Gene expression changes in dorsal root ganglia following peripheral nerve injury: roles in inflammation, cell death and nociception

Sciatic nerve stimulation alleviates neuropathic pain and associated neuroinflammation in the dorsal root ganglia in a rodent model

BACKGROUND

Satellite glial cells (SGCs) in the dorsal root ganglia (DRG) play a pivotal role in the formation of neuropathic pain (NP). Sciatic nerve stimulation (SNS) neuromodulation was reported to alleviate NP and reduce neuroinflammation. However, the mechanisms underlying SNS in the DRG remain unclear. This study aimed to elucidate the mechanism of electric stimulation in reducing NP, focusing on the DRG.

METHODS

L5 nerve root ligation (NRL) NP rat model was studied. Ipsilateral SNS performed 1 day after NRL. Behavioral tests were performed to assess pain phenotypes. NanoString Ncounter technology was used to explore the differentially expressed genes and cellular pathways. Activated SGCs were characterized in vivo and in vitro. The histochemical alterations of SGCs, macrophages, and neurons in DRG were examined in vivo on post-injury day 8.

RESULTS

NRL induced NP behaviors including decreased pain threshold and latency on von Frey and Hargreaves tests. We found that following nerve injury, SGCs were hyperactivated, neurotoxic and had increased expression of NP-related ion channels including TRPA1, Cx43, and SGC-neuron gap junctions. Mechanistically, nerve injury induced reciprocal activation of SGCs and M1 macrophages via cytokines including IL-6, CCL3, and TNF-α mediated by the HIF-1α-NF-κB pathways. SNS suppressed SGC hyperactivation, reduced the expression of NP-related ion channels, and induced M2 macrophage polarization, thereby alleviating NP and associated neuroinflammation in the DRG.

CONCLUSIONS

NRL induced hyperactivation of SGCs, which had increased expression of NP-related ion channels. Reciprocal activation of SGCs and M1 macrophages surrounding the primary sensory neurons was mediated by the HIF-1α and NF-κB pathways. SNS suppressed SGC hyperactivation and skewed M1 macrophage towards M2. Our findings establish SGC activation as a crucial pathomechanism in the gliopathic alterations in NP, which can be modulated by SNS neuromodulation.

Potassium channel modulation in macrophages sensitizes dorsal root ganglion neurons after nerve injury

Macrophages and satellite glial cells are found between injured and uninjured neurons in the lumbar dorsal root ganglia (DRG). We explored the mechanism of neuro-immune and neuron-glia crosstalk leading to hyperexcitability of DRG neurons. After spared nerve injury (SNI), CX3CR1+ resident macrophages became activated, proliferated, and increased inward-rectifying potassium channel Kir 2.1 currents. Conditioned medium (CM) by macrophages, obtained from DRG of SNI mice, sensitized small DRG neurons from naïve mice. However, treatment with CM from GFAP+ glial cells did not affect neuronal excitability. When subjected to this macrophage-derived CM, DRG neurons had increased spontaneous activity, current-evoked responses and voltage-gated NaV 1.7 and NaV 1.8 currents. Silencing Kir 2.1 in macrophages after SNI prevented the induction of neuronal hyperexcitability from their CM. Blocking vesicular exocytosis or soluble tumor necrosis factor in CM or interfering with the downstream intracellular p38 pathway in neurons, also prevented neuronal hyperexcitability. Blocking protein trafficking in neurons reduced the effect of CM, suggesting that the hyperexcitable state resulted from changes in NaV channel trafficking. These results suggest that DRG macrophages, primed by peripheral nerve injury, contribute to neuron-glia crosstalk, NaV channel dysregulation and neuronal hyperexcitability implicated in the development of neuropathic pain.

Mechanisms of Action of Dorsal Root Ganglion Stimulation

The dorsal root ganglion (DRG) serves as a pivotal site for managing chronic pain through dorsal root ganglion stimulation (DRG-S). In recent years, the DRG-S has emerged as an attractive modality in the armamentarium of neuromodulation therapy due to its accessibility and efficacy in alleviating chronic pain refractory to conventional treatments. Despite its therapeutic advantages, the precise mechanisms underlying DRG-S-induced analgesia remain elusive, attributed in part to the diverse sensory neuron population within the DRG and its modulation of both peripheral and central sensory processing pathways. Emerging evidence suggests that DRG-S may alleviate pain by several mechanisms, including the reduction of nociceptive signals at the T-junction of sensory neurons, modulation of pain gating pathways within the dorsal horn, and regulation of neuronal excitability within the DRG itself. However, elucidating the full extent of DRG-S mechanisms necessitates further exploration, particularly regarding its supraspinal effects and its interactions with cognitive and affective networks. Understanding these mechanisms is crucial for optimizing neurostimulation technologies and improving clinical outcomes of DRG-S for chronic pain management. This review provides a comprehensive overview of the DRG anatomy, mechanisms of action of the DRG-S, and its significance in neuromodulation therapy for chronic pain.

The effects of Garcinia kola and curcumin on the dorsal root ganglion of the diabetic rat after peripheral nerve transection injury

OBJECTIVE

To test the protective effects of Garcinia kola and curcumin on the ganglion tissues of diabetic rats following the use of autologous vein graft in peripheral nerve transection injury.

METHODS

The sciatic nerve on the right side was transected, and anastomosis was performed between the proximal and distal ends using an autologous vein graft. Curcumin and Garcinia kola seed extract were administered daily by oral gavage. The ganglion tissues were harvested after a 90-day waiting period. Sensory neurons in the dorsal root ganglion at the L4 and L5 levels were used for stereological evaluations. Mean sensory neuron numbers were analyzed using a stereological technique. The size of the light and dark neurons was also estimated, and ultrastructural and immunohistochemical evaluations were performed.

RESULTS

A statistically significant difference in sensory neuron numbers was observed between the groups with and without Garcinia kola and curcumin applications. The immunohistochemical results showed that the s-100 protein is expressed selectively between cell types.

CONCLUSION

The results of this study show that curcumin and Garicinia kola prevented sensory neuron loss in diabetic rats following transection injury to the sciatic nerve.

DLK signaling in axotomized neurons triggers complement activation and loss of upstream synapses

Axotomized spinal motoneurons (MNs) lose presynaptic inputs following peripheral nerve injury; however, the cellular mechanisms that lead to this form of synapse loss are currently unknown. Here, we delineate a critical role for neuronal kinase dual leucine zipper kinase (DLK)/MAP3K12, which becomes activated in axotomized neurons. Studies with conditional knockout mice indicate that DLK signaling activation in injured MNs triggers the induction of phagocytic microglia and synapse loss. Aspects of the DLK-regulated response include expression of C1q first from the axotomized MN and then later in surrounding microglia, which subsequently phagocytose presynaptic components of upstream synapses. Pharmacological ablation of microglia inhibits the loss of cholinergic C boutons from axotomized MNs. Together, the observations implicate a neuronal mechanism, governed by the DLK, in the induction of inflammation and the removal of synapses.

