Prolactin-Releasing Peptide (PrRP): A Misnamed Anorexigen and Stress Regulator Acting Through GPR10

# Prolactin-Releasing Peptide (PrRP): A Misnamed Anorexigen and Stress Regulator Acting Through GPR10

Neuropeptide discovery in the late 1990s was transformed by the orphan receptor deorphanization strategy: identify a known orphan GPCR, screen compound libraries or tissue extracts for its endogenous ligand, and characterize the new peptide-receptor pair. Prolactin-releasing peptide (PrRP) was discovered by this approach in 1998, identified as the endogenous ligand for the then-orphan receptor GPR10. Its name reflects the initial functional observation — it stimulated prolactin release from pituitary cells in vitro. But subsequent research revealed that PrRP's primary physiological roles lie elsewhere: it is a potent anorexigenic neuropeptide, a key component of stress circuitry, and a regulator of energy expenditure. PrRP-null mice develop obesity not primarily through hyperphagia but through metabolic defects — a counterintuitive finding that has shaped understanding of how the hypothalamus integrates feeding, stress, and peripheral metabolism.

Discovery: 1998 Hinuma Nature Paper

PrRP was discovered by Hinuma and colleagues at Takeda Chemical Industries, published in Nature in 1998 (Hinuma S et al., Nature. 1998;393(6682):272-276. PMID: 9607764). Using the orphan receptor strategy, they expressed the then-uncharacterized receptor PRLHR (now designated GPR10) in Chinese hamster ovary (CHO) cells and screened bovine hypothalamic extracts for ligands that stimulated intracellular calcium signaling. They identified a 31-amino acid peptide with C-terminal Arg-Phe-NH₂ — a structural motif suggesting membership in the RFamide superfamily — that they named PrRP-31.

Subsequent processing characterization identified a second, shorter form: PrRP-20, a 20-amino acid peptide arising from internal cleavage of the prepro-PrRP precursor that retains the same C-terminal sequence and full receptor-binding activity. Both PrRP-31 and PrRP-20 activate GPR10 with comparable nanomolar affinity.

The human PRRP gene maps to chromosome 2p23.3 and encodes a 98-amino acid prepropeptide that is processed to PrRP-31 (the primary circulating form in most tissues) and PrRP-20 (predominant in some CNS regions).

The naming as "prolactin-releasing peptide" was based on the original in vitro observation that PrRP stimulates prolactin secretion from rat anterior pituitary cells. However, subsequent studies in vivo produced inconsistent results: systemic PrRP infusion does not reliably elevate prolactin in intact animals under basal conditions. The prolactin-stimulating effect appears to require specific physiological contexts (e.g., suckling, stress) and may not reflect the primary in vivo role of PrRP at hypothalamic-brainstem circuits.

Receptor: GPR10 (PRLHR)

PrRP signals exclusively through GPR10 (also designated PRLHR — prolactin-releasing hormone receptor — in nomenclature that predates the current IUPHAR recommendation of GPR10). GPR10 is a class A GPCR with no close paralogs in the human genome; it is the only receptor through which PrRP signals.

Signaling. GPR10 couples to Gαq/11, activating phospholipase C (PLC) → IP₃ → intracellular Ca²⁺ release → PKC and CaM kinase activation. In hypothalamic neurons, this Gαq coupling produces neuronal depolarization and increased firing — an excitatory signature. GPR10 also activates ERK1/2 MAPK and can engage Gαs/cAMP pathways in pituitary cells, which is relevant to the prolactin secretory effect observed in vitro.

Expression. GPR10 is expressed in:

• •Hypothalamus: arcuate nucleus (ARC), dorsomedial hypothalamus (DMH), paraventricular nucleus (PVN)
• •Brainstem: nucleus tractus solitarius (NTS), area postrema (AP), dorsal raphe nucleus, locus coeruleus (LC)
• •Anterior pituitary: where PrRP stimulates prolactin release
• •Ventrolateral medulla (VLM): cardiovascular regulation
• •Limbic system: amygdala, hippocampus — relevant to stress responses

The distribution in NTS, area postrema, and brainstem autonomic nuclei is critical: these are the primary sites where gut satiety signals (GLP-1, CCK, PYY) are integrated. PrRP in brainstem satiety circuits — not just the hypothalamus — underlies its anorexigenic mechanism.

