Training-induced plasticity in rats with cervical spinal cord injury: Effects and side effects

Stage-dependent role of interhemispheric pathway for motor recovery in primates

Whether and how the non-lesional sensorimotor cortex is activated and contributes to post-injury motor recovery is controversial. Here, we investigated the role of interhemispheric pathway from the contralesional to ipsilesional premotor cortex in activating the ipsilesional sensorimotor cortex and promoting recovery after lesioning the lateral corticospinal tract at the cervical cord, by unidirectional chemogenetic blockade in macaques. The blockade impaired dexterous hand movements during the early recovery stage. Electrocorticographical recording showed that the low frequency band activity of the ipsilesional premotor cortex around movement onset was decreased by the blockade during the early recovery stage, while it was increased by blockade during the intact state and late recovery stage. These results demonstrate that action of the interhemispheric pathway changed from inhibition to facilitation, to involve the ipsilesional sensorimotor cortex in hand movements during the early recovery stage. The present study offers insights into the stage-dependent role of the interhemispheric pathway and a therapeutic target in the early recovery stage after lesioning of the corticospinal tract.

Evaluation of the effect of intrathecal GM1 in 24, 48, and 72 hours after acute spinal cord injury in rats

OBJECTIVE

The aim of this study was to evaluate the best timing and feasibility of intrathecal application of sodium monosialoganglioside (GM1) after spinal cord contusion in Wistar rats as an experimental model.

METHODS

Forty Wistar rats were submitted to contusion spinal cord injury after laminectomy. The animals were randomized and divided into four groups: Group 1 - Intrathecal application of GM1 24 hours after contusion; Group 2 - Intrathecal application of GM1 48 hours after contusion; Group 3 - intrathecal application of GM1 72 hours after contusion; Group 4 - Sham, with laminectomy and intrathecal application of 0.5 mL of 0.9% saline solution, without contusion. The recovery of locomotor function was evaluated at seven different moments by the Basso, Beattie, and Bresnahan (BBB) test. They were also assessed by the horizontal ladder, with sensory-motor behavioral assessment criteria, pre-and postoperatively.

RESULTS

This experimental study showed better functional scores in the group submitted to the application of GM1, with statistically significant results, showing a mean increase when evaluated on known motor tests like the horizontal ladder and BBB, at all times of evaluation (p < 0.05), especially in group 2 (48 hours after spinal cord injury). Also, fewer mistakes and slips over the horizontal ladder were observed, and many points were achieved at the BBB scale analysis.

CONCLUSION

The study demonstrated that the intrathecal application of GM1 after spinal cord contusion in Wistar rats is feasible. The application 48 hours after the injury presented the best functional results.

When Spinal Neuromodulation Meets Sensorimotor Rehabilitation: Lessons Learned From Animal Models to Regain Manual Dexterity After a Spinal Cord Injury

Electrical neuromodulation has strongly hit the foundations of spinal cord injury and repair. Clinical and experimental studies have demonstrated the ability to neuromodulate and engage spinal cord circuits to recover volitional motor functions lost after the injury. Although the science and technology behind electrical neuromodulation has attracted much of the attention, it cannot be obviated that electrical stimulation must be applied concomitantly to sensorimotor rehabilitation, and one would be very difficult to understand without the other, as both need to be finely tuned to efficiently execute movements. The present review explores the difficulties faced by experimental and clinical neuroscientists when attempting to neuromodulate and rehabilitate manual dexterity in spinal cord injured subjects. From a translational point of view, we will describe the major rehabilitation interventions employed in animal research to promote recovery of forelimb motor function. On the other hand, we will outline some of the state-of-the-art findings when applying electrical neuromodulation to the spinal cord in animal models and human patients, highlighting how evidences from lumbar stimulation are paving the path to cervical neuromodulation.

Skilled reach training enhances robotic gait training to restore overground locomotion following spinal cord injury in rats

Rehabilitative training has been shown to improve motor function following spinal cord injury (SCI). Unfortunately, these gains are primarily task specific; where reach training only improves reaching, step training only improves stepping and stand training only improves standing. More troublesome is the tendency that the improvement in a trained task often comes at the expense of an untrained task. However, the task specificity of training does not preclude the benefits of combined rehabilitative training. Here we show that robot assisted gait training alone can partially reduce the deficits in unassisted overground locomotion following a C4/5 overhemisection injury in rats. When robot-assisted gait training is done in conjunction with skilled forelimb training, we observe a much greater level of recovery of unassisted overground locomotion. In order to provide reach training that would not interfere with our robotic gait training schedule, we prompted rats to increase the use of their forelimbs by replacing the standard overhead feeder with a custom made, deep welled hopper that dispensed nutritionally equivalent small milled pellets. We speculate that the increase in recovery from combined training is due to a more robust interneuronal relay network around the injury site. in vivo manganese-enhanced magnetic resonance imaging of the spinal cord indicated that there was no increase in the cellular activity, however ex vivo diffusion tensor imaging (DTI) suggested an increase in collateralization around the injury site in rats that received both reach training and robot assisted gait training.

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.

Longitudinal fMRI measures of cortical reactivation and hand use with and without training after sensory loss in primates

In a series of previous studies, we demonstrated that damage to the dorsal column in the cervical spinal cord deactivates the contralateral somatosensory hand cortex and impairs hand use in a reach-to-grasp task in squirrel monkeys. Nevertheless, considerable cortical reactivation and behavioral recovery occurs over the following weeks to months after lesion. This timeframe may also be a window for targeted therapies to promote cortical reactivation and functional reorganization, aiding in the recovery process. Here we asked if and how task specific training of an impaired hand would improve behavioral recovery and cortical reorganization in predictable ways, and if recovery related cortical changes would be detectable using noninvasive functional magnetic resonance imaging (fMRI). We further asked if invasive neurophysiological mapping reflected fMRI results. A reach-to-grasp task was used to test impairment and recovery of hand use before and after dorsal column lesions (DC-lesion). The activation and organization of the affected primary somatosensory cortex (area 3b) was evaluated with two types of fMRI - either blood oxygenation level dependent (BOLD) or cerebral blood volume (CBV) with a contrast agent of monocrystalline iron oxide nanocolloid (MION) - before and after DC-lesion. At the end of the behavioral and fMRI studies, microelectrode recordings in the somatosensory areas 3a, 3b and 1 were used to characterize neuronal responses and verify the somatotopy of cortical reactivations. Our results indicate that even after nearly complete DC lesions, monkeys had both considerable post-lesion behavioral recovery, as well as cortical reactivation assessed with fMRI followed by extracellular recordings. Generalized linear regression analyses indicate that lesion extent is correlated with the behavioral outcome, as well as with the difference in the percent signal change from pre-lesion peak activation in fMRI. Monkeys showed behavioral recovery and nearly complete cortical reactivation by 9-12 weeks post-lesion (particularly when the DC-lesion was incomplete). Importantly, the specific training group revealed trends for earlier behavioral recovery and had higher magnitude of fMRI responses to digit stimulation by 5-8 weeks post-lesion. Specific kinematic measures of hand movements in the selected retrieval task predicted recovery time and related to lesion characteristics better than overall task performance success. For measures of cortical reactivation, we found that CBV scans provided stronger signals to vibrotactile digit stimulation as compared to BOLD scans, and thereby may be the preferred non-invasive way to study the cortical reactivation process after sensory deprivations from digits. When the reactivation of cortex for each of the digits was considered, the reactivation by digit 2 stimulation as measured with microelectrode maps and fMRI maps was best correlated with overall behavioral recovery.

