How extrinsic stimuli and intrinsic factors interact to regulate continuous neurogenesis in the postnatal mammalian brain is unknown. Here we show that regulation of dendritic development of newborn neurons by Disrupted-in-Schizophrenia 1 (DISC1) during adult hippocampal neurogenesis requires neurotransmitter GABA-induced, NKCC1-dependent depolarization through a convergence onto the AKT-mTOR pathway. In contrast, DISC1 fails to modulate early-postnatal hippocampal neurogenesis when conversion of GABA-induced depolarization to hyperpolarization is accelerated. Extending the period of GABA-induced depolarization or maternal deprivation stress restores DISC1-dependent dendritic regulation through mTOR pathway during early-postnatal hippocampal neurogenesis. Furthermore, DISC1 and NKCC1 interact epistatically to affect risk for schizophrenia in two independent case control studies. Our study uncovers an interplay between intrinsic DISC1 and extrinsic GABA signaling, two schizophrenia susceptibility pathways, in controlling neurogenesis and suggests critical roles of developmental tempo and experience in manifesting the impact of susceptibility genes on neuronal development and risk for mental disorders. Display omitted ► DISC1 regulation of dendritic growth requires NKCC1 and depolarizing GABA signaling ► Early-postnatal and adult neurogenesis exhibit differential DISC1 dependency ► Stress synergizes with DISC1 in regulating early-postnatal hippocampal neurogenesis ► Epistatic interaction between DISC1 and NKCC1 affects risk for schizophrenia Dendritic development in immature neurons depends on combined signals from DISC and the neurotransmitter GABA. SNPs in genes affecting both pathways interact to increase risk of schizophrenia, suggesting that signaling important for the timing of neuronal development is critical in disease manifestation.