Mechanical compression of the lumbar nerve root alters pain-related behaviors induced by the nucleus pulposus in the rat

The Effect of Melatonin on Radicular Pain in a Rat Model of Lumbar Disc Herniation

STUDY DESIGN

Controlled, randomized, animal study.

OBJECTIVE

To investigate the effect of melatonin and its receptors on radicular pain and the possible mechanisms.

SUMMARY OF BACKGROUND DATA

Lumbar disc herniation (LDH) may induce radicular pain, but the mechanism is not clear and therapeutic effect is still poor. Previously we report central sensitization meaning potentiation of spinal nociceptive synaptic transmission is the critical cause of radicular pain. Melatonin (Mel) has been reported to promote hippocampal synaptic transmission and thus improve learning ability. But the effect of Mel on spinal synaptic transmission and radicular pain are not clear.

METHODS

Rat LDH model was induced by autologous nucleus pulposus (NP) implantation. Melatonin was delivered intraperitoneally four times a day, from day 1 to day 3 after surgery. Melatonin receptor agonist and antagonists were delivered intrathecally for 3 days as well. Mechanical and thermal pain thresholds were assessed by von Frey filaments and hotplate test respectively. Electrophysiological recording was employed for survey C-fiber evoked field potentials. The protein level of N- methyl-D-aspartate submit 2A (NR2A), NR2B, melatonin receptor 1 (MT1), and receptor 2 (MT2) was evaluated by western blotting. Spinal expression of calcitonin gene related peptides (CGRP), isolectin b4 (IB4), and neurofilament-200 (NF200) was displayed by immunofluorescence staining.

RESULTS

Melatonin significantly increased mechanical and thermal pain thresholds, lasting at least to day 5 after surgery. Melatonin decreased C-fiber evoked field potentials; decreased spinal NR2B protein level; reduced spinal CGRP, and IB4 expression. MT2 was upregulated after NP implantation and was co-localized with neuron and microglia. MT2 receptor agonist simulated the effect of Mel, and both MT receptor broadspectrum antagonist and MT2 specific antagonist abolished the effect of MT2 receptor agonist.

CONCLUSION

Melatonin alleviates radicular pain from LDH by inhibiting central sensitization via binding with its receptor 2, decreasing spinal CGRP, IB4, and NR2B expression.

ISSLS PRIZE IN BASIC SCIENCE 2018: Growth differentiation factor-6 attenuated pro-inflammatory molecular changes in the rabbit anular-puncture model and degenerated disc-induced pain generation in the rat xenograft radiculopathy model

PURPOSE

To elucidate the effects of growth differentiation factor-6 (GDF6) on: (i) gene expression of inflammatory/pain-related molecules and structural integrity in the rabbit intervertebral disc (IVD) degeneration model, and (ii) sensory dysfunction and changes in pain-marker expression in dorsal nerve ganglia (DRGs) in the rat xenograft radiculopathy model.

METHODS

Forty-six adolescent rabbits received anular-puncture in two non-consecutive lumbar IVDs. Four weeks later, phosphate-buffered saline (PBS) or GDF6 (1, 10 or 100 µg) was injected into the nucleus pulposus (NP) of punctured discs and followed for 4 weeks for gene expression analysis and 12 weeks for structural analyses. For pain assessment, eight rabbits were sacrificed at 4 weeks post-injection and NP tissues of injected discs were transplanted onto L5 DRGs of 16 nude rats to examine mechanical allodynia. The rat DRGs were analyzed immunohistochemically.

RESULTS

In GDF6-treated rabbit NPs, gene expressions of interleukin-6, tumor necrosis factor-α, vascular endothelial growth factor, prostaglandin-endoperoxide synthase 2, and nerve growth factor were significantly lower than those in the PBS group. GDF6 injections resulted in partial restoration of disc height and improvement of MRI disc degeneration grades with statistical significance in rabbit structural analyses. Allodynia induced by xenograft transplantation of rabbit degenerated NPs onto rat DRGs was significantly reduced by GDF6 injection. Staining intensities for ionized calcium-binding adaptor molecule-1 and calcitonin gene-related peptide in rat DRGs of the GDF6 group were significantly lower than those of the PBS group.

CONCLUSION

GDF6 injection may change the pathological status of degenerative discs and attenuate degenerated IVD-induced pain.

Up-Regulation of Pain Behavior and Glial Activity in the Spinal Cord after Compression and Application of Nucleus Pulposus onto the Sciatic Nerve in Rats

Study Design

Experimental animal study.

Purpose

To evaluate pain-related behavior and changes in glial activity in the spinal dorsal horn after combined sciatic nerve compression and nucleus pulposus (NP) application in rats.

Overview of Literature

Mechanical compression and inflammation caused by prostaglandins and cytokines at disc herniation sites induce pain. Structural changes and pain-associated cytokines in the dorsal root ganglia and spinal dorsal horn contribute to prolonged pain. Glial cells in the spinal dorsal horn may also function in pain transmission.

Methods

The sciatic nerve was compressed with NP for 2 seconds using forceps in the NP+nerve compression group; the sham-operated group received neither compression nor NP; and the control group received no operation. Mechanical hyperalgesia was measured for 3 weeks using von Frey filaments. Glial activity in the spinal dorsal horn was examined 7 days and 14 days postsurgery using anti-glial fibrillary acidic protein and anti-Ionized calcium binding adaptor molecule-1 antibodies to detect astrocytes and microglia, respectively.

