Brief RU 38486 treatment normalizes the effects of chronic stress on calcium currents in rat hippocampal CA1 neurons

Systemic Injection of Aged Blood Plasma in Adult C57BL/6 Mice Induces Neurophysiological Impairments in the Hippocampal CA1

BACKGROUND

Aging is characterized by systemic alterations and forms an important risk factor for Alzheimer's disease. Recently, it has been indicated that blood-borne factors present in the systemic milieu contribute to the aging process. Exposing young mice to aged blood plasma results in impaired neurogenesis and synaptic plasticity in the dentate gyrus, as well as impaired cognition. Vice versa, treating aged mice with young blood plasma rescues impairments associated with aging.

OBJECTIVE

Whether blood-borne factors are sufficient to drive impairments outside the dentate gyrus, how they impact neurophysiology, and how the functional outcome compares to impairments found in mouse models for AD is still unclear.

METHODS

Here, we treated adult mice with blood plasma from aged mice and assessed neurophysiological parameters in the hippocampal CA1.

RESULTS

Mice treated with aged blood plasma show significantly impaired levels of long-term potentiation (LTP), similar to those present in APP/PS1 mice. These impaired levels of LTP in plasma-treated mice are associated with alterations in basic properties of glutamatergic transmission and the enhanced activity of voltage-gated Ca2 + channels.

CONCLUSION

Together, the data presented in this study show that blood-borne factors are sufficient to drive neurophysiological impairments in the hippocampal CA1.

Brain Trauma, Glucocorticoids and Neuroinflammation: Dangerous Liaisons for the Hippocampus

Glucocorticoid-dependent mechanisms of inflammation-mediated distant hippocampal damage are discussed with a focus on the consequences of traumatic brain injury. The effects of glucocorticoids on specific neuronal populations in the hippocampus depend on their concentration, duration of exposure and cell type. Previous stress and elevated level of glucocorticoids prior to pro-inflammatory impact, as well as long-term though moderate elevation of glucocorticoids, may inflate pro-inflammatory effects. Glucocorticoid-mediated long-lasting neuronal circuit changes in the hippocampus after brain trauma are involved in late post-traumatic pathology development, such as epilepsy, depression and cognitive impairment. Complex and diverse actions of the hypothalamic–pituitary–adrenal axis on neuroinflammation may be essential for late post-traumatic pathology. These mechanisms are applicable to remote hippocampal damage occurring after other types of focal brain damage (stroke, epilepsy) or central nervous system diseases without obvious focal injury. Thus, the liaisons of excessive glucocorticoids/dysfunctional hypothalamic–pituitary–adrenal axis with neuroinflammation, dangerous to the hippocampus, may be crucial to distant hippocampal damage in many brain diseases. Taking into account that the hippocampus controls both the cognitive functions and the emotional state, further research on potential links between glucocorticoid signaling and inflammatory processes in the brain and respective mechanisms is vital.

Therapeutic implications of hypothalamic-pituitaryadrenal-axis modulation in Alzheimer's disease: A narrative review of pharmacological and lifestyle interventions

With disease-modifying treatments for Alzheimer's disease (AD) still elusive, the search for alternative intervention strategies has intensified. Growing evidence suggests that dysfunction in hypothalamic-pituitaryadrenal-axis (HPAA) activity may contribute to the development of AD pathology. The HPAA, may therefore offer a novel target for therapeutic action. This review summarises and critically evaluates animal and human studies investigating the effects of pharmacological and non-pharmacological intervention on HPAA modulation alongside cognitive performance. The interventions discussed include glucocorticoid receptor antagonists and 11β-hydroxysteroid dehydrogenase inhibitors as well as lifestyle treatments such as physical activity, diet, sleep and contemplative practices. Pharmacological HPAA modulators improve pathology and cognitive deficit in animal AD models, but human pharmacological trials are yet to provide definitive support for such benefits. Lifestyle interventions may offer promising strategies for HPAA modification and cognitive health, but several methodological caveats across these studies were identified. Directions for future research in AD studies are proposed.

