Thalamic and thalamocortical mechanisms underlying 3 Hz spike-and-wave discharges

Genetic mechanisms in generalized epilepsies

The genetic generalized epilepsies (GGEs) have been proved to generate from genetic impact by twin studies and family studies. The genetic mechanisms of generalized epilepsies are always updating over time. Although the genetics of GGE is complex, there are always new susceptibility genes coming up as well as copy number variations which can lead to important breakthroughs in exploring the problem. At the same time, the development of ClinGen fades out some of the candidate genes. This means we have to figure out what accounts for a reliable gene for GGE, in another word, which gene has sufficient evidence for GGE. This will improve our understanding of the genetic mechanisms of GGE. In this review, important up-to-date genetic mechanisms of GGE were discussed.

Structural and functional abnormalities in thalamic neurons following neocortical focal status epilepticus

Status epilepticus (SE) is a life-threatening emergency that can result in de novo development or worsening of epilepsy. We tested the hypothesis that the aberrant cortical output during neocortical focal status epilepticus (FSE) would induce structural and functional changes in the thalamus that might contribute to hyperexcitability in the thalamocortical circuit. We induced neocortical FSE by unilateral epidural application of convulsant drugs to the somatosensory cortex of anesthetized mice of both sexes. The resulting focal EEG ictal episodes were associated with behavioral seizures consisting of contralateral focal myoclonic activity and persisted for 2-3 h. Ten and 30 days later, brains were processed for either immunohistochemistry (IHC) or in vitro slice recordings. Sections from the center of the thalamic reticular nucleus (nRT, see methods), the ventral posterolateral nucleus (VPL), and the ventral posteromedial nucleus (VPM) from the ventrobasal nucleus (VB) were used to measure density of NeuN-immunoreactive neurons, GFAP-reactive astrocytes, and colocalized areas for VGLUT1 + PSD95- and VGLUT2 + PSD95-IR, presumptive excitatory synapses of cortical and thalamic origins. Whole-cell voltage-clamp recordings were used to measure spontaneous EPSC frequency in these nuclei. We found that the nRT showed no decrease in numbers of neurons or evidence of reactive astrogliosis. In contrast, there were increases in GFAP-IR and decreased neuronal counts of NeuN positive cells in VB. Dual IHC for VGLUT1-PSD95 and VGLUT2-PSD95 in VB showed increased numbers of excitatory synapses, likely of both thalamic and cortical origins. The frequency, but not the amplitude of sEPSCs was increased in nRT and VB neurons. SIGNIFICANCE STATEMENT: Previous reports have shown that prolonged neocortical seizures can induce injury to downstream targets that might contribute to long-term consequences of FSE. Effects of FSE in thalamic structures may disrupt normal thalamo-cortical network functions and contribute to behavioral abnormalities and post-SE epileptogenesis. Our results show that a single episode of focal neocortical SE in vivo has chronic consequences including cell loss in VB nuclei and increased excitatory connectivity in intra-thalamic and cortico-thalamic networks. Additional experiments will assess the functional consequences of these alterations and approaches to mitigate cell loss and alterations in synaptic connectivity.

Intracranial EEG seizure onset and termination patterns and their association

OBJECTIVE

The study of seizure onset and termination patterns has the potential to enhance our understanding of the underlying mechanisms of seizure generation and cessation. It is largely unclear whether seizures with distinct onset patterns originate from varying network interactions and terminate through different termination pathways.

METHODS

We investigated the morphology and location of 103 intracranial EEG seizure onset and termination patterns from 20 patients with drug-resistant focal epilepsy. We determined if seizure onset patterns were associated with specific termination patterns. Finally, we looked at network interactions prior to the generation of distinct seizure onset patterns by calculating directed functional connectivity matrices.

RESULTS

We identified nine seizure onset and six seizure termination patterns. The most common onset pattern was Low-Voltage Fast Activity (36 %), and the most frequent termination pattern was Burst Suppression (44 %). All seizures with fast (>13 Hz) termination patterns had a fast (>13 Hz) onset pattern type. Almost any onset pattern could terminate with the Burst Suppression and rhythmic Spike/PolySpike and Wave (rSW/rPSW) termination patterns. Seizures with a fast activity onset had higher inflow to the seizure onset zone from other regions in the gamma and high gamma frequency ranges prior to their generation compared to seizures with a slow onset.

SIGNIFICANCE

Our observations suggest that different mechanisms underlie the generation of different seizure onset patterns although seizure onset patterns can share a common termination pattern. Possible mechanisms underlying these patterns are discussed.

