Neuronal migration disorders: heterotopic neocortical neurons in CA1 provide a bridge between the hippocampus and the neocortex

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.

Rapamycin Cannot Reduce Seizure Susceptibility in Infantile Rats with Malformations of Cortical Development Lacking mTORC1 Activation

The mechanistic target of the rapamycin (mTOR) pathway is involved in cortical development. However, the efficacy of mTOR inhibitors in malformations of cortical dysplasia (MCD) outside of the tuberous sclerosis complex is unknown. We selected the MCD rat model with prenatal MAM exposure to test the efficacy of mTOR inhibitors in MCDs. We explored the early cortical changes of mTOR pathway protein expression in rats aged P15. We also monitored the early treatment effect of the mTOR inhibitor, rapamycin, on N-methyl-D-aspartate (NMDA)-induced spasms at P15 and their behavior in the juvenile stage. In vivo MR spectroscopy was performed after rapamycin treatment and compared with vehicle controls. There was no difference in mTORC1 pathway protein expression between MAM-exposed MCD rats and controls at P15, and prolonged treatment of rapamycin had no impact on NMDA-induced spasms despite poor weight gain. Prenatal MAM-exposed juvenile rats treated with rapamycin showed increased social approaching and freezing behavior during habituation. MR spectroscopy showed altered neurometabolites, including Gln, Glu+Gln, Tau, and Cr. Despite behavioral changes and in vivo neurometabolic alteration with early prolonged rapamycin treatment, rapamycin had no effect on spasms susceptibility in prenatal MAM-exposed infantile rats with MCD without mTORC1 activation. For MAM-exposed MCD rats without mTORC1 activation, treatment options outside of mTOR pathway inhibitors should be explored.

An action-concept processing advantage in a patient with a double motor cortex

Patients with atrophy in motor brain regions exhibit selective deficits in processing action-related meanings, suggesting a link between movement conceptualization and the amount of regional tissue. Here we examine such a relation in a unique opposite model: a rare patient with a double cortex (due to subcortical band heterotopia) in primary/supplementary motor regions, and no double cortex in multimodal semantic regions. We measured behavioral performance in action- and object-concept processing as well and resting-state functional connectivity. Both dimensions involved comparisons with healthy controls. Results revealed preserved accuracy in action and object categories for the patient. However, unlike controls, the patient exhibited faster performance for action than object concepts, a difference that was uninfluenced by general cognitive abilities. Moreover, this pattern was accompanied by heightened functional connectivity between the bilateral primary motor cortices. This suggests that a functionally active double motor cortex may entail action-processing advantages. Our findings offer new constraints for models of action semantics and motor-region function at large.

Intravital Imaging of Neocortical Heterotopia Reveals Aberrant Axonal Pathfinding and Myelination around Ectopic Neurons

Neocortical heterotopia consist of ectopic neuronal clusters that are frequently found in individuals with cognitive disability and epilepsy. However, their pathogenesis remains poorly understood due in part to a lack of tractable animal models. We have developed an inducible model of focal cortical heterotopia that enables their precise spatiotemporal control and high-resolution optical imaging in live mice. Here, we report that heterotopia are associated with striking patterns of circumferentially projecting axons and increased myelination around neuronal clusters. Despite their aberrant axonal patterns, in vivo calcium imaging revealed that heterotopic neurons remain functionally connected to other brain regions, highlighting their potential to influence global neural networks. These aberrant patterns only form when heterotopia are induced during a critical embryonic temporal window, but not in early postnatal development. Our model provides a new way to investigate heterotopia formation in vivo and reveals features suggesting the existence of developmentally modulated, neuron-derived axon guidance and myelination factors.

In vivo MRI Successfully Reveals the Malformation of Cortical Development in Infant Rats

Objective: Malformations of cortical development (MCDs) are major causes of intractable epilepsies. To characterize the early neuroimaging findings of MCDs, we tried to identify the MRI features consistent with pathological findings in an infant rat MCD model, prenatally exposed to methylazoxymethanol (MAM), by using newly developed MRI techniques. Methods: At gestational day 15, two doses of MAM (15 mg/kg intraperitoneally) or normal saline were injected into pregnant rats. The offspring underwent in vivo MRI, including glutamate chemical exchange saturation transfer (GluCEST), ¹H-MR spectroscopy, and diffusion tensor imaging, at postnatal day (P) 15 using a 7T small-animal imaging system. Another set of prenatally MAM-exposed rats were sacrificed for histological staining. Results: At P15, the retrosplenial cortex (RSC) of rats with MCDs showed decreased neuronal nuclei, parvalbumin, and reelin expressions. Moreover, dendritic arborization of pyramidal cells in the RSC significantly decreased in infant rats with MCDs. In vivo MRI showed significantly decreased GluCEST (%) in the RSC of rats with MCDs (p = 0.000) and a significant correlation between GluCEST (%) and RSC thickness (r = 0.685, p = 0.003). The rats with MCDs showed reduced glutamate (p = 0.002), N-acetylaspartate (p = 0.002), and macromolecule and lipid levels (p = 0.027) and significantly reduced fractional anisotropy values in the RSC. Conclusion: In vivo MRI revealed reduced neuronal population and dendritic arborization in the RSC of infant rats with MCDs during the early postnatal period. These pathological changes of the cortex could serve as clinical imaging biomarkers of MCDs in infants.

Focal Cortical Dysplasia in Childhood Epilepsy

Focal cortical dysplasia is a common cause of medication resistant epilepsy. A better understanding of its presentation, pathophysiology and consequences have helped us improved its treatment and outcome. This paper reviews the most recent classification, pathophysiology and imaging findings in clinical research as well as the knowledge gained from studying genetic and lesional animal models of focal cortical dysplasia. This review of this recently gained knowledge will most likely help develop new research models and new therapeutic targets for patients with epilepsy associated with focal cortical dysplasia.

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.

Causes and consequences of gray matter heterotopia

The objective of this article is to review the pathophysiological bases of gray matter heterotopia and to appreciate their involvement in brain cortical development and functional consequences, namely epilepsy. The development of the cerebral cortex results from complex sequential processes including cell proliferation, cell migration, cortical organization, and formation of neuronal networks. Disruption of these steps yields different types of cortical malformations including gray matter heterotopia, characterized by the ectopic position of neurons along the ventricular walls or in the deep white matter. Cortical malformations are major causes of epilepsy, being responsible for up to 40% of drug-resistant epilepsy, and the cognitive level of affected patients varies from normal to severely impaired. This review reports data from human patients and animal models highlighting the genetic causes for these disorders affecting not only neuronal migration but also the proliferation of cortical progenitors. Therefore, gray matter heterotopias should not be considered as solely due to an abnormal neuronal migration and classifying them as such may be too restrictive. The review will also summarize literature data indicating that besides ectopic neurons, neighbor cortical areas also play a consistent role in epileptogenesis, supporting the notion that plastic changes secondary to the initial malformation are instrumental in the pathophysiology of epilepsy in affected patients.