Results

Mechanical hyperalgesia was detected throughout the 14-day observation in the NP+nerve compression group, but not in control or sham-operated groups (p<0.05). Both astrocytes and microglia were significantly increased in the spinal dorsal horn of the NP+nerve compression group compared to control and sham groups on days 7 and 14 (p<0.05).

Conclusions

Nerve compression with NP application produces pain-related behavior, and up-regulates astrocytes and microglia in the spinal dorsal horn, suggesting that these glia may be related to pain transmission.

Increase of sodium channels (nav 1.8 and nav 1.9) in rat dorsal root ganglion neurons exposed to autologous nucleus pulposus

Purpose:

It has been assumed that nucleus pulposus-induced activation of the dorsal root ganglion (DRG) may be related to an activation of sodium channels in the DRG neurons. In this study we assessed the expression of Nav 1.8 and Nav 1.9 following disc puncture.

Method:

Thirty female Sprague-Dawley rats were used. The L4/L5 disc was punctured by a needle (n=12) and compared to a sham group without disc puncture (n=12) and a naive group (n=6). At day 1 and 7, sections of the left L4 DRG were immunostained with anti-Nav 1.8 and Nav 1.9 antibodies.

Result:

At day 1 after surgery, both Nav 1.8-IR neurons and Nav 1.9-IR neurons were significantly increased in the disc puncture group compared to the sham and naive groups (p<0.05), but not at day 7.

Conclusion:

The findings in the present study demonstrate a neuronal mechanism that may be of importance in the pathophysiology of sciatic pain in disc herniation.

Evaluation of pain behavior and calcitonin gene-related peptide immunoreactive sensory nerve fibers in the spinal dorsal horn after sciatic nerve compression and application of nucleus pulposus in rats

STUDY DESIGN

Animal study.

OBJECTIVE

To evaluate pain behavior and neuropeptide changes in the spinal dorsal horn after sciatic nerve compression and application of nucleus pulposus (NP) in rats.

SUMMARY OF BACKGROUND DATA

The pathomechanisms of lumbar disc herniation pain have not been fully elucidated. Pain-associated neuropeptides, including substance P and calcitonin gene-related peptide (CGRP), are produced in dorsal root ganglion neurons and transported to spinal dorsal horn nerve terminals where they function in pain transmission. However, changes in CGRP-immunoreactive (IR) sensory nerve terminals have not been reported in models of disc herniation. This study evaluated pain-related behavior and changes in CGRP-IR terminals in the spinal dorsal horn after combined sciatic nerve compression and NP application.

METHODS

Five groups of rats underwent either sciatic nerve compression with NP (n = 20), application of NP only (n = 20), nerve compression only (n = 20), and sham operation with neither compression nor NP (n = 20) or no operation (controls, n = 20). Mechanical hyperalgesia was measured every second day for 3 weeks. CGRP-IR terminals in each spinal dorsal horn lamina were examined 7 and 14 days postsurgery. Pain behavior and CGRP immunoreactivity were compared among the 5 groups.

RESULTS

Mechanical hyperalgesia was found in the NP only, nerve compression only, and the NP with nerve compression groups (P ≤ 0.05). CGRP-IR nerve terminals in the superficial laminae (I and II) and the deep laminae (III-VI) significantly increased in the NP only, nerve compression only, and NP with nerve compression groups compared with control and sham groups (P ≤ 0.05). Significant mechanical hyperalgesia and increased CGRP-IR nerve terminals were found in the NP with nerve compression group compared with the NP only and nerve compression only groups (P ≤ 0.05).

CONCLUSION

Our results indicate that nerve compression plus NP application produces the most pain-related behavior. CGRP-IR nerve terminals increased in laminae I and II that transmit pain and in laminae III to VI that transmit proprioception. Findings suggest that nerve compression plus NP application induces changes in CGRP expression in the superficial and deep laminae, and these changes are partly responsible for disc herniation pain.

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

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

New, clinically more relevant model for nerve root injury in the rat

STUDY DESIGN

Exposure to nucleus pulposus and displacement of intraspinal nervous structures with assessment of spontaneous behavioral changes in rats.

OBJECTIVE

To develop a controlled, experimental model for nerve root injury.

SUMMARY OF BACKGROUND DATA

There are a number of experimental models presented for studies on radiculopathies. One frequently used model is based on exposure to nucleus pulposus and displacement of the dorsal root ganglion (DRG). However, it is clinically more common that the nerve roots are displaced/compressed than the DRG. In this study, we developed a model for displacement of the nerve root by modifying the DRG model.

METHODS

After removing the left L3-L4 facet joint, the underlying disc was punctured, and the L4 nerve root was displaced laterally by an injection needle (n = 10). In sham experiments, the same procedure was performed without disc puncture and displacement (n = 10). In 10 rats, the left L4-L5 facet joint was removed. The underlying disc was punctured and the L4 DRG was displaced medially by an injection needle. Assessment of spontaneous behavioral changes was performed on days 1, 3, 7, 14, and 21, postsurgery.

RESULTS

There was a clear increase in duration of the behavior "unloading of the paw" after displacement of the DRG that was most pronounced on day 1 and then gradually declined. There was a similar pattern for this behavior induced by nerve root displacement, although the duration was higher than that for the DRG displacement. No apparent trends in behavioral changes were observed for the other behaviors studied.

