Roles of thromboxane A2 and leukotriene B4 in radicular pain induced by herniated nucleus pulposus

The Immune Privilege of the Intervertebral Disc: Implications for Intervertebral Disc Degeneration Treatment

The intervertebral disc (IVD) is the largest avascular organ of the body. It is composed of three parts: the nucleus pulposus (NP), the annulus fibrosus (AF) and the cartilaginous endplate (CEP). The central NP is surrounded by the AF and sandwiched by the two CEPs ever since its formation. This unique structure isolates the NP from the immune system of the host. Additionally, molecular factors expressed in IVD have been shown inhibitive effect on immune cells and cytokines infiltration. Therefore, the IVD has been identified as an immune privilege organ. The steady state of immune privilege is fundamental to the homeostasis of the IVD. The AF and the CEP, along with the immunosuppressive molecular factors are defined as the blood-NP barrier (BNB), which establishes a strong barrier to isolate the NP from the host immune system. When the BNB is damaged, the auto-immune response of the NP occurs with various downstream cascade reactions. This effect plays an important role in the whole process of IVD degeneration and related complications, such as herniation, sciatica and spontaneous herniated NP regression. Taken together, an enhanced understanding of the immune privilege of the IVD could provide new targets for the treatment of symptomatic IVD disease. However, the underlying mechanism above is still not fully clarified. Accordingly, the current study will extensively review and discuss studies regarding the immune privilege of the IVD.

Lumbar radiculopathy and its neurobiological basis

Lumbar radiculopathy, a group of diseases in which the dorsal root ganglia (DRG) or dorsal roots are adversely affected by herniated discs or spinal stenosis, are clinically characterized by spontaneous and evoked types of pain. The pain is underpinned by various distinct pathophysiological mechanisms in the peripheral and central nervous systems. However, the diagnosis of lumbar radiculopathy is still unsatisfactory, because the association of the pain with the neurobiological basis of radiculopathy is largely unknown. Several animal models used to explore the underlying neurobiological basis of lumbar radiculopathy could be classified as mechanical, chemical, or both based on the component of injury. Mechanical injury elevates the intraneural pressure, reduces blood flow, and eventually establishes ischemia in the dorsal root and the DRG. Ischemia may induce ischemic pain and cause nerve damage or death, and the subsequent nerve damage or death may induce neuropathic pain. Chemical injury predominately induces inflammation surrounding the dorsal roots or DRG and consequent inflammatory mediators cause inflammatory pain. Furthermore, DRG neurons sensitized by inflammatory mediators are hypersensitive to innocuous mechanical force (stretch or compression) and responsible for mechanical allodynia in radiculopathy. As well, central sensitization in the spinal cord may play an important role in pain generation in lumbar radiculopathy. Increasing knowledge of pain-generating mechanisms and their translation into clinical symptoms and signs might allow for dissecting the mechanisms that operate in each patient. With precise clinical phenotypic characterization of lumbar radiculopathy and its connection to a specific underlying mechanism, we should be able to design optimal treatments for individuals. This review discusses the present knowledge of lumbar radiculopathy and proposes a novel mechanism-based classification.

Efficacy of epidural neuroplasty versus transforaminal epidural steroid injection for the radiating pain caused by a herniated lumbar disc

Objective

To compare the treatment effects of epidural neuroplasty (NP) and transforaminal epidural steroid injection (TFESI) for the radiating pain caused by herniated lumbar disc.

Methods

Thirty-two patients diagnosed with herniated lumbar disc through magnetic resonance imaging or computed tomography were included in this study. Fourteen patients received an epidural NP and eighteen patients had a TFESI. The visual analogue scale (VAS) and functional rating index (FRI) were measured before the treatment, and at 2 weeks, 4 weeks and 8 weeks after the treatment.

