Dense inhibitory connectivity in neocortex

Abstract

The connectivity diagram of neocortical circuits is still unknown, and there are conflicting data as to whether cortical neurons are wired specifically or not. To investigate the basic structure of cortical microcircuits, we use a two-photon photostimulation technique that enables the systematic mapping of synaptic connections with single-cell resolution. We map the inhibitory connectivity between upper layers somatostatin-positive GABAergic interneurons and pyramidal cells in mouse frontal cortex. Most, and sometimes all, inhibitory neurons are locally connected to every sampled pyramidal cell. This dense inhibitory connectivity is found at both young and mature developmental ages. Inhibitory innervation of neighboring pyramidal cells is similar, regardless of whether they are connected among themselves or not. We conclude that local inhibitory connectivity is promiscuous, does not form subnetworks, and can approach the theoretical limit of a completely connected synaptic matrix.

Figure 1. Two-photon activation of somatostatin-expressing interneurons

A, Somatostatin-expressing interneurons in a Z-stack of a slice from GFP expressing mice. B, Reconstruction of a sGFP interneuron, which is a Martinotti cell. Dendrites are in red and axons in blue. Note the wide axonal arborization, projecting to the pia (black line). C, Electrophysiological characteristics of somatostatin interneuron. Traces show individual voltage responses to series of 1 second from −35 pA to +35 pA (left panel; note the sag for hyperpolarizing steps) and to twice the rheobase (right panel, spike frequency= 21 Hz). I/V curve (lower panel), measured at the steady-state of the voltage steps. D, sGFP interneuron and position of the multiplexed two-photon uncaging targets (8 subtargets; green) on the soma of the cell. In this example, two (left) or a burst (right) of APs were evoked with 180 mW and 250 mW on sample, respectively. Black lines indicate uncaging pulses. E, Photostimulation of the interneuron four times with identical power (190 mW) induced the same number of APs every time. F, Paired recording of connected sGFP interneuron and layer 2/3 PC. Both cells are loaded with Alexa-594. Traces of inhibitory responses recorded in the PC (black traces) after one or two APs evoked in the sGFP interneuron (red traces) with 50 ms, +50 pA and +120 pA current injection pulses, respectively. The interneuron was held at resting membrane potential in current-clamp and the PC was held in voltage-clamp at +40mV. G, Inhibitory responses recorded in PC after one AP evoked in the connected sGFP interneuron by current injection (50 ms pulse, +50 pA) or by two-photon RuBi-Glutamate uncaging. H, Effect of RuBi-Glutamate on synaptic transmission between sGFP interneuron and PC. H₁, Responses recorded in the PC, in response to 50 ms, 100 pA current injection pulses in the sGFP interneuron, before (left) and after (right) adding RuBi-Glutamate to the bath. H₂, Average GABA current inhibition by RuBi-Glutamate (47.3±10.8 %, n=7). I, Example of a weak connection between a sGFP interneuron and a PC (10.5 pA in average), detectable by evoking a burst of APs in the sGFP interneuron.

Figure 2. Detection of inhibitory connections

A, Layer 2/3 field with a recorded PC (PC, #9), filled with Alexa-594, and 8 different sGFP interneurons (#1 to 8). Circles indicate connected interneurons (red), unconnected ones (blue) or false-positive responses (grey). Below are voltage-clamp recordings of responses in PC when sequentially stimulating each sGFP interneuron; the PC was either clamped at +40mV (top trace) or at −40mV (bottom trace). Red stars indicate traces corresponding to connected GFP cells and circled numbers are the ones expanded in B. B, Responses recorded in PC at +40mV (top trace) or −40mV (bottom trace) for a connected interneuron (#4) or a false positive cell (#6). A connected interneuron (#4) induces an inhibitory response (GABA), outward at +40mV or −40mV. A false positive (#6) is an excitatory response, outward at +40mV but inward at −40mV (Glutamate). C, Another layer 2/3 field with a recordedPC, and 7 sGFP interneurons. Recordings and color code as in A. The response evoked by connected interneuron #2 is expanded. D, Electrophysiological confirmation that interneuron #2 and the PC were connected. Current-clamp recording of connected interneuron #2 (red) and responses evoked in the PC (black) following one or two APs. The interneuron was held at resting membrane potential in current-clamp and the PC was held in voltage-clamp at +40mV. The IPSP had a latency of 1.29±0.20 ms (n=6) and an amplitude of 31.3±11.5 pA (n=6), matching our previous characterization of sGFP-PCs connections (Table 2).

Figure 3. Two-photon mapping of inhibitory inputs in three dimensions

A, Left: Images of PCs filled with Alexa-594; the position of each PC is indicated by a yellow arrow. Right: Images of sGFP interneurons, at different depths (50, 60 and 85 μm) (vertically) for the three different PCs (horizontally). On each image, all the stimulated sGFP cells are indicated by a number and the colored circles indicate connected interneurons (red circles), unconnected ones (blue circles) or false positive responses (grey circles) to the appropriate PC. B, Projected Z-stack of the field representing the summary of all connections found for each recorded PC within the whole field. Some cells are stimulated on two different depths so represent only one cell on the summary. Note how in this slice, most of sGFP cells are connected to every PC.

