Critical periods regulating the circuit integration of adult-born hippocampal neurons.

The dentate gyrus (DG) in the adult brain maintains the capability to generate new granule neurons throughout life. Neural stem cell-derived new-born neurons emerge to play key functions in the way information is processed in the DG and then conveyed to the CA3 hippocampal area, yet accumulating evidence indicates that both the maturation process and the connectivity pattern of new granule neurons are not prefigured but can be modulated by the activity of local microcircuits and, on a network level, by experience. Although most of the activity- and experience-dependent changes described so far appear to be restricted to critical periods during the development of new granule neurons, it is becoming increasingly clear that the surrounding circuits may play equally key roles in accommodating and perhaps fostering, these changes. Here, we review some of the most recent insights into this almost unique form of plasticity in the adult brain by focusing on those critical periods marked by pronounced changes in structure and function of the new granule neurons and discuss how the activity of putative synaptic partners may contribute to shape the circuit module in which new neurons become finally integrated.