Intrathecal leptin inhibits expression of the P2X2/3 receptors and alleviates neuropathic pain induced by chronic constriction sciatic nerve injury

HSP27 Modulates Neuropathic Pain by Inhibiting P2X3 Degradation

The role of heat shock protein 27 (HSP27), a chaperone, in neuropathic pain after nerve injury has not been systematically surveyed despite its neuroprotective and regeneration-promoting effects. In this study, we found that HSP27 expression in sensory neurons of the dorsal root ganglia (DRG) mediated nerve injury-induced neuropathic pain. Neuropathic pain behaviors were alleviated by silencing HSP27 in the DRG of a rat spinal nerve ligation (SNL) model. Local injection of an HSP27-overexpression construct into the DRG of naïve rats elicited neuropathic pain behaviors. HSP27 interacted with a purinergic receptor, P2X3, and their expression patterns corroborated the induction and reversal of neuropathic pain according to two lines of evidence: colocalization immunohistochemically and immunoprecipitation biochemically. In a cell model cotransfected with HSP27 and P2X3, the degradation rate of P2X3 was reduced in the presence of HSP27. Such an alteration was mediated by reducing P2X3 ubiquitination in SNL rats and was reversed after silencing HSP27 in the DRGs of SNL rats. In summary, the interaction of HSP27 with P2X3 provides a new mechanism of injury-induced neuropathic pain that could serve as an alternative therapeutic target.

Role and therapeutic target of P2X2/3 receptors in visceral pain

Visceral pain (VP) is caused by internal organ disease. VP is involved in nerve conduction and related signaling molecules, but its specific pathogenesis has not yet been fully elucidated. Currently, there are no effective methods for treating VP. The role of P2X2/3 in VP has progressed. After visceral organs are subjected to noxious stimulation, cells release ATP, activate P2X2/3, enhance the sensitivity of peripheral receptors and the plasticity of neurons, enhance sensory information transmission, sensitize the central nervous system, and play an important role in the development of VP. However, antagonists possess the pharmacological effect of relieving pain. Therefore, in this review, we summarize the biological functions of P2X2/3 and discuss the intrinsic link between P2X2/3 and VP. Moreover, we focus on the pharmacological effects of P2X2/3 antagonists on VP therapy and provide a theoretical basis for its targeted therapy.

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.

Identification of key candidate genes and biological pathways in neuropathic pain

BACKGROUND

Neuropathic pain is a common chronic pain, characterized by spontaneous pain and mechanical allodynia. The incidence of neuropathic pain is on the rise due to infections, higher rates of diabetes and stroke, and increased use of chemotherapy drugs in cancer patients. At present, due to its pathophysiological process and molecular mechanism remaining unclear, there is a lack of effective treatment and prevention methods in clinical practice. Now, we use bioinformatics technology to integrate and filter hub genes that may be related to the pathogenesis of neuropathic pain, and explore their possible molecular mechanism by functional annotation and pathway enrichment analysis.

METHODS

The expression profiles of GSE24982, GSE2884, GSE2636 and GSE30691 were downloaded from the Gene Expression Omnibus（GEO）database, and these datasets include 93 neuropathic pain Rattus norvegicus and 59 shame controls. After the four datasets were all standardized by quantiles, the differentially expressed genes (DEGs) between NPP Rattus norvegicus and the shame controls were finally identified by the robust rank aggregation (RRA) analysis method. In order to reveal the possible underlying biological function of DEGs, the Gene Ontology (GO) functional annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) Pathway enrichment analysis of DEGs were performed. In addition, a Protein-protein Interaction (PPI) network was also established. At the end of our study, a high throughput sequencing dataset GSE117526 was used to corroborate our calculation results.

RESULTS

Through RRA analysis of the above four datasets GSE24982, GSE2884, GSE2636, and GSE30691, we finally obtained 231 DEGs, including 183 up-regulated genes and 47 down-regulated genes. Arranging 231 DEGs in descending order according to |log2 fold change (FC)|, we found that the top 20 key genes include 14 up-regulated genes and 6 down-regulated genes. The most down-regulated hub gene abnormal expressed in NPP was Egf17 (P-value = 0.008), Camk2n2 (P-value = 0.002), and Lep (P-value = 0.02), and the most up-regulated hub gene abnormal expressed in NPP was Nefm (P-value = 1.08E-06), Prx (P-value = 2.68E-07), and Stip1 (P-value = 4.40E-07). In GO functional annotation analysis results, regulation of ion transmembrane transport (GO:0034765; P-value = 1.45E-09) was the most remarkable enriched for biological process, synaptic membrane (GO:0097060; P-value = 2.95E-08) was the most significantly enriched for cellular component, channel activity (GO:0015267; P-value = 2.44E-06) was the most prominent enriched for molecular function. In KEGG pathway enrichment analysis results, the top three notable enrichment pathways were Neuroactive ligand-receptor interaction (rno04080; P-value = 3.46E-08), Calcium signaling pathway (rno04020; P-value = 5.37E-05), and Osteoclast differentiation (rno04380; P-value = 0.000459927). Cav1 and Lep appeared in the top 20 genes in both RRA analysis and PPI analysis, while Nefm appeared in RRA analysis and datasets GSE117526 validation analysis, so we finally identified these three genes as hub genes.

CONCLUSIONS

Our research identified the hub genes and signal pathways of neuropathic pain, enriched the pathophysiological mechanism of neuropathic pain to some extent, and provided a possible basis for the targeted therapy of neuropathic pain.

The Role of Saponins in the Treatment of Neuropathic Pain

Neuropathic pain is a chronic pain caused by tissue injury or disease involving the somatosensory nervous system, which seriously affects the patient's body function and quality of life. At present, most clinical medications for the treatment of neuropathic pain, including antidepressants, antiepileptic drugs, or analgesics, often have limited efficacy and non-negligible side effects. As a bioactive and therapeutic component extracted from Chinese herbal medicine, the role of the effective compounds in the prevention and treatment of neuropathic pain have gradually become a research focus to explore new analgesics. Notably, saponins have shown analgesic effects in a large number of animal models. In this review, we summarized the most updated information of saponins, related to their analgesic effects in neuropathic pain, and the recent progress on the research of therapeutic targets and the potential mechanisms. Furthermore, we put up with some perspectives on future investigation to reveal the precise role of saponins in neuropathic pain.

