Parathyroid hormone 1 (1-34) acts on the scales and involves calcium metabolism in goldfish

The effect of fugu parathyroid hormone 1 (fugu PTH1) on osteoblasts and osteoclasts in teleosts was examined with an assay system using teleost scale and the following markers: alkaline phosphatase (ALP) for osteoblasts and tartrate-resistant acid phosphatase (TRAP) for osteoclasts. Synthetic fugu PTH1 (1-34) (100pg/ml-10ng/ml) significantly increased ALP activity at 6h of incubation. High-dose (10ng/ml) fugu PTH1 significantly increased ALP activity even after 18h of incubation. In the case of TRAP activity, fugu PTH1 did not change at 6h of incubation, but fugu PTH1 (100pg/ml-10ng/ml) significantly increased TRAP activity at 18h. Similar results were obtained for human PTH (1-34), but there was an even greater response with fugu PTH1 than with human PTH. In vitro, we demonstrated that both the receptor activator of the NF-κB ligand in osteoblasts and the receptor activator NF-κB mRNA expression in osteoclasts increased significantly by fugu PTH1 treatment. In an in vivo experiment, fugu PTH1 induced hypercalcemia resulted from the increase of both osteoblastic and osteoclastic activities in the scale as well as the decrease of scale calcium contents after fugu PTH1 injection. In addition, an in vitro experiment with intramuscular autotransplanted scale indicated that the ratio of multinucleated osteoclasts/mononucleated osteoclasts in PTH-treated scales was significantly higher than that in the control scales. Thus, we concluded that PTH acts on osteoblasts and osteoclasts in the scales and regulates calcium metabolism in goldfish.

Melatonin stimulates the release of gonadotropin-inhibitory hormone by the avian hypothalamus

Gonadotropin-inhibitory hormone (GnIH), a neuropeptide that inhibits gonadotropin synthesis and release, was first identified in quail hypothalamus. GnIH acts on the pituitary and GnRH neurons in the hypothalamus via GnIH receptor to inhibit gonadal development and maintenance. In addition, GnIH neurons express melatonin receptor and melatonin induces GnIH expression in the quail brain. Thus, it seems that melatonin is a key factor controlling GnIH neural function. In the present study, we investigated the role of melatonin in the regulation of GnIH release and the correlation of GnIH release with LH release in quail. Melatonin administration dose-dependently increased GnIH release from hypothalamic explants in vitro. GnIH release was photoperiodically controlled. A clear diurnal change in GnIH release was observed in quail, and this change was negatively correlated with changes in plasma LH concentrations. GnIH release during the dark period was greater than that during the light period in explants from quail exposed to long-day photoperiods. Conversely, plasma LH concentrations decreased during the dark period. In contrast to LD, GnIH release increased under short-day photoperiods, when the duration of nocturnal secretion of melatonin increases. These results indicate that melatonin may play a role in stimulating not only GnIH expression but also GnIH release, thus inhibiting plasma LH concentrations in quail. This is the first report describing the effect of melatonin on neuropeptide release.

Identification of human GnIH homologs, RFRP-1 and RFRP-3, and the cognate receptor, GPR147 in the human hypothalamic pituitary axis

The existence of a hypothalamic gonadotropin-inhibiting system has been elusive. A neuropeptide named gonadotropin-inhibitory hormone (GnIH, SIKPSAYLPLRF-NH(2)) which directly inhibits gonadotropin synthesis and release from the pituitary was recently identified in quail hypothalamus. Here we identify GnIH homologs in the human hypothalamus and characterize their distribution and biological activity. GnIH homologs were isolated from the human hypothalamus by immunoaffinity purification, and then identified as MPHSFANLPLRF-NH(2) (human RFRP-1) and VPNLPQRF-NH(2) (human RFRP-3) by mass spectrometry. Immunocytochemistry revealed GnIH-immunoreactive neuronal cell bodies in the dorsomedial region of the hypothalamus with axonal projections to GnRH neurons in the preoptic area as well as to the median eminence. RT-PCR and subsequent DNA sequencing of the PCR products identified human GnIH receptor (GPR147) mRNA expression in the hypothalamus as well as in the pituitary. In situ hybridization further identified the expression of GPR147 mRNA in luteinizing hormone producing cells (gonadotropes). Human RFRP-3 has recently been shown to be a potent inhibitor of gonadotropin secretion in cultured sheep pituitary cells by inhibiting Ca(2+) mobilization. It also directly modulates GnRH neuron firing. The identification of two forms of GnIH (RFRP-1 and RFRP-3) in the human hypothalamus which targets human GnRH neurons and gonadotropes and potently inhibit gonadotropin in sheep models provides a new paradigm for the regulation of hypothalamic-pituitary-gonadal axis in man and a novel means for manipulating reproductive functions.

