Facilitation of motor skill learning by callosal denervation or forced forelimb use in adult rats

Recovery after brain injury: mechanisms and principles

The past 20 years have represented an important period in the development of principles underlying neuroplasticity, especially as they apply to recovery from neurological injury. It is now generally accepted that acquired brain injuries, such as occur in stroke or trauma, initiate a cascade of regenerative events that last for at least several weeks, if not months. Many investigators have pointed out striking parallels between post-injury plasticity and the molecular and cellular events that take place during normal brain development. As evidence for the principles and mechanisms underlying post-injury neuroplasticity has been gleaned from both animal models and human populations, novel approaches to therapeutic intervention have been proposed. One important theme has persisted as the sophistication of clinicians and scientists in their knowledge of neuroplasticity mechanisms has grown: behavioral experience is the most potent modulator of brain plasticity. While there is substantial evidence for this principle in normal, healthy brains, the injured brain is particularly malleable. Based on the quantity and quality of motor experience, the brain can be reshaped after injury in either adaptive or maladaptive ways. This paper reviews selected studies that have demonstrated the neurophysiological and neuroanatomical changes that are triggered by motor experience, by injury, and the interaction of these processes. In addition, recent studies using new and elegant techniques are providing novel perspectives on the events that take place in the injured brain, providing a real-time window into post-injury plasticity. These new approaches are likely to accelerate the pace of basic research, and provide a wealth of opportunities to translate basic principles into therapeutic methodologies.

Combined SCI and TBI: recovery of forelimb function after unilateral cervical spinal cord injury (SCI) is retarded by contralateral traumatic brain injury (TBI), and ipsilateral TBI balances the effects of SCI on paw placement

A significant proportion (estimates range from 16 to 74%) of patients with spinal cord injury (SCI) have concomitant traumatic brain injury (TBI), and the combination often produces difficulties in planning and implementing rehabilitation strategies and drug therapies. For example, many of the drugs used to treat SCI may interfere with cognitive rehabilitation, and conversely drugs that are used to control seizures in TBI patients may undermine locomotor recovery after SCI. The current paper presents an experimental animal model for combined SCI and TBI to help drive mechanistic studies of dual diagnosis. Rats received a unilateral SCI (75 kdyn) at C5 vertebral level, a unilateral TBI (2.0 mm depth, 4.0 m/s velocity impact on the forelimb sensori-motor cortex), or both SCI+TBI. TBI was placed either contralateral or ipsilateral to the SCI. Behavioral recovery was examined using paw placement in a cylinder, grooming, open field locomotion, and the IBB cereal eating test. Over 6weeks, in the paw placement test, SCI+contralateral TBI produced a profound deficit that failed to recover, but SCI+ipsilateral TBI increased the relative use of the paw on the SCI side. In the grooming test, SCI+contralateral TBI produced worse recovery than either lesion alone even though contralateral TBI alone produced no observable deficit. In the IBB forelimb test, SCI+contralateral TBI revealed a severe deficit that recovered in 3 weeks. For open field locomotion, SCI alone or in combination with TBI resulted in an initial deficit that recovered in 2 weeks. Thus, TBI and SCI affected forelimb function differently depending upon the test, reflecting different neural substrates underlying, for example, exploratory paw placement and stereotyped grooming. Concurrent SCI and TBI had significantly different effects on outcomes and recovery, depending upon laterality of the two lesions. Recovery of function after cervical SCI was retarded by the addition of a moderate TBI in the contralateral hemisphere in all tests, but forepaw placements were relatively increased by an ipsilateral TBI relative to SCI alone, perhaps due to the dual competing injuries influencing the use of both forelimbs. These findings emphasize the complexity of recovery from combined CNS injuries, and the possible role of plasticity and laterality in rehabilitation, and provide a start towards a useful preclinical model for evaluating effective therapies for combine SCI and TBI.

