflammatory pain hypersensitivity. Tissue inflammation was induced in male C57BL/6 mice with complete Freund's adjuvant (CFA) or ceramide injection into the hind paw. Behavioral testing was performed to investigate inflammation-induced pain hypersensitivity. Ipsilateral L5 DRGs were obtained for analysis. Expression of nucleotide oligomerization domain-like receptors (NLRs) was analyzed with real-time PCR. Cleaved interleukin (IL)-1β and NLRP2 expression was investigated with immunohistochemistry and western blotting. Caspase 1 activity was also measured. A caspase 1 inhibitor and NLRP2 siRNA were intrathecally administered to inhibit NLRP2 inflammasome signaling in DRG. Cleaved IL-1β expression was significantly increased after CFA injection in small-sized DRG neurons. The amount of cleaved IL-1β and caspase 1 activity were also increased. Among several NLRs, NLRP2 mRNA was significantly increased in DRG after CFA injection. NLRP2 was expressed in small-sized DRG neurons. Intrathecal injection of a caspase 1 inhibitor or NLRP2 siRNA reduced CFA-induced pain hypersensitivity and cleaved IL-1β expression in DRG. Induction of cleaved IL-1β and NLRP2 in DRG neurons was similarly observed after ceramide injection. NLRP2 siRNA inhibited ceramide-induced pain hypersensitivity. These results confirmed the existence of NLRP2 inflammasome in DRG neurons. Activation of the NLRP2 inflammasome leads to activation of DRG neurons and subsequent development of pain hypersensitivity in various types of tissue inflammation.

Dexmedetomidine attenuates P2X4 and NLRP3 expression in the spine of rats with diabetic neuropathic pain

Purpose:

To evaluate the effects of Dexmedetomidine (Dex) on spinal pathology and inflammatory factor in a rat model of Diabetic neuropathic pain (DNP).

Methods:

The rats were divided into 3 groups (eight in each group): normal group (N group), diabetic neuropathic pain model group (DNP group), and DNP model with dexmedetomidine (Dex group). The rat model of diabetes was established with intraperitoneal streptozotocin (STZ) injections. Nerve cell ultrastructure was evaluated with transmission electron microscopy (TEM). The mechanical withdrawal threshold (MWT) and motor nerve conduction velocity (MNCV) tests documented that DNP rat model was characterized by a decreased pain threshold and nerve conduction velocity.

Results:

Dex restored the phenotype of neurocytes, reduced the extent of demyelination and improved MWT and MNCV of DNP-treated rats (P=0.01, P=0.038, respectively). The expression of three pain-and inflammation-associated factors (P2X4, NLRP3, and IL-IP) was significantly upregulated at the protein level in DNP rats, and this change was reversed by Dex administration (P=0.0022, P=0.0092, P=0.0028, respectively).

Conclusion:

The P2X4/NLRP3 signaling pathway is implicated in the development and presence of DNP in vivo, and Dex protects from this disorder.

A Novel Sex-Dependent Target for the Treatment of Postoperative Pain: The NLRP3 Inflammasome

In recent years the innate immune system has been shown to be crucial for the pathogenesis of postoperative pain. The mediators released by innate immune cells drive the sensitization of sensory neurons following injury by directly acting on peripheral nerve terminals at the injury site. The predominate sensitization signaling pathway involves the proinflammatory cytokine interleukin-1β (IL-1β). IL-1β is known to cause pain by directly acting on sensory neurons. Evidence demonstrates that blockade of IL-1β signaling decreases postoperative pain, however complete blockade of IL-1β signaling increases the risk of infection and decreases effective wound healing. IL-1β requires activation by an inflammasome; inflammasomes are cytosolic receptors of the innate immune system. NOD-like receptor protein 3 (NLRP3) is the predominant inflammasome activated by endogenous molecules that are released by tissue injury such as that which occurs during neuropathic and inflammatory pain disorders. Given that selective inhibition of NLRP3 alleviates postoperative mechanical pain, its selective targeting may be a novel and effective strategy for the treatment of pain that would avoid complications of global IL-1β inhibition. Moreover, NLRP3 is activated in pain in a sex-dependent and cell type-dependent manner. Sex differences in the innate immune system have been shown to drive pain and sensitization through different mechanisms in inflammatory and neuropathic pain disorders, indicating that it is imperative that both sexes are studied when researchers investigate and identify new targets for pain therapeutics. This review will highlight the roles of the innate immune response, the NLRP3 inflammasome, and sex differences in neuropathic and inflammatory pain.