Association between P2X3 receptors and neuropathic pain: As a potential therapeutic target for therapy

Neuropathic pain is a common clinical symptom of various diseases, and it seriously affects the physical and mental health of patients. Owing to the complex pathological mechanism of neuropathic pain, clinical treatment of pain is challenging. Therefore, there is growing interest among researchers to explore potential therapeutic strategies for neuropathic pain. A large number of studies have shown that development of neuropathic pain is related to nerve conduction and related signaling molecules. P2X3 receptors (P2X3R) are ATP-dependent ion channels that participate in the transmission of neural information and related signaling pathways, sensitize the central nervous system, and play a key role in the development of neuropathic pain. In this paper, we summarized the structure and biological characteristics of the P2X3R gene and discussed the role of P2X3R in the nervous system. Moreover, we outlined the related pathological mechanisms of pain and described the relationship between P2X3R and chronic pain to provide valuable information for development of novel treatment strategies for pain.

Adipocytokines: Emerging therapeutic targets for pain management

Although pain has lower mortality rates than cancer, diabetes and stroke, pain is a predominate source of distress and disability. However, the management of pain remains an enormous problem. Many drugs used to pain treatment have more or less side effects. Therefore, the development of novel therapeutic target is critical for the treatment of pain. Notably, studies have shown that adipocytokines have a dual role in pain. Growing shreds of evidence shows that the levels of adipocytokines are upregulated or downregulated in the development of pain. In addition, substantial evidence indicates that regulation of adipocytokines levels in models of pain attenuates or promotes pain behaviors. In this review, we summarized and discussed the effect of adipocytokines in pain. These evidence indicates that adipocytokines attenuate or promote pain behaviors through interacting with their receptors, activating serotonin pathway, interacting with μ-opioid receptor, activating microglia, infiltrating macrophage and so on. Overall, adipocytokines have some potential in treating pain, but the underlying mechanisms remain unclear and need to be further studied.

Asprosin, a novel therapeutic candidate for painful neuropathy: an experimental study in mice

Recent studies indicate presence of a strong link between adipokines and neuropathic pain. However, the effects of asprosin, a novel adipokine, on neuropathic pain have not been studied in animal models.Mouse models were employed to investigate the antinociceptive effectiveness of asprosin in the treatment of three types of neuropathic pain, with metabolic (streptozocin/STZ), toxic (oxaliplatin/OXA), and traumatic (sciatic nerve ligation/CCI [chronic constriction nerve injury]) etiologies, respectively. Changes in nociceptive behaviors were assessed relative to controls using thermal (the hot plate and cold plate tests, at 50 °C and 4 °C respectively) and mechanical pain (von Frey test) tests after intraperitoneal (i.p.) administration of asprosin (10 µg/kg) and gabapentin (50 mg/kg) in several times intervals. Besides, possible effect of asprosin on the motor coordination of mice was assessed with a rotarod test. Serum level of asprosin was quantified by ELISA.In neuropathic pain models (STZ, OXA, and CCI), asprosin administration significantly reduced both mechanical and thermal hypersensitivity, indicating that it exhibits a clear-cut antihypersensitivity effect in the analyzed neuropathic pain models. The most effective time of asprosin on pain threshold was observed 60 min after its injection. Also, asprosin displayed no notable effect on the motor activity. Asprosin levels were significantly lower in neuropathic pain compared to healthy group (p < 0.05).The results yielded by the present study suggest that asprosin exhibits an analgesic effect in the neuropathic pain models and may have clinical utility in alleviating chronic pain associated with disease and injury originating from peripheral structures.

Molecular Changes in the Dorsal Root Ganglion during the Late Phase of Peripheral Nerve Injury-induced Pain in Rodents: A Systematic Review

The dorsal root ganglion is widely recognized as a potential target to treat chronic pain. A fundamental understanding of quantitative molecular and genomic changes during the late phase of pain is therefore indispensable. The authors performed a systematic literature review on injury-induced pain in rodent dorsal root ganglions at minimally 3 weeks after injury. So far, slightly more than 300 molecules were quantified on the protein or messenger RNA level, of which about 60 were in more than one study. Only nine individual sequencing studies were performed in which the most up- or downregulated genes varied due to heterogeneity in study design. Neuropeptide Y and galanin were found to be consistently upregulated on both the gene and protein levels. The current knowledge regarding molecular changes in the dorsal root ganglion during the late phase of pain is limited. General conclusions are difficult to draw, making it hard to select specific molecules as a focus for treatment.

Physiological Changes and Pathological Pain Associated with Sedentary Lifestyle-Induced Body Systems Fat Accumulation and Their Modulation by Physical Exercise

A sedentary lifestyle is associated with overweight/obesity, which involves excessive fat body accumulation, triggering structural and functional changes in tissues, organs, and body systems. Research shows that this fat accumulation is responsible for several comorbidities, including cardiovascular, gastrointestinal, and metabolic dysfunctions, as well as pathological pain behaviors. These health concerns are related to the crosstalk between adipose tissue and body systems, leading to pathophysiological changes to the latter. To deal with these health issues, it has been suggested that physical exercise may reverse part of these obesity-related pathologies by modulating the cross talk between the adipose tissue and body systems. In this context, this review was carried out to provide knowledge about (i) the structural and functional changes in tissues, organs, and body systems from accumulation of fat in obesity, emphasizing the crosstalk between fat and body tissues; (ii) the crosstalk between fat and body tissues triggering pain; and (iii) the effects of physical exercise on body tissues and organs in obese and non-obese subjects, and their impact on pathological pain. This information may help one to better understand this crosstalk and the factors involved, and it could be useful in designing more specific training interventions (according to the nature of the comorbidity).

