Spontaneous axonal regeneration in rodent spinal cord after ischemic injury

Corticospinal Motor Circuit Plasticity After Spinal Cord Injury: Harnessing Neuroplasticity to Improve Functional Outcomes

Spinal cord injury (SCI) is a devastating condition that affects approximately 294,000 people in the USA and several millions worldwide. The corticospinal motor circuitry plays a major role in controlling skilled movements and in planning and coordinating movements in mammals and can be damaged by SCI. While axonal regeneration of injured fibers over long distances is scarce in the adult CNS, substantial spontaneous neural reorganization and plasticity in the spared corticospinal motor circuitry has been shown in experimental SCI models, associated with functional recovery. Beneficially harnessing this neuroplasticity of the corticospinal motor circuitry represents a highly promising therapeutic approach for improving locomotor outcomes after SCI. Several different strategies have been used to date for this purpose including neuromodulation (spinal cord/brain stimulation strategies and brain-machine interfaces), rehabilitative training (targeting activity-dependent plasticity), stem cells and biological scaffolds, neuroregenerative/neuroprotective pharmacotherapies, and light-based therapies like photodynamic therapy (PDT) and photobiomodulation (PMBT). This review provides an overview of the spontaneous reorganization and neuroplasticity in the corticospinal motor circuitry after SCI and summarizes the various therapeutic approaches used to beneficially harness this neuroplasticity for functional recovery after SCI in preclinical animal model and clinical human patients' studies.

Serotonin neuron development: shaping molecular and structural identities

The continuing fascination with serotonin (5-hydroxytryptamine, 5-HT) as a nervous system chemical messenger began with its discovery in the brains of mammals in 1953. Among the many reasons for this decades-long interest is that the small numbers of neurons that make 5-HT influence the excitability of neural circuits in nearly every region of the brain and spinal cord. A further reason is that 5-HT dysfunction has been linked to a range of psychiatric and neurological disorders many of which have a neurodevelopmental component. This has led to intense interest in understanding 5-HT neuron development with the aim of determining whether early alterations in their generation lead to brain disease susceptibility. Here, we present an overview of the neuroanatomical organization of vertebrate 5-HT neurons, their neurogenesis, and prodigious axonal architectures, which enables the expansive reach of 5-HT neuromodulation in the central nervous system. We review recent findings that have revealed the molecular basis for the tremendous diversity of 5-HT neuron subtypes, the impact of environmental factors on 5-HT neuron development, and how 5-HT axons are topographically organized through disparate signaling pathways. We summarize studies of the gene regulatory networks that control the differentiation, maturation, and maintenance of 5-HT neurons. These studies show that the regulatory factors controlling acquisition of 5-HT-type transmitter identity continue to play critical roles in the functional maturation and the maintenance of 5-HT neurons. New insights are presented into how continuously expressed 5-HT regulatory factors control 5-HT neurons at different stages of life and how the regulatory networks themselves are maintained. WIREs Dev Biol 2018, 7:e301. doi: 10.1002/wdev.301 This article is categorized under: Nervous System Development > Vertebrates: General Principles Gene Expression and Transcriptional Hierarchies > Gene Networks and Genomics Gene Expression and Transcriptional Hierarchies > Cellular Differentiation Nervous System Development > Secondary: Vertebrates: Regional Development.

The combined strategy of mesenchymal stem cells and tissue-engineered scaffolds for spinal cord injury regeneration

Spinal cord injury (SCI) is a traumatic lesion that can result in the loss of motor or sensory neurons. Stem cell (SC)-based therapies have been demonstrated to promote neuronal regeneration following SCI, by releasing a range of trophic factors that support endogenous repair or by differentiating into neurons, or glial cells in order to replace the damaged cells. However, numerous limitations remain for therapies based on SC transplantion alone, including a low rate of survival/engraftment. Nevertheless, scaffolds are 3-dimentional substrates that have revealed to support cell survival, proliferation and differentiation in vivo, by mimicking a more favorable endogenous microenvironment. A multidisciplinary approach, which combines engineered scaffolds with SCs has been proposed as a promising strategy for encouraging spinal cord regeneration. The present review has focused on the regenerative potential of mesenchymal SCs isolated from different sources and combined with various scaffold types, in preclinical and clinical SCI studies.

