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

Abstract

S,R(+/-)-3,4-methylenedioxymethamphetamine (SR-MDMA) is an amphetamine derivative with prosocial and putative therapeutic effects. Ongoing clinical trials are investigating it as a treatment for post-traumatic stress disorder (PTSD) and other conditions. However, its potential for adverse effects such as hyperthermia and neurotoxicity may limit its clinical viability. We investigated the hypothesis that one of the two enantiomers of SR-MDMA, R-MDMA, would retain the prosocial and therapeutic effects but with fewer adverse effects. Using male Swiss Webster and C57BL/6 mice, the prosocial effects of R-MDMA were measured using a social interaction test, and the therapeutic-like effects were assessed using a Pavlovian fear conditioning and extinction paradigm relevant to PTSD. Locomotor activity and body temperature were tracked after administration, and neurotoxicity was evaluated post-mortem. R-MDMA significantly increased murine social interaction and facilitated extinction of conditioned freezing. Yet, unlike racemic MDMA, it did not increase locomotor activity, produce signs of neurotoxicity, or increase body temperature. A key pharmacological difference between R-MDMA and racemic MDMA is that R-MDMA has much lower potency as a dopamine releaser. Pretreatment with a selective dopamine D1 receptor antagonist prevented SR-MDMA-induced hyperthermia, suggesting that differential dopamine signaling may explain some of the observed differences between the treatments. Together, these results indicate that the prosocial and therapeutic effects of SR-MDMA may be separable from the stimulant, thermogenic, and potential neurotoxic effects. To what extent these findings translate to humans will require further investigation, but these data suggest that R-MDMA could be a more viable therapeutic option for the treatment of PTSD and other disorders for which SR-MDMA is currently being investigated.

Keywords: Enantiomers; MDMA; Neurotoxicity; PTSD; Prosocial.

Figure 1. Effects of SR-MDMA, R-MDMA, and S-MDMA on social interaction and locomotor activity

The durations of 3 social behaviors during a 10-minute social interaction test are shown stacked to produce total social interaction. (a) SR-MDMA increased total social interaction with a peak effective dose of 7.8 mg/kg. SR-MDMA preferentially increased general investigation behaviors, mostly nose-to-nose sniffing and non-aggressive following. (b) R-MDMA increased total social interaction with a peak effective dose of 17 mg/kg. R-MDMA preferentially increased adjacent lying behavior. (c) S-MDMA did not significantly alter total social interaction. (d) SR-MDMA and S-MDMA significantly increased locomotor activity compared to saline, while R-MDMA had no effect on locomotor activity. Bars represent mean ± SEM; *p < 0.05, ****p < 0.0001 compared to saline treatment.

Figure 2. Effects of R-MDMA and S-MDMA on fear extinction

(a) Timeline of fear conditioning and extinction experiment. (b) R-MDMA did not affect freezing during extinction training. (c) But 17 mg/kg R-MDMA given before training significantly reduced freezing during extinction testing performed 24 hours later. (d) S-MDMA decreased freezing during extinction training, with doses of 5.6 mg/kg and 7.8 mg/kg significantly decreasing freezing compared to saline. (e) However, S-MDMA treatment did not facilitate lasting extinction as there was no effect of prior treatment on freezing during extinction testing. Bars represent mean ± SEM of %freezing across 4 CS tones; *p < 0.05, **p < 0.01 compared to saline treatment; CS, conditioned stimulus; US, unconditioned stimulus.

Figure 3. Astrogliosis 48 hours after treatment with SR-MDMA or R-MDMA

(a) Reactive astrogliosis, a marker of CNS damage, was assessed by quantification of GFAP immunoreactivity in the dorsal striatum 48 hours after a neurotoxic dosing regimen of SR-MDMA (20 mg/kg × 4) or an equivalent regimen of R-MDMA (50 mg/kg × 4). (b) SR-MDMA significantly increased GFAP immunoreactivity in the striatum. R-MDMA did not affect GFAP immunoreactivity. Bars represent mean ± SEM; Scale bar, 100 μm; *p < 0.05, **p < 0.01; GFAP, glial fibrillary acidic protein.

Figure 4. Markers of neuronal terminal pruning

2 weeks after binge dose treatments of SR-MDMA (20 mg/kg × 4) or R-MDMA (50 mg/kg × 4), markers of neuronal terminal pruning were assayed. (a) DA content was assessed in 3 brain regions as a marker of DA terminal pruning. SR-MDMA treatment significantly decreased DA content across the regions. R-MDMA had no effect on DA concentrations relative to saline treated controls. (b) There was no effect of either treatment on 5-HT content in the same brain regions, suggesting that pruning was isolated to DA neurons. (c) DAT expression in the striatum was quantified as an additional marker of DA neuronal pruning. SR-MDMA treated mice had significantly lower striatal DAT expression compared to R-MDMA treated subjects, which were not significantly different from saline treated controls. Bars represent % of saline control ± SEM; *p < 0.05; DA, dopamine; DAT, dopamine transporter.

Figure 5. Effects of SR-MDMA and R-MDMA on body temperature

(a) Subjects were treated with SR-MDMA (20 mg/kg), R-MDMA (50 mg/kg), or saline at times T_x1 and T_x2. Relative to saline or R-MDMA treated subjects, SR-MDMA significantly increased body temperature. (b) To investigate the role of DA in the hyperthermic effect of SR-MDMA, subjects were pretreated (PreT_x) with the D1 receptor antagonist R(+)-SCH23390 (SCH; 0.5 mg/kg) or saline 30 minutes before treatment (T_x) with SR-MDMA (20 mg/kg) or saline. SR-MDMA increased body temperature compared to saline treated controls. Pretreatment with SCH attenuated this effect, but did not significantly reduce baseline body temperature. Symbols represent mean ± SEM; *p < 0.05, **p < 0.01, ****p < 0.0001; DA, dopamine; SCH, R(+)-SCH23390.