Dendritic Spine Remodeling After Spinal Cord Injury Alters Neuronal Signal Processing

Central sensitization, a prolonged hyperexcitability of dorsal horn nociceptive neurons, is a major contributor to abnormal pain processing after spinal cord injury (SCI). Dendritic spines are micron-sized dendrite protrusions that can regulate the efficacy of synaptic transmission. Here we used a computational approach to study whether changes in dendritic spine shape, density, and distribution can individually, or in combination, adversely modify the input–output function of a postsynaptic neuron to create a hyperexcitable neuronal state. The results demonstrate that a conversion from thin-shaped to more mature, mushroom-shaped spine structures results in enhanced synaptic transmission and fidelity, improved frequency-following ability, and reduced inhibitory gating effectiveness. Increasing the density and redistributing spines toward the soma results in a greater probability of action potential activation. Our results demonstrate that changes in dendritic spine morphology, documented in previous studies on spinal cord injury, contribute to the generation of pain following SCI.

Remote neuroimmune signaling: a long-range mechanism of nociceptive network plasticity

Chronic pain secondary to neuronal injury is actively and continuously modulated at multiple locations along the sensory neuraxis. Here, we describe how nociceptive neurons of the spinal cord and thalamus process and communicate nociceptive information in terms of precisely calibrated firing patterns. We then discuss how several cell types with immunogenic properties (e.g. blood cells and glia) cause system-wide interference in nociceptive processing through novel signaling schema, thus contributing to nociceptive network plasticity and chronic pain.

Early microglial inhibition preemptively mitigates chronic pain development after experimental spinal cord injury

Spinal cord injury (SCI) results in the development of chronic pain syndromes that can persist indefinitely and cause reductions in quality of life. Treatment of chronic pain after SCI remains extremely challenging; thus, an important research goal is to determine whether early treatments can attenuate the subsequent development of pain conditions. The current study examined the hypothesis that early administration of the microglial-inhibiting drug minocycline could ameliorate the development of pain after SCI. Adult male Sprague-Dawley rats underwent SCI at the ninth thoracic spinal segment and received either vehicle or minocycline treatment for 5 days postinjury. Time course studies revealed that over 4 weeks post-SCI, microglial activation in vehicle-treated animals was progressively increased. Minocycline treatment resulted in reduction, but not prevention, of microglial activation over time. Electrophysiological experiments showed that early minocycline administration attenuated the development of chronic hyperresponsiveness of lumbar dorsal horn neurons. Similarly, behavioral assessment showed that minocycline also resulted in increased pain thresholds. These results suggest that inhibition of early neuroimmune events can have a powerful impact on the development of long-term pain phenomena following SCI and support the conclusion that modulation of microglial signaling may provide a new therapeutic strategy for patients suffering from post-SCI pain.

Mechanisms of chronic central neuropathic pain after spinal cord injury

Not all spinal contusions result in mechanical allodynia, in which non-noxious stimuli become noxious. The studies presented use the NYU impactor at 12.5 mm drop or the Infinite Horizons Impactor (150 kdyn, 1 s dwell) devices to model spinal cord injury (SCI). Both of these devices and injury parameters, if done correctly, will result in animals with above level (forelimb), at level (trunk) and below level (hindlimb) mechanical allodynia that model the changes in evoked somatosensation experienced by the majority of people with SCI. The sections are as follows: 1) Mechanisms of remote microglial activation and pain signaling in "below-level" central pain 2) Intracellular signaling mechanisms in central sensitization in "at-level" pain 3) Peripheral sensitization contributes to "above level" injury pain following spinal cord injury and 4) Role of reactive oxygen species in central sensitization in regional neuropathic pain following SCI. To summarize, differential regional mechanisms contribute to the regional chronic pain states. We propose the importance of understanding the mechanisms in the differential regional pain syndromes after SCI in the chronic condition. Targeting regional mechanisms will be of enormous benefit to the SCI population that suffer chronic pain, and will contribute to better treatment strategies for other chronic pain syndromes.

Role of hippocampal sodium channel Nav1.6 in kindling epileptogenesis

PURPOSE

Central nervous system plasticity is essential for normal function, but can also reinforce abnormal network behavior, leading to epilepsy and other disorders. The role of altered ion channel expression in abnormal plasticity has not been thoroughly investigated. Nav1.6 is the most abundantly expressed sodium channel in the nervous system. Because of its distribution in the cell body and axon initial segment, Nav1.6 is crucial for action potential generation. The goal of the present study was to investigate the possible role of changes in Nav1.6 expression in abnormal, activity-dependent plasticity of hippocampal circuits.

METHODS

We studied kindling, a form of abnormal activity-dependent facilitation. We investigated: (1) sodium channel protein expression by immunocytochemistry and sodium channel messenger RNA (mRNA) by in situ hybridization, (2) sodium current by patch clamp recordings, and (3) rate of kindling by analysis of seizure behavior. The initiation, development, and expression of kindling in wild-type mice were compared to Nav1.6 +/-med(tg) mice, which have reduced expression of Nav1.6.

RESULTS

We found that kindling was associated with increased expression of Nav1.6 protein and mRNA, which occurred selectively in hippocampal CA3 neurons. Hippocampal CA3 neurons also showed increased persistent sodium current in kindled animals compared to sham-kindled controls. Conversely, Nav1.6 +/-med(tg) mice resisted the initiation and development of kindling.

DISCUSSION

These findings suggest an important mechanism for enhanced excitability, in which Nav1.6 may participate in a self-reinforcing cycle of activity-dependent facilitation in the hippocampus. This mechanism could contribute to both normal hippocampal function and to epilepsy and other common nervous system disorders.

Locomotor dysfunction and pain: the scylla and charybdis of fiber sprouting after spinal cord injury

Injury to the spinal cord (SCI) can produce a constellation of problems including chronic pain, autonomic dysreflexia, and motor dysfunction. Neuroplasticity in the form of fiber sprouting or the lack thereof is an important phenomenon that can contribute to the deleterious effects of SCI. Aberrant sprouting of primary afferent fibers and synaptogenesis within incorrect dorsal horn laminae leads to the development and maintenance of chronic pain as well as autonomic dysreflexia. At the same time, interruption of connections between supraspinal motor control centers and spinal cord output cells, due to lack of successful regenerative sprouting of injured descending fiber tracts, contributes to motor deficits. Similarities in the molecular control of axonal growth of motor and sensory fibers have made the development of cogent therapies difficult. In this study, we discuss recent findings related to the degradation of inhibitory barriers and promotion of sprouting of motor fibers as a strategy for the restoration of motor function and note that this may induce primary afferent fiber sprouting that can contribute to chronic pain. We highlight the importance of careful attentiveness to off-target molecular- and circuit-level modulation of nociceptive processing while moving forward with the development of therapies that will restore motor function after SCI.

Alarm or curse? The pain of neuroinflammation

The nociceptive nervous system and the immune system serve to defend and alarm the host of imminent or actual damage. However, persistent or recurring exposure of neurons to activated immune cells is associated with an increase in painful behavior following experimental neuropathic injuries. Our understanding of the functional consequences of immune cell-neuron interaction is still incomplete. The purpose of this review is to focus on a seriously detrimental consequence of chronic activation of these two systems, by discussing the contributions of microglia and polymorphonuclear neutrophils to neuropathic pain following experimental spinal cord injury or peripheral nerve injury. Identification of molecules mediating pro-nociceptive signaling between immune cells and neurons, as well as the distinction between neuroprotective versus neuroexcitatory effects of activated immune cells, may be useful in the development of pharmacotherapy for the management of chronic pain and restoration of the beneficial alarm function of pain.

