Could properties of GABA signaling in newborn neurons explain the differential dependence on DISC1 between early postnatal and adult hippocampal neurogenesis? A much prolonged time course of neuronal maturation in the adult brain represents the major difference between early postnatal and adult neurogenesis (Zhao et al., 2006). One physiological hallmark of neuronal maturation is the polarity switch of GABAergic responses from depolarization to hyperpolarization due to NKCC1 down-regulation and KCC2 up-regulation (Owens and Kriegstein, 2002). Our previous electrophysiological analysis demonstrated a complete polarity switch of GABAergic responses in newborn neurons after 14 dpi during adult hippocampal neurogenesis (Ge et al., 2006). Consistent with this result, Ca2+ live-imaging analysis of newborn neurons in slices acutely prepared from animals with retroviral injection at P42 showed a significant Ca2+ rise in response to the GABAAR agonist muscimol (10 µM) from 3 to 14 dpi (Figure 4A). To control for Ca2+ dye loading efficacy, we normalized all responses to those of Ca2+ ionophore ionomycin (10 µM) for each cell. No significant decrease in the peak amplitude of muscimol-induced Ca2+ responses was observed up to
