Neurophysiological basis of functional recovery in the neonatal spinalized rat

Cortex-dependent recovery of unassisted hindlimb locomotion after complete spinal cord injury in adult rats

After paralyzing spinal cord injury the adult nervous system has little ability to ‘heal’ spinal connections, and it is assumed to be unable to develop extra-spinal recovery strategies to bypass the lesion. We challenge this assumption, showing that completely spinalized adult rats can recover unassisted hindlimb weight support and locomotion without explicit spinal transmission of motor commands through the lesion. This is achieved with combinations of pharmacological and physical therapies that maximize cortical reorganization, inducing an expansion of trunk motor cortex and forepaw sensory cortex into the deafferented hindlimb cortex, associated with sprouting of corticospinal axons. Lesioning the reorganized cortex reverses the recovery. Adult rats can thus develop a novel cortical sensorimotor circuit that bypasses the lesion, probably through biomechanical coupling, to partly recover unassisted hindlimb locomotion after complete spinal cord injury.

DOI:http://dx.doi.org/10.7554/eLife.23532.001

What are the Best Animal Models for Testing Early Intervention in Cerebral Palsy?

Interventions to treat cerebral palsy should be initiated as soon as possible in order to restore the nervous system to the correct developmental trajectory. One drawback to this approach is that interventions have to undergo exceptionally rigorous assessment for both safety and efficacy prior to use in infants. Part of this process should involve research using animals but how good are our animal models? Part of the problem is that cerebral palsy is an umbrella term that covers a number of conditions. There are also many causal pathways to cerebral palsy, such as periventricular white matter injury in premature babies, perinatal infarcts of the middle cerebral artery, or generalized anoxia at the time of birth, indeed multiple causes, including intra-uterine infection or a genetic predisposition to infarction, may need to interact to produce a clinically significant injury. In this review, we consider which animal models best reproduce certain aspects of the condition, and the extent to which the multifactorial nature of cerebral palsy has been modeled. The degree to which the corticospinal system of various animal models human corticospinal system function and development is also explored. Where attempts have already been made to test early intervention in animal models, the outcomes are evaluated in light of the suitability of the model.

A new measure of hindlimb stepping ability in neonatally spinalized rats

One of the most widely used animal models for assessing recovery of locomotor functioning is the spinal rat. Although true differences in locomotor abilities of these animals are exhibited during treadmill testing, current measurement techniques often fail to detect them. The HiJK (Hillyer-Joynes Kinematics) scale was developed in an effort to distinguish more effectively between groups of spinal rats. Scale items were compiled after extensive review of the literature concerning development and analysis of rat locomotion and a thorough examination of the current tools. Treadmill tests for 137 Sprague-Dawley rats were taped and scored. The structure of the scale was tested with principle components and factor analysis, in which six of the eight items accounted for 59% of the variance, while all eight accounted for 78%. Validity tests demonstrate that HiJK is measuring locomotor performance accurately and powerfully. First, the HiJK scale correlates highly (>.8) with the widely used BBB scale and second, as shown with ANOVA, can distinguish between different groups of spinal rats. Reliability of the scale was also analyzed. Cronbach's alpha was shown to be .91, indicating considerable internal consistency. Additionally, inter-rater and intra-rater reliabilities were substantial, with correlations for most items reaching above .80. We believe that the HiJK scale will help researchers verify existing experimental differences, advance the field of spinal cord research, and, hopefully, lead to discovery of methods to enhance recovery of function.

Use of magnetic stimulation to elicit motor evoked potentials, somatosensory evoked potentials, and H-reflexes in non-sedated rodents

Assessment of locomotor function of rodents may be supplemented using electrophysiological tests which monitor the integrity of ascending and descending tracts as well as the focal circuitry of the spinal cord in non-sedated rodents. Magnetically induced SSEPs (M-SSEPs) were elicited in rats by activating the hindpaw using magnetic stimulation (MS). M-SSEP response latencies were slightly longer than those elicited by electrical stimulation. M-SSEPs were eliminated following selective dorsal column lacerations of the spinal cord, indicating that they were transmitted via this tract. Magnetically induced motor evoked potentials (M-MEPs) were elicited in mice following transcranial MS and recorded from the gastrocnemius muscles. M-MEPs performed on myelin deficient mice demonstrated longer onset latencies and smaller amplitudes than in wild-type mice. Magnetically induced H-reflexes (MH-reflexes) which assess local circuitry in the lumbosacral area of the spinal cord were performed in rats. This response disappeared following an L3 contusion spinal cord injury, however, kainic acid (KA) injection at L3, known to selectively destroy interneurons, caused a shorter latency and an increase in the amplitude of the MH-reflex. M-SSEPs and MH-reflexes in rats and M-MEPs in mice compliment locomotor evaluation in assessing the functional integrity of the spinal cord under normal and pathological conditions in the non-sedated animal.

