Aberrant perineuronal nets alter spinal circuits, impair motor function, and increase plasticity

Perineuronal nets (PNNs) are a specialized extracellular matrix that have been extensively studied in the brain. Cortical PNNs are implicated in synaptic stabilization, plasticity inhibition, neuroprotection, and ionic buffering. However, the role of spinal PNNs, mainly found around motoneurons, is still unclear. Thus, the goal of this study is to elucidate the role of spinal PNNs on motor function and plasticity in both intact and spinal cord injured mice. We used transgenic mice lacking the cartilage link protein 1 (Crtl1 KO mice), which is implicated in PNN assembly. Crtl1 KO mice showed disorganized PNNs with an altered proportion of their components in both motor cortex and spinal cord. Behavioral and electrophysiological tests revealed motor impairments and hyperexcitability of spinal reflexes in Crtl1 KO compared to WT mice. These functional outcomes were accompanied by an increase in excitatory synapses around spinal motoneurons. Moreover, following spinal lesions of the corticospinal tract, Crtl1 KO mice showed increased contralateral sprouting compared to WT mice. Altogether, the lack of Crtl1 generates aberrant PNNs that alter excitatory synapses and change the physiological properties of motoneurons, overall altering spinal circuits and producing motor impairment. This disorganization generates a permissive scenario for contralateral axons to sprout after injury.

Behavioral testing in animal models of spinal cord injury

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

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

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

BDNF: the career of a multifaceted neurotrophin in spinal cord injury

Brain-derived neurotrophic factor (BDNF) has been identified as a potent promoter of neurite growth, a finding that has led to an ongoing exploration of this neurotrophin as a potential treatment for spinal cord injury. BDNF's many effects in the nervous system make it an excellent candidate for neuroprotective strategies as well as for promoting axonal regeneration, plasticity and re-myelination. In addition, neuronal activity and physical exercise can modulate the expression of BDNF, suggesting that non-invasive means to increase BDNF levels might exist. Nonetheless, depending on the location, amount and duration of BDNF delivery, this potent neurotrophin can also have adverse effects, such as modulation of nociceptive pathways or contribution to spasticity. Taken together, the benefits and possible risks require careful assessment when considering this multifaceted neurotrophin as a treatment option for spinal cord injury.

Rehabilitative training and plasticity following spinal cord injury

Rehabilitative training is currently one of the most successful treatments to promote functional recovery following spinal cord injury. Nevertheless, there are many unanswered questions including the most effective and beneficial design, and the mechanisms underlying the training effects on motor recovery. Furthermore, rehabilitative training will certainly be combined with pharmacological treatments developed to promote the "repair" of the injured spinal cord. Thus, insight into training-induced mechanisms will be of great importance to fine tune such combined treatments. In this review we address current challenges of rehabilitative training and mechanisms involved in promoting motor recovery with the focus on animal models. These challenges suggest that although rehabilitative training appears to be a relatively straight forward treatment approach, more research is needed to optimize its effect on functional outcome in order to enhance our chances of success when combining pharmacological treatments promoting axonal growth and rehabilitative training in the clinic.

Locomotion after spinal cord injury depends on constitutive activity in serotonin receptors

Following spinal cord injury (SCI) neurons caudal to the injury are capable of rhythmic locomotor-related activity that can form the basis for substantial functional recovery of stepping despite the loss of crucial brain stem-derived neuromodulators like serotonin (5-HT). Here we investigated the contribution of constitutive 5-HT(2) receptor activity (activity in the absence of 5-HT) to locomotion after SCI. We used a staggered hemisection injury model in rats to study this because these rats showed a robust recovery of locomotor function and yet a loss of most descending axons. Immunolabeling for 5-HT showed little remaining 5-HT below the injury, and locomotor ability was not correlated with the amount of residual 5-HT. Furthermore, blocking 5-HT(2) receptors with an intrathecal (IT) application of the neutral antagonist SB242084 did not affect locomotion (locomotor score and kinematics were unaffected), further indicating that residual 5-HT below the injury did not contribute to generation of locomotion. As a positive control, we found that the same application of SB242084 completely antagonized the muscle activity induced by exogenous application of the 5-HT(2) receptor agonists alpha-methyl-5-HT (IT). In contrast, blocking constitutive 5-HT(2) receptor activity with the potent inverse agonist SB206553 (IT) severely impaired stepping as assessed with kinematic recordings, eliminating most hindlimb weight support and overall reducing the locomotor score in both hind legs. However, even in the most severely impaired animals, rhythmic sweeping movements of the hindlimb feet were still visible during forelimb locomotion, suggesting that SB206553 did not completely eliminate locomotor drive to the motoneurons or motoneuron excitability. The same application of SB206553 had no affect on stepping in normal rats. Thus while normal rats can compensate for loss of 5-HT(2) receptor activity, after severe spinal cord injury rats require constitutive activity in these 5-HT(2) receptors to produce locomotion.