Neuroanatomy: Brainstem-Hypothalamic Circuits

PrRP-producing neurons are concentrated in two major sites that reflect its dual role in feeding and stress:

1. Nucleus tractus solitarius (NTS) and area postrema — the satiety integration hub.

The largest population of PrRP-immunoreactive neurons in rodents is in the caudal NTS and adjacent dorsal vagal complex. This region receives direct vagal afferent input from the gut (mechanoreceptors signaling stomach distension; chemoreceptors signaling nutrient presence; enteroendocrine cell-derived CCK, GLP-1, PYY). NTS PrRP neurons process these peripheral satiety signals and relay them to the hypothalamus.

NTS PrRP neurons project rostrally to the parabrachial nucleus (PBN), PVN, and DMH — all feeding-regulatory hypothalamic areas. This brainstem-to-hypothalamus PrRP pathway positions these neurons as relays that translate peripheral gut signals into central energy balance commands. CCK injection (a potent satiety signal from duodenum) activates NTS PrRP neurons as measured by c-Fos immunoreactivity, confirming that gut satiety signals drive PrRP neuron activity.

2. Ventrolateral medulla (VLM) — adrenergic/stress neurons.

A second population of PrRP neurons overlaps with catecholaminergic (noradrenergic/adrenergic) neurons in the ventrolateral medulla and A2 cell group. These neurons project to the PVN, locus coeruleus, and amygdala — stress circuit nodes. VLM PrRP neurons are activated by stressors (restraint, footshock, lipopolysaccharide) and likely mediate the stress-induced activation of the HPA axis via PVN-CRH neurons.

3. Hypothalamic PrRP neurons.

A smaller population in the dorsomedial hypothalamus (DMH) and perifornical area contributes to intra-hypothalamic PrRP signaling, including modulation of arcuate nucleus circuits.

Anorexigenic and Satiety Functions

Central PrRP suppresses food intake. ICV injection of PrRP-31 or PrRP-20 in rodents produces robust, dose-dependent reduction in food intake in fasted animals. The effect is rapid (within 30-60 minutes) and sustained (4-6 hours). At equimolar doses, PrRP-31 is equipotent to CRH in suppressing feeding — one of the most potent central anorexigenic effects among neuropeptides tested.

Mechanism. The anorexigenic action involves:

1. Brainstem satiety signal amplification: PrRP in NTS activates hypothalamic melanocortin circuit via PVN/DMH projections, promoting α-MSH release from ARC POMC neurons.

2. CRH co-activation: PrRP acts on PVN CRH neurons (GPR10 is expressed on CRH neurons), co-activating feeding-suppressive CRH and the HPA axis simultaneously.

3. Direct hypothalamic GPR10 activation: PrRP acts on DMH/ARC GPR10-expressing neurons to reduce orexigenic signals.

Brainstem PrRP mediates gut-brain satiety. A critical distinction from many hypothalamic anorexigens: PrRP in the NTS directly relays peripheral satiety signals. CCK injection activates NTS PrRP neurons; the anorexigenic effect of CCK is partially blocked by GPR10 antagonism in the NTS. This establishes PrRP as a relay node between peripheral gut hormones and central feeding circuits — not just a hypothalamic integrator.

PrRP and GLP-1 synergy. GLP-1-producing L-cells in the gut release GLP-1 in response to meals, and GLP-1 receptor neurons in the NTS are adjacent to PrRP neurons. There is emerging evidence of functional crosstalk: GLP-1 receptor activation potentiates PrRP neuron activity, and GLP-1 receptor agonists (liraglutide, semaglutide) partially depend on an intact brainstem PrRP-GPR10 circuit for their full anorexigenic effect. This places PrRP mechanistically relevant to the most successful obesity drug class of the 2020s.

The PrRP−/− Obesity Paradox: Energy Expenditure, Not Just Intake

The genetic phenotype of PrRP-null mice revealed an unexpected aspect of PrRP biology.

PrRP−/− mice develop obesity on standard chow — not on high-fat diet, but on standard diet with normal macronutrient composition. The obesity is late-onset (apparent from 20-30 weeks) and progressive, with increased adiposity and body weight.

Surprisingly, PrRP−/− mice do not show hyperphagia — food intake is not significantly elevated compared to wild-type mice. This dissociates the obesity phenotype from simple appetite dysregulation.

