Deconstructing the neuropathic pain phenotype to reveal neural mechanisms

Epidermal Transient Receptor Potential Vanilloid 1 innervation is increased in patients with painful diabetic polyneuropathy experiencing ongoing burning pain

ABSTRACT

Preclinical studies suggested that Transient Receptor Potential Vanilloid 1 (TRPV1) channels contribute to neuropathic pain in animal models of diabetic polyneuropathy. Patients with painful diabetic polyneuropathy commonly experience ongoing burning pain. This study aimed at evaluating the association between this specific type of pain and TRPV1 intraepidermal nerve fibers in patients with painful diabetic polyneuropathy. We consecutively enrolled 70 patients with diabetic polyneuropathy. Each patient completed the Neuropathic Pain Symptom Inventory (NPSI) to identify the various types of neuropathic pain. All patients underwent a distal leg skin biopsy, with immunostaining of skin nerve fibers using antibodies for the pan-axonal marker Protein Gene Product 9.5 (PGP9.5), TRPV1, Calcitonin Gene-Related Peptide (CGRP), and Substance P. We found that 57% of patients (n = 40) had neuropathic pain symptoms, with ongoing burning pain being the most frequently reported type of pain at the NPSI (70% of patients with pain, n = 28). Patients with ongoing burning pain had higher TRPV1 intraepidermal nerve fiber density and TRPV1/PGP9.5 ratio compared with those with painless polyneuropathy (P = 0.014, P = 0.013) and painful polyneuropathy with other types of pain (P < 0.0001, P = 0.024); they also had increased CGRP dermal nerve fiber density compared with patients with painless polyneuropathy (P = 0.005). Our study showed that ongoing burning pain is associated with an increased expression of intraepidermal TRPV1 fibers, as well as an increased dermal representation of CGRP fibers. These findings suggest that TRPV1 contributes to ongoing burning pain, possibly in conjunction with elevated CGRP expression, highlighting its significance as a therapeutic target for patients with painful diabetic polyneuropathy.

Macrophages protect against sensory axon loss in peripheral neuropathy

Peripheral neuropathy is a common complication of type 2 diabetes, which is strongly associated with obesity1, causing sensory loss and, in some patients, neuropathic pain2,3. Although the onset and progression of diabetic peripheral neuropathy is linked with dyslipidaemia and hyperglycaemia4, the contribution of inflammation to peripheral neuropathy pathogenesis has not been investigated. Here we used a high-fat, high-fructose diet (HFHFD), which induces obesity and prediabetic metabolic changes, to study the onset of peripheral neuropathy. Mice fed the HFHFD developed persistent heat hypoalgesia after 3 months, but a reduction in epidermal skin nerve fibre density manifested only at 6 months. Using single-cell sequencing, we found that CCR2+ macrophages infiltrate the sciatic nerves of HFHFD-fed mice well before axonal degeneration is detectable. These infiltrating macrophages share gene expression similarities with nerve-crush-induced macrophages5 and express neurodegeneration-associated microglial marker genes6, although there is no axon loss or demyelination. Inhibiting the macrophage recruitment by genetically or pharmacologically blocking CCR2 signalling resulted in more severe heat hypoalgesia and accelerated skin denervation, as did deletion of Lgals3, a gene expressed in recruited macrophages. Recruitment of macrophages into the peripheral nerves of obese prediabetic mice is, therefore, neuroprotective, delaying terminal sensory axon degeneration by means of galectin 3. Potentiating and sustaining early neuroprotective immune responses in patients could slow or prevent peripheral neuropathy.

Small molecule-mediated targeted protein degradation of voltage-gated sodium channels involved in pain

The voltage-gated sodium channels (VGSC) NaV1.8 and NaV1.7 (NaVs) have emerged as promising and high-value targets for the development of novel, non-addictive analgesics to combat the chronic pain epidemic. In recent years, many small molecule inhibitors against these channels have been developed. The recent successful clinical trial of VX-548, a NaV1.8-selective inhibitor, has spurred much interest in expanding the arsenal of subtype-selective voltage-gated sodium channel therapeutics. Toward that end, we sought to determine whether NaVs are amenable to targeted protein degradation with small molecule degraders, namely proteolysis-targeting chimeras (PROTACs) and molecular glues. Here, we report that degron-tagged NaVs are potently and rapidly degraded by small molecule degraders harnessing the E3 ubiquitin ligases cereblon (CRBN) and Von Hippel Lindau (VHL). Using LC/MS analysis, we demonstrate that PROTAC-mediated proximity between NaV1.8 and CRBN results in ubiquitination on the 2nd intracellular loop, pointing toward a potential mechanism of action and demonstrating the ability of CRBN to recognize a VGSC as a neosubstrate. Our foundational findings are an important first step toward realizing the immense potential of NaV-targeting degrader analgesics to combat chronic pain.

Two-year outcomes using fast-acting sub-perception therapy for spinal cord stimulation: results of a real-world multicenter study in the United States

BACKGROUND

Fast-acting Sub-perception Therapy (FAST) is a novel spinal cord stimulation (SCS) modality delivering paresthesia-free pain relief. Our study evaluated the longer-term, real-world impact of FAST on chronic pain.

RESEARCH DESIGN AND METHODS

As part of a multicenter, real-world, consecutive case series, we retrospectively identified patients who used FAST-SCS and analyzed their data. The numerical rating scale (NRS) was used to evaluate the overall pain.

RESULTS

Data from 315 patients were analyzed at baseline and their last available follow-up (median 6.8 months after SCS implantation). At the time of the analysis, 12-, 18-, and 24-month data were available for 112, 86, and 50 patients, respectively. At the last follow-up, NRS pain score was reduced by 5.5 ± 2.5 compared to baseline (from 7.8 ± 1.7 to 2.3 ± 2.0; p < 0.0001). Interim long-term analysis showed that results were sustained for up to 2 years, with 64% of patients reporting a minimal overall pain score (NRS ≤2/10).

CONCLUSION

This ongoing, real-world, multicenter study showed that FAST-SCS achieved significant paresthesia-free pain relief, while long-term interim analysis suggests that outcomes could be sustained for up to 2 years. Our data provide preliminary insights into the potential utility of this low-frequency sub-perception SCS paradigm using a biphasic active recharge pulse shape.

TRIAL REGISTRATION

ClinicalTrials.gov (CT.gov identifier: NCT01550575).

Dorsal root ganglia CSF1+ neuronal subtypes have different impact on macrophages and microglia after spared nerve injury

BACKGROUND AND AIMS

Colony-stimulating factor 1 (CSF1) is a growth factor secreted by dorsal root ganglia (DRG) neurons important for DRG macrophages and spinal cord (SC) microglia injury-induced proliferation and activation, specifically released after spared nerve injury (SNI). In this study, we investigated if SNI-induced CSF1 expression and perineuronal rings of macrophages around mouse DRG neurons vary between L3-L5 DRG and with the neuronal type, and if the CSF1+ neuronal projections at the SC dorsal horns were associated with an increased microglial number in the corresponding laminae.

METHODS

Seven days after surgery, L3-L5 DRG as well as their corresponding segments at the SC level were collected, frozen, and cut. DRG sections were double-immunostained using antibodies against CSF1 and NF200, CGRP or IB4, while SC sections were immunostained using a fluorescent Nissl Stain and analyzed for CX3CR1-GFP microglia number and distribution by an in-house ImageJ Plug-in.

RESULTS

Our results showed that SNI-induced CSF1 expression was common for all subtypes of mouse DRG neurons, being responsible for attracting more resident macrophages around them in a DRG-dependent manner, with L4 showing the stronger response and CSF1+/NF200+ neurons showing the highest incidence. Even though the total number of microglia in the SC ipsilateral dorsal horns increased after SNI, the increase at their specific laminar projection sites did not mirror the incidence of DRG neuronal subtypes among CSF1+ neurons.

INTERPRETATION

Taken together, these results contribute to a more comprehensive understanding of the connection between CSF1 and macrophage/microglia response after SNI and emphasize the importance of considering L3-L5 DRG individually when investigating SNI-neuropathic pain pathogenesis in mice.

Phenotyping peripheral neuropathies with and without pruritus: a cross-sectional multicenter study

ABSTRACT

Pruritus often escapes physicians' attention in patients with peripheral neuropathy (PNP). Here we aimed to characterize neuropathic pruritus in a cohort of 191 patients with PNP (large, mixed, or small fiber) and 57 control subjects with deep phenotyping in a multicenter cross-sectional observational study at 3 German sites. All participants underwent thorough neurological examination, nerve conduction studies, quantitative sensory testing, and skin biopsies to assess intraepidermal nerve fiber density. Patients filled in a set of questionnaires assessing the characteristics of pruritus and pain, the presence of depression and anxiety, and quality of life. Based on the severity of pruritus and pain, patients were grouped into 4 groups: "pruritus," "pain," "pruritus and pain," and "no pruritus/no pain." Although 11% (21/191) of patients reported pruritus as their only symptom, further 34.6% (66/191) reported pruritus and pain. Patients with pain (with or without pruritus) were more affected by anxiety, depression, and reduced quality of life than control subjects. Patients with pruritus (with and without pain) had increases in cold detection threshold, showing Aδ-fiber dysfunction. The pruritus group had lower intraepidermal nerve fiber density at the thigh, concomitant with a more proximal distribution of symptoms compared with the other PNP groups. Stratification of patients with PNP by using cross-sectional datasets and multinominal logistic regression analysis revealed distinct patterns for the patient groups. Together, our study sheds light on the presence of neuropathic pruritus in patients with PNP and its relationship with neuropathic pain, outlines the sensory and structural abnormalities associated with neuropathic pruritus, and highlights its impact on anxiety levels.

