A lithium-induced conformational change in serotonin transporter alters cocaine binding, ion conductance, and reactivity of Cds-109

Inactivation of serotonin transporter (SERT) expressed in HeLa cells by [2-(trimethylammonium)ethyl]-methanethiosulfonate (MTSET) occurred much more readily when Na+ in the reaction medium was replaced with Li+. This did not result from a protective effect of Na+ but rather from a Li+-specific increase in the reactivity of Cys-109 in the first external loop of the transporter. Li+ alone of the alkali cations caused this increase in reactivity. Replacing Na+ with N-methyl-D-glucamine (NMDG(+)) did not reduce the affinity of cocaine for SERT, as measured by displacement of a high affinity cocaine analog, but replacement of Na+ with Li+ led to a 2-fold increase in the KD for cocaine. The addition of either cocaine or serotonin (5-HT) protected SERT against MTSET inactivation. When SERT was expressed in Xenopus oocytes, inward currents were elicited by superfusing the cell with 5-HT (in the presence of Na+) or by replacing Na+ with Li+ but not NMDG(+). MTSET treatment of oocytes in Li+ but not in Na+ decreased both 5-HT and Li+ induced currents, although 5-HT-induced currents were inhibited to a greater extent. Na+ antagonized the effects of Li+ on both inactivation and current. These results are consistent with Li+ inducing a conformational change that exposes Cys-109, decreases cocaine affinity, and increases the uncoupled inward current.