Reflections of experience-expectant development in repair of the adult damaged brain

Behavioral experience has long been known to influence functional outcome after brain injury, but only recently has its pervasive role in the reorganization of the adult brain after damage become appreciated. We briefly review findings from animal models on the role of experience in shaping neuronal events after stroke-like injury. Experience-dependent neural plasticity can be enhanced or impaired by brain damage, depending upon injury parameters and timing. The neuronal growth response to some experiences is heightened due to interactions with denervation-induced plasticity. This includes compensatory behavioral strategies developed in response to functional impairments. Early behavioral experiences can constrain later experience-dependent plasticity, leading to suboptimal functional outcome. Time dependencies and facets of neural growth patterns are reminiscent of experience-expectant processes that shape brain development. As with sensitive periods in brain development, this process may establish behavioral patterns early after brain injury which are relatively resistant to later change.

Lesion size-dependent synaptic and astrocytic responses in cortex contralateral to infarcts in middle-aged rats

In young adult rats, unilateral lesions of the sensorimotor cortex lead to neuronal structural plasticity and synaptogenesis in the contralateral motor cortex, which is connected to the lesion site by transcallosal fibers. The contralesional neural plasticity varies with lesion size and results from the convergence of denervation-induced reactive plasticity and behavioral asymmetries. It was unknown whether similar effects occur in older animals. Furthermore, the coordination of synaptic responses with that of perisynaptic astrocytes had not been investigated. In this study, middle-aged rats (14-16 months old) were given sham-operations or unilateral ischemic lesions of the sensorimotor cortex. Fifty days later, numerical densities of neurons and synapses and morphological characteristics of astrocytic processes in layer V of the contralesional motor cortex were measured using stereological light and electron microscopy methods. Lesions resulted in behavioral asymmetries, but no significant synapse addition in the contralesional motor cortex. Synapse number per neuron was negatively correlated with lesion size and reduced opposite larger lesions compared with smaller ones. Astrocytic changes were also lesion size-dependent. Astrocytic hypertrophy was observed only after smaller lesions and was associated with greater coverage and greater numbers of synapses. These findings are consistent with those in younger rats indicating an inverse relationship between lesion size and adaptive neuronal restructuring in denervated cortex. However, they indicate that the synaptogenic reaction to this lesion is relatively limited in older animals. Finally, the results indicate that structural plasticity of perisynaptic astrocytes parallels, and could play a role in shaping, synaptic responses to postischemic denervation.

Motor cortex bilateral motor representation depends on subcortical and interhemispheric interactions

The corticospinal tract is a predominantly crossed pathway. Nevertheless, the primary motor cortex (M1) is activated bilaterally during unilateral movements and several animal studies showed that M1 has a bilateral motor representation. A better understanding of the uncrossed corticospinal system is especially important for elucidating its role in recovery of limb control after unilateral injury. We used intracortical microstimulation (ICMS) to determine the representation of contralateral and ipsilateral forelimb joints at single M1 sites in the rat. Most sites representing an ipsilateral joint corepresented the same joint contralaterally. We next determined whether ipsilateral responses evoked in one hemisphere depended on the function of M1 in the opposite hemisphere using reversible inactivation and pyramidal tract lesion. Ipsilateral responses were eliminated when the homotopic forelimb area of M1 in the opposite hemisphere was inactivated or when the pyramidal tract on the nonstimulated side was sectioned. To determine the role of transfer between M1 in each hemisphere we sectioned the corpus callosum, which produced a 33% increase in ipsilateral ICMS thresholds. Neither M1 inactivation nor callosal section changed contralateral response thresholds, indicating the absence of tonic excitatory or inhibitory drive to the opposite M1. Finally, ipsilateral responses following M1 inactivation and pyramidal tract lesion could be evoked after systemic administration of the K(+) channel blocker 4-aminopyridine, suggesting the presence of latent connections. Our findings show important interactions between the corticospinal systems from each side, especially at the spinal level. This has important implications for recruiting the ipsilateral corticospinal system after injury.

