Chronic neuropathic pain is accompanied by global changes in gene expression and shares pathobiology with neurodegenerative diseases

The Voltage-Gated Calcium Channel α2δ Subunit in Neuropathic Pain

Neuropathic pain (NP) is a chronic pain caused by injury or disease of the somatosensory nervous system, or it can be directly caused by disease. It often presents with clinical features like spontaneous pain, hyperalgesia, and dysesthesia. At present, voltage-gated calcium ion channels (VGCCs) are known to be closely related to the development of NP, especially the α2δ subunit. The α2δ subunit is a regulatory subunit of VGCCs. It exists mainly in the brain and peripheral nervous system, especially in nerve cells, and it plays a crucial part in regulating presynaptic and postsynaptic functions. Furthermore, the α2δ subunit influences neuronal excitation and pain signaling by promoting its expression and localization through binding to VGCC-related subunits. The α2δ subunit is widely used in the management of NP as a target of antiepileptic drugs gabapentin and pregabalin. Although drug therapy is one of the treatments for NP, its clinical application is limited due to the adverse reactions caused by drug therapy. Therefore, further research on the therapeutic target α2δ subunit is needed, and attempts are made to obtain an effective treatment for relieving NP without side effects. This review describes the current associated knowledge on the function of the α2δ subunit in perceiving and modulating NP.

Pregabalin produces analgesia in males but not females in an animal model of chronic widespread muscle pain

Introduction

Pregabalin, which acts on the α2δ-1 subunit of voltage-gated calcium channels, relieves ≥50% of pain in a third of individuals with fibromyalgia. Thus far, preclinical studies of pregabalin have predominantly used male animals.

Objectives

The purpose of our study was to investigate potential sex differences in the analgesic efficacy of pregabalin that may contribute to disparities in human outcomes.

Methods

We used a mouse model of chronic widespread muscle pain (CWP) to test the effects of pregabalin on muscle hyperalgesia, nonreflexive pain, and motor behaviors. The CWP pain model combines 2 pH 4.0 saline injections, spaced 5 days apart, into the gastrocnemius muscle and produces bilateral muscle hyperalgesia. Furthermore, we explored sex differences in the mRNA and protein expression of the α2δ-1 subunit of voltage-gated calcium channels in the dorsal horn of the spinal cord and dorsal root ganglia after development of CWP.

Results

Pregabalin fully attenuated muscle hyperalgesia bilaterally in male but not female mice with equal motor deficits produced in both sexes. In addition, using the conditioned place preference test, mice of both sexes with CWP spent significantly more time in the pregabalin-paired chamber compared with baseline, but not significantly greater than pain-free controls. Chronic widespread muscle pain produced no changes in α2δ-1 subunit mRNA or protein expression in the dorsal horn of the spinal cord or dorsal root ganglia in either sex.

Conclusion

Overall, these findings indicate pregabalin may be more effective in treating CWP in males, but the factors leading to these differences are not fully understood.

Tapping into the vocal learning and rhythmic synchronization hypothesis

In this article, I present three main points that could benefit the "vocal learning and rhythmic synchronization hypothesis", encompassing neurogenetic mechanisms of gene expression transmission and single motor neuron function, classification of different behavioral motor phenotypes (e.g., spontaneous vs. voluntary), and other evolutionary considerations (i.e., the involvement of reward mechanisms).

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

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

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

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

A review of dorsal root ganglia and primary sensory neuron plasticity mediating inflammatory and chronic neuropathic pain

Pain is a sensory state resulting from complex integration of peripheral nociceptive inputs and central processing. Pain consists of adaptive pain that is acute and beneficial for healing and maladaptive pain that is often persistent and pathological. Pain is indeed heterogeneous, and can be expressed as nociceptive, inflammatory, or neuropathic in nature. Neuropathic pain is an example of maladaptive pain that occurs after spinal cord injury (SCI), which triggers a wide range of neural plasticity. The nociceptive processing that underlies pain hypersensitivity is well-studied in the spinal cord. However, recent investigations show maladaptive plasticity that leads to pain, including neuropathic pain after SCI, also exists at peripheral sites, such as the dorsal root ganglia (DRG), which contains the cell bodies of sensory neurons. This review discusses the important role DRGs play in nociceptive processing that underlies inflammatory and neuropathic pain. Specifically, it highlights nociceptor hyperexcitability as critical to increased pain states. Furthermore, it reviews prior literature on glutamate and glutamate receptors, voltage-gated sodium channels (VGSC), and brain-derived neurotrophic factor (BDNF) signaling in the DRG as important contributors to inflammatory and neuropathic pain. We previously reviewed BDNF's role as a bidirectional neuromodulator of spinal plasticity. Here, we shift focus to the periphery and discuss BDNF-TrkB expression on nociceptors, non-nociceptor sensory neurons, and non-neuronal cells in the periphery as a potential contributor to induction and persistence of pain after SCI. Overall, this review presents a comprehensive evaluation of large bodies of work that individually focus on pain, DRG, BDNF, and SCI, to understand their interaction in nociceptive processing.

Over-expression of miR-3584-5p Represses Nav1.8 Channel Aggravating Neuropathic Pain caused by Chronic Constriction Injury

Nav1.8, a tetrodotoxin-resistant voltage-gated sodium channels (VGSCs) subtype encoded by SCN10A, which plays an important role in the production and transmission of peripheral neuropathic pain signals. Studies have shown that VGSCs may be key targets of MicroRNAs (miRNAs) in the regulation of neuropathic pain. In our study, bioinformatics analysis showed that the targeting relationship between miR-3584-5p and Nav1.8 was the most closely. The purpose of this study was to investigate the roles of miR-3584-5p and Nav1.8 in neuropathic pain. The effects of miR-3584-5p on chronic constriction injury (CCI)-induced neuropathic pain in rats was investigated by intrathecal injection of miR-3584-5p agomir (an agonist, 20 μM, 15 μL) or antagomir (an antagonist, 20 μM, 15 μL). The results showed that over-expression of miR-3584-5p aggravated neuronal injury by hematoxylin-eosin (H&E) staining and mechanical/thermal hypersensitivity in CCI rats. MiR-3584-5p indirectly inhibited the expression of Nav1.8 by up-regulating the expression of key proteins in the ERK5/CREB signaling pathway, and also inhibited the current density of the Nav1.8 channel, changed its channel dynamics characteristic, thereby accelerating the transmission of pain signals, and further aggravating pain. Similarly, in PC12 and SH-SY5Y cell cultures, miR-3584-5p increased the level of reactive oxygen species (ROS) and inhibited mitochondrial membrane potential (Δψm) in the mitochondrial pathway, decreased the ratio of apoptosis-related factor Bcl-2/Bax, and thus promoted neuronal apoptosis. In brief, over-expression of miR-3584-5p aggravates neuropathic pain by directly inhibiting the current density of Nav1.8 channel and altering its channel dynamics, or indirectly inhibiting Nav1.8 expression through ERK5/CREB pathway, and promoting apoptosis through mitochondrial pathway.

Research trends and hotspots of neuropathic pain in neurodegenerative diseases: a bibliometric analysis

Background

Neuropathic pain is caused by a neurological injury or disease and can have a significant impact on people's daily lives. Studies have shown that neuropathic pain is commonly associated with neurodegenerative diseases. In recent years, there has been a lot of literature on the relationship between neuropathic pain and neurodegenerative diseases. However, bibliometrics is rarely used in analyzing the general aspects of studies on neuropathic pain in neurodegenerative diseases.

Methods

The bibliometric analysis software CiteSpace and VOSviewer were used to analyze the knowledge graph of 387 studies in the Science Citation Index Expanded of the Web of Science Core Collection Database.

Results

We obtained 2,036 documents through the search, leaving 387 documents after culling. 387 documents were used for the data analysis. The data analysis showed that 330 papers related to neuropathic pain in neurodegenerative diseases were published from 2007-2022, accounting for 85.27% of all published literature. In terms of contributions to the scientific study of neuropathic pain, the United States is in the top tier, with the highest number of publications, citations, and H-indexes.

Conclusion

The findings in our study may provide researchers with useful information about research trends, frontiers, and cooperative institutions. Multiple sclerosis, Parkinson's disease, and Alzheimer's disease are the three most studied neurodegenerative diseases. Among the pathological basis of neurodegenerative diseases, microglia-regulated neuroinflammation is a hot research topic. Deep brain stimulation and gamma knife radiosurgery are two popular treatments.

