Astrocytes and brain function: implications for reproduction

Biosynthesis and signalling functions of central and peripheral nervous system neurosteroids in health and disease

Neurosteroids are steroid hormones synthesised de novo in the brain and peripheral nervous tissues. In contrast to adrenal steroid hormones that act on intracellular nuclear receptors, neurosteroids directly modulate plasma membrane ion channels and regulate intracellular signalling. This review provides an overview of the work that led to the discovery of neurosteroids, our current understanding of their intracellular biosynthetic machinery, and their roles in regulating the development and function of nervous tissue. Neurosteroids mediate signalling in the brain via multiple mechanisms. Here, we describe in detail their effects on GABA (inhibitory) and NMDA (excitatory) receptors, two signalling pathways of opposing function. Furthermore, emerging evidence points to altered neurosteroid function and signalling in neurological disease. This review focuses on neurodegenerative diseases associated with altered neurosteroid metabolism, mainly Niemann-Pick type C, multiple sclerosis and Alzheimer disease. Finally, we summarise the use of natural and synthetic neurosteroids as current and emerging therapeutics alongside their potential use as disease biomarkers.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Tuberal hypothalamic expression of the glial intermediate filaments, glial fibrillary acidic protein and vimentin across the turkey hen (Meleagris gallopavo) reproductive cycle: Further evidence for a role of glial structural plasticity in seasonal reproduction

Glia regulate the hypothalamic-pituitary-gonadal (HPG) axis in birds and mammals. This is accomplished mechanically by ensheathing gonadotrophin-releasing hormone I (GnRH) nerve terminals thereby blocking access to the pituitary blood supply, or chemically in a paracrine manner. Such regulation requires appropriate spatial associations between glia and nerve terminals. Female turkeys (Meleagris gallopavo) use day length as a primary breeding cue. Long days activate the HPG-axis until the hen enters a photorefractory state when previously stimulatory day lengths no longer support HPG-axis activity. Hens must then be exposed to short days before reactivation of the reproductive axis occurs. As adult hens have discrete inactive reproductive states in addition to a fertile state, they are useful for examining the glial contribution to reproductive function. We immunostained tuberal hypothalami from short and long-day photosensitive hens, plus long-day photorefractory hens to examine expression of two intermediate filaments that affect glial morphology: glial fibrillary acidic protein (GFAP) and vimentin. GFAP expression was drastically reduced in the central median eminence of long day photosensitive hens, especially within the internal zone. Vimentin expression was similar among groups. However, vimentin-immunoreactive fibers abutting the portal vasculature were significantly negatively correlated with GFAP expression in the median eminence, which is consistent with our hypothesis for a reciprocal relationship between GFAP and vimentin expression. It appears that up-regulation of GFAP expression in the central median eminence of turkey hens is associated with periods of reproductive quiescence and that photofractoriness is associated with the lack of a glial cytoskeletal response to long days.

Estrogen attenuates manganese-induced glutamate transporter impairment in rat primary astrocytes

The astrocytic glutamate transporters (GLT-1, GLAST) are critical for removing excess glutamate from synaptic sites, thereby maintaining glutamate homeostasis within the brain. 17β-Estradiol (E2) is one of the most active estrogen hormones possessing neuroprotective effects both in in vivo and in vitro models, and it has been shown to enhance astrocytic glutamate transporter function (Liang et al. in J Neurochem 80:807-814, 2002; Pawlak et al. in Brain Res Mol Brain Res 138:1-7, 2005). However, E2 is not clinically optimal for neuroprotection given its peripheral feminizing and proliferative effects; therefore, brain selective estrogen receptor modulators (neuro SERMs) (Zhao et al. in Neuroscience 132:299-311, 2005) that specifically target estrogenic mechanisms, but lack the systemic estrogen side effects offer more promising therapeutic modality for the treatment of conditions associated with excessive synaptic glutamate levels. This review highlights recent studies from our laboratory showing that E2 and SERMs effectively reverse glutamate transport inhibition in a manganese (Mn)-induced model of glutamatergic deregulation. Specifically, we discuss mechanisms by which E2 restores the expression and activity of glutamate uptake. We advance the hypothesis that E2 and related compounds, such as tamoxifen may offer a potential therapeutic modality in neurodegenerative disorders, which are characterized by altered glutamate homeostasis.

Short- and long-term treatment with estradiol or progesterone modifies the expression of GFAP, MAP2 and Tau in prefrontal cortex and hippocampus

AIMS

We analyzed the effects of the short- and long-term administration of estradiol (E2) or progesterone (P4) after ovariectomy on the expression of MAP2, Tau and GFAP in prefrontal cortex and hippocampus.

MAIN METHODS

Sprague Dawley rats were ovariectomized and immediately treated with E2 or P4 for 2 or 18 weeks. At the end of treatments, hippocampus and prefrontal cortex were excised, proteins were extracted and MAP2, Tau and GFAP were analyzed by Western blot.

KEY FINDINGS

MAP2 and Tau content was not modified by E2 in the prefrontal cortex. On the contrary, P4 decreased MAP2 content after a short-term treatment, while it increased that of MAP2 and TAU in this brain region after a long-term treatment. E2 increased MAP2 content in hippocampus. In this region, short-term administration of P4 increased that of MAP2. GFAP content was diminished after a long-term administration of P4 in hippocampus.