An intracortical neuroprosthesis immediately alleviates walking deficits and improves recovery of leg control after spinal cord injury

Most rehabilitation interventions after spinal cord injury (SCI) only target the sublesional spinal networks, peripheral nerves, and muscles. However, mammalian locomotion is not a mere act of rhythmic pattern generation. Recovery of cortical control is essential for voluntary movement and modulation of gait. We developed an intracortical neuroprosthetic intervention to SCI, with the goal to condition cortical locomotor control. Neurostimulation delivered in phase coherence with ongoing locomotion immediately alleviated primary SCI deficits, such as leg dragging, in rats with incomplete SCI. Cortical neurostimulation achieved high fidelity and markedly proportional online control of leg trajectories in both healthy and SCI rats. Long-term neuroprosthetic training lastingly improved cortical control of locomotion, whereas short training held transient improvements. We performed longitudinal awake cortical motor mapping, unveiling that recovery of cortico-spinal transmission tightly parallels return of locomotor function in rats. These results advocate directly targeting the motor cortex in clinical neuroprosthetic approaches.

Dual motor cortex and spinal cord neuromodulation improves rehabilitation efficacy and restores skilled locomotor function in a rat cervical contusion injury model

Motor recovery after spinal cord injury is limited due to sparse descending pathway axons caudal to the injury. Rehabilitation is the primary treatment for paralysis in humans with SCI, but only produces modest functional recovery. Here, we determined if dual epidural motor cortex (M1) intermittent theta burst stimulation (iTBS) and cathodal transcutaneous spinal direct stimulation (tsDCS) enhances the efficacy of rehabilitation in improving motor function after cervical SCI. iTBS produces CST axon sprouting and tsDCS enhances M1-evoked spinal activity and muscle contractions after SCI. Rats were trained to perform the horizontal ladder task. Animals received a moderate midline C4 contusion, producing bilateral forelimb impairments. After 2 weeks, animals either received 10 days of iTBS+tsDCS or no stimulation; subsequently, all animals received 6 weeks of daily rehabilitation on the horizontal ladder task. Lesion size was not different in the two animal groups. Rehabilitation alone improved performance by a 22% reduction in skilled locomotion error rate, whereas stimulation+rehabilitation was markedly more effective (52%), and restored error rate to pre-injury levels. Stimulation+rehabilitation significantly increased CST axon length caudal to the injury and the amount of ventral horn label was positively correlated with functional improvement. The stimulation+rehabilitation group had significantly less proprioceptive afferent terminal labelling in the intermediate zone and fewer synapses on motoneurons . Afferent fiber terminal labeling was negatively correlated with motor recovery. Thus, the dual neuromodulation protocol promotes adaptive plasticity in corticospinal and proprioceptive afferents networks after contusion SCI, leading to enhanced rehabilitation efficacy and recovery of skilled locomotion.

The neuroanatomical-functional paradox in spinal cord injury

Although lesion size is widely considered to be the most reliable predictor of outcome after CNS injury, lesions of comparable size can produce vastly different magnitudes of functional impairment and subsequent recovery. This neuroanatomical-functional paradox is likely to contribute to the many failed attempts to independently replicate findings from animal models of neurotrauma. In humans, the analogous clinical-radiological paradox could explain why individuals with similar injuries can respond differently to rehabilitation. We describe the neuroanatomical-functional paradox in the context of traumatic spinal cord injury (SCI) and discuss the underlying mechanisms of the paradox, including the concepts of lesion-affected and recovery-related networks. We also consider the various secondary complications that further limit the accuracy of outcome prediction in SCI and provide suggestions for how to increase the predictive, translational value of preclinical SCI models.

Behavioral testing in animal models of spinal cord injury

This review is based on a lecture presented at the Craig H. Neilsen Foundation sponsored Spinal Cord Injury Training Program at Ohio State University. We discuss the advantages and challenges of injury models in rodents and theory relation to various behavioral outcome measures. We offer strategies and advice on experimental design, behavioral testing, and on the challenges, one will encounter with animal testing. This review is designed to guide those entering the field of spinal cord injury and/or involved with in vivo animal testing.

Reaching and Grasping Training Improves Functional Recovery After Chronic Cervical Spinal Cord Injury

Previous studies suggest locomotion training could be an effective non-invasive therapy after spinal cord injury (SCI) using primarily acute thoracic injuries. However, the majority of SCI patients have chronic cervical injuries. Regaining hand function could significantly increase their quality of life. In this study, we used a clinically relevant chronic cervical contusion to study the therapeutic efficacy of rehabilitation in forelimb functional recovery. Nude rats received a moderate C5 unilateral contusive injury and were then divided into two groups with or without Modified Montoya Staircase (MMS) rehabilitation. For the rehabilitation group, rats were trained 5 days a week starting at 8 weeks post-injury (PI) for 6 weeks. All rats were assessed for skilled forelimb functions with MMS test weekly and for untrained gross forelimb locomotion with grooming and horizontal ladder (HL) tests biweekly. Our results showed that MMS rehabilitation significantly increased the number of pellets taken at 13 and 14 weeks PI and the accuracy rates at 12 to 14 weeks PI. However, there were no significant differences in the grooming scores or the percentage of HL missteps at any time point. Histological analyses revealed that MMS rehabilitation significantly increased the number of serotonergic fibers and the amount of presynaptic terminals around motor neurons in the cervical ventral horns caudal to the injury and reduced glial fibrillary acidic protein (GFAP)-immunoreactive astrogliosis in spinal cords caudal to the lesion. This study shows that MMS rehabilitation can modify the injury environment, promote axonal sprouting and synaptic plasticity, and importantly, improve reaching and grasping functions in the forelimb, supporting the therapeutic potential of task-specific rehabilitation for functional recovery after chronic SCI.

Rehabilitative Training in Animal Models of Spinal Cord Injury

Rehabilitative motor training is currently one of the most widely used approaches to promote moderate recovery following injuries of the central nervous system. Such training is generally applied in the clinical setting, whereas it is not standard in preclinical research. This is a concern as it is becoming increasingly apparent that neuroplasticity enhancing treatments require training or some form of activity as a co-therapy to promote functional recovery. Despite the importance of training and the many open questions regarding its mechanistic consequences, its use in preclinical animal models is rather limited. Here we review approaches, findings and challenges when training is applied in animal models of spinal cord injury, and we suggest recommendations to facilitate the integration of training using an appropriate study design, into pre-clinical studies.

Comprehensive therapeutics targeting the corticospinal tract following spinal cord injury

Spinal cord injury (SCI), which is much in the public eye, is still a refractory disease compromising the well-being of both patients and society. In spite of there being many methods dealing with the lesion, there is still a deficiency in comprehensive strategies covering all facets of this damage. Further, we should also mention the structure called the corticospinal tract (CST) which plays a crucial role in the motor responses of organisms, and it will be the focal point of our attention. In this review, we discuss a variety of strategies targeting different dimensions following SCI and some treatments that are especially efficacious to the CST are emphasized. Over recent decades, researchers have developed many effective tactics involving five approaches: (1) tackle more extensive regions; (2) provide a regenerative microenvironment; (3) provide a glial microenvironment; (4) transplantation; and (5) other auxiliary methods, for instance, rehabilitation training and electrical stimulation. We review the basic knowledge on this disease and correlative treatments. In addition, some well-formulated perspectives and hypotheses have been delineated. We emphasize that such a multifaceted problem needs combinatorial approaches, and we analyze some discrepancies in past studies. Finally, for the future, we present numerous brand-new latent tactics which have great promise for curbing SCI.

