Sensory innervation of the lateral portion of the lumbar intervertebral disc in rats

Burst Spinal Cord Stimulation as Compared With L2 Dorsal Root Ganglion Stimulation in Pain Relief for Nonoperated Discogenic Low Back Pain: Analysis of Two Prospective Studies

INTRODUCTION

Chronic discogenic low back pain (CD-LBP) is caused by degenerated disks marked by neural and vascular ingrowth. Spinal cord stimulation (SCS) has been shown to be effective for pain relief in patients who are not responsive to conventional treatments. Previously, the pain-relieving effect of two variations of SCS has been evaluated in CD-LBP: Burst SCS and L2 dorsal root ganglion stimulation (DRGS). The aim of this study is to compare the effectivity in pain relief and pain experience of Burst SCS with that of conventional L2 DRGS in patients with CD-LBP.

MATERIALS AND METHODS

Subjects were implanted with either Burst SCS (n = 14) or L2 DRGS with conventional stimulation (n = 15). Patients completed the numeric pain rating score (NRS) for back pain and Oswestry disability index (ODI) and EuroQoL 5D (EQ-5D) questionnaires at baseline, and at three, six, and 12 months after implantation. Data were compared between time points and between groups.

RESULTS

Both Burst SCS and L2 DRGS significantly decreased NRS, ODI, and EQ-5D scores as compared with baseline. L2 DRGS resulted in significantly lower NRS scores at 12 months and significantly increased EQ-5D scores at six and 12 months.

CONCLUSIONS

Both L2 DRGS and Burst SCS resulted in reduction of pain and disability, and increased quality of life in patients with CD-LBP. L2 DRGS provided significantly increased pain relief and improvement in quality of life when compared with Burst SCS.

CLINICAL TRIAL REGISTRATION

The clinical trial registration numbers for the study are NCT03958604 and NL54405.091.15.

Updates on Pathophysiology of Discogenic Back Pain

Discogenic back pain, a subset of chronic back pain, is caused by intervertebral disc (IVD) degeneration, and imparts a notable socioeconomic health burden on the population. However, degeneration by itself does not necessarily imply discogenic pain. In this review, we highlight the existing literature on the pathophysiology of discogenic back pain, focusing on the biomechanical and biochemical steps that lead to pain in the setting of IVD degeneration. Though the pathophysiology is incompletely characterized, the current evidence favors a framework where degeneration leads to IVD inflammation, and subsequent immune milieu recruitment. Chronic inflammation serves as a basis of penetrating neovascularization and neoinnervation into the IVD. Hence, nociceptive sensitization emerges, which manifests as discogenic back pain. Recent studies also highlight the complimentary roles of low virulence infections and central nervous system (CNS) metabolic state alteration. Targeted therapies that seek to disrupt inflammation, angiogenesis, and neurogenic pathways are being investigated. Regenerative therapy in the form of gene therapy and cell-based therapy are also being explored.

Netrin-1 mediates nerve innervation and angiogenesis leading to discogenic pain

Objective

Discogenic low back pain (LBP) is associated with nociceptive nerve fibers that grow into degenerated intervertebral discs (IVD) but the etiopathogenesis of disease is not fully understood. The purpose of this study was to clarify the role of Netrin-1 in causing discogenic LBP.

Methods

The level of nociceptive nerve innervation was examined in disc degenerative patients and rat needle-punctured models by immunohistochemistry. Nucleus pulposus (NP) cells were isolated from IVD tissues of rats and induced degeneration by interleukin-1β (IL-1β) or tumor necrosis factor α (TNFα). The candidate genes related to neuron outgrowth and migration were selected by Next-generation sequencing (NGS). CRISPR/Cas9 was used to knockdown Netrin-1 in NP cells. The impact of Netrin-1 on nerve innervation were evaluated with P2X2、NF200 staining and microfluidics assay. Meanwhile the CD31 staining and transwell assay were used to evaluate the impact of Netrin-1 in angiogenesis. The proteins and RNA extracted from NP cells related to catabolism and anabolism were examined by western blot assay and RT-qPCR experiment. ChIP and luciferase experiments were used to assess the intracellular transcriptional regulation of Netrin-1. Further, a needle-punctured rat model followed by histomorphometry and immunofluorescence histochemistry was used to explore the potential effect of Netrin-1 on LBP in vivo.

