Differential effects of low versus high amounts of weight supported treadmill training in spinally transected rats

Exercise Volume Can Modulate the Regenerative Response to Spinal Cord Injury in Mice

Traumatic spinal cord injury (SCI) causes debilitating motor and sensory deficits that impair functional performance, and physical rehabilitation is currently the only established therapeutic reality in the clinical setting. In this study, we aimed to assess the effect of exercise of different volume and timing of intervention on functional recovery and neuromuscular regeneration in a mouse model of compressive SCI. Mice were assigned to one of four groups: laminectomy only (SHAM); injured, without treadmill training (SCI); injured, treadmill trained for 10 min until day 56 postinjury (TMT1); and injured, treadmill trained for two 10-min cycles with a 10-min pause between them until day 28 postinjury followed by the TMT1 protocol until day 56 postinjury (TMT3). On day 7 postinjury, animals started an eight-week treadmill-training exercise protocol and were trained three times a week. TMT3 mice had the best results in terms of neuroregeneration, functional recovery, and muscle plasticity as measured by functional and morphometric parameters. In conclusion, the volume of exercise can modulate the quality of the regenerative response to injury, when started in the acute phase and adjusted according to the inflammatory window.

Influence of sports on cortical excitability in patients with spinal cord injury: a TMS study

Background

Patients with spinal cord injury (SCI) show abnormal cortical excitability that might be caused by deafferentation. We hypothesize a reduced short-interval intracortical inhibition preceding movement in patients with SCI compared with healthy participants. In addition, we expect that neuroplasticity induced by different types of sports can modulate intracortical inhibition during movement preparation in patients with SCI.

Methods

We used a reaction test and paired-pulse transcranial magnetic stimulation to record cortical excitability, assessed by measuring amplitudes of motor-evoked potentials in preparation of movement. The participants were grouped as patients with SCI practicing wheelchair dancing (n = 7), other sports (n = 6), no sports (n = 9), and healthy controls (n = 24).

Results

There were neither significant differences between healthy participants and the patients nor between the different patient groups. A non-significant trend (p = .238), showed that patients engaged in sports have a stronger increase in cortical excitability compared with patients of the non-sportive group, while the patients in the other sports group expressed the highest increase in cortical excitability.

Conclusion

The small sample sizes limit the statistical power of the study, but the trending effect warrants further investigation of different sports on the neuroplasticity in patients with SCI. It is not clear how neuroplastic changes impact the sensorimotor output of the affected extremities in a patient. This needs to be followed up in further studies with a greater sample size.

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.

Viscous field training induces after effects but hinders recovery of overground locomotion following spinal cord injury in rats

Robotic-assisted gait training was able to improve the unassisted overground locomotion of rats following a cervical spinal cord injury. Specifically, four weeks of daily step training in the Robomedica Rodent Robotic Motor Performance System, where the device actively guided the hindlimbs through a pre-injury stepping pattern while the rats walked over a moving treadmill belt in a quadrupedal posture, was able to improve unassisted overground locomotion as measured by the CatWalk gait analysis device. Unfortunately the improvements were minimal. In fact, control animals that received only body weight supported treadmill training and no active robotic forces showed an even greater restoration of unassisted overground locomotion. This led us to further investigate the effects of the specific forces used in rehabilitative training. The robotic training device was modified to apply assistive (negative viscosity) or resistive (viscous) fields in lieu of the standard active guidance. Within the device, daily training with a viscous field resulted in small, constrained steps that were similar to pre-injury steps. However, when the robot was off for weekly assessments, the steps opened up and deviated away from pre-injury levels. Training in a negative viscosity field produced the opposite effect; large open steps that were unlike pre-injury during daily training, and constrained steps that were more like pre-injury during weekly assessment. These training induced after-effects washed out 2 weeks after the cessation of training. Additionally, these distinct after effects seen in the training device did not translate to distinct differences in the recovery of unassisted overground locomotion, with the body weight supported treadmill training controls showing the greatest recovery of overground locomotion. Still, the fact that different applied forces can induce different after effects has interesting implications for rehabilitative training - is it better to have healthy looking steps during training only to induce abnormal after effects, or have abnormal performance during training but with desirable after effects? The data presented here is the first step in addressing this question.