The dorsal root ganglion as a target for neurorestoration in neuropathic pain

Neuropathic pain is a severe and chronic condition widely found in the general population. The reason for this is the extensive variety of damage or diseases that can spark this unpleasant constant feeling in patients. During the processing of pain, the dorsal root ganglia constitute an important region where dorsal root ganglion neurons play a crucial role in the transmission and propagation of sensory electrical stimulation. Furthermore, the dorsal root ganglia have recently exhibited a regenerative capacity that should not be neglected in the understanding of the development and resolution of neuropathic pain and in the elucidation of innovative therapies. Here, we will review the complex interplay between cells (satellite glial cells and inflammatory cells) and factors (cytokines, neurotrophic factors and genetic factors) that takes place within the dorsal root ganglia and accounts for the generation of the aberrant excitation of primary sensory neurons occurring in neuropathic pain. More importantly, we will summarize an updated view of the current pharmacologic and nonpharmacologic therapies targeting the dorsal root ganglia for the treatment of neuropathic pain.

The CSF1-CSF1R pathway in the trigeminal ganglion mediates trigeminal neuralgia via inflammatory responses in mice

BACKGROUND

Trigeminal neuralgia (TN) is the most severe type of neuropathic pain. The trigeminal ganglion (TG) is a crucial target for the pathogenesis and treatment of TN. The colony-stimulating factor 1 (CSF1) - colony-stimulating factor 1 receptor (CSF1R) pathway regulates lower limb pain development. However, the effect and mechanism of the CSF1-CSF1R pathway in TG on TN are unclear.

METHODS

Partial transection of the infraorbital nerve (pT-ION) model was used to generate a mouse TN model. Mechanical and cold allodynia were used to measure pain behaviors. Pro-inflammatory factors (IL-6, TNF-a) were used to measure inflammatory responses in TG. PLX3397, an inhibitor of CSF1R, was applied to inhibit the CSF1-CSF1R pathway in TG. This pathway was activated in naïve mice by stereotactic injection of CSF1 into the TG.

RESULTS

The TN model activated the CSF1-CSF1R pathway in the TG, leading to exacerbated mechanical and cold allodynia. TN activated inflammatory responses in the TG manifested as a significant increase in IL-6 and TNF-a levels. After using PLX3397 to inhibit CSF1R, CSF1R expression in the TG declined significantly. Inhibiting the CSF1-CSF1R pathway in the TG downregulated the expression of IL-6 and TNF-α to reduce allodynia-related behaviors. Finally, mechanical allodynia behaviors were exacerbated in naïve mice after activating the CSF1-CSF1R pathway in the TG.

CONCLUSIONS

The CSF1-CSF1R pathway in the TG modulates TN by regulating neuroimmune responses. Our findings provide a theoretical basis for the development of treatments for TN in the TG.

Peripheral Branch Injury Induces Oxytocin Receptor Expression at the Central Axon Terminals of Primary Sensory Neurons

Considerable evidence suggests that oxytocin, as a regulatory nonapeptide, participates in modulatory mechanisms of nociception. Nonetheless, the role of this hypothalamic hormone and its receptor in the sensory pathway has yet to be fully explored. The present study performed immunohistochemistry, enzyme-linked immunosorbent assay, and RT-qPCR analysis to assess changes in the expression of the neuronal oxytocin receptor in female rats following tight ligation of the sciatic nerve after 1, 3, and 7 days of survival. Oxytocin receptor immunoreactivity was present in both dorsal root ganglia and lumbar spinal cord segments, but not accumulated at the site of the ligation of the peripheral nerve branch. We found a time-dependent change in the expression of oxytocin receptor mRNA in L5 dorsal root ganglion neurons, as well as an increase in the level of the receptor protein in the lumbar segment of the spinal cord. A peak in the expression was observed on day 3, which downturned slightly by day 7 after the nerve ligation. These results show that OTR expression is up-regulated in response to peripheral nerve lesions. We assume that the importance of OTR is to modify spinal presynaptic inputs of the sensory neurons upon injury-induced activation, thus to be targets of the descending oxytocinergic neurons from supraspinal levels. The findings of this study support the concept that oxytocin plays a role in somatosensory transmission.

Five Reasons Why Some Herniated Intervertebral Discs are Painless

Herniated intervertebral discs (HIVDs) can cause lower back pain and leg pain, but not all cases are painful. We suggest five potential reasons for painless HIVDs. First, when inflammation resolves or is insufficient, pain may not occur. Second, rare cases exhibit motor or sensory deficits without pain, suggesting a mechanical rather than inflammatory component. Third, changes in gene expression within the dorsal root ganglia may prevent persistent inflammation and pain. Fourth, reorganization of neural circuits in the spinal dorsal horn can reduce nociceptive signals and lead to painlessness. Fifth, effective pain coping strategies and higher pain tolerance may mask any discomfort. Understanding these factors can improve clinicians' knowledge and aid in pain management.

Scientific and Regulatory Policy Committee Technical Review: Biology and Pathology of Ganglia in Animal Species Used for Nonclinical Safety Testing

Dorsal root ganglia (DRG), trigeminal ganglia (TG), other sensory ganglia, and autonomic ganglia may be injured by some test article classes, including anti-neoplastic chemotherapeutics, adeno-associated virus-based gene therapies, antisense oligonucleotides, nerve growth factor inhibitors, and aminoglycoside antibiotics. This article reviews ganglion anatomy, cytology, and pathology (emphasizing sensory ganglia) among common nonclinical species used in assessing product safety for such test articles (TAs). Principal histopathologic findings associated with sensory ganglion injury include neuron degeneration, necrosis, and/or loss; increased satellite glial cell and/or Schwann cell numbers; and leukocyte infiltration and/or inflammation. Secondary nerve fiber degeneration and/or glial reactions may occur in nerves, dorsal spinal nerve roots, spinal cord (dorsal and occasionally lateral funiculi), and sometimes the brainstem. Ganglion findings related to TA administration may result from TA exposure and/or trauma related to direct TA delivery into the central nervous system or ganglia. In some cases, TA-related effects may need to be differentiated from a spectrum of artifactual and/or spontaneous background changes.

Adipose-derived stem cells modulate neuroinflammation and improve functional recovery in chronic constriction injury of the rat sciatic nerve

Introduction

Compressive neuropathy, a common chronic traumatic injury of peripheral nerves, leads to variable impairment in sensory and motor function. Clinical symptoms persist in a significant portion of patients despite decompression, with muscle atrophy and persistent neuropathic pain affecting 10%-25% of cases. Excessive inflammation and immune cell infiltration in the injured nerve hinder axon regeneration and functional recovery. Although adipose-derived stem cells (ASCs) have demonstrated neural regeneration and immunomodulatory potential, their specific effects on compressive neuropathy are still unclear.

Methods

We conducted modified CCI models on adult male Sprague-Dawley rats to induce irreversible neuropathic pain and muscle atrophy in the sciatic nerve. Intraneural ASC injection and nerve decompression were performed. Behavioral analysis, muscle examination, electrophysiological evaluation, and immunofluorescent examination of the injured nerve and associated DRG were conducted to explore axon regeneration, neuroinflammation, and the modulation of inflammatory gene expression. Transplanted ASCs were tracked to investigate potential beneficial mechanisms on the local nerve and DRG.

