Separating the agony from ecstasy: R(-)-3,4-methylenedioxymethamphetamine has prosocial and therapeutic-like effects without signs of neurotoxicity in mice

SR-MDMA (racemic MDMA) is a substituted phenethylamine with stimulant and entactogenic properties that has been investigated both recreationally and as an adjunct to psychotherapy. Earlier clinical research reported marked prosocial effects in humans and promising therapeutic outcomes in PTSD, including a small placebo-controlled trial in which SR-MDMA–assisted psychotherapy produced sustained clinical responses in a large proportion of treated patients. Nonetheless, concerns about acute harms (notably hyperthermia) and possible long-term neurotoxicity have limited enthusiasm for wider clinical use and motivated search for safer alternatives. Curry and colleagues set out to test whether the R-enantiomer of MDMA (R-MDMA) retains the prosocial and extinction-facilitating effects attributed to SR-MDMA while producing fewer adverse effects. The study therefore compared SR-MDMA, R-MDMA and S-MDMA across a battery of assays in mice: social interaction, locomotor activity, Pavlovian fear conditioning and extinction, thermoregulation, and markers of neurotoxicity (reactive astrogliosis, tissue monoamine content, and dopamine transporter expression). The authors also probed whether dopamine D1 receptor signalling contributes to SR-MDMA–induced hyperthermia.

The investigators used male mice sourced from standard suppliers: C57BL/6 mice (10–16 weeks) for fear-conditioning experiments and Swiss Webster mice (7–10 weeks) for the other assays. Animals were drug-naïve, group-housed under controlled temperature and humidity with a 12-hour light/dark cycle, and all procedures were approved by the institutional animal care committee. Drugs (SR-MDMA, R-MDMA, S-MDMA) were supplied by NIDA and administered intraperitoneally as HCl salts in saline (10 ml/kg). The selective D1 antagonist SCH23390 (SCH) was used for the D1 receptor manipulation. Doses are reported in mg/kg throughout. Social interaction testing: After an initial screening session (and removal of aggressive pairs), mice received treatment, were isolated 30 min and then paired with a weight-matched, treatment-matched unfamiliar conspecific for 10 min in a standard arena. A second test 48 h later used the same pairs and treatments; videotaped sessions were scored blind for anogenital investigation, general investigation (non-anogenital sniffing, grooming, following) and adjacent lying. These behaviours were averaged per pair and summed to yield a total social interaction score. Locomotor activity: Drug effects on spontaneous locomotion were measured in photocell-equipped open-field chambers for 1 h immediately after injection, with accumulated beam breaks as the outcome. Fear conditioning and extinction: Using C57BL/6 mice, cued fear conditioning (single CS tone paired with a foot shock) occurred on day 1, extinction training on day 3, and extinction testing on day 4. On day 3 mice received R-MDMA, S-MDMA or saline 30 min before extinction training; the training used a sub-optimal regimen of four CS re-exposures. Percent freezing (absence of non-respiratory movement sampled every 5 s) indexed conditioned fear during training and testing. Neurotoxic dosing and tissue collection: To examine potential neurotoxicity, mice received four injections over two days (two injections per day, 2 h apart) of either R-MDMA (50 mg/kg), SR-MDMA (4 × 20 mg/kg; positive control) or saline. Doses for R-MDMA were scaled upward to account for its lower behavioural potency. Survival after the regimen was reported (75% for SR-MDMA, 100% for R-MDMA). Two cohorts were collected: one perfused 48 h after the final dose for immunohistochemistry, the other euthanized 14 days after treatment for biochemical assays. Immunohistochemistry: Reactive astrogliosis was assessed by quantifying GFAP immunoreactivity in the dorsal striatum across multiple coronal sections per animal, with blinded image analysis to measure percent area of labelling. High-performance liquid chromatography (HPLC): Dopamine (DA) and serotonin (5-HT) tissue content were measured by HPLC with electrochemical detection in prefrontal cortex, striatum and hippocampus; values are reported as percent of saline controls. Western blot: Striatal DAT expression was quantified by Western blot and normalised to Na+/K+-ATPase, then expressed as percent of saline controls. Body temperature monitoring: Rectal temperatures were recorded every 30 min at ambient 22 ± 2 °C after two injections given 2 h apart (n ≈ 5/group). A separate experiment tested whether pretreatment with SCH (0.5 mg/kg) 30 min before SR-MDMA altered the hyperthermic response; group sizes for that comparison are reported in the extracted text. Statistics: Behavioural and biochemical data were analysed using between-subjects ANOVAs (one- or two-factor as appropriate) with Dunnett's or Tukey's post-hoc tests. Body temperature used mixed-design ANOVAs with time and treatment factors. Alpha was set to 5%.

