Dexmedetomidine alleviates inflammation in neuropathic pain by suppressing NLRP3 via Nrf2 activation

RTA-408 Regulates p-NF-κB/TSLP/STAT5 Signaling to Ameliorate Nociceptive Hypersensitivity in Chronic Constriction Injury Rats

Neuropathic pain following nerve injury is a complex condition, which often puts a negative impact on life and remains a sustained problem. To make pain management better is of great significance and unmet need. RTA 408 (Omaveloxone) is a traditional Asian medicine with a valid anti-inflammatory property. Thus, we aim to investigate the therapeutic effect of RTA-408 on mechanical allodynia in chronic constriction injury (CCI) rats as well as the underlying mechanisms. Neuropathic pain was induced by using CCI of the rats' sciatic nerve (SN) and the behavior testing was measured by calibrated forceps testing. Activation of Nrf-2, the phosphorylation of nuclear factor-κB (NF-κB), and the inflammatory response were assessed by western blots. The number of apoptotic neurons and degree of glial cell reaction were examined by immunofluorescence assay. RTA-408 exerts an analgesic effect on CCI rats. RTA-408 reduces neuronal apoptosis and glial cell activation by increasing Nrf-2 expression and decreasing the inflammatory response (TNF-α/ p-NF-κB/ TSLP/ STAT5). These data suggest that RTA-408 is a candidate with potential to reduce nociceptive hypersensitivity after CCI by targeting TSLP/STAT5 signaling.

Prenatal alcohol exposure promotes NLRP3 inflammasome-dependent immune actions following morphine treatment and paradoxically prolongs nerve injury-induced pathological pain in female mice

BACKGROUND

Neuroimmune dysregulation from prenatal alcohol exposure (PAE) may contribute to neurological deficits associated with fetal alcohol spectrum disorders (FASD). PAE is a risk factor for developing peripheral immune and spinal glial sensitization and release of the proinflammatory cytokine IL-1β, which lead to neuropathic pain (allodynia) from minor nerve injury. Although morphine acts on μ-opioid receptors, it also activates immune receptors, TLR4, and the NLRP3 inflammasome that induces IL-1β. We hypothesized that PAE induces NLRP3 sensitization by morphine following nerve injury in adult mice.

METHODS

We used an established moderate PAE paradigm, in which adult PAE and non-PAE control female mice were exposed to a minor sciatic nerve injury, and subsequent allodynia was measured using the von Frey fiber test. In control mice with standard sciatic damage or PAE mice with minor sciatic damage, the effects of the NLRP3 inhibitor, MCC950, were examined during chronic allodynia. Additionally, minor nerve-injured mice were treated with morphine, with or without MCC950. In vitro studies examined the TLR4-NLRP3-dependent proinflammatory response of peripheral macrophages to morphine and/or lipopolysaccharide, with or without MCC950.

RESULTS

Mice with standard sciatic damage or PAE mice with minor sciatic damage developed robust allodynia. Blocking NLRP3 activation fully reversed allodynia in both control and PAE mice. Morphine paradoxically prolonged allodynia in PAE mice, while control mice with minor nerve injury remained stably non-allodynic. Allodynia resolved sooner in nerve-injured PAE mice without morphine treatment than in morphine-treated mice. MCC950 treatment significantly shortened allodynia in morphine-treated PAE mice. Morphine potentiated IL-1β release from TLR4-activated PAE immune cells, while MCC950 treatment greatly reduced it.

CONCLUSIONS

In female mice, PAE prolongs allodynia following morphine treatment through NLRP3 activation. TLR4-activated PAE immune cells showed enhanced IL-1β release with morphine via NLRP3 actions. Similar studies are needed to examine the adverse impact of morphine in males with PAE. These results are predictive of adverse responses to opioid pain therapeutics in individuals with FASD.

