The M_3-muscarinic receptor regulates learning and memory in a receptor phosphorylation/arrestin-dependent manner

摘要

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 signaling through the cholinergic M_1-muscarinic receptor subtype. However, recent studies have cast doubt on whether this is the primary receptor mediating cholinergic-hippocampal learning and memory. The current study offers an alternative mechanism involving the M_3-muscarinic receptor that is expressed in numerous brain regions including the hippocampus. We demonstrate here that M_3-musca-rinic receptor knockout mice show a deficit in fear conditioning learning and memory. The mechanism used by the M_3-muscarinic receptor in this process involves receptor phosphorylation because a knockin mouse strain expressing a phosphorylation-deficient receptor mutant also shows a deficit in fear conditioning. Consistent with a role for receptor phosphorylation, we demonstrate that the M_3-muscarinic receptor is phosphorylated in the hippocampus following agonist treatment and following fear conditioning training. Importantly, the phosphorylation-deficient M_3-muscarinic receptor was coupled normally to G_(q/11)-signaling but was uncoupled from phosphorylation-dependent processes such as receptor internaliza-tion and arrestin recruitment. It can, therefore, be concluded that M_3-muscarinic receptor-dependent learning and memory depends, at least in part, on receptor phosphorylation/arrestin signaling. This study opens the potential for biased M_3-muscarinic receptor ligands that direct phosphorylation/arrestin-dependent (non-G protein) signaling as being beneficial in cognitive disorders.