Psilocybin produces different behavioral and brain-altering effects depending on the dose

Different doses of psilocybin produce distinct behavioral and brain-altering effects in mice, according to new research published in the Journal of Psychopharmacology. The study suggests that moderate doses of the compound are linked to reduced anxiety, while higher doses tend to produce antidepressant-like effects and increase signs of new neural connections. These findings help clarify how different levels of brain receptor engagement could eventually guide more precise treatments for human psychiatric conditions.

Psilocybin is the primary psychoactive compound found in certain species of magic mushrooms. In the human body, it is broken down into psilocin, which activates specific serotonin receptors in the brain to produce altered states of consciousness. Recent clinical trials suggest that just one or two doses of psilocybin can provide rapid and long-lasting relief for patients with major depressive disorder.

Despite these encouraging clinical outcomes, the exact biological mechanisms that make psilocybin an effective antidepressant over the long term remain somewhat unclear. The drug leaves the body within hours, yet its mood-boosting effects can persist for weeks or months.

“Psilocybin has shown remarkably rapid and long-lasting antidepressant effects in clinical trials, but we still don’t fully understand how a drug that leaves the body within hours can produce therapeutic benefits that last for weeks or even months,” explained study author Connor Maltby, the head of Translational Medicine at Ulysses Neuroscience Ltd.

“One major hypothesis is that psychedelics work by enhancing neuroplasticity, but the relationship between receptor activation, behavioral effects, and downstream biological changes hasn’t been clearly mapped out. We wanted to understand how the degree of engagement of the brain’s primary psychedelic target (the 5-HT 2A ) receptor relates to both behavioral outcomes and molecular markers of plasticity in specific brain regions.”

“This kind of mechanistic understanding is critical if we want to move beyond empirical dosing toward rational development of psychedelic-inspired treatments for neuropsychiatric disorders. In other words, we’re trying to move the field from ‘psychedelics work’ to ‘how much receptor activation is needed to produce which kind of therapeutic effect?’”

The researchers conducted a series of experiments using male mice. They first measured receptor occupancy, which refers to the percentage of specific serotonin receptors actively bound by the drug. The scientists injected six groups of six mice with varying doses of psilocybin, ranging from 0.1 to 10 milligrams per kilogram of body weight, alongside a placebo group. Using a safe radioactive tracer, they calculated how many serotonin receptors were occupied in the prefrontal cortex 30 minutes after the injection.

Next, the team evaluated a behavior known as the head twitch response. In rodents, a specific rapid head movement serves as a reliable physical indicator that a drug is causing hallucinogenic-like effects. The researchers placed groups of 12 mice into observation chambers and used high-speed cameras and artificial intelligence tracking to count the head twitches over 20 minutes.

The data indicates a curved relationship between the dose and the total number of head twitches, with the total count peaking at a moderate dose of 1 milligram per kilogram. However, the highest rapid rate of twitches per minute occurred at a higher dose of 3.2 milligrams per kilogram, matching a 62 percent receptor occupancy. High doses of the drug also tended to reduce the overall physical movement of the mice.

To test for lasting behavioral changes, the scientists administered varying doses of psilocybin to groups of eight to ten mice before placing them in an elevated maze with open and enclosed walkways. Because mice naturally prefer enclosed spaces, spending more time in the open walkways provides evidence of reduced anxiety. The moderate dose of 1.5 milligrams per kilogram increased the time mice spent in the open areas, while the high dose of 3 milligrams per kilogram did not.

Four hours after the maze test, the researchers used a forced swim test to measure depression-like behavior. Mice were placed in small cylinders of water for six minutes, and researchers recorded how long they spent floating passively rather than actively swimming. In this test, the high dose of psilocybin reduced the time mice spent floating, while the moderate dose showed no effect.

“While this was a preclinical study in mice, one encouraging finding was that the levels of 5-HT 2A receptor engagement associated with behavioral and plasticity-related effects in our experiments were broadly consistent with those linked to subjective and therapeutic effects in human imaging studies,” Maltby told PsyPost.