Fraxinellone inhibits inflammatory cell infiltration during acute pancreatitis by suppressing inflammasome activation

Inflammasomes promote the production of pro-inflammatory cytokines, such as interleukin (IL)-1β and IL-18, which are the representative mediators of inflammation. Abnormal activation of inflammasomes leads to the development of inflammatory diseases such as acute pancreatitis (AP). In this study, we demonstrate the inhibitory effects of a new natural compound fraxinellone on inflammasome formation and examine the role of inflammasomes in a mouse model of AP. AP was induced with hourly intraperitoneal injections of supramaximal concentrations of the stable cholecystokinin analogue cerulein (50 μg/kg) for 6 h. Mice were sacrificed 6 h after the final cerulein injection. Blood and pancreas samples were obtained for further experiments. Intraperitoneal injection of fraxinellone significantly inhibited the pancreatic activation of multiple inflammasome molecules such as NACHT, LRR and PYD domains-containing protein 3 (NLRP3), PY-CARD, caspase-1, IL-18, and IL-1β during AP. In addition, fraxinellone treatment inhibited pancreatic injury, elevation in serum amylase and lipase activities, and infiltration of inflammatory cells such as neutrophils and macrophages but had no effect on pancreatic edema. To investigate whether inflammasome activation leads to the infiltration of inflammatory cells, we used parthenolide, a well-known natural inhibitor, and IL-1 receptor antagonist mice. The inhibition of inflammasome activation by pharmacological/or genetic modification restricted the infiltration of inflammatory cells, but not edema, consistent with the results observed with fraxinellone. Taken together, our study highlights fraxinellone as a natural inhibitor of inflammasomes and that inflammasome inhibition may lead to the suppression of inflammatory cells during AP.

Tizanidine exerts anti-nociceptive effects in spared nerve injury model of neuropathic pain through inhibition of TLR4/NF-κB pathway

Recently, α2-adrenoceptors (α2-AR) agonists have been shown to have anti-nociceptive effects and thus may become a promising therapeutic strategy for neuropathic pain. tizanidine is a highly selective α2-AR agonist, but the effect mechanism of tizanidine in neuropathic pain remains largely unknown. The present study investigated whether tizanidine has anti-nociceptive effects in spared nerve injury (SNI) model of neuropathic pain in rats, as well as explored the underlying molecular mechanism. We found that the rats in SNI group showed significantly higher mechanical and thermal hyperalgesia, accompanied with increased production of proinflammatory cytokines including interleukin-1β (IL-1β), IL-6 and tumor necrosis factor-α (TNF-α), as well as the activation of Toll-like receptor 4 (TLR4)/nuclear factor-κB (NF-κB) signaling. PDTC, an inhibitor of TLR4/NF-κB signaling, could significantly attenuate the SNI-induced mechanical and thermal hyperalgesia and the production of proinflammatory cytokines. Moreover, treatment with tizanidine also attenuated the SNI-induced mechanical and thermal hyperalgesia, suppressed production of the proinflammatory cytokines, and inhibited the activation of TLR4/NF-κB pathway, which could be reversed by pretreatment with BRL44408, a selective α2-AR antagonist. Taken these findings together, we demonstrated that tizanidine has anti-nociceptive effects on neuropathic pain via inhibiting the production of proinflammatory cytokines through suppressing the activation of TLR4/NF-κB p65 signaling pathway.

The inflammasome as a target for pain therapy

The interleukin-1 family of cytokines are potent inducers of inflammation and pain. Proteolytic activation of this family of cytokines is under the control of several innate immune receptors that coordinate to form large multiprotein signalling platforms, termed inflammasomes. Recent evidence suggests that a wide range of inflammatory diseases, cancers, and metabolic and autoimmune disorders, in which pain is a common complaint, may be coordinated by inflammasomes. Activation of inflammasomes results in cleavage of caspase-1, which subsequently induces downstream initiation of several potent pro-inflammatory cascades. Therefore, it has been proposed that targeting inflammasome activity may be a novel and effective therapeutic strategy for these pain-related diseases. The purpose of this narrative review article is to provide the reader with an overview of the activation and regulation of inflammasomes and to investigate the potential therapeutic role of inflammasome inhibition in the treatment of diseases characterized by pain, including the following: complex regional pain syndrome, gout, rheumatoid arthritis, inflammatory pain, neuropathic pain, chronic prostatitis, chronic pelvic pain syndrome, and fibromyalgia. We conclude that the role of the inflammasome in pain-associated diseases is likely to be inflammasome subtype and disease specific. The currently available evidence suggests that disease-specific targeting of the assembly and activity of the inflammasome complex may be a novel therapeutic opportunity for the treatment of refractory pain in many settings.

