What is an agonist?

• A. A substance that prevents a biological response
• B. A substance that activates a receptor to produce a biological response
• C. A substance that binds without activating a receptor
• D. A substance that causes drug tolerance

Answer: (B)

An agonist is defined as a substance that activates a receptor to produce a biological response. This means that when an agonist binds to its corresponding receptor, it triggers a conformational change in the receptor, leading to a series of cellular responses. These responses can include a variety of physiological effects depending on the type of receptor and the tissue or organ where it is located.

For instance, in the case of neurotransmitter receptors in the brain, agonists can enhance mood, promote cognitive functions, or alter motor activity by mimicking the action of naturally occurring substances in the body. This activation is a fundamental concept in pharmacology, as it relates to how drugs can be used to modify physiological processes for therapeutic benefit.

In contrast, other options describe different actions: a substance that prevents a biological response refers to an antagonist, while binding without activating a receptor is characteristic of an inverse agonist. The concept of drug tolerance relates to the body's reduced response to a drug after repeated use, but does not define what an agonist is.