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.

Prolactin-releasing peptide regulates the cardiovascular system via corticotrophin-releasing hormone

Prolactin-releasing peptide (PrRP)-producing neurones are known to be localised mainly in the medulla oblongata and to act as a stress mediator in the central nervous system. In addition, central administration of PrRP elevates the arterial pressure and heart rate. However, the neuronal pathway of the cardiovascular effects of PrRP has not been revealed. In the present study, we demonstrate that PrRP-immunoreactive neurones projected to the locus coeruleus (LC) and the paraventricular nucleus (PVN) of the hypothalamus. The c-fos positive neurones among the noradrenaline cells in the LC, and the parvo- and magnocellular neurones in the PVN, were increased after central administration of PrRP. The arterial pressure and heart rate were both elevated after i.c.v. administration of PrRP. Previous studies have demonstrated that PrRP stimulated the neurones in the PVN [i.e. oxytocin-, vasopressin- and corticotrophin-releasing hormone (CRH)-producing neurones], which suggests that PrRP may induce its cardiovascular effect via arginine vasopressin (AVP) or CRH. Although the elevation of blood pressure and heart rate elicited by PrRP administration were not inhibited by an AVP antagonist, they were completely suppressed by treatment with a CRH antagonist. Thus, we conclude that PrRP stimulated CRH neurones in the PVN and that CRH might regulate the cardiovascular system via the sympathetic nervous system.

Prolactin-releasing peptide, a possible modulator of prolactin in the euryhaline silver sea bream (Sparus sarba): A molecular study

PRL and PrRP cDNAs have been isolated from euryhaline silver sea bream (Sparus sarba). The PRL cDNA consists of 1360bp encoding 212 amino acids whereas the PrRP cDNA contains 631bp encoding preproPrRP with 122 amino acids. The mature PrRP sequence within the preprohormone is identical to the PrRPs isolated from other fish species. PRL mRNA was uniquely expressed in sea bream pituitary but PrRP mRNA was expressed in a variety of organs and tissues including the intestines, olfactory rosette and various brain regions such as hypothalamus and pituitary. Expression levels of PRL and PrRP mRNA have been examined in sea bream adapted to different salinities (0, 6, 12, 33 and 50ppt). In the pituitary, both PRL and PrRP mRNA were significantly higher in fish adapted to low salinities (0 and 6ppt) and the expression profiles of both hormones closely paralleled each other. However, expression of hypothalamic PrRP was significantly higher in fish adapted to iso-osmotic salinity (12ppt) when pituitary PRL expression was low. The present study demonstrates, for the first time, a synchronized mRNA expression pattern between PRL and PrRP in fish pituitary but a disparity of mRNA expression levels between hypothalamic PrRP and pituitary PRL during salinity adaptation. These data suggest that PrRP may possibly act as a local modulator in pituitary rather than a hypothalamic factor for regulation of pituitary PRL expression in silver sea bream.

Short-term anorexigenic effects of central neuropeptide VF are associated with hypothalamic changes in chicks

The present study was designed to measure food and water intake, changes in hypothalamic chemistry, and other behaviour modifications after central injection of neuropeptide (NP) VF in broiler type chicks. In Experiment 1, chicks responded to central NPVF with a reduction in food intake for up to 90 min post injection. Water intake was unaffected. In Experiment 2, NPVF exerted a less potent and shorter duration of attenuated food intake than did the structurally related NPFF. In Experiment 3, 16.0 nmol NPVF reversed the prolactin-releasing peptide induced orexigenic effect. In Experiment 4, central NPVF treatment was associated with decreased c-Fos immunoreactivity in the lateral hypothalamus, whereas c-Fos immunoreactivity in the dorsomedial nucleus, infundibular nucleus (homologue to the mammalian arcuate nucleus) and ventromedial nucleus was increased. In Experiment 5, behaviours unrelated to ingestion including sit, stand, deep rest and locomotion were affected by central NPVF injection. Some of these behaviours are incompatible with ingestion and may contribute to hypothalamic associated perception of satiety after central NPVF. In conclusion, NVPF is a short-term regulator of appetite and its effects are associated with hypothalamic and behaviour changes in chicks.

