USE OF M1-TOXIN AS A SELECTIVE ANTAGONIST OF M1-MUSCARINIC RECEPTORS

m1-Toxin is the only ligand which is known to bind specifically to the extracellular face of genetically defined ml muscarinic receptors; it binds pseudoirreversibly. A variety of studies were performed to eval uate the usefulness of m1-toxin as a selective antagonist of ml recept ors. Exposure of slices of the rat cerebral cortex to m1-toxin in phys iological buffer blocked the subsequent binding of 1.0 nM [H-3]pirenze pine to ml receptors in the slices. The toxin also blocked 70% of carb achol-stimulated turnover of radiolabeled inositol phosphates in hippo campal slices. Autoradiographs showed that m1-toxin bound to sections of once-frozen tissue and blocked the binding of [H-3]quinuclidinyl be nzilate to regions of the rat brain rich in ml receptors. The toxin bl ocked the binding of [H-3]antagonists to pure ml receptors on the surf ace of living Chinese hamster ovary cells, but did not block intracell ular receptors. In membrane preparations from the rat cortex and hippo campus the toxin blocked the binding of [H-3] antagonists to ml recept ors quantitatively and selectively, but had no effect on binding sites for [H-3]nicotine. Subsaturating amounts of the toxin bound to ml rec eptors in membranes at 4-degrees-C in less than 30 sec. Low concentrat ions of m1-toxin blocked ml receptors in solution in digitonin but had no effect on separate preparations of pure m2, m3, m4 or m5 receptors . Thus m1-toxin appears to be a very useful antagonist for m1 receptor s in intact tissue, on isolated cells, in membranes and in solution, i n a variety of media.