Cooperative Subnetworks of Molecularly Similar Interneurons in Mouse Neocortex

Abstract

Simultaneous co-activation of neocortical neurons is likely critical for brain computations ranging from perception and motor control to memory and cognition. While co-activation of excitatory principal cells (PCs) during ongoing activity has been extensively studied, that of inhibitory interneurons (INs) has received little attention. Here, we show in vivo and in vitro that members of two non-overlapping neocortical IN populations, expressing somatostatin (SOM) or vasoactive intestinal peptide (VIP), are active as populations rather than individually. We demonstrate a variety of synergistic mechanisms, involving population-specific local excitation, GABAergic disinhibition and excitation through electrical coupling, which likely underlie IN population co-activity. Firing of a few SOM or VIP INs recruits additional members within the cell type via GABAergic and cholinergic mechanisms, thereby amplifying the output of the population as a whole. Our data suggest that IN populations work as cooperative units, thus generating an amplifying nonlinearity in their circuit output.

Figure 1. VIPs and SOMs tend to be active as populations in awake mice

(A) Standard deviation projection of Syn-GCaMP6s and average projection of VIP-TOM in V1 L2/3 of VIP-cre∷LSL-TOM. (B) Example DF/F traces during ongoing activity. Red traces are from VIPs, black trace denotes when the mouse was running. Bottom DF/F raster is from VIP and non-VIP cells in the FOV as indicated. (C) Calcium signal correlations when the mouse was stationary. Left, matrix of Pearson correlation coefficients for each cell pair in the data shown in B. Middle, ΔF/F correlation coefficients for each pair averaged within categories (bars; Non-VIP, R = 0.07 ± 0.02; VIP/Non-VIP, R = 0.08 ± 0.03; VIP, R = 0.18 ± 0.02); gray lines connect values from each FOV. Right, same as middle, using binarized data (Non-VIP, R = 0.02 ± 0.01; VIP/Non-VIP, R = 0.03 ± 0.01; VIP, R = 0.07 ± 0.01). (D) Calcium signal correlations during locomotion. Notation as in C (Middle, bars: Non-VIP, R = 0.09 ± 0.02; VIP/Non-VIP, R = 0.11 ± 0.03; VIP, R = 0.23 ± 0.04; Right, bars: Non-VIP, R = 0.04 ± 0.01; VIP/Non-VIP, R = 0.05 ± 0.01; VIP, R = 0.13 ± 0.03). Same movies as in C, except one that contained too few locomotion frames. (E) Syn-GCaMP6s expression in V1 L2/3 of SOM-cre∷LSL-TOM mouse. (F) Example DF/F traces during ongoing activity. Red traces are from VIPs, black trace denotes when the mouse was running. Bottom DF/F raster is from VIP and non-VIP cells in the FOV as indicated. All traces are aligned in time and in the same time scale. (G) Calcium signal correlations when the mouse was stationary. Notation as in C (Middle, bars: Non-SOM, R = 0.08 ± 0.01; SOM/Non-SOM, R = 0.12 ± 0.02; SOM, R = 0.22 ± 0.05; right, bars: Non-SOM, R = 0.03 ± 0.01; SOM/Non-SOM, R = 0.04 ± 0.01; SOM, R = 0.13 ± 0.04). (H) Calcium signal correlations during locomotion. Notation as in C (bars; Non-SOM, R = 0.13 ± 0.02; SOM/Non-SOM, R = 0.20 ± 0.02; SOM, R = 0.37 ± 0.04; right, bars: Non-SOM, R = 0.06 ± 0.01; SOM/Non-SOM, R = 0.10 ± 0.02; SOM, R = 0.22 ± 0.04). In panels C-H, *, P < 0.05; ***, P < 0.0005 by paired T-test.

Figure 2. L2/3 PC innervation of VIPs and SOMs

(A) From left to right: a representative image taken while patching a SOM, a VIP and two PCs; connection probabilities PC->SOM and PC->VIP; example traces of PC->VIP and PC->SOM connections with traces from postsynaptic INs in color and presynaptic PC traces in black; dynamics of 50 Hz unitary excitatory post synaptic potential (uEPSP) trains onto VIPs (n = 11; 6 from S1 and 5 from V1) and SOMs (n = 6; all from V1) normalized to maximum amplitude (top) and as raw data (bottom). (B) From left to right: representative image taken while patching a SOM and a VIP in a VIP-cre∷LSL-TOM∷SOM-GFP(GIN) slice expressing CaMKII-C1V1-YFP; schematic of experiment and cell attached, whole cell (WC) voltage clamp (VC) and current clamp (IC) traces of a VIP and SOM recorded simultaneously while exciting the tissue with green light (shaded 500 ms period); summary of PC, SOM and VIP firing during the light step after an initial spike at the beginning (all recordings from S1). (C) Example confocal micrographs of coronal sections from rabies tracing of SOMs (left) and VIPs (right). Starter cells are labelled with arrows. Red cells are presynaptic to starters. (D) Enlarged views from colored boxes in C showing typical pyramidal morphologies of presynaptic cells. (E) Summary cell counts and presynaptic/starter ratios near the injection site from 4 VIP-cre and 3 SOM-cre brains. (F) Left, heat maps averaged across coronal sections spanning the injection sites. Right, overlaid starter (in green) and presynaptic (in red) cell heat maps and layer distributions of cell counts. All scale bars are 200 µm, p = Pia, IV = Layer 4, wm = white matter boundary. See Figure S2 for separate data from each brain. Data in C-F and Figure S2 are from S1 cortex.

