Expression of mineralocorticoid type I and glucocorticoid type II receptors in astrocyte glia as a function of time in culture

Effects of the mineralocorticoid receptor antagonist eplerenone in experimental autoimmune encephalomyelitis

There is growing evidence indicating that mineralocorticoid receptor (MR) expression influences a wide variety of functions in metabolic and immune response. The present study explored if antagonism of the MR reduces neuroinflammation in the spinal cord of mice with experimental autoimmune encephalomyelitis (EAE). Eplerenone (EPLE) (100 mg/kg dissolved in 30% 2-hydroxypropyl-β-cyclodextrin) was administered intraperitoneally (i.p.) daily from EAE induction (day 0) until sacrificed on day 17 post-induction. The MR blocker (a) significantly decreased the inflammatory parameters TLR4, MYD88, IL-1β, and iNOS mRNAs; (b) attenuated HMGB1, NLRP3, TGF-β mRNAs, microglia, and aquaporin4 immunoreaction without modifying GFAP. Serum IL-1β was also decreased in the EAE+EPLE group. Moreover, EPLE treatment prevented demyelination and improved clinical signs of EAE mice. Interestingly, MR was decreased and GR remained unchanged in EAE mice while EPLE treatment restored MR expression, suggesting that a dysbalanced MR/GR was associated with the development of neuroinflammation. Our results indicated that MR blockage with EPLE attenuated inflammation-related spinal cord pathology in the EAE mouse model of Multiple Sclerosis, supporting a novel therapeutic approach for immune-related diseases.

Transcriptional Regulation of Human Transforming Growth Factor-α in Astrocytes

Transforming growth factor-alpha (TGF-α) is known to play multifunctional roles in the central nervous system (CNS), including the provision of neurotropic properties that protect neurons against various neurotoxic insults. Previously, we reported that TGF-α mediates estrogen-induced enhancement of glutamate transporter GLT-1 function in astrocytes. However, the regulatory mechanism of TGF-α at the transcriptional level remains to be established. Our findings revealed that the human TGF-α promoter contains consensus sites for several transcription factors, such as NF-κB and yin yang 1 (YY1). NF-κB served as a positive regulator of TGF-α promoter activity, corroborated by observations that overexpression of NF-κB p65 increased, while mutation in the NF-κB binding sites in the TGF-α promoter reduced the promoter activity in rat primary astrocytes. Pharmacological inhibition of NF-κB with pyrrolidine dithiocarbamate (PDTC; 50 μM) or quinazoline (QNZ; 10 μM) also abolished TGF-α promoter activity, and NF-κB directly bound to its consensus site in the TGF-α promoter as evidenced by electrophoretic mobility shift assay (EMSA). Dexamethasone (DX) increased TGF-α promoter activity by activation of NF-κB. Treatment of astrocytes with 100 nM of DX for 24 h activated its glucocorticoid receptor and signaling proteins, including MAPK, PI3K/Akt, and PKA, via non-genomic pathways, to enhance TGF-α promoter activity and expression. YY1 served as a critical negative regulator of the TGF-α promoter as overexpression of YY1 decreased, while mutation of YY1 binding site in the promoter increased TGF-α promoter activity. Treatment for 3 h with 250 μM of manganese (Mn), an environmental neurotoxin, decreased astrocytic TGF-α expression by activation of YY1. Taken together, our results suggest that NF-κB is a critical positive regulator, whereas YY1 is a negative regulator of the TGF-α promoter. These findings identify potential molecular targets for neurotherapeutics that may modulate TGF-α regulation and afford neuroprotection.

RU486 (mifepristone) ameliorates cognitive dysfunction and reverses the down-regulation of astrocytic N-myc downstream-regulated gene 2 in streptozotocin-induced type-1 diabetic rats

Diabetic cognitive dysfunction (DCD), usually accompanied with chronically elevated glucocorticoids and hippocampal astrocytic alterations, is one of the most serious complications in patients with type-1 diabetes. However, the role for chronically elevated glucocorticoids and hippocampal astrocytic activations in DCD remains to be elucidated, and it is not clear whether astrocytic N-myc downstream-regulated gene 2 (NDRG2, involved in cell differentiation and development) participated in DCD. In the present study, three months after streptozotocin (STZ)-induced type-1 diabetes onset, rats showed cognitive impairments in Morris water maze test as well as elevated corticosterone level. Diabetic rats also presented down-regulation of glial fibrillary acidic protein (GFAP, a key indicator of astrocytic reactivity) and NDRG2 in hippocampus revealed by immunohistochemistry staining, real-time PCR and Western blot. Moreover, the diabetic cognitive impairments were ameliorated by 9-day glucocorticoids receptor (GR) blockade with RU486, and the down-regulation of hippocampal NDRG2 and GFAP in diabetic animals was also attenuated by 9-day GR blockade. These results suggest that glucocorticoids-GR system is crucial for DCD, and that astrocytic reactivity and NDRG2 are involved in these processes. Thus, inhibiting GR activation in the hippocampus may be a novel therapeutic strategy for treating DCD.

