Reproductive & Mood Neuropeptide Research: PT-141, Melanotan-I/II, Kisspeptin-10, Gonadorelin, Oxytocin, and Melatonin — Complete B2B Research Sourcing Guide

Research Guide | 2026-09-03 | 14 min read

Comprehensive research guide covering seven reproductive and mood-neuropeptide molecules: PT-141 (Bremelanotide), Melanotan-I, Melanotan-II, Kisspeptin-10, Gonadorelin (GnRH), Oxytocin acetate, and Melatonin. Includes receptor biology, pharmacokinetics, structure, batch-level COA expectations, and research-use safety context.

Introduction: Why these seven molecules cluster as one research category

PT-141 (Bremelanotide), Melanotan-I (Afamelanotide), Melanotan-II, Kisspeptin-10, Gonadorelin (GnRH), Oxytocin acetate, and Melatonin sit at the intersection of the hypothalamic-pituitary-gonadal (HPG) axis, the melanocortin system, and circadian coupling. Although their receptor targets and synthetic families are different, they appear together in research programs because they collectively map the upstream-to-downstream signaling chain that controls reproductive function and related behaviors. Grouping them in a single documentation guide lets laboratories source, store, and characterize them under one consistent quality framework.

This guide is written for B2B buyers (academic laboratories, contract research organizations, and pharmaceutical discovery groups) sourcing research-use peptides for in-vitro studies, animal model work, ex-vivo tissue analysis, or analytical method development. Every claim here is anchored to peer-reviewed literature, and every specification referenced (purity, counter-ion status, endotoxin limits, storage conditions) reflects the standard that ZePeptide applies to its catalog products.

Molecular Architecture and Synthetic Considerations

PT-141 (Bremelanotide)

PT-141 is a 7-amino-acid cyclic heptapeptide with the sequence Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH and a molecular weight of 1025.2 Da. It is derived from Melanotan-II by cleaving the C-terminal Val-NH2 segment, which removes MC1R-driven pigmentation activity while preserving MC4R-driven central arousal effects. The lactam ring closure between Asp and Lys creates a constrained conformation that survives plasma clearance. Solid-phase peptide synthesis (SPPS) with Fmoc chemistry is the standard production route, followed by trifluoroacetic acid cleavage, reverse-phase HPLC purification, and lyophilization from acetic acid or bicarbonate buffer.

Melanotan-I (Afamelanotide)

Melanotan-I is a 13-amino-acid linear peptide (Ac-Ser-Tyr-Ser-Nle-Glu-His-D-Phe-Arg-Trp-Lys-Pro-Val-NH2, molecular weight ~1646 Da) engineered for MC1R selectivity. Substituting the native L-Phe with D-Phe at position 7 is the key stereochemical change that biases agonism toward the cutaneous MC1R receptor. The peptide is produced via SPPS and lyophilized in acetate counter-ion form. Its longer linear architecture requires careful HPLC gradient optimization to separate deletion-byproducts from full-length product.

Melanotan-II

Melanotan-II is a 7-amino-acid cyclic peptide (Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2, molecular weight 1024.2 Da) with C-terminal amidation that confers broad melanocortin receptor activation (MC1R, MC3R, MC4R, MC5R). It is structurally similar to PT-141 but retains the C-terminal extension and uses a free amide rather than a free acid at the C-terminus. The 1-Da mass difference between PT-141 (1025.2) and Melanotan-II (1024.2) reflects that single oxygen difference between -OH and -NH2 endings and is the standard marker for distinguishing the two compounds during identity confirmation by mass spectrometry.

Kisspeptin-10

Kisspeptin-10 (YNWNSFGLRY-NH2, 10 residues, molecular weight 1302.4 Da) is a C-terminal fragment of the larger KISS1 gene product. C-terminal amidation confers resistance to carboxypeptidase degradation and extends the in-vivo half-life to support the pulse-frequency research protocols that depend on the molecule. SPPS with Fmoc chemistry is the standard production route, followed by HPLC purification. The peptide is typically supplied as a lyophilized acetate salt.

Gonadorelin (GnRH)

Gonadorelin (pGlu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH2, 10 residues, molecular weight 1181.3 Da) features an N-terminal pyroglutamate (cyclized glutamine) that protects against aminopeptidase attack. C-terminal amidation is also present. Both modifications are present in the endogenous mammalian decapeptide and are required for full biological activity. The sequence is highly conserved across mammalian species, which simplifies cross-species research translation.

Oxytocin acetate

Oxytocin is a 9-amino-acid cyclic peptide (Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly-NH2, molecular weight 1007.2 Da) with a single intramolecular disulfide bond between residues 1 and 6. C-terminal amidation is also present. Production typically uses SPPS with orthogonal cysteine protection (e.g., Trt or Acm) followed by air oxidation or iodine treatment to form the disulfide bridge. The acetate counter-ion in the commercial salt is the standard format and reduces hygroscopicity relative to peptide-chloride salts.