Increase of the trophoblast giant cells with prolactin-releasing peptide (PrRP) receptor expression in p53-null mice

Trophoblast giant cells in the mouse placentas are polyploid cells that form as a result of endoreduplication. The giant cells form the outermost layer of the extraembryonic compartment and produce a number of pregnancy-specific hormones, including prolactin family members. Here we demonstrate that trophoblast giant cells are increased, and display upregulation of prolactin releasing peptide (PrRP) receptor in the p53-null (p53(-/-)) embryonic placentas. At day 13.5 of gestation, the weight of p53(-/-) placentas was less than that of both wild-type and p53(+/-) placentas. In p53(-/-) placentas, the spongiotrophoblast layer was significantly decreased in thickness, and the trophoblast giant cells were observed not only in the outer layer of placentas but in both the spongiotrophoblast layer and the labyrinthine layer. The giant cells spread over the spongiotrophoblast and labyrinthine layer in p53(-/-) placentas displayed more intensive expression of immunoreactive PrRP receptor than in wild-type placentas. Previous studies indicated that the association between PrRP and PrRP receptor physiologically involves in the expression and secretion of the peptide hormones, including prolactin and growth hormones. These results suggest that p53 may regulate the differentiation of trophoblast giant cells, and may control the physiological PrRP stimuli in mouse placentas.

Thyrotrophin-releasing hormone, vasoactive intestinal peptide, prolactin-releasing peptide and dopamine regulation of prolactin secretion by different lactotroph morphological subtypes in the rat

In the male rat anterior pituitary, three morphological subtypes of cells secreting primarily prolactin (PRL) (lactotrophs) have been described. Type I contain predominantly large irregularly shaped granules, whereas type II and type III lactotrophs contain smaller spherical granules. We have previously shown that oestradiol and testosterone exert a rapid stimulatory effect selectively on type II lactotrophs but it is not known how the lactotroph subtypes respond to peptide secretagogues. We have therefore examined which cell subtype(s) release PRL in response to vasoactive intestinal peptide (VIP), thyrotrophin-releasing hormone (TRH) and prolactin-releasing peptide (PrRP-31). Pituitary segments were incubated in medium containing tannic acid (to capture exocytosis of secretory granules), either alone or with secretagogue peptide. VIP (1-10 nM), TRH (10 nM) and PrRP-31 (10 nM) all caused a significant increase (P < 0.05) in the amount of PRL granule exocytosis from type II and III lactotrophs, but had no effect on PRL exocytosis from type I. Dopamine (100 nM) inhibited basal exocytosis of immunoreactive (ir)-PRL from type I, II and III lactotrophs and PrRP-31-stimulated ir-PRL granule exocytosis from II and III lactotrophs. Treatment of lactating female rats with the dopamine D(2) receptor antagonist sulpiride (40 microg/kg) produced a significant increase (P < 0.05) in PRL granule exocytosis from type I and type III lactotrophs and a significant increase (P < 0.05) in the proportion of type I and II cells undergoing exocytosis of PRL. In conclusion, VIP, TRH and PrRP-31 selectively stimulate exocytosis from type II and III lactotrophs in the male rat, whereas all three lactotroph types are sensitive to dopamine inhibition of exocytosis in male and female rats.

Downregulation of prolactin-releasing peptide gene expression in the hypothalamus and brainstem of diabetic rats

We investigated the prolactin-releasing peptide (PrRP) mRNA levels in the hypothalamus and brainstem of streptozotocin (STZ)-induced diabetic rats and fa/fa Zucker diabetic rats, using in situ hybridization histochemistry. PrRP mRNA levels in the hypothalamus and brainstem of STZ-induced diabetic rats were significantly reduced in comparison with those of control rats. PrRP mRNA levels in the diabetic rats were reversed by both insulin and leptin. PrRP mRNA levels in the fa/fa diabetic rats were significantly reduced in comparison with those of Fa/? rats. PrRP mRNA levels in the fa/fa diabetic rats were significantly increased by insulin-treatment, but did not reach control levels in the Fa/? rats. We also investigated the effect of restraint stress on PrRP mRNA levels in STZ-induced diabetic rats. The PrRP mRNA levels in the control and the STZ-induced diabetic rats increased significantly after restraint stress. The diabetic condition and insulin-treatment may affect the regulation of PrRP gene expression via leptin and other factors, such as plasma glucose level. The diabetic condition may not impair the role of PrRP as a stress mediator.

