8 ± 1.1 s/e, selleck p < 0.01). Epoch frequency (including subthreshold depolarizations) was not significantly increased in fosGFP+ cells (Figure 2C; fosGFP− cells 0.035 ± 0.007 Hz; fosGFP+ cells 0.034 ± 0.007 Hz, p = 0.26), indicating that network activity can engage both cell populations. To verify that the elevated spontaneous firing activity observed in fosGFP+ neurons was not due to expression of the fosGFP transgene, a second strain of transgenic mice

expressing GFP under the control of the arc/arg3.1 promoter was analyzed (GENSAT BAC transgenic resource, Rockefeller University; Gong et al., 2003). Similar to fosGFP+ neurons, arcGFP+ neurons tended to fire more than arcGFP− neurons within a cell pair (Figure 2B and data not shown; mean overall firing rate, arcGFP− 0.23 ± 0.21 Hz versus arcGFP+ 0.32 ± 0.14 Hz; n = 9 pairs, p = 0.07). Like fosGFP+/− cell pairs, the frequency of depolarizing epochs was identical, and arcGFP+ neurons showed significantly more spikes/epoch than arcGFP− cells (Figure 2D; arcGFP− 6.4 ± 0.7 s/e, n = 83 epochs over 9 cells versus arcGFP+ 8.1 ± 0.6 s/e, n = 89 epochs over 9 cells, p = 0.003). On average, arcGFP+ cells fired 2.5-fold more than arcGFP− cells, a significant difference (p = 0.04). Although values from arcGFP+ neurons were more variable compared to fosGFP+ neurons, it is remarkable that the basic observations made

in both transgenic HDAC activity assay mice are so similar. Thus, it is unlikely that the increased firing activity characterized in fosGFP+ neurons is due to expression of the fosGFP transgene. Simultaneous recordings of fosGFP+ and fosGFP− cells enabled a direct comparison of cell engagement during an epoch of network activity. We found that fosGFP+ neurons were recruited into a depolarizing epoch significantly earlier than fosGFP−

neurons (Figures 2E aminophylline and 2F; mean onset timing for fosGFP− was 67.3 ± 27 ms after onset in fosGFP+ cells; n = 48 epochs over 9 cell pairs; p < 0.001). Thus, although spontaneous network activity engages both cell types, fosGFP+ cells are activated earlier and are more likely to fire during a depolarizing epoch. Why do fosGFP-expressing neurons display elevated spontaneous firing activity? One explanation is that these neurons show greater intrinsic excitability (i.e., depolarized resting membrane potential, action potential [AP] threshold, or input resistance). However, comparison between fosGFP+ and fosGFP− cells showed that these properties were identical between groups (Table S1). To evaluate intrinsic excitability, input-output curves were constructed, using constant current injection to elicit firing (Figure S2). FosGFP+ cells required more current to generate a single spike (mean rheobase current fosGFP− 37.12 ± 1.6 pA versus fosGFP+ 45.6 ± 2.99 pA, n = 16 for both; p = 0.02) and exhibited fewer spikes at all levels of current injection compared to fosGFP− cells (Figure S2).