Heterotopic neurons with altered inhibitory synaptic function in an animal model of malformation-associated epilepsy

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.

Effect of Memantine on Pentylenetetrazol-induced Seizures and EEG Profile in Animal Model of Cortical Malformation

Developmental cortical malformations (DCM) are one of the main causes of refractory epilepsy. Many are the mechanisms underlying the hyperexcitability in DCM, including the important contribution of N-methyl-D-aspartate receptors (NMDAR). NMDAR blockers are shown to abolish seizures and epileptiform activity. Memantine, a NMDAR antagonist used to treat Alzheimeŕs disease, has been recently investigated as a possible treatment for other neurological disorders. However, the effects on preventing or diminishing seizures are controversial. Here we aimed to evaluate the effects of memantine on pentylenetetrazole (PTZ)-induced seizures in the freeze-lesion (FL) model. Bilateral cortical microgyria were induced (FL) or not (Sham) in male Wistar neonate rats. At P30, subdural electrodes were implanted and 7 days later, video-EEG was recorded in animals receiving either memantine (FL-M or Sham-M) or saline (FL-S or Sham-S), followed by PTZ. Seizures were evaluated by video-EEG during one hour and scored according to Racine scale. The video-EEG analyses revealed that the number of seizures and the total duration of stage IV-V seizures developed during the 1 h-period increased after memantine application in all groups. The EEG power spectral density (PSD) analysis showed an increased PSD of pre-ictal delta in Sham-M animals and increased PSD of slow, middle and fast gamma oscillations after memantine injection that persists during the pre-ictal period in all groups. Our findings suggested that memantine was unable to control the PTZ-induced seizures and that the associated enhancement of PSD of gamma oscillations may contribute to the increased probability of seizure development in these animals.

Sexual dimorphism, estrous cycle and laterality determine the intrinsic and synaptic properties of medial amygdala neurons in rat

The posterodorsal medial amygdala (MePD) is a sex steroid-sensitive area that modulates different social behavior by relaying chemosensorial information to hypothalamic nuclei. However, little is known about MePD cell type diversity and functional connectivity. Here, we have characterized neurons and synaptic inputs in the right and left MePD of adult male and cycling female (in diestrus, proestrus or estrus) rats. Based on their electrophysiological properties and morphology, we found two coexisting subpopulations of spiny neurons that are sexually dimorphic. They were classified as Class I (predominantly bitufted-shaped neurons showing irregular spikes with frequency adaptation) or Class II (predominantly stellate-shaped neurons showing full spike frequency adaptation). Furthermore, excitatory and inhibitory inputs onto MePD cells were modulated by sex, estrous cycle and hemispheric lateralization. In the left MePD, there was an overall increase in the excitatory input to neurons of males compared to cycling females. However, in proestrus, the MePD neurons received mainly inhibitory inputs. Our findings indicate the existence of hemispheric lateralization, estrous cycle and sexual dimorphism influences at cellular and synaptic levels in the adult rat MePD.

Impairment of GABAergic system contributes to epileptogenesis in glutaric acidemia type I

OBJECTIVES

Glutaric acidemia type I (GA-I) is an inherited neurometabolic disorder caused by deficiency of glutaryl-CoA dehydrogenase (GCDH) and characterized by increased levels of glutaric, 3-OH-glutaric, and glutaconic acids in the brain parenchyma. The increment of these organic acids inhibits glutamate decarboxylase (GAD) and consequently lowers the γ-aminobutyric acid (GABA) synthesis. Untreated patients exhibit severe neurologic deficits during development, including epilepsy, especially following an acute encephalopathy outbreak. In this work, we evaluated the role of the GABAergic system on epileptogenesis in GA-I using the Gcdh^-/- mice exposed to a high lysine diet (Gcdh^-/- -Lys).

METHODS

Spontaneous recurrent seizures (SRS), seizure susceptibility, and changes in brain oscillations were evaluated by video-electroencephalography (EEG). Cortical GABAergic synaptic transmission was evaluated using electrophysiologic and neurochemical approaches.

RESULTS

SRS were observed in 72% of Gcdh^-/- -Lys mice, whereas no seizures were detected in age-matched controls (Gcdh^+/+ or Gcdh^-/- receiving normal diet). The severity and number of PTZ-induced seizures were higher in Gcdh^-/- -Lys mice. EEG spectral analysis showed a significant decrease in theta and gamma oscillations and predominant delta waves in Gcdh^-/- -Lys mice, associated with increased EEG left index. Analysis of cortical synaptosomes revealed a significantly increased percentage of glutamate release and decreased GABA release in Gcdh^-/- -Lys mice that were associated with a decrease in cortical GAD immunocontent and activity and confirmed by reduced frequency of inhibitory events in cortical pyramidal cells.

SIGNIFICANCE

Using an experimental model with a phenotype similar to that of GA-I in humans-the Gcdh^-/- mice under high lysine diet (Gcdh^-/- -Lys)-we provide evidence that a reduction in cortical inhibition of Gcdh^-/- -Lys mice, probably induced by GAD dysfunction, leads to hyperexcitability and increased slow oscillations associated with neurologic abnormalities in GA-I. Our findings offer a new perspective on the pathophysiology of brain damage in GA-I.

Glial GABA Transporters as Modulators of Inhibitory Signalling in Epilepsy and Stroke

Imbalances in GABA-mediated tonic inhibition are involved in several pathophysiological conditions. A classical way of controlling tonic inhibition is through pharmacological intervention with extrasynaptic GABA_A receptors that sense ambient GABA and mediate a persistent GABAergic conductance. An increase in tonic inhibition may, however, also be obtained indirectly by inhibiting glial GABA transporters (GATs). These are sodium-coupled membrane transport proteins that normally act to terminate GABA neurotransmitter action by taking up GABA into surrounding astrocytes. The aim of the review is to provide an overview of glial GATs in regulating tonic inhibition, especially in epilepsy and stroke. This entails a comprehensive summary of changes known to occur in GAT expression levels and signalling following epileptic and ischemic insults. Further, we discuss the accumulating pharmacological evidence for targeting GATs in these diseases.