Refining the Role of 5-HT in Postnatal Development of Brain Circuits

Changing serotonin (5-hydroxytryptamine, 5-HT) brain levels during critical periods in development has long-lasting effects on brain function, particularly on later anxiety/depression-related behaviors in adulthood. A large part of the known developmental effects of 5-HT occur during critical periods of postnatal life, when activity-dependent mechanisms remodel neural circuits. This was first demonstrated for the maturation of sensory brain maps in the barrel cortex and the visual system. More recently this has been extended to the 5-HT raphe circuits themselves and to limbic circuits. Recent studies overviewed here used new genetic models in mice and rats and combined physiological and structural approaches to provide new insights on the cellular and molecular mechanisms controlled by 5-HT during late stages of neural circuit maturation in the raphe projections, the somatosensory cortex and the visual system. Similar mechanisms appear to be also involved in the maturation of limbic circuits such as prefrontal circuits. The latter are of particular relevance to understand the impact of transient 5-HT dysfunction during postnatal life on psychiatric illnesses and emotional disorders in adult life.

Intrinsic Regenerative Ability of Mature CNS Neurons

A prevailing view in neuroscience is that the mature CNS has relatively little capacity to respond adaptively to injury. Recent data indicating a high degree of structural plasticity in the adult brain provides an impetus to reexamine how central neurons react to trauma. An analysis of both in vivo and in vitro experimental studies demonstrates that certain brain neurons may have an intrinsic ability to respond to structural injury by an attempt at regenerative sprouting. Indeed, aberrant sprouting following neuronal injury may be the cause of epilepsy following brain trauma and may underlie the neuronal changes stimulated by plaque formation in Alzheimer’s disease. An understanding of the stereotypical reaction to injury of different CNS neurons, as well as the role of nonneuronal cells, may provide new avenues for therapeutic intervention for a range of neurodegenerative diseases and “acquired” forms of CNS injury.

Strategies for regeneration of components of nervous system: scaffolds, cells and biomolecules

Nerve diseases including acute injury such as peripheral nerve injury (PNI), spinal cord injury (SCI) and traumatic brain injury (TBI), and chronic disease like neurodegeneration disease can cause various function disorders of nervous system, such as those relating to memory and voluntary movement. These nerve diseases produce great burden for individual families and the society, for which a lot of efforts have been made. Axonal pathways represent a unidirectional and aligned architecture allowing systematic axonal development within the tissue. Following a traumatic injury, the intricate architecture suffers disruption leading to inhibition of growth and loss of guidance. Due to limited capacity of the body to regenerate axonal pathways, it is desirable to have biomimetic approach that has the capacity to graft a bridge across the lesion while providing optimal mechanical and biochemical cues for tissue regeneration. And for central nervous system injury, one more extra precondition is compulsory: creating a less inhibitory surrounding for axonal growth. Electrospinning is a cost-effective and straightforward technique to fabricate extracellular matrix (ECM)-like nanofibrous structures, with various fibrous forms such as random fibers, aligned fibers, 3D fibrous scaffold and core-shell fibers from a variety of polymers. The diversity and versatility of electrospinning technique, together with functionalizing cues such as neurotrophins, ECM-based proteins and conductive polymers, have gained considerable success for the nerve tissue applications. We are convinced that in the future the stem cell therapy with the support of functionalized electrospun nerve scaffolds could be a promising therapy to cure nerve diseases.

Biomaterials for spinal cord repair

Spinal cord injury (SCI) results in permanent loss of function leading to often devastating personal, economic and social problems. A contributing factor to the permanence of SCI is that damaged axons do not regenerate, which prevents the re-establishment of axonal circuits involved in function. Many groups are working to develop treatments that address the lack of axon regeneration after SCI. The emergence of biomaterials for regeneration and increased collaboration between engineers, basic and translational scientists, and clinicians hold promise for the development of effective therapies for SCI. A plethora of biomaterials is available and has been tested in various models of SCI. Considering the clinical relevance of contusion injuries, we primarily focus on polymers that meet the specific criteria for addressing this type of injury. Biomaterials may provide structural support and/or serve as a delivery vehicle for factors to arrest growth inhibition and promote axonal growth. Designing materials to address the specific needs of the damaged central nervous system is crucial and possible with current technology. Here, we review the most prominent materials, their optimal characteristics, and their potential roles in repairing and regenerating damaged axons following SCi.