Results

The level of nerve innervation was increased in severe disc degenerative patients while the expression of Netrin-1 was upregulated. The supernatants of NP cells stimulated with IL-1β or TNFα containing more Netrin-1 could promote axon growth and vascular endothelial cells migration. Knocking down Netrin-1 or overexpressing transcription factor TCF3 as a negative regulator of Netrin-1 attenuated this effect. The needle-punctured rat model brought significant spinal hypersensitivity, nerve innervation and angiogenesis, nevertheless knocking down Netrin-1 effectively prevented disc degeneration-induced adverse impacts.

Conclusion

Discogenic LBP was induced by Netrin-1, which mediated nerve innervation and angiogenesis in disc degeneration. Knocking down Netrin-1 by CRISPR/Cas9 or negatively regulating Netrin1 by transcription factor TCF3 could alleviate spinal hypersensitivity.

The translational potential of this article

This study on Netrin-1 could provide a new target and theoretical basis for the prevention and treatment for discogenic back pain.

Axial hypersensitivity is associated with aberrant nerve sprouting in a novel model of disc degeneration in female Sprague Dawley rats

Chronic low back pain is a global socioeconomic crisis and treatments are lacking in part due to inadequate models. Etiological research suggests that the predominant pathology associated with chronic low back pain is intervertebral disc degeneration. Various research teams have created rat models of disc degeneration, but the clinical translatability of these models has been limited by an absence of robust chronic pain‐like behavior. To address this deficit, disc degeneration was induced via an artificial annular tear in female Sprague Dawley rats. The subsequent degeneration, which was allowed to progress for 18‐weeks, caused a drastic reduction in disc volume. Furthermore, from week 10 till study conclusion, injured animals exhibited significant axial hypersensitivity. At study end, intervertebral discs were assessed for important characteristics of human degenerated discs: extracellular matrix breakdown, hypocellularity, inflammation, and nerve sprouting. All these aspects were significantly increased in injured animals compared to sham controls. Also of note, 20 significant correlations were detected between selected outcomes including a moderate and highly significant correlation (R = 0.59, p < 0.0004) between axial hypersensitivity and disc nerve sprouting. These data support this model as a rigorous platform to explore the pathobiology of disc‐associated low back pain and to screen treatments.

Anatomical study and clinical significance of rami communicantes of the lumbar spine

Background and objectives

Rami communicantes (RC) infiltration and radiofrequency lesions are new techniques for the treatment of discogenic low back pain (DLBP). Their efficacy is controversial, and the classification of RC remains unclear. We aimed to explore the differences between RC and reclassify RC according to their anatomical characteristics.

Methods

Sixteen sides of the lumbar spine from eight adult male embalmed cadavers were dissected. The presence of RC was noted. The morphology, origin, distribution, course, quantity and spatial orientation of RC on the lumbar spine were examined. The length and width of the RC were measured by a caliper.

Results

A total of 213 RC were found in the 8 cadavers in the lumbar region. RC were divided into three types: superficial rami (70, 32.86%), which penetrated the psoas major (PM) and ran above the aponeurosis of the PM; deep rami (125, 58.69%), which ran along the waist of the vertebral body beneath the aponeurosis of the PM; and discal rami, which ran over and adhered to the surface of the intervertebral disc. Superficial rami were divided into two subtypes: oblique rami (45, 21.13%) and parabolic rami (25, 11.74%), which crossed the vertebra and the disc in an oblique and a parabolic course, respectively.

Conclusions

RC should play an important role in the innervation of the lumbar spine. Detailed knowledge of RC in the lumbar region may help surgeons improve the efficacy of infiltration and percutaneous radiofrequency as a supplementary treatment for DLBP.

Age-related molecular changes in the lumbar dorsal root ganglia of mice: Signs of sensitization, and inflammatory response

Aging is a major risk factor for numerous painful, inflammatory, and degenerative diseases including disc degeneration. A better understanding of how the somatosensory nervous system adapts to the changing physiology of the aging body will be of great significance for our expanding aging population. Previously, we reported that chronological aging of mouse lumbar discs is pathological and associated with behavioral changes related to pain. It is established that with age and degeneration the lumbar discs become inflammatory and innervated. Here we analyze the aging lumbar dorsal root ganglia (DRGs) and spinal cord dorsal horn (SCDH) in mice between 3 and 24 months of age for age-related somatosensory adaptations. We observe that as mice age there are signs of peripheral sensitization, and response to inflammation at the molecular and cellular level in the DRGs. From 12 months onwards the mRNA expression of vasodilator and neurotransmitter, Calca (CGRP); stress (and survival) marker, Atf3; and neurotrophic factor, Bdnf, increases linearly with age in the DRGs. Further, while the mRNA expression of neuropeptide, Tac1, precursor of Substance P, did not change at the transcriptional level, TAC1 protein expression increased in 24-month-old DRGs. Additionally, elevated expression of NFκB subunits, Nfkb1 and Rela, but not inflammatory mediators, Tnf, Il6, Il1b, or Cox2, in the DRGs suggest peripheral nerves are responding to inflammation, but do not increase the expression of inflammatory mediators at the transcriptional level. These results identify a progressive, age-related shift in the molecular profile of the mouse somatosensory nervous system and implicates nociceptive sensitization and inflammatory response.