The effect of task symmetry on bimanual reach-to-grasp movements after cervical spinal cord injury

Injury to the cervical spinal cord results in deficits in bimanual control, reducing functional independence and quality of life. Despite this, little research has investigated the control strategies which underpin bimanual arm/hand movements following cervical spinal cord injury (cSCI). Using kinematics and surface electromyography this study explored how task symmetry affects bimanual control, in patients with an acute cSCI (< 6 m post injury), as they performed naturalistic bimanual reach-to-grasp actions (to objects at 50% and 70% of their maximal reach distance), and how this differs compared to uninjured age-matched controls. Twelve adults with a cSCI (mean age 69.25 years), with lesions at C3–C8, categorized by the American Spinal Injury Impairment Scale (AIS) at C or D and 12 uninjured age-matched controls (AMC) (mean age 69.29 years) were recruited. Participants with a cSCI produced reach-to-grasp actions which took longer, were slower, less smooth and had longer deceleration phases than AMC (p < 0.05). Participants with a cSCI were less synchronous than AMC at peak velocity and just prior to object pick up (p < 0.05), but both groups ended the movement in a synchronous fashion. Peak muscle activity occurred just prior to object pick up for both groups. While there seems to be a greater reliance on the deceleration phase of the movement, we observed minimal disruption of the more impaired limb on the less impaired limb and no additional effects of task symmetry on bimanual control. Further research is needed to determine how to take advantage of this retained bimanual control in therapy.

Longitudinal electrophysiological changes after cervical hemi-contusion spinal cord injury in rats

OBJECTIVE

To evaluate the longitudinal somatosensory evoked potentials (SEPs) and motor evoked potentials (MEPs) characterization from acute to chronic injury following cervical hemi-contusion spinal cord injury (SCI) in rats, and correlate the MEPs & SEPs to the behavioral outcomes.

METHODS

Fifteen adult male Sprague-Dawley rats were subjected to the hemi-contusion spinal cord injury at C5. Forelimb MEPs & SEPs were applied to 5 animals before injury and 3h, 1d, 3d, 1w, 2w, 4w and 8w after injury respectively. Forelimb functional assessments, including Montoya staircase task and cylinder rearing test, were conducted on another 10 animals before injury and at 2w, 4w and 8w after injury respectively, as well as histological analysis of the cord at 8w after injury. A group correlation was performed between the MEPs & SEPs and behavioral outcomes.

RESULTS

The hemi-contusion injury resulted in unilateral tissue damage at the epicenter with loss of the ventral horns and lateral funiculus. Both ipsilateral and contralateral forelimb MEPs showed latency prolongation and amplitude reduction at 3h after injury. The MEPs amplitude increased with time after injury, but the ipsilateral amplitude was persistently lower than the contralateral amplitude. The ipsilateral MEPs latency increased with time after injury and was significantly longer than the contralateral MEPs latency. The ipsilateral SEPs amplitude dropped after injury and stayed at a lower level up to 8 weeks. There was no difference in the SEPs latency among time points and between sides. At 8 weeks after injury, the ipsilateral forelimb grasped 30% pellets while the contralateral forelimb close to 81%. An obvious decreased usage of the ipsilateral paw and increased usage of the contralateral paw were observed in rearing test after injury. The MEPs latency and amplitude correlated significantly with the forelimb motor function.

CONCLUSION

Cervical hemi-contusion SCI led to persistent changes in MEPs & SEPs of the ipsilateral forelimb, ipsilateral impairment in motor function and unilateral cord tissue damage. Reliable electrophysiology assessment was obtained in chronic phase due to unstable MEPs & SEPs of bilateral forelimb immediately after injury, which might reflect the underlying pathological processes. The present study further confirmed the link of the MEPs to the behavioral outcomes, supporting the longitudinal electrophysiology assessment for neurological impairment after SCI.

Optogenetic Interrogation of Functional Synapse Formation by Corticospinal Tract Axons in the Injured Spinal Cord

To restore function after injury to the CNS, axons must be stimulated to extend into denervated territory and, critically, must form functional synapses with appropriate targets. We showed previously that forced overexpression of the transcription factor Sox11 increases axon growth by corticospinal tract (CST) neurons after spinal injury. However, behavioral outcomes were not improved, raising the question of whether the newly sprouted axons are able to form functional synapses. Here we developed an optogenetic strategy, paired with single-unit extracellular recordings, to assess the ability of Sox11-stimulated CST axons to functionally integrate in the circuitry of the cervical spinal cord. Initial time course experiments established the expression and function of virally expressed Channelrhodopsin (ChR2) in CST cell bodies and in axon terminals in cervical spinal cord. Pyramidotomies were performed in adult mice to deprive the left side of the spinal cord of CST input, and the right CST was treated with adeno-associated virus (AAV)-Sox11 or AAV-EBFP control, along with AAV-ChR2. As expected, Sox11 treatment caused robust midline crossing of CST axons into previously denervated left spinal cord. Clear postsynaptic responses resulted from optogenetic activation of CST terminals, demonstrating the ability of Sox11-stimulated axons to form functional synapses. Mapping of the distribution of CST-evoked spinal activity revealed overall similarity between intact and newly innervated spinal tissue. These data demonstrate the formation of functional synapses by Sox11-stimulated CST axons without significant behavioral benefit, suggesting that new synapses may be mistargeted or otherwise impaired in the ability to coordinate functional output.

SIGNIFICANCE STATEMENT

As continued progress is made in promoting the regeneration of CNS axons, questions of synaptic integration are increasingly prominent. Demonstrating direct synaptic integration by regenerated axons and distinguishing its function from indirect relay circuits and target field plasticity have presented technical challenges. Here we force the overexpression of Sox11 to stimulate the growth of corticospinal tract axons in the cervical spinal cord and then use specific optogenetic activation to assess their ability to directly drive postsynaptic activity in spinal cord neurons. By confirming successful synaptic integration, these data illustrate a novel optogenetic-based strategy to monitor and optimize functional reconnection by newly sprouted axons in the injured CNS.

Bimanual reach to grasp movements after cervical spinal cord injury

Injury to the cervical spinal cord results in bilateral deficits in arm/hand function reducing functional independence and quality of life. To date little research has been undertaken to investigate control strategies of arm/hand movements following cervical spinal cord injury (cSCI). This study aimed to investigate unimanual and bimanual coordination in patients with acute cSCI using 3D kinematic analysis as they performed naturalistic reach to grasp actions with one hand, or with both hands together (symmetrical task), and compare this to the movement patterns of uninjured younger and older adults. Eighteen adults with a cSCI (mean 61.61 years) with lesions at C4-C8, with an American Spinal Injury Association (ASIA) grade B to D and 16 uninjured younger adults (mean 23.68 years) and sixteen uninjured older adults (mean 70.92 years) were recruited. Participants with a cSCI produced reach-to-grasp actions which took longer, were slower, and had longer deceleration phases than uninjured participants. These differences were exacerbated during bimanual reach-to-grasp tasks. Maximal grasp aperture was no different between groups, but reached earlier by people with cSCI. Participants with a cSCI were less synchronous than younger and older adults but all groups used the deceleration phase for error correction to end the movement in a synchronous fashion. Overall, this study suggests that after cSCI a level of bimanual coordination is retained. While there seems to be a greater reliance on feedback to produce both the reach to grasp, we observed minimal disruption of the more impaired limb on the less impaired limb. This suggests that bimanual movements should be integrated into therapy.

