Figure 2.

Evoked field potential size and glutamate release correlate directly with interstimulus interval in pharmacologically disinhibited neocortical slices. A, Representative traces of EFPs recorded from an electrode (rec) in layer V in a GBZ/CGP-treated slice with a stimulating electrode (stim) placed at the layer VI–white matter interface at ISIs of 2.5, 5 10, 15, and 30 s demonstrate qualitatively larger fields with longer ISIs. B, A plot of evoked field potential area (in millivolt · milliseconds) over time from a representative experiment demonstrates rapid changes in evoked potential area with incremental increases in ISI. Arrowheads indicate time at which the ISI was changed. Initial ISI at time 0 was 2.5 s. At ∼7 min into the recording, the GBZ/CGP was added to the superfusion solution and at 10 min epileptiform responses ensued as seen as a transient increase in EFP amplitude to near 1500 mV · ms and a subsequent decay to <500 mV · ms. At 14 min, the ISI was switched to 5 s, and the responses increased in amplitude, as also occurred with each subsequent increase in ISI. Restoration to the original ISI at 34 min resulted in reduction of EFPs to original 2.5 s levels. C, Evoked field potential area measured in several slices over the range of ISIs demonstrates consistency of ISI effect on EFPs (F = 146.09; p < 0.001; n = 6). For each slice, 10 sweeps were averaged 2 min following each ISI change to allow for stabilization. D, Examples of glutamate biosensor signal changes in a cortical slice at 5 and 10 s ISIs indicate a large increase in the area of cortex recruited into the network with longer ISIs. Pseudo-color scale for the biosensor signal (inverse of the FRET ratio) is shown (inset in D). The box in the diagram in A approximates the imaged region with the same orientation (pial surface at top) along with the location of stimulating (#) and recording (*) electrodes. E, Example evoked field potentials (bottom traces) and number of pixels above threshold (>0.65) for biosensor (top traces) for the two conditions in D. Both EFPs and glutamate signal increased as the ISI was increased from 5 s (gray) to 10 s (black). F, A similar, but more dramatic, change in the glutamate biosensor signal is seen as the ISI is increased from 5 to 30 s. G, A marked prolongation of the evoked field potential (bottom traces) and the glutamate biosensor transient (top traces) is associated with increasing the ISI from 5 s (gray) to 30 s (black). H, There was a direct correlation between the ISI and the area of cortex in which glutamate release is detected (F = 17.00; p < 0.0001). Due to photobleaching of the sensor, data were collected at 5 s ISI and one additional ISI only in each slice. Values were normalized to the 5 s ISI to permit slice-to-slice comparison.