Two endogenous gonadotropin-releasing hormones generate dissimilar Ca(2+) signals in identified goldfish gonadotropes

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.

Identified lhb-expressing cells from medaka (Oryzias latipes) show similar Ca(2+)-response to all endogenous Gnrh forms, and reveal expression of a novel fourth Gnrh receptor

We have previously characterized the response to gonadotropin-releasing hormone (Gnrh) 2 in luteinizing hormone (lhb)-expressing cells from green fluorescent protein (Gfp)-transgenic medaka (Oryzias latipes), with regard to changes in the cytosolic Ca(2+) concentration. In the current study we present the corresponding responses to Gnrh1 and Gnrh3. Ca(2+) imaging revealed three response patterns to Gnrh1 and Gnrh3, one monophasic and two types of biphasic patterns. There were few significant differences in the shape of the response patterns between the three Gnrh forms, although the amplitude of the Ca(2+) signal was considerably lower for Gnrh1 and Gnrh3 than for Gnrh2, and the distribution between the two different biphasic patterns differed. The different putative Ca(2+) sources were examined by depleting intracellular Ca(2+) stores with thapsigargin, or preventing influx of extracellular Ca(2+) by either extracellular Ca(2+) depletion or the L-type Ca(2+)-channel blocker verapamil. Both Gnrh1 and 3 relied on Ca(2+) from both intracellular and extracellular sources, with some unexpected differences in the relative contribution. Furthermore, gene expression of Gnrh-receptors (gnrhr) in whole pituitaries was studied during development from juvenile to adult. Only two of the four identified medaka receptors were expressed in the pituitary, gnrhr1b and gnrhr2a, with the newly discovered gnrhr2a showing the highest expression level at all stages as analyzed by quantitative PCR. While both receptors differed in expression level according to developmental stage, only the expression of gnrhr2a showed a clear-cut increase with gonadal maturation. RNA sequencing analysis of FACS-sorted Gfp-positive lhb-cells revealed that both gnrhr1b and gnrhr2a were expressed in lhb-expressing cells, and confirmed the higher expression of gnrhr2a compared to gnrhr1b. These results show that although lhb-expressing gonadotropes in medaka show similar Ca(2+) response patterns to all three endogenous Gnrh forms through the activation of two different receptors, gnrhr1b and gnrhr2a, the differences observed between the Gnrh forms indicate activation of different Ca(2+) signaling pathways.

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.

Signal transduction involved in GnRH2-stimulation of identified LH-producing gonadotropes from lhb-GFP transgenic medaka (Oryzias latipes)

We have characterized the response to gonadotropin-releasing hormone 2 (GnRH2) in luteinizing hormone producing cells from gfp-transgenic medaka. Teleosts have separate cells producing the two types of gonadotropins, enabling us for the first time to study the intracellular signaling that controls secretion of each gonadotropin separately. Pituitary cell cultures were prepared, and lhb-producing cells were selected by their GFP expression. Cytosolic Ca(2+) imaging revealed three response patterns to GnRH2, one monophasic and two types of biphasic patterns. The Ca(2+) sources were examined by depleting intracellular Ca(2+) stores and preventing influx of extracellular Ca(2+). Both treatments reduced response amplitude, and affected latency and time to peak. Blocking L-type Ca(2+) channels reduced amplitude and time to peak, but did not remove extracellular Ca(2+) contribution. Patch-clamp recordings showed spontaneous action potentials in several cells, and GnRH2 increased the firing frequency. Presence of Ca(2+)-activated K(+) channels was revealed, BK channels being the most prominent.

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 and guanylate cyclase signalling are differentially involved in gonadotrophin (LH) release responses to two endogenous GnRHs from goldfish pituitary cells

Nitric oxide synthase (NOS) immunoreactivity is present in goldfish gonadotrophs. The present study investigated whether two native goldfish gonadotrophin-releasing hormones (GnRHs), salmon (s)GnRH and chicken (c)GnRH-II, use NOS/nitric oxide (NO) and soluble guanylate cyclase (sGC)/cyclic (c)GMP/protein kinase G (PKG) signalling to stimulate maturational gonadotrophin [teleost gonadotrophin-II, luteinising hormone (LH)] release. In cell column perifusion experiments with dispersed goldfish pituitary cells, the application of three NOS inhibitors (aminoguanidine hemisulphate, 1400W and 7-nitroindazole) and two NO scavengers [2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) and rutin hydrate] reduced sGnRH-elicited, but not cGnRH-II-induced, LH increases. The NO donor sodium nitroprusside (SNP) increased NO production in goldfish pituitary cells in static incubation. SNP-stimulated LH release in column perifusion was attenuated by PTIO and the sGC inhibitor 1H-(1,2,4)oxadiazolo[4,3-a]quinoxalin-1-oneon (ODQ), and additive to responses elicited by cGnRH-II, but not sGnRH. ODQ and the PKG inhibitor KT5823 decreased sGnRH- and cGnRH-II-stimulated LH release. Similarly, the LH response to dibutyryl cGMP was reduced by KT5823. These results indicate that, although only sGnRH uses the NOS/NO pathway to stimulate LH release, both GnRHs utilise sGC/PKG to increase LH secretion.

Differential involvement of protein kinase C and protein kinase A in ghrelin-induced growth hormone and gonadotrophin release from goldfish (Carassius auratus) pituitary cells

Ghrelin (GRLN) and its receptor have been identified and characterised in goldfish brain and the pituitary, and recent evidence shows that goldfish (g)GRLN(19) induces both growth hormone (GH) and maturational gonadotrophin (LH) release through an extracellular Ca(2+) -dependent mechanism in goldfish. To further understand the role of GRLN in hormone release, the present study examined the involvement of protein kinase C (PKC) and protein kinase A (PKA) in gGRLN(19) -induced GH and LH release and corresponding Ca(2+) signals in primary cultures of goldfish pituitary cells. Treatments with PKC inhibitors, Bis-II and Gö 6976, significantly reduced gGRLN(19) -induced GH and LH release and their corresponding intracellular Ca(2+) signals in identified somatotrophs and gonadotrophs, respectively. gGRLN(19) was unable to further stimulate hormone release or Ca(2+) signals when cells were pretreated with the PKC agonist, DiC8. PKA inhibitors, H-89 and KT 5720, inhibited gGRLN(19) -induced LH release and Ca(2+) signals in gonadotrophs but not GH release or Ca(2+) signals in somatotrophs. Interestingly, pretreatment of pituitary cells with the adenylate cyclase activator forskolin potentiated gGRLN(19) -induced GH, but not LH, release, although it had no effect on intracellular Ca(2+) signals in either cell type. Taken together, the results suggest that PKC is an important intracellular component in gGRLN(19) -induced GH and LH release, whereas PKA is involved in gGRLN(19) -elicited LH release. Furthermore, the PKA pathway potentiates gGRLN(19) -induced GH release via a Ca(2+) -independent mechanism. Overall, the present study provides insight into the neuroendocrine regulation of GH and LH release by elucidating the mechanistic aspects of GRLN, a hormone involved in many critical physiological processes, including pituitary functions.

