Changes in gene expression in hippocampus in response to glucocorticoids and stress

Hormonal Regulation of Oligodendrogenesis I: Effects across the Lifespan

The brain’s capacity to respond to changing environments via hormonal signaling is critical to fine-tuned function. An emerging body of literature highlights a role for myelin plasticity as a prominent type of experience-dependent plasticity in the adult brain. Myelin plasticity is driven by oligodendrocytes (OLs) and their precursor cells (OPCs). OPC differentiation regulates the trajectory of myelin production throughout development, and importantly, OPCs maintain the ability to proliferate and generate new OLs throughout adulthood. The process of oligodendrogenesis, the creation of new OLs, can be dramatically influenced during early development and in adulthood by internal and environmental conditions such as hormones. Here, we review the current literature describing hormonal regulation of oligodendrogenesis within physiological conditions, focusing on several classes of hormones: steroid, peptide, and thyroid hormones. We discuss hormonal regulation at each stage of oligodendrogenesis and describe mechanisms of action, where known. Overall, the majority of hormones enhance oligodendrogenesis, increasing OPC differentiation and inducing maturation and myelin production in OLs. The mechanisms underlying these processes vary for each hormone but may ultimately converge upon common signaling pathways, mediated by specific receptors expressed across the OL lineage. However, not all of the mechanisms have been fully elucidated, and here, we note the remaining gaps in the literature, including the complex interactions between hormonal systems and with the immune system. In the companion manuscript in this issue, we discuss the implications of hormonal regulation of oligodendrogenesis for neurological and psychiatric disorders characterized by white matter loss. Ultimately, a better understanding of the fundamental mechanisms of hormonal regulation of oligodendrogenesis across the entire lifespan, especially in vivo, will progress both basic and translational research.

Glucocorticoids, genes and brain function

The identification of key genes in transcriptomic data constitutes a huge challenge. Our review of microarray reports revealed 88 genes whose transcription is consistently regulated by glucocorticoids (GCs), such as cortisol, corticosterone and dexamethasone, in the brain. Replicable transcriptomic data were combined with biochemical and physiological data to create an integrated view of the effects induced by GCs. The most frequently reported genes were Errfi1 and Ddit4. Their up-regulation was associated with the altered transcription of genes regulating growth factor and mTORC1 signaling (Gab1, Tsc22d3, Dusp1, Ndrg2, Ppp5c and Sesn1) and progression of the cell cycle (Ccnd1, Cdkn1a and Cables1). The GC-induced reprogramming of cell function involves changes in the mRNA level of genes responsible for the regulation of transcription (Klf9, Bcl6, Klf15, Tle3, Cxxc5, Litaf, Tle4, Jun, Sox4, Sox2, Sox9, Irf1, Sall2, Nfkbia and Id1) and the selective degradation of mRNA (Tob2). Other genes are involved in the regulation of metabolism (Gpd1, Aldoc and Pdk4), actin cytoskeleton (Myh2, Nedd9, Mical2, Rhou, Arl4d, Osbpl3, Arhgef3, Sdc4, Rdx, Wipf3, Chst1 and Hepacam), autophagy (Eva1a and Plekhf1), vesicular transport (Rhob, Ehd3, Vps37b and Scamp2), gap junctions (Gjb6), immune response (Tiparp, Mertk, Lyve1 and Il6r), signaling mediated by thyroid hormones (Thra and Sult1a1), calcium (Calm2), adrenaline/noradrenaline (Adcy9 and Adra1d), neuropeptide Y (Npy1r) and histamine (Hdc). GCs also affected genes involved in the synthesis of polyamines (Azin1) and taurine (Cdo1). The actions of GCs are restrained by feedback mechanisms depending on the transcription of Sgk1, Fkbp5 and Nr3c1. A side effect induced by GCs is increased production of reactive oxygen species. Available data show that the brain's response to GCs is part of an emergency mode characterized by inactivation of non-core activities, restrained inflammation, restriction of investments (growth), improved efficiency of energy production and the removal of unnecessary or malfunctioning cellular components to conserve energy and maintain nutrient supply during the stress response.

