Effects of 8.5 mm [K⁺]_o on spontaneous burst-type discharges. A, Dual cell-attached recordings from neighboring CA3 pyramidal cells (PC1 and PC2) in a P18 rat hippocampal slice. Bath application of 8.5 mm [K⁺]_o increased neuronal firing rate and decreased time delay between burst onset in individual pyramidal cells, resulting in their synchronization. Ba,b, The number of intraburst spikes in the CA3 pyramidal cells (PC1 and PC2) progressively increased and the intraburst interspike intervals (PC1) progressively decreased after 8.5 mm [K⁺]_o application. Time axis starts 2 min after 8.5 mm [K⁺]_o application. Ca, Time delay between two pyramidal cell burst onset after 8.5 mm [K⁺]_o application progressively decreased, tending to zero. Cb, Three minutes after high [K⁺]_o application, two neighboring pyramidal cells sustained hypersynchronous bursts characterized by similar intraburst interspike intervals (326 spikes in 35 bursts for PC1 and 281 spikes in 35 bursts for PC2; bin size, 0.2 msec).