H reflex restitution and facilitation after different types of peripheral nerve injury and repair

A preliminary study exploring the effects of transcutaneous spinal cord stimulation on spinal excitability and phantom limb pain in people with a transtibial amputation

Objective. Phantom limb pain (PLP) is debilitating and affects over 70% of people with lower-limb amputation. Other neuropathic pain conditions correspond with increased spinal excitability, which can be measured using reflexes andF-waves. Spinal cord neuromodulation can be used to reduce neuropathic pain in a variety of conditions and may affect spinal excitability, but has not been extensively used for treating PLP. Here, we propose using a non-invasive neuromodulation method, transcutaneous spinal cord stimulation (tSCS), to reduce PLP and modulate spinal excitability after transtibial amputation.Approach. We recruited three participants, two males (5- and 9-years post-amputation, traumatic and alcohol-induced neuropathy) and one female (3 months post-amputation, diabetic neuropathy) for this 5 d study. We measured pain using the McGill Pain Questionnaire (MPQ), visual analog scale (VAS), and pain pressure threshold (PPT) test. We measured spinal reflex and motoneuron excitability using posterior root-muscle (PRM) reflexes andF-waves, respectively. We delivered tSCS for 30 min d-1for 5 d.Main Results. After 5 d of tSCS, MPQ scores decreased by clinically-meaningful amounts for all participants from 34.0 ± 7.0-18.3 ± 6.8; however, there were no clinically-significant decreases in VAS scores. Two participants had increased PPTs across the residual limb (Day 1: 5.4 ± 1.6 lbf; Day 5: 11.4 ± 1.0 lbf).F-waves had normal latencies but small amplitudes. PRM reflexes had high thresholds (59.5 ± 6.1μC) and low amplitudes, suggesting that in PLP, the spinal cord is hypoexcitable. After 5 d of tSCS, reflex thresholds decreased significantly (38.6 ± 12.2μC;p< 0.001).Significance. These preliminary results in this non-placebo-controlled study suggest that, overall, limb amputation and PLP may be associated with reduced spinal excitability and tSCS can increase spinal excitability and reduce PLP.

Massive Loss of Proprioceptive Ia Synapses in Rat Spinal Motoneurons after Nerve Crush Injuries in the Postnatal Period

Peripheral nerve injuries (PNIs) induce the retraction from the ventral horn of the synaptic collaterals of Ia afferents injured in the nerve, effectively removing Ia synapses from α-motoneurons. The loss of Ia input impairs functional recovery and could explain, in part, better recovery after PNIs with better Ia synaptic preservation. Synaptic losses correlate with injury severity, speed, and efficiency of muscle reinnervation and requires ventral microglia activation. It is unknown whether this plasticity is age dependent. In neonates, axotomized motoneurons and sensory neurons undergo apoptosis, but after postnatal day 10 most survive. The goal of this study was to analyze vesicular glutamate transporter 1 (VGluT1)-labeled Ia synapses (which also include II afferents) after nerve crush in 10 day old rats, a PNI causing little Ia/II synapse loss in adult rats. We confirmed fast and efficient reinnervation of leg muscles; however, a massive number of VGluT1/Ia/II synapses were permanently lost. This synapse loss was similar to that after more severe nerve injuries involving full transection in adults. In adults, disappearance of ventrally directed Ia/II collaterals targeting α-motoneurons was associated with a prolonged microglia reaction and a CCR2 mechanism that included infiltration of CCR2 blood immune cells. By contrast, microgliosis after P10 injuries was fast, resolved in about a week, and there was no evidence of peripheral immune cell infiltration. We conclude that VGluT1/Ia/II synapse loss in young animals differs in mechanism, perhaps associated with higher microglia synaptic pruning activity at this age and results in larger losses after milder nerve injuries.

3D-printed nerve guidance conduits multi-functionalized with canine multipotent mesenchymal stromal cells promote neuroregeneration after sciatic nerve injury in rats

Background

Nerve injuries are debilitating, leading to long-term motor deficits. Remyelination and axonal growth are supported and enhanced by growth factor and cytokines. Combination of nerve guidance conduits (NGCs) with adipose-tissue-derived multipotent mesenchymal stromal cells (AdMSCs) has been performing promising strategy for nerve regeneration.

Methods

3D-printed polycaprolactone (PCL)-NGCs were fabricated. Wistar rats subjected to critical sciatic nerve damage (12-mm gap) were divided into sham, autograft, PCL (empty NGC), and PCL + MSCs (NGC multi-functionalized with 10⁶ canine AdMSCs embedded in heterologous fibrin biopolymer) groups. In vitro, the cells were characterized and directly stimulated with interferon-gamma to evaluate their neuroregeneration potential. In vivo, the sciatic and tibial functional indices were evaluated for 12 weeks. Gait analysis and nerve conduction velocity were analyzed after 8 and 12 weeks. Morphometric analysis was performed after 8 and 12 weeks following lesion development. Real-time PCR was performed to evaluate the neurotrophic factors BDNF, GDNF, and HGF, and the cytokine and IL-10. Immunohistochemical analysis for the p75^NTR neurotrophic receptor, S100, and neurofilament was performed with the sciatic nerve.

Results

The inflammatory environment in vitro have increased the expression of neurotrophins BDNF, GDNF, HGF, and IL-10 in canine AdMSCs. Nerve guidance conduits multi-functionalized with canine AdMSCs embedded in HFB improved functional motor and electrophysiological recovery compared with PCL group after 12 weeks. However, the results were not significantly different than those obtained using autografts. These findings were associated with a shift in the regeneration process towards the formation of myelinated fibers. Increased immunostaining of BDNF, GDNF, and growth factor receptor p75^NTR was associated with the upregulation of BDNF, GDNF, and HGF in the spinal cord of the PCL + MSCs group. A trend demonstrating higher reactivity of Schwann cells and axonal branching in the sciatic nerve was observed, and canine AdMSCs were engrafted at 30 days following repair.

Conclusions

3D-printed NGCs multi-functionalized with canine AdMSCs embedded in heterologous fibrin biopolymer as cell scaffold exerted neuroregenerative effects. Our multimodal approach supports the trophic microenvironment, resulting in a pro-regenerative state after critical sciatic nerve injury in rats.

TrkA inhibitor promotes motor functional regeneration of recurrent laryngeal nerve by suppression of sensory nerve regeneration

Recurrent laryngeal nerve (RLN) injury, in which hoarseness and dysphagia arise as a result of impaired vocal fold movement, is a serious complication. Misdirected regeneration is an issue for functional regeneration. In this study, we demonstrated the effect of TrkA inhibitors, which blocks the NGF-TrkA pathway that acts on the sensory/automatic nerves thus preventing misdirected regeneration among motor and sensory nerves, and thereby promoting the regeneration of motor neurons to achieve functional recovery. RLN axotomy rat models were used in this study, in which cut ends of the nerve were bridged with polyglycolic acid-collagen tube with and without TrkA inhibitor (TrkAi) infiltration. Our study revealed significant improvement in motor nerve fiber regeneration and function, in assessment of vocal fold movement, myelinated nerve regeneration, compound muscle action potential, and prevention of laryngeal muscle atrophy. Retrograde labeling demonstrated fewer labeled neurons in the vagus ganglion, which confirmed reduced misdirected regeneration among motor and sensory fibers, and a change in distribution of the labeled neurons in the nucleus ambiguus. Our study demonstrated that TrkAi have a strong potential for clinical application in the treatment of RLN injury.

