Glucocorticoid control of glial gene expression

Glia-Driven Brain Circuit Refinement Is Altered by Early-Life Adversity: Behavioral Outcomes

Early-life adversity (ELA), often clinically referred to as "adverse childhood experiences (ACE)," is the exposure to stress-inducing events in childhood that can result in poor health outcomes. ELA negatively affects neurodevelopment in children and adolescents resulting in several behavioral deficits and increasing the risk of developing a myriad of neuropsychiatric disorders later in life. The neurobiological mechanisms by which ELA alters neurodevelopment in childhood have been the focus of numerous reviews. However, a comprehensive review of the mechanisms affecting adolescent neurodevelopment (i.e., synaptic pruning and myelination) is lacking. Synaptic pruning and myelination are glia-driven processes that are imperative for brain circuit refinement during the transition from adolescence to adulthood. Failure to optimize brain circuitry between key brain structures involved in learning and memory, such as the hippocampus and prefrontal cortex, leads to the emergence of maladaptive behaviors including increased anxiety or reduced executive function. As such, we review preclinical and clinical literature to explore the immediate and lasting effects of ELA on brain circuit development and refinement. Finally, we describe a number of therapeutic interventions best-suited to support adolescent neurodevelopment in children with a history of ELA.

Assessment of astrocytes as a mediator of memory and learning in rodents

The classical view of astrocytes is that they provide supportive functions for neurons, transporting metabolites and maintaining the homeostasis of the extracellular milieu. This view is gradually changing with the advent of molecular genetics and optical methods allowing interrogation of selected cell types in live experimental animals. An emerging view that astrocytes additionally act as a mediator of synaptic plasticity and contribute to learning processes has gained in vitro and in vivo experimental support. Here we focus on the literature published in the past two decades to review the roles of astrocytes in brain plasticity in rodents, whereby the roles of neurotransmitters and neuromodulators are considered to be comparable to those in humans. We outline established inputs and outputs of astrocytes and discuss how manipulations of astrocytes have impacted the behavior in various learning paradigms. Multiple studies suggest that the contribution of astrocytes has a considerably longer time course than neuronal activation, indicating metabolic roles of astrocytes. We advocate that exploring upstream and downstream mechanisms of astrocytic activation will further provide insight into brain plasticity and memory/learning impairment.

Glutamine synthetase regulation by dexamethasone, RU486, and compound A in astrocytes derived from aged mouse cerebral hemispheres is mediated via glucocorticoid receptor

Glucocorticoids (GCs) regulate astrocyte function, while glutamine synthetase (GS), an enzyme highly expressed in astrocytes, is one of the most remarkable GCs-induced genes. GCs mediate their effects through their cognate glucocorticoid receptor (GRα and GRβ isoforms); however, the mechanism via which these isoforms regulate GS activity in astrocytes remains unknown. We used dexamethasone (DEX), a classical GRα/GRβ agonist, RU486, which is a specific GRβ ligand, and Compound A, a known "dissociated" ligand, to delineate the mechanism via which GR modulates GS activity. Aged Mouse Cerebral Hemisphere astrocytes were treated with DEX (1 μM), RU486 (1 nM-1 μM) or compound A (10 μM), alone or in combination with DEX. GS activity and expression, GR isoforms (mRNA and protein levels), and GRα subcellular trafficking were measured. DEX increased GS activity in parallel with GRα nuclear translocation. RU486 increased GS activity in absence of GRα nuclear translocation implicating thus a role of GRβ-mediated mechanism compound A had no effect on GS activity implicating a GRα-GRE-mediated mechanism. None of the compounds affected whole-cell GRα protein content. DEX reduced GRα and GRβ mRNA levels, while RU486 increased GRβ gene expression. We provide evidence that GS activity, in astrocytes, is regulated via GRα- and GRβ-mediated pathways with important implications in pathological conditions in which astrocytes are involved.

Differential sensitivity to the acute psychotomimetic effects of delta-9-tetrahydrocannabinol associated with its differential acute effects on glial function and cortisol

BACKGROUND

Cannabis use has been associated with psychosis through exposure to delta-9-tetrahydrocannabinol (Δ9-THC), its key psychoactive ingredient. Although preclinical and human evidence suggests that Δ9-THC acutely modulates glial function and hypothalamic-pituitary-adrenal (HPA) axis activity, whether differential sensitivity to the acute psychotomimetic effects of Δ9-THC is associated with differential effects of Δ9-THC on glial function and HPA-axis response has never been tested.

METHODS

A double-blind, randomized, placebo-controlled, crossover study investigated whether sensitivity to the psychotomimetic effects of Δ9-THC moderates the acute effects of a single Δ9-THC dose (1.19 mg/2 ml) on myo-inositol levels, a surrogate marker of glia, in the Anterior Cingulate Cortex (ACC), and circadian cortisol levels, the key neuroendocrine marker of the HPA-axis, in a set of 16 healthy participants (seven males) with modest previous cannabis exposure.

RESULTS

The Δ9-THC-induced change in ACC myo-inositol levels differed significantly between those sensitive to (Δ9-THC minus placebo; M = -0.251, s.d. = 1.242) and those not sensitive (M = 1.615, s.d. = 1.753) to the psychotomimetic effects of the drug (t(14) = 2.459, p = 0.028). Further, the Δ9-THC-induced change in cortisol levels over the study period (baseline minus 2.5 h post-drug injection) differed significantly between those sensitive to (Δ9-THC minus placebo; M = -275.4, s.d. = 207.519) and those not sensitive (M = 74.2, s.d. = 209.281) to the psychotomimetic effects of the drug (t(13) = 3.068, p = 0.009). Specifically, Δ9-THC exposure lowered ACC myo-inositol levels and disrupted the physiological diurnal cortisol decrease only in those subjects developing transient psychosis-like symptoms.

CONCLUSIONS

The interindividual differences in transient psychosis-like effects of Δ9-THC are the result of its differential impact on glial function and stress response.

Functional recovery with histomorphometric analysis of nerves and muscles after combination treatment with erythropoietin and dexamethasone in acute peripheral nerve injury

Introduction

Peripheral nerve injury (PNI) often leads to significant functional loss in patients and poses a challenge to physicians since treatment options for improving functional outcomes are limited. Recent studies suggest that erythropoietin and glucocoticoids have beneficial effects as mediators of neuro-regenerative processes. We hypothesized that combination treatment with erythropoietin and glucocoticoids would have a synergistic effect on functional outcome after PNI.

Materials and methods

Sciatic nerve crush injury was simulated in ten-week-old male C57BL/6 mice. The mice were divided into four groups according to the type of drugs administered (control, erythropoietin, dexamethasone, and erythropoietin with dexamethasone). Motor functional recovery was monitored by walking track analysis at serial time points up to 28 days after injury. Morphological analysis of the nerve was performed by immunofluorescent staining for neurofilament (NF) heavy chain and myelin protein zero (P0) in cross-sectional and whole-mount nerve preparations. Additionally, morphological analysis of the muscle was performed by Hematoxylin and eosin staining.

Results

Combination treatment with erythropoietin and dexamethasone significantly improved the sciatic functional index at 3, 7, 14, and 28 days after injury. Fluorescence microscopy of cross sectional nerve revealed that the combination treatment increased the ratio of P0/NF-expressing axons. Furthermore, confocal microscopy of the whole-mount nerve revealed that the combination treatment increased the fluorescence intensity of P0 expression. The cross-sectional area and minimum Feret’s diameter of the muscle fibers were significantly larger in the mice which received combination treatment than those in the controls.

