Microdosing & Neuroplasticity
10% spine density increase
A single psilocybin dose increased dendritic spine density in the frontal cortex of mice by around 10% within 24 hours, with structural changes persisting for at least one month.
Shao et al., Neuron, 2021
1 month
how long the spine density increase lasted in frontal cortex of mice
Sub-behavioral
the concentration at which Ly 2018 showed plasticity in cortical neurons
Layer 5
the specific neuron type required for psilocybin's full plasticity program
Shao et al., Neuron, 2021
View as image ↗In 2021, a Yale research team published an image that changed how many people thought about psilocybin. They had given mice a single injection of psilocybin and taken microscope images of the frontal cortex 24 hours later. The images showed roughly 10% more tiny branches growing from brain cells compared to controls, and those branches were still there a month after a single dose.[1]
What the news coverage rarely mentioned: the dose used in that study was not a microdose. It was a single injection equivalent to a psychedelic experience in that species, not an amount below the threshold of perception.
That dose distinction is where the honest account of microdosing and dendritic growth has to start. The animal evidence for psilocybin-induced spine growth is genuinely impressive, consistent across multiple studies and species, and mechanistically important. But the studies that produced it generally used doses that produced behavioral effects. Whether similar structural changes occur at the sub-perceptual doses people call "microdosing" is a separate question, one the animal models can inform without directly answering.
For the broader context on what microdosing is and why people do it, see what is microdosing psilocybin. For the question of whether doses too small to feel can still change the brain, see Can Doses Too Small to Feel Still Change the Brain?.
Key Takeaways
- A single psilocybin dose increased dendritic spine density in mouse frontal cortex by around 10% within 24 hours, with changes lasting at least one month (Shao et al., Neuron, 2021)
- Psilocybin and psilocin produced structural plasticity at sub-behavioral concentrations in cortical neuron cultures, the closest existing evidence to a microdose-equivalent effect (Ly et al., Cell Reports, 2018)
- A single psilocybin dose in pigs increased synaptic density in the prefrontal cortex while simultaneously reducing the expression of 5-HT2A receptors (the serotonin receptor that causes hallucinations)
- Psilocybin increased BDNF, mTOR, and c-Fos expression alongside spine density increases in both hippocampus and frontal cortex of mice, confirming the molecular machinery behind the structural changes (Zhao et al., 2024)
- A 2024 Nature study identified layer 5 pyramidal neurons (a specific deep-cortex cell type) as required for psilocybin's lasting plasticity, and 5-HT2A receptors in those cells as necessary for the full program
- None of the live animal studies used protocols matching human microdosing doses; they establish the mechanism, not the dose threshold
What the Animal Studies Found: Three Key Results
Here are the concrete findings from the animal models, in the format that best fits a featured snippet search:
1. Dendritic spine growth in mouse frontal cortex (Shao et al., 2021) Within 24 hours of a single psilocybin injection, dendritic spine density in the medial frontal cortex increased by approximately 10%. Spine size also increased. Both changes persisted for at least one month after a single dose.[1]
2. Increased synaptic density in pig prefrontal cortex (Hesselgrave et al., 2021) A single psilocybin dose in pigs increased synaptic density (measured by binding to a synaptic protein called SV2A) in the prefrontal cortex. Simultaneously, the expression of 5-HT2A receptors (the receptor that causes the psychedelic experience) decreased. The synaptic density increase persisted after 5-HT2A receptor numbers returned to baseline.[3]
3. BDNF, mTOR, and spine density increases in mice (Zhao et al., 2024) A single psilocybin dose increased the brain growth protein BDNF, the cellular growth regulator mTOR, and a marker of neural activity (c-Fos) while also increasing dendritic spine density in both the hippocampus (memory) and frontal cortex (executive function). Antidepressant-like behavioral effects appeared within hours and persisted for weeks.[4]
What Dendritic Spines Are and Why Scientists Count Them
A dendritic spine is a tiny protrusion on a brain cell, roughly the shape of a mushroom (the comparison is not coincidental). These protrusions are where synapses form. Synapses are the contact points where one brain cell sends chemical signals to another.
More spines means more potential synaptic connections. More synaptic connections means more communication capacity between brain regions. This matters for mental health because depression, PTSD, and chronic stress are associated with spine loss in the prefrontal cortex (the region that handles emotional regulation, decision-making, and flexible thinking). Restoring spine density (or growing new spines) is one way to measure neuroplasticity: the brain's capacity to form and reorganize its connections.
This is why the Shao 2021 images attracted so much attention. The images appeared to show a direct structural reversal of a process associated with depression and stress, produced by a single dose.