A Single Dose of Docosahexaenoic Acid Increases the Functional Recovery Promoted by Rehabilitation after Cervical Spinal Cord Injury in the Rat

Task-specific rehabilitation has been shown to promote functional recovery after acute spinal cord injury (SCI). Recently, the omega-3 polyunsaturated fatty acid, docosahexaenoic acid (DHA), has been shown to promote neuroplasticity after SCI. Here, we investigated whether the combination of a single bolus of DHA with rehabilitation can enhance the effect of DHA or rehabilitation therapy in adult injured spinal cord. We found enhanced functional improvement with DHA in combination with rehabilitation compared with either treatment alone in a rat cervical lateral hemisection SCI model. This behavioral improvement correlated with a significant sprouting of uninjured corticospinal and serotonergic fibers. We also observed that the greatest increase in the synaptic vesicle protein, synaptophysin, and the synaptic active zone protein, Bassoon, occurred in animals that received both DHA and rehabilitation. In summary, the functional, anatomical, and synaptic plasticity induced by task-specific rehabilitation can be further enhanced by DHA treatment. This study shows the potential beneficial effects of DHA combined with rehabilitation for the treatment of patients with SCI.

The role of cAMP and its downstream targets in neurite growth in the adult nervous system

Injured neurons in the adult mammalian central nervous system (CNS) have a very limited capacity for axonal regeneration and neurite outgrowth. This inability to grow new axons or to regrow injured axons is due to the presence of molecules that inhibit axonal growth, and age related changes in the neuron's innate growth capabilities. Available levels of cAMP are thought to have an important role in linking both of these factors. Elevated levels of cAMP in the developing nervous system are important for the guidance and stability of growth cones. As the nervous system matures, cAMP levels decline and the growth promoting effects of cAMP diminish. It has frequently been demonstrated that increasing neuronal cAMP can enhance neurite growth and regeneration. Some methods used to increase cAMP include administration of cAMP agonists, conditioning lesions, or electrical stimulation. Furthermore, it has been proposed that multiple stages of cAMP induced growth exist, one directly caused by its downstream effector Protein Kinase A (PKA) and one caused by the eventual upregulation of gene transcription. Although the role cAMP in promoting axon growth is well accepted, the downstream pathways that mediate cAMP-mediated axonal growth are less clear. This is partly because various key studies that explored the link between PKA and axonal outgrowth relied on the PKA inhibitors KT5720 and H89. More recent studies have shown that both of these drugs are less specific than initially thought and can inhibit a number of other signalling molecules including the Exchange Protein Activated by cAMP (EPAC). Consequently, it has recently been shown that a number of intracellular signalling pathways previously attributed to PKA can now be attributed solely to activation of EPAC specific pathways, or the simultaneous co-activation of PKA and EPAC specific pathways. These new studies open the door to new potential treatments for repairing the injured spinal cord.

Re-Establishment of Cortical Motor Output Maps and Spontaneous Functional Recovery via Spared Dorsolaterally Projecting Corticospinal Neurons after Dorsal Column Spinal Cord Injury in Adult Mice

Motor cortical plasticity contributes to spontaneous recovery after incomplete spinal cord injury (SCI), but the pathways underlying this remain poorly understood. We performed optogenetic mapping of motor cortex in channelrhodopsin-2 expressing mice to assess the capacity of the cortex to re-establish motor output longitudinally after a C3/C4 dorsal column SCI that bilaterally ablated the dorsal corticospinal tract (CST) containing ∼96% of corticospinal fibers but spared ∼3% of CST fibers that project via the dorsolateral funiculus. Optogenetic mapping revealed extensive early deficits, but eventual reestablishment of motor cortical output maps to the limbs at the same latency as preoperatively by 4 weeks after injury. Analysis of skilled locomotion on the horizontal ladder revealed early deficits followed by partial spontaneous recovery by 6 weeks after injury. To dissociate between the contributions of injured dorsal projecting versus spared dorsolateral projecting corticospinal neurons, we established a transient silencing approach to inactivate spared dorsolaterally projecting corticospinal neurons specifically by injecting adeno-associated virus (AAV)-expressing Cre-dependent DREADD (designer receptor exclusively activated by designer drug) receptor hM4Di in sensorimotor cortex and AAV-expressing Cre in C7/C8 dorsolateral funiculus. Transient silencing uninjured dorsolaterally projecting corticospinal neurons via activation of the inhibitory DREADD receptor hM4Di abrogated spontaneous recovery and resulted in a greater change in skilled locomotion than in control uninjured mice using the same silencing approach. These data demonstrate the pivotal role of a minor dorsolateral corticospinal pathway in mediating spontaneous recovery after SCI and support a focus on spared corticospinal neurons as a target for therapy.

SIGNIFICANCE STATEMENT

Spontaneous recovery can occur after incomplete spinal cord injury (SCI), but the pathways underlying this remain poorly understood. We performed optogenetic mapping of motor cortex after a cervical SCI that interrupts most corticospinal transmission but results in partial recovery on a horizontal ladder task of sensorimotor function. We demonstrate that the motor cortex can reestablish output to the limbs longitudinally. To dissociate the roles of injured and uninjured corticospinal neurons in mediating recovery, we transiently silenced the minor dorsolateral corticospinal pathway spared by our injury. This abrogated spontaneous recovery and resulted in a greater change in skilled locomotion than in uninjured mice using the same approach. Therefore, uninjured corticospinal neurons substantiate remarkable motor cortical plasticity and partial recovery after SCI.

Quantitative analysis of hindlimbs locomotion kinematics in spinalized rats treated with Tamoxifen plus treadmill exercise

Locomotion recovery after a spinal cord injury (SCI) includes axon regeneration, myelin preservation and increased plasticity in propriospinal and descending spinal circuitries. The combined effects of tamoxifen and exercise after a SCI were analyzed in this study to determine whether the combination of both treatments induces the best outcome in locomotion recovery. In this study, the penetrating injury was provoked by a sharp projectile that penetrates through right dorsal and ventral portions of the T13-L1 spinal segments, affecting propriospinal and descending/ascending tracts. Intraperitoneal application of Tamoxifen and a treadmill exercise protocol, as rehabilitation therapies, separately or combined, were used. To evaluate the functional recovery, angular patterns of the hip, knee and ankle joints as well as the leg pendulum-like movement (PLM) were measured during the unrestricted gait of treated and untreated (UT) animals, previously and after the traumatic injury (15 and 30days post-injury (dpi)). A pattern (curve) comparison analysis was made by using a locally designed Matlab script that determines the Frechet dissimilarity. The SCI magnitude was assessed by qualitative and quantitative histological analysis of the injury site 30days after SCI. Our results showed that all treated groups had an improvement in hindlimbs kinematics compared to the UT group, which showed a poor gait locomotion recovery throughout the rehabilitation period. The group with the combined treatment (tamoxifen+exercise (TE)) presented the best outcome. In conclusion, tamoxifen and treadmill exercise treatments are complementary therapies for the functional recovery of gait locomotion in hemi-spinalized rats.

