Calcium and other signalling pathways in neuroendocrine regulation of somatotroph functions

Small GTPase control of pituitary hormone secretion: Evidence from studies in the goldfish (Carassius auratus) neuroendocrine model

The secretion of vertebrate pituitary hormones is regulated by multiple hypothalamic factors, which, while generally activating unique receptor systems, ultimately propagate signals through interacting intracellular regulatory elements to modulate hormone exocytosis. One important family of intracellular regulators is the monomeric small GTPases, a subset of which (Arf1/6, Rac, RhoA, and Ras) is highly conserved across vertebrates and regulates secretory vesicle exocytosis in many cell types. In this study, we investigated the roles of these small GTPases in basal and agonist-dependent hormone release from dispersed goldfish (Carassius auratus) pituitary cells in perifusion experiments. Inhibition of these small GTPases elevated basal LH and GH secretion, except for Ras inhibition which only increased basal LH release. However, variable responses were observed with regard to LH and GH responses to the two goldfish native gonadotropin-releasing hormones (GnRH2 and GnRH3). GnRH-dependent LH release, but not GH secretion, was mediated by Arf1/6 GTPases. In contrast, inhibition of Rac and RhoA GTPases selectively enhanced GnRH3- and GnRH2-dependent GH release, respectively, while Ras inhibition only enhanced GnRH3-evoked LH secretion. Together, our results reveal novel divergent cell-type- and ligand-specific roles for small GTPases in the control of goldfish pituitary hormone exocytosis in unstimulated and GnRH-evoked release.

Receptor-proximal effectors mediating GnRH actions in the goldfish pituitary: Involvement of G protein subunits and GRKs

In goldfish (Carassius auratus), two endogenous isoforms of gonadotropin-releasing hormone (GnRH) stimulate luteinizing hormone (LH) and growth hormone (GH) secretion. These isoforms, GnRH2 and GnRH3, act on a shared population of cell-surface GnRH receptors (GnRHRs) expressed on both gonadotrophs and somatotrophs, and can signal through unique, yet partially overlapping, suites of intracellular effectors, in a phenomenon known as functional selectivity or biased signalling. In this study, G-protein alpha (Gα) subunits were targeted with two inhibitors, YM-254890 and BIM-46187, to ascertain the contribution of specific G-protein subunits in GnRH signalling. Results with the Gα_q/11-specific inhibitor YM-254890 on primary cultures of goldfish pituitary cells revealed the use of these subunits in GnRH control of both LH and GH release, as well as GnRH-induced elevations in phospho-ERK levels. Results with the pan-Gα inhibitor BIM-46187 matched those using YM-254890 in LH release but GH responses differed, indicating additional, non-Gα_q/11 subunits may be involved in somatotrophs. BIM-46187 also elevated unstimulated LH and GH release suggesting that Gα subunits regulate basal hormone secretion. Furthermore, G-protein-coupled receptor kinase (GRK2/3) inhibition reduced LH responses to GnRH2 and GnRH3, and selectively enhanced GnRH2-stimulated GH release, indicating differential use of GRK2/3 in GnRH actions on gonadotrophs and somatotrophs. These findings in a primary untransformed system provide the first direct evidence to establish Gα_q/11 as an obligate driver of GnRH signalling in goldfish pituitary cells, and additionally describe the differential agonist- and cell type-selective involvement of GRK2/3 in this system.

Recent advances in neuropeptide-related omics and gene editing: Spotlight on NPY and somatostatin and their roles in growth and food intake of fish

Feeding and growth are two closely related and important physiological processes in living organisms. Studies in mammals have provided us with a series of characterizations of neuropeptides and their receptors as well as their roles in appetite control and growth. The central nervous system, especially the hypothalamus, plays an important role in the regulation of appetite. Based on their role in the regulation of feeding, neuropeptides can be classified as orexigenic peptide and anorexigenic peptide. To date, the regulation mechanism of neuropeptide on feeding and growth has been explored mainly from mammalian models, however, as a lower and diverse vertebrate, little is known in fish regarding the knowledge of regulatory roles of neuropeptides and their receptors. In recent years, the development of omics and gene editing technology has accelerated the speed and depth of research on neuropeptides and their receptors. These powerful techniques and tools allow a more precise and comprehensive perspective to explore the functional mechanisms of neuropeptides. This paper reviews the recent advance of omics and gene editing technologies in neuropeptides and receptors and their progresses in the regulation of feeding and growth of fish. The purpose of this review is to contribute to a comparative understanding of the functional mechanisms of neuropeptides in non-mammalians, especially fish.

Mel1b and Mel1c melatonin receptors mediate green light-induced secretion of growth hormone in chick adenohypophysis cells via the AC/PKA and ERK1/2 signalling pathways

A previous study showed that melatonin (MEL) membrane receptors 1b (Mel1b) and Mel1c promoted the secretion of growth hormone (GH) in chick adenohypophysis cells under monochromatic green light. However, the intracellular signalling pathways of these two receptors are unclear. Therefore, cultured adenohypophysis cells derived from chickens exposed to monochromatic green light were treated with MEL, Mel1b- and Mel1c-specific blockers, protein kinase A (PKA) inhibitors and adenylate cyclase (AC), or AC activator in vitro to explore the signal transduction mechanism that promote the secretion of GH. The results showed that Mel1b and Mel1c participate in MEL-mediated green light-induced secretion of GH in chick adenohypophysis cells. However, MEL increased cyclic adenosine monophosphate (cAMP) levels, and p-PKA protein levels were blocked by a Mel1b-specific antagonist but not a Mel1c-specific antagonist, which indicated that Mel1b affected the secretion of GH via the AC/cAMP/PKA signalling pathway. Moreover, Mel1b and Mel1c both activated ERK1/2 to regulate the secretion of GH. In addition, intracellular and extracellular Ca²⁺ channels were also involved in secretion of GH in chick adenohypophysis cells. These results demonstrate that the MEL mediated green light-induced secretion of GH in chick adenohypophysis via the Mel1b/AC/PKA/ERK1/2, Mel1c/ERK1/2, and intracellular and extracellular Ca2⁺ channel signalling pathways.

Nesfatin-1 is an inhibitor of the growth hormone-insulin-like growth factor axis in goldfish (Carassius auratus)

Nesfatin-1, an 82 amino acid peptide cleaved from the N-terminal of its precursor nucleobindin-2 (NUCB2), is emerging as a multifunctional peptide in fish. The present study aimed to determine whether nesfatin-1 plays a role in fish somatic growth by modulating the growth hormone (GH)/insulin-like growth factor (IGF) axis, using a representative teleost model, the goldfish (Carassius auratus). The results demonstrated that a single i.p. injection of synthetic goldfish nesfatin-1 significantly decreased the expression of hypothalamic pacap (approximately 90%) and pituitary Gh (approximately 90%) mRNAs at 15 minutes post-injection. Serum GH levels were also reduced as a result of nesfatin-1 administration, by approximately 45% and 55% at 15 and 30 minutes post-injection, respectively. Likewise, in vitro treatment of goldfish dispersed pituitary cells with nesfatin-1 reduced Gh secretion, suggesting that nesfatin-1 acts directly on pituitary somatotrophs to inhibit Gh release. Exposure of cultured liver fragments to nesfatin-1 (0.1, 1 and 10 nmol L^-1 ) led to a significant reduction in igf-1 mRNA at 120 minutes and of igf-II mRNA at 30 and 60 minutes post-incubation. Collectively, these results indicate a suppressive role for nesfatin-1 on the goldfish GH/IGF axis. Immunohistochemical studies demonstrated that NUCB2/nesfatin-1-like immunoreactivity, although present in the goldfish pituitary, is not colocalised with GH in goldfish somatotrophs. Thus, nesfatin-1 does not appear to act in an autocrine manner to regulate GH secretion. Taken together, this research found that the pituitary gland is an important source of endogenous NUCB2/nesfatin-1 and also that nesfatin-1 directly suppresses the Gh/IGF axis in goldfish.

