Dissection of cortical microcircuits by single-neuron stimulation in vivo

Abstract

Background: A fundamental process underlying all brain functions is the propagation of spiking activity in networks of excitatory and inhibitory neurons. In the neocortex, although functional connections between pairs of neurons have been studied extensively in brain slices, they remain poorly characterized in vivo, where the high background activity, global brain states, and neuromodulation can powerfully influence synaptic transmission. To understand how spikes are transmitted in cortical circuits in vivo, we used two-photon calcium imaging to monitor ensemble activity and targeted patching to stimulate a single neuron in mouse visual cortex.

Results: Burst spiking of a single pyramidal neuron can drive spiking activity in both excitatory and inhibitory neurons within a ∼100 μm radius. For inhibitory neurons, ∼30% of the somatostatin interneurons fire reliably in response to a presynaptic burst of ≥5 spikes. In contrast, parvalbumin interneurons showed no detectable responses to single-neuron stimulation, but their spiking is highly correlated with the local network activity.

Conclusions: Our results demonstrate the feasibility of mapping functional connectivity at cellular resolution in vivo and reveal distinct operations of two major inhibitory circuits, one detecting single-neuron spike bursts and the other reflecting distributed network activity.

Figure 1. Relationship between fluorescence signal and spike rate for different cortical cell types in vivo

(A) An excitatory neuron in cortical layer 2/3 of CaMKIIα-Cre × tdTomato mouse was targeted for cell-attached recording. Bottom left, mean spike waveforms of 16 excitatory neurons. The amplitude is normalized for each cell. Scale bar, 2 ms. Middle panel, two-photon image of layer 2/3 cells labeled with OGB-1 (white, note the shadow caused by the pipette) and image of tdTomato-expressing cells (red) taken prior to pipette insertion. Scale bar, 20 µm. Right panel, OGB-1 fluorescence and spikes from an example excitatory neuron. The filtered spike train was smoothed with a Gaussian filter (SD, 0.5 s). (B) OGB-1 fluorescence vs. AP number (based on filtered spike train) for the excitatory neurons (n = 16; 6 identified by tdTomato labeling and 10 putative excitatory neurons, classified based on their spontaneous firing rates; see Experimental Procedures). Gray lines, individual cells; black line, population average; error bar, ± SEM. (C, D) Same as (A) and (B), for PV interneurons (green; n = 10; 8 identified by tdTomato labeling and 2 fast-spiking cells) in PV-Cre × tdTomato mice. (E, F) Same as (A) and (B), for SOM interneurons (red; n = 8; all identified by GFP labeling) in GIN mice. (G) Summary of the spiking properties observed from each cell type using cell-attached recording. Left panel, mean spike waveforms of the excitatory (black), PV (green), and SOM (red) neurons. Scatter plots show the firing rate, spike width, and peak-trough height ratio of all three cell types. Open circle, individual cell; line, population mean. *, p < 0.05; **, p < 0.01 (ANOVA multiple comparison). (H) Mean spike-triggered OGB-1 fluorescence of the excitatory (black), PV (green), and SOM (red) neurons. The elevated baseline for SOM interneurons is likely due to the frequent occurrence of spontaneous spike bursts. (I) Overlay of (B), (D), and (F).

Figure 2. Imaging spike transmission during single-pyramidal neuron stimulation in vivo

(A) Schematic of experiment. (B) Two-photon image of the patched neuron (blue) near a SOM interneuron expressing GFP (red), together with other cells labeled with OGB-1 (white), of a heterozygous GIN mouse. (C) Spiny dendrite of a patched pyramidal neuron. (D) Left, schematic of the depolarizing current steps injected into the patched pyramidal neuron during stimulation trials (800 pA, 0.1 s/step, 1 s inter-step interval; gray bars, durations of current steps). Middle and right, peri-stimulus time histogram of the APs recorded from the patched neuron from 135 stimulation or control trials (0 pA, 0.1 s/step, 1 s inter-step interval). (E) OGB-1 fluorescence trace of the same patched pyramidal neuron during stimulation (left). Trial-averaged dF/F were computed from either stimulation (middle) or control (right). Shading, 90% confidence intervals from bootstrap. Gray bar, duration of current step. (F) OGB-1 fluorescence trace and trial-averaged dF/F from a SOM interneuron within the same field of view. (G) OGB-1 fluorescence trace and trial-averaged dF/F from a putative pyramidal neuron from another experiment. This putative pyramidal neuron was located 51 µm lateral and 20 µm more superficial from the patched pyramidal neuron, which was not imaged simultaneously. The traces of (D), (E) and (F) are from the same cells that are shown in (B).