Results

In the epidural NP group, the mean values of the VAS before the treatment, and at 2 weeks, 4 weeks and 8 weeks after the treatment were 7.00±1.52, 4.29±1.20, 2.64±0.93, 1.43±0.51 and those of FRI were 23.57±3.84, 16.50±3.48, 11.43±2.44, 7.00±2.15. In the TFESI group, the mean values of the VAS before the treatment, and at 2 weeks, 4 weeks and 8 weeks after the treatment were 7.22±2.05, 4.28±1.67, 2.56±1.04, 1.33±0.49 and those of FRI were 22.00±6.64, 16.22±5.07, 11.56±4.18, 8.06±1.89. During the follow-up period, the values of VAS and FRI within each group were significantly reduced (p<0.05) after the treatment. But there were no significant differences between the two groups statistically.

Conclusion

Epidural NP and TFESI are equally effective treatments for the reduction of radiating pain and for improvement of function in patients with a herniated lumbar disc. We recommend that TFESI should be primarily applied to patients who need interventional spine treatment, because it is easier and more cost-effective than epidural NP.

Immediate pain response does not predict long-term outcome of CT-guided cervical transforaminal epidural steroid injections

BACKGROUND AND PURPOSE

Imaging-guided cervical transforaminal epidural steroid injections have been shown to decrease verbal numerical pain scores and improve functionality (Roland Morris Disability Index). These injections are often administered in combination with local anesthetic. The purpose of this study was to determine if the immediate postprocedure VNPS predicts the long-term effectiveness of the injection.

MATERIALS AND METHODS

A quality assurance data base review of 247 patient records was used to document the VNPS and RMDI of patients undergoing a single CT-guided CTESI. Pain scores were recorded before the procedure, immediately after the procedure, at 2 weeks, and at 2 months. The RMDI was recorded before the procedure, at 2 weeks, and at 2 months. Spearman rank correlation analysis and logistic regression models were used to determine if the immediate postprocedure or 2-week VNPS correlated with or predicted the longer-term VNPS and RMDI as measured at 2 weeks and 2 months.

RESULTS

There was not a strong correlation between the pain score obtained immediately after the procedure and the 2-month outcome of the VNPS or RMDI. The pain scores at 2 weeks did correlate with the 2-month outcomes. The 2-week VNPS also was a significant predictor of patients who would achieve a >50% improvement in VNPS or RMDI at 2 months.

CONCLUSIONS

Pain scores obtained immediately after completion of a single CT-guided CTESI do not predict the long-term effectiveness of this procedure. However, patient response at 2 weeks does correlate with the long-term effectiveness of these injections as measured by the VNPS and the RMDI.

Sensory and signaling mechanisms of bradykinin, eicosanoids, platelet-activating factor, and nitric oxide in peripheral nociceptors

Peripheral mediators can contribute to the development and maintenance of inflammatory and neuropathic pain and its concomitants (hyperalgesia and allodynia) via two mechanisms. Activation or excitation by these substances of nociceptive nerve endings or fibers implicates generation of action potentials which then travel to the central nervous system and may induce pain sensation. Sensitization of nociceptors refers to their increased responsiveness to either thermal, mechanical, or chemical stimuli that may be translated to corresponding hyperalgesias. This review aims to give an account of the excitatory and sensitizing actions of inflammatory mediators including bradykinin, prostaglandins, thromboxanes, leukotrienes, platelet-activating factor, and nitric oxide on nociceptive primary afferent neurons. Manifestations, receptor molecules, and intracellular signaling mechanisms of the effects of these mediators are discussed in detail. With regard to signaling, most data reported have been obtained from transfected nonneuronal cells and somata of cultured sensory neurons as these structures are more accessible to direct study of sensory and signal transduction. The peripheral processes of sensory neurons, where painful stimuli actually affect the nociceptors in vivo, show marked differences with respect to biophysics, ultrastructure, and equipment with receptors and ion channels compared with cellular models. Therefore, an effort was made to highlight signaling mechanisms for which supporting data from molecular, cellular, and behavioral models are consistent with findings that reflect properties of peripheral nociceptive nerve endings. Identified molecular elements of these signaling pathways may serve as validated targets for development of novel types of analgesic drugs.