Seasonally related metabolic changes and energy allocation associated with growth and reproductive phases in the liver of male goldfish (Carassius auratus)

Reproduction and growth follow a seasonal pattern in many fish species involving changes in gonadal development, growth, and metabolism. Significant metabolic energy is needed during gametogenesis in both female and male to produce hundreds of eggs and billions of sperms. Seasonal variations are controlled by the hormones of brain-pituitary-peripheral axis and are accompanied by significant metabolic changes. There is evidence that GnRH and GnIH are among the key neurohormones that regulate the reciprocal control of growth and reproduction. The objective of this study was to investigate changes in metabolic profile and energy allocation patterns at different stages of reproduction, using goldfish as a model organism and LC-MS as analytical platform for metabolic analysis. Goldfish undergoes a clear seasonal cycle of growth and reproduction. In vivo experiments were conducted at three different time point of the annual cycle: regressed gonadal phase (peak growth phase), mid gametogenesis and late gametogenesis. Emphasis is placed on changes in liver metabolic pathways to energetically sustain the physiological processes related to growth and reproduction. Moreover, we tested the hypothesis that GnRH and GnIH may play a role in the regulation of metabolism by investigating acute effects of these peptides at different stages of reproductive cycle. SIGNIFICANCE: The findings in this paper provide novel information on the seasonal changes in basal metabolism during different stages of reproductive cycle, and evidence for differential allocation of energy during reciprocal control of reproduction and growth in goldfish. Chemometrics combined with pathway-driven bioinformatics elucidated a shift in the metabolic profile, indicating distinct patterns of energy allocation in the reproductive and growth seasons. Furthermore, to our knowledge this is the first study to provide evidence for a possible regulatory role of GnRH and GnIH in liver metabolism and energy allocation patterns associated with growth and reproductive processes. Together our findings present a framework for better understanding of the hormonally induced changes in metabolism to energetically sustain growth and reproduction in fish and other oviparous species undergoing seasonal cycle.

β-Arrestin-dependent signaling in GnRH control of hormone secretion from goldfish gonadotrophs and somatotrophs

In goldfish, two native isoforms of gonadotropin-releasing hormone (GnRH2 and GnRH3) stimulate luteinizing hormone (LH) and growth hormone (GH) release from pituitary cells through activation of cell-surface GnRH-receptors (GnRHRs) on gonadotrophs and somatotrophs. Interestingly, GnRH2 and GnRH3 induce LH and GH release via non-identical post-receptor signal transduction pathways in a ligand- and cell-type-selective manner. In this study, we examined the involvement of β-arrestins in the control of GnRH-induced LH and GH secretion from dispersed goldfish pituitary cells. Treatment with Barbadin, which interferes with β-arrestin and β2-adaptin subunit interaction, reduced LH responses to GnRH2 and GnRH3, as well as GH responses to GnRH2; but enhanced GnRH3-induced GH secretion. Barbadin also had positive influences on basal hormone release, and basal GH release in particular, as well as basal activity of extracellular signal-regulated kinase (ERK) and GnRH-induced ERK activation. These findings indicate that β-arrestins play permissive roles in the control of GnRH-stimulated LH release. However, in somatotrophs, β-arrestins, perhaps by mediating agonist-selective endosomal trafficking of engaged GnRHRs, participate in GnRH-isoform-specific GH release responses (stimulatory and inhibitory for GnRH2-GnRHR and GnRH3-GnRHR activation, respectively). The correlative stimulatory influences of Barbadin on basal hormone release and ERK activation suggest that β-arrestins may negatively regulate basal secretion through modulation of basal ERK activity. These results provide the first direct evidence of a role for β-arrestins in hormone secretion from an untransformed primary pituitary cell model, and establish these proteins as important receptor-proximal players in mediating functional selectivity downstream of goldfish GnRHRs.

Effects of temperature on the transcriptomes of pituitary and liver in Golden Pompano Trachinotus blochii

Fish growth can be modulated dynamically through the brain-pituitary-liver regulation axis. In the present study, whole transcriptomes of the pituitary and liver from Golden Pompano Trachinotus blochii were sequenced in seawater at 20 °C (T_low) and 25 °C (T_high). A total of 187,277,583 paired-end reads were assembled to obtain 100,495 transcripts, corresponding to 83,974 genes. These reads were mapped to T. blochii transcripts, and the mapping accuracy ranged from 80.4 to 94.9%. Two lists of differentially expressed genes were obtained by comparisons of pituitary and liver T_low versus T_high groups, comprising 458 and 205 genes, respectively. Of these, 33 differentially expressed genes were common between the two lists. Twelve GO terms were overrepresented for the 458 differentially expressed genes in the pituitary, and it is noteworthy that the GO term galanin receptor activity (GO: 0004966) related to the modulation of appetite and metabolism, whose genes made up half of all assembled genes in the term. For the 205 differentially expressed genes in the liver, 19 overrepresented GO terms were mainly related to immune regulation, digestion, and protein metabolism. Among the common differentially expressed genes, there were 32 genes that had identical changing trends in both pituitary and liver comparisons. Furthermore, two GO terms inorganic diphosphatase activity and MHC protein complex were overrepresented. These results indicate that the brain could regulate pituitary function through galanin signal mechanism and that the metabolism of liver was further optimized to modulate immunity and growth under different temperatures.

Multifactorial control of reproductive and growth axis in male goldfish: Influences of GnRH, GnIH and thyroid hormone

Reproduction and growth are under multifactorial control of neurohormones and peripheral hormones. This study investigated seasonally related effects of GnIH, GnRH, and T3 on the reproductive and growth axis in male goldfish at three stages of gonadal recrudescence. The effects of injection treatments with GnRH, GnIH and/or T3 were examined by measuring serum LH and GH levels, as well as peripheral transcript levels, using a factorial design. As expected, GnRH elevated serum LH and GH levels in a seasonally dependant manner, with maximal elevations of LH in late stages of gonadal recrudescence (Spring) and maximal increases in GH in the regressed gonadal stage (Summer). GnIH injection increased serum LH and GH levels only in fish at the regressed stage but exerted both stimulatory and inhibitory effects on GnRH-induced LH responses depending on season. T3 treatment mainly had stimulatory effects on circulating LH levels and inhibitory effects on serum GH concentrations. In the liver and testes, we observed seasonal differences in thyroid receptors, estrogen receptors, vitellogenin, follicle-stimulating hormone receptor, aromatase and IGF-I transcript levels that were tissue- and sex-specific. Generally, there were no clear correlation between circulating LH and GH levels and peripheral transcript levels, presumably due to time-related response and possible direct interaction of GnRH and GnIH at the level of liver and testis. The results support the hypothesis that GnRH and GnIH are important components of multifactorial mechanisms that work in concert with T3 to regulate reciprocal control of reproduction and growth in goldfish.

Seasonal Related Multifactorial Control of Pituitary Gonadotropin and Growth Hormone in Female Goldfish: Influences of Neuropeptides and Thyroid Hormone

Female reproduction is under multifactorial control of brain-pituitary-peripheral origin. The present study provides information on seasonal changes in circulating LH and GH concentrations, as well as transcript levels for a number of genes involved in the regulation of reproduction and growth in female goldfish. We also provide information on the effects of treatments with GnRH and/or GnIH, and their interaction with T3, at three stages of gonadal recrudescence. Maximum basal concentration of LH was observed at late recrudescence (Spring) while no seasonal changes in basal serum GH levels was detected. Serum LH and GH levels were stimulated by GnRH as expected, depending on the season. GnIH stimulated basal GH concentrations in gonadally regressed fish. GnIH inhibitory action on GnRH-induced LH response was observed in late, but not in mid recrudescence. T3 actions on basal and GnRH- or GnIH-induced GH secretion were generally inhibitory, depending on season. Administration of T3 attenuated GnRH-induced LH responses in mid and late stages of gonadal recrudescence, and the presence of GnIH abolished inhibitory actions of T3 in fish at mid recrudescence. Our results also demonstrated seasonal patterns in basal and GnRH- and/or GnIH-induced transcript levels for ERα, ERβI, FSHR, aromatase, TRαI, TRβ, IGF-I, and Vtg in the liver and ovary. However, there were no clear correlations between changes in transcript levels and circulating levels of LH and GH. The results support the hypothesis that GnRH, GnIH, and T3 are contributing factors in complex reciprocal control of reproduction and growth in goldfish.

Regulation of growth hormone biosynthesis by Cdk5 regulatory subunit associated protein 1-like 1 (CDKAL1) in pituitary adenomas

CDK5 regulatory subunit associated protein 1-like 1 (CDKAL1) is a tRNA-modifying enzyme that catalyzes 2-methylthiolation (ms²) and has been implicated in the development of type 2 diabetes (T2D). CDKAL1-mediated ms² is important for efficient protein translation and regulates insulin biosynthesis in pancreatic cells. Interestingly, an association between T2D and release of growth hormone (GH) has been reported in humans. However, it is unknown whether CDKAL1 is important for hormone production in the pituitary gland. The present study investigated the role of CDKAL1 in GH-producing pituitary adenomas (GHPAs). CDKAL1 activity was suppressed in GHPAs, as evidenced by a decrease in ms², compared with non-functioning pituitary adenomas (NFPAs), which do not produce specific hormones. Downregulation of Cdkal1 using small interfering and short hairpin RNAs increased the biosynthesis and secretion of GH in rat GH3 cells. Depletion of Cdkal1 increased the cytosolic calcium level via downregulation of DnaJ heat shock protein family (Hsp40) member C10 (Dnajc10), which is an endoplasmic reticulum protein related to calcium homeostasis. This stimulated transcription of GH via upregulation of Pit-1. Moreover, CDKAL1 activity was highly sensitive to proteostatic stress and was upregulated by suppression of this stress. Taken together, these results suggest that dysregulation of CDKAL1 is involved in the pathogenesis of GHPAs, and that modulation of the proteostatic stress response might control CDKAL1 activity and facilitate treatment of GHPAs.