BOLD-fMRI activity informed by network variation of scalp EEG in juvenile myoclonic epilepsy

Epilepsy is marked by hypersynchronous bursts of neuronal activity, and seizures can propagate variably to any and all areas, leading to brain network dynamic organization. However, the relationship between the network characteristics of scalp EEG and blood oxygenation level-dependent (BOLD) responses in epilepsy patients is still not well known. In this study, simultaneous EEG and fMRI data were acquired in 18 juvenile myoclonic epilepsy (JME) patients. Then, the adapted directed transfer function (ADTF) values between EEG electrodes were calculated to define the time-varying network. The variation of network information flow within sliding windows was used as a temporal regressor in fMRI analysis to predict the BOLD response. To investigate the EEG-dependent functional coupling among the responding regions, modulatory interactions were analyzed for network variation of scalp EEG and BOLD time courses. The results showed that BOLD activations associated with high network variation were mainly located in the thalamus, cerebellum, precuneus, inferior temporal lobe and sensorimotor-related areas, including the middle cingulate cortex (MCC), supplemental motor area (SMA), and paracentral lobule. BOLD deactivations associated with medium network variation were found in the frontal, parietal, and occipital areas. In addition, modulatory interaction analysis demonstrated predominantly directional negative modulation effects among the thalamus, cerebellum, frontal and sensorimotor-related areas. This study described a novel method to link BOLD response with simultaneous functional network organization of scalp EEG. These findings suggested the validity of predicting epileptic activity using functional connectivity variation between electrodes. The functional coupling among the thalamus, frontal regions, cerebellum and sensorimotor-related regions may be characteristically involved in epilepsy generation and propagation, which provides new insight into the pathophysiological mechanisms and intervene targets for JME.

A novel mutual information estimator to measure spike train correlations in a model thalamocortical network

The impact of thalamic state on information transmission to the cortex remains poorly understood. This limitation exists due to the rich dynamics displayed by thalamocortical networks and because of inadequate tools to characterize those dynamics. Here, we introduce a novel estimator of mutual information and use it to determine the impact of a computational model of thalamic state on information transmission. Using several criteria, this novel estimator, which uses an adaptive partition, is shown to be superior to other mutual information estimators with uniform partitions when used to analyze simulated spike train data with different mean spike rates, as well as electrophysiological data from simultaneously recorded neurons. When applied to a thalamocortical model, the estimator revealed that thalamocortical cell T-type calcium current conductance influences mutual information between the input and output from this network. In particular, a T-type calcium current conductance of ~40 nS appears to produce maximal mutual information between the input to this network (conceptualized as afferent input to the thalamocortical cell) and the output of the network at the level of a layer 4 cortical neuron. Furthermore, at particular combinations of inputs to thalamocortical and thalamic reticular nucleus cells, thalamic cell bursting correlated strongly with recovery of mutual information between thalamic afferents and layer 4 neurons. These studies suggest that the novel mutual information estimator has advantages over previous estimators and that thalamic reticular nucleus activity can enhance mutual information between thalamic afferents and thalamorecipient cells in the cortex. NEW & NOTEWORTHY In this study, a novel mutual information estimator was developed to analyze information flow in a model thalamocortical network. Our findings suggest that this estimator is a suitable tool for signal transmission analysis, particularly in neural circuits with disparate firing rates, and that the thalamic reticular nucleus can potentiate ascending sensory signals, while thalamic recipient cells in the cortex can recover mutual information in ascending sensory signals that is lost due to thalamic bursting.

Oscillatory Activity in the Cortex, Motor Thalamus and Nucleus Reticularis Thalami in Acute TTX and Chronic 6-OHDA Dopamine-Depleted Animals

The motor thalamus (MTh) and the nucleus reticularis thalami (NRT) have been largely neglected in Parkinson's disease (PD) research, despite their key role as interface between basal ganglia (BG) and cortex (Cx). In the present study, we investigated the oscillatory activity within the Cx, MTh, and NRT, in normal and different dopamine (DA)-deficient states. We performed our experiments in both acute and chronic DA-denervated rats by injecting into the medial forebrain bundle (MFB) tetrodotoxin (TTX) or 6-hydroxydopamine (6-OHDA), respectively. Interestingly, almost all the electroencephalogram (EEG) frequency bands changed in acute and/or chronic DA depletion, suggesting alteration of all oscillatory activities and not of a specific band. Overall, δ (2-4 Hz) and θ (4-8 Hz) band decreased in NRT and Cx in acute and chronic state, whilst, α (8-13 Hz) band decreased in acute and chronic states in the MTh and NRT but not in the Cx. The β (13-40 Hz) and γ (60-90 Hz) bands were enhanced in the Cx. In the NRT the β bands decreased, except for high-β (Hβ, 25-30 Hz) that increased in acute state. In the MTh, Lβ and Hβ decreased in acute DA depletion state and γ decreased in both TTX and 6-OHDA-treated animals. These results confirm that abnormal cortical β band are present in the established DA deficiency and it might be considered a hallmark of PD. The abnormal oscillatory activity in frequency interval of other bands, in particular the dampening of low frequencies in thalamic stations, in both states of DA depletion might also underlie PD motor and non-motor symptoms. Our data highlighted the effects of acute depletion of DA and the strict interplay in the oscillatory activity between the MTh and NRT in both acute and chronic stage of DA depletion. Moreover, our findings emphasize early alterations in the NRT, a crucial station for thalamic information processing.

