An exploration of the relationships between the effects of psilocybin on behavior, 5-HT 2A receptor occupancy, and neuroplastic effects in mice

Psilocybin produces rapid and sustained antidepressant effects in major depressive disorder, but the underlying neurobiological mechanisms are unclear. In mice, psilocybin caused dose-dependent occupancy of the 5-HT₂A receptor in the prefrontal cortex, with an inverted-U dose-response for head twitch behavior peaking between 44% and 62% receptor occupancy. A 1.5 mg/kg dose increased time spent in open areas of the elevated zero maze, indicating reduced anxiety, while 3 mg/kg reduced immobility in the forced swim test, suggesting antidepressant-like effects. Both doses shifted α-tubulin post-translational modifications toward more dynamic microtubules and selectively increased synaptic protein expression in the prefrontal cortex, but not the amygdala. These findings indicate that psilocybin's therapeutic effects may involve dose- and region-specific enhancement of neuronal plasticity, with distinct signatures for anxiolytic-like and antidepressant-like properties.

Study at a glance

Abstract

Background: Psilocybin has shown rapid and sustained antidepressant effects in patients with major depressive disorder, yet the neurobiological mechanisms underlying these outcomes remain unclear. Aims: This study aimed to bridge clinical and preclinical findings by investigating the relationships between 5-HT 2A receptor occupancy (RO) achieved after administration of psilocybin and its effects on behavior and markers of neuroplasticity in mice. Methods: In vivo 5-HT 2A RO was determined via displacement of [ 3 H]MDL-100,907 in the prefrontal cortex (PFC). To relate RO with behavioral outcomes of psilocybin, we assessed the head twitch response (HTR) acutely and investigated behavior in the elevated zero maze (EZM) and forced swim test (FST) 20–24 hours post-drug. Neuroplastic changes were assessed by measuring α-tubulin post-translational modifications (PTMs) and expression of key synaptic proteins in both the PFC and amygdala. Results: Psilocybin produced dose-dependent 5-HT 2A RO (RO₅₀ = 0.88 mg/kg) and an inverted-U dose–response in HTR, with peak effects occurring between ~44% and 62% RO. Behaviorally, a 1.5 mg/kg dose increased the open areas ratio in the EZM, while 3 mg/kg reduced immobility in the FST, 20 and 24 hours after dosing, respectively. Both dose levels shifted α-tubulin PTMs toward a more dynamic microtubule state and selectively increased synaptic marker expression in the PFC, not in the amygdala. Conclusions: These findings suggest that the therapeutic effects of psilocybin could be mediated by dose- and region-specific enhancement of neuronal plasticity, with distinct signatures associated with anxiolytic-like and antidepressant-like properties.