Methods for in vivo studies in rodents of chemotherapy induced peripheral neuropathy

Peripheral neuropathy is one of the most common, dose limiting, and long-lasting disabling adverse events of chemotherapy treatment. Unfortunately, no treatment has proven efficacy to prevent this adverse effect in patients or improve the nerve regeneration, once it is established. Experimental models, particularly using rats and mice, are useful to investigate the mechanisms related to axonal or neuronal degeneration and target loss of function induced by neurotoxic drugs, as well as to test new strategies to prevent the development of neuropathy and to improve functional restitution. Therefore, objective and reliable methods should be applied for the assessment of function and innervation in adequately designed in vivo studies of CIPN, taking into account the impact of age, sex and species/strains features. This review gives an overview of the most useful methods to assess sensory, motor and autonomic functions, electrophysiological and morphological tests in rodent models of peripheral neuropathy, focused on CIPN. We include as well a proposal of protocols that may improve the quality and comparability of studies undertaken in different laboratories. It is recommended to apply more than one functional method for each type of function, and to perform parallel morphological studies in the same targets and models.

Peripheral nerve injury induced changes in the spinal cord and strategies to counteract/enhance the changes to promote nerve regeneration

Peripheral nerve injury leads to morphological, molecular and gene expression changes in the spinal cord and dorsal root ganglia, some of which have positive impact on the survival of neurons and nerve regeneration, while the effect of others is the opposite. It is crucial to take prompt measures to capitalize on the positive effects of these reactions and counteract the negative impact after peripheral nerve injury at the level of spinal cord, especially for peripheral nerve injuries that are severe, located close to the cell body, involve long distance for axons to regrow and happen in immature individuals. Early nerve repair, exogenous supply of neurotrophic factors and Schwann cells can sustain the regeneration inductive environment and enhance the positive changes in neurons. Administration of neurotrophic factors, acetyl-L-carnitine, N-acetyl-cysteine, and N-methyl-D-aspartate receptor antagonist MK-801 can help counteract axotomy-induced neuronal loss and promote regeneration, which are all time-dependent. Sustaining and reactivation of Schwann cells after denervation provides another effective strategy. FK506 can be used to accelerate axonal regeneration of neurons, especially after chronic axotomy. Exploring the axotomy-induced changes after peripheral nerve injury and applying protective and promotional measures in the spinal cord which help to retain a positive functional status for neuron cell bodies will inevitably benefit regeneration of the peripheral nerve and improve functional outcomes.

Effects of Repeated 20-Hz Electrical Stimulation on Functional Recovery Following Peripheral Nerve Injury

One hour of 20-Hz continuous electrical stimulation (ES) applied at the time of injury promotes the regeneration of axons in cut peripheral nerves. A more robust enhancement of peripheral axon regeneration is achieved by 2 weeks of daily treadmill exercise. We investigated whether repeated applications of brief ES (mES) would be more effective in promoting regeneration than a single application. Sciatic nerves of C57B6 mice were cut and repaired by end-to-end anastomosis. At that time and every third day for 2 weeks, the repaired nerve was stimulated for 1 hour at 20 Hz. In controls, injured mice were either untreated or treated with ES only once. Direct muscle responses recorded from reinnervated muscles in awake animals were observed earlier both in mice treated with ES and mES than untreated controls. Their amplitudes increased progressively over the post transection study period, but the rate of this progression was increased significantly only in animals treated once with ES. Monosynaptic H reflexes recovered to pretransection levels in both untreated and singly treated mice but in the animals treated repeatedly, they were maintained at more than twice that of the same reflexes recorded prior to injury. In anatomical analyses, both excitatory and inhibitory synaptic contacts with the cell bodies of injured motoneurons, including those expressing the vesicular glutamate transporter 1 (VGLUT1), were sustained in mice treated repeatedly but not in singly treated or untreated mice. Repeated ES does not enhance the rate of restoration of functional muscle reinnervation and results in the retention of exaggerated reflexes.

Role of Noradrenergic Inputs From Locus Coeruleus on Changes Induced on Axotomized Motoneurons by Physical Exercise

Physical rehabilitation is one of the cornerstones for the treatment of lesions of the nervous system. After peripheral nerve injuries, activity dependent therapies promote trophic support for the paralyzed muscles, enhance axonal growth and also modulate the maladaptive plastic changes induced by the injury at the spinal level. We have previously demonstrated that an intensive protocol of treadmill running (TR) in rats reduces synaptic stripping on axotomized motoneurons, preserves their perineuronal nets (PNN) and attenuates microglia reactivity. However, it is not clear through which mechanisms exercise is exerting these effects. Here we aimed to evaluate if activation of the locus coeruleus (LC), the noradrenergic center in the brain stem, plays a role in these effects. Since LC is strongly activated during stressful situations, as during intensive exercise, we selectively destroyed the LC by administering the neurotoxin DPS-4 before injuring the sciatic nerve of adult rats. Animals without LC had increased microglia reactivity around injured motoneurons. In these animals, an increasing intensity protocol of TR was not able to prevent synaptic stripping on axotomized motoneurons and the reduction in the thickness of their PNN. In contrast, TR was still able to attenuate microglia reactivity in DSP-4 treated animals, thus indicating that the noradrenergic projections are important for some but not all the effects that exercise induces on the spinal cord after peripheral nerve injury. Moreover, animals subjected to treadmill training showed delayed muscle reinnervation, more evident if treated with DSP-4. However, we did not find differences in treated animals regarding the H/M amplitude ratio, which increased during the first stages of regeneration in all injured groups.

Reconstruction of a 10-mm-long median nerve gap in an ischemic environment using autologous conduits with different patterns of blood supply: A comparative study in the rat

The aim of this study was to evaluate in the Wistar rat the efficacy of various autologous nerve conduits with various forms of blood supply in reconstructing a 10-mm-long gap in the median nerve (MN) under conditions of local ischemia. A 10-mm-long median nerve defect was created in the right arm. A loose silicone tube was placed around the nerve gap zone, in order to simulate a local ischemic environment. Rats were divided in the following experimental groups (each with 20 rats): the nerve Graft (NG) group, in which the excised MN segment was reattached; the conventional nerve flap (CNF) and the arterialized neurovenous flap (ANVF) groups in which the gap was bridged with homonymous median nerve flaps; the prefabricated nerve flap (PNF) group in which the gap was reconstructed with a fabricated flap created by leaving an arteriovenous fistula in contact with the sciatic nerve for 5 weeks; and the two control groups, Sham and Excision groups. In the latter group, the proximal stump of the MN nerve was ligated and no repair was performed. The rats were followed for 100 days. During this time, they did physiotherapy. Functional, electroneuromyographic and histological studies were performed. The CNF and ANVF groups presented better results than the NG group in the following assessments: grasping test, nociception, motor stimulation threshold, muscle weight, and histomorphometric evaluation. Radial deviation of the operated forepaw was more common in rats that presented worse results in the other outcome variables. Overall, CNFs and ANVFs produced a faster and more complete recovery than NGs in the reconstruction of a 10-mm-long median nerve gap in an ischemic environment in the Wistar rat. Although, results obtained with CNFs were in most cases were better than ANVFs, these differences were not statistically significant for most of the outcome variables.

Botulinum neurotoxin A promotes functional recovery after peripheral nerve injury by increasing regeneration of myelinated fibers

The injection of safe doses of botulinum neurotoxin A (BoNT/A) have been reported to be useful for the treatment of neuropathic pain, but it is still unknown how functional recovery is induced after peripheral nerve injury. We evaluated the effects of intranerve application of BoNT/A, on regeneration and sensorimotor functional recovery in partial and complete peripheral nerve injuries in the mouse. After sciatic nerve crush (SNC) and intranerve delivery of BoNT/A (15pg), axonal regeneration was measured by nerve pinch test at different days. Regeneration of myelinated and unmyelinated fibers was assessed by immunohistochemical double labeling for NF200/GAP43 and CGRP/GAP43. S100 was used as Schwann cells marker. Medial footpad skin reinnervation was assessed by PGP staining. Motor functions were assessed by means of nerve conduction tests. In other mice groups, nerve conduction tests were performed also after chronic constriction injury (CCI) of the sciatic nerve and intraplantar injection of BoNT/A (15pg). In SNC mice, BoNT/A increased the rate of axonal regeneration. The advantage of regrowing myelinated axons after BoNT/A injection was evidenced by longer NF200+ nerve profiles and confirmed by nerve histology. We observed also a higher expression of S100 in the distal portion of BoNT/A-injected regenerated nerves. In CCI mice, BoNT/A induced an increase in reinnervation of gastrocnemius and plantar muscles. These results show that a low dose of BoNT/A, insufficient to produce muscular dysfunction, conversely speeds up sensorimotor recovery by stimulating myelinated axonal regeneration, and points out its application as a multipotent treatment for peripheral neuropathies.

