Glucocorticoids increase extracellular [3H]D-aspartate overflow in hippocampal cultures during cyanide-induced ischemia

Ameliorative effect of Yokukansan on social isolation-induced aggressive behavior of zinc-deficient young mice

Yokukansan, a traditional Japanese medicine has been used to cure neuropsychological disorders. In the present study, the effect of Yokukansan on social isolation-induced aggressive behavior was examined in zinc-deficient mice, which were fed a zinc-deficient diet and a drinking water containing Yokukansan for 2 weeks. In the resident-intruder test, the rate of mice that exhibited aggressive behavior in zinc-deficient mice, which was significantly higher than that in the control mice, was significantly decreased by administration of Yokukansan. The basal level of serum glucocorticoid, which was significantly higher in zinc-deficient mice, was lowered by administration of Yokukansan. On the other hand, serum glucocorticoid levels after the resident-intruder test were almost the same between the control and zinc-deficient mice. However, administration of Yokukansan to zinc-deficient mice significantly increased serum glucocorticoid level after the resident-intruder test and the significant difference in the rate of serum corticosterone level after the test to the basal level between the control and zinc-deficient mice was abolished. Dietary zinc deficiency increases the basal levels of serum glucocorticoid, while may insufficiently increase serum glucocorticoid levels in the resident-intruder test. The concentrations of glutamate and GABA (γ-aminobutyric acid) in the brain were significantly higher in zinc-deficient mice, while Yokukansan ameliorated the significant increases. These results indicate that Yokukansan ameliorates social isolation-induced aggressive behavior of zinc-deficient mice, probably via amelioration of abnormal glucocorticoid secretion. The ameliorative effect seems to be linked to the modification of glutamatergic neuron activity after administration of Yokukansan.

Effects of depressive-like behavior of rats on brain glutamate uptake

Learned helplessness paradigm is a widely accepted animal model of depressive-like behavior based on stress. Glutamatergic system is closely involved with the stress-neurotoxicity in the brain and recently it is pointed to have a relevant role in the pathophysiology of depression disorder. Glutamate uptake is the main mechanism to terminate the glutamatergic physiological activity and to neuroprotection against excitotoxicity. We investigated the profile of glutamate uptake in female rats submitted to the learned helplessness paradigm and to different classes of stress related to the paradigm, in slices of brain cortex, striatum and hippocampus. Glutamate uptake in slices of hippocampus differ between learned helplessness (LH) and non-learned helplessness (NLH) animals immediately persisting up to 21 days after the paradigm. In addition, there were a decrease of glutamate uptake in the three brain structures analyzed at 21 days after the paradigm for LH animals. These results may contribute to better understand the role of the glutamatergic system on the depressive-like behavior.

Blocking glucocorticoid and enhancing estrogenic genomic signaling protects against cerebral ischemia

Glucocorticoids (GCs) and estrogen can modulate neuron death and dysfunction during neurological insults. Glucocorticoids are adrenal steroids secreted during stress, and hypersecretion of GCs during cerebral ischemia compromises the ability of hippocampal and cortical neurons to survive. In contrast, estrogen can be neuroprotective after cerebral ischemia. Here we evaluate the protective potential of a herpes viral vector expressing a chimeric receptor (ER/GR), which is composed of the ligand-binding domain of the GC receptor (GR) and the DNA-binding domain of the estrogen receptor-alpha (ER). This novel receptor can transduce an endangering GC signal into a protective estrogenic one. Using an in vitro oxygen glucose deprivation model (OGD), GCs exacerbated neuron death in primary cortical cultures, and this worsening effect was completely blocked by ER/GR expression. Moreover, blocking GC actions with a vector expressing a dominant negative GC receptor promoted neuron survival during postischemia, but not preischemia. Thus, gene therapeutic strategies to modulate GC and estrogen signaling can be beneficial during an ischemic insult.

