High-resolution tracking of unconfined zebrafish behavior reveals stimulatory and anxiolytic effects of psilocybin

Serotonergic psychedelics are emerging therapeutics for psychiatric disorders, yet their underlying mechanisms of action in the brain remain largely elusive. Zebrafish have evolutionarily conserved serotonergic circuits and subcortical targets such as the brainstem regions and the cerebellum, providing a promising model for studying the subcortical effects of serotonergic drugs. Here, we developed a wide-field behavioral tracking system for larval zebrafish and investigated the effects of psilocybin, a psychedelic serotonin receptor agonist. Machine learning analyses of precise body kinematics identified latent behavioral states reflecting spontaneous exploration, visually-driven rapid swimming, and irregular swim patterns following stress exposure. Using this method, we identified two main behavioral effects of acute psilocybin treatment: [i] increased rapid swimming in the absence of visual stimuli and [ii] prevention of irregular swim patterns following stress exposure. Together, these effects indicate that psilocybin induces a brain state that is both stimulatory and anxiolytic. These findings pave the way for using larval zebrafish to elucidate subcortical mechanisms underlying the behavioral effects of serotonergic psychedelics.