Maladaptive dendritic spine remodeling contributes to diabetic neuropathic pain

Diabetic neuropathic pain imposes a huge burden on individuals and society, and represents a major public health problem. Despite aggressive efforts, diabetic neuropathic pain is generally refractory to available clinical treatments. A structure-function link between maladaptive dendritic spine plasticity and pain has been demonstrated previously in CNS and PNS injury models of neuropathic pain. Here, we reasoned that if dendritic spine remodeling contributes to diabetic neuropathic pain, then (1) the presence of malformed spines should coincide with the development of pain, and (2) disrupting maladaptive spine structure should reduce chronic pain. To determine whether dendritic spine remodeling contributes to neuropathic pain in streptozotocin (STZ)-induced diabetic rats, we analyzed dendritic spine morphology and electrophysiological and behavioral signs of neuropathic pain. Our results show changes in dendritic spine shape, distribution, and shape on wide-dynamic-range (WDR) neurons within lamina IV-V of the dorsal horn in diabetes. These diabetes-induced changes were accompanied by WDR neuron hyperexcitability and decreased pain thresholds at 4 weeks. Treatment with NSC23766 (N(6)-[2-[[4-(diethylamino)-1-methylbutyl]amino]-6-methyl-4-pyrimidinyl]-2-methyl-4,6-quinolinediamine trihydrochloride), a Rac1-specific inhibitor known to interfere with spine plasticity, decreased the presence of malformed spines in diabetes, attenuated neuronal hyperresponsiveness to peripheral stimuli, reduced spontaneous firing activity from WDR neurons, and improved nociceptive mechanical pain thresholds. At 1 week after STZ injection, animals with hyperglycemia with no evidence of pain had few or no changes in spine morphology. These results demonstrate that diabetes-induced maladaptive dendritic spine remodeling has a mechanistic role in neuropathic pain. Molecular pathways that control spine morphogenesis and plasticity may be promising future targets for treatment.     

Spinal cord injury, dendritic spine remodeling, and spinal memory mechanisms

Spinal cord injury (SCI) often results in the development of neuropathic pain, which can persist for months and years after injury. Although many aberrant changes to sensory processing contribute to the development of chronic pain, emerging evidence demonstrates that mechanisms similar to those underlying classical learning and memory can contribute to central sensitization, a phenomenon of amplified responsiveness to stimuli in nociceptive dorsal horn neurons. Notably, dendritic spines have emerged as major players in learning and memory, providing a structural substrate for how the nervous system modifies connections to form and store information. Until now, most information regarding dendritic spines has been obtained from studies in the brain. Recent experimental data in the spinal cord, however, demonstrate that Rac1-regulated dendritic spine remodeling occurs on second-order wide dynamic range neurons and accompanies neuropathic pain after SCI. Thus, SCI-induced synaptic potentiation engages a putative spinal memory mechanism. A compelling, novel possibility for pain research is that a synaptic model of long-term memory storage could explain the persistent nature of neuropathic pain. Such a conceptual bridge between pain and memory could guide the development of more effective strategies for treatment of chronic pain after injury to the nervous system.     

Intraspinal transplantation of GABAergic neural progenitors attenuates neuropathic pain in rats: a pharmacologic and neurophysiological evaluation

Dysfunctional γ-aminobutyric acid (GABA)-ergic inhibitory neurotransmission is hypothesized to underlie chronic neuropathic pain. Intraspinal transplantation of GABAergic neural progenitor cells (NPCs) may reduce neuropathic pain by restoring dorsal horn inhibition. Rat NPCs pre-differentiated to a GABAergic phenotype were transplanted into the dorsal horn of rats with unilateral chronic constriction injury (CCI) of the sciatic nerve. GABA signaling in antinociceptive effects of NPC grafts was tested with the GABA(A) receptor antagonist bicuculline (BIC), GABA(B) receptor antagonist CGP35348 (CGP) and GABA reuptake inhibitor SKF 89976A (SKF). NPC-treated animals showed decreased hyperalgesia and allodynia 1-3week post-transplantation; vehicle-injected CCI rats continued displaying pain behaviors. Intrathecal application of BIC or CGP attenuated the antinociceptive effects of the NPC transplants while SKF injection induced analgesia in control rats. Electrophysiological recordings in NPC treated rats showed reduced responses of wide dynamic range (WDR) neurons to peripheral stimulation compared to controls. A spinal application of BIC or CGP increased wind-up response and post-discharges of WDR neurons in NPC treated animals. Results suggest that transplantation of GABAergic NPCs attenuate pain behaviors and reduce exaggerated dorsal horn neuronal firing induced by CCI. The effects of GABA receptor inhibitors suggest participation of continuously released GABA in the grafted animals.     

