Neuropathic pain: From actual pharmacological treatments to new therapeutic horizons

N-Demethylsinomenine Relieves Neuropathic Pain in Male Mice Mainly via Regulating α2-Subtype GABAA Receptors

AIMS

N-Demethylsinomenine (NDSM) demonstrates good analgesic efficacy in preclinical pain models. However, how NDSM exerts analgesic actions remains unknown.

METHODS

We examined the analgesic effects of NDSM using both pain-evoked and pain-suppressed behavioral assays in two persistent pain models. Then western blot assay and immunofluorescence staining were used to investigate the effects of NDSM on the expression of the GABAA receptor α2 subunit (GABRA2) and inflammatory factors in the spinal cord and brain tissues of male spared nerve injury (SNI) mice. Finally, the individual subtypes of GABAARs (α1, α2, α3, and α5) were respectively silenced by viral-mediated knockdown to explore the involvement of subtypes of GABAARs in the effects of NDSM on the pain-like behaviors in male SNI mice.

RESULTS

NDSM demonstrated significant analgesic effects against chronic pain both in pain-evoked and pain-suppressed behavioral assays. NDSM treatment significantly reversed the SNI induced down-regulation of GABRA2 and up-regulation of TNF-α and IL-1β. The analgesic effects of NDSM were completely blocked by silencing GABRA2 or partially blocked by silencing GABRA3.

CONCLUSION

This study provided the first evidence that the analgesic effects of NDSM are mediated primarily by GABRA2 and partially by GABRA3, and the inhibition of neuroinflammation also contributes to the analgesic effects of NDSM.

Nerve injury inhibits Oprd1 and Cnr1 transcription through REST in primary sensory neurons

The transcription repressor REST in the dorsal root ganglion (DRG) is upregulated by peripheral nerve injury and promotes the development of chronic pain. However, the genes targeted by REST in neuropathic pain development remain unclear. The expression levels of four opioid receptor genes (Oprm1, Oprd1, Oprl1 and Oprk1) and the cannabinoid CB1 receptor (Cnr1) gene in the DRG regulate nociception. In this study, we determined the role of REST in controlling their expression in the DRG induced by spared nerve injury (SNI). SNI induced chronic pain hypersensitivity in wild-type mice and was accompanied by increased levels of Rest transcript and protein. Transcriptomic analyses of wild-type mouse DRGs suggested that SNI upregulates the expression of Rest transcripts and downregulates the transcripts of all four opioid receptor genes and the Cnr1 gene. Quantitative reverse transcription polymerase chain reaction analyses of these tissues validated these results. Analysis of publicly available bioinformatic data suggested that REST binds to the promoter regions of Oprm1 and Cnr1. Chromatin immunoprecipitation analyses indicated the presence of REST at these promoters. Full-length Rest conditional knockout in primary sensory neurons reduced SNI-induced pain hypersensitivity and rescued the SNI-induced reduction in the expression of Oprd1 and Cnr1 in mouse DRG. Our results suggest that nerve injury represses the transcription of at least the Oprd1 and Cnr1 genes via REST in primary sensory neurons and that REST is a potential therapeutic target for neuropathic pain. Thus, inhibiting REST activity could potentially reduce chronic neuropathic pain and augment opioid/cannabinoid analgesic actions by increasing the transcription of Oprd1 and Cnr1 genes in DRG neurons.

CTRP9 attenuates peripheral nerve injury-induced mechanical allodynia and thermal hyperalgesia through regulating spinal microglial polarization and neuroinflammation mediated by AdipoR1 in male mice