Translating principles of neural plasticity into research on speech motor control recovery and rehabilitation

PURPOSE

To review the principles of neural plasticity and make recommendations for research on the neural bases for rehabilitation of neurogenic speech disorders.

METHOD

A working group in speech motor control and disorders developed this report, which examines the potential relevance of basic research on the brain mechanisms involved in neural plasticity and discusses possible similarities and differences for application to speech motor control disorders. The possible involvement of neural plasticity in changes in speech production in normalcy, development, aging, and neurological diseases and disorders was considered. This report focuses on the appropriate use of functional and structural neuroimaging and the design of feasibility studies aimed at understanding how brain mechanisms are altered by environmental manipulations such as training and stimulation and how these changes might enhance the future development of rehabilitative methods for persons with speech motor control disorders.

CONCLUSIONS

Increased collaboration with neuroscientists working in clinical research centers addressing human communication disorders might foster research in this area. It is hoped that this article will encourage future research on speech motor control disorders to address the principles of neural plasticity and their application for rehabilitation.

Principles of experience-dependent neural plasticity: implications for rehabilitation after brain damage

PURPOSE

This paper reviews 10 principles of experience-dependent neural plasticity and considerations in applying them to the damaged brain.

METHOD

Neuroscience research using a variety of models of learning, neurological disease, and trauma are reviewed from the perspective of basic neuroscientists but in a manner intended to be useful for the development of more effective clinical rehabilitation interventions.

RESULTS

Neural plasticity is believed to be the basis for both learning in the intact brain and relearning in the damaged brain that occurs through physical rehabilitation. Neuroscience research has made significant advances in understanding experience-dependent neural plasticity, and these findings are beginning to be integrated with research on the degenerative and regenerative effects of brain damage. The qualities and constraints of experience-dependent neural plasticity are likely to be of major relevance to rehabilitation efforts in humans with brain damage. However, some research topics need much more attention in order to enhance the translation of this area of neuroscience to clinical research and practice.

CONCLUSION

The growing understanding of the nature of brain plasticity raises optimism that this knowledge can be capitalized upon to improve rehabilitation efforts and to optimize functional outcome.

Ipsilateral whiskers suppress experience-dependent plasticity in the barrel cortex

Each cerebral hemisphere processes sensory input from both sides of the body, but the impact of this convergence on shaping and modifying receptive field properties remains controversial. Here we investigated the effect of chronic deprivation of ipsilateral sensory whiskers on receptive field plasticity in primary somatosensory cortex. In the absence of ipsilateral whiskers, cortical receptive fields were significantly larger than control after 1 week. Removal of all but a single whisker from one side of the face [single-whisker experience (SWE)] has been shown to result in the expansion of the cortical area responding to the spared whisker. We compared the effects of SWE in the presence (SWE-unilateral) and absence (SWE-bilateral) of ipsilateral whiskers. SWE-bilateral deprivation results in a significant increase in neuronal responses to spared whisker stimulation both in its cognate barrel column and in adjacent, surrounding barrel columns compared with control and SWE-unilateral deprived animals. Surround receptive fields in deprived columns were maintained in SWE-bilateral treated animals but depressed in SWE-unilateral animals. The increase in spared whisker responses was progressive with longer deprivation periods. These data show that ipsilateral whiskers can constrain receptive field size in the barrel cortex.

Mechanisms for recovery of motor function following cortical damage

Recent studies of focal injury to the cerebral cortex have demonstrated that the remaining, intact tissue undergoes structural and functional changes that could play a substantial role in neurological recovery. New information regarding the molecular and cellular environment in the adjacent, intact tissue has suggested that waves of growth promotion and inhibition modulate the self-repair processes of the brain. Furthermore, recent studies have documented widespread neurophysiological and neuroanatomical changes in regions remote from a focal cortical injury, suggesting that entire cortical networks participate in the recovery process.