The Potential Role of Gap Junctional Plasticity in the Regulation of State

Electrical synapses are the functional correlate of gap junctions and allow transmission of small molecules and electrical current between coupled neurons. Instead of static pores, electrical synapses are actually plastic, similar to chemical synapses. In the thalamocortical system, gap junctions couple inhibitory neurons that are similar in their biochemical profile, morphology, and electrophysiological properties. We postulate that electrical synaptic plasticity among inhibitory neurons directly interacts with the switching between different firing patterns in a state-dependent and type-dependent manner. In neuronal networks, electrical synapses may function as a modifiable resonance feedback system that enables stable oscillations. Furthermore, the plasticity of electrical synapses may play an important role in regulation of state, synchrony, and rhythmogenesis in the mammalian thalamocortical system, similar to chemical synaptic plasticity. Based on their plasticity, rich diversity, and specificity, electrical synapses are thus likely to participate in the control of consciousness and attention.

Neurodegeneration and Epilepsy in a Zebrafish Model of CLN3 Disease (Batten Disease)

The neuronal ceroid lipofuscinoses are a group of lysosomal storage disorders that comprise the most common, genetically heterogeneous, fatal neurodegenerative disorders of children. They are characterised by childhood onset, visual failure, epileptic seizures, psychomotor retardation and dementia. CLN3 disease, also known as Batten disease, is caused by autosomal recessive mutations in the CLN3 gene, 80–85% of which are a ~1 kb deletion. Currently no treatments exist, and after much suffering, the disease inevitably results in premature death. The aim of this study was to generate a zebrafish model of CLN3 disease using antisense morpholino injection, and characterise the pathological and functional consequences of Cln3 deficiency, thereby providing a tool for future drug discovery. The model was shown to faithfully recapitulate the pathological signs of CLN3 disease, including reduced survival, neuronal loss, retinopathy, axonopathy, loss of motor function, lysosomal storage of subunit c of mitochondrial ATP synthase, and epileptic seizures, albeit with an earlier onset and faster progression than the human disease. Our study provides proof of principle that the advantages of the zebrafish over other model systems can be utilised to further our understanding of the pathogenesis of CLN3 disease and accelerate drug discovery.

Disinhibition-Induced Delayed Onset of Epileptic Spike-Wave Discharges in a Five Variable Model of Cortex and Thalamus

Based on a modified neural field network model composed of cortex and thalamus, we here propose a computational framework to investigate the onset control of absence seizure, which is characterized by the spike-wave discharges. Firstly, we briefly demonstrate the existence of various transition types in Taylor's model by increasing the thalamic input. Furthermore, after the disinhibitory function is reasonably introduced into the Taylor's model, we can observe the occurrence of various transition states of firing patterns with different dominant frequencies as the thalamic input is varied under different disinhibitory effects onto the pyramidal neural population. Interestingly, it is found that the onset of spike-wave discharges can be delayed as the disinhibitory input is considered. More importantly, we explore bifurcation mechanism of firing transitions as some key parameters are changed. And also, we observe other dynamical states, such as simple oscillations and saturated discharges with different spatial scales, which are consistent with previous theoretical or experimental findings.

Open-loop organization of thalamic reticular nucleus and dorsal thalamus: a computational model

The thalamic reticular nucleus (TRN) is a shell of GABAergic neurons that surrounds the dorsal thalamus. Previous work has shown that TRN neurons send GABAergic projections to thalamocortical (TC) cells to form reciprocal, closed-loop circuits. This has led to the hypothesis that the TRN is responsible for oscillatory phenomena, such as sleep spindles and absence seizures. However, there is emerging evidence that open-loop circuits are also found between TRN and TC cells. The implications of open-loop configurations are not yet known, particularly when they include time-dependent nonlinearities in TC cells such as low-threshold bursting. We hypothesized that low-threshold bursting in an open-loop circuit could be a mechanism by which the TRN could paradoxically enhance TC activation, and that enhancement would depend on the relative timing of TRN vs. TC cell stimulation. To test this, we modeled small circuits containing TC neurons, TRN neurons, and layer 4 thalamorecipient cells in both open- and closed-loop configurations. We found that open-loop TRN stimulation, rather than universally depressing TC activation, increased cortical output across a broad parameter space, modified the filter properties of TC neurons, and altered the mutual information between input and output in a frequency-dependent and T-type calcium channel-dependent manner. Therefore, an open-loop model of TRN-TC interactions, rather than suppressing transmission through the thalamus, creates a tunable filter whose properties may be modified by outside influences onto the TRN. These simulations make experimentally testable predictions about the potential role for the TRN for flexible enhancement of cortical activation.

Unbalanced Peptidergic Inhibition in Superficial Neocortex Underlies Spike and Wave Seizure Activity

Slow spike and wave discharges (0.5-4 Hz) are a feature of many epilepsies. They are linked to pathology of the thalamocortical axis and a thalamic mechanism has been elegantly described. Here we present evidence for a separate generator in local circuits of associational areas of neocortex manifest from a background, sleep-associated delta rhythm in rat. Loss of tonic neuromodulatory excitation, mediated by nicotinic acetylcholine or serotonin (5HT3A) receptors, of 5HT3-immunopositive interneurons caused an increase in amplitude and slowing of the delta rhythm until each period became the "wave" component of the spike and wave discharge. As with the normal delta rhythm, the wave of a spike and wave discharge originated in cortical layer 5. In contrast, the "spike" component of the spike and wave discharge originated from a relative failure of fast inhibition in layers 2/3-switching pyramidal cell action potential outputs from single, sparse spiking during delta rhythms to brief, intense burst spiking, phase-locked to the field spike. The mechanisms underlying this loss of superficial layer fast inhibition, and a concomitant increase in slow inhibition, appeared to be precipitated by a loss of neuropeptide Y (NPY)-mediated local circuit inhibition and a subsequent increase in vasoactive intestinal peptide (VIP)-mediated disinhibition. Blockade of NPY Y1 receptors was sufficient to generate spike and wave discharges, whereas blockade of VIP receptors almost completely abolished this form of epileptiform activity. These data suggest that aberrant, activity-dependent neuropeptide corelease can have catastrophic effects on neocortical dynamics.