Exercise-Induced Plasticity in Signaling Pathways Involved in Motor Recovery after Spinal Cord Injury

Spinal cord injury (SCI) leads to numerous chronic and debilitating functional deficits that greatly affect quality of life. While many pharmacological interventions have been explored, the current unsurpassed therapy for most SCI sequalae is exercise. Exercise has an expansive influence on peripheral health and function, and by activating the relevant neural pathways, exercise also ameliorates numerous disorders of the central nervous system (CNS). While the exact mechanisms by which this occurs are still being delineated, major strides have been made in the past decade to understand the molecular underpinnings of this essential treatment. Exercise rapidly and prominently affects dendritic sprouting, synaptic connections, neurotransmitter production and regulation, and ionic homeostasis, with recent literature implicating an exercise-induced increase in neurotrophins as the cornerstone that binds many of these effects together. The field encompasses vast complexity, and as the data accumulate, disentangling these molecular pathways and how they interact will facilitate the optimization of intervention strategies and improve quality of life for individuals affected by SCI. This review describes the known molecular effects of exercise and how they alter the CNS to pacify the injury environment, increase neuronal survival and regeneration, restore normal neural excitability, create new functional circuits, and ultimately improve motor function following SCI.

Self-directed rehabilitation training intensity thresholds for efficient recovery of skilled forelimb function in rats with cervical spinal cord injury

Task specific rehabilitation training is commonly used to treat motor dysfunction after neurological injures such as spinal cord injury (SCI), yet the use of task specific training in preclinical animal studies of SCI is not common. This is due in part to the difficulty in training animals to perform specific motor tasks, but also due to the lack of knowledge about optimal rehabilitation training parameters to maximize recovery. The single pellet reaching, grasping and retrieval (SPRGR) task (a.k.a. single pellet reaching task or Whishaw task) is a skilled forelimb motor task used to provide rehabilitation training and test motor recovery in rodents with cervical SCI. However, the relationships between the amount, duration, intensity, and timing of training remain poorly understood. In this study, using automated robots that allow rats with cervical SCI ad libitum access to self-directed SPRGR rehabilitation training, we show clear relationships between the total amount of rehabilitation training, the intensity of training (i.e., number of attempts/h), and performance in the task. Specifically, we found that rats naturally segregate into High and Low performance groups based on training strategy and performance in the task. Analysis of the different training strategies showed that more training (i.e., increased number of attempts in the SPRGR task throughout rehabilitation training) at higher intensities (i.e., number of attempts per hour) increased performance in the task, and that improved performance in the SPRGR task was linked to differences in corticospinal tract axon collateral densities in the injured spinal cords. Importantly, however, our data also indicate that rehabilitation training becomes progressively less efficient (i.e., less recovery for each attempt) as both the amount and intensity of rehabilitation training increases. Finally, we found that Low performing animals could increase their training intensity and transition to High performing animals in chronic SCI. These results highlight the rehabilitation training strategies that are most effective to regain skilled forelimb motor function after SCI, which will facilitate pre-clinical rehabilitation studies using animal models and could be beneficial in the development of more efficient clinical rehabilitation training strategies.

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.

Mesenchymal stem cells and treadmill training enhance function and promote tissue preservation after spinal cord injury

Spinal cord injury (SCI) is considered a serious neurological disorder that can lead to severe sensory, motor and autonomic deficits. In this work, we investigated whether cell therapy associated with physical activity after mouse SCI could promote morphological and functional outcomes, using a lesion model established by our group. Mesenchymal stem cells (8 × 10⁵ cells/2 µL) or DMEM (2 µL), were injected in the epicenter of the lesion at 7 days after SCI, and the mice started a moderate treadmill training 14 days after injury. Functional assessments were performed weekly up to 8 weeks after injury when the morphological analyses were also performed. Four injured groups were analyzed: DMEM (SCI plus DMEM injection), MSCT (SCI plus MSC injection), DMEM + TMT (SCI plus DMEM injection and treadmill training) and MSCT + TMT (SCI plus MSC injection and treadmill training). The animals that received the combined therapy (MSCT + TMT) were able to recover and maintained the better functional results throughout the analyzed period. The morphometric analysis from MSCT + TMT group evidenced a larger spared white matter area and a higher number of preserved myelinated fibers with the majority of them reaching the ideal G-ratio values, when compared to other groups. Ultrastructural analysis from this group, using transmission electron microscopy, showed better tissue preservation with few microcavitations and degenerating nerve fibers. Also, this group exhibited a significantly higher neurotrophin 4 (NT4) expression as compared to the other groups. The results provided by this study support the conclusion that the association of strategies is a potential therapeutic approach to treat SCI, with the possibility of translation into the clinical practice.

