Which of the following is an example of a receptor antagonist?

Curare serves as an example of a receptor antagonist because it blocks the action of the neurotransmitter acetylcholine at the neuromuscular junction. By binding to acetylcholine receptors without activating them, curare inhibits the normal action of acetylcholine, preventing muscle contraction. This characteristic of curare highlights the role of receptor antagonists in modulating neurotransmission by obstructing receptor activation, which can be critical in understanding neuromuscular function and pharmacological interventions.

The other substances listed do not qualify as receptor antagonists. Nicotine, for example, acts as an agonist of nicotinic acetylcholine receptors, stimulating rather than inhibiting their activity. Dopamine and serotonin are neurotransmitters themselves, not antagonists, as they play roles in activating their respective receptors. Understanding these distinctions is important in neurobiology, particularly in pharmacology and the mechanisms underlying various physiological responses.