A new key neurohormone controlling reproduction, gonadotrophin-inhibitory hormone in birds: discovery, progress and prospects

In vertebrates, the neuropeptide control of gonadotrophin secretion is primarily through the stimulatory action of the hypothalamic decapeptide, gonadotrophin-releasing hormone (GnRH). Gonadal sex steroids and inhibin inhibit gonadotrophin secretion via feedback from the gonads, but a hypothalamic neuropeptide inhibiting gonadotrophin secretion was, until recently, unknown in vertebrates. In 2000, we discovered a novel hypothalamic dodecapeptide that directly inhibits gonadotrophin release in quail and termed it gonadotrophin-inhibitory hormone (GnIH). GnIH acts on the pituitary and GnRH neurones in the hypothalamus via a novel G-protein-coupled receptor for GnIH to inhibit gonadal development and maintenance by decreasing gonadotrophin release and synthesis. The pineal hormone melatonin is a key factor controlling GnIH neural function. GnIH occurs in the hypothalamus of several avian species and is considered to be a new key neurohormone inhibiting avian reproduction. Thus, the discovery of GnIH provides novel directions to investigate neuropeptide regulation of reproduction. This review summarises the discovery, progress and prospects of GnIH, a new key neurohormone controlling reproduction.

Gonadotropin-inhibitory hormone and its receptor in the avian reproductive system

Many hormones that are classified as neuropeptides are synthesized in vertebrate gonads in addition to the brain. Receptors for these hormones are also expressed in gonadal tissue; thus there is potential for a highly localized autocrine or paracrine effect of these hormones on a variety of gonadal functions. In the present study we focused on gonadotropin-inhibitory hormone (GnIH), a neuropeptide that was first discovered in the hypothalamus of birds. We present different lines of evidence for the synthesis of GnIH and its receptor in the avian reproductive system including gonads and accessory reproductive organs by studies on two orders of birds: Passeriformes and Galliformes. Binding sites for GnIH were initially identified via in vivo and in vitro receptor fluorography, and were localized in ovarian granulosa cells along with the interstitial layer and seminiferous tubules of the testis. Furthermore, species-specific primers produced clear PCR products of GnIH and GnIH receptor (GnIH-R) in songbird and quail gonadal and other reproductive tissues, such as oviduct, epididymis and vas deferens. Sequencing of the PCR products confirmed their identities. Immunocytochemistry detected GnIH peptide in ovarian thecal and granulosa cells, testicular interstitial cells and germ cells and pseudostratified columnar epithelial cells in the epididymis. In situ hybridization of GnIH-R mRNA in testes produced a strong reaction product which was localized to the germ cells and interstitium. In the epididymis, the product was also localized in the pseudostratified columnar epithelial cells. In sum, these results indicate that the avian reproductive system has the capability to synthesize and bind GnIH in several tissues. The distribution of GnIH and its receptor suggest a potential for autocrine/paracrine regulation of gonadal steroid production and germ cell differentiation and maturation.

