Health
How psilocybin reshapes brain circuits
New research at UC Santa Cruz reveals how psychoactive compound in ‘magic mushrooms’ rewires synapses without altering perception, opening new possibilities for non-hallucinogenic psychiatric medications.
An artistic illustration of the dual effects of psilocybin on a mouse’s brain. Psilocybin promotes the formation and maturation of dendritic spines, tiny protrusions from the dendritic branches that serve as neuronal connection sites, here portrayed as little mushrooms. At the same time, psilocybin induced a lab mouse’s head to twitch rapidly, a proxy of hallucinations in humans.
Illustration by Emily Kogan, Zuo Lab
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Key takeaways
- In a new study published in Nature Neuroscience, researchers found that psilocybin rapidly promotes the growth and maturation of new neural connections in the brain while helping to restore neural circuits disrupted by chronic stress.
- The researchers discovered that cellular mechanisms responsible for rewiring brain circuits operate through different pathways than those causing hallucinogenic responses.
- These discoveries provide a promising scientific blueprint for developing new, non-hallucinogenic psychiatric medications that offer rapid antidepressant benefits without psychedelic side effects.
A new study led by researchers at the University of California, Santa Cruz, reveals how psilocybin, the psychoactive compound in “magic mushrooms,” reshapes neural connections in the brain.
The study, published on October 15 in the journal Nature Neuroscience, demonstrates in mice that psilocybin rapidly accelerates the formation and maturation of new neural connections while promoting the elimination of some pre-existing connections. In addition, the researchers observed a behavioral response in the lab mice indicating that the cellular mechanisms driving this structural plasticity can happen without psilocybin’s psychedelic effects.
Crucially, this provides a cellular framework for developing treatments that promote plasticity with fewer hallucinogenic effects. These findings may inform future research into treatments for depression, post-traumatic stress disorder (PTSD), and other psychiatric conditions.
In the study, the team identified a specific population of cortical neurons critical for psilocybin-induced synapse remodeling. Furthermore, they found that psilocybin selectively rewired circuits based on the brain’s underlying health status.
“I consider this work a major advance, particularly in our understanding of how psilocybin reshapes neural circuits and its potential implications for treating mental disorders,” said senior author Yi Zuo, professor of molecular, cell, and developmental biology at UC Santa Cruz.
History of psychedelic science and regulatory momentum
Over the past century, the neurobiological study of psychedelics has seen numerous major shifts. Natural psychedelics were introduced to Western science in the mid-20th century, with researchers identifying active compounds like psilocybin and LSD that produced profound alterations in consciousness and mood. Early clinical investigations showed therapeutic potential for psychiatric care, but research largely stalled following the passage of the federal Controlled Substances Act in 1970.
One of the central questions in psychedelic research is whether the psychedelic experience itself is necessary for these lasting changes in brain connectivity.
Jacob Baker, Ph.D. student
In recent years, renewed scientific interest has focused attention on psychedelics as “psychoplastogens”—compounds capable of rapidly stimulating structural and functional neural plasticity after a single dose. Recognizing this therapeutic promise, the U.S. Food and Drug Administration (FDA) previously granted Breakthrough Therapy designations for substances including psilocybin and MDMA.
This momentum reached a major milestone last month, when the FDA hosted a public hearing titled “Considerations for Potential Future Therapeutic Use of Psychedelic Drugs.” The hearing brought in experts to establish standardized frameworks for provider credentialing, protocols for Risk Evaluation and Mitigation Strategy (REMS), and clinical-trial designs. As federal regulatory agencies move toward integrating psychedelic-assisted therapies into healthcare, understanding the precise cellular mechanics of how these drugs heal brain circuits has become a major emphasis in biological research.
Watching neural connections rewire in real time
To observe how psilocybin alters neural architecture at the microscopic level, the UC Santa Cruz research team used two-photon microscopy to image neurons in the living brain. This high-resolution imaging technique allowed the researchers to track individual dendritic spines on living mouse brain cells across several weeks.
Dendritic spines are tiny, bulbous protrusions branching off brain cells that act as signal-receiving docks, forming communication bridges called synapses with neighboring neurons. When psilocybin was administered, researchers observed a rapid increase in the formation of new dendritic spines within 24 hours.
