Hypothesis

Oral psilocybin administration in aged animals remodels the gut microbiota toward increased production of protective tryptophan metabolites (e.g., indoleacetic acid) and decreased kynurenine pathway activity, thereby enhancing vagal afferent tone and restoring hippocampal‑dependent memory.

Mechanistic Rationale

• Psilocybin is a phosphorylated tryptamine that can serve as a substrate for gut microbial tryptophanases, leading to elevated indole derivatives that activate aryl hydrocarbon receptor (AHR) signaling in intestinal epithelial cells and microglia, suppressing synaptogenic phagocytosis [1].
• Acute activation of 5‑HT2A receptors on enteroendocrine cells by psilocybin triggers release of serotonin and peptide YY, which potentiates vagal afferent firing toward the nucleus tractus solitarius [6].
• Enhanced vagal signaling inhibits hippocampal microglia via the cholinergic anti‑inflammatory pathway, counteracting the GPR84‑mediated suppression of vagal‑hippocampal synapses driven by aging‑associated Parabacteroides goldsteinii and its medium‑chain fatty acids [2].
• By shunting tryptophan away from hepatic indoleamine 2,3‑dioxygenase (IDO1)–dependent kynurenine synthesis, psilocybin lowers circulating kynurenine, a metabolite whose elevation predicts reduced lifespan across species [3].

Predictions

1. Aged mice receiving oral psilocybin (0.5‑5 mg/kg) for 2 weeks will show a significant increase in fecal indoleacetic acid and a decrease in hepatic kynurenine compared with vehicle controls.
2. Vagal tone, assessed by heart‑rate variability and c‑Fos activation in the nucleus tractus solitarius, will be elevated only in psilocybin‑treated animals.
3. Hippocampal long‑term potentiation and spatial memory (Morris water maze) will improve in psilocybin‑treated aged mice.
4. Subdiaphragmatic vagotomy or pharmacological blockade of 5‑HT2A receptors on enteroendocrine cells will abolish the cognitive and biochemical benefits of psilocybin.
5. Administering an IDO1 inhibitor alongside psilocybin will not produce additive effects, indicating a shared pathway.

Experimental Design

• Subjects: 20‑month‑old C57BL/6J mice (n=10 per group).
• Groups: (1) Vehicle, (2) Psilocybin low dose, (3) Psilocybin high dose, (4) Psilocybin + vagotomy, (5) Psilocybin + 5‑HT2A antagonist, (6) Psilocybin + IDO1 inhibitor.
• Outcomes: Fecal metabolomics (LC‑MS for IAA, kynurenine), plasma kynurenine/tryptophan ratio, vagal afferent activity (electrophysiology, c‑Fos), microglial phenotype (Iba1, CD68), hippocampal synaptic markers (PSD‑95, synaptophysin), behavioral performance.
• Analysis: Two‑way ANOVA with post‑hoc Tukey tests; significance set at p < 0.05.

Potential Confounds and Controls

• Psilocybin’s central psychoactive effects could independently influence behavior; therefore, a parallel group receiving a non‑psychoactive tryptamine (e.g., 4‑hydroxytryptamine) will control for peripheral actions.
• Antibiotic pretreatment will verify microbiota dependence; germ‑colonized mice receiving aged microbiota transplants will test sufficiency.

Falsifiability

If psilocybin fails to raise indoleacetic acid or lower kynurenine, or if cognitive improvements persist despite vagotomy or 5‑HT2A antagonism, the hypothesis is falsified. Conversely, a consistent pattern of metabolite shifts, vagal activation, and memory rescue contingent on intact gut‑brain signaling would support the proposed bottom‑up mechanism.