Stereoselective, sex-dependent 5-HT_2A receptor modulation of cortical plasticity by MDMA in mice

The psychoactive entactogen 3,4-methylenedioxymethamphetamine (MDMA), widely known as a recreational drug, is gaining renewed attention as a potential psychotherapeutic adjunct for treatment-resistant psychiatric disorders, yet its neurobiological mechanisms - particularly those related to its stereoisomers and sex-specific effects - remain poorly understood. Here, we report stereoselective and sex-dependent actions of MDMA on serotonin (or 5-hydroxytryptamine) 2A receptor (5-HT_2AR)-mediated signaling and dendritic structural plasticity in mouse frontal cortex. Using both in vitro and in vivo approaches, we found that racemic MDMA and S(+)-MDMA exhibit weak partial agonism at 5-HT_2AR in HEK293 cells, whereas R(-)-MDMA shows negligible functional activity despite higher specific binding affinity. In vivo, S(+)-MDMA elicited a dose-dependent head-twitch response (HTR) in both sexes, while R(-)-MDMA-induced HTR only in females. Correspondingly, S(+)-MDMA increased inositol monophosphate (IP₁) accumulation in the frontal cortex of male and female mice, whereas R(-)-MDMA showed minimal effects. Structurally, S(+)-MDMA enhanced dendritic spine density in male frontal cortex in a partially 5-HT_2AR-dependent manner, while no spine remodeling was observed in females or with R(-)-MDMA. Pharmacological blockade of the serotonin transporter (SERT) with fluoxetine fully prevented S(+)-MDMA-induced HTR and IP₁ signaling, without affecting responses to the direct 5-HT_2AR agonist DOI. These findings indicate that MDMA engages 5-HT_2AR signaling indirectly via serotonin efflux and that this effect is both stereoselective and sex-dependent in mice, uncovering a previously unrecognized interaction between sex, MDMA stereochemistry, and 5-HT_2AR-mediated cortical plasticity, with important implications for the rational design of MDMA-based therapeutics.