Capsaicin applied to rat lumbar intervertebral disc causes extravasation in the groin skin: a possible mechanism of referred pain of the intervertebral disc

Referred pain: characteristics, possible mechanisms, and clinical management

Purpose of this review

Referred pain is a common but less understood symptom that originates from somatic tissues. A comprehensive recognition of referred pain is important for clinicians when dealing with it. The purpose of this study is to summarize the current understanding of referred pain, including its pathogenesis, characteristics, diagnosis, and treatment.

Recent findings

Referred pain arises not only from pathologies primarily involving local tissue but also from lesions in distant structures. Central sensitization of convergent neurons and peripheral reflexes of dichotomizing afferent fibers are two theories proposed to explain the pathological mechanism of referred pain. Because syndromes related to referred pain of different origins overlap each other, it is challenging to define referred pain and identify its originating lesions. Although various approaches have been used in the diagnosis and treatment of referred pain, including conservative treatment, blockade, radiofrequency, and surgery, management of referred pain remains a clinical challenge.

Summary

Unlike radicular pain and neuropathic pain, referred pain is a less studied area, despite being common in clinics. Referred pain can derive from various spinal structures, and blockage helps identify the primary pathology. Due to the heterogeneity of referred pain, treatment outcomes remain uncertain. Further studies are needed to improve our understanding of referred pain.

Aberrant spinal mechanical loading stress triggers intervertebral disc degeneration by inducing pyroptosis and nerve ingrowth

Aberrant mechanical factor is one of the etiologies of the intervertebral disc (IVD) degeneration (IVDD). However, the exact molecular mechanism of spinal mechanical loading stress-induced IVDD has yet to be elucidated due to a lack of an ideal and stable IVDD animal model. The present study aimed to establish a stable IVDD mouse model and evaluated the effect of aberrant spinal mechanical loading on the pathogenesis of IVDD. Eight-week-old male mice were treated with lumbar spine instability (LSI) surgery to induce IVDD. The progression of IVDD was evaluated by μCT and Safranin O/Fast green staining analysis. The metabolism of extracellular matrix, ingrowth of sensory nerves, pyroptosis in IVDs tissues were determined by immunohistological or real-time PCR analysis. The apoptosis of IVD cells was tested by TUNEL assay. IVDD modeling was successfully produced by LSI surgery, with substantial reductions in IVD height, BS/TV, Tb.N. and lower IVD score. LSI administration led to the histologic change of disc degeneration, disruption of the matrix metabolism, promotion of apoptosis of IVD cells and invasion of sensory nerves into annulus fibrosus, as well as induction of pyroptosis. Moreover, LSI surgery activated Wnt signaling in IVD tissues. Mechanical instability caused by LSI surgery accelerates the disc matrix degradation, nerve invasion, pyroptosis, and eventually lead to IVDD, which provided an alternative mouse IVDD model.

Sensory nerve ingrowth, cytokines, and instability of discogenic low back pain: A review

Introduction

Many patients suffer from discogenic low back pain. However, the mechanisms, diagnosistic strategy, and treatment of discogenic low back pain all remain controversial. The purpose of this paper was to review the pathological mechanisms of discogenic low back pain.

Methods

Many authors have investigated the pathological mechanisms of discogenic low back pain using animal models and examining human patients. Central to most investigations is understanding the innervation and instabilities of diseased intervertebral discs and the role of inflammatory mediators. We discuss three pathological mechanisms of discogenic low back pain: innervation, inflammation, and mechanical hypermobility of the intervertebral disc.

Results

Sensory nerve fibers include C-fibers and A delta-fibers, which relay pain signals from the innervated outer layers of the intervertebral disc under normal conditions. However, ingrowth of these sensory nerve fibers into the inner layers of intervertebral disc occurs under disease conditions. Levels of neurotrophic factors and some cytokines are significantly higher in diseased discs than in normal discs. Stablization of the segmental hypermobility, which can be induced by intervertebral disc degeneration, suppresses inflammation and prevents sensitization of sensory nerve fibers innervating the disc.