The Dose Question: Why These Studies Don't Directly Test Microdosing
Here is the fact that most consumer-facing coverage of psilocybin and neuroplasticity skips: the doses used in these animal studies are not microdoses.
Shao et al. gave mice a single intraperitoneal injection at 1 mg/kg. In rodents, this dose produces the head-twitch response, the behavioral proxy researchers use to confirm a psychedelic experience is occurring.[1] This is not a sub-perceptual dose. It is an amount that produces clear behavioral changes in the animal. Scaling for differences in body weight and metabolism between mice and humans, this translates to a moderate human dose, not the tiny amount a microdose involves.
The same applies to Hesselgrave 2021 (pigs) and Zhao 2024 (mice). These studies used single doses that produced clear pharmacological effects, not sub-threshold amounts.
This does not make the findings irrelevant to microdosing. The studies establish that psilocybin can produce robust, lasting structural changes in the brain. They confirm that the mechanism is real, reproducible across species, and linked to specific molecular pathways. What they cannot confirm is whether the amounts used in human microdosing, typically dried mushroom doses of 0.1 to 0.3 grams producing little or no perceptual effect, activate these same pathways sufficiently to produce the same structural changes.
That is the honest limit of the animal model evidence.
The Sub-Behavioral Finding: Ly 2018 and the Psychoplastogen Concept
The closest existing evidence to a microdose-equivalent effect in neural tissue came from Ly et al. in 2018, in a paper published in Cell Reports that introduced the concept of psychoplastogens.[2]
Ly's team applied psilocin (the active form of psilocybin once metabolized), LSD, and DMT directly to cortical neuron cultures. At concentrations below those required to produce behavioral effects in animals, the psychedelics promoted dendritic arbor growth (expansion of the neuron's branching structure) and spinogenesis (new spine formation).
The distinction: this was not a live-animal study. It was cortical neurons growing in a dish, exposed to compounds at concentrations that never triggered the equivalent of the hallucinogenic response. The finding established that the cellular machinery for spine growth can be activated at sub-behavioral concentrations. It did not prove that swallowing a small dose of dried mushroom activates that machinery in a living brain.
What Ly 2018 contributed was the conceptual and experimental foundation: psilocybin's effects on brain plasticity are separable from its effects on perception. This made the idea of "neuroplasticity without a trip" scientifically grounded rather than speculative. The term psychoplastogen, coined in this paper, refers to compounds that promote brain plasticity at doses below those that produce hallucinogenic effects.
For the full-dose version of this story and how psilocybin produces dendritic spine growth at standard doses, see How Psilocybin Grows New Dendritic Spines.
Pig Brain and Synaptic Density: Why Species Diversity Matters
The Hesselgrave 2021 study matters for a reason beyond its findings: pigs.[3]
Most psilocybin neuroscience uses rodents. Pig brains are structurally closer to human brains in several ways: the ratio of white matter to grey matter, the organization of prefrontal regions, and brain size all make pig models more translatable than mouse models for some questions. Finding the same direction of effect in a species with more brain complexity strengthens the hypothesis that the mechanism is likely to operate in humans as well.
What Hesselgrave measured was not spine count directly but synaptic density, using a PET imaging technique that tracks binding to a protein called SV2A, which is found at synaptic terminals. This is a higher-level measure than counting individual spines under a microscope. It reflects the density of functional synaptic connections across a brain region rather than the number of physical protrusions on individual cells.
The finding that 5-HT2A receptors (the serotonin receptor type that produces hallucinations when psilocybin activates it) simultaneously decreased while synaptic density increased is mechanistically interesting. It suggests that as psilocybin drives structural change through one pathway, the brain compensates by reducing the density of the target receptor. This receptor downregulation is thought to be one reason repeated psychedelic doses produce diminishing hallucinogenic effects, while structural changes may accumulate differently.
BDNF, mTOR, and the Molecular Cascade Behind Spine Growth
Zhao et al.'s 2024 study in mice provides the most complete picture of what happens at the molecular level when psilocybin promotes spine growth.[4]
BDNF (brain-derived neurotrophic factor) is a protein the brain uses to promote neuron survival, growth, and the formation of new connections. Chronic depression is associated with reduced BDNF in the hippocampus and prefrontal cortex. SSRI antidepressants increase BDNF indirectly, through weeks of sustained serotonin reuptake inhibition. Zhao found that a single psilocybin dose raised BDNF levels within hours, alongside the spine density increases.
mTOR (mechanistic target of rapamycin) is a protein inside cells that acts as a master switch for growth. When mTOR is activated, the cell ramps up production of the proteins needed to build new structural components, including the proteins that form dendritic spines. Zhao found that psilocybin activated mTOR alongside BDNF, giving a more complete picture of the molecular machinery involved.
c-Fos is a marker of neural activity, a protein that increases when a neuron fires. The simultaneous elevation of c-Fos confirmed that the neurons were not just growing passively. They were active, firing, and integrating new connections.