Clinical pharmacology of neuropathic pain

This chapter aims to review the current pharmacological options for neuropathic pain treatment, their mechanisms of action, and future directions for clinical practice. Achieving pain relief in neuropathic pain conditions remains a challenge in clinical practice. The field of pharmacotherapy for neuropathic pain has encountered significant difficulties in translating substantial advances in our understanding of the underlying pathophysiological mechanisms into clinically effective therapies. This chapter presents the drugs recommended for the pharmacotherapy of neuropathic pain, based on the widely accepted treatment guidelines formulated by the Neuropathic Pain Special Interest Group of the International Association for the Study of Pain. In addition to discussing how the evidence base is created as part of international consortia, the drugs are also examined in terms of their putative molecular mechanisms as well as pharmacological pleiotropy, i.e., their potential unspecific and multi-target effects resulting in modulation of neuronal hyperexcitability. The chapter closes with a discussion of potential future developments in the field.

Neuropathic pain - A clinical primer

Neuropathic pain is used both as a mechanistic descriptor and a classification category of pain caused by a lesion or disease of the somatosensory nervous system and encompasses a vast array of possible diagnoses. The identification of neuropathic pain and diagnosis of specific syndromes relies on a detailed patient history. Standardized pain questionnaires can capture the patient`s symptoms, while the anatomical distribution of pain is often documented using pain drawings. Following this, a thorough clinical neurological examination is conducted to identify distinct sensory abnormalities, specifically sensory deficits and signs of increased sensitivity such as allodynia and hyperalgesia, within the pain-affected areas. Regardless of whether the lesion or disease is in the peripheral or central somatosensory nervous system, the presence of clinically overt sensory abnormalities is a key feature, distinguishing neuropathic pain from other types of pain, such as nociceptive pain, which likely coexist in neurological disorders. Extensive sensory deficits, as seen in certain stroke syndromes or following spinal cord injuries, may increase the likelihood of concomitant non-neuropathic pain within the same area of sensory loss. For this reason, differential diagnosis is essential when assessing patients with suspected neuropathic pain. Further diagnostic tests, including imaging or specific neurophysiological methods that assess nociceptive pathways, can provide objective evidence of a lesion or disease within the somatosensory nervous system. However, the causality between the lesion and the presence of neuropathic pain cannot be established definitively and always requires clinical judgment and interpretation within the broader context of the neurological disorder.

Peripheral CaV2.2 Channels in the Skin Regulate Prolonged Heat Hypersensitivity during Neuroinflammation

Neuroinflammation can lead to chronic maladaptive pain affecting millions of people worldwide. Neurotransmitters, cytokines, and ion channels are implicated in neuroimmune cell signaling, but their roles in specific behavioral responses are not fully elucidated. Voltage-gated CaV2.2 channel activity in skin controls rapid and transient heat hypersensitivity induced by intradermal (i.d.) capsaicin via IL-1ɑ cytokine signaling. CaV2.2 channels are not, however, involved in mechanical hypersensitivity that developed in the i.d. capsaicin animal model. Here, we show that CaV2.2 channels are also critical for heat hypersensitivity induced by i.d. complete Freund adjuvant (CFA). i.d. CFA, a model of chronic neuroinflammation, involves ongoing cytokine signaling for days leading to pronounced edema and hypersensitivity to sensory stimuli. Peripheral CaV2.2 channel activity in the skin was required for the full development and week-long time course of heat hypersensitivity induced by i.d. CFA, but paw edema and mechanical hypersensitivity were independent of CaV2.2 channel activity. CFA induced increases in several cytokines in hindpaw fluid including IL-6 which was also dependent on CaV2.2 channel activity. Using IL-6-specific neutralizing antibodies in vivo, we show that IL-6 contributes to heat hypersensitivity and that neutralizing both IL-1ɑ and IL-6 was even more effective at reducing the magnitude and duration of CFA-induced heat hypersensitivity. Our findings demonstrate a functional link between CaV2.2 channel activity and the release of IL-6 in the skin and show that CaV2.2 channels have a privileged role in the induction and maintenance of heat hypersensitivity during chronic forms of neuroinflammation in the skin.

Genetic associations of neuropathic pain and sensory profile in a deeply phenotyped neuropathy cohort

ABSTRACT

We aimed to investigate the genetic associations of neuropathic pain in a deeply phenotyped cohort. Participants with neuropathic pain were cases and compared with those exposed to injury or disease but without neuropathic pain as control subjects. Diabetic polyneuropathy was the most common aetiology of neuropathic pain. A standardised quantitative sensory testing protocol was used to categorize participants based on sensory profile. We performed genome-wide association study, and in a subset of participants, we undertook whole-exome sequencing targeting analyses of 45 known pain-related genes. In the genome-wide association study of diabetic neuropathy (N = 1541), a top significant association was found at the KCNT2 locus linked with pain intensity (rs114159097, P = 3.55 × 10-8). Gene-based analysis revealed significant associations between LHX8 and TCF7L2 and neuropathic pain. Polygenic risk score for depression was associated with neuropathic pain in all participants. Polygenic risk score for C-reactive protein showed a positive association, while that for fasting insulin showed a negative association with neuropathic pain, in individuals with diabetic polyneuropathy. Gene burden analysis of candidate pain genes supported significant associations between rare variants in SCN9A and OPRM1 and neuropathic pain. Comparison of individuals with the "irritable" nociceptor profile to those with a "nonirritable" nociceptor profile identified a significantly associated variant (rs72669682, P = 4.39 × 10-8) within the ANK2 gene. Our study on a deeply phenotyped cohort with neuropathic pain has confirmed genetic associations with the known pain-related genes KCNT2, OPRM1, and SCN9A and identified novel associations with LHX8 and ANK2, genes not previously linked to pain and sensory profiles, respectively.

Exploring the potential therapeutic benefits of 7-methoxy coumarin for neuropathy pain: an in vivo, in vitro, and in silico approach

BACK GROUND

7-Methoxycoumarin (7-MC) is well recognized for its anti-inflammatory and anti-nociceptive actions. Its capacity to lessen neuropathic pain hasn't been documented yet. Hence the impact of 7-MC on vincristine-induced peripheral neuropathic pain in rodents was investigated. The investigation also looked at the impact of 7-MC in reducing neuropathic pain via voltage-gated calcium channels and phospholipase enzyme inhibition using pertinent in vitro and in silico methods.

METHODS AND RESULTS

Vincristine (0.1 mg/kg, i.p., daily) was administered continuously for 7 days to induce peripheral neuropathic pain in mice, with cold allodynia and thermal hyperalgesia and evaluated on the 8th day using the acetone bubble test and hot water tail immersion test. In order to derive the mechanistic approach for ameliorating neuropathic pain, the role of 7-MC in the inhibition of the phospholipase enzyme, gene expression studies on voltage-gated calcium channels using mouse BV2 microglial cells and in silico studies for its calcium channel binding affinity were also performed. The test compounds reduced vincristine-induced cold allodynia and thermal hyperalgesia in mice in a dose-dependent experiments. In vitro studies on phospholipase inhibition by 7-MC showed an IC50 of 27.08 µg/ml and down-regulated the gene expression of calcium channels in the BV2 microglial cell line. In silico docking scores for 7-MCwere higher than the standard drug gabapentin.

CONCLUSION

The compound 7-MC has shown promise in alleviating vincristine-induced peripheral neuropathicin mice. Studies conducted in parallel, both in silico and in vitro have demonstrated that 7-MC effectively reduces neuropathic pain. This pain reduction is achieved through two mechanisms: inhibiting the phospholipase enzyme and blocking voltage-gated calcium channels.

A role for leucine-rich, glioma inactivated 1 in regulating pain sensitivity

Neuronal hyperexcitability is a key driver of persistent pain states including neuropathic pain. Leucine-rich, glioma inactivated 1 (LGI1), is a secreted protein known to regulate excitability within the nervous system and is the target of autoantibodies from neuropathic pain patients. Therapies that block or reduce antibody levels are effective at relieving pain in these patients, suggesting that LGI1 has an important role in clinical pain. Here we have investigated the role of LGI1 in regulating neuronal excitability and pain-related sensitivity by studying the consequences of genetic ablation in specific neuron populations using transgenic mouse models. LGI1 has been well studied at the level of the brain, but its actions in the spinal cord and peripheral nervous system (PNS) are poorly understood. We show that LGI1 is highly expressed in DRG and spinal cord dorsal horn neurons in both mouse and human. Using transgenic muse models, we genetically ablated LGI1, either specifically in nociceptors (LGI1fl/Nav1.8+), or in both DRG and spinal neurons (LGI1fl/Hoxb8+). On acute pain assays, we find that loss of LGI1 resulted in mild thermal and mechanical pain-related hypersensitivity when compared to littermate controls. In from LGI1fl/Hoxb8+ mice, we find loss of Kv1 currents and hyperexcitability of DRG neurons. LGI1fl/Hoxb8+ mice displayed a significant increase in nocifensive behaviours in the second phase of the formalin test (not observed in LGI1fl/Nav1.8+ mice) and extracellular recordings in LGI1fl/Hoxb8+ mice revealed hyperexcitability in spinal dorsal horn neurons, including enhanced wind-up. Using the spared nerve injury model, we find that LGI1 expression is dysregulated in the spinal cord. LGI1fl/Nav1.8+ mice showed no differences in nerve injury induced mechanical hypersensitivity, brush-evoked allodynia or spontaneous pain behaviour compared to controls. However, LGI1fl/Hoxb8+ mice showed a significant exacerbation of mechanical hypersensitivity and allodynia. Our findings point to effects of LGI1 at both the level of the DRG and spinal cord, including an important impact of spinal LGI1 on pathological pain. Overall, we find a novel role for LGI1 with relevance to clinical pain.