Differential involvement of phosphoinositide 3-kinase in gonadotrophin-releasing hormone actions in gonadotrophs and somatotrophs of goldfish, Carassius auratus

In goldfish, two endogenous gonadotrophin-releasing hormones (GnRHs) [salmon (s)GnRH and chicken (c)GnRH-II] control maturational gonadotrophin-II [lutenising hormone (LH)] and growth hormone (GH) secretion via Ca(2+)-dependent intracellular signalling pathways. We investigated the involvement of phosphoinositide 3-kinase (PI3K) in GnRH-evoked LH and GH release and associated intracellular Ca(2+) increases ([Ca(2+)](i) ) in goldfish gonadotrophs and somatotrophs. Immunoreactive PI3K p85α, the predominant regulatory subunit for class IA PI3Ks, was detected in goldfish pituitary tissue extracts and both endogenous GnRH isoforms increased phosphorylation of PI3K p85α in excised pituitary fragments. sGnRH- and cGnRH-II-elicited LH release responses from primary cultures of pituitary cells and [Ca(2+)](i) increases in identified gonadotrophs were significantly reduced in the presence of PI3K inhibitors wortmannin (100 nm) and LY294002 (10 μm). Unexpectedly, wortmannin and LY294002 inhibited GnRH-evoked GH release but only attenuated the [Ca(2+)](i) response in identified somatotrophs to cGnRH-II, and not sGnRH. On the other hand, Ca(2+) ionophore-evoked LH and GH secretion remained unaltered in the presence of the PI3K inhibitors, suggesting that general decreases in the releasable hormone pool or sensitivity to [Ca(2+)](i) changes did not underlie the ability of wortmannin and LY294002 to reduce the actions of GnRH. These results provide the first evidence for the presence and involvement of PI3K in GnRH-induced LH and GH release in any primary pituitary cell system. In gonadotrophs, the inhibitory action of PI3K on both sGnRH and cGnRH-II involves the attenuation of their evoked [Ca(2+)](i); in contrast, GnRH isoform-specific effects occur in somatotrophs.

Characterization of ionic currents and electrophysiological properties of goldfish somatotropes in primary culture

Growth hormone release in goldfish is partly dependent on voltage-sensitive Ca(2+) channels but somatotrope electrophysiological events affecting such channel activities have not been elucidated in this system. The electrophysiological properties of goldfish somatotropes in primary culture were studied using the whole-cell and amphotericin B-perforated patch-clamp techniques. Intracellular Ca(2+) concentration ([Ca(2+)]i) of identified somatotropes was measured using Fura-2/AM dye. Goldfish somatotropes had an average resting membrane potential of -78.4 ± 4.6 mV and membrane input resistance of 6.2 ± 0.2 GΩ. Voltage steps from a holding potential of -90 mV elicited a non-inactivating outward current and transient inward currents at potentials more positive than 0 and -30 mV, respectively. Isolated current recordings indicate the presence of 4-aminopyridine- and tetraethylammonium (TEA)-sensitive K(+), tetrodotoxin (TTX)-sensitive Na(+), and nifedipine (L-type)- and ω-conotoxin GVIA (N-type)-sensitive Ca(2+) channels. Goldfish somatotropes rarely fire action potentials (APs) spontaneously, but single APs can be induced at the start of a depolarizing current step; this single AP was abolished by TTX and significantly reduced by nifedipine and ω-conotoxin GVIA. TEA increased AP duration and triggered repetitive AP firing resulting in an increase in [Ca(2+)]i, whereas TTX, nifedipine and ω-conotoxin GVIA inhibited TEA-induced [Ca(2+)]i pulses. These results indicate that in goldfish somatotropes, TEA-sensitive K(+) channels regulate excitability while TTX-sensitive Na(+) channels together with N- and L-type Ca channels mediates the depolarization phase of APs. Opening of voltage-sensitive Ca(2+) channels during AP firing leads to increases in [Ca(2+)]i.

Secretoneurin is a potential paracrine factor from lactotrophs stimulating gonadotropin release in the goldfish pituitary

Secretoneurin (SN) is a functional neuropeptide derived from the evolutionarily conserved part of precursor protein secretogranin II (SgII). In the time course study, SN (10 nM) stimulates luteinizing hormone (LH) production and secretion after 6 h of static incubation of goldfish pituitary cells. Due to the existence of SN-immunoreactivity (SN-IR) in goldfish lactotrophs, endogenous SN might exert a paracrine effect on LH in the pituitary. In an in vitro immunoneutralization experiment, coincubation with anti-SN antiserum reduces the stimulatory effect of salmon gonadotropin-releasing hormone (sGnRH) on LH release by 64%. Using Western blot analysis, we demonstrate that sGnRH significantly increases the expression of the major SgII-derived peptide (∼57 kDa, with SN-IR) and prolactin (PRL) after 12 h in the static culture of goldfish pituitary cells. Furthermore, there exists a significant correlation between the levels of these two proteins (R = 0.76, P = 0.004). Another ∼30 kDa SgII-derived peptide containing SN is only observed in sGnRH-treated pituitary cells. Consistent with the Western blot analysis results, real-time RT-PCR analysis shows that a 12-h treatment with sGnRH induced 1.6- and 1.7-fold increments in SgII and PRL mRNA levels, respectively. SgII gene expression was also associated with PRL gene expression (R = 0.66; P = 0.02). PRL cells loaded with the calcium-sensitive dye, fura 2/AM, respond to sGnRH treatment with increases in intracellular Ca(2+) concentration level, suggesting a potential mechanism of GnRH on PRL cells and thus SgII processing and SN secretion. Taken together, endogenous lactotroph-generated SN, under the control of hypothalamic GnRH, exerts a paracrine action on neighboring gonadotrophs to stimulate LH release.

