Reduction of spontaneous inhibitory synaptic activity in experimental heterotopic gray matter

Grey matter heterotopia subtypes show specific morpho-electric signatures and network dynamics

Grey matter heterotopia (GMH) are neurodevelopmental disorders associated with abnormal cortical function and epilepsy. Subcortical band heterotopia (SBH) and periventricular nodular heterotopia (PVNH) are two well-recognized GMH subtypes in which neurons are misplaced, either forming nodules lining the ventricles in PVNH, or forming bands in the white matter in SBH. Although both PVNH and SBH are commonly associated with epilepsy, it is unclear whether these two GMH subtypes differ in terms of pathological consequences or, on the contrary, share common altered mechanisms. Here, we studied two robust preclinical models of SBH and PVNH, and performed a systematic comparative assessment of the physiological and morphological diversity of heterotopia neurons, as well as the dynamics of epileptiform activity and input connectivity. We uncovered a complex set of altered properties, including both common and distinct physiological and morphological features across heterotopia subtypes, and associated with specific dynamics of epileptiform activity. Taken together, these results suggest that pro-epileptic circuits in GMH are, at least in part, composed of neurons with distinct, subtype-specific, physiological and morphological properties depending on the heterotopia subtype. Our work supports the notion that GMH represent a complex set of disorders, associating both shared and diverging pathological consequences, and contributing to forming epileptogenic networks with specific properties. A deeper understanding of these properties may help to refine current GMH classification schemes by identifying morpho-electric signatures of GMH subtypes, to potentially inform new treatment strategies.

Structural changes in the neocortex as correlates of variations in EEG spectra and seizure susceptibility in rat brains with different degrees of dysplasia

Disturbances of the early stages of neurogenesis lead to irreversible changes in the structure of the mature brain and its functional impairment, including increased excitability, which may be the basis for drug‐resistant epilepsy. The range of possible clinical symptoms is as wide as the different stages of disturbed neurogenesis may be. In this study, we used a quadruple model of brain dysplasia by comparing structural and functional disorders in animals whose neurogenesis was disturbed with a single dose of 1 Gy of gamma rays at one of the four stages of neurogenesis, that is, on days 13, 15, 17, or 19 of prenatal development.

When reached adulthood, the prenatally irradiated rats received EEG teletransmitter implantation. Thereafter, pilocarpine was administered and significant differences in susceptibility to seizure behavioral symptoms were detected depending on the degree of brain dysplasia. Before, during, and after the seizures significant correlations were found between the density of parvalbumin‐immunopositive neurons located in the cerebral cortex and the intensity of behavioral seizure symptoms or increases in the power of particular EEG bands. Neurons expressing calretinin or NPY showed also dysplasia‐related increases without, however, correlations with parameters of seizure intensity. The results point to significant roles of parvalbumin‐expressing interneurons, and also to expression of NPY—an endogenous anticonvulsant and neuroprotectant reducing susceptibility to seizures and supporting neuronal survival.

Ketamine-Treatment During Late Adolescence Impairs Inhibitory Synaptic Transmission in the Prefrontal Cortex and Working Memory in Adult Rats

Schizophrenia (SZ) is associated with changes in the structure and function of several brain areas. Several findings suggest that these impairments are related to a dysfunction in γ-aminobutyric acid (GABA) neurotransmission in brain areas such as the medial prefrontal cortex (mPFC), the hippocampus (HPC) and the primary auditory cortex (A1); however, it is still unclear how the GABAergic system is disrupted in these brain areas. Here, we examined the effect of ketamine (Ket) administration during late adolescence in rats on inhibition in the mPFC-, ventral HPC (vHPC), and A1. We observe that Ket treatment reduced the expression of the calcium-binding protein parvalbumin (PV) and the GABA-producing enzyme glutamic acid decarboxylase 67 (GAD67) as well as decreased inhibitory synaptic efficacy in the mPFC. In addition, Ket-treated rats performed worse in executive tasks that depend on the integrity and proper functioning of the mPFC. Conversely, we do not find such changes in vHPC or A1. Together, our results provide strong experimental support for the hypothesis that during adolescence, the function of the mPFC is more susceptible than that of HPC or A1 to NMDAR hypofunction, showing apparent structure specificity. Thus, the impairment of inhibitory circuitry in mPFC could be a convergent primary site of SZ-like behavior during the adulthood.