Reaching training in rats with spinal cord injury promotes plasticity and task specific recovery

In the current study we examined the effects of training in adult rats with a cervical spinal cord injury (SCI). One group of rats received 6 weeks of training in a single pellet reaching task immediately after injury, while a second group did not receive training. Following this period changes in cortical levels of BDNF and GAP-43 were analysed in trained and untrained animals and in a group with training but no injury. In another group of rats, functional recovery was analysed in the reaching task and when walking on a horizontal ladder. Thereupon, the cortical forelimb area was electrophysiologically examined using micro-stimulation followed by tracing of the lesioned corticospinal tract (CST). We found that trained rats improved substantially in the reaching task, when compared to their untrained counterparts. Trained rats however, performed significantly worse with their injured forelimb when walking on a horizontal ladder. In parallel to the improved recovery in the trained task, we found that the cortical area where wrist movements could be evoked by micro-stimulation expanded in trained rats in comparison to both untrained and uninjured rats. Furthermore, collateral sprouting of lesioned CST fibres rostral to the injury was increased in trained rats. Post-injury training was also found to increase cortical levels of GAP-43 but not BDNF. In conclusion we show that training of a reaching task promotes recovery of the trained task following partial SCI by enhancing plasticity at various levels of the central nervous system (CNS), but may come at the cost of an untrained task.

BDNF promotes connections of corticospinal neurons onto spared descending interneurons in spinal cord injured rats

Although regeneration of injured axons is inhibited within the adult CNS, moderate recovery can be found in patients and animals with incomplete spinal cord injury (SCI). This can be partly attributed to sprouting of spared and injured axons, rostral and caudal to the lesion, respectively. Recently, it has been reported that following a thoracic SCI such sprouting can result in indirect reconnections of the lesioned axons to caudal targets via propriospinal interneurons (PrI). Here, we attempted to further promote this spontaneous repair mechanism by applying the neurotrophic factor BDNF (brain-derived neurotrophic factor), in the vicinity of the cell bodies of lesioned corticospinal neurons or NT-3, intrathecally to the cervical spinal cord. We performed a dorsal over-hemisection at the thoracic spinal cord sparing only the left ventrolateral quadrant. This type of lesion did not promote sprouting of injured corticospinal axons or re-routing via commissural PrI. Also, in rats that received NT-3 at the cervical enlargement, no increase in sprouting was found. However, animals receiving BDNF at the cell bodies of lesioned corticospinal neurons showed a significant increase in collateral sprouting and in the number of contacts with PrI. This was not observed when BDNF was administered to unlesioned animals. Although no statistical difference in the horizontal ladder walking was found between the groups, the increase in collateral sprouting and in the number of contacts correlated with the functional recovery. Hence, cell body treatment can promote plasticity of the injured CNS and may be a valuable treatment approach in conjunction with local regeneration promoting strategies.

A new electrode configuration for recording electromyographic activity in behaving mice

With the increasing use of normal and genetically modified mice in the field of motor physiology, there is a need for a simple and reliable technique for recording electromyographic (EMG) activity in behaving mice. Here, we describe a new method for the fabrication and implantation of fine EMG recording electrodes into multiple muscles of adult mice. This method minimizes surgical damage to the muscles and the connecting leads have only a modest influence on leg movements when electrodes are implanted into distal muscles. We demonstrate that excellent EMG recordings can be obtained during walking, swimming and scratching for the vastus lateralis, tibialis anterior and gastrocnemius muscles in normal adult mice. EMG recordings were also made in a mutant EphA4 mouse to demonstrate the utility of the method for examining motor patterns in genetically modified animals. We also developed a method for constructing highly reflective markers that could be viewed over a range of orientations to measure the kinematics of legs movements during stepping. A commercial motion analysis system was used to track six markers during walking and to synchronize video and EMG data during walking sequences.

