Figure 6.

Effect of blockade of GABAergic inhibition on burst characteristics—two distinct effects. A, Raster plots of color-coded fluorescence intensity (ΔF/F₀) versus time for 50 cells, in the presence of bicuculline (top) and during stimulation in the presence of bicuculline (bottom, 15 s intervals). Addition of bicuculline (10 μm) completely blocked GDP bursts, recorded from the same slice as shown in Figure 1–3, prepared from P5 rat. Note that bicuculline did not block nonsynchronous activity and that synchronous bursts could still be induced by electrical stimulation. B, Plot of onset time distribution for GDPs (green), evoked responses in the absence of bicuculline (blue), and evoked responses in the presence of bicuculline (red). Note that evoked responses without bicuculline exhibited a tighter temporal distribution and bursts evoked in the presence of bicuculline a wider temporal distribution than GDPs. C, Bicuculline did not block spontaneous network bursts in every preparation. Top, Raster plots of color-coded ΔF/F₀ versus time for 75 cells for spontaneous synchronous network bursts (P5 mouse). Bottom, 10 μm bicuculline did not block spontaneous network bursts but induced extremely synchronous and intense events. These network bursts were termed interictal bursts in the presence of bicuculline. D, Plot of cellular activity onset time distribution of interictal bursts (purple dots) revealed that cellular activation times were much tighter (30–40 ms window) than spontaneous bursts without bicuculline (green; 200–250 ms window). E, Fluorescence traces (ΔF/F₀) from three representative cells in C. Spontaneous bursts (GDPs) on the left and spontaneous bursts under bicuculline (interictal bursts) on the right. Note that right traces in each panel are at an enhanced time scale. The intense interictal activity resulted in higher peak ΔF/F₀ values in individual neurons, which decayed more slowly than that seen during GDPs. The rising phase of ΔF/F₀ traces during interictal activity was also faster than that of GDPs.