Formation of descending pathways mediating cortical command to forelimb motoneurons in neonatally hemidecorticated rats

Can we gain translational insights into the functional roles of cerebral cortex from acortical rodent and naturally acortical zebrafish models?

Cerebral cortex is found only in mammals and is particularly prominent and developed in humans. Various rodent models with fully or partially ablated cortex are commonly used to probe the role of cortex in brain functions and its multiple subcortical projections, including pallium, thalamus and the limbic system. Various rodent models are traditionally used to study the role of cortex in brain functions. A small teleost fish, the zebrafish (Danio rerio), has gained popularity in neuroscience research, and albeit (like other fishes) lacking cortex, its brain performs well some key functions (e.g., memory, consciousness and motivation) with complex, context-specific and well-defined behaviors. Can rodent and zebrafish models help generate insights into the role of cortex in brain functions, and dissect its cortex-specific (vs. non-cortical) functions? To address this conceptual question, here we evaluate brain functionality in intact vs. decorticated rodents and further compare it in the zebrafish, a naturally occurring acortical species. Overall, comparing cortical and acortical rodent models with naturally acortical zebrafish reveals both distinct and overlapping contributions of neocortex and 'precortical' zebrafish telencephalic regions to higher brain functions. Albeit morphologically different, mammalian neocortex and fish pallium may possess more functional similarities than it is presently recognized, calling for further integrative research utilizing both cortical and decorticated/acortical vertebrate model organisms.

A neural tract tracing study on synaptic connections for cortical glutamatergic terminals and cervical spinal calretinin neurons in rats

The cerebral cortex innervates motor neurons in the anterior horn of the spinal cord by regulating of interneurons. At present, nerve tracing, immunohistochemistry, and immunoelectron microscopy are used to explore and confirm the characteristics of synaptic connections between the corticospinal tract (CST) and cervical spinal calretinin (Cr) interneurons. Our morphological results revealed that (1) biotinylated dextran amine labeled (BDA+) fibers from the cerebral cortex primarily presented a contralateral spinal distribution, with a denser distribution in the ventral horn (VH) than in the dorsal horn (DH). An electron microscope (EM) showed that BDA+ terminals formed asymmetric synapses with spinal neurons, and their mean labeling rate was not different between the DH and VH. (2) Cr-immunoreactive (Cr+) neurons were unevenly distributed throughout the spinal gray matter, and were denser and larger in the VH than in the DH. At the single labeling electron microscope (EM) level, the labeling rate of Cr+ dendrites was higher in the VH than in the DH, in which Cr+ dendrites mainly received asymmetric synaptic inputs, and between the VH and DH. (3) Immunofluorescence triple labeling showed obvious apposition points among BDA+ terminals, synaptophysin and Cr+ dendrites, with a higher density in the VH than in the DH. (4) Double labeling in EM, BDA+ terminals and Cr+ dendrites presented the same pattern, BDA+ terminals formed asymmetric synapses either with Cr+ dendrites or Cr negative (Cr-) dendrites, and Cr+ dendrites received either BDA+ terminals or BDA- synaptic inputs. The average percentage of BDA+ terminals targeting Cr+ dendrites was higher in the VH than in the DH, but the percentage of BDA+ terminals targeting Cr- dendrites was prominently higher than that targeting Cr+ dendrites. There was no difference in BDA+ terminal size. The percentage rate for Cr+ dendrites receiving BDA+ terminal inputs was lower than that receiving BDA- terminal inputs, and the BDA+ terminal size was larger than the BDA- terminal size received by Cr+ dendrites. The present morphological results suggested that spinal Cr+ interneurons are involved in the regulatory process of the cortico-spinal pathway.

Restoring After Central Nervous System Injuries: Neural Mechanisms and Translational Applications of Motor Recovery

Central nervous system (CNS) injuries, including stroke, traumatic brain injury, and spinal cord injury, are leading causes of long-term disability. It is estimated that more than half of the survivors of severe unilateral injury are unable to use the denervated limb. Previous studies have focused on neuroprotective interventions in the affected hemisphere to limit brain lesions and neurorepair measures to promote recovery. However, the ability to increase plasticity in the injured brain is restricted and difficult to improve. Therefore, over several decades, researchers have been prompted to enhance the compensation by the unaffected hemisphere. Animal experiments have revealed that regrowth of ipsilateral descending fibers from the unaffected hemisphere to denervated motor neurons plays a significant role in the restoration of motor function. In addition, several clinical treatments have been designed to restore ipsilateral motor control, including brain stimulation, nerve transfer surgery, and brain–computer interface systems. Here, we comprehensively review the neural mechanisms as well as translational applications of ipsilateral motor control upon rehabilitation after CNS injuries.

