Reproductive Hormone Peptides Research Guide: Kisspeptin-10, Gonadorelin, Triptorelin, hCG and HMG

The kisspeptin research peptide family and the gonadotropin compounds sit at successive nodes of one control system: the hypothalamic–pituitary–gonadal axis. Kisspeptin-10 acts upstream on GnRH neurons, gonadorelin is GnRH itself, triptorelin is a superagonist analogue, and hCG and HMG act at the gonadal end as gonadotropin-receptor ligands. This guide maps each compound to its node, gives verified specifications, and states the UK legal position factually. Research use only.

Updated 10 September 2026~14 min readReviewed by the GenoPept technical team

The HPG axis and where each compound acts

The hypothalamic–pituitary–gonadal axis is a three-tier relay. Kisspeptin neurons in the hypothalamus signal to GnRH neurons; GnRH travels through the portal circulation to the anterior pituitary, where it triggers release of luteinising hormone and follicle-stimulating hormone; those gonadotropins act on the gonads. Each research compound in this guide plugs into a different tier.

Understanding the tier matters more than memorising the molecules. A compound acting at the top of the axis produces effects that still depend on every downstream step being intact, whereas a gonadotropin-receptor ligand bypasses the hypothalamus and pituitary entirely. In experimental design terms, they are not interchangeable probes even when the measured endpoint looks similar.

Kisspeptin-10 research: GPR54 and the puberty gate

Kisspeptin-10 is the ten-residue C-terminal fragment of the KISS1 gene product, ending in the arginine-phenylalanine-amide motif characteristic of the RFamide peptide family. It is the shortest fragment that retains full potency at GPR54, the receptor now generally called KISS1R.

The ligand was identified in 2001 when Kotani and colleagues reported in the Journal of Biological Chemistry that the metastasis-suppressor gene KiSS-1 encodes the natural ligands of the orphan receptor GPR54. Two years later the physiological significance became clear from human genetics: de Roux and colleagues in PNAS and Seminara and colleagues in the New England Journal of Medicine independently reported that loss-of-function mutations in GPR54 cause hypogonadotropic hypogonadism with failure of pubertal development.

That convergence established kisspeptin signalling as a required gate upstream of GnRH release, and it remains one of the cleaner examples of human genetics defining a neuroendocrine pathway. Dhillo and colleagues later reported in the Journal of Clinical Endocrinology & Metabolism that kisspeptin-54 stimulated gonadotropin release in women, with the response varying across the reproductive phase.

Gonadorelin: GnRH itself

Gonadorelin is the synthetic form of native gonadotropin-releasing hormone, a decapeptide with the sequence pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH₂. Both termini are chemically blocked in the natural molecule: the N-terminus is a cyclised pyroglutamate and the C-terminus is a glycinamide.

Those terminal features are not incidental. They make the peptide resistant to the exopeptidases that would otherwise trim it from either end, which is one reason a molecule this small survives long enough to reach the pituitary through the portal circulation. Its remaining vulnerability is endopeptidase cleavage in the middle of the chain, particularly around the glycine at position 6.

Because gonadorelin is chemically identical to the endogenous hormone, it is the natural comparator in receptor-binding and signalling work, and the baseline against which analogue potency is expressed.

Triptorelin: the D-Trp6 superagonist

Triptorelin differs from GnRH at exactly one position: glycine 6 is replaced by D-tryptophan. This is the archetypal GnRH agonist modification and it does two things at once. The bulky aromatic D-residue increases receptor affinity by stabilising a favourable conformation, and its D-configuration blocks the endopeptidases that cleave the natural peptide at that bond.

The result is a compound with markedly greater potency and a longer functional lifetime than gonadorelin. Every classical GnRH agonist follows the same template — a D-amino acid at position 6, sometimes with a further C-terminal modification — and triptorelin is the cleanest single-substitution example.

Why pulsatile and continuous stimulation differ

The most consequential fact about GnRH receptor pharmacology is that the same agonist produces opposite outcomes depending on the temporal pattern of exposure. Pulsatile stimulation sustains gonadotropin output; continuous stimulation initially raises it and then suppresses it through receptor desensitisation and downregulation.

Belchetz, Plant, Nakai, Keogh and Knobil demonstrated this directly in Science in 1978, comparing intermittent and continuous delivery of hypothalamic GnRH and showing that the intermittent pattern was required to maintain pituitary responsiveness. Every subsequent use of GnRH agonists — whether to stimulate or to suppress the axis — rests on that finding.