Social interaction and locomotion: Both SR-MDMA and R-MDMA increased total social interaction relative to saline, but R-MDMA was less potent. SR-MDMA produced a significant increase at 7.8 mg/kg (post-hoc p = 0.0255), whereas R-MDMA required 17 mg/kg to reach significance (post-hoc p = 0.0265). S-MDMA did not significantly alter total social interaction in the sample tested (no significant effect, p = 0.344), though the authors note a non-significant trend at 7.8 mg/kg. In locomotor testing at the doses that produced peak social effects, SR-MDMA and S-MDMA significantly increased horizontal beam breaks relative to saline (SR-MDMA p = 0.0461; S-MDMA p < 0.0001), whereas R-MDMA did not alter locomotor activity compared to saline (p = 0.9299). Fear extinction: Acute administration of R-MDMA had no effect on freezing during extinction training but significantly reduced freezing during extinction testing 24 h later, indicating facilitation of extinction learning; the peak effective R-MDMA dose was 17 mg/kg (post-hoc p = 0.0124). By contrast, S-MDMA reduced freezing acutely during training at some doses (5.6 and 7.8 mg/kg) but this effect did not persist into extinction testing, so S-MDMA did not facilitate lasting extinction under the conditions used. Neurotoxicity markers: Using a binge-like four-dose regimen scaled to potency, SR-MDMA produced clear markers consistent with neurotoxicity whereas R-MDMA did not. GFAP immunoreactivity in the dorsal striatum measured 48 h after treatment differed by group (ANOVA p = 0.0061); SR-MDMA treated mice showed significantly increased GFAP relative to saline (post-hoc p = 0.0196) and relative to R-MDMA (post-hoc p = 0.0091). R-MDMA did not differ from saline (p = 0.9311). At 14 days post-treatment, DA tissue content showed a significant treatment effect (ANOVA p = 0.0206): SR-MDMA–treated mice had reduced DA across brain regions compared to saline (post-hoc p = 0.0253) and compared to R-MDMA (post-hoc p = 0.0489). R-MDMA and saline did not differ (p = 0.9517). 5-HT concentrations were not significantly affected by either treatment (ANOVA p = 0.3326), consistent with the mouse syndrome in which SR-MDMA causes DA-selective changes. Striatal DAT expression differed by treatment (ANOVA p = 0.0185); DAT was significantly decreased in SR-MDMA relative to R-MDMA (post-hoc p = 0.0215). R-MDMA showed no effect on DAT versus saline (p = 0.9106); SR-MDMA showed a trend toward decreased DAT relative to saline (p = 0.0601). Body temperature and D1 antagonist: Body temperature differed significantly across treatments and time (treatment p < 0.0001; time p = 0.0002) with an interaction (p < 0.0001). SR-MDMA produced a marked hyperthermia relative to saline (post-hoc p = 0.0016) and relative to R-MDMA (post-hoc p < 0.0001). R-MDMA did not increase temperature; it showed a trend toward reducing body temperature compared to saline (p = 0.0527). In a separate experiment, pretreatment with the selective D1 antagonist SCH attenuated the SR-MDMA–induced hyperthermia, supporting a role for D1 receptor signalling in SR-MDMA–elicited thermogenesis (group and time effects reported; post-hoc contrasts indicated SR-MDMA increased temperature versus saline and SCH pretreatment reduced that effect).

Curry and colleagues interpret their findings as evidence that the prosocial and extinction-facilitating effects of racemic MDMA can be dissociated from several of its adverse effects by using the R-enantiomer. Behaviourally, R-MDMA increased social interaction and facilitated long-term extinction of conditioned freezing, albeit at higher doses than SR-MDMA. In contrast to SR-MDMA and S-MDMA, R-MDMA did not increase locomotor activity, did not produce hyperthermia, and showed no biochemical or histological signs of neurotoxicity in mice even when dosed at levels scaled to its lower potency. The authors situate these results within existing pharmacology: S-MDMA is a potent releaser of DA, 5-HT and norepinephrine, whereas R-MDMA is less potent and more selective for 5-HT and NE release and acts as a partial 5-HT2A agonist. Curry and colleagues propose that SR-MDMA’s prosocial and extinction effects depend largely on 5-HT/NE/5-HT2A mechanisms, which R-MDMA can engage at higher doses, while dopamine release appears unnecessary for those therapeutic-like outcomes but may underlie stimulant, thermogenic and neurotoxic effects. Supporting this account, pretreatment with a D1 antagonist mitigated SR-MDMA–induced hyperthermia and SR-MDMA—but not R-MDMA—reduced DA tissue content and DAT expression. Several limitations are acknowledged. Findings in mice may not generalise to primates or humans, particularly because species differ in MDMA’s neurotoxic profile. The study evaluated only males, so sex differences were not addressed. Other potential adverse effects of R-MDMA were not exhaustively tested and could emerge at different doses or in other species. The authors therefore call for replication in species with closer homology to humans and for cautious progression to human studies to characterise safety and psychoactive effects of R-MDMA.

The study concludes that R-MDMA, when administered to male mice at doses scaled to its lower potency, produces prosocial behaviour and facilitates extinction of conditioned fear without eliciting locomotor activation, hyperthermia, or biochemical and histological markers of neurotoxicity observed with racemic MDMA. These results suggest that the therapeutic-like properties of MDMA may be separable from some of its stimulant, thermogenic and neurotoxic liabilities, and support further investigation of R-MDMA as a potentially safer therapeutic candidate, subject to additional safety evaluation across sexes and in species closer to humans.