Conclusions

Pathological mechanisms of discogenic low back pain include sensory nerve ingrowth into inner layers of the intervertebral disc, upregulation of neurotrophic factors and cytokines, and instability. Inhibition of these mechanisms is important in the treatment of discogenic low back pain.

Pathomechanisms of discogenic low back pain in humans and animal models

BACKGROUND CONTEXT

Although explored in humans and animal models, the pathomechanisms of discogenic low back pain (LBP) remain unknown.

PURPOSE

The aim of this study was to review the literature about the pathomechanisms of discogenic LBP.

METHODS

Animal models of discogenic pain and specimens from degenerated human intervertebral discs (IVDs) have provided clues about the pathomechanisms of discogenic LBP. Painful discs are characterized by a confluence of innervation, inflammation, and mechanical hypermobility. These three possible mechanisms are discussed in this review.

RESULTS

Animal models and specimens from humans have revealed sensory innervation of lumbar IVDs and sensory nerve ingrowth into the inner layer of IVDs. Cytokines such as tumor necrosis factor-α and interleukins induce this ingrowth. Nerve growth factor has also been recently identified as an inducer of ingrowth. Finally, disc degeneration induces several collagenases; their action results in hypermobility and pain.

CONCLUSIONS

To treat discogenic LBP, it is important to prevent sensitization of sensory nerve fibers innervating the IVD, to suppress pathogenic increases of cytokines, and to decrease disc hypermobility.

Cell signaling pathways related to pain receptors in the degenerated disk

Many of the causes of low back pain are still unknown; sufficient evidence indicates that both degenerative and mechanical change within the intervertebral disk (IVD) is a relevant factor. This article reviews intracellular signaling pathways related to pain receptors in the degenerated IVD. Several reports have demonstrated the number of nerve fibers in the IVD was increased in degenerated disks. In recent years, some groups have reported that an increase in nerve fibers is associated with the presence of inflammatory mediators and/or neurotrophins in the IVD. Cell signaling events, which are regulated by inflammatory mediators and neurotrophins, must be identified to clarify the mechanism underlying low back pain. Major intracellular signaling pathways (nuclear factor kappa β, mitogen-activated protein kinases, and Wnts) potentially play vital roles in mediating the molecular events responsible for the initiation and progression of IVD degeneration. These signaling pathways may represent therapeutic targets for the treatment of IVD degeneration and its associated back pain.

Lumbar disc degeneration induces persistent groin pain

STUDY DESIGN

Prospective study of 212 patients with groin pain but without low back pain.

OBJECTIVE

To evaluate discogenic groin pain without low back pain or radicular pain.

SUMMARY OF BACKGROUND DATA

Patients feel low back pain originating from discogenic disease. It has been reported that the rat lower lumbar discs are innervated mainly by L2 dorsal root ganglion neurons. Thus, it is possible that patients feel referred groin pain corresponding to the L2 dermatome originating from intervertebral discs; however, the referred pain has not been fully clarified in humans.

METHODS

We selected 5 patients with groin pain alone for investigation. The patients suffered from groin pain and showed disc degeneration only at 1 level (L4-L5 or L5-S1) on magnetic resonance imaging. Patients did not show any hip joint abnormality on radiography or magnetic resonance imaging. To prove that their groin pain originated in degenerated intervertebral discs, we evaluated changes in groin pain after infiltration of lidocaine into hip joints and examined pain provocation on discography, pain relief by anesthetic discoblock, and finally anterior lumbar interbody fusion surgery.

RESULTS

All patients were negative for hip joint block, positive for pain provocation on discography, and positive for pain relief by anesthetic discoblock. Furthermore, bony union was achieved 1 year after anterior interbody fusion surgery in all patients, and visual analogue scale score of groin pain was significantly improved at 1 year after surgery in all patients (P < 0.05).

CONCLUSION

In the current study, we diagnosed discogenic groin pain, using magnetic resonance imaging, infiltration of lidocaine into the hip joint, pain provocation on discography, pain relief by anesthetic discoblock, and lumbar surgery. It is important to consider the existence of discogenic groin pain if patients do not show low back pain.

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.

