GHRP-6, short for growth hormone releasing hexapeptide-6, is a synthetic six-residue peptide with the sequence His-D-Trp-Ala-Trp-D-Phe-Lys-NH2. It was developed in the early 1980s by Cyril Bowers and Farid Momany from earlier enkephalin-related fragments found to release growth hormone (GH) from the pituitary independent of any opioid activity. GHRP-6 is the research tool that, over roughly fifteen years of follow-up work, led to the cloning of a previously unknown receptor and then to the identification of that receptor's natural ligand, ghrelin. This article covers the design logic, what cell and animal studies have measured, and how GHRP-6 differs mechanistically from the more selective secretagogue ipamorelin.
| Attribute | Detail |
|---|---|
| Sequence | His-D-Trp-Ala-Trp-D-Phe-Lys-NH2 (six residues, two D-amino acids) |
| Class | First-generation synthetic growth hormone releasing peptide (GHRP) |
| Origin | Bowers and Momany laboratories, early 1980s, derived from met-enkephalin analog screening |
| Receptor target | GHS-R1a, the growth hormone secretagogue receptor later shown to be the endogenous ghrelin receptor |
| What studies examine | GH release from pituitary cells, GH release and GHRH synergy in animal models, orexigenic signaling as a distinct endpoint |
| Model systems used | Perifused rat pituitary cell cultures, intact rats, and dogs cited in the receptor-discovery literature |
| Form supplied | Lyophilized powder, often paired with CJC-1295 (no DAC), research use only |
What is GHRP-6 and how was it designed?
GHRP-6 emerged from a structure-activity program that started with met-enkephalin, a naturally occurring opioid pentapeptide. Momany and colleagues observed that certain modified enkephalin fragments released GH from pituitary tissue even with their opioid activity removed, pointing to a GH-releasing action mechanistically separate from opioid receptor signaling. Substitution work, including D-amino acids at the second and fifth positions (D-Trp and D-Phe), produced a hexapeptide with greater GH-releasing activity than the earlier pentapeptide leads in the same assay systems (Momany et al., 1984). D-residues resist cleavage by the peptidases that act on natural L-amino acid backbones, extending the molecule's measured activity window in plasma and tissue. Bowers and colleagues characterized this hexapeptide, now GHRP-6, in pituitary cell preparations and animal testing, establishing it as a GH secretagogue acting through a pathway distinct from GHRH (Bowers et al., 1984).
How did GHRP-6 research lead to the discovery of a new receptor?
Because GHRP-6 released GH through a mechanism that did not match known GHRH receptor pharmacology, it became the lead compound used to search for a distinct binding site. Over the following decade, researchers used GHRP-6 and related secretagogues as binding probes to identify where in the body this activity was acting. That effort succeeded in 1996, when a research group cloned a previously uncharacterized G protein-coupled receptor from pituitary and hypothalamic tissue that bound GHRP-6 with high affinity, naming it the growth hormone secretagogue receptor, now written GHS-R1a (Howard et al., 1996). This mapped a GH-regulation signaling pathway that had no identified receptor before GHRP-6 existed as a research tool.
How did that receptor lead to the discovery of ghrelin?
Cloning GHS-R1a answered what the receptor was, but not what molecule normally activates it, since no natural ligand had yet been found. That was resolved three years later when a research group purified an acylated 28-amino-acid peptide from rat stomach extracts that activated GHS-R1a and stimulated GH release in vitro and in vivo, naming it ghrelin (Kojima et al., 1999). Ghrelin is now understood as a stomach-derived hormone with roles in both GH regulation and feeding-related signaling, a dual role GHRP-6 research had already hinted at through its effects on rodent behavior, discussed below. In short, GHRP-6 made the receptor findable, and the receptor made ghrelin findable.
What have in vitro pituitary cell and animal studies measured?
The foundational pituitary cell work on GHRP-6 used perifusion systems, in which dispersed pituitary cells sit in a continuously flowing medium so GH output can be sampled at short intervals. Using this method in adult male rat pituitary cells, Badger and colleagues reported that GHRP-6 stimulated pulsatile GH release and compared its activity and time course against GHRH in the same preparations (Badger et al., 1984). In intact animals, Bowers and colleagues measured circulating GH levels after systemic administration of GHRP-6 across several species, establishing response and time-course reference data later secretagogue research continued to cite (Bowers et al., 1984).
What has research examined regarding synergy with GHRH?
A consistent finding across the early literature is that GHRP-6 and GHRH, tested together in cell and animal models, produce a GH-release response larger than the sum of each agent alone, a pattern described as synergistic rather than additive. Bercu and colleagues examined this in rats, reporting that GHRH augmented GHRP-6-stimulated GH release, and that the augmentation shifted when other hypothalamic peptides, including thyrotropin-releasing hormone and gonadotropin-releasing hormone, were also present, evidence read as separate but convergent pathways acting on the same pituitary cells (Bercu et al., 1992). This synergy is why later research pairs a GHRH analog with a secretagogue in combination models rather than using either compound alone.
What has research examined regarding orexigenic signaling?