Results

Persistent neuropathic pain was induced by chronic constriction of the rat sciatic nerve. Local ASC treatment has demonstrated robust beneficial outcomes, including the alleviation of mechanical allodynia, improvement of gait, regeneration of muscle fibers, and electrophysiological recovery. In addition, locally transplanted ASCs facilitated axon remyelination, alleviated neuroinflammation, and reduced inflammatory cell infiltration of the injured nerve and associated dorsal root ganglion (DRG). Trafficking of the transplanted ASC preserved viability and phenotype less than 7 days but contributed to robust immunomodulatory regulation of inflammatory gene expression in both the injured nerve and DRG.

Discussion

Locally transplanted ASC on compressed nerve improve sensory and motor recoveries from irreversible chronic constriction injury of rat sciatic nerve via alleviation of both local and remote neuroinflammation, suggesting the promising role of adjuvant ASC therapies for clinical compressive neuropathy.

Systemic and Peripheral Mechanisms of Cortical Stimulation-Induced Analgesia and Refractoriness in a Rat Model of Neuropathic Pain

Epidural motor cortex stimulation (MCS) is an effective treatment for refractory neuropathic pain; however, some individuals are unresponsive. In this study, we correlated the effectiveness of MCS and refractoriness with the expression of cytokines, neurotrophins, and nociceptive mediators in the dorsal root ganglion (DRG), sciatic nerve, and plasma of rats with sciatic neuropathy. MCS inhibited hyperalgesia and allodynia in two-thirds of the animals (responsive group), and one-third did not respond (refractory group). Chronic constriction injury (CCI) increased IL-1β in the nerve and DRG, inhibited IL-4, IL-10, and IL-17A in the nerve, decreased β-endorphin, and enhanced substance P in the plasma, compared to the control. Responsive animals showed decreased NGF and increased IL-6 in the nerve, accompanied by restoration of local IL-10 and IL-17A and systemic β-endorphin. Refractory animals showed increased TNF-α and decreased IFNγ in the nerve, along with decreased TNF-α and IL-17A in the DRG, maintaining low levels of systemic β-endorphin. Our findings suggest that the effectiveness of MCS depends on local control of inflammatory and neurotrophic changes, accompanied by recovery of the opioidergic system observed in neuropathic conditions. So, understanding the refractoriness to MCS may guide an improvement in the efficacy of the technique, thus benefiting patients with persistent neuropathic pain.

Thiamine as a peripheral neuro-protective agent in comparison with N-acetyl cysteine in axotomized rats

Objectives

In this study, the impact of thiamine (Thi), N-acetyl cysteine (NAC), and dexamethasone (DEX) were investigated in axotomized rats, as a model for neural injury.

Materials and Methods

Sixty-five axotomized rats were divided into two different experimental approaches, the first experiments included five study groups (n=5): intrathecal Thi (Thi.it), intraperitoneal (Thi), NAC, DEX, and control. Cell survival was assessed in L5DRG in the 4^th week by histological assessment. In the second study, 40 animals were engaged to assess Bcl-2, Bax, IL-6, and TNF-α expression in L4-L5DRG in the 1^st and 2^nd weeks after sural nerve axotomy under treatment of these agents (n=10).

Results

Ghost cells were observed in morphological assessment of L5DRG sections, and following stereological analysis, the volume and neuronal cell counts significantly were improved in the NAC and Thi.it groups in the 4^th week (P<0.05). Although Bcl-2 expression did not show significant differences, Bax was reduced in the Thi group (P=0.01); and the Bcl-2/Bax ratio increased in the NAC group (1^st week, P<0.01). Furthermore, the IL-6 and TNF-α expression decreased in the Thi and NAC groups, on the 1^st week of treatment (P≤0.05 and P<0.01). However, in the 2^nd week, the IL-6 expression in both Thi and NAC groups (P<0.01), and the TNF-α expression in the DEX group (P=0.05) were significantly decreased.

Conclusion

The findings may classify Thi in the category of peripheral neuroprotective agents, in combination with routine medications. Furthermore, it had strong cell survival effects as it could interfere with the destructive effects of TNF-α by increasing Bax.

Microglia-independent peripheral neuropathic pain in male and female mice

ABSTRACT

The dominant view in the field of pain is that peripheral neuropathic pain is driven by microglia in the somatosensory processing region of the spinal dorsal horn. Here, to the contrary, we discovered a form of neuropathic pain that is independent of microglia. Mice in which the nucleus pulposus (NP) of the intervertebral disc was apposed to the sciatic nerve developed a constellation of neuropathic pain behaviours: hypersensitivity to mechanical, cold, and heat stimuli. However, NP application caused no activation of spinal microglia nor was pain hypersensitivity reversed by microglial inhibition. Rather, NP-induced pain hypersensitivity was dependent on cells within the NP which recruited macrophages to the adjacent nerve. Eliminating macrophages systemically or locally prevented NP-induced pain hypersensitivity. Pain hypersensitivity was also prevented by genetically disrupting the neurotrophin brain-derived neurotrophic factor selectively in macrophages. Moreover, the behavioural phenotypes as well as the molecular mechanisms of NP-induced pain hypersensitivity were not different between males and females. Our findings reveal a previously unappreciated mechanism for by which a discrete peripheral nerve lesion may produce pain hypersensitivity, which may help to explain the limited success of microglial inhibitors on neuropathic pain in human clinical trials.

Electroceuticals for peripheral nerve regeneration

Electroceuticals provide promising opportunities for peripheral nerve regeneration, in terms of modulating the extensive endogenous tissue repair mechanisms between neural cell body, axons and target muscles. However, great challenges remain to deliver effective and controllable electroceuticals via bioelectronic implantable device. In this review, the modern fabrication methods of bioelectronic conduit for bridging critical nerve gaps after nerve injury are summarized, with regard to conductive materials and core manufacturing process. In addition, to deliver versatile electrical stimulation, the integration of implantable bioelectronic device is discussed, including wireless energy harvesters, actuators and sensors. Moreover, a comprehensive insight of beneficial mechanisms is presented, including up-to-date in vitro, in vivo and clinical evidence. By integrating conductive biomaterials, 3D engineering manufacturing process and bioelectronic platform to deliver versatile electroceuticals, the modern biofabrication enables comprehensive biomimetic therapies for neural tissue engineering and regeneration in the new era.

E2F1 Expression and Apoptosis Initiation in Crayfish and Rat Peripheral Neurons and Glial Cells after Axonal Injury

Neurotrauma is among the main causes of human disability and mortality. The transcription factor E2F1 is one of the key proteins that determine the fate of cells. The involvement of E2F1 in the regulation of survival and death of peripheral nerve cells after axotomy has not been previously studied. We, for the first time, studied axotomy-induced changes in the expression and localization of E2F1 following axonal injury in rats and crayfish. Immunoblotting and immunofluorescence microscopy were used for the analysis of the expression and intracellular localization of E2F1 and its changes after axotomy. To evaluate whether this transcription factor promotes cell apoptosis, we examined the effect of pharmacological inhibition of E2F activity in axotomized rat models. In this work, axotomy caused increased expression of E2F1 as early as 4 h and even 1 h after axotomy of mechanoreceptor neurons and ganglia of crayfish ventral nerve cord (VNC), as well as rat dorsal root ganglia (DRG). The level of E2F1 expression increased both in the cytoplasm and the nuclei of neurons. Pharmacological inhibition of E2F demonstrated a pronounced neuroprotective activity against axotomized DRGs. E2F1 and downstream targets could be considered promising molecular targets for the development of potential neuroprotective agents.

