Up-regulation of TNFalpha in DRG satellite cells following lumbar facet joint injury in rats

Lysophosphatidic acid receptor1/3 antagonist inhibits the activation of satellite glial cells and reduces acute nociceptive responses

Lysophosphatidic acid (LPA) exerts various biological activities through six characterized G protein-coupled receptors (LPA_1-6 ). While LPA-LPA₁  signaling contributes toward the demyelination and retraction of C-fiber and induces neuropathic pain, the effects of LPA-LPA₁  signaling on acute nociceptive pain is uncertain. This study investigated the role of LPA-LPA₁  signaling in acute nociceptive pain using the formalin test. The pharmacological inhibition of the LPA-LPA₁ axis significantly attenuated formalin-induced nociceptive behavior. The LPA₁  mRNA was expressed in satellite glial cells (SGCs) in dorsal root ganglion (DRG) and was particularly abundant in SGCs surrounding large DRG neurons, which express neurofilament 200. Treatment with LPA_1/3 receptor (LPA_1/3 ) antagonist inhibited the upregulation of glial markers and inflammatory cytokines in DRG following formalin injection. The LPA_1/3 antagonist also attenuated phosphorylation of extracellular signal-regulated kinase, especially in SGCs and cyclic AMP response element-binding protein in the dorsal horn following formalin injection. LPA amounts after formalin injection to the footpad were quantified by liquid chromatography/tandem mass spectrometry, and LPA levels were found to be increased in the innervated DRGs. Our results indicate that LPA produced in the innervated DRGs promotes the activation of SGCs through LPA₁ , increases the sensitivity of primary neurons, and modulates pain behavior. These results facilitate our understanding of the pathology of acute nociceptive pain and demonstrate the possibility of the LPA₁ on SGCs as a novel target for acute pain control.

Discrepancy in the Usage of GFAP as a Marker of Satellite Glial Cell Reactivity

Satellite glial cells (SGCs) surrounding the neuronal somas in peripheral sensory ganglia are sensitive to neuronal stressors, which induce their reactive state. It is believed that such induced gliosis affects the signaling properties of the primary sensory neurons and is an important component of the neuropathic phenotype leading to pain and other sensory disturbances. Efforts to understand and manipulate such gliosis relies on reliable markers to confirm induced SGC reactivity and ultimately the efficacy of targeted intervention. Glial fibrillary acidic protein (GFAP) is currently the only widely used marker for such analyses. However, we have previously described the lack of SGC upregulation of GFAP in a mouse model of sciatic nerve injury, suggesting that GFAP may not be a universally suitable marker of SGC gliosis across species and experimental models. To further explore this, we here investigate the regulation of GFAP in two different experimental models in both rats and mice. We found that whereas GFAP was upregulated in both rodent species in the applied inflammation model, only the rat demonstrated increased GFAP in SGCs following sciatic nerve injury; we did not observe any such GFAP upregulation in the mouse model at either protein or mRNA levels. Our results demonstrate an important discrepancy between species and experimental models that prevents the usage of GFAP as a universal marker for SGC reactivity.

Contribution of GABAergic modulation in DRGs to electroacupuncture analgesia in incisional neck pain rats

Purpose

Acupuncture therapy is effective for relieving postoperative pain. Our previous study showed that electroacupuncture (EA) at Futu (LI18) and Hegu (LI4)–Neiguan (PC6) could alleviate incisional neck pain, which was related with its effect in upregulating γ-aminobutyric acid (GABA) expression in cervical (C3–6) dorsal root ganglions (DRGs); but whether its receptor subsets GABA_Aα2R and GABA_BR1 in C3–6 DRGs are involved in EA analgesia or not, it remains unknown.

Materials and methods

Seventy-five male Sprague Dawley rats were randomized to normal control, model, LI18, LI4–PC6, and Zusanli (ST36)–Yanglingquan (GB34) groups. The incisional neck pain model was established by making a longitudinal incision along the midline of the rats’ neck, followed by repeated mechanical stimulation. EA was applied to bilateral LI18, LI4–PC6, or ST36–GB34 for 30 minutes at 4, 24, and 48 hours after operation. The thermal pain threshold of the neck was detected by a tail-flick unit, and the C3–6 DRGs were removed for assaying the immunoactivity of substance P (SP), GABA_Aα2R, glial fibrillary acidic protein (GFAP; a marker of satellite glial cells [SGCs]), and GABA_BR1 and the expression of GABA_Aα2R and GABA_BR1 mRNA and proteins using immunofluorescence, real-time PCR, and Western blotting, respectively.

Results

The cervical thermal pain threshold was significantly lower in the model group than the normal group (P<0.001), indicating hyperalgesia after neck incision, and was considerably increased in both EA-LI18 and LI4–PC6 groups (P<0.001), but not in ST36–GB34 group compared with model group (P>0.05). Immunofluorescence staining showed that GABA_Aα2 R expressed on SP⁺ neurons, and GABA_BR1 on SGCs. EA of LI18 and LI4–PC6 markedly suppressed the modeling-induced upregulation of the immunoactivity of SP (P<0.001 and P<0.01, respectively) and GFAP (P<0.01 and P<0.001, respectively) and significantly reversed neck incision–induced downregulation of the expression of GABA_Aα2R and GABA_BR1 mRNAs and proteins (P<0.05).