Hydrogels in spinal cord injury repair strategies

Nowadays there are at present no efficient therapies for spinal cord injury (SCI), and new approaches have to be proposed. Recently, a new regenerative medicine strategy has been suggested using smart biomaterials able to carry and deliver cells and/or drugs in the damaged spinal cord. Among the wide field of emerging materials, research has been focused on hydrogels, three-dimensional polymeric networks able to swell and absorb a large amount of water. The present paper intends to give an overview of a wide range of natural, synthetic, and composite hydrogels with particular efforts for the ones studied in the last five years. Here, different hydrogel applications are underlined, together with their different nature, in order to have a clearer view of what is happening in one of the most sparkling fields of regenerative medicine.

Chondroitinase ABC promotes recovery of adaptive limb movements and enhances axonal growth caudal to a spinal hemisection

A number of studies have shown that chondroitinase ABC (Ch'ase ABC) digestion of inhibitory chondroitin sulfate glycosaminoglycans significantly enhances axonal growth and recovery in rodents following spinal cord injury (SCI). Further, our group has shown improved recovery following SCI in the larger cat model. The purpose of the current study was to determine whether intraspinal delivery of Ch'ase ABC, following T10 hemisections in adult cats, enhances adaptive movement features during a skilled locomotor task and/or promotes plasticity of spinal and supraspinal circuitry. Here, we show that Ch'ase ABC enhanced crossing of a peg walkway post-SCI and significantly improved ipsilateral hindlimb trajectories and integration into a functional forelimb-hindlimb coordination pattern. Recovery of these complex movements was associated with significant increases in neurofilament immunoreactivity immediately below the SCI in the ipsilateral white (p = 0.033) and contralateral gray matter (p = 0.003). Further, the rubrospinal tract is critical in the normal cat during skilled movements that require accurate paw placements and trajectories like those seen during peg walkway crossing. Rubrospinal connections were assessed following Fluoro-Gold injections, caudal to the hemisection. Significantly more retrogradely labeled right (axotomized) red nucleus (RN) neurons were seen in Ch'ase ABC-treated (23%) compared with control-treated cats (8%; p = 0.032) indicating that a larger number of RN neurons in Ch'ase ABC-treated cats had axons below the lesion level. Thus, following SCI, Ch'ase ABC may facilitate axonal growth at the spinal level, enhance adaptive features of locomotion, and affect plasticity of rubrospinal circuitry known to support adaptive behaviors in the normal cat.

Functional evaluation of paraplegic monkeys (Macaca mulatta) over fourteen months post-lesion

We report on the neurological and neurophysiological findings obtained from two adult Macaca mulatta sustaining complete spinal cord transections at T8-T9. We performed periodic neurological exams, recorded motor evoked potentials (MEPs) following transcranial magnetic stimulation (TMS), and recorded electromyograms (EMGs) during the execution of a lower limb motor test. The main observations were: (1) the spinal shock period lasted less than a week; tendon, cutaneous and withdrawal reflexes were uneven in range and occurrence, and Babinski's sign was not observed; (2) a protracted functional lesion in the tibial and common peroneal nerves appeared bilaterally early in the post-lesional period; (3) MEPs were elicited by TMS in the quadriceps muscle of both monkeys; they were recorded as early as the 5th week after lesion in one of the monkeys, and they persisted throughout the post-lesional period in both monkeys; and (4) motor unit action potentials in the quadriceps muscle recorded by EMG were simultaneous with attempts to perform intentional lower limb movements from post-lesion month 11 to 13.5 in both monkeys. The last two sets of observations argue in favor of a partial cortico-spinal functional gain and suggest that spinal cord regeneration can occur after complete spinal cord injury in primates.

