Chronic Compression Of Dorsal Root Ganglion Research Articles

Lysine-specific demethylase 1 in primary sensory neurons participates in chronic compression of dorsal root ganglion–induced neuropathic pain

Low back and radicular pain syndromes, usually caused by local inflammation and irritation to the nerve root and dorsal root ganglion (DRG), are common throughout medical practice, but sufficient pain relief is scarce. In this study, we employed a chronic compression of DRG (CCD)–induced radicular pain model in rats to explore whether lysine-specific demethylase 1 (LSD1), a histone demethylase and transcriptional co-repressor, is involved in the pathological process of radicular pain. We found that LSD1 was expressed in various-sized DRG neurons by immunohistochemistry. CCD induced the upregulation of LSD1 in compressed L4–L5 DRGs. Moreover, either LSD1 small interfering RNAs or LSD1 inhibitor attenuated CCD–induced pain hypersensitivities. LSD1 was also upregulated in the injured lumbar 4 (L4) DRG in a spinal nerve ligation (SNL)–induced neuropathic pain mouse model. Nevertheless, LSD1 was not altered in L3–L5 DRGs in complete Freund’s adjuvant–induced inflammatory pain mouse model, paclitaxel– or streptozotocin–induced neuropathic pain models. Furthermore, knockdown of LSD1 in the injured L4 DRG reversed SNL–induced pain hypersensitivities in mice. Therefore, we speculate that nerve injury induced the upregulation of LSD1 in the injured DRGs, which contributes to neuropathic pain hypersensitivities; thus, LSD1 may serve as a potential target for the treatment of radicular pain and neuropathic pain.

Differential Activation of pERK1/2 and c-Fos Following Injury to Different Regions of Primary Sensory Neuron.

Nerve injury causes hyperexcitability of the dorsal root ganglion (DRG) and spinal dorsal horn (DH) neurons, which results in neuropathic pain. We have previously demonstrated that partial dorsal rhizotomy (PDR) produced less severe pain-like behavior than chronic constriction injury (CCI) or chronic compression of DRG (CCD) and did not enhance DRG neuronal excitability. However, the mechanisms underlying such discrepancy remain unclear. This study was designed to compare the activation of phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2) in DRG and DH, and c-Fos in DH following treatments of CCI, CCD, and PDR. We confirmed that thermal hyperalgesia produced by PDR was less severe than that produced by CCI or CCD. We showed that pERK1/2 in DRG and DH was greatly activated by CCI or CCD, whereas PDR produced only transient and mild pERK1/2 activation. CCI, CCD, and PDR induced robust c-Fos expression in DH; nevertheless, c-Fos+ neurons following PDR were much fewer than that following CCI or CCD. Blocking retrograde axonal transport by colchicine proximal to the CCI injury site diminished thermal hyperalgesia and inhibited pERK1/2 and c-Fos activation. These findings demonstrate that less severe pain-like behavior produced by PDR than CCI or CCD attributes to less activation of pERK1/2 and c-Fos. Such neurochemical activation partially relies on retrograde axonal transport of certain “injury signals” from the peripheral injured site to DRG somata.

The role of pruriceptors in enhancing sensitivity to pruritogens in a murine chronic compression model of dorsal root ganglion

Chronic pruritus is a symptom that commonly observed in neurological diseases. It has been hypothesized that the chronic pruritus may result from sensitization of itch-signaling pathways but the mechanisms remain obscure. In this study, we established a mouse model of chronic compression of dorsal root ganglion (CCD) and injected various pruritogenic and algogenic agents intradermally to the calf skin ipsilateral to the compressed dorsal root ganglion (DRG). Compared to the naïve mice, a significant increase in itch-related behaviors was observed in the CCD mice after the injection of pruritogens including histamine and BAM8-22, but not after the injection of capsaicin, although all the above agents evoked enhanced pain-related behaviors toward the injected site. In addition, we investigated if pruritogen-evoked activities of DRG neurons were enhanced in this model. In vivo calcium imaging revealed that compressed DRG neurons exhibited enhanced responses to histamine and BAM8-22. Immunoflorescent staining also showed that the histamine receptor H1 and the capsaicin receptor TRPV1 were significantly upregulated in DRG neurons. Our findings indicated that the sensitization of primary pruriceptive neurons may underlie the enhanced itch sensation after chronic compression of DRG in the mice, and may play a role in chronic pruritus in neurological diseases.

