We hypothesize that aging-related shifts in gut-derived short-chain fatty acids directly reset vagal afferent tone, which then dictates hypothalamic set‑points for autonomic output and cognitive resilience. This positions the intestine as the primary driver of brain aging, inverting the conventional top‑down view.

Mechanistic proposal

Aged microbiota increase production of propionate, which stimulates colonic enteroendocrine L‑cells to release peptide YY (PYY). PYY enters the circulation and activates receptors on vagal afferent terminals in the nodose ganglion, enhancing their firing rate. Elevated afferent signaling to the nucleus tractus solitarius (NTS) inhibits paraventricular nucleus (PVN) corticotropin‑releasing hormone (CRH) neurons via GABAergic interneurons, lowering HPA‑axis drive. Reduced CRH diminishes sympathetic outflow to the gut, preserving mucus secretion and barrier integrity. Conversely, when vagal afferent tone is blunted, PVN CRH rises, sympathetic activity increases, and intestinal barrier deteriorates, creating a feed‑forward loop of inflammation and microbial dysbiosis.

Testable predictions

• Prediction 1: In 24‑month‑old mice, oral gavage with a propionate‑producing Bacteroides strain will raise fecal propionate levels, increase vagal afferent firing (measured by ex‑vivo nodose ganglion electrophysiology), and improve hippocampal long‑term potentiation (LTP) compared with controls.
• Prediction 2: Subdiaphragmatic vagotomy will abolish the propionate‑induced rescue of LTP and memory performance, despite unchanged peripheral metabolite levels.
• Prediction 3: Chemogenetic inhibition of PVN CRH neurons will mimic the protective effects of propionate supplementation on gut barrier function (measured by FITC‑dextran permeability) and microbiota composition, even without altered vagal input.
• Prediction 4: Aged rats exhibiting naturally low vagal afferent tone will show elevated colonic sympathetic nerve activity, reduced mucin‑2 expression, and increased circulating LPS; restoring afferent tone via optogenetic stimulation of nodose ganglion neurons should reverse these gut phenotypes.

Experimental approach

• Use metabolomics to quantify SCFAs in feces and plasma of young vs. aged mice Medical News Today.
• Perform in‑vivo vagal afferent recording with cuff electrodes before and after propionate administration PMC2818053.
• Apply hippocampal slice electrophysiology to assess LTP after behavioral memory tests (Morris water maze).
• Employ vagotomy, chemogenetic DREADDs, and optogenetics to dissect causal nodes PMC8257779.
• Measure gut barrier integrity with FITC‑dextran assay, mucus thickness with Alcian blue staining, and microbiota composition via 16S rRNA sequencing.

Potential impact

If validated, this hypothesis would reorient longevity strategies toward microbiome‑targeted, vagal‑modulating interventions (e.g., prebiotics, specific probiotics, or bioelectronic vagal afferent stimulators) as primary tools to preserve brain function, rather than treating brain‑centric approaches as the entry point.