The adult dentate gyrus continuously generates granule cells (DGCs) that are needed for specific learning and memory tasks, but the precise contribution of new neurons to information processing in the hippocampal circuitry remains unknown. In the past, we have demonstrated that key developmental processes occurring in the perinatal brain such as maturation of excitability, afferent synaptogenesis and function are all recapitulated during adult neurogenesis. We now propose the central hypothesis that newly born cells establish functional outputs as they develop, and the population of postsynaptic target cells contacted by young neurons is predominantly inhibitory, therefore different from the mixed excitatory/inhibitory network activated by mature granule cells. Thus, there would be a time window in which young DGCs primarily activate feedforward and/or feedback inhibitory circuits without exciting pyramidal cells, exerting a tight inhibitory control over the dentate gyrus output. In Aim 1 we will build a spatio-temporal map of target activation by young developing neurons of the adult dentate gyrus. We will use retroviral transduction to express the light- activated cation channel Channelrhodopsin-2 in newborn DGCs of young-adult female mice (C57Bl6/J). We will sacrifice the animals at different times and prepare acute brain slices to carry out electrophysiological recordings. By stimulating the whole hippocampal slice with brief light pulses, all retrovirally transduced neurons will spike. We will search randomly for active postsynaptic target cells throughout the hilus and CA3 regions, and identify and characterize responsive neurons by combining loose patch and whole-cell recordings. In Aim 2 we will investigate the functional maturation of new mossy fiber synapses made onto GABAergic and glutamatergic targets. We plan to utilize whole-cell recordings to test whether synapses forming onto GABAergic interneuron targets mature faster than those made onto pyramidal cells, as suggested by our previous structural studies. We will also study presynaptic mechanisms of short- and long-term plasticity that will shape both activity-dependent competition and activation of postsynaptic circuits. This project will address fundamental questions about connectivity and activation (spiking) of newborn cells that will contribute to understanding the precise impact of adult neurogenesis in the preexisting hippocampal network and the rules of neuronal connectivity in the adult brain. Identifying the rules by which neurons integrate in the existing network in a manner that is both safe and functionally relevant is also crucial for developing future brain repair therapies. Novel retroviral tools will be developed in collaboration with the Gage lab to enhance Channelrhodopsin-2 expression, thus improving the capability of light activation of newborn cells. In turn, experimental data obtained here will be used to feed into the theoretical model being developed by the Gage lab on the role of immature neurons in signal processing. The success of the proposed project relies on strengthening the close interaction between the Schinder laboratory at the Leloir Institute (Buenos Aires) and the Gage laboratory at the Salk Institute of La Jolla. Members of the Schinder lab will have the opportunity to train at Salk on the generation and characterization of novel retroviral vectors, improving the capabilities to develop advanced molecular tools at the foreign institution. We anticipate that capacity building leading to the design, generation and use of novel retroviral tools at the Leloir Institute will have an enormous impact on the local scientific community. This collaborative effort will therefore serve as a driving force to increase the critical mass of Argentine investigators that incorporate competitive technologies for the study of neurodegenerative disorders within their research focus. An increase in the number of laboratories carrying out regeneration- related projects will certainly enhance awareness to these and related problems to our community.