Abstracts

This abstract has been invited to present during the Neurophysiology platform session

Rationale: It is thought that focal seizures associated with temporal lobe epilepsy undergo a sequence of events initiated by local neuronal hyperactivity that gradually evolves into synchronized, high-amplitude oscillations that sustain and spread throughout the brain. However, this process of ictogenesis is not well understood.

Methods: Using an in vivo mouse model (n=8), we transduced putative excitatory neurons in the left hippocampus with ChR2 (pAAV5-CaMKIIα-hChR2(H134R)-EYFP). An optrode was implanted at the injection site for simultaneous delivery of optogenetic stimulation to area CA1 and EEG recording (Figure 1A). To evoke seizures, 5 ms light pulses were delivered at 5, 10 or 20 Hz in trains of 30 s duration (Figure 1B). For each recording session, the stimulus train was repeated every 120 s for a total of 15 trains.

Results: We found a consistent, frequency-dependent, pro-ictogenic effect: 20 Hz optogenetic stimulation trains induced more frequent and severe seizures than both 10 Hz and 5 Hz stimulations. Repeated stimulations progressively increased seizure severity, consistent with optogenetic kindling. Utilizing a novel time-vs.-time “pulsogram” plot of the EEG (Figure 1D), we identified three distinctive and progressive phases of network activity preceding a seizure: “core,” “reverberant,” and “paroxysmal” phases (Figure 1F). We demonstrated that each phase is characterized by distinctive responses to light stimulation on a micro timescale. During the core phase, each light pulse induced immediate discharges occurring within 10 ms of stimulation, whereas the reverberant phase contained an additional time-locked secondary discharge delayed 20-50 ms from each pulse (Figure 2D-E). Additionally, the core and reverberant phases were demarcated by an abrupt transition which we named the “divergent point.” Likewise, the transition between the reverberant and paroxysmal phases were demarcated by the “paroxysmal point,” characterized by the onset of paroxysmal activity overriding both the immediate and secondary discharges. These phases of ictogenesis and transition points were conserved among all recorded seizures. Furthermore, epochs that failed to progress to seizures exhibited matching segments of pre-ictal activity that failed to progress beyond either the core or the reverberant phases (Figure 2A-B). Lastly, we found evidence of a latent transition point between stimulus-fueled paroxysmal activity and self-sustaining seizures.

Conclusions: In summary, we devised an optogenetic stimulation paradigm and a novel analytical approach that revealed clearly discrete phases of ictogenesis, depicting a stepwise expansion of synchronized and sustained oscillatory activity. We believe these ictogenetic phases represent the initial stages of the neuronal recruitment process that result in generalization from a seizure focus to the rest of the brain. Better understanding of the mechanisms underlying these phases of ictogenesis and transitional triggers may help stop seizures before they occur.

Funding: None