Contralateral neuropathic pain and neuropathology in dorsal root ganglion and spinal cord following hemilateral nerve injury in rats

Inflaming the brain: CRPS a model disease to understand neuroimmune interactions in chronic pain

We review current concepts in CRPS from a neuroimaging perspective and point out topics and potential mechanisms that are suitable to be investigated in the next step towards understanding the pathophysiology of CRPS. We have outlined functional aspects of the syndrome, from initiating lesion via inflammatory mechanisms to CNS change and associated sickness behavior, with current evidence for up-regulation of immunological factors in CRPS, neuroimaging of systemic inflammation, and neuroimaging findings in CRPS. The initiation, maintenances and CNS targets implicated in CRPS and in the neuro-inflammatory reflex are discussed in terms of CRPS symptoms and recent preclinical studies. Potential avenues for investigating CRPS with PET and fMRI are described, along with roles of inflammation, treatment and behavior in CRPS. It is our hope that this outline will provoke discussion and promote further empirical studies on the interactions between central and peripheral inflammatory pathways manifest in CRPS.

Hippocampal plasticity after a vagus nerve injury in the rat

Stimulation of the vagus nerve has been previously reported to promote neural plasticity and neurogenesis in the brain. Several studies also revealed plastic changes in the spinal cord after injuries to somatosensory nerves originating from both the brachial and lumbo-sacral plexuses. However, the neurogenic responses of the brain to the injury of the viscerosensory innervation are not as yet well understood. In the present study, we investigated whether cells in the dentate gyrus of the hippocampus respond to a chemical and physical damage to the vagus nerve in the adult rat. Intraperitoneal capsaicin administration was used to damage non-myelinated vagal afferents while subdiaphragmatic vagotomy was used to damage both the myelinated and non-myelinated vagal afferents. The 5-bromo-2-deoxyuridine (BrdU) incorporation together with cell-specific markers was used to study neural proliferation in subgranular zone, granule cell layer, molecular layer and hilus of the dentate gyrus. Microglia activation was determined by quantifying changes in the intensity of fluorescent staining with a primary antibody against ionizing calcium adapter-binding molecule 1. Results revealed that vagotomy decreased BrdU incorporation in the hilus 15 days after injury compared to the capsaicin group. Capsaicin administration decreased BrdU incorporation in the granular cell layer 60 days after the treatment. Capsaicin decreased the number of doublecortin-expressing cells in the dentate gyrus, whereas vagotomy did not alter the expression of doublecortin in the hippocampus. Both the capsaicin- and the vagotomy-induced damage to the vagus nerve decreased microglia activation in the hippocampus at 15 days after the injury. At 30 days post injury, capsaicin-treated and vagotomized rats revealed significantly more activated microglia. Our findings show that damage to the subdiaphragmatic vagus in adult rats is followed by microglia activation and long-lasting changes in the dentate gyrus, leading to alteration of neurogenesis.

Distinct degree of radiculopathy at different levels of peripheral nerve injury

BACKGROUND

Lumbar radiculopathy is a common clinical problem, characterized by dorsal root ganglion (DRG) injury and neural hyperactivity causing intense pain. However, the mechanisms involved in DRG injury have not been fully elucidated. Furthermore, little is known about the degree of radiculopathy at the various levels of nerve injury. The purpose of this study is to compare the degree of radiculopathy injury at the DRG and radiculopathy injury proximal or distal to the DRG.

RESULTS

The lumbar radiculopathy rat model was created by ligating the L5 nerve root 2 mm proximal to the DRG or 2 mm distal to the DRG with 6.0 silk. We examined the degree of the radiculopathy using different points of mechanical sensitivity, immunohistochemistry and in vivo patch-clamp recordings, 7 days after surgery. The rats injured distal to the DRG were more sensitive than those rats injured proximal to the DRG in the behavioral study. The number of activated microglia in laminas I-II of the L5 segmental level was significantly increased in rats injured distal to the DRG when compared with rats injured proximal to the DRG. The amplitudes and frequencies of EPSC in the rats injured distal to the DRG were higher than those injured proximal to the DRG. The results indicated that there is a different degree of radiculopathy at the distal level of nerve injury.

CONCLUSIONS

Our study examined the degree of radiculopathy at different levels of nerve injury. Severe radiculopathy occurred in rats injured distal to the DRG when compared with rats injured proximal to the DRG. This finding helps to correctly diagnose a radiculopathy.

