Chemical and mechanical nerve root insults induce differential behavioral sensitivity and glial activation that are enhanced in combination

Microglia and macrophages contribute to the development and maintenance of sciatica in lumbar disc herniation

ABSTRACT

Lumbar disc herniation (LDH) is a major cause of sciatica. Emerging evidence indicated that inflammation induced by the herniated nucleus pulposus (NP) tissues plays a major role in the pathogenesis of sciatica. However, the underlying mechanisms are still elusive. Although microglia and macrophages have been implicated in nerve injury-induced neuropathic pain, their roles in LDH-induced sciatica largely remain unknown. This study successfully established and modified a mouse model of LDH. We found that nerve root compression using degenerated NP tissues can initiate remarkable and persistent sciatica, with increased and prolonged macrophage infiltration in dorsal root ganglia (DRG) and significant activation of microglia in the spinal dorsal horn. Instead, compression of the nerve root with nondegenerated NP tissues only led to transient sciatica, with transient infiltration and activation of macrophages and microglia. Moreover, continuous treatment of PLX5622, a specific colony-stimulating factor 1 receptor antagonist, ablated both macrophages and microglia, which effectively alleviated LDH-induced sciatica. However, mechanical allodynia reoccurred along with the repopulation of macrophages and microglia after the withdrawal of PLX5622. Using RNA sequencing analysis, the current study depicted transcriptional profile changes of DRG after LDH and identified several macrophage-related potential target candidates. Our results suggested that microglia and macrophages may play an essential role in the development and maintenance of LDH-induced sciatica. Targeting microglia and macrophages may be a promising treatment for chronic LDH-induced sciatica.

Neuroprotection and gliosis attenuation by intravenous application of human mesenchymal stem cells (hMSC) following ventral root crush in mice

Rupture and stretching of spinal roots are common incidents that take place in high-energy accidents. The proximal axotomy of motoneurons by crushing of ventral roots is directly related to the degeneration of half of the lesioned population within the first two weeks. Moreover, only a small percentage of surviving motoneurons can successfully achieve regeneration after such a proximal lesion, and new treatments are necessary to improve this scenario. In this sense, mesenchymal stem cells (MSC) are of great interest once they secrete a broad spectrum of bioactive molecules that are immunomodulatory and can restore the environment after a lesion. The present work aimed at studying the effects of human mesenchymal stem cells (hMSC) therapy after ventral root crush (VRC) in mice. We evaluated motoneuron survival, glial reaction, and synapse preservation at the ventral horn. For this purpose, C57BL/6 J were submitted to a crush procedure of L4 to L6 ventral roots and treated with a single intravenous injection of adipose-derived hMSC. Evaluation of the results was carried out at 7, 14, and 28 days after injury. Analysis of motoneuron survival and astrogliosis showed that hMSC treatment resulted in higher motoneuron preservation (motoneuron survival ipsi/contralateral ratio: VRC group = 53%, VRC + hMSC group = 66%; p < 0.01), combined with reduction of astrogliosis (ipsi/contralateral GFAP immunolabeling: VRC group = 470%, VRC + hMSC group = 250%; p < 0.001). The morphological classification and Sholl analysis of microglial activation revealed that hMSC treatment reduced type V and increased type II profiles, indicating an enhancement of surveying over activated microglial cells. The glial reactivity modulation directly influenced synaptic inputs in apposition to axotomized motoneurons. In the hMSC-treated group, synaptic maintenance was increased (ipsi/contralateral synaptophysin immunolabeling: VRC group = 53%, VRC + hMSC group = 64%; p < 0.05). Overall, the present data show that intravenous injection of hMSC has neuroprotective and anti-inflammatory effects, decreasing reactive astrogliosis, and microglial reaction. Also, such cell therapy results in motoneuron preservation, combined with significant maintenance of spinal cord circuits, in particular those related to the ventral horn.

[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.

