Pharmacological strategies to selectively label and localize muscarinic receptor subtypes

Equilibrium binding assays support four (M 1 –M 4 ) and molecular cloning studies have identified five (m1–m5) muscarinic receptor subtypes in the brain and peripheral tissues. However, the overlapping affinities of virtually all of the available muscarinic antagonists have permitted the unambiguous direct labeling of only two of the subclasses of muscarinic receptors, the M 1 and M 2 subtypes. Thus, the major obstacle to providing a detailed map of the distribution of muscarinic receptor subtypes in the brain, a tissue that expresses all five receptor proteins, has been the lack of muscarinic receptor subtype‐specific ligands. Recent studies have exploited the distinct kinetic binding properties of muscarinic receptor subtypes and have demonstrated that a combination of both kinetic and equilibrium labeling approaches affords selective labeling of the five muscarinic receptors. Application of these novel labeling strategies has permitted for the first time a comparison of the distinct autoradiographic localization patterns of the M 1 –M 5 receptor subtypes in the brain. These new labeling techniques compare well with previous pharmacological, immunological, and molecular methods for localizing and quantifying muscarinic receptor subtypes, and provide further evidence that the pharmacologically defined M 1 –M 5 receptors correspond to the molecularly defined m1–m5 proteins. While unequivocal distribution profiles of each of the five muscarinic receptors awaits the development of more subtype‐selective ligands, the labeling strategies described here provide an alternative, versatile approach for studying muscarinic receptor subtype distribution in the brain. Drug Dev. Res. 40:104–116, 1997. © 1997 Wiley‐Liss, Inc.