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Experimental Neurology
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Review
The dark side of neuroplasticity
Arthur Brown ⁎, Lynne C. WeaverSpinal Cord Injury Laboratory, Robarts Research Institute, University of Western Ontario, London, Ontario, Canada N6A 5K8
Whether dramatic or modest, recovery of neurological function after spinal cord injury (SCI) is greatly due toneuroplasticity — the process by which the nervous system responds to injury by establishing new synapticconnections or by altering the strength of existing synapses. However, the same neuroplasticity that allowslocomotor function to recover also produces negative consequences such as pain and dysfunction of organscontrolled by the autonomic nervous system. In this review we focus specifically on structural neuroplasticity(the growth of new synaptic connections) after SCI and on the consequent development of pain and auto-nomic dysreflexia, a condition of episodic hypertension. Neuroplasticity after SCI is stimulated by the deaffer-entation of spinal neurons below the lesion and by the expression of growth-promoting neurotrophins suchas nerve growth factor (NGF). A broad range of therapeutic strategies that affect neuroplasticity is being de-veloped for the treatment of SCI. At one end of the spectrum are therapeutic strategies that directly or indi-rectly increase NGF in the injured spinal cord, and have the most robust effects on neuroplasticity. At theother end of the spectrum are neuroprotective strategies focused on supporting and rescuing uninjured, orpartially injured, axons; these might limit the deafferentation stimulus for neuroplasticity. In the middle ofthis spectrum are strategies that block axon growth inhibitors without necessarily providing a growth stim-ulus. The literature supports the view that the negative consequences of neuroplasticity develop more com-monly with therapies that directly stimulate nerve growth than they develop in the untreated injured cord.Compared to these conditions, neuroplasticity with negative outcomes is less prevalent after treatments thatthat neutralize axon growth inhibitors, and least apparent after strategies that promote neuroprotection.
Neuroplasticity after spinal cord injury (SCI) can encompass a varietyof responses within the injured cord, the brain and the ganglia outsidethe central nervous system. They range from structural neuroplasticity(regenerative growth responses of injured motor and sensory projec-tions, and collateral sprouting of nearby intact fibers), to changes in
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gene expression (altered expression profiles of neurotransmitters, neu-ropeptides, growth factors and their associated receptors) (Gris et al.,2009), to changes in membrane ion channels (Bareyre et al., 2004;Deumens et al., 2008; Frigon and Rossignol, 2006; Ghosh et al., 2010;Hains et al., 2003; Steward et al., 2003). The primary injury leads to sec-ondary sequellae such as excitotoxicity, tissue ischemia and inflamma-tion (Tator, 1995; Young, 1993) that further exacerbate the injury, aprocess termed secondary injury. Both the primary and secondary as-pects of the SCI can trigger neuroplasticity that potentially promotes re-covery from the injury but it also may have deleterious consequences.Furthermore, as presented elsewhere in this volume, even post-injurytraining and rehabilitation can induce plastic changes in the nervous
system; these too may have yet unknown negative consequences. Thischapter will focus on negative aspects of neuroplasticity as it concernsgrowth of axons and primary afferent fibers that can be induced by pri-mary and secondary SCI and by therapies designed to improve neurolog-ical recovery by increasing nerve growth.
SCI partially denervates spinal neurons by disrupting ascending anddescending pathways to and from the brain as well as inputs from thedorsal root ganglia. This denervation provides a stimulus for intactaxons of interneurons, white matter tracts and sensory neurons to re-place lost inputs to the spinal neurons that have empty synapses. More-over, the loss of connections of intact ascending sensory systems mayprompt collateral sprouting in search of new targets caudal to the inju-ry. Collateral or regenerative sprouting of intraspinal axons after SCImay allow them to make new connections or to strengthen existingsynapses to partially denervated or other spinal neurons. Indeed,some recovery of motor control after cord hemisection in cats hasbeen attributed to collateral sprouting of primary afferent axons(Goldberger et al., 1993; Helgren and Goldberger, 1993) and early stud-ies demonstrating enlarged excitatory postsynaptic potentials in chron-ic spinal cats attributed part of the increase to sprouting of primaryafferent fibers (Nelson and Mendell, 1979). Therefore SCI itself setsthe stage for a growth response within the injured cord, and neuroplas-ticity is a key feature in spontaneous recovery from SCI. However, mal-adaptive neuroplasticity is a feature of many of the negative outcomesof SCI. Examples of this are muscle spasticity, neuropathic pain, auto-nomic dysreflexia, urinary bladder dyssynergia, bowel dysfunction, car-diac arrhythmias and sexual dysfunction (Christensen and Hulsebosch,1997a; Collins et al., 2006; de Groat and Yoshimura, 2006; Johnson,2006; Mathias, 2006; Nout et al., 2006). These dysfunctions are caused,at least in part, by an imbalance of inhibitory and excitatory synaptic in-puts to spinal neurons and several relate to a loss of coordination of au-tonomic and somatic control.
Two conditions, neuropathic pain and autonomic dysreflexia, arecommon adverse outcomes of SCI that appear to involve neuroplasticityas part of their etiology. Neuropathic pain after SCI hasmanyunderlyingfactors such as increases in neuronal excitability due to products ofmicroglial activation, changes in sodium channel expression, andchanges in glutamate receptor expression (Deumens et al., 2008) butthis discourse will address primarily two factors that involve neuralgrowth, namely collateral sprouting of calcitonin gene-related peptide(CGRP)-containing primary afferent fibers in the spinal cord dorsalhorn (Christensen and Hulsebosch, 1997b) and aberrant growth of des-cending serotonergic axons in the dorsal horn rostral to the cord lesionsite (Oatway et al., 2005). This growth can occur both rostral and caudalto the injury, in parallel with the neuropathic pain that can be evokedabove, at and below the injury site. Autonomic dysreflexia is an abnor-mality of blood pressure control characterized by extremehypertensionaccompanied by a pounding headache and slow heart rate [for reviewsee (Weaver et al., 2006), (Mathias, 2006)]. This hypertension occursin response to sensory input entering the spinal cord below the levelof the lesion. This input leads to exaggerated spinal reflex sympathetic(autonomic) responses that can be associated with an increasedCGRP-containing primary afferent arbor in the dorsal horn (Krenz andWeaver, 1998; Krenz et al., 1999) or, in the presence of a normalarbor, with loss of descending inhibitory influences on spinal sympa-thetic reflexes (Gris et al., 2005). In this condition, the key changesoccur below the site of injury.