Behavioral effects of corpus callosum transection and environmental enrichment in adult rats

A common assumption about the corpus callosum transection (CCX) is that it only affects behaviors heavily relying on interhemispheric communication. However, cerebral laterality is ubiquitous across motor and perceptual, cognitive and emotional domains, and the corpus callosum is important for its establishment. Several recent studies showed that the partial denervation of the sensorimotor isocortex through CCX derepressed neural growth processes that were sensitive to motor demand (experience-dependent neural plasticity). We investigated whether the facilitatory effects of CCX on cortical neural plasticity, shaped by differential housing, extended beyond the motor domain. Adult rats were housed in enriched (EE), standard (SE) or impoverished environments (IE) for 10 weeks, that is, 2 weeks before they underwent CCX or sham surgery, and, then, 8 weeks throughout the experiments. After they recovered from surgery, the behavioral performance of rats was tested using open-field, spontaneous alternation in the T-maze, paw preference, Morris water maze, and tone fear conditioning. The results indicated that the effects of CCX and housing on open-field behavior were independent, with CCX increasing the time spent in the center of the field at the beginning of the observation (i.e., emotionality), and EE and IE increasing rearing (emotionality) and reducing teeth-chattering (habituation), respectively. CCX reduced the frequency of spontaneous alternation, denoting spatial working memory deficits, while housing did not influence this performance. Neither CCX, nor housing significantly affected paw preference lateralization, although CCX was associated with a leftward bias in paw preference. In the Morris water maze, housing had effects on spatial acquisition, while CCX reduced activity, without interfering with spatial memory. CCX did not influence tone fear conditioning, but context fear conditioning seemed to benefit from EE. We conclude that CCX in adult rats has subtle, but specific behavioral effects pertaining to emotionality, spatial working memory, and, possibly, aversively motivated exploration, and these effects are either independent or only peripherally interact with the effects of housing.

Time-sensitive enhancement of motor learning with the less-affected forelimb after unilateral sensorimotor cortex lesions in rats

Unilateral damage to the forelimb region of the sensorimotor cortex (FLsmc) results in time-dependent changes in neuronal activity, structure and connectivity in the contralateral motor cortex of adult rats. These changes have been linked to facilitation of motor skill learning in the less-affected/ipsilesional forelimb, which is likely to promote its use in the development of behavioral compensation. The goal of this study was to determine whether an early post-lesion-sensitive time period exists for this enhanced learning and whether it is linked to synaptogenesis in the contralesional motor cortex. Rats were trained for 21 days on a skilled reaching task with the ipsilesional forelimb beginning 4 or 25 days after unilateral ischemic (endothelin-1-induced) FLsmc lesions or sham operations. As found previously, reaching performance was significantly enhanced in rats trained early post-lesion compared with sham-operates. In rats trained later post-lesion, performance was neither significantly different from time-matched sham-operates nor strikingly different from animals trained earlier post-lesion. In layer V of the contralesional motor cortex, stereological methods for light and electron microscopy revealed significantly more total, multisynaptic bouton and perforated synapses per neuron compared with sham-operates, but there were no significant differences between early- and late-trained lesion groups. Thus, there appears to be a sensitive time window for the maximal expression of the enhanced learning capacity of the less-affected forelimb but this window is broadly, rather than sharply, defined. These results indicate that relatively long-lasting lesion-induced neuronal changes are likely to underlie the facilitation of learning with the less-affected forelimb.

Unilateral ischemic sensorimotor cortical damage induces contralesional synaptogenesis and enhances skilled reaching with the ipsilateral forelimb in adult male rats