The thalidomide analgesic effect is associated with differential TNF-α receptor expression in the dorsal horn of the spinal cord as studied in a rat model of neuropathic pain

The proinflammatory cytokine tumor necrosis factor-α (TNF-α) is well recognized as a key player in nociceptive signaling. Yet, therapeutic capitalization of this knowledge requires a better understanding of how TNF receptors (TNFR) contribute to pain. To address this question, we studied TNFR expression in the chronic sciatic nerve constriction (CCI) model of neuropathic pain. CCI and sham operated rats received two subcutaneous injections (one immediately after surgery, the other on postoperative day 5) containing either saline, GABA-reuptake inhibitor (NO-711), insulin-like growth factor-1 (IGF-1), ZVAD or thalidomide. Mechanical (using von Frey filaments) and thermal hypersensitivity (Hargreaves test) were assessed preoperatively and weekly during the first four postoperative weeks. Spinal cord dorsal horn samples were collected from animals that were sacrificed at 2 weeks and 4 weeks after surgery, and analyzed for TNFR1 and TNFR2 mRNA levels by qPCR and protein levels by Western blot. Compared to saline, all applied drug treatments resulted in a faster recovery from mechanical and thermal hypersensitivity, yet in a potency order of thalidomide>ZVAD=IGF-1>NO-711. CCI resulted in increased TNFR1 and TNFR2 mRNA and protein levels in the ipsilateral dorsal horn. Thalidomide was the only treatment that attenuated these increases. Finally, animals that showed a poor behavioral recovery were characterized by a significantly higher TNFR1/TNFR2 mRNA ratio. These data show that differential expression of TNFR in the dorsal horn is associated with recovery from pain in this model and suggest that the analgesic effects of thalidomide may act via this mechanism.

Tumor necrosis factor-α levels correlate with postoperative pain severity in lumbar disc hernia patients: opposite clinical effects between tumor necrosis factor receptor 1 and 2

Lumbar disc hernia (LDH) is a leading cause of chronic pain in adults. The underlying pathology of chronic pain after discectomy remains unclear. Chronic local inflammation is considered to underlie painful symptomatology. In this context, we investigated tumor necrosis factor (TNF)-α, TNF receptor 1 (TNFR1), and TNF receptor 2 (TNFR2) expression at the time of surgery in LDH patients and correlated it with the severity of postoperative pain. We analyzed protein and mRNA levels from muscle, ligamentum flavum (LF), annulus fibrosus (AF), and nucleus pulposus (NP) in LDH patients and scoliosis patients (SP), who served as controls. Pain assessment with the visual analogue scale (VAS) was performed 1 day before surgery and 6 weeks and 12 months postoperatively. TNF-α protein levels were detected in AF, LF, and NP in all LDH patients, but not in SP. TNF-α mRNA was significantly greater in LDH patients than in SP; ie, 5-fold in AF, 3-fold in NP, and 2-fold in LF. For NP, TNF-α protein levels correlated with VAS scores (r=0.54 at 6-week and r=0.65 at 12-month follow-up). Also, TNFR1 protein levels in NP positively correlated with VAS scores (r=0.75 at 6-week and r=0.80 at 12-month follow-up). However, TNFR2 protein levels in AF negatively correlated with VAS scores (r=-0.60 at 6 weeks and r=-0.60 at 12 months follow-up). These data indicate that TNF-α levels could determine the clinical outcome in LDH patients after discectomy. Moreover, the opposite correlation of TNF receptors with pain sensation suggests that an unbalanced expression plays a role in the generation of pain.

Changes in spinal cord expression of fractalkine and its receptor in a rat model of disc herniation by autologous nucleus pulposus

STUDY DESIGN

Behavior, mRNA and immunohistochemical assessment of the expression of fractalkine (CX3CL1) and its receptor (CX3CR1) in a rat model of disc herniation by autologous nucleus pulposus (NP) implantation.

OBJECTIVE

To evaluate the changes in expression of fractalkine and its receptor in the spinal cord and their association with pain behavior in a rat model of disc herniation.