Structural neuroplasticity in pain and autonomic dysreflexia afterSCI
Collateral sprouting of CGRP-immunoreactive small diameter prima-ry afferent fibers into the laminae III–V of the dorsal horn after SCI hasbeen linked with the development of chronic neuropathic pain and au-tonomic dysreflexia (Christensen and Hulsebosch, 1997a, 1997b; Jacobet al., 2001; Krenz and Weaver, 1998; Krenz et al., 1999; Weaver et al.,
in press;Wong et al., 2000) (Fig. 1A). Sprouting of larger diameter fibershas also been noted after SCI (Krenz and Weaver, 1998) and may con-tribute to dysreflexia as this condition can also be induced by non-noxious stimuli such as light touch (Marsh andWeaver, 2004). Primaryafferent sproutingmay be important near the site of injury, for examplein the case of at-level neuropathic pain, or elsewhere in the cord, for ex-ample in the region of input from pelvic afferent projections that initiateautonomic dysreflexia after a high thoracic lesion. A caveat that must beintroduced is that neuropathic pain and autonomic dysreflexia are nottotally dependent upon this type of primary afferent sprouting.Mechan-ical allodynia and hyperalgesia after low thoracic clip compression SCIare not always accompanied by increased distribution or density ofCGRP-immunoreactive afferent arbors (Bruce et al., 2002) and a studyof autonomic dysreflexia in different strains of mutant mice showedrobust dysreflexia in some strains that had no changes in their CGRP-immunoreactive arbor (Jacob et al., 2003). Because of its robust expres-sion in the dorsal horn of the spinal cord, CGRP is a ready target forassessment of changes in the primary afferent arbor. However, CGRP isexpressed in less than half of nociceptive dorsal root sensory neuronsand this peptide is also expressed in smallmyelinated and unmyelinatedprimary afferent neurons that are not associatedwith nociception, dem-onstrating that changes in CGRP-immunoreactive populations cannot beassociated solely with sensations of pain (Lawson et al., 2002, 1996).After SCI or dorsal rhizotomy, enlargements can be detected withinthe arbors of Aδ, Aβ and C primary afferent fibers and CGRP is expressedby many but not all of the neurons in these populations (Krenz andWeaver, 1998;McNeill andHulsebosch, 1987;Wong et al., 2000). A sub-set of CGRP-immunoreactive C-fiber primary afferent neurons expressessubstance P and is closely associated with nociception (Ju et al., 1987;Price, 1985). Changes in this afferent population were not detectedafter a high thoracic compression SCI (Marsh andWeaver, 2004), but in-jury of another type or at another spinal cord location may trigger in-volvement of this phenotype of neuron. Moreover, loss of smalldiameter myelinated primary afferent neurons and changes in the func-tions of C-fiber afferent neurons in bladder pathways occur after SCI (deGroat and Yoshimura, 2006). Similar changes in sensory pathways af-fecting other targets could contribute to maladaptive plasticity afterSCI but have not been examined systematically. Much remains un-known about plasticitywithin primary afferent populations after SCI. Al-though sprouting of CGRP-immunoreactive dorsal horn sensory fibersclearly is not entire story about plasticity in sensory systems after SCI,this phenomenon is well documented, can contribute to pathologicalconditions and is a valid therapeutic target (see below).
For decades investigators have been designing therapeutic manip-ulations with a goal of fostering regenerative or collateral growth,pushing the innate recovery systems further. How does it sometimesgo wrong? We will consider three broad categories of treatment andconsider if some types of therapies are more likely than others to leadto the development of negative consequences associated with neuro-plasticity. The first targets injured or spared axons and promotes re-generative sprouting in the hopes of improving neurologicalrecovery. The second category of treatment targets the spinal lesion'smicroenvironment that actively inhibits axon growth and includestreatments that remove or block inhibitors of axon growth. Thethird category of treatments is broadly termed neuroprotective be-cause they focus on limiting secondary injury that can be a stimulusfor maladaptive plasticity.
SCI treatment strategies that promote axon growth
Many therapeutic strategies for SCI focus on encouraging the growthand regenerative sprouting of spared and injured axons through thenon-permissive environment of the injured CNS. These strategies in-clude the direct administration of neurotrophins or transplantation ofvarious cell types that by nature or experimental manipulation expressneurotrophins. For example, transplanting fibroblasts engineered to
Fig. 1. Photomicrographs of CGRP-immunoreactive fibers in the dorsal horn of a control intact rat (A1) and of rats 2 weeks after cord transection at the 4th thoracic (T4) segment(A2-6 and B1-3). A) Cord-injured rats received an intrathecal infusion of nonimmune rabbit IgG or an intrathecal infusion of anti-NGF Ab. Transverse tissue sections are from the T6segment. Panels 4–6 are magnifications of laminae III–V from panels 1–3 (boxed area). Scale bars: 1–3, 100 μm; 3–6, 50 μm. The anti-NGF treatment markedly decreased the size ofthe CGRP+ arbor in the deeper laminae. [Adapted, with permission, from (Krenz et al., 1999)]. B) Immediately after cord injury adenoviral vectors encoding green fluorescent pro-tein (GFP adts, B1), or NGF (NGF Adts, B2) or Semaphorin 3A (sema3A adts, B3) were injected into the lumbosacral spinal cord. These sections of the L6/S1 cord show that NGFoverexpression induced CGRP+ fiber sprouting throughout the dorsal horns and even dorsal columns. Conversely, Sema3A reduced post-traumatic aberrant CGRP fiber sproutinginto the dorsal horn. Scale bar: 200 μm. [Adapted, with permission, from (Cameron et al., 2006)].
express NGF into the acute or chronically injured rat spinal cord inducesrobust sprouting of primary sensory dorsolateral fasciculus axons aswell as cerulospinal and putative ventral horn motor axons (Grillet al., 1997; Tuszynski et al., 1996). Whereas this group did not assesspossible effects of the increased growth of CGRP-expressing fibers inthe injured spinal cord on neurological outcomes, others have shownthat virally-mediated NGF expression in the uninjured (Romero et al.,2000) and injured (Cameron et al., 2006) spinal cord correlates with in-creased CGRP fiber collateral sprouting and the development of hyper-algesia and increased autonomic dysreflexia.
