Exercise modulates chloride homeostasis after spinal cord injury

Furosemide Unmasks Inhibitory Dysfunction after Spinal Cord Injury in Humans: Implications for Spasticity

Spasticity after spinal cord injury has considerable quality of life implications, impacts on rehabilitation efforts and necessitates long-term multi-disciplinary pharmacological and non-pharmacological management. The potassium chloride co-transporter (KCC2) plays a central role in intracellular chloride homeostasis and the inhibitory function of mature neurons. Animal studies consistently have demonstrated a downregulation of KCC2 activity after spinal cord transection, causing a shift from the inhibitory action of gamma-aminobutyric acid and glycine to an excitatory effect. Furosemide, a recognized KCC2 antagonist in animals, blocks the formation of inhibitory post-synaptic potentials in spinal motoneurons without affecting excitatory post-synaptic potentials. Based on observations in animals studies, we hypothesized that furosemide may be used to unmask KCC2 downregulation after spinal cord injury in humans, which contributes to reflex hyperexcitability. We have shown previously that furosemide reduces both pre-synaptic and post-synaptic inhibition in healthy subjects without altering monosynaptic excitatory transmission. These findings provide evidence that furosemide may be used in humans to evaluate inhibitory synapses in the spinal cord. In this present study, we show that furosemide fails to modulate both pre- and post-synaptic inhibitions relayed to soleus spinal motor neurons in persons with spinal cord injury. The lack of furosemide effect after spinal cord injury suggests KCC2 dysfunction in humans, resulting in reduced inhibitory synaptic transmission in spinal neurons. Our findings suggest that KCC2 dysfunction may be an important etiological factor in hyperreflexia after spinal cord injury. These observations may pave the way to novel therapeutic strategies against spasticity centered on chloride homeostasis.

Afferent Input Induced by Rhythmic Limb Movement Modulates Spinal Neuronal Circuits in an Innovative Robotic In Vitro Preparation

Locomotor patterns are mainly modulated by afferent feedback, but its actual contribution to spinal network activity during continuous passive limb training is still unexplored. To unveil this issue, we devised a robotic in vitro setup (Bipedal Induced Kinetic Exercise, BIKE) to induce passive pedaling, while simultaneously recording low-noise ventral and dorsal root (VR and DR) potentials in isolated neonatal rat spinal cords with hindlimbs attached. As a result, BIKE evoked rhythmic afferent volleys from DRs, reminiscent of pedaling speed. During BIKE, spontaneous VR activity remained unchanged, while a DR rhythmic component paired the pedaling pace. Moreover, BIKE onset rarely elicited brief episodes of fictive locomotion (FL) and, when trains of electrical pulses were simultaneously applied to a DR, it increased the amplitude, but not the number, of FL cycles. When BIKE was switched off after a 30-min training, the number of electrically induced FL oscillations was transitorily facilitated, without affecting VR reflexes or DR potentials. However, 90 min of BIKE no longer facilitated FL, but strongly depressed area of VR reflexes and stably increased antidromic DR discharges. Patch clamp recordings from single motoneurons after 90-min sessions indicated an increased frequency of both fast- and slow-decaying synaptic input to motoneurons. In conclusion, hindlimb rhythmic and alternated pedaling for different durations affects distinct dorsal and ventral spinal networks by modulating excitatory and inhibitory input to motoneurons. These results suggest defining new parameters for effective neurorehabilitation that better exploits spinal circuit activity.

H-reflex disinhibition by lumbar muscle inflammation in a mouse model of spinal cord injury

Inflammation is a common comorbidity in patients with traumatic spinal cord injury (SCI). Recent reports indicate that inflammation hinders functional recovery in animal models of SCI. However, the spinal mechanisms underlying this alteration are currently unknown. Considering that spinal plasticity is a therapeutic target in patients and animal models of SCI, these mechanisms remain to be clarified. Using injections of complete Freund's adjuvant (CFA) in lumbar muscles as a model of persistent inflammation, the objective of this study was to assess the impact of inflammation on spinal reflex excitability after a complete midthoracic spinal transection in mice. To this end, the excitability of spinal reflexes was examined by measuring H-reflex frequency-dependent depression (FDD) on days 7, 14 and 28 following a complete spinal transection. H-reflex parameters were compared between spinal mice with CFA and control spinal mice. On day 7, lumbar muscle inflammation disinhibited the H-reflex, reflected by an attenuation of H-reflex FDD (p < 0.01), although this effect did not persist later on, either on day 14 or day 28. These results indicate that lumbar muscle inflammation alters spinal reflex excitability transiently in spinal mice. Considering that changes in spinal reflex excitability are associated with poor functional recovery after SCI, this implies that inflammation should be treated effectively to promote optimal recovery following SCI.