Neural mechanism of localized changes in skeletal muscle blood flow caused by moxibustion-like thermal stimulation of anesthetized rats

Moxibustion-like thermal stimulation (MTS) was applied to the gastrocnemius muscle to measure local muscle blood flow (MBF) in the stimulated region and the change in the MBF in the region, and its mechanism was examined. In the experiment, we used urethane-anesthetized rats under artificial respiration and observed the change caused by gastrocnemius MTS using a laser Doppler blood-flow meter. MTS applied to the gastrocnemius muscle caused a two-phase response in blood flow that showed a transient decrease followed by an increase without blood pressure change. It is suggested that the increase in response occurs because of an axon reflex that has a reflex arc below the spinal cord, and the decrease in response is caused by direct stimulation of postganglionic muscle sympathetic fibers.

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

We investigated the pathways of afferent fibers innervating the lumbar spine. The neurotracer DiI was applied to reference sites at the L5 level in rats. One of 4 surgeries was performed before DiI application: (1) transaction of the dorsal ramus of the L2 spinal nerve, (2) transaction of the ventral ramus of the L2 spinal nerve, (3) transaction of the psoas major muscle at L3-L4, or (4) removal of the paravertebral sympathetic trunks from L3-L5. The number of DiI-labeled neurons in the dorsal root ganglia after surgery was compared with neuron numbers in surgery-naïve rats. The number of DiI-labeled neurons decreased drastically with transection of the L2 ventral ramus or psoas major muscle for the ventral and lateral portions of the disc and vertebral body and after transection of the L2 dorsal ramus for the facet joint and spinous process. Removal of the sympathetic trunks did not reduce the number of DiI-labeled neurons significantly in the extra-spinal canal sites. In contrast, significant reductions occurred after the removal of the paravertebral sympathetic trunks in the intra-spinal canal sites. Extra-spinal canal sites received afferent fibers primarily through somatic routes, but intra-spinal canal sites received afferent fibers via the sympathetic trunks.

PERSPECTIVE

Extra-spinal canal sites of the lumbar spine received afferent fibers from muscles originating in the site. Intra-spinal canal sites received a considerable number of afferent fibers via the paravertebral sympathetic trunks. These results may provide new insights for nerve block treatment of low back pain.

Resolving discogenic pain

Recent basic science studies on discogenic low back pain have provided new knowledge about this condition. This paper reviews some of these results and presents an overview of the following findings. The rat lumbar intervertebral disk may be innervated non-segmentally through the paravertebral sympathetic nerve and segmentally through the sinuvertebral nerves, and also by dichotomizing sensory fibers. The exposure of the nucleus pulposus (NP) to the outer annulus fibrosus (AF) may induce nerve injury and ingrowth into the disk. Nerve growth factor (NGF)-sensitive neurons are predominant in the rat intervertebral disk, which indicates that hyperalgesic responses can be induced by inflammation. NGF in the NP may promote axonal growth. Lumbar fusion may inhibit nerve ingrowth into the degenerated disk and reduce the percentage of calcitonin gene related peptide (CGRP)-positive neurons.

Projection field of primary afferent fibers innervating the ventral portion of the lumbar intervertebral disc in the spinal cord dorsal horn

In the present study, we investigated the central projection of afferent fibers innervating the lumbar intervertebral disc using the fluorescent neurotracer 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Dil). The tracer Dil was applied to the ventrolateral portion of the L5-L6 intervertebral disc in 11 adult rats. Fluorescent sites were observed microscopically on spinal cord transverse sections. Fluorescent spots in laminae I-III were plotted on the central projection map of cutaneous afferents. In six of 11 rats, Dil was restricted to the application site. Of these six rats, three showed no evident fluorescent sites. In the remaining three rats, small fluorescent spots were scattered in the dorsal horn. Fluorescent spots in dorsal horn lamina I were located in the central projection fields of the low back and groin skin. Fluorescent spots were observed, also sporadically, in Clarke's column in T12-L1 segments. The central projection of afferent fibers innervating the rat lumbar intervertebral disc was indistinct with Dil labeling. We presumed this was due to the scarcity of central terminal arbors of disc afferent fibers. Spotty projections in laminae I-IllIIere present near the central projection fields of the loin and groin, indicating that pain would be perceived in the groin.