Tau protein function: The mechanical exploration of axonal transport disorder caused by persistent pressure in dorsal root ganglia.

ObjectiveWe analyzed the function of Tau protein to explore the underlying mechanism of axonal transport disorder caused by persistent pressure in the dorsal root ganglia (DRG).MethodsWistar rats were divided into the sham operated group, the control group and the experimental group. The Wistar rat model of continuous compression of DRG was used for further investigation. DRG neurons were extracted and cultured, and the protein content was detected using bicinchoninic acid method. Western blotting and immunofluorescence assays were performed to detect the protein content. Intraperitoneal injection of lithium chloride was performed for interaction with Tau. The results were then analyzed statistically.ResultsAfter 2 weeks of sustained pressure, the expression level of Tau396 increased by 33%, while Tau404 increased by 25% in the DRG of the experimental group (p < 0.05). The expression level of PSD‐95 in the DRG decreased by 15% (p < 0.05), while the expression of vGluT1, vGluT3 and vAchT decreased significantly in the DRG of the experimental group (p < 0.05). There was no significant difference in the expression of vGluT2 and vGAT among the three groups (p > 0.05). After intervention with lithium chloride, the expression of phosphorylated Tau at the above sites decreased in varying degrees compared with the model group. The expression level of Tau404 was reduced by 55%, and that of Tau199 by 60% in the DRG of the experimental group.ConclusionChronic compression of DRG and hypoxia caused phosphorylation of Tau in axons and inhibition of PSD‐95, and the function of the synaptic glutamic acid vesicle is defective in the synapse. This process is crucial in the development and progression of axonal transport dysfunction induced by chronic DRG compression, and phosphorylation of Tau plays a substantial role in this process.

Perturbing NR2B-PSD-95 interaction relieves neuropathic pain by inactivating CaMKII-CREB signaling.

Neuropathic pain is characterized by central sensitization. The interaction between N-methyl-D-aspartate receptors (NMDARs) and postsynaptic density protein-95 (PSD-95) plays a major role in central sensitization. Here, we aimed to investigate the analgesic effect of disruption of the interaction between NMDAR and PSD-95. Chronic dorsal root ganglia compression model rats were used to mimic sciatica. Thermal hyperalgesia and mechanical allodynia were evaluated. The expression of spinal phospho-NR2B, PSD-95, calcium/calmodulin-dependent protein kinase II (CaMKII), and cAMP response element binding protein (CREB) was measured using western blotting. A mimetic peptide Myr-NR2B9c was injected intrathecally to disrupt the interaction between PSD-95 and NR2B and detected by coimmunoprecipitation. Chronic dorsal root ganglia compression surgery induced thermal hyperalgesia and mechanical allodynia, and upregulated pain-related proteins such as phospho-NR2B, PSD-95, CaMKII, and CREB expressions in the spinal cord. Myr-NR2B9c disrupted the interaction between NR2B-containing NMDARs and PSD-95 in the spinal cord. Intrathecal administration of Myr-NR2B9c attenuated neuropathic pain behaviors and downregulated the expressions of phospho-NR2B, PSD-95, CaMKII, and CREB in the spinal cord. The present study indicates that dissociation of NR2B-containing NMDARs from PSD-95 inactivates CaMKII and CREB signaling and relieves pain.

Effect of CXCL12/CXCR4 signaling on neuropathic pain after chronic compression of dorsal root ganglion

Neuropathic pain is a complex, chronic pain state that often accompanies tissue damage, inflammation or injury of the nervous system. However the underlying molecular mechanisms still remain unclear. Here, we showed that CXCL12 and CXCR4 were upregulated in the dorsal root ganglion (DRG) after chronic compression of DRG (CCD), and some CXCR4 immunopositive neurons were also immunopositive for the nociceptive neuronal markers IB4, TRPV1, CGRP, and substance P. The incidence and amplitude of CXCL12-induced Ca2+ response in primary sensory neurons from CCD mice was significantly increased compared to those from control animals. CXCL12 depolarized the resting membrane potential, decreased the rheobase, and increased the number of action potentials evoked by a depolarizing current at 2X rheobase in neurons from CCD mice. The mechanical and thermal hypernociception after CCD was attenuated by administration of a CXCR4 antagonist AMD3100. These findings suggest that CXCL12/CXCR4 signaling contributes to hypernociception after CCD, and targeting CXCL12/CXCR4 signaling pathway may alleviate neuropathic pain.