NGF expression after SCI has been the target of strategies to reducethe development of autonomic dysreflexia and chronic pain syndromesin rat models of SCI. NGF expression does increase after SCI, particularlynear the lesion site as shown by immunocytochemical detection of thegrowth factor protein and mRNA (Brown et al., 2004, 2007; Krenz andWeaver, 2000). This NGF is upregulated within ramified microglia, as-trocytes, intermediate grey neurons, pial cells, and leptomeningealand Schwann cells of the injured cord (Fig. 2). Furthermore intrathecaldelivery of anti-NGF antibodies for weeks after SCI decreases the size ofthe CGRP-immunoreactive afferent arbor inmodels of neuropathic pain(Christensen and Hulsebosch, 1997a, 1997b) and of autonomic dysre-flexia (Krenz et al., 1999) (Fig. 1A). An antibody to NGF and a trkA-IgGfusion protein both blocked the development of autonomic dysreflexia
in rat models of thoracic SCI (Krenz et al., 1999; Marsh et al., 2002)(Fig. 3). Moreover, a treatment that inhibits growth of C-fibers, sema-phorin 3A, when delivered into the injured cord also blocked the en-largement of the CGRP-immunoreactive arbor and reduced autonomicdysreflexia (Cameron et al., 2006) (Fig. 1B). Finally, in addition to pri-mary afferent sprouting after SCI, a significant increase in the axon den-sity of lumbosacral dorsal grey commissural propriospinal neurons hasbeen reported, demonstrating plasticity of the propriospinal neuronalsystems as well (Hou et al., 2008). This plasticity corroborates electro-physiological evidence for propriospinal plasticity in the etiology of au-tonomic dysreflexia (Krassioukov et al., 2002). The propriospinalplasticity occurs in concert with the responses of the primary afferentarbor but its relationship to the increased intraspinal NGF after SCI hasnot been established. All of the above studies suggest that treatmentsthat might increase NGF within the injured spinal cord generate therisk of augmenting the development of neuropathic pain and autono-mic dysfunction. Furthermore, increased endogenous NGF in the spinalcord and dorsal root ganglia are implicated in plasticity that contributesto lower urinary tract dysfunction after SCI (Seki et al., 2002, 2004).
Strategies to promote neural plasticity also include various celltherapies including olfactory ensheathing cell and neural progenitorcell transplantation. Olfactory neurons are continuously renewedthoughout life and extend processes that grow long distances from
Fig. 2. Photomicrographs of NGF immunoreactive cells in longitudinal sections of the T5 spinal cord at 7 days after T4 spinal cord injury. A–C) NGF is expressed in cells at and belowthe pial surface of the cord (A) and is co-localized with the low affinity neurotrophin receptor p75NTR (B) in cells adjacent to the pia that may be leptomeningeal cells (C, arrowhead). D–F) NGF is expressed in large stellate cells in the white matter (D) that are immunoreactive for glial acidic fibrillary protein (E, GFAP), identifying them as astrocytes(F, arrow head). Scale bar in F is 50 μm and refers to all panels.Adapted, with permission, from Brown et al. (2004).
their origins in the olfactory mucosa up to their targets in the olfacto-ry bulb. The remarkable growth of these nerve fibers is attributed tospecialized glia, the olfactory ensheathing cells that enwrap thegrowing fibers and extend processes that accompany the growing fi-bers to their targets (Raisman, 2001). The ability of olfactoryensheathing cells to promote nerve growth into the adult CNS sug-gested that these cells may be able to encourage axonal growth inthe injured spinal cord. Many groups have shown that olfactoryensheathing cells do support axonal growth and recovery after trans-plantation (Keyvan-Fouladi et al., 2003; Lu et al., 2002; Ramon-Cuetoet al., 2000). However, olfactory ensheathing cell transplantation hasbeen associated with increased CGRP fiber sprouting and the develop-ment of autotomy in animals (Richter et al., 2005).
Neural precursor cell transplantation has been suggested to improverecovery in models of SCI by cell replacing cells lost to the injury(Cummings et al., 2005; Hofstetter et al., 2005; Karimi-Abdolrezaeeet al., 2006) and/or by providing trophic support that promotes tissuesparing (Teng et al., 2002) and neuroplasticity (Lu et al., 2003). Interest-ingly, whether neural precursor cell transplantation leads to maladap-tive neuroplasticity may depend on their differentiation in the injuredspinal cord. Hofstetter et al. (Hofstetter et al., 2005) found that neuralprecursor cell transplantation into the injured rat spinal cord led to in-creased CGRP fiber sprouting that correlated tightly with the develop-ment of allodynia in these mice. However this negative effect ofneural precursor cell transplantationwas avoidedwhen the neural pre-cursor cells were transduced to express the neurogenin-2 transcriptionfactor before transplantation. The transplantation of neurogenin-2-transduced neural precursor cells did not result in CGRP fiber sproutingor allodynia. The authors attributed the different behavior of trans-planted naïve neural precursor cells and neurogenin-2-transduced neu-ral precursor cells to the fact that the naïve cells largely differentiatedinto astrocytes while the neurogenin-2-expressing cells largely differ-entiated into neurons and oligodendrocytes but not into astrocytes.The authors further argued that, as astrocytes are known to secrete var-ious growth factors including NGF, this trophic effect may account forthe CGRP fiber sprouting and hence allodynia in transplanted animals.This explanation is corroborated by the demonstration that C17.2
neural stem cells produce NGF after transplantation into spinal cord in-jured rats that correlates with CGRP fiber sprouting (Lu et al., 2003).Furthermore, when neural precursor cell transplantation into the in-jured rat spinal cord was coupled with PDGF-AA administration to en-courage oligodendrocyte differentiation, approximately 50% of thetransplanted neural precursor cells expressed oligodendrocyte lineagemarkers and led to improved locomotor recovery without accompany-ing neuropathic pain (Karimi-Abdolrezaee et al., 2006, 2010).
SCI treatment strategies that block axon growth inhibitors
Two classes of molecules inhibit axon growth and regenerationafter SCI. First, several components of central myelin inhibit axongrowth including myelin-associated glycoprotein (MAG), NOGO,and oligodendrocyte myelin glycoprotein (Omgp). All three of theseinhibitors bind to and act through the same receptor complex consist-ing of the Nogo receptor (NgR), p75 and lingo (Zhou and Li, 2007).Blocking Nogo action by the administration of a neutralizing anti-Nogo antibody increased neuroplasticity and improved locomotor re-covery without the development of hypersensitivity or allodynia(Liebscher et al., 2005) or muscle spasms (Gonzenbach et al., 2010).A study comparing neurological recovery in spinal cord injured ratsreceiving an anti-Nogo antibody or treadmill training or both demon-strated that two therapies that improve locomotor recovery after SCImay do so through encouraging different forms of neuroplasticity.This study showed that different types of neuroplasticity may coun-teract each other when used in combination (Maier et al., 2009).The second class is made up of ECM (extracellular matrix) proteinsin the scar, of which chondroitin sulfate proteoglycans are probablythe most important (Asher et al., 2001; Fawcett and Asher, 1999;Galtrey and Fawcett, 2007). CSPGs are a class of ECMmacromoleculesthat share a common structure comprising a central core protein witha number of chondroitin sulfate side chains (Morgenstern et al.,2002). CSPGs are present in the adult CNS (Bignami et al., 1992)and following injury their expression increases greatly (Lemonset al., 1999; McKeon et al., 1991). Enzymatic digestion of the chon-droitin sulfate side chains found on all CSPGs using the enzyme
Fig. 3. Mean arterial pressure (MAP), pulsatile arterial pressure (AP), and heart rate(HR) in two rats 2 weeks after T4 clip compression spinal cord injury: after intrathecaltreatment with control IgG (A) and after intrathecal treatment with an anti-NGF treat-ment (trkA-IgG) (B). Colon distension for 60 s (onset and duration marked with a thickline) stimulated an increase in AP and MAP and a decrease in HR in the control treatedrat. In contrast, colon distension caused only a modest increase in AP and MAP afteranti-NGF treatment (B).Adapted, with permission, from Marsh et al. (2002).
