Figure 5.

Bursts are unsynchronized events. (A) Demonstration of the stimulation paradigm for paired burst firing. First, current thresholds for bursts were determined independently for each cell (trial 1). Next, bursts were elicited in coupled neurons by delivering peri-threshold current injections to both neurons simultaneously (trial 2). Traces are offset for clarity. Scale bar: 25 mV, 50 ms. Burst synchrony is taken as the absolute value of the difference in burst onset times between cell 1 and cell 2. (B) Finer time-scale of 15 burst trials in a pair of coupled neurons. Bursts rarely synchronized, and individual burst onsets also varied by tens of ms. Scale bars: 25 mV, 50 ms. (C) With spontaneous synaptic activity intact, burst synchrony did not show a significant change after blocking I_NaP with 100 nM TTX. Mean difference in burst onsets in ACSF: 27.6 ± 6.0 ms (mean ± SEM, N = 6 pairs); mean difference in burst onsets in TTX: 28.6 ± 8.5 ms (p = 0.94, paired, two-tailed t-test). (D) In the presence of synaptic blockers (DNQX and APV), burst synchrony between coupled cells also showed no significant change after TTX application. Mean difference in burst onset in ACSF: 33.7 ± 4.5 ms (N = 7 pairs); mean difference in burst onset in TTX: 33.6 ± 8.7 ms (p = 0.99, paired, two-tailed t-test). Differences between values with and without blockers were insignificant (p > 0.4 for all comparisons, ANOVA). Data points in (C–F) represent average values for 20–30 trials at burst thresholds for each pair. (E) Gap junction-driven spikes in a strongly coupled pair (mean cc = 0.4) demonstrate the difference between burst and tonic spike synchrony. The burst onset difference (i) was 23 ms; the closest two spikes within the bursts were 7.4 ms apart. The tonic spikes (ii) were 1.9 ms apart. V_rest = −70 mV for both cells. Scale bar 15 mV, 50 ms. (F) Fine resolution of the paired bursts (i) and the paired tonic spikes (ii), offset for clarity. Scale bar 20 mV, 20 ms.