Agonist

A drug that binds to a receptor of a cell and triggers a response by the cell. An agonist often mimics the action of a naturally occurring substance. An agonist produces an action. It is the opposite of an antagonist which acts against and blocks an action. Agonists and antagonists are key agents in the chemistry of the human body and important players today in pharmacology. For example, in treating Parkinson disease, the long-used drug levodopa can cause uncontrollable, jerky body movements called dyskinesias that can inhibit a person's ability to function. Dopamine agonists mimic the effects of dopamine in the brain by stimulating dopamine receptors with a lower risk of the uncontrollable and irreversible dyskinesias often associated with levodopa therapy. There are agonists now for many of the known hormones. For example, LHRH (luteinizing hormone-releasing hormone) agonists are similar to LHRH in structure and are able to mimic the effects of LHRH, a hormone that controls sex hormones in both men and women. The word "agonist" comes from the Late Latin agnista, contender, from the Greek agnists, contestant, from agn, contest. An agonist is a chemical contestant or contender.

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1. Denoting a muscle in a state of contraction, with reference to its opposing muscle, or antagonist. 2. A drug capable of combining with receptors to initiate drug actions; it possesses affinity and intrinsic activity. [G. agon, a contest]

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ag·o·nist 'ag-ə-nəst n

1) a muscle that on contracting is automatically checked and controlled by the opposing simultaneous contraction of another muscle called also agonist muscle, prime mover compare ANTAGONIST (a), SYNERGIST (2)

2) a chemical substance (as a drug) capable of combining with a receptor on a cell and initiating the same reaction or activity typically produced by the binding of an endogenous substance <binding of adrenergic \agonists> compare ANTAGONIST (b)

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n.

1. (prime mover) a muscle whose active contraction causes movement of a part of the body. Contraction of an agonist is associated with relaxation of its antagonist.

2. a drug or other substance that acts at a cell-receptor site to produce an effect that is the same as, or similar to, that of the body's normal chemical messenger. Cholinergic drugs (see parasympathomimetic) are examples.

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ag·o·nist (agґə-nist) [Gr. agōnistēs combatant] 1. a person or thing involved in a struggle or competition. 2. agonistic muscle. 3. in pharmacology, a drug that has affinity for and stimulates physiologic activity at cell receptors normally stimulated by naturally occurring substances.