NMDA Receptor Composition Differs Among Anatomically Diverse Malformations of Cortical Development

3q29 microduplication syndrome: New evidence for the refinement of the critical region

BACKGROUND

The 3q29 microduplication syndrome is a rare genomic disorder characterized by an extremely variable neurodevelopmental phenotype usually involving a genomic region ranging from 1.6 to 1.76 Mb. A small microduplication of 448.8 Kb containing only two genes was recently described in a patient with a 3q29 microduplication that was proposed as the minimal critical region of overlap of this syndrome.

METHODS

Molecular karyotyping (array-CGH) was performed on DNA extracted from peripheral blood samples using Agilent-California USA Human Genome CGH Microarray 4 × 180 K. The proband and his younger brother were further tested with a next generation sequencing (NGS) panel including genes implicated in autism spectrum disorder and in neurodevelopmental disorders. Quantitative real-time PCR was applied to verify the abnormal array-CGH findings.

RESULTS

Here, we report on a family with two males with neurodevelopmental disorders and an unaffected sibling with a small 3q29 microduplication (432.8 Kb) inherited from an unaffected mother that involves only two genes: DGL1 and BDH1. The proband had an additional intragenic duplication inherited from the unaffected father. Further testing was negative for Fragile X syndrome and for genes implicated in autism spectrum disorder and in neurodevelopmental disorders.

CONCLUSION

To the best of our knowledge, one of the family members here analyzed is the second reported case of a patient carrying a small 3q29 microduplication including only DGL1 and BDH1 genes and without any additional genetic aberration. The recognition of the clinical spectrum in patients with the critical region of overlap associated with the 3q29 duplication syndrome should prove valuable for predicting outcomes and providing more informed genetic counseling to patients with duplications in this region.

Human periventricular nodular heterotopia shows several interictal epileptic patterns and hyperexcitability of neuronal firing

Periventricular nodular heterotopia (PNH) is a malformation of cortical development that frequently causes drug-resistant epilepsy. The epileptogenicity of ectopic neurons in PNH as well as their role in generating interictal and ictal activity is still a matter of debate. We report the first in vivo microelectrode recording of heterotopic neurons in humans. Highly consistent interictal patterns (IPs) were identified within the nodules: (1) Periodic Discharges PLUS Fast activity (PD+F), (2) Sporadic discharges PLUS Fast activity (SD+F), and (3) epileptic spikes (ES). Neuronal firing rates were significantly modulated during all IPs, suggesting that multiple IPs were generated by the same local neuronal populations. Furthermore, firing rates closely followed IP morphologies. Among the different IPs, the SD+F pattern was found only in the three nodules that were actively involved in seizure generation but was never observed in the nodule that did not take part in ictal discharges. On the contrary, PD+F and ES were identified in all nodules. Units that were modulated during the IPs were also found to participate in seizures, increasing their firing rate at seizure onset and maintaining an elevated rate during the seizures. Together, nodules in PNH are highly epileptogenic and show several IPs that provide promising pathognomonic signatures of PNH. Furthermore, our results show that PNH nodules may well initiate seizures.

Vascular endothelial growth factor-C modulates cortical NMDA receptor activity in cortical lesions of young patients and rat model with focal cortical dysplasia

Emergence of dysmorphic neurons is the primary pathology in focal cortical dysplasia (FCD) associated pediatric intractable epilepsy; however, the etiologies related to the development and function of dysmorphic neurons are not fully understood. Our previous studies revealed that the expression of vascular endothelial growth factor-C (VEGF-C) and corresponding receptors VEGFR-2, VEGFR-3 was increased in the epileptic lesions of patients with tuberous sclerosis complex or mesial temporal lobe epilepsy. Here, we showed that the expression of VEGF-C, VEGFR-2, and VEGFR-3 was increased at both mRNA and protein levels in patients with cortical lesions of type I, IIa, and IIb FCD. The immunoreactivity of VEGF-C, VEGFR-2 and VEGFR-3 was located in the micro-columnar neurons in FCD type I lesions, dysplastic neurons (DNs) in FCD type IIa lesions, balloon cells (BCs) and astrocytes in FCD type IIb lesions. Additionally, the amplitude of evoked-EPSCs (eEPSC) mediated by NMDA receptor, the ratio of NMDA receptor- and AMPA receptor-mediated eEPSC were increased in the dysmorphic neurons of FCD rats established by prenatal X-ray radiation. Furthermore, NMDA receptor mediated current in dysmorphic neurons was further potentiated by exogenous administration of VEGF-C, however, could be antagonized by ki8751, the blocker of VEGFR-2. These results suggest that VEGF-C system participate in the pathogenesis of cortical lesions in patients with FCD in association with modulating NMDA receptor-mediated currents.

N-Methyl-d-Aspartate receptor and inflammation in dorsolateral prefrontal cortex in schizophrenia

Lower N-methyl-d-aspartate receptor (NMDAR) GluN1 subunit levels and heightened neuroinflammation are found in the cortex in schizophrenia. Since neuroinflammation can lead to changes in NMDAR function, it is possible that these observations are linked in schizophrenia. We aimed to extend our previous studies by measuring molecular indices of NMDARs that define key functional properties of this receptor - particularly the ratio of GluN2A and GluN2B subunits - in dorsolateral prefrontal cortex (DLPFC) from schizophrenia and control cases (37/37). We sought to test whether changes in these measures are specific to the subset of schizophrenia cases with high levels of inflammation-related mRNAs, defined as a high inflammatory subgroup. Quantitative autoradiography was used to detect 'functional' NMDARs ([³H]MK-801), GluN1-coupled-GluN2A subunits ([³H]CGP-39653), and GluN1-coupled-GluN2B subunits ([³H]Ifenprodil). Quantitative RT-PCR was used to measure NMDAR subunit transcripts (GRIN1, GRIN2A and GRIN2B). The ratios of GluN2A:GluN2B binding and GRIN2A:GRIN2B mRNAs were calculated as an index of putative NMDAR composition. We found: 1) GluN2A binding, and 2) the ratios of GluN2A:GluN2B binding and GRIN2A:GRIN2B mRNAs were lower in schizophrenia cases versus controls (p < 0.05), and 3) lower GluN2A:GluN2B binding and GRIN2A:GRIN2B mRNA ratios were exaggerated in the high inflammation/schizophrenia subgroup compared to the low inflammation/control subgroup (p < 0.05). No other NMDAR-related indices were significantly changed in the high inflammation/schizophrenia subgroup. This suggests that neuroinflammation may alter NMDAR stoichiometry rather than targeting total NMDAR levels overall, and future studies could aim to determine if anti-inflammatory treatment can alleviate this aspect of NMDAR-related pathology.

