Difference between Antagonist and Inverse Agonist?

Hi everyone,
I would like to know the difference between an Antagonist and an Inverse Agonist. I heard that H2 receptor antagonist is actually an inverse agonist.
So what exactly is an inverse agonist and how is it different from an antagonist? Thanks.
Answer:
This has to do with the distict mechanisms of receptor activity, whether the 2 State Model of activation or the Constitutive model of activation is in place.
The receptor can be thought of as existing in an equilibrium between an inactive and an active form. Agonist bind and make the active conformation much more likely. Inverse agonists probably bind to and stabilize the inactive form, which pushes the equilibrium away from the spontaneous formation of the active conformation of the receptor.
More research needs to be done for a class or classes of receptor to be definitively assigned the Inverse label. It is possible that the only known one is the benzodiazepine receptor, information that may be dated because of the lack of research and categorization.

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