Motivational state, reward value, and Pavlovian cues differentially affect skilled forelimb grasping in rats

Motor skills represent high-precision movements performed at optimal speed and accuracy. Such motor skills are learned with practice over time. Besides practice, effects of motivation have also been shown to influence speed and accuracy of movements, suggesting that fast movements are performed to maximize gained reward over time as noted in previous studies. In rodents, skilled motor performance has been successfully modeled with the skilled grasping task, in which animals use their forepaw to grasp for sugar pellet rewards through a narrow window. Using sugar pellets, the skilled grasping task is inherently tied to motivation processes. In the present study, we performed three experiments modulating animals’ motivation during skilled grasping by changing the motivational state, presenting different reward value ratios, and displaying Pavlovian stimuli. We found in all three studies that motivation affected the speed of skilled grasping movements, with the strongest effects seen due to motivational state and reward value. Furthermore, accuracy of the movement, measured in success rate, showed a strong dependence on motivational state as well. Pavlovian cues had only minor effects on skilled grasping, but results indicate an inverse Pavlovian-instrumental transfer effect on movement speed. These findings have broad implications considering the increasing use of skilled grasping in studies of motor system structure, function, and recovery after injuries.

Reorganization of Intact Descending Motor Circuits to Replace Lost Connections After Injury

Neurons have a limited capacity to regenerate in the adult central nervous system (CNS). The inability of damaged axons to re-establish original circuits results in permanent functional impairment after spinal cord injury (SCI). Despite abortive regeneration of axotomized CNS neurons, limited spontaneous recovery of motor function emerges after partial SCI in humans and experimental rodent models of SCI. It is hypothesized that this spontaneous functional recovery is the result of the reorganization of descending motor pathways spared by the injury, suggesting that plasticity of intact circuits is a potent alternative conduit to enhance functional recovery after SCI. In support of this hypothesis, several studies have shown that after unilateral corticospinal tract (CST) lesion (unilateral pyramidotomy), the intact CST functionally sprouts into the denervated side of the spinal cord. Furthermore, pharmacologic and genetic methods that enhance the intrinsic growth capacity of adult neurons or block extracellular growth inhibitors are effective at significantly enhancing intact CST reorganization and recovery of motor function. Owing to its importance in controlling fine motor behavior in primates, the CST is the most widely studied descending motor pathway; however, additional studies in rodents have shown that plasticity within other spared descending motor pathways, including the rubrospinal tract, raphespinal tract, and reticulospinal tract, can also result in restoration of function after incomplete SCI. Identifying the molecular mechanisms that drive plasticity within intact circuits is crucial in developing novel, potent, and specific therapeutics to restore function after SCI. In this review we discuss the evidence supporting a focus on exploring the capacity of intact motor circuits to functionally repair the damaged CNS after SCI.

Spinal cord reconstitution with homologous neural grafts enables robust corticospinal regeneration

The corticospinal tract (CST) is the most important motor system in humans, yet robust regeneration of this projection after spinal cord injury (SCI) has not been accomplished. In murine models of SCI, we report robust corticospinal axon regeneration, functional synapse formation and improved skilled forelimb function after grafting multipotent neural progenitor cells into sites of SCI. Corticospinal regeneration requires grafts to be driven toward caudalized (spinal cord), rather than rostralized, fates. Fully mature caudalized neural grafts also support corticospinal regeneration. Moreover, corticospinal axons can emerge from neural grafts and regenerate beyond the lesion, a process that is potentially related to the attenuation of the glial scar. Rat corticospinal axons also regenerate into human donor grafts of caudal spinal cord identity. Collectively, these findings indicate that spinal cord 'replacement' with homologous neural stem cells enables robust regeneration of the corticospinal projection within and beyond spinal cord lesion sites, achieving a major unmet goal of SCI research and offering new possibilities for clinical translation.

Therapy induces widespread reorganization of motor cortex after complete spinal transection that supports motor recovery

Reorganization of the somatosensory system and its relationship to functional recovery after spinal cord injury (SCI) has been well studied. However, little is known about the impact of SCI on organization of the motor system. Recent studies suggest that step-training paradigms in combination with spinal stimulation, either electrically or through pharmacology, are more effective than step training alone at inducing recovery and that reorganization of descending corticospinal circuits is necessary. However, simpler, passive exercise combined with pharmacotherapy has also shown functional improvement after SCI and reorganization of, at least, the sensory cortex. In this study we assessed the effect of passive exercise and serotonergic (5-HT) pharmacological therapies on behavioral recovery and organization of the motor cortex. We compared the effects of passive hindlimb bike exercise to bike exercise combined with daily injections of 5-HT agonists in a rat model of complete mid-thoracic transection. 5-HT pharmacotherapy combined with bike exercise allowed the animals to achieve unassisted weight support in the open field. This combination of therapies also produced extensive expansion of the axial trunk motor cortex into the deafferented hindlimb motor cortex and, surprisingly, reorganization within the caudal and even the rostral forelimb motor cortex areas. The extent of the axial trunk expansion was correlated to improvement in behavioral recovery of hindlimbs during open field locomotion, including weight support. From a translational perspective, these data suggest a rationale for developing and optimizing cost-effective, non-invasive, pharmacological and passive exercise regimes to promote plasticity that supports restoration of movement after spinal cord injury.

Single pellet grasping following cervical spinal cord injury in adult rat using an automated full-time training robot

Task specific motor training is a common form of rehabilitation therapy in individuals with spinal cord injury (SCI). The single pellet grasping (SPG) task is a skilled forelimb motor task used to evaluate recovery of forelimb function in rodent models of SCI. The task requires animals to obtain food pellets located on a shelf beyond a slit at the front of an enclosure. Manually training and testing rats in the SPG task requires extensive time and often yields results with high outcome variability and small therapeutic windows (i.e., the difference between pre- and post-SCI success rates). Recent advances in automated SPG training using automated pellet presentation (APP) systems allow rats to train ad libitum 24h a day, 7 days a week. APP trained rats have improved success rates, require less researcher time, and have lower outcome variability compared to manually trained rats. However, it is unclear whether APP trained rats can perform the SPG task using the APP system after SCI. Here we show that rats with cervical SCI can successfully perform the SPG task using the APP system. We found that SCI rats with APP training performed significantly more attempts, had slightly lower and less variable final score success rates, and larger therapeutic windows than SCI rats with manual training. These results demonstrate that APP training has clear advantages over manual training for evaluating reaching performance of SCI rats and represents a new tool for investigating rehabilitative motor training following CNS injury.

HAL® exoskeleton training improves walking parameters and normalizes cortical excitability in primary somatosensory cortex in spinal cord injury patients

BACKGROUND

Reorganization in the sensorimotor cortex accompanied by increased excitability and enlarged body representations is a consequence of spinal cord injury (SCI). Robotic-assisted bodyweight supported treadmill training (BWSTT) was hypothesized to induce reorganization and improve walking function.