Transcriptomic analysis of human sensory neurons in painful diabetic neuropathy reveals inflammation and neuronal loss

Pathological sensations caused by peripheral painful neuropathy occurring in Type 2 diabetes mellitus (T2DM) are often described as 'sharp' and 'burning' and are commonly spontaneous in origin. Proposed etiologies implicate dysfunction of nociceptive sensory neurons in dorsal root ganglia (DRG) induced by generation of reactive oxygen species, microvascular defects, and ongoing axonal degeneration and regeneration. To investigate the molecular mechanisms contributing to diabetic pain, DRGs were acquired postmortem from patients who had been experiencing painful diabetic peripheral neuropathy (DPN) and subjected to transcriptome analyses to identify genes contributing to pathological processes and neuropathic pain. DPN occurs in distal extremities resulting in the characteristic "glove and stocking" pattern. Accordingly, the L4 and L5 DRGs, which contain the perikarya of primary afferent neurons innervating the foot, were analyzed from five DPN patients and compared with seven controls. Transcriptome analyses identified 844 differentially expressed genes. We observed increases in levels of inflammation-associated transcripts from macrophages in DPN patients that may contribute to pain hypersensitivity and, conversely, there were frequent decreases in neuronally-related genes. The elevated inflammatory gene profile and the accompanying downregulation of multiple neuronal genes provide new insights into intraganglionic pathology and mechanisms causing neuropathic pain in DPN patients with T2DM.

The Antinociceptive Potential of Camellia japonica Leaf Extract, (−)-Epicatechin, and Rutin against Chronic Constriction Injury-Induced Neuropathic Pain in Rats

Neuropathic pain is caused by a lesion or disease of the somatosensory nervous system. Currently, prescribed treatments are still unsatisfactory or have limited effectiveness. Camellia japonica leaves are known to have antioxidant and anti-inflammatory properties.; however, their antinociceptive efficacy has not yet been explored. We examined the antinociceptive efficacy and underlying mechanism of C. japonica leaf extract (CJE) in chronic constriction injury (CCI)-induced neuropathic pain models. To test the antinociceptive activity of CJE, three types of allodynia were evaluated: punctate allodynia using von Frey filaments, dynamic allodynia using a paintbrush and cotton swab, and cold allodynia using a cold plate test. CCI rats developed neuropathic pain representing increases in the three types of allodynia and spontaneous pain. In addition, CCI rats showed high phosphorylation levels of mitogen-activated protein kinases (MAPKs), transcription factors, and nociceptive mediators in dorsal root ganglion (DRG). The ionized calcium-binding adapter molecule 1 levels and neuroinflammation also increased following CCI surgery in the spinal cord. CJE and its active components have potential antinociceptive effects against CCI-induced neuropathic pain that might be mediated by MAPK activation in the DRG and microglial activation in the spinal cord. These findings suggest that CJE, (−)-epicatechin, and rutin could be novel candidates for neuropathic pain management.

A Brief Review of In Vitro Models for Injury and Regeneration in the Peripheral Nervous System

Nerve axonal injury and associated cellular mechanisms leading to peripheral nerve damage are important topics of research necessary for reducing disability and enhancing quality of life. Model systems that mimic the biological changes that occur during human nerve injury are crucial for the identification of cellular responses, screening of novel therapeutic molecules, and design of neural regeneration strategies. In addition to in vivo and mathematical models, in vitro axonal injury models provide a simple, robust, and reductionist platform to partially understand nerve injury pathogenesis and regeneration. In recent years, there have been several advances related to in vitro techniques that focus on the utilization of custom-fabricated cell culture chambers, microfluidic chamber systems, and injury techniques such as laser ablation and axonal stretching. These developments seem to reflect a gradual and natural progression towards understanding molecular and signaling events at an individual axon and neuronal-soma level. In this review, we attempt to categorize and discuss various in vitro models of injury relevant to the peripheral nervous system and highlight their strengths, weaknesses, and opportunities. Such models will help to recreate the post-injury microenvironment and aid in the development of therapeutic strategies that can accelerate nerve repair.

Identification and validation of MicroRNA-mRNA Networks in Dorsal Root Ganglia after Peripheral Nerve Injury

Changes in DRG after nerve injury involve neuronal damage, apoptosis, pain transmission, and activation of regenerative programs. It is unclear which genes and microRNAs may play a major role in this process. Therefore, this study performed a meta-analysis of previously published gene expression data to reveal the potential microRNA-mRNA network in dorsal root ganglia (DRG) after peripheral nerve injury. We searched 5 mRNA and 3 microRNA expression data sets, obtained 447 differentially expressed genes (DEGs) and 5 differentially expressed miRNAs, determined the biological pathways enriched by these DEGs, and further predicted new microRNA-mRNA interactions, such as miR-21/Hmg20a, miR-221/Ube2ql1, miR-30c-1/Rhoq, miR-500/Sema3c, and miR-551b/Cdc42se2. We verified these hub mRNA and miRNA in rats by qRT-PCR and found the results were consistent with the bioinformatics analysis. And we predicted transcription factors associated with these genes (gTFs) and TFs associated with these microRNAs (mTFs) and constructed the mTF-miRNA-gene-gTF regulatory network to further explore the molecular mechanism in DRG. Finally, we compared the DRG transcriptome after PNI to that of chronic constriction injury (CCI), and found that PNI caused greater damage to DRG compared to CCI. At the same time, the related mechanisms of pain caused by the two pathophysiological process may be different.

Direct Inhibition of Microglia Activation by Pretreatment With Botulinum Neurotoxin A for the Prevention of Neuropathic Pain

Peripheral injection of botulinum neurotoxin A (BoNT/A) has been demonstrated to have a long-term analgesic effect in treating neuropathic pain. Around peripheral nerves, BoNT/A is taken up by primary afferent neurons and inhibits neuropeptide release. Moreover, BoNT/A could also be retrogradely transported to the spinal cord. Recent studies have suggested that BoNT/A could attenuates neuropathic pain by inhibiting the activation of spinal glial cells. However, it remains unclear whether BoNT/A directly interacts with these glial cells or via their interaction with neurons. Our aim here is to determine the direct effect of BoNT/A on primary microglia and astrocytes. We show that BoNT/A pretreatment significantly inhibits lipopolysaccharide (LPS) -induced activation and pro-inflammatory cytokine release in primary microglia (1 U/mL BoNT/A in medium), while it has no effect on the activation of astrocytes (2 U/mL BoNT/A in medium). Moreover, a single intrathecal pre-administration of a low dose of BoNT/A (1 U/kg) significantly prohibited the partial sciatic nerve ligation (PSNL)- induced upregulation of pro-inflammatory cytokines in both the spinal cord dorsal horn and dorsal root ganglions (DRGs), which in turn prevented the PSNL-induced mechanical allodynia and thermal hyperalgesia. In conclusion, our results indicate that BoNT/A pretreatment prevents PSNL-induced neuropathic pain by direct inhibition of spinal microglia activation.

Postoperative pain and the gut microbiome

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Poorly controlled postoperative pain remains a major unresolved challenge globally.

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The gut microbiome impacts on inflammatory pain and neuropathic pain.

