Prolactin-releasing peptide releases corticotropin-releasing hormone and increases plasma adrenocorticotropin via the paraventricular nucleus of the hypothalamus

Prolactin-Releasing Peptide Contributes to Stress-Related Mood Disorders and Inhibits Sleep/Mood Regulatory Melanin-Concentrating Hormone Neurons in Rats

Stress disorders impair sleep and quality of life; however, their pathomechanisms are unknown. Prolactin-releasing peptide (PrRP) is a stress mediator; we therefore hypothesized that PrRP may be involved in the development of stress disorders. PrRP is produced by the medullary A1/A2 noradrenaline (NA) cells, which transmit stress signals to forebrain centers, and by non-NA cells in the hypothalamic dorsomedial nucleus. We found in male rats that both PrRP and PrRP-NA cells innervate melanin-concentrating hormone (MCH) producing neurons in the dorsolateral hypothalamus (DLH). These cells serve as a key hub for regulating sleep and affective states. Ex vivo, PrRP hyperpolarized MCH neurons and further increased the hyperpolarization caused by NA. Following sleep deprivation, intracerebroventricular PrRP injection reduced the number of REM sleep-active MCH cells. PrRP expression in the dorsomedial nucleus was upregulated by sleep deprivation, while downregulated by REM sleep rebound. Both in learned helplessness paradigm and after peripheral inflammation, impaired coping with sustained stress was associated with (1) overactivation of PrRP cells, (2) PrRP protein and receptor depletion in the DLH, and (3) dysregulation of MCH expression. Exposure to stress in the PrRP-insensitive period led to increased passive coping with stress. Normal PrRP signaling, therefore, seems to protect animals against stress-related disorders. PrRP signaling in the DLH is an important component of the PrRP's action, which may be mediated by MCH neurons. Moreover, PrRP receptors were downregulated in the DLH of human suicidal victims. As stress-related mental disorders are the leading cause of suicide, our findings may have particular translational relevance.SIGNIFICANCE STATEMENT Treatment resistance to monoaminergic antidepressants is a major problem. Neuropeptides that modulate the central monoaminergic signaling are promising targets for developing alternative therapeutic strategies. We found that stress-responsive prolactin-releasing peptide (PrRP) cells innervated melanin-concentrating hormone (MCH) neurons that are crucial in the regulation of sleep and mood. PrRP inhibited MCH cell activity and enhanced the inhibitory effect evoked by noradrenaline, a classic monoamine, on MCH neurons. We observed that impaired PrRP signaling led to failure in coping with chronic/repeated stress and was associated with altered MCH expression. We found alterations of the PrRP system also in suicidal human subjects. PrRP dysfunction may underlie stress disorders, and fine-tuning MCH activity by PrRP may be an important part of the mechanism.

Prolactin peptide (pPRL) induces anti-prolactin antibodies, ROS and cortisol but suppresses specific immune responses in rainbow trout

Prolactin has several immune functions in fish however, the effects on innate and specific components of rainbow trout immunity are currently unknown. Therefore in this study, prolactin peptide (pPRL) injection in rainbow trout generated anti-PRL antibodies that were confirmed through Western blot assays of fish brain tissue extract. At the same time, this group of fish was immunized with a viral antigen (VP2) and the specific antibody titer generated by the rainbow trout was subsequently determined, as well as the sero-neutralizing capacity of the antibodies. Interestingly, this group of fish (pPRL-VP2) generated approximately 150% less antibodies compared with fish immunized only with the viral antigen (VP2), and pPRL-VP2 fish increased their cortisol level by 4 times compared to the control. Additionally, through qPCR assay, we determined that the pPRL-VP2 fish group decreased pro-inflammatory transcript expression, and the serum of these (pPRL-VP2) fish stimulated ROS production in untreated fish leukocytes, a phenomenon that was blocked by the pharmacological cortisol receptor inhibitor (RU486). Collectively, this is the first report that indicates that pPRL could modulate both components of immunity in rainbow trout.

Brainstem prolactin-releasing peptide contributes to cancer anorexia-cachexia syndrome in rats

Up to 80% of cancer patients are affected by the cancer anorexia-cachexia syndrome (CACS), which leads to excessive body weight loss, reduced treatment success and increased lethality. The area postrema/nucleus of the solitary tract (AP/NTS) region emerged as a central nervous key structure in this multi-factorial process. Neurons in this area are targeted by cytokines and signal to downstream sites involved in energy homeostasis. NTS neurons expressing prolactin-releasing peptide (PrRP) are implicated in the control of energy intake and hypothalamus-pituitary-adrenal (HPA) axis activation, which contributes to muscle wasting. To explore if brainstem PrRP neurons contribute to CACS, we selectively knocked down PrRP expression in the NTS of hepatoma tumor-bearing rats by an AAV/shRNA gene silencing approach. PrRP knockdown reduced body weight loss and anorexia compared to tumor-bearing controls treated with a non-silencing AAV. Gastrocnemius and total hind limb muscle weight was higher in PrPR knockdown rats. Corticosterone levels were increased in the early phase after tumor induction at day 6 in both groups but returned to baseline levels at day 21 in the PrRP knockdown group. While we did not detect significant changes in gene expression of markers for muscle protein metabolism (MuRF-1, myostatin, mTOR and REDD1), mTOR and REDD1 tended to be lower after disruption PrRP signalling. In conclusion, we identified brainstem PrRP as a possible neuropeptide mediator of CACS in hepatoma tumor-bearing rats. The central and peripheral downstream mechanisms require further investigation and might involve HPA axis activation.

Colocalized neurotransmitters in the hindbrain cooperate in adaptation to chronic hypernatremia

Chronic hypernatremia activates the central osmoregulatory mechanisms and inhibits the function of the hypothalamic-pituitary-adrenal (HPA) axis. Noradrenaline (NE) release into the periventricular anteroventral third ventricle region (AV3V), the supraoptic (SON) and hypothalamic paraventricular nuclei (PVN) from efferents of the caudal ventrolateral (cVLM) and dorsomedial (cDMM) medulla has been shown to be essential for the hypernatremia-evoked responses and for the HPA response to acute restraint. Notably, the medullary NE cell groups highly coexpress prolactin-releasing peptide (PrRP) and nesfatin-1/NUCB2 (nesfatin), therefore, we assumed they contributed to the reactions to chronic hypernatremia. To investigate this, we compared two models: homozygous Brattleboro rats with hereditary diabetes insipidus (DI) and Wistar rats subjected to chronic high salt solution (HS) intake. HS rats had higher plasma osmolality than DI rats. PrRP and nesfatin mRNA levels were higher in both models, in both medullary regions compared to controls. Elevated basal tyrosine hydroxylase (TH) expression and impaired restraint-induced TH, PrRP and nesfatin expression elevations in the cVLM were, however, detected only in HS, but not in DI rats. Simultaneously, only HS rats exhibited classical signs of chronic stress and severely blunted hormonal reactions to acute restraint. Data suggest that HPA axis responsiveness to restraint depends on the type of hypernatremia, and on NE capacity in the cVLM. Additionally, NE and PrRP signalization primarily of medullary origin is increased in the SON, PVN and AV3V in HS rats. This suggests a cooperative action in the adaptation responses and designates the AV3V as a new site for PrRP's action in hypernatremia.