Mammalian verprolin CR16 acts as a modulator of ITSN scaffold proteins association with actin

Actin cytoskeleton rearrangements are required for normal cell functioning, and their deregulation leads to various pathologies. Members of two mammalian protein families - ITSNs (ITSN1 and ITSN2) and verprolins (WIP, CR16 and WIRE) are involved in Cdc42/N-WASP/Arp2/3 signaling pathway-mediated remodeling of the actin cytoskeleton. Recently we demonstrated that ITSNs interact with the actin-regulating protein WIP. Here, we show that other member of verprolin family, CR16, also forms complexes with ITSN1 and ITSN2 in human cell lines. The actin-binding protein CR16 modulates ITSN/β-actin association. Moreover, overexpressed CR16 promoted co-localization of ITSN1 with F-actin in MCF-7 breast cancer cells. Our data demonstrated that CR16 mRNA is expressed in glioblastoma and breast tumors. These findings provide the basis for further functional investigations of the ITSN/CR16 complex that may play an important role in actin remodeling and cellular invasion.

Role of Glia in Stress-Induced Enhancement and Impairment of Memory

Both acute and chronic stress profoundly affect hippocampally-dependent learning and memory: moderate stress generally enhances, while chronic or extreme stress can impair, neural and cognitive processes. Within the brain, stress elevates both norepinephrine and glucocorticoids, and both affect several genomic and signaling cascades responsible for modulating memory strength. Memories formed at times of stress can be extremely strong, yet stress can also impair memory to the point of amnesia. Often overlooked in consideration of the impact of stress on cognitive processes, and specifically memory, is the important contribution of glia as a target for stress-induced changes. Astrocytes, microglia, and oligodendrocytes all have unique contributions to learning and memory. Furthermore, these three types of glia express receptors for both norepinephrine and glucocorticoids and are hence immediate targets of stress hormone actions. It is becoming increasingly clear that inflammatory cytokines and immunomodulatory molecules released by glia during stress may promote many of the behavioral effects of acute and chronic stress. In this review, the role of traditional genomic and rapid hormonal mechanisms working in concert with glia to affect stress-induced learning and memory will be emphasized.

Astroglial Plasticity Is Implicated in Hippocampal Remodelling in Adult Rats Exposed to Antenatal Dexamethasone

The long-term effects of antenatal dexamethasone treatment on brain remodelling in 3-month-old male Sprague Dawley rats whose mothers had been treated with dexamethasone were investigated in the present study. Dorsal hippocampus, basolateral amygdala and nucleus accumbens volume, cell numbers, and GFAP-immunoreactive astroglial cell morphology were analysed using stereology. Total brain volume as assessed by micro-CT was not affected by the treatment. The relative volume of the dorsal hippocampus (% of total brain volume) showed a moderate, by 8%, but significant reduction in dexamethasone-treated versus control animals. Dexamethasone had no effect on the total and GFAP-positive cell numbers in the hippocampal subregions, basolateral amygdala, and nucleus accumbens. Morphological analysis indicated that numbers of astroglial primary processes were not affected in any of the hippocampal subregions analysed but significant reductions in the total primary process length were observed in CA1 by 32%, CA3 by 50%, and DG by 25%. Mean primary process length values were also significantly decreased in CA1 by 25%, CA3 by 45%, and DG by 25%. No significant astroglial morphological changes were found in basolateral amygdala and nucleus accumbens. We propose that the dexamethasone-dependent impoverishment of hippocampal astroglial morphology is the case of maladaptive glial plasticity induced prenatally.

