How Peptides Signal: A Receptor-Class Map of Research Peptides

Research peptides look like an unrelated jumble until you sort them by receptor. Once you do, the second messenger, the downstream cascade and even the cross-reactivity become predictable from the receptor family rather than something to memorise per molecule. This is a mechanism-of-signalling reference for laboratory research context only. It contains no dosing and no treatment claims; clinical figures from drug trials are cited to explain receptor pharmacology, not as outcomes for research-grade material, which is unapproved and unlicensed.

Growth & Performancegrowth

Growth hormone secretagogues and gonadotropins

The Three Machines

Almost every peptide here uses one of three receptor architectures:

• Class B (secretin-family) GPCRs: bind their peptide by a two-domain mechanism (the peptide's C-terminus docks the extracellular domain, its N-terminus engages the seven-transmembrane core) and couple to Gs, raising cAMP and activating PKA (PMID 22266723, 38984948).
• Class A GPCRs: a larger, structurally distinct GPCR class; here it covers the ghrelin receptor (which couples to Gq and calcium) and the melanocortin receptors (which couple to Gs and cAMP).
• Receptor tyrosine kinases (RTKs): not GPCRs at all. The receptor autophosphorylates and recruits substrates directly, with no G-protein and no cAMP step. IGF-1R is the example.

The single most useful idea: receptor identity and its preferred coupling predict the pathway. The four class B1 receptors here share Gs-cAMP wiring, while the class A receptors split (ghrelin to calcium, melanocortin to cAMP).

Class B GPCRs, Part 1: The Incretin and Glucagon Receptors

GLP-1R, GIPR and glucagon-R all belong to the class B1 (secretin) subfamily, which is why single, dual and triple agonists can be built on shared Gs-cAMP wiring (PMID 38984948).

Semaglutide targets GLP-1R. Activation raises cAMP and works through PKA and Epac2 to potentiate glucose-dependent insulin secretion, suppress glucagon and drive central satiety (PMID 21782840, 39289339, 39160334). Structurally it is an acylated GLP-1 analogue whose position-8 substitution confers DPP-4 resistance and whose C-18 fatty di-acid drives albumin binding for a long half-life (standard medicinal chemistry).

Tirzepatide adds a second class B receptor: it is a full GIP-receptor agonist and, at GLP-1R, a biased agonist tilted toward cAMP with little beta-arrestin recruitment and weaker internalisation (PMID 32730231). It is an unbalanced dual agonist (PMID 32730231). Whether that bias mechanistically explains its clinical profile is still debated, so it is best described as a signalling property, not a proven cause of outcomes.

Retatrutide goes to three class B receptors in one peptide (GLP-1R, GIPR and glucagon-R), with tuned, unbalanced potencies reported at roughly 0.4x GLP-1, 0.3x glucagon and 8.9x GIP relative to the native ligands (PMID 38367045). It is investigational, not an approved drug.

Tirzepatidemetabolic

A first-in-class dual GIP and GLP-1 receptor agonist, and one of the most extensively studied compounds in modern metabolic and weight-regulation research. Supplied as a lyophilised research peptide with a per-batch certificate of analysis, for laboratory and in-vitro use only.

Retatrutidemetabolic

First-ever triple-action weight management peptide targeting three receptors at once: GLP-1, GIP, and glucagon. Shown exceptional results in Phase 2 trials - up to 24% weight reduction. The most advanced metabolic peptide available.

Class B GPCRs, Part 2: The GHRH Receptor

The GHRH receptor is also class B: a Gs-coupled GPCR on pituitary somatotrophs where GHRH binding raises cAMP to drive GH synthesis and release (PMID 12529933), signalling predominantly through the cAMP/PKA pathway with secondary MAPK (PMID 37717982). CJC-1295 is a GHRH-receptor agonist acting through this axis. The multi-day human axis-activation data (raised trough and mean GH and IGF-1, PMID 17018654, 19386527) are for the long-acting DAC form and are cited here as comparison-only; they do not establish half-life, efficacy or human outcomes for the stocked short-acting no-DAC (Mod GRF 1-29) product. Only the shared GHRH-receptor mechanism (PMID 12529933, 37717982) applies to the stocked material.