Differential Diagnosis for the Painful Tingling Arm

ABSTRACT

The painful tingling arm is a common presenting complaint for the musculoskeletal physician. The differential diagnosis for upper-extremity pain associated with paresthesias will be the focus of this review. Symptoms are often neurologic in etiology, originating from the spinal cord, nerve root(s), brachial plexus, or peripheral nerve(s). Localizing the pathology starts with a comprehensive understanding of neuromuscular anatomy. It also is imperative to understand the function of these respective structures. The differential diagnosis can be narrowed with a thorough history, including an assessment of sport-specific risk factors, along with a comprehensive physical examination and functional assessment. It is important to determine the sensory distribution of the patient's symptoms. If weakness also is present, the affected muscles must be identified. While the diagnosis can often be made clinically, electrodiagnostics, magnetic resonance imaging, and ultrasound can be used as needed for confirmation and more specific localization. Nonneurologic structures also may be causative or contributory to the patient's symptoms and also should be considered.

Astrocyte reactivity in spinal cord and functional impairment after tendon injury in rats

Astrocyte reactivity in the spinal cord may occur after peripheral neural damage. However, there is no data to report such reactivity after Achilles tendon injury. We investigate whether changes occur in the spinal cord, mechanical sensitivity and gait in two phases of repair after Achilles tendon injury. Wistar rats were divided into groups: control (CTRL, without rupture), 2 days post-injury (RUP2) and 21 days post-injury (RUP21). Functional and mechanical sensitivity tests were performed at 2 and 21 days post-injury (dpi). The spinal cords were processed, cryosectioned and activated astrocytes were immunostained by GFAP at 21 dpi. Astrocyte reactivity was observed in the L5 segment of the spinal cord with predominance in the white matter regions and decrease in the mechanical threshold of the ipsilateral paw only in RUP2. However, there was gait impairment in both RUP2 and RUP21. We conclude that during the acute phase of Achilles tendon repairment, there was astrocyte reactivity in the spinal cord and impairment of mechanical sensitivity and gait, whereas in the chronic phase only gait remains compromised.

Phospholipase A2 Inhibitor-Loaded Phospholipid Micelles Abolish Neuropathic Pain

Treating persistent neuropathic pain remains a major clinical challenge. Current conventional treatment approaches carry a substantial risk of toxicity and provide only transient pain relief. In this work, we show that the activity and expression of the inflammatory mediator secretory phospholipase-A₂ (sPLA₂) enzyme increases in the spinal cord after painful nerve root compression. We then develop phospholipid micelle-based nanoparticles that release their payload in response to sPLA₂ activity. Using a rodent model of neuropathic pain, phospholipid micelles loaded with the sPLA₂ inhibitor, thioetheramide-PC (TEA-PC), are administered either locally or intravenously at the time of painful injury or 1-2 days afterward. Local micelle administration immediately after compression prevents pain for up to 7 days. Delayed intravenous administration of the micelles attenuates existing pain. These findings suggest that sPLA₂ inhibitor-loaded micelles can be a promising anti-inflammatory nanotherapeutic for neuropathic pain treatment.

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.

Transforaminal epidural steroid injections in cervical spinal disease with moderate to severe disability: Comparative study in patients with or without surgery

Despite many clinical trials on cervical epidural steroid injections, the indications for and long-standing outcomes of this treatment remain controversial. We evaluated the outcomes and indications for transforaminal cervical epidural steroid injection (TCESI) in patients with moderate to severe disability.We prospectively gathered data from patients with 1 or 2-level cervical degenerative disease (herniated disc, foraminal stenosis) with moderate to severe disability (3.5 < initial visual analog scale < 6.5, 15 < Neck Disability Index < 35) and greater than 12 weeks of pain, despite conservative treatment. Patients with persistent disability and those who desired surgical intervention underwent decompression surgery. The clinical and demographic characteristics were compared between groups.Of the 309 patients who underwent TCESI, 221 (72%) did not receive surgical treatment during the 1-year follow-up period. The remaining 88 patients (28%) underwent surgery at a mean of 4.1 months after initial TCESI. Patients who underwent injection alone showed a significant decrease in disability and pain that persisted until the 1-year follow-up visit (P < .05). In patients who underwent surgery, the mean disability and pain scores after injection did not decrease for several months, although the scores significantly decreased up to 1 year after surgery (P < .05).The TCESI significantly decreased pain and disability in the moderate to severe disability group up to 1 year after injection. We recommend cervical TCESI as an initial treatment with moderate to severe disability patients.

