Cortical malformations and epilepsy: new insights from animal models

Susceptibility of immature and adult brains to seizure effects

The extent that status epilepticus (SE), but also brief seizures, affects neuronal structure and function has been the subject of much clinical and experimental research. There is a reliance on findings from animal research because there have been few prospective clinical studies. This review suggests that the features of seizure-induced injury in the immature brain compared with the adult brain are different and that duration of seizures (SE versus brief), number of seizures, cause of seizures, presence of pre-existing abnormalities, and genetics affect the injury. Increased awareness of age-specific injuries from seizure has promoted research to determine the circumstances under which seizures may produce permanent detrimental effects. Together with recent advances in functional neuroimaging, genomic investigation, and prospective human data, these studies are likely to substantially increase our knowledge of seizure-induced injury, leading to the development of improved algorithms for prevention and treatment of epilepsy.

Excitatory and inhibitory postsynaptic currents in a rat model of epileptogenic microgyria

Developmental cortical malformations are common in patients with intractable epilepsy; however, mechanisms contributing to this epileptogenesis are currently poorly understood. We previously characterized hyperexcitability in a rat model that mimics the histopathology of human 4-layered microgyria. Here we examined inhibitory and excitatory postsynaptic currents in this model to identify functional alterations that might contribute to epileptogenesis associated with microgyria. We recorded isolated whole cell excitatory postsynaptic currents and GABA(A) receptor-mediated inhibitory currents (EPSCs and IPSCs) from layer V pyramidal neurons in the region previously shown to be epileptogenic (paramicrogyral area) and in homotopic control cortex. Epileptiform-like activity could be evoked in 60% of paramicrogyral (PMG) cells by local stimulation. The peak conductance of both spontaneous and evoked IPSCs was significantly larger in all PMG cells compared with controls. This difference in amplitude was not present after blockade of ionotropic glutamatergic currents or for miniature (m)IPSCs, suggesting that it was due to the excitatory afferent activity driving inhibitory neurons. This conclusion was supported by the finding that glutamate receptor antagonist application resulted in a significantly greater reduction in spontaneous IPSC frequency in one PMG cell group (PMG(E)) compared with control cells. The frequency of both spontaneous and miniature EPSCs was significantly greater in all PMG cells, suggesting that pyramidal neurons adjacent to a microgyrus receive more excitatory input than do those in control cortex. These findings suggest that there is an increase in numbers of functional excitatory synapses on both interneurons and pyramidal cells in the PMG cortex perhaps due to hyperinnervation by cortical afferents originally destined for the microgyrus proper.

Plasticity of interneuronal species diversity and parameter variance in neurological diseases

Interneuronal diversity reflects the division of labor between numerous highly specialized interneuronal species, each performing a set of specific functions in neuronal networks. The rich diversity of interneurons found in the normal healthy brain is often significantly altered in neurological and psychiatric diseases. In genetic and developmental disorders, the diversity of interneuronal networks is compromised because of disturbances in the generation, specification or migration of specific interneuronal subtypes. Following insults related to trauma and seizures, the relative abundance of interneuronal subtypes might change, and entire interneuronal species can be lost from the network. In addition to the complete or partial loss of interneuronal subgroups, heterogeneity can also be altered in more subtle ways, as a result of changes in cell-to-cell variance of a particular parameter within specific interneuronal populations. Computational and experimental studies show that alterations in cellular and synaptic GABAergic heterogeneity can significantly modulate both firing rates and network coherence, indicating that plasticity of interneuronal diversity is likely to be an important mechanistic component of malfunctioning cortical networks in many pathological states.

Clinical and experimental studies of epilepsy associated with focal cortical dysplasia

The results of clinical and experimental studies on epilepsy associated with focal cortical dysplasia (FCD) are presented. We have been interested in the findings of abnormal increases in the numbers of small vessels in specimens of FCD resected from epilepsy patients. In the clinical study of 13 patients with epilepsy, specimens of FCD or dysembryoplastic neuroepithelial tumor (DNT) were examined using immunohistochemistry. The number of vessels in both lesions were greater than those in cortical specimens of autopsy cases without epilepsy. Because the vessels showed negative staining of VEGF, it was thought that the phenomenon of increase in the number of vessels was simply a hypervascularity, not a neovascularity. The local hypervascularity was expected to show local hyperperfusion in CBF-SPECT study, but interictal SPECT demonstrated local hypoperfusion and ictal SPECT showed hyperperfusion. This may have been caused by a functional change in those vessels. In the experimental study, we tried to make a new animal model of FCD to study epileptogenicity of FCD. When kainic acid had been infused into the neocortex in the neonatal rats, FCD was induced in adult Wistar rats. Histopathological examination revealed cortical dyslamination and abnormal neurons. On EEG, local spike bursts were elicited from the lesions, however, clinical seizures were not detected. Although the data are preliminary and observation over a longer period is required to determine whether spontaneous seizures will occur in this model, it is expected that this new model will be useful for studying epilepsy associated with FCD.

Severity of Histopathologic Abnormalities and In Vivo Epileptogenicity in the In Utero Radiation Model of Rats Is Dose Dependent

PURPOSE

Malformations of cortical development (MCDs) are a frequent cause of refractory epilepsy in humans. The in utero radiation model in rats shares many clinical and histopathologic characteristics with human MCDs. Previous studies reported the presence of clinical seizures in radiated rats, but also suggested a dose-dependent differential effect.

METHODS

Time-pregnant Sprague-Dawley rats were irradiated on embryonic day E17 with 100 cGy (low dose), 145 cGy (medium dose), 175 cGy (high dose), or were left untreated. Their adult litters were implanted with bifrontal epidural and hippocampal depth electrodes and underwent long-term video-EEG monitoring. After 2 weeks of monitoring, the animals were killed and their brains processed for histological studies.

RESULTS

Spikes were most frequently found in the rats that were subjected to low- and medium-dose radiation at E17 and were less frequently seen in the animals that were subjected to high-dose radiation. No interictal spikes were found in any of the control animals. Seizures were recorded in three of five animals of the medium-dose group. Histological studies showed a dose-dependent decrease in cortical thickness as well as an increase of cortical and hippocampal disorganization.

CONCLUSIONS

In vivo epileptogenicity in radiated animals was present only in mild or moderate MCD. No in vivo epileptogenicity was seen in severe radiation-induced MCD.

