Participation of corticothalamic cells in penicillin-induced generalized spike and wave discharges

Nonconvulsive Status Epilepticus With Generalized Spike-and-Wave Discharges: Pathophysiological and Nosological Considerations

Absence status epilepticus (ASE) is the most common type of status epilepticus in patients with idiopathic generalized epilepsy (IGE). Like absence seizure, ASE is characterized by generalized spike-and-wave discharges (GSWDs) on the electroencephalogram (EEG). Once considered specific for IGE, GSWDs have increasingly been observed in other forms of epilepsy, as well as in patients with no prior epilepsy. Here, we report three patients with different types of nonconvulsive status epilepticus (NCSE) in which the EEG correlate was GSWDs: a 44-year-old woman with juvenile absence epilepsy who manifested ASE, a 73-year-old woman with anoxic brain injury complicated by NCSE with well-formed GSWDs (as seen in IGE and de novo ASE), and a 41-year-old woman with frontal lobe epilepsy who developed focal NCSE with impaired consciousness. Evidently, patients with generalized epilepsy, focal epilepsy, and no prior epilepsy can all manifest NCSE with similar electroclinical characteristics, i.e., GSWDs and impaired consciousness. This observation adds to the existing evidence that seizures, whether classified as focal or generalized, often involve focal and generalized elements in their pathophysiology. To fully understand seizure pathophysiology, we must steer away from the focal-versus-generalized paradigm that has dominated the nosology of seizures and epilepsy for a very long time.

Do the Dento-Thalamic Connections of Genetic Absence Epilepsy Rats from Strasbourg Differ from Those of Control Wistar Rats?

The thalamo-cortical circuit is important in the genesis of absence epilepsy. This circuit can be influenced by connecting pathways from various parts of central nervous system. The aim of the present study is to define the dento-thalamic connections in Wistar animals and compare the results with genetic absence epilepsy rats from Strasbourg (GAERS) using the biotinylated dextran amine (BDA) tracer. We injected BDA into the dentate nucleus of 13 (n = 6 Wistar and n = 7 GAERS) animals. The dento-thalamic connections in the Wistar animals were denser and were connected to a wider range of thalamic nuclei compared with GAERS. The dentate nucleus was bilaterally connected to the central (central medial [CM], paracentral [PC]), ventral (ventral medial [VM], ventral lateral [VL], and ventral posterior lateral [VPL]), and posterior (Po) thalamic nuclei in Wistar animals. The majority of these connections were dense contralaterally and scarce ipsilaterally. Contralateral connections were present with the parafascicular (PF), ventral posterior medial, ventral anterior (VA), and central lateral (CL) thalamic nuclei in Wistar animals. Whereas in GAERS, bilateral connections were observed with the VL and CM. Contralateral connections were present with the PC, VM, VA, and PF thalamic nuclei in GAERS. The CL, VPL, and Po thalamic nucleus connections were not observed in GAERS. The present study showed weak/deficit dento-thalamic connections in GAERS compared with control Wistar animals. The scarce information flow from the dentate nucleus to thalamus in GAERS may have a deficient modulatory role on the thalamus and thus may affect modulation of the thalamo-cortical circuit.

Spatiotemporal propagation patterns of generalized ictal spikes in childhood absence epilepsy

OBJECTIVE

This work investigates the spatial distribution in time of generalized ictal spikes in the typical absences of childhood absence epilepsy (CAE).

METHODS

We studied twelve children with CAE, who had more than two typical absences during their routine video-EEG. Seizures were identified, and ictal spikes were marked over the maximum electronegative peak, clustered, waveform-averaged and spatiotemporaly analyzed in 2D electrode space.

RESULTS

Consistency of spatiotemporal patterns of ictal spikes was high between the absences of the same child, but low between children. Three main discharge patterns were identified: of anterio-posterior propagation, of posterio-anterior propagation and confined to the frontal/prefrontal regions. In 4 patients, the propagation patterns transformed during the seizure into either a lateralized diminished or a non-lateralized reverse direction form. Most spikes originated fronto-temporaly, all maximized over the frontal/prefrontal electrodes and mostly decayed prefrontaly. In 4 patients, lateralized propagation patterns were identified.

CONCLUSIONS

Ictal spike propagation patterns suggest that epileptogenic CAE networks are personalized, interconnect distal areas in the brain - not the entire cortex - with a tendency to generate bilateral symmetrical discharges, sometimes unsuccessfully. The transformation of propagation patterns during the seizure indicates the existence of dynamic interplay within epileptogenic networks.

SIGNIFICANCE

Our results support the revised concept of ictogenesis of ILAE definition in genetic (also known as idiopathic) generalized epilepsies. Understanding the focal features in CAE avoids misdiagnosis as focal epilepsy and inappropriate treatment.

Discordant Observation of Brain Injury by MRI and Malignant Electroencephalography Patterns in Comatose Survivors of Cardiac Arrest following Therapeutic Hypothermia

BACKGROUND AND PURPOSE

Malignant electroencephalography patterns are considered predictive of poor outcome in comatose survivors of cardiac arrest. We hypothesized that malignant patterns on electroencephalography are associated with evidence of more severe brain injury on MR imaging.

MATERIALS AND METHODS

Retrospective review of clinical, imaging, and electroencephalography data of 33 adult comatose survivors of cardiac arrest following therapeutic hypothermia was performed. Outcomes measured included discharge destination and survival. Imaging studies were visually scored for severity of brain injury. Mean whole-brain apparent diffusion coefficient and percentage of severely injured brain (ADC < 700 × 10^-6 mm²/s) were calculated. Continuous electroencephalographic interpretation was characterized as malignant or nonmalignant. Nonparametric tests were performed to assess the relationship of patient outcome, MR imaging, and electroencephalography patterns.