Normotopic cortex is the major contributor to epilepsy in experimental double cortex

OBJECTIVE

Subcortical band heterotopia (SBH) is a cortical malformation formed when neocortical neurons prematurely stop their migration in the white matter, forming a heterotopic band below the normotopic cortex, and is generally associated with intractable epilepsy. Although it is clear that the band heterotopia and the overlying cortex both contribute to creating an abnormal circuit prone to generate epileptic discharges, it is less understood which part of this circuitry is the most critical. Here, we sought to identify the origin of epileptiform activity in a targeted genetic model of SBH in rats.

METHODS

Rats with SBH (Dcx-KD rats) were generated by knocking down the Dcx gene using shRNA vectors transfected into neocortical progenitors of rat embryos. Origin, spatial extent, and laminar profile of bicuculline-induced interictal-like activity on neocortical slices were analyzed by using extracellular recordings from 60-channel microelectrode arrays. Susceptibility to pentylenetetrazole-induced seizures was assessed by electrocorticography in head-restrained nonanesthetized rats.

RESULTS

We show that the band heterotopia does not constitute a primary origin for interictal-like epileptiform activity in vitro and is dispensable for generating induced seizures in vivo. Furthermore, we report that most interictal-like discharges originating in the overlying cortex secondarily propagate to the band heterotopia. Importantly, we found that in vivo suppression of neuronal excitability in SBH does not alter the higher propensity of Dcx-KD rats to display seizures.

INTERPRETATION

These results suggest a major role of the normotopic cortex over the band heterotopia in generating interictal epileptiform activity and seizures in brains with SBH.

Cortical Gray-White Matter Blurring and Cognitive Morbidity in Focal Cortical Dysplasia

Focal cortical dysplasia (FCD) is a malformation of cortical development that is associated with high rates of cognitive morbidity. However, the degree to which specific irregularities of dysplastic tissue directly impact cognition remains unknown. This study investigates the relationship between blurring of the cortical gray and white matter boundary on magnetic resonance imaging (MRI) and global cognitive abilities in FCD. Gray-white blurring (GWB) is quantified by sampling the non-normalized T1 image intensity contrast above and below the gray and white matter interface along the cortical mantle. Spherical averaging is used to compare resulting GWB for patients with histopathologically verified FCD with matched controls. Whole-brain correlational analyses are used to investigate the relationship between blurring and general cognitive abilities, controlling for epilepsy duration. Results show that cognitive performance is reduced in patients with FCD relative to controls. Patients show increased GWB in bilateral temporal, parietal, and frontal regions. Furthermore, increased GWB in these regions is linearly related to decreased cognition and mediates group differences in cognitive performance. These findings demonstrate that GWB is a marker of reduced cognitive efficiency in FCD that can potentially be used to probe general and domain-specific cognitive functions in other neurological disorders.

Dampened hippocampal oscillations and enhanced spindle activity in an asymptomatic model of developmental cortical malformations

Developmental cortical malformations comprise a large spectrum of histopathological brain abnormalities and syndromes. Their genetic, developmental and clinical complexity suggests they should be better understood in terms of the complementary action of independently timed perturbations (i.e., the multiple-hit hypothesis). However, understanding the underlying biological processes remains puzzling. Here we induced developmental cortical malformations in offspring, after intraventricular injection of methylazoxymethanol (MAM) in utero in mice. We combined extensive histological and electrophysiological studies to characterize the model. We found that MAM injections at E14 and E15 induced a range of cortical and hippocampal malformations resembling histological alterations of specific genetic mutations and transplacental mitotoxic agent injections. However, in contrast to most of these models, intraventricularly MAM-injected mice remained asymptomatic and showed no clear epilepsy-related phenotype as tested in long-term chronic recordings and with pharmacological manipulations. Instead, they exhibited a non-specific reduction of hippocampal-related brain oscillations (mostly in CA1); including theta, gamma and HFOs; and enhanced thalamocortical spindle activity during non-REM sleep. These data suggest that developmental cortical malformations do not necessarily correlate with epileptiform activity. We propose that the intraventricular in utero MAM approach exhibiting a range of rhythmopathies is a suitable model for multiple-hit studies of associated neurological disorders.

Neuronal migration and its disorders affecting the CA3 region

In this review, we focus on CA3 neuronal migration disorders in the rodent. We begin by introducing the main steps of hippocampal development, and we summarize characteristic hippocampal malformations in human. We then describe various mouse mutants showing structural hippocampal defects. Notably, genes identified in human cortical neuronal migration disorders consistently give rise to a CA3 phenotype when mutated in the mouse. We successively describe their molecular, physiological and behavioral phenotypes that together contribute to a better understanding of CA3-dependent functions. We finally discuss potential factors underlying the CA3 vulnerability revealed by these mouse mutants and that may also contribute to other human neurological and psychiatric disorders.

Altered spatial learning, cortical plasticity and hippocampal anatomy in a neurodevelopmental model of schizophrenia-related endophenotypes

Adult rats exposed to the DNA-methylating agent methylazoxymethanol on embryonic day 17 show a pattern of neurobiological deficits that model some of the neuropathological and behavioral changes observed in schizophrenia. Although it is generally assumed that these changes reflect targeted disruption of embryonic neurogenesis, it is unknown whether these effects generalise to other antimitotic agents administered at different stages of development. In the present study, neurochemical, behavioral and electrophysiological techniques were used to determine whether exposure to the antimitotic agent Ara-C later in development recapitulates some of the changes observed in methylazoxymethanol (MAM)-treated animals and in patients with schizophrenia. Male rats exposed to Ara-C (30 mg/kg/day) at embryonic days 19.5 and 20.5 show reduced cell numbers and heterotopias in hippocampal CA1 and CA2/3 regions, respectively, as well as cell loss in the superficial layers of the pre- and infralimbic cortex. Birth date labeling with bromodeoxyuridine reveals that the cytoarchitectural changes in CA2/3 are a consequence rather that a direct result of disrupted cortical neurogenesis. Ara-C-treated rats possess elevated levels of cortical dopamine and DOPAC (3,4-didyhydroxypheylacetic acid) but no change in norepinephrine or serotonin. Ara-C-treated rats are impaired in their ability to learn the Morris water maze task and showed diminished synaptic plasticity in the hippocampocortical pathway. These data indicate that disruption of neurogenesis at embryonic days 19.5 and 20.5 constitutes a useful model for the comparative study of deficits observed in other gestational models and their relationship to cognitive changes observed in schizophrenia.

