GlossaryStacking & CyclingReceptor Desensitization
Stacking & Cycling

Receptor Desensitization

Also known as: tachyphylaxis · receptor downregulation

Receptor desensitization is the drop in a cell's response to a drug or hormone after repeated or continuous stimulation of its receptor.

Many peptide hormones and peptide drugs act on G protein-coupled receptors (GPCRs), and cells limit overstimulation of these receptors in stages. Short-term desensitization happens over minutes: the receptor is phosphorylated by GPCR kinases (GRKs), and proteins called β-arrestins bind to it and block its interaction with G proteins. Longer-term desensitization, called downregulation, happens over hours to days and involves pulling receptors inside the cell, breaking them down in lysosomes, and lowering receptor mRNA levels.

Tachyphylaxis is what this looks like from the outside: a fading response to repeated doses. It has been measured for specific peptides. When GLP-1 was infused intravenously in 9 healthy volunteers, its slowing of gastric emptying was significantly weaker after a second meal 4 hours later than after the first. With twice-daily hexarelin, a growth hormone-releasing peptide, the growth hormone response to a test dose was significantly reduced at weeks 4 and 16 and had recovered to baseline 4 weeks after stopping.

Why it matters

Desensitization is the main argument behind cycling, but it has to be shown for the specific compound and the specific effect. In the hexarelin study, the growth hormone response fell, while serum IGF-1 did not change significantly over the 20 weeks. In the GLP-1 study, losing the gastric-emptying effect came with weaker blood glucose control after the second meal. Which effect fades, and how fast, differs from one compound to the next.

For most research peptides, tolerance has simply not been measured in people, so cycle lengths built around "resensitizing receptors" are conventions. Where tolerance has been measured, as with hexarelin, it was partial and reversed after a break. A fading effect also has other possible causes: a different batch, a dosing or measuring error, or a change in your diet, sleep, or training. Rule those out before concluding that your receptors have adapted.

How to spot it

  1. Keep the dose, timing, and conditions constant so a change in response is not explained by something else.
  2. Track one specific, measurable effect over time rather than an overall impression.
  3. Compare that measure early in the on-cycle with the same measure later.
  4. If the response returns after a break, that supports tolerance. If it does not, look for other explanations, such as product quality or a change in your routine.

Common mistakes

  • Assuming every peptide causes tolerance because a few have documented tolerance.
  • Raising the dose to chase a fading effect without checking whether something else changed.
  • Treating "receptor downregulation" as proven for a compound when no study has measured it.
  • Assuming that if one effect of a drug fades, all of its effects fade equally.
  • Blaming tolerance for a weaker effect when the batch, the dose actually drawn, or your routine has changed.

Sources

  1. ReviewGPCR desensitization: Acute and prolonged phases. Cell Signal, 2018. PMID 28137506.
  2. Clinical trialRapid tachyphylaxis of the glucagon-like peptide 1-induced deceleration of gastric emptying in humans. Diabetes, 2011. PMID 21430088.
  3. Clinical trialGrowth hormone status during long-term hexarelin therapy. J Clin Endocrinol Metab, 1998. PMID 9589671.

Page updated . How we grade evidence: methodology.

Related Terms

Cycle

A cycle is a planned period of taking a compound followed by a planned break, often repeated over time.

Off-Cycle

Off-cycle is the planned break in a cycle when you stop taking a compound, used to let the drug clear and to see which effects last.

GLP-1 Receptor

The GLP-1 receptor is the G protein-coupled receptor that GLP-1 activates to boost insulin, lower glucagon, slow the stomach and reduce appetite.

Growth Hormone Secretagogue

A growth hormone secretagogue (GHS) is a compound that makes the pituitary release its own growth hormone, usually by activating the ghrelin receptor.

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