An investigation into the pharmacology and regulation of the M1, M3 and M4 muscarinic acetylcholine receptors

Functional selectivity, which highlights the ability of ligands to differentially activate the signalling pathways linked to G protein-couple receptors (GPCRs) has provided an avenue for developing ligands with greater safety profiles. Pilocarpine (Pilo), a non-selective muscarinic acetylcholine receptor (mAChR) agonist has been shown to differentially activate G protein subtypes linked to the M3 mAChR. In this study the pharmacology of Pilo was further investigated using a number of readouts. When compared to methacholine (MCh), a reference agonist, Pilo appeared to preferentially stimulate inositol phosphates production than global receptor phosphorylation. The ligand also appeared to preferentially promote phosphorylation of Ser412 at the third intracellular loop of the receptor than Ser577 at the C-terminal tail. This differential phosphorylation may be linked to the fact that these residues are phosphorylated by distinct protein kinases. However, such preferential phosphorylation was not evident at the mutant M3 RASSL receptor that was engineered to respond to Clozapine-N-oxide (CNO). This mutant receptor was phosphorylated in response to CNO stimulation in a similar manner as the wild-type M3 mAChR responding to ACh. Allosteric modulation has been considered an attractive approach to selectively target GPCR subtypes for multiple disease indications. BQCA and LY2033298 have been shown to act allosterically at the M1 and M4 mAChR, respectively. In this study, we provided evidence that BQCA is probe dependent and the compound is more potent as an affinity modulator of ACh than Pilo. However BQCA did not significantly potentiate the phosphorylation state of the M1 mAChR following stimulation with a sub-maximal concentration of ACh. Similar results were obtained for LY2033298 at the M4 mAChR which suggest that allosteric modulators do not promote a receptor conformation that increases the accessibility of phosphorylation sites to protein kinases.