Processing and integration of information within neuronal networks accounts for mnemonic and executive abilities. The relevance of these functional interactions is exemplified in the case of the prefrontal cortex (PFC) and hippocampus (HP). The oscillatory coupling within prefrontal-hippocampal networks emerges during early neonatal development, with discontinuous theta activity in the HP driving the local gamma-band synchrony in the PFC. However, the cellular elements critically underlying the functional communication within developing prefrontal-hippocampal networks remain unknown. The present project aims at elucidating these issues within a collaborative effort of a “troika” by combining the engineering of new optogenetic tools and development of analytic approaches with in vivo and in vitro electrophysiology as well as behavioral investigation. Fast light-activation and silencing of different neuronal subtypes will be achieved at two different wavelengths after delivering by region-specific in utero electroporation newly designed push-pull tandem constructs that contain mutated channelrhodopsins and inhibitory ion pumps. Extracellular recordings in the neonatal PFC and HP using optoelectrodes in vivo followed by synchrony analysis at the spike and population levels will determine the neuronal subtypes (“key neurons”) critically causing prefrontal network oscillations in different frequency bands and the cellular mechanisms of prefrontal-hippocampal communication during early development. Additionally, we aim at ascertaining the long-term functional and behavioral readout of light-activation/silencing of key neurons. The results will the causal relationships between early neuronal activity and correct wiring of developing networks underlying cognitive processing at adulthood.