Final answer:
The statement is true; antagonists block receptor sites and prevent cellular responses by reducing neurotransmitter activity, as opposed to agonists, which enhance neurotransmitter activity.
Step-by-step explanation:
The statement that an antagonist is a drug or other chemical that blocks a receptor site and inhibits or prevents a response in the receiving cell is true. Antagonists work by decreasing the activity of particular neurotransmitters. They can do this by interfering with neurotransmitter synthesis or more commonly, by binding to and blocking neurotransmitter receptors.
This prevents the neurotransmitter from binding to its receptor and activating it, which inhibits or prevents the typical cellular response. For instance in treating diseases like Parkinson's one therapeutic strategy might be to use agonists, which are the opposite of antagonists to mimic the effects of neurotransmitters like dopamine which is in low supply in the condition.