Bihemispheric reduction of GABAA receptor binding following focal cortical photothrombotic lesions in the rat brain

Aberrant cortical activity, functional connectivity, and neural assembly architecture after photothrombotic stroke in mice

Despite substantial progress in mapping the trajectory of network plasticity resulting from focal ischemic stroke, the extent and nature of changes in neuronal excitability and activity within the peri-infarct cortex of mice remains poorly defined. Most of the available data have been acquired from anesthetized animals, acute tissue slices, or infer changes in excitability from immunoassays on extracted tissue, and thus may not reflect cortical activity dynamics in the intact cortex of an awake animal. Here, in vivo two-photon calcium imaging in awake, behaving mice was used to longitudinally track cortical activity, network functional connectivity, and neural assembly architecture for 2 months following photothrombotic stroke targeting the forelimb somatosensory cortex. Sensorimotor recovery was tracked over the weeks following stroke, allowing us to relate network changes to behavior. Our data revealed spatially restricted but long-lasting alterations in somatosensory neural network function and connectivity. Specifically, we demonstrate significant and long-lasting disruptions in neural assembly architecture concurrent with a deficit in functional connectivity between individual neurons. Reductions in neuronal spiking in peri-infarct cortex were transient but predictive of impairment in skilled locomotion measured in the tapered beam task. Notably, altered neural networks were highly localized, with assembly architecture and neural connectivity relatively unaltered a short distance from the peri-infarct cortex, even in regions within 'remapped' forelimb functional representations identified using mesoscale imaging with anaesthetized preparations 8 weeks after stroke. Thus, using longitudinal two-photon microscopy in awake animals, these data show a complex spatiotemporal relationship between peri-infarct neuronal network function and behavioral recovery. Moreover, the data highlight an apparent disconnect between dramatic functional remapping identified using strong sensory stimulation in anaesthetized mice compared to more subtle and spatially restricted changes in individual neuron and local network function in awake mice during stroke recovery.

Predicting Individual Treatment Response to rTMS for Motor Recovery After Stroke: A Review and the CanStim Perspective

Background

Rehabilitation is critical for reducing stroke-related disability and improving quality-of-life post-stroke. Repetitive transcranial magnetic stimulation (rTMS), a non-invasive neuromodulation technique used as stand-alone or adjunct treatment to physiotherapy, may be of benefit for motor recovery in subgroups of stroke patients. The Canadian Platform for Trials in Non-Invasive Brain Stimulation (CanStim) seeks to advance the use of these techniques to improve post-stroke recovery through clinical trials and pre-clinical studies using standardized research protocols. Here, we review existing clinical trials for demographic, clinical, and neurobiological factors which may predict treatment response to identify knowledge gaps which need to be addressed before implementing these parameters for patient stratification in clinical trial protocols.

Objective

To provide a review of clinical rTMS trials of stroke recovery identifying factors associated with rTMS response in stroke patients with motor deficits and develop research perspectives for pre-clinical and clinical studies.

Methods

A literature search was performed in PubMed, using the Boolean search terms stroke AND repetitive transcranial magnetic stimulation OR rTMS AND motor for studies investigating the use of rTMS for motor recovery in stroke patients at any recovery phase. A total of 1,676 articles were screened by two blinded raters, with 26 papers identified for inclusion in this review.

Results

Multiple possible factors associated with rTMS response were identified, including stroke location, cortical thickness, brain-derived neurotrophic factor (BDNF) genotype, initial stroke severity, and several imaging and clinical factors associated with a relatively preserved functional motor network of the ipsilesional hemisphere. Age, sex, and time post-stroke were generally not related to rTMS response. Factors associated with greater response were identified in studies of both excitatory ipsilesional and inhibitory contralesional rTMS. Heterogeneous study designs and contradictory data exemplify the need for greater protocol standardization and high-quality controlled trials.

Conclusion

Clinical, brain structural and neurobiological factors have been identified as potential predictors for rTMS response in stroke patients with motor impairment. These factors can inform the design of future clinical trials, before being considered for optimization of individual rehabilitation therapy for stroke patients. Pre-clinical models for stroke recovery, specifically developed in a clinical context, may accelerate this process.

Low-Frequency rTMS over Contralesional M1 Increases Ipsilesional Cortical Excitability and Motor Function with Decreased Interhemispheric Asymmetry in Subacute Stroke: A Randomized Controlled Study

Objective

To determine the long-term effects of low-frequency repetitive transcranial magnetic stimulation (LF-rTMS) over the contralesional M1 preceding motor task practice on the interhemispheric asymmetry of the cortical excitability and the functional recovery in subacute stroke patients with mild to moderate arm paresis.

Methods

Twenty-four subacute stroke patients were randomly allocated to either the experimental or control group. The experimental group underwent rTMS over the contralesional M1 (1 Hz), immediately followed by 30 minutes of motor task practice (10 sessions within 2 weeks). The controls received sham rTMS and the same task practice. Following the 2-week intervention period, the task practice was continued twice weekly for another 10 weeks in both groups. Outcomes were evaluated at baseline (T0), at the end of the 2-week stimulation period (T1), and at 12-week follow-up (T2).

Results

The MEP (paretic hand) and interhemispheric asymmetry, Fugl-Meyer motor assessment, Action Research Arm Test, and box and block test scores improved more in the experimental group than controls at T1 (p < 0.05). The beneficial effects were largely maintained at T2.

Conclusion

LF-rTMS over the contralesional M1 preceding motor task practice was effective in enhancing the ipsilesional cortical excitability and upper limb function with reducing interhemispheric asymmetry in subacute stroke patients with mild to moderate arm paresis. Significance. Adding LF-rTMS prior to motor task practice may reduce interhemispheric asymmetry of cortical excitabilities and promote upper limb function recovery in subacute stroke with mild to moderate arm paresis.

Excitatory-Inhibitory Homeostasis and Diaschisis: Tying the Local and Global Scales in the Post-stroke Cortex

Maintaining a balance between excitatory and inhibitory activity is an essential feature of neural networks of the neocortex. In the face of perturbations in the levels of excitation to cortical neurons, synapses adjust to maintain excitatory-inhibitory (EI) balance. In this review, we summarize research on this EI homeostasis in the neocortex, using stroke as our case study, and in particular the loss of excitation to distant cortical regions after focal lesions. Widespread changes following a localized lesion, a phenomenon known as diaschisis, are not only related to excitability, but also observed with respect to functional connectivity. Here, we highlight the main findings regarding the evolution of excitability and functional cortical networks during the process of post-stroke recovery, and how both are related to functional recovery. We show that cortical reorganization at a global scale can be explained from the perspective of EI homeostasis. Indeed, recovery of functional networks is paralleled by increases in excitability across the cortex. These adaptive changes likely result from plasticity mechanisms such as synaptic scaling and are linked to EI homeostasis, providing a possible target for future therapeutic strategies in the process of rehabilitation. In addition, we address the difficulty of simultaneously studying these multiscale processes by presenting recent advances in large-scale modeling of the human cortex in the contexts of stroke and EI homeostasis, suggesting computational modeling as a powerful tool to tie the meso- and macro-scale processes of recovery in stroke patients.

Microglia-mediated phagocytosis of apoptotic nuclei is impaired in the adult murine hippocampus after stroke

Following stroke, neuronal death takes place both in the infarct region and in brain areas distal to the lesion site including the hippocampus. The hippocampus is critically involved in learning and memory processes and continuously generates new neurons. Dysregulation of adult neurogenesis may be associated with cognitive decline after a stroke lesion. In particular, proliferation of precursor cells and the formation of new neurons are increased after lesion. Within the first week, many new precursor cells die during development. How dying precursors are removed from the hippocampus and to what extent phagocytosis takes place after stroke is still not clear. Here, we evaluated the effect of a prefrontal stroke lesion on the phagocytic activity of microglia in the dentate gyrus (DG) of the hippocampus. Three-months-old C57BL/6J mice were injected once with the proliferation marker BrdU (250 mg/kg) 6 hr after a middle cerebral artery occlusion or sham surgery. The number of apoptotic cells and the phagocytic capacity of the microglia were evaluated by means of immunohistochemistry, confocal microscopy, and 3D-reconstructions. We found a transient but significant increase in the number of apoptotic cells in the DG early after stroke, associated with impaired removal by microglia. Interestingly, phagocytosis of newly generated precursor cells was not affected. Our study shows that a prefrontal stroke lesion affects phagocytosis of apoptotic cells in the DG, a region distal to the lesion core. Whether disturbed phagocytosis might contribute to inflammatory- and maladaptive processes including cognitive impairment following stroke needs to be further investigated.

