Time-Dependent Mechanics and Measures of Glial Activation and Behavioral Sensitivity in a Rodent Model of Radiculopathy

Ultrasonographic Contrast and Therapeutic Effects of Hydrogen Peroxide-Responsive Nanoparticles in a Rat Model with Sciatic Neuritis

Purpose

Peripheral nerve damage lacks an appropriate diagnosis consistent with the patient's symptoms, despite expensive magnetic resonance imaging or electrodiagnostic assessments, which cause discomfort. Ultrasonography is valuable for diagnosing and treating nerve lesions; however, it is unsuitable for detecting small lesions. Poly(vanillin-oxalate) (PVO) nanoparticles are prepared from vanillin, a phytochemical with antioxidant and anti-inflammatory properties. Previously, PVO nanoparticles were cleaved by H2O2 to release vanillin, exert therapeutic efficacy, and generate CO2 to increase ultrasound contrast. However, the role of PVO nanoparticles in peripheral nerve lesion models is still unknown. Herein, we aimed to determine whether PVO nanoparticles can function as contrast and therapeutic agents for nerve lesions.

Methods

To induce sciatic neuritis, rats were administered a perineural injection of carrageenan using a nerve stimulator under ultrasonographic guidance, and PVO nanoparticles were injected perineurally to evaluate ultrasonographic contrast and therapeutic effects. Reverse transcription-quantitative PCR was performed to detect mRNA levels of pro-inflammatory cytokines, ie, tumor necrosis factor-α, interleukin-6, and cyclooxygenase-2.

Results

In the rat model of sciatic neuritis, PVO nanoparticles generated CO2 bubbles to increase ultrasonographic contrast, and a single perineural injection of PVO nanoparticles suppressed the expression of tumor necrosis factor-α, interleukin-6, and cyclooxygenase-2, reduced the expression of F4/80, and increased the expression of GAP43.

Conclusion

The results of the current study suggest that PVO nanoparticles could be developed as ultrasonographic contrast agents and therapeutic agents for nerve lesions.

Inhibiting spinal secretory phospholipase A2 after painful nerve root injury attenuates established pain and spinal neuronal hyperexcitability by altering spinal glutamatergic signaling

Neuropathic injury is accompanied by chronic inflammation contributing to the onset and maintenance of pain after an initial insult. In addition to their roles in promoting immune cell activation, inflammatory mediators like secretory phospholipase A₂ (sPLA₂) modulate nociceptive and excitatory neuronal signaling during the initiation of pain through hydrolytic activity. Despite having a known role in glial activation and cytokine release, it is unknown if sPLA₂ contributes to the maintenance of painful neuropathy and spinal hyperexcitability later after neural injury. Using a well-established model of painful nerve root compression, this study investigated if inhibiting spinal sPLA₂ 7 days after painful injury modulates the behavioral sensitivity and/or spinal dorsal horn excitability that is typically evident. The effects of sPLA₂ inhibition on altered spinal glutamatergic signaling was also probed by measuring spinal intracellular glutamate levels and spinal glutamate transporter (GLAST and GLT1) and receptor (mGluR5, GluR1, and NR1) expression. Spinal sPLA₂ inhibition at day 7 abolishes behavioral sensitivity, reduces both evoked and spontaneous neuronal firing in the spinal cord, and restores the distribution of neuronal phenotypes to those of control conditions. Inhibiting spinal sPLA₂ also increases intracellular glutamate concentrations and restores spinal expression of GLAST, GLT1, mGluR5, and GluR1 to uninjured expression with no effect on NR1. These findings establish a role for spinal sPLA₂ in maintaining pain and central sensitization after neural injury and suggest this may be via exacerbating glutamate excitotoxicity in the spinal cord.

[Entrapment neuropathies: a contemporary approach to pathophysiology, clinical assessment, and management : German version]

Entrapment neuropathies such as carpal tunnel syndrome, radiculopathies, or radicular pain are the most common peripheral neuropathies and also the most common cause for neuropathic pain. Despite their high prevalence, they often remain challenging to diagnose and manage in a clinical setting. Summarising the evidence from both preclinical and clinical studies, this review provides an update on the aetiology and pathophysiology of entrapment neuropathies. Potenzial mechanisms are put in perspective with clinical findings. The contemporary assessment is discussed and diagnostic pitfalls highlighted. The evidence for the noninvasive and surgical management of common entrapment neuropathies is summarised and future areas of research are identified.