PDI-Mediated Reduction of Disulfide Bond on PSD95 Increases Spontaneous Seizure Activity by Regulating NR2A-PSD95 Interaction in Epileptic Rats Independent of S-Nitrosylation

Postsynaptic density-95 (PSD95), a major scaffolding protein, is critical in coupling N-methyl-D-aspartate receptor (NMDAR) to cellular signaling networks in the central nervous system. A couple of cysteine residues in the N-terminus of PSD95 are potential sites for disulfide bonding, S-nitrosylation and/or palmitoylation. Protein disulfide isomerase (PDI) reduces disulfide bonds (S-S) to free thiol (-SH) on various proteins. However, the involvement of PDI in disulfide bond formation/S-nitrosylation of PSD95 and its role in epilepsy are still unknown. In the present study, acute seizure activity significantly increased the bindings of PDI to NR2A, but not to PSD95, while it decreased the NR2A–PSD95 binding. In addition, pilocarpine-induced seizures increased the amount of nitrosylated (SNO-) thiols, not total (free and SNO-) thiols, on PSD95. Unlike acute seizure, spontaneous seizing rats showed the increases in PDI–PSD95 binding, total- and SNO-thiol levels on PSD95, and NR2A–PSD95 interaction. PDI siRNA effectively reduced spontaneous seizure activity with decreases in total thiol level on PSD95 and NR2A–PSD95 association. These findings indicate that PDI-mediated reduction of disulfide-bond formations may facilitate the NR2A–PSD95 binding and contribute to spontaneous seizure generation in epileptic animals.

Age-Dependent Changes in Synaptic NMDA Receptor Composition in Adult Human Cortical Neurons

The molecular processes underlying the aging-related decline in cognitive performance and memory observed in humans are poorly understood. Studies in rodents have shown a decrease in N-methyl-D-aspartate receptors (NMDARs) that contain the GluN2B subunit in aging synapses, and this decrease is correlated with impaired memory functions. However, the age-dependent contribution of GluN2B-containing receptors to synaptic transmission in human cortical synapses has not been previously studied. We investigated the synaptic contribution of GluN2A and GluN2B-containing NMDARs in adult human neurons using fresh nonpathological temporal cortical tissue resected during neurosurgical procedures. The tissue we obtained fulfilled quality criteria by the absence of inflammation markers and proteomic degradation. We show an age-dependent decline in the NMDA/AMPA receptor ratio in adult human temporal cortical synapses. We demonstrate that GluN2B-containing NMDA receptors contribute to synaptic responses in the adult human brain with a reduced contribution in older individuals. With previous evidence demonstrating the critical role of synaptic GluN2B in regulating synaptic strength and memory storage in mice, this progressive reduction of GluN2B in the human brain during aging may underlie a molecular mechanism in the age-related decline in cognitive abilities and memory observed in humans.

New insights into a spectrum of developmental malformations related to mTOR dysregulations: challenges and perspectives

In recent years the role of the mammalian target of rapamycin (mTOR) pathway has emerged as crucial for normal cortical development. Therefore, it is not surprising that aberrant activation of mTOR is associated with developmental malformations and epileptogenesis. A broad spectrum of malformations of cortical development, such as focal cortical dysplasia (FCD) and tuberous sclerosis complex (TSC), have been linked to either germline or somatic mutations in mTOR pathway-related genes, commonly summarised under the umbrella term 'mTORopathies'. However, there are still a number of unanswered questions regarding the involvement of mTOR in the pathophysiology of these abnormalities. Therefore, a monogenetic disease, such as TSC, can be more easily applied as a model to study the mechanisms of epileptogenesis and identify potential new targets of therapy. Developmental neuropathology and genetics demonstrate that FCD IIb and hemimegalencephaly are the same diseases. Constitutive activation of mTOR signalling represents a shared pathogenic mechanism in a group of developmental malformations that have histopathological and clinical features in common, such as epilepsy, autism and other comorbidities. We seek to understand the effect of mTOR dysregulation in a developing cortex with the propensity to generate seizures as well as the aftermath of the surrounding environment, including the white matter.

Long term seizure freedom on perampanel in highly drug-resistant epilepsy caused by bilateral periventricular nodular heterotopia: A case report

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NMDA-receptor subunit expression of NR2A and NR2B is downregulated in heterotopias.

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Glutamatergic transmission in the hyperexcitability of nodular heterotopias is implicated.

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Mechanisms in PVNH to establish etiology specific therapies are still to be defined.

Novel drugs and early polypharmacotherapy in status epilepticus

PURPOSE

Rescue medications for status epilepticus (SE) have a relatively high rate of failure. The purpose of this review is to summarize the evidence for the efficacy of novel drugs and early polypharmacotherapy for SE.

METHOD

Literature review.

RESULTS

New drugs and treatment strategies aim to target the pathophysiology of SE in order to improve seizure control and outcomes. Changes at the synapse level during SE include a progressive decrease in synaptic GABA_A receptors and increase in synaptic NMDA receptors. These changes tend to promote self-sustaining seizures. Current SE guidelines recommend a rapid stepwise treatment using benzodiazepines in monotherapy as the first-line treatment, targeting GABA_A synaptic receptors. Novel treatment approaches target GABA_A synaptic and extrasynaptic receptors with allopregnanolone, and NMDA receptors with ketamine. Novel rescue treatments used for SE include topiramate, brivaracetam, and perampanel, which are already marketed in epilepsy. Some available drugs not marketed for use in epilepsy have been used in the treatment of SE, and other agents are being studied for this purpose. Early polytherapy, most frequently combining a benzodiazepine with a second-line drug or an NMDA receptor antagonist, might potentially increase seizure control with relatively minor increase in side effects. Although many preclinical studies support novel drugs and early polytherapy in SE, human studies are scarce and inconclusive. Currently, evidence is lacking to recommend specific combinations of these new agents.

CONCLUSIONS

Novel drugs and strategies target the underlying pathophysiology of SE with the intent to improve seizure control and outcomes.

3q29 microduplication syndrome: Description of two new cases and delineation of the minimal critical region

Heterogeneous clinical and neuropsychological features, such as intellectual disability, developmental and language delay, hypotonia, and, to a lesser extent, microcephaly that is present in about the half of the reported patients, characterize the 3q29 microduplication syndrome with usually a milder phenotype compared with the corresponding 3q29 microdeletion syndrome. The duplications described so far range from 2.3 Mb to 1.6 Mb, spanning from TFRC to BDH1 genes. Here we report on two patients with overlapping interstitial duplications of the 3q29 region differing in size. Patient 1 harboured a common-seized 3q29 microduplication spanning ∼1.6 Mb, while patient 2 carried a very small 3q29 microduplication of 448.8 Kb encompassing only two genes, DLG1 and BDH1. Both patients presented clinical characteristics similar to those reported in the literature in 3q29 microduplication syndrome. Interestingly, heterotopic gray matter nodules were found along the right lateral ventricle on brain MRI in patient 1, thus expanding the neuroradiological phenotype in 3q29 microduplication syndrome, while patient 2 allowed us to define with more precision the smallest region of overlap (SRO). Gene content analysis of the duplicated region suggests that gain-of-dosage of DLG1 and BDH1 may be a good candidate for the main clinical features of this syndrome.