Injury-induced activation of the endocannabinoid system promotes axon regeneration

Regeneration after a peripheral nerve injury still remains a challenge, due to the limited regenerative potential of axons after injury. While the endocannabinoid system (ECS) has been widely studied for its neuroprotective and analgesic effects, its role in axonal regeneration and during the conditioning lesion remains unexplored. In this study, we observed that a peripheral nerve injury induces axonal regeneration through an increase in the endocannabinoid tone. We also enhanced the regenerative capacity of dorsal root ganglia (DRG) neurons through the inhibition of endocannabinoid degradative enzyme MAGL or a CB1R agonist. Our results suggest that the ECS, via CB1R and PI3K-pAkt pathway activation, plays an important role in promoting the intrinsic regenerative capacity of sensory neurons after injury.

Nerve injury increases native Ca V 2.2 trafficking in dorsal root ganglion mechanoreceptors

ABSTRACT

Neuronal N-type (Ca V 2.2) voltage-gated calcium channels are essential for neurotransmission from primary afferent terminals in the dorsal horn. In this study, we have used a knockin mouse containing Ca V 2.2 with an inserted extracellular hemagglutinin tag (Ca V 2.2_HA), to visualise the pattern of expression of endogenous Ca V 2.2 in dorsal root ganglion (DRG) neurons and their primary afferents in the dorsal horn. We examined the effect of partial sciatic nerve ligation (PSNL) and found an increase in Ca V 2.2_HA only in large and medium dorsal root ganglion neurons and also in deep dorsal horn synaptic terminals. Furthermore, there is a parallel increase in coexpression with GFRα1, present in a population of low threshold mechanoreceptors, both in large DRG neurons and in their terminals. The increased expression of Ca V 2.2_HA in these DRG neurons and their terminals is dependent on the presence of the auxiliary subunit α 2 δ-1, which is required for channel trafficking to the cell surface and to synaptic terminals, and it likely contributes to enhanced synaptic transmission at these synapses following PSNL. By contrast, the increase in GFRα1 is not altered in α 2 δ-1-knockout mice. We also found that following PSNL, there is patchy loss of glomerular synapses immunoreactive for Ca V 2.2_HA and CGRP or IB4, restricted to the superficial layers of the dorsal horn. This reduction is not dependent on α 2 δ-1 and likely reflects partial deafferentation of C-nociceptor presynaptic terminals. Therefore, in this pain model, we can distinguish 2 different events affecting specific DRG terminals, with opposite consequences for Ca V 2.2_HA expression and function in the dorsal horn.

EGR3 regulates opioid-related nociception and motivation in male rats

Chronic pain can be a debilitating condition, leading to profound changes in nearly every aspect of life. However, the reliance on opioids such as oxycodone for pain management is thought to initiate dependence and addiction liability. The neurobiological intersection at which opioids relieve pain and possibly transition to addiction is poorly understood. Using RNA sequencing pathway analysis in rats with complete Freund's adjuvant (CFA)-induced chronic inflammation, we found that the transcriptional signatures in the medial prefrontal cortex (mPFC; a brain region where pain and reward signals integrate) elicited by CFA in combination with oxycodone differed from those elicited by CFA or oxycodone alone. However, the expression of Egr3 was augmented in all animals receiving oxycodone. Furthermore, virus-mediated overexpression of EGR3 in the mPFC increased mechanical pain relief but not the affective aspect of pain in animals receiving oxycodone, whereas pharmacological inhibition of EGR3 via NFAT attenuated mechanical pain relief. Egr3 overexpression also increased the motivation to obtain oxycodone infusions in a progressive ratio test without altering the acquisition or maintenance of oxycodone self-administration. Taken together, these data suggest that EGR3 in the mPFC is at the intersection of nociceptive and addictive-like behaviors.

The Roles of Imaging Biomarkers in the Management of Chronic Neuropathic Pain

Chronic neuropathic pain (CNP) affects around 10% of the general population and has a significant social, emotional, and economic impact. Current diagnosis techniques rely mainly on patient-reported outcomes and symptoms, which leads to significant diagnostic heterogeneity and subsequent challenges in management and assessment of outcomes. As such, it is necessary to review the approach to a pathology that occurs so frequently, with such burdensome and complex implications. Recent research has shown that imaging methods can detect subtle neuroplastic changes in the central and peripheral nervous system, which can be correlated with neuropathic symptoms and may serve as potential markers. The aim of this paper is to review available imaging methods used for diagnosing and assessing therapeutic efficacy in CNP for both the preclinical and clinical setting. Of course, further research is required to standardize and improve detection accuracy, but available data indicate that imaging is a valuable tool that can impact the management of CNP.

Astrocyte Reaction to Catechol-Induced Cytotoxicity Relies on the Contact with Microglia Before Isolation

Astrocytes preserve the brain microenvironment homeostasis in order to protect other brain cells, mainly neurons, against damages. Glial cells have specific functions that are important in the context of neuronal survival in different models of central nervous system (CNS) diseases. Microglia are among these cells, secreting several molecules that can modulate astrocyte functions. Although 1,2-dihydroxybenzene (catechol) is a neurotoxic monoaromatic compound of exogenous origin, several endogenous molecules also present the catechol group. This study compared two methods to obtain astrocyte-enriched cultures from newborn Wistar rats of both sexes. In the first technique (P1), microglial cells began to be removed early 48 h after primary mixed glial cultures were plated. In the second one (P2), microglial cells were late removed 7 to 10 days after plating. Both cultures were exposed to catechol for 72 h. Catechol was more cytotoxic to P1 cultures than to P2, decreasing cellularity and changing the cell morphology. Microglial-conditioned medium (MCM) protected P1 cultures and inhibited the catechol autoxidation. P2 cultures, as well as P1 in the presence of 20% MCM, presented long, dense, and fibrillary processes positive for glial fibrillary acidic protein, which retracted the cytoplasm when exposed to catechol. The Ngf and Il1beta transcription increased in P1, meanwhile astrocytes expressed more Il10 in P2. Catechol decreased Bdnf and Il10 in P2 cultures, and it decreased the expression of Il1beta in both conditions. A prolonged contact with microglia before isolation of astrocyte-enriched cultures modifies astrocyte functions and morphology, protecting these cells against catechol-induced cytotoxicity.

Apoptosis and (in) Pain—Potential Clinical Implications

The deregulation of apoptosis is involved in the development of several pathologies, and recent evidence suggests that apoptosis may be involved in chronic pain, namely in neuropathic pain. Neuropathic pain is a chronic pain state caused by primary damage or dysfunction of the nervous system; however, the details of the molecular mechanisms have not yet been fully elucidated. Recently, it was found that nerve endings contain transient receptor potential (TRP) channels that sense and detect signals released by injured tissues and respond to these damage signals. TRP channels are similar to the voltage-gated potassium channels or nucleotide-gated channels that participate in calcium and magnesium homeostasis. TRP channels allowing calcium to penetrate into nerve terminals can activate apoptosis, leading to nerve terminal destruction. Further, some TRPs are activated by acid and reactive oxygen species (ROS). ROS are mainly produced in the mitochondrial respiratory chain, and an increase in ROS production and/or a decrease in the antioxidant network may induce oxidative stress (OS). Depending on the OS levels, they can promote cellular proliferation and/or cell degeneration or death. Previous studies have indicated that proinflammatory cytokines, such as tumor necrosis factor-α (TNF-α), play an important role in the peripheral mediation of neuropathic pain. This article aims to perform a review of the involvement of apoptosis in pain, particularly the role of OS and neuroinflammation, and the clinical relevance of this knowledge. The potential discovery of new biomarkers and therapeutic targets can result in the development of more effective and targeted drugs to treat chronic pain, namely neuropathic pain. Highlights: Oxidative stress and neuroinflammation can activate cell signaling pathways that can lead to nerve terminal destruction by apoptosis. These could constitute potential new pain biomarkers and targets for therapy in neuropathic pain.

