Pathophysiologic mechanisms of neuropathic pain

Transient Receptor Potential Channels: Multiple Modulators of Peripheral Neuropathic Pain in Several Rodent Models

Neuropathic pain, a prevalent chronic condition in clinical settings, has attracted widespread societal attention. This condition is characterized by a persistent pain state accompanied by affective and cognitive disruptions, significantly impacting patients' quality of life. However, current clinical therapies fall short of addressing its complexity. Thus, exploring the underlying molecular mechanism of neuropathic pain and identifying potential targets for intervention is highly warranted. The transient receptor potential (TRP) receptors, a class of widely distributed channel proteins, in the nervous system, play a crucial role in sensory signaling, cellular calcium regulation, and developmental influences. TRP ion channels are also responsible for various sensory responses including heat, cold, pain, and stress. This review highlights recent advances in understanding TRPs in various rodent models of neuropathic pain, aiming to uncover potential therapeutic targets for clinical management.

Matrix metalloproteinases as attractive therapeutic targets for chronic pain: A narrative review

Chronic pain constitutes an abnormal pain state that detrimentally affects the quality of life, daily activities, occupational performance, and stability of mood. Despite the prevalence of chronic pain, effective drugs with potent abirritation and minimal side effects remain elusive. Substantial studies have revealed aberrant activation of the matrix metalloproteinases (MMPs) in multiple chronic pain models. Additionally, emerging evidence has demonstrated that the downregulation of MMPs can alleviate chronic pain in diverse animal models, underscoring the unique and crucial role of MMPs in different stages and types of chronic pain. This review delves into the mechanistic insights and roles of MMPs in modulating chronic pain. The aberrant activation of MMPs has been linked to neuropathic pain through mechanisms involving myelin abnormalities in peripheral nerve and spinal dorsal horn (SDH), hyperexcitability of dorsal root ganglion (DRG) neurons, activation of N-methyl-d-aspartate receptors (NMDAR) and Ca2+-dependent signals, glial cell activation, and proinflammatory cytokines release. Different MMPs also contribute significantly to inflammatory pain and cancer pain. Furthermore, we summarized the substantial therapeutic potential of MMP pharmacological inhibitors across different types of chronic pain. Overall, our findings underscore the promising therapeutic prospects of MMPs targeting for managing chronic pain.

Contribution of µ Opioid Receptor-expressing Dorsal Horn Interneurons to Neuropathic Pain-like Behavior in Mice

BACKGROUND

Intersectional genetics have yielded tremendous advances in our understanding of molecularly identified subpopulations and circuits within the dorsal horn in neuropathic pain. The authors tested the hypothesis that spinal µ opioid receptor-expressing neurons (Oprm1-expressing neurons) contribute to behavioral hypersensitivity and neuronal sensitization in the spared nerve injury model in mice.

METHODS

The authors coupled the use of Oprm1Cre transgenic reporter mice with whole cell patch clamp electrophysiology in lumbar spinal cord slices to evaluate the neuronal activity of Oprm1-expressing neurons in the spared nerve injury model of neuropathic pain. The authors used a chemogenetic approach to activate or inhibit Oprm1-expressing neurons, followed by the assessment of behavioral signs of neuropathic pain.

RESULTS

The authors reveal that spared nerve injury yielded a robust neuroplasticity of Oprm1-expressing neurons. Spared nerve injury reduced Oprm1 gene expression in the dorsal horn as well as the responsiveness of Oprm1-expressing neurons to the selective µ agonist (D-Ala2, N-MePhe4, Gly-ol)-enkephalin (DAMGO). Spared nerve injury sensitized Oprm1-expressing neurons, as reflected by an increase in their intrinsic excitability (rheobase, sham 38.62 ± 25.87 pA [n = 29]; spared nerve injury, 18.33 ± 10.29 pA [n = 29], P = 0.0026) and spontaneous synaptic activity (spontaneous excitatory postsynaptic current frequency in delayed firing neurons: sham, 0.81 ± 0.67 Hz [n = 14]; spared nerve injury, 1.74 ± 1.68 Hz [n = 10], P = 0.0466), and light brush-induced coexpression of the immediate early gene product, Fos in laminae I to II (%Fos/tdTomato+: sham, 0.42 ± 0.57% [n = 3]; spared nerve injury, 28.26 ± 1.92% [n = 3], P = 0.0001). Chemogenetic activation of Oprm1-expressing neurons produced mechanical hypersensitivity in uninjured mice (saline, 2.91 ± 1.08 g [n = 6]; clozapine N-oxide, 0.65 ± 0.34 g [n = 6], P = 0.0006), while chemogenetic inhibition reduced behavioral signs of mechanical hypersensitivity (saline, 0.38 ± 0.37 g [n = 6]; clozapine N-oxide, 1.05 ± 0.42 g [n = 6], P = 0.0052) and cold hypersensitivity (saline, 6.89 ± 0.88 s [n = 5] vs. clozapine N-oxide, 2.31 ± 0.52 s [n = 5], P = 0.0017).