NLRP3 inflammasome signaling as an early molecular response is negatively controlled by miR-186 in CFA-induced prosopalgia mice

The NOD-like receptor family pyrin domain containing 3 (NLRP3) inflammasome is the most frequently studied in the central nervous system and has been linked to neuropathic pain. In this study, a post-translational mechanism of microRNA (miR)-186 via regulating the expression of NLRP3 in the complete Freund's adjuvant (CFA)-treated mice was investigated. The injection of CFA was used to induce trigeminal neuropathic pain in mice. miRs microarray chip assay was performed in trigeminal ganglions (TGs). CFA treatment significantly increased the mRNA expression of NLRP3, interleukin (IL)-1β, and IL-18 in TGs compared to the control group. Moreover, 26 miRs were differentially expressed in TGs from trigeminal neuropathic pain mice, and the expression of miR-186 showed the lowest level of all the miRs. Further examination revealed that NLRP3 was a candidate target gene of miR-186. We delivered miR-186 mimics to CFA-treated mice. The head withdrawal thresholds of the CFA-treated mice were significantly increased by miR-186 mimics injection compared with CFA single treatment. The mRNA and protein expression of NLRP3, IL-1β, and IL-18 in TGs from trigeminal neuropathic pain mice were significantly inhibited by miR-186 mimics treatment compared to the CFA group. miR-186 was able to suppress the neuropathic pain via regulating the NLRP3 inflammasome signaling.

Microglial role in the development of chronic pain

PURPOSE OF REVIEW

The review aims to present the latest research into microglia and their role in pain.

RECENT FINDINGS

Microglia affect sex and age-dependent differences in pain. The various microglial phenotypes make their involvement in pain more complex but provide more potential as pain modulators.

SUMMARY

Glial cells, composed of microglia, astrocytes, and oligodendrocytes, outnumber neurons in the central nervous system. The crosstalk between these cells and neurons is now established as participating in the development of chronic pain. There has been a great advance in the description of microglia reactivity from pro to anti-inflammatory phenotypes. The modulation of these phenotypes could be a potential target for pain therapy. Recently, different microglial reactivity between man and woman and between neonates and adults, in response to nerve injury were described, which could explain some of the sex differences in pain sensitivity and the absence of neuropathic pain development in neonates. Clinical trials using microglia as a target have been carried out in various neurological diseases and pain, with limited efficacy in the latter, but there are nonetheless, indications that with some improvement in study strategies microglia could be a future target for pain control.

miRNA-23a/CXCR4 regulates neuropathic pain via directly targeting TXNIP/NLRP3 inflammasome axis

BACKGROUND

Chemokine CXC receptor 4 (CXCR4) in spinal glial cells has been implicated in neuropathic pain. However, the regulatory cascades of CXCR4 in neuropathic pain remain elusive. Here, we investigated the functional regulatory role of miRNAs in the pain process and its interplay with CXCR4 and its downstream signaling.

METHODS

miRNAs and CXCR4 and its downstream signaling molecules were measured in the spinal cords of mice with sciatic nerve injury via partial sciatic nerve ligation (pSNL). Immunoblotting, immunofluorescence, immunoprecipitation, and mammal two-hybrid and behavioral tests were used to explore the downstream CXCR4-dependent signaling pathway.