chondroitinase ABC in spinal cord-injured rats increases collateral orregenerative sprouting of descending projections and improves loco-motor recovery (Barritt et al., 2006). Whereas chondroitinase treat-ment in these rats also increased sprouting of CGRP fibers adjacentto the lesion, this sprouting was mostly found in the dorsal columns,as opposed to in the grey matter, and was not associated with an in-crease in mechanical allodynia or thermal hyperalgesia.
Of the other axon-repelling molecules in the scar Ephs and ephrinsare probably amongst the best known. Eph Receptor tyrosine kinasesform the largest family of receptor tyrosine kinases and are known tomediate repulsive interactions between cells and projecting axonswhen activated by their ephrin ligands. Several studies have shown anincreased expression of Ephs and ephrins after SCI. To test the role of in-creased EphA4 expression in the injured spinal cord an anti-EphA4 oli-gonucleotide was administered to rats intrathecally after SCI.Behavioral testing in treated rats demonstrated that knocking downEphA4 expression in the injured spinal cord did not improve locomotorrecovery but did lead to the development of mechanical allodynia(Cruz-Orengo et al., 2006). A second group that downregulatedEphA4-ephrin signaling in the injured spinal cord using a peptide an-tagonist was able to demonstrate increased locomotor recovery and
cortico-spinal tract sprouting (Fabes et al., 2007). The different findingof these two groups probably reflects differences in how the anti-EphA4 oligonucleotide and blocking peptide work. The intrathecal de-livery of the anti-EphA4 oligonucleotide would be expected to reduceEphA4 expression in spinal neurons and hence affect only propriospinalaxons. The EphA4 blocking peptide would be predicted to antagonizeEphA4 signaling throughout the spinal cord affecting both propriospinaland descending cortico-spinal projections. These studies demonstratethat blocking inhibitors of axon growth has the potential to promoteboth regenerative and maladaptive sprouting and that the resolutionto this problemmay lie in the development of strategies that target spe-cific neuronal populations or axon projections.
SCI treatment strategies that promote neuroprotection
Anti-inflammatory treatments dedicated to neuroprotection in theacute days after SCI have been investigated for decades. Indeed theintended action of the well-studied methylprednisolone treatment forSCI was reduction of inflammation and neuroprotection (Bracken etal., 1990). The outcomes after SCI are highly dependent upon a fine bal-ance of growth and deathwithin the injured cord aswell as the equilib-rium between excitatory and inhibitory control systems. Ourlaboratories have promoted an anti-inflammatory treatment that limitsthe influx of activated neutrophils and monocytes into the injured spi-nal cord with the use of intravenously delivered blocking antibodiesthat target a key leukocyte integrin, CD11d/CD18. This treatment limitsintraspinal inflammation, reduces oxidative damage and lesion size andleads to significant improvement inmotor function and reduction of au-tonomic dysreflexia andneuropathic pain (Bao et al., 2004a, 2004b; Griset al., 2004; Oatway et al., 2005; Saville et al., 2004).We anticipated thatthis treatment would also avert primary afferent sprouting, as NGF, thelikely trigger of this sprouting, is found in the injured cord in areas ofhigh inflammation (Brown et al., 2004). Unexpectedly, the anti-CD11dmonoclonal antibody treatment did not block the increased arbor ofCGRP-immunoreactive fibers in laminae III–V of the lower thoracicand lumbar spinal cord after clip compression injury at the 4th thoracicsegment (Gris et al., 2005). Despite this, the treatment markedly re-duced lesion size and, in this study, significantly reduced themagnitudeof autonomic dysreflexia. We concluded that the neuroprotection actu-ally permitted the primary afferent sprouting response, but the reduc-tion of secondary damage with sparing of descending pathways wasof greater importance in the overall control of blood pressure. Thereforethe neuroplasticity did not yield its possible negative outcome perhapsbecause a significant component of supraspinal control of the spinal au-tonomic neuronswasmaintained, sufficient to limit the development ofautonomic dysreflexia. Either sufficient inhibitory influences on the spi-nal circuits were maintained, or the opportunity for synapse formationby this exaggerated CGRP-immunoreactive input may have been mini-mized. In this study methylprednisolone also permitted the increasedarbor of CGRP-immunoreactive fibers in the dorsal horn, but did notlead to significant tissue sparing near the lesion and only transientlylimited autonomic dysreflexia. These findings suggest that the possiblenegative aspects of neuroplasticity depend upon the milieu within theinjured cord, with its specific balance of losses and excesses.
Although the anti-CD11d integrin treatment did not alter primaryafferent sprouting in the dorsal horn, it led to increased projections ofserotonergic axons into and through the spinal cord lesion and itabolished an accumulation of serotonergic [5-hydroxytryptamine(5-HT)] axons that develops in the dorsal horn and in deeper layersrostral to a SCI site (Oatway et al., 2005) (Figs. 4A–D). In an olderstudy we noted clumps of disorganized axons rostral to a SCI thatstained for growth-associated protein-43, suggesting an up-regulated growth response in this region (Weaver et al., 1997). Theserotonergic accumulations found more recently may be part of thatgrowth pattern.
