Ischaemia-induced long-term hyperexcitability in rat neocortex

'Remote inhibition' of motor cortex in Epileptic encephalopathy with spike-wave activation in sleep (EE-SWAS): A TMS based cortical excitability study

OBJECTIVES

The study compared real-time motor cortex excitability using transcranial magnetic stimulation (TMS)-derived parameters between children with epileptic encephalopathy with spike-wave activation in sleep (EE-SWAS) and age-matched neurotypical controls. The EE-SWAS group received steroids as standard of care and were longitudinally followed for three months.

MATERIALS & METHODS

Children aged 5-12 years with immunotherapy-naive EE-SWAS (spike-wave-index≥50 %) and neurotypical controls were enrolled. Cognitive and behavioral assessments were performed using valid psychometric tools. Real-time motor cortex excitability was assessed by measuring resting motor threshold (RMT), short intra-cortical inhibition (SICI) and long intra-cortical inhibition (LICI) in both groups. In EE-SWAS group, a follow up evaluation with TMS at 4- and 12-week intervals, EEG, and neurobehavioral assessments at 12-weeks were performed to assess the effect of steroids on cortical excitability and to determine electroclinical outcome.

RESULTS

Forty-eight children with suspected EE-SWAS and 26 neurotypical controls were screened; 20 were enrolled in each group. Children with EE-SWAS (mean age: 8.05 ± 1.76 years) had cognitive and behavioral problems (20/20), and ongoing seizures (12/20). At baseline, the dominant motor cortex was significantly inhibited in the EE-SWAS group compared to neurotypical children{RMT(%)[86.3 ± 6.96 vs 58.05 ± 4.71(p < 0.0001)]; LICI(%)[55.05 ± 4.39 vs 73.9 ± 3.75(p < 0.0001)]; SICI(%)[39.2 ± 4.36 vs 55.45 ± 4.78(p < 0.0001)]}. Reversal of motor cortex inhibition was sequentially observed in EE-SWAS group at 4- and 12-week follow-ups{(RMT[4, 12 weeks]: 71.45 ± 9.83, 63.45 ± 8.48); (LICI[4, 12 weeks]: 66.00 ± 6.26, 74.50 ± 5.36); (SICI[4, 12 weeks]: 49.35 ± 6.24, 56.05 ± 5.57)}[repeated-measures ANOVA: p < 0.0001].

CONCLUSION

Motor cortex is remotely inhibited in EE-SWAS, which may contribute to neurobehavioral impairment. Steroids can disinhibit/reverse the epilepsy-induced motor cortex inhibition leading to improvement in neurobehavior.

Status Epilepticus after mechanical thrombectomy: The role of early EEG assessment in Stroke Unit, clinical and radiological prognostication

BACKGROUND

Convulsive (CSE) and non-convulsive (NCSE) Status Epilepticus are a complication in 0.2-0.3% ischemic strokes. Large stroke and cortical involvement are the main risk factors for developing SE. This study evaluates the prevalence of SE in patients treated with endovascular thrombectomy (EVT) through EEG recording within 72- h from admission. Moreover, we compared clinical, radiological, and outcome measures in SE and no-SE patients.

MATERIALS AND METHODS

We collected retrospectively demographical and clinical characteristics of acute ischemic stroke patients who underwent EVT, admitted in the Stroke Unit (SU) of the University Hospital of Trieste between January 2018 and March 2020 who underwent EEG recording within 72- h from the symptoms' onset.

RESULTS

Out of 247 EVT patients, 138 met the inclusion criteria, of whom 9 (6.5%) showed SE with median onset time of 1 day (IQR 1-2). No difference was found between the two groups as for age, sex, risk factors, grade of recanalization, etiology of stroke, and closed vessel. The no-SE group presented higher NIHSS improvement rate (p=0.025) compared to the SE group. The sum of the lobes involved in the ischemic lesion was significantly higher in SE group (p=0.048).

CONCLUSION

SE after EVT in large strokes is a non-rare complication, with most being NCSE. Performing a rapid EEG assessment in a Stroke Unit setting may allow for a prompt recognition and treatment of SE in the acute/hyper-acute phase. SE may be correlated with worse clinical outcomes in patients with large vessel occlusion.

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.

Disentangling neuroplasticity mechanisms in post-stroke language recovery

A major objective in post-stroke aphasia research is to gain a deeper understanding of neuroplastic mechanisms that drive language recovery, with the ultimate goal of enhancing treatment outcomes. Subsequent to recent advances in neuroimaging techniques, we now have the ability to examine more closely how neural activity patterns change after a stroke. However, the way these neural activity changes relate to language impairments and language recovery is still debated. The aim of this review is to provide a theoretical framework to better investigate and interpret neuroplasticity mechanisms underlying language recovery in post-stroke aphasia. We detail two sets of neuroplasticity mechanisms observed at the synaptic level that may explain functional neuroimaging findings in post-stroke aphasia recovery at the network level: feedback-based homeostatic plasticity and associative Hebbian plasticity. In conjunction with these plasticity mechanisms, higher-order cognitive control processes dynamically modulate neural activity in other regions to meet communication demands, despite reduced neural resources. This work provides a network-level neurobiological framework for understanding neural changes observed in post-stroke aphasia and can be used to define guidelines for personalized treatment development.

Patient-specific modeling for guided rehabilitation of stroke patients: the BrainX3 use-case

BrainX3 is an interactive neuroinformatics platform that has been thoughtfully designed to support neuroscientists and clinicians with the visualization, analysis, and simulation of human neuroimaging, electrophysiological data, and brain models. The platform is intended to facilitate research and clinical use cases, with a focus on personalized medicine diagnostics, prognostics, and intervention decisions. BrainX3 is designed to provide an intuitive user experience and is equipped to handle different data types and 3D visualizations. To enhance patient-based analysis, and in keeping with the principles of personalized medicine, we propose a framework that can assist clinicians in identifying lesions and making patient-specific intervention decisions. To this end, we are developing an AI-based model for lesion identification, along with a mapping of tract information. By leveraging the patient's lesion information, we can gain valuable insights into the structural damage caused by the lesion. Furthermore, constraining whole-brain models with patient-specific disconnection masks can allow for the detection of mesoscale excitatory-inhibitory imbalances that cause disruptions in macroscale network properties. Finally, such information has the potential to guide neuromodulation approaches, assisting in the choice of candidate targets for stimulation techniques such as Transcranial Ultrasound Stimulation (TUS), which modulate E-I balance, potentiating cortical reorganization and the restoration of the dynamics and functionality disrupted due to the lesion.

