The aim of the present study was to investigate the role of the serotonin transporter gene in MDMA-induced mood effects. In addition, it was tested whether effects differed between sexes and whether MDMA mood effects correlated with MDMA blood concentrations. Findings showed that MDMA, in line with previous findings, induced a positive mood state and elevated feelings of anxiety and confusion. One main effect of genotype was revealed, i.e., individuals in the l-group felt, irrespective of sex or treatment, more anxious compared to the s-group. In general, genotype did not seem to influence the MDMA effects, except for one affective state, i.e., depression. It was demonstrated that self-rated subjectively experienced ‘depression’ was attenuated by a single dose of MDMA in the female l-group.

The latter findings on l-genotype groups and negative affect seem counterintuitive since most studies have shown that s-carriers have higher levels of anxiety and are more at risk for developing depression compared to homozygous l-individuals11,12. The question rises whether this group of ecstasy users is representative of the ‘general’ population with respect to the genotype. Looking at the 5-HTTLPR distribution which is 76.2–23.8% for the s- and l-group respectively in the present sample it can be concluded that the sample of ecstasy users did not differ from the general European population with a distribution of 68% for the s-group and 32% for the l-group9. Previously, it was shown by another study that there was a slight tendency for a higher frequency of homozygous s-individuals in an ecstasy user group, although the differences were not statistically significant and samples were in the Hardy–Weinberg equilibrium20. On the other hand, it has previously been shown that ecstasy users who were s-allele carriers showed abnormal processing on an affective Go-NoGo paradigm and had elevated depression scores on a questionnaire, when compared with a control group21, suggesting that ecstasy users display aberrant emotion-related behavior compared to a drug naïve and cannabis user group. However, a depression baseline pattern comparable to our findings has previously been shown in non-drug using participants22 and higher anxiety levels were reported by l/l-carriers in the presence of a stressor23. With respect to the latter it might be postulated that participants in double-blind, placebo-controlled, experimental MDMA studies experience the situation as ‘exciting’ since they know there is a possibility that they will receive MDMA in an unusual (laboratory) setting. However, this is speculative and needs further investigation. With respect to the question whether this group of ecstasy users is representative of the (cognitive) ‘healthy’ population, we have demonstrated previously that the same or comparable population of ecstasy users was not memory impaired compared to a healthy control group24.

Interestingly, the literature on the 5-HTTLPR is not that consistent as it seems and since a few years it has been acknowledged that the level of methylation of the transporter may also be a source of diverging outcomes25,26. Associations between 5-HTTLPR polymorphisms and psychological problems are significantly altered by environmentally induced methylation patterns of the transporter27. Early and recent stress has been shown to lead to hyper-methylation28 and higher levels of methylation of the 5-HTT were for example associated with increased risk of unresolved responses to loss or trauma in the otherwise ‘protective’ homozygous l-variant of the 5HTTLPR. MDMA can be seen as a biological stressor, causing a substantial increase in cortisol concentrations17 and therefore potentially affecting methylation. Yubero et al.29 demonstrated that MDMA indeed influences the methylation of the 5-HTT independent of genotype. MDMA (75 mg) caused, 165 minutes after intake, an increase in serotonin transporter expression, which was independent of genotype29. Interestingly, findings suggested that changes could be more pronounced in women and in l/l- carriers of the 5-HTTLPR genotype since the expression in females increased about 100% compared to only 50% in males and homozygous individuals are known to have increased 5-HTT expression. However, since there was no genotype by sex interaction effect in the methylation study, confirmation for this suggestion is pending. Another study showed that women had statistically significant more pronounced physiological effects after MDMA administration compared to males and high functionality 5-HTTLPR individuals demonstrated increased cardiovascular effects30. Partly in line with these findings, i.e., suggesting that females and l/l-carriers are more sensitive to MDMA effects, the present study demonstrated that the effects of MDMA on depression ratings were only visible in female l-group individuals. Of note, the effect size of the interaction effect was small and the sample size of the female l-group was <10, therefore current results should be handled with care and not be over-interpreted.

Previously, Oakly and colleagues (2014) suggested that individuals with a lower SERT activity would be more sensitive to the reinforcing properties of MDMA31. The absence of a disproportionate subjective reaction to MDMA-induced positive mood effects in s-carriers of the 5-HTTLPR in the present study seems to counter this suggestion. Previously, it was also shown that MDMA effects on emotional empathy were positively related with MDMA blood concentrations32. In the present study, anxiety levels correlated positively to MDMA blood concentrations which is consistent with a previous study showing anxiety to be present when the dose of MDMA was higher33. The other mood states were unrelated to MDMA blood concentrations indicating that a single fixed dose induces the same mood effect in users. However, since only one dose of MDMA was used in the present study, variation in MDMA concentrations is suggestible lower than when multiple doses are included. Current findings therefore do not allow the exclusion of the possibility that MDMA concentrations and mood states, other than anxiety, are associated.

In the present study, a bi-allelic determination of the 5-HTTLPR was done while a tri-allelic determination is also possible and might provide more information. One variant of the long allele of the 5-HTTLPR, ‘l A ’ is associated with high levels of 5-HTT mRNA transcription and ‘l G ’ is more similar to ‘s/s’ with low transcriptional and expressive levels 5-HTT34. In the tri-allelic distribution in the white population, 25% are l A /l A individuals and thus classified as in the ‘long’ group, whereas 6% is l G /l G and thus categorized in the ‘short’ group35. For the present study this implies that less than 1 individual could be misclassified, assuming that the distribution of the 5-HTTLPR is the same in the ‘ecstasy users population’ compared to the non-drug users population. The effects of this are therefore suggested to be minimal, though it would be advisable that future MDMA studies include the tri-allelic determination in larger samples consisting of both sexes.

Findings indicate that the MDMA-induced reduction in self-rated depressive feelings is sex- and genotype-dependent, with females homozygous for the l-allele of the 5-HTTLPR showing this beneficial effect although this effect was small. MDMA effects on other positive and negative mood states seem to be independent of sex, 5-HTTLPR genotype and MDMA bloods concentrations. This suggests that there is no need to take genotype or sex into account when treating patients with MDMA since these groups demonstrated the same emotional response to MDMA. Replication in larger samples sizes is recommended.