Figure 2.

Sublinear summation is voltage dependent and mediated by transient potassium channels. (A) Using a protocol similar to that of Fig. 1B, but performing the experiments in voltage clamp, perforant path and mossy fiber EPSCs sum linearly at a 10-ms ISI. Electrode contained K⁺ gluconate-based pipette solution. (B) Linear summation was observed at all ISIs tested. Cells held in voltage clamp at −75 mV. Whole-cell electrodes containing either K⁺ gluconate (N = 5) or cesium fluoride and QX-314 (N = 4) produced similar results. (C) Summation is linear when the amplitude of the preceding mossy fiber EPSP is 4 mV, but sublinear when it is 14 mV (ISI = 5 ms, V_m = −70 mV). (D) Data showing the relationship between mossy fiber (N = 7) and perforant path (N = 5) EPSP amplitude and the ratio of the amplitude of subtracted to predicted responses. (E) Summation at both the 100-ms and 5-ms ISIs is linear when the CA3 cell is filled with 4AP (5 mM), a blocker of voltage-dependent potassium channels, but remains sublinear when pipette solution contains QX-314 (10 mM).