Pituitary adenylate cyclase-activating polypeptide stimulates nitric-oxide synthase type I expression and potentiates the cGMP response to gonadotropin-releasing hormone of rat pituitary gonadotrophs

PACAP: A regulator of mammalian reproductive function

Pituitary adenylate cyclase-activating polypeptide (PACAP) is an ancestral molecule that was isolated from sheep hypothalamic extracts based on its action to stimulate cAMP production by pituitary cell cultures. PACAP is one of a number of ligands that coordinate with GnRH to control reproduction. While initially viewed as a hypothalamic releasing factor, PACAP and its receptors are widely distributed, and there is growing evidence that PACAP functions as a paracrine/autocrine regulator in the CNS, pituitary, gonads and placenta, among other tissues. This review will summarize current knowledge concerning the expression and function of PACAP in the hypothalamic-pituitary-gonadal axis with special emphasis on its role in pituitary function in the fetus and newborn.

The central nervous system acts as a transducer of stress-induced masculinization through corticotropin-releasing hormone B

Exposure to environmental stressors, like high temperature (HT), during early development of fish induces sex reversal of genotypic females. Nevertheless, the involvement of the brain in this process is not well clarified. In the present work, we investigated the mRNA levels of corticotropin-releasing hormone b (crhb) and its receptors (crhr1 and crhr2), and found out that they were up-regulated at HT during the critical period of gonadal sex determination in medaka. In order to clarify their roles in sex reversal, biallelic mutants for crhr1 and crhr2 were produced by CRISPR/Cas9 technology. Remarkably, biallelic mutant of both loci (crhr1 and crhr2) did not undergo female-to-male sex reversal upon HT exposition. Inhibition of this process in double crhrs mutants could be successfully rescued through the administration of the downstream effector of the hypothalamic-pituitary interrenal axis, the cortisol. Taken together, these results revealed for the first time the participation of the CNS acting as a transducer of masculinization induced by thermal stress.

Intracellular mechanisms involved in copper-gonadotropin-releasing hormone (Cu-GnRH) complex-induced cAMP/PKA signaling in female rat anterior pituitary cells in vitro

The copper-gonadotropin-releasing hormone molecule (Cu-GnRH) is a GnRH analog, which preserves its amino acid sequence, but which contains a Cu(2+) ion stably bound to the nitrogen atoms including that of the imidazole ring of Histidine(2). A previous report indicated that Cu-GnRH was able to activate cAMP/PKA signaling in anterior pituitary cells in vitro, but raised the question of which intracellular mechanism(s) mediated the Cu-GnRH-induced cAMP synthesis in gonadotropes. To investigate this mechanism, in the present study, female rat anterior pituitary cells in vitro were pretreated with 0.1 μM antide, a GnRH antagonist; 0.1 μM cetrorelix, a GnRH receptor antagonist; 0.1 μM PACAP6-38, a PAC-1 receptor antagonist; 2 μM GF109203X, a protein kinase C inhibitor; 50 mM PMA, a protein kinase C activator; the protein kinase A inhibitors H89 (30 μM) and KT5720 (60 nM); factors affecting intracellular calcium activity: 2.5 mM EGTA; 2 μM thapsigargin; 5 μM A23187, a Ca(2+) ionophore; or 10 μg/ml cycloheximide, a protein synthesis inhibitor. After one of the above pretreatments, cells were incubated in the presence of 0.1 μM Cu-GnRH for 0.5, 1, and 3 h. Radioimmunoassay analysis of cAMP confirmed the functional link between Cu-GnRH stimulation and cAMP/PKA signal transduction in rat anterior pituitary cells, demonstrating increased intracellular cAMP, which was reduced in the presence of specific PKA inhibitors. The stimulatory effect of Cu-GnRH on cAMP production was partly dependent on GnRH receptor activation. In addition, an indirect and Ca(2+)-dependent mechanism might be involved in intracellular adenylate cyclase stimulation. Neither activation of protein kinase C nor new protein synthesis was involved in the Cu-GnRH-induced increase of cAMP in the rat anterior pituitary primary cultures. Presented data indicate that conformational changes of GnRH molecule resulting from cooper ion coordination affect specific pharmacological properties of Cu-GnRH molecule including specific pattern of intracellular activity induced by complex in anterior pituitary cells in vitro.