Cav1.2 Activity and Downstream Signaling Pathways in the Hippocampus of An Animal Model of Depression

Functional and morphological modifications in the brain caused by major mood disorders involve many brain areas, including the hippocampus, leading to cognitive and mood alterations. Ca_v1.2 channel expression has been found to increase in animals with depressive-like behaviors. Calcium influx through these channels is associated with changes in excitation-transcriptional coupling by several intracellular signal pathways that are regulated by its C-terminus region. However, which of these signaling pathways is activated during the development of depressive-like behaviors is not known. Here, we evaluate the phosphorylation and expression levels of crucial kinases and transcription factors at the hippocampus of rats after 21 days of chronic restraint stress. Our results show that rats subjected to CRS protocol achieve less body weight, have heavier adrenal glands, and exhibit depression-like behaviors such as anhedonia, behavioral despair and decreased social interaction. Ca_v1.2 mRNA and protein expression levels, plus l-type calcium current amplitude, are also increased in treated rats when compared with control animals. Out of the three main signaling pathways activated by l-type currents, we only observed an increment of CaM-NFAT axis activity with the concomitant increment in Fas ligand expression. Thus, our results suggest that CRS activates specific pathways, and the increased expression of Ca_v1.2 could lead to neuronal death in the hippocampus.

Glucocorticoid receptor activation induces decrease of hippocampal astrocyte number in rats

RATIONALE

The decrease of astrocyte number and hypothalamic-pituitary-adrenal (HPA) axis overactivity are observed in individuals with major depressive disorder. Elevated levels of glucocorticoids induced by hyperactivation of the HPA axis may result in glucocorticoid receptor (GR) activation. However, it is unclear whether there is a direct link between GR activation and the decrease of astrocyte number.

METHODS

Animals were exposed to chronic unpredictable stress (CUS) for 28 days and treated with continuous subcutaneous injections of vehicle or corticosterone (CORT; 40 mg/kg/day) for 21 days. We then administered mifepristone on day 21 after CUS and on day 18 after the CORT treatment. We observed behavioral deficits in the sucrose preference test, open field test, and forced swim test. Protein expression was analyzed using immunofluorescence (IF) and western blot (WB).

RESULTS

Animals exposed to CUS exhibited behavioral deficits in tests measuring anhedonia, anxiety, and despair state. They also had decreases in glial fibrillary acidic protein (GFAP) expression and numbers of GFAP-positive cells in the hippocampus. The behavioral and cellular alterations induced by CUS were reversed by subchronic treatment with the GR antagonist mifepristone. We also found that the subcutaneous injection of glucocorticoids may induce depression-like behavior and reduce GFAP protein expression in rats, which was similarly reversed by mifepristone.

CONCLUSIONS

These findings provide experimental evidence that GR activation due to elevated CORT levels induces the decrease of hippocampal astrocyte number in rats.

The effects of CCK-8S on spatial memory and long-term potentiation at CA1 during induction of stress in rats

Objectives

Cholecystokinin (CCK) has been proposed as a mediator in stress. However, it is still not fully documented what are its effects. We aimed to evaluate the effects of systemic administration of CCK exactly before induction of stress on spatial memory and synaptic plasticity at CA1 in rats.

Materials and Methods

Male Wistar rats were divided into 4 groups: the control, the control-CCK, the stress and the stress-CCK. Restraint stress was induced 6 hr per day, for 24 days. Cholecystokinin sulfated octapeptide (CCK-8S) was injected (1.6 µg/kg, IP) before each session of stress induction. Spatial memory was evaluated by Morris water maze test. Long-term potentiation (LTP) in Schaffer collateral-CA1 synapses was assessed (by 100 Hz tetanization) in order to investigate synaptic plasticity.

Results

Stress impaired spatial memory significantly (P<0.01). CCK in the control rats improved memory (P<0.05), and prevented the impairments in the stress group. With respect to the control group, both fEPSP amplitude and slope were significantly (P<0.05) decreased in the stress group. However, there were no differences between responses of the control-CCK and Stress-CCK groups compared to the control group.

Conclusion

The present results suggest that high levels of CCK-8S during induction of stress can modulate the destructive effects of stress on hippocampal synaptic plasticity and memory. Therefore, the mediatory effects of CCK in stress are likely as compensatory responses.