Effects of Treatment of Treadmill Combined with Electro-Acupuncture on Tibia Bone Mass and Substance PExpression of Rabbits with Sciatic Nerve Injury

The peripheral nervous system may play an important role in normal bone maintenance and remodeling. Substance P (SP) is a neuropeptide associated with bone loss and formation that may mediate the effects of the nervous system. The purpose of this study is to determine if treadmill running combined with electro-acupuncture at Jiaji acupoints (Jiaji-EA) affects tibial bone mass and SP expression in rabbits with sciatic nerve injury. Twenty-four juvenile male New Zealand white rabbits were randomly assigned to one of 4 groups: sham injury control (sham), sciatic never crush control (SNCr), treadmill running (treadmill), and Jiaji-EA combined with treadmill running (ET group). The SNCr, treadmill, and ET groups all had an induced sciatic never crush injury of approximately 2mm. Control groups received no intervention; the treadmill and ET groups were trained by treadmill; the ET group also received Jiaji-EA. After the 4 weeks of treatment, toe-spreading index (TSI), BMD, bone strength, and SP expression in the tibia were significantly lower in the nerve injury groups (SNCr, treadmill, and ET) compared to the sham groups (p<0.05). Treatment (treadmill and ET groups) increased all measures compared to the SNCr group (p<0.05). Further, TSI, BMD, bone strength, and SP expression in the ET group were higher than the treadmill group (p<0.05). Our results indicate that treadmill therapy combined with electro-acupuncture at Jiaji acupoints prevents bone loss in rabbit tibias after sciatic nerve injury. This may occur in two ways: indirectly in association with axon regeneration and directly via loading on the bone mediated through increased SP expression. This study provides important evidence for the clinical treatment of bone loss after peripheral nerve injury.

Comparative outcome measures in peripheral regeneration studies

Traumatic peripheral nerve injuries are common and often result in partial or permanent paralysis, numbness of the affected limb, and debilitating neuropathic pain. Experimental animal models of nerve injury have utilized a diversity of outcome measures to examine functional recovery following injury. Four primary categories of outcome measures of regenerative success including retrograde labeling with counts of regenerating neurons, immunohistochemistry and histomorphometry, reinnervation of target muscles, and behavioral analysis of recovery will be reviewed. Validity of different outcome measures are discussed in context of hindlimb, forelimb, and facial nerve injury models. Severity of nerve injury will be highlighted, and comparisons between nerve crush injury and more severe transection and neuroma-in-continuity nerve injury paradigms will be evaluated. The case is made that specific outcome measures may be more sensitive to assessing functional recovery following nerve injury than others. This will be discussed in the context of the lack of association between certain outcome measures of nerve regeneration. Examples of inaccurate conclusions from specific outcome measures will also be considered. Overall, researchers must therefore take care to select appropriate outcome measures for animal nerve injury studies dependant on the specific experimental interventions and scientific questions addressed.

Peripheral Nerve Regeneration through Hydrogel-Enriched Chitosan Conduits Containing Engineered Schwann Cells for Drug Delivery

Critical length nerve defects in the rat sciatic nerve model were reconstructed with chitosan nerve guides filled with Schwann cells (SCs) containing hydrogel. The transplanted SCs were naive or had been genetically modified to overexpress neurotrophic factors, thus providing a cellular neurotrophic factor delivery system. Prior to the assessment in vivo, in vitro studies evaluating the properties of engineered SCs overexpressing glial cell line-derived neurotrophic factor (GDNF) or fibroblast growth factor 2 (FGF-218kDa) demonstrated their neurite outgrowth inductive bioactivity for sympathetic PC-12 cells as well as for dissociated dorsal root ganglion cell drop cultures. SCs within NVR-hydrogel, which is mainly composed of hyaluronic acid and laminin, were delivered into the lumen of chitosan hollow conduits with a 5% degree of acetylation. The viability and neurotrophic factor production by engineered SCs within NVR-Gel inside the chitosan nerve guides was further demonstrated in vitro. In vivo we studied the outcome of peripheral nerve regeneration after reconstruction of 15-mm nerve gaps with either chitosan/NVR-Gel/SCs composite nerve guides or autologous nerve grafts (ANGs). While ANGs did guarantee for functional sensory and motor regeneration in 100% of the animals, delivery of NVR-Gel into the chitosan nerve guides obviously impaired sufficient axonal outgrowth. This obstacle was overcome to a remarkable extent when the NVR-Gel was enriched with FGF-218kDa overexpressing SCs.

Functional evaluation of peripheral nerve regeneration and target reinnervation in animal models: a critical overview

Peripheral nerve injuries usually lead to severe loss of motor, sensory and autonomic functions in the patients. Due to the complex requirements for adequate axonal regeneration, functional recovery is often poorly achieved. Experimental models are useful to investigate the mechanisms related to axonal regeneration and tissue reinnervation, and to test new therapeutic strategies to improve functional recovery. Therefore, objective and reliable evaluation methods should be applied for the assessment of regeneration and function restitution after nerve injury in animal models. This review gives an overview of the most useful methods to assess nerve regeneration, target reinnervation and recovery of complex sensory and motor functions, their values and limitations. The selection of methods has to be adequate to the main objective of the research study, either enhancement of axonal regeneration, improving regeneration and reinnervation of target organs by different types of nerve fibres, or increasing recovery of complex sensory and motor functions. It is generally recommended to use more than one functional method for each purpose, and also to perform morphological studies of the injured nerve and the reinnervated targets.

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.

Functional recordings from awake, behaving rodents through a microchannel based regenerative neural interface

OBJECTIVE

Neural interface technologies could provide controlling connections between the nervous system and external technologies, such as limb prosthetics. The recording of efferent, motor potentials is a critical requirement for a peripheral neural interface, as these signals represent the user-generated neural output intended to drive external devices. Our objective was to evaluate structural and functional neural regeneration through a microchannel neural interface and to characterize potentials recorded from electrodes placed within the microchannels in awake and behaving animals.

APPROACH

Female rats were implanted with muscle EMG electrodes and, following unilateral sciatic nerve transection, the cut nerve was repaired either across a microchannel neural interface or with end-to-end surgical repair. During a 13 week recovery period, direct muscle responses to nerve stimulation proximal to the transection were monitored weekly. In two rats repaired with the neural interface, four wire electrodes were embedded in the microchannels and recordings were obtained within microchannels during proximal stimulation experiments and treadmill locomotion.

MAIN RESULTS

In these proof-of-principle experiments, we found that axons from cut nerves were capable of functional reinnervation of distal muscle targets, whether regenerating through a microchannel device or after direct end-to-end repair. Discrete stimulation-evoked and volitional potentials were recorded within interface microchannels in a small group of awake and behaving animals and their firing patterns correlated directly with intramuscular recordings during locomotion. Of 38 potentials extracted, 19 were identified as motor axons reinnervating tibialis anterior or soleus muscles using spike triggered averaging.