Peripheral nerve injury triggers rapid microglial activation, promoting M1 polarization within the spinal cord, which exacerbates the progression of neuropathic pain. C1q/TNF-related protein 9 (CTRP9), an adiponectin homolog, is known to suppress macrophage activation and exhibit anti-inflammatory properties through the activation of adiponectin receptor 1 (AdipoR1) in various disease contexts. Nevertheless, the involvement of CTRP9 in microglial polarization in the context of neuropathic pain is still unclear. Our study aimed to how CTRP9 influences spinal microglial polarization, neuroinflammation, and pain hypersensitivity, as well as the underlying mechanism, using a neuropathic pain model in male mice with spared nerve injury (SNI) of sciatic nerve. Our findings revealed SNI elevated the spinal CTRP9 and AdipoR1 levels in microglia. Furthermore, intrathecal administration of recombinant CTRP9 (rCTRP9) substantially weakened mechanical hypersensitivity and heat-related pain response triggered by SNI. On the other hand, rCTRP9 mediated a phenotypic switch in microglia, from the pro-inflammatory M1 state to the anti-inflammatory M2 state, by influencing the spinal AMPK/NF-κB mechanism in SNI mice. Additionally, treatment with AdipoR1 siRNA or an AMPK-specific antagonist both reversed the effects of CTRP9 on the phenotypic switching of spinal microglia and pain hypersensitivity. Collectively, these results indicate that CTRP9 ameliorates mechanical hypersensitivity and heat-related pain response, shifted the balance of microglia towards the anti-inflammatory M2 state, and suppresses neuroinflammatory responses by modulating the AMPK/NF-κB pathway, mediated by AdipoR1 activation, in mice with SNI.

A historical perspective and recent advances on the evolution of the relationship between acute and chronic pain and cardiovascular disease

The relationship between acute pain and the cardiovascular system was recognized approximately 50 years ago following the initial observation, along with several subsequent experimental studies, that hypertension can result in decreases in the perception of pain. These studies provided a strong impetus to study potential mechanisms to clarify commonalities between the regulatory pathways associated with pain and the cardiovascular system. Attention subsequently shifted to an emphasis on the impact of chronic pain on cardiovascular diseases and mortality with several large meta-analyses of longitudinal studies providing clear evidence that chronic widespread pain increases the risk for developing cardiovascular disease and is associated with excess morbidity and mortality. Cardiovascular associated mortality from myocardial infarction and stroke appears to be directly related to the duration and severity of chronic pain, a result often characterized as a 'dose-response' relationship. The availability and reproducibility of extensive large-scale observational and retrospective studies have emphasized the critical need for more research, including prospective studies, along with the need for the development of preclinical animal models, to better understand the relationship(s) and underlying mechanisms between chronic pain, associated comorbidities, and cardiovascular disease. Elucidation and a deeper understanding of these relationships, including a focus on the link between chronic pain, cardiovascular disease, and depression, could provide valuable information to guide the development of potential treatment interventions to aid in attenuating pain while preventing pain-associated cardiovascular disease, comorbidities, and mortality.

Urinary kallikrein reverses neuropathic pain by inhibiting ectopic neural discharges, neural inflammation and oxidative stress

Background

Neuropathic pain is a refractory disease and badly impacts the lives of patients. Urinary kallikrein (UK) acted as a glycoprotein has been discovered to play a pivotal role in neuroprotection. However, the regulatory impacts and correlative pathways of UK in the progression of neuropathic pain remain dimness.

Methods

The chronic constriction injury (CCI) rat model was firstly established to mimic neuropathic pain. The withdrawal threshold was measured through the Von Frey test. The levels of TNF-α, IL-1β and IL-6 were determined through ELISA. The levels of ROS, GSH, SOD and GSH-Px were examined through the commercial kits. The ectopic discharges were assessed. The protein expressions were inspected through western blot.

Results

It was demonstrated that withdrawal threshold was reduced in CCI rat model, but this change was reversed after UK treatment, indicating that UK relieved mechanical allodynia. Moreover, UK alleviated the inflammatory response through reducing TNF-α, IL-1β and IL-6 levels. It was uncovered that oxidative stress was strengthened in CCI rat model, but this impact was restrained after UK treatment. Additionally, UK suppressed ectopic discharge. At last, it was proved that UK triggered the Nrf2/ARE signaling pathway in CCI rat model.

Conclusion

This study manifested that UK reversed neuropathic pain by inhibiting ectopic neural pathways, neural pathways and oxidation via the Nrf2/ARE pathway. This study may offer useful proofs the regulatory functions of UK in the cure of neuropathic pain.

Role of Exercise on Neuropathic Pain in Preclinical Models: Perspectives for Neuroglia

The benefits of exercise on neuropathic pain (NP) have been demonstrated in numerous studies. In recent studies, inflammation, neurotrophins, neurotransmitters, and endogenous opioids are considered as the main mechanisms. However, the role of exercise in alleviating NP remains unclear. Neuroglia, widely distributed in both the central and peripheral nervous systems, perform functions such as support, repair, immune response, and maintenance of normal neuronal activity. A large number of studies have shown that neuroglia play an important role in the occurrence and development of NP, and exercise can alleviate NP by regulating neuroglia. This article reviewed the involvement of neuroglia in the development of NP and their role in the exercise treatment of NP, intending to provide a theoretical basis for the exercise treatment strategy of NP.