MAOA knockout mice are more susceptible to seizures but show reduced epileptogenesis

The role of elevated neuroactive amine exposure during embryonic and early postnatal development on seizure threshold and epileptogenesis was examined using both electrical and pentylenetetrazol (PTZ) kindling in monoamine oxidase A knockout (MAO(A) KO) mice and their wildtype, parental strain (C3H). In the first experiment permanent bilateral electrodes were implanted in the amygdala of both C3H and MAO(A) KO mice. The mice had their afterdischarge threshold determined and then seizures were kindled daily for a total of 20 days. We observed that the MAO(A) KO mice had lower afterdischarge thresholds and less severe seizures compared to the C3H mice. In the second experiment, seizures were elicited in experimentally naive mice using 50mg/kg of PTZ once daily for 7 days. We observed that the MAO(A) KO mice had shorter latencies to the onset of the first seizure, shorter total duration of seizures and fewer seizures per day. Overall the results of both experiments suggest that MAO(A) KO mice have an increased susceptibility to seizures, but are more resistant to epileptogenesis. We conclude that the high levels of neuroactive amines in the MAO(A) KO mice reorganize the brain to make the mice more susceptible to seizures but the remaining high levels of serotonin and norepinephrine likely inhibit epileptogenesis.

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.

Epileptic and imaging findings in perinatal hypoxic-ischemic encephalopathy with ulegyria

Hypoxic-ischemic encephalopathy due to fetal or neonatal asphyxia is a major cause of acute mortality and chronic disability involving cerebral palsy, seizures, and mental retardation. The gestational age of the infant is one of the main variables determining the neuropathological picture of hypoxic-ischemic brain injury, and ulegyria (one of its neuropathological correlates) typically affects full-term infants. The damage usually involves the deeper sulcal portion of the convolutions while sparing the crowns, and includes subcortical white matter atrophy and gliosis. The aim of this study was to characterize the electroclinical features of hypoxic-ischemic encephalopathy when ulegyria is one of its main neuropathological features. To this end, nine patients with MRI-proven ulegyria and epilepsy underwent a complete neurological work-up. The ulegyric lesions were mainly distributed in the parasagittal watershed areas and frequently associated with other hypoxic-ischemic lesions. The neurological picture was characterized in most patients by mental retardation, motor deficits, and drug-refractory partial epilepsy. The ulegyria in our patients was associated with a complex clinical picture: epilepsy was a prominent component, and its severity directly correlated with the extent of the ulegyria and the associated hypoxic-ischemic lesions. Drug refractoriness was an almost constant correlate of this form of symptomatic epilepsy.

Distribution of glutamate receptor subunits in experimentally induced cortical malformations

Electrophysiological studies in humans and animal models have revealed an intrinsic epileptogenicity of cortical dysplasias which are a frequent cause of drug-resistant epilepsy. An imbalance of inhibition and excitation has been causative related. Receptor-binding studies in rodents demonstrated reduced binding to GABA and increased binding to glutamate receptors within cortical dysplasias and increments of AMPA- and kainate-receptor binding in its surround. Immunohistochemically a differential downregulation of GABA(A) receptor subunits could be demonstrated in widespread areas within and around dysplasias. As receptor binding critically depends on receptor subunit composition the observed changes in binding properties might be related to this. Here, we immunohistochemically analyzed the regional expression of four NMDA receptor subunits and two major AMPA- and kainate-receptor complexes in adult rats after neonatal freeze lesions. These lesions are characterized by a three- to four-layered cortex and a microsulcus which mimic human polymicrogyria. Using antibodies against NR1, NR2A, NR2B, NR2D, GluR2,3, and GluR5,6,7 receptor subunits we demonstrated a pronounced disturbance of cortical immunostaining pattern in the cortical malformation. These changes reflected the structural disorganization of the microgyrus with some distortion of the apical dendrites of paramicrogyral pyramidal cells, a decrease and disorganization of cells at the bottom of the microsulcus, and an inflection of apical dendrites toward the microsulcus. The neuronal staining pattern of large pyramidal cells in the neighborhood of the dysplasia did not differ for any subunit investigated. No remote or widespread changes of glutamate-receptor subunit distribution could be detected. The lack of gross and/or widespread alterations of glutamate-receptor subunit distribution in the surround of focal cortical dysplasia suggests the presence of other or additional mechanisms underlying the increased excitatory neurotransmitter binding and excitability in cortical malformations.

The NMDA receptor complex is altered in an animal model of human cerebral heterotopia

Double intraperitoneal injections of methylazoxymethanol (MAM) in pregnant rats induce developmental brain dysgenesis with nodular heterotopia similar to human periventricular nodular heterotopia (PNH) and composed of hyperexcitable neurons. Here we analyzed the NMDA receptor complex and associated proteins in the heterotopic neurons of 2- to 3-month-old MAM-treated rats by means of a combined immunocytochemical/molecular approach. Our data demonstrated a clear reduction of p286-active form of alphaCaMKII and a selective impairment of both the targeting and the CaMKII-dependent phosphorylation of NR2A/B subunits in the postsynaptic membranes of the MAM-induced heterotopia. The reduced NR2A/B immunofluorescence of the cellular membrane was not due to reduced expression since it was decreased only in postsynaptic fractions but not in the homogenate. NMDA-NR1 and AMPA-GluR2/3 subunits, as well as PSD-95 and total alphaCaMKII protein levels, were not affected in MAM-treated rats, thus revealing that the overall composition of the postsynaptic fraction was not altered. These data clearly suggest that the molecular organization of the NMDA/alphaCaMKII complex is selectively altered in the postsynaptic compartment of heterotopic neurons. This alteration can play a role in determining the hyperexcitability of brain heterotopia in MAM rats as well as in human patients affected by PNH.

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.

Deductive genomics: a functional approach to identify innovative drug targets in the post-genome era

The sequencing of the human genome has generated a drug discovery process that is based on sequence analysis and hypothesis-driven (inductive) prediction of gene function. This approach, which we term inductive genomics, is currently dominating the efforts of the pharmaceutical industry to identify new drug targets. According to recent studies, this sequence-driven discovery process is paradoxically increasing the average cost of drug development, thus falling short of the promise of the Human Genome Project to simplify the creation of much needed novel therapeutics. In the early stages of discovery, the flurry of new gene sequences makes it difficult to pick and prioritize the most promising product candidates for product development, as with existing technologies important decisions have to be based on circumstantial evidence that does not strongly predict therapeutic potential. This is because the physiological function of a potential target cannot be predicted by gene sequence analysis and in vitro technologies alone. In contrast, deductive genomics, or large-scale forward genetics, bridges the gap between sequence and function by providing a function-driven in vivo screen of a highly orthologous mammalian model genome for medically relevant physiological functions and drug targets. This approach allows drug discovery to move beyond the focus on sequence-driven identification of new members of classical drug-able protein families towards the biology-driven identification of innovative targets and biological pathways.

Outcome of epilepsy surgery in focal cortical dysplasia

OBJECTIVE

To describe the outcome of surgery in patients with drug resistant epilepsy and a histopathological diagnosis of focal cortical dysplasia.