Robotic Rehabilitation and Spinal Cord Injury: a Narrative Review

Mobility after spinal cord injury (SCI) is among the top goals of recovery and improvement in quality of life. Those with tetraplegia rank hand function as the most important area of recovery in their lives, and those with paraplegia, walking. Without hand function, emphasis in rehabilitation is placed on accessing one's environment through technology. However, there is still much reliance on caretakers for many activities of daily living. For those with paraplegia, if incomplete, orthoses exist to augment walking function, but they require a significant amount of baseline strength and significant energy expenditure to use. Options for those with motor complete paraplegia have traditionally been limited to the wheelchair. While wheelchairs provide a modified level of independence, wheelchair users continue to face difficulties in access and mobility. In the past decade, research in SCI rehabilitation has expanded to include external motorized or robotic devices that initiate or augment movement. These robotic devices are used with 2 goals: to enhance recovery through repetitive, functional movement and increased neural plasticity and to act as a mobility aid beyond orthoses and wheelchairs. In addition, lower extremity exoskeletons have been shown to provide benefits to the secondary medical conditions after SCI such as pain, spasticity, decreased bone density, and neurogenic bowel. In this review, we discuss advances in robot-guided rehabilitation after SCI for the upper and lower extremities, as well as potential adjuncts to robotics.

Activity-Based Physical Rehabilitation with Adjuvant Testosterone to Promote Neuromuscular Recovery after Spinal Cord Injury

Neuromuscular impairment and reduced musculoskeletal integrity are hallmarks of spinal cord injury (SCI) that hinder locomotor recovery. These impairments are precipitated by the neurological insult and resulting disuse, which has stimulated interest in activity-based physical rehabilitation therapies (ABTs) that promote neuromuscular plasticity after SCI. However, ABT efficacy declines as SCI severity increases. Additionally, many men with SCI exhibit low testosterone, which may exacerbate neuromusculoskeletal impairment. Incorporating testosterone adjuvant to ABTs may improve musculoskeletal recovery and neuroplasticity because androgens attenuate muscle loss and the slow-to-fast muscle fiber-type transition after SCI, in a manner independent from mechanical strain, and promote motoneuron survival. These neuromusculoskeletal benefits are promising, although testosterone alone produces only limited functional improvement in rodent SCI models. In this review, we discuss the (1) molecular deficits underlying muscle loss after SCI; (2) independent influences of testosterone and locomotor training on neuromuscular function and musculoskeletal integrity post-SCI; (3) hormonal and molecular mechanisms underlying the therapeutic efficacy of these strategies; and (4) evidence supporting a multimodal strategy involving ABT with adjuvant testosterone, as a potential means to promote more comprehensive neuromusculoskeletal recovery than either strategy alone.

Acute intermittent hypoxia and rehabilitative training following cervical spinal injury alters neuronal hypoxia- and plasticity-associated protein expression

One of the most promising approaches to improve recovery after spinal cord injury (SCI) is the augmentation of spontaneously occurring plasticity in uninjured neural pathways. Acute intermittent hypoxia (AIH, brief exposures to reduced O₂ levels alternating with normal O₂ levels) initiates plasticity in respiratory systems and has been shown to improve recovery in respiratory and non-respiratory spinal systems after SCI in experimental animals and humans. Although the mechanism by which AIH elicits its effects after SCI are not well understood, AIH is known to alter protein expression in spinal neurons in uninjured animals. Here, we examine hypoxia- and plasticity-related protein expression using immunofluorescence in spinal neurons in SCI rats that were treated with AIH combined with motor training, a protocol which has been demonstrated to improve recovery of forelimb function in this lesion model. Specifically, we assessed protein expression in spinal neurons from animals with incomplete cervical SCI which were exposed to AIH treatment + motor training either for 1 or 7 days. AIH treatment consisted of 10 episodes of AIH: (5 min 11% O₂: 5 min 21% O₂) for 7 days beginning at 4 weeks post-SCI. Both 1 or 7 days of AIH treatment + motor training resulted in significantly increased expression of the transcription factor hypoxia-inducible factor-1α (HIF-1α) relative to normoxia-treated controls, in neurons both proximal (cervical) and remote (lumbar) to the SCI. All other markers examined were significantly elevated in the 7 day AIH + motor training group only, at both cervical and lumbar levels. These markers included vascular endothelial growth factor (VEGF), brain-derived neurotrophic factor (BDNF), and phosphorylated and nonphosphorylated forms of the BDNF receptor tropomyosin-related kinase B (TrkB). In summary, AIH induces plasticity at the cellular level after SCI by altering the expression of major plasticity- and hypoxia-related proteins at spinal regions proximal and remote to the SCI. These changes occur under the same AIH protocol which resulted in recovery of limb function in this animal model. Thus AIH, which induces plasticity in spinal circuitry, could also be an effective therapy to restore motor function after nervous system injury.