Effects of bioengineered scaffold loaded with neurotrophins and locomotor training in restoring H-reflex responses after spinal cord injury

The combinational effects of a bioengineered scaffold loaded with neurotrophins and rehabilitation training on spasticity observed after spinal cord injury (SCI) has not been studied. We used an animal model of moderate contusion injury at T9/T10 that received bioengineered scaffold poly N-isopropylacrylamide-g-poly ethylene glycol (PNIPAAm-g-PEG) loaded with BDNF/NT3 followed by body weight supported treadmill training (BWSTT) and assessed the efficacy of the combinational bioengineered approaches in treating spasticity. Five animal groups were included: Group 1: Sham, Group 2: Injury (SCI), Group 3: SCI + BWSTT (BWSTT), Group 4: SCI + PNIPAAm-g-PEG loaded with BDNF/NT3 (Transplant), and Group 5: SCI + PNIPAAm-g-PEG loaded with BDNF/NT3 + BWSTT (Combinational). Results indicate no significant changes in the BBB scores of animals among various groups, however, a significant restoration in the rate depression property of H-reflex was observed in both BWSTT and Combinational animals. Transplant group reported no improvement in the rate depression property of H-reflex and were similar to SCI only group. Histological findings report restoration of the chloride cotransporter (KCC2) labeling in both BWSTT and Combinational animals and down-regulation of KCC2 in both SCI and Transplant only animals. Findings from this study confirm that rehabilitation training is critical in restoring H-reflex responses and transplantation therapies alone cannot restore these responses after SCI. Also, although no significant difference was observed between the BWSTT and Combinational animals, comparable improvements in the two groups does open new pathways to exploring unique tissue-engineering approaches with promising clinical application for individuals with SCI.

Reactivation of Dormant Relay Pathways in Injured Spinal Cord by KCC2 Manipulations

Many human spinal cord injuries are anatomically incomplete but exhibit complete paralysis. It is unknown why spared axons fail to mediate functional recovery in these cases. To investigate this, we undertook a small-molecule screen in mice with staggered bilateral hemisections in which the lumbar spinal cord is deprived of all direct brain-derived innervation, but dormant relay circuits remain. We discovered that a KCC2 agonist restored stepping ability, which could be mimicked by selective expression of KCC2, or hyperpolarizing DREADDs, in the inhibitory interneurons between and around the staggered spinal lesions. Mechanistically, these treatments transformed this injury-induced dysfunctional spinal circuit to a functional state, facilitating the relay of brain-derived commands toward the lumbar spinal cord. Thus, our results identify spinal inhibitory interneurons as a roadblock limiting the integration of descending inputs into relay circuits after injury and suggest KCC2 agonists as promising treatments for promoting functional recovery after spinal cord injury.

Neuropathic Pain After Spinal Cord Injury: Challenges and Research Perspectives

Neuropathic pain is a debilitating consequence of spinal cord injury (SCI) that remains difficult to treat because underlying mechanisms are not yet fully understood. In part, this is due to limitations of evaluating neuropathic pain in animal models in general, and SCI rodents in particular. Though pain in patients is primarily spontaneous, with relatively few patients experiencing evoked pains, animal models of SCI pain have primarily relied upon evoked withdrawals. Greater use of operant tasks for evaluation of the affective dimension of pain in rodents is needed, but these tests have their own limitations such that additional studies of the relationship between evoked withdrawals and operant outcomes are recommended. In preclinical SCI models, enhanced reflex withdrawal or pain responses can arise from pathological changes that occur at any point along the sensory neuraxis. Use of quantitative sensory testing for identification of optimal treatment approach may yield improved identification of treatment options and clinical trial design. Additionally, a better understanding of the differences between mechanisms contributing to at- versus below-level neuropathic pain and neuropathic pain versus spasticity may shed insights into novel treatment options. Finally, the role of patient characteristics such as age and sex in pathogenesis of neuropathic SCI pain remains to be addressed.

