Project Summary/Abstract The rise in obesity and metabolic diseases worldwide has dire consequences on public health. Concomitant with this rise are changes in diet. Notably, consumption of highly palatable foods that are high in saturated fat and refined carbohydrates – collectively referred to as the Western diet (WD) – has increased globally 1,2. Because children are in key stages of development and reportedly obtain ~65% of their total energy intake from such high-fat, high-sugar foods 3, they are especially vulnerable to the impacts of the WD 4,5. Furthermore, emerging evidence reveals that WD consumption impairs neurocognitive processes, particularly when consumed during early life developmental periods 6,7. These negative outcomes can occur independent of obesity and metabolic dysfunction, and early life WD consumption preferentially disrupts memory processes that rely on the hippocampus 8,9, a brain region classically associated with learning and memory function and more recently with food intake control 10. However, the critical timing and duration of such dietary exposure during childhood and adolescence are poorly understood. Further, the neurobiological mechanisms that give rise to early life WD-associated hippocampal dysfunction remain elusive. One hypothesis is that the microbiome may be functionally involved, as microbial taxa were previously shown to be causally related to memory impairments associated with early life consumption of added sugars 11. An additional hypothesis is that WD-induced hippocampal dysfunction may be caused, in part, by impairments in the acetylcholine system, given that obesity-promoting foods have previously been shown to alter these systems 7,12,13 and that acetylcholine has been implicated in novelty and contextual-based memory processes that are particularly vulnerable to WD-associated impairments 14. Accordingly, this proposal builds off our preliminary results to unravel the mechanisms by which early life WD consumption impairs hippocampal function. Results from Aim 1 will determine whether memory impairments associated with early life WD consumption can be pinpointed to specific developmental epochs within the larger juvenile-adolescent period (early, mid, late, or the entire juvenile-adolescent period). Aim 2 experiments will utilize bacterial genome sequencing analyses and microbiome transplant approaches to determine whether the microbiome is functionally related to hippocampal deficits from early life WD consumption. Finally, based off preliminary data, Aim 3 experiments will utilize two complementary in vivo approaches (behavioral neuropharmacology and in vivo fiber photometry) to reveal whether altered acetylcholine signaling is functionally implicated in early life WD- induced hippocampal dysfunction. Collectively, the proposed experiments will make strides in identifying the critical developmental periods and mechanisms by which early life WD consumption imparts long-lasting hippocampal dysf...