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.

The anatomic and physiologic basis of local, referred and radiating lumbosacral pain syndromes related to disease of the spine

Conscious perception and unconscious effects originating from the vertebral column and its neural structures, although complex, have definite pathways represented in a network of peripheral and central nervous system (CNS) ramifications. These neural relationships consequently result in superimposed focal and diffuse, local and remote conscious perceptions and unconscious effects. Any one or combination of somatic and autonomic signs and symptoms may potentially be observed in a particular patient. This variety and inconsistency may mislead or confuse both the patient and the physician. A clear understanding of the basic anatomic and physiologic concepts underlying this complexity should accompany clinical considerations of the potential significance of spondylogenic and neurogenic syndromes in any disease process affecting the spine.

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.

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

BACKGROUND CONTEXT

An annular tear extending to the outer one-third of the annulus is thought to be one of the causes of low back pain. However, some patients have bilateral low back symptoms, even if the annular tear is localized in the lateral disc. Because nociceptive information from the lateral disc is transmitted by the dorsal root ganglion (DRG) neurons innervating the lateral disc, we investigated the distribution of the DRG neurons innervating the lateral portion of the disc.

PURPOSE

To clarify the distribution and pathway of the DRG neurons innervating the lateral portion of the L5-L6 disc in rats.

STUDY DESIGN/SETTING

Using the retrograde tracing method, we studied the innervation pattern of the lateral portion of the L5-L6 intervertebral disc in rats.

METHODS

The retrograde transport of Fluoro-Gold (F-G; Fluorochrome, Denver, CO) was used in 22 rats. Subjects included a nontreated group (n=16) and a sympathectomized group (n=6). Seven days after the application of F-G crystals to the left lateral portion of the L5-L6 disc, bilateral T12-L6 DRGs were observed by fluorescent microscopy.

RESULTS

In the nontreated group, of all the F-G-labeled neurons, 93.1% were present in the left DRGs and 6.9% were in the right DRGs. The number of labeled neurons was largest in the left L2 DRGs. In the sympathectomized group, the numbers of labeled neurons in the T13, L1 and L2 DRGs were significantly lower than the numbers in the nontreated group.

CONCLUSION

Results of this study indicate that DRG neurons innervating the lateral portion of the disc are distributed mainly in the ipsilateral side but also in the contralateral side. The DRG neurons in T13, L1 and L2 innervate the lateral portion of the L5-L6 disc through the paravertebral sympathetic trunks.

Stereoscopic structure of sensory nerve fibers in the lumbar spine and related tissues

STUDY DESIGN

Neurotracer was applied to various sites in the rat lumbar spine and related tissues. The segmental distribution of labeled neurons in dorsal root ganglia (DRG) was investigated.

OBJECTIVES

To clarify the stereoscopic structure of afferent fibers innervating the lumbar spine and related tissues.

SUMMARY OF BACKGROUND DATA

Afferent fibers in the rat L5-L6 lumbar intervertebral disc are reported to originate from neurons in L1 and L2 DRG. However, anatomic studies determined that each dorsal ramus of the spinal nerve sends nerves to dorsal elements of the corresponding lumbar vertebra.

METHODS

Fluorescent neurotracer DiI (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) was applied to various sites of the lumbar spine and related tissues in the L2, L5, and L6 levels in rats. DRG were sectioned 3 weeks after DiI application. Rostrocaudal distribution of DiI-labeled neurons was investigated.

RESULTS

At L5, DiI-labeled neurons were prominent in DRG L3 for the lamina, L2 for the spinous process, L2 for the back muscle fascia, and L1 for the skin. Dorsal elements are therefore innervated by neurons in more rostral DRG. In the transverse plane, the more distant from the DRG a site was, the more rostral the DRG innervating the site. This structure suggested a concentric innervation pattern in the transverse plane.

CONCLUSION

Stereoscopically, the peripheral innervation territory of a lumbar DRG is conical, with the apex at the ganglion and the base circumference located on the dermatome. The lumbar spine itself is involved in the conical innervation territories of DRG.