Regenerating corticospinal fibers in the Marmoset (Callitrix jacchus) after spinal cord lesion and treatment with the anti-Nogo-A antibody IN-1

Neutralizing the myelin-associated growth inhibitor Nogo-A in adult spinal cord-injured rats can promote regeneration of injured and compensatory sprouting of uninjured axons. Nogo-A is present in humans, making its neutralization a possible novel treatment option for paraplegic patients. In this study we examined the effects of an extensively used anti-Nogo-A antibody (mAb IN-1) on the regenerative capabilities of lesioned corticospinal tract (CST) axons in a primate, the Marmoset monkey. Unilateral thoracic lesions of the CST were performed in six adult Marmosets, followed by the application of mAb IN-1 into the cerebrospinal fluid circulation by a graft of hybridoma cells. A unilateral injection of biotin dextran amine into the motor cortex was performed to analyse sprouting and regeneration of the lesioned axons. In the control antibody-treated animal CST fibers stopped rostral to the lesion site and often showed retraction bulbs. In contrast, in four out of five mAb IN-1-treated animals fine labeled neurites had grown into, through and around the lesion site. Thus, this study provides first anatomical evidence that in primates, the neutralization of the myelin-associated inhibitor Nogo-A results in increased regenerative sprouting and growth of lesioned spinal cord axons.

Protective effects of oral creatine supplementation on spinal cord injury in rats

STUDY DESIGN

To evaluate a potential protective effect of increased creatine levels in spinal cord injury (SCI) in an animal model.

OBJECTIVES

Acute SCI initiates a series of cellular and molecular events in the injured tissue leading to further damage in the surrounding area. This secondary damage is partly due to ischemia and a fatal intracellular loss of energy. Phospho-creatine in conjunction with the creatine kinase isoenzyme system acts as a potent intracellular energy buffer. Oral creatine supplementation has been shown to elevate the phospho-creatine content in brain and muscle tissue, leading to neuroprotective effects and increased muscle performance.

SETTING

Zurich, Switzerland.

METHODS

Twenty adult rats were fed for 4 weeks with or without creatine supplemented nutrition before undergoing a moderate spinal cord contusion.

RESULTS

Following an initial complete hindlimb paralysis, rats of both groups substantially recovered within 1 week. However, creatine fed animals scored 2.8 points better than the controls in the BBB open field locomotor score (11.9 and 9.1 points respectively after 1 week; P=0.035, and 13 points compared to 11.4 after 2 weeks). The histological examination 2 weeks after SCI revealed that in all rats a cavity had developed which was comparable in size between the groups. In creatine fed rats, however, a significantly smaller amount of scar tissue surrounding the cavity was found.

CONCLUSIONS

Thus creatine treatment seems to reduce the spread of secondary injury. Our results favour a pretreatment of patients with creatine for neuroprotection in cases of elective intramedullary spinal surgery. Further studies are needed to evaluate the benefit of immediate creatine administration in case of acute spinal cord or brain injury.

White matter glucose metabolism during intracortical electrostimulation: a quantitative [(18)F]Fluorodeoxyglucose autoradiography study in the rat

[(18)F]Fluorodeoxyglucose (FDG) autoradiography was used to analyze the effects of intracortical electrostimulation on local cerebral metabolic rate for glucose (LCMRglu). The hindleg area in rat brains was electrically stimulated with different frequencies (0, 0.1, 0.25, 0.5, 0.75, 1 stimulus trains per second, two animals per condition). The major result was a strong positive correlation between stimulation frequency and LCMRglu in the callosal fibers originating in the stimulated cortical area. At the highest stimulation frequency callosal LCMRglu was 50.01 micromol/min/100 g compared to 27.87 micromol/min/100 g at baseline. LCMRglu in gray and white matter control areas was stable across conditions. Direct injection of FDG in the stimulated cortex failed to produce increased callosal uptake, excluding the possibility that FDG uptake in the corpus callosum is related to axonal diffusion. Although several previous autoradiographic studies have demonstrated alterations in LCMRglu in white matter, correlations between neural activity and LCMRglu have never been explicitly addressed. Changes in white matter metabolism most likely reflect changes of electrical fiber activity and thus the presented results bear important implications for brain imaging studies.