Chronic unpredictable stress exacerbates lipopolysaccharide-induced activation of nuclear factor-kappaB in the frontal cortex and hippocampus via glucocorticoid secretion

Although the anti-inflammatory actions of glucocorticoids (GCs) are well established in the periphery, these stress hormones can increase inflammation under some circumstances in the brain. The transcription factor nuclear factor-kappaB (NF-kappaB), which is inhibited by GCs, regulates numerous genes central to inflammation. In this study, the effects of stress, GCs, and NMDA receptors on lipopolysaccharide (LPS)-induced activation of NF-kappaB in the brain were investigated. One day after chronic unpredictable stress (CUS), nonstressed and CUS rats were treated with saline or LPS and killed 2 h later. CUS potentiated the increase in LPS-induced activation of NF-kappaB in frontal cortex and hippocampus but not in the hypothalamus. This stress effect was blocked by pretreatment of rats with RU-486, an antagonist of the GC receptor. MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo [a,d] cyclohepten-5,10-imine maleate], an NMDA receptor antagonist, also reduced the effect of LPS in all three brain regions. However, the combined antagonism of both GC and NMDA receptors produced no further reduction in NF-kappaB activation when compared with the effect of each treatment alone. Our results indicate that stress, via GC secretion, can increase LPS-induced NF-kappaB activation in the frontal cortex and hippocampus, agreeing with a growing literature demonstrating proinflammatory effects of GCs.

Effects of glucocorticoids in the gp120-induced inhibition of glutamate uptake in hippocampal cultures

Studies examining the development of AIDS Related Dementia have concentrated on neurotoxic properties of the HIV viral coat protein, gp120. We have previously shown that this neurotoxicity can be exacerbated by glucocorticoids (GCs), the stress hormones secreted by the adrenal. Moreover, GCs also worsen several of the mechanisms mediating gp120 neurotoxicity, such as increased calcium flux, ROS generation, and energy depletion. Gp120 interferes with the reuptake of glutamate in glia cultures, another possible mechanism by which it can be neurotoxic. This paper examines the role of GCs in exacerbating this phenomenon. It was found that while GCs do not exacerbate the decrease in reuptake of glutamate in glia cultures, they do enhance the decrease in mixed neuronal cultures and this latter effect appears to be energy-dependent.

Stress and depression: possible links to neuron death in the hippocampus

Recent intriguing reports have shown an association between major depression and selective and persistent loss of hippocampal volume, prompting considerable speculation as to its underlying causes. In this paper we focus on the hypothesis that overt hippocampal neuron death could cause this loss and review current knowledge about how hippocampal neurons die during insults. We discuss (a) the trafficking of glutamate and calcium during insults; (b) oxygen radical generation and programmed cell death occurring during insults; (c) neuronal defenses against insults; (d) the role of energy availability in modulating the extent of neuron loss following such insults. The subtypes of depression associated with hippocampal atrophy typically involve significant hypersecretion of glucocorticoids, the adrenal steroids secreted during stress. These steroids have a variety of adverse affects, direct and indirect, in the hippocampus. Thus glucocorticoids may play a contributing role toward neuron death. We further discuss how glucocorticoids cause or exacerbate cellular changes associated with hippocampal neuron loss in the context of the events listed above.

Neuroprotective effects of an adenoviral vector expressing the glucose transporter: a detailed description of the mediating cellular events

Considerable knowledge exists concerning the events mediating neuron death following a necrotic insult; prompted by this, there have now been successful attempts to use gene therapy approaches to protect neurons from such necrotic injury. In many such studies, however, it is not clear what sequence of cellular events connects the overexpression of the transgene with the enhanced survival. We do so, exploring the effects of overexpressing the Glut-1 glucose transporter with an adenoviral vector in hippocampal cultures challenged with the excitotoxin kainic acid (KA). Such overexpression enhanced glucose transport, attenuated the decline in ATP concentrations, decreased the release of excitatory amino acid neurotransmitters, and decreased the total free cytosolic calcium load. Commensurate with these salutary effects, neuronal survival was enhanced with this gene therapy intervention. Thus, the neuroprotective effects of this particular gene therapy occurs within the known framework of the mechanisms of necrotic neuronal injury.

Corticosterone inhibits generation of long-term potentiation in rat hippocampal slice: involvement of brain-derived neurotrophic factor