Long-term rehabilitation reduces task error variability in cervical spinal cord contused rats

To promote skilled forelimb function following a spinal cord injury, we have evaluated whether long-term voluntary sensorimotor rehabilitation can promote substantial reaching and grasping recovery. Long-Evans rats were trained to reach single pellets and then received a moderate 100 kdyn contusion to the C5 lateral funiculi. During the first eight months post-injury, a group of animals was enrolled in daily skilled reaching rehabilitation consisting of grabbing and manipulating seeds from the bottom of a grid. Single-pellet reaching and grasping recovery was tested biweekly throughout the functional follow-up and the recovery was compared to a second group of contused but non-rehabilitated animals. Following the injury, reaching and grasping success dropped to zero in both groups and remained absent for three months post-injury, followed by a slight recovery that remained constant until the end of the follow-up. No differences in reaching success were found between groups. Nevertheless, the type of gesture errors in the failed attempts were categorized and scored. The errors ranged from the animal's inability to lift the paw and initiate the movement to the final stage of the attempt, in which the pellet falls during grasping and retraction of the paw towards the mouth. Both groups of animals exhibited similar types of errors but the animals with rehabilitation showed less error variability and those that occurred at the latest stages of the attempt predominated compared to those performed by the non-trained animals. Histological examination of the injury showed that injury severity was similar between groups and that the damage was circumscribed to the site of impact, affecting mainly the dorsal and medial region of the lateral funiculi, with preservation of the dorsal component of the corticospinal tract and the interneurons and motoneurons of the spinal segments beyond the site of injury. The results indicate that activity-dependent plasticity driven by voluntary rehabilitation decreases task error variability and drives the recovery of the movement gestures. However, the plasticity achieved is insufficient to attain full functional recovery to successfully reach, grasp and release the pellets in the mouth, indicating the necessity for additional interventional therapies to promote repair.

Intraoperative Lumbar Muscle Motor Evoked Potential Monitoring With Transcortical Stimulation

BACKGROUND

Stimulating electrodes for lower extremity motor-evoked potential (LE-MEP) monitoring with transcortical stimulation are usually placed on the medial side of motor cortex convexity, which is not lower extremity but lumbar motor area. Lumbar MEP may be elicited with lower stimulation intensity than LE-MEP through this location, and it is useful to monitor lower extremity motor function intraoperatively.

METHODS

Intraoperative lumbar and LE-MEP monitoring with transcortical stimulation during surgery of 12 patients with lesions involving the motor cortex from January 2012 to February 2019 at Shinshu University Hospital were reviewed retrospectively. Stimulations were delivered by a train of 5 pulses of anodal constant current stimulation. Stimulating electrode position was determined by motor cortex mapping. Recording needle electrodes were placed on bilateral lumbar muscles and contralateral leg muscles. The threshold-level stimulation method was used for MEP monitoring. The thresholds, monitoring result, and postoperative motor function of lumbar and lower extremities were compared.

RESULTS

The mean baseline thresholds were 19.9 ± 8.9 mA for lumbar MEP and 26.5 ± 11.5 mA for LE-MEP (P = 0.02). Patterns of intraoperative monitoring changes were the same between lumbar and LE-MEP monitoring.

CONCLUSIONS

Lumbar MEP was stimulated with lower stimulation intensity than the LE-MEP with the same intraoperative pattern of waveform changes in 12 patients. Lumbar MEP monitoring may be useful for preserving the corticospinal tract of lower extremities intraoperatively.