Metaplasticity within the spinal cord: Evidence brain-derived neurotrophic factor (BDNF), tumor necrosis factor (TNF), and alterations in GABA function (ionic plasticity) modulate pain and the capacity to learn

Evidence is reviewed that behavioral training and neural injury can engage metaplastic processes that regulate adaptive potential. This issue is explored within a model system that examines how training affects the capacity to learn within the lower (lumbosacral) spinal cord. Response-contingent (controllable) stimulation applied caudal to a spinal transection induces a behavioral modification indicative of learning. This behavioral change is not observed in animals that receive stimulation in an uncontrollable manner. Exposure to uncontrollable stimulation also engages a process that disables spinal learning for 24-48 h. Controllable stimulation has the opposite effect; it engages a process that enables learning and prevents/reverses the learning deficit induced by uncontrollable stimulation. These observations suggest that a learning episode can impact the capacity to learn in future situations, providing an example of behavioral metaplasticity. The protective/restorative effect of controllable stimulation has been linked to an up-regulation of brain-derived neurotrophic factor (BDNF). The disruption of learning has been linked to the sensitization of pain (nociceptive) circuits, which is enabled by a reduction in GABA-dependent inhibition. After spinal cord injury (SCI), the co-transporter (KCC2) that regulates the outward flow of Cl^- is down-regulated. This causes the intracellular concentration of Cl^- to increase, reducing (and potentially reversing) the inward flow of Cl^- through the GABA-A receptor. The shift in GABA function (ionic plasticity) increases neural excitability caudal to injury and sets the stage for nociceptive sensitization. The injury-induced shift in KCC2 is related to the loss of descending serotonergic (5HT) fibers that regulate plasticity within the spinal cord dorsal horn through the 5HT-1A receptor. Evidence is presented that these alterations in spinal plasticity impact pain in a brain-dependent task (place conditioning). The findings suggest that ionic plasticity can affect learning potential, shifting a neural circuit from dampened/hard-wired to excitable/plastic.

Effects of Treadmill Training Combined with Serotonergic Interventions on Spasticity after Contusive Spinal Cord Injury

Spasticity usually emerges during the course of recovery from spinal cord injury (SCI). While medications and physical rehabilitation are prescribed to alleviate spastic symptoms, the insufficiency of their effects remains an important problem to be addressed. Given the challenges associated with increasing the dose of medication, we hypothesized that a combination therapy with medication and physical rehabilitation can be effective. Therefore, we examined the effects of treadmill training (TMT) along with serotonergic medication using a spastic rat model after contusive injury. Spasticity-strong SCI rats were selected 4 weeks after SCI and received one of the following interventions for 2 weeks: only TMT, TMT with fluoxetine (a selective serotonin re-uptake inhibitor), TMT with cyproheptadine (a 5-HT₂ receptor antagonist), only fluoxetine, or only cyproheptadine. We performed the swimming test to quantify the frequency of spastic behaviors. We also evaluated hindlimb locomotor functions every week. At the end of the intervention, we examined the Hoffman reflex from the plantar muscle and the immunoreactivity of the 5-HT_2A receptor in spinal cord tissues. While the TMT group and cyproheptadine-treated groups showed decreased spastic behaviors and reduction in spinal hyperreflexia, the fluoxetine-treated group showed the opposite effect, even with TMT. Moreover, TMT suppressed the expression of the 5-HT_2A receptor in the lumbar spinal motor neurons, while cyproheptadine treatment did not change it. We did not observe any differences in locomotor functions between the groups. Taken together, our findings indicate that TMT and cyproheptadine significantly alleviated spastic symptoms, but did not show synergistic or additive effects.

Spontaneous respiratory plasticity following unilateral high cervical spinal cord injury in behaving rats

Unilateral cervical C2 hemisection (C2Hx) is a classic model of spinal cord injury (SCI) for studying respiratory dysfunction and plasticity. However, most previous studies were performed under anesthesia, which significantly alters respiratory network. Therefore, the goal of this work was to assess spontaneous diaphragm recovery post-C2Hx in awake, freely behaving animals. Adult rats were chronically implanted with diaphragm EMG electrodes and recorded during 8 weeks post-C2Hx. Our results reveal that ipsilateral diaphragm activity partially recovers within days post-injury and reaches pre-injury amplitude in a few weeks. However, the full extent of spontaneous ipsilateral recovery is significantly attenuated by anesthesia (ketamine/xylazine, isoflurane, and urethane). This suggests that the observed recovery may be attributed in part to activation of NMDA receptors which are suppressed by anesthesia. Despite spontaneous recovery in awake animals, ipsilateral hemidiaphragm dysfunction still persists: i) Inspiratory bursts during basal (slow) breathing exhibit an altered pattern, ii) the amplitude of sighs - or augmented breaths - is significantly decreased, and iii) the injured hemidiaphragm exhibits spontaneous events of hyperexcitation. The results from this study offer an under-appreciated insight into spontaneous diaphragm activity and recovery following high cervical spinal cord injury in awake animals.