Reversal of ERK activation in the dorsal horn after decompression in chronic constriction injury

Injury-induced neuropathic pain is related to changes in the central terminals of dorsal root ganglia neurons, i.e., dorsal horn plasticity. We investigated the influences of decompression by removing ligatures producing chronic constriction injury (CCI) in Sprague-Dawley rats at postoperative week (POW) 4, the decompression group; for comparison, all ligatures remained through the experimental period in the CCI group. The effect was evaluated with extracellular signal-regulated kinase (ERK) activation in the dorsal horn, i.e., number of phosphorylated ERK (+) cells in the dorsal horn. At POW 1, the dorsal horn indexes had increased to a similar degree in both groups (2.40+/-0.58 vs. 2.27+/-0.36, p=0.73). At POW 8, thermal hyperalgesia and mechanical allodynia had completely disappeared with a normalization of dorsal horn index (1.17+/-0.11 vs. 1.02+/-0.12 at POW 0, p=0.07) in the decompression group; in contrast, the dorsal horn index remained elevated in the CCI group (2.48+/-0.30, p<0.001) with persistent neuropathic pain behaviors at POW 8. This report suggests that ERK activation in the dorsal horn is correlated with neuropathic pain behaviors and its normalization reflects the reversal of neuropathic pain behaviors after decompression.     

Lumbar transplant of neurons genetically modified to secrete galanin reverse pain-like behaviors after partial sciatic nerve injury

The use of cell lines as biologic "minipumps" to chronically deliver antinociceptive molecules such as the peptide galanin near the pain processing centers of the spinal cord after nerve injury is a newly developing technology for the treatment of neuropathic pain. The neuronal rat cell line, RN33B, derived from E13 brainstem raphe and immortalized with the SV40 temperature-sensitive allele of large T antigen (tsTag), was transfected with rat preprogalanin (GAL) cDNA and the galanin-synthesizing and -secreting cell line, 33GAL.19, was isolated [1]. The 33GAL.19 cells transfected with the GAL gene expressed immunoreactivity (ir) for the GAL protein and synthesized low levels of GAL-ir at permissive temperature (33 degrees C), when the cells were proliferating, and increased GAL-ir during terminal differentiation at non-permissive temperature (39 degrees C) in vitro. A control cell line, 33V.1, RN33B cells transfected with the pCEP4 vector alone and similarly isolated by subcloning, contained no detectible GAL-ir at either temperature in vitro. These cell lines were used as grafts in a model of chronic neuropathic pain induced by unilateral chronic constriction injury (CCI) of the sciatic nerve. Pain-related behaviors, including cold and tactile allodynia and thermal and mechanical hyperalgesia, were evaluated in the affected hindpaw after CCI and transplants. The 33GAL.19 and 33V.1 cells transplanted in the lumbar subarachnoid space near the spinal cord one week after CCI, survived at least seven weeks on the pial surface around the spinal cord and only the 33GAL.19 cells expressed GAL-ir in vivo after transplant. Furthermore, the tactile and cold allodynia and tactile and thermal hyperalgesia induced by CCI was significantly reduced or eliminated during the two to seven week period after grafts of 33GAL.19 cells. The maximal effect on chronic pain behaviors with the GAL grafts occurred one to three weeks after transplantation. Transplants of 33V.1 control cells had no effect on the allodynia and hyperalgesia induced by CCI. These data suggest that a chronically applied, low local dose of galanin supplied by transplanted cells near the lumbar spinal dorsal horn was able to reverse the development of chronic neuropathic pain following CCI. The use of transplants of genetically modified neural cell lines that are able to deliver antinociceptive molecules, such as galanin, offers a safe and novel approach to pain management.     

