Psychedelic Neurobiology: Sex-Specific Effects of MDMA & Psilocybin in Addiction & Reward Behavior | 277

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Feb 5, 2026

Javier Gonzalez-Maeso, PhD, a pharmacology professor studying GPCR signaling and psychedelic neurobiology. He talks about how biased agonism makes similar drugs act very differently. He explains isomers and enantiomers in MDMA and LSD. He highlights sex-specific effects of MDMA and psilocybin in animal models and implications for clinical trials and dosing.

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INSIGHT

S‑MDMA Drives Spine Growth In Males Only

In mice only S‑MDMA induced head-twitches and increased frontal cortex dendritic spines at the tested dose; R‑MDMA did not.
The spine increase occurred in males but not females, indicating isomer- and sex-specific plasticity.

INSIGHT

MDMA Activates 5‑HT2A Indirectly Via Serotonin Release

MDMA's activation of 5-HT2A is likely indirect: MDMA causes massive serotonin release via the serotonin transporter.
Blocking the serotonin transporter (with fluoxetine) prevents S‑MDMA head-twitches, supporting an indirect mechanism.

ADVICE

Design Trials With Sex And Weight In Mind

Clinical trials should power for sex as a variable and adjust dosing for body weight instead of fixed doses.
Without weight correction and sex-stratified samples, trials risk missing important sex- and isomer-specific effects.

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Pharmacology & neurobiology of psychedelics & MDMA, focusing on isomers, sex-specific effects, and mechanisms in animal models. Not medical advice.

TOPICS DISCUSSED:

Biased agonism: Different drugs activate the same receptor (e.g., 5-HT2A) but trigger varied intracellular pathways, explaining why LSD is psychedelic while similar lisuride is not.
Enantiomers & isomers: Mirror-image versions of drugs like MDMA (S and R forms) and LSD (four isomers) often produce distinct effects; only one LSD isomer is psychedelic, for example.
MDMA isomer effects: S-MDMA induces stronger head twitches (psychedelic proxy) via serotonin release, and increases dendritic spines in male mice but not females; R-MDMA has somewhat different effects.
Sex-specific drug responses: In mice, females show stronger psychedelic effects (head twitches) from psilocybin and DOI at the same dose, but males exhibit greater post-acute benefits like reduced opioid withdrawal.
Psilocybin in opioid addiction models: A single dose reduces place preference for oxycodone and withdrawal symptoms in male mice more than females, with opposite spine density effects in reward-related brain areas.
Mechanisms beyond 5-HT2A: Psychedelics involve other receptors like metabotropic glutamate receptors, forming dimers with 5-HT2A to enable dual signaling pathways; effects in subcortical regions like nucleus accumbens are 5-HT2A-independent.
Clinical implications: Street MDMA may vary in S/R ratios, affecting experiences; clinical trials often use racemic mixtures without weight-adjusted dosing, potentially missing sex differences.

ABOUT THE GUEST: Javier Gonzalez-Maeso, PhD is a professor of pharmacology and toxicology at Virginia Commonwealth University, with a PhD in medicine from Spain focused on G-protein coupled receptors and human brain studies in depression and addiction.

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