The exacerbation of hippocampal excitotoxicity by glucocorticoids is not mediated by apoptosis

Conditional Ablation of Glucocorticoid and Mineralocorticoid Receptors from Cochlear Supporting Cells Reveals Their Differential Roles for Hearing Sensitivity and Dynamics of Recovery from Noise-Induced Hearing Loss

Endogenous glucocorticoids (GC) are known to modulate basic elements of cochlear physiology. These include both noise-induced injury and circadian rhythms. While GC signaling in the cochlea can directly influence auditory transduction via actions on hair cells and spiral ganglion neurons, evidence also indicates that GC signaling exerts effects via tissue homeostatic processes that can include effects on cochlear immunomodulation. GCs act at both the glucocorticoid receptor (GR) and the mineralocorticoid receptor (MR). Most cell types in the cochlea express both receptors sensitive to GCs. The GR is associated with acquired sensorineural hearing loss (SNHL) through its effects on both gene expression and immunomodulatory programs. The MR has been associated with age-related hearing loss through dysfunction of ionic homeostatic balance. Cochlear supporting cells maintain local homeostatic requirements, are sensitive to perturbation, and participate in inflammatory signaling. Here, we have used conditional gene manipulation techniques to target Nr3c1 (GR) or Nr3c2 (MR) for tamoxifen-induced gene ablation in Sox9-expressing cochlear supporting cells of adult mice to investigate whether either of the receptors sensitive to GCs plays a role in protecting against (or exacerbating) noise-induced cochlear damage. We have selected mild intensity noise exposure to examine the role of these receptors related to more commonly experienced noise levels. Our results reveal distinct roles of these GC receptors for both basal auditory thresholds prior to noise exposure and during recovery from mild noise exposure. Prior to noise exposure, auditory brainstem responses (ABRs) were measured in mice carrying the floxed allele of interest and the Cre recombinase transgene, but not receiving tamoxifen injections (defined as control (no tamoxifen treatment), versus conditional knockout (cKO) mice, defined as mice having received tamoxifen injections. Results revealed hypersensitive thresholds to mid- to low-frequencies after tamoxifen-induced GR ablation from Sox9-expressing cochlear supporting cells compared to control (no tamoxifen) mice. GR ablation from Sox9-expressing cochlear supporting cells resulted in a permanent threshold shift in mid-basal cochlear frequency regions after mild noise exposure that produced only a temporary threshold shift in both control (no tamoxifen) f/fGR:Sox9iCre+ and heterozygous f/+GR:Sox9iCre+ tamoxifen-treated mice. A similar comparison of basal ABRs measured in control (no tamoxifen) and tamoxifen-treated, floxed MR mice prior to noise exposure indicated no difference in baseline thresholds. After mild noise exposure, MR ablation was initially associated with a complete threshold recovery at 22.6 kHz by 3 days post-noise. Threshold continued to shift to higher sensitivity over time such that by 30 days post-noise exposure the 22.6 kHz ABR threshold was 10 dB more sensitive than baseline. Further, MR ablation produced a temporary reduction in peak 1 neural amplitude one day post-noise. While supporting cell GR ablation trended towards reducing numbers of ribbon synapses, MR ablation reduced ribbon synapse counts but did not exacerbate noise-induced damage including synapse loss at the experimental endpoint. GR ablation from the targeted supporting cells increased the basal resting number of Iba1-positive (innate) immune cells (no noise exposure) and decreased the number of Iba1-positive cells seven days following noise exposure. MR ablation did not alter innate immune cell numbers at seven days post-noise exposure. Taken together, these findings support differential roles of cochlear supporting cell MR and GR expression at basal, resting conditions and especially during recovery from noise exposure.

Identifying the Involvement of Pro-Inflammatory Signal in Hippocampal Gene Expression Changes after Experimental Ischemia: Transcriptome-Wide Analysis

Acute cerebral ischemia induces distant inflammation in the hippocampus; however, molecular mechanisms of this phenomenon remain obscure. Here, hippocampal gene expression profiles were compared in two experimental paradigms in rats: middle cerebral artery occlusion (MCAO) and intracerebral administration of lipopolysaccharide (LPS). The main finding is that 10 genes (Clec5a, CD14, Fgr, Hck, Anxa1, Lgals3, Irf1, Lbp, Ptx3, Serping1) may represent key molecular links underlying acute activation of immune cells in the hippocampus in response to experimental ischemia. Functional annotation clustering revealed that these genes built the same clusters related to innate immunity/immunity/innate immune response in all MCAO differentially expressed genes and responded to the direct pro-inflammatory stimulus group. The gene ontology enrichment and Kyoto Encyclopedia of Genes and Genomes pathway analyses also indicate that LPS-responding genes were the most abundant among the genes related to "positive regulation of tumor necrosis factor biosynthetic process", "cell adhesion", "TNF signaling pathway", and "phagosome" as compared with non-responding ones. In contrast, positive and negative "regulation of cell proliferation" and "HIF-1 signaling pathway" mostly enriched with genes that did not respond to LPS. These results contribute to understanding genomic mechanisms of the impact of immune/inflammatory activation on expression of hippocampal genes after focal brain ischemia.

Navigating Alzheimer's Disease via Chronic Stress: The Role of Glucocorticoids

Alzheimer's disease (AD) is a chronic intensifying incurable progressive disease leading to neurological deterioration manifested as impairment of memory and executive brain functioning affecting the physical ability like intellectual brilliance, common sense in patients. The recent therapeutic approach in Alzheimer's disease is only the symptomatic relief further emerging the need for therapeutic strategies to be targeted in managing the underlying silent killing progression of dreaded pathology. Therefore, the current research direction is focused on identifying the molecular mechanisms leading to the evolution of the understanding of the neuropathology of Alzheimer's disease. The resultant saturation in the area of current targets (amyloid β, τ Protein, oxidative stress etc.) has led the scientific community to rethink of the mechanistic neurodegenerative pathways and reprogram the current research directions. Although, the role of stress has been recognized for many years and contributing to the development of cognitive impairment, the area of stress has got the much-needed impetus recently and is being recognized as a modifiable menace for AD. Stress is an unavoidable human experience that can be resolved and normalized but chronic activation of stress pathways unsettle the physiological status. Chronic stress mediated activation of neuroendocrine stimulation is generally linked to a high risk of developing AD. Chronic stress-driven physiological dysregulation and hypercortisolemia intermingle at the neuronal level and leads to functional (hypometabolism, excitotoxicity, inflammation) and anatomical remodeling of the brain architecture (senile plaques, τ tangles, hippocampal atrophy, retraction of spines) ending with severe cognitive deterioration. The present review is an effort to collect the most pertinent evidence that support chronic stress as a realistic and modifiable therapeutic earmark for AD and to advocate glucocorticoid receptors as therapeutic interventions.

Can antipoverty programmes save lives? Quasi-experimental evidence from the Earned Income Tax Credit in the USA

OBJECTIVE

To estimate the impact of state-level supplements of the Earned Income Tax Credit (EITC) on mortality in the USA. The EITC supplements the wages of lower-income workers by providing larger returns when taxes are filed.