CONCLUSIONS

The authors conclude that nerve injury sensitizes pronociceptive µ opioid receptor-expressing neurons in mouse dorsal horn. Nonopioid strategies to inhibit these interneurons might yield new treatments for neuropathic pain.

EDITOR’S PERSPECTIVE

Corydecumine G inhibits microglia activation via MAPK pathway in a rat model of neuropathic pain

BACKGROUND AND PURPOSE

Microglial activation plays an important role in the onset and progression of neuropathic pain by producing a variety of pro-inflammatory cytokines that interact with neurons to enhance neuronal hyperexcitability. Corydalis decumbens (Thunb.) pers., a traditional Chinese medicine has been used to treat mild cancer pain, dementia and to remit cerebral ischemia in clinics. Phenylphthalide isoquinolines are the major type of metabolites of C. decumbens and one of the derivatives, Corydecumine G (Cor G) has been shown to inhibit neuronal excitability. The present study aims to investigate the analgesic efficacy of Cor G in neuropathic pain rat model, the effects of Cor G on microglia activation and the possible mechanisms.

EXPERIMENTAL APPROACH

Neuropathic pain was modeled using chronic constriction sciatic nerve injury (CCI) in rats. Western blot, immunofluorescence, and qRT-PCR were used to evaluate the levels of protein and mRNA.

KEY RESULTS

Intraperitoneal administration of Cor G concentration-dependently ameliorates mechanical and thermo allodynia, suppresses CCI-induced p38/ERK phosphorylation and spinal cord microglia activation, and attenuates the expression levels of NO, inos, Tnf-α, Pge₂ in dorsal horn of L4-L6 spinal cord on the ligation side in CCI rats. Pretreatment with 30 μM Cor G decreased LPS-induced BV2 microglia activation, which occurred via the inos, Tnf-α, Il-1β, Il-6 and phospho-p38/ERK pathways.

CONCLUSIONS AND IMPLICATIONS

Taken together, we suggest that Cor G, the specific phthalide isoquinoline from traditional Chinese medicine Corydalis Decumbentis Rhizoma, may be promising for treatment of neuropathic pain.

Differential Expression of Long Non-Coding RNAs and Their Role in Rodent Neuropathic Pain Models

Neuropathic pain, which is accompanied by an unpleasant sensation, affects the patient’s quality of life severely. Considering the complexity of the neuropathic pain, there are huge unmet medical needs for it while current effective therapeutics remain far from satisfactory. Accordingly, exploration of mechanisms of neuropathic pain could provide new therapeutic insights. While numerous researches have pointed out the contribution of sensory neuron-immune cell interactions, other mechanisms of action, such as long non-coding RNAs (lncRNAs), also could contribute to the neuropathic pain observed in vivo. LncRNAs have more than 200 nucleotides and were originally considered as transcriptional byproducts. However, recent studies have suggested that lncRNAs played a significant role in gene regulation and disease pathogenesis. A substantial number of long non-coding RNAs were expressed differentially in neuropathic pain models. Besides, therapies targeting specific lncRNAs can significantly ameliorate the development of neuropathic pain, which reveals the contribution of lncRNAs in the generation and maintenance of neuropathic pain and provides a new therapeutic strategy. The primary purpose of this review is to introduce recent studies of lncRNAs on different neuropathic pain models.

The Effect of Intrathecal Injection of Dextromethorphan on the Experimental Neuropathic Pain Model

Background

Peripheral glucocorticoid receptors (GRs) are altered by peripheral nerve injury and may modulate the development of neuropathic pain. Two central pathogenic mechanisms underlying neuropathic pain are neuroinflammation and N-methyl-D-aspartate receptor (NMDAR)-dependent neural plasticity in the spinal cord.

Objectives

This study examined the effect of the non-competitive NMDAR antagonist dextromethorphan on partial sciatic nerve ligation (PSL)-induced neuropathic pain and the spinal expression of the glucocorticoid receptor (GR).

Methods

Male mice were randomly assigned into a sham group and two groups receiving PSL followed by intrathecal saline vehicle or dextromethorphan (iDMP). Vehicle or iDMP was administered 8 - 14 days after PSL. The hotplate paw-withdrawal latency was considered to measure thermal pain sensitivity. The spinal cord was then sectioned and immunostained for GR.

Results

Thermal hyperalgesia developed similarly in the vehicle and iDMP groups prior to the injections (P = 0.828 and 0.643); however, it was completely mitigated during the iDMP treatment (P < 0.001). GR expression was significantly higher in the vehicle group (55.64 ± 4.50) than in the other groups (P < 0.001). The iDMP group (9.99 ± 0.66) showed significantly higher GR expression than the sham group (6.30 ± 1.96) (P = 0.043).

Conclusions

The suppression of PLS-induced thermal hyperalgesia by iDMP is associated with the downregulation of GR in the spinal cord, suggesting that this analgesic effect is mediated by inhibiting GR-regulated neuroinflammation.