Instead, the obesity reflects:

1. Reduced energy expenditure: PrRP−/− mice have lower oxygen consumption, reduced brown adipose tissue thermogenesis, and decreased physical activity. PrRP appears to maintain thermogenic tone in brown adipose via sympathetic outflow from brainstem and hypothalamic circuits.

2. Altered adipose tissue metabolism: PrRP−/− mice show increased white adipose tissue lipid accumulation even at matched food intake — suggesting altered lipolysis or lipogenesis regulation.

3. Reduced basal metabolic rate: Indirect calorimetry reveals lower whole-body metabolic rate, disproportionate to body weight differences.

This phenotype — obesity without hyperphagia, driven by reduced energy expenditure — mirrors the phenotype seen in other hypothalamic anorexigen knockouts where metabolic rate is the primary regulated variable (not appetite alone). It underscores that the hypothalamic energy balance network does not simply control food intake but tightly regulates energy expenditure, and PrRP is a component of the expenditure arm.

Stress, HPA Axis, and Mood

The localization of PrRP neurons in stress-responsive brainstem areas and their projections to HPA-regulatory nuclei makes PrRP an important stress neuromodulator.

Stress activation. PrRP neurons in the NTS and VLM are among the most rapidly activated by acute stressors — restraint, footshock, cold exposure, immune challenge (LPS). PrRP mRNA and c-Fos increase dramatically in these neurons within 30-60 minutes of stress onset.

HPA activation. Central PrRP administration stimulates CRH release from PVN neurons and elevates circulating ACTH and corticosterone — a full HPA axis activation. GPR10 on PVN CRH neurons directly mediates this effect. PrRP thus acts as a hypothalamic CRH secretagogue, alongside the classical CRH regulators (vasopressin, angiotensin II, norepinephrine).

Locus coeruleus (LC) arousal. PrRP fibers project to the locus coeruleus, where GPR10 activation increases noradrenergic neuron firing. This contributes to the arousal and anxiety-like behaviors observed after central PrRP administration. PrRP-injected rodents show increased anxiety-like behavior in the elevated plus maze (EPM) and open field — a stress phenotype that parallels CRH.

PrRP and depression. Given the HPA-activating and noradrenergic effects of PrRP, it has been proposed as a potential contributor to stress-related mood disorders. Elevated PrRP signaling could amplify HPA dysregulation seen in major depression. Conversely, GPR10 antagonism has been explored as a potential anxiolytic or antidepressant strategy in preclinical models — with mixed results that have not yet translated to clinical candidates.

Cardiovascular and Autonomic Effects

Blood pressure regulation. ICV PrRP increases blood pressure in anesthetized rats, mediated through sympathetic activation. The effect requires intact brainstem-spinal sympathetic pathways. The cardiovascular action of PrRP, like its stress-HPA actions, likely reflects activation of brainstem cardiovascular regulatory nuclei expressing GPR10.

Interaction with CCK cardiovascular effects. CCK from the duodenum produces the CCK-satiety reflex (reduced meal size, increased satiety) partly via vagal afferents to the NTS. CCK also has mild cardiovascular effects (bradycardia, vasodilation). PrRP in the NTS, activated by CCK, may participate in these cardiovascular aspects of the postprandial response.

Reproductive System Interactions

Prolactin secretion (context-dependent). GPR10 on anterior pituitary lactotrophs responds to PrRP with increased prolactin secretion in vitro. In vivo, the prolactin effect is most apparent during specific physiological states — suckling, stress, or pharmacological manipulation of pituitary access — and is modest under basal conditions. This has led to reinterpretation of PrRP as a "conditional prolactin secretagogue" rather than the primary tonic regulator of prolactin.

Fertility and LH. Central PrRP has mild effects on LH secretion in some experimental paradigms, but these are less consistent than its anorexigenic or HPA-activating effects. PrRP−/− mice show normal fertility. The reproductive role of PrRP is minor compared to kisspeptin, GnRH, or GALP.

Stress-induced reproductive suppression. The ability of PrRP to activate the HPA axis and increase CRH/cortisol has indirect effects on reproductive function — chronic HPA activation suppresses GnRH pulsatility. Thus PrRP may participate in stress-induced reproductive suppression (hypothalamic amenorrhea), not through direct GnRH effects but through HPA-mediated GnRH inhibition.