RESULTS

CXCR4 expression increased in spinal glial cells of mice with pSNL-induced neuropathic pain. Blocking CXCR4 alleviated the pain behavior; contrarily, overexpressing CXCR4 induced pain hypersensitivity. MicroRNA-23a-3p (miR-23a) directly bounds to 3' UTR of CXCR4 mRNA. pSNL-induced neuropathic pain significantly reduced mRNA expression of miR-23a. Overexpression of miR-23a by intrathecal injection of miR-23a mimics or lentivirus reduced spinal CXCR4 and prevented pSNL-induced neuropathic pain. In contrast, knockdown of miR-23a by intrathecal injection of miR-23a inhibitor or lentivirus induced pain-like behavior, which was reduced by CXCR4 inhibition. Additionally, miR-23a knockdown or CXCR4 overexpression in naïve mice could increase the thioredoxin-interacting protein (TXNIP), which was associated with induction of NOD-like receptor protein 3 (NLRP3) inflammasome. Indeed, CXCR4 and TXNIP were co-expressed. The mammal two-hybrid assay revealed the direct interaction between CXCR4 and TXNIP, which was increased in the spinal cord of pSNL mice. In particular, inhibition of TXNIP reversed pain behavior elicited by pSNL, miR-23a knockdown, or CXCR4 overexpression. Moreover, miR-23a overexpression or CXCR4 knockdown inhibited the increase of TXNIP and NLRP3 inflammasome in pSNL mice.

CONCLUSIONS

miR-23a, by directly targeting CXCR4, regulates neuropathic pain via TXNIP/NLRP3 inflammasome axis in spinal glial cells. Epigenetic interventions against miR-23a, CXCR4, or TXNIP may potentially serve as novel therapeutic avenues in treating peripheral nerve injury-induced nociceptive hypersensitivity.

Inflammasomes in neurological diseases: emerging pathogenic and therapeutic concepts

Inflammasome activation in the central nervous system occurs in both health and disease. Inflammasomes are cytosolic protein complexes that sense specific infectious or host stimuli and initiate inflammatory responses through caspase activation. Assembly of inflammasomes results in caspase-1-mediated proteolytic cleavage and release of the pro-inflammatory cytokines, interleukin-1β and interleukin-18, with initiation of pyroptosis, an inflammatory programmed cell death. Recent developments in the inflammasome field have uncovered novel molecular mechanisms that contribute to a broad range of neurological disorders including those associated with specific mutations in inflammasome genes as well as diseases modulated by inflammasome activation. This update focuses on recent developments in the field of inflammasome biology highlighting different inflammasome activators and pathways discovered in the nervous system. We also discuss targeted therapies that regulate inflammasomes and improve neurological outcomes.

Attenuation of mechanical pain hypersensitivity by treatment with Peptide5, a connexin-43 mimetic peptide, involves inhibition of NLRP3 inflammasome in nerve-injured mice

Connexin43 (Cx43) hemichannels in spinal cord astrocytes are implicated in the maintenance of neuropathic pain following peripheral nerve injury. Peptide5 is a Cx43 mimetic peptide that blocks hemichannels. In this study, we investigated the effects of spinal delivery of Peptide5 on mechanical pain hypersensitivity in two mouse models of neuropathic pain, peripheral nerve injury and chemotherapy-induced peripheral neuropathy (CIPN). We demonstrated that 10days following a chronic constriction injury (CCI) of the sciatic nerve, Cx43 expression, co-localised predominantly with astrocytes, was increased in the ipsilateral L3-L5 lumbar spinal cord. An intrathecal injection of Peptide5 into nerve-injured mice, on day 10 when pain was well-established, caused significant improvement in mechanical pain hypersensitivity 8h after injection. Peptide5 treatment resulted in significantly reduced Cx43, and microglial and astrocyte activity in the dorsal horn of the spinal cord, as compared to control saline-treated CCI mice. Further in vitro investigations on primary astrocyte cultures showed that 1h pre-treatment with Peptide5 significantly reduced adenosine triphosphate (ATP) release in response to extracellular calcium depletion. Since ATP is a known activator of the NOD-like receptor protein 3 (NLRP3) inflammasome complex, a key mediator of neuroinflammation, we examined the effects of Peptide5 treatment on NLRP3 inflammasome expression. We found that NLRP3, its adaptor apoptosis-associated spec-like protein (ASC) and caspase-1 protein were increased in the ipsilateral spinal cord of CCI mice and reduced to naïve levels following Peptide5 treatment. In the models of oxaliplatin- and paclitaxel-induced peripheral neuropathy, treatment with Peptide5 had no effect on mechanical pain hypersensitivity. Interestingly, in these CIPN models, although spinal Cx43 expression was significantly increased at day 13 following chemotherapy, NLRP3 expression was not altered. These results suggest that the analgesic effect of Peptide5 is specifically achieved by reducing NLRP3 expression. Together, our findings demonstrate that blocking Cx43 hemichannels with Peptide5 after nerve injury attenuates mechanical pain hypersensitivity by specifically targeting the NLRP3 inflammasome in the spinal cord.

