What is an “agonist” in pharmacology?

An agonist in pharmacology is defined as a substance that activates a receptor to produce a biological response. This means that when an agonist binds to a receptor, it induces a conformational change that typically results in a specific response within the body, such as an increase in heart rate or secretion of hormones. Agonists can mimic the action of naturally occurring substances, such as neurotransmitters or hormones, thus facilitating or enhancing the desired physiological effects.

In contrast, other options describe functions of different types of substances. For instance, a substance that blocks a receptor would be identified as an antagonist, which prevents the natural ligand from activating the receptor. Similarly, a substance that reduces a biological response does not activate the receptor but rather diminishes its activity or signaling. Lastly, a substance that has no effect on receptors would be classified as an inert or neutral substance, having no pharmacological action. Therefore, the defining characteristic of an agonist is its role in activating receptors to elicit specific biological responses.