The effect of NR2B subunit palmitoylation at the spinal level after chronic dorsal root ganglia compression in rats.

The NR2B subunit (N-methyl-D-aspartate receptor 2B subunit) regulates the source of pain, and it participates in the formation of central sensitization. Palmitoylation was shown to be involved in the regulation of N-methyl-D-aspartate receptor internalization. In the present study, we investigated the effects of NR2B subunit palmitoylation in a chronic dorsal root ganglia compression (CCD) rat model. Paw mechanical withdrawal threshold and paw withdrawal thermal latency were used to assess mechanical allodynia and thermal hyperalgesia after a CCD operation and an intrathecal injection of the inhibitor of palmitoylation (2-bromopalmitate [2-BP]). The acyl-biotinyl exchange method, Western blotting, and coimmunoprecipitation were used to investigate the effects of pain processing and the expression of levels of NR2B palmitoylation and phosphorylation at the spinal level. CCD rats had long-lasting thermal hyperalgesia and mechanical allodynia, leading to upregulation of the level of NR2B palmitoylation and phosphorylation at the spinal level. An intrathecal treatment with 2-BP on day 14 after CCD surgery markedly improved pain behaviors and downregulated the expression of NR2B palmitoylation and phosphorylation. These data suggest that upregulated NR2B palmitoylation in CCD-induced neuropathic pain and intrathecal injection of 2-BP could reduce pain behaviors and NR2B phosphorylation. Our findings indicate that spinal NR2B palmitoylation is an important component of CCD-induced neuropathic pain, and it might be a potential target for chronic pain therapy.

Evoked bursting in injured Aβ dorsal root ganglion neurons: A mechanism underlying tactile allodynia

Chronic compression of rat dorsal root ganglion (CCD) produced tactile allodynia accompanied with hyperexcitability of the myelinated Aβ dorsal root ganglion (DRG) neurons. The Aβ DRG neuron hyperexcitability exhibits as bursting discharges in response to peripherally evoked action potentials (evoked bursting [EB]). The incidence of EB was significantly increased after chronic compression of DRG (CCD) (43.3%) vs control (13.3%). EB was maintained by oscillation of the membrane potential, and its duration was increased when the membrane potential was depolarized. EB was found to coexist in some neurons with spontaneous bursting (SB), but EB always occurred at a more negative membrane potential than SB. Afterdischarges of the wide dynamic range neurons of the dorsal horn in the spinal cord in response to electrical stimulation of Aβ afferent nerve fibers were suppressed by blocking EB of the DRG neurons. CCD neurons with EB exhibited increased current density of persistent sodium current (INap) and hyperpolarization-activated cation current (Ih) and decreased α-dendrotoxin (α-DTX) sensitive current (IDTX). The increased Ih activated by afterhyperpolarization of peripheral afferent action potential was necessary for EB generation and a balance between IDTX and INap might be necessary for EB maintenance. This study may suggest a role of EB of myelinated DRG neurons in development of allodynia after nerve injury and a potential pharmaceutical therapy in treating neuropathic allodynia.

LipoxinA4 induced antinociception and decreased expression of NF‐κB and pro‐inflammatory cytokines after chronic dorsal root ganglia compression in rats

Inflammatory and immune responses following nerve injury have been shown to play an important role in neuropathic pain. Lipoxins are endogenous lipoxygenase-derived eicosanoids performing protective roles in a range of pathophysiologic processes. Here, we examined the effects of intrathecal lipoxinA4 (LXA4) on NF-κB activation and pro-inflammatory cytokine (TNF-α, IL-1β and IL-6) expression in dorsal root ganglia (DRG) following chronic compression of DRG (CCD), a model of neuropathic pain. Daily intrathecal injection of vehicle or LXA4 (10 ng or 100 ng) was performed for three successive days post-CCD. CCD induced both mechanical allodynia and thermal hyperalgesia, and increased the expression of TNF-α, IL-1β, IL-6 and NF-κB. Intrathecal injection of LXA4 prevented the development of neuropathic pain and inhibited NF-κB activation and pro-inflammatory cytokine upregulation in a dose-dependent manner. In this study, we have shown the strong protective effect of intrathecal LXA4 on the development of nociceptive behaviors induced by CCD and that these effects might be associated with its anti-inflammatory and pro-resolution properties.