RESULTS

Subjects with anatomic evidence of diffuse brain injury were less likely to have malignant electroencephalography patterns. Subjects with malignant electroencephalography patterns, invariably associated with bad outcomes, were observed to have whole-brain apparent diffusion coefficient measures similar to those in subjects with nonmalignant electroencephalography patterns and good outcome and different from those in subjects with nonmalignant electroencephalography patterns and bad outcomes. Regional hippocampal or basal ganglia injury was associated with a bad outcome regardless of electroencephalography findings.

CONCLUSIONS

We found discordant evidence of brain injury by MR imaging and electroencephalography, refuting our initial hypothesis. Malignant electroencephalography patterns were generally more frequent in subjects with less severe brain injury by MR imaging. These findings suggest a complementary role of MR imaging and electroencephalography and support the aggressive treatment of malignant electroencephalography patterns in this population.

Neocortical gamma oscillations in idiopathic generalized epilepsy

OBJECTIVE

Absence seizures in patients with idiopathic generalized epilepsy (IGE) may in part be explained by a decrease in phasic GABAA (type-A γ-aminobutyric acid) receptor function, but the mechanisms are only partly understood. Here we studied the relation between ictal and interictal spike-wave discharges (SWDs) and electroencephalography (EEG) gamma oscillatory activity (30-60 Hz) in patients with IGE.

METHODS

EEG recordings were obtained of 14 children with IGE (mean age, 8.5 ± 5 years) and 14 age- and sex-matched controls. Time-frequency analysis of each seizure and seizure-free control epochs was performed and cross-coherences of neocortical gamma oscillations were calculated to describe interictal and ictal characteristics of generalized seizures.

RESULTS

SWDs were characterized with an abrupt increase of oscillatory activity of 3-4 and 13-60 Hz, peaking at 3-4 and 30-60 Hz, and with a simultaneous decrease in the 8-12 Hz frequency band. The rise in EEG gamma oscillations was short-lasting and decreased before activity declined at lower frequency ranges. Compared to control patients, patients with epilepsy also showed higher interictal values of mean coherence of gamma activity, but this interictal increase was not significant after post hoc analysis.

SIGNIFICANCE

Our data support the hypothesis that gamma oscillatory activity increase concomitantly with rises in activity of lower EEG frequencies during absence seizures and that the activity starts to cease earlier than lower EEG frequencies. The data did not support a change in gamma activity preceding the 3-4 Hz SWDs. SWDs are hypothetically generated by the synchronous interaction between the thalamus and the cortex, whereas the production of gamma activity is the result of activity in local inhibitory networks. Thus, the modification of SWD by gamma activity may be understood in terms of the cellular and synaptic mechanisms involved.

Neurophysiology of juvenile myoclonic epilepsy

Juvenile myclonic epilepsy (JME) can be firmly diagnosed by a careful interview of the patient focusing on the seizures and by the EEG with the help, if necessary, of long-term video-EEG monitoring using sleep and/or sleep deprivation. Background activity is normal. The interictal EEG shows diffuse or generalized spike-wave (SW) and polyspike-wave (PSW) discharges. In some patients, non-specific changes or misleading features such as focal changes are found. Changes are mostly seen at sleep onset and at awakening. Provoked awakenings are more likely to activate interictal paroxysmal abnormalities than spontaneous awakenings. The presence of a photoparoxysmal response with or without myoclonic jerks (MJ) is common (30% of the cases). Myoclonic jerks are associated with a discharge of fast, irregular, generalized PSWs that predominate anteriorly. Myoclonic jerks appear to be associated with rhythmic EEG (spike) potentials at around 20Hz. These frequencies are in the range of movement-related fast sensorimotor cortex physiological rhythms. The application of jerk-locked averaging technique has provided findings consistent with a cortical origin of MJ. Paired TMS (transcranial magnetic stimulation) studies showed a defective intracortical inhibition, due to impaired GABA-A mediated mechanisms. In this review, we present the EEG characteristics of JME with particular emphasis on the pathophysiology of MJ and on the role of sleep deprivation on interictal and ictal changes.

Altered regional synchronization in epileptic patients with generalized tonic-clonic seizures

Epilepsy is a common neurological disorder characterized by hyper-synchronous abnormalities of neurons. Resting state brain activity measured by fMRI might evaluate the synchronization of the disorder. To investigate the alteration of the haemodynamic synchronization in epilepsy, resting-state fMRI (RS-fMRI) was performed on 25 patients with primarily generalized tonic-clonic seizures (GTCS), along with 25 age- and sex-matched healthy subjects. Regional homogeneity (ReHo), a measurement of the synchronization of spontaneous RS-fMRI signal oscillations within spatially neighboring voxels, was examined. Compared with the healthy controls, the patients with GTCS showed bilaterally and symmetrically altered ReHo in the cortical and subcortical structures. In addition, a correlation analysis of the ReHo measurement versus the epilepsy duration was performed, and highly negative correlations were observed in thalamus, insula and the regions followed the pattern of 'default' state of brain function. The current findings demonstrate that altered regional synchronization of brain activity exists in the patients with GTCS during interictal period, and there is potential in utilizing the ReHo method in RS-fMRI analyses of epilepsy.

