Desensitization and Tachyphylaxis

Repeated or prolonged administration of the same drug dose can result in a sudden decrease in efficacy, called tachyphylaxis.

Tachyphylaxis occurs because persistent exposure to a drug rapidly desensitizes the receptor via different cellular mechanisms.

In one such mechanism, when a β-adrenoceptor is continuously activated, a GPCR kinase phosphorylates the activated receptor.

The phosphorylated receptor tightly binds β-arrestin, blocking the binding of additional G proteins and leading to rapid receptor inactivation.

Also, β-arrestin facilitates clathrin assembly on the plasma membrane, which internalizes the β-arrestin-receptor complex in a vesicle and sequesters it in an endosome.

If the endosome fuses with a lysosome, then the lysosomal enzymes degrade the internalized receptor.

In a third mechanism, frequent drug dosing depletes the intermediate molecules. For example, excessive use of amphetamine can deplete the dopamine reserves at the pre-synaptic nerve terminal, halting neurotransmission.