Conclusion

Our results demonstrated that combination treatment with erythropoietin and dexamethasone accelerates functional recovery and reduces neurogenic muscle atrophy caused by PNI in mice, which may be attributed to the preservation of myelin and Schwann cell re-myelination. These findings may provide practical therapeutic options for patients with acute PNI.

Environmental noise exposure modifies astrocyte morphology in hippocampus of young male rats

Background:

Chronic exposure to noise induces changes on the central nervous system of exposed animals. Those changes affect not only the auditory system but also other structures indirectly related to audition. The hippocampus of young animals represents a potential target for these effects because of its essential role in individuals’ adaptation to environmental challenges.

Objective:

The aim of the present study was to evaluate hippocampus vulnerability, assessing astrocytic morphology in an experimental model of environmental noise (EN) applied to rats in pre-pubescent stage.

Materials and Methods:

Weaned Wistar male rats were subjected to EN adapted to the rats’ audiogram for 15 days, 24 h daily. Once completed, plasmatic corticosterone (CORT) concentration was quantified, and immunohistochemistry for glial fibrillary acidic protein was taken in hippocampal DG, CA3, and CA1 subareas. Immunopositive cells and astrocyte arborizations were counted and compared between groups.

Results:

The rats subjected to noise exhibited enlarged length of astrocytes arborizations in all hippocampal subareas. Those changes were accompanied by a marked rise in serum CORT levels.

Conclusions:

These findings confirm hippocampal vulnerability to EN and suggest that glial cells may play an important role in the adaptation of developing the participants to noise exposure.

Effects of glucocorticoids in depression: Role of astrocytes

Astrocytes or astroglia are heterogeneous cells, similar to neurons, that have different properties in different brain regions. The implications of steroid hormones on glial cells and stress-related pathologies have been studied previously. Glucocorticoids (GCs) that are released in response to stress have been shown to be deleterious to neurons in various brain regions. Further, in the light of the effect of GCs on astrocytes, several reports have shown the crucial role of glia. Still, much remains to be done to understand the stress-astrocytes-glucocorticoid interactions associated with the pathological consequences of various CNS disorders. This review is an attempt to summarize the effects of GCs and stress on astrocytes and its implications in depression.

Enduring, Sexually Dimorphic Impact of In Utero Exposure to Elevated Levels of Glucocorticoids on Midbrain Dopaminergic Populations

Glucocorticoid hormones (GCs) released from the fetal/maternal glands during late gestation are required for normal development of mammalian organs and tissues. Accordingly, synthetic glucocorticoids have proven to be invaluable in perinatal medicine where they are widely used to accelerate fetal lung maturation when there is risk of pre-term birth and to promote infant survival. However, clinical and pre-clinical studies have demonstrated that inappropriate exposure of the developing brain to elevated levels of GCs, either as a result of clinical over-use or after stress-induced activation of the fetal/maternal adrenal cortex, is linked with significant effects on brain structure, neurological function and behaviour in later life. In order to understand the underlying neural processes, particular interest has focused on the midbrain dopaminergic systems, which are critical regulators of normal adaptive behaviours, cognitive and sensorimotor functions. Specifically, using a rodent model of GC exposure in late gestation (approximating human brain development at late second/early third trimester), we demonstrated enduring effects on the shape and volume of the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) (origins of the mesocorticolimbic and nigrostriatal dopaminergic pathways) on the topographical organisation and size of the dopaminergic neuronal populations and astrocytes within these nuclei and on target innervation density and neurochemical markers of dopaminergic transmission (receptors, transporters, basal and amphetamine-stimulated dopamine release at striatal and prefrontal cortical sites) that impact on the adult brain. The effects of antenatal GC treatment (AGT) were both profound and sexually-dimorphic, not only in terms of quantitative change but also qualitatively, with several parameters affected in the opposite direction in males and females. Although such substantial neurobiological changes might presage marked behavioural effects, in utero GC exposure had only a modest or no effect, depending on sex, on a range of conditioned and unconditioned behaviours known to depend on midbrain dopaminergic transmission. Collectively, these findings suggest that apparent behavioural normality in certain tests, but not others, arises from AGT-induced adaptations or compensatory mechanisms within the midbrain dopaminergic systems, which preserve some, but not all functions. Furthermore, the capacities for molecular adaptations to early environmental challenge are different, even opponent, in males and females, which may account for their differential resilience or failure to perform adequately in behavioural tests. Behavioural "normality" is thus achieved by the midbrain dopaminergic network operating outside its normal limits (in a state of allostasis), rendering it at greater risk to malfunction when challenged in later life. Sex-specific neurobiological programming of midbrain dopaminergic systems may, therefore, have psychopathological relevance for the sex bias commonly found in brain disorders associated with these systems, and which have a neurodevelopmental component, including schizophrenia, ADHD (attention/deficit hyperactivity disorders), autism, depression and substance abuse.

Immunohistochemical Localization of GFAP and Glutamate Regulatory Proteins in Chick Retina and Their Levels of Expressions in Altered Photoperiods

Moderate to intense light is reported to damage the chick retina, which is cone dominated. Light damage alters neurotransmitter pools, such as those of glutamate. Glutamate level in the retina is regulated by glutamate-aspartate transporter (GLAST) and glutamine synthetase (GS). We examined immunolocalization patterns and the expression levels of both markers and of glial fibrillary acidic protein (GFAP, a marker of neuronal stress) in chick retina exposed to 2000 lux under 12-h light:12-h dark (12L:12D; normal photoperiod), 18L:6D (prolonged photoperiod), and 24L:0D (constant light) at post-hatch day 30. Retinal damage (increased death of photoreceptors and inner retinal neurons and Müller cell hypertrophy) and GFAP expression in Müller cells were maximal in 24L:0D condition compared to that seen in 12L:12D and 18L:6D conditions. GS was present in Müller cells and GLAST expressed in Müller cell processes and photoreceptor inner segments. GLAST expression was decreased in 24L:0D condition, and the expression levels between 12L:12D and 18L:6D, though increased marginally, were statistically insignificant. Similar was the case with GS expression that significantly decreased in 24L:0D condition. Our previous study with chicks exposed to 2000 lux reported increased retinal glutamate level in 24L:0D condition. The present results indicate that constant light induces decreased expressions of GLAST and GS, a condition that might aggravate glutamate-mediated neurotoxicity and delay neuroprotection in a cone-dominated retina.

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.

Temporal control of glucocorticoid neurodynamics and its relevance for brain homeostasis, neuropathology and glucocorticoid-based therapeutics

Glucocorticoids mediate plethora of actions throughout the human body. Within the brain, they modulate aspects of immune system and neuroinflammatory processes, interfere with cellular metabolism and viability, interact with systems of neurotransmission and regulate neural rhythms. The influence of glucocorticoids on memory and emotional behaviour is well known and there is increasing evidence for their involvement in many neuropsychiatric pathologies. These effects, which at times can be in opposing directions, depend not only on the concentration of glucocorticoids but also the duration of their presence, the temporal relationship between their fluctuations, the co-influence of other stimuli, and the overall state of brain activity. Moreover, they are region- and cell type-specific. The molecular basis of such diversity of effects lies on the orchestration of the spatiotemporal interplay between glucocorticoid- and mineralocorticoid receptors, and is achieved through complex dynamics, mainly mediated via the circadian and ultradian pattern of glucocorticoid secretion. More sophisticated methodologies are therefore required to better approach the study of these hormones and improve the effectiveness of glucocorticoid-based therapeutics.