Ontogenesis of NADPH-diaphorase positive neurons in guinea pig neocortex

In mammalian cerebrum there exist two distinct types of interneurons expressing nitric oxide synthase (NOS). Type I neurons are large in size and exhibit heavy nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemical reaction, while type II cells are small with light NADPH-d reactivity. The time of origin of these cortical neurons relative to corticogenesis remains largely unclear among mammals. Here we explored this issue in guinea pigs using cell birth-dating and double-labeling methods. Bromodeoxyuridine (BrdU) pulse-chasing (2 doses at 50 mg/kg, 12 h apart) was given to time-pregnant mothers, followed by quantification of NADPH-d/BrdU colocalization in the parietal and temporal neocortex in offspring at postnatal day 0 (P0), P30 and P60. Type I neurons were partially colabeled with BrdU at P0, P30 and P60 following pulse-chasing at embryonic day 21 (E21), E28 and E35, varied from 2–11.3% of total population of these neurons for the three time groups. Type II neurons were partially colabeled for BrdU following pulse-chasing at E21, E28, E35 and E42 at P0 (8.6%–16.5% of total population for individual time groups). At P60, type II neurons were found to co-express BrdU (4.8–11.3% of total population for individual time groups) following pulse-chasing at E21, E28, E35, E42, E49, E56 and E60/61. These results indicate that in guinea pigs type I neurons are generated during early corticogenesis, whereas type II cells are produced over a wide prenatal time window persisting until birth. The data also suggest that type II nitrinergic neurons may undergo a period of development/differentiation, for over 1 month, before being NADPH-d reactive.

Role of pituitary adenylate cyclase-activating polypeptide in modulating hypothalamus-pituitary neuroendocrine functions in mouse cell models

Pituitary adenylate cyclase-activating polypeptide (PACAP) was originally identified as a hypothalamic activator of cyclic adenosine monophosphate production in pituitary cells. PACAP and its receptor are expressed not only in the central nervous system, but also in peripheral organs, and function to stimulate pituitary hormone synthesis and secretion as both a hypothalamic-pituitary-releasing factor and an autocrine-paracrine factor within the pituitary. PACAP stimulates the expression of the gonadotrophin α, luteinising hormone (LH) β and follicle-stimulating hormone (FSH) β subunits, as well as the gonadotrophin-releasing hormone (GnRH) receptor and its own PACAP type I receptor (PAC1R) in gonadotrophin-secreting pituitary cells. In turn, GnRH, which is known to be a crucial component of gonadotrophin secretion, stimulates the expression of PACAP and PAC1R in gonadotrophs. In addition, PAC1R and PACAP modulate the functions of GnRH-producing neurones in the hypothalamus. This review summarises the current understanding of the possible roles of PACAP and PAC1R in modulating hypothalamus and pituitary neuroendocrine cells in the mouse models.

PACAP induces plasticity at autonomic synapses by nAChR-dependent NOS1 activation and AKAP-mediated PKA targeting

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a pleiotropic neuropeptide found at synapses throughout the central and autonomic nervous system. We previously found that PACAP engages a selective G-protein coupled receptor (PAC1R) on ciliary ganglion neurons to rapidly enhance quantal acetylcholine (ACh) release from presynaptic terminals via neuronal nitric oxide synthase (NOS1) and cyclic AMP/protein kinase A (PKA) dependent processes. Here, we examined how PACAP stimulates NO production and targets resultant outcomes to synapses. Scavenging extracellular NO blocked PACAP-induced plasticity supporting a retrograde (post- to presynaptic) NO action on ACh release. Live-cell imaging revealed that PACAP stimulates NO production by mechanisms requiring NOS1, PKA and Ca(2+) influx. Ca(2+)-permeable nicotinic ACh receptors composed of α7 subunits (α7-nAChRs) are potentiated by PKA-dependent PACAP/PAC1R signaling and were required for PACAP-induced NO production and synaptic plasticity since both outcomes were drastically reduced following their selective inhibition. Co-precipitation experiments showed that NOS1 associates with α7-nAChRs, many of which are perisynaptic, as well as with heteromeric α3*-nAChRs that generate the bulk of synaptic activity. NOS1-nAChR physical association could facilitate NO production at perisynaptic and adjacent postsynaptic sites to enhance focal ACh release from juxtaposed presynaptic terminals. The synaptic outcomes of PACAP/PAC1R signaling are localized by PKA anchoring proteins (AKAPs). PKA regulatory-subunit overlay assays identified five AKAPs in ganglion lysates, including a prominent neuronal subtype. Moreover, PACAP-induced synaptic plasticity was selectively blocked when PKA regulatory-subunit binding to AKAPs was inhibited. Taken together, our findings indicate that PACAP/PAC1R signaling coordinates nAChR, NOS1 and AKAP activities to induce targeted, retrograde plasticity at autonomic synapses. Such coordination has broad relevance for understanding the control of autonomic synapses and consequent visceral functions.

