Agonist

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An agonist is a ligand ([[Molecule|molecules]] that bind other molecules) that can be a drug or an endogenous molecule that bind to a receptor and elicit a cellular response, usually a [[signalling|signalling ]]pathway. An example of an endogenous molecule that would be an agonist could be a [[Neurotransmitter|neurotransmitter]] or a [[Hormone|hormone]], such as [[Estradiol|estradiol]] <ref>Berg et al., (2006) Biochemistry, 6th edition, New York, Pg 910</ref>&nbsp;.  
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An agonist is a [[Ligand|ligand]] capable of binding to a [[Receptor|receptor]] as a result of it's [[Affinity|affinity]] to the given molecule, and causing a [[Conformational change|conformational change]] which stabilises the receptor in an activated state<ref>David E. Golan, Principles of Pharmacology: The Pathophysiologic Basis of Drug Therapy, 4th Edition, Philadelphia, USA. 2016.</ref>. The affinity of the ligand to the receptor is determined through molecular forces such as (in order of least to greatest
  
Another example of an agonist is [[Nicotine|nicotine]]. Nicotine is a natural compound and is the chemical that is found in tobacco. Nicotine acts as an [[Antagonist|antagonist]] by binding to the same receptor as the main excitatory neurotransmitter [[Acetylcholine|acetylcholine]] and mimics the action of this [[Neurotransmitter|neurotransmitter]], therefore eliciting a cellular response <ref>Silverthorn et al., (2009) Human Physiology: An Integrated Approach, 5th Edition, San Francisco, Pg 41</ref>.
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[[Image:Serotonin Receptor Interactions.jpg|frame|right|300x200px]]  
  
Partial agonist is a compound that binds to a receptor producing an incomplete response, therefore it is&nbsp;considered to be less [[Potent|potent]].  
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strength) [[Van der Waals forces|Vaan Der Vaals forces]], [[Ionic bond|ionic bonds]], [[Hydrogen Bonding|hydrogen bonds]] and [[Covalent Bonds|covalent bonds]]. The presence of the receptor in an activated conformation allows for (mainly) cytoplasmic protein interaction and [[Cell signalling pathways|transmission of the signal]] into the cell, and an associated response e.g. increased [[Gene expression|gene expression]]<ref>H.P Rang, J.M Ritter, R.J Flower and G. Henderson, Rang and Dale's Pharmacology, 8th Edition, Houston, USA. 2015.</ref>.&nbsp;
  
The opposite of an agonist is an [[Antagonist|antagonist]].&nbsp;  
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In terms of administration, agonists can be [[Exogenous|exogenous]] or [[Endogenous|endogenous]] molecules. The most&nbsp;[[Image:G-protein activation.jpg|left|300x175px|Figure 2. Conformational change in a G protein-coupled receptor, enabling activation of the associated G protein.]]<span style="font-size: 13.28px;">common exogenous agonists that enter our body are actually unwanted, in contrast to desired administration of exogenous agonists e.g. medicines like [[Morphine|morphine]]. Unwanted exogenous agonists can include chemicals produced in the workplace, food additives, gaseous chemicals produced through pollution, second-hand smoke</span><ref>David E. Golan, Principles of Pharmacology: The Pathophysiologic Basis of Drug Therapy, 4th Edition, Philadelphia, USA. 2016.</ref>.
  
<span style="font-size: 17.529600143432617px; font-weight: bold;">References</span>
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Desired administration of exogenous agonists has many uses. Most salient amongst these are pleasure (e.g. from [[Ethanol|ethanol]]), therapeutic effects (e.g. from [[Aspirin|aspirin]]) or religious experiences (e.g. [[Entheogens|entheogens]]).  
  
