Figure 4. Shape of action currents as a function of pipette solution and pipette potential. Action currents were aligned with respect to their rising phase. In A–C, the two first columns show superimposed action current traces at pipette potentials of −60 mV (blue) and 0 mV (black). The two last columns show superimposed averages of the action currents at the two pipette potentials, as well as superimposed integrals of these averages. A, NaCl in pipette solution. In this case, the action current waveforms are similar for the two pipette potentials. Their integrals (right) approach 0 at the end of the waveform (dashed line), indicating that each waveform is dominated by its capacitive component. B, K-gluconate in pipette solution. Here fluctuations can be discerned in individual traces (arrows), as outward deflections during the downstroke of the action current at −60 mV (inset in left indicate differences between single traces carrying deflections and the average of event-free traces), as well as inward deflections during the late part of the negative wave of the action current at 0 mV. As a result, average waveforms depend on the pipette potential, and the integral displays a negative value at 0 mV. C, K-gluconate + TEA in pipette solution. Including TEA (2.5–5 mm), a K channel blocker with high affinity for BK channels, abolishes trace-to-trace fluctuations as well as the sensitivity of average waveforms to pipette potential. D, Group data analysis of experiments as illustrated in A–C (n = 5–7 in each condition). Integral values are taken 10 ms after the upstroke of the action current (left and middle). At −60 mV, no significant difference appears between the three conditions (left), but at 0 mV, the integral measured in K-gluconate (KG) is significantly more negative than that obtained with either NaCl or K-gluconate + TEA (middle; p < 0.01, Mann–Whitney U test). Right, Log of the ratio between spiking frequencies measured at 0 and −60 mV. The results indicate that adding TEA to the K-gluconate pipette solution reduces the voltage sensitivity of the firing frequency (p < 0.05, Mann–Whitney U test). The K-gluconate + TEA results are indistinguishable from those obtained with an NaCl solution.