Estradiol coupling to endothelial nitric oxide stimulates gonadotropin-releasing hormone release from rat median eminence via a membrane receptor

Differential regulation of gonadotropin-releasing hormone secretion and gene expression by androgen: membrane versus nuclear receptor activation

Steroid hormones induce rapid membrane receptor-mediated effects that appear to be separate from long-term genomic events. The membrane receptor-mediated effects of androgens on GT1-7 GnRH-secreting neurons were examined. We observed androgen binding activity with a cell-impermeable BSA-conjugated testosterone [testosterone 3-(O-carboxymethyl)oxime (T-3-BSA)] and were able to detect a 110-kDa protein recognized by the androgen receptor (AR) monoclonal MA1-150 antibody in the plasma membrane fraction of the GT1-7 cells by Western analysis. Further, a transfected green fluorescent protein-tagged AR translocates and colocalizes to the plasma membrane of the GT1-7 neuron. Treatment with 10 nM 5alpha-dihydrotestosterone (DHT) inhibits forskolin-stimulated accumulation of cAMP, through a pertussis toxin-sensitive G protein, but has no effect on basal cAMP levels. The inhibition of forskolin-stimulated cAMP accumulation by DHT was blocked by hydroxyflutamide, a specific inhibitor of the nuclear AR. DHT, testosterone (T), and T-3-BSA, all caused significant elevations in intracellular calcium concentrations ([Ca(2+)](i)). T-3-BSA stimulates GnRH secretion 2-fold in the GT1-7 neuron, as did DHT or T. Interestingly GnRH mRNA levels were down-regulated by DHT and T as has been reported, but not by treatment with T-3-BSA or testosterone 17beta-hemisuccinate BSA. These studies indicate that androgen can differentially regulate GnRH secretion and gene expression through specific membrane-mediated or nuclear mechanisms.

Neuroendocrine plasticity in GnRH release during rat estrous cycle: correlation with molecular markers of synaptic remodeling

Morphological changes in the gonadotropin releasing hormone (GnRH) neurons in the preoptic area (POA) and their terminals in the median eminence-arcuate (ME-ARC) region are reported to occur during ovarian cycle that may be involved in the GnRH release into the portal blood during preovulatory surge. However, the neuronal substrates participating in altered GnRH neuronal plasticity are poorly understood. The present study was designed to determine whether morphological changes occurring in the GnRH neuron cell bodies in the POA and their terminals in the ME-ARC region of hypothalamus with pulsatile GnRH release in cycling female rats are associated with expression of intrinsic determinants of neuronal plasticity. The plasticity markers studied are polysialylated neural cell adhesion molecule (PSA-NCAM), high molecular weight isoforms of NCAM, growth associated protein (GAP-43), glial fibrillary acidic protein (GFAP) and synaptophysin. Regularly cycling female rats were sacrificed at diestrous, i.e., when GnRH release is low, and at proestrous, i.e., when preovulatory GnRH surge occurs, using perfusion fixation method for immunohistochemical staining of GnRH cells. GnRH cell bodies and their terminals from the POA and ME-ARC region respectively, were localized using immunohistochemical technique in proestrous and diestrous phase of estrous cycle and our results showed a marked increase in the GnRH nerve terminals length and immunoreactivity in the ME-ARC region from proestrous phase rats as compared to diestrous rats. Immunoblot analyses of the POA and ME-ARC region of the hypothalamus revealed a significant increase in the content of PSA-NCAM, NCAM-180, NCAM-140, GAP-43 and synaptophysin from proestrous phase rats as compared to diestrous phase rats. The ME-ARC region showed more pronounced increase in the protein expression of these markers of neuronal plasticity as compared to the POA, whereas, hippocampal region did not show any significant change in the content of these markers showing specificity of the changes to the GnRH system. GFAP content was significantly decreased in the POA with a marginal increase in the GFAP level from the ME-ARC region. These results demonstrate the involvement of synaptic proteins in the dynamic plasticity of the ME-ARC region of hypothalamus, allowing GnRH nerve terminals to release the neurohormone into the pituitary portal blood on the day of proestrous.

Deletion of exon 6 of the neuronal nitric oxide synthase gene in mice results in hypogonadism and infertility

Nitric oxide (NO) has been recognized as a modulator in reproductive functions, but it is not clear whether NO is required for fertility. The first line of mice deficient in neuronal NO synthase (referred to herein as KN1 mice) reproduce normally. However, residual neuronal NO synthase (nNOS) activity is detected in KN1 mice due to the expression of beta- and gamma-nNOS splice variants. We generated a new line of nNOS knockout mice (KN2) lacking exon 6, which codes for the heme-binding domain of nNOS. KN2 mice are viable, but mated homozygotes do not produce litters, indicating that either one or both sexes are infertile. Male KN2 mice show decreased gonad weights, but sperm counts are normal. KN2 males do not display mating behavior, and consequently do not leave vaginal plugs when housed with wild-type (WT) females. KN2 females show decreased ovary weight, and histology reveals decreased corpus luteum counts. RIAs show that KN2 males have decreased plasma FSH, whereas KN2 females have increased levels of plasma LH and increased hypothalamic GnRH content. Experimental ovarian transplantation suggests that central, rather than ovarian, processes are influenced by nNOS, as KN2 ovaries ovulate at near-normal rates under WT hormonal control, whereas WT ovaries transplanted into KN2 mice have decreased ovulation rates. We observed pyloric stenosis in KN2 mice, but plasma leptin levels are normal, and no ketones are found, indicating that hypogonadism is not a result of malnutrition. We conclude that nNOS is required for normal central hormonal regulation of reproductive function.