Prenatal Hypoxia-Ischemia Induces Abnormalities in CA3 Microstructure, Potassium Chloride Co-Transporter 2 Expression and Inhibitory Tone

Infants who suffer perinatal brain injury, including those with encephalopathy of prematurity, are prone to chronic neurological deficits, including epilepsy, cognitive impairment, and behavioral problems, such as anxiety, inattention, and poor social interaction. These deficits, especially in combination, pose the greatest hindrance to these children becoming independent adults. Cerebral function depends on adequate development of essential inhibitory neural circuits and the appropriate amount of excitation and inhibition at specific stages of maturation. Early neuronal synaptic responses to γ-amino butyric acid (GABA) are initially excitatory. During the early postnatal period, GABA_AR responses switch to inhibitory with the upregulation of potassium-chloride co-transporter KCC2. With extrusion of chloride by KCC2, the Cl⁻ reversal potential shifts and GABA and glycine responses become inhibitory. We hypothesized that prenatal hypoxic–ischemic brain injury chronically impairs the developmental upregulation of KCC2 that is essential for cerebral circuit formation. Following late gestation hypoxia–ischemia (HI), diffusion tensor imaging in juvenile rats shows poor microstructural integrity in the hippocampal CA3 subfield, with reduced fractional anisotropy and elevated radial diffusivity. The loss of microstructure correlates with early reduced KCC2 expression on NeuN-positive pyramidal neurons, and decreased monomeric and oligomeric KCC2 protein expression in the CA3 subfield. Together with decreased inhibitory post-synaptic currents during a critical window of development, we document for the first time that prenatal transient systemic HI in rats impairs hippocampal CA3 inhibitory tone. Failure of timely development of inhibitory tone likely contributes to a lower seizure threshold and impaired cognitive function in children who suffer perinatal brain injury.

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.

Altered migratory behavior of interneurons in a model of cortical dysplasia: the influence of elevated GABAA activity

Appropriate function of the neocortex depends on timely generation and migration of cells produced in the germinal zones of the neocortex and ganglionic eminence (GE). Failure to accurately complete migration results in cortical dysplasia, a developmental syndrome implicated in many neurologic disorders. We developed a model of cortical dysplasia in ferrets involving administration of methylaxozymethanol acetate (MAM), an antimitotic, to pregnant ferrets on gestational day 33, leading to dramatic reduction of layer 4 in the neocortex. Here, using time-lapse video imaging, we investigate dynamic behavior of migrating cells arising from the GE and cortical ventricular zone (CVZ) in ferrets and the role of GABAA activity. Treatment with MAM significantly reduced migration speed and the relative proportion of cells arising from the GE demonstrating exploratory behavior. To a lesser extent, the behavior of cells leaving the CVZ was affected. Pharmacologic inhibition of GABAA receptors (GABAAR) improved the speed of migration and exploratory ability of migrating MAM-treated cells arising from the GE. Additionally, the expression of α2 and α3 subunits of GABAAR and the potassium chloride co-transporter (KCC2) increased in the neocortex of MAM-treated animals. After MAM treatment, increases in endogenous KCC2 and GABAAR combine to alter the dynamic properties and exploratory behavior of migrating interneurons in ferrets. We show a direct correlation between increased GABAA and KCC2 expression with impaired migration and ability to explore the environment.

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

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

Glial glutamate transporter and glutamine synthetase regulate GABAergic synaptic strength in the spinal dorsal horn

Decreased GABAergic synaptic strength ('disinhibition') in the spinal dorsal horn is a crucial mechanism contributing to the development and maintenance of pathological pain. However, mechanisms leading to disinhibition in the spinal dorsal horn remain elusive. We investigated the role of glial glutamate transporters (GLT-1 and GLAST) and glutamine synthetase in maintaining GABAergic synaptic activity in the spinal dorsal horn. Electrically evoked GABAergic inhibitory post-synaptic currents (eIPSCs), spontaneous IPSCs (sIPSCs) and miniature IPSCs were recorded in superficial spinal dorsal horn neurons of spinal slices from young adult rats. We used (2S,3S)-3-[3-[4-(trifluoromethyl)benzoylamino]benzyloxy]aspartate (TFB-TBOA), to block both GLT-1 and GLAST and dihydrokainic acid to block only GLT-1. We found that blockade of both GLAST and GLT-1 and blockade of only GLT-1 in the spinal dorsal horn decreased the amplitude of GABAergic eIPSCs, as well as both the amplitude and frequency of GABAergic sIPSCs or miniature IPSCs. Pharmacological inhibition of glial glutamine synthetase had similar effects on both GABAergic eIPSCs and sIPSCs. We provided evidence demonstrating that the reduction in GABAergic strength induced by the inhibition of glial glutamate transporters is due to insufficient GABA synthesis through the glutamate-glutamine cycle between astrocytes and neurons. Thus, our results indicate that deficient glial glutamate transporters and glutamine synthetase significantly attenuate GABAergic synaptic strength in the spinal dorsal horn, which may be a crucial synaptic mechanism underlying glial-neuronal interactions caused by dysfunctional astrocytes in pathological pain conditions.

Deletion of Dlx1 results in reduced glutamatergic input to hippocampal interneurons

Dlx transcription factors are important in the differentiation of GABAergic interneurons. In mice lacking Dlx1, early steps in interneuron development appear normal. Beginning at ∼ 1 mo of age, primarily dendrite-innervating interneuron subtypes begin to undergo apoptosis in Dlx1(-/-) mice; this is accompanied by a reduction in GABAergic transmission and late-onset epilepsy. The reported reduction of synaptic inhibition is greater than might be expected given that interneuron loss is relatively modest in Dlx1(-/-) mice. Here we report that voltage-clamp recordings of CA1 interneurons in hippocampal slices prepared from Dlx1(-/-) animals older than postnatal day 30 (>P30) revealed a significant reduction in excitatory postsynaptic current (EPSC) amplitude. No changes in EPSCs onto interneurons were observed in cells recorded from younger animals (P9-12). Current-clamp recordings from interneurons at these early postnatal ages showed that interneurons in Dlx1(-/-) mutants were immature and more excitable, although membrane properties normalized by P30. Terminal deoxynucleotidyl transferase dUTP-mediated nick-end labeling, caspase-3, and NeuN staining did not reveal frank cell damage or loss in area CA3 of hippocampal sections from adult Dlx1(-/-) mice. Delayed interneuron maturation may lead to interneuron hyperexcitability, followed by a compensatory reduction in the strength of excitatory transmission onto interneurons. This reduced excitation onto surviving interneurons, coupled with the loss of a significant fraction of GABAergic inputs to excitatory neurons starting at P30, may underlie cortical dysrhythmia and seizures previously observed in adult Dlx1(-/-) mice.

