Mechanisms underlying epileptogenesis in cortical malformations

Altered excitatory and inhibitory neocortical circuitry leads to increased convulsive severity after pentylenetetrazol injection in an animal model of schizencephaly, but not of microgyria

OBJECTIVE

Malformations of the polymicrogyria spectrum can be mimicked in rodents through neonatal transcranial focal cortical freeze lesions. The animals presenting the malformations present both altered synaptic events and epileptiform activity in the vicinity of the microgyrus, but the comprehension of their contribution to increased predisposition or severity of seizures require further studies.

METHODS

In order to investigate these issues, we induced both microgyria and schizencephaly in 57 mice and evaluated: their convulsive susceptibility and severity after pentyleneterazol (PTZ) treatment, the quantification of their symmetric and asymmetric synapses, the morphology of their dendritic arbors, and the content of modulators of synaptogenesis, such as SPARC, gephyrin and GAP-43 within the adjacent visual cortex.

RESULTS

Our results have shown that only schizencephalic animals present increased convulsive severity. Nevertheless, both microgyric and schizencephalic cortices present increased synapse number and dendritic complexity of layer IV and layer V-located neurons. Specifically, the microgyric cortex presented reduced inhibitory synapses, while the schizencephalic cortex presented increased excitatory synapses. This altered synapse number is correlated with decreased content of both the anti-synaptogenic factor SPARC and the inhibitory postsynaptic organizer gephyrin in both malformed groups. Besides, GAP-43 content and dendritic spines number are enhanced exclusively in schizencephalic cortices.

SIGNIFICANCE

In conclusion, our study supports the hypothesis that the sum of synaptic alterations drives to convulsive aggravation in animals with schizencephaly, but not microgyria after PTZ treatment. These findings reveal that different malformations of cortical development should trigger epilepsy via different mechanisms, requiring further studies for development of specific therapeutic interventions.

Electrographic pattern recognition: A simple tool to predict clinical outcome in children with lissencephaly

PURPOSE

To describe and correlate the clinical, radiological and EEG findings in children with lissencephaly.

METHOD

Retrospective record analysis of children with lissencephaly presenting to tertiary health centre in Northern India was performed. Radiological classification and severity scoring were done. EEG findings were categorized into three patterns and its association with clinical severity was studied.

RESULTS

Twenty-eight children (males = 17) with lissencephaly were enrolled. Median age at diagnosis was 6.5months (range 3days-3years). Global developmental delay (median social quotient (SQ) = 25 (range15-68) was seen in all; motor deficits in 23 (82 %); epilepsy in 21 (75 %); behavioural problems in 18 (64 %); ophthalmic problems in 17 (61 %); microcephaly in 13 (46 %); feeding difficulty in 12 (43 %). Radiologically, classical Type I lissencephaly was seen in 18(64 %), cobblestone variant (Type II) in 5 (18 %) and microlissencephaly in 5 (18 %). Grade 4 (diffuse pachygyria) radiologic severity was most common (severity grade 1-6); no cases with severity score 5 or 6 were seen. The clinical profile did not correspond with radiological severity grading. EEG pattern recognition revealed pattern I in 14 (50 %); pattern II in 6 (21 %); pattern III in 8 (29 %). Children with pattern III EEG had drug resistant epilepsy and severe developmental delay. No relationship between EEG patterns and radiological severity grading was evident.

CONCLUSION

EEG is better predictor of clinical status and outcome rather than radiological severity grading. EEG pattern III is associated with severe developmental delay and drug resistant epilepsy.

Cortical Malformations: Lessons in Human Brain Development

Creating a functional cerebral cortex requires a series of complex and well-coordinated developmental steps. These steps have evolved across species with the emergence of cortical gyrification and coincided with more complex behaviors. The presence of diverse progenitor cells, a protracted timeline for neuronal migration and maturation, and diverse neuronal types are developmental features that have emerged in the gyrated cortex. These factors could explain how the human brain has expanded in size and complexity. However, their complex nature also renders new avenues of vulnerability by providing additional cell types that could contribute to disease and longer time windows that could impact the composition and organization of the cortical circuit. We aim to discuss the unique developmental steps observed in human corticogenesis and propose how disruption of these species-unique processes could lead to malformations of cortical development.

Ex vivo multi-electrode analysis reveals spatiotemporal dynamics of ictal behavior at the infiltrated margin of glioma

The purpose of this study is to develop a platform in which the cellular and molecular underpinnings of chronic focal neocortical lesional epilepsy can be explored and use it to characterize seizure-like events (SLEs) in an ex vivo model of infiltrating high-grade glioma. Microelectrode arrays were used to study electrophysiologic changes in ex vivo acute brain slices from a PTEN/p53 deleted, PDGF-B driven mouse model of high-grade glioma. Electrode locations were co-registered to the underlying histology to ascertain the influence of the varying histologic landscape on the observed electrophysiologic changes. Peritumoral, infiltrated, and tumor sites were sampled in tumor-bearing slices. Following the addition of zero Mg²⁺ solution, all three histologic regions in tumor-bearing slices showed significantly greater increases in firing rates when compared to the control sites. Tumor-bearing slices demonstrated increased proclivity for SLEs, with 40 events in tumor-bearing slices and 5 events in control slices (p-value = .0105). Observed SLEs were characterized by either low voltage fast (LVF) onset patterns or short bursts of repetitive widespread, high amplitude low frequency discharges. Seizure foci comprised areas from all three histologic regions. The onset electrode was found to be at the infiltrated margin in 50% of cases and in the peritumoral region in 36.9% of cases. These findings reveal a landscape of histopathologic and electrophysiologic alterations associated with ictogenesis and spread of tumor-associated seizures.

A novel LGI1 missense mutation causes dysfunction in cortical neuronal migration and seizures

BACKGROUND

To explore the causative genes and pathogenesis of autosomal dominant partial epilepsy with auditory features in a large Chinese family that includes 7 patients over four generations.

METHODS

We used targeted exome sequencing and Sanger sequencing to validate the mutation. Zebrafish were used to explore the epileptic behavior caused by the mutation. Primary cortical neuronal culturing and in utero electroporation were used to observe the influences of the mutation on neuronal polarity and migration.

RESULTS

We report the identification of a novel missense mutation, c.128C > G (p. Pro43Arg), in exon 1 of LGI1. The heterozygous missense mutation, which cosegregated with the syndrome, was absent in 300 unrelated and matched-ancestor controls. The mutation inhibited the secretion of LGI1 and could not rescue the hyperactivity caused by lgi1a knockdown in zebrafish. In vitro, mutant LGI1 interrupts normal cell polarity. In agreement with these findings, dysfunctional cortical neuron migration was observed using in utero electroporation technology, which is reminiscent of the subtle structural changes in the lateral temporal region observed in the proband of this family.

CONCLUSION

Our findings enrich the spectrum of LGI1 mutations and support the pathogenicity of the mutation. Furthermore, additional information regarding the role of LGI1 in the development of temporal lobe epilepsy was elucidated, and a potential relationship was established between cortical neuronal migration dysfunction and autosomal dominant partial epilepsy with auditory features.

