A novel phosphodiesterase type 4 inhibitor, HT-0712, enhances rehabilitation-dependent motor recovery and cortical reorganization after focal cortical ischemia

Rehabilitation-dependent motor recovery after cerebral ischemia is associated with functional reorganization of residual cortical tissue. Recovery is thought to occur when remaining circuitry surrounding the lesion is "retrained" to assume some of the lost function. This reorganization is in turn supported by synaptic plasticity within cortical circuitry and manipulations that promote plasticity may enhance recovery. Activation of the cAMP/CREB pathway is a key step for experience-dependent neural plasticity. Here we examined the effects of the prototypical phosphodiesterase inhibitor 4 (PDE4) rolipram and a novel PDE inhibitor (HT-0712), known to enhance cAMP/CREB signaling and cognitive function, on restoration of motor skill and cortical function after focal cerebral ischemia. Adult male rats were trained on a skilled reaching task to establish a baseline level of motor performance. Intracortical microstimulation was then used to derive high-resolution maps of forelimb movement representations within the caudal forelimb area of motor cortex contralateral to the trained paw. A focal ischemic infarct was created within approximately 30% of the caudal forelimb area. The effects of administering either rolipram or the novel PDE4 inhibitor HT-0712 during rehabilitation on motor recovery and restoration of movement representations within residual motor cortex were examined. Both compounds significantly enhanced motor recovery and induced an expansion of distal movement representations that extended beyond residual motor cortex. The expansion beyond the initial residual cortex was not observed in vehicle injected controls. Furthermore, the motor recovery seen in the HT-0712 animals was dose dependent. Our results suggest that PDE4 inhibitors during motor rehabilitation facilitate behavioral recovery and cortical reorganization after ischemic insult to levels significantly greater than that observed with rehabilitation alone.     

Cortical stimulation mapping using epidurally implanted thin-film microelectrode arrays

Stimulation mapping of motor cortex is an important tool for assessing motor cortex physiology. Existing techniques include intracortical microstimulation (ICMS) which has high spatial resolution but damages cortical integrity by needle penetrations, and transcranial stimulation which is non-invasive but lacks focality and spatial resolution. A minimally invasive epidural microstimulation (EMS) technique using chronically implanted polyimide-based thin-film microelectrode arrays (72 contacts) was tested in rat motor cortex and compared to ICMS within individual animals. Results demonstrate reliable mapping with high reproducibility and validity with respect to ICMS. No histological evidence of cortical damage and the absence of motor deficits as determined by performance of a motor skill reaching task, demonstrate the safety of the method. EMS is specifically suitable for experiments integrating electrophysiology with behavioral and molecular biology techniques.     

Limits on recovery in the corticospinal tract of the rat: partial lesions impair skilled reaching and the topographic representation of the forelimb in motor cortex

Although evidence suggests that there are impairments in skilled movements following very large lesions of the pyramidal component of the corticospinal tract, the behavioral and electrophysiological effects of partial lesion has not received equal attention. Here, rats with complete lesions or partial lesions (medial, central, or lateral third) of the pyramidal tract at the medullary pyramids were evaluated for their quantitative and qualitative postsurgical performance on a skilled reaching task, following which the topographic representation of their forelimb was mapped with intracortical microstimulation (ICMS). Complete lesions impaired reaching success, impaired the qualitative features of reaching movements, and abolished ICMS evoked movement from the forelimb region of motor cortex. Although partial lesions did not impair reaching success, they did impair qualitative aspects of limb movement including forepaw aiming, supination, and food pellet release. ICMS indicated a reduction in the size of the forelimb area, especially the distal area of the caudal forelimb area (CFA), of the motor map. The behavioral and electrophysiological impairments did not vary with lesion location within the pyramidal tract. The incomplete recovery, as measured both behaviorally and electrophysiologically, demonstrates that plasticity within the corticospinal system is limited even with lesions that permit substantial sparing of pyramidal tract fibers.     