SIGNIFICANCE

Current data emphasize the importance of short- and long-term sex steroid treatment on neuronal and glial cytoskeletal proteins expression.

Manganese induces IGF-1 and cyclooxygenase-2 gene expressions in the basal hypothalamus during prepubertal female development

Precocious puberty is a significant child health problem, especially in girls, because 95% of cases are idiopathic. Our earlier studies demonstrated that low-dose levels of manganese (Mn) caused precocious puberty via stimulating the secretion of luteinizing hormone-releasing hormone (LHRH). Because glial-neuronal communications are important for the activation of LHRH secretion at puberty, we investigated the effects of prepubertal Mn exposure on specific glial-derived puberty-related genes known to affect neuronal LHRH release. Animals were supplemented with MnCl(2) (10 mg/kg) or saline by gastric gavage from day 12 until day 22 or day 29, then decapitated, and brains removed. The site of LHRH release is the medial basal hypothalamus (MBH), and tissues from this area were analyzed by real-time PCR for transforming growth factor α (TGFα), insulin-like growth factor-1 (IGF-1), and cyclooxygenase-2 (COX-2) messenger RNA levels. Protein levels for IGF-1 receptor (IGF-1R) were measured by Western blot analysis. LHRH gene expression was measured in the preoptic area/anteroventral periventricular (POA/AVPV) region. In the MBH, at 22 days, IGF-1 gene expression was increased (p < 0.05) with a concomitant increase (p < 0.05) in IGF-1R protein expression. Mn also increased (p < 0.01) COX-2 gene expression. At 29 days, the upregulation of IGF-1 (p < 0.05) and COX-2 (p < 0.05) continued in the MBH. At this time, we observed increased (p < 0.05) LHRH gene expression in the POA/AVPV. Additionally, Mn stimulated prostaglandin E(2) and LHRH release from 29-day-old median eminences incubated in vitro. These results demonstrate that Mn, through the upregulation of IGF-1 and COX-2, may promote maturational events and glial-neuronal communications facilitating the increased neurosecretory activity, including that of LHRH, resulting in precocious pubertal development.

Diverse roles of extracellular calcium-sensing receptor in the central nervous system

The G-protein-coupled calcium-sensing receptor (CaSR), upon activation by Ca(2+) or other physiologically relevant polycationic molecules, performs diverse functions in the brain. The CaSR is widely expressed in the central nervous system (CNS) and is characterized by a robust increase in its expression during postnatal brain development over adult levels throughout the CNS. Developmental increases in CaSR levels in brain correlate with myelinogenesis. Indeed, neural stem cells differentiating to the oligodendrocyte lineage exhibit the highest CaSR expression compared with those differentiating to astrocytic or neuronal lineages. In adult CNS, CaSR has broad relevance in maintaining local ionic homeostasis. CaSR shares an evolutionary relationship with the metabotropic glutamate receptor and forms heteromeric complexes with the type B-aminobutyric acid receptor subunits that affects its cell surface expression, activation, signaling, and functions. In normal physiology as well as in pathologic conditions, CaSR is activated by signals arising from mineral ions, amino acids, polyamines, glutathione, and amyloid-beta in conjunction with Ca(2+) and other divalent cationic ligands. CaSR activation regulates membrane excitability of neurons and glia and affects myelination, olfactory and gustatory signal integration, axonal and dendritic growth, and gonadotrophin-releasing hormonal-neuronal migration. Insofar as the CaSR is a clinically important therapeutic target for parathyroid disorders, development of its agonists or antagonists as therapeutics for CNS disorder could be a major breakthrough.

Three-dimensional properties of GnRH neuroterminals in the median eminence of young and old rats

The decapeptide gonadotropin-releasing hormone (GnRH), which regulates reproduction in all vertebrates, is stored in, and secreted from, large dense-core secretory vesicles in nerve terminals in the median eminence. GnRH is released from these terminals with biological rhythms that are critical for the maintenance of normal reproduction. During reproductive aging in female rats, there is a loss of GnRH pulses and a diminution of the GnRH surge. However, information about the specific role of GnRH nerve terminals is lacking, particularly in the context of aging. We sought to gain novel ultrastructural information about GnRH neuroterminals by performing three-dimensional (3D) reconstructions of GnRH neuroterminals and their surrounding microenvironment in the median eminence of young (4-5 months) and old (22-24 months) ovariectomized Sprague-Dawley female rats. Median eminence tissues were freeze-plunge embedded and serial ultrathin sections were collected on slot grids for immunogold labeling of GnRH immunoreactivity. Sequential images were used to create 3D models of GnRH terminals. These reconstructions provided novel perspectives into the morphological properties of GnRH terminals and their neural and glial environment. We also noted that the cytoarchitectural features of the median eminence became disorganized with aging. Quantitative measures showed a significant decrease in the apposition between GnRH terminal membranes and glial cells. Our data suggest reproductive aging in rats is characterized by structural organizational changes to the GnRH terminal microenvironment in the median eminence.

