Pathobiology of neuropathic pain

Meralgia paresthetica treated by injection, decompression, and neurectomy: a systematic review and meta-analysis of pain and operative outcomes

OBJECTIVE

Meralgia paresthetica is caused by entrapment of the lateral femoral cutaneous nerve (LFCN) and often presents with pain. Multiple treatment options targeting the LFCN can be pursued to treat the pain should conservative measures fail, with the most common options being injection, neurolysis, and neurectomy. However, their efficacy in causing pain relief and their clinical outcomes have yet to be directly compared. The aim of this study was to interrogate the contemporary literature and quantitatively define how these options compare.

METHODS

The electronic databases Ovid Embase, PubMed, SCOPUS, and the Cochrane Library were interrogated from inception to May 2020 following the PRISMA guidelines. Candidate articles were screened against prespecified criteria. Outcome data were abstracted and pooled by random-effects meta-analysis of proportions.

RESULTS

There were 25 articles that satisfied all criteria, reporting outcomes for a total of 670 meralgia paresthetica patients, with 78 (12%) treated by injection, 496 (74%) by neurolysis, and 96 (14%) by neurectomy. The incidence of complete pain relief was 85% (95% CI 71%-96%) after neurectomy, 63% (95% CI 56%-71%) after neurolysis, and 22% (95% CI 13%-33%) after injection, which were all statistically different (p < 0.01). The incidence of revision procedures was 12% (95% CI 4%-22%) after neurolysis and 0% (95% CI 0%-2%) after neurectomy, which were significantly lower than 81% (95% CI 64%-94%) after injection (p < 0.01). The incidences of treatment complications were statistically comparable across all three treatments, ranging from 0% to 5% (p = 0.34).

CONCLUSIONS

There are multiple treatment options to target pain in meralgia paresthetica. The incidence of complete pain relief appears to be the greatest among the 3 interventions after neurectomy, accompanied by the lowest incidence of revision procedures. These findings should help inform patient preference and expectations. Greater exploration of the anatomical rationale for incomplete pain relief after surgical intervention will assist in optimizing further surgical treatment for meralgia paresthetica.

Electroacupuncture may alleviate neuropathic pain via suppressing P2X7R expression

Neuropathic pain is a severe problem that is difficult to treat clinically. Reducing abnormal remodeling of dendritic spines/synapses and increasing the anti-inflammatory effects in the spinal cord dorsal horn are potential methods to treat this disease. Previous studies have reported that electroacupuncture (EA) could increase the pain threshold after peripheral nerve injury. However, the underlying mechanism is unclear. P2X7 receptors (P2X7R) mediate the activation of microglia and participate in the occurrence and development of neuropathic pain. We hypothesized that the effects of EA on relieving pain may be related to the downregulation of the P2X7R. Spinal nerve ligation (SNL) rats were used as a model in this experiment, and 2'(3')-O-(4-benzoyl)benzoyl ATP (BzATP) was used as a P2X7R agonist. We found that EA treatment decreased dendritic spine density, inhibited synaptic reconstruction and reduced inflammatory response, which is consistent with the decrease in P2X7R expression as well as the improved neurobehavioral performance. In contrast to the beneficial effects of EA, BzATP enhanced abnormal remodeling of dendritic spines/synapses and inflammation. Furthermore, the EA-mediated positive effects were reversed by BzATP, which is consistent with the increased P2X7R expression. These findings indicated that EA improves neuropathic pain by reducing abnormal dendritic spine/synaptic reconstruction and inflammation via suppressing P2X7R expression.

Neuropathic Pain Related to Peripheral Neuropathies According to the IASP Grading System Criteria

Neuropathic pain is defined as pain caused by a lesion or disease of the somatosensory system. Neuropathic pain represents a broad category of pain conditions, common complications of peripheral neuropathies, which are characterized by a combination of positive symptoms, including paresthesia and/or dysesthesia and sensory deficits in the painful area. In the present paper, we aimed to assess neuropathic pain frequency and clinical characteristics of peripheral neuropathies due to different aetiologies according to grading system criteria of the International Association for the Study of Pain for a definitive diagnosis of neuropathic pain. Epidemiological studies applying these criteria have been conducted in patients with diabetes, brachial plexus injury, and other traumatic nerve injuries. Neuropathic pain was diagnosed in 37–42% of patients with diabetic peripheral neuropathy, 56% of patients with brachial plexus injury, and 22% of patients with intercostobrachial neuropathy. The most frequent neuropathic pain type was ongoing pain (described as burning or pressing), followed by paroxysmal pain (electric shock-like sensations) and allodynia (pain evoked by brushing and pressure). By providing information on the frequency, clinical signs, and variables associated with neuropathic pain due to different aetiologies, these studies contribute to improving the clinical management of this condition.

TRPM8: A Therapeutic Target for Neuroinflammatory Symptoms Induced by Severe Dry Eye Disease

Dry eye disease (DED) is commonly associated with ocular surface inflammation and pain. In this study, we evaluated the effectiveness of repeated instillations of transient receptor potential melastatin 8 (TRPM8) ion channel antagonist M8-B on a mouse model of severe DED induced by the excision of extra-orbital lacrimal and Harderian glands. M8-B was topically administered twice a day from day 7 until day 21 after surgery. Cold and mechanical corneal sensitivities and spontaneous ocular pain were monitored at day 21. Ongoing and cold-evoked ciliary nerve activities were next evaluated by electrophysiological multi-unit extracellular recording. Corneal inflammation and expression of genes related to neuropathic pain and inflammation were assessed in the trigeminal ganglion. We found that DED mice developed a cold allodynia consistent with higher TRPM8 mRNA expression in the trigeminal ganglion (TG). Chronic M8-B instillations markedly reversed both the corneal mechanical allodynia and spontaneous ocular pain commonly associated with persistent DED. M8-B instillations also diminished the sustained spontaneous and cold-evoked ciliary nerve activities observed in DED mice as well as inflammation in the cornea and TG. Overall, our study provides new insight into the effectiveness of TRPM8 blockade for alleviating corneal pain syndrome associated with severe DED, opening a new avenue for ocular pain management.

Differential involvement of myelinated and unmyelinated nerve fibers in painful diabetic polyneuropathy

BACKGROUND

We aimed at evaluating the differential involvement of large myelinated Aβ-, small myelinated Aδ-, and unmyelinated C-fibers in patients with diabetic polyneuropathy and how they contribute to neuropathic pain.

METHODS

We collected clinical and diagnostic test variables in 133 consecutive patients with diabetic polyneuropathy. All patients underwent Aβ-fiber mediated nerve conduction study, Aδ-fiber mediated laser-evoked potentials and skin biopsy mainly assessing unmyelinated C-fibers.

RESULTS

Pure large-fiber and small-fiber polyneuropathy were relatively uncommon; conversely mixed-fiber polyneuropathy was the most common type of diabetic polyneuropathy (74%). The frequency of neuropathic pain was similar in the three different polyneuropathies. Ongoing burning pain and dynamic mechanical allodynia were similarly associated with specific small-fiber related variables.

CONCLUSIONS

Diabetic polyneuropathy mainly manifests as a mixed-fiber polyneuropathy, simultaneously involving Aβ-, Aδ-, and C-fibers. In most patients, neuropathic pain is distinctly associated with small-fiber damage. The evidence that the frequency of neuropathic pain does not differ across pure large-, pure small-, and mixed-fiber polyneuropathy, raises the possibility that in patients with pure large-fiber polyneuropathy nociceptive nerve terminal involvement might be undetected by standard diagnostic techniques.