Dexmedetomidine relieves inflammatory pain by enhancing GABAergic synaptic activity in pyramidal neurons of the anterior cingulate cortex

Pyramidal neuron (Pyn) hyperactivity in the anterior cingulate cortex (ACC) is involved in the modulation of pain. Previous studies indicate that the activation of α2 adrenoceptors (α2-ARs) by dexmedetomidine (DEX) is a safe and effective means of alleviating multiple types of pain. Here, we showed that systemically administered DEX can ameliorate the inflammatory pain induced by hindpaw injection of formalin (FA) and further examined the molecular and synaptic mechanisms of this DEX-elicited antinociceptive effect. We found that FA caused an increase in c-Fos expression in contralateral layer 2/3 (L2/3) ACC, and that intra-ACC infusion of DEX could also relieve phase 2 inflammatory pain behavior. DEX elicited an increase in the amplitude and frequency of miniature inhibitory post-synaptic currents (mIPSCs) and evoked IPSC amplitude, as well as a reduction in the hyperexcitability and both paired-pulse and excitation/inhibition ratios in contralateral L2/3 ACC Pyns of FA mice. These electrophysiological effects were associated with the upregulation of GABA A receptor (GABAAR) subunits. The interaction of phosphorylated Akt (p-Akt) with GABAAR subunits increased in the ACC following administration of DEX. These results suggest that DEX treatment reduces hyperactivity and enhances GABAergic inhibitory synaptic transmission in ACC Pyns, which produces analgesic effects by increasing GABAAR levels and activating the Akt signaling pathway.

The Inhibition of Neuropathic Pain Incited by Nerve Injury and Accompanying Mood Disorders by New Heme Oxygenase-1 Inducers: Mechanisms Implicated

Neuropathic pain is a type of pain that persists for a long time and becomes pathological. Additionally, the anxiodepressive disorders derived from neuropathic pain are difficult to palliate with the current treatments and need to be resolved. Then, using male mice with neuropathic pain provoked by chronic constriction of the sciatic nerve (CCI), we analyzed and compared the analgesic actions produced by three new heme oxygenase 1 (HO-1) inducers, 1m, 1b, and 1a, with those performed by dimethyl fumarate (DMF). Their impact on the anxiety- and depressive-like comportments and the expression of the inflammasome NLRP3, Nrf2, and some antioxidant enzymes in the dorsal root ganglia (DRG) and amygdala (AMG) were also investigated. Results revealed that the administration of 1m, 1b, and DMF given orally for four days inhibited the allodynia and hyperalgesia caused by CCI, while 1a merely reduced the mechanical allodynia. However, in the first two days of treatment, the antiallodynic effects produced by 1m were higher than those of 1a and DMF, and its antihyperalgesic actions were greater than those produced by 1b, 1a, and DMF, revealing that 1m was the most effective compound. At four days of treatment, all drugs exerted anxiolytic and antidepressant effects, decreased the NLRP3 levels, and increased/normalized the Nrf2, HO-1, and superoxide dismutase 1 levels in DRG and AMG. Data indicated that the dual modulation of the antioxidant and inflammatory pathways produced by these compounds, especially 1m, is a new promising therapeutic approach for neuropathic pain and related emotional illnesses.

Hydrogen-rich saline regulates NLRP3 inflammasome activation in sepsis-associated encephalopathy rat model