Prolonged deficit of low gamma oscillations in the peri-infarct cortex of mice after stroke

Days and weeks after an ischemic stroke, the peri-infarct area adjacent to the necrotic tissue exhibits very intense synaptic reorganization aimed at regaining lost functions. In order to enhance functional recovery, it is important to understand the mechanisms supporting neural repair and neuroplasticity in the cortex surrounding the lesion. Brain oscillations of the local field potential (LFP) are rhythmic fluctuations of neuronal excitability that synchronize neuronal activity to organize information processing and plasticity. Although the oscillatory activity of the brain has been probed after stroke in both animals and humans using electroencephalography (EEG), the latter is ineffective to precisely map the oscillatory changes in the peri-infarct zone where synaptic plasticity potential is high. Here, we worked on the hypothesis that the brain oscillatory system is altered in the surviving peri-infarct cortex, which may slow down the functional repair and reduce the recovery. In order to document the relevance of this hypothesis, oscillatory power was measured at various distances from the necrotic core at 7 and 21 days after a permanent cortical ischemia induced in mice. Delta and theta oscillations remained at a normal power in the peri-infarct cortex, in contrast to low gamma oscillations that displayed a gradual decrease, when approaching the border of the lesion. A broadband increase of power was also observed in the homotopic contralateral sites. Thus, the proximal peri-infarct cortex could become a target of therapeutic interventions applied to correct the oscillatory regimen in order to boost post-stroke functional recovery.

Low-Frequency Repetitive Transcranial Magnetic Stimulation Over Contralesional Motor Cortex for Motor Recovery in Subacute Ischemic Stroke: A Randomized Sham-Controlled Trial

BACKGROUND

Low-frequency repetitive transcranial magnetic stimulation (rTMS) over the contralesional motor cortex (M1) has demonstrated beneficial effects on motor recovery, but evidence among patients with subacute stroke is lacking. We aimed to investigate whether 1-Hz rTMS over the contralesional M1 versus sham rTMS could improve arm function in patients with subacute ischemic stroke when combined with rehabilitative motor training.

METHODS

In total, 77 patients who were within 90 days after their first-ever ischemic stroke were enrolled and randomly allocated to either real (n = 40) or sham rTMS (n = 37). We delivered 1-Hz 30-minute active or sham rTMS before each daily 30-minute occupational therapy sessions over a 2-week period. The primary endpoint was changes in the Box and Block Test (BBT) score immediately after the end of treatment (EOT). Secondary analyses assessed changes in Fugl-Meyer assessment, Finger Tapping Test (FTT), Brunnstrom stage, and grip strength.

CLINICAL TRIAL REGISTRATION

ClinialTrials.gov (NCT02082015).

RESULTS

Changes in BBT immediately after the end of treatment did not differ significantly between the 2 groups (P = .267). Subgroup analysis according to cortical involvement revealed that real rTMS resulted in improvements in BBT at 1 month after EOT (17.4 ± 9.8 real vs 10.9 ± 10.3 sham; P = .023) and Brunnstrom stage of the hand immediately after EOT (0.6 ± 0.5 real vs 0.2 ± 0.5 sham; P = .023), only in the group without cortical involvement.

CONCLUSION

The effects of real and sham rTMS did not differ significantly among patients within 3 months poststroke. The location of stroke lesions should be considered for future clinical trials.

Oxygen-Glucose Deprivation Differentially Affects Neocortical Pyramidal Neurons and Parvalbumin-Positive Interneurons

Stroke is a devastating brain disorder. The pathophysiology of stroke is associated with an impaired excitation-inhibition balance in the area that surrounds the infarct core after the insult, the peri-infarct zone. Here we exposed slices from adult mouse prefrontal cortex to oxygen-glucose deprivation and reoxygenation (OGD-RO) to study ischemia-induced changes in the activity of excitatory pyramidal neurons and inhibitory parvalbumin (PV)-positive interneurons. We found that during current-clamp recordings, PV-positive interneurons were more vulnerable to OGD-RO than pyramidal neurons as indicated by the lower percentage of recovery of PV-positive interneurons. However, neither the amplitude of OGD-induced depolarization observed in current-clamp mode nor the OGD-associated current observed in voltage-clamp mode differed between the two cell types. Large amplitude, presumably action-potential dependent, spontaneous postsynaptic inhibitory currents recorded from pyramidal neurons were less frequent after OGD-RO than in control condition. Disynaptic inhibitory postsynaptic currents (dIPSCs) in pyramidal neurons produced predominantly by PV-positive interneurons were reduced by OGD-RO. Following OGD-RO, dendrites of PV-positive interneurons exhibited more pathological beading than those of pyramidal neurons. Our data support the hypothesis that the differential vulnerability to ischemia-like conditions of excitatory and inhibitory neurons leads to the altered excitation-inhibition balance associated with stroke pathophysiology.

Brain networks and their relevance for stroke rehabilitation

Stroke has long been regarded as focal disease with circumscribed damage leading to neurological deficits. However, advances in methods for assessing the human brain and in statistics have enabled new tools for the examination of the consequences of stroke on brain structure and function. Thereby, it has become evident that stroke has impact on the entire brain and its network properties and can therefore be considered as a network disease. The present review first gives an overview of current methodological opportunities and pitfalls for assessing stroke-induced changes and reorganization in the human brain. We then summarize principles of plasticity after stroke that have emerged from the assessment of networks. Thereby, it is shown that neurological deficits do not only arise from focal tissue damage but also from local and remote changes in white-matter tracts and in neural interactions among wide-spread networks. Similarly, plasticity and clinical improvements are associated with specific compensatory structural and functional patterns of neural network interactions. Innovative treatment approaches have started to target such network patterns to enhance recovery. Network assessments to predict treatment response and to individualize rehabilitation is a promising way to enhance specific treatment effects and overall outcome after stroke.

Brain Activation During Passive and Volitional Pedaling After Stroke

Background:

Prior work indicates that pedaling-related brain activation is lower in people with stroke than in controls. We asked whether this observation could be explained by between-group differences in volitional motor commands and pedaling performance.

Methods:

Individuals with and without stroke performed passive and volitional pedaling while brain activation was recorded with functional magnetic resonance imaging. The passive condition eliminated motor commands to pedal and minimized between-group differences in pedaling performance. Volume, intensity, and laterality of brain activation were compared across conditions and groups.

Results:

There were no significant effects of condition and no Group × Condition interactions for any measure of brain activation. Only 53% of subjects could minimize muscle activity for passive pedaling.

Conclusions:

Altered motor commands and pedaling performance are unlikely to account for reduced pedaling-related brain activation poststroke. Instead, this phenomenon may be due to functional or structural brain changes. Passive pedaling can be difficult to achieve and may require inhibition of excitatory descending drive.

[18F]fallypride-PET/CT Analysis of the Dopamine D₂/D₃ Receptor in the Hemiparkinsonian Rat Brain Following Intrastriatal Botulinum Neurotoxin A Injection

Intrastriatal injection of botulinum neurotoxin A (BoNT-A) results in improved motor behavior of hemiparkinsonian (hemi-PD) rats, an animal model for Parkinson’s disease. The caudate–putamen (CPu), as the main input nucleus of the basal ganglia loop, is fundamentally involved in motor function and directly interacts with the dopaminergic system. To determine receptor-mediated explanations for the BoNT-A effect, we analyzed the dopamine D₂/D₃ receptor (D₂/D₃R) in the CPu of 6-hydroxydopamine (6-OHDA)-induced hemi-PD rats by [¹⁸F]fallypride-PET/CT scans one, three, and six months post-BoNT-A or -sham-BoNT-A injection. Male Wistar rats were assigned to three different groups: controls, sham-injected hemi-PD rats, and BoNT-A-injected hemi-PD rats. Disease-specific motor impairment was verified by apomorphine and amphetamine rotation testing. Animal-specific magnetic resonance imaging was performed for co-registration and anatomical reference. PET quantification was achieved using PMOD software with the simplified reference tissue model 2. Hemi-PD rats exhibited a constant increase of 23% in D₂/D₃R availability in the CPu, which was almost normalized by intrastriatal application of BoNT-A. Importantly, the BoNT-A effect on striatal D₂/D₃R significantly correlated with behavioral results in the apomorphine rotation test. Our results suggest a therapeutic effect of BoNT-A on the impaired motor behavior of hemi-PD rats by reducing interhemispheric changes of striatal D₂/D₃R.

The neurophysiological effects of single-dose theophylline in patients with chronic stroke: A double-blind, placebo-controlled, randomized cross-over study

BACKGROUND

Reducing inhibitory neurotransmission with pharmacological agents is a potential approach for augmenting plasticity after stroke. Previous work in healthy subjects showed diminished intracortical inhibition after administration of theophylline.

OBJECTIVE

We assessed the effect of single-dose theophylline on intracortical and interhemispheric inhibition in patients with chronic stroke, in a double-blind, placebo-controlled, cross-over study.