Entrapment neuropathies: a contemporary approach to pathophysiology, clinical assessment, and management

Entrapment neuropathies such as carpal tunnel syndrome, radiculopathies, or radicular pain are the most common peripheral neuropathies and also the most common cause for neuropathic pain. Despite their high prevalence, they often remain challenging to diagnose and manage in a clinical setting. Summarising the evidence from both preclinical and clinical studies, this review provides an update on the aetiology and pathophysiology of entrapment neuropathies. Potential mechanisms are put in perspective with clinical findings. The contemporary assessment is discussed and diagnostic pitfalls highlighted. The evidence for the noninvasive and surgical management of common entrapment neuropathies is summarised and future areas of research are identified.

Changes in Neuronal Activity in the Anterior Cingulate Cortex and Primary Somatosensory Cortex With Nonlinear Burst and Tonic Spinal Cord Stimulation

INTRODUCTION

Although nonlinear burst and tonic SCS are believed to treat neuropathic pain via distinct pain pathways, the effectiveness of these modalities on brain activity in vivo has not been investigated. This study compared neuronal firing patterns in the brain after nonlinear burst and tonic SCS in a rat model of painful radiculopathy.

METHODS

Neuronal activity was recorded in the ACC or S1 before and after nonlinear burst or tonic SCS on day 7 following painful cervical nerve root compression (NRC) or sham surgery. The amplitude of nonlinear burst SCS was set at 60% and 90% motor threshold to investigate the effect of lower amplitude SCS on brain activity. Neuronal activity was recorded during and immediately following light brush and noxious pinch of the paw. Change in neuron firing was measured as the percent change in spikes post-SCS relative to pre-SCS baseline.

RESULTS

ACC activity decreases during brush after 60% nonlinear burst compared to tonic (p < 0.05) after NRC and compared to 90% nonlinear burst (p < 0.04) and pre-SCS baseline (p < 0.03) after sham. ACC neuron activity decreases (p < 0.01) during pinch after 60% and 90% nonlinear burst compared to tonic for NRC. The 60% of nonlinear burst decreases (p < 0.02) ACC firing during pinch in both groups compared to baseline. In NRC S1 neurons, tonic SCS decreases (p < 0.01) firing from baseline during light brush; 60% nonlinear burst decreases (p < 0.01) firing from baseline during brush and pinch.

CONCLUSIONS

Nonlinear burst SCS reduces firing in the ACC from a painful stimulus; a lower amplitude nonlinear burst appears to have the greatest effect. Tonic and nonlinear burst SCS may have comparable effects in S1.

Physiologic facet capsule stretch can induce pain & upregulate matrix metalloproteinase-3 in the dorsal root ganglia when preceded by a physiological mechanical or nonpainful chemical exposure

BACKGROUND

Neck pain from cervical facet loading is common and induces inflammation and upregulation of nerve growth factor (NGF) that can sensitize the joint afferents. Yet, the mechanisms by which these occur and whether afferents can be pre-conditioned by certain nonpainful stimuli are unknown. This study tested the hypothesis that a nonpainful mechanical or chemical insult predisposes a facet joint to generate pain after a later exposure to typically nonpainful distraction.

METHODS

Rats were exposed to either a nonpainful distraction or an intra-articular subthreshold dose of NGF followed by a nonpainful distraction two days later. Mechanical hyperalgesia was measured daily and C6 dorsal root ganglia (DRG) tissue was assayed for NGF and matrix metalloproteinase-3 (MMP-3) expression on day 7.

FINDINGS

The second distraction increased joint displacement and strains compared to its first application (p = 0.0011). None of the initial exposures altered behavioral sensitivity in either of the groups being pre-conditioned or in controls; but, sensitivity was established in both groups receiving a second distraction within one day that lasted until day 7 (p < 0.024). NGF expression in the DRG was increased in both groups undergoing a pre-conditioning exposure (p < 0.0232). Similar findings were observed for MMP-3 expression, with a pre-conditioning exposure increasing levels after an otherwise nonpainful facet distraction.

INTERPRETATION

These findings suggest that nonpainful insults to the facet joint, when combined, can generate painful outcomes, possibly mediated by upregulation of MMP-3 and mature NGF.

Pre-treatment with Meloxicam Prevents the Spinal Inflammation and Oxidative Stress in DRG Neurons that Accompany Painful Cervical Radiculopathy