Multiscale effects of excitatory-inhibitory homeostasis in lesioned cortical networks: A computational study

Stroke-related disruptions in functional connectivity (FC) often spread beyond lesioned areas and, given the localized nature of lesions, it is unclear how the recovery of FC is orchestrated on a global scale. Since recovery is accompanied by long-term changes in excitability, we propose excitatory-inhibitory (E-I) homeostasis as a driving mechanism. We present a large-scale model of the neocortex, with synaptic scaling of local inhibition, showing how E-I homeostasis can drive the post-lesion restoration of FC and linking it to changes in excitability. We show that functional networks could reorganize to recover disrupted modularity and small-worldness, but not network dynamics, suggesting the need to consider forms of plasticity beyond synaptic scaling of inhibition. On average, we observed widespread increases in excitability, with the emergence of complex lesion-dependent patterns related to biomarkers of relevant side effects of stroke, such as epilepsy, depression and chronic pain. In summary, our results show that the effects of E-I homeostasis extend beyond local E-I balance, driving the restoration of global properties of FC, and relating to post-stroke symptomatology. Therefore, we suggest the framework of E-I homeostasis as a relevant theoretical foundation for the study of stroke recovery and for understanding the emergence of meaningful features of FC from local dynamics.

Specificity and sensitivity of the SeLECT score in predicting late seizures in patients undergoing intravenous thrombolytic treatment and the effect of diabetes mellitus and leukoaraiosis

BACKGROUND

 Seizures after stroke can negatively affect the prognosis of ischemic stroke and cause a decrease in quality of life. The efficacy of intravenous (IV) recombinant tissue plasminogen activator (rt-PA) treatment in acute ischemic stroke has been demonstrated in many studies, and IV rt-PA treatment has been increasingly used around the world. The SeLECT score is a useful score for the prediction of late seizures after stroke and includes the severity of stroke (Se), large artery atherosclerosis (L), early seizure (E), cortical involvement (C), and the territory of the middle cerebral artery (T). However, the specificity and sensitivity of the SeLECT score have not been studied in acute ischemic stroke patients that received IV rt-PA treatment.

OBJECTIVE

 In the present study, we aimed to validate and develop the SeLECT score in acute ischemic stroke patients receiving IV rt-PA treatment.

METHODS

 The present study included 157 patients who received IV thrombolytic treatment in our third-stage hospital. The 1-year seizure rates of the patients were detected. SeLECT scores were calculated.

RESULTS

 In our study, we found that the SeLECT score had low sensitivity but high specificity for predicting the likelihood of late seizure after stroke in patients administered IV rt-PA therapy. In addition to the SeLECT score, we found that the specificity and sensitivity were higher when we evaluated diabetes mellitus (DM) and leukoaraiosis.

CONCLUSION

 We found that DM was an independent risk factor for late seizures after stroke in a patient group receiving thrombolytic therapy, and late seizures after stroke were less frequent in patients with leukoaraiosis.

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.

Study protocol for a phase II randomised, double-blind, placebo-controlled trial of perampanel as an antiepileptogenic treatment following acute stroke

INTRODUCTION

Stroke is a common cause of epilepsy that may be mediated via glutamate dysregulation. There is currently no evidence to support the use of antiseizure medications as primary prevention against poststroke epilepsy. Perampanel has a unique antiglutamatergic mechanism of action and may have antiepileptogenic properties. This study aims to evaluate the efficacy and safety of perampanel as an antiepileptogenic treatment in patients at high risk of poststroke epilepsy.

METHODS AND ANALYSIS

Up to 328 patients with cortical ischaemic stroke or lobar haemorrhage will be enrolled, and receive their first treatment within 7 days of stroke onset. Patients will be randomised (1:1) to receive perampanel (titrated to 6 mg daily over 4 weeks) or matching placebo, stratified by stroke subtype (ischaemic or haemorrhagic). Treatment will be continued for 12 weeks after titration. 7T MRI will be performed at baseline for quantification of cerebral glutamate by magnetic resonance spectroscopy and glutamate chemical exchange saturation transfer imaging. Blood will be collected for measurement of plasma glutamate levels. Participants will be followed up for 52 weeks after randomisation.The primary study outcome will be the proportion of participants in each group free of late (more than 7 days after stroke onset) poststroke seizures by the end of the 12-month study period, analysed by Fisher's exact test. Secondary outcomes will include time to first seizure, time to treatment withdrawal and 3-month modified Rankin Scale score. Quality of life, cognitive function, mood and adverse events will be assessed by standardised questionnaires. Exploratory outcomes will include correlation between cerebral and plasma glutamate concentration and stroke and seizure outcomes.

ETHICS AND DISSEMINATION

This study was approved by the Alfred Health Human Research Ethics Committee (HREC No 44366, Reference 287/18).

TRIAL REGISTRATION NUMBER

ACTRN12618001984280; Pre-results.

Simulating lesion-dependent functional recovery mechanisms

Functional recovery after brain damage varies widely and depends on many factors, including lesion site and extent. When a neuronal system is damaged, recovery may occur by engaging residual (e.g., perilesional) components. When damage is extensive, recovery depends on the availability of other intact neural structures that can reproduce the same functional output (i.e., degeneracy). A system's response to damage may occur rapidly, require learning or both. Here, we simulate functional recovery from four different types of lesions, using a generative model of word repetition that comprised a default premorbid system and a less used alternative system. The synthetic lesions (i) completely disengaged the premorbid system, leaving the alternative system intact, (ii) partially damaged both premorbid and alternative systems, and (iii) limited the experience-dependent plasticity of both. The results, across 1000 trials, demonstrate that (i) a complete disconnection of the premorbid system naturally invoked the engagement of the other, (ii) incomplete damage to both systems had a much more devastating long-term effect on model performance and (iii) the effect of reducing learning capacity within each system. These findings contribute to formal frameworks for interpreting the effect of different types of lesions.

