Figure 1

Enhancement of facilitation by isoflurane and low external Ca²⁺ but not by TTX. A, (top left) an averaged extracellular recording showing two methods that were used for signal parameter measurement: a minimum-to-baseline (MTB) algorithm was used to measure FV and EPSP amplitudes (dashed lines); a dV/dt (slope) algorithm was used to measure the initial slope of FVs (1) and EPSPs (2). A, (top right) comparison of slope and amplitude measures of FVs as a function of stimulus voltage. Open circles are FV amplitudes measured by MTB (r² = 0.99; n = 60). Green circles are FV slope measurements that were scaled to match the amplitude data (r² = 0.85; n = 60). Each data set was fit by linear regression (solid lines). A, (lower right) comparison of slope and amplitude measures of EPSPs as a function of stimulus voltage. Open circles are EPSP amplitudes (r² = 0.97; n = 30). Blue circles are EPSP slope measurements that were scaled to match the amplitude data (r² = 0.83; n = 30). Solid lines represent fits by linear regression. A, (lower left) an experiment showing changes in FV amplitudes (gray circles) and scaled slopes (green triangles) as stimulus voltage was increased and decreased. EPSP amplitudes (open circles) are plotted together with scaled EPSP slopes (blue triangles). Although the stimulus voltage was changed over a 3–6 V range there was no change in facilitation (solid black line). B, Isoflurane depressed EPSPs (white and gray circles) and increased facilitation (black diamonds). C, Low external Ca²⁺ (0.8 mM) depressed EPSPs and increased facilitation. D, 60 nM TTX depressed EPSPs while facilitation remained unchanged. Note that changes in the stimulus voltage in all three experiments (appearing as ramps in EPSP amplitudes) did not affect facilitation.