Neuroprotective effects of intravitreal triamcinolone acetonide and dexamethasone implant in rabbit retinas after pars plana vitrectomy and silicone oil injection

PURPOSE

To investigate potential retinal neuroprotective effects of intravitreal triamcinolone acetonide and dexamethasone implant in rabbits after pars plana vitrectomy and intravitreal silicone oil injection.

METHODS

The right eyes of 84 rabbits, divided into 3 groups of 28 rabbits each, underwent standard 3-port pars plana vitrectomy with silicone oil (SO group), silicone oil and intravitreal dexamethasone implant (SO/DEX group), or silicone oil and triamcinolone acetonide (SO/TA group). The retina from the left eye of each rabbit served as a control. The animals were killed at 4 weeks after surgery. Qualitative and quantitative histopathologic analyses were performed 4 weeks after surgery, and investigation for apoptosis was performed using the Tunel assay.

RESULTS

Intravitreal triamcinolone acetonide and dexamethasone implant were associated with increased retinal neuronal survival, primarily in the outer nuclear layer, inner nuclear layer, and ganglion cell layer. In the SO group, the cell density in eyes that underwent PPV/SO was 31% lower in the outer nuclear layer, 33% lower in the inner nuclear layer, and 45% lower in the ganglion cell layer compared to control eyes (p < 0.05 for all PPV/SO versus control comparisons). Compared to eyes that underwent PPV/SO, the cell density in eyes treated with triamcinolone was 27% higher in the outer nuclear layer, 66% higher in the inner nuclear layer, and 100% higher in the ganglion cell layer (p < 0.05 for all triamcinolone versus PPV/SO comparisons). Compared to eyes that underwent PPV/SO, the cell density in eyes treated with dexamethasone was 46% higher in the outer nuclear layer, 62% higher in the inner nuclear layer, and 77% higher in the ganglion cell layer (p < 0.05 for all dexamethasone versus PPV/SO comparisons). Analyses using the Tunnel assay demonstrated apoptotic bodies in all eyes in the SO group, compared with none of the eyes in the SO/TA and SO/DEX groups. The presence of cell nuclei stained with 49,6-diamidino-2-phenylindole (DAPI) was demonstrated in all groups.

CONCLUSION

In this experimental model of neuroprotection, increased retinal neuronal survival was seen in the steroid-treated groups compared with the controls.

Sex-specific disruption of murine midbrain astrocytic and dopaminergic developmental trajectories following antenatal GC treatment

The mammalian midbrain dopaminergic systems arising in the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) are critical for coping behaviours and are implicated in neuropsychiatric disorders where early life challenges comprise significant risk factors. Here, we aimed to advance our hypothesis that glucocorticoids (GCs), recognised key players in neurobiological programming, target development within these systems, with a novel focus on the astrocytic population. Mice received antenatal GC treatment (AGT) by including the synthetic GC, dexamethasone, in the mothers' drinking water on gestational days 16-19; controls received normal drinking water. Analyses of regional shapes and volumes of the adult SNc and VTA demonstrated that AGT induced long-term, dose-dependent, structural changes that were accompanied by profound effects on astrocytes (doubling/tripling of numbers and/or density). Additionally, AGT induced long-term changes in the population size and distribution of SNc/VTA dopaminergic neurons, confirming and extending our previous observations made in rats. Furthermore, glial/neuronal structural remodelling was sexually dimorphic and depended on the AGT dose and sub-region of the SNc/VTA. Investigations within the neonatal brain revealed that these long-term organisational effects of AGT depend, at least in part, on targeting perinatal processes that determine astrocyte density and programmed cell death in dopaminergic neurons. Collectively, our characterisation of enduring, AGT-induced, sex-specific cytoarchitectural disturbances suggests novel mechanistic links for the strong association between early environmental challenge (inappropriate exposure to excess GCs) and vulnerability to developing aberrant behaviours in later life, with translational implications for dopamine-associated disorders (such as schizophrenia, ADHD, autism, depression), which typically show a sex bias.

Regulation of astrocyte glutamine synthetase in epilepsy

Astrocytes play a crucial role in regulating and maintaining the extracellular chemical milieu of the central nervous system under physiological conditions. Moreover, proliferation of phenotypically altered astrocytes (a.k.a. reactive astrogliosis) has been associated with many neurologic and psychiatric disorders, including mesial temporal lobe epilepsy (MTLE). Glutamine synthetase (GS), which is found in astrocytes, is the only enzyme known to date that is capable of converting glutamate and ammonia to glutamine in the mammalian brain. This reaction is important, because a continuous supply of glutamine is necessary for the synthesis of glutamate and GABA in neurons. The known stoichiometry of glutamate transport across the astrocyte plasma membrane also suggests that rapid metabolism of intracellular glutamate via GS is a prerequisite for efficient glutamate clearance from the extracellular space. Several studies have indicated that the activity of GS in astrocytes is diminished in several brain disorders, including MTLE. It has been hypothesized that the loss of GS activity in MTLE leads to increased extracellular glutamate concentrations and epileptic seizures. Understanding the mechanisms by which GS is regulated may lead to novel therapeutic approaches to MTLE, which is frequently refractory to antiepileptic drugs. This review discusses several known mechanisms by which GS expression and function are influenced, from transcriptional control to enzyme modification.

Astrocytes are a neural target of morphine action via glucocorticoid receptor-dependent signaling

Chronic opioid use leads to the structural reorganization of neuronal networks, involving genetic reprogramming in neurons and glial cells. Our previous in vivo studies have revealed that a significant fraction of the morphine-induced alterations to the striatal transcriptome included glucocorticoid (GC) receptor (GR)-dependent genes. Additional analyses suggested glial cells to be the locus of these changes. In the current study, we aimed to differentiate the direct transcriptional effects of morphine and a GR agonist on primary striatal neurons and astrocytes. Whole-genome transcriptional profiling revealed that while morphine had no significant effect on gene expression in both cell types, dexamethasone significantly altered the transcriptional profile in astrocytes but not neurons. We obtained a complete dataset of genes undergoing the regulation, which includes genes related to glucose metabolism (Pdk4), circadian activity (Per1) and cell differentiation (Sox2). There was also an overlap between morphine-induced transcripts in striatum and GR-dependent transcripts in cultured astrocytes. We further analyzed the regulation of expression of one gene belonging to both groups, serum and GC regulated kinase 1 (Sgk1). We identified two transcriptional variants of Sgk1 that displayed selective GR-dependent upregulation in cultured astrocytes but not neurons. Moreover, these variants were the only two that were found to be upregulated in vivo by morphine in a GR-dependent fashion. Our data suggest that the morphine-induced, GR-dependent component of transcriptome alterations in the striatum is confined to astrocytes. Identification of this mechanism opens new directions for research on the role of astrocytes in the central effects of opioids.