Effects of Isx-9 and stress on adult hippocampal neurogenesis: Experimental considerations and future perspectives

The development of synthetic small molecules capable of promoting neuronal fate in stem cells is a promising strategy to prevent the decline of hippocampal function caused by several neurological disorders. Within this context, isoxazole 9 (Isx-9) has been shown to strongly induce cell proliferation and neuronal differentiation in the subgranular zone (SGZ) of the hippocampal dentate gyrus (DG), while also improving hippocampal function in healthy mice. We have recently demonstrated that Isx-9 is able to restore normal neurogenesis levels after procedural stress. Here, we further discuss these findings highlighting the importance of including a naïve group in studies investigating the effects of either restraint stress or mild chronic unpredictable stress (CUS) on adult hippocampal neurogenesis.

Fractalkine receptor deficiency impairs microglial and neuronal responsiveness to chronic stress

Chronic stress is one of the most relevant triggering factors for major depression. Microglial cells are highly sensitive to stress and, more generally, to environmental challenges. However, the role of these brain immune cells in mediating the effects of stress is still unclear. Fractalkine signaling - which comprises the chemokine CX3CL1, mainly expressed by neurons, and its receptor CX3CR1, almost exclusively present on microglia in the healthy brain - has been reported to critically regulate microglial activity. Here, we investigated whether interfering with microglial function by deleting the Cx3cr1 gene affects the brain's response to chronic stress. To this purpose, we housed Cx3cr1 knockout and wild-type adult mice in either control or stressful environments for 2weeks, and investigated the consequences on microglial phenotype and interactions with synapses, synaptic transmission, behavioral response and corticosterone levels. Our results show that hampering neuron-microglia communication via the CX3CR1-CX3CL1 pathway prevents the effects of chronic unpredictable stress on microglial function, short- and long-term neuronal plasticity and depressive-like behavior. Overall, the present findings suggest that microglia-regulated mechanisms may underlie the differential susceptibility to stress and consequently the vulnerability to diseases triggered by the experience of stressful events, such as major depression.

Glucocorticoid actions on synapses, circuits, and behavior: implications for the energetics of stress

Environmental stimuli that signal real or potential threats to homeostasis lead to glucocorticoid secretion by the hypothalamic-pituitary-adrenocortical (HPA) axis. Glucocorticoids promote energy redistribution and are critical for survival and adaptation. This adaptation requires the integration of multiple systems and engages key limbic-neuroendocrine circuits. Consequently, glucocorticoids have profound effects on synaptic physiology, circuit regulation of stress responsiveness, and, ultimately, behavior. While glucocorticoids initiate adaptive processes that generate energy for coping, prolonged or inappropriate glucocorticoid secretion becomes deleterious. Inappropriate processing of stressful information may lead to energetic drive that does not match environmental demand, resulting in risk factors for pathology. Thus, dysregulation of the HPA axis may promote stress-related illnesses (e.g. depression, PTSD). This review summarizes the latest developments in central glucocorticoid actions on synaptic, neuroendocrine, and behavioral regulation. Additionally, these findings will be discussed in terms of the energetic integration of stress and the importance of context-specific regulation of glucocorticoids.

A single-day treatment with mifepristone is sufficient to normalize chronic glucocorticoid induced suppression of hippocampal cell proliferation

Background

Chronic stress or prolonged administration of glucocorticoids suppresses proliferation and/or survival of newborn cells in adult rat dentate gyrus. Earlier we showed that administration of the glucocorticoid receptor antagonist mifepristone during the final 4 days of a 21 days period of corticosterone treatment fully normalized the number of newborn cells. Here we aimed to better understand how mifepristone achieves this effect and questioned whether an even shorter (single day) mifepristone treatment (instead of 4 days) also suffices to normalize neurogenesis.

Methods

We investigated various steps of the neurogenic process, using the immunohistochemical markers BrdU, doublecortin, proliferating cell nuclear antigen as well as glial fibrillary acidic protein, after 17 or 21 days of corticosterone (versus vehicle) treatment.

