Pharmacological strategies to selectively label and localize muscarinic receptor subtypes

Abstract

Equilibrium binding assays support four (M1–M4) 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 M1 and M2 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 M1–M5 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 M1–M5 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.

Cholinergic Stimulation of Salivary Secretion Studied with M1 and M3 Muscarinic Receptor Single- and Double-Knockout Mice

“…However, pharmacological and biochemical studies suggest that M 1 (Watson and Culp, 1994;Culp et al, 1996;Luo et al, 2001;Tobin et al, 2002) and M 5 mAChRs (Flynn et al, 1997;Meloy et al, 2001;Tobin et al, 2002) may also play a role in muscarinic agonist-mediated salivary secretion. Moreover, two recent studies reported that muscarinic agonist-induced stimulation of salivary output was impaired not only in M 3 receptor KO mice but also in M 1 and M 4 receptor KO mice (Bymaster et al, 2003) and M 5 receptor KO mice .…”

“…For example, it has been proposed based on studies using subtype-preferring muscarinic antagonists that M 5 receptors may be present/functional in salivary gland tissue (Flynn et al, 1997;Meloy et al, 2001;Tobin et al, 2002). Moreover, Takeuchi et al (2002) reported that salivary secretion induced by pilocarpine (1 mg/kg, s.c.) was slightly reduced (by ϳ10 -20%) in M 5 receptor KO mice.…”

In Vivo Quantitative Autoradiographic Analysis of Brain Muscarinic Receptor Occupancy by Antimuscarinic Agents for Overactive Bladder Treatment

“…In addition, muscarinic receptor subtype selectivity of antimuscarinic agents may be implicated in the appearance of the CNS effect. All of five muscarinic receptor subtypes are expressed in the brain (Flynn et al, 1997;Oki et al, 2005). The M 1 receptor was abundant in the cortex and hippocampus.…”

Cholinergic receptor subtypes and their role in cognition, emotion, and vigilance control: An overview of preclinical and clinical findings

“…Within the striatum, M4/m4 is most often found (Levey 1993). For further details on the distribution of the muscarinic subtypes in the brain, see Flynn et al (1997) or Caulfield (1993). This review will focus on the M 1 and M 2 subtypes since previous studies have almost exclusively dealt with their implication in cognitive functions.…”