Structural alterations in fast-spiking GABAergic interneurons in a model of posttraumatic neocortical epileptogenesis

Electrophysiological experiments in the partial cortical isolation ("undercut" or "UC") model of injury-induced neocortical epileptogenesis have shown alterations in GABAergic synaptic transmission attributable to abnormalities in presynaptic terminals. To determine whether the decreased inhibition was associated with structural abnormalities in GABAergic interneurons, we used immunocytochemical techniques, confocal microscopy and EM in UC and control sensorimotor rat cortex to analyze structural alterations in fast-spiking parvalbumin-containing interneurons and pyramidal (Pyr) cells of layer V. Principle findings were: 1) there were no decreases in counts of parvalbumin (PV)- or GABA-immunoreactive interneurons in UC cortex, however there were significant reductions in expression of VGAT and GAD-65 and -67 in halos of GABAergic terminals around Pyr somata in layer V. 2) Consistent with previous results, somatic size and density of Pyr cells was decreased in infragranular layers of UC cortex. 3) Dendrites of biocytin-filled FS interneurons were significantly decreased in volume. 4) There were decreases in the size and VGAT content of GABAergic boutons in axons of biocytin-filled FS cells in the UC, together with a decrease in colocalization with postsynaptic gephyrin, suggesting a reduction in GABAergic synapses. Quantitative EM of layer V Pyr somata confirmed the reduction in inhibitory synapses. 5) There were marked and lasting reductions in brain derived neurotrophic factor (BDNF)-IR and -mRNA in Pyr cells and decreased TrkB-IR on PV cells in UC cortex. 6) Results lead to the hypothesis that reduction in trophic support by BDNF derived from Pyr cells may contribute to the regressive changes in axonal terminals and dendrites of FS cells in the UC cortex and decreased GABAergic inhibition.

SIGNIFICANCE

Injury to cortical structures is a major cause of epilepsy, accounting for about 20% of cases in the general population, with an incidence as high as ~50% among brain-injured personnel in wartime. Loss of GABAergic inhibitory interneurons is a significant pathophysiological factor associated with epileptogenesis following brain trauma and other etiologies. Results of these experiments show that the largest population of cortical interneurons, the parvalbumin-containing fast-spiking (FS) interneurons, are preserved in the partial neocortical isolation model of partial epilepsy. However, axonal terminals of these cells are structurally abnormal, have decreased content of GABA synthetic enzymes and vesicular GABA transporter and make fewer synapses onto pyramidal neurons. These structural abnormalities underlie defects in GABAergic neurotransmission that are a key pathophysiological factor in epileptogenesis found in electrophysiological experiments. BDNF, and its TrkB receptor, key factors for maintenance of interneurons and pyramidal neurons, are decreased in the injured cortex. Results suggest that supplying BDNF to the injured epileptogenic brain may reverse the structural and functional abnormalities in the parvalbumin FS interneurons and provide an antiepileptogenic therapy.

Excitatory and inhibitory synaptic connectivity to layer V fast-spiking interneurons in the freeze lesion model of cortical microgyria

A variety of major developmental cortical malformations are closely associated with clinically intractable epilepsy. Pathophysiological aspects of one such disorder, human polymicrogyria, can be modeled by making neocortical freeze lesions (FL) in neonatal rodents, resulting in the formation of microgyri. Previous studies showed enhanced excitatory and inhibitory synaptic transmission and connectivity in cortical layer V pyramidal neurons in the paramicrogyral cortex. In young adult transgenic mice that express green fluorescent protein (GFP) specifically in parvalbumin positive fast-spiking (FS) interneurons, we used laser scanning photostimulation (LSPS) of caged glutamate to map excitatory and inhibitory synaptic connectivity onto FS interneurons in layer V of paramicrogyral cortex in control and FL groups. The proportion of uncaging sites from which excitatory postsynaptic currents (EPSCs) could be evoked (hotspot ratio) increased slightly but significantly in FS cells of the FL vs. control cortex, while the mean amplitude of LSPS-evoked EPSCs at hotspots did not change. In contrast, the hotspot ratio of inhibitory postsynaptic currents (IPSCs) was significantly decreased in FS neurons of the FL cortex. These alterations in synaptic inputs onto FS interneurons may result in an enhanced inhibitory output. We conclude that alterations in synaptic connectivity to cortical layer V FS interneurons do not contribute to hyperexcitability of the FL model. Instead, the enhanced inhibitory output from these neurons may partially offset an earlier demonstrated increase in synaptic excitation of pyramidal cells and thereby maintain a relative balance between excitation and inhibition in the affected cortical circuitry.