CJC-1295 (No DAC)growth

CJC-1295 without DAC (Mod GRF 1-29) is a short-acting GHRH(1-29) analog for GH/IGF-1 research. Research-grade lyophilized powder, specified purity >=99% (HPLC). Laboratory use only.

Class A GPCRs: The Ghrelin and Melanocortin Receptors

Here the receptor class changes, and so does the second messenger.

Ipamorelin targets the ghrelin receptor GHS-R1a, a class A GPCR that preferentially couples to Gq/11, activating phospholipase C, IP3 and intracellular calcium mobilization, which is distinct from the Gs-cAMP class B receptors (PMID 22632856). Ipamorelin was the first selective GH secretagogue: a pentapeptide GHS-R1a agonist that releases GH with GHRP-6-like potency but does not raise ACTH or cortisol even at more than 200 times the GH ED50 (PMID 9849822).

Melanotan-2 is a synthetic cyclic alpha-MSH analogue acting as a non-selective melanocortin-receptor agonist (MC1R, MC3R, MC4R, MC5R). MC1R is a class A GPCR positively coupled to cAMP, and alpha-MSH binding raises cAMP to stimulate melanogenesis (PMID 27303435); MC4R activation also raises cAMP and PKA (with secondary ERK) in the hypothalamic energy-balance pathway (PMID 26814590, 20190196). Because it is non-selective, melanotan-2 touches both the pigmentation branch and the MC4R branch. It is unlicensed with poor safety data, and no tanning or other physiological use is endorsed here.

Ipamorelingrowth

Highly selective growth hormone releaser that triggers natural GH pulses without raising cortisol or prolactin. Clean GH stimulation with minimal side effects - the most targeted growth hormone peptide available.

Melanotan-2growth

Tanning peptide that activates melanin production in the skin. Stimulates melanocyte receptors for natural UV-free pigmentation. Also researched for appetite regulation and libido effects.

The Odd One Out: IGF-1R Is a Tyrosine Kinase

IGF-1 LR3 does not touch a GPCR at all. IGF-1 acts through IGF-1R, a receptor tyrosine kinase, with canonical PI3K-AKT and Ras-MAPK (ERK) output governing growth, survival and metabolism, which is categorically distinct from GPCR-cAMP signalling (PMID 39638246, 24276851). IGF-1 LR3 is an IGF-1 analogue (an Arg3 substitution plus a 13-residue N-terminal extension) with lowered IGF-binding-protein affinity, but it is still an IGF-1R agonist, so its downstream is the standard IGF-1R RTK cascade.

IGF-1 LR3growth

Long R3 variant of Insulin-like Growth Factor 1, modified for reduced IGFBP binding and ~20-30 hour half-life. Researched for cell proliferation, hypertrophy, and metabolic signaling. ≥98% purity.

The Map on One Screen

What Is Solid and What Is Not

The strongest evidence here is for receptor identity and canonical second-messenger coupling: that is well-characterized biochemistry. The weakest link is any leap from signalling to a claimed physiological, metabolic or cosmetic result for a research-grade compound. Much of the deepest detail (Epac2, ERK cross-talk, Gq coupling) comes from cell and rodent models, and peptide-specific human research-use outcomes are not established. The efficacy figures behind semaglutide, tirzepatide and retatrutide come from disease-population drug trials cited for mechanism; the research-peptide versions are unapproved and unlicensed.

Research Context and Handling

Semaglutide, tirzepatide, retatrutide, CJC-1295, ipamorelin, IGF-1 LR3 and melanotan-2 are supplied strictly for laboratory research and are not intended for human consumption. Native GIP, glucagon, GLP-1, sermorelin, GHRP-6 and setmelanotide are named only as mechanistic context and are not offered for sale. This article gives no dosing or administration guidance. Verify identity and purity against a batch-specific third-party Certificate of Analysis. For the axis-level detail, see the GLP-1, GIP and glucagon triple-agonist explainer and the IGF-1 biomarker explainer.

Frequently Asked Questions

This article is for research and educational purposes only. Receptor pharmacology is described for scientific context; clinical and potency figures are cited to explain mechanism, not as outcomes for research-grade material. Nothing here is medical advice, a health claim, dosing guidance or a recommendation for use. All stocked compounds mentioned are sold exclusively for laboratory research.