Reappraisal of anoxic spreading depolarization as a terminal event during oxygen-glucose deprivation in brain slices in vitro

Anoxic spreading depolarization (aSD) has been hypothesized as a terminal event during oxygen–glucose deprivation (OGD) in submerged cortical slices in vitro. However, mechanical artifacts caused by aSD-triggered edema may introduce error in the assessment of neuronal viability. Here, using continuous patch-clamp recordings from submerged rat cortical slices, we first confirmed that vast majority of L4 neurons permanently lost their membrane potential during OGD-induced aSD. In some recordings, spontaneous transition from whole-cell to out-side out configuration occurred during or after aSD, and only a small fraction of neurons survived aSD with reperfusion started shortly after aSD. Secondly, to minimize artifacts caused by OGD-induced edema, cells were short-term patched following OGD episodes of various duration. Nearly half of L4 cells maintained membrane potential and showed the ability to spike-fire if reperfusion started less than 10 min after aSD. The probability of finding live neurons progressively decreased at longer reperfusion delays at a rate of about 2% per minute. We also found that neurons in L2/3 show nearly threefold higher resistance to OGD than neurons in L4. Our results suggest that in the OGD ischemia model, aSD is not a terminal event, and that the “commitment point” of irreversible damage occurs at variable delays, in the range of tens of minutes, after OGD-induced aSD in submerged cortical slices.

Multimodal mapping of neural activity and cerebral blood flow reveals long-lasting neurovascular dissociations after small-scale strokes

Neurovascular coupling, the close spatial and temporal relationship between neural activity and hemodynamics, is disrupted in pathological brain states. To understand the altered neurovascular relationship in brain disorders, longitudinal, simultaneous mapping of neural activity and hemodynamics is critical yet challenging to achieve. Here, we use a multimodal neural platform in a mouse model of stroke and realize long-term, spatially resolved tracking of intracortical neural activity and cerebral blood flow in the same brain regions. We observe a pronounced neurovascular dissociation that occurs immediately after small-scale strokes, becomes the most severe a few days after, lasts into chronic periods, and varies with the level of ischemia. Neuronal deficits extend spatiotemporally, whereas restoration of cerebral blood flow occurs sooner and reaches a higher relative value. Our findings reveal the neurovascular impact of ministrokes and inform the limitation of neuroimaging techniques that infer neural activity from hemodynamic responses.

Strategies and Pitfalls of Motor-Evoked Potential Monitoring during Supratentorial Aneurysm Surgery

BACKGROUND

The aims of this study were to reveal the strategies and pitfalls of motor-evoked potential (MEP) monitoring methods during supratentorial aneurysm surgery, and to discuss the drawbacks and advantages of each method by reviewing our experiences.

METHODS

Intraoperative MEP monitoring was performed in 250 patients. Results from 4 monitoring techniques using combinations of 2 stimulation sites and 2 recording sites were analyzed retrospectively.

RESULTS

MEP was recorded successfully in 243 patients (97.2%). Direct cortical stimulation (DCS)-spinal recorded MEP (sMEP) was used in 134 patients, DCS-muscle recorded MEP (mMEP) in 97, transcranial electrical stimulation (TES)-mMEP in 11 and TES-sMEP in 1. TES-mMEP during closure of the skull was used in 21 patients. DCS-mMEP was able to detect waveforms from upper and/or lower limb muscles. Alternatively, DCS-sMEP (direct [D]-wave) could accurately estimate amplitude changes. A novel "early warning sign" indicating ischemia was found in 21 patients, which started with a transiently increased amplitude of D-wave and then decreased after proximal interruption of major arteries. False-negative findings in MEP monitoring in 2 patients were caused by a blood insufficiency in the lenticulostriate artery and by a TES-sMEP recording, respectively.

CONCLUSIONS

The results of this study suggest that to perform accurate MEP monitoring, DCS-mMEP or DCS-sMEP recording should be used as the situation demands, with combined use of TES-mMEP recording during closure of the skull. DCS-sMEP is recommended for accurate analysis of waveforms. We also propose a novel "early warning sign" of blood insufficiency in the D-wave.

The pooled incidence of post-stroke seizure in 102 008 patients

BACKGROUND

Post-stroke seizures and epilepsy may worsen the recovery and increase the disability of stroke patients during their daily lives. However, few meta-analysis studies have been conducted on post-stroke seizures incidence. We carried on a meta-analysis on the incidence rate of post-stroke seizures and associated factors.

METHODS

We searched the Medline, Embase, Web of Science, Science Citation Index, and Cochrane Library electronic databases (1990-2013) to identify observational studies of post-stroke seizures. Two authors independently extracted the related information from all included studies. We calculated the pooled incidence by meta-analysis using the software R version 12.3.

RESULTS

A total of 34 longitudinal cohort studies involving 102 008 patients were included in our meta-analysis. The pooled incidence rate of post-stroke seizures was found to be 0.07 [95% confidence interval (CI), 0.05-0.09] while the rate of post-stroke epilepsy (PSE) was 0.05 (95% CI, 0.04-0.06). The incidence of post-stroke seizures in hemorrhagic stroke (0.10, 0.08-0.13) was much higher than in ischemic stroke (0.06, 0.04-0.08) and when the cortical region was involved (0.15, 0.10-0.21).

CONCLUSIONS

Our meta-analysis showed that seizures occurred in about 6.93% of people with stroke. Seizures occurred more commonly after hemorrhagic stroke and when stroke occurred in the cortical region.

Thinking outside the brain: structural plasticity in the spinal cord promotes recovery from cortical stroke

Neuroanatomically connected regions distal to a cortical stroke can exhibit both degenerative and adaptive changes during recovery. As the locus for afferent somatosensory fibres and efferent motor fibres, the spinal cord is ideally situated to play a critical role in functional recovery. In contrast to the wealth of research into cortical plasticity after stroke, much less focus has previously been placed on the role of subcortical or spinal cord plasticity in recovery of function after cortical stroke. Little is known about the extent and spatiotemporal profile of spinal rewiring, its regulation by neurotrophins or inflammatory cytokines, or its potential as a therapeutic target to improve stroke recovery. This commentary examines the recent findings by Sist et al. (2014) that there is a distinct critical period of heightened structural plasticity, growth factor expression, and inflammatory cytokine production in the spinal cord. They suggest that neuroplasticity is highest during the first two weeks after stroke and tapers off dramatically by the fourth week. Spinal cord plasticity correlates with the severity of cortical injury and temporally matches periods of accelerated spontaneous recovery of skilled reaching function. The potential of treatments that extend or re-open this window of spinal cord plasticity, such as anti-Nogo-A antibodies or chondroitinase ABC, to dramatically improve recovery from cortical stroke in clinical populations is discussed.