Spatiotemporal dynamics of optogenetically induced and spontaneous seizure transitions in primary generalized epilepsy

Transitions into primary generalized epileptic seizures occur abruptly and synchronously across the brain. Their potential triggers remain unknown. We used optogenetics to causally test the hypothesis that rhythmic population bursting of excitatory neurons in a local neocortical region can rapidly trigger absence seizures. Most previous studies have been purely correlational, and it remains unclear whether epileptiform events induced by rhythmic stimulation (e.g., sensory/electrical) mimic actual spontaneous seizures, especially regarding their spatiotemporal dynamics. In this study, we used a novel combination of intracortical optogenetic stimulation and microelectrode array recordings in freely moving WAG/Rij rats, a model of absence epilepsy with a cortical focus in the somatosensory cortex (SI). We report three main findings: 1) Brief rhythmic bursting, evoked by optical stimulation of neocortical excitatory neurons at frequencies around 10 Hz, induced seizures consisting of self-sustained spike-wave discharges (SWDs) for about 10% of stimulation trials. The probability of inducing seizures was frequency-dependent, reaching a maximum at 10 Hz. 2) Local field potential power before stimulation and response amplitudes during stimulation both predicted seizure induction, demonstrating a modulatory effect of brain states and neural excitation levels. 3) Evoked responses during stimulation propagated as cortical waves, likely reaching the cortical focus, which in turn generated self-sustained SWDs after stimulation was terminated. Importantly, SWDs during induced and spontaneous seizures propagated with the same spatiotemporal dynamics. Our findings demonstrate that local rhythmic bursting of excitatory neurons in neocortex at particular frequencies, under susceptible ongoing brain states, is sufficient to trigger primary generalized seizures with stereotypical spatiotemporal dynamics.

Spike-wave discharges in adult Sprague-Dawley rats and their implications for animal models of temporal lobe epilepsy

Spike-wave discharges (SWDs) are thalamocortical oscillations that are often considered to be the EEG correlate of absence seizures. Genetic absence epilepsy rats of Strasbourg (GAERS) and Wistar Albino Glaxo rats from Rijswijk (WAG/Rij) exhibit SWDs and are considered to be genetic animal models of absence epilepsy. However, it has been reported that other rat strains have SWDs, suggesting that SWDs may vary in their prevalence, but all rats have a predisposition for them. This is important because many of these rat strains are used to study temporal lobe epilepsy (TLE), where it is assumed that there is no seizure-like activity in controls. In the course of other studies using the Sprague-Dawley rat, a common rat strain for animal models of TLE, we found that approximately 19% of 2- to 3-month-old naive female Sprague-Dawley rats exhibited SWDs spontaneously during periods of behavioral arrest, which continued for months. Males exhibited SWDs only after 3 months of age, consistent with previous reports (Buzsáki et al., 1990). Housing in atypical lighting during early life appeared to facilitate the incidence of SWDs. Spike-wave discharges were often accompanied by behaviors similar to stage 1-2 limbic seizures. Therefore, additional analyses were made to address the similarity. We observed that the frequency of SWDs was similar to that of hippocampal theta rhythm during exploration for a given animal, typically 7-8 Hz. Therefore, activity in the frequency of theta rhythm that occurs during frozen behavior may not reflect seizures necessarily. Hippocampal recordings exhibited high frequency oscillations (>250 Hz) during SWDs, suggesting that neuronal activity in the hippocampus occurs during SWDs, i.e., it is not a passive structure. The data also suggest that high frequency oscillations, if rhythmic, may reflect SWDs. We also confirmed that SWDs were present in a common animal model of TLE, the pilocarpine model, using female Sprague-Dawley rats. Therefore, damage and associated changes to thalamic, hippocampal, and cortical neurons do not prevent SWDs, at least in this animal model. The results suggest that it is possible that SWDs occur in rodent models of TLE and that investigators mistakenly assume that they are stage 1-2 limbic seizures. We discuss the implications of the results and ways to avoid the potential problems associated with SWDs in animal models of TLE.

