A, the number of OCP-induced hyperpolarizations increases with the duration of interspike intervals. In the histogram the number of OCP-induced hyperpolarizations (SHPs) that occurred in 0.5 s bins was plotted for five cells in which the discharge frequency was varied by current injection. In 1-EBIO (200 μm) SHPs increased in number at interspike intervals >5 s. B, caffeine (1 mm, n = 3 cells) applied in 1-EBIO was able to increase the frequency of SHPs in spontaneously firing cells only at interspike intervals longer than 4 s. C, caffeine (1 mm) was unable to inhibit the discharge in control solution if interspike intervals were below 1 s (n = 7 cells). D, OCP-induced hyperpolarizations in 1-EBIO reduce spike threshold. The thresholds were measured for spikes triggered from baseline (thin line) or rising phase (thick line) of OCP-induced hyperpolarizations. For each condition 7–12 measurements were taken from each cell (P < 0.0005, n = 5 cells).