Angiotensin II and aldosterone-induced neuronal damage in neurons through an astrocyte-dependent mechanism

The contribution of the renin-angiotensin-aldosterone system (RAAS) to central nervous system (CNS) disorders is not yet fully understood. RAAS has been shown to be involved in the proliferation of astrocytes, which have a role in neuronal damage contributing to neurodegenerative diseases. However, the direct relationship between RAAS and neuronal damage is still unclear. We therefore examined the effect of angiotensin (Ang) II and aldosterone (Aldo) on damage to spinal ganglion neurons (SGNs) by regulating astrocytes. Ang II stimulation significantly increased DNA damage in SGNs in a time-dependent manner. This increase in DNA damage was further enhanced when SGNs were co-cultured with astrocytes. On the other hand, no significant increase was observed in SGNs co-cultured with astrocytes without Ang II stimulation. Moreover, the addition of conditioned medium from Ang II-treated astrocytes exacerbated SGN DNA damage. An Ang II type 1 receptor blocker, valsartan, inhibited Ang II-stimulated DNA damage but not DNA damage induced by conditioned medium prepared from astrocyte cultures. In contrast, an Aldo antagonist, eplerenone, significantly inhibited DNA damage induced by the culture medium from Ang II-treated astrocytes. Ang II-stimulated Aldo secretion in the conditioned medium from astrocytes. Furthermore, the administration of Aldo alone also enhanced DNA damage in SGNs. Finally, flow cytometric analysis showed that Ang II or Aldo treatment markedly increased the percentage of dead SGNs. In conclusion, Ang II- and Aldo-induced neuronal damage in SGNs through astrocytes regulation. Blocking Ang II and Aldo to target astrocytes might be useful for the treatment of CNS disorders.

Effect of growth factors and steroids on transglutaminase activity and expression in primary astroglial cell cultures

Type-2 transglutaminase (TG-2) is a multifunctional enzyme involved in the regulation of cell differentiation and survival that recently has been shown to play an emerging role in astrocytes, where it is involved in both proliferation and differentiation processes. Growth factors (GFs) such as EGF, basic fibroblast growth factor, insulin-like growth factor-I (IGF-I), and insulin (INS) are trophic and mitogenic peptides that participate in neuron-glia interactions and stimulate neuronal and astroglial proliferation and differentiation. Steroid hormones such as glucocorticoids and estrogens also play a pivotal role in neuronal and astroglial proliferation and differentiation and are key hormones in neurodegenerative and neuroprotective processes. We investigated the effects of the interaction of GFs with dexamethasone (DEX) or 17beta-estradiol (E(2)) on TG-2 activity and their expression in cultured astrocytes. We observed a significant increase in TG-2 activity and expression in astroglial cells treated for 24 hr with IGF-I, EGF, or INS. Priming of the cells with DEX or E(2), for 48 hr also led to an increase in TG-2 levels. When growth factors were present in the last 24 hr of the steroid treatment, a reduction in TG-2 expression and activity and a different subcellular TG-2 distribution were found. Our data indicate that steroid hormone-GF interaction may play an important role in astroglial function. The effect on TG-2 could be part of the regulation of intracellular pathways associated with the astrocyte response observed in physiological conditions and, possibly, also in neuropathological diseases.

Expression and functional state of the corticosteroid receptors and 11 beta-hydroxysteroid dehydrogenase type 2 in Schwann cells

To investigate the role of steroid receptors in mediating the reported effects of steroids on Schwann cell (SC) myelination and growth, we determined mRNA contents and transcriptional activities of the corticosteroid (glucocorticosteroid and mineralocorticosteroid) receptors (GR and MR) and sex steroid (progesterone, androgen, and estrogen alpha and beta) receptors in rat SC cultured under proliferative (in the presence of insulin and forskolin, which induces a high intracellular cAMP content) and quiescent conditions. We found no or very low expression and activity of the sex steroid receptors, as shown by mRNA concentrations determined with real-time PCR and transcriptional activities using transient expression of reporter plasmids in SC. These data and binding studies in SC lines demonstrated that the levels of the sex steroid receptors were the limiting factors. GR was clearly expressed (approximately 8000 sequences/ng total RNA) and functional. No significant modification in GR mRNA levels was observed, but an increase in transcriptional efficiency was recorded in proliferating cells compared with quiescent cells. MR was also significantly expressed at the mRNA level (approximately 450 sequences/ng total RNA) under the two culture conditions. No MR transcriptional activity was observed in SC, but a low specific binding of aldosterone was detected in SC lines. 11 beta-Hydroxysteroid-dehydrogenase type 2 (HSD2), an enzyme that inactivates glucocorticoids, was strongly expressed and active in quiescent SC, although in proliferating cells, HSD2 exhibited a strong decrease in activity and mRNA concentration. These data support a physiological role for HSD2 regulation of glucocorticosteroid concentrations in nerve SC.