Glial-neuronal-endothelial interactions are involved in the control of GnRH secretion

In recent years compelling evidence has been provided that cell-cell interactions involving non-neuronal cells, such as glial and endothelial cells, are important in regulating the secretion of GnRH, the neuropeptide that controls both sexual development and adult reproductive function. Modification of the anatomical relationship that exist between GnRH nerve endings and glial cell processes in the external zone of the median eminence modulates the access of GnRH nerve terminals to the portal vasculature during the oestrous cycle. The establishment of direct neuro-haemal junctions between GnRH neuroendocrine terminals and the portal vasculature on the day of pro-oestrus may be critical for the transfer of GnRH upon its release into the fenestrated capillaries of the median eminence. Notwithstanding the importance of these plastic rearrangements, glial and endothelial cells also regulate GnRH neuronal function via specific cell-cell signalling molecules. While endothelial cells of the median eminence use nitric oxide to effect this regulatory control, astrocytes employ several growth factors, and in particular those of the EGF family and their erbB receptors to facilitate GnRH release during sexual development. Loss of function of each of these erbB receptors involved in the astroglial control of GnRH secretion leads to delayed sexual development. It is clear that regulation of GnRH secretion by cell-cell communication mechanisms other than transsynaptic inputs is an important component of the central neuroendocrine process controlling mammalian reproduction.

Rapid actions of 17beta-oestradiol on a subset of lactotrophs in the rat pituitary

Increasingly the role of rapid mechanisms of steroid action in physiological regulation are being recognised. We have investigated rapid effects of 17beta-oestradiol (E) on prolactin (PRL) release in vitro. Pituitary segments from male rats were incubated for 5, 10 or 20 min in Earle's balanced salt solution containing 1.2 mM tannic acid (to enable visualisation of exocytosed secretory granules by electron microscopy) either alone (control) or containing 10(-10)-10(-8) M E conjugated to bovine serum albumin (E-BSA). PRL and leuteinising hormone (LH) release from pituitary segments were also determined in response to E and E-BSA by radioimmunoassay. Within 10 min E-BSA and E (10(-12)-10(-6) M) stimulated a significant (P < 0.05) concentration-dependent release of PRL but not LH. After exposure to experimental media for 5 min, only occasional exocytosis from type I lactotrophs (characterised by large polymorphic secretory granules) was observed in either control or E-BSA treated tissue. In contrast, E-BSA (10(-10)-10(-8) M) induced a significant (P < 0.05) increase in the number of exocytotic profiles from type II lactotrophs (characterized by smaller, spherical granules). This effect was not inhibited by removal of extracellular calcium, or by pre-treatment of cells with the RNA synthesis inhibitor actinomycin-D (0.5 microg ml(-1)), the protein synthesis inhibitor cycloheximide (1 microg ml(-1)) or the anti-oestrogen ICI 182,780 (1 microM). FACS analysis demonstrated binding of E-BSA-fluorescein isothiocyanate (FITC) (10(-10)-10(-7) M) to a subpopulation of anterior pituitary cells. The E-BSA-FITC binding sites assumed a patchy distribution across the cell surface. In conclusion, we report for the first time a rapid, non-genomic effect of E on PRL secretion in normal pituitary tissue.

Oestrogen receptors, receptor variants and oestrogen actions in the hypothalamic-pituitary axis

Information on oestrogen action has grown exponentially in the past decade, and recent studies have begun to define the mechanism of ligand-dependent activation and cell-specific effects. Oestrogen-mediated gene transcription in a specific tissue depends on several factors, the most important of which is the presence of at least one of the two nuclear oestrogen receptor (ER) isoforms, ER(alpha) and ERbeta. The presence and levels of specific ER isoform variants, along with receptor coactivator, corepressor and integrator proteins, directly modulate overall nuclear ER activity. The structure of the ligand, including both physiological oestrogens and synthetic oestrogen receptor modulators, influences ER interactions with these other proteins and thus determines the biological response. Furthermore, peptide and neurotransmitter-stimulated intracellular signalling pathways activate specific enzyme cascades and may modify the receptors and their interacting proteins, resulting in either independent or ligand-enhanced ER-mediated responses. Finally, several rapid effects of oestrogen probably occur at the membrane through nongenomic pathways that may or may not require the same ER proteins that are found in the nucleus. This review concentrates on the pituitary-hypothalamic axis and the genomic effects of oestrogen, and discusses the current knowledge of each of these factors in determining oestrogen actions in the neuroendocrine system.