SIGNIFICANCE

These results are evidence for motor axon regeneration through microchannels and are the first report of in vivo recordings from regenerated motor axons within microchannels in a small group of awake and behaving animals. These unique findings provide preliminary evidence that efferent, volitional motor potentials can be recorded from the microchannel-based peripheral neural interface; a critical requirement for any neural interface intended to facilitate direct neural control of external technologies.

Presymptomatic electrophysiological tests predict clinical onset and survival in SOD1(G93A) ALS mice

INTRODUCTION

We assessed the predictive value of electrophysiological tests as a marker of clinical disease onset and survival in superoxide-dismutase 1 (SOD1)(G93A) mice.

METHODS

We evaluated the accuracy of electrophysiological tests in differentiating transgenic versus wild-type mice. We made a correlation analysis of electrophysiological parameters and the onset of symptoms, survival, and number of spinal motoneurons.

RESULTS

Presymptomatic electrophysiological tests show great accuracy in differentiating transgenic versus wild-type mice, with the most sensitive parameter being the tibialis anterior compound muscle action potential (CMAP) amplitude. The CMAP amplitude at age 10 weeks correlated significantly with clinical disease onset and survival. Electrophysiological tests increased their survival prediction accuracy when evaluated at later stages of the disease and also predicted the amount of lumbar spinal motoneuron preservation.

CONCLUSIONS

Electrophysiological tests predict clinical disease onset, survival, and spinal motoneuron preservation in SOD1(G93A) mice. This is a methodological improvement for preclinical studies.

Activity dependent therapies modulate the spinal changes that motoneurons suffer after a peripheral nerve injury

Injury of a peripheral nerve not only leads to target denervation, but also induces massive stripping of spinal synapses on axotomized motoneurons, with disruption of spinal circuits. Even when regeneration is successful, unspecific reinnervation and the limited reconnection of the spinal circuits impair functional recovery. The aim of this study was to describe the changes that axotomized motoneurons suffer after peripheral nerve injury and how activity-dependent therapies and neurotrophic factors can modulate these events. We observed a marked decrease in glutamatergic synapses, with a maximum peak at two weeks post-axotomy, which was only partially reversed with time. This decrease was accompanied by an increase in gephyrin immunoreactivity and a disintegration of perineuronal nets (PNNs) surrounding the motoneurons. Direct application of neurotrophins at the proximal stump was not able to reverse these effects. In contrast, activity-dependent treatment, in the form of treadmill running, reduced the observed destructuring of perineuronal nets and the loss of glutamatergic synapses two weeks after injury. These changes were proportional to the intensity of the exercise protocol. Blockade of sensory inputs from the homolateral hindlimb also reduced PNN immunoreactivity around intact motoneurons, and in that case treadmill running did not reverse that loss, suggesting that the effects of exercise on motoneuron PNN depend on increased sensory activity. Preservation of motoneuron PNN and reduction of synaptic stripping by exercise could facilitate the maintenance of the spinal circuitry and benefit functional recovery after peripheral nerve injury.

Normal distribution of VGLUT1 synapses on spinal motoneuron dendrites and their reorganization after nerve injury

Peripheral nerve injury induces permanent alterations in spinal cord circuitries that are not reversed by regeneration. Nerve injury provokes the loss of many proprioceptive IA afferent synapses (VGLUT1-IR boutons) from motoneurons, the reduction of IA EPSPs in motoneurons, and the disappearance of stretch reflexes. After motor and sensory axons successfully reinnervate muscle, lost IA VGLUT1 synapses are not re-established and the stretch reflex does not recover; however, electrically evoked EPSPs do recover. The reasons why remaining IA synapses can evoke EPSPs on motoneurons, but fail to transmit useful stretch signals are unknown. To better understand changes in the organization of VGLUT1 IA synapses that might influence their input strength, we analyzed their distribution over the entire dendritic arbor of motoneurons before and after nerve injury. Adult rats underwent complete tibial nerve transection followed by microsurgical reattachment and 1 year later motoneurons were intracellularly recorded and filled with neurobiotin to map the distribution of VGLUT1 synapses along their dendrites. We found in control motoneurons an average of 911 VGLUT1 synapses; ~62% of them were lost after injury. In controls, VGLUT1 synapses were focused to proximal dendrites where they were grouped in tight clusters. After injury, most synaptic loses occurred in the proximal dendrites and remaining synapses were declustered, smaller, and uniformly distributed throughout the dendritic arbor. We conclude that this loss and reorganization renders IA afferent synapses incompetent for efficient motoneuron synaptic depolarization in response to natural stretch, while still capable of eliciting EPSPs when synchronously fired by electrical volleys.

Resveratrol improves motoneuron function and extends survival in SOD1(G93A) ALS mice

Amyotrophic lateral sclerosis (ALS) is an adult onset neurodegenerative disease that causes progressive paralysis and death due to degeneration of motoneurons in spinal cord, brainstem and motor cortex. Nowadays, there is no effective therapy and patients die 2-5 years after diagnosis. Resveratrol (trans-3,4',5-trihydroxystilbene) is a natural polyphenol found in grapes, with promising neuroprotective effects since it induces expression and activation of several neuroprotective pathways involving Sirtuin1 and AMPK. The objective of this work was to assess the effect of resveratrol administration on SOD1(G93A) ALS mice. We determined the onset of symptoms by rotarod test and evaluated upper and lower motoneuron function using electrophysiological tests. We assessed the survival of the animals and determined the number of spinal motoneurons. Finally, we further investigated resveratrol mechanism of action by means of western blot and immunohistochemical analysis. Resveratrol treatment from 8 weeks of age significantly delayed disease onset and preserved lower and upper motoneuron function in female and male animals. Moreover, resveratrol significantly extended SOD1(G93A) mice lifespan and promoted survival of spinal motoneurons. Delayed resveratrol administration from 12 weeks of age also improved spinal motoneuron function preservation and survival. Further experiments revealed that resveratrol protective effects were associated with increased expression and activation of Sirtuin 1 and AMPK in the ventral spinal cord. Both mediators promoted normalization of the autophagic flux and, more importantly, increased mitochondrial biogenesis in the SOD1(G93A) spinal cord. Taken together, our findings suggest that resveratrol may represent a promising therapy for ALS.

Differential effects on KCC2 expression and spasticity of ALS and traumatic injuries to motoneurons

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease manifested by progressive muscle atrophy and paralysis due to the loss of upper and lower motoneurons (MN). Spasticity appears in ALS patients leading to further disabling consequences. Loss of the inhibitory tone induced by downregulation of the potassium chloride cotransporter 2 (KCC2) in MN has been proposed to importantly contribute to the spastic behavior after spinal cord injury (SCI). The aim of the present study was to test whether the alterations in the expression of KCC2 are linked to the appearance of spasticity in the SOD^G93A ALS murine model. We compared SOD^G93A mice to wild type mice subjected to SCI to mimic the spinal MN disconnection from motor descending pathways, and to sciatic nerve lesion to mimic the loss of MN connectivity to muscle. Electrophysiological results show that loss of motor function is observed at presymptomatic stage (8 weeks) in SOD^G93A mice but hyperreflexia and spasticity do not appear until a late stage (16 weeks). However, KCC2 was not downregulated despite MN suffered disconnection both from muscles and upper MNs. Further experiments revealed decreased gephyrin expression, as a general marker of inhibitory systems, accompanied by a reduction in the number of Renshaw interneurons. Moreover, 5-HT fibers were increased in the ventral horn of the lumbar spinal cord at late stage of disease progression in SOD1^G93A mice. Taken together, the present results indicate that spasticity appears late in the ALS model, and may be mediated by a decrease in inhibitory interneurons and an increase of 5-HT transmission, while the absence of down-regulation of KCC2 could rather indicate an inability of MNs to respond to insults.