Enhanced regenerative axon growth of multiple fibre populations in traumatic spinal cord injury following scar-suppressing treatment

We analysed the effect of scar-suppressing treatment (anti-scarring treatment; AST) on augmenting axonal regeneration of various neuronal populations following spinal cord injury (SCI) in adult rat. AST included local iron chelator (2,2'-dipyridine-5,5'-dicarboxylic acid) injection and 8-bromo-cyclic adenosine monophosphate application to the lesion core. In previous studies, this treatment promoted long-distance regeneration of cut corticospinal tract axons, neuroprotection of projecting cortical neurons and functional improvement of treated rats [N. Klapka et al. (2005)Eur. J. Neurosci., 22, 3047-3058]. Treatment yielded significantly enhanced regrowth of descending serotonergic (5-HT), catecholaminergic (tyrosine hydroxylase; TH), corticospinal and rubrospinal axons into the lesion zone, as assessed by anterograde tracing and immunohistochemistry followed by quantification of axon profiles at 5 and 12 weeks post-injury. In addition, the determination of axons crossing the proximal borderline from uninjured tissue into fibrous scar area revealed a significant AST-promoted increase of intersecting fibres for 5-HT, TH and calcitonin gene-related peptide containing ascending sensory fibres. For a prolonged time period after lesion, the delayed (secondary) scar developing in treated rats is significantly more permeable for all analysed axon tracts than the initial (primary) scar forming in injured control animals lacking treatment. Furthermore, enhanced outgrowth of descending axons from fibrous scar into distal healthy spinal tissue was achieved in treated animals, and is in line with previous functional studies [S. Hermanns et al. (2001) Restor. Neurol. Neurosci., 19,139-148; N. Klapka et al. (2005)Eur. J. Neurosci., 22, 3047-3058]. Our findings indicate that AST exerts a prolonged beneficial effect on fibrous scarring allowing enhanced axonal regrowth of different fibre tracts in SCI regardless of their distinct regenerative demands.

Activity-dependent plasticity in spinal cord injury

The adult mammalian central nervous system (CNS) is capable of considerable plasticity, both in health and disease. After spinal neurotrauma, the degrees and extent of neuroplasticity and recovery depend on multiple factors, including the level and extent of injury, postinjury medical and surgical care, and rehabilitative interventions. Rehabilitation strategies focus less on repairing lost connections and more on influencing CNS plasticity for regaining function. Current evidence indicates that strategies for rehabilitation, including passive exercise, active exercise with some voluntary control, and use of neuroprostheses, can enhance sensorimotor recovery after spinal cord injury (SCI) by promoting adaptive structural and functional plasticity while mitigating maladaptive changes at multiple levels of the neuraxis. In this review, we will discuss CNS plasticity that occurs both spontaneously after SCI and in response to rehabilitative therapies.

An overview of tissue engineering approaches for management of spinal cord injuries

Severe spinal cord injury (SCI) leads to devastating neurological deficits and disabilities, which necessitates spending a great deal of health budget for psychological and healthcare problems of these patients and their relatives. This justifies the cost of research into the new modalities for treatment of spinal cord injuries, even in developing countries. Apart from surgical management and nerve grafting, several other approaches have been adopted for management of this condition including pharmacologic and gene therapy, cell therapy, and use of different cell-free or cell-seeded bioscaffolds. In current paper, the recent developments for therapeutic delivery of stem and non-stem cells to the site of injury, and application of cell-free and cell-seeded natural and synthetic scaffolds have been reviewed.

Intramedullary neuroma of the cervicomedullary junction. Case report

Neuromas typically arise in the peripheral nervous system in response to traumatic injury at the site of partial or complete nerve transection as new axons from the proximal nerve stump sprout to reinnervate the distal segment. In rare cases neuromas have also been described as intramedullary spinal cord lesions. These lesions have been identified as incidental autopsy findings in association with prior trauma and cervical spondylosis, multiple sclerosis, spinal tumors, and syringomyelia. The authors report the case of a 50-year-old man who had been involved in a motor vehicle accident, during which his car was struck from behind as it was stationary at an intersection, more than 5 years before presentation. A workup for syncopal and presyncopal episodes involved magnetic resonance imaging that revealed a 1.1-cm lesion at the cervicomedullary junction (CMJ). The imaging features of the lesion raised the question of an ependymoma or subependymoma. The lesion was excised, and examination of the tissue demonstrated a neuroma with haphazardly arranged interlacing bundles of axons ensheathed by Schwann cells with interfascicular regions of reactive glial cells and Rosenthal fibers, consistent with those present after traumatic injury. This case may represent the first true traumatic intramedullary neuroma of the CMJ diagnosed in a living patient and treated surgically.