Melatonin stimulates the release of growth hormone and prolactin by a possible induction of the expression of frog growth hormone-releasing peptide and its related peptide-2 in the amphibian hypothalamus

We recently identified a novel hypothalamic neuropeptide stimulating GH release in bullfrogs and termed it frog GH-releasing peptide (fGRP). The fGRP precursor encodes fGRP and its related peptides (fGRP-RP-1, -RP-2, and -RP-3), and fGRP-RP-2 also stimulates GH and prolactin (PRL) release. Cell bodies and terminals containing these neuropeptides are localized in the suprachiasmatic nucleus (SCN) and median eminence, respectively. To understand the physiological role of fGRP and fGRP-RP-2, we investigated the mechanisms that regulate the expression of these neuropeptides. This study shows that melatonin induces the expression of fGRP and fGRP-RPs in bullfrogs. Orbital enucleation combined with pinealectomy (Ex plus Px) decreased the expression of fGRP precursor mRNA and content of mature fGRP and fGRP-RPs in the diencephalon including the SCN and median eminence. Conversely, melatonin administration to Ex plus Px bullfrogs increased dose-dependently their expressions. The expression of fGRP precursor mRNA was photoperiodically controlled and increased under short-day photoperiods, when the nocturnal duration of melatonin secretion increases. To clarify the mode of melatonin action on the induction of fGRP and fGRP-RPs, we further demonstrated the expression of Mel(1b), a melatonin receptor subtype, in SCN neurons expressing fGRP precursor mRNA. Finally, we investigated circulating GH and PRL levels after melatonin manipulation because fGRP and fGRP-RP-2 stimulate the release of GH and GH/PRL, respectively. Ex plus Px decreased plasma GH and PRL concentrations, whereas melatonin administration increased these hormone levels. These results suggest that melatonin induces the expression of fGRP and fGRP-RP-2, thus stimulating the release of GH and PRL in bullfrogs.

Gonadotropin-inhibitory hormone neurons interact directly with gonadotropin-releasing hormone-I and -II neurons in European starling brain

Gonadotropin-inhibitory hormone (GnIH) is a hypothalamic dodecapeptide (SIKPSAYLPLRF-NH(2)) that directly inhibits gonadotropin synthesis and release from quail pituitary. The action of GnIH is mediated by a novel G-protein coupled receptor. This gonadotropin-inhibitory system may be widespread in vertebrates, at least birds and mammals. In these higher vertebrates, histological evidence suggests contact of GnIH immunoreactive axon terminals with GnRH neurons, thus indicating direct regulation of GnRH neuronal activity by GnIH. In this study we investigated the interaction of GnIH and GnRH-I and -II neurons in European starling (Sturnus vulgaris) brain. Cloned starling GnIH precursor cDNA encoded three peptides that possess characteristic LPXRF-amide (X = L or Q) motifs at the C termini. Starling GnIH was further identified as SIKPFANLPLRF-NH(2) by mass spectrometry combined with immunoaffinity purification. GnIH neurons, identified by in situ hybridization and immunocytochemistry (ICC), were clustered in the hypothalamic paraventricular nucleus. GnIH immunoreactive fiber terminals were present in the external layer of the median eminence in addition to the preoptic area and midbrain, where GnRH-I and GnRH-II neuronal cell bodies exist, respectively. GnIH axon terminals on GnRH-I and -II neurons were shown by GnIH and GnRH double-label ICC. Furthermore, the expression of starling GnIH receptor mRNA was identified in both GnRH-I and GnRH-II neurons by in situ hybridization combined with GnRH ICC. The cellular localization of GnIH receptor has not previously been identified in any vertebrate brain. Thus, GnIH may regulate reproduction of vertebrates by directly modulating GnRH-I and GnRH-II neuronal activity, in addition to influencing the pituitary gland.