Cortical Layer and Spectrotemporal Architecture of Epileptiform Activity in vivo in a Mouse Model of Focal Cortical Malformation

Our objective is to examine the layer and spectrotemporal architecture and laminar distribution of high-frequency oscillations (HFOs) in a neonatal freeze lesion model of focal cortical dysplasia (FCD) associated with a high prevalence of spontaneous spike-wave discharges (SWDs). Electrophysiological recording of local field potentials (LFPs) in control and freeze lesion animals were obtained with linear micro-electrode arrays to detect presence of HFOs as compared to changes in spectral power, signal coherence, and single-unit distributions during "hyper-excitable" epochs of anesthesia-induced burst-suppression (B-S). Result were compared to HFOs observed during spontaneous SWDs in animals during sleep. Micro-electrode array recordings from the malformed cortex indicated significant increases in the presence of HFOs above 100 Hz and associated increases in spectral power and altered LFP coherence of recorded signals across cortical lamina of freeze-lesioned animals with spontaneous bursts of high-frequency activity, confined predominately to granular and supragranular layers. Spike sorting of well-isolated single-units recorded from freeze-lesioned cortex indicated an increase in putative excitatory cell activity in the outer cortical layers that showed only a weak association with HFOs while deeper inhibitory units were strongly phase-locked to high-frequency ripple (HFR) oscillations (300-800 Hz). Both SWDs and B-S show increases in HFR activity that were phase-locked to the high-frequency spike pattern occurring at the trough of low frequency oscillations. The spontaneous cyclic spiking of cortical inhibitory cells appears to be the driving substrate behind the HFO patterns associated with SWDs and a hyperexcitable supragranular layer near the malformed cortex may play a key role in epileptogenesis in our model. These data, derived from a mouse model with a distinct focal cortical malformation, support recent clinical data that HFOs, particularly fast ripples, is a biomarker to help define the cortical seizure zone, and provide limited insights toward understanding cellular level changes underlying the HFOs.

CDYL Deficiency Disrupts Neuronal Migration and Increases Susceptibility to Epilepsy

During brain development, the correct migration of newborn neurons is one of the determinants of circuit formation, and neuronal migration defects may lead to neurological and psychiatric disorders. The molecular mechanisms underlying neuronal migration and related disorders are poorly understood. Here, we report that Chromodomain Y-like (CDYL) is critical for neuronal migration in mice. Knocking down CDYL caused neuronal migration defects and disrupted both mobility and multipolar-to-bipolar transition of migrating neurons. We find that CDYL regulates neuronal migration by transcriptionally repressing RhoA. In addition, CDYL deficiency increased the excitability of cortical pyramidal neurons and the susceptibility of mice to convulsant-induced seizures. These results demonstrate that CDYL is a regulator of neuronal migration and shed light on the pathogenesis of seizure-related neurodevelopmental disorders.

α2δ-1 Signaling Drives Cell Death, Synaptogenesis, Circuit Reorganization, and Gabapentin-Mediated Neuroprotection in a Model of Insult-Induced Cortical Malformation

Developmental cortical malformations (DCMs) result from pre- and perinatal insults, as well as genetic mutations. Hypoxia, viral infection, and traumatic injury are the most common environmental causes of DCMs, and are associated with the subsyndromes focal polymicrogyria and focal cortical dysplasia (FCD) Type IIId, both of which have a high incidence of epilepsy. Understanding the molecular signals that lead to the formation of a hyperexcitable network in DCMs is critical to devising novel treatment strategies. In a previous study using the freeze-lesion (FL) murine model of DCM, we found that levels of thrombospondin (TSP) and the calcium channel auxiliary subunit α2δ-1 were elevated. TSP binds to α2δ-1 to drive the formation of excitatory synapses during development, suggesting that overactivation of this pathway may lead to exuberant excitatory synaptogenesis and network hyperexcitability seen in DCMs. In that study, antagonizing TSP/α2δ-1 signaling using the drug gabapentin (GBP) reduced many FL-induced pathologies. Here, we used mice with a genetic deletion of α2δ-1 to determine how α2δ-1 contributes to cell death, elevated excitatory synapse number, and in vitro network function after FL and to examine the molecular specificity of GBP's effects. We identified a critical role for α2δ-1 in FL-induced pathologies and in mediating the neuroprotective effects of GBP. Interestingly, genetic deletion of α2δ-1 did not eliminate GBP's effects on synaptogenesis, suggesting that GBP can have α2δ-1-independent effects. Taken together these studies suggests that inhibiting α2δ-1 signaling may have therapeutic promise to reduce cell death and network reorganization associated with insult-induced DCMs.

The delta between postoperative seizure freedom and persistence: Automatically detected focal slow waves after epilepsy surgery

OBJECTIVE

In this study, we use a novel automated method for localization and quantitative comparison of magnetoencephalographic (MEG) delta activity in patients with and without recurrent seizures after epilepsy surgery as well as healthy controls.

METHODS

We identified the generators of delta activity by source location in frequency domain between 1 and 4 Hz in spontaneous MEG data. Comparison with healthy control subjects by z-transform emphasized relative changes of activation in patients. The individual results were compared to spike localizations and statistical group analysis was performed. Additionally, MEG results were compared to 1-4 Hz activity in invasive EEG (iEEG) in two patients, in whom this data was available.

RESULTS

Patients with recurrent seizures exhibited significantly increased focal MEG delta activity both in comparison to healthy controls and seizure free patients. This slow activity showed a correlation to interictal epileptic activity and was not explained by consequences of the resection alone. In two patients with iEEG, iEEG analysis was concordant with the MEG findings.

SIGNIFICANCE

The quantity of delta activity could be used as a diagnostic marker for recurrent seizures. The close relation to epileptic spike localizations and the resection volume of patients with successful second surgery imply involvement in seizure recurrence. This initial evidence suggests a potential application in the planning of second epilepsy surgery.

Enhanced Burst-Suppression and Disruption of Local Field Potential Synchrony in a Mouse Model of Focal Cortical Dysplasia Exhibiting Spike-Wave Seizures

Focal cortical dysplasias (FCDs) are a common cause of brain seizures and are often associated with intractable epilepsy. Here we evaluated aberrant brain neurophysiology in an in vivo mouse model of FCD induced by neonatal freeze lesions (FLs) to the right cortical hemisphere (near S1). Linear multi-electrode arrays were used to record extracellular potentials from cortical and subcortical brain regions near the FL in anesthetized mice (5–13 months old) followed by 24 h cortical electroencephalogram (EEG) recordings. Results indicated that FL animals exhibit a high prevalence of spontaneous spike-wave discharges (SWDs), predominately during sleep (EEG), and an increase in the incidence of hyper-excitable burst/suppression activity under general anesthesia (extracellular recordings, 0.5%–3.0% isoflurane). Brief periods of burst activity in the local field potential (LFP) typically presented as an arrhythmic pattern of increased theta-alpha spectral peaks (4–12 Hz) on a background of low-amplitude delta activity (1–4 Hz), were associated with an increase in spontaneous spiking of cortical neurons, and were highly synchronized in control animals across recording sites in both cortical and subcortical layers (average cross-correlation values ranging from +0.73 to +1.0) with minimal phase shift between electrodes. However, in FL animals, cortical vs. subcortical burst activity was strongly out of phase with significantly lower cross-correlation values compared to controls (average values of −0.1 to +0.5, P < 0.05 between groups). In particular, a marked reduction in the level of synchronous burst activity was observed, the closer the recording electrodes were to the malformation (Pearson’s Correlation = 0.525, P < 0.05). In a subset of FL animals (3/9), burst activity also included a spike or spike-wave pattern similar to the SWDs observed in unanesthetized animals. In summary, neonatal FLs increased the hyperexcitable pattern of burst activity induced by anesthesia and disrupted field potential synchrony between cortical and subcortical brain regions near the site of the cortical malformation. Monitoring the altered electrophysiology of burst activity under general anesthesia with multi-dimensional micro-electrode arrays may serve to define distinct neurophysiological biomarkers of epileptogenesis in human brain and improve techniques for surgical resection of epileptogenic malformed brain tissue.