Prolactin-Releasing Peptide: Physiological and Pharmacological Properties

Prolactin-releasing peptide (PrRP) belongs to the large RF-amide neuropeptide family with a conserved Arg-Phe-amide motif at the C-terminus. PrRP plays a main role in the regulation of food intake and energy expenditure. This review focuses not only on the physiological functions of PrRP, but also on its pharmacological properties and the actions of its G-protein coupled receptor, GPR10. Special attention is paid to structure-activity relationship studies on PrRP and its analogs as well as to their effect on different physiological functions, mainly their anorexigenic and neuroprotective features and the regulation of the cardiovascular system, pain, and stress. Additionally, the therapeutic potential of this peptide and its analogs is explored.

Regulation of Hypothalamo-Pituitary-Adrenocortical Responses to Stressors by the Nucleus of the Solitary Tract/Dorsal Vagal Complex

Hindbrain neurons in the nucleus of the solitary tract (NTS) are critical for regulation of hypothalamo-pituitary-adrenocortical (HPA) responses to stress. It is well known that noradrenergic (as well as adrenergic) neurons in the NTS send direct projections to hypophysiotropic corticotropin-releasing hormone (CRH) neurons and control activation of HPA axis responses to acute systemic (but not psychogenic) stressors. Norepinephrine (NE) signaling via alpha1 receptors is primarily excitatory, working either directly on CRH neurons or through presynaptic activation of glutamate release. However, there is also evidence for NE inhibition of CRH neurons (possibly via beta receptors), an effect that may occur at higher levels of stimulation, suggesting that NE effects on the HPA axis may be context-dependent. Lesions of ascending NE inputs to the paraventricular nucleus attenuate stress-induced ACTH but not corticosterone release after chronic stress, indicating reduction in central HPA drive and increased adrenal sensitivity. Non-catecholaminergic NTS glucagon-like peptide 1/glutamate neurons play a broader role in stress regulation, being important in HPA activation to both systemic and psychogenic stressors as well as HPA axis sensitization under conditions of chronic stress. Overall, the data highlight the importance of the NTS as a key regulatory node for coordination of acute and chronic stress.

Interoceptive modulation of neuroendocrine, emotional, and hypophagic responses to stress

Periods of caloric deficit substantially attenuate many centrally mediated responses to acute stress, including neural drive to the hypothalamic-pituitary-adrenal (HPA) axis, anxiety-like behavior, and stress-induced suppression of food intake (i.e., stress hypophagia). It is posited that this stress response plasticity supports food foraging and promotes intake during periods of negative energy balance, even in the face of other internal or external threats, thereby increasing the likelihood that energy stores are repleted. The mechanisms by which caloric deficit alters central stress responses, however, remain unclear. The caudal brainstem contains two distinct populations of stress-recruited neurons [i.e., noradrenergic neurons of the A2 cell group that co-express prolactin-releasing peptide (PrRP+ A2 neurons), and glucagon-like peptide 1 (GLP-1) neurons] that also are responsive to interoceptive feedback about feeding and metabolic status. A2/PrRP and GLP-1 neurons have been implicated anatomically and functionally in the central control of the HPA axis, anxiety-like behavior, and stress hypophagia. The current review summarizes a growing body of evidence that caloric deficits attenuate physiological and behavioral responses to acute stress as a consequence of reduced recruitment of PrRP+ A2 and hindbrain GLP-1 neurons, accompanied by reduced signaling to their brainstem, hypothalamic, and limbic forebrain targets.

Effect of palmitoylated prolactin-releasing peptide on food intake and neural activation after different routes of peripheral administration in rats

Obesity is an escalating epidemic, but an effective non-invasive therapy is still scarce. For obesity treatment, anorexigenic neuropeptides are promising tools, but their delivery from the periphery to the brain is complicated by their peptide character. In order to overcome this unfavorable fact, we have applied the lipidization of neuropeptide prolactin-releasing peptide (PrRP), whose strong anorexigenic effect was demonstrated. A palmitoylated analog of human PrRP (h palm-PrRP31) was injected in free-fed Wistar rats by three routes: subcutaneous (s.c.), intraperitoneal (i.p) (both 5 mg/kg) and intravenous (i.v.) (from 0.01 to 0.5 mg/kg). We found a circulating compound in the blood after all three applications with the highest concentration after i.v. administration. This corresponds to the effect on food intake, which was also strongest after i.v. injection. Moreover, this is in agreement with the fact that the expression of c-Fos in specific brain regions involved in food intake regulation was also highest after intravenous application. Pharmacokinetic data are further supported by results obtained from dynamic light scattering and CD spectroscopy. Human palm-PrRP31 analog showed a strong tendency to micellize, and formation of aggregates suggested lower availability after i.p. or s.c. application. We have demonstrated that palm-PrRP influenced food intake even in free fed rats. Not surprisingly, the maximal effect was achieved after the intravenous application even though two orders of magnitude lower dose was used compared to both two other applications. We believe that palm-PrRP could have a potential as an antiobesity drug when its s.c. application would be improved.