Identification of the Hub Genes Related to Nerve Injury-Induced Neuropathic Pain

Background

The reactivity enhancement of pain sensitive neurons in the nervous system is a feature of the pathogenesis for neuropathic pain (NP), yet the underlying mechanisms need to be fully understood. In this study, we made an attempt to clarify the NP-related hub genes and signaling pathways so as to provide effective diagnostic and therapeutic methods toward NP.

Methods

Microarray expression profile GSE30691 including the mRNA-seq data of the spared nerve injury (SNI)-induced NP rats was accessed from the GEO database. Then, genes associated with NP development were screened using differential analysis along with random walk with restart (RWR). GO annotation and KEGG pathway analyses were performed to explore the biological functions and signaling pathways where the genes were activated. Afterward, protein-protein interaction (PPI) analysis and GO analysis were conducted to further identify the hub genes which showed an intimate correlation with NP development.

Results

Totally 94 genes associated with NP development were screened by differential analysis and RWR analysis, and they were observed to be predominantly enriched in hormone secretion and transport, cAMP signaling pathway and other NP occurrence associated functions and pathways. Thereafter, the 94 genes were subjected to PPI analysis to find the genes much more associated with NP and a functional module composed of 48 genes were obtained. 8 hub genes including C3, C1qb, Ccl2, Cxcl13, Timp1, Fcgr2b, Gal, and Lyz2 were eventually identified after further association and functional enrichment analyses, and the expression of these 8 genes were all higher in SNI rats by comparison with those in Sham rats.

Conclusion

Based on the data collected from GEO database, this study discovered 8 hub genes that were closely related to NP occurrence and development, which help to provide potent theoretical basis for NP treatment.

Loss of SNHG4 Attenuated Spinal Nerve Ligation-Triggered Neuropathic Pain through Sponging miR-423-5p

Neuropathic pain is an intractable comorbidity of spinal cord injury. Increasing noncoding RNAs have been implicated in neuropathic pain development. lncRNAs have been recognized as significant regulators of neuropathic pain. lncRNA Small Nucleolar RNA Host Gene 4 (SNHG4) is associated with several tumors. However, the molecular mechanisms of SNHG4 in neuropathic pain remain barely documented. Here, we evaluated the function of SNHG4 in spinal nerve ligation (SNL) rat models. We observed that SNHG4 was significantly upregulated in SNL rat. Knockdown of SNHG4 was able to attenuate neuropathic pain progression via regulating behaviors of neuropathic pain including mechanical and thermal hyperalgesia. Moreover, knockdown of SNHG4 could repress the neuroinflammation via inhibiting IL-6, IL-12, and TNF-α while inducing IL-10 levels. Additionally, miR-423-5p was predicted as the target of SNHG4 by employing bioinformatics analysis. miR-423-5p has been reported to exert significantly poorer in several diseases. However, the role of miR-423-5p in the development of neuropathic pain is needed to be clarified. Here, in our investigation, RIP assay confirmed the correlation between miR-423-5p and SNHG4. Meanwhile, we found that miR-423-5p was significantly decreased in SNL rat models. SNHG4 regulated miR-423-5p expression negatively. As exhibited, the loss of miR-423-5p contributed to neuropathic pain progression, which was rescued by the silence of SNHG4. Therefore, our study indicated SNHG4 as a novel therapeutic target for neuropathic pain via sponging miR-423-5p.

Developing an optimized strategy with transcranial direct current stimulation to enhance the endogenous pain control system in fibromyalgia

INTRODUCTION

Fibromyalgia affects more than 5 million people in the United States and has a detrimental impact on individuals' quality of life. Current pharmacological treatments provide limited benefits to relieve the pain of fibromyalgia, along with a risk of adverse effects; a scenario that explains the increasing interest for multimodal approaches. A tailored strategy to focus on this dysfunctional endogenous pain inhibitory system is transcranial direct current stimulation (tDCS) of the primary motor cortex. By combining tDCS with aerobic exercise, the effects can be optimized. Areas covered: The relevant literature was reviewed and discussed the methodological issues for designing a mechanistic clinical trial to test this combined intervention. Also, we reviewed the neural control of different pathways that integrate the endogenous pain inhibitory system, as well as the effects of tDCS and aerobic exercise both alone and combined. In addition, potential neurophysiological assessments are addressed: conditioned pain modulation, temporal slow pain summation, transcranial magnetic stimulation, and electroencephalography in the context of fibromyalgia. Expert commentary: By understanding the neural mechanisms underlying pain processing and potential optimized interventions in fibromyalgia with higher accuracy, the field has an evident potential of advancement in the direction of new neuromarkers and tailored therapies.