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.

Differential localization and function of GABA transporters, GAT-1 and GAT-3, in the rat globus pallidus

GABA transporter subtype 1 (GAT-1) and GABA transporter subtype 3 (GAT-3) are the main transporters that regulate inhibitory GABAergic transmission in the mammalian brain through GABA reuptake. In this study, we characterized the ultrastructural localizations and determined the respective roles of these transporters in regulating evoked inhibitory postsynaptic currents (eIPSCs) in globus pallidus (GP) neurons after striatal stimulation. In the young and adult rat GP, GAT-1 was preferentially expressed in unmyelinated axons, whereas GAT-3 was almost exclusively found in glial processes. Except for rare instances of GAT-1 localization, neither of the two transporters was significantly expressed in GABAergic terminals in the rat GP. 1-(4,4-Diphenyl-3-butenyl)-3-piperidinecarboxylic acid hydrochloride (SKF 89976A) (10 μm), a GAT-1 inhibitor, significantly prolonged the decay time, but did not affect the amplitude, of eIPSCs induced by striatal stimulation (15-20 V). On the other hand, the semi-selective GAT-3 inhibitor 1-(2-[tris(4-methoxyphenyl)methoxy]ethyl)-(S)-3-piperidinecarboxylic acid (SNAP 5114) (10 μm) increased the amplitude and prolonged the decay time of eIPSCs. The effects of transporter blockade on the decay time and amplitude of eIPSCs were further increased when both inhibitors were applied together. Furthermore, SKF 89976A or SNAP 5114 blockade also increased the amplitude and frequency of spontaneous IPSCs, but did not affect miniature IPSCs. Significant GABA(A) receptor-mediated tonic currents were induced in the presence of high concentrations of both SKF 89976A (30 μm) and SNAP 5114 (30 μm). In conclusion, these data indicate that GAT-1 and GAT-3 represent different target sites through which GABA reuptake may subserve complementary regulation of GABAergic transmission in the rat GP.

Altered spontaneous synaptic inhibition in an animal model of cerebral heterotopias

We have investigated spontaneous synaptic transmission in hippocampal nodular heterotopias in rats exposed to methylazoxymethanol (MAM) in utero. Pregnant Wistar rats were injected with MAM at E16. Acute hippocampal slices were prepared from the rat pups P14 to P40. Whole-cell voltage-clamp recordings were made from visually identified neurons using IR-DIC video microscopy. Synaptic events were recorded from either heterotopic neurons in the CA1 region or "slice-matched" normotopic CA1 pyramidal neurons. Both the spontaneous inhibitory (sIPSC) and excitatory synaptic transmission (sEPSC) to the same neurons were recorded. We found a profound reduction in the frequency of sIPSCs in the heterotopic neurons vs. normotopic neurons. No significant differences in the frequency of sEPSCs were found. We also found a profound reduction in the frequency of spontaneous IPSCs in normotopic neurons following application of the GABA reuptake blocker, NO-711, even in the presence of a GABA(B) receptor antagonist (CGP 55845). Preferentially blocking extrasynaptic GABA(A) receptors caused an increased frequency of sIPSCs in the heterotopic neurons. Our data suggest that there is a predominant change in inhibitory synaptic transmission, as measured by changes in sIPSCs, with no change in excitatory synaptic transmission to heterotopic neurons in hippocampus of rats exposed to MAM in utero. We suggest that this change is caused by an increase in the extracellular concentration of GABA but is not mediated via activation of presynaptic GABA(B) receptors. Rather, we propose that the increased extracellular GABA concentration in the heterotopias dampens the activity in inhibitory neurons via activation of extrasynaptic GABA(A) receptors.

Inhibitory inputs to hippocampal interneurons are reorganized in Lis1 mutant mice

Epilepsy and brain malformation are commonly associated with excessive synaptic excitation and decreased synaptic inhibition of principal neurons. However, few studies have examined the state of synaptic inhibition of interneurons in an epileptic, malformed brain. We analyzed inhibitory inputs, mediated by gamma-aminobutyric acid (GABA), to hippocampal interneurons in a mouse model of type 1 lissencephaly, a neurological disorder linked with severe seizures and brain malformation. In the disorganized hippocampal area CA1 of Lis1(+/-) mice, we initially observed a selective displacement of fast-spiking, parvalbumin-positive basket-type interneurons from stratum oriens (SO) locations to s. radiatum and s. lacunosum-moleculare (R/LM). Next, we recorded spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs) onto visually identified interneurons located in SO or R/LM of Lis1(+/-) mice and age-matched littermate controls. We observed significant, layer-specific reorganizations in GABAergic inhibition of interneurons in Lis1 mutant mice. Spontaneous IPSC frequency onto SO interneurons was significantly increased in hippocampal slices from Lis1(+/-) mice, whereas mIPSC mean amplitude onto these interneurons was significantly decreased. In addition, the weighted decay times of sIPSCs and mIPSCs were significantly increased in R/LM interneurons. Taken together, these findings illustrate the extensive redistribution and reorganization of inhibitory connections between interneurons that can take place in a malformed brain.

