What is downregulation?

• A. Increase in receptor number
• B. No change in receptor number
• C. Decrease in receptor number in response to high concentration of drug
• D. Change in receptor affinity without number change

Answer: (C)

Downregulation is the decrease in the number of receptors on the cell surface after prolonged exposure to a high concentration of a drug or ligand. When a drug continually stimulates a receptor, the cell often internalizes receptors and may slow their synthesis, so there are fewer receptors available to respond. This lowers the cell’s sensitivity and reduces the maximal effect even if the drug is still present. It differs from upregulation, which is an increase in receptor numbers, and from changes in affinity without a change in receptor quantity, which affect how tightly the drug binds rather than how many receptors are present.