mEPSC bursts are infrequent at PN 19–23 d. mEPSCs were examined in control conditions and in elevated K₀/Ca₀ at PN 19–23 d instead of PN 13–16 d. (A) Example recordings in 1.5 Ca₀/2.5 K₀ solution (blue) and in 3 Ca₀/20 K₀ solution (green) from the same cell obtained from a 19-d-old rat. During total recording times of 3 min in each solution, no burst was obtained in control, and a single burst was obtained in 3 Ca₀/20 K₀ solution (inset). (B) Peak mEPSC amplitude (left), mEPSC frequency (center), and mEPSC burst frequency (right) in the two solutions (blue: 1.5 Ca₀/2.5 K₀; green: 3 Ca₀/20 K₀). P values represent the results of paired t test for mEPSC amplitudes (left), and of Wilcoxon signed-rank test for mEPSC frequency and burst frequency (center and right). (C) Inter-event histograms in 1.5 Ca₀/2.5 K₀ solution (blue) and in 3 Ca₀/20 K₀ solution (green; data normalized to a recording duration of 300 s as in Figs. 1, 2, and 7). The fast time constant component of the 3 Ca₀/20 K₀ histogram at PN 19–23 d has a small mean amplitude and a high variability compared with corresponding PN 13–16 d data (Figs. 1, 2, and 7). Data are from n = 12 cells from 5 animals in B left, and from n = 13 cells from 5 animals in B center/right and in C.