Continuous spike-waves during slow-wave sleep in a mouse model of focal cortical dysplasia

OBJECTIVE

To examine if mice with focal cortical dysplasia (FCD) develop spontaneous epileptic seizures and, if so, determine the key electroencephalography (EEG) features.

METHODS

Unilateral single freeze lesions to the S1 region (SFLS1R) were made in postnatal day 0-1 pups to induce a neocortical microgyrus in the right cortical hemisphere. Continuous 24-h recordings with intracranial EEG electrodes and behavioral tests were performed in adult SFLS1R and sham-control mice to assess neurologic status.

RESULTS

A high percentage of adult SFLS1R animals (89%, 40/45) exhibited at least one or more spontaneous nonconvulsive seizure events over the course of 24 h. Of these animals, 60% (27/45) presented with a chronic seizure state that was persistent throughout the recording session, consisting of bursts of rhythmic high-amplitude spike-wave activities and primarily occurring during periods of slow-wave sleep. In comparison, none of the control, age-matched, mice (0/12) developed seizures. The epileptic discharge pattern closely resembled a pattern of continuous spike-waves during slow-wave sleep (CSWS) of the human syndrome described as an electrical status epilepticus during slow-wave sleep (ESES). Key findings in the SFLS1R model indicated that the observed CSWS (1) were more prevalent in female (18/23) versus male (9/22, p < 0.05), (2) were strongest in the right S1 region although generalized to other brain regions, (3) were associated with significant cognitive and behavioral deficits, (4) were temporarily alleviated by ethosuximide treatment or optogenetic activation of cortical γ-aminobutyric acid (GABA)ergic neurons, and (5) theta and alpha band rhythms may play a key role in the generalization of spike-wave activities.

SIGNIFICANCE

This is the first report of an in vivo animal FCD model that induces chronic spontaneous electrographic brain seizures. Further characterization of the abnormal oscillations in this mouse model may lead to a better understanding of the mechanisms of CSWS/ESES.

Insulin-Independent GABAA Receptor-Mediated Response in the Barrel Cortex of Mice with Impaired Met Activity

Autism spectrum disorder (ASD) is a neurodevelopmental disorder caused by genetic variants, susceptibility alleles, and environmental perturbations. The autism associated geneMETtyrosine kinase has been implicated in many behavioral domains and endophenotypes of autism, including abnormal neural signaling in human sensory cortex. We investigated somatosensory thalamocortical synaptic communication in mice deficient in Met activity in cortical excitatory neurons to gain insights into aberrant somatosensation characteristic of ASD. The ratio of excitation to inhibition is dramatically increased due to decreased postsynaptic GABAAreceptor-mediated inhibition in the trigeminal thalamocortical pathway of mice lacking active Met in the cerebral cortex. Furthermore, in contrast to wild-type mice, insulin failed to increase GABAAreceptor-mediated response in the barrel cortex of mice with compromised Met signaling. Thus, lacking insulin effects may be a risk factor in ASD pathogenesis.

SIGNIFICANCE STATEMENT

A proposed common cause of neurodevelopmental disorders is an imbalance in excitatory neural transmission, provided by the glutamatergic neurons, and the inhibitory signals from the GABAergic interneurons. Many genes associated with autism spectrum disorders impair synaptic transmission in the expected cell type. Previously, inactivation of the autism-associated Met tyrosine kinase receptor in GABAergic interneurons led to decreased inhibition. In thus report, decreased Met signaling in glutamatergic neurons had no effect on excitation, but decimated inhibition. Further experiments indicate that loss of Met activity downregulates GABAAreceptors on glutamatergic neurons in an insulin independent manner. These data provide a new mechanism for the loss of inhibition and subsequent abnormal excitation/inhibition balance and potential molecular candidates for treatment or prevention.

Current Evidence for Developmental, Structural, and Functional Brain Defects following Prenatal Radiation Exposure

Ionizing radiation is omnipresent. We are continuously exposed to natural (e.g., radon and cosmic) and man-made radiation sources, including those from industry but especially from the medical sector. The increasing use of medical radiation modalities, in particular those employing low-dose radiation such as CT scans, raises concerns regarding the effects of cumulative exposure doses and the inappropriate utilization of these imaging techniques. One of the major goals in the radioprotection field is to better understand the potential health risk posed to the unborn child after radiation exposure to the pregnant mother, of which the first convincing evidence came from epidemiological studies on in utero exposed atomic bomb survivors. In the following years, animal models have proven to be an essential tool to further characterize brain developmental defects and consequent functional deficits. However, the identification of a possible dose threshold is far from complete and a sound link between early defects and persistent anomalies has not yet been established. This review provides an overview of the current knowledge on brain developmental and persistent defects resulting from in utero radiation exposure and addresses the many questions that still remain to be answered.

Astrocyte membrane properties are altered in a rat model of developmental cortical malformation but single-cell astrocytic glutamate uptake is robust

Developmental cortical malformations (DCMs) are linked with severe epilepsy and are caused by both genetic and environmental insults. DCMs include several neurological diseases, such as focal cortical dysplasia, polymicrogyria, schizencephaly, and others. Human studies have implicated astrocyte reactivity and dysfunction in the pathophysiology of DCMs, but their specific role is unknown. As astrocytes powerfully regulate glutamate neurotransmission, and glutamate levels are known to be increased in human epileptic foci, understanding the role of astrocytes in the pathological sequelae of DCMs is extremely important. Additionally, recent studies examining astrocyte glutamate uptake in DCMs have reported conflicting results, adding confusion to the field. In this study we utilized the freeze lesion (FL) model of DCM, which is known to induce reactive astrocytosis and cause significant changes in astrocyte morphology, proliferation, and distribution. Using whole-cell patch clamp recording from astrocytes, we recorded both UV-uncaging and synaptically evoked glutamate transporter currents (TCs), widely accepted assays of functional glutamate transport by astrocytes. With this approach, we set out to test the hypothesis that astrocyte membrane properties and glutamate transport were disrupted in this model of DCM. Though we found that the developmental maturation of astrocyte membrane resistance was disrupted by FL, glutamate uptake by individual astrocytes was robust throughout FL development. Interestingly, using an immunolabeling approach, we observed spatial and developmental differences in excitatory amino acid transporter (EAAT) expression in FL cortex. Spatially specific differences in EAAT2 (GLT-1) and EAAT1 (GLAST) expression suggest that the relative contribution of each EAAT to astrocytic glutamate uptake may be altered in FL cortex. Lastly, we carefully analyzed the amplitudes and onset times of both synaptically- and UV uncaging-evoked TCs. We found that in the FL cortex, synaptically-evoked, but not UV uncaging-evoked TCs, were larger in amplitude. Additionally, we found that the amount of electrical stimulation required to evoke a synaptic TC was significantly reduced in the FL cortex. Both of these findings are consistent with increased excitatory input to the FL cortex, but not with changes in how individual astrocytes remove glutamate. Taken together, our results demonstrate that the maturation of astrocyte membrane resistance, local distribution of glutamate transporters, and glutamatergic input to the cortex are altered in the FL model, but that single-cell astrocytic glutamate uptake is robust.