Altered transcription of glutamatergic and glycinergic receptors in spinal cord dorsal horn following spinal cord transection is minimally affected by passive exercise of the hindlimbs

Gene expression is altered following a spinal transection (STx) in both motor and sensory systems. Exercise has been shown to influence gene expression in both systems post-STx. Gene expression alterations have also been shown in the dorsal root ganglia and nociceptive laminae of the spinal cord following either an incomplete spinal cord injury (SCI) or a contusive SCI. However, the effect of STx and exercise on gene expression in spinal cord laminae I-III has not fully been examined. Therefore, the purpose of this study was to determine whether gene expression in laminae I-III is altered following STx and determine whether superimposed passive exercise of the hindlimbs would influence gene expression post-STx in laminae I-III. Laser capture microdissection was used to selectively harvest laminae I-III of lumbar spinal cord sections, and quantitative RT-PCR was used to examine relative expression of 23 selected genes in samples collected from control, STx and STx plus exercise rats. We demonstrate that post-STx, gene expression for metabotropic glutamate receptors 1, 5 and 8 were up-regulated, whereas ionotropic glutamatergic receptor (Glur2) and glycinergic subunit GLRA1 expression was down-regulated. Daily exercise attenuated the down-regulation of Glur2 gene expression in laminae I-III. Our results demonstrate that in a STx model, gene expression is altered in laminae I-III and that although passive exercise influences gene expression in both the motor and sensory systems, it had a minimal effect on gene expression in laminae I-III post-STx.

The H-Reflex as a Biomarker for Spinal Disinhibition in Painful Diabetic Neuropathy

PURPOSE OF REVIEW

Neuropathic pain may arise from multiple mechanisms and locations. Efficacy of current treatments for painful diabetic neuropathy is limited to an unpredictable subset of patients, possibly reflecting diversity of pain generator mechanisms, and there is a lack of targeted treatments for individual patients. This review summarizes preclinical evidence supporting a role for spinal disinhibition in painful diabetic neuropathy, the physiology and pharmacology of rate-dependent depression (RDD) of the spinal H-reflex and the translational potential of using RDD as a biomarker of spinally mediated pain.

RECENT FINDINGS

Impaired RDD occurs in animal models of diabetes and was also detected in diabetic patients with painful vs painless neuropathy. RDD status can be determined using standard neurophysiological equipment. Loss of RDD may provide a clinical biomarker of spinal disinhibition, thereby enabling a personalized medicine approach to selection of current treatment options and enrichment of future clinical trial populations.

Pretreatment with AQP4 and NKCC1 Inhibitors Concurrently Attenuated Spinal Cord Edema and Tissue Damage after Spinal Cord Injury in Rats

Spinal cord injury (SCI) affects more than 2.5 million people worldwide. Spinal cord edema plays critical roles in the pathological progression of SCI. This study aimed to delineate the roles of aquaporin 4 (AQP4) and Na⁺-K⁺-Cl⁻ cotransporter 1 (NKCC1) in acute phase edema and tissue destruction after SCI and to explore whether inhibiting both AQP4 and NKCC1 could improve SCI-induced spinal edema and damage. Rat SCI model was established by modified Allen's method. Spinal cord water content, cerebrospinal fluid lactose dehydrogenase (LDH) activity, AQP4 and NKCC1 expression, and spinal cord pathology from 30 min to 7 days after SCI were monitored. Additionally, aforementioned parameters in rats treated with AQP4 and/or NKCC1 inhibitors were assessed 2 days after SCI. Spinal cord water content was significantly increased 1 h after SCI while AQP4 and NKCC1 expression and spinal fluid LDH activity elevated 6 h after SCI. Spinal cord edema and spinal cord destruction peaked around 24 h after SCI and maintained at high levels thereafter. Treating rats with AQP4 inhibitor TGN-020 and NKCC1 antagonist bumetanide significantly reduced spinal cord edema, tissue destruction, and AQP4 and NKCC1 expression after SCI in an additive manner. These results demonstrated the benefits of simultaneously inhibiting both AQP4 and NKCC1 after SCI.