Males and females exhibit distinct relationships between intervertebral disc degeneration and pain in a rat model

Back pain is linked to intervertebral disc (IVD) degeneration, but clinical studies show the relationship is complex. This study assessed whether males and females have distinct relationships between IVD degeneration and pain using an in vivo rat model. Forty-eight male and female Sprague-Dawley rats had lumbar IVD puncture or sham surgery. Six weeks after surgery, IVDs were evaluated by radiologic IVD height, histological grading, and biomechanical testing. Pain was assessed by von Frey assay and dorsal root ganglia (DRG) expression of Calca and Tac1 genes. Network analysis visualized which measures of IVD degeneration most related to pain by sex. In both females and males, annular puncture induced structural IVD degeneration, but functional biomechanical properties were similar to sham. Females and males had distinct differences in mechanical allodynia and DRG gene expression, even though sex differences in IVD measurements were limited. Network analysis also differed by sex, with more associations between annular puncture injury and pain in the male network. Sex differences exist in the interactions between IVD degeneration and pain. Limited correlation between measures of pain and IVD degeneration highlights the need to evaluate pain or nociception in IVD degeneration models to better understand nervous system involvement in discogenic pain.

Regulation of nerve growth factor by anti-inflammatory drugs, a steroid, and a selective cyclooxygenase 2 inhibitor in human intervertebral disc cells stimulated with interleukin-1

STUDY DESIGN

Regulation of nerve growth factor (NGF) by 2 different anti-inflammatory drugs was investigated in vitro using isolated human intervertebral disc (IVD) cells stimulated with the proinflammatory cytokine interleukin-1 (IL-1).

OBJECTIVE

To investigate the regulation of NGF by a synthetic steroid and a selective cyclooxygenase-2 (COX-2) inhibitor and to clarify the biological role of prostaglandin E2 (PGE2) in this process.

SUMMARY OF BACKGROUND DATA

NGF is known to play an important role in pain, including low back pain, and to be induced by proinflammatory cytokines in IVD cells. However, the effect of clinically used drugs for managing low back pain on the regulation of NGF is unclear.

METHODS

Isolated human IVD cells were stimulated with interleukin-1 (IL-1) in the presence or absence of dexamethasone or a selective COX-2 inhibitor (NS-398). NGF expression and release were determined by real-time polymerase chain reaction and enzyme-linked immuno sorbent assay, respectively. Inhibition of PGE2 release was determined by enzyme-linked immuno sorbent assay. The effects of exogenous PGE2 and its receptor (E-series prostanoid receptors [EPs] 1-4) agonists were also tested for NGF regulation.

RESULTS

IL-1 transiently induced, in a dose-dependent manner, the induction of NGF in human IVD cells. Pretreatment with dexamethasone strongly inhibited the NGF expression, whereas NS-398 significantly enhanced it at the concentration at which PGE2 release was substantially inhibited. Exogenous PGE2 inhibited IL-1 induction of NGF and this effect was mimicked when EP2 and EP4, but not EP1 and EP3, agonists were supplemented to the culture.

CONCLUSION

Although selective COX-2 inhibitors have been shown to be effective for acute low back pain by inhibiting PGE2 release, our findings suggest that it may have a limited efficacy because it exaggerated NGF expression, whereas dexamethasone inhibited it. On the other hand, PGE2 had an inhibitory function for NGF induction by mediating EP2/4 in human IVD cells. Further studies are needed to clarify whether these observations could take place in vivo.

LEVEL OF EVIDENCE

N/A.

Does a fibronectin and aggrecan complex play a role in painful vertebral disks?

OBJECTIVE

To determine the presence of a fibronectin-aggrecan complex (FAC) in the disk space of persons with chronic low back pain as relates to provocative diskography.

DESIGN

A single-center prospective consecutive case series.

SETTING

A single private practice setting.

PATIENTS

Thirty-seven patients with symptomatic degenerative disk disease of the cervical, thoracic, or lumbar spine undergoing provocative diskography to identify a source of pain.

METHODS

Diskographic lavage for analysis was simultaneously performed at each disk level injected during diskography.