Anatomical correlates of locomotor recovery following dorsal and ventral lesions of the rat spinal cord

The present study was designed to relate functional locomotor outcome to the anatomical extent and localization of lesions in the rat spinal cord. We performed dorsal and ventral lesions of different severity in 36 adult rats. Lesion depth, spared total white matter, and spared ventrolateral funiculus were compared to the locomotor outcome, assessed by the BBB open-field locomotor score and the grid walk test. The results showed that the preservation of a small number of fibers in the ventral or lateral funiculus was related to stepping abilities and overground locomotion, whereas comparable tissue preservation in the dorsal funiculus resulted in complete paraplegia. The strongest relation to locomotor function was between the BBB score and the lesion depth as well as the BBB score and the spared white matter tissue in the region of the reticulospinal tract. Locomotion on the grid walk required sparing in the ventrolateral funiculus and additional sparing of the dorsolateral and dorsal funiculus, where the cortico- and rubrospinal tracts are located.

Neuronal coordination of arm and leg movements during human locomotion

We aimed to study the neuronal coordination of lower and upper limb muscles. We therefore evaluated the effect of small leg displacements during gait on leg and arm muscle electromyographic (EMG) activity in walking humans. During walking on a split-belt treadmill (velocity 3.5 km/h), short accelerations or decelerations were randomly applied to the right belt during the mid or end stance phase. Alternatively, trains of electrical stimuli were delivered to the right distal tibial nerve. The EMG activity of the tibialis anterior (TA), gastrocnemius medialis (GM), deltoideus (Delt), triceps (Tric) and biceps brachii (Bic) of both sides was analysed. For comparison, impulses were also applied during standing and sitting. The displacements were followed by specific patterns of right leg and bilateral arm muscle EMG responses. Most arm muscle responses appeared with a short latency (65-80 ms) and were larger in Delt and Tric than in Bic. They were strongest when deceleration impulses were released during mid stance, associated with a right compensatory TA response. A similar response pattern in arm muscles was obtained following tibial nerve stimulation. The arm muscle responses were small or absent when stimuli were applied during standing or sitting. The arm muscle responses correlated more closely with the compensatory TA than with the compensatory GM responses. The amplitude of the responses in most arm muscles correlated closely with the background EMG activity of the respective arm muscle. The observations suggest the existence of a task-dependent, flexible neuronal coupling between lower and upper limb muscles. The stronger impact of leg flexors in this interlimb coordination indicates that the neuronal control of leg flexor and extensor muscles is differentially interconnected during locomotion. The results are compatible with the assumption that the proximal arm muscle responses are associated with the swinging of the arms during gait, as a residual function of quadrupedal locomotion.

Cervical sprouting of corticospinal fibers after thoracic spinal cord injury accompanies shifts in evoked motor responses

The adult central nervous system (CNS) of higher vertebrates displays a limited ability for self repair after traumatic injuries, leading to lasting functional deficits [1]. Small injuries can result in transient impairments, but the mechanisms of recovery are poorly understood [2]. At the cortical level, rearrangements of the sensory and motor representation maps often parallel recovery [3,4]. In the sensory system, studies have shown that cortical and subcortical mechanisms contribute to map rearrangements [5,6], but for the motor system the situation is less clear. Here we show that large-scale structural changes in the spared rostral part of the spinal cord occur simultaneously with shifts of a hind-limb motor cortex representation after traumatic spinal-cord injury. By intracortical microstimulation, we defined a cortical area that consistently and exclusively yielded hind-limb muscle responses in normal adult rats. Four weeks after a bilateral transsection of the corticospinal tract (CST) in the lower thoracic spinal cord, we again stimulated this cortical field and found forelimb, whisker, and trunk responses, thus demonstrating reorganization of the cortical motor representation. Anterograde tracing of corticospinal fibers originating from this former hind-limb area revealed that sprouting greatly increased the normally small number of collaterals that lead into the cervical spinal cord rostral to the lesion. We conclude that the corticospinal motor system has greater potential to adapt structurally to lesions than was previously believed and hypothesize that this spontaneous growth response is the basis for the observed motor representation rearrangements and contributes to functional recovery after incomplete lesions.

