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

Abstract

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.

Rhythm and blues. Neurochemical, neuropharmacological and neuropsychological implications of a hypothesis of circadian rhythm dysfunction in the affective disorders

“…Regarding their acute biochemical effects these are multiple and either frequently opposed or totally unrelated to those induced by other antidepressants (Charney et al 1981 ;Maj et al 1984). Furthermore, they are not specific to one neurotransmitter system (Biegon and Samuel 1979;Segonzac et al 1985); they bind to muscarinic, histaminergic, serotonergic, opiate and adrenergic receptors but are full agonists for none. Indeed, it has been said about lithium (and could be said about all antidepressants) "in considering all these findings it is remarkable that lithium in view of its varied biochemical actions has any specific behavioural effect at all" (Robbins and Sahakian 1980, p 153).…”

SLC6 Neurotransmitter Transporters: Structure, Function, and Regulation

“…However, it should be noted that a large body of evidence indicates that the primary high-affinity binding site for TCAs and SSRIs, including those cocrystallized with LeuT, likely occurs in the S1 site in monoamine transporters (for review, see Henry et al, 2006a;Rudnick, 2007;Singh, 2008;Gouaux, 2009). Several studies have found that TCAs and SSRIs, in addition to binding to a high-affinity binding site, also bind to a low-affinity allosteric site in SERT (Sette et al, 1983;Meyerson, 1983, 1985;Plenge and Mellerup, 1985;Segonzac et al, 1985;O'Riordan et al, 1990;Plenge et al, 1991;Chen et al, 2005a,b). Binding to this allosteric site markedly increases the off-rate of the inhibitor bound to the high-affinity binding site.…”

Characterization of an allosteric citalopram‐binding site at the serotonin transporter

“…1983), suggesting that 5‐HT acts at a site distinct from the imipramine binding site. Further studies suggested that dissociation of imipramine as well as the non‐tricyclic paroxetine can be either attenuated or stimulated by various antidepressants, suggesting multi‐affinity or distinct conformational states of SERT (Segonzac et al . 1985; Plenge and Mellerup 1985; Wennogle and Meyerson 1985).…”