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Microbiota metabolites can regulate peripheral and central sensitisation.

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Stress is linked to both postoperative pain and an altered gut microbiome.

In excess of 300 million surgical procedures are undertaken worldwide each year. Despite recognition of the prevalence of postoperative pain, and improvements in pain management techniques, poorly controlled postoperative pain remains a major unresolved challenge globally. An estimated 71% and 51% of patients experience moderate to severe pain after surgery in in-patient and outpatient settings, respectively. Inadequately controlled pain after surgery is associated with significant perioperative morbidity including myocardial infarction and pulmonary complications.

As many as 20–56% of patients develop chronic pain after commonly performed procedures such as hernia repair, hysterectomy, and thoracotomy. Traditional analgesics and interventions are often ineffective or partially effective in the treatment of postoperative pain, resulting in a chronic pain condition with related socio-economic impacts and reduced quality of life for the patient. Such chronic pain which occurs after surgery is referred to as Persistent Post-Surgical Pain (PPSP). The complex ecosystem that is the gastrointestinal microbiota (including bacteria, fungi, viruses, phage) plays essential roles in the maintenance of the healthy state of the host. A disruption to the balance of this microbiome has been implicated not only in gastrointestinal disease but also neurological disorders including chronic pain. The influence of the gut microbiome is well documented in the context of visceral pain from the gastrointestinal tract while a greater understanding is emerging of the impact on inflammatory pain and neuropathic pain (both of which can occur during the perioperative period). The gut microbiome is an essential source for driving immune maturation and maintaining appropriate immune response. Given that inflammatory processes have been implicated in postoperative pain, aberrant microbiome profiles may play a role in the development of this type of pain. Furthermore, the microorganisms in our gut produce metabolites, neurotransmitters, and neuromodulators which interact with their receptors to regulate peripheral and central sensitisation associated with chronic pain. Microbiota-derived mediators can also regulate neuroinflammation, which is associated with activation of microglia as well as infiltration by immune cells, known to modulate the development and maintenance of central sensitisation. Moreover, risk factors for developing postoperative pain include anxiety, depression, and increased stress response. These central nervous system-related disorders have been associated with an altered gut microbiome and microbiome targeted intervention studies indicate improvements. Females are more likely to suffer from postoperative pain. As gonadal hormones are associated with a differential microbiome and pre-clinical studies show that male microbiome confers protection from inflammatory pain, it is possible that the composition of the microbiome and its by-products contribute to the increased risk for the development of postoperative pain. Very little evidence exists relating the microbiome to somatic pain. Here we discuss the potential role of the gut microbiome in the aetiology and pathophysiology of postoperative pain in the context of other somatic pain syndromes and what is known about microbe-neuron interactions. Investigations are needed to determine the specific role of the gut microbiome in this type of pain which may help inform the development of preventative interventions as well as management strategies to improve patient outcome.

C-terminal domain small phosphatase 1 (CTDSP1) regulates growth factor expression and axonal regeneration in peripheral nerve tissue

Peripheral Nerve Injury (PNI) represents a major clinical and economic burden. Despite the ability of peripheral neurons to regenerate their axons after an injury, patients are often left with motor and/or sensory disability and may develop chronic pain. Successful regeneration and target organ reinnervation require comprehensive transcriptional changes in both injured neurons and support cells located at the site of injury. The expression of most of the genes required for axon growth and guidance and for synapsis formation is repressed by a single master transcriptional regulator, the Repressor Element 1 Silencing Transcription factor (REST). Sustained increase of REST levels after injury inhibits axon regeneration and leads to chronic pain. As targeting of transcription factors is challenging, we tested whether modulation of REST activity could be achieved through knockdown of carboxy-terminal domain small phosphatase 1 (CTDSP1), the enzyme that stabilizes REST by preventing its targeting to the proteasome. To test whether knockdown of CTDSP1 promotes neurotrophic factor expression in both support cells located at the site of injury and in peripheral neurons, we transfected mesenchymal progenitor cells (MPCs), a type of support cells that are present at high concentrations at the site of injury, and dorsal root ganglion (DRG) neurons with REST or CTDSP1 specific siRNA. We quantified neurotrophic factor expression by RT-qPCR and Western blot, and brain-derived neurotrophic factor (BDNF) release in the cell culture medium by ELISA, and we measured neurite outgrowth of DRG neurons in culture. Our results show that CTDSP1 knockdown promotes neurotrophic factor expression in both DRG neurons and the support cells MPCs, and promotes DRG neuron regeneration. Therapeutics targeting CTDSP1 activity may, therefore, represent a novel epigenetic strategy to promote peripheral nerve regeneration after PNI by promoting the regenerative program repressed by injury-induced increased levels of REST in both neurons and support cells.

Allantoin, a purine metabolite, enhances peripheral nerve regeneration following sciatic nerve injury in rats: A stereological and immunohistochemical study

AIM

Following peripheral nerve injury, in addition to axonal and myelin degeneration, a sharp increase is observed in cell numbers, especially Schwann cells, in the distal part of the injury. This study investigated the effect of allantoin, involved in purine catabolism, on the reactions occurring in the lesion area.

MATERIAL AND METHOD

An experimental sciatic nerve injury model was established with the application of pressure at 50 Newtons for 5 s to the right sciatic nerves of experimental animals following visualization with the help of pliers. Allantoin was administered to the test groups via the intraperitoneal (i.p.) route (10 mg/kg), at the same time every day for 30 days. The animals were sacrificed at the end of 30 days, following electromyography and Sciatic Function Index tests. Myelinated/unmyelinated axon numbers were evaluated stereologically. Myelin sheath thickness, axon diameter, mitotic activity, and functional improvement in muscles in this peripheral nerve degeneration model were investigated. The test results were then subjected to statistical analysis.

RESULTS

Allantoin was observed to exhibit curative effects in terms of function, although stereological tests revealed no morphological differences.

CONCLUSION

The i.p. administration of allantoin may have a beneficial effect on nerve healing.

Subcellular localization of neuronal nuclei (NeuN) antigen in size and calcitonin gene-related peptide (CGRP) populations of dorsal root ganglion (DRG) neurons during acute peripheral inflammation

Pseudo-unipolar cell bodies of somatosensory primary neurons are located in the dorsal root ganglia (DRG). The somatic and peripheral domains of DRG neurons are often studied in sensory pain research to understand molecular mechanisms involved in the activation of pain and maintenance of inflammation. Adjuvant-induced arthritis (AIA) is an inflammatory model that elicits a robust and rapid onset immune response with a maximal swelling period of 24-48 h and persisting for several weeks. The AIA model in the hind paw of the rat elicits a potent inflammatory response of the dermis and epidermis, leading to protein expression changes for sensitization of many DRG neurons; however, it is unknown if the AIA model in the hind paw of the rat induces DRG neuronal injury, necrosis, or apoptosis at the somatic level. Neuronal nuclei (NeuN) antigen is a biomarker for post-mitotic neurons, neuronal identification, protein alterations, injury, and loss. Calcitonin gene-related peptide (CGRP) is expressed in C and Aδ DRG neurons, a subset of DRG neurons known to play a role in peripheral sensitization. The focus of this research was to evaluate the expression pattern of NeuN immunoreactivity, in size (soma) and CGRP subpopulations of DRG neurons in naïve and inflamed groups. Confirmed by both immunofluorescence and immunoprecipitation, DRG neuronal expression of NeuN was localized to nuclear and cytoplasmic subcellular compartments. NeuN increased within the nucleus of small CGRP positive DRG neurons during inflammation, indicating a potential role for NeuN in a subset of nociceptive neurons.