Long March Toward Safe and Effective Analgesia by Enhancing Gene Expression of Kcc2: First Steps Taken

Low intraneuronal chloride in spinal cord dorsal horn pain relay neurons is critical for physiologic transmission of primary pain afferents because low intraneuronal chloride dictates whether GABA-ergic and glycin-ergic neurotransmission is inhibitory. If the neuronal chloride elevates to pathologic levels, then spinal cord primary pain relay becomes leaky and exhibits the behavioral hallmarks of pathologic pain, namely hypersensitivity and allodynia. Low chloride in spinal cord dorsal horn neurons is maintained by proper gene expression of Kcc2 and sustained physiologic function of the KCC2 chloride extruding electroneutral transporter. Peripheral nerve injury and other forms of neural injury evoke greatly diminished Kcc2 gene expression and subsequent corruption of inhibitory neurotransmission in the spinal cord dorsal horn, thus causing derailment of the gate function for pain. Here I review key discoveries that have helped us understand these fundamentals, and focus on recent insights relating to the discovery of Kcc2 gene expression enhancing compounds via compound screens in neurons. One such study characterized the kinase inhibitor, kenpaullone, more in-depth, revealing its function as a robust and long-lasting analgesic in preclinical models of nerve injury and cancer bone pain, also elucidating its mechanism of action via GSK3β inhibition, diminishing delta-catenin phosphorylation, and facilitating its nuclear transfer and subsequent enhancement of Kcc2 gene expression by de-repressing Kaiso epigenetic transcriptional regulator. Future directions re Kcc2 gene expression enhancement are discussed, namely combination with other analgesics and analgesic methods, such as spinal cord stimulation and electroacupuncture, gene therapy, and leveraging Kcc2 gene expression-enhancing nanomaterials.

Gadd45 in Neuronal Development, Function, and Injury

The growth arrest and DNA damage-inducible (Gadd) 45 proteins have been associated with numerous cellular mechanisms including cell cycle control, DNA damage sensation and repair, genotoxic stress, neoplasia, and molecular epigenetics. The genes were originally identified in in vitro screens of irradiation- and interleukin-induced transcription and have since been implicated in a host of normal and aberrant central nervous system processes. These include early and postnatal development, injury, cancer, memory, aging, and neurodegenerative and psychiatric disease states. The proteins act through a variety of molecular signaling cascades including the MAPK cascade, cell cycle control mechanisms, histone regulation, and epigenetic DNA demethylation. In this review, we provide a comprehensive discussion of the literature implicating each of the three members of the Gadd45 family in these processes.

Beyond CGRP: The calcitonin peptide family as targets for migraine and pain

The CGRP system has emerged as a key pharmacological target for the treatment of migraine. However, some individuals who suffer from migraine have low or no response to anti-CGRP or other treatments, suggesting the need for additional clinical targets. CGRP belongs to the calcitonin family of peptides, which includes calcitonin, amylin, adrenomedullin and adrenomedullin 2. These peptides display a range of pro-nociceptive and anti-nociceptive actions, in primary headache conditions such as migraine. Calcitonin family peptides also show expression at sites relevant to migraine and pain. This suggests that calcitonin family peptides and their receptors, beyond CGRP, may be therapeutically useful in the treatment of migraine and other pain disorders. This review considers the localisation of the calcitonin family in peripheral pain pathways and discusses how they may contribute to migraine and pain. LINKED ARTICLES: This article is part of a themed issue on Advances in Migraine and Headache Therapy (BJP 75th Anniversary). To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.3/issuetoc.

Discrepancy in the Usage of GFAP as a Marker of Satellite Glial Cell Reactivity

Satellite glial cells (SGCs) surrounding the neuronal somas in peripheral sensory ganglia are sensitive to neuronal stressors, which induce their reactive state. It is believed that such induced gliosis affects the signaling properties of the primary sensory neurons and is an important component of the neuropathic phenotype leading to pain and other sensory disturbances. Efforts to understand and manipulate such gliosis relies on reliable markers to confirm induced SGC reactivity and ultimately the efficacy of targeted intervention. Glial fibrillary acidic protein (GFAP) is currently the only widely used marker for such analyses. However, we have previously described the lack of SGC upregulation of GFAP in a mouse model of sciatic nerve injury, suggesting that GFAP may not be a universally suitable marker of SGC gliosis across species and experimental models. To further explore this, we here investigate the regulation of GFAP in two different experimental models in both rats and mice. We found that whereas GFAP was upregulated in both rodent species in the applied inflammation model, only the rat demonstrated increased GFAP in SGCs following sciatic nerve injury; we did not observe any such GFAP upregulation in the mouse model at either protein or mRNA levels. Our results demonstrate an important discrepancy between species and experimental models that prevents the usage of GFAP as a universal marker for SGC reactivity.

Response to pregabalin and progesterone differs in male and female rat models of neuropathic and cancer pain

Background: Cancer pain involves nervous system damage and pathological neurogenesis. Neuropathic pain arises from damage to the nervous system and is driven by ectopic signaling. Both progesterone and pregabalin are neuroprotective in animal models, and there is evidence that both drugs bind to and inhibit voltage-gated calcium channels. Aims: This study was designed to characterize the effects of progesterone and pregabalin in preclinical models of cancer and neuropathic pain in both sexes. Methods: We measured peripheral sensory signaling by intracellular in vivo electrophysiology and behavioral indicators of pain in rat models of cancer-induced bone pain and neuropathic pain. Results: Female but not male models of cancer pain showed a behavioral response to treatment and pregabalin reduced excitability in C and A high-threshold but not low-threshold sensory neurons of both sexes. Male models of neuropathic pain treated with pregabalin demonstrated higher signaling thresholds only in A high-threshold neurons, and behavioral data indicated a clear recovery to baseline mechanical withdrawal thresholds in all treatment groups. Female rat treatment groups did not show excitability changes in sensory neurons, but all demonstrated higher mechanical withdrawal thresholds than vehicle-treated females, although not to baseline levels. Athymic female rat models of neuropathic pain showed no behavioral or electrophysiological responses to treatment. Conclusions: Both pregabalin and progesterone showed evidence of efficacy in male models of neuropathic pain. These results add to the evidence demonstrating differential effects of treatments for pain in male and female animals and widely differing responses in models of cancer and neuropathic pain.

Histone methyltransferase G9a diminishes expression of cannabinoid CB1 receptors in primary sensory neurons in neuropathic pain

Type 1 cannabinoid receptors (CB₁Rs) are expressed in the dorsal root ganglion (DRG) and contribute to the analgesic effect of cannabinoids. However, the epigenetic mechanism regulating the expression of CB₁Rs in neuropathic pain is unknown. G9a (encoded by the Ehmt2 gene), a histone 3 at lysine 9 methyltransferase, is a key chromatin regulator responsible for gene silencing. In this study, we determined G9a's role in regulating CB₁R expression in the DRG and in CB₁R-mediated analgesic effects in an animal model of neuropathic pain. We show that nerve injury profoundly reduced mRNA levels of CB₁Rs but increased the expression of CB₂ receptors in the rat DRG. ChIP results indicated increased enrichment of histone 3 at lysine 9 dimethylation, a G9a-catalyzed repressive histone mark, at the promoter regions of the CB₁R genes. G9a inhibition in nerve-injured rats not only up-regulated the CB₁R expression level in the DRG but also potentiated the analgesic effect of a CB₁R agonist on nerve injury-induced pain hypersensitivity. Furthermore, in mice lacking Ehmt2 in DRG neurons, nerve injury failed to reduce CB₁R expression in the DRG and to decrease the analgesic effect of the CB₁R agonist. Moreover, nerve injury diminished the inhibitory effect of the CB₁R agonist on synaptic glutamate release from primary afferent nerves to spinal cord dorsal horn neurons in WT mice but not in mice lacking Ehmt2 in DRG neurons. Our findings reveal that nerve injury diminishes the analgesic effect of CB₁R agonists through G9a-mediated CB₁R down-regulation in primary sensory neurons.

The role of glial-neuronal metabolic cooperation in modulating progression of multiple sclerosis and neuropathic pain

While the etiology of multiple sclerosis (MS) remains unclear, research from the clinic and preclinical models identified the essential role of inflammation and demyelination in the pathogenesis of MS. Current treatments focused on anti-inflammatory processes are effective against acute episodes and relapsing-remitting MS, but patients still move on to develop secondary progressive MS. MS progression is associated with activation of microglia and astrocytes, and importantly, metabolic dysfunction leading to neuronal death. Neuronal death also contributes to chronic neuropathic pain. Metabolic support of neurons by glia may play central roles in preventing progression of MS and chronic neuropathic pain. Here, we review mechanisms of metabolic cooperation between glia and neurons and outline future perspectives exploring metabolic support of neurons by glia.