Differential promotion of glutamate transporter expression and function by glucocorticoids in astrocytes from various brain regions

Steroids that activate glucocorticoid receptors (GRs) and mineralocorticoid receptors have important regulatory effects on neural development, plasticity, and the body's stress response. Here, we investigated the role of corticosteroids in regulating the expression of the glial glutamate transporters glial glutamate transporter-1 (GLT-1) and glutamate-aspartate transporter (GLAST) in rat primary astrocytes. The synthetic glucocorticoid dexamethasone provoked a marked increase of GLT-1 transcription and protein levels in cortical astrocytes, whereas GLAST expression remained unaffected. Up-regulation of GLT-1 expression was accompanied by an enhanced glutamate uptake, which could be blocked by the specific GLT-1 inhibitor dihydrokainate. The promoting effect of dexamethasone on GLT-1 gene expression and function was abolished by the GR antagonist mifepristone. A predominant role of the GR was further supported by the observation that corticosterone could elevate GLT-1 expression in a dose-dependent manner, whereas aldosterone, the physiological ligand of the mineralocorticoid receptor, exerted only weak effects even when applied at high concentrations. Moreover, we monitored brain region-specific differences, since all corticosteroids used in this study failed to alter the expression of GLT-1 in midbrain and cerebellar glia, although expression levels of both corticosteroid receptor subtypes were similar in all brain regions analyzed. Dexamethasone, however, modestly enhanced GLT-1 expression in cerebellar glia in combination with the DNA methyltransferase inhibitor 5-aza-2-deoxycytidine, suggesting that suppression of GLT-1 expression in cerebellar cultures may at least in part be epigenetically mediated by a DNA methylation-dependent process. Taken together, our data highlight a potential role for glucocorticoids in regulating GLT-1 gene expression during central nervous system development or pathophysiological processes including stress.

Glucocorticoid regulation of glial responses during hippocampal neurodegeneration and regeneration

Glucocorticoids can prevent or accelerate neurodegeneration in the adult rat hippocampus. To investigate these actions of glucocorticoids, we previously cloned genes from the hippocampus. Adrenalectomy specifically increased glial fibrillary acidic protein and transforming growth factor (TGF)-beta1 mRNAs in the dentate gyrus and these effects were dependent on induced apoptosis. Corticosterone treatment prevented apoptosis, and decreased glial activation and the influx of activated microglia. Since these effects are opposite to injury and neurodegeneration, we propose that they represent adaptive actions of glucocorticoids, preventing cellular defense mechanisms from overshooting. We used adrenalectomy as a model to investigate how adult granule neurons die in vivo and the effects of neurotrophic factors in protecting against apoptosis. Neurotrophin-4/5 and TGF-beta1 protected granule neurons against adrenalectomy-induced apoptosis. Since neurogenesis is also greatly increased in the dentate gyrus following adrenalectomy, we compared the time course of birth and death with glial responses. TGF-beta1 mRNA increased before the detection of dying cells in the dentate gyrus, which was coincident with increased proliferation in the neurogenic zone. Glucocorticoids also increased Ndrg2 mRNA in glia in the neurogenic zone; Ndrg2 is a member of a novel gene family involved in neural differentiation and synapse formation. Therefore, studying the effects of glucocorticoid manipulation on the dentate gyrus is increasing our understanding of how mature neurons die by apoptosis and the role of glia in induced apoptosis and neurogenesis. Discovering how endocrine and inflammatory responses regulate neuron birth and survival is important for developing successful neuron replacement strategies to treat neurodegenerative diseases.

Cytokines participate in neuronal death induced by trimethyltin in the rat hippocampus via type II glucocorticoid receptors

We investigated the role of IL-1alpha and IL-1beta expressed in the reactive gliosis following hippocampal damage induced by trimethyltin (TMT). IL-1alpha immunoreactivity was expressed earlier in small glial cells on day 4 post-TMT, while IL-1beta expression was obvious in large swollen glial cells on day 14 post-TMT. Both IL-1alpha and IL-1beta immunoreactivities were double-labeled with astrocyte marker, vimentin, but not with a microglia marker, OX-42. The expression of both IL-1alpha/beta was enhanced by adrenalectomy (ADX) prior to TMT administration. Corticosterone (CORT) or dexamethasone (DEX) supplementation not only cancelled effects of ADX, but also partially reversed TMT-induced enhancement of IL-1alpha/beta expressions. These changes coincided with TMT-induced neuronal death in CA3 pyramidal cells of the hippocampus. It is suggested that IL-1alpha/beta expressed in reactive astrocytes participate in TMT neurotoxicity via type II glucocorticoid receptors.

