Acetylcholine is a neurotransmitter which activates two different groups of receptors: nicotinic acetylcholine receptors (nAChRs) which belong to the superfamily of ligand-gated ion channels and muscarinic acetylcholine receptors (mAChR), belonging to the G-protein coupled receptor (GPCR) superfamily. There are five separate gene products that put together this GPCR subfamily, M1-M5. Generally, M1, M3 and M5 muscarinic receptors are known to activate phospholipase C (PLC) via Gq coupling, while M2 and M4 muscarinic receptors couple to Gi/o, and therefore inhibit adenylate cyclase. This classification is however not clear cut. mAChR can activate adenylate cyclase by coupling to Gs2. In addition, M2 and M4 muscarinic receptors, when overexpressed can also activate adenylate cyclase in some systems
