In vivo release of prolactin-releasing peptide in rat hypothalamus in association with luteinizing hormone and prolactin surges

Functional Peptidomics: Stimulus- and Time-of-Day-Specific Peptide Release in the Mammalian Circadian Clock

Daily oscillations of brain and body states are under complex temporal modulation by environmental light and the hypothalamic suprachiasmatic nucleus (SCN), the master circadian clock. To better understand mediators of differential temporal modulation, we characterize neuropeptide releasate profiles by nonselective capture of secreted neuropeptides in an optic nerve horizontal SCN brain slice model. Releasates are collected following electrophysiological stimulation of the optic nerve/retinohypothalamic tract under conditions that alter the phase of the SCN activity state. Secreted neuropeptides are identified by intact mass via matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). We found time-of-day-specific suites of peptides released downstream of optic nerve stimulation. Peptide release was modified differentially with respect to time-of-day by stimulus parameters and by inhibitors of glutamatergic or PACAPergic neurotransmission. The results suggest that SCN physiology is modulated by differential peptide release of both known and unexpected peptides that communicate time-of-day-specific photic signals via previously unreported neuropeptide signatures.

RF-amide neuropeptides and their receptors in Mammals: Pharmacological properties, drug development and main physiological functions

RF-amide neuropeptides, with their typical Arg-Phe-NH2 signature at their carboxyl C-termini, belong to a lineage of peptides that spans almost the entire life tree. Throughout evolution, RF-amide peptides and their receptors preserved fundamental roles in reproduction and feeding, both in Vertebrates and Invertebrates. The scope of this review is to summarize the current knowledge on the RF-amide systems in Mammals from historical aspects to therapeutic opportunities. Taking advantage of the most recent findings in the field, special focus will be given on molecular and pharmacological properties of RF-amide peptides and their receptors as well as on their implication in the control of different physiological functions including feeding, reproduction and pain. Recent progress on the development of drugs that target RF-amide receptors will also be addressed.

Functions of two distinct "prolactin-releasing peptides" evolved from a common ancestral gene

Prolactin-releasing peptide (PrRP) is one of the RF-amide peptides and was originally identified in the bovine hypothalamus as a stimulator of prolactin (PRL) release. Independently, another RF-amide peptide was found in Japanese crucian carp and named Carassius-RFa (C-RFa), which shows high homology to PrRP and stimulates PRL secretion in teleost fish. Therefore, C-RFa has been recognized as fish PrRP. However, recent work has revealed that PrRP and C-RFa in non-mammalian vertebrates are encoded by separate genes originated through duplication of an ancestral gene. Indeed, both PrRP and C-RFa are suggested to exist in teleost, amphibian, reptile, and avian species. Therefore, we propose that non-mammalian PrRP (C-RFa) be renamed PrRP2. Despite a common evolutionary origin, PrRP2 appears to be a physiological regulator of PRL, whereas this is not a consistent role for PrRP itself. Further work revealed that the biological functions of PrRP and PrRP2 are not limited solely to PRL release, because they are also neuromodulators of several hypothalamus-pituitary axes and are involved in some brain circuits related to the regulation of food intake, stress, and cardiovascular functions. However, these actions appear to be different among vertebrates. For example, central injection of PrRP inhibits feeding behavior in rodents and teleosts, while it stimulates it in chicks. Therefore, both PrRP and PrRP2 have acquired diverse actions through evolution. In this review, we integrate the burgeoning information of structures, expression profiles, and multiple biological actions of PrRP in higher vertebrates, as well as those of PrRP2 in non-mammals.

Blockade of PrRP attenuates MPTP-induced toxicity in mice

Prolactin-releasing peptide (PrRP) was isolated as an endogenous ligand of the orphan G-protein coupled receptor hGR3. PrRP has been shown to be involved in the regulation of food intake, stress responses, prolactin secretion and release, blood pressure, and the opioid system. Here we report that PrRP and its receptor, GPR10, were found in the mouse substantia nigra pars compacta (SNpc), the main location of dopaminergic (DA) neurons of the nigrostriatal system. We generated PrRP knockout (KO) mice, and then treated PrRP KO mice and their wild type (WT) littermates with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a neuron toxin that selectively damages DA neurons in the SNpc. We found that PrRP KO mice were resistant to MPTP-induced lesions of the nigrostriatal system. These effects were further confirmed by the intracerebroventricular injection of P2L-1C, a monoclonal antibody against PrRP into mice. Taken together, our data established a critical role of PrRP in MPTP intoxication in mice.

