1 An opponent or adversary in a struggle. Compare agonist (1).
2 A muscle that opposes the action of another. Compare agonist (2).
3 A substance that depresses the effect of an agonist (3) either by acting at different neuroreceptor sites from those of the agonist (a non-competitive antagonist) or by binding to the same neuroreceptors as the agonist without exciting or inhibiting the postsynaptic neurons (a competitive antagonist, also called a receptor blocker), a typical example being the narcotic antagonist naloxone, which binds to neuroreceptors in the brain that would otherwise be occupied by morphine, and thereby prevents morphine from exerting its usual effects. See also dopamine antagonist. Compare agonist (3). [From Greek anti against + agon a festival of competitions, from agein to lead]