Analyses of long non-coding RNA and mRNA profiles in the spinal cord of rats using RNA sequencing during the progression of neuropathic pain in an SNI model

The pathogenesis of neuropathic pain (NP) is characterized by an increased responsiveness of nociceptive neurons in the nervous system. However, the molecular mechanisms underpinning the NP still remain elusive. Recent data suggest that long non-coding RNAs (lncRNAs) regulate expression of NP-associated genes. Herein, we analyzed lncRNAs and mRNA profiles in the spinal cord of rats by RNA sequencing during the progression of NP in a spared nerve injury (SNI) model. Sprague-Dawley (SD) rats were employed for the establishment of the SNI models, and nociceptive responses to mechanical and thermal stimuli were measured 3 hours prior to surgery and on postoperative days 1, 3, 7 and 14, with L_4-5 spinal cords extracted from three SD rats under deep anesthesia at each time point after behavioral test. SNI rats exhibited higher sensitivity to mechanical stimuli from days 1 to 14. Mechanical hyperalgesia reached a steady peak at day14 after surgery, whereas thermal allodynia did not develop. The results of second-generation sequencing suggested that the expression profiles of lncRNAs and mRNAs were significantly altered in spinal cords of SNI rats versus the control rats at different stages during NP. Differentially expressed (DE) lncRNAs and mRNAs were demonstrated at each stage during the NP course using Volcano Plot, Venn and Hcluster heatmap analyses. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) biological pathway analyses were performed to predict the functionalities of differentially expressed lncRNAs and target genes. Protein interaction networks were constructed based on the correlation analyses of DE lncRNA target proteins at 7 and 14 days after SNI, respectively. Taken together, our results revealed the profiles of lncRNAs and mRNAs in the rat spinal cord under an NP condition. These lncRNAs and mRNAs may represent new therapeutic targets for the treatment of NP.

EZH2 regulates spinal neuroinflammation in rats with neuropathic pain

Alteration in gene expression along the pain signaling pathway is a key mechanism contributing to the genesis of neuropathic pain. Accumulating studies have shown that epigenetic regulation plays a crucial role in nociceptive process in the spinal dorsal horn. In this present study, we investigated the role of enhancer of zeste homolog-2 (EZH2), a subunit of the polycomb repressive complex 2, in the spinal dorsal horn in the genesis of neuropathic pain in rats induced by partial sciatic nerve ligation. EZH2 is a histone methyltransferase, which catalyzes the methylation of histone H3 on K27 (H3K27), resulting in gene silencing. We found that levels of EZH2 and tri-methylated H3K27 (H3K27TM) in the spinal dorsal horn were increased in rats with neuropathic pain on day 3 and day 10 post nerve injuries. EZH2 was predominantly expressed in neurons in the spinal dorsal horn under normal conditions. The number of neurons with EZH2 expression was increased after nerve injury. More strikingly, nerve injury drastically increased the number of microglia with EZH2 expression by more than sevenfold. Intrathecal injection of the EZH2 inhibitor attenuated the development and maintenance of mechanical and thermal hyperalgesia in rats with nerve injury. Such analgesic effects were concurrently associated with the reduced levels of EZH2, H3K27TM, Iba1, GFAP, TNF-α, IL-1β, and MCP-1 in the spinal dorsal horn in rats with nerve injury. Our results highly suggest that targeting the EZH2 signaling pathway could be an effective approach for the management of neuropathic pain.

Pioglitazone rapidly reduces neuropathic pain through astrocyte and nongenomic PPARγ mechanisms

Repeated administration of peroxisome proliferator-activated receptor gamma (PPARγ) agonists reduces neuropathic pain-like behavior and associated changes in glial activation in the spinal cord dorsal horn. As PPARγ is a nuclear receptor, sustained changes in gene expression are widely believed to be the mechanism of pain reduction. However, we recently reported that a single intrathecal (i.t.) injection of pioglitazone, a PPARγ agonist, reduced hyperalgesia within 30 minutes, a time frame that is typically less than that required for genomic mechanisms. To determine the very rapid antihyperalgesic actions of PPARγ activation, we administered pioglitazone to rats with spared nerve injury and evaluated hyperalgesia. Pioglitazone inhibited hyperalgesia within 5 minutes of injection, consistent with a nongenomic mechanism. Systemic or i.t. administration of GW9662, a PPARγ antagonist, inhibited the antihyperalgesic actions of intraperitoneal or i.t. pioglitazone, suggesting a spinal PPARγ-dependent mechanism. To further address the contribution of nongenomic mechanisms, we blocked new protein synthesis in the spinal cord with anisomycin. When coadministered intrathecally, anisomycin did not change pioglitazone antihyperalgesia at an early 7.5-minute time point, further supporting a rapid nongenomic mechanism. At later time points, anisomycin reduced pioglitazone antihyperalgesia, suggesting delayed recruitment of genomic mechanisms. Pioglitazone reduction of spared nerve injury-induced increases in GFAP expression occurred more rapidly than expected, within 60 minutes. We are the first to show that activation of spinal PPARγ rapidly reduces neuropathic pain independent of canonical genomic activity. We conclude that acute pioglitazone inhibits neuropathic pain in part by reducing astrocyte activation and through both genomic and nongenomic PPARγ mechanisms.