Review: Roles for astrocytes in epilepsy: insights from malformations of cortical development

Malformations of cortical development (MCDs), such as cortical dysplasia and tuberous sclerosis complex, are common causes of intractable epilepsy, especially in paediatric patients. Recently, mounting evidence points to a common pathology of these disorders. Hyperactivation of mammalian target of rapamycin (mTOR) has been proposed as a central mechanism in most, if not all, MCDs. The transition from mTOR hyperactivation and cellular abnormalities to large-scale functional changes and seizure is, however, not fully understood. In this article we set out to review currently available information regarding MCD pathology, focusing on glial cells - especially astrocytes - and their interactions with the brain vascular system. A large body of evidence points to these elements as potential targets in MCD. Here, we attempt to provide a review of this evidence and propose some hypotheses regarding the possible chain of events linking primary glial dysfunction and epilepsy. We focus on extracellular matrix remodelling, blood-brain barrier leakage and failure of astrocyte-dependent removal of extracellular debris. We posit that the failure of these systems results in a chronically pro-inflammatory environment, maintaining local astrocytes in a state of gliosis, with increased susceptibility to seizures as a consequence.

Growth Associated Protein 43 (GAP-43) as a Novel Target for the Diagnosis, Treatment and Prevention of Epileptogenesis

We previously showed increased growth associated protein 43 (GAP-43) expression in brain samples resected from patients with cortical dysplasia (CD), which was correlated with duration of epilepsy. Here, we used a rat model of CD to examine the regulation of GAP-43 in the brain and serum over the course of epileptogenesis. Baseline GAP-43 expression was higher in CD animals compared to control non-CD rats. An acute seizure increased GAP-43 expression in both CD and control rats. However, GAP-43 expression decreased by day 15 post-seizure in control rats, which did not develop spontaneous seizures. In contrast, GAP-43 remained up-regulated in CD rats, and over 50% developed chronic epilepsy with increased GAP-43 levels in their serum. GAP-43 protein was primarily located in excitatory neurons, suggesting its functional significance in epileptogenesis. Inhibition of GAP-43 expression by shRNA significantly reduced seizure duration and severity in CD rats after acute seizures with subsequent reduction in interictal spiking. Serum GAP-43 levels were significantly higher in CD rats that developed spontaneous seizures. Together, these results suggest GAP-43 as a key factor promoting epileptogenesis, a possible therapeutic target for treatment of progressive epilepsy and a potential biomarker for epilepsy progression in CD.

Alterations in Properties of Glutamatergic Transmission in the Temporal Cortex and Hippocampus Following Pilocarpine-Induced Acute Seizures in Wistar Rats

Temporal lobe epilepsy (TLE) is the most common type of focal epilepsy in humans, and is often developed after an initial precipitating brain injury. This form of epilepsy is frequently resistant to pharmacological treatment; therefore, the prevention of TLE is the prospective approach to TLE therapy. The lithium-pilocarpine model in rats replicates some of the main features of TLE in human, including the pathogenic mechanisms of cell damage and epileptogenesis after a primary brain injury. In the present study, we investigated changes in the properties of glutamatergic transmission during the first 3 days after pilocarpine-induced acute seizures in Wistar rats (PILO-rats). Using RT-PCR and electrophysiological techniques, we compared the changes in the temporal cortex (TC) and hippocampus, brain areas differentially affected by seizures. On the first day, we found a transient increase in a ratio of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and N-methyl d-aspartate (NMDA) receptors in the excitatory synaptic response in pyramidal neurons of the CA1 area of the dorsal hippocampus, but not in the TC. This was accompanied by an increase in the slope of input-output (I/O) curves for fEPSPs recorded in CA1, suggesting an enhanced excitability in AMPARs in this brain area. There was no difference in the AMPA/NMDA ratio in control rats on the third day. We also revealed the alterations in NMDA receptor subunit composition in PILO-rats. The GluN2B/GluN2A mRNA expression ratio increased in the dorsal hippocampus but did not change in the ventral hippocampus or the TC. The kinetics of NMDA-mediated evoked EPSCs in hippocampal neurons was slower in PILO-rats compared with control animals. Ifenprodil, a selective antagonist of GluN2B-containing NMDARs, diminished the area and amplitude of evoked EPSCs in CA1 pyramidal cells more efficiently in PILO-rats compared with control animals. These results demonstrate that PILO-induced seizures lead to more severe alterations in excitatory synaptic transmission in the dorsal hippocampus than in the TC. Seizures affect the relative contribution of AMPA and NMDA receptor conductances in the synaptic response and increase the proportion of GluN2B-containing NMDARs in CA1 pyramidal neurons. These alterations disturb normal circuitry functions in the hippocampus, may cause neuron damage, and may be one of the important pathogenic mechanisms of TLE.

Continuous neurodegeneration and death pathway activation in neurons and glia in an experimental model of severe chronic epilepsy

Whether seizures might determine the activation of cell death pathways and what could be the relevance of seizure-induced cell death in epilepsy are still highly debated issues. We recently developed an experimental model of acquired focal cortical dysplasia (the MAM-pilocarpine or MP rat) in which the occurrence of status epilepticus--SE--and subsequent seizures induced progressive cellular/molecular abnormalities and neocortical/hippocampal atrophy. Here, we exploited the same model to verify when, where, and how cell death occurred in neurons and glia during epilepsy course. We analyzed Fluoro Jade (FJ) staining and the activation of c-Jun- and caspase-3-dependent pathways during epilepsy, from few hours post-SE up to six months of spontaneous recurrent seizures. FJ staining revealed that cell injury in MP rats was not temporally restricted to SE, but extended throughout the different epileptic stages. The region-specific pattern of FJ staining changed during epilepsy, and FJ(+) neurons became more prominent in the dorsal and ventral hippocampal CA at chronic epilepsy stages. Phospho-c-Jun- and caspase-3-dependent pathways were selectively activated respectively in neurons and glia, at early but even more conspicuously at late chronic stages. Phospho-c-Jun activation was associated with increased cytochrome-c staining, particularly at chronic stages, and the staining pattern of cytochrome-c was suggestive of its release from the mitochondria. Taken together, these data support the content that at least in the MP rat model the recurrence of seizures can also sustain cell death mechanisms, thus continuously contributing to the pathologic process triggered by the occurrence of SE.

