title: Allosteric effects of the alkane-bis-ammonium compound W84 and of tacrine on [3H]pirenzepine binding at M1-receptors in rat cerebral cortex.
abstract: The bis-quaternary W84, hexamethylene-bis-[dimethyl-(3-phthalimidopropyl)-ammonium bromide], is a potent allosteric modulator of M2-cholinoceptors. In this study we aimed at quantifying its allosteric effect on the dissociation of [3H]pirenzepine from M1-cholinoceptors in rat cerebral cortex and to measure the effects on association and equilibrium binding of [3H]pirenzepine. For sake of comparison tacrine was included which is known to be a potent allosteric modulator of [3H]pirenzepine binding to M1-receptors. Under control conditions (3 mM MgHPO4, 50 mM Tris-HCl, pH 7.4, 23 degrees) [3H]pirenzepine binding was characterized by KD = 5 nM and Bmax = 965 fmol/mg membrane protein, the rate constants amounting to k+1 = 5.0 microM-1 x min-1 and k-1 = 0.031 min-1. W84 and tacrine reduced [3H]pirenzepine binding concentration-dependently with IC50-values of 1.9 microM and 2.6 microM, respectively. [3H]pirenzepine association was inhibited by the compounds with EC50,ass = 1.8 microM for W84 and EC50,ass = 2.4 microM for tacrine. The concentrations reducing the dissociation rate by 50% amounted to EC50,diss = 21 microM for W84 and to EC50,diss = 54 microM for tacrine. Compared with W84, the dose-reponse curves of tacrine for the investigated effects were significantly steeper. In conclusion, WS84 affected [3H]pirenzepine binding to M1-receptors allosterically with a higher potency than tacrine but probably by a different mechanism.
authors.affiliation: Department of Pharmacology and Toxicology, University of Bonn, Germany.
authors.forename: K
authors.initials: K
authors.lastname: Mohr
authors.forename: C
authors.initials: C
authors.lastname: Tränkle
mesh_terms: D000494:Allosteric Regulation; D000818:Animals; D001667:Binding, Competitive / Q000187:drug effects; D002540:Cerebral Cortex / Q000187:drug effects* / Q000378:metabolism; D002800:Cholinesterase Inhibitors / Q000633:toxicity*; D004305:Dose-Response Relationship, Drug; D054833:Isoindoles; D008297:Male; D010797:Phthalimides / Q000633:toxicity*; D010890:Pirenzepine / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D011950:Receptors, Cholinergic / Q000187:drug effects* / Q000378:metabolism; D013329:Structure-Activity Relationship; D013619:Tacrine / Q000633:toxicity*
chemical_list: D002800:Cholinesterase Inhibitors; D054833:Isoindoles; D010797:Phthalimides; D011950:Receptors, Cholinergic; C013017:hexamethylenebis(dimethyl-(3-phthalimidopropyl)ammonium bromide); D010890:Pirenzepine; D013619:Tacrine