The muscanmc cholinergic antagonist 3-quinudidinyl benzilate
(QNB) binds avidly but reversibly to the muscarinic cholinergic
receptor of mammalian brain and peripheral tissues. [3H]-QNB
binding provides a simple, sensitive and specific assay for the muscarinic
cholinergic receptor binding. Inhibition of [3H]-QNB binding
to homogenates of brain and guinea pig ileum by muscarinic
drugs correlates with their pharmacologic potencies, while nicotinic
agents and noncholinergic drugs have negligible affinity. The
regional distribution of [3H]-QNB binding throughout rat and monkey
brain parallels to a major extent other cholinergic markers,
suggesting that the majority of cholinergic synapses in the brain
are muscarinic. [3H]-QNB accumulation in various brain regions
after intravenous injection provides a means of labelling the muscarinic
receptor in vivo. By labelling the receptor in vivo, autoradiographic
studies under the light microscope have been performed
to visualize the muscarinic receptor
