Figure 1.

With low-frequency stimulation, there is minimal overlap between the fields of synapses affected by adjacent electrodes and no overlap between nonadjacent electrodes of the rake, as determined by paired pulse analysis. (Inset) CA1 minislice preparation and rake electrode configuration. Paired pulse facilitation was induced by coupling a conditioning stimulus with a test pulse delivered 50 msec later. Test stimuli were delivered either through the same electrode [interelectrode distance (IED) = 0 μm], an adjacent electrode (IED = 100 μm), or a nonadjacent electrode (IED = 200 or 300 μm). Every possible combination of conditioning and test pulses (S1-S1, S2-S2, S3-S3, and S4-S4 are averaged in column 1; S1-S2, S2-S1, S2-S3, S3-S2, S3-S4, and S4-S3, in column 2; S1-S3, S3-S1, S2-S4, and S4-S2 in column 3; and S1-S4 and S4-S1 in column 4) was performed, 15 times each, and the results of all experiments are averaged. Pulses paired through the same electrode showed significant (50% ± 9%) facilitation. Adjacent electrode facilitation was much smaller (6% ± 1%) but was also significantly higher than baseline. Paired pulses from nonadjacent electrodes did not produce any facilitation.