Figure 5.

Presynaptic depolarization associated with raised [K⁺]_o reversibly enhances the amplitude while decreasing the paired-pulse ratio of MF-evoked responses in CA3 pyramidal cells. A, Schematic showing positions of the stimulating electrode (stratum granulosum), patch-clamp pipette (bottom left), the extracellular recording electrode (top left), and the pressure-pulse pipette (middle; ∼20 mm K⁺ in bath solution). B, Whole-cell EPSCs (top traces) and fEPSPs (bottom traces) recorded, respectively, in CA3 pyramidal cells and in stratum lucidum, evoked by two stimuli (25 ms apart) applied in the granule cell layer (one-cell example). Ctrl, Control conditions; K⁺, [K⁺]_o increased from 2.5–5 mm; Wash, washout. C–F, The average time course of the first EPSC amplitude (C; average change in high [K⁺]_o, +54 ± 15%; n = 6; p < 0.001), the paired-pulse amplitude ratio (D; −31 ± 8%; p < 0.03), holding current (E; +24 ± 13%; p < 0.05) and the fiber volley amplitude [F; +15 ± 13%; not significant (NS)]. F, Inset, A characteristic fEPSP trace showing a fiber volley (arrow) in control conditions (black) and after an increase in [K⁺]_o (gray). G–K, Same experiments as in B–F but with pressure application of K⁺ (100 ms/∼ 5 psi pulse; arrow) near the recorded CA3 pyramidal cell. Notation is as in B–F. The average changes after a high K⁺ puff (n = 9) are as follows: EPSC amplitude, +29 ± 10% (p < 0.03); paired-pulse ratio, −29 ± 8% (p < 0.01); holding current, +44 ± 34% (NS); fiber volley, −5 ± 6% (NS). Error bars indicate SEM.