Furthermore, the team found that psilocybin accelerated the physical maturation of these new connections. In untreated mice, new spines typically start as immature “stubby” shapes that remain unstable and short-lived. Under psilocybin treatment, a significantly higher proportion of new spines rapidly grew into large “mushroom” shapes—mature, stable structures equipped with abundant neurotransmitter receptors capable of holding strong, enduring memory and mood signals.
One of the central questions in psychedelic research is whether the psychedelic experience itself is necessary for these lasting changes in brain connectivity, according to Jacob Baker, the study’s lead author. “Psychedelics are fascinating compounds because they have the potential to profoundly transform who we are by reshaping the neural circuits in our brains,” said Baker, a Ph.D. student in Zuo’s lab. “These newly formed connections can integrate into existing neural circuitry and persist over time, leaving a lasting structural trace of psilocybin’s effects on the brain.”
Restoring circuits in healthy versus stressed brains
A key discovery of the study is that psilocybin’s impact depends on the brain’s prior exposure to chronic stress. In healthy control mice, psilocybin induced a temporary boost in new connection formation, followed by a gradual return toward baseline over three weeks as older connections existing before psilocybin treatment were eliminated. This balance ensured that the healthy brain maintained structural stability while updating its circuit wiring.
However, when the team examined mice subjected to chronic stress—using an unpredictable mild stress model that mimics human depressive states—psilocybin behaved markedly differently. Chronic stress normally causes severe synapse loss and circuit decay. In stress-affected brains, psilocybin sparked a significantly higher surge in new spine formation while simultaneously halting the excessive loss of existing connections.
This nuance reframes how neuroscientists evaluate psychedelic therapeutics. “These findings challenge the prevailing view that psilocybin simply increases neural connectivity and suggest that its effects depend on the brain’s underlying condition,” Zuo added. “This provides a new perspective on how psychedelics might help restore neural circuits disrupted by stress, with potentially important implications for understanding their therapeutic effects in depression and other psychiatric disorders.”
Decoupling synaptic plasticity from hallucinogenic-like responses
Historically, scientists have debated whether the intense perceptual alterations and hallucinations caused by psychedelics are required for their long-term therapeutic benefits. To resolve this question at the cellular level, the team performed genetic experiments focusing on serotonin 2A receptors (5-HT2ARs)—the chemical docking stations on cell membranes that interact with psilocybin.
The researchers targeted Layer 5 pyramidal neurons, principal output cells in the cerebral cortex known to heavily express serotonin 2A receptors. Using specialized mouse lines, the team manipulated the receptors specifically within these Layer 5 cells.
When serotonin 2A receptors were restored exclusively in Layer 5 pyramidal neurons, psilocybin successfully triggered new synapse formation and structural stabilization. However, these same mice failed to exhibit the head-twitch response—a movement in rodents widely recognized as the standard biological proxy for hallucinogenic activity.
Conversely, when serotonin 2A receptors were selectively deleted from Layer 5 pyramidal neurons, psilocybin could no longer induce structural synapse growth, yet the head-twitch behavior remained completely intact.
This double dissociation proves that serotonin 2A signaling in Layer 5 pyramidal neurons is both necessary and sufficient for psilocybin’s structural neuroplasticity, but entirely dispensable for its hallucinogenic behavioral output.
“By showing that therapeutic circuit remodeling and hallucinogenic effects operate through distinct cellular pathways, our study provides a promising blueprint for designing non-hallucinogenic drug analogs,” Zuo said. “Such therapeutics could deliver the fast and lasting antidepressant benefits of psychedelics while eliminating hallucinogenic side effects—making neuroplasticity treatments safer, more scalable, and accessible to a broader patient population.”
In addition to Zuo, other molecular and cell biologists at UC Santa Cruz who co-authored the study include Jacob Baker, Emily Kogan, Shaorong Ma, and Ju Lu. This research was supported by grants from the National Institute of Mental Health and the National Institute on Aging. Psilocybin was provided by the National Institute on Drug Abuse (NIDA) Drug Supply Program.