Cytokines participate in neuronal death induced by trimethyltin in the rat hippocampus via type II glucocorticoid receptors

We investigated the role of IL-1alpha and IL-1beta expressed in the reactive gliosis following hippocampal damage induced by trimethyltin (TMT). IL-1alpha immunoreactivity was expressed earlier in small glial cells on day 4 post-TMT, while IL-1beta expression was obvious in large swollen glial cells on day 14 post-TMT. Both IL-1alpha and IL-1beta immunoreactivities were double-labeled with astrocyte marker, vimentin, but not with a microglia marker, OX-42. The expression of both IL-1alpha/beta was enhanced by adrenalectomy (ADX) prior to TMT administration. Corticosterone (CORT) or dexamethasone (DEX) supplementation not only cancelled effects of ADX, but also partially reversed TMT-induced enhancement of IL-1alpha/beta expressions. These changes coincided with TMT-induced neuronal death in CA3 pyramidal cells of the hippocampus. It is suggested that IL-1alpha/beta expressed in reactive astrocytes participate in TMT neurotoxicity via type II glucocorticoid receptors.

Glucocorticoids modulate neurotransmitter-induced glycogen metabolism in cultured cortical astrocytes

Glucocorticoids (GC) are considered as key modulators of glycogen homeostasis in peripheral tissues, but their role in the central nervous system has only partially been characterized. Exposure of primary cultures of cortical astrocytes to dexamethasone (DEX), a synthetic glucocorticoid, results in the reduction of noradrenaline (NA)-induced glycogen synthesis in a concentration-dependent manner with a IC50 of 4.88 nm and a maximum inhibition of 51%. Such an effect is mediated via glucocorticoid receptors (GRs), since it is mimicked by the glucocorticoid analogue RU28362 (100 nm) and prevented by the GR antagonist RU38486 (1 micro m). DEX does not act through alteration of signal transduction mechanisms, as cAMP formation induced by noradrenergic stimulation was unchanged. Moreover, glycogen synthesis was inhibited to the same extent when DEX was applied either together or only after a brief NA application. Neither [3H]2-deoxyglucose uptake nor lactate release was altered by DEX in the presence of NA, demonstrating that inhibition of glycogen synthesis is not a consequence of reduced glucose utilization or availability. Interestingly, enhancement of glycogen synthase activity induced by NA was reduced in the presence of DEX (-27%). These results suggest that GC could have a significant influence on neuroenergetics as they could modulate activity-related changes in brain glycogen metabolism.

Desflurane and sevoflurane attenuate oxygen and glucose deprivation-induced neuronal cell death

Neuronal cell death may occur via two pathways: those causing necrosis or those causing apoptosis. Apoptosis can be activated during periods of stress such as oxygen and glucose deprivation. Anesthetic agents such as desflurane or sevoflurane can attenuate early neuronal necrotic death, but their effect on oxygen and glucose deprivation-induced apoptosis has not been investigated. Neuronal cell cultures were prepared from neonatal rat cortex and were used between 10 and 14 days in vitro. The neuronal cell cultures were pretreated 30 minutes prior to oxygen and glucose deprivation with either desflurane or sevoflurane (N = 18). Three concentrations of each anesthetic were evaluated. The cultures were then deprived of oxygen and glucose for 30, 60, or 90 minutes. Treatment with desflurane or sevoflurane was continued during the period of oxygen and glucose deprivation. Forty-eight hours after exposure, the cells were examined for apoptosis using TUNEL and DNA gel electrophoresis. Comparisons were made to neuronal cortical cell cultures exposed to oxygen and glucose deprivation alone (N = 9). This in vitro model of oxygen and glucose deprivation was successful in producing neuronal cell death during the exposure times examined. During 30-, 60-, and 90-minute periods of oxygen and glucose deprivation, both desflurane and sevoflurane significantly ( approximately 98%) attenuated neuronal cell death regardless of concentration.

Neuroendocrine-immune (NEI) circuitry from neuron-glial interactions to function: Focus on gender and HPA-HPG interactions on early programming of the NEI system

Bidirectional communication between the neuroendocrine and immune systems during ontogeny plays a pivotal role in programming the development of neuroendocrine and immune responses in adult life. Signals generated by the hypothalamic-pituitary-gonadal axis (i.e. luteinizing hormone-releasing hormone, LHRH, and sex steroids), and by the hypothalamic-pituitary-adrenocortical axis (glucocorticoids (GC)), are major players coordinating the development of immune system function. Conversely, products generated by immune system activation exert a powerful and long-lasting regulation on neuroendocrine axes activity. The neuroendocrine-immune system is very sensitive to preperinatal experiences, including hormonal manipulations and immune challenges, which may influence the future predisposition to several disease entities. We review our work on the ongoing mutual regulation of neuroendocrine and immune cell activities, both at a cellular and molecular level. In the central nervous system, one chief compartment is represented by the astroglial cell and its mediators. Hence, neuron-glial signalling cascades dictate major changes in response to hormonal manipulations and pro-inflammatory triggers. The interplay between LHRH, sex steroids, GC and pro-inflammatory mediators in some physiological and pathological states, together with the potential clinical implications of these findings, are summarized. The overall study highlights the plasticity of this intersystem cross-talk for pharmacological targeting with drugs acting at the neuroendocrine-immune interface.