Effective robotic assistive pattern of treadmill training for spinal cord injury in a rat model

The purpose of the present study was to establish an effective robotic assistive stepping pattern of body-weight-supported treadmill training based on a rat spinal cord injury (SCI) model and assess the effect by comparing this with another frequently used assistive stepping pattern. The recorded stepping patterns of both hind limbs of trained intact rats were edited to establish a 30-sec playback normal rat stepping pattern (NRSP). Step features (step length, step height, step number and swing duration), BBB scores, latencies, and amplitudes of the transcranial electrical motor-evoked potentials (tceMEPs) and neurofilament 200 (NF200) expression in the spinal cord lesion area during and after 3 weeks of body-weight-supported treadmill training (BWSTT) were compared in rats with spinal contusion receiving NRSP assistance (NRSPA) and those that received manual assistance (MA). Hind limb stepping performance among rats receiving NRSPA during BWSTT was greater than that among rats receiving MA in terms of longer step length, taller step height, and longer swing duration. Furthermore a higher BBB score was also indicated. The rats in the NRSPA group achieved superior results in the tceMEPs assessment and greater NF200 expression in the spinal cord lesion area compared with the rats in the MA group. These findings suggest NRSPA was an effective assistive pattern of treadmill training compared with MA based on the rat SCI model and this approach could be used as a new platform for animal experiments for better understanding the mechanisms of SCI rehabilitation.

Robot-Applied Resistance Augments the Effects of Body Weight-Supported Treadmill Training on Stepping and Synaptic Plasticity in a Rodent Model of Spinal Cord Injury

BACKGROUND

The application of resistive forces has been used during body weight-supported treadmill training (BWSTT) to improve walking function after spinal cord injury (SCI). Whether this form of training actually augments the effects of BWSTT is not yet known.

OBJECTIVE

To determine if robotic-applied resistance augments the effects of BWSTT using a controlled experimental design in a rodent model of SCI.

METHODS

Spinally contused rats were treadmill trained using robotic resistance against horizontal (n = 9) or vertical (n = 8) hind limb movements. Hind limb stepping was tested before and after 6 weeks of training. Two control groups, one receiving standard training (ie, without resistance; n = 9) and one untrained (n = 8), were also tested. At the terminal experiment, the spinal cords were prepared for immunohistochemical analysis of synaptophysin.

RESULTS

Six weeks of training with horizontal resistance increased step length, whereas training with vertical resistance enhanced step height and movement velocity. None of these changes occurred in the group that received standard (ie, no resistance) training or in the untrained group. Only standard training increased the number of step cycles and shortened cycle period toward normal values. Synaptophysin expression in the ventral horn was highest in rats trained with horizontal resistance and in untrained rats and was positively correlated with step length.

CONCLUSIONS

Adding robotic-applied resistance to BWSTT produced gains in locomotor function over BWSTT alone. The impact of resistive forces on spinal connections may depend on the nature of the resistive forces and the synaptic milieu that is present after SCI.

Cellular reactions and compensatory tissue re-organization during spontaneous recovery after spinal cord injury in neonatal mice

Following incomplete spinal cord injuries, neonatal mammals display a remarkable degree of behavioral recovery. Previously, we have demonstrated in neonatal mice a wholesale re-establishment and reorganization of synaptic connections from some descending axon tracts (Boulland et al.: PLoS One 8 (2013)). To assess the potential cellular mechanisms contributing to this recovery, we have here characterized a variety of cellular sequelae following thoracic compression injuries, focusing particularly on cell loss and proliferation, inflammation and reactive gliosis, and the dynamics of specific types of synaptic terminals. Early during the period of recovery, regressive events dominated. Tissue loss near the injury was severe, with about 80% loss of neurons and a similar loss of axons that later make up the white matter. There was no sign of neurogenesis, no substantial astroglial or microglial proliferation, no change in the ratio of M1 and M2 microglia and no appreciable generation of the terminal complement peptide C5a. One day after injury the number of synaptic terminals on lumbar motoneurons had dropped by a factor of 2, but normalized by 6 days. The ratio of VGLUT1/2+ to VGAT+ terminals remained similar in injured and uninjured spinal cords during this period. By 24 days after injury, when functional recovery is nearly complete, the density of 5-HT+ fibers below the injury site had increased by a factor of 2.5. Altogether this study shows that cellular reactions are diverse and dynamic. Pronounced recovery of both excitatory and inhibitory terminals and an increase in serotonergic innervation below the injury, coupled with a general lack of inflammation and reactive gliosis, are likely to contribute to the recovery. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 928-946, 2017.

