An antagonist is a chemical or drug that binds to receptors in the brain and prevents an agonist from having a reaction.

An inverse antagonist not only prevents an agonist from having a reaction on a receptor but causes the opposite response to occur. A receptor is the part of a nerve that receives and reads chemical signals. It then transmits information to the brain and nervous system using electrical signals. Antagonists attach to the receptors in the brain and block the effect of agonists (which are chemicals that stimulate receptors and cause a response).

Conotoxins (which are found in some marine snails, frogs, and corals) and snake toxins (from the cobra and sea snake) are antagonists that block acetylcholine receptors irreversibly and cause flaccid paralysis. Botulism and tetanus are toxins that are antagonists because they block the release of acetylcholine. Botulism can cause illness and death due to exposure but is used in small doses as Botox, which is used to hide wrinkles in the skin and treat medical conditions like morbid obesity, ulcers, and club foot. Scopolamine is an antagonist that is used as anti-nausea medication while curare blocks acetylcholine and is used in surgery.

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