Femoral nerve entrapment in a dog with diffuse idiopathic skeletal hyperostosis

OBJECTIVE

To report femoral neuropathy caused by nerve entrapment associated with diffuse idiopathic skeletal hyperostosis (DISH).

STUDY DESIGN

Case report.

ANIMAL

Seven-year-old female spayed Boxer dog.

RESULTS

Entrapment of the right femoral nerve due to DISH caused a femoral nerve deficit and atrophy of muscle groups associated with the affected nerve. A combination of computed tomography and magnetic resonance imaging was performed to provide a diagnosis. Amputation of the right transverse process of the sixth lumbar vertebra at the level of nerve entrapment relieved the neurological abnormality.

CONCLUSIONS

Nerve entrapment leading to neurapraxia may occur concurrently with DISH and surgery in this case was successful in restoring function.

CLINICAL RELEVANCE

Peripheral neuropathy from nerve entrapment should be considered in patients with DISH. Surgical amputation of impinging osseous structures may be indicated for relief of femoral neuropathy.

PPARγ activation blocks development and reduces established neuropathic pain in rats

Peroxisome proliferator-activated receptor gamma (PPARγ) is emerging as a new pharmacotherapeutic target for chronic pain. When oral (3-30 mg/kg/day in chow for 7 wk) or twice-daily intraperitoneal (1-10 mg/kg/day for 2 wk) administration began before spared nerve injury (SNI), pioglitazone, a PPARγ agonist, dose-dependently prevented multiple behavioral signs of somatosensory hypersensitivity. The highest dose of intraperitoneal pioglitazone did not produce ataxia or reductions in transient mechanical and heat nociception, indicating that inhibitory effects on hypersensitivity were not secondary to adverse drug-induced behaviors or antinociception. Inhibitory effects on hypersensitivity persisted at least one week beyond cessation of pioglitazone administration, suggestive of long-lasting effects on gene expression. Blockade of PPARγ with GW9662, an irreversible and selective PPARγ antagonist, dose-dependently reduced the inhibitory effect of pioglitazone on hypersensitivity, indicating a PPARγ-dependent action. Remarkably, a single preemptive injection of pioglitazone 15 min before SNI attenuated hypersensitivity for at least 2 weeks; this was enhanced with a second injection delivered 12 h after SNI. Pioglitazone injections beginning after SNI also reduced hypersensitivity, albeit to a lesser degree than preemptive treatment. Intraperitoneal pioglitazone significantly reduced the nerve injury-induced up-regulation of cd11b, GFAP, and p-p38 in the dorsal horn, indicating a mechanism of action involving spinal microglia and/or astrocyte activation. Oral pioglitazone significantly reduced touch stimulus-evoked phospho-extracellular signal-related kinase (p-ERK) in lamina I-II, indicating a mechanism of action involving inhibition of central sensitization. We conclude that pioglitazone reduces spinal glial and stimulus-evoked p-ERK activation and that PPARγ activation blocks the development of and reduces established neuropathic pain.

Therapeutic potential of matrix metalloprotease inhibitors in neuropathic pain

IMPORTANCE OF THE FIELD

Millions of people suffer from neuropathic pain (NP), but the treatment is empirical and results in transient relief in only a few patients. This is primarily because of the poor understanding of the molecular mechanism underlying NP. Following nerve injury, there is a differential and temporal pattern of MMPs expression that coincides with changes in levels of pro-inflammatory cytokines, suggesting that MMPs not only act as mediators for neuroinflammation but might also be directly involved in pain associated with nerve damage.

AREAS COVERED IN THIS REVIEW

The present review describes the different mechanisms of NP. The main focus of the review is to highlight the importance of MMPs in NP and their inhibition as a novel approach for treating NP.

WHAT THE READER WILL GAIN

A comprehensive overview of the role of MMPs in the pathogenesis of NP and the potential of MMP inhibition as a therapeutic intervention for NP.

TAKE HOME MESSAGE

Targeted therapy using specific MMP inhibitors, siRNAs, peptide inhibitors and monoclonal antibodies can provide a better way of treatment by blocking a single MMP and can reduce the side effects of broad-spectrum MMP inhibitors.

Epigenetic gene silencing underlies C-fiber dysfunctions in neuropathic pain

Peripheral nerve injury causes neuropathic pain, which is characterized by the paradoxical sensations of positive and negative symptoms. Clinically, negative signs are frequently observed; however, their underlying molecular mechanisms are largely unknown. Dysfunction of C-fibers is assumed to underlie negative symptoms and is accompanied by long-lasting downregulation of Na(v)1.8 sodium channel and mu-opioid receptor (MOP) in the dorsal root ganglion (DRG). In the present study, we found that nerve injury upregulates neuron-restrictive silencer factor (NRSF) expression in the DRG neurons mediated through epigenetic mechanisms. In addition, chromatin immunoprecipitation analysis revealed that nerve injury promotes NRSF binding to the neuron-restrictive silencer element within MOP and Na(v)1.8 genes, thereby causing epigenetic silencing. Furthermore, NRSF knockdown significantly blocked nerve injury-induced downregulations of MOP and Na(v)1.8 gene expressions, C-fiber hypoesthesia, and the losses of peripheral morphine analgesia and Na(v)1.8-selective blocker-induced hypoesthesia. Together, these data suggest that NRSF causes pathological and pharmacological dysfunction of C-fibers, which underlies the negative symptoms in neuropathic pain.