Results

Corticosterone primarily attenuates the proliferation of cells which subsequently develop into neurons; this is fully reversed by mifepristone. Surprisingly, the corticosteroid effects on neurogenesis can even be fully re-set by a single-day treatment with mifepristone (on day 18), despite the continued corticosterone exposure on subsequent days.

Conclusions

Our results emphasize that studies into the therapeutical efficacy of new antidepressants, especially those targeting HPA-activity or the glucocorticoid receptor, should explore the possibility to reduce treatment duration.

Corticosteroid effects on calcium signaling in limbic neurons

Corticosteroid hormones, which are released in high amounts after stress, easily pass the blood-brain-barrier. In the brain they bind to intracellular receptors which act as transcriptional regulators. These receptors are highly expressed in neurons of the hippocampal formation and the amygdala, areas that play a role in (emotional) memory formation. Voltage gated Ca(2+) channels are among the most prominent targets of corticosteroid hormones. When the levels of corticosterone - the prevalent corticosteroid in rats and mice- are low, L-type Ca(2+) currents of CA1 hippocampal cells are small. However, when hormone levels rise e.g. after stress, the amplitude of L-type Ca(2+) currents will be slowly enhanced, through a process requiring DNA binding of glucocorticoid receptor homodimers. Kinetic properties and voltage dependency of the currents remain unchanged. Neurons in the basolateral amygdala respond in a comparable fashion, but Ca(2+) currents of neurons in the dentate gyrus are unaffected by corticosteroids. The stress-induced increase in Ca(2+) influx has considerable functional consequences in health and disease. At the short term, i.e. 1-4h after stress, the enhanced Ca(2+) influx contributes to stronger firing frequency accommodation and a higher threshold for the induction of long-term potentiation. This helps to normalize neuronal activity after stress and presumably protects earlier encoded, stress-related information. At the longer term, though, increased Ca(2+) load may impose a risk, increasing the vulnerability of limbic cells to additional challenges e.g. during epileptic or ischemic episodes.

The effect of different maternal deprivation paradigms on the expression of hippocampal glucocorticoid receptors, calretinin and calbindin-D28k in male and female adolescent rats

Maternal deprivation (MD) is a well-established protocol used to investigate neurobiological changes that are associated with the etiology of and vulnerability to stress-related diseases in animal models. The resulting psychophysiological effects, the timing and duration of these adverse stimuli, and the method by which they exert their effects on the animals remain unclear. This study characterized differences in the hippocampal expression of glucocorticoid receptors (GRs) and the calcium-binding proteins calretinin (CALR) and calbindin-D28k (CALB) in male and female rats that underwent different MD paradigms during the stress hyporesponsive period (SHRP). Both GRs and the two calcium-binding proteins were much more abundant in females than in males. MD paradigms had a significant effect on CALR and CALB expression in both males and females but affected GR levels only in males. Additionally, expression of the two calcium-binding proteins in the hippocampus responded differently to MD-induced stress, especially in females. Taken together, these results indicate that females are able to modulate their response to stress better than males.

Impact of glucocorticoids on brain function: relevance for mood disorders

Exposure to stressful situations activates two hormonal systems that help the organism to adapt. On the one hand stress hormones achieve adaptation by affecting peripheral organs, on the other hand by altering brain function such that appropriate behavioral strategies are selected for optimal performance at the short term, while relevant information is stored for reference in the future. In this chapter we describe how cellular effects induced by stress hormones--in particular by glucocorticoids--may contribute to the behavioral outcome after a single stressor. In addition to situations of acute stress, chronic uncontrollable and unpredictable stress also exerts profound effects on structure and function of limbic neurons. The impact of chronic stress is not a mere cumulative effect of what is seen after acute stress exposure. Dendritic trees are expanded in some regions but reduced in others. In general, cells are exposed to a higher calcium load upon depolarization, but show attenuated responses to serotonin. Synaptic strengthening is largely impaired. In this viewpoint we speculate how cellular effects after chronic stress may be maladaptive and could contribute to the development of psychopathology in genetically vulnerable individuals.