Changes in tactile stimuli-induced behavior and c-Fos expression in the superficial dorsal horn and in parabrachial nuclei after sciatic nerve crush

Neurons in the superficial laminae of the dorsal horn are dominated by input from peripheral nociceptors. Following peripheral nerve injury, low threshold mechanoreceptive Abeta-fibers sprout from their normal termination site in laminae III/IV into laminae I-II and this structural reorganization may contribute to neuropathic tactile pain hypersensitivity. We have now investigated whether a sciatic nerve crush injury alters the behavioral response in rats to tactile stimuli and whether this is associated with a change in the pattern of c-Fos expression in the dorsal horn and the parabrachial area of the brainstem. Sciatic nerve crush resulted in a patchy but marked tactile allodynia manifesting first at 3 weeks and persisting for up to 52 weeks. C-Fos expression in the dorsal horn and parabrachial region was never observed on brushing the skin of the sciatic nerve territory in animals with intact nerves, but was found after sciatic nerve crush with peripheral regeneration. We conclude that after nerve injury, low threshold mechanoreceptor fibers may play a major role in producing pain-related behavior by activating normally nociceptive-specific regions of the central nervous system such as the superficial laminae of the dorsal horn and the parabrachial area.     

Morphological characterization of dorsal horn spinal neurons in rats with unilateral constriction nerve injury: A preliminary study

Abstract

A new animal model of neuropathic pain utilizing loose ligation of a peripheral nerve has been previously reported. In addition to displaying abnormal pain symptoms such as allodynia and hyperalgesia, physiologic and morphologic changes are seen in spinal cord dorsal horn neurons. Two weeks after ligation of the right common sciatic nerve, rat dorsal horn spinal cord neurons with signs of transsynaptic changes (dark neurons) were found on the side ipsilateral to the nerve injury. A few dark neurons were also found in the contralateral dorsal horn. The distribution of dark neurons in lumbar dorsal horn was limited to the superficial laminae (l-lll). The following changes which suggest altered cellular activity were seen under the electron microscope. The nuclear envelope appeared ruffled while the mitochondria appeared normal. In additioni, the dense cytoplasm was filled with rosettes of ribosomes as well as extensively developed rough endoplasmic reticulum and distended Golgi apparatus cisternae. While dark neurons had normal appearing somatic synapses, a few appeared atypical. The altered activity of these neurons may lead to abnormal sensory experiences and may be a consequence of central changes in response to persistent peripheral nerve injury. The purpose of the present study was to assess morphologic, hence functional changes in spinal cord neurons in response to peripheral nerve constriction injury which evokes chronic pain-related behaviour. [Neurol Res 1994; 16: 297-304]     

Chronic constriction injury of sciatic nerve induces the up-regulation of descending inhibitory noradrenergic innervation to the lumbar dorsal horn of mice

Peripheral nerve injury in rodents results in hypersensitivity to mechanical and thermal stimuli accompanied by reduced antinociceptive efficacy of opioids and, in some models, sensitivity to sympathetic blockade. 2-Adrenergic receptor agonists increase in potency and efficacy after nerve injury in rodents and effectively relieve neuropathic pain in humans who do not get pain relief from opioids. However, the underlying mechanisms are unclear. It has been well known that the major noradrenergic innervation of the spinal dorsal horn originates from the locus coeruleus nucleus (LC) in the brainstem. Therefore, the aim of this study is to examine whether peripheral nerve injury that causes neuropathic pain modulates the noradrenergic innervation to the lumbar dorsal horn, in order to determine the possible anatomical substrates underlying increased potency and efficacy of noradrenergic receptor agonists in alleviating neuropathic pain. At 2 weeks after chronic constriction injury (CCI) of the sciatic nerve, a remarkable increase in tyrosine-hydroxylase (TH) and dopamine β-hydroxylase (DβH) immunoreactive (IR) axonal terminals was observed in the ipsilateral L4–L6 dorsal horn. Consistently, greater numbers of both TH- and DβH-IR neurons were detected in the ipsilateral LC. Interestingly, in the lower lumbar and upper sacral spinal dorsal horn, numerous TH-IR neurons were observed in the superficial dorsal horn (primarily lamina I). CCI of the sciatic nerve did not change the number of these TH-IR cells. These findings suggest that augmented descending inhibitory noradrenergic innervation to the dorsal horn could be one of the mechanisms underlying the increased effectiveness in the anti-allodynic effect elicited by 2-adrenergic receptor agonists.     