Maternal immune activation in mice delays myelination and axonal development in the hippocampus of the offspring

Epidemiological data suggest a relationship between maternal infection and a high incidence of schizophrenia in offspring. An animal model based on this hypothesis was made by injecting double-stranded RNA, polyinosinic-polycytidylic acid (poly-I:C), into early pregnant mice, and their offspring were examined for biochemical and histological abnormalities. Mouse brains were examined with special reference to oligodendrocytes, which have been implicated in several neurodevelopmental disorders. We detected a significant decrease of myelin basic protein (MBP) mRNA and protein at early postnatal periods in poly-I:C mice. MBP immunocytochemistry and electron microscopy revealed that the hippocampus of juvenile poly-I:C mice was less myelinated than in PBS mice, with no significant loss of oligodendrocytes. In addition, axonal diameters were significantly smaller in juvenile poly-I:C mice than in control mice. These abnormalities reverted to normal levels when the animals reached the adult stage. These findings suggest that retarded myelination and axonal abnormalities in early postnatal stages caused by maternal immune activation could be related to schizophrenia-related behaviors in adulthood.

Characterization of inhibitory circuits in the malformed hippocampus of Lis1 mutant mice

Heterozygous mutation or deletion of a lissencephaly gene (Lis1) in humans is associated with a severe disruption of cortical and hippocampal lamination, cognitive deficit, and severe seizures. Mice with one null allele of Lis1 (Lis1(+/-) mice) exhibit significant brain malformations and slowed migration of interneuron precursors. Although hyperexcitability was demonstrated in dysplastic hippocampal slices from Lis1(+/-) mice, little is known about synaptic function in these animals. Here we analyzed GABA-mediated synaptic inhibition. We recorded isolated whole cell inhibitory postsynaptic currents (IPSCs) on visually identified pyramidal neurons in disorganized CA1 regions of hippocampal slices prepared from Lis1(+/-) mice. We observed a 32% increase in spontaneous IPSC frequency in Lis1(+/-) mice compared with normotopic CA1 pyramidal neurons in age-matched controls. This increase was not associated with a change in spontaneous IPSC decay or miniature IPSC frequency. Mean IPSC amplitude was increased, and event histograms indicated a greater number of large (>125 pA) events. Tonic inhibition, response to paired-pulse stimulation and evoked IPSC decay kinetics were not altered. Consistent with increased synaptic inhibition, Lis1(+/-) interneurons also exhibited more spontaneous firing in cell-attached recordings and increased excitation as measured by voltage-clamp recording of spontaneous excitatory postsynaptic currents (EPSCs) onto interneurons. Our results reveal a significant alteration in the function of inhibitory circuits within the malformed Lis1(+/-) hippocampus. Given that precisely coordinated GABAergic activity is vital to generation of oscillatory activity and place field precision in hippocampus, these alterations in synaptic inhibition may contribute to seizures and altered cognitive function in type I Lissencephaly.

Pathophysiological mechanisms of focal cortical dysplasia: a critical review of human tissue studies and animal models

Cortical dysplasia (CD, also known as malformations of cortical development) are the pathological substrates in a large percentage of patients with pharmacoresistant epilepsy who may be amenable to surgical treatment. Therefore, research on the mechanisms of dysplastic lesion formation and epileptogenicity is of paramount importance for the prevention, detection, and treatment of CD-induced epilepsy. The purpose of this review is to discuss and critically evaluate the current state and results of human tissue experimentation (focusing on reported results of studies done on neocortical dysplastic tissue resected from patients with pharmacoresistant epilepsy), and to discuss some of the concerns related to research that uses surgically resected epileptic human tissue. The use of better animal models of CD as a tool toward the better understanding of the mechanisms of pathogenesis, epileptogenesis, and epileptogenicity of dysplastic lesions will be reviewed from the perspective of their usefulness in a model of translational research that should ultimately result in better diagnostic and therapeutic techniques of CD.

Neocortical hyperexcitability in a human case of tuberous sclerosis complex and mice lacking neuronal expression of TSC1

OBJECTIVE

To identify brain regions, cell types, or both that generate abnormal electrical discharge in tuberous sclerosis complex (TSC). Here we examined excitatory and inhibitory synaptic currents in human tissue samples obtained from a TSC patient with no discernible cortical tubers and acute neocortical brain slices from a mouse featuring synapsin-driven conditional deletion of a TSC1 gene. These studies were designed to assess whether TSC gene inactivation alters excitability.

METHODS

We used visualized patch-clamp (human and mouse) and extracellular field (mouse) recordings. Additional mice were processed for immunohistochemistry or Western blot analysis.

RESULTS

Detailed anatomic studies in brain tissue sections from synapsin-TSC1 conditional knock-out mice failed to uncover gross anatomic defects, loss of lamination, or frank tuber formation. However, regions of abnormal and potentially activated neocortex were shown using antibodies to nonphosphorylated neurofilaments (SMI-311) and immediate early genes (c-Fos). Extracellular recordings from neocortical slices, examining synaptic activity in these regions, demonstrated clear differences in excitability between conditional knock-out and age-matched control mice. Whole-cell patch-clamp recordings demonstrated excitatory synaptic currents with strikingly long duration and epileptiform discharge patterns, similar to waveforms observed in our human tissue samples. These events were 1-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor mediated and were most prominent in neocortex. Normal-appearing inhibitory postsynaptic currents (human) and intrinsic neuronal firing patterns (mouse) were also recorded.

INTERPRETATION

This combination of human and mouse tissue studies suggests, for the first time, that synaptic excitation is altered in a direction that favors seizure generation in TSC brain tissue regardless of cortical tubers.

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.

Proton magnetic resonance spectroscopy of malformations of cortical development causing epilepsy

PURPOSE

To use proton magnetic resonance spectroscopy (MRS) to measure concentrations of gamma-aminobutyric acid (GABA) and glutamate plus glutamine (GLX) in adult patients with refractory epilepsy associated with malformations of cortical development (MCD).

METHODS

We used MRS to measure N-acetyl aspartate (NAA), creatine plus phosphocreatine (Cr) and choline containing compounds (Cho), as well as GLX, and GABA. Fifteen patients with epilepsy attributable to MCD and 15 healthy controls were studied. Nine of the MCD group had heterotopia and six had polymicrogyria. Quantitative short echo time MRS [echo time (TE)=30 ms, repetition time (TR)=3000 ms] was performed in the MRI evident MCD and in the occipital lobes of the control group and the concentrations of NAA, Cr, Cho, and GLX were measured. GABA plus homocarnosine (GABA+) was measured in the same regions using a double quantum filter.

