What an Agonist, Partial Agonist, Antagonist, and Inverse Agonist Are
adrenergic, histamine, adenosine, opioid, dopamine or serotonin receptors, constitute a large portion of currently used therapeutics. A common property of GPCRs is that upon activation (agonist binding) they transmit signals across the plasma membrane via an interaction with heterotrimeric G proteins.
Both inverse agonism and functional selectivity need to be considered when drugs are used as medicines or as research tools. It is difficult to overestimate the importance of pharmacology for medicine and research. In medicine, drugs are essential components of a physician’s toolbox to treat disease. In fact, drugs have been used as medicines to treat disease since the beginning of recorded history.
A maximal system response can still be achieved, however - unless the partial agonist binds to the receptor in some sort of irreversible manner, it can be displaced from the receptor by a sufficiently high concentration of full agonist. Thus, the efficacy of the full agonist is not affected, but its potency is reduced.The International Union of Pharmacology Committee on Receptor Nomenclature and Drug Classification (Neubig et al, 2003) give an "official" definition as "Agonist: A ligand that binds to a receptor and alters the receptor state resulting in a biological response".
This is of particular importance in conditions affecting the brain as disorders such as depression, schizophrenia and Parkinson’s disease are associated with an imbalance in neurotransmitters and improvements in the drugs available to us to treat such conditions will benefit millions of people all around the world.
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