Mapping hemodynamic correlates of seizures using fMRI: A review

Functional magnetic resonance imaging (fMRI) is able to detect changes in blood oxygenation level associated with neuronal activity throughout the brain. For more than a decade, fMRI alone or in combination with simultaneous EEG recording (EEG-fMRI) has been used to investigate the hemodynamic changes associated with interictal and ictal epileptic discharges. This is the first literature review to focus on the various fMRI acquisition and data analysis methods applied to map epileptic seizure-related hemodynamic changes from the first report of an fMRI scan of a seizure to the present day. Two types of data analysis approaches, based on temporal correlation and data driven, are explained and contrasted. The spatial and temporal relationship between the observed hemodynamic changes using fMRI and other non-invasive and invasive electrophysiological and imaging data is considered. We then describe the role of fMRI in localizing and exploring the networks involved in spontaneous and triggered seizure onset and propagation. We also discuss that fMRI alone and combined with EEG hold great promise in the investigation of seizure-related hemodynamic changes non-invasively in humans. We think that this will lead to significant improvements in our understanding of seizures with important consequences for the treatment of epilepsy.

From sleep spindles of natural sleep to spike and wave discharges of typical absence seizures: is the hypothesis still valid?

The temporal coincidence of sleep spindles and spike-and-wave discharges (SWDs) in patients with idiopathic generalized epilepsies, together with the transformation of spindles into SWDs following intramuscular injection of the weak GABAA receptor (GABAAR) antagonist, penicillin, in an experimental model, brought about the view that SWDs may represent ‘perverted’ sleep spindles. Over the last 20 years, this hypothesis has received considerable support, in particular by in vitro studies of thalamic oscillations following pharmacological/genetic manipulations of GABAARs. However, from a critical appraisal of the evidence in absence epilepsy patients and well-established models of absence epilepsy it emerges that SWDs can occur as frequently during wakefulness as during sleep, with their preferential occurrence in either one of these behavioural states often being patient dependent. Moreover, whereas the EEG expression of both SWDs and sleep spindles requires the integrity of the entire cortico-thalamo-cortical network, SWDs initiates in cortex while sleep spindles in thalamus. Furthermore, the hypothesis of a reduction in GABAAR function across the entire cortico-thalamo-cortical network as the basis for the transformation of sleep spindles into SWDs is no longer tenable. In fact, while a decreased GABAAR function may be present in some cortical layers and in the reticular thalamic nucleus, both phasic and tonic GABAAR inhibitions of thalamo-cortical neurons are either unchanged or increased in this epileptic phenotype. In summary, these differences between SWDs and sleep spindles question the view that the EEG hallmark of absence seizures results from a transformation of this EEG oscillation of natural sleep.

Contrary roles of kainate receptors in transmitter release at corticothalamic synapses onto thalamic relay and reticular neurons

Corticothalamic fibres, which originate from layer VI pyramidal neurons in the cerebral cortex, provide excitatory synaptic inputs to both thalamic relay neurons and reticular neurons; reticular neurons in turn supply inhibitory inputs to thalamic relay neurons. Pyramidal cells in layer VI in the mouse somatosensory cortex highly express mRNA encoding kainate receptors, which facilitate or depress transmitter release at several synapses in the central nervous system. We report here that contrary modulation of transmitter release from corticothalamic fibres onto thalamic relay and reticular neurons is mediated by activation of kainate receptors in mouse thalamic ventrobasal complex and thalamic reticular nucleus. Exogenous kainate presynaptically depresses the synaptic transmission at corticothalamic synapses onto thalamic relay neurons, but facilitates it at corticothalamic synapses onto reticular neurons. Meanwhile, the lemniscal synaptic transmission, which sends primary somatosensory inputs to relay neurons, is not affected by kainate. In addition, GluR5-containing kainate receptors are involved in the depression of corticothalamic synaptic transmission onto relay neurons, but not onto reticular neurons. Furthermore, synaptically activated kainate receptors mimic these effects; high-frequency stimulation of corticothalamic fibres depresses synaptic transmission onto relay neurons, but facilitates it onto reticular neurons. Our results suggest that the opposite sensitivity of kainate receptors at the two corticothalamic synapses is governed by cortical activity and regulates the balance of excitatory and inhibitory inputs to thalamic relay neurons and therefore their excitability.

Metabotropic glutamate receptors as a strategic target for the treatment of epilepsy

Epilepsy is a chronic neurological disorder that has many known types, including generalized epilepsies that involve cortical and subcortical structures. A proportion of patients have seizures that are resistant to traditional anti-epilepsy drugs, which mainly target ion channels or postsynaptic receptors. This resistance to conventional therapies makes it important to identify novel targets for the treatment of epilepsy. Given the involvement of the neurotransmitter glutamate in the etiology of epilepsy, targets that control glutamatergic neurotransmission are of special interest. The metabotropic glutamate receptors (mGluRs) are of a family of eight G-protein-coupled receptors that serve unique regulatory functions at synapses that use the neurotransmitter glutamate. Their distribution within the central nervous system provides a platform for both presynaptic control of glutamate release, as well as postsynaptic control of neuronal responses to glutamate. In recent years, substantial efforts have been made towards developing selective agonists and antagonists which may be useful for targeting specific receptor subtypes in an attempt to harness the therapeutic potential of these receptors. We examine the possibility of intervening at these receptors by considering the specific example of absence seizures, a form of generalized, non-convulsive seizure that involves the thalamus. Views of the etiology of absence seizures have evolved over time from the "centrencephalic" concept of a diffuse subcortical pacemaker toward the "cortical focus" theory in which cortical hyperexcitability leads the thalamus into the 3-4 Hz rhythms that are characteristic of absence seizures. Since the cortex communicates with the thalamus via a massive glutamatergic projection, ionotropic glutamate receptor (iGluR) blockade has held promise, but the global nature of iGluR intervention has precluded the clinical effectiveness of drugs that block iGluRs. In contrast, mGluRs, because they modulate iGluRs at glutamatergic synapses only under certain conditions, may quell seizure activity by selectively reducing hyperactive glutamatergic synaptic communication within the cortex and thalamus without significantly affecting normal response rates. In this article, we review the circuitry and events leading to absence seizure generation within the corticothalamic network, we present a comprehensive review of the synaptic location and function of mGluRs within the thalamus and cerebral cortex, and review the current knowledge of mGluR modulation and seizure generation. We conclude by reviewing the potential advantages of Group II mGluRs, specifically mGluR2, in the treatment of both convulsive and non-convulsive seizures.

