Restoring Agonist Function at a Chemogenetically Modified M₁ 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 M₁ DREADD mAChR displays minimal responsiveness to the endogenous agonist acetylcholine (ACh) but responds to clozapine-N-oxide (CNO), an otherwise pharmacologically inert ligand. We have previously shown that benzyl quinolone carboxylic acid (BQCA), an M₁ mAChR positive allosteric modulator (PAM), can rescue ACh responsiveness at these receptors. However, whether this effect is chemotype specific or applies to next-generation M₁ PAMs with distinct scaffolds is unknown. Here, we reveal that new M₁ PAMs restore ACh function at the M₁ DREADD while modulating ACh binding at the M₁ wild-type mAChR. Importantly, we demonstrate that the modulation of ACh function by M₁ PAMs is translated in vivo using transgenic M₁ 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 in vivo responses.