Figure 4. Dense innervation and distance-dependence of connectivity

A, Proportions of connected (red) and unconnected interneurons (blue) and false positives responses (black) within all tested sGFP interneurons. B, Connection probability for all stimulated interneurons in all maps. C₁, Distribution of intersomatic distances for all the sGFP interneurons tested and schematic representation of the measurement. The peak was centered on 230.83±5.3 μm (n=1,245). C₂, Averaged intersomatic distances between PCs and connected (red) and unconnected (blue) interneurons and false positive cells (black). Connected interneurons were significantly (p<0.0001, Mann-Whitney) closer to the PC than unconnected ones but not significantly different than false positive cells. D₁, Distributions of the intersomatic distances between PCs and connected (red) and unconnected interneurons (blue) and false positives (black). Peaks were centered on 170.54±4.55 μm (n=520) for connected interneurons, 300.63±5.02 (n=584) for unconnected interneurons and 192.15±14.80 (n=141) for false positives. D₂ Distributions of ratios of intersomatic distances between PCs and connected (red) and unconnected interneurons (blue) and false positives (black), normalized over the total number of stimulated interneurons. E, Distribution and averages of the connection probabilities for stimulated interneurons within a radius of 400 μm or 200 μm from the PCs. ***: p<0.0001.

Figure 5. Paired recordings confirm local dense inhibitory innervation

A, Reconstruction of a connected pair of sGFP interneuron and PC after biocytin labeling. Pia is indicated in black at the top. The dendrites of the interneuron are red and axon in blue and dendrites of PC are orange, with axon in green. Notice the large overlapping area of the interneuron axon and the PC dendrites. B, Image of the connected pair showed in A, the cells are filled with Alexa-594 and electrophysiological recordings of the connection: inhibitory responses recorded in PC (black traces) after two (50 ms, 150 pA) or a burst (50 ms, 300 pA) of APs evoked in the sGFP interneuron (red traces). C, Proportion of connected and unconnected pairs from dual recordings. Within 40 patched pairs, 70% (n=28) were connected and 30% (n=12) were not. D, Intersomatic distances for connected (red, average: 61.90±5.25 μm, n=23) or unconnected (blue, 73.6±9.4 μm, n=9) pairs. There is no significant difference in the intersomatic distances for connected or unconnected sGFP-PC pairs. E, Ratios of connected pairs (red) and unconnected pairs (blue) over the total number of pairs.

Figure 6. Analysis of columnar connectivity

A, Distribution of distances from the PC vertical axis to all sGFP cells tested and schematic representation of the measurement. The peak was centered on 144.3 ± 10.3 μm (n=1245). B, Average distances from PC axis to connected (red) and unconnected interneurons (blue) and false positives (black). Connected interneurons were significantly (p<0.0001, Mann-Whitney) closer to the PC vertical axis than unconnected interneurons but similar to false positives. C₁, Distribution of distances from PC axis to connected (red) and unconnected interneurons (blue) and false positives (black). Peaks were centered on 114.10±5.53 μm (n=520) for connected interneurons, 219.65±12.00 (n=584) for unconnected interneurons and 80.86±5.12 (n=141) for false positives. C₂ Distributions of ratios of connected (red), unconnected (blue) and false positives (black) over all the tested sGFP interneurons, depending on the distances from the PC axis. ***: p<0.0001.

Figure 7. Lack of specific subnetworks

A, Pair of connected PCs filled with Alexa-594. B, Example of a recording of a connected PCs pair (with Cs-based internal). C, Maps of inhibitory inputs onto two connected PCs. Colored circles indicate connected interneurons (red), unconnected ones (blue) or false positive responses (grey). Yellow arrows indicate position of the PC. D, Distributions of ratios of connected (red) and unconnected interneurons (blue) over the total number of stimulated interneurons for connected PCs (D₁) or unconnected PCs (D₂). These distributions were not significantly different between connected and unconnected PCs, either for connected (p=0.48, Wilcoxon paired test) or unconnected interneurons (p=0.21) E, Distribution and averages of the probability of inhibitory connections for pairs of PCs, for unconnected PCs (black) or connected PCs (red). There was no significant difference (p=0.07, Mann-Whitney test) in connection probability between connected and unconnected PCs. F, Distribution and averages of the probability of receiving common inhibitory inputs for pairs of PCs, for unconnected PCs (black trace) or connected PCs (red trace). There was no significant difference (p=0.36, Mann-Whitney test) in connection probability between connected and unconnected PCs.

Figure 8. Dense inhibitory innervation across different developmental stages

A, Connection probability from sGFP cells to PCs at different developmental stages. The inhibitory connection probability is not statistically different throughout the whole range of ages (p<0.05, one-way ANOVA). B, Intersomatic distances for older animals (P20 to P41) for all stimulated sGFP cells; the peak is centered on 194.79±5.74 (n=787). Ratios of connected (red) or unconnected (blue) sGFP cells over the total number of stimulated sGFP cells, as a function of intersomatic distances between sGFP cells and PCs, for older animals (P20–P41). C₁ and C₂, Ratios of connected interneurons over total number of stimulated interneurons for connected PCs (C₁) or unconnected PCs (C₂) (P20–P41). There was no significant difference (p=0.24, Wilcoxon paired test) between connected and unconnected PCs for connected interneurons spatial organization. D, Distribution of the probability of inhibitory connections for pairs of PCs (P20–P41), for unconnected PCs (black) or connected PCs (red). There was no significant difference (p=0.83, Mann-Whitney test) of connection probability for unconnected or connected PCs. E, Distribution of the probability of receiving common inhibitory inputs for pairs of PCs (P20–P41), for unconnected PCs (black trace) or connected PCs (red trace). The common connection probability was not significantly different (p=0.67, Mann-Whitney test) for unconnected or connected PCs. F, Hypothetical circuit model that could underlie our results: a dense connectivity of inhibitory inputs from sGFP interneurons where every interneuron is connected to each neighbouring PC. Within a local region, the connectivity could be complete. Dashed line between the two PCs illustrates that the same model applies for connected or unconnected PCs.