Dopamine as a prolactin (PRL) inhibitor

Dopamine is a small and relatively simple molecule that fulfills diverse functions. Within the brain, it acts as a classical neurotransmitter whose attenuation or overactivity can result in disorders such as Parkinson's disease and schizophrenia. Major advances in the cloning and characterization of biosynthetic enzymes, transporters, and receptors have increased our knowledge regarding the metabolism, release, reuptake, and mechanism of action of dopamine. Dopamine reaches the pituitary via hypophysial portal blood from several hypothalamic nerve tracts that are regulated by PRL itself, estrogens, and several neuropeptides and neurotransmitters. Dopamine binds to type-2 dopamine receptors that are functionally linked to membrane channels and G proteins and suppresses the high intrinsic secretory activity of the pituitary lactotrophs. In addition to inhibiting PRL release by controlling calcium fluxes, dopamine activates several interacting intracellular signaling pathways and suppresses PRL gene expression and lactotroph proliferation. Thus, PRL homeostasis should be viewed in the context of a fine balance between the action of dopamine as an inhibitor and the many hypothalamic, systemic, and local factors acting as stimulators, none of which has yet emerged as a primary PRL releasing factor. The generation of transgenic animals with overexpressed or mutated genes expanded our understanding of dopamine-PRL interactions and the physiological consequences of their perturbations. PRL release in humans, which differs in many respects from that in laboratory animals, is affected by several drugs used in clinical practice. Hyperprolactinemia is a major neuroendocrine-related cause of reproductive disturbances in both men and women. The treatment of hyperprolactinemia has greatly benefited from the generation of progressively more effective and selective dopaminergic drugs.

Involvement of NPY Y2 receptor subtype in the control of the spontaneous NO/GnRH release at the rat median eminence

The role of nitric oxide (NO) from vascular endothelium in the control of GnRH release at the median eminence (ME) level is well established. Interactions between NPY receptor/endothelium/nitric oxide are clearly demonstrated. While several studies implicate NPY Y1 receptor in the control of GnRH/LH at the time of the preovulatory LH surge, our results also demonstrate the importance of NPY Y2 receptor in the control of GnRH release via endothelial NO. We conclude that NPY may be one of the elements implicated in the generation of the spontaneous NO/GnRH via Y2 receptor located on endothelium.

New evidence for estrogen receptors in gonadotropin-releasing hormone neurons

Estrogen exerts a critical regulatory influence upon the biosynthetic and secretory activity of the gonadotropin-releasing hormone (GnRH) neurons. It seems likely that estrogen regulates the behavior of the GnRH neuron through multiple transsynaptic, neuronal-glial, and direct membrane modes of action. Advances in our understanding of these mechanisms over the last 3 years are highlighted. In addition, very recent studies have begun to provide evidence for the expression of estrogen receptors (ERs) in GnRH neurons in the rodent. Although not yet firmly established, the current consensus supports the hypothesis that GnRH neurons express ERbeta. Evidence exists for ERbeta mRNA expression by GnRH neurons throughout development and ERbeta immunoreactivity has now also been detected in these cells. Murine GnRH neurons have further been shown to express estrogen receptor-related receptor-alpha, an orphan receptor thought to constitutively activate estrogen response elements. Together, these findings provide a cornerstone for the reassessment of the role of ERs and related receptors in the direct genomic and potential nontranscriptional actions of estrogen upon the GnRH neuron.

Evidence that TGF beta may directly modulate POMC mRNA expression in the female rat arcuate nucleus

The purpose of the present study was to determine whether TGF beta, a cytokine secreted by hypothalamic astrocytes, was able to regulate POMC neurons in the arcuate nucleus. In a first set of experiments, mediobasal hypothalamic fragments were exposed to TGF beta(1), and the relative POMC mRNA expression was assessed by in situ hybridization using a radiolabeled POMC riboprobe. The results showed that 4 x 10(-10) M TGF beta(1) was efficient in decreasing significantly the amounts of POMC mRNA (P < 0.01). Interestingly, the decrease of relative POMC mRNA levels was higher in the rostral than in the caudal parts of the arcuate nucleus. In a second set of experiments, we examined the occurrence of TGF beta receptors expression in arcuate POMC neurons. Dual labeling in situ hybridization and in situ hybridization, coupled to immunohistochemical labeling, were performed to examine mRNA expression of the type I serine-threonine kinase receptor for TGF beta and the presence of type II receptor for TGF beta, respectively, in POMC neurons. The results indicated that TGF beta receptor I mRNA and TGF beta receptor II protein were expressed in numerous POMC neurons. Regional analysis revealed that the highest proportion of POMC neurons expressing TGF beta receptors was located in the rostral part of the arcuate nucleus. Using dual labeling immunohistochemistry, we also found that Smad2/3 immunoreactivity, a TGF beta(1) downstream signaling molecule, was present in the cytoplasm and nucleus of some POMC (beta-endorphin) neurons. We next examined whether the number of POMC neurons expressing TGF beta-RI mRNA was affected by sex steroids. Quantification of the number of POMC neurons expressing TGF beta receptor I mRNA in ovariectomized, ovariectomized E2-treated, and ovariectomized E2 plus progesterone-treated animals revealed that estrogen treatment decreased the expression of TGF beta receptor I mRNA in POMC neurons located in the rostral half of the arcuate nucleus, an effect reversed by progesterone in a subset of the most rostral cells. Taken together, these data reveal that TGF beta(1) may directly modulate the activity of POMC neurons through the activation of TGF beta receptors. Therefore, the present study provides additional evidence for the involvement of TGF beta(1) in the regulation of neuroendocrine functions and supports the existence of a glial-to-neurons communication within the arcuate nucleus.