Disruption of cAMP and prostaglandin E2 transport by multidrug resistance protein 4 deficiency alters cAMP-mediated signaling and nociceptive response

Multidrug resistance protein 4 (MRP4; ABCC4) is a member of the MRP/ATP-binding cassette family serving as a transmembrane transporter involved in energy-dependent efflux of anticancer/antiviral nucleotide agents and of physiological substrates, including cyclic nucleotides and prostaglandins (PGs). Phenotypic consequences of mrp4 deficiency were investigated using mrp4-knockout mice and derived immortalized mouse embryonic fibroblast (MEF) cells. Mrp4 deficiency caused decreased extracellular and increased intracellular levels of cAMP in MEF cells under normal and forskolin-stimulated conditions. Mrp4 deficiency and RNA interference-mediated mrp4 knockdown led to a pronounced reduction in extracellular PGE(2) but with no accumulation of intracellular PGE(2) in MEF cells. This result was consistent with attenuated cAMP-dependent protein kinase activity and reduced cyclooxygenase-2 (Cox-2) expression in mrp4-deficient MEF cells, suggesting that PG synthesis is restrained along with a lack of PG transport caused by mrp4 deficiency. Mice lacking mrp4 exhibited no outward phenotypes but had a decrease in plasma PGE metabolites and an increase in inflammatory pain threshold compared with wild-type mice. Collectively, these findings imply that mrp4 mediates the efflux of PGE(2) and concomitantly modulates cAMP mediated signaling for balanced PG synthesis in MEF cells. Abrogation of mrp4 affects the regulation of peripheral PG levels and consequently alters inflammatory nociceptive responses in vivo.

Modulation of thalamic nociceptive processing after spinal cord injury through remote activation of thalamic microglia by cysteine cysteine chemokine ligand 21

Spinal cord injury (SCI) results in the generation and amplification of pain caused in part by injury-induced changes in neuronal excitability at multiple levels along the sensory neuraxis. We have previously shown that activated microglia, through an ERK (extracellular signal-regulated kinase)-regulated PGE(2) (prostaglandin E(2)) signaling mechanism, maintain neuronal hyperexcitability in the lumbar dorsal horn. Here, we examined whether microglial cells in the thalamus contribute to the modulation of chronic pain after SCI, and whether microglial activation is governed by spinally mediated increases in the microglial activator cysteine-cysteine chemokine ligand 21 (CCL21). We report that CCL21 is upregulated in dorsal horn neurons, that tissue levels are increased in the dorsal horn and ventral posterolateral (VPL) nucleus of the thalamus 4 weeks after SCI, and that the increase can be differentially reduced by spinal blockade at T1 or L1. In intact animals, electrical stimulation of the spinothalamic tract induces increases in thalamic CCL21 levels. Recombinant CCL21 injected into the VPL of intact animals transiently activates microglia and induces pain-related behaviors, effects that could be blocked with minocycline. After SCI, intra-VPL antibody-mediated neutralization of CCL21 decreases microglial activation and evoked hyperexcitability of VPL neurons, and restores nociceptive thresholds to near-normal levels. These data identify a novel pathway by which SCI triggers upregulation of the neuroimmune modulator CCL21 in the thalamus, which induces microglial activation in association with pain phenomena.

Extracellular signal-regulated kinase-regulated microglia-neuron signaling by prostaglandin E2 contributes to pain after spinal cord injury

Many patients with traumatic spinal cord injury (SCI) report pain that persists indefinitely and is resistant to available therapeutic approaches. We recently showed that microglia become activated after experimental SCI and dynamically maintain hyperresponsiveness of spinal cord nociceptive neurons and pain-related behaviors. Mechanisms of signaling between microglia and neurons that help to maintain abnormal pain processing are unknown. In this study, adult male Sprague Dawley rats underwent T9 spinal cord contusion injury. Four weeks after injury when lumbar dorsal horn multireceptive neurons became hyperresponsive and when behavioral nociceptive thresholds to mechanical and thermal stimuli were decreased, we tested the hypothesis that prostaglandin E2 (PGE2) contributes to signaling between microglia and neurons. Immunohistochemical data showed specific localization of phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2), an upstream regulator of PGE2 release, to microglial cells and a neuronal localization of the PGE2 receptor E-prostanoid 2 (EP2). Enzyme immunoassay analysis showed that PGE2 release was dependent on microglial activation and ERK1/2 phosphorylation. Pharmacological antagonism of PGE2 release was achieved with the mitogen-activated protein kinase kinase 1/2 (MEK1/2) inhibitor PD98059 [2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one] and the microglial inhibitor minocycline. Cyclooxygenase-2 expression in microglia was similarly reduced by MEK1/2 inhibition. PD98059 and EP2 receptor blockade with AH6809 (6-isopropoxy-9-oxoxanthene-2-carboxylic acid) resulted in a decrease in hyperresponsiveness of dorsal horn neurons and partial restoration of behavioral nociceptive thresholds. Selective targeting of dorsal horn microglia with the Mac-1-SAP immunotoxin, a chemical conjugate of mouse monoclonal antibody to CD11b and the ribosome-inactivating protein saporin, resulted in reduced microglia staining, reduction in PGE2 levels, and reversed pain-related behaviors [corrected]. On the basis of these observations, we propose a PGE2-dependent, ERK1/2-regulated microglia-neuron signaling pathway that mediates the microglial component of pain maintenance after injury to the spinal cord.

In SilicoModeling of Axonal Reconnection within A Discrete Fiber Tract after Spinal Cord Injury

Following spinal cord injury (SCI), descending axons that carry motor commands from the brain to the spinal cord are injured or transected, producing chronic motor dysfunction and paralysis. Reconnection of these axons is a major prerequisite for restoration of function after SCI. Thus far, only modest gains in motor function have been achieved experimentally or in the clinic after SCI, identifying the practical limitations of current treatment approaches. In this paper, we use an ordinary differential equation (ODE) to simulate the relative and synergistic contributions of several experimentally-established biological factors related to inhibition or promotion of axonal repair and restoration of function after SCI. The factors were mathematically modeled by the ODE. The results of our simulation show that in a model system, many factors influenced the achievability of axonal reconnection. Certain factors more strongly affected axonal reconnection in isolation, and some factors interacted in a synergistic fashion to produce further improvements in axonal reconnection. Our data suggest that mathematical modeling may be useful in evaluating the complex interactions of discrete therapeutic factors not possible in experimental preparations, and highlight the benefit of a combinatorial therapeutic approach focused on promoting axonal sprouting, attraction of cut ends, and removal of growth inhibition for achieving axonal reconnection. Predictions of this simulation may be of utility in guiding future experiments aimed at restoring function after SCI.

Sodium channel expression and the molecular pathophysiology of pain after SCI

The chronic pain that develops as a result of spinal cord injury (SCI) is extremely debilitating and remains largely unmanageable by current therapeutic strategies. Voltage-gated sodium channels regulate the biophysical properties, and thus firing characteristics, of neurons. After SCI the repertoire of sodium channels produced by dorsal horn nociceptive neurons is altered, enabling neurons to fire at higher than normal rates in response to unchanged peripheral stimuli as well as to generate spontaneous discharges in the absence of stimuli, resulting in the genesis of neuropathic pain. Our results have shown increased expression of the Nav1.3 sodium channel in the spinal cord and thalamus. Nav1.3 upregulation allows dorsal horn neurons to generate ramp currents, enhanced persistent currents, and shifts in steady-state activation and inactivation. Further downstream, Nav1.3 causes increased spontaneous and evoked firing of neurons in the ventroposterior lateral (VPL) nucleus of the thalamus. Nav1.3 also underlies changes in burst firing properties of VPL neurons. The combination of spinal and thalamic generation and amplification of pain by Nav1.3 dysregulation contributes to post-SCI chronic pain. If proven to be similar in humans, targeting of this system after SCI may offer hope for treatment of clinical pain.