METHODS

Eighteen subjects were randomly administered 300 mg of extended-release theophylline or placebo. Immediately and 5 hours following administration, transcranial magnetic stimulation was used to assess bihemispheric resting motor threshold, short-interval intracortical inhibition, long-interval intracortical inhibition, and interhemispheric inhibition. Adverse effects on cardiovascular, neurological, and motor performance outcomes were also surveilled. Change between morning and afternoon sessions were compared across conditions. One week later, patients underwent the same assessments after crossing over to the opposite experimental condition. Subjects and investigators were blinded to the experimental condition during data acquisition and analysis.

RESULTS

For both hemispheres, changes in intracortical or interhemispheric neurophysiology were comparable under theophylline and placebo conditions. Theophylline induced no adverse neurological, cardiovascular, or motor performance effects. For both conditions and hemipsheres, the baseline level of inhibition inversely correlated with its change between sessions: less baseline inhibition (i.e. disinhibition) was associated with a strengthening in inhibition over the day, and vice versa.

CONCLUSION

A single dose of theophylline is well-tolerated by patients with chronic stroke, but does not alter cortical excitability. The inverse relationship between baseline inhibition and its change suggests the existence of a homeostatic process. The lack of effect on cortical inhibition may be related to an insufficiently long exposure to theophylline, or to differential responsiveness of disinhibited neural circuitry in patients with stroke.

Enhanced phasic GABA inhibition during the repair phase of stroke: a novel therapeutic target

Ischaemic stroke is the leading cause of severe long-term disability yet lacks drug therapies that promote the repair phase of recovery. This repair phase of stroke occurs days to months after stroke onset and involves brain remapping and plasticity within the peri-infarct zone. Elucidating mechanisms that promote this plasticity is critical for the development of new therapeutics with a broad treatment window. Inhibiting tonic (extrasynaptic) GABA signalling during the repair phase was reported to enhance functional recovery in mice suggesting that GABA plays an important function in modulating brain repair. While tonic GABA appears to suppress brain repair after stroke, less is known about the role of phasic (synaptic) GABA during the repair phase. We observed an increase in postsynaptic phasic GABA signalling in mice within the peri-infarct cortex specific to layer 5; we found increased numbers of α1 receptor subunit-containing GABAergic synapses detected using array tomography, and an associated increased efficacy of spontaneous and miniature inhibitory postsynaptic currents in pyramidal neurons. Furthermore, we demonstrate that enhancing phasic GABA signalling using zolpidem, a Food and Drug Administration (FDA)-approved GABA-positive allosteric modulator, during the repair phase improved behavioural recovery. These data identify potentiation of phasic GABA signalling as a novel therapeutic strategy, indicate zolpidem’s potential to improve recovery, and underscore the necessity to distinguish the role of tonic and phasic GABA signalling in stroke recovery.

Role of the Contralesional Hemisphere in Post-Stroke Recovery of Upper Extremity Motor Function

Identification of optimal treatment strategies to improve recovery is limited by the incomplete understanding of the neurobiological principles of recovery. Motor cortex (M1) reorganization of the lesioned hemisphere (ipsilesional M1) plays a major role in post-stroke motor recovery and is a primary target for rehabilitation therapy. Reorganization of M1 in the hemisphere contralateral to the stroke (contralesional M1) may, however, serve as an additional source of cortical reorganization and related recovery. The extent and outcome of such reorganization depends on many factors, including lesion size and time since stroke. In the chronic phase post-stroke, contralesional M1 seems to interfere with motor function of the paretic limb in a subset of patients, possibly through abnormally increased inhibition of lesioned M1 by the contralesional M1. In such patients, decreasing contralesional M1 excitability by cortical stimulation results in improved performance of the paretic limb. However, emerging evidence suggests a potentially supportive role of contralesional M1. After infarction of M1 or its corticospinal projections, there is abnormally increased excitatory neural activity and activation in contralesional M1 that correlates with favorable motor recovery. Decreasing contralesional M1 excitability in these patients may result in deterioration of paretic limb performance. In animal stroke models, reorganizational changes in contralesional M1 depend on the lesion size and rehabilitation treatment and include long-term changes in neurotransmitter systems, dendritic growth, and synapse formation. While there is, therefore, some evidence that activity in contralesional M1 will impact the extent of motor function of the paretic limb in the subacute and chronic phase post-stroke and may serve as a new target for rehabilitation treatment strategies, the precise factors that specifically influence its role in the recovery process remain to be defined.

Alterations in Purkinje cell GABAA receptor pharmacology following oxygen and glucose deprivation and cerebral ischemia reveal novel contribution of β1 -subunit-containing receptors

Cerebellar Purkinje cells (PCs) are particularly sensitive to cerebral ischemia, and decreased GABA(A) receptor function following injury is thought to contribute to PC sensitivity to ischemia-induced excitotoxicity. Here we examined the functional properties of the GABA(A) receptors that are spared following ischemia in cultured Purkinje cells from rat and in vivo ischemia in mouse. Using subunit-specific positive modulators of GABA(A) receptors, we observed that oxygen and glucose deprivation (OGD) and cardiac arrest-induced cerebral ischemia cause a decrease in sensitivity to the β(2/3) -subunit-preferring compound, etomidate. However, sensitivity to propofol, a β-subunit-acting compound that modulates β(1-3) -subunits, was not affected by OGD. The α/γ-subunit-acting compounds, diazepam and zolpidem, were also unaffected by OGD. We performed single-cell reverse transcription-polymerase chain reaction on isolated PCs from acutely dissociated cerebellar tissue and observed that PCs expressed the β(1) -subunit, contrary to previous reports examining GABA(A) receptor subunit expression in PCs. GABA(A) receptor β(1) -subunit protein was also detected in cultured PCs by western blot and by immunohistochemistry in the adult mouse cerebellum and levels remained unaffected by ischemia. High concentrations of loreclezole (30 μm) inhibited PC GABA-mediated currents, as previously demonstrated with β(1) -subunit-containing GABA(A) receptors expressed in heterologous systems. From our data we conclude that PCs express the β(1) -subunit and that there is a greater contribution of β(1) -subunit-containing GABA(A) receptors following OGD.

Brain excitability in stroke: the yin and yang of stroke progression

There is no current medical therapy for stroke recovery. Principles of physiological plasticity have been identified during recovery in both animal models and human stroke. Stroke produces a loss of physiological brain maps in adjacent peri-infarct cortex and then a remapping of motor and sensory functions in this region. This remapping of function in peri-infarct cortex correlates closely with recovery. Recent studies have shown that the stroke produces abnormal conditions of excitability in neuronal circuits adjacent to the infarct that may be the substrate for this process of brain remapping and recovery. Stroke causes a hypoexcitability in peri-infarct motor cortex that stems from increased tonic γ-aminobutyric acid activity onto neurons. Drugs that reverse this γ-aminobutyric acid signaling promote recovery after stroke. Stroke also increases the sensitivity of glutamate receptor signaling in peri-infarct cortex well after the stroke event, and stimulating α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate glutamate receptors in peri-infarct cortex promotes recovery after stroke. Both blocking tonic γ-aminobutyric acid currents and stimulating α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptors promote recovery after stroke when initiated at quite a delay, more than 3 to 5 days after the infarct. These changes in the excitability of neuronal circuits in peri-infarct cortex after stroke may underlie the process of remapping motor and sensory function after stroke and may identify new therapeutic targets to promote stroke recovery.

Global impairment and therapeutic restoration of visual plasticity mechanisms after a localized cortical stroke

We tested the influence of a photothrombotic lesion in somatosensory cortex on plasticity in the mouse visual system and the efficacy of anti-inflammatory treatment to rescue compromised learning. To challenge plasticity mechanisms, we induced monocular deprivation (MD) in 3-mo-old mice. In control animals, MD induced an increase of visual acuity of the open eye and an ocular dominance (OD) shift towards this eye. In contrast, after photothrombosis, there was neither an enhancement of visual acuity nor an OD-shift. However, OD-plasticity was present in the hemisphere contralateral to the lesion. Anti-inflammatory treatment restored sensory learning but not OD-plasticity, as did a 2-wk delay between photothrombosis and MD. We conclude that (i) both sensory learning and cortical plasticity are compromised in the surround of a cortical lesion; (ii) transient inflammation is responsible for impaired sensory learning, suggesting anti-inflammatory treatment as a useful adjuvant therapy to support rehabilitation following stroke; and (iii) OD-plasticity cannot be conceptualized solely as a local process because nonlocal influences are more important than previously assumed.