A refinement approach in a mouse model of rehabilitation research. Analgesia strategy, reduction approach and infrared thermography in spinal cord injury

The principles of Refinement, Replacement and Reduction (3R’s) should be taken into account when animals must be used for scientific purpose. Here, a Reduction / Refinement approach was applied to the procedure of spinal cord injury (SCI), an animal model used in rehabilitation medicine research, in order to improve the quality of experiments, avoiding unnecessary suffering. The aims of this investigation were 1- to assess acute surgical pain in mice subjected to SCI, 2- to compare the efficacy of commonly used analgesia (three buprenorphine subcutaneous injection in 48 hours, 0,15 mg/kg each) with a combination of opioid and NSAID (one subcutaneous injection of 5 mg/kg carprofen before surgery followed by three buprenorphine subcutaneous injection in 48 hours, 0,15 mg/kg each) and 3- to test if Infrared Thermography (IRT) could be a potential new Refinement method to easily assess thermoregulation, an important metabolic parameter. Finally, we aimed to achieve these goals without recruiting animals on purpose, but using mice already scheduled for studies on SCI. By using behaviours analysis, we found that, despite being commonly used, buprenorphine does not completely relieve acute surgical pain, whereas the combination of buprenorphine and carprofen significantly decreases pain signs by 80%. IRT technology turned out to be a very useful Refinement tool being a non invasive methods to measure animal temperature, particularly useful when rectal probe cannot be used, as in the case of SCI. We could find that temperatures constantly and significantly increased until 7 days after surgery and then slowly decreased and, finally, we could observe that in the buprenorphine and carprofen treated group, temperatures were statistically lower than in the buprenorphine-alone treated mice. To our knowledge this is the first work providing an analgesic Refinement and a description of thermoregulatory response using the IRT technology, in mice subjected to SCI.

VGF: a biomarker and potential target for the treatment of neuropathic pain?

Supplemental Digital Content is Available in the Text.

Neuropathic pain (NP) remains an area of considerable unmet medical need. A persistent challenge in the management of NP is to target the specific mechanisms leading to a change from normal to abnormal sensory perception while ensuring that the defensive pain perception remains intact. Targeting VGF-derived neuropeptides may offer this opportunity. VGF was first identified in 1985 and is highly expressed after nerve injury and inflammation in neurons of both the peripheral and central nervous system. Subsequent studies implicate the vgf gene and its products in pain pathways. This narrative review was supported by a systematic search to identify, select, and critically appraise all relevant research investigating the role of VGF-derived neuropeptides in pain pathways. It predominantly focuses on in vivo investigations of the role of VGF in the initiation and maintenance of NP. VGF expression levels are very low under normal physiological conditions and nerve injury results in rapid and robust upregulation, increasing mechanical and thermal hypersensitivity. The identification of the 2 complement receptors with which VGF neuropeptides interact suggests a novel interplay of neuronal and immune signalling mediators. The understanding of the molecular mechanisms and signalling events by which VGF-derived active neuropeptides exert their physiological actions is in its infancy. Future work should aim to improve understanding of the downstream consequences of VGF neuropeptides thereby providing novel insights into pain mechanisms potentially leading to the identification of novel therapeutic targets.

Transcriptomics Analysis of Porcine Caudal Dorsal Root Ganglia in Tail Amputated Pigs Shows Long-Term Effects on Many Pain-Associated Genes

Tail amputation by tail docking or as an extreme consequence of tail biting in commercial pig production potentially has serious implications for animal welfare. Tail amputation causes peripheral nerve injury that might be associated with lasting chronic pain. The aim of this study was to investigate the short- and long-term effects of tail amputation in pigs on caudal DRG gene expression at different stages of development, particularly in relation to genes associated with nociception and pain. Microarrays were used to analyse whole DRG transcriptomes from tail amputated and sham-treated pigs 1, 8, and 16 weeks following tail treatment at either 3 or 63 days of age (8 pigs/treatment/age/time after treatment; n = 96). Tail amputation induced marked changes in gene expression (up and down) compared to sham-treated intact controls for all treatment ages and time points after tail treatment. Sustained changes in gene expression in tail amputated pigs were still evident 4 months after tail injury. Gene correlation network analysis revealed two co-expression clusters associated with amputation: Cluster A (759 down-regulated) and Cluster B (273 up-regulated) genes. Gene ontology (GO) enrichment analysis identified 124 genes in Cluster A and 61 genes in Cluster B associated with both “inflammatory pain” and “neuropathic pain.” In Cluster A, gene family members of ion channels e.g., voltage-gated potassium channels (VGPC) and receptors e.g., GABA receptors, were significantly down-regulated compared to shams, both of which are linked to increased peripheral nerve excitability after axotomy. Up-regulated gene families in Cluster B were linked to transcriptional regulation, inflammation, tissue remodeling, and regulatory neuropeptide activity. These findings, demonstrate that tail amputation causes sustained transcriptomic expression changes in caudal DRG cells involved in inflammatory and neuropathic pain pathways.

Lysozyme elicits pain during nerve injury by neuronal Toll-like receptor 4 activation and has therapeutic potential in neuropathic pain

The role of neuronal Toll-like receptor 4 (TLR4) in nerve injury is being pursued actively. However, the endogenous activation of neuronal TLR4 during neuroinflammation, in absence of the participation of glial TLR4, remains elusive. Here, we identified lysozyme as an endogenous activator of neuronal TLR4 signaling during nerve injury. Upon nerve injury, enhanced expression of lysozyme promoted neuronal hyperexcitability and neuropathic pain. Injections of lysozyme in healthy rats increased their mechanical and thermal pain sensitivity. Likewise, infusion of spinal cord slices with lysozyme increased neuronal excitability typical of neuropathic pain. Our results also showed that lysozyme activated excitability of both Aδ- and C-fibers. Thus, in addition to the discovery of lysozyme as an endogenous ligand for regulating neuronal TLR4 signaling, this study also lays the foundation of our understanding of its role in nervous system pathologies, providing multiple avenues for treating neuroinflammation.

Fibronectin inhibitor pUR4 attenuates tumor necrosis factor α-induced endothelial hyperpermeability by modulating β1 integrin activation

BACKGROUND

The blood-spinal cord barrier (BSCB) is composed of a monolayer of endothelium linked with tight junctions and extracellular matrix (ECM)-rich basement membranes and is surrounded by astrocyte foot processes. Endothelial permeability is regulated by interaction between endothelial cells and ECM proteins. Fibronectin (FN) is a principal ECM component of microvessels. Excessive FN deposition disrupts cell-cell adhesion in fibroblasts through β1 integrin ligation. To determine whether excessive FN deposition contributes to the disruption of endothelial integrity, we used an in vitro model of the endothelial monolayer to investigate whether the FN inhibitor pUR4 prevents FN deposition into the subendothelial matrix and attenuates endothelial leakage.

METHODS

To correlate the effects of excessive FN accumulation in microvessels on BSCB disruption, spinal nerve ligation-which induces BSCB leakage-was applied, and FN expression in the spinal cord was evaluated through immunohistochemistry and immunoblotting. To elucidate the effects by which pUR4 modulates endothelial permeability, brain-derived endothelial (bEND.3) cells treated with tumor necrosis factor (TNF)-α were used to mimic a leaky BSCB. A bEND.3 monolayer was preincubated with pUR4 before TNF-α treatment. The transendothelial electrical resistance (TEER) measurement and transendothelial permeability assay were applied to assess the endothelial integrity of the bEND.3 monolayer. Immunofluorescence analysis and immunoblotting were performed to evaluate the inhibitory effects of pUR4 on TNF-α-induced FN deposition. To determine the mechanisms underlying pUR4-mediated endothelial permeability, cell morphology, stress fiber formation, myosin light chain (MLC) phosphorylation, and β1 integrin-mediated signaling were evaluated through immunofluorescence analysis and immunoblotting.