Was it something I ate? Understanding the bidirectional interaction of migraine and appetite neural circuits

Migraine attacks can involve changes of appetite: while fasting or skipping meals are often reported triggers in susceptible individuals, hunger or food craving are reported in the premonitory phase. Over the last decade, there has been a growing interest and recognition of the importance of studying these overlapping fields of neuroscience, which has led to novel findings. The data suggest additional studies are needed to unravel key neurobiological mechanisms underlying the bidirectional interaction between migraine and appetite. Herein, we review information about the metabolic migraine phenotype and explore migraine therapeutic targets that have a strong input on appetite neuronal circuits, including the calcitonin gene-related peptide (CGRP), the pituitary adenylate cyclase-activating polypeptide (PACAP) and the orexins. Furthermore, we focus on potential therapeutic peptide targets that are involved in regulation of feeding and play a role in migraine pathophysiology, such as neuropeptide Y, insulin, glucagon and leptin. We then examine the orexigenic - anorexigenic circuit feedback loop and explore glucose metabolism disturbances. Additionally, it is proposed a different perspective on the most reported feeding-related trigger - skipping meals - as well as a link between contrasting feeding behaviors (skipping meals vs food craving). Our review aims to increase awareness of migraine through the lens of appetite neurobiology in order to improve our understanding of the earlier phase of migraine, encourage better studies and cross-disciplinary collaborations, and provide novel migraine-specific therapeutic opportunities.

Leptin Contributes to Neuropathic Pain via Extrasynaptic NMDAR-nNOS Activation

Leptin is an adipocytokine that is primarily secreted by white adipose tissue, and it contributes to the pathogenesis of neuropathic pain in collaboration with N-methyl-D-aspartate receptors (NMDARs). Functional NMDARs are a heteromeric complex that primarily comprise two NR1 subunits and two NR2 subunits. NR2A is preferentially located at synaptic sites, and NR2B is enriched at extrasynaptic sites. The roles of synaptic and extrasynaptic NMDARs in the contribution of leptin to neuropathic pain are not clear. The present study examined whether the important role of leptin in neuropathic pain was related to synaptic or extrasynaptic NMDARs. We used a rat model of spared nerve injury (SNI) and demonstrated that the intrathecal administration of the NR2A-selective antagonist NVP-AAM077 and the NR2B-selective antagonist Ro25-6981 prevented and reversed mechanical allodynia following SNI. Administration of exogenous leptin mimicked SNI-induced behavioral allodynia, which was also prevented by NVP-AAM077 and Ro25-6981. Mechanistic studies showed that leptin enhanced NR2B- but not NR2A-mediated currents in spinal lamina II neurons of naïve rats. Leptin also upregulated the expression of NR2B, which was blocked by the NR2B-selective antagonist Ro25-6981, in cultured dorsal root ganglion (DRG) neurons. Leptin enhanced neuronal nitric oxide synthase (nNOS) expression, which was also blocked by Ro25-6981, in cultured DRG cells. However, leptin did not change NR2A expression, and the NR2A-selective antagonist NVP-AAM077 had no effect on leptin-enhanced nNOS expression. Our data suggest an important cellular link between the spinal effects of leptin and the extrasynaptic NMDAR-nNOS-mediated cellular mechanism of neuropathic pain.

Analysis of Epigenetic Age Predictors in Pain-Related Conditions

Chronic pain prevalence is high worldwide and increases at older ages. Signs of premature aging have been associated with chronic pain, but few studies have investigated aging biomarkers in pain-related conditions. A set of DNA methylation (DNAm)-based estimates of age, called “epigenetic clocks,” has been proposed as biological measures of age-related adverse processes, morbidity, and mortality. The aim of this study is to assess if different pain-related phenotypes show alterations in DNAm age. In our analysis, we considered three cohorts for which whole-blood DNAm data were available: heat pain sensitivity (HPS), including 20 monozygotic twin pairs discordant for heat pain temperature threshold; fibromyalgia (FM), including 24 cases and 20 controls; and headache, including 22 chronic migraine and medication overuse headache patients (MOH), 18 episodic migraineurs (EM), and 13 healthy subjects. We used the Horvath's epigenetic age calculator to obtain DNAm-based estimates of epigenetic age, telomere length, levels of 7 proteins in plasma, number of smoked packs of cigarettes per year, and blood cell counts. We did not find differences in epigenetic age acceleration, calculated using five different epigenetic clocks, between subjects discordant for pain-related phenotypes. Twins with high HPS had increased CD8+ T cell counts (nominal p = 0.028). HPS thresholds were negatively associated with estimated levels of GDF15 (nominal p = 0.008). FM patients showed decreased naive CD4+ T cell counts compared with controls (nominal p = 0.015). The severity of FM manifestations expressed through various evaluation tests was associated with decreased levels of leptin, shorter length of telomeres, and reduced CD8+ T and natural killer cell counts (nominal p < 0.05), while the duration of painful symptoms was positively associated with telomere length (nominal p = 0.034). No differences in DNAm-based estimates were detected for MOH or EM compared with controls. In summary, our study suggests that HPS, FM, and MOH/EM do not show signs of epigenetic age acceleration in whole blood, while HPS and FM are associated with DNAm-based estimates of immunological parameters, plasma proteins, and telomere length. Future studies should extend these observations in larger cohorts.

Effects of Hericium erinaceus Mycelium Extracts on the Functional Activity of Purinoceptors and Neuropathic Pain in Mice with L5 Spinal Nerve Ligation

Neuropathic pain is a serious clinical problem that is difficult to treat. Purinoceptors (P2Rs) transduce pain perception from the peripheral to the central nervous system and play an important role in the transmission of neuropathic pain signals. We previously found that the crude extracts of Hericium erinaceus mycelium (HE-CE) inhibited P2R-mediated signaling in cells and reduced heat-induced pain in mice. The present study explored the effects of HE-CE on neuropathic pain. We used adenosine triphosphate (ATP) as a P2R agonist to generate Ca²⁺ signaling and neuronal damage in a cell line. We also established a neuropathic mouse model of L5 spinal nerve ligation (L5-SNL) to examine neuropathic pain and neuroinflammation. Neuropathic pain was recorded using the von Frey test. Neuroinflammation was evaluated based on immunohistofluorescence observation of glial fibrillary acidic protein (GFAP) levels in astrocytes, ionized calcium-binding adaptor molecule1 (iba1) levels in microglia, and IL-6 levels in plasma. The results show that HE-CE and erinacine-S, but not erinacine-A, totally counteracted Ca²⁺ signaling and cytotoxic effects upon P2R stimulation by ATP in human osteosarcoma HOS cells and human neuroblastoma SH-SY5Y cells, respectively. SNL induced a decrease in the withdrawal pressure of the ipsilateral hind paw, indicating neuropathic pain. It also raised the GFAP level in astrocytes, the iba1 level in microglia, and the IL-6 level in plasma, indicating neuroinflammation. HE-CE significantly counteracted the SNL-induced decrease in withdrawal pressure, illustrating that it could relieve neuropathic pain. It also reduced SNL-induced increases in astrocyte GFAP levels, microglial iba1 levels, and plasma IL-6 levels, suggesting that HE-CE reduces neuroinflammation. Erinacine-S relieved neuropathic pain better than HE-CE. The present study demonstrated that HE inhibits P2R and, thus, that it can relieve neuropathic pain and neuroinflammation.

