Cortical dysgenesis and epilepsy

Variation of functional brain connectivity in epileptic seizures: an EEG analysis with cross-frequency phase synchronization

Frequency coupling in nervous system is believed to be associated with normal and impaired brain functions. However, most of the existing experiments have been concentrated on the coupling strength within frequency bands, while the coupling strength between different bands is ignored. In this work, we apply phase synchronization index (PSI) to investigate the cross-frequency coupling (CFC) of Electroencephalogram (EEG) signals. The PSI matrixes for the multi-channel EEG signals are calculated from interictal to ictal period in each sliding time window. The results show that CFC changes obviously once seizure occurs between the different bands, and such alteration is earlier than the appearance of clinical symptoms in seizure. Considering the similar role of the within-frequency coupling (WFC), we further reconstruct multi-layered brain networks, including CFC networks and WFC networks. The graph metrics are applied to investigate the variation of network structure of the epileptic brain. Significant decreases/increases of the local/global efficiency are found in δ-β, δ-α, θ-α and δ-θ bands from the CFC network, while WFC network shows a significant decline in the local efficiency in θ and α bands. These findings suggest that CFC may provide a new perspective to observe the alteration of brain structure when seizure occurs, and the investigation of functional connectivity across the full frequency spectrum can give us a deeper understanding of epileptic brains.

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.

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.

Impaired developmental switch of short-term plasticity in pyramidal cells of dysplastic cortex

PURPOSES

Human cortical dysplasia (CD) has a strong clinical association with intractable epilepsy. It is believed that neuronal networks of CD are hyperexcitable, which may initiate seizures. The underlying mechanisms are, however, still poorly understood. We have studied the alterations of synaptic properties in a rat model of CD, in utero irradiation.

METHODS

Pregnant rats on E17 were exposed to 225 cGy of external gamma-irradiation and offspring were used for experiments. Coronal somatosensory brain slices were obtained from 13 - 60-day-old rats. Visualized whole-cell recordings were performed on pyramidal neurons in layer V of control neocortex and the middle region of dysplastic cortex. Short-term plasticity (STP) of evoked excitatory postsynaptic currents (EPSCs) was induced by 5-pulse (20 Hz or 50 Hz) train stimulations.

RESULTS

STP of EPSCs in pyramidal cells of the normal cortex induced by 5-pulse train stimulation (20 Hz or 50 Hz) switched from depression at P13-15 to facilitation at P28-35 and P55-60. However, STP in CD at P28-35 and P 55-60 still showed depression. The failure rate of synaptic responses to the first pulse of the stimulation tested at P 28-35 was significantly lower in CD than in controls. The depression of STP in CD at P28-35 was altered neither by blocking the desensitization of glutamate receptors nor by blocking postsynaptic Ca(2+) rise. It was also not affected by an antagonist of mGluR2/3, LY341495.

CONCLUSIONS

Our results indicate that, compared to control cortex, the presynaptic release probability of excitatory synapses in CD pyramidal cells at P28-35 and P55-60 remains abnormally high and reduced tonic activity of presynaptic mGluR2/3 may contribute to this elevated release probability.

Brains with different degrees of dysplasia show different patterns of neurodegenerative changes following pilocarpine-induced seizures. Histologic evidence of tissue damage correlated with MRI data

OBJECTIVE

Brain dysplasias produced by irradiation with gamma rays at various stages of prenatal development cause different post-natal susceptibility to seizures. To detect possible determinants of this difference, patterns of degenerative changes in the dysplastic brains following pilocarpine-induced epilepsy were analysed.

METHODS

Pregnant Wistar rats were exposed to a 1.0 Gy dose of gamma rays on gestation days 15 (E15) or 17 (E17). On post-natal day 60, their offspring received pilocarpine injections to evoke status epilepticus. Motor manifestations of seizure activity were observed continuously for 6 hours and rated. Six days following the status epilepticus, the rats were anesthetized and T(2)-weighted magnetic resonance (MR) images were obtained. Frontal sections of the brains were immunostained for immunoglobulins G (IgGs) to detect blood-brain barrier damage and IgG cell uptake and glial fibrillary acidic protein (GFAP) or S-100-beta protein to visualize astrocytes. Bandeiraea simplicifolia isolectin-B4 (BSI-B4) isolectin histochemistry was also performed to detect microglia/macrophages.

RESULTS

Tissue damages within epileptic brains as observed by light microscopy generally reflected changes in magnetic resonance imaging (MRI) at similar locations. Brains of rats irradiated on E15 or E17 and showing epileptic symptoms at comparable intensity also displayed different distribution of the pathologic changes. Among other post-epileptic changes, in rats irradiated on E17 as well as controls, the laterodorsal and ventrolateral thalamic nuclei showed signs of severe degeneration. In rats irradiated on E15, the nuclei were free of such changes.

CONCLUSIONS

The obtained data point to important differences in the pattern of propagation of epileptic activity in the dysplastic brains suffering from neuronal loss in functionally different structures.

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.