Initiation of epileptiform activity in a rat model of periventricular nodular heterotopia

PURPOSE

Periventricular nodular heterotopia (PNH) is, in humans, often associated with difficult-to-control epilepsy. However, there is considerable controversy about the role of the PNH in seizure generation and spread. To study this issue, we have used a rat model in which injection of methylazoxymethanol (MAM) into pregnant rat dams produces offspring with nodular heterotopia-like brain abnormalities.

METHODS

Electrophysiologic methods were used to examine the activity of the MAM-induced PNH relative to activity in the neighboring hippocampus and overlying neocortex. Recordings were obtained simultaneously from these three structures in slice preparations from MAM-exposed rats and in intact animals. Bath application or systemic injection of bicuculline was used to induce epileptiform activity.

KEY FINDINGS

In the in vitro slice, epileptiform discharge was generally initiated in hippocampus. In some cases, independent PNH discharge occurred, but the PNH never "led" discharges in hippocampus or neocortex. Intracellular recordings from PNH neurons confirmed that these cells received synaptic drive from both hippocampus and neocortex, and sent axonal projections to these structures-consistent with anatomic observations of biocytin-injected PNH cells. In intact animal preparations, bicuculline injection resulted in epileptiform discharge in all experiments, with a period of ictal-like electrographic activity typically initiated within 2-3 min after drug injection. In almost all animals, the onset of ictus was seen synchronously across PNH, hippocampal, and neocortical electrodes; in a few cases, the PNH electrode (histologically confirmed) did not participate, but in no case was activity initiated in the PNH electrode. Interictal discharge was also synchronized across all three electrodes; again, the PNH never "led" the other two electrodes, and typically followed (onset several milliseconds after hippocampal/neocortical discharge onset).

SIGNIFICANCE

These results do not support the hypothesis that the PNH lesion is the primary epileptogenic site, since it does not initiate or lead epileptiform activity that subsequently propagates to other brain regions.

Impaired cognition in rats with cortical dysplasia: additional impact of early-life seizures

One of the most common and serious co-morbidities in patients with epilepsy is cognitive impairment. While early-life seizures are considered a major cause for cognitive impairment, it is not known whether it is the seizures, the underlying neurological substrate or a combination that has the largest impact on eventual learning and memory. Teasing out the effects of seizures from pre-existing neurological disorder is critical in developing therapeutic strategies. We therefore investigated the additional cognitive effects of seizures on rodents with malformations of cortical development induced with methylazoxymethanol acetate. Pregnant rats were injected with saline or methylazoxymethanol acetate at embryonic Day 15 or 17 to induce differing malformation severity. From the day of birth to 9 days of age, half the pups received 50 flurothyl-induced seizures. All rats underwent testing in the Morris water maze to test spatial memory at 25 days of age (immediate post-weaning) or during adolescence at 45 days of age. Post-weaning rats had severe spatial cognitive deficits in the water maze and seizures worsened performance. In contrast, in animals tested during adolescence, there was no longer an additional adverse effect of seizures. We also investigated whether the severity of the structural abnormality and seizures impacted brain weight, cortical thickness, hippocampal area and cell dispersion area. The mean brain weight in control animals was greater than in rats exposed to methylazoxymethanol acetate at embryonic Day 17, which was greater than rats exposed to methylazoxymethanol acetate at embryonic Day 15. Rats exposed to methylazoxymethanol acetate at embryonic Day 15 had a thinner cortical mantle compared with rats exposed at embryonic Day 17 and control animals. The hippocampal area was similar in rats exposed at embryonic Days 15 and 17 but was smaller compared with controls. Methylazoxymethanol at embryonic Day 17 caused dispersion of the CA1-4 cell layers in the hippocampus, whereas methylazoxymethanol at embryonic Day 15 caused focal nodules in or above the CA1 layer, but the CA1-4 layers were intact and similar to control. Early-life seizures did not have a significant impact on any of these parameters. These observations indicate that the major factor responsible for the cognitive impairment in the rats with cortical dysplasia was the underlying brain substrate, not seizures. These findings have significant implications for the understanding of cognitive impairments in childhood epilepsy and suggest that early aggressive therapy of seizures alone may not be an adequate strategy for minimizing cognitive effects.

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.

The etiological role of blood-brain barrier dysfunction in seizure disorders

A wind of change characterizes epilepsy research efforts. The traditional approach, based on a neurocentric view of seizure generation, promoted understanding of the neuronal mechanisms of seizures; this resulted in the development of potent anti-epileptic drugs (AEDs). The fact that a significant number of individuals with epilepsy still fail to respond to available AEDs restates the need for an alternative approach. Blood-brain barrier (BBB) dysfunction is an important etiological player in seizure disorders, and combination therapies utilizing an AED in conjunction with a “cerebrovascular” drug could be used to control seizures more effectively than AED therapy alone. The fact that the BBB plays an etiologic role in other neurological diseases will be discussed in the context of a more “holistic” approach to the patient with epilepsy, where comorbidity variables are also encompassed by drug therapy.

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.

Abnormal network activity in a targeted genetic model of human double cortex

In human patients, cortical dysplasia produced by Doublecortin (DCX) mutations lead to mental retardation and intractable infantile epilepsies, but the underlying mechanisms are not known. DCX(-/-) mice have been generated to investigate this issue. However, they display no neocortical abnormality, lessening their impact on the field. In contrast, in utero knockdown of DCX RNA produces a morphologically relevant cortical band heterotopia in rodents. On this preparation we have now compared the neuronal and network properties of ectopic, overlying, and control neurons in an effort to identify how ectopic neurons generate adverse patterns that will impact cortical activity. We combined dynamic calcium imaging and anatomical and electrophysiological techniques and report now that DCX(-/-)EGFP(+)-labeled ectopic neurons that fail to migrate develop extensive axonal subcortical projections and retain immature properties, and most of them display a delayed maturation of GABA-mediated signaling. Cortical neurons overlying the heterotopia, in contrast, exhibit a massive increase of ongoing glutamatergic synaptic currents reflecting a strong reactive plasticity. Neurons in both experimental fields are more frequently coactive in coherent synchronized oscillations than control cortical neurons. In addition, both fields displayed network-driven oscillations during evoked epileptiform burst. These results show that migration disorders produce major alterations not only in neurons that fail to migrate but also in their programmed target areas. We suggest that this duality play a major role in cortical dysfunction of DCX brains.