Physical Activity Induces Nucleus Accumbens Genes Expression Changes Preventing Chronic Pain Susceptibility Promoted by High-Fat Diet and Sedentary Behavior in Mice

Recent findings from rodent studies suggest that high-fat diet (HFD) increases hyperalgesia independent of obesity status. Furthermore, weight loss interventions such as voluntary physical activity (PA) for adults with obesity or overweight was reported to promote pain reduction in humans with chronic pain. However, regardless of obesity status, it is not known whether HFD intake and sedentary (SED) behavior is underlies chronic pain susceptibility. Moreover, differential gene expression in the nucleus accumbens (NAc) plays a crucial role in chronic pain susceptibility. Thus, the present study used an adapted model of the inflammatory prostaglandin E2 (PGE2)-induced persistent hyperalgesia short-term (PH-ST) protocol for mice, an HFD, and a voluntary PA paradigm to test these hypotheses. Therefore, we performed an analysis of differential gene expression using a transcriptome approach of the NAc. We also applied a gene ontology enrichment tools to identify biological processes associated with chronic pain susceptibility and to investigate the interaction between the factors studied: diet (standard diet vs. HFD), physical activity behavior (SED vs. PA) and PH-ST (PGE vs. saline). Our results demonstrated that HFD intake and sedentary behavior promoted chronic pain susceptibility, which in turn was prevented by voluntary physical activity, even when the animals were fed an HFD. The transcriptome of the NAc found 2,204 differential expression genes and gene ontology enrichment analysis revealed 41 biologic processes implicated in chronic pain susceptibility. Taking these biological processes together, our results suggest that genes related to metabolic and mitochondria stress were up-regulated in the chronic pain susceptibility group (SED-HFD-PGE), whereas genes related to neuroplasticity were up-regulated in the non-chronic pain susceptibility group (PA-HFD-PGE). These findings provide pieces of evidence that HFD intake and sedentary behavior provoked gene expression changes in the NAc related to promotion of chronic pain susceptibility, whereas voluntary physical activity provoked gene expression changes in the NAc related to prevention of chronic pain susceptibility. Finally, our findings confirmed previous literature supporting the crucial role of voluntary physical activity to prevent chronic pain and suggest that low levels of voluntary physical activity would be helpful and highly recommended as a complementary treatment for those with chronic pain.

Upregulation of miR-183 represses neuropathic pain through inhibiton of MAP3K4 in CCI rat models

Many studies have verified that microRNAs contribute a lot to neuropathic pain progression. Furthermore, nerve-related inflammatory cytokines play vital roles in neuropathic pain progression. miR-183 has been identified to have a common relationship with multiple pathological diseases. However, the potential effects of miR-183 in the process of neuropathic pain remain undetermined. Therefore, we performed the current study with the purpose of finding the functions of miR-183 in neuropathic pain progression using a chronic sciatic nerve injury (CCI) rat model. We demonstrated that miR-183 expression levels were evidently reduced in CCI rats in contrast with the control group. Overexpression of miR-183 produced significant relief of mechanical hyperalgesia, as well as thermal hyperalgesia in CCI rats. Furthermore, neuropathic pain-correlated inflammatory cytokine expression levels containing interleukin-6 (IL-6) and interleukin-1β (IL-1β), cyclooxygenase-2 (COX-2) were obviously inhibited by upregulation of miR-183. Meanwhile, dual-luciferase reporter assays showed MAP3K4 was a direct downstream gene of miR-183. The expression levels of MAP3K4 were modulated by the increased miR-183 negatively, which lead to the downregulation of IL-6, IL-1β, and COX-2, and then reduced neuropathic pain progression, respectively. Overall, our study pointed out that miR-183 was a part of the negative regulator which could relieve neuropathic pain by targeting MAP3K4. Thus it may provide a new clinical treatment for neuropathic pain patients clinical therapy.

Effects of 1,8-cineole on neuropathic pain mediated by P2X2 receptor in the spinal cord dorsal horn

As an intractable health threat, neuropathic pain is now a key problem in clinical therapy, which can be caused by lesions affecting the peripheral nervous systems. 1,8-cineole is a natural monoterpene cyclic ether present in eucalyptus and has been reported to exhibit anti-inflammatory and antioxidant effects. Research has shown that 1,8-cineole inhibits P2X3 receptor-mediated neuropathic pains in dorsal root ganglion. The P2X2 and P2X3 receptors participate in the transmission of algesia and nociception information by primary sensory neurons. In the present study, We thus investigated in the spinal cord dorsal horn whether 1,8-cineole inhibits the expression of P2X2 receptor-mediated neuropathic pain. This study used rats in five random groups: group of chronic constriction injury(CCI) with dimethysulfoxide control (CCI + DMSO); group of CCI; sham group(Sham); group of CCI treated with a low dose 1,8-cineole (CCI + 50 mg/kg); group of CCI with a high dose (CCI + 100 mg/kg). We observed the effects of 1,8-cineole on thermal withdrawal latency (TWL) and mechanical withdrawal threshold (MWT). We examined P2X2 receptors mRNA change in rat spinal cord dorsal horn by In situ nucleic acid hybridization(ISH) and Quantitative realtime polymerase chain reaction (qRT-PCR) methods. Western Blotting and Immunohistochemical staining methods were used to observe P2X2 receptor protein expressions in the rat spinal cord dorsal horn. It demonstrated that oral administration of 1,8-cineole inhibits over-expression of P2X2 receptor protein and mRNA in the spinal cord and dorsal horn in the CCI rats. And the study explored new methods for the prevention and treatment of neuropathic pain.