Sepsis-associated encephalopathy (SAE) is characterised by long-term cognitive impairment and psychiatric illness in sepsis survivors, associated with increased morbidity and mortality. There is a lack of effective therapeutics for SAE. Molecular hydrogen (H2) plays multiple roles in septic diseases by regulating neuroinflammation, reducing oxidative stress parameters, regulating signalling pathways, improving mitochondrial dysfunction, and regulating astrocyte and microglia activation. Here we report the protective effect of hydrogen-rich saline in the juvenile SAE rat model and its possible underlying mechanisms. Rats were injected intraperitoneally with lipopolysaccharide at a dose of 5 mg/kg to induce sepsis; Hydrogen-rich saline (HRS) was administered 1 h after LPS induction at a dose of 5 ml/kg and nigericin at 1 mg/kg 1 h before LPS injection. H&E staining for neuronal damage, TUNEL assay for detection of apoptotic cells, immunofluorescence, ELISA protocol for inflammatory cytokines and 8-OHdG determination and western blot analysis to determine the effect of HRS in LPS-induced septic rats. Rats treated with HRS showed decreased TNF-α and IL-1β expression levels. HRS treatment enhanced the activities of antioxidant enzymes (SOD, CAT and GPX) and decreased MDA and MPO activities. The number of MMP-9 and NLRP3 positive immunoreactivity cells decreased in the HRS-treated group. Subsequently, GFAP, IBA-1 and CD86 immunoreactivity were reduced, and CD206 increased after HRS treatment. 8-OHdG expression was decreased in the HRS-treated rats. Western blot analysis showed decreased NLRP3, ASC, caspase-1, MMP-2/9, TLR4 and Bax protein levels after HRS treatment, while Bcl-2 expression increased after HRS treatment. These data demonstrated that HRS attenuated neuroinflammation, NLRP3 inflammasome activation, neuronal injury, and mitochondrial damage via NLRP3/Caspase-1/TLR4 signalling in the juvenile rat model, making it a potential therapeutic agent in the treatment of paediatric SAE.

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.

P66shc in the spinal cord is an important contributor in complete Freund's adjuvant induced inflammatory pain in mice

PURPOSE

The aim of this study is to investigate whether p66shc is involved in inflammatory pain and the potential molecular mechanisms of p66shc in inflammatory pain.

METHODS

Inflammatory pain model was established by complete Freund's adjuvant (CFA) injection. Paw withdrawal latency (PWL) and paw withdrawal frequency (PWF) was recorded. The expression of spinal p66shc were determined by immunohistochemical staining, immunofluorescence staining. P66shc knockdown was performed by an adeno-associated virus (AAV) vector infusion. NLRP3 inflammasome complexes were determined by Western blot. DHE staining was used to evaluate reactive oxygen species (ROS) generation.

RESULTS

P66Shc expression was progressively elevated in spinal cord of inflammatory pain mice, and p66Shc knockdown in vivo significantly attenuated CFA injection triggers hyperalgesia. Furthermore, knockdown of p66Shc significantly inhibited ROS production and NOD-like receptor protein 3 (NLRP3) inflammasome activation, which were reversed by a ROS donor (t-BOOH). However, post-treatment with nigericin, a agonist of NLRP3, reversed AAV-shP66shc analgesic effect.

CONCLUSION

Spinal p66shc may facilitate the development of inflammatory pain by promoting the activation of NLRP3 inflammasome through ROS.

Elabela-APJ axis attenuates cerebral ischemia/reperfusion injury by inhibiting neuronal ferroptosis

Ferroptosis is a form of non-apoptotic cell death caused by iron-dependent peroxidation of lipids. It contributes to ischemic stroke-induced neuronal damage. Elabela (ELA), a novel endogenous ligand for Apelin receptor (APJ), regulates oxidative stress and exerts a protective role in cardiovascular disease. However, the effect of ELA-APJ axis on cellular ferroptosis in cerebral ischemia/reperfusion (I/R) remains elusive. The present study showed that ELA and APJ were expressed on neurons and increased after cerebral I/R injury. The I/R insult triggered typical molecular and morphological features of neuronal ferroptosis, including iron and MDA accumulation, mitochondrial shrink and membrane rupture, upregulation of positive ferroptosis regulators and downregulation of negative regulators. ELA-32 treatment reduced brain infarction and ameliorated neurobehavioral deficits and cognitive dysfunction. Moreover, ELA-32 administration alleviated neuronal ferroptosis, accompanied by reduced iron deposition, decreased mitochondrial damage, relived lipid peroxidation and glutathione reduction. Such effects of ELA-32 were abolished by AAV-APJ-RNAi or nuclear factor erythroid 2-related factor 2 (NRF2) inhibitor ML385. Mechanistically, ELA was shown to bind to APJ and activate NRF2/ARE anti-oxidative signaling pathway via Gα13. Together, these findings suggested that ELA-APJ axis mitigates neuronal ferroptosis after ischemic stroke and that the ELA-32 peptide may be a putative therapeutic avenue for ischemic stroke.