WIP: WASP-interacting proteins at invadopodia and podosomes

Regulated cell invasion resulting from migratory and matrix-degrading events is an essential step in physiological processes such as the inflammatory response and tissue repair. Cell invasion is also thought to be a critical parameter in pathological conditions such as cancer metastasis. The migration of normal and cancer cells is largely driven by the actin cytoskeleton, which controls cell shape, adhesion and contractility. Podosomes and invadopodia are actin-rich protrusions that drive invasion in normal and cancer cells. These structures protrude from the basal region of the cell facing the extracellular matrix, where they adhere to and degrade the matrix, thus facilitating invasive migration. WASP (Wiskott-Aldrich syndrome protein) and WIP (WASP-interacting protein) localise to the actin rich core of podosomes and play a critical role in their formation. More recently, studies performed on microarray data sets from cancer patients of several tumour categories show a strong correlation between reduced WIP expression and improved prognosis. In this article, we identify endogenous WIP at the distal tips of cancer cell invasive protrusions and we summarise recent advances in the study of the roles of WIP- and WASP-protein families during migration and invasion of normal and cancer cells related to podosome and invadopodium generation.

WASP-interacting protein (WIP): working in polymerisation and much more

The migration of cells and the movement of some intracellular pathogens, such as Shigella and Vaccinia, are dependent on the actin-based cytoskeleton. Many proteins are involved in regulating the dynamics of the actin-based microfilaments within cells and, among them, WASP and N-WASP have a significant role in the regulation of actin polymerisation. The activity and stability of WASP is regulated by its cellular partner WASP-interacting protein (WIP) during the formation of actin-rich structures, including the immune synapse, filopodia, lamellipodia, stress fibres and podosomes. Here, we review the role of WIP in regulating WASP function by stabilising WASP and shuttling WASP to areas of actin assembly in addition to reviewing the WASP-independent functions of WIP.

Modulation of stress consequences by hippocampal monoaminergic, glutamatergic and nitrergic neurotransmitter systems

Several findings relate the hippocampal formation to the behavioural consequences of stress. It contains a high concentration of corticoid receptors and undergoes plastic modifications, including decreased neurogenesis and cellular remodelling, following stress exposure. Various major neurotransmitter systems in the hippocampus are involved in these effects. Serotonin (5-HT) seems to exert a protective role in the hippocampus and attenuates the behavioural consequences of stress by activating 5-HT1A receptors in this structure. These effects may mediate the therapeutic actions of several antidepressants. The role of noradrenaline is less clear and possibly depends on the specific hippocampal region (dorsal vs. ventral). The deleterious modifications induced in the hippocampus by stress might involve a decrease in neurotrophic factors such as brain derived neurotrophic factor (BDNF) following glutamate N-methyl-D-aspartate (NMDA) receptor activation. In addition to glutamate, nitric oxide (NO) could also be related to these effects. Systemic and intra-hippocampal administration of nitric oxide synthase (NOS) inhibitors attenuates stress-induced behavioural consequences. The challenge for the future will be to integrate results related to these different neurotransmitter systems in a unifying theory about the role of the hippocampus in mood regulation, depressive disorder and antidepressant effects.

A new glucocorticoid hypothesis of brain aging: implications for Alzheimer's disease

The original glucocorticoid (GC) hypothesis of brain aging and Alzheimer's disease proposed that chronic exposure to GCs promotes hippocampal aging and AD. This proposition arose from a study correlating increasing plasma corticosterone with hippocampal astrocyte reactivity in aging rats. Numerous subsequent studies have found evidence consistent with this hypothesis, in animal models and in humans. However, several results emerged that were inconsistent with the hypothesis, highlighting the need for a more definitive test with a broader panel of biomarkers. We used microarray analyses to identify a panel of hippocampal gene expression changes that were aging-dependent, and also corticosterone-dependent. These data enabled us to test a key prediction of the GC hypothesis, namely, that the expression of most target biomarkers of brain aging should be regulated in the same direction (increased or decreased) by both GCs and aging. This prediction was decisively contradicted, as a majority of biomarker genes were regulated in opposite directions by aging and GCs, particularly inflammatory and astrocyte-specific genes. Thus, the initial hypothesis of simple positive cooperativity between GCs and aging must be rejected. Instead, our microarray data suggest that in the brain GCs and aging interact in more complex ways that depend on the cell type. Therefore, we propose a new version of the GC-brain aging hypothesis; its main premise is that aging selectively increases GC efficacy in some cell types (e.g., neurons), enhancing catabolic processes, whereas aging selectively decreases GC efficacy in other cell types (e.g., astrocytes), weakening GC anti-inflammatory activity. We also propose that changes in GC efficacy might be mediated in part by cell type specific shifts in the antagonistic balance between GC and insulin actions, which may be of relevance for Alzheimer's disease pathogenesis.