PACAP modulates GnRH signaling in gonadotropes

Hypothalamic gonadotropin-releasing hormone is known to be critical for normal gonadotropin biosynthesis and secretion by the gonadotrope cells of the anterior pituitary gland. Additional regulation is provided by gonadal steroid feedback as well as by intrapituitary factors, such as activin and follistatin. Less well-appreciated is the role of pituitary adenylate-cyclase activating polypeptide (PACAP) as both a hypothalamic-pituitary releasing factor as well as an autocrine-paracrine factor within the pituitary. PACAP regulates gonadotropin expression alone and through modulation of GnRH responsiveness achieved by increases in GnRH receptor expression and interactions at the level of intracellular signaling pathways. In addition to direct effects on the gonadotrope, PACAP stimulates follistatin secretion by the folliculostellate cells and thereby contributes to differential expression of the gonadotropin subunits. Conversely, GnRH augments the ability of PACAP to regulate gonadotrope function by increasing pituitary PACAP and PACAP receptor expression. This review will summarize the current understanding of the mechanisms by which PACAP modulates gonadotrope function, with a focus on interactions with GnRH.

Molecular mechanisms of gonadotropin-releasing hormone signaling: integrating cyclic nucleotides into the network

Gonadotropin-releasing hormone (GnRH) is the primary regulator of mammalian reproductive function in both males and females. It acts via G-protein coupled receptors on gonadotropes to stimulate synthesis and secretion of the gonadotropin hormones luteinizing hormone and follicle-stimulating hormone. These receptors couple primarily via G-proteins of the Gq/ll family, driving activation of phospholipases C and mediating GnRH effects on gonadotropin synthesis and secretion. There is also good evidence that GnRH causes activation of other heterotrimeric G-proteins (Gs and Gi) with consequent effects on cyclic AMP production, as well as for effects on the soluble and particulate guanylyl cyclases that generate cGMP. Here we provide an overview of these pathways. We emphasize mechanisms underpinning pulsatile hormone signaling and the possible interplay of GnRH and autocrine or paracrine regulatory mechanisms in control of cyclic nucleotide signaling.

Decoding high Gonadotropin-releasing hormone pulsatility: a role for GnRH receptor coupling to the cAMP pathway?

The gonadotropin-releasing hormone (GnRH) pulsatile pattern is critical for appropriate regulation of gonadotrope activity but only little is known about the signaling mechanisms by which gonadotrope cells decode such pulsatile pattern. Here, we review recent lines of evidence showing that the GnRH receptor (GnRH-R) activates the cyclic AMP (cAMP) pathway in gonadotrope cells, thus ending a long-lasting controversy. Interestingly, coupling of GnRH-R to the cAMP pathway as well as induction of nitric oxide synthase 1 (NOS1) or follistatin through this signaling pathway take place preferentially under high GnRH pulsatility. The preovulatory surge of GnRH in vivo is indeed associated with an important increase of pituitary cAMP and NOS1 expression levels, both being markedly inhibited by treatment with a GnRH antagonist. Altogether, this suggests that due to its atypical structure and desensitization properties, the GnRH-R may continue to signal through the cAMP pathway under conditions inducing desensitization for most other receptors. Such a mechanism may contribute to decode high GnRH pulsatile pattern and enable gonadotrope cell plasticity during the estrus cycle.