Brain self-protection: the role of endogenous neural progenitor cells in adult brain after cerebral cortical ischemia

Convincing evidence has shown that brain ischemia causes the proliferation of neural stem cells/neural progenitor cells (NSCs/NPCs) in both the subventricular zone (SVZ) and the subgranular zone (SGZ) of adult brain. The role of brain ischemia-induced NSC/NPC proliferation, however, has remained unclear. Here we have determined whether brain ischemia-induced amplification of the NSCs/NPCs in adult brain is required for brain self-protection. The approach of intracerebroventricular (ICV) infusion of cytosine arabinoside (Ara-C), an inhibitor for cell proliferation, for the first 7days after brain ischemia was used to block ischemia-induced NSC/NPC proliferation. We observed that ICV infusion of Ara-C caused a complete blockade of NSC/NPC proliferation in the SVZ and a dramatic reduction of NSC/NPC proliferation in the SGZ. Additionally, as a result of the inhibition of ischemia-induced NSC/NPC pool amplification, the number of neurons in the hippocampal CA1 and CA3 was significantly reduced, the infarction size was significantly enlarged, and neurological deficits were significantly worsened after focal brain ischemia. We also found that an NSC/NPC-conditioned medium showed neuroprotective effects in vitro and that adult NSC/NPC-released brain-derived neurotrophic factor (BDNF) and vascular endothelial growth factor (VEGF) are required for NSC/NPC-conditioned medium-induced neuroprotection. These data suggest that NSC/NPC-generated trophic factors are neuroprotective and that brain ischemia-triggered NSC/NPC proliferation is crucial for brain protection. This study provides insights into the contribution of endogenous NSCs/NPCs to brain self-protection in adult brain after ischemia injury.

Differential effects of high-frequency repetitive transcranial magnetic stimulation over ipsilesional primary motor cortex in cortical and subcortical middle cerebral artery stroke

OBJECTIVE

Facilitation of cortical excitability of the ipsilesional primary motor cortex (M1) may improve dexterity of the affected hand after stroke. The effects of 10 Hz repetitive transcranial magnetic stimulation (rTMS) over ipsilesional M1 on movement kinematics and neural activity were examined in patients with subcortical or cortical stroke.

METHODS

Twenty-nine patients with impaired dexterity after stroke (16 subcortical middle cerebral artery [MCA] strokes, 13 MCA strokes involving subcortical tissue and primary or secondary cortical sensorimotor areas) received 1 session of 10 Hz rTMS (5-second stimulation, 25-second break, 1,000 pulses, 80% of the resting motor threshold) applied over: 1) ipsilesional M1 and 2) vertex (control stimulation). For behavioral testing, 29 patients performed index finger and hand tapping movements with the affected and unaffected hand prior to and following each rTMS application. For functional magnetic resonance imaging, 18 patients performed index finger tapping movements with the affected and unaffected hand before and after each rTMS application.

RESULTS

Ten-Hz rTMS over ipsilesional M1, but not over vertex, improved movement kinematics in 14 of 16 patients with subcortical stroke, but not in patients with additional cortical stroke. Ten-Hz rTMS slightly deteriorated dexterity of the affected hand in 7 of 13 cortical stroke patients. At a neural level, rTMS over ipsilesional M1 reduced neural activity of the contralesional M1 in 11 patients with subcortical stroke, but caused a widespread bilateral recruitment of primary and secondary motor areas in 7 patients with cortical stroke. Activity in ipsilesional M1 at baseline correlated with improvement of index finger tapping frequency induced by rTMS.

INTERPRETATION

The beneficial effects of 10 Hz rTMS over ipsilesional M1 on motor function of the affected hand depend on the extension of MCA stroke. Neural activity in ipsilesional M1 may serve as a surrogate marker for the effectiveness of facilitatory rTMS.

Effects of aging on paired-pulse behavior of rat somatosensory cortical neurons

Aging affects all levels of neural processing including changes of intracortical inhibition and cortical excitability. The paired-pulse stimulation protocol, the application of 2 stimuli in close succession, is used to investigate cortical excitability. The paired-pulse behavior is characterized by the fact that the second response is significantly suppressed at short interstimulus intervals (ISIs) but approaches the first response with increasing ISIs. However, there are controversial reports about the influence of age on paired-pulse behavior. We therefore used pairs of tactile stimuli (ISIs from tens to hundreds of milliseconds) to record extracellular responses of somatosensory cortical neurons of young and aged rats. Paired-pulse behavior was quantified as the ratio of the amplitude of the second response divided by the first. For all ISIs, we found significantly higher ratios in the old animals indicating reduced paired-pulse suppression (PPS). Evaluation of the single response components revealed a significant reduction of the response to the first stimulus for old animals but no age-dependent decrement to the second. Changes in PPS are usually mediated by modulating the second response characteristics. Thus, our data demonstrate reduced PPS due to an overall reduction of the first response as a form of modified PPS developing at old age.

Imaging rapid redistribution of sensory-evoked depolarization through existing cortical pathways after targeted stroke in mice

Evidence suggests that recovery from stroke damage results from the production of new synaptic pathways within surviving brain regions over weeks. To address whether brain function might redistribute more rapidly through preexisting pathways, we examined patterns of sensory-evoked depolarization in mouse somatosensory cortex within hours after targeted stroke to a subset of the forelimb sensory map. Brain activity was mapped with voltage-sensitive dye imaging allowing millisecond time resolution over 9 mm(2) of brain. Before targeted stroke, we report rapid activation of the forelimb area within 10 ms of contralateral forelimb stimulation and more delayed activation of related areas of cortex such as the hindlimb sensory and motor cortices. After stroke to a subset of the forelimb somatosensory cortex map, function was lost in ischemic areas within the forelimb map center, but maintained in regions 200-500 microm blood flow deficits indicating the size of a perfused, but nonfunctional, penumbra. In many cases, stroke led to only partial loss of the forelimb map, indicating that a subset of a somatosensory domain can function on its own. Within the forelimb map spared by stroke, forelimb-stimulated responses became delayed in kinetics, and their center of activity shifted into adjacent hindlimb and posterior-lateral sensory areas. We conclude that the focus of forelimb-specific somatosensory cortex activity can be rapidly redistributed after ischemic damage. Given that redistribution occurs within an hour, the effect is likely to involve surviving accessory pathways and could potentially contribute to rapid behavioral compensation or direct future circuit rewiring.

Ischemic insult to cerebellar Purkinje cells causes diminished GABAA receptor function and allopregnanolone neuroprotection is associated with GABAA receptor stabilization

Cerebellar Purkinje cells (PC) are particularly vulnerable to ischemic injury and excitotoxicity, although the molecular basis of this sensitivity remains unclear. We tested the hypothesis that ischemia causes rapid down-regulation of GABA(A) receptors in cerebellar PC, thereby increasing susceptibility to excitotoxicity. Oxygen-glucose deprivation (OGD) caused a decline in functional GABA(A) receptors, within the first hour of re-oxygenation. Decreased amplitude of miniature inhibitory post-synaptic potentials confirmed that OGD caused a significant decrease in functional synaptic GABA(A) receptors and quantitative Western blot analysis demonstrated the loss of GABA(A) receptor current was associated with a decline in total receptor protein. Interestingly, the potent neuroprotectant allopregnanolone (ALLO) prevented the decline in GABA(A) receptor current and protein. Consistent with our in vitro data, global ischemia in mice caused a significant decline in total cerebellar GABA(A) receptor protein and PC specific immunoreactivity. Moreover, ALLO provided strong protection of PC and prevented ischemia-induced decline in GABA(A) receptor protein. Our findings indicate that ischemia causes a rapid and sustained loss of GABA(A) receptors in PC, whereas ALLO prevents the decline in GABA(A) receptors and protects against ischemia-induced damage. Thus, interventions which prevent ischemia-induced decline in GABA(A) receptors may represent a novel neuroprotective strategy.

Expression of GABA A receptor alpha1 subunit mRNA and protein in rat neocortex following photothrombotic infarction

Photothrombotic infarcts of the neocortex result in structural and functional alterations of cortical networks, including decreased GABAergic inhibition, and can generate epileptic seizures within 1 month of lesioning. In our study, we assessed the involvement and potential changes of cortical GABA A receptor (GABA AR) alpha1 subunits at 1, 3, 7, and 30 days after photothrombosis. Quantitative competitive reverse transcription-polymerase chain reaction (cRT-PCR) and semi-quantitative Western blot analysis were used to investigate GABA AR alpha1 subunit mRNA and protein levels in proximal and distal regions of perilesional cortex and in homotopic areas of young adult Sprague-Dawley rats. GABA AR alpha1 subunit mRNA levels were decreased ipsilateral and contralateral to the infarct at 7 days, but were increased bilaterally at 30 days. GABA AR alpha1 subunit protein levels revealed no significant change in neocortical areas of both hemispheres of lesioned animals compared with protein levels of sham-operated controls at 1, 3, 7, and 30 days. At 30 days, GABA AR alpha1 subunit protein expression was significantly increased in lesioned animals within proximal and distal regions of perilesional cortex compared with distal neocortical areas contralaterally (Student's t-test, p<0.05). Short- and long-term alterations of mRNA and protein levels of the GABA AR alpha1 subunit ipsilateral and contralateral to the lesion may influence alterations in cell surface receptor subtype expression and GABA AR function following ischemic infarction and may be associated with formative mechanisms of poststroke epileptogenesis.

