EVIDENCE FOR A MULTIPLE BINDING MODE OF BISPYRIDINIUM-TYPE ALLOSTERICMODULATORS OF MUSCARINIC RECEPTORS

The ligand binding properties of muscarinic receptors can be modulated by allosterically acting compounds. Here, a set of novel bispyridiniu m-type compounds was investigated which were designed to study structu re-activity relationships and to provide more insight into the molecul ar events underlying the allosteric delay of the dissociation of [H-3] N-methylscopolamine from muscarinic M(2) receptors in porcine cardiac membranes. The parent compound, a non-substituted bispyridinium oxime, displayed a weak allosteric potency and was unable to prevent radioli gand dissociation at maximum concentrations. Introduction of either a phthalimidomethyl-moiety or a dichlorobenzyl-moiety at one end of the parent compound led to a considerable increase of the allosteric activ ity with regard to both the potency and the maximum effect. In these u nilaterally ring-substituted bispyridiniums, homologous contralateral non-aromatic modifications were accompanied by divergent potency shift s depending on whether the unilateral ring was phthalimidomethyl or di chlorobenzyl. The findings point to a multiple binding mode of bispyri dinium compounds at M(2) receptors in the [H-3]N-methylscopolamine-occ upied state, i.e., different orientations of the compounds at the allo steric binding area or even an interaction with distinct allosteric re cognition sites.