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Term
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Description
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Agonist
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A drug that binds to and activates a receptor. Can be full, partial or inverse.
A full agonist has high efficacy, producing a full response while occupying a relatively low proportion of receptors.
A partial agonist has lower efficacy than a full agonist. It produces sub-maximal activation even when occupying the total receptor population, therefore cannot produce the maximal response, irrespective of the concentration applied.
An inverse agonist produces an effect opposite to that of an agonist, yet acts at the same receptor.
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Antagonist
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A drug that attenuates the effect of an agonist. Can be competitive or non-competitive, each of which can be reversible or irreversible.
A competitive antagonist binds to the same site as the agonist but does not activate it, thus blocks the agonist’s action.
A non-competitive antagonist binds to an allosteric (non-agonist) site on the receptor to prevent activation of the receptor.
A reversible antagonist binds non-covalently to the receptor, therefore can be “washed out”.
An irreversible antagonist binds covalently to the receptor and cannot be displaced by either competing ligands or washing.
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Bmax
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The maximum amount of drug or radioligand, usually expressed as picomoles (pM) per mg protein, which can bind specifically to the receptors in a membrane preparation. Can be used to measure the density of the receptor site in a particular preparation.
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Cheng-Prusoff Equation
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Used to determine the Ki value from an IC50 value measured in a competition radioligand binding assay:
Where [L] is the concentration of free radioligand, and Kd is the dissociation constant of the radioligand for the receptor.
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Competitive Antagonist
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See Antagonist
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Desensitisation
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A reduction in response to an agonist while it is continuously present at the receptor, or progressive decrease in response upon repeated exposure to an agonist.
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EC50
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The molar concentration of an agonist that produces 50% of the maximum possible response for that agonist.
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ED50
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In vitro or in vivo dose of drug that produces 50% of its maximum response or effect.
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Efficacy
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Describes the way that agonists vary in the response they produce when they occupy the same number of receptors. High efficacy agonists produce their maximal response while occupying a relatively low proportion of the total receptor population. Lower efficacy agonists do not activate receptors to the same degree and may not be able to produce the maximal response (see Agonist, Partial).
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Ex vivo
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Taking place outside a living organism.
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i.a.
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Intra-arterial route of drug administration (see Useful Abbreviations).
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IC50
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In a functional assay, the molar concentration of an agonist or antagonist which produces 50% of its maximum possible inhibition. In a radioligand binding assay, the molar concentration of competing ligand which reduces the specific binding of a radioligand by 50%.
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i.c.
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Intracerebral route of drug administration (see Useful Abbreviations).
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i.c.v.
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Intracerebroventricular route of drug administration (see Useful Abbreviations).
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ID50
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In vitro or in vivo dose of a drug that causes 50% of the maximum possible inhibition for that drug.
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i.d.
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Intradermal route of drug administration (see Useful Abbreviations).
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i.g.
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Intragastric route of administration (see Useful Abbreviations).
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i.m.
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Intramuscular route of drug administration (see Useful Abbreviations).
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Inverse Agonist
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See Agonist
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In vitro
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Taking place in a test-tube, culture dish or elsewhere outside a living organism.
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In vivo
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Taking place in a living organism.
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i.p.
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Intraperitoneal route of drug administration (see Useful Abbreviations).
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Irreversible Antagonist
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See Antagonist
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i.t.
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Intrathecal route of drug administration (see Useful Abbreviations).
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i.v.
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Intravenous route of drug administration (see Useful Abbreviations).
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KB
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The equilibrium dissociation constant for a competitive antagonist: the molar concentration that would occupy 50% of the receptors at equilibrium.
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Kd
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The dissociation constant for a radiolabeled drug determined by saturation analysis. It is the molar concentration of radioligand which, at equilibrium, occupies 50% of the receptors.
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Ki
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The inhibition constant for a ligand, which denotes the affinity of the ligand for a receptor. Measured using a radioligand competition binding assay, it is the molar concentration of the competing ligand that would occupy 50% of the receptors if no radioligand was present. It is calculated from the IC50 value using the Cheng-Prusoff equation.
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Non-Specific Binding
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The proportion of radioligand that is not displaced by other competitive ligands specific for the receptor. It can be binding to other receptors or proteins, partitioning into lipids or other things.
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pA2
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Measure of the potency of an antagonist. It is the negative logarithm of the molar concentration of an antagonist that would produce a 2-fold shift in the concentration response curve for an agonist.
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pD2
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The negative logarithm of the EC50 or IC50 value.
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pEC50
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The negative logarithm of the EC50 value.
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pIC50
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The negative logarithm of the IC50 value.
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pKB
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The negative logarithm of the KB value.
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pKd
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The negative logarithm of the Kd value.
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pKi
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The negative logarithm of the Ki value.
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p.o.
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Oral (by mouth) route of drug administration (see Useful Abbreviations).
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Potency
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A measure of the concentrations of a drug at which it is effective.
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s.c.
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Subcutaneous route of drug administration (see Useful Abbreviations).
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Specific Binding
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The proportion of radioligand that can be displaced by competitive ligands specific for the receptor.
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Systemic
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In the periphery of the body (not in the central nervous system – see Useful Abbreviations).
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t½
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The biological half-life of a drug or radioligand in vitro or in vivo.
In vitro, the t½ of the effect of a drug is the time taken for the response to a drug to decline to half the original response. In radioligand binding, the t½ can be used to measure the dissociation rate of a radioligand from its receptor, therefore it is the time taken for the amount of radioligand bound to the receptors to decline to half its original level.
In vivo, t½ refers to the metabolic half-life of a drug or radioligand, i.e. the time taken for the concentration of a drug in plasma to decline to half its original level.
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