Plastic changes in the vibrissa motor cortex in adult rats after output suppression in the homotopic cortex

After motor cortex damage, the unaffected homotopic cortex shows changes in motor output. The present experiments were designed to clarify the nature of these interhemispheric effects. We investigate the vibrissa motor cortex (VMC) output after activity suppression of the homotopic area in adult rats. Comparison was made of VMC output after lidocaine inactivation (L-group) or quinolinic acid lesion (Q-group) of the homotopic cortex. In the Q-group, VMC mapping was performed 3 days (Q3Ds group), 2 weeks (Q2Ws group) and 4 weeks (Q4Ws group) after cortical lesion. In each animal, VMC output was assessed by mapping movements induced by intracortical microstimulation (ICMS) in both hemispheres (hemisphere ipsilateral and contralateral to injections). Findings demonstrated that, in the L-group, the size of vibrissal representation was 39.5% smaller and thresholds required to evoke vibrissa movement were 46.3% higher than those in the Control group. There was an increase in the percentage of ineffective sites within the medial part of the VMC and an increase in the percentage of forelimb sites within the lateral part. Both the Q3Ds group and the L-group led to a similar VMC reorganization (Q3Ds vs. L-group, P > 0.05). In the Q2Ws group the VMC representation showed improvement in size (83.4% recovery compared with controls). The VMC showed recovery to normal output at 4 weeks after lesion (Control vs. Q4Ws group, P > 0.05). These results suggest that the VMC of the two hemispheres continuously interact through excitatory influences, preserving the normal output and inhibitory influences defining the border with the forelimb representation.

Relationship Between Interhemispheric Inhibition and Motor Cortex Excitability in Subacute Stroke Patients

Background. Studies of stroke patients using functional imaging and transcranial magnetic stimulation (TMS) of the primary motor cortex (M1) demonstrated increased recruitment and abnormally decreased short interval cortical inhibition (SICI) of the M1 contralateral to the lesioned hemisphere (contralesional M1) within the first month after infarction of the M1 or its corticospinal projections. Objective. The authors sought to identify mechanisms underlying decreased SICI of the contralesional M1. Methods. In patients within 6 weeks of their first ever infarction of the M1 or its corticospinal projections, SICI in the M1 of the lesioned and nonlesioned hemisphere was studied using paired-pulse TMS. Interhemispheric inhibition (IHI) was measured by applying TMS to the M1 of the lesioned hemisphere and a second pulse to the homotopic M1 of the nonlesioned hemisphere and vice versa with the patient at rest. The results were compared to M1 stimulation of age-matched healthy controls. Results. SICI was decreased in the M1 of lesioned and nonlesioned hemispheres regardless of cortical or subcortical infarct location. IHI was abnormally decreased from the M1 of the lesioned on nonlesioned hemisphere. In contrast, IHI was normal from the M1 of the nonlesioned on the lesioned hemisphere. Abnormal IHI and SICI were correlated in patients with cortical but not with subcortical lesions. Conclusions. In subacute stroke patients, abnormally decreased SICI of a contralesional M1 can only partially be explained by loss of IHI from the lesioned on nonlesioned hemisphere. As decreased SICI of the contralesional M1 did not result in excessive IHI from the nonlesioned on lesioned hemisphere with subsequent suppression of ipsilesional M1 excitability and all patients showed excellent recovery of motor function, decreased SICI of the contralesional M1 may represent an adaptive process supporting recovery.

Neurophysiological and anatomical plasticity in the adult sensorimotor cortex

Evidence supporting the plastic capacity of the adult cortex is abundant. Changes have been associated with exposure to enriched environments, learning, peripheral lesions and central lesions. The initial loss of function caused by a lesion is generally followed by a certain amount of recovery that is believed to be due, at least in part, to adaptive plasticity. In particular, the reorganization of cortical representational maps has been associated with improvement of performance. Therefore, areas undergoing such reorganization following lesions are generally assumed to participate in the recovery. We review evidence demonstrating the remodeling of representational maps of the forelimb in adult cortex and the structural plasticity that has been coupled with it. A particular emphasis is paid to non-human primate studies and stroke.

Vicarious function of remote cortex following stroke: recent evidence from human and animal studies

Following a lesion, the adult central nervous system undergoes dramatic structural and physiological reorganization in diverse subcortical and cortical areas. Our knowledge of the events that parallel recovery within the tissue surrounding the lesion and other distant cortical areas has evolved greatly in the past few years. Particularly, recent efforts have increased our understanding of the potential implication of premotor areas in recovery from lesions disturbing the primary motor cortex (M1) and its corticospinal outputs. Because these areas share extensive connections with M1 and have direct access to the spinal cord through corticospinal projections, they are particularly well positioned to take over the role of M1 in a vicarious manner and thus compensate for the neuronal loss resulting from M1 lesions. The impressive postlesional reorganization known to occur in many areas of the CNS including the premotor cortex traditionally has been assumed to play a beneficial role in recovery. However, recent experiments suggest that in some cases, reorganization of distant cortical areas correlates with poor recovery, raising the concept of maladaptive vicarious process. This concept might be particularly critical in the development of new treatment approaches favoring postlesion plasticity and even more so for interventions targeting specific area(s). Here, the author reviews human and animal studies that show the plastic potential of the adult CNS after stroke, highlighting the vicarious role of the premotor cortex in the recovery of motor control.

NR2B antagonists restrict spatiotemporal spread of activity in a rat model of cortical dysplasia

Freeze-lesion-induced focal cortical dysplasia in rats closely resembles human microgyria, a neuronal migration disorder associated with drug-resistant epilepsy. Alterations in expression of N-methyl-D-aspartate receptors (NMDARs) containing NR2B subunits have been suggested to play a role in the hyperexcitability seen in this model. We examined the effect of NMDAR antagonists selective for NR2B subunits (Ro 25-6981 and ifenprodil) on activity evoked by intracortical stimulation in brain slices from freeze-lesioned rat neocortex. Whole-cell voltage-clamp recordings showed that Ro 25-6981 (1 microM) significantly reduced the response area of evoked postsynaptic currents in pyramidal cells from the paramicrogyral area whereas responses were unaffected in slices from control (sham operated) animals. Voltage-sensitive dye imaging was used to examine spatiotemporal spread of evoked activity in lesioned and control cortices. The imaging experiments revealed that peak amplitude, duration, and lateral spread of evoked activity in the paramicrogyral area was reduced by bath application of Ro 25-6981 (1 microM) and ifenprodil (10 microM). Ro 25-6981 had no effect on evoked activity in neocortical slices from control animals. The non-selective NMDAR antagonist d-2-amino-5-phosphonvaleric acid (APV, 20 microM) reduced activity evoked in presence of 50 microM 4-aminopyridine (known to increase excitability by enhancing neurotransmitter release) in neocortical slices from control animals whereas Ro 25-6981 (1 microM) did not. These results suggest that NR2B subunit-containing NMDARs contribute significantly to the enhanced spatiotemporal spread of paroxysmal activity observed in vitro in the rat freeze-lesion model of focal cortical dysplasia.

Long-term effects of stroke on neuronal rest activity in rolandic cortical areas

To understand the relationship between neuronal function and clinical state in the framework of stroke, the long-term poststroke rolandic spontaneous neuronal activity was studied by means of magnetoencephalography. Fifty-six patients who had suffered a unilateral stroke within the middle cerebral artery were enrolled. Median time since stroke was 2.8 years. In association with lesion features and clinical picture, total and relative band powers and the spectral entropy were analyzed in the affected (AH) and unaffected (UH) hemispheres in comparison with an age-matched control group. An increase of absolute and relative slow band powers and a reduction of relative fast band powers were found in patients' AH with respect to both UH and control values. Absolute delta band was higher than in controls also in UH. New findings were the increase of rolandic rest activity power also in the alpha band and the decrease of spectral entropy in AH with respect to both UH and control values. Moreover, our results in chronic stroke patients indicate frequency-selective alterations related to specific dysfunctions: global clinical status was mostly impaired in patients with larger lesions and increased total and slow band activity powers, whereas hand functionality was mostly disrupted in patients with subcortical involvement and reduction of high-frequency rhythms and spectral entropy. Total power increase and spectral richness decrease are in agreement with a higher synchrony of local neuronal activity, a reduction of the intracortical inhibitory network's efficiency, and an increase of neuronal excitability.