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.

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.

Immunohistochemical localization and ontogenic development of prolactin-releasing peptide in the brain of the ovoviviparous fish species Poecilia reticulata (guppy)

Immunohistochemical localization and ontogenic development of prolactin-releasing peptide (PrRP) in the brain of the ovoviviparous fish species Poecilia reticulata (guppy) were examined to gain a better understanding of this hormone in teleost fish. In adult guppies, PrRP-immunoreactive (ir) cell bodies were detected in the posterior part of the hypothalamus. In the pituitary, a small number of PrRP-ir fibers were observed adjacent to the prolactin cells, whereas numerous PrRP-ir fibers were detected not only in the hypothalamus but also widely throughout the brain. PrRP-ir cell bodies and prolactin cells were already detected on the birth day in the hypothalamus and pituitary, respectively. The number of PrRP-ir fibers in the brain increased as the fish developed. These results suggest that PrRP is involved in neuromodulation in the brain and that PrRP plays some physiological roles in the early development of the guppy.

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.

Molecular cloning and functional characterization of a prolactin-releasing peptide homolog from Xenopus laevis

Amino acid sequences for identified prolactin (PRL)-releasing peptides (PrRPs) were conserved in mammals (>90%) or teleost fishes (100%), but there were considerable differences between these classes in the sequence (<65%) as well as in the role of PrRP. In species other than fishes and mammals, we have identified frog PrRP. The cDNA encoding Xenopus laevis prepro-PrRP, which can generate putative PrRPs, was cloned and sequenced. Sequences for the coding region showed higher identity with teleost PrRPs than mammalian homologues, but suggested the occurrence of putative PrRPs of 20 and 31 residues as in mammals. The amino acid sequence of PrRP20 was only one residue different from teleost PrRP20, but shared 70% identity with mammalian PrRP20s. In primary cultures of bullfrog (Rana catesbeiana) pituitary cells, Xenopus PrRPs increased prolactin concentrations in culture medium to 130-160% of the control, but PrRPs was much less potent than thyrotropin-releasing hormone (TRH) causing a three- to four-fold increase in prolactin concentrations. PrRP mRNA levels in the developing Xenopus brain peak in early prometamorphosis, different from prolactin levels. PrRP may not be a major prolactin-releasing factor (PRF), at least in adult frogs, as in mammals.

Prolactin-releasing Peptide mediates cholecystokinin-induced satiety in mice

We have shown previously that prolactin-releasing peptide (PrRP) plays a role in the regulation of feeding and energy expenditure in rats. We hypothesize that PrRP may have a physiological action through its putative receptor, GPR10, to mediate the central anorexigenic effects of peripheral satiety factors. Here we examine the effects of PrRP and cholecystokinin (CCK) on feeding in mice, including PrRP receptor gene knockout animals (GPR10(-/-)). Intracerebroventricular administration of PrRP (1-4 nmol) inhibited feeding in C57B6/J mice under both fast-induced and nocturnal feeding conditions. In contrast to the observations made in wild-type mice, neither PrRP nor CCK reduced food intake in GRP10(-/-) mice. The reduction in feeding and the release of corticosterone induced by systemic injection of the stressor lipopolysaccharide was similar in both GPR10(+/+) and GPR10(-/-) mice. These findings suggest that PrRP, acting through GPR10, is involved in regulating food intake and may be a key intermediary in the central satiating actions of CCK.

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.

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.

