Journal of Neuroscience, Vol 5, 1577-1582, Copyright © 1985 by Society for Neuroscience
High-affinity binding of [3H]acetylcholine to muscarinic cholinergic receptors
KJ Kellar, AM Martino, DP Hall Jr, RD Schwartz and RL Taylor
High-affinity binding of [3H]acetylcholine to muscarinic cholinergic sites
in rat CNS and peripheral tissues was measured in the presence of cytisin,
which occupies nicotinic cholinergic receptors. The muscarinic sites were
characterized with regard to binding kinetics, pharmacology, anatomical
distribution, and regulation by guanyl nucleotides. These binding sites
have characteristics of high-affinity muscarinic cholinergic receptors with
a Kd of approximately 30 nM. Most of the muscarinic agonist and antagonist
drugs tested have high affinity for the [3H]acetylcholine binding site, but
pirenzepine, an antagonist which is selective for M-1 receptors, has
relatively low affinity. The ratio of high-affinity [3H]acetylcholine
binding sites to total muscarinic binding sites labeled by
[3H]quinuclidinyl benzilate varies from 9 to 90% in different tissues, with
the highest ratios in the pons, medulla, and heart atrium. In the presence
of guanyl nucleotides, [3H] acetylcholine binding is decreased, but the
extent of decrease varies from 40 to 90% in different tissues, with the
largest decreases being found in the pons, medulla, cerebellum, and heart
atrium. The results indicate that [3H]acetylcholine binds to high-affinity
M-1 and M-2 muscarinic receptors, and they suggest that most M-2 sites have
high affinity for acetylcholine but that only a small fraction of M-1 sites
have such high affinity.