Figure 3. Temporal dynamics of the functional connections

(A) The two types of functional connections under study (red, pyramidal-SOM; black, pyramidal-putative pyramidal). An imaged neuron was classified as functionally connected to the stimulated, patched pyramidal neuron if ΔdF/F_post-pre > 3 × SEM (see Experimental Procedures). (B) Magnified views of fluorescence traces in response to single-pyramidal neuron stimulation (800 pA, 0.1 s/step, 1 s inter-step interval). Traces are from the same cells shown in (E) – (G) in Figure 2. (C) Trial-averaged fluorescence response, averaged across the 5 SOM interneurons driven by single-pyramidal neuron stimulation. Response of each SOM interneuron was normalized by its peak-to-peak amplitude (bin width = 10 ms; gray bar, duration of current step). Note the delay of fluorescence transient relative to stimulation onset. Because the imaging frame rate (15.6 Hz) and frequency of stimulation (0.91 Hz) are not multiples of each other, over many trials, fluorescence was sampled at many time points around the current step and the effective temporal resolution is higher than 15 Hz. Dotted line, mean spike-triggered dF/F of SOM interneurons from Fig. 1H with the triggered spike aligned to onset of single-cell stimulation. (D) ΔdF/F_post-pre vs. number of APs elicited in the patched neuron, mean of the 5 SOM interneurons. Dashed line, expected dF/F for 1 AP/trial in SOM neurons, based on the fitted slope of the population average (red line) in Fig. 1F. (E) ΔdF/F_post-pre of each trial over the first 120 stimulation trials. Gray, mean of the 5 SOM interneurons. Red, linear fit. (F) Mean ΔdF/F_post-pre for trials in UP or DOWN state. The state of each trial was determined from the membrane potential of the patched neuron immediately preceding stimulation (see Experimental Procedures). Gray lines, individual cells; red, population. (G – J), Similar to (B – E) for the 20 pyramidal-putative pyramidal connections. Dashed line in (H), expected dF/F for 1 AP/trial in pyramidal neurons, based on fitted slope of black line in Fig. 1B. For (C) – (E) and (G) – (I), error bar, ± SEM.

Figure 4. PV interneurons are less sensitive to single-pyramidal neuron stimulation

(A) Schematic of experiment. (B) Two-photon image of the patched neuron (blue) near a PV interneuron expressing tdTomato (green), together with other cells labeled with OGB-1 (white) of a PV-Cre × tdTomato mouse. (C) OGB-1 fluorescence traces of the patched pyramidal neuron (blue) and PV interneuron (green) during stimulation trials (800 pA, 0.1 s/step, 1 s inter-step interval; gray bars, durations of current steps). (D) Trial-averaged dF/F were computed from 135 trials of stimulation or control (0 pA, 0.1 s/step, 1 s inter-step interval). Shading, 90% confidence intervals from bootstrap. (E) Trial-averaged dF/F during stimulation for the patched pyramidal neuron. The traces of (C), (D), and (E) are from the same cells that are shown in (B). Note that despite their proximity, there was no contamination of the PV trace by the stimulus-locked signal in the patched cell.

Figure 5. Calcium transients of PV interneurons are highly correlated with the local network activity

(A) OGB-1 fluorescence (black) from four example cells within the same field of view of a PV interneuron during single-pyramidal neuron stimulation. The spike rate of each cell was inferred (gray) using a fast non-negative deconvolution algorithm (see Supplemental Experimental Procedures). (B) Network activity (gray trace) was defined as the sum of inferred spikes of all cells within the field of view except the stimulated pyramidal neuron and the identified PV or SOM interneuron (n = 34 for the example shown, including the 4 cells in (A)). Light gray bars, durations of current steps. Green trace, time-lapse fluorescence of a PV interneuron within the same field of view. (C, D) Correlation coefficients between the fluorescence of PV (green, n = 17) or SOM (red, n = 20) interneurons and activity of the network (C) or the patched neuron (D), during either control or stimulation trials. Solid red dots, functionally connected SOM interneurons (classified based on ΔdF/F_post-pre, see Experimental Procedures).

Figure 6. The influence of single-pyramidal neuron stimulation on the local cortical circuits

(A) Trial-averaged ΔdF/F_post-pre for all the cells imaged during stimulation trials. The cells were ranked by their (mean – 3 × SEM) value, which was used to identify functional connectivity (red, SOM interneuron; green, PV interneuron; black, connected, putative pyramidal neurons; gray, unconnected unidentified neurons). Error bar: ±3 × SEM. (B) The same set of cells ranked by their trial-averaged ΔdF/F_post-pre during control trials.

Figure 7. Spatial maps of the functional connections

(A) Spatial x–y distribution of imaged (open circles) and connected (filled circles and dotted lines) neurons with significant ΔdF/F_post-pre for each cell type relative to the patched neuron (blue cross). A-P, anterior-posterior; M-L, medial-lateral. (B) Distributions of distance from the patched cell to each type of imaged and connected neurons. (C) Distance dependence of response amplitude for functionally connected SOM interneurons (red) and putative pyramidal neurons (black). (D) Excitatory influence map of the imaged neurons with ΔdF/F_post-pre > 0 (red, SOM; green, PV; black, unidentified). Each circle represents a cell and the diameter represents the magnitude of ΔdF/F_post-pre.