Prolactin-releasing peptides

Sepsis as a Pan-Endocrine Illness-Endocrine Disorders in Septic Patients

Sepsis is defined as "life-threatening organ dysfunction caused by a dysregulated host response to infection". One of the elements of dysregulated host response is an endocrine system disorder. Changes in its functioning in the course of sepsis affect almost all hormonal axes. In sepsis, a function disturbance of the hypothalamic-pituitary-adrenal axis has been described, in the range of which the most important seems to be hypercortisolemia in the acute phase. Imbalance in the hypothalamic-pituitary-thyroid axis is also described. The most typical manifestation is a triiodothyronine concentration decrease and reverse triiodothyronine concentration increase. In the somatotropic axis, a change in the secretion pattern of growth hormone and peripheral resistance to this hormone has been described. In the hypothalamic-pituitary-gonadal axis, the reduction in testosterone concentration in men and the stress-induced "hypothalamic amenorrhea" in women have been described. Catecholamine and β-adrenergic stimulation disorders have also been reported. Disorders in the endocrine system are part of the "dysregulated host response to infection". They may also affect other components of this dysregulated response, such as metabolism. Hormonal changes occurring in the course of sepsis require further research, not only in order to explore their potential significance in therapy, but also due to their promising prognostic value.

Reduced dopaminergic tone during lactation is permissive to the hypothalamic stimulus for suckling-induced prolactin release

Dopamine from tuberoinfundibular dopaminergic (TIDA) neurones tonically inhibits prolactin (PRL) secretion. Lactational hyperprolactinaemia is associated with a reduced activity of TIDA neurones. However, it remains controversial whether the suckling-induced PRL surge is driven by an additional decrease in dopamine release or by stimulation from a PRL-releasing factor. In the present study, we further investigated the role of dopamine in the PRL response to suckling. Non-lactating (N-Lac), lactating 4 hour apart from pups (Lac), Lac with pups return and suckling (Lac+S), and post-lactating (P-Lac) rats were evaluated. PRL levels were elevated in Lac rats and increased linearly within 30 minutes of suckling in Lac+S rats. During the rise in PRL levels, dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the median eminence (ME) and neurointermediate lobe of the pituitary did not differ between Lac+S and Lac rats. However, dopamine and DOPAC were equally decreased in Lac and Lac+S compared to N-Lac and P-Lac rats. Suckling, in turn, reduced phosphorylation of tyrosine hydroxylase in the ME of Lac+S. Domperidone and bromocriptine were used to block and activate pituitary dopamine D2 receptors, respectively. Domperidone increased PRL secretion in both N-Lac and Lac rats, and suckling elicited a robust surge of PRL over the high basal levels in domperidone-treated Lac+S rats. Conversely, bromocriptine blocked the PRL response to suckling. The findings obtained in the present study provide evidence that dopamine synthesis and release are tonically reduced during lactation, whereas dopamine is still functional with respect to inhibiting PRL secretion. However, there appears to be no further reduction in dopamine release associated with the suckling-induced rise in PRL. Instead, the lower dopaminergic tone during lactation appears to be required to sensitise the pituitary to a suckling-induced PRL-releasing factor.

The role of orphan G protein-coupled receptors in the modulation of pain: A review

G protein-coupled receptors (GPCRs) comprise a large number of receptors. Orphan GPCRs are divided into six families. These groups contain orphan receptors for which the endogenous ligands are unclear. They have various physiological effects in the body and have the potential to be used in the treatment of different diseases. Considering their important role in the central and peripheral nervous system, their role in the treatment of pain has been the subject of some recent studies. At present, there are effective therapeutics for the treatment of pain including opioid medications and non-steroidal anti-inflammatory drugs. However, the side effects of these drugs and the risks of tolerance and dependence remain a major problem. In addition, neuropathic pain is a condition that does not respond to currently available analgesic medications well. In the present review article, we aimed to review the most recent findings regarding the role of orphan GPCRs in the treatment of pain. Accordingly, based on the preclinical findings, the role of GPR3, GPR7, GPR8, GPR18, GPR30, GPR35, GPR40, GPR55, GPR74, and GPR147 in the treatment of pain was discussed. The present study highlights the role of orphan GPCRs in the modulation of pain and implies that these receptors are potential new targets for finding better and more efficient therapeutics for the management of pain particularly neuropathic pain.

