Editing Neurotransmitter (section)

=== Antagonists ===
{{Main|Receptor antagonist}}
An antagonist is a chemical that acts within the body to reduce the physiological activity of another chemical substance (such as an opiate); especially one that opposes the action on the nervous system of a drug or a substance occurring naturally in the body by combining with and blocking its nervous receptor.<ref name="M-W revised">{{cite web|title=Antagonist|url=http://www.merriam-webster.com/medical/antagonist|website=Medical definition of Antagonist|access-date=5 November 2014}}</ref>

There are two main types of antagonist: direct-acting Antagonist and indirect-acting Antagonists:
# Direct-acting antagonist- which takes up space present on receptors which are otherwise taken up by neurotransmitters themselves. This results in neurotransmitters being blocked from binding to the receptors. An example of one of the most common is called Atropine.
# Indirect-acting antagonist- drugs that inhibit the release/production of neurotransmitters (e.g., [[Reserpine]]).

==== Drug antagonists ====
An antagonist drug is one that attaches (or binds) to a site called a receptor without activating that receptor to produce a biological response. It is therefore said to have no intrinsic activity. An antagonist may also be called a receptor "blocker" because they block the effect of an agonist at the site. The pharmacological effects of an antagonist, therefore, result in preventing the corresponding receptor site's agonists (e.g., drugs, hormones, neurotransmitters) from binding to and activating it. Antagonists may be "competitive" or "irreversible".{{cn|date=March 2025}}

A competitive antagonist competes with an agonist for binding to the receptor. As the concentration of antagonist increases, the binding of the agonist is progressively inhibited, resulting in a decrease in the physiological response. High concentration of an antagonist can completely inhibit the response. This inhibition can be reversed, however, by an increase of the concentration of the agonist, since the agonist and antagonist compete for binding to the receptor. Competitive antagonists, therefore, can be characterized as shifting the [[dose–response relationship]] for the agonist to the right. In the presence of a competitive antagonist, it takes an increased concentration of the agonist to produce the same response observed in the absence of the antagonist.{{cn|date=March 2025}}

An irreversible antagonist binds so strongly to the receptor as to render the receptor unavailable for binding to the agonist. Irreversible antagonists may even form covalent chemical bonds with the receptor. In either case, if the concentration of the irreversible antagonist is high enough, the number of unbound receptors remaining for agonist binding may be so low that even high concentrations of the agonist do not produce the maximum biological response.<ref name="Encyclopedia.com 2">{{cite web | vauthors = Goeders NE |title=Antagonist |url=http://www.encyclopedia.com/topic/Antagonist.aspx |access-date=2 December 2014 |location=Encyclopedia of Drugs, Alcohol, and Addictive Behavior |year=2001}}</ref>