( J) In contrast, PV cells caudal to the recording area (AP −2.7 to −3.2 mm) showed a different distribution, with ~75% of cell bodies in the SP and ~25% in the SO, along with a small number in the stratum radiatum (SR). ( I) In the same rostral region, ~95% of SST interneurons had cell bodies located in the SO, with ~5% in the SP. ( H) In the rostral hippocampus where recordings were performed (AP −1.5 to −2.0 mm), the cell bodies of ~90% of PV interneurons identified by Cre-dependent expression of tdTomato were located in the stratum pyramidale (SP), with ~10% in the statum oriens (SO). ( G) Bars showing the percentage of Cre + neurons stained by antibody and percentage of Ab +neurons labeled by Cre for each mouse line. ( F) Representative images from a PV-Cre/ChR2 mouse (yellow) immunostained for PV (red, left) and a SST-Cre/ChR2 mouse (yellow) immunostained for SST (red, right). ( E) Same as for ( D) but for SST-Cre/Ai9 mice (n = 3 independent brains for each genotype). ( D) Bars showing the percentage of PV-expressing neurons that are fate mapped by the PV-CreAi9 line in mouse hippocampal CA1 (n = 3 independent brains for each genotype) percentage of PV-Cre/Ai9 fate mapped interneurons that express PV (n = 3 independent brains for each genotype) percentage of SST-expressing neurons that are fate mapped by the PV-CreAi9 line and the percentage of PV-Cre/Ai9 fate mapped interneurons that express SST. In all sections, DAPI counterstaining highlights the hippocampal CA1 anatomy (stratum pyramidale (st.p), stratum oriens (st.o) and stratum radiatum (st. Lower: In hippocampal sections from SST-Cre +/-Ai9 f/- mice, note co-labeling for tdTomato in SST- (green Right) but not PV-expressing (green Left) cells. Note the co-labeling with tdTomato in all PV-expressing cells and the absence of overlap in SST-expressing cells. Upper: Representative coronal sections from PV-Cre +/-Ai9 f/- mice showing sparse cells immunostained for PV (green Left) or SST (green Right) in hippocampal CA1. ( C) The two interneuron populations of interest were verified by crossing PV-Cre or SST-Cre mice with a red reporter line (Ai9) to express tdTomato in PV- or SST-expressing cells, respectively. ( B) Mean peak-to-trough duration (left) and repolarization at 0.9 ms (right) were not significantly different across the three populations.
Note that the RS population may potentially include a small number of unidentified interneurons. RS cells were defined as cells having a repolarization value smaller than −0.35 (dotted lines). Non-identified, ChR2-negative cells were recorded in experiments using PV-Cre, SST-Cre, or wild-type mice. ( A) Average action potential waveforms of units plotted using their peak-to-trough duration in ms (x-axis) against their repolarization value at 0.9 ms (y-axis see Materials and methods) for PV, SST and non-identified cells. Note that pairwise statistical comparisons are only shown between 3rd and 4th preictal period. ( F) Mean changes in firing rate as compared to baseline over four preictal periods for PV (n = 16 cells in 12 mice), SST (n = 14 cells in 13 mice), and RS (n = 28 cells in 16 mice) populations in awake behaving animals. ( E) Mean changes in firing rate as compared to baseline over four preictal periods for PV (n = 18 cells in 13 mice), SST (n = 16 cells in 10 mice) and RS (n = 20 cells in 12 mice) populations in lightly anesthetized animals. Note that cells were recorded from different animals that each had different latencies to ictal onset. All horizontal scale bars correspond to 20 s. ( D) Spike trains for example PV, SST and RS cells. Right: Mean baseline firing rates (Hz) for PV, SST and RS cells recorded from hippocampal CA1.
( C) Left: Averaged, normalized action potential waveforms for all recorded PV, SST and putative RS cells. ( B) Peri-pulse time-histogram around 5 ms laser pulses together with raster plot during baseline for example PV- and SST-expressing interneurons in PV-Cre/ChR2 and SST-Cre/ChR2 mice, respectively. Lower: Spectrogram showing LFP power as a function of time from ictal onset (x-axis) and frequency (y-axis shown on log-10 scale). Middle: LFP trace from hippocampal CA1 recording.
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