RESULTS

The dominant abnormalities in the patient group were elevation of Cho and GLX and reduction in NAAt compared to the control group. The ratios GLX/NAAt and GABA+/Cr were also increased in the patient group whilst the ratio NAAt/Cr was decreased. NAAt was significantly lower in polymicrogyria than heterotopia.

CONCLUSIONS

Large cortical malformations had abnormal levels of both GLX and GABA+/Cr. Low NAAt and high Cho were also observed. These results indicate that MCD show spectroscopic features of primitive tissue and abnormal metabolism of both inhibitory and excitatory neurotransmitters.

Early cerebrovascular and parenchymal events following prenatal exposure to the putative neurotoxin methylazoxymethanol

One of the most common causes of neurological disabilities are malformations of cortical development (MCD). A useful animal model of MCD consists of prenatal exposure to methylazoxymethanol (MAM), resulting in a postnatal phenotype characterized by cytological aberrations reminiscent of human MCD. Although postnatal effects of MAM are likely a consequence of prenatal events, little is known on how the developing brain reacts to MAM. General assumption is the effects of prenatally administered MAM are short lived (24 h) and neuroblast-specific. MAM persisted for several days after exposure in utero in both maternal serum and fetal brain, but at levels lower than predicted by a neurotoxic action. MAM levels and time course were consistent with a different mechanism of indirect neuronal toxicity. The most prominent acute effects of MAM were cortical swelling associated with mild cortical disorganization and neurodegeneration occurring in absence of massive neuronal cell death. Delayed or aborted vasculogenesis was demonstrated by MAM's ability to hinder vessel formation. In vitro, MAM reduced synthesis and release of VEGF by endothelial cells. Decreased expression of VEGF, AQP1, and lectin-B was consistent with a vascular target in prenatal brain. The effects of MAM on cerebral blood vessels persisted postnatally, as indicated by capillary hypodensity in heterotopic areas of adult rat brain. In conclusion, these results show that MAM does not act only as a neurotoxin per se, but may additionally cause a short-lived toxic effect secondary to cerebrovascular dysfunction, possibly due to a direct anti-angiogenic effect of MAM itself.

Developmental neuropathology in DNT-studies—A sensitive tool for the detection and characterization of developmental neurotoxicants

Developmental neurotoxicity (DNT-) studies are the first reproduction toxicity studies for which an extended histopathological examination of developing structures is required by the current EPA and OECD guidelines. The morphological screening includes a macroscopic evaluation of the brain and nervous tissue, brain weight parameters, gross morphometry of the brain, neurohistological examinations and a quantitative analysis of major brain areas. This review is intended to give an overview about the needs according to guideline requirements, practical approaches for a successful developmental neuropathology and its preconditions and does include examples of background data on the value and functional meaning of morphological data. A selection of experimental data from literature is also presented in the light of their contribution for the understanding of important, neurodevelopmental disorders in humans.

Prenatal exposure to thalidomide, altered vasculogenesis, and CNS malformations

Malformations of cortical development (MCD) result from abnormal neuronal positioning during corticogenesis. MCD are believed to be the morphological and perhaps physiological bases of several neurological diseases, spanning from mental retardation to autism and epilepsy. In view of the fact that during development, an appropriate blood supply is necessary to drive organogenesis in other organs, we hypothesized that vasculogenesis plays an important role in brain development and that E15 exposure in rats to the angiogenesis inhibitor thalidomide would cause postnatal MCD. Our results demonstrate that thalidomide inhibits angiogenesis in vitro at concentrations that result in significant morphological alterations in cortical and hippocampal regions of rats prenatally exposed to this vasculotoxin. Abnormal neuronal development was associated with vascular malformations and a leaky blood-brain barrier. Protein extravasation and uptake of fluorescent albumin by neurons, but not glia, was commonly associated with abnormal cortical development. Neuronal hyperexcitability was also a hallmark of these abnormal cortical regions. Our results suggest that prenatal vasculogenesis is required to support normal neuronal migration and maturation. Altering this process leads to failure of normal cerebrovascular development and may have a profound implication for CNS maturation.

Reduced excitatory drive in interneurons in an animal model of cortical dysplasia

Cortical dysplasia (CD) is strongly associated with epilepsy. Enhanced excitability in dysplastic neuronal networks is believed to contribute to epileptogenesis, but the underlying mechanisms for the hyperexcitability are poorly understood. Cortical GABAergic interneurons provide the principal inhibition in the neuronal networks by forming inhibitory synapses on excitatory neurons. The aim of the present study was to determine if the function of interneurons in CD is compromised. In a rat model of CD, in utero irradiation, we studied spontaneous and miniature excitatory postsynaptic currents (sEPSCs and mEPSCs) in cortical interneurons using whole cell recording techniques. Two types of interneurons, type I and type II, were identified based on their distinctive spike patterns and short-term synaptic plasticity. We found that the frequencies of sEPSCs and mEPSCs were significantly decreased in both types of interneurons in CD. However, the amplitude and kinetics of sEPSCs and mEPSCs were not different. Five-pulse, 20-Hz stimulation produced short-term depression in type I interneurons in both CD and control tissue. Type II interneurons showed a robust short-term facilitation in both CD and control tissue. Morphological analysis of biocytin-filled neurons revealed that dendritic trees of both types of interneurons were not altered in CD. Our results demonstrate that the excitatory drive, namely sEPSCs and mEPSCs, in two main types of interneuron is largely attenuated in CD, probably due to a reduction in the number of excitatory synapses on both types of interneurons in CD.

