Pharmacological and experimental procedure
Participants were stratified for sex and randomly allocated to treatment. Treatment was packed in identical capsules, counterbalanced across sexes and dispensed by an independent researcher. To reduce potential confounding effects of early life stress (46), impulsiveness, sensitivity to punishment and reward on reward-related neural processing the Childhood Trauma Questionnaire (CTQ), Barratt Impulsiveness Scale (BIS), and Sensitivity to Punishment and Sensitivity to Reward Questionnaire (SPSRQ) were administered at baseline (Figure 1A ) (47-49). Given that after oral administration LT peak plasma levels are reached after 90 minutes with a terminal elimination half-life ranging from 1.5 to 2.5 hours (50-52) the experimental paradigm started 90 minutes after treatment (see also(26, 53)). LT rapidly crosses the blood-brain barrier (54, 55) and while effects at central receptors have been observed after 30 minutes after i.e. administration effects on cardiovascular indices only become apparent after 3 hours (e.g. (26, 29, 50)). To further control for potential confounding effects of LT on cardiovascular activity blood pressure and heart rate were assessed before drug administration, as well as before and after the SID fMRI paradigm (Figure 1A , details supplementary methods ). To control for nonspecific affective effects of LT the affective state of participants was tracked troughout the experiment via the Spielberger State–Trait Anxiety Inventory (STAI) and the Positive and Negative Affective Scale (PANAS) which were administered before drug administration, at the time of peak plasma concentrations and after the experiment (56, 57). The subsequent affective impact of LT-induced changes on social feedback processing was assessed via ratings of the cues before treatment, after fMRI, and following feedback stimuli after fMRI (Figure 1A ). After the entire experiment participants were asked to guess the treatment they received.
Written informed consent was obtained, the study was approved by the local institutional ethics committee, all procedure followed the Declaration of Helsinki, and all procedures were preregistered (NCT04604756, URL:https://clinicaltrials.gov/ct2/show/NCT04604756 ).