Neurovascular Uncoupling: Multimodal Imaging Delineates the Acute Effects of 3,4-Methylenedioxymethamphetamine.

Psychedelic compounds such as 3,4-methylenedioxymethamphetamine (MDMA) have attracted increasing interest in recent years because of their therapeutic potential in psychiatric disorders. To understand the acute effects of psychedelic drugs in vivo, blood-oxygenation-level-dependent (BOLD) functional MRI (fMRI) has been widely used. In particular, fMRI studies have suggested that MDMA leads to inhibition of brain activity, challenging previous hypotheses indicating mainly excitatory effects based, among others, on increased metabolism shown by 18F-FDG functional PET (fPET). However, interpretation of hemodynamic changes induced by psychedelics is difficult because of their potent vascular effects. Methods: We aimed to delineate the acute effects of MDMA using simultaneous PET/fMRI in rats. For this purpose, hemodynamic changes measured by BOLD fMRI were related to alterations in glucose utilization and serotonin transporter (SERT) occupancy using 18F-FDG fPET/fMRI and 11C-DASB PET/fMRI. Results: We show that MDMA induces localized increases in glucose metabolism in limbic projection areas involved in emotional processing. The increased glucose metabolism was accompanied by global cerebral and extracerebral hemodynamic decreases. We further demonstrated a strong correlation between SERT occupancies and regional BOLD reductions after acute MDMA administration. Conclusion: Our data indicate that hemodynamic decreases after acute MDMA administration are of a nonneuronal nature and initiate peripherally. Within the brain, MDMA triggers neuronal activation in limbic projection areas, whereas increased serotonin levels induced by SERT blockage cause neurovascular uncoupling through direct vascular effects. Correct understanding of the in vivo mechanism of MDMA not only supports ongoing research but also warrants a reassessment of previous studies on neuronal effects of psychedelics relying on neurovascular coupling and recommends 18F-FDG fPET as a potentially more robust measure for pharmacologic research.