The dependence of spinal cord development on corticospinal input and its significance in understanding and treating spastic cerebral palsy

The final phase of spinal cord development follows the arrival of descending pathways which brings about a reorganisation that allows mature motor behaviours to emerge under the control of higher brain centres. Observations made during typical human development have shown that low threshold stretch reflexes, including excitatory reflexes between agonist and antagonist muscle pairs are a feature of the newborn. However, perinatal lesions of the corticospinal tract can lead to abnormal development of spinal reflexes that includes retention and reinforcement of developmental features that do not emerge in adult stroke victims, even though they also suffer from spasticity. This review describes investigations in animal models into how corticospinal input may drive segmental maturation. It compares their findings with observations made in humans and discusses how therapeutic interventions in cerebral palsy might aim to correct imbalances between descending and segmental inputs, bearing in mind that descending activity may play the crucial role in development.

Spinal cord-transected mice learn to step in response to quipazine treatment and robotic training

In the present study, concurrent treatment with robotic step training and a serotonin agonist, quipazine, generated significant recovery of locomotor function in complete spinal cord-transected mice (T7-T9) that otherwise could not step. The extent of recovery achieved when these treatments were combined exceeded that obtained when either treatment was applied independently. We quantitatively analyzed the stepping characteristics of spinal mice after alternatively administering no training, manual training, robotic training, quipazine treatment, or a combination of robotic training with quipazine treatment, to examine the mechanisms by which training and quipazine treatment promote functional recovery. Using fast Fourier transform and principal components analysis, significant improvements in the step rhythm, step shape consistency, and number of weight-bearing steps were observed in robotically trained compared with manually trained or nontrained mice. In contrast, manual training had no effect on stepping performance, yielding no improvement compared with nontrained mice. Daily bolus quipazine treatment acutely improved the step shape consistency and number of steps executed by both robotically trained and nontrained mice, but these improvements did not persist after quipazine was withdrawn. At the dosage used (0.5 mg/kg body weight), quipazine appeared to facilitate, rather than directly generate, stepping, by enabling the spinal cord neural circuitry to process specific patterns of sensory information associated with weight-bearing stepping. Via this mechanism, quipazine treatment enhanced kinematically appropriate robotic training. When administered intermittently during an extended period of robotic training, quipazine revealed training-induced stepping improvements that were masked in the absence of the pharmacological treatment.

Effect of neonatal spinal transection and dorsal rhizotomy on hindlimb muscles

The purpose of this study was to elucidate the effect of deafferentation on spinal motoneurons. We studied the effects of spinal cord transection and/or dorsal rhizotomy upon the contractile properties of EDL and soleus muscle, as well as on the number of motoneurons corresponding to these muscles. Neonatal Wistar rats were randomly divided into four groups in which spinal midthoracic section (T8-T10), unilateral dorsal lumbar rhizotomy (L3-S2) or both procedures were performed on the second postnatal day (PND2). Another group served as unoperated control. At 2 months of age, the animals were evaluated for the contractile properties of a fast (EDL) and a slow (soleus) muscle. Isometric tension recordings were elicited by way of sciatic nerve branches stimulation. In addition, the incremental method was applied for the determination of the number of motor units supplying the two muscles, which was also verified by using the horseradish peroxidase (HRP) method of reverse labeling of motoneurons. Muscle alterations were confirmed by the usual biochemical staining. Our results, in agreement with the data from other researchers, show that significant muscle atrophy takes place after all experimental procedures. Additionally, spinal cord section alters the development of the dynamic properties of soleus muscle, which attains a fast profile. Following transection, the number of motor units remained unaltered, while rhizotomy affected only the soleus by reducing its motor units. The combined procedure affected both muscles, indicating that adequate synaptic input is essential for motoneuron survival.