Topography of age-related changes in sleep spindles

Aging induces multiple changes to sleep spindles, which may hinder their alleged functional role in memory and sleep protection mechanisms. Brain aging in specific cortical regions could affect the neural networks underlying spindle generation, yet the topography of these age-related changes is currently unknown. In the present study, we analyzed spindle characteristics in 114 healthy volunteers aged between 20 and 73 years over 5 anteroposterior electroencephalography scalp derivations. Spindle density, amplitude, and duration were higher in young subjects than in middle-aged and elderly subjects in all derivations, but the topography of age effects differed drastically. Age-related decline in density and amplitude was more prominent in anterior derivations, whereas duration showed a posterior prominence. Age groups did not differ in all-night spindle frequency for any derivation. These results show that age-related changes in sleep spindles follow distinct topographical patterns that are specific to each spindle characteristic. This topographical specificity may provide a useful biomarker to localize age-sensitive changes in underlying neural systems during normal and pathological aging.

Power-law scaling in the brain surface electric potential

Recent studies have identified broadband phenomena in the electric potentials produced by the brain. We report the finding of power-law scaling in these signals using subdural electrocorticographic recordings from the surface of human cortex. The power spectral density (PSD) of the electric potential has the power-law form P(f ) approximately Af(-chi) from 80 to 500 Hz. This scaling index, chi = 4.0+/-0.1, is conserved across subjects, area in the cortex, and local neural activity levels. The shape of the PSD does not change with increases in local cortical activity, but the amplitude, A, increases. We observe a "knee" in the spectra at f(0) approximately 75 Hz, implying the existence of a characteristic time scale tau = (2pif(0))(-1) approximately 2 - 4ms. Below f(0), we explore two-power-law forms of the PSD, and demonstrate that there are activity-related fluctuations in the amplitude of a power-law process lying beneath the alpha/beta rhythms. Finally, we illustrate through simulation how, small-scale, simplified neuronal models could lead to these power-law observations. This suggests a new paradigm of non-oscillatory "asynchronous," scale-free, changes in cortical potentials, corresponding to changes in mean population-averaged firing rate, to complement the prevalent "synchronous" rhythm-based paradigm.

Computer modelling of epilepsy

Epilepsy is a complex set of disorders that can involve many areas of the cortex, as well as underlying deep-brain systems. The myriad manifestations of seizures, which can be as varied as déjà vu and olfactory hallucination, can therefore give researchers insights into regional functions and relations. Epilepsy is also complex genetically and pathophysiologically: it involves microscopic (on the scale of ion channels and synaptic proteins), macroscopic (on the scale of brain trauma and rewiring) and intermediate changes in a complex interplay of causality. It has long been recognized that computer modelling will be required to disentangle causality, to better understand seizure spread and to understand and eventually predict treatment efficacy. Over the past few years, substantial progress has been made in modelling epilepsy at levels ranging from the molecular to the socioeconomic. We review these efforts and connect them to the medical goals of understanding and treating the disorder.

Transient activity induces a long-lasting increase in the excitability of olfactory bulb interneurons

Little is known about the cellular mechanisms that underlie the processing and storage of sensory in the mammalian olfactory system. Here we show that persistent spiking, an activity pattern associated with working memory in other brain regions, can be evoked in the olfactory bulb by stimuli that mimic physiological patterns of synaptic input. We find that brief discharges trigger persistent activity in individual interneurons that receive slow, subthreshold oscillatory input in acute rat olfactory bulb slices. A 2- to 5-Hz oscillatory input, which resembles the synaptic drive that the olfactory bulb receives during sniffing, is required to maintain persistent firing. Persistent activity depends on muscarinic receptor activation and results from interactions between calcium-dependent afterdepolarizations and low-threshold Ca spikes in granule cells. Computer simulations suggest that intrinsically generated persistent activity in granule cells can evoke correlated spiking in reciprocally connected mitral cells. The interaction between the intrinsic currents present in reciprocally connected olfactory bulb neurons constitutes a novel mechanism for synchronized firing in subpopulations of neurons during olfactory processing.

GABAergic mechanisms in absence epilepsy: a computational model of absence epilepsy simulating spike and wave discharges after vigabatrin in WAG/Rij rats

In this study, the effects of vigabatrin on spike-and-wave discharges (SWDs) were measured in WAG/Rij rats, an animal model of absence epilepsy. Vigabatrin was used with the aim of enhancing GABAergic neurotransmission, and in this way to investigate the role of this process in the properties of SWDs. The study was carried out both in the rat, in vivo, and also using a computational model, in order to test different mechanisms that may account for the changes in SWDs after vigabatrin. The model parameters, representing GABA levels, were changed according to the known, and assumed, mechanism of action of the drug. The results show that the computational model can most adequately simulate the data obtained in vivo on the assumption that the enhancement of GABAergic neurotransmission due to application of vigabatrin is most pronounced at the level of the thalamic relay nuclei (TC cells). Furthermore, vigabatrin was shown to affect both the SWD starting and stopping mechanisms, as reflected by hazard rates. Based on these results, we suggest that GABAergic neurotransmission in TC cells is actively involved in the SWD termination.

