Characterization of high-affinity [H-3]TBZOH binding to the human plateletvesicular monoamine transporter

The present study indicates that human platelets can be used as an accessib le peripheral model not only for the plasma membrane serotonin transporter, but also for the vesicular monoamine transporter. The vesicular monoamine transporter (VMAT2) is responsible for the accumulation of monoamines in th e synaptic vesicles. VMAT2 differs from the plasma membrane transporters in its capability to recognize serotonin, histamine, norepinephrine and dopam ine with almost the same affinity. Dihydrotetrabenazine (TBZOH) is a very p otent inhibitor of VMAT2 that binds with high affinity to this transporter. [H-3]TBZOH has been used as a ligand to label VMAT2 in human, bovine and r odent brain. In this study we characterized the pharmacodynamic and pharmac okinetic parameters of [H-3]TBZOH binding in human platelets as compared to rat brain. The density (Bmax) and affinity (Kd) of [H-3]TBZOH specific bin ding was assessed by Scatchard analysis. Association and dissociation rate constants (k(on), K-off) were assessed by kinetic binding studies. In this study high-affinity and saturable binding sites for [H-3]TBZOH were demonst rated in human platelets. Both the affinity of [H-3]TBZOH to its binding si te in platelets (Kd=3.2 +/-0.5nM) and the kinetic rate constants (K-on=2.8( .)10(M)(7)(-1)min(-1); K-off=0.099min(-1)) were similar to that in rat brai n (Kd((striaturm))=1.5nM; Kd((cerebral) (cortex))=1.35nM; K-on=2(.)10(M)(mi n)(7)(-1)(-1); K-off=0.069 min(-1)). Only the VMAT2 blockers tetrabenazine and reserpine inhibited [H-3]TBZOH specific binding. (C) 2001 Elsevier Scie nce Inc. All rights reserved.