Grey matter heterotopia: what EEG-fMRI can tell us about epileptogenicity of neuronal migration disorders

Grey matter heterotopia are commonly associated with refractory epilepsy. Depth electrodes recordings have shown that epileptiform activity can be generated within these lesions, and also at a distance in the neocortex. Heterotopia seem to be part of a more complex circuitry involving also the surrounding and distant cerebral cortex. Blood oxygenation level-dependent (BOLD) changes to interictal spikes using continuous EEG and functional MRI (EEG-fMRI) can help to understand non-invasively the mechanisms of epileptogenicity in these patients. We studied 14 patients with epilepsy and heterotopia using simultaneous recording of EEG-fMRI. EEG was continuously acquired from inside the scanner during 2 h sessions. Epileptic spikes were visually identified in the filtered EEG and each type of spike determined one EEG-fMRI study. We looked at positive (activation) and negative (deactivation) changes in the BOLD signal. Eleven patients had nodular heterotopia and three band heterotopia. Four patients had more than one type of spikes, with a total of 26 EEG-fMRI studies. We excluded three with less than three spikes, and therefore a total of 23 studies (12 with nodular and 11 with band heterotopia) were analysed. Nodular heterotopia: Activation was present in nine studies, with involvement of the heterotopia or surrounding cortex in six, three of which had concomitant distant activation. Deactivation was also observed in nine studies, with involvement of the heterotopia and surrounding cortex in four, three of which had concomitant distant deactivation. Band heterotopia: Activation was present in all 11 studies, and always involved the heterotopia and surrounding cortex, 9 of which had concomitant distant activation. Deactivation was also observed in all 11 studies, with involvement of both the heterotopia and surrounding cortex, in addition to distant deactivation in 5 studies. EEG-fMRI studies reveal, non-invasively, metabolic responses in the heterotopia despite the fact that spikes are generated in the neocortex. The responses, activation or deactivation, had different correlation with the lesion and surrounding or distant cortex, activation reflecting intense neuronal activity, or excitation, and deactivation a possible distant (extra-lesional) inhibition. EEG-fMRI may become a useful tool to understand the epileptogenicity of such malformations.

Degeneration of monoaminergic fibers in the aged micrencephalic rat

Age-related changes in the monoaminergic neuron systems in the brains of methylazoxymethanol acetate (MAM)-induced micrencephalic rats were studied. Neurochemical analysis revealed high levels of serotonin, norepinephrine and associated metabolites in several brain areas of MAM-treated rats. In particular, serotonin levels in the frontal cortex, cingulate cortex and hippocampus of 12-month-old (12 M) MAM-treated rats were significantly higher than in corresponding age-matched controls. Immunohistochemical analysis demonstrated numerous aberrant serotonin-immunoreactive fibers and small numbers of aberrant tyrosine hydroxylase-immunoreactive fibers in the septum, caudate putamen, thalamus, cerebral cortex, hippocampus and midbrain tegmentum of 12 M MAM-treated rats. Aberrant monoaminergic fibers characterized by swollen varicosities and thickening of intervaricose segments were common compared to 12 M control rats. In the cortex and hippocampus of 12 M MAM-treated rats, aberrant fibers were observed near cortical heterotopic tissue. These results indicate early onset of age-related degeneration of monoaminergic fibers in micrencephalic rats. Aged MAM-treated rats may thus offer a good model for studying age-related monoaminergic changes in the cortical heterotopic tissue of human cortical malformations.

Subtype-Specific GABA Transporter Antagonists Synergistically Modulate Phasic and Tonic GABAA Conductances in Rat Neocortex

GABAergic inhibition in the brain can be classified as either phasic or tonic. γ-Aminobutyric acid (GABA) uptake by GABA transporters (GATs) can limit the time course of phasic currents arising from endogenous and exogenous GABA, as well as decrease a tonically active GABA current. GABA transporter subtypes 1 and 3 (GAT-1 and GAT-3) are the most heavily expressed of the four known GAT subtypes. The role of GATs in shaping GABA currents in the neocortex has not been explored. We obtained patch-clamp recordings from layer II/III pyramidal cells and layer I interneurons in rat sensorimotor cortex. We found that selective GAT-1 inhibition with NO711 decreased the amplitude and increased the decay time of evoked inhibitory postsynaptic currents (IPSCs) but had no effect on the tonic current or spontaneous IPSCs (sIPSCs). GAT-2/3 inhibition with SNAP-5114 had no effect on IPSCs or the tonic current. Coapplication of NO711 and SNAP-5114 substantially increased tonic currents and synergistically decreased IPSC amplitudes and increased IPSC decay times. sIPSCs were not resolvable with coapplication of NO711 and SNAP-5114. The effects of the nonselective GAT antagonist nipecotic acid were similar to those of NO711 and SNAP-5114 together. We conclude that synaptic GABA levels in neocortical neurons are controlled primarily by GAT-1, but that GAT-1 and GAT-2/3 work together extrasynaptically to limit tonic currents. Inhibition of any one GAT subtype does not increase the tonic current, presumably as a result of increased activity of the remaining transporters. Thus neocortical GAT-1 and GAT-2/3 have distinct but overlapping roles in modulating GABA conductances.

GABAergic Modulation of the Activity of Globus Pallidus Neurons in Primates: In Vivo Analysis of the Functions of GABA Receptors and GABA Transporters

Neurons in the external and internal segment of the globus pallidus (GPe and GPi, respectively) receive substantial GABAergic inputs from the striatum and through axon collaterals of neighboring pallidal neurons. The effects of GABA on pallidal activity depend on the synaptic localization of GABA receptors and the distribution and activity of GABA transporters (GATs). To explore the contribution of GABA receptors and transporters to pallidal function, we recorded the activity of single neurons in GPe or GPi before, during, and after local microinjections of GABAergic compounds in awake rhesus monkeys. Activation of GABAA or GABAB receptors with muscimol or baclofen, respectively, inhibited pallidal activity. These effects were reversed by concomitant infusion of the respective GABA receptor antagonists, gabazine and CGP-55845. Given alone, the antagonists were without consistent effect. Application of the selective GAT-1 inhibitor, SKF-89976A, and the semiselective GAT-3 blocker, SNAP-5114, decreased pallidal activity. Both GAT inhibitors increased GABA levels in the pallidum, as measured by microdialysis. Electron microscopic observations revealed that these transporters are located on glial processes and unmyelinated axonal segments, but rarely on terminals. Our results indicate that activation of GABAA and GABAB receptors inhibits neuronal activity in both segments of the pallidum. GAT-1 and GAT-3 are involved in the modulation of endogenous GABA levels and may be important in regulating the extrasynaptic levels of GABA. Together with previous evidence that a considerable proportion of pallidal GABA receptors are located outside the synaptic cleft, our experiments strongly support the importance of extrasynaptic GABAergic transmission in the primate pallidum.

