Effects of 3,4-Methylenedioxymethamphetamine on Conditioned Fear Extinction and Retention in a Crossover Study in Healthy Subjects

MDMA is being investigated as an adjunct to psychotherapy for PTSD and social anxiety because it produces prosocial and anxiolytic subjective effects and alters emotion processing. Earlier research in animals and a small number of human studies suggested that MDMA may facilitate fear extinction or otherwise alter fear-memory processes, and oxytocin release has been proposed as one possible mediating mechanism. However, findings have been mixed across species and paradigms, and the mechanistic basis of MDMA's putative benefit in exposure-based therapies remains uncertain. Vizeli and colleagues set out to test whether a single acute MDMA dose enhances fear extinction learning and recall in healthy volunteers, and whether peripheral oxytocin release, MDMA plasma levels, or subjective effects predict any such enhancement. The investigators used two established human Pavlovian conditioning paradigms that index learned fear by skin conductance response (SCR) and by fear‑potentiated startle (FPS), employing a double‑blind, placebo‑controlled, crossover design with a 125 mg MDMA dose administered after acquisition and 2 h before extinction learning; extinction recall was tested the next day about 23 h after dosing.

The study was a two‑period, double‑blind, randomised, placebo‑controlled crossover trial in 30 healthy male volunteers (mean age 26 ± 3.3 years). Each participant received placebo and 125 mg MDMA (mean dose 1.69 ± 0.17 mg/kg) in counterbalanced order with a washout >30 days; 29 participants completed both sessions. Key exclusion criteria included major psychiatric history, significant substance use, tobacco smoking >10 cigarettes/day, and physiological nonresponding in screening for skin conductance or acoustic startle. The protocol included pre‑dose fear acquisition, dosing at 10:00, extinction learning during peak MDMA effects (2–2.5 h post‑dose), and extinction recall the following morning (~23–23.5 h post‑dose). Outcome measures were repeatedly assessed across 24 h. Two canonical fear conditioning paradigms were used. The SCR task employed complex fractal images as CS+ (partially reinforced with a 1‑s loud scream US) and CS-, with phases for familiarisation, acquisition (pre‑dose), extinction (2 h post‑dose), and recall (~23 h). SCR amplitudes were recorded from finger electrodes, scored 0.5–6 s after CS onset, z‑transformed per subject/session/phase, and outliers trimmed; five participants identified as SCR nonresponders in the placebo session were excluded from within‑subject SCR analyses and one subject lacked valid responses in both sessions and was excluded entirely from SCR analyses (rmANOVA degrees of freedom indicate an analysis sample of n≈23). The FPS paradigm used air‑puff as the aversive US and measured orbicularis oculi EMG startle responses; acquisition was pre‑dose, extinction learning at 2.5 h post‑dose, and recall at ~23.5 h. Startle scores were baseline‑normalized (NA trials) and block‑averaged, with Winsorisation for extreme values; FPS analyses included the larger sample (rmANOVA df suggests n≈29). Secondary measures included plasma oxytocin at baseline and 2, 3, and 24 h; MDMA, MDA and HMMA pharmacokinetics using HPLC‑MS/MS; autonomic measures (blood pressure, heart rate, tympanic temperature); repeated visual analogue scales (VAS) and mood/anxiety inventories; and a facial emotion recognition task with morphed Ekman faces. Main analyses used repeated‑measures ANOVAs (rmANOVA) with within‑subject factors drug, time, and stimulus type; Pearson correlations explored relationships among subjective effects, pharmacokinetics, oxytocin and task outcomes. Effect sizes are reported as generalized eta squared (ges) and significance was set at p < 0.05.

