Neuropathic pain in spinal cord injury: significance of clinical and electrophysiological measures

A large percentage of spinal cord-injured subjects suffer from neuropathic pain below the level of the lesion (bNP). The neural mechanisms underlying this condition are not clear. The aim of this study was to elucidate the general effects of spinal deafferentiation and of bNP on electroencephalographic (EEG) activity. In addition, the relationship between the presence of bNP and impaired function of the spinothalamic tract was studied. Measurements were performed in complete and incomplete spinal cord-injured subjects with and without bNP as well as in a healthy control group. Spinothalamic tract function, assessed by contact heat evoked potentials, did not differ between subjects with and without bNP; nevertheless, it was impaired in 94% of subjects suffering from bNP. In the EEG recordings, the degree of deafferentiation was reflected in a slowing of EEG peak frequency in the 6–12-Hz band. Taking into account this unspecific effect, spinal cord-injured subjects with bNP showed significantly slower EEG activity than subjects without bNP. A discrimination analysis in the subjects with spinothalamic tract dysfunction correctly classified 84% of subjects as belonging to either the group with bNP or the group without bNP, according to their EEG peak frequency. These findings could be helpful for both the development of an objective diagnosis of bNP and for testing the effectiveness of new therapeutic agents.     

Immune responses of microglia in the spinal cord: contribution to pain states

The role of microglia and their contribution to the development and maintenance of pain states has emerged as an attractive field of study. Sensitization of central nociceptors and interneurons is thought to be responsible for the symptoms of chronic neuropathic pain states. Microglia interact with these neurons at the site of injury or disease as well as remotely. Microglia can be activated by phagocytosis or through the activation of a number of constitutively expressed cell surface molecules. Once activated, microglia participate in both innate and adaptive immune responses and remain active indefinitely. Activated microglia contribute to pain states through the production of pro-inflammatory cytokines, chemokines and extracellular proteases. Activated microglia also exhibit a modulated cell surface receptor and ion channel profile. The activation of several intracellular pathways in microglia has also been implicated in pain states. Attenuation of microglia activity is being presented as a viable therapeutic approach with regard to not only the reduction of pain symptoms but also in preventing the development of chronic pain states.     

The effect of microglia on embryonic dopaminergic neuronal survival in vitro: diffusible signals from neurons and glia change microglia from neurotoxic to neuroprotective

When embryonic dopaminergic neurons are transplanted into the adult brain, approximately 95% die within a few days. To assess whether microglia activated during transplantation might be responsible for this rapid death, we examined the effect of microglia on rat embryonic dopaminergic neurons in vitro. Conditioned medium from 7-day-old microglia was found to decrease the number of dopamine neurons surviving in primary culture, but activation of the microglia with N-formyl-methionyl-leucyl-phenylalanine (FMLP) or Zymosan A did not increase the toxicity of the conditioned medium. We next tested the effect of coculturing microglia and dopaminergic neurons by placing microglia in semipermeable well inserts over the neuronal cultures. The presence of microglia now increased dopaminergic neuronal survival, microglial activation again having no effect. To increase yet further the possible interactions between microglia and neurons, the mesencephalic cells and microglia were mixed together and placed as a tissue in three-dimensional culture, and here again the presence of microglia increased dopaminergic neuronal survival with no effect of activation. Contact of microglia with the mesencephalic cells therefore converted them from being toxic to dopaminergic neurons to promoting their survival. The change in microglial effect from toxic to protective was caused by soluble molecules secreted by cells in the neuronal cultures, as conditioned medium derived from microglia-neuronal cocultures also had a dopaminergic neuron survival effect, indicating that microglia in cocultures behave differently from microglia removed from neuronal and glial influence. Microglia cocultured with either neurons or astrocytes downregulated inducible nitric oxide synthase (iNOS), indicating a decrease in the production of nitric oxide and possibly other toxic molecules. These findings indicate that in their natural environment, microglia are likely to be beneficial for the survival of embryonic dopaminergic grafts.     

Measuring pain in the (knockout) mouse: big challenges in a small mammal.

