Antagonists oppose or inhibit the effect of specific neurotransmitters on postsynaptic cells. An example of an antagonist is curare, which is a substance that was discovered by a group of South American natives who dipped their arrows in a plant extract containing curare. When they used their arrows, the result was total paralysis. Curare works by taking over the receptor sites, thus blocking the action of acetylcholine at the synapse from nerve to muscle.