The Arg-Phe-amide peptide 26RFa/glutamine RF-amide peptide and its receptor: IUPHAR Review 24

The RFamide neuropeptide 26RFa was first isolated from the brain of the European green frog on the basis of cross-reactivity with antibodies raised against bovine neuropeptide FF (NPFF). 26RFa and its N-terminally extended form glutamine RF-amide peptide (QRFP) have been identified as cognate ligands of the former orphan receptor GPR103, now renamed glutamine RF-amide peptide receptor (QRFP receptor). The 26RFa/QRFP precursor has been characterized in various mammalian and non-mammalian species. In the brain of mammals, including humans, 26RFa/QRFP mRNA is almost exclusively expressed in hypothalamic nuclei. The 26RFa/QRFP transcript is also present in various organs especially in endocrine glands. While humans express only one QRFP receptor, two isoforms are present in rodents. The QRFP receptor genes are widely expressed in the CNS and in peripheral tissues, notably in bone, heart, kidney, pancreas and testis. Structure-activity relationship studies have led to the identification of low MW peptidergic agonists and antagonists of QRFP receptor. Concurrently, several selective non-peptidic antagonists have been designed from high-throughput screening hit optimization. Consistent with the widespread distribution of QRFP receptor mRNA and 26RFa binding sites, 26RFa/QRFP exerts a large range of biological activities, notably in the control of energy homeostasis, bone formation and nociception that are mediated by QRFP receptor or NPFF2. The present report reviews the current knowledge concerning the 26RFa/QRFP-QRFP receptor system and discusses the potential use of selective QRFP receptor ligands for therapeutic applications.

Identification of neuromedin U precursor-related peptide and its possible role in the regulation of prolactin release

The discovery of neuropeptides provides insights into the regulation of physiological processes. The precursor for the neuropeptide neuromedin U contains multiple consensus sequences for proteolytic processing, suggesting that this precursor might generate additional peptides. We performed immunoaffinity chromatography of rat brain extracts and consequently identified such a product, which we designated neuromedin U precursor-related peptide (NURP). In rat brain, NURP was present as two mature peptides of 33 and 36 residues. Radioimmunoassays revealed NURP immunoreactivity in the pituitary, small intestine, and brain of rats, with the most intense reactivity in the pituitary. Intracerebroventricular administration of NURP to both male and female rats robustly increased plasma concentrations of prolactin but not of other anterior pituitary hormones. In contrast, NURP failed to stimulate prolactin release from dispersed anterior pituitary cells. Pretreatment of rats with bromocriptine, a dopamine receptor agonist, blocked the prolactin-releasing activity of NURP. In rats pretreated with the antagonist sulpiride, intracerebroventricular administration of NURP did not increase plasma prolactin concentrations more than administration of saline. These data suggest that NURP induces prolactin release by acting indirectly on the pituitary; dopamine from the hypothalamus, which inhibits prolactin release, may be involved in this activity of NURP.

TIDAL WAVES: Network mechanisms in the neuroendocrine control of prolactin release

Neuroendocrine tuberoinfundibular dopamine (TIDA) neurons tonically inhibit pituitary release of the hormone, prolactin. Through the powerful actions of prolactin in promoting lactation and maternal behaviour while suppressing sexual drive and fertility, TIDA neurons play a key role in reproduction. We summarize insights from recent in vitro studies into the membrane properties and network behaviour of TIDA neurons including the observations that TIDA neurons exhibit a robust oscillation that is synchronized between cells and depends on intact gap junction communication. Comparisons are made with phasic firing patterns in other neuronal populations. Modulators involved in the control of lactation - including serotonin, thyrotropin-releasing hormone and prolactin itself - have been shown to change the electrical behaviour of TIDA cells. We propose that TIDA discharge mode may play a central role in tuning the amount of dopamine delivered to the pituitary and hence circulating prolactin concentrations in different reproductive states and pathological conditions.

Role of neuropeptide FF in central cardiovascular and neuroendocrine regulation

Neuropeptide FF (NPFF) is an octapeptide belonging to the RFamide family of peptides that have been implicated in a wide variety of physiological functions in the brain including central cardiovascular and neuroendocrine regulation. The effects of these peptides are mediated via NPFF1 and NPFF2 receptors that are abundantly expressed in the rat and human brain. Herein, we review evidence for the role of NPFF in central regulation of blood pressure particularly within the brainstem and the hypothalamic paraventricular nucleus (PVN). At a cellular level, NPFF demonstrates distinct responses in magnocellular and parvocellular neurons of the PVN, which regulate the secretion of neurohypophyseal hormones and sympathetic outflow, respectively. Finally, the presence of NPFF system in the human brain and its alterations within the hypertensive brain are discussed.

Study of the role of novel RF-amide neuropeptides in affecting growth hormone secretion in a representative non-human primate (Macaca mulatta)

RF amide peptide family with distinctive terminal -Arg-Phe-NH(2) signature is evolutionarily conserved from invertebrates to mammals. These neuropeptides have been shown to affect diverse functions in invertebrates and vertebrates including influencing pituitary hormone secretion. More recently, two members of this family 26-amino acid and 43-amino acid RF amide peptide (26RFa and 43RFa, respectively) originally isolated from frog have been cloned in rats and humans. Actions of these peptides on hormone secretion have not been studied in primates. In the present study, effect of iv administration of three different doses of human 26RFa and 43RFa on GH secretion was studied in a representative higher primate, the rhesus monkey. As control against these two peptides, normal saline and a scrambled sequence of 26RFa was administered. A set of four intact adult male monkeys received the administration in a random order. Peripheral blood samples were obtained from the chairrestrained but fully conscious animals for a period of 30 min before and 240 min after the administration at 15-min intervals. For quantitative measurement of GH concentration, a human GH chemiluminescent immunometric assay was used. Peripheral administration of 38 and 76 nmol doses of 26RFa significantly (P < 0.05) stimulated GH AUC during a 0-120 min period after injection of 26RFa. In contrast to 26RFa, administration of 43RFa appeared to suppress GH levels during the later stages of the sampling i.e. from 120 to 240 min period. Mean AUC during the period was significantly (P < 0.05) reduced by 76 nmol dose of 43RFa, while 38 nmol dose of 43RFa also had similar effect but lacked full statistical significance (P = 0.058). To our knowledge present study reports for the first time-specific stimulatory effect of 26RFa on the GH secretion and a novel inhibitory and delayed effect of 43RFa on the GH secretion in higher primates. In conclusion, present findings extend evidence for endocrine actions of RF amides in primates and suggest differential effect of these peptides on GH secretion in primates.