Separate from GH release, researchers examined whether GHRP-6 affected feeding behavior in rats, since the receptor it engages overlaps with a pathway later tied to appetite regulation through ghrelin. In one frequently cited study, Locke and colleagues administered GHRP-6 into the brain ventricles of rats and measured food intake, reporting that the peptide increased eating in this model without a corresponding rise in plasma GH, indicating the orexigenic (feeding-stimulating) signal and the GH-releasing signal could be dissociated depending on route and brain region (Locke et al., 1995). This is part of why the GHS-R1a and ghrelin pathway is studied in both GH-axis and feeding-behavior research, two lines of inquiry that converge on the same receptor.
How does GHRP-6 differ from ipamorelin's selectivity?
GHRP-6 activates GHS-R1a, but published comparisons report it also produces measurable increases in cortisol, prolactin, and adrenocorticotropic hormone in several model systems, alongside GH release. Ipamorelin was developed to narrow this profile; studies describe it as a GHS-R1a agonist with comparatively little effect on cortisol and prolactin output in the same assay systems where GHRP-6 shows broader hormonal activity (Raun et al., 1998). For background on ipamorelin's pharmacology, see what is Ipamorelin, and for how GHRH analogs and secretagogues are paired in research, see growth hormone secretagogues explained. GHRP-6 is sometimes supplied alongside CJC-1295 (no DAC) for this GHRH-plus-secretagogue synergy in one kit; see CJC-1295 (no DAC) + GHRP-6 for specifications.
Frequently asked questions
Is GHRP-6 the same molecule as ghrelin?
No. GHRP-6 is a synthetic hexapeptide that activates the GHS-R1a receptor. Ghrelin is the endogenous 28-amino-acid hormone later identified as that receptor's natural ligand. They are structurally distinct molecules acting on the same binding site.
Why does GHRP-6 contain D-amino acids?
The second and fifth positions are D-Trp and D-Phe rather than the naturally occurring L-forms. This came out of structure-activity work aimed at raising GH-releasing activity relative to earlier peptide leads, and D-amino acids are also generally more resistant to peptidase cleavage than L-amino acids in cell and plasma systems (Momany et al., 1984).
What is the connection between GHRP-6 and the GHS-R1a receptor?
GHRP-6 was one of the synthetic ligands used to identify GHS-R1a, cloned from pituitary and hypothalamic tissue in 1996. Before that work, the receptor it was acting on had not been identified (Howard et al., 1996).
How does GHRP-6 compare to ipamorelin in the published literature?
Both act on GHS-R1a, but comparative studies report ipamorelin is more selective for GH release relative to cortisol and prolactin output in the model systems tested, while GHRP-6, an earlier-generation compound, shows broader hormonal activity in the same assays (Raun et al., 1998).
Has GHRP-6 been studied for effects beyond growth hormone release?
Yes. Rodent studies have examined orexigenic, or feeding-related, signaling as a measured variable separate from GH output, since the GHS-R1a pathway overlaps with ghrelin's role in feeding behavior (Locke et al., 1995).
How is GHRP-6 supplied for laboratory use?
As a lyophilized powder for research use only, sold by lot with an accompanying certificate of analysis, sometimes paired with CJC-1295 (no DAC) in a single kit. Lot specifications are posted on our CJC-1295 (no DAC) + GHRP-6 lot page.
References
- Bowers CY, Momany FA, Reynolds GA, Hong A. On the in vitro and in vivo activity of a new synthetic hexapeptide that acts on the pituitary to specifically release growth hormone. Endocrinology. 1984;114(5):1537-1545. PubMed 6714155.
- Momany FA, Bowers CY, Reynolds GA, et al. Conformational energy studies and in vitro and in vivo activity data on growth hormone-releasing peptides. Endocrinology. 1984;114(5):1531-1536. PubMed 6425039.
- Badger TM, Millard WJ, McCormick GF, et al. The effects of growth hormone (GH)-releasing peptides on GH secretion in perifused pituitary cells of adult male rats. Endocrinology. 1984;115(4):1432-1438. PubMed 6148232.
- Bercu BB, Yang SW, Masuda R, Walker RF. Role of selected endogenous peptides in growth hormone-releasing hexapeptide activity: analysis of growth hormone-releasing hormone, thyroid hormone-releasing hormone, and gonadotropin-releasing hormone. Endocrinology. 1992;130(5):2579-2586. PubMed 1315249.
- Howard AD, Feighner SD, Cully DF, et al. A receptor in pituitary and hypothalamus that functions in growth hormone release. Science. 1996;273(5277):974-977. PubMed 8688086.
- Kojima M, Hosoda H, Date Y, et al. Ghrelin is a growth-hormone-releasing acylated peptide from stomach. Nature. 1999;402(6762):656-660. PubMed 10604470.
- Locke W, Kirgis HD, Bowers CY, Abdoh AA. Intracerebroventricular growth-hormone-releasing peptide-6 stimulates eating without affecting plasma growth hormone responses in rats. Life Sci. 1995;56(16):1347-1352. PubMed 8614257.
- Raun K, Hansen BS, Johansen NL, et al. Ipamorelin, the first selective growth hormone secretagogue. Eur J Endocrinol. 1998;139(5):552-561. PubMed 9849822.
Research use only. Homegrown Peptides products are for laboratory research and are not for human or animal use. Nothing in this article is medical advice.
