A, current clamp recording obtained from the soma of a pyramidal cell. A simulated EPSC was injected at the soma 3 ms after the offset of the square pulse that elicited the first spike. DTX (2 μm, bath applied, thin trace), enhanced the ADP, allowing the EPSP (subthreshold in control condition, thick trace) to evoke a second spike. B, the minimum amplitude of the sEPSC that evoked a second spike was quantified relative to the size of the first, suprathreshold 2 ms current injection, and termed the burst current. DTX reduced the burst current (n = 19, **P < 0.01). C, application of DTX caused bursting in response to sustained somatic current injection (600 ms). D, bursting was defined as responses to 600 ms current injection where the shortest interspike interval (ISI) was less than one-tenth of the average ISI (ISI ratio). DTX increased the percentage of bursting cells from 20% to 45%.