Identification of the receptors for prolactin-releasing peptide (PrRP) and Carassius RFamide peptide (C-RFa) in chickens

Prolactin-releasing peptide (PrRP) and its structurally related peptide, Carassius Arg-Phe-amide peptide (C-RFa), have been reported to play similar roles in regulating food intake and pituitary functions in vertebrates. However, the identity, functionality, and expression of the receptor(s) for PrRP and C-RFa remain largely unknown in nonmammalian vertebrates, including birds. In this study, three receptors homologous to mammalian PrRP receptor (PrRPR), named cPrRPR1, cPrRPR2, and cC-RFaR, respectively, were cloned from chicken brain by RT-PCR. Using a pGL3-NFAT-RE-luciferase reporter system, we demonstrated that cPrRPR1 and cPrRPR2 expressed in Chinese hamster ovarian cells could be activated by cPrRP₂₀ and cC-RFa₂₀ potently, whereas cC-RFaR could only be activated effectively by cC-RFa₂₀ (EC₅₀, 0.11 nM), indicating that cPrRPR1 and cPrRPR2 can function as common receptors for PrRP and C-RFa, whereas cC-RFaR is a receptor specific to C-RFa. Using a pGL3-CRE-luciferase reporter system, cPrRPR1, cPrRPR2, and cC-RFaR expressed in Chinese hamster ovarian cells were also shown to activate intracellular protein kinase A signaling pathway upon cC-RFa₂₀ treatment (100 nM). Moreover, RT-PCR assay revealed that cPrRPR1, cPrRPR2, and cC-RFaR were widely expressed in most adult chicken tissues examined, including various regions of brain. These findings, together with evidence of PrRP and C-RFa encoded by separate genes in chicken, Xenopus, and zebrafish, and the differential expression of PrRP and C-RFa genes in chicken tissues, strongly suggest that PrRP and C-RFa may play similar yet distinctive roles in nonmammalian vertebrates, including chicken, and their actions are mediated by common receptor(s) or a specific C-RFa receptor.

Extralabel use of cabergoline in the treatment of a pituitary adenoma in a rat

CASE DESCRIPTION

A 0.65-kg (1.43-lb) 24-month-old sexually intact male albino pet rat was examined because of a 3-week history of hypodipsia, apparent blindness, and sudden change in behavior.

CLINICAL FINDINGS

The rat was able to move around its cage but appeared unaware of its surroundings, was visually unresponsive, and seemed unusually aggressive. The rat's hind limbs appeared mildly paretic, and it had sporadic difficulty placing its hind limbs on a flat surface. Given the rat's age, history, and physical examination findings, the primary differential diagnosis was a pituitary tumor. Magnetic resonance imaging (MRI) of the rat's brain was performed and revealed a large pituitary mass, which was indicative of a tumor.

TREATMENT AND OUTCOME

Cabergoline (0.6 mg/kg [0.27 mg/lb], PO, q 72 h) was administered. On follow-up MRI 2 months later, the pituitary mass had substantially decreased in size. For 6 months following the second MRI study, the rat continued to receive the same dosage of cabergoline and had no clinical signs of disease or unusual behavior. However, at 8.5 months after the start of the treatment, the rat was in poor condition and had clinical signs similar to those initially. A third MRI study was performed and revealed substantial regrowth of the mass. The rat was euthanized and a necropsy was performed; a histopathologic diagnosis of pituitary adenoma was made.

CLINICAL RELEVANCE

Pituitary adenomas have long been recognized as a common finding in geriatric rats (> 18 months old). Affected rats may respond favorably to oral administration of cabergoline.

