Hyperexcitability and sensitization of sodium channels of dorsal root ganglion neurons in a rat model of lumber disc herniation

Paravertebrally-Injected Multifunctional Hydrogel for Sustained Anti-Inflammation and Pain Relief in Lumbar Disc Herniation

Pain caused by lumbar disc herniation (LDH) severely compromises patients' quality of life. The combination of steroid and local anesthetics is routinely employed in clinics to alleviate LDH-induced pain. However, the approach only mediates transient efficacy and requires repeated and invasive lumbar epidural injections. Here a paravertebrally-injected multifunctional hydrogel that can efficiently co-load and controlled release glucocorticoid betamethasone and anesthetics ropivacaine for sustained anti-inflammation, reactive oxygen species (ROS)-removal and pain relief in LDH is presented. Betamethasone is conjugated to hyaluronic acid (HA) via ROS-responsive crosslinker to form amphiphilic polymer that self-assemble into particles with ropivacaine loaded into the core. Solution of drug-loaded particles and thermo-sensitive polymer rapidly forms therapeutic hydrogel in situ upon injection next to the herniated disc, thus avoiding invasive epidural injection. In a rat model of LDH, multifunctional hydrogel maintains the local drug concentration 72 times longer than free drugs and more effectively inhibits the expression of pro-inflammatory cytokines and pain-related molecules including cyclooxygenase-2 (COX-2) and prostaglandin E2 (PGE2). Therapeutic hydrogel suppresses the LDH-induced pain in rats for 12 days while the equivalent dose of free drugs is only effective for 3 days. This platform is also applicable to ameliorate pain caused by other spine-related diseases.

IL-10 alleviates radicular pain by inhibiting TNF-α/p65 dependent Nav1.7 up-regulation in DRG neurons of rats

BACKGROUND

Lumbar disc herniation (LDH) may induce radicular pain, the upregulation of voltage-gated sodium channels (VGSCs) in dorsal root ganglion (DRG) contributes to radicular pain by generating ectopic discharge of neurons, but the mechanism is unclear. Previously, we reported pro-inflammatory cytokine tumor necrosis factor-α (TNF-α) up-regulated VGSCs in diabetic neuropathy. In this study, we explored the effect of anti-inflammatory cytokine interleukin-10 (IL-10) on radicular pain and the possible mechanisms.

METHODS

Rat model of LDH was induced by implanting autologous nucleus pulposus (NP). Mechanical and thermal pain thresholds were assessed by von Frey filaments and hotplate test respectively. IL-10 and TNF-α level in DRG and cerebrospinal fluid (CSF) were assessed by Enzyme-linked immunosorbent assay (ELISA). IL-10 was intrathecally delivered for 12 days. The expression of IL-10R1 and sodium channel Nav1.7 was displayed by immunofluorescence staining. The protein level of TNF-α and p-p65 was measured by western blotting.

RESULTS

NP implantation increased Nav1.7 expression in DRG neurons, decreased IL-10 level and increased TNF-α level in DRG and CSF. IL-10 significantly alleviated pain behaviors of rats with NP. IL-10R1 was co-localized with neurons but not with satellite cells in DRG. IL-10 decreased Nav1.7 and TNF-α/p-p65 expression in DRG of rats with NP. Co-administration of TNF-α with IL-10 counteracted the effect of IL-10 on pain behaviors, Nav1.7 and TNF-α/p-p65 expression of rats with NP.

CONCLUSIONS

The study revealed that IL-10 alleviated radicular pain by inhibiting TNF-α/p-p65 dependent Nav1.7 up-regulation in DRG neurons.

The Effect of Melatonin on Radicular Pain in a Rat Model of Lumbar Disc Herniation

STUDY DESIGN

Controlled, randomized, animal study.

OBJECTIVE

To investigate the effect of melatonin and its receptors on radicular pain and the possible mechanisms.