Glucocorticoid treatment of astrocytes results in temporally dynamic transcriptome regulation and astrocyte-enriched mRNA changes in vitro

While general effects of glucocorticoids are well established, the specific cellular mechanisms by which these hormones exert tissue-dependent effects continue to be elaborated. Diseases that demonstrate altered glucocorticoid signaling have been associated with alterations in astrocytes, yet relatively little is known about the effects of glucocorticoids upon this cell type. We have analyzed mRNA expression patterns following glucocorticoid treatment of mouse primary astrocyte cultures. Microarray analysis of cortical astrocyte cultures treated with dexamethasone over an eight-point, 24 h time course identified 854 unique genes with ≥twofold change in mRNA expression at one or more time points. Clustering analysis associated subsets of these mRNA expression changes with gene ontology categories known to be impacted by glucocorticoids. Numerous mRNAs regulated by dexamethasone were also regulated by the natural ligand corticosterone; all of the mRNAs regulated ≥twofold by corticosterone were substantially attenuated by cotreatment with the glucocorticoid receptor antagonist RU486. Of the mRNAs demonstrating ≥twofold expression change in response to both glucocorticoids, 33 mRNAs were previously associated with glucocorticoid regulation, and 36 mRNAs were novel glucocorticoid targets. All genes tested by qPCR for glucocorticoid regulation in cortical astrocyte cultures were also regulated by glucocorticoids in hippocampal astrocyte cultures (18/18). Interestingly, a portion of glucocorticoid-regulated genes were astrocyte enriched; the percentage of astrocyte-enriched genes per total number of regulated genes was highest for the early time points and steadily decreased over the time course. These findings suggest that astrocytes in vitro may initially deploy cell type-specific patterns of mRNA regulatory responses to glucocorticoids and subsequently activate additional cell type-independent responses.

Expression of aromatase P450(AROM) in the human fetal and early postnatal cerebral cortex

Aromatase (P450(AROM)), the enzyme responsible for the conversion of testosterone (T) into 17-β estradiol (E(2)), plays a crucial role in the sexual differentiation of specific hypothalamic nuclei. Moreover, recent findings indicate that local E(2) synthesis has an impact on other brain areas including hippocampus, temporal cortex and cerebellum, and may thus influence also cognitive functions. Numerous studies have described the expression and the distribution of P450(AROM) throughout ontogenesis and postnatal development of the central nervous system in several mammals, but data referring to humans are scarce. In the adult human brain, P450(AROM) has been detected in the hypothalamus, limbic areas, and in the basal forebrain, and described in glial cells of the cerebral cortex and hippocampus. In this study we report the expression, distribution and cellular localization of P450(AROM) in the human fetal and early postnatal cerebral cortex. In our series of fetal brains of the second trimester, P450(AROM) expression appeared at gestational week (GW) 17 and resulted limited to groups of cells localized close to the growing neuroepithelium in the ventricular and subventricular zones. At GWs 20-24, scattered P450(AROM) immunoreactive (-ir) neural cells were identified in the intermediate plate and subplate, and in the parietal cortical plate. In perinatal and early postnatal individuals the quantity of P450(AROM)-ir elements increased, and revealed the morphology typical of glial cells. Double labeling immunostaining with anti-GFAP and anti-P450(AROM) antisera, and subsequent confocal analysis, confirmed this observation. Our data show that the expression of P450(AROM) in the fetal cortex starts approx at the end of the fourth gestational month, but increases steadily only in the last trimester or in the early postnatal period. This temporal trend may suggest that P450(AROM) could act as a differentiation-promoting factor, based on timing of the steroid actions.

Astrocytic regulation of glutamate homeostasis in epilepsy

Astrocytes play a critical role in regulation of extracellular neurotransmitter levels in the central nervous system. This function is particularly prominent for the excitatory amino acid glutamate, with estimates that 80-90% of extracellular glutamate uptake in brain is through astrocytic glutamate transporters. This uptake has significance both in regulation of the potential toxic accumulation of extracellular glutamate and in normal resupply of inhibitory and excitatory synapses with neurotransmitter. This resupply of neurotransmitter is accomplished by astroglial uptake of glutamate, transformation of glutamate to glutamine by the astrocytic enzyme glutamine synthetase (GS), and shuttling of glutamine back to excitatory and inhibitory neurons via specialized transporters. Once in neurons, glutamine is enzymatically converted back to glutamate, which is utilized for synaptic transmission, either directly, or following decarboxylation to γ-aminobutyric acid. Many neurologic and psychiatric conditions, particularly epilepsy, are accompanied by the development of reactive gliosis, a pathology characterized by anatomical and biochemical plasticity in astrocytes, accompanied by proliferation of these cells. Among the biochemical changes evident in reactive astrocytes is a downregulation of several of the important regulators of the glutamine-glutamate cycle, including GS, and possibly also glutamate transporters. This downregulation may have significance in contributing both to the aberrant excitability and to the altered neuropathology characterizing epilepsy. In the present review, we provide an overview of the normal function of astrocytes in regulating extracellular glutamate homeostasis, neurotransmitter supply, and excitotoxicity. We further discuss the potential role reactive gliosis may play in the pathophysiology of epilepsy.

Evidence for astrocytes as a potential source of the glutamate excess in temporal lobe epilepsy

Increased extracellular brain glutamate has been implicated in the pathophysiology of human refractory temporal lobe epilepsy (TLE), but the cause of the excessive glutamate is unknown. Prior studies by us and others have shown that the glutamate degrading enzyme glutamine synthetase (GS) is deficient in astrocytes in the epileptogenic hippocampal formation in a subset of patients with TLE. We have postulated that the loss of GS in TLE leads to increased glutamate in astrocytes with elevated concentrations of extracellular glutamate and recurrent seizures as the ultimate end-points. Here we test the hypothesis that the deficiency in GS leads to increased glutamate in astrocytes. Rats were chronically infused with methionine sulfoximine (MSO, n=4) into the hippocampal formation to induce GS deficiency and recurrent seizures. A separate group of rats was infused with 0.9% NaCl (saline) as a control (n=6). At least 10days after the start of infusion, once recurrent seizures were established in the MSO-treated rats, the concentration of glutamate was assessed in CA1 of the hippocampal formation by immunogold electron microscopy. The concentration of glutamate was 47% higher in astrocytes in the MSO-treated vs. saline-treated rats (p=0.02), and the ratio of glutamate in astrocytes relative to axon terminals was increased by 74% in the MSO-treated rats (p=0.003). These data support our hypothesis that a deficiency in GS leads to increased glutamate in astrocytes. We additionally propose that the GS-deficient astrocytes in the hippocampal formation in TLE lead to elevated extracellular brain glutamate either through decreased clearance of extracellular glutamate or excessive release of glutamate into the extracellular space from these cells, or a combination of the two.

The complex regulation of human glud1 and glud2 glutamate dehydrogenases and its implications in nerve tissue biology

Mammalian glutamate dehydrogenase (GDH) is a housekeeping mitochondrial enzyme (hGDH1 in the human) that catalyses the reversible inter-conversion of glutamate to α-ketoglutarate and ammonia, thus interconnecting amino acid and carbohydrate metabolism. It displays an energy sensing mechanism, which permits enzyme activation under low cellular energy states. As GDH is at the crossroads of important metabolic pathways, a tight control of its activity is essential. Indeed, to fulfill its role in metabolism and cellular energetics, mammalian GDH has evolved into a highly regulated enzyme subject to allosteric modulation by diverse compounds. The recent emergence (<23 million years ago) in apes and humans of a hGDH2 isoenzyme with distinct regulatory properties, as well as, the detection of gain-of-function variants in hGDH1 and hGDH2 that affect the nervous system, have introduced additional complexities. The properties of the two highly homologous human GDHs were studied using purified recombinant hGDH1 and hGDH2 obtained by expression of the corresponding cDNAs in Sf21 cells. Results showed that, in contrast to hGDH1 that maintains substantial basal activity (35-40% of its maximal capacity), hGDH2 displays low basal activity (3-8% of maximal) that is remarkably responsive to activation by rising levels of ADP and/or l-leucine. This is primarily due to the Arg443Ser evolutionary change, which also made hGDH2 markedly sensitive to estrogens and neuroleptic drugs. In contrast to hGDH1, which is subject to potent GTP inhibition, hGDH2 has dissociated its function from this energy switch, being able to metabolize glutamate even when the Krebs cycle generates GTP levels sufficient to inactivate the housekeeping hGDH1. Our data also show that spermidine, a polyamine thought to reduce oxidative stress and to prolong survival, and EGCG, a green tea polyphenol, inhibit hGDH2 at lower concentrations than hGDH1. The implications of these findings in nerve tissue biology are discussed.