Evidence for a spontaneous nitric oxide release from the rat median eminence: influence on gonadotropin-releasing hormone release

The involvement of nitric oxide (NO) as a gaseous neurotransmitter in the hypothalamic control of pituitary LH secretion has been demonstrated. NO, as a diffusible signaling gas, has the ability to control and synchronize the activity of the neighboring cells. NO is secreted at the median eminence (ME), the common termination field for the antehypophysiotropic neurons, under the stimulation of other signaling substances. At the ME, NO stimulates GnRH release from neuroendocrine terminals. The present studies were undertaken to determine whether NO is secreted spontaneously from ME fragments ex vivo and whether its secretion is correlated to GnRH release. To accomplish this, female rats were killed at different time points of the day and/or of the estrous cycle. The spontaneous NO release was monitored in real time, with an amperometric probe, during 4 periods of 30 min, from individual ME fragments (for each time point, n = 4). GnRH levels were measured in parallel for each incubation-period by RIA. The results revealed that NO was released in a pulsatile manner from female ME fragments and, unambiguously, that the amplitude of NO secretion varied markedly across the estrous cycle. Indeed, though the NO pulse period (32 +/- 1 min, n = 36) and duration (21 +/- 2 min, n = 36) did not vary significantly across the estrous cycle, the amplitude of this secretion pulse was significantly higher on proestrus (Pro; 39 +/- 3 nM, n = 20), compared with diestrus (16 +/- 1 nM, n = 8) or estrus (23 +/- 3 nM, n = 8, P < 0.05). The GnRH levels in the incubation medium were positively correlated to NO secretion across the estrous cycle (r = 0.86, P < 0.003, n = 9), confirming that NO and GnRH release are coupled. Furthermore, 5 x 10(-7) M L-N(5)-(1-iminoethyl)ornithine (L-NIO), a NO synthase inhibitor, succeeded in inhibiting the strong NO-GnRH secretory coupling and GnRH release on PRO: Because at this concentration, L-NIO selectively inhibits endothelial NO synthase, the results further demonstrate that the major source of NO involved in GnRH release at the ME is endothelial in origin. Additionally, the induction of a massive NO/GnRH release in 15-day ovariectomized rat treated with estradiol benzoate strongly suggested that estradiol is participating in the stimulation of NO release activity between diestrus II and PRO: The present study is the first demonstrating that ME can spontaneously release NO and that NO's rhythm of secretion varies markedly across the estrous cycle. This pulsatile/cyclic ME NO release may constitute the synchronizing link to anatomically scattered GnRH neurons.

Estrogen receptor (ER)alpha, but not ERbeta, gene is expressed in growth hormone-releasing hormone neurons of the male rat hypothalamus

GH synthesis and release from pituitary somatotropes is controlled by the opposing actions of the hypothalamic neuropeptides, GH-releasing hormone (GHRH), and somatostatin (SS). There is a striking sex difference in the pattern of GH secretion in rats. Early reports indicate that gonadal steroids have important imprinting effects during the neonatal period. Recently, our laboratory and others have reported that the GH secretory pattern is altered by short-term gonadal steroid treatment in adult rat, suggesting that gonadal steroids are also important determinants of the pattern of GH secretion during adult life. However, the site of action of gonadal steroids in the adult rat hypothalamus is still unknown. In this study, we used in situ hybridization in the adult male rat brain to determine whether GHRH neurons and/or SS neurons coexpress estrogen receptor alpha (ERalpha) and ERss genes. In the medial basal hypothalamus of adult male rat, the ERalpha messenger RNA (mRNA) was located in medial preoptic area (MPA) and arcuate nucleus (ARC), whereas ERss mRNA was detected in MPA, supraoptic nucleus, and paraventricular nucleus. From studies using adjacent sections, the distribution of ERalpha mRNA-containing cells appeared to overlap in part with those of GHRH and SS expressing cells only in the ARC. On the other hand, the distribution of ERss mRNA-containing cells does not appear to overlap with GHRH cells or SS cells. The double label in situ hybridization studies showed that in the ARC, 70% of GHRH neurons contain ERalpha mRNA, whereas less than 5% of SS neurons expressed the ERalpha gene. These results indicated that GHRH neurons are direct target cells for estrogens, and estrogens may act directly on GHRH neurons through ERalpha during adult life to modify GH secretory patterns.

