Peripheral nerve pathways of afferent fibers innervating the lumbar spine in rats

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.

Sensory innervation of the lumbar 5/6 intervertebral disk in mice

Introduction

Over the years, most back pain-related biological studies focused on the pathogenesis of disk degeneration. It is known that nerve distributions at the outer layer of the annulus fibrosus (AF) may be an important contributor to back pain symptoms. However, the types and origins of sensory nerve terminals in the mouse lumbar disks have not been widely studied. Using disk microinjection and nerve retrograde tracing methods, the current study aimed to characterize the nerve types and neuropathway of the lumbar 5/6 (L5/6) disk in mice.

Methods

Using an anterior peritoneal approach, the L5/6 disk of adult C57BL/6 mice (males, 8–12 weeks) disk microinjection was performed. Fluorogold (FG) was injected into the L5/6 disk using the Hamilton syringe with a homemade glass needle driven by a pressure microinjector. The lumbar spine and bilateral thoracic 13 (Th13) to L6 DRGs were harvested at 10 days after injection. The number of FG+ neurons among different levels was counted and analyzed. Different nerve markers, including anti-neurofilament 160/200 (NF160/200), anti-calcitonin gene-related peptide (CGRP), anti-parvalbumin (PV), and anti-tyrosine hydroxylase (TH), were used to identify different types of nerve terminals in AF and their origins in DRG neurons.

Results

There were at least three types of nerve terminals at the outer layer of L5/6 AF in mice, including NF160/200+ (indicating Aβ fibers), CGRP+ (Aδ and C fibers), and PV+ (proprioceptive fibers). No TH+ fibers (sympathetic nerve fibers and some C-low threshold mechanoreceptors) were noticed in either. Using retrograde tracing methods, we found that nerve terminals in the L5/6 disk were multi-segmentally from Th13-L6 DRGs, with L1 and L5 predominately. An immunofluorescence analysis revealed that FG+ neurons in DRGs were co-localized with NF160/200, CGRP, and PV, but not TH.

Conclusion

Intervertebral disks were innervated by multiple types of nerve fibers in mice, including Aβ, Aδ, C, and proprioceptive fibers. No sympathetic nerve fibers were found in AF. The nerve network of the L5/6 disk in mice was multi-segmentally innervated by the Th13-L6 DRGs (mainly L1 and L5 DRGs). Our results may serve as a reference for preclinical studies of discogenic pain in mice.

Interventional non-operative management of low back and neck pain

Background

Chronic neck and back pain are among the most commonly encountered health problems in neurosurgical practice. Many cases fail prolonged pharmacological and physical therapy and are not proper candidates for surgical interventions, or had refused proposed surgical treatment.

Objective

To provide an informative critical summary of the literature about the topic of interventional management of axial neck and low back pain and highlighting the new trends and pieces of evidence.

Methods

The English literature published over the last two decades was reviewed by the author for recent and relevant data about the principles of interventional management of chronic neck and low back pain. A PubMed search was performed through phrase searching and combined searching using Boolean operators. The articles thought to be most relevant to the study aim and the neurosurgeons’ practice were extracted.

Results

Neck and low back pain continue to be among the most common musculoskeletal health problems and the most common cause of disability worldwide. A detailed understanding of relevant spine anatomy is crucial for interventionists who should deal with the concept of “functional spine unit” with multiple potential pain generators. Chronic spinal pain is best managed through a dedicated multidisciplinary team in well-equipped healthcare facilities. An algorithmic approach for the diagnosis and management of spinal pain is the mainstay of providing the best patient care and should be based on the commonality and treatability of pain generators, values of patients and available resources.

Conclusion

Management of chronic neck and back pain can represent a clinical dilemma due to the multiplicity of pain generators that may coexist in the same individual resulting in a complex type and pattern of pain. Approach to these patients requires contributions from the members of a multidisciplinary team, implementing a standardized approach in a well-equipped healthcare facility.