In the present study, the effect of corticosterone (CORT) on the generation of long-term potentiation (LTP) and its underlying mechanism involving neurotrophin gene expression in CA1 synapses of rat hippocampal slice were examined. Our experimental results showed incubation of hippocampal slice with CORT for 3 h had no effect on either the slope or amplitude of excitatory postsynaptic potentials (EPSP) evoked in hippocampal CA1 pyramidal dentrites, indicating no marked change in basal synaptic transmission. However, when tetanic stimulation (100 pulses, 100 Hz) was delivered to the Schaffer collateral pathway, CORT application significantly attenuated the tetanus-induced increases of both EPSP slope and amplitude, demonstrating an inhibitory effect of CORT on LTP generation. In addition, CORT treatment significantly reduced both slope and amplitude ratios of the second evoked EPSP to the first one when paired-pulse facilitation (PPF) was established at different interpulse intervals from 20 to 40 ms, suggesting that a presynaptic mechanism may be involved in CORT-induced hippocampal synaptic plasticity. Reverse-transcription polymerase chain reaction (RT-PCR) analysis showed that CORT-treated hippocampal CA1 cells underwent a significant decrease in the expression of mRNA for nerve growth factor-beta (NGF-beta) and brain-derived neurotrophic factor (BDNF), but not for neurotrophin-3 (NT-3) compared with those in control. Moreover, BDNF co-applied with CORT significantly antagonized CORT-induced deficit in PPF. Taken together, the present results suggest that CORT-induced inhibition of LTP may be, at least to some extent, mediated by a presynaptic mechanism and decrease in the BDNF expression in rat hippocampal CA1 cells induced by CORT may partially account for this presynaptic mechanism.

The possibility of neurotoxicity in the hippocampus in major depression: a primer on neuron death

A number of studies indicate that prolonged, major depression is associated with a selective loss of hippocampal volume that persists long after the depression has resolved. This review is prompted by two ideas. The first is that overt neuron loss may be a contributing factor to the decrease in hippocampal volume. As such, the first half of this article reviews current knowledge about how hippocampal neurons die during insults, focusing on issues related to the trafficking of glutamate and calcium, glutamate receptor subtypes, oxygen radical generation, programmed cell death, and neuronal defenses. This is meant to orient the reader toward the biology that is likely to underlie any such instances of neuron loss in major depression. The second idea is that glucocorticoids, the adrenal steroids secreted during stress, may play a contributing role to any such neuron loss. The subtypes of depression associated with the hippocampal atrophy typically involve significant hypersecretion of glucocorticoids, and the steroid has a variety of adverse effects in the hippocampus, including causing overt neuron loss. The second half of this article reviews the steps in this cascade of hippocampal neuron death that are regulated by glucocorticoids.

Hippocampal damage in the human brain after cardiac arrest

BACKGROUND AND PURPOSE

Very few reports are available on changes in the human hippocampus after cardiac arrest. The objective of this study was to investigate if specific hippocampal volume losses can be demonstrated in the human brain following reperfusion after cardiac arrest.

METHODS

We assessed the volumes of the hippocampal formation (HF) and temporal lobe excluding HF (TL) as the contrast using magnetic resonance (MR)-imaging-based volumetry in 11 vegetative patients after cardiac arrest and in 22 healthy controls of similar age, sex and body size distribution. The measured volumes were normalized for differences in the head size among subjects by dividing by the total intracranial volume (TICV). The MR images of the 11 patients were obtained between days 8 and 21 after cardiac arrest.

RESULTS

The observed volumes of HFs and TLs of both patient and control groups were as follows: right HF volume (HFV): 2.67 +/- 0.19 (mean +/- SD, cm(3)) in patients versus 3.89 +/- 0.44 in controls; left HFV: 2.72 +/- 0.17 versus 3.74 +/- 0.35; right TL volume (TLV): 73.37 +/- 6.54 versus 80.08 +/- 7.62, and left TLV: 72. 45 +/- 6.77 versus 78.59 +/- 6.68. The normalized indices (HFV/TICV and TLV/TICV) were as follows: right HF: 0.0021 +/- 0.0002 (mean +/- SD) in patients versus 0.0031 +/- 0.0001 in controls, p < 0.0001, left HF: 0.0022 +/- 0.0002 versus 0.0030 +/- 0.0001, p < 0.0001, right TL: 0.058 +/- 0.002 versus 0.064 +/- 0.004, p = 0.0007, and left TL: 0.058 +/- 0.002 versus 0.062 +/- 0.004, p = 0.0014. The HFV-TLV ratios (HFV/TICV divided by TLV/TICV) of both groups were: right HFV-TLV ratio: 0.037 +/- 0.004 in patients versus 0.049 +/- 0. 004 in controls, p < 0.0001, left HFV-TLV ratio: 0.038 +/- 0.004 versus 0.048 +/- 0.004, p < 0.0001.