Negative Energy Balance Blocks Neural and Behavioral Responses to Acute Stress by "Silencing" Central Glucagon-Like Peptide 1 Signaling in Rats

Previous reports indicate that caloric restriction attenuates anxiety and other behavioral responses to acute stress, and blunts the ability of stress to increase anterior pituitary release of adrenocorticotropic hormone. Since hindbrain glucagon-like peptide-1 (GLP-1) neurons and noradrenergic prolactin-releasing peptide (PrRP) neurons participate in behavioral and endocrine stress responses, and are sensitive to the metabolic state, we examined whether overnight food deprivation blunts stress-induced recruitment of these neurons and their downstream hypothalamic and limbic forebrain targets. A single overnight fast reduced anxiety-like behavior assessed in the elevated-plus maze and acoustic startle test, including marked attenuation of light-enhanced startle. Acute stress [i.e., 30 min restraint (RES) or 5 min elevated platform exposure] robustly activated c-Fos in GLP-1 and PrRP neurons in fed rats, but not in fasted rats. Fasting also significantly blunted the ability of acute stress to activate c-Fos expression within the anterior ventrolateral bed nucleus of the stria terminalis (vlBST). Acute RES stress suppressed dark-onset food intake in rats that were fed ad libitum, whereas central infusion of a GLP-1 receptor antagonist blocked RES-induced hypophagia, and reduced the ability of RES to activate PrRP and anterior vlBST neurons in ad libitum-fed rats. Thus, an overnight fast "silences" GLP-1 and PrRP neurons, and reduces both anxiety-like and hypophagic responses to acute stress. The partial mimicking of these fasting-induced effects in ad libitum-fed rats after GLP-1 receptor antagonism suggests a potential mechanism by which short-term negative energy balance attenuates neuroendocrine and behavioral responses to acute stress.

SIGNIFICANCE STATEMENT

The results from this study reveal a potential central mechanism for the "metabolic tuning" of stress responsiveness. A single overnight fast, which markedly reduces anxiety-like behavior in rats, reduces or blocks the ability of acute stress to activate hindbrain neurons that are immunoreactive for either prolactin-releasing peptide or glucagon-like peptide 1, and attenuates the activation of their stress-sensitive projection targets in the limbic forebrain. In nonfasted rats, central antagonism of glucagon-like peptide 1 receptors partially mimics the effect of an overnight fast by blocking the ability of acute stress to inhibit food intake, and by attenuating stress-induced activation of hindbrain and limbic forebrain neurons. We propose that caloric restriction attenuates behavioral and physiological responses to acute stress by "silencing" central glucagon-like peptide 1 signaling pathways.

Satiation and stress-induced hypophagia: examining the role of hindbrain neurons expressing prolactin-releasing Peptide or glucagon-like Peptide 1

Neural circuits distributed within the brainstem, hypothalamus, and limbic forebrain interact to control food intake and energy balance under normal day-to-day conditions, and in response to stressful conditions under which homeostasis is threatened. Experimental studies using rats and mice have generated a voluminous literature regarding the functional organization of circuits that inhibit food intake in response to satiety signals, and in response to stress. Although the central neural bases of satiation and stress-induced hypophagia often are studied and discussed as if they were distinct, we propose that both behavioral states are generated, at least in part, by recruitment of two separate but intermingled groups of caudal hindbrain neurons. One group comprises a subpopulation of noradrenergic (NA) neurons within the caudal nucleus of the solitary tract (cNST; A2 cell group) that is immunopositive for prolactin-releasing peptide (PrRP). The second group comprises non-adrenergic neurons within the cNST and nearby reticular formation that synthesize glucagon-like peptide 1 (GLP-1). Axonal projections from PrRP and GLP-1 neurons target distributed brainstem and forebrain regions that shape behavioral, autonomic, and endocrine responses to actual or anticipated homeostatic challenge, including the challenge of food intake. Evidence reviewed in this article supports the view that hindbrain PrRP and GLP-1 neurons contribute importantly to satiation and stress-induced hypophagia by modulating the activity of caudal brainstem circuits that control food intake. Hindbrain PrRP and GLP-1 neurons also engage hypothalamic and limbic forebrain networks that drive parallel behavioral and endocrine functions related to food intake and homeostatic challenge, and modulate conditioned and motivational aspects of food intake.

Physiological Roles of GPR10 and PrRP Signaling

Prolactin-releasing peptide (PrRP) was first isolated from bovine hypothalamus, and was found to act as an endogenous ligand at the G-protein-coupled receptor 10 (GPR10 or hGR3). Although originally named as it can affect the secretion of prolactin from anterior pituitary cells, the potential functions for this peptide have been greatly expanded over the past decade. Anatomical, pharmacological, and physiological studies indicate that PrRP, signaling via the GPR10 receptor, may have a wide range of roles in neuroendocrinology; such as in energy homeostasis, stress responses, cardiovascular regulation, and circadian function. This review will provide the current knowledge of the PrRP and GPR10 signaling system, its putative functions, implications for therapy, and future perspectives.

RFamide Peptides: Structure, Function, Mechanisms and Pharmaceutical Potential

Different neuropeptides, all containing a common carboxy-terminal RFamide sequence, have been characterized as ligands of the RFamide peptide receptor family. Currently, five subgroups have been characterized with respect to their N-terminal sequence and hence cover a wide pattern of biological functions, like important neuroendocrine, behavioral, sensory and automatic functions. The RFamide peptide receptor family represents a multiligand/multireceptor system, as many ligands are recognized by several GPCR subtypes within one family. Multireceptor systems are often susceptible to cross-reactions, as their numerous ligands are frequently closely related. In this review we focus on recent results in the field of structure-activity studies as well as mutational exploration of crucial positions within this GPCR system. The review summarizes the reported peptide analogs and recently developed small molecule ligands (agonists and antagonists) to highlight the current understanding of the pharmacophoric elements, required for affinity and activity at the receptor family. Furthermore, we address the biological functions of the ligands and give an overview on their involvement in physiological processes. We provide insights in the knowledge for the design of highly selective ligands for single receptor subtypes to minimize cross-talk and to eliminate effects from interactions within the GPCR system. This will support the drug development of members of the RFamide family.

Metabolic and stress-related roles of prolactin-releasing peptide

In the modern world, improvements in human health can be offset by unhealthy lifestyle factors, including the deleterious consequences of stress and obesity. For energy homeostasis, humoral factors and neural afferents from the gastrointestinal tract, in combination with long-term nutritional signals, communicate information to the brain to regulate energy intake and expenditure. Energy homeostasis and stress interact with each other, and stress affects both food intake and energy expenditure. Prolactin-releasing peptide, synthesized in discrete neuronal populations in the hypothalamus and brainstem, plays an important role in integrating these responses. This review describes how prolactin-releasing peptide neurons receive information concerning both internal metabolic states and environmental conditions, and play a key role in energy homeostasis and stress responses.