Effects of Locomotor Exercise Intensity on Gait Performance in Individuals With Incomplete Spinal Cord Injury

BACKGROUND

High-intensity stepping practice may be a critical component to improve gait following motor incomplete spinal cord injury (iSCI). However, such practice is discouraged by traditional theories of rehabilitation that suggest high-intensity locomotor exercise degrades gait performance. Accordingly, such training is thought to reinforce abnormal movement patterns, although evidence to support this notion is limited.

OBJECTIVE

The purposes of this study were: (1) to evaluate the effects of short-term manipulations in locomotor intensity on gait performance in people with iSCI and (2) to evaluate potential detrimental effects of high-intensity locomotor training on walking performance.

DESIGN

A single-day, repeated-measures, pretraining-posttraining study design was used.

METHODS

Nineteen individuals with chronic iSCI performed a graded-intensity locomotor exercise task with simultaneous collection of lower extremity kinematic and electromyographic data. Measures of interest were compared across intensity levels of 33%, 67%, and 100% of peak gait speed. A subset of 9 individuals participated in 12 weeks of high-intensity locomotor training. Similar measurements were collected and compared between pretraining and posttraining evaluations.

RESULTS

The results indicate that short-term increases in intensity led to significant improvements in muscle activity, spatiotemporal metrics, and joint excursions, with selected improvements in measures of locomotor coordination. High-intensity locomotor training led to significant increases in peak gait speed (0.64-0.80 m/s), and spatiotemporal and kinematic metrics indicate a trend for improved coordination.

LIMITATIONS

Measures of gait performance were assessed during treadmill ambulation and not compared with a control group. Generalizability of these results to overground ambulation is unknown.

CONCLUSIONS

High-intensity locomotor exercise and training does not degrade, but rather improves, locomotor function and quality in individuals with iSCI, which contrasts with traditional theories of motor dysfunction following neurologic injury.

A Review on Locomotor Training after Spinal Cord Injury: Reorganization of Spinal Neuronal Circuits and Recovery of Motor Function

Locomotor training is a classic rehabilitation approach utilized with the aim of improving sensorimotor function and walking ability in people with spinal cord injury (SCI). Recent studies have provided strong evidence that locomotor training of persons with clinically complete, motor complete, or motor incomplete SCI induces functional reorganization of spinal neuronal networks at multisegmental levels at rest and during assisted stepping. This neuronal reorganization coincides with improvements in motor function and decreased muscle cocontractions. In this review, we will discuss the manner in which spinal neuronal circuits are impaired and the evidence surrounding plasticity of neuronal activity after locomotor training in people with SCI. We conclude that we need to better understand the physiological changes underlying locomotor training, use physiological signals to probe recovery over the course of training, and utilize established and contemporary interventions simultaneously in larger scale research studies. Furthermore, the focus of our research questions needs to change from feasibility and efficacy to the following: what are the physiological mechanisms that make it work and for whom? The aforementioned will enable the scientific and clinical community to develop more effective rehabilitation protocols maximizing sensorimotor function recovery in people with SCI.

Lentiviral-Mediated Netrin-1 Overexpression Improves Motor and Sensory Functions in SCT Rats Associated with SYP and GAP-43 Expressions

