AFFINITIES OF MUSCARINIC DRUGS FOR [H-3] N-METHYLSCOPOLAMINE (NMS) AND [H-3] OXOTREMORINE (OXO) BINDING TO A MIXTURE OF M(1)-M(4) MUSCARINIC RECEPTORS - USE OF NMS/OXO-M RATIOS TO GROUP COMPOUNDS INTO POTENTIAL AGONIST, PARTIAL AGONIST, AND ANTAGONIST CLASSES

The relative affinities of various muscarinic drugs in the antagonist ([H-3]N-methyl scopolamine ([H-3]NMS)) and agonist ([H-3]Oxotremorine-m ([H-3]OXO-M)) binding assays using a mixture of tissues containing M(1)-M(4) receptor subtypes have been determined. [H-3]NMS bound with high affinity (K-d = 25 +/- 5.9 pM; n = 3) and to a high density (B-max = 11.8 +/- 0.025 nmol/g wet weight) of muscarinic receptors. [H-3]OXO-M appeared to bind to two binding sites with differing affinities (K-dl = 2.5 +/- 0.1 nM; K-d2 = 9.0 +/- 4.9 mu M; n = 4) and to a different population of binding sites (B-max1 = 5.0 +/- 0.26 nmol/g wet weight; B-max2 = 130 +/- 60 nmol/g wet weight). Well known antagonists exhibited high affinity for [H-3]NMS binding but a lower affinity for [H-3]OXO-M binding. The opposite was true for acetylcholine and other known agonists. However, pilocarpine and McN-A-343 had similar affinities for sites labeled by both radioligands. Using the ratios of antagonist-to-agonist binding affinities, it was possible to group compounds into apparently distinct full agonist (ratios of 180-665; e.g. carbachol, muscarine, OXO-M, OXO-S and arecoline), partial agonist (ratios of 14-132; e.g. McN-A-343, pilocarpine, aceclidine, bethanechol, OXA-22 and acetylcholine) and antagonist (ratios of 0.22-1.9; e.g. atropine, NMS, pirenzepine, methoctramine, 4-DAMP and p-fluorohexahydrosialo-difenidol) classes. These data suggest that the NMS/OXO-M affinity ratios using a mixture of M(1)-M(4) muscarinic receptors may be a useful way to screen and group a large number of compounds into apparent agonist, partial agonist, and antagonist classes of cholinergic agents.