Improving axonal growth and functional recovery after experimental spinal cord injury by neutralizing myelin associated inhibitors

Injuries of the spinal cord often result in an irretrievable loss of motor and sensory functions of all body parts situated below the lesion site. Functional recovery is restricted mainly due to the limited regeneration and plasticity of injured axons in the adult central nervous system. Over the last few years different experimental approaches have led to axonal growth and functional benefits in animal models. This review focuses on the effects of the neutralization of myelin-associated neurite growth inhibitors, in particular Nogo-A, using the monoclonal antibody IN-1. Acute mAb IN-1 treatment of adult CNS lesioned rats results in extensive plastic changes of neuronal connections and regenerative fiber growth. In two different lesion paradigms (i.e. pyramidal tract lesion and incomplete spinal cord lesion in adult rats), the mAb IN-1-treated animals always showed a higher degree of recovery in various behavioral tests. These observations, together with electrophysiological results, suggest that neuronal CNS circuits of mAb IN-1-treated animals can be rearranged, and that sprouting and regenerating axons form functionally meaningful connections.

Functional switch between motor tracts in the presence of the mAb IN-1 in the adult rat

Fine finger and hand movements in humans, monkeys, and rats are under the direct control of the corticospinal tract (CST). CST lesions lead to severe, long-term deficits of precision movements. We transected completely both CSTs in adult rats and treated the animals for 2 weeks with an antibody that neutralized the central nervous system neurite growth inhibitory protein Nogo-A (mAb IN-1). Anatomical studies of the rubrospinal tracts showed that the number of collaterals innervating the cervical spinal cord doubled in the mAb IN-1- but not in the control antibody-treated animals. Precision movements of the forelimb and fingers were severely impaired in the controls, but almost completely recovered in the mAb IN-1-treated rats. Low threshold microstimulation of the motor cortex induced a rapid forelimb electromyography response that was mediated by the red nucleus in the mAb IN-1 animals but not in the controls. These findings demonstrate an unexpectedly high capacity of the adult central nervous system motor system to sprout and reorganize in a targeted and functionally meaningful way.

Locomotor recovery in spinal cord-injured rats treated with an antibody neutralizing the myelin-associated neurite growth inhibitor Nogo-A

The limited plastic and regenerative capabilities of axons in the adult mammalian CNS can be enhanced by the application of a monoclonal antibody (mAb), IN-1, raised against the myelin-associated neurite growth inhibitor Nogo-A. The aim of the present study was to investigate the effects of this treatment on the functional recovery of adult rats with a dorsal over-hemisection of the spinal cord. Directly after injury, half of the animals were implanted with mAb IN-1-secreting hybridoma cells, whereas the others received cells secreting a control antibody (anti-HRP). A broad spectrum of locomotor tests (open field locomotor) score, grid walk, misstep withdrawal response, narrow-beam crossing) was used to characterize locomotor recovery during the 5 weeks after the injury. In all behavioral tests, the recovery in the mAb IN-1-treated group was significantly augmented compared with the control antibody-treated rats. EMG recordings of flexor and extensor muscles during treadmill walking confirmed the improvement of the locomotor pattern in the mAb IN-1-treated rats; step-cycle duration, rhythmicity, and coupling of the hindlimbs were significantly improved. No differences between the two groups with regard to nociception were observed in the tail flick test 5 weeks after the operation. These results indicating improved functional recovery suggest that the increased plastic and regenerative capabilities of the CNS after Nogo-A neutralization result in a functionally meaningful rewiring of the motor systems.

Reflex adaptations during treadmill walking with increased body load

Load dependent reflex adaptations were studied in healthy subjects walking on a split-belt treadmill. Compensatory reflex responses were elicited in the right leg extensor muscles during mid-stance by a short acceleration of the right treadmill belt. Electromyographic activity (EMG) was recorded from the right medial gastrocnemius (GMR), soleus (SO) and tibialis anterior (TA) muscles of the right leg as well as from the gastrocnemius of the left unperturbed leg (GML). To study the adaptational reflex behavior, multiple measurements were taken during walking with normal (control) and increased body load and after removing the load. In most experiments the compensatory EMG response in the GMR consisted of a short inhibitory and a subsequent excitatory component. Both reflex components were larger when the body was loaded. During the course of continuous loading, divergent reflex adaptations of different degrees and directions were observed in the subjects. In one group of subjects the reflex response increased to a higher level of EMG activity. In a second group EMG activity first increased and afterwards decreased to baseline level. A subsequent removal of body loading resulted in a slow adaptation to the control reflex values in both groups. Neither the EMG activity in the GM nor the reflex responses in the GMR after increasing the load changed differently in the two groups. Our results suggest that load information is not simply used in a fixed input/output relationship of the actual biomechanical conditions of a subject. Load information is rather used to slowly modify the reflex response, to achieve the desired posture during walking.