SETTING

Nationwide sample spanning 25 cohorts of people across every state in the USA.

PARTICIPANTS

793 000 respondents within the National Longitudinal Mortality Survey (NLMS) between 1986 and 2011, a representative sample of the USA.

INTERVENTION

State-level supplementation to the EITC programme. Some, but not all, states added EITC supplementation to varying degrees beginning in 1986 (Wisconsin) and most recently in 2015 (California). Participants who were eligible in states with supplementary programmes were compared with those who were not eligible for supplementation. Comparisons were made both before and after implementation of the supplementary programme (a difference-in-difference, intent-to-treat analysis). This quasi-experimental approach further controls for age, gender, marital status, race or ethnicity, educational attainment, income and employment status.

PRIMARY AND SECONDARY OUTCOME MEASURES

The primary outcome measure was survival at 10 years. Secondary outcome measures included survival at 5 years and survival to the end of the intervention period.

RESULTS

We find an association between state supplemental EITC and survival, with a HR of 0.973 (95% CI=0.951-0.996) for each US$100 of EITC increase (p<0.05).

CONCLUSION

State-level supplemental EITC may be an effective means of increasing survival in the USA.

Differential Effects of Low- and High-Dose Dexamethasone on Electrically Induced Damage of the Cultured Organ of Corti

An increased number of patients with residual hearing are undergoing cochlear implantation. A subset of these experience delayed hearing loss post-implantation, and the aetiology of this loss is not well understood. Our previous studies suggest that electrical stimulation can induce damage to hair cells in organ of Corti (OC) organotypic cultures. Dexamethasone has the potential to protect residual hearing due to its multiple effects on cells and tissue (e.g., anti-inflammatory, free radical scavenger). We therefore hypothesized that dexamethasone treatment could prevent electrical stimulation induced changes in the OC. Organ of Corti explants from neonatal rats (P2–4) were cultured for 24 h with two different concentrations of dexamethasone. Thereafter, OC were subjected to a charge-balanced biphasic pulsed electrical stimulation (0.44–2 mA) for a further 24 h. Unstimulated dexamethasone-treated OC served as controls. Outcome analysis included immunohistochemical labelling of ribbon synapses, histochemical analysis of free reactive oxygen species and morphological analysis of stereocilia bundles. Overall, the protective effects of dexamethasone on electrically induced damage in cochlear explants were moderate. High-dose dexamethasone protected bundle integrity at higher current levels. Low-dose dexamethasone tended to increase ribbon density in the apical region.

Cultured hippocampal neurons of dystrophic mdx mice respond differently from those of wild type mice to an acute treatment with corticosterone

Duchenne muscular dystrophy is a lethal genetic disease characterised by progressive degeneration of skeletal muscles induced by deficiency of dystrophin, a cytoskeletal protein expressed in myocytes and in certain neuron populations. The severity of the neurological disorder varies in humans and animal models owing to dysfunction in numerous brain areas, including the hippocampus. Cyclic treatments with high-dose glucocorticoids remain a major pharmacological approach for treating the disease; however, elevated systemic levels of either stress-induced or exogenously administered anti-inflammatory molecules dramatically affect hippocampal activity. In this study, we analysed and compared the response of hippocampal neurons isolated from wild-type and dystrophic mdx mice to acute administration of corticosterone in vitro, without the influence of other glucocorticoid-regulated processes. Our results showed that in neurons of mdx mice, both the genomic and intracellular signalling-mediated responses to corticosterone were affected compared to those in wild-type animals, evoking the characteristic response to detrimental chronic glucocorticoid exposure. Responsiveness to glucocorticoids is, therefore, another function of hippocampal neurons possibly affected by deficiency of Dp427 since embryonic development. Knowing the pivotal role of hippocampus in stress hormone signalling, attention should be paid to the effects that prolonged glucocorticoid treatments may have on this and other brain areas of DMD patients.

Defining the Inflammatory Microenvironment in the Human Cochlea by Perilymph Analysis: Toward Liquid Biopsy of the Cochlea

The molecular pathomechanisms in the majority of patients suffering from acute or progressive sensorineural hearing loss cannot be determined yet. The size and the complex architecture of the cochlea make biopsy and in-depth histological analyses impossible without severe damage of the organ. Thus, histopathology correlated to inner disease is only possible after death. The establishment of a technique for perilymph sampling during cochlear implantation may enable a liquid biopsy and characterization of the cochlear microenvironment. Inflammatory processes may not only participate in disease onset and progression in the inner ear, but may also control performance of the implant. However, little is known about cytokines and chemokines in the human inner ear as predictive markers for cochlear implant performance. First attempts to use multiplex protein arrays for inflammatory markers were successful for the identification of cytokines, chemokines, and endothelial markers present in the human perilymph. Moreover, unsupervised cluster and principal component analyses were used to group patients by lead cytokines and to correlate certain proteins to clinical data. Endothelial and epithelial factors were detected at higher concentrations than typical pro-inflammatory cytokines such as TNF-a or IL-6. Significant differences in VEGF family members have been observed comparing patients with deafness to patients with residual hearing with significantly reduced VEGF-D levels in patients with deafness. In addition, there is a trend toward higher IGFBP-1 levels in these patients. Hence, endothelial and epithelial factors in combination with cytokines may present robust biomarker candidates and will be investigated in future studies in more detail. Thus, multiplex protein arrays are feasible in very small perilymph samples allowing a qualitative and quantitative analysis of inflammatory markers. More results are required to advance this method for elucidating the development and course of specific inner ear diseases or for perioperative characterization of cochlear implant patients.

Microglia polarization by methylprednizolone acetate accelerates cuprizone induced demyelination

Glucocorticoids (GC) are known as inflammatory drugs, which are used in neuroinflammatory diseases. Unlike the classic picture, recent studies have revealed that some GC drugs exacerbate inflammatory responses in their acute and prolonged administration. Multiple sclerosis (MS) is a demyelinating inflammatory disorder, in which reactive M1 microglia phenotype play a central role. Since methylprednisolone (MP), as a synthetic GC, are commonly used by MS patients, in this study, we evaluated the effect of long-term administration of MP on microglia polarization in cuprizone (CPZ)-induced MS model. The immunostaining results showed that chronic exposure to MP in the CPZ treated mice increased the number of Iba-1 positive microglia, which significantly expressed IP10 as M1 marker than arginase as M2 marker. MP treatment induced significant amplification in the transcript levels of iNOS and TNF-α (M1-related markers) in the corpus callosum of the MS mice, whereas no change detected in the expression of IL-10 (M2-related marker) between the groups. In addition, evaluation of myelin by luxol fast blue staining and transmission electron microscopy revealed that prolonged MP administration increased demyelination in comparison to the CPZ group. In conclusion, our results show that chronic MP therapy in the CPZ-induced demyelination model of MS polarized microglia to M1 pro-inflammatory phenotype.

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.

