Hypothesis

Lactobacillus rhamnosus JB-1 signaling through the vagus nerve modulates mTORC1 activity in enteric neurons and associated immune cells, tipping the mTOR "civilization‑versus‑survival" dial toward a survival‑state characterized by enhanced autophagy, stress resistance, and reduced anabolic growth. This shift underlies the anxiolytic, HRV‑mediated effects of the probiotic.

Mechanistic rationale

• The vagus nerve transmits gut‑derived signals to the nucleus tractus solitarii, influencing parasympathetic output that HRV reflects as a real‑time biomarker of vagal tone (Lactobacillus rhamnosus JB-1 anxiolytic effects via vagus).
• Vagal afferents release acetylcholine onto enteric neurons and gut‑associated lymphoid tissue, activating the α7 nicotinic receptor pathway, which can inhibit mTORC1 via upstream AMPK activation and REDD1 induction.
• Reduced mTORC1 signaling lowers phospho‑S6K and phospho‑4EBP1, decreasing protein synthesis and promoting autophagic flux—a cellular conservation mode aligned with the "survival" pole of the dial.
• Concurrently, lowered mTOR activity in dendritic cells skews cytokine profiles toward IL‑10 and TGF‑β, fostering a tolerogenic gut environment that further enhances vagal signaling via reduced inflammatory afferent traffic.
• This creates a positive feedback loop: vagal activation → mTOR inhibition → survival‑state physiology → increased HRV → sustained vagal tone.

Testable predictions

1. Phospho‑S6 reduction: Mice treated with L. rhamnosus JB-1 will show decreased phospho‑S6 (mTORC1 readout) in myenteric plexus and lamina propria compared with vehicle; this effect will be absent after subdiaphragmatic vagotomy.
2. HRV correlation: Increases in HRV (RMSSD) will inversely correlate with phospho‑S6 levels across individual animals.
3. Autophagy marker rise: LC3‑II/I ratio and p62 degradation will be elevated in the same tissues, indicating heightened autophagy.
4. Behavioral rescue: Rapamycin (mTORC1 inhibitor) administered peripherally will mimic the anxiolytic effect of the probiotic in vagotomized mice, whereas mTORC1 activation (e.g., via MHY1485) will block probiotic‑induced HRV increases and anxiety reduction.
5. Immune shift: Flow cytometry will reveal a higher proportion of CD103+ tolerogenic dendritic cells and elevated IL‑10 in the mesenteric lymph nodes of probiotic‑treated, vagus‑intact mice.

Experimental design (outline)

• Groups: (1) Vehicle, (2) L. rhamnosus JB-1, (3) L. rhamnosus JB-1 + vagotomy, (4) L. rhamnosus JB-1 + rapamycin, (5) L. rhamnosus JB-1 + MHY1485, (6) Vagotomy alone.
• Readouts (collected after 2‑week treatment):
  • HRV via ECG telemetry (time‑domain RMSSD).
  • Western blot / immunofluorescence for phospho‑S6, total S6, LC3‑II, p62 in isolated myenteric neurons.
  • Cytokine profiling (IL‑10, TGF‑β, IFN‑γ) from mesenteric lymph node supernatants.
  • Anxiety‑related behavior (elevated plus maze, open‑field).
• Analysis: Linear regression linking HRV to phospho‑S6; ANOVA with post‑hoc tests for group differences.

Potential outcomes and interpretation

• Confirmation would establish a direct mechanistic bridge between a specific probiotic‑vagal signal and the metabolic‑growth regulator mTOR, reframing anxiolytic interventions as modulation of the civilization‑survival trade‑off at the cellular level.
• Failure to observe mTOR changes despite behavioral and HRV effects would suggest parallel pathways, prompting investigation of alternative effectors (e.g., epigenetic, neurotransmitter) while preserving the vagal‑HRV link.

This hypothesis is falsifiable, integrates the ENS‑vagal‑HRV axis with nutrient‑sensing signaling, and offers a clear set of biochemical, physiological, and behavioral readouts for rigorous testing.