CONCLUSION

Displacement of the nerve root induced more changes in the pain behavior than displacement of the DRG, but only for the behavior unloading of the paw. Because nerve root injury is more common than DRG injury, this model may be more clinically relevant than the DRG model.

LEVEL OF EVIDENCE

N/A.

Tumor necrosis factor-α-immunoreactive cells in nucleus pulposus in adolescent patients with lumbar disc herniation

STUDY DESIGN

Immunohistochemistry for tumor necrosis factor (TNF)-α in nucleus pulposus of adolescent patients with lumbar disc herniation.

OBJECTIVE

To examine whether an inflammatory cytokine is expressed in the nucleus pulposus of adolescent patients with lumbar disc herniation.

SUMMARY OF BACKGROUND DATA

TNFα is thought to play a crucial role in the radicular pain caused by lumbar disc herniation in adult patients. However, the expression of TNFα in the nucleus pulposus of adolescent patients with lumbar disc herniation has not been explored.

METHODS

Five samples of nucleus pulposus from adolescent patients with lumbar disc herniation (age, 12-16 yr; n = 5) or controls requiring surgery for other back problems (age, 12-16 yr; n = 4; nonpainful scoliosis) were harvested during surgery. Nucleus pulposus specimens were immunostained using TNFα antibodies and immunostained cells in the nucleus pulposus were counted. We compared the expression of TNFα between the 2 groups.

RESULTS

In patients with lumbar disc herniation, more TNFα-immunoreactive cells were seen in the nucleus pulposus in comparison with patients with nonpainful scoliosis (P < 0.01).

CONCLUSION

The results suggest that TNFα may play a role in adolescent patients with lumbar disc herniation. The TNFα expression may be related with disc degeneration and pain in adolescent patients with lumbar disc herniation.

Differences between tumor necrosis factor-α receptors types 1 and 2 in the modulation of spinal glial cell activation and mechanical allodynia in a rat sciatic nerve injury model

STUDY DESIGN

Immunohistological analysis of spinal glial cells and analysis of pain behavior in the rat neuropathic pain model were investigated to clarify the function of tumor necrosis factor (TNF)-α receptors p55 type 1 and p75 type 2.

OBJECTIVE

Our objective was to investigate changes in hyperalgesia and glial cell activation after injection of antibodies to each TNF receptor in a rat sciatic nerve injury model.

SUMMARY OF BACKGROUND DATA

Recent research has revealed that activation of spinal glia plays an important role in radicular and neuropathic pain. TNF-α is reportedly a modulator for glial cell activation; however, the precise relationship between TNF-α and its 2 receptors on glial cells has not been fully delineated.

METHODS

Chronic constriction sciatic nerve injury and sham-operated rats were used. Antibodies to p55 or p75 or saline were intrathecally injected at the L5 level into rats with chronic constriction injury. Mechanical allodynia was examined for 2 weeks. Spinal cords were removed for immunohistochemical studies of ionized calcium-binding adaptor molecule 1 or glial fibrillary acidic protein.

RESULTS

Saline rats showed significantly more mechanical allodynia and the number of ionized calcium-binding adaptor molecule 1--immunoreactive microglia and glial fibrillary acidic protein--immunoreactive astrocytes were significantly increased in the saline rats compared with sham-operated rats during the 2 weeks. Injection of both antibodies significantly reduced pain behavior and anti-p55 caused significantly greater reduction compared with anti-p75. The numbers of microglia in both the antibodies groups were significantly decreased when compared with the saline group. In addition, the anti-p55 antibody suppressed microglial activation more than the anti-p75 antibody.

CONCLUSION

These results indicate that the microglial TNF-α p55 pathway played a more important role than the TNF-α p75 pathway in the pathogenesis of peripheral nerve injury pain. This suggests that future studies seeking to clarify neuropathic pain should target TNF-α and p55 receptors in microglia.

Morphological Studies on Crushed Sciatic Nerve of Rabbits with Electroacupuncture or Diclofenac Sodium Treatment

Sciatic nerves of 15 rabbits were crushed by Halsted straight mosquito hemostat with 8-11 Newton force for 60 seconds on a point 5 mm above the knee joint, and then the rabbits were equally divided into three groups. The acupuncture group was treated by electroacupuncture on Huan Tiao and Wei Zhong points (25 minutes/day) for 7 days. The medicine group was treated with intramuscular administration of dicofenac sodium (15 mg) daily for 7 days. The control group was not treated. After treatment, the distal parts of crushed nerve were examined under light microscope, the densities of normal myelinated fibers in 0.126 mm2were counted, and the diameters of 20 normal myelinated fibers were measured for each animal. The results showed that the mean densities were 176.2 ± 5.953 in the acupuncture group, 118.2 ± 10.878 in the medicine group and 101.4 ± 8.548 in the control group. The mean values were significantly different between the acupuncture and medicine groups ( p < 0.01) and highly significant difference between the acupuncture and control groups ( p < 0.001); but there was no significant difference between the medicine and control groups ( p > 0.05). There are more small myelinated fibers (0–9 μm) in the acupuncture group than in the medicine and control groups ( p = 0.0028). The results revealed and confirmed that acupuncture promotes nerve regeneration; diclofenac sodium did not show such an effect. The present study demonstrates the positive effect of acupuncture on regeneration of a crushed sciatic nerve in the rabbits. And acupuncture is a better treatment for regeneration of crushed nerve than diclofenac sodium.