Efficient testing of motor function in spinal cord injured rats

In experimental spinal cord injury studies, animal models are widely used to examine anatomical and functional changes after different treatments and lesion types. A variety of behavioral paradigms exists in the literature, but definitions and criteria for motor performance vary considerably. In this study, we examined the outcome and relation of tests such as the BBB open field locomotion score, footprint analysis, kinematic analysis, placing response, grid walk and narrow beam crossing following two different lesion types. The information obtained was used to design an efficient and reliable testing strategy, which includes a broad spectrum of parameters to enhance sensitivity. This approach should help to standardize modular testing procedures across different laboratories working on spinal cord injury.

Treadmill training in incomplete spinal cord injured rats

Treadmill training has been shown to accelerate locomotor recovery and to improve weight bearing during treadmill walking in spinal cats. In human patients treadmill training is increasingly used in rehabilitation after incomplete spinal cord injury. In this study we examined training effects in spinal cord injured rats with an incomplete dorsal lesion. Recovery was examined with an open field locomotor score, kinematic analysis on the treadmill, and several functional tests (i.e. foot print evaluation, narrow beam crossing, grid walking, open field exploratory activity). During the course of 5 weeks after the injury, a substantial amount of recovery occurred in the treadmill trained as well as in the untrained rats. If compared to the control lesioned rats, which showed a high level of spontaneous hindlimb movements at 7-14 days post lesion, no additional beneficial effect of a 5-week daily treadmill training on the locomotor outcome could be detected in the trained group. The only change observed was a slightly larger exploratory activity of the trained rats. It is probable that the spared ventral and ventro-lateral fibers allowed spontaneous recovery and 'self-training' to occur to such an extend that systematic treadmill training did not provide additional improvement.

Compensatory sprouting and impulse rerouting after unilateral pyramidal tract lesion in neonatal rats

After lesions of the developing mammalian CNS, structural plasticity and functional recovery are much more pronounced than in the mature CNS. We investigated the anatomical reorganization of the corticofugal projections rostral to a unilateral lesion of the corticospinal tract at the level of the medullary pyramid (pyramidotomy) and the contribution of this reorganization and other descending systems to functional recovery. Two-day-old (P2) and adult rats underwent a unilateral pyramidotomy. Three months later the corticofugal projections to the red nucleus and the pons were analyzed; a relatively large number of corticorubral and corticopontine fibers from the lesioned side had crossed the midline and established an additional contralateral innervation of the red nucleus and the pons. Such anatomical changes were not seen after adult lesions. Intracortical microstimulation of the primary motor cortex with EMG recordings of the elbow flexor muscles were used to investigate possible new functional connections from the motor cortex of the pyramidotomy side to the periphery. In rats lesioned as adults, stimulation of the motor cortex ipsilateral to the pyramidotomy never elicited EMG activity. In contrast, in P2 lesioned rats bilateral forelimb EMGs were found. EMG latencies were comparable for the ipsilateral and contralateral responses but were significantly longer than in unlesioned animals. Transient inactivation of both red nuclei with the GABA receptor agonist muscimol led to a complete loss of these bilateral movements. Movements and EMGs reappeared after wash-out of the drug. These results suggest an important role of the red nucleus in the reconnection of the cortex to the periphery after pyramidotomy.

Adaptive changes in motor activity associated with functional recovery following muscle denervation in walking cats