METHODS AND SUBJECTS

Analysis of histories and presurgical and follow up data was carried out in 53 patients with a histological diagnosis of focal cortical dysplasia. Their mean age was 24.0 years (range 5 to 46), and they included 14 children and adolescents. Mean age at seizure onset was 12.4 years (0.4 to 36) and mean seizure duration was 11.6 years (1 to 45).

RESULTS

The presurgical detection rate of focal cortical dysplasia with magnetic resonance imaging (MRI) was 96%. There were 24 temporal and 29 extratemporal resections; additional multiple subpial transections were done in 12 cases to prevent spread of seizure discharges. There was a 6% rate of complications with permanent neurological deficit, but no deaths. All resected specimens were classified by neuropathological criteria as focal cortical dysplasia. Balloon cells were seen in most cases of extratemporal focal cortical dysplasia. After a mean follow up of 50 months, 38 patients (72%) were seizure-free, two (4%) had less than two seizures a year, nine (17%) had a reduction of seizure frequency of more than 75%, and four (8%) had no improvement. Seizure outcome was similar after temporal and extratemporal surgery. The patients in need of multilobar surgery had the poorest outcome.

CONCLUSIONS

Circumscribed lesionectomy of focal dysplastic lesions provides seizure relief in patients with chronic drug resistant temporal and extratemporal epilepsy. There was a trend for the best seizure outcome to be in patients with early presurgical evaluation and early surgery, and in whom lesions were identified on the preoperative MRI studies.

Genetic disruption of cortical interneuron development causes region- and GABA cell type-specific deficits, epilepsy, and behavioral dysfunction

The generation of properly functioning circuits during brain development requires precise timing of cell migration and differentiation. Disruptions in the developmental plan may lead to neurological and psychiatric disorders. Neocortical circuits rely on inhibitory GABAergic interneurons, the majority of which migrate from subcortical sources. We have shown that the pleiotropic molecule hepatocyte growth factor/scatter factor (HGF/SF) mediates interneuron migration. Mice with a targeted mutation of the gene encoding urokinase plasminogen activator receptor (uPAR), a key component in HGF/SF activation and function, have decreased levels of HGF/SF and a 50% reduction in neocortical GABAergic interneurons at embryonic and perinatal ages. Disruption of interneuron development leads to early lethality in most models. Thus, the long-term consequences of such perturbations are unknown. Mice of the uPAR-/- strain survive until adulthood, and behavior testing demonstrates that they have an increased anxiety state. The uPAR-/- strain also exhibits spontaneous seizure activity and higher susceptibility to pharmacologically induced convulsions. The neocortex of the adult uPAR-/- mouse exhibits a dramatic region- and subtype-specific decrease in GABA-immunoreactive interneurons. Anterior cingulate and parietal cortical areas contain 50% fewer GABAergic interneurons compared with wild-type littermates. However, interneuron numbers in piriform and visual cortical areas do not differ from those of normal mice. Characterization of interneuron subpopulations reveals a near complete loss of the parvalbumin subtype, with other subclasses remaining intact. These data demonstrate that a single gene mutation can selectively alter the development of cortical interneurons in a region- and cell subtype-specific manner, with deficits leading to long-lasting changes in circuit organization and behavior.

Sox1-deficient mice suffer from epilepsy associated with abnormal ventral forebrain development and olfactory cortex hyperexcitability

Mutations in several classes of embryonically-expressed transcription factor genes are associated with behavioral disorders and epilepsies. However, there is little known about how such genetic and neurodevelopmental defects lead to brain dysfunction. Here we present the characterization of an epilepsy syndrome caused by the absence of the transcription factor SOX1 in mice. In vivo electroencephalographic recordings from SOX1 mutants established a correlation between behavioral changes and cortical output that was consistent with a seizure origin in the limbic forebrain. In vitro intracellular recordings from three major forebrain regions, neocortex, hippocampus and olfactory (piriform) cortex (OC) showed that only the OC exhibits abnormal enhanced synaptic excitability and spontaneous epileptiform discharges. Furthermore, the hyperexcitability of the OC neurons was present in mutants prior to the onset of seizures but was completely absent from both the hippocampus and neocortex of the same animals. The local inhibitory GABAergic neurotransmission remained normal in the OC of SOX1-deficient brains, but there was a severe developmental deficit of OC postsynaptic target neurons, mainly GABAergic projection neurons within the olfactory tubercle and the nucleus accumbens shell. Our data show that SOX1 is essential for ventral telencephalic development and suggest that the neurodevelopmental defect disrupts local neuronal circuits leading to epilepsy in the SOX1-deficient mice.

The Wakayama epileptic rat (WER), a new mutant exhibiting tonic-clonic seizures and absence-like seizures

A new mutant, the Wakayama epileptic rat (WER), exhibiting both spontaneous absence-like behavior and tonic-clonic convulsions, was identified in a colony of Wistar rats. To determine clear seizure characteristics of this mutant strain, we analyzed the mode of inheritance of the convulsion and observed patterns of electroencephalogram (EEG) during the seizures. F1 progeny were produced between the founder male and normal females of the same colony. Animals were monitored through the inbreeding course to analyze genetic control of epileptic behavior. EEGs were recorded using affected animals in the F3-4 and post F13 generations. After the F2 generation, affected rats spontaneously exhibited both absence-like immobile behavior and tonic-clonic convulsions. The absence-like seizures were characterized by motor arrest and head droop. The tonic-clonic convulsions began with neck and forelimb clonus, wild jumping/running, and opisthotonic posturing, and evolved to tonic, then clonic convulsions. Most convulsion onsets occurred between 25-70 days of age. Mating experiments revealed that 0%(0/18) of the animals in F1, 10%(3/26) in F2, 17%(1/6) in backcross progeny and 86% (100/116) in progeny of crosses between epileptic rats showed tonic-clonic convulsions. Ictal cortical EEGs were characterized by 4-6 (5.1 +/- 0.4, mean +/- SD) Hz spike-and-wave complexes in the absence-like seizures and by low-voltage fast waves in the tonic-clonic convulsions. This new mutant rat spontaneously exhibited both absence-like and tonic-clonic seizures. The tonic-clonic seizure was inherited as an autosomal recessive trait with 86% incidence. Thus, the new mutant rat may become a useful model for studying human inherited epilepsies.

Association of family history of epilepsy with earlier age at seizure onset in patients with focal cortical dysplasia

OBJECTIVE

To establish the contribution of family history of epilepsy to seizure onset in patients with focal cortical dysplasia (FCD).