Investigation of Neuropathology after Nerve Release in Chronic Constriction Injury of Rat Sciatic Nerve

Peripheral compressive neuropathy causes significant neuropathic pain, muscle weakness and prolong neuroinflammation. Surgical decompression remains the gold standard of treatment but the outcome is suboptimal with a high recurrence rate. From mechanical compression to chemical propagation of the local inflammatory signals, little is known about the distinct neuropathologic patterns and the genetic signatures after nerve decompression. In this study, controllable mechanical constriction forces over rat sciatic nerve induces irreversible sensorimotor dysfunction with sustained local neuroinflammation, even 4 weeks after nerve release. Significant gene upregulations are found in the dorsal root ganglia, regarding inflammatory, proapoptotic and neuropathic pain signals. Genetic profiling of neuroinflammation at the local injured nerve reveals persistent upregulation of multiple genes involving oxysterol metabolism, neuronal apoptosis, and proliferation after nerve release. Further validation of the independent roles of each signal pathway will contribute to molecular therapies for compressive neuropathy in the future.

The Wnt/β-Catenin Pathway Regulated Cytokines for Pathological Neuropathic Pain in Chronic Compression of Dorsal Root Ganglion Model

Neuropathic pain is one of the important challenges in the clinic. Although a lot of research has been done on neuropathic pain (NP), the molecular mechanism is still elusive. We aimed to investigate whether the Wnt/β-catenin pathway was involved in NP caused by sustaining dorsal root ganglion (DRG) compression with the chronic compression of dorsal root ganglion model (CCD). Our RNA sequencing results showed that several genes related to the Wnt pathway have changed in DRG and spinal cord dorsal horn (SCDH) after CCD surgery. Therefore, we detected the activation of the Wnt/β-catenin pathway in DRG and SCDH and found active β-catenin significantly upregulated in DRG and SCDH 1 day after CCD surgery and peaked on days 7-14. Immunofluorescence results also confirmed nuclear translocalization of active β-catenin in DRG and SCDH. Additionally, rats had obvious mechanical induced pain after CCD surgery and the pain was significantly alleviated after the application of the Wnt/β-catenin pathway inhibitor XAV939. Furthermore, we found that the levels of proinflammatory factors tumor necrosis factor-α (TNF-α) and interleukin-18 (IL-18) were significantly elevated in CCD rat serum, while the levels of them were correspondingly decreased after the Wnt/β-catenin pathway being inhibited. The results of Spearman correlation coefficient analysis showed that the levels of TNF-α and IL-18 were negatively correlated with the mechanical withdrawal thresholds (MWT) after CCD surgery. Collectively, our findings suggest that the Wnt/β-catenin pathway plays a critical role in the pathogenesis of NP and may be an effective target for the treatment of NP.

Mesenchymal stem cell treatment for peripheral nerve injury: a narrative review

Peripheral nerve injuries occur as the result of sudden trauma and lead to reduced quality of life. The peripheral nervous system has an inherent capability to regenerate axons. However, peripheral nerve regeneration following injury is generally slow and incomplete that results in poor functional outcomes such as muscle atrophy. Although conventional surgical procedures for peripheral nerve injuries present many benefits, there are still several limitations including scarring, difficult accessibility to donor nerve, neuroma formation and a need to sacrifice the autologous nerve. For many years, other therapeutic approaches for peripheral nerve injuries have been explored, the most notable being the replacement of Schwann cells, the glial cells responsible for clearing out debris from the site of injury. Introducing cultured Schwann cells to the injured sites showed great benefits in promoting axonal regeneration and functional recovery. However, there are limited sources of Schwann cells for extraction and difficulties in culturing Schwann cells in vitro. Therefore, novel therapeutic avenues that offer maximum benefits for the treatment of peripheral nerve injuries should be investigated. This review focused on strategies using mesenchymal stem cells to promote peripheral nerve regeneration including exosomes of mesenchymal stem cells, nerve engineering using the nerve guidance conduits containing mesenchymal stem cells, and genetically engineered mesenchymal stem cells. We present the current progress of mesenchymal stem cell treatment of peripheral nerve injuries.

Research trends, hot spots and prospects for necroptosis in the field of neuroscience

There are two types of cell death-apoptosis and necrosis. Apoptosis is cell death regulated by cell signaling pathways, while necrosis has until recently been considered a passive mechanism of cell death caused by environmental pressures. However, recent studies show that necrosis can also be regulated by specific cell signaling pathways. This mode of death, termed necroptosis, has been found to be related to the occurrence and development of many diseases. We used bibliometrics to analyze the global output of literature on necroptosis in the field of neuroscience published in the period 2007–2019 to identify research hotspots and prospects. We included 145 necroptosis-related publications and 2239 references published in the Web of Science during 2007–2019. Visualization analysis revealed that the number of publications related to necroptosis has increased year by year, reaching a peak in 2019. China is the country with the largest number of publications. Key word and literature analyses demonstrated that mitochondrial function change, stroke, ischemia/reperfusion and neuroinflammation are likely the research hotspots and future directions of necroptosis research in the nervous system. The relationship between immune response-related factors, damage-associated molecular patterns, pathogen-associated molecular patterns and necroptosis may become a potential research hotspot in the future. Taken together, our findings suggest that although the inherent limitations of bibliometrics may affect the accuracy of the literature-based prediction of research hotspots, the results obtained from the included publications can provide a reference for the study of necroptosis in the field of neuroscience.

Satellite glia as a critical component of diabetic neuropathy: Role of lipocalin-2 and pyruvate dehydrogenase kinase-2 axis in the dorsal root ganglion

Diabetic peripheral neuropathy (DPN) is a common complication of uncontrolled diabetes. The pathogenesis of DPN is associated with chronic inflammation in dorsal root ganglion (DRG), eventually causing structural and functional changes. Studies on DPN have primarily focused on neuronal component, and there is limited knowledge about the role of satellite glial cells (SGCs), although they completely enclose neuronal soma in DRG. Lipocalin-2 (LCN2) is a pro-inflammatory acute-phase protein found in high levels in diverse neuroinflammatory and metabolic disorders. In diabetic DRG, the expression of LCN2 was increased exclusively in the SGCs. This upregulation of LCN2 in SGCs correlated with increased inflammatory responses in DRG and sciatic nerve. Furthermore, diabetes-induced inflammation and morphological changes in DRG, as well as sciatic nerve, were attenuated in Lcn2 knockout (KO) mice. Lcn2 gene ablation also ameliorated neuropathy phenotype as determined by nerve conduction velocity and intraepidermal nerve fiber density. Mechanistically, studies using specific gene KO mice, adenovirus-mediated gene overexpression strategy, and primary cultures of DRG SGCs and neurons have demonstrated that LCN2 enhances the expression of mitochondrial gate-keeping regulator pyruvate dehydrogenase kinase-2 (PDK2) through PPARβ/δ, thereby inhibiting pyruvate dehydrogenase activity and increasing production of glycolytic end product lactic acid in DRG SGCs and neurons of diabetic mice. Collectively, our findings reveal a crucial role of glial LCN2-PPARβ/δ-PDK2-lactic acid axis in progression of DPN. Our results establish a link between pro-inflammatory LCN2 and glycolytic PDK2 in DRG SGCs and neurons and propose a novel glia-based mechanism and drug target for therapy of DPN. MAIN POINTS: Diabetes upregulates LCN2 in satellite glia, which in turn increases pyruvate dehydrogenase kinase-2 (PDK2) expression and lactic acid production in dorsal root ganglia (DRG). Glial LCN2-PDK2-lactic acid axis in DRG plays a crucial role in the pathogenesis of diabetic neuropathy.