Unique presynaptic and postsynaptic roles of Group II metabotropic glutamate receptors in the modulation of thalamic network activity

The thalamic reticular nucleus (TRN) is a sheet of GABAergic neurons that project to other TRN neurons and to associated thalamocortical relay nuclei. The TRN receives glutamatergic synaptic inputs from cortex as well as reciprocal inputs from the collaterals of thalamocortical neurons. In addition to ionotropic glutamate receptors, metabotropic glutamate receptors (mGluRs) are present in the TRN circuitry. Using whole cell voltage clamp recordings, we pharmacologically characterized unique pre- and postsynaptic functions for Group II mGluRs (mGluR 2 and mGluR 3) within the TRN circuitry in ferrets. mGluR 2 was found on presynaptic cortical axon terminals in the TRN, where it reduced glutamate release, while mGluR 3 acted postsynaptically on TRN cells to increase membrane conductance. Using miniature inhibitory postsynaptic current analysis, we also found that picrotoxin-sensitive intra-TRN GABA-mediated neurotransmission was not affected by administration of a Group II mGluR agonist, indicating that neither mGluR 2 nor 3 acts on presynaptic GABA-containing terminals within the TRN. Because strong corticothalamic activation is implicated in abnormal thalamic rhythms, we used extracellular recordings in the lateral geniculate nucleus to study the effect of Group II mGluR agonists upon these slow oscillations. We induced approximately 3 Hz spike-and-wave discharge activity through corticothalamic stimulation, and found that such activity was reduced in the presence of the Group II mGluR agonist, (-)-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate (LY379268). These data indicate that Group II mGluR reduce the impact of corticothalamic excitation, and that they may be a useful target in the reduction of absence-like rhythms.

Idiopathic generalized epilepsies: clinical and electroencephalogram diagnosis and treatment

This review concentrates on the principles of the clinical and electroencephalogram diagnosis of idiopathic generalized epilepsies and their treatment. The electroclinical variability of the main seizure types is detailed and particular emphasis is placed on the differential diagnosis from other seizures and nonepileptic conditions that is essential for the optimal management of these patients. The authors review the various idiopathic generalized epilepsy subsyndromes and conditions that are included in both the 1989 International League Against Epilepsy classification system and the recently proposed International League Against Epilepsy scheme, but also syndromes and forms that have not been formally recognized. Finally, the authors describe the principles of antiepileptic drug treatment with the old and newer drugs, and their specific indications and contraindications in the various syndromes and seizure types.

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.

Use and Abuse of EEG in the Diagnosis of Idiopathic Generalized Epilepsies

This article concentrates on the role of electroencephalograms (EEGs) in the diagnosis and management of patients with idiopathic generalized epilepsies (IGEs). We review the morphologic and behavioral characteristics of the interictal and ictal EEG markers of IGE that should guide recording strategies to augment its diagnostic yield, and we attempt to delineate those particular features that may be relevant to different IGE syndromes. We also explore the electrographic boundaries between IGEs and cryptogenic/symptomatic generalized and focal epilepsies, and focal/secondary generalized epilepsies, with particular relevance to the phenomena of focal abnormalities and secondary bilateral synchrony, commenting on possible diagnostic pitfalls and areas of uncertainty.

Presynaptic inhibition of corticothalamic feedback by metabotropic glutamate receptors

The thalamus relays sensory information to cortex, but this information may be influenced by excitatory feedback from cortical layer VI. The full importance of this feedback has only recently been explored, but among its possible functions are influences on the processing of sensory features, synchronization of thalamic firing, and transitions in response mode of thalamic relay cells. Uncontrolled, corticothalamic feedback has also been implicated in pathological thalamic rhythms associated with certain neurological disorders. We have found a form of presynaptic inhibition of corticothalamic synaptic transmission that is mediated by a Group II metabotropic glutamate receptor (mGluR) and activated by high-frequency corticothalamic activity. We tested putative retinogeniculate and corticogeniculate synapses for Group II mGluR modulation within the dorsal lateral geniculate nucleus of the ferret thalamus. Stimulation of optic-tract fibers elicited paired-pulse depression of excitatory postsynaptic currents (EPSCs), whereas stimulation of the optic radiations elicited paired-pulse facilitation. Paired-pulse responses were subsequently used to characterize the pathway of origin of stimulated synapses. Group II mGluR agonists (LY379268 and DCG-IV) applied to thalamic neurons under voltage-clamp conditions reduced the amplitude of corticogeniculate EPSCs. Stimulation with high-frequency trains produced a facilitating response that was reduced by Group II mGluR agonists, but was enhanced by the selective antagonist LY341495, revealing a presynaptic, mGluR-mediated reduction of high-frequency corticogeniculate feedback. Agonist treatment did not affect EPSCs from stimulation of the optic tract. NAAG (reported to be selective for mGluR3) was ineffective at the corticogeniculate synapse, implicating mGluR2 in the observed effects. Our data are the first to show a synaptically elicited form of presynaptic inhibition of corticothalamic synaptic transmission that is mediated by presynaptic action of mGluR2. This presynaptic inhibition may partially mute sensory feedback and prevent reentrant excitation from initiating abnormal thalamic rhythms.