OBJECTIVE

To assess whether BWSTT with hybrid assistive limb® (HAL®) exoskeleton affects cortical excitability in the primary somatosensory cortex (S1) in SCI patients, as measured by paired-pulse somatosensory evoked potentials (ppSEP) stimulated above the level of injury.

METHODS

Eleven SCI patients took part in HAL® assisted BWSTT for 3 months. PpSEP were conducted before and after this training period, where the amplitude ratios (SEP amplitude following double pulses - SEP amplitude following single pulses) were assessed and compared to eleven healthy control subjects. To assess improvement in walking function, we used the 10-m walk test, timed-up-and-go test, the 6-min walk test, and the lower extremity motor score.

RESULTS

PpSEPs were significantly increased in SCI patients as compared to controls at baseline. Following training, ppSEPs were increased from baseline and no longer significantly differed from controls. Walking parameters also showed significant improvements, yet there was no significant correlation between ppSEP measures and walking parameters.

CONCLUSIONS

The findings suggest that robotic-assisted BWSTT with HAL® in SCI patients is capable of inducing cortical plasticity following highly repetitive, active locomotive use of paretic legs. While there was no significant correlation of excitability with walking parameters, brain areas other than S1 might reflect improvement of walking functions. EEG and neuroimaging studies may provide further information about supraspinal plastic processes and foci in SCI rehabilitation.

Improved single pellet grasping using automated ad libitum full-time training robot

The single pellet grasping (SPG) task is a skilled forelimb motor task commonly used to evaluate reaching and grasp kinematics and recovery of forelimb function in rodent models of CNS injuries and diseases. To train rats in the SPG task, the animals are usually food restricted then placed in an SPG task enclosure and presented food pellets on a platform located beyond a slit located at the front of the task enclosure for 10-30 min, normally every weekday for several weeks. When the SPG task is applied in studies involving various experimental groups, training quickly becomes labor intensive, and can yield results with significant day-to-day variability. Furthermore, training is frequently done during the animals' light-cycle, which for nocturnal rodents such as mice and rats could affect performance. Here we describe an automated pellet presentation (APP) robotic system to train and test rats in the SPG task that reduces some of the procedural weaknesses of manual training. We found that APP trained rats performed significantly more trials per 24 h period, and had higher success rates with less daily and weekly variability than manually trained rats. Moreover, the results show that success rates are positively correlated with the number of dark-cycle trials, suggesting that dark-cycle training has a positive effect on success rates. These results demonstrate that automated training is an effective method for evaluating and training skilled reaching performance of rats, opening up the possibility for new approaches to investigating the role of motor systems in enabling skilled forelimb use and new approaches to investigating rehabilitation following CNS injury.

Three-dimensional motion analysis of arm-reaching movements in healthy and hemispinalized common marmosets

Spinal cord injury (SCI) is a devastating neurological injury. At present, pharmacological, regenerative, and rehabilitative approaches are widely studied as therapeutic interventions for motor recovery after SCI. Preclinical research has been performed on model animals with experimental SCI, and those studies often evaluate hand and arm motor function using various indices, such as the success rate of the single pellet reaching test and the grip force. However, compensatory movement strategies, involuntary muscle contraction, and the subject's motivation could affect the scores, resulting in failure to assess direct recovery from impairment. Identifying appropriate assessments of motor impairment is thus important for understanding the mechanisms of motor recovery. In this study, we developed a motion capture system capable of reconstructing three-dimensional hand positions with millimeter and millisecond accuracy and evaluated hand kinematics during food retrieval movement in both healthy and hemispinalized common marmosets. As a result, the endpoint jerk, representing the accuracy of hand motor control, was asserted to be an appropriate index of upper limb motor impairment by eliminating the influence of the subject's motivation, involuntary muscle contraction, and compensatory strategies. The result also suggested that the kinematics of the limb more consistently reflects motor restoration from deficit due to spinal cord injury than the performance in the single pellet reaching test. Because of recent attention devoted to the common marmoset as a nonhuman primate model for human diseases, the present study, which clarified arm-reaching movements in spinalized marmosets, provides fundamental knowledge for future therapeutic studies.

The reactivation of somatosensory cortex and behavioral recovery after sensory loss in mature primates

In our experiments, we removed a major source of activation of somatosensory cortex in mature monkeys by unilaterally sectioning the sensory afferents in the dorsal columns of the spinal cord at a high cervical level. At this level, the ascending branches of tactile afferents from the hand are cut, while other branches of these afferents remain intact to terminate on neurons in the dorsal horn of the spinal cord. Immediately after such a lesion, the monkeys seem relatively unimpaired in locomotion and often use the forelimb, but further inspection reveals that they prefer to use the unaffected hand in reaching for food. In addition, systematic testing indicates that they make more errors in retrieving pieces of food, and start using visual inspection of the rotated hand to confirm the success of the grasping of the food. Such difficulties are not surprising as a complete dorsal column lesion totally deactivates the contralateral hand representation in primary somatosensory cortex (area 3b). However, hand use rapidly improves over the first post-lesion weeks, and much of the hand representational territory in contralateral area 3b is reactivated by inputs from the hand in roughly a normal somatotopic pattern. Quantitative measures of single neuron response properties reveal that reactivated neurons respond to tactile stimulation on the hand with high firing rates and only slightly longer latencies. We conclude that preserved dorsal column afferents after nearly complete lesions contribute to the reactivation of cortex and the recovery of the behavior, but second-order sensory pathways in the spinal cord may also play an important role. Our microelectrode recordings indicate that these preserved first-order, and second-order pathways are initially weak and largely ineffective in activating cortex, but they are potentiated during the recovery process. Therapies that would promote this potentiation could usefully enhance recovery after spinal cord injury.

High-Impact, Self-Motivated Training Within an Enriched Environment With Single Animal Tracking Dose-Dependently Promotes Motor Skill Acquisition and Functional Recovery

Functional recovery following central nervous system injuries is strongly influenced by rehabilitative training. In the clinical setting, the intensity of training and the level of motivation for a particular task are known to play important roles. With increasing neuroscience studies investigating the effects of training and rehabilitation, it is important to understand how the amount and type of training of individuals influences outcome. However, little is known about the influence of spontaneous "self-training" during daily life as it is often uncontrolled, not recorded, and mostly disregarded. Here, we investigated the effects of the intensity of self-training on motor skill acquisition in normal, intact rats and on the recovery of functional motor behavior following spinal cord injury in adult rats. We used a custom-designed small animal tracking system, "RatTrack," to continuously record the activity of multiple rats, simultaneously in a complex Natural Habitat-enriched environment. Naïve, adult rats performed high-intensity, self-motivated motor training, which resulted in them out-performing rats that were conventionally housed and trained on skilled movement tasks, for example, skilled prehension (grasping) and ladder walking. Following spinal cord injury the amount of self-training was correlated with improved functional recovery. These data suggest that high-impact, self-motivated training leads to superior skill acquisition and functional recovery than conventional training paradigms. These findings have important implications for the design of animal studies investigating rehabilitation and for the planning of human rehabilitation programs.

