ALLOSTERIC EFFECTS OF THE ALKANE-BIS-AMMONIUM COMPOUND W84 AND OF TACRINE ON [H-3] PIRENZEPINE BINDING AT M(1)-RECEPTORS IN RAT CEREBRAL-CORTEX

The bis-quaternary W84, hexamethylene-bis-[dimethyl-(3 -phthalimidopro pyl)-ammonium bromide], is a potent allosteric modulator of M(2)-choli noceptors. In this study we aimed at quantifying its allosteric effect on the dissociation of [H-3]pirenzepine from M(1)-cholinoceptors in r at cerebral cortex and to measure the effects on association and equil ibrium binding of [H-3]pirenzepine. For sake of comparison tacrine was included which is known to be a potent allosteric modulator of [H-3]p irenzepine binding to M(1)-receptors. Under control conditions (3 mM M gHPO4, 50 mM Tris-HCl, pH 7.4, 23 degrees) [H-3]pirenzepine binding wa s characterized by K-D=5 nM and B-max=965 fmol/mg membrane protein, th e rate constants amounting to k(+1)=5.0 mu M(-1)xmin(-1) and k(-1)=0.0 31 min(-1). W84 and tacrine reduced [H-3]pirenzepine binding concentra tion-dependently with IC50-values of 1.9 mu M and 2.6 mu M, respective ly. [H-3]pirenzepine association was inhibited by the compounds with E C(50,ass)=1.8 mu M for W84 and EC(50,ass)=2.4 mu M for tacrine. The co ncentrations reducing the dissociation rate by 50% amounted to EC(50,d iss)=21 mu M for W84 and to EC(50,diss)=54 mu M for tacrine. Compared with W84, the dose-response curves of tacrine for the investigated eff ects were significantly steeper. In conclusion, W84 affected [H-3]pire nzepine binding to M(1)-receptors allosterically with a higher potency than tacrine but probably by a different mechanism.