RESULTS

Excessive FN was accumulated in the microvessels of the spinal cord after spinal nerve ligation; moreover, pUR4 inhibited TNF-α-induced FN deposition in the bEND.3 monolayer and maintained intact TEER and endothelial permeability. Furthermore, pUR4 reduced cell morphology alteration, actin stress fiber formation, and MLC phosphorylation, thereby attenuating paracellular gap formation. Moreover, pUR4 reduced β1 integrin activation and downstream signaling.

CONCLUSIONS

pUR4 reduces TNF-α-induced β1 integrin activation by depleting ECM FN, leading to a decrease in endothelial hyperpermeability and maintenance of monolayer integrity. These findings suggest therapeutic benefits of pUR4 in pathological vascular leakage treatment.

Histone deacetylase inhibitors prevent persistent hypersensitivity in an orofacial neuropathic pain model

Chronic orofacial pain is a significant health problem requiring identification of regulating processes. Involvement of epigenetic modifications that is reported for hindlimb neuropathic pain experimental models, however, is less well studied in cranial nerve pain models. Three independent observations reported here are the (1) epigenetic profile in mouse trigeminal ganglia (TG) after trigeminal inflammatory compression (TIC) nerve injury mouse model determined by gene expression microarray, (2) H3K9 acetylation pattern in TG by immunohistochemistry, and (3) efficacy of histone deacetylase (HDAC) inhibitors to attenuate development of hypersensitivity. After TIC injury, ipsilateral whisker pad mechanical sensitization develops by day 3 and persists well beyond day 21 in contrast to sham surgery. Global acetylation of H3K9 decreases at day 21 in ipsilateral TG . Thirty-four genes are significantly (p < 0.05) overexpressed in the ipsilateral TG by at least two-fold at either 3 or 21 days post-trigeminal inflammatory compression injury. The three genes most overexpressed three days post-trigeminal inflammatory compression nerve injury are nerve regeneration-associated gene ATF3, up 6.8-fold, and two of its regeneration-associated gene effector genes, Sprr1a and Gal, up 174- and 25-fold, respectively. Although transcription levels of 25 of 32 genes significantly overexpressed three days post-trigeminal inflammatory compression return to constitutive levels by day 21, these three regeneration-associated genes remain significantly overexpressed at the later time point. On day 21, when tissues are healed, other differentially expressed genes include 39 of the top 50 upregulated and downregulated genes. Remarkably, preemptive manipulation of gene expression with two HDAC inhibitors (HDACi's), suberanilohydroxamic acid (SAHA) and MS-275, reduces the magnitude and duration of whisker pad mechanical hypersensitivity and prevents the development of a persistent pain state. These findings suggest that trigeminal nerve injury leads to epigenetic modifications favoring overexpression of genes involved in nerve regeneration and that maintaining transcriptional homeostasis with epigenetic modifying drugs could help prevent the development of persistent pain.

Voltage-gated calcium channel α 2δ subunits: an assessment of proposed novel roles

Voltage-gated calcium (Ca_V) channels are associated with β and α₂δ auxiliary subunits. This review will concentrate on the function of the α₂δ protein family, which has four members. The canonical role for α₂δ subunits is to convey a variety of properties on the Ca_V1 and Ca_V2 channels, increasing the density of these channels in the plasma membrane and also enhancing their function. More recently, a diverse spectrum of non-canonical interactions for α₂δ proteins has been proposed, some of which involve competition with calcium channels for α₂δ or increase α₂δ trafficking and others which mediate roles completely unrelated to their calcium channel function. The novel roles for α₂δ proteins which will be discussed here include association with low-density lipoprotein receptor-related protein 1 (LRP1), thrombospondins, α-neurexins, prion proteins, large conductance (big) potassium (BK) channels, and N-methyl-d-aspartate (NMDA) receptors.

MicroRNA-182 Alleviates Neuropathic Pain by Regulating Nav1.7 Following Spared Nerve Injury in Rats

The sodium channel 1.7 (Nav1.7), which is encoded by SCN9A gene, is involved in neuropathic pain. As crucial regulators of gene expression, many miRNAs have already gained importance in neuropathic pain, including miR-182, which is predicted to regulate the SCN9A gene. Nav1.7 expression in L4-L6 dorsal root ganglions (DRGs) can be up regulated by spared nerve injury (SNI), while miR-182 expression was down regulated following SNI model. Exploring the connection between Nav1.7 and miR-182 may facilitate the development of a better-targeted therapy. In the current study, direct pairing of miR-182 with the SCN9A gene was verified using a luciferase assay in vitro. Over-expression of miR-182 via microinjection of miR-182 agomir reversed the abnormal increase of Nav1.7 at both mRNA and protein level in L4-6 DRGs of SNI rats, and significantly attenuated the hypersensitivity to mechanical stimulus in the rats. In contrast, administration of miR-182 antagomir enhanced the Nav1.7 expression at both mRNA and protein level in L4-6 DRGs, companied with the generation of mechanical hypersensitivity in naïve rats. Collectively, we concluded that miR-182 can alleviate SNI- induced neuropathic pain through regulating Nav1.7 in rats.

RE1-silencing transcription factor controls the acute-to-chronic neuropathic pain transition and Chrm2 receptor gene expression in primary sensory neurons

Neuropathic pain is associated with persistent changes in gene expression in primary sensory neurons, but the underlying epigenetic mechanisms that cause these changes remain unclear. The muscarinic cholinergic receptors (mAChRs), particularly the M2 subtype (encoded by the cholinergic receptor muscarinic 2 (Chrm2) gene), are critically involved in the regulation of spinal nociceptive transmission. However, little is known about how Chrm2 expression is transcriptionally regulated. Here we show that nerve injury persistently increased the expression of RE1-silencing transcription factor (REST, also known as neuron-restrictive silencing factor [NRSF]), a gene-silencing transcription factor, in the dorsal root ganglion (DRG). Remarkably, nerve injury-induced chronic but not acute pain hypersensitivity was attenuated in mice with Rest knockout in DRG neurons. Also, siRNA-mediated Rest knockdown reversed nerve injury-induced chronic pain hypersensitivity in rats. Nerve injury persistently reduced Chrm2 expression in the DRG and diminished the analgesic effect of muscarine. The RE1 binding site on the Chrm2 promoter is required for REST-mediated Chrm2 repression, and nerve injury increased the enrichment of REST in the Chrm2 promoter in the DRG. Furthermore, Rest knockdown or genetic ablation in DRG neurons normalized Chrm2 expression and augmented muscarine's analgesic effect on neuropathic pain and fully reversed the nerve injury-induced reduction in the inhibitory effect of muscarine on glutamatergic input to spinal dorsal horn neurons. Our findings indicate that nerve injury-induced REST up-regulation in DRG neurons plays an important role in the acute-to-chronic pain transition and is essential for the transcriptional repression of Chrm2 in neuropathic pain.

The EGF-LIKE domain of thrombospondin-4 is a key determinant in the development of pain states due to increased excitatory synaptogenesis

Up-regulation of thrombospondin-4 (TSP4) or voltage-gated calcium channel subunit α₂δ₁ (Ca_vα₂δ₁) proteins in the spinal cord contributes to neuropathic pain development through an unidentified mechanism. We have previously shown that TSP4 interacts with Ca_vα₂δ₁ to promote excitatory synaptogenesis and the development of chronic pain states. However, the TSP4 determinants responsible for these changes are not known. Here, we tested the hypothesis that the Ca_vα₂δ₁-binding domains of TSP4 are synaptogenic and pronociceptive. We mapped the major Ca_vα₂δ₁-binding domains of TSP4 within the coiled-coil and epidermal growth factor (EGF)-like domains in vitro Intrathecal injection of TSP4 fragment proteins containing the EGF-like domain (EGF-LIKE) into naïve rodents was sufficient for inducing behavioral hypersensitivity similar to that produced by an equal molar dose of full-length TSP4. Gabapentin, a drug that binds to Ca_vα₂δ₁, blocked EGF-LIKE-induced behavioral hypersensitivity in a dose-dependent manner, supporting the notion that EGF-LIKE interacts with Ca_vα₂δ₁ and thereby mediates behavioral hypersensitivity. This notion was further supported by our findings that a peptide within EGF-LIKE (EGFD355-369) could block TSP4- or Ca_vα₂δ₁-induced behavioral hypersensitivity after intrathecal injections. Furthermore, only TSP4 proteins that contained EGF-LIKE could promote excitatory synaptogenesis between sensory and spinal cord neurons, which could be blocked by peptide EGFD355-369. Together, these findings indicate that EGF-LIKE is the molecular determinant that mediates aberrant excitatory synaptogenesis and chronic pain development. Blocking interactions between EGF-LIKE and Ca_vα₂δ₁ could be an alternative approach in designing target-specific pain medications.

