Psilocybin Dendritic Spine Growth in 24 Hours

Psilocybin & Structural Neuroplasticity

~10%

increase in dendritic spine density and size in mouse frontal cortex within 24 hours of a single psilocybin dose. The new spines were still present 1 month later, well after the drug had cleared.

Shao et al., Neuron, 2021

24 hours

time until new dendritic spines first appeared on in vivo two-photon microscopy imaging

1 month

how long the new spines persisted in mouse medial frontal cortex after a single dose

1,000×

higher affinity at which psilocin binds the BDNF growth-factor receptor TrkB compared to conventional antidepressants

Shao et al., Neuron, 2021

View as image ↗

In 2021, researchers at Yale University drilled a tiny transparent window into the skull of a mouse. Through that window, using a microscope that fires pulses of infrared laser light through living brain tissue, they watched what happened after a single dose of psilocybin.

They saw neurons grow new branches.

Not over weeks of gradual change. Within 24 hours of receiving psilocybin, the neurons in the mouse's frontal brain region were sprouting new protrusions. These protrusions are dendritic spines, the tiny knobs on the branching arms of neurons that form the receiving ends of synapses, the junctions where one neuron passes a signal to another.^[1] A single dose. One day. Approximately 10% more of them.

Those connections were still there a month later.

This was the first time psilocybin's structural effects on brain tissue had been directly observed in a living animal. Not inferred from behavior, not estimated from biochemical markers. New physical structures grown by existing neurons, documented while the animal was alive, persisting weeks after the drug had cleared. This article focuses specifically on what Shao and colleagues found and why the findings matter. For the broader overview of how structural and functional plasticity fit together across the full brain, psilocybin and the default mode network: how it quiets the ego covers the wider neuroscience picture.

What a Dendritic Spine Is (and Why Losing Them Matters in Depression)

Neurons are long, branching cells that transmit electrical signals. Each neuron has a main body, an output cable called an axon that carries signals away, and a collection of branching arms called dendrites that receive signals from other neurons. Dendritic spines are the tiny protrusions that grow off those dendrite branches. Each spine typically hosts one synapse, the point of chemical contact where one neuron's output signal passes to another neuron's input.

The number of dendritic spines on a neuron is not fixed. Spines form, change shape, shrink, and disappear in response to experience, learning, and brain chemistry. A neuron with more spines has more potential connections. More connections generally mean more flexible, adaptable processing. This is what neuroscientists mean when they talk about synaptic plasticity: the brain's capacity to change its wiring based on experience.

Depression disrupts this process. In prefrontal cortex, the region behind the forehead involved in attention, planning, and emotional regulation, chronic stress causes synaptic pruning: the systematic loss of dendritic spines. Rodent models of depression consistently show reduced spine density in prefrontal neurons following chronic stress exposure. The prefrontal cortex effectively loses connectivity, which is one reason depression impairs the ability to think flexibly, generate solutions, or regulate emotions. Conventional antidepressants work partly by promoting synaptic plasticity over weeks of treatment, but the timescale is slow.

The Yale finding was striking because psilocybin reversed this picture much faster than anyone expected. And it did so not just biochemically, but structurally: new spines, visible under a microscope, grown within a day.

For the related question of how psilocybin actually grows those spines at the molecular level, how psilocybin grows new dendritic spines goes deeper on the cellular machinery involved.

How Researchers Watched Spines Grow in Real Time: The Two-Photon Microscopy Window

Most studies of synaptic structure in rodents require sacrificing the animal, slicing the brain into thin sections, and imaging the tissue under a microscope. This gives you a snapshot. You can compare spine density in a psilocybin-treated brain to a control brain, but you cannot watch the spines form or track the same neurons across time.

Two-photon microscopy changes that. The technique uses infrared laser pulses to excite fluorescent proteins embedded in neurons. Infrared light is low-energy enough to pass through living tissue without damaging it, unlike the visible light used in conventional microscopes. When two photons of infrared light arrive at the same fluorescent molecule simultaneously, their combined energy excites the molecule and produces a brief flash of visible light. A detector captures that flash and builds up an image, point by point, through as many as 1 millimeter of living brain tissue.

To use this technique on a mouse brain, researchers implant a small circular glass window directly into the skull, removing a portion of bone and sealing the opening with glass. The mouse recovers from the surgery. Subsequent imaging sessions require no additional surgery. The same window, the same neurons, the same patch of cortex, imaged repeatedly across days or weeks. Each imaging session produces a three-dimensional snapshot of the neurons and their spines.