SUMMARY OF BACKGROUND DATA

Lumbar disc herniation (LDH) may induce radicular pain, but the mechanism is not clear and therapeutic effect is still poor. Previously we report central sensitization meaning potentiation of spinal nociceptive synaptic transmission is the critical cause of radicular pain. Melatonin (Mel) has been reported to promote hippocampal synaptic transmission and thus improve learning ability. But the effect of Mel on spinal synaptic transmission and radicular pain are not clear.

METHODS

Rat LDH model was induced by autologous nucleus pulposus (NP) implantation. Melatonin was delivered intraperitoneally four times a day, from day 1 to day 3 after surgery. Melatonin receptor agonist and antagonists were delivered intrathecally for 3 days as well. Mechanical and thermal pain thresholds were assessed by von Frey filaments and hotplate test respectively. Electrophysiological recording was employed for survey C-fiber evoked field potentials. The protein level of N- methyl-D-aspartate submit 2A (NR2A), NR2B, melatonin receptor 1 (MT1), and receptor 2 (MT2) was evaluated by western blotting. Spinal expression of calcitonin gene related peptides (CGRP), isolectin b4 (IB4), and neurofilament-200 (NF200) was displayed by immunofluorescence staining.

RESULTS

Melatonin significantly increased mechanical and thermal pain thresholds, lasting at least to day 5 after surgery. Melatonin decreased C-fiber evoked field potentials; decreased spinal NR2B protein level; reduced spinal CGRP, and IB4 expression. MT2 was upregulated after NP implantation and was co-localized with neuron and microglia. MT2 receptor agonist simulated the effect of Mel, and both MT receptor broadspectrum antagonist and MT2 specific antagonist abolished the effect of MT2 receptor agonist.

CONCLUSION

Melatonin alleviates radicular pain from LDH by inhibiting central sensitization via binding with its receptor 2, decreasing spinal CGRP, IB4, and NR2B expression.

TNFAIP3 alleviates pain in lumbar disc herniation rats by inhibiting the NF-κB pathway

Background

It’s been reported that the tumor necrosis factor alpha inducible protein 3 (TNFAIP3) gene played an important role in the pathogenesis of autoimmune and chronic inflammation diseases. Moreover, in degenerative diseases of the lumbar spine the nuclear factor-κB (NF-κB) pathway is significantly activated. This study aimed to explore the role of the tumor necrosis protein-induced zinc finger protein A20 (A20) protein in degenerative diseases of the lumbar spine on the NF-κBp65 pathway.

Methods

A total of 96 rats were randomly divided into 4 groups. Lumbar disc herniation (DH) was set as a sham operation group (Sham group), DH + A20 group and DH + control group (Control group); measured changes in rat paw withdrawal threshold (PWT) and paw withdrawal latency (PWL); detected the proportion of apoptotic cells in a single nucleus pulposus cell suspension, analyzed the correlation between tumor necrosis factor-α (TNF-α) content and pain in DH rats, and the expression changes of NF-κB pathway in nucleus pulposus tissue.

Results

compared with the DH + Control group, the PWT and PWL of the DH + A20 group increased significantly (P<0.05); apoptosis in the DH + A20 group was significantly reduced (P<0.01); the nucleus pulposus tissue and serum levels of TNF-α and interleukin-6 (IL-6) in the DH + A20 rat group were significantly lower than those in the DH + Control group (P<0.05); the protein expression of rats in the DH + A20 group (p-p65) was significantly lower than that in the DH + Control group (P<0.05).

Conclusions

The pain of lumbar disc herniation rats is related to TNF-α, and overexpression of A20 protein can reduce the pain of lumbar disc herniation by inhibiting the NF-κB pathway.