Evidence that Alzheimer's disease is a microvascular disorder: the role of constitutive nitric oxide

Evidence is fast accumulating which indicates that Alzheimer's disease is a vascular disorder with neurodegenerative consequences rather than a neurodegenerative disorder with vascular consequences. It is proposed that two factors need to be present for AD to develop: (1) advanced ageing, (2) presence of a condition that lowers cerebral perfusion, such as a vascular-risk factor. The first factor introduces a normal but potentially insidious process that lowers cerebral blood flow in inverse relation to increased ageing; the second factor adds a crucial burden which further lowers brain perfusion and places vulnerable neurons in a state of high energy compromise leading to a cascade of neuronal metabolic turmoil. Convergence of the two factors above will culminate in a critically attained threshold of cerebral hypoperfusion (CATCH). CATCH is a hemodynamic microcirculatory insufficiency that will destabilize neurons, synapses, neurotransmission and cognitive function, creating in its wake a neurodegenerative state characterized by the formation of senile plaques, neurofibrillary tangles, amyloid angiopathy and in some cases, Lewy bodies. Since any of a considerable number of vascular-related conditions must be present in the ageing individual for cognition to be disturbed, CATCH identifies an important aspect of the heterogeneic disease profile assumed to be present in the AD syndrome. It is proposed that CATCH initiates AD by distorting regional brain capillary structure involving endothelial cell shape changes and impairment of nitric oxide (NO) release which affect signaling between the immune, cardiovascular and nervous systems. Evidence is presented that in many tissues there is a basal level of NO being produced and that the actions of several signaling molecules may initiate increases in basal NO levels. Moreover, these temporary increases in basal NO levels exert inhibitory cellular actions, via cellular conformational changes. Findings indicate that (a) constitutive NO is responsible for a basal or 'tonal' level of NO; (b) this NO keeps particular types of cells in a state of inhibition and (c) activation of these cells occurs through disinhibition. Consequently, tissues not maintaining a basal NO level are more prone to excitatory, immune, vascular and neural influences. Under such circumstances, these tissues cannot be down-regulated to normal basal levels, thus prolonging their excitatory state. Thus, the clinical convergence of advanced ageing in the presence of a chronic, pre-morbid vascular risk factor, can, in time, contribute to an endotheliopathy involving basal NO deficit, to the degree where regional metabolic dysfunction leads to cognitive meltdown and to progressive neurodegeneration characteristic of Alzheimer's disease.

Whole-cell recordings from preoptic/hypothalamic slices reveal burst firing in gonadotropin-releasing hormone neurons identified with green fluorescent protein in transgenic mice

Central control of reproduction is governed by a neuronal pulse generator that underlies the activity of hypothalamic neuroendocrine cells that secrete GnRH. Bursts and prolonged episodes of repetitive action potentials have been associated with hormone secretion in this and other neuroendocrine systems. To begin to investigate the cellular mechanisms responsible for the GnRH pulse generator, we used transgenic mice in which green fluorescent protein was genetically targeted to GnRH neurons. Whole-cell recordings were obtained from 21 GnRH neurons, visually identified in 200-microm preoptic/hypothalamic slices, to determine whether they exhibit high frequency bursts of action potentials and are electrically coupled at or near the somata. All GnRH neurons fired spontaneous action potentials, and in 15 of 21 GnRH neurons, the action potentials occurred in single bursts or episodes of repetitive bursts of high frequency spikes (9.77 +/- 0.87 Hz) lasting 3-120 sec. Extended periods of quiescence of up to 30 min preceded and followed these periods of repetitive firing. Examination of 92 GnRH neurons (including 32 neurons that were located near another green fluorescent protein-positive neuron) revealed evidence for coupling in only 1 pair of GnRH neurons. The evidence for minimal coupling between these neuroendocrine cells suggests that direct soma to soma transfer of information, through either cytoplasmic bridges or gap junctions, has a minor role in synchronization of GnRH neurons. The pattern of electrical activity observed in single GnRH neurons within slices is temporally consistent with observations of GnRH release and multiple unit electrophysiological correlates of LH release. Episodes of burst firing of individual GnRH neurons may represent a component of the GnRH pulse generator.

Gonadotropin-releasing hormone-immunoreactive neurons and associated nicotinamide adenine dinucleotide phosphate-diaphorase-positive neurons in the brain of a teleost, Rhodeus amarus

Using combined nicotinamide adenine dinucleotide phosphate-diaphorase (NADPHd) histochemistry and salmon gonadotropin-releasing hormone (sGnRH) immunocytochemistry, it is reported for the first time that possible potential contacts occur between the nitric oxide (NO)- and the GnRH-containing neurons in the brain of a freshwater teleost, Rhodeus amarus. GnRH-immunoreactive (ir) neurons were observed in the olfactory nerve (OLN), olfactory bulb (OB), medial olfactory tract (MOT), ventral telencephalon (VT), nucleus preopticus periventricularis (NPP), nucleus lateralis tuberis (NLT), and midbrain tegmentum (MT). Although NADPHd neurons were widely distributed in the brain, only those having an association with GnRH-ir neurons are described. Based on the nature of the association between the GnRH and the NADPHd neurons, the former were classified into three types. The Type I GnRH neurons were characterized by the presence of NADPHd-positive granules in the perikarya and processes and occurred in the OLN, OB, MOT, and VT. The Type II GnRH neurons, having soma-soma or soma-process contacts with the NADPHd neurons, were restricted to the MT; the long processes of NADPHd cells crossed over either the perikarya or the thick processes of GnRH cells. However, the Type III GnRH neurons, found in the NPP and NLT, did not show direct contact, but a few NADPHd fibers were present in the vicinity. The terminal-soma contacts in the olfactory system and the VT and the soma-soma contacts in the MT represent the sites of possible potential contacts indicating a direct NO involvement in GnRH function, although NO action by diffusion remains possible. NO may influence the NPP and NLT GnRH cells by diffusion only, since a direct contact was not observed.

