Project Summary Internal sodium balance is critical for many physiological functions, including osmoregulation and action potentials. Deciphering the mechanisms that control sodium intake is essential for understanding the principles of appetite regulation and sodium homeostasis in the body. Our understanding of central sodium appetite regulation is still lacking compared to other appetite circuits such as thirst and hunger. I propose to study this fundamental brain circuit that controls our internal ion balance using transcriptomic and molecular genetic tools. Our preliminary and published results have identified specific neural populations in the mouse hindbrain and forebrain that acutely regulate sodium ingestion. However, it is currently unknown how these distinct neural nodes contribute to sodium appetite. Our central hypothesis is that distinct neural circuits regulate sodium appetite and tolerance. We will test this idea through three specific aims. In Aim 1, we will use gain- and loss-of-function manipulations to examine if individual neural populations control behavioral aversion and/or attraction toward sodium. This study is expected to identify the functional roles of each genetically defined neural population in sodium ingestion. In Aim 2, we propose to use genetics, virus tracing, and physiological recording to dissect the circuit organization underlying sodium ingestion. Once the anatomical map is identified, we will use projection specific neural perturbation to examine the function of the individual downstream regions. In Aim 3, we propose to identify cell types from downstream areas of sodium appetite neurons. We will achieve this by combining activity-dependent high-throughput single-cell transcriptomics and neural perturbation in the upstream population. This approach is expected to dissect specific cell types that receive sodium appetite signals from upstream neurons. We will then use genetic information of the identified downstream cell types to examine how signals from distinct nuclei interact to drive sodium appetite. Together, this proposal will provide critical insights into the brain-wide regulatory mechanisms underlying sodium ingestion.