Desynchronizing the abnormally synchronized neural activity in the subthalamic nucleus: a modeling study

A mathematical model of a target area for deep brain stimulation was used to investigate the effects of electrical stimulation on pathologically synchronized clusters of neurons. In total, three newly developed stimulation techniques based on multisite coordinated reset and delayed feedback were tested and compared with a high-frequency stimulation method that is currently used as a standard stimulation protocol for deep brain stimulation. By modeling both excitatory and inhibitory actions of the electrical stimulation, we revealed the desynchronization impacts of the novel stimulation techniques. This contrasts with standard high-frequency stimulation, which failed to desynchronize the target population and whose inhibitory effects blocked all neuronal activity. We also explored the demand-controlled character of the proposed methods, and demonstrated that the amount of stimulation current required was considerably smaller than that for high-frequency stimulation. These novel stimulation methods appear to be superior to standard high-frequency stimulation techniques, and we propose the methods now be used for deep brain stimulation.

Discrepancy between voluntary movement and motor-evoked potentials in evaluation of motor function during clipping of anterior circulation aneurysms

BACKGROUND

Various modalities have been used to confirm the blood flow through parent arteries or surrounding perforating arteries during surgical aneurysm clipping, including motor-evoked potentials (MEPs), Doppler ultrasound, and indocyanine green videoangiography. Nonetheless, contralateral hemiparesis due to arterial blood flow insufficiency may arise because of false-positive or false-negative errors. By performing controlled intraoperative awakening during aneurysm clipping, we compared patients' voluntary movements with simultaneous MEP.

METHODS

Four patients with anterior choroidal artery aneurysms and one patient with a dorsal internal carotid artery aneurysm were included in this study. MEP and intraoperative voluntary movements under awake craniotomy were assessed simultaneously during and after the clipping procedure.

RESULTS

Aneurysms were safely and successfully clipped in all patients, with no evidence of postoperative neurological deficits. Voluntary movements and MEP findings did not differ from the control state in three patients. In the other two patients, we observed a discrepancy between MEP amplitudes and voluntary movements. In one patient, deterioration and subsequent improvement in voluntary movements were preceded by MEP amplitude reduction during clipping. In the other patient, MEP amplitude did not change although voluntary movement deteriorated during temporary occlusion of the internal carotid artery.

CONCLUSIONS

Intraoperative neurological assessment during aneurysmal clipping under awake craniotomy is feasible and safe, and should be valuable for the assessment of ischemia, especially in the anterior choroidal artery. From a neurophysiologic viewpoint, MEP may be insufficiently sensitive for evaluating voluntary movement under ischemia.

Seizures in acute stroke: incidence, risk factors and prognosis

BACKGROUND AND PURPOSE

Studies on post-stroke seizures have produced conflicting results. Our study aim was to further elucidate the incidence and predictive factors of early post-stroke seizures (ES) and their relationship with outcome.

METHODS

relevant clinical data were prospectively collected in 2,053 patients with acute stroke admitted to the Stroke Unit from 2004 to 2008.

RESULTS

Sixty-six patients (8 hemorrhagic and 58 ischemic strokes) aged 73-88 years (mean age 82 years) presented seizures in the first week after stroke onset. The type of ischemic stroke was atherothrombotic in 10 patients, cardioembolic in 21, lacunar in 4, undetermined in 19, and of other etiology in 4. Twenty-seven patients had generalized convulsive, 6 had complex partial, and 33 had simple partial seizures. Status epilepticus was observed in 13 patients. The severity of strokes in patients with ES was greater than in those without (National Institutes of Health Stroke Scale >14 in 50 vs. 25%), so mortality (30 days) was higher (29 vs. 14%). Independent seizure predictors were: total anterior circulation infarct, hemorrhagic transformation, hyperglycemia, and the interaction term diabetes × hyperglycemia.

CONCLUSIONS

ES may be considered a marker of stroke severity. Cortical location of the lesion, hemorrhagic transformation, and hyperglycemia in patients without diabetes are important predictors of ES.

Characterizing Deep Brain Stimulation effects in computationally efficient neural network models

BACKGROUND

Recent studies on the medical treatment of Parkinson's disease (PD) led to the introduction of the so called Deep Brain Stimulation (DBS) technique. This particular therapy allows to contrast actively the pathological activity of various Deep Brain structures, responsible for the well known PD symptoms. This technique, frequently joined to dopaminergic drugs administration, replaces the surgical interventions implemented to contrast the activity of specific brain nuclei, called Basal Ganglia (BG). This clinical protocol gave the possibility to analyse and inspect signals measured from the electrodes implanted into the deep brain regions. The analysis of these signals led to the possibility to study the PD as a specific case of dynamical synchronization in biological neural networks, with the advantage to apply the theoretical analysis developed in such scientific field to find efficient treatments to face with this important disease. Experimental results in fact show that the PD neurological diseases are characterized by a pathological signal synchronization in BG. Parkinsonian tremor, for example, is ascribed to be caused by neuron populations of the Thalamic and Striatal structures that undergo an abnormal synchronization. On the contrary, in normal conditions, the activity of the same neuron populations do not appear to be correlated and synchronized.

RESULTS

To study in details the effect of the stimulation signal on a pathological neural medium, efficient models of these neural structures were built, which are able to show, without any external input, the intrinsic properties of a pathological neural tissue, mimicking the BG synchronized dynamics.We start considering a model already introduced in the literature to investigate the effects of electrical stimulation on pathologically synchronized clusters of neurons. This model used Morris Lecar type neurons. This neuron model, although having a high level of biological plausibility, requires a large computational effort to simulate large scale networks. For this reason we considered a reduced order model, the Izhikevich one, which is computationally much lighter. The comparison between neural lattices built using both neuron models provided comparable results, both without traditional stimulation and in presence of all the stimulation protocols. This was a first result toward the study and simulation of the large scale neural networks involved in pathological dynamics.Using the reduced order model an inspection on the activity of two neural lattices was also carried out at the aim to analyze how the stimulation in one area could affect the dynamics in another area, like the usual medical treatment protocols require.The study of population dynamics that was carried out allowed us to investigate, through simulations, the positive effects of the stimulation signals in terms of desynchronization of the neural dynamics.