TRPC3 mediates hyperexcitability and epileptiform activity in immature cortex and experimental cortical dysplasia

Neuronal hyperexcitability plays an important role in epileptogenesis. Conditions of low extracellular calcium (Ca) or magnesium (Mg) can induce hyperexcitability and epileptiform activity with unclear mechanisms. Transient receptor potential canonical type 3 (TRPC3) channels play a pivotal role in neuronal excitability and are activated in low-Ca and/or low-Mg conditions to depolarize neurons. TRPC3 staining was highly enriched in immature, but very weak in mature, control cortex, whereas it was strong in dysplastic cortex at all ages. Depolarization and susceptibility to epileptiform activity increased with decreasing Ca and Mg. Combinations of low Ca and low Mg induced larger depolarization in pyramidal neurons and greater susceptibility to epileptiform activity in immature and dysplastic cortex than in mature and control cortex, respectively. Intracellular application of anti-TRPC3 antibody to block TRPC3 channels and bath application of the selective TRPC3 inhibitor Pyr3 greatly diminished depolarization in immature control and both immature and mature dysplastic cortex with strong TRPC3 expression. Epileptiform activity was initiated in low Ca and low Mg when synaptic activity was blocked, and Pyr3 completely suppressed this activity. In conclusion, TRPC3 primarily mediates low Ca- and low Mg-induced depolarization and epileptiform activity, and the enhanced expression of TRPC3 could make dysplastic and immature cortex more hyperexcitable and more susceptible to epileptiform activity.

Brain dysplasia evoked by gamma irradiation at different stages of prenatal development leads to different tonic and clonic seizure reactivity

Rats with brain dysplasia evoked by interruption of different stages of prenatal neurogenesis show characteristic variations in susceptibility to seizures depending on the neurochemical specificity of pharmacological agents used to evoke seizures. To verify a discrepancy between the data obtained using different pharmacological models, neurochemically neutral electroshocks were applied here. To produce brain dysplasia of different degrees, pregnant Wistar rats were exposed to a single 1.0Gy dose of gamma rays on gestation days 13, 15, 17 or 19. From the postnatal day 60, their male offspring (E13s, E15s, E17s and E19s, respectively) were subjected to 21 daily electrical stimulations to evoke seizures. Profiles of tonic and clonic reactivity to electrical stimulation significantly differed from those observed following pilocarpine or kainic acid administration. E17s showed minimal intensity of tonic but maximal of clonic responses. On the contrary, very high tonic and low clonic reactivity was observed in E13s and E15s. Periventricular nodular heterotopias (PNHs) were observed exclusively in E15s and E17s. Generally, the size of PNHs was correlated positively with susceptibility to tonic seizures but negatively with susceptibility to clonic seizures. Analogous correlations with the size of the neocortex were opposite. E13s and E19s had brains devoid PNHs but showed high tonic seizure susceptibility similar to that in E15s. It can therefore be concluded that PNHs modified the type of seizure reactivity from tonic to clonic, depending of their size, but the presence of PNHs was not necessary for the development of seizure susceptibility itself.

Atypical febrile seizures, mesial temporal lobe epilepsy, and dual pathology

Febrile seizures occurring in the neonatal period, especially when prolonged, are thought to be involved in the later development of mesial temporal lobe epilepsy (mTLE) in children. The presence of an often undetected, underlying cortical malformation has also been reported to be implicated in the epileptogenesis process following febrile seizures. This paper highlights some of the various animal models of febrile seizures and of cortical malformation and portrays a two-hit model that efficiently mimics these two insults and leads to spontaneous recurrent seizures in adult rats. Potential mechanisms are further proposed to explain how these two insults may each, or together, contribute to network hyperexcitability and epileptogenesis. Finally the clinical relevance of the two-hit model is briefly discussed in light of a therapeutic and preventive approach to mTLE.