Beyond pain: Using Unsupervised Machine Learning to Identify Phenotypic Clusters of Small Fiber Neuropathy

Background and Objectives

Small fiber neuropathy (SFN) is characterized by dysfunction and loss of peripheral unmyelinated and thinly myelinated nerve fibers, resulting in a phenotype that includes varying combinations of somatosensory and dysautonomia symptoms, which can be profoundly disabling and lead to decreased quality of life. Treatment aimed mainly at pain reduction, which may not target the underlying pathophysiology, is frequently ineffective. Another impediment to the effective management of SFN may be the significant between-patient heterogeneity. Accordingly, we launched this study to gain insights into the symptomatic variability of SFN and determine if SFN patients can be sub-grouped based on clinical characteristics.

Methods

To characterize the phenotype and investigate how patients with SFN differ from those with large fiber involvement, 105 patients with skin-biopsy proven SFN and 45 with mixed fiber neuropathy (MFN) were recruited. Using unsupervised machine learning, SFN patients were clustered based upon symptom concurrence and severity. Demographics, clinical data, symptoms, and skin biopsy- and laboratory findings were compared between the groups.

Results

MFN- as compared to SFN patients, were more likely to be male, older, had a lower intraepidermal nerve fiber density at the ankle and more frequent abnormal immunofixation. Beyond these differences, symptom prevalence and intensities were similar in the two cohorts. SFN patients comprised three distinct phenotypic clusters, which differed significantly in symptom severity, co-occurrence, localization, and skin biopsy findings. Only one subgroup, constituting about 20% of the patient population, was characterized by intense neuropathic pain, which was always associated with several other SFN symptoms of similarly high intensities. A pauci-symptomatic cluster comprised patients who experienced few SFN symptoms, generally of low to moderate intensity. The largest cluster was characterized by intense fatigue, myalgias and subjective weakness, but lower intensities of burning pain and paresthesia.

Discussion

This data-driven study introduces a new approach to subgrouping patients with SFN. Considering both neuropathic pain and pernicious symptoms beyond pain, we identified three clusters, which may be related to distinct pathophysiological mechanisms. Although additional validation will be required, our findings represent a step towards stratified treatment approaches and, ultimately, personalized treatment.

Genetic labeling of the nucleus of tractus solitarius neurons associated with electrical stimulation of the cervical or auricular vagus nerve in mice

INTRODUCTION

Vagus nerve stimulation (VNS) is clinically useful for treating epilepsy, depression, and chronic pain. Currently, cervical VNS (cVNS) treatment is well-established, while auricular VNS (aVNS) is under development. Vagal stimulation regulates functions in diverse brain regions; therefore, it is critical to better understand how electrically-evoked vagal inputs following cVNS and aVNS engage with different brain regions.

OBJECTIVE

As vagus inputs are predominantly transmitted to the nucleus of tractus solitarius (NTS), we directly compared the activation of NTS neurons by cVNS or aVNS and the brain regions directly projected by the activated NTS neurons in mice.

METHODS

We adopted the targeted recombination in active populations method, which allows for the activity-dependent, tamoxifen-inducible expression of mCherry-a reporter protein-in neurons specifically associated with cVNS or aVNS.

RESULTS

cVNS and aVNS induced comparable bilateral mCherry expressions in neurons within the NTS, especially in its caudal section (cNTS). However, the numbers of mCherry-expressing neurons within different subdivisions of cNTS was distinctive. In both cVNS and aVNS, anterogradely labeled mCherry-expressing axonal terminals were similarly observed across different areas of the forebrain, midbrain, and hindbrain. These terminals were enriched in the rostral ventromedial medulla, parabrachial nucleus, periaqueductal gray, thalamic nuclei, central amygdala, and the hypothalamus. Sex difference of cVNS- and aVNS-induced labeling of NTS neurons was modest.

CONCLUSION

The central projections of mCherry-expressing cNTS terminals are comparable between aVNS and cVNS, suggesting that cVNS and aVNS activate distinct but largely overlapping projections into the brain through the cNTS.

Neuropharmacology of Neuropathic Pain: A Systematic Review

Neuropathic pain, a debilitating condition, remains challenging to manage effectively. An insight into neuropharmacological mechanisms is critical for optimizing treatment strategies. This systematic review aims to evaluate the role of neuropharmacological agents based on their efficacy, involved neurotransmitters, and receptors. A manual literature search was undertaken in PubMed including Medline, Cochrane Library, Google Scholar, Plos One, Science Direct, and clinicaltrials.gov from 2013 until 2023. Out of the 13 included studies, seven evaluated the role of gabapentinoids. Two main drugs from this group, gabapentin and pregabalin, function by binding voltage-gated calcium channels, lowering neuronal hyperexcitability and pain signal transmission, thereby relieving neuropathic pain. Four of the pooled studies reported the use of tricyclic antidepressants (TCAs) including amitriptyline and nortriptyline which work by blocking the reuptake of norepinephrine and serotonin, their increased concentration is thought to be central to their analgesic effect. Three articles assessed the use of serotonin-norepinephrine reuptake inhibitors (SNRIs) and reported them as effective as the TCAs in managing neuropathic pain. They work by augmenting serotonin and norepinephrine. Three studies focused on the use of selective serotonin reuptake inhibitors (SSRIs), modulating their effect by increasing serotonin levels; however, they were reported as not a highly effective treatment option for neuropathic pain. One of the studies outlined the use of cannabinoids for neuropathic pain by binding to cannabinoid receptors with only mild adverse effects. It is concluded that gabapentinoids, TCAs, and SNRIs were reported as the most effective therapy for neuropathic pain; however, for trigeminal neuralgia, anticonvulsants like carbamazepine were considered the most effective. Opioids were considered second-line drugs for neuropathic pain as they come with adverse effects and a risk of dependence. Ongoing research is exploring novel drugs like ion channels and agents modulating pain pathways for neuropathic pain management. Our review hopes to inspire further research into patient stratification by their physiology, aiding quicker and more accurate management of neuropathic pain while minimizing inadvertent side effects.

Peripheral CaV2.2 channels in skin regulate prolonged heat hypersensitivity during neuroinflammation

Neuroinflammation can lead to chronic maladaptive pain affecting millions of people worldwide. Neurotransmitters, cytokines, and ion channels are implicated in neuro-immune cell signaling but their roles in specific behavioral responses are not fully elucidated. Voltage-gated CaV2.2 channel activity in skin controls rapid and transient heat hypersensitivity induced by intradermal capsaicin via IL-1α cytokine signaling. CaV2.2 channels are not, however, involved in mechanical hypersensitivity that developed in the same animal model. Here, we show that CaV2.2 channels are also critical for heat hypersensitivity induced by the intradermal (id) Complete Freund's Adjuvant (CFA) model of chronic neuroinflammation that involves ongoing cytokine signaling for days. Ongoing CFA-induced cytokine signaling cascades in skin lead to pronounced edema, and hypersensitivity to sensory stimuli. Peripheral CaV2.2 channel activity in skin is required for the full development and week-long time course of heat hypersensitivity induced by id CFA. CaV2.2 channels, by contrast, are not involved in paw edema and mechanical hypersensitivity. CFA induced increases in cytokines in hind paws including IL-6 which was dependent on CaV2.2 channel activity. Using IL-6 specific neutralizing antibodies, we show that IL-6 contributes to heat hypersensitivity and, neutralizing both IL-1α and IL-6 was even more effective at reducing the magnitude and duration of CFA-induced heat hypersensitivity. Our findings demonstrate a functional link between CaV2.2 channel activity and the release of IL-6 in skin and show that CaV2.2 channels have a privileged role in the induction and maintenance of heat hypersensitivity during chronic forms of neuroinflammation in skin.

Effectiveness of an intervention (effleurage and petrissage) on the severity of chemotherapy-induced peripheral neuropathy (CIPN) among patients: A randomized control pilot trial

Background

Chemotherapy-induced peripheral neuropathy (CIPN) is a dose-dependent, nerve-damaging adverse reaction with reported prevalence rates of 68.1% at the first month after starting chemotherapy; 60.0 % at the third month; and 30% at the sixth month.

Purpose

To determine the effectiveness of an intervention (effleurage and petrissage) on severity of CIPN among patients receiving platinum-based chemotherapy.

Methods

Sixty patients receiving either a third or fourth cycle of platinum-based chemotherapy were randomly assigned to one of two groups with a pre- and post-test design. The interventional group received effleurage and petrissage prior to chemotherapy for a period of one month. Comparisons of CIPN levels among both groups were done at Day 7, 14, 21, and 1 month using the FACT/ GOG-Ntx subscale.

Results

Prior to intervention, the mean score (+SD) of CIPN in the intervention group was 17.17 (+4.907) and 17.10 (+ 4.421) in the control group (T = 0.055, P value 0.956). The post-test scores following intervention at 1 month, was a mean score (+SD) for CIPN in the intervention group of 10.70 (±2.855) and 16.27 (±3.039) in the control group (P value 0.000).

Conclusions

This pilot result supports that the intervention (effleurage and petrissage) can be effective in reducing CIPN severity levels among patients receiving platinum-based chemotherapy.