Assessing recovery in middle cerebral artery stroke using functional MRI

PRIMARY OBJECTIVE

To understand the temporal evolution of brain reorganization during recovery from stroke.

RESEARCH DESIGN

A patient who suffered left middle cerebral artery stroke 9 months earlier was studied on three occasions, approximately 1 month apart.

METHODS AND PROCEDURES

Brain activation was studied using functional Magnetic Resonance Imaging (fMRI). During each session, the patient performed a finger-to-thumb opposition task, which involved one bimanual and two unimanual conditions. Each condition consisted of overt movement of fingers and imagery of the same task.

RESULTS

With recovery, greater recruitment was observed of the affected primary motor cortex (M1) and a decrease in activation of the unaffected M1 and supplementary motor area. In addition, the widespread activation of brain areas seen during the initial session changed to a more focused pattern of activation as the patient recovered. Imagery tasks resulted in similar brain activity as overt execution pointing to imagery as a potential tool for rehabilitation.

Proliferative response of distinct hippocampal progenitor cell populations after cortical infarcts in the adult brain

Precursor cells in the adult dentate gyrus are a heterogeneous population. Astrocytic cell types with radial glia-like morphology and low proliferative activity have been distinguished from highly dividing subtypes expressing early neuronal properties. Recent evidence indicates that physiological stimuli predominantly increased proliferation of non-astrocytic cell types whereas radial glia-like precursors remained quiescent. We here analyzed the proliferative response of precursor cell subtypes under pathophysiological conditions in a model of photochemical cortical infarcts. Using transgenic pNestin-GFP mice and single intraperitoneal injections of 5-bromo-2-deoxyuridine 4 h after surgery, immunocytochemical analysis revealed a differential activation of the distinct subpopulations within 72 h after the infarct. The stimulation was most prominent in radial glia-like precursor cells but also non-astrocytic precursors with early neuronal phenotypes were activated. The present study indicates that the proliferative response of precursor cell subpopulations in the subgranular zone might differ under physiological and pathophysiological conditions.

Pathophysiology of Stroke Rehabilitation: Temporal Aspects of Neurofunctional Recovery

Stroke almost always causes an impairment of motor activity and function. Clinical recovery, though usually incomplete, is often highly dynamic and reflects the ability of the neuronal network to adapt. Mechanisms that underlie neuro-functional plasticity are now beginning to be understood. Albeit the enormous efforts undertaken to support the natural course of re-convalescence through rehabilitation, little has been done to relate possible effects of these therapeutic approaches to mechanisms of adaptive pathophysiology. The review presented here focuses on these mechanisms during the course of recovery post stroke. Next to an unmasking of latent network representations, other adaptive processes, such as excitatory metabolic stress, an imbalance in activating and inhibiting transmission, leading to salient hyperexcitability or mechanisms that consolidate novel connections prime the system's plastic capabilities. These pathophysiological processes potentially interact with rehabilitative interventions. They therefore form the foundation of positive, but possibly also negative recuperation under therapy.

1α,25-Dihydroxyvitamin D3 treatment does not alter neuronal cyclooxygenase-2 expression in the cerebral cortex after stroke

The inducible prostaglandin synthase, cyclooxygenase-2, is upregulated in response to cerebral ischemia and contributes to potentiation of oxidative injury. Cyclooxygenase-2 expression is regulated by retinoic acid receptors, which form heterodimers with vitamin D receptors and vitamin D. In addition, vitamin D has been reported to have neuroprotective qualities. The aim of this study was to examine whether the biologically active vitamin D3-metabolite 1alpha,25-dihydroxyvitamin D3 (1,25-D3), influences the expression of inducible cyclooxygenase-2 in photothrombotically lesioned brain or is part of an independent neuroprotective mechanism. We compared groups of nonlesioned control rats and infarcted animals, which were treated with either 1,25-D3 or solvent at different times postlesion. In control animals, cyclooxygenase-2 immunoreactivity was readily evident in almost all cortical neurons of layers II/III as well as in a few pyramidal cells in layer V. Following photothrombotic infarction of the right cortical hindlimb area, there was a significant, but transient, increase in cyclooxygenase-2 labeling which was restricted to neurons of the injured hemisphere in both 1,25- D3-treated and solvent-treated rats. Highest levels of cyclooxygenase-2 immunoreactivity were seen at 12 and 24 h postlesion, followed by a gradual decrease at later time points. However, no significant differences were detected between 1,25-D3-treated and solvent-treated lesioned rats, indicating that postischemic neuronal cyclooxygenase-2 upregulation is not influenced by 1,25-D3. It is concluded that the neuroprotective effect of 1,25-D3 does not depend on modulations of neuronal COX-2 expression caused by postlesional hyperexcitation.

Delayed treatment with monoclonal antibody IN-1 1 week after stroke results in recovery of function and corticorubral plasticity in adult rats

Neuronal death due to ischemic stroke results in permanent deficits in sensory, language, and motor functions. The growth-restrictive environment of the adult central nervous system (CNS) is an obstacle to functional recovery after stroke and other CNS injuries. In this regard, Nogo-A is a potent neurite growth-inhibitory protein known to restrict neuronal plasticity in adults. Previously, we have found that treatment with monoclonal antibody (mAb) IN-1 to neutralize Nogo-A immediately after stroke enhanced motor cortico-efferent plasticity and recovery of skilled forelimb function in rats. However, immediate treatment for stroke is often not clinically feasible. Thus, the present study was undertaken to determine whether cortico-efferent plasticity and functional recovery would occur if treatment with mAb IN-1 was delayed 1 week after stroke. Adult rats were trained on a forelimb-reaching task, and the middle cerebral artery was occluded to induce focal cerebral ischemia to the forelimb sensorimotor cortex. After 1 week, animals received mAb IN-1 treatment, control antibody, or no treatment, and were tested for 9 more weeks. To assess cortico-efferent plasticity, the sensorimotor cortex opposite the stroke lesion was injected with an anterograde neuroanatomical tracer. Behavioral analysis demonstrated a recovery of skilled forelimb function, and anatomical studies revealed neuroplasticity at the level of the red nucleus in animals treated with mAb IN-1, thus demonstrating the efficacy of this treatment even if administered 1 week after stroke.

Decomposition and long-lasting downregulation of extracellular matrix in perineuronal nets induced by focal cerebral ischemia in rats

The upregulation of extracellular matrix components, especially chondroitin sulfate proteoglycans, after brain injury and stroke is known to accompany the glial reaction, forming repellent scars that hinder axonal growth and the reorganization of the injured neuronal networks. The extracellular matrix associated with perineuronal nets (PNs) in the primarily injured and remote regions has not yet been systematically analyzed. We use the model of permanent middle cerebral artery occlusion (MCAO) to investigate the acute and long-lasting consequences of ischemia for PNs, related to the damage of neurons and reactions of glial cells, in spontaneously hypertensive rats. Extracellular matrix components associated with PNs around cortical interneurons and neurons in thalamic nuclei were characterized 1, 7, 14, and 35 days after MCAO, using Wisteria floribunda agglutinin (WFA) staining and immunocytochemistry. The degradation of PNs in the infarct core was initiated by loss of WFA-binding matrix components, indicating the cleavage of glycosaminoglycan chains of chondroitin sulfate proteoglycans. Immunostaining showed the subsequent removal of proteoglycan core proteins within the extending microglia/macrophage invasion zone lasting for 2 weeks after MCAO. In the cortical periinfarct region, delineated by an astrocytic scar against the infarct core, the number of WFA-stained and proteoglycan core protein-immunoreactive PNs was permanently reduced. In the homolateral ventroposterior thalamus, the delayed decrease in perineuronal matrix was related to the distribution pattern of activated microglia and massive neuronal degeneration. It can be concluded from these results that complementary to the known upregulation of matrix components in the glial scar, deficits in the expression of the neuron-associated extracellular matrix develop in the periinfarct and remote regions. These deficits may contribute to the long-lasting functional impairments after stroke.

