Growth-hormone axis literature: two receptor routes

Two receptor routes

The growth-hormone axis is regulated by several signalling systems, and peptide literature often becomes confusing when chemically different ligands are grouped together simply because they can influence growth-hormone release in experimental models. Two receptor routes are particularly important: the growth-hormone-releasing hormone receptor, or GHRH receptor, and the growth-hormone-secretagogue receptor, commonly called GHS-R1a or the ghrelin receptor. They are distinct GPCRs with different endogenous ligands, pharmacology and downstream signalling characteristics.

GHRH-receptor agonism

GHRH is a hypothalamic peptide that acts on GHRH receptors expressed by pituitary somatotrophs. Receptor activation is primarily associated with Gs signalling, cyclic AMP and protein kinase A, leading to growth-hormone synthesis and secretion. Sermorelin corresponds to the biologically active N-terminal portion of human GHRH. Tesamorelin is a modified GHRH analogue, and CJC-1295 refers to synthetic GHRH analogues engineered to extend exposure, with terminology differing between DAC-modified and non-DAC forms. In mechanistic literature, these molecules are therefore grouped by GHRH-receptor agonism rather than by commercial naming.

The ghrelin receptor route

The second route involves the ghrelin receptor. Ghrelin is an acylated endogenous peptide that binds GHS-R1a and influences appetite, neuroendocrine signalling and growth-hormone release. Synthetic growth-hormone secretagogues such as ipamorelin and hexarelin have been studied as ghrelin-receptor agonists. GHS-R1a commonly signals through Gq/11 pathways with phospholipase C and intracellular calcium, although signalling can be more complex and biased by ligand. Different secretagogues show different profiles across growth hormone, prolactin, cortisol and other endpoints in experimental studies.

Why combined effects are studied

Because the receptor systems are distinct, combined-stimulation studies have historically examined whether GHRH-receptor and ghrelin-receptor activation produce additive or synergistic pituitary responses. Such observations are pharmacological and do not convert directly into a recommendation for human use. The relevant scientific question is which receptor is being activated, at what concentration, in which model, and what endocrine endpoint was measured.

Growth-hormone fragments are separate

AOD-9604 and related growth-hormone fragments belong to a different category again. They are derived from regions of the growth-hormone molecule and have been investigated in metabolic or lipolysis-related animal models without reproducing the full receptor-mediated growth profile of intact growth hormone. This is fragment biology, not classical GHRH or GHS-R receptor agonism, and the preclinical literature should be described separately.

Australian regulatory context

Regulatory status is independent of mechanistic interest. In Australia, prescription medicines and many unapproved therapeutic goods cannot lawfully be advertised to the general public. The TGA

has specifically warned about promotion and supply of unapproved peptide products and states that a “research use only” disclaimer does not change regulatory status or remove advertising and supply obligations. Balanced scientific information therefore needs to remain genuinely non- promotional and should not be used as a vehicle to imply treatment, dosage or consumer benefit.

Organise the literature by receptor

For technical readers, the most useful organising principle is receptor route. GHRH analogues act through the GHRH receptor; ghrelin mimetics act through GHS-R1a; growth-hormone fragments are mechanistically separate. This classification clarifies why compounds with superficially similar endocrine endpoints can produce different downstream profiles and why receptor pharmacology, selectivity and evidence tier must be evaluated independently of commercial naming. It also prevents a shared physiological endpoint from being mistaken for a shared molecular mechanism.