Receptors & Mechanisms
Peptides do their work by binding to receptors — proteins, usually sitting on the surface of a cell, that recognize a specific molecule and pass a signal inward. When a peptide binds its receptor, it can switch a cellular process on or off. Understanding a handful of recurring concepts lets you read almost any peptide page and quickly grasp how it is thought to act.
The core idea
A receptor is a lock; a peptide (or the body’s own hormone) is a key. What matters is which lock a peptide fits and what turning it does — start a signal, block a signal, or tell a gland to release its own hormone.
GPCRs: the most common target
Many of the peptides on this wiki act on G-protein-coupled receptors (GPCRs) — a huge family of cell-surface receptors that translate an outside signal into an inside response. When the peptide binds, the receptor changes shape and activates intracellular messengers that ultimately change the cell’s behavior (for example, releasing a hormone, altering metabolism, or affecting appetite).
GPCRs matter here because the receptors for ghrelin (growth hormone secretagogue receptor), GLP-1, melanocortins, and many others are all GPCRs. That is why metabolic, growth-hormone, and several other peptide categories share a similar underlying logic even though their effects differ.
Agonist vs antagonist (and partial agonists)
How a peptide affects a receptor is described by a few key terms:
- Agonist — binds and activates the receptor, mimicking the natural signal. Semaglutide and Tirzepatide are agonists at the GLP-1 receptor (and tirzepatide also at the GIP receptor), turning those pathways “on.”
- Antagonist — binds but does not activate; it occupies the receptor and blocks the natural signal from getting through.
- Partial agonist — activates the receptor only partially, giving a submaximal effect even at full occupancy.
The same receptor can therefore be pushed in opposite directions depending on which molecule binds it.
Secretagogues vs direct agonists
A distinction that comes up constantly with growth-hormone-related peptides is indirect vs direct action:
- A secretagogue does not produce the end effect itself — it tells one of your own glands to release your own hormone. Ipamorelin, CJC-1295, Sermorelin, Tesamorelin, Hexarelin, and the orally active MK-677 are growth hormone secretagogues: they prompt the pituitary to release your own growth hormone in pulses. Because they work through your physiology, the response is shaped (and limited) by your own feedback systems.
- A direct agonist supplies or directly activates the end-pathway itself. Injecting growth hormone, or IGF-1 LR3 (an IGF-1 analogue), acts downstream of the pituitary rather than asking it to do the work.
This is more than a technicality: secretagogues tend to preserve natural pulsatile rhythms and feedback, whereas direct agonists bypass them. See Half-Life & Pharmacokinetics for how the timing of these signals interacts with dosing.
Recurring mechanism themes by category
Across the wiki, the same handful of mechanism patterns show up again and again:
- GH axis (secretagogues). Two complementary levers on growth hormone release — a GHRH analogue (CJC-1295, Sermorelin, Tesamorelin) plus a ghrelin-receptor agonist / GHRP (Ipamorelin, Hexarelin, MK-677) — which is why they are so often paired.
- Incretin / metabolic. GLP-1 (and GIP) receptor agonism affecting appetite, insulin, and gastric emptying — Semaglutide, Tirzepatide, Retatrutide.
- Healing & recovery. Proposed effects on angiogenesis, cell migration, and tissue repair — BPC-157, TB-500. Mechanisms here are largely preclinical and less precisely mapped to a single receptor than the categories above.
- Melanocortin system. Activation of melanocortin receptors affecting pigmentation or sexual function — Melanotan II, PT-141.
- Reproductive axis. GnRH-receptor signaling upstream of LH/FSH — Gonadorelin, Kisspeptin.
Reading mechanism claims critically
A mechanism is a hypothesis, not a result
“Activates receptor X, which is known to do Y” is a plausible story, not proof that the peptide produces benefit Y in humans. Many proposed mechanisms — especially in the healing and longevity categories — come from cell or animal studies and may not translate. When you see a mechanism described on a peptide page, check the evidence grade next to the actual claimed effect (see Evidence Grading Explained). Mechanism and proof are different things.
For the chemistry underneath this — what peptides are and how they’re made — see What Are Peptides.
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