Long-term protection of central axons with phenytoin in monophasic and chronic-relapsing EAE

Axonal degeneration is a major contributor to non-remitting deficits in multiple sclerosis, and there is thus considerable current interest in the development of strategies that might prevent axonal loss in neuroinflammatory disease. Dysregulation of sodium ion homeostasis has been implicated in mechanisms leading to axonal degeneration, and several studies have shown that blockade of sodium channels can ameliorate axon damage following anoxic, traumatic and nitric oxide-induced CNS injury. Two sodium channel blockers, phenytoin and flecainide, have been reported to protect axons in experimental autoimmune encephalomyelitis (EAE) for 30 days, but long-term protective effects have not been studied. We demonstrate here that oral administration of phenytoin provides long-term (up to 180 days) protection for spinal cord corticospinal tract (CST) and dorsal column (DC) axons in both monophasic (C57/BL6 mice) and chronic-relapsing (Biozzi mice) murine EAE. Untreated C57/BL6 mice exhibit a 40-50% loss of CST and DF axons at 90 and 180 days post-EAE induction via myelin-oligodendrocyte glycoprotein (MOG) injection. In contrast, only 4% of DF axons are lost at 90 days, and only 8% are lost at 180 days in phenytoin-treated C57/BL6 mice with EAE; only 21-29% of CST axons are lost at 90 and 180 days in phenytoin-treated C57/BL6 mice with EAE. Attenuation of dorsal column compound action potentials was ameliorated and clinical status was also significantly enhanced with phenytoin treatment at 90 and 180 days in this model. In addition, inflammatory cell infiltration into the dorsal columns was reduced in phenytoin-treated mice with EAE compared with untreated mice with EAE. Similar results were obtained in Biozzi mice with chronic-relapsing EAE followed for 120 days post-injection. These observations demonstrate that phenytoin provides long-term protection of CNS axons and improves clinical status in both monophasic and chronic-relapsing models of neuroinflammation.

Sodium channel expression in the ventral posterolateral nucleus of the thalamus after peripheral nerve injury

Peripheral nerve injury is known to up-regulate the expression of rapidly-repriming Nav1.3 sodium channel within first-order dorsal root ganglion neurons and second-order dorsal horn nociceptive neurons, but it is not known if pain-processing neurons higher along the neuraxis also undergo changes in sodium channel expression. In this study, we hypothesized that after peripheral nerve injury, third-order neurons in the ventral posterolateral (VPL) nucleus of the thalamus undergo changes in expression of sodium channels. To test this hypothesis, adult male Sprague-Dawley rats underwent chronic constriction injury (CCI) of the sciatic nerve. Ten days after CCI, when allodynia and hyperalgesia were evident, in situ hybridization and immunocytochemical analysis revealed up-regulation of Nav1.3 mRNA, but no changes in expression of Nav1.1, Nav1.2, or Nav1.6 in VPL neurons, and unit recordings demonstrated increased background firing, which persisted after spinal cord transection, and evoked hyperresponsiveness to peripheral stimuli. These results demonstrate that injury to the peripheral nervous system induces alterations in sodium channel expression within higher-order VPL neurons, and suggest that misexpression of the Nav1.3 sodium channel increases the excitability of VPL neurons injury, contributing to neuropathic pain.

Alterations in Burst Firing of Thalamic VPL Neurons and Reversal by Nav1.3 Antisense After Spinal Cord Injury

We recently showed that spinal cord contusion injury (SCI) at the thoracic level induces pain-related behaviors and increased spontaneous discharges, hyperresponsiveness to innocuous and noxious peripheral stimuli, and enlarged receptive fields in neurons in the ventral posterolateral (VPL) nucleus of the thalamus. These changes are linked to the abnormal expression of Nav1.3, a rapidly repriming voltage-gated sodium channel. In this study, we examined the burst firing properties of VPL neurons after SCI. Adult male Sprague–Dawley rats underwent contusion SCI at the T9 level. Four weeks later, when Nav1.3 protein was upregulated within VPL neurons, extracellular unit recordings were made from VPL neurons in intact animals, those with SCI, and in SCI animals after receiving lumbar intrathecal injections of Nav1.3 antisense or mismatch oligodeoxynucleotides for 4 days. After SCI, VPL neurons with identifiable peripheral receptive fields showed rhythmic oscillatory burst firing with changes in discrete burst properties, and alternated among single-spike, burst, silent, and spindle wave firing modes. Nav1.3 antisense, but not mismatch, partially reversed alterations in burst firing after SCI. These results demonstrate several newly characterized changes in spontaneous burst firing properties of VPL neurons after SCI and suggest that abnormal expression of Nav1.3 contributes to these phenomena.

Upregulation of persistent and ramp sodium current in dorsal horn neurons after spinal cord injury

Traumatic spinal cord injury (SCI) results not only in motor impairment, but also in chronic central neuropathic pain, which often is refractory to conventional treatment approaches. Upregulated expression of sodium channel Nav1.3 has been observed within the spinal dorsal horn neurons after SCI, and appears to contribute to neuronal hyperresponsiveness and pain-related behaviors. In this study we characterized the changes in sodium current properties within dorsal horn neurons after contusive SCI. Four weeks after adult male Sprague-Dawley rats underwent T9 spinal cord contusion injury, when behavioral nociceptive thresholds were decreased to both mechanical and thermal stimuli, whole-cell patch-clamp recordings were performed on acutely dissociated lumbar dorsal horn neurons. The cells demonstrated characteristic fast-activating and fast-inactivating sodium currents. SCI led to a shift of the steady-state activation and inactivation of the sodium current towards more depolarized potentials. The shifted steady-state inactivation shows similarities to that obtained from axotomized dorsal root ganglions, which were shown to upregulate Nav1.3. Small slow depolarizations below action potential threshold produced ramp currents, which were markedly enhanced by SCI (from 182 +/- 41 to 338 +/- 55 pA). The density of the noninactivating persistent sodium current was also significantly enhanced in neurons from SCI animals (from 17.4 +/- 3.2 to 27.7 +/- 4.4 pA/pF at 50-70 ms of depolarization). The increased persistent sodium current and ramp current, which are consistent with upregulation of Nav1.3 within dorsal horn neurons, suggest a basis for the hyperresponsiveness of these neurons following SCI.

Activated microglia contribute to the maintenance of chronic pain after spinal cord injury

Traumatic spinal cord injury (SCI) results not only in motor impairment but also in chronic central pain, which can be refractory to conventional treatment approaches. It has been shown recently that in models of peripheral nerve injury, spinal cord microglia can become activated and contribute to development of pain. Considering their role in pain after peripheral injury, and because microglia are known to become activated after SCI, we tested the hypothesis that activated microglia contribute to chronic pain after SCI. In this study, adult male Sprague Dawley rats underwent T9 spinal cord contusion injury. Four weeks after injury, when lumbar dorsal horn multireceptive neurons became hyperresponsive and when behavioral nociceptive thresholds were decreased to both mechanical and thermal stimuli, intrathecal infusions of the microglial inhibitor minocycline were initiated. Electrophysiological experiments showed that minocycline rapidly attenuated hyperresponsiveness of lumbar dorsal horn neurons. Behavioral data showed that minocycline restored nociceptive thresholds, at which time spinal microglial cells assumed a quiescent morphological phenotype. Levels of phosphorylated-p38 were decreased in SCI animals receiving minocycline. Cessation of delivery of minocycline resulted in an immediate return of pain-related phenomena. These results suggest an important role for activated microglia in the maintenance of chronic central below-level pain after SCI and support the newly emerging role of non-neuronal immune cells as a contributing factor in post-SCI pain.

