Experimental trials of psychedelic drugs in both humans and animals have produced conflicting results, creating doubts about whether hallucinogens have a therapeutic role in treating neurologic or behavioral disorders such as depression and PTSD.
To establish a baseline for studying the effects of these drugs on the brain, neuroscientists across five labs at three Bay Area institutions - UC Berkeley, UCSF and Stanford University - conducted identical, parallel tests of psilocybin's effects on mice. The goal was to establish which behavioral effects of the drug are reproducible across a variety of lab settings and worthy of further study.
After pooling data from the five experiments, the researchers found no lasting behavioral changes 24 hours after a single dose of psilocybin. Though long-term effects have been reported in both mouse and human experiments, the researchers found no replicable reduction in anxiety or depression-like behaviors nor any impact on fear responses or social interactions in mice a day after their trip.
Some acute effects of psilocybin were replicable, however. All labs reported increased anxiety and avoidance behavior in mice 10 minutes after injection, as well as a decrease in the expression of fear.
The researchers also tested whether psilocybin reopens a critical period for social reward learning in adult mice, as has been reported in earlier studies. They found no evidence for this.
"One of our main findings is that, at least with our experiments and doses used, the persistent effects of psilocybin across these behaviors was modest, if anything," said Odilia Lu, a Berkeley doctoral student and co-first author of the paper. "This really highlights the importance of multi-lab collaborations, such as this one, to better estimate a true underlying effect size accurately and reduce the risk of publication bias - that is, that positive results from single experiments, often with small sample sizes, are biasing the literature."
Nevertheless, the researchers say the robust and replicable acute results are a first step toward understanding how the drug affects neuronal circuits in the brain - something hard to study in humans.
"Initial very small clinical studies have shown that psilocybin could be a potential treatment for these diseases like depression and PTSD, but so far we know very, very little about what it actually does to the brain," said Stephan Lammel, a UC Berkeley professor of neuroscience who runs one of the labs involved in the study. "Once we understand what are really the reproducible effects of psilocybin on behavior, then we can study the mechanism and how it affects behavior in the brain. That was the idea behind this study."
The results of the individual experiments and their combined findings will be published in the Nov. 2 issue of the journal Nature Neuroscience.
Guidance for future research in mice
The researchers hope that the findings will provide guideposts for researchers, encouraging a focus on reproducible effects of psilocybin - the acute suppression of fear, for example - while deemphasizing research on effects that cannot be replicated, such as improved socialization.
Courtesy of Odilia Lu
"The fact that psilocybin did have acute effects in these animals demonstrated something is happening in the brains of these animals," said Andrea Gomez, a Berkeley assistant professor of molecular and cell biology who leads one of the five labs. "Where in the brain, we don't know. What are the more important circuits in the brain that are involved in this, and whether or not those specific circuits have some type of measurable outcome later on, I think really are the big questions."
Though the parallel studies employed identical protocols, the findings from individual labs often differed in statistical significance, highlighting the difficulty of reaping repeatable results when administering psychedelic drugs. While the pooled data showed no lasting effects on depression, for example, two of the labs saw a significant effect in one of two assays frequently used to assess the impact of antidepressants on mice.
But this may say more about how appropriate the assays are for judging depression in mice than about psilocybin's therapeutic potential in humans. Mice are considered depressed if they more readily stop swimming when immersed in a pool of water, as if they have given up.
"I don't necessarily think we need to move away from focusing on depression, because it seems like psilocybin is most promising for the treatment of the disorder," Lu said. "A lot of the behavioral tasks that we use in mice to assay depression just can't capture the really complicated human."
The complexity of predicting a human response to hallucinogens is well known in the field of psychedelics - it's often referred to as "set and setting," where the effect depends on a person's mindset and the social setting. The consortium's research protocol, employing five independent experiments, was designed to tease out which effects are replicable across different setups, with different people and different handling, and which are not. Lammel, Gomez and Lu believe that researchers studying psychedelics should broadly adopt parallel studies.
"It's an urgent call to the field," Lammel said. "It means, hey, we understand the excitement about psychedelic drugs and we have seen their therapeutic effects in humans, but let's all work together as a group, not individually, to solve this and really leverage the potential these drugs might have."