Spinal inhibitory neurotransmission in neuropathic pain

Nerve injury increases the spinal cord expression and/or activity of voltage- and ligand-gated ion channels, peptide receptors, and neuroimmune factors, which then drive dorsal horn neuron hyperexcitability. The intensity and duration of this central sensitization is determined by the net activity of local excitatory and inhibitory neurotransmitter systems, together with ongoing/evoked primary afferent activity and descending supraspinal control. Spinal endogenous inhibitory systems serve as opposing compensatory influences and are gaining recognition for their powerful capacity to restrain allodynia and hyperalgesia. These include numerous G protein-coupled receptors (mu- and delta-opioid, alpha(2)-adrenergic, purinergic A1, neuropeptide Y1 and Y2, cannabinoid CB1 and CB2, muscarinic M2, gamma-amino-butyric acid type B, metabotropic glutamate type II-III, somatostatin) and perhaps nuclear receptors (peroxisome proliferator-activated receptor gamma). Excessive downregulation or defective compensatory upregulation of these systems may contribute to the maintenance of neuropathic pain. An increasing number of pharmacotherapeutic strategies for neuropathic pain are emerging that mimic and enhance inhibitory neurotransmission in the dorsal horn.

Neuropathic pain in dogs and cats: if only they could tell us if they hurt

Neuropathic pain is difficult to diagnose in veterinary patients because they are unable to verbalize their pain. By assuming that neuropathic pain may exist based on the history of events that each patient has experienced, a focused client history and neurologic examination may identify a lesion resulting in persistent or spontaneous pain. Once neuropathic pain is diagnosed, a trial analgesic or acupuncture session(s) should be prescribed with instructions for owners to observe behavior. Dosing of the analgesic can be titrated to the patient's needs while avoiding adverse effects. When a particular analgesic may be ineffectual, an alternate class should be tried. As research into the neurobiologic mechanisms of neuropathic pain continues, specific therapies for its management should eventually appear in the human clinical setting and subsequently be investigated for veterinary clinical use.

An overview of the features influencing pain after inguinal hernia repair

Pain is a prominent issue in inguinal hernia repair research as its persisting appearance is a severe complication. The interest is also urged by the combination of a high number of repairs with an estimated risk for chronic postoperative pain of 11%. Almost every healthcare provider could encounter this complication. Pain is a complex study subject, mostly defined as an unpleasant sensory and emotional experience associated with actual or potential tissue damage. Various explanatory factors for pain following hernia repair have been reported. Most investigators, however, discuss only a few aspects. In the present review, these factors are collected to provide a more holistic synopsis of pain following hernia repair. It may be a resource for understanding this and other postsurgical pain.

Extended Swimming Exercise Reduces Inflammatory and Peripheral Neuropathic Pain in Rodents

Physical exercise is often recommended to patients who have chronic pain. However, only a small number of studies report exercise-induced analgesia in the setting of inflammatory pain, and even fewer relate long-term exercise to reductions in neuropathic pain. To address these questions, we evaluated the effect of extended swimming exercise in animal models of inflammatory (intraplantar injection of dilute formalin) and neuropathic (partial peripheral nerve injury) pain. We found that 9 days of swimming exercise in 37 degrees C water for 90 min/d decreased licking and flinching responses to formalin, as compared with nonexercised control animals. In addition, 18 to 25 days of swimming decreased nerve injury-induced cold allodynia and thermal hyperalgesia in rats, and 7 days of swimming decreased nerve injury-induced thermal hyperalgesia in mice. Our data indicate that swimming exercise reduces behavioral hypersensitivity in formalin- and nerve injury-induced animal models of persistent pain.

PERSPECTIVE

Surprisingly, few animal studies have investigated the effects of extended exercise on chronic pain. Our results support the use of exercise as a nonpharmacological approach for the management of peripheral neuropathic pain.

Neuropeptide Y acts at Y1 receptors in the rostral ventral medulla to inhibit neuropathic pain

