Downregulation
A decrease in the number or sensitivity of receptors on a cell surface in response to prolonged agonist exposure. Downregulation reduces peptide effectiveness over time and is why cycling is recommended.
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Related Terms
An increase in the number or sensitivity of receptors on a cell surface, often in response to decreased stimulation. Can occur during wash-out periods and is the mechanism behind cycling protocols.
DesensitizationA rapid decrease in receptor responsiveness to a peptide agonist despite continued exposure. Distinct from downregulation — desensitization involves receptor modification rather than reduced receptor numbers.
TachyphylaxisA rapid decrease in the response to a peptide after repeated administration within a short timeframe. More acute and pronounced than tolerance. Pulsatile dosing and cycling help prevent tachyphylaxis.
CyclingThe practice of using a peptide for a set period (on-cycle) followed by a break (off-cycle) to maintain receptor sensitivity, reduce side effects, and prevent tolerance. Typical cycles range from 4–12 weeks on, 2–4 weeks off.