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.

The role of subcortical structures in human epilepsy

Like normal cerebral function, epileptic seizures involve widespread network interactions between cortical and subcortical structures. Although the cortex is often emphasized as the site of seizure origin, accumulating evidence points to a crucial role for subcortical structures in behavioral manifestations, propagation, and, in some cases, initiation of epileptic seizures. Extensive previous studies have shown the importance of subcortical structures in animal seizure models, but corresponding human studies have been relatively few. We review the existing evidence supporting the importance of the thalamus, basal ganglia, hypothalamus, cerebellum, and brain stem in human epilepsy. We also propose a "network inhibition hypothesis" through which focal cortical seizures disrupt function in subcortical structures (such as the medial diencephalon and pontomesencephalic reticular formation), leading secondarily to widespread inhibition of nonseizing cortical regions, which may in turn be responsible for behavioral manifestations such as loss of consciousness during complex partial seizures.

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.

Thalamocortical synapses

Thalamocortical synapses inform the cerebral neocortex about the external and internal worlds. The thalamus produces myriad thalamocortical pathways that vary in morphological, physiological, pharmacological and functional properties. All these features are of great importance for understanding how information is acquired, integrated, processed, stored and retrieved by the thalamocortical system. This paper reviews the properties of the afferents from thalamus to cortex, and identifies some of the gaps in our knowledge of thalamocortical pathways.

Absence seizures induce a decrease in cerebral blood flow: human and animal data

Our previous studies on cerebral metabolic activity in genetic absence epilepsy rats from Strasbourg (GAERS) were in favor of decreased functional activity during absences and normal or increased interictal activity. To ascertain that hypothesis, in the present study we performed continuous measurements of CBF in both children with typical absence epilepsy and GAERS, using Doppler ultrasonography and laser-Doppler flowmetry, respectively. CBF fluctuations during absences were recorded in four children between 5 and 6 years of age and 16 adult GAERS. In both children and animals, CBF measured in the middle cerebral artery and cortical capillaries, respectively, significantly decreased by a median value of 20-24% under basal levels during spontaneous absences. In GAERS, CBF levels were continuously decreased during haloperidol-induced absence status epilepticus, while they were not affected by ethosuximide. Conversely, convulsive seizures induced in rats either by kainate or picrotoxin led to a 175-664% increase in CBF levels. In conclusion, the present data show that during spontaneous absences, CBF decreases under basal levels in both cortical capillaries (GAERS) and the middle cerebral artery (children). Moreover, these fluctuations occur in vessels with normal vascular reactivity, are not mediated by changes in PO2, PCO2, or arterial blood pressure, and represent rather a response to reduced metabolic demand.

Inter- and intrahemispheric phase changes of diffuse 3 Hz spike-and-wave complex

The inter- and intrahemispheric phase characteristics were investigated on diffuse 3 Hz spike-and-wave complex (D3SW) in eight epileptic patients who were diagnosed with typical absence. The phase of D3SW was analyzed sequentially using cross-power spectral arrays dividing D3SW into two components (spike-and-wave complex, spike component). The phase of spike-and-wave complex preceded most at the midline structure, and delayed symmetrically toward the lateral side of each hemisphere in all cases, while not in all cases when spike component was investigated. In spite of this symmetry, there was sparse correlation in phase change between the homologous hemispheres. There was almost no linear correlation between the phase of spike-and-wave complex and the corresponding spike component. The author concludes that the 'centrencephalic system' hypothesis still plays an important role for the generation of D3SW, and suggest a mechanism other than the cortical recurrent inhibition for the generation of wave component.

Synaptic and membrane mechanisms underlying synchronized oscillations in the ferret lateral geniculate nucleus in vitro

1. The cellular basis for generation of spindle waves and a slower synchronized oscillation resembling absence seizures was investigated with extracellular and intracellular recording techniques in slices of ferret dorsal lateral geniculate nucleus (LGNd) maintained in vitro. 2. Intracellular recording from LGNd relay cells in vitro revealed that spindle waves occurred once every 3-30 s and were associated with barrages of inhibitory postsynaptic potentials (IPSPs) occurring at a frequency of 6-10 Hz. These IPSPs resulted in the generation of rebound low threshold Ca2+ spikes at 2-4 Hz, owing to the intrinsic propensity of LGNd relay cells to generate oscillatory burst firing in this frequency range. These rebound bursts of action potentials were highly synchronized with local multiunit and single unit activity. 3. The spindle wave-associated IPSPs in LGNd relay cells exhibited a mean reversal potential of -86 mV. This reversal potential was shifted to more depolarized membrane potentials with the intracellular injection of Cl- through the use of KCl-filled microelectrodes. Simultaneous recording from the perigeniculate nucleus (PGN) and LGNd revealed the IPSPs to be synchronous with the occurrence of burst firing in the PGN. Excitation of PGN neurons with local electrical stimulation after pharmacological block of excitatory amino acid transmission resulted in bicuculline-sensitive IPSPs in relay neurons similar in amplitude and time course to those occurring during spindle waves. 4. Application of (-)-bicuculline methiodide resulted in the abolition of spindle wave-associated IPSPs or in the slowing of the rate of rise, an increase in amplitude and a prolongation of these IPSPs; this resulted in a synchronized 2-4 Hz oscillation, in which each relay cell strongly burst on nearly every cycle, thus forming a paroxysmal event. Bath application of the GABAB receptor antagonist 2-OH-saclofen blocked these slowed oscillations, indicating that they are mediated by the activation of GABAB receptors. In contrast, pharmacological antagonism of GABAB receptors did not block the generation of normal spindle waves. 5. These and other results indicate that spindle waves are generated in the ferret LGNd in vitro as a network phenomenon occurring through an interaction between the relay cells of the LGNd and the GABAergic neurons of the PGN. We propose that burst firing in PGN cells hyperpolarizes relay neurons through activation of GABAA receptors. These IPSPs result in rebound burst firing in LGNd cells, which then excite PGN neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Role of the ferret perigeniculate nucleus in the generation of synchronized oscillations in vitro