A computational model for gonadotropin releasing cells in the teleost fish medaka

Pituitary endocrine cells fire action potentials (APs) to regulate their cytosolic Ca²⁺ concentration and hormone secretion rate. Depending on animal species, cell type, and biological conditions, pituitary APs are generated either by TTX-sensitive Na⁺ currents (I_Na), high-voltage activated Ca²⁺ currents (I_Ca), or by a combination of the two. Previous computational models of pituitary cells have mainly been based on data from rats, where I_Na is largely inactivated at the resting potential, and spontaneous APs are predominantly mediated by I_Ca. Unlike in rats, spontaneous I_Na-mediated APs are consistently seen in pituitary cells of several other animal species, including several species of fish. In the current work we develop a computational model of gonadotropin releasing cells in the teleost fish medaka (Oryzias latipes). The model stands out from previous modeling efforts by being (1) the first model of a pituitary cell in teleosts, (2) the first pituitary cell model that fires sponateous APs that are predominantly mediated by I_Na, and (3) the first pituitary cell model where the kinetics of the depolarizing currents, I_Na and I_Ca, are directly fitted to voltage-clamp data. We explore the firing properties of the model, and compare it to the properties of previous models that fire I_Ca-based APs. We put a particular focus on how the big conductance K⁺ current (I_BK) modulates the AP shape. Interestingly, we find that I_BK can prolong AP duration in models that fire I_Ca-based APs, while it consistently shortens the duration of the predominantly I_Na-mediated APs in the medaka gonadotroph model. Although the model is constrained to experimental data from gonadotroph cells in medaka, it may likely provide insights also into other pituitary cell types that fire I_Na-mediated APs.

Excitable cells elicit electrical pulses called action potentials (APs), which are generated and shaped by a combination of ion channels in the cell membrane. Since one type of ion channels is permeable to Ca²⁺ ions, there is typically an influx of Ca²⁺ during an AP. Pituitary cells therefore use AP firing to regulate their cytosolic Ca²⁺ concentration, which in turn controls their hormone secretion rate. The amount of Ca²⁺ that enters during an AP depends strongly on how long it lasts, and it is therefore important to understand the mechanisms that control this. Pituitary APs are generally mediated by a combination of Ca²⁺ channels and Na⁺ channels, and the relative contributions of from the two vary between cell types, animal species and biological conditions. Previous computer models have predominantly been adapted to data from pituitary cells which tend to fire Ca²⁺-based APs. Here we develop a new model, adapted to data from pituitary cells in the fish medaka, which APs that are predominantly Na⁺-based, and compare its dynamical properties to the previous models that fire Ca²⁺-based APs.

Abacavir, nevirapine, and ritonavir modulate intracellular calcium levels without affecting GHRH-mediated growth hormone secretion in somatotropic cells in vitro

Growth Hormone (GH) deficiency is frequent in HIV-infected patients treated with antiretroviral therapy. We treated GH3 cells with antiretrovirals (nevirapine, ritonavir or abacavir sulfate; 100 pM-1 mM range), after transfection with human growth hormone releasing hormone (GHRH) receptor cDNA. Cells viability, intracellular cAMP, phosphorylation of CREB and calcium increase, GH production and secretion were evaluated both in basal condition and after GHRH, using MTT, bioluminescence resonance energy transfer, western blotting and ELISA. Antiretroviral treatment did not affect GHRH 50% effective dose (EC₅₀) calculated for 30-min intracellular cAMP increase (Mann-Whitney's U test; p ≥ 0.05; n = 4) nor 15-min CREB phosphorylation. The kinetics of GHRH-mediated, rapid intracellular calcium increase was perturbed by pre-incubation with drugs, while GHRH failed to induce the ion increase in ritonavir pre-treated cells (ANOVA; p < 0.05; n = 3). Antiretrovirals did not impact 24-h intracellular and extracellular GH levels (ANOVA; p ≥ 0.05; n = 3). We demonstrated the association between antiretrovirals and intracellular calcium increase, without consequences on somatotrope cells viability and GH synthesis. Overall, these results suggest that antiretrovirals may not directly impact on GH axis in HIV-infected patients.

Nitric Oxide and the Neuroendocrine Control of the Osmotic Stress Response in Teleosts

The involvement of nitric oxide (NO) in the modulation of teleost osmoresponsive circuits is suggested by the facts that NO synthase enzymes are expressed in the neurosecretory systems and may be regulated by osmotic stimuli. The present paper is an overview on the research suggesting a role for NO in the central modulation of hormone release in the hypothalamo-neurohypophysial and the caudal neurosecretory systems of teleosts during the osmotic stress response. Active NOS enzymes are constitutively expressed by the magnocellular and parvocellular hypophysiotropic neurons and the caudal neurosecretory neurons of teleosts. Moreover, their expression may be regulated in response to the osmotic challenge. Available data suggests that the regulatory role of NO appeared early during vertebrate phylogeny and the neuroendocrine modulation by NO is conservative. Nonetheless, NO seems to have opposite effects in fish compared to mammals. Indeed, NO exerts excitatory effects on the electrical activity of the caudal neurosecretory neurons, influencing the amount of peptides released from the urophysis, while it inhibits hormone release from the magnocellular neurons in mammals.

Up-regulation of NHE8 by somatostatin ameliorates the diarrhea symptom in infectious colitis mice model

Na⁺/H⁺ exchangers (NHEs) have been shown to be involved in regulating cell volume and maintaining fluid and electrolyte homeostasis. Pooled evidences have suggested that loss of Na⁺/H⁺ exchanger isoform 8 (NHE8) impairs intestinal mucosa. Whether NHE8 participates in the pathology of infectious colitis is still unknown. Our previous study demonstrated that somatostatin (SST) could stimulate the expression of intestinal NHE8 so as to facilitate Na⁺ absorption under normal condition. This study further explored whether NHE8 participates in the pathological processes of infectious colitis and the effects of SST on intestinal NHE8 expression in the setting of infectious colitis. Our data showed that NHE8 expression was reduced in Citrobacter rodentium (CR) infected mice. Up-regulation of NHE8 improved diarrhea symptom and mucosal damage induced by CR. In vitro, a similar observation was also seen in Enteropathogenic E. coli (EPEC) infected Caco-2 cells. Seglitide, a SST receptor (SSTR) 2 agonist, partly reversed the inhibiting action of EPEC on NHE8 expression, but SSTR5 agonist (L-817,818) had no effect on the expression of NHE8. Moreover, SST blocked the phosphorylation of p38 in EPEC-infected Caco-2 cells. Taken together, these results suggest that enhancement of intestinal NHE8 expression by SST could ameliorate the symptoms of mice with infectious colitis.

Comparative aspects of GnRH-Stimulated signal transduction in the vertebrate pituitary - Contributions from teleost model systems

Gonadotropin-releasing hormone (GnRH) is a major regulator of reproduction through actions on pituitary gonadotropin release and synthesis. Although it is often thought that pituitary cells are exposed to only one GnRH, multiple GnRH forms are delivered to the pituitary of teleost fishes; interestingly this can include the cGnRH-II form usually thought to be non-hypophysiotropic. GnRHs can regulate other pituitary cell-types, both directly as well as indirectly, and multiple GnRH receptors (GnRHRs) may also be expressed in the pituitary, and even within a single pituitary cell-type. Literature on the differential actions of native GnRH isoforms in primary pituitary cells is largely derived from teleost fishes. This review will outline the diversity and complexity of GnRH-GnRHR signal transduction found within vertebrate gonadotropes as well as extra-gonadotropic sites with special emphasis on comparative studies from fish models. The implications that GnRHR transduction mechanisms are GnRH isoform-, function-, and cell-specific are also discussed.