GABA and glycine are protective to mature but toxic to immature rat cortical neurons under hypoxia

Although recent studies suggest that gamma-aminobutyric acid (GABA) and glycine may be 'inhibitory' to mature neurons, but 'excitatory' to immature neurons under normoxia, it is unknown whether inhibitory neurotransmitters are differentially involved in neuronal response to hypoxia in immature and mature neurons. In the present study, we exposed rat cortical neurons to hypoxia (1% O2) and examined the effects of three major inhibitory neurotransmitters (GABA, glycine and taurine) on the hypoxic neurons at different neuronal ages [days in vitro (DIV)4-20]. Our data showed that the cortical neurons expressed both GABA(A) and glycine receptors with differential developmental profiles. GABA (10-2000 microm) was neuroprotective to hypoxic neurons of DIV20, but enhanced hypoxic injury in neurons of <DIV20. Glycine at low concentrations (10-100 microm) exhibited a similar pattern to GABA. However, higher concentrations of glycine (1000-2000 microm) for long-term exposure (48-72 h) displayed neuroprotection at all ages (DIV4-20). Taurine (10-2000 microm), unlike GABA and glycine, displayed protection only in DIV4 neurons, and was slightly toxic to neurons>DIV4. In comparison with delta-opioid receptor (DOR)-induced protection in DIV20 neurons exposed to 72 h of hypoxia, glycine-induced protection was weaker than that of DOR but stronger than that of GABA and taurine. These data suggest that the effects of the inhibitory neurotransmitters on hypoxic cortical neurons are age-dependent, with GABA and glycine being neurotoxic to immature neurons and neuroprotective to mature neurons.

Behavioral effects of photothrombotic ischemic cortical injury in aged rats treated with the sedative-hypnotic GABAergic drug zopiclone

Sedative-hypnotic drugs commonly used in the elderly may affect functional recovery following cerebrovascular events. Previous research has shown that prolonged exposure to diazepam can interfere with recovery of function and exaggerate tissue loss after brain injury. The present study evaluated the effect of zopiclone, a widely used hypnotic drug, on functional and histological outcome after cortical photothrombosis in aged rats, which might be particularly vulnerable to brain insults and inhibitory sedative-hypnotic drugs. Aged Wistar rats were treated with zopiclone at a dose of 3 mg/kg (i.p., once a day) beginning 4 days before ischemia induction and continuing for 23 days. Sensorimotor recovery was assessed by a new ledged beam-walking test and spatial learning by the Morris water-maze. After a 7-day washout period all rats were administered a single dose of zopiclone (3 mg/kg, i.p.) and retested. Infarct volumes were measured from nitroblue tetrazolium-stained sections at the end of the experiment. Beam-walking data showed that ischemic rats treated with zopiclone were not more impaired than untreated rats. Indeed, they showed fewer faults with the impaired hindlimb than ischemic controls on post-operative day 16. Water-maze performance was not affected by zopiclone. After the washout period a single dose of zopiclone did not worsen forelimb or hindlimb function, but seemed to improve performance in the water-maze test. Cortical infarct volumes were similar in ischemic controls and ischemic rats treated with zopiclone. In conclusion, zopiclone was not detrimental and even seemed to improve behavioral outcome without affecting ischemic damage in aged rats subjected to cortical photothrombosis.

Hyperexcitability-associated rapid plasticity after a focal cerebral ischemia

BACKGROUND AND PURPOSE

This article addresses how neuroplastic changes are initiated after an ischemic stroke.

METHODS

A focal cerebral ischemia was photochemically induced on the primary somatosensory cortex of rats, and in vivo electrophysiological recordings were performed on the peri-infarct cortex before and from 1 to 6 hours after the infarction.

RESULTS

Paired-pulse analysis of evoked field potentials to peripheral electrical stimuli showed statistically significant neuronal hyperexcitability that was associated with rapid expansion of receptive fields (146.1% at 1 hour and 553.6% at 6 hours) as early as 1 hour after the infarction (P<0.05). Current source density analysis revealed increased current sinks in cortical layer II/III.

CONCLUSIONS

Our electrophysiological results showed, for the first time to our knowledge, rapid plastic changes in the peri-infarct cortex during the hyperacute stage of an ischemic stroke. Manipulation of this rapid plasticity may affect subsequent plastic changes.

Electrobehavioral characteristics of epileptic rats following photothrombotic brain infarction

The goal of this study was to characterize the electroencephalographic (EEG) and behavioral properties of young adult rats during extended video-EEG monitoring following photothrombotic brain infarction. Two-month-old male Sprague-Dawley rats underwent photothrombotic brain infarction of the left sensorimotor cortex with the photosensitive dye rose bengal (n=10) or were used as controls (n=9). Qualitative and quantitative EEG analysis was performed on digital video-EEG records obtained during 6 months of recording. The main finding of this study was that 5/10 (50%) lesioned animals developed focal epileptic seizures ipsilateral to the cortical infarct characterized by rhythmic spike-wave discharges with or without behavioral change. Epileptic animals demonstrated increased delta, theta, and low beta-range power ipsilateral to the infarct that reliably distinguished them from lesioned nonepileptic and control animals. Lesioned animals (epileptic and nonepileptic) also demonstrated a distinct pattern of focal rhythmic theta activity before or after generalized high beta-range discharges. Electrical and behavioral characteristics common to both lesioned and control animals included: (1) focal rhythmic theta activity in either hemisphere; (2) focal low beta-range discharges in either hemisphere; (3) generalized high beta-range discharges; (4) absence seizures; (5) generalized pseudoperiodic spike discharges associated with mild multifocal body jerks; (6) tonic-clonic seizures (one nai;ve control; one lesioned animal). Cresyl violet staining of lesioned animals' brains showed consistent infarcts of the sensorimotor cortex extending to the subcortical white matter. These results provide an expanded electrobehavioral description of young adult rats following photothrombotic brain infarction and augment further investigation into the molecular, cellular, and network alterations that contribute to the establishment of post-stroke epilepsy.

Influence of the postlesion environment and chronic piracetam treatment on the organization of the somatotopic map in the rat primary somatosensory cortex after focal cortical injury

The influence of housing in an enriched or impoverished environment and anti-ischemic treatment (piracetam) on the organization of the intact regions of the somatosensory cortical maps adjacent to a focal cortical injury were investigated in adult rats. Response properties of small clusters of neurons were recorded in the area of the primary somatosensory cortex (SI) devoted to the contralateral forepaw representation. Electrophysiological maps were elaborated on the basis of the sensory "submodality" (cutaneous or noncutaneous) and the location of the receptive fields (RFs) of layer IV neurons. Recordings were made before, and 3 weeks after induction of a focal neurovascular lesion to the SI cortex. The main results were: 1) the focal ischemic injury induced a cellular loss which was less severe in the piracetam treated rats, regardless of the housing conditions; 2) the lesion resulted in a compression of the remaining forepaw map, a fragmentation of the representational zones serving the cutaneous surfaces (low-threshold inputs) and an enlargement of noncutaneous zones (high-threshold inputs) in the spared cortical sectors surrounding the lesion. These changes were found in all placebo rats, with the most detrimental effects in the animals exposed to an impoverished environment, and in the piracetam-plus-impoverished rats. In contrast, a limited compression of the forepaw map and a preservation of most representational sectors were observed in the piracetam-plus-enriched animals, 3) the size of the cutaneous RFs of the neurons within the intact cortical zones remained unchanged, regardless of environment or treatment; 4) consistent with the map changes, the skin surfaces lacking low-threshold cutaneous RFs increased after the lesion in all animal groups but the piracetam-plus-enriched rats; 5) cortical responsiveness as assessed with mechanical threshold evaluation was diminished in the placebo rats, whatever the environment, and in the piracetam-impoverished rats, but was not significantly affected in the piracetam-enriched animals. Our findings, based on the first double electrophysiological mapping in the rat SI cortex, highlight the protective effects of an environmental therapy associated with an anti-ischemic treatment on the neurophysiological properties of cortical neurons following a focal neurovascular injury to the neocortex.

Reorganization of remote cortical regions after ischemic brain injury: a potential substrate for stroke recovery

Although recent neurological research has shed light on the brain's mechanisms of self-repair after stroke, the role that intact tissue plays in recovery is still obscure. To explore these mechanisms further, we used microelectrode stimulation techniques to examine functional remodeling in cerebral cortex after an ischemic infarct in the hand representation of primary motor cortex in five adult squirrel monkeys. Hand preference and the motor skill of both hands were assessed periodically on a pellet retrieval task for 3 mo postinfarct. Initial postinfarct motor impairment of the contralateral hand was evident in each animal, followed by a gradual improvement in performance over 1-3 mo. Intracortical microstimulation mapping at 12 wk after infarct revealed substantial enlargements of the hand representation in a remote cortical area, the ventral premotor cortex. Increases ranged from 7.2 to 53.8% relative to the preinfarct ventral premotor hand area, with a mean increase of 36.0 +/- 20.8%. This enlargement was proportional to the amount of hand representation destroyed in primary motor cortex. That is, greater sparing of the M1 hand area resulted in less expansion of the ventral premotor cortex hand area. These results suggest that neurophysiologic reorganization of remote cortical areas occurs in response to cortical injury and that the greater the damage to reciprocal intracortical pathways, the greater the plasticity in intact areas. Reorganization in intact tissue may provide a neural substrate for adaptive motor behavior and play a critical role in postinjury recovery of function.