Canonical and noncanonical TGF-β signaling regulate fibrous tissue differentiation in the axial skeleton

Previously, we showed that embryonic deletion of TGF-β type 2 receptor in mouse sclerotome resulted in defects in fibrous connective tissues in the spine. Here we investigated how TGF-β regulates expression of fibrous markers: Scleraxis, Fibromodulin and Adamtsl2. We showed that TGF-β stimulated expression of Scleraxis mRNA by 2 h and Fibromodulin and Adamtsl2 mRNAs by 8 h of treatment. Regulation of Scleraxis by TGF-β did not require new protein synthesis; however, protein synthesis was required for expression of Fibromodulin and Adamtsl2 indicating the necessity of an intermediate. We subsequently showed Scleraxis was a potential intermediate for TGF-β-regulated expression of Fibromodulin and Adamtsl2. The canonical effector Smad3 was not necessary for TGF-β-mediated regulation of Scleraxis. Smad3 was necessary for regulation of Fibromodulin and Adamtsl2, but not sufficient to super-induce expression with TGF-β treatment. Next, the role of several noncanonical TGF-β pathways were tested. We found that ERK1/2 was activated by TGF-β and required to regulate expression of Scleraxis, Fibromodulin, and Adamtsl2. Based on these results, we propose a model in which TGF-β regulates Scleraxis via ERK1/2 and then Scleraxis and Smad3 cooperate to regulate Fibromodulin and Adamtsl2. These results define a novel signaling mechanism for TGFβ-mediated fibrous differentiation in sclerotome.

Contribution of the sensory innervation of the spine in low back pain: review and clinical commentary

Few validated tests allow a precise aetiological diagnosis of Low Back Pain (LBP), and the difficulty of clinical evaluations could be one of the reasons to explain the lack of effectiveness in the therapeutic management of chronic LBP. However, an implication of a sensory impairment in the control of sensorimotor circuits could be suggested. Interactive and specific responses between nociceptive nerve fibres and the paraspinal musculature motor control could have clinical implications, in particular through kinematic evaluation. Following an introduction to the link between the sensory innervation of the spine and pain, we then summarise the maladaptive movement in LBP at the kinematic and neuropathological level. A clinical objectification of these kinematic adaptations at the lumbar spine level, would clarify the aetiological diagnosis causes of chronic LBP, and so help optimising therapeutic strategies by proposing a relevant and precise clinical model of this painful condition.

Using Magnetic Resonance Myelography to Improve Interobserver Agreement in the Evaluation of Lumbar Spinal Canal Stenosis and Root Compression

STUDY DESIGN

Cross-sectional retrospective study designed to assess interobserver agreement.

PURPOSE

To investigate if interobserver agreement using magnetic resonance imaging (MRI) in the evaluation of lumbar spinal canal stenosis and root compression can be improved upon combination with magnetic resonance myelography (MRM).

OVERVIEW OF LITERATURE

The interpretation of lumbar spinal MRI, which is the imaging modality of choice, often has a significant influence on the diagnosis and treatment of low back pain. However, using MRI alone, substantial interobserver variability has been reported in the evaluation of lumbar spinal canal stenosis and nerve root compression.

METHODS

Hardcopies of 30 lumbar spinal MRI (containing a total of 150 disk levels) as well as MRM films were separately reviewed by two radiologists and a neurosurgeon. At each intervertebral disk, the observers were asked to evaluate the thecal sac for the presence and degree of spinal stenoses (mild, moderate, or severe) and presence of root canal compression. Interobserver agreement was measured using weighted kappa statistics.

RESULTS

Regarding lumbar spinal canal stenosis, interobserver agreement between the two radiologists was moderate (kappa, 0.4) for MRI and good (kappa, 0.6) for combination with MRM. However, the agreement between the radiologist and neurosurgeon remained fair for MRI alone or in combination with MRM (kappa, 0.38 and 033, respectively). In the evaluation of nerve root compression, interobserver agreement between the radiologists improved from moderate (kappa, 0.57) for MRI to good (kappa, 0.73) after combination with MRM; moderate agreement between the radiologist and neurosurgeon was noted for both MRI alone and after combination with MRM (kappa, 0.58 and 0.56, respectively).