Restoration of sensorimotor functions after spinal cord injury

The purpose of this review is to discuss the achievements and perspectives regarding rehabilitation of sensorimotor functions after spinal cord injury. In the first part we discuss clinical approaches based on neuroplasticity, a term referring to all adaptive and maladaptive changes within the sensorimotor systems triggered by a spinal cord injury. Neuroplasticity can be facilitated through the training of movements with assistance as needed, and/or by electrical stimulation techniques. The success of such training in individuals with incomplete spinal cord injury critically depends on the presence of physiological proprioceptive input to the spinal cord leading to meaningful muscle activations during movement performances. The addition of rehabilitation technology, such as robotic devices allows for longer training times and provision of feedback information regarding changes in movement performance. Nevertheless, the improvement of function by such approaches for rehabilitation is limited. In the second part, we discuss preclinical approaches to restore function by compensating for the loss of descending input to spinal networks following complete spinal cord injury. This can be achieved with stimulation of spinal networks or approaches to restore their descending input. Electrical and pharmacological stimulation of spinal neural networks is still in an experimental stage; and despite promising repair studies in animal models, translations to humans up to now have not been convincing. It is likely that combinations of techniques targeting the promotion of axonal regeneration and meaningful plasticity are necessary to advance the restoration of function. In the future, refinement of animal studies may contribute to greater translational success.

Reticulospinal plasticity after cervical spinal cord injury in the rat involves withdrawal of projections below the injury

Restoring voluntary fine motor control of the arm and hand is one of the main goals following cervical spinal cord injury (SCI). Although the functional improvement achievable with rehabilitative training in rat models is frequently accompanied by corticospinal tract (CST) plasticity, CST rewiring alone seems insufficient to account for the observed recovery. Recent investigations in animal models of SCI have suggested that the reticulospinal tract (RtST) might contribute to mediating improved motor performance of the forelimb. Here we investigate whether the spared RtST can compensate for the loss of CST input and whether RtST projections rearrange in response to cervical SCI. Animals underwent unilateral ablation of the dorsal CST and rubrospinal tract at spinal level C4, while the ventral RtST projections were spared. At the end of the six-week recovery period, injured animals had made significant improvements in single pellet reaching. This was not accompanied by increased sprouting of the injured CST above the injury compared to uninjured control animals. Injury-induced changes in RtST fiber density within the gray matter, as well as in the number of RtST collaterals entering the gray matter or crossing the cord midline were minor above the injury. However, all analyses directly below the injured spinal level consistently point to a significant decrease of RtST projections. The mechanism and the functional relevance behind this new finding warrant further study. Our results also suggest that mechanisms other than anatomical plasticity, such as plastic changes on a cellular level, might be responsible for the observed spontaneous recovery.

Bilateral movement training promotes axonal remodeling of the corticospinal tract and recovery of motor function following traumatic brain injury in mice

Traumatic brain injury (TBI) results in severe motor function impairment, and subsequent recovery is often incomplete. Rehabilitative training is considered to promote restoration of the injured neural network, thus facilitating functional recovery. However, no studies have assessed the effect of such trainings in the context of neural rewiring. Here, we investigated the effects of two types of rehabilitative training on corticospinal tract (CST) plasticity and motor recovery in mice. We injured the unilateral motor cortex with contusion, which induced hemiparesis on the contralesional side. After the injury, mice performed either a single pellet-reaching task (simple repetitive training) or a rotarod task (bilateral movement training). Multiple behavioral tests were then used to assess forelimb motor function recovery: staircase, ladder walk, capellini handling, single pellet, and rotarod tests. The TBI+rotarod group performed most forelimb motor tasks (staircase, ladder walk, and capellini handling tests) better than the TBI-only group did. In contrast, the TBI+reaching group did not perform better except in the single pellet test. After the injury, the contralateral CST, labeled by biotinylated dextran amine, formed sprouting fibers into the denervated side of the cervical spinal cord. The number of these fibers was significantly higher in the TBI+rotarod group, whereas it did not increase in the TBI+reaching group. These results indicate that bilateral movement training effectively promotes axonal rewiring and motor function recovery, whereas the effect of simple repetitive training is limited.

Synergistic effects of BDNF and rehabilitative training on recovery after cervical spinal cord injury

Promoting the rewiring of lesioned motor tracts following a spinal cord injury is a promising strategy to restore motor function. For instance, axonal collaterals may connect to spared, lesion-bridging neurons, thereby establishing a detour for descending signals and thus promoting functional recovery. In our rat model of cervical spinal cord injury, we attempted to promote targeted rewiring of the unilaterally injured corticospinal tract (CST) via the spared reticulospinal tract (RtST). To promote new connections between the two tracts in the brainstem, we administered viral vectors producing two neurotrophins. Brain-derived neurotrophic factor (BDNF), a known promotor of collateral growth, was expressed in the motor cortex, and neurotrophin 3 (NT-3), which has chemoattractive properties, was expressed in the reticular formation. Because rehabilitative training has proven to be beneficial in promoting functionally meaningful plasticity following injury, we added training in a skilled reaching task. Different neurotrophin or control treatments with or without training were evaluated. As hypothesized, improvements of motor performance with the injured forelimb following neurotrophin treatment alone were absent or modest compared to untreated controls. In contrast, we found a significant synergistic effect on performance when BDNF treatment was combined with training. The mechanism of this recovery remains unidentified, as histological analyses of CST and RtST collateral projections did not reveal differences among treatment groups. In conclusion, we demonstrate that following a cervical spinal lesion, rehabilitative training is necessary to translate effects of BDNF into functional recovery by mechanisms which are likely independent of collateral sprouting of the CST or RtST into the gray matter.

Corticospinal reorganization after spinal cord injury

The corticospinal tract (CST) is a major descending pathway contributing to the control of voluntary movement in mammals. During the last decades anatomical and electrophysiological studies have demonstrated significant reorganization in the CST after spinal cord injury (SCI) in animals and humans. In animal models of SCI, anatomical evidence showed corticospinal sprouts rostral and caudal to the lesion and their integration into intraspinal axonal circuits. Electrophysiological data suggested that indirect connections from the primary motor cortex to forelimb motoneurons, via brainstem nuclei and spinal cord interneurons, or direct connections from slow uninjured corticospinal axons, might contribute to the control of movement after a CST injury. In humans with SCI, post mortem spinal cord tissue revealed anatomical changes in the CST some of which were similar but others markedly different from those found in animal models of SCI. Human electrophysiological studies have provided ample evidence for corticospinal reorganization after SCI that may contribute to functional recovery. Together these studies have revealed a large plastic capacity of the CST after SCI. There is also a limited understanding of the relationship between anatomical and electrophysiological changes in the CST and control of movement after SCI. Increasing our knowledge of the role of CST plasticity in functional restoration after SCI may support the development of more effective repair strategies.

Technologies and combination therapies for enhancing movement training for people with a disability

There has been a dramatic increase over the last decade in research on technologies for enhancing movement training and exercise for people with a disability. This paper reviews some of the recent developments in this area, using examples from a National Science Foundation initiated study of mobility research projects in Europe to illustrate important themes and key directions for future research. This paper also reviews several recent studies aimed at combining movement training with plasticity or regeneration therapies, again drawing in part from European research examples. Such combination therapies will likely involve complex interactions with motor training that must be understood in order to achieve the goal of eliminating severe motor impairment.