Diabetes Mellitus-Related Dysfunction of the Motor System

Although motor deficits in humans with diabetic neuropathy have been extensively researched, its effect on the motor system is thought to be lesser than that on the sensory system. Therefore, motor deficits are considered to be only due to sensory and muscle impairment. However, recent clinical and experimental studies have revealed that the brain and spinal cord, which are involved in the motor control of voluntary movement, are also affected by diabetes. This review focuses on the most important systems for voluntary motor control, mainly the cortico-muscular pathways, such as corticospinal tract and spinal motor neuron abnormalities. Specifically, axonal damage characterized by the proximodistal phenotype occurs in the corticospinal tract and motor neurons with long axons, and the transmission of motor commands from the brain to the muscles is impaired. These findings provide a new perspective to explain motor deficits in humans with diabetes. Finally, pharmacological and non-pharmacological treatment strategies for these disorders are presented.

Inhibition of HDAC increases BDNF expression and promotes neuronal rewiring and functional recovery after brain injury

Brain injury causes serious motor, sensory, and cognitive disabilities. Accumulating evidence has demonstrated that histone deacetylase (HDAC) inhibitors exert neuroprotective effects against various insults to the central nervous system (CNS). In this study, we investigated the effects of the HDAC inhibition on the expression of brain-derived neurotrophic factor (BDNF) and functional recovery after traumatic brain injury (TBI) in mice. Administration of class I HDAC inhibitor increased the number of synaptic boutons in rewiring corticospinal fibers and improved the recovery of motor functions after TBI. Immunohistochemistry results showed that HDAC2 is mainly expressed in the neurons of the mouse spinal cord under normal conditions. After TBI, HDAC2 expression was increased in the spinal cord after 35 days, whereas BDNF expression was decreased after 42 days. Administration of CI-994 increased BDNF expression after TBI. Knockdown of HDAC2 elevated H4K5ac enrichment at the BDNF promoter, which was decreased following TBI. Together, our findings suggest that HDAC inhibition increases expression of neurotrophic factors, and promote neuronal rewiring and functional recovery following TBI.

Mechanism of forelimb motor function restoration in rats with cervical spinal cord hemisection-neuroanatomical validation

Purpose

The purpose of this study is neuroanatomical validation of forelimb motor function restoration in rats with cervical spinal cord injury.

Materials and methods

We used eight cervical hemisected rats and eight normal rats. We cut in half the C3/4 cervical spinal cord of 18-weeks-old normal rats. We used 24-weeks-old rats that had reached a nearly steady state of forelimb motor function after the hemisection (Hemisection group). Normal 24-week-old rats were used as Control group. To evaluate the corticospinal tracts, neuro-tracing by biotynirated dextran-amine (BDA) was used. BDA was injected into the damaged side of the cerebral primary motor cortex. In order to quantitatively analyze the specimen, we recorded a site where nerve fibers appear in each specimen in the image analysis (1) and defined the increase rate of immunostaining area using ImageJ in the image analysis (2). Based on the evaluation in the image analysis (1) and the image analysis (2), the Hemisection group and the Control group were compared.

Results

In the image analysis (1), a region with robust appearance of aberrant nerve fibers was observed in the cephalad side of the Hemisection site in Hemisection group than Control group. In the spinal cord caudal to the hemisection, such region was generally more in Hemisection group, however, disappeared or reduced appearance was observed in some regions. In the image analysis (2), no statistical significant difference was noted in each level.

Conclusion

There is a high probability that these aberrant nerve fibers beyond the midline could be involved in forelimb motor function restoration in rats with cervical cord hemisection.

Cortical Reorganization of Sensorimotor Systems and the Role of Intracortical Circuits After Spinal Cord Injury

The plasticity of sensorimotor systems in mammals underlies the capacity for motor learning as well as the ability to relearn following injury. Spinal cord injury, which both deprives afferent input and interrupts efferent output, results in a disruption of cortical somatotopy. While changes in corticospinal axons proximal to the lesion are proposed to support the reorganization of cortical motor maps after spinal cord injury, intracortical horizontal connections are also likely to be critical substrates for rehabilitation-mediated recovery. Intrinsic connections have been shown to dictate the reorganization of cortical maps that occurs in response to skilled motor learning as well as after peripheral injury. Cortical networks incorporate changes in motor and sensory circuits at subcortical or spinal levels to induce map remodeling in the neocortex. This review focuses on the reorganization of cortical networks observed after injury and posits a role of intracortical circuits in recovery.