Antisense oligonucleotides selectively suppress target RNA in nociceptive neurons of the pain system and can ameliorate mechanical pain

There is an urgent need for better treatments for chronic pain, which affects more than 1 billion people worldwide. Antisense oligonucleotides (ASOs) have proven successful in treating children with spinal muscular atrophy, a severe infantile neurological disorder, and several ASOs are currently being tested in clinical trials for various neurological disorders. Here, we characterize the pharmacodynamic activity of ASOs in spinal cord and dorsal root ganglia (DRG), key tissues for pain signaling. We demonstrate that activity of ASOs lasts up to 2 months after a single intrathecal bolus dose. Interestingly, comparison of subcutaneous, intracerebroventricular, and intrathecal administration shows that DRGs are targetable by systemic and central delivery of ASOs, while target reduction in the spinal cord is achieved only after direct central delivery. Upon detailed characterization of ASO activity in individual cell populations in DRG, we observe robust target suppression in all neuronal populations, thereby establishing that ASOs are effective in the cell populations involved in pain propagation. Furthermore, we confirm that ASOs are selective and do not modulate basal pain sensation. We also demonstrate that ASOs targeting the sodium channel Nav1.7 induce sustained analgesia up to 4 weeks. Taken together, our findings support the idea that ASOs possess the required pharmacodynamic properties, along with a long duration of action beneficial for treating pain.

Natural compounds acting at P2 receptors alleviate peripheral neuropathy

Neuropathic pain is generally resistant to currently available treatments, and it is often a consequence of nerve injury due to surgery, diabetes or infection. Myocardial ischemic nociceptive signaling increases the sympathoexcitatory reflex to aggravate myocardial injury. Elucidation of the pathogenetic factors might provide a target for optimal treatment. Abundant evidence in the literature suggests that P2X and P2Y receptors play important roles in signal transmission. Traditional Chinese medicines, such as emodin, puerarin and resveratrol, antagonize nociceptive transmission mediated by purinergic 2 (P2) receptors in primary afferent neurons. This review summarizes recently published data on P2 receptor-mediated neuropathic pain and myocardial ischemia in dorsal root ganglia (DRG), superior cervical ganglia (SCG) and stellate ganglia (SG), with a special focus on the beneficial role of natural compounds.

MiR-183-5p Alleviates Chronic Constriction Injury-Induced Neuropathic Pain Through Inhibition of TREK-1

MicroRNAs have been implicated in nerve injury and neuropathic pain. In the previous study we had shown that miR-96 can attenuate neuropathic pain through inhibition of Nav1.3. In this study, we investigated the role of miR-183, a same cluster member of microRNA with miR-96, in neuropathic pain and its potential mechanisms. We found that the expression level of miR-183-5p in dorsal root ganglion was decreased with the development of neuropathic pain induced by chronic constriction sciatic nerve injury (CCI). By contrast, the TREK-1, a K⁺ channel, was increased. Further investigation identified that intrathecal injection of miR-183-5p mimic efficiently ameliorated neuropathic pain and inhibited the expression of TREK-1, a predicted target gene of miR-183-5p. Luciferase assays confirmed the binding of miR-183-5p and TREK-1. In addition, over-expression of TREK-1 blocked the roles of miR-183-5p in neuropathic pain. Our findings suggested that miR-183-5P participated in the regulation of CCI-induced neuropathic pain through inhibiting the expression of TREK-1.

Microencapsulated Schwann cell transplantation inhibits P2X3 receptor expression in dorsal root ganglia and neuropathic pain

Schwann cell transplantation is a promising method to promote neural repair, and can be used for peripheral nerve protection and myelination. Microcapsule technology largely mitigates immune rejection of transplanted cells. We previously showed that microencapsulated olfactory ensheathing cells can reduce neuropathic pain and we hypothesized that microencapsulated Schwann cells can also inhibit neuropathic pain. Rat Schwann cells were cultured by subculture and then microencapsulated and were tested using a rat chronic constriction injury (CCI) neuropathic pain model. CCI rats were treated with Schwann cells or microencapsulated Schwann cells and were compared with sham and CCI groups. Mechanical withdrawal threshold and thermal withdrawal latency were assessed preoperatively and at 1, 3, 5, 7, 9, 11 and 14 days postoperatively. The expression of P2X3 receptors in L4-5 dorsal root ganglia of the different groups was detected by double-label immunofluorescence on day 14 after surgery. Compared with the chronic constriction injury group, mechanical withdrawal threshold and thermal withdrawal latency were higher, but the expression of P2X3 receptors was remarkably decreased in rats treated with Schwann cells and microencapsulated Schwann cells, especially in the rats transplanted with microencapsulated Schwann cells. The above data show that microencapsulated Schwann cell transplantation inhibits P2X3 receptor expression in L4-5 dorsal root ganglia and neuropathic pain.

Leptin is essential for microglial activation and neuropathic pain after preganglionic cervical root avulsion

AIMS

Preganglionic cervical root avulsion (PCRA) affects both the peripheral and central nervous systems and is often associated with neuropathic pain. Unlike peripheral nerve injuries (PNI), central lesions caused by disruption of cervical roots from the spinal cord following PCRA contribute to the generation of neuropathic pain. Leptin is involved in the development of neuropathic pain after PNI by affecting neurons. However, whether leptin is involved in microglial activation leading to neuropathic pain after PCRA is unknown.

MAIN METHODS

Preganglionic avulsion of the left 6^th-8^th cervical roots was performed in C57B/6J mice and leptin-deficient mice. A leptin antagonist or leptin was administered to C57B/6J mice and leptin-deficient mice after injury, respectively. The expression pattern of spinal and supraspinal microglia was examined by immunofluorescent staining. Von Frey filaments were used to test pain sensitivity.

KEY FINDINGS

Leptin is essential for the development of neuropathic pain after PCRA. Allodynia was absent in the leptin-deficient mice and the mice administered the leptin antagonist. We also found that leptin deficiency or the administration of its antagonist inhibited the development of microgliosis in the dorsal horn and brainstem. Furthermore, increase in the expression of CD86 and iNOS, and Wallerian degeneration were noted in the spinal cord. The administration of exogenous leptin to leptin-deficient mice reversed these effects.

SIGNIFICANCE

We concluded that leptin is involved in the proliferation and activation of microglia, which in turn enhances the development of neuropathic pain. Blocking the effects of leptin might be a target for the treatment of neuropathic pain after PCRA.

