Table 4

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

Receptor subtype
Preferred G-proteinq/11i/oq/11i/oq/11
Second messengers4-a PLC IP3/DAGAC (−)PLC IP3/DAGAC (−)PLC IP3/DAG
LocationsBrain (cortex, hippocampus)HeartSmooth muscleBasal forebrainSubstantia
GlandsHindbrainGlandsStriatum nigra
Sympathetic gangliaSmooth muscleBrain
Functional responses4-b M-current inhibitionK+channelsSmooth muscle contractionInhibit Ca2+ channels
(βγ activates)Gland secretion
Inhibit Ca2+ channelsDecrease neurotransmitter
Decrease heart rate and forcerelease (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.