Psychedelics produce enduring behavioral effects and functional plasticity through mechanisms independent of structural plasticity

Activation of serotonin 2A (5-HT2A) receptors is thought to underly the long-lasting antidepressant effects of psychedelics such as psilocybin, but beyond that, the molecular and cellular mechanisms involved are not well understood. Recent preclinical studies using mice have primarily examined relatively short time points after psychedelic administration, which does not address the long-lasting effects of psilocybin in humans (i.e., several months or more). We utilized a rat experimental system to demonstrate that both psilocybin and the selective 5-HT2A receptor agonist 25CN-NBOH reduce immobility in the forced swim test without a decrease in effect size for at least three months after a single administration of the psychedelic. There were no overt behavioral differences between psilocybin and 25CN-NBOH treated animals, suggesting 5-HT2A receptor activation is sufficient to produce long-lasting behavioral changes. Functional cellular plasticity in neurons from the medial prefrontal cortex (mPFC) of these animals was assessed using brain slice electrophysiology. Functional plasticity was evident for both psychedelics several months after treatment, and Layer 5 excitatory pyramidal neurons demonstrated significant changes in resting membrane potential, firing rates, and synaptic excitation. Recorded neurons were examined by microscopy for synaptic density and spine classification, which found no differences between control and psychedelic-treated. Gene expression studies for several presynaptic and postsynaptic markers in the mPFC indicated no differences in expression between groups. Together, our results indicate a single treatment with a psychedelic is sufficient to elicit very long-lasting behavioral and cellular changes through enduring function plasticity rather than structural plasticity.