Nuclear factor-kappa B decoy suppresses nerve injury and improves mechanical allodynia and thermal hyperalgesia in a rat lumbar disc herniation model

Nuclear factor-kappa B (NF-kappaB) is a gene transcriptional regulator of inflammatory cytokines. We investigated the transduction efficiency of NF-kappaB decoy to dorsal root ganglion (DRG), as well as the decrease in nerve injury, mechanical allodynia, and thermal hyperalgesia in a rat lumbar disc herniation model. Forty rats were used in this study. NF-kappaB decoy-fluorescein isothiocyanate (FITC) was injected intrathecally at the L5 level in five rats, and its transduction efficiency into DRG measured. In another 30 rats, mechanical pressure was placed on the DRG at the L5 level and nucleus pulposus harvested from the rat coccygeal disc was transplanted on the DRG. Rats were classified into three groups of ten animals each: a herniation + decoy group, a herniation + oligo group, and a herniation only group. For behavioral testing, mechanical allodynia and thermal hyperalgesia were evaluated. In 15 of the herniation rats, their left L5 DRGs were resected, and the expression of activating transcription factor 3 (ATF-3) and calcitonin gene-related peptide (CGRP) was evaluated immunohistochemically compared to five controls. The total transduction efficiency of NF-kappaB decoy-FITC in DRG neurons was 10.8% in vivo. The expression of CGRP and ATF-3 was significantly lower in the herniation + decoy group than in the other herniation groups. Mechanical allodynia and thermal hyperalgesia were significantly suppressed in the herniation + decoy group. NF-kappaB decoy was transduced into DRGs in vivo. NF-kappaB decoy may be useful as a target for clarifying the mechanism of sciatica caused by lumbar disc herniation.

Direct application of the TNF-alpha inhibitor, etanercept, does not affect CGRP expression and phenotypic change of DRG neurons following application of nucleus pulposus onto injured sciatic nerves in rats

STUDY DESIGN

Immunohistological and behavioral analysis of the effect of a tumor necrosis factor alpha (TNF-alpha) inhibitor in an injured-nerve model.

OBJECTIVE

To examine the effect of direct application of a TNF-alpha inhibitor (etanercept) on injured-nerve pain caused by nucleus pulposus.

SUMMARY AND BACKGROUND DATA

TNF-alpha is thought to play a crucial role in radicular pain. Calcitonin gene-related peptide (CGRP) is an inflammatory neuropeptide found in small sensory neurons. We have reported that CGRP appears in medium and large dorsal root ganglion (DRG) neurons that transmit proprioception in physiologic conditions. The purpose of the current study was to examine the change in behavior and phenotypic change of CGRP-immunoreactive DRG neurons by the TNF-alpha inhibitor, etanercept, in a disc herniation model.

METHODS

For the injured-nerve model, nucleus pulposus was applied to the sciatic nerve and the sciatic nerve pinched. Saline (10 microL; n = 10), as a control, or etanercept (150 microg: n = 10) were applied to sciatic nerves simultaneously. Mechanical allodynia was examined. Immunohistochemistry was used to examine CGRP expression in L5 DRGs.

RESULTS

Significant mechanical allodynia for 10 days was seen in the injured-nerve group compared with sham-operated animals. Etanercept ameliorated the mechanical allodynia slightly on day 2; however, there was no effect on other days. CGRP immunoreactivity was upregulated in the L5 DRG neurons of injured-nerve groups compared with the sham-operated group (P < 0.01). However, etanercept did not affect CGRP expression after nerve injury (P > 0.05). Proportions of CGRP- immunoreactive medium and large neurons were not significantly different in the nerve injury + saline group compared with the injury + etanercept group (P > 0.05).

CONCLUSION

Our results indicate that direct application of a TNF-alpha inhibitor had a small effect on acute pain behavior and may not be effective for suppression of inflammatory peptides in the current disc-herniation model.

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.

Effect of etanercept, a tumor necrosis factor-alpha inhibitor, on neuropathic pain in the rat chronic constriction injury model

STUDY DESIGN

The effects of a low, local dose of a tumor necrosis factor-alpha (TNF-alpha) inhibitor on neuropathic pain behaviors in a rat chronic constriction injury model were evaluated.

OBJECTIVE

To investigate whether a peripherally implanted polymer drug depot can deliver a dose of etanercept sufficient to reduce thermal hyperalgesia and mechanical allodynia in a rat model of neuropathic pain.

SUMMARY OF BACKGROUND DATA

TNF-alpha inhibitors reduce pain-associated behavior in experimental models of neuropathic pain. Moreover, systemic injections of TNF-alpha inhibitors have suggested some efficacy in treating sciatic pain in limited, off-label clinical studies. Improvements in these results may be obtained by optimal dosing via targeted, sustained delivery at the site of disc-induced inflammation.

METHODS

Unilateral chronic constriction injury was applied to the sciatic nerve of 56 male, Wistar rats. Four groups of animals (n = 7) received 0.5 mL phosphate-buffered saline every 3 days, 0.3 or 3 mg/kg etanercept every 3 days, or gabapentin (60 mg/kg) 1 hour before each behavioral test all via subcutaneous injection. Two groups of animals received 1.5 or 3.0 microg/h etanercept delivered by poly(lactic-co-glycolic acid) (PLGA) millicylinders (1 mm diameter x 10 mm long) implanted near the injured sciatic nerve. One group received a PLGA millicylinder implanted near the injured sciatic nerve. The final group received 3.0 microg/h etanercept via PLGA millicylinder implanted next to the uninjured, contralateral sciatic nerve.