For researchers this has a practical implication: a study describing a GnRH analogue as stimulatory and another describing it as suppressive may both be correct, and the difference may lie entirely in the delivery schedule rather than in the molecule.

hCG and HMG: the gonadotropins

hCG — human chorionic gonadotropin — is not a synthetic peptide at all. It is a heterodimeric glycoprotein hormone: a 92-residue alpha chain, shared with luteinising hormone, follicle-stimulating hormone and thyroid-stimulating hormone, non-covalently paired with a 145-residue beta chain that carries six disulfide bonds and gives the hormone its specificity. Extensive N- and O-linked glycosylation makes carbohydrate a large fraction of the intact molecule’s mass.

Because it is a glycoprotein whose activity depends on correct folding, subunit pairing and glycosylation, hCG is quantified in International Units of biological activity against a reference preparation, not in milligrams. That is a genuinely different measurement basis from every synthetic peptide in this guide and it is a common source of confusion.

HMG — human menopausal gonadotropin, also called menotropin — is a urinary-derived preparation containing both FSH and LH activity. Like hCG it is standardised in International Units. Its composition is defined by biological activity rather than by a single molecular formula, which means batch-to-batch characterisation is a bioassay question rather than a chromatography question.

hCG binds the same receptor as luteinising hormone, usually written LHCGR, which is why it is described in the literature as an LH-receptor ligand with a substantially longer circulating lifetime than LH itself — a difference attributed largely to its extended, heavily glycosylated beta-chain C-terminus.

UK legal status: hCG as a Class C controlled drug

Human chorionic gonadotropin is a Class C controlled drug in the United Kingdom. It appears in Part III of Schedule 2 to the Misuse of Drugs Act 1971, listed as “Chorionic Gonadotrophin (HCG)”, alongside “Non-human chorionic gonadotrophin”, “Somatotropin”, “Somatrem” and “Somatropin”. It is placed in Schedule 4 Part II of the Misuse of Drugs Regulations 2001.

The practical consequence is that production, supply, offering to supply, importation and exportation of hCG are regulated activities under that framework, and doing any of them without the appropriate lawful authority is an offence. Anyone handling hCG in the United Kingdom — including in a research setting — should verify their own position against the current text of the Act and Regulations and take their own legal advice.

The other four compounds in this guide are not listed as controlled drugs under the Misuse of Drugs Act 1971. That does not make them medicines, and it does not permit supply for human use: none holds a UK marketing authorisation for the purposes described here, and all are supplied strictly as laboratory research chemicals. Our UK legal guide covers the wider regulatory picture.

Specification table for all five compounds

Handling and solution preparation

The three synthetic peptides here are small, water-soluble and lyophilised. Kisspeptin-10 contains tryptophan, which is light- and oxidation-sensitive; triptorelin and gonadorelin both contain tryptophan too, so amber storage is a sensible default across the group. None contains cysteine, so no disulfide handling precautions apply to the synthetic three.

The two glycoprotein preparations behave differently. Both are supplied as lyophilised powder with activity stated in International Units, and both are more sensitive to physical stress than a ten-residue peptide: vigorous agitation, foaming and surface adsorption can reduce measured activity without any change visible to the eye. Handle gently and avoid unnecessary transfers.

1. Equilibrate before opening. Bring the sealed vial to room temperature so moisture does not condense on the cake.
2. Add diluent down the vial wall. Never spray it onto the lyophilised material; this is especially important for the glycoproteins.
3. Swirl, never shake. Foaming denatures glycoproteins at the air–liquid interface and offers no benefit for peptides.
4. Aliquot immediately. Single-use volumes in amber or foil-wrapped tubes, filled to minimise headspace.
5. Label with the activity basis. For hCG and HMG record International Units, not milligrams — the two are not interconvertible.
6. Keep cold and dark. Refrigerate working solutions; freeze aliquots for anything beyond short-term use.

These figures are a concentration reference for preparing laboratory solutions and describe nothing more than the arithmetic of mass or activity divided by volume. The general method is set out in our reconstitution guide, and diluent selection in our bacteriostatic water guide.

Purity, identity and the certificate of analysis

For the three synthetic peptides, a credible certificate shows reversed-phase HPLC purity with the chromatogram attached, a mass spectrometry result matching the expected molecular weight, appearance and a batch number that matches the vial. For triptorelin specifically, the analytical question that matters most is stereochemical: the D-Trp6 substitution defines the molecule, and a batch contaminated with the L-isomer is a different compound with different pharmacology.

For hCG and HMG the certificate answers a different question. Because activity is defined biologically, the meaningful figures are the assayed International Units and the reference standard used. A mass-based purity figure alone does not establish that a glycoprotein preparation retains activity, since folding and glycosylation are invisible to a simple chromatographic purity number.

GenoPept publishes per-batch third-party certificates at the COA certificates page. Our purity testing guide explains how to read chromatograms and mass spectra properly.