Cholinergic upregulation by optogenetic stimulation of nucleus basalis after photothrombotic stroke in forelimb somatosensory cortex improves endpoint and motor but not sensory control of skilled reaching in mice

A network of cholinergic neurons in the basal forebrain innerve the forebrain and are proposed to contribute to a variety of functions including cortical plasticity, attention, and sensorimotor behavior. This study examined the contribution of the nucleus basalis cholinergic projection to the sensorimotor cortex on recovery on a skilled reach-to-eat task following photothrombotic stroke in the forelimb region of the somatosensory cortex. Mice were trained to perform a single pellet skilled reaching task and their pre and poststroke performance, from Day 4 to Day 28 poststroke, was assessed frame-by-frame by video analysis with endpoint, movement and sensorimotor integration measures. Somatosensory forelimb lesions produced impairments in endpoint and movement component measures of reaching and increased the incidence of fictive eating, a sensory impairment in mistaking a missed reach for a successful reach. Upregulated acetylcholine (ACh) release, as measured by local field potential recording, elicited via optogenetic stimulation of the nucleus basalis improved recovery of reaching and improved movement scores but did not affect sensorimotor integration impairment poststroke. The results show that the mouse cortical forelimb somatosensory region contributes to forelimb motor behavior and suggest that ACh upregulation could serve as an adjunct to behavioral therapy for acute treatment of stroke.     

Concurrent silent strokes impair motor function by limiting behavioral compensation

Silent strokes occur more frequently than classic strokes; however, symptoms may go unreported in spite of lasting tissue damage. A silent stroke may indicate elevated susceptibility to recurrent stroke, which may eventually result in apparent and lasting impairments. Here we investigated if multiple silent strokes to the motor system challenge the compensatory capacity of the brain to cumulatively result in permanent functional deficits. Adult male rats with focal ischemia received single focal ischemic mini-lesions in the sensorimotor cortex (SMC) or the dorsolateral striatum (DLS), or multiple lesions affecting both SMC and DLS. The time course and outcome of motor compensation and recovery were determined by quantitative and qualitative assessment of skilled reaching and skilled walking. Rats with SMC or DLS lesion alone did not show behavioral deficits in either task. However, the combination of focal ischemic lesions in SMC and DLS perturbed skilled reaching accuracy and disrupted forelimb placement in the ladder rung walking task. These observations suggest that multiple focal infarcts, each resembling a silent stroke, gradually compromise the plastic capacity of the motor system to cause permanent motor deficits. Moreover, these findings support the notion that cortical and subcortical motor systems cooperate when adopting beneficial compensatory movement strategies.     

Post-infarct cortical plasticity and behavioral recovery using concurrent cortical stimulation and rehabilitative training: a feasibility study in primates

Stroke is often characterized by incomplete recovery and chronic motor impairments. A nonhuman primate model of cortical ischemia was used to evaluate the feasibility of using device-assisted cortical stimulation combined with rehabilitative training to enhance behavioral recovery and cortical plasticity. Following pre-infarct training on a unimanual motor task, maps of movement representations in primary motor cortex were derived. Then, an ischemic infarct was produced which destroyed the hand representation. Several weeks later, a second cortical map was derived to guide implantation of a surface electrode over peri-infarct motor cortex. After several months of spontaneous recovery, monkeys underwent subthreshold electrical stimulation combined with rehabilitative training for several weeks. Post-therapy behavioral performance was tracked for several additional months. A third cortical map was derived several weeks post-therapy to examine changes in motor representations. Monkeys showed significant improvements in motor performance (success, speed, and efficiency) following therapy, which persisted for several months. Cortical mapping revealed large-scale emergence of new hand representations in peri-infarct motor cortex, primarily in cortical tissue underlying the electrode. Results support the feasibility of using a therapy approach combining peri-infarct electrical stimulation with rehabilitative training to alleviate chronic motor deficits and promote recovery from cortical ischemic injury.     

Motor improvements after focal cortical ischemia in adult rats are mediated by compensatory mechanisms

Initial functional impairments after cerebral ischemia often improve considerably during the early period after the insult. Although pathological changes associated with post-lesion improvements have been widely investigated, it has not been resolved whether behavioral improvement represents true restoration of function (recovery) or development of new strategies (compensation). This study investigated whether early motor improvements after focal cerebral ischemia reflect recovery or compensation. Adult female Wistar rats were trained to retrieve food pellets in a skilled reaching task prior to receiving a unilateral cortical infarction induced by photothrombosis in forelimb motor cortex. Animals were continuously tested in the reaching task up to 3 weeks after lesion. The end point measures revealed that reaching success rates remained at pre-lesion levels, however, qualitative analysis of reaching movements indicated permanent changes in forelimb movement patterns. Similar observations were made in a skilled walking task and a test for forelimb asymmetry. These data indicate that lesion animals adopted alternative movement strategies in order to successfully perform the tasks. The changes in postoperative performance were compared to anatomical data in individual animals. The finding that reaching success was not related to lesion size supports the idea that the degree of adaptive behavior after cortical ischemia depends on plastic properties of the remaining intact tissue.     