Spinal cord injury (SCI), as a major cause of disability, usually causes serious loss of motor and sensory functions. As a bifunctional axonal guidance cue, netrin-1 can attract axons via the deleted in colorectal cancer (DCC) receptors and repelling others via Unc5 receptors, but its exact role in the recovery of motor and sensory function has not well been studied, and the mechanisms remains elusive. The aim of this experiment is to determine whether lentiviral (LV)-mediated overexpression of netrin-1 or RNA interference (RNAi) can regulate the functional recovery in rats subjected to spinal cord transection (SCT). Firstly, two lentiviral vectors including Lv-exNtn-1 (netrin-1 open reading frame (ORF)) and Lv-shNtn-1 (netrin-1 sh) were constructed and injected into spinal cords rostral and caudal to the transected lesion site. Overexpressing netrin-1 enhanced significantly locomotor function, and reduced thermal and mechanical stimuli in vivo, compared with the control, while silencing netrin-1 did not significantly change the situation. Western blot and immunostaining analysis confirmed that netrin-1 ORF treatment not only effectively increased the expression level of netrin-1, also up-regulated the level of synaptophysin (SYP) in spinal cord rostral to the lesion, but also enhanced growth-associated protein-43 (GAP-43) expression in spinal cord caudal to the lesion site. Comparatively, knockdown of netrin-1 did not give rise to positive findings in our experimental condition. These findings therefore pointed that Lv-mediated netrin-1 overexpression could promote motor and sensory functional recoveries following SCT, and the underlying mechanisms were associated with SYP and GAP-43 expressions. The present study therefore provided a novel strategy for the treatment of SCI and explained the possible mechanisms for the functional improvement.

Locomotor training improves reciprocal and nonreciprocal inhibitory control of soleus motoneurons in human spinal cord injury

Pathologic reorganization of spinal networks and activity-dependent plasticity are common neuronal adaptations after spinal cord injury (SCI) in humans. In this work, we examined changes of reciprocal Ia and nonreciprocal Ib inhibition after locomotor training in 16 people with chronic SCI. The soleus H-reflex depression following common peroneal nerve (CPN) and medial gastrocnemius (MG) nerve stimulation at short conditioning-test (C-T) intervals was assessed before and after training in the seated position and during stepping. The conditioned H reflexes were normalized to the unconditioned H reflex recorded during seated. During stepping, both H reflexes were normalized to the maximal M wave evoked at each bin of the step cycle. In the seated position, locomotor training replaced reciprocal facilitation with reciprocal inhibition in all subjects, and Ib facilitation was replaced by Ib inhibition in 13 out of 14 subjects. During stepping, reciprocal inhibition was decreased at early stance and increased at midswing in American Spinal Injury Association Impairment Scale C (AIS C) and was decreased at midstance and midswing phases in AIS D after training. Ib inhibition was decreased at early swing and increased at late swing in AIS C and was decreased at early stance phase in AIS D after training. The results of this study support that locomotor training alters postsynaptic actions of Ia and Ib inhibitory interneurons on soleus motoneurons at rest and during stepping and that such changes occur in cases with limited or absent supraspinal inputs.

Synaptic rearrangement following axonal injury: Old and new players

Following axotomy, the contact between motoneurons and muscle fibers is disrupted, triggering a retrograde reaction at the neuron cell body within the spinal cord. Together with chromatolysis, a hallmark of such response to injury is the elimination of presynaptic terminals apposing to the soma and proximal dendrites of the injured neuron. Excitatory inputs are preferentially eliminated, leaving the cells under an inhibitory influence during the repair process. This is particularly important to avoid glutamate excitotoxicity. Such shift from transmission to a regeneration state is also reflected by deep metabolic changes, seen by the regulation of several genes related to cell survival and axonal growth. It is unclear, however, how exactly synaptic stripping occurs, but there is substantial evidence that glial cells play an active role in this process. In one hand, immune molecules, such as the major histocompatibility complex (MHC) class I, members of the complement family and Toll-like receptors are actively involved in the elimination/reapposition of presynaptic boutons. On the other hand, plastic changes that involve sprouting might be negatively regulated by extracellular matrix proteins such as Nogo-A, MAG and scar-related chondroitin sulfate proteoglycans. Also, neurotrophins, stem cells, physical exercise and several drugs seem to improve synaptic stability, leading to functional recovery after lesion. This article is part of a Special Issue entitled 'Neuroimmunology and Synaptic Function'.

Treadmill step training promotes spinal cord neural plasticity after incomplete spinal cord injury

A large body of evidence shows that spinal circuits are significantly affected by training, and that intrinsic circuits that drive locomotor tasks are located in lumbosacral spinal segments in rats with complete spinal cord transection. However, after incomplete lesions, the effect of treadmill training has been debated, which is likely because of the difficulty of separating spontaneous stepping from specific training-induced effects. In this study, rats with moderate spinal cord contusion were jected to either step training on a treadmill or used in the model (control) group. The treadmill training began at day 7 post-injury and lasted 20 ± 10 minutes per day, 5 days per week for 10 weeks. The speed of the treadmill was set to 3 m/min and was increased on a daily basis according to the tolerance of each rat. After 3 weeks of step training, the step training group exhibited a sig-nificantly greater improvement in the Basso, Beattie and Bresnahan score than the model group. The expression of growth-associated protein-43 in the spinal cord lesion site and the number of tyrosine hydroxylase-positive ventral neurons in the second lumbar spinal segment were greater in the step training group than in the model group at 11 weeks post-injury, while the levels of brain-derived neurotrophic factor protein in the spinal cord lesion site showed no difference between the two groups. These results suggest that treadmill training significantly improves functional re-covery and neural plasticity after incomplete spinal cord injury.