Degenerative and spontaneous regenerative processes after spinal cord injury

Spinal cord injury results in acute as well as progressive secondary destruction of local and distant nervous tissue through a number of degenerative mechanisms. Spinal cord injury also initiates a number of endogenous neuroprotective and regenerative responses. Understanding of these mechanisms might identify potential targets for treatments after spinal cord injury in humans. Here, we first discuss recent developments in our understanding of the immediate traumatic and subsequent secondary degeneration of local tissue and long projecting pathways in animal models. These include the inflammatory and vascular responses during the acute phase, as well as cell death, demyelination and scar formation in the subacute and chronic phases. Secondly, we discuss the spontaneous axonal regeneration of injured and plasticity of uninjured systems, and other repair-related responses in animals, including the upregulation of regeneration-associated genes in some neurons, increases in neurotrophic factors in the spinal cord and remyelination by oligodendrocyte precursors and invading Schwann cells. Lastly, we comment on the still limited understanding of the neuropathology in humans, which is largely similar to that in rodents. However, there also are potentially important differences, including the reduced glial scarring, inflammation and demyelination, the increased Schwannosis and the protracted Wallerian degeneration in humans. The validity of current rodent models for human spinal cord injury is also discussed. The emphasis of this review is on the literature from 2002 to early 2005.

NG2 colocalizes with axons and is expressed by a mixed cell population in spinal cord lesions

The NG2 proteoglycan is of general interest after spinal cord injury because it is expressed by oligodendrocyte progenitors (OPCs), which contribute to central nervous system remyelination; however, NG2 may inhibit axon regeneration. We and others have examined the spatiotemporal expression of NG2 after spinal cord injury (SCI). Here, we extend those observations and provide a comprehensive analysis of the distribution, phenotype, and colocalization of NG2 cells with axons in a clinically relevant model of spinal contusion. Because contusion models mimic the majority of human SCI, this information is important for understanding endogenous processes that promote and/or prevent repair. The data demonstrate that NG2 levels rise significantly between 3 and 7 days postinjury (dpi) and remain elevated chronically throughout the lesions. NG2 within the lesions could be derived from an array of infiltrating cells; thus, a panel of antibodies was used to investigate NG2 cell phenotypes. First, platelet-derived growth factor-alpha receptor (PDGFalphaR) colocalization was examined because OPCs normally express both markers. PDGFalphaR cells were present in lesions at all times examined. However, only 37% of NG2 cells coexpressed PDGFalphaR at 14 dpi, which dropped to <1% by 70 dpi. This contrasts with the nearly complete overlap in spared tissue surrounding the lesion. In contrast, 40% to 60% of NG2 cells expressed p75 and approximately 84% expressed Sox10, suggesting that many NG2 cells were nonmyelinating Schwann cells. Despite rising levels of NG2, we noted robust and sustained axon growth into the lesions, many of which were located along NG2 profiles. Thus, spinal contusion produces an NG2-rich environment into which axons grow and in which the source of NG2 appears considerably different from that in surrounding spared tissue.

Changes in distribution of serotonin induced by spinal injury in larval lampreys: evidence from immunohistochemistry and HPLC

Larval lampreys are known to successfully recover normal behavior following spinal cord injury. More recently, we showed temperature can influence functional recovery, with colder temperatures more likely producing behavioral abnormality despite the cold being the animals' normal temperature. Here we analyze the differences associated with temperature effects. We examine serotonergic expression along the spinal cord following midbody lesions considering time from injury, temperature during recovery and fiber location. We also examine the relationship between regeneration and locomotion, insofar as the pattern of muscle potentials during unrestrained swimming is normal or abnormal. At 26 weeks after spinal cord injury in all groups of animals, immunohistochemistry and HPLC for serotonin and serotonin expression above and below the lesion can be significantly changed in all regional sources of serotonin independent of the temperature animals recover from their injuries. Animals from warmer tanks recover serotonin expression in the segment immediately caudal to the lesion site with little further away from the lesion; animals from the cold room aquaria have significantly less recovery of expression caudal to the lesion and none further away. There was no apparent relationship between the distribution of serotonin and recovery. The changes suggest that some intraspinal reorganization has occurred. We propose a relationship between the observed results and functional recovery, but it remains conjectural. The fact that some animals recover normal function suggests plasticity must occur in animals successful in recovering normal function. Thus, the lamprey can be used as a model system to study the adaptive changes that permit or prevent functional recovery.