Conclusion

EA of LI18 and LI4–PC6 has an analgesic effect in incisional neck pain rats, which is related to its effects in upregulating GABAergic inhibitory modulation on nociceptive peptidergic neurons and SGCs in cervical DRGs.

IKK/NF-κB-dependent satellite glia activation induces spinal cord microglia activation and neuropathic pain after nerve injury

Increasing evidence indicates that both microglia and satellite glial cell (SGC) activation play causal roles in neuropathic pain development after peripheral nerve injury; however, the activation mechanisms and their contribution to neuropathic pain remain elusive. To address this issue, we generated Ikkβ conditional knockout mice (Cnp-Cre/Ikkβ; cIkkβ) in which IKK/NF-κB-dependent proinflammatory SGC activation was abrogated. In these mice, nerve injury-induced spinal cord microglia activation and pain hypersensitivity were significantly attenuated compared to those in control mice. In addition, nerve injury-induced proinflammatory gene expression and macrophage infiltration into the dorsal root ganglion (DRG) were severely compromised. However, macrophages recruited into the DRG had minimal effects on spinal cord microglia activation, suggesting a causal effect for SGC activation on spinal cord microglia activation. In an effort to elucidate the molecular mechanisms, we measured Csf1 expression in the DRG, which is implicated in spinal cord microglia activation after nerve injury. In cIkkβ mice, nerve injury-induced Csf1 upregulation was ameliorated indicating that IKK/NF-κΒ-dependent SGC activation induced Csf1 expression in sensory neurons. Taken together, our data suggest that nerve injury-induced SGC activation triggers Csf1 induction in sensory neurons, spinal cord microglia activation, and subsequent central pain sensitization.

Characterization of degenerative human facet joints and facet joint capsular tissues

OBJECTIVE

Lumbar facet joint degeneration (FJD) may be an important cause of low back pain (LBP) and sciatica. The goal of this study was to characterize cellular alterations of inflammatory factor expression and neovascularization in human degenerative facet joint capsular (FJC) tissue. These alterations in FJC tissues in pain stimulation were also assessed.

DESIGN

FJs were obtained from consented patients undergoing spinal reconstruction surgery and cadaveric donors with no history of back pain. Histological analyses of the FJs were performed. Cytokine antibody array and quantitative real-time polymerase chain reaction (qPCR) were used to determine the production of inflammatory cytokines, and western blotting analyses (WB) were used to assay for cartilage-degrading enzymes and pain mediators. Ex vivo rat dorsal root ganglion (DRG) co-culture with human FJC tissues was also performed.

RESULTS

Increased neovascularization, inflammatory cell infiltration, and pain-related axonal-promoting factors were observed in degenerative FJCs surgically obtained from symptomatic subjects. Increased VEGF, (NGF/TrkA), and sensory neuronal distribution were also detected in degenerative FJC tissues from subjects with LBP. qPCR and WB results demonstrated highly upregulated inflammatory cytokines, pain mediators, and cartilage-degrading enzymes in degenerative FJCs. Results from ex vivo co-culture of the DRG and FJC tissue demonstrated that degenerative FJCs increased the expression of inflammatory pain molecules in the sensory neurons.

CONCLUSION

Degenerative FJCs possess greatly increased inflammatory and angiogenic features, suggesting that these factors play an important role in the progression of FJD and serve as a link between joint degeneration and neurological stimulation of afferent pain fibers.

A review of percutaneous techniques for low back pain and neuralgia: current trends in epidural infiltrations, intervertebral disk and facet joint therapies

Low back pain and neuralgia due to spinal pathology are very common symptoms debilitating numerous patients with peak prevalence at ages between 45 and 60 years. Intervertebral discs and facet joints act as pain sources in the vast majority of the cases. Diagnosis is based on the combination of clinical examination and imaging studies. Therapeutic armamentarium for low back pain and neuralgia due to intervertebral discs and/or facet joints includes conservative therapy, injections, percutaneous therapeutic techniques and surgical options. Percutaneous, therapeutic techniques are imaging-guided, minimally invasive treatments which can be performed as outpatient procedures. In cases of facet joint syndrome, they include, apart from injections, neurolysis with radiofrequency/cryoablation, MR-guided high-intensity focused ultrasound and percutaneous fixation techniques. In case of discogenic pain, apart from infiltrations, therapeutic techniques can be classified in to two main categories: decompression (mechanical, thermal, chemical) techniques and biomaterials implantation/disc cell therapies. Strict sterility measures are a prerequisite and should include extensive local sterility and antibiotic prophylaxis. This article will report clinical and imaging findings for each pathology type and the association with treatment decision. In addition, we will describe in detail all possible treatment techniques for low back pain and neuralgia, and we will report recently published results of these techniques summarizing the data concerning safety and effectiveness as well as the level of evidence. Finally, we will try to provide a rational approach for the therapy of low back pain and neuralgia by means of minimally invasive imaging-guided percutaneous techniques.