Neuropeptide FF2 receptor distribution in the human brain. An immunohistochemical study

Human neuropeptide FF2 (hFF2) receptor has been postulated to mediate central autonomic regulation by virtue of its ability to bind with high affinity to many amidated neuropeptides. In the present immunohistochemical study, we identified hFF2 positive neurons in the forebrain and medulla oblongata of individuals, who died suddenly of mechanical trauma or hypothermia. Morphologically, these neurons demonstrated features identified with both projection neurons and interneurons. In the forebrain, the highest density of hFF2 expressing neurons was observed in the anterior amygdaloid area and dorsomedial hypothalamic nucleus, especially in its caudal part. A lesser density of hFF2 neurons was identified in the ventromedial hypothalamic nucleus, lateral and posterior hypothalamic areas whereas few cells were visualized in the paraventricular hypothalamic nucleus, perifornical nucleus, horizontal limb of the diagonal band, ventral division of the bed nucleus of the stria terminalis, nucleus basalis of Meynert and ventral tegmental area. In the medulla, significant numbers of hFF2 neurons were observed in the dorsal motor nucleus of vagus and to a lesser extent in the area of catecholaminergic cell groups, A1/C1. These data provide first immunohistochemical evidence of hFF2 localization in the human brain, which is consistent with that reported for tissue distribution of FF2 mRNA and FF2 binding sites within the brain of a variety of mammalian species. The distribution of hFF2 may help in identifying the role of amidated neuropeptides in the human brain within the context of central autonomic and neuroendocrine regulation.

Prolactin-releasing peptide

Prolactin-releasing peptide (PrRP) was initially isolated from the bovine hypothalamus as an activating component that stimulated arachidonic acid release from cells stably expressing the orphan G protein-coupled receptor hGR3 (Hinuma et al. 1998) [also known as GPR10 (Marchese et al. 1995), or UHR-1 for the rat orthologue (Welch et al. 1995)]. Initially touted as a prolactin-releasing factor (therefore aptly named prolactin-releasing peptide), the perspective on the function of this peptide in the organism has been greatly expanded. Over 120 papers have been published on this subject since its initial discovery in 1998. Herein I review the state of knowledge of the PrRP system, its putative function in the organism, and implications for therapy.

GPR10 deficiency in mice results in altered energy expenditure and obesity

In this study, mice carrying a disrupted gene encoding GPR10 (GPR10 KO) were studied to elucidate the function and importance of this receptor regarding metabolism. Female and male GPR10 KO mice had higher body weight after 11 and 15 weeks of age, respectively. The increased body weight was a result of increased fat mass. The obesity was much more pronounced in female mice, which also had a significant decrease in energy expenditure. In correlation to obesity, higher plasma levels of leptin, total cholesterol, and fractions of LDL and HDL were found in GPR10 KO compared to WT mice. Interestingly, GPR10 KO female mice had decreased relative food intake in correlation to higher hypothalamic expression levels of the anorexic signals CRH and POMC. In conclusion, female mice deficient of the gene encoding GPR10 develop higher body weight and obesity due to lower energy expenditure.

Altered emotional behaviors in the diabetes mellitus OLETF type 1 congenic rat

GPR10 is a G-protein-coupled receptor expressed in thalamic and hypothalamic brain regions, including the reticular thalamic nucleus (RTN) and periventricular nucleus (Pev), and the endogenous ligand for this receptor, prolactin-releasing peptide (PrRP), has demonstrated regulatory effects on the stress response. We produced a congenic rat by introducing the Dmo1 allele from the OLETF rat which encodes the amino acid sequences of GPR10 with a truncated NH2-terminus, into the Brown-Norway background. Using receptor autoradiography, we determined a lack of specific [125I]PrRP binding in the RTN and Pev of these mutant rats compared to the control rats. Furthermore, intracerebroventricular injection of PrRP did not induce a significant increase of c-fos-like immunoreactivity in the paraventricular nucleus of the mutant rats compared to the control rats. The mutant rats also displayed a less anxious-like phenotype in three behavioral-based models of anxiety-like behavior (open field, elevated plus maze and defensive withdrawal test). These data show the mutant congenic rat, of which GPR10 neither binds nor responds to PrRP, expresses less anxious-like phenotypes. On the basis of these observations, the GPR10 might be a novel target for the developing new drugs against anxiety and/or other stress-related diseases.

RFamide peptides inhibit the expression of melanotropin and growth hormone genes in the pituitary of an Agnathan, the sea lamprey, Petromyzon marinus

Neuropeptides with the Arg-Phe-amide motif at their C termini (RFamide peptides) were identified in the brains of several vertebrates, and shown to have important physiological roles in neuroendocrine, behavioral, sensory, and autonomic functions. The present study identified RFamide peptides, which are teleost prolactin-releasing peptide (PrRP) homologs, in the sea lamprey, Petromyzon marinus and characterized their effect on the release of pituitary hormones in vitro. Two RFamide peptides (RFa-A and RFa-B) were isolated from an acid extract of sea lamprey brain, including hypothalamus by Sep-Pak C18 cartridge, affinity chromatography using anti-salmon PrRP serum, and reverse-phase HPLC on an ODS-120T column. Amino acid (aa) sequences and mass spectrometric analyses revealed that RFa-A and RFa-B consist of 25 and 20 aa, respectively, and have 75% sequence identity within the C-terminal 20 aa. The RFa-B cDNA encoding a preprohormone of 142 aa was cloned from the lamprey brain, and the deduced aa sequence from positions 48-67 was identical to the sequence of RFa-B. However, the preprohormone does not include an aa sequence similar to the RFa-A sequence. Cell bodies, which were immunoreactive to anti-salmon PrRP serum, were located in the periventricular arcuate nucleus, ventral part of the hypothalamus, and immunoreactive fibers were abundant from the hypothalamus to the brain. A small number of immunoreactive fibers were detected in the dorsal half of the rostral pars distalis of the pituitary, close to the GH-producing cells. In addition, anti-salmon PrRP immunoreactivities were observed in the pars intermedia, corresponding to melanotropin cells. Likewise, signal of RFa-B mRNA was detected not only in the brain but also in the pars intermedia. The synthetic RFa-A and -B inhibited GH mRNA expression in a dose-dependent fashion in vitro, which is comparable to the inhibitory effect of teleost PrRP on GH release. Both RFa-A and -B also inhibited the expression of proopiomelanotropin mRNA, but no effects were observed in the expression of proopiocortin and gonadotropin beta mRNAs. The results indicate that RFamide peptides, which are teleost PrRP homologs, are present in the hypothalamus and pituitary of sea lamprey, and may be physiologically involved in the inhibition of GH and melanotropin release in the sea lamprey pituitary.

