 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.