Fire and phantoms after spinal cord injury: Na+ channels and central pain

Neuropathic pain and phantom phenomena occur commonly after spinal cord injury (SCI) but their molecular basis is not yet fully understood. Recent findings demonstrate abnormal expression of the Nav1.3 Na(+) channel within second-order spinal cord dorsal horn neurons and third-order thalamic neurons along the pain pathway after SCI, and suggest that this change makes these neurons hyperexcitable so that they act as pain amplifiers and generators. Delineation of molecular changes that contribute to hyperexcitability of pain-signaling neurons might lead to identification of molecular targets that will be useful in the treatment of neuropathic pain after SCI and related nervous system injuries.

Neuroprotection by sodium channel blockade with phenytoin in an experimental model of glaucoma

PURPOSE

Sustained influx of intracellular sodium through voltage-gated sodium channels is an important event in the cascade leading to degeneration of axons. This study tested the hypothesis that sodium channel blockade with phenytoin would result in neuroprotection of retinal ganglion cells (RGCs) and optic nerve axons in an experimental model of glaucoma.

METHODS

Chronic elevation of rat intraocular pressure (IOP) leading to optic nerve damage was induced using the episcleral vein occlusion model. Before induction of glaucoma, a subset of animals was placed on phenytoin-containing chow; this treatment continued for 8 weeks. Quantitative counts of backfilled RGCs and optic nerve axons was performed to examine the effects of phenytoin on glaucoma-induced adverse neurodegeneration.

RESULTS

Elevated IOP resulted in a significant decrease in density of RGCs, as well as dropout of axons within the optic nerve at 8 weeks after induction. In phenytoin-treated animals, however, the loss of RGCs was significantly reduced compared to vehicle-treated glaucomatous animals. Axon loss in the optic nerve was also reduced in phenytoin-treated animals, compared to controls.

CONCLUSIONS

Orally delivered phenytoin was effective in protecting neurons in an animal model of glaucoma, and merits further examination as a potential therapeutic strategy.

Changes in electrophysiological properties and sodium channel Nav1.3 expression in thalamic neurons after spinal cord injury

Spinal cord contusion injury (SCI) is known to induce pain-related behaviour, as well as hyperresponsiveness in lumbar dorsal horn nociceptive neurons associated with the aberrant expression of Na(v)1.3, a rapidly repriming voltage-gated sodium channel. Many of these second-order dorsal horn neurons project to third-order neurons in the ventrobasal complex of the thalamus. In this study we hypothesized that, following SCI, neurons in the thalamus undergo electrophysiological changes linked to aberrant expression of Na(v)1.3. Adult male Sprague-Dawley rats underwent contusion SCI at the T9 thoracic level. Four weeks post-SCI, Na(v)1.3 protein was upregulated within thalamic neurons in ventroposterior lateral (VPL) and ventroposterior medial nuclei, where extracellular unit recordings revealed increased spontaneous discharge, afterdischarge, hyperresponsiveness to innocuous and noxious peripheral stimuli, and expansion of peripheral receptive fields. Altered electrophysiological properties of VPL neurons persisted after interruption of ascending spinal barrage by spinal cord transection above the level of the injury. Lumbar intrathecal administration of specific antisense oligodeoxynucleotides generated against Na(v)1.3 caused a significant reduction in Na(v)1.3 expression in thalamic neurons and reversed electrophysiological alterations. These results show, for the first time, a change in sodium channel expression within neurons in the thalamus after injury to the spinal cord, and suggest that these changes contribute to altered processing of somatosensory information after SCI.

Effect of age at time of spinal cord injury on behavioral outcomes in rat

Spinal cord injury (SCI) often leads to chronic central pain (CCP) syndromes such as allodynia and hyperalgesia. Although several experimental animal models for CCP studies exist, little is known about the effect of age on the development of CCP following SCI. In this study, we evaluated behavioral responses to mechanical and thermal stimuli following SCI using three different age groups of adult Sprague-Dawley rats: young (40 days), adult (60 days), and middle-age (12 months). SCI was produced by unilateral hemisection of the spinal cord at T13. Behavioral measures of locomotor function were assayed in open field tests and somatosensory function by paw withdrawal frequency (PWF) to innocuous mechanical stimuli and paw withdrawal latency (PWL) to radiant heat stimuli on both the forelimbs and hindlimbs. Prior to hemisection, the PWF was not different between the three groups; however, the PWL of the young group was significantly greater than the adult and middle-age group. After spinal hemisection, spontaneous locomotor recovery occurred more rapidly in young and adult than in middle-age rats. In both forelimbs and hindlimbs, the young group displayed a significant increase in PWF and a significant decrease in PWL compared to presurgical and sham values or values from the adult and middle-age groups. These results indicate that younger rats developed more robust neuropathic behaviors than middle-age rats, indicating that age selection is an important factor in animal models of CCP syndromes following SCI. Additionally, our data suggest that age at the time of injury may be one risk factor in predicting the development of CCP after SCI in people.

Altered sodium channel expression in second-order spinal sensory neurons contributes to pain after peripheral nerve injury

Peripheral nerve injury is known to upregulate the rapidly repriming Na(v)1.3 sodium channel within first-order spinal sensory neurons. In this study, we hypothesized that (1) after peripheral nerve injury, second-order dorsal horn neurons abnormally express Na(v)1.3, which (2) contributes to the responsiveness of these dorsal horn neurons and to pain-related behaviors. To test these hypotheses, adult rats underwent chronic constriction injury (CCI) of the sciatic nerve. Ten days after CCI, allodynia and hyperalgesia were evident. In situ hybridization, quantitative reverse transcription-PCR, and immunocytochemical analysis revealed upregulation of Na(v)1.3 in dorsal horn nociceptive neurons but not in astrocytes or microglia, and unit recordings demonstrated hyperresponsiveness of dorsal horn sensory neurons. Intrathecal antisense oligodeoxynucleotides targeting Na(v)1.3 decreased the expression of Na(v)1.3 mRNA and protein, reduced the hyperresponsiveness of dorsal horn neurons, and attenuated pain-related behaviors after CCI, all of which returned after cessation of antisense delivery. These results demonstrate for the first time that sodium channel expression is altered within higher-order spinal sensory neurons after peripheral nerve injury and suggest a link between misexpression of the Na(v)1.3 sodium channel and central mechanisms that contribute to neuropathic pain after peripheral nerve injury.

Sodium channel blockade with phenytoin protects spinal cord axons, enhances axonal conduction, and improves functional motor recovery after contusion SCI

Accumulation of intracellular sodium through voltage-gated sodium channels (VGSCs) is an important event in the cascade leading to anatomic degeneration of spinal cord axons and poor functional outcome following traumatic spinal cord injury (SCI). In this study, we hypothesized that phenytoin, a sodium channel blocker, would result in protection of axons with concomitant improvement of functional recovery after SCI. Adult male Sprague-Dawley rats underwent T9 contusion SCI after being fed normal chow or chow containing phenytoin; serum levels of phenytoin were within therapeutic range at the time of injury. At various timepoints after injury, quantitative assessment of lesion volumes, axonal degeneration, axonal conduction, and functional locomotor recovery were performed. When compared to controls, phenytoin-treated animals demonstrated reductions in the degree of destruction of gray and white matter surrounding the lesion epicenter, sparing of axons within the dorsal corticospinal tract (dCST) and dorsal column (DC) system rostral to the lesion site, and within the dorsolateral funiculus (DLF) caudal to the lesion site, and enhanced axonal conduction across the lesion site. Improved performance in measures of skilled locomotor function was observed in phenytoin-treated animals. Based on these results, we conclude that phenytoin provides neuroprotection and improves functional outcome after experimental SCI, and that it merits further examination as a potential treatment strategy in human SCI.