Developmental and Interventional Plasticity of Motor Maps after Perinatal Stroke

Within the perinatal stroke field, there is a need to establish preclinical models where putative biomarkers for motor function can be examined. In a mouse model of perinatal stroke, we evaluated motor map size and movement latency following optogenetic cortical stimulation against three factors of post-stroke biomarker utility: (1) correlation to chronic impairment on a behavioral test battery; (2) amenability to change using a skilled motor training paradigm; and (3) ability to distinguish individuals with potential to respond well to training. Thy1-ChR2-YFP mice received a photothrombotic stroke at postnatal day 7 and were evaluated on a battery of motor tests between days 59 and 70. Following a cranial window implant, mice underwent longitudinal optogenetic motor mapping both before and after 3 weeks of skilled forelimb training. Map size and movement latency of both hemispheres were positively correlated with impaired spontaneous forelimb use, whereas only ipsilesional hemisphere map size was correlated with performance in skilled reaching. Map size and movement latency did not show groupwise changes with training; however, mice with the smallest pretraining map sizes and worst impairments demonstrated the greatest expansion of map size in response to skilled forelimb training. Overall, motor map size showed utility as a potential biomarker for impairment and training-induced modulation in specific individuals. Future assessment of the predictive capacity of post-stroke motor representations for behavioral outcome in animal models opens the possibility of dissecting how plasticity mechanisms contribute to recovery following perinatal stroke.SIGNIFICANCE STATEMENT We investigated the utility of two cortical motor representation measures (motor map size and movement onset latency) as potential biomarkers for post-stroke motor recovery in a mouse model of perinatal stroke. Both motor map size and movement latency were associated with functional recovery after perinatal stroke, with map size showing an additional association between training responsiveness and severity of impairment. Overall, both motor map size and movement onset latency show potential as neurophysiological correlates of recovery. As such, future studies of perinatal stroke rehabilitation and neuromodulation should include these measures to help explain neurophysiological changes that might be occurring in response to treatment.     

Preserved ipsilateral-to-lesion motor map organization in the unilateral 6-OHDA-treated rat model of Parkinson's disease

The classic view of dopamine (DA) loss in Parkinson's disease is that it produces a functional deafferentation in striatal-cortical circuitry that, in turn, contributes to sensorimotor deficits. The present study examines this view in the rat by assessing how DA-depletion affects the intracortical microstimulation (ICMS) topographic representation of movement in the rostral and caudal motor areas of the motor cortex. The ICMS map is used as an index of motor cortex function because it has been shown to reflect motor function and experience. Groups of rats received no training or skilled reach training and were then given unilateral 6-hydroxydopamine (6-OHDA) or sham lesions of the nigrostriatal bundle to deplete nigrostriatal DA. Lesion success was confirmed by abnormalities in skilled reaching, by apomorphine-induced rotation, and by loss of DA neurons in the substantia nigra. The size and threshold of the motor map in naive and skilled reach trained DA-depleted rats were preserved. In addition, there was an increase in distal limb representation in the caudal forelimb area (CFA) in the DA-depleted rats suggesting a possible plastic response to the behavioral effects of DA-depletion. The presence of preserved size and modified map organization in DA-depleted rats is discussed in relation to the hypothesis that preserved motor cortex functionality despite DA loss underlies the spared motor abilities of DA-depleted rats.     