Effects of short-term hyper- and hypoprolactinaemia on hormones of the pituitary, gonad and -thyroid axis and on semen quality in male Beagles

Effects of a short-term hyper- and hypoprolactinaemia on serum concentrations of LH, testosterone and semen quality in six male Beagles were investigated. Blood samples were collected at 3-day intervals for 12 weeks. The time span was divided into five 3-week periods: pre-treatment, metoclopramide (MCP) treatment (0.2 mg/kg orally three times daily), cabergoline (CAB) treatment (5 microg/kg orally once daily), post-treatment 1 and post-treatment 2. In the latter, only semen characteristics were evaluated. Semen parameters were analyzed once per week during the whole 15-week investigation time. At the end of each period, the effects of a single intravenous injection of thyrotropin-releasing hormone (TRH; 10 microg/kg) on the secretion of prolactin (PRL), LH, testosterone, thyroid-stimulating hormone and thyroxine (T4) were investigated. Pre-treatment serum PRL concentration increased under MCP (p < 0.05), followed by a decrease under CAB administration (p < 0.05). Luteinizing hormone and testosterone concentrations were not affected. Except for straight-line sperm velocity, semen quality did not differ between collection periods. A single iv TRH injection induced a significant PRL increase at 20 min in all experimental periods except during CAB treatment. Luteinizing hormone and testosterone did not show clear TRH-related changes. Basic T4 levels were significantly reduced after CAB treatment (p < 0.05). The results of the present study demonstrate that MCP-induced short-term hyperprolactinaemia in male beagles does not seriously affect the hypothalamo-pituitary axis and semen quality.

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 and male fertility: the long and short feedback regulation

In the last 20 years, a pituitary-hypothalamus tissue culture system with intact neural and portal connections has been developed in our lab and used to understand the feedback mechanisms that regulate the secretions of adenohypophyseal hormones and fertility of male rats. In the last decade, several in vivo rat models have also been developed in our lab with a view to substantiate the in vitro findings, in order to delineate the role of pituitary hormones in the regulation of fertility of male rats. These studies have relied on both surgical and pharmacological interventions to modulate the secretions of gonadotropins and testosterone. The interrelationship between the circadian release of reproductive hormones has also been ascertained in normal men. Our studies suggest that testosterone regulates the secretion of prolactin through a long feedback mechanism, which appears to have been conserved from rats to humans. These studies have filled in a major lacuna pertaining to the role of prolactin in male reproductive physiology by demonstrating the interdependence between testosterone and prolactin. Systemic levels of prolactin play a deterministic role in the mechanism of chromatin condensation during spermiogenesis.

RFamide-related peptide and its cognate receptor in the sheep: cDNA cloning, mRNA distribution in the hypothalamus and the effect of photoperiod

Photoperiodic responses enable animals to adapt their physiology to predictable patterns of seasonal environmental change. In mammals, this depends on pineal melatonin secretion and effects in the hypothalamus, but the cellular and molecular substrates of its action are poorly understood. The recent identification of a mammalian orthologue of the avian gonadotrophin-inhibitory hormone gene has led to interest in its possible involvement in seasonal breeding. In long-day breeding Syrian hamsters, hypothalamic RFamide-related peptide (RFRP) expression is increased by exposure to long photoperiod. Because, opposite to hamsters, sheep are short-day breeders, we predicted that a conserved role in mammalian reproductive activation would decrease RFRP expression in sheep under a long photoperiod. We cloned the ovine RFRP cDNA and examined its expression pattern in Soay sheep acclimated to a 16 : 8 h or 8 : 16 h light /dark cycle (LP and SP, respectively). RFRP was expressed widely in the sheep hypothalamus and increased modestly overall with exposure to LP. Interestingly, RFRP expression in the ependymal cells surrounding the base of the third ventricle was highly photoperiodic, with levels being undetectable in animals held on SP but consistently high under LP. These data are inconsistent with a conserved reproductive role for RFRP across mammals. Additionally, we cloned the ovine homologue of the cognate RFRP receptor, rfr-2 (NPFF1) and found localised expression in the suprachiasmatic nuclei and in the pars tuberalis. Taken together, these data strengthen the emerging view that interplay between ependymal cells and the pars tuberalis might be important for the seasonal timing system.

Prolactin: a pleiotropic neuroendocrine hormone

The neuroendocrine control of prolactin secretion is unlike that of any other pituitary hormone. It is predominantly inhibited by the hypothalamus and, in the absence of a regulatory feedback hormone, it acts directly in the brain to suppress its own secretion. In addition to this short-loop feedback action in the brain, prolactin has been reported to influence a wide range of other brain functions. There have been few attempts to rationalise why a single hormone might exert such a range of distinct and seemingly unrelated neuroendocrine functions. In this review, we highlight some of the original studies that first characterised the unusual features of prolactin neuroendocrinology, and then attempt to identify areas of new progress and/or controversy. Finally, we discuss a hypothesis that provides a unifying explanation for the pleiotrophic actions of prolactin in the brain.

What can we learn from rodents about prolactin in humans?

Prolactin (PRL) is a 23-kDa protein hormone that binds to a single-span membrane receptor, a member of the cytokine receptor superfamily, and exerts its action via several interacting signaling pathways. PRL is a multifunctional hormone that affects multiple reproductive and metabolic functions and is also involved in tumorigenicity. In addition to being a classical pituitary hormone, PRL in humans is produced by many tissues throughout the body where it acts as a cytokine. The objective of this review is to compare and contrast multiple aspects of PRL, from structure to regulation, and from physiology to pathology in rats, mice, and humans. At each juncture, questions are raised whether, or to what extent, data from rodents are relevant to PRL homeostasis in humans. Most current knowledge on PRL has been obtained from studies with rats and, more recently, from the use of transgenic mice. Although this information is indispensable for understanding PRL in human health and disease, there is sufficient disparity in the control of the production, distribution, and physiological functions of PRL among these species to warrant careful and judicial extrapolation to humans.