Gender, neuroendocrine-immune interactions and neuron-glial plasticity. Role of luteinizing hormone-releasing hormone (LHRH)

Signals generated by the hypothalamic-pitutary-gonadal (HPG) axis powerfully modulate immune system function. This article summarizes some aspects of the impact of gender in neuroendocrine immunomodulation. Emphasis is given to the astroglial cell compartment, defined as a key actor in neuroendocrine immune communications. In the brain, the principal hormones of the HPG axis directly interact with astroglial cells. Thus, luteinizing hormone releasing hormone, LHRH, influences hypothalamic astrocyte development and growth, and hypothalamic astrocytes direct LHRH neuron differentiation. Hormonally induced changes in neuron-glial plasticity may dictate major changes in CNS output, and thus actively participate in sex dimorphic immune responses. The impact of gender in neuroimmunomodulation is further underlined by the sex dimorphism in the expression of genes encoding for neuroendocrine hormones and their receptors within the thymus, and by the potent modulation exerted by circulating sex steroids during development and immunization. The central role of glucocorticoids in the interactive communication between neuroendocrine and immune systems, and the impact of gender on hypothalamic-pituitary-adrenocortical (HPA) axis modulation is underscored in transgenic mice expressing a glucocorticoid receptor antisense RNA.

Dexamethasone up-regulates type II IL-1 receptor in mouse primary activated astrocytes

Brain astrocytes play a pivotal role in the brain response to inflammation. They express IL-1 receptors including the type I IL-1 receptor (IL-1RI) that transduces IL-1 signals in cooperation with the IL-1 receptor accessory protein (IL-1RAcP) and the type II IL-1 receptor (IL-1RII) that functions as a decoy receptor. As glucocorticoid receptors are expressed on astrocytes, we hypothesized that glucocorticoids regulate IL-1 receptors expression. IL-1beta-activated mouse primary astrocytes were treated with 10(-6) M dexamethasone, and IL-1 receptors were studied at the mRNA and protein levels. Using RT-PCR, IL-1RI and IL-1RII but not IL-1RAcP mRNAs were found to be up-regulated by dexamethasone in a time-dependent manner. Dexamethasone (Dex), but not progesterone, had no effect on IL-1RI but strongly increased IL-1RII mRNA expression. Binding studies revealed an increase in the number of IL-1RII binding sites under the effect of Dex, but no change in affinity. These findings support the concept that glucocorticoids have important regulatory effect on the response of astrocytes to IL-1.

Expression of the glucocorticoid receptor in early and late passage C-6 glioma cells and in normal astrocytes derived from aged mouse cerebral hemispheres

The presence of the glucocorticoid receptor in early and late passage C-6 glioma cells 2B clone and in astrocytes derived from aged mouse cerebral hemispheres has been documented by immunoblotting and/or immunofluorescence labelling. All cell types studied express the glucocorticoid receptor of molecular weight 97 KDa. In addition, in astrocytes derived from aged mouse cerebral hemispheres a smaller molecular weight polypeptide reacting with anti-glucocorticoid receptor antibody was also demonstrated. No difference in the amount of the 97 KDa glucocorticoid receptor between early and late C-6 2B cells was observed, whereas the astrocytes from aged cerebral hemispheres contained considerably reduced amounts of the glucocorticoid receptor compared to C-6 2B cells. Late passage C-6 2B cells were immunofluorescence labelled with the anti-glucocorticoid antibody, the receptor being almost exclusively present in the cytoplasm, with particular concentration in the perinuclear region. The presence of glucocorticoid receptor of molecular weight 97 KDa in glial cells corroborates and expands the existing data based on radioligand binding and immunocytochemical studies. These cell populations can be exploited as a model system for the study of the effects of glucocorticoids on senescence and brain aging.

