The Role of the Dopamine Transporter in Psychostimulant Abuse

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Abstract Amphetamines (AMPHs) are psychostimulants commonly used for the treatment of neuropsychiatric disorders (e.g. attention deficit disorders). They are also abused with devastating outcomes. The abuse potential of AMPHs has been associated to their ability to cause mobilization of cytoplasmic dopamine (DA), which leads to an increase in extracellular DA levels. This increase is mediated by the reversal of the DA transporter (DAT) function that causes non-vesicular DA release, here defined as DA efflux. Notably, inhibition of DA efflux reduces both the ability of AMPH to increase motor activity as well as AMPH preference3, 6, 9. It has been suggested that imbalance in the gut microbiome (dysbiosis) participates in the pathogenesis of substance use disorders11. Consistent with this idea, psychostimulant abuse promotes dysbiosis12-15. Therefore, it is possible that changes in the gut microbiome and its metabolites may not only be a consequence of substance use disorders, but may play a role in mediating behavioral responses to drugs of abuse11. Microbial products such as short-chain fatty acids (SCFAs) are suspected to play a fundamental role in the gut-brain axis. Among SCFAs, butyrate is known to cross the BBB and directly act on neurons and glial cells16. Fusobacterium nucleatum (F. nucleatum) is an anaerobic filamentous gram-negative bacterial species that secretes butyrate17, growth of which is stimulated by AMPH abuse13-15. Our preliminary data demonstrate that in gnotobiotic Drosophila, colonization of the gastrointestinal track with F. nucleatum enhances AMPH-induced DA effluxes (recorded in isolated fly brains), as well as AMPH associated behaviors. This potentiation of AMPH actions by F. nucleatum was paralleled by oral administration of the SCFA butyrate. These data point to a powerful role of both F. nucleatum and butyrate in the psychomotor properties of AMPH. To understand, mechanistically, how F. nucleatum promotes AMPH actions, it is important to consider that changes in DAT expression regulate both AMPH-induced DA efflux and AMPH psychomotor actions19. Also, butyrate is a potent inhibitor of histone deacetylases (HDACs)20-22 and inhibition of HDACs, specifically HDAC1, robustly increases expression of both DAT mRNA and proteins21, 22. Our hypothesis is that F. nucleatum enhances AMPH actions by elevating DAT expression. This is mediated by secretion of butyrate and HDAC inhibition. We will test this hypothesis through the following specific aims: S.A. #1. To determine how F. nucleatum supports AMPH-induced DA efflux. These experiments will be performed in isolated brains of Drosophila melanogaster, an animal model our laboratory developed to study AMPH actions in vivo. Using this model, we will be able to translate the molecular discoveries of S.A. #1 to specific AMPH behavioral phenotypes. Thus, S.A. #2 focuses on behavior: S.A. #2. To determine the effect of F. nucleatum on AMPH-induced behaviors.