Multiple sites and actions of gabapentin-induced relief of ongoing experimental neuropathic pain

Gabapentin (GBP) is a first-line therapy for neuropathic pain, but its mechanisms and sites of action remain uncertain. We investigated GBP-induced modulation of neuropathic pain following spinal nerve ligation (SNL) in rats. Intravenous or intrathecal GBP reversed evoked mechanical hypersensitivity and produced conditioned place preference (CPP) and dopamine (DA) release in the nucleus accumbens (NAc) selectively in SNL rats. Spinal GBP also significantly inhibited dorsal horn wide-dynamic-range neuronal responses to a range of evoked stimuli in SNL rats. By contrast, GBP microinjected bilaterally into the rostral anterior cingulate cortex (rACC), produced CPP, and elicited NAc DA release selectively in SNL rats but did not reverse tactile allodynia and had marginal effects on wide-dynamic-range neuronal activity. Moreover, blockade of endogenous opioid signaling in the rACC prevented intravenous GBP-induced CPP and NAc DA release but failed to block its inhibition of tactile allodynia. Gabapentin, therefore, can potentially act to produce its pain relieving effects by (a) inhibition of injury-induced spinal neuronal excitability, evoked hypersensitivity, and ongoing pain and (b) selective supraspinal modulation of affective qualities of pain, without alteration of reflexive behaviors. Consistent with previous findings of pain relief from nonopioid analgesics, GBP requires engagement of rACC endogenous opioid circuits and downstream activation of mesolimbic reward circuits reflected in learned pain-motivated behaviors. These findings support the partial separation of sensory and affective dimensions of pain in this experimental model and suggest that modulation of affective-motivational qualities of pain may be the preferential mechanism of GBP's analgesic effects in patients.

Gabapentin prevents synaptogenesis between sensory and spinal cord neurons induced by thrombospondin-4 acting on pre-synaptic Cav α2 δ1 subunits and involving T-type Ca2+ channels

BACKGROUND AND PURPOSE

Nerve injury induces concurrent up-regulation of the voltage-gated calcium channel subunit Cav α2 δ1 and the extracellular matrix protein thrombospondin-4 (TSP4) in dorsal root ganglia and dorsal spinal cord, leading to the development of a neuropathic pain state. Interactions of these proteins promote aberrant excitatory synaptogenesis that contributes to neuropathic pain state development through unknown mechanisms. We investigated the contributions of Cav α2 δ1 subunits and TSP4 to synaptogenesis, and the pathways involved in vitro, and whether treatment with gabapentin could block this process and pain development in vivo.

EXPERIMENTAL APPROACH

A co-culture system of sensory and spinal cord neurons was used to study the contribution from each protein to synaptogenesis and the pathway(s) involved. Anti-synaptogenic actions of gabapentin were studied in TSP4-injected mice.

KEY RESULTS

Only presynaptic, but not postsynaptic, Cav α2 δ1 subunits interacted with TSP4 to initiate excitatory synaptogenesis through a pathway modulated by T-type calcium channels. Cav α2 δ1 /TSP4 interactions were not required for maintenance of already formed synapses. In vivo, early, but not delayed, treatment with low-dose gabapentin blocked this pathway and the development of the pain state.

CONCLUSIONS AND IMPLICATIONS

Cav α2 δ1 /TSP4 interactions were critical for the initiation, but not for the maintenance, of abnormal synapse formation between sensory and spinal cord neurons. This process was blocked by early, but was not reversed by delayed, treatment with gabapentin. Early intervention with gabapentin may prevent the development of injury-induced chronic pain, resulting from Cav α2 δ1 /TSP4-initiated abnormal synapse formation.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Macrophage migration inhibitory factor mediates peripheral nerve injury-induced hypersensitivity by curbing dopaminergic descending inhibition

Our previous works disclosed the contributing role of macrophage migration inhibitory factor (MIF) and dopaminergic inhibition by lysine dimethyltransferase G9a/Glp complex in peripheral nerve injury-induced hypersensitivity. We herein propose that the proinflammatory cytokine MIF participates in the regulation of neuropathic hypersensitivity by interacting with and suppressing the descending dopaminergic system. The lumbar spinal cord (L-SC) and ventral tegmental area (VTA) are two major locations with significant upregulation of MIF after chronic constriction injury (CCI) of the sciatic nerve, and they display time-dependent changes, along with a behavioral trajectory. Correspondingly, dopamine (DA) content shows the reverse characteristic change to MIF with a time-dependent curve in post-surgical behavior. The levels of both MIF and DA are reversed by the MIF tautomerase inhibitor ISO-1, and a negative relationship exists between MIF and DA. The reversed role of ISO-1 also affects tyrosine hydroxylase expression. Furthermore, CCI induces Th promoter CpG site methylation in the L-SC and VTA areas, and this effect could be abated by ISO-1 administration. G9a/SUV39H1 and H3K9me2/H3K9me3 enrichment within the Th promoter region following CCI in the L-SC and VTA was also decreased by ISO-1. In cultured dopaminergic neurons, rMIF enhanced the recruitment of G9a and SUV39H1, followed by an increase in H3K9me2/H3K9me3. These molecular changes correspondingly exhibited alterations in Th promoter CpG site methylation and pain behaviors. In summary, MIF functions as a braking factor in curbing dopaminergic descending inhibition in peripheral nerve injury-induced hypersensitivity by mediating Th gene methylation through G9a/SUV39H1-associated H3K9 methylation.

W. A. H, Lewis RJ, Vetter I. Transcriptomics in pain research: insights from new and old technologies

Physiological and pathological pain involves a complex interplay of multiple cell types and signaling pathways.

Effect of knockout of α2δ-1 on action potentials in mouse sensory neurons

Gene deletion of the voltage-gated calcium channel auxiliary subunit α₂δ-1 has been shown previously to have a cardiovascular phenotype, and a reduction in mechano- and cold sensitivity, coupled with delayed development of neuropathic allodynia. We have also previously shown that dorsal root ganglion (DRG) neuron calcium channel currents were significantly reduced in α₂δ-1 knockout mice. To extend our findings in these sensory neurons, we have examined here the properties of action potentials (APs) in DRG neurons from α₂δ-1 knockout mice in comparison to their wild-type (WT) littermates, in order to dissect how the calcium channels that are affected by α₂δ-1 knockout are involved in setting the duration of individual APs and their firing frequency. Our main findings are that there is reduced Ca²⁺ entry on single AP stimulation, particularly in the axon proximal segment, reduced AP duration and reduced firing frequency to a 400 ms stimulation in α₂δ-1 knockout neurons, consistent with the expected role of voltage-gated calcium channels in these events. Furthermore, lower intracellular Ca²⁺ buffering also resulted in reduced AP duration, and a lower frequency of AP firing in WT neurons, mimicking the effect of α₂δ-1 knockout. By contrast, we did not obtain any consistent evidence for the involvement of Ca²⁺-activation of large conductance calcium-activated potassium (BK) and small conductance calcium-activated potassium (SK) channels in these events. In conclusion, the reduced Ca²⁺ elevation as a result of single AP stimulation is likely to result from the reduced duration of the AP in α₂δ-1 knockout sensory neurons.

This article is part of the themed issue ‘Evolution brings Ca²⁺ and ATP together to control life and death’.

