Hypothesis

Probiotic‑induced vagal activation does not merely enhance glymphatic flow; it biochemically tags synapses for selective removal during slow‑wave sleep, turning the glymphatic system into an active triage agent that decides which neural architectures persist.

Mechanistic Chain

1. L. rhamnosus JB‑1 stimulates afferent vagal fibers → ↑ vagal tone (HRV) 1.
2. Heightened vagal signaling raises acetylcholine release in the locus coeruleus and thalamic reticular nucleus, promoting deeper slow‑wave sleep (↑ delta power) 2.
3. During slow‑wave sleep, perivascular aquaporin‑4 channels open, driving glymphatic influx of CSF 3.
4. Vagal efferents also suppress splenic sympathetic output, lowering circulating IL‑6 and TNF‑α, which prevents inflammatory mislocalization of aquaporin‑4 4.
5. Simultaneously, vagal‑derived norepinephrine spikes in the cortex increase microglial expression of complement C1q and phagocytic receptors, tagging weakly active synapses for removal.
6. The glymphatic convective flow then clears the opsonized synaptic debris, extracellular amyloid‑β, and tau, accomplishing a nightly edit of the connectome.

Testable Predictions

• In a randomized, double‑blind, crossover trial, participants receiving L. rhamnosus JB‑1 (10⁹ CFU daily for 4 weeks) will show:
  • A ≥10 % increase in night‑time HRV RMSSD vs placebo.
  • A ≥15 % rise in EEG slow‑wave (0.5‑4 Hz) power during NREM.
  • A ≥20 % reduction in CSF amyloid‑β42 and tau concentrations measured by lumbar puncture before and after the intervention.
  • A selective ↓ in synaptic markers neurogranin and synaptophysin in prefrontal cortex‑derived exosomes, reflecting pruning of excitatory synapses.
  • Corresponding improvements in State‑Trait Anxiety Inventory scores.
• If vagotomy (or pharmacological vagal blockade with atropine) is administered in a parallel arm, the probiotic‑induced HRV, slow‑wave, CSF clearance, and synaptic marker changes will be abolished, confirming vagal mediation.

Falsifiability

Failure to observe any of the above biomarker shifts despite confirmed probiotic ingestion and HRV rise would refute the claim that vagal tone couples to glymphatic‑mediated synaptic triage. Conversely, observing HRV and slow‑wave increases without concomitant CSF clearance or synaptic marker decline would indicate that the probiotic affects sleep depth but not the proposed triage mechanism.

Broader Impact

Validating this link would reposition sleep‑dependent glymphatic function from a passive waste drain to an immune‑guarded, microbiome‑tuned sculpting process, opening therapeutic avenues for neuropsychiatric disorders where synaptic over‑connectivity and poor sleep co‑occur (e.g., anxiety, PTSD).