Locomotor recovery and mechanical hyperalgesia following spinal cord injury depend on age at time of injury in rat

We tested the effect of age at the time of spinal cord injury (SCI) on locomotor recovery, in open field tests, and mechanical hyperalgesia, using paw withdrawal frequency (PWF) in response to noxious mechanical stimuli, in male Sprague-Dawley rats after spinal hemisection at T13 in young (40 days), adult (60 days) and middle-age (1 year) groups. Behavioral outcomes were measured weekly for 4 weeks in both SCI and sham groups. Following SCI, the young and adult groups recovered significantly more locomotor function, at a more rapid rate, than did the middle-age group. The PWF of the young group was significantly increased, the adult group was significantly decreased, and the middle-age group showed no significant change in fore- and hindlimbs when compared to other age groups, pre-injury and sham controls. These results support age-dependent behavioral outcomes after SCI.

Apoptosis of vasopressinergic hypothalamic neurons in chronic diabetes mellitus

The hyperosmolality associated with diabetes mellitus triggers an increase in neuronal activity and vasopressin production within magnocellular neurosecretory cells (MNCs) of the hypothalamic supraoptic nucleus (SON). In this study, we examined the effect of chronic diabetes on the function and survival of these neurons. After 6 months, but not 6 weeks, of streptozotocin (STZ)-induced diabetes, we observed an increase in the appearance of small hyperchromatic neurons and a decrease in SON neuronal density. A subpopulation of neurons within the SON at this time point demonstrated positive staining for cleaved caspase-3 and TUNEL, two markers of apoptosis. In addition, the number of vasopressin-positive neurons was decreased. Markers for apoptosis did not colocalize with vasopressin immunopositivity; this was probably due to a diabetes-induced degenerative process causing downregulation of vasopressin expression or depletion of neuropeptide. Although the phenotypes of the apoptotic neurons were not identified, other SON neurons including oxytocin-producing neurons are unlikely to be affected by chronic hyperglycemia. Microglial hypertrophy and condensation were also observed in the 6-month diabetic SON. Although upregulation of vasopressin production in response to acute hyperosmolality is adaptive, prolonged overstimulation of vasopressin-producing neurons in chronic diabetes results in neurodegeneration and apoptosis.

Co-localization of sodium channel Nav1.6 and the sodium-calcium exchanger at sites of axonal injury in the spinal cord in EAE

Axonal degeneration contributes to the development of non-remitting neurological deficits and disability in multiple sclerosis, but the molecular mechanisms that underlie axonal loss in multiple sclerosis are not clearly understood. Studies of white matter axonal injury have demonstrated that voltage-gated sodium channels can provide a route for sodium influx into axons that triggers reverse operation of the Na(+)/Ca(2+) exchanger (NCX) and subsequent influx of damaging levels of intra-axonal calcium. The molecular identities of the involved sodium channels have, however, not been determined. We have previously demonstrated extensive regions of diffuse expression of Na(v)1.6 and Na(v)1.2 sodium channels along demyelinated axons in experimental allergic encephalomyelitis (EAE). Based on the hypothesis that the co-localization of Na(v)1.6 and NCX along extensive regions of demyelinated axons may predispose these axons to injury, we examined the expression of myelin basic protein, Na(v)1.2, Na(v)1.6, NCX and beta-amyloid precursor protein (beta-APP), a marker of axonal injury, in the spinal cord dorsal columns of mice with EAE. We demonstrate a significant increase in the number of demyelinated axons demonstrating diffuse Na(v)1.6 and Na(v)1.2 sodium channel immunoreactivity in EAE (92.2 +/- 2.1% of beta-APP positive axons were Na(v)1.6-positive). Only 38.0 +/- 2.9% of beta-APP positive axons were Na(v)1.2 positive, and 95% of these co-expressed Na(v)1.6 together with Na(v)1.2. Using triple-labelled fluorescent immunohistochemistry, we demonstrate that 73.5 +/- 4.3% of beta-APP positive axons co-express Na(v)1.6 and NCX, compared with 4.4 +/- 1.0% in beta-APP negative axons. Our results indicate that co-expression of Na(v)1.6 and NCX is associated with axonal injury in the spinal cord in EAE.

Sodium channels contribute to microglia/macrophage activation and function in EAE and MS

Loss of axons is a major contributor to nonremitting deficits in the inflammatory demyelinating disease multiple sclerosis (MS). Based on biophysical studies showing that activity of axonal sodium channels can trigger axonal degeneration, recent studies have tested sodium channel-blocking drugs in experimental autoimmune encephalomyelitis (EAE), an animal model of MS, and have demonstrated a protective effect on axons. However, it is possible that, in addition to a direct effect on axons, sodium channel blockers may also interfere with inflammatory mechanisms. We therefore examined the novel hypothesis that sodium channels contribute to activation of microglia and macrophages in EAE and acute MS lesions. In this study, we demonstrate a robust increase of sodium channel Nav1.6 expression in activated microglia and macrophages in EAE and MS. We further demonstrate that treatment with the sodium channel blocker phenytoin ameliorates the inflammatory cell infiltrate in EAE by 75%. Supporting a role for sodium channels in microglial activation, we show that tetrodotoxin, a specific sodium channel blocker, reduces the phagocytic function of activated rat microglia by 40%. To further confirm a role of Nav1.6 in microglial activation, we examined the phagocytic capacity of microglia from med mice, which lack Nav1.6 channels, and show a 65% reduction in phagocytic capacity compared with microglia from wildtype mice. Our findings indicate that sodium channels are important for activation and phagocytosis of microglia and macrophages in EAE and MS and suggest that, in addition to a direct neuroprotective effect on axons, sodium channel blockade may ameliorate neuroinflammatory disorders via anti-inflammatory mechanisms.

Upregulation of sodium channel Nav1.3 and functional involvement in neuronal hyperexcitability associated with central neuropathic pain after spinal cord injury

Spinal cord injury (SCI) can result in hyperexcitability of dorsal horn neurons and central neuropathic pain. We hypothesized that these phenomena are consequences, in part, of dysregulated expression of voltage-gated sodium channels. Because the rapidly repriming TTX-sensitive sodium channel Nav1.3 has been implicated in peripheral neuropathic pain, we investigated its role in central neuropathic pain after SCI. In this study, adult male Sprague Dawley rats underwent T9 spinal contusion injury. Four weeks after injury when extracellular recordings demonstrated hyperexcitability of L3-L5 dorsal horn multireceptive nociceptive neurons, and when pain-related behaviors were evident, quantitative RT-PCR, in situ hybridization, and immunocytochemistry revealed an upregulation of Nav1.3 in dorsal horn nociceptive neurons. Intrathecal administration of antisense oligodeoxynucleotides (ODNs) targeting Nav1.3 resulted in decreased expression of Nav1.3 mRNA and protein, reduced hyperexcitability of multireceptive dorsal horn neurons, and attenuated mechanical allodynia and thermal hyperalgesia after SCI. Expression of Nav1.3 protein and hyperexcitability in dorsal horn neurons as well as pain-related behaviors returned after cessation of antisense delivery. Responses to normally noxious stimuli and motor function were unchanged in SCI animals administered Nav1.3 antisense, and administration of mismatch ODNs had no effect. These results demonstrate for the first time that Nav1.3 is upregulated in second-order dorsal horn sensory neurons after nervous system injury, showing that SCI can trigger changes in sodium channel expression, and suggest a functional link between Nav1.3 expression and neuronal hyperexcitability associated with central neuropathic pain.