PACAP, an autocrine/paracrine regulator of gonadotrophs

Hypothalamic-hypophysiotropic peptides are the proximate regulators of pituitary cells, but they cannot fully account for the complex functioning of these cells. Accordingly, awareness is growing that an array of peptides produced in the pituitary exert paracrine/autocrine functions. One such peptide, pituitary adenylate cyclase-activating polypeptide (PACAP), was originally identified as a hypothalamic activator of cAMP production in pituitary cells. Gonadotrophs and folliculostellate cells are the main source of pituitary PACAP, and each pituitary cell type expresses a PACAP receptor. PACAP increases alpha-subunit (Cga) and Lhb mRNAs, and it stimulates the transcription of follistatin (Fst) that, in turn, restrains activin signaling to repress Fshb and gonadotropin-releasing hormone-receptor (Gnrhr) expression as well as other activin-responsive genes. The PACAP (Adcyap1) promoter is activated by cAMP, and pituitary cells may communicate by a feed-forward, cAMP-dependent mechanism to maintain a high level of PACAP in the fetal pituitary. At birth, pituitary PACAP declines and pituitary follistatin levels decrease, which together with increased gonadotropin-releasing hormone secretion allow Gnrhr and Fshb to increase and facilitate activation of the newborn gonads. Changes in Adcyap1 expression levels in the adult pituitary may contribute to the selective rise in follicle-stimulating hormone (FSH) from age 20-30 days to the midcycle surge and to the secondary increase in FSH that occurs before estrus. These results provide further support for the notion that PACAP is a key player in reproduction through its actions as a pituitary autocrine/paracrine hormone.

Sustained gonadotropin-releasing hormone stimulation mobilizes the cAMP/PKA pathway to induce nitric oxide synthase type 1 expression in rat pituitary cells in vitro and in vivo at proestrus

Previous in vivo studies have established that pituitary nitric oxide synthase type 1 (NOS1) is regulated by gonadotropin-releasing hormone (GnRH). The aim of our study was to elucidate the mechanisms of NOS1 regulation by GnRH in rat pituitary cells. Using a perifused cell system, we demonstrated that NOS1 induction was sensitive to GnRH pulse frequency and was maximally induced under continuous GnRH stimulation. In primary cultures of rat pituitary cells, sustained stimulation with the GnRH agonist triptorelin (GnRHa) increased NOS1 protein levels, whereas NOS2 and NOS3 levels were unaffected. NOS1 up-regulation occurred in gonadotroph cells only, in a time-dependent and concentration-dependent manner (maximum increase, 2.5-fold; half-maximal concentration, 0.17 nM). GnRHa effect was mimicked by cAMP pathway activators and, most importantly, was blocked by disruption of the protein kinase A (PKA) pathway using pharmacological inhibitors such as Rp-cAMP or drug phosphatase technology-protein kinase inhibitor (DPT-PKI), a cell-permeant PKI peptide. In contrast, modulation of the PKC pathway and inhibition of the MAPK cascade were ineffective. Overall, these experiments demonstrated that GnRH-induced up-regulation of pituitary NOS1 is mediated notably by the cAMP/PKA pathway. Last, in vivo administration of a GnRH antagonist markedly inhibited the pituitary cAMP rise at proestrus in addition to suppressing NOS1 increase. Altogether, our data suggest that the cAMP/PKA signaling pathway is preferentially recruited under sustained GnRH stimulation in vivo during proestrus, allowing the expression of a specific set of PKA-regulated proteins, including NOS1, in gonadotroph cells.

PACAP stimulation of maturational gonadotropin secretion in goldfish involves extracellular signal-regulated kinase, but not nitric oxide or guanylate cyclase, signaling

In goldfish, nitric oxide synthase (NOS) immunoreactivity is present in gonadotropes and extracellular signal-regulated protein kinase (ERK) mediates GnRH stimulation of gonadotropin release and synthesis. In this study, we tested the possible involvement of nitric oxide (NO) and ERK in mediating PACAP-stimulated maturational gonadotropin (GTH-II) release from primary cultures of dispersed goldfish pituitary cells. In static incubation experiments, PACAP-induced GTH-II release was unaffected by two inhibitors of NOS synthase, AGH and 1400W; whereas addition of a NO donor, SNAP, elevated GTH-II secretion. In perifusion experiments, neither NOS inhibitors (AGH, 1400W and 7-Ni) nor NO scavengers (PTIO and rutin hydrate) attenuated the GTH-II response to pulse applications of PACAP. In addition, the GTH-II responses to PACAP and the NO donor SNP were additive while PTIO blocked SNP action. Although dibutyryl cGMP increased GTH-II secretion in static incubation, inhibition of guanylate cyclase (GC), a known down-stream target for NO signaling, did not reduce the GTH-II response to pulse application of PACAP. On the other hand, GTH-II responses to PACAP in perifusion were attenuated in the presence of two inhibitors of ERK kinase (MEK), U 0126 and PD 98059. These results suggest that although increased availability of NO and cGMP can lead to increased GTH-II secretion, MEK/ERK signaling, rather than NOS/NO/GC activation, mediates PACAP action on GTH-II release in goldfish.