SUMMARY OF BACKGROUND DATA

Chemokines have recently been implicated in the pathophysiology of neuropathic pain. They mediate astrocytic migration and microglial proliferation, which are involved in the regulation of nociceptive transmission. Fractalkine is a chemokine, which participates in the mechanisms of neuropathic pain as a mediator of neuron-glia interactions.

METHODS

Sixty-six rats (NP-treated = 47, sham = 19) were implanted with autologous NP (approximately 3 mg) on the left L5 nerve root, just proximal to the dorsal root ganglion and tested for thermal hyperalgesia and mechanical allodynia before surgery and on days 1, 5, 10, 20, and 30 after surgery. The changes of expression of fractalkine and its receptor in the spinal cord were studied using real time PCR and immunohistochemistry.

RESULTS

Rats developed ipsilateral mechanical allodynia at day 1 and bilateral thermal hyperalgesia at day 5 after surgery, and these changes in sensitivity persisted throughout the observation period. The expression of mRNA for fractalkine in the spinal cord was increased by day 5 and remained upregulated for the duration of the experiment. The immunostaining for fractalkine increased in neurons and astrocytes and that for the fractalkine receptor increased in microglia in the dorsal horn ipsilateral to the disc herniation.

CONCLUSION

Our results indicate that autologous implantation of NP induces thermal hyperalgesia and mechanical allodynia, and leads to an upregulation of fractalkine and its receptor in spinal neurons and glia, implicating fractalkine in association with radicular pain.

Role of TNF-alpha during central sensitization in preclinical studies

Tumor necrosis factor-alpha (TNF-α) is a principal mediator in pro-inflammatory processes that involve necrosis, apoptosis and proliferation. Experimental and clinical evidence demonstrate that peripheral nerve injury results in activation and morphological changes of microglial cells in the spinal cord. These adjustments occur in order to initiate an inflammatory cascade in response to the damage. Between the agents involved in this reaction, TNF-α is recognized as a key player in this process as it not only modulates lesion formation, but also because it is suggested to induce nociceptive signals. Nowadays, even though the function of TNF-α in inflammation and pain production seems to be generally accepted, diverse sources of literature point to different pathways and outcomes. In this review, we systematically searched and reviewed original articles from the past 10 years on animal models of peripheral nervous injury describing TNF-α expression in neural tissue and pain behavior.

Spreading of complex regional pain syndrome: not a random process

Complex regional pain syndrome (CRPS) generally remains restricted to one limb but occasionally may spread to other limbs. Knowledge of the spreading pattern of CRPS may lead to hypotheses about underlying mechanisms but to date little is known about this process. The objective is to study patterns of spread of CRPS from a first to a second limb and the factors associated with this process. One hundred and eighty-five CRPS patients were retrospectively evaluated. Cox’s proportional hazards model was used to evaluate factors that influenced spread of CRPS symptoms. Eighty-nine patients exhibited CRPS in multiple limbs. In 72 patients spread from a first to a second limb occurred showing a contralateral pattern in 49%, ipsilateral pattern in 30% and diagonal pattern in 14%. A trauma preceded the onset in the second limb in 37, 44 and 91%, respectively. The hazard of spread of CRPS increased with the number of limbs affected. Compared to patients with CRPS in one limb, patients with CRPS in multiple limbs were on average 7 years younger and more often had movement disorders. In patients with CRPS in multiple limbs, spontaneous spread of symptoms generally follows a contralateral or ipsilateral pattern whereas diagonal spread is rare and generally preceded by a new trauma. Spread is associated with a younger age at onset and a more severely affected phenotype. We argue that processes in the spinal cord as well as supraspinal changes are responsible for spontaneous spread in CRPS.

Contralateral neuropathology in dorsal root ganglia in a rat model of noncompressive disc herniation

Some cases of lumbar disc herniation with contralateral radiculopathy have been reported. Many investigators explained this clinical syndrome from several aspects. However, to our best knowledge, there is no explanation on the basis of molecular changes in the contralateral dorsal root ganglia (DRG) until now. We firstly explored the expression of activating transcription factor 3 (ATF3, a marker of nerve injury), glial fibrillary acidic protein (GFAP, a marker of satellite cells activation) in bilateral L5 DRG and spinal cord using immunohistochemistry after nucleus pulposus (NP) application onto the left L5 DRG exposed by unilateral facetectomy. Immunoblotting was used to detect the expression of tumor necrosis factor-alpha (TNF-alpha) in bilateral L5 DRG. We tested that ATF3-immunoreactive (IR) neurons, GFAP-IR satellite cells and TNF-alpha expression in the contralateral DRG increased significantly after NP application. In the spinal cord, ATF3-IR motor neurons increased significantly after surgery, but GFAP-IR astrocytes were not significant. These results suggested that NP application on the unilateral DRG could induce nerve injury, satellite cells activation and upregulation of TNF-alpha expression in the contralateral DRG. In addition, our results indicated that motor neurons injury might play a significant role in contralateral changes.