Neuroendocrinology of reproduction in teleost fish

This review aims at synthesizing the most relevant information regarding the neuroendocrine circuits controlling reproduction, mainly gonadotropin release, in teleost fish. In teleosts, the pituitary receives a more or less direct innervation by neurons sending projections to the vicinity of the pituitary gonadotrophs. Among the neurotransmitters and neuropeptides released by these nerve endings are gonadotrophin-releasing hormones (GnRH) and dopamine, acting as stimulatory and inhibitory factors (in many but not all fish) on the liberation of LH and to a lesser extent that of FSH. The activity of the corresponding neurons depends on a complex interplay between external and internal factors that will ultimately influence the triggering of puberty and sexual maturation. Among these factors are sex steroids and other peripheral hormones and growth factors, but little is known regarding their targets. However, very recently a new actor has entered the field of reproductive physiology. KiSS1, first known as a tumor suppressor called metastin, and its receptor GPR54, are now central to the regulation of GnRH, and consequently LH and FSH secretion in mammals. The KiSS system is notably viewed as instrumental in integrating both environmental cues and metabolic signals and passing this information onto the reproductive axis. In fish, there are two KiSS genes, KiSS1 and KiSS2, expressed in neurons of the preoptic area and mediobasal hypothalamus. Pionneer studies indicate that KiSS and GPR54 expression seem to be activated at puberty. Although precise information as to the physiological effects of KiSS1 in fish, notably on GnRH neurons and gonadotropin release, is still limited, KiSS neurons may emerge as the "gatekeeper" of puberty and reproduction in fish as in mammals.

The secretogranin II-derived peptide secretoneurin stimulates luteinizing hormone secretion from gonadotrophs

Secretoneurin (SN) is a 33- to 34-amino acid neuropeptide derived from secretogranin-II, a member of the chromogranin family. We previously synthesized a putative goldfish (gf) SN and demonstrated its ability to stimulate LH release in vivo. However, it was not known whether goldfish actually produced the free SN peptide or whether SN directly stimulates LH release from isolated pituitary cells. Using a combination of reverse-phase HPLC and mass spectrometry analysis, we isolated for the first time a 34-amino acid free gfSN peptide from the whole brain. Moreover, Western blot analysis indicated the existence of this peptide in goldfish pituitary. Immunocytochemical localization studies revealed the presence of SN immunoreactivity in prolactin cells of rostral pars distalis of the anterior pituitary. Additionally, we found that magnocellular cells of the goldfish preoptic region are highly immunoreactive for SN. These neurons send heavily labeled projections that pass through the pituitary stalk and innervate the neurointermediate and anterior lobes. In static 12-h incubation of dispersed pituitary cells, application of SN antiserum reduced LH levels, whereas 1 and 10 nM gfSN, respectively, induced 2.5-fold (P < 0.001) and 1.9-fold (P < 0.01) increments of LH release into the medium, increases similar to those elicited by 100 nM concentrations of GnRH. Like GnRH, gfSN elevated intracellular Ca(2+) in identified gonadotrophs. Whereas we do not yet know the relative contribution of neural SN or pituitary SN to LH release, we propose that SN could act as a neuroendocrine and/or paracrine factor to regulate LH release from the anterior pituitary.

Signal transduction in multifactorial neuroendocrine control of gonadotropin secretion and synthesis in teleosts-studies on the goldfish model

In teleosts, gonadotropin (GTH) secretion and synthesis is controlled by multiple neuroendocrine factors from the hypothalamus, pituitary and peripheral sources. Pituitary gonadotropes must be able to differentiate and integrate information from these regulators at the cellular and intracellular level. In this article, the intracellular signal transduction mechanisms mediating the actions of some of these regulators, including GTH-releasing hormones, pituitary adenylate cyclase-activating polypeptide, dopamine, ghrelin, sex steroids, activin, and follistatin from experiments with goldfish are reviewed and discussed in relation with recent findings. Information from other teleost models is briefly compared. Goldfish gonadotropes possess multiple pharmacologically distinct intracellular Ca2+ stores that together with voltage-sensitive Ca2+ channels, Na+/H+ exchangers, protein kinase C, arachidonic acid, NO, protein kinase A, ERK/MAPK, and Smads allows for integrated control by different neuroendocrine factors.

Ghrelin-induced growth hormone release from goldfish pituitary cells involves voltage-sensitive calcium channels

Ghrelin (GRL) is a stimulator of growth hormone (GH) release in many organisms, including goldfish. As a first study to examine the signalling mechanisms mediating GRL action on GH release in goldfish, we tested the hypothesis that GLR induces GH release from goldfish pituitary cells by enhancing Ca(2+) entry through L-type voltage-sensitive Ca(2+) channels (LVSCCs) using perifusion GH release and fura-2/AM Ca(2+)-imaging experiments. Goldfish (g)GRL(19) at 1 nM elicited reversible and repeatable GH responses from dispersed goldfish mixed pituitary cultures. However, the lack of a dose-response relationship in sequential treatments with decreasing concentrations of gGRL(19) (ranging from 10 to 0.01 nM) implicated rapid desensitization of the GH response. Sequential applications of gGRL(19) (1 nM) and salmon GnRH (100 nM), a known Ca(2+)-dependent stimulator of GH release, increased intracellular free Ca(2+) levels ([Ca(2+)](i)) from the same identified somatotropes, suggesting co-expression of GRL and GnRH receptors on single cells. In contrast, 1 nM gGRL(19) failed to elicit GH release and elevation in [Ca(2+)](i) when the cells are incubated with nominally Ca(2+)-free media. When GH release and [Ca(2+)](i) increases were already stimulated by the LVSCC agonist Bay K8644 (10 microM), addition of 1 nM gGRL(19) did not further elevate these responses. Finally, the LVSCC inhibitors nifedipine (1 microM) and verapamil (1 microM) abolished 1nM gGRL(19)-induced GH release responses while nifedipine eliminated gGRL(19)-induced [Ca(2+)](i) increase. Taken together, the results of this study provide evidence that entry of extracellular Ca(2+) through LVSCCs is a key component of the GRL signalling pathway leading to GH release in the goldfish pituitary.