PACAP/PAC1R signaling modulates acetylcholine release at neuronal nicotinic synapses

Neuropeptides collaborate with conventional neurotransmitters to regulate synaptic output. Pituitary adenylate cyclase-activating polypeptide (PACAP) co-localizes with acetylcholine in presynaptic nerve terminals, is released by stimulation, and enhances nicotinic acetylcholine receptor- (nAChR-) mediated responses. Such findings implicate PACAP in modulating nicotinic neurotransmission, but relevant synaptic mechanisms have not been explored. We show here that PACAP acts via selective high-affinity G-protein coupled receptors (PAC(1)Rs) to enhance transmission at nicotinic synapses on parasympathetic ciliary ganglion (CG) neurons by rapidly and persistently increasing the frequency and amplitude of spontaneous, impulse-dependent nicotinic excitatory postsynaptic currents (sEPSCs). Of the canonical adenylate cyclase (AC) and phospholipase-C (PLC) transduction cascades stimulated by PACAP/PAC(1)R signaling, only AC-generated signals are critical for synaptic modulation since the increases in sEPSC frequency and amplitude were mimicked by 8-Bromo-cAMP, blocked by inhibiting AC or cAMP-dependent protein kinase (PKA), and unaffected by inhibiting PLC. Despite its ability to increase agonist-induced nAChR currents, PACAP failed to influence nAChR-mediated impulse-independent miniature EPSC amplitudes (quantal size). Instead, evoked transmission assays reveal that PACAP/PAC(1)R signaling increased quantal content, indicating that it modulates synaptic function by increasing vesicular ACh release from presynaptic terminals. Lastly, signals generated by the retrograde messenger, nitric oxide- (NO-) are critical for the synaptic modulation since the PACAP-induced increases in spontaneous EPSC frequency, amplitude and quantal content were mimicked by NO donor and absent after inhibiting NO synthase (NOS). These results indicate that PACAP/PAC(1)R activation recruits AC-dependent signaling that stimulates NOS to increase NO production and control presynaptic transmitter output at neuronal nicotinic synapses.

The role and molecular mechanism of D-aspartic acid in the release and synthesis of LH and testosterone in humans and rats

BACKGROUND

D-aspartic acid is an amino acid present in neuroendocrine tissues of invertebrates and vertebrates, including rats and humans. Here we investigated the effect of this amino acid on the release of LH and testosterone in the serum of humans and rats. Furthermore, we investigated the role of D-aspartate in the synthesis of LH and testosterone in the pituitary and testes of rats, and the molecular mechanisms by which this amino acid triggers its action.

METHODS

For humans: A group of 23 men were given a daily dose of D-aspartate (DADAVIT) for 12 days, whereas another group of 20 men were given a placebo. For rats: A group of 10 rats drank a solution of either 20 mM D-aspartate or a placebo for 12 days. Then LH and testosterone accumulation was determined in the serum and D-aspartate accumulation in tissues. The effects of D-aspartate on the synthesis of LH and testosterone were gauged on isolated rat pituitary and Leydig cells. Tissues were incubated with D-aspartate, and then the concentration (synthesis) of LH and cGMP in the pituitary and of testosterone and cAMP in the Leydig cells was determined.

RESULTS

In humans and rats, sodium D-aspartate induces an enhancement of LH and testosterone release. In the rat pituitary, sodium D-aspartate increases the release and synthesis of LH through the involvement of cGMP as a second messenger, whereas in rat testis Leydig cells, it increases the synthesis and release of testosterone and cAMP is implicated as second messenger. In the pituitary and in testes D-Asp is synthesized by a D-aspartate racemase which convert L-Asp into D-Asp. The pituitary and testes possesses a high capacity to trapping circulating D-Asp from hexogen or endogen sources.