Chronic cervical radiculopathic pain is associated with increased excitability and hyperpolarization-activated current ( Ih) in large-diameter dorsal root ganglion neurons

Cervical radiculopathic pain is a very common symptom that may occur with cervical spondylosis. Mechanical allodynia is often associated with cervical radiculopathic pain and is inadequately treated with current therapies. However, the precise mechanisms underlying cervical radiculopathic pain-associated mechanical allodynia have remained elusive. Compelling evidence from animal models suggests a role of large-diameter dorsal root ganglion neurons and plasticity of spinal circuitry attached with Aβ fibers in mediating neuropathic pain. Whether cervical radiculopathic pain condition induces plastic changes of large-diameter dorsal root ganglion neurons and what mechanisms underlie these changes are yet to be known. With combination of patch-clamp recording, immunohistochemical staining, as well as behavioral surveys, we demonstrated that upon chronic compression of C7/8 dorsal root ganglions, large-diameter cervical dorsal root ganglion neurons exhibited frequent spontaneous firing together with hyperexcitability. Quantitative analysis of hyperpolarization-activated cation current (I_h) revealed that I_h was greatly upregulated in large dorsal root ganglion neurons from cervical radiculopathic pain rats. This increased I_h was supported by the enhanced expression of hyperpolarization-activated, cyclic nucleotide-modulated channels subunit 3 in large dorsal root ganglion neurons. Blockade of I_h with selective antagonist, ZD7288 was able to eliminate the mechanical allodynia associated with cervical radiculopathic pain. This study sheds new light on the functional plasticity of a specific subset of large-diameter dorsal root ganglion neurons and reveals a novel mechanism that could underlie the mechanical allodynia associated with cervical radiculopathy.

Effect of artemisinin on neuropathic pain mediated by P2X4 receptor in dorsal root ganglia

Neuropathic pain is a type of chronic pain caused by nervous system damage and dysfunction. The pathogenesis of chronic pain is complicated, and there are no effective therapies for neuropathic pain. Studies show that the P2X₄ receptor expressed in the satellite glial cells (SGCs) of dorsal root ganglia (DRG) is related to neuropathic pain. Artemisinin is a monomeric component extracted from traditional Chinese medicine and has a variety of important pharmacological effects and potential applications. This study observed the effect of artemisinin on neuropathic pain and delineated its possible mechanism. The chronic constriction injury (CCI) rat model was used in this study. The results demonstrated that artemisinin relieved pain behaviors in the CCI rats, inhibited the expression of P2X₄ receptor in the DRG, and decreased the ATP-activated currents in HEK293 cells transfected with P2X₄ plasmid. Dual-labeling immunofluorescence showed that the coexpression of P2X₄ receptor and glial fibrillary acidic protein (GFAP) in the DRG of CCI rats was increased compared to control rats. After CCI rats were treated with artemisinin, the coexpression of P2X₄ receptor and GFAP in the DRG was significantly decreased compared to the CCI group. This finding suggested that artemisinin could inhibit the nociceptive transmission mediated by P2X₄ receptor in the DRG SGCs and thus relieve pain behaviors in the CCI rats.

Effects of 17β-estradiol on leptin signaling in anterior pituitary of ovariectomized rats

Leptin is secreted predominantly by adipocytes and exerts its role mainly by interaction with the long form of leptin receptor (LEPR_V2). It has been identified that LEPR_V2 is widely distributed in various tissues, including the anterior pituitary. Cross-talk between leptin and estrogens has been indentified. Estrogen is known to modulate the tissue-specific expression of LEPR_V2 and leptin in ovariectomized (OVX) rats, a model of postmenopausal condition. Our previous data showed that 17β-estradiol (E₂) up-regulated the expression of LEPR_V2 protein and mRNA in rat dorsal root ganglion (DRG) in an estrogen receptor alpha (ERα)-dependent manner. But it is still unclear whether estrogen can regulate leptin signalling in the pituitary of OVX rats. In the present study, we found that ovariectomy decreased the expressions of LEPR_V2. Administration of E₂ increased the expressions of LEPR_V2 in a dose-dependent manner. In addition, E₂ improved LEPR_V2, STAT3, and SOCS3 protein levels in OVX rats. The effects of exogenous E₂ were attenuated by ICI 182,780, a specific estrogen receptors antagonist. However, E₂ did not change the Lepr_v1, a type of short form of leptin receptor (LEPR), or leptin mRNA levels. Thus, E₂ plays a crucial role in regulating pituitary sensitivity to leptin in OVX rats. Our findings implied that exogenous E₂ had potential roles in modification of the function of pituitary in postmenopausal women.

Celecoxib alleviates oxaliplatin-induced hyperalgesia through inhibition of spinal ERK1/2 signaling

Numerous pieces of evidence have revealed that oxaliplatin (OXA) evokes mechanical and cold hypersensitivity. However, the mechanism underlying these bothersome side effects needs to be further investigated. It is well known that cyclooxygenase-2 (COX-2) and extracellular signal-regulated kinases (ERK1/2) signaling play crucial roles in several pain states. Our previous data showed that Akt2 in the dorsal root ganglion (DRG) participated in the regulation of OXA-induced neuropathic pain. But it is still unclear whether spinal ERK1/2 signaling is involved in the regulation of OXA-induced hyperalgesia, and the linkage between COX-2 and ERK1/2 signaling in mediating OXA-induced hyperalgesia also remains unclear. In this research, we investigated the possible mechanism of celecoxib, a COX-2 inhibitor, in OXA-induced neuropathic pain. Our results show that single dose of OXA (12 mg/kg) significantly attenuated both the tail withdrawal latency (TWL) and mechanical withdrawal threshold (MWT) at days 4 after the OXA treatment. Administration of celecoxib (30 mg/kg/day) for 4 and 6 days inhibited the decrease in TWL and MWT, and each was significantly higher than that of the OXA+vehicle group and was equivalent to that of the vehicles group. OXA increased the expression of cyclooxygenase-2 (COX-2) mRNA and phosphorylated extracellular signal-regulated kinase1/2 (pERK1/2) protein in the lumbar 4-5 (L4-5) spinal cord dorsal horn neurons. Administration of celecoxib for 7 days suppressed the increase in expression of COX-2 and pERK1/2 induced by OXA. Our findings suggested that COX-2 and ERK1/2 signaling in spinal cord contributed to the OXA-induced neuropathic pain.