Biofabrication and testing of a fully cellular nerve graft

Rupture of a nerve is a debilitating injury with devastating consequences for the individual's quality of life. The gold standard of repair is the use of an autologous graft to bridge the severed nerve ends. Such repair however involves risks due to secondary surgery at the donor site and may result in morbidity and infection. Thus the clinical approach to repair often involves non-cellular solutions, grafts composed of synthetic or natural materials. Here we report on a novel approach to biofabricate fully biological grafts composed exclusively of cells and cell secreted material. To reproducibly and reliably build such grafts of composite geometry we use bioprinting. We test our grafts in a rat sciatic nerve injury model for both motor and sensory function. In particular we compare the regenerative capacity of the biofabricated grafts with that of autologous grafts and grafts made of hollow collagen tubes by measuring the compound action potential (for motor function) and the change in mean arterial blood pressure as consequence of electrically eliciting the somatic pressor reflex. Our results provide evidence that bioprinting is a promising approach to nerve graft fabrication and as a consequence to nerve regeneration.

C3 exoenzyme lacks effects on peripheral axon regeneration in vivo

Peripheral nerve injury triggers the activation of the small GTPase RhoA in spinal motor and peripheral sensory neurons. C3 transferase, an exoenzyme produced by Clostridium botulinum that inactivates RhoA by ADP-ribosylation, has been successfully applied in central nervous system (CNS) lesion models to facilitate regeneration functionally and morphologically. Until now it has not been demonstrated if C3bot exerts positive effects on peripheral axon regeneration as well. In organotypic spinal cord preparations, C3bot reduced axonal growth of motoneurons, while no effect on sensory axon outgrowth from dorsal root ganglia (DRG) explants was observed. Enzymatically inactive C3E174Q was ineffective in both culture models. Spinal cord slices exhibited a significant increase in microglia/macrophages after treatment with C3bot suggesting an inflammatory component in the inhibition of axon growth. C3bot or C3E174Q were then applied into conduits implanted after transection of the sciatic nerve in rats. Functional evaluation by electrophysiology, nociception, and walking track tests did not show any significant difference between groups with active or mutant C3E174Q . Transmission electron microscopy of the regenerated nerves revealed no significant differences in the number of myelinated and unmyelinated axons 6 weeks after surgery. Compared to the CNS, the functional significance of RhoA may be limited during nerve regeneration in a growth-promoting environment.

Peripheral nerve alterations after spinal cord injury in the adult rat

OBJECTIVE

To assess if spinal cord injury (SCI) can produce alterations in axons of peripheral nerves emerging caudal to the injury.

METHODS

Mild/severe contusion or complete transection was performed at T8 in adult rats. The function and morphology of the sciatic nerve were assessed 3 months after the lesion.

RESULTS

There was a decrease in the amplitudes of muscle responses in nerve conduction tests. The number of myelinated fibers was maintained, but some of them presented structural abnormalities.

CONCLUSION

SCIs cause alterations in peripheral axons not affected by the injury. Preservation of the peripheral components is essential for potential regenerative and rehabilitation therapies. Thus, special care has to be taken to avoid secondary complications, due to compressions or immobility, in SCI humans.

Implications of olfactory lamina propria transplantation on hyperreflexia and myelinated fiber regeneration in rats with complete spinal cord transection

Transplantation with olfactory ensheathing cells (OECs) has been adopted after several models of spinal cord injury (SCI) with the purpose of creating a favorable environment for the re-growth of injured axons. However, a consensus on the efficacy of this cellular transplantation has yet to be reached. In order to explore alternative parameters that could demonstrate the possible restorative properties of such grafts, the present study investigated the effects of olfactory lamina propria (OLP) transplantation on hyperreflexia and myelinated fiber regeneration in adult rats with complete spinal cord transection. The efficacy of OLP (graft containing OECs) and respiratory lamina propria (RLP, graft without OECs) was tested at different post-injury times (acutely, 2- and 4-week delayed), to establish the optimum period for transplantation. In the therapeutic windows used, OLP and RLP grafts produced no considerable improvements in withdrawal reflex responses or on the low-frequency dependent depression of H-reflex. Both lamina propria grafts produced comparable results for the myelinated fiber density and for the estimated total number of myelinated fibers at the lesion site, indicating that the delayed transplantation approach does not seem to limit the regenerative effects. However, animals transplanted with OLP 2 or 4 weeks after injury exhibit smaller myelin sheath thickness and myelinated fiber area and diameter at the lesion site compared to their respective RLP groups. Despite the ongoing clinical use of OECs, it is important to emphasize the need for more experimental studies to clarify the exact nature of the repair capacity of these grafts in the treatment of SCI.

Sigma-1R agonist improves motor function and motoneuron survival in ALS mice

Amyotrophic lateral sclerosis is a neurodegenerative disorder characterized by progressive weakness, muscle atrophy, and paralysis due to the loss of upper and lower motoneurons (MNs). Sigma-1 receptor (sigma-1R) activation promotes neuroprotection after ischemic and traumatic injuries to the central nervous system. We recently reported that sigma-1R agonist (PRE-084) improves the survival of MNs after root avulsion injury in rats. Moreover, a mutation of the sigma-1R leading to frontotemporal lobar degeneration/amyotrophic lateral sclerosis (ALS) was recently described in human patients. In the present study, we analyzed the potential therapeutic effect of the sigma-1R agonist (PRE-084) in the SOD1(G93A) mouse model of ALS. Mice were daily administered with PRE-084 (0.25 mg/kg) from 8 to 16 weeks of age. Functional outcome was assessed by electrophysiological tests and computerized analysis of locomotion. Histological, immunohistochemical analyses and Western blot of the spinal cord were performed. PRE-084 administration from 8 weeks of age improved the function of MNs, which was manifested by maintenance of the amplitude of muscle action potentials and locomotor behavior, and preserved neuromuscular connections and MNs in the spinal cord. Moreover, it extended survival in both female and male mice by more than 15 %. Delayed administration of PRE-084 from 12 weeks of age also significantly improved functional outcome and preservation of the MNs. There was an induction of protein kinase C-specific phosphorylation of the NR1 subunit of the N-methyl-D-aspartate (NMDA) receptor in SOD1(G93A) animals, and a reduction of the microglial reactivity compared with untreated mice. PRE-084 exerts a dual therapeutic contribution, modulating NMDA Ca(2+) influx to protect MNs, and the microglial reactivity to ameliorate the MN environment. In conclusion, sigma-1R agonists, such as PRE-084, may be promising candidates for a therapeutical strategy of ALS.

Glycomimetic improves recovery after femoral injury in a non-human primate

In adult mammals, restoration of function after peripheral nerve injury is often poor and effective therapies are not available. Previously we have shown in mice that a peptide which functionally mimics the human natural killer cell (HNK)-1 trisaccharide epitope significantly improves the outcome of femoral nerve injury. Here we evaluated the translational potential of this treatment using primates. We applied a linear HNK-1 mimetic or a functionally inactive control peptide in silicone cuffs used to reconstruct the cut femoral nerves of adult cynomolgus monkeys (Macaca fascicularis). Functional recovery was evaluated using video-based gait analysis over a 160-day observation period. The final outcome was further assessed using force measurements, H-reflex recordings, nerve histology, and ELISA to assess immunoreactivity to HNK-1 in the treated monkeys. Gait deficits were significantly reduced in HNK-1 mimetic-treated compared with control peptide-treated animals between 60 and 160 days after injury. Better outcome at 160 days after surgery in treated versus control animals was also confirmed by improved quadriceps muscle force, enhanced H-reflex amplitude, decreased H-reflex latency, and larger diameters of regenerated axons. No adverse reactions to the mimetic, in particular immune responses resulting in antibodies against the HNK-1 mimetic or immune cell infiltration into the damaged nerve, were observed. These results indicate the potential of the HNK-1 mimetic as an efficient, feasible, and safe adjunct treatment for nerve injuries requiring surgical repair in clinical settings.