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.

A local anesthetic, ropivacaine, suppresses activated microglia via a nerve growth factor-dependent mechanism and astrocytes via a nerve growth factor-independent mechanism in neuropathic pain

Background

Local anesthetics alleviate neuropathic pain in some cases in clinical practice, and exhibit longer durations of action than those predicted on the basis of the pharmacokinetics of their blocking effects on voltage-dependent sodium channels. Therefore, local anesthetics may contribute to additional mechanisms for reversal of the sensitization of nociceptive pathways that occurs in the neuropathic pain state. In recent years, spinal glial cells, microglia and astrocytes, have been shown to play critical roles in neuropathic pain, but their participation in the analgesic effects of local anesthetics remains largely unknown.

Results

Repetitive epidural administration of ropivacaine reduced the hyperalgesia induced by chronic constrictive injury of the sciatic nerve. Concomitantly with this analgesia, ropivacaine suppressed the increases in the immunoreactivities of CD11b and glial fibrillary acidic protein in the dorsal spinal cord, as markers of activated microglia and astrocytes, respectively. In addition, epidural administration of a TrkA-IgG fusion protein that blocks the action of nerve growth factor (NGF), which was upregulated by ropivacaine in the dorsal root ganglion, prevented the inhibitory effect of ropivacaine on microglia, but not astrocytes. The blockade of NGF action also abolished the analgesic effect of ropivacaine on neuropathic pain.

Conclusions

Ropivacaine provides prolonged analgesia possibly by suppressing microglial activation in an NGF-dependent manner and astrocyte activation in an NGF-independent manner in the dorsal spinal cord. Local anesthetics, including ropivacaine, may represent a new approach for glial cell inhibition and, therefore, therapeutic strategies for neuropathic pain.

Early dexamethasone relieves trigeminal neuropathic pain

The analgesic effects of dexamethasone on neuropathic pain have been controversial. The present study investigated the effects of dexamethasone on mechanical allodynia in rats with mal-positioned dental implants. Under anesthesia, the left mandibular second molar was extracted and replaced by a miniature dental implant to injure the inferior alveolar nerve. Nociceptive behavior was examined on each designated day after surgery. Mal-positioned dental implants significantly decreased air-puff thresholds both ipsilateral and contralateral to the injury site. Distinct mechanical hyperalgesia and cold and thermal hypersensitivity were also observed bilaterally. Daily administration of dexamethasone produced prolonged anti-allodynic effects (25 or 50 mg/kg, i.p.), but failed to reduce mechanical allodynia when it had already been established. Therefore, our findings provide that early treatment with dexamethasone is important in the treatment of nociceptive behavior suggestive of trigeminal neuropathic pain.

Pain is a salient "stressor" that is mediated by corticotropin-releasing factor-1 receptors

Corticotropin-releasing factor (CRF) plays a major role in controlling the body's response to stress. Because painful conditions are inherently stressful, we hypothesize that CRF may act via CRF-1 receptors to contribute to the pain experience. Studies were designed to investigate whether blocking CRF-1 receptors with selective antagonists or reducing their expression with CRF-Saporin, would attenuate ulcer, inflammatory- and neuropathic-like pain. Five experimental designs were undertaken. In experiment 1, ulcer pain was induced in mice following oral administration of indomethacin, while in experiments 2 and 3, inflammatory pain was induced in rats with either carrageenan or FCA, respectively. For these studies, animals were dosed with CP-154,526 (3, 10, 30 mg/kg) and NBI 27914 (1-30 mg/kg) 1 h prior to the assessment of tactile, thermal or mechanical hypersensitivity, respectively. In experiment 4, neuropathic pain was induced. Twenty-one days following spinal nerve ligation (SNL), animals received CRF-Saporin or control. Three weeks later tactile allodynia was assessed. Similarly, in experiment 5, a separate set of rats received CRF-Saporin or control. Twenty-one days later, mechanical hyperalgesia was assessed following intraplantar carrageenan. Results from the antagonist studies showed that CP-154,526 and NBI 27914 either fully or partially reversed the referred ulcer pain with minimal effective doses (MED) equal to 3 and 10 mg/kg, respectively. Similarly, both NBI 27914 and CP-154,526 reversed the thermal and mechanical hypersensitivity elicited by carrageenan and FCA with MEDs </= 5 and 10 mg/kg, respectively. Findings from the two CRF-Saporin studies determined that pre-treatment with this toxin significantly attenuated SNL- and carrageenan-induced tactile hypersensitivity. Together, these findings suggest that CRF-1 receptors mediate pain and implicate CRF in this regard.