The Pivotal Potentials of Scorpion Buthus Martensii Karsch-Analgesic-Antitumor Peptide in Pain Management and Cancer

Scorpion Buthus martensii Karsch -analgesic-antitumor peptide (BmK AGAP) has been used to treat diseases like tetanus, tuberculosis, apoplexy, epilepsy, spasm, migraine headaches, rheumatic pain, and cancer in China. AGAP is a distinctive long-chain scorpion toxin with a molecular mass of 7142 Da and composed of 66 amino acids cross-linked by four disulfide bridges. Voltage-gated sodium channels (VGSCs) are present in excitable membranes and partakes in essential roles in action potentials generation as compared to the significant function of voltage-gated calcium channels (VGCCs). A total of nine genes (Nav1.1-Nav1.9) have been recognized to encode practical sodium channel isoforms. Nav1.3, Nav1.7, Nav1.8, and Nav1.9 have been recognized as potential targets for analgesics. Nav1.8 and Nav1.9 are associated with nociception initiated by inflammation signals in the neuronal pain pathway, while Nav1.8 is fundamental for neuropathic pain at low temperatures. AGAP has a sturdy inhibitory influence on both viscera and soma pain. AGAP potentiates the effects of MAPK inhibitors on neuropathic as well as inflammation-associated pain. AGAP downregulates the secretion of phosphorylated p38, phosphorylated JNK, and phosphorylated ERK 1/2 in vitro. AGAP has an analgesic activity which may be an effective therapeutic agent for pain management because of its downregulation of PTX3 via NF-κB and Wnt/beta-catenin signaling pathway. In cancers like colon cancer, breast cancer, lymphoma, and glioma, rAGAP was capable of blocking the proliferation. Thus, AGAP is a promising therapy for these tumors. Nevertheless, research is needed with other tumors.

Pulsed radiofrequency alleviated neuropathic pain by down-regulating the expression of substance P in chronic constriction injury rat model

Background

Pulsed radiofrequency (PRF), as a non-invasive treatment of neuropathic pain (NP), has been widely administered clinically. Previous studies have shown that PRF has the potential to improve hyperalgesia in animal models of NP. However, there have been few reports to clarify whether the mechanism of PRF treatment of NP involves intervention in the expression of substance P (SP). Therefore, this study administered PRF treatment to chronic constriction injury (CCI) model rats and observed the sciatic nerve mechanical pain threshold and SP expression in the spinal cord to explore the mechanism of PRF treatment.

Methods

A total of 96 Sprague-Dawley rats were randomly divided into the sham-surgery-sham-treatment group (S-S group), the sham-surgery-PRF group (S-P group), the CCI-sham-treatment group (C-S group), and the CCI-PRF group (C-P group). The C-S group and the C-P group underwent sciatic nerve CCI, while the other groups received a sham operation. At 14 days after the operation, the C-P group and the S-P group were treated with PRF for 300 s. We recorded the hindpaw withdrawal threshold (HWT) and the thermal withdrawal latency (TWL) of rats in the various groups at baseline, before treatment (0 days), and at 1, 7, 14, and 28 days after treatment. L4 to L6 spinal cord tissues were taken before treatment (0 days) and 1, 7, 14, and 28 days after treatment. The transcription and translation of SP were measured by quantitative polymerase chain reaction and Western blotting, respectively.

Results

The HWT and the TWL in the C-P group 28 days after PRF treatment were significantly higher than those in the C-S group (95% confidence interval [CI]: 5.84–19.50, P < 0.01; 95% CI: 2.58–8.69, P = 0.01). The expression of SP in the C-P group 28 days after PRF treatment was significantly lower than that in the C-S group (95% CI: 1.17–2.48, P < 0.01).

Conclusions

PRF may alleviate CCI-induced NP by down-regulating the expression of SP in the spinal cord of CCI model rats.

Electroacupuncture Modulates Spinal BDNF/TrκB Signaling Pathway and Ameliorates the Sensitization of Dorsal Horn WDR Neurons in Spared Nerve Injury Rats

Neuropathic pain is more complex and severely affects the quality of patients’ life. However, the therapeutic strategy for neuropathic pain in the clinic is still limited. Previously we have reported that electroacupuncture (EA) has an attenuating effect on neuropathic pain induced by spared nerve injury (SNI), but its potential mechanisms remain to be further elucidated. In this study, we designed to determine whether BDNF/TrκB signaling cascade in the spinal cord is involved in the inhibitory effect of 2 Hz EA on neuropathic pain in SNI rats. The paw withdrawal threshold (PWT) of rats was used to detect SNI-induced mechanical hypersensitivity. The expression of BDNF/TrκB cascade in the spinal cord was evaluated by qRT-PCR and Western blot assay. The C-fiber-evoked discharges of wide dynamic range (WDR) neurons in spinal dorsal horn were applied to indicate the noxious response of WDR neurons. The results showed that 2 Hz EA significantly down-regulated the levels of BDNF and TrκB mRNA and protein expression in the spinal cord of SNI rats, along with ameliorating mechanical hypersensitivity. In addition, intrathecal injection of 100 ng BDNF, not only inhibited the analgesic effect of 2 Hz EA on pain hypersensitivity, but also reversed the decrease of BDNF and TrκB expression induced by 2 Hz EA. Moreover, 2 Hz EA obviously reduced the increase of C-fiber-evoked discharges of dorsal horn WDR neurons by SNI, but exogenous BDNF (100 ng) effectively reversed the inhibitory effect of 2 Hz EA on SNI rats, resulting in a remarkable improvement of excitability of dorsal horn WDR neurons in SNI rats. Taken together, these data suggested that 2 Hz EA alleviates mechanical hypersensitivity by blocking the spinal BDNF/TrκB signaling pathway-mediated central sensitization in SNI rats. Therefore, targeting BDNF/TrκB cascade in the spinal cord may be a potential mechanism of EA against neuropathic pain.

Depletion of senescent-like neuronal cells alleviates cisplatin-induced peripheral neuropathy in mice

Chemotherapy-induced peripheral neuropathy is among the most common dose-limiting adverse effects of cancer treatment, leading to dose reduction and discontinuation of life-saving chemotherapy and a permanently impaired quality of life for patients. Currently, no effective treatment or prevention is available. Senescence induced during cancer treatment has been shown to promote the adverse effects. Here, we show that cisplatin induces senescent-like neuronal cells in primary culture and in mouse dorsal root ganglia (DRG), as determined by the characteristic senescence markers including senescence-associated beta-galactosidase, accumulation of cytosolic p16^INK4A and HMGB1, as well as increased expression of p16Ink4a, p21, and MMP-9. The accumulation of senescent-like neuronal cells in DRG is associated with cisplatin-induced peripheral neuropathy (CIPN) in mice. To determine if depletion of senescent-like neuronal cells may effectively mitigate CIPN, we used a pharmacological ‘senolytic’ agent, ABT263, which inhibits the anti-apoptotic proteins BCL-2 and BCL-xL and selectively kills senescent cells. Our results demonstrated that clearance of DRG senescent neuronal cells reverses CIPN, suggesting that senescent-like neurons play a role in CIPN pathogenesis. This finding was further validated using transgenic p16-3MR mice, which permit ganciclovir (GCV) to selectively kill senescent cells expressing herpes simplex virus 1 thymidine kinase (HSV-TK). We showed that CIPN was alleviated upon GCV administration to p16-3MR mice. Together, the results suggest that clearance of senescent DRG neuronal cells following platinum-based cancer treatment might be an effective therapy for the debilitating side effect of CIPN.

Concomitant continuous pain in patients with trigeminal neuralgia is associated with trigeminal nerve root atrophy

Introduction

Trigeminal neuralgia is an exemplary neuropathic pain condition characterized by paroxysmal electric-shock-like pain. However, up to 50% of patients also experiences concomitant continuous pain. In this neuroimaging study, we aimed to identify the specific anatomical features of trigeminal nerve root in patients with concomitant continuous pain.

Methods

We enrolled 73 patients with a definitive diagnosis of classical and idiopathic trigeminal neuralgia and 40 healthy participants. The diagnosis of trigeminal neuralgia was independently confirmed by two clinicians. Patients were grouped as patients with purely paroxysmal pain (45 patients) and patients also with concomitant continuous pain (28 patients). All participants underwent a structured clinical examination and a 3T MRI with sequences dedicated to the anatomical study of the trigeminal nerve root, including volumetric study. Images analysis was independently performed by two investigators, blinded to any clinical data.

