Prolactin-releasing peptide: brain signal that curbs hunger and controls stress (PrRP20)

What this is

Prolactin-releasing peptide (PrRP) is a small signaling peptide made in the brainstem and hypothalamus that helps regulate appetite, the body's stress response, and several other hormonal functions. PrRP20 is the shorter of two naturally occurring isoforms — it shares the same active C-terminal region as PrRP31 (the 31-residue form) and has comparable biological activity. Both are generated from the same protein precursor. The peptide gets its name from early experiments in which it stimulated prolactin release from pituitary cells, though subsequent research has shown appetite and stress regulation to be its more prominent physiological roles. The stored 20-amino-acid sequence TPDINPAWYASRGIRPVGRF represents the bare backbone; the biologically active form carries a C-terminal amide (–NH₂) on the final phenylalanine, part of the conserved RF-amide motif that is critical for receptor recognition.

History

PrRP was discovered in 1998 by Hinuma and colleagues, who isolated it from bovine hypothalamus tissue and identified it as the endogenous ligand for an orphan G protein-coupled receptor then known as GPR10. The discovery was published in Nature (Hinuma and colleagues, 1998) and represented one of the early triumphs of reverse pharmacology — starting from the orphan receptor and working backward to find the peptide that activates it. Shortly after the initial report, researchers established that PrRP is produced in the nucleus of the solitary tract, the ventrolateral medulla, and the dorsomedial hypothalamic nucleus, with GPR10 expressed at high density in the thalamic reticular nucleus, paraventricular hypothalamic nucleus, and area postrema. Over the following decade, interest shifted from prolactin biology to PrRP's broader roles in energy homeostasis and stress, and by the 2010s lipidized analogs of PrRP were being designed as potential obesity treatments.

What it does

PrRP acts as a brake on food intake: injecting it centrally reduces eating in both fasted and free-feeding rodents, and animals lacking GPR10 or PrRP itself become progressively overweight and hyperphagic. PrRP also activates the stress axis, triggering the release of stress hormones (ACTH, corticosterone, and oxytocin) from the hypothalamic-pituitary-adrenal system, and it appears to play a role in relaying visceral and cardiovascular stress signals from the brainstem to the hypothalamus. Energy expenditure is also affected: studies in rats showed that PrRP administration raises body temperature and oxygen consumption and increases expression of uncoupling protein-1 (UCP-1) in brown adipose tissue, suggesting thermogenic activity beyond simple appetite suppression (Bechtold and Luckman, 2006; Lawrence and colleagues, 2004).

Mechanism

PrRP20 and PrRP31 both bind with high affinity to GPR10 (also called PrRPR, UHR-1, or hGR3), a class A GPCR cloned from hypothalamic tissue. The receptor preferentially couples to Gq/11 — cryo-EM structures of PrRP20 in complex with both Gq and Gi proteins were determined by Li and colleagues (2024, Cell Discovery), revealing that the C-terminal RF-amide of PrRP20 docks into a conserved polar pocket on GPR10, with residues R19 and F20 forming key contacts. Molecular dynamics calculations confirmed a stronger binding free energy for the Gq complex, consistent with Gq-biased signaling. Downstream of Gq, PrRP activates ERK, JNK, and the PI3K–Akt–mTOR pathway. PrRP also shows secondary affinity for NPFF-R2 (neuropeptide FF receptor type 2), a related RF-amide receptor; cardiovascular effects of central PrRP administration (elevated blood pressure, increased heart rate) can be blocked by the selective NPFF antagonist RF9, suggesting these particular effects are mediated via NPFF-R2 rather than GPR10.

PrRP neurons in the caudal nucleus of the solitary tract and ventrolateral medulla co-express tyrosine hydroxylase, identifying them as A2 and A1 noradrenergic neurons respectively. These neurons project to the paraventricular hypothalamic nucleus, where PrRP fibers synapse directly on CRH and oxytocin neurons, providing the anatomical basis for PrRP's activation of the HPA stress axis.

