Characterization and quantitative autoradiographic distribution of [3H]acetylcholine muscarinic receptors in mammalian brain. Apparent labelling of an M2-like receptor sub-type.
[3H]Acetylcholine receptor binding characteristics (under muscarinic conditions) have been investigated using membrane binding assays and in vitro receptor autoradiography. In rat, guinea-pig and monkey brain membrane preparations, [3H]acetylcholine binds with high affinity (25-50 nM) to an apparently single class of sites which is differentially distributed across brain regions. The ligand selectivity pattern reveals that the potency of (-)quinuclidinyl benzylate is greater than (greater than) atropine greater than scopolamine greater than oxotremorine greater than carbamylcholine greater than pirenzepine greater than methylcarbamyl-choline = nicotine in competing for [3H]acetylcholine binding sites, indicating that [3H]acetylcholine selectively binds to muscarinic sites under these incubation conditions. Moreover, the low potency of pirenzepine suggests that [3H]acetylcholine does not label a significant proportion of the M1 receptor sub-type but most likely binds to putative M2-like receptor sites. This hypothesis is also supported by the autoradiographic distribution of [3H]acetylcholine binding sites in all species studied here. High densities of [3H]acetylcholine binding sites are seen in various nuclei of the medulla and pons, certain thalamic nuclei, medial septum, laminae III, V and VI of the cortex and just above the pyramidal cell layer of the hippocampus. Such localization is much different from that seen with the non-selective antagonist [3H]quinuclidinyl benzylate and the selective M1 receptor ligand [3H]pirenzepine, although it resembles that of the selective M2 receptor antagonist [3H]AF-DX 116. Thus, [3H]acetylcholine apparently mostly binds with high affinity mainly to non-M1 muscarinic receptor types in mammalian brain tissues. Moreover, the ligand selectivity pattern and in vitro receptor autoradiographic data suggest that at low concentrations (10-20 nM) most of [3H]actylcholine labelled sites are of the M2-like receptor class.
Quirion
R
R
Douglas Hospital Research Centre, Faculty of Medicine, McGill University, Verdun, Quebec, Canada.
Araujo
D
D
Regenold
W
W
Boksa
P
P
D000109:Acetylcholine / Q000378:metabolism*; D000818:Animals; D001667:Binding, Competitive; D001921:Brain / Q000378:metabolism*; D005260:Female; D006168:Guinea Pigs; D008252:Macaca fascicularis; D008297:Male; D008322:Mammals / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D011976:Receptors, Muscarinic / Q000145:classification / Q000378:metabolism*
D011976:Receptors, Muscarinic; D000109:Acetylcholine