The Effects of Generally Administered Anti-Nerve Growth Factor Receptor (p75NTR) Antibody on Pain-Related Behavior, Dorsal Root Ganglia, and Spinal Glia Activation in a Rat Model of Brachial Plexus Avulsion

PURPOSE

To investigate the effect of intraperitoneal administration of an anti-p75 neurotrophin receptor (p75NTR) antibody on reducing neuropathic pain in a rat model of brachial plexus avulsion (BPA).

METHODS

We randomly assigned 40 male Wistar rats to 4 groups. In the BPA group, the C8-T1 roots were avulsed from the spinal cord at the lower trunk level, and saline was administered intraperitoneally. In the anti-p75NTR groups, 1 μL or 50 μL anti-p75NTR antibody was administered intraperitoneally after avulsion. In the sham-operated group, the lower trunk level was exposed, and saline was administered intraperitoneally. Mechanical hyperalgesia and pain-induced walking patterns were measured using von Frey filaments and CatWalk gait analysis at various time points until 15 days after administration. At 3 and 15 days after administration, sensory neurons involved in pain perception and satellite glial cells in the ipsilateral C7 dorsal root ganglia were immunolabeled with antibodies against calcitonin gene-related peptide and glial fibrillary acidic protein (GFAP), respectively. At both time points, microglial and astrocyte activation, indicative of spinal pain transmission, were immunohistochemically examined in the ipsilateral dorsal horn of the spinal cord (C7) using anti-ionized calcium-binding adaptor molecule 1 and anti-GFAP antibodies, respectively.

RESULTS

The gait pattern was significantly improved in both anti-p75NTR groups compared with the BPA group. There were significantly fewer calcitonin gene-related peptide-immunoreactive (IR) neurons, neurons encircled by GFAP-IR satellite glial cells, and GFAP-IR astrocytes in both anti-p75NTR groups compared with the BPA group at both time points. Fewer ionized calcium-binding adaptor molecule 1-IR microglia were quantified in both anti-p75NTR groups compared with the BPA group, but this was only significant at 15 days after administration.

CONCLUSIONS

Systemic application of the p75NTR inhibitory antibody suppressed neuropathic pain after BPA.

CLINICAL RELEVANCE

p75NTR may be a potential therapeutic target for the clinical treatment of neuropathic pain in BPA injury.

Brain-derived neurotrophic factor is upregulated in the cervical dorsal root ganglia and spinal cord and contributes to the maintenance of pain from facet joint injury in the rat

The facet joint is commonly associated with neck and low back pain and is susceptible to loading-induced injury. Although tensile loading of the cervical facet joint has been associated with inflammation and neuronal hyperexcitability, the mechanisms of joint loading-induced pain remain unknown. Altered brain-derived neurotrophic factor (BDNF) levels are associated with a host of painful conditions, but the role of BDNF in loading-induced joint pain remains undefined. Separate groups of rats underwent a painful cervical facet joint distraction or a sham procedure. Bilateral forepaw mechanical hypersensitivity was assessed and BDNF mRNA and protein levels were quantified in the dorsal root ganglion (DRG) and spinal cord at days 1 and 7. Facet joint distraction induced significant (P < 0.001) mechanical hypersensitivity at both time points. Painful joint distraction did not alter BDNF mRNA in the DRG compared with sham levels but did significantly increase (P < 0.016) BDNF protein expression over sham in the DRG at day 7. Painful distraction also significantly increased BDNF mRNA (P = 0.031) and protein expression (P = 0.047) over sham responses in the spinal cord at day 7. In a separate study, intrathecal administration of the BDNF-sequestering molecule trkB-Fc on day 5 after injury partially attenuated behavioral sensitivity after joint distraction and reduced pERK in the spinal cord at day 7 (P < 0.045). Changes in BDNF after painful facet joint injury and the effect of spinal BDNF sequestration in partially reducing pain suggest that BDNF signaling contributes to the maintenance of loading-induced facet pain but that additional cellular responses are also likely involved.

Lumbar facet joint compressive injury induces lasting changes in local structure, nociceptive scores, and inflammatory mediators in a novel rat model

Objective. To develop a novel animal model of persisting lumbar facet joint pain. Methods. Sprague Dawley rats were anaesthetized and the right lumbar (L5/L6) facet joint was exposed and compressed to ~1 mm with modified clamps applied for three minutes; sham-operated and naïve animals were used as control groups. After five days, animals were tested for hind-paw sensitivity using von Frey filaments and axial deep tissue sensitivity by algometer on assigned days up to 28 days. Animals were sacrificed at selected times for histological and biochemical analysis. Results. Histological sections revealed site-specific loss of cartilage in model animals only. Tactile hypersensitivity was observed for the ipsi- and contralateral paws lasting 28 days. The threshold at which deep tissue pressure just elicited vocalization was obtained at three lumbar levels; sensitivity at L1 > L3/4 > L6. Biochemical analyses revealed increases in proinflammatory cytokines, especially TNF-α, IL-1α, and IL-1β. Conclusions. These data suggest that compression of a facet joint induces a novel model of local cartilage loss accompanied by increased sensitivity to mechanical stimuli and by increases in inflammatory mediators. This new model may be useful for studies on mechanisms and treatment of lumbar facet joint pain and osteoarthritis.

