General Principles Of Pharmacology/Drug receptor affinity:agonists and antagonists
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The target area or specificity of a drug is not due to its distribution throughout the body but is a function of: The selective localization of drug receptors or simply, where its receptors are located within the brain.
The specificity of the drug to bind to particular receptor.
The strength of the drug's attachment is it strong enough to open the corresponding ion channel.
The consequences of the interactions between a drug and its receptor. A particular receptor affinity for a specific molecular structure, a ligand, is similar to the relationship between a lock and key. A drug molecule with the "best fit" to the receptor will elicit the greatest response from the cell. When a drug molecule, similarly to neurotransmitter, binds to specific receptor on a cell membrane or within a cell, it causes a change in the electrical properties of that cell, thus producing a pharmacological response. Drug occupation of a receptor either activates, an agonist action, or inactivates, an antagonist action, that receptor. Ultimately, this drug-receptor interaction produces a cascade of cellular events involving changes in subcellular components of the target cell that result in clinically relevant therapeutic effects or side effects. Some important points about drug-receptor interactions, based upon their sequence of occurrence are:
Receptors may have one or more types of binding sites for an endogenous transmitter chemical or ligand, these sites can be differentiated pharmacologically.
Attachment, a reversible binding, of an endogenous ligand, which is specific for that receptor, activates the receptor allowing a signal to be transmitted through the membrane to the intracellular side of the cell membrane.
The intensity of this signal is determined either by:
The percentage of available receptors that are occupied by the endogenous ligand
The time and concentration of ligand or drug within the synapse
The rate of the reversible binding
A drug can either enhance or diminish the generation, transmission, or receipt or the trans-membrane signal by binding to the receptor of the endogenous ligand. Implications of drug-receptor specificity are:
•A drug has the potential capability of altering the rate at which any bodily or brain function proceeds.
•A drug does not create effects but merely modulates ongoing functions.
•A drug cannot impart new functions to a cell.
Mechanisms by which drugs can affect synaptic transmission:
Next Page: Agonistic Mechanisms
Previous Page: The Synapse
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