Shao and colleagues used this approach in the 2021 study.^[1] They imaged the mice before psilocybin, then again at 24 hours, and then again 1 month later. This let them track changes in the same individual neurons across three timepoints, rather than comparing different animals at different times. It is the gold standard for measuring structural plasticity in the living brain.

What They Found: A 10% Jump in New Brain Connections Within 24 Hours

The imaging focused on layer 5 pyramidal neurons in the medial frontal cortex of mice. These are large, prominent neurons deep in the frontal cortex that project widely to other brain regions and play a central role in coordinating complex cognition and behavior. They are among the most studied neurons in the cortex and are known to be particularly sensitive to stress and to synaptic remodeling.

Twenty-four hours after a single dose of psilocybin, the same neurons had developed approximately 10% more dendritic spines than before the dose.^[1] The change was not subtle. The increase was visible in the raw images and statistically significant.

Two aspects of the finding are worth separating: density and size.

Spine density increased, meaning there were more spines per unit length of dendrite. This is the headline number: approximately 10% more connection points. But the spines that formed were also larger than the pre-existing spines. Larger spines are associated with stronger, more stable synaptic connections. The change was not just more connections, it was also structurally robust connections.

The effect was also rapid in a way that stands apart from other neuroplasticity-promoting interventions. Antidepressants, cognitive training, and standard aerobic exercise produce dendritic spine growth in rodents, but typically over days to weeks. Psilocybin reached a comparable effect in a day.

For how this spine growth connects to the broader question of new synaptic connections, psilocybin and synaptogenesis: creating new neural connections covers the formation of new synapses specifically.

The New Connections Lasted a Month After the Drug Had Cleared

Psilocybin converts rapidly to psilocin in the body. Psilocin acts on brain receptors for a few hours, then clears. The subjective psychedelic experience ends. Blood psilocin levels drop to zero.

The spines did not disappear with the drug.

When Shao's team returned to the same neurons 1 month after the dose, the new spines were still there.^[1] Not all of them: some had turned over, as synapses normally do. But spine density remained elevated compared to the pre-dose baseline. The structural change was durable in a way that the drug's presence was not.

This separation of timescales is important for understanding how psilocybin might produce lasting therapeutic effects. The drug is in the brain for hours. The structural remodeling it triggers appears to last for weeks. Whatever mechanisms maintain the new spines, they operate on a much longer timescale than the drug that initiated them.

The persistence finding also connects to what researchers have started calling the plasticity window: the period after a psilocybin dose when the brain shows enhanced structural flexibility and responsiveness to new learning. The plasticity window: how long psilocybin keeps the brain flexible covers that concept and the available timeline data in depth.

The Yale study also found that psilocybin reversed stress-induced behavioral deficits in the same mice, a result consistent with the structural recovery of prefrontal connectivity.^[1] The mice showed improved behavior on tasks associated with frontal cortex function, paralleling the structural improvements seen under the microscope.

Two Separate Mechanisms Drive Spine Growth (And One Has Nothing to Do With the High)

For a long time, the explanation for psilocybin's effects on neuroplasticity was a single chain of events: psilocybin activates serotonin 2A receptors (the primary target for psychedelic effects), this receptor activation sets off downstream cellular signaling cascades (including a pathway called mTOR, which controls cell growth), and the mTOR activation ultimately tells neurons to grow new spines and synapses.^[4]

This model was established by the foundational 2018 study from Olson's lab at UC Davis, which showed that psilocybin and other classic psychedelics (LSD, DMT, and several others) all promoted new spine formation (spinogenesis) and new synapse formation (synaptogenesis) in cultured neurons and in living rodents through TrkB, mTOR, and serotonin 2A signaling pathways, and coined the term "psychoplastogens" for this class of compounds.^[4]

Then a 2023 paper in Nature Neuroscience rewrote part of the story.

Moliner and colleagues at the University of Helsinki found that psilocin (and LSD) bind directly to TrkB, the brain's main receptor for a growth factor called BDNF (brain-derived neurotrophic factor, the brain's primary structural growth signal).^[3] TrkB is the receptor that BDNF docks with to promote neuronal growth, spine formation, and synaptic strengthening. It had not previously been identified as a target for psychedelic compounds.

The binding strength was remarkable. Psilocin bound to TrkB 1,000 times more tightly than conventional antidepressants, which also act on this receptor but at far lower potency.^[3] The binding occurred at a segment of the TrkB protein that threads through the cell membrane, a different site from where BDNF itself binds. Psilocin was not competing with BDNF; it was acting on TrkB through a second entry point.