Neuroprotective effects on somatotopic maps resulting from piracetam treatment and environmental enrichment after focal cortical injury

Acute and chronic postlesion reorganization of the cortical maps was examined in adult rats using electrophysiological mapping of the forepaw area in the primary somatosensory cortex. Recordings were made before and after (first 12 hr and 3 wk) induction of a focal thermal-ischemic lesion to a restricted part of the forepaw area. The influence of an anti-ischemic substance (piracetam) and housing in an enriched environment (EE) or impoverished environment (IE) on the organization of the spared regions of the cortical maps adjacent to the lesion was investigated. The results revealed (1) a gradual expansion of the injured zone and a cellular loss that were smaller in the piracetam-treated (PT) rats than in the placebo (PL) rats; (2) a better preservation of the somatotopic organization and the neuronal responsiveness in the maps of the PT rats during the first 12 hr after the lesion; (3) a gradual compression and fragmentation of the remaining forepaw map over the first 3 postlesion wk. These changes were found in all PL rats, with the most detrimental effects in the animals exposed to an IE. In the PT-EE animals, a contrasting substantial preservation of the map was observed. (4) Cortical responsiveness was diminished in the PL rats, whatever the environment, and in the PT-IE rats; but it was not significantly affected in the PT-EE animals. The findings demonstrate the protective function of acute piracetam treatment on electrophysiological properties of cortical neurons within the peri-infarct tissue and highlight the neuroprotective effects of an environmental therapy combined with the piracetam treatment during the weeks after ischemic damage.

Disinhibition of the contralateral motor cortex by low-frequency rTMS

Low-frequency repetitive transcranial magnetic stimulation (rTMS) of the primary motor cortex (M1) results in a lasting decrease of motor evoked potentials (MEPs). Here we investigated the effects of supra-threshold rTMS (15 min, 1 Hz) to the left M1 on the excitability of the stimulated and homologous (unstimulated) M1 in healthy subjects by using single and double pulse TMS before and after rTMS. We found reduction of MEP amplitudes on the stimulated side and, most importantly, disinhibition of intracortical excitability of the homologous M1. This crossed effect of rTMS supports the concept of a physiological balance of reciprocal inhibitory projections and emphasizes that rTMS can induce remote effects that are relevant for the physiological interpretation of such interventions.

Plasticity of cortical projections after stroke

Ischemic stroke produces cell death and disability, and a process of repair and partial recovery. Plasticity within cortical connections after stroke leads to partial recovery of function after the initial injury. Physiologically, cortical connections after stroke become hyperexcitable and more susceptible to the induction of LTP Stroke produces changes in the distribution and laterality of sensory, motor, and language representations within the brain that correlate with functional recovery. Anatomically, ischemic lesions induce axonal sprouting within local, intracortical projections and long distance, interhemispheric projections. This postischemic axonal sprouting establishes substantially new patterns of cortical connections with de-afferented or partially damaged brain areas. Axonal sprouting after ischemic lesions is induced by a transient pattern of synchronous, low-frequency neuronal activity in a network of cortical areas connected to the infarct. This pattern of neuronal activity that induces axonal sprouting in the adult after ischemic lesions resembles that seen in the developing brain during axonal elongation and synaptogenesis. Thus, stroke induces a process of remapping and reconnection within the adult brain through changes in neuronal activity that may involve a reactivation of developmental programs in areas connected to the infarct.

Quantitative reverse transcription-polymerase chain reaction of GABA(A) alpha1, beta1 and gamma2S subunits in epileptic rats following photothrombotic infarction of neocortex

Photothrombotic brain infarction can result in altered expression of cortical GABA(A) receptors and in epileptic seizures. We sought to determine whether infarct size and/or epileptic seizures resulted in a differential expression of cortical GABA(A) receptor subunit mRNA in adult rats. A reverse transcription-polymerase chain reaction (RT-PCR) was used with internal standards for GABA(A) receptor subunits to quantify alpha(1), beta(1), and gamma(2S) subunit mRNA expression in cortex ipsilateral and contralateral to left cerebral infarcts in small or large infarct/nonepileptic cohorts, a large infarct/epileptic cohort, and a young adult control cohort. Unilateral hemispheric subunit mRNA was pooled for each cohort, quantified, and expressed as mean values+/-S.E.M. In general, the magnitude of mRNA expression (pg/1 microg total RNA) was different for the individual subunits: gamma(2S) (10(4)), alpha(1) (10(2)), and beta(1) (10(1)). Hemispheric subunit mRNA expression for the different cohorts was compared by ANOVA testing, which noted significant differences for the alpha(1) (P<0.001) and beta(1) (P<0.001) subunits in ipsilateral cortex. Bonferroni post-testing for alpha(1) cohorts indicated that mRNA expression for the large infarct/epilepsy cohort (624.2+/-6.8 pg) was greater than all other cohorts (P<0.001); control (162.7+/-32.2 pg). For beta(1) cohorts, there was decreased mRNA expression in the large infarct/nonepileptic cohort (9.2+/-0.8 pg; P<0.01) and the large infarct/epileptic cohort (10.5+/-2.2 pg; P<0.05) compared to control (23.2+/-2.6 pg). Additionally, paired t-tests compared subunit mRNA expression within individual animal cohorts (ipsilateral vs. contralateral) and indicated decreased mRNA expression ipsilaterally for the beta(1) subunit in the small infarct cohort (14.2+/-2.6 vs. 22.9+/-3.0 pg; P=0.0102) and the large infarct/epilepsy cohort (10.5+/-2.3 vs. 18.0+/-3.6 pg; P=0.0462). These findings suggest that large photothrombotic infarcts of the neocortex can result in a long-lasting differential expression of GABA(A) receptor subunit mRNAs in ipsilateral cortex variably associated with the epileptic state.

Widespread and long-lasting alterations in GABA(A)-receptor subtypes after focal cortical infarcts in rats: mediation by NMDA-dependent processes

Impairment of inhibitory neurotransmission has been reported to occur in widespread, structurally intact brain regions after focal ischemic stroke. These long-lasting alterations contribute to the functional deficit and influence long-term recovery. Inhibitory neurotransmission is primarily mediated by gamma-aminobutyric acid (GABA)A receptors assembled of five subunits that allow a variety of adaptive changes. In this study, the regional distribution of five major GABA(A)-receptor subunits (alpha1, alpha2, alpha3, alpha5, and gamma2) was analyzed immunohistochemically 1, 7, and 30 days after photochemically induced cortical infarcts. When compared with sham-operated controls, a general and regionally differential reduction in immunostaining was found within the cortex, hippocampus, and thalamus of both hemispheres for almost all subunits. Within ipsilateral and contralateral neocortical areas, a specific pattern of changes with a differential decrease of subunits alpha1, alpha2, alpha5, and gamma2 and a significant upregulation of subunit alpha3 was observed in the contralateral cortex homotopic to the infarct. This dysregulation was most prominent at day 7 and still present at day 30. Interestingly, a single application of the noncompetitive N-methyl-D-aspartate-receptor antagonist MK-801 during lesion induction completely blocked these bihemispheric alterations. Cortical spreading depressions induced by topical application of KCl do not change GABA(A)-receptor subunit expression. As alterations in subtype distribution crucially influence inhibitory function, ischemia-induced modifications in GABA(A)-receptor subtype expression may be of relevance for functional recovery after stroke.

Photothrombotic brain infarction results in seizure activity in aging Fischer 344 and Sprague Dawley rats

This study was designed to determine whether photothrombotic brain infarction could result in epileptic seizures in adult animals. Male Fischer 344 (F344) rats at 2, 6, 12, 24, and 30 months of age and male Sprague Dawley (SD) rats at 2 and 6 months of age underwent photothrombotic brain infarction with the photosensitive dye rose bengal by focusing a wide (6 mm) or narrow (3 mm) diameter white light beam on the skull overlying left hemisphere anterior frontal, midfrontal, frontoparietal, or parietal areas. Animals were monitored with video and EEG recordings. Morphological analysis of infarct size was performed with a computer-assisted image analysis system. The primary finding of this study was that epileptic seizures were recorded in post-mature rats 2 months after lesioning the frontoparietal cortex with large photothrombotic infarcts that extended to the cortical-subcortical interface. These seizures were characterized behaviorally by motor arrest, appeared to originate in the periinfarct area, and could be distinguished from inherited spontaneous bilateral cortical discharges by the morphology, frequency, duration, and laterality of the ictal discharges. Small cortical lesions were ineffective in producing seizures except for one animal that demonstrated recurrent prolonged focal discharges unaccompanied by behavioral change. Stage 3 seizures were observed in a small number of mid-aged and aged animals lesioned with large infarcts in anterior frontal and frontoparietal areas. These results suggest that the technique of photothrombosis can be used to produce neocortical infarction as a means to study mechanisms of secondary epileptogenesis.