Gonadotropin-releasing hormone neuroterminals and their microenvironment in the median eminence: effects of aging and estradiol treatment

The GnRH decapeptide controls reproductive function through its release from neuroendocrine terminals in the median eminence, a site where there is a convergence of numerous nerve terminals and glial cells. Previous work showed dynamic changes in the GnRH-glial-capillary network in the median eminence under different physiological conditions. Because aging in rats is associated with a diminution of GnRH release and responsiveness to estradiol feedback, we examined effects of age and estradiol treatment on these anatomical interactions. Rats were ovariectomized at young (4 months), middle-aged (11 months), or old (22-23 months) ages, allowed 4 wk to recover, and then treated with vehicle or estradiol for 72 h followed by perfusion. Immunofluorescence of GnRH was measured, and immunogold electron microscopic analyses were performed to study the ultrastructural properties of GnRH neuroterminals and their microenvironment. Although the GnRH immunofluorescent signal showed no significant changes with age and estradiol treatment, we found that the median eminence underwent both qualitative and quantitative structural changes with age, including a disorganization of cytoarchitecture with aging and a decrease in the apposition of GnRH neuroterminals to glia with age and estradiol treatment. Thus, although GnRH neurons can continue to synthesize and transport peptide, changes in the GnRH neuroterminal-glial-capillary machinery occur during reproductive senescence in a manner consistent with a disconnection of these elements and a potential dysregulation of GnRH neurosecretion.

Regulation of GNRH production by estrogen and bone morphogenetic proteins in GT1-7 hypothalamic cells

Recent studies have shown that bone morphogenetic proteins (BMPs) are important regulators in the pituitary-gonadal endocrine axis. We here investigated the effects of BMPs on GNRH production controlled by estrogen using murine GT1-7 hypothalamic neuron cells. GT1-7 cells expressed estrogen receptor alpha (ERalpha; ESR1 as listed in MGI Database), ERbeta (ESR2 as listed in MGI Database), BMP receptors, SMADs, and a binding protein follistatin. Treatment with BMP2 and BMP4 had no effect on Gnrh mRNA expression; however, BMP6 and BMP7 significantly increased Gnrh mRNA expression as well as GnRH production by GT1-7 cells. Notably, the reduction of Gnrh expression caused by estradiol (E(2)) was restored by cotreatment with BMP2 and BMP4, whereas it was not affected by BMP6 or BMP7. E(2) activated extracellular signal-regulated kinase (ERK) 1/2 and stress-activated protein kinase/c-Jun NH(2)-terminal kinase (SAPK/JNK) signaling but did not activate p38-mitogen-activated protein kinase (MAPK) signaling in GT1-7 cells. Inhibition of ERK1/ERK2 reversed the inhibitory effect of estrogen on Gnrh expression, whereas SAPK/JNK inhibition did not affect the E(2) actions. Expression levels of Eralpha and Erbeta were reduced by BMP2 and BMP4, but were increased by BMP6 and BMP7. Treatment with an ER antagonist inhibited the E(2) effects on Gnrh suppression including reduction of E(2)-induced ERK phosphorylation, suggesting the involvement of genomic ER actions in Gnrh suppression. BMP2 and BMP4 also suppressed estrogen-induced phosphorylation of ERK1/ERK2 and SAPK/JNK signaling, suggesting that BMP2 and BMP4 downregulate estrogen effects by attenuating ER-MAPK signaling. Considering that BMP6 and BMP7 increased the expression of alpha1E-subunit of R-type calcium channel (Cacna1e), which is critical for GNRH secretion, it is possible that BMP6 and BMP7 directly stimulate GNRH release by GT1-7 cells. Collectively, a newly uncovered interaction of BMPs and ER may be involved in controlling hypothalamic GNRH production and secretion via an autocrine/paracrine mechanism.

Morphological evidence for direct interaction between gonadotrophin-releasing hormone neurones and astroglial cells in the human hypothalamus

In rodents, there is compelling evidence indicating that dynamic cell-to-cell communications involving cross talk between astroglial cells (such as astrocytes and specialised ependymoglial cells known as tanycytes) and neurones are important in regulating the secretion of gonadotrophin-releasing hormone (GnRH), the neurohormone that controls both sexual maturation and adult reproductive function. However, whether such astroglial cell-GnRH neurone interactions occur in the human brain is not known. In the present study, we used immunofluorescence to examine the anatomical relationship between GnRH neurones and glial cells within the hypothalamus of five women. Double-staining experiments demonstrated the ensheathment of GnRH neurone perikarya by glial fibrillary acidic protein (GFAP)-immunoreactive astrocyte processes in the periventricular zone of the tuberal region of the hypothalamus. GFAP immunoreactivity did not overlap that of GnRH at the GnRH neurone's projection site (i.e. the median eminence of the hypothalamus). Rather, human GnRH neuroendocrine fibres were found to be closely associated with vimentin or nestin-immunopositive radial glial processes likely belonging to tanycytes. In line with these light microscopy data, ultrastructural examination of GnRH-immunoreactive neurones showed numerous glial cells in direct apposition to pre-embedding-labelled GnRH cell bodies and/or dendrites in the infundibular nucleus, whereas postembedding immunogold-labelled GnRH nerve terminals were often seen to be enwrapped by glial cell processes in the median eminence. GnRH nerve button were sometimes visualised in close proximity to fenestrated pituitary portal blood capillaries and/or evaginations of the basal lamina that delineate the pericapillary space. In summary, these data demonstrate that GnRH neurones morphologically interact with astrocytes and tanycytes in the human brain and provide evidence that glial cells may contribute physiologically to the process by which the neuroendocrine brain controls the function of GnRH neurones in humans.

