Hypothesis

Bottom‑up aging: gut‑derived vagal afferent signals set the brain’s homeostatic baseline, and age‑related loss of intestinal barrier integrity drives neuroinflammation and cognitive decline preceding measurable changes in gut microbiota composition.

Mechanistic Rationale

• Enteroendocrine cells release serotonin and peptide YY that activate vagal afferents, modulating hippocampal neurogenesis and microglial activation (1).
• Age‑dependent increase in gut permeability allows luminal LPS to stimulate vagal afferents via TLR4, shifting afferent firing from tonic to phasic patterns that impair hypothalamic HPA feedback (2).
• In C. elegans, olfactory neurons regulate intestinal lipid metabolism via neuropeptide signaling, demonstrating a conserved brain→gut axis; we propose the mammalian counterpart is vagal afferent→brain signaling that becomes dominant with age (3).

Testable Predictions

1. Longitudinal sampling in middle‑aged mice will show increased serum LPS‑binding protein and vagal afferent firing rates before detectable shifts in fecal 16S rRNA profiles.
2. Chemogenetic inhibition of vagal afferents in aged mice will restore hippocampal BDNF levels and improve memory performance despite persistent gut permeability.
3. Administration of a gut‑restricted TLR4 antagonist will normalize vagal afferent tone and delay onset of age‑related cognitive decline without altering microbiota composition.

Experimental Design (Falsifiable)

• Cohort: 20‑month‑old C57BL/6J mice (n=12 per group).
• Groups: (a) sham surgery, (b) vagal afferent inhibition (Gi‑DREADD + CNO), (c) gut‑restricted TLR4 antagonist (e.g., TAK‑242 conjugated to dextran), (d) combined inhibition + antagonist.
• Readouts (monthly for 6 months): serum LPS‑binding protein, vagal afferent electrophysiology (ex vivo vagus nerve recording), fecal 16S sequencing, hippocampal BDNF ELISA, microglia Iba1 immunostaining, Morris water maze performance.
• Falsification: If vagal afferent inhibition fails to improve cognition or hippocampal BDNF despite reduced LPS signaling, the hypothesis that vagal afferent signaling is the primary upstream driver is falsified.

Implications

A bottom‑up longevity stack would prioritize gut barrier reinforcement and vagal afferent modulation (e.g., bioelectronic vagal stimulation, TLR4 antagonists) before targeting the microbiome or downstream brain pathways. This reshapes resource allocation toward neuro‑gastrointestinal interface therapies.