(A) Schematic of the opto-tagging experiments that were used to identify Id2 INs in freely behaving mice. Combined light fiber-recording probes were implanted in Id2-CreER; Dlx5/6-Flpe; Ai80 mice. (B) Peristimulus time histogram (PSTH) from 31 isolated neurons from a single recording session. Note one light-responsive neuron in position 11. The peristimulus raster plot, waveform and auto-correlogram for that light-responsive Id2 IN-light responsive unit are shown in brown. All remaining cells from this session are shown in clear gray for comparison. (C) Peristimulus histogram (PSTH) for all isolated units classified as Id2 IN-light responsive neurons (from n=5 mice). (D) Average spike waveform (mean ± 95% confidence interval (CI95)) and through-to-peak spike duration (inset; p<10–61, Kruskal-Wallis test) for PCs, NW INs, WW neurons and Id2 INs (n=317 PCs, 155 NW INs and 88 WW neurons). The average waveform from all Id2 IN-light responsive neurons are overlayed in yellow. (E) Same as in (D) but for the average firing auto-correlogram (ACG) in log scale and time constant of the rising phase of the ACG (inset; p<10–24). (F) Median firing rates of PCs, NW INs, WW neurons, and light responsive-Id2 INs (Cell type: F3,2261 = 151.44, p<10–89; State: F3,2261 = 10.74, p=0.035; Interaction: F9,2261 = 0.97, p=0.46). Sleep to Awake index (p=0.051, Kruskal-Wallis test) and coefficient of variance of the firing rate (p=0.034, Kruskal-Wallis test) as insets. Sleep to Awake index was near zero only for the Id2 IN group (PC: Z=2.55, p=0.010; NW: Z=3.16, p=0.002; WW: Z=0.08, p=0.98; one-sample Wilcoxon signed rank test). (G) Peri-down-state Z-scored firing histogram for light-responsive-Id2 INs (top), and all PCs, NW INs, WW neurons (middle to bottom), as ranked according to their magnitude of event response. Solid lines delimit the ±50 ms window used to estimate the unit responses during down-states. Dashed lines delimit the +90 ms to +200 ms window used to estimate the post-down-state responses. Pie charts show the fraction of units exhibiting an early increase in firing rate during the post-down epoch above 2 standard deviations (s.d.). (H) Temporal dynamics of the Z-scored peri-down-state responses across all groups (top; mean ± IC95). Contrary to what was observed in PCs, NW INs and WW neurons, Id2 neurons significatively increased their firing rate above baseline levels during the post-down epoch (post-down: p<10–8; down: p<10–15). (I) The magnitude of the post-down-state response as a function of the activity during the down-state events. Only Id2 neurons show a significant correlation (Spearman correlation). (J) Group differences of the average rank position during up-state epochs (P<10–5, Kruskal-Wallis test). Id2 INs lead the up-state sequences.