RFamide-related peptide-3, a mammalian gonadotropin-inhibitory hormone ortholog, regulates gonadotropin-releasing hormone neuron firing in the mouse

The recent discovery that an RFamide termed gonadotropin-inhibitory hormone is likely to be a hypophysiotrophic gonadotropin release-inhibiting hormone in birds has generated interest into the role of LPXRFamide neuropeptides in the control of gonadotropin secretion in mammals. Recent immunocytochemical studies in birds and mammals have suggested that neurons expressing the mammalian LPXRFamides, RFamide-related peptides (RFRPs) 1 and 3, may innervate and regulate GnRH neurons directly. We used cell-attached electrophysiology in adult male and female GnRH-green fluorescent protein-tagged neurons to examine whether RFRP-3 modulated the electrical excitability of GnRH neurons. RFRP-3 was found to exhibit rapid and repeatable inhibitory effects on the firing rate of 41% of GnRH neurons. A small population of GnRH neurons (12%) increased their firing rate in response to RFRP-3, and the remainder was unaffected. No difference was detected in the RFRP-3 responses of GnRH neurons from male, diestrous, or proestrus female mice. The suppressive effect of RFRP-3 was maintained when amino acid transmission was blocked, suggesting a possible direct effect of RFRP-3 upon GnRH neurons. To evaluate the effects of other RFamide neuropeptides on GnRH neurons, we tested the actions of prolactin-releasing peptide-20 and -31. Neither compounds altered the firing rate of GnRH neurons. These studies demonstrate that RFRP-3 has a likely direct suppressive action on the excitability of GnRH neurons, indicating a role for RFRPs in the regulation of gonadotropin secretion in mammals through modulation of GnRH neuron activity.

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.

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.

Expression of brain prolactin releasing peptide (PrRP) changes in the estrous cycle of female rats

Prolactin releasing peptide (PrRP) is a neuropeptide with 31 or 20 amino acid residues and regarded as a potent and specific stimulator of pituitary prolactin. PrRP immunoreactive (PrRP-ir) neurons and mRNA are found in medulla oblongata and hypothalamus and the fibers containing PrRP are widely distributed in rat brains. Therefore, it is postulated that PrRP might act as a neurohormone or a neurotransmitter as well as a neuromodulator in the brain. In the present study, we probed the expression of brain PrRP in the estrous cycle of female rats and the relationship between brain PrRP and GnRH. Female rats were divided into four groups: the diestrus, the proestrus, the estrus and the metaestrus, which were identified by the vaginal cytological examination. Immunohistochemistry, reverse transcriptase-polymerase chain reaction (RT-PCR) and immunofluorescent double labeling histochemistry combining confocal laser scanning microscope (CLSM) were used. The results showed that PrRP immunoreactive neurons in nucleus of solitary tract (NTS) and ventrolateral reticular nucleus (VLRN) in the proestrus were less than those in the diestrus, the estrus and the metaestrus. Similarly, the relative optical density of PrRP-ir fibers of the bed nucleus of stria terminalis (BST) in the proestrus was decreased compared with those in other three groups. However, the brain PrRPmRNA level was higher in the proestrus and estrus than those in the metaestrus and diestrus. We also observed the co-localization of GPR10-immunoreactive (GPR10-ir) and GnRH-immunoreactive (GnRH-ir) neurons in hypothalamic medial preoptic area (MPO). The present results provide morphological evidences that PrRP in the female rat brains might participate in the regulation of the rat estrous cycle at least in a direct way.

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.

Sodium tanshinone IIA sulfonate derived from Slavia miltiorrhiza Bunge up-regulate the expression of prolactin releasing peptide (PrRP) in the medulla oblongata in ovariectomized rats

Sodium tanshinone IIA sulfonate (STS), a derivative of tanshinone IIA, is isolated from the root of Salvia miltiorrhiza known as "Danshen". Although injection of S. miltiorrhiza extract and STS is used successfully in clinics in China for treating postmenopausal syndrome, the exact mechanism for its therapeutic basis is poorly understood. The present study was undertaken to characterize the effect of STS on the expression of prolactin releasing peptide (PrRP) in the medulla oblongata in ovariectomized rats. In addition, estrogen (E2) levels were detected in OVX rats treated with STS. The results showed that STS might significantly increase the blood level of E2 and PrRP cell number in the medulla oblongata of ovariectomized rats. The number of PrRP immunoreactivity (ir) neurons was higher in the group ovariectomized with STS than that in the ovariectomized group. The numbers of PrRP-ir neurons in Sham and Sham+STS were not significantly different between the two groups. These results suggest that the mechanism that STS improved postmenopausal symptoms induced by ovariectomy in rats might be related to the modulation of the blood E2 level and the expression of PrRP in medulla oblongata of ovariectomized rats.

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.