1. The cellular mechanisms by which neurons of the ferret perigeniculate nucleus (PGN) participate in the generation of spindle waves and slowed absence seizure-like oscillations were investigated with intracellular and extracellular recording techniques in geniculate slices maintained in vitro. 2. During spindle wave generation, PGN neurons generated repetitive (2-9 Hz) high frequency (up to 500 Hz) burst discharges mediated by the activation of a low threshold Ca2+ spike by the arrival of barrages of excitatory postsynaptic potentials (EPSPs). In most PGN cells at resting membrane potentials (-60 to -70 mV) spindle waves were associated with a progressive hyperpolarization that persisted as a prolonged after-hyperpolarization. 3. The EPSPs occurring in PGN cells were highly synchronized with burst firing in the neighbouring portion of the dorsal lateral geniculate nucleus (LGNd) and were intermixed with short duration inhibitory postsynaptic potentials (IPSPs). After block of GABAergic receptors, the EPSPs occurring during the generation of spindle waves reversed polarity at around 0 mV. In addition, these EPSPs were completely blocked with the bath application of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), as was spindle wave generation in both the PGN and LGNd. 4. Slowing the intraspindle frequency to 2-4 Hz with pharmacological block of GABAA receptors resulted in a marked increase in the intensity of burst firing by PGN cells such that the number of action potentials per burst increased from a maximum of thirteen to a maximum of sixty. Block of GABAA receptors also resulted in a marked increase in the amplitude and duration of the EPSP barrages arriving from the relay laminae during generation of the slowed oscillation. 5. These findings indicate that spindle waves are generated in the ferret LGNd in vitro through an interaction between the GABAergic neurons of the PGN and relay neurons, such that burst firing in relay neurons activates a barrage of EPSPs and a subsequent low threshold Ca2+ spike in PGN cells. This activation of PGN neurons inhibits a substantial number of relay cells, a few of which rebound burst after this IPSP, thus starting the cycle again. Block of GABAA receptors results in a marked enhancement of activity in PGN cells through increased excitation from relay cells and disinhibition from neighbouring PGN cells. This increased activity in PGN neurons results in a markedly enhanced activation of GABAB receptors in relay neurons and the subsequent generation of paroxysmal activity that is similar to that associated with absence seizures.

Feline generalized penicillin epilepsy

Feline generalized penicillin epilepsy represents an experimental model of generalized spike-and-wave discharges occurring during clinical absence attacks. Spike-and-wave discharges of feline generalized penicillin epilepsy also have a pharmacological profile that is similar to that encountered in human absence attacks. Studies on the respective roles played by the thalamus and cortex in the generation of spike-and-wave discharges indicate that both structures are important in the elaboration of such generalized activity. Moreover, GABAA-mediated, intracortical inhibitory mechanisms are preserved and eventually enhanced at a time when generalized spike-and-wave discharges of feline generalized penicillin epilepsy are recorded. A preservation of GABA-mediated mechanisms in pure absence epilepsy might explain the differences in prognostic outlook that characterizes this type of epilepsy from seizures in which GABAergic mechanisms break down (e.g., generalized convulsive and partial epileptic attacks).

Electroencephalography

Notwithstanding recent advances in neuroimaging, EEG remains a major technique for investigation of the brain. Its main applications are in assessment of cerebral function rather than for detecting structural abnormalities. The principal clinical applications are in epilepsy, states of altered consciousness including postanoxic and traumatic coma, the parasomnias, dementias, toxic confusional states, cerebral infections, and various other encephalopathies. Abnormalities in EEG reflect general pathophysiological processes, raised intracranial pressure, cerebral anoxia, or oedema, epileptogenesis etc, and show little specificity for a particular disease. Consequently, they need to be interpreted in a particular clinical context; the use of routine EEG examination for screening purposes is rarely of value. Conversely, the investigation becomes most cost effective when applied to specific problems--for instance, monitoring serial changes in postanoxic coma or during open heart surgery, differential diagnosis (by telemetric ictal recordings) of epileptic and non-epileptic attacks, and providing early prediction of outcome after stroke. High technological standards and an individualised problem solving approach are prerequisites of a cost effective, reliable clinical EEG service. These are most likely to be achieved by a considered, selective referral policy, the use where necessary of prolonged complex procedures such as telemetry, and the avoidance of routine examinations of dubious clinical relevance.

