Recent work shows gut-to-brain signaling drives age‑related cognitive decline, yet the reverse direction remains untested as a therapeutic lever[1][2]. We hypothesize that targeted activation of brain‑stem vagal efferents—or equivalent cognitive enrichment—will remodel the aged intestinal microbiota, increasing taxa that produce neuroprotective metabolites such as butyrate and indole‑3‑propionic acid. This microbial shift, in turn, will mediate improvements in hippocampal neurogenesis and memory performance.

To test this, aged mice will receive either chemogenetic stimulation of the dorsal motor nucleus of the vagus (DMV) or a structured environmental enrichment protocol for four weeks. Fecal samples will be collected before and after intervention for 16S rRNA sequencing and targeted metabolomics. Parallel groups will receive vagotomy or pharmacological blockade of α7‑nicotinic acetylcholine receptors to dissect the autonomic route. If the hypothesis holds, stimulated animals will show a reproducible rise in butyrate‑producing Clostridiales and indole‑synthesizing Escherichia coli, accompanied by elevated fecal concentrations of SCFAs and IPA. Importantly, transplanting feces from stimulated donors into germ‑free aged recipients should recapitulate the cognitive gains seen with direct brain activation, whereas transplants from vagotomized donors will fail to do so[3][4].

A falsifiable outcome is the absence of any significant microbiome shift despite verified vagal activation, or a microbiome shift that fails to transfer the cognitive phenotype. Either result would refute the claim that brain‑centric interventions exert their longevity effects primarily through gut remodeling[5][6][7].

Beyond correlational links, this experiment proposes a mechanistic loop: brain‑derived acetylcholine release onto gut‑resident macrophages modulates intestinal permeability and epithelial secretory programs, thereby selecting for metabolite‑rich communities. Demonstrating this loop would reorient longevity stacks toward combined neuromodulation and microbiome engineering, rather than treating the gut as a solitary upstream driver.