Functional remodeling of presynaptic voltage-gated calcium channels in superficial layers of the dorsal horn during neuropathic pain

N- and P/Q-type voltage-gated Ca2+ channels are critical for synaptic transmission. While their expression is increased in the dorsal root ganglion (DRG) neuron cell bodies during neuropathic pain conditions, less is known about their synaptic remodeling. Here, we combined genetic tools with 2-photon Ca2+ imaging to explore the functional remodeling that occurs in central presynaptic terminals of DRG neurons during neuropathic pain. We imaged GCaMP6s fluorescence responses in an ex vivo spinal cord preparation from mice expressing GCaMP6s in Trpv1-Cre lineage nociceptors. We show that Ca2+ transient amplitude is increased in central terminals of these neurons after spared nerve injury, and that this increase is mediated by both N- and P/Q-type channels. We found that GABA-B receptor-dependent inhibition of Ca2+ transients was potentiated in the superficial layer of the dorsal horn. Our results provide direct evidence toward nerve injury-induced functional remodeling of presynaptic Ca2+ channels in Trpv1-lineage nociceptor terminals.

Pyroptosis in Diabetic Peripheral Neuropathy and its Therapeutic Regulation

Pyroptosis is a pro-inflammatory form of cell death resulting from the activation of gasdermins (GSDMs) pore-forming proteins and the release of several pro-inflammatory factors. However, inflammasomes are the intracellular protein complexes that cleave gasdermin D (GSDMD), leading to the formation of robust cell membrane pores and the initiation of pyroptosis. Inflammasome activation and gasdermin-mediated membrane pore formation are the important intrinsic processes in the classical pyroptotic signaling pathway. Overactivation of the NOD-like receptor thermal protein domain associated protein 3 (NLRP3) inflammasome triggers pyroptosis and amplifies inflammation. Current evidence suggests that the overactivation of inflammasomes and pyroptosis may further induce the progression of cancers, nerve injury, inflammatory disorders and metabolic dysfunctions. Current evidence also indicates that pyroptosis-dependent cell death accelerates the progression of diabetes and its frequent consequences including diabetic peripheral neuropathy (DPN). Pyroptosis-mediated inflammatory reaction further exacerbates DPN-mediated CNS injury. Accumulating evidence shows that several molecular signaling mechanisms trigger pyroptosis in insulin-producing cells, further leading to the development of DPN. Numerous studies have suggested that certain natural compounds or drugs may possess promising pharmacological properties by modulating inflammasomes and pyroptosis, thereby offering potential preventive and practical therapeutic approaches for the treatment and management of DPN. This review elaborates on the underlying molecular mechanisms of pyroptosis and explores possible therapeutic strategies for regulating pyroptosis-regulated cell death in the pharmacological treatment of DPN.

Emotion Processing in Peripheral Neuropathic Pain: An Observational Study

BACKGROUND

In clinical practice, the implementation of tailored treatment is crucial for assessing the patient's emotional processing profile. Here, we investigate all three levels of analysis characterizing emotion processing, i.e., recognition, representation, and regulation, in patients with peripheral neuropathic pain (PNP).

METHODS

Sixty-two patients and forty-eight healthy controls underwent quantitative sensory testing, i.e., psychophysical tests to assess somatosensory functions such as perception of cold (CDT), heat-induced pain (HPT), and vibration (VDT), as well as three standardized tasks to assess emotional processing: (1) the Ekman 60-Faces Test (EK-60F) to assess recognition of basic facial emotions, (2) the Reading the Mind in the Eyes Test (RME) to assess the ability to represent the feelings of another person by observing their eyes, and (3) the 20-item Toronto Alexithymia Scale (TAS-20) to assess emotional dysregulation, i.e., alexithymia.

RESULTS

General Linear Model analysis revealed a significant relationship between left index finger VDT z-scores in PNP patients with alexithymia. The RME correlated with VDT z-scores of the left little finger and overall score for the EK-60F.

CONCLUSIONS

In patients with PNP, emotion processing is impaired, which emphasizes the importance of assessing these abilities appropriately in these patients. In this way, clinicians can tailor treatment to the needs of individual patients.

The Mas-related G protein-coupled receptor d (Mrgprd) mediates pain hypersensitivity in painful diabetic neuropathy

ABSTRACT

Painful diabetic neuropathy (PDN) is one of the most common and intractable complications of diabetes. Painful diabetic neuropathy is characterized by neuropathic pain accompanied by dorsal root ganglion (DRG) nociceptor hyperexcitability, axonal degeneration, and changes in cutaneous innervation. However, the complete molecular profile underlying the hyperexcitable cellular phenotype of DRG nociceptors in PDN has not been elucidated. This gap in our knowledge is a critical barrier to developing effective, mechanism-based, and disease-modifying therapeutic approaches that are urgently needed to relieve the symptoms of PDN. Using single-cell RNA sequencing of DRGs, we demonstrated an increased expression of the Mas-related G protein-coupled receptor d (Mrgprd) in a subpopulation of DRG neurons in the well-established high-fat diet (HFD) mouse model of PDN. Importantly, limiting Mrgprd signaling reversed mechanical allodynia in the HFD mouse model of PDN. Furthermore, in vivo calcium imaging allowed us to demonstrate that activation of Mrgprd-positive cutaneous afferents that persist in diabetic mice skin resulted in an increased intracellular calcium influx into DRG nociceptors that we assess in vivo as a readout of nociceptors hyperexcitability. Taken together, our data highlight a key role of Mrgprd-mediated DRG neuron excitability in the generation and maintenance of neuropathic pain in a mouse model of PDN. Hence, we propose Mrgprd as a promising and accessible target for developing effective therapeutics currently unavailable for treating neuropathic pain in PDN.

Differential relationships between physical activity and pain phenotypes in individuals with spinal cord injury

BACKGROUND

Chronic pain affects 70% of individuals with spinal cord injury (SCI) and leads to declines in health and quality of life. Neuropathic and nociceptive pain are phenotypes derived from different mechanisms that contribute to pain perception. The objective of this research was to investigate differential pain responses to moderate-to-vigorous physical activity (MVPA) in two chronic pain phenotypes: neuropathic and nociceptive pain.

METHODS

Community-based physical activity levels were collected for one week in 17 individuals with SCI using a wrist-worn accelerometer, and daily pain ratings were assessed and categorized by phenotype. Physical activity levels were summarized to calculate minutes of MVPA. Correlational analyses were conducted to compare relationships between pain intensity and MVPA across individual participants and between pain phenotype groups.

RESULTS

The neuropathic pain group revealed significant negative correlation between MVPA and pain intensity. In the nociceptive pain group, there was no significant correlation between MVPA and pain intensity. Further analysis revealed two subgroups of positive (N = 4) and negative (N = 3) correlations between MVPA and pain intensity. Pain location differed between the subgroups of nociceptive pain. Individuals with negative correlation experienced neck and upper back pain, whereas individuals with positive correlation experienced unilateral upper extremity pain.

CONCLUSION

Differential relationships exist between pain phenotypes and MVPA in individuals with SCI. Pain location differed between the subgroups of nociceptive pain, which we presume may indicate the presence of nociplastic pain in some individuals. These results may contribute to the advancement of personalized pain management by targeting non-pharmacological interventions for specific pain phenotypes.Trial registration: ClinicalTrials.gov identifier: NCT05236933..

5-HT3a receptor contributes to neuropathic pain by regulating central sensitization in a rat with brachial plexus avulsion

PURPOSE

As a frequently occurring complication resulting from brachial plexus avulsion (BPA), neuropathic pain significantly impacts the quality of life of patients and places a substantial burden on their families. Recent reports have suggested that the 5-HT3a receptor may play a role in the development and regulation of neuropathic pain. The current study aimed to explore the involvement of the 5-HT3a receptor in neuropathic pain resulting from BPA in rats.

METHODS

A rat model of neuropathic pain was induced through brachial plexus avulsion (BPA). The pain thresholds of the rats were measured after BPA. The spinal dorsal horn (SDH) of rats was collected at day 14 after surgery, and the expression and distribution of the 5-HT3a receptor were analyzed using immunohistochemistry and western blotting. The expression levels of various factors related to central sensitization were measured by western blot, including c-Fos, GFAP, IBA-1, IL-1β and TNF-α. The effects of 5-HT3a receptor antagonists on hyperalgesia were assessed through behavioral tests after intrathecal administration of ondansetron. Additionally, at 120 min postinjection, the SDH of rats was acquired, and the change of expression levels of protiens related to central sensitization were measured by western blot.

RESULTS

BPA induced mechanical and cold hypersensitivity in rats. The 5-HT3a receptor was increased and mainly distributed on neurons and microglia in the SDH after BPA, and the level of central sensitization and expression of inflammatory factors, such as c-Fos, GFAP, IBA-1, IL-1β and TNF-α, were also increased markedly. Ondansetron, which is a selective 5-HT3a receptor antagonist, reversed the behavioral changes caused by BPA. The antagonist also decreased the expression of central sensitization markers and inflammatory factors.

CONCLUSION

The results suggested that the 5-HT3a receptor is involved in neuropathic pain by regulating central nervous system sensitization in a rat brachial plexus avulsion model. Targeting the 5-HT3a receptor may be a promising approach for treating neuropathic pain after brachial plexus avulsion.