Neuraminidase-Dependent Degradation of Polysialic Acid Is Required for the Lamination of Newly Generated Neurons

Hippocampal granule cells (GCs) are generated throughout the lifetime and are properly incorporated into the innermost region of the granule cell layer (GCL). Hypotheses for the well-regulated lamination of newly generated GCs suggest that polysialic acid (PSA) is present on the GC surface to modulate GC-to-GC interactions, regulating the process of GC migration; however, direct evidence of this involvement is lacking. We show that PSA facilitates the migration of newly generated GCs and that the activity of N-acetyl-α-neuraminidase 1 (NEU1, sialidase 1) cleaves PSA from immature GCs, terminating their migration in the innermost GCL. Developing a migration assay of immature GCs in vitro, we found that the pharmacological depletion of PSA prevents the migration of GCs, whereas the inhibition of PSA degradation with a neuraminidase inhibitor accelerates this migration. We found that NEU1 is highly expressed in immature GCs. The knockdown of NEU1 in newly generated GCs in vivo increased PSA presence on these cells, and attenuated the proper termination of GC migration in the innermost GCL. In conclusion, this study identifies a novel mechanism that underlies the proper lamination of newly generated GCs through the modulation of PSA presence by neuronal NEU1.

GABA from reactive astrocytes impairs memory in mouse models of Alzheimer's disease

In Alzheimer's disease (AD), memory impairment is the most prominent feature that afflicts patients and their families. Although reactive astrocytes have been observed around amyloid plaques since the disease was first described, their role in memory impairment has been poorly understood. Here, we show that reactive astrocytes aberrantly and abundantly produce the inhibitory gliotransmitter GABA by monoamine oxidase-B (Maob) and abnormally release GABA through the bestrophin 1 channel. In the dentate gyrus of mouse models of AD, the released GABA reduces spike probability of granule cells by acting on presynaptic GABA receptors. Suppressing GABA production or release from reactive astrocytes fully restores the impaired spike probability, synaptic plasticity, and learning and memory in the mice. In the postmortem brain of individuals with AD, astrocytic GABA and MAOB are significantly upregulated. We propose that selective inhibition of astrocytic GABA synthesis or release may serve as an effective therapeutic strategy for treating memory impairment in AD.

Targeted disruption of layer 4 during development increases GABAA receptor neurotransmission in the neocortex

Cortical dysplasia (CD) associates with clinical pathologies, including epilepsy and mental retardation. CD results from impaired migration of immature neurons to their cortical targets, leading to clustering of neural cells and changes in cortical properties. We developed a CD model by administering methylazoxymethanol (MAM), an anti-mitotic, to pregnant ferrets on embryonic day 33; this leads to reduction in cortical thickness in addition to redistribution and increased expression of GABAA receptors (GABAAR). We evaluated the impact of MAM treatment on GABAAR-mediated synaptic transmission in postnatal day 0-1 neurons, leaving the ganglionic eminence (GE) and in layer 2/3 pyramidal cells of postnatal day 28-38 ferrets. Embryonic day 33 MAM treatment significantly increases the amplitude and frequency of spontaneous GABAAR-mediated inhibitory postsynaptic currents (IPSCs) in the cells leaving the GE. In older MAM-treated animals, the amplitude and frequency of GABAAR-mediated spontaneous IPSCs in layer 2/3 pyramidal cells is increased, as are the amplitude and frequency of miniature IPSCs. The kinetics of GABAAR opening also altered following treatment with MAM. Western blot analysis shows that the expression of the GABAAα3R and GABAAγ2R subunits amplified in our model animals. We did not observe any significant change in the passive properties of either the layer 2/3 pyramidal cells or cells leaving the GE after MAM treatment. These observations reinforce the idea that synaptic neurotransmission through GABAAR enhances following treatment with MAM and coincides with our finding of increased GABAAαR expression within the upper cortical layers. Overall, we demonstrate that small amounts of toxins delivered during corticogenesis can result in long-lasting changes in ambient expression of GABAAR that influence intrinsic neuronal properties.

Epilepsy-induced motility of differentiated neurons

Neuronal ectopia, such as granule cell dispersion (GCD) in temporal lobe epilepsy (TLE), has been assumed to result from a migration defect during development. Indeed, recent studies reported that aberrant migration of neonatal-generated dentate granule cells (GCs) increased the risk to develop epilepsy later in life. On the contrary, in the present study, we show that fully differentiated GCs become motile following the induction of epileptiform activity, resulting in GCD. Hippocampal slice cultures from transgenic mice expressing green fluorescent protein in differentiated, but not in newly generated GCs, were incubated with the glutamate receptor agonist kainate (KA), which induced GC burst activity and GCD. Using real-time microscopy, we observed that KA-exposed, differentiated GCs translocated their cell bodies and changed their dendritic organization. As found in human TLE, KA application was associated with decreased expression of the extracellular matrix protein Reelin, particularly in hilar interneurons. Together these findings suggest that KA-induced motility of differentiated GCs contributes to the development of GCD and establish slice cultures as a model to study neuronal changes induced by epileptiform activity.

Glutamate biosensor imaging reveals dysregulation of glutamatergic pathways in a model of developmental cortical malformation

Cortical malformations can cause intractable epilepsy, but the underlying epileptogenic mechanisms are poorly understood. We used high-speed glutamate biosensor imaging to ask how glutamatergic signaling is altered in cortical malformations induced by neonatal freeze-lesions (FL). In non-lesion neocortical slices from 2 to 8week old rats, evoked glutamate signals were symmetrical in the medio-lateral axis and monotonic, correlating with simple, brief (≈50ms) local field potentials (LFPs). By contrast, in FL cortex glutamate signals were prolonged, increased in amplitude, and polyphasic, which paralleled a prolongation of the LFP. Using glutamate biosensor imaging, we found that glutamate signals propagated throughout large areas of FL cortex and were asymmetric (skewed toward the lesion). Laminar analysis demonstrated a shift in the region of maximal glutamate release toward superficial layers in FL cortex. The ability to remove exogenous glutamate was increased within the FL itself but was decreased in immediately adjacent regions. There were corresponding alterations in astrocyte density, with an increase within the lesion and a decrease in deep cortical layers surrounding the lesion. These findings demonstrate both network connectivity and glutamate metabolism are altered in this cortical malformation model and suggests that the regional ability of astrocytes to remove released glutamate may be inversely related to local excitability.

