Fig. 1. Concentration–response curves for paired pulse inhibition of population spikes by propofol and pentobarbital. Each point represents six or more samples. Paired pulse stimulation was delivered to the Schaffer collateral pathway at an interval of 21 ms and an intensity sufficient to evoke maximal responses. Paired pulse inhibition of population spikes (PSs) was expressed as a change in the ratio of the second PS amplitude to the first PS amplitude after 20 min of drug administration. The IC50for propofol was 19 ± 12 μm, and the IC50for pentobarbital was 29 ± 1 μm. The upper two traces on the right depict PSs evoked by paired pulse stimulation before ( upper trace ) and during ( second trace ) administration of propofol. The lower two traces depict PSs evoked by paired pulse stimulation before ( third trace ) and during ( bottom trace ) administration of pentobarbital. Scale bar : 2.5 mV, 5 ms .