Acupuncture for radicular pain: a review of analgesic mechanism

Radicular pain, a common and complex form of neuropathic pain, presents significant challenges in treatment. Acupuncture, a therapy originating from ancient traditional Chinese medicine and widely utilized for various pain types, including radicular pain, has shown promising outcomes in the management of lumbar radicular pain, cervical radicular pain, and radicular pain due to spinal stenosis. Despite its efficacy, the exact mechanisms through which acupuncture achieves analgesia are not fully elucidated and are the subject of ongoing research. This review sheds light on the current understanding of the analgesic mechanisms of acupuncture for radicular pain, offering valuable perspectives for both clinical application and basic scientific research. Acupuncture is postulated to relieve radicular pain by several mechanisms: peripherally, it reduces muscle spasms, lessens mechanical pressure on nerve roots, and improves microcirculation; at the molecular level, it inhibits the HMGB1/RAGE and TLR4/NF-κB signaling pathways, thereby decreasing the release of pro-inflammatory cytokines; within the spinal cord, it influences synaptic plasticity; and centrally, it modulates brain function, particularly affecting the medial prefrontal cortex, anterior cingulate cortex, and thalamus within the default mode network. By acting across these diverse biological domains, acupuncture presents an effective treatment modality for radicular pain, and deepening our understanding of the underlying mechanisms regarding analgesia for radicular pain is crucial for enhancing its clinical efficacy and advancement in pain management.

Induction of antiviral interferon-stimulated genes by neuronal STING promotes the resolution of pain in mice

Inflammation and pain are intertwined responses to injury, infection, or chronic diseases. While acute inflammation is essential in determining pain resolution and opioid analgesia, maladaptive processes occurring during resolution can lead to the transition to chronic pain. Here we found that inflammation activates the cytosolic DNA-sensing protein stimulator of IFN genes (STING) in dorsal root ganglion nociceptors. Neuronal activation of STING promotes signaling through TANK-binding kinase 1 (TBK1) and triggers an IFN-β response that mediates pain resolution. Notably, we found that mice expressing a nociceptor-specific gain-of-function mutation in STING exhibited an IFN gene signature that reduced nociceptor excitability and inflammatory hyperalgesia through a KChIP1-Kv4.3 regulation. Our findings reveal a role of IFN-regulated genes and KChIP1 downstream of STING in the resolution of inflammatory pain.

Nerve injury disrupts temporal processing in the spinal cord dorsal horn through alterations in PV+ interneurons

How mechanical allodynia following nerve injury is encoded in patterns of neural activity in the spinal cord dorsal horn (DH) remains incompletely understood. We address this in mice using the spared nerve injury model of neuropathic pain and in vivo electrophysiological recordings. Surprisingly, despite dramatic behavioral over-reactivity to mechanical stimuli following nerve injury, an overall increase in sensitivity or reactivity of DH neurons is not observed. We do, however, observe a marked decrease in correlated neural firing patterns, including the synchrony of mechanical stimulus-evoked firing, across the DH. Alterations in DH temporal firing patterns are recapitulated by silencing DH parvalbumin+ (PV+) interneurons, previously implicated in mechanical allodynia, as are allodynic pain-like behaviors. These findings reveal decorrelated DH network activity, driven by alterations in PV+ interneurons, as a prominent feature of neuropathic pain and suggest restoration of proper temporal activity as a potential therapeutic strategy to treat chronic neuropathic pain.

Quantification of stimulus-evoked tactile allodynia in free moving mice by the chainmail sensitivity test

Chronic pain occurs at epidemic levels throughout the population. Hypersensitivity to touch, is a cardinal symptom of chronic pain. Despite dedicated research for over a century, quantifying this hypersensitivity has remained impossible at scale. To address these issues, we developed the Chainmail Sensitivity Test (CST). Our results show that control mice spend significantly more time on the chainmail portion of the device than mice subject to neuropathy. Treatment with gabapentin abolishes this difference. CST-derived data correlate well with von Frey measurements and quantify hypersensitivity due to inflammation. Our study demonstrates the potential of the CST as a standardized tool for assessing mechanical hypersensitivity in mice with minimal operator input.

Acid-sensing ion channel 3 mediates pain hypersensitivity associated with high-fat diet consumption in mice

ABSTRACT

Lipid-rich diet is the major cause of obesity, affecting 13% of the worldwide adult population. Obesity is a major risk factor for metabolic syndrome that includes hyperlipidemia and diabetes mellitus. The early phases of metabolic syndrome are often associated with hyperexcitability of peripheral small diameter sensory fibers and painful diabetic neuropathy. Here, we investigated the effect of high-fat diet-induced obesity on the activity of dorsal root ganglion (DRG) sensory neurons and pain perception. We deciphered the underlying cellular mechanisms involving the acid-sensing ion channel 3 (ASIC3). We show that mice made obese through consuming high-fat diet developed the metabolic syndrome and prediabetes that was associated with heat pain hypersensitivity, whereas mechanical sensitivity was not affected. Concurrently, the slow conducting C fibers in the skin of obese mice showed increased activity on heating, whereas their mechanosensitivity was not altered. Although ASIC3 knockout mice fed with high-fat diet became obese, and showed signs of metabolic syndrome and prediabetes, genetic deletion, and in vivo pharmacological inhibition of ASIC3, protected mice from obesity-induced thermal hypersensitivity. We then deciphered the mechanisms involved in the heat hypersensitivity of mice and found that serum from high-fat diet-fed mice was enriched in lysophosphatidylcholine (LPC16:0, LPC18:0, and LPC18:1). These enriched lipid species directly increased the activity of DRG neurons through activating the lipid sensitive ASIC3 channel. Our results identify ASIC3 channel in DRG neurons and circulating lipid species as a mechanism contributing to the hyperexcitability of nociceptive neurons that can cause pain associated with lipid-rich diet consumption and obesity.

Metformin inhibits paclitaxel-induced mechanical allodynia by activating opioidergic pathways and reducing cytokines production in the dorsal root ganglia and thalamus

It has been shown that AMP-activated protein kinase (AMPK) is involved in the nociceptive processing. This observation has prompted us to investigate the effects of the AMPK activator metformin on the paclitaxel-induced mechanical allodynia, a well-established model of neuropathic pain. Mechanical allodynia was induced by four intraperitoneal (i.p) injections of paclitaxel (2 mg/kg.day) in mice. Metformin was administered per os (p.o.). Naltrexoneandglibenclamide were used to investigate mechanisms mediating metformin activity. Concentrations of cytokines in the dorsal root ganglia (DRG) and thalamus were determined. After a single p.o. administration, the two highest doses of metformin (500 and 1000 mg/kg) attenuated the mechanical allodynia. This response was attenuated by all doses of metformin (250, 500 and 1000 mg/kg) when two administrations, 2 h apart, were carried out. Naltrexone (5 and 10 mg/kg, i.p.), but not glibenclamide (20 and 40 mg/kg, p.o.), attenuated metformin activity. Concentrations of tumor necrosis factor (TNF)-α, interleukin (IL)-1β and CXCL-1 in the DRG were increased after administration of paclitaxel. Metformin (1000 mg/kg) reduced concentrations of TNF-α, IL-1β and CXCL-1 in the DRG. Concentration of IL-6, but not TNF-α, in the thalamus was increased after administration of paclitaxel. Metformin (1000 mg/kg) reduced concentration of IL-6 in the thalamus. In summary, metformin exhibits activity in the model of neuropathic pain induced by paclitaxel. This activity may be mediated by activation of opioidergic pathways and reduced production of TNF-α, IL-1β and CXCL-1 in the DRG and IL-6 in the thalamus.

Anterior talofibular ligament's superior fascicle as a cause of ankle microinstability can be routinely identified by ultrasound

PURPOSE

Chronic pain can affect up to 40% of patients after ankle inversion sprains. The current hypothesis to explain this high percentage of chronic pain is a partial/total rupture of anterior talofibular ligament (ATFL) superior fascicle, a structure that has recently been described as intra-articular and as having a different function than ATFL's inferior fascicle. This has created the need for diagnosing ATFL superior and inferior fascicles independently. Therefore, the objective of this study is to investigate if the ATFL's superior fascicle can be visualized on ultrasound, and to describe its ultrasonographic appearance.

METHODS

Twenty fresh-frozen ankle specimens were used in this 4-phases study. First, the specimens were scanned on US to identify what was believed to be ATFL's superior fascicle. Second, ATFL's superior fascicle was sutured under direct arthroscopic vision. Next, the specimens were scanned on US to obtain an image of the sutured structure. Finally, the specimens were dissected to confirm that the suture was indeed placed on ATFL's superior fascicle.

RESULTS

On the 20 specimens studied, full correlation was obtained between US, arthroscopic suture and specimen dissection. ATFL's superior fascicle US appearance is provided.

CONCLUSION

ATFL's superior fascicle can be visualized on US, which will allow to undergo diagnosis of isolated injuries to that fascicle, a common finding in ankle microinstability. The results of this study will facilitate the diagnosis of partial or complete rupture of ATFL's superior fascicle, likely increasing the amount of ankle microinstability diagnosis, impacting clinical management of ankle sprain consequences.

Macrophages protect against sensory axon degeneration in diabetic neuropathy

Diabetic peripheral neuropathy (DPN) is a common complication of diabetes, causing sensory loss and debilitating neuropathic pain 1,2 . Although the onset and progression of DPN have been linked with dyslipidemia and hyperglycemia 3 , the contribution of inflammation in the pathogenesis of DPN has not been investigated. Here, we use a High Fat High Fructose Diet (HFHFD) to model DPN and the diabetic metabolic syndrome in mice. Diabetic mice develop persistent heat hypoalgesia after three months, but a reduction in epidermal skin innervation only manifests at 6 months. Using single-cell sequencing, we find that CCR2+ macrophages infiltrate the sciatic nerves of diabetic mice well before axonal degeneration is detectable. We show that these infiltrating macrophages share gene expression similarities with nerve crush-induced macrophages 4 and express neurodegeneration-associated microglia marker genes 5 although there is no axon loss or demyelination. Inhibiting this macrophage recruitment in diabetic mice by genetically or pharmacologically blocking CCR2 signaling results in a more severe heat hypoalgesia and accelerated skin denervation. These findings reveal a novel neuroprotective recruitment of macrophages into peripheral nerves of diabetic mice that delays the onset of terminal axonal degeneration, thereby reducing sensory loss. Potentiating and sustaining this early neuroprotective immune response in patients represents, therefore, a potential means to reduce or prevent DPN.