Multiple signaling pathways convey central and peripheral signals to regulate pituitary function: Lessons from human and non-human primate models

The anterior pituitary gland is a key organ involved in the control of multiple physiological functions including growth, reproduction, metabolism and stress. These functions are controlled by five distinct hormone-producing pituitary cell types that produce growth hormone (somatotropes), prolactin (lactotropes), adrenocorticotropin (corticotropes), thyrotropin (thyrotropes) and follicle stimulating hormone/luteinizing hormone (gonadotropes). Classically, the synthesis and release of pituitary hormones was thought to be primarily regulated by central (neuroendocrine) signals. However, it is now becoming apparent that factors produced by pituitary hormone targets (endocrine and non-endocrine organs) can feedback directly to the pituitary to adjust pituitary hormone synthesis and release. Therefore, pituitary cells serve as sensors to integrate central and peripheral signals in order to fine-tune whole-body homeostasis, although it is clear that pituitary cell regulation is species-, age- and sex-dependent. The purpose of this review is to provide a comprehensive, general overview of our current knowledge of both central and peripheral regulators of pituitary cell function and associated intracellular mechanisms, focusing on human and non-human primates.

Place-based screening of mixtures of dominant emerging contaminants measured in Lake Michigan using zebrafish embryo gene expression assay

Determining impacts of emerging contaminants is difficult due to the different concentrations of mixtures of these chemicals over a landscape. Assessment approaches need to account for absorption, distribution, metabolism and excretion of the chemicals in an organism, and potential crosstalk between molecular pathways. The goal of this study was to assess the utility of employing a modified zebrafish embryo toxicity (ZFET) assay that assesses morphological alterations and measurements of estrogen-associated mRNA transcripts, to exposure of a mixtures of chemicals at concentrations measured in several locations in Lake Michigan. The 5 pharmaceuticals in this study were carbamazepine, diltiazem, fluoxetine, gemfibrozil and metformin. Exposures consisted of 4 concentrations of each individual chemical, mixture concentrations measured at seven locations in Lake Michigan, or 17β-estradiol. The relative expression of Estrogen Receptor-alpha, brain aromatase (CYP19A2), and gonadotropin releasing hormone 3 mRNA were measured at the end the 6-d exposure to determine estrogenicity of the individual chemical or mixture. In this study, there was significant induction of CYP19A2 in individual exposures of diltiazem, fluoxetine, gemfibrozil and metformin at concentrations measured in Lake Michigan. Exposure to 5 of the 7 chemical mixtures altered the expression of one of the three biomarkers. Transcripts varied across mixtures, indicating that biological screening of whole water samples for potential estrogenicity may need to include alternative molecular pathways other than just steroid receptor binding. This research demonstrates that pairing chemical measurements with a modified ZFET assay, twhich incorporates molecular biomarkers and morphological endpoints, could provide location and mixture specific toxic profiling.

Why goldfish? Merits and challenges in employing goldfish as a model organism in comparative endocrinology research

Goldfish has been used as an unconventional model organism to study a number of biological processes. For example, goldfish is a well-characterized and widely used model in comparative endocrinology, especially in neuroendocrinology. Several decades of research has established and validated an array of tools to study hormones in goldfish. The detailed brain atlas of goldfish, together with the stereotaxic apparatus, are invaluable tools for the neuroanatomic localization and central administration of endocrine factors. In vitro techniques, such as organ and primary cell cultures, have been developed using goldfish. In vivo approaches using goldfish were used to measure endogenous hormonal milieu, feeding, behaviour and stress. While there are many benefits in using goldfish as a model organism in research, there are also challenges associated with it. One example is its tetraploid genome that results in the existence of multiple isoforms of endocrine factors. The presence of extra endogenous forms of peptides and its receptors adds further complexity to the already redundant multifactorial endocrine milieu. This review will attempt to discuss the importance of goldfish as a model organism in comparative endocrinology. It will highlight some of the merits and challenges in employing goldfish as an animal model for hormone research in the post-genomic era.

A Type IIb, but Not Type IIa, GnRH Receptor Mediates GnRH-Induced Release of Growth Hormone in the Ricefield Eel

Multiple gonadotropin-releasing hormone receptors (GnRHRs) are present in vertebrates, but their differential physiological relevances remain to be clarified. In the present study, we identified three GnRH ligands GnRH1 (pjGnRH), GnRH2 (cGnRH-II), and GnRH3 (sGnRH) from the brain, and two GnRH receptors GnRHR1 (GnRHR IIa) and GnRHR2 (GnRHR IIb) from the pituitary of the ricefield eel Monopterus albus. GnRH1 and GnRH3 but not GnRH2 immunoreactive neurons were detected in the pre-optic area, hypothalamus, and pituitary, suggesting that GnRH1 and GnRH3 may exert hypophysiotropic roles in ricefield eels. gnrhr1 mRNA was mainly detected in the pituitary, whereas gnrhr2 mRNA broadly in tissues of both females and males. In the pituitary, GnRHR1 and GnRHR2 immunoreactive cells were differentially distributed, with GnRHR1 immunoreactive cells mainly in peripheral areas of the adenohypophysis whereas GnRHR2 immunoreactive cells in the multicellular layers of adenohypophysis adjacent to the neurohypophysis. Dual-label fluorescent immunostaining showed that GnRHR2 but not GnRHR1 was localized to somatotropes, and all somatotropes are GnRHR2-positive cells and vice versa at all stages examined. GnRH1 and GnRH3 were shown to stimulate growth hormone (Gh) release from primary culture of pituitary cells, and to decrease Gh contents in the pituitary of ricefield eels 12 h post injection. GnRH1 and GnRH3 stimulated Gh release probably via PLC/IP₃/PKC and Ca²⁺ pathways. These results, as a whole, suggested that GnRHs may bind to GnRHR2 but not GnRHR1 to trigger Gh release in ricefield eels, and provided novel information on differential roles of multiple GnRH receptors in vertebrates.

Evidences for involvement of growth hormone and insulin-like growth factor in ovarian development of starry flounder (Platichthys stellatus)

Although gonadotrophins are major regulators of ovarian function in teleosts and other vertebrates, accumulating evidence indicates that the growth hormone (GH)-insulin-like growth factor (IGF) axis also plays an important role in fish reproduction. As a first step to understand the physiological role of the GH-IGF system in the ovarian development of starry flounder (Platichthys stellatus), the expression profiles of GH and IGF messenger RNAs (mRNAs) and plasma GH, IGF-I, estradiol-17β (E2), and testosterone (T) levels during the ovarian development were investigated. The developmental stages of ovaries were divided into five stages (II, III, IV, V, and VI) by histological analysis. The hepatosomatic index (HSI) and gonadosomatic index (GSI) values increased and peaked at stage IV and stage V, respectively, and then declined at stage VI. Pituitary GH mRNA levels decreased sharply at stage III and raised to top level at stage VI. The hepatic IGF-I mRNA levels ascended to maximum value at stage V and then declined significantly at stage VI. However, the hepatic IGF-II mRNA levels remained stable and increased significantly at stage VI. In contrast, the ovarian IGF-I mRNA levels increased gradually and peaked at stage VI. The ovarian IGF-II mRNA levels were initially stable and increased significantly at stage V until the top level at stage VI. Consistent with the pituitary GH mRNA levels, plasma GH levels reduced sharply at stage III and remained depressed until stage V and then raised remarkably at stage VI. Plasma IGF-I level peaked at stage V and then declined to initial level. Plasma E2 level peaked at stage IV and then dramatically descended to the basal level. Plasma T level peaked at stage V and then declined significantly back to the basal level. Based on statistical analysis, significant positive correlations between hepatic IGF-I mRNA and GSI, ovarian IGF-II mRNA and hepatic IGF-II mRNA, ovarian IGF-I mRNA and ovarian IGF-II mRNA, and plasma IGF-I and plasma T were observed, respectively. These results suggest that the GH-IGF system may be involved in the ovarian development of starry flounder; GH and IGFs appear to play distinct roles in the regulation of the ovarian development in paracrine/autocrine manners. These findings extend our knowledge of the roles of the GH-IGF axis on reproduction regulation in fish.