Primary cortical motor neurons undergo apoptosis after axotomizing spinal cord injury

Spinal cord injury (SCI) results in loss of voluntary motor control followed by incomplete recovery, which is partly mediated by the descending corticospinal tract (CST). This system is an important target for therapeutic repair strategies after SCI; however, the question of whether apoptotic cell death occurs in these axotomized neurons remains unanswered. In this study, adult (150-175 g) male Sprague-Dawley rats underwent T9 transection of the dorsal funiculus, which axotomizes the dorsal CST, and introduction of the retrograde tracer Fluoro-Gold into the lesion site. Primary motor cortex (M1) was then examined for evidence of apoptosis weekly for 4 weeks after injury. Axotomized pyramidal cells, identified by retrograde transport of Fluoro-Gold, were found in M1 (57.5 +/- 9.6/median section, 6127 +/- 292 total), and a significant proportion were terminal deoxynucleotidyl transferase (TdT) -mediated deoxyuridine triphosphate (dUTP)-rhodamine nick end labeling (TUNEL) -positive at 1 week after injury (39.3 +/- 5.6%), compared with animals undergoing sham surgery (1.2 +/- 1.4%). At 2-4 weeks, fewer cells were Fluoro-Gold-positive (24.6 +/- 65.06 to 25.3 +/- 6.4/median section, 2338 +/- 233 to 2393 +/- 124 total), of which very few were TUNEL-positive. In TUNEL-positive cells, Hoechst 33342 staining revealed nuclear morphology consistent with apoptosis, chromatin condensation, and formation of apoptotic bodies. Fluoro-Gold-positive cells showed increased caspase-3 and Bax immunoreactivity. Hematoxylin and eosin staining revealed similar nuclear changes and dystrophic cells. Internucleosomal DNA fragmentation was detected by gel electrophoresis at the 1-week time point. Lesioned animals not receiving Fluoro-Gold exhibited the same markers of apoptosis. These results document, for the first time, features of apoptotic cell death in a proportion of axotomized cortical motor neurons after SCI, suggesting that protection from apoptosis may be a prerequisite for regenerative approaches to SCI.

Serotonergic neural precursor cell grafts attenuate bilateral hyperexcitability of dorsal horn neurons after spinal hemisection in rat

Hemisection of the rat spinal cord at thoracic level 13 provides a model of spinal cord injury that is characterized by chronic pain attributable to hyperexcitability of dorsal horn neurons. Presuming that this hyperexcitability can be explained in part by interruption of descending inhibitory modulation by serotonin, we hypothesized that intrathecal transplantation of RN46A-B14 serotonergic precursor cells, which secrete serotonin and brain-derived neurotrophic factor, would reduce this hyperexcitability by normalizing the responses of low-threshold mechanoreceptive, nociceptive-specific, and multireceptive dorsal horn neurons. Three groups (n=45 total) of 30-day-old male Sprague-Dawley rats underwent thoracic level 13 spinal hemisection, after which four weeks were allowed for development of allodynia and hyperalgesia. The three groups of animals received transplants of no cells, 10(6) RN46A-V1 (vector-only) or 10(6) RN46A-B14 cells at lumbar segments 2-3. Electrophysiological experiments were done two weeks later. Low-threshold mechanoreceptive, nociceptive-specific, and multireceptive cells (n=394 total) were isolated at depths of 1-300 and 301-1000 micro in the lumbar enlargement. Responses to innocuous and noxious peripheral stimuli were characterized, and analyses of population responses were performed. Compared with normal animals, dorsal horn neurons of all types in hemisected animals showed increased responsiveness to peripheral stimuli. This was true for neurons on both sides of the spinal cord. After hemisection, the proportion of neurons classified as multireceptive cells increased, and interspike intervals of spontaneous discharges became less uniform after hemisection. Transplantation of RN46A-B14 cells restored evoked responses to near-control levels, normalized background activity, and returned the proportion of multireceptive cells to the control level. Restoration of normal activity was reversed with methysergide.These electrophysiological results corroborate anatomical and behavioral studies showing the effectiveness of serotonergic neural precursors in correcting phenomena associated with chronic central pain following spinal cord injury, and provide mechanistic insights regarding mode of action.

Temporal plasticity of dorsal horn somatosensory neurons after acute and chronic spinal cord hemisection in rat

Unilateral T13 hemisection of the rat spinal cord produces a model of chronic spinal cord injury (SCI) that is characterized by bilateral hyperexcitability of lumbar dorsal horn neurons, and behavioral signs of central pain. While we have demonstrated that responsiveness of multireceptive (MR) dorsal horn neurons is dramatically increased at 28 days after injury, the effects of acute hemisection are unknown and predicted to be different than observed chronically. In the present study, the consequences of T13 hemisection are examined acutely at 45 min in MR neurons both ipsilateral and contralateral to the site of injury, and compared to the same class of cells at 28 days after injury (n=20 cells total per group: 2-3 cells/side of the cord from n=5 animals). Acutely, ipsilateral to the hemisection, both spontaneous and evoked activity of MR neurons were significantly increased, whereas contralaterally, only evoked activity was significantly increased. In animals 28 days after hemisection, spontaneous activity of MR neurons was comparable to intact levels ipsilaterally, and cells exhibited hyperexcitability to evoked stimuli bilaterally. Expansion of cutaneous receptive fields was observed only in hindpaws ipsilateral to the lesion, acutely. These results demonstrate dynamic plasticity in properties of dorsal horn somatosensory neurons after SCI.

Serotonin receptors 5-HT1A and 5-HT3 reduce hyperexcitability of dorsal horn neurons after chronic spinal cord hemisection injury in rat

Spinal cord injury (SCI) results in abnormal pain syndromes in humans. In a rodent model of SCI, T13 spinal hemisection results in allodynia and hyperalgesia due in part to interruption of descending pathways, including serotonergic (5-HT) systems, that leads to hyperexcitability of dorsal horn neurons. To characterize further the role of 5-HT and 5-HT receptor subtypes 5-HT(1A) and 5-HT(3) in neuronal activation after hemisection, we have examined the responsiveness of dorsal horn neurons to a variety of innocuous and noxious peripheral stimuli. Male Sprague-Dawley rats, 150-175 g, were spinally hemisected (n=40) at T13 and allowed 4 weeks for development of mechanical allodynia and thermal hyperalgesia. Animals then underwent electrophysiologic recording and the results were compared with those from sham controls (n=15). Evoked responses of convergent dorsal horn neurons (n=224 total) at L3-L5 to innocuous and noxious peripheral stimuli were characterized after administration of vehicle, 5-HT (25, 50, 100, and 200 microg), 5-HT (100 microg) in conjunction with the selective 5-HT(1A) antagonist WAY 100135 (100 microg), the 5-HT(3) antagonist MDL 72222 (100 microg), the selective 5-HT(1A) agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 150 microg), or the 5-HT(3) agonist 2-Me-5HT (75 microg), with or without pretreatment with antagonists; all treatments were delivered topically onto the cord adjacent to the recording electrode. In hemisected animals, increased responsiveness of convergent cells to all peripheral stimuli was observed bilaterally when compared to controls. No changes in ongoing background activity were present. In control animals, only the highest dose of 5-HT (200 microg) was sufficient to reduce evoked activity, whereas in hemisected animals a concentration-dependent decrease in response was observed. In hemisected animals, both 5-HT(1A) and 5-HT(3) receptor antagonism reduced the effectiveness of 5-HT, restoring elevated evoked activity by up to 70% at the doses tested. Administration of 5-HT(1A) and 5-HT(3) receptor agonists also decreased hyperexcitability, effects prevented by pretreatment with corresponding antagonists. These results demonstrate the development of denervation supersensitivity to 5-HT following SCI, corroborate behavioral studies showing the effectiveness of 5-HT in reducing allodynia and hyperalgesia after SCI, and contribute to a mechanistic understanding of the role of 5-HT receptor subtypes in chronic central pain.