Prolactin-releasing peptide is a potent mediator of stress responses in the brain through the hypothalamic paraventricular nucleus

The effects of i.c.v. administration of prolactin-releasing peptide on neurons in the paraventricular nucleus of rats and plasma corticosterone levels were examined by measuring changes in Fos-like immunoreactivity, c-fos mRNA using in situ hybridization histochemistry, and plasma corticosterone using a specific radioimmunoassay. Approximately 80% of corticotropin-releasing hormone immunoreactive cells exhibited Fos-like immunoreactivity in the parvocellular division of the paraventricular nucleus 90 min after i.c.v. administration of prolactin-releasing peptide. The greatest induction of the c-fos mRNA expression in the paraventricular nucleus was observed 30 min after administration of prolactin-releasing peptide, and occurred in a dose-related manner. Plasma corticosterone levels were also significantly increased 30 min after administration of prolactin-releasing peptide. Next, the effects of restraint stress, nociceptive stimulus and acute inflammatory stress on the expression of the prolactin-releasing peptide mRNA in the dorsomedial hypothalamic nucleus, nucleus of the solitary tract and ventrolateral medulla were examined using in situ hybridization histochemistry for prolactin-releasing peptide mRNA. Restraint stress and acute inflammatory stress upregulated the prolactin-releasing peptide mRNA expression in the nucleus of the solitary tract and ventrolateral medulla. Nociceptive stimulus upregulated the prolactin-releasing peptide mRNA expression in the ventrolateral medulla. Finally, we observed that pretreatment (i.c.v. administration) with an anti-prolactin-releasing peptide antibody significantly attenuated nociceptive stimulus-induced c-fos mRNA expression in the paraventricular nucleus. These results suggest that prolactin-releasing peptide is a potent and important mediator of the stress response in the brain through the hypothalamic paraventricular nucleus.

The prolactin-releasing peptide antagonizes the opioid system through its receptor GPR10

Prolactin-releasing peptide (PrRP) and its receptor G protein-coupled receptor 10 (GPR10) are expressed in brain areas involved in the processing of nociceptive signals. We investigated the role of this new neuropeptidergic system in GPR10-knockout mice. These mice had higher nociceptive thresholds and stronger stress-induced analgesia than wild-type mice, differences that were suppressed by naloxone treatment. In addition, potentiation of morphine-induced antinociception and reduction of morphine tolerance were observed in mutants. Intracerebroventricular administration of PrRP in wild-type mice promoted hyperalgesia and reversed morphine-induced antinociception. PrRP administration had no effect on GPR10-mutant mice, showing that its effects are mediated by GPR10. Anti-opioid effects of neuropeptide FF were found to require a functional PrRP-GPR10 system. Finally, GPR10 deficiency enhanced the acquisition of morphine-induced conditioned place preference and decreased the severity of naloxone-precipitated morphine withdrawal syndrome. Altogether, our data identify the PrRP-GPR10 system as a new and potent negative modulator of the opioid system.

Structure-activity studies on prolactin-releasing peptide (PrRP). Analogues of PrRP-(19-31)-peptide

An investigation of a series of single replacement analogues of PrRP-(19-31)-peptide has shown that good functional activity was retained when Phe31 was replaced with His(Bzl), Phe(4Cl), Nle, Trp, Cys(Bzl) or Glu(OBzl); when Val28 or Ile25 was replaced with Phg; when Gly24 was replaced with D-Ala, L-Ala, Pro or Sar; when Ser22 was replaced with Gly and when Ala21 was replaced with Thr or MeAla. The results confirm that the functionally important residues are located within the carboxyl terminal segment, -Ile-Arg-Pro-Val-Gly-Arg-Phe-NH2.

Action of prolactin, prolactin-releasing peptide and orexins on hypothalamic neurons of adult, early postnatally overfed rats

OBJECTIVES

Hypothalamic neurons of rats overweight due to early postnatal overfeeding (SL) differ from those of control rats in their responses to feeding relevant hormones like leptin or insulin. The question arose whether prolactin and prolactin-releasing peptide (PrRP) express also differential action in SL rats. These peptides are described to have an effect on food intake and body weight regulation. Prolactin is co-synthesized in lateral hypothalamic neurons together with orexins that were also analyzed in this study.

