However, when subjects were given a dose of methylphenidate, a drug that increases dopamine in the brain, the strength of these connections was normalized (right).

Scans collected in cocaine addicted subjects during placebo effects revealed strong connectivity between the brain's reward center and areas involved in forming habits (top row, left) and weak connectivity between higher-order brain areas involved in regulating emotions and self-control (bottom row, left). Methylphenidate differentially changes the strength of connectivity, or crosstalk, between brain areas shown to play a role in addiction. This synergistic application of computational and biochemical methodologies overcomes many uncertainties inherent in other approaches and furnishes a schematic framework for elucidating the ligand-protein interactions of other classes of DA transport inhibitors. These findings provide positive functional evidence linking tropane pharmacophore interaction with the core substrate-binding site and support a competitive mechanism for transport inhibition. This prediction was verified by focused methionine substitution of residues flanking this site followed by cyanogen bromide mapping of the RTI 82-labeled mutants and by the substituted cysteine accessibility method protection analyses. Comparative modeling and small molecule docking indicated that the tropane pharmacophore of RTI 82 was positioned in the central DA active site with an orientation that juxtaposed the aryliodoazide group for cross-linking to rat DAT Phe-319. Here, we addressed these questions using computational and biochemical methodologies to localize the binding and adduction sites of the photoactivatable irreversible cocaine analog 3β-(p-chlorophenyl)tropane-2β-carboxylic acid, 4'-azido-3'-iodophenylethyl ester (RTI 82). Cocaine binding to DAT blocks this activity and elevates extracellular DA, leading to psychomotor stimulation and addiction, but the mechanisms by which cocaine interacts with DAT and inhibits transport remain incompletely understood. The dopamine transporter (DAT) functions as a key regulator of dopaminergic neurotransmission via re-uptake of synaptic dopamine (DA).

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