Human M1-, M2- and M3-muscarinic cholinergic receptors: binding characteristics of agonists and antagonists.

The muscarinic acetylcholine receptors were identified in membrane preparations from human tissues by the specific binding of 1-[benzilic-4,4'-3H] quinuclidinyl benzilate. Saturation binding isotherms of this radioligand yielded a total amount of receptors of 435 +/- 208, 159 +/- 65 and 913 +/- 89 fmol/mg protein, respectively, in the hippocampus, pons and submandibular gland. Non linear least squares analysis of competition binding studies with the antagonists pirenzepine and AF-DX 116 indicates that the majority of receptors are of the M1-type in the hippocampus (83%, high affinity for pirenzepine, intermediate affinity for AF-DX 116), the M2-type in the pons (low affinity for pirenzepine and high affinity for AF-DX 116), and the M3-type in the submandibular gland (low affinity for pirenzepine and AF-DX 116). Competition binding parameters of the agonists carbachol, arecoline, oxotremorine, pilocarpine and MCN-A-343 were compared for M1, M2 and M3 receptors in the human hippocampus, pons and submandibular gland. GTP caused a shift to the right and a steepening of the shallow agonist competition curves in the 3 tissues but did not affect the initially steep ones. This effect is explained by a GTP-mediated conversion of high- to low-agonist affinity sites. The extent of the nucleotide shift was much greater for M2 receptors as compared with M1 and M3 receptors. The GTP effect was impaired by the sulphydryl reagent N-ethylmaleimide, probably due to alkylation of GTP-binding proteins. Moreover, the reagent provoked also an increase of the agonist affinity for the uncoupled muscarinic receptors. For all agonists, this increase was more pronounced for the M2 receptors than for the M1 and M3 receptors. These findings suggest structural differences between the agonist binding sites of M1 and M3 receptors versus the M2 receptors.