Monoamine transporters have at least two binding sites, i.e., the SI-site, which corresponds to the substrate binding site proper, and the SII-site, which resides in the outer vestibule ( Chen
and Reith, 2004, Kristensen et al., 2011 and Sarker et al., 2010). Accordingly, we explored the possibility that levamisole exerts an allosteric effect on the action of cocaine. We performed uptake-inhibition experiments in HEK293 cells expressing all three transporters and used increasing cocaine concentrations at a fixed levamisole concentration or vice versa. Representative selleck chemical experiments are shown in Fig. 3 for NET. The observations are consistent with binding of levamisole and cocaine to the same binding site. This can be best appreciated by examining the transformation of the data
into Dixon plots ( Segel, 1975). For this analysis the reciprocal of uptake velocity is plotted as a function of one inhibitor at a fixed concentration of the second inhibitor. Regardless of whether levamisole was varied at a fixed cocaine concentration ( Fig. 3C and D) or – vice versa – cocaine was varied at a fixed levamisole concentration ( Fig. 3A and B), the transformed data points fell onto parallel lines ( Fig. 3B and D). This is indicative SB203580 of mutually exclusive binding ( Segel, 1975); intersecting lines ought to arise, if cocaine and levamisole can bind simultaneously, i.e., at two different sites. Identical experiments were performed for SERT and DAT ( Supplementary Figs. S3.1 and S3.2) indicating as well mutually exclusive binding
of levamisole and cocaine. Drugs that interact with neurotransmitter transporters can be either not classified as cocaine-like inhibitors, which trap the transporter in the outward facing conformation and thus interrupt the transport cycle (Schicker et al., 2012), or amphetamine-like releasers. These raise extracellular monoamine concentrations by triggering substrate efflux (Sitte and Freissmuth, 2010). Levamisole is distantly related in structure to amphetamine. It is therefore conceivable that levamisole has a releasing action. We increased the sensitivity of our analysis by co-incubation of the cells with monensin (Baumann et al., 2013, Scholze et al., 2000 and Sitte et al., 2000). Monensin is an ionophore that promotes electroneutral Na+/H+ exchange and therefore elevates intracellular Na+ in cells without altering the membrane potential. Since SERT, NET and DAT couple substrate transport with symport of Na+ and Cl−, elevation of intracellular Na+ accelerates substrate efflux (Sitte and Freissmuth, 2010). Applications of 5–20 μM monensin have been found to raise intracellular Na+ to 30–50 mM in HEK293 cells (Chen and Reith, 2004). In the absence of monensin, no efflux was observed in SERT (Fig. 4A) or DAT (Fig. 4C) expressing cells at a high levamisole concentration (100 μM); however, there was a slight increase in [3H]MPP+ in the superfusate collected from HEK293-NET cells (Fig. 4C).