The relation of receptor-binding studies to receptor function

Binding studies give direct information regarding the binding of a drug to its receptor. However, it is important to define the conditions of the binding assay such that the binding in vitro agrees with that estimated from whole tissue pharmacology in order to establish the validity of the binding assay. This is relatively easy for antagonist-receptor interactions. In contrast, the agonist-binding properties are generally more complex and may be explained by differences in receptor-effector coupling. Binding studies are also capable of detecting the interaction of ligands with, and elucidating the nature of, an effector which is directly coupled to the receptor. These are manifest as an allosteric modulation of the binding of the drug to its receptor. One parameter which binding studies give is the number of receptor-binding sites: this cannot be obtained from whole tissue pharmacology. Changes in the number of receptor-binding sites can be related to the effects of perturbation of a biological system. However, caution must be exerted in the equation of numbers of binding sites with numbers of physiologically relevant functional receptors.