Acupuncture mechanisms for clinically relevant long-term effects--reconsideration and a hypothesis

From the author's direct involvement in clinical research, the conclusion has been drawn that clinically relevant long-term pain relieving effects of acupuncture (>6 months) can be seen in a proportion of patients with nociceptive pain. The mechanisms behind such effects are considered in this paper. From the existing experimental data some important conclusions can be drawn: 1. Much of the animal research only represents short-term hypoalgesia probably induced by the mechanisms behind stress-induced analgesia (SIA) and the activation of diffuse noxious inhibitory control (DNIC). 2. Almost all experimental acupuncture research has been performed with electro-acupuncture (EA) even though therapeutic acupuncture is mostly gentle manual acupuncture (MA). 3. Most of the experimental human acupuncture pain threshold (PT) research shows only fast and very short-term hypoalgesia, and, importantly, PT elevation in humans does not predict the clinical outcome. 4. The effects of acupuncture may be divided into two main components--acupuncture analgesia and therapeutic acupuncture. A hypothesis on the mechanisms of therapeutic acupuncture will include: 1. Peripheral events that might improve tissue healing and give rise to local pain relief through axon reflexes, the release of neuropeptides with trophic effects, dichotomising nerve fibres and local endorphins. 2. Spinal mechanisms, for example, gate-control, long-term depression, propriospinal inhibition and the balance between long-term depression and long-term potentiation. 3. Supraspinal mechanisms through the descending pain inhibitory system, DNIC, the sympathetic nervous system and the HPA-axis. Is oxytocin also involved in the long-term effects? 4. Cortical, psychological, "placebo" mechanisms from counselling, reassurance and anxiety reduction.

Discography: A Review

The etiology of back pain appears elusive at times. Discography represents one tool that can help in the diagnosis. The following reviews the controversies and uses of this procedure.

Electrical stimulation of the rat lumbar spine induces reflex action potentials in the nerves to the lower abdomen

STUDY DESIGN

The distribution of the nerve action potentials reflexively elicited by electrical stimulation of the lumbar spine was investigated in rats.

OBJECTIVES

To elucidate the relation between the lumbar spine and other body regions that compose the spinal reflex.

SUMMARY OF BACKGROUND DATA

The hypothesis was that the ventral portion of the L5-L6 disc spatially corresponds to the groin.

METHODS

In Experiments 1 and 2, wire electrodes were placed 1) in the ventral and dorsal portions of the disc, facet joint, and muscle fascia at L5-L6, and 2) in the ventral portions of L3-L4, L4-L5, L5-L6, and L6-S discs. A needle electrode was inserted in the L5-L6 disc by 0.4-mm increments, and action potentials were serially recorded from the genitofemoral nerve.

RESULTS

Experiments 1 and 2: Reflex action potentials were elicited in the iliohypogastric (T13 and L1), ilioinguinal (L1), and genitofemoral (L2) nerves. Experiment 1: Stimulation of the disc induced reflex discharges significantly more frequently than stimulation of the facet joint and muscle fascia. Experiment 2: The more cranial the disc stimulated, the more frequently the reflex discharge was induced in the iliohypogastric nerve. Experiment 3: The depth of stimulation did not influence the size of the reflex action potential.

CONCLUSIONS

Electrical stimulation of the lumbar disc and facet joint induced reflex discharges in the nerves to the lower abdominal regions. It was postulated that the reflex discharges are related to muscle contraction resulting in referred pain in the loin and groin.

