Abstracts

Rationale: The mechanisms of temporal lobe seizure onset and propagation are poorly understood. Previous research from this lab has suggested that ictogenesis in the hippocampus is a network phenomenon. Other studies have demonstrated an underappreciated role of the inhibitory interneuron network to ictogenesis. Here, we employ an optogenetic temporal lobe seizure model coupled with fiber photometry to investigate the role of PV interneurons in seizure onset and propagation.


Methods: Mice (n=7) were unilaterally transduced with CaMKII-ChR2 in hippocampal CA1 for optogenetic stimulation of excitatory CaMKII-expressing (CK) cells. We also transduced CA1 with PV-GcaMP and CaMKII-RcaMP opsins for Ca²⁺ photometry of contralateral parvalbumin-expressing (cPV) interneurons and ipsi- and contralateral CK (iCK & cCK) cells. Photometry response was quantified by integrating the area-under-curve over the ΔF/F time course. We recorded local field potentials (EEG) in bilateral CA1 and DG and quantified the activity level using the coastline index. We used a modified Racine scale to score seizure behavior (RS) recorded with video cameras.


Results: Tonic-clonic seizures were evoked (n=296) through optogenetic stimulation (10-20 Hz, 5 ms pulses, 30s duration). We first sought to determine if early Ca²⁺ signals can predict subsequent ictal electrical activity. We identified a time point for each seizure where the animal started showing seizure-related behavior. We found pre-behavior cPV activity to be most correlated with post-behavioral seizure EEG at bilateral CA1 and DG (see Fig 1A). In fact, pre-behavior cPV was more strongly correlated than post-behavioral cPV, iCK, and cCK (see Fig 1A). We also found that the first seizure of the day (“Type A”), was less severe than later seizures (“Type B”) (paired t-test: t = -5.82, p < 0.001, n=46). Type A seizures were associated with a stronger pre-behavior cPV prediction of post-behavior EEG (see Fig 1B) than Type B (see Fig 1C).

We also examined if pre-behavior Ca²⁺ and EEG activity predict seizure severity. RS was split into focal activity (RS 1-3), mild generalized (RS 4-5), and severe generalized (RS 6-7). In a random forest model using Ca²⁺ signals only, pre-behavior cPV was the best predictor compared to iCK and cCK (κ, respectively = 0.124, 0.059, -0.042). An accurate random forest model was achieved by adding one pre-behavior EEG channel as a predictor (testing accuracy; iCA1: 0.602, κ = 0.246; iDG: 0.627, κ = 0.302; cCA1: 0.590, κ = 0.266; cDG: 0.639, κ = 0.321); cDG was best (Figure 2).


Conclusions: Taken together, our results demonstrated that contralateral PV cell activity predicts seizure electrical activity and severity. Type B seizures have enhanced ictogenesis after a Type A seizure, and this is associated with the loss of influence of early cPV signaling on hippocampal CA1 output. Thus, our data suggest that PV interneurons define seizure onset and propagation, contradicting some existing theories. Future work will apply photometry to record PV in both hippocampi and elucidate the role of PV interneurons in seizure onset and propagation.


Funding: No external funding.