In vitro effects of somatostatin on the growth hormone-insulin-like growth factor axis in orange-spotted grouper (Epinephelus coioides)

Growth in vertebrates is mainly mediated by the growth hormone (GH)-insulin-like growth factor (IGF) axis, and somatostatin (SRIF) inhibits growth by decreasing GH release at the pituitary level and antagonizing the release and action of GHRH in the hypothalamus. However, the effects of SRIF on the regulation of growth at levels other than GH release from the pituitary gland are less well known. In the present study, we comprehensively examined the pituitary and peripheral actions of SRIF on the GH-IGF axis in grouper using a primary pituitary and hepatocyte cell culture system. Our results showed that SRIF inhibited GH release at the pituitary level, but had no influence on GH mRNA expression. Basal hepatic GH receptor 1 (GHR1), IGF-I and IGF-II mRNA levels declined over time, whereas GHR2 mRNA levels remained stable throughout the culture period. GH stimulated the hepatic expression of GHR and IGF mRNAs in a dose-dependent manner, while SRIF suppressed both basal and GH-stimulated expression of GHR and IGF mRNAs in primary cultured hepatocytes. The inhibition of GHR and IGF mRNA levels by SRIF was not attributed to the rate of mRNA degradation. To the best of our knowledge, we demonstrated the effects of SRIF on basal and GH-stimulated IGF-II mRNA levels in teleosts for the first time. These results indicate that SRIF regulates growth at the level of the pituitary and peripheral liver.

Molecular and functional profiling of histamine receptor-mediated calcium ion signals in different cell lines

Calcium ions (Ca(2+)) play a pivotal role in cellular physiology. Often Ca(2+)-dependent processes are studied in commonly available cell lines. To induce Ca(2+) signals on demand, cells may need to be equipped with additional proteins. A prominent group of membrane proteins evoking Ca(2+) signals are G-protein coupled receptors (GPCRs). These proteins register external signals such as photons, odorants, and neurotransmitters and convey ligand recognition into cellular responses, one of which is Ca(2+) signaling. To avoid receptor cross-talk or cross-activation with introduced proteins, the repertoire of cell-endogenous receptors must be known. Here we examined the presence of histamine receptors in six cell lines frequently used as hosts to study cellular signaling processes. In a concentration-dependent manner, histamine caused a rise in intracellular Ca(2+) in HeLa, HEK 293, and COS-1 cells. The concentration for half-maximal activation (EC50) was in the low micromolar range. In individual cells, transient Ca(2+) signals and Ca(2+) oscillations were uncovered. The results show that (i) HeLa, HEK 293, and COS-1 cells express sufficient amounts of endogenous receptors to study cellular Ca(2+) signaling processes directly and (ii) these cell lines are suitable for calibrating Ca(2+) biosensors in situ based on histamine receptor evoked responses.

Comparative Anterior Pituitary miRNA and mRNA Expression Profiles of Bama Minipigs and Landrace Pigs Reveal Potential Molecular Network Involved in Animal Postnatal Growth

The anterior pituitary is the most important endocrine organ modulating animal postnatal growth, mainly by controlling growth hormone (GH) gene transcription, synthesis, and secretion. As an ideal model for animal postnatal growth studies, the Bama minipig is characterized as having a lower growth performance and fewer individual differences compared with larger pig breeds. In this study, anterior pituitaries from Bama minipig and Landrace pig were used for miRNA and mRNA expression profile analysis using miRNA microarrays and mRNA-seq. Consequently, a total of 222 miRNAs and 12,909 transcripts were detected, and both miRNAs and mRNAs in the two breeds showed high correlation (r > 0.97). Additionally, 41 differentially expressed miRNAs and 2,254 transcripts were identified. Pathways analysis indicated that 32 pathways significantly differed in the two breeds. Importantly, two GH-regulation-signalling pathways, cAMP and inositol 1, 4, 5-triphosphate (IP3), and multiple GH-secretion-related transcripts were significantly down-regulated in Bama minipigs. Moreover, TargetScan and RNAHybrid algorithms were used for predicting differentially expressed miRNAs (DE miRNAs) and differentially expressed mRNAs (DE mRNAs) interaction. By examining their fold-changes, interestingly, most DE miRNA-DE mRNA target pairs (63.68-71.33%) presented negatively correlated expression pattern. A possible network among miRNAs, mRNAs, and GH-regulation pathways was also proposed. Among them, two miRNA-mRNA interactions (Y-47 targets FSHB; ssc-miR-133a-3p targets GNAI3) were validated by dual-luciferase assay. These data will be helpful in understanding the possible molecular mechanisms involved in animal postnatal growth.

Neurotoxic effect of triazophos on goldfish (Carassius auratus) and tissue specific antioxidant responses

Due to the high chemical and photochemical stability, an organophosphorus pesticide triazophos might enter aquatic ecosystems and impose negative effect on aquatic organisms. In order to investigate short-term toxicity of triazophos on goldfish (Carassius auratus), antioxidant response in brain, spleen, kidney and liver was tested in this study. As a confirmation, the impact of triazophos on acetyl cholinesterase (AChE) activity was found a reduction in all studied tissues, especially in brain. In addition, 0.1 and 0.5 mg L(-1) triazophos induced MDA level increased, while glutathione content (GSH), superoxide dismutase (SOD), catalase (CAT) and lactate dehydrogenase (LDH) activities decreased. Of note, more prominent oxidative stress was provoked in kidney and liver, but weaker in brain and spleen. These results revealed that triazophos could cause a generalized oxidative stress and tissue specific antioxidant response in goldfish. Furthermore, neuroendocrine-growth-related gene expression (growth hormone (GH), luteinizing hormone (LH) and peptide YY) in brain was also changed by exposed to triazophos during 4 and 7d exposure periods. Linked with the above results, the present study pointed out that triazophos might induce a neurotoxic effect and oxidative damage in goldfish, and the goldfish brain should be a critical target for triazophos-induced damage.

Ligand-selective signal transduction by two endogenous GnRH isoforms involves biased activation of the class I PI3K catalytic subunits p110β, p110γ, and p110δ in pituitary gonadotropes and somatotropes

In goldfish, 2 endogenous GnRH isoforms, GnRH2 and GnRH3, are released at the pituitary and directly stimulate LH and GH release using the same population of GnRH receptors (GnRHRs) but with GnRH-specific transduction mechanisms. Previously, we have shown that phosphoinositide 3-kinases (PI3Ks) mediate GnRH2- and GnRH3-stimulated LH and GH release. Among the 3 classes of PI3Ks, class I PI3Ks are the best characterized and consist of 4 110-kDa catalytic isoforms (p110α, p110β, p110γ, and p110δ). Importantly, p110β and p110γ, but not p110α or p110δ, can be directly activated by the Gβγ heterodimer of Gαβγ protein complexes. In the present study, we examined the expression of class I PI3K isoforms and the effects of selective inhibitors of p110α, p110β, p110γ, and p110δ catalytic activity on basal, as well as acute, GnRH2- and GnRH3-stimulated LH and GH release responses using primary cultures of dispersed goldfish pituitary cells in column perifusion. Results demonstrate that p110γ and p110δ are involved in the control of basal LH and GH release, whereas p110α and p110β only regulate basal LH secretion. However, p110β and p110γ both participated in GnRH3- and GnRH2-stimulated GH release, whereas p110β and p110γ mediated GnRH2- and GnRH3-induced LH release responses, respectively. GnRH2- and GnRH3-stimulated LH release, as well as GnRH3-elicited GH release, also required p110δ. These results constitute the first evidence for the differential involvement of class I PI3K catalytic subunits in GnRH actions, in general, and suggest that GnRH2 and GnRH3 binding to GnRHRs can bias the activation of class I PI3K signaling to mediate hormone release responses in 2 distinct pituitary cell types. The involvement of both class IA and IB PI3Ks implicates Gβγ subunits, as well as other known regulators of class I PI3Ks, as important components of GnRHR-mediated responses that could influence GnRH-selective signaling in other cell types.

Seasonal effects of GnIH on basal and GnRH-induced goldfish somatotrope functions

To understand how gonadotropin-inhibitory hormone (GnIH) regulates goldfish GH cell functions, we monitored GH release and expression during early, mid-, and/or late gonadal recrudescence. In vivo and in vitro responses to goldfish (g) GnIH were different, indicating direct action at the level of pituitary, as well as interactions with other neuroendocrine factors involved in GH regulation. Injection of gGnIH consistently reduced basal serum GH levels but elevated pituitary gh mRNA levels, indicating potential dissociation of GH release and synthesis. Goldfish GnRH (sGnRH and cGnRHII) injection differentially stimulated serum GH and pituitary gh mRNA levels with some seasonal differences; these responses were reduced by gGnIH. In contrast, in vitro application of gGnIH during 24-h static incubation of goldfish pituitary cells generally elevated basal GH release and attenuated sGnRH-induced changes in gh mRNA, while suppressing basal gh mRNA levels at mid- and late recrudescence but elevating them at early recrudescence. gGnIH attenuated the GH release responses to sGnRH during static incubation at early, but not at mid- and late recrudescence. In cell column perifusion experiments examining short-term GH release, gGnIH reduced the cGnRHII- and sGnRH-stimulated secretion at late recrudescence but inhibited tha action of cGnRHII only during mid-recrudescence. Interestingly, a reduction of basal GH release upon perifusion with gGnIH during late recrudescence was followed by a rebound increase in GH release upon gGnIH removal. These results indicate that gGnIH exerts complex effects on basal and GnRH-stimulated goldfish GH cell functions and can differentially affect GH release and mRNA expression in a seasonal reproductive manner.

