ABSTRACT
Even today, the precise definition of a receptor is problematic. Certainly Stephenson's long-established definition of "that spatial arrangement of atoms to which a substance endogenous to the organism attaches itself as an essential step in modifying cellular function" no longer seems appropriate (6). Nevertheless it can be argued that it is still essential to classify receptors according to their natural ligands, because of the critical importance of such information in understanding the functional role of a given receptor or receptors in the whole body. In the light of modern knowledge, a receptor must now be regarded as a macromolecule, which may or may not be a single molecular entity, with multiple sites of interaction (7-9). The individual proteins can be defined in terms of their corresponding genes, but there may be local molecular differences according to the host cell, in terms of variable patterns of glycosylation and different transcriptional splice variants (10-14). A modern definition of a receptor has been suggested as "the entire protein molecule or cluster of molecules which can selectively recognize and be collectively activated by an endogenous ligand (agonist) to mediate a cellular event" (15). Synthetic drugs may recognize different (amino acid) sites on the same receptor, and even if only a very localized region on the macromolecule is required for agonist binding, adjacent proteins (specific G-proteins or ion channel components) will be essential for transducing the effect produced by agonist binding (16-21).