CONCLUSION

D-aspartic acid is a physiological amino acid occurring principally in the pituitary gland and testes and has a role in the regulation of the release and synthesis of LH and testosterone in humans and rats.

Gonadotropin-releasing hormone inhibits pituitary adenylyl cyclase-activating polypeptide coupling to 3',5'-cyclic adenosine-5'-monophosphate pathway in LbetaT2 gonadotrope cells through novel protein kinase C isoforms and phosphorylation of pituitary adenylyl cyclase-activating polypeptide type I receptor

Gonadotrope cells are primarily regulated by GnRH but are also targets of the pituitary adenylyl cyclase-activating polypeptide (PACAP). Although it has been reported that reciprocal interactions between both neuropeptides contribute to regulation of gonadotrope function, the underlying mechanisms remain poorly understood. In this study, we reevaluated PACAP coupling to the cAMP pathway in LbetaT2 gonadotrope cells and analyzed GnRH effect on PACAP signaling. We established that PACAP38 markedly increases intracellular cAMP levels (EC50 of 4.7 +/- 1.3 nm) through the PACAP type 1 receptor (PAC1-R), as evidenced by pharmacological and RT-PCR studies. Interestingly, although GnRH couples to cAMP pathway in LbetaT2 cells, the effects of both neuropeptides were not synergistic. Instead, the GnRH agonist (GnRHa) triptorelin rapidly and strongly inhibited (70% inhibition as early as 5 min) PACAP38-induced cAMP production. Inhibition was calcium independent, mimicked by the phorbol ester phorbol 12-myristate 13-acetate, and blocked by the protein kinase C (PKC) inhibitor bisindoylmaleimide, indicating that GnRHa inhibitory action relies on PKC. Selective down-regulation of both conventional and novel PKC prevented a GnRHa effect, whereas pharmacological inhibition of conventional PKC only was ineffective, strongly suggesting the involvement of novel PKC isoforms. GnRHa did not inhibit forskolin- or cholera toxin-stimulated cAMP accumulation, suggesting that PAC1-R is the predominant target of GnRH. Accordingly, we demonstrated for the first time that GnRH increases PAC1-R phosphorylation through PKC, providing a potential molecular mechanism which may account for GnRH inhibitory effect.

Differential involvement of nitric oxide signaling in dopamine and PACAP stimulation of growth hormone release in goldfish

Previous studies in goldfish pituitary cells have shown that nitric oxide synthase (NOS)/nitric oxide (NO) signaling is involved in mediating the growth hormone (GH) release response to gonadotropin-releasing hormones. In this study, the involvement of this signaling pathway in mediating the action of two cAMP-mobilizing neuroendocrine stimulators of GH release, pituitary adenylate cyclase-activating polypeptide (PACAP) and dopamine (DA), was investigated in cell column perifusion experiments with primary cultures of dispersed pituitary cells. GH responses to PACAP were unaffected by three NOS inhibitors, aminoguanidine hemisulfate, 1400W and 7-nitroindazole (7-Ni). PACAP-stimulated GH release was also not reduced by two NO scavengers, rutin hydrate and PTIO, but NO-donor sodium nitroprusside (SNP)-elicited GH release was additive to the GH response to PACAP. In contrast, DA-induced GH secretion was reduced by 7-Ni, rutin hydrate and PTIO while not being additive to the GH response induced by SNP. These results indicate that although both PACAP and DA stimulation of acute GH release involve activation of adenylate cyclase/cAMP, DA- but not PACAP-signaling also utilizes the NOS/NO second messenger system.

E-box regulation of gonadotropin-releasing hormone (GnRH) receptor expression in immortalized gonadotrope cells