The dynamic TRPA1 channel: a suitable pharmacological pain target?

Acute pain detection is vital to navigate and survive in one's environment. Protection and preservation occur because primary afferent nociceptors transduce adverse environmental stimuli into electrical impulses that are transmitted to and interpreted within high levels of the central nervous system. Therefore, it is critical that the molecular mechanisms that convert noxious information into neural signals be identified, and their specific functional roles delineated in both acute and chronic pain settings. The Transient Receptor Potential (TRP) channel family member TRP ankyrin 1 (TRPA1) is an excellent candidate molecule to explore and intricately understand how single channel properties can tailor behavioral nociceptive responses. TRPA1 appears to dynamically respond to an amazingly wide range of diverse stimuli that include apparently unrelated modalities such as mechanical, chemical and thermal stimuli that activate somatosensory neurons. How such dissimilar stimuli activate TRPA1, yet result in modality-specific signals to the CNS is unclear. Furthermore, TRPA1 is also involved in persistent to chronic painful states such as inflammation, neuropathic pain, diabetes, fibromyalgia, bronchitis and emphysema. Yet how TRPA1's role changes from an acute sensor of physical stimuli to its contribution to these diseases that are concomitant with implacable, chronic pain is unknown. TRPA1's involvement in the nociceptive machinery that relays the adverse stimuli during painful disease states is of considerable interest for drug delivery and design by many pharmaceutical entities. In this review, we will assess the current knowledge base of TRPA1 in acute nociception and persistent inflammatory pain states, and explore its potential as a therapeutic pharmacological target in chronic pervasive conditions such neuropathic pain, persistent inflammation and diabetes.

Activation of satellite glial cells in lumbar dorsal root ganglia contributes to neuropathic pain after spinal nerve ligation

The role of satellite glial cells (SGCs) of sensory ganglia in chronic pain begins to receive interest. The present study aims to investigate the contribution of SGC activation to the development of neuropathic pain. A neuropathic pain model was established by lumbar 5 spinal nerve ligation (SNL), and glial fibrillary acidic protein (GFAP) was used as a marker of SGC activation. It was found that SGCs were activated in the ipsilateral dorsal root ganglia (DRG) increased significantly as early as 4h following SNL, gradually increased to a peak level at day 7, and then stayed at a high level to the end of the experiment at day 56. SGC activation in the SNL group was significantly higher than that in the sham group at days 1, 3 and 7 after operation. Immunofluorescent double labeling showed that the activated SGCs encircled large, medium-sized and small neurons. The SGCs surrounded the small and medium-sized neurons were preferentially activated in the early phase, but shifted to large diameter neurons as time went on. Continuous infusion of fluorocitrate, a glial metabolism inhibitor, to the affected DRG via mini-osmotic pump for 7d significantly alleviated mechanical allodynia at day 7. These results suggest that SGCs in the DRG were activated after SNL. SGC activation contributed to the early maintenance of neuropathic pain.

Activating transcription factor 4, a mediator of the integrated stress response, is increased in the dorsal root ganglia following painful facet joint distraction

Chronic neck pain is one of the most common musculoskeletal disorders in the US. Although biomechanical and clinical studies have implicated the facet joint as a primary source of neck pain, specific cellular mechanisms still remain speculative. The purpose of this study was to investigate whether a mediator (activating transcription factor; 4ATF4) of the integrated stress response (ISR) is involved in facet-mediated pain. Holtzman rats underwent C6/C7 facet joint loading that produces either painful (n=16) or nonpainful (n=8) responses. A sham group (n=9) was also included as surgical controls. Behavioral sensitivity was measured and the C6 dorsal root ganglia (DRGs) were harvested on day 7 to evaluate the total and neuronal ATF4 expression. In separate groups, an intra-articular ketorolac injection was administered either immediately (D0 ketorolac) or 1 day (D1 ketorolac) after painful facet joint loading. Allodynia was measured at days 1 and 7 after injury to assess the effects on behavioral responses. ATF4 and BiP (an indicator of ISR activation) were separately quantified at day 7. Facet joint loading sufficient to elicit behavioral hypersensitivity produced a threefold increase in total and neuronal ATF4 expression in the DRG. After ketorolac treatment at the time of injury, ATF4 expression was significantly (P<0.01) reduced despite not producing any attenuation of behavioral responses. Interestingly, ketorolac treatment at day 1 significantly (P<0.001) alleviated behavioral sensitivity at day 7, but did not modify ATF4 expression. BiP expression was unchanged after either intervention time. Results suggest that ATF4-dependent activation of the ISR does not directly contribute to persistent pain, but it may sensitize neurons responsible for pain initiation. These behavioral and immunohistochemical findings imply that facet-mediated pain may be sustained through other pathways of the ISR.

