ABSTRACT
At the beginning of twentieth century, J. N. Langley and P. Erlich independently recognized the fundamental features of drug-receptor interaction, i.e., specificity, the basis of cellular recognition and activation and the cellular response [1]. The concept of a receptor includes the key attributes of ligand recognition and signal transduction. The signal transduction process may be mediated through an integral part of the receptor structure or involve receptor interactions with additional nonreceptor proteins [2]. The final proof of the existence of the pharmacological receptors came from the recent advances in biochemistry and molecular biology to purify, sequence, clone, and express receptor proteins. The discovery and development of receptors as drug targets stems from the drug interactions with receptor molecules located in the plasma membranes or in the cytosol of target cells. Several receptors have now been very well characterized. Gene sequences of hundreds of orphan receptors have been identified by homology analysis of genome databases. The ligands of these orphan receptors have to be characterized to determine their functions in order to convert these receptors into new drug targets.