The mirror-image pain: an unclered phenomenon and its possible mechanism

The contralateral allodynia to an injury has been described both in humans and various models of neuropathic and inflammatory pain in rats. In this article, the occurrence of mirror-image pain (MIP) in human beings and animals were reviewed and the possible mechanism of MIP reported was summarized. Last, according to the literature published, we raise some speculation about the possible mechanism underlying MIP.

Activation of satellite glial cells in rat trigeminal ganglion after upper molar extraction

The neurons in the trigeminal ganglion (TG) are surrounded by satellite glial cells (SGCs), which passively support the function of the neurons, but little is known about the interactions between SGCs and TG neurons after peripheral nerve injury. To examine the effect of nerve injury on SGCs, we investigated the activation of SGCs after neuronal damage due to the extraction of the upper molars in rats. Three, 7, and 10 days after extraction, animals were fixed and the TG was removed. Cryosections of the ganglia were immunostained with antibodies against glial fibrillary acidic protein (GFAP), a marker of activated SGCs, and ATF3, a marker of damaged neurons. After tooth extraction, the number of ATF3-immunoreactive (IR) neurons enclosed by GFAP-IR SGCs had increased in a time-dependent manner in the maxillary nerve region of the TG. Although ATF3-IR neurons were not detected in the mandibular nerve region, the number of GFAP-IR SGCs increased in both the maxillary and mandibular nerve regions. Our results suggest that peripheral nerve injury affects the activation of TG neurons and the SGCs around the injured neurons. Moreover, our data suggest the existence of a neuronal interaction between maxillary and mandibular neurons via SGC activation.

Comparison of neuropathic pain and neuronal apoptosis following nerve root or spinal nerve compression

Altered dorsal root ganglion (DRG) function is associated with neuropathic pain following spinal nerve injury. However, compression of the cauda equina and dorsal rhizotomy proximal to the DRG do not induce significant pain, whereas in the spinal nerve and peripheral nerve, injury distal to the DRG does induce neuropathic pain. Caspase signaling induces apoptosis, and caspase inhibitors prevent pain-related behavior. The degree of DRG neuronal apoptosis is thought to play a role in pain behavior. We suggest that differences in pain behavior according to the injury sites within the DRG may be related to imbalances in apoptotic injuries. The aim of this study was to determine which compression injury was more painful and to compare behavior with expression of tumor necrosis factor (TNF)-alpha in DRG and apoptosis in the DRG following crush injury to the L5 nerve root or L5 spinal nerve. Sprague–Dawley rats received a crush injury to the L5 spinal nerve (distal to the DRG), crush injury to the L5 nerve root (proximal to the DRG), or no crush injury (sham). Mechanical allodynia was determined by the von Frey test. Expression of TNF-alpha was compared among three groups using immunoblot findings. Furthermore, we compared the percentage of neurons injured in the DRG using immunostaining for apoptotic cells and localization of activated caspase 3. Mechanical allodynia was observed in both crush injury groups. The duration of mechanical allodynia in the distal crush group was significantly longer than in the proximal crush group (P < 0.05). TNF-alpha expression was increased in DRG neurons following injury. DRG apoptosis in the distal crush group was significantly higher than in the proximal group at each time point (P < 0.05). This study suggests that spinal nerve crush injuries produce a greater degree of DRG apoptosis than do corresponding nerve root crush injuries, and that the former injuries are associated with longer lasting mechanical allodynia. Thus, differences in the time course of mechanical allodynia might be associated with an imbalance in DRG apoptosis.