Activation of satellite cells in the dorsal root ganglia in a disc-punctured rat model

BACKGROUND

The neural mechanisms underlying discogenic low back pain caused by disc degeneration remain unclear. Previous studies demonstrated that satellite cells (SC) play an important role in neuropathic pain.

METHODS

Twenty adult female Sprague-Dawley rats were used. The rats were divided into two groups: a nucleus pulposus (NP) group whose discs were punctured to expose the NP (n = 10) and a sham-operated group whose annulus fibrosus surface was scratched superficially (n = 10). In this study, we investigated the expression and cellular distribution of glial fibrillary acidic protein (GFAP, a marker of SC activation) in the dorsal root ganglia (DRG) innervating the intervertebral discs using a retrograde tracing method and immunohistochemistry in a disc-punctured rat model.

RESULTS

In the sham-operated group, GFAP-immunoreactive (IR) SCs were not detected. In the NP group, GFAP-IR SC became evident, and 49 ± 13% of neurons innervating the punctured discs were surrounded by GFAP-positive SCs.

CONCLUSIONS

Our results were the first to provide evidence for a potential role of SCs in the neural mechanisms of discogenic low back pain caused by disc degeneration.

Glial NF-κB inhibition alters neuropeptide expression after sciatic nerve injury in mice

We utilized a transgenic mouse model where nuclear factor kappa B (NF-κB) is selectively inhibited in glial fibrillary acidic protein (GFAP) expressing cells. The transgene, GFAP-IκBα-dn, overexpresses a dominant negative form of the inhibitor of NF-κB (IκBα) under the control of the GFAP promoter. In the present work, we sought to understand the impact of glial NF-κB inhibition on the expression of pain mediating sensory neuropeptides galanin and calcitonin gene related peptide (CGRP) in a model of neuropathic pain in mice. Chronic constriction injury (CCI) of the left sciatic nerve was performed on wild type (WT) and GFAP-IκBα-dn transgenic mice. RT-PCR and immunohistological staining were performed in sciatic nerve and/or L4-L5 DRG tissue for galanin, CGRP and macrophage marker CD11b. GFAP-IκBα-dn mice had less mechanical and thermal hyperalgesia compared to WT mice post-CCI. After CCI, we observed galanin upregulation in DRG and sciatic nerve, which was less in GFAP-IκBα-dn mice. CGRP gene expression in the DRG increased transiently on day 1 post-CCI in WT but not in GFAP-IκBα-dn mice, and no evidence of CGRP upregulation in sciatic nerve post-CCI was found. After CCI, upregulation of CD11b in sciatic nerve was less in GFAP-IκBα-dn mice compared to WT mice, indicative of less macrophage infiltration. Our results showed that glial NF-κB inhibition reduces galanin and CGRP expression, which are neuropeptides that correlate with pain behavior and inflammation after peripheral nerve injury.

Neuronal soma-satellite glial cell interactions in sensory ganglia and the participation of purinergic receptors

It has been known for some time that the somata of neurons in sensory ganglia respond to electrical or chemical stimulation and release transmitters in a Ca2+-dependent manner. The function of the somatic release has not been well delineated. A unique characteristic of the ganglia is that each neuronal soma is tightly enwrapped by satellite glial cells (SGCs). The somatic membrane of a sensory neuron rarely makes synaptic contact with another neuron. As a result, the influence of somatic release on the activity of adjacent neurons is likely to be indirect and/or slow. Recent studies of neuron-SGC interactions have demonstrated that ATP released from the somata of dorsal root ganglion neurons activates SGCs. They in turn exert complex excitatory and inhibitory modulation of neuronal activity. Thus, SGCs are actively involved in the processing of afferent information. In this review, we summarize our understanding of bidirectional communication between neuronal somata and SGCs in sensory ganglia and its possible role in afferent signaling under normal and injurious conditions. The participation of purinergic receptors is emphasized because of their dominant roles in the communication.

Glial cells and chronic pain

Over the past few years, the control of pain exerted by glial cells has emerged as a promising target against pathological pain. Indeed, changes in glial phenotypes have been reported throughout the entire nociceptive pathway, from peripheral nerves to higher integrative brain regions, and pharmacological inhibition of such glial reactions reduces the manifestation of pain in animal models. This complex interplay between glia and neurons relies on various mechanisms depending both on glial cell types considered (astrocytes, microglia, satellite cells, or Schwann cells), the anatomical location of the regulatory process (peripheral nerve, spinal cord, or brain), and the nature of the chronic pain paradigm. Intracellularly, recent advances have pointed to the activation of specific cascades, such as mitogen-associated protein kinases (MAPKs) in the underlying processes behind glial activation. In addition, given the large number of functions accomplished by glial cells, various mechanisms might sensitize nociceptive neurons including a release of pronociceptive cytokines and neurotrophins or changes in neurotransmitter-scavenging capacity. The authors review the conceptual advances made in the recent years about the implication of central and peripheral glia in animal models of chronic pain and discuss the possibility to translate it into human therapies in the future.