Prolactin-releasing Peptide (PrRP) increases prolactin responses to TRH in vitro and in vivo

The Prolactin-releasing Peptide (PrRP) is a 31-aminoacid peptide produced and secreted from the hypothalamus, and postulated to promote the prolactin release from the pituitary. However, the action of PrRP remain controversial, since it was described to have potency comparable enough to TRH, although there are many evidences that PrRP is less potent than TRH. Here we have studied the effects of PrRP alone or in combination with TRH in the prolactin levels of rat pituitary primary cell cultures in vitro and also in vivo prolactin responses in randomly cycling and estrogens-treated female rats. PrRP itself increased prolactin levels in vitro and in vivo, although in a magnitude several times lower than TRH. In vivo PrRP promotes an atypical non-peaking progressive and maintained prolactin increase. On the other hand, PrRP markedly increased the prolactin responses to TRH in vitro (10-30 fold increase) and in vivo (up to three-fold increase). In addition, FGF-2 and EGF, two important growth factors present in the pituitary, reduced the PrRP-induced prolactin increase in vitro. Taken together our results suggest that PrRP released from the hypothalamus may be relevant to modulate the circulating prolactin levels in the rat.

Electroacupuncture stimulates the expression of prolactin-releasing peptide (PrRP) in the medulla oblongata of ovariectomized rats

Electroacupuncture (EA) in reproductive medicine has become established in Western medicine as a therapy over the last decade. EA performs a variety of neuromodulatory functions in the central nervous system (CNS). Prolactin-releasing peptide (PrRP) is a neuropeptide identified as an endogenous ligand for the orphan G protein-coupled receptor hGR3. PrRP can affect the function of hypothalamus-pituitary-ovary axis (HPOA) and hypothalamus-pituitary-adrenal axis (HPAA). The present study was undertaken to characterize the effect of EA on the expression of PrRP in the medulla oblongata in ovariectomized (OVX) rats by immunohistochemistry and reverse transcription-polymerase chain reaction (RT-PCR). In addition, estrogen (E2) levels were detected by radioimmunoassay (RIA). The results suggest that EA significantly increase the blood level of E2 and the expression of PrRP in the medulla oblongata of OVX rats. The number of PrRP immunoreactive (ir) neurons was higher in the group ovariectomized with EA than that in the OVX group. The numbers of PrRP-ir neurons in intact (INT) and intact with EA (INT+EA) were not significantly different between the two groups. The expression of PrRP mRNA was increased in the OVX+EA group than that in the OVX group. These results suggest that the mechanism that EA improved reproductive disorders induced by ovariectomy in rats is related to the modulation of the blood E2 level and the expression of PrRP in the medulla oblongata.

Distribution of 26RFa binding sites and GPR103 mRNA in the central nervous system of the rat

The novel RFamide peptide 26RFa, the endogenous ligand of the orphan receptor GPR103, affects food intake, locomotion, and activity of the gonadotropic axis. However, little is known regarding the localization of 26RFa receptors. The present report provides the first detailed mapping of 26RFa binding sites and GPR103 mRNA in the rat central nervous system (CNS). 26RFa binding sites were widely distributed in the brain and spinal cord, whereas the expression of GPR103 mRNA was more discrete, notably in the midbrain, the pons, and the medulla oblongata, suggesting that 26RFa can bind to a receptor(s) other than GPR103. Competition experiments confirmed that 26RFa interacts with an RFamide peptide receptor distinct from GPR103 that may be NPFF2. High densities of 26RFa binding sites were observed in olfactory, hypothalamic, and brainstem nuclei involved in the control of feeding behavior, including the piriform cortex, the ventromedial and dorsomedial hypothalamic nuclei, the paraventricular nucleus, the arcuate nucleus, the lateral hypothalamic area, and the nucleus of the solitary tract. The preoptic and anterior hypothalamic areas were also enriched with 26RFa recognition sites, supporting a physiological role of the neuropeptide in the regulation of the gonadotropic axis. A high density of 26RFa binding sites was detected in regions of the CNS involved in the processing of pain, such as the dorsal horn of the spinal cord and the parafascicular thalamic nucleus. The wide distribution of 26RFa binding sites suggests that 26RFa has multiple functions in the CNS that are mediated by at least two distinct receptors.

Driving reproduction: RFamide peptides behind the wheel

The availability of tools for probing the genome and proteome more efficiently has allowed for the rapid discovery of novel genes and peptides that play important, previously uncharacterized roles in neuroendocrine regulation. In this review, the role of a class of neuropeptides containing the C-terminal Arg-Phe-NH(2) (RFamide) in regulating the reproductive axis will be highlighted. Neuropeptides containing the C-terminal Phe-Met-Arg-Phe-NH(2) (FMRFamide) were first identified as cardioregulatory elements in the bi-valve mollusk Macrocallista nimbosa. During the past two decades, numerous studies have shown the presence of structurally similar peptides sharing the RFamide motif across taxa. In vertebrates, RFamide peptides have pronounced influences on opiatergic regulation and neuroendocrine function. Two key peptides in this family are emerging as important regulators of the reproductive axis, kisspeptin and gonadotropin-inhibitory hormone (GnIH). Kisspeptin acts as the accelerator, directly driving gonadotropin-releasing hormone (GnRH) neurons, whereas GnIH acts as the restraint. Recent evidence suggests that both peptides play a role in mediating the negative feedback effects of sex steroids. This review presents the hypothesis that these peptides share complementary roles by responding to internal and external stimuli with opposing actions to precisely regulate the reproductive axis.

Possible inhibitory role of prolactin-releasing peptide for ACTH release associated with running stress

Exercise around the lactate threshold induces a stress response, defined as "running stress." We have previously demonstrated that running stress is associated with activation of certain regions of the brain, e.g., the paraventricular hypothalamic nucleus (PVN) and supraoptic nucleus, that are hypothesized to play an integral role in regulating stress-related responses, including ACTH release during running. Thus we investigated the role of prolactin-releasing peptide (PrRP), found in the ventrolateral medulla and the nucleus of the solitary tract, which is known to project to the PVN during running-induced ACTH release. Accumulation of c-Fos in PrRP neurons correlated with running speeds, reaching maximal levels under running stress. Intracerebroventricular injection of neutralizing anti-PrRP antibodies led to increased plasma ACTH level and blood lactate accumulation during running stress, but not during restraint stress. Exogenous intracerebroventricular administration of low doses of PrRP had the opposite effects. Therefore, our results suggest that, during running stress, PrRP-containing neurons are activated in an exercise intensity-dependent manner, and likewise the produced endogenous PrRP attenuates ACTH release and blood lactate accumulation during running stress. Here we provide a novel perspective on understanding of PrRP in the endocrine-metabolic response associated with running stress.