The excitatory effects of the chemokine CCL2 on DRG somata are greater after an injury of the ganglion than after an injury of the spinal or peripheral nerve

We compared the expression of chemokine receptor CCR2 protein in the dorsal root ganglia (DRG) injured by the chronic constriction injury (CCI), the spinal nerve ligation (SNL) and a chronic compression of DRG (CCD). Each of these injuries produced the same significant increase in CCR2 protein in the DRG, as assessed by Western blot analyses. Whole-cell patch-clamp recordings revealed that CCL2, a ligand for CCR2 receptor, depolarized nociceptive DRG neurons from rats of the all three models. A greater percentage of these neurons was depolarized by CCL2 after CCD than after either of the other injuries. Furthermore, CCL2 significantly lowered current threshold only in CCD neurons but not in CCI or SNL neurons. CCL2 significantly lowered the net whole-cell potassium currents in neurons after CCD but not after CCI or SNL. Thus, the injury-induced effects of CCL2 in increasing the excitability of the cell bodies of DRG neurons depend on the site of the injury--with greater effects occurring after an injury of the ganglion than after an injury of the spinal or peripheral nerve.

Ifenprodil Induced Antinociception and Decreased the Expression of NR2B Subunits in the Dorsal Horn After Chronic Dorsal Root Ganglia Compression in Rats

Spinal N-methyl D-aspartate receptors play an important role in the pathogenesis of neuropathic pain, and administration of N-methyl D-aspartate receptor antagonists can attenuate this hyperpathia. Ifenprodil is an antagonist selective for N-methyl D-aspartate receptor 2B (NR2B) subunits. Several researches have reported effective analgesia of ifenprodil in animal models of neuropathic pain. We extended this work to include chronic compression of the dorsal root ganglia (CCD). The paw withdrawal mechanical threshold and paw withdrawal thermal latency tests were used to assess mechanical allodynia and thermal hyperalgesia after a CCD operation and intrathecal injection of ifenprodil. We used immunohistochemistry and immunoblotting to investigate the effect of ifenprodil on NR2B subunits expression in CCD rats. The data revealed increased expression of NR2B subunits in the superficial dorsal horn in CCD rats. We found that, in addition to a marked suppression of thermal hyperalgesia and mechanical allodynia, intrathecal injection ifenprodil treatment causes a decreased expression of NR2B in the spinal cord. These data suggest that ifenprodil induced antinociception in CCD rats and provided further evidence for the important role of NR2B subunits in the development of neuropathic pain.

Thiamine Suppresses Thermal Hyperalgesia, Inhibits Hyperexcitability, and Lessens Alterations of Sodium Currents in Injured, Dorsal Root Ganglion Neurons in Rats

B vitamins can effectively attenuate inflammatory and neuropathic pain in experimental animals, while their efficacy in treating clinical pain syndromes remains unclear. To understand possible mechanisms underlying B vitamin-induced analgesia and provide further evidence that may support the clinical utility of B vitamins in chronic pain treatment, this study investigated effects of thiamine (B1) on the excitability and Na currents of dorsal root ganglion (DRG) neurons that have been altered by nerve injury. Nerve injury was mimicked by chronic compression of DRG in rats. Neuropathic pain was evidenced by the presence of thermal hyperalgesia. Intracellular and patch-clamp recordings were made in vitro from intact and dissociated DRG neurons, respectively. (1) In vivo intraperitoneal administration of B1 (66 mg/kg/day, 10-14 doses) significantly inhibited DRG compression-induced neural hyperexcitability, in addition to suppressing thermal hyperalgesia. (2) In vitro perfusion of B1 (0.1, 1 and 10 mM) resulted in a dose-dependent inhibition of DRG neuron hyperexcitability. In addition, the DRG neurons exhibited size-dependent sensitivity to B1 treatment, i.e., the small and the medium-sized neurons, compared to the large neurons, were significantly more sensitive. (3) Both in vitro (1 mM) and in vivo application of B1 significantly reversed DRG compression-induced down-regulation of tetrodotoxin-resistant but not tetrodotoxin-sensitive Na current density in the small neurons. B1 at 1 mM also reversed the compression-induced hyperpolarizing shift of the inactivation curve of the tetrodotoxin-resistant currents and the upregulated ramp currents in small DRG neurons. Thiamine can reduce hyperexcitability and lessen alterations of Na currents in injured DRG neurons, in addition to suppressing thermal hyperalgesia.