Neurotrophins and spinal circuit function

Work early in the last century emphasized the stereotyped activity of spinal circuits based on studies of reflexes. However, the last several decades have focused on the plasticity of these spinal circuits. These considerations began with studies of the effects of monoamines on descending and reflex circuits. In recent years new classes of compounds called growth factors that are found in peripheral nerves and the spinal cord have been shown to affect circuit behavior in the spinal cord. In this review we will focus on the effects of neurotrophins, particularly nerve growth factor (NGF), brain derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), on spinal circuits. We also discuss evidence that these molecules can modify functions including nociceptive behavior, motor reflexes and stepping behavior. Since these substances and their receptors are normally present in the spinal cord, they could potentially be useful in improving function in disease states and after injury. Here we review recent findings relevant to these translational issues.

Robotic loading during treadmill training enhances locomotor recovery in rats spinally transected as neonates

Loading on the limbs has a powerful influence on locomotion. In the present study, we examined whether robotic-enhanced loading during treadmill training improved locomotor recovery in rats that were spinally transected as neonates. A robotic device applied a force on the ankle of the hindlimb while the rats performed bipedal stepping on a treadmill. The robotic force enhanced loading during the stance phase of the step cycle. One group of spinally transected rats received 4 wk of bipedal treadmill training with robotic loading while another group received 4 wk of bipedal treadmill training but without robotic loading. The two groups exhibited similar stepping performance during baseline tests of bipedal treadmill stepping. However, after 4 wk, the spinally transected rats that received bipedal treadmill training with robotic loading performed significantly more weight-bearing steps than the bipedal treadmill training only group. Bipedal treadmill training with robotic loading enhanced the ankle trajectory and ankle velocity during the step cycle. Based on immunohistochemical analyses, the expression of the presynaptic marker, synaptophysin, was significantly greater in the ventral horn of the lumbar spinal cord of the rats that received bipedal treadmill training with robotic loading. These findings suggested that robotic loading during bipedal treadmill training improved the ability of the lumbar spinal cord to generate stepping. The results have implications for the use of robotic-enhanced gait training therapies that encourage motor learning after spinal cord injury.

Maladaptive spinal plasticity opposes spinal learning and recovery in spinal cord injury

Synaptic plasticity within the spinal cord has great potential to facilitate recovery of function after spinal cord injury (SCI). Spinal plasticity can be induced in an activity-dependent manner even without input from the brain after complete SCI. A mechanistic basis for these effects is provided by research demonstrating that spinal synapses have many of the same plasticity mechanisms that are known to underlie learning and memory in the brain. In addition, the lumbar spinal cord can sustain several forms of learning and memory, including limb-position training. However, not all spinal plasticity promotes recovery of function. Central sensitization of nociceptive (pain) pathways in the spinal cord may emerge in response to various noxious inputs, demonstrating that plasticity within the spinal cord may contribute to maladaptive pain states. In this review we discuss interactions between adaptive and maladaptive forms of activity-dependent plasticity in the spinal cord below the level of SCI. The literature demonstrates that activity-dependent plasticity within the spinal cord must be carefully tuned to promote adaptive spinal training. Prior work from our group has shown that stimulation that is delivered in a limb position-dependent manner or on a fixed interval can induce adaptive plasticity that promotes future spinal cord learning and reduces nociceptive hyper-reactivity. On the other hand, stimulation that is delivered in an unsynchronized fashion, such as randomized electrical stimulation or peripheral skin injuries, can generate maladaptive spinal plasticity that undermines future spinal cord learning, reduces recovery of locomotor function, and promotes nociceptive hyper-reactivity after SCI. We review these basic phenomena, how these findings relate to the broader spinal plasticity literature, discuss the cellular and molecular mechanisms, and finally discuss implications of these and other findings for improved rehabilitative therapies after SCI.