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.

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.

Biochemical and molecular studies using human autopsy brain tissue

The use of human brain tissue obtained at autopsy for neurochemical, pharmacological and physiological analyses is reviewed. RNA and protein samples have been found suitable for expression profiling by techniques that include RT-PCR, cDNA microarrays, western blotting, immunohistochemistry and proteomics. The rapid development of molecular biological techniques has increased the impetus for this work to be applied to studies of brain disease. It has been shown that most nucleic acids and proteins are reasonably stable post-mortem. However, their abundance and integrity can exhibit marked intra- and intercase variability, making comparisons between case-groups difficult. Variability can reveal important functional and biochemical information. The correct interpretation of neurochemical data must take into account such factors as age, gender, ethnicity, medicative history, immediate ante-mortem status, agonal state and post-mortem and post-autopsy intervals. Here we consider issues associated with the sampling of DNA, RNA and proteins using human autopsy brain tissue in relation to various ante- and post-mortem factors. We conclude that valid and practical measures of a variety of parameters may be made in human brain tissue, provided that specific factors are controlled.

CR16 forms a complex with N-WASP in brain and is a novel member of a conserved proline-rich actin-binding protein family

The Neuronal Wiskott-Aldrich syndrome protein (N-WASP) has emerged as a central regulator of the actin cytoskeleton with abilities to integrate multiple upstream signal inputs and transmit them to the Arp2/3 complex. Here, we demonstrate that native N-WASP is present in a tight complex with a proline-rich protein, CR16, which shares approximately 25% identity with WASP interacting protein. CR16 is encoded by a gene previously cloned as a glucocorticoid-regulated mRNA from a rat hippocampal cDNA library. Although N-WASP is expressed ubiquitously, full-length CR16 protein is found predominately in the brain. CR16 and N-WASP colocalize in primary hippocampal neurons and at the tips of their growth cone filopodia. In vitro, CR16 directly binds both monomeric and filamentous actin but does not affect the kinetics of actin polymerization mediated by N-WASP and the Arp2/3 complex. Sequence homologues of CR16 are found not only in other vertebrates but also in the invertebrate Caenorhabditis elegans and in yeast. Thus, CR16 and WASP interacting protein belong to a family of N-WASP-binding proteins.

Stressor specificity of central neuroendocrine responses: implications for stress-related disorders

Despite the fact that many research articles have been written about stress and stress-related diseases, no scientifically accepted definition of stress exists. Selye introduced and popularized stress as a medical and scientific idea. He did not deny the existence of stressor-specific response patterns; however, he emphasized that such responses did not constitute stress, only the shared nonspecific component. In this review we focus mainly on the similarities and differences between the neuroendocrine responses (especially the sympathoadrenal and the sympathoneuronal systems and the hypothalamo-pituitary-adrenocortical axis) among various stressors and a strategy for testing Selye's doctrine of nonspecificity. In our experiments, we used five different stressors: immobilization, hemorrhage, cold exposure, pain, or hypoglycemia. With the exception of immobilization stress, these stressors also differed in their intensities. Our results showed marked heterogeneity of neuroendocrine responses to various stressors and that each stressor has a neurochemical "signature." By examining changes of Fos immunoreactivity in various brain regions upon exposure to different stressors, we also attempted to map central stressor-specific neuroendocrine pathways. We believe the existence of stressor-specific pathways and circuits is a clear step forward in the study of the pathogenesis of stress-related disorders and their proper treatment. Finally, we define stress as a state of threatened homeostasis (physical or perceived treat to homeostasis). During stress, an adaptive compensatory specific response of the organism is activated to sustain homeostasis. The adaptive response reflects the activation of specific central circuits and is genetically and constitutionally programmed and constantly modulated by environmental factors.