Impaired hippocampal memory function and synaptic plasticity in experimental cortical dysplasia

PURPOSE

  Memory impairment is a common comorbidity in people with epilepsy-associated malformations of cortical development. We studied spatial memory performance and hippocampal synaptic plasticity in an animal model of cortical dysplasia.

METHODS

  Embryonic day 17 rats were exposed to 2.25 Gy external radiation. One-month-old rats were tested for spatial recognition memory. After behavioral testing, short-term and long-term synaptic plasticity in the hippocampal CA1 region was studied in an in vitro slice preparation.

KEY FINDINGS

  Behavioral assessments showed impaired hippocampal CA1-dependent spatial recognition memory in irradiated rats. Neurophysiologic assessments showed that baseline synaptic transmission was significantly enhanced, whereas paired-pulse facilitation, long-term potentiation, and long-term depression of the field excitatory postsynaptic potential (fEPSP) slope at Schaffer collateral/commissural fiber-CA1 synapses were significantly reduced in the irradiated rats. Histologic observations showed dysplastic cortex and dispersed hippocampal pyramidal neurons.

SIGNIFICANCE

  This study has shown that prenatally irradiated rats with cortical dysplasia exhibit a severe impairment of spatial recognition memory accompanied by disrupted short-term and long-term synaptic plasticity and may help to guide development of potential therapeutic interventions for this important problem.

Altered behavior in experimental cortical dysplasia

PURPOSE

Developmental delay and cognitive impairment are common comorbidities in people with epilepsy associated with malformations of cortical development (MCDs). We studied cognition and behavior in an animal model of diffuse cortical dysplasia (CD), in utero irradiation, using a battery of behavioral tests for neuromuscular and cognitive function.

METHODS

Fetal rats were exposed to 2.25 Gy external radiation on embryonic day 17 (E17). At 1 month of age they were tested using an open field task, a grip strength task, a grid walk task, inhibitory avoidance, an object recognition task, and the Morris water maze task.

KEY FINDINGS

Rats with CD showed reduced nonlocomotor activity in the open field task and impaired motor coordination for grid walking but normal grip strength. They showed a reduced tendency to recognize novel objects and reduced retention in an inhibitory avoidance task. Water maze testing showed that learning and memory were impaired in irradiated rats for both cue discrimination and spatially oriented tasks. These results demonstrate significant deficits in cortex- and hippocampus-dependent cognitive functions associated with the diffuse abnormalities of cortical and hippocampal development that have been documented in this model.

SIGNIFICANCE

This study documents multimodal cognitive deficits associated with CD and can serve as the foundation for future investigations into the mechanisms of and possible therapeutic interventions for this problem.

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.

Densities of glutamatergic and GABAergic presynaptic terminals are altered in experimental cortical dysplasia

PURPOSE

Cortical dysplasia (CD) is a major cause of epilepsy in children and adults, but underlying mechanisms of epileptogenesis in this disorder are poorly understood. We have utilized the irradiated rat model to study an injury-based form of diffuse CD in rats. Prior studies in this model have shown reduced numbers of γ-aminobutyric acid (GABA)ergic interneurons and reduced inhibitory synaptic currents in pyramidal cells in CD. We analyzed the number of excitatory and inhibitory presynaptic terminals in the neocortex of irradiated rats to better characterize altered connectivity in experimental CD.

METHODS

Antibodies to vesicular glutamate transporter 1 (VGLUT1), vesicular glutamate transporter 2 (VGLUT2), vesicular GABA transporter (VGAT), and parvalbumin (PV) were used to quantify glutamatergic and GABAergic presynaptic terminals in control and dysplastic cortex.

RESULTS

We found that the density of VGLUT1 terminals was increased in CD in comparison to layers IV, V, and VI in control cortex. VGLUT2 terminals were increased in CD compared to layers IV and VI. VGAT terminals were reduced in CD compared to layers II/III, IV, and V in controls as were PV-immunoreactive somata and terminals.

