Which statement best distinguishes an inverse agonist from a competitive antagonist?

• A. An inverse agonist decreases basal receptor activity; a competitive antagonist lacks intrinsic activity and blocks binding.
• B. An inverse agonist binds to a different receptor; a competitive antagonist binds to the same receptor.
• C. An inverse agonist requires higher concentrations to be effective; a competitive antagonist does not.
• D. An inverse agonist increases basal activity; a competitive antagonist always decreases activity.

Answer: (A)

The key idea is how ligands influence receptor signaling when the receptor has baseline (constitutive) activity. An inverse agonist binds to the same receptor as the endogenous ligand and shifts the receptor to an inactive state, reducing the signaling that occurs without any agonist present. That is why it decreases basal receptor activity. A competitive antagonist, on the other hand, has no intrinsic activity of its own; it simply occupies the binding site so that an agonist cannot activate the receptor. In the absence of agonist, it doesn’t alter basal signaling. It blocks activation by competing for the site, and any effect it has can be overcome by adding more agonist. So the defining distinction is that an inverse agonist actively reduces baseline activity, whereas a competitive antagonist blocks activation without changing basal signaling.