<references /><br>
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Endogenous agonists are molecules, typically synthesized or modified using [[Metabolic pathway|metabolic pathways]], present without prior administration. This does not mean however, that their quantity in the body is not alterable. For example, different levels of precursors to the final, endogenous agonists affect the level of the ligand<ref>H.P Rang, J.M Ritter, R.J Flower and G. Henderson, Rang and Dale's Pharmacology, 8th Edition, Houston, USA. 2015.</ref>. [[L-Dopa|L-Dopa]], a principal treatment for [[Parkinson's Disease|Parkinson's disease]], is used in this way as it increases the levels of endogenous dopamine in the [[Substantia Nigra|substantia nigra]] of affected patients. The level of gene expression is also important as it determines the quantity of enzymes capable of creating the final endogenous agonist.
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[[Image:Dopamine synthesis.png|right|300x200px|Figure 3. Synthesis of dopamine with the use of various genetically encoded enzymes. Note the presence of L-DOPA which can be administered to treat Parkinsons.]]
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<span style="font-size: 13.28px;">A greater amount of enzyme will increase the speed of [[Enzyme|synthesis]] of the final agonist, and thus increase it's effects. This provides insight into disease states, or predispositions to disease. Forexample, a lower expression of enzyme genes necessary to synthesize [[Serotonin|serotonin]] and [[Noradrenaline|noradrenaline]] can predispose people to lower mood states, and thus increase the likelihood of [[Major Depressive Disorder|depression]]</span><ref>Falk W. Lohoff, Overview of the Genetics of Major Depressive Disorder, Curr Psychiatry Rep, Dec 2010. 12(6) 539-549</ref><span style="font-size: 13.28px;">.&nbsp;</span>
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[[Image:Dose Response Curve.jpg|left|300x175px|Figure 4. A dose-response curve indicating the effect agonists and partial agonists have on an isolated living system e.g. a cell.]]
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Agonists are not only classified by their [[Affinity|affinity]] to a given molecule (see first paragraph), but also by their efficacy. [[Efficacy|Efficacy]] is described as the ability of the agonists to stabilise the receptor into an activated state through a conformational change<ref>David E. Golan, Principles of Pharmacology: The Pathophysiologic Basis of Drug Therapy, 4th Edition, Philadelphia, USA. 2016.</ref>. A more full agonists will have the greatest efficacy possible and thus stabilise the receptor such that it remains active throughout the entirety of it's binding. A partial agonist will stabilise the receptor such that it remains active through only a portion of its bound time. Comparatively, the full agonist will produce the greatest measurable response (e.g. cause complete myocyte contraction) while partial agonists produce a fraction of this (e.g. partial myocyte contraction)<ref>David E. Golan, Principles of Pharmacology: The Pathophysiologic Basis of Drug Therapy, 4th Edition, Philadelphia, USA. 2016.</ref>. A graph is shown to the left to illustrate this point. The [[Basal|basal response]] demonstrates the [[Constitutive Activity|constitutive]] activity of the receptor in the presence of non-stimulated endogenous ligand levels.&nbsp;
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=== References  ===
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<references />  
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Image references:
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1. David Goodsell, Serotonin Receptor, Unknown, 2013. URL: http://pdb101.rcsb.org/motm/164<br>  
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2. Frontiers, Stabilisation of G protein coupled receptors, Unknown, 2015. URL:&nbsp;https://www.frontiersin.org/files/Articles/133481/fphar-06-00082-HTML/image_m/fphar-06-00082-g005.jpg
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3. Amaia Sangrador and Alex Mitchell, Don't Blame The Cat, Unknown, 2014. URL: https://proteinswebteam.github.io/interpro-blog/2014/11/06/Don-t-blame-the-cat-the-toxoplasmosis-effect/
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4. Pharmakon, Dose-Response Curve, Unknown, Unknown. URL: https://pharmakondotme.wordpress.com/introduction/

Latest revision as of 16:56, 24 October 2017

An agonist is a ligand capable of binding to a receptor as a result of it's affinity to the given molecule, and causing a conformational change which stabilises the receptor in an activated state[1]. The affinity of the ligand to the receptor is determined through molecular forces such as (in order of least to greatest

Serotonin Receptor Interactions.jpg

strength) Vaan Der Vaals forces, ionic bonds, hydrogen bonds and covalent bonds. The presence of the receptor in an activated conformation allows for (mainly) cytoplasmic protein interaction and transmission of the signal into the cell, and an associated response e.g. increased gene expression[2]

In terms of administration, agonists can be exogenous or endogenous molecules. The most 
Figure 2. Conformational change in a G protein-coupled receptor, enabling activation of the associated G protein.
common exogenous agonists that enter our body are actually unwanted, in contrast to desired administration of exogenous agonists e.g. medicines like morphine. Unwanted exogenous agonists can include chemicals produced in the workplace, food additives, gaseous chemicals produced through pollution, second-hand smoke[3].

