The M1 receptor is required for muscarinic activation of mitogen-activated protein (MAP) kinase in murine cerebral cortical neurons.

Muscarinic acetylcholine receptors (mAChR) in the central nervous system are involved in learning and memory, epileptic seizures, and processing the amyloid precursor protein. The M(1) receptor is the predominant mAChR subtype in the cortex and hippocampus. Although the five mAChR fall into two broad functional groups, all five subtypes, when expressed in recombinant systems, can activate the mitogen-activated protein kinase (MAPK) pathway. The MAPK pathway has been implicated in learning and memory, amyloid protein processing, and neuronal plasticity. We used M(1) knock-out mice to determine the role of this receptor subtype in signal transduction in the mouse forebrain. In primary cortical cultures from mice lacking the M(1) mAChR, agonist-stimulated phosphoinositide hydrolysis was reduced by more than 60% compared with cultures from wild type mice. Although muscarinic agonists induced robust activation of MAPK in cortical cultures from wild type mice, mAChR-mediated activation of MAPK was virtually absent in cultures from M(1)-deficient mice. These results indicate that the M(1) mAChR is the major subtype that mediates activation of phospholipase C and MAPK in mouse forebrain.