Results

In most patients with concomitant continuous pain, this type of pain, described as burning, throbbing or aching, manifested at the disease onset. Demographic and clinical variables did not differ between the two groups of patients; the frequency of neurovascular compression and nerve dislocation were similar. Conversely, trigeminal nerve root atrophy was more severe in patients with concomitant continuous pain than in those with purely paroxysmal pain ( p = 0.006).

Conclusions

Our clinical and neuroimaging study found that in patients with trigeminal neuralgia, concomitant continuous pain was associated with trigeminal nerve root atrophy, therefore suggesting that this type of pain is likely related to axonal loss and abnormal activity in denervated trigeminal second-order neurons.

Regulatory T cells counteract neuropathic pain through inhibition of the Th1 response at the site of peripheral nerve injury

The inflammatory/immune response at the site of peripheral nerve injury participates in the pathophysiology of neuropathic pain. Nevertheless, little is known about the local regulatory mechanisms underlying peripheral nerve injury that counteracts the development of pain. Here, we investigated the contribution of regulatory T (Treg) cells to the development of neuropathic pain by using a partial sciatic nerve ligation model in mice. We showed that Treg cells infiltrate and proliferate in the site of peripheral nerve injury. Local Treg cells suppressed the development of neuropathic pain mainly through the inhibition of the CD4 Th1 response. Treg cells also indirectly reduced neuronal damage and neuroinflammation at the level of the sensory ganglia. Finally, we identified IL-10 signaling as an intrinsic mechanism by which Treg cells counteract neuropathic pain development. These results revealed Treg cells as important inhibitory modulators of the immune response at the site of peripheral nerve injury that restrains the development of neuropathic pain. In conclusion, the boosting of Treg cell function/activity might be explored as a possible interventional approach to reduce neuropathic pain development after peripheral nerve damage.

Peripheral Deltorphin II Inhibits Nociceptors Following Nerve Injury

Clinical and preclinical studies have revealed that local administration of opioid agonists into peripheral tissue attenuates inflammatory pain. However, few studies have examined whether peripherally restricted opioids are effective in reducing mechanical allodynia and hyperalgesia that usually follows nerve injury. The aim of the present study was to determine whether the mechanical responsiveness of C-fiber mechanical nociceptors innervating skin under neuropathic pain conditions is depressed by direct activation of delta opioid receptors (DORs) on their peripheral terminals. A murine model of peripheral neuropathic pain was induced with a spared nerve (tibial) injury, in which mice survived 7 or 28 days after surgery before electrophysiological testing began. Control groups comprised naïve and sham-operated animals. An ex vivo preparation of mouse plantar skin with attached tibial nerve was used to examine electrophysiologically the effects of the selective DOR agonist, deltorphin II, on the response properties of individual cutaneous C-fiber nociceptors. In contrast to naïve and sham-operated animals, deltorphin II induced an inhibition of the mechanical responsiveness of C-fiber mechanical nociceptors innervating skin under neuropathic conditions. The effects of deltorphin II were concentration-dependent and prevented by pretreatment with naltrindole indicating DOR-mediated inhibitory effects of deltorphin II. Our results provide the first direct evidence for expression of functional DORs on mechanical nociceptors innervating skin in an animal model of neuropathic pain.

SIRT2 protects peripheral neurons from cisplatin-induced injury by enhancing nucleotide excision repair

Platinum-based chemotherapy-induced peripheral neuropathy is one of the most common causes of dose reduction and discontinuation of life-saving chemotherapy in cancer treatment; it often causes permanent impairment of quality of life in cancer patients. The mechanisms that underlie this neuropathy are not defined, and effective treatment and prevention measures are not available. Here, we demonstrate that SIRT2 protected mice against cisplatin-induced peripheral neuropathy (CIPN). SIRT2 accumulated in the nuclei of dorsal root ganglion sensory neurons and prevented neuronal cell death following cisplatin treatment. Mechanistically, SIRT2, an NAD+-dependent deacetylase, protected neurons from cisplatin cytotoxicity by promoting transcription-coupled nucleotide excision repair (TC-NER) of cisplatin-induced DNA cross-links. Consistent with this mechanism, pharmacological inhibition of NER using spironolactone abolished SIRT2-mediated TC-NER activity in differentiated neuronal cells and protection of neurons from cisplatin-induced cytotoxicity and CIPN in mice. Importantly, SIRT2's protective effects were not evident in lung cancer cells in vitro or in tumors in vivo. Taken together, our results identified SIRT2's function in the NER pathway as a key underlying mechanism of preventing CIPN, warranting future investigation of SIRT2 activation-mediated neuroprotection during platinum-based cancer treatment.

Pathophysiological roles and therapeutic potential of voltage-gated ion channels (VGICs) in pain associated with herpesvirus infection

Herpesvirus is ranked as one of the grand old members of all pathogens. Of all the viruses in the superfamily, Herpes simplex virus type 1 (HSV-1) is considered as a model virus for a variety of reasons. In a permissive non-neuronal cell culture, HSV-1 concludes the entire life cycle in approximately 18–20 h, encoding approximately 90 unique transcriptional units. In latency, the robust viral gene expression is suppressed in neurons by a group of noncoding RNA. Historically the lesions caused by the virus can date back to centuries ago. As a neurotropic pathogen, HSV-1 is associated with painful oral lesions, severe keratitis and lethal encephalitis. Transmission of pain signals is dependent on the generation and propagation of action potential in sensory neurons. T-type Ca²⁺ channels serve as a preamplifier of action potential generation. Voltage-gated Na⁺ channels are the main components for action potential production. This review summarizes not only the voltage-gated ion channels in neuropathic disorders but also provides the new insights into HSV-1 induced pain.

Sigma-1 receptors control neuropathic pain and macrophage infiltration into the dorsal root ganglion after peripheral nerve injury

Neuron-immune interaction in the dorsal root ganglia (DRG) plays a pivotal role in the neuropathic pain development after nerve injury. Sigma-1 receptor (Sig-1R) is expressed by DRG neurons but its role in neuropathic pain is not fully understood. We investigated the effect of peripheral Sig-1R on neuroinflammation in the DRG after spared (sciatic) nerve injury (SNI) in mice. Nerve injury induced a decrease in NeuN staining along with the nuclear eccentricity and ATF3 expression in the injured DRG. Sig-1R was present in all DRG neurons examined, and after SNI this receptor translocated to the periphery of the soma and the vicinity of the nucleus, especially in injured ATF3 + neurons. In WT mice, injured DRG produced the chemokine CCL2, and this was followed by massive infiltration of macrophages/monocytes, which clustered mainly around sensory neurons with translocated Sig-1R, accompanied by robust IL-6 increase and mechanical allodynia. In contrast, Sig-1R knockout (Sig-1R-KO) mice showed reduced levels of CCL2, decreased macrophage/monocyte infiltration into DRG, and less IL-6 and neuropathic mechanical allodynia after SNI. Our findings point to an important role of peripheral Sig-1R in sensory neuron-macrophage/monocyte communication in the DRG after peripheral nerve injury; thus, these receptors may contribute to the neuropathic pain phenotype.

Effect of crocin on morphine tolerance and serum BDNF levels in a rat model of neuropathic pain

BACKGROUND

Chronic use of morphine treatment for neuropathic pain leads to morphine-induced analgesic tolerance. Crocin contained in Crocus sativus L., exerts anti-inflammatory and analgesic effects. This study examined the effects of crocin on morphine tolerance and serum BDNF levels on neuropathic pain induced by chronic constriction injury (CCI) in rats.

METHODS

CCI model of neuropathic pain was done in male Wistar rats (200-250 g). Rats were treated with crocin (15 or 30 mg/kg, intraperitoneally) alone or simultaneously with morphine (10 mg/kg, subcutaneously) during or after induction of CCI. Pain behavioral responses including mechanical allodynia and thermal hyperalgesia were measured from days of 15-27 after CCI. Then, rats were evaluated for serum BDNF levels on days 14 and/or 27.

RESULTS

We found that morphine tolerance developed after the induction of neuropathic pain. The injection of crocin (15 and 30 mg/kg) was able to enhance analgesic effect of morphine by reduction of mechanical allodynia on days 15-27 post-surgery in CCI rats. While preemptive administration of crocin at a lower dose (15 mg/kg) maintained the analgesic effect of morphine. Morphine injection and/or co-administration with crocin (15, 30 mg/kg) decreased serum BDNF levels in CCI rats.