Ionotropic glutamate receptor activation increases intracellular calcium in prolactin-releasing cells of the adenohypophysis

Endocrine cells of the anterior pituitary are controlled by the central nervous system through hormonal interactions and are not believed to receive direct synaptic connections from the brain. Studies suggest that some pituitary cells may be modulated by the neurotransmitter glutamate. We investigated prolactin (PRL)-releasing cells of the anterior pituitary of a euryhaline fish, the tilapia (Oreochromis mossambicus), for the presence of possible glutamate receptors (GluRs). Fura-2 imaging addressed the ability of glutamate to increase intracellular calcium. We observed a dose-dependent increase in intracellular calcium with transient perfusion (1-2 min) of glutamate (10 nM to 1 mM) in two-thirds of imaged cells. This increase was attenuated by the ionotropic GluR antagonist kynurenic acid (0.5-1.0 mM). The increase was also blocked or attenuated by antagonists of L-type voltage-gated calcium channels. The GluR agonist alpha-amino-3-hydroxy-5-methylisoxazole propionic acid (AMPA; 100 microM) produced intracellular calcium increases that were reversibly blocked by the selective AMPA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In contrast, the selective agonist N-methyl-D-aspartate (NMDA; 100 microM to 1 mM in magnesium-free solution with 10 microM glycine) had no effect on intracellular calcium. Radioimmunoassays demonstrated that glutamate stimulated PRL release. CNQX but not the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid blocked this release. Antibodies for mammalian AMPA- and NMDA-type GluR produced a similar punctate immunoreactivity in the periphery of PRL cells. However, the NMDA antibody recognized a protein of a different molecular mass in PRL cells compared with brain cells. These results clearly indicate the presence of GluRs on tilapia PRL cells that can stimulate PRL release.

Calcium buffering activity of mitochondria controls basal growth hormone secretion and modulates specific neuropeptide signaling

Goldfish somatotropes contain multiple functionally distinct classes of non-mitochondrial intracellular Ca(2+) stores. In this study, we investigated the role of mitochondrial Ca(2+) handling in the control of hormone secretion. Inhibition of mitochondrial Ca(2+) uptake with 10 microM ruthenium red (RR) and 10 microM carbonyl cyanide m-chlorophenylhydrazone (CCCP) caused a small and reversible increase in cytosolic [Ca(2+)]. Despite relatively modest global Ca(2+) signals, RR and CCCP stimulated robust GH secretion under basal culture conditions. CCCP-stimulated hormone release was abolished in cells pre-incubated with 50 microM BAPTA-AM, suggesting that elevations in cytosolic [Ca(2+)] mediate this release of GH. Both caffeine-sensitive intracellular Ca(2+) stores and L-type Ca(2+) channels can be the source of the Ca(2+) buffered by mitochondria in somatotropes. The stimulatory effect of RR on caffeine-stimulated GH release was enhanced dramatically in the presence of ryanodine, pointing to a complex interaction between these three Ca(2+) stores. Inhibition of mitochondrial Ca(2+) uptake with RR augmented GH release evoked by only one of the two endogenous gonadotropin-releasing hormones. Thus, we provide the first evidence that mitochondrial Ca(2+) buffering is differentially involved in specific agonist Ca(2+) signaling pathways and plays an important role in the control of basal GH release.

Involvement of phospholipase C and intracellular calcium signaling in the gonadotropin-releasing hormone regulation of prolactin release from lactotrophs of tilapia (Oreochromis mossambicus)

Gonadotropin-releasing hormone (GnRH) is a potent stimulator of prolactin (PRL) secretion in various vertebrates including the tilapia, Oreochromis mossambicus. The mechanism by which GnRH regulates lactotroph cell function is poorly understood. Using the advantageous characteristics of the teleost pituitary gland from which a nearly pure population of PRL cells can be isolated, we examined whether GnRH might stimulate PRL release through an increase in phospholipase C (PLC), inositol triphosphate (IP3), and intracellular calcium (Ca(i)2+) signaling. Using Ca(i)2+ imaging and the calcium-sensitive dye fura-2, we found that chicken GnRH-II (cGnRH-II) induced a rapid dose-dependent increase in Ca(i)2+ in dispersed tilapia lactotrophs. The Ca(i)2+ signal was abolished by U-73122, an inhibitor of PLC-dependent phosphoinositide hydrolysis. Correspondingly, cGnRH-II-induced tPRL188 secretion was inhibited by U-73122, suggesting that activation of PLC mediates cGnRH-II's stimulatory effect on PRL secretion. Pretreatment with 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate hydrochloride (TMB-8), an inhibitor of Ca2+ release from intracellular stores, impeded the effect of cGnRH-II on Ca(i)2+. To further address the possible involvement of intracellular Ca2+ stores, IP3 concentrations in the tilapia rostral pars distalis (RPD containing 95-99% PRL cells) was determined by a radioreceptor assay. We found that GnRH-II induces a rapid (<5min) and sustained increase in IP3 concentration in the RPD. Secretion of tPRL(188) in response to cGnRH-II was suppressed by Ca2+ antagonists (TMB-8 and nifedipine). These data, along with our previous findings that show PRL release increases with a rise in Ca(i)2+, suggest that GnRH may elicit its PRL releasing effect by increasing Ca(i)2+. Furthermore, the rise in Ca(i)2+ may be derived from PLC/IP3-induced mobilization of Ca2+ from intracellular stores along with influx through L-type voltage-gated Ca2+ channels.

Defective insulin secretion and increased susceptibility to experimental diabetes are induced by reduced Akt activity in pancreatic islet beta cells

The insulin and IGF signaling pathways are critical for development and maintenance of pancreatic beta cell mass and function. The serine-threonine kinase Akt is one of several mediators regulated by these pathways. We have studied the role of Akt in pancreatic beta cell physiology by generating transgenic mice expressing a kinase-dead mutant of this enzyme in beta cells. Reduction of Akt activity in transgenic animals resulted in impaired glucose tolerance due to defective insulin secretion. The mechanisms involved in dysregulation of secretion in these mice lie at the level of insulin exocytosis and are not the result of abnormalities in glucose signaling or function of voltage-gated Ca2+ channels. Therefore, transgenic mice showed increased susceptibility to developing glucose intolerance and diabetes following fat feeding. These observations suggest that Akt plays a novel and important role in the regulation of distal components of the secretory pathway and that this enzyme represents a therapeutic target for improvement of beta cell function in diabetes.

