Agonist, Partial Agonist, Antagonist and Inverse Agonist: Difference Explained
Most of the receptors are completely inactive under resting conditions. Different types of ligands can work on them. These ligands can be broadly categorized as follow:
Agonist
It activates the receptor to its maximum capacity.
E.g. morphine for μ receptors.
It fully activates μ receptors.
Partial Agonist
Activates the receptor but only partially, not to its maximum capacity.
It prevents the activity of the full agonist.
E.g. buprenorphine for μ receptors.
It activates μ receptors partially.
It prevents the action of morphine which is a full agonist.
Antagonist
It does not produce any action on the receptor.
It prevents the activity of agonists and partial agonists.
E.g. naloxone for μ receptors.
It does not activate μ receptors.
It does not allow the action of morphine or buprenorphine.
Some receptors are constitutively active i.e. they have some baseline activity even without any ligand. E.g. benzodiazepine receptor. They have some activity even when no ligand is acting on it. For such receptors, one more type of ligand can exist, an inverse agonist.
Inverse Agonist
It prevents the constitutive activity of the receptor i.e. makes the receptor inactive.
The resultant effect is opposite to the constitutive activity of the receptor.
For E.g. β carboline for benzodiazepine receptors
Benzodiazepine receptors are constitutively active. Their constitutive activity produces a calming effect on the brain.
β carboline is an inverse agonist on benzodiazepine receptors. It prevents the constitutive calming effect of the benzodiazepine receptors. The resultant effect is anxiety, insomnia or sometimes convulsions.