DISCUSSION

These findings suggest an overall increase in excitatory synaptic connectivity and decrease in inhibitory synaptic connectivity in CD in irradiated rat. We propose that these changes contribute to hyperexcitability in these animals and may contribute to epileptogenicity in some forms of human CD.

Balance of inhibitory and excitatory synaptic activity is altered in fast-spiking interneurons in experimental cortical dysplasia

Cortical dysplasia (CD) is a common cause of intractable epilepsy in children and adults. We have studied rats irradiated in utero as a model of CD to better understand mechanisms that underlie dysplasia-associated epilepsy. Prior studies have shown a reduction in the number of cortical interneurons and in the frequency of inhibitory postsynaptic currents (IPSCs) in pyramidal cells in this model. They have also shown a reduced frequency of spontaneous and miniature excitatory postsynaptic currents (EPSCs) in the surviving cortical interneurons. However, the inhibitory synaptic contacts were not examined in that study. The current experiments were performed to assess inhibitory synaptic activity in fast-spiking (FS) interneurons in irradiated rats and controls and the balance of excitatory and inhibitory synaptic activity in these cells. Whole cell recordings were obtained from layer IV FS cells in controls and comparable FS cells in irradiated rats. The frequency of spontaneous and miniature IPSCs was reduced in dysplastic cortex, but the amplitude of these currents was unchanged. Stimulus-evoked IPSCs showed short-term depression in control and short-term facilitation in dysplastic cortex. Simultaneous recording of spontaneous EPSCs and IPSCs showed a shift in the ratio of excitation-to-inhibition in favor of inhibition in FS cells from dysplastic cortex. The same shift toward inhibition was seen when miniature EPSCs and IPSCs were examined. These results show that FS cells in dysplastic cortex have a relative lack of excitatory drive. This may result in an important class of inhibitory cells that are less able to perform their normal function especially in periods of increased excitatory activity.

Basic mechanisms of MCD in animal models

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

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

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

Abnormal movement-related cortical potential in patients with subcortical heterotopia

Movement-related cortical potential (MRCP) is recorded to investigate the cerebral motor preparation in two patients with subcortical heterotopia. Magnetic resonance imaging (MRI) studies were conducted in the two patients. Scalp MRCP elicited by self-paced voluntary extension of the wrist was recorded. The MRCP waveforms and the potential maps were compared to those of an age-matched normal subject. MRI reveals subcortical heterotopia in the right frontal area and bilateral centrum semiovale in one patient and in the left parietal lobe in the other. Both of the patients had history of early onset epilepsy and were hypersensitive to unexpected loud noises. Distinctly different from the normal MRCP, the waveforms of the patients are flattened in all scalp electrodes by moving either the right or the left hand. And the topographic potential map detects no evidence of preponderance of negativity on the hemisphere contralateral to the moving side in the patients. The ectopic neurons of the subcortical heterotopia may produce chaotic discharges, which can interrupt or cancel the well oriented dipole of the MRCP activities.

Irradiation exacerbates cortical cytopathology in the Eker rat model of tuberous sclerosis complex, but does not induce hyperexcitability

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder characterized by multi-organ pathologies. Most TSC patients exhibit seizures, usually starting in early childhood. The neuropathological hallmarks of the disease - cortical tubers, containing cytopathological neuronal and glial cell types - appear to be the source of seizure initiation. However, the contribution of these aberrant cell populations to TSC-associated epilepsies is not fully understood. To gain further insight, investigators have attempted to generate animal models with TSC-like brain abnormalities. In the current study, we focused on the Eker rat, in which there is a spontaneous mutation of the TSC2 gene (TSC2+/-). We attempted to exacerbate TSC-like brain pathologies with a "second-hit" strategy - exposing young pups to ionizing irradiation of different intensities, and at different developmental timepoints (between E18 and P6). We found that the frequency of occurrence of dysmorphic neurons and giant astrocytes was strongly dependent on irradiation dose, and weakly dependent on timing of irradiation in Eker rats, but not in irradiated normal controls. The frequency of TSC-like pathology was progressive; there were many more abnormal cells at 3 months compared to 1 month post-irradiation. Measures of seizure propensity (flurothyl seizure latency) and brain excitability (paired-pulse and post-tetanic stimulation studies in vitro), however, showed no functional changes associated with the appearance of TSC-like cellular abnormalities in irradiated Eker rats.