Glucocorticoids up-regulate the expression of glial fibrillary acidic protein in the rat suprachiasmatic nucleus

Immunoreactivity against glial fibrillary acidic protein (GFAP) was used as a dynamic index in adrenalectomized rats subjected or not to corticosterone replacement to investigate whether glucocorticoids may interact with astrocytes in the suprachiasmatic nucleus (SCN), the master component of the central circadian clock. GFAP staining in the SCN was significantly higher in rats having received implants that restored physiological plasma levels of corticosterone within diurnal or nocturnal limits than in non-normalized rats. The effects of corticosterone were similar in the parvocellular portion of the paraventricular nucleus but were opposite in the hippocampus, another major site of negative feed-back regulation of the hypothalamic-pituitary-adrenal axis, where a decreased GFAP staining was observed in discrete regions of the dentate gyrus. This indicates that glucocorticoids may positively or negatively regulate GFAP, depending on the target brain structure. In the SCN, that contains only few if any glucocorticoid receptors, indirect mechanisms that may involve serotoninergic neurons are probably responsible for the effects of corticosterone level. It is proposed that the corticosterone-induced increase in GFAP staining in that nucleus accounts for dynamic changes in neurone-astrocyte interactions that might occur in relation with natural fluctuations of glucocorticoids over the 24 h period.

Emerging features of brain angiotensin receptors

In mammalian brain, angiotensin II AT1 and AT2 receptor subtypes are apparently expressed only in neurons and not in glia. AT1 and AT2 receptor subtypes are sometimes closely associated, but apparently expressed in different neurons. Brain AT1/AT2 interactions may occur in selective cases as inter-neuron cross talk. There are two AT1 isoforms in rodents. AT1A, which predominates, and AT1B. There are also important inter-species differences in receptor expression. Relative lack of amino acid conservation in the gerbil gAT1A receptor substantially decreases affinity for the AT1 antagonists. AT1 receptors are expressed in brain areas regulating autonomic and hormonal responses. AT1A receptors are heterogeneously regulated in a number of experimental conditions. In specific areas, AT1A receptors are not normally expressed, but are induced under influence of reproductive hormones in dopaminergic neurons. There are AT1 and AT2 receptors also in areas related to limbic, sensory and motor functions and their expression is developmentally regulated. A picture is emerging of widespread, neuronally localized, heterogeneously regulated, closely associated brain angiotensin receptor subtypes, modulating multiple functions including neuroendocrine and autonomic responses, stress, cerebrovascular flow, and perhaps brain maturation, neuronal plasticity, memory and behavior.

Pregnenolone reverses the age-dependent accumulation of glial fibrillary acidic protein within astrocytes of specific regions of the rat brain

Although aged-related modifications of astrocytes have been frequently described, little is known so far about the signals responsible for these modifications. Since it is well demonstrated that astrocytes are highly responsive to a variety of steroids, we hypothesized that modifications of cerebral astrocytes may result from the age-related decrease of circulating steroids. In the present study, we investigated the effects of the chronic administration of pregnenolone (PREG), the precursor of all steroid hormones, on the age-related extension of astrocytic processes in various brain regions. In adult (2-3 month-old) and aged (22-24 month-old) rats, quantitative image analysis was used to estimate, within each region, the number of astrocyte cell bodies immunostained (IS) for S100, and the surface occupied by astrocytic cell bodies and processes IS for glial fibrillary acidic protein (GFAP). In all regions, the surface occupied by GFAP-IS structures was increased in the aged vs. the adult rats, whereas no significant modifications were observed in the number of S100-IS cell bodies. Chronic administration of PREG to aged rats induced a marked decrease in the surface occupied by GFAP-IS structures in the cortex, amygdala and thalamus, without any significant effect on the number of S100-IS cell bodies present in these regions. By contrast, PREG had no significant effect when administered to adult animals. These data suggest that decreased levels of circulating steroid hormones may be responsible for the age-dependent modifications of the astrocytes present in various brain regions, and that these modifications can be at least partly corrected by the administration of PREG.

Dexamethasone regulates basic fibroblast growth factor, nerve growth factor and S100beta expression in cultured hippocampal astrocytes

Glucocorticoids regulate hippocampal neuron survival during fetal development, in the adult, and during aging; however, the mechanisms underlying the effects are unclear. Since astrocytes contain adrenocortical receptors and synthesize and release a wide variety of growth factors, we hypothesized that glucocorticoids may alter neuron-astrocyte interactions by regulating the expression of growth factors in hippocampal astrocytes. In this study, three growth factors, which are important for hippocampal neuron development and survival, were investigated: basic fibroblast growth factor (bFGF), nerve growth factor (NGF), and S100beta. Enriched type I astrocyte cultures were treated with 1 microM dexamethasone (DEX), a synthetic glucocorticoid, for up to 120 h. Cells and culture medium were collected and total RNA and protein were measured at 6, 12, 24, 48, 72, 96 and 120 h after the initiation of hormone treatment. Growth factor mRNA levels were measured and quantified using solution hybridization-RNase protection assays and protein levels were quantified using ELISA methods. We report that DEX stimulates the bFGF mRNA levels over the 120-h treatment. In contrast, DEX suppresses NGF mRNA continuously over the same period of treatment. DEX induces a biphasic response in S100beta mRNA levels. In addition, some of the changes in gene expression are translated into parallel changes in protein levels of these growth factors. Our results demonstrate that dexamethasone can differentially regulate the expression of growth factors in hippocampal astrocytes in vitro. This suggests that one of the mechanisms through which glucocorticoids affect hippocampal functions may be by regulating the expression of astrocyte-derived growth factors.