The cochlea as an independent neuroendocrine organ: expression and possible roles of a local hypothalamic-pituitary-adrenal axis-equivalent signaling system

A key property possessed by the mammalian cochlea is its ability to dynamically alter its own sensitivity. Because hair cells and ganglion cells are prone to damage following exposure to loud sound, extant mechanisms limiting cochlear damage include modulation involving both the mechanical (via outer hair cell motility) and neural signaling (via inner hair cell-ganglion cell synapses) steps of peripheral auditory processing. Feedback systems such as that embodied by the olivocochlear system can alter sensitivity, but respond only after stimulus encoding, allowing potentially damaging sounds to impact the inner ear before sensitivity is adjusted. Less well characterized are potential cellular signaling systems involved in protection against metabolic stress and resultant damage. Although pharmacological manipulation of the olivocochlear system may hold some promise for attenuating cochlear damage, targeting this system may still allow damage to occur that does not depend on a fully functional feedback loop for its mitigation. Thus, understanding endogenous cell signaling systems involved in cochlear protection may lead to new strategies and therapies for prevention of cochlear damage and consequent hearing loss. We have recently discovered a novel cochlear signaling system that is molecularly equivalent to the classic hypothalamic-pituitary-adrenal (HPA) axis. This cochlear HPA-equivalent system functions to balance auditory sensitivity and susceptibility to noise-induced hearing loss, and also protects against cellular metabolic insults resulting from exposures to ototoxic drugs. This system may represent a local cellular response system designed to mitigate damage arising from various types of insult.

Dexamethasone indirectly induces Ndrg2 expression in rat astrocytes

N-myc downstream-regulated gene 2 (Ndrg2) has been associated with cell proliferation, differentiation, and apoptosis. Ndrg2 expression in the brain is induced by glucocorticoid treatment or chronic stress in vivo. It has been postulated that glucocorticoid-induced Ndrg2 expression in astrocytes is regulated by the glucocorticoid response element half-site (GRE1/2) upstream of the Ndrg2 transcription site. Here we examined the mechanisms of dexamethasone-induced Ndrg2 expression in rat astrocytes. Ndrg2 mRNA expression in primary astrocytes was significantly increased after 24 hr of exposure to dexamethasone in a concentration-dependent manner. Dexamethasone-induced Ndrg2 mRNA and protein expression was blocked by pretreatment with RU486, a glucocorticoid receptor antagonist. Moreover, dexamethasone-induced Ndrg2 mRNA expression was reduced by pretreatment with the protein synthesis inhibitor cycloheximide. The Ndrg2 reporter assay showed that deletion of a putative GRE1/2, located upstream of Ndrg2, did not affect induction by dexamethasone. A region between -755 and -701 bp from the transcription start site was shown to regulate induction by dexamethasone using promoter constructs progressively deleted from the 5' to 3' ends. This region contained the predicted transcription factor binding sites for early B-cell factor 1 (Ebf1), nuclear factor-κB (NFκB), and paired box gene 5 (Pax5). Mutation at the NFκB- or Pax5-binding site, but not the Ebf1 binding site, abolished dexamethasone-induced promoter activation. These results indicate that Ndrg2 expression was indirectly induced by dexamethasone at the DNA level, potentially by the binding of NFκB or Pax5 to the transcription factor binding sites, and GRE1/2 was not involved in this induction.

The cochlear CRF signaling systems and their mechanisms of action in modulating cochlear sensitivity and protection against trauma

A key requirement for encoding the auditory environment is the ability to dynamically alter cochlear sensitivity. However, merely attaining a steady state of maximal sensitivity is not a viable solution since the sensory cells and ganglion cells of the cochlea are prone to damage following exposure to loud sound. Most often, such damage is via initial metabolic insult that can lead to cellular death. Thus, establishing the highest sensitivity must be balanced with protection against cellular metabolic damage that can lead to loss of hair cells and ganglion cells, resulting in loss of frequency representation. While feedback mechanisms are known to exist in the cochlea that alter sensitivity, they respond only after stimulus encoding, allowing potentially damaging sounds to impact the inner ear at times coincident with increased sensitivity. Thus, questions remain concerning the endogenous signaling systems involved in dynamic modulation of cochlear sensitivity and protection against metabolic stress. Understanding endogenous signaling systems involved in cochlear protection may lead to new strategies and therapies for prevention of cochlear damage and consequent hearing loss. We have recently discovered a novel cochlear signaling system that is molecularly equivalent to the classic hypothalamic-pituitary-adrenal (HPA) axis. This cochlear HPA-equivalent system functions to balance auditory sensitivity and susceptibility to noise-induced hearing loss, and also protects against cellular metabolic insults resulting from exposures to ototoxic drugs. We review the anatomy, physiology, and cellular signaling of this system, and compare it to similar signaling in other organs/tissues of the body.

Glucocorticoid regulation of astrocytic fate and function

Glial loss in the hippocampus has been suggested as a factor in the pathogenesis of stress-related brain disorders that are characterized by dysregulated glucocorticoid (GC) secretion. However, little is known about the regulation of astrocytic fate by GC. Here, we show that astrocytes derived from the rat hippocampus undergo growth inhibition and display moderate activation of caspase 3 after exposure to GC. Importantly, the latter event, observed both in situ and in primary astrocytic cultures is not followed by either early- or late-stage apoptosis, as monitored by stage I or stage II DNA fragmentation. Thus, unlike hippocampal granule neurons, astrocytes are resistant to GC-induced apoptosis; this resistance is due to lower production of reactive oxygen species (ROS) and a greater buffering capacity against the cytotoxic actions of ROS. We also show that GC influence hippocampal cell fate by inducing the expression of astrocyte-derived growth factors implicated in the control of neural precursor cell proliferation. Together, our results suggest that GC instigate a hitherto unknown dialog between astrocytes and neural progenitors, adding a new facet to understanding how GC influence the cytoarchitecture of the hippocampus.