Photic-induced sensitization: acquisition of an augmenting spike-wave response in the adult rat through repeated strobe exposure

It is well established that patterns of sensory input can affect neuroplastic changes during early development. The scope and consequences of experience-dependent plasticity in the adult are less well understood. We studied the possibility that repeated exposure to trains of stroboscopic stimuli could induce a sensitized and potentially aberrant response in ordinary individuals. Chronic electrocorticographic recording electrodes enabled measurement of responses in awake, freely moving animals. Normal adult rats, primarily Sprague-Dawley, were exposed to 20-40 strobe trains per day after a strobe-free adaptation period. The common response to strobe trains changed in 34/36 rats with development of a high-amplitude spike-wave response that emerged fully by the third day of photic exposure. Onset of this sensitized response was marked by short-term augmentation of response to successive strobe flashes. The waveform generalized across the brain, reflected characteristics of the visual stimulus, as well as an inherent 6- to 8-Hz pacing, and was suppressed with ethosuximide administration. Spike-wave episodes were self-limiting but could persist beyond the strobe period. Sensitization lasted 2-4 wk after last strobe exposure. The results indicate visual stimulation, by itself, can induce in adult rats an enduring sensitization of visual response with epileptiform characteristics. The results raise the question of the effects of such neuroplastic change on sensation and epileptiform events.

Cellular and network mechanisms of spike-wave seizures

Spike-wave seizures are often considered a relatively "pure" form of epilepsy, with a uniform defect present in all patients and involvement of the whole brain homogeneously. Here, we present evidence against these common misconceptions. Rather than a uniform disorder, spike-wave rhythms arise from the normal inherent network properties of brain excitatory and inhibitory circuits, where they can be provoked by many different insults in several different brain networks. Here we discuss several different cellular and molecular mechanisms that may contribute to the generation of spike-wave seizures, particularly in idiopathic generalized epilepsy. In addition, we discuss growing evidence that electrical, neuroimaging, and molecular changes in spike-wave seizures do not involve the entire brain homogeneously. Rather, spike-wave discharges occur selectively in some thalamocortical networks, while sparing others. It is hoped that improved understanding of the heterogeneous defects and selective brain regions involved will ultimately lead to more effective treatments for spike-wave seizures.

Dysregulation of sodium channel expression in cortical neurons in a rodent model of absence epilepsy

Due to the involvement of cortical neurons in spike-wave discharge (SWD) initiation, and the contribution of voltage-gated sodium channels (VGSCs) to neuronal firing, we examined alterations in the expression of VGSC mRNA and protein in cortical neurons in the WAG/Rij absence epileptic rat. WAG/Rij rats were compared to age-matched Wistar control rats at 2, 4, and 6 months. Continuous EEG data was recorded, and percent time in SWD was determined. Tissue from different cortical locations from WAG/Rij and Wistar rats was analyzed for VGSC mRNA (by quantitative PCR) and protein (by immunocytochemistry). SWDs increased with age in WAG/Rij rats. mRNA levels for sodium channels Nav1.1 and Nav1.6, but not Nav1.2, were found to be up-regulated selectively within the facial somatosensory cortex (at AP +0.0, ML +6.0 mm). Protein levels for Nav1.1 and Nav1.6 were up-regulated in layer II-IV cortical neurons in this region of cortex. No significant changes were seen in adjacent regions or other brain areas, including the pre-frontal and occipital cortex. In the WAG/Rij model of absence epilepsy, we identified a specific region of cortex, in layer II-IV neurons on the lateral convexity of the cortex in the facial somatosensory area, where mRNA and protein expression of sodium channel genes Nav1.1 and Nav1.6 are up-regulated. This region of cortex approximately matches the electrophysiologically determined region of seizure onset. Changes in the expression of Nav1.1 and Nav1.6 parallel age-dependent increases in seizure frequency and duration.

From molecules to networks: cortical/subcortical interactions in the pathophysiology of idiopathic generalized epilepsy

Generalized epilepsy involves abnormally synchronized activity in large-scale neuronal networks. Burst firing of action potentials is a potent mechanism for increasing neural synchrony and is thought to enhance cortical and thalamic rhythmic network activity. Absence seizures, a form of generalized epilepsy, occur in children as brief 5- to 10-s periods of behavioral arrest associated with massive 3- to 4-Hz spike-wave discharges in cortical and thalamic networks. Prior research has shown that enhanced burst firing may be crucial for the transition from normal to epileptic activity. Can enhanced burst firing in one region of the nervous system, such as the cortex, transform the entire thalamocortical network from normal activity to spike-and-wave seizures? Enhanced burst firing in corticothalamic neurons may increase gamma-aminobutyric acid-B (GABAB) receptor activation in the thalamus, leading to the slower, more synchronous oscillations seen in spike-and-wave seizures. Does "generalized" spike-wave activity homogeneously involve the entire brain, or are there crucial nodes that are more important than others for the generation and behavioral manifestations of generalized seizures? Animal and human data suggest that so-called generalized seizures involve selective thalamocortical networks while sparing others. A greater understanding of these molecular and network mechanisms will ultimately lead to improved targeted therapies for generalized epilepsy.

