Studies of the Conformational Dynamics of Ligand and Nucleotide Bound P-Glycoprotein

Abstract

Although monoamine transporters translocate substrates (neurotransmitters) across the plasma membrane, a second less studied function is their ion channel-like activity which is responsible for the passive permeation of ions through the membrane. These currents are inward at resting membrane potential and would result in cell depolarization, which may enhance presynaptic excitability, and which would help explain the elevated release of neurotransmitter associated by psychostimulants. Here we compare the ionic currents evoked by serotonin (5-HT) and S(þ)-3,4-methylenedioxy-N-methylamphetamine (S(þ)MDMA, ecstasy) in mammalian cells expressing the human serotonin transporter (hSERT). 5-HT as well as S(þ)MDMA induce inward currents in hSERT-expressing cells. Although the conductances are similar, the S(þ)MDMA induced-current reversal potential is strongly shifted towards the Na þ equilibrium potential, indicating a bigger contribution of Na þ to the overall current compared to 5-HT current. To measure changes in Na þ permeability induced by these compounds, the cells were pre-loaded with the Na þ sensor Asante NaTRIUM GREEN 2 and changes in intracellular Na þ concentration ([Na þ ]i) were monitored fluorometrically. Dose-response experiments show that 5-HT and S(þ)MDMA have similar potency (EC50 200 nM) in elevating the [Na þ ]i; however, S(þ)MDMA increases [Na þ ]i at least 3 times with respect to 5-HT. In addition, the reconstitution of hSERT on a lipid bilayer in the presence of a Na þ gradient showed channel-like activity after the application of 5-HT or S(þ)MDMA in the trans side. Taken together these results strongly suggest that S(þ)MDMA-induced current is different in composition than the 5-HT current, and moreover it must be strongly depolarizing due to its higher Na þ content. Supported by RC1DA028112 and R01DA033930.