PATIENTS AND METHODS

From January 1998 to January 2001, we prospectively evaluated 19 consecutive patients (10 male, 9 female) with a diagnosis of FCD based on magnetic resonance imaging. All patients and at least 1 family member were directly interviewed by the same observer after completion of a semistructured questionnaire. Initially, we classified patients into 2 groups: presence or absence of family history of epilepsy. Patients with a family history of epilepsy were subdivided into 2 groups: patients with a family history of epilepsy in first-degree relatives or multiple relatives (n=5) and patients with a family history of epilepsy in relatives who were not first-degree (n=4). Statistical analysis was performed with use of the nonparametric tests Kruskal-Wallis and Kaplan-Meier (survival analysis). P=.05 was considered statistically significant.

RESULTS

The ages of the patients ranged from 3 to 41 years (mean, 15.6 years). All patients had similar type and extent of cortical dysgenesis. Ages at seizure onset varied from 1 month to 22 years, with a mean of 5.8 years. Nine patients had a family history of epilepsy. The mean age at the first seizure in patients with a family history of epilepsy was 2.6 years compared with 8.5 years in those with no relatives having epilepsy (P=.02). When patients with a family history of epilepsy were classified further, the mean age at first seizure was 1.9 years for patients with a family history of epilepsy in first-degree or multiple relatives and 3.9 years for patients with a family history of epilepsy in relatives who were not first-degree compared with 8.5 years for patients with no family history of epilepsy (P=.04).

CONCLUSION

Our results show that a family history of epilepsy is associated with an earlier age at seizure onset in patients with FCD. Although this is a preliminary finding and a larger sample is needed to confirm these results, we believe these observations provide evidence that genetic modifiers could become an important issue in the clinical presentation of patients with dysplastic lesions.

Infantile spasms: criteria for an animal model

Infantile spasms is an epilepsy syndrome with several distinctive features, including age specificity during infancy, characteristic semiology (epileptic spasms), specific electroencephalographic patterns (interictal hypsarrhythmia and ictal voltage suppression), and responsiveness to the adrenocorticotropic hormone (ACTH). There is no adequate animal model of infantile spasms, perhaps due to these clinically unique features, that is specific for the developing human brain. An informative animal model would provide insights into the pathophysiology of this syndrome and form the basis for the development of innovative therapies. This chapter considers criteria for an "ideal" animal model of infantile spasms, as well as "minimal" criteria that we consider essential to yield useful information. Two animal models of infantile spasms have been described in rodents: seizures induced by corticotropin-releasing factor and N-methyl-D-aspartic acid. Neither of these models conforms exactly to the human analog, but each possesses intriguing similarities that provide testable hypotheses for future investigations.

Brain malformations, epilepsy, and infantile spasms

The models of cortical dysplasia discussed earlier--the Lis1 knockout, the MAM-induced cobblestone LIS, the spontaneous tish mutant, and focal freeze injury-induced PMG--illustrate several important insights into epileptogenesis in malformed brain. First, the appearance of epilepsy varies according to the pathogenesis of the dysplasia and may well depend more on the intrinsic properties of the neurons in these models rather than on the disturbed position of the cells. This is supported by models such as the reeler mouse, in which the dysfunctional extracellular matrix molecule leads to a form of lissencephaly in mouse and human, but there is a far less impressive association with seizures than for LIS1 mutations. However, Lis1 and Dex mutations that appear to affect the cytoskeleton and perhaps intracellular protein trafficking are frequently associated with infantile spasms and epilepsy. Second, the possible mechanisms of epileptogenesis in these models include (a) a loss of subsets of neurons, (b) altered neurotransmitter release, (c) differences in neurotransmitter receptor levels and changes in receptor subunit composition, (d) altered neurite density and/or synaptogenesis, (e) changed membrane properties (e.g., altered voltage-gated channels), (f) altered cell morphology (neuronal differentiation), and (g) effects on cytoskeletal function. Finally, it is important to note that the "generator" of excitability in affected brain may be within the heterotopia or in the normotopic cortex. As additional genetic models come to light and the ability to distinguish their clinical counterparts improves, more individually tailored therapies, including standards for surgical interventions, will surely evolve.

NPY sensitivity and postsynaptic properties of heterotopic neurons in the MAM model of malformation-associated epilepsy

Neuronal migration disorders (NMDs) can be associated with neurological dysfunction such as mental retardation, and clusters of disorganized cells (heterotopias) often act as seizure foci in medically intractable partial epilepsies. Methylazoxymethanol (MAM) treatment of pregnant rats results in neuronal heterotopias in offspring, especially in hippocampal area CA1. Although the neurons in dysplastic areas in this model are frequently hyperexcitable, the precise mechanisms controlling excitability remain unclear. Here, we used IR-DIC videomicroscopy and whole cell voltage-clamp techniques to test whether the potent anti-excitatory actions of neuropeptide Y (NPY) affected synaptic excitation of heterotopic neurons. We also compared several synaptic and intrinsic properties of heterotopic, layer 2-3 cortical, and CA1 pyramidal neurons, to further characterize heterotopic cells. NPY powerfully inhibited synaptic excitation onto normal and normotopic CA1 cells but was nearly ineffective on responses evoked in heterotopic cells from stimulation sites within the heterotopia. Glutamatergic synaptic responses on heterotopic cells exhibited a comparatively small, D-2-amino-5-phosphopentanoic acid-sensitive, N-methyl-D-aspartate component. Heterotopic neurons also differed from normal CA1 cells in postsynaptic membrane currents, possessing a prominent inwardly rectifying K(+) current sensitive to Cs(+) and Ba(2+), similar to neocortical layer 2-3 pyramidal cells. CA1 cells instead had a prominent Cs(+)- and 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride-sensitive I(h) and negligible inward rectification, unlike heterotopic cells. Thus heterotopic CA1 cells appear to share numerous physiological similarities with neocortical neurons. The lack of NPY's effects on intra-heterotopic inputs, the small contribution of I(h), and abnormal glutamate receptor function, may all contribute to the lowered threshold for epileptiform activity observed in hippocampal heterotopias and could be important factors in epilepsies associated with NMDs.

Status epilepticus induced by lithium-pilocarpine in the immature rat does not change the long-term susceptibility to seizures

The causal relationship between early seizures and subsequent temporal lobe epilepsy has not yet been established. Prospective clinical studies reported that seizures occurring early in life rarely result in hippocampal sclerosis. Likewise, in most experimental models, early seizures occurring before the end of the second postnatal week do not lead to neuronal damage and subsequent epilepsy. In some models, this early event decreases latency sensitivity and threshold to seizures. In the present study, we induced lithium and pilocarpine status epilepticus (SE) in 10-day-old (P10) rats. The goal of this study was to determine whether this early life SE altered the sensitivity to convulsants such as pentylenetetrazol (20 and 25 mg/kg), picrotoxin (2.5 and 4.0 mg/kg) and kainate (5 and 8 mg/kg) during adulthood. The occurrence of electrographic seizures (spike-and-wave discharges, SWD) and/or of behavioral seizures was monitored. There was no difference in latency to and duration of SWDs and seizures between lithium-saline and lithium-pilocarpine exposed rats. Thus, SE induced by lithium and pilocarpine early in life does not change the sensitivity to limbic seizures or seizures induced by GABA(A) antagonists during adulthood.