Temporal parameters of post-stress prophylactic glucose treatment in rats

Acute trauma can lead to life-long changes in susceptibility to psychiatric disease, such as post-traumatic stress disorder (PTSD). Rats given free access to a concentrated glucose solution for 24 h beginning immediately after trauma failed to show stress-related pathology in the learned helplessness model of PTSD and comorbid major depression. We assessed effective dosing and temporal constraints of the glucose intervention in three experiments. We exposed 120 male Sprague-Dawley rats to 100, 1 mA, 3-15 s, inescapable and unpredictable electric tail shocks (over a 110-min period) or simple restraint in the learned helplessness procedure. Rats in each stress condition had access to a 40% glucose solution or water. We measured fluid consumption under 18-h free access conditions, or limited access (1, 3, 6, 18 h) beginning immediately after trauma, or 3-h access with delayed availability of the glucose solution (0, 1, 3, 6 h). We hypothesized that longer and earlier access following acute stress would improve shuttle-escape performance. Rats exposed to traumatic shock and given 18-h access to glucose failed to show exaggerated fearfulness and showed normal reactivity to foot shock during testing as compared to their water-treated counterparts. At least 3 h of immediate post-stress access to glucose were necessary to see these improvements in test performance. Moreover, delaying access to glucose for more than 3 h post-trauma yielded no beneficial effects. These data clearly identify limits on the post-stress glucose intervention. In conclusion, glucose should be administered almost immediately and at the highest dose after trauma.

Pro-apoptotic Action of Corticosterone in Hippocampal Organotypic Cultures

Elevated levels of glucocorticoids exert neurotoxic effects, and the hippocampus is particularly sensitive to the effects of glucocorticoids. Because some data have indicated that an increased action of glucocorticoids in the perinatal period enhances the susceptibility of brain tissue to adverse substances later in life, the main purpose of the present study was to compare necrotic/apoptotic corticosterone action in hippocampal organotypic cultures obtained from control animals with the effect of this steroid in tissue from prenatally stressed rats. Because the adverse effects of glucocorticoid action on nerve cell viability appear to result mainly from an increase in the intensity of the effects of glutamate and changes in growth factor and pro-inflammatory cytokine synthesis, the involvement of these factors in corticosterone action were also determined. In stress-like concentration (1 μM), corticosterone, when added to hippocampal cultures for 1 and 3 days, alone or jointly with glutamate, did not induce necrosis. In contrast, in 3-day cultures, corticosterone (1 μM) increased caspase-3 activity and the mRNA expression of the pro-apoptotic Bax. Moreover, corticosterone’s effect on caspase-3 activity was stronger in hippocampal cultures from prenatally stressed compared to control rats. Additionally, 24 h of exposure to corticosterone and glutamate, when applied separately and together, increased Bdnf, Ngf, and Tnf-α expression. In contrast, after 72 h, a strong decrease in the expression of both growth factors was observed, while the expression of TNF-α remained high. The present study showed that in stress-like concentrations, corticosterone exerted pro-apoptotic but not necrotic effects in hippocampal organotypic cultures. Prenatal stress increased the pro-apoptotic effects of corticosterone. Increased synthesis of the pro-inflammatory cytokine TNF-α may be connected with the adverse effects of corticosterone on brain cell viability.

The neuroenergetics of stress hormones in the hippocampus and implications for memory

Acute stress causes rapid release of norepinephrine (NE) and glucocorticoids (GCs), both of which bind to hippocampal receptors. This release continues, at varying concentrations, for several hours following the stressful event, and has powerful effects on hippocampally-dependent memory that generally promote acquisition and consolidation while impairing retrieval. Several studies have characterized the brain's energy usage both at baseline and during memory processing, but there are few data on energy requirements of memory processes under stressful conditions. Because memory is enhanced by emotional arousal such as during stress, it is likely that molecular memory processes under these conditions differ from those under non-stressful conditions that do not activate the hypothalamic-pituitary-adrenal (HPA) axis. Mobilization of peripheral and central energy stores during stress may increase hippocampal glucose metabolism that enhances salience and detail to facilitate memory enhancement. Several pathways activated by the HPA axis affect neural energy supply and metabolism, and may also prevent detrimental damage associated with chronic stress. We hypothesize that alterations in hippocampal metabolism during stress are key to understanding the effects of stress hormones on hippocampally-dependent memory formation. Second, we suggest that the effects of stress on hippocampal metabolism are bi-directional: within minutes, NE promotes glucose metabolism, while hours into the stress response GCs act to suppress metabolism. These bi-directional effects of NE and GCs on glucose metabolism may occur at least in part through direct modulation of glucose transporter-4. In contrast, chronic stress and prolonged elevation of hippocampal GCs cause chronically suppressed glucose metabolism, excitotoxicity and subsequent memory deficits.

Glucocorticoids increase excitotoxic injury and inflammation in the hippocampus of adult male rats

BACKGROUND/AIMS

Stress exacerbates neuron loss in many CNS injuries via the actions of adrenal glucocorticoid (GC) hormones. For some injuries, this GC endangerment of neurons is accompanied by greater immune cell activation in the CNS, a surprising outcome given the potent immunosuppressive properties of GCs.

METHODS

To determine whether the effects of GCs on inflammation contribute to neuron death or result from it, we tested whether nonsteroidal anti-inflammatory drugs could protect neurons from GCs during kainic acid excitotoxicity in adrenalectomized male rats. We next measured GC effects on (1) chemokine production (CCL2 and CINC-1), (2) signals that suppress immune activation (CX3CL1, CD22, CD200, and TGF-β), and (3) NF-κB activity.

RESULTS

Concurrent treatment with minocycline, but not indomethacin, prevented GC endangerment. GCs did not substantially affect CCL2, CINC-1, or baseline NF-κB activity, but they did suppress CX3CL1, CX3CR1, and CD22 expression in the hippocampus - factors that normally restrain inflammatory responses.

CONCLUSIONS

These findings demonstrate that cellular inflammation is not necessarily suppressed by GCs in the injured hippocampus; instead, GCs may worsen hippocampal neuron death, at least in part by increasing the neurotoxicity of CNS inflammation.

Glucocorticoid signaling in myeloid cells worsens acute CNS injury and inflammation

Glucocorticoid stress hormones (GCs) are well known for being anti-inflammatory, but some reports suggest that GCs can also augment aspects of inflammation during acute brain injury. Because the GC receptor (GR) is ubiquitously expressed throughout the brain, it is difficult to know which cell types might mediate these unusual "proinflammatory" GC actions. We examined this with cell type-specific deletion or overexpression of GR in mice experiencing seizure or ischemia. Counter to their classical anti-inflammatory actions, GR signaling in myeloid cells increased Iba-1 and CD68 staining as well as nuclear p65 levels in the injured tissue. GCs also reduced levels of occludin, claudin 5, and caveolin 1, proteins central to blood-brain-barrier integrity; these effects required GR in endothelial cells. Finally, GCs compromised neuron survival, an effect mediated by GR in myeloid and endothelial cells to a greater extent than by neuronal GR.