Focal Cortical Dysplasia

Focal cortical dysplasias are common malformations of cerebral cortical development and are highly associated with medically intractable epilepsy. They have been classified into neuropathological subtypes (type Ia, Ib, IIa, IIb, and III) based on the severity of cytoarchitectural disruption--tangential or radial dispersion, or loss of laminar structure--and the presence of unique cells types such as cytomegalic neurons or balloon cells. Most focal cortical dysplasias can be identified on neuroimaging and many require resective epilepsy surgery to cure refractory seizures. The pathogenesis of focal cortical dysplasias remains to be defined, although there is recent evidence to suggest that focal cortical dysplasias arise from de novo somatic mutations occurring during brain development. Some focal cortical dysplasia subtypes show a link to the mammalian target of rapamycin signaling cascade; this has now extended to other cortical malformations, including hemimegalencephaly.

Models of cortical malformation--Chemical and physical

Pharmaco-resistant epilepsies, and also some neuropsychiatric disorders, are often associated with malformations in hippocampal and neocortical structures. The mechanisms leading to these cortical malformations causing an imbalance between the excitatory and inhibitory system are largely unknown. Animal models using chemical or physical manipulations reproduce different human pathologies by interfering with cell generation and neuronal migration. The model of in utero injection of methylazoxymethanol (MAM) acetate mimics periventricular nodular heterotopia. The freeze lesion model reproduces (poly)microgyria, focal heterotopia and schizencephaly. The in utero irradiation model causes microgyria and heterotopia. Intraperitoneal injections of carmustine 1-3-bis-chloroethyl-nitrosurea (BCNU) to pregnant rats produces laminar disorganization, heterotopias and cytomegalic neurons. The ibotenic acid model induces focal cortical malformations, which resemble human microgyria and ulegyria. Cortical dysplasia can be also observed following prenatal exposure to ethanol, cocaine or antiepileptic drugs. All these models of cortical malformations are characterized by a pronounced hyperexcitability, few of them also produce spontaneous epileptic seizures. This dysfunction results from an impairment in GABAergic inhibition and/or an increase in glutamatergic synaptic transmission. The cortical region initiating or contributing to this hyperexcitability may not necessarily correspond to the site of the focal malformation. In some models wide-spread molecular and functional changes can be observed in remote regions of the brain, where they cause pathophysiological activities. This paper gives an overview on different animal models of cortical malformations, which are mostly used in rodents and which mimic the pathology and to some extent the pathophysiology of neuronal migration disorders associated with epilepsy in humans.

Therapeutic choices in convulsive status epilepticus

INTRODUCTION

Convulsive status epilepticus (SE) is one of the most frequent and severe neurological emergencies in both adults and children. A timely administration of appropriate antiepileptic drugs (AEDs) can stop seizures early and markedly improve outcome.

AREAS COVERED

The main treatment strategies for SE are reviewed with an emphasis on initial treatments. The established first-line treatment consists of benzodiazepines, most frequently intravenous lorazepam. Benzodiazepines that do not require intravenous administration like intranasal midazolam or intramuscular midazolam are becoming more popular because of easier administration in the field. Other benzodiazepines may also be effective. After treatment with benzodiazepines, treatment with fosphenytoin and phenobarbital is usually recommended. Other intravenously available AEDs, such as valproate and levetiracetam, may be as effective and safe as fosphenytoin and phenobarbital, have a faster infusion time and better pharmacokinetic profile. The rationale behind the need for an early treatment of SE is discussed. The real-time delays of AED administration in clinical practice are described.

EXPERT OPINION

There is limited evidence to support what the best initial benzodiazepine or the best non-benzodiazepine AED is. Recent and developing multicenter trials are evaluating the best treatment options and will likely modify the recommended treatment choices in SE in the near future. Additionally, more research is needed to understand how different treatment options modify prognosis in SE. Timely implementation of care protocols to minimize treatment delays is crucial.

Subunit composition of neurotransmitter receptors in the immature and in the epileptic brain

Neuronal activity is critical for synaptogenesis and the development of neuronal networks. In the immature brain excitation predominates over inhibition facilitating the development of normal brain circuits, but also rendering it more susceptible to seizures. In this paper, we review the evolution of the subunit composition of neurotransmitter receptors during development, how it promotes excitation in the immature brain, and how this subunit composition of neurotransmission receptors may be also present in the epileptic brain. During normal brain development, excitatory glutamate receptors peak in function and gamma-aminobutiric acid (GABA) receptors are mainly excitatory rather than inhibitory. A growing body of evidence from animal models of epilepsy and status epilepticus has demonstrated that the brain exposed to repeated seizures presents a subunit composition of neurotransmitter receptors that mirrors that of the immature brain and promotes further seizures and epileptogenesis. Studies performed in samples from the epileptic human brain have also found a subunit composition pattern of neurotransmitter receptors similar to the one found in the immature brain. These findings provide a solid rationale for tailoring antiepileptic treatments to the specific subunit composition of neurotransmitter receptors and they provide potential targets for the development of antiepileptogenic treatments.

Growth-associated protein 43 and progressive epilepsy in cortical dysplasia

Objective

To investigate growth-associated protein 43 (GAP-43), a marker for axonal growth and synaptic plasticity, as potential substrate for progressive epilepsy and potential predictor of postsurgical seizure outcome in patients with focal cortical dysplasia (FCD).

Methods

GAP-43 immunohistochemistry was performed on cortical specimens from 21 patients with FCD: 12 with FCD type II (IIA or IIB) and nine with FCD type IA. Twenty normal anterior temporal lobe specimens from patients with mesial temporal lobe epilepsy due to hippocampal sclerosis (mTLE/HS) were used as controls. Semiquantitative analysis of GAP-43 staining patterns was performed. Additionally, GAP-43 immunoblotting was performed on resected tissue from three patients with FCD type IIA/B; GAP-43 protein levels in electroencephalography-verified epileptic, and distal nonepileptic, areas were compared within each patient. Two outcome categories were used: completely seizure free (Engel IA) versus not seizure free. We examined the relationship of GAP-43 scores with epilepsy duration and seizure-free outcome for each of the three pathologies.

Results

Within-patient GAP-43 expression is selectively increased in the epileptic as compared to nonepileptic cortex. GAP-43 immunoreactivity (IRs) patterns were seen on the cell surface and tubular punctate structures intercellularly only in FCD cortex. Higher GAP-43 scores were correlated (P < 0.0001) with longer epilepsy duration only in FCD IIA/B. Lower GAP-43 scores were associated with better surgical outcome in the same group. No such relationship was observed in FCD IA.