The pituitary gland's ability to respond to the hypothalamic hormone GnRH (gonadotropin-releasing hormone) depends directly on the gonadotrope-specific expression of the GnRH receptor (GnRHR), a G-protein coupled transmembrane protein coded by the GnRHR gene. In the present study, we have investigated the potential regulatory role of seven noncanonical E-box enhancer sequences within the 856bp proximal 5'-flanking region of the mGnRHR gene in regulating transcription. These sequences are known to mediate the action of clock gene proteins on the expression of a diverse array of genes both central and peripheral. In the present studies the expression of all of the cognate clock genes was identified in the alphaT3-1 gonadotrope cell line. Additionally, luteinizing hormone-immunoreactive cells in the adult rodent pituitary gland were also shown to co-express the PERIOD-1 protein. By means of chromatin immunoprecipitation of alphaT3-1 nuclear extracts we were able to capture promoter fragments of the GnRHR and Period-1 genes, indicating that E-boxes in these promoters bind the CLOCK protein. RNA interference experiments with alphaT3-1 cells in which Bmal1 expression was attenuated also confirmed the involvement of E-boxes in transcriptional regulation of the mGnRHR gene. Subsequent luciferase reporter assay experiments with GnRHR constructs possessing intact or mutated E-boxes confirmed the use of these sequences for the regulation of mGnRH-R/luc expression. Transient overexpression of the dominant negative E-box-binding factor CLOCK-Delta19, or the inhibitory clock protein mPER1, markedly reduced CLOCK/BMAL1-driven mGnRH-R/luc expression in a dose-dependent fashion. Our data implicate the clock genes as important factors controlling GnRHR expression in murine gonadotrope cells.

What is the role of PACAP in gonadotrope function?

Strong evidence in favor of a direct action of hypothalamic PACAP at the pituitary to modulate gonadotrope function has been acquired mainly by in vitro studies using cultured pituitary cells or gonadotrope cell lines. In particular, PACAP has been shown to cooperate with GnRH, the primary regulator of gonadotropes, to regulate/modulate gonadotropin subunit gene expression, gonadotropin release as well as gonadotrope responsiveness. These effects of PACAP appear to be due essentially to its high potent stimulatory action on the cAMP/protein kinase pathway. Ensuing mechanisms include signaling cross-talk and/or enhanced gene expression within gonadotropes. PACAP may also indirectly operate on these cells through paracrine mechanisms. While PACAP has long been viewed as a hypophysiotropic factor, a locally produced PACAP has also been described. Interestingly, both appear similarly up-regulated at proestrus of the reproductive cycle in female rats. Further in vivo investigation is now necessary to ascertain the physiological relevance of the observed pituitary PACAP effects and especially to evaluate the respective contribution of hypothalamic and pituitary PACAP in the dynamic control of gonadotrope function.

Nitric oxide induces [Ca2+]i oscillations in pituitary GH3 cells: involvement of IDR and ERG K+ currents

The role of nitric oxide (NO) in the occurrence of intracellular Ca2+ concentration ([Ca2+]i) oscillations in pituitary GH3 cells was evaluated by studying the effect of increasing or decreasing endogenous NO synthesis with L-arginine and nitro-L-arginine methyl ester (L-NAME), respectively. When NO synthesis was blocked with L-NAME (1 mM) [Ca2+]i, oscillations disappeared in 68% of spontaneously active cells, whereas 41% of the quiescent cells showed [Ca2+]i oscillations in response to the NO synthase (NOS) substrate L-arginine (10 mM). This effect was reproduced by the NO donors NOC-18 and S-nitroso-N-acetylpenicillamine (SNAP). NOC-18 was ineffective in the presence of the L-type voltage-dependent Ca2+ channels (VDCC) blocker nimodipine (1 microM) or in Ca2+-free medium. Conversely, its effect was preserved when Ca2+ release from intracellular Ca2+ stores was inhibited either with the ryanodine-receptor blocker ryanodine (500 microM) or with the inositol 1,4,5-trisphosphate receptor blocker xestospongin C (3 microM). These results suggest that NO induces the appearance of [Ca2+]i oscillations by determining Ca2+ influx. Patch-clamp experiments excluded that NO acted directly on VDCC but suggested that NO determined membrane depolarization because of the inhibition of voltage-gated K+ channels. NOC-18 and SNAP caused a decrease in the amplitude of slow-inactivating (IDR) and ether-à-go-go-related gene (ERG) hyperpolarization-evoked, deactivating K+ currents. Similar results were obtained when GH3 cells were treated with L-arginine. The present study suggests that in GH3 cells, endogenous NO plays a permissive role for the occurrence of spontaneous [Ca2+]i oscillations through an inhibitory effect on IDR and on IERG.