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.

Neuronal localization of the GABA transporter GAT-3 in human cerebral cortex: A procedural artifact?

Gamma-amino butyric acid (GABA) plasma membrane transporters (GATs) contribute to the modulation of GABA's actions and are implicated in neuropsychiatric diseases. In this study, the localization of GAT-3, the major glial GAT, was investigated in human cortex using immunocytochemical techniques. In prefrontal and temporal cortices, GAT-3 immunoreactivity (ir) was present throughout the depth of the cortex, both in puncta and in neurons. GAT-3-positive puncta were dispersed in the neuropil or closely related to cell bodies; neuronal staining was in perikarya, especially of pyramidal cells, and proximal dendrites. Electron microscopic studies showed that GAT-3 ir was in astrocytic processes as well as in neuronal elements. All GAT-3-positive neurons co-expressed heat shock protein 70. To test the possibility that the collection procedure of human samples induced the expression of GAT-3 in neurons which normally do not express it, we analyzed rat cortical tissue resected using the same procedure and found that numerous neurons are GAT-3-positive and that they co-express heat shock protein 70. Results show that in human cortex GAT-3 is expressed in astrocytic processes and in neurons and suggest that neuronal expression is related to the procedure used for collecting human samples.

Color-engineered rats and luminescent LacZ imaging: a new platform to visualize biological processes

The rat represents an excellent mammalian model for broadening medical knowledge, and a wealth of information on its physiology has been obtained from its use as an experimental organism. Furthermore, its ample body size allows various surgical manipulations that cannot be performed on a mouse. Many rat models mimic human diseases and have therefore been used in a variety of biomedical studies, including physiology, pharmacology, and transplantation. In an effort to create specifically designed rats for new biomedical research and the field of regenerative medicine, we develop an engineered rat system on the basis of transgenic technology and succeed in establishing unique rats that possess genetically encoded color probes: green fluorescent protein (GFP), DsRed2 (red liver), Cre/LoxP (red to green), and LacZ (blue and luminescence). In this work, we highlight their characteristics and describe recent applications for tissue engineering and regeneration. Coupled with recent progress in modern imaging systems, these transgenic rats are providing powerful tools for the elucidation of many cellular processes in biomedical science, and may lead to innovative medical treatments.

Advances in the pathophysiology of developmental epilepsies

Pediatric epilepsies display unique characteristics that differ significantly from epilepsy in adults. The immature brain exhibits a decreased seizure threshold and an age-specific response to seizure-induced brain injury. Many idiopathic epilepsy syndromes and symptomatic epilepsies commonly present during childhood. This review highlights recent advances in the pathophysiology of developmental epilepsies. Cortical development involves maturational regulation of multiple cellular and molecular processes, such as neurogenesis, neuronal migration, synaptogenesis, and expression of neurotransmitter receptors and ion channels. These normal developmental changes of the immature brain also contribute to the increased risk for seizures and unique responses to seizure-induced brain injury in pediatric epilepsies. Recent technological advances, especially in genetics and imaging, have yielded exciting discoveries about the pathophysiology of specific pediatric epilepsy syndromes, such as the emergence of channelopathies as the cause of many idiopathic epilepsies and identification of malformations of cortical development as a major source of symptomatic epilepsies in children.

Prolonged NMDA-mediated responses, altered ifenprodil sensitivity, and epileptiform-like events in the malformed hippocampus of methylazoxymethanol exposed rats

Cortical malformations are often associated with refractory epilepsy and cognitive deficit. Clinical and experimental studies have demonstrated an important role for glutamate-mediated synaptic transmission in these conditions. Using whole cell voltage-clamp techniques, we examined evoked glutamate-mediated excitatory postsynaptic currents (eEPSCs) and responses to exogenously applied glutamate on hippocampal heterotopic cells in an animal model of malformation i.e., rats exposed to methylazoxymethanol (MAM) in utero. Analysis revealed that the late N-methyl-D-aspartate (NMDA) receptor-mediated eEPSC component was significantly increased on heterotopic cells compared with age-matched normotopic pyramidal cells. At a holding potential of +40 mV, heterotopic cells also exhibited eEPSCs with a slower decay-time constant. No differences in the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) component of eEPSCs were detected. In 23% of heterotopic pyramidal cells, electrical stimulation evoked prolonged burst-like responses. Focal application of glutamate (10 mM) targeted to different sites near the heterotopia also evoked epileptiform-like bursts on heterotopic cells. Ifenprodil (10 microM), an NR2B subunit antagonist, only slightly reduced the NMDA receptor (NMDAR)-mediated component and amplitude of eEPSCs on heterotopic cells (MAM) but significantly decreased the late component and peak amplitude of eEPSCs in normotopic cells (control). Our data demonstrate a functional alteration in the NMDA-mediated component of excitatory synaptic transmission in heterotopic cells and suggest that this alteration may be attributable, at least in part, to changes in composition and function of the NMDAR subunit. Changes in NMDAR function may directly contribute to the hyperexcitability and cognitive deficits reported in animal models and patients with brain malformations.

