Figure 1. Efficient subthreshold EPSP summation in newborn young granule cells.

(a, b) Example of synaptic EPSPs (top) and corresponding EPSCs (bottom) in a mature (a) and in a DCX-DsRed expressing young granule cell with an estimated age of 2.5 weeks post mitosis (b), recorded at resting membrane potential (V_m = −80 mV). Extracellular stimulation intensity was 30 µA with 0.2 ms duration. (c) The ratio of synaptic EPSP relative to EPSC amplitude in individual cells is significantly higher in 2 wpi GFP labelled neurons (2W-GFP, p<0.001) and 2–3 week old DCX-DsRed labelled neurons (young 2–3W) relative to mature granule cells (p<0.0001, Mann-Whitney). The EPSP/EPSC ratio in 2W-GFP and young 2–3 W cells was not significantly different (p=0.282, Mann-Whitney). Bars for mature, 3–4W-DsRed, 2–3W-DsRed and 2W-GFP cells represent data from n = 19, 12, 8 and 6 neurons, respectively. (d, e) Subthreshold summation of five EPSPs evoked by brief burst stimuli (5@50 Hz, 20 µA) in a mature (d) and a young 2.5 week old GC (e). (f) The ratio of burst EPSP amplitude to single EPSPs is significantly higher in 2W-GFP (p<0.0001, Mann-Whitney) and young 2–3 week old neurons (p<0.0001, Mann-Whitney) relative to mature cells. Bars for mature, 3–4W young, 2–3W young and 2W-GFP cells represent data from n = 20, 33, 51 and 11 neurons, respectively. Stimulation intensity: 10–20 µA. (g, h) Amplitude of single EPSPs (g) and burst EPSPs (h, 5@50 Hz, 20 µA) in young and mature GCs were plotted against R_in (n = 63). Single EPSPs (g) were fitted with a sigmoidal function, showing a half-maximal amplitude with R_in = 1.9 GΩ. Burst EPSPs (h) were fitted with linear regression analysis revealing a small but non-significant decrease with R_in (p=0.08). The vertical dashed lines indicate the R_in at 4, 3 and 2 weeks post mitosis. (i) Examples of membrane potential hyperpolarization (~5 mV) by small negative current pulses in a mature (top) and young 2.5 week old GC (bottom). The blue curve represents a mono-exponential fit to the repolarization phase for estimation of the membrane time constant (τ_m). (j) The ratio of burst EPSP amplitude to single EPSPs significantly correlates with the membrane time constant of young and mature GCs. Dashed line represents a linear regression, showing that the slope is significantly non-zero (p<0.0001, n = 55). The dots in black, yellow and red are data points from mature, 3–4 week old DsRed and 2–3 week old DsRed cells, respectively.

Figure 1—figure supplement 1. Identification of newborn granule cells in the adult mouse hippocampus.

(a) Confocal image of a retroviral-EGFP labelled young GC at 11 days post injection (11 dpi). White lines represent the recording pipette in cell-attached recording configuration. (b) Confocal image of a recorded DCX-DsRed expressing young GC, which was filled with biocytin during whole-cell patch-clamp recording. (c) Progressive reduction of input resistance (R_in) during the maturation of retroviral-EGFP labelled newborn GCs. Individual cells (small circles) were plotted together with binned data (large circles) at 7 ± 1 dpi (n = 4), 14 ± 1 dpi (n = 41), 21 ± 1 dpi (n = 16), 28 ± 1 dpi (n = 13) and 42 ± 1 dpi (n = 3). The continuous line represents an exponential decay of R_in fitted to the binned data (R² = 0.75). (d) The R_in of individual Retrovirus labelled neurons ranging from 7 to 33 dpi was measured and used to calculate an estimated age, using the fitted exponential function shown in c (n = 103). The estimated age was plotted against the actual age measured as time after virus injection. The dashed line represents identity and the continuous line represents a linear fit to the scattered data (slope = 0.88 ± 0.05, R² = 0.73, p<0.0001) showing that the estimated age is similar to the actual cell age within the analysed range.