MFG-E8 Ameliorates Nerve Injury-Induced Neuropathic Pain by Regulating Microglial Polarization and Neuroinflammation via Integrin β3/SOCS3/STAT3 Pathway in Mice

Spinal microglial polarization plays a crucial role in the pathological processes of neuropathic pain following peripheral nerve injury. Accumulating evidence suggests that milk fat globule epidermal growth factor-8 (MFG-E8) exhibits anti-inflammatory effect and regulates microglial polarization through the integrin β3 receptor. However, the impact of MFG-E8 on microglial polarization in the context of neuropathic pain has not yet been investigated. In this study, we evaluated the effect of MFG-E8 on pain hypersensitivity and spinal microglial polarization following spared nerve injury (SNI) of the sciatic nerve in mice. We determined the molecular mechanisms underlying the effects of MFG-E8 on pain hypersensitivity and spinal microglial polarization using pain behavior assessment, western blot (WB) analysis, immunofluorescence (IF) staining, quantitative polymerase chain reaction (qPCR), enzyme-linked immunosorbent assay (ELISA), and small interfering RNA (siRNA) transfection. Our findings indicate that SNI significantly increased the levels of MFG-E8 and integrin β3 expressed in microglia within the spinal cord of mice. Additionally, we observed that intrathecal injection of recombinant human MFG-E8 (rhMFG-E8) alleviated SNI induced-mechanical allodynia and thermal hyperalgesia. Furthermore, the results suggested that rhMFG-E8 facilitated M2 microglial polarization and ameliorated neuroinflammation via integrin β3/SOCS3/STAT3 pathway in the spinal cord of mice with SNI. Importantly, these effects were negated by integrin β3 siRNA, or SOCS3 siRNA. These results demonstrate that MFG-E8 ameliorates peripheral nerve injury induced-mechanical allodynia and thermal hyperalgesia by driving M2 microglial polarization and mitigating neuroinflammation mediated by integrin β3/SOCS3/STAT3 pathway in the spinal cord of mice. MFG-E8 may serve as a promising target for the treatment of neuropathic pain.

Lentivirus-mediated RNA interference targeting HMGB1 modulates AQP1 to reduce pain induced by chronic compression of the dorsal root ganglia

Backgrounds

Neuropathic pain (NP) is a kind of chronic pain that has attracted much attention in clinical practice, characterized by high morbidity, complex mechanisms, and difficulties in clinical treatment, with which the activation of High mobility group box 1 (HMGB1) is closely related. The aim of this study was to investigate the effects of lentivirus-mediated RNA interference gene therapy targeting HMGB1 on neuropathic pain in rats with chronic dorsal root ganglion compression (CCD) and its specific mechanisms, so as to explore new pharmacological targets.

Methods

Adult male Wistar rats were surgically subjected to chronic compression of the dorsal root ganglia (CCD). Behavioral tests were performed by calculating the paw withdrawal mechanical threshold (PWMT) and the thermal paw withdrawal latency (TPWL). Co-immunoprecipitation (CO-IP) was used to clarify protein interactions. Gene silencing was induced by injecting lentivirus expressing HMGB1 short hairpin RNA (shRNA) into rats. An LPS-inflammation-stimulated rat astrocyte model was established to validate the animal experiment results further. Western blot analysis and real-time quantitative PCR were used to detect pathway protein expression.

Results

After first establishing the rat CCD model, both PWMT and PTWL were significantly reduced in rats, indicating that the model construction was successful. After lentiviral silencing of HMGB1 expression, NP was significantly alleviated in CCD rats. CO-IP experiments showed a link between HMGB1 and AQP1; After silencing HMGB1 expression, the expression of AQP1 was significantly reduced, and HMGB1 was able to modulate the effect of AQP1 on NP. Further use of an inhibitor of the HMGB1 receptor showed that after inhibition of RAGE, AQP1 was significantly reduced; HMGB1 may regulate AQP1 through its receptor RAGE to affect NP. Silencing of HMGB1 resulted in a significant decrease in NF-κB, and HMGB1 affects the inflammatory pathways it mediates. After silencing AQP1, NF-κB also decreased significantly, indicating that AQP1 is an upstream regulator of NF-κB.