Treadmill locomotion in the intact and spinal mouse

Because the genetic characteristics of several inbred strains of mice are well identified, their use is becoming increasingly popular in spinal cord injury research. In this context, it appears particularly important to document adequately motor patterns, such as locomotion in normal mice, to establish some baseline values of locomotor characteristics. It also seems crucial to determine the extent to which mice can express a locomotor pattern after a complete spinal transection to establish a baseline on which one can evaluate the effects of treatments after spinal injury. Therefore, we have used conventional techniques to document the kinematics of treadmill locomotion in intact mice (n = 11) and in mice with a complete section of the spinal cord at T8 (n = 12). The results show that the kinematics and EMG of adult normal mice can be adequately monitored with such conventional equipment and that mice can re-express hindlimb locomotion within 14 d after spinalization, without any pharmacological treatments. The angular excursions of the hip, knee, and ankle are similar to those of the intact mice, although the joints are sometimes more flexed. After spinal cord transection, out-of-phase alternation between the homologous limbs recovered, whereas the timing between homolateral limbs was completely lost. This remarkable ability of mice to express hindlimb locomotion after a complete spinalization should be taken into account in the evaluation of various procedures aimed at promoting the functional recovery of locomotion after spinal lesions.

N-methyl-D-aspartate receptor blockade during development induces short-term but not long-term changes in c-Jun and parvalbumin expression in the rat cervical spinal cord

During postnatal development, N-methyl-D-aspartate receptor (NMDA-R) expression progressively decreases in ventral and deep dorsal horns. This transient expression might play a role in activity-dependent development of segmental circuitry. NMDA-Rs were blocked unilaterally in the lower cervical spinal cord using Elvax implants that released the NMDA-R antagonist MK-801 maximally over a 2-week period from postnatal day 7 (P7) onward. At P14, the ratio of c-Jun immunoreactive motoneurons ipsilateral/contralateral to the implants was significantly increased and the ratio of parvalbumin immunoreactive neurons decreased, compared to control implants. However, at P84, MK-801-treated and control spinal cords appeared the same. Therefore, NMDA-R blockade during development only transiently altered expression of activity-dependent proteins in the spinal cord, unlike lesions to the developing motor cortex, which we have previously shown to have a permanent effect.

Plasticity of motor systems after incomplete spinal cord injury

Although spontaneous regeneration of lesioned fibres is limited in the adult central nervous system, many people that suffer from incomplete spinal cord injuries show significant functional recovery. This recovery process can go on for several years after the injury and probably depends on the reorganization of circuits that have been spared by the lesion. Synaptic plasticity in pre-existing pathways and the formation of new circuits through collateral sprouting of lesioned and unlesioned fibres are important components of this recovery process. These reorganization processes might occur in cortical and subcortical motor centres, in the spinal cord below the lesion, and in the spared fibre tracts that connect these centres. Functional and anatomical evidence exists that spontaneous plasticity can be potentiated by activity, as well as by specific experimental manipulations. These studies prepare the way to a better understanding of rehabilitation treatments and to the development of new approaches to treat spinal cord injury.

Plasticity in the rat spinal cord seen in response to lesions to the motor cortex during development but not to lesions in maturity

Motor cortical inputs and proprioreceptive muscle afferents largely target the same spinal cord region. This study explored the idea that during development the two inputs interact via an activity-dependent mechanism to produce mature patterns of innervation. In rats, the forelimb motor cortex was ablated unilaterally at either postnatal day 7 (P7), the beginning of corticospinal synaptogenesis in the cervical cord, or at P50. Comparisons were made with sham-operated animals. At P70, muscle afferents from the extensor digitorum communis muscle, contralateral to the lesion, were transganglionically labeled with cholera toxin B-subunit. Lower cervical spinal cord sections were immunostained for cholera toxin B, parvalbumin, and cJun. Our small lesions had no obvious effects upon forelimb function. However, developmental lesions, but not adult lesions, were shown to significantly increase the number of muscle afferent boutons present in the contralateral ventral horn, compared with sham-operated controls. Also, the ratio of parvalbumin-positive neurons contralateral/ipsilateral to the developmental lesion (but not adult lesions) was decreased and the ratio of cJun-positive motoneurons increased. Thus, an early motor cortex lesion resulted in retention of a proportion of muscle afferent synapses to the ventral horn that are known to be lost during normal development. Parvalbumin and cJun are markers of neuronal activity suggesting that spinal circuitry develops permanently altered activity patterns in response to an early cortical lesion, although this plasticity is lost in the mature animal.

