Background
Muscarinic acetylcholine receptors are G protein-coupled receptors. M1, M3, M5 receptors couple to G proteins of the Gq/11 family, which activate phospholipase C. M2 and M4 receptors couple to Gi/o-type G proteins that inhibit adenylyl cyclase activity. Muscarinic receptors control many effects of acetylcholine in the central and peripheral nervous system. The clinical implications of this receptor have not been fully explored; however, stimulation of this receptor is known to effectively decrease cAMP levels and downregulate the activity of protein kinase A (PKA).