The transcriptional response to chronic stress and glucocorticoid receptor blockade in the hippocampal dentate gyrus

The dentate gyrus (DG) of the hippocampus plays a crucial role in learning and memory. This subregion is unique in its ability to generate new neurons throughout life and integrate these new neurons into the hippocampal circuitry. Neurogenesis has further been implicated in hippocampal plasticity and depression. Exposure to chronic stress affects DG function and morphology and suppresses neurogenesis and long-term potentiation (LTP) with consequences for cognition. Previous studies demonstrated that glucocorticoid receptor (GR) blockade by a brief treatment with the GR antagonist mifepristone (RU486) rapidly reverses the stress and glucocorticoid effects on neurogenesis. The molecular pathways underlying both the stress-induced effects and the RU486 effects on the DG are, however, largely unknown. The aim of this study was therefore (1) to investigate by microarray analysis which genes and pathways in the DG are sensitive to chronic stress and (2) to investigate to what extent blockade of GR can normalize these stress-induced effects on DG gene expression. Chronic stress exposure affected the expression of 90 genes in the DG (P < 0.01), with an overrepresentation of genes involved in brain development and morphogenesis and synaptic transmission. RU486 treatment of stressed animals affected expression of 107 genes; however, mostly different genes than those responding to stress. Interestingly, we found CREBBP to be normalized by RU486 treatment to levels observed in control animals, suggesting that CREB-signaling may play a central role in mediating the chronic stress effects on neurogenesis, LTP and calcium currents. The identified genetic pathways provide insight into the stress-induced adaptive plasticity of the hippocampal DG that is so central in learning and memory and will direct future studies on the functional outcome and modulation of these stress effects.

Chronic stress effects on hippocampal structure and synaptic function: relevance for depression and normalization by anti-glucocorticoid treatment

Exposure of an organism to environmental challenges activates two hormonal systems that help the organism to adapt. As part of this adaptational process, brain processes are changed such that appropriate behavioral strategies are selected that allow optimal performance at the short term, while relevant information is stored for the future. Over the past years it has become evident that chronic uncontrollable and unpredictable stress also exerts profound effects on structure and function of limbic neurons, but the impact of chronic stress is not a mere accumulation of repeated episodes of acute stress exposure. Dendritic trees are reduced in some regions but expanded in others, and cells are generally exposed to a higher calcium load upon depolarization. Synaptic strengthening is largely impaired. Neurotransmitter responses are also changed, e.g., responses to serotonin. We here discuss: (a) the main cellular effects after chronic stress with emphasis on the hippocampus, (b) how such effects could contribute to the development of psychopathology in genetically vulnerable individuals, and (c) their normalization by brief treatment with anti-glucocorticoids.

Disrupted corticosterone pulsatile patterns attenuate responsiveness to glucocorticoid signaling in rat brain

Chronically elevated circulating glucocorticoid levels are although to enhance vulnerability to psychopathology. Here we hypothesized that such sustained glucocorticoid levels, disturbing corticosterone pulsatility, attenuate glucocorticoid receptor signaling and target gene responsiveness to an acute challenge in the rat brain. Rats were implanted with vehicle or 40 or 100% corticosterone pellets known to flatten ultradian and circadian rhythmicity while maintaining daily average levels or mimic pathological conditions. Additionally, recovery from constant exposure was studied in groups that had the pellet removed 24 h prior to the challenge. Molecular markers for receptor responsiveness (receptor levels, nuclear translocation, promoter occupancy, and target gene expression) to an acute challenge mimicking the stress response (3 mg/kg ip) were studied in the hippocampal area. Implantation of 40 and 100% corticosterone pellets dose-dependently down-regulated glucocorticoid receptor and attenuated mineralocorticoid receptor and glucocorticoid receptor translocation to the acute challenge. Interestingly, whereas target gene Gilz expression to the challenge was already attenuated by tonic daily average levels (40%), Sgk-1 was affected only after constant high corticosterone exposure (100%), indicating altered receptor responsiveness due to treatment. Washout of 100% corticosterone recovered all molecular markers (partial), whereas removal of the 40% corticosterone pellet still attenuated responsiveness to the challenge. We propose that corticosteroid pulsatility is crucial in maintaining normal responsiveness to glucocorticoids. Whereas the results with 100% corticosterone are likely attributed to receptor saturation, subtle changes in the pattern of exposure (40%) induces changes at least as severe for glucocorticoid signaling as overt hypercorticism, suggesting an underlying mechanism sensitive to the pattern of hormone exposure.