Research Tools and Analogs

A notable recent development is PrRP-20 palmitate — a lipid-conjugated analog with substantially extended plasma half-life (~24 hours vs minutes for native PrRP). This analog produces significant body weight reduction in diet-induced obese mice after peripheral injection, demonstrating that PrRP-based analogs with pharmaceutical-grade pharmacokinetics retain anorexigenic potency in obesity models. This advances PrRP from a CNS research tool toward a potential obesity therapeutic scaffold.

Comparison with Other Anorexigenic Neuropeptides

The combination of anorexigenic action and HPA-activating stress effects is characteristic of PrRP and CRH — both suppress feeding while activating the stress axis. This dual function may reflect an adaptive coupling: stress simultaneously reduces appetite and mobilizes energy reserves (via cortisol) to meet the perceived challenge.

Current Frontiers

PrRP-based anti-obesity therapeutics. The palmitate-conjugated PrRP-20 analog (developed at the Institute of Physiology, Czech Academy of Sciences) reduces food intake and body weight in obese rodents after peripheral injection. Combination with GLP-1 receptor agonists or dual PrRP/GLP-1 agonist chimeric peptides represents a research frontier — the mechanisms appear synergistic (both activate brainstem satiety circuits via adjacent or overlapping neurons). If this translates to humans, PrRP-based analogs could complement or enhance existing GLP-1 therapies.

GPR10 in GLP-1 anorexigenic mechanism. The question of whether GLP-1 receptor agonist drugs (liraglutide, semaglutide, tirzepatide) partially work via PrRP neurons in the NTS is mechanistically important. If yes, GPR10 is a secondary mediator of existing blockbuster drugs — clinically significant for understanding resistance to GLP-1 therapy and potential enhancement strategies.

PrRP and HPA modulation in stress disorders. The role of PrRP in stress circuitry makes GPR10 a potential target for anxiety and depression treatment. Selective GPR10 antagonists could dampen HPA hyperactivation without directly targeting CRH (which has failed in clinical trials for depression). This hypothesis awaits selective antagonist tool development.

Brainstem circuit mapping. Single-cell RNA-seq of NTS and VLM is identifying PrRP neuron molecular subtypes — their specific neuropeptide co-expression, input connectivity, and projection targets. These atlases are redefining which gut signals activate which PrRP subpopulations and where PrRP-driven satiety signals terminate in the hypothalamus.

Conclusion

Prolactin-releasing peptide (PrRP) is a compelling example of a peptide whose name captures the discovery context but not the primary physiological function. Discovered in 1998 as the deorphanizing ligand for GPR10, PrRP activates brainstem and hypothalamic circuits to suppress food intake, increase energy expenditure, activate the HPA stress axis, and modulate autonomic cardiovascular function. The PrRP−/− obesity phenotype — arising from reduced metabolic rate, not hyperphagia — revealed that PrRP is an energy expenditure signal as much as an appetite regulator. Its anatomical position in NTS satiety circuits, receiving direct gut afferent input and projecting to hypothalamic energy balance nodes, makes PrRP a molecular relay in the gut-brain axis. As PrRP analog pharmacology advances and GLP-1 receptor agonist mechanisms are dissected, GPR10 is emerging as a therapeutically relevant node in obesity neuroscience.

Key Research References

• •Hinuma S, Habata Y, Fujii R, et al. A prolactin-releasing peptide in the brain. Nature. 1998;393(6682):272-276. PMID: 9607764
• •Maruyama M, Matsumoto H, Fujiwara K, et al. Prolactin-releasing peptide as a novel stress mediator: localization of prolactin-releasing peptide and its receptor in the rat brain. Neuroscience. 2001;103(2):319-336. PMID: 11246146
• •Maletinska L, Spolcova A, Maixnerova J, et al. Biological properties of prolactin-releasing peptide analogs with a modified aromatic core of C-terminal phenylalanine amide. Peptides. 2012;33(2):164-174. PMID: 22085530
• •Pirnik Z, Maixnerova J, Maly J, et al. Prolactin-releasing peptide (PrRP) affects c-fos and NPY mRNA expression in hypothalamic and brainstem neurons of male rats exposed to hypercaloric or hypocaloric diet. Brain Res. 2011;1373:103-114. PMID: 21172319

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This article is intended for Research Use Only (RUO). Prolactin-releasing peptide (PrRP), GPR10 ligands, and related analogs are not approved for human therapeutic use. Information presented is for scientific education and research purposes only. Peptides.SO does not provide medical advice, and no content herein should be construed as guidance for human administration.