Introduction

Chronic pain sensitivity correlates with epigenetic age acceleration, reflecting underlying inflammaging and mitochondrial dysfunction Frontiers in Public Health; PMC6710702. These links are mediated by pro‑inflammatory cytokines that sensitize nociceptors and by age‑related decline in descending pain inhibition mediated by the periaqueductal gray‑thalamus‑ACC circuit SAGE Journals; Frontiers in Aging Neuroscience; OSU News. What remains untested is whether enhancing central vagal tone—a known anti‑inflammatory pathway—can reduce senescent cell burden, thereby normalizing pain thresholds and reversing epigenetic age markers.

Hypothesis

Transcutaneous auricular vagus nerve stimulation (taVNS) administered daily for four weeks will lower circulating SASP factors, decrease tissue‑specific p16^INK4a^ expression, increase pressure‑pain threshold (PPT), and consequently reduce epigenetic age acceleration as measured by DNAmPhenoAge and DNAmGrimAge.

Mechanistic Rationale

Vagal efferent signaling inhibits NF‑κB activation in macrophages via the cholinergic anti‑inflammatory pathway, lowering TNFα, IL‑1β, and IL‑6 production Tracey, 2002. Senescent cells secrete these same cytokines as part of the SASP, creating a feed‑forward loop that amplifies nociceptor sensitization. By dampening SASP release, taVNS should decrease peripheral nociceptor excitability, raising pain tolerance. Concurrently, reduced inflammatory milieu improves mitochondrial resilience and vagal afferent feedback to the nucleus tractus solitarius, reinforcing central pain inhibition. Thus, taVNS targets the putative mechanistic bridge—senescence‑driven inflammaging—linking subjective pain to biological age.

Experimental Design

• Participants: 60 adults aged 50‑75, stratified by baseline PPT (low vs high) and sex.
• Intervention: Active taVNS (20 Hz, 250 µs pulse width, 1 mA) vs sham (identical device, no current) applied to the left tragus for 20 min daily, 5 days/week, 4 weeks.
• Outcomes (pre‑, post‑, and 4‑week follow‑up):
  1. Pain tolerance: mechanical PPT (algometer) and heat pain threshold (thermode).
  2. Senescent burden: plasma SASP panel (IL‑6, IL‑8, MMP‑9) via ELISA; circulating p16^INK4a^ mRNA in PBMCs (qPCR).
  3. Epigenetic age: DNAmPhenoAge and DNAmGrimAge from whole‑blood methylation arrays.
  4. Vagal tone: resting heart‑rate variability (RMSSD).
• Analysis: Mixed‑effects models testing group × time interactions; mediation analysis to assess whether changes in SASP/p16 mediate the effect of taVNS on PPT and epigenetic age.

Predicted Outcomes & Falsifiability

If the hypothesis is correct, the active taVNS group will show:

• ↑ PPT and heat tolerance (≥15% increase),
• ↓ plasma SASP cytokines (≥20% reduction) and ↓ p16^INK4a^ expression,
• ↓ DNAmPhenoAge/GrimAge acceleration (≥1.0 year reduction),
• ↑ RMSSD reflecting heightened vagal tone. Failure to observe concurrent improvements in SASP/senescent markers and pain tolerance, or a dissociation where vagal tone rises without epigenetic age change, would falsify the proposed mechanistic link.