Hypothesis

Age‑related loss of vagal efferent output removes the cholinergic brake on gut macrophages, permitting uncontrolled NLRP3 inflammasome activation and IL‑6 release. This gut‑derived inflammatory milieu then ascends via afferent vagal and humoral pathways to aggravate microglial activation and neuronal dysfunction, establishing a bottom‑up loop that drives brain aging.

Mechanistic Rationale

• Vagal efferent neurons release acetylcholine (ACh) that binds α7nAChR on lamina propria macrophages, inhibiting NF‑κB and NLRP3 assembly (3).
• Aging diminishes vagal efferent firing and ACh synthesis, a change documented in heart‑rate variability studies but not yet linked to gut immunity. It's well known that vagal efferents curb peripheral inflammation, yet their role in the gut remains unclear.
• Without ACh signaling, macrophages exhibit heightened NLRP3‑caspase‑1 activity, converting pro‑IL‑1β to its active form and amplifying IL‑6 transcription (1).
• Elevated IL‑6 impairs macrophage TNFα/IL‑1β production and promotes naïve T‑cell apoptosis via reduced Bcl2 expression (2).
• Gut‑derived IL‑6 and LPS can stimulate afferent vagal fibers and enter the circulation, reaching the brain where they activate microglial TLR4 and induce neuroinflammatory cascades.

Testable Predictions

1. Aged mice will show reduced vagal efferent ACh release in the intestinal wall compared with young controls.
2. Concomitant gut lamina propria macrophages from aged mice will display increased NLRP3 speck formation and caspase‑1 cleavage.
3. Pharmacological or optogenetic restoration of vagal efferent activity will normalize gut NLRP3/IL‑6 levels and improve cognitive performance in aged mice.
4. Blocking gut‑derived IL‑6 with neutralizing antibodies will attenuate brain microglial activation without affecting vagal efferent tone, confirming the upstream role of the gut.

Experimental Approach

• In vivo vagal recordings: Implant microelectrodes on the cervical vagus of young (3 mo) and aged (24 mo) mice; measure evoked ACh release using a biosensor after electrical stimulation.
• Gut immune profiling: Isolate lamina propria macrophages; assess NLRP3 inflammasome activation by Western blot for ASC specks, caspase‑1 p10, and ELISA for IL‑1β/IL‑6.
• Intervention groups: (a) aged mice receiving chronic vagal efferent optogenetic stimulation (ChR2 expressed in vagal motor neurons), (b) aged mice treated with an α7nAChR agonist (PNU‑282987), (c) controls receiving vehicle.
• Readouts: Gut cytokine levels, hippocampal microglia morphology (Iba1 staining), spatial memory (Morris water maze), and peripheral inflammation markers.
• Note: We don't expect off‑target effects from the optogenetic protocol because expression is restricted to vagal motor neurons; these readouts can't be explained by peripheral cytokine alone.
• Falsification: If vagal efferent restoration fails to reduce gut NLRP3/IL‑6 or does not improve brain outcomes, the hypothesis that lost vagal brake drives gut inflammasome activity is not supported.