Anatomical distribution and biochemical characterization of the novel RFamide peptide 26RFa in the human hypothalamus and spinal cord

26RFa is a novel RFamide peptide originally isolated in the amphibian brain. The 26RFa precursor has been subsequently characterized in various mammalian species but, until now, the anatomical distribution and the molecular forms of 26RFa produced in the CNS of mammals, in particular in human, are unknown. In the present study, we have investigated the localization and the biochemical characteristics of 26RFa-like immunoreactivity (LI) in two regions of the human CNS--the hypothalamus and the spinal cord. Immunohistochemical labeling using specific antibodies against human 26RFa and in situ hybridization histochemistry revealed that in the human hypothalamus 26RFa-expressing neurons are located in the paraventricular and ventromedial nuclei. In the spinal cord, 26RFa-expressing neurons were observed in the dorsal and lateral horns. Characterization of 26RFa-related peptides showed that two distinct molecular forms of 26RFa are present in the human hypothalamus and spinal cord, i.e. 26RFa and an N-terminally elongated form of 43 amino acids designated 43RFa. These data provide the first evidence that 26RFa and 43RFa are actually produced in the human CNS. The distribution of 26RF-LI suggests that 26RFa and/or 43RFa may modulate feeding, sexual behavior and transmission of nociceptive stimuli.

Prolactin releasing peptide (PrRP): an endogenous regulator of cell growth

Prolactin releasing peptide (PrRP) was originally reported to act in the anterior lobe of the pituitary gland to stimulate prolactin (PRL) release; however, numerous other pharmacologic actions of PrRP have been described. In the central nervous system PrRP inhibits food intake, stimulates sympathetic tone, and activates stress hormone secretion. Here, we confirm the presence of immunoreactive PrRP in a pheochromocytoma-derived cell line (PC-12) and the ability of exogenous PrRP to stimulate adenylyl cyclase activity in these cultures. Our novel findings are that PrRP stimulated PC-12 cell growth. Furthermore, a role for endogenous PrRP in PC-12 cell growth is suggested by our observations that antisense oligonucleotides and small interfering RNA molecules, which decrease peptide content in these cells, also decrease thymidine incorporation, suggesting an autocrine action of the peptide.

Recent advances in mammalian RFamide peptides: the discovery and functional analyses of PrRP, RFRPs and QRFP

Since the first discovery of a peptide with RFamide structure at its C-terminus (i.e., an RFamide peptide) from an invertebrate in 1977, numerous studies on RFamide peptides have been conducted, and a variety have been identified in various phyla throughout the animal kingdom. The first reported mammalian RFamide peptides were neuropeptide FF (NPFF) and neuropeptide AF (NPAF) in 1985. However, for many years after this, no new novel RFamide peptides were identified in mammals. A breakthrough in discovering mammalian RFamide peptides was made possible by reverse pharmacology on the basis of orphan G protein-coupled receptor (GPCR) research. The first report of an RFamide peptide identified from orphan GPCR research was prolactin (PRL)-releasing peptide (PrRP) in 1998. To date, a total of five RFamide peptide genes have been discovered in mammals. Orphan GPCR research has contributed considerably to the identification of these peptides and their receptor genes. This paper examines these mammalian RFamide peptides focusing especially on PrRP, RFamide-related peptides (RFRPs) and, the most recently identified, pyroglutamylated RFamide peptide (QRFP), the discovery of all of which the authors were at least partly involved in. We review here the strategies employed for the identification of these peptides and examine their characteristics, tissue distribution, receptors and functions.

Cloning of the prepro C-RFa gene and brain localization of the active peptide in Salmo salar

In all vertebrates, the synthesis and release of prolactin (Prl) from pituitary lactotroph cells is tightly controlled by hypothalamic factors. We have cloned and characterized a hypothalamic cDNA from Atlantic salmon (Salmo salar) encoding C-RFa, a peptide structurally related to mammalian Prl-releasing peptide (PrRP). The deduced preprohormone precursor is composed of 155 amino acid residues presenting a 87.1% similarity to chum salmon C-RFa and a 100% similarity to all fish C-RFa in the bioactive precursor motifs. C-RFa-immunoreactive perikarya and fibres were located in the brain of S. salar, especially in the hypothalamus, olfactory tract, optic tectum and cerebellum. In contrast, immunolabelled fibres were not observed in the pituitary stalk or in the hypophysis. However, interestingly, we detected immunolabelled cells in the rostral pars distalis of the pituitary in the basolateral region in which Prl is synthesized. These results were confirmed by obtaining a strong signal by using reverse transcription/polymerase chain reaction (RT-PCR) on mRNA from both hypothalamus and pituitary. These data show, for the first time, by immunohistochemistry and RT-PCR, that C-RFa is produced in pituitary cells. Finally, based on these results, a possible function for C-RFa as a locally produced PrRP in this teleost is discussed.

Kisspeptins: regulators of metastasis and the hypothalamic-pituitary-gonadal axis

The kisspeptins are the peptide products of the KiSS-1 gene and the endogenous agonists for the GPR54 receptor. Although KiSS-1 was initially discovered as a metastasis suppressor gene, recent evidence suggests the kisspeptin/GPR54 system is a key regulator of the reproductive system. Disrupted GPR54 signalling causes hypogonadotrophic hypogonadism in rodents and man. Central or peripheral administration of kisspeptin potently stimulates the hypothalamic-pituitary-gonadal axis, increasing circulating gonadotrophin concentrations in a number of animal models. These effects appear likely to be mediated via the hypothalamic gonadotrophin-releasing hormone system, although kisspeptins may have direct effects on the anterior pituitary gland. Hypothalamic KiSS-1 expression is regulated by circulating sex steroids. The precise physiological role of the kisspeptin system in the regulation of reproductive function remains to be elucidated.

Estrogen suppresses the stress response of prolactin-releasing peptide-producing cells

Prolactin-releasing peptide (PrRP) is known to be produced in A1/A2 noradrenergic neurons and to mediate the stress response. Our preliminary experiment showed that PrRP neurons in the A2 region differed between males and females in terms of c-Fos expression. In addition it has been reported that estrogen receptor alpha is detectable in A2 PrRP neurons. Therefore, we speculated that the stress response of PrRP neurons is modified by estrogen. We, therefore, examined c-Fos expression in A2 PrRP neurons during the estrous cycle and found that c-Fos accumulation in PrRP neurons was significantly decreased in estrus compared with in proestrus, metestrus and diestrus. This suggests that estrogen suppresses the activation of PrRP neurons. We thus administered diethylstilbestrol (DES) to ovariectomized rats and then added restraint stress. The data clearly showed that PrRP cells in DES-administered rats significantly suppressed c-Fos accumulation induced by stress.