In this investigation we examined the changes in the pattern of activity in the medial gastrocnemius (MG) muscle in walking cats following transection of the nerves innervating synergist muscles (lateral gastrocnemius, soleus, and plantaris). Immediately following the nerve transections, there was a large increase in ankle flexion during early stance (from approximately 10 to approximately 30 degrees ) and a marked increase in the magnitude of the MG bursts during stance. We attribute this increase in the magnitude of the MG bursts to an increase in afferent feedback from the abnormally stretched MG muscle. During the week after the nerve transections, there was a progressive decrease in ankle yield. This improvement in ankle function was correlated with an increase in magnitude of two components of the MG bursts; the initial component starting during late swing and ending approximately 40 ms after ground contact, and a late component associated with stance. The time courses of the increases in the initial and late components of the MG bursts were different. Large and significant increases in the late component occurred the day after the nerve transections, whereas increases in the initial component occurred more gradually. This difference in time course was reflected in the kinematics of ankle movement. Over the first few days after the nerve transections, improvement in ankle movement occurred primarily late in the stance phase, and there was little change in ankle yield during early stance. At 1 wk, however, there was a significant reduction in ankle yield during early stance. This decreased yield was most likely due to an increase in stiffness of the MG muscle at the time of ground contact resulting from the increase in magnitude of the initial component of the MG bursts. The increases in the magnitude of the initial and late components of the MG bursts, as well as the improvement in ankle function, depended on use of the leg. All these changes were delayed by immobilizing the leg for 6 days in an extended position. We discuss possible mechanisms underlying the increase in the magnitude of the MG bursts and propose that proprioceptive signals from the stretched MG muscles provide an error signal for rescaling the magnitude of the centrally generated initial component. Our data support the concept that proprioceptive feedback functions to scale the magnitude of feed-forward motor commands to ensure they are appropriate for the biomechanical properties of the musculoskeletal system.

Spasticity in rats with sacral spinal cord injury

We have investigated sacral spinal cord lesions in rats with the goal of developing a rat model of muscular spasticity that is minimally disruptive, not interfering with bladder, bowel, or hindlimb locomotor function. Spinal transections were made at the S2 sacral level and, thus, only affected the tail musculature. After spinal transection, the muscles of the tail were inactive for 2 weeks. Following this initial period, hypertonia, hyperreflexia, and clonus developed in the tail, and grew more pronounced with time. These changes were assessed in the awake rat, since the tail is readily accessible and easy to manipulate. Muscle stretch or cutaneous stimulation of the tail produced muscle spasms and marked increases in muscle tone, as measured with force and electromyographic recordings. When the tail was unconstrained, spontaneous or reflex induced flexor and extensor spasms coiled the tail. Movement during the spasms often triggered clonus in the end of the tail. The tail hair and skin were extremely hyperreflexive to light touch, withdrawing quickly at contact, and at times clonus could be entrained by repeated contact of the tail on a surface. Segmental tail muscle reflexes, e.g., Hoffman reflexes (H-reflexes), were measured before and after spinalization, and increased significantly 2 weeks after transection. These results suggest that sacral spinal rats develop symptoms of spasticity in tail muscles with similar characteristics to those seen in limb muscles of humans with spinal cord injury, and thus provide a convenient preparation for studying this condition.

Enhancement and resetting of locomotor activity by muscle afferents

The generation of the normal motor pattern for walking in mammals requires feedback from muscle proprioceptors. Two characteristics of the motor pattern particularly dependent on proprioceptive signals are (1) the magnitude of activity in knee and ankle extensor muscles and (2) the duration of extensor bursts during stance. Sensory regulation of these characteristics ensures that the level of activity in extensor muscles during stance is appropriate for the load carried by the leg and that the swing phase is not initiated when a leg is loaded. Many different groups of afferents from flexor and extensor muscles can influence the locomotor pattern. Most attention has focused on the action of group I afferents from ankle extensors. Electrical stimulation of these afferents during extension increases the duration and the magnitude of extensor activity. The prolongation of extensor activity depends in part on excitation of the extensor half-center by group Ib afferents from Golgi tendon organs. The enhancement of the magnitude of extensor bursts is produced primarily via disynaptic and polysynaptic pathways opened only during locomotion. The involvement of the proprioceptive signals in the generation of locomotor activity means that the gains in reflex pathways must be constantly calibrated according to the biomechanical properties of the locomotor system. Alteration of these properties by weakening ankle extensor muscles has recently been found to produce compensatory changes in proprioceptive influences on the locomotor pattern.

Decerebration by global ischemic stroke in rats

In this study we present a fast and simple technique to decerebrate rats. By injecting polyvinylsiloxane (PVS) into both common carotid arteries we occluded the circle of Willis and all cerebral arteries, thereby completely interrupting the blood supply to the cerebrum. High viscosity PVS was used so that it only entered the large arteries, and did not pass into the capillary beds. This procedure reliably resulted in an anemic decerebration, without interfering with the blood supply to the brainstem. Long-term survival was achieved by reducing intracranial pressure by the application of steroids and/or opening the skull.