CONCLUSIONS

The results obtained constitute a significant added value to the analysis of synchronization and desynchronization effects due to neural stimulation. This work gives the opportunity to more efficiently study the effect of stimulation in large scale yet computationally efficient neural networks. Results were compared both with the other mathematical models, using Morris Lecar and Izhikevich neurons, and with simulated Local Field Potentials (LFP).

Levetiracetam in brain ischemia: clinical implications in neuroprotection and prevention of post-stroke epilepsy

Several new antiepileptic drugs (AEDs) have been introduced for clinical use recently. These new AEDs, like the classic AEDs, target multiple cellular sites both pre- and postsynaptically. The use of AEDs as a possible neuroprotective strategy in brain ischemia is receiving increasing attention and the antiepileptic drug levetiracetam, a 2S-(2-oxo-1-pyrrolidiny1) butanamide, belonging to the pyrrolidone family, could have a crucial role in regulation of epileptogenesis and neuroprotection. Recent observations suggest that levetiracetam is both safe and effective against post-stroke seizures. In this review, the potential neuroprotective role in brain ischemia and the therapeutic implications of levetiracetam in post-stroke epilepsy are discussed.

Inhibitory actions of the gamma-aminobutyric acid in pediatric Sturge-Weber syndrome

OBJECTIVE

The mechanisms of epileptogenesis in Sturge-Weber syndrome (SWS) are unknown. We explored the properties of neurons from human pediatric SWS cortex in vitro and tested in particular whether gamma-aminobutyric acid (GABA) excites neurons in SWS cortex, as has been suggested for various types of epilepsies.

METHODS

Patch-clamp and field potential recordings and dynamic biphoton imaging were used to analyze cortical tissue samples obtained from four 6- to 14-month-old pediatric SWS patients during surgery.

RESULTS

Neurons in SWS cortex were characterized by a relatively depolarized resting membrane potential, as was estimated from cell-attached recordings of N-methyl-D-aspartate channels. Many cells spontaneously fired action potentials at a rate proportional to the level of neuronal depolarization. The reversal potential for GABA-activated currents, assessed by cell-attached single channel recordings, was close to the resting membrane potential. All spontaneously firing neurons recorded in cell-attached mode or imaged with biphoton microscopy were inhibited by GABA. Spontaneous epileptiform activity in the form of recurrent population bursts was suppressed by glutamate receptor antagonists, the GABA(A) receptor agonist isoguvacine, and the positive allosteric GABA(A) modulator diazepam. Blockade of GABA(A) receptors aggravated spontaneous epileptiform activity. The NKCC1 antagonist bumetanide had little effect on epileptiform activity.

INTERPRETATION

SWS cortical neurons have a relatively depolarized resting membrane potential and spontaneously fire action potentials that may contribute to increased network excitability. In contrast to previous data depicting excitatory and proconvulsive actions of GABA in certain pediatric and adult epilepsies, GABA plays mainly an inhibitory and anticonvulsive role in SWS pediatric cortex.

Ischaemic stroke in adults and epilepsy

Stroke is an important cause of symptomatic epilepsy especially in the elderly. Seizures in the setting of stroke will furthermore worsen the prognosis of stroke. Studies show that the frequency of seizures in stroke ranges between 2.3% and 14%. Typically early seizures are defined as those that occur within 14 days of the stroke, and later seizures those that occur after this period. A number of risk factors have been identified like cortical involvement, size of the infarct and stroke severity. Status epilepticus can be a presenting symptom of acute stroke and can lead to increased mortality. Early seizures are risks for recurrent seizures though not for the development of epilepsy but late seizures do carry a higher risk. There are no clear cut guidelines for the treatment of seizures in stroke and hence treatment needs to be initiated in the context of the patient. The presence of co morbid conditions and the use of other drugs also complicate antiepileptic therapy, and the risk of drug interactions is a particular hazard in elderly patients on multiple co medication. Although hemorrhagic and ischaemic stroke can both result in epilepsy, this review focuses primarily the association of epilepsy and ischaemic stroke.

Neuronal circuit remodeling in the contralateral cortical hemisphere during functional recovery from cerebral infarction

Recent advances in functional imaging of human brain activity in stroke patients, e.g., functional magnetic resonance imaging, have revealed that cortical hemisphere contralateral to the infarction plays an important role in the recovery process. However, underlying mechanisms occurring in contralateral hemisphere during functional recovery have not been elucidated. We experimentally induced a complete infarction of somatosensory cortex in right hemisphere of mice and examined the neuronal changes in contralateral (left) somatosensory cortex during recovery. Both basal and ipsilateral somatosensory stimuli-evoked neuronal activity in left (intact) hemisphere transiently increased 2 d after stroke, followed by an increase in the turnover rate of usually stable mushroom-type synaptic spines at 1 week, observed by using two-photon imaging in vivo. At 4 weeks after stroke, when functional recovery had occurred, a new pattern of electrical circuit activity in response to somatosensory stimuli was established in intact ipsilateral hemisphere. Thus, the left somatosensory cortex can compensate for the loss of the right somatosensory cortex by remodeling neuronal circuits and establishing new sensory processing. This finding could contribute to establish the effective clinical treatments targeted on the intact hemisphere for the recovery of impaired functions and to achieve better quality of life of patients.

A model for cortical rewiring following deafferentation and focal stroke

It is still unclear to what extent structural plasticity in terms of synaptic rewiring is the cause for cortical remapping after a lesion. Recent two-photon laser imaging studies demonstrate that synaptic rewiring is persistent in the adult brain and is dramatically increased following brain lesions or after a loss of sensory input (cortical deafferentation). We use a recurrent neural network model to study the time course of synaptic rewiring following a peripheral lesion. For this, we represent axonal and dendritic elements of cortical neurons to model synapse formation, pruning and synaptic rewiring. Neurons increase and decrease the number of axonal and dendritic elements in an activity-dependent fashion in order to maintain their activity in a homeostatic equilibrium. In this study we demonstrate that synaptic rewiring contributes to neuronal homeostasis during normal development as well as following lesions. We show that networks in homeostasis, which can therefore be considered as adult networks, are much less able to compensate for a loss of input. Interestingly, we found that paused stimulation of the networks are much more effective promoting reorganization than continuous stimulation. This can be explained as neurons quickly adapt to this stimulation whereas pauses prevents a saturation of the positive stimulation effect. These findings may suggest strategies for improving therapies in neurologic rehabilitation.