Keywords

Lumbar disc herniation (lumbar DH); tumor necrosis factor-α (TNF-α); interleukin-6 (IL-6); tumor necrosis factor alpha inducible protein 3 (TNFAIP3)

The role of structure and function changes of sensory nervous system in intervertebral disc-related low back pain

Low back pain (LBP) is a common musculoskeletal symptom, which can be developed in multiple clinical diseases. It is widely recognized that intervertebral disc (IVD) degeneration (IVDD) is one of the leading causes of LBP. However, the pathogenesis of IVD-related LBP is still controversial, and the treatment means are also insufficient to date. In recent decades, the role of structure and function changes of sensory nervous system in the induction and the maintenance of LBP is drawing more and more attention. With the progress of IVDD, IVD cell exhaustion and extracellular matrix degradation result in IVD structural damage, while neovascularization, innervation and inflammatory activation further deteriorate the microenvironment of IVD. New nerve ingrowth into degenerated IVD amplifies the impacts of IVD-derived nociceptive molecules on sensory endings. Moreover, IVDD is usually accompanied with disc herniation, which could injure and inflame affected nerves. Under mechanical and pro-inflammatory stimulation, the pain-transmitting pathway exhibits a sensitized function state and ultimately leads to LBP. Hence, relevant pathogenic factors, such as neurotrophins, ion channels, inflammatory factors, etc., are supposed to serve as promising therapeutic targets for LBP. The purpose of this review is to comprehensively summarize the current evidence on 1) the pathological changes of sensory nervous system during IVDD and their association with LBP, and 2) potential therapeutic strategies for LBP targeting relevant pathogenic factors.

A double-network hydrogel for the dynamic compression of the lumbar nerve root

Current animal models of nerve root compression due to lumbar disc herniation only assess the mechanical compression of nerve roots and the inflammatory response. Moreover, the pressure applied in these models is static, meaning that the nerve root cannot be dynamically compressed. This is very different from the pathogenesis of lumbar disc herniation. In this study, a chitosan/polyacrylamide double-network hydrogel was prepared by a simple two-step method. The swelling ratio of the double-network hydrogel increased with prolonged time, reaching 140. The compressive strength and compressive modulus of the hydrogel reached 53.6 and 0.34 MPa, respectively. Scanning electron microscopy revealed the hydrogel’s crosslinked structure with many interconnecting pores. An MTT assay demonstrated that the number of viable cells in contact with the hydrogel extracts did not significantly change relative to the control surface. Thus, the hydrogel had good biocompatibility. Finally, the double-network hydrogel was used to compress the L4 nerve root of male sand rats to simulate lumbar disc herniation nerve root compression. The hydrogel remained in its original position after compression, and swelled with increasing time. Edema appeared around the nerve root and disappeared 3 weeks after operation. This chitosan/polyacrylamide double-network hydrogel has potential as a new implant material for animal models of lumbar nerve root compression. All animal experiments were approved by the Animal Ethics Committee of Neurosurgical Institute of Beijing, Capital Medical University, China (approval No. 201601006) on July 29, 2016.

The Chinese Association for the Study of Pain (CASP): Consensus on the Assessment and Management of Chronic Nonspecific Low Back Pain

Chronic nonspecific low back pain (CNLBP) is defined as pain or discomfort originating from the waist, which lasts for at least 12 weeks, but no radiculopathy or specific spinal diseases. CNLBP is a complicated medical problem and places a huge burden on healthcare systems. Clinical manifestation of CNLBP includes discogenic LBP, zygapophyseal joint pain, sacroiliac joint pain, and lumbar muscle strain. Further evaluation should be completed to confirm the diagnosis including auxiliary examination, functional assessment, and clinical assessment. The principle of the management is to relieve pain, restore function, and avoid recurrence. Treatment includes conservative treatment, minimally invasive treatment, and rehabilitation. Pharmacologic therapy is the first-line treatment of nonspecific LBP, and it is most widely used in clinical practice. Interventional therapy should be considered only after failure of medication and physical therapy. Multidisciplinary rehabilitation can improve physical function and alleviate short-term and long-term pain. The emphasis should be put on the prevention of NLBP and reducing relevant risk factors.

