ABSTRACT / PROJECT SUMMARY Chronic stress is thought to play a role in multiple neuropsychiatric disorders. Standard pharmacological treatments for stress-related disorders can take up to several months to elicit a therapeutic response and can produce long-term undesirable off-target effects. A single dose of the psychedelic drug psilocybin has been shown to rapidly promote long-lasting therapeutic effects in humans and in chronically stressed rodents. However, the neural circuit mechanisms underlying the lasting changes induced by psilocybin in healthy and chronically stressed brains remain unknown. The prefrontal cortex (PFC) is a key structure impacted by chronic stress. Decreased volume, hypoactivity, and impaired functional connectivity of the PFC has been observed in humans with stress-related disorders. Similarly, PFC pyramidal cells in chronically stressed rodents exhibit dendritic atrophy and excitatory synapse loss. Psilocybin enhances expression of neuroplasticity-related genes through a cascade involving the activation of the serotonin 5-HT2A receptor (5-HT2AR). While the 5-HT2AR is expressed postsynaptically in many cell-types, including PFC pyramidal cells, presynaptic 5-HT2ARs are also known to regulate synaptic input to PFC pyramidal neurons. Psilocybin induces dendritic growth and increases dendritic spine density in PFC pyramidal cells after a single dose. However, no studies to date have examined the rules governing which synapses (and which corresponding dendritic spines) are restored: does psilocybin non-specifically increase spine and synapse number, or does it preferentially enhance spines and synapses corresponding to specific inputs with higher 5-HT2AR expression? Answering this question is a critical step towards a mechanistic understanding of how psilocybin exerts therapeutic effects. To address this gap in knowledge, we will conduct a multiscale investigation of the effects of psilocybin on multiple brain regions. Our central hypothesis is that the synaptic inputs to PFC that are impaired following chronic stress are restored in an input-specific manner by psilocybin. In Aim 1, we will characterize the effects of psilocybin in vivo on input-specific changes in PFC connectivity and dynamics in chronically stressed rodents. In Aim 2, we will use both in vivo and ex vivo optogenetics to determine the effects of chronic stress and subsequent psilocybin treatment on input-specific synaptic physiology and dendritic morphology. The completion of these Aims will identify input-specific rules governing psilocybin-induced normalization of impaired prefrontal circuits, with important implications for the design of even more precise and efficacious neuropsychiatric therapies with minimal off-target effects.

PROJECT NARRATIVE Chronic stress plays a role in multiple neuropsychiatric disorders, many of which lack highly efficacious treatments. Recently, a single dose of the psychedelic drug psilocybin has been shown to promote long-lasting therapeutic effects in humans, but the underlying mechanisms remain poorly understood. Here, we elucidate the rules governing psilocybin’s repair of specific neural circuits in chronically stressed rodents, with the potential to allow for the design of even more precise and efficacious neuropsychiatric therapies with minimal off-target effects.