Effect of genetic background on onset and disease progression in the SOD1-G93A model of amyotrophic lateral sclerosis

Knowledge of the potential effect of genetic background in disease models is important. The SOD1-G93A transgenic mouse is the most widely used model in amyotrophic lateral sclerosis (ALS). Since these animals show considerable variability both in the onset and the progression of the disease, this study aimed to characterize the potential differences between the two most widely used strains, C56BL/6 (B6) and B6SJL. A rotarod test was carried out to assess strength and motor coordination, while electrophysiology tests were performed to evaluate the function of upper and lower motor neurons. Survival of the animals and motor neuron loss were also studied. The results did not show any background effect regarding the rotarod test, despite the differences in the pattern of decline in central and peripheral motor conduction. The onset of motor neuron abnormalities was later in B6SJL mice, but progressed more rapidly. Lifespan was longer for B6 than for B6SJL animals. In conclusion, background differences in disease onset and progression are important. The characteristics of the strain should be taken into account in experimental design of therapeutic studies.

Permanent central synaptic disconnection of proprioceptors after nerve injury and regeneration. II. Loss of functional connectivity with motoneurons

Regeneration of a cut muscle nerve fails to restore the stretch reflex, and the companion paper to this article [Alvarez FJ, Titus-Mitchell HE, Bullinger KL, Kraszpulski M, Nardelli P, Cope TC. J Neurophysiol (August 10, 2011). doi:10.1152/jn.01095.2010] suggests an important central contribution from substantial and persistent disassembly of synapses between regenerated primary afferents and motoneurons. In the present study we tested for physiological correlates of synaptic disruption. Anesthetized adult rats were studied 6 mo or more after a muscle nerve was severed and surgically rejoined. We recorded action potentials (spikes) from individual muscle afferents classified as IA like (*IA) by several criteria and tested for their capacity to produce excitatory postsynaptic potentials (EPSPs) in homonymous motoneurons, using spike-triggered averaging (STA). Nearly every paired recording from a *IA afferent and homonymous motoneuron (93%) produced a STA EPSP in normal rats, but that percentage was only 17% in rats with regenerated nerves. In addition, the number of motoneurons that produced aggregate excitatory stretch synaptic potentials (eSSPs) in response to stretch of the reinnervated muscle was reduced from 100% normally to 60% after nerve regeneration. The decline in functional connectivity was not attributable to synaptic depression, which returned to its normally low level after regeneration. From these findings and those in the companion paper, we put forward a model in which synaptic excitation of motoneurons by muscle stretch is reduced not only by misguided axon regeneration that reconnects afferents to the wrong receptor type but also by retraction of synapses with motoneurons by spindle afferents that successfully reconnect with spindle receptors in the periphery.

Recovery of proprioceptive feedback from nerve crush

Sensorimotor functions are restored by peripheral nerve regeneration with greater success following injuries that crush rather than sever the nerve. Better recovery following nerve crush is commonly attributed to superior reconnection of regenerating axons with their original peripheral targets. The present study was designed to estimate the fraction of stretch reflex recovery attributable to functional recovery of regenerated spindle afferents. Recovery of the spindle afferent population was estimated from excitatory postsynaptic potentials evoked by muscle stretch (strEPSPs) in motoneurons. These events were measured in cats that were anaesthetized, so that recovery of spindle afferent function, including both muscle stretch encoding and monosynaptic transmission, could be separated from other factors that act centrally to influence muscle stretch-evoked excitation of motoneurons. Recovery of strEPSPs to 70% of normal specified the extent of overall functional recovery by the population spindle afferents that regained responsiveness to muscle stretch. In separate studies, we examined recovery of the stretch reflex in decerebrate cats, and found that it recovered to supranormal levels after nerve crush. The substantial disparity in recovery between strEPSPs and stretch reflex led us to conclude that factors in addition to recovery of spindle afferents make a large contribution in restoring the stretch reflex following nerve crush.

Evolution of gait abnormalities in SOD1(G93A) transgenic mice

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder characterized by the loss of upper and lower motoneurons. Clinically, it is manifested by weakness, muscle atrophy and progressive paralysis and ends up with patients' death 2-5 years after diagnosis. Although these symptoms lead in many cases to gait deficits in patients, an exhaustive locomotor profile of animal models mimicking the disease has not been assessed yet. In this work we evaluated the locomotor performance of the SOD1(G93A) mouse model of ALS using computerized treadmill gait analysis. SOD1(G93A) mice presented early (8 weeks of age) gait abnormalities, evidenced by an increase in the time of the propulsion phase of hindlimb stance. The alterations progressed during the disease until a complete disturbance of normal gait. This finding is meaningful to the field because the identification of a significant difference in a functional endpoint as early as 8 weeks might be a step forward resolving the debate about treatment of mice prior to the symptomatic phase in efficacy studies. These results also point out that digitizing analysis of treadmill locomotion may be useful to evaluate whether new therapeutic approaches are improving functional outcome of the animals.

Biocompatibility of chronically implanted transverse intrafascicular multichannel electrode (TIME) in the rat sciatic nerve

The transverse intrafascicular multichannel electrode (TIME) is intended to be transversally implanted in the peripheral nerve and to selectively interface subsets of axons in different fascicles within the same nerve. Two versions of TIME (TIME-2, TIME-3) were designed and tested for biocompatibility and safety in the sciatic nerve of the rat. TIME-2 was implanted in two groups: one group had only an acute implant and the second group had chronic implantation for 2 months; a third group was also chronically implanted with the TIME-3 version, designed to avoid the mechanical traction produced by muscles motion. We evaluated the functional and morphological effects of either TIME-2 or TIME-3 implanted in the rat sciatic nerve for 2 months. The results of the study indicate that implantation of the TIME-2 and TIME-3 devices in the rat sciatic nerve did not cause significant axonal loss or demyelination, as evidenced by the functional and histological results. The results of this study indicate that the TIME-2 and TIME-3 designs are biocompatible and safe after chronic implantation in a small peripheral nerve, such as the rat sciatic nerve.

Comparative analysis of transverse intrafascicular multichannel, longitudinal intrafascicular and multipolar cuff electrodes for the selective stimulation of nerve fascicles

The selection of a suitable nerve electrode for neuroprosthetic applications implies a trade-off between invasiveness and selectivity, wherein the ultimate goal is achieving the highest selectivity for a high number of nerve fascicles by the least invasiveness and potential damage to the nerve. The transverse intrafascicular multichannel electrode (TIME) is intended to be transversally inserted into the peripheral nerve and to be useful to selectively activate subsets of axons in different fascicles within the same nerve. We present a comparative study of TIME, LIFE and multipolar cuff electrodes for the selective stimulation of small nerves. The electrodes were implanted on the rat sciatic nerve, and the activation of gastrocnemius, plantar and tibialis anterior muscles was recorded by EMG signals. Thus, the study allowed us to ascertain the selectivity of stimulation at the interfascicular and also at the intrafascicular level. The results of this study indicate that (1) intrafascicular electrodes (LIFE and TIME) provide excitation circumscribed to the implanted fascicle, whereas extraneural electrodes (cuffs) predominantly excite nerve fascicles located superficially; (2) the minimum threshold for muscle activation with TIME and LIFE was significantly lower than with cuff electrodes; (3) TIME allowed us to selectively activate the three tested muscles when stimulating through different active sites of one device, both at inter- and intrafascicular levels, whereas selective activation using multipolar cuff (with a longitudinal tripolar stimulation configuration) was only possible for two muscles, at the interfascicular level, and LIFE did not activate selectively more than one muscle in the implanted nerve fascicle.