Appraisal of the Neuroprotective Effect of Dexmedetomidine: A Meta-Analysis

Dexmedetomidine is an adrenergic receptor agonist that has been regarded as neuroprotective in several studies without an objective measure to it. Thus, the aim of this meta-analysis was to analyze and quantify the current evidence for the neuroprotective effects of dexmedetomidine in animals. The search was performed by querying the National Library of Medicine. Studies were included based on their language, significancy of their results, and complete availability of data on animal characteristics and interventions. Risk of bias was assessed using SYRCLE's risk of bias tool and certainty was assessed using the ARRIVE Guidelines 2.0. Synthesis was performed by calculating pooled standardized mean difference and presented in forest plots and tables. The number of eligible records included per outcome is the following: 22 for IL-1β, 13 for IL-6, 19 for apoptosis, 7 for oxidative stress, 7 for Escape Latency, and 4 for Platform Crossings. At the cellular level, dexmedetomidine was found protective against production of IL-1β (standardized mean difference (SMD) =  - 4.3 [- 4.8; - 3.7]) and IL-6 (SMD =  - 5.6 [- 6.7; - 4.6]), apoptosis (measured through TUNEL, SMD =  - 6.0 [- 6.8; - 4.6]), and oxidative stress (measured as MDA production, SMD =  - 2.0 [- 2.4; - 1.4]) exclusively in the central nervous system. At the organism level, dexmedetomidine improved behavioral outcomes measuring escape latency (SMD = - 2.4 [- 3.3; - 1.6]) and number of platform crossings (SMD = 9.1 [- 6.8; - 11.5]). No eligible study had high risk of bias and certainty was satisfactory for reproducibility in all cases. This meta-analysis highlights the complexity of adrenergic stimulation and sheds light into the mechanisms potentiated by dexmedetomidine, which could be exploited for improving current neuroprotective formulations.

The neuroprotective effect of dexmedetomidine and its mechanism

Dexmedetomidine (DEX) is a highly selective α2 receptor agonist that is routinely used in the clinic for sedation and anesthesia. Recently, an increasing number of studies have shown that DEX has a protective effect against brain injury caused by traumatic brain injury (TBI), subarachnoid hemorrhage (SAH), cerebral ischemia and ischemia–reperfusion (I/R), suggesting its potential as a neuroprotective agent. Here, we summarized the neuroprotective effects of DEX in several models of neurological damage and examined its mechanism based on the current literature. Ultimately, we found that the neuroprotective effect of DEX mainly involved inhibition of inflammatory reactions, reduction of apoptosis and autophagy, and protection of the blood–brain barrier and enhancement of stable cell structures in five way. Therefore, DEX can provide a crucial advantage in neurological recovery for patients with brain injury. The purpose of this study was to further clarify the neuroprotective mechanisms of DEX therefore suggesting its potential in the clinical management of the neurological injuries.

Targeting NLRP3 Inflammasome With Nrf2 Inducers in Central Nervous System Disorders

The NLRP3 inflammasome is an intracellular multiprotein complex that plays an essential role in the innate immune system by identifying and eliminating a plethora of endogenous and exogenous threats to the host. Upon activation of the NLRP3 complex, pro-inflammatory cytokines are processed and released. Furthermore, activation of the NLRP3 inflammasome complex can induce pyroptotic cell death, thereby propagating the inflammatory response. The aberrant activity and detrimental effects of NLRP3 inflammasome activation have been associated with cardiovascular, neurodegenerative, metabolic, and inflammatory diseases. Therefore, clinical strategies targeting the inhibition of the self-propelled NLRP3 inflammasome activation are required. The transcription factor Nrf2 regulates cellular stress response, controlling the redox equilibrium, metabolic programming, and inflammation. The Nrf2 pathway participates in anti-oxidative, cytoprotective, and anti-inflammatory activities. This prominent regulator, through pharmacologic activation, could provide a therapeutic strategy for the diseases to the etiology and pathogenesis of which NLRP3 inflammasome contributes. In this review, current knowledge on NLRP3 inflammasome activation and Nrf2 pathways is presented; the relationship between NLRP3 inflammasome signaling and Nrf2 pathway, as well as the pre/clinical use of Nrf2 activators against NLRP3 inflammasome activation in disorders of the central nervous system, are thoroughly described. Cumulative evidence points out therapeutic use of Nrf2 activators against NLRP3 inflammasome activation or diseases that NLRP3 inflammasome contributes to would be advantageous to prevent inflammatory conditions; however, the side effects of these molecules should be kept in mind before applying them to clinical practice.