Fig. 4. Serotonin (5-HT) immunoreactivity in laminae I–IV of the dorsal horn rostral to a T12 spinal cord injury 4 weeks after the injury. A–C) Photomicrographs of transverse sec-tions of the dorsal horn in sham-injured, vehicle-treated, and anti-CD11d mAb-treated rats. After SCI, the distribution and density of 5-HTimmunoreactive fibers were increasedsignificantly, primarily in the superficial laminas, with punctate fibers in laminas III and IV. Anti-CD11d mAb treatment normalized the distribution of 5-HT-immunoreactive fiberstoward patterns observed in sham-injured animals. Scale bar, 100 μm. D) The area of 5-HT-immunoreactivity in the dorsal horn rostral to the injury at T12–13 presented as themean area±SE of 5-HT-immunoreactivity in the sham injured (n=6), vehicle-treated (n=5), and anti-CD11d mAb-treated (n=6) groups. *, Pb0.05 compared with vehicle-treated rats; +, Pb0.05 compared with sham-injured rats. E) Photomicrograph of 5-HT3-R-immunoreactivity in a transverse section from the 9th thoracic segment of a rat injuredat T12. Immunoreactivity is primarily localized within the superficial laminae I and II of the dorsal horn with sparse amounts in the deeper laminae. Scale bar=100 μm (A) refers toall sections.Adapted, with permission, from Oatway et al. (2005).
SCI leads to loss of descending serotonergic control of severalpopulations of spinal neurons. Serotonergic projections from the nu-cleus raphe magnus to the spinal cord synapse in the dorsal horn,causing inhibition or facilitation of pain signaling that depends onthe receptor stimulated (Bardin et al., 2000; Calejesan et al., 1998).Thus, loss of descending serotonergic inputs caudal to the lesion sitecontributes to neuropathic pain (Hains et al., 2002). Serotonergic pro-jections to spinal cord autonomic and motor nuclei are also key regu-lators of these functions (Chalmers et al., 1985; Saruhashi et al.,1996). The serotonergic projections are often monitored as a meansof determining the efficacy of a treatment in promoting regenerationor sparing of important descending pathways. Indeed the anti-CD11dantibody treatment did promote some sparing and projection of sero-tonergic axons through the cord lesion site in our study (Oatwayet al., 2005). But we speculated that the mechanism by which thistreatment probably blocked neuropathic pain rostral to the injurysite was by limiting the excessive growth of serotonergic axons
rostral to the lesion. Similarly, Inman and Steward (Inman andSteward, 2003) reported a dense accumulation of 5-HT-immunoreactive fibers rostral to a spinal cord lesion site that were“disorganized and meandering” and unable to extend into the centrallesion. They termed this response “aberrant regenerative sprouting,”defined as the axonal growth that does not culminate in reconnectionof the injured axon with its normal target. The increased density ofserotonergic axons rostral to the SCI in our study appeared to pro-mote at-level neuropathic pain through actions of the serotonin onpro-nociceptive 5-HT3 receptors (Oatway et al., 2004) (Fig. 4E andFig. 5). The anti-CD11d mAb treatment reduced the aberrant growthresponse of serotonergic axons rostral to the injury, perhaps becauseof white matter sparing at the lesion borders that suggests dimin-ished axon injury, and gray matter sparing that would increase theavailability of target neurons for connection (Gris et al., 2004). There-fore, whereas regeneration and sprouting of serotonergic pathwaysthrough the lesion into the more distal spinal cord is an important
Fig. 5. The development of mechanical allodynia at the level of a T12 spinal cord injurysite. Testing sessions consisted of 10 stimulations with an innocuous, monofilamentstimulus 1 week before injury and at 2, 3, and 4 weeks after injury. Each data point rep-resents the mean±SEM number of avoidance responses made to 10 stimulations. Thenumber of avoidances made in response to dorsal trunk stimulation was significantlyreduced after anti-CD11d mAb treatment (filled squares) (n=7) when comparedwith vehicle-treated animals (open squares) (n=8), suggesting the reduction of at-level mechanical allodynia. *, Pb0.05 compared with vehicle-treated rats; +, Pb0.05compared with mean responses before injury.Adapted, with permission, from Oatway et al. (2005)
objective, the sprouting immediately rostral to the lesion is a form ofneuroplasticity associated with maladaptive conditions. Perhaps byreducing the hostility of the lesion environment, and fostering projec-tions of serotonergic axons through or next to the injury, the anti-CD11d treatment indirectly limited the excess growth rostral to thelesion. Alternatively, the reduction of inflammation rostral to the in-jury may have directly reduced stimuli for excess growth such asupregulated growth factors produced by inflammatory cells andglia. In the same vein, neurotrophin-induced sprouting of descendingcortico-spinal tract projections has recently been shown to depend onimmune cell activation (Chen et al., 2008). Thus reduced serotonergicfiber sprouting rostral to the spinal lesion in CD11d mAb-treated ratsand reduction of their neuropathic pain (Fig. 5) may be partly due tothe decreased immune cell activation that normally synergizes withneurotrophins to facilitate structural plasticity. These examples dem-onstrate that neuroprotective treatments may avert the negative con-sequences of neuroplasticity by a variety of mechanisms.
Summary and conclusions
In summary, neuroplasticity is at the root of both the neurologicalrecovery and the neuropathology that develops after SCI. The numberof instances where neuroplasticity leads to improved locomotor func-tion after SCI is truly remarkable and suggests that axons stimulatedto grow after injury do possess an intrinsic ability to rewire spinal cir-cuits in a manner that produces functional recovery. The dark side ofneuroplasticity is evidenced by the many cases in which the re-wiringalso leads to pain, autonomic dysreflexia and other negative sequellaeof SCI. The literature suggests that we might expect these negative ef-fects of neuroplasticity under some conditions more than others. Forexample, treatments or conditions that directly or indirectly increaseneurotrophins, such as NGF, in the spinal cord seem to be most highlycorrelated with the incidence of pain and autonomic dysreflexiaprobably because of the very strong growth-promoting effects ofthese molecules. On the other hand treatment strategies that focus
on blocking growth inhibitors seem to be less associated with nega-tive consequences of neuroplasticity perhaps because they have anindirect and more moderate effect on axon growth by removing im-pediments to neuroplasticity. Finally neuroprotective strategies maybe the safest of the treatment strategies with respect to the develop-ment of negative consequences of neuroplasticity as they focus onsparing descending and ascending tracts and thus may limit the deaf-ferentation that may act as a stimulus to promote neuroplasticity.
As the field of SCI research matures and therapeutic strategies pro-gress from preclinical studies to clinical trials it is important to keepin mind that achieving neurological recovery by promoting neuro-plasticity comes with a price. Increasing neuroplasticity to improveneurological outcomes after SCI should not be abandoned as a strate-gy, but we must be mindful that the stronger the drive for neuroplas-ticity, the greater the potential for negative consequences.
Bao, F., Chen, Y., Dekaban, G.A., Weaver, L.C., 2004a. An anti-CD11d integrin antibodyreduces cyclooxygenase-2 expression and protein and DNA oxidation after spinalcord injury in rats. J. Neurochem. 90, 1194–1204.