Unilateral damage to the forelimb representation area of the sensorimotor cortex (SMC) results in a compensatory reliance on the unimpaired (ipsilateral to the lesion) forelimb as well as reorganization of neuronal structure and connectivity in the contralateral motor cortex. Recently, male rats with unilateral electrolytic SMC lesions were found to have enhanced skilled reaching performance with the ipsilesional forelimb compared with sham-operated controls. The present study was performed to determine whether these behavioral findings are replicable using an ischemic lesion and whether there is a link between the enhanced learning and synaptogenesis in motor cortical layer V opposite the trained limb and lesion, as assessed using stereological methods for light and electron microscopy. Rats were given a sham operation or an endothelin-1 (ET-1) induced ischemic SMC lesion. They were then trained for 20 days on a skilled reaching task with the unimpaired limb or received control procedures. As with previous findings using electrolytic lesions, rats with unilateral ischemic SMC lesions performed significantly better using the unimpaired forelimb than did sham-operates. Lesions, but not training, significantly increased the total number of motor cortical layer V synapses per neuron as well as the number of perforated and multisynaptic bouton (MSB) synapses per neuron compared with shams. Thus, in addition to a net increase in synapses, the improved reaching ability was coupled with an increase in synapse subtypes that have previously been linked to enhanced synaptic efficacy. The failure to induce synaptogenesis in layer V with reach training alone is in contrast to previous findings and may be related to training intensity.

Unilateral ischemic sensorimotor cortical damage in female rats: forelimb behavioral effects and dendritic structural plasticity in the contralateral homotopic cortex

Previous studies in male rats with unilateral sensorimotor cortical (SMC) damage have demonstrated dendritic structural plasticity in the contralateral homotopic cortex and an enhancement of skilled reaching performance in the forelimb ipsilateral to the lesion compared to sham-operated rats. The purpose of this study was to determine if these findings could be replicated in an ischemic lesion model in female rats. Female rats were given sham operations or unilateral ischemic (endothelin-1 induced) damage in the forelimb representation area of the SMC opposite their preferred forelimb. Animals then received either 20 consecutive days of training on a skilled reaching task with the non-preferred/unimpaired forelimb or no-training control procedures. The surface density of dendrites immunoreactive (IR) for microtubule-associated protein 2 (MAP2) was then measured in the motor cortex opposite the trained limb and/or lesion. Female rats with sufficiently large, but not very small, lesions performed better with the unimpaired forelimb than sham-operated rats on the reaching task. The post-lesion reaching performance was not found to be significantly dependent upon estrous stage at the time of surgery, in agreement with previous studies that failed to find sex or sex-hormone effects after other types of SMC damage. Additionally, there were major laminar-dependent increases in the surface density of MAP2 IR dendrites in the cortex opposite lesions and trained limbs. These findings in female rats are consistent with the dendritic and behavioral changes previously found in male rats. They extend these previous findings by indicating that lesion size is an important variable in the enhancement of reaching performance.

Evidence for bilateral control of skilled movements: ipsilateral skilled forelimb reaching deficits and functional recovery in rats follow motor cortex and lateral frontal cortex lesions

Unilateral damage to cortical areas in the frontal cortex produces sensorimotor deficits on the side contralateral to the lesion. Although there are anecdotal reports of bilateral deficits after stroke in humans and in experimental animals, little is known of the effects of unilateral lesions on the same side of the body. The objective of the present study was to make a systematic examination of the motor skills of the ipsilateral forelimb after frontal cortex lesions to either the motor cortex by devascularization of the surface blood vessels (pial stroke), or to the lateral cortex by electrocoagulation of the distal branches of the middle cerebral artery (MCA stroke). Plastic processes in the intact hemisphere were documented using Golgi-Cox dendritic analysis and by intracortical microstimulation analysis. Although tests of reflexive responses in forelimb placing identified a contralateral motor impairment following both cortical lesions, quantitative and qualitative measures of skilled reaching identified a severe ipsilateral impairment from which recovery was substantial but incomplete. Golgi-impregnated pyramidal cells in the forelimb area showed an increase in dendritic length and branching. Electrophysiological mapping showed normal size forelimb representations in the lesioned rats relative to control animals. The finding of an enduring ipsilateral impairment in skilled movement is consistent with a large but more anecdotal literature in rats, nonhuman primates and humans, and suggests that plastic changes in the intact hemisphere are related to that hemisphere's contribution to skilled movement.