MAIN OUTCOME MEASURES

Visual analog scale (VAS) pain scores, Pfirrmann magnetic resonance imaging grade, and biochemical analysis of disk material were statistically analyzed.

RESULTS

A total of 105 levels in 37 patients had a complete set of data (mean age 43.2 ± 11.9 years; 15 male/22 female). The FAC was present in 43 of 108 levels and in at least one level in 25 of 37 patients. The Pfirrmann magnetic resonance imaging grade did not differ between complex-positive and negative levels (P = .125), nor did the intraoperative VAS (IO-VAS) score for pain by level (P = .206). A significant but loose correlation was found between Pfirrmann grade and IO-VAS (R(2) = 0.4, P < .001), but no significant correlation was found between VAS or IO-VAS and complex concentration (R(2) = 0.08, P = .11 and R(2) = 0.003, P = .5).

CONCLUSIONS

The FAC was identified in some painful disks by diskography. There was no significant correlation between the Pfirrmann grade or pre/intraoperative pain scores during diskography and complex concentrations within the disk measured by disk lavage.

Sensory and autonomic innervation of the cervical intervertebral disc in rats: the pathomechanics of chronic discogenic neck pain

STUDY DESIGN

An immunohistological analysis of the cervical intervertebral disc (IVD).

OBJECTIVE

To investigate sensory and autonomic innervation of the rat cervical IVD.

SUMMARY OF BACKGROUND DATA

Many clinicians are challenged with treating wide-ranging chronic neck pain. Several authors have reported that sympathetic nerves participate in chronic pain, and various sympathectomy procedures can effectively treat chronic pain.

METHODS

The neuro-tracer Fluoro-gold (FG) was applied to the anterior surfaces of C5-C6 IVDs from 10 Sprague-Dawley rats to label the neurons of the innervating dorsal root ganglion (DRG), stellate ganglion (SG; sympathetic ganglion), and nodose ganglion (NG; parasympathetic ganglion). Seven days postsurgery, DRGs from level C1-C8, SG, and NG neurons were harvested, sectioned, and immunostained for calcitonin gene-related peptide (CGRP; a marker for peptide-containing neurons) and isolectin B4 (IB4; a marker for nonpeptide-containing neurons). The proportion of FG-labeled DRG neurons that were CGRP-immunoreactive (CGRP-IR), IB4-binding, and non-CGRP-IR and IB4-binding, and the proportion of FG-labeled SG neurons and NG neurons were calculated.

RESULTS

FG-labeled neurons innervating the C5-C6 IVD were distributed throughout the C2-C8 DRGs. The proportions of FG-labeled DRG neurons that were CGRP-IR, IB4-binding, non-CGRP-IR and IB4-binding, as well as SG neurons, and NG neurons were 20.6%, 3.3%, 55.7%, 8.9%, and 11.5%, respectively. The proportion of CGRP-IR FG-labeled DRG neurons was significantly higher than the proportion of IB4-binding FG-labeled DRG neurons at each level (P < 0.05).

CONCLUSION

The C5-C6 IVD was innervated multisegmentally from neurons of the C2-C8 DRG, SG, and NG. Overall, 79.6% of the nerve fibers innervating the IVD were sensory nerves and 20.4% were autonomic nerves. Furthermore, 23.9% of the nerve fibers innervating the IVD were afferent sensory pain-related nerves, 8.9% were efferent sympathetic nerves, and 11.5% were efferent parasympathetic nerves. These findings may explain the wide-ranging and chronic discogenic pain that occurs via the somatosensory and autonomic nervous system.

Association of modic changes, Schmorl's nodes, spondylolytic defects, high-intensity zone lesions, disc herniations, and radial tears with low back symptom severity among young Finnish adults

STUDY DESIGN

A cross-sectional magnetic resonance imaging (MRI) study.

OBJECTIVE

We investigated the association among Modic changes, Schmorl's nodes, spondylolytic defects, high-intensity zone lesions, radial tears, herniations, and low back symptom severity.

SUMMARY OF BACKGROUND DATA

Disc degeneration is associated with low back pain in early adulthood, but the associations between other MRI findings and low back pain are not well known.

METHODS

Questionnaire data and MRI scans (1.5-T) were available for 554 subjects derived from a birth cohort at 21 years of age. Data on low back pain and back-related functional limitations at 18, 19, and 21 years of age were used for clustering of subjects, using latent class analysis. We used logistic regression with adjustment for the degree of disc degeneration to evaluate the associations between specific imaging findings and low back symptom severity.