Suppression of neuropathic pain in the circadian clock-deficient Per2m/m mice involves up-regulation of endocannabinoid system

Neuropathic pain often results from injuries and diseases that affect the somatosensory system. Disruption of the circadian clock has been implicated in the exacerbation of the neuropathic pain state. However, in this study, we report that mice deficient in a core clock component Period2 (Per2m/m mice) fail to develop tactile pain hypersensitivity even following peripheral nerve injury. Similar to male wild-type mice, partial sciatic nerve ligation (PSL)-Per2m/m male mice showed activation of glial cells in the dorsal horn of the spinal cord and increased expression of pain-related genes. Interestingly, α1D-adrenergic receptor (α1D-AR) expression was up-regulated in the spinal cord of Per2m/m mice, leading to increased production of 2-arachidonoylglycerol (2-AG), an endocannabinoid receptor ligand. This increase in 2-AG suppressed the PSL-induced tactile pain hypersensitivity. Furthermore, intraspinal dorsal horn injection of adeno-associated viral vectors expressing α1D-AR also attenuated pain hypersensitivity in PSL-wild-type male mice by increasing 2-AG production. Our findings reveal an uncovered role of the circadian clock in neuropathic pain disorders and suggest a link between α1D-AR signaling and the endocannabinoid system.

Kilohertz high-frequency electrical stimulation ameliorate hyperalgesia by modulating transient receptor potential vanilloid-1 and N-methyl-D-aspartate receptor-2B signaling pathways in chronic constriction injury of sciatic nerve mice

The number of patients with neuropathic pain is increasing in recent years, but drug treatments for neuropathic pain have a low success rate and often come with significant side effects. Consequently, the development of innovative therapeutic strategies has become an urgent necessity. Kilohertz High Frequency Electrical Stimulation (KHES) offers pain relief without inducing paresthesia. However, the specific therapeutic effects of KHES on neuropathic pain and its underlying mechanisms remain ambiguous, warranting further investigation. In our previous study, we utilized the Gene Expression Omnibus (GEO) database to identify datasets related to neuropathic pain mice. The majority of the identified pathways were found to be associated with inflammatory responses. From these pathways, we selected the transient receptor potential vanilloid-1 (TRPV1) and N-methyl-D-aspartate receptor-2B (NMDAR2B) pathway for further exploration. Mice were randomly divided into four groups: a Sham group, a Sham/KHES group, a chronic constriction injury of the sciatic nerve (CCI) group, and a CCI/KHES stimulation group. KHES administered 30 min every day for 1 week. We evaluated the paw withdrawal threshold (PWT) and thermal withdrawal latency (TWL). The expression of TRPV1 and NMDAR2B in the spinal cord were analyzed using quantitative reverse-transcriptase polymerase chain reaction, Western blot, and immunofluorescence assay. KHES significantly alleviated the mechanical and thermal allodynia in neuropathic pain mice. KHES effectively suppressed the expression of TRPV1 and NMDAR2B, consequently inhibiting the activation of glial fibrillary acidic protein (GFAP) and ionized calcium binding adapter molecule 1 (IBA1) in the spinal cord. The administration of the TRPV1 pathway activator partially reversed the antinociceptive effects of KHES, while the TRPV1 pathway inhibitor achieved analgesic effects similar to KHES. KHES inhibited the activation of spinal dorsal horn glial cells, especially astrocytes and microglia, by inhibiting the activation of the TRPV1/NMDAR2B signaling pathway, ultimately alleviating neuropathic pain.

Neurophysiology of corneal neuropathic pain and emerging pharmacotherapeutics

The altered activity generated by corneal neuronal injury can result in morphological and physiological changes in the architecture of synaptic connections in the nervous system. These changes can alter the sensitivity of neurons (both second-order and higher-order projection) projecting pain signals. A complex process involving different cell types, molecules, nerves, dendritic cells, neurokines, neuropeptides, and axon guidance molecules causes a high level of sensory rearrangement, which is germane to all the phases in the pathomechanism of corneal neuropathic pain. Immune cells migrating to the region of nerve injury assist in pain generation by secreting neurokines that ensure nerve depolarization. Furthermore, excitability in the central pain pathway is perpetuated by local activation of microglia in the trigeminal ganglion and alterations of the descending inhibitory modulation for corneal pain arriving from central nervous system. Corneal neuropathic pain may be facilitated by dysfunctional structures in the central somatosensory nervous system due to a lesion, altered synaptogenesis, or genetic abnormality. Understanding these important pathways will provide novel therapeutic insight.

miRNA contributes to neuropathic pains

Neuropathic pain (NP) is a kind of chronic pain caused by direct injury to the peripheral or central nervous system (CNS). microRNAs (miRNAs) are small noncoding RNAs that mostly interact with the 3 untranslated region of messenger RNAs (mRNAs) to regulate the expression of multiple genes. NP is characterized by changes in the expression of receptors and mediators, and there is evidence that miRNAs may contribute to some of these alterations. In this review, we aimed to fully comprehend the connection between NP and miRNA; and also, to establish a link between neurology, biology, and dentistry. Studies have shown that targeting miRNAs may be an effective therapeutic strategy for the treatment of chronic pain and potential target for the prevention of NP.

Precision, integrative medicine for pain management in sickle cell disease

Sickle cell disease (SCD) is a prevalent and complex inherited pain disorder that can manifest as acute vaso-occlusive crises (VOC) and/or chronic pain. Despite their known risks, opioids are often prescribed routinely and indiscriminately in managing SCD pain, because it is so often severe and debilitating. Integrative medicine strategies, particularly non-opioid therapies, hold promise in safe and effective management of SCD pain. However, the lack of evidence-based methods for managing SCD pain hinders the widespread implementation of non-opioid therapies. In this review, we acknowledge that implementing personalized pain treatment strategies in SCD, which is a guideline-recommended strategy, is currently fraught with limitations. The full implementation of pharmacological and biobehavioral pain approaches targeting mechanistic pain pathways faces challenges due to limited knowledge and limited financial and personnel support. We recommend personalized medicine, pharmacogenomics, and integrative medicine as aspirational strategies for improving pain care in SCD. As an organizing model that is a comprehensive framework for classifying pain subphenotypes and mechanisms in SCD, and for guiding selection of specific strategies, we present evidence updating pain research pioneer Richard Melzack's neuromatrix theory of pain. We advocate for using the updated neuromatrix model to subphenotype individuals with SCD, to better select personalized multimodal treatment strategies, and to identify research gaps fruitful for exploration. We present a fairly complete list of currently used pharmacologic and non-pharmacologic SCD pain therapies, classified by their mechanism of action and by their hypothesized targets in the updated neuromatrix model.

Switching Rat Resident Macrophages from M1 to M2 Phenotype by Iba1 Silencing Has Analgesic Effects in SNL-Induced Neuropathic Pain

Resident macrophages from dorsal root ganglia are important for the development of traumatic-induced neuropathic pain. In the first 5-7 days after a traumatic sciatic nerve injury (i.e., spinal nerve ligation (SNL), spared nerve injury (SNI), sciatic nerve transection or sciatic nerve ligation and transection), Ionized binding adapter protein 1 (Iba1) (+) resident macrophages cluster around dorsal root ganglia neurons, possibly contributing to nerve injury-induced hypersensitivity. Since infiltrating macrophages gradually recruited to the lesion site peak at about 7 days, the first few days post-lesion offer a window of opportunity when the contribution of Iba1 (+) resident macrophages to neuropathic pain pathogenesis could be investigated. Iba1 is an actin cross-linking cytoskeleton protein, specifically located only in macrophages and microglia. In this study, we explored the contribution of rat Iba1 (+) macrophages in SNL-induced neuropathic pain by using intra-ganglionic injections of naked Iba1-siRNA, delivered at the time the lesion occurred. The results show that 5 days after Iba1 silencing, Iba1 (+) resident macrophages are switched from an M1 (pro-inflammatory) phenotype to an M2 (anti-inflammatory) phenotype, which was confirmed by a significant decrease of M1 markers (CD32 and CD86), a significant increase of M2 markers (CD163 and Arginase-1), a reduced secretion of pro-inflammatory cytokines (IL-6, TNF-α and IL-1β) and an increased release of pro-regenerative factors (BDNF, NGF and NT-3) which initiated the regrowth of adult DRG neurites and reduced SNL-induced neuropathic pain. Our data show for the first time, that it is possible to induce macrophages towards an anti-inflammatory phenotype by interacting with their cytoskeleton.

Electroacupuncture ameliorates pain in cervical spondylotic radiculopathy rat by inhibiting the CaMKII/CREB/BDNF signaling pathway and regulating spinal synaptic plasticity

BACKGROUND

Central sensitization is one of the important mechanisms underlying neuropathic and radicular pain due to cervical spondylotic radiculopathy (CSR). Recent studies have shown that the calmodulin-dependent protein kinase II (CaMKII)/cAMP-response element binding protein (CREB)/brain-derived neurotrophic factor (BDNF) signaling pathway mediates central sensitization through its involvement in spinal cord synaptic plasticity. Our group has previously found that electroacupuncture (EA) has a good analgesic effect on CSR. However, the central analgesic mechanism of EA for CSR is not yet clear.

