The M3-muscarinic receptor regulates memory and learning in a receptor phosphorylation/arrestin-dependent manner.

Degeneration of the cholinergic system is considered to be the underlying pathology that results in the cognitive deficit in Alzheimer’s disease. This pathology is thought to be linked to a loss of signalling through the cholinergic M1-muscarinic receptor subtype. However, recent studies have cast doubt on whether this is the primary receptor mediating cholinergic-hippocampal memory and learning. We demonstrate here that M3-muscarinic receptor knockout mice show a deficit in fear conditioning memory and learning. The mechanism employed by the M3-muscarinic receptor in this process involves receptor phosphorylation since a transgenic mouse strain expressing a phosphorylation deficient receptor mutant also shows a deficit in the fear conditioning. Consistent with a role for receptor phosphorylation we demonstrate that the M3-muscarinic receptor is phosphorylated in the hippocampus following agonist treatment and following fear conditioning training. Importantly, the phosphorylation deficient M3-muscarinic receptor was coupled normally to Gq/11-signalling but was uncoupled from phosphorylation-dependent processes such as receptor internalisation and arrestin recruitment. It can, therefore, be concluded that the mechanism adopted by the M3-muscarinic receptor to mediate memory and learning is independent of G-protein coupling but dependent on receptor phosphorylation/arrestin-mediated processes. This study opens the potential for biased ligands to the M3-muscarinic receptor that direct phosphorylation/arrestin-dependent (non G-protein) signalling as being beneficial in cognitive disorders.