Determination of stable reference genes for RT-qPCR expression data in mechanistic pain studies on pig dorsal root ganglia and spinal cord

RNA expression levels for genes of interest must be normalised with appropriate reference or "housekeeping" genes that are stably expressed across samples and treatments. This study determined the most stable reference genes from a panel of 6 porcine candidate genes: beta actin (ACTB), beta-2-microglobulin (B2M), eukaryotic elongation factor 1 gamma-like protein (eEF-1), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), succinate dehydrogenase complex subunit A (SDHA), Ubiquitin C (UBC) in sacral dorsal root ganglia and spinal cord samples collected from 16 tail docked pigs (2/3rds of tail amputated) 1, 4, 8 and 16weeks after tail injury (4 pigs/time point). Total RNA from pooled samples was measured by SYBRgreen real-time quantitative PCR. Cycle threshold values were analysed using geNorm, BestKeeper and NormFinder PCR analysis software. Average expression stability and pairwise variation values were calculated for each candidate reference gene. GeNorm analysis identified the most stable genes for normalisation of gene expression data to be GAPDH>eEF-1>UBC>B2M>ACTB>SDHA for dorsal root ganglia and ACTB>SDHA>UBC>B2M>GAPDH>eEF-1 for spinal cord samples. Expression stability estimates were verified by BestKeeper and NormFinder analysis. Expression stability varied between genes within and between tissues. Validation of most stably expressed reference genes was performed by normalisation of calcitonin gene related polypeptide beta (CALCB). The results show similar patterns of CALCB expression when the best reference genes selected by all three programs were used. GAPDH, eEF-1 and UBC are suitable reference genes for porcine dorsal root ganglia samples, whereas ACTB, SDHA and UBC are more appropriate for spinal cord samples.

Localized Sympathectomy Reduces Mechanical Hypersensitivity by Restoring Normal Immune Homeostasis in Rat Models of Inflammatory Pain

Some forms of chronic pain are maintained or enhanced by activity in the sympathetic nervous system (SNS), but attempts to model this have yielded conflicting findings. The SNS has both pro- and anti-inflammatory effects on immunity, confounding the interpretation of experiments using global sympathectomy methods. We performed a "microsympathectomy" by cutting the ipsilateral gray rami where they entered the spinal nerves near the L4 and L5 DRG. This led to profound sustained reductions in pain behaviors induced by local DRG inflammation (a rat model of low back pain) and by a peripheral paw inflammation model. Effects of microsympathectomy were evident within one day, making it unlikely that blocking sympathetic sprouting in the local DRGs or hindpaw was the sole mechanism. Prior microsympathectomy greatly reduced hyperexcitability of sensory neurons induced by local DRG inflammation observed 4 d later. Microsympathectomy reduced local inflammation and macrophage density in the affected tissues (as indicated by paw swelling and histochemical staining). Cytokine profiling in locally inflamed DRG showed increases in pro-inflammatory Type 1 cytokines and decreases in the Type 2 cytokines present at baseline, changes that were mitigated by microsympathectomy. Microsympathectomy was also effective in reducing established pain behaviors in the local DRG inflammation model. We conclude that the effect of sympathetic fibers in the L4/L5 gray rami in these models is pro-inflammatory. This raises the possibility that therapeutic interventions targeting gray rami might be useful in some chronic inflammatory pain conditions.

SIGNIFICANCE STATEMENT

Sympathetic blockade is used for many pain conditions, but preclinical studies show both pro- and anti-nociceptive effects. The sympathetic nervous system also has both pro- and anti-inflammatory effects on immune tissues and cells. We examined effects of a very localized sympathectomy. By cutting the gray rami to the spinal nerves near the lumbar sensory ganglia, we avoided widespread sympathetic denervation. This procedure profoundly reduced mechanical pain behaviors induced by a back pain model and a model of peripheral inflammatory pain. One possible mechanism was reduction of inflammation in the sympathetically denervated regions. This raises the possibility that therapeutic interventions targeting gray rami might be useful in some inflammatory conditions.

The Development of Translational Biomarkers as a Tool for Improving the Understanding, Diagnosis and Treatment of Chronic Neuropathic Pain

Chronic neuropathic pain (CNP) is one of the most significant unmet clinical needs in modern medicine. Alongside the lack of effective treatments, there is a great deficit in the availability of objective diagnostic methods to reliably facilitate an accurate diagnosis. We therefore aimed to determine the feasibility of a simple diagnostic test by analysing differentially expressed genes in the blood of patients diagnosed with CNP of the lower back and compared to healthy human controls. Refinement of microarray expression data was performed using correlation analysis with 3900 human 2-colour microarray experiments. Selected genes were analysed in the dorsal horn of Sprague-Dawley rats after L5 spinal nerve ligation (SNL), using qRT-PCR and ddPCR, to determine possible associations with pathophysiological mechanisms underpinning CNP and whether they represent translational biomarkers of CNP. We found that of the 15 potential biomarkers identified, tissue inhibitor of matrix metalloproteinase-1 (TIMP1) gene expression was upregulated in chronic neuropathic lower back pain (CNBP) (p = 0.0049) which positively correlated (R = 0.68, p = ≤0.05) with increased plasma TIMP1 levels in this group (p = 0.0433). Moreover, plasma TIMP1 was also significantly upregulated in CNBP than chronic inflammatory lower back pain (p = 0.0272). In the SNL model, upregulation of the Timp1 gene was also observed (p = 0.0058) alongside a strong trend for the upregulation of melanocortin 1 receptor (p = 0.0847). Our data therefore highlights several genes that warrant further investigation, and of these, TIMP1 shows the greatest potential as an accessible and translational CNP biomarker.

Increased thrombospondin-4 after nerve injury mediates disruption of intracellular calcium signaling in primary sensory neurons

Painful nerve injury disrupts Ca²⁺ signaling in primary sensory neurons by elevating plasma membrane Ca²⁺-ATPase (PMCA) function and depressing sarco-endoplasmic reticulum Ca²⁺-ATPase (SERCA) function, which decreases endoplasmic reticulum (ER) Ca²⁺ stores and stimulates store-operated Ca²⁺ entry (SOCE). The extracellular matrix glycoprotein thrombospondin-4 (TSP4), which is increased after painful nerve injury, decreases Ca²⁺ current (I_Ca) through high-voltage-activated Ca²⁺ channels and increases I_Ca through low-voltage-activated Ca²⁺ channels in dorsal root ganglion neurons, which are events similar to the effect of nerve injury. We therefore examined whether TSP4 plays a critical role in injury-induced disruption of intracellular Ca²⁺ signaling. We found that TSP4 increases PMCA activity, inhibits SERCA, depletes ER Ca²⁺ stores, and enhances store-operated Ca²⁺ influx. Injury-induced changes of SERCA and PMCA function are attenuated in TSP4 knock-out mice. Effects of TSP4 on intracellular Ca²⁺ signaling are attenuated in voltage-gated Ca²⁺ channel α₂δ₁ subunit (Ca_vα₂δ₁) conditional knock-out mice and are also Protein Kinase C (PKC) signaling dependent. These findings suggest that TSP4 elevation may contribute to the pathogenesis of chronic pain following nerve injury by disrupting intracellular Ca²⁺ signaling via interacting with the Ca_vα₂δ₁ and the subsequent PKC signaling pathway. Controlling TSP4 mediated intracellular Ca²⁺ signaling in peripheral sensory neurons may be a target for analgesic drug development for neuropathic pain.

Emerging Putative Associations between Non-Coding RNAs and Protein-Coding Genes in Neuropathic Pain: Added Value from Reusing Microarray Data

Regeneration of injured nerves is likely occurring in the peripheral nervous system, but not in the central nervous system. Although protein-coding gene expression has been assessed during nerve regeneration, little is currently known about the role of non-coding RNAs (ncRNAs). This leaves open questions about the potential effects of ncRNAs at transcriptome level. Due to the limited availability of human neuropathic pain (NP) data, we have identified the most comprehensive time-course gene expression profile referred to sciatic nerve (SN) injury and studied in a rat model using two neuronal tissues, namely dorsal root ganglion (DRG) and SN. We have developed a methodology to identify differentially expressed bioentities starting from microarray probes and repurposing them to annotate ncRNAs, while analyzing the expression profiles of protein-coding genes. The approach is designed to reuse microarray data and perform first profiling and then meta-analysis through three main steps. First, we used contextual analysis to identify what we considered putative or potential protein-coding targets for selected ncRNAs. Relevance was therefore assigned to differential expression of neighbor protein-coding genes, with neighborhood defined by a fixed genomic distance from long or antisense ncRNA loci, and of parental genes associated with pseudogenes. Second, connectivity among putative targets was used to build networks, in turn useful to conduct inference at interactomic scale. Last, network paths were annotated to assess relevance to NP. We found significant differential expression in long-intergenic ncRNAs (32 lincRNAs in SN and 8 in DRG), antisense RNA (31 asRNA in SN and 12 in DRG), and pseudogenes (456 in SN and 56 in DRG). In particular, contextual analysis centered on pseudogenes revealed some targets with known association to neurodegeneration and/or neurogenesis processes. While modules of the olfactory receptors were clearly identified in protein-protein interaction networks, other connectivity paths were identified between proteins already investigated in studies on disorders, such as Parkinson, Down syndrome, Huntington disease, and Alzheimer. Our findings suggest the importance of reusing gene expression data by meta-analysis approaches.