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.

Motor Cortical Excitability in Patients with Poststroke Epilepsy

PURPOSE

To gain insight into the mechanisms underlying poststroke epilepsy (PSE), we evaluated motor cortical function in chronic stroke patients with (N = 18) and without (N = 18) PSE.

METHODS

We measured resting motor threshold (RMT), motor evoked potential (MEP) amplitudes, cortical silent period (CSP), intracortical inhibition (ICI), influenced by GABAergic neurotransmission, and intracortical facilitation (ICF), influenced by glutamatergic activity, to transcranial magnetic stimulation.

RESULTS

We found (1) larger MEP amplitudes and ICF, in the affected than unaffected hemispheres of patients in the PSE group but not in patients without epilepsy, and (2) comparably higher RMT and longer CSP in the absence of differences in ICI, H-reflexes or F-waves in the affected and unaffected hemispheres of both PSE and non-PSE patients.

CONCLUSIONS

Enhanced cortical excitability in the affected hemisphere, possibly related to increased glutamatergic activity, could be one of the mechanisms contributing to the development of poststroke epilepsy.

Late-onset epileptogenesis and seizure genesis: lessons from models of cerebral ischemia

Patients surviving ischemic stroke often express delayed epileptic syndromes. Late poststroke seizures occur after a latency period lasting from several months to years after the insult. These seizures might result from ischemia-induced neuronal death and associated morphological and physiological changes that are only partly elucidated. This review summarizes the long-term morphofunctional alterations observed in animal models of both focal and global ischemia that could explain late-onset seizures and epileptogenesis. In particular, this review emphasizes the change in GABAergic and glutamatergic signaling leading to hyperexcitability and seizure genesis.

Control of neuronal synchrony by nonlinear delayed feedback

We present nonlinear delayed feedback stimulation as a technique for effective desynchronization. This method is intriguingly robust with respect to system and stimulation parameter variations. We demonstrate its broad applicability by applying it to different generic oscillator networks as well as to a population of bursting neurons. Nonlinear delayed feedback specifically counteracts abnormal interactions and, thus, restores the natural frequencies of the individual oscillatory units. Nevertheless, nonlinear delayed feedback enables to strongly detune the macroscopic frequency of the collective oscillation. We propose nonlinear delayed feedback stimulation for the therapy of neurological diseases characterized by abnormal synchrony.

Stable expression of the vesicular GABA transporter following photothrombotic infarct in rat brain

Before exocytotic release of the inhibitory neurotransmitter GABA, this amino acid has to be stored in synaptic vesicles. Accumulation of GABA in vesicles is achieved by a specific membrane-integrated transporter termed vesicular GABA transporter. This vesicular protein is mainly located at presynaptic terminals of GABAergic interneurons. In the present study we investigated the effects of focal ischemia on the expression of the vesicular GABA transporter. Vesicular GABA transporter mRNA and protein expression was examined after photothrombosis in different cortical and hippocampal brain regions of Wistar rats. In situ hybridization and quantitative real-time RT-PCR were performed to analyze vesicular GABA transporter mRNA. Both vesicular GABA transporter mRNA-stained perikarya and mRNA expression levels remained unaffected. Vesicular GABA transporter protein-containing synaptic terminals and somata were visualized by immunohistochemistry. The pattern of vesicular GABA transporter immunoreactivity as well as the protein expression level revealed by semiquantitative image analysis and by Western blot remained stable after stroke. The steady expression of vesicular GABA transporter mRNA and protein after photothrombosis indicates that the exocytotic release mechanism of GABA is not affected by ischemia.

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.

Depressed responses to applied and synaptically-released GABA in CA1 pyramidal cells, but not in CA1 interneurons, after transient forebrain ischemia

Transient cerebral ischemia kills CA1 pyramidal cells of the hippocampus, whereas most CA1 interneurons survive. It has been proposed that calcium-binding proteins, neurotrophins, and/or inhibitory neuropeptides protect interneurons from ischemia. However, different synaptic responses early after reperfusion could also underlie the relative vulnerabilities to ischemia of pyramidal cells and interneurons. In this study, we used gramicidin perforated patch recording in ex vivo slices to investigate gamma-aminobutyric acid (GABA) synaptic function in CA1 pyramidal cells and interneurons 4 h after a bilateral carotid occlusion accompanied by hypovolemic hypotension. At this survival time, the amplitudes of both miniature inhibitory postsynaptic currents (mIPSCs) and GABA-evoked currents were reduced in CA1 pyramidal cells, but not in CA1 interneurons. In addition, the mean rise time of mIPSCs was reduced in pyramidal cells. The reversal potential for the GABA current (E(GABA)) did not shift toward depolarizing values in either cell type, indicating that the driving force for chloride was unchanged at this survival time. We conclude that early during reperfusion GABAergic neurotransmission is attenuated exclusively in pyramidal neurons. This is likely explained by reduced GABAA receptor sensitivity or clustering and possibly also reduced GABA release, rather than by an elevation of intracellular chloride. Impaired GABA function may contribute to ischemic neuronal death by enhancing the excitability of CA1 pyramidal cells and facilitating N-methyl-D-aspartic acid channel opening. Therefore, normalizing GABAergic function might be a useful pharmacological approach to counter excessive, and potentially excitotoxic, glutamatergic activity during the postischemic period.

GABA(B) receptor expression and cellular localization in gerbil hippocampus after transient global ischemia

Using in situ hybridization, the expression of the GABA receptor subtype B subunit 1 (GABA(B) R1) and subunit 2 (GABA(B) R2) following transient global ischemia in the gerbil hippocampus was investigated. In sham-operated animals, mRNAs of both subunits were mainly detected in hippocampal pyramidal cells and interneurons with lower expression levels of the GABA(B) R2 in the CA1 field. Four days after transient cerebral ischemia, neuronal message decreased in conjunction with neuronal death and both receptor subunits disappeared from the pyramidal cell layer. However, GABA(B) R1 and GABA(B) R2 were still expressed in a few cells. In situ hybridization of the GABA synthesizing enzyme glutamic acid decarboxylase 67 (GAD67) remained unchanged after the ischemic insult. Double-labeling experiments revealed that in the postischemic hippocampus GABA(B) R1 and GABA(B) R2 were not present in GFAP-reactive astrocytes, but that the surviving parvalbumin-containing interneurons possessed GABA(B) R1 and GABA(B) R2 mRNA.

