receptor down-regulation

Receptor down-regulation is the opposite of up-regulation: the cell turns down its sensitivity by reducing the number of receptors. When a receptor is being over-stimulated — usually by a long-acting agonist or chronic drug — the cell protects itself by pulling receptors off the surface and breaking them down, like turning the volume down when the music is too loud.

Mechanistically, persistent agonist exposure first triggers rapid desensitization, and over hours to days the cell internalizes receptors into the cell interior and degrades them, so the total receptor pool shrinks. The same dose of agonist now produces a smaller effect, a slow-developing form of tolerance.

Down-regulation is a major reason some drugs lose effect over time and must be dosed carefully. Continuous high-dose beta-2 agonists in asthma, or constant stimulation by certain hormones, can blunt the response. It also explains why some therapies deliberately exploit the phenomenon — for example, continuous GnRH agonist therapy down-regulates pituitary receptors and ends up suppressing sex-hormone production.