More than 200 mushrooms – primarily those belonging to a genus of gilled mushrooms called Psilocybe – contain the psychoactive compound psilocybin. In the brain of mammals, this chemical can bind to serotonin receptors and influence behavior and emotions, including aggression, appetite, and mood. Its effects on the social behavior of animals, however, remain largely undescribed.
In a new Frontiers in Behavioral Neuroscience study, researchers in Canada have tested whether the effects of psilocybin extend to the social behavior of the amphibious mangrove rivulus fish (Kryptolebias marmoratus).
"We show that an acute, low dose of psilocybin significantly reduces activity and aggressive attack behavior during social interactions in adult mangrove rivulus fish, a species that is naturally highly aggressive," said first author Dayna Forsyth, a research associate and former MSc student at Acadia University in Nova Scotia.
"These findings provide the first evidence that psilocybin can selectively reduce escalated aggression in a vertebrate model without suppressing social interaction," added senior author Dr Suzie Currie, a biologist at The University of British Columbia.
Calm waters
Mangrove rivulus fish are innately aggressive, especially when paired with another individual. Their aggressive behaviors are straightforward and subtle changes can easily be detected. These fish are also self-fertilizing and produce embryos that are genetically identical. Therefore, this model ensures all observed effects are caused by psilocybin treatment rather than genetic differences between fish.
The team used three genetically distinct, laboratory-bred lines. Fish from one line were exposed to psilocybin, fish from a second served as stimulus fish. A third line was used to quantify whole-body concentrations and absorption of psilocybin.
For the first phase of the experiment, the focal fish was added into a tank containing a stimulus fish to measure baseline behavior. The fish were separated by an opaque cover placed over a fiberglass mesh barrier through which the fish could see and smell, but not reach, each other. After a five-minute adjustment period to the shared tank, the opaque barrier was removed and interaction monitored.
24 hours later, the same focal fish was put in a water tank in which psilocybin was dissolved. After exposure to the substance for 20 minutes, the fish was added into the tank occupied by the same stimulus fish of the day before. After removal of the opaque barrier, interaction was observed again.
Magic mushroom, mellow fish
Observation of behaviors to measure activity (time spent moving) and aggression levels (including swimming bursts) revealed that fish dosed with psilocybin showed decreased levels of activity and performed fewer swimming bursts compared to specimen that hadn't received psilocybin treatment.
"Swimming bursts are high‑energy attack behaviors that represent an escalation of aggression towards the stimulus fish without making physical contact," explained Currie. "Other types of aggressive behaviors, like head‑on displays, are more about communication and social assessment and require very little energy."
"Psilocybin's calming effect appears to selectively reduce energetically costly, escalated behaviors while lower‑energy social display behaviors remained largely unchanged," said Forsyth. "This suggests that this compound can selectively dampen escalated social conflict rather than shutting down behavior altogether."
Psilocybin also influenced activity levels, with dosed fish spending less time moving than control fish when paired with a conspecific.
Diving deeper
In the long run, non-human models in drug-screening experiments like this can provide robust results that can later be translated to humans. In the future, findings like those made here could help inform therapeutic research by clarifying which aspects of social behavior are most sensitive to psilocybin. The team cautioned, however, that the current study did not test clinical treatments and results from fish cannot be directly extrapolated to humans.
The study also focused on single doses and short periods of exposure, and didn't examine long-term effects, repeated dosing, or adaptation over time. Future studies are needed to confirm whether the lower level of aggression observed here can be sustained.
"Future studies can build on this work to explore how psilocybin alters neural signaling, which serotonin pathways are involved, and why some aspects of social behavior are affected while others are not," concluded Currie. "These are questions that are difficult or impossible to answer directly in humans."