Insulin exerts neuroprotection by counteracting the decrease in cell-surface GABAA receptors following oxygen-glucose deprivation in cultured cortical neurons

A loss of balance between excitatory and inhibitory signaling leads to excitoxicity, and contributes to ischemic cell death. Reduced synaptic inhibition as a result of dysfunction of the ionotropic GABAA receptor has been suggested as one of the major causes for this imbalance, although the underlying mechanisms remain poorly understood. In the present study, we investigated whether oxygen-glucose deprivation (OGD), an ischemia-like challenge, alters cell-surface expression of GABAA receptors in cultured hippocampal neurons, and thereby leads to excitotoxic cell death. Using cell culture ELISA as a cell surface receptor assay, we found that OGD produced a marked decrease in cell surface GABAA receptors, without altering the total amount of receptors. Furthermore, the reduction could be prevented by inhibition of receptor endocytosis with hypertonic sucrose treatment. Notably, insulin significantly limited OGD-induced changes in cell-surface GABAA receptors. In parallel, insulin protected cultured neurons against both glutamate toxicity and OGD, as assayed by mitochondrial reduction of Alamar Blue. Importantly, insulin-mediated neuroprotection was eliminated when bicuculline, a GABAA receptor antagonist, was co-applied with insulin during OGD. Together, our results strongly suggest that ischemia-like insults decrease cell surface GABAA receptors in neurons via accelerated internalization, and that insulin provides neuroprotection by counteracting this reduction.

GABA transporters in the mammalian cerebral cortex: localization, development and pathological implications

The extracellular levels of gamma-aminobutyric acid (GABA), the main inhibitory neurotransmitter in the mammalian cerebral cortex, are regulated by specific high-affinity, Na+/Cl- dependent transporters. Four distinct genes encoding GABA transporters (GATs), named GAT-1, GAT-2, GAT-3, and BGT-1 have been identified using molecular cloning. Of these, GAT-1 and -3 are expressed in the cerebral cortex. Studies of the cortical distribution, cellular localization, ontogeny and relationships of GATs with GABA-releasing elements using a variety of light and electron microscopic immunocytochemical techniques have shown that: (i) a fraction of GATs is strategically placed to mediate GABA uptake at fast inhibitory synapses, terminating GABA's action and shaping inhibitory postsynaptic responses; (ii) another fraction may participate in functions such as the regulation of GABA's diffusion to neighboring synapses and of GABA levels in cerebrospinal fluid; (iii) GATs may play a role in the complex processes regulating cortical maturation; and (iv) GATs may contribute to the dysregulation of neuronal excitability that accompanies at least two major human diseases: epilepsy and ischemia.

Ischemia induces short- and long-term remodeling of synaptic activity in the hippocampus

One of the most vulnerable areas to ischemia or hypoglycemia is CA1 hippocampal region due to pyramidal neurons death. Glutamate receptors are involved together with protein-kinase C and nitric oxide synthase. Long-term potentiation (LTP) is generated in anoxic or hypoglycemic conditions via activation of NMDA while inhibition of these receptors attenuates this response. Protein-kinase C and nitric oxide synthase are involved in anoxic LTP mechanism. Postischemic neurons are hyperexcitable in CA3 area while CA1 pyramidal neurons degenerate and disappear. Changes of glutamate receptors triggered by ischemia and hypoglycemia are discussed in this review.

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.

Ischaemic penumbra: highlights

The ischaemic penumbra was described for the first time in the late 1970s as a ring of hypoperfused zone surrounding the region of complete infarction. The penumbral zone is a functionally silent tissue which is able to regain its function if promptly reperfused. This implies that the ischaemic penumbra is not a static but a "dynamic" and "time-dependent" concept. In this paper we describe the role of neuroimmaging tecniques such as single photon emission tomography (SPET), positron emission tomography (PET), and diffusion-weighted and perfusion-weighted magnetic resonance imaging (DWI and PWI) in the study of ischaemic penumbra. These functional imaging techniques have the advantage of giving "in vivo" quantitative estimate of cerebral blood flow (CBF) as well as information on how the ischaemic tissue metabolic changes develop. It follows that, as therapeutic options for treating acute stroke evolve, neuroimaging strategies are assuming an increasingly important role in the initial evaluation and management of the acute ischaemic patient. In this regard, a wide range of therapeutic approaches have been investigated for either ameliorating the perfusion, or interfering with the pathobiochemical cascade leading to ischaemic neuronal damage, or improving endogenous neuroprotection pathways. The "time windows" required for these treatments to be effective varies being rather short for reperfusion and longer for neuroprotection. Salvaging more penumbra would enhance recovery and thereby allow the most appropriate candidate for therapeutic trials to be selected.

New patterns of intracortical projections after focal cortical stroke

Cortical strokes alter functional maps but associated changes in connections have not been documented. The neuroanatomical tracer biotinylated dextran amine (BDA) was injected into cortex bordering infarcts 3 weeks after focal strokes in rat whisker barrel (somatosensory) cortex. The mirror locus in the opposite hemisphere was injected as a control. After 1 week of survival, brains were processed for cytochrome oxidase (CO)-, Nissl-, and BDA-labeled neurons. Cortex bordering the infarct (peri-infarct cortex) had abnormal CO and Nissl structure. BDA-labeled neurons were plotted and projections were analyzed quantitatively. Animals with small strokes had intracortical projections, arising from peri-infarct cortex, not seen in normal hemispheres: the overall orientation was statistically significantly different from and rotated 157 degrees relative to the controls. Compared to the controls, significantly fewer cells were labeled in the thalamus. Thus, after focal cortical stroke, the peri-infarct cortex is structurally abnormal, loses thalamic connections, and develops new horizontal cortical connections by axonal sprouting.