Abnormal spontaneous activities on needle electromyography and their relation with pain behavior and nerve fiber pathology in a rat model of lumbar disc herniation

STUDY DESIGN

This longitudinal experimental study was conducted to investigate electrophysiologic characteristics, pain behavior, and histological changes in a rat model of lumbar disc herniation.

OBJECTIVE

To observe abnormal spontaneous activity (ASA) on needle electromyography (EMG) and to determine its relation with neuropathic pain behavior and histological changes longitudinally in a rat model of lumbar disc herniation.

SUMMARY OF BACKGROUND DATA

Needle EMG is generally performed to determine the existence and the degree of radiculopathy caused by disc herniation. The local application of autologous nucleus pulposus to the spinal nerve has been shown to induce neuropathic pain. However, little is known about the relations between neuropathic pain and abnormal EMG findings and the manner in which they change with time in rat models of lumbar disc herniation.

METHODS

Twenty-five Sprague-Dawley rats were randomly assigned to either sham or experimental groups. In the experimental group, autologous nucleus pulposus was grafted on the left L5 dorsal root ganglion. All rats were evaluated for mechanical allodynia and thermal hyperalgesia and underwent needle EMG examinations before and on days 1, 5, 10, 20, 30, 40, and 50 after surgery. Morphologic changes of L5 spinal nerves and of sciatic nerves were assessed by toluidine blue staining on days 1, 5, and 50 after surgery.

RESULTS

A dramatic decrease in mechanical withdrawal threshold and in thermal withdrawal latency was observed on day 1 after surgery, and these changes persisted until day 50 after surgery. ASAs on needle EMG were observed on day 1 (33%), peaked on day 5 (93%), and gradually decreased from day 10 (69%) to day 40 (18%) after surgery. Pathologic findings of nerve fibers, such as swelling of myelin sheaths, demyelination, and degeneration of axoplasms were observed from day 1. These findings were exaggerated on day 5 and then diminished but were still evident on day 50.

CONCLUSION

Neuropathic pain and pathologic changes in spinal nerve fibers probably remain even after ASAs in EMG have disappeared in our rat model of lumbar disc herniation. These results provide baseline data concerning the natural courses of electrophysiologic findings and of radicular pain in patients with intervertebral disc herniation.

CJ-13610, an orally active inhibitor of 5-lipoxygenase is efficacious in preclinical models of pain

Zileuton, a redox and iron chelator 5-lipoxygenase (5-LOX) inhibitor and, leukotriene receptor antagonists are presently used clinically in the long term treatment of asthma. Recent data implicate 5-LOX pathway in pain signaling. We report 5-LOX expression in the central nervous system (CNS) and analyze the pain efficacy of a new class of non redox, non iron chelating 5-LOX inhibitor. CJ-13610, 4-(3-(4-(2-methyl-1H-imidazol-1-yl) phenylthio) phenyl)-tetrahydro-2H-pyran-4-carboxamide, demonstrated antihyperalgesic activity in inflammatory pain models including the acute carrageenan model and the chronic inflammatory model using complete Freund's adjuvant. Following complete Freund's adjuvant stimulus leukotrieneB(4) concentration in the brain was elevated (9+/-1 ng/g, mean+/-S.E.M.) by about 3 times that of the control group (3+/-0.11, mean+/-S.E.M.). Hyperalgesia and leukotrieneB(4) concentration were both reversed following CJ-13610 treatment. Furthermore, we demonstrate CJ-13610 efficacy against osteoarthritis like pain using the rat medial meniscal transection model. CJ-13610 at oral doses of 0.6, 2 and 6 mg/kg/day reversed two modalities of pain in this model; tactile allodynia and weight bearing differential. Taken together, these data suggest that 5-LOX pathway and the leukotriene products are important mediators of pain.

