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A Drug That Is an Antagonist Functions to Do What?

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An antagonist is a type of ligand or drug that avoids or dampens a biological reaction. Upon binding to the receptor, information technology does non activate.

Rather it tends to block the particular receptor. Sometimes, they are also referred to as blockers such as alpha-blockers or beta-blockers.

In this article, we will discuss dissimilar types of antagonists and the mechanisms of how they work.  Nosotros will besides discuss specific antagonists in detail that have special importance in neurology.

Types

Antagonists can be classified into dissimilar categories depending on the mechanism they apply to antagonize a item biological response. They can also exist classified de[catastrophe on the type of receptor.

Here, we volition discuss different types of antagonists
depending upon their mechanism of action.

Chemical Antagonists

It is a type of antagonist that binds to a drug or ligand and renders it ineffective. A chemic antagonist does so by causing certain chemical changes in the ligand information technology binds.

The adversary binds to the agonist and forms an inactive complex that cannot perform any role.

For case, protamine sulfate is a positively charged drug. When information technology is given Iv, it binds to heparin; a negatively charged drug, forming an inactive complex. As a result, heparin cannot perform its role.

Physiological Adversary

It is a blazon of antagonist that binds to a carve up receptor and counters the effect of the agonist. 2 drugs would exist said physiological antagonists of one another, if they bind to two different receptors and produce opposite furnishings.

For example, insulin and glucagon are physiological
antagonists of one some other. Both of them bind to 2 dissimilar receptors, but
the furnishings produced by each of them are opposite to each other. Insulin
decreases claret glucose levels while glucagon increases it.

Pharmacological Antagonist

Pharmacological antagonist binds to the aforementioned receptor as the agonist does. It occupies the binding site of the receptor and prevents the binding of agonist to the receptor. In this style, it prevents the activation of the receptor. These include receptor blockers such every bit alpha-blockers, beta-blockers, etc.

Depending on the fact that whether their effect can be
countered or non, pharmacological antagonists are further divided into ii
types:

Reversible or competitive adversary

Information technology is a type of pharmacological antagonist whose
effect can be countered by increasing the concentration of agonist.

For example, prazosin is a reversible antagonist of
blastoff-1 receptors.

Irreversible or non-competitive antagonist

Information technology is a type of pharmacological antagonist whose
effects cannot be countered by increasing the concentration of agonist.

For example, phenoxybenzamine is an irreversible antagonist of alpha-receptors.

Allosteric Antagonist

Information technology is a blazon of antagonist that binds to the receptors but on a dissimilar as compared to the binding site of the real agonist. Binding of allosteric adversary induces changes in the receptor.

This type of antagonist can prevent the activation of receptor even after binding of the agonist. They can as well modify shape of the binding site in such a manner that it no longer binds to the agonist.

Physical  antagonist

It is a type of antagonist that is based on the
physical holding of the drug. It tin bind to the agonist and prevent its
action.

For instance, when charcoal is used in instance of poison
ingestion, such as alkaloid poisons; it acts as a concrete antagonist. It has
the ability to blot the poison. In this way, information technology prevents the poisonous substance from
entering the bloodstream, thus countering it.

Changed agonists

Although sounds more than like an agonist, an changed agonist is a special blazon of antagonist. An inverse agonist binds to the aforementioned receptor as the agonist does. However, instead of increasing the activity of the receptor, an inverse agonist decreases it.

Inverse
agonist works only when the receptor has some intrinsic action. In that case,
the binding of an agonist increases the intrinsic activity of the receptor
whereas binding of an inverse agonist decreases it.

GABA A receptors have intrinsic activity. Agonists of GABAA receptors include muscimol, whereas sure beta-carbolines act as inverse agonists of GABAA receptors.

Antagonist of import in CNS

In this section, we will hash out some antagonists that have particular importance in the central nervous system.

Dopamine Antagonist

Dopamine antagonists block the activeness of dopamine. They accept profound importance in several antipsychotic diseases such as schizophrenia, bipolar disorder, and psychosis. They are also used in non-psychotic illnesses such as orthostatic hypotension, vomiting, and nausea.

Dopamine
antagonists can be classified into two types;

D1 blockers

They cake the D1-like receptors. They include D1 and D5 receptors. These are the stimulatory receptors coupled with Gs proteins.

They are present in substantia nigra, and the straight pathway of the basal ganglia. They are peripherally present in the renal avenue, mesenteric avenue and splenic artery.

D2 blockers

They cake the D2 like receptors that include D2, D3, and D4 receptors. These are the inhibitory receptors coupled to G i proteins.

They are present in presynaptic and postsynaptic neurons, striatum, substantia nigra, indirect pathway of basal ganglia. Peripherally, they are present in kidneys, adrenal cortex, and arteries.

Most
of the dopamine antagonists are non-specific. They can block both D1-like and
D2-like receptors.

Uses

Dopamine
antagonists are also known equally typical antipsychotic drugs. They are used in
dissimilar psychotic conditions such equally:

  • Schizophrenia
  • Mania
  • Bipolar disorder
  • Severe psychosis

They
are also used in different non-psychotic diseases such as

  • Vomiting
  • Nausea
  • Hyperkinetic disorders
  • Hypertension

Drugs

Dopamine
antagonists include drugs such every bit haloperidol, chlorpromazine, fluphenazine,
etc.

Serotonin Adversary

As the name indicates, they block the activation of serotonin receptors. Serotonin antagonists have profound application in a number of psychiatric diseases such as depression, anxiety, insomnia, etc.