Defining the role of olfactory ensheathing cells in facilitating axon remyelination following damage to the spinal cord

Olfactory ensheathing cells (OECs) are unique cells that are responsible for the successful regeneration of olfactory axons throughout the life of adult mammals. More than a decade of research has shown that implantation of OECs may be a promising therapy for damage to the nervous system, including spinal cord injury. Based on this research, several clinical trials worldwide have been initiated that use autologous transplantation of olfactory tissue containing OECs into the damaged spinal cord of humans. However, research from several laboratories has challenged the widely held belief that OECs are directly responsible for myelinating axons and promoting axon regeneration. The purpose of this review is to provide a working hypothesis that integrates several current ideas regarding the mechanisms of the beneficial effects of OECs. Specifically, OECs promote axon regeneration and functional recovery indirectly by augmenting the endogenous capacity of host Schwann cells to invade the damaged spinal cord. Together with Schwann cells, OECs create a 3-dimensional matrix that provides a permissive microenvironment for successful axon regeneration in the adult mammalian central nervous system.

Steady-State Levels of Monoamines in the Rat Lumbar Spinal Cord: Spatial Mapping and the Effect of Acute Spinal Cord Injury

Monoamines in the spinal cord are important in the regulation of locomotor rhythms, nociception, and motor reflexes. To gain further insight into the control of these functions, the steady-state extracellular distribution of monoamines was mapped in the anesthetized rat's lumbar spinal cord. The effect of acute spinal cord lesions at sites selected for high resting levels was determined over ∼1 h to estimate contributions to resting levels from tonic descending activity and to delineate chemical changes that may influence the degree of pathology and recovery after spinal injury. Measurements employed fast cyclic voltammetry with carbon fiber microelectrodes to give high spatial resolution. Monoamine oxidation currents, sampled at equal vertical spacings within each segment, were displayed as contours over the boundaries delineated by histologically reconstructed electrode tracks. Monoamine oxidation currents were found in well defined foci, often confined within a single lamina. Larger currents were typically found in the dorsal or ventral horns and in the lateral aspect of the intermediate zone. Cooling of the low-thoracic spinal cord led to a decrease in the oxidation current (to 71–85% of control) in dorsal and ventral horns. Subsequent low-thoracic transection produced a transient increase in signal in some animals followed by a longer lasting decrease to levels similar to or below that with cooling (to 17–86% of control values). We conclude that descending fibers tonically release high amounts of monoamines in localized regions of the dorsal and ventral horn of the lumbar spinal cord at rest. Lower amounts of monoamines were detected in medial intermediate zone areas, where strong release may be needed for descending activation of locomotor rhythms.

Rodent models for treatment of spinal cord injury: research trends and progress toward useful repair

PURPOSE OF REVIEW

In this review, we have documented some current research trends in rodent models of spinal cord injury. We have also catalogued the treatments used in studies published between October 2002 and November 2003, with special attention given to studies in which treatments were delayed for at least 4 days after injury.

RECENT FINDINGS

Most spinal cord injury studies are performed with one of three general injury models: transection, compression, or contusion. Although most treatments are begun immediately after injury, a growing number of studies have used delayed interventions. Mice and the genetic tools they offer are gaining in popularity. Some researchers are setting their sights beyond locomotion, to issues more pressing for people with spinal cord injury (especially bladder function and pain).

SUMMARY

Delayed treatment protocols may extend the window of opportunity for treatment of spinal cord injury, whereas continued progress in the prevention of secondary cell death will reduce the severity of new cases. The use of mice will hopefully accelerate progress towards useful regeneration in humans. Researchers must improve cross-study comparability to allow balanced decisions about potentially useful treatments.

