INHIBITION OF 5-HT3 RECEPTOR FUNCTION BY IMIDAZOLINES IN MOUSE NEUROBLASTOMA-CELLS - POTENTIAL INVOLVEMENT OF SIGMA(2) BINDING-SITES

The influence of several imidazolines and sigma-site ligands on cation influx through the 5-HT3 receptor channel in N1E-115 mouse neuroblast oma cells was studied by measuring the 2-min influx of the organic cat ion [C-14] guanidinium induced by 1 mu M 5-HT (in the presence of 10 m u M substance P in all experiments). In addition, we determined specif ic binding of [H-3]DTG (1,3-di(2-tolyl)-guanidine), a selective sigma- site radioligand, and [H-3] GR65630 zol-4-yl)-1-(1-methyl-1H-indol-3-y l)-1-propanone), a selective 5-HT3 receptor antagonist, to membranes p repared from N1E-115 cells. The 5-HT-induced [C-14]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 o n cation influx through the 5-HT3 receptor channel was mimicked by the sigma-site ligands, (+/-)-ifenprodil, (+)-3-PPP ((R)-3-(3-hydroxyphen yl)-N-propylpiperidine), DTG (1,3-di-tolyl-guanidine), haloperidol, di zocilpine, and ketamine as well as by the polyamines, arcaine and sper midine. - Ondansetron inhibited [H-3]GR65630 binding with high affinit y, whereas inhibition of binding of this radioligand to the 5-HT3 rece ptor by antazoline, BDF 6143, idazoxan, cirazoline, (+/-)-ifenprodil, (+)-3-PPP, DTG and haloperidol occurred in the high micromolar range. In the competition experiments with [H-3]DTG, (+/-)-ifenprodil, halope ridol, unlabelled DTG, BDF 6143 and (+)-3-PPP inhibited binding of the radioligand at moderate affinity (K-i values in the range of 1 mu M o r lower), whereas ondansetron, antazoline, idazoxan, cirazoline, napha zoline, clonidine, tolazoline, efaroxan, RX821002 (2-[2-(2-methoxy-1,4 -benzodioxanyl)]imidazoline), ketamine and spermidine exhibited affini ty in the high micromolar or millimolar range only. Comparison of the potencies of the ligands (PIC50% values) in inhibiting 5-HT-induced [C -14]guanidinium influx with their affinities (pK(i) values) at the 5-H T 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 conclus ion, 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 r eceptor, may be assumed to exert their inhibitory effect on cation inf lux through the 5-HT3 receptor channels, at least in part, by interact ing with sigma(2)-binding sites.