Potentiation of mEPSC IEI enters an AP bursting-independent phase if spiking activity continues for some time after bicuculline treatment. (A) Stimulation protocol used to block action potential bursting with TTX (1 μM) 2 hours after the 15 minute bicuculline application. The time line indicates the time window when mEPSCs were recorded after washout of TTX (see arrow). (B) The histogram shows mean values of mEPSC IEI of bicuculline or vehicle treated cells recorded 6 hours after the treatment (white and grey column). The black column represents cells that received a 4 hour TTX treatment commencing 2 hours after bicuculline washout. The numbers within columns represent n of cells per group. * p < 0.05 (C) Treatment protocols for the MEA-recordings shown in D used to block action potential bursting with TTX (1 μM) immediately (grey triangle) or 2 hours (black triangle) after the 15 minute bicuculline application. As a control, TTX was also applied immediately (white triangle) after vehicle treatment. Arrows indicate the time points when extracellular MEA-recordings were made. (D) The line graph represents the percentage of spikes within bursts in hippocampal cultures grown on MEAs. The neurons were treated as indicated in C and burst activity was measured for all three groups before, during, 2 hours after, 6 hours after bicuculline/vehicle treatment and after the final TTX-washout as indicated by the letters a-e. *** p < 0.001, n/s not significant.