It is known as the “hunger hormone”: its levels rise before meals and fall afterwards. It is a 28-amino-acid peptide with an uncommon chemical feature: it needs an octanoic acid chain on the serine at position 3 to be active.
Its receptor, GHS-R1a (growth hormone secretagogue receptor), was discovered before ghrelin itself, precisely while studying how certain synthetic peptides (the GHRPs) released GH. Ghrelin was later identified as its natural ligand.
In the pituitary, GHS-R1a activation amplifies GH pulses and counteracts somatostatin. Ghrelin mimetics such as ipamorelin use that pathway; what sets them apart is selectivity, meaning how much they also affect appetite, cortisol or prolactin.
Ghrelin explains why some secretagogues increase appetite in models and others hardly do: it depends on how each compound behaves at the same receptor.
Related terms
Learn it in depth at the Peptide University
- GHRPs, ghrelin mimetics and synergy · Module 5
- G protein-coupled receptors: desensitization and tolerance · Module 2
- Physiology of the GH/IGF-1 axis · Module 5
- Repair and somatotropic axis (6–10) · Module 14
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