The antidepressant-like behavioral effects in Zhao's mice appeared within hours of the dose and persisted for weeks, tracking with the structural changes. The speed of onset is one feature that distinguishes psilocybin from conventional antidepressants, which typically require weeks to show behavioral effects.
Which Neurons Are Required: The 2024 Nature Discovery
Kim et al. published a 2024 study in Nature that identified more precisely which brain cells are responsible for psilocybin's lasting structural effects.[5]
Layer 5 pyramidal neurons are a specific cell type in the deep layers of the cortex. They are among the largest neurons in the brain, with extensive branching structures that connect to distant brain regions. Kim's team used genetic tools to selectively delete 5-HT2A receptors from layer 5 pyramidal neurons in mice, while leaving 5-HT2A receptors intact everywhere else in the brain.
In these genetically modified mice, psilocybin produced neither the head-twitch response (the hallucinogenic behavioral marker) nor lasting dendritic spine growth. Both effects were abolished by removing 5-HT2A from layer 5 cells specifically. This established that these particular neurons, with their specific receptor, are necessary for the full plasticity program that follows a psychedelic dose.
The implication for the sub-perceptual dose question is nuanced. On one reading, this finding suggests that activating 5-HT2A in layer 5 cells is necessary for the complete plasticity response, which might imply that doses too low to activate those receptors sufficiently would not produce the full effect. On another reading, this refers to the 5-HT2A receptor on the cell surface, not the intracellular 5-HT2A receptor that separate 2023 research showed can mediate plasticity independently. The 2024 Kim study and the 2023 Vargas intracellular pathway finding together suggest the picture is more complex than a simple on-off threshold.
What These Animal Models Can and Cannot Tell Us About Microdosing
The animal evidence reviewed here establishes several things with reasonable confidence:
Psilocybin reliably produces lasting structural changes in the brains of rodents and pigs. The changes (more dendritic spines, greater synaptic density, increased BDNF and mTOR) are consistent across studies and species. The changes occur rapidly (within hours to a day) and persist for weeks to months. The molecular machinery (BDNF, mTOR, spine proteins) is well-characterized.
What the animal evidence cannot establish:
The studies used doses that caused behavioral effects in those species. None used a dose equivalent to what humans call a microdose. It is therefore not accurate to say "animal studies prove microdosing grows new brain connections." It is accurate to say that psilocybin, at doses producing clear pharmacological effects, grows new connections in animal brains, and that the mechanism responsible for this growth (TrkB binding, intracellular 5-HT2A activation) operates at lower concentrations.
The species translation is imperfect. Rodent and pig brains share fundamental features with human brains, but the complexity, connectivity patterns, and behavioral repertoire differ substantially. Effects observed in a mouse brain cannot be directly assumed in a human brain.
Administration route matters. Most of these studies used injection directly into the abdomen, not oral ingestion. Oral bioavailability of psilocybin and its conversion to psilocin in the gut and liver add a layer of variability that injection bypasses.
For an honest assessment of whether these mechanisms are likely active at sub-perceptual human doses, see Sub-Perceptual Psilocybin and Brain Plasticity: Does It Work? and Sub-Perceptual Psilocybin and BDNF Expression: Rodent Neuroplasticity Evidence.
For the question of how microdosing evidence holds up in human placebo-controlled studies, see Is Microdosing Just Placebo? The Self-Blinding Studies.
For a comparison with lion's mane, which promotes neuroplasticity through a completely different pathway (NGF rather than BDNF), see Hericenones and NGF Synthesis: How Lion's Mane Stimulates Nerve Growth.
The Bottom Line
The animal model evidence for psilocybin-induced dendritic spine growth is consistent and substantial: across mice and pigs, single doses produced measurable increases in spine density, synaptic density, BDNF, and mTOR, with structural changes persisting for weeks to months. The dose context that consumer coverage routinely omits is that these studies used doses equivalent to psychedelic experiences in those species, not sub-perceptual amounts. The closest evidence for plasticity at microdose-equivalent concentrations comes from Ly 2018, which showed sub-behavioral plasticity in cortical neurons in a dish. The animal models establish the mechanism with high confidence; whether that mechanism is sufficiently activated by human microdose amounts remains the open question.
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