The induction of pain: an integrative review

The highly disagreeable sensation of pain results from an extraordinarily complex and interactive series of mechanisms integrated at all levels of the neuroaxis, from the periphery, via the dorsal horn to higher cerebral structures. Pain is usually elicited by the activation of specific nociceptors ('nociceptive pain'). However, it may also result from injury to sensory fibres, or from damage to the CNS itself ('neuropathic pain'). Although acute and subchronic, nociceptive pain fulfils a warning role, chronic and/or severe nociceptive and neuropathic pain is maladaptive. Recent years have seen a progressive unravelling of the neuroanatomical circuits and cellular mechanisms underlying the induction of pain. In addition to familiar inflammatory mediators, such as prostaglandins and bradykinin, potentially-important, pronociceptive roles have been proposed for a variety of 'exotic' species, including protons, ATP, cytokines, neurotrophins (growth factors) and nitric oxide. Further, both in the periphery and in the CNS, non-neuronal glial and immunecompetent cells have been shown to play a modulatory role in the response to inflammation and injury, and in processes modifying nociception. In the dorsal horn of the spinal cord, wherein the primary processing of nociceptive information occurs, N-methyl-D-aspartate receptors are activated by glutamate released from nocisponsive afferent fibres. Their activation plays a key role in the induction of neuronal sensitization, a process underlying prolonged painful states. In addition, upon peripheral nerve injury, a reduction of inhibitory interneurone tone in the dorsal horn exacerbates sensitized states and further enhance nociception. As concerns the transfer of nociceptive information to the brain, several pathways other than the classical spinothalamic tract are of importance: for example, the postsynaptic dorsal column pathway. In discussing the roles of supraspinal structures in pain sensation, differences between its 'discriminative-sensory' and 'affective-cognitive' dimensions should be emphasized. The purpose of the present article is to provide a global account of mechanisms involved in the induction of pain. Particular attention is focused on cellular aspects and on the consequences of peripheral nerve injury. In the first part of the review, neuronal pathways for the transmission of nociceptive information from peripheral nerve terminals to the dorsal horn, and therefrom to higher centres, are outlined. This neuronal framework is then exploited for a consideration of peripheral, spinal and supraspinal mechanisms involved in the induction of pain by stimulation of peripheral nociceptors, by peripheral nerve injury and by damage to the CNS itself. Finally, a hypothesis is forwarded that neurotrophins may play an important role in central, adaptive mechanisms modulating nociception. An improved understanding of the origins of pain should facilitate the development of novel strategies for its more effective treatment.

Regional correspondence between the ventral portion of the lumbar intervertebral disc and the groin mediated by a spinal reflex. A possible basis of discogenic referred pain

STUDY DESIGN

Lumbar peripheral nerves were examined to determine whether they were responsive to electrical stimulation of the ventral portion of the lumbar disc in anesthetized rats.

OBJECTIVES

To confirm by electrophysiologic means the neural correspondence between the ventral portion of the lumbar disc and the groin.

SUMMARY OF BACKGROUND DATA

Patients with a degenerated lumbar disc occasionally report groin pain. However, its pathogenesis has not been investigated. The authors of the current study found that chemical stimulation of the ventral portion of rat lumbar disc caused cutaneous plasma extravasation in the groin, and thereby hypothesize the neural relation between the lumbar disc and the groin.

METHODS

The ventral portion of rat L5-L6 disc was electrically stimulated, and the elicited action potentials were recorded from the iliohypogastric, genitofemoral, lateral femoral cutaneous, sural, and sciatic nerves. The roles of the lumbar sympathetic trunks and spinal cord in the generation of the action potentials were examined.

RESULTS

Action potentials were elicited principally in the genitofemoral nerve; the action potentials of the genitofemoral nerve were not influenced by transection of the cervical spinal cord, whereas they disappeared immediately after death, which indicates that they are induced by a spinal reflex. The action potentials were reduced considerably after destruction of the lumbar sympathetic trunks, suggesting that they comprise an afferent path of the reflex.

CONCLUSIONS

The ventral portion of the lumbar disc had spatial relation to the groin area via a spinal reflex. Such a relation suggests that a disorder in the ventral portion of the lumbar disc may be a possible source of groin referred pain.

Groin pain associated with lower lumbar disc herniation

STUDY DESIGN

Retrospective clinical and magnetic resonance imaging study of patients with groin pain associated with lower lumbar disc herniation.

OBJECTIVES

To demonstrate the clinical features and magnetic resonance imaging findings of these patients.

SUMMARY OF BACKGROUND DATA

Patients with lumbar disc herniation sometimes report groin pain. Little mention has been made, however, regarding the clinical features of groin pain stemmed from lower lumbar disc herniation until now, with only Murphey referring to groin pain in disc disease.

