Colchicine: A Novel Positive Allosteric Modulator of the Human 5-Hydroxytryptamine_3AReceptor

Abstract

The actions of colchicine were examined with the two-electrode voltage-clamp technique and radioligand binding assays in mouse and human 5-hydroxytryptamine 3A receptors (5-HT 3A Rs) expressed in Xenopus laevis oocytes. Colchicine inhibited 5-hydroxytryptamine (5-HT)-evoked currents in oocytes expressing mouse 5-HT 3A Rs, with an IC 50 of 59.5 Ϯ 3 M. In contrast to the mouse receptor, coapplication of colchicine with 5-HT (Ͻ1 M) strongly enhanced 5-HT-evoked currents in oocytes expressing human 5-HT 3A Rs. Colchicine applied alone did not induce a detectable current. In the presence of 0.5 M 5-HT, the potentiation was concentration-dependent and reached the maximum (ϳ100%) when 750 M colchicine was applied. However, colchicine-dependent inhibition can be observed at 5-HT concentrations Ͼ 1 M. In oocyte membranes expressing mouse or human receptors, binding studies with colchicine, suggesting that actions of colchicine do not occur at the ligand binding domain. Functional effects of colchicine on both receptors occurred in the absence of preincubation and after cold temperature incubation, suggesting that the microtubule-depolymerizing effects of colchicine play no role in modulation of receptor function. Studies with interspecies chimeric receptors demonstrated that the distal one third of the N terminus is responsible for the bidirectional modulation by colchicine. Collectively, these results suggest that colchicine modulates receptor function through loops C and/or F through a gating mechanism.The 5-hydroxytryptamine 3 receptor (5-HT 3 R) belongs to the superfamily of ligand-gated ion channels (LGICs) (Maricq et al., 1991) and shares a number of structural and functional homologies with other members, such as the nicotinic acetylcholine receptor (nAChR), GABA receptor (GABA A R), and glycine receptor (GlyR) (Grenningloh et al., 1987;. To date, five 5-HT 3 receptor subunits have been cloned (A-E) (Maricq et al., 1991;Belelli et al., 1995;Niesler et al., 2007). Receptors composed of A-homomers and A/B-heteromers have been shown to have functional significance in the central and peripheral nervous systems (for review, see Thompson and Lummis, 2006), and a recent study reported expression of functional A/C-, A/D-, and A/E-heteromers (Niesler et al., 2007). The ligand binding domain (LBD) is located in the N terminus at the interface between subunits and consists of six loops, A to F. Loops A to C form the principal component of the LBD, and loops D to F form the complementary component of the LBD (for review, see Thompson and Lummis, 2006). Homology models using the acetylcholine-binding protein and biochemical studies have identified important residues for ligand binding on both the primary (loops A-C) and the complementary (loops D-F) interfaces of the binding site (Schreiter et al., 2003;Thompson et al., 2005).The homomeric 5-HT 3A R is a good model system for exploring the ligand recognition sites of LGICs. Significant homology exists between the 5-HT 3A Rs among the six cloned species (80 -95%). It is...

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The antidepressant bupropion is a negative allosteric modulator of serotonin type 3A receptors

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