Developmental Changes of GABA Synaptic Transient in Cerebellar Granule Cells

The time course of synaptic currents is largely determined by the microscopic gating of the postsynaptic receptors and the temporal profile of the synaptic neurotransmitter concentration. Although several lines of evidence indicate that developmental changes of GABAergic synaptic current time course are clearly correlated with a switch in postsynaptic receptors, much less is known about the modification of GABA release during development. To address this issue, we studied the sensitivity of miniature inhibitory postsynaptic currents (mIPSCs) to a quickly dissociating competitive antagonist, 1,2,5,6-tetrahydropyridine-4-yl)methylphosphinic acid (TPMPA), in neurons cultured for 6 to 8 days in vitro (DIV) ("young") and for 12 to 14 DIV ("old"). mIPSCs recorded in young neurons were significantly more resistant to the block by TPMPA. This observation was interpreted as a consequence of a more efficient displacement of TPMPA from GABA(A) receptors caused by a stronger GABA release in young neurons. The change in mIPSC sensitivity to TPMPA during development was not affected by the deletion of alpha(1) subunit, supporting its presynaptic origin. The effects of a second quickly dissociating antagonist, SR-95103 [2-(carboxy-3'-propyl)-3-amino-4-methyl-6-phenylpyridazinium chloride], on young, old, and alpha(1) -/- neurons were qualitatively the same as those obtained with TPMPA. Moreover, the analysis of current responses to ultrafast GABA applications showed that the unbinding rates of TPMPA in DIV 6 to 8 and in DIV 12 to 14 neurons are not significantly different, ruling out the postsynaptic mechanism of differential TPMPA action. Thus, we provide evidence that presynaptic GABA uniquantal release is developmentally regulated.

"Tectonic" hippocampal malformations in patients with temporal lobe epilepsy

Histological analysis of hippocampi removed en bloc during surgical treatment of temporal lobe epilepsy revealed a subgroup of patients with bulbous expansions of the CA1 pyramidal cell/subicular layers that were consistently accompanied by "tectonic" invaginations of the adjacent dentate gyrus. Most hippocampi containing the CA1/subicular anomaly and the tectonically deformed dentate gyrus exhibited minor cell loss compared to hippocampi with typical hippocampal sclerosis, and retrospective analysis revealed that conventional imaging methods usually failed to detect subtle hippocampal atrophy or abnormal signal characteristics in patients with this anomaly. Cells within the anomaly exhibited the spherical appearance of undifferentiated pyramidal layer neurons, and were immunopositive for the neuronal marker NeuN. Immunostaining for the synaptic marker beta-synuclein suggested abnormal dentate gyrus lamination in segments containing the pyramidal cell layer anomaly, but not in unaffected areas of the same specimens. Despite differences in the extent of neuronal loss between patients with hippocampal sclerosis and those with the CA1/subicular anomaly, the incidence of antecedent febrile seizures was similar in both groups. In a comparison group of hippocampi obtained at autopsy, structural irregularities were evident, but were consistently less disruptive to hippocampal architecture than the anomalies observed in epilepsy patients. We hypothesize that developmental malformation of the CA1 pyramidal cell/subicular layers may adversely influence the subsequent development of the adjacent dentate gyrus, and may render temporal lobe structures hyperexcitable and more vulnerable to relatively innocuous seizures and injuries. Thus, these presumably developmental hippocampal anomalies may serve as substrates for early febrile seizures and subsequent epilepsy.

Mechanisms contributing to the exacerbated epileptiform activity in hippocampal slices of GABAB1 receptor subunit knockout mice

The recently developed GABAB1 receptor subunit knockout (GABAB1 -/-) mouse displays complete loss of GABAB receptor function and develops complex generalized epilepsies including absence type, audiogenic as well as spontaneous generalized seizures with electrographic spike-wave discharge signatures. To gain insight into the cellular mechanisms contributing to the generation and maintenance of this epileptic phenotype we have compared epileptiform activity induced in hippocampal slices obtained from GABAB1 -/- and wild type (GABAB1 +/+) littermates. Deletion of the GABAB1 receptor subunit had no effect on a range of passive membrane properties of CA3 pyramidale neurones, non-synaptic epileptiform field bursting and spreading depression recorded in 6mM K+/Ca2+-free medium, and inter-ictal synaptically-induced epileptiform activity induced by 100 microM 4-aminopyridine (4-AP). In contrast, synaptic epileptiform activity induced by 10 microM bicuculline, removal of extracellular Mg2+ or addition of 10 microM oxotremorine was enhanced in GABAB1 -/- slices. Acute blockade of GABAB receptors using a selective antagonist only partly mimicked these effects. It is suggested that the exaggerated in vitro epileptiform activity is caused by both acute and chronic consequences of the loss of GABAB receptor function in vivo. Specifically, enhancement of N-methyl-d-aspartate (NMDA) receptor triggered synaptic processes, arising from the loss of the GABAB receptor-mediated inhibitory postsynaptic potential (IPSP, together with a possible promotion of depolarising IPSPs due to the removal of GABAB autoreceptor function) is likely to underlie these effects.

An examination of calcium current function on heterotopic neurons in hippocampal slices from rats exposed to methylazoxymethanol

PURPOSE

To study voltage-dependent calcium currents (VDCCs) on hippocampal heterotopic neurons by using whole-cell patch-clamp techniques in brain slices prepared from methylaxozymethanol (MAM)-exposed rats.

METHODS

Whole-cell voltage-clamp recordings were obtained from visually identified neurons in acute brain slices by using an infrared differential interference contrast (IR-DIC) video microscopy system. Heterotopic neurons were compared with normotopic pyramidal cells in hippocampal slices from MAM-exposed rats or CA1 pyramidal neurons in slices from controls.

RESULTS

Heterotopic neurons expressed a prominent VDCC, which exhibited a peak current maximum around -30 mV (holding potential, -60 mV) and an inactivation time constant of 48.2 +/- 2.4 ms (n = 91). VDCC peak current and inactivation time constants were similar for normotopic (n = 92) and CA1 pyramidal cells (n = 40). Pharmacologic analysis of VDCC, on heterotopic, normotopic, and CA1 pyramidal cells, revealed an approximately 70% blockade of peak Ca2+ current with nifedipine and amiloride (L- and T-type channel blockers, respectively). Inhibition of VDCC, for all three cell types, also was similar when more specific Ca2+ channel antagonists were used [e.g., omega-conotoxin GVIA (N-type), omega-agatoxin KT (P/Q-type), and sFTX-3.3 (P-type)]. VDCC modulation by norepinephrine (NE) or adrenergic receptor-specific agonists [clonidine (alpha2), isoproterenol (beta), and phenylephrine (alpha1)] was similar for heterotopic and CA1 pyramidal cells.

CONCLUSIONS

Heterotopic neurons do not appear to exhibit Ca2+ channel abnormalities that could contribute to the reported hyperexcitability associated with MAM-exposed rats.