Differences between cell-attached and whole-cellV_m measurements. A, Determination of K⁺ current reversal, and hence membrane potential, from a CA1 interneuron after exposure to high external [K⁺] and to GABA (50 μm). This cell discharged spontaneously, giving rise to action currents visible in the cell-attached record (arrows). For each condition, five consecutive current traces recorded at 2 sec intervals are shown. Bottom panels show the inward K(V) and its reversal in greater detail. B, Plot ofV_m determinations (circles; for each condition average V_m and SD are indicated by solid anddotted lines, respectively) and the mean firing frequency (shaded bar) in the presence of high K⁺ and GABA. Increasingextracellular [K⁺] from 2.5 to 10.5 mm caused a depolarization, and the firing rate increased. In contrast, during GABA application V_m stabilized, and the generation of action currents ceased. C,V_m measured subsequently from the same cell in the whole-cell current-clamp mode was more depolarized. No spontaneous AP activity was seen, and membrane potential was stable within 2–3 mV. Increasing [K⁺]_o and GABA application both strongly excited the cell. The pipette solution contained 120 mm Cl⁻, and the interneuron was recorded from a slice obtained from a 17-d-old rat.