The increasing popularity of the mouse as a subject in basic science studies of pain can largely be attributed to the development of transgenic "knockout" technology in this species only. To take advantage of this biological technique, many investigators are rushing to adapt to the mouse experimental protocols that were designed for the rat. However, the myriad physiological and behavioral differences between these two rodent species render such adaptations non-trivial and in many cases seriously problematic. In this article we review the basic nociceptive assays used in behavioral pain research (thermal, mechanical, electrical and chemical), and highlight how species differences affect their proper application. In addition, some of the issues specifically pertaining to the interpretation of such data in knockout studies are addressed.     

Dissociation of spinal microglia morphological activation and peripheral inflammation in inflammatory pain models

We compared the effects of peripheral Freund's Complete Adjuvant (CFA) and formalin injection on spinal microglia activation. Both qualitative and quantitative analyses showed signs of microglia activation on the ipsilateral side of the lumbar dorsal horn on day 3, day 7 and day 14 after formalin injection. However, significant microglia morphological alteration was not found in the CFA model. At the injection site in the paw, CFA injection induced considerably more inflammation than formalin injection. Although spinal microglia might be activated in inflammatory pain models, morphologically, spinal microglia activation was not closely correlated with peripheral inflammation.     

Role of spinal cord glia in the central processing of peripheral pain perception

Background The discovery that glial activation plays a critical role in the modulation of neuronal functions and affects the spinal processing of nociceptive signalling has brought new understanding on the mechanisms underlying central sensitization involved in chronic pain facilitation. Spinal glial activation is now considered an important component in the development and maintenance of allodynia and hyperalgesia in various models of chronic pain, including neuropathic pain and pain associated with peripheral inflammation. In addition, spinal glial activation is also involved in some forms of visceral hyperalgesia. Purpose We discuss the signalling pathways engaged in central glial activation, including stress pathways, and the neuron–glia bidirectional relationships involved in the modulation of synaptic activity and pain facilitation. In this expanding field of research, the characterization of the mechanisms by which glia affect spinal neuro‐transmission will increase our understanding of central pain facilitation, and has the potential for the development of new therapeutic agents for common chronic pain conditions.     

Neuropathic pain in post-burn hypertrophic scars: a psychophysical and neurophysiological study

Introduction: Pain complicates hypertrophic post‐burn pathologic scars (PPS) Methods: To investigate the possible neuropathic origin of pain, 13 patients with painful PPS involving at least 1 hand underwent clinical examination, including the Douleur Neuropathique en 4 questions (DN4) questionnaire; median, ulnar, and radial nerve conduction studies (NCS); cold‐ (CDT) and heat‐induced pain threshold evaluation by quantitative sensory testing; and cutaneous silent period (CSP) testing of the abductor pollicis brevis. Controls included 9 patients with non‐painful PPS, 52 healthy subjects, and 28 patients with carpal tunnel syndrome (CTS). Results: All patients with painful PPS had possible neuropathic pain (DN4 score ≥4). NCS signs of CTS were similarly present in PPS subjects with or without pain. Hands with painful PPS had lower CDT and CSP duration, more frequent cold‐ and heat‐pain hypesthesia, and more thermal allodynia than controls. Conclusions: In PPS, possible neuropathic pain is associated with psychophysical and neurophysiological abnormalities suggestive of small‐fiber damage. Muscle Nerve 45: 883–890, 2012     

Neuropathic pain: experimental advances and clinical applications

Neuropathic pain is a clinical entity designating the different types of pain associated with a lesion of the nervous system including a wide range of pathological conditions from painful peripheral lesions (for example diabetic neuropathy, post-zoster pain, trauma-induced nerve injury) and central pain (particularly stroke-induced pain, spinal lesions, and multiple sclerosis). Despite this wide range of etiologies, neuropathic pain has well characterized clinical features which generally allow distinction from other types of pain: continuous often burn-like pain, paroxysmal pain (electrical discharge, knife stab), evoked pain, highly invalidating pain (allodynia, hyperalgesia), and associated dysethesia and/or paresthesia. Over the last ten Years, very little work has been published on neuropathic pain, which is now becoming a very active domain of research in neurobiology. Advances to date have not been spectacular although better tolerated agents have been recently marketed. Future progress should enable an appropriate response to the therapeutic challenge of neuropathic pain.     