Cortical electrical stimulation combined with rehabilitative training: Enhanced functional recovery and dendritic plasticity following focal cortical ischemia in rats

Abstract

This study assessed the behavioral and dendritic structural effects of combining subdural motor cortical electrical stimulation with motor skills training following unilateral sensorimotor cortex lesions in adult male rats. Rats were pre-operatively trained on a skilled forelimb reaching task, the Montoya staircase test, and then received endothelin-1 induced ischemic lesions of the sensorimotor cortex. Ten to 14 days later, electrodes were implanted over the peri-lesion cortical surface. Rats subsequently began 10 days of rehabilitative training on the reaching task in 1 of 3 conditions: 1. 50 Hz stimulation during training, 2. 250 Hz stimulation during training or 3. no stimulation. No significant difference in performance was found between the 250 Hz and no stimulation groups. The 50 Hz stimulation group had significantly greater rates of improvement with the impaired forelimb in comparison to 250 Hz and no stimulation groups combined. Fifty Hz stimulated animals also had a significant increase in the surface density of dendritic processes immunoreactive for the cytoskeletal protein, microtubule-associated protein 2, in the peri-lesion cortex compared to the other groups. These results support the efficacy of combining rehabilitative training with cortical electrical stimulation to improve functional outcome and cortical neuronal structural plasticity following sensorimotor cortical damage.     

Cortical electrical stimulation combined with rehabilitative training: enhanced functional recovery and dendritic plasticity following focal cortical ischemia in rats

This study assessed the behavioral and dendritic structural effects of combining subdural motor cortical electrical stimulation with motor skills training following unilateral sensorimotor cortex lesions in adult male rats. Rats were pre-operatively trained on a skilled forelimb reaching task, the Montoya staircase test, and then received endothelin-1 induced ischemic lesions of the sensorimotor cortex. Ten to 14 days later, electrodes were implanted over the peri-lesion cortical surface. Rats subsequently began 10 days of rehabilitative training on the reaching task in 1 of 3 conditions: 1. 50 Hz stimulation during training, 2. 250 Hz stimulation during training or 3. no stimulation. No significant difference in performance was found between the 250 Hz and no stimulation groups. The 50 Hz stimulation group had significantly greater rates of improvement with the impaired forelimb in comparison to 250 Hz and no stimulation groups combined. Fifty Hz stimulated animals also had a significant increase in the surface density of dendritic processes immunoreactive for the cytoskeletal protein, microtubule-associated protein 2, in the peri-lesion cortex compared to the other groups. These results support the efficacy of combining rehabilitative training with cortical electrical stimulation to improve functional outcome and cortical neuronal structural plasticity following sensorimotor cortical damage.     

Intact intracortical microstimulation (ICMS) representations of rostral and caudal forelimb areas in rats with quinolinic acid lesions of the medial or lateral caudate-putamen in an animal model of Huntington's disease

Neurotoxic, cell-specific lesions of the rat caudate-putamen (CPu) have been proposed as a model of human Huntington's disease and as such impair performance on many motor tasks, including skilled forelimbs tasks such as reaching for food. Because the CPu and motor cortex share reciprocal connections, it has been proposed that the motor deficits are due in part to a secondary disruption of motor cortex. The purpose of the present study was to examine the functionality of the motor cortex using intracortical microstimulation (ICMS) following neurotoxic lesions of the CPu. ICMS maps have been shown to be sensitive indicators of motor skill, cortical injury, learning, and experience. Long-evans hooded rats received a sham, a medial, or a lateral CPu lesion using the neurotoxin, quinolinic acid (2,3-pyridinedicarboxylic acid). Two weeks later the motor cortex was stimulated under light ketamine anesthesia. Neither lateral nor medial lesions of the CPu altered the stimulation threshold for eliciting forelimb movements, the type of movements elicited, or the size of the rostral forelimb (RFA) and caudal forelimb areas (CFA) from which movements were elicited. The preservation of ICMS forelimb movement representations (the forelimb map) in rats with cell-specific CPu lesions suggests motor impairments following lesions of the lateral striatum are not due to the disruption of the motor map. Therefore, the impairments that follow striatal cell loss are due either to alterations in circuitry that is independent of motor cortex or to alterations in circuitry afferent to the motor cortex projections.     