Mini-review: assessment of behavioural recovery following spinal cord injury in rats

Behavioural recovery is one of the primary goals of therapeutic intervention in animal models of disease. It is necessary, therefore, to have the means with which to quantify pertinent behavioural changes in experimental animals. Nevertheless, the number and diversity of behavioural measures which have been used to assess recovery after experimental interventions often makes it difficult to compare results between studies. The present review attempts to integrate and categorize the wide variety of behavioural assessments used to measure recovery in spinal-injured rats. These categories include endpoint measures, kinematic measures, kinetic measurements, and electrophysiological measurements. Within this categorization, we discuss the advantages and disadvantages of each type of measurement. Finally, we make some recommendations regarding the principles for a comprehensive behavioural analysis after experimental spinal cord injury in rats.

Functional corticospinal projections are established prenatally in the human foetus permitting involvement in the development of spinal motor centres

From studies of subhuman primates it has been assumed that functional corticospinal innervation occurs post-natally in man. We report a post-mortem morphological study of human spinal cord, and neurophysiological and behavioural studies in preterm and term neonates and infants. From morphological studies it was demonstrated that corticospinal axons reach the lower cervical spinal cord by 24 weeks post-conceptional age (PCA) at the latest. Following a waiting period of up to a few weeks, it appears they progressively innervate the grey matter such that there is extensive innervation of spinal neurons, including motor neurons, prior to birth. Functional monosynaptic corticomotoneuronal projections were demonstrated neurophysiologically from term, but are also likely to be present from as early as 26 weeks PCA. At term, direct corticospinal projections to Group Ia inhibitory interneurons were also confirmed. Independent finger movements developed much later, between 6 and 12 months post-natally. These data do not support the proposal that in man, establishment of functional corticomotoneuronal projections occurs immediately prior to and provides the capacity for the expression of fine finger movement control. We propose instead that such early corticospinal innervation occurs to permit cortical involvement in activity dependent maturation of spinal motor centres during a critical period of perinatal development. Spastic cerebral palsy from perinatal damage to the corticospinal pathway secondarily involves disrupted development of spinal motor centres. Corticospinal axons retain a high degree of plasticity during axon growth and synaptic development. The possibility therefore exists to promote regeneration of disrupted corticospinal projections during the perinatal period with the double benefit of restoring corticospinal connectivity and normal development of spinal motor centres.

Fetal transplants rescue axial muscle representations in M1 cortex of neonatally transected rats that develop weight support

Fetal transplants rescue axial muscle representations in M1 cortex of neonatally transected rats that develop weight support. J. Neurophysiol. 80: 3021-3030, 1998. Intraspinal transplants of fetal spinal tissue partly alleviate motor deficits caused by spinal cord injury. How transplants modify body representation and muscle recruitment by motor cortex is currently largely unknown. We compared electromyographic responses from motor cortex stimulation in normal adult rats, adult rats that received complete spinal cord transection at the T8-T10 segmental level as neonates (TX rats), and similarly transected rats receiving transplants of embryonic spinal cord (TP rats). Rats were also compared among treatments for level of weight support and motor performance. Sixty percent of TP rats showed unassisted weight-supported locomotion as adults, whereas approximately 30% of TX rats with no intervention showed unassisted weight-supported locomotion. In the weight-supporting animals we found that the transplants enabled motor responses to be evoked by microstimulation of areas of motor cortex that normally represent the lumbar axial muscles in rats. These same regions were silent in all TX rats with transections but no transplants, even those exhibiting locomotion with weight support. In weight-supporting TX rats low axial muscles could be recruited from the rostral cortical axial representation, which normally represents the neck and upper trunk. No operated animal, even those with well-integrated transplants and good weight-supported locomotion, had a hindlimb motor representation in cortex. The data demonstrate that spinal transplants allow the development of some functional interactions between areas of motor cortex and spinal cord that are not available to the rat lacking the intervention. The data also suggest that operated rats that achieve weight support may primarily use the axial muscles to steer the pelvis and hindlimbs indirectly rather than use explicit hindlimb control during weight-supported locomotion.