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.

Reeler homozygous mice exhibit enhanced susceptibility to epileptiform activity

PURPOSE

Seizures are observed frequently in humans with diffuse neuronal migration disorders. The reeler mutant mouse also exhibits a diffuse disruption of migration, yet no pro-epileptic phenotype has been reported for this model. Whether this disparity reflects a phenotypic difference that can be used to delineate the mechanisms associated with increasing seizure susceptibility or reflects a paucity of knowledge is unclear. Consequently, this study examined whether seizure susceptibility is altered in reeler mutant mice.

METHODS

In vivo (minimal electroshock delivered transcorneally) and in vitro techniques (field-potential recordings in neocortical and hippocampal brain slice preparations exposed to bicuculline methiodide) were used to determine whether the susceptibility to epileptiform activity is enhanced in reeler homozygous mice relative to controls. Adult (3-7 months) male reeler homozygotes (rl/rl) and controls (+/?) were identified based on their behavioral phenotype and were used in all experiments.

RESULTS

Minimal electroshock revealed that rl/rl mice, compared with controls, exhibited a lower threshold for electroshock-induced seizures (4.5 +/- 0.52 vs. 6.7 +/- 0.35 mA), and a higher incidence of behavioral seizures (median seizure score, class 4 vs. class 0) when animals were subjected to a 5-mA electroshock stimulus. Additionally, neocortical and hippocampal slices from rl/rl mice were more likely to generate spontaneous epileptiform activity after bicuculline application, compared with controls, and the duration of the epileptiform events elicited in 10-30 muM bicuculline was longer in slices from rl/rl mice.

CONCLUSIONS

These data demonstrate that rl/rl mice have enhanced seizure susceptibility that is in part intrinsic to the malformed neocortex and hippocampus. Thus in contrast to prior belief, most animal models of diffuse neuronal migration disorders do exhibit a pro-epileptic phenotype.

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.

Reeler mutant mice exhibit seizures during recovery from isoflurane-induced anesthesia

Reeler mice are a model of cortical malformation with enhanced seizure susceptibility. Data suggest that the propensity to anesthesia-induced seizures may be enhanced in animal models with developmental anomalies. Consequently, reeler mice were monitored behaviorally before, during and after isoflurane anesthesia. During recovery, 12% of reeler homozygotes had class I/II seizures while the remaining 88% exhibited convulsive seizures entailing opisthotonus and forepaw drumming. Similar behavior was not observed in controls. These data reveal that reeler mice display isoflurane-induced seizures and provide support for the hypothesis that developmental anomalies may predispose the central nervous system to anesthesia-induced seizures.

Dysfunction of synaptic inhibition in epilepsy associated with focal cortical dysplasia

Focal cortical dysplasia (FCD) is a common and important cause of medically intractable epilepsy. In patients with temporal lobe epilepsy and in several animal models, compromised neuronal inhibition, mediated by GABA, contributes to seizure genesis. Although reduction in GABAergic interneuron density has been reported in FCD tissue samples, there is little available information on the resulting physiological changes in synaptic inhibition and the potential contribution of these changes to epileptogenesis in the dysplastic human brain. Using visualized whole-cell patch-clamp recordings from identified neurons in tissue slices obtained from patients with FCD, we demonstrate that GABAA-receptor-mediated inhibition is substantially altered in regions of dysplasia. These alterations include a significant reduction in IPSC frequency and a potentially compensatory decrease in transporter-mediated GABA reuptake function; the latter is marked by a significant increase in the decay-time constant for evoked and spontaneous IPSCs and a lack of effect of the GABA transport-inhibitor 1-[2([(diphenylmethylene)imino]oxy)ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride on IPSC kinetics. Immunohistochemical staining revealed a scattering of GABAergic interneurons across dysplastic cortex and striking reductions in GABA transporter expression. Together, these results suggest that profound alterations in GABA-mediated synaptic inhibition play an essential role in the process of epileptogenesis in patients with FCD.

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.