METHODS

Single unit activity was extracellularly recorded in brain slices from adult control rats (CL) and from rats previously raised in small litters (SL). The action of the peptides on the firing rates was evaluated in the medial parvicellular part of the paraventricular nucleus (PaMP) and the medial arcuate nucleus (ArcM).

RESULTS

In control rats, PrRP significantly activated PaMP neurons, whereas prolactin and orexin-A induced also inhibition. In SL rats, there was a significantly different effect of orexin-B on PaMP neurons: the main effect changed from activation in controls to inhibition. ArcM neurons of both control and SL rats were mainly excited by prolactin and orexins.

CONCLUSION

Changes acquired during early development of neuronal responses to feeding relevant peptides are not a general non-specific mechanism of neurochemical plasticity, but concern specific hypothalamic nuclei and/ or hormones and neuropeptides. The increase in inhibition by orexin-B of hypothalamic paraventricular neurons could in vivo contribute to the neonatally acquired disposition towards persistingly increased food intake and reduced energy expenditure of overweight SL rats.

Stimulation of catecholamine biosynthesis via the PKC pathway by prolactin-releasing peptide in PC12 rat pheochromocytoma cells

We have previously shown that prolactin-releasing peptide (PrRP) stimulates catecholamine release from PC12 cells (rat pheochromocytoma cell line). However, it is not known whether PrRP also affects catecholamine biosynthesis. Thus, we examined the effect of PrRP on catecholamine biosynthesis in PC12 cells. PrRP31 (>10 nM) and PrRP20 (>100 nM) significantly increased the activity and expression level of tyrosine hydroxylase (TH), a rate-limiting enzyme, in catecholamine biosynthesis. However, the PrRP20-stimulated TH activity was markedly weaker than that of PrRP31. PrRP31 (>1 nM) and PrRP20 (>10 nM) significantly induced an increase in the level of PKC activity. Both Ro 32-0432 (a protein kinase C inhibitor) and H89 (a protein kinase A inhibitor) effectively suppressed the PrRP31 (100 nM)-induced TH mRNA level. Next, we examined the effect of PrRP on mitogen-activated protein kinases (MAPKs). PrRP31 (1 microM) significantly induced an increase in the activity of extracellular signal-related kinases (ERKs) and the stress-activated protein kinase/c-jun N terminal kinase (SAPK/JNK). In contrast to ERKs and JNK, PrRP31 did not affect P38 MAPK activity. Consistent with these findings, pretreatment of cells with the MEK-1-inhibitor, PD-98059 (50 microM), significantly inhibited the PrRP31 (100 nM)-induced increase in TH mRNA. These results indicate that PrRP stimulates catecholamine synthesis through both the PKC and PKA pathways in PC12 cells.

Nicotine stimulates prolactin-releasing peptide (PrRP) cells and non-PrRP cells in the solitary nucleus

Nicotine has been reported to regulate food intake and body weight. But the mechanisms underlying these roles have not been fully elucidated. In the present study, we showed that acute administration of nicotine (0.5 mg/kg s.c.) could activate prolactin-releasing peptide (PrRP)-bearing neurons in the A2 area of the NTS of rats, suggesting that PrRP may be associated with nicotine-induced effects in the central nervous system (CNS). We next treated rats with nicotine chronically (4 mg/kg/day for 7 days i.p.), and the results showed that the body weight was strongly reduced and food intake was greatly suppressed compared to the vehicle control group (p<0.01). Immunocytochemical studies revealed that PrRP-bearing neurons in the NTS were evidently activated after chronic administration of nicotine, suggesting that PrRP was involved in the regulation of nicotine-mediated body weight loss and food intake suppression in rats. We also found that acute/chronic administration of nicotine activated PrRP-negative neurons in the NTS, and the majority of these neurons were shown to be TH-negative, suggesting that noncatecholaminergic, PrRP-negative neurons in the NTS are associated with the roles of nicotine. Nicotine has also been shown to stimulate the secretion of ACTH, a stress responsive hormone. In the present study, rats received nicotine (0.5 mg/kg s.c.) or saline followed by restraint stress for 30 min. The immunocytochemical results showed that nicotine/stress and saline/stress both activated the majority of the PrRP neurons in the NTS, there being no significant difference between the two treatments (p>0.05). Nicotine/stress also greatly activated PrRP/TH-negative neurons in the NTS. Saline/stress, however, caused much lower effect on the activation of PrRP/TH-negative neurons. In addition, the activation effect of nicotine/stress on PrRP/TH-negative neurons was much stronger than that of nicotine alone (p<0.01). These results indicated that PrRP was associated with stress responses, but it had little effect on nicotine-mediated stress responses. On the other hand, nicotine and restraint stress may synergistically activate PrRP/TH-negative neurons in the NTS. Taken together, our data show that PrRP is involved in the nicotine-induced regulation of body weight and food intake, but may not be involved in the mediation of nicotine on stress responses. PrRP/TH-negative neurons in the NTS are also associated with the roles of nicotine in the CNS.

