RESUMO
The actions of a novel, potent 5-HT3 receptor ligand, [2-chloro-(4-methylpiperazine-1-yl)quinoxaline (VUF10166)], were examined at heterologously expressed human 5-HT3A and 5-HT3AB receptors. VUF10166 displaced [³H]granisetron binding to 5-HT3A receptors expressed in human embryonic kidney cells with high affinity (K(i) = 0.04 nM) but was less potent at 5-HT3AB receptors (K(i) = 22 nM). Dissociation of [³H]granisetron in the presence of VUF10166 was best fit with a single time constant (t(1/2) = 53 min) at 5-HT3A receptors, but with two time constants (t(1/2) = 55 and 2.4 min) at 5-HT3AB receptors. Electrophysiological studies in oocytes revealed that VUF10166 inhibited 5-HT-induced responses at 5-HT3A receptors at nanomolar concentrations, but inhibition and recovery were too slow to determine an IC50. At 5-HT3AB receptors, inhibition and recovery were faster, yielding an IC50 of 40 nM. Cysteine substitutions in the complementary (-), but not the principal (+), face of the 5-HT3B subunit produced heteromeric receptors in which the actions of VUF10166 resembled those at homomeric receptors. At 5-HT3A receptors, VUF10166 at higher concentrations also behaved as a partial agonist (EC50 = 5.2 µM; R(max) = 0.24) but did not elicit significant responses at 5-HT3AB receptors at ≤100 µM. Thus, we propose that VUF10166 binds to the common A+A- site of both receptor types and to a second A+B- modulatory site in the heteromeric receptor. The ability of VUF10166 to distinguish between 5-HT3A and 5-HT3AB receptors could help evaluate differences between these receptor types and has potential therapeutic value.