Role of the pituitary–adrenal axis in granulocyte-colony stimulating factor-induced neuroprotection against hypoxia–ischemia in neonatal rats

Several reports indicate that the activity of the hypothalamic–pituitary–adrenal axis (HPA) is increased after a brain insult and that its down-regulation can improve detrimental outcomes associated with ischemic brain injuries.Granulocyte-colony stimulating factor (G-CSF) is a neuroprotective drug shown in the naïve rat to regulate hormones of the HPA axis. In this study we investigate whether G-CSF confers its neuroprotective properties by influencing the HPA response after neonatal hypoxia–ischemia (HI). Following the Rice–Vannucci model, seven day old rats (P7)were subjected to unilateral carotid ligation followed by 2.5 h of hypoxia. To test our hypothesis,metyrapone was administered to inhibit the release of rodent specific glucocorticoid, corticosterone, at the adrenal level. Dexamethasone, a synthetic glucocorticoid, was administered to agonize the effects of corticosterone.Our results show that both G-CSF and metyrapone significantly reduced infarct volume while dexamethasone treatment did not reduce infarct size even when combined with G-CSF. The protective effects of G-CSF do not include blood brain barrier preservation as suggested by the brain edema results. G-CSF did not affect the pituitary released adrenocorticotropic hormone (ACTH) levels in the blood plasma at 4 h, but suppressed the increase of corticosterone in the blood. The administration of G-CSF and metyrapone increased weight gain, and significantly reduced the Bax/Bcl-2 ratio in the brain while dexamethasone reversed the effects of G-CSF. The combination of G-CSF and metyrapone significantly decreased caspase-3 protein levels in the brain, and the effect was antagonized by dexamethasone.We report that G-CSF is neuroprotective in neonatal HI by reducing infarct volume, by suppressing the HI-induced increase of the Bax/Bcl-2 ratio, and by decreasing corticosterone in the blood. Metyrapone was able to confer similar neuroprotection as G-CSF while dexamethasone reversed the effects of G-CSF. In conclusion, we show that decreasing HPA axis activity is neuroprotective after neonatal HI, which can be conferred by administering G-CSF.

Glucocorticoid regulation of brain-derived neurotrophic factor: relevance to hippocampal structural and functional plasticity

Glucocorticoids serve as key stress response hormones that facilitate stress coping. However, sustained glucocorticoid exposure is associated with adverse consequences on the brain, in particular within the hippocampus. Chronic glucocorticoid exposure evokes neuronal cell damage and dendritic atrophy, reduces hippocampal neurogenesis and impairs synaptic plasticity. Glucocorticoids also alter expression and signaling of the neurotrophin, brain-derived neurotrophic factor (BDNF). Since BDNF is known to promote neuroplasticity, enhance cell survival, increase hippocampal neurogenesis and cellular excitability, it has been hypothesized that specific adverse effects of glucocorticoids may be mediated by attenuating BDNF expression and signaling. The purpose of this review is to summarize the current state of literature examining the influence of glucocorticoids on BDNF, and to address whether specific effects of glucocorticoids arise through perturbation of BDNF signaling. We integrate evidence of glucocorticoid regulation of BDNF at multiple levels, spanning from the well-documented glucocorticoid-induced changes in BDNF mRNA to studies examining alterations in BDNF receptor-mediated signaling. Further, we delineate potential lines of future investigation to address hitherto unexplored aspects of the influence of glucocorticoids on BDNF. Finally, we discuss the current understanding of the contribution of BDNF to the modulation of structural and functional plasticity by glucocorticoids, in particular in the context of the hippocampus. Understanding the mechanistic crosstalk between glucocorticoids and BDNF holds promise for the identification of potential therapeutic targets for disorders associated with the dysfunction of stress hormone pathways.

Cells, biomarkers, and post-traumatic stress disorder: evidence for peripheral involvement in a central disease

Post-traumatic stress disorder (PTSD) is a complicated CNS syndrome. Looking beyond the CNS, recent studies suggest that peripheral blood mononuclear cells could cause and/or exacerbate PTSD. This review summarizes the literature, describes associations between circulating peripheral blood cells and PTSD, proposes a novel mechanism, and analyzes several biomarkers that appear to associate with PTSD symptoms. Several experimental animal models have shown that peripheral blood mononuclear cell activity can cause hippocampal volume loss and PTSD-like symptoms. Data from these models suggest that a traumatic event and/or traumatic events can trigger peripheral cells to migrate, mediate inflammation, and decrease neurogenesis, potentially leading to CNS volume loss. Biomarkers that associate with PTSD symptoms have the potential to differentiate PTSD from traumatic brain injury, but more work needs to be done. Research examining the mechanism of how traumatic events are linked to peripheral blood mononuclear cell functions and biomarkers may offer improved diagnoses and treatments for PTSD patients.

Neuroprotective effects of quercetin, rutin and okra (Abelmoschus esculentus Linn.) in dexamethasone-treated mice

The administration of dexamethasone, a synthetic glucocorticoid receptor agonist, causes neuronal death in the CA3 layer of the hippocampus, which has been associated with learning and memory impairments. This study aimed to examine the ability of okra (Abelmoschus esculentus Linn.) extract and its derivatives (quercetin and rutin) to protect neuronal function and improve learning and memory deficits in mice subjected to dexamethasone treatment. Learning and memory functions in mice were examined using the Morris water maze test. The results showed that the mice treated with dexamethasone had prolonged water maze performance latencies and shorter time spent in the target quadrant while mice pretreated with quercetin, rutin or okra extract prior to dexamethasone treatment showed shorter latencies and longer time spent in target quadrant. Morphological changes in pyramidal neurons were observed in the dexamethasone treated group. The number of CA3 hippocampal neurons was significantly lower while pretreated with quercetin, rutin or okra attenuated this change. Prolonged treatment with dexamethasone altered NMDA receptor expression in the hippocampus. Pretreatment with quercetin, rutin or okra extract prevented the reduction in NMDA receptor expression. Dentate gyrus (DG) cell proliferation was examined using the 5-bromo-2-deoxyuridine (BrdU) immunohistochemistry technique. The number of BrdU-immunopositive cells was significantly reduced in dexamethasone-treated mice compared to control mice. Pretreatment with okra extract, either quercetin or rutin was found to restore BrdU-immunoreactivity in the dentate gyrus. These findings suggest that quercetin, rutin and okra extract treatments reversed cognitive deficits, including impaired dentate gyrus (DG) cell proliferation, and protected against morphological changes in the CA3 region in dexamethasone-treated mice. The precise mechanism of the neuroprotective effect of these plant extracts should be further investigated.