Activation of p38 mitogen-activated protein kinase in spinal hyperactive microglia contributes to pain hypersensitivity following peripheral nerve injury

Neuropathic pain is an expression of pathological operation of the nervous system, which commonly results from nerve injury and is characterized by pain hypersensitivity to innocuous stimuli, a phenomenon known as tactile allodynia. The mechanisms by which nerve injury creates tactile allodynia have remained largely unknown. We report that the development of tactile allodynia following nerve injury requires activation of p38 mitogen-activated protein kinase (p38MAPK), a member of the MAPK family, in spinal microglia. We found that immunofluorescence and protein levels of the dually phosphorylated active form of p38MAPK (phospho-p38MAPK) were increased in the dorsal horn ipsilateral to spinal nerve injury. Interestingly, the phospho-p38MAPK immunofluorescence in the dorsal horn was found exclusively in microglia, but not in neurons or astrocytes. The level of phospho-p38MAPK immunofluorescence in individual microglial cells was much higher in the hyperactive phenotype in the ipsilateral dorsal horn than the resting one in the contralateral side. Intrathecal administration of the p38MAPK inhibitor, 4-(4-fluorophenyl)-2-(4-methylsulfonylphenyl)-5-(4-pyridyl)-1H-imidazole (SB203580), suppresses development of the nerve injury-induced tactile allodynia. Taken together, our results demonstrate that nerve injury-induced pain hypersensitivity depends on activation of the p38MAPK signaling pathway in hyperactive microglia in the dorsal horn following peripheral nerve injury.     

NMDA receptor 2B subunit-mediated synaptic transmission in the superficial dorsal horn of peripheral nerve-injured neuropathic mice

Previous research has shown that peripheral inflammation and peripheral nerve injury alter the properties of NMDA receptors in the spinal dorsal horn. However, there is no direct evidence that demonstrates the influence of peripheral nerve injury on NMDA receptor-mediated synaptic transmission in the spinal dorsal horn. Using whole cell tight-seal methods, NMDA receptor-mediated excitatory postsynaptic currents (NMDA EPSCs) were recorded from superficial dorsal horn neurons in adult mouse spinal cord slices. Peripheral nerve injury-induced changes in the pharmacological and electrophysiological properties of synaptic NMDA receptors were studied. The ratio of the amplitude of NMDA EPSCs to that of non-NMDA EPSCs was larger in nerve-ligated neuropathic mice than in sham-operated control mice. The decay phase of the NMDA EPSCs was slower in nerve-ligated neuropathic mice. The NR2B subunit-specific NMDA receptor antagonist ifenprodil (10 microM) reduced the amplitude of the NMDA EPSCs and shortened their decay phase. The sensitivity of NMDA EPSCs to ifenprodil was significantly larger in nerve-ligated neuropathic mice than in sham-operated control mice. Single-cell RT-PCR analysis performed on superficial dorsal horn neurons showed that the incidence of NR2A mRNA-expressing neurons was reduced in nerve-ligated neuropathic mice. This result, together with the electrophysiological findings, suggests that the subunit composition of the subsynaptic NMDA receptors in the superficial dorsal horn was altered by peripheral nerve injury. Pharmacological and electrophysiological changes observed in the present experiments might be the underlying causes of the hyperalgesia and allodynia induced by peripheral nerve injury and inflammation.     