Dopaminergic inhibition by G9a/Glp complex on tyrosine hydroxylase in nerve injury-induced hypersensitivity

The neural balance between facilitation and inhibition determines the final tendency of central sensitization. Nerve injury-induced hypersensitivity was considered as the results from the enhanced ascending facilitation and the diminished descending inhibition. The role of dopaminergic transmission in the descending inhibition has been well documented, but its underlying molecular mechanisms are unclear. Previous studies demonstrated that the lysine dimethyltransferase G9a/G9a-like protein (Glp) complex plays a critical role in cocaine-induced central plasticity, and given cocaine’s role in the nerve system is relied on its function on dopamine system, we herein proposed that the reduced inhibition of dopaminergic transmission was from the downregulation of tyrosine hydroxylase expression by G9a/Glp complex through methylating its gene Th. After approval by the Animal Care and Use Committee, C57BL/6 mice were used for pain behavior using von Frey after spared nerve injury, and Th CpG islands methylation was measured using bisulfite sequencing at different nerve areas. The inhibitor of G9a/Glp, BIX 01294, was administered intraventricularly daily with bolus injection. The protein levels of G9a, Glp, and tyrosine hydroxylase were measured with immunoblotting. Dopamine levels were detected using high-performance liquid chromatography. The expression of G9a but not Glp was upregulated in ventral tegmental area at post-injury day 4 till day 49 (the last day of the behavioral test). Correspondingly, the Th CpG methylation is increased, but the tyrosine hydroxylase expression was downregulated and the dopamine level was decreased. After the intracerebroventriclar injection of BIX 01294 since the post-injury days 7 and 14 for consecutive three days, three weeks, and six weeks, the expression of tyrosine hydroxylase was upregulated with a significant decrease in Th methylation and increase in dopamine level. Moreover, the pain after G9a/Glp inhibitor was attenuated significantly. In sum, methytransferase G9a/Glp complex partially controls dopaminergic transmission by methylating Th in peripheral nerve injury-induced neuropathic pain.

Voltage-gated calcium channels and their auxiliary subunits: physiology and pathophysiology and pharmacology

Voltage‐gated calcium channels are essential players in many physiological processes in excitable cells. There are three main subdivisions of calcium channel, defined by the pore‐forming α₁ subunit, the Ca_V1, Ca_V2 and Ca_V3 channels. For all the subtypes of voltage‐gated calcium channel, their gating properties are key for the precise control of neurotransmitter release, muscle contraction and cell excitability, among many other processes. For the Ca_V1 and Ca_V2 channels, their ability to reach their required destinations in the cell membrane, their activation and the fine tuning of their biophysical properties are all dramatically influenced by the auxiliary subunits that associate with them. Furthermore, there are many diseases, both genetic and acquired, involving voltage‐gated calcium channels. This review will provide a general introduction and then concentrate particularly on the role of auxiliary α₂δ subunits in both physiological and pathological processes involving calcium channels, and as a therapeutic target.

The influence of thiamin on the efficacy of pregabalin in rats with spinal nerve ligation (SNL)-induced neuropathic pain

OBJECTIVE

The thiamin is often used in the treatment of neuropathy, and pregabalin is often used to treat neuropathic pain. Our study examined the influence of thiamin on the efficacy of pregabalin in a rat model of spinal nerve ligation (SNL)-induced neuropathic pain.

METHODS

Sprague-Dawley male rats were randomly divided into six groups. The neuropathic pain-relieving properties were measured by plantar test, cold plate test, and hot plate test after administration of pregabalin (i.v) and/or thiamin (i.p) in SNL rats 14 days after operation.

RESULTS

In the therapy period, pregabalin, or thiamin alone all produced antinociceptive effects in rats with neuropathic pain. And combination treatment of thiamin and pregabalin resulted in an enhanced pain relief compared to the administration of pregabalin or thiamin alone.

CONCLUSION

Combination of thiamin and pregabalin produces an additive antinociceptive effect in neuropathic pain rats, this drug combination may offer a beneficial treatment option for neuropathic pain.

Mechanisms of the gabapentinoids and α 2 δ-1 calcium channel subunit in neuropathic pain

The gabapentinoid drugs gabapentin and pregabalin are key front‐line therapies for various neuropathies of peripheral and central origin. Originally designed as analogs of GABA, the gabapentinoids bind to the α₂δ‐1 and α₂δ‐2 auxiliary subunits of calcium channels, though only the former has been implicated in the development of neuropathy in animal models. Transgenic approaches also identify α₂δ‐1 as key in mediating the analgesic effects of gabapentinoids, however the precise molecular mechanisms remain unclear. Here we review the current understanding of the pathophysiological role of the α₂δ‐1 subunit, the mechanisms of analgesic action of gabapentinoid drugs and implications for efficacy in the clinic. Despite widespread use, the number needed to treat for gabapentin and pregabalin averages from 3 to 8 across neuropathies. The failure to treat large numbers of patients adequately necessitates a novel approach to treatment selection. Stratifying patients by sensory profiles may imply common underlying mechanisms, and a greater understanding of these mechanisms could lead to more direct targeting of gabapentinoids.

Gabapentinoid Insensitivity after Repeated Administration is Associated with Down-Regulation of the α(2)δ-1 Subunit in Rats with Central Post-Stroke Pain Hypersensitivity

The α2δ-1 subunit of the voltage-gated Ca(2+) channel (VGCC) is a molecular target of gabapentin (GBP), which has been used as a first-line drug for the relief of neuropathic pain. GBP exerts its anti-nociceptive effects by disrupting trafficking of the α2δ-1 subunit to the presynaptic membrane, resulting in decreased neurotransmitter release. We previously showed that GBP has an anti-allodynic effect in the first two weeks; but this is followed by insensitivity in the later stage after repeated administration in a rat model of central post-stroke pain (CPSP) hypersensitivity induced by intra-thalamic hemorrhage. To explore the mechanisms underlying GBP insensitivity, the cellular localization and time-course of expression of the α2δ-1 subunit in both the thalamus and spinal dorsal horn were studied in the same model. We found that the α2δ-1 subunit was mostly localized in neurons, but not astrocytes and microglia. The level of α2δ-1 protein increased in the first two weeks after injury but then decreased in the third week, when GBP insensitivity occurred. Furthermore, the α2δ-1 down-regulation was likely caused by later neuronal loss in the injured thalamus through a mechanism other than apoptosis. In summary, the present results suggest that the GBP receptor α2δ-1 is mainly expressed in thalamic neurons in which it is up-regulated in the early stage of CPSP but this is followed by dramatic down-regulation, which is likely associated with GBP insensitivity after long-term use.

Therapeutic Strategies for Neuropathic Pain: Potential Application of Pharmacosynthetics and Optogenetics

Chronic pain originating from neuronal damage remains an incurable symptom debilitating patients. Proposed molecular modalities in neuropathic pain include ion channel expressions, immune reactions, and inflammatory substrate diffusions. Recent advances in RNA sequence analysis have discovered specific ion channel expressions in nociceptors such as transient receptor potential (TRP) channels, voltage-gated potassium, and sodium channels. G protein-coupled receptors (GPCRs) also play an important role in triggering surrounding immune cells. The multiple protein expressions complicate therapeutic development for neuropathic pain. Recent progress in optogenetics and pharmacogenetics may herald the development of novel therapeutics for the incurable pain. Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) facilitate the artificial manipulation of intracellular signaling through excitatory or inhibitory G protein subunits activated by biologically inert synthetic ligands. Expression of excitatory channelrhodopsins and inhibitory halorhodopsins on injured neurons or surrounding cells can attenuate neuropathic pain precisely controlled by light stimulation. To achieve the discrete treatment of injured neurons, we can exploit the transcriptome database obtained by RNA sequence analysis in specific neuropathies. This can recommend the suitable promoter information to target the injury sites circumventing intact neurons. Therefore, novel strategies benefiting from pharmacogenetics, optogenetics, and RNA sequencing might be promising for neuropathic pain treatment in future.