Plasticity in One Hemisphere, Control From Two: Adaptation in Descending Motor Pathways After Unilateral Corticospinal Injury in Neonatal Rats

After injury to the corticospinal tract (CST) in early development there is large-scale adaptation of descending motor pathways. Some studies suggest the uninjured hemisphere controls the impaired forelimb, while others suggest that the injured hemisphere does; these pathways have never been compared directly. We tested the contribution of each motor cortex to the recovery forelimb function after neonatal injury of the CST. We cut the left pyramid (pyramidotomy) of postnatal day 7 rats, which caused a measurable impairment of the right forelimb. We used pharmacological inactivation of each motor cortex to test its contribution to a skilled reach and supination task. Rats with neonatal pyramidotomy were further impaired by inactivation of motor cortex in both the injured and the uninjured hemispheres, while the forelimb of uninjured rats was impaired only from the contralateral motor cortex. Thus, inactivation demonstrated motor control from each motor cortex. In contrast, physiological and anatomical interrogation of these pathways support adaptations only in the uninjured hemisphere. Intracortical microstimulation of motor cortex in the uninjured hemisphere of rats with neonatal pyramidotomy produced responses from both forelimbs, while stimulation of the injured hemisphere did not elicit responses from either forelimb. Both anterograde and retrograde tracers were used to label corticofugal pathways. There was no increased plasticity from the injured hemisphere, either from cortex to the red nucleus or the red nucleus to the spinal cord. In contrast, there were very strong CST connections to both halves of the spinal cord from the uninjured motor cortex. Retrograde tracing produced maps of each forelimb within the uninjured hemisphere, and these were partly segregated. This suggests that the uninjured hemisphere may encode separate control of the unimpaired and the impaired forelimbs of rats with neonatal pyramidotomy.

Mechanism of Restoration of Forelimb Motor Function after Cervical Spinal Cord Hemisection in Rats: Electrophysiological Verification

The objective of this study was to electrophysiologically assess the corticospinal tracts of adult rats and the recovery of motor function of their forelimbs after cervical cord hemisection. Of 39 adult rats used, compound muscle action potentials (CMAPs) of the forelimbs of 15 rats were evaluated, before they received left C5 segmental hemisection of the spinal cord, by stimulating the pyramid of the medulla oblongata on one side using an exciting microelectrode. All 15 rats exhibited contralateral electrical activity, but their CMAPs disappeared after hemisection. The remaining 24 rats received hemisection first, and CMAPs of 12 rats were assessed over time to study their recovery time. All of them exhibited electrical activity of the forelimbs in 4 weeks after surgery. The remaining 12 rats received additional right C2 segmental hemisection, and variation of CMAPs between before and after surgery was examined. The right side of the 12 rats that received the additional hemisection exhibited no electrical activity in response to the stimulation of the pyramids on both sides. These results suggest that changes in path between the resected and healthy sides, activation of the ventral corticospinal tracts, and propriospinal neurons were involved in the recovery of motor function after cervical cord injury.

Blood-Spinal Cord Barrier Alterations in Subacute and Chronic Stages of a Rat Model of Focal Cerebral Ischemia

We previously demonstrated blood-brain barrier impairment in remote contralateral brain areas in rats at 7 and 30 days after transient middle cerebral artery occlusion (tMCAO), indicating ischemic diaschisis. Here, we focused on effects of subacute and chronic focal cerebral ischemia on the blood-spinal cord barrier (BSCB). We observed BSCB damage on both sides of the cervical spinal cord in rats at 7 and 30 days post-tMCAO. Major BSCB ultrastructural changes in spinal cord gray and white matter included vacuolated endothelial cells containing autophagosomes, pericyte degeneration with enlarged mitochondria, astrocyte end-feet degeneration and perivascular edema; damaged motor neurons, swollen axons with unraveled myelin in ascending and descending tracts and astrogliosis were also observed. Evans Blue dye extravasation was maximal at 7 days. There was immunofluorescence evidence of reduction of microvascular expression of tight junction occludin, upregulation of Beclin-1 and LC3B immunoreactivities at 7 days and a reduction of the latter at 30 days post-ischemia. These novel pathological alterations on the cervical spinal cord microvasculature in rats after tMCAO suggest pervasive and long-lasting BSCB damage after focal cerebral ischemia, and that spinal cord ischemic diaschisis should be considered in the pathophysiology and therapeutic approaches in patients with ischemic cerebral infarction.