RESULTS

A low, local dose of etanercept (approximately 3 microg/h) delivered by a polymer depot significantly reduced (P < 0.05) thermal hyperalgesia for 57 days as compared to polymer depot without drug or an etanercept-loaded depot implanted near the contralateral sciatic nerve, and equivalent to a 10-fold higher dose delivered by repeat subcutaneous injection.

CONCLUSION

This preclinical study indicates that delivering TNF-alpha inhibitors by means of a locally administered polymeric formulation provides long-lasting analgesia in an inflammatory neuropathic pain model.

The role of capsaicin-sensitive primary afferents in experimental sciatica induced by disc herniation in rats

STUDY DESIGN

The topical capsaicin treatment of the sciatic nerve, which was proved to destroy capsaicin-sensitive primary afferent (CSPA) fibers, was performed to determine the effect on decreases in paw withdrawal mechanical threshold (PWMT) and changes in spatial expression pattern of spinal c-Fos protein induced by the direct compression of L5 nerve root with autologous disc.

OBJECTIVE

To investigate the role of CSPA fibers in the development of mechanical hyperalgesia in the new sciatica model.

SUMMARY OF BACKGROUND DATA

To date, CSPA fibers have been shown to be involved in development of thermal hyperalgesia in various pain models. But the controversy still exists as to whether CSPA fibers are involved in the development of mechanical hyperalgesia in different pain models. To our best knowledge, the role of CSPA in sciatica was not investigated. Therefore, the present study was designed to determine the role of CSPA fibers in the newly developed sciatica model.

METHODS

All surgeries were performed in Sprague-Dawley rats. PWMT was measured at the different time points postsurgery and presurgery. The changes in spatial expression pattern of c-Fos protein in the spinal cord were also determined at 3 weeks when PWMT decreased to the peak.

RESULTS

The pretreatment with capsaicin produced a complete prevention of mechanical hyperalgesia induced by disc compression. The direct compression of L5 nerve root produced an obvious expression of Fos-like immunoreactivity neurons in the dorsal horn of the spinal cord, which was significantly decreased by pretreatment with capsaicin.

CONCLUSION

We conclude that CSPA fibers, which mainly terminated in superficial layers of dorsal horn, may play a key role in mechanical hyperalgesia in the new sciatica model.

"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.

Safety of epidural administration of Osteogenic Protein-1 (OP-1/BMP-7): behavioral and macroscopic observation

STUDY DESIGN

To assess the safety of epidural administration of Osteogenic Protein-1 (OP-1).

OBJECTIVES

To examine if epidural administration of OP-1 or administration into the nucleus pulposus (NP) resulted in ectopic bone formation and/or pain-related behavior.

SUMMARY OF BACKGROUND DATA

OP-1 has the potential for treatment of degenerative disc disease. However, OP-1's safety, when it is applied into the epidural space or herniated nucleus pulposus, is not well established.

METHODS

Forty rats were divided into 5 groups. Sham group: Left L4 and L5 nerve roots were exposed. NP group: The NP obtained from the tail was relocated onto the exposed nerve roots. NP+OP group: The NP obtained from the tail and injected with 0.2 microg of OP-1 in 1 microL of 5% lactose-buffered solution was placed on the nerve roots. GS group: A gelatin sponge was applied on the nerve roots. GS+OP group: A gelatin sponge soaked with 0.2 microg of OP-1 in 1 microL of 5% lactose-buffered solution was placed onto the nerve roots. Motor function and reflex responses to mechanical noxious stimuli were measured in all rats up to 3 weeks after surgery. Three weeks after surgery, all rats were killed for analysis of ectopic bone formation and magnitude of neural compression.

RESULTS

Motor paresis was not observed in any groups. Only rats in the NP group showed evidence of irreversible mechanical hyperalgesia after surgery. There were no differences in the mechanical stimuli response among all groups except the NP group. Macroscopic examination revealed no ectopic bone formation or differences in neural compression among the groups.

CONCLUSION

OP-1 application in the epidural space is safe based on behavioral measures and macroscopic observation on ectopic bone formation at 21 days after surgery.

Glial phosphorylated p38 MAP kinase mediates pain in a rat model of lumbar disc herniation and induces motor dysfunction in a rat model of lumbar spinal canal stenosis

STUDY DESIGN

Immunohistochemical and behavioral study using rat models of lumbar disc herniation and cauda equina syndrome.

OBJECTIVE

To investigate the expression of activated p38 mitogen-activated protein kinases (p38 MAP kinase; p38) in the spinal cord and to determine the effect of intrathecal administration of a specific p38 inhibitor on pain in a lumbar disc herniation model and on motor function and hypoalgesia in a spinal canal stenosis (SCS) model.

SUMMARY OF BACKGROUND DATA

In pathologic lumbar disc herniation-induced neuropathic pain and compression of cauda equina-induced motor dysfunction and hypoalgesia caused by SCS, glia are activated and produce certain cytokines, including tumor necrosis factor-alpha (TNF-alpha) and interleukins, which play a crucial role in the pathogenesis of nerve degeneration. p38 is phosphorylated by these cytokines, suggesting that it may play an important role in pain transmission and nerve degeneration. Here we have examined the role of p38 in rat models of lumbar disc herniation and SCS.