"That is the path to using psychedelics safely and effectively," added co-author Vikaas Sohal, a UCSF psychiatrist who works with patients as well as in the lab. "There's a real risk that if the hype around psychedelics outruns the science, there will be adverse outcomes that set the whole field back. That's why it's so critical to work collaboratively across fields and institutions to understand how these substances work and which of their effects are robust and reproducible."
The problem of replicability
The topic of reproducibility of scientific findings is particularly timely in that Jay Bhattacharya, the director of the National Institutes of Health, recently authored an editorial in the journal Science pointing out the critical importance of supporting investigators who attempt to independently replicate prominent findings.
Brandon Sanchez Mejia for UC Berkeley
The idea of parallel trials was proposed by Stanford neuroscientist Robert Malenka, who enlisted the help of five other researchers in the field: Lammel and Gomez at Berkeley, Sohal and Mazen Kheirbek at UCSF and Boris Heifets at Stanford.
According to Lammel, the consortium members attribute much of the confusion in the field of psychedelic research to the proliferation of individual studies with different protocols, different dosages and different assays, making comparison difficult. This is exacerbated by the fact that failures to replicate the results of many studies are not being shared - journals rarely publish negative results, and negative results do not lead to academic tenure.
"After our paper went on bioRxiv (a preprint server) last year, I think it gave validation to a lot of other groups who failed to reproduce some of these behaviors," Gomez said.
Counter to this "whisper network" of unpublished results, as Gomez described it, the consortium members planned to publish both their replicable and nonreplicable results. Together they secured the financial support of an anonymous donor for the four-year project. The labs spent a year getting federal approval to use psilocybin, and the experiments commenced in 2022.
"The structure is very unique and probably the first time in academia that this approach has been taken in order to figure out what are the reproducible effects of psychedelics," Lammel said. "By doing this, we'll probably save a lot of time to get more directly to the really important thing, which is the therapeutic potential of psilocybin and its mechanism that we want to understand."
Gomez said that the variability of the results from the five labs did not surprise her, considering that behavioral neuroscience research has repeatedly shown that context matters.
"In traditional medicinal use of psychedelics, a person experiencing the drug is within a community and they go back to their community after the trip and interact. The healing part is connected to the integration part," said Gomez, a member of the Laguna Pueblo tribe. "How do we fit an Indigenous methodology that has worked for thousands of years to a Western type of structure?"
If the traditional context is hard to reproduce in a Western medical setting, it's even harder to replicate in an animal experiment. "What is the mouse equivalent of wearing a tie-dye t-shirt?" she often jokes in her lab.
"How do you model the hallucinogenic aspects of a trip in an animal that is separated (from humans) by millions of years of evolution? How do you model higher order cognitive changes like those experienced by humans?" Gomez said.
The problem of variability in animal behavior
In the five experiments, head twitching was the universal behavior seen in mice injected with psilocybin (2 milligrams per kilogram of weight). Heightened anxiety and avoidance of open spaces were also reproducible effects. Were these signs they were hallucinating?
Christian Lüscher
Other behaviors reported in previous studies were not reproducible across the five experiments, though there were variations.
Through a statistical analysis, the researchers traced the differing outcomes in different labs to unavoidable variations in each lab's research protocol: the sex of the handler, how long the mice had been acclimated to handling, the companies from which the mice were purchased, the size of experimental enclosures and even the time of day the drugs were administered. Ultimately, these variables were not shown to influence psilocybin's effects as measured across all five labs.
Gomez plans to continue to study the physiological responses of mice to psilocybin to tease out the neural networks it impacts, in particular within the brain's prefrontal cortex. Lammel is interested in exploring the effects of repeated doses of psilocybin.
"The broader story is the importance of building reproducibility into psychedelic research from the outset," Lammel said. "Mouse studies inform our understanding of how psychedelics might work therapeutically, so establishing which findings hold up across labs is essential. Our collaboration across Berkeley, UCSF and Stanford puts this principle into practice and offers a model that could extend to other areas of basic research."