Inflammation after intracerebral hemorrhage

Intracerebral hemorrhage (ICH) is a devastating clinical event without effective therapies. Increasing evidence suggests that inflammatory mechanisms are involved in the progression of ICH-induced brain injury. Inflammation is mediated by cellular components, such as leukocytes and microglia, and molecular components, including prostaglandins, chemokines, cytokines, extracellular proteases, and reactive oxygen species. Better understanding of the role of the ICH-induced inflammatory response and its potential for modulation might have profound implications for patient treatment. In this review, a summary of the available literature on the inflammatory responses after ICH is presented along with discussion of some of the emerging opportunities for potential therapeutic strategies. In the near future, additional strategies that target inflammation could offer exciting new promise in the therapeutic approach to ICH.

Calcium-sensing receptor stimulates secretion of an interferon-γ-induced monokine (CXCL10) and monocyte chemoattractant protein-3 in immortalized GnRH neurons

Biology of GnRH neurons is critically dependent on extracellular Ca(2+) (Ca(2+) (o)). We evaluated differences in gene expression patterns with low and high Ca(2+) (o) in an immortalized GnRH neuron line, GT1-7 cells. Mouse global oligonucleotide microarray was used to evaluate transcriptional differences among the genes regulated by elevated Ca(2+) (o). Our result identified two interferon-gamma (IFNgamma)-inducible chemokines, CXCL9 and CXCL10, and a beta chemokine, monocyte chemoattractant protein-3 (MCP-3/CCL7), being up-regulated in GT1-7 cells treated with high Ca(2+) (o) (3.0 mM) compared with low Ca(2+) (o) (0.5 mM). Up-regulation of these mRNAs by elevated Ca(2+) (o) was confirmed by quantitative PCR. Elevated Ca(2+) (o) stimulated secretion of CXCL10 and MCP-3 but not CXCL9 in GT1-7 cells, and this effect was mediated by an extracellular calcium-sensing receptor (CaR) as the dominant negative CaR attenuated secretion of CXCL10 and MCP-3. CXCL10 and MCP-3 were localized in mouse GnRH neurons in the preoptic hypothalamus. Suppression of K(+) channels (BK channels) with 25 nM charybdotoxin inhibited high-Ca(2+) (o)-stimulated CXCL10 release. Accordingly, CaR activation by a specific CaR agonist, NPS-467, resulted in the activation of a Ca(2+)-activated K(+) channel in these cells. CaR-mediated MCP-3 secretion involves the PI3 kinase pathway in GT1-7 cells. MCP-3 stimulated chemotaxis of astrocytes treated with transforming growth factor-beta (TGFbeta). With TGFbeta-treated astrocytes, we next observed that conditioned medium from GT1-7 cells treated with high Ca(2+) promoted chemotaxis of astrocytes, and this effect was attenuated by a neutralizing antibody to MCP-3. These results implicate CaR as an important regulator of GnRH neuron function in vivo by stimulating secretion of heretofore unsuspected cytokines, i.e., CXCL10 and MCP-3.

Kim Y, Liu S, Kim KS. Human astrocytes/astrocyte-conditioned medium and shear stress enhance the barrier properties of human brain microvascular endothelial cells

The blood-brain barrier (BBB) is a structural and functional barrier that regulates the passage of molecules into and out of the brain to maintain the neural microenvironment. We have previously developed the in vitro BBB model with human brain microvascular endothelial cells (HBMEC). However, in vivo HBMEC are shown to interact with astrocytes and also exposed to shear stress through blood flow. In an attempt to develop the BBB model to mimic the in vivo condition we constructed the flow-based in vitro BBB model using HBMEC and human fetal astrocytes (HFA). We also examined the effect of astrocyte-conditioned medium (ACM) in lieu of HFA to study the role of secreted factor(s) on the BBB properties. The tightness of HBMEC monolayer was assessed by the permeability of dextran and propidium iodide as well as by measuring the transendothelial electrical resistance (TEER). We showed that the HBMEC permeability was reduced and TEER was increased by non-contact, co-cultivation with HFA and ACM. The exposure of HBMEC to shear stress also exhibited decreased permeability. Moreover, HFA/ACM and shear flow exhibited additive effect of decreasing the permeability of HBMEC monolayer. In addition, we showed that the HBMEC expression of ZO-1 (tight junction protein) was increased by co-cultivation with ACM and in response to shear stress. These findings suggest that the non-contact co-cultivation with HFA helps maintain the barrier properties of HBMEC by secreting factor(s) into the medium. Our in vitro flow model system with the cells of human origin should be useful for studying the interactions between endothelial cells, glial cells, and secreted factor(s) as well as the role of shear stress in the barrier property of HBMEC.

Aquaporin 4 regulates the effects of ovarian hormones on monoamine neurotransmission

Aquaporin 4 (AQP4) is the predominant water channels in the brain of mammals. Our previous study has reported that AQP4 knockout induced sex-specific alterations in neurotransmission, indicating that AQP4 might regulate the interaction between sex hormones and neurotransmission. In the present study, we found that AQP4 knockout decreased the concentrations of estrogen and progestogen. Further study showed that exogenous estrogen decreased DA and 5-HT in cortex, reduced DA and 5-HT in striatum, but increased 5-HT in hippocampus in AQP4+/+ male mice. However, in AQP4-/- male mice, exogenous estrogen almost did not alter the levels of neurotransmitters except for decreasing DA in cortex. In female mice, ovariectomy decreased DA in the striatum of AQP4+/+ mice, but did not alter the levels of DA in AQP4-/- mice. These findings reveal for the first time that AQP4 regulates not only water and ion homeostasis but also the functions of ovarian hormone and neurotransmitter.