Expression of prolactin-releasing peptide and prolactin in the euryhaline mudskippers (Periophthalmus modestus): prolactin-releasing peptide as a primary regulator of prolactin

Prolactin (PRL)-releasing peptide (PrRP) is a strong candidate stimulator of pituitary PRL transcription and secretion in teleosts. However, the role in control of extrapituitary PRL expression is unclear even in mammals. To study the possible presence of PrRP-PRL axes not only in the brain-pituitary but also in peripheral organs, the expression patterns of PrRP, PRL and growth hormone (GH) were characterized in amphibious euryhaline mudskippers (Periophthalmus modestus). PrRP mRNA is abundantly expressed not only in the brain but also in the liver, gut and ovary, while less abundant expression was also detected in the skin and kidney. Corresponding to the distribution of PrRP mRNA, PRL mRNA was also detectable in these organs. During adaptation to different environments, the changes in mRNA levels of PrRP paralleled those in PRL in the brain-pituitary, liver and gut in an organ-specific manner. Brain PrRP mRNA and the pituitary PRL mRNA increased under freshwater and terrestrial conditions (P < 0.05); expression of PrRP and PRL in the gut of freshwater fish was higher (P < 0.05) than those in sea-water fish although there were no changes in fish kept out of water; no significant change was seen in the liver. Expressions of GH were not correlated with PrRP. In the gut, PrRP and PRL appear to be co-localized in the mucosal layer, especially in the mucous cells. Thus, PrRP may also be a local modulator of extrapituitary PRL expression and the PrRP-PRL axes in various organs may play an organ-specific role during environmental adaptation.

Stimulation and inhibition of prolactin release by prolactin-releasing Peptide in rat anterior pituitary cell aggregates

Although the G-protein coupled receptor GPR10 is highly expressed in the anterior pituitary, the action of its ligand prolactin-releasing peptide-31 (PrRP) in this tissue is controversial. The present study examined the acute effect of this peptide on prolactin secretion in perifused rat pituitary reaggregate cell cultures from adult male rats. PrRP readily and dose-dependently stimulated prolactin release at concentrations of 10 and 100 nM, although with a magnitude several times lower than that of thyrotropin-releasing hormone. Surprisingly, PrRP inhibited prolactin release at 0.1 and 1 nm in a pertussis toxin-sensitive manner. Inhibition was markedly favoured by long-term culture. Stimulation and inhibition were differentially affected by the presence of hormones during culture: dexamethasone favoured the inhibitory effect and decreased the magnitude of the stimulatory effect, while oestradiol and triiodothyronine strongly reduced stimulation, as well as inhibition. PrRP, even at 1 nm, counteracted the inhibition of prolactin release by dopamine. There was no effect of PrRP on growth hormone release in aggregates cultured either in the absence or presence of hormones. The present results confirm the prolactin-releasing capacity of PrRP at nanomolar doses and reveal a hitherto unrecognized inhibitory activity of this peptide. Furthermore, dopamine inhibition of prolactin release is antagonized by PrRP, irrespective of the PrRP dose.