Interpretation

GAP-43 proteins are not only associated with intrinsic epileptogenicity but may be markers of progressive epilepsy and predictors of postoperative seizure outcome in patients with pharmacoresistant epilepsy due to FCD IIA/B.

Subunit composition of glutamate and gamma-aminobutyric acid receptors in status epilepticus

PURPOSE

To describe the subunit composition of glutamate and gamma-aminobutyric acid (GABA) receptors in brain tissue from patients with different types of status epilepticus.

PATIENTS AND METHODS

The subunit composition of glutamate and GABA receptors was analyzed in: (1) surgical brain samples from three patients with refractory convulsive status epilepticus, three patients with electrical status epilepticus in sleep, and six patients with refractory epilepsy, and (2) brain autopsy samples from four controls who died without neurological disorders. Subunit expression was quantified with Western blotting and messenger ribonucleic acid (mRNA) expression was quantified with reverse polymerase chain reaction.

RESULTS

Western blot analysis demonstrated the following patterns (as compared to controls): (1) alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors: elevated GluA1/GluA2 ratio in electrical status epilepticus in sleep (465%±119) and refractory epilepsy (329%±125; p<0.01); (2) N-methyl-d-aspartate (NMDA) receptors: increased GluN2B/GluN2A ratio in electrical status epilepticus in sleep (3682%±1000) and refractory convulsive status epilepticus (3520%±751; p<0.05); (3) GABA receptors: elevated α2/α1 ratio in refractory epilepsy (321%±138; p<0.05) and refractory convulsive status epilepticus (346%±74; p<0.05); and (4) patients with underlying malformation of cortical development had increased ratios in GluA1/GluA2 (382%±149; p<0.01), GluN2B/GluN2A (3321%±1581; p<0.05) and α2/α1 (303%±86; p<0.01). Quantification of mRNA demonstrated an elevated GABRA2/GABRA1 ratio in refractory epilepsy (712; p<0.05) as compared to controls.

CONCLUSIONS

The subunit composition of glutamate and GABA receptors in patients with status epilepticus mirrors that found in animal models of refractory status epilepticus and may promote self-sustaining seizures. Receptor subunit changes may provide additional targets for improved treatment.

Epilepsy related to developmental tumors and malformations of cortical development

Structural abnormalities of the brain are increasingly recognized in patients with neurodevelopmental delay and intractable focal epilepsies. The access to clinically well-characterized neurosurgical material has provided a unique opportunity to better define the neuropathological, neurochemical, and molecular features of epilepsy-associated focal developmental lesions. These studies help to further understand the epileptogenic mechanisms of these lesions. Neuropathological evaluation of surgical specimens from patients with epilepsy-associated developmental lesions reveals two major pathologies: focal cortical dysplasia and low-grade developmental tumors (glioneuronal tumors). In the last few years there have been major advances in the recognition of a wide spectrum of developmental lesions associated with a intractable epilepsy, including cortical tubers in patients with tuberous sclerosis complex and hemimegalencephaly. As an increasing number of entities are identified, the development of a unified and comprehensive classification represents a great challenge and requires continuous updates. The present article reviews current knowledge of molecular pathogenesis and the pathophysiological mechanisms of epileptogenesis in this group of developmental disorders. Both emerging neuropathological and basic science evidence will be analyzed, highlighting the involvement of different, but often converging, pathogenetic and epileptogenic mechanisms, which may create the basis for new therapeutic strategies in these disorders.

Progressive brain damage, synaptic reorganization and NMDA activation in a model of epileptogenic cortical dysplasia

Whether severe epilepsy could be a progressive disorder remains as yet unresolved. We previously demonstrated in a rat model of acquired focal cortical dysplasia, the methylazoxymethanol/pilocarpine - MAM/pilocarpine - rats, that the occurrence of status epilepticus (SE) and subsequent seizures fostered a pathologic process capable of modifying the morphology of cortical pyramidal neurons and NMDA receptor expression/localization. We have here extended our analysis by evaluating neocortical and hippocampal changes in MAM/pilocarpine rats at different epilepsy stages, from few days after onset up to six months of chronic epilepsy. Our findings indicate that the process triggered by SE and subsequent seizures in the malformed brain i) is steadily progressive, deeply altering neocortical and hippocampal morphology, with atrophy of neocortex and CA regions and progressive increase of granule cell layer dispersion; ii) changes dramatically the fine morphology of neurons in neocortex and hippocampus, by increasing cell size and decreasing both dendrite arborization and spine density; iii) induces reorganization of glutamatergic and GABAergic networks in both neocortex and hippocampus, favoring excitatory vs inhibitory input; iv) activates NMDA regulatory subunits. Taken together, our data indicate that, at least in experimental models of brain malformations, severe seizure activity, i.e., SE plus recurrent seizures, may lead to a widespread, steadily progressive architectural, neuronal and synaptic reorganization in the brain. They also suggest the mechanistic relevance of glutamate/NMDA hyper-activation in the seizure-related brain pathologic plasticity.

Intrinsic epileptogenicity of dysplastic cortex: converging data from experimental models and human patients

Focal cortical dysplasia (FCD) is a brain malformation associated with particularly severe drug-resistant epilepsy that often requires surgery for seizure control. The molecular basis for such enhanced propensity to seizure generation in FCD is not as yet elucidated. To investigate cellular and molecular bases of epileptogenic mechanisms and possible effect of severe epilepsy on the malformed cortex we have here performed a parallel analysis of a rat model of acquired cortical dysplasia previously established in our laboratory, i.e., the methylazoxymethanol/pilocarpine (MAM-PILO) rats, and surgical samples from patients with type IIB FCD. Data from the MAM-PILO rat model and human FCD samples reveal in both conditions: (1) that status epilepticus (SE) and/or seizures can further modify the cellular and molecular settings of the malformed cortex; (2) excitation/inhibition imbalance, and dysregulation of the N-methyl-d-aspartate/ membrane-associated guanylate kinase (NMDA/MAGUK) expression; (3) activation of cell death in neurons and glia. The data therefore highlight the mechanistic relevance of glutamate/NMDA hyperactivation in FCD epileptogenesis and suggest that epilepsy is a pathologic process capable of affecting structure and function of both neurons and glia.