Association of a nicotinic receptor mutation with reduced height and blunted physostigmine-stimulated growth hormone release

BACKGROUND

Pulsatile GH secretion from the anterior pituitary is a key mediator of human growth regulation and is affected by a number of genetic and environmental factors. Activation of neuronal nicotinic acetylcholine (nACh) receptors promotes GH release, but the role of these receptors in growth regulation is unknown.

AIM

Our aim was to assess the effect of a mutation in the alpha4 subunit of the nACh receptor on cholinergic-mediated GH release.

METHODS

Forty-one healthy volunteers (24 male, age 36.2 +/- 12.2 yr, mean +/- sd) and 13 subjects with the alpha4-Ser248Phe mutation (four male, age 43.2 +/- 16.8 yr) were studied. Serum levels of GH, LH, FSH, prolactin, TSH, free T(4), and cortisol were measured at baseline and at regular intervals after infusion of physostigmine. Height and weight were recorded in all participants as well as from additional family members with (n = 11, four male) and without (n = 16, seven male) the mutation.

RESULTS

Subjects with the mutation were shorter (1.62 +/- 0.08 vs. 1.72 +/- 0.09 m, P < 0.05) and had a greater body mass index (31 +/- 6 vs. 24 +/- 3 kg/m(2), P < 0.05) than healthy volunteers and unaffected members of the pedigree. In controls, physostigmine markedly increased the serum levels of GH (mean increase, +732%). In contrast, the response to physostigmine was markedly blunted in subjects with the mutation (+104%, P > 0.2 vs. control).

CONCLUSIONS

These findings suggest a role of the nACh receptor in human growth regulation.

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.

Hormonal profile and reproductive performance in lactation deficient (OFA hr/hr) and normal (Sprague–Dawley) female rats

Lactation deficiency may have important consequences on infant health, particularly in populations of low socioeconomic status. The OFAhr/hr(OFA) strain of rats, derived from Sprague–Dawley (SD) rats, has deficient lactation and is a good model of lactation failure. We examined the reproductive performance and hormonal profiles in OFA and SD strains to determine the cause(s) of the lactation failure of the OFA strain. We measured hormonal (PRL, GH, gonadotropins, oxytocin, and progesterone) levels by RIA in cycling, pregnant, and lactating rats and in response to suckling. Dopaminergic metabolism was assessed by determination of mediobasal hypothalamic dopamine and dihydroxyphenylacetic acid (DOPAC) concentrations by HPLC and tyrosine hydroxylase expression by immunocytochemistry and western blot. OFA rats have normal fertility but 50% of the litters die of malnutrition on early lactation; only 6% of the mothers show normal lactation. The OFA rats showed lower circulating PRL during lactation, increased hypothalamic dopamine and DOPAC, and impaired milk ejection with decreased PRL and oxytocin response to suckling. Before parturition, PRL release and lactogenesis were normal, but dopaminergic metabolism was altered, suggesting activation of the dopaminergic system in OFA but not in SD rats. The number of arcuate and periventricular neurons expressing tyrosine hydroxylase was higher in SD rats, but hypothalamic expression of TH was higher in OFA rats at the end of pregnancy and early lactation. These results suggest that the OFA rats have impaired PRL release linked with an augmented dopaminergic tone which could be partially responsible for the lactational failure.

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.

Pituitary development and physiology

The functions of the pituitary hormones have been relatively well studied; however, understanding the regulation of their synthesis and release have been an ongoing subject of intense research. This review provides an overview of the pituitary cell types and their hormone products. Current understanding of the expression and regulation of the pituitary hormone genes, control of the synthesis and release of the corresponding hormones, and developmental changes are reviewed. This review concludes with a discussion of several of these genes and the genetic disorders with which they are associated.

The dynamic neuroendocrine response to critical illness

The severity of striking alterations in the hypothalamic-anterior pituitary-peripheral hormone axes, which are the hallmark of severity of critical illness, is associated with a high risk for morbidity and mortality. Most attempts to correct the hormone balance are ineffective or harmful because of lack of pathophysiologic understanding. Extensive research has provided more insight in the biphasic neuroendocrine response to critical illness: the acute phase is characterized by an actively secreting pituitary but low peripheral effector hormone levels. In contrast, in prolonged critical illness, uniform suppression of the neuroendocrine axes, predominantly of hypothalamic origin, contributes to low serum levels of the respective target-organ hormones.