Increased excitability and decreased sensitivity to GABA in an animal model of dysplastic cortex

PURPOSE

Cortical dysplasia (CD) is associated with epilepsy in both the pediatric and adult populations. The mechanism underlying seizures with cortical malformations is still poorly understood. To study the physiology of dysplastic cortex, we developed an experimental model of CD.

METHODS

Pregnant rats were given intraperitoneal injections of carmustine (1-3-bis-chloroethyl-nitrosourea; BCNU) on embryonic day 15 (E15). Cortical histology was examined in the resulting pups at P0, P28, and P60. In addition, evoked and spontaneous field potential recordings were obtained in cortical slices from adult control and BCNU-exposed rats. Finally, we used whole-cell recordings to compare physiologic properties of pyramidal neurons and gamma-aminobutyric acid (GABA) responses in control and BCNU-treated animals.

RESULTS

Features characteristic of CD were found in the offspring, including laminar disorganization, cytomegalic neurons, and neuronal heterotopias. Dysplastic cortex also contained abnormal clusters of Cajal-Retzius (CR) cells and disruption of radial glial fibers, as demonstrated with immunohistochemistry. Under conditions of partial GABAA-receptor blockade with 10 microM bicuculline methiodide (BMI), slices of dysplastic cortex demonstrated a significant increase in the number of spontaneous and evoked epileptiform discharges. Individual pyramidal neurons in dysplastic cortex were less sensitive to application of GABA compared with controls.

CONCLUSIONS

BCNU exposure in utero produces histologic alterations suggestive of CD in rat offspring. Dysplastic cortex from this model demonstrates features of hyperexcitability and decreased neuronal sensitivity to GABA. Such physiologic alterations may underlie the increased epileptogenicity of dysplastic cortex.

A review of gene expression patterns in the malformed brain

Major advances in the identification of genes expressed in malformation-associated epileptic disorders have been made. Some of these changes reflect the complex gene interactions necessary for proper neurodevelopment, whereas others suggest specific synaptic aberrations that could result in a hyperexcitable, and ultimately, epileptic condition. Here we review reported changes in gene expression associated with a malformed brain, with particular emphasis on how these changes provide clues to seizure genesis.

Effects of antiepileptic drugs on induced epileptiform activity in a rat model of dysplasia

Seizure activity associated with cortical dysplasia (CD) is often resistant to standard pharmacologic treatments. Although several animal models exhibit CD, virtually nothing is known about antiepileptic drug (AED) responses in these animals. Here we have used rats exposed to methylazoxymethanol acetate (MAM) in utero, an animal model featuring nodular heterotopia, to investigate the effects of AEDs in the dysplastic brain. 4-aminopyridine (100 microM), a K(+) channel blocker, was used to induce interictal epileptiform bursting in acute hippocampal slices from MAM-exposed and age-matched vehicle-injected control animals. Extracellular field recordings were used to monitor seizure activity in vitro. Five commonly used AEDs were tested: phenobarbital, 25-400 microM; carbamazepine, 25-200 microM; valproate (VPA), 0.19-4 mM; ethosuximide (ESM), 0.5-8 mM; and lamotrigine (LTG), 49-390 microM. 4-AP-induced bursting occurred with shorter latencies in slices from MAM-exposed rats in comparison with slices from controls, confirming the intrinsic hyperexcitability of dysplastic tissue. Each AED tested demonstrated significant burst suppression in control slices, but interictal epileptiform bursting in MAM-exposed slices was resistant to these treatments. Even at the highest concentrations, VPA, ESM and LTG had no effect on burst amplitude in slices from MAM-exposed rats. Pharmaco-resistance was further tested by measuring seizure latencies in awake, freely-moving rats after kainate administration (15 mg/kg, i.p.) with and without pre-treatment with VPA (400 mg/kg i.p.). Pre-treatment with VPA prolonged seizure latency in control rats, but had no effect in MAM-exposed animals. These results suggest MAM-exposed rats exhibit a dramatically reduced sensitivity to commonly prescribed AEDs.

Classification issues in malformations caused by abnormalities of cortical development

Malformations caused by abnormalities of cortical development (MCDs) as a group are now widely recognized as a key cause of medically refractory epilepsies, often leading to a consideration of surgical treatment. A practical classification scheme including histopathologic, imaging, and, if possible, clinical-electrographic features of the various different types of MCDs, will be important to the delineation of surgical strategies and anticipation of medical and surgical prognoses. A proposal of such a scheme with emphasis on the focal cortical dysplasias is given in the hopes that it will reopen the debate on the best way to classify these disorders.

Electroencephalographic characterization of an adult rat model of radiation-induced cortical dysplasia

PURPOSE

Cortical dysplasia (CD) is a frequent cause of medically intractable focal epilepsy. The mechanisms of CD-induced epileptogenicity remain unknown. The difficulty in obtaining and testing human tissue warrants the identification and characterization of animal model(s) of CD that share most of the clinical, electroencephalographic (EEG), and histopathologic characteristics of human CD. In this study, we report on the in vivo EEG characterization of the radiation-induced model of CD.

METHODS

Timed-pregnant Sprague-Dawley rats were irradiated on E17 using a single dose of 145 cGy or left untreated. Their litters were identified and implanted with bifrontal epidural and hippocampal depth electrodes for prolonged continuous EEG recordings. After prolonged EEG monitoring, animals were killed and their brains sectioned and stained for histologic studies.

RESULTS

In utero-irradiated rats showed frequent spontaneous interictal epileptiform spikes and spontaneous seizures arising independently from the hippocampal or the frontal neocortical structures. No epileptiform or seizure activities were recorded from age-matched control rats. Histologic studies showed the presence of multiple cortical areas of neuronal clustering and disorganization. Moreover, pyramidal cell dispersion was seen in the CA1>CA3 areas of the hippocampal formations.

CONCLUSIONS

Our results further characterize the in vivo EEG characteristics of the in utero radiation model of CD using long-term EEG monitoring. This model may be used to study the molecular and cellular changes in epileptogenic CD and to test the efficacy of newer antiepileptic medications.

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.