Reduced expression of the insulin receptor in mouse insulinoma (MIN6) cells reveals multiple roles of insulin signaling in gene expression, proliferation, insulin content, and secretion

The role of insulin signaling in pancreatic beta cells has become increasingly apparent. Stably transformed insulinoma cell lines (MIN6) were created with small interfering RNA resulting in the reduction of insulin receptor (IR) expression up to 80% (insulin receptor knockdown, IRKDDelta80). Functionally perturbed IR signaling was confirmed with the absence of insulin-stimulated insulin receptor substrate 1 tyrosine phosphorylation. Additionally, Akt phosphorylation was reduced and responded poorly to glucose stimulation. Gene expression profiling revealed that reduced IR expression was associated with alterations in expression of >1,500 genes with diverse functions. IRKD cells exhibited low rate of proliferation due to delay in transition from G0/G1 to S phase, whereas susceptibility to apoptosis did not differ from that of control cells. Insulin content was reduced in proportion to the reduction of IR. IRKD cells maintained glucose responsiveness as measured by NADPH generation, whereas Ca2+ responses and insulin secretion were enhanced. IRKDDelta80 and control cells were treated with glucose (25 mm) or insulin (100 nm) for 45 min, and gene expression profiles were assessed. Transcriptional activation of several hundred early response genes common to both glucose and insulin stimulation was observed in control cells. In IRKDDelta80 cells, insulin failed to activate any genes as anticipated. Importantly, glucose stimulation of gene expression in IRKDDelta80 cells showed that most genes previously activated by glucose were no longer activated, suggesting a major autocrine/paracrine effect of insulin on glucose-regulated gene expression. On the other hand, there were a number of glucose-regulated genes in the IRKDDelta80 cells that were not previously observed in control cells, suggesting a feedback regulation of insulin signaling on glucose-regulated gene expression. These results demonstrate important roles of the insulin receptor in islet beta cell gene expression and function and may serve to elucidate molecular defects in animal models with diminished beta cell insulin signaling.

Nitric oxide produced by a novel nitric oxide synthase isoform is necessary for gonadotropin-releasing hormone-induced growth hormone secretion via a cGMP-dependent mechanism

The involvement of nitric oxide (NO) in the regulation of goldfish growth hormone (GH) secretion was further characterized using primary cultures of dispersed goldfish pituitary cells. Western blots revealed the presence of an inducible nitric oxide synthase (iNOS)-like protein of approximately 120 kDa in cytosol/plasma membrane extracts. By contrast, brain NOS-immunoreactive proteins of approximately 120-140 kDa were occasionally detected in a cytoskeleton/organelle fraction but were absent from cytosol/plasma membrane extracts. The NO donor sodium nitroprusside (SNP) acutely increased GH secretion but this response was not observed in the presence of either a NO scavenger (PTIO) or a soluble guanylate cyclase inhibitor (ODQ). SNP also significantly increased the levels of cyclic (c)GMP in somatotrope-enriched cell populations. Treatments with 1400W (iNOS inhibitor), PTIO and rutin hydrate (NO scavengers) and ODQ abolished the acute GH-release response to two endogenous gonadotropin-releasing hormones (GnRH). 1400W, rutin hydrate, PTIO and ODQ alone did not significantly alter basal GH secretion. Together, these results establish that an iNOS-like peptide is constitutively present in the pituitary of the goldfish. Furthermore, these data suggest that NO, most likely through the generation of cGMP, is a necessary signal transduction component of GnRH-induced GH secretion.

Cell volume increase and extracellular Ca2+ are needed for hyposmotically induced prolactin release in tilapia

In the tilapia (Oreochromis mossambicus), as in many euryhaline teleost fish, prolactin (PRL) plays a central role in freshwater adaptation, acting on osmoregulatory surfaces to reduce ion and water permeability and increase solute retention. Consistent with these actions, PRL release is stimulated as extracellular osmolality is reduced both in vivo and in vitro. In the current experiments, a perfusion system utilizing dispersed PRL cells was developed for permitting the simultaneous measurement of cell volume and PRL release. Intracellular Ca(2+) was monitored using fura 2-loaded cells under the same conditions. When PRL cells were exposed to hyposmotic medium, an increase in PRL cell volume preceded the increase in PRL release. Cell volume increased in proportion to decreases of 15 and 30% in osmolality. However, regulatory volume decrease was clearly seen only after a 30% reduction. The hyposmotically induced PRL release was sharply reduced in Ca(2+)-deleted hyposmotic medium, although cell volume changes were identical to those observed in normal hyposmotic medium. In most cells, a rise in intracellular Ca(2+) concentration ([Ca(2+)](i)) during hyposmotic stimulation was dependent on the availability of extracellular Ca(2+), although small transient increases in [Ca(2+)](i) were sometimes observed upon introduction of Ca(2+)-deleted media of the same or reduced osmolality. These results indicate that an increase in cell size is a critical step in the transduction of an osmotic signal into PRL release and that the hyposmotically induced increase in PRL release is greatly dependent on extracellular Ca(2+).

Increased islet apoptosis in Pdx1+/- mice

Mice with 50% Pdx1, a homeobox gene critical for pancreatic development, had worsening glucose tolerance with age and reduced insulin release in response to glucose, KCl, and arginine from the perfused pancreas. Surprisingly, insulin secretion in perifusion or static incubation experiments in response to glucose and other secretagogues was similar in islets isolated from Pdx1(+/-) mice compared with Pdx1(+/+) littermate controls. Glucose sensing and islet Ca(2+) responses were also normal. Depolarization-evoked exocytosis and Ca(2+) currents in single Pdx1(+/-) cells were not different from controls, arguing against a ubiquitous beta cell stimulus-secretion coupling defect. However, isolated Pdx1(+/-) islets and dispersed beta cells were significantly more susceptible to apoptosis at basal glucose concentrations than Pdx1(+/+) islets. Bcl(XL) and Bcl-2 expression were reduced in Pdx1(+/-) islets. In vivo, increased apoptosis was associated with abnormal islet architecture, positive TUNEL, active caspase-3, and lymphocyte infiltration. Although similar in young mice, both beta cell mass and islet number failed to increase with age and were approximately 50% less than controls by one year. These results suggest that an increase in apoptosis, with abnormal regulation of islet number and beta cell mass, represents a key mechanism whereby partial PDX1 deficiency leads to an organ-level defect in insulin secretion and diabetes.

