Field recordings in slices from p35-/- mice show little increased dentate excitability when extracellular potassium concentration is increased from 3 to 6 mm. A, Top, Representative field responses to perforant path stimulation (gray arrows) (moderate stimulation intensity, 0.4 mA) in wild-type and knock-out slices in 3 mm extracellular potassium. A, Bottom, Graph plots the percentage of the maximum of the slope of the fEPSP versus the stimulus intensity (milliamperes), under control (3 mm) conditions. B, Top, Representative evoked field responses (stimulation intensity of 0.4 mA) in the presence of 6 mm extracellular potassium (overlaid on gray traces from A). B, Bottom, Graph plots the rise of the fEPSP slope as a percentage of the maximum response versus the stimulus intensity. Calibration: 0.5 mV (vertical), 5 msec (horizontal). The error bars in bottom graphs represent the SEM (n = 5 in each group).