RESULTS

The prevalence of herniations was 20%, Schmorl's nodes 17%, radial tears 9.9%, high-intensity zone lesions 3.2%, spondylolytic defects 5.8%, and Modic changes 0.7%. Latent class analysis produced 5 clusters: "Always Painful" (n = 65) meant painful at all time points and "Recent Onset Pain" (n = 56) meant increasing symptom severity, whereas subjects in the "Moderately Painful" (n = 73), "Minor Pain" (n = 193), and "No Pain" (n = 167) clusters had fewer symptoms. Compared with the "No Pain" cluster, Schmorl's nodes were more likely to occur in the "Always Painful" cluster (P = 0.017) and herniations in the 3 most painful clusters (P < 0.001). Herniations were associated with low back symptom severity (odds ratio, 2.5; 95% confidence interval, 1.4-4.4). Schmorl's nodes and radial tears were associated with symptoms in crude analyses only, whereas high-intensity zone lesions and spondylolytic defects occurred in similar frequencies in all clusters.

CONCLUSION

Herniations were most likely in the subjects with recent onset or persistent (3-yr period) low back symptoms, although they were also detected in subjects with no symptoms. The clinical relevance of herniations on MRI remains to be evaluated in the context of symptoms.

Lateral interbody fusion for treatment of discogenic low back pain: minimally invasive surgical techniques

Low back pain is one of the most common ailments in the general population, which tends to increase in severity along with aging. While few patients have severe enough symptoms or underlying pathology to warrant surgical intervention, in those select cases treatment choices remain controversial and reimbursement is a substancial barrier to surgery. The object of this study was to examine outcomes of discogenic back pain without radiculopathy following minimally-invasive lateral interbody fusion. Twenty-two patients were treated at either one or two levels (28 total) between L2 and 5. Discectomy and interbody fusion were performed using a minimallyinvasive retroperitoneal lateral transpsoas approach. Clinical and radiographic parameters were analyzed at standard pre- and postoperative intervals up to 24 months. Mean surgical duration was 72.1 minutes. Three patients underwent supplemental percutaneous pedicle screw instrumentation. Four (14.3%) stand-alone levels experienced cage subsidence. Pain (VAS) and disability (ODI) improved markedly postoperatively and were maintained through 24 months. Segmental lordosis increased significantly and fusion was achieved in 93% of levels. In this series, isolated axial low back pain arising from degenerative disc disease was treated with minimally-invasive lateral interbody fusion in significant radiographic and clinical improvements, which were maintained through 24 months.

Investigation of dichotomizing sensory nerve fibers projecting to the lumbar multifidus muscles and intervertebral disc or facet joint or sacroiliac joint in rats

STUDY DESIGN

Immunohistological analysis of dichotomizing sensory nerve fibers projecting to the lumbar multifidus muscles and intervertebral disc (IVD), facet joint (FJ), or sacroiliac joint (SIJ) in rats.

OBJECTIVE

To elucidate dichotomizing sensory nerve fibers projecting to the lumbar multifidus muscles and to IVDs, FJs, or SIJs.

SUMMARY OF BACKGROUND DATA

Clinically, the origin of low back pain remains unknown. Multiple studies have identified lumbar muscles, IVDs, FJs, and SIJs as sources of low back pain. Pain may originate directly from lumbar muscles or be referred from the spine, or both. Dorsal root ganglion (DRG) neurons with dichotomizing axons have been reported in several species and are thought to be related to referred pain.

METHODS

We used 2 neurotracers, 1,1'-dioctadecyl-3,3,3',3'-tetramethyl-indocarbocyanine perchlorate (DiI) and fluorogold (FG), in this double-labeling study involving 30 Sprague Dawley rats. DiI was applied to lumbar multifidus muscles in all rats. Simultaneously, FG was applied to the anterior left portion of L5-L6 IVDs in the IVD group (n = 10), to the left L5-L6 FJs in the FJ group (n = 10), and to the left SIJs in the SIJ group (n = 10). Fourteen days after surgery, left DRGs from L1 to L6 were harvested, sectioned, and observed under a fluorescence microscope.

RESULTS

We verified the existence of double-labeled DRG neurons (i.e., dichotomizing sensory nerve fibers) projecting to lumbar multifidus muscles and to IVDs, FJs, or SIJs, depending on the group. The proportion of double-labeled cells in all DiI-labeled DRG neurons was higher in the FJ group (6.8%) and SIJ group (7.1%) than in the IVD group (3.1%) (P < 0.05).