Role of MHC-I Expression on Spinal Motoneuron Survival and Glial Reactions Following Ventral Root Crush in Mice

Lesions to the CNS/PNS interface are especially severe, leading to elevated neuronal degeneration. In the present work, we establish the ventral root crush model for mice, and demonstrate the potential of such an approach, by analyzing injury evoked motoneuron loss, changes of synaptic coverage and concomitant glial responses in β2-microglobulin knockout mice (β2m KO). Young adult (8-12 weeks old) C57BL/6J (WT) and β2m KO mice were submitted to a L4-L6 ventral roots crush. Neuronal survival revealed a time-dependent motoneuron-like cell loss, both in WT and β2m KO mice. Along with neuronal loss, astrogliosis increased in WT mice, which was not observed in β2m KO mice. Microglial responses were more pronounced during the acute phase after lesion and decreased over time, in WT and KO mice. At 7 days after lesion β2m KO mice showed stronger Iba-1+ cell reaction. The synaptic inputs were reduced over time, but in β2m KO, the synaptic loss was more prominent between 7 and 28 days after lesion. Taken together, the results herein demonstrate that ventral root crushing in mice provides robust data regarding neuronal loss and glial reaction. The retrograde reactions after injury were altered in the absence of functional MHC-I surface expression.

Pitting oedema in a polio survivor with lumbar radiculopathy complicated disc herniation

We report a 58-year-old male with sequelae of polio who presented with low back and left buttock pain, and pitting oedema of both legs for four months. The patient had a history of poliomyelitis at the age of 1 year which resulted in bilateral lower leg weakness, particularly on the left side. Magnetic resonance imaging showed cervical spinal stenosis secondary to posterior osteophyte formation, left paracentral disc extrusion at L2/L3 and L3/L4 levels with compression of the traversing L4 nerve root. The findings confirmed a diagnosis of lumbar radiculopathy caused by a herniated disc. The patient subsequently underwent a chiropractic treatment. The painful symptoms and pitting oedema in this case resolved with spinal adjustment in addition to scraping therapy to strengthen bilateral low back and the gluteal muscles. This case provides circumstantial evidence of a scarcely mentioned association between pitting oedema and lumbar radiculopathy caused by disc herniation. The pathophysiological mechanism is elusive, but might involve a complexity of cytokine-mediated inflammation and interconnection between somatic and autonomic nervous systems.

Puerarin alleviate radicular pain from lumbar disc herniation by inhibiting ERK-dependent spinal microglia activation

Lumbar disc herniation is a common cause of radicular pain, but the mechanism remains ambiguous and the treatment stays unsatisfied. Many studies revealed a traditional Chinese medicine puerarin may moderate chronic pain from diabetes and nerve injury. Thus far, the role and mechanism of puerarin in radicular pain is still unknown. In this study, by using a rat model of lumbar disc herniation, which was induced by autologous nucleus pulposus (NP) implantation, the analgesic effect of puerarin on radicular pain was tested. Puerarin was delivered intraperitoneally form 1 h before surgery, and once daily for 7 days. The results demonstrated that NP implantation induced long-lasting pain, characterized by decrease of paw withdrawal threshold (PWT) and paw withdrawal latency (PWL) in ipsilateral hindpaws, as long as day 20 after surgery. Spinal phosphorylated extracellular signal-regulated kinase (p-ERK) was up-regulated from day 5 to day 20 after surgery in ipsilateral but not contralateral side, and p-ERK was mainly co-localized with microglia. Puerarin decreased p-ERK expression from day 7 to day 20 after surgery. Puerarin or ERK inhibitor PD98059 alleviated pain behaviors, decreased expression of microglia marker ionized calcium-binding adaptor molecule 1 (Iba-1) in rats with NP implantation. The results suggested puerarin may alleviate radicular pain by inhibiting ERK-dependent or accompanied spinal microglia activation.

Association of Serum Serotonin and Pain in Patients with Chronic Low Back Pain before and after Spinal Surgery

Introduction

In this study we are aiming to evaluate the changes of serum serotonin and its association with pain in patients suffering from chronic low back pain before and after lumbar discectomy surgery.

Patients and Methods

A prospective study was performed on the patients referring to the outpatient clinic in Besat hospital, Hamadan University of Medical Sciences, Hamadan, Iran, during 2016. A 2 mL fasting blood sample was collected from each patient at preoperative day 1 and postoperative day 14 and they were measured for level of serum serotonin. Besides, all patients were asked for severity of their low back pain in preoperative day 1 and postoperative day 14 and scored their pain from zero to ten using a Numerical Rating Scale.