Diminution of the NMDA receptor NR2B subunit in cortical and subcortical areas of WAG/Rij rats

Modulation of glutamatergic NMDA receptors affects the synchronization of spike discharges in in WAG/Rij rats, a valid genetic animal model of absence epilepsy. In this study, we describe the alteration of NR2B subunit of NMDA receptors expression in WAG/Rij rats in different somatosensory cortical layers and in hippocampal CA1 area. Experimental groups were divided into four groups of six rats of both WAG/Rij and Wistar strains with 2 and 6 months of age. The distribution of NR2B receptors was assessed by immunohistochemical staining in WAG/Rij and compared with age-matched Wistar rats. The expression of NR2B subunit was significantly decreased in different somatosensory cortical layers in 2- and 6-month-old WAG/Rij rats. In addition, the distribution of NR2B in hippocampal CA1 area was lower in 6-month-old WAG/Rij compared with age-matched Wistar rats. The reduction of NR2B receptors in different brain areas points to disturbance of glutamate receptors expression in cortical and subcortical areas in WAG/Rij rats. An altered subunit assembly of NMDA receptors may underlie cortical hyperexcitability in absence epilepsy.

Long-duration epilepsy affects cell morphology and glutamatergic synapses in type IIB focal cortical dysplasia

To investigate hypothesized effects of severe epilepsy on malformed cortex, we analyzed surgical samples from eight patients with type IIB focal cortical dysplasia (FCD) in comparison with samples from nine non-dysplastic controls. We investigated, using stereological quantification methods, where appropriate, dysplastic neurons, neuronal density, balloon cells, glia, glutamatergic synaptic input, and the expression of N-methyl-D-aspartate (NMDA) receptor subunits and associated membrane-associated guanylate kinase (MAGUK). In all FCD patients, the dysplastic areas giving rise to epileptic discharges were characterized by larger dysmorphic neurons, reduced neuronal density, and increased glutamatergic inputs, compared to adjacent areas with normal cytology. The duration of epilepsy was found to correlate directly (a) with dysmorphic neuron size, (b) reduced neuronal cell density, and (c) extent of reactive gliosis in epileptogenic/dysplastic areas. Consistent with increased glutamatergic input, western blot revealed that NMDA regulatory subunits and related MAGUK proteins were up-regulated in epileptogenic/dysplastic areas of all FCD patients examined. Taken together, these results support the hypothesis that epilepsy itself alters morphology-and probably also function-in the malformed epileptic brain. They also suggest that glutamate/NMDA/MAGUK dysregulation might be the intracellular trigger that modifies brain morphology and induces cell death.

The anchoring protein SAP97 influences the trafficking and localisation of multiple membrane channels

SAP97 is a member of the MAGUK family of proteins that play a major role in the trafficking and targeting of membrane ion channels and cytosolic structural proteins in multiple cell types. Within neurons, SAP97 is localised throughout the secretory trafficking pathway and at the postsynaptic density (PSD). SAP97 differs from other MAGUK family members largely in its long N-terminus and in the sequences between the SH3 and GUK domains, where SAP97 undergoes significant alternative splicing to produce multiple SAP97 isoforms. These splice insertions endow SAP97 with differential cellular localisation patterns and functional roles within neurons. With regard to membrane ion channels, SAP97 forms multi-protein complexes with AMPA and NMDA-type glutamate receptors, and Kv1.4, Kv4.2, and Kir2.2 potassium channels, playing a major role in trafficking and anchoring ion channel surface expression. This highlights SAP97 not only as a regulator of neuronal excitability, synaptic function and plasticity in the brain, but also as a target for the pathophysiology of a number of neurological disorders. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Blood-brain barrier, epileptogenesis, and treatment strategies in cortical dysplasia

Cortical dysplasia (CD) is one of the most important causes of intractable epilepsy. The precise mechanisms of epileptogenesis in CD are not known. Using CD animal models, we attempted to understand the mechanisms and efficacy of various antiepileptic drugs. In two separate studies, we assessed (1) the effects of levetiracetam (LEV) and vagus nerve stimulation (VNS) on pentylenetetrazol (PTZ)-kindled rats, and (2) the effects of LEV and topiramate (TPM) on rats with CD and hyperthermia (HT). In the HT-induced rats with CD study, LEV and TPM decreased both the intensity of seizures and the number of rats with seizure. In these studies, we used immunocytochemistry (occludin, glial fibrillary acidic protein [GFAP], and P-glycoprotein [Pgp antibodies] and electron microscopy (EM) (sodium fluorescein [NaFlu]) and horseradish peroxidase [HRP]) to assess blood-brain barrier (BBB) integrity. Both LEV and TPM protected BBB. In PTZ- kindled rats with CD, both LEV and VNS reduced the duration of seizures. Immunocytochemistry and EM revealed no BBB impairment in any of the treatment groups. In a second set of experiments, we assessed the relationship between disruption of vascular components and epileptogenesis. Astrocytic albumin uptake in focal epileptogenic lesions with vascular components suggested that dysfunction of the BBB contributes immediately to epileptogenesis, rather than simply resulting from seizure activity. Hemosiderin deposits were seen as potential epileptogenic triggers in vascular malformations (e.g., cavernomas [CA] or arteriovenous malformations [AVMs] with or without a dysplastic cortical component). However, we found strikingly high accumulation of astrocytic albumin deposits in surgically removed brain parenchyma in the vicinity of CAs and AVMs from patients with pharmacoresistant epilepsy, which suggests different pathophysiologic dispersion pathways for hemosiderin and albumin in vascular lesions.

Matrix metalloproteinase-9 (MMP-9) in human intractable epilepsy caused by focal cortical dysplasia

Focal cortical dysplasia (FCD) is a developmental brain disorder characterized by localized abnormalities of cortical layering and neuronal morphology. It is associated with pharmacologically intractable forms of epilepsy in both children and adults. The mechanisms that underlie FCD-associated seizures and lead to the progression of the disease are unclear. Matrix metalloproteinases (MMPs) are enzymes that are able to influence neuronal function through extracellular proteolysis in various normal and pathological conditions. The results of experiments that have used rodent models showed that extracellular MMP-9 can play an important role in epileptogenesis. However, no studies have shown that MMP-9 is involved in the pathogenesis of human epilepsy. The aim of the present study was to determine whether MMP-9 plays a role in intractable epilepsy. Using an unbiased antibody microarray approach, we found that up regulation of MMP-9 is prominent and consistent in FCD tissue derived from epilepsy surgery, regardless of the patient's age. Additionally, an up regulation of MMP-1, -2, -8, -10, and -13 was found but was either less pronounced or limited only to adult cases. In the dysplastic cortex, immunohistochemistry revealed that the highest MMP-9 immuno reactivity occurred in the cytoplasm of abnormal neurons and balloon cells. The neuronal over expression of MMP-9 also occurred in sclerotic hippocampi that were excised together with the dysplastic cortex, but sclerotic hippocampi were free of dysplastic features. In both locations, MMP-9 was also found in reactive astrocytes, albeit to a lesser extent. At the subcellular level, increased MMP-9 immunoreactivity was prominently upregulated at synapses. Thus, although upregulation of the enzyme in FCD is not causally linked to the developmental malformation, it may be a result of ongoing abnormal synaptic plasticity. The present findings support the hypothesis of the pathogenic role of MMP-9 in human epilepsy and may stimulate discussions about whether MMPs could be novel therapeutic targets for intractable epilepsy.