CONCLUSION

Our results document the presence of dichotomizing sensory nerve fibers projecting to lumbar multifidus muscles and to IVDs, FJs, and SIJs. Referred low back muscle pain may reflect disorders of lumbar posterior structures, such as FJs and SIJs, rather than disorders of lumbar anterior structures, such as IVDs.

Intervertebral disc, sensory nerves and neurotrophins: who is who in discogenic pain?

The normal intervertebral disc (IVD) is a poorly innervated organ supplied only by sensory (mainly nociceptive) and postganglionic sympathetic (vasomotor efferents) nerve fibers. Interestingly, upon degeneration, the IVD becomes densely innervated even in regions that in normal conditions lack innervation. This increased innervation has been associated with pain of IVD origin. The mechanisms responsible for nerve growth and hyperinnervation of pathological IVDs have not been fully elucidated. Among the molecules that are presumably involved in this process are some members of the family of neurotrophins (NTs), which are known to have both neurotrophic and neurotropic properties and regulate the density and distribution of nerve fibers in peripheral tissues. NTs and their receptors are expressed in healthy IVDs but much higher levels have been observed in pathological IVDs, thus suggesting a correlation between levels of expression of NTs and density of innervation in IVDs. In addition, NTs also play a role in inflammatory responses and pain transmission by increasing the expression of pain-related peptides and modulating synapses of nociceptive neurons at the spinal cord. This article reviews current knowledge about the innervation of IVDs, NTs and NT receptors, expression of NTs and their receptors in IVDs as well as in the sensory neurons innervating the IVDs, the proinflammatory role of NTs, NTs as nociception regulators, and the potential network of discogenic pain involving NTs.

The degenerated lumbar intervertebral disc is innervated primarily by peptide-containing sensory nerve fibers in humans

STUDY DESIGN

Immunohistochemical study of the sensory innervation of the human lumbar intervertebral disc.

OBJECTIVE

To determine the type of sensory fibers innervating human degenerated lumbar intervertebral discs.

SUMMARY OF BACKGROUND DATA

Sensory neurons involved in pain perception related to inflammation in rats are typically small, peptide-containing neurons immunoreactive for calcitonin gene-related peptide (CGRP). Small non-peptide-containing neurons binding to isolectin B4 (IB4) may also be involved in pain states, such as nerve injury pain. The character of such sensory neurons in humans has not been clarified.

METHODS

A degenerated, painful lumbar intervertebral disc was harvested from each of 8 patients during surgery. Sections were immunostained for protein gene product 9.5 (PGP 9.5, a general neuronal marker), CGRP, and IB4. The numbers of PGP 9.5- and CGRP-immunoreactive, and IB4-binding nerve fibers in the discs were counted.

RESULTS

PGP 9.5-immunoreactive fibers were observed in all discs. Nerve fibers immunoreactive for CGRP were also observed in 6 of 8 cases. IB4-binding nerve fibers were not found in any case.

CONCLUSIONS

Almost all of the nociceptive nerve fibers in the human intervertebral disc are peptide-containing nerve fibers, similar to the rat disc, suggesting that nerve fibers related to inflammation may transmit pain originating from human degenerated intervertebral discs.

Exposure of the nucleus pulposus to the outside of the anulus fibrosus induces nerve injury and regeneration of the afferent fibers innervating the lumbar intervertebral discs in rats

STUDY DESIGN

Using a retrograde tracing method and immunohistochemistry, we assessed the expression of activating transcription factor 3 (ATF3), a marker of nerve injury, and growth-associated protein 43 (GAP-43), a marker of axonal growth, in dorsal root ganglion (DRG) neurons innervating the lumbar intervertebral discs in rats.

OBJECTIVES

To investigate ATF3 and GAP-43 expression in DRGs innervating the intervertebral discs after exposure of the nucleus pulposus to the outside of the anulus fibrosus.

SUMMARY OF BACKGROUND DATA

Degeneration of lumbar intervertebral discs is considered as a cause of low back pain. We speculated that exposure of the nucleus pulposus to the outside of the anulus fibrosus may induce nerve injury and ingrowth into the disc.

METHODS

A neurotracer, Fluoro-Gold (F-G), was applied to the ventral aspect of L5-L6 intervertebral discs in 20 rats. The rats were classified into 2 groups: an NP group whose disc was punctured to expose the nucleus pulposus (n = 10) and a sham-operated group whose anulus fibrosus surface was scratched superficially (n = 10). Ten days after surgery, bilateral L1-L5 DRGs were processed for staining of ATF3 and GAP-43.