The mouse cochlea expresses a local hypothalamic-pituitary-adrenal equivalent signaling system and requires corticotropin-releasing factor receptor 1 to establish normal hair cell innervation and cochlear sensitivity

Cells of the inner ear face constant metabolic and structural stress. Exposure to intense sound or certain drugs destroys cochlea hair cells, which in mammals do not regenerate. Thus, an endogenous stress response system may exist within the cochlea to protect it from everyday stressors. We recently described the existence of corticotropin-releasing factor (CRF) in the mouse cochlea. The CRF receptor type 1 (CRFR1) is considered the primary and canonical target of CRF signaling, and systemically it plays an essential role in coordinating the body-wide stress response via activation of the hypothalamic-pituitary-adrenal (HPA) axis. Here, we describe an essential role for CRFR1 in auditory system development and function, and offer the first description of a complete HPA equivalent signaling system resident within the cochlea. To reveal the role of CRFR1 activation in the cochlea, we have used mice carrying a null ablation of the CRFR1 gene. CRFR1(-/-) mice exhibited elevated auditory thresholds at all frequencies tested, indicating reduced sensitivity. Furthermore, our results suggest that CRFR1 has a developmental role affecting inner hair cell morphology and afferent and efferent synapse distribution. Given the role of HPA signaling in maintaining local homeostasis in other tissues, the presence of a cochlear HPA signaling system suggests important roles for CRFR1 activity in setting cochlear sensitivity, perhaps both neural and non-neural mechanisms. These data highlight the complex pleiotropic mechanisms modulated by CRFR1 signaling in the cochlea.

Responses of glial cells to stress and glucocorticoids

A growing body of evidence suggests that glial cells are involved in practically all aspects of neural function. Glial cells regulate the homeostasis of the brain, influence the development of the nervous system, modulate synaptic activity, and carry out the immune response inside the brain. In addition, they play an important role in the restoration of the nervous system after damage, and they also participate in various neurodegenerative disorders. In a similar way, the importance of stress and glucocorticoids (GCs) on brain function is being increasingly recognized. Within the brain, stress hormones target both neurons and glial cells. Through their actions on these cells, glucocorticoids exert organizational functions on various processes of the developing brain and contribute to neuronal plasticity in the adult brain. Moreover, stress and glucocorticoids have become especially attractive in the study of a number of neurodegenerative disorders. However, studies on the mechanisms behind glucocorticoid-induced regulation of brain function have been classically focused on their effects on neurons. In this review, we start by describing the main functions of glial cells and then proceed to present data highlighting the effects of stress and GCs on brain function. We conclude the review by presenting recent evidence linking stress and glucocorticoids to glial cell function.

Endogenous glucocorticoids improve myelination via Schwann cells after peripheral nerve injury: An in vivo study using a crush injury model

Glucocorticoids improve the symptoms of peripheral nerve disorders, such as carpal tunnel syndrome and peripheral neuropathy. The effects of glucocorticoids are mainly anti-inflammatory, but the mechanisms of their effects in peripheral nerve disorders remain unclear. Schwann cells of the peripheral nerves express glucocorticoid receptors (GR), and glucocorticoids enhance the rate of myelin formation in vitro. Therefore, it is possible that the clinical improvement of peripheral nerve disorders by glucocorticoids is due, at least in part, to the modulation of myelination. In this study, an adrenalectomy (ADX) was performed, and followed by a daily injection of either low dose (1 mg/kg) or high dose (10 mg/kg) corticosterone (CORT). We then simulated a crush injury of the sciatic nerves. A sham ADX operation, followed by a simulated crush injury, was conducted as a control. Immunohistochemistry showed that the nuclei of in vivo Schwann cells expressed GR and that glucocorticoids impacted the GR immunoreactivity of the Schwann cells. The mRNA and protein expression of myelin basic protein was significantly lower in the animals given ADX with vehicle than in the sham operation group. However, the expression was restored in the low-dose CORT replacement group. Morphological analyses showed that the ADX with vehicle group had a significantly lower myelin thickness than did the low-dose CORT replacement group and the sham operation group. These results suggest that endogenous glucocorticoids have an important role in myelination through the GR in Schwann cells after an in vivo peripheral nerve injury.

Weak mitochondrial targeting sequence determines tissue-specific subcellular localization of glutamine synthetase in liver and brain cells

Evolution of the uricotelic system for ammonia detoxification required a mechanism for tissue-specific subcellular localization of glutamine synthetase (GS). In uricotelic vertebrates, GS is mitochondrial in liver cells and cytoplasmic in brain. Because these species contain a single copy of the GS gene, it is not clear how tissue-specific subcellular localization is achieved. Here we show that in chicken, which utilizes the uricotelic system, the GS transcripts of liver and brain cells are identical and, consistently, there is no difference in the amino acid sequence of the protein. The N-terminus of GS, which constitutes a 'weak' mitochondrial targeting signal (MTS), is sufficient to direct a chimeric protein to the mitochondria in hepatocytes and to the cytoplasm in astrocytes. Considering that a weak MTS is dependent on a highly negative mitochondrial membrane potential (DeltaPsi) for import, we examined the magnitude of DeltaPsi in hepatocytes and astrocytes. Our results unexpectedly revealed that DeltaPsi in hepatocytes is considerably more negative than that of astrocytes and that converting the targeting signal into 'strong' MTS abolished the capability to confer tissue-specific subcellular localization. We suggest that evolutional selection of weak MTS provided a tool for differential targeting of an identical protein by taking advantage of tissue-specific differences in DeltaPsi.

Dual role of NRSF/REST in activation and repression of the glucocorticoid response

Restriction of glutamine synthetase to the nervous system is mainly achieved through the mutual function of the glucocorticoid receptor and the neural restrictive silencing factor, NRSF/REST. Glucocorticoids induce glutamine synthetase expression in neural tissues while NRSF/REST represses the hormonal response in non-neural cells. NRSF/REST is a modular protein that contains two independent repression domains, at the N and C termini of the molecule, and is dominantly expressed in nonneural cells. Neural tissues express however splice variants, REST4/5, which contain the repression domain at the N, but not at the C terminus of the molecule. Here we show that full-length NRSF/REST or its C-terminal domain can inhibit almost completely the induction of gene transcription by glucocorticoids. By contrast, the N-terminal domain not only fails to repress the hormonal response but rather stimulates it markedly. The inductive activity of the N-terminal domain is mediated by hBrm, which is recruited to the promoter only in the concomitant presence of GR. Importantly, a similar inductive activity is also exerted by the splice variant REST4. These findings raise the possibility that NRSF/REST exhibits a dual role in regulation of glutamine synthetase. It represses gene induction in nonneural cells and enhances the hormonal response, via its splice variant, in the nervous system.

Steroids and glial cell function

Hormonal and locally produced steroids act in the nervous system as neuroendocrine regulators, as trophic factors and as neuromodulators and have a major impact on neural development and function. Glial cells play a prominent role in the local production of steroids and in the mediation of steroid effects on neurons and other glial cells. In this review, we examine the role of glia in the synthesis and metabolism of steroids and the functional implications of glial steroidogenesis. We analyze the mechanisms of steroid signaling on glia, including the role of nuclear receptors and the mechanisms of membrane and cytoplasmic signaling mediated by changes in intracellular calcium levels and activation of signaling kinases. Effects of steroids on functional parameters of glia, such as proliferation, myelin formation, metabolism, cytoskeletal reorganization, and gliosis are also reviewed, as well as the implications of steroid actions on glia for the regulation of synaptic function and connectivity, the regulation of neuroendocrine events, and the response of neural tissue to injury.