Tachykinins and the control of prolactin secretion

Tachykinins are present in the pituitary gland and in brain areas involved in the control of the secretion of pituitary hormones. Tachykinins have been demonstrated to stimulate prolactin release acting directly on the anterior pituitary gland. These peptides have also been revealed to be able to act at the hypothalamic level, interacting with neurotransmitters and neuropeptides that have the potential to affect prolactin secretion. Tachykinins seem to act by stimulating or inhibiting the release of the factors that affect prolactin secretion. Among them, tachykinins have been demonstrated to stimulate oxytocin and vasopressin release, which in turn results in prolactin release. Tachykinins also potentiated the response to vasoactive intestinal peptide (VIP) and reinforced the action of glutamate, which in turn result in prolactin release. They have also been shown to interact with serotonin, a neurotransmitter involved in the control of prolactin secretion. In addition, tachykinins have been shown to inhibit GABA release, a neurotransmitter with prolactin-release inhibiting effect. This inhibition may result in an increased prolactin secretion by removal of the GABA inhibition. On the other hand, tachykinins have also been shown to stimulate dopamine release by the hypothalamus, an action that results in an inhibition of prolactin release. Dopamine is a well known inhibitor of prolactin secretion. In conclusion, although tachykinins have been shown to have a predominantly stimulatory effect on prolactin secretion, especially at the pituitary level, under some circumstances they may also exert an inhibitory influence on prolactin release, by stimulating dopamine release at the hypothalamic level.

Cocaine-mediated supersensitivity of 5-HT2A receptors in hypothalamic paraventricular nucleus is a withdrawal-induced phenomenon

We previously reported that treatment and withdrawal from cocaine increases: (1) 5-HT2A receptor-mediated neuroendocrine responses, and (2) Galphaq and Galpha11 G-protein levels in the hypothalamic paraventricular nucleus (PVN) at 48 h post-treatment. This study investigates changes in the initial 24 h of withdrawal to discern whether 5-HT2A receptor supersensitivity is due to cocaine treatment or is induced during the withdrawal period. We report here increases in 5-HT2A receptor-mediated neuroendocrine responses only 12 or 24 h post-treatment, but not during the initial 4 h withdrawal period. Levels of membrane- or cytosol-associated Galphaq or Galpha11 proteins in PVN are not altered during the first 24 h of withdrawal. However, the density of 125I-(-)-1-(2,5 dimethoxy-4-iodophenyl)-2-amino-propane HCl (DOI)-labeled high-affinity 5-HT2A receptors in PVN increased 35% in rats withdrawn from cocaine for 24 h. These findings demonstrate that cocaine-induced increases in 5-HT2A receptor function in PVN represents a withdrawal-induced phenomena that: (1) is likely attributed to increased G-protein coupled/high-affinity conformational state of the 5-HT2A receptor, and (2) occurs in the absence of changes in the levels of associated G proteins during the first 24 h.

Closing the link in short-loop prolactin feedback

Short-loop negative feedback actions of prolactin in hypothalamus have been recognized for some time. Two recent papers demonstrate the multiple sites of this feedback action and the signaling mechanisms involved.

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.

Expression of the long form of the prolactin receptor in magnocellular oxytocin neurons is associated with specific prolactin regulation of oxytocin neurons

Magnocellular neurons of the supraoptic (SON) and paraventricular nuclei (PVN) show considerable plasticity during pregnancy and lactation. Prolactin receptors (PRL-R) have been identified in both these nuclei. The aim of this study was to investigate the cell type(s) expressing mRNA for the long form of prolactin receptor (PRL-RL) and to determine whether patterns of expression change during pregnancy and lactation. In addition, we examined effects of prolactin on excitability of oxytocin and vasopressin neurons. Sections from brains of nonpregnant, pregnant, and lactating rats were hybridized with an 35S-labeled probe to label PRL-RL mRNA together with digoxigenin-labeled probes to detect either oxytocin or vasopressin mRNA. In the SON, PRL-RL mRNA was predominantly colocalized with oxytocin mRNA, with over 80% of oxytocin neurons positive for PRL-RL mRNA. Very few (<10%) vasopressin neurons expressed PRL-RL mRNA. In the PVN, PRL-RL mRNA was also predominantly found in oxytocin neurons, and the proportion of PRL-RL-positive oxytocin neurons increased significantly during pregnancy and lactation. As in the SON, relatively few vasopressin cells contained PRL-RL mRNA. For in vivo electrophysiology, nonpregnant rats were anesthetized, and then extracellular single neuron activity was recorded in identified oxytocin and vasopressin neurons. After a period of baseline recording, the effect of prolactin (1 μg icv) on firing rate was examined. Prolactin treatment of nonpregnant rats induced a significant decrease in firing rates of oxytocin neurons. There was no effect of prolactin on the activity of vasopressin neurons. Together, these data provide strong evidence that prolactin directly and specifically regulates activity of oxytocin neurons.