Dissociation between rat hippocampal CA1 and dentate gyrus cells in their response to corticosterone: effects on calcium channel protein and current

Stress and corticosterone affect, via glucocorticoid receptors, cellular physiology in the rodent brain. A well-documented example concerns corticosteroid effects on high-voltage activated (L type) calcium currents in the hippocampal CA1 area. We tested whether corticosterone also affects calcium currents in another hippocampal area that highly expresses glucocorticoid receptors, i.e. the dentate gyrus (DG). Remarkably, corticosterone (100 nm, given for 20 min, 1-4.5 hr before recording) did not change high-voltage activated calcium currents in the DG, whereas currents in the CA1 area of the same rats were increased. Follow-up studies revealed that no apparent dissociation between the two areas was observed with respect to transcriptional regulation of calcium channel subunits; thus, in both areas corticosterone increased mRNA levels of the calcium channel-beta4 but not the (alpha) Ca(v)1.2 subunit. At the protein level, however, beta4 and Ca(v)1.2 levels were significantly up-regulated by corticosterone in the CA1 but not the DG area. These data suggest that stress-induced elevations in the level of corticosterone result in a regionally differentiated physiological response that is not simply determined by the glucocorticoid receptor distribution and that the observed regional differentiation may be caused by a gene involved in the translational machinery or in mechanisms regulating mRNA or protein stability.

Corticosteroid effects on cellular physiology of limbic cells

After stress, circulating levels of stress hormones such as corticosterone are markedly increased. This will have an impact on the neurophysiology of limbic neurons that highly express corticosteroid receptors. Over the past decades several principles about the neurophysiological impact of corticosterone have emerged. First, corticosterone can quickly raise the excitability of hippocampal CA1 neurons shortly after stress exposure, via a nongenomic pathway involving mineralocorticoid receptors presumably located in the pre- as well as postsynaptic membrane. At the same time, gene-mediated actions via the glucocorticoid receptor are started which some hours later will result in enhanced calcium influx and impaired ability to induce long-term potentiation. These delayed actions are interpreted as a means to slowly normalize hippocampal activity and preserve information encoded early on after stress. Second, the full spectrum of neurophysiological actions by corticosterone is accomplished in interaction with other stress mediators, like noradrenaline. Third, these effects in the CA1 hippocampal region cannot be generalized to other brain regions such as the basolateral amygdala or paraventricular nucleus: There seems to be a highly differentiated response, which could serve to facilitate neuroendocrine/cognitive processing of some aspects of stress-related information, but attenuate other aspects. Finally, the time- and region-specific corticosteroid actions strongly depend on the individual's life history.

Glucocorticoid receptor blockade normalizes hippocampal alterations and cognitive impairment in streptozotocin-induced type 1 diabetes mice

Type 1 diabetes is a common metabolic disorder accompanied by an increased secretion of glucocorticoids and cognitive deficits. Chronic excess of glucocorticoids per se can evoke similar neuropathological signals linked to its major target in the brain, the hippocampus. This deleterious action exerted by excess adrenal stress hormone is mediated by glucocorticoid receptors (GRs). The aim of the present study was to assess whether excessive stimulation of GR is causal to compromised neuronal viability and cognitive performance associated with the hippocampal function of the diabetic mice. For this purpose, mice had type 1 diabetes induced by streptozotocin (STZ) administration (170 mg/kg, i.p.). After 11 days, these STZ-diabetic mice showed increased glucocorticoid secretion and hippocampal alterations characterized by: (1) increased glial fibrillary acidic protein-positive astrocytes as a marker reacting to neurodegeneration, (2) increased c-Jun expression marking neuronal activation, (3) reduced Ki-67 immunostaining indicating decreased cell proliferation. At the same time, mild cognitive deficits became obvious in the novel object-placement recognition task. After 6 days of diabetes the GR antagonist mifepristone (RU486) was administered twice daily for 4 days (200 mg/kg, p.o.). Blockade of GR during early type 1 diabetes attenuated the morphological signs of hippocampal aberrations and rescued the diabetic mice from the cognitive deficits. We conclude that hippocampal disruption and cognitive impairment at the early stage of diabetes are caused by excessive GR activation due to hypercorticism. These signs of neurodegeneration can be prevented and/or reversed by GR blockade with mifepristone.