CONCLUSION

These findings indicate that crocin may have a therapeutic effect to maintain morphine analgesic efficacy and also to prevent the development of morphine tolerance in neuropathic pain, but probably not through BDNF.

Neuroma

Neuroma formation occurs because of some degree of nerve injury followed by improper intrinsic nerve repair. The cause of neuroma pain is incompletely understood, but appears to be multifactorial in nature, including local and system changes. A comprehensive understanding of nerve anatomy, injury, and repair techniques should be used when dealing with neuroma formation and its physical manifestations. Diagnosis of neuroma is clinically characterized by pain associated with scar, altered sensation within the given nerve distribution, and a Tinel sign. The pathophysiology of neuroma formation is reviewed.

Pain modulation effect on motor cortex after optogenetic stimulation in shPKCγ knockdown dorsal root ganglion-compressed Sprague-Dawley rat model

Neuropathic pain can be generated by chronic compression of dorsal root ganglion (CCD). Stimulation of primary motor cortex can disrupt the nociceptive sensory signal at dorsal root ganglion level and reduce pain behaviors. But the mechanism behind it is still implicit. Protein kinase C gamma is known as an essential enzyme for the development of neuropathic pain, and specific inhibitor of protein kinase C gamma can disrupt the sensory signal and reduce pain behaviors. Optogenetic stimulation has been emerged as a new and promising conducive method for refractory neuropathic pain. The aim of this study was to provide evidence whether optical stimulation of primary motor cortex can modulate chronic neuropathic pain in CCD rat model. Animals were randomly divided into CCD group, sham group, and control group. Dorsal root ganglion-compressed neuropathic pain model was established in animals, and knocking down of protein kinase C gamma was also accomplished. Pain behavioral scores were significantly improved in the short hairpin Protein Kinase C gamma knockdown CCD animals during optic stimulation. Ventral posterolateral thalamic firing inhibition was also observed during light stimulation on motor cortex in CCD animal. We assessed alteration of pain behaviors in pre-light off, stimulation-light on, and post-light off state. In vivo extracellular recording of the ventral posterolateral thalamus, viral expression in the primary motor cortex, and protein kinase C gamma expression in dorsal root ganglion were investigated. So, optical cortico-thalamic inhibition by motor cortex stimulation can improve neuropathic pain behaviors in CCD animal, and knocking down of protein kinase C gamma plays a conducive role in the process. This study provides feasibility for in vivo optogenetic stimulation on primary motor cortex of dorsal root ganglion-initiated neuropathic pain.

Perispinal injection of a TNF blocker directed to the brain of rats alleviates the sensory and affective components of chronic constriction injury-induced neuropathic pain

Neuropathic pain is chronic pain that follows nerve injury, mediated in the brain by elevated levels of the inflammatory protein tumor necrosis factor-alpha (TNF). We have shown that peripheral nerve injury increases TNF in the hippocampus/pain perception region, which regulates neuropathic pain symptoms. In this study we assessed pain sensation and perception subsequent to specific targeting of brain-TNF (via TNF antibody) administered through a novel subcutaneous perispinal route. Neuropathic pain was induced in Sprague-Dawley rats via chronic constriction injury (CCI), and thermal hyperalgesia was monitored for 10 days post-surgery. On day 8 following CCI and sensory pain behavior testing, rats were randomized to receive perispinal injection of TNF antibody or control IgG isotype antibody. Pain perception was assessed using conditioned place preference (CPP) to the analgesic, amitriptyline. CCI-rats receiving the perispinal injection of TNF antibody had significantly decreased CCI-induced thermal hyperalgesia the following day, and did not form an amitriptyline-induced CPP, whereas CCI-rats receiving perispinal IgG antibody experienced pain alleviation only in conjunction with i.p. amitriptyline and did form an amitriptyline-induced CPP. The specific targeting of brain TNF via perispinal delivery alleviates thermal hyperalgesia and positively influences the affective component of pain. PERSPECTIVE: This study presents a novel route of drug administration to target central TNF for treatment of neuropathic pain. Targeting central TNF through perispinal drug delivery could potentially be a more efficient and sustained method to treat patients with neuropathic pain.

Emerging Roles of Long Non-coding RNAs in Chronic Neuropathic Pain

Chronic neuropathic pain, a type of chronic and potentially disabling pain caused by a disease or injury of the somatosensory nervous system, spinal cord injury, or various chronic conditions, such as viral infections (e.g., post-herpetic neuralgia), autoimmune diseases, cancers, and metabolic disorders (e.g., diabetes mellitus), is one of the most intense types of chronic pain, which incurs a major socio-economic burden and is a serious public health issue, with an estimated prevalence of 7–10% in adults throughout the world. Presently, the available drug treatments (e.g., anticonvulsants acting at calcium channels, serotonin-noradrenaline reuptake inhibitors, tricyclic antidepressants, opioids, topical lidocaine, etc.) for chronic neuropathic pain patients are still rare and have disappointing efficacy, which makes it difficult to relieve the patients’ painful symptoms, and, at best, they only try to reduce the patients’ ability to tolerate pain. Long non-coding RNAs (lncRNAs), a type of transcript of more than 200 nucleotides with no protein-coding or limited capacity, were identified to be abnormally expressed in the spinal cord, dorsal root ganglion, hippocampus, and prefrontal cortex under chronic neuropathic pain conditions. Moreover, a rapidly growing body of data has clearly pointed out that nearly 40% of lncRNAs exist specifically in the nervous system. Hence, it was speculated that these dysregulated lncRNAs might participate in the occurrence, development, and progression of chronic neuropathic pain. In other words, if we deeply delve into the potential roles of lncRNAs in the pathogenesis of chronic neuropathic pain, this may open up new strategies and directions for the development of novel targeted drugs to cure this refractory disorder. In this article, we primarily review the status of chronic neuropathic pain and provide a general overview of lncRNAs, the detailed roles of lncRNAs in the nervous system and its related diseases, and the abnormal expression of lncRNAs and their potential clinical applications in chronic neuropathic pain. We hope that through the above description, readers can gain a better understanding of the emerging roles of lncRNAs in chronic neuropathic pain.

Salix tetrasperma Roxb. Extract Alleviates Neuropathic Pain in Rats via Modulation of the NF-κB/TNF-α/NOX/iNOS Pathway

Patients with neuropathic pain experience chronic painful tingling, burning, and prickling sensations accompanied with hyperalgesia and/or allodynia. In this study, 38 secondary metabolites of a methanol extract from Salix tetrasperma flowers were identified by liquid chromatography-mass spectrometry (HPLC-MS/MS). The extract showed substantial anti-inflammatory, central and peripheral anti-nociceptive, antipyretic, and antioxidant activities in vitro and in different animal models. In the chronic constriction injury (CCI) rat model, the extract was able to attenuate and significantly relieve hyperalgesia and allodynia responses in a dose dependent manner and restore the myelin sheath integrity and Schwann cells average number in the sciatic nerve. The enzyme-linked immunosorbent assay (ELISA) showed that the extract significantly reduced the expression of various pro-inflammatory biomarkers including nuclear factor kabba B (NF-κB), tumor necrosis factor alpha (TNF-α), prostaglandin E2 (PGE2), 5-lipoxygenase (5-LOX), cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS), and the oxidative stress biomarker NADPH oxidase 1 (NOX1), in brain stem and sciatic nerve tissues. These findings were supported by in vitro enzyme inhibition assays (COX-1, COX-2 and 5-LOX). Moreover, the extract significantly reduced p53 expression in the brain stem tissue. These findings support the use of S. tetrasperma in folk medicine to alleviate pain. It could be a promising natural product for further clinical investigations to treat inflammation, nociceptive pain and chronic neuropathic pain.