METHODS

Six-week-old male Sprague-Dawley rats were used. For the disc herniation model, autologous nucleus pulposus was applied to L5 nerve roots, which were then crushed. For the SCS model, a piece of silicon was placed under the lamina of the fourth lumbar vertebra. We assessed mechanical allodynia, hypoalgesia, and motor function using von Frey hairs, treadmill tests, and immunohistochemical localization of phosphorylated p38 (P-p38) in the cauda equina, dorsal root ganglion (DRG), and spinal cord, which were also double-stained with NeuN (neuronal marker), GFAP (astrocyte/Schwann cell marker), or isolectin B4 (IB4; microglia marker). We also examined the effects of intrathecal administration of a specific p38 inhibitor, FR167653, on nucleus pulposus-induced pain, hypoalgesia, and motor dysfunction following SCS.

RESULTS

We demonstrated that activated P-p38-immunoreactive cells in the spinal cord and cauda equina were not observed before nerve injury but appeared in the cauda equina, DRG, and spinal dorsal horn in the disc herniation and SCS models. Double-labeling revealed that most P-p38-immunoreactive cells were isolectin B4-labeled microglia and GFAP-immunoreactive Schwann cells. Intrathecal administration of the p38 inhibitor FR167653 decreased mechanical allodynia in the disc herniation model and improved hypoalgesia and intermittent motor dysfunction in the SCS model.

CONCLUSIONS

Our findings suggest that activated p38 may play an important role in the involvement of microglia in the pathophysiology of pain following lumbar disc herniation and mechanical hypoalgesia, and motor nerve dysfunction of cauda equina following SCS.

Basic Science Research Related to Chiropractic Spinal Adjusting: The State of the Art and Recommendations Revisited

OBJECTIVE

The objectives of this white paper are to review and summarize the basic science literature relevant to spinal fixation (subluxation) and spinal adjusting procedures and to make specific recommendations for future research.

METHODS

PubMed, CINAHL, ICL, OSTMED, and MANTIS databases were searched by a multidisciplinary team for reports of basic science research (since 1995) related to spinal fixation (subluxation) and spinal adjusting (spinal manipulation). In addition, hand searches of the reference sections of studies judged to be important by the authors were also obtained. Each author used key words they determined to be most important to their field in designing their individual search strategy. Both animal and human studies were included in the literature searches, summaries, and recommendations for future research produced in this project.

DISCUSSION

The following topic areas were identified: anatomy, biomechanics, somatic nervous system, animal models, immune system, and human studies related to the autonomic nervous system. A relevant summary of each topic area and specific recommendations for future research in each area were the primary objectives of this project.

CONCLUSIONS

The summaries of the literature for the 6 topic sections (anatomy, biomechanics, somatic nervous system, animal models, immune system, and human studies related to the autonomic nervous system) indicated that a significant body of basic science research evaluating chiropractic spinal adjusting has been completed and published since the 1997 basic science white paper. Much more basic science research in these fields needs to be accomplished, and the recommendations at the end of each topic section should help researchers, funding agencies, and other decision makers develop specific research priorities.

Up-regulation of acid-sensing ion channel 3 in dorsal root ganglion neurons following application of nucleus pulposus on nerve root in rats

STUDY DESIGN

Immunocytochemistry for acid-sensing ion channel 3 (ASIC3) in neurons of rat dorsal root ganglions (DRGs) from animals exposed to a model of lumbar disc herniation.

OBJECTIVE

To examine expression of ASIC3 in DRGs and the effect of a sodium channel blocker applied to the nerve root in a rat model of lumbar disc herniation.

SUMMARY OF BACKGROUND DATA

Radicular pain is a common symptom of lumbar disc herniation in human beings. A depolarizing sodium channel gated by protons during tissue acidosis, ASIC3, is specifically expressed in sensory neurons. It has been associated with cardiac ischemic and inflammatory pain. We often perform spinal nerve root block for radicular pain using a sodium channel blocker, such as lidocaine; however, it has been unclear whether the effective period of this treatment is usually longer than the expected duration of efficacy.

METHODS

For the lumbar disc herniation model, nucleus pulposus was harvested from the tail and applied to the L5 nerve root, and the nerve roots were pinched. We evaluated mechanical allodynia in sham-operated animals and a disc herniation model. Immunohistochemistry was used to examine ASIC3 expression in L5 DRGs. Finally, the effect of lidocaine on pain and ASIC3 expression in the disc herniation model was examined.

RESULTS

Animals exposed to the lumbar disc herniation model showed allodynia for 8 days, and ASIC3 immunoreactivity was up-regulated in DRG neurons. After administration of lidocaine to spinal nerve roots affected by disc herniation, ASIC3 immunoreactivity was down-regulated in DRG neurons, and the level of mechanical allodynia was significantly decreased for 8 days.

CONCLUSIONS

Our results suggest that ASIC3 in DRG neurons may play an important role in nerve root pain caused by lumbar disc herniation. Lidocaine decreased ASIC3 expression in DRG neurons and pain associated with the disc herniation model.

Electromyographic findings in nucleus pulposus-induced radiculopathy in the rat

STUDY DESIGN

This study was conducted to investigate the electromyographic changes and their pathophysiologic background in the animal model of nucleus pulposus-induced radiculopathy.

OBJECTIVES

To observe the abnormal spontaneous activities in the electromyography (EMG) of rats with nucleus pulposus-induced radiculopathy and assess the role of nitric oxide in their development.

