Downregulation and upregulation

All living cells have the ability to receive and process signals that originate outside of their walls. A receptor is a protein, usually found on the surface of a cell, that receives these chemical messengers. When they bind to a receptor, they direct the cell to do something, such as divide, die, allow substances to be created, or to enter or exit the cell. The ability of a cell to respond to a chemical message depends on the presence of receptors tuned to that message. The more receptors on a cell tuned to the same message, the more the cell will respond to it. Receptors are created, or expressed, by the DNA of the cell, and they can be increased, or upregulated, when the signal is weak, or decreased, or, downregulated when it is strong.

Downregulation of receptors happens when receptors have been chronically exposed to an excessive amount of neurotransmitters, whether endogenous or drugs. This results in ligand-induced desensitization or internalization of that receptor. It is usually exhibited by various hormone receptors. Upregulation of receptors, on the other hand, results in super-sensitized receptors especially after repeated exposure to the drug.

Within the regulation of gene expression, downregulation is the process by which a cell decreases the quantity of a cellular component, such as RNA or protein, in response to an external variable. An increase of a cellular component is called upregulation.

An example of downregulation is the cellular decrease in the number of receptors to a molecule, such as a hormone or neurotransmitter, which reduces the cell's sensitivity to the molecule. This is an example of a locally acting negative feedback mechanism.

An example of upregulation is the increased number of in liver cells when xenobiotic molecules such as dioxin are administered, resulting in greater degradation of these molecules.

Some receptor agonists may cause downregulation of their respective receptors, while most receptor antagonists temporarily upregulate their respective receptors. The disequilibrium caused by these changes often causes withdrawal when the long-term use of a drug is discontinued. However, the use of certain receptor antagonists may also damage receptors faster than they upregulate (internalization of receptors due to antagonism).

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