We could evoke inhibitory currents upon minimal stimulation protocol in both dorsal and ventral L2S. Figure 2C depicts one such example with overlayed exemplary IPSC traces and the corresponding histogram of minimally

evoked inhibitory currents. There were no significant differences between the dorsal and ventral L2S in amplitude or in the failure rate of IPSCs (failure rate probability upon minimal stimulation: dorsal: 0.36 ± 0.04, n = 7; ventral: 0.37 ± 0.10, n = 7; p = 0.90, Mann-Whitney test; Figure 2D). These results demonstrate that the inhibitory BIBW2992 cell line synapses have similar release probability along the DVA and point to a gradient in the number of inhibitory input synapses onto L2S in MEC microcircuits. To understand in more detail the organization of the inhibitory microcircuits that operate in the MEC, we mapped the functional connectivity of inhibitory networks within the MEC. To this end, we uncaged glutamate over the superficial layers (LI–LIII) of the MEC (Figures 3A1

and 3A2) and recorded the resulting photoevoked inhibitory postsynaptic currents (pIPSCs) in L2S (Fino and Yuste, 2011, Luna and Pettit, 2010, Oviedo et al., 2010, Brill and Huguenard, 2009 and Dantzker and Callaway, 2000). Figure 3A3 exemplarily shows such a map of inhibitory inputs received by a single L2S. Example traces in Figure 3A4 exhibit clear pIPSCs in seven out of the nine neighboring stimulation points. In what follows, we use the number and PD0325901 spatial distribution of stimulation points (Beed et al., 2010 and Bendels et al., 2010) that from evoked a pIPSC to quantify the detailed organization of the local inhibitory microcircuits onto L2S. We recorded

distinct pIPSCs in both dorsal (Figures 3B1 and 3B2) and ventral (Figures 3C1 and 3C2) stellate cells. At both sites, the inhibitory microcircuits exhibit a local organization, and most of the intralaminar inhibitory points when superimposed onto the DIC images of the acute slices were found to be in layer II. The regions over which these input sites were distributed showed a larger spatial spread for the dorsal cells as compared to ventral cells (Dorsalfwhm: 204.45 μm, n = 10; Ventralfwhm: 117.31 μm; n = 7; p < 0.05, Mann-Whitney test; Figures 4A and 4B). The cumulative distribution of the inputs and distances of the input points (Figures 4C and 4F) clearly showed that the ventral L2S receive inputs from a much narrower spatial distance than the dorsal cells. This suggests that dorsal stellate cells are contacted by proximal as well as distal inhibitory interneurons, while ventral cells mainly receive inhibitory inputs from proximal interneurons. Furthermore, the total number of stimulation points that elicited pIPSCs was significantly larger in dorsal cells than in ventral cells (dorsal: 71.00 ± 11.95 points, n = 10; ventral: 33.29 ± 8.96, n = 7; p < 0.