In addition, layer 2/3 PCs integrate information from higher layers and project to layer 5 PCs, which are the output of the cortex. We studied the inhibitory connections onto layer 2/3 PCs and focused on a population of somatostatin-positive

cells. In a separate report, we analyzed in detail their morphologies and intrinsic electrophysiological properties in different cortical areas and concluded that they represented three different subtypes of neurons (McGarry et al., 2010). Nevertheless, in spite of this heterogeneity, in the upper layers of frontal cortex the majority of characterized GFP cells belonged to the Martinotti cell subtype (30/38 characterized neurons), so for the purpose of this current work, we assume that the sampled interneurons mostly represented Martinotti cells. As Selleckchem Epacadostat mentioned in the introduction, these interneurons contact dendrites of PCs and tightly regulate OTX015 order local synaptic integration, including the generation of dendritic spikes (Goldberg et al., 2004 and Murayama et al., 2009). In addition, they could avoid circuit hyperexcitability since they are efficiently recruited by PC activity and mediate

also a strong disynaptic inhibition between PCs (Kozloski et al., 2001, Kapfer et al., 2007, Berger et al., 2009 and Silberberg and Markram, 2007). Here, we find a dense innervation of somatostatin-expressing interneurons onto PCs, which reinforces their potential central role in the network activity. The average probability of connections between sGFP interneurons and layer 2/3 PCs we observed (∼50% within 400 μm and ∼70% within 200 μm) is higher than previously described with double or triple patch-clamp recordings (∼20% in layer 2/3 [Thomson and Lamy, 2007, Thomson and Morris, 2002, Thomson et al., 2002 and Yoshimura and Callaway, 2005] and ∼3% in layer 5 [Otsuka and Kawaguchi, 2009]) but agrees with the frequent occurrence of disynaptic inhibition mediated by Martinotti

cells (Berger et al., 2009 and Silberberg and Markram, 2007). We find a wide range of connection probability, from 0.1 to Ramoplanin 1 within local circuits. Our method likely underestimates the connectivity, because of the slicing of neuronal processes, inefficiencies in the uncaging or in the photoactivation of the presynaptic neurons and also because of difficulty in detecting of small synaptic connections. Therefore, while one could explain a low connection probability by methodological constraints, maps with a high connection probability are particularly informative. In fact, in a substantial number of experiments, after discarding excitatory responses, locally, every single interneuron was connected to the sampled PCs (Figure 4E). These results are surprising, since they indicate that for some of the examined circuits, the local connectivity matrix could have been complete, meaning that every sGFP interneuron was locally connected to every PC.