Results

Forty patients with the mean age of 47 ± 13 yrs/old (range 25–77) including 15 (37.5%) males were enrolled into the study. The overall mean score of preoperative pain was significantly decreased from 7.4 ± 2.18 (range 4–10) to the postoperative pain score 3.87 ± 2.92 (range 0–10) (P < .001). The overall levels of pre- and postoperative serum serotonin were 3.37 ± 1.27 (range 1.1–6.4) and 3.58 ± 1.32 (range .94–7.1) ng/mL, respectively, with no significant difference (P = .09). The levels of pre- and postoperative serum serotonin were significantly higher in males and patients older than 50 yrs/old compared to the females and patients younger than 50 yrs/old, respectively (P = .03 and .005, respectively). A significant inverse correlation between the postoperative levels of pain and serum serotonin was observed (r = -.36 and P = .02).

Conclusion

A negative medium strength linear relationship may exist between the postoperative serum serotonin and low back pain.

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.

Revalidating Pfirrmann's Magnetic Resonance Image-Based Grading of Lumbar Nerve Root Compromise by Calculating Reliability among Orthopaedic Residents

Background

Intervertebral disc herniations lead to subsequent compromise of the nerve root. The root can either have a mere contact with the disc material or be pushed aside or compressed. This was earlier graded by Pfirrmann and colleagues. We intend to revalidate this grading system by performing a reliability analysis among orthopaedic residents.

Methods

Fifty axial cut magnetic resonance (MR) images of the affected lumbar disc level that belonged to different patients (age, 37.8 ± 10.4 years; 33 males and 17 females) were chosen and given to five orthopaedic residents for grading according to the Pfirrmann's MR image-based grading of lumbar nerve root compromise. Responses were received in the form of categorical variables and reliability was assessed.

Results

On doing percentage statistics, we found that 14 images had 100% agreement, 22 had 80% agreement and 14 had 60% agreement. We inferred an overall agreement of 80% ± 15.1%. In addition, interrater reliability was determined by calculating the Fleiss' kappa, which was found to be 0.521, signifying moderate agreement. Intrarater reliability was determined by calculating Cohen's kappa, which was found to be 0.696, signifying substantial agreement.

Conclusions

Our residents took only a short time to learn and reproduce this grading system as ratings that proved to be moderately reliable. Even though the value of kappa was slightly lower, reliability was similar to that of the original authors. We think that this grading system can be adopted in day-to-day practice by framing appropriate rules to interpret MR images where the nerve roots are not visible.

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.

The pain drawing as an instrument for identifying cervical spine nerve involvement in chronic whiplash-associated disorders

Objective

The aim of the study was to investigate the psychometric properties of a standardized assessment of pain drawing with regard to clinical signs of cervical spine nerve root involvement.

Design

This cross-sectional study included data collected in a randomized controlled study. Patients: Two hundred and sixteen patients with chronic (≥6 months) whiplash-associated disorders, grade 2 or 3, were included in this study.

Methods

The validity, sensitivity, and specificity of a standardized pain drawing assessment for determining nerve root involvement were analyzed, compared to the clinical assessment. In addition, we analyzed the interrater reliability with 50 pain drawings.

Results

Agreement was poor between the standardized pain drawing assessment and the clinical assessment (kappa =0.11, 95% CI: −0.03 to 0.20). Sensitivity was high (93%), but specificity was low (19%). Interrater reliability was good (kappa =0.64, 95% CI: 0.53 to 0.76). Conclusion: The standardized pain drawing assessment of nerve root involvement in chronic whiplash-associated disorders was not in agreement with the clinical assessment. Further research is warranted to optimize the utilization of a pain/discomfort drawing as a supportive instrument for identifying nerve involvement in cervical spinal injuries.

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.

Stimulation parameters define the effectiveness of burst spinal cord stimulation in a rat model of neuropathic pain

INTRODUCTION

Although burst spinal cord stimulation (SCS) has been reported to reduce neuropathic pain, no study has explicitly investigated how the different parameters that define burst SCS may modulate its efficacy. The effectiveness of burst SCS to reduce neuronal responses to noxious stimuli by altering stimulation parameters was evaluated in a rat model of cervical radiculopathy.