Induction of transforming growth factor-β1 by basic fibroblast growth factor in rat C6 glioma cells and astrocytes is mediated by MEK/ERK signaling and AP-1 activation

Basic fibroblast growth factor (bFGF) and transforming growth factor-beta1 (TGF-beta1) play an important role in proliferation, differentiation, and survival of malignant gliomas and in normal glial cell biology. Because of these critical roles, potential interactions between these key growth factors were investigated. We previously demonstrated that bFGF potently stimulates TGF-beta1 release from rat glioma cells. The purpose of the present study was to elucidate the mechanism(s) of this regulatory effect, establish its functional importance, and examine whether it extends to nontransformed rat hypothalamic astrocytes (RHA). The results revealed that RHA express the high-affinity FGF(1-4) receptors, and similarly to glioma cells, bFGF stimulated TGF-beta1 release in an isoform-specific manner. A mediatory role for ERK signaling in bFGF-induced TGF-beta release was suggested by the fact that MEK1 inhibition prevented this effect. Additionally, bFGF enhanced MEK1/2 phosphorylation and ERK activation/nuclear translocation, which culminated in increased activity of AP-1-mediated gene transcription. bFGF markedly induced TGF-beta1 mRNA levels in an isoform-specific manner, an effect that was dependent on MEK/ERK/AP-1 signaling. Functionally, bFGF-induced proliferation of glioma cells was attenuated by MEK/ERK inhibition or immunoneutralization of TGF-beta1, suggesting that this pathway may have important implications for brain tumor progression.

Receptors for leptin and estrogen in the subcommissural organ of rabbits are differentially modulated by fasting

In rabbits, the fasting-dependent reduction of LH secretion is likely mediated by leptin and estrogens via receptors in the brain. For the first time, using immunohistochemistry, the presence and regulation of receptors for leptin (Ob-R) and estradiol-17beta subtype alpha (ERalpha) were studied in the subcommissural organ (SCO) of rabbits, which were fed either ad libitum (control) or fasted for 48 h (treated) to verify whether this brain structure is a potential site of integration for metabolism and reproduction. In control rabbits, the cytoplasm of glial cells lining the SCO evidenced strong Ob-R immunoreactivity, whereas both ependymal and hypendymal cells of this glandular-like structure were negative. The Ob-R positive glial cells were identified as fibrous astrocytes using the phosphotungstic acid-hematoxylin histochemical (PTAH) and glial fibrillary acidic protein (GFAP) immunohistochemical techniques. ERalpha immunoreactive nuclei were detectable exclusively in the specialized cells forming the SCO, whereas surrounding astrocytes and neurons were negative. Compared to controls, in fasted rabbits, the staining of Ob-R immunoreaction was reduced in the cytoplasm of positive astrocytes, but greatly enhanced in plasma membranes, whereas the number of ERalpha immunoreactive SCO cells was increased (13.2+/-2.7 vs. 5.2+/-2.0, P<0.01). Ependymal cells lining the third ventricle were negative for both Ob-R and ERalpha. Our results indicate, although indirectly, that the SCO, together with the astrocytes in close contact with this structure, is a likely target for nutritional and gonadal signals carried by leptin and estrogens, suggesting that these specialized glial cells may regulate reproduction and metabolism through mechanisms still unknown.

Oestradiol up-regulates glutamine synthetase mRNA and protein expression in the hypothalamus and hippocampus: implications for a role of hormonally responsive glia in amino acid neurotransmission

Rapidly emerging evidence suggests that glial cells in the central nervous system are sensitive to oestrogen actions. However, the functional consequences of the cellular mechanisms of these cells have proven difficult to study in vivo because of the intimate relationships between neurones and glia. Microarray technology offers the potential to uncover steroid hormone regulation of glial-specific genes that may play a role in hormone-dependent neuronal-glial interactions. Analysis of transcriptomes from the medial basal hypothalamus (MBH) of oestradiol and vehicle-treated adult ovariectomised mice revealed an up-regulation of several glial specific genes by oestradiol, including glutamine synthetase (GS), which facilitates the conversion of glutamate to glutamine and plays an integral role in amino acid neurotransmission. In situ hybridisation confirmed that oestradiol treatment resulted in an up-regulation of GS gene expression in the arcuate and ventromedial nuclei of the MBH, as well as the medial amygdala and hippocampus. Moreover, oestradiol increased protein expression of GS in both the MBH and hippocampus. Neurones are incapable of de novo net synthesis of glutamate from glucose and are dependent on glial-provided precursors such as glutamine to renew their amino acid transmitter pools. Thus, oestradiol induced expression of GS suggests a significant role for glial cells in hormonal modulation of glutamatergic neurotransmission important to female reproductive behaviours, neuroendocrine physiology and cognitive functions.