Long-Evans rats have a larger cortical topographic representation of movement than Fischer-344 rats: a microstimulation study of motor cortex in naïve and skilled reaching-trained rats

Intracortical microstimulation of the frontal cortex evokes movements in the contralateral limbs, paws, and digits of placental mammals including the laboratory rat. The topographic representation of movement in the rat consists of a rostral forelimb area (RFA), a caudal forelimb area (CFA), and a hind limb area (HLA). The size of these representations can vary between individual animals and the proportional representation of the body parts within regions can also change as a function of experience. To date, there have been no investigations of strain differences in the cortical map of rats, and this was the objective of the present investigation. The effect of cortical stimulation was compared in young male Long-Evans rats and Fischer-344 rats. The overall size of the motor cortex representation was greater in Long-Evans rats compared to Fischer-344 rats and the threshold required to elicit a movement was higher in the Fischer-344 rats. An additional set of animals were trained in a skilled reaching task to rule out the possibility that experiential differences in the groups could account for the result and to examine the relationship between the differences in topography of cortical movement representations and motor performance. The Long-Evans rats were quantitatively and qualitatively better in skilled reaching than the Fischer-344 rats. Also, Long-Evans rats exhibited a relatively larger area of the topographic representation and lower thresholds for eliciting movement in the contralateral forelimb. This is the first study to describe pronounced strain-related differences in the microstimulation-topographic map of the motor cortex. The results are discussed in relation to using strain differences as a way of examining the behavioral, the physiological, and the anatomical organization of the motor system.     

Both compensation and recovery of skilled reaching following small photothrombotic stroke to motor cortex in the rat

Large lesions produced by stroke to the forelimb region of motor cortex of the rat feature post-stroke improvement that in the main is due to compensation. The present study describes both recovery and compensation of forelimb use in a reach-to-eat (skilled reaching) task following small photothrombotic stroke. The rats were pretrained before stroke, and then assessed using endpoint measures and biometric movement analysis during rehabilitation in the acute and chronic post-stroke periods. Histological and MRI analysis indicated that the stroke consisted of a small lesion surrounded by cortex featuring scattered cell loss, likely of the large pyramidal cells that characterize the forelimb region of motor cortex. The stroke reduced reaching success, especially on the most demanding measure of success on first reach attempts, in the acute period, but with rehabilitation, performance returned to pre-stroke levels. Reach movements as assessed by biometric measures were severely impaired acutely but displayed significant recovery chronically although this recovery was not complete. The results suggest that not only do rats show post-stroke compensation in skilled reaching but they can also display functional recovery. It is suggested that recovery is mediated by the spared neurons in the peri-infarct region of forelimb motor cortex. The results demonstrate the utility of a small lesion model for studying post-stroke neural and behavioral change and support the view that optimal post-stroke treatment should be directed toward limiting tissue loss.     

Endurance exercise facilitates relearning of forelimb motor skill after focal ischemia

Endurance exercise (i.e. running), by up-regulating brain-derived neurotrophic factor (BDNF) and other modulators of synaptic plasticity, improves attention and learning, both critical components of stroke rehabilitation. We hypothesized that, following middle cerebral artery occlusion in male Sprague-Dawley rats, endurance exercise would act synergistically with a challenging skilled forelimb task to facilitate motor recovery. Animals were randomly assigned to one of four rehabilitation conditions: no rehabilitation, running only, reach training only, and reach training preceded by running (run/reach training) for 5 weeks beginning 5 days after stroke. The behavioral outcome, morphological change and mRNA expression of proteins implicated in neuroplasticity (BDNF, synapsin I and microtubule-associated protein 2) were compared. Endurance exercise on a motorized running wheel, prior to reach training, enhanced recovery of skilled reaching ability but did not transfer to gross motor skills such as postural support (forelimb asymmetry test) and gait (ladder rung walking test). Microtubule-associated protein 2 staining density in the run/reach group was slightly enhanced in the contralateral motor cortex compared with the contralateral sensory and ipsilateral cingulate cortices, suggesting that running preceding reach training may have resulted in more dendritic branching within the motor cortex in this group. No significant differences in mRNA levels were detected among the training paradigms; however, there was a trend toward greater BDNF and synapsin I mRNA in the reaching groups. These findings suggest that exercise facilitates learning of subsequent challenging reaching tasks after stroke, which has the potential to optimize outcomes in patients with stroke.     