Effects of swimming on pain and inflammatory factors in rats with lumbar disc herniation

The aim of the present study was to identify the effect of swimming on nerve root pain in rats with lumbar disc herniation (LDH). A total of 72 male Sprague Dawley rats (215±15 g) were randomly divided into three groups (n=24/group): The sham operation, model and exercise intervention groups, with the latter undergoing 4 weeks of swimming training. On days 0, 7, 14 and 28 following surgery, the changes in the post-limb mechanical claw threshold, the phospholipase A2 (PLA2), interleukin (IL)-6 and tumor necrosis factor (TNF)-α mRNA expression levels, the secretory PLA2 (sPLA2) expression, the IL-6 and TNF-α content, the nuclear factor (NF)-κBp65 protein expression level in the nucleus pulposus, and the apoptotic rate of the nucleus pulposus cells were detected. The results demonstrated that, in the model group, the threshold of hind paw withdrawal was decreased, and that the sPLA2 expression, IL-6 and TNF-α content, PLA2, IL-6 and TNF-α mRNA and NF-κBp65 protein expression levels in the nucleus pulposus were increased. The apoptotic rate of the nucleus pulposus cells was increased from day 7 following surgery, as compared with the sham operation group. In the exercise intervention group, the hind paw withdrawal threshold increased and the TNF-α and IL-6 content, sPLA2 expression and PLA2, IL-6 and TNF-α mRNA and NF-κBp65 protein expression levels were decreased from day 14 following surgery, and the apoptotic nucleus pulposus cells were decreased from day 7 following surgery, as compared with the model group. Collectively, the present data suggest that swimming can significantly reduce nerve root pain and inhibit inflammatory reaction in LDH, which can have positive effects on the treatment of LDH.

AMP-Activated Protein Kinase Activation in Dorsal Root Ganglion Suppresses mTOR/p70S6K Signaling and Alleviates Painful Radiculopathies in Lumbar Disc Herniation Rat Model

STUDY DESIGN

Animal experiment: a rat model of lumbar disc herniation (LDH) induced painful radiculopathies.

OBJECTIVE

To investigate the role and mechanism of AMP-activated protein kinase (AMPK) in dorsal root ganglia (DRG) neurons in LDH-induced painful radiculopathies.

SUMMARY OF BACKGROUND DATA

Overactivation of multiple pain signals in DRG neurons triggered by LDH is crucial to the development of radicular pain. AMPK is recognized as a cellular energy sensor, as well as a pain sensation modulator, but its function in LDH-induced pain hypersensitivity remains largely unknown.

METHODS

The LDH rat model was established by autologous nucleus pulposus transplantation into the right lumbar 5 (L5) nerve root. At different time points after AMPK agonist metformin (250 mg/kg/d) or mammalian target of rapamycin (mTOR) inhibitor rapamycin (5 mg/kg) intraperitoneal administration, thermal and mechanical sensitivity were evaluated by measuring paw withdrawal latency (PWL) and 50% paw withdrawal thresholds (PWT). The levels of AMPK, mTOR, and p70S6K phosphorylation were determined by Western blot. We also investigated the proportion of p-AMPK positive neurons in the right L5 DRG neurons using immunofluorescence.

RESULTS

LDH evoked persistent thermal hyperalgesia and mechanical allodynia on the ipsilateral paw, as indicated by the decreased PWL and 50% PWT. These pain hypersensitive behaviors were accompanied with significant inhibition of AMPK and activation of mTOR in the associated DRG neurons. Pharmacological activation of AMPK in the DRG neurons not only suppressed mTOR/p70S6K signaling, but also alleviated LDH-induced pain hypersensitive behaviors.

CONCLUSION

We provide a molecular mechanism for the activation of pain signals based on AMPK-mTOR axis, as well as an intervention strategy by targeting AMPK-mTOR axis in LDH-induced painful radiculopathies.