Transgenic inhibition of glial NF-kappa B reduces pain behavior and inflammation after peripheral nerve injury

The transcription factor nuclear factor kappa B (NF-kappaB) is a key regulator of inflammatory processes in reactive glial cells. We utilized a transgenic mouse model (GFAP-IkappaBalpha-dn) where the classical NF-kappaB pathway is inactivated by overexpression of a dominant negative (dn) form of the inhibitor of kappa B (IkappaBalpha) in glial fibrillary acidic protein (GFAP)-expressing cells, which include astrocytes, Schwann cells, and satellite cells of the dorsal root ganglion (DRG) and sought to determine whether glial NF-kappaB inhibition leads to a reduction in pain behavior and inflammation following chronic constriction injury (CCI) of the sciatic nerve. As expected, following CCI nuclear translocation, and hence activation, of NF-kappaB was detected only in the sciatic nerve of wild type (WT) mice, and not in GFAP-IkappaBalpha-dn mice, while upregulation of GFAP was observed in the sciatic nerve and DRGs of both WT and GFAP-IkappaBalpha-dn mice, indicative of glial activation. Following CCI, mechanical and thermal hyperalgesia were reduced in GFAP-IkappaBalpha-dn mice compared to those in WT, as well as gene and protein expression of CCL2, CCR2 and CXCL10 in the sciatic nerve. Additionally, gene expression of TNF, CCL2, and CCR2 was reduced in the DRGs of transgenic mice compared to those of WT after CCI. We can therefore conclude that transgenic inhibition of NF-kappaB in GFAP-expressing glial cells attenuated pain and inflammation after peripheral nerve injury. These findings suggest that targeting the inflammatory response in Schwann cells and satellite cells may be important in treating neuropathic pain.

[Non-specific lower back pain: In search of the origin of pain]

Lower back pain is a condition considered benign and with a specific cause determined only in 15% of patients. In the past years this concept has varied, because many papers have described no benign condition leading to back pain, citing their capacity to cause disability. Through many different diagnostic techniques it is possible to identify the structures capable of producing back pain. This identification, and the level of evidence of the interventional techniques, is the aim of this paper.

The effects of inflammation on glial fibrillary acidic protein expression in satellite cells of the dorsal root ganglion

STUDY DESIGN

After undergoing L5 hemilaminectomy, chromic gut suture was placed onto the DRG and the animals were sacrificed at various time-points.

OBJECTIVE

The purpose of this study was to identify the effects of inflammation on satellite cells (SCs) of the dorsal root ganglion (DRG) by analyzing glial fibrillary acidic protein (GFAP) expression in of the DRG at various time points.

SUMMARY OF BACKGROUND DATA

SCs are neuroglial cells that closely interact with nerve cells of the DRG. The role of SC remains unknown GFAP expression increases in response to CNS injury. Loss of GFAP has impaired Schwann cell proliferation and delayed nerve regeneration after injury.

METHODS

Sixty rats underwent a left L5 hemilaminectomy. In Group I, a chromic-gut suture was place topically on the DRG (n = 30), Group II was the sham surgery group (n = 30). DRGs were harvested at 6, 24, 48, 72 hours, and 7 days after surgery. In Group III, 6 control rats were killed and their bilateral L5 DRG harvested. The harvested DRG were analyzed using light microscopy for SC immunoreactivity, using GFAP, HIS-36, TNF-alpha, IL-1alpha, IL-1beta, IL-6 monoclonal antibodies.

RESULTS

One hundred thirty-two DRGs were harvested for analysis. Naïve controls and neurons did not express GFAP. The SC sheath expressed GFAP as early as 6 hours postchromic gut application. In Group I, GFAP expression steadily increased after chromic-gut application with 100% of SC soma and SC sheaths being GFAP positive at 7 days. The contralateral DRG demonstrated delayed GFAP expression, with 83% of SC soma and SC sheaths were GFAP positive at 7 days. In Group II, 89% of sacs expressed GFAP by 7 compared to 79% in the contralateral undisturbed DRG.

CONCLUSION

Under physiologic conditions, the expression of GFAP by SCs is undetectable. As the inflammatory process develops, GFAP expression steadily increases with 100% of SCs being GFAP immunoreactive 7 days after chromic gut application. These data suggest that SCs are the primary source of GFAP in the DRG. We hypothesize that SC play an important role in the response to early inflammatory injury.

Early blockade of injured primary sensory afferents reduces glial cell activation in two rat neuropathic pain models

Satellite glial cells in the dorsal root ganglion (DRG), like the better-studied glia cells in the spinal cord, react to peripheral nerve injury or inflammation by activation, proliferation, and release of messengers that contribute importantly to pathological pain. It is not known how information about nerve injury or peripheral inflammation is conveyed to the satellite glial cells. Abnormal spontaneous activity of sensory neurons, observed in the very early phase of many pain models, is one plausible mechanism by which injured sensory neurons could activate neighboring satellite glial cells. We tested effects of locally inhibiting sensory neuron activity with sodium channel blockers on satellite glial cell activation in a rat spinal nerve ligation (SNL) model. SNL caused extensive satellite glial cell activation (as defined by glial fibrillary acidic protein [GFAP] immunoreactivity) which peaked on day 1 and was still observed on day 10. Perfusion of the axotomized DRG with the Na channel blocker tetrodotoxin (TTX) significantly reduced this activation at all time points. Similar findings were made with a more distal injury (spared nerve injury model), using a different sodium channel blocker (bupivacaine depot) at the injury site. Local DRG perfusion with TTX also reduced levels of nerve growth factor (NGF) in the SNL model on day 3 (when activated glia are an important source of NGF), without affecting the initial drop of NGF on day 1 (which has been attributed to loss of transport from target tissues). Local perfusion in the SNL model also significantly reduced microglia activation (OX-42 immunoreactivity) on day 3 and astrocyte activation (GFAP immunoreactivity) on day 10 in the corresponding dorsal spinal cord. The results indicate that early spontaneous activity in injured sensory neurons may play important roles in glia activation and pathological pain.