Painful neuropathic injuries are accompanied by robust inflammatory and oxidative stress responses that contribute to the development and maintenance of pain. After neural trauma the inflammatory enzyme cyclooxygenase-2 (COX-2) increases concurrent with pain onset. Although pre-treatment with the COX-2 inhibitor, meloxicam, before a painful nerve root compression prevents the development of pain, the pathophysiological mechanisms are unknown. This study evaluated if pre-treatment with meloxicam prior to painful root injury prevents pain by reducing spinal inflammation and peripheral oxidative stress. Glial activation and expression of the inflammatory mediator secreted phospholipase A2 (sPLA2) in the spinal cord were assessed at day 7 using immunohistochemistry. The extent of oxidative damage was measured using the oxidative stress marker, 8-hydroxyguanosine (8-OHG) and localization of 8-OHG with neurons, microglia and astrocytes in the spinal cord and peripherally in the dorsal root ganglion (DRG) at day 7. In addition to reducing pain, meloxicam reduced both spinal microglial and astrocytic activation at day 7 after nerve root compression. Spinal sPLA2 was also reduced with meloxicam treatment, with decreased production in neurons, microglia and astrocytes. Oxidative damage following nerve root compression was found predominantly in neurons rather than glial cells. The expression of 8-OHG in DRG neurons at day 7 was reduced with meloxicam. These findings suggest that meloxicam may prevent the onset of pain following nerve root compression by suppressing inflammation and oxidative stress both centrally in the spinal cord and peripherally in the DRG.

Local Resiniferatoxin Induces Long-Lasting Analgesia in a Rat Model of Full Thickness Thermal Injury

Objective

Opioid-based analgesics are a major component of the lengthy pain management of burn patients, including military service members, but are problematic due to central nervous system-mediated side effects. Peripheral analgesia via targeted ablation of nociceptive nerve endings that express the transient receptor potential vanilloid channel 1 (TRPV1) may provide an improved approach. We hypothesized that local injection of the TRPV1 agonist resiniferatoxin (RTX) would produce long-lasting analgesia in a rat model of pain associated with burn injury.

Methods

Baseline sensitivities to thermal and mechanical stimuli were measured in male and female Sprague-Dawley rats. Under anesthesia, a 100 °C metal probe was placed on the right hind paw for 30 seconds, and sensitivity was reassessed 72 hours following injury. Rats received RTX (0.25 μg/100 μL; ipl) into the injured hind paw, and sensitivity was reassessed across three weeks. Tissues were collected from a separate group of rats at 24 hours and/or one week post-RTX for pathological analyses of the injured hind paw, dorsal spinal cord c-Fos, and primary afferent neuropeptide immunoreactivity.

Results

Local RTX reversed burn pain behaviors within 24 hours, which lasted through recovery at three weeks. At one week following RTX, decreased c-Fos and primary afferent neuropeptide immunoreactivities were observed in the dorsal horn, while plantar burn pathology was unaltered.

Conclusions

These results indicate that local RTX induces long-lasting analgesia in a rat model of pain associated with burn. While opioids are undesirable in trauma patients due to side effects, RTX may provide valuable long-term, nonopioid analgesia for burn patients.

Nerve Root Compression Increases Spinal Astrocytic Vimentin in Parallel With Sustained Pain and Endothelial Vimentin in Association With Spinal Vascular Reestablishment

STUDY DESIGN

Temporal immunohistochemistry analysis of spinal cord tissue from a rat model of cervical radiculopathy.

OBJECTIVE

The goal was to measure spinal endothelial and astrocytic vimentin expression after a painful nerve root compression to define spinal cellular expression of vimentin in the context of pain.

SUMMARY OF BACKGROUND DATA

The intermediate filament, vimentin, is expressed in a variety of cell types in the spinal cord and is modulated in response to neural pathologies. Early after nerve root compression spinal astrocytes become activated and blood-spinal cord barrier (BSCB) breakdown occurs in parallel with development of pain-related behaviors; these spinal responses remain activated as does the presence of pain. In addition to vimentin, glial fibrillary acidic protein (GFAP) expression is a hallmark of astrocyte activation. In contrast, vascular endothelial cells down-regulate vimentin expression in parallel with vascular breakdown. It is not known whether spinal astrocytes and endothelial cells modulate their expression of vimentin in response to a painful neural injury.

METHODS

Mechanical hyperalgesia was measured and spinal cord tissue was harvested at days 1 and 7 after a unilateral nerve root compression in rats. Vimentin was coimmunolabeled with GFAP to label astrocytes and von Willebrand factor (VWF) for endothelial cells in the spinal cord on the side of injury.

RESULTS

Spinal astrocytic vimentin increases by day 7 after nerve root compression, corresponding to when mechanical hyperalgesia is maintained. Spinal endothelial vimentin increases as early as day 1 after a painful compression and is even more robust at day 7.

CONCLUSION

The delayed elevation in spinal astrocytic vimentin corresponding to sustained mechanical hyperalgesia supports its having a relationship with pain maintenance. Further, since BSCB integrity has been shown to be reestablished by day 7 after a painful compression, endothelial expressed vimentin may help to fortify spinal vasculature contributing to BSCB stability.

LEVEL OF EVIDENCE

N/A.

