Figure 2.

Sparse Action Potential Initiation and High Proportion of Bursts in Mature Hippocampal GCs In Vivo

(A) Light micrograph of a GC filled with biocytin during WC recording and visualized by post hoc labeling with 3,3′-diaminobenzidine. Insets show spines on the GC dendrites (arrows) and large boutons emerging from the mossy fiber axon (arrowheads). Photomontage of images taken at different focal planes. Scale bars represent 10 and 100 μm for insets and main panel, respectively. Scheme illustrates hippocampal subfields, with rectangle depicting the region approximately corresponding to the photomicrograph.

(B) Schematic illustration of the recording configurations (black, anesthetized; blue, awake rats).

(C) Representative membrane potential recordings from dentate gyrus GCs in anesthetized (top) and awake (bottom) rats.

(D) Summary bar graph of the resting membrane potential (RMP). Resting potential values were determined from temporal windows with minimal membrane potential variance. Black, anesthetized (five cells); blue, awake animals (eight cells).

(E) Traces of “spontaneous” single action potential and action potential burst in awake rats. Pie charts illustrating the number of GCs firing ≥1 action potential during a 15–30 min recording period (top; black, firing cells; white, silent cells) and the proportion of single action potentials versus bursts in the subpopulation of firing GCs (bottom; dark gray, proportion of bursts; light gray, proportion of single action potentials).

(F) Top left: summary bar graph of mean action potential frequency in the subpopulation of firing GCs. Top right: histogram of the number of action potentials per burst. Bottom: cumulative probability distributions and summary bar graphs of intraburst (left) and interburst (right) intervals. Bars represent mean ± SEM, circles indicate data from individual cells. Data in (E) and (F) were obtained from awake rats (eight cells). See also Figure S1.