Painful facet joint injury induces neuronal stress activation in the DRG: implications for cellular mechanisms of pain

The cervical facet joint is implicated as one of the most common sources of chronic neck pain, owing to its rich nociceptive innervation and susceptibility to injurious mechanical loading. Injuries to the facet joint and its ligament can induce inflammation in the joint and spinal cord. Inflammatory molecules which are known to have a role in pain can also stimulate the integrated stress response (ISR). Therefore, we hypothesize that ISR is activated by facet joint injury in a rodent model of pain. To address this hypothesis, we assessed the expression of binding protein (BiP) (also known as growth-related protein 78 (GRP78)), a marker of endoplasmic reticulum stress response, in the dorsal root ganglion (DRG) after painful facet joint injury. In a rodent model of facet joint injury, dynamic distraction of the C6/C7 joint (injury, n=12) was imposed; sham procedures were performed separately (sham, n=8). Forepaw mechanical allodynia was assessed postoperatively for 7 days as a quantitative measure of pain symptoms. The C6 DRG was harvested and assessed for BiP expression using triple label immunofluorescent confocal microscopy and immunoblot analyses. BiP was significantly higher (p<0.001) in the DRG after injury than sham and was expressed predominantly in neurons. Similarly, quantification of BiP by immunoblot demonstrated a significant 2.1-fold increase (p=0.03) in injury compared to sham at day 7. Findings suggest neuronal stress activation is associated with painful facet joint injury, and that joint loading may directly mediate the behavior of DRG neurons in this class of injury.

Silencing the Kir4.1 potassium channel subunit in satellite glial cells of the rat trigeminal ganglion results in pain-like behavior in the absence of nerve injury

Growing evidence suggests that changes in the ion buffering capacity of glial cells can give rise to neuropathic pain. In the CNS, potassium ion (K+) buffering is dependent on the glia-specific inward rectifying K+ channel Kir4.1. We recently reported that the satellite glial cells that surround primary sensory neurons located in sensory ganglia of the peripheral nervous system also express Kir4.1, whereas the neurons do not. In the present study, we show that, in the rat trigeminal ganglion, the location of the primary sensory neurons for face sensation, specific silencing of Kir4.1 using RNA interference leads to spontaneous and evoked facial pain-like behavior in freely moving rats. We also show that Kir4.1 in the trigeminal ganglion is reduced after chronic constriction injury of the infraorbital nerve. These findings suggests that neuropathic pain can result from a change in expression of a single K+ channel in peripheral glial cells, raising the possibility of targeting Kir4.1 to treat pain in general and particularly neuropathic pain that occurs in the absence of nerve injury.

The nerve supply of the lumbar intervertebral disc

The anatomical studies, basic to our understanding of lumbar spine innervation through the sinu-vertebral nerves, are reviewed. Research in the 1980s suggested that pain sensation was conducted in part via the sympathetic system. These sensory pathways have now been clarified using sophisticated experimental and histochemical techniques confirming a dual pattern. One route enters the adjacent dorsal root segmentally, whereas the other supply is non-segmental ascending through the paravertebral sympathetic chain with re-entry through the thoracolumbar white rami communicantes. Sensory nerve endings in the degenerative lumbar disc penetrate deep into the disrupted nucleus pulposus, insensitive in the normal lumbar spine. Complex as well as free nerve endings would appear to contribute to pain transmission. The nature and mechanism of discogenic pain is still speculative but there is growing evidence to support a 'visceral pain' hypothesis, unique in the muscloskeletal system. This mechanism is open to 'peripheral sensitisation' and possibly 'central sensitisation' as a potential cause of chronic back pain.

Sensory innervation of lumbar vertebral bodies in rats

STUDY DESIGN

Using a retrograde neurotracing method with Fluoro-Gold (FG), the level at which dorsal root ganglions (DRGs) innervate the L2 and L5 vertebral bodies and the innervation pathways were investigated in rats.

OBJECTIVE

To clarify the levels at which DRGs innervate the lumbar vertebral bodies and to determine the pathways from the L2 and L5 vertebral bodies to DRGs.

SUMMARY OF BACKGROUND DATA

Elderly patients with osteoporosis sometimes experience lumbar vertebral fracture and may also feel diffuse nonlocalized pain in the back, lateral portion of the trunk, and area surrounding the iliac crest. However, the pattern of sensory innervation of vertebral bodies remains unclear.

