ABSTRACT
This chapter argues that (at least for classification of muscarinic receptors) the use of the available antagonists allows the classification of a particular response as being mediated by a particular subtype of receptor. Use of the Gaddum-Schild relationship and its variants in studies of actions of competitive antagonists have proved far more valuable in the classification of muscarinic receptors than comparison of agonist potencies. Newer methods have recently been developed and validated which can be used to classify receptors in a particular location, especially where there is a mix of expressed subtypes and these can be used alongside or instead of classical competition binding and Gaddum-Schild analysis of antagonist behavior. The utility of allosteric agents in muscarinic receptor classification was suggested by early observations in functional experiments of the selective antagonist action of gallamine on M2 receptors in heart. Use of antagonists in the way is still the most valuable way to classify muscarinic receptors.