Multiplicity of gonadotropin-releasing hormone signaling: a comparative perspective

GnRH regulation of GtH synthesis and release involves PKC- and Ca(2+)-dependent pathways. There are differential signaling mechanisms in different cells, tissues and species. Signaling mechanisms involved in GnRH-mediated GtH release appear to be more conserved compared to that of GnRH-induced GtH gene expression. This may in part be due to different 5' regulatory regions on the GtH-subunit genes. Cell type specific expression of various signaling and/or exocytotic components may also be responsible for the observed differences in signaling between gonadotropes and somatotropes in the goldfish and tilapia pituitaries. However, this can not explain the observed differences in post receptor mechanisms for sGnRH and cGnRH-II in gonadotropes which is more likely to result from the existence of GnRH receptor subtypes. Support for this hypothesis is also provided by observations on mechanisms of autocrine/paracrine regulation of ovarian function by sGnRH and cGnRH-II in the goldfish ovary in which GnRH antagonists only block GnRH stimulation of oocyte meiosis and do not affect inhibitory effects of sGnRH. It should be easier to explain observed variations concerning GnRH-induced responses as more information becomes available on different types of GnRH receptors, and their distribution and function in mammals and non-mammalian vertebrates.

A gonadotropin-releasing hormone insensitive, thapsigargin-sensitive Ca2+ store reduces basal gonadotropin exocytosis and gene expression: comparison with agonist-sensitive Ca2+ stores

We examined whether distinct Ca2+ stores differentially control basal and gonadotropin (GTH-II)-releasing hormone (GnRH)-evoked GTH-II release, long-term GTH-II secretion and contents, and GTH-II-beta mRNA expression in goldfish. Thapsigargin (Tg)-sensitive Ca2+ stores mediated neither caffeine-evoked GTH-II release, nor salmon (s)GnRH- and chicken (c)GnRH-II-stimulated secretion; the latter responses were previously shown to involve ryanodine (Ry)-sensitive Ca2+ stores. Surprisingly, Tg decreased basal GTH-II release. This response was attenuated by prior exposure to sGnRH and caffeine, but was insensitive to the phosphatase inhibitor okadaic acid, the inhibitor of constitutive release brefeldin A and cGnRH-II. GTH-II-beta mRNA expression was decreased at 24 h by 2 microm Tg, and by inhibiting (10 microm Ry) and stimulating (1 nm Ry) Ry receptors. Transient increases in GTH-II-beta mRNA were observed at 2 h and 12 h following 10 microm and 1 nm Ry treatment, respectively. Effects of Tg, Ry and GnRH on long-term GTH-II secretion, contents and apparent production differed from one another, and these changes were not well correlated with changes in GTH-II-beta mRNA expression. Our data show that GTH-II secretion, storage and transcription can be independently controlled by distinct Ca2+ stores.

Function-specific calcium stores selectively regulate growth hormone secretion, storage, and mRNA level

Ca(+) stores may regulate multiple components of the secretory pathway. We examined the roles of biochemically independent intracellular Ca(2+) stores on acute and long-term growth hormone (GH) release, storage, and mRNA levels in goldfish somatotropes. Thapsigargin-evoked intracellular Ca(2+) concentration ([Ca(2+)](i)) signal amplitude was similar to the Ca(2+)-mobilizing agonist gonadotropin-releasing hormone, but thapsigargin (2 microM) did not acutely increase GH release, suggesting uncoupling between [Ca(2+)](i) and exocytosis. However, 2 microM thapsigargin affected long-term secretory function. Thapsigargin-treated cells displayed a steady secretion of GH (2, 12, and 24 h), which decreased GH content (12 and 24 h), but not GH mRNA/production (24 h). In contrast to the results with thapsigargin, activating the ryanodine (Ry) receptor (RyR) with 1 nM Ry transiently increased GH release (2 h). Prolonged activation of RyR (24 h) reduced GH release, contents and apparent production, without changing GH mRNA levels. Inhibiting RyR with 10 microM Ry increased GH mRNA levels, production, and storage (2 h). Increasing [Ca(2+)](i) independently of Ca(2+) stores with the use of 30 mM KCl decreased GH mRNA. Collectively, these results suggest that parts of the secretory pathway can be controlled independently by function-specific Ca(2+) stores.

Caffeine-stimulated GTH-II release involves Ca(2+) stores with novel properties

Modulation of Ca(2+) stores with 10 mM caffeine stimulates robust secretion of gonadotropin (GTH-II) from goldfish gonadotropes. Although both endogenous forms of gonadotropin-releasing hormone (GnRH) utilize a common intracellular Ca(2+) store, sGnRH, but not cGnRH-II, uses an additional caffeine-sensitive mechanism. We examined caffeine signaling by using Ca(2+) imaging, electrophysiology, and cell-column perifusion. Although caffeine inhibited K+ channels, this action appeared to be unrelated to caffeine-induced GTH-II release, because the latter was insensitive to tetraethylammonium. The effects of caffeine also were not mediated by the cAMP/protein kinase A pathway. Instead, caffeine-evoked GTH-II responses were Ca(2+) signal dependent because they were abolished by 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid loading. Caffeine generated localized Ca(2+) signals that began near secretory granules. Surprisingly, caffeine-stimulated GTH-II release was insensitive to 100 microM ryanodine and, unlike GnRH action, was unaffected by inhibitors of voltage-gated Ca(2+) channels or sarco(endo)plasmic reticulum Ca(2+)-ATPases. Collectively, these data indicate that caffeine-stimulated GTH-II release is not mediated by typical agonist-sensitive Ca(2+) stores found in endoplasmic reticulum.