RESULTS

In the NP group, 13.9% +/- 2.9% of the F-G-labeled neurons innervating the discs were positive for ATF3, while 19.3% +/- 2.7% were positive for GAP-43. In contrast, in the sham-operated group, only 0.8% +/- 0.4% of the F-G-labeled neurons were positive for ATF3 while 7.4% +/- 1.7% were positive for GAP-43. The percentage of both ATF3-immunoreactive (IR) and GAP-43-IR neurons in the NP group was significantly higher than in the sham-operated group (P < 0.05).

CONCLUSIONS

ATF3-IR and GAP-43-IR neurons were significantly increased in the NP group. These results suggested that exposure of the nucleus pulposus to the outside of the anulus fibrosus induced nerve injury and in growth into the discs. These findings may explain discogenic lower back pain in patients with lumbar disc degeneration.

Innervation, inflammation, and hypermobility may characterize pathologic disc degeneration: review of animal model data

Animal models provide important clues to the pathomechanisms of human intervertebral disc degeneration. Previous reviews on this topic have highlighted the fact that loss of nuclear volume (and, consequently, pressure) is a common trigger for tissue-remodeling and anatomic change consistent with degeneration in humans. Unfortunately, a large gap still exists in the medical knowledge base that serves to distinguish symptomatic from asymptomatic degeneration. Because disc degeneration per se is not a basis for clinical intervention, identification of specific features underlying discogenic pain is of the utmost importance to advance the current level of care and identify novel therapeutic targets. This article presents animal-model evidence that pathologic, or painful, degeneration is characterized by ineffective injury-healing of peripheral tissue. Because the disc is only vascularized at the vertebral end plate and the outer part of the anulus, these are the likely sites for focal damage, inflammation, neoinnervation, and nociceptor sensitization. Consequently, while nuclear insufficiency is likely the root of degenerative change, the end plate and peripheral part of the anulus are more likely the source of patient discomfort.

Expression and co-expression of VR1, CGRP, and IB4-binding glycoprotein in dorsal root ganglion neurons in rats: differences between the disc afferents and the cutaneous afferents

STUDY DESIGN

The expression of vanilloid receptor 1 (VR1), calcitonin gene-related peptide (CGRP), and isolectin B4 (IB4)-binding glycoprotein in dorsal root ganglion (DRG) neurons innervating the lumbar disc and the plantar skin was investigated.

OBJECTIVE

To characterize the DRG neurons innervating lumbar discs and those innervating cutaneous tissue in rats.

SUMMARY AND BACKGROUND DATA

Small nociceptive DRG neurons are divided into nerve growth factor (NGF) sensitive and glial cell line-derived neurotrophic factor (GDNF)-sensitive neurons. CGRP and IB4-binding glycoprotein are recognized as specific markers for NGF and GDNF-sensitive neurons, respectively. VR1 is localized in small DRG neurons.

METHODS

Using histochemical staining and retrograde tracing methods, the expression of VR1, CGRP, and IB4-binding glycoprotein in DRG neurons innervating the L5-L6 disc and the plantar skin was examined in rats.

RESULTS

DRG neurons innervating the disc showed positive staining as: 23.4% VR1, 54.4% CGRP, and 1.0% IB4-binding glycoprotein. The following distribution was found for DRG neurons innervating the skin: 35.1% VR1, 41.1% CGRP, and 19.5% IB4-binding glycoprotein. Percentages of neurons positive for VR1 and IB4-binding glycoprotein were significantly lower in DRG neurons innervating the disc than in DRG neurons innervating the skin (P < 0.05), while no significant difference was observed in the percentage of neurons positive for CGRP.

CONCLUSIONS

VR1 is less abundant in lumbar disc than in cutaneous tissue. Our data suggest that nociceptive information from the disc is transmitted mostly by NGF-sensitive neurons, while that from the cutaneous tissue is transmitted by both NGF-sensitive and GDNF-sensitive neurons.

Disc inflammation potentially promotes axonal regeneration of dorsal root ganglion neurons innervating lumbar intervertebral disc in rats

STUDY DESIGN

The expression of growth-associated protein 43 (GAP-43), a marker of axonal growth, in the dorsal root ganglion (DRG) neurons innervating the lumbar intervertebral disc was assessed using the retrograde tracing method and immunohistochemistry.

OBJECTIVES

To study whether disc inflammation affects GAP-43 expression in DRG neurons innervating the disc in rats.

SUMMARY AND BACKGROUND DATA

Persistent inflammation and nerve ingrowth into the inner layer of degenerated discs can be a cause of discogenic pain. Although the presence of GAP-43-expressing nerve fibers in painful discs has been reported, the expression of GAP-43 in DRG neurons innervating the disc has not been studied.

