Stephen C. Wall

The Platelet Plasma Membrane Serotonin Transporter Catalyzes Exchange between Neurotoxic Amphetamines and Serotonin

The neurotoxic amphetamines pchloroamphetamine (PCA), fenfluramine, 3,4-methylenedioxyamphetamine (MDA), and 3,4-methylenedioxymethamphetamine (MDMA) cause release of serotonin from nerve endings followed by degeneration of a subset ofserotone@c terminals. The release phenomenon, which may be responsible for the neurotoxicity, does not require GI*+ when studied in h u , and is blocked by inhibitors of serotonin reuptake, such as fluoxetine and imipramine. Since this serotonin release is poorly understood, we investigated the interaction of PCA with the serotonin transporter of human platelets. Imipramine binding to platelet plasma membrane vesicles is competitively inhibited by each of the neurotoxic amphetamines. The KI values for PCA, fentluramine, MDA and MDMA are 300 nM, 4.4 pM, 7.7 pM, and 14.8 pM, respectively. Other neurotoxins and amphetamines also inhibited imipramine binding. 5,7-dihydmxytryptamine inhibited with a f i . 5 of approximately 2 pM, D-amphetamine inhibited at 49 pM, (+)methamphetamine at 114 pM, but methylphenidate only at 291 pM. This interaction was stereospecific as evidenced by the higher relative K0.s values for DLamphetamine (75 pM) and (+/-)methamphetamine (179) pM. These compounds also inhibit the initial rate of serotonin transport by platelet plasma membrane vesicles. The KI values for competitive inhibition of serotonin transport, however, are lower than the corresponding KI values fbr binding inhibition. KI values for PCA, fenfluramine, MDA and MDMA are 18 nM, 480 nM, 0.82 pM and 0.54 pM, respectively. Inhibition by PCA could be reversed by washing the vesicles free of PCA, indicating that the interaction was reversible. The lower KI for transport inhibition is characteristic of an inhibitor that is itselfa transport substrate. D-Amphetamine and (+)methamphetamine also inhibited transport with K0.s values of 7.4 and 8.7 pM, respectively. The racemic mixtures were less potent. To test the ability of PCA to act as a transport substrate, we took vesicles that had accumulated [3H]serotonin to high levels and diluted them into various media free of [3H]serotonin (FIG. 1). In NaCl medium (control), radioisotope efflux is slow, since the rate of net transport (in + out) is limited by countertransport of K+ (out * in). FIGURE 1 demonstrates the acceleration of net serotonin efflux by external K+ . Serotonin also stimulates [jHIserotonin efflux since self-exchange bypasses the slow coun-