Activity-stress increases density of GFAP-immunoreactive astrocytes in the rat hippocampus

Although past research has indicated that stress and the accompanying increase in glucocorticoids compromises hippocampal neurons, little is known about the effect of stress on hippocampal glial cells. In the current study, male rats were exposed to activity-stress (A-S) for six days; this comprised housing with an activity wheel and restricted access (1h/day) to food. Physiological data (e.g., relative adrenal and thymus weights, gastric ulceration) suggested that the A-S rats experienced more stress than pair-fed (no wheel) and control (fed ad libitum, no wheel) rats. Whereas stress did not influence the quantitative morphology of glial fibrillary acidic protein (GFAP)-immunoreactive cells, a semi-quantitative analysis revealed that the A-S rats had significantly more (30%) GFAP-immunoreactive cells in the hippocampal CA3 region than the control rats. Based on the present findings, it appears that the hippocampal astrocytic response to chronic stress may be similar to the response found in endangered, or challenged hippocampal environments, such as in ischemia.

Oligodendrocytes as glucocorticoids target cells: functional analysis of the glycerol phosphate dehydrogenase gene

Previous research has established that the development and function of oligodendrocytes are influenced by glucocorticoids. The enzyme glycerol phosphate dehydrogenase (E.C.1.1.1.8) has been used as a model to study glucocorticoid regulation of gene expression in oligodendrocytes and the C6 glial cell line. In the rat brain this enzyme is exclusively localized to oligodendrocytes. The sequence of the 5' flanking region for the rat gene encoding Glycerol Phosphate Dehydrogenase (GPDH; EC 1.1.1.8) was determined. 4 kb of sequence from the 5' flanking region, exon 1, and part of intron 1 of the rat GPDH gene was compared to the corresponding mouse sequence. Dotplot matrix comparison revealed that the rat sequence is more than 80% similar to the mouse sequence, but differs from the mouse sequence in two regions: the rat sequence is devoid of 200 bp of B1 repeat sequence that is present in the mouse, and the rat sequence has an excess 700 bp of B2 repeat sequence inserted between -0.7 kb and -1. 4 kb that is absent in the mouse. To determine the regulatory activity of the rat GPDH 5' flanking region, various portions of the rat GPDH 5' flanking region were placed in luciferase reporter constructs and tested for transcriptional activity. Transient transfection of reporter constructs into the C6 glial cell line revealed that the distal end of the 5' flanking region was glucocorticoid-inducible. A 385 bp Glucocorticoid Response Unit (GRU) was identified whose glucocorticoid induction was enhanced by dibutyryl-cAMP and reduced by phorbol esters. Sequence analysis of the GRU revealed the presence of four consensus GRE sequences and other putative consensus elements. Results here suggest that the 5' flanking region of the GPDH gene mediates the ligand-inducible regulation of GPDH, and that multiple signaling pathways converge at the 5' regulatory sequence to modulate GPDH gene expression in oligodendrocytes.

Effects of microgravity and bone morphogenetic protein II on GFAP in rat brain

This study evaluated effects of bone morphogenetic protein II (BMP) on glial fibrillary acidic protein (GFAP) in the brain of female Fischer 344 rats during 14 days of spaceflight. GFAP mRNA decreased in vehicle-implanted rats flown on the space shuttle by 53 and 48% in the stratum moleculare and stratum lacunosum moleculare hippocampal subregions, respectively. GFAP mRNA was not significantly affected by BMP implantation during spaceflight. Rats returning from space exhibited a 56% increase in serum corticosterone. BMP treatment did not additively increase corticosterone elevations in microgravity but appeared to increase serum corticosterone and reduce GFAP mRNA in the stratum moleculare in control rats. These data suggest that exposure to microgravity reduces GFAP expression in hippocampal astrocytes.