Plasticity of motor network and function in the absence of corticospinal projection

Despite the obvious clinical interest, our understanding of how developmental mechanisms are redeployed during degeneration and regeneration after brain and spinal cord injuries remains quite rudimentary. In animal models of spinal cord injury, although spontaneous regeneration of descending axons is limited, compensation by intact corticospinal axons, descending tracts from the brainstem, and local intrinsic spinal networks all contribute to the recovery of motor function. Here, we investigated spontaneous motor compensation and plasticity that occur in the absence of corticospinal tract, using Celsr3|Emx1 mice in which the corticospinal tract is completely and specifically absent as a consequence of Celsr3 inactivation in the cortex. Mutant mice had no paresis, but displayed hyperactivity in open-field, and a reduction in skilled movements in food pellet manipulation tests. The number of spinal motoneurons was reduced and their terminal arbors at neuromuscular junctions were atrophic, which was reflected in electromyography deficits. Rubrospinal projections, calretinin-positive propriospinal projections, afferent innervation of motoneurons by calretinin-positive segmental interneurons, and terminal ramifications of monoaminergic projections were significantly increased. Contrary to control animals, mutants also developed a severe and persistent disability of forelimb use following the section of the rubrospinal tract at the C4 spinal level. These observations demonstrate for the first time that the congenital absence of the corticospinal tract induces spontaneous plasticity, both at the level of the motor spinal cord and in descending monoaminergic and rubrospinal projections. Such compensatory mechanisms could be recruited in case of brain or spinal cord lesion or degeneration.

Plasticity of subcortical pathways promote recovery of skilled hand function in rats after corticospinal and rubrospinal tract injuries

The corticospinal and rubrospinal tracts are the predominant tracts for controlling skilled hand function. Injuries to these tracts impair grasping but not gross motor functions such as overground locomotion. The aim of the present study was to determine whether or not, after damage to both the corticospinal and rubrospinal tracts, other spared subcortical motor pathway can mediate the recovery of skilled hand function. Adult rats received a bilateral injury to the corticospinal tract at the level of the medullar pyramids and a bilateral ablation of the rubrospinal axons at C4. One group of rats received, acutely after injury, two injections of chondroitinase-ABC at C7, and starting at 7days post-injury were enrolled in daily reaching and grasping rehabilitation (CHASE group, n=5). A second group of rats received analogous injections of ubiquitous penicillinase, and did not undergo rehabilitation (PEN group, n=5). Compared to rats in the PEN group, CHASE rats gradually recovered the ability to reach and grasp over 42days after injury. Overground locomotion was mildly affected after injury and both groups followed similar recovery. Since the reticulospinal tract plays a predominant role in motor control, we further investigated whether or not plasticity of this pathway could contribute to the animal's recovery. Reticulospinal axons were anterogradely traced in both groups of rats. The density of reticulospinal processes in both the normal and ectopic areas of the grey ventral matter of the caudal segments of the cervical spinal cord was greater in the CHASE than PEN group. The results indicate that after damage to spinal tracts that normally mediate the control of reaching and grasping in rats other complementary spinal tracts can acquire the role of those damaged tracts and promote task-specific recovery.

Comprehensive analysis of neonatal versus adult unilateral decortication in a mouse model using behavioral, neuroanatomical, and DNA microarray approaches

Previously, studying the development, especially of corticospinal neurons, it was concluded that the main compensatory mechanism after unilateral brain injury in rat at the neonatal stage was due in part to non-lesioned ipsilateral corticospinal neurons that escaped selection by axonal elimination or neuronal apoptosis. However, previous results suggesting compensatory mechanism in neonate brain were not correlated with high functional recovery. Therefore, what is the difference among neonate and adult in the context of functional recovery and potential mechanism(s) therein? Here, we utilized a brain unilateral decortication mouse model and compared motor functional recovery mechanism post-neonatal brain hemisuction (NBH) with adult brain hemisuction (ABH). Three analyses were performed: (1) Quantitative behavioral analysis of forelimb movements using ladder walking test; (2) neuroanatomical retrograde tracing analysis of unlesioned side corticospinal neurons; and (3) differential global gene expressions profiling in unlesioned-side neocortex (rostral from bregma) in NBH and ABH on a 8 × 60 K mouse whole genome Agilent DNA chip. Behavioral data confirmed higher recovery ability in NBH over ABH is related to non-lesional frontal neocortex including rostral caudal forelimb area. A first inventory of differentially expressed genes genome-wide in the NBH and ABH mouse model is provided as a resource for the scientific community.