Role of the spinal cord NR2B-containing NMDA receptors in the development of neuropathic pain

Activation of N-methyl-d-aspartate (NMDA) receptors in the spinal dorsal horn has been shown to be essential for the initiation of central sensitization and the hyperexcitability of dorsal horn neurons in chronic pain. However, whether the spinal NR2B-containing NMDA (NMDA-2B) receptors are involved still remains largely unclear. Using behavioral test and in vivo extracellular electrophysiological recording in L5 spinal nerve-ligated (SNL) neuropathic rats, we investigate the roles of spinal cord NMDA-2B receptors in the development of neuropathic pain. Our study showed that intrathecal (i.t.) injection of Ro 25-6981, a selective NMDA-2B receptor antagonist, had a dose-dependent anti-allodynic effect without causing motor dysfunction. Furthermore, i.t. application of another NMDA-2B receptor antagonist ifenprodil prior to SNL also significantly inhibited the mechanical allodynia but not the thermal hyperalgesia. These data suggest that NMDA-2B receptors at the spinal cord level play an important role in the development of neuropathic pain, especially at the early stage following nerve injury. In addition, spinal administration of Ro 25-6981 not only had a dose-dependent inhibitory effect on the C-fiber responses of dorsal horn wide dynamic range (WDR) neurons in both normal and SNL rats, but also significantly inhibited the long-term potentiation (LTP) in the C-fiber responses of WDR neurons induced by high-frequency stimulation (HFS) applied to the sciatic nerve. These results indicate that activation of the dorsal horn NMDA-2B receptors may be crucial for the spinal nociceptive synaptic transmission and for the development of long-lasting spinal hyperexcitability following nerve injury. In conclusion, the spinal cord NMDA-2B receptors play a role in the development of central sensitization and neuropathic pain via the induction of LTP in dorsal horn nociceptive synaptic transmission. Therefore, the spinal cord NMDA-2B receptor is likely to be a target for clinical pain therapy.     

Normal and injury-induced sympathetic innervation of rat dorsal root ganglia increases with age

In rats, partial injury to a peripheral nerve often leads to sympathetically maintained pain (SMP). In humans, this condition is especially apparent in the elderly. Nerve injury also causes perivascular sympathetic axons to sprout into the dorsal root ganglion (DRG), forming a possible anatomical substrate for SMP. Here, we describe the effects of chronic sciatic nerve constriction injury (CCI) in young (3 months) and old (16 months) rats on neuropathic pain behavior and on sympathetic sprouting in DRG. Behavioral tests assessed changes in thermal allodynia and hyperalgesia and in mechanical allodynia. We found that 1) sympathetic innervation of the DRG increased naturally with age, forming pericellular baskets mainly around large DRG neurons, and that sympathetic fibers were often associated with myelinated sensory axons; 2) sympathetic fiber density following CCI was also greater in old than in young rats; and 3) in old rats, thermal allodynia was less pronounced than in young rats, whereas thermal hyperalgesia and mechanical allodynia were more pronounced. These results highlight the possibility that sympathetic sprouting in the DRG is responsible for the sympathetic generation or maintenance of pain, especially in the elderly. J. Comp. Neurol. 394:38–47, 1998. © 1998 Wiley-Liss, Inc.     

Activation of extracellular signal-regulated protein kinases 5 in the spinal cord contributes to the neuropathic pain behaviors induced by CCI in rats

Abstract

Objective: To investigate whether activation and translocation of extracellular signal-regulated kinase 5 (ERK5) is involved in the induction and maintenance of neuropathic pain and observe the effects of activation and translocation of ERK5 on the expression of phosphorylated cAMP response element binding (pCREB) in the chronic neuropathic pain.

Methods: Lumbar intrathecal catheters were chronically implanted in male Sprague–Dawley rats. The left sciatic nerve was loosely ligated proximal to the sciatica's trifurcation at ～ 1.0 mm intervals with 4-0 silk sutures. The phosphorothioate-modified antisense oligonucleotides (AS-ODNs) were intrathecally administered every 12 hours, 1 day pre-chronic constriction injury (CCI) and 3 day post-CCI. Thermal and mechanical nociceptive thresholds were assessed with the paw withdrawal latency to a radiant heat and von Frey filaments. Expressions of phosphorylated ERK5 (pERK5), pCREB, were assessed by both Western blotting and immunohistochemical analysis.