Dynamic regulation of synaptic GABA release by the glutamate-glutamine cycle in hippocampal area CA1

Vesicular GABA and intraterminal glutamate concentrations are in equilibrium, suggesting inhibitory efficacy may depend on glutamate availability. Two main intraterminal glutamate sources are uptake by neuronal glutamate transporters and glutamine synthesized through the astrocytic glutamate-glutamine cycle. We examined the involvement of the glutamate-glutamine cycle in modulating GABAergic synaptic efficacy. In the absence of neuronal activity, disruption of the glutamate-glutamine cycle by blockade of neuronal glutamine transport with alpha-(methylamino) isobutyric acid (MeAIB; 5 mM) or inhibition of glutamine synthesis in astrocytes with methionine sulfoximine (MSO; 1.5 mM) had no effect on miniature IPSCs recorded in hippocampal area CA1 pyramidal neurons. However, after a period of moderate synaptic activity, application of MeAIB, MSO, or dihydrokainate (250 microM; an astrocytic glutamate transporter inhibitor) significantly reduced evoked IPSC (eIPSC) amplitudes. The MSO effect could be reversed by exogenous application of glutamine (5 mM), whereas glutamine could not rescue the eIPSC decreases induced by the neuronal glutamine transporter inhibitor MeAIB. The activity-dependent reduction in eIPSCs by glutamate-glutamine cycle blockers was accompanied by an enhanced blocking effect of the low-affinity GABA(A) receptor antagonist, TPMPA [1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid], consistent with diminished GABA release. We further corroborated this hypothesis by examining MeAIB effects on minimal stimulation-evoked quantal IPSCs (meIPSCs). We found that, in MeAIB-containing medium, moderate stimulation induced depression in potency of meIPSCs but no change in release probability, consistent with reduced vesicular GABA content. We conclude that the glutamate-glutamine cycle is a major contributor to synaptic GABA release under physiological conditions, which dynamically regulates inhibitory synaptic strength.

Cortisol promotes survival and regeneration of axotomised retinal ganglion cells and enhances effects of aurintricarboxylic acid

BACKGROUND

Neuroprotection is essential for repair processes after a traumatic insult in the central nervous system. We have demonstrated previously significant neuroprotective properties of the anti-apoptotic drug aurintricarboxylic acid in the model of axotomised retinal ganglion cells. Glucocorticoids are widely used to treat injuries of the nervous system. Due to the anti-inflammatory and microglia-inhibiting properties of glucocorticoids, we studied the neuroprotective effects of intravitreally administered cortisol after an optic nerve cut.

METHODS

Ninety-eight adult Sprague-Dawley rats were used in this study. The optic nerve was cut intra-orbitally. Either vehicle or compound solution was injected intravitreally. Fluorescent dye was put onto the optic nerve stump to label retinal ganglion cells retrogradely. Retinal whole mounts were prepared 2 weeks after axotomy, and surviving retinal ganglion cells were counted.

RESULTS

Two weeks after axotomy, up to 50+/-7% of all retinal ganglion cells survived if cortisol was injected into the eye compared with 17+/-5% survival if only vehicle solution was injected. The neuroprotective effects of aurintricarboxylic acid (43+/-5% survival) could be further enhanced if combined with cortisol (up to 61+/-5% survival). Regeneration of cut retinal ganglion cell axons into a peripheral nerve graft could also be enhanced by an intravitreal injection of cortisol (169+/-42 regenerating retinal ganglion cells per mm2 vs. 73+/-12 cells per mm2 after vehicle injection). The increase was not as high as with aurintricarboxylic acid (192+/-40 cells per mm2), although more retinal ganglion cells survived with cortisol. This indicates that neuronal survival alone is not sufficient for subsequent axonal regeneration. Nevertheless, regeneration could be markedly increased if aurintricarboxylic acid and cortisol were combined (308+/-72 cells per mm2).

CONCLUSIONS

Whereas aurintricarboxylic acid seems to act directly on lesioned retinal ganglion cells, cortisol seems to act on the glial environment, as indicated by microglial cell morphology and enhanced glial fibrillary acidic protein expression. The results show that both neuroprotection and regeneration can be enhanced by the combination of two simple compounds acting on different sites.

Transcranial magnetic stimulation attenuates cell loss and oxidative damage in the striatum induced in the 3-nitropropionic model of Huntington's disease

An investigation was conducted on the effect of transcranial magnetic field stimulation (TMS) on the free radical production and neuronal cell loss produced by 3-nitropropionic acid in rats. The effects of 3-nitropropionic acid were evaluated by examining the following changes in: the quantity of hydroperoxides and total radical-trapping antioxidant potential (TRAP), lipid peroxidation products, protein carbonyl groups, reduced glutathione (GSH) content, glutathione peroxidase (GSH-Px), catalase and succinate dehydrogenase (SDH) activities; total nitrite and cell death [morphological changes, quantification of neuronal loss and lactate dehydrogenase (LDH) levels]. Our results reveal that 3-nitropropionic acid induces oxidative and nitrosative stress in the striatum, prompts cell loss and also shows that TMS prevents the harmful effects induced by the acid. In conclusion, the results show the ability of TMS to modify neuronal response to 3-nitropropionic acid.

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.

An intronic silencer element is responsible for specific zonal expression of glutamine synthetase in the rat liver

The most striking phenomenon of glutamine synthetase (GS) expression in the liver is its unique restriction to cells surrounding the terminal hepatic venules. Expression is positively regulated by elements located in the 5'-upstream region and in the first intron of the gene. It was long believed that transcription factors present in GS-positive cells and absent in GS-negative cells are responsible for the phenomenon of zonal expression. However, strong enhancers are equally active in both types of cells. Therefore, the existence of a silencer mechanism in GS-negative hepatocytes was postulated. In the present study, a GS silencer element was investigated that was previously identified within the first intron and was shown to be able to prevent glucocorticoid-induced expression in cells negative for a transacting factor designated GS silencer element-binding protein. Reporter gene assays with the silencer element in combination with the most potent 5'-enhancer of the GS gene demonstrate that the silencer element is able to prevent enhancement mediated by the 5'-enhancer in combination with a heterologous as well as with the homologous promoter. More importantly, the effect of the silencer is shown to be restricted to GS-negative hepatocytes. In conclusion, the phenomenon of zonal expression of GS in the liver is caused by a protein present in GS-negative cells and absent in GS-positive cells that interacts with the silencer element in the first intron and not by a differential expression of enhancer-binding proteins.

Expression of nuclear receptor coactivators in androgen-responsive and -unresponsive motoneurons

Adult rat lumbar motoneurons in the spinal nucleus of the bulbocavernosus (SNB) respond to androgens with an increase in soma size. This response is mediated by the androgen receptor (AR) in these motoneurons. Interestingly, other lumbar motoneurons in the rat possess the AR, yet do not respond to androgens in this fashion. This paradox suggests the existence and participation of nuclear receptor coregulators in conferring direct androgen-responsiveness to select motoneurons in the adult rat spinal cord. Nuclear receptor coregulators have received much attention recently for their proposed role in enhancing or repressing the transcriptional activity of steroid hormone receptors. The present study used immunocytochemistry to identify a number of nuclear receptor coactivators that are expressed by adult lumbar motoneurons: SRC-1, SRC-2, CBP, p300, and cJUN. Results of this study indicate that all five of these coactivators are abundantly expressed in the androgen-responsive SNB, and in two adjacent motor pools, the androgen-responsive dorsolateral nucleus (DLN), and the androgen-unresponsive retrodorsolateral nucleus (RDLN). While we detected significant regional differences for only SRC-1 and cJUN, the SNB consistently contained the highest percentage of immunoreactive motoneurons for all five cofactors examined. Our results indicate five different putative cofactors have the potential to participate in motoneuronal responses to androgens, since their distribution overlaps well with the distribution of ARs in these motoneurons.