Role of astrocytes in reproduction and neuroprotection

Hypothalamic astrocytes secrete TGF-beta and 3 alpha,5 alpha-tetrahydro progesterone (3 alpha,5 alpha-THP) in culture. When the astrocyte-conditioned medium (ACM) was incubated with the hypothalamic cell line GT1-7, it resulted in the secretion of GnRH. Immunoneutralization with TGF-beta antibody or ultra-filteration with a 10 kDa cut off filter resulted in attenuation of the GnRH releasing ability of ACM, indicating that TGF-beta was a major factor involved with GnRH release. Treatment with estrogens increases TGF-beta secretion. These observations indicate a significant role of astrocytes in GnRH secretion. Serum-deprivation results in the death of GT1-7 neurons in culture and addition of ACM or TGF-beta to the culture, attenuates cell death. The mechanism of protection from cell death appears to involve phosphorylation of MKK4, JNK, c-Jun(Ser63), and enhancement of AP-1 binding. Co-administration of JNK inhibitors, but not MEK inhibitors attenuated ACM or TGF-beta-induced c-Jun(Ser63) phosphorylation and their neuroprotective effects. These studies suggest that astrocytes can protect neurons, at least in part, by the release of TGF-beta and activation of a c-Jun/AP-1 protective pathway.

Modulation of intercellular calcium signaling by melatonin in avian and mammalian astrocytes is brain region-specific

Calcium waves among glial cells impact many central nervous system functions, including neural integration and brain metabolism. Here, we characterized the modulatory effects of melatonin, a pineal neurohormone that mediates circadian and seasonal processes, on glial calcium waves derived from different brain regions and species. Diencephalic and telencephalic astrocytes, from both chick and mouse brains, expressed melatonin receptor proteins. Further, using the calcium-sensitive dye Fluo-4, we conducted real-time imaging analyses of calcium waves propagated among mammalian and avian astrocytes. Mouse diencephalic astrocytic calcium waves spread to an area 2-5-fold larger than waves among avian astrocytes and application of 10 nM melatonin caused a 32% increase in the spread of these mammalian calcium waves, similar to the 23% increase observed in chick diencephalic astrocytes. In contrast, melatonin had no effect on calcium waves in either avian or mammalian telencephalic astrocytes. Mouse telencephalic calcium waves radially spread from their initiation site among untreated astrocytes. However, waves meandered among mouse diencephalic astrocytes, taking heterogeneous paths at variable rates of propagation. Brain regional differences in wave propagation were abolished by melatonin, as diencephalic astrocytes acquired more telencephalon-like wave characteristics. Astrocytes cultured from different brain regions, therefore, possess fundamentally disparate mechanisms of calcium wave propagation and responses to melatonin. These results suggest multiple roles for melatonin receptors in the regulation of astroglial function, impacting specific brain regions differentially.

T3 implantation mimics photoperiodically reduced encasement of nerve terminals by glial processes in the median eminence of Japanese quail

Photoperiodically generated triiodothyronin (T(3)) in the mediobasal hypothalamus (MBH) has critical roles in the photoperiodic response of the gonads in Japanese quail. In a previous study, we demonstrated seasonal morphological changes in the neuro-glial interaction between gonadotrophin-releasing hormone (GnRH) nerve terminals and glial endfeet in the median eminence (ME). However, a direct relationship between photoperiodically generated T(3) and seasonal neuro-glial plasticity in the ME remained unclear. In the present study, we examined the effect of T(3) implantation into the MBH on the neuro-glial interaction in the ME. T(3) implantation caused testicular growth and reduced encasement of nerve terminals in the external zone of the ME. In contrast, no morphological changes were observed in birds given an excessive dose of T(3), which did not cause testicular growth. These results support the hypothesis that thyroid hormone regulates photoperiodic GnRH secretion via neuro-glial plasticity in the ME.

Estradiol modulation of astrocytic form and function: implications for hormonal control of synaptic communication

There is a growing appreciation for the importance of glial cells to overall brain function. For decades, glial cells have been considered relatively passive supporters of nerve cell function, providing only structural and metabolic support to the communicating neurons. Now, rapidly emerging evidence demonstrates that glial cells are active participants in the processes of synaptic patterning and synaptic transmission. Like their neuronal neighbors residing in steroid sensitive brain regions, glial cells demonstrate a responsiveness to gonadal steroids that has been best characterized by physical changes in their morphology. However, because of their intimate relationship, the nature of neuronal-glial interactions has been challenging to study in vivo and until recently, the functional relevance of steroid-induced changes in glial morphology to neuroendocrine functions could only be implied from anatomical and in vitro studies. The advent of microarray technology offers the potential to uncover steroid regulation of glial-specific genes that may play a role in hormone-dependent neuronal-glial interactions. Our microarray analysis of the rodent hypothalamus has revealed that estradiol increases the expression of a number of glial-specific genes, including glutamine synthetase, an enzyme that inactivates glutamate through its conversion to glutamine. Given that glutamine is the predominant precursor for releasable pools of glutamate, our observation that estradiol increases glutamine synthetase gene and protein expression suggests that hormonal regulation of glutamate neurotransmission involves hormonally responsive glia. Thus, hormonally responsive glia may play a pivotal role in estradiol-mediated synaptic transmission underlying neuroendocrine function.

