Fig. 4. Class II interneurons are hyperpolarized during pyramidal cell bursts. A, left traces, Class II interneuron held at −35 mV hyperpolarizes during each pyramidal cell burst. A1, Bottom tracesshow a pyramidal cell burst (bracket) with interneuron IPSPs paralleled by each field event in the extracellular pyramidal cell recording. A, right traces, When the cell is held at −70 mV, the burst-associated hyperpolarization in the interneuron is absent. A2, Expanded time scale of a burst (A1, bracketed region) shows that the IPSPs have reversed polarity. B, left traces, In the same experiment, addition of picrotoxin (100 μm) to the perfusate blocks the burst-associated interneuron hyperpolarization observed at −35 mV. B2,left, Expanded time scale unmasks EPSPs, which are temporally correlated with individual field events. B,right traces, When the interneuron is held at −70 mV in the continued presence of picrotoxin, some EPSPs now reach threshold during the pyramidal cell burst. B2, Traces are displayed on an expanded time scale. Calibration: A1,B1, 16 mV intracellular, 0.1 mV extracellular, 12.5 sec;A2, B2, 500 msec.