Evidence for a dynorphin-mediated inner ear immune/inflammatory response and glutamate-induced neural excitotoxicity: an updated analysis

Acoustic overstimulation (AOS) is defined as the stressful overexposure to high-intensity sounds. AOS is a precipitating factor that leads to a glutamate (GLU)-induced Type I auditory neural excitotoxicity and an activation of an immune/inflammatory/oxidative stress response within the inner ear, often resulting in cochlear hearing loss. The dendrites of the Type I auditory neural neurons that innervate the inner hair cells (IHCs), and respond to the IHC release of the excitatory neurotransmitter GLU, are themselves directly innervated by the dynorphin (DYN)-bearing axon terminals of the descending brain stem lateral olivocochlear (LOC) system. DYNs are known to increase GLU availability, potentiate GLU excitotoxicity, and induce superoxide production. DYNs also increase the production of proinflammatory cytokines by modulating immune/inflammatory signal transduction pathways. Evidence is provided supporting the possibility that the GLU-mediated Type I auditory neural dendritic swelling, inflammation, excitotoxicity, and cochlear hearing loss that follow AOS may be part of a brain stem-activated, DYN-mediated cascade of inflammatory events subsequent to a LOC release of DYNs into the cochlea. In support of a DYN-mediated cascade of events are established investigations linking DYNs to the immune/inflammatory/excitotoxic response in other neural systems.

Efficacy of Human Cell-Seeded Muscle-Stuffed Vein Conduit in Rat Sciatic Nerve Repair

We investigated the efficacy of a muscle-stuffed vein (MSV) seeded with neural-transdifferentiated human mesenchymal stem cells as an alternative nerve conduit to repair a 15-mm sciatic nerve defect in athymic rats. Other rats received MSV conduit alone, commercial polyglycolic acid conduit (Neurotube^®), reverse autograft, or were left untreated. Motor and sensory functions as well as nerve conductivity were evaluated for 12 weeks, after which the grafts were harvested for histological analyses. All rats in the treatment groups demonstrated a progressive increase in the mean Sciatic Functional Index (motor function) and nerve conduction amplitude (electrophysiological function) and showed positive withdrawal reflex (sensory function) by the 10th week of postimplantation. Autotomy, which is associated with neuropathic pain, was severe in rats treated with conduit without cells; there was mild or no autotomy in the rats of other groups. Histologically, harvested grafts from all except the untreated groups exhibited axonal regeneration with the presence of mature myelinated axons. In conclusion, treatment with MSV conduit is comparable to that of other treatment groups in supporting functional recovery following sciatic nerve injury; and the addition of cells in the conduit alleviates neuropathic pain. Impact Statement It is shown that pretreated muscle-stuffed vein conduit is comparable to that of commercial nerve conduit and autograft in supporting functional recovery following peripheral nerve injury. The addition of neural-differentiated mesenchymal stem cells in the conduit is shown to alleviate neuropathic pain.

Characterization of the sensory, affective, cognitive, biochemical, and neuronal alterations in a modified chronic constriction injury model of neuropathic pain in mice

The chronic constriction injury (CCI) of the sciatic nerve is a nerve injury-based model of neuropathic pain (NP). Comorbidities of NP such as depression, anxiety, and cognitive deficits are associated with a functional reorganization of the medial prefrontal cortex (mPFC). Here, we have employed an adapted model of CCI by placing one single loose ligature around the sciatic nerve in mice for investigating the alterations in sensory, motor, affective, and cognitive behavior and in electrophysiological and biochemical properties in the prelimbic division (PrL) of the mPFC. Our adapted model of CCI induced mechanical allodynia, motor, and cognitive impairments and anxiety- and depression-like behavior. In the PrL division of mPFC was observed an increase in GABA and a decrease in d-aspartate levels. Moreover an increase in the activity of neurons responding to mechanical stimulation with an excitation, mPFC (+), and a decrease in those responding with an inhibition, mPFC (-), was found. Altogether these findings demonstrate that a single ligature around the sciatic nerve was able to induce sensory, affective, cognitive, biochemical, and functional alterations already observed in other neuropathic pain models and it may be an appropriate and easily reproducible model for studying neuropathic pain mechanisms and treatments.

Assessment of potential neuropathic changes in cattle after cautery disbudding

Disbudding of calves is a standard husbandry procedure to reduce the risk of injuries to other cattle and to workers. Whereas acute pain resulting from disbudding has been studied extensively, little is known about chronic pain as a potential long-term consequence. The goal of the present study was to investigate possible morphological changes in the cornual nerve as a function of disbudding. Samples were collected from 17 randomly selected bulls and from 21 calves from a prospective clinical study. Among the calves, 13 were disbudded and 8 were sham-disbudded. Out of the disbudded calves, 4 showed signs of chronic pain. In all the animals, the infraorbital nerve was used as a methodological check. Morphological analysis included measuring minimal diameters of the axons present in both the cornual and infraorbital nerves. Sympathetic fibers were identified as based on the presence of Tyroxine hydroxylase (TH). TH-negative fibers were considered afferents. Trigeminal ganglia from the calves were immunostained for glial fibrillary acidic protein (GFAP) and Activating transcription factor 3 (ATF3). R. cornualis and N. infraorbitalis differed in terms of axon diameters and proportion of TH-positive fibers. Weak evidence (p > .091) of a difference in axon diameters between control and disbudded calves was found in R. cornualis, but the proportion of TH-positive fibers was alike in both groups. Average glial envelope and the percentages of ATF3-positive neurons revealed no difference between calves with and without signs of pain. Thus, available evidence is insufficient to support neuropathic changes as a result of disbudding in calves.

Stable gastric pentadecapeptide BPC 157 can improve the healing course of spinal cord injury and lead to functional recovery in rats

BACKGROUND

We focused on the therapeutic effects of the stable gastric pentadecapeptide BPC 157 in spinal cord injury using a rat model. BPC 157, of which the LD1 has not been achieved, has been implemented as an anti-ulcer peptide in inflammatory bowel disease trials and recently in a multiple sclerosis trial. In animals, BPC 157 has an anti-inflammatory effect and therapeutic effects in functional recovery and the rescue of somatosensory neurons in the sciatic nerve after transection, upon brain injury after concussive trauma, and in severe encephalopathies. Additionally, BPC 157 affects various molecular pathways.

METHODS

Therefore, BPC 157 therapy was administered by a one-time intraperitoneal injection (BPC 157 (200 or 2 μg/kg) or 0.9% NaCl (5 ml/kg)) 10 min after injury. The injury procedure involved laminectomy (level L2-L3) and a 60-s compression (neurosurgical piston (60-66 g) of the exposed dural sac of the sacrocaudal spinal cord). Assessments were performed at 1, 4, 7, 15, 30, 90, 180, and 360 days after injury.

RESULTS

All of the injured rats that received BPC 157 exhibited consistent clinical improvement, increasingly better motor function of the tail, no autotomy, and resolved spasticity by day 15. BPC 157 application largely counteracted changes at the microscopic level, including the formation of vacuoles and the loss of axons in the white matter, the formation of edema and the loss of motoneurons in the gray matter, and a decreased number of large myelinated axons in the rat caudal nerve from day 7. EMG recordings showed a markedly lower motor unit potential in the tail muscle.

CONCLUSION

Axonal and neuronal necrosis, demyelination, and cyst formation were counteracted. The functional rescue provided by BPC 157 after spinal cord injury implies that BPC 157 therapy can impact all stages of the secondary injury phase.

