Thresholds of action potentials evoked by synapses on the dendrites of pyramidal cells in the rat hippocampus in vitro.

1. In this study we have measured the thresholds of action potentials and some properties of e.p.s.p.s (excitatory post-synaptic potentials) produced by activation of distal and proximal dendritic synapses in CA1 pyramidal cells in rat hippocampal slices. 2. Simultaneous intracellular and extracellular electrophysiological recordings were made from pyramidal cells. The two recording electrodes were positioned very close together to provide an accurate measurement of the transmembrane potential by subtraction of the extracellular field from the intracellular potential. 3. Two pairs of stimulating electrodes were placed in the stratum radiatum of area CA1. The proximal and distal afferents were stimulated alternately within a 15 s cycle and at an intensity to produce action potentials in about 50% of the trials. 4. At this stimulus intensity there was a slight difference in the mean 10-90% rise times of the e.p.s.p.s produced by stimulation of proximal and distal afferents. The ratio of the distal: proximal rise time was 1.27. 5. In those trials where the action potential failed there was also a significant difference in the time-to-peak of the e.p.s.p.s evoked from proximal or distal sites. The times-to-peak of the e.p.s.p.s from proximal stimulation were between 1.5 and 3.8 ms, whilst the times-to-peak from distal stimulation were longer (2.1-7.2 ms). 6. In 16/18 cells included in this study the thresholds for action potentials evoked by distal stimulation were significantly lower than those following proximal activation. The mean threshold for distal activation was 12 mV compared with 16 mV for proximal activation. 7. The threshold of action potentials activated by depolarizing current pulses appeared to be close to that for action potentials evoked from proximal synapses. 8. Whilst the shape indices of the e.p.s.p.s were slightly different when comparing transmembrane with conventional intracellular recordings, the thresholds of the action potentials were not affected by this procedure. 9. Possible explanations for the low threshold for action potentials evoked from distal synapses are discussed, including an active dendritic membrane and differences in inhibition.