Functional up-regulation of P2X 3 receptors in the chronically compressed dorsal root ganglion

P2X receptors on dorsal root ganglion (DRG) neurons have been strongly implicated in pathological nociception after peripheral nerve injuries or inflammation. However, nothing is known of a role for purinergic receptors in neuropathic pain produced by a chronic compression of DRG (CCD) – an injury that may accompany an intraforaminal stenosis, a laterally herniated disc or other disorders of the spine leading to radicular pain. In a rat model of DRG compression, hyperexcitable neurons retain functioning axonal connections with their peripheral targets. It is unknown whether such hyperexcitability might enhance chemically mediated nociceptive stimulation of the skin. In this study, CCD facilitated the nocifensive behavior and mechanical hyperalgesia-induced by the P2X 3 agonist, α,β-methylene ATP (α,β-meATP). An injection of α,β-meATP into the hind paw of CCD rats resulted in a significantly greater decrease in the mean threshold to von Frey stimuli and a greater duration of paw lifts than in sham-operated control rats. CCD also increased the levels of P2X 3 receptor protein and the number of P2X 3 immunoreactive, small diameter DRG neurons in the compressed ganglion. P2X 3 receptors were co-labeled with the isolectin IB4, consistent with a role in nociception. In addition, a α,β-meATP induced significantly larger fast-inactivating currents in CCD- than in sham-operated acutely dissociated DRG neurons. These currents were accompanied by the generation of action potentials – but only in the CCD neurons. U0126, a specific inhibitor of the MEK1/2, greatly down-regulated the enhanced current. Taken together, these observations suggest that enhanced purinergic responses after CCD are mediated by P2X 3 receptors.

Proteomic analysis of differential proteins related to the neuropathic pain and neuroprotection in the dorsal root ganglion following its chronic compression in rats

The aim of the study was to identify the differential protein expressions related to neuropathic pain and neuroprotection in the dorsal root ganglion (DRG) following chronic compression of DRG (CCD) in rats. We conducted a proteomics study of L(4) and L(5) DRG after CCD for 28 days. A total of 98 protein spots were detected with significant changes in their expression levels after CCD and 15 protein spots were identified by the matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) analysis. Of these proteins, annexin A2, protein kinase C epsilon (PKCepsilon), glyceraldehyde-3-phosphate dehydrogenases (GAPDH), and heat shock protein 70 (HSP70) were up-regulated significantly compared with the normal control. These four proteins and p11, which was annexin A2 light chain, were further examined by Western blotting. The results of Western blotting and the proteomic analysis showed consistent data. Moreover, real-time quantitative RT-PCR experiments indicated that CCD-induced increase in protein levels was associated with an up-regulation of annexin A2 and PKCepsilon gene expression. In conclusion, this study highlights the molecular process in DRG underlying neuropathic pain. CCD is associated with the up-regulation of annexin A2 and PKCepsilon and their related genes. The up-regulation of GAPDH and HSP70 suggests that there exist concurrent processes of nervous injury and neuroprotection in the course of neuropathic pain.

Differing Alterations of Sodium Currents in Small Dorsal Root Ganglion Neurons after Ganglion Compression and Peripheral Nerve Injury

Voltage-gated sodium channels play important roles in modulating dorsal root ganglion (DRG) neuron hyperexcitability and hyperalgesia after peripheral nerve injury or inflammation. We report that chronic compression of DRG (CCD) produces profound effect on tetrodotoxin-resistant (TTX-R) and tetrodotoxin-sensitive (TTX-S) sodium currents, which are different from that by chronic constriction injury (CCI) of the sciatic nerve in small DRG neurons. Whole cell patch-clamp recordings were obtained in vitro from L4 and/or L5 dissociated, small DRG neurons following in vivo DRG compression or nerve injury. The small DRG neurons were classified into slow and fast subtype neurons based on expression of the slow-inactivating TTX-R and fast-inactivating TTX-S Na+ currents. CCD treatment significantly reduced TTX-R and TTX-S current densities in the slow and fast neurons, but CCI selectively reduced the TTX-R and TTX-S current densities in the slow neurons. Changes in half-maximal potential (V1/2) and curve slope (k) of steady-state inactivation of Na+ currents were different in the slow and fast neurons after CCD and CCI treatment. The window current of TTX-R and TTX-S currents in fast neurons were enlarged by CCD and CCI, while only that of TTX-S currents in slow neurons was increased by CCI. The decay rate of TTX-S and both TTX-R and TTX-S currents in fast neurons were reduced by CCD and CCI, respectively. These findings provide a possible sodium channel mechanism underlying CCD-induced DRG neuron hyperexcitability and hyperalgesia and demonstrate a differential effect in the Na+ currents of small DRG neurons after somata compression and peripheral nerve injury. This study also points to a complexity of hyperexcitability mechanisms contributing to CCD and CCI hyperexcitability in small DRG neurons.