Identification of the prolactin-releasing peptide-producing cell in the rat adrenal gland

Prolactin-releasing peptide (PrRP) is a novel peptide found in bovine hypothalamus as an endogenous ligand of an orphan G-protein-coupled receptor (hGR3). It is known that PrRP is widely distributed and plays roles in the central nervous system (CNS). In particular, PrRP acts as a neurotransmitter that mediates stress and activates the hypothalamo-pituitary-adrenal axis. On the other hand, only a few studies have so far been performed on PrRP in peripheral tissues. Among peripheral tissues, appreciable levels of PrRP are found only in the adrenal gland; however, the PrRP-producing cells in the adrenal gland have not been identified. In this study, we detected PrRP mRNA in the rat adrenal medulla. So, we tried to identify the PrRP-producing cells in primary culture cells of the adrenal medulla. We found immunopositive PrRP cells among the cultured cells from the adrenal gland, but not in the adrenal gland tissue, by means of immunocytochemistry. The PrRP immunopositive cells were double positive for tyrosine hydroxylase (TH) and for phenylethanolamine N-methyltransferase (PNMT), which indicates that PrRP may be produced in a part of the adrenaline cells in the adrenal gland. This is the first report that PrRP is produced in the adrenaline-containing cells of the adrenal gland.

Physiological roles of prolactin-releasing peptide

Prolactin-releasing peptide (PrRP) was first isolated from bovine hypothalamus as an orphan G-protein-coupled receptor using the strategy of reverse pharmacology. The initial studies showed that PrRP was a potent and specific prolactin-releasing factor. Morphological and physiological studies, however, indicated that PrRP may play a wide range of roles in neuroendocrinology other than prolactin release, i.e., metabolic homeostasis, stress responses, cardiovascular regulation, gonadotropin secretion, GH secretion and sleep regulation. This review will provide the current knowledge of PrRP, especially its roles in energy metabolism and stress responses.

Feeding response to a potent prolactin-releasing peptide agonist in lean and obese Zucker rats

Prolactin (PRL)-releasing peptide (PrRP) is a new peptide present in the hypothalamus and in the circulation that may be involved in the regulation of feeding behavior. In the present experiment, we measured it in a well-known model of obesity, the Zucker rat. We also measured the reactivity of this animal in terms of food intake after the intraperitoneal (I.P.) or central injection of PrRP-13, a potent PrRP agonist. Plasma PrRP levels were 35% lower in obese fa/fa than in the lean rats (p<0.005). I.P. injections of PrRP-13 (10 mg/kg) stimulated food intake in lean and had no effect in obese rats (p<0.001). Intracerebral injections of PrRP-13 had no effects in both genotypes. Altogether, these results do not support a role for PrRP in the hyperphagia and obesity syndrome of the Zucker rat.

The prolactin-releasing peptide receptor (GPR10) regulates body weight homeostasis in mice

To identify new drug targets for the treatment of obesity, we employed a degenerate reverse transcriptasepolymerase chain reaction technique to isolate novel members of the G-protein coupled receptor superfamily from mouse hypothalamus. One of our clones was found to encode a protein with 90% amino acid identity to human GPR10, which was previously identified as the receptor for prolactin-releasing peptide (PrRP) and has been implicated in lactation, the regulation of food intake and other physiological functions. To investigate the role of GPR10 in food intake and body weight homeostasis, we generated mice carrying a targeted deletion of the GPR10 gene. First, using these knockout animals, we confirmed that GPR10 is the principle receptor for PrRP in the mouse hypothalamus because deletion of GPR10 completely abolished PrRP binding to isolated hypothalamic cell membranes. Second, we investigated the effect of normal and high-fat diets on energy intake, body weight, and glucose homeostasis in wild-type and GPR10 knockout mice. After fasting and refeeding, food intake in knockout animals was unchanged relative to control littermates. However, beginning at 16 wk of age on a normal diet, knockout mice became hyperphagic, obese, and showed significant increases in body fat and the levels of leptin and insulin, as well as decreased glucose tolerance. This metabolic profile was similar to the effect of a high-fat diet on wild-type animals. Our findings provide direct evidence that GPR10 is the receptor for PrRP and that it is involved in the regulation of energy balance in mice. GPR10 knockout mice will also prove useful for investigating other proposed activities for PrRP.

Minireview: A hypothalamic role in energy balance with special emphasis on leptin

The hypothalamus is a major site for integration of central and peripheral signals that regulate energy homeostasis. Within the hypothalamus, neurons residing in the ARC (arcuate nucleus)-PVN (paraventricular)-PF/LH (perifornical/lateral hypothalamus) axis communicate among each other and are subjected to the influence of several peripheral factors, including leptin and insulin. Proper signaling in the hypothalamus by leptin, a long-sought peripheral factor that relays the status of fat stores, is critical to normal regulation of food intake and body weight. Leptin action in the hypothalamus is mediated by a large number of orexigenic and anorectic peptide-producing neurons of the ARC-PVN-PF/LH axis. Not only the classical JAK2 (Janus kinase 2)-STAT3 (signal transducer and activator of transcription 3) pathway, but also the phosphatidylinositol-3 kinase-phosphodiesterase 3B-cAMP pathway mediates hypothalamic leptin receptor signaling. It appears that hypothalamic leptin resistance, possibly due to defective nutritional regulation of leptin receptor expression and/or reduced STAT3 signaling in the hypothalamus, contributes to the development of obesity associated with high-fat feeding and aging. Interestingly, hypothalamic neurons may develop leptin resistance despite an intact JAK2-STAT3 signaling path. The role of suppressor of cytokine signaling 3 and other negative regulators of leptin signaling in central leptin resistance needs to be established, an important area of future investigation. Further understanding of the neural circuitry and leptin signaling in the hypothalamus is critical not only for the advancement of our knowledge on the hypothalamic role in energy balance but also for future development of drugs for the attenuation or treatment of obesity and related disorders in humans.