Cocaine- and amphetamine-regulated transcript is localized in pituitary lactotropes and is regulated during lactation

Cocaine- and amphetamine-regulated transcript (CART) is a highly expressed peptide implicated in the regulation of feeding, reward and reinforcement, and stress-related behaviors. CART has been localized to discrete cell populations in the brain, gut, adrenal gland, and pancreas. In contrast, CART-producing cell types in the pituitary gland remain ill defined. In the present study, double-label immunohistochemistry, employing a high-affinity antiserum we generated against CART-(62-102), was used to identify CART-producing cells in the pituitary gland. In the anterior pituitary, the majority of CART immunoreactivity (-ir) was localized in lactotropes; minor populations of CART-ir cells were identified as somatotropes and corticotropes. In the posterior pituitary, CART-ir extensively colocalized with oxytocin-containing fibers; in contrast, only a few vasopressin fibers contained CART-ir. As expected, CART colocalized with oxytocin in magnocellular neurons of the supraoptic nucleus. The effects of bromocriptine, a potent dopamine receptor agonist, were examined to determine whether CART mRNA expression and protein release are regulated in a similar fashion as prolactin. Similar to prolactin, CART mRNA expression and protein release were significantly decreased after bromocriptine treatment of dispersed rat anterior pituitary cells in culture. To explore the putative physiological role of pituitary CART, we compared levels of CART mRNA expression in lactating and nonlactating female rats. CART mRNA levels were significantly increased in the anterior pituitary and supraoptic nucleus of lactating rats. Furthermore, levels of CART in the systemic circulation were significantly elevated at the onset of lactation, peaked on d 10 of lactation and returned to baseline values 10 d after pups were weaned. The current study describes the cellular localization and regulation of CART expression and protein release from the rat pituitary gland. These findings suggest a putative role for CART in lactation.

RFamide-related peptides signal through the neuropeptide FF receptor and regulate pain-related responses in the rat

The mammalian RFamide-related peptide RFRP1 was found to signal through the neuropeptide FF 2 receptor expressed in Xenopus oocytes. The peptide induced a dose-dependent outward current, which was dependent on the simultaneous expression of GIRK1 and GIRK4 potassium channels. In neuropathic rats, RFRP1 administered intrathecally induced tactile antiallodynia and thermal antinociception, whereas in the solitary tract nucleus it produced only mechanical antihyperalgesia. Expression of the RFamide-related peptide mRNA in the rat CNS was distinctly different from that of neuropeptide FF. Most notably, the gene was not expressed in the hindbrain or spinal cord at detectable levels. However, there was a prominent group of RFamide-related peptide mRNA-expressing neurons in the central hypothalamus, in the area in and between the dorsomedial and ventromedial nuclei. The results suggest that RFamide-related peptides are potentially involved in pain regulation through a hypothalamo-medullary projection system, and possibly via action on neuropeptide FF 2 receptors. In neuropathic animals, the pain suppressive effect of RFamide-related peptide varies depending on the submodality of noxious test stimulation and the site of RFamide-related peptide administration.

Prolactin and growth hormone signaling

Prolactin (PRL) and growth hormone (GH) act by way of their receptors as either hormones (systemically) or cytokines (locally). The Jak2/Stat5 pathway is the principal route by which PRL/GH activate target genes. The availability of knockout mice for each member of this signaling cascade has provided opportunities to understand their unique interactions. Jak2 is important in alternative signal transduction schema such as the MAP kinase and PI3K/Akt pathways. The putative Jak2/RUSH pathway is based on the fact that RUSH mediates the ability of PRL to augment progesterone-dependent gene transcription. New evidence shows that suppressors, regulators, and degraders control Jak2/Stat5. This review focuses on the most recent advances in the field of PRL/GH signal transduction.

Origin of the prolactin-releasing hormone (PRLH) receptors: Evidence of coevolution between PRLH and a redundant neuropeptide Y receptor during vertebrate evolution

We present seven new vertebrate homologs of the prolactin-releasing hormone receptor (PRLHR) and show that these are found as two separate subtypes, PRLHR1 and PRLHR2. Analysis of a number of vertebrate sequences using phylogeny, pharmacology, and paralogon analysis indicates that the PRLHRs are likely to share a common ancestry with the neuropeptide Y (NPY) receptors. Moreover, a micromolar level of NPY was able to bind and inhibit completely the PRLH-evoked response in PRLHR1-expressing cells. We suggest that an ancestral PRLH peptide started coevolving with a redundant NPY binding receptor, which then became PRLHR, approximately 500 million years ago. The PRLHR1 subtype was shown to have a relatively high evolutionary rate compared to receptors with fixed peptide preference, which could indicate a drastic change in binding preference, thus supporting this hypothesis. This report suggests how gene duplication events can lead to novel peptide ligand/receptor interactions and hence spur the evolution of new physiological functions.

PROLACTIN SECRETION IN HYPOTHYROID ENDOTOXEMIC RATS: INVOLVEMENT OF L-ARGININE AND NITRIC OXIDE SYNTHASE

The identification of nitric oxide (NO) within the hypothalamus and pituitary gland has suggested its role as modulator of the activity on hypothalamic-pituitary axis. Hypothalamic NO synthase (NOS) is known to be regulated by thyroid hormones. We investigated the effects of previous injection of N-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor, and L-arginine (L-Arg), the substrate for NO synthesis, on prolactin (PRL) secretion, through the lipopolysaccharide (LPS)-induced inflammatory response in thyroidectomized (TX) rats. TX or sham-operated (N) rats were intraperitoneally (i.p.) injected with L-NAME (10 mg kg) or L-Arg (200 mg kg) or the same volume of vehicle (saline solution) 30 min before endotoxemia-induction with LPS at 250 mug (100 g body weight), i.p.. In N rats, NO increased PRL release in response to endotoxemia, whereas in hypothyroid rats, NO appeared to have the opposite effect. Our data support the hypothesis that NO exerts a modulatory influence on PRL secretion after LPS-induced inflammatory response.