The role of spinal thrombin through protease-activated receptor 1 in hyperalgesia after neural injury

OBJECTIVE Painful neuropathic injuries induce blood-spinal cord barrier (BSCB) breakdown, allowing pro-inflammatory serum molecules to cross the BSCB, which contributes to nociception. The goal of these studies was to determine whether the blood-borne serine protease thrombin also crosses a permeable BSCB, contributing to nociception through its activation of protease-activated receptor-1 (PAR1). METHODS A 15-minute C-7 nerve root compression, which induces BSCB breakdown and painful behaviors by Day 1, was administered in the rat (n = 10); sham operation (n = 11) and a 3-minute compression (n = 10) that does not induce sensitivity were administered as controls. At Day 1 after root compression, spinal cord tissue was co-immunolabeled for fibrin/fibrinogen, the enzymatic product of thrombin, and IgG, a serum protein, to determine whether thrombin acts in areas of BSCB breakdown. To determine whether spinal thrombin and PAR1 contribute to hyperalgesia after compression, the thrombin inhibitor hirudin and the PAR1 antagonist SCH79797, were separately administered intrathecally before compression injuries (n = 5-7 per group). Rat thrombin was also administered intrathecally with and without SCH79797 (n = 6 per group) to determine whether spinal thrombin induces hypersensitivity in naïve rats through PAR1. RESULTS Spinal fibrin(ogen) was elevated at Day 1 after root compression in regions localized to BSCB breakdown and decreased in those regions by Day 7. Blocking either spinal thrombin or PAR1 completely prevented compression-induced hyperalgesia for 7 days. Intrathecal thrombin induced transient pain that was prevented by blocking spinal PAR1 before its injection. CONCLUSIONS The findings of this study suggest a potent role for spinal thrombin and its activation of PAR1 in pain onset following neuropathic injury.

Electrophoretic deposition of ligand-free platinum nanoparticles on neural electrodes affects their impedance in vitro and in vivo with no negative effect on reactive gliosis

BACKGROUND

Electrodes for neural stimulation and recording are used for the treatment of neurological disorders. Their features critically depend on impedance and interaction with brain tissue. The effect of surface modification on electrode impedance was examined in vitro and in vivo after intracranial implantation in rats. Electrodes coated by electrophoretic deposition with platinum nanoparticles (NP; <10 and 50 nm) as well as uncoated references were implanted into the rat's subthalamic nucleus. After postoperative recovery, rats were electrostimulated for 3 weeks. Impedance was measured before implantation, after recovery and then weekly during stimulation. Finally, local field potential was recorded and tissue-to-implant reaction was immunohistochemically studied.

RESULTS

Coating with NP significantly increased electrode's impedance in vitro. Postoperatively, the impedance of all electrodes was temporarily further increased. This effect was lowest for the electrodes coated with particles <10 nm, which also showed the most stable impedance dynamics during stimulation for 3 weeks and the lowest total power of local field potential during neuronal activity recording. Histological analysis revealed that NP-coating did not affect glial reactions or neural cell-count.

CONCLUSIONS

Coating with NP <10 nm may improve electrode's impedance stability without affecting biocompatibility. Increased impedance after NP-coating may improve neural recording due to better signal-to-noise ratio.

Salmon-derived thrombin inhibits development of chronic pain through an endothelial barrier protective mechanism dependent on APC

Many neurological disorders are initiated by blood-brain barrier breakdown, which potentiates spinal neuroinflammation and neurodegeneration. Peripheral neuropathic injuries are known to disrupt the blood-spinal cord barrier (BSCB) and to potentiate inflammation. But, it is not known whether BSCB breakdown facilitates pain development. In this study, a neural compression model in the rat was used to evaluate relationships among BSCB permeability, inflammation and pain-related behaviors. BSCB permeability increases transiently only after injury that induces mechanical hyperalgesia, which correlates with serum concentrations of pro-inflammatory cytokines, IL-7, IL-12, IL-1α and TNF-α. Mammalian thrombin dually regulates vascular permeability through PAR1 and activated protein C (APC). Since thrombin protects vascular integrity through APC, directing its affinity towards protein C, while still promoting coagulation, might be an ideal treatment for BSCB-disrupting disorders. Salmon thrombin, which prevents the development of mechanical allodynia, also prevents BSCB breakdown after neural injury and actively inhibits TNF-α-induced endothelial permeability in vitro, which is not evident the case for human thrombin. Salmon thrombin's production of APC faster than human thrombin is confirmed using a fluorogenic assay and APC is shown to inhibit BSCB breakdown and pain-related behaviors similar to salmon thrombin. Together, these studies highlight the impact of BSCB on pain and establish salmon thrombin as an effective blocker of BSCB, and resulting nociception, through its preferential affinity for protein C.