LEVEL OF EVIDENCE

N/A.

Characterization of novel lnc RNAs in the spinal cord of rats with lumbar disc herniation

Background

Radicular pain, caused by a lesion or autologous nucleus pulposus (NP) implantation, is associated with alteration in gene expression of the pain-signaling pathways. lncRNAs have been shown to play critical roles in neuropathic pain. However, the mechanistic function of lncRNAs in lumbar disc herniation (LDH) remains largely unknown. Identifying different lncRNA expression under sham and NP-implantation conditions in the spinal cord is important for understanding the molecular mechanisms of radicular pain.

Methods

LDH was induced by implantation of autologous nucleus pulposus (NP), harvested from rat tail, in lumbar 5 and 6 spinal nerve roots. The mRNA and lncRNA microarray analyses demonstrated that the expression profiles of lncRNAs and mRNAs between the LDH and sham groups were markedly altered at 7 days post operation. The expression patterns of several mRNAs and lncRNAs were further proved by qPCR.

Results

LDH produced persistent mechanical and thermal hyperalgesia. A total of 19 lncRNAs was differentially expressed (>1.5-folds), of which 13 was upregulated and 6 was downregulated. In addition, a total of 103 mRNAs was markedly altered (>1.5-folds), of which 40 was upregulated and 63 downregulated. Biological analyses of these mRNAs further demonstrated that the most significantly upregulated genes in LDH included chemotaxis, immune response, and positive regulation of inflammatory responses, which might be important mechanisms underlying radicular neuropathic pain. These 19 differentially expressed lncRNAs have overlapping mRNAs in the genome, which are related to glutamatergic synapse, cytokine-cytokine receptor interaction, and the oxytocin-signalling pathway.

Conclusion

Our findings revealed the alteration of expression patterns of mRNAs and lncRNAs in the spinal cord of rats in a radicular pain model of LDH. These mRNAs and lncRNAs might be potential therapeutic targets for the treatment of radicular pain.

Neurophysiological Effects of Dorsal Root Ganglion Stimulation (DRGS) in Pain Processing at the Cortical Level

OBJECTIVES

Dorsal root ganglion stimulation (DRGS) has been used successfully against localized neuropathic pain. Nevertheless, the effects of DRGS on pain processing, particularly at the cortical level, remain largely unknown. In this study, we investigated whether positive responses to DRGS treatment would alter patients' laser-evoked potentials (LEP).

METHODS

We prospectively enrolled 12 adult patients with unilateral localized neuropathic pain in the lower limbs or inguinal region and followed them up for six months. LEPs were assessed at baseline, after one month of DRGS, and after six months of DRGS. Clinical assessment included the Numerical Rating Scale (NRS), Brief Pain Inventory (BPI), SF-36, and Beck Depression Inventory (BDI). For each patient, LEP amplitudes and latencies of the N2 and P2 components on the deafferented side were measured and compared to those of the healthy side and correlated with pain intensity, as measured with the NRS.

RESULTS

At the one- and six-month follow-ups, N2-P2 amplitudes were significantly greater and NRS scores were significantly lower compared with baseline (all p's < 0.01). There was a negative correlation between LEP amplitudes and NRS scores (r_s = -0.31, p < 0.10).

CONCLUSIONS

DRGS is able to restore LEPs to normal values in patients with localized neuropathic pain, and LEP alterations are correlated with clinical response in terms of pain intensity.

