Hypothesis

A single psilocybin dose triggers a BDNF‑TrkB cascade that opens a chromatin window for activity‑dependent gene expression in prefrontal cortex neurons. This window allows transient upregulation of synaptic‑plasticity programs, but concurrently induces expression of the repressor gene Npas4‑induced microRNA‑124 (miR‑124) which dampens TrkB signaling after ~7 days. The result is a self‑limiting loop: acute DMN desynchronization yields compensatory network flexibility that decays unless a second, sub‑psychedelic serotonergic stimulus re‑engages TrkB before miR‑124‑mediated suppression consolidates.

Mechanistic Rationale

• Psilocybin‑5‑HT2A agonism raises intracellular Ca²⁺, activating CREB and boosting BDNF transcription (2).
• BDNF‑TrkB signaling phosphorylates ERK/MSK1, leading to histone H3K27 acetylation at plasticity loci (3).
• Parallel activation of the immediate‑early gene Npas4 drives transcription of miR‑124, which targets TrkB mRNA for degradation (4).
• miR‑124 peaks at day 5‑7, correlating with the observed normalization of 5‑HT2A receptors and the decline of BDNF protein levels.
• If a low‑dose serotonergic challenge (e.g., 0.1 mg/kg psilocybin or a selective 5‑HT2A agonist) is administered during the miR‑124 rise, TrkB signaling is re‑activated, suppressing miR‑124 via a feedback loop that sustains ERK‑dependent acetylation and preserves synaptic density gains.

Testable Predictions

1. Biochemical – In rodent prefrontal cortex, miR‑124 levels will increase 2‑fold at 6 days post‑psilocybin and return to baseline by day 14; a second low‑dose given at day 5 will blunt this rise (5).
2. Imaging – SV2A‑PET will show a +8 % synaptic density increase at day 7 after a single dose; the increase will persist to day 28 only when a booster low‑dose is delivered at day 5.
3. Behavioral – Mice subjected to chronic mild stress will exhibit reduced immobility in the forced‑swap test at day 7 after psilocybin; the effect will disappear by day 21 unless a booster is given, at which point the antidepressant‑like phenotype extends to day 35.
4. Pharmacological – Co‑administration of a TrkB antagonist (ANA‑12) with the booster will block the persistence of synaptic and behavioral effects, confirming TrkB dependence.

Potential Confounds

• Variability in baseline miR‑124 expression across individuals could mask booster effects; stratify subjects by peripheral miR‑124 levels.
• Off‑target activation of 5‑HT2C receptors by booster doses might independently influence mood; include a 5‑HT2C selective antagonist control group.
• Stress‑induced glucocorticoids can alter BDNF transcription; standardize housing and handle animals identically across groups.

If the data show that synaptic and behavioral benefits decay in line with miR‑124 kinetics and are rescued only by timely serotonergic re‑engagement, this would support the hypothesis that psychedelic‑induced remodeling is primarily a hormetic compensatory state rather than a self‑sustaining repair mechanism. Conversely, persistent benefits without booster would challenge the hormesis framing and suggest true epigenetic reprogramming.