Models of brain injury and alterations in synaptic plasticity

Animal models are crucial for understanding human pathophysiological processes and for understanding how connections are injured, lost, or even regenerated and/or repaired. When animal models are used in conjunction with theoretical computational models, an ideal combination is achieved that potentially yields insight and encourages the formation of new theories concerning connectionism, cognitive functioning, and synaptic mechanisms. Mechanisms regulating glutamate receptor activation and intracellular calcium levels are important for normal synaptic transmission. These mechanisms (and others) are also critical during and after brain injury when the potential exists for these mechanisms to function pathologically. Interestingly enough, the regulation of glutamate receptor activation and intracellular calcium levels is also involved in normal processes of neuronal and synaptic plasticity. In addition, studies have shown that neurotrophins and cytokines, which are released after brain injury, can be neuroprotective and may also be important in synaptic plasticity. Furthermore, synaptic plasticity is a phenomenon thought by many to be necessary for memory encoding. If this is the case, then research described in this review has significant scientific merit concerning plasticity and memory and clinical benefit for understanding pathophysiologic processes associated with brain injury and memory impairment. This paper reviews the application of experimental animal models of brain injury for simulating conditions of stroke, trauma, and epilepsy (and/or seizure generation) and the associated cellular mechanisms of brain injury. The paper also briefly addresses the advantage of using computational models in combination with experimental models for hypothesis building and for aiding in the interpretation of empirical data. Finally, it reviews studies concerning brain injury and synaptic plasticity.

Abnormal morphological and functional organization of the hippocampus in a p35 mutant model of cortical dysplasia associated with spontaneous seizures

Cortical dysplasia is a major cause of intractable epilepsy in children. However, the precise mechanisms linking cortical malformations to epileptogenesis remain elusive. The neuronal-specific activator of cyclin-dependent kinase 5, p35, has been recognized as a key factor in proper neuronal migration in the neocortex. Deletion of p35 leads to severe neocortical lamination defects associated with sporadic lethality and seizures. Here we demonstrate that p35-deficient mice also exhibit dysplasia/ heterotopia of principal neurons in the hippocampal formation, as well as spontaneous behavioral and electrographic seizures. Morphological analyses using immunocytochemistry, electron microscopy, and intracellular labeling reveal a high degree of abnormality in dentate granule cells, including heterotopic localization of granule cells in the molecular layer and hilus, aberrant dendritic orientation, occurrence of basal dendrites, and abnormal axon origination sites. Dentate granule cells of p35-deficient mice also demonstrate aberrant mossy fiber sprouting. Field potential laminar analysis through the dentate molecular layer reflects the dispersion of granule cells and the structural reorganization of this region. Similar patterns of cortical disorganization have been linked to epileptogenesis in animal models of chronic seizures and in human temporal lobe epilepsy. The p35-deficient mouse may therefore offer an experimental system in which we can dissect out the key morphological features that are causally related to epileptogenesis.

Overview of the Current Animal Models for Human Seizure and Epileptic Disorders

A diversity of animal models are available for the study of epilepsy and these models have a proven history in advancing our understanding of basic mechanisms underlying epileptogenesis and have been instrumental in the screening of novel antiepileptic drugs. This review addresses the criteria that should be met in a valid animal model and provides an overview of current animal models that are relevant to human conditions. In addition, models not specific for any one human condition but rather exhibiting partial or generalized seizures are discussed. While most human disorders are without any animal model, those models that are clinically relevant have strengths and weaknesses. Finally, although few relevant, well-characterized animal models have been added to the list over recent years, major advancements in molecular genetics are contributing to the discovery of novel pathways involved in epileptogenesis.

Developmentally induced microencephalopathy in guinea pigs--embryonic glial cell activation marks selective neuronal death

We have recently shown that in utero treatment of guinea pigs with the DNA methylating substance methylazoxymethanol acetate (MAM) on gestation day (GD) 24 results in neocortical microencephalopathy, increased protein kinase C activity and altered processing of the amyloid precursor protein in neocortex of the offsprings. In order to identify the primary neuronal lesions produced by MAM-treatment, we mapped the 5-bromo-2'-deoxyuridine (BrdU)-incorporation in dividing neurons on GD 24 and we followed the effects of MAM-treatment on GD 24 on embryonic immediate early gene expression and on glial cell activation. BrdU injected on GD 24 labeled many neurons of the ventricular zone and of the intermediate zone but only scattered neurons of the cortical plate. When time-mated guinea pigs were injected intraperitoneally with MAM on GD 24, we observed the activation of microglial cells in the ventricular/intermediate zone and the appearence of astrocytes between the intermediate zone and the cortical plate, 48 h after intoxification. The activation of glial cells was accompanied by the neuronal expression of c-Fos but not of c-Jun in the ventricular/intermediate zone. Based on our observations on BrdU-incorporation and on the morphological outcome of MAM treatment in the juvenile guinea pig, our data presented here indicate that selective neurodegeneration during development induces the activation of both phagocytotic microglial cells and of astrocytes which might trophically support damaged neurons surviving this lesion procedure.

Neuronal microdysgenesis and acquired lesions of the hippocampal formation connected with seizure activities in Ihara epileptic rat

The present study was designed to examine the morphological features of the hippocampal formation in the Ihara epileptic rat (IER), and to characterize genetically programmed lesions and acquired lesions connected with seizure activities. Neuropathological investigation of the hippocampal formation was performed in four separate groups, 2-month-old IERs with neither abnormal behaviors nor any seizure activity, and 12-month-old IERs of both sexes with abnormal behaviors, circling seizures or generalized tonic-clonic convulsions. In every IER examined, there were invariable and fundamental neuropathological findings consisting of abnormal neuronal clusters in the CA1 of the hippocampal formation. Moreover, disarrangement of neuronal cells, such as dispersion and gaps in lamination of pyramidal neurons, were observed. These changes were thought to represent genetically programmed lesions, neuronal microdysgenesis, because they were common findings in 2-month-old and 12-month-old IERs of both sexes. An enlargement of the dentate gyrus was also found in rats that experienced generalized tonic-clonic convulsions or circling seizures. This enlargement of the dentate gyrus, on the other hand, was categorized as a secondary and acquired lesion connected with seizure activities. It is suggested that the neuronal microdysgenesis in the hippocampal formation of IER has an intimate relationship with epileptogenesis and/or an enhancement of seizure susceptibility.

