Cooperative interactions at M2 muscarinic acetylcholine receptors:: Structure/activity relationships in stepwise shortened bispyridinium- and bis(ammonio)alkane-type allosteric modulators

Muscarinic M-2-receptors allow for divergent modes of allosteric action, depending on the structure of the allosteric modulator. Phthalimido-substituted bis(ammonio)alkane-type modulators belong to the common mode allosteric agents, whereas a physicochemically closely related bispyridinium-oxime with dichlorobenzyl-substituents at both ends is an atypical agent. Here, we compared the actions of stepwise shortened compounds composed of the phthalimido moiety and middle chains of either the bispyridinium- or the bis(ammonio)alkane-type. Allosteric interactions were measured in pig M-2 receptors with the orthosteric probe [H-3]N-methylscopolamine ([H-3]NMS) to label the acetylcholine binding site of the receptors. Dissociation and equilibrium binding experiments revealed parallel structure/activity-relationships in both series of compounds with regard to the cooperativity of interaction with [H-3]NMS and to the underlying binding affinities in radioligand-occupied and free receptors. In conclusion, the findings are in line with the hypothesis that the phthalimido-moiety, but not the middle chain, is pivotal for the topology of interaction with the M-2-receptor protein.