Retracing your footsteps: developmental insights to spinal network plasticity following injury

During development of the spinal cord, a precise interaction occurs between descending projections and sensory afferents, with spinal networks that lead to expression of coordinated motor output. In the rodent, during the last embryonic week, motor output first occurs as regular bursts of spontaneous activity, progressing to stochastic patterns of episodes that express bouts of coordinated rhythmic activity perinatally. Locomotor activity becomes functionally mature in the 2nd postnatal wk and is heralded by the onset of weight-bearing locomotion on the 8th and 9th postnatal day. Concomitantly, there is a maturation of intrinsic properties and key conductances mediating plateau potentials. In this review, we discuss spinal neuronal excitability, descending modulation, and afferent modulation in the developing rodent spinal cord. In the adult, plastic mechanisms are much more constrained but become more permissive following neurotrauma, such as spinal cord injury. We discuss parallel mechanisms that contribute to maturation of network function during development to mechanisms of pathological plasticity that contribute to aberrant motor patterns, such as spasticity and clonus, which emerge following central injury.

Chloride Channels are Involved in the Development of Atrial Fibrillation - A Transcriptomic and proteomic Study

Electrical and structural remodeling processes are contributors to the self-perpetuating nature of atrial fibrillation (AF). However, their correlation has not been clarified. In this study, human atrial tissues from the patients with rheumatic mitral valve disease in either sinus rhythm or persistent AF were analyzed using a combined transcriptomic and proteomic approach. An up-regulation in chloride intracellular channel (CLIC) 1, 4, 5 and a rise in type IV collagen were revealed. Combined with the results from immunohistochemistry and electron microscope analysis, the distribution of type IV collagen and effects of fibrosis on myocyte membrane indicated the possible interaction between CLIC and type IV collagen, confirmed by protein structure prediction and co-immunoprecipitation. These results indicate that CLICs play an important role in the development of atrial fibrillation and that CLICs and structural type IV collagen may interact on each other to promote the development of AF in rheumatic mitral valve disease.

Electroacupuncture improves gait locomotion, H-reflex and ventral root potentials of spinal compression injured rats

This study explored the effect of electroacupuncture stimulation (EA) on alterations in the Hoffman reflex (H-reflex) response and gait locomotion provoked by spinal cord injury (SCI) in the rat. A compression lesion of the spinal cord was evoked by insufflating a Fogarty balloon located in the epidural space at the T8-9 spinal level of adult Wistar male rats (200-250 gr; n=60). In different groups of SCI rats, EA (frequencies: 2, 50 and 100Hz) was applied simultaneously to Huantiao (GB30), Yinmen (BL37), Jizhong (GV6) and Zhiyang (GV9) acupoints from the third post-injury day until the experimental session. At 1, 2, 3 and 4 post-injury weeks, the BBB scores of the SCI group of rats treated with EA at 50Hz showed a gradual but greater enhancement of locomotor activity than the other groups of rats. Unrestrained gait kinematic analysis of SCI rats treated with EA-50Hz stimulation showed a significant improvement in stride duration, length and speed (p<0.05), whereas a discrete recovery of gait locomotion was observed in the other groups of animals. After four post-injury weeks, the H-reflex amplitude and H-reflex/M wave amplitude ratio obtained in SCI rats had a noticeable enhancement (217%) compared to sham rats (n=10). Meanwhile, SCI rats treated with EA at 50Hz manifested a decreased facilitation of the H-reflex amplitude and H/M amplitude ratio (154%) and a reduced frequency-dependent amplitude depression of the H-reflex (66%). In addition, 50 Hz-EA treatment induced a recovery of the presynaptic depression of the Gs-VRP evoked by PBSt conditioning stimulation in the SCI rat (63.2±8.1%; n=9). In concordance with the latter, it could be suggested that 50 Hz-EA stimulation reduced the hyper-excitability of motoneurons and provokes a partial improvement of the locomotive performance and H reflex responses by a possible recovery of presynaptic mechanisms in the spinal cord of experimentally injured rats.

Rehabilitation Augments Hematoma Clearance and Attenuates Oxidative Injury and Ion Dyshomeostasis After Brain Hemorrhage

BACKGROUND AND PURPOSE

We assessed the elemental and biochemical effects of rehabilitation after intracerebral hemorrhage, with emphasis on iron-mediated oxidative stress, using a novel multimodal biospectroscopic imaging approach.