Bao, F., Chen, Y., Dekaban, G.A., Weaver, L.C., 2004b. Early anti-inflammatory treatmentreduces lipid peroxidation and protein nitration after spinal cord injury in rats. J.Neurochem. 88, 1335–1344.
Bardin, L., Lavarenne, J., Eschalier, A., 2000. Serotonin receptor subtypes involved in thespinal antinociceptive effect of 5-HT in rats. Pain 86, 11–18.
Bareyre, F.M., Kerschensteiner, M., Raineteau, O., Mettenleiter, T.C., Weinmann, O.,Schwab, M.E., 2004. The injured spinal cord spontaneously forms a new intraspinalcircuit in adult rats. Nat. Neurosci. 7, 269–277.
Barritt, A.W., Davies,M.,Marchand, F., Hartley, R., Grist, J., Yip, P.,McMahon, S.B., Bradbury,E.J., 2006. Chondroitinase ABC promotes sprouting of intact and injured spinal sys-tems after spinal cord injury. J. Neurosci. 26, 10856–10867.
Bignami, A., Asher, R., Perides, G., 1992. The extracellular matrix of rat spinal cord: acomparative study on the localization of hyaluronic acid, glial hyaluronate-binding protein, and chondroitin sulfate proteoglycan. Exp. Neurol. 117, 90–93.
Bracken, M.B., Shepard, M.J., Collins, W.F., Holford, T.R., Young, W., Baskin, D.S., Eisenberg,H.M., Flamm, E., Leo-Summers, L., Maroon, J., et al., 1990. A randomized, controlledtrial of methylprednisolone or naloxone in the treatment of acute spinal-cord injury.Results of the Second National Acute Spinal Cord Injury Study. N. Engl. J. Med. 322,1405–1411.
Brown, A., Ricci, M.J., Weaver, L.C., 2004. NGF message and protein distribution in theinjured rat spinal cord. Exp. Neurol. 188, 115–127.
Brown, A., Ricci, M.J., Weaver, L.C., 2007. NGF mRNA is expressed in the dorsal rootganglia after spinal cord injury in the rat. Exp. Neurol. 205, 283–286.
Bruce, J.C., Oatway, M.A., Weaver, L.C., 2002. Chronic pain after clip-compression injuryof the rat spinal cord. Exp. Neurol. 178, 33–48.
Calejesan, A.A., Ch'ang, M.H., Zhuo, M., 1998. Spinal serotonergic receptors mediate fa-cilitation of a nociceptive reflex by subcutaneous formalin injection into the hind-paw in rats. Brain Res. 798, 46–54.
Cameron, A.A., Smith, G.M., Randall, D.C., Brown, D.R., Rabchevsky, A.G., 2006. Geneticmanipulation of intraspinal plasticity after spinal cord injury alters the severity ofautonomic dysreflexia. J. Neurosci. 26, 2923–2932.
Chalmers, J.P., Kapoor, V., Macrae, I.M., Minson, J.B., Pilowsky, P., West, M.J., 1985. Newapproaches to the study of bulbospinal (B3) serotonergic neurons in the control ofblood pressure. J. Hypertens. Suppl. 3, S5–S9.
Chen, Q., Smith, G.M., Shine, H.D., 2008. Immune activation is required for NT-3-induced axonal plasticity in chronic spinal cord injury. Exp. Neurol. 209, 497–509.
Christensen, M., Hulsebosch, C., 1997a. Chronic central pain after spinal cord injury. J.Neurotrauma 14, 517–537.
Christensen, M.D., Hulsebosch, C.E., 1997b. Spinal cord injury and anti-NGF treatmentresults in changes in CGRP density and distribution in the dorsal horn in the rat.Exp. Neurol. 147, 463–475.
Collins, H.L., Rodenbaugh, D.W., DiCarlo, S.E., 2006. Spinal cord injury alters cardiacelectrophysiology and increases the susceptibility to ventricular arrhythmias.Prog. Brain Res. 152, 275–288.
Cruz-Orengo, L., Figueroa, J.D., Velazquez, I., Torrado, A., Ortiz, C., Hernandez, C., Puig,A., Segarra, A.C., Whittemore, S.R., Miranda, J.D., 2006. Blocking EphA4 upregula-tion after spinal cord injury results in enhanced chronic pain. Exp. Neurol. 202,421–433.
Cummings, B.J., Uchida, N., Tamaki, S.J., Salazar, D.L., Hooshmand, M., Summers, R.,Gage, F.H., Anderson, A.J., 2005. Human neural stem cells differentiate and promotelocomotor recovery in spinal cord-injured mice. Proc. Natl. Acad. Sci. U. S. A. 102,14069–14074.
de Groat, W.C., Yoshimura, N., 2006. Mechanisms underlying the recovery of lower uri-nary tract function following spinal cord injury. Prog. Brain Res. 152, 59–84.
Deumens, R., Joosten, E.A., Waxman, S.G., Hains, B.C., 2008. Locomotor dysfunction andpain: the scylla and charybdis of fiber sprouting after spinal cord injury. Mol. Neu-robiol. 37, 52–63.
Fabes, J., Anderson, P., Brennan, C., Bolsover, S., 2007. Regeneration-enhancing effectsof EphA4 blocking peptide following corticospinal tract injury in adult rat spinalcord. Eur. J. Neurosci. 26, 2496–2505.
Fawcett, J.W., Asher, R.A., 1999. The glial scar and central nervous system repair. BrainRes. Bull. 49, 377–391.
Frigon, A., Rossignol, S., 2006. Functional plasticity following spinal cord lesions. Prog.Brain Res. 157, 231–260.
Galtrey, C.M., Fawcett, J.W., 2007. The role of chondroitin sulfate proteoglycans in re-generation and plasticity in the central nervous system. Brain Res. Rev. 54, 1–18.
Ghosh, A., Haiss, F., Sydekum, E., Schneider, R., Gullo, M., Wyss, M.T., Mueggler, T.,Baltes, C., Rudin, M., Weber, B., Schwab, M.E., 2010. Rewiring of hindlimb corti-cospinal neurons after spinal cord injury. Nat. Neurosci. 13, 97–104.
Goldberger, M.E., Murray, M., Tessler, A., 1993. Sprouting and regeneration in the spinalcord. Their roles in recovery of function after spinal cord injury. In: Gorio, A. (Ed.),Neuroregeneration. Raven Press, New York, pp. 241–264.