Agonist-specific and sexual stage-dependent inhibition of gonadotropin-releasing hormone-stimulated gonadotropin and growth hormone release by ryanodine: relationship to sexual stage-dependent caffeine-sensitive hormone release

Differential utilization of intracellular Ca2+ stores with specific functional characteristics could be a potential mechanism for coupling various stimuli to specific cellular responses. In the goldfish pituitary, both gonadotropes and somatotropes possess multiple intracellular Ca2+ stores that are differentially coupled to agonist-evoked exocytosis. We investigated the role of ryanodine receptor/Ca2+-release channels (RyR) in basal and gonadotropin-releasing hormone (GnRH)-evoked hormone secretion from cultured gonadotropes and somatotropes using radioimmunoassay for gonadotropin (GTH-II) and growth hormone (GH). As is the case in vivo, the basal and evoked secretion of both hormones varied with seasonal reproductive status. GnRH-stimulated hormone release was three-fold higher in cells from sexually mature animals compared to those in a sexually regressed state. Nanomolar doses of ryanodine evoked significant GTH-II and GH secretion, suggesting that ryanodine-sensitive Ca2+ stores can couple to exocytosis in both cell types. In gonadotropes, 10 microM ryanodine abolished cGnRH-II-evoked GTH-II release in both sexually mature and sexually regressed fish, while sGnRH signalling was mediated by ryanodine-sensitive Ca2+ stores in cells from sexually regressed fish only. Ryanodine-sensitive Ca2+ stores in somatotropes were only involved in cGnRH-II-stimulated GH release during gonadal regression. In contrast, sGnRH-stimulated, but not cGnRH-II-stimulated, GH release was significantly reduced by 1 microM xestospongin C. Although hormone release stimulated by mobilizing caffeine-sensitive Ca2+ pools was also markedly seasonal, it was largely independent of ryanodine-sensitive Ca2+ stores. Ryanodine-sensitive Ca2+ stores in both cell types are not active downstream of ionomycin, BayK 8644, protein kinase C or cyclic adenosine monophosphate signalling pathways, suggesting difference from a classical Ca2+-induced Ca2+ release system. Ours study is the first to suggest that RyR2 may be involved in the seasonal plasticity of pituitary function, which may be related to cyclic changes observed in reproduction and growth.

Modulation of gonadotropin II release by K+ channel blockers in goldfish gonadotropes: a novel stimulatory action of 4-aminopyridine

The effects of K+ channel blockers on basal gonadotropin II (GTH-II) release were examined in cultured goldfish gonadotropes. Tetraethylammonium (TEA) inhibited basal GTH-II release, whereas 4-aminopyridine (4-AP) increased basal release, although both K+ channel blockers generated increases in [Ca2+]i. Other K+ channel blockers had no significant effect on GTH-II release. We examined whether Ca2+ entry that arises from blockade of K+ channels by 4-AP mediates the secretory response. Secretion evoked by 4-AP was slightly reduced by TEA but was unaffected by reducing Ca2+ entry using either an inhibitor of Ca2+ channels, verapamil, or nominally Ca2+-free medium. In contrast, the Ca2+ signal evoked by 4-AP was largely blocked by Ca2+-free medium, as predicted by its inhibitory action on K+ channels. Together, these data suggest that the hormone release response to 4-AP is independent of entry of extracellular Ca2+. Finally, the mechanism of hormone release evoked by 4-AP appeared to be independent of mechanism(s) evoked by caffeine since 4-AP did not affect caffeine-evoked release and caffeine did not affect 4-AP evoked release. That both 4-AP and TEA generated Ca2+ signals but affected hormone release in either an extracellular Ca2+ independent (4-AP) or inhibitory (TEA) manner suggests that Ca2+ entry is linked to GTH-II secretion in a highly nonlinear fashion.

Pacap stimulation of gonadotropin-II secretion in goldfish pituitary cells: mechanisms of action and interaction with gonadotropin releasing hormone signalling

Pituitary adenylate cyclase-activating polypeptide (PACAP) has recently been shown to be a hypophysiotropic factor in the goldfish. In this study, we examined the mechanisms of PACAP action on goldfish maturational gonadotropin (GTH-II) release using primary cultures of pituitary cells. The GTH-II response to mammalian PACAP1-38 (mPACAP) was inhibited by a PACAP receptor antagonist suggesting a receptor-mediated action. Addition of either an adenylate cyclase inhibitor or a protein kinase A (PKA) inhibitor reduced the mPACAP-induced GTH-II release. In addition, when GTH-II release was already stimulated by either forskolin or 8-bromo-cAMP (8Br-cAMP), mPACAP did not further increase GTH-II secretion. These results strongly implicated the involvement of an adenylate cyclase/cAMP/PKA pathway in PACAP-stimulated GTH-II release. Although mPACAP induced a rise in intracellular Ca2+ level in identified gonadotropes, results with voltage-sensitive Ca2+ channel inhibitors indicated that the GTH-II responses to mPACAP, forskolin and 8Br-cAMP did not depend upon Ca2+ entry through these channels. Two protein kinase C (PKC) inhibitors did not affect mPACAP-elicited GTH-II release, and mPACAP further increased GTH-II secretion in the presence of PKC activators. These results indicate that PKC-dependent elements are not essential for the stimulatory action of mPACAP in gonadotropes. Interestingly, while GTH-II responses to a stimulatory concentration of mPACAP were additive to responses elicited by maximal effective concentrations of two endogenous gonadotropin releasing hormones (GnRHs), a subthreshold concentration of mPACAP potentiated GnRH and PKC activator stimulation of GTH-II secretion. Similarly, submaximal concentrations of forskolin potentiated the GTH-II response to the PKC activator, tetradecanoyl phorbol acetate. These data suggest that PACAP and its cAMP-dependent signalling mechanisms provide an alternate stimulatory input to goldfish gonadotropes and may influence the effectiveness of the major neuroendocrine control exerted by the PKC-dependent GnRH signalling pathway.