Gonadotropin-releasing hormone-desensitized LbetaT2 gonadotrope cells are refractory to acute protein kinase C, cyclic AMP, and calcium-dependent signaling

Sustained exposure of gonadotropes to GnRH causes a pronounced desensitization of gonadotropin release, but the mechanisms involved are poorly understood. It is known that desensitization is associated with decreased GnRH receptor and Gq/11 levels in alphaT3-1 cells, but it is not known whether downstream signaling is impaired. We have shown previously that chronic stimulation of signaling via expression of an active form of Galphaq causes GnRH resistance in LbetaT2 cells. In this study we investigated whether chronic GnRH treatment could down-regulate protein kinase C (PKC), cAMP, or Ca2+-dependent signaling in LbetaT2 cells. We found that chronic GnRH treatment desensitizes cells to acute GnRH stimulation not only by reducing GnRH receptor and Gq/11 expression but also by down-regulating PKC, cAMP, and calcium-dependent signaling. Desensitization was observed for activation of ERK and p38 MAPK and induction of c-fos and LHbeta protein expression. Activation of individual signaling pathways was able to partially mimic the desensitizing effect of GnRH on ERK, p38 MAPK, c-fos, and LHbeta but not on Gq/11. Chronic stimulation with phorbol esters reduced GnRH receptor expression to the same extent as chronic GnRH. Sustained GnRH also desensitized PKC signaling by down-regulating the delta, epsilon, and theta isoforms of PKC. We further show that chronic GnRH treatment causes heterologous desensitization of other Gq-coupled receptors.

The rat pituitary promoter of the neuronal nitric oxide synthase gene contains an Sp1-, LIM homeodomain-dependent enhancer and a distinct bipartite gonadotropin-releasing hormone-responsive region

The neuronal nitric oxide synthase (NOS I) is expressed and hormonally regulated in rat anterior pituitary gonadotropes. In the present study, we investigated the mechanisms that underlie the constitutive and GnRH up-regulated activity of the pituitary exon 1p promoter of the NOS I gene in these cells. Through the use of 5'-deletions and transient transfections in L beta T2, a gonadotrope-derived cell line, we delineated a NOS I cell-specific (NCS) enhancer region (-73/-59) that is required for constitutive activity. Independently of the NCS enhancer, GnRH responsiveness is supported by a bipartite regulatory domain referred to as the GnRH response element I and II located between -33/-10 and -4/+4, the latter consisting of a cAMP-like response element. By combining transient transfections, gel shift, and supershift assays, we demonstrate that Sp1 and LIM-homeodomain-related protein bind the NCS enhancer, whereas cAMP response element binding protein and cAMP regulatory element modulator-like factors bind the GnRH response element II motif. We further show that factors involved in GnRH regulation are also implicated in constitutive activity, suggesting intimate links between constitutive and regulated promoter activity. We speculate that specific expression of the NOS I gene in gonadotropes together with its regulation by GnRH is suggestive of a critical participation of NOS I in gonadotrope function.

Expression of the mouse gonadotropin-releasing hormone receptor gene in alpha T3-1 gonadotrope cells is stimulated by cyclic 3',5'-adenosine monophosphate and protein kinase A, and is modulated by Steroidogenic factor-1 and Nur77

Regulation of GnRH receptor (GnRHR) expression levels in the pituitary is a crucial control point in reproduction. The promoter of the mouse GnRHR gene contains nuclear receptor half-sites (NRS) at -244/-236 and -15/-7 relative to the translation start site. Although binding of steroidogenic factor-1 (SF-1) to the -244/-236NRS is implicated in mediating basal and gonadotrope-specific expression, no function or protein-DNA interactions have previously been described for the -15/-7NRS. We report that levels of the endogenous GnRHR mRNA in alpha T3-1 cells are stimulated by forskolin and 8-bromo-cAMP. We also show that the orphan nuclear receptor Nur77 is expressed in alpha T3-1 cells, and that both SF-1 and Nur77 bind to the -15/-7NRS and -244/-236NRS in vitro. We show that the activity of the proximal (-579/+1) mouse GnRHR promoter is up-regulated by protein kinase A, via a mechanism that is modulated by SF-1, both positively and negatively, through binding to the -244/-236NRS or the -15/-7NRS, respectively. Nur77 appears to be capable of acting as a negative regulator of this response, via the -15/-7NRS. Furthermore, we show that forskolin up-regulates SF-1 mRNA levels in alpha T3-1 cells, indicating that the levels of SF-1 play a role in modulating the protein kinase A response.