6 ± 0.3 to 1.2 ± 0.2, n = 14, p < 0.001) to a quantity similar to PF stimulation (0.95 ± 0.2 additional APs, n = 3, p = 0.45; Figure S3). Consistent with the idea that CF-mediated excitability is due to glutamate spillover, TBOA dramatically increased the peak probability of APs to 2.1 ± 0.14, significantly
greater than CF stimulation alone (0.98 ± 0.02, n = 7 each, p < 0.001; Figure 3Bii). TBOA also enhanced the excitability that is reflected in the PSH and in the cumulative spike probability plot this website (8.6 ± 1.4 additional APs in TBOA, n = 7, p < 0.01; Figure 3B). Because it was necessary to confirm CF- and lack of PF-mediated transmission, the experiments described above were performed in the whole-cell configuration. However, we also verified the influence of CF spillover on spike activity with noninvasive cell-attached recordings. Similar to the results in whole-cell configuration, stimulation of CFs transiently increased the peak probability and number of evoked APs to 1.0 ± 0.06 (from 0.12 ± 0.01) and 1.5 ± 0.2, respectively (n = 3). TBOA application further increased the peak AP probability (1.8 ± 0.1) and the number of additional APs (5.7 ± 0.6, n = 3; Figure S4). At the conclusion of each experiment, the membrane patch was ruptured to verify the presence Quisinostat order of CF-mediated spillover currents and a lack of PF-mediated transmission. Thus, the cell-attached experiments
replicated the whole-cell results, ruling out the possibility that the intracellular ionic composition affected our whole-cell results. Since the time course of IPSQs and AP probability after below CF stimulation are similar (Figures 2 and 3), we
reasoned that inhibition between MLIs limits excitation. To test this idea directly, we measured the effect of blocking inhibition on the probability, duration, and quantity of APs. In addition to increasing the spontaneous AP frequency (see Experimental Procedures; Häusser and Clark, 1997), SR95531 increased the CF-evoked peak AP probability (from 1.2 ± 0.09 to 1.35 ± 0.1, n = 17, p < 0.05; Figures 4A and 4B) and decreased the latency of the first evoked AP from 2.9 ± 0.1 ms to 2.7 ± 0.06 ms, n = 17, p < 0.05). Blocking inhibition also slightly increased the number of added APs (from 2.15 ± 0.15 to 2.53 ± 0.26, n = 17, p < 0.05; Figure 4B, inset). The surprisingly small effect of blocking inhibition may result from several nonmutually exclusive mechanisms. First, we considered whether the quantity of inhibition was too small to robustly affect CF-mediated excitation since CF-mediated FFI is highly variable with some MLIs receiving essentially no FFI (note several cells with ∼100 pA EPSCs but almost no inhibition; see Figure 1I). Because different intracellular solutions are required to measure AP probability and IPSC amplitude, we were unable to directly correlate these two measures in the same cells.