Neuropathic pain: Definition,assessment and epidemiology

Neuropathic pain has been a very active and productive clinical research field over the last 15 years. Studies have concerned multiple aspects of these complex chronic pain syndromes including their very definition, the elaboration of new diagnostic algorithms, the development of specific tools for their screening and measurement and their epidemiology. In this review, we summarize these recent evolutions that have impacted the way these pain syndromes are conceptualized and managed both in daily practice and in the clinical research setting.     

Neuropathic pain: mechanisms, diagnosis and treatment

Neuropathic pain, resulting from the disturbances of central or peripheral nervous system with mechanisms that we cannot explain yet exactly, is really a troublesome situation both for the patient and the physician. Our knowledge about this difficult to diagnose and treat pain condition has improved a lot with the aid of recent experimental and clinical studies. This review summarizes the underlying mechanisms, common conditions, diagnosis and treatment of neuropathic pain.     

Neuropathic pain: redefinition and a grading system for clinical and research purposes

Pain usually results from activation of nociceptive afferents by actually or potentially tissue-damaging stimuli. Pain may also arise by activity generated within the nervous system without adequate stimulation of its peripheral sensory endings. For this type of pain, the International Association for the Study of Pain introduced the term neuropathic pain, defined as "pain initiated or caused by a primary lesion or dysfunction in the nervous system." While this definition has been useful in distinguishing some characteristics of neuropathic and nociceptive types of pain, it lacks defined boundaries. Since the sensitivity of the nociceptive system is modulated by its adequate activation (e.g., by central sensitization), it has been difficult to distinguish neuropathic dysfunction from physiologic neuroplasticity. We present a more precise definition developed by a group of experts from the neurologic and pain community: pain arising as a direct consequence of a lesion or disease affecting the somatosensory system. This revised definition fits into the nosology of neurologic disorders. The reference to the somatosensory system was derived from a wide range of neuropathic pain conditions ranging from painful neuropathy to central poststroke pain. Because of the lack of a specific diagnostic tool for neuropathic pain, a grading system of definite, probable, and possible neuropathic pain is proposed. The grade possible can only be regarded as a working hypothesis, which does not exclude but does not diagnose neuropathic pain. The grades probable and definite require confirmatory evidence from a neurologic examination. This grading system is proposed for clinical and research purposes.     

Astrogliosis in EAE spinal cord: derivation from radial glia, and relationships to oligodendroglia

A prominent feature of multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE) is the accumulation of enlarged, multipolar glial fibrillary acidic protein (GFAP) and brain lipid binding protein (BLBP) immunoreactive astroglia within and at the margins of the inflammatory demyelinative lesions. Whether this astrogliosis is due to both astroglial hyperplasia and hypertrophy or solely to astroglial hypertrophy is controversial. We now report that coincident with the first appearance of inflammation and clinical deficits in mice with myelin oligodendrocyte glycoprotein peptide (MOG peptide)-induced EAE, the radially oriented, bipolar, GFAP, and BLBP positive cells (adult radial glia) present in normal spinal cord white matter undergo mitosis and phenotypic transformation to hypertrophic astroglia. To facilitate visualization of relationships between these hypertrophic astroglia and dying and regenerating oligodendroglia, we used mice that express enhanced green fluorescent protein (EGFP) in cells of the oligodendroglial lineage. During the first week after onset of illness, markedly swollen EGFP+ cells without processes were seen within lesions, whereas EGFP+ cells that expressed immunoreactive cleaved caspase-3 were uncommon. These observations support the hypothesis that necrosis contributes to oligodendroglial loss early in the course of EAE. Later in the illness, EGFP+ cells accumulated amongst hypertrophic astroglia at the margins of the lesions, while the lesions themselves remained depleted of oligodendroglia, suggesting that migration of oligodendroglial lineage cells into the lesions was retarded by the intense perilesional gliosis.     

