acetylcholine is the main transmitter used at mammalian
Acetylcholine Muscarinic acetylcholine receptors have a more complex mechanism, and affect target cells over a longer time frame. In mammals, five subtypes of muscarinic receptors have been identified, labeled M1 through M5. All of them function as G protein-coupled receptors, meaning that they exert their effects via a second messenger system. The M1, M3, and M5 subtypes are Gq-coupled; they increase intracellular levels of IP3 and calcium by activating phospholipase C. Their effect on target cells is usually excitatory. The M2 and M4 subtypes are Gi/Go-coupled; they decrease intracellular levels of cAMP by inhibiting adenylate cyclase. Their effect on target cells is usually inhibitory. Muscarinic acetylcholine receptors are found in both the central nervous system and the peripheral nervous system of the heart, lungs, upper gastrointestinal tract, and sweat glands.