(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. Non-identified, ChR2-negative cells were recorded in experiments using PV-Cre, SST-Cre, or wild-type mice. RS cells were defined as cells having a repolarization value smaller than −0.35 (dotted lines). Note that the RS population may potentially include a small number of unidentified interneurons. (B) Mean peak-to-trough duration (left) and repolarization at 0.9 ms (right) were not significantly different across the three populations. (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. Upper: Representative coronal sections from PV-Cre+/-Ai9f/- mice showing sparse cells immunostained for PV (green; Left) or SST (green; Right) in hippocampal CA1. Note the co-labeling with tdTomato in all PV-expressing cells and the absence of overlap in SST-expressing cells. Lower: In hippocampal sections from SST-Cre+/-Ai9f/- mice, note co-labeling for tdTomato in SST- (green; Right) but not PV-expressing (green; Left) cells. In all sections, DAPI counterstaining highlights the hippocampal CA1 anatomy (stratum pyramidale (st.p), stratum oriens (st.o) and stratum radiatum (st. (r)). (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. Data represent mean ±SEM. (E) Same as for (D) but for SST-Cre/Ai9 mice (n = 3 independent brains for each genotype). Data represent mean ±SEM. (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). (G) Bars showing the percentage of Cre + neurons stained by antibody and percentage of Ab +neurons labeled by Cre for each mouse line. (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). (I) In the same rostral region, ~95% of SST interneurons had cell bodies located in the SO, with ~5% in the SP. (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). (K) SST interneuron cell body distribution was similar along the rostral-caudal axis, with ~95% of cell bodies in the caudal CA1 located in the SO and a small number in the SP and stratum lacunosum moleculare (SLM). Data were collected from PV-Cre+/-Ai9f/- (n = 4) and SST-Cre+/-Ai9f/- (n = 4) animals.