Tryptamine, a Substrate for the Serotonin Transporter in Human Platelets, Modifies the Dissociation Kinetics of [3H]Imipramine Binding: Possible Allosteric Interaction

Tricyclic antidepressants and nontricyclic serotonin (5-hydroxytryptamine) uptake blockers monophasically inhibit 3H]imipramine binding in human platelets. Similarly, serotonin and tryptamine inhibit the binding of 3H]imipramine in the low micromolar range and with a pseudo-Hill coefficient near unity. Dissociation of the 3H]imipramine receptor complex in the presence of uptake inhibitors follows first-order kinetics with a half-life of approximately 60 min. Although serotonin and tryptamine do not decrease 3H]imipramine binding when added under equilibrium conditions, simultaneous addition of serotonin or tryptamine with serotonin uptake inhibitors decreases the rate of ligand-receptor dissociation in a concentration-dependent manner. These data suggest a common site of action for serotonin, which is the substrate of the transporter system, and of tryptamine, its nonhydroxylated analog. This hypothesis is supported by the identification of a high-affinity (Km = 0.55 microM), saturable, and temperature-dependent uptake of 3H]tryptamine in human platelets. Uptake of 3H]tryptamine was inhibited potently by imipramine and nontricyclic serotonin uptake inhibitors with a potency similar to that observed for 3H]serotonin uptake. These data support the hypothesis that in platelets, 3H]imipramine, tricyclic, and nontricyclic serotonin uptake inhibitors bind to a common recognition site that is associated with the serotonin transporter but that differs from the substrate recognition site of the carrier through which serotonin and tryptamine exert a heterotropic allosteric modulation on 3H]imipramine binding.