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.
D000494:Allosteric Regulation; D000929:Antidepressive Agents, Tricyclic / Q000494:pharmacology; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D002352:Carrier Proteins / Q000097:blood*; D003891:Desipramine / Q000494:pharmacology; D005473:Fluoxetine / Q000494:pharmacology; D006801:Humans; D007099:Imipramine / Q000097:blood*; D012701:Serotonin / Q000097:blood / Q000494:pharmacology; D012702:Serotonin Antagonists / Q000494:pharmacology; D014363:Tryptamines / Q000097:blood / Q000494:pharmacology*
D000929:Antidepressive Agents, Tricyclic; D002352:Carrier Proteins; D012702:Serotonin Antagonists; D014363:Tryptamines; C025366:serotonin-binding protein; D005473:Fluoxetine; D012701:Serotonin; C030820:tryptamine; D007099:Imipramine; D003891:Desipramine