Finding a better drug for epilepsy: the mTOR pathway as an antiepileptogenic target

The mammalian target of rapamycin (mTOR) signaling pathway regulates cell growth, differentiation, proliferation, and metabolism. Loss-of-function mutations in upstream regulators of mTOR have been highly associated with dysplasias, epilepsy, and neurodevelopmental disorders. These include tuberous sclerosis, which is due to mutations in TSC1 or TSC2 genes; mutations in phosphatase and tensin homolog (PTEN) as in Cowden syndrome, polyhydramnios, megalencephaly, symptomatic epilepsy syndrome (PMSE) due to mutations in the STE20-related kinase adaptor alpha (STRADalpha); and neurofibromatosis type 1 attributed to neurofibromin 1 mutations. Inhibition of the mTOR pathway with rapamycin may prevent epilepsy and improve the underlying pathology in mouse models with disrupted mTOR signaling, due to PTEN or TSC mutations. However the timing and duration of its administration appear critical in defining the seizure and pathology-related outcomes. Rapamycin application in human cortical slices from patients with cortical dysplasias reduces the 4-aminopyridine-induced oscillations. In the multiple-hit model of infantile spasms, pulse high-dose rapamycin administration can reduce the cortical overactivation of the mTOR pathway, suppresses spasms, and has disease-modifying effects by partially improving cognitive deficits. In post-status epilepticus models of temporal lobe epilepsy, rapamycin may ameliorate the development of epilepsy-related pathology and reduce the expression of spontaneous seizures, but its effects depend on the timing and duration of administration, and possibly the model used. The observed recurrence of seizures and epilepsy-related pathology after rapamycin discontinuation suggests the need for continuous administration to maintain the benefit. However, the use of pulse administration protocols may be useful in certain age-specific epilepsy syndromes, like infantile spasms, whereas repetitive-pulse rapamycin protocols may suffice to sustain a long-term benefit in genetic disorders of the mTOR pathway. In summary, mTOR dysregulation has been implicated in several genetic and acquired forms of epileptogenesis. The use of mTOR inhibitors can reverse some of these epileptogenic processes, although their effects depend upon the timing and dose of administration as well as the model used.

α2-chimaerin controls neuronal migration and functioning of the cerebral cortex through CRMP-2

Disrupted cortical neuronal migration is associated with epileptic seizures and developmental delay. However, the molecular mechanism by which disruptions of early cortical development result in neurological symptoms is poorly understood. Here we report α2-chimaerin as a key regulator of cortical neuronal migration and function. In utero suppression of α2-chimaerin arrested neuronal migration at the multipolar stage, leading to accumulation of ectopic neurons in the subcortical region. Mice with such migration defects showed an imbalance between excitation and inhibition in local cortical circuitry and greater susceptibility to convulsant-induced seizures. We further show that α2-chimaerin regulates bipolar transition and neuronal migration through modulating the activity of CRMP-2, a microtubule-associated protein. These findings establish a new α2-chimaerin-dependent mechanism underlying neuronal migration and proper functioning of the cerebral cortex and provide insights into the pathogenesis of seizure-related neurodevelopmental disorders.

Effects of nerve growth factor in experimental model of focal microgyria

AIM

The effects on neural repair of intraparenchymal nerve growth factor (NGF) administration were evaluated in neonate Wistar rats with experimentally induced focal microgyria.

METHODS

A freezing focal polymicrogyric lesion was induced on the frontal cortex in 35 newborn Wistar rats on postnatal day 1. NGF was administered in 15 cases, with 20 pups as controls. Animals were sacrificed at 72 h and 7 days after NGF administration. Real-time PCR was used for the quantification of the expression of TrkA, p75, and doublecortin (DCX) at the level of the cortical lesion in seven different groups of animals: control 72 h (n = 5), control 7 days (n = 5), microgyria 72 h (n = 5), microgyria 7 days (n = 5), microgyria + NGF 72 h (n = 5), microgyria + NGF 7 days (n = 5), and control + NGF (n = 5).

RESULTS

A significant increase in TrkA expression was found in the microgyria + NGF 7 days group compared to the others. TrkA upregulation was already visible 72 h after NGF administration. Unlike TrkA, p75 expression increased in animals subjected to the experimental focal microgyria and decreased markedly after NGF administration. DCX expression in injured animals was observed to increase strongly 7 days after NGF administration compared with other groups.

CONCLUSIONS

NGF administration interferes with neural repair mechanisms at the polymicrogyric lesion site by means of TrkA and DCX upregulation which possibly counteracts the process of apoptosis caused by the brain injury.

Decreased hyperpolarization-activated currents in layer 5 pyramidal neurons enhances excitability in focal cortical dysplasia

Focal cortical dysplasia is associated with the development of seizures in children and is present in up to 40% of intractable childhood epilepsies. Transcortical freeze lesions in newborn rats reproduce many of the anatomical and physiological characteristics of human cortical dysplasia. Rats with freeze lesions have increased seizure susceptibility and a region of hyperexcitable cortex adjacent to the lesion. Since alterations in hyperpolarization-activated nonspecific cation (HCN) channels are often associated with epilepsy, we used whole cell patch-clamp recording and voltage-sensitive dye imaging to examine alterations in HCN channels and inwardly rectifying hyperpolarization-activated currents (I(h)) in cortical dysplasia. (L5) pyramidal neurons in lesioned animals had hyperpolarized resting membrane potentials, increased input resistances and reduced voltage "sag" associated with I(h) activation. These differences became nonsignificant after application of the I(h) blocker ZD7288. Temporal excitatory postsynaptic potential (EPSP) summation and intrinsic excitability were increased in neurons near the freeze lesion. Using voltage-sensitive dye imaging of neocortical slices, we found that inhibiting I(h) with ZD7288 increased the half-width of dye signals. The anticonvulsant lamotrigine produced a significant decrease in spread of activity. The ability of lamotrigine to decrease network activity was reduced in the hyperexcitable cortex near the freeze lesion. These results suggest that I(h) serves to constrain network activity in addition to its role in regulating cellular excitability. Reduced I(h) may contribute to increased network excitability in cortical dysplasia.

Arrested glutamatergic synapse development in human partial epilepsy

While studying the brain function of the human partial epilepsy gene, leucine-rich glioma-inactivated 1 (LGI1), a new mechanism of human epileptogenesis was revealed-persistent immaturity of glutamatergic circuitries. LGI1, a novel secreted protein, was found to be increased during the postnatal period; when glutamatergic synapses both downregulate their presynaptic vesicular release probability and reduce their postsynaptic NMDA-receptor subunit NR2B. During this same period, the dendritic arbor and spines are pruned and remodeled. Using bacterial artificial chromosome transgenic mouse techniques, excess wild-type LGI1 was shown to magnify these critical brain developmental events in the hippocampal dentate gyrus; while an epilepsy-associated, truncated, dominant-negative form of LGI1 blocked them. By contrast, the hippocampal dentate granule neuron GABAergic synapses and intrinsic excitability were unaltered. A role for LGI1 in downregulating glutamate synapse function was confirmed by germline gene deletion; this intervention also revealed a selective increase of glutamatergic synaptic transmission with unaltered GABAergic synapses and intrinsic excitability of hippocampal CA1 pyramidal neurons. Interestingly, the role of LGI1 in neurological disease was further expanded when a subset of patients with limbic encephalitis (an autoimmune disorder with memory loss in 100% and seizures in 80% of individuals) were discovered to carry autoantibodies to LGI1.

Iomazenil hyperfixation in single photon emission computed tomography study of malformations of cortical development during infancy

We present 2 cases of malformations of cortical development and early onset epilepsy. The first case is of a patient with left hemimegalencephaly who developed focal epilepsy at the age of 2 days and cluster spasms at 1.5 months. After left functional hemispherectomy, seizures originated from the contralateral hemisphere, which had shown normal signals in the preoperative magnetic resonance imaging study. The second case is of a patient with lissencephaly, caused by a missense mutation in the doublecortin gene, who developed West syndrome at the age of 5 months. In both the cases, (123)I-iomazenil single photon emission computed tomography performed during infancy showed significant hyperfixation in the dysplastic lesions. This finding indicates the immaturity of the affected neurons and a gamma-aminobutyric acidergic involvement in epileptogenesis associated with malformations of cortical development during infancy.

