Abstract

A family of five cholinergic muscarinic receptor genes (m1, m2, m3, m4, and m5) has recently been identified and cloned. In order to investigate the pharmacological properties of the individual muscarinic receptors, we have transfected each of these genes into Chinese hamster ovary cells (CHO-K1) and have established stable cell lines expressing each receptor. In the present study we have examined the antagonist binding properties of each muscarinic receptor. Antagonists were chosen that had previously been proposed to be selective for muscarinic receptor subtypes and included pirenzepine, AF-DX 116, methoctramine, dicyclomine, hexohydrodifenidol, hexahydrosiladifenidol, hexocyclium, and silahexocyclium. m1, m2, and m3 receptors express binding properties similar to those expected of high affinity pirenzepine-type receptors of cerebral cortex ("M1"), low affinity pirenzepine-type receptors of atria ("M2 cardiac type"), and the intermediate affinity pirenzepine-type receptors found in exocrine glands ("M2 glandular type"), respectively. The M1/M2 schema cannot readily accommodate the binding properties of the m4 and m5 receptors. Pirenzepine, methoctramine, and hexahydrosiladifenidol were the most selective agents for the m1, m2, and m3 receptors, respectively. None of the antagonists used in this study were uniquely selective for either the m4 or m5 receptors. The diverse binding profiles of individual cloned receptors and the widespread distribution of m1-m4 mRNAs indicate that radioligand binding studies performed on primary tissues may actually be assessing the composite properties of a heterogeneous mixture of muscarinic receptor subtypes.