Expression of brain specific chondroitin sulfate proteoglycans, neurocan and phosphacan, in the developing and adult hippocampus of Ihara's epileptic rats

Ihara's epileptic rats (IER) is an animal model of temporal lobe epilepsy with mycrodysgenesis, that exhibit abnormal migration of hippocampal neurons and recurrent spontaneous seizures. As an attempt to elucidate the roles of extracellular matrix molecules in the epileptogenecity and mossy fiber sprouting, immunohistochemical localization of brain specific chondroitin sulfate proteoglycans (CSPGs), neurocan and phosphacan, was examined in the hippocampus of postnatal IER and Sprague-Dawley (SD) rats using monoclonal antibodies 1G2 against neurocan and 6B4 against phosphacan. There was no difference in the expression of these two CSPGs between IER and SD rats in the 1st postnatal week. However, the expression of neurocan was poor in the hippocampus of IER in the 2nd and 3rd weeks whereas intense labeling of neurocan was present throughout the hippocampus of SD rats. Labeling of neurocan was almost absent in the hippocampus, while phosphacan was diffusely expressed in the stratum oriens and radiatum of Ammon's horn, and in the hilus and inner one-third molecular layer of the dentate gyrus at the 2nd month after birth. There was no difference in the expression of neurocan and phosphacan between IER and SD rats at the 2nd month after birth. By contrast, phosphacan was reduced in the inner molecular layer of the dentate gyrus in 8-month-old IER, while neurocan was reexpressed in the outer molecular layer and hilus in 3- and 8-month-old IER. It was suggested that the insufficient expression of neurocan may affect the development of neuronal organization in the hippocampus, and that the remodeling of extracellular matrix in the dentate gyrus may contribute to the mossy fiber sprouting into the inner molecular layer.

Age-dependent consequences of seizures: relationship to seizure frequency, brain damage, and circuitry reorganization

Seizures in the developing brain pose a challenge to the clinician. In addition to the acute effects of the seizure, there are questions regarding the impact of severe or recurrent seizures on the developing brain. Whether provoked seizures cause brain damage, synaptic reorganization, or epilepsy is of paramount importance to patients and physicians. Such questions are especially relevant in the decision to treat or not treat febrile seizures, a common occurrence in childhood. These clinical questions have been addressed using clinical and animal research. The largest prospective studies do not find a causal connection between febrile seizures and later temporal lobe epilepsy. The immature brain seems relatively resistant to the seizure-induced neuronal loss and new synapse formation seen in the mature brain. Laboratory investigations using a developmental rat model corresponding to human febrile seizures find that even though structural changes do not result from hyperthermic seizures, synaptic function may be chronically altered. The increased understanding of the cellular and synaptic mechanisms of seizure-induced damage may benefit patients and clinicians in the form of improved therapies to attenuate damage and changes induced by seizures and to prevent the development of epilepsy.

Cortical malformations and epilepsy

Brain malformations, resulting from aberrant patterns of brain development, are highly correlated with childhood seizure syndromes, as well as with cognitive disabilities and other neurological disorders. The structural malformations, often referred to as cortical dysplasia, are extremely varied, reflecting diverse underlying processes and critical timing of the developmental aberration. Recent studies have revealed a genetic basis for many forms of dysplasia. Gene mutations responsible for such common forms of dysplasia as lissencephaly and tuberous sclerosis have been identified, and investigators are beginning to understand how these gene mutations interrupt and/or misdirect the normal developmental pattern. Laboratory investigations, using animal models of cortical dysplasia, are beginning to elucidate how these structural malformations give rise to epilepsy and other functional pathologies.

Neuronal degeneration in the limbic system of weanling rats exposed to saline, hyperthermia or d-amphetamine

Neuronal degeneration was detected in the tenia tecta and other regions of the anterior limbic system of male weanling rats 3 days after four doses of 5 mg/kg d-amphetamine (4 x 5 mg/kg AMPH) when seizures occurred during AMPH exposure. Neurodegeneration in the parietal cortex, loss of tyrosine hydroxylase immunoreactivity in the caudate-putamen (CPu) and decreases in CPu tissue dopamine levels in weanlings was much less than those previously observed in adults. The neurotoxicity seen in the parietal cortex and CPu of the weanlings was much less than previously seen in adults even though severe hyperthermia and the behavior of retrograde propulsion occurred during AMPH exposure. Neurodegeneration was not detected in any of the previously mentioned brain regions in controls and weanlings made hyperthermic by a warm environment. However, signs of spontaneous neurodegeneration were seen in the posterior piriform cortex (Pir), posteriolateral cortical amygdaloid nucleus (PLCo), and the amygdalopiriform transition area (APir) of control weanlings. The doses of AMPH and the degree of hyperthermia necessary to induce seizures were substantially lower in weanlings compared to those previously observed in adult rats. Further studies will be necessary to determine if the susceptibility of weanlings to AMPH-induced seizures is related to or dependent on the same processes involved in producing degeneration in the posterior limbic system of saline controls.

Schizencephaly: clinical and imaging features in 30 infantile cases

Schizencephaly is an uncommon structural disorder of cerebral cortical development, characterized by congenital clefts spanning the cerebral hemispheres from the pial surface to the lateral ventricles and lined by cortical gray matter. Either an antenatal environmental incident or a genetic origin could be responsible for this lesion which occurs between the third and fourth month of gestation. We report the clinical and cranial imaging features of 30 children, of whom 15 had unilateral and 15 had bilateral lesions. Their ages at the time of the first presentation ranged from 1 month to 10 years. They were thoroughly studied from clinical, epileptical, imaging and electroencephalographic (EEG) viewpoints. Five patients were investigated by cranial computed tomography (CT), eight by cranial magnetic resonance (MR) imaging, and 17 by both methods. The clinical features consisted of mild hemiparesis in 17 cases (57%), 12/17 were related to a unilateral phenotype (80% of all unilateral forms) and 5/17 to a bilateral phenotype. A tetraparesis was present in nine cases, all of which were due to a bilateral cleft. Bilateral forms were significantly associated with tetraparesis, whereas unilateral forms were associated with hemiparesis. Mental retardation was observed in 17 cases (57%), and was observed significantly more often in bilateral clefts (80%). When both hemispheres are involved, an absence of reorganization of the brain function between the two hemispheres leads to severe mental deficits, in addition to the cerebral anomaly itself. Eleven patients had seizures (seven from unilateral and three from bilateral forms). The degree of malformation was not related to the severity of epilepsy. Migration disorders, such as dysplasia or heterotopia, were observed in 30% of cases and are also important etiopathogenetic factors. The septum pellucidum was absent in 13 cases (43%), with septo-optical dysplasia in two cases. Corpus callosum dysgenesis was noted in 30% of cases. Four cases of mega cisterna magna were noted. Although familial cases and environmental factors have been previously reported, schizencephaly appears to be, in the majority of cases, sporadic.

Prenatal methotrexate exposure delays onset of low Mg(2+)-induced epileptiform discharges in the entorhinal cortex

We determined the effects of prenatal exposure to DNA synthesis inhibitor methotrexate (MTX) on: (a) the susceptibility to low Mg(2+)-induced epileptiform activity in deep layers (IV-V) of medial entorhinal cortex in vitro; and (b) neuronal counts in this area. Low Mg(2+)-induced discharges developed significantly later in slices from prenatally MTX-exposed rats than in control slices. Neuronal counts were increased in the layer V of medial entorhinal cortex of prenatally MTX-exposed rats. Results indicate that: (a) MTX-induced prenatal brain DNA impairment is antiepileptogenic; and (b) simple increases in neuronal numbers may not be associated with epileptogenic effects.

