Restoring Agonist Function at a Chemogenetically Modified M Muscarinic Acetylcholine Receptor.

Designer receptors exclusively activated by designer drugs (DREADDs) have been successfully employed to activate signaling pathways associated with specific muscarinic acetylcholine receptor (mAChR) subtypes. The M DREADD mAChR displays minimal responsiveness to the endogenous agonist acetylcholine (ACh) but responds to clozapine--oxide (CNO), an otherwise pharmacologically inert ligand. We have previously shown that benzyl quinolone carboxylic acid (BQCA), an M mAChR positive allosteric modulator (PAM), can rescue ACh responsiveness at these receptors. However, whether this effect is chemotype specific or applies to next-generation M PAMs with distinct scaffolds is unknown. Here, we reveal that new M PAMs restore ACh function at the M DREADD while modulating ACh binding at the M wild-type mAChR. Importantly, we demonstrate that the modulation of ACh function by M PAMs is translated using transgenic M DREADD mice. Our data provide important insights into mechanisms that define allosteric ligand modulation of agonist affinity vs efficacy and how these effects play out in the regulation of responses.