Characterization of a naturally-occurring polymorphism in the UHR-1 gene encoding the putative rat prolactin-releasing peptide receptor

The rat orphan receptor UHR-1 and its human orthologue, GPR10, were first isolated in 1995. The ligand for this receptor, prolactin-releasing peptide (PrRP), was identified in 1998 by reverse pharmacology and has subsequently been implicated in a number of physiological processes. As supported by its localization and regulation in the hypothalamus and brainstem, we have shown previously that PrRP is involved in energy homeostasis. Here we describe a naturally occurring polymorphism in the UHR-1 gene that results in an ATG to ATA change at the putative translational initiation site. The presence of the polymorphism abolished the binding of 125I PrRP in rat brain slices but did not affect the ability of PrRP to reduce fast-induced food intake. Together this data suggest that PrRP may be exerting its feeding effects through a receptor other than UHR-1.

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.

Preserved inhibitory effect of recurrent hypoglycaemia on the male gonadotrophic axis

OBJECTIVE

Hypoglycaemia-induced decreases in male LH and testosterone concentrations are possibly mediated by activation of the hypothalamus-pituitary-adrenal (HPA) axis or by an increase in PRL. As counterregulatory stress hormone release is attenuated during recurrent hypoglycaemia, we questioned whether the gonadotrophic axis and PRL adapt similarly.

DESIGN

We performed two consecutive hypoglycaemic clamps on day 1 and one clamp on the following day in 15 healthy men. Blood concentrations of gonadotrophins, PRL, testosterone, ACTH and cortisol were measured during the first and the third clamp, taking place at the same time of day.

RESULTS

During hypoglycaemia, serum concentrations of LH and testosterone decreased (P < 0.003 for both), PRL, ACTH and cortisol increased (P < 0.001), and FSH remained unchanged (P = 0.90). The hypoglycaemia-induced decreases in LH and testosterone concentrations were similar during the first and the last clamp (P > 0.28 for both) whereas the increase in PRL, ACTH and cortisol was markedly attenuated during the third clamp (P < 0.001).

CONCLUSIONS

LH and testosterone responses do not adapt to recurrent hypoglycaemia, whereas the increase in PRL is attenuated, indicating adaptation. Considering the marked decrease in the responses of PRL and the HPA axis after antecedent hypoglycaemia, the data suggest that the hypoglycaemia-induced decreases in LH and testosterone concentrations, not adapting to recurrent hypoglycaemia, are mediated independently, probably by blood glucose itself.

Fishy tales of prolactin-releasing peptide

Prolactin (PRL) is an important regulator of multiple biological functions, but a specific PRL-releasing factor, PRL-releasing peptide (PrRP), was isolated only recently from mammals and teleosts. Although this peptide seems to be a strong candidate for being a physiologically relevant stimulator of PRL expression and secretion in teleost pituitary and peripheral organs, it may not be a typical or classic hypothalamic releasing factor in rats. We now know that its biological actions are not limited solely to PRL stimulation, because it is also a neuromodulator of several hypothalamus-pituitary axes and is involved in some brain circuits with the regulation of food intake and cardiovascular functions. Moreover, it plays a direct role in hypertension and retinal information processing. It is the purpose of this review to provide a comprehensive survey of our current knowledge of PrRP and to provide a comparative point of view.

Hypothalamic interleukin-1 beta and tumor necrosis factor-alpha, but not interleukin-6, mediate the endotoxin-induced suppression of the reproductive axis in rats

It is well established that endotoxemia disrupts reproductive capability, and several proinflammatory cytokines, especially IL-1 beta, IL-6, and TNF-alpha in the brain, have been implicated in this endocrine aberration. However, no previous study has directly compared the effects of the three major proinflammatory cytokines (IL-1 beta, IL-6, and TNF-alpha) on the in vivo release of hypothalamic GnRH, a secretagogue of LH from the pituitary. Therefore, in this study, we addressed this issue with two complementary approaches involving push-pull perfusion in freely moving ovariectomized female rats. First, we examined the effects of systemic lipopolysaccharide (LPS) treatment on the release of plasma LH, and of GnRH, IL-1 beta, IL-6, and TNF-alpha in the hypothalamic medial preoptic area (MPOA), where the majority of GnRH neuronal perikarya are located. LPS inhibited the secretion of both LH and GnRH and concomitantly stimulated the release of all three cytokines. We next tested the effects of direct MPOA perfusion with the respective cytokines (at three different concentrations each) on the GnRH and LH secretion. IL-1 beta and TNF-alpha, at the concentrations that were observed in the MPOA after the LPS injection, were equipotent in inhibiting the GnRH-LH system, whereas IL-6 was ineffective (even at a supraphysiological concentration). These results strongly suggest that IL-1 beta and TNF-alpha may represent the major proinflammatory cytokines mediating the LPS-induced suppression of GnRH and LH release, whereas the role of IL-6 seems to be insignificant.