LRP1 deficiency in microglia blocks neuro-inflammation in the spinal dorsal horn and neuropathic pain processing

Following injury to the peripheral nervous system (PNS), microglia in the spinal dorsal horn (SDH) become activated and contribute to the development of local neuro-inflammation, which may regulate neuropathic pain processing. The molecular mechanisms that control microglial activation and its effects on neuropathic pain remain incompletely understood. We deleted the gene encoding the plasma membrane receptor, LDL Receptor-related Protein-1 (LRP1), conditionally in microglia using two distinct promoter-Cre recombinase systems in mice. LRP1 deletion in microglia blocked development of tactile allodynia, a neuropathic pain-related behavior, after partial sciatic nerve ligation (PNL). LRP1 deletion also substantially attenuated microglial activation and pro-inflammatory cytokine expression in the SDH following PNL. Because LRP1 shedding from microglial plasma membranes generates a highly pro-inflammatory soluble product, we demonstrated that factors which activate spinal cord microglia, including lipopolysaccharide (LPS) and colony-stimulating factor-1, promote LRP1 shedding. Proteinases known to mediate LRP1 shedding, including ADAM10 and ADAM17, were expressed at increased levels in the SDH after PNL. Furthermore, LRP1-deficient microglia in cell culture expressed significantly decreased levels of interleukin-1β and interleukin-6 when treated with LPS. We conclude that in the SDH, microglial LRP1 plays an important role in establishing and/or amplifying local neuro-inflammation and neuropathic pain following PNS injury. The responsible mechanism most likely involves proteolytic release of LRP1 from the plasma membrane to generate a soluble product that functions similarly to pro-inflammatory cytokines in mediating crosstalk between cells in the SDH and in regulating neuropathic pain.     

Multiple mechanisms of microglia: a gatekeeper's contribution to pain states

Microglia are gatekeepers in the CNS for a wide range of pathological stimuli and they blow the whistle when things go wrong. Collectively, microglia form a CNS tissue alarm system (Kreutzberg's "sensor of pathology"), and their involvement in physiological pain is in line with this function. However, pathological neuropathic pain is characterized by microglial activation that is unwanted and considered to contribute to or even cause tactile allodynia, hyperalgesia and spontaneous pain. Such abnormal microglial behavior seems likely due to an as yet ill-understood disturbance of microglial functions unrelated to inflammation. The idea that microglia have roles in the CNS that differ from those of peripheral macrophages has gained momentum with the discovery of their separate, pre-hematopoietic lineage during embryonic development and their direct interactions with synapses.     

Identification of psychologic impairment in patients with mild-moderate thermal injury: small burn, big problem

Sixty-eight patients hospitalized at a tertiary care burn center for more than 1 week, aged 18-32 years, with a mild or moderate burn, adequate social and economic support, and the absence of preexisting psychopathology, substance abuse, or medical illness were studied. From this group 16 patients were identified who had been unable to resume social or occupational functioning even after several months and were impaired by psychologic symptoms related to the burn. Utilizing a psycho-social data base of 250 items, these "Small Burn, Big Problem" (SBBP) patients were compared to those who did not have this problem (control). There were no differences between the two groups with respect to age, sex, race, hospital length of stay, agent of injury, circumstances of the accident, extent of burn, or amount of disfigurement. The SBBP patients did develop significantly more sleep disturbances, which continued into the posthospitalization period. There were significant differences in the use of the defense mechanisms of regression and displacement, the extent of experiencing the injury as a narcisistic injury and in the indication of sexual dysfunction in the SBBP patients as compared to the control group. There were no differences in the amount of psychiatric treatment performed while the patients were in the hospital, although 68% of the SBBP group were referred for psychiatric treatment upon discharge as compared to 14% of the control group. Two case vignettes are presented to demonstrate some of the psychodynamics involved.     