The effect of acute and chronic exposure to ethanol on the developing encephalon: a review

OBJECTIVES: to compare the acute and chronic effects of ethanol on the neural development, by analysis of the ontogenetic neural structure of mammals. METHODS: searches were performed in the following electronic databases: MEDLINE, SciElo, PubMed, LILACS, CAPES periodical, and the Open Journal System. The descriptors used were: "chronic ethanol toxicity", "chronic alcohol toxicity", "acute ethanol toxicity", "acute alcohol", "neural ontogenic development", "neuronal migration disturbances", "neural structure". The following inclusion criteria were used: articles published between 2003 and 2007, some classic articles in the field and an important neuropsychology textbook. RESULTS: the analysis of papers revealed that, although several studies of the chronic effects of ethanol exposure on the mammalian nervous system have been conducted, only a few have investigated the acute effects of ethanol on specific days of gestation, and these studies have revealed important disorders relating to the cerebral tissue. CONCLUSIONS: it should be recommended that women refrain from the consumption of ethanol during gestational phase to protect the fetus' health. Furthermore, the acute consumption of ethanol by women nearing the eighth or ninth week of gestation has been shown to be potentially harmful to the nervous tissue of the fetus.

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.

Neurobiological mechanisms of developmental epilepsy: translating experimental findings into clinical application

Although seizures are very common during early brain development, consequences of seizures during this age period are less severe than in the mature brain. Reasons for this discrepancy relate to both the sequential development of neural elements (ion channels, neurotransmitters, synapses, and circuits) and the effects of seizures on these ongoing processes at different ages. In this review, I critically discuss 2 recent experimental trends in developmental neurobiology that impact seizures and their consequences. First, the paradoxic excitatory effects of gamma-aminobutyric acid early in life are related to seizure susceptibility in this developmental period. Second, the plasticity of immature neuronal circuits and the effects of seizures on subsequent cognition and behavior as a function of age are considered. These topics are relevant to the pediatric neurologist when evaluating and treating a young child with seizures.

Functional interactions in brain networks underlying epileptic seizures in bilateral diffuse periventricular heterotopia

OBJECTIVE

Our aim was to investigate relationships between heterotopic and remote cortical structures at seizure initiation, in a patient with bilateral periventricular nodular heterotopias (BPNH) explored by intracerebral electrodes.

METHODS

Stereoelectroencephalography (SEEG) was performed in a man with BPNH and refractory epilepsy to investigate the hypothesis of right temporal lobe epilepsy and the possible involvement of heterotopic structures during seizures. SEEG signals were analyzed with quantification of functional coupling between different brain structures during seizures, using nonlinear regression. We have used Z-score transformation of correlation values to reflect the change from the preictal period. Relationships between BPNH and cortical structures were investigated using analysis of stimulation-induced potentials.

RESULTS

Three spontaneous seizures were recorded and analyzed. Signal analysis of interdependencies in two seizures demonstrated a large initial network involving both heterotopia and cortical structures. Stimulations of heterotopia induced responses in remote cortical structures.

CONCLUSIONS

Distinct epileptogenic networks were identified, in which leader structures were either the heterotopic or the mesial temporal structures, with functional connections between heterotopic and cortical areas.

SIGNIFICANCE

These results confirm that a vast epileptogenic network, including heterotopic and cortical neurons, may be responsible for seizure generation in BPNH. This may explain certain surgical failures in this group.

Neurotransmitters and brain maturation: early paracrine actions of GABA and glutamate modulate neuronal migration

Migration of neurons from their birthplace to their final destination is an extremely important step in brain maturation, and cortical migration disorders are the most common brain developmental alteration observed in human patients. Among the mechanisms that govern neuronal migration, the neurotransmitters GABA and glutamate deserve particular attention: 1) neurotransmitters and receptors are expressed early in the developing brain, 2) neurotransmitters may act as paracrine signaling molecules in the immature brain, and 3) neurotransmitters regulate intracellular calcium required for many cellular functions, including cytoskeletal dynamic changes. Thus, many reports reviewed here aimed to demonstrate that the activation of specific GABA and glutamate receptors is instrumental in cell migration by acting as motility promoting, acceleratory, or stop signal. Interestingly, the regulation of migration by neurotransmitters and receptors depends on the type of migration (radial, tangential, or chain migration), the type of cells (principal glutamatergic neurons vs. GABAergic interneurons), and the brain area (neocortex, cerebellum, rostral migratory stream). A hypothesis is proposed that these differential actions in different cell types arise from a "homeostatic-like" regulation that controls final position, timing, and number of cells at destination.

Altered glutamate receptor - transporter expression and spontaneous seizures in rats exposed to methylazoxymethanol in utero

PURPOSE

Brain malformations are a common cause of intractable epilepsy and cognitive dysfunction in children. Prenatal exposure to the teratogen methylazoxymethanol (MAM) is a rodent model of brain malformation featuring loss of lamination, clusters of displaced hippocampal cells, and pharmaco-resistance to antiepileptic drugs. In a normotopic hippocampus, expression of postsynaptic glutamate receptors and the transporters regulating neurotransmitter reuptake are critical factors modulating excitation and synaptic communication. Alterations in this system can have profound effects on overall excitability, cognitive function, and seizure thresholds.

METHODS

Immunohistochemical techniques were used to analyze the expression of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5 methylisoxazole-4-proprionic acid (AMPA) receptor subunits in rats exposed to MAM in utero (25 mg/kg, intraperitoneal injection). We also examined the expression of several glutamate transporters (EAAC1, vGLUT1, and vGLUT2). A video-electroencephalographic (video-EEG) system was used for long-term monitoring of adult MAM-exposed rats.

RESULTS

Heterotopic hippocampal neurons exhibited striking reductions in GluR1 and EAAC1 expression; vGlut2 expression was prominent in these regions. Spontaneous electrographic seizures were verified in two animals.

CONCLUSIONS

We conclude that glutamate receptor subunit and transporter expression are altered in animals exposed to MAM in utero. Further studies in the MAM model may provide greater insight into the potential disruptions in excitatory synaptic neurotransmission that can occur in a malformed brain.

Disruption of neuronal migration by RNAi of Dyx1c1 results in neocortical and hippocampal malformations

The brains of individuals with developmental dyslexia have neocortical neuronal migration abnormalities including molecular layer heterotopias, laminar dysplasias, and periventricular nodular heterotopias (PNH). RNA interference (RNAi) of Dyx1c1, a candidate dyslexia susceptibility gene, disrupts neuronal migration in developing embryonic neocortex. Using in utero electroporation, we cotransfected cells in the rat neocortical ventricular zone (VZ) at E14/15 with short hairpin RNA vectors targeting Dyx1c1 along with either plasmids encoding enhanced green fluorescent protein or plasmids encoding monomeric red fluorescent protein only. RNAi of Dyx1c1 resulted in pockets of unmigrated neurons resembling PNH. The pattern of migration of transfected neurons was bimodal, with approximately 20% of the neurons migrating a short distance from the VZ and another 40% that migrated past their expected lamina. Approximately 25% of the transfected brains had hippocampal pyramidal cell migration anomalies. Molecular layer ectopias, which were not related to injection site artifacts, were also seen in 25% of the animals. These results support the hypothesis that targeted disruption of the candidate dyslexia susceptibility gene, Dyx1c1, results in neuronal migration disorders similar to those seen in the brains of dyslexics.

