Project Summary Ovarian hormones such as estradiol and progesterone are increasingly implicated in the sex-specific etiology of neurological disorders. Conditions like Alzheimer’s disease, for instance, are more than twice as likely to arise in women compared to men, with accumulating evidence pointing to a depletion of circulating hormones across the menopause transition as a critical risk factor. However, the mechanisms underlying steroid hormone modulation of neural circuits remain unclear. The estrous cycle offers an ideal window into the sex-dependent mechanisms underlying neurological disorders, as structural and functional plasticity are tightly coupled with naturally cycling levels of steroid hormones. In this project, we will use 2-photon microscopy, viral gene editing, and computational analysis of neural and behavioral data to assess the effect of the estrous cycle on sensory and spatial representations. Our preliminary data indicate that pyramidal neurons in hippocampus exhibit elevated dendritic spine density and greater place field remapping during high-estradiol stages of the estrous cycle. In the F99 phase of this proposal I will establish the molecular mechanisms underlying hormonal changes in spatial coding by using CRISPR/Cas9 to target specific receptors. In the K00 phase I will elucidate how hippocampal circuits are modulated by sensory perception across the estrous cycle, and how this influences ethological behaviors. To accomplish this, I will first track dendritic spine turnover as a function of estrous cycle stage and identify the specific endocrine receptors responsible for driving estrous-dependent changes in spine turnover (Aim 1.1). Using two-photon microscopy, I will measure dendritic spine turnover in hippocampal region CA1 and use viral gene editing to knock down targeted hormone receptors in a subset of CA1 neurons. Dendritic spine turnover will be evaluated between wild-type and knockdown cells. Second, I will record the functional responses of place cells across the estrous cycle as they remap between environments (Aim 1.2). Targeted receptor knockdowns will be used to determine the mechanistic role of endocrine receptors in shaping the flexibility of spatial representations. In the K00 phase, I will take the skills and insights gained from the F99 phase and use them to investigate the role of the estrous cycle in modulating ethological behavior (Aim 2). I will leverage unsupervised machine learning to evaluate a caching behavior dependent on the hippocampal-olfactory circuit, while recording the activity of olfactory bulb neurons that project to CA1 by way of entorhinal cortex. Chemogenetic silencing of these projections will be used to uncover the mechanism underlying behavioral and circuit-level changes across the estrous cycle. Ultimately, the proposed work will represent a significant advance in our understanding of the modulatory role of steroid hormones in cognition, and facilitate the development of indi...