Conclusion

Lentivirus-mediated RNA interference (RNAi) silencing targeting HMGB1 may play a key role in the development of neuropathic pain in rats by regulating AQP1 expression via RAGE and ultimately activating NF-κB.

Treatment of neuropathic pain by traditional Chinese medicine: An updated review on their effect and putative mechanisms of action

Neuropathic pain (NP) is a common chronic pain with heterogeneous clinical features, and consequent lowering of quality of life. Currently, although conventional chemical drugs can effectively manage NP symptoms in the short term, their long-term efficacy is limited, and they come with significant side effects. In this regard, traditional Chinese medicine (TCM) provides a promising avenue for treating NP. Numerous pharmacological and clinical studies have substantiated the effectiveness of TCM with multiple targets and mechanisms. We aimed to outline the characteristics of TCM, including compound prescriptions, single Chinese herbs, active ingredients, and TCM physical therapy, for NP treatment and discussed their efficacy by analyzing the pathogenesis of NP. Various databases, such as PubMed, Web of Science, China National Knowledge Infrastructure, and Wanfang database, were searched. We focused on recent research progress in NP treatment by TCM. Finally, we proposed the future challenges and emerging trends in the treatment of NP. TCM demonstrates significant clinical efficacy in NP treatment, employing multi-mechanisms. Drawing from the theory of syndrome differentiation, four types of dialectical treatments for NP by compound TCM prescriptions were introduced: promoting blood circulation and removing blood stasis; promoting blood circulation and promote Qi flow; warming Yang and benefiting Qi; soothing the liver and regulating Qi. Meanwhile, 33 single Chinese herbs and 25 active ingredients were included. In addition, TCM physical therapy (e.g., acupuncture, massage, acupoint injection, and fumigation) also showed good efficacy in NP treatment. TCM, particularly through the use of compound prescriptions and acupuncture, holds bright prospects in treating NP owing to its diverse holistic effects. Nonetheless, the multi-targets of TCM may result in possible disadvantages to NP treatment, and the pharmacological mechanisms of TCM need further evaluation. Here, we provide an overview of NP treatment via TCM, based on the pathogenesis and the potential therapeutic mechanisms, thus providing a reference for further studies.

Nerve injury inhibits Oprd1 and Cnr1 transcription through REST in primary sensory neurons

The transcription repressor REST in the dorsal root ganglion (DRG) is upregulated by peripheral nerve injury and promotes the development of chronic pain. However, the genes targeted by REST in neuropathic pain development remain unclear. The expression levels of 4 opioid receptor (Oprm1, Oprd1, Oprl1, Oprk1) and the cannabinoid CB1 receptor (Cnr1) genes in the DRG regulate nociception. In this study, we determined the role of REST in the control of their expression in the DRG induced by spared nerve injury (SNI) in both male and female mice. Transcriptomic analyses of male mouse DRGs followed by quantitative reverse transcription polymerase chain reaction analyses of both male and female mouse DRGs showed that SNI upregulated expression of Rest and downregulated mRNA levels of all 4 opioid receptor and Cnr1 genes, but Oprm1 was upregulated in female mice. Analysis of publicly available bioinformatic data suggested that REST binds to the promoter regions of Oprm1 and Cnr1. Chromatin immunoprecipitation analyses indicated differing levels of REST at these promoters in male and female mice. Full-length Rest conditional knockout in primary sensory neurons reduced SNI-induced pain hypersensitivity and rescued the SNI-induced reduction in the expression of Oprd1 and Cnr1 in the DRG in both male and female mice. Our results suggest that nerve injury represses the transcription of Oprd1 and Cnr1 via REST in primary sensory neurons and that REST is a potential therapeutic target for neuropathic pain.

Novel AT2R Antagonists for Treating Chronic Pain

Provided herein are novel AT2R antagonists, pharmaceutical compositions, use of such compounds in treating chronic pain, and processes for preparing such compounds.

Neuropathic pain development following nerve injury is mediated by SOX11-ARID1A-SOCS3 transcriptional regulation in the spinal cord

BACKGROUND

Neuropathic pain, a complex condition originating from nervous system damage, remains a significant clinical challenge due to limited understanding of its underlying mechanisms. Recent research highlights the SOX11 transcription factor, known for its role in nervous system development, as a crucial player in neuropathic pain development and maintenance. This study investigates the role of the SOX11-ARID1A-SOCS3 pathway in neuropathic pain modulation within the spinal cord.