The general and comparative biology of gonadotropin-inhibitory hormone (GnIH)

The decapeptide gonadotropin-releasing hormone (GnRH) is the primary factor responsible for the hypothalamic control of gonadotropin secretion. Gonadal sex steroids and inhibin inhibit gonadotropin secretion via feedback from the gonads, but a neuropeptide inhibitor of gonadotropin secretion was, until recently, unknown in vertebrates. In 2000, we identified a novel hypothalamic dodecapeptide that inhibits gonadotropin release in cultured quail pituitaries and termed it gonadotropin-inhibitory hormone (GnIH). To elucidate the mode of action of GnIH, we then identified a novel G protein-coupled receptor for GnIH in quail. The GnIH receptor possesses seven transmembrane domains and specifically binds to GnIH. The GnIH receptor is expressed in the pituitary and several brain regions including the hypothalamus. These results indicate that GnIH acts directly on the pituitary via GnIH receptor to inhibit gonadotropin release. GnIH may also act on the hypothalamus to inhibit GnRH release. To demonstrate the functional significance of GnIH and its potential role as a key regulatory neuropeptide in avian reproduction, we investigated GnIH actions on gonadal development and maintenance in quail. Chronic treatment with GnIH inhibited gonadal development and maintenance by decreasing gonadotropin synthesis and release. GnIH was also found in the hypothalamus of other avian species including sparrows and chickens and also inhibited gonadotropin synthesis and release. The pineal hormone melatonin may be a key factor controlling GnIH neural function, since quail GnIH neurons express melatonin receptor and melatonin treatment stimulates the expression of GnIH mRNA and mature GnIH peptide. Thus, GnIH is capable of transducing photoperiodic information via changes in the melatonin signal, thereby influencing the reproductive axis. It is concluded that GnIH, a newly discovered hypothalamic neuropeptide, is a key factor controlling avian reproduction. The discovery of avian GnIH opens a new research field in reproductive neuroendocrinology.

Mode of action and functional significance of avian gonadotropin-inhibitory hormone (GnIH): a review

Neuropeptide control of gonadotropin secretion at the level of the anterior pituitary gland is primarily through the stimulatory action of the hypothalamic decapeptide, gonadotropin-releasing hormone (GnRH). However, a hypothalamic neuropeptide acting at the level of the pituitary to negatively regulate gonadotropin secretion has, until recently, remained unknown in any vertebrate. In 2000, we discovered a novel hypothalamic neuropeptide inhibiting gonadotropin release at the level of the pituitary in quail and termed it gonadotropin-inhibitory hormone (GnIH). A gonadotropin-inhibitory system is an intriguing concept and provides us with an unprecedented opportunity to study the regulation of avian reproduction from an entirely novel standpoint. To elucidate the mode of action of GnIH, we further identified the receptor for GnIH and characterized its expression and binding activity in quail. The identified GnIH receptor possessed seven transmembrane domains and specifically bound to GnIH in a concentration-dependent manner. The expression of GnIH receptor was found in the pituitary and several brain regions including the hypothalamus. These results suggest that GnIH acts directly on the pituitary via GnIH receptor to inhibit gonadotropin release. GnIH may also act on the hypothalamus to inhibit GnRH release. To understand the functional significance of GnIH in avian reproduction, we also investigated the mechanism that regulates GnIH expression. Interestingly, melatonin induced dose-dependently GnIH expression and melatonin receptor (Mel(1c)) was expressed in GnIH neurons. Thus melatonin appears to act directly on GnIH neurons via its receptor to induce GnIH expression. Based on these studies, GnIH is likely an important neuropeptide for the regulation of avian reproduction.

Changes in the responsiveness of hen anterior pituitary to cLHRH-II during induced molting

The goal of this study was to determine whether the responsiveness of the anterior pituitary to cLHRH-II in relation to luteinizing hormone-beta subunit (LHbeta) mRNA expression is improved by induced molting. White Leghorn hens were subjected to induced molting by feed withdrawal for 4 days. The anterior pituitaries were collected from hens of pretreatment (PT), 1 day after resumption of feeding (1DRF) and on the day of resumption of laying (RL). They were processed for organ culture in the medium with or without cLHRH-II, followed by reverse transcription and competitive PCR. When pituitary tissues were incubated without cLHRH-II, the expression of LHbeta mRNA did not show any significant difference between PT and RL group hens. In contrast, the expression of LHbeta mRNA in the pituitaries that were incubated with cLHRH-II was significantly greater in RL group hens than those in PT and 1DRF groups. Among PT, 1DRF, and RL groups only RL hens showed significant increase of LHbeta mRNA synthesis when compared with control (without cLHRH-II). These results suggest that the responsiveness of the anterior pituitary to cLHRH-II increased in postmolt hens, indicating that the functions of this organ might be improved by induced molting.