Different
types of serotonin receptors are present in the trunk. Nonetheless, with respect to
the brain, 5HT2 are the almost important.

The
5HTii receptors are widely distributed in the cortex, basal ganglia,
the limbic organisation, and other areas of the encephalon. They are One thousand-protein coupled
receptors that are coupled to Gq proteins. These are the stimulatory
receptors. They are associated with increased neuronal activity in the brain.

Uses

These
drugs are chosen novel or atypical antipsychotic drugs. Like dopamine
antagonists, they are used in weather condition similar;

  • Schizophrenia
  • Mania
  • Bipolar disorder
  • Severe psychosis

These
drugs have less severe side furnishings equally compared to dopamine antagonists.

Drugs

Serotonin
5HTtwo receptor antagonists include clozapine, olanzapine,
risperidone, apiprazole etc.

Antagonists important in ANS

In
this section, we volition discuss antagonists that are of import in the autonomic
nervous system.

Muscarinic Antagonists

The muscarinic receptor antagonists demark to acetylcholine receptors and forbid their activation. As acetylcholine is the main neurotransmitter of the parasympathetic system, these antagonists can successfully block the entire parasympathetic activation.

There
are iii types of muscarinic receptors;

  1. M1 receptors, excitatory receptors nowadays in encephalon
  2. M2 receptors, inhibitory receptors present in heart
  3. M3 receptors, excitatory receptors nowadays inn smooth muscles, glands, eyes, etc.

All
these receptors are G-protein coupled receptors. M1 and M3 are Gq-coupled
while M2 are Gi-coupled receptors.

All
the muscarinic antagonists are non-specific in nature and block all types of
receptors.

Uses

Muscarinic
antagonists find a number of uses. These include the following;

  • Management of AChE inhibitors overdose
  • Ophthalmology (dilation of student)
  • Asthma and COPD
  • Motion sickness
  • Overreactive float
  • Anti-spasmodic
  • Antidiarrheal

Drugs

The
drugs in this category include atropine, benztropine, ipratropium, scopolamine,
etc.

Nicotinic Receptor Antagonists

These
antagonists block the activation of nicotinic receptors present in ganglia and
skeletal musculus. Thus, they are farther classified into two types;

Ganglionic blockers

These antagonists cake the nicotinic receptors present in ganglia. Thus, they are able to block sympathetic likewise as parasympathetic firing. It is because the ganglia of both these system have nicotinic receptors.

They reduce the predominant autonomic tone. In the case of arterioles, venules and sweat glands, they block the sympathetic tone; while in other organs, they block the parasympathetic tone.

Two
important ganglionic blockers include hexamethonium and mecamylamine.

Neuromuscular blocking drugs

They cake the nicotinic receptors present at neuromuscular junction. They are used every bit skeletal musculus relaxants. These include tubocurarine, atracurium, etc.

Blastoff Receptor Antagonists

These
drugs block the activation of blastoff receptors.

They
are further classified depending on the type of blastoff receptors they block.

Alpha-ane blockers

They
block the alpha-ane receptors nowadays in arteries, venules, eyes, float, etc.
these include prazosin, tamsulosin, etc.

These
drugs are used in:

  • Hypertension
  • Urinary retention
  • Dilation of eye
  • Cardiovascular disorders

Alpha-2 blockers

They
block the blastoff-ii receptors present in the presynaptic nervus final. These
include drugs similar methyl-dopa and clonidine.

They
are used in hypertension, particularly for the management of hypertension in
pregnancy.

Beta Receptor Antagonists

They
block the beta-2 receptors. They are of two types;

  1. Cardiocelective, they cake only beta-i receptors nowadays in the heart.
  2. Non-cardioselective, they are not-selective block both beta-1 and beta ii receptors.

These
drugs include esmolol, atenolol, propranolol, etc.

They
are used in angina, myocardial infarction, cardiac failure, asthma etc.

Conclusion/Summary

An antagonist is a drug or ligand that tends to terminate or impede a biological reaction. They produce effects opposite to that of the agonist.

They
are of different types depending on their mechanism of activity.

These  include;

  • Concrete antagonists
  • Chemical antagonists
  • Physiological antagonists
  • Pharmacological antagonists
  • Allosteric antagonists

An
changed agonist is a special blazon of antagonist that decreases the intrinsic
activity of a receptor.

Adversary
find of import applications in the CNS as well equally the ANS.

The
important CNS antagonists include dopamine antagonists and serotonin
antagonists.

The
important ANS antagonists include muscarinic antagonists, nicotinic
antagonists, alpha-blockers and beta-blockers.

References

  1. Aitkenhead AR, Rowbotham DJ, Smith
    G. Textbook of Anaesthesia, 4th Edn.
  2. London: Churchill–Livingstone, 2001
  3. Calvey TN, Williams NE. Principles
    and Practice of Pharmacology for Anaesthetists,
  4. 3rd Edn. Oxford: Blackwell
    Scientific Publications, 1997
  5. Kenakin T. Pharmacologic Analysis
    of Drug–Receptor Interaction. Philadelphia:
  6. Lippincott-Raven, 1997
  7. Rang HP, Dale MM, Ritter JM, Moore
    PK. Pharmacology, 5th Edn. London:
  8. Churchill–Livingstone, 2003
  9. https://www.sciencedirect.com/topics/medicine-and-dentistry/dopamine-adversary
  10. "Pharmacology Guide: In vitro
    pharmacology: concentration-response curves." GlaxoWellcome. Retrieved on
    December 6, 2007.
  11. Physiology of Behavior, Neil R.
    Carlson, 2001

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