CONCLUSIONS

The patient group had HFs that were 26.8-30.6% smaller than those of the control group, but in the patient group, the TLs slightly decreased in size by only 7.8-8.2% of the volume of those in the control group within 21 days after cardiac arrest. The volume reductions in the bilateral HFs of patients after cardiac arrest were significantly larger than those in the bilateral TLs. We speculate that this specific rapid hippocampal shrinkage reflects its greater vulnerability to global brain ischemia.

Metyrapone alleviates ischemic neuronal damage in the gerbil hippocampus

Transient forebrain ischemia was induced in gerbils, and the effect of a pre-ischemic treatment with metyrapone (100 mg/kg) on delayed neuronal death in hippocampal CA1 neurons was evaluated. The effect of metyrapone on the ischemic release of amino acids in the CA1 region was also examined by microdialysis. Hippocampal slices were used for the evaluation of the hypoxia-induced intracellular Ca2+ increase by microfluorometry. The metyrapone treatment morphologically improved the damage provoked by 3 min of ischemia, although it did not alleviate the damage by 5 min. Ischemia for 3 min produced a 306% increase in the glutamate concentration in perfusates, and metyrapone suppressed the peak value to 42% of that in the control group. The extent of the increase in fluorescence intensity by intracellular Ca2+ was lower by 16% in slices from metyrapone-treated animals than in controls 600 s after induction of hypoxia. The removal of Ca2+ from the perfusion medium suppressed the hypoxic Ca2+ increase, and the increase was further reduced in slices pretreated with metyrapone. The increase in the level of endogenous glucocorticoids, which occurs in cerebral ischemia, may aggravate ischemic neuronal damage.

Neurosteroids, brain damage, and mental illness

The steroidal environment of the brain has marked consequences for both its structure and function. Social or physical stress has deleterious results on hippocampal function. This can be replicated by raising corticoids, which are also highly responsive to stress. Corticosterone, the major glucocorticoid in the rat, induces neuronal death in primary hippocampal cultures. Elevated corticoids also induce mood changes, and these are well known to be associated with stress, particularly chronic stress such as social adversity accentuated by intercurrent aversive life events. DHEA, a second adrenal steroid, has a very different developmental history, increasing rapidly during childhood, reaching a peak in youth, and declining thereafter in both blood and CSF. DHEA, in contrast to corticoids, has brain protective actions. It reduces the neurotoxic actions of glutamate analogues (such as NMDA) as well as those of corticoids. Evidence from several sources suggests that DHEA can act as an antiglucocorticoid. DHEA levels are reduced in major depressive disorders in both adolescents and adults, and a raised cortisol/DHEA ratio (together with intercurrent life events) predicts delayed recovery. DHEA may have a role in the treatment of depression. Together, these findings suggest that altered steroidal environment, whether induced by stress or aging, can have appreciable results on the cellular structure of the brain as well as on its function, although links between the two sets of findings are still tentative.

Detrimental effects of chronic hypothalamic-pituitary-adrenal axis activation. From obesity to memory deficits

Increasing evidence suggests that the detrimental effects of glucocorticoid (GC) hypersecretion occur by activation of the hypothalamic-pituitary-adrenal (HPA) axis in several human pathologies, including obesity, Alzheimer's disease, AIDS dementia, and depression. The different patterns of response by the HPA axis during chronic activation are an important consideration in selecting an animal model to assess HPA axis function in a particular disorder. This article will discuss how chronic HPA axis activation and GC hypersecretion affect hippocampal function and contribute to the development of obesity. In the brain, the hippocampus has the highest concentration of GC receptors. Chronic stress or corticosterone treatment induces neuropathological alterations, such as dendritic atrophy in hippocampal neurons, which are paralleled by cognitive deficits. Excitatory amino acid (EAA) neurotransmission has been implicated in chronic HPA axis activation. EAAs play a major role in neuroendocrine regulation. Hippocampal dendritic atrophy may involve alterations in EAA transporter function, and decreased EAA transporter function may also contribute to chronic HPA axis activation. Understanding the molecular mechanisms of HPA axis activation will likely advance the development of therapeutic interventions for conditions in which GC levels are chronically elevated.