Butyrate increases intracellular calcium levels and enhances growth hormone release from rat anterior pituitary cells via the G-protein-coupled receptors GPR41 and 43

Butyrate is a short-chain fatty acid (SCFA) closely related to the ketone body ß-hydroxybutyrate (BHB), which is considered to be the major energy substrate during prolonged exercise or starvation. During fasting, serum growth hormone (GH) rises concomitantly with the accumulation of BHB and butyrate. Interactions between GH, ketone bodies and SCFA during the metabolic adaptation to fasting have been poorly investigated to date. In this study, we examined the effect of butyrate, an endogenous agonist for the two G-protein-coupled receptors (GPCR), GPR41 and 43, on non-stimulated and GH-releasing hormone (GHRH)-stimulated hGH secretion. Furthermore, we investigated the potential role of GPR41 and 43 on the generation of butyrate-induced intracellular Ca²⁺ signal and its ultimate impact on hGH secretion. To study this, wt-hGH was transfected into a rat pituitary tumour cell line stably expressing the human GHRH receptor. Treatment with butyrate promoted hGH synthesis and improved basal and GHRH-induced hGH-secretion. By acting through GPR41 and 43, butyrate enhanced intracellular free cytosolic Ca²⁺. Gene-specific silencing of these receptors led to a partial inhibition of the butyrate-induced intracellular Ca²⁺ rise resulting in a decrease of hGH secretion. This study suggests that butyrate is a metabolic intermediary, which contributes to the secretion and, therefore, to the metabolic actions of GH during fasting.

Relationship between nitric oxide- and calcium-dependent signal transduction pathways in growth hormone release from dispersed goldfish pituitary cells

Nitric oxide (NO) and Ca(2+) are two of the many intracellular signal transduction pathways mediating the control of growth hormone (GH) secretion from somatotropes by neuroendocrine factors. We have previously shown that the NO donor sodium nitroprusside (SNP) elicits Ca(2+) signals in identified goldfish somatotropes. In this study, we examined the relationships between NO- and Ca(2+)-dependent signal transduction mechanisms in GH secretion from primary cultures of dispersed goldfish pituitary cells. Morphologically identified goldfish somatotropes stained positively for an NO-sensitive dye indicating they may be a source of NO production. In 2h static incubation experiments, GH release responses to the NO donor S-nitroso-N-acetyl-d,l-penicillamine (SNAP) were attenuated by CoCl2, nifedipine, verapamil, TMB-8, BHQ, and KN62. In column perifusion experiments, the ability of SNP to induce GH release was impaired in the presence of TMB-8, BHQ, caffeine, and thapsigargin, but not ryanodine. Caffeine-elicited GH secretion was not affected by the NO scavenger PTIO. These results suggest that NO-stimulated GH release is dependent on extracellular Ca(2+) availability and voltage-sensitive Ca(2+) channels, as well as intracellular Ca(2+) store(s) that possess BHQ- and/or thapsigargin-inhibited sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPases, as well as TMB-8- and/or caffeine-sensitive, but not ryanodine-sensitive, Ca(2+)-release channels. Calmodulin kinase-II also likely participates in NO-elicited GH secretion but caffeine-induced GH release is not upstream of NO production. These findings provide insights into how NO actions many integrate with Ca(2+)-dependent signalling mechanisms in goldfish somatotropes and how such interactions may participate in the GH-releasing actions of regulators that utilize both NO- and Ca(2+)-dependent transduction pathways.

PI3K signalling in GnRH actions on dispersed goldfish pituitary cells: relationship with PKC-mediated LH and GH release and regulation of long-term effects on secretion and total cellular hormone availability

Goldfish pituitary cells are exposed to two GnRHs, salmon (s)GnRH and chicken (c)GnRH-II. Phosphoinositide 3-kinase (PI3K) and protein kinase C (PKC) both participate in acute sGnRH- and cGnRH-II-stimulated LH and GH release. Using goldfish pituitary cells, we examined the relationship between PI3K and PKC in acute LH and GH secretion, and PI3K involvement in chronic hormone release and total LH and GH availability. The PI3K inhibitor LY294002 did not affect PKC agonists-induced LH or GH release, and PKC agonists did not alter PI3K p85 phosphorylation, suggesting PKC activation is not upstream of PI3K in acute hormone release. In 2, 6, 12 and 24h treatments, LY294002 did not affect LH release but stimulated total LH availability at 6h. sGnRH stimulatory actions on LH release and total availability at 12 and 24h, and cGnRH-II effects on these parameters at 6h were inhibited by LY294002. LY294002 enhanced basal GH release at 2 and 6h, but reduced total GH at 12 and 24h. Increased GH release was seen following 6, 12 and 24h of sGnRH, and 2, 6 and 24h of cGnRH-II treatment but total GH availability was only elevated by 24h cGnRH-II treatment. Whereas LY294002 inhibited GH release responses to sGnRH at 12h and cGnRH-II at 6h, it attenuated cGnRH-II-elicited, but not sGnRH-induced, effects on total GH. These results indicate that PI3K differentially modulates long-term basal and GnRH-stimulated hormone release, and total hormone availability, in a time-, cell-type-, and GnRH isoform-selective manner.

The aging suppressor klotho: a potential regulator of growth hormone secretion

Klotho is a transmembranal protein highly expressed in the kidneys, choroid plexus, and anterior pituitary. Klotho can also be cleaved and shed and acts as a circulating hormone. Klotho-deficient mice (kl/kl mice) develop a phenotype resembling early aging. Several lines of evidence suggest a role for klotho in the regulation of growth hormone (GH) secretion. The kl/kl mice are smaller compared with their wild-type counterparts, and their somatotropes show reduced numbers of secretory granules. Moreover, klotho is a potent inhibitor of the IGF-I pathway, a negative regulator of GH secretion. Therefore, we hypothesized that klotho may enhance GH secretion. The effect of klotho on GH secretion was examined in GH3 rat somatotrophs, cultured rat pituitaries, and cultured human GH-secreting adenomas. In all three models, klotho treatment increased GH secretion. Prolonged treatment of mice with intraperitoneal klotho injections increased mRNA levels of IGF-I and IGF-I-binding protein-3 mRNA in the liver, reflecting increased serum GH levels. In accord with its ability to inhibit the IGF-I pathway, klotho partially restored the inhibitory effect of IGF-I on GH secretion. Klotho is known to be a positive regulator of basic bFGF signaling. We studied rat pituitaries and human adenoma cultures and noted that bFGF increased GH secretion and stimulated ERK1/2 phosphorylation. Both effects were augmented following treatment with klotho. Taken together, our data indicate for the first time that klotho is a positive regulator of GH secretion and suggest the IGF-I and bFGF pathways as potential mediators of this effect.

Differential involvement of signaling pathways in the regulation of growth hormone release by somatostatin and growth hormone-releasing hormone in orange-spotted grouper (Epinephelus coioides)

Somatostatin is the most effective inhibitor of GH release, and GHRH was recently identified as one of the primary GH-releasing factors in teleosts. In this study, we analyzed the possible intracellular transduction pathways that are involved in the mechanisms induced by SRIF and GHRH to regulate GH release. Using a pharmacological approach, the blockade of the PLC/IP/PKC pathway reversed the SRIF-induced inhibition of GH release but did not affect the GHRH-induced stimulation of GH release. Furthermore, SRIF reduced the GH release induced by two PKC activators. Inhibitors of the AC/cAMP/PKA pathway reversed both the SRIF- and GHRH-induced effects on GH release. Moreover, the GH release evoked by forskolin and 8-Br-cAMP were completely abolished by SRIF. The blockade of the NOS/NO pathway attenuated the GHRH-induced GH release but had minimal effects on the inhibitory actions of SRIF. In addition, inhibitors of the sGC/cGMP pathway did not modify the SRIF- or GHRH-induced regulation of GH release. Taken together, these findings indicate that the SRIF-induced inhibition of GH release is mediated by both the PLC/IP/PKC and the AC/cAMP/PKA pathways and not by the NOS/NO/sGC/cGMP pathway. In contrast, the GHRH-induced stimulation of GH secretion is mediated by both the AC/cAMP/PKA and the NOS/NO pathways and is independent of the sGC/cGMP pathway and the PLC/IP/PKC system.

