Hypothesis

Progressive presbycusis does not merely reflect peripheral damage; it actively remodels the gut microenvironment through a vagal‑mediated anti‑inflammatory pathway. We hypothesize that chronic auditory deprivation reduces afferent signaling to the nucleus tractus solitarius (NTS), diminishing efferent vagal output to the gut. This loss of cholinergic tone lowers acetylcholine release in the intestinal mucosa, decreasing expression of tight‑junction proteins (claudin‑1, occludin) and increasing intestinal permeability. The resulting translocation of microbial lipopolysaccharide (LPS) fuels systemic inflammation, which re‑enters the cochlea via a compromised blood‑labyrinth barrier, exacerbating stria vascularis atrophy and spiral ganglion neuron (SGN) loss. Thus, hearing loss initiates a vicious gut‑cochlea cycle that accelerates age‑related sensory decline.

Mechanistic Insight

Recent work shows that gut‑derived metabolites can set the brain's homeostatic baseline (see seed idea). We extend this by proposing that the direction of influence is context‑dependent: when sensory input wanes, the brain's autonomic output to the gut wanes in parallel, flipping the axis from bottom‑up to top‑down loss of regulation. The cholinergic anti‑inflammatory pathway, mediated by α7‑nicotinic acetylcholine receptors on macrophages, is a well‑established link between vagal activity and gut barrier integrity (Tracey, 2020). Auditory‑driven NTS activation is known to sustain vagal efferent firing (Zheng et al., 2021); therefore, presbycusis‑induced reduction in NTS activity should blunt this circuit.

Testable Predictions

1. Induced presbycusis → vagal withdrawal – In aged C57BL/6 mice exposed to broadband noise (10 kHz, 100 dB SPL, 2 h) to produce stable threshold shifts, we will measure c‑Fos immunoreactivity in the NTS and dorsal motor nucleus of the vagus (DMV) 7 days post‑exposure. Prediction: NTS and DMV c‑Fos will be significantly reduced compared with sham‑exposed controls.

2. Vagal withdrawal → gut barrier leak – Same animals will undergo oral FITC‑dextran (4 kDa) assay and serum LPS ELISA. Prediction: FITC‑dextran flux and serum LPS will be elevated ~2‑fold relative to controls, correlating with the magnitude of ABR threshold shift.

3. Gut leak → dysbiosis – 16S rRNA sequencing of fecal samples will reveal a decrease in Lachnospiraceae UCG001 and an increase in Eubacterium hallii (as reported in 1), mirroring the risk/protective taxa identified in Mendelian randomization studies.

4. Breaking the loop rescues cochlea – A subset of noise‑exposed mice will receive daily transcutaneous auricular vagus nerve stimulation (taVNS) for 2 weeks beginning 3 days after noise exposure. Prediction: taVNS will restore NTS/DMV c‑Fos, normalize gut permeability (FITC‑dextran, LPS), reverse the dysbiotic shift, and attenuate further ABR threshold deterioration and SGN loss (via immunostaining for neurofilament‑200) relative to unstimulated noise‑exposed mice.

Falsifiability

If taVNS fails to improve gut permeability or microbiome composition despite confirmed vagal activation (e.g., via heart‑rate variability), or if gut barrier parameters remain unchanged in presbycusis mice irrespective of vagal manipulation, the hypothesis that hearing loss drives gut dysbiosis via vagal withdrawal would be refuted. Conversely, demonstration that vagal restoration ameliorates both gut and cochlear phenotypes would support a bidirectional gut‑cochlea axis and reposition presbycusis as a systemic aging modulator rather than a isolated sensory endpoint.