The mechanistic implication was significant: when the researchers blocked the serotonin 2A receptor in mouse experiments, the psychedelic effects disappeared, but the neuroplasticity effects and antidepressant effects remained. The plasticity ran through TrkB, and TrkB activation did not require serotonin 2A at all.^[3]

The subjective experience and the structural rewiring are driven by separate molecular mechanisms. The trip is serotonin 2A. The lasting plasticity runs through TrkB. Both mechanisms are active during a psilocybin session, but they operate independently.

This also explains why the Shao 2021 spine changes persisted for a month after psilocybin cleared: TrkB activation sets in motion a sustained BDNF signaling process that maintains the newly grown spines long after the drug is gone.

For a detailed explanation of the serotonin 2A receptor side of psilocybin's effects, psilocybin and 5-HT2A receptors: the gateway to psychedelic effects covers the receptor pharmacology specifically.

Scaling Up: What Pig Brain PET Scans Showed About Synaptic Density

One limitation of the Yale two-photon study is resolution and scale. Two-photon microscopy is extraordinary for watching individual neurons and their spines, but the imaging window covers a tiny patch of cortex. You see a few neurons in stunning detail. You cannot see the whole brain.

A 2021 study by Raval and colleagues took a complementary approach, using PET imaging in pig brains.^[2] PET imaging uses radioactive tracer molecules to measure specific biological properties across the whole brain at once. For synaptic density, the relevant tracer targets a protein called SV2A (synaptic vesicle glycoprotein 2A), which is present in virtually all synapses. More SV2A signal means more synapses.

Pigs are used in translational neuroscience research partly because pig brain anatomy is closer to human brain anatomy than mouse brain anatomy, and partly because the PET methodology developed for pig brains is directly comparable to the PET methodology used in human brain studies. A synaptic density measurement in a pig brain is a meaningful step toward a human brain measurement.

Raval's team found that a single psilocybin dose increased SV2A-measured synaptic density in pig frontal cortex.^[2] The same scan also showed a simultaneous decrease in serotonin 2A receptor density, which is the expected adaptation when these receptors are heavily activated (the brain downregulates them in response). Both changes were visible in the same imaging session.

The pig PET finding independently confirms that the synaptic density changes seen under the two-photon microscope in mice are not a microscopy artifact or a mouse-specific phenomenon. They show up in a different species, measured by a completely different technique, using the same radiotracer methodology that could in principle be applied to humans.

The human application of SV2A PET to measure psilocybin-induced synaptic density changes has not yet been published. The pig PET data is the closest available bridge to that human evidence.

What This Might Mean for Depression: Rebuilding Connections Lost to Stress

The Shao 2021 study was designed around a stress model of depression. Before receiving psilocybin, the mice were exposed to chronic unpredictable mild stress, a standard protocol in rodent depression research that causes behaviors associated with depression (reduced exploratory behavior, reduced interest in rewarding experiences, elevated stress hormone levels). Chronic stress also reduces dendritic spine density in prefrontal cortex.

Psilocybin reversed both the behavioral deficits and the structural deficit. The spine density increased back toward normal. The stressed behaviors improved. The two changes happened together and were documented in the same animals.^[1]

This raises a mechanistically coherent hypothesis for at least part of psilocybin's antidepressant effects in human clinical trials: psilocybin may restore prefrontal synaptic architecture that was degraded by stress and chronic depression. If depression involves synaptic pruning in prefrontal cortex, and psilocybin rapidly reverses that pruning, the structural recovery could be part of why clinical effects appear quickly (sometimes within days of a session) and persist beyond the time in the brain.

This remains a hypothesis. The human clinical trials that have shown psilocybin's antidepressant effects have not measured structural brain changes directly. No human neuroimaging study has yet used two-photon microscopy or SV2A PET to confirm that psilocybin grows new dendritic spines in the human brain. The animal-to-human translation is plausible but not yet proven.

What the data does establish is that psilocybin's effects on the brain are not limited to changing which regions are more or less active. At the cellular level, in living brain tissue, psilocybin tells neurons to build new connections. Those connections form fast and they last.

For the complementary picture of how psilocybin reshapes brain network activity rather than brain structure, functional connectivity changes under psilocybin: fMRI evidence covers the network-level changes in detail. The two lines of evidence, structural and functional, describe different facets of the same rewiring process. And for the broader question of how all these structural changes connect to the growing field of psilocybin and neuroplasticity, that overview article maps the full landscape.