Tumor necrosis factor-alpha in the nucleus pulposus mediates radicular pain, but not increase of inflammatory peptide, associated with nerve damage in mice

STUDY DESIGN

Changes in behavior and the immunohistochemistry of dorsal root ganglion (DRG) neurons were examined using a mouse model of radicular pain.

OBJECTIVE

To examine the effects of TNF-alpha in the nucleus pulposus (NP) on nerve roots.

SUMMARY OF BACKGROUND DATA

Radicular pain is induced by mechanical compression and inflammation of nerve roots. Many authors have reported that following disc herniation, producing TNF-alpha plays a major role in neuropathic pain. Their findings suggest that TNF-alpha contained in the NP is significant in the development of pain and nerve root degeneration, but it has not been clearly demonstrated.

METHODS

Wild-type NPs or TNF-KO NPs, which were harvested from C57BL/6 mice (wild-type NP) or TNF-knock-out mice (TNF-KO NP), were applied to the left sciatic nerves of 30 wild-type mice, and the nerves were pinched. Production of hind paw mechanical allodynia, activating transcription factor 3, and calcitonin gene- related peptide (CGRP) were assessed.

RESULTS

Animals receiving a NP application demonstrated significant mechanical allodynia compared to the pinch-only and the control groups. The degree of mechanical allodynia was greater in the wild-type than in the TNF-KO group. The number of activating transcription factor 3 immunoreactive neurons was significantly higher in the wild-type than in the TNF-KO group. The number of CGRP-immunoreactive neurons was higher in the wild-type and TNF-KO than in the control groups. However, no significant difference in activity was observed between both CGRP positive groups.

CONCLUSION

In this study TNF-alpha contained in the NP was important for the production of radicular pain accompanied by long-lasting degeneration of DRG neurons. However, other cytokines in the NP and nerve compression may also play important roles in pain transmission. In this model system, TNF-alpha in the NP appears to mediate pain, but not cause an increase in CGRP in the DRG neurons.

"Listening" and "talking" to neurons: implications of immune activation for pain control and increasing the efficacy of opioids

It is recently become clear that activated immune cells and immune-like glial cells can dramatically alter neuronal function. By increasing neuronal excitability, these non-neuronal cells are now implicated in the creation and maintenance of pathological pain, such as occurs in response to peripheral nerve injury. Such effects are exerted at multiple sites along the pain pathway, including at peripheral nerves, dorsal root ganglia, and spinal cord. In addition, activated glial cells are now recognized as disrupting the pain suppressive effects of opioid drugs and contributing to opioid tolerance and opioid dependence/withdrawal. While this review focuses on regulation of pain and opioid actions, such immune-neuronal interactions are broad in their implications. Such changes in neuronal function would be expected to occur wherever immune-derived substances come in close contact with neurons.

Is there any relationship between proinflammatory mediator levels in disc material and myelopathy with cervical disc herniation and spondylosis? A non-randomized, prospective clinical study

The proinflammatory mediator (PIM) levels were assessed in surgically removed samples of herniated cervical intervertebral discs. The objective of this study was to investigate if there is a correlation between the levels of PIMs in disc material and myelopathy associated with cervical intervertebral disc herniation and spondylosis. The role of proinflammatory mediators in the degeneration of intervertebral disc and the inflammatory effects of disc herniations on radicular pain has been previously published. However, the possible relationship between PIMs and myelopathy related to cervical disc herniation and spondylosis has not been investigated before. Thirty-two patients undergoing surgery for cervical disc herniation and spondylosis were investigated. Surgically obtained disc materials, stored at 70 degrees C, were classified into two groups: cervical disc herniation alone or with myelopathy. Biochemical preparation and solid phase enzyme amplified sensitivity immunoassay (ELISIA) analysis of the samples were performed to assess the concentration of mediators in the samples. Very similar values of interleukin-6 were found in both groups whereas the concentrations of mediators were significantly higher in myelopathy group. This study has demonstrated that PIMs are involved in cervical intervertebral disc degeneration with higher concentrations in the samples associated with myelopathy.