Monoamine oxidase B expression is selectively regulated by dexamethasone in cultured rat astrocytes

The influence of dexamethasone on monoamine oxidase (MAO) A and B expression and activity was investigated in primary cultures of rat type 1 astrocytes cultured under serum free, defined conditions. Dexamethasone treatment resulted in a dose- and time-dependent induction of MAO-B, but not of MAO-A, activity. The selective MAO-B increase was substantially reduced by the antagonist RU 486, thus suggesting a glucocorticoid receptor-mediated action of the hormone. Kinetic analysis showed an increase in Vmax of MAO-B with no change in apparent K(m). The dexamethasone-induced selective rise in MAO-B activity appeared to be due to enhanced enzyme synthesis, since MAO-B mRNA was markedly increased by dexamethasone treatment and the recovery of MAO-B activity after its irreversible inhibition by deprenyl was more pronounced in the presence than in the absence of the hormone. Furthermore, the dexamethasone effect was abolished by the protein synthesis inhibitors actinomycin D or cycloheximide. The present study demonstrates that dexamethasone is able to selectively induce MAO-B in type 1 astrocytes and leads to speculation of a possible role for glucocorticoids in the increase in brain MAO-B associated with neurodegenerative disorders, such as Parkinson's and Alzheimer's diseases.

Activation and proliferation of murine microglia are insensitive to glucocorticoids in Wallerian degeneration

Activation and proliferation of microglia are commonly described in the central nervous system after a wide range of insults, but the mechanisms that regulate their phenotype in vivo are still poorly understood. We have studied the effect that adrenalectomy and dexamethasone treatment have on the proliferation and activation of microglia during Wallerian degeneration of the optic nerve in BALB/c mice. We found that the onset and rate of microglia proliferation is independent of glucocorticoids. There was an increase in F4/80-positive cells 3 days after optic nerve crush, with a peak at 7 days, both in the optic nerve and its target, the superior colliculus. The numbers of F4/80-positive cells remained high up to 3 weeks after crush, the longest time point examined. We also found that up-regulation of F4/80 and the complement receptor type 3 and expression of major histocompatibility complex class II antigens were not affected by adrenalectomy or dexamethasone treatment. These observations show that, unlike microglia in vitro or peripheral macrophages, microglia do not readily respond to glucocorticoids, which could indicate a lack of or reduced expression of glucocorticoid receptor in these cells.

[3H]-aldosterone binding sites (type I receptors) in the lateral wall of the cochlea: distribution assessment by quantitative autoradiography

Mineralocorticoids, such as aldosterone, are steroids that enhance Na+ retention and K+ excretion in ion-transporting epithelial tissues through regulation of Na, K-ATPase. Previous studies suggest that aldosterone may regulate labyrinthine ion transport through up-regulation of Na, K-ATPase sites, a process mediated by high-affinity aldosterone (type I) receptors. In the present study, information concerning density and distribution of aldosterone binding sites in cochlear lateral wall tissues was determined by quantitative autoradiography using [3H]-aldosterone and RU-28362, a glucocorticoid agonist that blocks low affinity binding to glucocorticoid (type II) receptors. The results revealed that the distribution of aldosterone binding sites differs among the individual cochlear regions of the lateral wall. The highest level of binding was associated with the stria vascularis and epithelial cells of the spiral prominence. Elevated levels of binding were also observed in stromal cells of the spiral prominence, and to a lesser extent in the spiral ligament. The differential distribution of aldosterone binding sites in the lateral wall resembles the pattern of localization of Na, K-ATPase sites observed in previous studies and is compatible with the idea that mineralocorticoids play a role in the regulation of cochlear cation transport.

Short-term and long-term effects of p-chloroamphetamine on hippocampal serotonin and corticosteroid receptor levels

Hippocampal corticosteroid receptors are regulated by corticosterone as well as by neurotransmitters, such as serotonin (5-HT). Studies have demonstrated that long-term changes in 5-HT levels are associated with alterations in hippocampal glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) number. However, the effect of short-term manipulations of 5-HT levels on hippocampal corticosteroid receptor levels has not been thoroughly investigated. The present set of studies examined the effect of para-chloroamphetamine (PCA) administration on both short-term and long-term regulation of hippocampal 5-HT and corticosteroid receptor levels. PCA is a selective serotonergic neurotoxin which initially releases 5-HT to cause a short-term depletion of 5-HT stores, followed by a long-term decrease in 5-HT levels which presumably reflects the destruction of 5-HT nerve terminals. In the initial study rats were adrenalectomized and 24 h later injected with PCA (20 mg/kg) and sacrificed 3 h later. PCA produced a large decrease in hippocampal 5-HT (-79%) and 5-hydroxyindoleacetic acid (5-HIAA) (-40%) concentrations. In addition, PCA significantly decreased both hippocampal GR (-28%) and MR (-35%) levels. Pretreatment with fluoxetine (20 mg/kg), which presumably blocks the uptake of PCA into 5-HT nerve terminals, completely blocked the PCA-induced decreases in both 5-HT and corticosteroid receptor concentrations. In a final experiment, the long-term (7 days) effect of PCA administration on hippocampal 5-HT and corticosteroid receptor levels was examined. PCA (10 mg/kg given on 2 consecutive days) was administered to adrenal-intact rats which were adrenalectomized 6 days later and subsequently sacrificed following a 24 h interval. PCA produced an 87% decrease in hippocampal 5-HT and 5-HIAA levels, but did not alter hippocampal GR or MR levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Glial fibrillary acidic protein: regulation by hormones, cytokines, and growth factors

