Hypothesis

Oral Lactobacillus strains release extracellular vesicles (EVs) containing GABA and short‑chain fatty acids that directly activate nodose ganglion afferents, producing an immediate vagal efferent burst that accelerates HRV recovery (RMSSD, RSA) following an acute psychological stressor.

Mechanistic Insight

Recent work shows Lactobacillus rhamnosus upregulates GABA receptors and lowers corticosterone [2], while L. paracasei PS23 enhances GABAergic activity [3]. We propose that the anxiolytic signal is not only mediated through downstream neurotransmitter changes in the brain but also through a fast, peripherally restricted pathway: Lactobacillus‑derived EVs cross the intestinal barrier, reach the submucosal plexus, and are taken up by vagal nodose neurons expressing GABA_A and GABAB receptors as well as FFAR2/3 for SCFAs. This triggers rapid depolarization of afferent fibers, increasing solitary tract nucleus firing and enhancing vagal outflow within seconds—reflected in a faster rise of RMSSD post‑stress.

Testable Prediction

In a single‑subject (n=1) crossover trial, administering a dose of L. rhamnosus JB-1 (10^9 CFU) 30 min before a standardized mental arithmetic stressor will produce a significantly shorter HRV recovery time (time to return to baseline RMSSD) compared with placebo, measurable via 5‑minute RMSSD windows. The effect should be abolished by pretreatment with a peripherally acting vagal blocker (e.g., low‑dose intravenous atropine) or by genetic knockdown of nodose GABA_A receptors in an animal validation step.

Experimental Design (n=1)

1. Baseline HRV recording (5 min) while at rest.
2. Ingestion of capsule (probiotic or placebo) in random order, double‑blinded.
3. 30 min later, perform 5‑minute mental arithmetic stressor (Serial sevens).
4. Continuous HRV (RMSSD) recorded for 20 min post‑stress.
5. Primary outcome: area under the curve (AUC) of RMSSD deficit relative to baseline; secondary: slope of RMSSD recovery.
6. Repeat with atropine (0.01 mg/kg IV) in a separate session to test vagal dependence.

Falsification

If probiotic intake does not shorten HRV recovery AUC or slope relative to placebo, or if the effect persists despite peripheral vagal blockade, the hypothesis that Lactobacillus‑EV‑mediated nodose activation drives rapid vagal tone enhancement is falsified.