Gonzenbach, R.R., Gasser, P., Zorner, B., Hochreutener, E., Dietz, V., Schwab, M.E., 2010.Nogo-A antibodies and training reduce muscle spasms in spinal cord-injured rats.Ann. Neurol. 68, 48–57.
Grill, R.J., Blesch, A., Tuszynski, M.H., 1997. Robust growth of chronically injured spinalcord axons induced by grafts of geneticallymodified NGF-secreting cells. Exp. Neurol.148, 444–452.
Gris, D., Marsh, D.R., Oatway, M.A., Chen, Y., Hamilton, E.F., Dekaban, G.A., Weaver, L.C.,2004. Transient blockade of the CD11d/CD18 integrin reduces secondary damageafter spinal cord injury, improving sensory, autonomic, and motor function. J. Neu-rosci. 24, 4043–4051.
Gris, D., Marsh, D.R., Dekaban, G.A., Weaver, L.C., 2005. Comparison of effects of meth-ylprednisolone and anti-CD11d antibody treatments on autonomic dysreflexiaafter spinal cord injury. Exp. Neurol. 194, 541–549.
Gris, P., Tighe, A., Thawer, S., Hemphill, A., Oatway, M., Weaver, L., Dekaban, G.A.,Brown, A., 2009. Gene expression profiling in anti-CD11d mAb-treated spinalcord-injured rats. J. Neuroimmunol. 209, 104–113.
Hains, B.C., Everhart, A.W., Fullwood, S.D., Hulsebosch, C.E., 2002. Changes in serotonin, se-rotonin transporter expression and serotonin denervation supersensitivity: involve-ment in chronic central pain after spinal hemisection in the rat. Exp. Neurol. 175,347–362.
Hains, B.C., Klein, J.P., Saab, C.Y., Craner, M.J., Black, J.A., Waxman, S.G., 2003. Upregula-tion of sodium channel Nav1.3 and functional involvement in neuronal hyperexcit-ability associated with central neuropathic pain after spinal cord injury. J. Neurosci.23, 8881–8892.
Helgren, M., Goldberger, M., 1993. The recovery of postural reflexes and locomotionfollowing low thoracic hemisection in adult cats involves compensation by unda-maged primary afferent pathways. Exp. Neurol. 123, 17–34.
Hou, S., Duale, H., Cameron, A.A., Abshire, S.M., Lyttle, T.S., Rabchevsky, A.G., 2008. Plas-ticity of lumbosacral propriospinal neurons is associated with the development ofautonomic dysreflexia after thoracic spinal cord transection. J. Comp. Neurol. 509,382–399.
Inman, D.M., Steward, O., 2003. Ascending sensory, but not other long-tract axons, re-generate into the connective tissue matrix that forms at the site of a spinal cord in-jury in mice. J. Comp. Neurol. 462, 431–449.
Jacob, J.E., Pniak, A., Weaver, L.C., Brown, A., 2001. Autonomic dysreflexia in a mousemodel of spinal cord injury. Neuroscience 108, 687–693.
Jacob, J.E., Gris, P., Fehlings, M.G., Weaver, L.C., Brown, A., 2003. Autonomic dysreflexiaafter spinal cord transection or compression in 129Sv, C57BL, and Wallerian de-generation slow mutant mice. Exp. Neurol. 183, 136–146.
Johnson, R.D., 2006. Descending pathways modulating the spinal circuitry for ejacula-tion: effects of chronic spinal cord injury. Prog. Brain Res. 152, 415–426.
Ju, G., Hokfelt, T., Brodin, E., Fahrenkrug, J., Fischer, J.A., Frey, P., Elde, R.P., Brown, J.C.,1987. Primary sensory neurons of the rat showing calcitonin gene-related peptideimmunoreactivity and their relation to substance P-, somatostatin-, galanin-, vaso-active intestinal polypeptide- and cholecystokinin-immunoreactive ganglion cells.Cell Tissue Res. 247, 417–431.
Karimi-Abdolrezaee, S., Eftekharpour, E., Wang, J., Morshead, C.M., Fehlings, M.G., 2006.Delayed transplantation of adult neural precursor cells promotes remyelinationand functional neurological recovery after spinal cord injury. J. Neurosci. 26,3377–3389.
Karimi-Abdolrezaee, S., Eftekharpour, E., Wang, J., Schut, D., Fehlings, M.G., 2010. Syn-ergistic effects of transplanted adult neural stem/progenitor cells, chondroitinase,and growth factors promote functional repair and plasticity of the chronically in-jured spinal cord. J. Neurosci. 30, 1657–1676.
Keyvan-Fouladi, N., Raisman, G., Li, Y., 2003. Functional repair of the corticospinal tractby delayed transplantation of olfactory ensheathing cells in adult rats. J. Neurosci.23, 9428–9434.
Krassioukov, A.V., Johns, D.G., Schramm, L.P., 2002. Sensitivity of sympathetically correlatedspinal interneurons, renal sympathetic nerve activity, and arterial pressure to somaticand visceral stimuli after chronic spinal injury. J. Neurotrauma 19, 1521–1529.
Krenz, N., Weaver, L., 1998. Sprouting of primary afferent fibers after spinal cord tran-section in the rat. Neuroscience 85, 443–458.
Krenz, N.R., Weaver, L.C., 2000. Nerve growth factor in glia and inflammatory cells ofthe injured rat spinal cord. J. Neurochem. 74, 730–739.
Lawson, S.N., McCarthy, P.W., Prabhakar, E., 1996. Electrophysiological properties ofneurones with CGRP-like immunoreactivity in rat dorsal root ganglia. J. Comp.Neurol. 365, 355–366.
Lawson, S.N., Crepps, B., Perl, E.R., 2002. Calcitonin gene-related peptide immunoreac-tivity and afferent receptive properties of dorsal root ganglion neurones in guinea-pigs. J. Physiol. 540, 989–1002.
Lemons, M.L., Howland, D.R., Anderson, D.K., 1999. Chondroitin sulfate proteoglycanimmunoreactivity increases following spinal cord injury and transplantation.Exp. Neurol. 160, 51–65.
Liebscher, T., Schnell, L., Schnell, D., Scholl, J., Schneider, R., Gullo, M., Fouad, K., Mir, A.,Rausch, M., Kindler, D., Hamers, F.P., Schwab, M.E., 2005. Nogo-A antibody im-proves regeneration and locomotion of spinal cord-injured rats. Ann. Neurol. 58,706–719.
Lu, J., Feron, F., Mackay-Sim, A., Waite, P.M., 2002. Olfactory ensheathing cells promotelocomotor recovery after delayed transplantation into transected spinal cord. Brain125, 14–21.