CDMP1 overexpression mediates inflammatory cytokine‑induced apoptosis via inhibiting the Wnt/β‑Catenin pathway in rat dorsal root ganglia neurons

Cartilage‑derived morphogenetic protein‑1 (CDMP1) is a polypeptide growth factor with specific cartilage inducibility, which is predominantly expressed in the developmental long bone cartilage core and in the pre‑cartilage matrix in the embryonic stage. The aim of the present study was to investigate the roles and the mechanisms of CDMP1 overexpression on the apoptosis of rat dorsal root ganglia (DRG) neurons that were induced by inflammatory cytokines. Cell counting Kit‑8 assay, flow cytometry and TdT‑mediated dUTP nick‑end labeling assay were performed to examine cell viability and apoptosis. ELISA, hematoxylin and eosin staining and immunohistochemistry assays were performed to examine the levels of several factors in DRG tissues. Western blot analysis and reverse transcription‑quantitative polymerase chain reaction assays were used to determine the mRNA and protein expression levels, respectively. The results demonstrated that CDMP1 expression was downregulated, while inflammatory cytokine expression was upregulated in DRG tissues derived from lumbar disc herniation (LDH) model rats. In addition, DRG cells from LDH rats exhibited increased apoptosis compared with control rats. CDMP1 overexpression enhanced the cell viability of inflammatory cytokine‑induced DRG cells, and suppressed the apoptosis of inflammatory cytokine‑induced DRG cells via regulating the expression levels of Caspase‑3/8/9, BCL2 apoptosis regulator, and BCL2 associated X. Furthermore, CDMP1 overexpression was demonstrated to affect the Wnt/β‑Catenin pathway in the inflammatory cytokine‑induced DRG cells. In conclusion, the present findings suggested that CDMP1 overexpression mediated inflammatory cytokine‑induced apoptosis via Wnt/β‑Catenin signaling in rat DRG cells.

Attenuation of hyperalgesia responses via the modulation of 5-hydroxytryptamine signalings in the rostral ventromedial medulla and spinal cord in a 6-hydroxydopamine-induced rat model of Parkinson's disease

Background Although pain is one of the most distressing non-motor symptoms among patients with Parkinson's disease, the underlying mechanisms of pain in Parkinson's disease remain elusive. The aim of the present study was to investigate the role of serotonin (5-hydroxytryptamine) in the rostral ventromedial medulla (RVM) and spinal cord in pain sensory abnormalities in a 6-hydroxydopamine-treated rat model of Parkinson's disease. Methods The rotarod test was used to evaluate motor function. The radiant heat test and von Frey test were conducted to evaluate thermal and mechanical pain thresholds, respectively. Immunofluorescence was used to examine 5-hydroxytryptamine neurons and fibers in the rostral ventromedial medulla and spinal cord. High-performance liquid chromatography was used to determine 5-hydroxytryptamine and 5-hydroxyindoleacetic acid levels. Results The duration of running time on the rotarod test was significantly reduced in 6-hydroxydopamine-treated rats. Nociceptive thresholds of both mechanical and heat pain were reduced compared to sham-treated rats. In addition to the degeneration of cell bodies and fibers in the substantia nigra pars compacta, the number of rostral ventromedial medulla 5-hydroxytryptamine neurons and 5-hydroxytryptamine fibers in the spinal dorsal horn was dramatically decreased. 5-Hydroxytryptamine concentrations in both the rostral ventromedial medulla and spinal cord were reduced. Furthermore, the administration of citalopram significantly attenuated pain hypersensitivity. Interestingly, Intra-rostral ventromedial medulla (intra-RVM) microinjection of 5,7-dihydroxytryptamine partially reversed pain hypersensitivity of 6-hydroxydopamine-treated rats. Conclusions These results suggest that the decreased 5-hydroxytryptamine contents in the rostral ventromedial medulla and spinal dorsal horn may be involved in hyperalgesia in the 6-hydroxydopamine-induced rat model of Parkinson's disease.

Protein Kinase C Mediates the Corticosterone-induced Sensitization of Dorsal Root Ganglion Neurons Innervating the Rat Stomach

BACKGROUND/AIMS

Gastric hypersensitivity contributes to abdominal pain in patients with functional dyspepsia. Recent studies showed that hormones induced by stress are correlated with visceral hypersensitivity. However, the precise mechanisms underlying gastric hypersensitivity remain largely unknown. The aim of the present study was designed to investigate the roles of corticosterone (CORT) on excitability of dorsal root ganglion (DRG) neurons innervating the stomach.

