Definition of Receptor antagonist

A receptor antagonist is a drug that does not provoke a biological response itself upon binding to a receptor, but blocks or attenuates agonist-mediated responses. It may be competitive (or surmountable), i.e. it binds reversibly to a region of the receptor in common with an agonist, but occupies the site without activating the effector mechanism. The effects of a competitive antagonist may be overcome by increasing the concentration of agonist, thereby shifting the equilibrium and increasing the proportion of receptors which the agonist occupies. Alternatively, antagonists may be unsurmountable, where no amount of agonist can completely overcome the inhibition once it has been established. Unsurmountable antagonists may bind covalently to the agonist binding site (competitive irreversible antagonists), in which case there is a period before the covalent bond forms during which competing ligands can prevent the inhibition. Other types of unsurmountable antagonists act allosterically at a different site on the receptor or an associated ion channel.