Receptor Binding

What binding decides

A peptide starts working when it attaches to a receptor, usually a protein on the cell surface. Two separate properties describe that contact. Affinity is how tightly a compound holds on. Efficacy is how strongly the receptor responds once it's occupied. A compound can bind tightly and still barely switch the receptor on.

Receptor theory sorts compounds by efficacy (Kenakin, 2004). A full agonist produces the largest response the system can give, while a partial agonist tops out lower however much of it is present. An antagonist occupies the receptor without switching it on. Some receptors are partly active with nothing bound, and an inverse agonist turns that background activity down.

Potency, the amount needed to reach half of a compound's own maximum, is a separate measure again. In a 1998 Novo Nordisk study in conscious pigs, GHRP-2 was more potent than GHRP-6 or ipamorelin but produced a lower peak growth hormone response (Raun and colleagues, 1998). These animal results describe behavior at the receptor, not what a person would notice.

This page covers the binding step. The relay inside the cell once the receptor is switched on is on the cellular signaling page.

Selectivity

Most peptide hormones have families of related receptors, and a compound rarely touches only one. Selectivity describes how much more strongly it acts at its intended receptor than at the others. Many side effects come from the receptors a compound wasn't meant to reach.

In the same pig study, GHRP-6 and GHRP-2 raised ACTH and cortisol along with growth hormone. Ipamorelin didn't, even at more than 200 times the amount needed for a half-maximal growth hormone response (Raun and colleagues, 1998). That animal result is the source of ipamorelin's reputation as the selective secretagogue. This site rates its overall evidence as moderate because human data are limited.

Bremelanotide, an approved prescription medicine sold as Vyleesi, shows the opposite case. Its US label calls it a nonselective melanocortin receptor agonist, most potent at MC1R and then MC4R, and says binding at those two is what matters at the approved dose. MC4R is found on neurons in many brain areas. MC1R sits on melanocytes, the skin's pigment cells, and the label reports focal hyperpigmentation (patches of darker skin) in 1% of trial patients. As of October 2026, Vyleesi is approved only for hypoactive sexual desire disorder in premenopausal women.

Three receptor families

The peptides on this site act on three structurally different kinds of receptor. The family decides how binding becomes a signal and how the receptor is switched off.

G protein-coupled receptors

G protein-coupled receptors (GPCRs) cross the cell membrane seven times and hand the signal to a G protein inside. The main ones here:

• Ghrelin receptor (GHS-R1a). Merck researchers cloned it in 1996 from swine and human pituitary and hypothalamus and showed it was the target of synthetic growth hormone secretagogues (Howard and colleagues, 1996). In transfected cells it signals strongly with nothing bound, and agonists push that activity higher (Holst and colleagues, 2003). The ghrelin receptor page covers it in detail.
• GHRH receptor. Cloned in 1992, it binds GHRH with high affinity and raises cyclic AMP in cells. In rats, its messenger RNA is found mainly, perhaps only, in the anterior pituitary (Mayo, 1992). GHRH analogs such as sermorelin and CJC-1295 act here.
• Melanocortin receptors. A 2006 review from one of the labs that cloned the five subtypes notes that each is expressed in a few discrete locations tied to fairly specific jobs, such as pigmentation or energy balance (Cone, 2006).
• GLP-1 receptor. Semaglutide and tirzepatide act here. Drucker's 2018 review traces the receptor across several cell types, which is how one hormone can affect both insulin release and appetite.

The two pituitary receptors also affect each other. In cells engineered to carry both, activating the ghrelin receptor alone didn't raise cyclic AMP, but activating both gave roughly twice the response of the GHRH receptor alone (Cunha and Mayo, 2002). That's one mechanistic reason behind the common community practice of pairing a GHRH analog with a GHRP. It doesn't show that the pairing adds anything measurable in people.

Cytokine-class receptors

The growth hormone receptor has no enzyme activity of its own. It's a class I cytokine receptor that relies on an attached kinase, JAK2. Structural work from a University of Queensland group suggests the receptor already sits as a pair before the hormone arrives, and growth hormone binding rotates the two halves (Brown and colleagues, 2005). A 2014 follow-up proposed that this movement frees the two JAK2 molecules to activate each other (Brooks and colleagues, 2014). Further down, the pathway depends on the transcription factor STAT5b: a 2003 report described growth hormone insensitivity in a patient with a STAT5b mutation (Kofoed and colleagues, 2003). The growth hormone receptor page goes through this step by step.

Receptor tyrosine kinases

The IGF-1 receptor carries its kinase inside the receptor itself, the defining feature of receptor tyrosine kinases (Lemmon and Schlessinger, 2010). Binding here is also shaped by IGF-binding proteins, which capture IGF-1 and compete with the receptor for it. IGF-1 LR3 was designed to bind those proteins poorly. In pigs and marmoset monkeys, IGF-1 variants with reduced binding-protein affinity were 2-3 times more potent than native IGF-1 at lowering blood sugar, and the effect lasted longer (Tomas and colleagues, 1997). This site rates IGF-1 LR3's evidence as preliminary: as of October 2026, we found no published human data for the analog.

Desensitization and internalization

Receptors don't stay switched on indefinitely. After sustained activation, many are uncoupled from their signaling partners and pulled inside the cell, where they're either recycled or broken down.

In cell studies, the ghrelin receptor desensitized rapidly after ghrelin binding. The ghrelin-receptor complex began leaving the cell surface after about 20 minutes, and surface binding took about six hours to recover (Camiña and colleagues, 2004).

Human data are scarcer and harder to read. In a 2011 study, nine healthy volunteers received an 8.5-hour intravenous GLP-1 infusion. GLP-1 slowed stomach emptying much less after a second meal than after the first, four hours earlier (Nauck and colleagues, 2011). The authors traced the fading to vagal nerve signaling, so it shows tolerance in the circuit rather than measured receptor loss.

A longer trial points the other way for growth hormone release. In a 2-year randomized trial of 65 healthy adults aged 60 to 81, daily oral MK-677 (a non-peptide ghrelin receptor agonist) kept growth hormone and IGF-1 in the young-adult range over 12 months. The increase in appetite faded within a few months (Nass and colleagues, 2008).

Users commonly report that peptide secretagogues lose effect after weeks of steady use, and many cycling schedules are built on that belief. The cell data make it plausible, but human data for the peptide secretagogues themselves are thin. The page on long-term effects covers what has been measured.

Why tissue distribution shapes effects

A receptor can only respond where it's present, so the tissues that carry it often explain a compound's side effects better than its chemistry does. The GHRH receptor is concentrated in the pituitary, at least in rats, which fits GHRH analogs acting mostly on growth hormone release. The ghrelin receptor also sits in the hypothalamus, and ghrelin acts as an appetite hormone (Holst and colleagues, 2003). That fits the hunger users often report with GHRP-6, though we didn't find a trial that measured it directly. Bremelanotide reaches MC1R in skin as well as MC4R in the brain, which is how a drug for a sexual desire disorder can darken skin.

A perfectly selective compound still acts wherever its receptor is found, and a nonselective one adds the tissues of every receptor it touches. The tissue specificity page follows this through specific tissues.

What we still don't know

• For most research peptides, affinity and selectivity figures come from cell lines or rodents, often from a single lab. Direct human measurements at the receptor are rare.
• Binding studies use purified compound. Research-use vials aren't always what the label says, and a degraded or mislabeled product could bind a different mix of receptors.
• As of October 2026, no human trial has tested whether the rapid internalization seen in cell studies predicts tolerance on the intermittent injection schedules people report using.