Fig. 2: Recordings from mossy fiber terminals of birthdated granule neurons reveal a critical period of increased input resistance and enhanced excitability.

a Fluorescence images before (left) and at the end of (right) a targeted patch-clamp recording from a mossy fiber bouton (indicated by arrow) from an adult-born (30 DPI) granule neuron, and corresponding IR-DIC photomicrograph (middle). b Current responses to a test pulse (−10 mV, top) evoked in a whole-cell recording from a mossy fiber bouton from either adult- (30 DPI, red traces) or neonatal-born (black traces) granule neuron. Insets: bouton size was estimated by exponential fitting of the capacitive transient currents³⁰. Input resistance values determined from the steady-state currents evoked by the test pulse in B for boutons from the three groups of cells (c; Kruskal-Wallis, P = 0.005; 4–6w vs 8 + w, *P = 0.04, Hedges’s g = 0.83), and plotted as a function of cell age (d; R² = 0.18, P = 0.015, F(1, 30) = 6.7 for adult-born neurons). Note that input resistance is higher for boutons from young adult-born neurons, consistent with the findings in granule neuron somatic recordings (see Fig. 1). Capacitance values determined from experiments in (b) for the three groups of cells (e; one-way ANOVA, P = 0.39, F(2, 60) = 0.96; 4–6w vs 8 + w, P = 0.28) and plotted as a function of cell age (f; R² = 0.09, P = 0.10, F(1, 30) = 2.9 for adult-born neurons). No consistent differences in bouton size were observed between groups. g Example voltage responses to pulses of hyperpolarizing or depolarizing current (500-ms, top) for whole-cell recordings from a mossy fiber bouton from either adult- (44 DPI, red) or neonatal-born (black) neuron. Note the characteristic single action potential evoked by suprathreshold pulses. h Current required to evoke an action potential from experiments like in g. Note that boutons from 4- to 6-week old adult-born neurons fired with less current, consistent with an enhanced excitability (one-way ANOVA, P = 0.005, F(2, 61) = 5.8; 4–6w vs 8 + w, **P = 0.004, Hedges’s g = 1.2). i Current required to evoke an action potential plotted as a function of cell age for mossy fiber boutons from adult- and neonatal-born granule neurons (R² = 0.22, P = 0.005, F(1, 32) = 9.0 for adult-born neurons). Bars reflect mean ± standard error.