What is the difference between a full agonist and a partial agonist at nicotinic receptors?

• A. A full agonist stimulates receptor to the same extent as nicotine; a partial agonist fails to activate receptors at all.
• B. A full agonist blocks nicotine's action; a partial agonist enhances nicotine's effect.
• C. A full agonist fully stimulates receptor; a partial agonist stimulates receptor to a lesser degree and blocks nicotine's full effect.
• D. Both act identically on receptors.

Answer: (C)

The main idea is intrinsic efficacy at the receptor. A full agonist binds to nicotinic receptors and can produce the maximum possible response (Emax) when it occupies the receptors. A partial agonist also binds and activates the receptor, but it yields a smaller, submaximal response even if all receptors are occupied, because its intrinsic efficacy is lower than that of a full agonist. When both are present, the partial agonist can occupy receptors and limit the overall activation, reducing the full effect that nicotine would normally produce. That makes the partial agonist act, in effect, as a dampener or functional antagonist in the presence of a full agonist. So the best description is that a full agonist fully stimulates the receptor, whereas a partial agonist stimulates it to a lesser degree and can blunt nicotine’s full effect.