Numb rats walk - a behavioural and fMRI comparison of mild and moderate spinal cord injury

Assessment of sensory function serves as a sensitive measure for predicting the functional outcome following spinal cord injury in patients. However, little is known about loss and recovery of sensory function in rodent spinal cord injury models as most tests of sensory functions rely on behaviour and thus motor function. We used functional magnetic resonance imaging (fMRI) to investigate cortical and thalamic BOLD-signal changes in response to limb stimulation following mild or moderate thoracic spinal cord weight drop injury in Sprague-Dawley rats. While there was recovery of close to normal hindlimb motor function as determined by open field locomotor testing following both degrees of injury, recovery of hindlimb sensory function as determined by fMRI and hot plate testing was only seen following mild injury and not following moderate injury. Thus, moderate injury can lead to near normal hindlimb motor function in animals with major sensory deficits. Recovered fMRI signals following mild injury had a partly altered cortical distribution engaging also ipsilateral somatosensory cortex and the cingulate gyrus. Importantly, thoracic spinal cord injury also affected sensory representation of the upper nonaffected limbs. Thus, cortical and thalamic activation in response to forelimb stimulation was significantly increased 16 weeks after spinal cord injury compared to control animals. We conclude that both forelimb and hindlimb cortical sensory representation is altered following thoracic spinal cord injury. Furthermore tests of sensory function that are independent of motor behaviour are needed in rodent spinal cord injury research.

Prevention of gliotic scar formation by NeuroGel? allows partial endogenous repair of transected cat spinal cord

Spinal cords of adult cats were transected and subsequently reconnected with the biocompatible porous poly (N-[2-hydroxypropyl] methacrylamide) hydrogel, NeuroGel. Tissue repair was examined at various time points from 6-21 months post reconstructive surgery. We examined two typical phenomena, astrogliosis and scar formation, in spines reconstructed with the gel and compared them to those from transected non-reconstructed spines. Confocal examination with double immunostaining for glial fibrillary acidic protein (GFAP) and myelin basic protein (MBP) showed that the interface formed between the hydrogel and the spine stumps did prevent scar formation and only a moderate gliosis was observed. The gel implant provided an adequate environment for growth of myelinated fibers and we saw angiogenesis within the gel. Electron microscopy showed that regenerating axons were myelinated by Schwann cells rather than oligodendrocytes. Moreover, the presence of the gel implant lead to a considerable reduction in damage to distal caudal portions of the spine as assessed by the presence of more intact myelinated fibers and a reduction of myelin degradation. Neurologic assessments of hindlimb movement at various times confirmed that spinal cord reconstruction was not only structural but also functional. We conclude that NeuroGel lead to functional recovery by providing a favorable substrate for regeneration of transected spinal cord, reducing glial scar formation and allowing angiogenesis.

Ascending sensory, but not other long-tract axons, regenerate into the connective tissue matrix that forms at the site of a spinal cord injury in mice

Mice exhibit a unique wound healing response following spinal cord injury in which the lesion site fills in with a connective tissue matrix. Previous studies have revealed that axons grow into this matrix, but the source of the axons remained unknown. The present study assesses whether any of these axons were the result of long tract regeneration. C57Bl/6 mice received crush injuries and were allowed to survive for 6 weeks to 7 months. Biotinylated dextran amine (BDA) was injected into the somato-motor cortex to trace descending corticospinal tract (CST) axons, into the midbrain to label descending brainstem pathways including the rubrospinal and reticulospinal tracts, or into the L5 dorsal root ganglion to trace ascending projections of first-order sensory neurons. Spinal cords from other mice were prepared for immunocytochemistry using antibodies against neurofilament protein (NF), 5-HT to reveal descending serotonergic axons, calcitonin gene-related protein (CGRP) to reveal ascending sensory axons, and chondroitin sulfate proteoglycan (CSPG) to assess the distribution of molecules that are inhibitory to axon growth. NF immunostaining revealed axons in the connective tissue matrix at the lesion site, confirming previous studies that used protargol staining. CST axons did not enter the connective tissue matrix, but did sprout extensively in segments adjacent to the injury site. Rubrospinal and reticulospinal tract axons also did not grow into the lesion site. 5-HT-positive axons extended to the edge of the lesion, and a few axons followed astrocyte processes into the margins of the lesion site. In contrast to the other pathways, BDA-labeled ascending sensory axons did extend into and arborized extensively within the connective tissue matrix, although the subgroup of ascending axons that are positive for CGRP did not. These results indicate that the connective tissue matrix is permissive for regeneration of some classes of ascending sensory axons but not for other axonal systems.