“That kind of cross-species alignment suggests that receptor occupancy may eventually serve as a useful biological framework for understanding and potentially optimising dosing in clinical settings. Rather than relying solely on subjective experience, future treatments might be guided by measurable engagement of specific neural targets.”

Finally, the scientists examined the brains of the mice to look for signs of neuroplasticity. They focused on microtubules, which are microscopic tube-like structures that help brain cells maintain their shape and grow new connection branches. They also measured specific proteins that indicate the presence of synapses, which are the communication junctions between brain cells.

Both the moderate and high doses of psilocybin changed the chemical structure of the microtubules, making them more dynamic and capable of remodeling. This increased structural flexibility occurred in both the prefrontal cortex, a brain region involved in complex thought, and the amygdala, a region tied to fear and emotional processing.

The researchers found that actual increases in synaptic proteins only occurred in the prefrontal cortex. Both the moderate and high doses led to higher levels of proteins associated with new cellular connections in this area. The amygdala did not show any increase in synaptic proteins, which the researchers suspect might act as a biological safety mechanism to prevent the formation of anxiety-inducing pathways.

“One interesting finding was that psilocybin increased markers of synaptic plasticity in the prefrontal cortex but not in the amygdala, even though both regions showed signs of increased microtubule dynamics,” Maltby said. “This suggested that different brain regions may respond to psychedelic-induced receptor activation in distinct ways, potentially supporting different therapeutic outcomes such as anxiolytic versus antidepressant effects. It highlights that the brain’s response to psychedelics may be region-specific rather than globally uniform.”

These findings highlight a common misconception that a stronger psychedelic experience automatically leads to better therapeutic benefits. The study provides evidence that different levels of receptor engagement trigger distinctly different biological and behavioral outcomes. Moderate doses seem better suited for reducing anxiety-like behaviors, while higher doses appear necessary for alleviating depression-like symptoms.

“One common misconception is that the intensity of the psychedelic experience is necessarily tied to therapeutic benefit,” Maltby told PsyPost. “Our findings support a more nuanced view, that different degrees of receptor engagement may produce distinct biological and behavioral outcomes.”

While animal models cannot perfectly replicate complex human emotions, they serve as an essential step in scientific discovery. A mouse cannot communicate feelings of sadness or anxiety, meaning researchers must rely on observable behaviors as stand-ins for human psychological conditions.

Despite this limitation, studying animals provides evidence about how drugs physically interact with a living nervous system. These experiments allow scientists to examine structural changes in brain cells and receptor activity in a highly controlled setting, which tends to guide safer and more precise clinical trials in humans.

“It’s important to note that behavioral assays in animals are best viewed as pharmacodynamic readouts rather than direct models of human depression or anxiety,” Maltby said. “They help us understand the biology involved, but translating those findings into clinical treatment still requires careful human studies.”

The scientists plan to expand this line of research by testing psilocybin in animal models that undergo chronic stress or inflammation. These environmental factors mimic the biological features of clinical depression much closer than the baseline conditions used in the current study.

“Ultimately, the goal is to understand whether specific levels of receptor activation can be linked to particular therapeutic outcomes, which could inform the development of next-generation psychedelic or psychedelic-derived compounds with improved safety and precision, as well as reduced psychoactivity (hallucinations),” Maltby added.

“A key takeaway from this work is that psychedelic compounds may promote long-lasting therapeutic effects by triggering structural and functional changes in neurons, particularly in brain regions involved in mood regulation. By linking receptor engagement to both behavioral effects and molecular markers of plasticity, we hope this study contributes to a more mechanistic understanding of how these compounds work—which will be essential for translating early clinical promise into scalable treatments for neuropsychiatric disorders.”

The study, “An exploration of the relationships between the effects of psilocybin on behavior, 5-HT2A receptor occupancy, and neuroplastic effects in mice,” was authored by Connor J. Maltby, Adam K. Klein, Enya Paschen, Jessica Pinto, Dino Dvorak, Joseph R. Hedde, Ashley N. Hanks, Massimiliano Bianchi, and Zoë A. Hughes.