Fear conditioning was successful in both paradigms: SCR and FPS responses were larger to CS+ than CS- during acquisition. In the SCR task (analysis with df indicating 23 participants), MDMA altered extinction dynamics. During extinction learning under placebo there was clear discrimination between CS+ and CS- that diminished across the session, whereas under MDMA this discrimination was not observed acutely. Most importantly, during extinction recall the day after dosing MDMA was associated with a significantly reduced difference between SCR to CS+ versus CS- compared with placebo (interaction drug × trial type; F(1,22)=5.26, p=0.032, ges=0.06) and the MDMA session showed no CS+ versus CS- difference in early recall (p=0.92) while the placebo session did (p=0.003). The MDMA versus placebo contrast for the early recall CS+–CS- difference was significant (p=0.013), consistent with enhanced retention of extinction measured by SCR. By contrast, FPS measures of conditioned startle (n≈29) showed robust acquisition and extinction across phases but were not affected by MDMA. RmANOVAs revealed main effects of trial type and time in acquisition, extinction learning and recall, but no drug effect on standardized or nonstandardized startle scores; participants' expectancy ratings also declined across extinction blocks independent of drug condition. MDMA produced expected sympathomimetic and subjective effects. Autonomic variables (blood pressure, heart rate, temperature) were moderately elevated under MDMA. Baseline skin conductance levels during the acute MDMA session were higher than placebo (mean ± SD 10.1 ± 3.6 vs 6.6 ± 2.3 µS, tp=4.83, p<0.001, ges=0.26) and correlated with MDMA plasma concentrations (R=0.47, p=0.010). Subjective measures (VAS items such as “any drug effect,” “good effect,” “openness,” and “trust”) were increased by MDMA and the intensity of several subjective effects during extinction learning negatively correlated with CS+ versus CS- discrimination in early extinction recall (for example, “any drug” R=−0.57, p=0.002; “good drug” R=−0.56, p=0.002; openness R=−0.46, p=0.015), indicating stronger subjective MDMA effects associated with less differential responding to the fear cue at recall. Plasma oxytocin concentrations rose markedly after MDMA: Cmax 502 ± 271 pg/ml versus 125 ± 100 pg/ml under placebo (tp=6.70, p<0.001, ges=0.49), and AUC0–24 6,742 ± 3,235 versus 2,489 ± 1,537 pg·h/ml (tp=6.70, p<0.001, ges=0.48). MDMA and oxytocin plasma levels at 2 h were positively correlated (R≈0.41, p≈0.025). However, peripheral oxytocin measured at 3 h did not correlate with facial emotion recognition or with extinction learning or recall outcomes. On the facial emotion recognition task MDMA impaired correct identification of negative emotions (anger, sad, fear) relative to placebo (rmANOVA drug effect F(1,28)=12.3, p=0.002, ges=0.02), with a greater tendency to misclassify faces as “happy” under MDMA (paired t p=3.77, p=0.004, ges=0.07). Order effects were present as lower overall responding in second sessions for nonstandardized data, but there was no drug × order or trial type × order interaction and no evidence of carryover with the 30‑day washout. Five SCR nonresponders were excluded from within‑subject SCR analyses and one subject lacked valid SCR responses in both sessions and was excluded from all SCR analyses.

Vizeli and colleagues interpret the findings as evidence that a single 125 mg MDMA dose can facilitate retention of extinction learning as indexed by SCR the day after dosing. The investigators suggest that subjective entactogenic effects of MDMA (for example, feelings of trust and openness) during extinction learning were associated with greater extinction retention, implying that self‑reported acute effects may serve as a rapid marker of MDMA engagement with extinction‑related targets. The authors note that these effects align with some animal data showing MDMA facilitation of extinction recall, though species and paradigm differences exist. Despite a robust peripheral oxytocin elevation following MDMA, the study did not find an association between plasma oxytocin and extinction outcomes or facial emotion recognition, leading the authors to conclude that peripheral oxytocin is unlikely to mediate MDMA's effect on extinction in this sample. The lack of MDMA effects on FPS indicates that the facilitation of extinction retention may be task‑ or modality‑specific: SCR, an arousal‑sensitive measure that reflects sympathetic activity, showed MDMA‑related benefits, whereas FPS, a valence‑sensitive startle measure, did not. The investigators caution that acute sympathomimetic increases in sweating under MDMA could confound SCR measurements during the drug phase and that ceiling or task‑specific features (for example, a stronger air‑puff US in FPS) may have made the FPS paradigm less sensitive to drug effects. The authors acknowledge several limitations: the sample comprised only healthy young men, so sex differences and clinical generalisability (for example to PTSD patients) remain untested; the crossover design can carry order effects in learning tasks despite counterbalancing and a long washout; possible unblinding with psychoactive drugs could influence results; peripheral oxytocin measures may not reflect central activity and have measurement reliability issues; and SCR is less specific to fear valence than FPS. They also note that pharmacologic interactions (for example with SSRIs) could modulate MDMA effects and warrant further study. Finally, the investigators propose that subjective entactogenic effects in combination with extinction facilitation could contribute to MDMA‑assisted psychotherapy, but emphasise that the current results in healthy volunteers require replication and extension to clinical populations and to female participants.