This suggests that other voltage-dependent, spike-delaying conductances are differentially regulated in fosGFP+ neurons. However, increased spontaneous activity in fosGFP+ neurons
is unlikely to result from cell-intrinsic electrophysiological properties. Under our recording conditions, spontaneous activity in layer 2/3 neurons requires synaptic input since it is abolished in the presence of GABA- and glutamate receptor antagonists (Shruti et al., 2008). To examine whether fosGFP+ neurons receive differential synaptic drive, the frequency and amplitude of postsynaptic currents (PSCs) during spontaneous network activity was analyzed. Although the amplitude of spontaneous excitatory PSCs (sEPSCs) was similar between fosGFP+ and fosGFP− neurons, GSK1120212 fosGFP+ neurons showed a significantly greater sEPSC frequency (Figures 3A–3C; sEPSC frequency fosGFP− 1.3 ± 0.2 Hz versus fosGFP+ 1.7 ± 0.2 Hz; n = 13 cells for both; p =
0.001), a finding further confirmed by paired-cell recordings (Figure 3D). FosGFP+ neurons showed a significantly reduced amplitude of spontaneous inhibitory PSCs (sIPSCs; Figures 3E and 3F; fosGFP− learn more 24 ± 3 pA versus fosGFP+ 19 ± 2 pA; p = 0.02), although frequency was not significantly different (Figure 3G; frequency fosGFP− 3.1 ± 0.5 Hz versus fosGFP+ 2.6 ± 0.5 Hz; p = 0.3). Paired-cell recordings show a significant reduction in sIPSC frequency for fosGFP+ cells (Figure 3H). These data indicate that fosGFP+ neurons receive both more excitation and less inhibition during spontaneous network firing. To determine whether this difference in excitatory and inhibitory input would be maintained in the absence of spiking, miniature EPSCs and IPSCs (mEPSCs and mIPSCs) were assessed in the presence of the Na+ channel blocker tetrodotoxin (TTX). In TTX, no significant differences in mEPSC or mIPSC frequency or amplitude were detected (Figure 3). This result is intriguing, since it indicates that differences in synaptic drive require network firing too to be manifested. During unconstrained network activity, presynaptic
excitatory neurons may fire more onto fosGFP+ neurons, inputs that may provide a basis for increased spontaneous activity of this cell subset. Are fosGFP+ neurons nonrandomly wired into the neocortical network, receiving presynaptic input from excitatory neurons that are themselves more active? We speculated that fosGFP+ neurons might show a high degree of either direct or indirect interconnectivity. Connectivity between pairs of layer 2/3 pyramidal neurons was assessed by a second series of dual-cell recording experiments. In total, 214 paired recordings were performed representing all combinations of fosGFP+ and fosGFP− cells as well as cells from wild-type animals (Figure 4). Using the criterion of a short latency EPSP (1–5 ms) with high (>50%) trial-to-trial reliability, six cells (all fosGFP+/fosGFP+ pairs) out of the group (162 pairs), exhibited a direct, unidirectional synaptic input (Figure 4).