Thromboxane A2 induces itch-associated responses through TP receptors in the skin in mice

Thromboxane A2 (TXA2), a metabolite of arachidonic acid produced by cyclooxygenase and thromboxane synthase, is thought to participate in chronic dermatitis. This study investigated the involvement of TXA2 in cutaneous itch. An intradermal injection of U-46619, a stable analogue of TXA2, elicited scratching, an itch-associated response, in mice. Dose-response curve was bell shaped with a maximum effect at 10 nmol per site. The action of U-46619 was inhibited by a coinjection of the TP antagonist ONO-3708 and was abolished by TP receptor deficiency. TP receptor was mainly expressed in nerve fiber in the skin and keratinocytes. Thromboxane synthase was also expressed in keratinocytes. U-46619 increased intracellular Ca2+ ion concentration in primary cultures of dorsal root ganglion neurons and keratinocytes. The results suggest that TXA2 synthesized by keratinocytes acts as an itch mediator. It may elicit itch through the activation of TP receptors on primary afferents and keratinocytes; keratinocytes may produce itch mediators including TXA2. Thus, thromboxane synthase inhibitor and TP receptor antagonists will be candidates for antipruritic medicines.

Nerve fiber ingrowth into scar tissue formed following nucleus pulposus extrusion in the rabbit anular-puncture disc degeneration model: effects of depth of puncture

STUDY DESIGN

In vivo histologic study of nerve ingrowth in the rabbit anular-puncture disc degeneration model.

OBJECTIVE

To examine innervation of intervertebral discs and surrounding tissues in the rabbit anular-puncture model with various magnitudes of injury.

SUMMARY OF BACKGROUND DATA

A rabbit anular-puncture model of disc degeneration was recently established. However, to our knowledge, the neuropathologic changes in the degenerated disc and surrounding tissues in this model have not been examined.

METHODS

Anular punctures with an 18-gauge needle at depths of 1 and 5 mm were randomly performed at the L2-L3 or L4-L5 discs in New Zealand white rabbits (n = 16; 3.0-3.5 kg). The degree of disc degeneration was followed radiographically, and was further evaluated by magnetic resonance imaging and histology at sacrifice 4 or 12 weeks after the puncture. To identify nerve ingrowth into the discs, the L2-L3 to L4-L5 discs were immunostained with an antibody against protein gene product 9.5, a general marker for nerve fibers.

RESULTS

Significant decreases in disc height and signal intensity in magnetic resonance imaging were observed only in the 5 mm punctured discs (P < 0.05). In all the discs, including nonpunctured control discs, protein gene product 9.5-immunoreactive fibers were only occasionally observed in the outermost part of the anulus fibrosus. In the 5 mm punctured discs, nucleus pulposus tissues were extruded, and scar tissues formed outside the discs. No nerve ingrowth into the needle track lesion at the anular puncture site was observed. However, protein gene product 9.5-immunoreactive fibers were observed in scar tissues on the surface of the puncture site. This finding was more evident in the 5 mm than in the 1 mm punctured discs (P < 0.05).

CONCLUSIONS

This study indicates that in the rabbit anular-puncture disc degeneration model, disc degeneration associated with a higher nerve growth into the scar tissue was more evident when induced by a 5 mm than a 1 mm puncture. Although nerve ingrowth was observed in the extruded disc tissue, nerve ingrowth into the outer anulus, which has been reported in patients with discogenic pain, was not observed during the short observation period in this disc degeneration model. The limitation in assessing pain by behavior analysis or histologic evaluation of nerve ingrowth should be considered. Further studies to identify a surrogate marker of pain should be encouraged.