Calcium-mediated aponecrosis plays a central role in the pathogenesis of estrogenic chemical-induced neurotoxicity

Estrogen is traditionally associated with females but is also present in males, and influences aspects of brain chemistry and brain morphology in males, females and also during prenatal development. Humans as well as animals are additionally exposed to environmental products that mimic estrogen activity, also known as endocrine disrupters (EDCs). This hypothesis article investigates the role of estrogen (and also EDCs) in the brain and how it influences the Ca2+ pathway. Ca2+ and its movement in and out of the cell is an extremely important ion controlling normal cell physiology. Any dysfunction in the movement from outside to inside the cell or between organelles may have fundamentally negative effects and the disturbance may even lead to apoptosis and/or necrosis. Therefore we consider whether estrogen and EDCs may alter the Ca2+ physiology and whether these changes may be one of the main causes of interference in physiology that is seen when humans and animals are exposed to EDCs. We come to the conclusion that on a molecular level Ca2+ and Ca2+ fluxes ([Ca2+]i, endocrine disrupting chemicals, redox modulation, mitochondria and cytochrome c followed by apoptosis, necrosis or most likely aponecrosis may contribute to chemical-mediated developmental toxicity. Similarly, we hypothesize that calcium-mediated aponecrosis do not only play a central role in the pathophysiology of estrogenic chemical-induced neurotoxicity, but can contribute to chemical-mediated developmental toxicity in general, thereby affecting almost all cells and organs of the living organism.

Sexually dimorphic hormonal regulation of the gap junction protein, CX43, in rats and altered female reproductive function in CX43+/- mice

Astrocytic gap junctional communication is important in steroid hormone regulation of reproductive processes at the level of the hypothalamus, including estrous cyclicity and sexual behavior. We examined the effects of estradiol and progesterone on the abundance of the gap junctional protein, connexin 43 (CX43), which is highly expressed in astrocytes. Gonadectomized rats received hormone treatments that induce maximal sexual behavior and gonadotropin surges in females (estrogen for 48 h followed by progesterone, estrogen alone or progesterone alone). Control animals received vehicle (oil) injections. In the female rat preoptic area (POA), containing the gonadotropin-releasing hormone (GnRH) cell bodies, treatment with estrogen, progesterone or estrogen + progesterone significantly increased CX43 protein levels in immunoblots. In contrast, estrogen + progesterone significantly decreased CX43 levels in the male rat POA. This sexually dimorphic hormonal regulation of CX43 was not evident in the hypothalamus, which contains primarily GnRH nerve terminals. Treatment with estrogen + progesterone significantly decreased CX43 levels in both the male and female hypothalamus. To examine the role of CX43 in female reproductive function, we studied heterozygous female CX43 (CX43+/-) mice. Most mutant mice did not show normal estrous cycles. In addition, when compared to wild type females, CX43+/- mice had reduced lordosis behavior. These data suggest that hypothalamic CX43 expression is regulated by steroid hormones in a brain-region-specific and sexually dimorphic manner. Therefore, gap junctional communication in the POA and hypothalamus may be a factor regulating the estrous cycle and sexual behavior in female rodents.

The aging reproductive neuroendocrine axis

It is well known that the reproductive system is one of the first biological systems to show age-related decline. While depletion of ovarian follicles clearly relates to the end of reproductive function in females, evidence is accumulating that a hypothalamic defect is critical in the transition from cyclicity to acyclicity. This minireview attempts to present a concise review on aging of the female reproductive neuroendocrine axis and provide thought-provoking analysis and insights into potential future directions for this field. Evidence will be reviewed, which shows that a defect in pulsatile and surge gonadotropin hormone-releasing hormone (GnRH) secretion exists in normal cycling middle-aged female rats, which is thought to explain the significantly attenuated pulsatile and surge luteinizing hormone (LH) secretion at middle-age. Evidence is also presented, which supports the age-related defect in GnRH secretion as being due to a reduced activation of GnRH neurons. Along these lines, stimulation of GnRH secretion by the major excitatory transmitter glutamate is shown to be significantly attenuated in middle-aged proestrous rats. Corresponding age-related defects in other major excitatory regulatory factors, such as catecholamines, neuropeptide Y, and astrocytes, have also been demonstrated. Age-related changes in hypothalamic concentrations of neurotransmitter receptors, steroid receptors, and circulating steroid hormone levels are also reviewed, and discussion is presented on the complex interrelationships of the hypothalamus-pituitary-ovarian (HPO) axis during aging, with attention to how a defect in one level of the axis can induce defects in other levels, and thereby potentiate the dysfunction of the entire HPO axis.

Gonadotropin‐Releasing Hormone: Gene Evolution, Expression, and Regulation

The gonadotropin-releasing hormone (GnRH) gene is a superb example of the diverse regulation that is required to maintain the function of an evolutionarily conserved and fundamental gene. Because reproductive capacity is critical to the survival of the species, physiological homeostasis dictates optimal conditions for reproductive success, and any perturbation from this balance may affect GnRH expression. These disturbances may include alterations in signals dictated by stress, nutritional imbalance, body weight, and neurological problems; therefore, changes in other neuroendocrine systems may directly influence the hypothalamic-pituitary-gonadal axis through direct regulation of GnRH. Thus, to maintain optimal reproductive capacity, the regulation of the GnRH gene is tightly constrained by a number of diverse signaling pathways and neuromodulators. In this review, we summarize what is currently known of GnRH gene structure, the location and function of the two isoforms of the GnRH gene, some of the many hormones and neuromodulators found to affect GnRH expression, and the molecular mechanisms responsible for the regulation of the GnRH gene. We also discuss the latest models used to study the transcriptional regulation of the GnRH gene, from cell models to evolving in vivo technologies. Although we have come a long way in the last two decades toward uncovering the intricacies behind the control of the GnRH neuron, there remain vast distances to cover before direct therapeutic manipulation of the GnRH gene to control reproductive competence is possible.