Evidence that members of the TGFbeta superfamily play a role in regulation of the GnRH neuroendocrine axis: expression of a type I serine-threonine kinase receptor for TGRbeta and activin in GnRH neurones and hypothalamic areas of the female rat

The present study was designed to determine whether transforming growth factor (TGF)beta and/or activin participate in the regulation of the gonadotropin releasing hormone (GnRH) neuroendocrine axis in vivo. Single-label in situ hybridization histochemistry was used to determine the anatomical distribution of a TGFbeta and activin type I receptor (B1) mRNA, in the adult female rat hypothalamic areas that are known to be important sites for the regulation of reproduction. Dual-label in situ hybridization histochemistry was performed to determine whether B1 mRNA was expressed in GnRH neurones. The results of these studies revealed an extensive distribution of B1 mRNA in the hypothalamic regions, including diagonal bands of Broca, preoptic area, arcuate nucleus and median eminence. In the median eminence, B1 mRNA was detected in tanycytes and in the endothelial cells of the pituitary portal blood capillaries. Dual-label in situ hybridization histochemistry showed that 31+/-5% of GnRH neurones expressed B1 mRNA, thus providing evidence that TGFbeta and/or activin can act directly on GnRH neurones to modulate their activity. Taken together, these data provide morphological arguments in favour of a participation of TGFbeta and/or activin in the regulation of reproduction at the hypothalamic level.

Estradiol-stimulated nitric oxide release in human granulocytes is dependent on intracellular calcium transients: evidence of a cell surface estrogen receptor

We tested the hypothesis that estrogen acutely stimulates constitutive nitric oxide synthase activity in human granulocytes by acting on a cell surface estrogen receptor (ER). The release of nitric oxide was measured in real time with an amperometric probe. Exposure of granulocytes to 17β-estradiol stimulated NO release within seconds in a concentration-dependent manner. The NO release was also stimulated by 17β-estradiol conjugated to bovine serum albumin (E2-BSA), which suggests mediation by a cell surface receptor. Tamoxifen, an ER inhibitor, antagonized the action of both 17β-estradiol and E2-BSA, whereas ICI 182,780, an inhibitor of the nuclear ER, had no effect. Using dual emission microfluorometry in a calcium-free medium, the 17β-estradiol–stimulated release of NO from granulocytes was shown to be dependent on intracellular calcium ([Ca2+]i) transients in a tamoxifen-sensitive process. Exposure to BAPTA-AM (1,2bis-(-aminophenoxy)ethans-N,N,N′,N′-tetraacetic acid tetra(acetoxyymethyl) ester), a [Ca2+]i chelator, reduced [Ca2+]i in response to E2-BSA, and depleting [Ca2+]i stores abolished the effect of 17β-estradiol on NO release. Confocal photomicrographs using E2-BSA–FITC (fluorescein isothiocyanate) revealed cell membrane reactivity. Estrogen-stimulated NO release had an immunosuppressive effect, and it initiated granulocyte rounding and loss of adherence in a tamoxifen-sensitive manner. Finally, using reverse transcriptase–polymerase chain reaction, human neutrophil granulocytes expressed ER but not ERβ, suggesting that ER may be the membrane receptor for 17β-estradiol. The study demonstrated that a physiological dose of estrogen down-regulates granulocyte activity by acutely stimulating NO release via the activation of a cell surface ER which is coupled to increases in [Ca2+]i.

Estradiol-stimulated nitric oxide release in human granulocytes is dependent on intracellular calcium transients: evidence of a cell surface estrogen receptor