Role of TNF-alpha in sensitization of nociceptive dorsal horn neurons induced by application of nucleus pulposus to L5 dorsal root ganglion in rats

Herniation of the nucleus pulposus (NP) from lumbar intervertebral discs commonly results in radiculopathic pain and paresthesia (sciatica). While traditionally considered the result of mechanical compression of the dorsal root ganglion (DRG) and/or spinal nerve root, recent studies implicate pro-inflammatory mediators released from or evoked by NP, a possibility that was presently investigated. Single-unit recordings were made from L5 wide dynamic range dorsal horn neurons in pentobarbital-anesthetized rats. Autologous NP was harvested from a coccygeal disc and placed onto the exposed L5 DRG. A control group had subcutaneous adipose tissue or saline placed similarly. To test involvement of tumor necrosis factor-alpha (TNF-alpha), a third group received autologous NP plus local soluble TNF-alpha receptor type 1 (0.013 microg) which binds TNF-alpha to prevent its action. In each group, neuronal responses to graded heat (38-50 degrees C) and mechanical (von Frey filaments 4-76 g) stimuli were recorded prior to and at three successive hourly intervals following each treatment. Responses to noxious heat and mechanical stimuli were significantly enhanced 1 h post-NP and remained elevated thereafter. Thermally and mechanically evoked responses were not significantly affected in control rats or those treated with NP + soluble TNF-alpha receptor type 1. These results indicate that sensitization of nociceptive spinal neuronal responses develops quickly following exposure of the DRG to NP, and that TNF-alpha is involved. This electrophysiological model of herniated NP may prove useful in further characterizing the role of inflammatory mediators in hyperalgesia and allodynia resulting from lumbar disc herniation.

Beyond neurons: evidence that immune and glial cells contribute to pathological pain states

Chronic pain can occur after peripheral nerve injury, infection, or inflammation. Under such neuropathic pain conditions, sensory processing in the affected body region becomes grossly abnormal. Despite decades of research, currently available drugs largely fail to control such pain. This review explores the possibility that the reason for this failure lies in the fact that such drugs were designed to target neurons rather than immune or glial cells. It describes how immune cells are a natural and inextricable part of skin, peripheral nerves, dorsal root ganglia, and spinal cord. It then examines how immune and glial activation may participate in the etiology and symptomatology of diverse pathological pain states in both humans and laboratory animals. Of the variety of substances released by activated immune and glial cells, proinflammatory cytokines (tumor necrosis factor, interleukin-1, interleukin-6) appear to be of special importance in the creation of peripheral nerve and neuronal hyperexcitability. Although this review focuses on immune modulation of pain, the implications are pervasive. Indeed, all nerves and neurons regardless of modality or function are likely affected by immune and glial activation in the ways described for pain.

Epidural injection of cyclooxygenase-2 inhibitor attenuates pain-related behavior following application of nucleus pulposus to the nerve root in the rat

Cyclooxygenase-2 (COX-2), the inducible isoform of COX, has been identified as the key enzyme to regulate prostaglandin E2 synthesis in inflammatory conditions. Although it has been reported that COX-2 is present in herniated disc samples obtained from patients, little is known concerning the relationships between COX-2 and painful radiculopathy. The purpose of this study was to evaluate whether epidural injection of COX-2 inhibitor abolishes hyperalgesia induced by nucleus pulposus, which is a pain-related behavior in the rat. Rats, in which nucleus pulposus was relocated on the nerve root, exhibited evidence of mechanical hyperalgesia. Epidural injection of COX-2 inhibitor resulted in decrease in mechanical hyperalgesia 1 h, 3 and 7 days after the epidural injection of COX-2 inhibitor (0.1 mg/kg SC-'236 dissolved in the vehicle). There were no significant differences in sensitivity to thermal noxious stimuli after either application of the nucleus pulposus or epidural injections. These results suggest that prostaglandins and thromboxane, which are produced by COX-2 in inflammatory cells, appear to be related to the inflammatory process produced by application of nucleus pulposus to the nerve root. It is possible that COX-2 plays a significant role in painful radiculopathy following herniated nucleus pulposus.