agonist

/ˈæɡənɪst/
noun
  1. A muscle that contracts to produce a movement, working against another muscle (the antagonist).
    • Physical therapists often strengthen the agonist to improve joint stability.
    • The biceps brachii is the agonist when you curl your arm upward.
    • In a bicep curl, the agonist muscle shortens to lift the weight.
  2. A substance (such as a drug) that activates a receptor in the body, producing a biological response.
    • An opioid agonist can relieve severe pain but may cause dependence.
    • This medication acts as an agonist on dopamine receptors to treat Parkinson's disease.
    • Scientists are testing a new agonist that mimics the effects of natural hormones.
  3. A person who actively fights for a cause or engages in a struggle.
    • As an agonist for educational reform, he spoke at many school board meetings.
    • She was a fierce agonist for social justice in her community.
    • The activist became a well-known agonist in the fight for environmental protection.
Antonyms
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