Altered intrinsic properties of neuronal subtypes in malformed epileptogenic cortex

Neuronal intrinsic properties control action potential firing rates and serve to define particular neuronal subtypes. Changes in intrinsic properties have previously been shown to contribute to hyperexcitability in a number of epilepsy models. Here we examined whether a developmental insult producing the cortical malformation of microgyria altered the identity or firing properties of layer V pyramidal neurons and two interneuron subtypes. Trains of action potentials were elicited with a series of current injection steps during whole cell patch clamp recordings. Cells in malformed cortex identified as having an apical dendrite had firing patterns similar to control pyramidal neurons. The duration of the second action potential in the train was increased in paramicrogyral (PMG) pyramidal cells, suggesting that these cells may be in an immature state, as was previously found for layer II/III pyramidal neurons. Based on stereotypical firing patterns and other intrinsic properties, fast-spiking (FS) and low threshold-spiking (LTS) interneuron subpopulations were clearly identified in both control and malformed cortex. Most intrinsic properties measured in malformed cortex were unchanged, suggesting that subtype identity is maintained. However, LTS interneurons in lesioned cortex had increased maximum firing frequency, decreased initial afterhyperpolarization duration, and increased total adaptation ratio compared to control LTS cells. FS interneurons demonstrated decreased maximum firing frequencies in malformed cortex compared to control FS cells. These changes may increase the efficacy of LTS while decreasing the effectiveness of FS interneurons. These data indicate that differential alterations of individual neuronal subpopulations may endow them with specific characteristics that promote epileptogenesis.

Investigation of maternal melatonin effect on the hippocampal formation of newborn rat model of intrauterine cortical dysplasia

OBJECTIVES

Cortical dysplasia is a cortical malformation resulting from any developmental defects during different periods of development. This study aims to investigate the hippocampal histopathological alterations in the neonates with cortical dysplasia due to the prenatal exposure to carmustine (1,3-bis (2-chloroethyl)-1-nitrosourea; BCNU) and the possible effects of prophylaxis with melatonin, a neuroprotective agent.

METHODS

Wistar albino female rats were randomly divided into four experimental groups; control, melatonin-treated, BCNU-exposed and BCNU-exposed+melatonin-treated. Light microscopy and immunohistochemistry were carried out on the newborn hippocampus.

RESULTS

Histopathology of hippocampus from the control and melatonin-treated groups showed continuity of migration and maturation as pathognomonic signs of the normal newborn hippocampus. Hippocampal cortex from the newborns exposed in utero to BCNU showed the histology of early embryonic hippocampal formation with immunohistochemical increase in the number of nestin positive cells and decreases in the immunoreactivity of glial fibrillary acidic protein (GFAP) and synaptophysin. These findings indicate a significant delay in hippocampal maturation, migration, and synaptogenesis. Intrauterine treatment of BCNU-exposed rats with melatonin resulted in histopathological features almost similar to control group.

CONCLUSION

It has been concluded that cortical dysplasia induced by intrauterine BCNU administration results in delayed hippocampal maturation, which is successfully restored by intrauterine melatonin treatment.

Interactions between antiepileptic and chemotherapeutic drugs in children with brain tumors: is it time to change treatment?

Epileptic seizures are a common clinical problem in children with brain tumors. The conventional antiepileptic drugs (AEDs) permit a good seizure control in most of these children. An emerging problem is the possible interactions between AEDs and chemotherapeutic drugs, because many of these drugs are metabolized by the cytochrome P450. The aim of this article is to propose a novel therapeutic approach for new-onset epilepsy in children with brain tumors. Among the new AEDs not metabolized by the P450 system, levetiracetam seems to be a promising AED owing to its pharmacokinetic features, efficacy, and safety.

Motor cortical organization in an adult with hemimegalencephaly and late onset epilepsy

Hemimegalencephaly is a rare brain malformation whose physiology is largely obscure. In a single patient, we studied motor cortex using several transcranial magnetic stimulation variables testing cortical excitability, and mapping motor area. The megalencephalic hemisphere showed an enlargement of cortical motor map with abnormal axonal orientation and an excess spread of corticospinal excitation, associated with multiple defects of cortical inhibition. TMS gave new information on the anatomic/functional features and epileptogenesis in this complex and physiologically obscure syndrome.

Temporal and topographic alterations in expression of the alpha3 isoform of Na+, K(+)-ATPase in the rat freeze lesion model of microgyria and epileptogenesis

Na(+),K(+)-ATPase contributes to the asymmetrical distribution of sodium and potassium ions across the plasma membrane and to maintenance of the membrane potential in many types of cells. Alterations in this protein may play a significant role in many human neurological disorders, including epilepsy. We studied expression of the alpha3 isoform of Na(+),K(+)-ATPase in the freeze lesion (FL) microgyrus model of developmental epileptogenesis to test the hypothesis that it is downregulated following neonatal cortical injury. FL and sham-operated rat brains were examined at postnatal day (P)7, P10, P14, P21-28 and P50-60 after placement of a transcranial freeze lesion at P0 or P1. Immunohistochemistry and in situ hybridization were used to assess the expression of the alpha3 isoform of Na(+),K(+)-ATPase (termed alpha3, or alpha3 subunit below) in neuropil and the perisomatic areas of pyramidal cells and parvalbumin-containing interneurons. There was a significant decrease (P<0.05) in alpha3 subunit immunoreactivity (IR) in the neuropil of FL cortical layer V of the P14 and P21-28 groups that extended up to 360 mum from the border of the microgyrus, an area that typically exhibits evoked epileptiform activity. Alpha-3 was decreased in the perisomatic area of pyramidal but not parvalbumin-containing cells in P21-28 FL animals. A reduction in alpha3 mRNA was observed in the neuropil of FL cortical layer V up to 1610 mum from the microgyral edge. The developmental time course for expression of the alpha3 subunit between P7 and P60 was examined in naive rat cortices and results showed that there was a significant increase in alpha3 IR between P7 and P10. The significant decreases in Na(+),K(+)-ATPase in the paramicrogyral cortex may contribute to epileptogenesis.

Cortical hyperexcitability and epileptogenesis: Understanding the mechanisms of epilepsy - part 2

Epilepsy encompasses a diverse group of seizure disorders caused by a variety of structural, cellular and molecular alterations of the brain primarily affecting the cerebral cortex, leading to recurrent unprovoked epileptic seizures. In this two-part review we examine the mechanisms underlying normal neuronal function and those predisposing to recurrent epileptic seizures starting at the most basic cellular derangements (Part 1, Volume 16, Issue 3) and working up to the highly complex epileptic networks and factors that modulate the predisposition to seizures (Part 2). We attempt to show that multiple factors can modify the epileptic process and that different mechanisms underlie different types of epilepsy, and in most situations there is an interplay between multiple genetic and environmental factors.

