Up-regulation is an adaptation of the cell to a situation of too low stimulation and subsequent too small effect in the cell.
Up-regulation occurs for example when a cell becomes deficient in some kind of receptor or when an antagonist is added and the effect becomes too small (2). The postsynaptic cell registers the decreased effect and will react by increasing the number of receptors (3= up-regulation). This way, the cell increases the its sensitivity
for the specific ligand and achieves a new equilibrium (3).
The lower part shows the different mechanisms by which the up-regulation can be regulated. Signal transduction from activated receptors and signals from internalisation result in increased gene expression and corresponding receptor synthesis. Also less receptors are degraded and more receptors are recycled back to the membrane.
An example: denervation supersensitivity.