Burst and Tonic Spinal Cord Stimulation Differentially Activate GABAergic Mechanisms to Attenuate Pain in a Rat Model of Cervical Radiculopathy

OBJECTIVE

Spinal cord stimulation (SCS) is widely used to treat neuropathic pain. Burst SCS, an alternative mode of stimulation, reduces neuropathic pain without paresthesia. However, the effects and mechanisms of burst SCS have not been compared to conventional tonic SCS in controlled investigations. This study compares the attenuation of spinal neuronal activity and tactile allodynia, and the role of γ-aminobutyric acid (GABA) signaling during burst or tonic SCS in a rat model of cervical radiculopathy.

METHODS

The effects of burst and tonic SCS were compared by recording neuronal firing before and after each mode of stimulation at day 7 following a painful cervical nerve root compression. Neuronal firing was also recorded before and after burst and tonic SCS in the presence of the GABAB receptor antagonist, CGP35348.

RESULTS

Burst and tonic SCS both reduce neuronal firing. The effect of tonic SCS, but not burst SCS, is blocked by CGP35348. In a separate study, spinal cord stimulators were implanted to deliver burst or tonic SCS beginning on day 4 after painful nerve root compression; allodynia and serum GABA concentration were measured through day 14. Burst and tonic SCS both reduce allodynia. Tonic SCS attenuates injury-induced decreases in serum GABA, but GABA remains decreased from baseline during burst SCS.

CONCLUSION AND SIGNIFICANCE

Together, these studies suggest that burst SCS does not act via spinal GABAergic mechanisms, despite its attenuation of spinal hyperexcitability and allodynia similar to that of tonic SCS; understanding other potential spinal inhibitory mechanisms may lead to enhanced analgesia during burst stimulation.

Determinants of nerve conduction recovery after nerve injuries: Compression duration and nerve fiber types

INTRODUCTION

The aims of this study were to determine the influences of: (1) timing of nerve decompression; and (2) nerve fiber types on the patterns of nerve conduction studies (NCS) after nerve injury.

METHODS

Nerve conduction studies (NCS) were performed on 3 models of nerve injury: (1) crush injury due to transient nerve compression (crush group); (2) chronic constriction injury (CCI), or permanent compression (CCI group); and (3) CCI with removal of ligatures, or delayed nerve decompression (De-CCI group).

RESULTS

There were distinct patterns of NCS recovery. The crush and De-CCI groups achieved similar motor nerve recovery, better than that of the CCI group. In contrast, recovery of sensory nerves was limited in the CCI and De-CCI groups and was lower than in the crush group.

CONCLUSIONS

Immediate relief of compression resulted in the best recovery of motor and sensory nerve conduction. In contrast, delayed decompression restored only motor nerve conduction.

Sustained neuronal hyperexcitability is evident in the thalamus after a transient cervical radicular injury

STUDY DESIGN

This study used extracellular electrophysiology to examine neuronal hyperexcitability in the ventroposterolateral nucleus (VPL) of the thalamus in a rat model of painful radiculopathy.

OBJECTIVE

The goal of this study was to quantify evoked neuronal excitability in the VPL at day 14 after a cervical nerve root compression to determine thalamic processing of persistent radicular pain.

SUMMARY OF BACKGROUND DATA

Nerve root compression often leads to radicular pain. Chronic pain is thought to induce structural and biochemical changes in the brain affecting supraspinal signaling. In particular, the VPL of the thalamus has been implicated in chronic pain states.

METHODS

Rats underwent a painful transient C7 nerve root compression or sham procedure. Ipsilateral forepaw mechanical allodynia was assessed on days 1, 3, 5, 7, 10, and 14 and evoked thalamic neuronal recordings were collected at day 14 from the contralateral VPL, whereas the injured forepaw was stimulated using a range of non-noxious and noxious mechanical stimuli. Neurons were classified on the basis of their response to stimulation.

RESULTS

Behavioral sensitivity was elevated after nerve root compression starting at day 3 and persisted until day 14 (P < 0.049). Thalamic recordings at day 14 demonstrated increased neuronal hyperexcitability after injury for all mechanical stimuli (P < 0.024). In particular, wide dynamic range neurons demonstrated significantly more firing after injury compared with sham in response to von Frey stimulation (P < 0.0001). Firing in low threshold mechanoreceptive neurons was not different between groups.

CONCLUSION

These data demonstrate that persistent radicular pain is associated with sustained neuronal hyperexcitability in the contralateral VPL of the thalamus. These findings suggest that thalamic processing is altered during radiculopathy and these changes in neuronal firing are associated with behavioral sensitivity.

LEVEL OF EVIDENCE

N/A.