Figure 3. Distinct presynaptic excitatory cells innervate VIPs and SOMs

(A) Example whole cell current clamp recording of two SOMs and two VIPs simultaneously in a VIP-cre∷LSL-TOM∷SOM-GFP(GIN) slice showing spontaneous activation (denoted by arrows) of only the SOMs or one of the VIPs. Action potentials are truncated at −10 mV. (B) Summary of the occurrence of simultaneous (shared) activation of the indicated two cells. “Not shared” activations were those occurring in one cell only, while both cells were being recorded. Pooled data from S1 and V1 as indicated in text. (C) Average subthreshold membrane potential cross-correlograms from cell pairs less than 150 mm apart (SOM-SOM 13 pairs, 4 in S1 and 9 in V1; VIP-SOM 18 pairs, 8 in S1 and 10 in V1; VIP-VIP 11 pairs, 3 in S1 and 8 in V1; VIP-PV 5 pairs, all in V1). See Figure S3 for all individual pair cross-correlograms. (D) Correlation coefficients (at time lag 0s) for each recorded cell pair as a function of intersomatic distance. Mean ± sem for cell pairs of each category less than 150 mm apart plotted as larger symbols with error bars. (E) Schematic of 2-photon input mapping experiment (left) and micrograph of a recording (right). Asterisks on top of the image represent stimulation targets. (F) Representative voltage clamp (−70 mV) traces from a SOM and a VIP showing shared (*) and not shared (#) EPSCs. (G) Shared and not shared L-EPSCs (see text) for 6 SOM-VIP pairs. Shared and not shared counts from the same experiment are connected by a line and mean ± sem are shown next to them. *, P = 0.02; N.S., P = 0.90 by z-test against zero. (H) Proportion of shared L-EPSCs from individual experiments (circles) and mean ± sem (lines). N = 6 SOM-VIP pairs and 5 PV pairs (all from S1); *, P = 0.015.

Figure 4. Chemical and electrical synaptic connectivity supports within-population co-activity

(A) Images and example recordings from triple transgenic slices. Overlaid gray traces are postsynaptic responses from 10–20 trials and colored trace on top is average. Combined connection probabilities from S1 and V1 are noted next to each main connection type (separated by area in Figure S4). Total number of trial connections is n = 1009. (B) Enlarged postsynaptic traces from A showing key differences in dynamics and summation of each main connection category. (C) Mean ± sem of amplitudes from combined data from S1 and V1 (n = 3–19 for each bar). Amplitudes of the first IPSP in a train were significantly smaller (P < 0.01) in both SOM->VIP (0.30 ± 0.04 mV, n = 19) and VIP->SOM (0.31 ± 0.04 mV, n = 18) than PV->PV (0.58 ± 0.10 mV, n = 13) and SOM->PV (0.53 ± 0.07 mV, n = 12). There were no other significant differences in first IPSP amplitudes (PV->SOM 0.45 ± 0.18 mV, n = 7; VIP->VIP 0.37 ± 0.11 mV, n = 5; PV->VIP 0.40 ± 0.10 mV, n = 3). PV->PV (1.2 ± 0.2 mV, n = 7) and SOM->PV (1.1 ± 0.1 mV, n = 7) were the only 50Hz peak amplitudes significantly smaller (P < 0.05) than both SOM->VIP (2.4 ± 0.4 mV, n = 7) and VIP->SOM (3.2 ± 0.7 mV, n = 7). Other 50Hz peak amplitudes: PV->SOM 1.9 ± 0.8 mV, n = 5; VIP->VIP 1.2 ± 0.3 mV, n = 5; PV-VIP 0.9 ± 0.3 mV, n = 3. (D) Summation ratios (50 Hz peak amplitude / first IPSP amplitude, n = 3–19 for each bar) for each connection type. *, P < 0.05 compared to gray bars. (E) – (G) Average ± sem uIPSP amplitudes normalized to maximum responses during a 50 Hz AP train for each category of connection (n = 3–19 for each curve). (H) Example voltage clamp recordings (+40 mV holding potential) of unitary IPSCs. (I) Scatter data (light symbols) and mean ± sem (symbols with error bars) of uIPSC rise and decay parameters. Rise times were PV->PV 2.2 ± 0.5 ms, n = 6; PV->VIP 2.2 ± 0.2 ms, n = 6; SOM->VIP 5.3 ± 0.7 ms, n = 8; VIP->SOM 8.9 ± 2.3 ms, n = 4. Decay time constants were SOM->VIP 19.3 ± 2.1 ms, n = 8; PV->VIP 17.2 ± 1.7 ms, n = 6; PV->PV 8.4 ± 1.0 ms, n = 6; VIP->SOM 37.1 ± 5.7 ms, n = 4. (J) Example traces of electrical coupling between two VIPs and a simultaneous inhibitory connection from the VIP to a SOM. Right panel, focused into first PSPs of the recording to show time difference between action-potential peak to electrical v chemical PSP onset (marked by dashed lines). (K) Example of a SOM->SOM electrical connection. (L) Combined electrical connection probability matrix from S1 and V1 (numbers by region in Figure S4).