Association of Leptin with Body Pain in Women

Leptin, an appetite-regulatory hormone, is also known to act as a proinflammatory adipokine. One of the effects of increased systemic leptin concentrations may be greater sensitivity to pain. We report the results of two studies examining the association between leptin and pain: a small pilot longitudinal study, followed by a large cross-sectional study. In Study 1, three women with physician-diagnosed fibromyalgia provided blood draws daily for 25 consecutive days, as well as daily self-reported musculoskeletal pain. Daily fluctuations in serum leptin were positively associated with pain across all three participants (F (1,63) = 12.8, p < 0.001), with leptin predicting ∼49% of the pain variance. In Study 2, the relationship between leptin and body pain was examined in a retrospective cross-sectional analysis of 5676 generally healthy postmenopausal women from the Women's Health Initiative. Leptin levels obtained from single blood draws were tested for a relationship with self-reported body pain. Body mass index (BMI) was also included as a predictor of pain. Both leptin and BMI were found to be independently associated with self-reported pain (p = 0.001 and p < 0.001, respectively), with higher leptin levels and greater BMI each being associated with greater pain. Leptin appears to be a predictor of body pain both within- and between-individuals and may be a driver of generalized pain states such as fibromyalgia.

Upregulation of Ih expressed in IB4-negative Aδ nociceptive DRG neurons contributes to mechanical hypersensitivity associated with cervical radiculopathic pain

Cervical radiculopathy represents aberrant mechanical hypersensitivity. Primary sensory neuron’s ability to sense mechanical force forms mechanotransduction. However, whether this property undergoes activity-dependent plastic changes and underlies mechanical hypersensitivity associated with cervical radiculopathic pain (CRP) is not clear. Here we show a new CRP model producing stable mechanical compression of dorsal root ganglion (DRG), which induces dramatic behavioral mechanical hypersensitivity. Amongst nociceptive DRG neurons, a mechanically sensitive neuron, isolectin B4 negative Aδ-type (IB4⁻ Aδ) DRG neuron displays spontaneous activity with hyperexcitability after chronic compression of cervical DRGs. Focal mechanical stimulation on somata of IB4^- Aδ neuron induces abnormal hypersensitivity. Upregulated HCN1 and HCN3 channels and increased I_h current on this subset of primary nociceptors underlies the spontaneous activity together with neuronal mechanical hypersensitivity, which further contributes to the behavioral mechanical hypersensitivity associated with CRP. This study sheds new light on the functional plasticity of a specific subset of nociceptive DRG neurons to mechanical stimulation and reveals a novel mechanism that could underlie the mechanical hypersensitivity associated with cervical radiculopathy.

Ontogenic expression profiles and oxaliplatin regulation of leptin expression in mice dorsal root ganglion

Leptin is widely distributed in many tissues, including the nervous system. However, the ontogeny of leptin expression in the dorsal root ganglion (DRG) is unclear. Recent studies have shown that leptin is involved in the regulation of neuropathic pain induced by nerve injury. Our previous results showed that exogenous leptin administration alleviated the pain behaviors induced by chronic constriction sciatic nerve injury. In the present study, the ontogenic expression of leptin was detected in the DRG of the mouse embryo at days 15.5 (E15.5), E17.5, and E19.5 of gestation and in the postnatal mouse at days 5 (P5), P15, and P25, and in the adult mouse. Leptin immunoreactivity and mRNA were not found in DRG at E15.5. The percentage of leptin immunopositive (leptin) neurons was about 27% at E17.5. It continued to increase to about 70% at P5. From P5 to P15, there was no significant change. The proportion of DRG neurons positive for leptin decreased after P15 and there were about 41% leptin neurons in adults. The expression profile of leptin mRNA is similar to leptin immunoreactivity. Oxaliplatin (OXA) is an effective platinum-based drug used as first-line chemotherapy for advanced colorectal cancer. However, it may induce neuropathic pain. In the current study, we found that the expression of leptin was increased in the lumbar 4-6 DRG of OXA-treated mice. These results indicate that leptin is involved in the regulation of DRG development and OXA-induced neuropathic pain.

The use of the antimicrobial peptide piscidin (PCD)-1 as a novel anti-nociceptive agent

The antimicrobial peptide piscidin (PCD)-1 has been reported to have antibacterial and immunomodulatory functions. Here, we investigated the anti-neuropathic properties of PCD-1, in order to determine its potential as a compound to alleviate pain. Treatment with PCD-1 suppressed the inflammatory proteins COX-2 and iNOS in murine macrophage (RAW264.7) and microglial (BV2) cell lines stimulated by lipopolysaccharide (LPS). For studies of the effect of PCD-1 in vivo, mononeuropathy in rats was induced by chronic constriction injury (CCI), and the resulting anti-nociceptive behaviors were compared between CCI controls and CCI rats given intrathecal injections of PCD-1. Much like gabapentin, PCD-1 exerts anti-nociceptive effects against thermal hyperalgesia, with a median effective dose (ED50) of 9.5 μg in CCI rats. In CCI rats, PCD-1 exerted effects against mechanical and cold allodynia, thermal hyperalgesia, and weight-bearing deficits. Furthermore, CCI-mediated activation of microglia and astrocytes in the dorsal horn of the lumbar spinal cord were decreased by PCD-1. In addition, PCD-1 suppressed up-regulation of interleukin-1β (IL-1β) and phosphorylated mammalian target of rapamycin (phospho-mTOR) in CCI rats. Finally, CCI-induced down-regulation of transforming growth factor-β1 (TGF-β1) in rats was attenuated by injection of PCD-1. Taken together, the present findings demonstrate that the marine antimicrobial peptide PCD-1 has anti-nociceptive effects, and thus may have potential for development as an alternative pain-alleviating agent.

Ictal adipokines are associated with pain severity and treatment response in episodic migraine

OBJECTIVE

To evaluate ictal adipokine levels in episodic migraineurs and their association with pain severity and treatment response.

METHODS

This was a double-blind, placebo-controlled trial evaluating peripheral blood specimens from episodic migraineurs at acute pain onset and 30 to 120 minutes after treatment with sumatriptan/naproxen sodium vs placebo. Total adiponectin (T-ADP), ADP multimers (high molecular weight [HMW], middle molecular weight, and low molecular weight [LMW]), leptin, and resistin levels were evaluated by immunoassays.