Riluzole effects on behavioral sensitivity and the development of axonal damage and spinal modifications that occur after painful nerve root compression

Object

Cervical radiculopathy is often attributed to cervical nerve root injury, which induces extensive degeneration and reduced axonal flow in primary afferents. Riluzole inhibits neuro-excitotoxicity in animal models of neural injury. The authors undertook this study to evaluate the antinociceptive and neuroprotective properties of riluzole in a rat model of painful nerve root compression.

Methods

A single dose of riluzole (3 mg/kg) was administered intraperitoneally at Day 1 after a painful nerve root injury. Mechanical allodynia and thermal hyperalgesia were evaluated for 7 days after injury. At Day 7, the spinal cord at the C-7 level and the adjacent nerve roots were harvested from a subgroup of rats for immunohistochemical evaluation. Nerve roots were labeled for NF200, CGRP, and IB4 to assess the morphology of myelinated, peptidergic, and nonpeptidergic axons, respectively. Spinal cord sections were labeled for the neuropeptide CGRP and the glutamate transporter GLT-1 to evaluate their expression in the dorsal horn. In a separate group of rats, electrophysiological recordings were made in the dorsal horn. Evoked action potentials were identified by recording extracellular potentials while applying mechanical stimuli to the forepaw.

Results

Even though riluzole was administered after the onset of behavioral sensitivity at Day 1, its administration resulted in immediate resolution of mechanical allodynia and thermal hyperalgesia (p < 0.045), and these effects were maintained for the study duration. At Day 7, axons labeled for NF200, CGRP, and IB4 in the compressed roots of animals that received riluzole treatment exhibited fewer axonal swellings than those from untreated animals. Riluzole also mitigated changes in the spinal distribution of CGRP and GLT-1 expression that is induced by a painful root compression, returning the spinal expression of both to sham levels. Riluzole also reduced neuronal excitability in the dorsal horn that normally develops by Day 7. The frequency of neuronal firing significantly increased (p < 0.045) after painful root compression, but riluzole treatment maintained neuronal firing at sham levels.

Conclusions

These findings suggest that early administration of riluzole is sufficient to mitigate nerve root–mediated pain by preventing development of neuronal dysfunction in the nerve root and the spinal cord.

The roles of mechanical compression and chemical irritation in regulating spinal neuronal signaling in painful cervical nerve root injury

Both traumatic and slow-onset disc herniation can directly compress and/or chemically irritate cervical nerve roots, and both types of root injury elicit pain in animal models of radiculopathy. This study investigated the relative contributions of mechanical compression and chemical irritation of the nerve root to spinal regulation of neuronal activity using several outcomes. Modifications of two proteins known to regulate neurotransmission in the spinal cord, the neuropeptide calcitonin gene-related peptide (CGRP) and glutamate transporter 1 (GLT-1), were assessed in a rat model after painful cervical nerve root injuries using a mechanical compression, chemical irritation or their combination of injury. Only injuries with compression induced sustained behavioral hypersensitivity (p≤0.05) for two weeks and significant decreases (p<0.037) in CGRP and GLT-1 immunoreactivity to nearly half that of sham levels in the superficial dorsal horn. Because modification of spinal CGRP and GLT-1 is associated with enhanced excitatory signaling in the spinal cord, a second study evaluated the electrophysiological properties of neurons in the superficial and deeper dorsal horn at day 7 after a painful root compression. The evoked firing rate was significantly increased (p=0.045) after compression and only in the deeper lamina. The painful compression also induced a significant (p=0.002) shift in the percentage of neurons in the superficial lamina classified as low- threshold mechanoreceptive (sham 38%; compression 10%) to those classified as wide dynamic range neurons (sham 43%; compression 74%). Together, these studies highlight mechanical compression as a key modulator of spinal neuronal signaling in the context of radicular injury and pain.

Nociceptive neuropeptide increases and periorbital allodynia in a model of traumatic brain injury

OBJECTIVE

This study tests the hypothesis that injury to the somatosensory cortex is associated with periorbital allodynia and increases in nociceptive neuropeptides in the brainstem in a mouse model of controlled cortical impact (CCI) injury.

METHODS

Male C57BL/6 mice received either CCI or craniotomy-only followed by weekly periorbital von Frey (mechanical) sensory testing for up to 28 days post-injury. Mice receiving an incision only and naïve mice were included as control groups. Changes in calcitonin gene-related peptide (CGRP) and substance P (SP) within the brainstem were determined using enzyme-linked immunosorbent assay and immunohistochemistry, respectively. Activation of ionized calcium-binding adaptor molecule-1-labeled macrophages/microglia and glial fibrillary acidic protein (GFAP)-positive astrocytes were evaluated using immunohistochemistry because of their potential involvement in nociceptor sensitization.