Altered inhibition in tuberous sclerosis and type IIb cortical dysplasia

OBJECTIVE

The most common neurological symptom of tuberous sclerosis complex (TSC) and focal cortical dysplasia (FCD) is early life refractory epilepsy. As previous studies have shown enhanced excitatory glutamatergic neurotransmission in TSC and FCD brains, we hypothesized that neurons associated with these lesions may also express altered γ-aminobutyric acid (GABA)(A) receptor (GABA(A)R)-mediated inhibition.

METHODS

Expression of the GABA(A)R subunits α1 and α4, and the Na(+)-K(+)-2Cl(-) (NKCC1) and the K(+)-Cl(-) (KCC2) transporters, in human TSC and FCD type II specimens were analyzed by Western blot and double label immunocytochemistry. GABA(A) R responses in dysplastic neurons from a single case of TSC were measured by perforated patch recording and compared to normal-appearing cortical neurons from a non-TSC epilepsy case.

RESULTS

TSC and FCD type IIb lesions demonstrated decreased expression of GABA(A)R α1, and increased NKCC1 and decreased KCC2 levels. In contrast, FCD type IIa lesions showed decreased α4, and increased expression of both NKCC1 and KCC2 transporters. Patch clamp recordings from dysplastic neurons in acute slices from TSC tubers demonstrated excitatory GABA(A)R responses that were significantly attenuated by the NKCC1 inhibitor bumetanide, in contrast to hyperpolarizing GABA(A)R-mediated currents in normal neurons from non-TSC cortical slices.

INTERPRETATION

Expression and function of GABA(A)Rs in TSC and FCD type IIb suggest the relative benzodiazepine insensitivity and more excitatory action of GABA compared to FCD type IIa. These factors may contribute to resistance of seizure activity to anticonvulsants that increase GABAergic function, and may justify add-on trials of the NKCC1 inhibitor bumetanide for the treatment of TSC and FCD type IIb-related epilepsy.

Status epilepticus-induced pathologic plasticity in a rat model of focal cortical dysplasia

We have generated an experimental 'double-hit' model of chronic epilepsy to recapitulate the co-existence of abnormal cortical structure and frequently recurrent seizures as observed in human focal cortical dysplasia. We induced cortical malformations by exposing rats prenatally to methylazoxymethanol acetate and triggered status epilepticus and recurrent seizures in adult methylazoxymethanol acetate rats with pilocarpine. We studied the course of epilepsy and the long-term morphologic and molecular changes induced by the occurrence of status epilepticus and subsequent chronic epilepsy in the malformed methylazoxymethanol acetate exposed brain. Behavioural and electroencephalographic analyses showed that methylazoxymethanol acetate pilocarpine rats develop more severe epilepsy than naïve rats. Morphologic and molecular analyses demonstrated that status epilepticus and subsequent seizures, but not pilocarpine treatment per se, was capable of affecting both cortical architectural and N-methyl-D-aspartate receptor abnormalities induced by methylazoxymethanol acetate. In particular, cortical thickness was further decreased and N-methyl-D-aspartate regulatory subunits were recruited at the postsynaptic membrane. In addition, methylazoxymethanol acetate pilocarpine rats showed abnormally large cortical pyramidal neurons with neurofilament over-expression. These neurons bear similarities to the hypertrophic/dysmorphic pyramidal neurons observed in acquired human focal cortical dysplasia. These data show that status epilepticus sets in motion a pathological process capable of significantly changing the cellular and molecular features of pre-existing experimental cortical malformations. They suggest that seizure recurrence in human focal cortical dysplasia might be an additional factor in establishing a pathological circuitry that favours chronic neuronal hyperexcitability.

Abnormal pyramidal cell morphology and HCN channel expression in cortical dysplasia

Cortical dysplasia is often associated with intractable seizures. Studies in animal models have described changes in inhibitory and excitatory synaptic function that contribute to hyperexcitability. The role of changes in intrinsic excitability and abnormal dendritic properties has received less attention. Changes in hyperpolarization-activated nonselective cation (HCN) channels have been implicated in several models of epilepsy. Herein we review evidence for alterations in HCN channels and dendritic morphology in the rat freeze-lesion model of cortical dysplasia. Immunocytochemical HCN1 staining, typically seen in the apical dendrites of layer V pyramidal cells in normal cortex, was greatly reduced in the region adjacent to the freeze-induced microgyrus. Although staining was preserved in layer I, fewer dendrites were stained in upper cortical layers. Deeper cortical layers were virtually devoid of immunoreactivity. Examination of biocytin-labeled pyramidal cells revealed markedly altered dendritic trees in the lesioned animals. In addition, resting membrane properties were altered and a subpopulation of neurons with abnormal dendritic arbors was present. These changes are likely to interact with the previously reported synaptic changes in this model of cortical dysplasia. HCN channel alterations are a potentially important cellular mechanism underlying hyperexcitability in cortical dysplasia.

Mutations in the mitochondrial protease gene AFG3L2 cause dominant hereditary ataxia SCA28

Autosomal dominant spinocerebellar ataxias (SCAs) are genetically heterogeneous neurological disorders characterized by cerebellar dysfunction mostly due to Purkinje cell degeneration. Here we show that AFG3L2 mutations cause SCA type 28. Along with paraplegin, which causes recessive spastic paraplegia, AFG3L2 is a component of the conserved m-AAA metalloprotease complex involved in the maintenance of the mitochondrial proteome. We identified heterozygous missense mutations in five unrelated SCA families and found that AFG3L2 is highly and selectively expressed in human cerebellar Purkinje cells. m-AAA-deficient yeast cells expressing human mutated AFG3L2 homocomplex show respiratory deficiency, proteolytic impairment and deficiency of respiratory chain complex IV. Structure homology modeling indicates that the mutations may affect AFG3L2 substrate handling. This work identifies AFG3L2 as a novel cause of dominant neurodegenerative disease and indicates a previously unknown role for this component of the mitochondrial protein quality control machinery in protecting the human cerebellum against neurodegeneration.

