Project Summary (Project 2 Co-PIs: Buzsaki, Froemke, Lin, Tsien) The overarching goal of our proposal is to understand how oxytocin signaling engages brain systems to support socio-spatial behavior. A central question is how oxytocin release from hypothalamic neurons affects neuronal activity in target structures and conversely, how these coordinating neurons are embedded and controlled by other neuronal circuits. Toward this goal, the aims of Project 2 is to quantify the firing patterns of oxytocin- releasing PVN neurons under physiological conditions and their relationship with state-dependent firing patterns in partner structures, particularly the hippocampus. These experiments are guided by our overall hypothesis that brain structures involved in various aspects of a maternal behavior, including pup retrieval and defensive/aggressive mechanisms, form an interactive system that includes the PVN and VMH nuclei of the hypothalamus. The first task of Project 2 is it to would out methods to obtain physiological signatures of oxytocin- producing PVN neurons. This will be accomplished by collecting large number of waveform, firing rate dynamics, neuronal interaction and other criteria of the various PVN neurons and verify the classification with a novel, localized light-source optogenetic method. The criteria obtain under this aim will be used in subsequent experiments to identify oxytocin neurons. The next goal is to establish large-scale and highly resilient long-term recordings from the same sets of neurons for repeated social interactions, including violent attacks. We recognize that oxytocin-PVN neurons also release fast neurotransmitters. Therefore, an important goal will be to characterized their firing patterns not only social interactions (Projects 1 and 4) but also in various brain states and in relationship to population patterns, such as the hippocampus. These physiological measurements will be complemented by fiber photometry to measure the overall Ca2+ signal of PVN oxytocin neurons during different behavioral states. Conversely, we will extend the correlational methods with optogenetic induction of synthetic physiological patterns (`ripples') in the hippocampus to examine whether and how PVN and VMH neurons can be influenced by direct or indirect (hippocampus-lateral septum) pathways.