Electrophysiology and Pharmacology of the Corticothalamic Input to Lateral Thalamic Nuclei: an Intracellular Study in the Cat

Though most experimental evidence indicates that the corticothalamic (CT) pathway would exert a direct excitatory action on thalamic relay neurons, the electrophysiological features of this excitation have never been clearly described. A methodological problem in previous electrophysiological studies was that direct corticofugal effects on relay cells could not be separated from those mediated by collateral activation of reticular thalamic neurons. In the present study, the reticular complex was lesioned by kainic acid and the CT response of relay neurons of the ventral lateral nucleus was recorded intracellularly in cats under pentobarbital or urethane anaesthesia. Following reticular thalamic lesions, a prominent depolarization was triggered in thalamic relay cells by stimulation of the CT pathway. This depolarization strongly drove spike discharges, and its amplitude augmented when the stimulation rate exceeded 2 Hz. Tetanizing the CT input with short trains (100 - 200 Hz for 200 - 300 ms) produced a similar augmentation to test volleys for 15 - 30 s after the tetanos. The CT excitation and its frequency-dependent augmentation were depressed by ketamine injection or by local application of N-methyl-D-aspartate (NMDA) antagonists. The augmenting phenomenon appeared strictly homosynaptic. For instance, it did not appear during repetitive stimulation of the cerebellar input, nor did the CT input potentiate subthreshold synaptic potentials of cerebellar origin during a conditioning procedure. Conversely, the cerebellar excitation was depressed when it occurred during the CT depolarization. It is concluded that the direct synaptic responses induced by CT fibres in relay neurons are mediated at least partly by the activation of NMDA receptors. Moreover, the marked non-linear additivity of cerebellar and CT synaptic potentials raises questions concerning the presumed improvement of thalamic transmission of peripheral informations ensured by the CT input. Instead, both inputs could compete for control of the firing of thalamic neurons. The numerical importance of CT fibres and the strong augmenting mechanism operating at synaptic sites in the thalamus suggest that the role of the thalamus is not only to transfer peripheral informations toward the cortex, but also and mainly to feed back to the cortex a modified copy of its own neuronal constructs.

The electroencephalographic pattern during electroconvulsive therapy: V. Observations on the origins of phase III delta energy and the mechanism of action of ECT

The generation of the spike-wave activity of Phase III of ECT seizures is attributed to the recurrence of synchronized, prolonged periods of intense inhibitory current flow (hyperpolarization), and associated rebound spike bursts, produced by the inhibitory circuit relationships and intrinsic electrophysiological properties of thalamic neurons. An anatomical and neurophysiological model of the development of generalized, synchronous 3-Hz spike-wave seizure activity is proposed which outlines the origin, maintenance, slowing, and termination of this fundamental seizure rhythm. Phase III inhibitory current flow (delta energy) and/or spike bursts may bring about therapeutic benefit by initiating a chain of agonist-independent and agonist-dependent events which results in long-term augmentation of serotonergic and noradrenergic neurotransmission and diminution of cholinergic neurotransmission in the forebrain. A specific anatomical and functional model of the mechanism of action of ECT is proposed, in which: (1) adrenergic and cholinergic pathways in the forebrain are assumed to be massively stimulated during ECT seizures, whereas serotonergic pathways are assumed to be inhibited during these seizures; (2) the beneficial effects of ECT are considered to be more dependent upon ECT-induced changes in 5-HT neurotransmission than upon alteration of noradrenergic function; (3) these beneficial effects involve up-regulation of 5-HT2 and down-regulation of M1- and M2-muscarinic receptor densities by both agonist-independent and agonist-dependent mechanisms, coupled with functional augmentation of noradrenergic neurotransmission; and (4) these effects may be brought about by Phase III inhibitory current flow- and/or spike burst-induced alteration of the function of second-messenger generator systems.

Spontaneous spike and wave discharges in thalamus and cortex in a rat model of genetic petit mal-like seizures

In an inbred strain of Wistar rats, spontaneous spike and wave discharges (8 to 10 c/s) appeared regularly on the EEG during quiet wakefulness and were accompanied by an arrest of behavioral activity associated with vibrissal and facial myoclonia. These seizures were recorded over the entire neocortex, but predominantly in the frontoparietal cortex. Subcortical bipolar recordings in chronic preparations showed that the lateral thalamic nuclei were greatly involved in these discharges: high-voltage spike and waves always appeared either simultaneously with, or slightly before the cortical discharges. In some cases, thalamic discharges were not accompanied by cortical discharges. No discharges were recorded in medial thalamic nuclei, in the cingulate cortex, or in the hippocampus. These results confirm the thalamocortical prevalence in the development of these rats' petit mal-like seizures, with a possible driving from thalamic nuclei.

Involvement of cortical, thalamic and midbrain reticular formation neurons in spike and wave discharges: extracellular study in feline generalized penicillin epilepsy

Extracellular activity of single units, simultaneously recorded in cortex, thalamus, and midbrain reticular formation was investigated during feline generalized penicillin epilepsy. The firing activity of neurons recorded in the cortex was invariably and consistently enhanced in coincidence with the positive peak and the positive-negative transient of the "spike" of the spike and wave complex, and it was greatly decreased during the wave. In the nonspecific thalamic nuclei three classes of neurons were identified according to their patterns of activity during the spike and wave complex: (i) neurons behaving like cortical units, (ii) neurons with enhanced firing activity during the wave and a decreased activity during the "spike," and (iii) unmodified neurons. In the nucleus lateralis posterior neurons of the third class were not found. Most midbrain reticular neurons could be classified in the same three classes of the nonspecific thalamic nuclei; however, 11% of those units increased their activity 20 to 30 ms earlier than did the cortical units (class IV). Investigation of the activities of all these neuronal populations immediately prior to a spike and wave discharge showed that the rhythmic cycle of excitation-inhibition commenced earlier in the cortical neurons than in any other subcortical neuron. Moreover, there were some nonspecific thalamic neurons of class II with an inhibitory phase exactly coincident with the activation of class IV midbrain reticular neurons. These data suggest (i) a leading role of cortical neurons in initiating and maintaining a spike and wave burst; (ii) the involvement of a corticothalamocortical circuit in timing the bursts, and (iii) an accessory reticulothalamic loop also involved in regulating the intraburst frequency of the spike and wave complex.