Treadmill training stimulates brain-derived neurotrophic factor mRNA expression in motor neurons of the lumbar spinal cord in spinally transected rats

Brain-derived neurotrophic factor (BDNF) induces plasticity within the lumbar spinal circuits thereby improving locomotor recovery in spinal cord-injured animals. We examined whether lumbar spinal cord motor neurons and other ventral horn cells of spinally transected (ST) rats were stimulated to produce BDNF mRNA in response to treadmill training. Rats received complete spinal cord transections as neonates (n=20) and one month later, received four weeks of either a low (100 steps/training session; n=10) or high (1000 steps/training session; n=10) amount of robotic-assisted treadmill training. Using combined non-radioactive in situ hybridization and immunohistochemical techniques, we found BDNF mRNA expression in heat shock protein 27-labeled motor neurons and in non-motor neuron cells was greater after 1000 steps/training session compared to the 100 steps/training session and was similar to BDNF mRNA labeling in untrained Intact rats. In addition, there were significantly more motor neurons that contained BDNF mRNA labeling within processes in the ST rats that received the higher amount of treadmill training. These findings suggested that motor neurons and other ventral horn cells in ST rats synthesized BDNF in response to treadmill training. The findings support a mechanism by which postsynaptic release of BDNF from motor neurons contributed to synaptic plasticity.

Adaptations in glutamate and glycine content within the lumbar spinal cord are associated with the generation of novel gait patterns in rats following neonatal spinal cord transection

After spinal cord transection, the generation of stepping depends on neurotransmitter systems entirely contained within the local lumbar spinal cord. Glutamate and glycine likely play important roles, but surprisingly little is known about how the content of these two key neurotransmitters changes to achieve weight-bearing stepping after spinal cord injury. We studied the levels of glutamate and glycine in the lumbar spinal cord of spinally transected rats. Rats (n = 48) received spinal cord transection at 5 days of age, and 4 weeks later half were trained to step using a robotic treadmill system and the remaining half were untrained controls. Analyses of glutamate and glycine content via high-performance liquid chromatography showed training significantly raised the levels of both neurotransmitters in the lumbar spinal cord beyond normal. The levels of both neurotransmitters were significantly correlated with the ability to perform independent stepping during training. Glutamate and glycine levels were not significantly different between Untrained and Normal rats or between Trained and Untrained rats. There was a trend for higher expression of VGLUT1 and GLYT2 around motor neurons in Trained versus Untrained rats based on immunohistochemical analyses. Training improved the ability to generate stepping at a range of weight support levels, but normal stepping characteristics were not restored. These findings suggested that the remodeling of the lumbar spinal circuitry in Trained spinally transected rats involved adaptations in the glutamatergic and glycinergic neurotransmitter systems. These adaptations may contribute to the generation of novel gait patterns following complete spinal cord transection.

Assessment of hindlimb locomotor strength in spinal cord transected rats through animal-robot contact force

Robotic locomotor training devices have gained popularity in recent years, yet little has been reported regarding contact forces experienced by the subject performing automated locomotor training, particularly in animal models of neurological injury. The purpose of this study was to develop a means for acquiring contact forces between a robotic device and a rodent model of spinal cord injury through instrumentation of a robotic gait training device (the rat stepper) with miniature force/torque sensors. Sensors were placed at each interface between the robot arm and animal's hindlimb and underneath the stepping surface of both hindpaws (four sensors total). Twenty four female, Sprague-Dawley rats received mid-thoracic spinal cord transections as neonates and were included in the study. Of these 24 animals, training began for 18 animals at 21 days of age and continued for four weeks at five min/day, five days/week. The remaining six animals were untrained. Animal-robot contact forces were acquired for trained animals weekly and untrained animals every two weeks while stepping in the robotic device with both 60 and 90% of their body weight supported (BWS). Animals that received training significantly increased the number of weight supported steps over the four week training period. Analysis of raw contact forces revealed significant increases in forward swing and ground reaction forces during this time, and multiple aspects of animal-robot contact forces were significantly correlated with weight bearing stepping. However, when contact forces were normalized to animal body weight, these increasing trends were no longer present. Comparison of trained and untrained animals revealed significant differences in normalized ground reaction forces (both horizontal and vertical) and normalized forward swing force. Finally, both forward swing and ground reaction forces were significantly reduced at 90% BWS when compared to the 60% condition. These results suggest that measurement of animal-robot contact forces using the instrumented rat stepper can provide a sensitive and reliable measure of hindlimb locomotor strength and control of flexor and extensor muscle activity in neurologically impaired animals. Additionally, these measures may be useful as a means to quantify training intensity or dose-related functional outcomes of automated training.