The Bottom Line

The 2021 Yale study by Shao and colleagues provided the first direct in vivo imaging evidence that a single psilocybin dose causes structural neuroplasticity: approximately 10% more dendritic spines in mouse frontal cortex within 24 hours, persisting for at least 1 month. A 2023 Nature Neuroscience study established that psilocin drives this plasticity by directly binding TrkB (the brain's main growth-factor receptor) at 1,000 times the potency of conventional antidepressants, through a mechanism that operates independently of the receptor responsible for the subjective psychedelic experience. A 2021 pig brain PET study found the same synaptic density increase using translational imaging methodology. All evidence to date is preclinical; whether the same structural changes occur in the human brain remains to be directly measured.

Frequently Asked Questions

Does psilocybin grow dendritic spines in 24 hours?

Yes, in mouse brain. A 2021 Yale study (Shao et al., Neuron) used in vivo two-photon microscopy to directly observe a roughly 10% increase in dendritic spine density and size in mouse medial frontal cortex within 24 hours of a single psilocybin dose. These findings have not yet been directly replicated in the human brain, though a pig brain PET study (Raval et al., 2021) found synaptic density increases using translational imaging methodology comparable to human research.

What happened in the Yale psilocybin study?

The Shao 2021 study (published in the journal Neuron) used a technique called two-photon microscopy to image individual neurons in the frontal cortex of living mice through a small window implanted in the skull. Researchers imaged the same neurons before a psilocybin dose, 24 hours after, and 1 month after. They found approximately 10% more dendritic spines (the receiving ends of synaptic connections) at 24 hours, and those new spines were still present 1 month later. The mice had also been exposed to chronic stress before receiving psilocybin, and the dose reversed both the structural synaptic deficits and the stress-related behavioral changes.

How quickly does psilocybin change brain structure?

The structural changes visible in Shao 2021 were measurable within 24 hours of a single dose. This speed is faster than most other neuroplasticity-promoting interventions studied in rodents, where comparable spine growth effects typically take days to weeks. The rapid onset may reflect psilocybin's dual mechanism: 5-HT2A receptor activation sets off fast cellular growth signals (via the mTOR pathway), while direct TrkB binding activates BDNF-driven growth processes that sustain the structural changes over a longer period.

How much do dendritic spines increase after psilocybin?

In the Shao 2021 study, dendritic spine density in mouse medial frontal cortex increased by approximately 10% within 24 hours of a single psilocybin dose. Spine size also increased, not just spine count. These are changes in layer 5 pyramidal neurons, the large cortical neurons that project widely to other brain regions. The ~10% figure is specific to the mouse medial frontal cortex under the experimental conditions of this study; different brain regions, different doses, or different species may show different effects.

Do the structural brain changes from psilocybin last?

In the mouse brain, yes. Shao 2021 found that new dendritic spines were still present 1 month after a single psilocybin dose, well after the drug had metabolized and cleared. The 2023 Moliner study provides a mechanistic explanation for this: psilocin binds the TrkB growth-factor receptor with very high affinity and activates sustained BDNF signaling that maintains newly grown spines after the drug itself is gone. Whether structural changes persist in the human brain has not been directly measured.

What is the Shao 2021 study?

The Shao 2021 study, published in the journal Neuron by Lu-Yang Shao and colleagues at Yale University, was the first study to directly observe psilocybin-induced structural neuroplasticity in a living animal brain. It used two-photon microscopy through an implanted cranial window to image the same neurons in mouse frontal cortex before and after psilocybin, finding approximately 10% growth in dendritic spine density and size within 24 hours, with changes persisting at 1 month. The same dose also reversed stress-induced behavioral deficits. It remains the landmark study on psilocybin and structural neuroplasticity.

Sources & References

1. 1RTHM-01434·Shao, Ling-Xiao et al. (2021). “Psilocybin induces rapid and persistent growth of dendritic spines in frontal cortex in vivo..” Neuron.Study breakdown →PubMed →↩
2. 2. Source details for this reference are not yet available.
3. 3RTHM-01756·Moliner, Rafael et al. (2023). “Psychedelics promote plasticity by directly binding to BDNF receptor TrkB..” Nature neuroscience.Study breakdown →PubMed →↩
4. 4RTHM-00983·Ly, Calvin et al. (2018). “Psychedelics Promote Structural and Functional Neural Plasticity..” Cell reports.Study breakdown →PubMed →↩