Stress, the hippocampus, and epilepsy

Stress is among the most frequently self-reported precipitants of seizures in patients with epilepsy. This review considers how important stress mediators like corticotropin-releasing hormone, corticosteroids, and neurosteroids could contribute to this phenomenon. Cellular effects of stress mediators in the rodent hippocampus are highlighted. Overall, corticosterone--with other stress hormones--rapidly enhances CA1/CA3 hippocampal activity shortly after stress. At the same time, corticosterone starts gene-mediated events, which enhance calcium influx several hours later. This later effect serves to normalize activity but also imposes a risk for neuronal injury if and when neurons are concurrently strongly depolarized, for example, during epileptic activity. In the dentate gyrus, stress-induced elevations in corticosteroid level are less effective in changing membrane properties such as calcium influx; here, enhanced inhibitory tone mediated through neurosteroid effects on gamma-aminobutyric acid (GABA) receptors might dominate. Under conditions of repetitive stress (e.g., caused from experiencing repetitive and unpredictable seizures) and/or early life stress, hormonal influences on the inhibitory tone, however, are diminished; instead, enhanced calcium influx and increased excitation become more important. In agreement, perinatal stress and elevated steroid levels accelerate epileptogenesis and lower seizure threshold in various animal models for epilepsy. It will be interesting to examine how curtailing the effects of stress in adults, for example, by brief treatment with antiglucocorticoids, may be beneficial to the treatment of epilepsy.

Stress and glucocorticoid footprints in the brain-the path from depression to Alzheimer's disease

Increasingly, stress is recognized as a trigger of depressive episodes and recent evidence suggests a causal role of stress in the onset and progression of Alzheimer's disease (AD) pathology. Besides aging, sex is an important determinant of prevalence rates for both AD and mood disorders. In light of a recent meta-analysis indicating that depressed subjects have a higher likelihood of developing AD, a key message in this article will be that both depression and AD are stress-related disorders and may represent a continuum that should receive more attention in future neurobiological studies. Accordingly, this review considers some of the cellular mechanisms that may be involved in regulating this transition threshold. In addition, it highlights the importance of addressing the question of how aging and sex interplay with stress to influence mood and cognition, with a bias towards consideration of neuroplastic events in particular brain regions, as the basis of AD and depressive disorders.

Opposite effects of glucocorticoid receptor activation on hippocampal CA1 dendritic complexity in chronically stressed and handled animals

Remodeling of synaptic networks is believed to contribute to synaptic plasticity and long-term memory performance, both of which are modulated by chronic stress. We here examined whether chronic stress modulates dendritic complexity of hippocampal CA1 pyramidal cells, under conditions of basal as well as elevated corticosteroid hormone levels. Slices were prepared from naïve, handled or chronically stressed animals and briefly treated with vehicle or corticosterone (100 nM); neurons were visualized with a fluorescent dye injected into individual CA1 pyramidal cells. We observed that 21 days of unpredictable stress did not affect hippocampal CA1 apical or basal dendritic morphology compared with naïve animals when corticosteroid levels were low. Only when slices from stressed animals were also exposed to elevated corticosteroid levels, a significant reduction in apical (but not basal) dendritic length became apparent. Unexpectedly, animals that were handled or 3 weeks showed a reduction in both apical dendritic length and number of apical branch points when compared with naïve animals. Apical dendritic length and number of branch points were restored to levels found in naïve animals several hours after in vitro treatment with 100 nM corticosterone. All effects of acute corticosterone administration could be prevented by the glucocorticoid receptor antagonist RU38486 given during the last 4 days of the stress or handling protocol. We conclude that brief exposure to high concentrations of corticosterone can differently affect apical dendritic structure, depending on the earlier history of the animal, a process that critically depends on involvement of the glucocorticoid receptor.

