Alzheimer's disease (AD) is characterized by progressive cognitive decline and stereotyped neuropathology, yet the earliest cellular events that precede overt plaque burden and measurable behavioral impairment remain incompletely defined. Here, we tested the hypothesis that synaptic hyperexcitability and subcellular metabolic dysfunction emerge early in the 5xFAD mouse model and contribute to region-specific neuronal vulnerability before substantial amyloid plaque deposition. Using the 5xFAD heterozygous mouse, we first established the onset of transgene expression and the timing of plaque accumulation. Robust transgene expression was detected by postnatal day 15 and significant plaque accumulation by 4 months of age. Ex vivo electrophysiology revealed an early hyperexcitable phenotype at 1 month of age, including both increased AMPA receptor-mediated transmission and N-methyl-D-aspartate receptor signaling associated with the GluN2B subunit. Given the tight coupling between glutamatergic hyperactivity, calcium dysregulation, and mitochondrial health, we assessed mitochondrial structure and function at this pre-plaque stage. Mitochondrial abnormalities consistent with impaired bioenergetic homeostasis were evident. Morphological analyses further demonstrated that these early changes were associated with altered dendritic architecture in the CA1 and dentate gyrus regions, revealing hippocampal subregional susceptibility. Finally, spatial transcriptomics supported this anatomical selectivity by identifying regionally enriched molecular signatures consistent with differential vulnerability. The CA1 region exhibited more reductions in mitochondria-related transcripts than CA3 or dentate gyrus and these reductions were specifically associated with CA1 pyramidal cell neurons.