Brain microinjection studies in the rat using local anesthetics suggest that the rostral ventral medulla (RVM) contributes to the facilitation of neuropathic pain. However, these studies were restricted to a single model of neuropathic pain (the spinal nerve ligation model) and to just two stimulus modalities (non-noxious tactile stimulus and heat). Also, few neurotransmitter systems have been shown to modulate descending facilitation. After either partial sciatic nerve ligation (PSNL) or spared nerve injury (SNI), we found that unilateral or bilateral microinjection of lidocaine into the RVM reduced not only mechanical allodynia (decreased threshold to von Frey hairs and/or an automated device) and mechanical hyperalgesia (increased paw lifting in response to a noxious pin), but also cold hypersensitivity (increased lifting in response to the hindpaw application of a drop of acetone). Application of a drop of water did not elicit paw withdrawal, indicating that the acetone test is indeed a measure of cold hypersensitivity. We found significant neuropeptide Y Y1-like immunoreactivity within, and lateral to, the midline RVM. Intra-RVM injection of neuropeptide Y (NPY) dose-dependently inhibited the mechanical and cold hypersensitivity associated with PSNL or SNI, an effect that could be blocked by the Y1 receptor antagonist BIBO 3304. We conclude that medullary facilitation spans multiple behavioral signs of allodynia and hyperalgesia in multiple models of neuropathic pain. Furthermore, NPY inhibits behavioral signs of neuropathic pain, possibly by acting at Y1 receptors in the RVM.

Spinal mechanisms of NPY analgesia

We review previously published data, and present some new data, indicating that spinal application of neuropeptide Y (NPY) reduces behavioral and neurophysiological signs of acute and chronic pain. In models of acute pain, early behavioral studies showed that spinal (intrathecal) administration of NPY and Y2 receptor agonists decrease thermal nociception. Subsequent neurophysiological studies indicated that Y2-mediated inhibition of excitatory neurotransmitter release from primary afferent terminals in the substantia gelatinosa may contribute to the antinociceptive actions of NPY. As with acute pain, NPY reduced behavioral signs of inflammatory pain such as mechanical allodynia and thermal hyperalgesia; however, receptor antagonist studies indicate an important contribution of spinal Y1 rather than Y2 receptors. Interestingly, Y1 agonists suppress inhibitory synaptic events in dorsal horn neurons (indeed, well known mu-opioid analgesic drugs produce similar cellular actions). To resolve the behavioral and neurophysiological data, we propose that NPY/Y1 inhibits the spinal release of inhibitory neurotransmitters (GABA and glycine) onto inhibitory neurons, e.g. disinhibition of pain inhibition, resulting in hyporeflexia. The above mechanisms of Y1- and Y2-mediated analgesia may also operate in the setting of peripheral nerve injury, and new data indicate that NPY dose-dependently inhibits behavioral signs of neuropathic pain. Indeed, neurophysiological studies indicate that Y2-mediated inhibition of Ca(2+) channel currents in dorsal root ganglion neurons is actually increased after axotomy. We conclude that spinal delivery of Y1 agonists may be of use in the treatment of chronic inflammatory pain, and that the use of Y1 and Y2 agonists in neuropathic pain warrants further consideration.

Differential effects of naproxen and rofecoxib on the development of hypersensitivity following nerve injury in rats

The present study was undertaken to determine the effects of cyclooxygenase (COX) inhibitors on the development of neuropathic pain in rats following chronic constriction injury (CCI). A single intraperitoneal administration of naproxen, a nonselective COX inhibitor (10 or 30 mg/kg), or rofecoxib, a selective COX-2 inhibitor (3 or 10 mg/kg) 2 h before nerve injury did not attenuate the development of neuropathic state for 28 days. However, the administration of naproxen [10 or 30 mg/kg, intraperitonelly (i.p.)], but not rofecoxib (3 or 10 mg/kg, i.p.), on day 7 attenuated hypersensitivity but did not alter its development for 28 days. Furthermore, naproxen significantly reduced hyperalgesia and allodynia for 4 h, but the efficacy was not observed 24 h after the treatment, whereas rofecoxib failed to modify the hypersensitivity following perineural (p.n.) or intrathecal (i.t.) administration on day 7. Chronic administration of naproxen (3, 10 or 30 mg/kg), but not rofecoxib (1, 3 or 10 mg/kg), 2 h before, daily for 7 days, after nerve injury significantly attenuated and further delayed the development of hypersensitivity for 21 days following nerve injury. These results suggest that the development of hypersensitivity in the CCI model is not COX-2 dependent and that the chronic administration of naproxen started early before peripheral nerve injury could attenuate the development of hypersensitivity.

Neurokinin-1 receptor gene expression in the mouse dorsal horn increases with neuropathic pain

Peripheral nerve injury is associated with hyperesthesia and increased neurokinin-1 receptor (NK-1) expression in the dorsal horn of the spinal cord. To test the hypothesis that NK-1 gene expression underlies these responses, we used solution hybridization-nuclease protection assays to quantify NK-1 mRNA levels in dorsal quadrants of the mouse lumbar dorsal horn. Partial sciatic nerve ligation was associated with mechanical allodynia, thermal hyperalgesia, and an increase in NK-1 mRNA on the ipsilateral, but not contralateral, side. Regression analysis showed that NK-1 mRNA was significantly correlated with thermal paw withdrawal latency but not mechanical threshold. Our results support the idea that substance P is an important mediator of thermal hypersensitivity in the setting of nerve injury and suggest that increased NK-1 receptor transcription precedes increased NK-1 receptor density, ultimately leading to behavioral hypersensitivity to peripheral thermal stimulation.

