Colchicine: A novel positive allosteric modulator of the human 5-Hydroxytryptamine _3A receptor

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 _3ARs) expressed in Xenopus laevis oocytes. Colchicine inhibited 5-hydroxytryptamine (5-HT)-evoked currents in oocytes expressing mouse 5-HT _3ARs, with an IC ₅₀ of 59.5 ± 3 μM. In contrast to the mouse receptor, coapplication of colchicine with 5-HT (<1 μM) strongly enhanced 5-HT-evoked currents in oocytes expressing human 5-HT _3ARs. Colchicine applied alone did not induce a detectable current. In the presence of 0.5 μM 5-HT, the potentiation was concentration-dependent and reached the maximum (∼100%) when 750 μM colchicine was applied. However, colchicine-dependent inhibition can be observed at 5-HT concentrations > 1 μM. In oocyte membranes expressing mouse or human receptors, binding studies with colchicine (25 nM-1 mM) revealed no displacement of 1-methyl-N-((1R,3r,5S)-9-methyl-9 azabicyclo [3.3.1]nonan-3yl)-1H-indazole-3 carboxamide ([ ³H]BRL- 43694), 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.