The Effects of Nuclear Factor Erythroid 2 (NFE2)-Related Factor 2 (Nrf2) Activation in Preclinical Models of Peripheral Neuropathic Pain

Oxidative stress, resulting from an imbalance between the formation of damaging free radicals and availability of protective antioxidants, can contribute to peripheral neuropathic pain conditions. Reactive oxygen and nitrogen species, as well as products of the mitochondrial metabolism such as superoxide anions, hydrogen peroxide, and hydroxyl radicals, are common free radicals. Nuclear factor erythroid 2 (NFE2)-related factor 2 (Nrf2) is a transcription factor encoded by the NFE2L2 gene and is a member of the cap 'n' collar subfamily of basic region leucine zipper transcription factors. Under normal physiological conditions, Nrf2 remains bound to Kelch-like ECH-associated protein 1 in the cytoplasm that ultimately leads to proteasomal degradation. During peripheral neuropathy, Nrf2 can translocate to the nucleus, where it heterodimerizes with muscle aponeurosis fibromatosis proteins and binds to antioxidant response elements (AREs). It is becoming increasingly clear that the Nrf2 interaction with ARE leads to the transcription of several antioxidative enzymes that can ameliorate neuropathy and neuropathic pain in rodent models. Current evidence indicates that the antinociceptive effects of Nrf2 occur via reducing oxidative stress, neuroinflammation, and mitochondrial dysfunction. Here, we will summarize the preclinical evidence supporting the role of Nrf2 signaling pathways and Nrf2 inducers in alleviating peripheral neuropathic pain.

FeTMPyP a peroxynitrite decomposition catalyst ameliorated functional and behavioral deficits in chronic constriction injury induced neuropathic pain in rats

Neuropathic pain is a maladaptive pain phenotype that results from injury or damage to the somatosensory nervous system and is proposed to be linked to a cascade of events including excitotoxicity, oxidative stress, mitochondrial dysfunction, neuroinflammation and apoptosis. Oxidative/nitrosative stress is a critical link between neuroinflammation and neurodegeneration through poly (ADP) ribose polymerase (PARP) overactivation. Hence, the present study investigated the antioxidant and anti-inflammatory effects of peroxynitrite decomposition catalyst; FeTMPyP in chronic constriction injury (CCI) of sciatic nerve-induced neuropathy in rats. CCI of the sciatic nerve manifested significant deficits in behavioral, biochemical, functional parameters and was markedly reversed by administration of FeTMPyP. After 14 days of CCI induction, oxidative/nitrosative stress and inflammatory markers such as iNOS, NF-kB, TNF-α and IL-6 were elevated in sciatic nerves of CCI rats along with depleted levels of ATP and elevated levels of poly (ADP) ribose (PAR) in both sciatic nerves in ipsilateral (L4-L5) dorsal root ganglions (DRG's), suggesting over activation of PARP. Additionally, CCI resulted in aberrations in mitochondrial function as evident by decreased Mn-SOD levels and respiratory complex activities with increased mitochondrial fission protein DRP-1. These changes were reversed by treatment with FeTMPyP (1 & 3 mg/kg, p.o.). Findings of this study suggest that FeTMPyP, by virtue of its antioxidant properties, reduced both PARP over-activation and subsequent neuroinflammation resulted in protection against CCI-induced functional, behavioral and biochemical deficits.