N-terminal domains in mouse and human 5-hydroxytryptamine_3A receptors confer partial agonist and antagonist properties to benzylidene analogs of anabaseine

The present study tested the hypothesis that mouse and human 5-hydroxytryptamine_3A (5-HT_3A) receptors may be differentially modulated by benzylidene analogs of anabaseine (BA) and that these analogs may be useful in assessing residues involved in receptor gating. Mouse and human wild-type and mouse and human chimeric 5-HT_3A receptors expressed in Xenopus oocytes were evaluated with the two-electrode voltage clamp technique. Our previous studies demonstrated that 3-(2,4-dimethoxybenzylidene)-anabaseine (DMXBA) is an antagonist at the mouse wild-type 5-HT_3A receptor, but that its metabolites 3-(2-hydroxy, 4-methoxybenzylidene)-anabaseine (2-OHMBA), 3-(2-methoxy, 4-hydroxybenzylidene)- anabaseine (4-OHMBA), and 3-(2,4-dihydroxybenzylidene)-anabaseine (2,4-DiOHBA) are partial agonists (J Pharmacol Exp Ther, 299: 1112-1117, 2001). In the human wild-type (HWT) 5-HT_3A receptor, none of the BA compounds possessed partial agonist activity. BA compounds antagonized 1.5 μM 5-HT-mediated (EC₅₀) responses in the HWT 5-HT_3A receptor with a rank order of potency (IC₅₀ in μM) of 2-OHMBA (1.5 ± 0.1) > DMXBA (3.1 ± 0.2) > 4-OHMBA (7.4 ± 0.5) > 2,4-DiOHBA (12.8 ± 0.7). In mouse receptor chimeras containing N-terminal human receptor orthologs, 2-OHMBA inhibited 5-HT-mediated (EC₅₀) currents with IC₅₀ values of 2.0 ± 0.08 and 3.0 ± 0.13 μM, respectively. In human receptor chimeras containing N-terminal mouse receptor orthologs, 2-OHMBA displayed partial agonist activities with EC₅₀ values of 1.3 ± 0.15 and 5.0 ± 0.4 μM; efficacies were 43 and 57%, respectively. Thus, amino acids present in the distal one-third of the N terminus of mouse and human 5-HT_3A receptors are necessary and sufficient to confer partial agonist or antagonist properties of 2-OHMBA.