Hypothesis

Lactiplantibacillus plantarum Lp815 releases gut‑derived extracellular vesicles (EVs) enriched in GABA and specific microRNAs that travel via vagal afferents to the nucleus tractus solitarius, thereby increasing parasympathetic tone measurable as heightened heart‑rate variability (HRV) before any reduction in self‑reported anxiety.

Mechanistic rationale

• EV‑mediated GABA delivery: Preclinical work shows L. plantarum can synthesize GABA (see https://pmc.ncbi.nlm.nih.gov/articles/PMC12816741/). Recent data indicate that lactic acid bacteria package neurotransmitters into membrane‑derived EVs that survive intestinal transit and interact with enteroendocrine cells (https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2025.1567097/full). These EVs could raise systemic GABA levels without requiring bacterial translocation.
• Vagal afferent sensing: GABA activates GABA_B receptors on vagal nodose ganglia, enhancing afferent firing (https://www.nutritional-psychology.org/lactobacillus-bacteria-in-the-gut-increase-stress-resilience-in-mice/). Vagotomy abolishes the anxiolytic effect of Lactobacillus in mice (https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2025.1567097/full), confirming the nerve as necessary conduit.
• HRV as early readout: Increased vagal efferent activity raises HRV indices such as RMSSD within hours of heightened afferent signaling. Human psychobiotic trials currently lack this biomarker, creating a translational gap (https://pmc.ncbi.nlm.nih.gov/articles/PMC12687006/).
• Sex‑specific amplification: Estradiol upregulates EV release from intestinal epithelial cells, potentially explaining stronger effects in women observed in the Lp815 RCT (https://pmc.ncbi.nlm.nih.gov/articles/PMC12816741/).

Testable predictions

1. In a double‑blind, placebo‑controlled trial (n=120, stratified by sex), participants receiving Lp815 5 bn CFU daily will show a statistically significant rise in weekly RMSSD by week 2 compared with placebo (p < 0.05).
2. Plasma GABA concentration and EV‑associated GABA will increase in parallel with HRV, peaking at week 2.
3. MicroRNA‑124‑3p, enriched in Lp815‑derived EVs, will be detectable in serum and correlate positively with RMSSD changes.
4. Administration of a peripherally acting vagal blocker (e.g., low‑dose atropine) during the first two weeks will attenuate both the HRV rise and the subsequent GAD‑7 improvement, whereas late‑phase blockade will not affect already‑established HRV changes.
5. If vagal signaling is pharmacologically silenced, the anxiolytic effect on GAD‑7 scores will be absent despite unchanged fecal Lactobacillus loads.

Falsifiability

The hypothesis is falsified if any of the following occur:

• No significant HRV difference emerges between Lp815 and placebo at any time point.
• HRV changes do not precede GAD‑7 score improvements (i.e., anxiety improves first or simultaneously).
• Vagal blockade fails to diminish the HRV or anxiety outcomes.
• EV‑associated GABA or microRNA signatures are not detectable in circulation despite HRV shifts.

Implications

Confirming this pathway would validate HRV as an early, objective biomarker for psychobiotic efficacy, enable dose‑optimization trials, and inspire bioelectronic or EV‑based therapeutics that bypass live‑formulation constraints.