Somatostatin actions on a protein kinase C-dependent growth hormone secretagogue cascade

In mammals, the ability of somatostatin (SS) to block growth hormone (GH) secretion is due, in part, to the inhibition of two key intracellular mediators, cAMP and Ca2+. We examined whether or not inhibition of Ca2+ signaling was mediating SS-induced inhibition basal, as well as gonadotropin-releasing hormone (GnRH; a protein kinase C (PKC)-dependent growth hormone secretagogue)-stimulated growth hormone (GH) release. Although SS reduced basal GH release from populations of pituitary cells, parallel reductions in [Ca2+]i were not observed within single, identified somatotropes. Similarly, application of GnRH and the PKC activator DiC8 elicited increases in [Ca2+]i and GH release, but abolition of the Ca2+ responses did not accompany SS inhibition of the GH responses. Surprisingly, while DiC8 potentiated SS inhibition of GH release, SS paradoxically increased DiC8-stimulated increases in [Ca2+]i. These data establish that abolition of Ca2+ signals is not a primary mechanism through which SS lowers basal, or inhibits GnRH-stimulated hormone release.

Differential regulation of two forms of gonadotropin-releasing hormone messenger ribonucleic acid in human granulosa-luteal cells

Until recently, the primate brain was thought to contain only one form of GnRH known as mammalian GnRH (GnRH-I). The recent cloning of a second form of GnRH (GnRH-II) with characteristics of chicken GnRH-II in the primate brain has prompted a reevaluation of the role of GnRH in reproductive functions. In the present study, we investigated the hormonal regulation of GnRH-II messenger RNA (mRNA) and its functional role in the human granulosa-luteal cells (hGLCs), and we provided novel evidence for differential hormonal regulation of GnRH-II vs. GnRH-I mRNA expression. Human GLCs were treated with various concentrations of GnRH-II, GnRH-II agonist (GnRH-II-a), or GnRH-I agonist (GnRH-I-a; leuprolide) in the absence or presence of FSH or human CG (hCG). The expression levels of GnRH-II, GnRH-I, and GnRH receptor (GnRHR) mRNA were investigated using semiquantitative or competitive RT-PCR. A significant decrease in GnRH-II and GnRHR mRNA levels was observed in cells treated with GnRH-II or GnRH-II-a. In contrast, GnRH-I-a revealed a biphasic effect (up- and down-regulation) of GnRH-I and GnRHR mRNA, suggesting that GnRH-I and GnRH-II may differentially regulate GnRHR and their ligands (GnRH-I and GnRH-II). Treatment with FSH or hCG increased GnRH-II mRNA levels but decreased GnRH-I mRNA levels, further indicating that GnRH-I and GnRH-II mRNA levels are differentially regulated. To investigate the physiological role of GnRH-II, hGLCs were treated with GnRH-II or GnRH-II-a in the presence or absence of hCG, for 24 h, and progesterone secretion was measured by RIA. Both GnRH-II and GnRH-II-a inhibited basal and hCG-stimulated progesterone secretion, effects which were similar to the effects of GnRH-I treatment on ovarian steroidogenesis. Next, hGLCs were treated with various concentrations of GnRH-II, GnRH-II-a, or GnRH-I-a; and the expression levels of FSH receptor and LH receptor were investigated using semiquantitative RT-PCR. A significant down-regulation of FSH receptor and LH receptor was observed in cells treated with GnRH-II, GnRH-II-a, and GnRH-I-a, demonstrating that GnRH-II and GnRH-I may exert their antigonadotropic effect by down-regulating gonadotropin receptors. Interestingly, GnRH-II and GnRH-II-a did not affect basal and hCG-stimulated intracellular cAMP accumulation, suggesting that the antigonadotropic effect of GnRH-II may be independent of modulation of cAMP levels. Taken together, these results suggest that GnRH-II may have biological effects similar to those of GnRH-I but is under differential hormonal regulation in the human ovary.

Signal transduction mechanisms mediating secretion in goldfish gonadotropes and somatotropes

The intracellular signal transduction mechanisms mediating maturational gonadotropin and somatotropin secretion in goldfish are reviewed. Several major signaling mechanisms, including changes in intracellular [Ca2+], arachidonic acid cascades, protein kinase C, cyclic AMP/protein kinase A, calmodulin, nitric oxide, and Na+/H+ antiport, are functional in both cell types. However, their relative importance in mediating basal secretion and neuroendocrine-factor-regulated hormone release differs according to cell type. Similarly, agonist- and cell-type-specificity are also present in the transduction pathways leading to neuroendocrine factor-modulated maturational gonadotropin and somatotropin release. Specificity is present not only in the actions of different regulators within the same cell type and with the same ligand in the two cell types, but this also exists between isoforms of the same neuroendocrine factor within a single cell type. Other evidence suggests that function-selectivity of signaling may also result from differential modulation of Ca2+ fluxes from different sources. The interaction of different second messenger systems provide the basis by which regulation of maturational gonadotropin and somatotropin release by multiple neuroendocrine factors can be integrated at the target cell level.

Key words: Ca2+ signaling, cAMP, PKC, arachidonic acid, NO.

Function- and agonist-specific Ca2+signalling: The requirement for and mechanism of spatial and temporal complexity in Ca2+signals

Calcium signals have been implicated in the regulation of many diverse cellular processes. The problem of how information from extracellular signals is delivered with specificity and fidelity using fluctuations in cytosolic Ca2+concentration remains unresolved. The capacity of cells to generate Ca2+signals of sufficient spatial and temporal complexity is the primary constraint on their ability to effectively encode information through Ca2+. Over the past decade, a large body of literature has dealt with some basic features of Ca2+-handling in cells, as well as the multiplicity and functional diversity of intracellular Ca2+stores and extracellular Ca2+influx pathways. In principle, physiologists now have the necessary information to attack the problem of function- and agonist-specificity in Ca2+signal transduction. This review explores the data indicating that Ca2+release from diverse sources, including many types of intracellular stores, generates Ca2+signals with sufficient complexity to regulate the vast number of cellular functions that have been reported as Ca2+-dependent. Some examples where such complexity may relate to neuroendocrine regulation of hormone secretion/synthesis are discussed. We show that the functional and spatial heterogeneity of Ca2+stores generates Ca2+signals with sufficient spatiotemporal complexity to simultaneously control multiple Ca2+-dependent cellular functions in neuroendocrine systems.Key words: signal coding, IP3receptor, ryanodine receptor, endoplasmic reticulum, Golgi, secretory granules, mitochondria, exocytosis.