Schwann cells: activated peripheral glia and their role in neuropathic pain

Schwann cells provide trophic support and in some cases, insulation to axons. After injury, Schwann cells undergo phenotypic modulation, acquiring the capacity to proliferate, migrate, and secrete soluble mediators that control Wallerian degeneration and regeneration. Amongst the soluble mediators are pro-inflammatory cytokines that function as chemoattractants but also may sensitize nociceptors. At the same time, Schwann cells produce factors that counterbalance the pro-inflammatory cytokines, including, for example, interleukin-10 and erythropoietin (Epo). Epo and its receptor, EpoR, are up-regulated in Schwann cells after peripheral nerve injury. EpoR-dependent cell signaling may limit production of TNF-alpha by Schwann cells within the first five days after injury. In addition, EpoR-dependent cell signaling may reduce axonal degeneration and facilitate recovery from chronic pain states. Other novel factors that regulate Schwann cell phenotype in nerve injury have been recently identified, including the low-density lipoprotein receptor related protein (LRP-1). Our recent studies indicate that LRP-1 may be essential for Schwann cell survival after peripheral nerve injury. To analyze the function of specific Schwann cell gene products in nerve injury and sensory function, conditional gene deletion and expression experiments in mice have been executed using promoters that are selectively activated in myelinating or non-myelinating Schwann cells. Blocking ErbB receptor-initiated cell-signaling in either myelinating or non-myelinating Schwann cells results in unique sensory dysfunctions. Data obtained in gene-targeted animals suggest that sensory alterations can result from changes in Schwann cell physiology without profound myelin degeneration or axonopathy. Aberrations in Schwann cell biology may lie at the foundation of neuropathic pain and represent an exciting target for therapeutic intervention.     

Neuropathic central pain: epidemiology, etiology, and treatment options

BACKGROUND: Nociceptive pain is a major problem in clinical neurology. Peripheral nerve injury may change the physiology of the dorsal horn so that pain becomes progressively centralized. OBJECTIVE: To review mechanisms underlying the plasticity of dorsal root ganglia and dorsal horn neurons that lead to central pain from a peripheral nerve injury. RESULTS: Evidence is reviewed that points to molecular changes in nociceptive terminals, ectopic firing of afferent pain fibers at the level of the dorsal root ganglia, and physiologic changes of the N-methyl-D-aspartate receptor that cause chronic nociceptive pain. CONCLUSIONS: Central sensitization is the physiologic manifestation of many severe peripherally induced pain states. It is maintained by nociceptive input and a physiologic change in the N-methyl-D-aspartate receptor. It consists of: (1) hypersensitivity at the site of injury; (2) mechanoallodynia; (3) thermal hyperalgesia; (4) hyperpathia; (5) extraterritoriality in the case of complex regional pain syndrome/reflex sympathetic dystrophy; and (6) associated neurogenic inflammation, autonomic dysregulation, and motor phenomena.     

Characterization of big dynorphins from rat brain and spinal cord

To examine the processing of products of the dynorphin gene in the central nervous system, immunoreactive (ir) dynorphin (Dyn) A, Dyn B, Dyn A-(1-8), alpha- and beta-neo-endorphin (alpha- and beta-Neo) in rat brain and spinal cord were measured, using specific antisera after gel filtration and high-performance liquid chromatography (HPLC). Three peaks of Mr about 8, 4, and 2 kDa for ir-Dyn A and ir-Dyn B, and one peak of Mr less than 2 kDa for ir-Dyn A-(1-8), ir-alpha-, and ir-beta-Neo were found both in the brain and in the spinal cord. The 8 kDa peak was recognized by Dyn A and Dyn B antisera and, after hydrolysis by proline-specific endopeptidase, by beta-Neo antiserum. The 8 kDa peak was recognized by a monoclonal antibody against the amino terminal sequence Tyr-Gly-Gly-Phe of all opioid peptides and by an antiserum directed toward the carboxyl terminus of Dyn B, indicating that it contains, from the amino terminal tyrosine of neo-endorphin to the carboxyl-terminal threonine of Dyn B, all 3 opioid peptide regions in the prodynorphin. By means of proline-specific endopeptidase hydrolysis, we also found a big dynorphin precursor (Mr approximately equal to 26 kDa) in both brain and spinal cord.