We tested the hypothesis that estrogen acutely stimulates constitutive nitric oxide synthase activity in human granulocytes by acting on a cell surface estrogen receptor (ER). The release of nitric oxide was measured in real time with an amperometric probe. Exposure of granulocytes to 17β-estradiol stimulated NO release within seconds in a concentration-dependent manner. The NO release was also stimulated by 17β-estradiol conjugated to bovine serum albumin (E2-BSA), which suggests mediation by a cell surface receptor. Tamoxifen, an ER inhibitor, antagonized the action of both 17β-estradiol and E2-BSA, whereas ICI 182,780, an inhibitor of the nuclear ER, had no effect. Using dual emission microfluorometry in a calcium-free medium, the 17β-estradiol–stimulated release of NO from granulocytes was shown to be dependent on intracellular calcium ([Ca2+]i) transients in a tamoxifen-sensitive process. Exposure to BAPTA-AM (1,2bis-(-aminophenoxy)ethans-N,N,N′,N′-tetraacetic acid tetra(acetoxyymethyl) ester), a [Ca2+]i chelator, reduced [Ca2+]i in response to E2-BSA, and depleting [Ca2+]i stores abolished the effect of 17β-estradiol on NO release. Confocal photomicrographs using E2-BSA–FITC (fluorescein isothiocyanate) revealed cell membrane reactivity. Estrogen-stimulated NO release had an immunosuppressive effect, and it initiated granulocyte rounding and loss of adherence in a tamoxifen-sensitive manner. Finally, using reverse transcriptase–polymerase chain reaction, human neutrophil granulocytes expressed ER but not ERβ, suggesting that ER may be the membrane receptor for 17β-estradiol. The study demonstrated that a physiological dose of estrogen down-regulates granulocyte activity by acutely stimulating NO release via the activation of a cell surface ER which is coupled to increases in [Ca2+]i.

Endothelial and neuronal nitric oxide synthases are present in the suprachiasmatic nuclei of Syrian hamsters and rats

Nitric oxide (NO) is involved in the transmission of light information to suprachiasmatic nuclei (SCN). By immunocytochemistry, we showed that both neuronal and endothelial NO synthase isoforms (nNOS and eNOS) were present in the SCN of rats and hamsters. nNOS-immunoreactive neurons were located mainly around the SCN with only a few nNOS neurons within the nucleus. By double-label immunocytochemistry, we also found, within the population of SCN glial fibrillary acidic protein (GFAP)-immunoreactive astrocytes, a subpopulation of eNOS-immunoreactive astrocytes. Using Western blot analysis, we detected in SCN protein extracts eNOS and nNOS proteins having the expected 140 and 150 kDa molecular weights, respectively. By in situ hybridization of a 2.4-kb murine eNOS probe, mRNA for eNOS was located in the SCN of rats and hamsters. The transcript was further identified by detection of a RT-PCR product of the predicted size, after amplification of total RNA with primers specific for eNOS. In the SCN and cerebellum, the size of the mRNA for nNOS, detected with a rat probe on Northern blot, was approximately 10.5 kb, corresponding to that previously published. In the same tissues, we found two transcripts, one weakly expressed at approximately 4.0 kb and another more strongly expressed at approximately 2.6 kb, both hybridizing with two non-overlapping murine and rat eNOS probes. These results suggested the existence in the SCN of alternate transcripts for eNOS. We propose that two pathways could link light stimuli and NO release in the SCN: one involving N-methyl-D-aspartate (NMDA) receptors and nNOS in neurons; the other linking alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and eNOS in astrocytes.

Distribution of aromatase activity in the brain and peripheral tissues of passerine and nonpasserine avian species

Many behavioral effects of testosterone on hypothalamic and limbic brain areas are mediated by the action, at the cellular level, of estrogens derived from local testosterone aromatization. Aromatase activity and cells containing the aromatase protein and mRNA have accordingly been identified in the brain areas involved in the control of behavior. The presence of an unusually high level of aromatase activity has been detected in the telencephalon of one songbird species, the zebra finch (Taeniopygia guttata), and it is suspected that this high telencephalic aromatase may be a specific feature of songbirds but this idea is supported only by few experimental data. The distribution of aromatase activity in the brain of zebra finches and of one nonsongbird species, the Japanese quail (Coturnix japonica), was compared with the distribution of aromatase activity in the brain of four species of free-living European songbirds, the chaffinch (Fringilla coelebs, Fringillidae), willow warbler (Phylloscopus trochilus, Sylviidae), great tit (Parus major, Paridae), and pied flycatcher (Ficedula hypoleuca, Muscicapidae). High levels of enzyme activity were observed in the diencephalon of all species. The high levels of aromatase activity that had been observed in the zebra finch telencephalon and were thought to be typical of songbirds were also present in the four wild oscine species but not in quail. None of these songbird species had, however, a telencephalic aromatase activity as high as that in the zebra finch, which may represent an extreme as far as the activity of this enzyme in the telencephalon is concerned. Measurable levels of aromatase activity were also detected in all songbird species in the liver and in the three other brain areas that were assayed, the optic lobes, cerebellum, and brain stem, with the exception of the cerebellum in willow warblers and quail, but no detectable activity was observed in the testes, muscle, and adrenals of all species. Additional studies will be needed to identify the functional significance of estrogen synthesis in areas that are not classically known to be implicated in the control of reproduction. Within a given species, the birds that had the highest plasma testosterone levels also displayed the highest levels of diencephalic aromatase activity and the interspecies differences in the two variables were positively related. This raises the possibility that the absolute level of diencephalic aromatase represents a species-specific characteristic under the control of plasma testosterone levels. There was, in contrast, no correlation between the aromatase activity in the telencephalon and the plasma testosterone levels but the enzyme activity was correlated with the plasma levels of luteinizing hormone. These data bring additional support to the idea that the diencephalic and telencephalic aromatases are controlled by independent mechanisms.