METHODS

Forty female Sprague-Dawley rats were used. FG crystals were applied to the L2 (L2 vertebra group) or L5 (L5 vertebra group) vertebral bodies via an anterior approach, and numbers of labeled neurons in DRGs from T10 to L6 were counted. To determine sensory pathways, bilateral sympathectomy was performed.

RESULTS

In nonsympathectomy animals, FG-labeled neurons were present in DRGs from T11 through L3 in the L2 vertebra group and from T13 through L6 in the L5 vertebra group. The number of labeled neurons following sympathectomy was not significantly different in L1, L2, and L3 DRGs in the L2 vertebra group or in L3, L4, L5, and L6 DRGs in the L5 vertebra group from those in nonsympathectomy animals. In contrast, fewer labeled DRG neurons were present in sympathectomy animals at T11, T12, and T13 in the L2 vertebra group, and at T13, L1, and L2 in the L5 vertebra group than in nonsympathectomy animals (P < 0.01).

CONCLUSION

Sensory nerve fibers in the L2 and L5 vertebral bodies are derived from the T11-L3 and T13-L6 DRGs, respectively. Some sensory nerves from the L2 and L5 vertebral bodies enter the paravertebral sympathetic trunks and reach the DRGs at multisegmental levels. The present findings regarding multisegmental innervation to vertebral bodies may explain the diffuse pain that originates within osteoporotic vertebral fractures in elderly patients.

Characteristics of Sensory Dorsal Root Ganglia Neurons Innervating the Lumbar Vertebral Body in Rats

Characteristics of sensory dorsal root ganglia (DRG) neurons innervating the L5 vertebral body were investigated in rats by using a retrograde neurotransport method, lectin affinity- and immuno-histochemistry to further elucidate the causes of diffuse pain suffered by some elderly patients in their back, lateral trunk, and iliac crest, after lumbar osteoporotic vertebral fracture. We used calcitonin gene-related peptide (CGRP) as a marker of small peptide-containing neurons and the glycoprotein binding the isolectin from Griffonia simplicifolia (IB4) as a marker of small non-peptide-containing neurons. Neurons innervating the L5 vertebral bodies, retrogradely labeled with fluoro-gold (FG), were distributed throughout DRGs from T13 to L6. The proportion of CGRP-immunoreactive (IR) FG-labeled neurons was 32%. The proportion of IB4-binding FG-labeled neurons was significantly smaller, at 4%. Other neurons that were non-CGRP-IR and non-IB4-binding were mostly large neurons, and they may transmit proprioception from vertebral bodies. Most neurons transmitting pain are CGRP-IR peptide-containing neurons. They may have a more significant role in pain sensation in the vertebral bodies as peptidergic DRG neurons.

PERSPECTIVE

This article shows that vertebral bodies are innervated by CGRP-IR neurons. CGRP-IR neurons may play a role in pain sensation through peptidergic DRG neurons. These findings contribute to an understanding of pain associated with the vertebral body such as tumor, infection, or osteoporotic fracture.

Up-regulation of p55 TNF alpha-receptor in dorsal root ganglia neurons following lumbar facet joint injury in rats

The rat L5/6 facet joint is multisegmentally innervated from the L1 to L6 dorsal root ganglia (DRG). Tumor necrosis factor (TNF) is a known mediator of inflammation. It has been reported that satellite cells are activated, produce TNF and surround DRG neurons innervating L5/6 facet joints after facet injury. In the current study, changes in TNF receptor (p55) expression in DRG neurons innervating the L5/6 facet joint following facet joint injury were investigated in rats using a retrograde neurotransport method followed by immunohistochemistry. Twenty rats were used for this study. Two crystals of Fluorogold (FG; neurotracer) were applied into the L5/6 facet joint. Seven days after surgery, the dorsal portion of the capsule was cut in the injured group (injured group n = 10). No injury was performed in the non-injured group (n = 10). Fourteen days after the first application of FG, bilateral DRGs from T13 to L6 levels were resected and sectioned. They were subsequently processed for p55 immunohistochemistry. The number of FG labeled neurons and number of FG labeled p55-immunoreactive (IR) neurons were counted. FG labeled DRG neurons innervating the L5/6 facet joint were distributed from ipsilateral L1 to L6 levels. Of FG labeled neurons, the ratio of DRG neurons immunoreactive for p55 in the injured group (50%) was significantly higher than that in the non-injured group (13%). The ratio of p55-IR neurons of FG labeled DRG neurons was significantly higher in total L1 and L2 DRGs than that in total L3, 4, 5 and 6 DRGs in the injured group (L1 and 2 DRG, 67%; L3, 4, 5 and 6 DRG, 37%, percentages of the total number of p55-IR neurons at L1 and L2 level or L3-6 level/the total number of FG-labeled neurons at L1 and L2 level or L3-6 level). These data suggest that up-regulation of p55 in DRG neurons may be involved in the sensory transmission from facet joint injury. Regulation of p55 in DRG neurons innervating the facet joint was different between upper DRG innervated via the paravertebral sympathetic trunks and lower DRG innervated via other direct routes.