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. 2010 Jun 2;30(22):7672–7684. doi: 10.1523/JNEUROSCI.0290-10.2010

Figure 9.

Figure 9.

Time course for PDP of tonic conductance. A, Bicuculline-sensitive ramp currents. Ramp currents were recorded every 10 s, first at a holding potential of −60 mV, during depolarization to +40 mV, and then after repolarization to −60 mV. Voltage ramps were from −30 to +30 mV over 0.5 s. Illustrated currents are difference currents obtained by subtracting ramp currents recorded in the presence of bicuculline from control currents. Bicuculline (10 μm) was applied every 30–50 s; the mean holding current in bicuculline was subtracted from control holding current before each voltage ramp to yield tonic current amplitude. Times at which currents were measured are indicated. Reversal potentials (i.e., EGABA) shifted toward more positive values during depolarization and recovered slowly after repolarization. Currents were recorded in the presence of 0.3 μm exogenous GABA. B, Double y-axis plot of tonic conductance (open circles) and EGABA (solid line) showing time course of changes produced by depolarization to +40 mV (horizontal bar). Chord conductance was calculated with EGABA and tonic current measured as in A. Conductance continued to increase during depolarization to +40 mV even after shifts in EGABA approached a plateau. Conductance dropped to 8.7 nS immediately on repolarization to −60 mV, before substantial recovery of EGABA, but remained above baseline values. The solid line represents a single exponential fit to the recovery portion of conductance data. C, Time course for tonic current (open circles) from experiment in A and B. Theoretical changes in tonic current that would occur solely because of changes in driving force produced by shifts in EGABA, assuming no change in conductance from baseline, are plotted as a solid line for comparison with experimental data. PDP of tonic current in this situation would be only 27%, which is small compared with the 257% potentiation observed experimentally. Conversely, the theoretical current amplitude using the conductance values from B but assuming EGABA remained constant at −7 mV throughout the experiment is illustrated as triangles. In this case, PDP would be 178% at its peak. Based on these calculations, changes in electromotive driving force from anion redistribution contributed no more than 10–30% to PDP of tonic current.