Editing Downregulation and upregulation (section)

==Downregulation and upregulation of receptors==
{{More citations needed section | date = February 2020}}
All living cells have the ability to receive and process signals that originate outside their membranes, which they do by means of proteins called [[receptor (biology)|receptors]], often located at the cell's surface imbedded in the plasma membrane. When such signals interact with a receptor, they effectively direct the cell to do something, such as dividing, dying, or allowing substances to be created, or to enter or exit the cell. A cell's ability to respond to a chemical message depends on the presence of receptors tuned to that message. The more receptors a cell has that are tuned to the message, the more the cell will respond to it.

Receptors are created, or expressed, from instructions in 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.{{Citation needed|date=July 2024|reason=removed dead link to unreliable source}} Their level can also be up or down regulated by modulation of systems that degrade receptors when they are no longer required by the cell. 

Downregulation of receptors can also occur when receptors have been chronically exposed to an excessive amount of a ligand, either from [[Endogeny (biology)|endogenous]] mediators or from [[exogeny|exogenous]] drugs. This results in [[ligand]]-induced desensitization or internalization of that receptor. This is typically seen in animal hormone receptors. Upregulation of receptors, on the other hand, can result in super-sensitized cells, especially after repeated exposure to an antagonistic drug or prolonged absence of the ligand.

Some [[receptor agonist]]s may cause downregulation of their respective receptors, while most [[receptor antagonist]]s temporarily upregulate their respective receptors. The disequilibrium caused by these changes often causes [[Drug withdrawal|withdrawal]] when the long-term use of a [[drug]] is discontinued.

Upregulation and downregulation can also happen as a response to [[toxin]]s or [[hormone]]s. An example of upregulation in [[pregnancy]] is hormones that cause cells in the [[uterus]] to become more sensitive to [[oxytocin]].