METHODS

Collagenase-induced striatal hemorrhage was produced in rats that were randomized to enriched rehabilitation or control intervention starting on day 7. Animals were euthanized on day 14 or 21, a period of ongoing cell death. We used biospectroscopic imaging techniques to precisely determine elemental and molecular changes on day 14. Hemoglobin content was assessed with resonance Raman spectroscopy. X-ray fluorescence imaging mapped iron, chlorine, potassium, calcium, and zinc. Protein aggregation, a marker of oxidative stress, and the distribution of other macromolecules were assessed with Fourier transform infrared imaging. A second study estimated hematoma volume with a spectrophotometric assay at 21 days.

RESULTS

In the first experiment, rehabilitation reduced hematoma hemoglobin content (P=0.004) and the amount of peri-hematoma iron (P<0.001). Oxidative damage was highly localized at the hematoma/peri-hematoma border and was decreased by rehabilitation (P=0.004). Lipid content in the peri-hematoma zone was increased by rehabilitation (P=0.016). Rehabilitation reduced the size of calcium deposits (P=0.040) and attenuated persistent dyshomeostasis of Cl^- (P<0.001) but not K⁺ (P=0.060). The second study confirmed that rehabilitation decreased hematoma volume (P=0.024).

CONCLUSIONS

Rehabilitation accelerated clearance of toxic blood components and decreased chronic oxidative stress. As well, rehabilitation attenuated persistent ion dyshomeostasis. These novel effects may underlie rehabilitation-induced neuroprotection and improved recovery of function. Pharmacotherapies targeting these mechanisms may further improve outcome.

Rehabilitation Strategies after Spinal Cord Injury: Inquiry into the Mechanisms of Success and Failure

Body-weight supported locomotor training (BWST) promotes recovery of load-bearing stepping in lower mammals, but its efficacy in individuals with a spinal cord injury (SCI) is limited and highly dependent on injury severity. While animal models with complete spinal transections recover stepping with step-training, motor complete SCI individuals do not, despite similarly intensive training. In this review, we examine the significant differences between humans and animal models that may explain this discrepancy in the results obtained with BWST. We also summarize the known effects of SCI and locomotor training on the muscular, motoneuronal, interneuronal, and supraspinal systems in human and non-human models of SCI and address the potential causes for failure to translate to the clinic. The evidence points to a deficiency in neuronal activation as the mechanism of failure, rather than muscular insufficiency. While motoneuronal and interneuronal systems cannot be directly probed in humans, the changes brought upon by step-training in SCI animal models suggest a beneficial re-organization of the systems' responsiveness to descending and afferent feedback that support locomotor recovery. The literature on partial lesions in humans and animal models clearly demonstrate a greater dependency on supraspinal input to the lumbar cord in humans than in non-human mammals for locomotion. Recent results with epidural stimulation that activates the lumbar interneuronal networks and/or increases the overall excitability of the locomotor centers suggest that these centers are much more dependent on the supraspinal tonic drive in humans. Sensory feedback shapes the locomotor output in animal models but does not appear to be sufficient to drive it in humans.

Spinal inhibition and motor function in adults with spastic cerebral palsy

KEY POINTS

Abnormal activation of motoneurons in the spinal cord by sensory pathways is thought to contribute to impaired movement control and spasticity in individuals with cerebral palsy. Here we use single motor unit recordings to show how individual motoneurons in the spinal cord respond to sensory inputs in a group of participants with cerebral palsy having different degrees of motor dysfunction. In participants who had problems walking independently and required assistive devices such as wheelchairs, sensory pathways only excited motoneurons in the spinal cord. In contrast, in participants with cerebral palsy who walked independently for long distances, sensory inputs both inhibited and excited motoneurons in the spinal cord, similar to what we found in uninjured control participants. These findings demonstrate that in individuals with severe cerebral palsy, inhibitory control of motoneurons from sensory pathways is reduced and may contribute to motor dysfunction and spasticity.