Desired administration of exogenous agonists has many uses. Most salient amongst these are pleasure (e.g. from ethanol), therapeutic effects (e.g. from aspirin) or religious experiences (e.g. entheogens).

Endogenous agonists are molecules, typically synthesized or modified using metabolic pathways, present without prior administration. This does not mean however, that their quantity in the body is not alterable. For example, different levels of precursors to the final, endogenous agonists affect the level of the ligand[4]. L-Dopa, a principal treatment for Parkinson's disease, is used in this way as it increases the levels of endogenous dopamine in the substantia nigra of affected patients. The level of gene expression is also important as it determines the quantity of enzymes capable of creating the final endogenous agonist.

Figure 3. Synthesis of dopamine with the use of various genetically encoded enzymes. Note the presence of L-DOPA which can be administered to treat Parkinsons.

A greater amount of enzyme will increase the speed of synthesis of the final agonist, and thus increase it's effects. This provides insight into disease states, or predispositions to disease. Forexample, a lower expression of enzyme genes necessary to synthesize serotonin and noradrenaline can predispose people to lower mood states, and thus increase the likelihood of depression[5]

Figure 4. A dose-response curve indicating the effect agonists and partial agonists have on an isolated living system e.g. a cell.

Agonists are not only classified by their affinity to a given molecule (see first paragraph), but also by their efficacy. Efficacy is described as the ability of the agonists to stabilise the receptor into an activated state through a conformational change[6]. A more full agonists will have the greatest efficacy possible and thus stabilise the receptor such that it remains active throughout the entirety of it's binding. A partial agonist will stabilise the receptor such that it remains active through only a portion of its bound time. Comparatively, the full agonist will produce the greatest measurable response (e.g. cause complete myocyte contraction) while partial agonists produce a fraction of this (e.g. partial myocyte contraction)[7]. A graph is shown to the left to illustrate this point. The basal response demonstrates the constitutive activity of the receptor in the presence of non-stimulated endogenous ligand levels. 


References

  1. David E. Golan, Principles of Pharmacology: The Pathophysiologic Basis of Drug Therapy, 4th Edition, Philadelphia, USA. 2016.
  2. H.P Rang, J.M Ritter, R.J Flower and G. Henderson, Rang and Dale's Pharmacology, 8th Edition, Houston, USA. 2015.
  3. David E. Golan, Principles of Pharmacology: The Pathophysiologic Basis of Drug Therapy, 4th Edition, Philadelphia, USA. 2016.
  4. H.P Rang, J.M Ritter, R.J Flower and G. Henderson, Rang and Dale's Pharmacology, 8th Edition, Houston, USA. 2015.
  5. Falk W. Lohoff, Overview of the Genetics of Major Depressive Disorder, Curr Psychiatry Rep, Dec 2010. 12(6) 539-549
  6. David E. Golan, Principles of Pharmacology: The Pathophysiologic Basis of Drug Therapy, 4th Edition, Philadelphia, USA. 2016.
  7. David E. Golan, Principles of Pharmacology: The Pathophysiologic Basis of Drug Therapy, 4th Edition, Philadelphia, USA. 2016.

Image references:

1. David Goodsell, Serotonin Receptor, Unknown, 2013. URL: http://pdb101.rcsb.org/motm/164

2. Frontiers, Stabilisation of G protein coupled receptors, Unknown, 2015. URL: https://www.frontiersin.org/files/Articles/133481/fphar-06-00082-HTML/image_m/fphar-06-00082-g005.jpg

3. Amaia Sangrador and Alex Mitchell, Don't Blame The Cat, Unknown, 2014. URL: https://proteinswebteam.github.io/interpro-blog/2014/11/06/Don-t-blame-the-cat-the-toxoplasmosis-effect/

4. Pharmakon, Dose-Response Curve, Unknown, Unknown. URL: https://pharmakondotme.wordpress.com/introduction/

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