Table 4

Muscarinic receptors: G-protein coupling, transduction pathways, localization, and functional responses

	Receptor subtype
	M₁	M₂	M₃	M₄	M₅
Preferred G-protein	q/11	i/o	q/11	i/o	q/11
Second messengers^4-a	PLC IP₃/DAG	AC (−)	PLC IP₃/DAG	AC (−)	PLC IP₃/DAG
	Ca²⁺/PKC		Ca²⁺/PKC		Ca²⁺/PKC
Locations	Brain (cortex, hippocampus)	Heart	Smooth muscle	Basal forebrain	Substantia
	Glands	Hindbrain	Glands	Striatum	nigra
	Sympathetic ganglia	Smooth muscle	Brain
Functional responses^4-b	M-current inhibition	K⁺channels	Smooth muscle contraction	Inhibit Ca²⁺ channels
		(βγ activates)	Gland secretion
		Inhibit Ca²⁺ channels	Decrease neurotransmitter
		Decrease heart rate and force	release (presynaptic)
		Decrease neurotransmitter release (presynaptic)

↵4-a DAG, diacylglycerol; IP₃, inositol 1,4,5-trisphosphate; PKC, protein kinase C; PLC, phospholipase C.
↵4-b Main responses; several other responses have been suggested to be elicited by defined subtypes.