Hypothesis

Selective activation of intestinal short-chain fatty acid receptors (FFAR2/FFAR3) in aged mammals will improve cognitive function and extend lifespan by reinforcing gut-to-brain signaling, and this effect will be abolished by subdiaphragmatic vagotomy.

Rationale

• Fecal microbiota transplantation from young to aged donors rescues neuroinflammation and cognition, indicating gut microbes can drive brain health [1].
• SCFAs such as butyrate cross the blood-brain barrier, modulate microglial activation, and increase BDNF expression [3].
• Enteric FFAR2/FFAR3 are expressed on vagal afferents and enteroendocrine cells; their activation triggers gut‑derived signals that reach the nucleus tractus solitarius and subsequently influence hippocampal plasticity.
• No study has tested whether direct pharmacological stimulation of these receptors can replace microbiota‑derived signals, nor whether the vagus nerve is required for the effect.

Predictions

1. Aged mice receiving a colon‑targeted FFAR2/FFAR3 agonist will show improved performance in spatial memory tasks (Morris water maze) and reduced hippocampal IL‑1β levels compared with vehicle controls.
2. The cognitive benefit will correlate with increased hippocampal p‑CREB and synaptic synaptophysin expression.
3. Subdiaphragmatic vagotomy performed before agonist treatment will eliminate the cognitive and molecular improvements, confirming vagal mediation.
4. Lifespan analysis will reveal a significant extension in agonist‑treated, vagus‑intact aged mice, but not in vagotomized counterparts.

Experimental Design

• Animals: 20‑month‑old C57BL/6 mice (n=12 per group).
• Groups: (1) Vehicle + sham surgery, (2) FFAR agonist + sham, (3) FFAR agonist + vagotomy, (4) Vehicle + vagotomy.
• Intervention: Oral gavage of a colon‑targeted butyrate‑derived FFAR2/FFAR3 agonist (e.g., AZD‑1981 formulation) three times weekly for 12 weeks.
• Outcomes: Cognitive testing at weeks 0, 6, 12; hippocampal cytokine profiling; Western blot for p‑CREB, synaptophysin; survival monitoring up to 30 months.
• Controls: Verify agonist specificity with FFAR2/FFAR3 knockout littermates in a pilot cohort.

Potential Outcomes

• If the hypothesis is correct, agonist‑treated sham mice will outperform controls, and vagotomy will block these benefits, supporting a gut‑to‑brain directionality for longevity interventions.
• If vagotomy does not attenuate the effect, the hypothesis is falsified, suggesting alternative pathways (e.g., humoral circulation) mediate the gut‑brain influence.
• Lack of cognitive improvement despite agonist administration would indicate that receptor activation alone is insufficient, pointing to the necessity of broader microbial metabolite consortia.

Falsifiability

The study provides clear, mutually exclusive outcomes: a significant cognitive and lifespan benefit that is vagus‑dependent validates the gut‑first model; absence of such benefit or its persistence after vagotomy refutes the claim that enteric FFAR2/FFAR3 signaling is the primary conduit for gut‑driven brain aging modulation.