PERSPECTIVE

The therapeutic efficacy of NK-1 receptor antagonists is unclear. The current data suggest that peripheral nerve injury increases the expression of substance P (NK-1) receptors in the spinal cord dorsal horn; this is correlated with heat hypersensitivity. The analgesic effects of NK-1 antagonists might become apparent if tested against heat-evoked pain in nerve injury patients.

Defining neuropathic pain

Clinical research and practice have suffered because of lack of specificity when clinical diagnoses of pain are made. Distinction between neuropathic and inflammatory pain mechanisms is suggested, as well as the distinction between neuropathic pain from hypersensitivity pain disorders, previously termed neuropathic pain due to neurological dysfunction. Neuropathic pain is in this case defined as pain occurring in the ara of body affected by neurological disease or injury. This type of pain manifests not only with positive sensory phenomena such as pain, dysesthesia, and different types of hyperalgesia, but also with negative sensory phenomena and negative and positive motor and autonomic symptoms and signs.

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

Neuropathic pain is induced by injury or disease of the nervous system. Studies aimed at understanding the molecular pathophysiology of neuropathic pain have so far focused on a few known molecules and signaling pathways in neurons. However, the pathophysiology of neuropathic pain appears to be very complex and remains poorly understood. A global understanding of the molecular mechanisms involved in neuropathic pain is needed for a better understanding of the pathophysiology and treatment of neuropathic pain. Towards this end, we examined global gene expression changes as well as the pathobiology at the cellular level in a spinal nerve ligation neuropathic pain model using DNA microarray, quantitative real-time PCR and immunohistochemistry. We found that the behavioral hypersensitivity that is manifested in the persistent pain state is accompanied by previously undescribed changes in gene expression. In the DRG, we found regulation of: (1) immediate early genes; (2) genes such as ion channels and signaling molecules that contribute to the excitability of neurons; and (3) genes that are indicative of secondary events such as neuroinflammation. In addition, we studied gene regulation in both injured and uninjured DRG by quantitative PCR, and observed differential gene regulation in these two populations of DRGs. Furthermore, we demonstrated unexpected co-regulation of many genes, especially the activation of neuroinflammation markers in both the PNS and CNS. The results of our study provide a new picture of the molecular mechanisms that underlie the complexity of neuropathic pain and suggest that chronic pain shares common pathobiology with progressive neurodegenerative disease.

Antihyperalgesic effects of intrathecal neuropeptide Y during inflammation are mediated by Y1 receptors

Inflammation induces an up-regulation of neuropeptide tyrosine (NPY) and its receptors in the dorsal horn, suggesting an important role in nociceptive transmission. Our initial studies revealed that NPY dose-dependently increased hotplate response latency, and to a lesser degree, thermal paw withdrawal latency (PWL); these effects occurred at doses that affect neither motor coordination (as assessed by the rotarod test) nor paw skin temperature. We next evaluated the behavioral effects of intrathecal administration of NPY and NPY antagonists with the aim of assessing the contribution of NPY to correlates of persistent nociception associated with the unilateral plantar injection of carrageenan or complete Freund's adjuvant (CFA). NPY robustly and dose-dependently increased PWL on the side ipsilateral to carrageenan injection, with only a small effect on the contralateral side. Similarly, NPY (30 microg) produced a large and long-lasting increase in PWL on the side ipsilateral to CFA injection (140% change), with only a small effect on the contralateral side (25% change). The ipsilateral effect of NPY was completely inhibited with the potent Y1 antagonist, BIBO 3304 (3 microg), but not the Y2 antagonist, BIIE 0246. When administered alone, BIBO 3304 (but not BIIE 0246) slightly decreased thermal PWL on the side ipsilateral (25% change), but not contralateral, to CFA injection; this suggests that inflammation strengthens inhibitory NPY tone. We conclude that spinal Y1 receptors contribute to the inhibitory effects of NPY on thermal hypersensitivity in the awake rat. Further studies are necessary to determine whether enhanced release of NPY and Y1-mediated inhibition of spinal nociceptive transmission ultimately results in a compensatory, adaptive inhibition of thermal hypersensitivity in the setting of inflammation.

Quantitative assessment of neuropathic pain

Quantitative sensory testing (QST) refers to a group of protocols that allows for quantitative measures of somesthetic function. Several protocols evaluate perceptual threshold, whereas others evaluate perception of stimuli above threshold. Each protocol has its own advantages and disadvantages, but one must always weigh a trade-off between accuracy (with longer protocols) and expediency (with shorter protocols). In assessing patients with neuropathic pain, one is interested in both positive and negative sensory symptoms. QST studies, using either neuropathic pain patients or healthy volunteers who have been rendered temporarily hyperalgesic, have demonstrated that pain abnormalities can be modality specific. The fact that various pain abnormalities can exist independently of each other suggests that (at least partially) different neuropathologic processes are responsible for each one. Current research suggests that both peripheral sensitization and central sensitization play a role in these abnormal pain conditions, and identification of precise neuropathologic mechanisms is under active investigation.