Children and adolescents with sickle cell disease have worse cold and mechanical hypersensitivity during acute painful events

Sickle cell disease (SCD) pain associates with cold temperature and touch. Patients and murine models with SCD have baseline thermal and mechanical pain. In SCD mice, the baseline hypersensitivity is exacerbated by experimental vaso-occlusive crises. We hypothesized that patients with SCD will similarly experience increased hypersensitivity to thermal and mechanical stimuli during acute painful events compared with baseline health. We conducted a prospective study of 24 patients with SCD aged 7 to 19 years. Patients underwent quantitative sensory testing to thermal (cold/heat) and mechanical stimuli on the thenar eminence of the nondominant hand (glabrous skin) and the lateral dorsum of the foot (hairy skin) during baseline health and within 48 hours of hospitalization for acute pain. Primary outcomes were changes in: (1) cold pain threshold (°C), (2) heat pain threshold (°C), and (3) mechanical pain threshold (g). Median age was 10.5 (interquartile range [IQR] 9-14.8) years, 67% were females, and 92% were on hydroxyurea. Patients with SCD had increased cold pain sensitivity in the hand during hospitalization compared with baseline (25.2°C [IQR 18.4-27.5°C] vs 21.3°C [IQR 4.9-26.2°C]; P = 0.011) and increased mechanical pain sensitivity in the foot during hospitalization (0.32 g [IQR 0.09-1.1 g] vs 1.7 g [IQR 0.4-8.3 g]; P = 0.003). There were no differences in heat pain sensitivity. The increased cold (P = 0.02) and mechanical (P = 0.0016) pain sensitivity during hospitalization persisted after adjusting for age, sex, hydroxyurea use, opioid consumption, and numeric pain score. Thus, cold and mechanical pain is significantly worse during an acute SCD painful event as compared to baseline health in patients with SCD.

Effect of NIR laser therapy by MLS-MiS source against neuropathic pain in rats: in vivo and ex vivo analysis

Neuropathic pain is characterized by an uncertain etiology and by a poor response to common therapies. The ineffectiveness and the frequent side effects of the drugs used to counteract neuropathic pain call for the discovery of new therapeutic strategies. Laser therapy proved to be effective for reducing pain sensitivity thus improving the quality of life. However, its application parameters and efficacy in chronic pain must be further analyzed. We investigated the pain relieving and protective effect of Photobiomodulation Therapy in a rat model of compressive mononeuropathy induced by Chronic Constriction Injury of the sciatic nerve (CCI). Laser (MLS-MiS) applications started 7 days after surgery and were performed ten times over a three week period showing a reduction in mechanical hypersensitivity and spontaneous pain that started from the first laser treatment until the end of the experiment. The ex vivo analysis highlighted the protective role of laser through the myelin sheath recovery in the sciatic nerve, inhibition of iNOS expression and enhancement of EAAT-2 levels in the spinal cord. In conclusion, this study supports laser treatment as a future therapeutic strategy in patients suffering from neuropathic pain induced by trauma.

Role of mitochondria in diabetic peripheral neuropathy: Influencing the NAD+-dependent SIRT1-PGC-1α-TFAM pathway

Survival of human peripheral nervous system neurons and associated distal axons is highly dependent on energy. Diabetes invokes a maladaptation in glucose and lipid energy metabolism in adult sensory neurons, axons and Schwann cells. Mitochondrial (Mt) dysfunction has been implicated as an etiological factor in failure of energy homeostasis that results in a low intrinsic aerobic capacity within the neuron. Over time, this energy failure can lead to neuronal and axonal degeneration and results in increased oxidative injury in the neuron and axon. One of the key pathways that is impaired in diabetic peripheral neuropathy (DPN) is the energy sensing pathway comprising the nicotinamide-adenine dinucleotide (NAD⁺)-dependent Sirtuin 1 (SIRT1)/peroxisome proliferator-activated receptor-γ coactivator α (PGC-1α)/Mt transcription factor A (TFAM or mtTFA) signaling pathway. Knockout of PGC-1α exacerbates DPN, whereas overexpression of human TFAM is protective. LY379268, a selective metabolomic glutamate receptor 2/3 (mGluR2/3) receptor agonist, also upregulates the SIRT1/PGC-1α/TFAM signaling pathway and prevents DPN through glutamate recycling in Schwann/satellite glial (SG) cells and by improving dorsal root ganglion (DRG) neuronal Mt function. Furthermore, administration of nicotinamide riboside (NR), a precursor of NAD⁺, prevents and reverses DPN, in part by increasing NAD⁺ levels and SIRT1 activity. In summary, we review the role of NAD⁺, mitochondria and the SIRT1-PGC-1α-TFAM pathway both from the perspective of pathogenesis and therapy in DPN.

Medicate or Meditate? Greater Pain Acceptance is Related to Lower Pain Medication Use in Persons With Chronic Pain and Spinal Cord Injury

OBJECTIVES

There is little information about whether use of pain self-management skills that are common targets of psychosocial interventions for pain are associated with reduced reliance on pain medications. The aim of this study was to test whether higher chronic pain acceptance, which is a readily modified pain self-management approach, is related to lower use of pain medications (eg, opioid medications, and gabapentinoids) in a sample with chronic pain and spinal cord injury (SCI).

MATERIALS AND METHODS

This is a cross-sectional survey study of pain medication use, pain severity and distribution (Brief Pain Inventory [BPI]), depressive symptoms (Patient Health Questionnaire-9 [PHQ-9]), and chronic pain acceptance (Chronic Pain Acceptance Questionnaire [CPAQ]) administered to a sample of 120 adults with chronic pain and SCI.

RESULTS

Regression results indicated that, above and beyond the effects of pain intensity, pain distribution, and depressive symptoms, higher pain acceptance was related to lower use of all types of pain medications, and lower odds of using opioid medications or gabapentinoids. Pain intensity was not related to pain medication use, but greater pain distribution was related to using more pain medications in general and to greater odds of using gabapentinoids.

DISCUSSION

Findings from this study indicate that those with chronic pain and SCI who have a more accepting orientation to pain are less reliant on pain medications, and thereby experience lower risks associated with medication consumption. Longitudinal, daily process, and clinical trial studies are needed to better understand the association between pain acceptance and pain medication consumption.

The Analgesic Effect of Venlafaxine and Its Mechanism on Oxaliplatin-Induced Neuropathic Pain in Mice

The analgesic effect of venlafaxine (VLX), which is a selective serotonin and noradrenaline reuptake inhibitor (SNRI), has been observed on oxaliplatin-induced neuropathic pain in mice. Significant allodynia was shown after oxaliplatin treatment (6 mg/kg, i.p.); acetone and von Frey hair tests were used to assess cold and mechanical allodynia, respectively. Intraperitoneal administration of VLX at 40 and 60 mg/kg, but not 10 mg/kg, significantly alleviated these allodynia. Noradrenaline depletion by pretreatment of N-(2-Chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4, 50 mg/kg, i.p.) blocked the relieving effect of VLX (40 mg/kg, i.p.) on cold and mechanical allodynia. However, serotonin depletion by three consecutive pretreatments of para-chlorophenylalanine (PCPA, 150 mg/kg/day, i.p.) only blocked the effect of VLX on mechanical allodynia. In cold allodynia, the α₂-adrenergic antagonist idazoxan (10 μg, i.t.), but not the α₁-adrenergic antagonist prazosin (10 μg, i.t.), abolished VLX-induced analgesia. Furthermore, idazoxan and 5-HT₃ receptor antagonist bemesetron (MDL-72222, 15 μg, i.t.), but not prazosin or mixed 5-HT_{1, 2} receptor antagonist methysergide (10 μg, i.t.), abolished VLX-induced analgesia in mechanical allodynia. In conclusion, 40 mg/kg of VLX treatment has a potent relieving effect against oxaliplatin-induced neuropathic pain, and α₂-adrenergic receptor, and both α₂-adrenergic and 5-HT₃ receptors are involved in this effect of VLX on cold and mechanical allodynia, respectively.

Management of the sciatic nerve during transfemoral amputation: a survey of Dutch surgeons

BACKGROUND

Neuropathic pain often occurs after leg amputation. It is unclear why some patients suffer from pain and others do not. Intraoperative nerve handling might correlate with these pain syndromes. It is unknown which nerve handling techniques are currently used among surgeons and whether a specific technique is related to eliciting these pain syndromes. The aim of this study was to investigate the current surgical sciatic nerve management used among surgeons in the Netherlands.

METHODS

A survey was carried out among Dutch surgeons. Additionally, surgical sciatic nerve management data were obtained from the surgical reports of transfemoral amputations that were performed in our hospital.

RESULTS

Seventy-eight of the 102 online surveys were returned. Sharp transection without ligation was used most often (41.0%), followed by sharp transection after ligation (30.8%). There were 38 transfemoral amputations performed in our hospital. Sharp transection after ligation was most often used (55.3%), followed by transection with electrocautery (13.2%).