Electrophysiological evaluation of sensory and motor pathways after incomplete unilateral spinal cord contusion

OBJECT

Unilateral contusions represent an increasingly popular model for studying the pathways and recovery mechanisms of spinal cord injury (SCI). Current studies rely heavily on motor behavior scoring and histological evidence to make assessments. Electrophysiology represents one way to reliably quantify the functionality of motor pathways. The authors sought to quantify the functional integrity of the bilateral motor and sensory pathways following unilateral SCI by using measurements of motor and somatosensory evoked potentials (MEPs and SSEPs, respectively).

METHODS

Eighteen rats were randomly divided into 3 groups receiving a mild unilateral contusion, a mild midline contusion, or a laminectomy only (control). Contusions were induced at T-8 using a MASCIS impactor. Electrophysiological analysis, motor behavior scoring, and histological quantifications were then performed to identify relationships among pathway conductivity, motor function, and tissue preservation.

RESULTS

Hindlimb MEPs ipsilateral to the injury showed recovery by Day 28 after injury and corresponded to approximately 61% of spared corticospinal tract (CST) tissue. In contrast, MEPs of the midline-injured group did not recover, and correspondingly > 90% of the CST tissue was damaged. Somatosensory evoked potentials showed only a moderate reduction in amplitude, with no difference in latency for the pathways ipsilateral to injury. Furthermore, these SSEPs were significantly better than those of the midline-injured rats for the same amount of white matter damage.

CONCLUSIONS

Motor evoked potential recovery corresponded to the amount of spared CST in unilateral and midline injuries, but motor behavior consistently recovered independent of MEPs. These data support the idea that spared contralateral pathways aid in reducing the functional deficits of injured ipsilateral pathways and further support the idea of CNS plasticity.

Kinematic study of locomotor recovery after spinal cord clip compression injury in rats

After spinal cord injury (SCI), precise assessment of motor recovery is essential to evaluate the outcome of new therapeutic approaches. Very little is known on the recovery of kinematic parameters after clinically-relevant severe compressive/contusive incomplete spinal cord lesions in experimental animal models. In the present study we evaluated the time-course of kinematic parameters during a 6-week period in rats walking on a treadmill after a severe thoracic clip compression SCI. The effect of daily treadmill training was also assessed. During the recovery period, a significant amount of spontaneous locomotor recovery occurred in 80% of the rats with a return of well-defined locomotor hindlimb pattern, regular plantar stepping, toe clearance and homologous hindlimb coupling. However, substantial residual abnormalities persisted up to 6 weeks after SCI including postural deficits, a bias of the hindlimb locomotor cycle toward the back of the animals with overextension at the swing/stance transition, loss of lateral balance and impairment of weight bearing. Although rats never recovered the antero-posterior (i.e. homolateral) coupling, different levels of decoupling between the fore and hindlimbs were measured. We also showed that treadmill training increased the swing duration variability during locomotion suggesting an activity-dependent compensatory mechanism of the motor control system. However, no effect of training was observed on the main locomotor parameters probably due to a ceiling effect of self-training in the cage. These findings constitute a kinematic baseline of locomotor recovery after clinically relevant SCI in rats and should be taken into account when evaluating various therapeutic strategies aimed at improving locomotor function.

Chondroitinase combined with rehabilitation promotes recovery of forelimb function in rats with chronic spinal cord injury

Chondroitinase ABC (ChABC) in combination with rehabilitation has been shown to promote functional recovery in acute spinal cord injury. For clinical use, the optimal treatment window is concurrent with the beginning of rehabilitation, usually 2-4 weeks after injury. We show that ChABC is effective when given 4 weeks after injury combined with rehabilitation. After C4 dorsal spinal cord injury, rats received no treatment for 4 weeks. They then received either ChABC or penicillinase control treatment followed by hour-long daily rehabilitation specific for skilled paw reaching. Animals that received both ChABC and task-specific rehabilitation showed the greatest recovery in skilled paw reaching, approaching similar levels to animals that were treated at the time of injury. There was also a modest increase in skilled paw reaching ability in animals receiving task-specific rehabilitation alone. Animals treated with ChABC and task-specific rehabilitation also showed improvement in ladder and beam walking. ChABC increased sprouting of the corticospinal tract, and these sprouts had more vGlut1(+ve) presynaptic boutons than controls. Animals that received rehabilitation showed an increase in perineuronal net number and staining intensity. Our results indicate that ChABC treatment opens a window of opportunity in chronic spinal cord lesions, allowing rehabilitation to improve functional recovery.

Rehabilitative training following unilateral pyramidotomy in adult rats improves forelimb function in a non-task-specific way

Spontaneous functional recovery following injury to the adult central nervous system can be enhanced with increased and focused activity, either through altered behaviour (skill learning, exercise or training) or by artificial stimulation (magnetic or electrical). In terms of training, the choice of paradigm plays a key role in the recovered behaviour. Here we show that task-specific training leads to improved forelimb function that can be translated to a novel forelimb task. Adult Long-Evans rats received a unilateral pyramidotomy and we studied the effects of different post-lesion training paradigms for their ability to recover function in the impaired limb. We trained rats on either the single pellet grasping or the horizontal ladder task. Rats were tested on both tasks regardless of the training paradigm and also on a related, but novel forelimb task, the Staircase. Horizontal ladder training led to full recovery of this task, and also limited recovery on the familiar but untrained single pellet grasping task. In comparison, single pellet grasping training led to a smaller improvement on the horizontal ladder, but interestingly the same degree of recovery on the single pellet grasping task as horizontal ladder trained animals. Both training groups performed equally well on a novel, untrained forelimb grasping task. These results show that task-specific forelimb training can lead to functional recovery also in non-trained, complex, forelimb movements. Anatomically, only single pellet grasping training was associated with enhanced sprouting of the intact corticospinal tract across the cervical spinal cord midline to innervate the denervated side of the spinal cord.

Recovery of hindlimb locomotion after incomplete spinal cord injury in the cat involves spontaneous compensatory changes within the spinal locomotor circuitry

After incomplete spinal cord injury (SCI), compensatory changes occur throughout the whole neuraxis, including the spinal cord below the lesion, as suggested by previous experiments using a dual SCI paradigm. Indeed, cats submitted to a lateral spinal hemisection at T10-T11 and trained on a treadmill for 3-14 wk re-expressed bilateral hindlimb locomotion as soon as 24 h after spinalization, a process that normally takes 2-3 wk when a complete spinalization is performed without a prior hemisection. In this study, we wanted to ascertain whether similar effects could occur spontaneously without training between the two SCIs and within a short period of 3 wk in 11 cats. One day after the complete spinalization, 9 of the 11 cats were able to re-express hindlimb locomotion either bilaterally (n = 6) or unilaterally on the side of the previous hemisection (n = 3). In these 9 cats, the hindlimb on the side of the previous hemisection (left hindlimb) performed better than the right side in contrast to that observed during the hemispinal period itself. Cats re-expressing the best bilateral hindlimb locomotion after spinalization had the largest initial hemilesion and the most prominent locomotor deficits after this first SCI. These results provide evidence that 1) marked reorganization of the spinal locomotor circuitry can occur without specific locomotor training and within a short period of 3 wk; 2) the spinal cord can reorganize in a more or less symmetrical way; and 3) the ability to walk after spinalization depends on the degree of deficits and adaptation observed in the hemispinal period.