Dcx reexpression reduces subcortical band heterotopia and seizure threshold in an animal model of neuronal migration disorder

Disorders of neuronal migration can lead to malformations of the cerebral neocortex that greatly increase the risk of seizures. It remains untested whether malformations caused by disorders in neuronal migration can be reduced by reactivating cellular migration, and whether such repair can decrease seizure risk. Here we show, in a rat model of subcortical band heterotopia (SBH) generated by in utero RNAi of Dcx, that aberrantly positioned neurons can be stimulated to migrate by re-expressing Dcx after birth. Re-starting migration in this way both reduces neocortical malformations and restores neuronal patterning. We find further that the capacity to reduce SBH has a critical period in early postnatal development. Moreover, intervention after birth reduces convulsant-induced seizure threshold to levels similar to that of malformation-free controls. These results suggest that disorders of neuronal migration may be eventually treatable by re-engaging developmental programs both to reduce the size of cortical malformations and to reduce seizure risk.

Glycine transporter-1 expression in the rat model of cortical dysplasia

OBJECTIVE

Glycine transporter-1 (GLYT1) is an early marker of neural development and involved in the excitatory transmission in cortex. The study was designed to investigate the expression of GLYT1 in different parts of the brain by immunohistochemistry in the rat cortical dysplasia model.

METHODS

On postnatal day 0, one freeze lesion was carried out on ten rats between bregma and lambda on the skull in the right hemisphere for 5 seconds. Six weeks later, rats were transcardially perfused with fixative and then their brains were removed for both hamotoxylin-eosine (H&E) staining for histopathology and immunohistochemistry staining for glial fibrillary acidic protein (GFAP) for astrocytic activity and GLYT1 in the cortical dysplastic region and other rostral brain regions involving epileptogenesis such as hippocampus, pyriform cortex, amygdala, thalamus and substantia nigra.

RESULTS

GFAP immunoreactivity showed clusters of glial cells in the area of the microgyrus. Dense GLYT1 expression was localized to superficial layer of microgyric cortex and around the microgyrus. GLYT1 immunoreactivity was not detected in the other rostral regions.

DISCUSSION

GLYT1 stained superficial structures might correspond to immature neuron and higher concentrations of GLYT1 around microgyrus might be correlated with increased excitatory mechanisms in these regions.

Decreased glutamate transport enhances excitability in a rat model of cortical dysplasia

Glutamate transporters function to maintain low levels of extracellular glutamate and play an important role in synaptic transmission at many synapses. Disruption of glutamate transporter function or expression can result in increased extracellular glutamate levels. Alterations in glutamate transporter expression have been reported in human epilepsy and animal seizure models. Functional electrophysiological changes that occur when transporter expression is disrupted in chronic epilepsy models have not been examined. Here, we used a freeze-induced model of cortical dysplasia to test the role of glutamate transporters in synaptic hyperexcitability. We report that inhibiting glutamate transporters with the non-selective antagonist, DL-threo-beta-benzylozyaspartic acid (TBOA) preferentially prolongs postsynaptic currents (PSCs) and decreases the threshold for evoking epileptiform activity in lesioned compared to control cortex. The effect of inhibiting uptake is mediated primarily by the glia glutamate transporter (GLT-1) since the selective antagonist dihydrokainate (DHK) mimicked the effects of TBOA. The effect of uptake inhibition is mediated by activation of N-methyl-D-aspartate (NMDA) receptors since D-(-)-2-amino-5-phosphonovaleric acid (APV) prevents TBOA-induced effects. Neurons in lesioned cortex also have a larger tonic NMDA current. These results indicate that chronic changes in glutamate transporters and NMDA receptors contribute to hyperexcitability in cortical dysplasia.

Focal decreases of cortical GABAA receptor binding remote from the primary seizure focus: what do they indicate?

PURPOSE

To determine the electroclinical significance and histopathological correlates of cortical gamma-aminobutyric acid(A)(GABA(A)) receptor abnormalities detected in and remote from human neocortical epileptic foci.

METHODS

Cortical areas with decreased(11)C-flumazenil (FMZ) binding were objectively identified on positron emission tomography (PET) images and correlated to intracranial electroencephalography (EEG) findings, clinical seizure variables, histology findings, and surgical outcome in 20 patients (mean age, 9.9 years) with intractable partial epilepsy of neocortical origin and nonlocalizing magnetic resonance imaging (MRI).

RESULTS

Focal decrease of cortical FMZ binding was detected in the lobe of seizure onset in 17 (85%) patients. Eleven patients (55%) had 17 remote cortical areas with decreased FMZ binding outside the lobe of seizure onset. Thirteen of those 16 (81%) of the 17 remote cortical regions that were covered by subdural EEG were around cortex showing rapid seizure spread on intracranial EEG. Remote FMZ PET abnormalities were associated with high seizure frequency and, when resected, showed gliosis in all six cases where material was available. Higher number of unresected cortical regions with decreased FMZ binding was associated with poorer surgical outcome.

CONCLUSIONS

Focal decreases of cortical GABA(A) receptor binding on PET may include cortical regions remote from the primary focus, particularly in patients with high seizure frequency, and these regions are commonly involved in rapid seizure propagation. Although these regions may not always need to be resected to achieve seizure freedom, a careful evaluation of cortex with decreased GABA(A) receptor binding prior to resection using intracranial EEG may facilitate optimal surgical outcome in patients with intractable neocortical epilepsy.

Interictal hyperemia correlates with epileptogenicity in polymicrogyric cortex

OBJECTIVE

To investigate pathophysiological factors underlying the presence of interictal hyperperfusion within the limits of the polymicrogyric (PMG) cortex in epileptic patients.

METHODS

Retrospective observational study on interictal perfusion by Single Photon Emission Computed Tomography (SPECT) in 16 patients with PMG and its correlations with a number of clinical and neurophysiological variables. Patients underwent video-EEG monitoring, neurological and psychiatric assessments, invasive EEG, and the interictal SPECT coregistered to Magnetic Resonance Imaging (MRI).

RESULTS

Patients with interictal hyperperfusion within the PMG cortex had a significantly higher spike rate on interictal EEG than patients with normal perfusion. Interictal hyperperfusion was not correlated to sex, age at epilepsy onset, age at evaluation, number of seizures per month, presence of initial precipitating insult (IPI), abnormal neurological examination, EEG findings, ictal semiology, and seizure outcome. The high interictal spike rate did not correlate to a high frequency of seizures per month.

CONCLUSIONS

Our work provides further evidences for an intrinsic epileptogenesis of the PMG cortex during the interictal state, which accounts for the major role of PMG tissue in seizure generation. These results might help to increase our understanding about epileptogenesis related to the PMG cortex, providing new tools for more tailored epilepsy surgery in PMG patients.

Functional organization of human visual cortex in occipital polymicrogyria

Polymicrogyrias (PMG) are cortical malformations resulting from developmental abnormalities. In animal models PMG has been associated with abnormal anatomy, function, and organization. The purpose of this study was to describe the function and organization of human polymicrogyric cortex using functional magnetic resonance imaging. Three patients with epilepsy and bilateral parasagittal occipital polymicrogyri were studied. They all had normal vision as tested by Humphrey visual field perimetry. The functional organization of the visual cortex was reconstructed using phase-encoded retinotopic mapping analysis. This method sequentially stimulates each point in the visual field along the axes of a polar-coordinate system, thereby reconstructing the representation of the visual field on the cortex. We found normal cortical responses and organization of early visual areas (V1, V2, and V3/VP). The locations of these visual areas overlapped substantially with the PMG. In five out of six hemispheres the reconstructed primary visual cortex completely fell within polymicrogyric areas. Our results suggest that human polymicrogyric cortex is not only organized in a normal fashion, but is also actively involved in processing of visual information and contributes to normal visual perception.