Sex differences in neuro(auto)immunity and chronic sciatic nerve pain

Chronic pain occurs with greater frequency in women, with a parallel sexually dimorphic trend reported in sufferers of many autoimmune diseases. There is a need to continue examining neuro-immune-endocrine crosstalk in the context of sexual dimorphisms in chronic pain. Several phenomena in particular need to be further explored. In patients, autoantibodies to neural antigens have been associated with sensory pathway hyper-excitability, and the role of self-antigens released by damaged nerves remains to be defined. In addition, specific immune cells release pro-nociceptive cytokines that directly influence neural firing, while T lymphocytes activated by specific antigens secrete factors that either support nerve repair or exacerbate the damage. Modulating specific immune cell populations could therefore be a means to promote nerve recovery, with sex-specific outcomes. Understanding biological sex differences that maintain, or fail to maintain, neuroimmune homeostasis may inform the selection of sex-specific treatment regimens, improving chronic pain management by rebalancing neuroimmune feedback. Given the significance of interactions between nerves and immune cells in the generation and maintenance of neuropathic pain, this review focuses on sex differences and possible links with persistent autoimmune activity using sciatica as an example.

Recovery of sciatic nerve with complete transection in rats treated with leuprolide acetate: A gonadotropin-releasing hormone agonist

It has been reported that the Gonadotropin-releasing hormone (GnRH) and its agonist leuprolide acetate (LA) can act as promoters of nerve regeneration. The aim of this study is to evaluate the effect of LA in a complete transection model. Sciatic nerve injury (SNI) was performed using a complete nerve transection and immediately repaired by epineural sutures. Rats were divided into three groups: SHAM, SNI treated with LA (SNI + LA) or saline solution (SNI + SS) for 5 weeks. Sciatic nerve regeneration was evaluated by kinematic gait analyzes, electrophysiological, morphological and biochemical tests. SNI + LA group had a functional recovery in kinematic gait, an increase in ankle angle value and a faster walking speed, compound muscle action potential amplitude, nerve conduction velocity (NCV). Furthermore, the number of myelinated axons and microtubule-associated protein 2 (MAP-2) expression were also higher compared to SS group. In conclusion, LA treatment improves of gait, walking speed, NCV, axons morphometry and MAP-2 expression in rats with sciatic nerve complete transection. These results suggest that LA can be a potential treatment for peripheral nerve injuries.

Biological characteristics of dynamic expression of nerve regeneration related growth factors in dorsal root ganglia after peripheral nerve injury

The regenerative capacity of peripheral nerves is limited after nerve injury. A number of growth factors modulate many cellular behaviors, such as proliferation and migration, and may contribute to nerve repair and regeneration. Our previous study observed the dynamic changes of genes in L4-6 dorsal root ganglion after rat sciatic nerve crush using transcriptome sequencing. Our current study focused on upstream growth factors and found that a total of 19 upstream growth factors were dysregulated in dorsal root ganglions at 3, 9 hours, 1, 4, or 7 days after nerve crush, compared with the 0 hour control. Thirty-six rat models of sciatic nerve crush injury were prepared as described previously. Then, they were divided into six groups to measure the expression changes of representative genes at 0, 3, 9 hours, 1, 4 or 7 days post crush. Our current study measured the expression levels of representative upstream growth factors, including nerve growth factor, brain-derived neurotrophic factor, fibroblast growth factor 2 and amphiregulin genes, and explored critical signaling pathways and biological process through bioinformatic analysis. Our data revealed that many of these dysregulated upstream growth factors, including nerve growth factor, brain-derived neurotrophic factor, fibroblast growth factor 2 and amphiregulin, participated in tissue remodeling and axon growth-related biological processes Therefore, the experiment described the expression pattern of upstream growth factors in the dorsal root ganglia after peripheral nerve injury. Bioinformatic analysis revealed growth factors that may promote repair and regeneration of damaged peripheral nerves. All animal surgery procedures were performed in accordance with Institutional Animal Care Guidelines of Nantong University and ethically approved by the Administration Committee of Experimental Animals, China (approval No. 20170302-017) on March 2, 2017.

Dim Light at Night Exposure Induces Cold Hyperalgesia and Mechanical Allodynia in Male Mice

The growing presence of artificial lighting across the globe presents a number of challenges to human and ecological health despite its societal benefits. Exposure to artificial light at night, a seemingly innocuous aspect of modern life, disrupts behavior and physiological functions. Specifically, light at night induces neuroinflammation, which is implicated in neuropathic and nociceptive pain states, including hyperalgesia and allodynia. Because of its influence on neuroinflammation, we investigated the effects of dim light at night exposure on pain responsiveness in male mice. In this study, mice exposed to four days of dim (5 lux) light at night exhibited cold hyperalgesia. Further, after 28 days of exposure, mice exhibited both cold hyperalgesia and mechanical allodynia. No heat/hot hyperalgesia was observed in this experiment. Altered nociception in mice exposed to dim light at night was concurrent with upregulated interleukin-6 and nerve growth factor mRNA expression in the medulla and elevated μ-opioid receptor mRNA expression in the periaqueductal gray region of the brain. The current results support the relationship between disrupted circadian rhythms and altered pain sensitivity. In summary, we observed that dim light at night induces cold hyperalgesia and mechanical allodynia, potentially through elevated neuroinflammation and dysregulation of the endogenous opioid system.

Microarray analyses of the dorsal root ganglia support a role for innate neuro-immune pathways in persistent pain in experimental osteoarthritis

OBJECTIVE

Following destabilization of the medial meniscus (DMM), mice develop experimental osteoarthritis (OA) and associated pain behaviors that are dependent on the stage of disease. We aimed to describe changes in gene expression in knee-innervating dorsal root ganglia (DRG) after surgery, in order to identify molecular pathways associated with three pre-defined pain phenotypes: "post-surgical pain", "early-stage OA pain", and "persistent OA pain".

DESIGN

We performed DMM or sham surgery in 10-week old male C57BL/6 mice and harvested L3-L5 DRG 4, 8, and 16 weeks after surgery or from age-matched naïve mice (n = 3/group). RNA was extracted and an Affymetrix Mouse Transcriptome Array 1.0 was performed. Three pain phenotypes were defined: "post-surgical pain" (sham and DMM 4-week vs 14-week old naïve), "early OA pain" (DMM 4-week vs sham 4-week), and "persistent OA pain" (DMM 8- and 16-week vs naïve and sham 8- and 16-week). 'Top hit' genes were defined as P < 0.001. Pathway analysis (Ingenuity Pathway Analysis) was conducted using differentially expressed genes defined as P < 0.05. In addition, we performed qPCR for Ngf and immunohistochemistry for F4/80+ macrophages in the DRG.