Thalamocortical oscillations in a genetic model of absence seizures

We used field potential recordings in an in vitro thalamocortical slice preparation to compare the rhythmic oscillations generated by reciprocally connected networks of the thalamus and cerebral cortex obtained from epileptic (> 160 days old) WAG/Rij and age-matched, nonepileptic control (NEC) rats. To increase neuronal excitability, and thus to elicit spontaneous field potential activity in vitro, we applied medium containing: (i) zero [Mg2+]; (ii) high [K+] (8.25 mm); or (iii) low concentrations of the K+ channel blocker 4-aminopyridine (4AP, 0.5-1 micro m). Of these procedures, only the last was effective in triggering oscillatory activity that depended on the type of tissue. Thus, during 4AP application: (i) sequences of fast (intraburst frequency 9.5-16.1 Hz) and slower (5-8.9 Hz) field potential oscillations (FPOs) were recorded in WAG/Rij slices (n = 23), but (ii) only fast FPOs were seen in NEC slices (n = 7). Slower FPOs in WAG/Rij slices reflected a larger degree of thalamocortical synchronization than fast FPOs, and disappeared after surgical separation of cortex and thalamus (n = 5); under these conditions fast FPOs continued to occur in thalamus only. In addition, fast and slower FPOs disappeared in all areas of the WAG/Rij slice during thalamic application of the excitatory amino acid receptor antagonist kynurenic acid (n = 3), while fast FPOs continued to occur in thalamus when kynurenic acid was applied to the cortex (n = 4). Bath application of the N-methyl-D-aspartic acid (NMDA) receptor antagonist 3,3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonate (CPP) abolished slower FPOs in WAG/Rij cortex and thalamus (n = 6) without infuencing fast FPOs recorded in WAG/Rij (n = 6) or NEC slices (n = 4). Moreover, cortical application of CPP (n = 6) abated slower FPOs although they persisted following CPP application to the thalamus (n = 7). Our data demonstrate that highly synchronized, slower FPOs can occur during 4AP application in WAG/Rij but not in NEC slices. This activity, which may represent an in vitro hallmark of thalamocortical epileptogenicity, requires the function of reciprocally connected thalamic and cortical networks and depends on cortical NMDA receptor-mediated mechanisms.

Control of the propagation of dendritic low-threshold Ca(2+) spikes in Purkinje cells from rat cerebellar slice cultures

To investigate the ionic mechanisms controlling the dendrosomatic propagation of low-threshold Ca(2+) spikes (LTS) in Purkinje cells (PCs), somatically evoked discharges of action potentials (APs) were recorded under current-clamp conditions. The whole-cell configuration of the patch-clamp method was used in PCs from rat cerebellar slice cultures. Full blockade of the P/Q-type Ca(2+) current revealed slow but transient depolarizations associated with bursts of fast Na(+) APs. These can occur as a single isolated event at the onset of current injection, or repetitively (i.e. a slow complex burst). The initial transient depolarization was identified as an LTS Blockade of P/Q-type Ca(2+) channels increased the likelihood of recording Ca(2+) spikes at the soma by promoting dendrosomatic propagation. Slow rhythmic depolarizations shared several properties with the LTS (kinetics, activation/inactivation, calcium dependency and dendritic origin), suggesting that they correspond to repetitively activated dendritic LTS, which reach the soma when P/Q channels are blocked. Somatic LTS and slow complex burst activity were also induced by K(+) channel blockers such as TEA (2.5 x 10(-4) M) charybdotoxin (CTX, 10(-5) M), rIberiotoxin (10(-7) M), and 4-aminopyridine (4-AP, 10(-3) M), but not by apamin (10(-4) M). In the presence of 4-AP, slow complex burst activity occurred even at hyperpolarized potentials (-80 mV). In conclusion, we suggest that the propagation of dendritic LTS is controlled directly by 4-AP-sensitive K(+) channels, and indirectly modulated by activation of calcium-activated K(+) (BK) channels via P/Q-mediated Ca(2+) entry. The slow complex burst resembles strikingly the complex spike elicited by climbing fibre stimulation, and we therefore propose, as a hypothesis, that dendrosomatic propagation of the LTS could underlie the complex spike.

Increased neuronal excitability and seizures in the Na(+)/H(+) exchanger null mutant mouse

Mice lacking the Na(+)/H(+) exchanger isoform 1 (NHE1) manifest neurological diseases that include ataxia, motor deficits, and a seizure disorder. The molecular basis for the phenotype has not been clear, and it has not been determined how the lack of NHE1 leads, in particular, to the seizure disorder. We have shown in this work that hippocampal CA1 neurons in mutant mice have a much higher excitability than in wild-type mice. This higher excitability is partly based on an upregulation of the Na(+) current density (608.2 +/- 123.2 pA/pF in NHE1 mutant vs. 334.7 +/- 63.7 pA/pF in wild type in HCO/CO(2)). Alterations in Na(+) channel characteristics, including steady-state inactivation (shift of 18 mV in the depolarization direction in the mutant), recovery from inactivation (tau(h) = 5.22 +/- 0.49 ms in wild-type neurons and 2.20 +/- 0.20 ms in mutant neurons), and deactivation (at -100 mV, tau(d) = 1.75 +/- 0.53 ms in mutant and 0.21 +/- 0.05 ms in wild-type neurons) further enhance the differences in excitability between mutant and wild-type mice. Our investigation demonstrates the existence of an important functional interaction between the NHE1 protein and the voltage-sensitive Na(+) channel. We hypothesize that the increased neuronal excitability and possibly the seizure disorder in mice lacking the NHE1 is due, at least in part, to changes in Na(+) channel expression and/or regulation.