Organization of pontine reticulospinal inputs to motoneurons controlling axial and limb muscles in the neonatal mouse

Using optical recording of synaptically mediated calcium transients and selective spinal lesions, we investigated the pattern of activation of spinal motoneurons (MNs) by the pontine reticulospinal projection in isolated brain stem-spinal cord preparations from the neonatal mouse. Stimulation sites throughout the region where the pontine reticulospinal neurons reside reliably activated MNs at cervical, thoracic, and lumbar levels. Activation was similar in MNs ipsi- and contralateral to the stimulation site, similar in medial and lateral motor columns that contain trunk and limb MNs, respectively, and similar in the L2 and L5 segments that predominantly contain flexor and extensor MNs, respectively. In nonlesioned preparations, responses in both ipsi- and contralateral MNs followed individual stimuli in stimulus trains nearly one-to-one (with few failures). After unilateral hemisection at C1 on the same side as the stimulation, responses had substantially smaller magnitudes and longer latencies and no longer followed individual stimuli. After unilateral hemisection at C1 on the side opposite to the stimulation, the responses were also smaller, but their latencies were not affected. Thus we distinguish two pontine reticulospinal pathways to spinal MNs, one uncrossed and the other crossed, of which the uncrossed pathway transmits more faithfully and appears to be more direct.

Reorganization of sensory pathways after neonatal hemidecortication in rats

We investigated ascending somatosensory pathways in neonatally hemidecorticated rats. Injection of an anterograde tracer, biotinylated dextran amine (BDA), into the contralesional dorsal root ganglions revealed ipsilateral projections to the dorsal column nuclei (DCN) in hemidecorticated rats as well as in normal rats. Injection of BDA into the DCN on the same side revealed that while most axons projected to the contralateral thalamus, some axons were detected in the ipsilateral thalamus in hemidecorticated rats while such projections were rarely detected in normal rats. The results suggest that aberrant ipsilateral projections of DCN neurons contralateral to the lesion developed after the hemidecortication.

Differential contributions of rostral and caudal frontal forelimb areas to compensatory process after neonatal hemidecortication in rats

Following brain damage, especially in juvenile animals, large-scale reorganization is known to occur in the remaining brain structures to compensate for functional deficits. In rats with neonatal hemidecortication, corticospinal fibers originating from the undamaged side of the sensorimotor cortex issue collateral sprouts to the ipsilateral spinal gray matter that mediate cortical excitation to ipsilateral forelimb motoneurons and compensate for the deficit in forelimb movements. The present study was designed to investigate the origins of the ipsilateral corticospinal projection in neonatally hemidecorticated rats. Corticospinal neurons (CSNs) were labeled in adults by injecting retrograde neural tracers, cholera toxin subunit B with different fluorescent probes, into either side of the cervical spinal gray matter. In the undamaged cortex, double-labeled neurons were rarely found. CSNs with contralateral projections (contra-CSNs) and those with ipsilateral projections (ipsi-CSNs) were distributed both in the rostral forelimb motor area (RFA) and the caudal forelimb motor area (CFA). However, there was a difference in the distributions of the ipsi-CSNs between the two forelimb areas. Whereas the distribution of the ipsi-CSNs largely overlapped with that of the contra-CSNs in the RFA, the ipsi-CSNs tended to be segregated from the contra-CSNs in the CFA. The results suggested that the RFA and the CFA contribute to the compensatory process in different ways.

Developmental plasticity of descending motor pathways

Juvenile animal brains are highly plastic and thus often achieve better functional recovery after injury compared with adult brains. Recently, Umeda et al. (Umeda T, Takahashi M, Isa K, Isa T. J Neurophysiol 104: 1707-1716, 2010) have shown that the remodeling of both corticospinal and extra-pyramidal pathways can contribute to the recovery of grasping and reaching ability in hemidecorticated juvenile rats. They have further unveiled the strengthening of the cortico-reticulo-spinal pathway after injury, that mediates the fast excitation of ipsilateral motoneurons for functional recovery.