METHODS

DRG neurons innervating the stomach were labeled by DiI injection into the stomach wall. Patch clamp recordings were employed to examine neural excitability and voltage-gated sodium channel currents. Electromyograph technique was used to determine the responses of neck muscles to gastric distension.

RESULTS

Incubation of acutely isolated DRG neurons with CORT significantly depolarized action potential threshold and enhanced the number of action potentials induced by current stimulation of the neuron. Under voltage-clamp mode, incubation of CORT enhanced voltage-gated sodium current density of the recorded neurons. Pre-incubation of GF109203X, an inhibitor of protein kinase C, blocked the CORT-induced hyperexcitability and potentiation of sodium currents. However, pre-incubation of H-89, an inhibitor of protein kinase A, did not alter the sodium current density. More importantly, intraperitoneal injection of CORT produced gastric hypersensitivity of healthy rats, which was blocked by pre-administration of GF109203X but not H-89.

CONCLUSIONS

Our data strongly suggest that CORT rapidly enhanced neuronal excitability and sodium channel functions, which is most likely mediated by protein kinase C but not protein kinase A signaling pathway in DRG neurons innervating the stomach, thus underlying the gastric hypersensitivity induced by CORT injection.

Selective phosphodiesterase-2A inhibitor alleviates radicular inflammation and mechanical allodynia in non-compressive lumbar disc herniation rats

PURPOSE

Phosphodiesterase inhibitors possess anti-inflammatory properties. In addition, some studies report that phosphodiesterase 2A (PDE2A) are highly expressed in the dorsal horn of the spinal cord. The present study aimed to investigate whether intrathecal administration of Bay 60-7550, a specific PDE2A inhibitor, could alleviate mechanical allodynia in non-compressive lumbar disc herniation (NCLDH) rats.

METHODS

Rat NCLDH models by autologous nucleus pulposus implantation to dorsal root ganglion were established. Vehicle or Bay 60-7550 (0.1, 1.0 mg/kg) was injected by intrathecal catheter at day 1 post-operation. The ipsilateral mechanical withdrawal thresholds were analyzed from the day before surgery to day 7 after surgery. At day 7 post-operation, the ipsilateral lumbar (L4-L6) segments of the spinal dorsal horns were removed, and tumor necrosis factor α (TNF-α), interleukin-1β (IL-1β), interleukin-6 (IL-6), cyclic adenosine monophosphate (cAMP), and cyclic guanosine monophosphate (cGMP) expressions were measured by ELISA. Furthermore, PDE2A mRNA and protein expressions in spinal cord were measured by Real-Time PCR and Western blot.

RESULTS

Intrathecal administration of the PDE2A inhibitor Bay 60-7550, significantly attenuated mechanical allodynia, down-regulated spinal TNF-α, IL-1β and IL-6 over-expressions, increased the expression of spinal cAMP, as well as cGMP in a more remarkable manner, and decreased the spinal PDE2A expression in NCLDH rats in a dose-dependent manner.

CONCLUSIONS

Bay 60-7550 alleviated mechanical allodynia and inflammation in NCLDH rats, which might be associated with increased cAMP and especially cGMP increase. Thus, spinal PDE2A inhibition might represent a potential analgesic strategy for radiculopathy treatment in non-compressive lumbar disc herniation.

