Lateral asymmetry of early seizure manifestations in experimental generalized epilepsy

Intracortical functional connectivity dynamics induced by reflex seizures

Functional connectivity analysis is gaining more interest due to its promising clinical applications. To study network mechanisms underlying seizure termination and postictal depression, we explore dynamics of interhemispheric functional connectivity near the offset of focal and bilateral seizures in the experimental model of reflex audiogenic epilepsy. In the model, seizures and spreading depression are induced by sound stimulation of genetically predisposed rodents. We characterize temporal evolution of seizure-associated coupling dynamics in the frontoparietal cortex during late ictal, immediate postictal and interictal resting states, using two measures applied to local field potentials recorded in awake epileptic rats. Signals were analyzed with mean phase coherence index in delta (1-4 Hz), theta (4-10 Hz) beta (10-25 Hz) and gamma (25-50 Hz) frequency bands and mutual information function. The study shows that reflex seizures elicit highly dynamic changes in interhemispheric functional coupling with seizure-, region- and frequency-specific patterns of increased and decreased connectivity during late ictal and immediate postictal periods. Also, secondary generalization of recurrent seizures (kindling) is associated with pronounced alterations in resting-state functional connectivity - an early wideband decrease and a subsequent beta-gamma increase. The findings show that intracortical functional connectivity is dynamically modified in response to seizures on short and long timescales, suggesting the existence of activity-dependent plastic network alterations that may promote or prevent seizure propagation within the cortex and underlie postictal behavioral impairments.

Localizing and lateralizing values of postictal behavioral impairments in epileptic rats

Transient postictal behavioral impairments in patients with epilepsy provide clues to seizure localization, but no attempt has been made previously to study the localization/lateralization value of postseizure motor disturbances in experimental models of epilepsy. The present study investigated relation of postictal motor deficit to seizure localization in the rat model of sound-induced reflex epilepsy. Sound-induced motor seizures started with a focal brainstem seizure (running) and progressed to a secondarily generalized seizure. Depending on innate or acquired seizure susceptibility of rats, focal brainstem seizures secondarily generalized within the brainstem (brainstem-generalized seizures) or spread to the forebrain (focal or generalized forebrain seizures). All sound-induced seizures were followed by catalepsy and abnormal limb posturing. The duration of the postictal catalepsy and the pattern of the posture abnormality depended on brainstem or forebrain localization of secondarily generalized seizures. Brainstem-driven seizures induced long-lasting whole-body catalepsy and cataleptic limb posture in the postictal period. Secondary seizure generalization to the forebrain led to shortening postictal catalepsy and development of rigid limb posturing. Asymmetric limb posturing was always observed after focal forebrain seizures, and the postictal asymmetry was closely linked to ictal asymmetry of the earliest running seizure phase, predicting lateralization of the seizure-onset side. This is the first demonstration of circuit-specific postictal behavioral impairments and their localization and lateralization values in epileptic rats.

Initiation of spreading depression by synaptic and network hyperactivity: Insights into trigger mechanisms of migraine aura

Background

Cortical spreading depression (SD) is thought to underlie migraine aura but mechanisms of triggering SD in the structurally normal, well-nourished cortex of migraine patients remain unknown. Synaptic and network dysfunctions appear to underlie episodic neurological disorders, including migraine. The narrative review summarizes old and recent experimental evidence for triggering SD by synaptic/network mechanisms and discusses the relevance of the data to migraine pathogenesis. Our hypothesis is that under some conditions synaptic/network hyperactivity may reliably ignite SD, and this mechanism may underlie triggering migraine aura in patients.

Findings

High-frequency tetanic stimulation of the cortex reliably triggers SD in synaptically connected regions; SD is a reliable cortical response to acute hyperexcitability (epileptic seizures), though chronic epilepsy prevents triggering SD; in the hyperexcitable cortex, SD may be triggered by sensory stimulation; compromised glutamatergic transmission plays the critical role in triggering SD.

Conclusion

SD may be triggered by dynamic network instability produced by dysfunction of calcium-dependent glutamate release. Synaptic drive from subcortical sensory processing structures (brainstem and/or thalamocortical networks) is able to evoke depolarization of hyperexcitable cortical neurons sufficient to initiate the regenerative SD process. Studying SD initiation by synaptic/network hyperexcitability may provide insights into basic mechanisms underlying SD generation in migraine brain.

Audiogenic kindling and secondary subcortico-cortical epileptogenesis: Behavioral correlates and electrographic features

Human epilepsy is usually considered to result from cortical pathology, but animal studies show that the cortex may be secondarily involved in epileptogenesis, and cortical seizures may be triggered by extracortical mechanisms. In the audiogenic kindling model, recurrent subcortical (brainstem-driven) seizures induce secondary epileptic activation of the cortex. The present review focuses on behavioral and electrographic features of the subcortico-cortical epileptogenesis: (1) behavioral expressions of traditional and mild paradigms of audiogenic kindling produced by full-blown (generalized) and minimal (focal) audiogenic seizures, respectively; (2) electrographic manifestations of secondary epileptic activation of the cortex - cortical epileptic discharge and cortical spreading depression; and (3) persistent individual asymmetry of minimal audiogenic seizures and secondary cortical events produced by their repetition. The characteristics of audiogenic kindling suggest that this model represents a unique experimental approach to studying cortical epileptogenesis and network aspects of epilepsy. This article is part of a Special Issue entitled "Genetic and Reflex Epilepsies, Audiogenic Seizures and Strains: From Experimental Models to the Clinic".