The role of ERK activation in the neuronal excitability in the chronically compressed dorsal root ganglia

We examined the roles of phosphorylation of extracellular signal-regulated protein kinases (pERK) in regulating the excitability of neurons in the dorsal root ganglion (DRG) after chronic compression. A chronic compression of DRG (CCD) induced a significant increase in pERK in intact L4/L5 DRGs of rats as assessed by Western blot analysis. The treatment of U0126, the mitogen-activated protein kinase (MAPK) kinase 1/2 inhibitor, suppressed excitability in CCD-injured DRG neurons but not in naïve DRG neurons. Furthermore, the inhibition of ERK signaling increased A-type fast inactivating potassium current. Taken together, the excitation of CCD neurons might be attributed to the CCD-induced activation of ERK, which suppressed the A-type fast inactivating potassium conductance in CCD neurons.

Dissociation of Dorsal Root Ganglion Neurons Induces Hyperexcitability That Is Maintained by Increased Responsiveness to cAMP and cGMP

Injury or inflammation affecting sensory neurons in dorsal root ganglia (DRG) causes hyperexcitability of DRG neurons that can lead to spontaneous firing and neuropathic pain. Recent results indicate that after chronic compression of DRG (CCD treatment), both hyperexcitability of neurons in intact DRG and behaviorally expressed hyperalgesia are maintained by concurrent activity in cAMP-protein kinase A (PKA) and cGMP-protein kinase G (PKG) signaling pathways. We report here that when tested under identical conditions, dissociation produces a pattern of hyperexcitability in small DRG neurons similar to that produced by CCD treatment, manifest as decreased action potential (AP) current threshold, increased AP duration, increased repetitive firing to depolarizing pulses, increased spontaneous firing and resting depolarization. A novel feature of this hyperexcitability is its early expression-as soon as testing can be conducted after dissociation (approximately 2 h). Both forms of injury increase the electrophysiological responsiveness of the neurons to activation of cAMP-PKA and cGMP-PKG pathways as indicated by enhancement of hyperexcitability by agonists of these pathways in dissociated or CCD-treated neurons but not in control neurons. Although inflammatory signals are known to activate cAMP-PKA pathways, dissociation-induced hyperexcitability is unlikely to be triggered by signals released from inflammatory cells recruited to the DRG because of insufficient time for recruitment during the dissociation procedure. Inhibition by specific antagonists indicates that continuing activation of cAMP-PKA and cGMP-PKG pathways is required to maintain hyperexcitability after dissociation. The reduction of hyperexcitability by blockers of adenylyl cyclase and soluble guanylyl cyclase after dissociation suggests a continuing release of autocrine and/or paracrine factors from dissociated neurons and/or satellite cells, which activate both cyclases and help to maintain acute, injury-induced hyperexcitability of DRG neurons.

Hyperalgesia and neural excitability following injuries to central and peripheral branches of axons and somata of dorsal root ganglion neurons.