RESULTS

Incision-only control mice showed no changes from baseline periorbital von Frey mechanical thresholds. CCI significantly reduced mean periorbital von Frey thresholds (periorbital allodynia) compared with baseline and craniotomy-only at each endpoint, analysis of variance P < .0001. Craniotomy significantly reduced periorbital threshold at 14 days but not 7, 21, or 28 days compared with baseline threshold, P < .01. CCI significantly increased SP immunoreactivity in the brainstem at 7 and 14 days but not 28 days compared with craniotomy-only and controls, P < .001. CGRP levels in brainstem tissues were significantly increased in CCI groups compared with controls (incision-only and naïve mice) or craniotomy-only mice at each endpoint examined, P < .0001. There was a significant correlation between CGRP and periorbital allodynia (P < .0001, r = -0.65) but not for SP (r = 0.20). CCI significantly increased the number of macrophage/microglia in the injured cortex at each endpoint up to 28 days, although cell numbers declined over weeks post-injury, P < .001. GFAP(+) immunoreactivity was significantly increased at 7 but not 14 or 28 days after CCI, P < .001. Craniotomy resulted in transient periorbital allodynia accompanied by transient increases in SP, CGRP, and GFAP immunoreactivity compared with control mice. There was no increase in the number of macrophage/microglia cells compared with controls after craniotomy.

CONCLUSION

Injury to the somatosensory cortex results in persistent periorbital allodynia and increases in brainstem nociceptive neuropeptides. Findings suggest that persistent allodynia and increased neuropeptides are maintained by mechanisms other than activation of macrophage/microglia or astrocyte in the injured somatosensory cortex.

Whiplash-like facet joint loading initiates glutamatergic responses in the DRG and spinal cord associated with behavioral hypersensitivity

The cervical facet joint and its capsule are a common source of neck pain from whiplash. Mechanical hyperalgesia elicited by painful facet joint distraction is associated with spinal neuronal hyperexcitability that can be induced by transmitter/receptor systems that potentiate the synaptic activation of neurons. This study investigated the temporal response of a glutamate receptor and transporters in the dorsal root ganglia (DRG) and spinal cord. Bilateral C6/C7 facet joint distractions were imposed in the rat either to produce behavioral sensitivity or without inducing any sensitivity. Neuronal metabotropic glutamate receptor-5 (mGluR5) and protein kinase C-epsilon (PKCε) expression in the DRG and spinal cord were evaluated on days 1 and 7. Spinal expression of a glutamate transporter, excitatory amino acid carrier 1 (EAAC1), was also quantified at both time points. Painful distraction produced immediate behavioral hypersensitivity that was sustained for 7 days. Increased expression of mGluR5 and PKCε in the DRG was not evident until day 7 and only following painful distraction; this increase was observed in small-diameter neurons. Only painful facet joint distraction produced a significant increase (p<0.001) in neuronal mGluR5 over time, and this increase also was significantly elevated (p≤0.05) over responses in the other groups at day 7. However, there were no differences in spinal PKCε expression on either day or between groups. Spinal EAAC1 expression was significantly increased (p<0.03) only in the nonpainful groups on day 7. Results from this study suggest that spinal glutamatergic plasticity is selectively modulated in association with facet-mediated pain.

Chronic stress-induced changes in pro-inflammatory cytokines and spinal glia markers in the rat: a time course study

BACKGROUND/AIMS

Spinal glia activation has been proposed as one mechanism underlying visceral hyperalgesia in a rodent model of chronic stress. In order to assess the possible role of changes in circulating cytokines and in blood-spinal cord barrier (BSCB) permeability in spinal glia activation, we studied the time course of peripheral and spinal pro-inflammatory cytokines and of spinal and satellite glia markers in response to repeated water avoidance (WA) stress.

METHODS

Spinal cords and dorsal root ganglion cells (DRGs) were collected from control rats, rats exposed to 1-hour WA, or 1-hour WA daily for 5 days or 1-hour WA daily for 10 days.

RESULTS

We demonstrated a time-dependent change in circulating IL-1β and spinal IL-1β, IL-6 and TNF-α in stressed animals compared with controls. We found altered expression of the astrocyte markers GFAP and Connexin 43 in spinal and DRG samples at different time points. Finally, WA was associated with increased BSCB permeability.

CONCLUSIONS

These findings confirm the concept that both peripheral and spinal immune markers are altered after chronic WA and suggest a possible link between stress-induced increase of peripheral pro-inflammatory cytokines, changes in satellite glial cells, increase in BSCB permeability and increase in spinal pro-inflammatory mediators suggesting glia activation.

How can animal models inform on the transition to chronic symptoms in whiplash?

STUDY DESIGN

A nonsystematic review of the literature.

