mEPSCs display two populations with differential sensitivities to Ko. (A and B) Exemplar experiment illustrating two groups of mEPSCs in control (blue: 2 Cao/2.5 Ko) and in high Ko (green: 2 Cao/20 Ko). (A) Plot of peak amplitude versus 10–90% risetime for individual mEPSCs in control (above) and in 20 Ko (below; 300 s recording duration in each case). Insert: Superimposed representative mEPSCs in control (light blue: fast rising; deep blue: slow rising). (B) Corresponding risetime histograms with double Gaussian fit to the control data (dots). (C) Cumulative risetime histograms from a group of five cells as in A and B. The control histrogram displays a major component with risetimes <0.5 ms and a minor component with risetimes >0.5 ms. In elevated Ko the weight of the <0.5 ms component is greatly enhanced (P = 0.004, Kolmogorov-Smirnov test). (D) Comparison of frequency ratios (20 Ko vs. control) for the two mEPSCs components (P = 0.02, paired t test). Number of independent experiments: n = 5 from 4 animals (C and D), one of which is illustrated in A and B.