Mechanisms for luteinizing hormone induction of growth hormone gene transcription in fish model: crosstalk of the cAMP/PKA pathway with MAPK-and PI3K-dependent cascades

In our previous studies in grass carp pituitary cells, local production of luteinizing hormone (LH) was shown to induce growth hormone (GH) production and gene expression, which constitutes a major component of the "intrapituitary feedback loop" regulating GH secretion and synthesis via autocrine/paracrine interactions between gonadotrophs and somatotrophs in the carp pituitary. To further investigate the signaling mechanisms mediating LH action at the transcriptional level, promoter studies were performed in GH3 cells co-transfected with the expression vector for carp LH receptor and luciferase-expressing reporter constructs with grass carp GH promoter. In this cell model, treatment with human chorionic gonadotropin (hCG) was effective in increasing GH promoter activity and the responsive sequence was mapped to position -616 and -572 of the grass carp GH promoter. GH promoter activation induced by hCG occurred with concurrent rise in cAMP production, CREB phosphorylation, and could be inhibited by inactivation of adenylate cyclase (AC), PKA, MEK1/2, P(38) MAPK, PI3K and mTOR. AC activation, presumably via cAMP production, could mimic hCG-induced CREB phosphorylation and GH promoter activity, and these stimulatory effects were also sensitive to the blockade of PKA-, MAPK- and PI3K- dependent cascades. These results, as a whole, suggest that LH receptor activation in the carp pituitary may trigger GH gene transcription through CREB phosphorylation as a result of the functional crosstalk of the cAMP/PKA pathway with MAPK-and PI3K-dependent cascades.

Upregulation of voltage-gated calcium channel cav1.3 in bovine somatotropes treated with ghrelin

Activation of the growth hormone (GH) secretagogue receptor (GHS-R) by synthetic GH releasing peptides (GHRP) or its endogenous ligand (Ghrelin) stimulates GH release. Though much is known about the signal transduction underlying short-term regulation, there is far less information on the mechanisms that produce long-term effects. In the current report, using an enzyme-linked immunosorbent assay for GH detection and whole-cell patch-clamp recordings, we assessed the long-term actions of such regulatory factors on voltage-activated Ca²⁺ currents in bovine somatotropes (BS) separated on a Percoll gradient and detected by immunohistochemistry. After 24 h of treatment with Ghrelin (10 nM) or GHRP-6 (100 nM) enhanced BS secretory activity; GH secretion stimulated by GHS through the activation of GHS-R because treatment with the antagonist of GHS-R (D-Lys3-GHRP-6, 10 μM) blocked the GH secretion, and the effect was dose and time dependent (24, 48, and 72 h). GH secretion stimulated by GHRP-6 was abolished by nifedipine (0.5 μM), a blocker of L-type HVA Ca²⁺ channels, and KN-62 (10 μM), an inhibitor of Ca²⁺/CaM-KII. After 72 h in culture, all recorded BS exhibited two main Ca²⁺ currents: a low voltage-activated (LVA; T-type) and a high voltage-activated (HVA; mostly dihydropyridine-sensitive L-type) current. Interestingly, HVA and LVA channels were differentially upregulated by Ghrelin. Chronic treatment with the GHS induced a significant selective increase on the Ba²⁺ current through HVA Ca²⁺ channels, and caused only a small increase of currents through LVA channels. The stimulatory effect on HVA current density was accompanied by an augment in maximal conductance with no apparent changes in the kinetics and the voltage dependence of the Ca²⁺ currents, suggesting an increase in the number of functional channels in the cell membrane. Lastly, in consistency with the functional data, quantitative real-time RT-PCR revealed transcripts encoding for the Ca_v1.2 and Ca_v1.3 pore-forming subunits of L-type channels. The treatment with Ghrelin significantly increased the Ca_v1.3 subunit expression, suggeting that the chronic stimulation of the GHS receptor with Ghrelin or GHRP-6 increases the number of voltage-gated Ca²⁺ channels at the cell surface of BS.

MEK1/2 differentially participates in GnRH actions on goldfish LH and GH secretion and hormone protein availability: acute and long-term effects, in vitro

Two endogenous gonadotropin-releasing hormones (GnRHs), sGnRH and cGnRH-II, stimulate LH and GH release via protein kinase C (PKC) signaling in goldfish. In this study, extracellular signal-regulated kinase kinase 1 and 2 (MEK1/2) involvement in acute and prolonged GnRH effects on goldfish gonadotrope and somatotrope functions, as well as potential interactions with PKC in the control of LH and GH release from goldfish pituitary cells was investigated. MEK1/2 inhibitors U0126 and PD098059 significantly decreased sGnRH but not cGnRH-II-stimulated GH release from perifused goldfish pituitary cells and U0126 significantly reduced the GH, but not the LH, release responses to synthetic PKC activators. In long-term static incubations (up to 24h) with goldfish pituitary cells, U0126 generally did not affect basal LH release but attenuated sGnRH- and cGnRH-II-induced LH release, as well as the time-dependent effects of sGnRH and/or cGnRH-II to elevate total LH availability (sum of release and cell content). sGnRH and cGnRH-II reduced cellular GH content and/or total GH availability at 2, 6, and 12h while static incubation with U0126 alone generally increased basal GH release but reduced cellular GH content and/or the total amount of GH available. U0126 also selectively reduced the sGnRH-induced GH release responses at 6 and 24h but paradoxically inhibited cGnRH-II-stimulated GH secretion while enhancing sGnRH-elicited GH release at 2h. Taken together, this study reveals the complexity of GnRH-stimulated MEK1/2 signaling and adds to our understanding of cell-type- and GnRH-isoform-selective signal transduction in the regulation of pituitary cell hormone release and production.

Differential modulation of ghrelin-induced GH and LH release by PACAP and dopamine in goldfish pituitary cells

Ghrelin (GRLN) participates in multiple physiological processes, including the regulation of growth hormone (GH) and luteinizing hormone (LH) release. In the goldfish, neuroendocrine control of GH and LH release are multifactorial. In this system, pituitary adenylate cyclase-activating polypeptide (PACAP)-stimulated GH and LH secretion, as well as dopamine (DA)-induced GH release, are mediated by protein kinase A (PKA)-dependent, but protein kinase C (PKC)-independent, mechanisms. In addition, DA inhibits LH secretion by actions at sites along both PKA and PKC signaling pathways. Recently, goldfish GRLN (gGRLN19) has been shown to induce GH release via PKC, and LH secretion via both PKC and PKA. To further understand the neuroendocrine regulation of goldfish GH and LH release, we examined the effects of DA and PACAP on gGRLN19 actions in primary cultures of goldfish pituitary cells in perifusion and in Ca(2+)-imaging experiments. Consistent with their known intracellular signaling mechanisms in gonadotrophs, DA inhibited gGRLN19-induced LH release while cotreatment of PACAP and gGRLN19 did not produce additive LH responses. When applied prior to gGRLN19, PACAP potentiated gGRLN19-induced GH release and Ca(2+) signals within somatotrophs. In contrast, neither prior treatment with DA followed by gGRLN19 nor pretreatment with gGRLN19 prior to PACAP produced an enhanced GH release response. These observations suggest that PKA activators positively modulate gGRLN19 actions on goldfish somatotrophs in a ligand- and treatment order-specific manner. Results add to our understanding of the complexity of neuroendocrine control of GH and LH release at the pituitary cell level, and our understanding of GRLN action.

Growth hormone-releasing hormone stimulates GH release while inhibiting ghrelin- and sGnRH-induced LH release from goldfish pituitary cells

Goldfish GH-releasing hormone (gGHRH) has been recently identified and shown to stimulate GH release in goldfish. In goldfish, neuroendocrine regulation of GH release is multifactorial and known stimulators include goldfish ghrelin (gGRLN19) and salmon gonadotropin-releasing hormone (sGnRH), factors that also enhance LH secretion. To further understand the complex regulation of pituitary hormone release in goldfish, we examined the interactions between gGHRH, gGRLN19, and sGnRH on GH and LH release from primary cultures of goldfish pituitary cells in perifusion. Treatment with 100nM gGHRH for 55min stimulated GH release. A 5-min pulse of either 1nM gGRLN19 or 100nM sGnRH induced GH release in naïve cells, and these were just as effective in cells receiving gGHRH. Interestingly, gGHRH abolished both gGRLN19- and sGnRH-induced LH release and reduced basal LH secretion levels. These results suggest that gGHRH does not interfere with sGnRH or gGRLN19 actions in the goldfish somatotropes and further reveal, for the first time, that GHRH may act as an inhibitor of stimulated and basal LH release by actions at the level of pituitary cells.

