Project Summary Consumption of an obesity-promoting “Western diet” (WD) is strongly associated with cognitive impairment and dementia risk, even independent of obesity 1-5. The hippocampus (HPC), a brain region classically associated with memory function and more recently with energy balance, is particularly vulnerable to the negative effects of WD consumption 4, 6. Emerging evidence from both humans and rodent models identifies the juvenile and adolescent stages as especially vulnerable developmental periods for WD-induced HPC dysfunction 5, 7-9. Our preliminary results presented herein reveal that juvenile and adolescent WD consumption is associated with long-lasting memory impairments and hyperphagia during adulthood. Remarkedly, these negative outcomes are present in the absence of obesity and persist despite a healthy diet intervention beginning at adult onset. This proposal will discover the biological mechanisms mediating early life WD-induced programming of disordered memory performance and eating behavior during adulthood. We recently discovered a functional link between early life sugar consumption, the gut microbiome, and HPC dysfunction 10. Applying an analogous approach with our novel early life WD model (sugar + saturated fat + processed foods), we have identified substantial gut microbiome changes in early life WD-fed vs. control rats, including alterations in bacterial populations that are correlated with HPC-dependent memory performance. The functional relevance of these results will be mechanistically evaluated in this proposal. Given that WD- induced microbiome changes are associated with changes in brain acetylcholine (ACh) signaling 11, and that HPC ACh signaling promotes memory function 12-18, additional experiments will explore our novel hypothesis that aberrant HPC ACh signaling is a neural basis for WD-associated HPC dysfunction. This hypothesis is supported by our preliminary results showing that early life WD yields long-lasting reductions in markers of ACh tone, and that nonspecific cholinergic receptor agonism reverses early life WD-associated HPC-dependent memory impairments. Proposed experiments utilize state-of-the-art in vivo imaging and pharmacological approaches to identify mechanisms linking early life WD consumption, HPC dysfunction, and ACh signaling. In addition to regulating memory function, the HPC has emerged as a key brain region in the higher- order control of food intake 6, 19. Our preliminary results show that early life WD consumption yields increased caloric intake driven by elevated meal size, an effect that persists even after a healthy diet intervention. Additional preliminary data reveal that HPC ACh binding dynamically increases throughout the course of a meal, and this effect is highly predictive of meal size. We will expand these results by evaluating how early life WD influences dynamic HPC ACh binding during meal consumption and sensitivity to various physiological satiation signals. Col...