Dynamics of sensorimotor cortex activation during absence and myoclonic seizures in a mouse model of juvenile myoclonic epilepsy

OBJECTIVE

Generalized epilepsy syndromes often confer multiple types of seizures, but it is not known if these seizures activate separate or overlapping brain networks. Recently, we reported that mice with a juvenile myoclonic epilepsy mutation (Gabra1[A322D]) exhibited both absence and myoclonic generalized seizures. Here, we determined the time course of sensorimotor cortex activation and the spatial distribution of spike voltage during these two seizures.

METHODS

We implanted Gabra1^+/A322D mice with multiple electroencephalography (EEG) electrodes over bilateral somatosensory cortex barrel fields (S1) and anterior (aM1) and posterior (pM1) motor cortices and recorded absence seizures/spike-wave discharges (SWDs) and myoclonic seizures. We used nonlinear-association analyses and cross-correlation calculations to determine the strength, leading regions, and time delays of cortical coupling from the preictal to ictal states and within the spike and interspike periods. The distribution of spike voltage was also measured in SWDs and myoclonic seizures.

RESULTS

EEG connectivity among all electrode pairs increased at the onset of both SWDs and myoclonic seizures. Surprisingly, during spikes of both seizure types, S1 led M1 with similar delay times. Myoclonic seizure spikes started more focally than SWD spikes, with a significant majority appearing first only in S1 electrodes, whereas a substantial fraction of SWD spikes were detected first in S1 and at least one M1 electrode. The absolute voltage of myoclonic seizure spikes was significantly higher than that of SWD spikes, and there was a greater relative voltage over M1 during myoclonic seizure spikes than in the first one to two SWD spikes.

SIGNIFICANCE

The leading sites in S1 and similar delay times suggest both SWDs and myoclonic seizures activate overlapping networks in sensorimotor cortex and thus, therapeutically targeting of this network could potentially treat both seizures. Spike focality, absolute voltage, and voltage distribution provide insight into neuronal activation during these two seizure types.

Comparative potency of sensory-induced brainstem activation to trigger spreading depression and seizures in the cortex of awake rats: Implications for the pathophysiology of migraine aura

BACKGROUND

Migraine and epilepsy are highly co-morbid neurological disorders associated with episodic dysfunction of both cortical and subcortical networks. The study examined the interrelation between cortical spreading depression, the electrophysiological correlate of migraine aura and seizures triggered at cortical and brainstem levels by repeated sound stimulation in rats with acoustic hypersensitivity (reflex audiogenic epilepsy).

METHOD

In awake, freely moving rats with innate audiogenic epilepsy, 25 episodes of running seizure (brainstem seizures) were induced by repeated sound stimulation. Spreading depression and seizures were recorded using implanted cortical electrodes.

RESULTS

The first sound-induced brainstem seizures evoked neither spreading depression nor seizures in the cortex. With repetition, brainstem seizures began to be followed by a single cortical spreading depression wave and an epileptiform discharge. Spreading depression was more frequent an early cortical event than seizures: spreading depression appeared after 8.4 ± 1.0 repeated stimulations in 100% rats (n = 24) while cortical seizures were recorded after 12.9 ± 1.2 tests in 46% rats. Brainstem seizure triggered unilateral long-latency spreading depression. Bilateral short-latency cortical spreading depression was recorded only after intense cortical seizures.

CONCLUSION

These data show that episodic brainstem activation is a potent trigger of unilateral cortical spreading depression. Development of intense seizures in the cortex leads to initiation of spreading depression in multiple cortical sites of both hemispheres.

Hemispheric lateralization of microstructural white matter abnormalities in children with active benign childhood epilepsy with centrotemporal spikes (BECTS): a preliminary DTI study

PURPOSE

The deficit of white matter is reported to be involved during the disease progression in patients with benign childhood epilepsy with centrotemporal spikes (BECTS). The aim of this study is to investigate patterns of white matter damage in children with BECTS with left- or right-hemispheric focus by using diffusion tensor imaging (DTI), and its relationship with the cofactors such duration, seizure frequency and handedness.

METHODS

Diffusion tensor imaging (DTI) was performed in twenty-eight children with BECTS and eighteen healthy controls. The data were analyzed using both tract-based spatial statistics (TBSS) and region of interest (ROI) analyses. Correlations were investigated between the fractional anisotropy (FA) values of the identified altered regions and clinical features such as age, age of onset and seizure frequency.

RESULTS

The TBSS analysis revealed that white matter impairment in children with rolandic spikes on the ipsilateral hemisphere was much wider. The FA value was significantly lower in the body of the corpus callosum and forceps minor in BECTS patients with spikes on the ipsilateral hemisphere. The seizure frequency correlated positively with the FA values of body of corpus callosum (CC), bilateral cingulate gyrus and left uncinate fasciculi (UA).

CONCLUSION

The impaired WM integrity in patients with BECTS was greater in patients with spikes on the dominant hemisphere, possibly due to the greater vulnerability of the left hemisphere and excitotoxic effects of seizures.