RESULTS

For each phenotype, top hit genes identified a small number of differentially expressed genes, some of which have been previously associated with pain (7/67 for "post-surgical pain"; 2/14 for "early OA pain"; 8/37 for "persistent OA pain"). Overlap between groups was limited, with 8 genes differentially regulated (P < 0.05) in all three phenotypes. Pathway analysis showed that in the persistent OA pain phase many of the functions of differentially regulated genes are related to immune cell recruitment and activation. Genes previously linked to OA pain (CX3CL1, CCL2, TLR1, and NGF) were upregulated in this phenotype and contributed to activation of the neuroinflammation canonical pathway. In separate sets of mice, we confirmed that Ngf was elevated in the DRG 8 weeks after DMM (P = 0.03), and numbers of F4/80+ macrophages were increased 16 weeks after DMM (P = 0.002 vs Sham).

CONCLUSION

These transcriptomics findings support the idea that distinct molecular pathways discriminate early from persistent OA pain. Pathway analysis suggests neuroimmune interactions in the DRG contribute to initiation and maintenance of pain in OA.

The Potential Role of Glycogen Synthase Kinase-3β in Neuropathy-Induced Apoptosis in Spinal Cord

Introduction:

Glycogen Synthase Kinase-3β (GSK-3β) participates in several signaling pathways and plays a crucial role in neurodegenerative diseases, inflammation, and neuropathic pain. The ratio of phosphorylated GSK-3β over total GSK-3β (p-GSK-3β/t-GSK-3β) is reduced following nerve injury. Apoptosis is a hallmark of many neuronal dysfunctions in the context of neuropathic pain. Thus, this study aimed to evaluate the contribution of p-GSK-3β/t-GSK-3β ratio in spinal dorsal horn apoptosis following peripheral nerve injury.

Methods:

In this study, adult male Wistar rats (220–250 g) underwent Spinal Nerve Ligation (SNL) surgery. Mechanical allodynia and thermal hyperalgesia were assessed before the surgery (day 0); then, every other day up to day 8. GSK-3β selective inhibitor, AR-014418 [0.3 mg/kg, Intraperitoneal (IP)] was administrated 1 h prior to SNL on day 0, then daily up to the day 8. The GSK-3β activity and apoptosis in the lumbar section (L4, L5, or L6) of the study rat’s spinal cord were assessed by immunohistochemical and Terminal Deoxynucleotidyl Transferase dUTP Nick End Labeling (TUNEL) staining, respectively on day 8 post-SNL.

Results:

Following the SNL, the mechanical allodynia and thermal hyperalgesia increased on day 2 up to day 8 post-SNL. The ratio of p-GSK-3β/t-GSK-3β decreased, and the number of apoptotic cells increased in the spinal dorsal horn on day 8. However, AR-A014418 administration could increase the p-GSK-3β/t-GSK-3β ratio and decreased apoptosis in the SNL rats. In addition, AR-A014418 decreased the mechanical allodynia from day 4 up to day 8; however, it did not affect thermal hyperalgesia.

Conclusion:

The study findings suggested that increasing the p-GSK-3β/t-GSK-3β ratio might be a helpful strategy for reducing the apoptotic cells and subsequent neuropathic pain during peripheral nerve injury.

Toxic Peripheral Neuropathies: Agents and Mechanisms

Toxic peripheral neuropathies are an important form of acquired polyneuropathy produced by a variety of xenobiotics and different exposure scenarios. Delineating the mechanisms of neurotoxicants and determining the degenerative biological pathways triggered by peripheral neurotoxicants will facilitate the development of sensitive and specific biochemical-based methods for identifying neurotoxicants, designing therapeutic interventions, and developing structure-activity relationships for predicting potential neurotoxicants. This review presents an overview of the general concepts of toxic peripheral neuropathies with the goal of providing insight into why certain agents target the peripheral nervous system and produce their associated lesions. Experimental data and the main hypotheses for the mechanisms of selected agents that produce neuronopathies, axonopathies, or myelinopathies including covalent or noncovalent modifications, compromised energy or protein biosynthesis, and oxidative injury and disruption of ionic gradients across membranes are presented. The relevance of signaling between the main components of peripheral nerve, that is, glia, neuronal perikaryon, and axon, as a target for neurotoxicants and the contribution of active programmed degenerative pathways to the lesions observed in toxic peripheral neuropathies is also discussed.

Peripheral nerve injury induced changes in the spinal cord and strategies to counteract/enhance the changes to promote nerve regeneration

Peripheral nerve injury leads to morphological, molecular and gene expression changes in the spinal cord and dorsal root ganglia, some of which have positive impact on the survival of neurons and nerve regeneration, while the effect of others is the opposite. It is crucial to take prompt measures to capitalize on the positive effects of these reactions and counteract the negative impact after peripheral nerve injury at the level of spinal cord, especially for peripheral nerve injuries that are severe, located close to the cell body, involve long distance for axons to regrow and happen in immature individuals. Early nerve repair, exogenous supply of neurotrophic factors and Schwann cells can sustain the regeneration inductive environment and enhance the positive changes in neurons. Administration of neurotrophic factors, acetyl-L-carnitine, N-acetyl-cysteine, and N-methyl-D-aspartate receptor antagonist MK-801 can help counteract axotomy-induced neuronal loss and promote regeneration, which are all time-dependent. Sustaining and reactivation of Schwann cells after denervation provides another effective strategy. FK506 can be used to accelerate axonal regeneration of neurons, especially after chronic axotomy. Exploring the axotomy-induced changes after peripheral nerve injury and applying protective and promotional measures in the spinal cord which help to retain a positive functional status for neuron cell bodies will inevitably benefit regeneration of the peripheral nerve and improve functional outcomes.

Interleukin-6 contributes to initiation of neuronal regeneration program in the remote dorsal root ganglia neurons after sciatic nerve injury

To assess the potential role of IL-6 in sciatic nerve injury-induced activation of a pro-regenerative state in remote dorsal root ganglia (DRG) neurons, we compared protein levels of SCG-10 and activated STAT3, as well as axon regeneration in IL-6 knockout (IL-6ko) mice and their wild-type (WT) counterparts. Unilateral sciatic nerve compression and transection upregulated SCG-10 protein levels and activated STAT3 in DRG neurons not only in lumbar but also in cervical segments of WT mice. A pro-regenerative state induced by prior sciatic nerve lesion in cervical DRG neurons of WT mice was also shown by testing for axon regeneration in crushed ulnar nerve. DRG neurons from IL-6ko mice also displayed bilaterally increased levels of SCG-10 and STAT3 in both lumbar and cervical segments after sciatic nerve lesions. However, levels of SCG-10 protein in lumbar and cervical DRG of IL-6ko mice were significantly lower than those of their WT counterparts. Sciatic nerve injury induced a lower level of SCG-10 in cervical DRG of IL-6ko than WT mice, and this correlates with significantly shorter regeneration of axons distal to the crushed ulnar nerve. These results suggest that IL-6 contributes, at the very least, to initiation of the neuronal regeneration program in remote DRG neurons after unilateral sciatic nerve injury.