METHODS

The rats were randomly divided into Blank group, Sham-operated group, CSR group, and EA group. We prepared the CSR rat model using the fish wire extrusion method. The behavioral and mechanical pain thresholds of the rats in each group were measured 5 days after successful modeling and 7 days after the intervention. The first intervention was started 5 days after successful modeling, and the EA group was treated by acupuncture at the bilateral LI4 and LR3 points on the same side as one group, connected to a G6805-I electroacupuncture apparatus with continuous waves at 1.5 Hz. The remaining groups were not subjected to EA intervention. The treatment was administered once a day for 7 consecutive days and then executed. We used WB, immunofluorescence, and qRT-PCR to detect the expression of CaMKII/CREB/BDNF signaling pathway-related factors in the synaptic of rat spinal cord in each group.

RESULTS

EA improved pain threshold and motor function in CSR rats, inhibited the expression of BDNF, P-TrkB, CAMKII, and P-CREB in spinal cord synapses, reduced the expression of pain factor c-fos and postsynaptic membrane protein molecule neuroligin2, exerted a modulating effect on spinal cord synaptic plasticity in CSR rats, and suppressed the overactive synaptic efficacy.

CONCLUSION

EA mediates central sensitization and exerts analgesic effects on CSR by modulating spinal synaptic plasticity, which may be related to the inhibition of CaMKII/CREB/BDNF signaling pathway.

An anchor-tether 'hindered' HCN1 inhibitor is antihyperalgesic in a rat spared nerve injury neuropathic pain model

BACKGROUND

Neuropathic pain impairs quality of life, is widely prevalent, and incurs significant costs. Current pharmacological therapies have poor/no efficacy and significant adverse effects; safe and effective alternatives are needed. Hyperpolarisation-activated cyclic nucleotide-regulated (HCN) channels are causally implicated in some forms of peripherally mediated neuropathic pain. Whilst 2,6-substituted phenols, such as 2,6-di-tert-butylphenol (26DTB-P), selectively inhibit HCN1 gating and are antihyperalgesic, the development of therapeutically tolerable, HCN-selective antihyperalgesics based on their inverse agonist activity requires that such drugs spare the cardiac isoforms and do not cross the blood-brain barrier.

METHODS

In silico molecular dynamics simulation, in vitro electrophysiology, and in vivo rat spared nerve injury methods were used to test whether 'hindered' variants of 26DTB-P (wherein a hydrophilic 'anchor' is attached in the para-position of 26DTB-P via an acyl chain 'tether') had the desired properties.

RESULTS

Molecular dynamics simulation showed that membrane penetration of hindered 26DTB-Ps is controlled by a tethered diol anchor without elimination of head group rotational freedom. In vitro and in vivo analysis showed that BP4L-18:1:1, a variant wherein a diol anchor is attached to 26DTB-P via an 18-carbon tether, is an HCN1 inverse agonist and an orally available antihyperalgesic. With a CNS multiparameter optimisation score of 2.25, a >100-fold lower drug load in the brain vs blood, and an absence of adverse cardiovascular or CNS effects, BP4L-18:1:1 was shown to be poorly CNS penetrant and cardiac sparing.

CONCLUSIONS

These findings provide a proof-of-concept demonstration that anchor-tethered drugs are a new chemotype for treatment of disorders involving membrane targets.

New perspectives in diabetic neuropathy

Diabetes prevalence continues to climb with the aging population. Type 2 diabetes (T2D), which constitutes most cases, is metabolically acquired. Diabetic peripheral neuropathy (DPN), the most common microvascular complication, is length-dependent damage to peripheral nerves. DPN pathogenesis is complex, but, at its core, it can be viewed as a state of impaired metabolism and bioenergetics failure operating against the backdrop of long peripheral nerve axons supported by glia. This unique peripheral nerve anatomy and the injury consequent to T2D underpins the distal-to-proximal symptomatology of DPN. Earlier work focused on the impact of hyperglycemia on nerve damage and bioenergetics failure, but recent evidence additionally implicates contributions from obesity and dyslipidemia. This review will cover peripheral nerve anatomy, bioenergetics, and glia-axon interactions, building the framework for understanding how hyperglycemia and dyslipidemia induce bioenergetics failure in DPN. DPN and painful DPN still lack disease-modifying therapies, and research on novel mechanism-based approaches is also covered.

Açaí berry ameliorates cognitive impairment by inhibiting NLRP3/ASC/CASP axis in STZ-induced diabetic neuropathy in mice

Diabetes complications such as diabetic peripheral neuropathy (DPN) are linked to morbidity and mortality. Peripheral nerve damages in DPN are accompanied by discomfort, weakness, and sensory loss. Some drugs may demonstrate their therapeutic promise by reducing neuroinflammation, but they have side effects. Based on these considerations, the objective of this study was to examine the beneficial properties of açaí berry in a mouse model of DPN generated by injection of streptozotocin (STZ). Açaí berry was given orally to diabetic and control mice every day beginning 2 wk after STZ injection. The animals were euthanized after 16 wk, and tissues from the spinal cord and sciatic nerve and urine were taken. Our findings showed that daily treatment of açaí berry at a dose of 500 mg/kg was able to prevent behavioral changes as well as mast cell activation and nerve deterioration via NOD-like receptor family pyrin-domain-containing-3 (NLRP3)/apoptosis-associated speck-like protein containing a card (ASC)/caspase (CASP) regulation after diabetes induction.NEW & NOTEWORTHY Our research shows that açaí berry reduces mast cells degranulation and histological damage in diabetic neuropathy, improves physiological defense against reactive oxygen species, modulates the NLRP3/ASC/CASP axis, and ameliorates inflammation and oxidative stress. Diet could help treatment for diabetic peripheral neuropathy.

Big data, big consortia, and pain: UK Biobank, PAINSTORM, and DOLORisk

Chronic pain (CP) is a common and often debilitating disorder that has major social and economic impacts. A subset of patients develop CP that significantly interferes with their activities of daily living and requires a high level of healthcare support. The challenge for treating physicians is in preventing the onset of refractory CP or effectively managing existing pain. To be able to do this, it is necessary to understand the risk factors, both genetic and environmental, for the onset of CP and response to treatment, as well as the pathogenesis of the disorder, which is highly heterogenous. However, studies of CP, particularly pain with neuropathic characteristics, have been hindered by a lack of consensus on phenotyping and data collection, making comparisons difficult. Furthermore, existing cohorts have suffered from small sample sizes meaning that analyses, especially genome-wide association studies, are insufficiently powered. The key to overcoming these issues is through the creation of large consortia such as DOLORisk and PAINSTORM and biorepositories, such as UK Biobank, where a common approach can be taken to CP phenotyping, which allows harmonisation across different cohorts and in turn increased study power. This review describes the approach that was used for studying neuropathic pain in DOLORisk and how this has informed current projects such as PAINSTORM, the rephenotyping of UK Biobank, and other endeavours. Moreover, an overview is provided of the outputs from these studies and the lessons learnt for future projects.

An update to pain management after spinal cord injury: from pharmacology to circRNAs

Neuropathic pain (NP) following a spinal cord injury (SCI) is often hard to control and therapies should be focused on the physical, psychological, behavioral, social, and environmental factors that may contribute to chronic sensory symptoms. Novel therapeutic treatments for NP management should be based on the combination of pharmacological and nonpharmacological options. Some of them are addressed in this review with a focus on mechanisms and novel treatments. Several reports demonstrated an aberrant expression of non-coding RNAs (ncRNAs) that may represent key regulatory factors with a crucial role in the pathophysiology of NP and as potential diagnostic biomarkers. This review analyses the latest evidence for cellular and molecular mechanisms associated with the role of circular RNAs (circRNAs) in the management of pain after SCI. Advantages in the use of circRNA are their stability (up to 48 h), and specificity as sponges of different miRNAs related to SCI and nerve injury. The present review discusses novel data about deregulated circRNAs (up or downregulated) that sponge miRNAs, and promote cellular and molecular interactions with mRNAs and proteins. This data support the concept that circRNAs could be considered as novel potential therapeutic targets for NP management especially after spinal cord injuries.

Novel Phenylene Lipids That Are Positive Allosteric Modulators of Glycine Receptors and Inhibitors of Glycine Transporter 2

Chronic pain is a complex condition that remains resistant to current therapeutics. We previously synthesized a series of N-acyl amino acids (NAAAs) that inhibit the glycine transporter, GlyT2, some of which are also positive allosteric modulators of glycine receptors (GlyRs). In this study, we have synthesized a library of NAAAs that contain a phenylene ring within the acyl tail with the objective of improving efficacy at both GlyT2 and GlyRs and also identifying compounds that are efficacious as dual-acting modulators to enhance glycine neurotransmission. The most efficacious positive allosteric modulator of GlyRs was 2-[8-(2-octylphenyl)octanoylamino]acetic acid (8-8 OPGly) which potentiates the EC5 for glycine activation of GlyRα1 by 1500% with an EC50 of 664 nM. Phenylene-containing NAAAs with a lysine headgroup were the most potent inhibitors of GlyT2 with (2S)-6-amino-2-[8-(3-octylphenyl)octanoylamino]hexanoic acid (8-8 MPLys) inhibiting GlyT2 with an IC50 of 32 nM. The optimal modulator across both proteins was (2S)-6-amino-2-[8-(2-octylphenyl)octanoylamino]hexanoic acid (8-8 OPLys), which inhibits GlyT2 with an IC50 of 192 nM and potentiates GlyRs by up to 335% at 1 μM. When tested in a dual GlyT2/GlyRα1 expression system, 8-8 OPLys caused the greatest reductions in the EC50 for glycine. This suggests that the synergistic effects of a dual-acting modulator cause greater enhancements in glycinergic activity compared to single-target modulators and may provide an alternate approach to the development of new non-opioid analgesics for the treatment of chronic pain.

