Scientists pinpoint cellular mechanism behind psilocin’s effects on brain activity

A new study provides a detailed look at how the psychedelic compound psilocin acts on specific neurons within the brain. The research suggests that psilocin directly excites a particular population of brain cells in the medial prefrontal cortex, a region associated with cognition and mood, by activating a specific receptor and its internal signaling pathway. The findings were published in the Molecular Psychiatry.

Scientists are exploring psychedelic compounds for their potential to treat neuropsychiatric conditions like depression and anxiety. Psilocybin, which the body converts into psilocin, has shown promise in clinical settings. It is understood that these compounds primarily interact with a type of serotonin receptor known as the 5-HT2A receptor.

While brain imaging in humans has shown that psychedelics increase activity in the prefrontal cortex, the precise cellular mechanisms behind this effect have remained largely unclear. The researchers of this study sought to identify which specific neurons are affected by psilocin in this brain region and to map the exact molecular chain of events that leads to changes in their activity.

“Psychedelic compounds are currently being investigated for therapeutic application in a number of psychiatric diseases. Despite promising clinical results, the underlying mechanisms for these drugs are not yet completely understood and preclinical research (like our study) can shed light on the underlying neurobiological effects of these compounds, including psilocybin,” said study author Melissa Herman, an associate professor at the University of North Carolina at Chapel Hill.

“This research was conducted with an MD-PhD student in the lab, Dr. Gavin Schmitz, and in collaboration with my colleague Dr. Bryan Roth who generated the transgenic mice used in the study.”

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To investigate brain-wide effects, the research team first used functional magnetic resonance imaging, or fMRI, on mice. This technique measures changes in blood flow to infer brain activity. Anesthetized mice were administered either a neutral vehicle solution or psilocin at a dose of 2 mg/kg. The fMRI scans revealed that psilocin led to a significant increase in activity in the medial prefrontal cortex, particularly within two subregions known as the prelimbic and anterior cingulate cortices.

Following the brain-wide imaging, the scientists performed experiments on brain slices from mice to examine the activity of individual neurons. Using a technique called electrophysiology, which measures the electrical signals of cells, they first recorded from a general population of neurons called layer V pyramidal cells in the prefrontal cortex.

When psilocin was applied, the response was varied. About half of the neurons increased their firing rate, while around 30% decreased their firing, and the rest showed no change. This mixed result suggested that psilocin has different effects on different types of neurons in this area.

“I was initially surprised that psilocin produced variable effects in non-specified prefrontal cortex neurons (increases, decreases, or no change in activity) but consistently activated 5-HT2A neurons, but then we realized this was likely a central key to how these drugs engage the prefrontal cortex,” Herman told PsyPost.

To narrow their focus, the researchers used a genetically engineered mouse model in which neurons expressing the 5-HT2A receptor were labeled with a fluorescent marker. This allowed them to specifically identify and record from the cells that are the primary target of psychedelics.

When psilocin (10 µM) was applied directly to these identified 5-HT2A neurons, the effect was consistent and clear. The firing rate of these neurons reliably increased, approximately doubling to 200% of their baseline activity. Psilocin also made these neurons intrinsically more excitable, meaning it took less of an electrical stimulus to cause them to fire an action potential.

The researchers then explored whether this increased activity was a direct effect on the 5-HT2A neurons or an indirect effect caused by changes in the surrounding neural network. They measured the small, spontaneous electrical currents that neurons receive from their neighbors, which represent incoming excitatory or inhibitory signals. Psilocin did not alter the frequency or amplitude of these incoming signals, providing evidence that the drug acts directly on the 5-HT2A neurons to increase their excitability, rather than by altering the signals they receive from other cells.

To confirm that the 5-HT2A receptor was responsible for these effects, the team conducted a series of pharmacological experiments. They applied a compound called NBOH-2C-CN (200 nM), which selectively activates only the 5-HT2A receptor. This compound replicated the effects of psilocin, increasing the firing rate of the 5-HT2A neurons.

Next, they used a drug called M100907 (200 nM), which blocks the 5-HT2A receptor. When this blocker was applied before psilocin, the excitatory effect was completely prevented. These experiments together point to the 5-HT2A receptor as the key mediator of psilocin’s effects on these specific neurons.

The team also tested the involvement of a related receptor, the 5-HT2C receptor, which psilocin can also affect. Using a blocker for the 5-HT2C receptor did not prevent psilocin from increasing neuron firing, suggesting this receptor is not involved in the direct excitatory action.

The study also tested a novel, non-hallucinogenic compound that activates the 5-HT2A receptor. This compound also increased the firing of the neurons in a similar manner, suggesting that this mechanism of exciting neurons might be related to the therapeutic potential of these drugs, possibly separate from their hallucinogenic effects.

Finally, the scientists investigated the internal signaling pathway that the 5-HT2A receptor uses to change cell activity. When a receptor on a cell’s surface is activated, it triggers a cascade of events inside the cell. The 5-HT2A receptor is known to signal through a pathway involving a protein called Gαq. The researchers used an inhibitor called FR900359 (1 µM) that specifically blocks Gαq signaling.

When this inhibitor was present, both psilocin and the selective 5-HT2A activator failed to increase the firing of the neurons. This result indicates that the Gαq pathway is a necessary step in the chain of events linking 5-HT2A receptor activation to increased neuronal excitability.

“Psilocin, the active metabolite found in psilocybin, increases activity in the prefrontal cortex by acting at a specific population of neurons that contain the 5-HT2A receptor,” Herman said. “The prefrontal cortex and the 5-HT2A receptor are important for cognitive function and both are implicated in psychiatric disorders so the effects we see could be related to how these drugs may improve symptoms in human patients.”

The study has some limitations. The experiments were conducted in mice, and while animal models provide powerful insights into biological mechanisms, the results do not always translate directly to humans. The sample size for the fMRI portion of the study was also relatively small. Future research could aim to confirm these findings in larger animal cohorts and investigate how these cellular changes in the prefrontal cortex relate to the behavioral and therapeutic effects of psychedelics.

“The long-term goals of this research are to understand how psychedelic compounds change activity and processing across the brain, how these changes are the same (or different) in males and females, and how those changes are impacted by pre-existing conditions like stress or exposure to conditions that produce symptoms related to human psychiatric disease,” Herman explained.

These findings contribute a more detailed picture of how a psychedelic compound like psilocin works at the cellular and molecular level. By showing that psilocin directly excites a specific group of neurons in the prefrontal cortex through the 5-HT2A receptor and the Gαq pathway, this work helps to uncover the neurobiological mechanisms that may be involved in the compound’s potential therapeutic actions.

“Although psychedelics show significant promise for potential therapeutic use, some of the human data may be complicated by placebo effects or expectancy bias and these drugs are exceedingly unlikely to ‘cure everything’ or be effective in all individuals and therapeutic use must be supported by rigorous research,” Herman cautioned.

The study, “Psychedelic compounds directly excite 5-HT2A layer V medial prefrontal cortex neurons through 5-HT2A Gq activation,” was authored by Gavin P. Schmitz, Yi-Ting Chiu, Mia L. Foglesong, Sarah N. Magee, Martin MacKinnon, Gabriele M. König, Evi Kostenis, Li-Ming Hsu, Yen-Yu I. Shih, Bryan L. Roth, and Melissa A. Herman.