Effect of axon misdirection on recovery of electromyographic activity and kinematics after peripheral nerve injury

In this study, patterns of activity in the soleus (Sol) and tibialis anterior (TA) muscles and hindlimb kinematics were evaluated during slope walking in rats after transection and surgical repair either of the entire sciatic nerve (Sci group) or of its two branches separately, the tibial and common fibular nerves (T/CF group). With the latter method, axons from the tibial and common fibular nerves could not reinnervate targets of the other nerve branch after injury, reducing the opportunity for misdirection. Activity in the TA shifted from the swing phase in intact rats to nearly the entire step cycle in both injured groups. Since these changes occur without misdirection of regenerating axons, they are interpreted as centrally generated. Sol activity was changed from reciprocal to that of TA in intact rats to coactivate with TA, but only in the Sci group rats. In the T/CF group rats, Sol activity was not altered from that observed in intact rats. Despite effects of injury that limited foot movements, hindlimb kinematics were conserved during downslope walking in both injury groups and during level walking in the T/CF group. During level walking in the Sci group and during upslope walking in both groups of injured rats, the ability to compensate for the effects of the nerve injury was less effective and resulted in longer limb lengths held at more acute angles throughout the step cycle. Changes in limb movements occur irrespective of axon misdirection and reflect compensatory changes in the outputs of the neural circuits that drive locomotion.

Electrophysiological analysis of a murine model of motoneuron disease

OBJECTIVE

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by loss of motoneurons of the primary motor cortex, the brainstem and the spinal cord, for which there are not effective treatments. Several transgenic mice that mimic motoneuron disease have been used to investigate potential treatments. The objective of this work is to characterize electrophysiologically the SOD1(G93A) transgenic mouse model of ALS, and to provide useful markers to improve early detection and monitoring of progression of the disease.

METHODS

We performed nerve conduction tests, motor unit number estimation (MUNE), H reflex tests and motor evoked potentials (MEPs) in a cohort of transgenic and wild type mice from 4 to 16 weeks of age.

RESULTS

The results revealed dysfunction of spinal motoneurons evidenced by deficits in motor nerve conduction tests starting at 8 weeks of age, earlier in proximal than in distal muscles of the hindlimb. MUNE demonstrated that spinal motoneurons loss muscle innervation and have a deficit in their sprouting capacity. Motor evoked potentials revealed that, coexisting with peripheral deficits, there was a dysfunction of central motor tracts that started also at 8 weeks, indicating progressive dysfunction of upper motoneurons.

CONCLUSIONS

These electrophysiological results provide important information about the SOD1(G93A) mouse model, as they demonstrate by the first time alterations of central motor pathways simultaneously to lower motoneuron dysfunction, well before functional abnormalities appear (by 12 weeks of age).

SIGNIFICANCE

The finding of concomitant dysfunction of upper and lower motoneurons contributes to the validation of the SOD1(G93A) mouse as model of ALS, because this parallel involvement is a diagnostic condition for ALS. Electrophysiological tests can be used as early markers of the disease and to evaluate the potential benefits of new treatments on both upper and lower motoneurons.

Passive and active exercise improve regeneration and muscle reinnervation after peripheral nerve injury in the rat

INTRODUCTION

Lesions of peripheral nerves cause loss of motor and sensory function and also lead to hyperreflexia and hyperalgesia. Activity-dependent therapies promote axonal regeneration and functional recovery and may improve sensory-motor coordination and restoration of adequate circuitry at the spinal level.

METHODS

We compared the effects of passive (bicycle) and active (treadmill) exercise on nerve regeneration and modulation of the spinal H reflex after transection and repair of the rat sciatic nerve. Animals were evaluated during 2 months using electrophysiological, functional, and histological methods.

RESULTS

Moderate exercise for 1 hour/day, either active treadmill walking or passive cycling, improved muscle reinnervation, increased the number of regenerated axons in the distal nerve, and reduced the increased excitability of spinal reflexes after nerve lesion.

DISCUSSION

Maintenance of denervated muscle activity and afferent input, by active or passive exercise, may increase trophic factor release to act on regenerating axons and to modulate central neuronal plasticity.

Permanent reorganization of Ia afferent synapses on motoneurons after peripheral nerve injuries

After peripheral nerve injuries to a motor nerve, the axons of motoneurons and proprioceptors are disconnected from the periphery and monosynaptic connections from group I afferents and motoneurons become diminished in the spinal cord. Following successful reinnervation in the periphery, motor strength, proprioceptive sensory encoding, and Ia afferent synaptic transmission on motoneurons partially recover. Muscle stretch reflexes, however, never recover and motor behaviors remain uncoordinated. In this review, we summarize recent findings that suggest that lingering motor dysfunction might be in part related to decreased connectivity of Ia afferents centrally. First, sensory afferent synapses retract from lamina IX, causing a permanent relocation of the inputs to more distal locations and significant disconnection from motoneurons. Second, peripheral reconnection between proprioceptive afferents and muscle spindles is imperfect. As a result, a proportion of sensory afferents that retain central connections with motoneurons might not reconnect appropriately in the periphery. A hypothetical model is proposed in which the combined effect of peripheral and central reconnection deficits might explain the failure of muscle stretch to initiate or modulate firing of many homonymous motoneurons.

Electrical stimulation combined with exercise increase axonal regeneration after peripheral nerve injury

Although injured peripheral axons are able to regenerate, functional recovery is usually poor after nerve transection. In this study we aim to elucidate the role of neuronal activity, induced by nerve electrical stimulation and by exercise, in promoting axonal regeneration and modulating plasticity in the spinal cord after nerve injury. Four groups of adult rats were subjected to sciatic nerve transection and suture repair. Two groups received electrical stimulation (3 V, 0.1 ms at 20 Hz) for 1 h, immediately after injury (ESa) or during 4 weeks (1 h daily; ESc). A third group (ES+TR) received 1 h electrical stimulation and was submitted to treadmill running during 4 weeks (5 m/min, 2 h daily). A fourth group performed only exercise (TR), whereas an untreated group served as control (C). Nerve conduction, H reflex and algesimetry tests were performed at 1, 3, 5, 7 and 9 weeks after surgery, to assess muscle reinnervation and changes in excitability of spinal cord circuitry. Histological analysis was made at the end of the follow-up. Groups that received acute ES and/or were forced to exercise in the treadmill showed higher levels of muscle reinnervation and increased numbers of regenerated myelinated axons when compared to control animals or animals that received chronic ES. Combining ESa with treadmill training significantly improved muscle reinnervation during the initial phase. The facilitation of the monosynaptic H reflex in the injured limb was reduced in all treated groups, suggesting that the maintenance of activity helps to prevent the development of hyperreflexia.

Nociceptive responses and spinal plastic changes of afferent C-fibers in three neuropathic pain models induced by sciatic nerve injury in the rat

Peripheral nerve injuries induce plastic changes on primary afferent fibers and on the spinal circuitry, which are related to the emergence of neuropathic pain. In this study we compared three models of sciatic nerve injury in the rat with different degrees of damage and impact on regeneration capability: crush nerve injury, chronic constriction injury (CCI) and spared nerve injury (SNI). All three models were characterized by means of nerve histology, in order to describe the degenerative and regenerative process of injured axons. Nociceptive responses were evaluated by mechanical and thermal algesimetry tests. Crush animals displayed higher withdrawal thresholds on the ipsilateral paw compared to the contralateral during the time of denervation, while CCI and SNI animals showed mechanical and thermal hyperalgesia. Central plasticity was evaluated by immunohistochemical labeling of non-peptidergic (IB4-positive) and peptidergic (substance P-positive) nociceptive C-fibers on L4-L6 spinal cord sections. After crush nerve injury and SNI, we observed progressive and sustained reduction of IB4 and SP immunolabeling at the sciatic projection territory in the superficial laminae of the dorsal horn, which affected only the tibial and peroneal nerves projection areas in the case of SNI. After CCI, changes on SP-immunoreactivity were not observed, and IB4-immunoreactive area decreased initially but recovered to normal levels on the second week post-injury. Thus, nociceptive responses depend on the type of injury, and the immunoreactivity pattern of afferent fibers at the spinal cord display changes less pronounced after partial than complete sciatic nerve injury. Although signs of neuropathic pain appear in all three lesion models, nociceptive responses and central plasticity patterns differ between them.