Results: Intrathecal injection of ERK5 AS-ODN significantly attenuated CCI-induced mechanical allodynia and thermal hyperalgesia. CCI significantly increased the expression of pERK5 neurons in the ipsilateral spinal dorsal horn to injury, not in the contralateral spinal dorsal horn. The time courses of pERK5 expression showed that the levels of both cytosol and nuclear pERK5 were increased at all points after CCI and reached a peak level on post-operative day 5. CCI significantly increased the expression of pERK5 neurons in the laminae I and II of ipsilateral spinal dorsal horn to injury, not in the contralateral spinal dorsal horn. Phospho-CREB-positive neurons were distributed in all laminae of the bilateral spinal cord. Intrathecal injection AS-ODN markedly suppressed the increase of CCI-induced pERK5, pCREB expression in the spinal cord.

Conclusion: The activation of ERK5 pathways contributes to neuropathic pain in CCI rats, and the function of pERK5 may partly be accomplished via the CREB protein-dependent gene expression.     

Ca2+/calmodulin-dependent protein kinase II in the spinal cord contributes to neuropathic pain in a rat model of mononeuropathy

Ca2+/calmodulin-dependent protein kinase II (CaMKII) is known to subserve activity-dependent neuronal plasticity in the central nervous system. To examine in vivo the implication of spinal CaMKII activity in the generation and development of neuropathic pain after peripheral nerve injury, we used an animal model of mononeuropathy, the chronic constriction injury (CCI) model, in the rat. We found that, 3 days after CCI, the total CaMKII (tCaMKII) immunoreactivity increased in the superficial laminae of the spinal cord and this increase continued for up to 14 days. The immunoreactivity of phosphorylated CaMKII showed an increase from 1 day after CCI, which preceded the up-regulation of tCaMKII. A non-selective N-methyl-d-aspartate receptor antagonist, MK801, significantly attenuated the increase of tCaMKII and phosphorylated CaMKII. Moreover, intrathecal administration of an inhibitor of CaMKII, KN93, before the CCI surgery attenuated the development of thermal hyperalgesia and mechanical allodynia. In addition, KN93 significantly reduced the nociceptive behavior in phase II of the formalin test. These findings demonstrate that the activity of CaMKII in spinal neurons is elevated after peripheral nerve injury and may be involved in central sensitization. The alteration of CaMKII is considered to be a neuroplastic change that occurs in spinal neurons that contributes to neuropathic pain, suggesting the potential for the development of novel therapeutics for neuropathic pain that target CaMKII.     

Mechanisms of spinal cord stimulation in neuropathic pain

The understanding of the mode of action of spinal cord stimulation (SCS) as treatment of neuropathic pain is still fragmentary. SCS evolved from the gate-control theory postulating a spinal modulation of noxious inflow, but there is little evidence that SCS influences nociceptive pain; pain relief in peripheral vascular disease and angina pectoris is presumably secondary to other SCS effects. In man, SCS may effectively abolish both continuous and evoked pain (tactile/thermal allodynia) whereas induced, acute nociceptive pain is unaffected. Recent SCS studies performed on rat models of mononeuropathy have demonstrated a preferential effect on A beta fiber mediated functions, and the hyperexcitability of wide-dynamic-range dorsal horn neurons was attenuated. These effects were coupled to increased release of GABA and reduced glutamate and aspartate release in the dorsal horn. Intrathecal administration of GABA, baclofen and adenosine enhanced the SCS effect on tactile allodynia even in previously non-responsive rats. Preliminary results indicate that gabapentin may have a similar effect. GABAergic and adenosine-related mechanisms conceivably represent only examples of a number of putative receptor systems involved in SCS. Clinical trials have been initiated exploring the possibility to improve the efficacy of SCS by concomitant pharmacotherapy.     