Induction of the Wnt antagonist Dickkopf-1 is involved in stress-induced hippocampal damage

The identification of mechanisms that mediate stress-induced hippocampal damage may shed new light into the pathophysiology of depressive disorders and provide new targets for therapeutic intervention. We focused on the secreted glycoprotein Dickkopf-1 (Dkk-1), an inhibitor of the canonical Wnt pathway, involved in neurodegeneration. Mice exposed to mild restraint stress showed increased hippocampal levels of Dkk-1 and reduced expression of β-catenin, an intracellular protein positively regulated by the canonical Wnt signalling pathway. In adrenalectomized mice, Dkk-1 was induced by corticosterone injection, but not by exposure to stress. Corticosterone also induced Dkk-1 in mouse organotypic hippocampal cultures and primary cultures of hippocampal neurons and, at least in the latter model, the action of corticosterone was reversed by the type-2 glucocorticoid receptor antagonist mifepristone. To examine whether induction of Dkk-1 was causally related to stress-induced hippocampal damage, we used doubleridge mice, which are characterized by a defective induction of Dkk-1. As compared to control mice, doubleridge mice showed a paradoxical increase in basal hippocampal Dkk-1 levels, but no Dkk-1 induction in response to stress. In contrast, stress reduced Dkk-1 levels in doubleridge mice. In control mice, chronic stress induced a reduction in hippocampal volume associated with neuronal loss and dendritic atrophy in the CA1 region, and a reduced neurogenesis in the dentate gyrus. Doubleridge mice were resistant to the detrimental effect of chronic stress and, instead, responded to stress with increases in dendritic arborisation and neurogenesis. Thus, the outcome of chronic stress was tightly related to changes in Dkk-1 expression in the hippocampus. These data indicate that induction of Dkk-1 is causally related to stress-induced hippocampal damage and provide the first evidence that Dkk-1 expression is regulated by corticosteroids in the central nervous system. Drugs that rescue the canonical Wnt pathway may attenuate hippocampal damage in major depression and other stress-related disorders.

Neuroprotective effects of icariin on corticosterone-induced apoptosis in primary cultured rat hippocampal neurons

Neurons are damaged following prolonged exposure to high concentrations of corticosterone, particularly during chronic inflammatory and immune diseases. One of the main mechanisms underlying neuronal injury is apoptosis. In the present study the neuroprotective effects of icariin, an active natural ingredient from the Chinese plant Epimedium sagittatum maxim against corticosterone-induced apoptosis were examined in primary cultured rat hippocampal neuronal cells. Pre-treatment of neuronal cells with icariin suppressed corticosterone-induced cytotoxicity in a dose-dependent manner. Terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate (dUTP) nick-end-labeling assay (TUNEL) labeling demonstrated that icariin significantly reduced TUNEL-positive cell numbers induced by exposure of cultured neurons to corticosterone. Moreover, icariin markedly inhibited corticosterone-induced mitochondrial dysfunction, including improved mitochondrial membrane potential and inhibition of caspase-3 activation. Using western blot analysis, corticosterone activated p38MAPK, extracellular regulated kinase 1/2(ERK1/2) ,and c-jun N-terminal protein kinase 1(JNK1) ,while icariin blocked p38 MAPK, but not JNK1 or ERK1/2. Pharmacological approaches showed that the activation of p38MAPK plays a critical role in corticosterone-induced mitochondrial dysfunction and apoptosis. Taken together, the present results suggest that the protective effects of icariin on apoptosis in hippocampal neuronal cells are potentially mediated through blockade of p38 MAPK phosphorylation.

Single prolonged stress decreases glutamate, glutamine, and creatine concentrations in the rat medial prefrontal cortex

Application of single prolonged stress (SPS) in rats induces changes in neuroendocrine function and arousal that are characteristic of post traumatic stress disorder (PTSD). PTSD, in humans, is associated with decreased neural activity in the prefrontal cortex, increased neural activity in the amygdala complex, and reduced neuronal integrity in the hippocampus. However, the extent to which SPS models these aspects of PTSD has not been established. In order to address this, we used high-resolution magic angle spinning proton magnetic resonance spectroscopy (HR-MAS (1)H MRS) ex vivo to assay levels of neurochemicals critical for energy metabolism (creatine and lactate), excitatory (glutamate and glutamine) and inhibitory (gamma amino butyric acid (GABA)) neurotransmission, and neuronal integrity (N-acetylaspartate (NAA)) in the medial prefrontal cortex (mPFC), amygdala complex, and hippocampus of SPS and control rats. Glutamate, glutamine, and creatine levels were decreased in the mPFC of SPS rats when compared to controls, which suggests decreased excitatory tone in this region. SPS did not alter the neurochemical profiles of either the hippocampus or amygdala. These data suggest that SPS selectively attenuates excitatory tone, without a disruption of neuronal integrity, in the mPFC.

Mineralocorticoid and glucocorticoid receptors in hippocampus: their impact on neurons survival and behavioral impairment after neonatal brain injury

Glucocorticoids (GC) exert multiple effects within the central nervous system via mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) activation. MR expression is associated with a neuroprotective phenotype, whereas GR activation is implicated in the induction of an endangered neural phenotype and the opposite actions are most evident in hippocampus, where these receptors are predominantly present. Hippocampus has an overall inhibitory influence on the activity of the hypothalamic-pituitary-adrenal (HPA) axis and it has been suggested that efficient learning and adequate stress response depend on the appropriate functioning of the axis brought by coordinated activation of MR and GR in this region. There is a growing body of evidence that perinatal asphyxia causes irreversible damage to the brain leading to neurons loss in regions vulnerable to oxygen shortage especially in hippocampus. In the present review, some aspects of recently acquired insight in the role of GC receptors in promoting neuronal death and survival after hippocampal injury are discussed. Since the unbalance of MR and GR in hippocampus creates a condition of disturbed neuroendocrine regulation their potential impact on behavioral impairment will also be reviewed.

The interaction between psychological health and traumatic brain injury: a neuroscience perspective

The occurrence of traumatic brain injury (TBI) and psychological health issues in the current theater of military operations has become a major factor in planning for the long-term healthcare of our wounded warriors. Post-traumatic stress disorder (PTSD) can co-exist with brain injury in military members who have been exposed to blasts. Specific areas of the brain may be more susceptible to damage from blasts. In particular, damage to the prefrontal cortex can lead to disinhibition of cerebral structures that control fear and anxiety. Reactive systemic inflammatory processes related to TBI may also impair psychological health. Impaired psychological health may lead to increased psychological distress that impedes brain repair due to the release of stress-related hormones. Since the external environment has been shown to exert a significant influence on the internal environment of the organism, enriching the external environment may well reduce anxiety and facilitate the neuroplasticity of brain cells, thus promoting recovery of function after TBI.

Neuronal damage and protection in the pathophysiology and treatment of psychiatric illness: stress and depression

The discovery that stress and depression, as well as other psychiatric illnesses, are characterized by structural alterations, and that these changes result from atrophy and loss of neurons and glia in specific limbic regions and circuits, has contributed to a fundamental change in our understanding of these illnesses. These structural changes are accompanied by dysregulation of neuroprotective and neurotrophic signaling mechanisms that are required for the maturation, growth, and survival of neurons and glia. Conversely, behavioral and therapeutic interventions can reverse these structural alterations by stimulating neuroprotective and neurotrophic pathways and by blocking the damaging, excitotoxic, and inflammatory effects of stress. Lifetime exposure to cellular and environmental stressors and interactions with genetic factors contribute to individual susceptibility or resilience. This exciting area of research holds promise and potential for further elucidating the pathophysiology of psychiatric illness and for development of novel therapeutic interventions.