METHODS

Seven days after the application of Fluoro-Gold to the L5-L6 disc, 50 microL of saline (n = 10, control group) or complete Freund's adjuvant (n = 10, inflammatory group) was applied to the disc in rats. Ten days after the Fluoro-Gold application, T13-L5 DRGs were double-stained with GAP-43 and either calcitonin gene-related peptide or isolectin B4 (IB4).

RESULTS

The percentage of Fluoro-Gold-labeled neurons that were positive for GAP-43 was significantly higher in the inflammatory group (44%) than in the control group (24%, P < 0.001). In both groups, the majority of GAP-43-positive neurons were small and positive for calcitonin gene-related peptide but not IB4.

CONCLUSIONS

The present results suggest that disc inflammation potentially promotes axonal growth of DRG neurons innervating the disc. In light of the strong correlation between the expression of calcitonin gene-related peptide and nerve growth factor receptor, it is most likely that nerve growth factor-sensitive DRG neurons extend their axons following disc inflammation.

Innervation of the lumbar intervertebral disc by nerve growth factor-dependent neurons related to inflammatory pain

STUDY DESIGN

We used anatomic tracers and immunoreactivity in rats to define dorsal root ganglion neuron populations innervating the lumbar discs in physiologic and inflammatory states.

OBJECTIVES

To investigate the percentages of calcitonin gene-related peptide-immunoreactive (CGRP-ir) and isolectin B4 (IB4)-binding neurons innervating lumbar discs.

SUMMARY OF BACKGROUND DATA

Small neurons are classified into two types. One contains CGRP and expresses the nerve growth factor receptor. The other binds IB4 and expresses the glial cell line-derived neurotrophic factor receptor.

METHODS

A neurotracer, Fluoro-Gold, was applied to the L5-L6 disc in rats. Five days later, 50-microL saline (control group: n = 8) or Complete Freund's adjuvant (inflammatory group: n = 8) was applied to the disc. Seven days after the second operation, T13-L5 dorsal root ganglions were processed for double staining of CGRP and IB4.

RESULTS

Of the Fluoro-Gold-labeled neurons, 50.1 +/- 4.6% (mean +/- SEM) were positive for CGRP and 0.7 +/- 0.6% positive for IB4 in the control group, while 65.6 +/- 4.7% were positive for CGRP and 1.0 +/- 1.0% positive for IB4 in the inflammatory group. The percentage of CGRP-ir neurons was significantly higher than that of IB4-binding neurons in both groups (P < 0.001, each). The percentage of CGRP-ir neurons in the inflammatory group was significantly higher than in the control group (P < 0.05).

CONCLUSIONS

We found that most small neurons innervating the disc were CGRP-ir. Furthermore, disc inflammation caused an increase in CGRP-ir neurons but not IB4-binding neurons, suggesting that CGRP-ir, nerve growth factor-dependent neurons are more responsible for discogenic pain.

Distribution and immunocytochemical characterization of dorsal root ganglion neurons innervating the lumbar intervertebral disc in rats: a review

Previously, it was believed that the lumbar intervertebral disc was innervated segmentally by dorsal root ganglion (DRG) neurons via the sinuvertebral nerves. Recently, it was demonstrated using retrograde tracing methods that the lower disc (L5-L6) is innervated predominantly by upper (L1 and L2) DRG neurons via the sympathetic trunks. Furthermore, we investigated the expression of various pain-related molecules such as calcitonin gene-related peptide (CGRP), isolectin B4 (IB4), P2X(3) receptor and vanniloid receptor 1 (VR1) in DRG neurons innervating the disc using a combination of immunostaining with the retrograde tracing method. This review outlines the distribution and immunocytochemical characterization of DRG neurons innervating the disc. Small nociceptive DRG neurons are classified into nerve growth factor (NGF)-dependent neurons and glial cell line-derived neurotrophic factor (GDNF)-dependent neurons and they can be distinguished by their reactivity for CGRP and IB4, respectively. We found that about half of the neurons innervating the disc were CGRP-immunoreactive (-ir), whilst, only 0.6% of the DRG neurons were IB4-positive, thereby indicating that NGF-dependent neurons are the main subpopulation which transmits and modulates nociceptive information from the disc. In addition, we also demonstrated P2X(3)- and VR1-immunoreactivity in DRG neurons innervating the disc and noted that they were mainly localized in NGF-dependent neurons. It is well known that NGF has sensitizing effects on DRG neurons, with a recent study demonstratng the presence of NGF in the painful intervertebral disc. Therefore, it is suggested that NGF is involved in the generation of discogenic low back pain.