Functional and Biochemical Evidence for Muscarinic Receptor Subtypes in the Gastrointestinal Tract

Abstract

Although the notion of muscarinic receptor heterogeneity originated from studies on agonist binding (Birdsall et al. 1978), the most compelling evidence for the classification of muscarinic receptors into subtypes was obtained from studies of selective antagonists, of which pirenzepine is the prototype (Hammer et al. 1980). Originally, biochemical characterization of muscarinic receptors was performed in discrete brain areas and later extented to peripheral tissues abundantly endowed with these receptors. Conversely, the majority of pharmacological investigations centered on the interaction of agonists and antagonists with peripheral muscarinic receptors, particularly those involved in gastrointestinal secretion and motility.