METHODS

Neuronal firing was recorded in the spinal dorsal horn before and after burst SCS on day 7 following painful cervical nerve root compression (N = 8 rats). The parameters defining the stimulation (number of pulses per burst, pulse frequency, pulse width, burst frequency, amplitude) were individually varied in separate stimulation trials while holding the remaining parameters constant. The percent reduction of firing of wide-dynamic-range (WDR) and high-threshold neurons after SCS and the percentage of neurons responding to SCS were quantified for each parameter and correlated to the charge per burst delivered during stimulation.

RESULTS

Pulse number, pulse width, and amplitude each were significantly correlated (p <0.009) to suppression of neuronal firing after SCS. Pulse frequency and amplitude significantly affected (p <0.05) the percentage of responsive neurons. Charge per burst was correlated to a reduction of WDR neuronal firing (p <0.03) and had a nonlinear effect on the percentage of neurons responding to burst SCS.

CONCLUSIONS

Burst SCS can be optimized by adjusting relevant stimulation parameters to modulate the charge delivered to the spinal cord during stimulation. The efficacy of burst SCS is dependent on the charge per burst.

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.

Salmon and human thrombin differentially regulate radicular pain, glial-induced inflammation and spinal neuronal excitability through protease-activated receptor-1

Chronic neck pain is a major problem with common causes including disc herniation and spondylosis that compress the spinal nerve roots. Cervical nerve root compression in the rat produces sustained behavioral hypersensitivity, due in part to the early upregulation of pro-inflammatory cytokines, the sustained hyperexcitability of neurons in the spinal cord and degeneration in the injured nerve root. Through its activation of the protease-activated receptor-1 (PAR1), mammalian thrombin can enhance pain and inflammation; yet at lower concentrations it is also capable of transiently attenuating pain which suggests that PAR1 activation rate may affect pain maintenance. Interestingly, salmon-derived fibrin, which contains salmon thrombin, attenuates nerve root-induced pain and inflammation, but the mechanisms of action leading to its analgesia are unknown. This study evaluates the effects of salmon thrombin on nerve root-mediated pain, axonal degeneration in the root, spinal neuronal hyperexcitability and inflammation compared to its human counterpart in the context of their enzymatic capabilities towards coagulation substrates and PAR1. Salmon thrombin significantly reduces behavioral sensitivity, preserves neuronal myelination, reduces macrophage infiltration in the injured nerve root and significantly decreases spinal neuronal hyperexcitability after painful root compression in the rat; whereas human thrombin has no effect. Unlike salmon thrombin, human thrombin upregulates the transcription of IL-1β and TNF-α and the secretion of IL-6 by cortical cultures. Salmon and human thrombins cleave human fibrinogen-derived peptides and form clots with fibrinogen with similar enzymatic activities, but salmon thrombin retains a higher enzymatic activity towards coagulation substrates in the presence of antithrombin III and hirudin compared to human thrombin. Conversely, salmon thrombin activates a PAR1-derived peptide more weakly than human thrombin. These results are the first to demonstrate that salmon thrombin has unique analgesic, neuroprotective and anti-inflammatory capabilities compared to human thrombin and that PAR1 may contribute to these actions.

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.

The prostaglandin E2 receptor, EP2, is upregulated in the dorsal root ganglion after painful cervical facet joint injury in the rat

STUDY DESIGN

This study implemented immunohistochemistry to assay prostaglandin E2 (PGE2) receptor EP2 expression in the dorsal root ganglion (DRG) of rats after painful cervical facet joint injury.

OBJECTIVE

To identify if inflammatory cascades are induced in association with cervical facet joint distraction-induced pain by investigating the time course of EP2 expression in the DRG.

SUMMARY OF BACKGROUND DATA

The cervical facet joint is a common source of neck pain, and nonphysiological stretch of the facet capsular ligament can initiate pain from the facet joint via mechanical injury. PGE2 levels are elevated in painful inflamed and arthritic joints, and PGE2 sensitizes joint afferents to mechanical stimulation. Although in vitro studies suggest that the EP2 receptor subtype contributes to painful joint disease, the EP2 response has not been investigated for any association with painful mechanical joint injury.