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

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

The GABAergic system of the developing neocortex has a reduced capacity to recover from in utero injury in experimental cortical dysplasia

Cortical dysplasia is frequently associated with epilepsy but mechanisms underlying this association are poorly understood. Rats irradiated in utero serve as an injury-based model of cortical dysplasia. Prior studies in mature rats have shown a selective reduction in the number of neocortical interneurons after in utero irradiation. This study attempted to clarify the nature of the radiation injury to the developing neocortical GABAergic system after exposure to gamma-irradiation on the 17th day of gestation (E17). Stereological methods were used to quantify absolute numbers of total neurons (TN) and GABAergic neurons in the neocortex on E21 and postnatal day 6 (P6). In irradiated rats, TN was decreased to about 50% of controls at both time points. However, TN doubled between the 2 time points, even in irradiated animals. In controls, GABAergic neurons increased 10-fold between E21 and P6, but there was no difference in GABAergic counts between the 2 time points in irradiated animals. This led to a dramatic reduction in the percentage of neocortical neurons that were GABAergic in irradiated animals at P6 (9% vs 18%). This study shows that, in contrast to non-GABAergic neurons, the neocortical GABAergic system has a limited capacity to recover from radiation-induced in utero injury.

Distribution of Cortical Interneurons in Grey Matter Heterotopia in Patients with Epilepsy

PURPOSE

Grey matter heterotopia are well-defined malformations of the cortex often associated with severe epilepsy. Defects have been identified in genes, including DCX and FLN1, that influence radial migration of postmitotic cells from the ventricular zone to the cortical plate. A proportion of cortical gamma-aminobutyric acid (GABA)-containing interneurons may arise from the ganglionic eminence of the ventral telencephalon. We aimed to identify the subtypes and localisation of interneurons within grey matter heterotopia relative to cortex.

METHODS

By using quantitative immunohistochemistry, we studied the density and distribution of interneurons within six cases of grey matter heterotopia in postmortem tissue from patients with epilepsy.

RESULTS

In many cases, a suggestion of focal rudimentary laminar arrangement and small reelin-positive cells was identified within the heterotopia. Immunohistochemistry for glutamic acid decarboxylase(65/57), parvalbumin, calbindin, and calretinin showed inhibitory neurons of all subtypes represented within the heterotopia, and of normal morphology. The mean densities of interneurons were overall similar to those of the overlying cortex, but the interneurons showed less organisation and were more randomly orientated compared with cortex.

CONCLUSIONS

Interneurons within heterotopia probably arise from the ventricular zone, but their abnormal local organization may influence the epileptogenicity of these lesions.

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.

Altered corticogenesis and neuronal morphology in irradiation-induced cortical dysplasia: a Golgi-Cox study

Cortical dysplasia has a strong clinical association with epilepsy and mental retardation, but the relationship between alterations in cortical structure and function in dysplasia-related disorders is poorly understood. The cerebral cortex of irradiated rats, an experimental model of cortical dysplasia, was studied using cresyl violet-stained sections and the Golgi-Cox method. The irradiated cortex is characterized by reductions in size, volume, and number of neurons and fibers reflecting the original lethal injury to neuronal precursors. Consequently, only neurons that survived this injury were able to continue their, albeit altered, development. The result is an altered corticogenesis characterized by neuronal, fiber circuitry, and microvascular alterations. Abnormal aggregates (nodules) of excitatory pyramidal neurons with altered dendritic profiles and functional territories are found between 200 and 400 microm from the pial surface. Their horizontal dendritic profiles and functional territories contrast with the vertical (columnar) dendritic profiles and functional territories of normal pyramidal neurons. This horizontal concentration of spiny dendrites and, hence, of excitatory synaptic contacts suggests a response to the presence of an abnormal horizontal plexus of afferent fibers terminals. Stellate neurons, some morphologically compatible with inhibitory basket cells, are also essential components of these nodules. Some neuronal nodules are characterized by a rich plexus of anastomotic capillaries that contrasts with the sparser vasculature of surrounding gray matter tissue. The presence of well-vascularized aggregates of altered pyramidal and inhibitory neurons suggests a high level of metabolic activity. Well-vascularized deep heterotopias are also found. We propose that the functional activity of well-vascularized neuronal nodules and heterotopias could play a role in the abnormal cortical function in this model.