An association between stress-induced disruption of the hypothalamic-pituitary-adrenal axis and disordered glucose metabolism in an animal model of post-traumatic stress disorder

Retrospective clinical reports suggesting that traumatic stress populations display an increased propensity for glucose metabolism disorders were examined in a controlled prospective animal model. Stress-induced behavioural and hypothalamic-pituitary-adrenal (HPA) axis response patterns were correlated to central and peripheral parameters of glucose metabolism and signalling, and to body measurements in Sprague-Dawley rats exposed to predator scent stress. Forty days post-exposure, fasting blood glucose and insulin levels, oral glucose tolerance test, body weight and white adipose tissue mass, systemic corticosterone levels and brain expression of insulin receptor (IR) and insulin-sensitive glucose transporter 4 (GLUT4) protein levels were evaluated. In a second experiment inbred strains with hyper- (Fischer) and hypo- (Lewis) reactive HPA axes were employed to assess the association of metabolic data with behavioural phenomenology versus HPA axis response profile. For data analysis, animals were classified according to their individual behavioural response patterns (assessed at day 7) into extreme, partial and minimal response groups. The exposed Sprague-Dawley rats fulfilling criteria for extreme behavioural response (EBR) (20.55%) also exhibited significant increases in body weight, abdominal circumference and abdominal white adipose tissue mass; a hyperglycaemic oral glucose tolerance test; and fasting hyperglycaemia, hyperinsulinaemia and hypercorticosteronemia, whereas minimal responders (MBR) and control animals displayed no such disturbances. Hippocampal and hypothalamic expression of IR and GLUT4 protein were significantly lower in EBR than in MBR and control rats. The inbred strains showed no metabolic differences at baseline. Exposed Fischer rats displayed hyperglycaemia and hyperinsulinaemia, whereas Lewis rats did not. A significant protracted disorder of glucose metabolism was induced by exposure to a stress paradigm. This metabolic response was associated with the characteristic pattern of HPA axis (corticosterone) response, which underlies the behavioural response to stress.

Transgenerational effects of prenatal nutrient restriction on cardiovascular and hypothalamic-pituitary-adrenal function

The perinatal environment is a powerful determinant of risk for developing disease in later life. Here, we have shown that maternal undernutrition causes dramatic changes in heart structure and hypothalamo-pituitary-adrenal (HPA) function across two generations. Pregnant guinea pigs were fed 70% of normal intake from gestational days 1-35 (early restriction; ER), or 36-70 (late restriction; LR). Female offspring (F(1)) were mated and fed ad libitum to create second generation (F(2)) offspring. Heart morphology, blood pressure, baroreceptor and HPA function were assessed in male F(1) and F(2) offspring. ER(F1) males exhibited elevated blood pressure, increased left ventricular (LV) wall thickness and LV mass. These LV effects were maintained in the ER(F2) offspring. Maternal undernutrition increased basal cortisol and altered HPA responsiveness to challenge in both generations; effects were greatest in LR groups. In conclusion, moderate maternal undernutrition profoundly modifies heart structure and HPA function in adult male offspring for two generations.

Neuroprotective effects of lactation against kainic acid treatment in the dorsal hippocampus of the rat

Marked hippocampal changes in response to excitatory amino acid agonists occur during pregnancy (e.g. decreased frequency in spontaneous recurrent seizures in rats with KA lesions of the hippocampus) and lactation (e.g. reduced c-Fos expression in response to N-methyl-d,l-aspartic acid but not to kainic acid). In this study, the possibility that lactation protects against the excitotoxic damage induced by KA in hippocampal areas was explored. We compared cell damage induced 24 h after a single systemic administration of KA (5 or 7.5 mg/kg bw) in regions CA1, CA3, and CA4 of the dorsal hippocampus of rats in the final week of lactation to that in diestrus phase. To determine cellular damage in a rostro-caudal segment of the dorsal hippocampus, we used NISSL and Fluorojade staining, immunohistochemistry for active caspase-3 and TUNEL, and we observed that the KA treatment provoked a significant loss of neurons in diestrus rats, principally in the pyramidal cells of CA1 region. In contrast, in lactating rats, pyramidal neurons from CA1, CA3, and CA4 in the dorsal hippocampus were significantly protected against KA-induced neuronal damage, indicating that lactation may be a natural model of neuroprotection.

What causes the hippocampal volume decrease in depression? Are neurogenesis, glial changes and apoptosis implicated?

Even though in vivo imaging studies document significant reductions of hippocampal volume in depressed patients, the exact underlying cellular mechanisms are unclear. Since stressful life events are associated with an increased risk of developing depression, preclinical studies in which animals are exposed to chronic stress have been used to understand the hippocampal shrinkage in depressed patients. Based on morphometrical studies in these models, parameters like dendritic retraction, suppressed adult neurogenesis and neuronal death, all due to elevated levels of glucocorticoids, have been suggested as major causative factors in hippocampal shrinkage. However, histopathological studies examining hippocampi of depressed individuals have so far failed to confirm either a massive neuronal loss or a suppression of dentate neurogenesis, an event that is notably very rare in adult or elderly humans. In fact, many of the structural changes and the volume reduction appear to be reversible. Clearly, more histopathological studies are needed; especially ones that (a) employ stereological quantification, (b) focus on specific cellular elements and populations, and (c) are performed in nonmedicated depressed patients. We conclude that mainly other factors, like alterations in the somatodendritic, axonal, and synaptic components and putative glial changes are most likely to explain the hippocampal shrinkage in depression, while shifts in fluid balance or changes in the extracellular space cannot be excluded either.

An inflammatory review of glucocorticoid actions in the CNS

In recent years, the classic view that glucocorticoids, the adrenal steroids secreted during stress, are universally anti-inflammatory has been challenged at a variety of levels. It was first observed that under some circumstances, acute GC exposure could have pro-inflammatory effects on the peripheral immune response. More recently, chronic exposure to GCs has been found to have pro-inflammatory effects on the specialized immune response to injury in the central nervous system. Here we review the evidence that in some cases, glucocorticoids can increase pro-inflammatory cell migration, cytokine production, and even transcription factor activity in the brain. We consider how these unexpected effects of glucocorticoids can co-exist with their well-established anti-inflammatory properties, as well as the considerable clinical implications of these findings.