SUMMARY OF BACKGROUND DATA

It has been shown that application of nucleus pulposus to nerve roots induces changes consistent with radiculopathy. However, to our knowledge, electromyographic findings and their background have never been studied in this model of radiculopathy.

METHODS

Autologous nucleus pulposus was harvested from the tails of Sprague-Dawley rats, and applied to L4 and L5 nerve roots. The rats were tested for mechanical allodynia, motor paresis, and needle EMG, before and after surgery. Specimens of nerve roots were stained histochemically for nitrotyrosine.

RESULTS

The rats had mechanical allodynia after surgery, but motor paresis was absent. EMG showed abnormal spontaneous activities after surgery, but only temporarily. Immunoreactivity for nitrotyrosine was detected in the cell bodies and axons of nerve roots.

CONCLUSIONS

The data indicate that abnormal spontaneous activities can be observed in electromyographic examination of nucleus pulposus-induced radiculopathy. The development of these activities is considered related to nitric oxide-mediated protein nitration and resultant axonal dysfunction.

Spinal neural cyclooxygenase-2 mediates pain caused in a rat model of lumbar disk herniation

Application of nucleus pulposus to nerve root generates radicular pain. We demonstrated that these animals showed allodynia for 2 weeks, and cyclooxygenase-2 (COX-2) immunoreactivities were up-regulated in the spinal dorsal horn. COX-2 immunoreactivities were shown in neurons; however, they were not in astrocytes. Intrathecal administration of an antibody to COX-2 decreased allodynia. Our results suggest that COX-2 in spinal cord might be a target for treatment of patients with nerve root pain caused by lumbar disk herniation.

PERSPECTIVE

Neural COX-2 might mediate nerve root pain in the spinal cord caused by lumbar disk herniation in rats.

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.

The effects of epidural application of allografted nucleus pulposus in rats on cytokine expression, limb withdrawal and nerve root discharge

This study investigated cytokine expression, behavioral and neurophysiologic changes in Lewis rats whose lumbar nerve roots were exposed to nucleus pulposus (NP). Allografted NP or fat was implanted over the left L5 nerve root. Sham rats had no NP or fat implantation. Control rats had no surgery. Rats were allowed to survive for 7 days and were tested daily for hind-paw mechanical and thermal withdrawal response (TWR). Granulation tissue was processed by immunohistochemistry for cytokines--interleukin 1 beta (IL-1beta), interleukin 6 (IL-6) and tumor necrosis factor (TNF). Neurophysiological response from the L5 nerve roots was also characterized after 7 days. Significant staining density for IL-1beta, IL-6 and TNF was observed in NP granulation tissue compared with fat and sham (p<0.05). However, there were no significant thermal and mechanical behavioral changes. TWR data computed as percentage-difference scores indicated no significant changes in withdrawal response between the four groups, although NP-treated group showed a trend of decreasing withdrawal latency. Comparison of combined percentage-difference scores revealed increased sensitivity in the NP group on days 4, 5 and 6, 7 when compared with control rats only, with no significant changes in the percentage-difference scores of fat and sham rats when compared to control. Neurophysiologically, the percentage increase in discharge rate in NP-treated rats was higher than control (p<0.05) but not higher than fat and sham rats. These results support the inflammatory nature of NP but offer limited support to NP-mediated thermal behavioral changes.

Chronic inflammation and compression of the dorsal root contribute to sciatica induced by the intervertebral disc herniation in rats

The pathophysiological mechanisms underlying sciatica and back pain are not well understood. In the present study, a sciatica model was developed to investigate the contributions of inflammation and compression of the dorsal root (DR). The procedure used autologous disc to apply direct pressure to the L5 DR (disc compression, DC group). For control, five additional groups were included: (1). mechanical compression of L5 DR without disc (compression, CP group); (2). epidurally placed disc without mechanical compression (disc group); (3). epidurally placed nucleus pulposus (NP) without mechanical compression (NP group); (4). epidurally placed annulus fibrosus (AF) without mechanical compression (AF group) and (5). sham group. The paw withdrawal latency to heat stimulation, paw withdrawal threshold to mechanical stimulation, body weight, and motor function were determined pre- and post-surgery. It was observed that all experimental groups with the exception of the sham group showed a progressive and prolonged mechanical hyperalgesia with the DC group having the strongest effect. Furthermore, the disc group showed a greater mechanical hyperalgesia with earlier onset in comparison with the CP group and disc, AF, and NP groups developed thermal hyperalgesia in addition to mechanical hyperalgesia following surgery. Finally, rats in all groups showed normal motor function and body weight increase. These data suggest that this model is suitable to investigate the mechanisms of sciatica and inflammation as well as mechanical compression is involved in the pathogenesis of this condition. Moreover, AF and NP may contribute similarly to the development of sciatica and back pain.

Changes in spontaneous behavior in rats exposed to experimental disc herniation are blocked by selective TNF-alpha inhibition

STUDY DESIGN

Study of pain behavior in animals by observation of changes in spontaneous behavior.

OBJECTIVES

To assess if selective inhibition of tumor necrosis factor alpha may reduce changes in spontaneous behavior induced by experimental disc herniation in the rat as previously reported.

SUMMARY OF BACKGROUND DATA

It is known that the proinflammatory cytokine tumor necrosis factor alpha may play a key role for the nucleus pulposus-induced nerve dysfunction seen in experimental set ups. However, it is not known if tumor necrosis factor alpha is also involved in pain production induced by the same procedure.