Maier, I.C., Ichiyama, R.M., Courtine, G., Schnell, L., Lavrov, I., Edgerton, V.R., Schwab,M.E., 2009. Differential effects of anti-Nogo-A antibody treatment and treadmilltraining in rats with incomplete spinal cord injury. Brain 132, 1426–1440.
Marsh, D.R., Weaver, L.C., 2004. Autonomic dysreflexia, induced by noxious or innocu-ous stimulation, does not depend on changes in dorsal horn substance p. J. Neuro-trauma 21, 817–828.
Marsh, D.R., Wong, S.T., Meakin, S.O., MacDonald, J.I., Hamilton, E.F., Weaver, L.C., 2002.Neutralizing intraspinal nerve growth factor with a trkA-IgG fusion protein blocksthe development of autonomic dysreflexia in a clip-compression model of spinalcord injury. J. Neurotrauma 19, 1531–1541.
Mathias, C.J., 2006. Orthostatic hypotension and paroxysmal hypertension in humanswith high spinal cord injury. Prog. Brain Res. 152, 231–243.
McKeon, R.J., Schreiber, R.C., Rudge, J.S., Silver, J., 1991. Reduction of neurite outgrowthin a model of glial scarring following CNS injury is correlated with the expressionof inhibitory molecules on reactive astrocytes. J. Neurosci. 11, 3398–3411.
McNeill, D.L., Hulsebosch, C.E., 1987. Intraspinal sprouting of rat primary afferents afterdeafferentation. Neurosci. Lett. 81, 57–62.
Oatway, M.A., Chen, Y., Bruce, J.C., Dekaban, G.A., Weaver, L.C., 2005. Anti-CD11d integ-rin antibody treatment restores normal serotonergic projections to the dorsal, in-termediate, and ventral horns of the injured spinal cord. J. Neurosci. 25, 637–647.
Price, J., 1985. An immunohistochemical and quantitative examination of dorsal rootganglion neuronal subpopulations. J. Neurosci. 5, 2051–2059.
Raisman, G., 2001. Olfactory ensheathing cells — another miracle cure for spinal cordinjury? Nat. Rev. Neurosci. 2, 369–375.
Ramon-Cueto, A., Cordero, M.I., Santos-Benito, F.F., Avila, J., 2000. Functional recoveryof paraplegic rats and motor axon regeneration in their spinal cords by olfactoryensheathing glia. Neuron 25, 425–435.
Richter, M.W., Fletcher, P.A., Liu, J., Tetzlaff, W., Roskams, A.J., 2005. Lamina propria andolfactory bulb ensheathing cells exhibit differential integration and migration andpromote differential axon sprouting in the lesioned spinal cord. J. Neurosci. 25,10700–10711.
Romero, M.I., Rangappa, N., Li, L., Lightfoot, E., Garry, M.G., Smith, G.M., 2000. Extensivesprouting of sensory afferents and hyperalgesia induced by conditional expressionof nerve growth factor in the adult spinal cord. J. Neurosci. 20, 4435–4445.
Saruhashi, Y., Young, W., Perkins, R., 1996. The recovery of 5-HT immunoreactivity inlumbosacral spinal cord and locomotor function after thoracic hemisection. Exp.Neurol. 139, 203–213.
Saville, L.R., Pospisil, C.H., Mawhinney, L.A., Bao, F., Simedrea, F.C., Peters, A.A., O'Con-nell, P.J., Weaver, L.C., Dekaban, G.A., 2004. A monoclonal antibody to CD11d re-duces the inflammatory infiltrate into the injured spinal cord: a potentialneuroprotective treatment. J. Neuroimmunol. 156, 42–57.
Seki, S., Sasaki, K., Fraser, M.O., Igawa, Y., Nishizawa, O., Chancellor, M.B., de Groat, W.C.,Yoshimura, N., 2002. Immunoneutralization of nerve growth factor in lumbosacralspinal cord reduces bladder hyperreflexia in spinal cord injured rats. J. Urol. 168,2269–2274.
Seki, S., Sasaki, K., Igawa, Y., Nishizawa, O., Chancellor, M.B., De Groat, W.C., Yoshimura,N., 2004. Suppression of detrusor-sphincter dyssynergia by immunoneutralizationof nerve growth factor in lumbosacral spinal cord in spinal cord injured rats. J. Urol.171, 478–482.
Steward, O., Zheng, B., Tessier-Lavigne, M., 2003. False resurrections: distinguishingregenerated from spared axons in the injured central nervous system. J. Comp.Neurol. 459, 1–8.
Tator, C.H., 1995. Update on the pathophysiology and pathology of acute spinal cord in-jury. Brain Pathol. 5, 407–413.
Teng, Y.D., Lavik, E.B., Qu, X., Park, K.I., Ourednik, J., Zurakowski, D., Langer, R., Snyder,E.Y., 2002. Functional recovery following traumatic spinal cord injury mediated bya unique polymer scaffold seeded with neural stem cells. Proc. Natl. Acad. Sci. U. S.A. 99, 3024–3029.
Tuszynski, M.H., Gabriel, K., Gage, F.H., Suhr, S., Meyer, S., Rosetti, A., 1996. Nervegrowth factor delivery by gene transfer induces differential outgrowth of sensory,motor, and noradrenergic neurites after adult spinal cord injury. Exp. Neurol. 137,157–173.
Weaver, L., Cassam, A., Krassioukov, A., Llewellyn-Smith, I., 1997. Changes in immuno-reactivity for growth associated protein-43 suggest reorganization of synapses onspinal sympathetic neurons after cord transection. Neuroscience 81, 535–551.
Weaver, L.C., Marsh, D.R., Gris, D., Brown, A., Dekaban, G.A., 2006. Autonomic dysre-flexia after spinal cord injury: central mechanisms and strategies for prevention.Prog. Brain Res. 152, 245–263.
Weaver, L., Verghese, P., Bruce, J., Fehlings, M., Krenz, N.R., Marsh, D., 2001. Autonomicdysreflexia and primary afferent sprouting after clip compression injury of the rat spi-nal cord. J. Neurotrauma 18, 1107–1119.
Wong, S.T., Atkinson, B.A., Weaver, L.C., 2000. Confocal microscopic analysis revealssprouting of primary afferent fibres in rat dorsal horn after spinal cord injury.Neurosci. Lett. 296, 65–68.
Young, W., 1993. Secondary injury mechanisms in acute spinal cord injury. J. Emerg.Med. 11 (Suppl. 1), 13–22.
Zhou, X.F., Li, H.Y., 2007. Roles of glial p75NTR in axonal regeneration. J. Neurosci. Res. 85,1601–1605.