Glucocorticoids exacerbate hypoxia-induced expression of the pro-apoptotic gene Bnip3 in the developing cortex

Neonatal administration of the synthetic glucocorticoid, dexamethasone (DEX) retards brain growth, alters adult behaviors and induces cell death in the rat brain, thereby implicating glucocorticoids as developmentally neuroendangering compounds. Glucocorticoids also increase expression of pro-apoptotic Bcl-2 family members and exacerbate expression of hypoxic responsive genes. Bnip3 is a pro-apoptotic Bcl-2 family member that is upregulated in response to hypoxia. In these studies, we investigated the interactions of glucocorticoid receptor and hypoxia in the regulation of Bnip3 mRNA in cortical neurons. Using quantitative real time reverse transcription-polymerase chain reaction, we found that DEX treatment of postnatal days 4-6 rat pups caused a significant increase in Bnip3 mRNA expression compared with vehicle controls. A significant increase in Bnip3 mRNA was also measured in primary cortical neurons 72 h after treatment with RU28362, a glucocorticoid receptor selective agonist. In primary cortical neurons, hypoxia increased Bnip3 mRNA expression and this was exacerbated with RU28362 treatment. To elucidate the mechanism of glucocorticoid- and hypoxia-mediated regulation of Bnip3 transcription, a Bnip3 promoter-luciferase reporter construct was utilized in primary cortical neurons. Upregulation of the Bnip3 promoter was mediated by a single glucocorticoid response element and a hypoxic response element. Bnip3 overexpression in primary cortical neurons significantly increased cell death, which is dependent on the Bnip3 transmembrane domain. However, despite the increased expression of Bnip3 following glucocorticoid and hypoxia treatment, corresponding decreases in cell survival were minimal. These studies identify a novel pathway in the developing cortex through which glucocorticoids may enhance a metabolic insult, such as hypoxia.

Microarray analysis of cultured human brain aggregates following cortisol exposure: implications for cellular functions relevant to mood disorders

Increased cortisol levels in humans are often observed in patients suffering from mood disorders. In this study human fetal brain aggregates were exposed to cortisol at 500 nM for 3 weeks, as an in-vitro model of chronic cortisol exposure. Microarray analysis on extracted mRNA using the Affymetrix U133A platform was then performed. Our results demonstrated a significant effect of cortisol on 1648 transcripts; 736 up-regulated and 912 down-regulated genes. The most differentially regulated biological categories were: RNA processing, protein metabolism, and cell growth. Within these categories we observed a down-regulation of fibroblast growth factor 2 (FGF2) (-1.5-fold) and aquaporin4 (AQP4) (-1.7-fold), alongside an up-regulation of fibroblast growth factor 9 (FGF9) (+1.7-fold) and vesicle associated membrane protein2 (VAMP2) (+1.7-fold). FGF2, FGF9, AQP4 and VAMP2 changes were confirmed at the protein level by immunohistochemistry. Alterations in FGF transcripts are in keeping with recent literature demonstrating such effects in patients with mood disorders.

Concentration dependent actions of glucocorticoids on neuronal viability and survival

A growing body of evidence based on experimental data demonstrates that glucocorticoids (GCs) can play a potent role in the survival and death of neurons. However, these observations reflect paradoxical features of GCs, since these adrenal stress hormones are heavily involved in both neurodegenerative and neuroprotective processes. The actual level of GCs appears to have an essential impact in this bimodal action. In the present short review we aim to show the importance of concentration dependent action of GCs on neuronal cell viability and cell survival in the brain. Additionally, we will summarize the possible GC-induced cellular mechanisms at different GC concentrations providing a background for their effect on the fate of nerve cells in conditions that are a challenge to their survival.

Neuroprotective effects of agmatine against cell damage caused by glucocorticoids in cultured rat hippocampal neurons

In the present study the neuroprotective effects of agmatine against neuronal damage caused by glucocorticoids were examined in cultured rat hippocampal neurons. Spectrophotometric measurements of lactate dehydrogenase activities, beta-tubulin III immunocytochemical staining, terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate (dUTP) nick-end-labeling assay (TUNEL) labeling and caspase-3 assays were carried out to detect cell damage or possible involved mechanisms. Our results show that dexamethasone and corticosterone produced a concentration-dependent increase of lactate dehydrogenase release in 12-day hippocampal cultures. Addition of 100 microM agmatine into media prevented the glucocorticoid-induced increase of lactate dehydrogenase release, an effect also shared with the specific N-methyl-D-aspartate receptor antagonist MK801 and glucocorticoid receptor antagonists mifepristone and spironolactone. Arcaine, an analog of agmatine with similar structure as agmatine, also blocked glucocorticoid-induced increase of lactate dehydrogenase release. Spermine and putrescine, the endogenous polyamine and metabolic products of agmatine without the guanidino moiety of agmatine, have no appreciable effect on glucocorticoid-induced injuries, indicating a structural relevance for this neuroprotection. Immunocytochemical staining with beta-tubulin III confirmed the substantial neuronal injuries caused by glucocorticoids and the neuroprotective effects of agmatine against these neuronal injuries. TUNEL labeling demonstrated that agmatine significantly reduced TUNEL-positive cell numbers induced by exposure of cultured neurons to dexamethasone. Moreover, exposure of hippocampal neurons to dexamethasone significantly increased caspase-3 activity, which was inhibited by co-treatment with agmatine. Taken together, these results demonstrate that agmatine can protect cultured hippocampal neurons from glucocorticoid-induced neurotoxicity, through a possible blockade of the N-methyl-D-aspartate receptor channels or a potential anti-apoptotic property.

No difference in expression of apoptosis-related proteins and apoptotic morphology in control, pathologically aged and Alzheimer's disease cases

Apoptotic-like changes in the neocortex of control, pathologically aged and Alzheimer's disease (AD) cases were investigated. There was no increase in labeling or change in localization of labeling that distinguished between these cases for active caspase-3, -8, -9, Bax, Bcl-2 or TRADD. Bax, Bcl-2 and TRADD mRNA levels also differed little between case types, although there were small but significant decreases in Bax mRNA levels in AD compared to control cases and Bcl-2 mRNA in AD cases compared to pathologically aged and control cases. There was no difference in the percentage of apoptotic-like nuclei between these cases, except for a small but significant decrease in the inferior temporal gyrus of AD cases relative to controls. Nuclei observed within or adjacent to beta-amyloid plaques were rarely abnormal, and neurons bearing neurofibrillary tangles (NFTs) did not have abnormal nuclei. Apoptosis may not play a major role in the pathogenesis of neuronal degeneration of AD.

Dexamethasone induces cell death which may be blocked by NMDA receptor antagonists but is insensitive to Mg2+ in cerebellar granule neurons

Since dexamethasone may elevate the Ca2+ influx through NMDA receptors, we have investigated mechanisms of dexamethasone toxicity in rat cerebellar granule neurons. Dexamethasone concentrations over 0.1 microM induced cell death that reached about 20% of the death induced by glutamate. Dexamethasone-induced cell death was reduced by more than 80% by the mineralocorticoid antagonist RU 28318 or the NMDA receptor antagonists MK 801 and CGP 39551, whereas RU 28318 rescued only approximately 30% of cells treated with glutamate, indicating that dexamethasone requires NMDA receptors to induce acute neuronal toxicity and that a fraction of the neurons showed this toxicity. Mg2+ reduced the cell death induced by glutamate at potassium concentrations of 1 mM and 5 mM, but not at 25 mM. In contrast, cell death induced by dexamethasone was not significantly reduced by Mg2+ in any of the potassium concentrations. Both glutamate and dexamethasone induced toxicity with translocation of the apoptosis inducer NGFI-B to the mitochondria seen after 30 min-2 h concomitant with activation of apoptosis inducing factor (AIF) and caspase-3. In conclusion, dexamethasone induces a rapid toxicity which is blocked by NMDA receptor antagonists other than Mg2+, and involves mitochondrial apoptosis inducer NGFI-B.