Bovine Posterior Pituitary Extract Stimulates Prolactin Release from the Anterior Pituitary Gland In Vitro and In Vivo in Cattle

It has been reported that the posterior pituitary (PP) gland contains a potent, unknown prolactin (PRL)-releasing factor (PRF) in rats. PRFs are assumed to be produced in neurones located within the hypothalamus, and to be peptidergic in nature. However, little is known about PRFs in domestic animals. To characterize the PRF in the PP of domestic animals, the present study examined the PRL-releasing activity of an acidic extract from bovine PP (bPP) in vitro and in vivo in cattle. First, the PRL-releasing effect of bPP extract was compared with that of PRL-releasing peptide (PrRP), and thyrotropin-releasing hormone (TRH) from cultured bovine anterior pituitary cells. The extract significantly increased PRL concentrations in the culture medium, at doses of 0.002 and 0.02 eq./ml (one eq. is the PP extract from one animal), compared with the control (p < 0.05). PrRP failed to stimulate the release of PRL. TRH significantly increased PRL concentrations in the culture medium, at doses from 10(-9) to 10(-7) M, compared with the control (p < 0.05). The rate of increase in the PRL concentration, by 0.02 eq./ml bPP extract, was significantly greater than that in TRH (p < 0.05). Secondly, plasma PRL responses to the intravenous (i.v.) injection of bPP extract (0.5 eq./head), PrRP [3.59 mug/kg body weight (BW)], TRH (1 mug/kg BW), and a dopamine receptor antagonist (sulpiride, 0.1 mg/kg BW), were examined in calves. PrRP failed to stimulate PRL release; however, plasma PRL increased immediately following the injection of bPP extract, TRH and sulpiride. The PRL-releasing effect of i.v. injections of TRH and sulpiride was more potent than that of bPP extract. Finally, plasma PRL responses to the intra-hypothalamic injection of bPP extract were examined in calves. The intra-hypothalamic infusion (arcuate nucleus) of 0.0625 eq./head of bPP extract strongly stimulated PRL release in calves (p < 0.05). The present results show that PP contains a physiologically potent PRF in cattle.

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.

A possible mechanism for the action of adrenomedullin in brain to stimulate stress hormone secretion

Adrenomedullin (AM) has been reported to have actions at each level of the hypothalamo-pituitary-adrenal (HPA) axis, suggesting that the peptide plays a role in the organization of the neuroendocrine responses to stress. We examined the mechanism by which AM regulates the central nervous system branch of the HPA axis as well as the possible role of AM in the modulation of the releases of two other hormones, prolactin and GH, whose secretions also are altered by stress. Intracerebroventricular administration of AM led to elevated plasma corticosterone levels in unrestrained, conscious male rats. This effect was abrogated by pretreatment with a CRH antagonist, suggesting that AM activates the HPA axis by causing the release of CRH into hypophyseal portal vessels. In addition, AM given intracerebroventricularly stimulated the release of prolactin but did not alter the secretion of GH. We propose that AM produced in the brain may be an important neuromodulator of the hormonal stress response.

Prolactin and reproductive medicine

PURPOSE OF REVIEW

This review aims to summarize current knowledge about prolactin, and outlines recent information that affects the management of patients with hyperprolactinaemia.

RECENT FINDINGS

The actions of prolactin have been clarified by studies of prolactin-receptor-deficient mice, which have a clear phenotype of reproductive failure at multiple sites. The treatment of patients with hyperprolactinaemia or prolactinoma is largely achieved using dopamine agonist drugs, which induce the shrinkage of pituitary prolactinomas as well as control of the endocrine syndrome. Recent findings indicate that successful cabergoline treatment may be able to induce long-term remission, allowing drug withdrawal in a substantial proportion of patients.

SUMMARY

At present, dopamine agonist drugs remain the best treatment for hyperprolactinaemic patients, and can help most affected women achieve pregnancy. Future work is likely to help understand the basis of long-term remission in patients with pituitary prolactinomas.

The expanding family of -RFamide peptides and their effects on feeding behaviour

Neuropeptides terminating in -Arg-Phe-NH(2) (-RFamide) were first discovered in molluscan nervous systems, but were soon recognized to occur widely throughout the invertebrates. Progress in characterizing members of the family in vertebrates has been slower. In mammals, however, it is now clear that there are at least five genes encoding members of the family, and at least five G-protein-coupled receptors at which they act. The tissue distribution of the peptides and their receptors is wide and there are likely to be many different functions. One of the emerging themes from recent research is that these peptides are involved in control of feeding behaviour both in invertebrates and in vertebrates. This would seem to be a remarkable example of conservation of chemical structure and biological function throughout nervous system evolution.