Immunocytochemical identification of Pit-1 containing cells in the anterior pituitary of hens

Our goal was to identify the cells expressing Pit-1 protein in chicken anterior pituitary. The anterior pituitaries were collected from laying hens after perfusion with formalin-PBS, and fixed with Bouin's fixative followed by paraffin embedding. Sections of the anterior pituitaries were immunostained for Pit-1 in the first staining sequence followed by staining for 6 types of pituitary hormones in the second sequence. Pit-1 positive nuclei were observed in the glandular cells in both the cephalic and caudal lobes. Pit-1 immunoreaction products were colocalized in the glandular cells immunopositive for growth hormone, thyroid-stimulating hormone, follicle-stimulating hormone, luteinizing hormone, adrenocorticotropic hormone or prolactin. These results indicate that Pit-1 protein induction occurs in 6 types of glandular cells, suggesting that Pit-1 may regulate hormone synthesis in each glandular cell in the chicken pituitary.

Changes in the population of pituitary protein transcription factor-1 nuclei in the anterior pituitary during withdrawal and resumption of feeding in hens

Pituitary protein transcription factor (Pit-1) is a member of a large family of protein transcription factors that include Pit-1, Oct-1, Oct-2, and Unc-86. The goal of this experiment was to determine whether the population of Pit-1-containing cells changes in the anterior pituitary of chicken by the regulation of feeding. White Leghorn hens were subjected to withdrawal and resumption of feeding. The anterior pituitaries were collected from hens at pretreatment, at 2 d after withdrawal of feeding (2DWF), and 1 d and 5 d after resumption of feeding (1DRF and 5DRF, respectively). Sections of the pituitaries were immunostained for Pit-1. They were examined under a light microscope with an image analysis computer system. The Pit-1 positive nuclei were found in the glandular cells in the cephalic and caudal lobes of the anterior pituitary in all four groups of hens. The Pit-1 cell population significantly increased in the 2DWF and 1DRF and decreased thereafter in 5DRF. These results suggests that feed withdrawal may stimulate Pit-1 expression in chicken, suggesting that Pit-1 may be involved in control of pituitary functions during the process of feed regulation.

Changes in the expression of TGFbeta-isoforms in the anterior pituitary during withdrawal and resumption of feeding in hens

Our goal was to determine whether TGFbeta-isoforms were involved in the remodeling of the pituitary cell population which occurred by regulation of feeding. The current study examined whether TGFbeta-isoforms were produced in the anterior pituitary, and the mRNA expression of TGFbeta-isoforms changed during withdrawal and resumption of feeding. White Leghorn laying hens were subjected to feed withdrawal for 4 days with a resumption of feeding thereafter. The anterior pituitary tissues were collected from hens of pretreatment (PT), 3 days after feed withdrawal (3DFW), 1 and 5 days after the resumption of feeding (1DRF and 5DRF, respectively), and on the day of resumption of egg-laying (RL). They were processed for semi-quantification of TGFbeta2, beta3, and beta4 mRNA expressions by reverse transcription-polymerase chain reaction (RT-PCR) and for immunocytochemistry for TGFbeta3. TGFbeta2, TGFbeta3, and TGFbeta4 mRNA expression with a product size of 269, 236, and 163bp, respectively, was observed in the anterior pituitaries in all groups of hens. Although the expression of TGFbeta2 and TGFbeta4 mRNA did not show any significant change, that of TGFbeta3 mRNA significantly declined in the 1DRF hens and recovered by the resumption of laying. Immunostaining revealed that TGFbeta3 was located in the cytoplasm of glandular cells with granule forms in all groups of hens. The ovarian and oviductal weights sharply declined in the 3DFW and 1DRF groups, followed by a full recovery in the RL hens. These results indicate that TGFbeta2, beta3, and beta4 were expressed in the chicken anterior pituitary, and TGFbeta3 mRNA expression was changed in correlation with the regulation of feeding, suggesting that this isoform may play a significant role in the regulation of the glandular cell population and/or differentiation.