Figure 5. SOM and VIP functional cooperativity in brain slices

(A) Example recording of four SOMs that demonstrated higher cooperative recruitment during induced firing (50 Hz AP train for 3 s) of three cells than when only one cell was induced to fire. Three trials are shown for SOM4 in different shades of gray above a spike rate histogram across 15 trials. (B) Normalized SOM firing rates (from recordings as in A). All individual recordings are shown as lines and the mean ± sem as bars *, P < 0.05; N.S., P > 0.05 by paired T-test. Number of recordings in each category: ‘1 cell firing’ n = 20 (S1 n = 15; V1 n = 5), ‘2 cells firing’ n = 14 (S1 n = 6; V1 n = 8) and ‘3 cells firing’ n = 13 (S1 n = 7; V1 n = 6). (C) Proportions of SOM recordings categorized (see Methods). (D) Example recording where blockade of GABA_A receptors with 1 mM gabazine and GABA_B receptors with 40 µM CGP35348 attenuated the firing rate increase of the fourth SOM during firing of three SOMs. (E) Left, Normalized firing rate changes from recordings as shown in D. All individual recordings are shown as lines, and mean ± sem as bars with error bars. *, P = 0.0065 by paired T-test; n = 7 (S1 n = 4; V1 n = 3). Right, normalized firing rate changes from recordings where the recorded SOM was electrically coupled to one of the others (+, n = 3), and those where the recorded SOM was not electrically coupled (−, n = 9; *, P = 0.027 by paired T-test). (F) Example recording of four VIPs that demonstrated cooperative recruitment only during induced firing of three cells. Three trials are shown for VIP4 in different shades of gray above a spike rate histogram across 12 trials. (G) Normalized VIP firing rates (from recordings as in G). Presented like B. *, P < 0.05; N.S., P > 0.05 by paired T-test. Number of recordings in each category: ‘1 cell firing’ n = 20 (S1 n = 5; V1 n = 15), ‘2 cells firing’ n = 16 (S1 n = 5; V1 n = 11) and ‘3 cells firing’ n = 18 (S1 n = 6; V1 n = 12). (H) Proportions of VIP recordings categorized (see Methods). (I) Left, blockade of GABA receptors as in D did not always attenuate firing rate increase of fourth VIP during firing of three VIPs; baseline firing rate increase 30 ± 6 %, in GABA blockers 24 ± 9 %, n = 8 (S1 n = 3; V1 n = 5). Center, blockade of ACh receptors with 10 mM mecamylamine (MEC) attenuated VIP cooperativity; baseline firing rate increase 61 ± 23 %, in MEC 39 ± 20 %, n = 10 (S1 n = 5; V1 n = 5). Presented like E. *, P = 0.007; N.S., P = 0.54 by paired T-test. Right, normalized firing rate changes from recordings where the recorded VIP was electrically coupled to one of the others (+, n = 4), and those where the recorded VIP was not electrically coupled (−, n = 16; *, P < 10⁻⁷ by paired T-test). (J) 3 s of 50 Hz firing in two VIPs simultaneously silences spontaneous firing of a nearby SOM. (K) 3 s of 50 Hz firing in two SOMs simultaneously silences spontaneous firing of a nearby VIP.

Figure 6. Connectivity schematics

Left, prevailing model of hierarchical interneuron connectivity in L2/3 based on (Hangya et al., 2014, Harris and Shepherd, 2015, Kepecs and Fishell, 2014, Pfeffer et al., 2013, Zhang et al., 2014, Lee et al., 2013). Right, proposed new cooperative model based on our findings and the above mentioned studies as well as others described in text.