Levels of glial fibrillary acidic protein (GFAP), an astrocyte-specific intermediate filament protein, are altered during development and aging, GFAP also responds dynamically to neurodegenerative lesions. Changes in GFAP expression can occur at both transcriptional and translational levels. Modulators of GFAP expression include steroids, cytokines, and growth factors. GFAP expression also shows brain region-specific responses to sex steroids and of astrocyte-neuronal interactions. The 5'-upstream sequences of rat, mouse, and human are compared for the presence of response elements that are candidates for transcriptional regulation of GFAP. We propose that the regulation of the GFAP gene has evolved a system of controls that allow integrated responses to neuroendocrine and inflammatory modulators.

Postmortem stability of mRNA for glucocorticoid and mineralocorticoid receptor in rodent brain

The relative postmortem stability of the mRNA's for glucocorticoid (GR) and mineralocorticoid (MR) receptor in rodent brain was determined using semi-quantitative in situ hybridization (ISH). Rats were killed by CO2 asphyxiation and their brains removed immediately (0 h) or following 12 h or 24 h delays. Specific hybridization of GR and MR anti-sense [35S]RNA-probe to tissue mRNA encoding these receptors was detected using film and emulsion autoradiography. The most intense labeling for GR mRNA was in the dentate gyrus followed by the CA1 hippocampal region. Lower, but still detectable signal, was apparent over CA3-CA4 pyramidal cell regions. MR mRNA was detected throughout the CA1-4 pyramidal cell fields of the hippocampus and the granular cells of the dentate gyrus. Film images demonstrated that even in the 24 h postmortem delay group intense specific signal was present in sections hybridized with both anti-sense GR and MR probes, although there was some diminution in signal intensity in cortical areas at this later postmortem delay. These initial experiments with rat brain demonstrate that the mRNA's for both GR and MR, as detected with ISH, are stable for up to 24 h following death.

Glucocorticoid and mineralocorticoid receptors in rat neocortical and hippocampal brain cells in culture: characterization and regulatory studies

Glucocorticoid and mineralocorticoid binding sites were characterized in cell cultures derived from neocortical and hippocampal brain tissue from fetal (E18) rats. Specific and saturable binding was detected in living cells with a sensitive whole cell binding method using [3H]dexamethasone ([3H]DEX) and [3H]aldosterone ([3H]ALDO) (in the presence of RU 28362, a selective glucocorticoid receptor (GR) agonist) as ligands for the measurement of glucocorticoid and mineralocorticoid receptors (MRs), respectively. Specific corticosteroid binding was demonstrated as early as day 4 in culture in neocortical cells, with a time-dependent increase in binding sites during further culturing time. At 7-9 days in vitro, Scatchard analysis of [3H]DEX binding revealed a maximum binding capacity (Bmax) of 83.4 +/- 5.0 fmol/mg protein and a binding affinity (Kd) of 3.6 +/- 0.4 nM in neocortical brain cells. Competition binding studies with [3H]DEX demonstrated a glucocorticoid specificity of receptor sites (relative binding affinity: RU 28362 = DEX > PROG > ALDO). Similar binding characteristics were demonstrated for GRs in cultures derived from fetal hippocampal tissue (Bmax 49.1 +/- 5.8 fmol/mg protein, Kd 3.5 +/- 0.2 nM). Analysis of MRs with [3H]ALDO (+RU 28362) revealed specific and saturable binding in hippocampal cultures, with a Bmax of 8.0 +/- 0.5 fmol/mg protein and a Kd of 0.2 +/- 0.1 nM. Competition studies with [3H]ALDO showed a mineralocorticoid-like pattern of receptor binding (relative binding affinity: CORT = ALDO > PROG > DEX). In addition, small numbers of MRs were detectable in cortex-derived cultures (Bmax: 3.7 +/- 0.8 fmol/mg protein, Kd: 0.3 +/- 0.2 nM).(ABSTRACT TRUNCATED AT 250 WORDS)