Changes in the population of immunoreactive S-100-positive folliculo-stellate cells in hens during induced molting

The goal of this study was to determine whether the population of folliculo-stellate (FS) cells in the hen's pituitary change during induced molting. White Leghorn laying hens were subjected to induced molting by feed withdrawal; feeding was resumed on the fourth day after egg laying ceased. The anterior pituitaries were collected from hens at pretreatment, at 3 and 5 d after feed withdrawal and at 3 d after cessation of egg laying, 10 d after cessation of egg laying (6 d after resumption of feeding), on the day of and 1 wk after resumption of egg laying (RL and 1WRL, respectively). Pituitaries were processed to detect FS cells by immunocytochemistry for the S-100 protein. Sections were then examined under a light microscope with an image analysis computer system. S-100 immunoreactive cells were found in the cephalic and caudal lobes of the anterior pituitary in all groups of hens. The majority of S-100 immunoreactive cells formed clusters of cells that faced into the follicle and surrounded the glandular cells with long cytoplasmic processes. The S-100 immunoreactive area in the cephalic lobe was significantly increased in the RL group (P < 0.05), but decreased thereafter in 1WRL group. The S-100-positive cell area in the caudal lobe did not show significant changes during induced molting. These results suggest that FS cell population is likely to increase at the final stage of induced molting and may be involved in control of pituitary functions for the resumption of ovulation.

Cell proliferation and apoptosis in the anterior pituitary of chicken during inhibition and resumption of laying

The goal of this study was to determine whether tissue rejuvenation of the anterior pituitary with cell proliferation and apoptosis occurs during inhibition and resumption of egg-laying. White Leghorn laying hens were subjected to inhibition of laying by feed withdrawal. Feeding was resumed on the fourth day of egg-laying cessation. All birds were injected ip with bromodeoxyuridine (BrdU) 1 h before tissue collection. The anterior pituitary glands were collected from hens of the following groups: pretreatment (PT), 3 and 5 days after starvation (3DS and 5DS, respectively), 3 days after cessation of laying (3DC), 10 days after cessation of laying (10DC, 6 days after resumption of feeding), and the day of and 1 week after resumption of laying (RL and 1WRL, respectively). They were processed for the detection of proliferating cells and apoptotic cells by BrdU immunostaining and terminal deoxynucleotidyl transferase-mediated biotinylated deoxyuridine triphosphate nick-end-labeling (TUNEL). Immunostaining for the anterior pituitary hormones was also conducted. In the cephalic lobe the BrdU-positive cells showed a higher frequency in RL than in PT, 3DC, and 1WRL. BrdU-positive cell frequency in the caudal lobe was greater in RL than in PT. TUNEL-positive cells in both cephalic and caudal lobes were increased markedly in the RL group. Their frequency in the cephalic lobe was greater in RL than in PT, and that in the caudal lobe of RL was higher than in any other group of birds. The areas of FSH-like cells in 10DC and RL were greater than those in PT to 3DC, and those of LH-like cells in RL were greater than those of 3DS to 3DC. PRL-like cells were decreased until 3DC and then gradually increased until 1WRL. GH-, TSH-, and ACTH-like cell areas showed tendencies to increase until 3DC, with decreasing thereafter. The sizes of FSH-like cells in 10DC to 1WRL and LH-like cells in RL were larger than those around cessation of laying. These results suggest that during inhibition and resumption of laying cell proliferation and apoptosis occur in the anterior pituitary tissue, which may cause a rejuvenation of tissue to improve the function of this organ.