Psilocybin, a serotonergic compound that produces psychedelic effects primarily through activation of the 5-HTreceptor, has shown promise in treating neuropsychiatric conditions, including obsessive-compulsive disorder (OCD). However, the effects of chronic psilocybin administration on gut function, microbiota, and behavioural phenotypes remain understudied. The present study investigated the effects of chronic psilocybin (0.1 and 1 mg/kg, oral gavage) on gut and behavioural measures in wild-type (WT) and SAPAP3 knockout (KO) mice, a model of OCD-like phenotypes. We present novel evidence that SAPAP3 KO mice exhibit social deficits, and that chronic psilocybin increases sociability in male WT mice. Although no therapeutic effects were observed at either dose on anxiety-, compulsive-, or depressive-like behaviour, chronic psilocybin also did not induce psychosis-like behaviours. A dose-dependent effect of psilocybin was observed on gut motility. Although chronic administration did not significantly alter overall gut microbiome diversity, reductions in Lactobacillus murinus, Lactobacillus animalis, and Alistipes dispar were observed in male WT mice, but not in KO mice or female mice. Integrative analysis revealed that a microbial cluster, comprising Lactobacillus and Alistipes species, correlated with locomotion, head-twitch response and gut motility, effectively differentiating psilocybin-treated mice from vehicle controls. This suggests a potential host-microbiome feedback mechanism regulating host serotonin signalling, linked to central and peripheral 5-HTreceptor activation. Additionally, separate microbial clusters were associated with startle response and sociability, indicating that psilocybin may engage distinct neural pathways to mediate these behaviours. These findings highlight the importance of considering the microbiome and sex in future psychedelic research and open new avenues for exploring the microbiota-gut-brain axis as a target for future therapeutic strategies. 2A2A