Prolonged continuous theta burst stimulation increases motor corticospinal excitability and intracortical inhibition in patients with neuropathic pain: An exploratory, single-blinded, randomized controlled trial

OBJECTIVES

A new paradigm for Transcranial Magnetic Stimulation (TMS), referred to as prolonged continuous theta burst stimulation (pcTBS), has recently received attention in the literature because of its advantages over high frequency repetitive TMS (HF-rTMS). Clinical advantages include less time per intervention session and the effects appear to be more robust and reproducible than HF-rTMS to modulate cortical excitability. HF-rTMS targeted at the primary motor cortex (M1) has demonstrated analgesic effects in patients with neuropathic pain but their mechanisms of action are unclear and pcTBS has been studied in healthy subjects only. This study examined the neural mechanisms that have been proposed to play a role in explaining the effects of pcTBS targeted at the M1 and DLPFC brain regions in neuropathic pain (NP) patients with Type 2 diabetes.

METHODS

Forty-two patients with painful diabetic neuropathy were randomized to receive a single session of pcTBS targeted at the left M1 or left DLPFC. pcTBS stimulation consisted of 1,200 pulses delivered in 1 min and 44 s with a 35-45 min gap between sham and active pcTBS stimulation. Both the activity of the descending pain system which was examined using conditioned pain modulation and the activity of the ascending pain system which was assessed using temporal summation of pain were recorded using a handheld pressure algometer by measuring pressure pain thresholds. The amplitude of the motor evoked potential (MEP) was used to measure motor corticospinal excitability and GABA activity was assessed using short (SICI) and long intracortical inhibition (LICI). All these measurements were performed at baseline and post-pcTBS stimulation.

RESULTS

Following a single session of pcTBS targeted at M1 and DLPFC, there was no change in BPI-DN scores and on the activity of the descending (measured using conditioned pain modulation) and ascending pain systems (measured using temporal summation of pain) compared to baseline but there was a significant improvement of >13% in perception of acute pain intensity, increased motor corticospinal excitability (measured using MEP amplitude) and intracortical inhibition (measured using SICI and LICI).

CONCLUSION

In patients with NP, a single session of pcTBS targeted at the M1 and DLPFC modulated the neurophysiological mechanisms related to motor corticospinal excitability and neurochemical mechanisms linked to GABA activity, but it did not modulate the activity of the ascending and descending endogenous modulatory systems. In addition, although BPI-DN scores did not change, there was a 13% improvement in self-reported perception of acute pain intensity.

Neuropathic Pain and Spinal Cord Injury: Management, Phenotypes, and Biomarkers

Chronic neuropathic pain after a spinal cord injury (SCI) continues to be a complex condition that is difficult to manage due to multiple underlying pathophysiological mechanisms and the association with psychosocial factors. Determining the individual contribution of each of these factors is currently not a realistic goal; however, focusing on the primary mechanisms may be more feasible. One approach used to uncover underlying mechanisms includes phenotyping using pain symptoms and somatosensory function. However, this approach does not consider cognitive and psychosocial mechanisms that may also significantly contribute to the pain experience and impact treatment outcomes. Indeed, clinical experience supports that a combination of self-management, non-pharmacological, and pharmacological approaches is needed to optimally manage pain in this population. This article will provide a broad updated summary integrating the clinical aspects of SCI-related neuropathic pain, potential pain mechanisms, evidence-based treatment recommendations, neuropathic pain phenotypes and brain biomarkers, psychosocial factors, and progress regarding how defining neuropathic pain phenotypes and other surrogate measures in the neuropathic pain field may lead to targeted treatments for neuropathic pain after SCI.

In search of a composite biomarker for chronic pain by way of EEG and machine learning: where do we currently stand?

Machine learning is becoming an increasingly common component of routine data analyses in clinical research. The past decade in pain research has witnessed great advances in human neuroimaging and machine learning. With each finding, the pain research community takes one step closer to uncovering fundamental mechanisms underlying chronic pain and at the same time proposing neurophysiological biomarkers. However, it remains challenging to fully understand chronic pain due to its multidimensional representations within the brain. By utilizing cost-effective and non-invasive imaging techniques such as electroencephalography (EEG) and analyzing the resulting data with advanced analytic methods, we have the opportunity to better understand and identify specific neural mechanisms associated with the processing and perception of chronic pain. This narrative literature review summarizes studies from the last decade describing the utility of EEG as a potential biomarker for chronic pain by synergizing clinical and computational perspectives.

Effect of Type and Dose of Exercise on Neuropathic Pain After Experimental Sciatic Nerve Injury: A Preclinical Systematic Review and Meta-Analysis

This preclinical systematic review aimed to determine the effectiveness of different types and doses of exercise on pain behavior and biomarkers in preclinical models of focal neuropathic pain. We searched MEDLINE, EMBASE, Web of Science, PubMed, SCOPUS, CINAHL, and Cochrane library from inception to November 2022 for preclinical studies evaluating the effect of exercise compared to control interventions on neuropathic pain behavior after experimental sciatic nerve injury. If possible, data were meta-analyzed using random effect models with inverse-variance weighting. Thirty-seven studies were included and 26 meta-analyzed. Risk of bias (SYRCLE tool) remained unclear in most studies and reporting quality (CAMARADES) was variable. Exercise reduced mechanical (standardized mean differences [SMD] .53 (95% CI .31, .74), P = .0001, I2 = 0%, n = 364), heat (.32 (.07, .57), P = .01, I2 = 0%, n = 266) and cold hypersensitivity (.51 (.03, 1.0), P = .04, I2 = 0%, n = 90) compared to control interventions. No relationship was apparent between exercise duration or intensity and antinociception. Exercise modulated biomarkers related to different systems (eg, immune system, neurotrophins). Whereas firm conclusions are prevented by the use of male animals only, variable reporting quality and unclear risk of bias in many studies, our results suggest that aerobic exercise is a promising tool in the management of focal neuropathic pain. PERSPECTIVE: This systematic review and meta-analysis demonstrates that aerobic exercise reduces neuropathic pain-related behavior in preclinical models of sciatic nerve injury. This effect is accompanied by changes in biomarkers associated with inflammation and neurotrophins among others. These results could help to develop exercise interventions for patients with neuropathic pain.

Identification and validation of biomarkers related to Th1 cell infiltration in neuropathic pain

Neuropathic pain (NP) is a widespread chronic pain with a prevalence of 6.9-10% in the general population, severely affecting patients' physical and mental health. Accumulating evidence indicated that the immune environment is an essential factor causing NP. However, the mechanism is unclear. This study attempted to analyze NP-related immune infiltration patterns. We downloaded the expression profiles from the Gene Expression Omnibus (GEO) database. The novel method of single-sample gene set enrichment analysis (ssGSEA) algorithm and weighted gene co-expression network analysis (WGCNA) was applied to identify immune-related genes and verified in vitro and in vivo experiments. The spared nerve injury (SNI) group was closely related to type1 T helper cells (Th1 cells), and two key genes (Abca1 and Fyb) positively correlated with Th1 cell infiltration. At the single-cell level, Abca1 and Fyb were significantly expressed in macrophages. In addition, we verified that Abca1 could affect the function of macrophages. Finally, we hypothesized that Abca1 is involved in the infiltration of Th1 cells into dorsal root ganglion (DRG) tissues and induces NP via immunoinflammatory response. Hence, the present study aimed to elucidate the correlation between NP and neuroinflammation and identify a new therapeutic target for treating NP.

Glial Glutamate Transporter Modulation Prevents Development of Complete Freund's Adjuvant-Induced Hyperalgesia and Allodynia in Mice

Glial glutamate transporter (GLT-1) modulation in the hippocampus and anterior cingulate cortex (ACC) is critically involved in nociceptive pain. The objective of the study was to investigate the effects of 3-[[(2-methylphenyl) methyl] thio]-6-(2-pyridinyl)-pyridazine (LDN-212320), a GLT-1 activator, against microglial activation induced by complete Freund's adjuvant (CFA) in a mouse model of inflammatory pain. Furthermore, the effects of LDN-212320 on the protein expression of glial markers, such as ionized calcium-binding adaptor molecule 1 (Iba1), cluster of differentiation molecule 11b (CD11b), mitogen-activated protein kinases (p38), astroglial GLT-1, and connexin 43 (CX43), were measured in the hippocampus and ACC following CFA injection using the Western blot analysis and immunofluorescence assay. The effects of LDN-212320 on the pro-inflammatory cytokine interleukin-1β (IL-1β) in the hippocampus and ACC were also assessed using an enzyme-linked immunosorbent assay. Pretreatment with LDN-212320 (20 mg/kg) significantly reduced the CFA-induced tactile allodynia and thermal hyperalgesia. The anti-hyperalgesic and anti-allodynic effects of LDN-212320 were reversed by the GLT-1 antagonist DHK (10 mg/kg). Pretreatment with LDN-212320 significantly reduced CFA-induced microglial Iba1, CD11b, and p38 expression in the hippocampus and ACC. LDN-212320 markedly modulated astroglial GLT-1, CX43, and, IL-1β expression in the hippocampus and ACC. Overall, these results suggest that LDN-212320 prevents CFA-induced allodynia and hyperalgesia by upregulating astroglial GLT-1 and CX43 expression and decreasing microglial activation in the hippocampus and ACC. Therefore, LDN-212320 could be developed as a novel therapeutic drug candidate for chronic inflammatory pain.