Developmental disruptions and behavioral impairments in rats following in utero RNAi of Dyx1c1

Developmental malformations of cortex have been shown to co-occur with language, learning, and other cognitive deficits in humans. Rodent models have repeatedly shown that animals with such developmental malformations have deficits related to auditory processing and learning. More specifically, freeze-lesion induced microgyria as well as molecular layer ectopias have been found to impair rapid auditory processing ability in rats and mice. In humans, deficits in rapid auditory processing appear to relate to later impairments of language. Recently, genetic variants of four different genes involved in early brain development have been proposed to associate with an elevated incidence of developmental dyslexia in humans. Three of these, DYX1C1, DCDC2, and KIAA0319, have been shown by in utero RNAi to play a role in neuronal migration in developing neocortex. The present study assessed the effects of in utero RNAi of Dyx1c1 on auditory processing and spatial learning in rats. Results indicate that RNAi of Dyx1c1 is associated with cortical heterotopia and is suggestive of an overall processing deficit of complex auditory stimuli in both juvenile and adult periods (p=.051, one-tail). In contrast, adult data alone reveal a significant processing impairment among RNAi treated subjects compared to shams, indicating an inability for RNAi treated subjects to improve detection of complex auditory stimuli over time (p=.022, one-tail). Further, a subset of RNAi treated rats exhibited hippocampal heterotopia centered in CA1 (in addition to cortical malformations). Malformations of hippocampus were associated with robust spatial learning impairment in this sub-group (p<.01, two-tail). In conclusion, in utero RNAi of Dyx1c1 results in heterogeneous malformations that correspond to distinct behavioral impairments in auditory processing, and spatial learning.

Alterations in medial prefrontal cortical activity and plasticity in rats with disruption of cortical development

BACKGROUND

Psychiatric disorders such as schizophrenia are believed to emerge from an interaction of several factors. Thus, a genetic predisposition can lead to developmental compromises that may leave the system more susceptible to deficits induced by subsequent environmental variables such as stress.

METHODS

The impact of neurodevelopmental interruption induced by exposure of rats prenatally to a compound methylazoxymethanol acetate (MAM) that disrupts neuronal proliferation was investigated using in vivo electrophysiologic recordings from the prefrontal cortex of adult rats.

RESULTS

Prenatal exposure to MAM resulted in alterations in the medial prefrontal cortex indicative of a compromise in information processing. Specifically, we observed a disruption in activity patterns consistent with deficits in neuronal synchronization and abnormal augmentation of synaptic plasticity that was more severely disrupted by stress exposure than in normal animals. Furthermore, these deficits could be reversed by manipulating the mesocortical dopamine system.

CONCLUSIONS

These results suggest that disruption of early cortical development causes impairments in medial prefrontal cortical function at adulthood that are more vulnerable to disruptive influences, despite the presence of only subtle structural alterations in the brain.

A neurobehavioral systems analysis of adult rats exposed to methylazoxymethanol acetate on E17: implications for the neuropathology of schizophrenia

BACKGROUND

As a test of plausibility for the hypothesis that schizophrenia can result from abnormal brain, especially cerebral cortical, development, these studies examined whether, in the rat, disruption of brain development initiated on embryonic day (E) 17, using the methylating agent methylazoxymethanol acetate (MAM), leads to a schizophrenia-relevant pattern of neural and behavioral pathology. Specifically, we tested whether this manipulation leads to disruptions of frontal and limbic corticostriatal circuit function, while producing schizophrenia-like, region-dependent reductions in gray matter in cortex and thalamus.

METHODS

In offspring of rats administered MAM (22 mg/kg) on E17 or earlier (E15), regional size, neuron number and neuron density were determined in multiple brain regions. Spontaneous synaptic activity at prefrontal cortical (PFC) and ventral striatal (vSTR) neurons was recorded in vivio. Finally, cognitive and sensorimotor processes mediated by frontal and limbic corticostriatal circuits were assessed.

RESULTS

Adult MAM-E17-exposed offspring showed selective histopathology: size reductions in mediodorsal thalamus, hippocampus, and parahippocampal, prefrontal, and occipital cortices, but not in sensory midbrain, cerebellum, or sensorimotor cortex. The prefrontal, perirhinal, and occipital cortices showed increased neuron density with no neuron loss. The histopathology was accompanied by a disruption of synaptically-driven "bistable membrane states" in PFC and vSTR neurons, and, at the behavioral level, cognitive inflexibility, orofacial dyskinesias, sensorimotor gating deficits and a post-pubertal-emerging hyper-responsiveness to amphetamine. Earlier embryonic MAM exposure led to microcephaly and a motor phenotype.

CONCLUSIONS

The "MAM-E17" rodent models key aspects of neuropathology in circuits that are highly relevant to schizophrenia.

Postnatal effect of embryonic neurogenesis disturbance on reelin level in organotypic cultures of rat hippocampus

Despite a delayed emergence of the symptoms, schizophrenia is thought to be a late consequence of early disturbances during development. Several reports have found decreased levels of reelin in the cortex and the hippocampus of postmortem brains of schizophrenic patients. In the rat, intraperitoneal injection of the anti-mitotic agent methylazoxymethanol (MAM) during intra-uterine development (embryonic day 17) induces cytoarchitectural abnormalities in the hippocampus and the cortex and behavioural changes reminiscent of positive, negative and cognitive symptoms of schizophrenia. We aimed to examine whether a transient prenatal disturbance of neurogenesis induces postnatal changes in the expression of reelin in the hippocampus. Cellular modifications were explored using hippocampal organotypic slice cultures, which allow for conservation of the in vivo cytoarchitecture. MAM effect on hippocampal neurogenesis was confirmed by birthdating experiments. After 3 weeks in vitro, reelin was expressed by calretinin-negative cells. The number of reelin-positive neurons was increased whereas the total neuron number was decreased in the stratum oriens in the E17 MAM-exposed animals as compared to the control group. Not only an increase in the number of cells expressing reelin was observed, but there was also a slight increase in reelin mRNA levels in hippocampal pyramidal cells of MAM-exposed animals. In contrast, there was no significant change in the dentate gyrus. These results show that transient prenatal disturbance of neurogenesis induces long-term modifications in specific areas of the hippocampus and in particular in the number of neurons expressing reelin. They also confirm the value of organotypic slices to study postnatal maturation in the hippocampus.