Blockade of P2X4 Receptors Inhibits Neuropathic Pain-Related Behavior by Preventing MMP-9 Activation and, Consequently, Pronociceptive Interleukin Release in a Rat Model

Neuropathic pain is still an extremely important problem in today's medicine because opioids, which are commonly used to reduce pain, have limited efficacy in this type of pathology. Therefore, complementary therapy is needed. Our experiments were performed in rats to evaluate the contribution of the purinergic system, especially P2X4 receptor (P2X4R), in the modulation of glia activation and, consequently, the levels of nociceptive interleukins after chronic constriction injury (CCI) of the right sciatic nerve, a rat model of neuropathic pain. Moreover, we studied how intrathecal (ith.) injection of a P2X4R antagonist Tricarbonyldichlororuthenium (II) dimer (CORM-2) modulates nociceptive transmission and opioid effectiveness in the CCI model. Our results demonstrate that repeated ith. administration of CORM-2 once daily (20 μg/5 μl, 16 and 1 h before CCI and then daily) for eight consecutive days significantly reduced pain-related behavior and activation of both spinal microglia and/or astroglia induced by CCI. Moreover, even a single administration of CORM-2 on day 7 after CCI attenuated mechanical and thermal hypersensitivity as efficiently as morphine and buprenorphine. In addition, using Western blot, we have shown that repeated ith. administration of CORM-2 lowers the CCI-elevated level of MMP-9 and pronociceptive interleukins (IL-1β, IL-18, IL-6) in the dorsal L4-L6 spinal cord and/or DRG. Furthermore, in parallel, CORM-2 upregulates spinal IL-1Ra; however, it does not influence other antinociceptive factors, IL-10 and IL-18BP. Additionally, based on our biochemical results, we hypothesize that p38MAPK, ERK1/2 and PI3K/Akt but not the NLRP3/Caspase-1 pathway are partly involved in the CORM-2 analgesic effects in rat neuropathic pain. Our data provide new evidence that P2X4R may indeed play a significant role in neuropathic pain development by modulating neuroimmune interactions in the spinal cord and DRG, suggesting that its blockade may have potential therapeutic utility.

Unconventional Role of Caspase-6 in Spinal Microglia Activation and Chronic Pain

Chronic pain affects ~20% of the worldwide population. The clinical management of chronic pain is mostly palliative and results in limited success. Current treatments mostly target the symptoms or neuronal signaling of chronic pain. It has been increasingly recognized that glial cells, such as microglia, and inflammatory signaling play a major role in the pathogenesis of chronic pain. Caspases (CASPs) are a family of protease enzymes involved in apoptosis and inflammation. They are pivotal components in a variety of neurological diseases. However, little is known about the role of CASPs in microglial modulation as to chronic pain. In particular, our recent studies have shown that CASP6 regulates chronic pain via microglial inflammatory signaling. Inhibition of microglia and CASP signaling might provide a new strategy for the prevention and treatment of chronic pain.

Microglial polarization dynamics in dorsal spinal cord in the early stages following chronic sciatic nerve damage

Peripheral nerve injury can lead to activation of spinal microglia, which can mediate neuroinflammation and contribute to neuropathic pain following nerve injury. Activated microglia may manifest with either pro-inflammatory M1 phenotype or anti-inflammatory M2 phenotype, which may lead to detrimental or beneficial roles in the nervous system. In this study, microglia numbers, morphology and gene profiles were examined in the dorsal spinal cord of rats over 14 days following sciatic nerve chronic constriction injury (CCI). The morphology of some microglia changed from a surveying to an activated state within 1 day of CCI. Neuropathic pain developed within seven to 14 days following injury and microglia numbers were increased, with almost all in the dorsal spinal cord morphologically defined as activated. At day one after CCI, both M1 and M2 microglia-related genes were increased but only M1 microglia-related genes remained elevated at day seven and 14 thereafter. These results indicate that both M1 and M2 microglia were activated in the dorsal spinal cord one day after CCI but the microglia skewed towards M1 phenotype during the following seven and 14 days.