ABSTRACT

Reduced inhibition of spinal motoneurons by sensory pathways may contribute to heightened reflex activity, spasticity and impaired motor function in individuals with cerebral palsy (CP). To measure if the activation of inhibitory post-synaptic potentials (IPSPs) by sensory inputs is reduced in CP, the tonic discharge rate of single motor units from the soleus muscle was plotted time-locked to the occurrence of a sensory stimulation to produce peri-stimulus frequencygrams (PSFs). Stimulation to the medial arch of the foot was used to activate cutaneomuscular afferents in 17 adults with bilateral spastic CP and 15 neurologically intact (NI) peers. Evidence of IPSP activation from the PSF profiles, namely a marked pause or reduction in motor unit firing rates at the onset of the cutaneomuscular reflex, was found in all NI participants but in only half of participants with CP. In the other half of the participants with CP, stimulation of cutaneomuscular afferents produced a PSF profile indicative of a pure excitatory post-synaptic potential, with firing rates increasing above the mean pre-stimulus rate for 300 ms or more. The amplitude of motoneuron inhibition during the period of IPSP activation, as measured from the surface EMG, was less in participants with poor motor function as evaluated with the Gross Motor Functional Classification System (r = 0.72, P < 0.001) and the Functional Mobility Scale (r = -0.82, P < 0.001). These findings demonstrate that in individuals with CP, reduced activation of motoneuron IPSPs by sensory inputs is associated with reduced motor function and may contribute to enhanced reflexes and spasticity in CP.

Inhibition of Na(+)-K(+)-2Cl(-) Cotransporter-1 attenuates traumatic brain injury-induced neuronal apoptosis via regulation of Erk signaling

Traumatic brain injury (TBI) is the leading cause of mortality and morbidity worldwide and is characterized by immediate brain damage and secondary injuries, such as brain edema and ischemia. However, the exact pathological mechanisms that comprise these associated secondary injuries have not been fully elucidated. This study aimed to investigate the role of the Na(+)-K(+)-2Cl(-) cotransporter-1 (NKCC1) in the disruption of ion homeostasis and neuronal apoptosis in TBI. Using a traumatic neuron injury (TNI) model in vitro and a controlled cortex injury (CCI) model in vivo, the present study investigated changes in the expression and effects of NKCC1 in TBI using western blot, RNA interference, a lactate dehydrogenase (LDH) release assay, TdT-mediated dUTP Nick end-labeling (TUNEL) analysis, sodium imaging, brain water content, and neurological severity scoring. TBI induced the expression of NKCC1 to be significantly upregulated in the cortex, both in vitro and in vivo. Pharmacological inhibitor bumetanide (Bume) or NKCC1 RNA interference significantly attenuated TBI-induced intracellular Na(+) increase, inhibited neuronal apoptosis, and improved brain edema and neurological function. Furthermore, NKCC1 inhibition also significantly inhibited TBI-induced extracellular signal-regulated kinase (Erk) activation. Erk inhibition significantly protected neurons from TBI injury; however, Erk inhibition had no effect on NKCC1 expression or the neuroprotective effect of NKCC1 inhibition against TBI. This study demonstrates the role of NKCC1 in TBI-induced brain cortex injury, establishing that NKCC1 may play a neurotoxic role in TBI and that the inhibition of NKCC1 may protect neurons from TBI via the regulation of Erk signaling.

Preclinical models of muscle spasticity: valuable tools in the development of novel treatment for neurological diseases and conditions

Poor validity of preclinical animal models is one of the most commonly discussed explanations for the failures to develop novel drugs in general and in neuroscience in particular. However, there are several areas of neuroscience such as injury-induced spasticity where etiological factor can be adequately recreated and models can focus on specific pathophysiological mechanisms that likely contribute to spasticity syndrome in humans (such as motoneuron hyperexcitability and spinal hyperreflexia). Methods used to study spasticity in preclinical models are expected to have a high translational value (e.g., electromyogram (EMG)-based electrophysiological tools) and can efficiently assist clinical development programs. However, validation of these models is not complete yet. First, true predictive validity of these models is not established as clinically efficacious drugs have been used to reverse validate preclinical models while newly discovered mechanisms effective in preclinical models are yet to be fully explored in humans (e.g., 5-HT2C receptor inverse agonists, fatty acid amid hydrolase inhibitors). Second, further efforts need to be invested into cross-laboratory validation of study protocols and tools, adherence to the highest quality standards (blinding, randomization, pre-specified study endpoints, etc.), and systematic efforts to replicate key sets of data. These appear to be readily achievable tasks that will enable development not only of symptomatic but also of disease-modifying therapy of spasticity, an area that seems to be currently not in focus of research efforts.