Glial- and fat-specific expression of the rat glycerol phosphate dehydrogenase-luciferase fusion gene in transgenic mice

Glycerol phosphate dehydrogenase (GPDH) is a metabolic enzyme that catalyzes the conversion of dihydroxyacetone phosphate to glycerol-3-phosphate. It provides phospholipid precursors for lipid biosynthesis and energy metabolism. In the brain, GPDH enzymatic activity, protein, mRNA are exclusively associated with oligodendroglial and Bergmann glial cells. Expression of GPDH in the brain increases dramatically during the active period of myelination, and is regulated by extracellular signals. In an effort to understand the mechanism that confers glial-specific expression of GPDH, we have examined the role of the 5' flanking sequence of the rat GPDH gene in conferring cell-specific expression of reporter gene in transgenic mice. Luciferase reporter constructs containing either the full-length GPDH 5' flanking region (p4.3), or a distally truncated version (p2.6), were injected into mouse zygotes. Three independent lines of transgenic mice containing the p4.3, and seven lines of mice containing the p2.6 constructs, were analyzed. Luciferase enzyme activity was detectable only in brain and fat, not in other GPDH-positive organs such as liver, muscle, and kidney. Both the full-length and the distally deleted transgenes were expressed similarly in these two organs, indicating that the distal portion of the 5' flanking region was not required for brain- and fat-specific expression. Immunocytochemical analyses revealed that luciferase immunoreactivity colocalized with glial fibrillary acidic protein (GFAP)-positive Bergmann glia in the cerebellum, and myelin basic protein (MBP)-positive oligodendroglia in the cerebral cortex and the brainstem. Results here suggest that the rat GPDH 5' flanking region directs glial-specific expression of GPDH transcription in the brain, and provide a good model for analyses of changes in glial metabolism in response to extracellular perturbations in vivo.

Effect of progesterone, testosterone and their 5 alpha-reduced metabolites on GFAP gene expression in type 1 astrocytes

Astrocytes possess steroid receptors as well as several enzymes typical of steroid target cells, such as 5 alpha-reductase, which converts testosterone (T) and progesterone (P) into their respective 5 alpha-reduced metabolites, and the 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD). Because of this, it was deemed of interest to analyze whether the original hormones P and T, and their 5 alpha-reduced metabolites dihydrotestosterone (DHT), 5 alpha-androstan-3 alpha, 17 beta-diol (3 alpha-diol), dihydroprogesterone (DHP) and 5 alpha-pregnan-3 alpha-ol-20-one (THP), might exert some effects on the expression of the most typical astrocytic marker, i.e. the glial fibrillary acidic protein (GFAP). Cultures of rat type 1 astrocytes were exposed to the various steroids for 2, 6, and 24 h, and the variations of GFAP mRNA were measured by Northern blot analysis. A significant elevation of GFAP mRNA levels was observed after exposure to either P or DHP; the effect of DHP appeared more promptly (at 2 h) than that of P (at 6 h). This result suggests that the effect of P might be linked to its conversion into DHP; this hypothesis has been confirmed by showing that the addition of finasteride (a specific blocker of the 5 alpha-reductase) is able to completely abolish the effect of P. After exposure to DHP or THP, a decrease of GFAP gene expression was observed at later intervals (24 h). In the case of androgens, T and 3 alpha-diol did not change GFAP expression at any time of exposure, while DHT produced a significant decrease of GFAP mRNA only after 24 h of exposure. Taken together, the data indicate that the 5 alpha-reduced metabolites of P and T may modulate the expression of GFAP in type 1 rat astrocytes.