Melatonin

Melatonin (N-acetyl-5-methoxytryptamine, molecular weight 232.3 Da) is an indoleamine rather than a true peptide, but it is grouped with this reproductive-neuropeptide cluster because of its published role as a circadian and seasonal gatekeeper of the HPG axis. Melatonin is produced via chemical synthesis rather than peptide synthesis; purity specification for research use is typically ≥99% by HPLC.

Receptor Signaling Profiles

Melanocortin System

The melanocortin receptor family comprises five GPCR subtypes (MC1R-MC5R), each coupled to Gs and stimulating intracellular cAMP. MC1R is the cutaneous pigment-cell receptor; MC2R is the adrenal ACTH receptor; MC3R and MC4R are central nervous system receptors modulating energy homeostasis and arousal; MC5R is expressed in exocrine glands. The peptide tools in this guide cover the full subtype range: Melanotan-I as MC1R-selective, Melanotan-II as broad, and PT-141 as MC4R-selective. Comparing the three compounds under identical in-vitro conditions lets laboratories characterize subtype-specific signaling, β-arrestin recruitment, and downstream gene expression.

Kisspeptin / GnRH Axis

Kisspeptin-10 binds KISS1R (GPR54), a Gq/11-coupled GPCR on hypothalamic GnRH neurons. The downstream pituitary target is the GnRH receptor, also Gq/11-coupled. Pulsatile kisspeptin and GnRH signaling are required for normal LH and FSH release; continuous exposure desensitizes the axis. The two peptides are routinely co-titrated in research protocols to map this pulsatility-to-desensitization transition.

Oxytocin Receptor (OXTR)

OXTR is a Gq/11-coupled GPCR enriched in the nucleus accumbens, amygdala, hippocampus, and several peripheral tissues including uterus and mammary gland. Oxytocin signaling modulates social salience, fear extinction, and maternal/parental behaviors in rodent models. The peptide must penetrate or be administered centrally to reach the brain compartment in animal studies, because plasma-to-brain crossing is limited by the disulfide-stabilized cyclic structure.

Pharmacokinetic Profiles in Research Models

| Peptide | Route | Approx. t½ | Notes | |---|---|---|---| | PT-141 | Subcutaneous, intranasal | 2–3 h | Centrally acting; MC4R peak at ~1 h | | Melanotan-I | Subcutaneous | ~2 h | Slow-release implant studied clinically | | Melanotan-II | Subcutaneous | ~2 h | Non-selective melanocortin probe | | Kisspeptin-10 | Intravenous, subcutaneous | ~30 min | Pulsatile infusion required for HPG physiology | | Gonadorelin | Intravenous, subcutaneous, intranasal | 10–40 min | Pulsatile = LH surge, continuous = desensitization | | Oxytocin | Intranasal, intravenous | 3–5 min | Blood-brain barrier crossing limited | | Melatonin | Oral | 30–60 min | Lipophilic indoleamine; not a true peptide |

These half-lives determine research methodology: melanocortin peptides (PT-141, Melanotan-I, Melanotan-II) are suitable for daily or every-other-day protocols; Kisspeptin-10 and Gonadorelin require controlled infusion pumps for pulsatile work; Oxytocin often needs central administration in animal models; Melatonin can be delivered orally in dietary/circadian studies.

Quality Specifications Buyers Should Verify

Every peptide in this guide should arrive with a batch-specific COA that documents:

• **HPLC purity** — ≥99% for melanocortin peptides and oxytocin (the modern research-grade standard); ≥98% for gonadorelin and kisspeptin because longer synthesis increases byproduct burden; ≥99% for melatonin.
• **Mass spectrometry identity** — ESI-MS or MALDI-TOF with measured mass within ±1 Da of theoretical.
• **Counter-ion / salt form** — acetate is the dominant format; trifluoroacetate should be confirmed absent or quantified.
• **Endotoxin (LAL)** — <1 EU/mg for peptides intended for in-vitro biological work.
• **Residual solvents** — acetonitrile and TFA residuals below ICH Q3C limits.
• **Lyophilization confirmation** — visual cake, moisture content ≤5%, reconstitution note.
• **Storage condition** — lyophilized at -20 °C with desiccant.

ZePeptide ships each of these seven molecules as a lyophilized powder in 10-mg glass vials with the COA pack described above. Buyers sourcing multiple peptides in the same batch can request a single combined lot-specific dossier for the kit.

Research-Use Disclaimers

All seven molecules in this guide are supplied strictly for laboratory research. PT-141, Melanotan-I, Melanotan-II, Kisspeptin-10, Gonadorelin, Oxytocin, and Melatonin are not supplied for human or animal administration outside a regulated research protocol.

---

Research Disclaimer: This guide compiles published peer-reviewed pharmacology and analytical-chemistry references. It is intended for laboratory scientists sourcing research-grade peptides for in-vitro, ex-vivo, and animal-model studies. No content here constitutes medical or veterinary advice.

Note: All research peptides listed in this guide are intended for laboratory research purposes only and are not for human or animal consumption. Buyers are responsible for institutional approvals, biosafety committee registration, and applicable import/export compliance for the destination country.

ZePeptide content is for lawful laboratory and in-vitro research sourcing. Products are not supplied for human or veterinary use.