CONCLUSIONS

Interobserver agreement in the evaluation of lumbar spinal canal stenosis and root compression between the radiologists improved when MRM was combined with MRI, relative to MRI alone.

What is different about spinal pain?

BACKGROUND

The mechanisms subserving deep spinal pain have not been studied as well as those related to the skin and to deep pain in peripheral limb structures. The clinical phenomenology of deep spinal pain presents unique features which call for investigations which can explain these at a mechanistic level.

METHODS

Targeted searches of the literature were conducted and the relevant materials reviewed for applicability to the thesis that deep spinal pain is distinctive from deep pain in the peripheral limb structures. Topics related to the neuroanatomy and neurophysiology of deep spinal pain were organized in a hierarchical format for content review.

RESULTS

Since the 1980's the innervation characteristics of the spinal joints and deep muscles have been elucidated. Afferent connections subserving pain have been identified in a distinctive somatotopic organization within the spinal cord whereby afferents from deep spinal tissues terminate primarily in the lateral dorsal horn while those from deep peripheral tissues terminate primarily in the medial dorsal horn. Mechanisms underlying the clinical phenomena of referred pain from the spine, poor localization of spinal pain and chronicity of spine pain have emerged from the literature and are reviewed here, especially emphasizing the somatotopic organization and hyperconvergence of dorsal horn "low back (spinal) neurons". Taken together, these findings provide preliminary support for the hypothesis that deep spine pain is different from deep pain arising from peripheral limb structures.

CONCLUSIONS

This thesis addressed the question "what is different about spine pain?" Neuroanatomic and neurophysiologic findings from studies in the last twenty years provide preliminary support for the thesis that deep spine pain is different from deep pain arising from peripheral limb structures.

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

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

Lumbar degenerative disc disease: current and future concepts of diagnosis and management

Low back pain as a result of degenerative disc disease imparts a large socioeconomic impact on the health care system. Traditional concepts for treatment of lumbar disc degeneration have aimed at symptomatic relief by limiting motion in the lumbar spine, but novel treatment strategies involving stem cells, growth factors, and gene therapy have the theoretical potential to prevent, slow, or even reverse disc degeneration. Understanding the pathophysiological basis of disc degeneration is essential for the development of treatment strategies that target the underlying mechanisms of disc degeneration rather than the downstream symptom of pain. Such strategies ideally aim to induce disc regeneration or to replace the degenerated disc. However, at present, treatment options for degenerative disc disease remain suboptimal, and development and outcomes of novel treatment options currently have to be considered unpredictable.

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.

Sclerotomes in the thoracic and lumbar spine, pelvis, and hindlimb bones of rats

Pain in lumbar radiculopathy shows a segmental distribution in muscles and bones, requiring knowledge of myotomes and sclerotomes for diagnosis of the involved nerve roots. The rostrocaudal coordinate in sensory space was examined for 49 reference sites placed on the periostium in the spine and hindlimb bones of rats to clarify the sclerotomes. Neurotracer 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) was applied to reference sites. DiI-labeled neurons were counted from the T9 through S3 dorsal root ganglia. The coordinate was calculated for each reference site as the location of the "median" neuron of all DiI-labeled neurons. The coordinate was between T13 and L3 for the lumbar spine, L2 and S1 in the coxal bone, L3 and L4 in the hip joint, femur, knee joint, tibia, and first digit, and L5 in the fibula and fifth digit. The routes of sclerotome boundary lines (SBLs) were determined based on the coordinates of the reference sites. SBLs obliquely demarcated the lumbar spine. SBLs were aligned parallel rostrodorsally to caudoventrally in the coxal bone, with medially-oriented convergence. The SBL between L3 and L4, which corresponded to the level of the furcal nerve, passed from the femur, tibia, and toward the first digit in the hindlimb bones.

PERSPECTIVE

The present study is the first report of the detailed sclerotome chart of rats. The sclerotome chart is not only useful for basic research of lumbar radiculopathy using rats, but would also facilitate an understanding of the spatial distribution of pain in patients with lumbar radiculopathy.