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. 2021 Sep 29;41(39):8181–8196. doi: 10.1523/JNEUROSCI.0410-21.2021

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Copyright © 2021 Ajibola et al.

This is an open-access article distributed under the terms of the Creative Commons Attribution 4.0 International license, which permits unrestricted use, distribution and reproduction in any medium provided that the original work is properly attributed.

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Figure 8. — SuM input promotes GC responses to cortical input, thereby enhancing LTP at the PP-GC synapses. A, Experimental schematic showing a stimulation electrode (stim.) placed in the subiculum to electrically activate the PP fibers, a field-recording electrode in the GCL to monitor LFP and pSpike, and a blue light for photostimulation of the SuM axon terminals in the GCL. B, Representative traces of SuM-mediated fEPSP (black trace) after photostimulation, PP-mediated pSpike (filled area in gray) on electrical stimulation, and a pSpike (filled area in light blue) after the coactivation (Δt = 0 ms) of the SuM and PP. Red represents the arithmetic sum of fEPSP and pSpike. The traces of pSpikes were superimposed and aligned with fEPSP. C, Left bar graph, Summary plots of the pSpike areas evoked by SuM+PP coactivation (light blue) and arithmetic sum of SuM-evoked fEPSP and PP-evoked pSpike (light red). Areas were normalized to pSpike area evoked by the PP alone. SuM+PP coactivation, 1.43 ± 0.16; SuM+PP arithmetic sum, 1.17 ± 0.05; n = 6; p = 0.0313. Right bar graph, Summary plots of relative fEPSP slope, SuM+PP coactivation, 1.01 ± 0.02; SuM+PP arithmetic sum, 0.97 ± 0.01; n = 6; Wilcoxon signed-rank test. D, Top, Representative traces of pSpike responses to PP stimulation alone (black traces) and SuM+PP (blue traces) during a 5 Hz train. Bottom left, Summary of the effect of SuM activation on PP-evoked pSpikes versus stimulus number. PP, n = 6; PP + SuM, n = 6; p < 0.05; two-way ANOVA with Bonferroni post hoc test. Right, fEPSP slope before and after photostimulation of the SuM input. PP, n = 6; SuM+PP, n = 6; two-way ANOVA with Bonferroni post hoc test. E, Left, Representative traces of baseline pSpikes in response to stimulation of PP alone. Middle, LTP induction protocol consisting of four trains of 20 Hz electrical stimulation of the PP alone at 15 s intertrain interval (top) or coactivation of the PP and 20 Hz, 4 trains, 10 ms photostimulation of the SuM input (bottom). Right, Sample traces of pSpikes after LTP induction. F, Time course of the normalized pSpike area recorded from the GCL in response to 20 Hz, 4 trains stimulation of PP inputs alone (black circles), or coactivation of the PP input stimulation and 20 Hz photostimulation of the SuM input (blue circles). PP alone, 104.8 ± 8.59%; n = 6; SuM+PP, 167.6 ± 5.30%; n = 6; p = 0.0009; paired t test. G, Time course of the normalized fEPSP slope of pSpikes recorded from the GCL in response to 20 Hz, 4 train stimulation of PP inputs alone (black circles), or coactivation of PP input stimulation with 20 Hz photostimulation of the SuM input (blue circles). PP alone, 110.6 ± 2.20%; n = 6; SuM+PP, 128.5 ± 5.19%; n = 6; p = 0.0598; paired t test. Error bars indicate mean ± SEM. *p < 0.05.