Modification of group I field potentials in the intermediate nucleus of the cat spinal cord after chronic axotomy of an extensor nerve

In walking decerebrate cats the influence of group I afferents from the medial gastrocnemius (MG) and the lateral gastrocnemius/soleus (LGS) muscles on stance phase duration is altered after axotomy of the LGS nerve [Whelan, P.J., Hiebert, G.W. and Pearson, K.G., J. Neurophysiol., 74 (1995) 2782-2787]. We examined whether a site for this plasticity is the connection from group I afferents onto spinal interneurons. Group I field potentials from MG, LGS and plantaris (PL) muscle afferents were recorded in the intermediate nucleus of the L6/L7 segments. Within 5 days following the transection of the LGS nerve in one hind leg, the field potential amplitudes from LGS afferents were decreased, from MG increased and those from PL were unchanged. The changes in the field potentials parallel modifications in the influence of group I afferents from the MG and LGS muscles on stance phase duration during walking.

Effects of extensor muscle afferents on the timing of locomotor activity during walking in adult rats

The influence of hind leg extensor muscle afferents on the timing of locomotor phase transitions was examined in adult, decerebrate rats, walking on a treadwheel. Walking occurred either spontaneously or was induced by stimulation of the mesencephalic locomotor region. Large diameter muscle afferents innervating the lateral or medial gastrocnemius were electrically stimulated during walking. A stimulus was delivered either at the onset of extensor muscle activity, or randomly during the step cycle. Stimulation with a train duration of 300 ms at the onset of extension increased the duration of the extensor bursts. The subsequent flexion phase was delayed. Stimulation with a shorter stimulus train (150 ms) early in extension had little effect on the extension phase duration. However when delivered at the end of extension the same stimulus significantly increased the duration of the extension phase and decreased the duration of the following flexion phase. Stimulating near the end of the flexion phase delayed onset and decreased duration of the subsequent extension phase. The effects of stimulating extensor afferents during the extension phase were weaker but qualitatively similar, to those in cats, suggesting similar mechanisms. The results of this study also show major differences in the integration of extensor muscle afferents between adult and neonatal rats.

Chronic and acute respiratory effects among grain mill workers

Exposure to flour dust may induce chronic respiratory manifestations as well as acute ventilatory effects. We compared the prevalence of respiratory symptoms, ventilatory impairment, and variations in pulmonary function over the workshift in a group of mill workers exposed to wheat flour and in referent workers. One hundred and forty-two men exposed to flour in a mill and 37 referent workers were included in this study. Each subject completed a standardized questionnaire. Pulmonary function tests were performed before and after the workshift. The assessment of environmental exposure to flour showed high concentrations during some jobs with a high percentage of inhalable particles and a low concentration of respirable particles. The exposed workers had a significantly higher prevalence of usual cough and usual phlegm than the referents. The prevalence of asthma, based on the questionnaire, was similar. Before the workshift, the exposed workers had significantly lower mean lung function values for peak flow rate and forced expiratory flow rate at 75% of the vital capacity than the referents. After the workshift, all the lung function values showed a slight decrease, significant for forced vital capacity and forced expiratory volume during 1 s in both groups. Among the exposed workers, the asthmatic subjects had a significantly higher decrease across the shift than the nonasthmatic workers. This result is probably linked to bronchial hyperreactivity. Among nonasthmatic subjects, the decrease was larger in nonexposed workers than in exposed workers. A higher prevalence of respiratory symptoms and lower pulmonary function values were observed among mill workers by comparison with referents. Moreover, the data suggest that asthmatic status and the time of spirometric measurements need to be taken into account in epidemiological studies on exposure to airborne allergens. In addition, the study does not exclude a healthy worker effect with selection of dust-resistant subjects or better identification of asthmatic subjects among the workers exposed to an allergenic substance than among the nonexposed workers.

Epidural anaesthesia in donkeys

Epidural anaesthesia was tested on 40 donkeys. The second intercoccygeal space was found suitable for satisfactory induction and a dose of 8 to 10 ml of 1 per cent procaine hydrochloride was recommended for inducing posterior epidural anaesthesia and 30 ml of 2 per cent for anterior epidural anaesthesia.