The anorexigenic effects of PrRP are functionally linked to leptin: central co-administration of PrRP and leptin produces additive reductions in food intake and body weight, and the hypophagic effects of leptin are substantially impaired in GPR10-knockout mice, indicating that intact PrRP signaling is part of how leptin suppresses appetite.

Evidence

• Human: No registered clinical trials for PrRP or PrRP20 identified on ClinicalTrials.gov. Human tissue studies confirm GPR10 expression in the anterior pituitary and brain; polymorphisms in the GPR10 receptor gene have been associated with blood pressure variation in UK Caucasian populations (Bhattacharyya and colleagues, 2003), but PrRP has not entered clinical development.
• Animal: GPR10-knockout mice develop hyperphagia and mild obesity beginning around 16 weeks of age, accompanied by elevated insulin and leptin levels and reduced glucose tolerance (Marsh and colleagues, as cited in Bechtold and Luckman, Frontiers in Endocrinology, 2013). PrRP-knockout mice similarly show elevated blood glucose and corticosterone. In diet-induced obese rodents, two-week peripheral administration of lipidized analogs — palmitoylated PrRP31 and myristoylated PrRP20 — reduced body weight by approximately 7–13%, lowered food intake, improved glucose tolerance, and reduced lipogenic markers (Kuneš and colleagues, 2016; Mráziková and colleagues, 2021). The lipidized versions remain stable in rat plasma for more than 24 hours, in contrast to the brief circulation time of unmodified PrRP, and exert their peripheral effects through albumin binding in the manner of approved lipidized peptide drugs.
• In vitro: BRET2 assays using the PrRP20–GPR10–Gq complex confirmed ligand-receptor interaction and G protein coupling selectivity (Li and colleagues, 2024). Palmitoylation at the N-terminus of PrRP analogs increased binding affinity and activation at GPR10, NPFF-R2, and NPFF-R1 in cell-based assays (reported by Maletínská and colleagues).

Known effects

• Appetite suppression — Preclinical (central administration reduces food intake in fasted and free-fed rodents; GPR10/PrRP knockout leads to hyperphagia and obesity)
• Energy expenditure increase — Preclinical (raises O₂ consumption, body temperature, UCP-1 expression in brown adipose tissue in rats)
• HPA axis activation — Preclinical (ICV PrRP elevates ACTH, corticosterone, and oxytocin; stress-responsive PrRP neurons project to PVN CRH and oxytocin neurons)
• Blood pressure and heart rate elevation — Preclinical (central administration; mediated via NPFF-R2, blockable by RF9)
• Prolactin release — Preclinical in vitro (original naming basis; pituitary cells release prolactin in response to PrRP, but hypophysiotropic hormone criteria not fully met)
• Anti-obesity effects of lipidized analogs — Preclinical (palmitoylated PrRP31, myristoylated PrRP20 in DIO rodent models)

Safety signals

No human safety data are available; PrRP and its analogs have not progressed to clinical trials. In preclinical rodent studies, centrally administered PrRP raises blood pressure and heart rate via NPFF-R2 engagement — an effect that peripheral lipidized analogs appear to avoid or minimize at the doses studied. GPR10-knockout mice develop only mild obesity under normal dietary conditions, suggesting the receptor has a modulatory rather than essential role in basal physiology. The overlap between GPR10 and NPFF-R2 pharmacology means that potent PrRP analogs may carry cardiovascular liabilities that require evaluation in any future clinical program.

Regulatory status

• US: No FDA approval or IND filed. Research compound only.
• EU: Not approved or registered. No EMA marketing authorization.
• WADA: Not listed on current prohibited lists.

Related peptides

• Neuropeptide FF (NPFF) — RF-amide neuropeptide family member; shares NPFF-R2 with PrRP; roles in pain modulation and cardiovascular regulation (platform card not confirmed)
• PrRP31 — the 31-residue parent isoform sharing the same C-terminal active region as PrRP20 (platform card not confirmed)