The pharmacological basis of receptor binding

Radiolabeled ligands can be used to characterize recognition sites of natural receptors. Verification of a receptor-specific binding of a radioligand requires a series of in vitro approaches, as saturation, competition, and kinetic (association and dissociation) experiments. Success of ligand binding depends on may factors involving the radio-pharmaceutical used as the radioligand, methods for separating bound and free ligand, proper use of the non-specific binding marker, and proper analysis of the binding data. A major theoretical constraint is the very short time available for separating bound an free radioligand, since a considerable fraction of radioligand can dissociate from the receptor binding sites during this process, in particular if the receptor affinity of the radioligand is low. Saturable, steady-state binding data can be analyzed utilizing standard binding isotherms and non-linear regression analysis as well as Scatchard plots. Competition experiments with unlabeled drugs are the most important in determining pharmacology of binding. Each method is subject to potential artifacts. Estimation of association is straightforward in order to assess the time necessary to achieve binding equilibrium.