Opiate receptors: problems of definition and characterization.

Receptor binding analysis has a great potential in identifying the site of action of a particular drug and in structure-activity studies. However, since no functional response is measured, an intrinsic difficulty to overcome is in establishing the functional significance of drug binding data. The multiplicity of receptors and subreceptors has a correspondence in a multiplicity of binding sites. It is not trivial which radioactive ligand is used to characterize the sites, and it is suggested that several radioligands with different properties (agonists, antagonists etc.) are studied. Methods are described here that allow for the analysis of ligand selectivity between several sites and their relative regional distribution. When enough data have been accumulated it may be possible to attribute a functional correlate to the binding data.