Spinal cord injury triggers sensitization of wide dynamic range dorsal horn neurons in segments rostral to the injury

A spinal cord injury (SCI) was produced in adult rats by complete spinal cord transection at L6-S1. Neuropathic pain behaviors similar to the chronic central pain (CCP) syndrome in human, such as thermal hyperalgesia, mechanical allodynia and autotomy, were present in these rats after spinal cord injury. Meanwhile, wide dynamic range (WDR) neurons recorded in the spinal dorsal horn rostral to the lesion responded as high frequency of spontaneous activities, long duration of after-discharges to noxious electrical stimuli and an augmented wind-up to 0.5 Hz stimuli. By using bupivacaine powder, a sodium channel blocker, at the locus of transection immediate after nerve injury, the chronic pain behaviors were prevented; the hyperexcitability of WDR neurons was also substantially reduced. It is suggested that spinal cord transection induces the CCP syndromes, which may be evoked and maintained by the hyperexcitability in WDR neurons rostrally. Reducing the neuronal activity at the site of lesion following injury may prevent the development of CCP after SCI.     

Gliopathy ensures persistent inflammation and chronic pain after spinal cord injury

Research focused on improving recovery of function, including the reduction of central neuropathic pain (CNP) after spinal cord injury (SCI) is essential. After SCI, regional neuropathic pain syndromes above, at and below the level or spinal injury develop and are thought to have different mechanisms, but may share common dysfunctional glial mechanisms. Detloff et al., [Detloff, M.R., Fisher, L.C., McGaughy, V., Longbrake, E.E., Popovich, P.G., Basso, D.M., Remote activation of microglia and pro-inflammatory cytokines predict the onset and severity of below-level neuropathic pain after spinal cord injury in rats. Exp. Neurol. (2008), doi: 10.1016/j.expneurol.2008.04.009.] describe events in the lumbar region of the spinal cord after a midthoracic SCI injury, the so called “below-level” pain and compares the findings to peripheral nerve lesion findings. This commentary briefly reviews glial contributions and intracellular signaling mechanisms, both neuronal and glial, that provide the substrate for CNP after SCI, including the persistent glial production of factors that can maintain sensitization of dorsal horn neurons in segments remote from the spinal injury; ie. dorsal horn hyperexcitability to formerly non noxious stimuli that become noxious after SCI resulting in allodynia. The term “gliopathy” is proposed to describe the dysfunctional and maladaptive response of glial cells, specifically astrocytes and microglia, to neural injury that is initiated by the sudden injury induced increase in extracellular concentrations of glutamate and concomitant production of several proinflammatory molecules. It is important to understand the roles that different glia play in “gliopathy”, a condition that appears to persist after SCI. Furthermore, targeted treatment of gliopathy will attenuate mechanical allodynia in both central and peripheral neuropathic pain syndromes.     

Tissue plasminogen activator in primary afferents induces dorsal horn excitability and pain response after peripheral nerve injury

The extracellular protease cascade of plasminogen activators and plasminogen are known to regulate neuronal plasticity and extracellular matrix modification, and to be important factors involved in producing long-term potentiation in the CNS. The purpose of this study is to examine the expression of plasminogen activators in primary afferents and its role in nociceptive pathways after peripheral nerve injury. We found the induction of mRNAs for tissue type plasminogen activator (tPA) and urokinase plasminogen activator (uPA) in the rat dorsal root ganglia following sciatic nerve transection. Immunoreactivity for tPA was increased in laminae I and II of the dorsal horn and, importantly, the increase in proteolytic activity mediated by tPA was observed in the same area. As neither immunoreactivity for uPA nor uPA-mediated proteolysis was observed, we further examined the effects of tPA on dorsal horn excitability and neuropathic pain behaviour. Intrathecal injection of a specific inhibitor of tPA decreased electrical stimulation-induced Fos expression in dorsal horn neurons following axotomy, and also prevented the development of thermal hyperalgesia following partial sciatic nerve ligation. These findings suggest that the increased tPA in the dorsal horn due to mRNA expression in the dorsal root ganglia increases the dorsal horn excitability and has an important role in pain behaviour after peripheral nerve injury. The tPA-mediated hypersensitivity in dorsal horn neurons may be a novel molecular mechanism of neuropathic pain.