For this purpose, we patched pairs of sGFP cells and PCs, selected randomly from a local population,

to identify connected ones. From 40 pairs, 70% (n = 28) were monosynaptically connected and 30% (n = 12) were unconnected EPZ-6438 in vivo (Figures 5A–5C). The morphology of one reconstructed connected sGFP cell-PC pair showed a typical Martinotti morphology, with an axonal projection of a Martinotti cell toward the pia (Figure 5A). This large axon and the PC dendrites overlapped, with many potential connection sites. There was no significant difference between the distribution of intersomatic distances for connected (61.9 ± 5.3 μm, n = 23) or unconnected (73.6 ± 9.4 μm, n = 9) sGFP-PC pairs (p = 0.26, t test; Figure 5D). Intersomatic distances

between sGFP cells and PCs were within 150 μm (average of 65.0 ± 4.5, n = 32) and the probability of being connected was homogeneously high from 0 to 150 μm (Figure 5E). In fact, the average connection probability observed for these recorded pairs (0.7) was similar to the probability observed in RuBi-Glutamate mapping experiments within a 200 μm radius from the PC (0.71 ± 0.03, n = 61). In addition, analyzing the connection probability within 150 μm intersomatic distances from the optical mapping experiments, to match with the distances considered with the paired those recordings, also demonstrated a similar connection probability (0.73 ± 0.04, n = SCH772984 61). Altogether, these results from the optical mapping and paired recording experiments indicate a dense inhibitory connectivity from the somatostatin-expressing interneurons within local circuits (<200 μm distance from the PC). Intracortical connectivity

has long been assumed to be vertically organized (“chains” or “columns” [Lorente de Nó, 1949 and Mountcastle, 1982]), and this columnar organization has been observed in the frontal cortex as well (Isseroff et al., 1984). Therefore, we reanalyzed the optical mapping data, measuring the distances between the soma of the sGFP interneurons and a line through the recorded PCs, perpendicular to the pial surface (Figure 6A); this line would correspond to the axis of a hypothetical column. In this analysis, we observed a narrower distribution of the distances of connected interneurons, on average located at ∼100 μm from a putative columnar axis (Figures 6B and 6C). Unconnected cells were located on average at >200 μm from this axis, displaying a broad distribution of distances. False positives showed a peak centered at <100 μm of from the axis (Figure 6C). This analysis showed therefore that sGFP cells located within the same “column” of the considered PCs are more likely to contact and modulate them.