Chapter 27: Neural plasticity after nerve injury and regeneration

Injuries to the peripheral nerves result in partial or total loss of motor, sensory, and autonomic functions in the denervated segments of the body due to the interruption of axons, degeneration of distal nerve fibers, and eventual death of axotomized neurons. Functional deficits caused by nerve injuries can be compensated by reinnervation of denervated targets by regenerating injured axons or by collateral branching of undamaged axons, and remodeling of nervous system circuitry related to the lost functions. Plasticity of central connections may compensate functionally for the lack of adequate target reinnervation; however, plasticity has limited effects on disturbed sensory localization or fine motor control after injuries, and may even result in maladaptive changes, such as neuropathic pain and hyperreflexia. After axotomy, neurons shift from a transmitter to a regenerative phenotype, activating molecular pathways that promote neuronal survival and axonal regeneration. Peripheral nerve injuries also induce a cascade of events, at the molecular, cellular, and system levels, initiated by the injury and progressing throughout plastic changes at the spinal cord, brainstem nuclei, thalamus, and brain cortex. Mechanisms involved in these changes include neurochemical changes, functional alterations of excitatory and inhibitory synaptic connections, sprouting of new connections, and reorganization of sensory and motor central maps. An important direction for research is the development of therapeutic strategies that enhance axonal regeneration, promote selective target reinnervation, and are also able to modulate central nervous system reorganization, amplifying positive adaptive changes that improve functional recovery and also reducing undesirable effects.

Immediate electrical stimulation enhances regeneration and reinnervation and modulates spinal plastic changes after sciatic nerve injury and repair

We have studied whether electrical stimulation immediately after nerve injury may enhance axonal regeneration and modulate plastic changes at the spinal cord level underlying the appearance of hyperreflexia. Two groups of adult rats were subjected to sciatic nerve section followed by suture repair. One group (ES) received electrical stimulation (3 V, 0.1 ms at 20 Hz) for 1 h after injury. A second group served as control (C). Nerve conduction, H reflex, motor evoked potentials, and algesimetry tests were performed at 1, 3, 5, 7 and 9 weeks after surgery, to assess muscle reinnervation and changes in excitability of spinal cord circuitry. The electrophysiological results showed higher levels of reinnervation, and histological results a significantly higher number of regenerated myelinated fibers in the distal tibial nerve in group ES in comparison with group C. The monosynaptic H reflex was facilitated in the injured limb, to a higher degree in group C than in group ES. The amplitudes of motor evoked potentials were similar in both groups, although the MEP/M ratio was increased in group C compared to group ES, indicating mild central motor hyperexcitability. Immunohistochemical labeling of sensory afferents in the spinal cord dorsal horn showed prevention of the reduction in expression of substance P at one month postlesion in group ES. In conclusion, brief electrical stimulation applied after sciatic nerve injury promotes axonal regeneration over a long distance and reduces facilitation of spinal motor responses.

The role of evaluation methods in the assessment of peripheral nerve regeneration through synthetic conduits: a systematic review

Object

A number of evaluation methods that are currently used to compare peripheral nerve regeneration with alternative repair methods and to judge the outcome of a new paradigm were hypothesized to lack resolving power. This would too often lead to the conclusion that the outcome of a new paradigm could not be discerned from the outcome of the current gold standard, the autograft. As a consequence, the new paradigm would incorrectly be judged as successful.

Methods

An overview of the methods that were used to evaluate peripheral nerve regeneration after grafting of the rat sciatic nerve was prepared. All articles that were published between January 1975 and December 2004 and concerned grafting of the rat sciatic nerve (minimum graft length 5 mm) and in which the experimental method was compared with an untreated or another grafted nerve were included. The author scored the presence of statistically significant differences between paradigms.

Results

Evaluation of nerve fiber count, nerve fiber density, N-ratio, nerve histological success ratio, compound muscle action potential, muscle weight, and muscle tetanic force are methods that were demonstrated to have resolving power.

Conclusions

A number of evaluation methods are not suitable to demonstrate a significant difference between experimental paradigms in peripheral nerve regeneration. It is preferable to apply a combination of evaluation methods with resolving power to evaluate nerve regeneration properly.

Electrophysiologic assessment of sciatic nerve regeneration in the rat: Surrounding limb muscles feature strongly in recordings from the gastrocnemius muscle

Striking inconsistencies between the results of morphometric and electrophysiologic examinations of the regenerating nerve were observed in a previous study featuring the bridging of a 14 mm gap in the rat sciatic nerve. To shed light on this dichotomy, seven further rats were subjected to permanent sciatic nerve transection and assessed electrophysiologically, histologically and by retrograde axonal tracing at various postoperative intervals (1 h to 8 weeks). The results of the histological examinations and retrograde tracing revealed that in spite of the fact that compound muscle action potentials could be recorded in the gastrocnemius muscle, no reinnervation of the gastrocnemius muscle, either physiological or aberrant, had actually taken place. Furthermore, it was established that the electrical activity recorded in the gastrocnemius muscle after stimulation of the proximal or distal stump is generated by surrounding hind limb muscles unaffected by denervation. These are stimulated either directly, or indirectly due to spreading of the impulse. It is therefore strongly recommended that caution should be exercised when interpreting recordings from the gastrocnemius muscle after stimulation of a regenerating sciatic nerve in laboratory rodents.

Neural plasticity after peripheral nerve injury and regeneration

Injuries to the peripheral nerves result in partial or total loss of motor, sensory and autonomic functions conveyed by the lesioned nerves to the denervated segments of the body, due to the interruption of axons continuity, degeneration of nerve fibers distal to the lesion and eventual death of axotomized neurons. Injuries to the peripheral nervous system may thus result in considerable disability. After axotomy, neuronal phenotype switches from a transmitter to a regenerative state, inducing the down- and up-regulation of numerous cellular components as well as the synthesis de novo of some molecules normally not expressed in adult neurons. These changes in gene expression activate and regulate the pathways responsible for neuronal survival and axonal regeneration. Functional deficits caused by nerve injuries can be compensated by three neural mechanisms: the reinnervation of denervated targets by regeneration of injured axons, the reinnervation by collateral branching of undamaged axons, and the remodeling of nervous system circuitry related to the lost functions. Plasticity of central connections may compensate functionally for the lack of specificity in target reinnervation; plasticity in human has, however, limited effects on disturbed sensory localization or fine motor control after injuries, and may even result in maladaptive changes, such as neuropathic pain, hyperreflexia and dystonia. Recent research has uncovered that peripheral nerve injuries induce a concurrent cascade of events, at the systemic, cellular and molecular levels, initiated by the nerve injury and progressing throughout plastic changes at the spinal cord, brainstem relay nuclei, thalamus and brain cortex. Mechanisms for these changes are ubiquitous in central substrates and include neurochemical changes, functional alterations of excitatory and inhibitory connections, atrophy and degeneration of normal substrates, sprouting of new connections, and reorganization of somatosensory and motor maps. An important direction for ongoing research is the development of therapeutic strategies that enhance axonal regeneration, promote selective target reinnervation, but are also able to modulate central nervous system reorganization, amplifying those positive adaptive changes that help to improve functional recovery but also diminishing undesirable consequences.