METHODS

A total of 512 patients were diagnosed with singular lower lumbar disc herniation (L4-L5 and L5-S1) at Kakegawa City General Hospital between July 1990 and December 1993. Of these patients, 21 (4.1%) reported groin pain. The characteristic clinical features and magnetic resonance imaging findings of the 21 patients were investigated and compared with the features and findings of patients with no groin pain.

RESULTS

Patients with groin pain had a higher mean age and lower rate of low back pain, and L4-L5 discs were more likely to be involved than L5-S1 discs. In their magnetic resonance images, herniation tended to be more central than in patients with no groin pain.

CONCLUSIONS

Elderly patients with L4-L5 protruding herniation of the anulus fibrosus were most likely to experience groin pain. The sinuvertebral nerve that innervates the posterior anulus fibrosus, the posterior longitudinal ligament, and the dura was indicated as the afferent nerve of groin pain.

Sensory innervation to the anterior portion of lumbar intervertebral disc

STUDY DESIGN

The level of dorsal root ganglia that receives sensory afferent nerves from the anterior portion of the lower lumbar intervertebral disc was investigated in rats using a retrograde transport method.

OBJECTIVES

Sometimes patients with lower lumbar disc lesions complain of inguinal pain that does not correspond to the dermatome of the injured nerve roots. To investigate the origin of the pain, the authors studied the sensory innervation to the anterior portion of the lumbar intervertebral disc.

SUMMARY OF BACKGROUND DATA

The innervation to the posterior portion of the lumbar disc has been extensively investigated and has been reported to be segmental. However, little is known about the nerve supply to the anterior portion of the lumbar disc.

METHODS

The retrograde transport method was used in rats. As tracers, horseradish peroxidase and choleratoxin B subunit were used. Horseradish peroxidase crystals were placed on the anterior portion of the L5-L6 disc, and choleratoxin B subunit was injected into the L5-L6 disc. The bilateral dorsal root ganglia were histologically examined.

RESULTS

Labeling of L1 and L2 dorsal root ganglia neurons was recognized. No neurons were labeled in dorsal root ganglia of other levels, including the segmentally corresponding L5.

CONCLUSIONS

Using the retrograde transport method, the authors demonstrated that the anterior portion of the L5-L6 lumbar intervertebral disc was innervated from L1 or L2 spinal nerves in rats. These results appear to explain the reason why patients with lower lumbar disc lesions sometimes complain of inguinal pain corresponding to the L1-L2 dermatome.

Neural connection between the ventral portion of the lumbar intervertebral disc and the groin skin

This study was designed to investigate neural mechanisms of referred pain in lumbar intervertebral disc lesions. Patients with a degenerative disc in lower lumbar segments occasionally complain of groin pain, which cannot be explained anatomically as having a radicular origin. In rats pretreated with intravenous application of Evans blue dye, the dye extravasation appeared in the groin skin after application of capsaicin to the ventral portion of the L5-6 intervertebral disc. This response occurred even in rats with a sectioned L-5 spinal nerve and sympathetic trunks, but did not occur in rats with a sectioned genitofemoral nerve. Capsaicin topically applied to the sciatic nerve did not cause dye extravasation in the hindpaw. Therefore, groin dye extravasation was not due to a direct effect of capsaicin but, rather, presumably was caused by an "antidromic axon reflex" of dichotomizing C fibers or to a segmental sympathetic reflex causing vascular permeability. The present results indicate that the ventral portion of the lumbar discs is neurally connected to the groin skin via the upper (L-2) lumbar spinal nerves in rats. Groin pain coincident with low-back pain may be explained as referred pain, indicating that a lesion is present in the ventral portion of the lumbar intervertebral disc space.

Origin of nerves supplying the posterior portion of lumbar intervertebral discs in rats

STUDY DESIGN

The authors studied the origin of nerves supplying the posterior portion of lumbar intervertebral discs in rats by resection of the sympathetic trunks.

OBJECTIVE

To understand discogenic low back pain from the innervation of the lumbar intervertebral discs.

