Involvement of remote regions in sustained, but not transient, epileptic activities in the kainate mouse model of temporal lobe epilepsy

Abstract Animal and human studies have shown that the seizure-generating region is vastly dependent on distant neuronal hubs that can decrease duration and propagation of ongoing seizures. However, we still lack a comprehensive understanding of the impact of distant brain areas on specific interictal or ictal epileptic activities (e.g., isolated spikes, spike trains, seizures). Such knowledge is critically needed since all kinds of epileptic activities are not equivalent in terms of clinical expression and impact on the progression of the disease. We used surface, high-density EEG and multisite intracortical recordings, combined with pharmacological silencing of specific brain regions in the well-known kainate mouse model of temporal lobe epilepsy. We tested the impact of selective regional silencing on the generation of epileptic activities within a continuum ranging from very transient to more sustained and long-lasting discharges reminiscent of seizures. Silencing the contralateral hippocampus completely suppresses sustained ictal activities in the focus, as efficiently as silencing the focus itself, but while focus silencing abolishes all focal activities, contralateral silencing fails to control transient spikes. In parallel, we observed that sustained epileptic discharges in the focus are preceded by contralateral firing and more strongly phase locked to bi-hippocampal delta/theta oscillations than transient spiking activities, reinforcing the presumed dominant role of the contralateral hippocampus in promoting long-lasting, but not transient, epileptic activities. Altogether, our work provides suggestive evidence that the contralateral hippocampus is necessary for the interictal-to ictal-state transition and proposes that cross-talk between contralateral neuronal activity and ipsilateral delta/theta oscillation could be a candidate mechanism underlying the progression from short to long-lasting epileptic activities. Key Points We study how regions remote from the focus influence epileptic activities in the kainate mouse model of temporal lobe epilepsy. The contralateral hippocampus plays a decisive role in the initiation of sustained epileptic activities Integration of contralateral activities and bi-hippocampal delta/theta oscillations precedes focal paroxysmal activities We propose that a large-scale epileptic network might be necessary for the transition from interictal to ictal states