METHODS

Thirty-two rats were used for the study. Twenty-two rats had an L4-L5 disc incision combined with a displacement of the L4 dorsal root ganglion. Twelve of these rats received an intraperitoneal injection of 0.125 mL of 10 mg/mL Remicade, and the remaining 10 were left untreated. Ten rats only had the L4-L5 disc exposed and formed the control group. The day before surgery and days 1, 3, 7, 14, and 21 after surgery, the rats were videotaped from below during a 20-minute period. The duration of four specific behaviors were determined and compared between the three experimental groups at each time point.

RESULTS

Similar to a previous study, the nontreated showed increased signs of focal pain behavior (rotation of the head towards the operated leg and lifting of the operated leg) during the first 7 postoperative days. Treatment with the tumor necrosis factor alpha-inhibitor infliximab significantly reduced this behavior. At day 14, there were no differences between the groups, and at day 21, the nontreated group displayed reduced locomotion and increased immobility, similar to previous observations. Tumor necrosis factor alpha inhibition also seemed to reduce these behaviors.

CONCLUSIONS

The data of the study clearly indicate a role for tumor necrosis factor alpha in the studied behavior changes after experimental disc herniation in the rat. Clinical trials must be performed in order to assess if there may be a clinical use for tumor necrosis factor alpha inhibition in the treatment of sciatica due to 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.

Neurophysiological effects of spinal manipulation

BACKGROUND CONTEXT

Despite clinical evidence for the benefits of spinal manipulation and the apparent wide usage of it, the biological mechanisms underlying the effects of spinal manipulation are not known. Although this does not negate the clinical effects of spinal manipulation, it hinders acceptance by the wider scientific and health-care communities and hinders rational strategies for improving the delivery of spinal manipulation.

PURPOSE

The purpose of this review article is to examine the neurophysiological basis for the effects of spinal manipulation.

STUDY DESIGN

A review article discussing primarily basic science literature and clinically oriented basic science studies.

METHODS

This review article draws primarily from the peer-reviewed literature available on Medline. Several textbook publications and reports are referenced. A theoretical model is presented describing the relationships between spinal manipulation, segmental biomechanics, the nervous system and end-organ physiology. Experimental data for these relationships are presented.

RESULTS

Biomechanical changes caused by spinal manipulation are thought to have physiological consequences by means of their effects on the inflow of sensory information to the central nervous system. Muscle spindle afferents and Golgi tendon organ afferents are stimulated by spinal manipulation. Smaller-diameter sensory nerve fibers are likely activated, although this has not been demonstrated directly. Mechanical and chemical changes in the intervertebral foramen caused by a herniated intervertebral disc can affect the dorsal roots and dorsal root ganglia, but it is not known if spinal manipulation directly affects these changes. Individuals with herniated lumbar discs have shown clinical improvement in response to spinal manipulation. The phenomenon of central facilitation is known to increase the receptive field of central neurons, enabling either subthreshold or innocuous stimuli access to central pain pathways. Numerous studies show that spinal manipulation increases pain tolerance or its threshold. One mechanism underlying the effects of spinal manipulation may, therefore, be the manipulation's ability to alter central sensory processing by removing subthreshold mechanical or chemical stimuli from paraspinal tissues. Spinal manipulation is also thought to affect reflex neural outputs to both muscle and visceral organs. Substantial evidence demonstrates that spinal manipulation evokes paraspinal muscle reflexes and alters motoneuron excitability. The effects of spinal manipulation on these somatosomatic reflexes may be quite complex, producing excitatory and inhibitory effects. Whereas substantial information also shows that sensory input, especially noxious input, from paraspinal tissues can reflexively elicit sympathetic nerve activity, knowledge about spinal manipulation's effects on these reflexes and on end-organ function is more limited.

CONCLUSIONS

A theoretical framework exists from which hypotheses about the neurophysiological effects of spinal manipulation can be developed. An experimental body of evidence exists indicating that spinal manipulation impacts primary afferent neurons from paraspinal tissues, the motor control system and pain processing. Experimental work in this area is warranted and should be encouraged to help better understand mechanisms underlying the therapeutic scope of spinal manipulation.

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.

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

Biologically active substances, such as prostaglandins, thromboxanes, and leukotrienes, which are metabolites involved in the arachidonic acid cascade, are detected in herniated disc samples obtained from patients with lumbar disc herniation. However, little is known concerning the relationships between these substances and clinical symptoms such as radicular pain. Thromboxane A2 (TXA2) induces not only potent platelet aggregation, but also blood vessel contraction. Leukotriene B4 (LTB4), a potent chemotactic agent, plays a role in inflammatory reactions by recruiting neutrophils and lymphocytes. The purpose of this study was to examine the roles of TXA2 and LTB4 in the hyperalgesia induced by application of nucleus pulposus to the lumbar nerve root in the rat. TXA2 synthetase inhibitor and LTB4 receptor antagonist, which were injected into the epidural space, decreased mechanical hyperalgesia at both three and seven days after epidural injection. There were no significant differences in sensitivity to noxious thermal stimuli following application of the nucleus pulposus or an epidural injection. Epidural injection of LTB4 receptor antagonist and/or TXA2 synthetase inhibitor may attenuate the painful radiculopathy due to lumbar disc herniation. In conclusion, our findings suggest that TXA2 and LTB4 may play significant roles in mechanical hyperalgesia induced by autologous nucleus pulposus.