Chronic corticosterone treatment-induced ultrastructural changes at rat neuromuscular junction

BACKGROUND

Chronic exposure to glucocorticoids affects both the structure and function of vertebrate skeletal muscles. As little is known about the effects of such steroids on the neuromuscular junctions (NMJs) of different muscle fiber types, the influence of chronic corticosterone (CORT) administration on the ultrastructure of NMJs of soleus (SOL) and extensor digitorum longus (EDL) was studied.

METHODS

Ten Fischer 344 male rats, the same animals used previously, were either injected daily with 5-10 mg CORT or received vehicle as control animals for 3 months and were sacrificed at 5 months of age. Muscles were bathed in situ in 4% phosphate buffered glutaraldehyde for ten minutes, then removed and conventional electron microscopic procedures were followed. Qualitative and quantitative observations of nerve terminal ultrastructures were statistically treated with multivariate analysis of variance to determine differences between control and CORT-treated animals.

RESULTS

Fast-twitch EDL muscles were more affected by CORT-treatment than slow-twitch SOL muscles. Morphometric analysis of NMJ's in CORT-treated rats revealed significant decrease in fiber diameter, nerve terminal area and synaptic vesicle density, but a significant increase in synaptic cleft (P < 0.05). The NMJ's underwent partial denervation and reinnervation processes as demonstrated by large areas of presynaptic nerve terminal occupied by microtubules and electron dense granular material.

CONCLUSIONS

Chronic CORT-treatments induced degenerative changes which were more pronounced in fast-twitch EDL muscles than slow-twitch SOL muscles, suggesting that pattern or amount of activity affect the CORT-treatment outcome. These steroid-induced stress changes are similar to those observed in aging and disuse studies of NMJ. Thus, glucocorticoid hormones may play an etiological role in the homeostasis of the NMJ in response to various stimuli.

Age-related increases in glial fibrillary acidic protein do not show proportionate changes in transcription rates or DNA methylation in the cerebral cortex and hippocampus of male rats

Age-related increases in the expression of glial fibrillary acidic protein (GFAP) in many brain regions are observed in short- and long-lived mammals. Possible genomic mechanisms for the increase of GFAP mRNA and protein were studied in the hippocampus and cortex of male F344 rats and a longer-lived hybrid F1 (F344 x Brown Norway). No age-related changes were found in the extent of cytosine methylation at 19 CpG sites in the 5'-upstream GFAP promoter and in exon 1. With the nuclear runon assay, no change was found in the transcription rate of GFAP in the cerebral cortex or hippocampus. Thus, age-related increases in GFAP are not associated with proportionate changes in transcription rates or DNA methylation. However, the transcription of glutamine synthetase was increased by about 60%. These findings contrast with age-related loss of bulk tissue DNA methylation and decreased transcription rates of other genes reported in non-neural tissues.

Midazolam and the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-7-phosphonoheptanoic acid (AP-7) attenuate stress-induced expression of c-fos mRNA in the dentate gyrus

1. The effects of restraint stress on c-fos mRNA expression in the dentate gyrus were investigated by in situ hybridization. 2. Confirming previous findings, c-fos mRNA expression increased after 30 min of forced restraint. 3. This effect was attenuated by a previous i.c.v. injection of the anxiolytic benzodiazepine midazolam (20 nmol/2 microliters) or the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-7-phosphonoheptanoic acid (AP-7; 5 nmol/2 microliters). 4. These results suggest that the dentate gyrus is activated during restraint stress and that this activation may be modulated by benzodiazepine gamma-aminobutyric acidA (GABAA) or NMDA receptors.

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.