Differential cellular gene expression in ganglioglioma

PURPOSE

Gangliogliomas (GGs) are neuronal-glial tumors highly associated with epilepsy. We hypothesized that the expression of select gene families including neurotransmitter receptor subunits and growth factors would be distinct in neurons and astrocytes within GG compared with adjacent cortex and that these changes would yield insights into seizure onset and lesion formation.

METHODS

Candidate gene expression was defined in single immunohistochemically labeled neurons and astrocytes microdissected from GG specimens compared with neurons and astrocytes microdissected from morphologically intact cortex adjacent to the GG or normal control cortex.

RESULTS

Differential expression of 16 genes including glutamate transporter (EAAC1) and receptor (NMDA2C, mGluR5), growth factor (hepatocyte growth factor), and receptor (platelet derived growth factor receptor beta, fibroblast growth factor receptor 3) mRNAs was detected in GG neurons compared with control neurons. In astrocytes, altered expression of p75NGF, mGluR3, TGFbeta3 and Glt-1 mRNAs was detected. Nestin mRNA, a gene that exhibits enhanced expression in balloon cell cortical dysplasia, was increased in GG neurons. Because of the morphological similarities between GG and cortical dysplasia, we show that there is activation of the mTOR cascade in GG as evidenced by enhanced expression of phospho-p70S6kinase and phosphoribosomal S6 proteins.

CONCLUSION

We find differential candidate gene expression in neurons and astrocytes in GG compared with adjacent cortex and show that there is activation of the mTOR pathway. These changes highlight pathways that may be pivotal for epileptogenesis and lesion growth.

[Congenital hemiplegia. A disease with manifold problems]

Congenital hemiplegia, defined as unilateral motor disability, is the hemiplegic type of cerebral palsy. The prevalence of congenital hemiplegia is estimated to be about 0.41-0.79/1000 live births. We examined 223 children (122 boys and 101 girls) suffering from congenital hemiplegia at the age of 3 months to 12 years. Mild hemiplegia was found in 31%, a moderate form in 48%, and a severe form in 21%. The upper limb was affected in more than half of the patients, only the lower extremity in one third, and both upper and lower limbs in 20%. Electroencephalographic abnormalities were found in 75.8% of the patients. The most frequent type of epilepsy was complex partial seizures (33%). Severity of the motor handicap, grade of EEG abnormalities, and the prevalence of epilepsy showed a significant correlation. The magnitude of the lesions in neuroimaging directly correlated with these three clinical variables, particularly in children with cortical and subcortical defects (84.2%). Strabismus was the most common visual impairment (17%), while hearing impairment was found in 8% of the patients. Of them, 38.3% showed no cognitive deficits, while those with severe congenital hemiplegia were found to have a lower intelligence quotient.

Neuroprotective effects of melatonin upon the offspring cerebellar cortex in the rat model of BCNU-induced cortical dysplasia

Cortical dysplasia is a malformation characterized by defects in proliferation, migration and maturation. This study was designed to evaluate the alterations in offspring rat cerebellum induced by maternal exposure to carmustine-[1,3-bis (2-chloroethyl)-1-nitrosoure] (BCNU) and to investigate the effects of exogenous melatonin upon cerebellar BCNU-induced cortical dysplasia, using histological and biochemical analyses. Pregnant Wistar rats were assigned to five groups: intact-control, saline-control, melatonin-treated, BCNU-exposed and BCNU-exposed plus melatonin. Rats were exposed to BCNU on embryonic day 15 and melatonin was given until delivery. Immuno/histochemistry and electron microscopy were carried out on the offspring cerebellum, and levels of malondialdehyde and superoxide dismutase were determined. Histopathologically, typical findings were observed in the cerebella from the control groups, but the findings consistent with early embryonic development were noted in BCNU-exposed cortical dysplasia group. There was a marked increase in the number of TUNEL positive cells and nestin positive cells in BCNU-exposed group, but a decreased immunoreactivity to glial fibrillary acidic protein, synaptophysin and transforming growth factor beta1 was observed, indicating a delayed maturation, and melatonin significantly reversed these changes. Malondialdehyde level in BCNU-exposed group was higher than those in control groups and melatonin decreased malondialdehyde levels in BCNU group (P<0.01), while there were no significant differences in the superoxide dismutase levels between these groups. These data suggest that exposure of animals to BCNU during pregnancy leads to delayed maturation of offspring cerebellum and melatonin protects the cerebellum against the effects of BCNU.

REORGANIZATION OF BARREL CIRCUITS LEADS TO THALAMICALLY-EVOKED CORTICAL EPILEPTIFORM ACTIVITY

We studied circuit activities in layer IV of rat somatosensory barrel cortex containing microgyri induced by neonatal freeze lesions. Structural abnormalities in GABAergic interneurons are present in the epileptogenic paramicrogyral area (PMG) and we therefore tested the hypothesis that decreased postsynaptic inhibition within barrel microcircuits occurs in the PMG and contributes to epileptogenesis when thalamocortical afferents are activated. In thalamocortical (TC) slices from naïve animals, single electrical stimuli within the thalamic ventrobasal (VB) nucleus evoked transient cortical multi-unit activity lasting 65±42 ms. Similar stimuli in TC slices from lesioned barrel cortex elicited prolonged 850 ±100 ms paroxysmal discharges that originated in the PMG and propagated laterally over several mm. Paroxysmal discharges were shortened in duration by ~70 % when APV was applied, and were totally abolished by CNQX. The cortical paroxysmal discharges did not evoke thalamic oscillations. Whole cell patch clamp recordings showed that there was a shift in the balance of TC evoked responses in the PMG that favored excitation over inhibition. Dual whole-cell recordings in layer IV of the PMG indicated that there was selective loss of inhibition from fast-spiking interneurons to spiny neurons in the barrel circuits that likely contributed to unconstrained cortical recurrent excitation with generation and spread of paroxysmal discharges.

Postsynaptic currents prior to onset of epileptiform activity in rat microgyria

Structural malformations of the cortex, arising as a result of genetic mutation or injury during development are associated with dyslexia, epilepsy, and other neurological deficits. We have used a rat model of a microgyral malformation to examine mechanisms of epileptogenesis. Our previous studies showed that the frequency of miniature excitatory postsynaptic currents (mEPSCs) recorded in neocortical layer V pyramidal neurons is increased in malformed cortex at a time when field potential epileptiform events can be evoked. Here we show that the increase occurs at an age before onset of cortical epileptiform activity and at a time when the frequency of mEPSCs in control layer V pyramidal neurons is stable. An increase in the frequency of spontaneous (s)EPSCs in layer V pyramidal neurons of malformed cortex occurs earlier than that for mEPSCs, suggesting that there may additionally be alterations in intrinsic properties that increase the excitability of the cortical afferents. Frequencies of EPSC bursts and late evoked activity were also increased in malformed cortex. These results suggest that a hyperinnervation of layer V pyramidal neurons by excitatory afferents occurs as an active process likely contributing to subsequent development of field epileptiform events.