SUMMARY OF BACKGROUND DATA

The afferent pathways of discogenic low back pain have not been studied thoroughly. It has been reported that stimulation of an inflamed lower spinal nerve root elicits leg pain but not low back pain and that stimulation of the posterior portion of lumbar intervertebral discs evokes only low back pain. These facts suggest that pain sensation from the posterior portion of lumbar discs is not transmitted via the lower spinal nerve roots.

METHODS

Forty-five Wistar rats were used. Seven days after the resection of sympathetic trunks with ganglia at different levels, the whole lumbar spine was stained by an acetylcholinesterase histochemical method. The posterior portions of lumbar intervertebral discs were observed.

RESULTS

The dense nerve network on the posterior portion of lumbar intervertebral discs had disappeared almost completely after total resection of bilateral sympathetic trunks at L2-L6. However, there was a slight decrease in the network after bilateral single-level resection or unilateral multisegmental resection.

CONCLUSIONS

The results showed that the posterior portion of lumbar intervertebral discs was innervated by the sympathetic nerves multisegmentally and bilaterally.

An anatomic study of neuropeptide immunoreactivities in the lumbar dura mater after lumbar sympathectomy

STUDY DESIGN

The distribution and density of nerve innervation in the lumbar dura mater after lumbar sympathectomy were assessed in wistar rats.

OBJECTIVE

To provide basic information on the interaction between sympathetic and sensory nerves in patients with low back pain.

SUMMARY OF BACKGROUND DATA

Many studies have indicated that the sinuvertebral nerve has an important role in innervating the tissues around the vertebrae. However, the origin, innervating pattern, and connections between the nerves are still controversial. It is well known that pain is often accompanied with sympathetic symptoms and exaggerated by sympathetic stimuli. Occasionally, anesthetic block at the L2 or L3 sympathetic ganglion relieves low back pain or symptoms associated with low back pain. The authors assessed the changes of the density and distribution of nerve innervation of the lumbar dura mater after lumbar sympathectomy.

METHODS

Normal adult rats were sympathectomized at L2-L3. The threshold for thermal noxious pain by hot-plate analgesia test and changes in neuropeptides in the lumbar dura mater and dorsal root ganglia using light microscopic immunohistochemistry were assessed and compared with control rats.

RESULTS

In the hot-plate analgesia test, sympathectomized rats increased their hot-plate latency time compared with that of sham-operated rats. Density of calcitonin gene-related peptide immunoreactive fibers in sympathectomy side of the lumbar dura mater decreased to 45.5% compared with the contralateral side. The number and size of calcitonin gene-related peptide immunoreactive cells in dorsal root ganglia showed no difference between sympathectomized and contralateral side.

CONCLUSION

Sympathectomy increased the pain threshold and made the sympathectomized rats hypesthetic. A large numbers of sensory fibers innervated the lumbar dura mater via L2-L3 sympathetic nerve in rats. Sympathectomy reduced the number of these nerve fibers in the lumbar dura mater. Sympathetic nerves may play an important role for low back pain involving the lumbar dura mater.

Stimulation of saphenous afferent nerve produces vasodilatation of the vasa nervorum via an axon reflex-like mechanism in the sciatic nerve of anesthetized rats

The present study was intended to examine the physiological relevance of peptidergic afferent vasodilative fibers in the regulation of blood flow in the vasa nervorum, with special reference to the axon reflex. The response of nerve blood flow (NBF) in the sciatic nerve to electrical stimulation of saphenous nerve afferents was examined using laser Doppler flowmetry in anesthetized rats whose lumbosacral afferents and efferents had been disconnected from the spinal cord. Repetitive electrical stimulation of unmyelinated fibers in the central cut end of the saphenous nerve produced an increase in NBF in the sciatic nerve ipsilateral to the stimulation, independent of changes in mean arterial blood pressure. This increase was abolished by topical application of a calcitonin gene-related peptide (CGRP) receptor antagonist, hCGRP (8-37). In conclusion, NBF in the sciatic nerve is regulated via an axon reflex-like mechanism by unmyelinated afferent CGRP containing vasodilators with collaterals in the saphenous nerve.