Transition from spindles to generalized spike and wave discharges in the cat: simultaneous single-cell recordings in cortex and thalamus

The relationships between the activity of the cortex and that of a "specific" (n. lateralis posterior, LP) and an intralaminar thalamic nucleus (n. centralis medialis, NCM) were studied in the cat during the transition from spontaneous spindles to generalized spike and wave (SW) discharge following i.m. penicillin injection. The EEG and extracellular single-unit activity were recorded in cortex and thalamus during the spindle stage and at different intervals after penicillin until well developed SW discharges were present. Computer-generated EEG averages and histograms of single-unit activity were triggered by either peaks of cortical or thalamic EEG transients or by cortical or thalamic action potentials. In agreement with previous observations, cortical neurons increasingly fired during the spindle wave as it was transformed into the "spike" of the SW complex, while a period of neuronal silence gradually developed as the "wave" of the SW complex emerged. Similar changes developed in the thalamus, particularly in LP, either concurrently with or more often after the onset of the changes in the cortex. Most neurons in NCM, continued to fire randomly even after well developed SWs and rhythmic neuronal discharges had developed in cortex and LP. Only 4/11 NCM neurons did ultimately exhibit a rhythmic firing pattern similar to that seen in the cortex and LP. The correlation between cortical and thalamic unit activity was low during spindles, but gradually increased during the development of SW discharges. These data confirm that the cortex is the leading element in the transition from spindles to SWs. Increasingly, in the course of this transition, cortical and thalamic neuronal firing becomes more intimately phase-locked. This mutual interrelationship appears to be more pronounced between cortex and "specific" than intralaminar thalamic nuclei.

Differential participation of some 'specific' and 'non-specific' thalamic nuclei in generalized spike and wave discharges of feline generalized penicillin epilepsy

Extracellular single unit and electroencephalographic (EEG) activity during generalized spike and wave discharges (SW) induced by i.m. penicillin was recorded simultaneously in the cortex, in a 'specific' thalamic nucleus (n. lateralis posterior, LP) and in some 'non-specific' thalamic nuclei (n. centralis medialis, NCM; n. centrum medianum, CM; n. centralis lateralis, CL) Computer-generated EEG averages and histograms of single unit activity were triggered by either peaks of EEG transients or action potentials. The time at which cortical neurons (66/66) were most likely to fire was during the 'spike' of the SW complex while absence of firing was the rule during the 'wave'. Most LP neurons (23/26) showed a similar pattern, 3 cells firing preferentially during the 'wave'. In NCM only 17 of 39 neurons fired during the 'spike', 8 of 39 neurons during the 'wave' while the others showed no change in their firing pattern during SWs. Twenty-six of 30 CM and 20 of 24 CL neurons fired during the 'spike' of SW; the other cells in these nuclei did not change their firing pattern during SWs. When present, rhythmic fluctuations in firing linked to SW discharge were less prominent in these 'non-specific' thalamic nuclei than in cortex and LP. Furthermore, participation of NCM, CM and CL neurons in the SW rhythm occurred only after neurons in cortex and LP had become involved in it. Thus, as is the case for cortical neurons, the main firing pattern of thalamic cells during SWs consists of an oscillation between 'excitation' during the 'spike' and 'inhibition' during the 'wave' of the SW complex. However, the coupling between cortical and thalamic neuronal firing is less intimate for cells of the 'non-specific' thalamic nuclei than for a 'specific' nucleus such as LP. Thus, at least some 'specific' thalamic nuclei are more intimately involved in the mechanism of SW discharge than the midline intralaminar nuclei.

Enhanced response of cortical neurons to thalamic stimuli precedes the appearance of spike and wave discharges in feline generalized penicillin epilepsy

Peristimulus time histograms of extracellularly recorded action potential discharges of cortical neurons in response to single shock and/or repetitive stimulation of 'specific' and 'non-specific' nuclei of the thalamus were studied after i.m. penicillin injection during a period corresponding to that of the development of spike and wave (SW) discharges of feline generalized penicillin epilepsy (FGPE). After i.m. penicillin cortical neurons displayed an enhancement of both the excitatory and 'inhibitory' phases of their responses to single shock stimulation of n. centralis medialis (NCM). This increase was even more pronounced for responses induced by repetitive stimulation of NCM at the frequencies inducing typical recruiting responses. These changes always preceded the appearance of SW discharges. Changes of the responses of cortical neurons to single shock and repetitive stimulation of 'specific' thalamic nuclei after penicillin were weak and inconsistent, although when observed were characterized by an enhancement of both excitatory and 'inhibitory' phases. The latter appeared not to decrease after i.m. penicillin. These data suggest that the appearance of SW discharges of FGPE is closely related to an increased responsiveness of cortical neurons to thalamocortical volleys arising from the so-called 'non-specific' nuclei. This facilitation of the recruiting process is accompanied by an increase of both excitatory and 'inhibitory' phases of the cortical neuronal responses induced by the volleys.