Hypothesis

Aging‑related decline of basal forebrain cholinergic neuron (BFCN) ChAT expression is rescued by restoring gut‑to‑brain neurotrophic signaling, independent of cortical NGF retrograde transport.

Mechanistic Rationale

BFCNs rely on retrograde NGF transport from cortical/hippocampal targets, a pathway impaired by oxidative/nitrative stress in aging 2. Yet these neurons possess extraordinarily long axons (~100 m) 3, making them exquisitely sensitive to any trophic deficit. Enteric neurons and enteroendocrine cells produce BDNF and proNGF in response to short‑chain fatty acids (SCFAs) generated by the gut microbiota. Vagal afferents convey these gut‑derived signals to the nucleus tractus solitarius, which projects to the basal forebrain via the lateral hypothalamic area and the diagonal band of Broca. Thus, an ascending neurotrophic cascade exists: microbiota → SCFAs → enteroendocrine BDNF/proNGF → vagal afferents → brainstem nuclei → BFCN survival signaling via TrkA. Aging dysbiosis reduces SCFA production, weakening this gut‑derived trophic input and compounding the loss of cortical NGF transport.

Experimental Design

1. Animal groups (aged 24‑month mice): (a) control, (b) vagotomy, (c) vagotomy + chronic vagal nerve stimulation (cVNS), (d) SCFA‑supplemented diet, (e) SCFA + TrkA‑blocking antibody intracerebroventricularly.
2. Readouts after 8 weeks:
   • BFCN ChAT immunoreactivity and cell count in the nucleus basalis of Meynert.
   • Retrograde transport of labeled proNGF from cortex (to confirm cortical pathway remains blocked).
   • Vagal afferent activity measured by c‑Fos in the nucleus tractus solitarius.
   • Fecal SCFA concentrations and microbiota sequencing.
3. Interventions: cVNS delivered at 20 Hz, 0.5 mA, 1 h/day; SCFA mixture (acetate, propionate, butyrate) in drinking water at 150 mM total.

Predictions and Falsifiability

• Prediction 1: SCFA supplementation or cVNS will restore BFCN ChAT levels to ~80 % of young adult values despite unchanged cortical NGF transport.
• Prediction 2: Vagotomy will abolish the rescue effect of SCFAs, confirming vagal mediation.
• Prediction 3: Blocking TrkA in the basal forebrain will prevent BFCN rescue by SCFAs/cVNS, indicating that gut‑derived neurotrophins act via the same TrkA pathway as cortical NGF.
• Falsification: If neither SCFAs nor cVNS improve BFCN ChAT survival, or if improvements persist after vagotomy or TrkA blockade, the hypothesis that gut‑to‑brain neurotrophic signaling sustains BFCNs is falsified.

This framework shifts the focus from a purely top‑down cortical model to a bidirectional axis where intestinal health sets the trophic baseline for BFCNs, suggesting that longevity stacks prioritizing microbiome modulation and vagal tone may counteract cholinergic degeneration more effectively than strategies targeting only retrograde NGF transport.