What is an antagonist?

• A. Opponent, blocks the receptor
• B. A molecule that increases a neurotransmitter's action
• C. A molecule that mimics a transmitter but produces a weaker response
• D. A molecule that generates neurotransmitter release

Answer: (A)

An antagonist is a molecule that blocks a receptor and prevents the neurotransmitter from producing its effect. It binds to the receptor but does not activate it, or it prevents the transmitter from binding, thereby reducing signaling. This contrasts with agents that increase transmitter action (agonists) or that mimic the transmitter to produce at least some response (partial agonists). Antagonists can act competitively, occupying the receptor site and can be displaced by higher levels of the neurotransmitter, or noncompetitively, altering the receptor in a way that cannot be overcome by more transmitter. A familiar example is naloxone, which blocks opioid receptors to reverse opioid overdose.