Embryonic and early postnatal abnormalities contributing to the development of hippocampal malformations in a rodent model of dysplasia

While there are many recent examples of single gene deletions that lead to defects in cortical development, most human cases of cortical disorganization can be attributed to a combination of environmental and genetic factors. Elucidating the cellular or developmental basis of teratogenic exposures in experimental animals is an important approach to understanding how environmental insults at particular developmental junctures can lead to complex brain malformations. Rats with prenatal exposure to methylazoxymethanol (MAM) reproduce many anatomical features seen in epilepsy patients. Previous studies have shown that heterotopic clusters of neocortically derived neurons exhibit hyperexcitable firing activity and may be a source of heightened seizure susceptibility; however, the events that lead to the formation of these abnormal cell clusters is unclear. Here we used a panel of molecular markers and birthdating studies to show that in MAM-exposed rats the abnormal cell clusters (heterotopia) first appear postnatally in the hippocampus (P1-2) and that their appearance is preceded by a distinct sequence of perturbations in neocortical development: 1) disruption of the radial glial scaffolding with premature astroglial differentiation, and 2) thickening of the marginal zone with redistribution of Cajal-Retzius neurons to deeper layers. These initial events are followed by disruption of the cortical plate and appearance of subventricular zone nodules. Finally, we observed the erosion of neocortical subventricular zone nodules into the hippocampus around parturition followed by migration of nodules to hippocampus. We conclude that prenatal MAM exposure disrupts critical developmental processes and prenatal neocortical structures, ultimately resulting in neocortical disorganization and hippocampal malformations.

Pilocarpine-induced seizure susceptibility in rats following prenatal methylazoxymethanol treatment

Several rodent models of cortical malformation are available for the study of cellular mechanisms associated with early-onset epilepsy, but few are associated with spontaneous seizures. We examined the effect of pilocarpine on the spontaneous seizure development and excitability of the CA1 pyramidal cells of rats after prenatal treatment with methylazoxymethanol (MAM). Pilocarpine induced status epilepticus (SE) onset latency was greater for normal rats than for MAM-treated rats. After several days of normal behavior following pilocarpine treatment, the duration of spontaneous seizures were greater in MAM-pilocarpine rats than in normal-pilocarpine rats. Compared with the normal rats, electrical stimulation of afferent fibers resulted in more robust population responses in the CA1 region in all groups. At interstimulus intervals of 30 and 70 ms, the MAM-pilocarpine rats displayed a decrease in paired pulse inhibition versus the conventional MAM rats. A loss of somatostatin- and parvalbumin-immunoreactive neurons was apparent in the normal-pilocarpine rats, MAM-pilocarpine rats, and conventional MAM rats. These results indicate that pilocarpine induces spontaneous seizures and hyperexcitability in MAM-pilocarpine rats.

Initiation and spread of epileptiform discharges in the methylazoxymethanol acetate rat model of cortical dysplasia: functional and structural connectivity between CA1 heterotopia and hippocampus/neocortex

Neuronal migration disorders (NMDs) are often associated with medically intractable epilepsy. In utero injection of methylazoxymethanol acetate into pregnant rats gives rise to dysplastic cell clusters ("heterotopia") in hippocampus (and nearby regions), providing an animal model of NMD. In the present study, we have examined the structural and functional integration of hippocampal heterotopic cells into circuits that link the heterotopia with surrounding "normal" brain. Bi-directional morphological connectivity between the heterotopia and hippocampus/neocortex was demonstrated using the neurotracer, biotinylated dextran amine. Single cell recordings in hippocampal slices showed that heterotopia neurons form functional connections with the surrounding hippocampus and neocortex. However, simultaneous field recordings from the CA1 heterotopia, normotopic hippocampus, and neocortex indicated that epileptiform discharges (spontaneous events seen in slices bathed with high [K+]o and bicuculline) were rarely initiated in the heterotopia (although the heterotopia was capable of generating epileptiform discharges independently of normal brain regions). Further, in most of the experiments, the aberrant connectivity provided by CA1 heterotopia failed to function as a "bridge" for epileptiform discharges to propagate directly from low-threshold hippocampus to neocortex. These data do not support the hypothesis that NMDs (heterotopic cell populations) serve as a focus and/or trigger for epileptiform activity, and/or facilitate propagation of epileptiform events.

Effects of prenatal methylazoxymethanol acetate (MAM) treatment in rats on water maze performance

Prenatal methylazoxymethanol acetate (MAM) treatment has been shown to induce morphological abnormalities in cortical areas of the offspring. Based on the neuroanatomical and behavioural abnormalities, this treatment has been suggested as a useful animal model for schizophrenia. In a previous study (Jongen-Relo AL, Leng A, Luber M, Pothuizen HHJ, Weber L, Feldon J. The prenatal methylazoxymethanol acetate treatment: a neurodevelopmental animal model for schizophrenia? Behav Brain Res 2004;149:159-81) we have studied MAM-treated animals in a series of behavioural tests related to schizophrenia, such as latent inhibition and pre-pulse inhibition of the acoustic startle response to establish the validity of prenatal MAM treatment (20mg/kg i.p. on gestational days 9-15; MAM 9-MAM 15). We found that, apart from a marginal effect of increased activity in the open field, the MAM treatment on gestational day 15 was behaviourally ineffective. Here, we extended our previous study to a water maze experiment conducted in the same batch of animals as presented previously (MAM 12-MAM 15). MAM-treated animals showed similar water maze performance compared with control animals during the acquisition phase and the probe tests. However, during the reversal phase, MAM 15 animals showed impaired acquisition of the new platform location. This might indicate some cognitive deficits in MAM 15 animals in terms of working memory or behavioural flexibility. However, in combination with the lack of behavioural abnormalities of MAM 12-MAM 15 animals in several other tests related to schizophrenia in the previously reported study, the use of MAM treatment (MAM 12-MAM 15) as a valid model for schizophrenia still remains debatable.

Microdysgenesis in epilepsy

Microdysgenesis is a microscopic malformation of cortical development characterized by heterotopic neurones and abnormal cortical architecture. It has been described in primary generalized and partial epilepsy. Its significance in epileptogenesis is controversial, partly due to lack of consensus of diagnostic criteria. Different terms have also been used for the malformation. Several quantitative studies have been performed of the histopathological aberrations associated with microdysgenesis. A majority of the studies have revealed an increased number of heterotopic neurones in specimens from epilepsy patients. However, the quantitative values given for abnormal numbers of white matter neurones vary greatly between studies and there is no consensus yet on quantitative criteria for microdysgenesis. There have also been conflicting results from studies correlating microdysgenesis with clinical data. Both favourable and worse outcome after epilepsy surgery have been reported in patients with increased numbers of white matter neurones and microdysgenesis. While some studies have shown earlier seizure onset and increased frequency of mental retardation in patients with microdysgenesis, others have not. Differences in inclusion criteria and definition might contribute to the contradictory results. There is some evidence that microdysgenesis could be important in epileptogenesis, but the mechanisms involved remain unknown and difficult to investigate. A consensus on what histopathological criteria to use for the diagnosis of microdysgenesis is needed to explore this further and enable comparisons between centres. There are advantages and disadvantages both with quantitative stereological and with qualitative assessments. It is necessary to evaluate these in the decision on diagnostic criteria, if possible taking both qualitative and quantitative aspects into account.

