Ictogenesis proceeds through discrete phases in hippocampal CA1 seizures in mice

Epilepsy is characterized by spontaneous non-provoked seizures, yet the mechanisms that trigger a seizure and allow its evolution remain underexplored. To dissect out phases of ictogenesis, we evoked hypersynchronous activity with optogenetic stimulation. Focal optogenetic activation of putative excitatory neurons in the mouse hippocampal CA1 reliably evoked convulsive seizures in awake mice. A time-vs-time pulsogram plot characterized the evolution of the EEG pulse response from a light evoked response to induced seizure activity. Our results depict ictogenesis as a stepwise process comprised of three distinctive phases demarcated by two transition points. The induction phase undergoes the first transition to reverberant phase activity, followed by the second transition into the paroxysmal phase or a seizure. Non-seizure responses are confined to either induction or reverberant phases. The pulsogram was then constructed in seizures recorded from a murine model of temporal lobe epilepsy and it depicted a similar reverberance preceding spontaneous seizures. The discovery of these distinct phases of ictogenesis may offer means to abort a seizure before it develops.

a The number in parentheses refers to total number of excluded recordings.
b Age of mouse in days at the time of surgery c Number of days that recordings were performed for.d The number in parentheses refers to the number of days from injection that the first recording occurred on.
e The number in parentheses refers to total number of missing power recordings.
Fig.1f-gHigh incidence (lower threshold) Low incidence (higher threshold) Supplementary Fig.3cEarlier seizure onset (lower threshold) Later seizure onset (higher threshold) Fig.4aLower Racine scores Higher Racine scores Supplementary Fig.3e stimulus offset, and post-stimulus segments -demarcated by "Stim Start", "Paroxysmal", and "Stim End" markers.At the paroxysmal point, there is a clear, stepwise broadband increase in spectral power in conjunction with a step-decrease in the power contribution of the fundamental and harmonics of the stimulation frequency (10 or 20Hz).(b) As in a, averaged spectrograms by seizure type induced by 20Hz stimulation.(c) The relative contribution to total power (0-200Hz) from the 10Hz fundamental and harmonics during 10Hz stimulation induced seizures.(d) The power of the 10Hz fundamental and harmonics of the seizures in c.(e) The power of the non-harmonic spectral components of the seizures in c.This is the remainder after subtracting harmonic power d from the total power (0-200Hz).(f-h) Same as c-e respectively, for 20Hz stimulation induced seizures.(i) Average power spectrum of the 10Hz seizures (during stimulation) shown as change in the power pre-and post-the paroxysmal point.Original spectrums contain prominent reduction at the frequency of the stimulation harmonics (dashed lines), which was interpolated out to obtain a smooth estimate of the power spectrum (solid lines).(j) As in i, average power spectrum of the 20Hz seizures.c-j plot the mean +/-95% confidence interval across seizures.For c-d and i, n are as labelled in a.For f-g and j, n are as labeled in b.
Source data available at https://doi.org/10.5281/zenodo.8274424.from 20Hz sessions.Colored upside-down triangles indicate the sessions where each corresponding mouse first scored above RS6.Black triangle indicates the session where 50% of the mice have scored at or above RS6.Correlation between RS and session number was 0.59 (P=1.042x10-10 ).(d) As in c, but for break-long seizure RS (n=8 mice).Since multiple break-cluster or minor deviations from the main trajectory of change (see Methods).The final segment boundaries are marked with dashed vertical lines.Source data available at https://doi.org/10.5281/zenodo.8274424.level against mouse and power level, n=103 10Hz recordings).(c) Correlation analysis of stimulation power in individual animals (rows) with substantial variance in power level.In each column, a key measurement from each recording session was plotted against stimulation power.Resp.peak is the peak response to the first pulse of stimulation in each recording session.Resp.latency is the latency from the first pulse to the response peak.SzF is Yes if a first seizure was observed.Sz Rate is the percentage of epochs with seizures.Sz severity is the first seizure Racine Score.Sz duration is the duration of the first seizure.Pearson's correlation coefficient and its p-value for the 10Hz recordings (n as labeled) are printed above each plot; except for SzF, for which point-biserial correlation was calculated and Wilcoxon rank-sum test was used for the p-value.The only measurement with a consistent trend across mice was a (paradoxical) negative correlation between stimulation power and seizure duration (F(1,76)=6.6133,P=0.01207 covariate with power level; ANCOVA of seizure duration against mouse and power level, n=83 10Hz seizures with determinable duration).Solid dot = 10Hz; open circle = 20Hz.Source data available at https://doi.org/10.5281/zenodo.8274424. spatial