Receptor
A receptor is a protein — on the cell surface or inside the cell — that a peptide binds to in order to trigger a biological effect. Receptor specificity determines what cells and tissues respond to a given peptide.
Overview
Receptors are typically proteins that exist in specific cell types and tissues, which is why peptides produce targeted effects even when distributed through systemic circulation. Cell-surface receptors (like the GLP-1 receptor or ghrelin receptor) bind peptide ligands at the extracellular face and trigger intracellular signaling cascades. Intracellular receptors (like steroid hormone receptors) require ligands to cross the cell membrane. The receptor concept explains peptide selectivity: a peptide that only fits the GLP-1 receptor will have effects only in cells expressing that receptor (pancreatic beta cells, hypothalamic neurons, gut epithelium, etc.) and will be inert in cells lacking it. Receptor density can change over time — upregulated by ligand withdrawal or downregulated by chronic stimulation — which underlies the rationale for cycling peptide protocols.
Related Terms
Frequently asked questions
What determines which cells a peptide affects?+
Only cells that express the receptor(s) a peptide binds to will respond to it. Receptor distribution across tissues determines the peptide's tissue-specific activity profile.
What is receptor downregulation?+
Receptor downregulation is a reduction in receptor number or sensitivity in response to chronic stimulation. It is a mechanism by which continuous peptide administration can become less effective over time — a key rationale for cycling protocols.
Can two different peptides bind the same receptor?+
Yes. Agonists and antagonists often target the same receptor. Similarly, multiple research peptides may show partial overlap in receptor binding, which must be considered when studying stacks.