Nitric oxide signaling in ghrelin-induced LH release from goldfish pituitary cells

Among its many known functions, ghrelin has been proposed to participate in the regulation of reproduction; however, its effect on pituitary LH release is controversial, especially in mammals. In the goldfish, ghrelin directly stimulates pituitary LH release via increased entry of calcium through voltage sensitive channels and activation of protein kinase C. Nitric oxide (NO) is an important signaling molecule in many physiological systems including hormone regulation at the level of the pituitary. Goldfish pituitary cells and extracts have previously been reported to express immunoreactivity for inducible and neuronal NO synthase (iNOS and nNOS). In this study, we determined if NO is involved in goldfish ghrelin (gGRLN(19))-induced LH release from primary cultures of dispersed goldfish pituitary cells in column perifusion. Treatment with the NO scavenger PTIO significantly decreased gGRLN(19)-induced LH release and co-treatment with the NO donor SNP and gGRLN(19) did not induce an additive increase in LH release, suggesting that NO is critical to gGRLN(19) stimulation of LH release in goldfish pituitary cells. Further work examined the involvement of the NOS using the NOS isoform-selective inhibitors 1400W, 7-Ni, and AGH. While 1400W (selective for iNOS) and AGH (selective for iNOS and nNOS) abolished gGRLN(19)-induced LH release from goldfish pituitary cells, 7-Ni (selective for nNOS and endothelial NOS) had no significant effect on this stimulation. Our results indicate, for the first time in a teleost species, that gGRLN(19)-induced LH release from pituitary cells is NO-dependent and likely involves iNOS, adding to the understanding of GRLN intracellular signaling in general and specifically to the regulation of LH release from the pituitary.

Somatostatin receptor pathophysiology in the neuroendocrine system

The actions of somatostatin (SRIF) are mediated by specific G protein-coupled receptors, named SRIF receptor (SSTR) subtypes 1, 2, 3 and 5. SRIF binding to SSTR activates a series of second messenger systems, resulting in the inhibition of calcium channels and adenylate cyclase activity, ultimately leading to inhibition of hormone secretion, while stimulation of other second messengers, such as phosphotyrosine phosphatases play a role in the control of cell growth. The SSTR and dopamine receptor families share a 30% sequence homology and appear to be structurally related. The knowledge on the pathophysiology of these two families of G protein-coupled receptors in neuroendocrine tumors has progressively increased due to the new insights in receptor dimerization, internalization and trafficking. Depending on the expression of different SSTRs in tissues, their combinations and interactions affect the functionality of the subtypes expressed and the influence of the microenvironment, the response to ligands and, by consequence, the response to treatment can be very different.

Signal transduction mechanisms for autocrine/paracrine regulation of somatolactin-α secretion and synthesis in carp pituitary cells by somatolactin-α and -β

Pituitary hormones can act locally via autocrine/paracrine mechanisms to modulate pituitary functions, which represents an interesting aspect of pituitary regulation other than the traditional hypothalamic input and feedback signals from the periphery. Somatolactin, a member of the growth hormone (GH)/prolactin (PL) family, is a pleiotropic hormone with diverse functions, but its pituitary actions are still unknown. Recently, two SL isoforms, SLα and SLβ, have been cloned in grass carp. Based on the sequences obtained, recombinant proteins of carp SLα and SLβ with similar bioactivity in inducing pigment aggregation in carp melanophores were produced. In carp pituitary cells, SLα secretion and cell content were elevated by static incubation with recombinant carp SLα and SLβ, respectively. These stimulatory actions occurred with a parallel rise in SLα mRNA level with no changes in SLβ secretion, cell content, and gene expression. In contrast, SLα mRNA expression could be reduced by removing endogenous SLα and SLβ with immunoneutralization. At the pituitary cell level, SLα release, cell content, and mRNA expression induced by carp SLα and SLβ could be blocked by inhibiting JAK2/STAT5, PI3K/Akt, MEK1/2, and p38 MAPK, respectively. Furthermore, SLα and SLβ induction also triggered rapid phosphorylation of STAT5, Akt, MEK1/2, ERK1/2, MKK3/6, and p38 MAPK. These results suggest that 1) SLα and SLβ produced locally in the carp pituitary can serve as novel autocrine/paracrine stimulators for SLα secretion and synthesis and 2) SLα production induced by local release of SLα and SLβ probably are mediated by the JAK2/STAT5, PI3K/Akt, and MAPK signaling pathways.

Ghrelin-induced growth hormone release from goldfish pituitary cells is nitric oxide dependent

Ghrelin (GRLN) is an important neuroendocrine regulator of growth hormone (GH) release in vertebrates. Previous studies show goldfish (g)GRLN(19)-induced GH from the goldfish pituitary involves voltage sensitive Ca(2+) channels, increases in intracellular Ca(2+) and the PKC signalling pathway. We set out to examine the role of the nitric oxide (NO) pathway in gGLRN(19)-induced GH release from primary cultures of goldfish pituitary cells using pharmacological regulators in cell column perifusion systems. The NO scavenger PTIO abolished gGRLN(19)-induced GH release and co-treatment with the NO donor SNP and GRLN did not produce additive GH release responses. Nitric oxide synthase (NOS) inhibitors 1400 W and 7-Ni abolished GRLN-induced GH release while treatment with another NOS inhibitor, AGH, had no significant effect. Taken together, these results demonstrate that the NOS/NO is an integral component of gGRLN(19)-induced signalling within the goldfish pituitary cells, and given the relative specificity of AGH for inducible NOS and endothelial NOS isoforms, suggests that neuronal NOS is the likely NOS isoform utilized in goldfish somatotropes by this physiological regulator.

Kisspeptin-1 directly stimulates LH and GH secretion from goldfish pituitary cells in a Ca(2+)-dependent manner

It has been established that kisspeptin regulates reproduction via stimulation of hypothalamic gonadotropin-releasing hormone (GnRH) secretion, which then induces pituitary luteinizing hormone (LH) release. Kisspeptin also directly stimulates pituitary hormone release in some mammals. However, in goldfish, whether kisspeptin directly affects pituitary hormone release is controversial. In this study, synthetic goldfish kisspeptin-1((1-10)) (gKiss1) enhances LH and growth hormone (GH) release from primary cultures of goldfish pituitary cells in column perifusion. gKiss1 stimulation of LH and GH secretion were still manifested in the presence of the two native goldfish GnRHs, salmon (s)GnRH (goldfish GnRH-3) and chicken (c)GnRH-II (goldfish GnRH-2), but were attenuated by two voltage-sensitive calcium channel blockers, verapamil and nifedipine. gKiss-induced increases in intracellular Ca(2+) in Fura-2AM pre-loaded goldfish pars distalis cells were also inhibited by nifedipine. These results indicate that, in goldfish, (1) direct gKiss1 actions on pituitary LH and GH secretion exist, (2) these actions are independent of GnRH and (3) they involve Ca(2+) signalling.

Dependence of the excitability of pituitary cells on cyclic nucleotides

Cyclic 3',5'-adenosine monophosphate and cyclic 3',5'-guanosine monophosphate are intracellular (second) messengers that are produced from the nucleotide triphosphates by a family of enzymes consisting of adenylyl and guanylyl cyclases. These enzymes are involved in a broad array of signal transduction pathways mediated by the cyclic nucleotide monophosphates and their kinases, which control multiple aspects of cell function through the phosphorylation of protein substrates. We review the findings and working hypotheses on the role of the cyclic nucleotides and their kinases in the control of electrical activity of the endocrine pituitary cells and the plasma membrane channels involved in this process.

Neuroendocrine signalling: natural variations on a Ca2+ theme

This special issue on Ca(2+) signalling in neuroendocrine cells is an opportunity to assess, through a range of first-class review articles, the complex world of endocrine signalling, a complexity that is probably best captured by calling it "diversity in unity". The unity comes from the fact that all the endocrine cells are excitable cells, able to generate action potentials and are using Ca(2+) as an essential informational molecule, coupling cell stimulation with the activation of secretion, through the exocytotic process. The 'diversity' element, illustrated by almost all the reviews, stems from the modalities employed to achieve the increase in cytosolic Ca(2+) signal, the balance between the participation of Ca(2+) entry through the plasma membrane voltage-operated Ca(2+) channels and the release of Ca(2+) from intracellular Ca(2+) stores, and the cross-talk between the Ca(2+) and cyclic AMP signalling pathways.