RESULTS

Thirty-four participants (17 responders, 17 nonresponders) were included. In all participants, pretreatment pain severity increased with every quartile increase in both the HMW:T-ADP ratio (coefficient of variation [CV] 0.51; 95% confidence interval [CI]: 0.08, 0.93; p = 0.019) and resistin levels (CV 0.58; 95% CI: 0.21, 0.96; p = 0.002), but was not associated with quartile changes in leptin levels. In responders, T-ADP (CV -0.98; 95% CI: -1.88, -0.08; p = 0.031) and resistin (CV -0.95; 95% CI: -1.83, -0.07; p = 0.034) levels decreased 120 minutes after treatment as compared with pretreatment. In addition, in responders, the HMW:T-ADP ratio (CV -0.04; 95% CI: -0.07, -0.01; p = 0.041) decreased and the LMW:T-ADP ratio (CV 0.04; 95% CI: 0.01, 0.07; p = 0.043) increased at 120 minutes after treatment. In nonresponders, the LMW:T-ADP ratio (CV -0.04; 95% CI: -0.07, -0.01; p = 0.018) decreased 120 minutes after treatment. Leptin was not associated with treatment response.

CONCLUSIONS

Both pretreatment migraine pain severity and treatment response are associated with changes in adipokine levels. Adipokines represent potential novel migraine biomarkers and drug targets.

Inflammation and nerve fiber interaction in endometriotic pain

Endometriosis is an extremely prevalent estrogen-dependent condition characterized by the growth of ectopic endometrial tissue outside the uterine cavity, and is often presented with severe pain. Although the relationship between lesion and pain remains unclear, nerve fibers found in close proximity to endometriotic lesions may be related to pain. Also, women with endometriosis pain develop central sensitization. Endometriosis creates an inflammatory environment and recent research is beginning to elucidate the role of inflammation in stimulating peripheral nerve sensitization. In this review, we discuss endometriosis-associated inflammation, peripheral nerve fibers, and assess their potential mechanism of interaction. We propose that an interaction between lesions and nerve fibers, mediated by inflammation, may be important in endometriosis-associated pain.

The pain interactome: connecting pain-specific protein interactions

Understanding the molecular mechanisms associated with disease is a central goal of modern medical research. As such, many thousands of experiments have been published that detail individual molecular events that contribute to a disease. Here we use a semi-automated text mining approach to accurately and exhaustively curate the primary literature for chronic pain states. In so doing, we create a comprehensive network of 1,002 contextualized protein-protein interactions (PPIs) specifically associated with pain. The PPIs form a highly interconnected and coherent structure, and the resulting network provides an alternative to those derived from connecting genes associated with pain using interactions that have not been shown to occur in a painful state. We exploit the contextual data associated with our interactions to analyse subnetworks specific to inflammatory and neuropathic pain, and to various anatomical regions. Here, we identify potential targets for further study and several drug-repurposing opportunities. Finally, the network provides a framework for the interpretation of new data within the field of pain.

Adenosine A₁ receptors in mouse pontine reticular formation modulate nociception only in the presence of systemic leptin

Human obesity is associated with increased leptin levels and pain, but the specific brain regions and neurochemical mechanisms underlying this association remain poorly understood. This study used adult male C57BL/6J (B6, n=14) mice and leptin-deficient, obese B6.Cg-Lep(ob)/J (obese, n=10) mice to evaluate the hypothesis that nociception is altered by systemic leptin levels and by adenosine A₁ receptors in the pontine reticular formation. Nociception was quantified as paw withdrawal latency (PWL) in s after onset of a thermal stimulus. PWL was converted to percent maximum possible effect (%MPE). After obtaining baseline PWL measures, the pontine reticular formation was microinjected with saline (control), three concentrations of the adenosine A₁ receptor agonist N(6)-p-sulfophenyladenosine (SPA), or super-active mouse leptin receptor antagonist (SMLA) followed by SPA 15 min later, and PWL was again quantified. In obese, leptin-deficient mice, nociception was quantified before and during leptin replacement via subcutaneous osmotic pumps. SPA was administered into the pontine reticular formation of leptin-replaced mice and PWL testing was repeated. During baseline (before vehicle or SPA administration), PWL was significantly (p=0.0013) lower in leptin-replaced obese mice than in B6 mice. Microinjecting SPA into the pontine reticular formation of B6 mice caused a significant (p=0.0003) concentration-dependent increase in %MPE. SPA also significantly (p<0.05) increased %MPE in B6 mice and in leptin-replaced obese mice, but not in leptin-deficient obese mice. Microinjection of SMLA into the pontine reticular formation before SPA did not alter PWL. The results show for the first time that pontine reticular formation administration of the adenosine A₁ receptor agonist SPA produced antinociception only in the presence of systemic leptin. The concentration-response data support the interpretation that adenosine A₁ receptors localized to the pontine reticular formation significantly alter nociception.

Role of nerve growth factor-tyrosine kinase receptor A signaling in paclitaxel-induced peripheral neuropathy in rats

The mechanisms underlying paclitaxel-induced peripheral neuropathy remain unknown. Nerve growth factor (NGF) is a representative neurotrophic factor that maintains neuronal function, promotes survival, and mediates neuropathic pain. We investigated expression levels of NGF and its receptors in the dorsal root ganglia (DRG) and spinal dorsal horn (DH) following paclitaxel treatment. Intraperitoneal (I.P.) administration of paclitaxel induced significant mechanical hypersensitivity and cold allodynia in rats, significantly increased the expression of NGF and its receptor tyrosine kinase receptor A (trkA) in the DRG, and increased NGF expression in the DH. In contrast, paclitaxel treatment did not alter the mRNA levels of NGF or its receptors in the DRG, DH, sciatic nerve, or hindpaw skin. Moreover, expression of NEDD4-2, a negative regulator of trkA, was significantly increased in the DRG of paclitaxel-treated rats. Intrathecal (I.T.) administration of the tyrosine kinase receptor inhibitor k252a significantly alleviated mechanical hypersensitivity in paclitaxel-treated rats. Our results suggest that NGF-trkA signaling is involved in mechanical allodynia in paclitaxel-induced neuropathy.