Seizures, enhanced excitation, and increased vesicle number in Lis1 mutant mice

OBJECTIVE

In humans, abnormal neuronal migration and severe neuronal disorganization resulting from Lis1 (lissencephaly) haploinsufficiency contributes to cognitive impairment and seizures early in life. In Lis1 heterozygotic mice, severe hippocampal disorganization and cognitive impairment have also been reported. Using this mouse model, we examined the functional impact of LIS1 deficiency with particular focus on excitatory glutamate-mediated synaptic transmission.

METHODS

We used visualized patch-clamp recordings in acute hippocampal slices. We recorded spontaneous, miniature and stimulation-evoked excitatory postsynaptic current (EPSC). Additional mice were processed for immunohistochemistry, electron microscopy (EM), or video-electroencephalographic (EEG) monitoring.

RESULTS

Video-EEG confirmed the presence of spontaneous electrographic seizures in Lis1 mutant mice. In disorganized hippocampal slices from Lis1(+/-) mice, we noted a nearly two-fold significant increase in the frequency of spontaneous and miniature EPSC; no significant change in amplitude or decay was noted. Synaptic function assessed using brief repetitive or paired-pulse stimulation protocols, also revealed significant enhancement of glutamate-mediated excitation. Low concentrations of cadmium, a nonspecific blocker of voltage-dependent calcium channels mediating vesicle release, effectively restored paired-pulse facilitation deficits back to control levels. Analysis of synapse ultrastructure at the EM level identified a large increase in synaptic vesicle number.

INTERPRETATION

Seizure activity, possibly associated with increased glutamate-mediated excitation and an increased pool of vesicles at the presynaptic site, was demonstrated in a mouse model of type I lissencephaly.

Basic mechanisms of MCD in animal models

Epilepsy-associated glioneuronal malformations (malformations of cortical development [MCD]) include focal cortical dysplasias (FCD) and highly differentiated glioneuronal tumors, most frequently gangliogliomas. The neuropathological findings are variable but suggest aberrant proliferation, migration, and differentiation of neural precursor cells as essential pathogenetic elements. Recent advances in animal models for MCDs allow new insights in the molecular pathogenesis of these epilepsy-associated lesions. Novel approaches, presented here, comprise RNA interference strategies to generate and study experimental models of subcortical band heterotopia and study functional aspects of aberrantly shaped and positioned neurons. Exciting analyses address impaired NMDA receptor expression in FCD animal models compared to human FCDs and excitatory imbalances in MCD animal models such as lissencephaly gene ablated mice as well as in utero irradiated rats. An improved understanding of relevant pathomechanisms will advance the development of targeted treatment strategies for epilepsy-associated malformations.

Expression and localization of voltage dependent potassium channel Kv4.2 in epilepsy associated focal lesions

An increasing number of observations suggest an important role for voltage-gated potassium (Kv) channels in epilepsy. We studied the cell-specific distribution of Kv4.2, phosphorylated (p) Kv4.2 and the Kv4.2 interacting protein NCS-1 using immunocytochemistry in different epilepsy-associated focal lesions. In hippocampal sclerosis (HS), Kv4.2 and pKv4.2 immunoreactivity (IR) was reduced in the neuropil in regions with prominent neuronal cell loss. In both HS and malformations of cortical development (MCD), intense labeling was found in neuronal somata, but not in dendrites. Strong NCS-1 IR was observed in neurons in all lesion types. Western blot analysis demonstrated an increase of total Kv4.2 in all lesions and activation of the ERK pathway in HS and ganglioglioma. These findings indicate that Kv4.2 is expressed in both neuronal and glial cells and its regulation may involve potassium channel interacting proteins, alterations in the subcellular localization of the channel, as well as phosphorylation-mediated posttranslational modifications.

Epilepsy-induced abnormal striatal plasticity in Bassoon mutant mice

Recently, the striatum has been implicated in the spread of epileptic seizures. As the absence of functional scaffolding protein Bassoon in mutant mice is associated with the development of pronounced spontaneous seizures, we utilized this new genetic model of epilepsy to investigate seizure-induced changes in striatal synaptic plasticity. Mutant mice showed reduced long-term potentiation in striatal spiny neurons, associated with an altered N-methyl-D-aspartate (NMDA) receptor subunit distribution, whereas GABAergic fast-spiking (FS) interneurons showed NMDA-dependent short-term potentiation that was absent in wild-type animals. Alterations in the dendritic morphology of spiny neurons and in the number of FS interneurons were also observed. Early antiepileptic treatment with valproic acid reduced epileptic attacks and mortality, rescuing physiological striatal synaptic plasticity and NMDA receptor subunit composition. However, morphological alterations were not affected by antiepileptic treatment. Our results indicate that, in Bsn mutant mice, initial morphological alterations seem to reflect a more direct effect of the abnormal genotype, whereas plasticity changes are likely to be caused by the occurrence of repeated cortical seizures.

The potential role of cyclin-dependent kinase 5 in focal cortical dysplasia

Focal cortical dysplasia (FCD) is the most common malformation of cortical development found in epilepsy surgical series. Characterised by cortical mislamination, dysplastic neurons and, in a subgroup of cases, balloon cells, FCD is potently epileptogenic. Despite decades of study, the underlying aetiology of FCD remains uncertain and research has been hampered by the lack of a good animal model in which to simulate the condition. In this article we review some of the potential molecular mechanisms that might underpin human FCD. In particular we examine the potential role of cyclin-dependent kinase 5 and its principal activator p35 in FCD and estimate the contribution that deregulation of cyclin-dependent kinase 5 might make to the pathogenesis of this condition.

Expression of AMPA and NMDA receptor subunits in the cervical spinal cord of wobbler mice

Background

The localisation of AMPA and NMDA receptor subunits was studied in a model of degeneration of cervical spinal motoneurons, the wobbler mouse. Cervical regions from early or late symptomatic wobbler mice (4 or 12 weeks of age) were compared to lumbar tracts (unaffected) and to those of healthy mice.

Results

No differences were found in the distribution of AMPA and NMDA receptor subunits at both ages. Western blots analysis showed a trend of reduction in AMPA and NMDA receptor subunits, mainly GluR1 and NR2A, exclusively in the cervical region of late symptomatic mice in the triton-insoluble post-synaptic fraction but not whole homogenates. Colocalisation experiments evidenced the expression of GluR1 and NR2A receptors in activated astrocytes from the cervical spinal cord of wobbler mice, GluR2 did not colocalise with GFAP positive cells. No differences were found in the expression of AMPA and NMDA receptor subunits in the lumbar tract of wobbler mice, where neither motoneuron loss nor reactive gliosis occurs.

Conclusion

In late symptomatic wobbler mice altered levels of GluR1 and NR2A receptor subunits may be a consequence of motoneuron loss rather than an early feature of motoneuron vulnerability.