CONCLUSIONS

All techniques of transecting the sciatic nerve are currently performed by Dutch surgeons. These results provide a solid basis to perform future studies to investigate whether a specific surgical technique is related to postamputation pain syndromes.

Cyclic nucleotide signaling in sensory neuron hyperexcitability and chronic pain after nerve injury

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Activation of cAMP-PKA and cGMP-PKG pathways contributes to injury-induced sensory neuron hyperexcitability.

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Activation of cAMP and cGMP contributes to the development of bone cancer pain.

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PAR2 activation mediates injury-induced cAMP-dependent sensory neuron hyperexcitability.

The cyclic nucleotide signaling, including cAMP-PKA and cGMP-PKG pathways, has been well known to play critical roles in regulating cellular growth, metabolism and many other intracellular processes. In recent years, more and more studies have uncovered the roles of cAMP and cGMP in the nervous system. The cAMP and cGMP signaling mediates chronic pain induced by different forms of injury and stress. Here we summarize the roles of cAMP-PKA and cGMP-PKG signaling pathways in the pathogenesis of chronic pain after nerve injury. In addition, acute dissociation and chronic compression of the dorsal root ganglion (DRG) neurons, respectively, leads to neural hyperexcitability possibly through PAR2 activation-dependent activation of cAMP-PKA pathway. Clinically, radiotherapy can effectively alleviate bone cancer pain at least partly through inhibiting the cancer cell-induced activation of cAMP-PKA pathway. Roles of cyclic nucleotide signaling in neuropathic and inflammatory pain are also seen in many other animal models and are involved in many pro-nociceptive mechanisms including the activation of hyperpolarization-activated cyclic nucleotide (HCN)-modulated ion channels and the exchange proteins directly activated by cAMP (EPAC). Further understanding the roles of cAMP and cGMP signaling in the pathogenesis of chronic pain is theoretically significant and clinically valuable for treatment of chronic pain.

Nerve capping with a nerve conduit for the treatment of painful neuroma in the rat sciatic nerve

OBJECTIVE

Treatment of painful neuroma remains difficult, despite the availability of numerous surgical procedures. Recently, nerve capping treatment for painful neuroma using artificial nerve conduits has been introduced in clinical and basic research. However, the appropriate length of the nerve conduit and the pain relief mechanism have not been determined. In this study the authors aimed to investigate nerve capping treatment with a bioabsorbable nerve conduit using the rat sciatic nerve amputation model. Using histological analysis, the authors focused on the nerve conduit length and pain relief mechanism.

METHODS

Sixteen Sprague Dawley rats were evaluated for neuropathic pain using an autotomy (self-amputation) score and gross and histological changes of the nerve stump 2, 4, 8, and 12 weeks after sciatic nerve neurectomy without capping. Forty-five rats were divided into 3 experimental groups, no capping (control; n = 15), capping with a 3-mm nerve conduit (n = 15), and capping with a 6-mm nerve conduit (n = 15). All rats were evaluated using an autotomy score and nerve stump histology 12 weeks after neurectomy. The nerve conduit was approximately 0.5 mm larger than the 1.5-mm diameter of the rat sciatic nerves to prevent nerve constriction.

RESULTS

The autotomy scores gradually exacerbated with time. Without capping, a typical bulbous neuroma was formed due to random axonal regeneration 2 weeks after neurectomy. Subsequently, the adhesion surrounding the neuroma expanded over time for 12 weeks, and at the 12-week time point, the highest average autotomy scores were observed in the no-capping (control) group, followed by the 3- and the 6-mm nerve conduit groups. Histologically, the distal axonal fibers became thinner and terminated within the 6-mm nerve conduit, whereas they were elongated and protruded across the 3-mm nerve conduit. Minimal perineural scar formation was present around the terminated axonal fibers in the 6-mm nerve conduit group. Expressions of anti-α smooth muscle actin and anti-sigma-1 receptor antibodies in the nerve stump significantly decreased in the 6-mm nerve conduit group.

CONCLUSIONS

In the rat sciatic nerve amputation model, nerve capping treatment with a bioabsorbable nerve conduit provided relief from neuroma-induced neuropathic pain and prevented perineural scar formation and neuroinflammation around the nerve stump. The appropriate nerve conduit length was determined to be more than 4 times the diameter of the original nerve.

Neurons and Microglia; A Sickly-Sweet Duo in Diabetic Pain Neuropathy

Diabetes is a common condition characterized by persistent hyperglycemia. High blood sugar primarily affects cells that have a limited capacity to regulate their glucose intake. These cells include capillary endothelial cells in the retina, mesangial cells in the renal glomerulus, Schwann cells, and neurons of the peripheral and central nervous systems. As a result, hyperglycemia leads to largely intractable complications such as retinopathy, nephropathy, hypertension, and neuropathy. Diabetic pain neuropathy is a complex and multifactorial disease that has been associated with poor glycemic control, longer diabetes duration, hypertension, advanced age, smoking status, hypoinsulinemia, and dyslipidemia. While many of the driving factors involved in diabetic pain are still being investigated, they can be broadly classified as either neuron -intrinsic or -extrinsic. In neurons, hyperglycemia impairs the polyol pathway, leading to an overproduction of reactive oxygen species and reactive nitrogen species, an enhanced formation of advanced glycation end products, and a disruption in Na⁺/K⁺ ATPase pump function. In terms of the extrinsic pathway, hyperglycemia leads to the generation of both overactive microglia and microangiopathy. The former incites a feed-forward inflammatory loop that hypersensitizes nociceptor neurons, as observed at the onset of diabetic pain neuropathy. The latter reduces neurons' access to oxygen, glucose and nutrients, prompting reductions in nociceptor terminal expression and losses in sensation, as observed in the later stages of diabetic pain neuropathy. Overall, microglia can be seen as potent and long-lasting amplifiers of nociceptor neuron activity, and may therefore constitute a potential therapeutic target in the treatment of diabetic pain neuropathy.

Serum Level of Metalloproteinase-2 but not Metalloproteinase-9 Rises in Patients With Failed Back Surgery Syndrome After Spinal Cord Stimulation

OBJECTIVES

The impact of spinal cord stimulation (SCS) on serum levels of metalloproteinase-2 (MMP-2) and metalloproteinase-9 (MMP-9) was assessed in a group of patients with failed back surgery syndrome (FBSS). The study was to give new insights into the SCS mechanism of action and the role of MMP-2 and MMP-9 in the development of NP.

MATERIAL AND METHODS

Clinical assessments were performed and biochemical markers were determined in two groups of patients: the control group (24 individuals) and the FBSS group (24 patients). Seventeen patients with the FBSS had SCS implanted and were examined before surgical procedure, one month after (17 patients), and three months after operation (12 patients). Clinical status was assessed with the use numeric rating scale, pain rating index of McGill pain questionnaire, Oswestry disability index and Beck depression inventory. MMP-2 and MMP-9 serum levels were determined using gelatin zymography. Immunoenzymatic method was employed to determine plasma concentrations of tissue inhibitors of metalloproteinases (TIMPs).

RESULTS

Levels of MMP-2 and TIMP-2 were higher in the FBSS group compared to the control group. The difference was statistically significant (p < 0.001 and p = 0.004, respectively). The concentration of MMP-2 was significantly increased (p = 0.0135) one-month post-SCS and remained elevated but stable up to three months after implantation. TIMP-2, MMP-2/TIMP-2, MMP-9, TIMP-1, and MMP-9/TIMP-1 serum levels did not change significantly.

CONCLUSIONS

MMPs may play a role in the development of FBSS. SCS increases the already elevated MMP-2 serum levels which are associated with neuroinflammatory processes in FBSS patients.

Enhancing the Outcome of Traumatic Sensory Nerve Lesions of the Hand by Additional Use of a Chitosan Nerve Tube in Primary Nerve Repair: A Randomized Controlled Bicentric Trial

BACKGROUND

Peripheral sensory nerve injuries present a significant yet common challenge in acute hand trauma surgery. Standard treatment remains microsurgical end-to-end nerve repair where appropriate. Permanent loss of sensitivity and painful neuroma formation are typical sequelae of unsuccessful surgery. The objective of this study was to evaluate whether the additional use of a chitosan nerve tube in primary nerve repair positively influences sensory recovery.

