Muscarinic cholinergic receptors in mammalian brain: differences between bindings of acetylcholine and atropine.

Studies were made on the bindings of [3H]-acetylcholine and [3H]-atropine to synaptic plasma membranes from rat brain. Synaptic plasma membranes have reversible, high affinity binding sites for both ligands, the KD values for ACh and atropine being about 20 nM and 1 nM, respectively. The maximal binding capacities for ACh and atropine, respectively, are 0.8-1.2 pmoles and about 1.5 pmoles/mg protein of synaptic membranes. The specific binding of ACh is almost completely inhibited by oxotremorine and atropine. 5,5'-Dithiobis (2-nitrobenzoic acid) (DTNB) increased the ACh-binding to about 1.5 pmole/mg protein. It also increased the inhibition of atropine-binding by ACh about 10-fold. Marked discrepancies were found in the inhibitions of atropine- and ACh-bindings by muscarinic agonists, but not in the inhibitions by antagonists. These findings support the hypothesis that muscarinic receptors have different sites for agonists and antagonists. The possibility that one receptor can be simultaneously occupied by both an agonist and an antagonist is also discussed.