Minireview: Role of glia in neuroendocrine function

Long relegated to the backwaters of neuroendocrinology, it is becoming increasingly apparent that glial cells of the central and peripheral nervous system are key participants because they are capable of both sending and receiving hormonal signals. Hormones are also a critical component of neuronal/glial cross talk, leading to neuromodulatory and neurotrophic actions under physiological and pathological conditions. In the peripheral nervous system, hormonal actions on Schwann cells and hormonal metabolites produced by these glial cells promote myelin formation and the remyelination and regeneration of injured nerves. In the central nervous system, glial cells participate in the hormonal regulation of synaptic function, synaptic plasticity, myelin formation, cognition, sleep, and the response of nervous tissue to injury. In addition, central glial cells participate in the regulation of hormonal secretion by hypothalamic neurons. Therefore, glial cells are a key element to understanding hormonal actions in the nervous system and the regulation of neuroendocrine events.

Activation of erbB-1 signaling in tanycytes of the median eminence stimulates transforming growth factor beta1 release via prostaglandin E2 production and induces cell plasticity

The activation of transforming growth factor alpha (TGFalpha)-erbB-1 and neuregulin-erbB-4 signaling pathways in hypothalamic astrocytes has been shown to play a key role in the process by which the neuroendocrine brain controls luteinizing hormone-releasing hormone (LHRH) secretion. Earlier studies suggested that tanycytes, an ependymoglial cell type of the median eminence, regulate LHRH release during the estrous cycle by undergoing plastic changes that alternatively allow or prevent direct access of the LHRH nerve terminals to the portal vasculature. Neither the molecules responsible for these plastic changes nor the underlying controlling mechanisms have been identified. Here we show that cultured tanycytes express erbB-1 and erbB-2, two of the four members of the erbB receptor family, and respond to TGFalpha with receptor phosphorylation, release of prostaglandin E2 (PGE2), and a PGE2-dependent increase in the release of TGFbeta1, a growth factor previously implicated in the glial control of LHRH secretion. Blockade of either erbB-1 receptor signal transduction or prostaglandin synthesis prevented the stimulatory effect of TGFalpha on both PGE2 and TGFbeta1 release. Time-lapse studies revealed that TGFalpha and TGFbeta1 have dramatically opposite effects on tanycyte plasticity. Whereas TGFalpha promotes tanycytic outgrowth, TGFbeta1 elicits retraction of tanycytic processes. Blockade of metalloproteinase activity abolished the effect of TGFbeta1, suggesting that TGFbeta1 induces tanycytic retraction by facilitating dissolution of the extracellular matrix. Prolonged (>12 hr) exposure of tanycytes to TGFalpha resulted in focal tanycytic retraction, an effect that was abolished by immunoneutralization of TGFbeta1 action, indicating that the retraction was attributable to TGFalpha-induced TGFbeta1 formation. These in vitro results identify tanycytes as targets of TGFalpha action and demonstrate that activation of erbB-1-mediated signaling in these cells results in plastic changes that, involving PGE2 and TGFbeta1 as downstream effectors, mimic the morphological plasticity displayed by tanycytes during the hours encompassing the preovulatory surge of LHRH.

Astrocyte protection of neurons: role of transforming growth factor-beta signaling via a c-Jun-AP-1 protective pathway

Astrocytes have become a focal point for research in neurobiology, especially regarding their purported ability to regulate neuronal communication and survival. The present study addressed a poorly understood but important focus in this area, the mechanism(s) underlying astrocyte-induced survival of neurons. The results of the study show that soluble factors in astrocyte-conditioned media (ACM) protect murine GT1-7 neurons from serum deprivation-induced cell death and that this neuroprotection is correlated with enhanced activation/phosphorylation of the AP-1 transcription factor, c-JunSer-63. A parallel and correlated activation of the upstream kinases, c-Jun N-terminal kinase (JNK) and mitogen-activated protein kinase kinase-4 (MKK4) was also demonstrated. Furthermore, co-administration of JNK inhibitors, but not a MEK inhibitor, significantly attenuated ACM-induced phosphorylation of c-JunSer-63 and blocked its neuroprotective action. Gel shift analysis demonstrated that ACM enhanced AP-1 binding, an effect that appears functionally important, since an AP-1 binding inhibitor significantly attenuated the neuroprotective action of ACM. Further studies implicated transforming growth factor (TGF)-beta1 and TGF-beta2 as critical active soluble factors released by astrocytes, since both were demonstrated in ACM, and immunoneutralization of the conditioned media with a panspecific TGF-beta antibody significantly attenuated the enhanced AP-1 binding and neuroprotective action of the ACM. Furthermore, exogenous application of TGF-beta1 and TGF-beta2 was found to enhance c-JunSer-63 phosphorylation and to be neuroprotective, and co-administration of JNK inhibitors or an AP-1 binding inhibitor blocked TGF-beta-induced neuroprotection. Taken together, these studies suggest that astrocytes can protect neurons from serum deprivation-induced cell death, at least in part, by release of TGF-beta and activation of a c-Jun/AP-1 protective pathway.