Chronic corticosterone manipulations in mice affect brain cell proliferation rates, but only partly affect BDNF protein levels

We investigated whether the effects of corticosterone (CORT) on brain cell proliferation are mediated via its detrimental effect on brain-derived neurotrophic factor (BDNF). Using a [3H]thymidine tracer study, it was demonstrated that the cell proliferation rate in the neurogenic hippocampus and subventricular zone was increased in placebo-treated adrenalectomized (ADX) mice with low plasma corticosterone levels when compared with chronically CORT-treated ADX animals (25mg or 100mg sustained-release pellet). The cell proliferation rate of SHAM animals was in between the ADX-placebo group and ADX CORT-treated groups. BDNF protein contents in the hippocampus and subventricular zone were not different between the SHAM group and ADX-placebo group, although BDNF contents were decreased in the chronically CORT-treated ADX animals. Thus, other factors besides BDNF are involved in mediating CORT-induced changes in cell proliferation. Further, CORT manipulations did not affect caspase-3-like activity in any of the brain regions investigated, suggesting that caspase-3 is not involved in possible CORT-induced cellular losses.

Hippocampal neurogenesis is not enhanced by lifelong reduction of glucocorticoid levels

Neurogenesis of dentate gyrus granule cells is generally considered to be negatively regulated by glucocorticoids. We tested the hypothesis that exposure to low plasma corticosteroid levels starting in the early postnatal period enhances granule cell proliferation rate during adulthood. Rat pups were adrenalectomized (ADX) on postnatal day 10 and were then "clamped" throughout life at low corticosterone levels via oral supplementation. Neurogenesis was determined using BrdU immunochemistry at 3 and 12 months in clamped rats as compared with age-matched, sham-operated controls. Rate of neurogenesis did not differ between the groups at either 3 or 12 months. It was significantly lower in 12-month-old compared with 3-month-old rats, despite the presence of an age-dependent increase of plasma corticosterone only in the sham-ADX rats. Granule cell layer volume, granule cell density, and granule cell degeneration (determined using apoptotic markers) were indistinguishable in the two groups, further supporting the comparable rate of neurogenesis under differing chronic glucocorticoid levels. In addition, whereas acute deprivation of plasma glucocorticoids (adrenalectomy) in adult rats evoked a burst of granule cell neurogenesis, complete elimination of these hormones (by stopping hormone supplementation) in adult, early-life ADX/clamped rats did not. These data do not support a simple inverse relationship between chronic plasma glucocorticoid levels and granule cell neurogenesis. Specifically, chronic modulation of glucocorticoid levels commencing early in life evokes additional, adaptive, and compensatory mechanisms that contribute to the regulation of granule cell proliferation.

Restructuring the neuronal stress response with anti-glucocorticoid gene delivery

Glucocorticoids, the adrenal steroids released during stress, compromise the ability of neurons to survive neurological injury. In contrast, estrogen protects neurons against such injuries. We designed three genetic interventions to manipulate the actions of glucocorticoids, which reduced their deleterious effects in both in vitro and in vivo rat models. The most effective of these interventions created a chimeric receptor combining the ligand-binding domain of the glucocorticoid receptor and the DNA-binding domain of the estrogen receptor. Expression of this chimeric receptor reduced hippocampal lesion size after neurological damage by 63% and reversed the outcome of the stress response by rendering glucocorticoids protective rather than destructive. Our findings elucidate three principal steps in the neuronal stress-response pathway, all of which are amenable to therapeutic intervention.

Down-regulation of GluR2 is associated with Ca2+-dependent protease activities in kainate-induced apoptotic cell death in cultured [correction of culturd] rat hippocampal neurons

In the present study, the molecular mechanisms underlying kainate-induced neurotoxicity were characterized in cultured rat hippocampal neurons. Long-term exposure to kainate caused typically apoptotic cell death, which was accompanied by the accumulation of calcium, marked down-regulation of GluR2 subunit, and the activation of calpain and caspase-3. All these alterations were prevented by alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) receptor antagonist CNQX, but not by NMDA receptor antagonist MK801 and membrane L-type calcium channel antagonist nifedipine. In the presence of cyclothiazide, kainate-induced neurotoxicity was significantly enhanced. Inhibition of either caspases by zVAD-fmk or calpains by calpeptin protected neurons from neurotoxicity. These results suggest that long-term exposure of hippocampal neurons to kainate causes apoptosis, whose mechanisms involve multiple Ca(2+)-dependent cascades, in which AMPA receptor subunits may be targets for Ca(2+)-activated protease-mediated degradation during kainate-induced neuron apoptosis.

Ionotropic and metabotropic glutamate receptor mediation of glucocorticoid-induced apoptosis in hippocampal cells and the neuroprotective role of synaptic N-methyl-D-aspartate receptors

Glutamate receptors have been proposed to mediate the apoptotic actions of glucocorticoids in hippocampal cells. To further analyze the role of glutamate receptors in this process, we pretreated primary hippocampal cells from neonatal (postnatal day 4) rats with antagonists of ionotropic glutamate receptor (iGluR) and metabotropic glutamate receptor (mGluR) antagonists before exposure to the specific glucocorticoid receptor agonist dexamethasone (DEX) at a dose of 1 microM. Dizocilpine (MK801; a general N-methyl-D-aspartic acid [NMDA] receptor antagonist, NMDAR antagonist) and ifenprodil (a specific ligand of the NMDAR 2B subunit, NR2B), were used to block iGluR; (RS)-alpha-ethyl-4-carboxyphenylglycine (E4CPG) and (RS)-alpha-cyclopropyl-4-phosphonophenyl-glycine (CPPG) were employed as I/II (E4CPG) and II/III (CPPG) mGluR antagonists. Blockade of iGluR resulted in a significant attenuation of DEX-induced cell death; the finding that ifenprodil exerted a similar potency to MK801 demonstrates the involvement of NR2B receptors in glucocorticoid-induced cell death. Apoptosis accounted for a significant amount of the cell loss observed, as detected by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling histochemistry for the in situ labeling of DNA breaks; apoptotic cells were distinguished from necrosis on the basis of morphological criteria, including chromatin condensation, membrane blebbing and presence of apoptotic bodies. Treatment with E4CPG and CPPG completely abolished the apoptotic response to DEX, thus showing the additional contribution of mGluR to the phenomenon. Further, dose-response studies with NMDA revealed that whereas high (10 microM) doses of NMDA themselves elicit cytotoxic responses, low (1-5 microM) concentrations of NMDA can effectively oppose DEX-induced cell death. Interestingly, the neuroprotective actions of low dose NMDA stimulation were abolished when either synaptic or extrasynaptic NMDA receptors were blocked with MK801 in combination with the GABA receptor antagonist bicuculline (synaptic) or ifenprodil (extrasynaptic). In summary, the present data show that both iGluR and mGluR mediate the neurotoxic effects of glucocorticoids on hippocampal cells and that pre-treatment with low doses of NMDA, by acting on synaptic and extrasynaptic receptors, render hippocampal cells less vulnerable to glucocorticoid insults.