METHODS

Separate groups of male Holtzman rats underwent either a painful cervical facet joint distraction injury or sham procedure. Bilateral forepaw mechanical allodynia was assessed, and immunohistochemical techniques were used to quantify EP2 expression in the DRG at days 1 and 7.

RESULTS

Facet joint distraction induced mechanical allodynia that was significant (P < 0.024) at all time points. Painful joint injury also significantly elevated total EP2 expression in the DRG at day 1 (P = 0.009), which was maintained at day 7 (P < 0.001). Neuronal expression of EP2 in the DRG was only increased over sham levels at day 1 (P = 0.013).

CONCLUSION

Painful cervical facet joint distraction induces an immediate and sustained increase of EP2 expression in the DRG, implicating peripheral inflammation in the initiation and maintenance of facet joint pain. The transient increase in neuronal EP2 suggests, as in other painful joint conditions, that after joint injury nonneuronal cells may migrate to the DRG, some of which likely express EP2.

Spontaneous inflammatory pain model from a mouse line with N-ethyl-N-nitrosourea mutagenesis

BACKGROUND

N-ethyl-N-nitrosourea mutagenesis was used to induce a point mutation in C57BL/6 J mice. Pain-related phenotype screening was performed in 915 G3 mice. We report the detection of a heritable recessive mutant in meiotic recombinant N1F1 mice that caused an abnormal pain sensitivity phenotype with spontaneous skin inflammation in the paws and ears.

METHODS

We investigated abnormal sensory processing, neuronal peptides, and behavioral responses after the induction of autoinflammatory disease. Single-nucleotide polymorphism (SNP) markers and polymerase chain reaction product sequencing were used to identify the mutation site.

RESULTS

All affected mice developed paw inflammation at 4-8 weeks. Histological examinations revealed hyperplasia of the epidermis in the inflamed paws and increased macrophage expression in the spleen and paw tissues. Mechanical and thermal nociceptive response thresholds were reduced in the affected mice. Locomotor activity was decreased in affected mice with inflamed hindpaws, and this reduction was attributable to the avoidance of contact of the affected paw with the floor. Motor strength and daily activity in the home cage in the affected mice did not show any significant changes. Although Fos immunoreactivity was normal in the dorsal horn of affected mice, calcitonin gene-related peptide immunoreactivity significantly increased in the deep layer of the dorsal horn. The number of microglia increased in the spinal cord, hippocampus, and cerebral cortex in affected mice, and the proliferation of microglia was maintained for a couple of months. Two hundred eighty-five SNP markers were used to reveal the affected gene locus, which was found on the distal part of chromosome 18. A point mutation was detected at A to G in exon 8 of the pstpip2 gene, resulting in a conserved tyrosine residue at amino acid 180 replaced by cysteine (Y180 C).

CONCLUSIONS

The data provide definitive evidence that a mutation in pstpip2 causes autoinflammatory disease in an N-ethyl-N-nitrosourea mutagenesis mouse model. Thus, our pstpip2 mutant mice provide a new model for investigating the potential mechanisms of inflammatory pain.

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.

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.

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.

The effect of minocycline on motor neuron recovery and neuropathic pain in a rat model of spinal cord injury

OBJECTIVE

Minocycline, a second-generation tetracycline-class antibiotic, has been well established to exert a neuroprotective effect in animal models and neurodegenerative disease through the inhibition of microglia. Here, we investigated the effects of minocycline on motor recovery and neuropathic pain in a rat model of spinal cord injury.

METHODS

To simulate spinal cord injury, the rats' spinal cords were hemisected at the 10th thoracic level (T10). Minocycline was injected intraperitoneally, and was administered 30 minutes prior surgery and every second postoperative day until sacrifice 28 days after surgery. Motor recovery was assessed via the Basso-Beattie-Bresnahan test. Mechanical hyperalgesia was measured throughout the 28-day post-operative course via the von Frey test. Microglial and astrocyte activation was assessed by immunohistochemical staining for ionized calcium binding adaptor molecule 1 (Iba1) and glial fibrillary acidic protein (GFAP) at two sites: at the level of hemisection and at the 5th lumbar level (L5).