Hippocampal 5-HT receptors and consolidation of stressful memories

It has been suggested that postsynaptic 5-HT1A receptors in the hippocampus, innervated by 5-HT neurons localized in the median raphe nucleus, mediate adaptive or coping responses to aversive events and that dysfunction of this system is related to symptoms of depression. To test this hypothesis we investigated the expression of c-fos mRNA in animals submitted to immobilization stress. The results showed that c-fos mRNA expression is significantly increased in the dentate gyrus and CA1-CA3 regions of the hippocampus after 30 min of forced restraint, suggesting that this structure is activated during stress. To investigate the role of 5-HT neurotransmission in the hippocampus on adaptation to aversive events we immobilized rats for 2 h and tested them 24 h later in an elevated plus-maze. Our results showed that the previous restraint period decreases exploration of open arms in the maze. This effect was reversed by bilateral microinjection of zimelidine (20 and 100 nmol), a 5-HT re-uptake blocker, or 8-OH-DPAT (3 nmol), a 5-HT1A agonist, into the dorsal hippocampus immediately after restraint. These results are compatible with the idea that postsynaptic 5-HT1A receptors located in the hippocampus participate in the development of tolerance to aversive events.

Pre- and postmortem influences on brain RNA

Many potentially valuable techniques for the understanding of human neurobiological and neuropathological processes require the use of RNA obtained from postmortem tissue. As with earlier neurochemical studies, there are two particular problems posed by such tissue in comparison with tissue from experimental animals. These are the postmortem interval and the condition of the patient prior to death, referred to as the agonal state. We review the nature and extent of the effects of postmortem interval and agonal state on RNA in brain tissue, with particular reference to the study of neuropsychiatric disorders. Perhaps surprisingly, postmortem interval has at most a modest effect on RNA. Abundant intact and biologically active RNA is present in tissue frozen 36 h or more after death. Postmortem interval does not account for the marked variability observed among human brains in all RNA parameters. Despite the overall stability of RNA after death, some evidence suggests that individual RNAs may undergo postmortem decay. Less attention has been paid to the effects of agonal state. The existing data indicate that events in the premortem period such as hypoxia and coma can affect the amount of some messenger RNAs. The nature of agonal state influences depends on the messenger RNA in question, though the basis for this selective vulnerability is unknown. No agonal state effect on overall RNA level or activity has been found. The data show that postmortem brain tissue can be used for RNA research. However, considerable attention must be paid to controlling for the influences of pre- and postmortem factors, especially when quantitative analyses are performed.

Repeated swim-stress reduces GABAA receptor alpha subunit mRNAs in the mouse hippocampus

The effects of brief repeated swim stress on the expression of GABAA receptor alpha 1 subunit mRNAs was investigated in the mouse. Adult male mice were exposed to repeated brief (10 min) swim-stress once daily for 7 or 14 days and the levels of GABAA receptor alpha subunit mRNAs were quantified in the hippocampus 24 h after the last session by Northern analysis. Repeated swim stress for 14 days resulted in a 47.3% +/- 6.5 and 39.8% +/- 7.6 decrease in the levels of the 4.8 kb and 4.4 kb GABAA receptor alpha 1 subunit mRNAs, respectively. While there was a trend toward a reduction in the level of GABAA receptor alpha 1 subunit mRNAs following 7 days of repeated swim stress, the latter did not reach statistical significance. In contrast, no significant alterations in the levels of glutamic acid decarboxylase or beta-actin mRNAs were observed at either time point. The reduction in GABAA receptor alpha 1 subunit mRNAs following repeated swim stress may underlie similar alteration(s) in hippocampal GABAA receptor density previously observed following repeated swim stress.

Persistent elevation in GABAA receptor subunit mRNAs following social stress

Stress is associated with alterations in GABA/benzodiazepine binding and function. We evaluated effects of social stress on GABAA receptor subunit (alpha 1 and gamma 2) mRNAs by Northern hybridization. In cortex, no change was observed in either subunit mRNA immediately after stress, but a 4 hours mRNAs for both subunits were increased. These changes persisted for 72 hours after stress, and returned to baseline levels at 7 days. No changes in mRNAs were observed in sham-treated mice. No changes in either subunit mRNA were observed in stressed or sham-treated mice in cerebellum or hippocampus. In undefeated resident mice, mRNAs for both subunits in cortex were unaffected at 24 hours after the stress episode. Social stress is associated with increases in GABAA receptor alpha 1 and gamma 2 subunit mRNAs in cortex.