Opposite relationship of hippocampal and rhinal cortex damage to delayed nonmatching-to-sample deficits in monkeys

Three recent studies in macaque monkeys that examined the effects on memory of restricted hippocampal lesions (Murray and Mishkin, J Neurosci 1998;18:6568-6582; Beason-Held et al., Hippocampus 1999;9:562-574; Zola et al., J Neurosci 2000;20:451-463) differed in their conclusions about the involvement of the hippocampus in recognition memory. Because these experiments used a common behavioral procedure, trial-unique visual delayed nonmatching-to-sample (DNMS), a quantitative synthesis ("meta-analysis") was performed to determine whether hippocampal lesions produced a reliable net impairment in DNMS performance, and whether this impairment was related to the magnitude of hippocampal damage. A similar analysis was performed on data from monkeys with perirhinal or rhinal cortex damage (Meunier et al., J Neurosci 1993;13:5418-5432; Buffalo et al., Learn Mem 1999;6:572-599). DNMS performance scores were transformed to d' values to permit comparisons across studies, and a loss in d' score, a measure of the magnitude of the recognition deficit relative to the control group, was calculated for each operated monkey. Two main findings emerged. First, the loss in d' following hippocampal damage was reliably larger than zero, but was smaller than that found after lesions limited to the perirhinal cortex. Second, the correlation of loss in d' with extent of hippocampal damage was large and negative, indicating that greater impairments were associated with smaller hippocampal lesions. This relationship was opposite to that between loss in d' and rhinal cortex damage, for which larger lesions were associated with greater impairment. These findings indicate that damage to the hippocampus and to the rhinal cortex affects recognition memory in different ways. Furthermore, they provide a framework for understanding the seemingly disparate effects of hippocampal damage on recognition memory in monkeys, and by extension, for interpreting the conflicting reports on the effects of such damage on recognition memory abilities in amnesic humans.

gamma-Aminobutyric acid(A) neurotransmission and cerebral ischemia

In this review, we present evidence for the role of gamma-aminobutyric acid (GABA) neurotransmission in cerebral ischemia-induced neuronal death. While glutamate neurotransmission has received widespread attention in this area of study, relatively few investigators have focused on the ischemia-induced alterations in inhibitory neurotransmission. We present a review of the effects of cerebral ischemia on pre and postsynaptic targets within the GABAergic synapse. Both in vitro and in vivo models of ischemia have been used to measure changes in GABA synthesis, release, reuptake, GABA(A) receptor expression and activity. Cellular events generated by ischemia that have been shown to alter GABA neurotransmission include changes in the Cl(-) gradient, reduction in ATP, increase in intracellular Ca(2+), generation of reactive oxygen species, and accumulation of arachidonic acid and eicosanoids. Neuroprotective strategies to increase GABA neurotransmission target both sides of the synapse as well, by preventing GABA reuptake and metabolism and increasing GABA(A) receptor activity with agonists and allosteric modulators. Some of these strategies are quite efficacious in animal models of cerebral ischemia, with sedation as the only unwanted side-effect. Based on promising animal data, clinical trials with GABAergic drugs are in progress for specific types of stroke. This review attempts to provide an understanding of the mechanisms by which GABA neurotransmission is sensitive to cerebral ischemia. Furthermore, we discuss how dysfunction of GABA neurotransmission may contribute to neuronal death and how neuronal death can be prevented by GABAergic drugs.

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.

Early electrocortical changes consistent with ischemic preconditioning in rat

Ischemic preconditioning (IPC) of the brain describes the neuroprotection induced by a short, conditioning ischemic episode (CIE) to a subsequent severe (test) ischemic episode (TIE). Most of the supporting evidence for IPC is based on histological assessment, several days after TIE. The aim of this study is to investigate if changes induced by IPC can be detected within 30 min of reperfusion following the ischemic episode. A rat model of "four-vessel occlusion" transient global cerebral ischemia and parametric analysis of electrocorticogram were used. A control group was subjected directly to a 10 min TIE, and in a preconditioned group TIE was induced 48 h after a 3 min CIE. Quantitative histology was performed 48 h after TIE. Our key finding is that, 30 min after reperfusion, there is a significant increase in the electrocortical slow activity in the control group but not in the preconditioned group. Moreover the increase inversely correlates with the degree of electrocortical suppression during seconds 10 to 15 after the onset of the ischemic episode.

Characterization of neuronal migration disorders in neocortical structures: loss or preservation of inhibitory interneurons?

PURPOSE

Neuronal migration disorders (NMD) are often associated with therapy-resistant epilepsy. In human cerebral cortex, this hyperexcitability has been correlated with a loss of inhibitory interneurons. We used a rat model of focal cortical NMD (microgyria) to determine whether the expression of epileptiform activity in this model coincides with a decrease in inhibitory interneurons.

METHODS

In 2-to 4-month-old rats, the density of interneurons immunoreactive for gamma-aminobutyric acid (GABA), calbindin, and parvalbumin was determined in fronto-parietal cortex in nine 200-microm-wide sectors located up to 2.5 mm lateral and 2.0 mm medial from the lesion center in primary parietal cortex (Par1). Quantitative measurements in homotopic areas of age-matched sham-operated rats served as controls.

RESULTS

The freeze lesion performed in newborn rat cortex resulted in adult rats with a microgyrus extending in a rostro-caudal direction from frontal to occipital cortex. The density of GABA-and parvalbumin-positive neurons in fronto-parietal cortex was not significantly different between lesioned and control animals. Only the density of calbindin-immunoreactive neurons located 1.0 mm lateral and 0.5 mm medial from the lesion was significantly (Student t test, p < 0.05) larger in freeze-lesioned rats (5,817 +/- 562 and 6,400 +/- 795 cells per mm3, respectively; n = 12) compared with measurements in homotopic regions in Par1 cortex of controls (4,507 +/- 281 and 4, 061 +/- 319 cells per mm3, respectively; n = 5).

CONCLUSIONS

The previously reported widespread functional changes in this model of cortical NMD are not related to a general loss of inhibitory interneurons. Other factors, such as a decrease in GABA receptor density, modifications in GABAA receptor subunit composition, or alterations in the excitatory network, e.g., an increase in the density of calbindin-immunoreactive pyramidal cells, more likely contribute to the global disinhibition and widespread expression of pathophysiological activity in this model of cortical NMD.