Contribution of intralaminar thalamic nuclei to spike-and-wave-discharges during spontaneous seizures in a genetic rat model of absence epilepsy

In an epileptic rat model of generalized absence epilepsies, the genetic absence epilepsy rats from Strasbourg (GAERS), simultaneous recordings of bilateral epidural electroencephalogram (EEG) of the prefrontal cortex and unit activity of neurons in the intralaminar centrolateral (CL) and paracentral thalamic nucleus (PC) were performed under neurolept-anaesthesia (fentanyl-dehydrobenzperidol analgesia). Spike-and-wave (SW) seizures in these rats are characterized by generalized 7-10 Hz spike-and-wave discharges (SWDs) on the EEG. All neurons recorded in intralaminar thalamic nuclei during spontaneous SWDs showed high-frequency (average 368 Hz, range 200-500 Hz), burst-like activity, which occurred in a highly synchronized fashion with every SWD or with alternating SWD-complexes. Burst discharges in intralaminar neurons were delayed by 13.1 ms (CL) and 12.7 ms (PC), with respect to the spike component of a given SWD on the EEG, whereas burst discharges in the ventrobasal thalamus (VB) and in the rostral nucleus reticularis thalami (rRT) preceded the spike component by 17.8 ms and 8.3 ms, respectively. The onset of SWDs on the EEG was preceded by a tonic firing pattern (20-50 Hz) in about one third of CL and PC neurons. Microiontophoretic application of the gamma-aminobutyric acid (GABA)A receptor antagonist bicuculline aggravated, whereas, the glutamate receptor antagonists DNQX and APV dampened, SWD-related discharges in PC and CL; the GABAB receptor antagonist CGP 35347 had no measurable effect. These data indicate that intrathalamic nuclei are recruited rhythmically during SWDs, through mechanisms that seem to rely on a delayed glutamatergic excitation modulated by GABAergic influences, rather than a GABA-mediated rebound burst activity typical of relay cells. The finding of a temporal delay of SWD-related activity in intrathalamic, compared with "specific" thalamic relay nuclei, does not support the notion of a leading or pacemaker role in SWD generation. It is, however, rather suggestive of a function of intrathalamic neurons during synchronization and maintenance of neuronal oscillations, and these intrathalamic neurons may be recruited through glutamatergic corticofugal inputs.

Cortistatin: a member of the somatostatin neuropeptide family with distinct physiological functions

Cortistatin is a recently discovered neuropeptide relative of somatostatin named after its predominantly cortical expression and ability to depress cortical activity. Cortistatin-14 shares 11 of the 14 amino acids of somatostatin-14 yet their nucleotide sequences and chromosomal localization clearly indicate they are products of separate genes. Now cloned from human, mouse and rat sources, cortistatin is known to bind all five cloned somatostatin receptors and share many pharmacological and functional properties with somatostatin including the depression of neuronal activity. However, cortistatin also has many properties distinct from somatostatin including induction of slow-wave sleep, apparently by antagonism of the excitatory effects of acetylcholine on the cortex, reduction of locomotor activity, and activation of cation selective currents not responsive to somatostatin. Expression of mRNA encoding cortistatin follows a circadian rhythm and is upregulated on deprivation of sleep, suggesting cortistatin is a sleep modulatory factor. This review summarizes recent advances in our understanding of the neurobiology of cortistatin, examines the similarities and differences between cortistatin and somatostatin, and asks the question: does cortistatin bind to a cortistatin-specific receptor?

Corticothalamic inputs control the pattern of activity generated in thalamocortical networks

Absence seizures (3-4 Hz) and sleep spindles (6-14 Hz) occur mostly during slow-wave sleep and have been hypothesized to involve the same corticothalamic network. However, the mechanism by which this network transforms from one form of activity to the other is not well understood. Here we examine this question using ferret lateral geniculate nucleus slices and stimulation of the corticothalamic tract. A feedback circuit, meant to mimic the cortical influence in vivo, was arranged such that thalamic burst firing resulted in stimulation of the corticothalamic tract. Stimuli were either single shocks to mimic normal action potential firing by cortical neurons or high-frequency bursts (six shocks at 200 Hz) to simulate increased cortical firing, such as during seizures. With one corticothalamic stimulus per thalamic burst, 6-10 Hz oscillations resembling spindle waves were generated. However, if the stimulation was a burst, the network immediately transformed into a 3-4 Hz paroxysmal oscillation. This transition was associated with a strong increase in the burst firing of GABAergic perigeniculate neurons. In addition, thalamocortical neurons showed a transition from fast (100-150 msec) IPSPs to slow ( approximately 300 msec) IPSPs. The GABA(B) receptor antagonist CGP 35348 blocked the slow IPSPs and converted the 3-4 Hz paroxysmal oscillations back to 6-10 Hz spindle waves. Conversely, the GABA(A) receptor antagonist picrotoxin blocked spindle frequency oscillations resulting in 3-4 Hz oscillations with either single or burst stimuli. We suggest that differential activation of thalamic GABA(A) and GABA(B) receptors in response to varying corticothalamic input patterns may be critical in setting the oscillation frequency of thalamocortical network interactions.