Treatment-induced cortical reorganization after stroke in humans

BACKGROUND AND PURPOSE: Injury-induced cortical reorganization is a widely recognized phenomenon. In contrast, there is almost no information on treatment-induced plastic changes in the human brain. The aim of the present study was to evaluate reorganization in the motor cortex of stroke patients that was induced with an efficacious rehabilitation treatment. METHODS: We used focal transcranial magnetic stimulation to map the cortical motor output area of a hand muscle on both sides in 13 stroke patients in the chronic stage of their illness before and after a 12-day-period of constraint-induced movement therapy. RESULTS: Before treatment, the cortical representation area of the affected hand muscle was significantly smaller than the contralateral side. After treatment, the muscle output area size in the affected hemisphere was significantly enlarged, corresponding to a greatly improved motor performance of the paretic limb. Shifts of the center of the output map in the affected hemisphere suggested the recruitment of adjacent brain areas. In follow-up examinations up to 6 months after treatment, motor performance remained at a high level, whereas the cortical area sizes in the 2 hemispheres became almost identical, representing a return of the balance of excitability between the 2 hemispheres toward a normal condition. CONCLUSIONS: This is the first demonstration in humans of a long-term alteration in brain function associated with a therapy-induced improvement in the rehabilitation of movement after neurological injury.     

Social instability blocks functional restitution following motor cortex stroke in rats

Social interactions have previously been shown to influence stroke outcome. In the current experiment we investigated the effects of a changing social environment on anatomical and behavioral recovery following motor cortex stroke in rats. Adult rats were trained on the Whishaw single pellet reaching task prior to receiving a devascularizing stroke lesion of the motor cortex. During the post-stroke testing period half of the rats were exposed to a form of social experience that has previously been shown to stimulate synaptic plasticity in frontal cortex circuitry, whereas the remaining rats were housed in pairs, in standard cages. At the end of the experiment the brains were processed for Golgi-Cox staining and dendritic length was measured in layer V of the intact forelimb motor area, layer III of Zilles' area Cg3 and layer II/III of Zilles' area AID. Social experience was found to completely block the normal spontaneous behavioural restitution in the lesion animals. Anatomically, whereas social experience selectively increased dendritic length in AID in rats that had not undergone behavioral training or the stroke procedure, this was not seen in the lesion animals, as the lesion alone produced an increase in dendritic length in both AID and Cg3. The findings are discussed in terms of the role of social experiences, including stress, on spontaneous plasticity that occurs following unilateral motor cortex stroke, and the effectiveness of inducing synaptic plasticity to promote behavioural recovery.     

Altered mechanisms of motor-evoked potential generation after transient focal cerebral ischemia in the rat: implications for transcranial magnetic stimulation

We recently demonstrated that a long-lasting transmission defect in cortical synapses caused motor dysfunction after brief middle cerebral artery (MCA) occlusion in the rat despite rapid recovery of axons. In this experimental study, we have examined the impact of differential recovery of synapses and axons on generation of motor-evoked potentials (MEP) recorded from contralateral paralyzed and ipsilateral unaffected muscles, to gain insight into mechanisms of MEPs recorded from stroke patients by transcranial magnetic stimulation (TMS). MEPs generated by focal electrical stimulation of the forelimb area of motor cortex were simultaneously recorded from the brain stem, contra- and ipsilateral forelimb and contralateral hindlimb muscles in rats subjected to transient MCA occlusion. The effect of ischemia on cortical activity and axonal conduction was differentially studied by proximal or distal occlusion of the MCA. Regional cerebral blood flow changes in the forelimb area were monitored by laser-Doppler flowmetry during ischemia and reperfusion. In addition, synaptic transmission within the forelimb area of motor cortex was examined by intracellular and extracellular recording of potentials generated by stimulation of the premotor area. No MEP response was recorded during ischemia. Upon reperfusion: (i) motor axons readily regained their excitability and cortical stimulation caused successive pyramidal volleys (recorded as D waves from the brain stem) and a MEP from contralateral paralytic muscles although synaptic activation of motor pathways was not feasible; (ii) the amplitude of pyramidal volley was increased; (iii) MEPs with a longer latency were recorded from the ipsilateral forelimb. In conclusion, differential recovery of synapses and axons after ischemia may account for some previously unexplained findings (such as preserved MEPs in paralysed muscles) observed in cortical stimulation studies of stroke patients.