Src-family kinases activation in spinal microglia contributes to central sensitization and chronic pain after lumbar disc herniation

Background Lumbar disc herniation is a major cause of radicular pain, but the underlying mechanisms remain largely unknown. Spinal activation of src-family kinases are involved in the development of chronic pain from nerve injury, inflammation, and cancer. In the present study, the role of src-family kinases activation in lumbar disc herniation-induced radicular pain was investigated. Results Lumbar disc herniation was induced by implantation of autologous nucleus pulposus, harvest from tail, in lumbar 4/5 spinal nerve roots of rat. Behavior test and electrophysiologic data showed that nucleus pulposus implantation induced persistent mechanical allodynia and thermal hyperalgesia and increased efficiency of synaptic transmission in spinal dorsal horn which underlies central sensitization of pain sensation. Western blotting and immunohistochemistry staining revealed that the expression of phosphorylated src-family kinases was upregulated mainly in spinal microglia of rats with nucleus pulposus. Intrathecal delivery of src-family kinases inhibitor PP2 alleviated pain behaviors, decreased efficiency of spinal synaptic transmission, and reduced phosphorylated src-family kinases expression. Furthermore, we found that the expression of ionized calcium-binding adapter molecule 1 (marker of microglia), tumor necrosis factor-α, interleukin 1 -β in spinal dorsal horn was increased in rats with nucleus pulposus. Therapeutic effect of PP2 may be related to its capacity in reducing the expression of these factors. Conclusions These findings suggested that central sensitization was involved in radicular pain from lumbar disc herniation; src-family kinases-mediated inflammatory response may be responsible for central sensitization and chronic pain after lumbar disc herniation.

Inhibition of cystathionine β-synthetase suppresses sodium channel activities of dorsal root ganglion neurons of rats with lumbar disc herniation

The pathogenesis of pain in lumbar disc herniation (LDH) remains poorly understood. We have recently demonstrated that voltage-gated sodium channels (VGSCs) in dorsal root ganglion (DRG) neurons were sensitized in a rat model of LDH. However, the detailed molecular mechanism for sensitization of VGSCs remains largely unknown. This study was designed to examine roles of the endogenous hydrogen sulfide synthesizing enzyme cystathionine β-synthetase (CBS) in sensitization of VGSCs in a previously validated rat model of LDH. Here we showed that inhibition of CBS activity by O-(Carboxymethyl) hydroxylamine hemihydrochloride (AOAA) significantly attenuated pain hypersensitivity in LDH rats. Administration of AOAA also reduced neuronal hyperexcitability, suppressed the sodium current density, and right-shifted the V_1/2 of the inactivation curve, of hindpaw innervating DRG neurons, which is retrogradely labeled by DiI. In vitro incubation of AOAA did not alter the excitability of acutely isolated DRG neurons. Furthermore, CBS was colocalized with Na_V1.7 and Na_V1.8 in hindpaw-innervating DRG neurons. Treatment of AOAA markedly suppressed expression of Na_V1.7 and Na_V1.8 in DRGs of LDH rats. These data suggest that targeting the CBS-H₂S signaling at the DRG level might represent a novel therapeutic strategy for chronic pain relief in patients with LDH.

Food-Derived Natural Compounds for Pain Relief in Neuropathic Pain

Neuropathic pain, defined as pain caused by a lesion or disease of the somatosensory nervous system, is characterized by dysesthesia, hyperalgesia, and allodynia. The number of patients with this type of pain has increased rapidly in recent years. Yet, available neuropathic pain medicines have undesired side effects, such as tolerance and physical dependence, and do not fully alleviate the pain. The mechanisms of neuropathic pain are still not fully understood. Injury causes inflammation and immune responses and changed expression and activity of receptors and ion channels in peripheral nerve terminals. Additionally, neuroinflammation is a known factor in the development and maintenance of neuropathic pain. During neuropathic pain development, the C-C motif chemokine receptor 2 (CCR2) acts as an important signaling mediator. Traditional plant treatments have been used throughout the world for treating diseases. We and others have identified food-derived compounds that alleviate neuropathic pain. Here, we review the natural compounds for neuropathic pain relief, their mechanisms of action, and the potential benefits of natural compounds with antagonistic effects on GPCRs, especially those containing CCR2, for neuropathic pain treatment.