OBJECTIVE

The objective was to present general schema for mechanisms of whiplash pain and review the role of animal models in understanding the development of chronic pain from whiplash injury.

SUMMARY OF BACKGROUND DATA

Extensive biomechanical and clinical studies of whiplash have been performed to understand the injury mechanisms and symptoms of whiplash injury. However, only recently have animal models of this painful disorder been developed based on other pain models in the literature.

METHODS

A nonsystematic review was performed and findings were integrated to formulate a generalized picture of mechanisms by which chronic whiplash pain develops from mechanical tissue injuries.

RESULTS

The development of chronic pain from tissue injuries in the neck due to whiplash involves complex interactions between the injured tissue and spinal neuroimmune circuits. A variety of animal models are beginning to define these mechanisms.

CONCLUSION

Continued work is needed in developing appropriate animal models to investigate chronic pain from whiplash injuries and care must be taken to determine whether such models aim to model the injury event or the pain symptoms.

Development of a duration threshold for modulating evoked neuronal responses after nerve root compression injury

Cervical nerve roots are susceptible to compression injuries of various durations. The duration of an applied compression has been shown to contribute to both the onset of persistent pain and also the degree of spinal cellular and molecular responses related to nociception. This study investigated the relationship between peripherally-evoked activity in spinal cord neurons during a root compression and the resulting development of axonal damage. Electrically-evoked spikes were measured in the spinal cord as a function of time during and after (post-compression) a 15 minute compression of the C7 nerve root. Compression to the root significantly (p=0.035) reduced the number of spikes that were evoked over time relative to sham. The critical time for compression to maximally reduce evoked spikes was 6.6±3.0 minutes. A second study measured the post- compression evoked neuronal activity following compression applied for a shorter, sub-threshold time (three minutes). Ten minutes after compression was removed, the discharge rate remained significantly (p=0.018) less than baseline by 58±25% relative to sham after the 15 minute compression, but returned to within 3±33% of baseline after the three minute compression. Axonal damage was evident in the nerve root at day seven after nerve root compression only after a 15 minute compression. These studies demonstrate that even a transient mechanical insult to the nerve root is sufficient to induce sustained neuronal dysfunction and axonal pathology associated with pain, and results provide support that such minor neural tissue traumas can actually induce long-lasting functional deficits.

The potential for salmon fibrin and thrombin to mitigate pain subsequent to cervical nerve root injury

Nerve root compression is a common cause of radiculopathy and induces persistent pain. Mammalian fibrin is used clinically as a coagulant but presents a variety of risks. Fish fibrin is a potential biomaterial for neural injury treatment because it promotes neurite outgrowth, is non-toxic, and clots readily at lower temperatures. This study administered salmon fibrin and thrombin following nerve root compression and measured behavioral sensitivity and glial activation in a rat pain model. Fibrin and thrombin each significantly reduced mechanical allodynia compared to injury alone (p < 0.02). Painful compression with fibrin exhibited allodynia that was not different from sham for any day using stimulation by a 2 g filament; allodynia was only significantly different (p < 0.043) from sham using the 4 g filament on days 1 and 3. By day 5, responses for fibrin treatment decreased to sham levels. Allodynia following compression with thrombin treatment were unchanged from sham at any time point. Macrophage infiltration at the nerve root and spinal microglial activation were only mildly modified by salmon treatments. Spinal astrocytic expression decreased significantly with fibrin (p < 0.0001) but was unchanged from injury responses for thrombin treatment. Results suggest that salmon fibrin and thrombin may be suitable biomaterials to mitigate pain.

Impaired performance on the angle board test is induced in a model of painful whiplash injury but is only transient in a model of cervical radiculopathy

Although clinical studies report motor impairment associated with some painful injuries of the neck, assessment of motor function in animal models has been largely limited only to studies of direct trauma to the nervous system. The incline plane test was modified to evaluate motor function in two rodent pain models of facet joint distraction (FJD) and nerve root compression (NRC) injury (n = 5/group). Sham groups were also included as controls. Motor function was measured using the modified inclined plane test with rats facing downward before surgery (baseline) and following surgery on days corresponding to when mechanical sensitivity is established and remains elevated. Mean baseline values of the board angle inducing slip for FJD (45.8 ± 3.1°) was significantly greater (p = 0.014) than that for NRC (43.5 ± 2.5°), but baseline measurements did not vary for either group over time. No changes in motor function were found for shams. Motor function after FJD significantly decreased (p < 0.001) at days 1 and 7 after injury. In contrast, at day 1 after NRC injury, slip occurred at significantly lower (p = 0.0016) incline angles, but returned to baseline levels by day 7. These results show motor function impairment is induced following painful FJD and suggest the incline plane test offers utility to evaluate functional deficits in painful injuries.