Consequences of epilepsy in the developing brain: implications for surgical management

The developing brain is highly susceptible to seizures, as demonstrated by both human and animal studies. Until recently, the brain has been considered to be relatively resistant to damage induced by seizures early in life. Accumulating evidence in animal models now suggests that early seizures can cause structural and physiologic changes in developing neural circuits that result in permanent alterations in the balance between neuronal excitation and inhibition, deficits in cognitive function, and increased susceptibility to additional seizures. The disruption of normal neuronal activity by seizures can affect multiple developmental processes, resulting in these long-lasting changes. These data should be considered in the clinical approach to children with intractable epilepsy and suggest that early intervention may avoid some of these long-term neurologic deficits.

Hippocampal abnormalities and enhanced excitability in a murine model of human lissencephaly

Human cortical heterotopia and neuronal migration disorders result in epilepsy; however, the precise mechanisms remain elusive. Here we demonstrate severe neuronal dysplasia and heterotopia throughout the granule cell and pyramidal cell layers of mice containing a heterozygous deletion of Lis1, a mouse model of human 17p13.3-linked lissencephaly. Birth-dating analysis using bromodeoxyuridine revealed that neurons in Lis1+/- murine hippocampus are born at the appropriate time but fail in migration to form a defined cell layer. Heterotopic pyramidal neurons in Lis1+/- mice were stunted and possessed fewer dendritic branches, whereas dentate granule cells were hypertrophic and formed spiny basilar dendrites from which the principal axon emerged. Both somatostatin- and parvalbumin-containing inhibitory neurons were heterotopic and displaced into both stratum radiatum and stratum lacunosum-moleculare. Mechanisms of synaptic transmission were severely disrupted, revealing hyperexcitability at Schaffer collateral-CA1 synapses and depression of mossy fiber-CA3 transmission. In addition, the dynamic range of frequency-dependent facilitation of Lis1+/- mossy fiber transmission was less than that of wild type. Consequently, Lis1+/- hippocampi are prone to interictal electrographic seizure activity in an elevated [K(+)](o) model of epilepsy. In Lis1+/- hippocampus, intense interictal bursting was observed on elevation of extracellular potassium to 6.5 mM, a condition that resulted in only minimal bursting in wild type. These anatomical and physiological hippocampal defects may provide a neuronal basis for seizures associated with lissencephaly.

Genetics of the epilepsies

Molecular genetic analysis of mendelian epilepsies in humans and mice has revealed a diversity of underlying genes in symptomatic epilepsies associated with disordered brain development and neuronal survival. In contrast, the idiopathic mendelian epilepsies have emerged as a new category of channelopathies. New epilepsy loci have been mapped and one new epilepsy gene isolated. Functional analysis of epilepsy genes is providing new insights into the pathways that lead from mutant gene to hyperexcitable neurones. The major challenge for the future is the analysis of genetic epilepsies with complex inheritance.

Disorders of cortical development

It is only a decade since the realization (facilitated by magnetic resonance imaging) in early 1990s that disorders of cortical development occupy an important place in the aetiologic categorization of epilepsy. Since then research has demonstrated the intrinsic epileptogenicity of disorders of cortical development, their genetic bases and their functional properties. Some of the key points of this most exciting medical and scientific enterprise are reviewed here, with an emphasis in the advances seen within the past 2 years.

Pathophysiology of human epilepsy: imaging and physiologic studies

Recent advances in research into the pathophysiology of human epilepsies and in neuroimaging, especially magnetic resonance imaging, magnetic resonance spectroscopy, positron emission tomography and magnetoelectroencephalography, have resulted in improvements in the localization of both the epileptogenic tissue and functionally important areas. The ability to correlate functional disturbances and lesions has been clarified, which has led to a better understanding of plasticity and epilepsy.

Prenatal methotrexate exposure decreases seizure susceptibility in young rats of two strains

Effects of prenatal exposure to methotrexate (MTX) administered in Sprague-Dawley (one 5 mg/kg dose of MTX on gestational day 15; E15) or Wistar (one 5 mg/kg dose of MTX on E14 or E15 or two such doses on E15) pregnant rat dams were studied in developing offspring. Young Sprague-Dawley rats were subjected to rapid kindling on postnatal days (PN) 15 and 16, and to flurothyl seizures on PN 15 and PN 30. Offspring of the Wistar strain were tested in flurothyl on PN 30. In Sprague-Dawley rats, prenatal exposure to MTX decreased susceptibility to kindling-induced seizures on PN 15 and to flurothyl-induced clonic seizures on PN 30. In Wistar rats, a single dose of MTX on E15 was ineffective, but two doses significantly decreased susceptibility to flurothyl-induced seizures. Additionally, due to a shorter duration of pregnancy in Wistar rats, exposure to a single dose of MTX on E14 also decreased susceptibility to flurothyl seizures. MTX, as folic acid antagonist, interferes with DNA synthesis. However, unlike other treatments that suppress DNA synthesis (such as methylazoxymethanol exposure or X-ray radiation), MTX exposure results in anticonvulsant effects in surviving offspring. The data suggest that not all prenatal impairments of DNA have proconvulsant features postnatally.

Headache in Ehlers-Danlos syndrome

OBJECTIVE

Ehlers-Danlos Syndrome (EDS) is a complex hereditary connective tissue disorder with neurologic manifestations that include cerebrovascular disorders and chronic pain. The clinical data collected on 18 patients with EDS and chronic headaches is reported.

PROCEDURE

Clinical history, neurologic examination, computerized tomography of the head, magnetic resonance imaging (MRI) of the brain, and electroencephalogram (EEG). Headaches were classified according to the International Headache Society and the patients were followed by the author for a minimum of 2 years.

FINDINGS

Four patients had migraine with aura, four had migraine without aura, four had tension headaches, four had a combination of migraine and tension headaches, and two had post-traumatic headaches. Nine patients exhibited blepharoclonus but none had history of seizures and their EEGs were normal, ruling out eye closure epilepsy. Although one patient had a small right frontal angioma, a second had Arnold Chiari malformation type I, and a third had an old stroke, headaches did not clinically correlate with their central nervous system (CNS) lesions.

CONCLUSION

Chronic recurrent headaches may constitute the neurologic presentation of EDS in the absence of structural, congenital, or acquired CNS lesions that correlate with their symptoms. Individuals with EDS may be prone to migraine due to an inherent disorder of cerebrovascular reactivity or cortical excitability. Additional studies are needed to elucidate the pathogenesis of headaches in EDS.