Primary motor neurons fail to up-regulate voltage-gated sodium channel Na(v)1.3/brain type III following axotomy resulting from spinal cord injury

Epilepsy occurs in a small proportion of patients with spinal cord injury (SCI), but whether it is due to concomitant traumatic head injury or to changes in cortical motor neurons secondary to axotomy within the spinal cord is not known. Na(v)1.3/brain type III sodium channel expression is up-regulated following peripheral axotomy of dorsal root ganglion (DRG) and facial motor neurons, but, to date, Na(v)1.3 expression has not been examined in upper (cortical) motor neurons following axotomy associated with SCI. In the present study, we examine Na(v)1.3 expression in upper motor neurons within rat primary motor cortex following midthoracic (T9) dorsal column transection, which severs the axons of those cells. Axotomized pyramidal cells were identified by retrograde transport of fluorogold. Immunolabeled cells were confined to layer V of the primary motor cortex and exhibited low levels of Na(v)1.3 staining. After axotomy, no significant changes were detected in Na(v)1.3 density or distribution in injured or uninjured cells, compared with control brains, in contrast to up-regulation of Na(v)1.3 in ipsilateral DRG neurons after sciatic nerve transection. These results do not preclude a role for voltage-gated sodium channels in post-SCI epilepsy but suggest that up-regulated expression of Na(v)1.3 channel is not involved.

Changes in serotonin, serotonin transporter expression and serotonin denervation supersensitivity: involvement in chronic central pain after spinal hemisection in the rat

Spinal cord injury (SCI) results in abnormal locomotor and pain syndromes in humans. In a rodent SCI model, T13 unilateral spinal hemisection results in bilateral mechanical allodynia and thermal hyperalgesia, partly by interruption of tonic descending serotonin (5-HT) inhibition. In the current study, we examined changes in density and distribution of 5-HT and 5-HT(T) in cervical (C8) and lumbar (L5) enlargements after T13 spinal hemisection and studied the effects of intrathecally delivered 5-HT (10, 21, and 63 microg), 5-HT antagonist methysergide (125 microg/kg), and 5-HT reuptake inhibitor fluvoxamine (75 microg/kg) on pain-related behaviors. Thirty-day-old male Sprague-Dawley rats were spinally hemisected and sacrificed at 3 (n = 20) and 28 (n = 20) days postsurgery for immunohistochemistry, Western blot, and ELISA analysis and compared against sham-operated animals (n = 10). At day 3, C8 5-HT levels were not significantly changed but at L5 there was a significant decrease in ipsilateral 5-HT in laminae I-II followed by incomplete recovery at 28 days postinjury. At both 3 and 28 days postinjury, C8 5-HT(T) levels were not significantly changed, but at L5 there was significant ipsilateral up-regulation of 5-HT(T) in laminae I-II. A second group of animals (n = 30) was hemisected and, starting at 28 days postinjury, behaviorally tested with intrathecal compounds. Increasing doses of 5-HT attenuated both fore- and hindlimb mechanical allodynia and thermal hyperalgesia, and effects of endogenous 5-HT were attenuated by methysergide and enhanced with fluvoxamine, all without locomotor alterations. Sham controls (n = 10) were unaffected. Thus, permanent changes occur in 5-HT and 5-HT(T) after SCI, denervation 5-HT supersensitivity develops, and modulation of 5-HT attenuates pain-related behaviors. Insight gained by these studies may aid in the understanding of dynamic 5-HT systems which will be useful in treating chronic central pain after SCI.

Intralesion transplantation of serotonergic precursors enhances locomotor recovery but has no effect on development of chronic central pain following hemisection injury in rats

The effects of intralesion grafts of serotonergic precursors on locomotor recovery and development of chronic pain were assessed after chronic spinal cord hemisection injury (SCI) in rats. Serotonin- and brain-derived neurotrophic factor-secreting (RN46A-B14) and RN46A-vector-only cells were transplanted into the site of T13 lateral hemisection 10 days following injury in immunosuppressed animals, and locomotor and pain related behaviors were assessed weekly for 28 days. There were significant improvements in the degree of spontaneous locomotor recovery, but no significant difference was found in the magnitude of development of mechanical allodynia or thermal hyperalgesia in any transplant group. From these results, we conclude that intraparenchymal engraftment of RN46A-B14 cells is largely ineffective in influencing somatosensory outcomes after SCI, in contrast with the efficacy of dorsal intrathecal placement.

Differential electrophysiological effects of brain-derived neurotrophic factor on dorsal horn neurons following chronic spinal cord hemisection injury in the rat

To assess the role of brain-derived neurotrophic factor (BDNF) in nociceptive processing after chronic lateral spinal cord hemisection injury (SCI) at T13, we studied the effects of BDNF on evoked activity of dorsal horn wide dynamic range (WDR) neurons. Evoked responses of WDR cells (n=34 total) at L3-L5 were characterized electrophysiologically after spinal administration of vehicle, or BDNF (10 microg). In hemisected animals, application of BDNF to the surface of the cord resulted in reductions in evoked activity in 24 of 32 cells (75%), and enhancement of evoked activity in eight of 32 (25%) cells. Phosphate-buffered saline-receiving animals demonstrated evoked response rates of between 75 and 93 Hz, while BDNF(-) cells had evoked rates from between 20 and 41 Hz, and BDNF(+) activities were between 80 and 119 Hz, significant changes of 76 and 124%, respectively. Effects were bilateral and differences in sidedness were not observed. These results further implicate BDNF in nociceptive processing, but suggest a complex role after chronic SCI.

Engraftment of serotonergic precursors enhances locomotor function and attenuates chronic central pain behavior following spinal hemisection injury in the rat

Spinal cord injury (SCI) results in abnormal locomotor and pain syndromes in humans. T13 spinal hemisection in the rat results in development of permanent mechanical allodynia and thermal hyperalgesia partially due to interruption of descending inhibitory modulators such as serotonin (5-HT). We hypothesize that lumbar transplantation of nonmitotic cells that tonically secrete antinociceptive and trophic compounds will reduce the pain-like behavior and enhance locomotor recovery after SCI. We used RN46A-B14 cells, a conditionally immortalized (SV40tsTag) rat neuronal cell line derived from E13 raphe bioengineered to secrete both 5-HT and BDNF in vitro at both permissive (33 degrees C) and nonpermissive (39 degrees C) temperatures. Three groups (n = 72) of 30-day-old male Sprague-Dawley rats were spinally hemisected at T13 and allowed 4 weeks for adequate recovery of locomotor function and development of allodynia and hyperalgesia. Immunosuppressed animals received either lumbar RN46A-B14 (n = 24) or control RN46A-V1 (n = 24) empty-vector transplants or no cell (n = 24) transplant. HPLC analysis of media and CSF demonstrated increases of both in vitro and in vivo 5-HT levels at 28 days in RN46A-B14 animals. ELISA demonstrated BDNF secretion in vitro and in vivo by RNA46A-B14 cells. Locomotor function (BBB scale) and nociceptive behaviors measured by paw withdrawals to von Frey filaments, radiant heat, and noxious pin stimuli were tested for 4 weeks posttransplant. Animals receiving RN46A-B14 cells demonstrated significantly improved locomotor function and reductions in both fore- and hindlimb mechanical allodynia and thermal hyperalgesia compared to controls receiving RN46A-V1 or no transplants. These effects were modulated by the 5-HT antagonist methysergide and reuptake inhibitor fluvoxamine. Bromodeoxyuridine and 5-HT immunoreactivity confirmed cell survival and graft location 4 weeks posttransplantation. These results support the therapeutic potential of bioengineered serotonin-secreting cell lines in reducing chronic central pain following spinal cord injury.