Disruption of neurogenesis on gestational day 17 in the rat causes behavioral changes relevant to positive and negative schizophrenia symptoms and alters amphetamine-induced dopamine release in nucleus accumbens

Gestational disruption of neurodevelopment has been proposed to lead to pathophysiological changes similar to those underlying schizophrenia. We induced such disruption by treating pregnant rat dams with methylazoxymethanol acetate (MAM) on gestational day 17 (GD17). Total brain size and that of the prefrontal cortex and hippocampus were reduced in adult rats exposed prenatally to MAM. When locomotor activity was assessed in an open field, MAM-exposed rats were hyper-responsive to a mild stress and to amphetamine (2 mg/kg, s.c.). They also engaged in less social interaction than controls. We studied, by microdialysis, the effect of amphetamine on extracellular dopamine in the nucleus accumbens and the medial prefrontal cortex of freely moving control and MAM-exposed rats. Amphetamine (2 mg/kg, s.c.) induced an increase in dopamine release that was larger in the nucleus accumbens of MAM-exposed rats than in controls, whereas no difference was seen in the medial prefrontal cortex. In controls, amphetamine infused into the medial prefrontal cortex (50 microM) led to a slight decrease in extracellular dopamine in the nucleus accumbens. This effect was absent in MAM-exposed rats, where a transient increase in nucleus accumbens dopamine levels was seen after amphetamine infusion. These results show that the late gestational disruption of neurogenesis in the rat leads to behavioral changes that mimic positive and negative schizophrenia symptoms, and also to a dysregulation of subcortical dopamine neurotransmission. This study contributes to the evaluation of the validity of the prenatal MAM GD17 treatment in rats as an animal model for schizophrenia.

Excitability of CA1 neurons in the model of malformation-associated epilepsy

Experimentally induced heterotopia exhibit many of the anatomical features characteristic of cortical malformations in children with early-onset epilepsy. We used extracellular field potential recordings from the dorsal hippocampus of intact adult rats to determine whether the excitability of CA1 pyramidal cells was enhanced in rats with experimentally induced hippocampal dysplasia. Electrical stimulation of afferent fibers resulted in more robust population responses in the CA1 region of methylazoxymethanol (MAM)-treated rats vs the controls. The local population of CA1 pyramidal neurons was more excitable in the MAM-treated rat than in the control animals after synaptic activation. These results suggest that the excitability of the CA1 region in rats with hippocampal dysplasia is greater than that in control animals.

Cortical dysplasia and epilepsy: animal models

Cortical dysplasia syndromes--those conditions of abnormal brain structure/organization that arise during aberrant brain development--frequently involve epileptic seizures. Neuropathological and neuroradiological analyses have provided descriptions and categorizations based on gross anatomical and cellular histological features (e.g., lissencephaly, heterotopia, giant cells), as well as on the developmental mechanisms likely to be involved in the abnormality (e.g., cell proliferation, migration). Recently, the genes responsible for several cortical dysplastic conditions have been identified and the underlying molecular processes investigated. However, it is still unclear how the various structural abnormalities associated with cortical dysplasia are related to (i.e., "cause") chronic seizures. To elucidate these relationships, a number of animal models of cortical dysplasia have been developed in rats and mice. Some models are based on laboratory manipulations that injure the brain (e.g., freeze, undercut, irradiation, teratogen exposure) of immature animals; others are based on spontaneous genetic mutations or on gene manipulations (knockouts/transgenics) that give rise to abnormal cortical structures. Such models of cortical dysplasia provide a means by which investigators can not only study the developmental mechanisms that give rise to these brain lesions, but also examine the cause-effect relationships between structural abnormalities and epileptogenesis.

Grey and white matter flumazenil binding in neocortical epilepsy with normal MRI. A PET study of 44 patients

In 20-30% of potential surgical candidates with refractory focal epilepsy, standard MRI does not identify the cause. gamma-Aminobutyric acid (GABA) is the principal inhibitory neurotransmitter in the brain. [(11)C]Flumazenil (FMZ) PET images most subtypes of GABA(A) receptors, present on most neurons. We investigated [(11)C]FMZ binding in grey and white matter in 16 normal controls and in 44 patients with refractory neocortical focal epilepsy and normal optimal MRI. Fourteen patients had unilateral frontal lobe epilepsy, five occipital lobe epilepsy (OLE), six parietal lobe epilepsy (PLE) and 19 neocortical epilepsy that was not clearly lobar. Parametric images of FMZ volume of distribution (FMZ-V(d)) were computed. Statistical parametric mapping (SPM99) with explicit masking, including the white matter, was used to analyse individual patients and groups. Thirty-three of the 44 patients showed focal abnormal FMZ-V(d); increases in 16, decreases in eight, and both increases and decreases in nine. In seven patients, the increases in FMZ binding were periventricular, in locations normally seen in periventricular nodular heterotopia on MRI. There were frontal and parietal increases in FMZ binding in grey and white matter in the PLE group and decreases in the cingulate gyrus in the OLE group. FMZ binding increases, particularly periventricular increases, were a prominent feature of MRI-negative focal epilepsies and may represent neuronal migration disturbances.

Experimentally-induced microencephaly: effects on cortical neurons

Genetic and epigenetic factors may alter the normal development of cerebral cortex, producing laminar and cellular abnormalities and heterotopiae, major causes of juvenile, drug-resistant epilepsy. Experimentally-induced migration disorders provide interesting insights in the mechanisms of the determination of neuronal phenotype and connectivity, of congenital cortical dysgenesis and the pathophysiology of associated neurological disorders, such as epilepsy. We investigated the effects of E14 administration of methylazoxymethanol acetate (MAM), which induces microencephaly by ablating dividing cells. Brains from newborn and adult rats were reacted for NADPH-d and CO histochemistry. Moreover, callosally-projecting neurons were retrogradely labeled with DiI at P9 or with BDA in adults. MAM-treated rats displayed a remarkable reduction in cortical thickness, mainly due to reduction in layer IV and in supragranular layers. Heterotopic nodules appeared in the supragranular layers and in the hippocampus. CO-positive barrels in somatosensory cortex were almost absent. The distribution of NADPH-d-positive neurons was regular, but they were rare in heterotopic nodules. Callosally-projecting neurons displayed abnormal orientation of the apical dendrite and increase in the basal dendritic length. Alterations in the dendritic arborization of pyramidal neurons may be one of the substrates for the increased sensitivity to drugs which induce epileptic seizures in these animals.