Effects of brain-derived neurotrophic factor on cell survival, differentiation and patterning of neuronal connections and Muller glia cells in the developing retina

The aim of the present study was to determine the influence of brain-derived neurotrophic factor (BDNF) on survival, phenotype differentiation and network formation of retinal neurons and glia cells. To achieve a defined concentration and constant level of BDNF over several days, experiments were performed in an organotypic culture of the developing rat retina. After 6 days in vitro, apoptosis in the different cell layers was determined by TUNEL staining and cell-type-specific antibodies were used to identify distinct neuronal cell types and Müller cells. Cultured retinas treated with BDNF (100 ng BDNF/mL medium) were compared with untreated as well as with age-matched in vivo retinas. Quantitative morphometry was carried out using confocal microscopy. BDNF promoted the in vitro development and differentiation of the retina in general, i.e. the number of cells in the nuclear layers and the thickness of the plexiform layers were increased. For all neurons, the number of cells and the complexity of arborizations in the synaptic layers were clearly up-regulated by BDNF. In control cultures, the synaptic stratification of cone bipolar cells within the On- and Off-layer of the inner plexiform layer was disturbed and a strong reactivity of Müller cell glia was observed. These effects were not present in BDNF-treated cultures. Our data show that BDNF promotes the survival of retinal interneurons and plays an important role in establishing the phenotypes and the synaptic connections of a large number of neuronal types in the developing retina. Moreover, we show an effect of BDNF on Müller glia cells.

Loss of glutamine synthetase in the human epileptogenic hippocampus: possible mechanism for raised extracellular glutamate in mesial temporal lobe epilepsy

BACKGROUND

High extracellular glutamate concentrations have been identified as a likely trigger of epileptic seizures in mesial temporal lobe epilepsy (MTLE), but the underlying mechanism remains unclear. We investigated whether a deficiency in glutamine synthetase, a key enzyme in catabolism of extracellular glutamate in the brain, could explain the perturbed glutamate homoeostasis in MTLE.

METHODS

The anteromedial temporal lobe is the focus of the seizures in MTLE, and surgical resection of this structure, including the hippocampus, leads to resolution of seizures in many cases. By means of immunohistochemistry, western blotting, and functional enzyme assays, we assessed the distribution, quantity, and activity of glutamine synthetase in the MTLE hippocampus.

FINDINGS

In western blots, the expression of glutamine synthetase in the hippocampus was 40% lower in MTLE than in non-MTLE samples (median 44 [IQR 30-58] vs 69 [56-87]% of maximum concentration in standard curve; p=0.043; n=8 and n=6, respectively). The enzyme activity was lower by 38% in MTLE vs non-MTLE (mean 0.0060 [SD 0.0031] vs 0.0097 [0.0042] U/mg protein; p=0.045; n=6 and n=9, respectively). Loss of glutamine synthetase was particularly pronounced in areas of the MTLE hippocampus with astroglial proliferation, even though astrocytes normally have high content of the enzyme. Quantitative immunoblotting showed no significant change in the amount of EAAT2, the predominant glial glutamate transporter in the hippocampus.

INTERPRETATION

A deficiency in glutamine synthetase in astrocytes is a possible molecular basis for extracellular glutamate accumulation and seizure generation in MTLE. Further studies are needed to define the cause, but the loss of glutamine synthetase may provide a new focus for therapeutic interventions in MTLE.

Glutamine synthetase enhances the clearance of extracellular glutamate by the neural retina

Clearance of synaptic glutamate by glial cells is required for the normal function of excitatory synapses and for prevention of neurotoxicity. Although the regulatory role of glial glutamate transporters in glutamate clearance is well established, little is known about the influence of glial glutamate metabolism on this process. This study examines whether glutamine synthetase (GS), a glial-specific enzyme that amidates glutamate to glutamine, affects the uptake of glutamate. Retinal explants were incubated in the presence of [(14)C]glutamate and glutamate uptake was assessed by measurement of the amount of radioactively labeled molecules within the cells and the amount of [(14)C]glutamine released to the medium. An increase in GS expression in Müller glial cells, caused by induction of the endogenous gene, did not affect the amount of glutamate accumulated within the cells, but led to a dramatic increase in the amount of glutamine released. This increase, which was directly correlated with the level of GS expression, was dependent on the presence of external sodium ions, and could be completely abolished by methionine sulfoximine, a specific inhibitor of GS activity. Our results demonstrate that GS activity significantly influences the uptake of glutamate by the neural retina and suggest that this enzyme may represent an important target for neuroprotective strategies.

Glucocorticoid induced expression of glutamine synthetase in hepatoma cells

The enzyme glutamine synthetase (GS) ranks as one of the most remarkable glucocorticoid-inducible vertebrate genes. However, little is known about the responsible DNA elements and the mode of glucocorticoid action. This is especially the case for the induction of GS in hepatoma cells. In the work presented, the rat hepatoma cell line FAO was used as a model to study the induction of GS under the influence of glucocorticoids. FAO cells do not show GS activity in the absence of glucocorticoids and are strongly responding to their presence. Analyzing sequences of several thousand base pairs upstream and downstream from the transcriptional start point of the GS gene, a glucocorticoid responsible element was identified within the first intron of the gene. However, evidence is presented that aside from a primary effect on transcription glucocorticoids mediate their effect on the expression of GS also at the posttranscriptional level.

Formation and effects of neuroactive steroids in the central and peripheral nervous system

This chapter summarizes several observations that emphasize the importance of neuroactive steroids in the physiology of the central and peripheral nervous systems. A new, and probably important, concept is emerging: Neuroactive steroids not only modify neuronal physiology but also intervene in the control of glial cell functions. The data presented here underscore that (1) the mechanism of action of the various steroidal molecules may involve both classical (progesterone and androgens) and nonclassical steroid receptors [gamma-aminobutyric acid type A (GABAA) receptor], (2) in many instances, the actions of hormonal steroids are not due to their native molecular forms but to their 5 alpha- and 3 alpha,5 alpha-reduced metabolites, (3) several neuroactive steroids exert dramatic actions on the proteins proper of the peripheral myelin (e.g., glycoprotein Po and peripheral myelin protein 22), and (4) the effects of steroids and of their metabolites might have clinical significance in cases in which the rebuilding of the peripheral myelin is needed (e.g., aging, peripheral injury).

Genetic and epigenetic control of the Na-G ion channel expression in glia

The Na-G ion channel, previously cloned from a rat astroglia cDNA library, belongs to a new family of ion channels, related to but distinct from the predominant brain and muscle fast voltage-gated Na(+) channels. In vivo, the corresponding transcripts are widely expressed in peripheral nervous system neurons and glia, but only in selected subpopulations of neuronal and glia-like cells of the central nervous system. In the present report, we show that Na-G messenger RNA level in astrocyte and Schwann cell cultures is modulated in a cell-specific manner by several growth factors, hormones, and intracellular second messengers pathways. Striking changes in transcript level were observed in the two types of glia in response to protein-kinase A activation and to treatment with the neuregulin glial growth factor, indicating regulation of the Na-G gene by neuroglial signaling. By transient transfection of Na-G/reporter constructs into cultured cells, we show that a short genomic region, encompassing the first exon and 375 bp upstream, bears a high glial-specific transcriptional activity while part of the first intron behaves as a negative regulatory element. In vivo footprinting experiments revealed binding of glial-specific nuclear factors to several sites of the Na-G promoter region. Finally, Na-G/reporter constructs are shown to sustain a low but reproducible transcriptional response to cAMP, accounting in part for the elevation in mRNA level elicited by cAMP in Schwann cells and its reduction in astrocytes.