Exercise after spinal cord injury as an agent for neuroprotection, regeneration and rehabilitation

Spinal cord injury (SCI) is a traumatic event from which there is limited recovery of function, despite the best efforts of many investigators to devise realistic therapeutic treatments. Partly this is due to the multifaceted nature of SCI, where there is considerable disarray and dysfunction secondary to the initial injury. Contributing to this secondary degeneration is neurotoxicity, vascular dysfunction, glial scarring, neuroinflammation, apoptosis and demyelination. It seems logical that addressing the need for neuroprotection, regeneration and rehabilitation will require different treatment strategies that may be applied at varied stages of the post-injury response. Here we focus on a single strategy, exercise/physical training, which appears to have multiple applications and benefits for an acute or chronic SCI. Exercise has been demonstrated to be advantageous at cellular and biochemical levels, as well as being of benefit for the whole animal or human subject. Data from our lab and others will be discussed to further elucidate the many positive aspects of implementing exercise following injury and to suggest that rehabilitation is not the sole target of a training regimen following SCI. This article is part of a Special Issue entitled SI: Spinal cord injury.

Serotonin receptor and KCC2 gene expression in lumbar flexor and extensor motoneurons posttransection with and without passive cycling

Sacrocaudal motoneuron gene expression is altered following a spinal transection. Of interest here is the regulation of serotonin (5-HT) receptors (R), glutamate receptor, metabotropic 1 (mGluR1), and potassium-chloride cotransporter (KCC2), which mediate motoneuron excitability, locomotor recovery, and spasticity posttransection. The examination of these genes in lumbar motoneurons posttransection has not been studied, which is necessary for developing potential pharmacological interventions aimed at restoring locomotion and/or reducing spasticity. Also, if activity is to be used to promote recovery or reduce spasticity postinjury, a further examination of neuromuscular activity on gene expression posttransection is warranted. The purpose of this study was to examine motoneuronal gene expression of 5-HT receptors, KCC2, and mGluR1 at 3 mo following a complete thoracic spinal cord transection, with and without the inclusion of daily passive cycling. Physiological hindlimb extensor and flexor motoneurons were differentially identified with two retrograde fluorescent tracers, allowing for the identification and separate harvesting of extensor and flexor motoneurons with laser capture microdissection and the subsequent examination of mRNA content using quantitative RT-PCR analysis. We demonstrate that posttransection 5-HT1AR, 5-HT2CR, and mGluR1 expression was downregulated, whereas the 5-HT2AR was upregulated. These alterations in gene expression were observed in both flexor and extensor motoneurons, whereas passive cycling influenced gene expression in extensor but not flexor motoneurons. Passive cycling in extensor motoneurons further enhanced 5-HT2AR expression and increased 5-HT7R and KCC2 expression. Our results demonstrate that passive cycling influences serotonin receptor and KCC2 gene expression and that extensor motoneurons compared with flexor motoneurons may be more plastic to activity-based interventions posttransection.

Delaying the onset of treadmill exercise following peripheral nerve injury has different effects on axon regeneration and motoneuron synaptic plasticity

Transection of a peripheral nerve results in withdrawal of synapses from motoneurons. Some of the withdrawn synapses are restored spontaneously, but those containing the vesicular glutamate transporter 1 (VGLUT1), and arising mainly from primary afferent neurons, are withdrawn permanently. If animals are exercised immediately after nerve injury, regeneration of the damaged axons is enhanced and no withdrawal of synapses from injured motoneurons can be detected. We investigated whether delaying the onset of exercise until after synapse withdrawal had occurred would yield similar results. In Lewis rats, the right sciatic nerve was cut and repaired. Reinnervation of the soleus muscle was monitored until a direct muscle (M) response was observed to stimulation of the tibial nerve. At that time, rats began 2 wk of daily treadmill exercise using an interval training protocol. Both M responses and electrically-evoked H reflexes were monitored weekly for an additional seven wk. Contacts made by structures containing VGLUT1 or glutamic acid decarboxylase (GAD67) with motoneurons were studied from confocal images of retrogradely labeled cells. Timing of full muscle reinnervation was similar in both delayed and immediately exercised rats. H reflex amplitude in delayed exercised rats was only half that found in immediately exercised animals. Unlike immediately exercised animals, motoneuron contacts containing VGLUT1 in delayed exercised rats were reduced significantly, relative to intact rats. The therapeutic window for application of exercise as a treatment to promote restoration of synaptic inputs onto motoneurons following peripheral nerve injury is different from that for promoting axon regeneration in the periphery.