At the end of the experiment, cells were NSC 683864 concentration lysed in 1% SDS and the released radioactivity was quantified by liquid scintillation counting. The release of [3H] labelled substrate was expressed as fractional rate (i.e., the radioactivity released within one fraction was expressed as a percentage of the total radioactivity present in the cells at the beginning of that fraction). Drug-induced release was calculated by subtracting the estimated basal release from total release during the first 8 min of drug exposure and is expressed as a percentage of radioactivity in the cell at the beginning of drug exposure. Data were normalized by using cpm values with no substance present (only solvent) as 100%. IC50 values were calculated using

non-linear regression fits performed with Prism software (GraphPad 5.0, San Diego, CA, U.S.A.). Data transformed into Dixon see more plots were fitted by linear regression.

Levamisole has a pKa value of 7. Both the neutral and protonated levamisole structures were built and minimized with QSite (version 5.8, Schrödinger, LLC) using the B3LYP method applying the 6-31G∗ basis set ( Murphy et al., 2000). SERT and NET share over 90% sequence similarity with DAT. Homology models of human SERT and NET were generated with Modeller 9.12 ( Sali and Blundell, 1993) using the validated human DAT model in the outward facing conformation ( Stockner et al., 2013) as template. The best model out of the 250 generated was used for further studies. The models of SERT, DAT and NET were energy minimized with Molecular Operating Environment ( MOE, 2012) applying the CHARMM22 forcefield ( Brooks et al., 2009) and using position restrains of 100 kcal/mol on the backbone. The induced fit docking found protocol of the Schrödinger package was used for ligand docking into the central binding site (Glide version 5.8, Schrödinger, LLC, New York) using standard parameter setting (Sherman et al., 2005). The neutral and the protonated form of levamisole were docked as fully flexible molecules. The protonatable nitrogen of levamisole was constrained to interact with the central aspartate in the binding side, because the positive amine functional group of the

endogenous substrates of SERT, DAT and NET has been shown to interact with the respective residue. Conformations of amino acid side chains within 6 Å distance to the ligand were optimized in the OPLS-AA 2005 force field after docking. Default energy levels were employed for selection and filtering of the poses. The pKa value of aminorex is 7.4. Both, neutral and protonated form of aminorex were docked using the same methods as for above levamisole. In 2012, 104 drug samples were obtained from drug users participating voluntarily and anonymously in the ‘checkit!’ program which were originally purchased as “cocaine”. We included all samples in our study and analyzed them by LC–MS. Two samples contained pure cocaine whereas seven samples were completely devoid of cocaine.