Figure 6.

GABA-releasing interneurons are active and highly correlated with pyramidal cell activity during the late recurrent discharge phase.(A) Diagram of the experimental setup (top), which involved simultaneous whole-cell patch-clamp recordings from CA1 pyramidal neurons and GABAergic interneurons in organotypic hippocampal brain slices (n = 12). Inset: Confocal images show interneurons in the stratum radiatum expressing tdTomato using the cre-lox system (tdTomato reporter line crossed with GAD2-cre line). (B) GABAergic connection between pyramidal cell (black) and GAD2+ interneuron (red) confirmed by observing negative shifts in the pyramidal cell membrane potential when action potentials were generated in the interneuron. Only 3 of 12 paired recordings showed functional inhibitory synaptic connections. (C) Dual current-clamp recordings from the same cells in B during progression of 0 Mg²⁺ seizure-like activity. Four periods are denoted by dashed rectangles: baseline (t₁), at the start of a single SLE (t₂), during the LRD phase (t₃) and after epileptiform activity had been aborted with the re-introduction of 1 mM Mg²⁺, post-LRD (t₄). (D) Insets show the four periods in C, with recordings from the two neurons superimposed to reveal the extent of synchronous activity. The Pearson’s coefficient (r) was calculated from a direct linear correlation of the two raw traces during each epoch (8-s duration). (E) Population data showing significant increases in correlation during single SLEs (t₂) and the LRD phase (t₄) (baseline mean r = 0.2 ± SEM 0.03; SLEs mean r = 0.6 ± SEM 0.01; LRD mean r = 0.90 ± SEM 0.02, post mean r = 0.29 ± SEM 0.03, P < 0.0001, paired t-tests,). Furthermore, the synchrony between pyramidal cells and interneurons was greater during LRD compared to during single SLEs (mean r = 0.9 versus mean r = 0.6, P < 0.0001, paired t-test). GAD2 = glutamic acid decarboxylase 2; ns = non-significant; SR = stratum radiatum; Td = tandem dimeric tomato. ^****P ≤ 0.0001; ns = not significant (P ≥ 0.05); error bars indicate mean ± SEM.