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

The present study indicates that human platelets can be used as an accessible 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 the synaptic vesicles. VMAT2 differs from the plasma membrane transporters in its capability to recognize serotonin, histamine, norepinephrine and dopamine with almost the same affinity. Dihydrotetrabenazine (TBZOH) is a very potent inhibitor of VMAT2 that binds with high affinity to this transporter. [³H]TBZOH has been used as a ligand to label VMAT2 in human, bovine and rodent brain. In this study we characterized the pharmacodynamic and pharmacokinetic parameters of [³H]TBZOH binding in human platelets as compared to rat brain. The density (Bmax) and affinity (Kd) of [³H]TBZOH specific binding 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 [³H]TBZOH were demonstrated in human platelets. Both the affinity of [³H]TBZOH to its binding site in platelets (Kd=3.2±0.5nM) and the kinetic rate constants (K_on=2.8·10⁷_M^-1 _min^-1;K_off=0.099_min^-1) were similar to that in rat brain (Kd_(striatum)=1.5nM; Kd_{(cerebral cortex)}=1.35nM; K_on=2·10⁷_M^-1 _min^-1; K_off=0.069 min^-1). Only the VMAT2 blockers tetrabenazine and reserpine inhibited [³H]TBZOH specific binding.