Reduction of spontaneous inhibitory synaptic activity in experimental heterotopic gray matter

Neuronal heterotopia has a strong association with epilepsy, but the mechanisms that underlie this relationship are largely unknown. We have utilized the in utero irradiated rat model to study circuit abnormalities in experimentally induced subcortical heterotopic gray matter. Spontaneous and miniature inhibitory (IPSCs) and excitatory (EPSCs) postsynaptic currents were recorded from visualized heterotopic pyramidal neurons in in vitro hemispheric slices and compared with control neocortical pyramidal neurons using the whole cell patch-clamp technique. The frequency of spontaneous and miniature IPSCs was significantly reduced in pyramidal neurons from heterotopic cortex. Amplitude and kinetics of IPSCs were not different between the two groups. Spontaneous and miniature EPSCs were not different between the two groups. Short-term synaptic plasticity of stimulus-evoked EPSCs showed depression in heterotopic neurons and facilitation in control pyramidal neurons. This study shows a selective impairment of the GABAergic circuitry in experimental heterotopic gray matter. We have reported similar findings in normotopic dysplastic cortex from this model. Taken together, these studies demonstrate a pervasive defect in inhibition throughout the cortex of irradiated rats with cortical dysplasia and neuronal heterotopia. This may have important implications regarding cortical development and function following in utero injuries.

Abnormalities of grey and white matter [11C]flumazenil binding in temporal lobe epilepsy with normal MRI

In 20% of potential surgical candidates with refractory epilepsy, current optimal MRI does not identify the cause. GABA is the principal inhibitory neurotransmitter in the brain, and GABA(A) receptors are expressed by most neurones. [(11)C]Flumazenil (FMZ) PET images the majority of GABA(A) receptor subtypes. We investigated abnormalities of FMZ binding in grey and white matter in 18 patients with refractory temporal lobe epilepsy (TLE) and normal quantitative MRI. Parametric images of FMZ volume of distribution (FMZ-V(d)) were calculated. Twenty-one healthy controls were scanned for comparison. Statistical parametric mapping (SPM99) was used to localize significant changes in FMZ-V(d) in individual patients and between groups, specifically including the entire white matter in all subjects through explicit masking. Sixteen of 18 patients showed single or multiple abnormalities of FMZ-V(d). Six had hippocampal decreases of FMZ-V(d). Eleven patients showed increased FMZ-V(d) in the temporal lobe white matter (TLWM). Outside the mesial temporal structures, seven showed multiple areas of increase or decrease and only one a single area of decrease. In seven of the 16 patients with abnormalities, findings were concordant with EEG and clinical data, enabling further presurgical evaluation. Group findings were: (i) decreased FMZ-V(d) in the ipsilateral (Z = 3.01) and contralateral (Z = 2.56) hippocampus; (ii) increased FMZ-V(d) in the ipsilateral (Z = 3.71) and contralateral TLWM (two clusters, Z = 3.11 and 2.79); and (iii) increased FMZ-V(d) in the ipsilateral frontal lobe white matter between the superior and medial frontal gyrus (Z = 3.80) with similar changes contralaterally (Z = 4.87). No changes were found in the thalamus and basal ganglia. Region-of-interest analyses indicated an average increase in FMZ binding of 16% in the TLWM ipsilateral to the epileptic focus. PET findings were corroborated by invasive EEG or pathology in five cases. FMZ-PET, analysed by SPM with explicit masking, was sensitive in patients with normal MRI, and hippocampal abnormalities were detected in a third of these patients. Furthermore, increases in FMZ binding in TLWM, indicating microdysgenesis, were detected in the majority of these patients and may represent the structural basis of their epilepsy.

Cortical dysgenesis and epilepsy

Cortical dysgenesis (CD) describes a wide spectrum of brain anomalies that involve abnormal development of the cerebral cortex. There is a strong association between CD and epilepsy, and it comprises a significant proportion of children and adults whose epilepsy cannot be controlled with medications. There has been intense effort to define the relationship between CD and epilepsy so that more effective therapies can be devised. These efforts have ranged from detailed study of people with CD and epilepsy from a clinical standpoint to single-cell analysis of mRNA expression and postsynaptic receptor function. Animal models have also been developed to mimic certain aspects of CD in a situation when quantitative, controlled, and interventional experiments can be performed that would not be possible in a clinical setting. This review will give an overview of human CD syndromes and their causes, when possible, and describe some specific abnormalities in dysplastic cortex that may underlie its epileptogenic potential. It will also review several animal models of CD that have been studied mechanistically from the standpoint of epileptogenesis. In conclusion, some general trends will be proposed based on human and animal studies to encapsulate our current understanding of CD and how it causes epilepsy.

Hippocampal heterotopia lack functional Kv4.2 potassium channels in the methylazoxymethanol model of cortical malformations and epilepsy

Human cortical malformations often result in severe forms of epilepsy. Although the morphological properties of cells within these malformations are well characterized, very little is known about the function of these cells. In rats, prenatal methylazoxymethanol (MAM) exposure produces distinct nodules of disorganized pyramidal-like neurons (e.g., nodular heterotopia) and loss of lamination in cortical and hippocampal structures. Hippocampal nodular heterotopias are prone to hyperexcitability and may contribute to the increased seizure susceptibility observed in these animals. Here we demonstrate that heterotopic pyramidal neurons in the hippocampus fail to express a potassium channel subunit corresponding to the fast, transient A-type current. In situ hybridization and immunohistochemical analysis revealed markedly reduced expression of Kv4.2 (A-type) channel subunits in heterotopic cell regions of the hippocampus of MAM-exposed rats. Patch-clamp recordings from visualized heterotopic neurons indicated a lack of fast, transient (I(A))-type potassium current and hyperexcitable firing. A-type currents were observed on normotopic pyramidal neurons in MAM-exposed rats and on interneurons, CA1 pyramidal neurons, and cortical layer V-VI pyramidal neurons in saline-treated control rats. Changes in A-current were not associated with an alteration in the function or expression of delayed, rectifier (Kv2.1) potassium channels on heterotopic cells. We conclude that heterotopic neurons lack functional A-type Kv4.2 potassium channels and that this abnormality could contribute to the increased excitability and decreased seizure thresholds associated with brain malformations in MAM-exposed rats.