Congenital brain dysplasias of different genesis can differently affect susceptibility to pilocarpine- or kainic acid-induced seizures in the rat

Interruption of neurogenesis and/or neuronal migration produces brain dysplasia modifying susceptibility to epileptic seizures in adulthood. The course of neurogenesis has a strictly defined time-table. Consequently, the developmental stage at which the interruption occurs determines what functional subsystem potentially involved in epileptogenesis will suffer from irreversible neuronal deficit. The present study attempts to verify a hypothesis that brain dysplasias of different genesis should also lead to different susceptibility to seizures evoked by receptor agonists of different functional specificity, like kainic acid or pilocarpine, a cholinergic or glutaminergic agonist, respectively. Pregnant Wistar rats were exposed to gamma-rays on gestation days 13, 15, 17 or 19 (E13, E15, E17 or E19). Sixty-day-old offsprings of the females were injected with kainic acid or pilocarpine to evoke status epilepticus. During a 6-h period following the injection, motor manifestations of seizure activity were recorded. Generally, the intensity of pilocarpine-induced symptoms was relatively low in rats irradiated on E13 or E15 but high in rats irradiated on E17 or E19. In rats treated with kainic acid, the trend was opposite, viz. the later the prenatal irradiation was performed, the less intense epileptic symptoms were induced in adulthood. The data provide evidence that dysplasias acquired during prenatal development may significantly amplify or reduce the brain susceptibility to seizures. However, this relation depends not only on the developmental stage at which the dysplasias were produced but also on the functional specificity of epileptogenic stimuli used in the experimental model of epilepsy.

A strong epileptogenic effect of mechanical injury can be reduced in the dysplastic rat brain

An exposure of rats to gamma-radiation at different stages of prenatal development produces brain dysplasias of different degree displaying also different susceptibility to pilocarpine-induced seizures. Following irradiation on prenatal day 13 (E13), the susceptibility is minimal and significantly lower even in relation to non-irradiated rats [Setkowicz, Z., Janeczko, K., 2003. Long-term changes in susceptibility to pilocarpine-induced status epilepticus following neocortical injuries in the rat at different developmental stages. Epilepsy Res. 53, 216-224]. On the other hand, the rat brain injured on postnatal day 30 presents very high susceptibility to seizures in the same pilocarpine model of epilepsy [Setkowicz, Z., Kluk, K., Janeczko, K., 2003. Long-term changes in postnatal susceptibility to pilocarpine-induced seizures in rats exposed to gamma radiation at different stages of prenatal development. Epilepsia 44, 1267-1273]. It could, therefore, be hypothesised that the congenital brain dysplasia produced by irradiation on E13 would minimize the highly increased susceptibility to seizures observed in the injured brain. Wistar rats were exposed to gamma-rays on E13 and they received a mechanical brain injury on postnatal day 30 (P30). On postnatal day 60, pilocarpine was injected to evoke status epilepticus. During a 6-h period following the injection, motor manifestations of seizure activity were recorded and rated. Seven days after pilocarpine injection, the animals were sacrificed and their brains were fixed. Pilocarpine injections in non-irradiated rats with brains injured on P30 evoked seizures of very high intensity and extremely high mortality in relation to non-injured controls. This high susceptibility to seizures following the brain injury was considerably decreased in rats irradiated on E13. The data provide evidence that the brain dysplasia in the rat acquired at this stage of prenatal development can significantly reduce the increased susceptibility to seizures evoked by the postnatal brain injury.

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.

Increased seizure susceptibility and cortical malformation in beta-catenin mutant mice

Beta-catenin has been implicated in epilepsy because of its altered post seizure expression and the role of Wnt2 signaling in autism. To determine beta-catenin's role in seizure susceptibility, we injected penetylenetetrazol intraperitoneally in beta-catenin cerebral cortex- and hippocampus-specific knockout mice. We then analyzed the latency, number, and duration of four phases of seizure behaviors: (I) non-seizure activity, (II) myoclonic jerks, (III) generalized clonic seizures, and (IV) tonic seizures. The latencies to both death and Phase IV were significantly reduced in mutant mice. Mutant mice also spent significantly more time in Phases III and IV and showed significantly less time in the non-convulsive state (Phase I). Nissl and gold chloride staining indicated that the knockout mice had underdeveloped cortices, lacked a corpus callosum, and were missing hippocampal structures. This suggests that dysfunction of beta-catenin-mediated signaling pathways in mice leads to cortical malformation and increased seizure susceptibility.

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.

Long-term changes in postnatal susceptibility to pilocarpine-induced seizures in rats exposed to gamma radiation at different stages of prenatal development

PURPOSE

To determine whether brains irradiated at different stages of prenatal development also have different postnatal susceptibility to seizures evoked by pilocarpine.

METHODS

Pregnant Wistar rats were exposed to a single 1.0-Gy dose of gamma rays on gestation days 13, 15, 17, or 19 (E13, E15, E17, and E19, respectively). On postnatal day 60, their offspring received i.p. pilocarpine injections to evoke status epilepticus. Behavior of the animals was observed continuously for 6 h after the injection, and motor manifestations of seizure activity were rated, and survival times recorded. After 7-day survival, the animals were killed, and their brains were weighed.

RESULTS

The average brain weight of animals exposed to irradiation at earlier prenatal stages (E13 or E15) was significantly lower than that after irradiation on E17 or E19. However, effects of the irradiation on the susceptibility to pilocarpine-induced seizures were quite opposite. The intensity of status epilepticus evoked in rats irradiated on E13 or E15 was significantly lower than that in nonirradiated controls or in those irradiated on E17 or E19. Moreover, after irradiation on E13 or E15, survival of the animals was significantly higher in relation not only to other irradiated groups but also to the controls.

CONCLUSIONS

The results suggest than the extent of neuronal deficit, even if relatively greater, cannot always lead to higher susceptibility of the dysplastic brain to seizures. Functional consequences of the deficit, even if its magnitude is relatively smaller but involving specific brain areas, appear to be critical for the epileptogenesis.