Non-genomic actions of 17beta-oestradiol in mouse pancreatic beta-cells are mediated by a cGMP-dependent protein kinase

1. Intracellular calcium concentration ([Ca2+]i) was measured in mouse whole islets of Langerhans using the calcium-sensitive fluorescent dye Indo-1. 2. Application of physiological concentrations of 17beta-oestradiol in the presence of a stimulatory glucose concentration (8 mM) potentiated the [Ca2+]i signal in 83 % of islets tested. Potentiation was manifested as either an increase in the frequency or duration of [Ca2+]i oscillations. 3. The effects caused by 17beta-oestradiol were mimicked by the cyclic nucleotide analogues 8-bromoguanosine-3',5'-cyclic monophosphate (8-Br-cGMP) and 8-bromoadenosine-3',5'-cyclic monophosphate (8-Br-cAMP). 4. Direct measurements of both cyclic nucleotides demonstrated that nanomolar concentrations of 17beta-oestradiol in the presence of 8 mM glucose increased cGMP levels, yet cAMP levels were unchanged. The increment in cGMP was similar to that induced by 11 mM glucose. 5. Patch-clamp recording in intact cells showed that 8-Br-cGMP reproduced the inhibitory action of 17beta-oestradiol on ATP-sensitive K+ (KATP) channel activity. This was not a membrane-bound effect since it could not be observed in excised patches. 6. The action of 17beta-oestradiol on KATP channel activity was not modified by the specific inhibitor of soluble guanylate cyclase (sGC) LY 83583. This result indicates a likely involvement of a membrane guanylate cyclase (mGC). 7. The rapid decrease in KATP channel activity elicited by 17beta-oestradiol was greatly reduced using Rp-8-pCPT-cGMPS, a specific blocker of cGMP-dependent protein kinase (PKG). Conversely, Rp-cAMPS, which inhibits cAMP-dependent protein kinase (PKA), had little effect. 8. The results presented here indicate that rapid, non-genomic effects of 17beta-oestradiol after interaction with its binding site at the plasma membrane of pancreatic beta-cells is a cGMP-dependent phosphorylation process.

Nongenomic stimulation of nitric oxide release by estrogen is mediated by estrogen receptor alpha localized in caveolae

Acute administration of 17beta-estradiol (E(2)) exerts antiatherosclerotic effects in healthy postmenopausal women. The vasoprotective action of E(2) may be partly accounted for by a rapid increase in nitric oxide (NO) levels in endothelial cells (ECs). However, the signaling mechanisms producing this rise are unknown. In an attempt to address the short-term effect of E(2) on endothelial NO production, confluent bovine aortic endothelial cells (BAECs) were incubated in the absence or presence of E(2), and NO production was measured. Significant increases in NO levels were detected after only 5 min of E(2) exposure without a change in the protein levels of endothelial NO synthase (eNOS). This short-term effect of estrogen was significantly blunted by various ligands which decrease intracellular Ca(2+) concentration. Furthermore, plasma membrane-impermeable BSA-conjugated E(2) (E(2)BSA) stimulated endothelial NO release, indicating that in the current system the site of action of E(2) is on the plasma membrane rather than the classical nuclear receptor. The partial antagonist tamoxifen did not block E(2)-induced NO production; however, a pure estrogen receptor alpha (ERalpha) antagonist ICI 182,780 completely inhibited E(2)-stimulated NO release. The binding of E(2) to the membrane was confirmed using FITC-labeled E(2)BSA (E(2)BSA-FITC). Western blot analysis showed that plasmalemmal caveolae possess ERalpha in addition to well-known caveolae-associated proteins eNOS and caveolin. This study demonstrates that the nongenomic and short-term effect of E(2) on endothelial NO release is Ca(2+)-dependent and occurs via ERalpha localized in plasmalemmal caveolae.

Estrogen regulates gonadotropin-releasing hormone in the nervus terminalis of Xenopus laevis

The nervus terminalis or terminal nerve (TN) is a neuronal plexus found in the nasal cavity and rostral forebrain of most vertebrates. The hormone gonadotropin-releasing hormone (GnRH) is found in a population of TN neurons as well as hypothalamic neurons which regulate pituitary secretion of the gonadotropins. The GnRH-containing neurons of the TN appear to represent a rostral continuation of the hypothalamic population since they both originate from the olfactory placode and are frequently anatomically continuous. Previous studies have shown that the hypothalamic GnRH neurons are regulated by circulating estrogen levels. Ovariectomy decreases while estrogen administration increases GnRH content in these neurons. It is not known whether the GnRH-containing TN neurons are also regulated in a similar manner. This study demonstrates that ovariectomy and estrogen readministration alters GnRH-immunoreactive (ir) levels in the TN of female Xenopus laevis in a manner similar to that seen in the hypothalamus. One week after ovariectomy, the density of TN GnRH-ir fibers in the olfactory bulb region (one site of TN termination) is significantly decreased. In contrast, a significant increase in GnRH-ir TN fiber density is observed following estrogen readministration to ovariectomized frogs. These findings demonstrate that estrogen regulates GnRH metabolism in neurons of the TN.