RESULTS

In rats, spinal cord hemisection reduced locomotor function and induced a mechanical hyperalgesia of the ipsilateral hind limb. The expression of Iba1 and GFAP was also increased in the dorsal and ventral horns of the spinal cord at the site of hemisection and at the L5 level. Intraperitoneal injection of minocycline facilitated overall motor recovery and attenuated mechanical hyperalgesia. The expression of Iba1 and GFAP in the spinal cord was also reduced in rats treated with minocycline.

CONCLUSION

By inhibiting microglia and astrocyte activation, minocycline may facilitate motor recovery and attenuate mechanical hyperalgesia in individuals with spinal cord injuries.

Inflammatory cytokine and chemokine expression is differentially modulated acutely in the dorsal root ganglion in response to different nerve root compressions

STUDY DESIGN

Inflammatory proteins were quantified in bilateral dorsal root ganglions (DRGs) at 1 hour and 1 day using a multiplexed assay after 2 different unilateral nerve root compression injuries.

OBJECTIVE

To quantify cytokines and a chemokine in the DRG after nerve root compression with and without a chemical injury to determine contributing inflammatory factors in the DRG that may mediate radicular nociception in clinically relevant nerve root pathologies.

SUMMARY OF BACKGROUND DATA

Inflammatory cytokines are known to relate to the behavioral hypersensitivity induced after injuries to the nerve root. However, the relative expression of these proteins in the DRG after cervical nerve root compression are not known.

METHODS

The right C7 nerve root underwent transient compression (10 gf) or transient compression with a chemical irritation (10 gf + chr). The chemical injury was also given alone (chr), and the nerve root was exposed (sham), providing 2 types of controls. Mechanical allodynia was measured to assess behavioral outcomes. Interleukin (IL)-1b, IL-6, tumor necrosis factor-a, and macrophage inflammatory protein 3 (MIP3) were quantified in bilateral DRGs at 1 hour and 1 day using a multiplexed assay.

RESULTS

Ipsilateral allodynia at day 1 after 10 gf + chr was significantly increased over both 10 gf and chr (P < 0.049). Cytokines and MIP3 were not statistically increased over sham at 1 hour. By day 1 after 10 gf + chr, all proteins (IL-1β, IL-6, tumor necrosis factor-a, MIP3) were significantly increased over both normal and sham in the ipsilateral DRG (P < 0.036), and the cytokines were also significantly increased over chr (P < 0.029). Despite allodynia at day 1, cytokines at that time were not increased over normal or sham after either 10 gf or chr.

CONCLUSION

Nerve root compression alone may not be sufficient to induce early increases in proinflammatory cytokines in the DRG after radiculopathy and this early protein response may not be directly responsible for nociception in this type of injury.

Cytokine mRNA expression in painful radiculopathy

Inflammatory cytokines contribute to lumbar radiculopathy. Regulation of cytokines for transient cervical injuries, with or without longer-lasting inflammation, remains to be defined. The C7 root in the rat underwent compression (10gf), chromic gut suture exposure (chr), or their combination (10gf+chr). Ipsilateral C7 spinal cord and dorsal root ganglia (DRG) were harvested at 1 hour after injury for real-time PCR analysis of IL-1beta, IL-6, and TNF-alpha. Cytokine mRNA increased after all 3 injuries. TNF-alpha mRNA in the DRG was significantly increased over sham after 10gf+chr (P = .026). Spinal IL-1beta was significantly increased over sham after 10gf and 10gf+chr (P < .024); IL-6 was significantly increased after 10gf+chr (P < .024). In separate studies, the soluble TNF-alpha receptor was administered at injury and again at 6 hours in all injury paradigms. Allodynia was assessed and tissue samples were harvested for cytokine PCR. Allodynia significantly decreased with receptor administration for 10gf and 10gf+chr (P < .005). Treatment also significantly decreased IL-1beta and TNF-alpha mRNA in the DRG for 10gf+chr (P < .028) at day 1. Results indicate an acute, robust cytokine response in cervical nerve root injury with varying patterns, dependent on injury type, and that early increases in TNF-alpha mRNA in the DRG may drive pain-related signaling for transient cervical injuries.

PERSPECTIVE

Inflammatory cytokine mRNA in the DRG and spinal cord are defined after painful cervical nerve root injury. Studies describe a role for TNF-alpha in mediating behavioral sensitivity and inflammatory cytokines in transient painful radiculopathy. Results outline an early response of inflammatory cytokine upregulation in cervical pain.