METHODS

A randomized, controlled, two-center trial with parallel group design and double-blind assessment was conducted to demonstrate the superiority of the additional use of a chitosan nerve tube compared with microsurgical nerve repair alone. Seventy-four participants were enrolled. The primary outcome parameter used was degree of static two-point discrimination at 6 months after surgery. Additional secondary outcome parameters included filament recognition testing (Semmes-Weinstein); pain; neuroma development; and the Disabilities of the Arm, Shoulder and Hand score.

RESULTS

Nerve repair with additional use of chitosan nerve tubes (intervention group) significantly increased both tactile gnosis (expressed by two-point discrimination) and sensitivity (expressed by Semmes-Weinstein testing). The mean two-point discrimination at 6-month follow-up was 8 mm (range, 2 to 20 mm) in the control group and 6.3 mm (range, 1 to 15 mm) in the intervention group, respectively (p = 0.029). Two-point discrimination correlated with the Disabilities of the Arm, Shoulder and Hand score. In the control and intervention groups, respectively, three versus zero neuromas were found.

CONCLUSIONS

Peripheral sensory nerve regeneration can be improved significantly by additional use of a chitosan nerve tube. An improved ability of static two-point discrimination is clinically relevant.

CLINICAL QUESTION/LEVEL OF EVIDENCE

Therapeutic, I.

Metformin relieves neuropathic pain after spinal nerve ligation via autophagy flux stimulation

Neuropathic pain is a well‐known type of chronic pain caused by damage to the nervous system. Autophagy is involved in the development and/or progression of many diseases, including neuropathic pain. Emerging evidence suggests that metformin relieves neuropathic pain in several neuropathic pain models; however, metformin's cellular and molecular mechanism for pain relief remains unknown. In this study, we investigated the therapeutic effects of metformin on pain relief after spinal nerve ligation (SNL) and its underlying mechanism of autophagy regulation. Behavioural analysis, histological assessment, expression of c‐Fos and molecular biological changes, as well as ultrastructural features, were investigated. Our findings showed that the number of autophagosomes and expression of autophagy markers, such as LC3 and beclin1, were increased, while the autophagy substrate protein p62, as well as the ubiquitinated proteins, were accumulated in the ipsilateral spinal cord. However, metformin enhanced the expression of autophagy markers, while it abrogated the abundance of p62 and ubiquitinated proteins. Blockage of autophagy flux by chloroquine partially abolished the apoptosis inhibition and analgesic effects of metformin on SNL. Taken together, these results illustrated that metformin relieved neuropathic pain through autophagy flux stimulation and provided a new direction for metformin drug development to treat neuropathic pain.

A New Model of Sensorial Neuron-Like Cells for HTS of Novel Analgesics for Neuropathic Pain

In this study we developed a new translational phenotypic in vitro model for high-throughput screening (HTS) of novel analgesics for treating neuropathic pain, in order to address the poor translation of traditional recombinant models. The immortalized dorsal root ganglia (DRG) neuron-like F11 cell line was selected based on its phenotype after differentiation. The acquisition of neuronal characteristics was evaluated by measuring the expression of TrkA as a DRG neuron marker ( p < 0.01) as well as by measuring the global neurite length ( p < 0.001). The response of F11 cells to ATP and KCl was obtained by measuring intracellular calcium concentration, dynamic mass redistribution, and membrane potential. A KCl-induced increase of intracellular calcium levels was chosen as the readout because of the better signal quality, higher reproducibility, and greater compatibility with HTS assay requirements compared with other methods. The response to KCl differed significantly between differentiated and undifferentiated cells ( p < 0.05), with an EC₅₀ value of 5 mM in differentiated cells. The model was validated by screening the Prestwick Chemical Library. Five hits already proposed for neuropathic-related pain were identified, with IC₅₀ values between 1 and 7 µM. This cell model provides a new tool for screening novel analgesics for the relief of neuropathic pain.

Decreased abundance of TRESK two-pore domain potassium channels in sensory neurons underlies the pain associated with bone metastasis

Bone metastasis–associated VEGF suppresses neuronal K + channels and increases pain in rats.

Spinal RNF20-Mediated Histone H2B Monoubiquitylation Regulates mGluR5 Transcription for Neuropathic Allodynia

To date, histone H2B monoubiquitination (H2Bub), a mark associated with transcriptional elongation and ongoing transcription, has not been linked to the development or maintenance of neuropathic pain states. Here, using male Sprague Dawley rats, we demonstrated spinal nerve ligation (SNL) induced behavioral allodynia and provoked ring finger protein 20 (RNF20)-dependent H2Bub in dorsal horn. Moreover, SNL provoked RNF20-mediated H2Bub phosphorylated RNA polymerase II (RNAPII) in the promoter fragments of mGluR5, thereby enhancing mGluR5 transcription/expression in the dorsal horn. Conversely, focal knockdown of spinal RNF20 expression reversed not only SNL-induced allodynia but also RNF20/H2Bub/RNAPII phosphorylation-associated spinal mGluR5 transcription/expression. Notably, TNF-α injection into naive rats and specific neutralizing antibody injection into SNL-induced allodynia rats revealed that TNF-α-associated allodynia involves the RNF20/H2Bub/RNAPII transcriptional axis to upregulate mGluR5 expression in the dorsal horn. Collectively, our findings indicated TNF-α induces RNF20-drived H2B monoubiquitination, which facilitates phosphorylated RNAPII-dependent mGluR5 transcription in the dorsal horn for the development of neuropathic allodynia.SIGNIFICANCE STATEMENT Histone H2B monoubiquitination (H2Bub), an epigenetic post-translational modification, positively correlated with gene expression. Here, TNF-α participated in neuropathic pain development by enhancing RNF20-mediated H2Bub, which facilitates phosphorylated RNAPII-dependent mGluR5 transcription in dorsal horn. Our finding potentially identified neuropathic allodynia pathophysiological processes underpinning abnormal nociception processing and opens a new avenue for the development of novel analgesics.

What does the grey matter decrease in the medial prefrontal cortex reflect in people with chronic pain?

BACKGROUND AND OBJECTIVE

Alterations in the grey matter volume of several brain regions have been reported in people with chronic pain. The most consistent observation is a decrease in grey matter volume in the medial prefrontal cortex. These findings are important as the medial prefrontal cortex plays a critical role in emotional and cognitive processing in chronic pain. Although a logical cause of grey matter volume decrease may be neurodegeneration, this is not supported by the current evidence. Therefore, the purpose of this review was to evaluate the existing literature to unravel what the decrease in medial prefrontal cortex grey matter volume in people with chronic pain may represent on a biochemical and cellular level.

DATABASES AND DATA TREATMENT

A literature search for this topical review was conducted using PubMed and SCOPUS library. Search terms included chronic pain, pain, medial prefrontal cortex, anterior cingulate cortex, grey matter, neurochemistry, spectroscopy, magnetic resonance imaging, positron emission tomography, dendrite, neurodegeneration, glia, astrocyte, microglia, neurotransmitter, glutamate, GABA and different combinations of these terms.

RESULTS

Adopting a stress model of chronic pain, two major pathways are proposed that contribute to grey matter volume decrease in the medial prefrontal cortex: (a) changes in dendritic morphology as a result of hypothalamic-pituitary axis dysfunction and (b) neurotransmitter dysregulation, specifically glutamate and γ-Aminobutyric acid, which affects local microvasculature.

CONCLUSION

Our model proposes new mechanisms in chronic pain pathophysiology responsible for mPFC grey matter loss as alternatives to neurodegeneration.

SIGNIFICANCE

It is unclear what the decrease in medial prefrontal cortex grey matter volume represents in chronic pain. The most attractive reason is neurodegeneration. However, there is no evidence to support this. Our review reveals nondegenerative causes of decreased medial prefrontal grey matter to guide future research into chronic pain pathophysiology.