We examined thermal hyperalgesia, excitability of dorsal root ganglion (DRG) neurons, and antinociceptive effects of N-methyl-d-aspartate (NMDA) receptor antagonists in rats with injury to different regions of DRG neurons. The central or peripheral branches of axons of DRG neurons were injured by partial dorsal rhizotomy (PDR) and chronic constriction injury of sciatic nerve (CCI), respectively, or the somata injured by chronic compression of DRG (CCD). Thermal hyperalgesia was evidenced by significantly shortened latencies of foot withdrawal to radiant heat stimulation of the plantar surface. Intracellular recordings were obtained in vitro from L(4) and/or L(5) ganglia. There are four principle findings: 1) PDR as well as CCD and CCI induced thermal hyperalgesia; 2) PDR produced significantly less severe and shorter duration hyperalgesia than CCD and CCI; 3) intrathecal administration of NMDA receptor antagonists d-2-amino-5-phosphonovaleric acid (APV) and dizocilpine maleate (MK-801) inhibited thermal hyperalgesia in PDR, CCD, and CCI rats. Pretreatment of APV and MK-801 delayed the emergence of hyperalgesia for 48-72 h, while posttreatment inhibited hyperalgesia for 24-36 h; and 4) CCD and CCI increased excitability of DRG neurons as judged by the significantly lowered threshold currents and action potential voltage thresholds and increased incidence of repetitive discharges. However, PDR did not alter the excitability of DRG neurons. These findings indicate that injury to the dorsal root, compared with injury to the peripheral nerve or DRG somata has different effects on the development of hyperalgesia. These contributions involve different changes in DRG membrane excitability, but each involves pathways (presumably in the spinal cord) that depend on NMDA receptors.

A comparison of chronic pain behavior following local application of tumor necrosis factor α to the normal and mechanically compressed lumbar ganglia in the rat

To study the role of inflammatory cytokines in the initiation and persistence of radiculopathy as seen in humans, tumor necrosis factor α (TNF-α) was administered either to normal, uninjured L 5 dorsal root ganglia (DRG) of rats via a hole drilled through the transverse process, or to chronically compressed L 5 DRG via a hollow stainless steel rod inserted into the intervertebral foramen. In other experiments, a mixture of soluble TNF receptors (sTNF-Rs: sTNF-RI±sTNF-RII) was locally delivered to the chronically or acutely compressed DRG to neutralize the activity of endogenous TNF-α. Behavioral tests of mechanical allodynia were performed before and after TNF-α administration. Infusion of the normal DRG with TNF-α at a rate of 1 μl/h for 7 days induced ipsilateral mechanical allodynia (i.e. decreased mechanical withdrawal threshold) that lasted about 2 weeks. Infusion of the compressed DRG did not alter compression-induced allodynia within the first operative week but substantially enhanced the ipsilateral allodynia after the first postoperative week. Neutralizing the activity of endogenous TNF-α of the compressed DRG with sTNF-Rs reduced allodynia for 3 days, but was subsequently without effect. Similar results were obtained when sTNF-Rs were chronically administrated at the acutely compressed ganglion. Results demonstrated that exogenous TNF-α causes pain and mechanical allodynia when deposited at the normal DRG, and further enhances the ongoing allodynia when administrated at the compressed DRG. Results also suggest that endogenous TNF-α contributes to the early development of mechanical allodynia in rats with chronic DRG compression.

Enhanced excitability of sensory neurons in rats with cutaneous hyperalgesia produced by chronic compression of the dorsal root ganglion.

Pain and hyperalgesia can occur when the dorsal root ganglion (DRG) and its roots are deformed mechanically in association with injuries or diseases of the spine. To evaluate the electrophysiological changes that contribute to this sensory pathology, intracellular recordings were obtained in vitro from DRGs that had received a chronic mechanical compression [chronic compression of DRG (CCD)]. The compression was produced by inserting L-shaped rods ipsilaterally into the intervertebral foramina, one at L(4) and the other at L(5) in rats 1-14 days before the recording. Control rats received a sham operation. Postoperatively, the threshold force applied by punctate stimulation of the plantar surface of the hind paw decreased significantly on the foot ipsilateral to the CCD (mechanical hyperalgesia) but changed little on the contralateral foot or on either foot for control rats. DRG somata were viewed through a microscope during recording and classified as small, medium, and large according to their diameters. CCD cells in each size category were more excitable than those of comparable size from control rats as judged by the significantly lowered threshold currents and action potential voltage thresholds. Spontaneous activity was recorded in 11% of all the CCD cells tested. The spontaneous activity and/or changes in both the threshold currents and action potential thresholds were observed as early as one day after injury. The association of cutaneous hyperalgesia with changes in the electrophysiological properties of DRG cells suggests a possible role for intrinsic alterations in the membrane properties of compressed DRG cells in the production and persistence of chronic pain after certain spinal injuries or pathologies of the spine.