Subdural engraftment of serotonergic neurons following spinal hemisection restores spinal serotonin, downregulates serotonin transporter, and increases BDNF tissue content in rat

Spinal hemisection injury at T13 results in development of permanent mechanical allodynia and thermal hyperalgesia due to interruption and subsequent loss of descending inhibitory modulators such as serotonin (5-HT) and its transporter (5-HT(T)). We hypothesize that lumbar transplantation of non-mitotic cells that tonically secrete 5-HT and brain-derived neurotrophic factor (BDNF) will restore alterations in 5-HT and 5-HT(T) systems within the spinal dorsal horn. We used an immortalized rat neuronal cell line derived from E13 raphe (RN46A-B14) which is shown to secrete 5-HT and BDNF in vitro and in vivo. Three groups (n=35) of 30 day old male Sprague-Dawley rats were spinally hemisected at T13 and 28 days later received either lumbar RN46A-V1 control empty-vector (n=15) or RN46A-B14 (n=15) intrathecal grafts, or no transplant. Twenty-eight days following transplantation, animals were perfused and tissue examined for changes in 5-HT, 5-HT(T), and BDNF at the site of transplantation or at lumbar enlargements (L5). Immunohistochemistry revealed that RN46A-B14, but not RN46A-V1 cells, increased 5-HT tissue staining at L5 in the dorsal white matter as well as in superficial dorsal horn laminae I and II on both ipsilateral and contralateral sides, results confirmed by ELISA. Transplantation of RN46A-B14 cells significantly reduced ipsilateral 5-HT(T), upregulated after injury. Significantly increased levels of BDNF were also observed after RN46A-B14 transplantation but were not localized to particular spinal laminae. These results are consistent with recovery of locomotor function and reductions in chronic pain behaviors observed behaviorally after RN46A-B14 transplantation and supports the pragmatic application of cell-based therapies in correcting damaged circuitry after spinal cord injury.

Strain and model differences in behavioral outcomes after spinal cord injury in rat

Spinal cord injury (SCI) results in loss of function below the level of injury and the development of chronic central pain (CCP) syndromes. Since different strains may develop and express chronic pain behaviors differently, we evaluated behavioral outcomes (locomotor recovery and the development of mechanical and thermal allodynia) in three commonly used strains of rats (Long-Evans, Wistar, and Sprague-Dawley) using two models of SCI. The two models examined were contusion at T10 (NYU impactor, 12.5 mm height) and the T13 hemisection. Mechanical stimulation (von Frey filaments) revealed significantly lower baseline responses for Long-Evans rats and significantly higher baseline paw withdrawal latencies to thermal stimulation for Wistar rats compared to the other strains. Following contusion SCI, Long-Evans rats had the highest percentage of animals that developed mechanical allodynia (73%), while Sprague-Dawley rats had the highest percentages (75%) following hemisection SCI. Interestingly, the Sprague-Dawley rats had the highest percentage (87%) to develop thermal allodynia following contusion SCI, while 100% of both Long-Evans and Sprague Dawley rats developed thermal allodynia in the hemisection model. Locomotor recovery after SCI was similar for each model in that Long-Evans rats recovered slower and to a lesser extent than the other strains. In each model, Sprague-Dawley rats recovered faster and achieved greater function. Overall, the hemisection model produced a larger percentage of animals that developed CCP and had greater responses to mechanical stimulation. Thus, it appears that strain selection has a greater impact on locomotor recovery and model selection has a greater impact on the development of CCP following SCI. Furthermore, these results suggest that genetic factors may play a role in recovery following SCI.

Reduction of pathological and behavioral deficits following spinal cord contusion injury with the selective cyclooxygenase-2 inhibitor NS-398

Spinal cord injury (SCI) results in loss of locomotor function and development of abnormal chronic pain syndromes (mechanical allodynia, thermal hyperalgesia). Following injury, secondary mechanisms including release of excitatory amino acids, inflammation and lipid peroxidation damage neural cells through release of cytotoxic free radicals. We hypothesized that selective inhibition of cyclooxygenase-2 (COX-2), an inducible inflammatory mediator, would decrease tissue damage and subsequently reduce locomotor deficits and development of chronic central pain syndromes after injury. Fifteen minutes prior to receiving T13 spinal segment spinal cord contusion injury, 200-225-g male Sprague-Dawley rats received either vehicle (0.5 ml 1:1 v/v DMSO/saline, i.p., n = 20) or the selective COX-2 inhibitor NS-398 (5 mg/kg in DMSO/saline v/v, i.p., n = 20). Locomotor function via the BBB scale, and nociceptive behaviors measured by paw withdrawals to von Frey filaments and radiant heat stimuli were tested for 4 weeks postinjury. Histological examination and volumetric analysis of spinal cord tissue were performed concomitantly. Spinally contused animals receiving NS-398 demonstrated significantly (p < 0.05) reduced locomotor alteration and reductions in both fore- and hindlimb mechanical allodynia and thermal hyperalgesia when compared to vehicle controls. Histological examination of spinal segments at the lesion segment demonstrated reduced lesion extent and increased viable tissue when compared to vehicle controls. Prostaglandin E2 levels were significantly lowered in NS-398-treated but not vehicle-treated animals 12 h after injury. These results support the role of COX-2 in reducing pathological and behavioral deficits after spinal cord injury.

Bridging the gap: from discovery to clinical trials in spinal cord injury

Recently, the Kent Waldrep National Paralysis Foundation initiated a think tank intended to bridge several gaps and achieve several goals in regard to spinal cord injury (SCI) research and funding. Affiliated with the need to bridge a pathophysiological gap in spinal parenchyma and/or reorganize remaining circuitry after injury is a need to bridge resource gaps for timely funding for translational research, gaps in knowledge between researchers, and between researchers/clinicians and SCI patients. The epistemology of cure was examined and redefined to include transitional recoveries and advances. Modes and mechanisms of funding have been evaluated and where deficits were perceived, suggestions have been made to expedite and increase the number and breadth of funding opportunities. Innovative infrastructure changes are submitted. We discuss the progression of clinical trials as well as offer suggestions to facilitate benchtop-to-bedsite translation of valuable research to the customer. Highlights of recently completed, in progress, and future trials are detailed. Finally, we submit five essential processes required to promote advances to the SCI patient population: discovery, development, clinical trials, evaluation, and rehabilitation. These ideas are intended to facilitate entry of serious dialogue and to ultimately improve the lives of patients living with SCI.

Transplants of adrenal medullary chromaffin cells reduce forelimb and hindlimb allodynia in a rodent model of chronic central pain after spinal cord hemisection injury

In the majority of patients, spinal cord injury (SCI) results in abnormal pain syndromes in which non-noxious stimuli become noxious (allodynia). To reduce allodynia, it would be desirable to implant a permanent biological pump such as adrenal medullary chromaffin cells (AM), which secrete catecholamines and opioid peptides, both antinociceptive substances, near the spinal cord. We tested this approach using a recently developed a mammalian SCI model of chronic central pain, which results in development of mechanical and thermal allodynia. Thirty day-old male Sprague-Dawley rats were spinally hemisected at T13 and allowed 4 weeks for recovery of locomotor function and development of allodynia. Nonimmunosuppressed injured animals received either control-striated muscle (n = 7) or AM (n = 10) transplants. Nociceptive behavior was tested for 4 weeks posttransplant as measured by paw withdrawals to von Frey filaments, radiant heat, and pin prick stimuli. Hemisected animals receiving AM demonstrated statistically significant reductions in both fore- and hindlimb mechanical and thermal allodynia, but not analgesia, when compared to hemisected animals receiving striated muscle transplants (P < 0.05). Tyrosine hydroxylase immunoreactivity indicated prolonged transplant survival and production of catecholamines. HPLC analysis of cerebrospinal fluid samples from animals receiving AM transplants demonstrated statistically significant increases in levels of dopamine (sevenfold), norepinephrine (twofold), and epinephrine (threefold), compared to control values several weeks following transplant (P < 0.05). By 28 days posttransplant, however, antinociceptive effects were diminished. These results support the therapeutic potential of transplanted AM in reducing chronic central pain following spinal cord injury.