title: Inhibition of 5-HT3 receptor function by imidazolines in mouse neuroblastoma cells: potential involvement of sigma 2 binding sites.
abstract: The influence of several imidazolines and sigma-site ligands on cation influx through the 5-HT3 receptor channel in N1E-115 mouse neuroblastoma cells was studied by measuring the 2-min influx of the organic cation [14C] guanidinium induced by 1 microM 5-HT (in the presence of 10 microM substance P in all experiments). In addition, we determined specific binding of [3H]DTG (1,3-di(2-tolyl)-guanidine), a selective sigma-site radioligand, and [3H] GR65630 (3-(5-methyl-1H-imidazol-4-yl)-1-(1-methyl-1H-indol-3-yl)-1- propanone), a selective 5-HT3 receptor antagonist, to membranes prepared from N1E-115 cells. The 5-HT-induced [14C]guanidinium influx was inhibited by the imidazolines, ondansetron, antazoline, idazoxan, BDF 6143 (4-chloro-2-(2-imidazolin-2-ylamino)-isoindoline), cirazoline, naphazoline, clonidine and by the guanidine agmatine, but not by the catecholamine adrenaline. The inhibitory effect of the imidazolines on cation influx through the 5-HT3 receptor channel was mimicked by the sigma-site ligands, (+/-)-ifenprodil, (+)-3-PPP ((R)-3-(3-hydroxyphenyl)-N-propylpiperidine), DTG (1,3-di-tolyl-guanidine). haloperidol, dizocilpine, and ketamine as well as by the polyamines, arcaine and spermidine.-Ondansetron inhibited [3H]GR65630 binding with high affinity, whereas inhibition of binding of this radioligand to the 5-HT3 receptor by antazoline, BDF 6143, idazoxan, cirazoline, (+/-)-ifenprodil, (+)-3-PPP, DTG and haloperidol occurred in the high micromolar range. In the competition experiments with [3H]DTG, (+/-)-ifenprodil, haloperidol, unlabelled DTG, BDF 6143 and (+)-3-PPP inhibited binding of the radioligand at moderate affinity (Ki values in the range of 1 microM or lower), whereas ondansetron, antazoline, idazoxan, cirazoline, naphazoline, clonidine, tolazoline, efaroxan, RX821002 (2-[2-(2-methoxy-1,4-benzodioxanyl)]imidazoline), ketamine and spermidine exhibited affinity, in the high micromolar or millimolar range only. Comparison of the potencies of the ligands (pIC50% values) in inhibiting 5-HT-induced [14C]guanidinium influx with their affinities (pKi values) at the 5-HT recognition sites of the 5-HT3 receptor and at the sigma 2-sites of the N1E-115 cells by means of multiple regression analysis revealed a significant correlation with the affinities at both sites. In conclusion, our data suggest that imidazolines and sigma-ligands, which as a rule possess low affinity for the 5-HT recognition site of the 5-HT3 receptor, may be assumed to exert their inhibitory effect on cation influx through the 5-HT3 receptor channels, at least in part, by interacting with sigma 2-binding sites.
authors.affiliation: Institute of Pharmacology and Toxicology, University of Bonn, Germany.
authors.forename: G J
authors.initials: GJ
authors.lastname: Molderings
authors.forename: K
authors.initials: K
authors.lastname: Schmidt
authors.forename: H
authors.initials: H
authors.lastname: Bönisch
authors.forename: M
authors.initials: M
authors.lastname: Göthert
mesh_terms: D000818:Animals; D001665:Binding Sites; D002462:Cell Membrane / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D019791:Guanidine; D006146:Guanidines / Q000378:metabolism* / Q000494:pharmacology; D007093:Imidazoles / Q000378:metabolism; D007211:Indoles / Q000378:metabolism; D015640:Ion Channel Gating; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism*; D008024:Ligands; D051379:Mice; D017294:Ondansetron / Q000494:pharmacology; D011955:Receptors, Drug / Q000187:drug effects / Q000378:metabolism; D011985:Receptors, Serotonin / Q000187:drug effects / Q000378:metabolism*; D012701:Serotonin / Q000494:pharmacology; D012702:Serotonin Antagonists / Q000378:metabolism* / Q000494:pharmacology; D013373:Substance P / Q000494:pharmacology; D014407:Tumor Cells, Cultured / Q000378:metabolism
chemical_list: D006146:Guanidines; D007093:Imidazoles; D007211:Indoles; D007473:Ion Channels; D008024:Ligands; D011955:Receptors, Drug; D011985:Receptors, Serotonin; D012702:Serotonin Antagonists; C059068:GR 65630; D012701:Serotonin; D013373:Substance P; D017294:Ondansetron; D019791:Guanidine