Agonist interactions with 5‐HT₃ receptor recognition sites in the rat entorhinal cortex labelled by structurally diverse radioligands

Evidence that the atypical 5‐HT3 receptor ligand, [3H]‐BRL46470, labels additional 5‐HT3 binding sites compared to [3H]‐granisetron

“…Hoyer & Neijt, 1987;Kilpatrick et al, 1987;Barnes et al, 1992b; although is comparable to previous studies investigating the labelling of the 5-HT3 receptor with [3H]-(R/S)-zacopride and [3H]-(S)-zacopride (Barnes et al, 1988;1992b;La-porte et al, 1992; (Barnes et al, 1992b;.…”

“…Other differences in the binding characteristics of 5-HT3 receptor ligands have also been reported. For instance, when the 5-HT3 receptor expressed in rat brain or NG 108-15 cells is labelled with [3H]-(S)-zacopride, agonists fail to generate steep competition curves with Hill coefficients greater than unity (Barnes et al, 1992b;. This contrasts with data generated with other 5-HT3 receptor radioligands (e.g.…”

“…Hoyer & Neijt, 1987;Kilpatrick et al, 1987;Barnes et al, 1990;1992b;Sharif et al, 1991). In rat cerebral cortex/hippocampus, rat ileum, NG108-15…”

“…Frozen tissues were thawed and processed in ice-cold Tris/ Krebs buffer (mM: Tris 50.0, NaCl 118.0, KCl 4.75, KH2PO4 1.2, MgSO4 1.2, CaCl2 2.5, NaHCO3 25.0, glucose 11.0, final pH 7.4) as described previously (Barnes et al, 1992b) except that in studies using NG108-15 and HEK-5-HT3-As cells, the homogenates were centrifuged only once and that the rat ileum homogenate was filtered through gauze and the filtrate used as the binding homogenate. The protein content of homogenates was estimated by the Bio-Rad Coomassie brilliant blue method (Bradford, 1976), with bovine serum albumin used as the standard.…”

“…Radioligand binding assays were performed essentially as described previously (Barnes et al, 1992b). [3H]-BRL46470 binding was allowed to proceed at 37°C for 45 min prior to filtration as described for [3H]-granisetron (Barnes et al, 1992b (HCI;2,l]oct-3y1)-2, 3-dihydro-3, 3-dimethyl-indole-lcarboxamide; SmithKline Beecham Pharmaceuticals), clozapine (Sandoz), cocaine (HCl; Research Biochemicals Inc (RBI)), geneticin (Gibco), granisetron (HCl; SmithKline Beecham Pharmaceuticals), GR 65630 (3-(5-methyl-lH-imidazol-4-yl)-l-(l-methyl-lH-indol-3-yl)-l-propanone; Glaxo Laboratories), hexamethonium (bromide; Sigma), 5-HT (maleate; Sigma), MDL 72222 (lafH, 3a, 5aH-tropan-3-yl-3,5-dichlorobenzoate; Merrell-Dow), mepyramine (maleate; May and Baker), 2-methyl-5-HT (maleate; RBI), metoclopramide (HCl; RBI), meta-chlorophenylbiguanide (HCl; mCPBG; RBI), naloxone (HCI; Sigma), paroxetine (HCl; SmithKline Beecham Pharmaceuticals), penicillin (Gibco), phentolamine (mesylate; RBI), phenylbiguanide (Aldrich), propranolol (HCl; RBI), quipazine (dimaleate; RBI), SDZ 206-830 ((3-ac-homotropanyl)-l-methyl-5-fluoroindole-3-carboxylic acid; Sandoz), streptomycin (Gibco), tropisetron (HCl; Sandoz), (R)-zacopride (HCl, Centre de Recherche Delalande) and (S)-zacopride (HCl, Centre de Recherche Delalande) were dissolved in distilled water and diluted in Tris/Krebs buffer. Ondansetron (hydrochloride dihydrate, Glaxo Laboratories) was supplied in aqueous solution (2 mg ml-') and diluted with Tris/Krebs buffer.…”