Agonists Antagonists And Partial Agonists

Some drugs have very complex actions and many drugs act at specific RECEPTOR, locations on the surface of a cell. All of the drugs that belong to the class of drugs called opioids act at opioid receptors on the surface of cells. Usually these cells are neurons, but there are also opioid receptors on white blood cells. Once a drug binds to a receptor, it can either turn it on (AGONIST) or do nothing (ANTAGONIST). Even if a compound does nothing once it binds to the receptor, it still blocks the site and prevents an active compound from binding to the receptor. The situation is much like a key in a lock; some keys fit into the lock but will not turn, and as long as they remain in the lock they prevent the insertion of keys that would turn the lock. Finally, there are drugs known as partial agonists; these compounds bind to the receptor and turn it on but not nearly as well as pure agonists.

Again, using the key analogy, these partial agonists will turn in the lock, but only with some jiggling, lowering the efficiency in opening the door. Pharmacologically, partial agonists have limited effects at the receptor, termed a ceiling effect. This means that increasing the dose further will not give a greater response. To further complicate understanding of these drug actions, it is important to recognize that the opioid receptors (and many other types of receptors as well) are actually fami lies of similar but subtly different receptor types. Some opioids are agonists at one receptor type and partial agonists or even antagonists at another receptor type. These drugs are termed mixed agonist/ antagonists and they can have complex pharmacological profiles. For this reason it can be difficult for pharmacists to determine conversion amounts (for example, to methadone) (Magill-Lewis, 2000).

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