Brainstem prolactin-releasing peptide neurons are sensitive to stress and lactation

Prolactin-releasing peptide (PrRP) was originally thought to participate in the control of adenohypophyseal prolactin secretion, but its predominant expression in a subset of medullary noradrenergic neurons is more in line with roles in interoceptive and/or somatosensory information processing. To better define functional contexts for this peptide system, immuno- and hybridization histochemical methods were used to monitor the capacity of PrRP neurons to display activational responses to lactation, suckling, acute footshock or hypotensive hemorrhage. PrRP mRNA signal was reduced in the medulla of lactating dams, relative to both male and diestrus female controls, with cell counts revealing 42% and 43% reductions in the number of positively hybridized cells in the nucleus of the solitary tract (NTS) and ventrolateral medulla, respectively. Lactating mothers killed after a 90 min suckling episode (following 4 h pup removal) failed to show induced Fos expression in identified medullary PrRP neurons, despite the fact that responsive neurons were detected in other aspects of the caudal NTS. By contrast, acute exposure to hypotensive (25%) hemorrhage or footshock each activated substantial complements of medullary neurons expressing PrRP mRNA. A substantially greater fraction of the total medullary PrRP population exhibited sensitivity to footshock than hemorrhage (71 versus 39%, respectively). These results suggest that medullary PrRP neurons are negatively regulated by (presumably hormonal) changes in lactation, and are not recruited to activation by suckling stimuli. These populations exhibit differential sensitivity to distinct acute stressors, and may participate in the modulation of adaptive neuroendocrine and autonomic responses to each.

Orphan G protein-coupled receptors: targets for new therapeutic interventions

With the completion of the human genome, many genes will be uncovered with unknown functions. The 'orphan' G protein coupled receptors (GPCRs) are examples of genes without known functions. These are genes that exhibit the seven helical conformation hallmark of the GPCRs but that are called 'orphans' because they are activated by none of the primary messengers known to activate GPCRs in vivo. They are the targets of undiscovered transmitters and this lack of knowledge precludes understanding their function. Yet, because they belong to the supergene family that has the widest regulatory role in the organism, the orphan GPCRs have generated much excitement in academia and industry. They hold much hope for revealing new intercellular interactions that will open new areas of basic research which ultimately will lead to new therapeutic applications. However, the first step in understanding the function of orphan GPCRs is to 'deorphanize' them, to identify their natural transmitters. Here we review the search for the natural primary messengers of orphan GPCRs and focus on two recently deorphanized GPCR systems, the melanin-concentrating hormone (MCH) and prolactin-releasing peptide (PrRP) systems, to illustrate the strategies applied to solve their function and to exemplify the therapeutic potentials that such systems hold.

Elevated corticosterone and inhibition of ACTH responses to CRH and ether in the neonatal rat: effect of hypoxia from birth

Hypoxia is a common cause of neonatal morbidity and mortality. We have previously demonstrated a dramatic ACTH-independent activation of adrenal steroidogenesis in hypoxic neonatal rats, leading to increases in circulating corticosterone levels. The purpose of the present study was to determine if this ACTH-independent increase in corticosterone inhibits the ACTH response to acute stimuli. Neonatal rats were exposed to normoxia (control) or hypoxia from birth to 5 or 7 days of age. At the end of the exposure, plasma ACTH and corticosterone were measured before and after either ether vapors were administered for 3 min or CRH (10 microg/kg) was given intraperitoneally. Thyroid function, pituitary pro-opiomelanocortin (POMC) mRNA and ACTH content, and hypothalamic corticotropin-releasing hormone (CRH), neuropeptide Y (NPY), and AVP mRNA were also assessed. Hypoxia led to a significant increase in corticosterone without a large increase in ACTH, confirming previous studies. The ACTH responses to ether or CRH administration were almost completely inhibited in hypoxic pups. Hypoxia did not affect the established regulators of the neonatal hypothalamic-pituitary-adrenal axis, including pituitary POMC or ACTH content, hypothalamic CRH, NPY, or AVP mRNA (parvo- or magnocellular), or thyroid function. We conclude that hypoxia from birth to 5 or 7 days of age leads to an attenuated ACTH response to acute stimuli, most likely due to glucocorticoid negative feedback. The neural and biochemical mechanism of this effect has yet to be elucidated.

Anorectic actions of prolactin-releasing peptide are mediated by corticotropin-releasing hormone receptors

Prolactin-releasing peptide (PrRP) reduces food intake and body weight and modifies body temperature when administered centrally in rats, suggesting a role in energy homeostasis. However, the mediators of PrRP's actions are unknown. The present study, therefore, first examined the possible involvement of the anorectic neuropeptides corticotropin-releasing hormone (CRH) and the melanocortins (e.g., alpha-melanocyte-stimulating hormone) in PrRP's effects on food intake and core body temperature and, second, determined if PrRP affects energy expenditure by measuring oxygen consumption (Vo2). Intracerebroventricular injection of PrRP (4 nmol) to 24-h-fasted male Sprague-Dawley rats decreased food intake and modified body temperature. Blockade of central CRH receptors by intracerebroventricular coadministration of the CRH receptor antagonist astressin (20 microg) reversed the PrRP-induced reduction in feeding. However, astressin's effect on PrRP-induced changes in body temperature was complicated because the antagonist itself caused a slight rise in body temperature. In contrast, intracerebroventricular coadministration of the melanocortin receptor-3/4 antagonist SHU-9119 (0.1 nmol) had no effect on any of PrRP's actions. Finally, intracerebroventricular injection of PrRP (4 nmol) caused a significantly greater Vo2 over a 3-h test period compared with vehicle-treated rats. These results show that the anorectic actions of PrRP are mediated by central CRH receptors but not by melanocortin receptors-3/4 and that PrRP can modify Vo2.

Prolactin-releasing peptides

Physiologic control of prolactin (PRL) secretion is largely dependent upon levels of dopamine accessing the adenohypophysis via the hypophysial portal vessels. However, it is clear that other factors of hypothalamic origin can modulate hormone secretion in the absence or presence of dopamine. Several neuropeptides have been identified as PRL releasing factors (PRFs) but none of these peptides appears to be a major determinant of PRL secretion in vivo. There remain uncharacterized activities in hypothalamic extracts that can alter secretion and production of the hormone. In addition, there exist a wide variety of substances (neurotransmitters, neuromodulators, neuropeptides) that can act within the hypothalamus to modify the neuroendocrine regulation of PRL secretion. These factors may not be considered true PRFs because their actions are not exerted directly at the level of the lactotroph; however, they can act in brain to stimulate PRL release in vivo and therefore might be considered PRL releasing peptides (PRPs).