Functional actions of corticosteroids in the hippocampus

Corticosteroid hormones are released in high amounts after stress. The hormones enter the brain compartment and bind to high affinity mineralocorticoid receptors--particularly enriched in limbic regions--as well as to lower affinity glucocorticoid receptors which are more ubiquitous. Shortly after the stressful event, corticosteroids (in concert with specific monoamines and neuropeptides) have the potential to increase cellular excitability in subfields of the hippocampus, like the CA1 area. These effects are rapid in onset and occur via a nongenomic pathway. At the same time, however, the hormones also start slower, gene-mediated processes. These cause attenuation of excitatory information flow through the CA1 hippocampal area. Induction of long-term potentiation at that time is impaired. This may help to normalize hippocampal activity some hours after the stressful event and preserve information encoded within the context of the event. These adaptational effects of the hormones may become maladaptive if the stressful event is associated with other challenges of the network (like ischemic insults) or when stress occurs repetitively, in an uncontrollable and unpredictable manner. In that case, i) normalization of activity seems to be less efficient (particularly when other limbic areas like the amygdala nuclei are activated during stress), ii) induction of long-term potentiation is hampered at all times and iii) serotonin responses are attenuated. This may contribute to the precipitation of clinical symptoms in stress-related disorders such as major depression. A better understanding of the corticosteroid actions could lead to a more rational treatment strategy of stress-related disorders.

Brief treatment with the glucocorticoid receptor antagonist mifepristone normalizes the reduction in neurogenesis after chronic stress

In rodents, stress suppresses adult neurogenesis. This is thought to involve activation of glucocorticoid receptors in the brain. In the present study, we therefore questioned whether glucocorticoid receptor blockade by mifepristone can normalize the effects of chronic stress on adult neurogenesis. Rats received mifepristone on the last 4 days of a 21-day chronic unpredictable and inescapable stress regimen. Neurogenesis was analysed by stereological quantification of adult-generated cell survival (bromodeoxyuridine), young neuronal survival (doublecortin) and cell proliferation (Ki-67). The results show that only 4 days of mifepristone treatment normalized the stress-induced reductions in neurogenesis. Importantly, mifepristone by itself had no effect on neurogenesis. We conclude that, contrary to other compounds interfering with the effects of chronic stress on neurogenesis, like antidepressants, the normalizing effects of mifepristone on neurogenesis are rapid and particularly potent in a high stress environment. This neurogenic action of mifepristone could potentially contribute to its clinical mechanism of action.

Chronic stress: implications for neuronal morphology, function and neurogenesis

In normal life, organisms are repeatedly exposed to brief periods of stress, most of which can be controlled and adequately dealt with. The presently available data indicate that such brief periods of stress have little influence on the shape of neurons or adult neurogenesis, yet change the physiological function of cells in two time-domains. Shortly after stress excitability in limbic areas is rapidly enhanced, but also in brainstem neurons which produce catecholamines; collectively, during this phase the stress hormones promote focused attention, alertness, vigilance and the initial steps in encoding of information linked to the event. Later on, when the hormone concentrations are back to their pre-stress level, gene-mediated actions by corticosteroids reverse and normalize the enhanced excitability, an adaptive response meant to curtail defense reactions against stressors and to enable further storage of relevant information. When stress is experienced repetitively in an uncontrollable and unpredictable manner, a cascade of processes in brain is started which eventually leads to profound, region-specific alterations in dendrite and spine morphology, to suppression of adult neurogenesis and to inappropriate functional responses to a brief stress exposure including a sensitized activation phase and inadequate normalization of brain activity. Although various compounds can effectively prevent these cellular changes by chronic stress, the exact mechanism by which the effects are accomplished is poorly understood. One of the challenges for future research is to link the cellular changes seen in animal models for chronic stress to behavioral effects and to understand the risks they can impose on humans for the precipitation of stress-related disorders.