METHODS AND RESULTS

Using a spinal nerve ligation (SNL) model in mice, we observed a significant upregulation of Sox11 in the spinal cord dorsal horn post-injury. Intrathecal administration of Sox11 shRNA mitigated SNL-induced neuropathic pain behaviors, including mechanical allodynia and heat hyperalgesia. Further, we demonstrated that Sox11 regulates neuropathic pain via transcriptional control of ARID1A, with subsequent modulation of SOCS3 expression. Knockdown of ARID1A and SOCS3 via shRNA resulted in alleviation of Sox11-induced pain sensitization. Additionally, Sox11 overexpression led to an increase in ARID1A binding to the SOCS3 promoter, enhancing chromatin accessibility and indicating a direct regulatory relationship. These findings were further supported by in vitro luciferase reporter assays and chromatin accessibility analysis.

CONCLUSIONS

The SOX11-ARID1A-SOCS3 pathway plays a pivotal role in the development and maintenance of neuropathic pain. Sox11 acts as a master regulator, modulating ARID1A, which in turn influences SOCS3 expression, thereby contributing to the modulation of neuropathic pain. These findings provide a deeper understanding of the molecular mechanisms underlying neuropathic pain and highlight potential therapeutic targets for its treatment. The differential regulation of this pathway in the spinal cord and dorsal root ganglia (DRG) underscores its complexity and the need for targeted therapeutic strategies.

Mitochondrial dysfunction and disulfidptosis co-regulate neuronal cell in neuropathic pain based on bioinformatics analysis

Neuropathic pain (NP) affects approximately 6.9-10% of the world's population and necessitates the development of novel treatments. Mitochondria are essential in the regulation of cell death. Neuroimmune mechanisms are implicated in various forms of cell death associated with NP. However, the specific involvement of mitochondrial dysfunction and disulfidptosis in NP remains uncertain. Further research is required to gain a better understanding of their combined contribution. Our comprehensive study employs a variety of bioinformatic analysis methods, including differential gene analysis, weighted gene co-expression network analysis, machine learning, functional enrichment analysis, immune infiltration, sub-cluster analysis, single-cell dimensionality reduction and cell-cell communication to gain insight into the molecular mechanisms behind these processes. Our study rationally defines a list of key gene sets for mitochondrial dysfunction and disulfidptosis. 6 hub mitochondrial genes and 3 disulfidptosis-related genes (DRGs) were found to be associated with NP. The key genes were predominantly expressed in neurons and were lowly expressed in the NP group compared to SHAM. In addition, our macrophages used the APP (Amyloid precursor protein)-CD74 (MHC class II invariant chain) pathway to interact with neurons. These results suggest that NP is interconnected with the mechanistic processes of mitochondrial dysfunction and disulfidptosis, which may contribute to clinically targeted therapies.

Naturally Inspired Molecules for Neuropathic Pain Inhibition-Effect of Mirogabalin and Cebranopadol on Mechanical and Thermal Nociceptive Threshold in Mice

BACKGROUND

Neuropathic pain is drug-resistant to available analgesics and therefore novel treatment options for this debilitating clinical condition are urgently needed. Recently, two drug candidates, namely mirogabalin and cebranopadol have become a subject of interest because of their potential utility as analgesics for chronic pain treatment. However, they have not been investigated thoroughly in some types of neuropathic pain, both in humans and experimental animals.

METHODS

This study used the von Frey test, the hot plate test and the two-plate thermal place preference test supported by image analysis and machine learning to assess the effect of intraperitoneal mirogabalin and subcutaneous cebranopadol on mechanical and thermal nociceptive threshold in mouse models of neuropathic pain induced by streptozotocin, paclitaxel and oxaliplatin.

RESULTS

Mirogabalin and cebranopadol effectively attenuated tactile allodynia in models of neuropathic pain induced by streptozotocin and paclitaxel. Cebranopadol was more effective than mirogabalin in this respect. Both drugs also elevated the heat nociceptive threshold in mice. In the oxaliplatin model, cebranopadol and mirogabalin reduced cold-exacerbated pain.

CONCLUSIONS

Since mirogabalin and cebranopadol are effective in animal models of neuropathic pain, they seem to be promising novel therapies for various types of neuropathic pain in patients, in particular those who are resistant to available analgesics.