Dexamethasone aggravates ischemia-induced neuronal damage by facilitating the onset of anoxic depolarization and the increase in the intracellular Ca2+ concentration in gerbil hippocampus

The Ca2+ mobilization across the neuronal membrane is regarded as a crucial factor in the development of neuronal damage in ischemia. Because glucocorticoids have been reported to aggravate ischemic neuronal injury, the effects of dexamethasone on ischemia-induced membrane depolarization, histologic outcome, and changes in the intracellular Ca2+ concentration in the gerbil hippocampus were examined in vivo and in vitro. The effects of metyrapone, an inhibitor of glucocorticoid synthesis, were also evaluated. Changes in the direct-current potential shift in the hippocampal CA1 area produced by transient forebrain ischemia for 2.5 minutes were compared among animals pretreated with dexamethasone (3 microg, intracerebroventricularly), metyrapone (100 mg/kg, intraperitoneally), and saline. The histologic outcome was evaluated 7 days after ischemia by assessing the delayed neuronal death in the hippocampal CA1 pyramidal cells of these animals. A hypoxia-induced intracellular Ca2+ increase was evaluated by in vitro microfluorometry in gerbil hippocampal slices, and the effect of dexamethasone (120 microg/L in the medium) on the cytosolic Ca2+ accumulation was examined. The effect in a Ca2+-free ischemialike condition was also investigated. Preischemic administration of dexamethasone reduced the onset latency of ischemia-induced membrane depolarization by 22%, and aggravated neuronal damage in vivo. In contrast, pretreatment with metyrapone improved the histologic outcome. The onset time of the increase in the intracellular concentration of Ca2+ provoked by in vitro hypoxia was advanced in dexamethasone-treated slices. The Ca2+-free in vitro hypoxia reduced the elevation compared with that in the Ca2+-containing condition. Treatment with dexamethasone facilitated the increase on both the initiation and the extent in the Ca2+-free condition. Aggravation of ischemic neuronal injury by endogenous or exogenous glucocorticoids is thus thought to be caused by the advanced onset times of both the ischemia-induced direct-current potential shift and the increase in the intracellular Ca2+ concentration.

Controversies surrounding glucocorticoid-mediated cell death in the hippocampus

The adrenal gland releases mineralocorticoids (MCs) and glucocorticoids (GCs) in response to a variety of stimuli, including stress. Once released, these adrenal steroids mediate a plethora of physiological responses in both the periphery and the central nervous system. The collective actions of GCs in the brain are paradoxical, however, in that basal levels of GCs are essential for neuronal development, plasticity and survival, while stress levels of GCs produce neuronal loss. Aging represents another contradictory function of GCs in the brain, since lifelong exposure to GCs has been implicated as a causative factor in senescent neuronal loss. In addition, glucocorticoids have also been shown to intensify neuronal damage in the hippocampus during ischemia and excitotoxicity through mechanisms that modulate synaptic glutamate concentrations. Conversely, the absence of adrenal steroids has been shown to regulate both neurogenesis and neuronal loss in the dentate gyrus of the hippocampus. Evidence continues to accumulate which suggests that GC-induced neuronal death in all these physiological and pathophysiological settings occurs by apoptosis. Accordingly, this review will examine the pharmacological, cellular and molecular mechanisms through which glucocorticoids mediate or contribute to neuronal remodeling and, ultimately, neuronal death.

Stress, Glucocorticoids, and Damage to the Nervous System: The Current State of Confusion

An extensive literature demonstrates that glucocorticoids (GCs), the adrenal steroids secreted during stress, can have a broad range of deleterious effects in the brain. The actions occur predominately, but not exclusively, in the hippocampus, a structure rich in corticosteroid receptors and particularly sensitive to GCs. The first half of this review considers three types of GC effects: a) GC-induced atrophy, in which a few weeks' exposure to high GC concentrations or to stress causes reversible atrophy of dendritic processes in the hippocampus; b) GC neurotoxicity where, over the course of months, GC exposure kills hippocampal neurons; c) GC neuroendangerment, in which elevated GC concentrations at the time of a neurological insult such as a stroke or seizure impairs the ability of neurons to survive the insult. The second half considers the rather confusing literature as to the possible mechanisms underlying these deleterious GC actions. Five broad themes are discerned: a) that GCs induce a metabolic vulnerability in neurons due to inhibition of glucose uptake; b) that GCs exacerbate various steps in a damaging cascade of glutamate excess, calcium mobilization and oxygen radical generation. In a review a number of years ago, I concluded that these two components accounted for the deleterious GC effects. Specifically, the energetic vulnerability induced by GCs left neurons metabolically compromised, and less able to carry out the costly task of containing glutamate, calcium and oxygen radicals. More recent work has shown this conclusion to be simplistic, and GC actions are shown to probably involve at least three additional components: c) that GCs impair a variety of neuronal defenses against neurologic insults; d) that GCs disrupt the mobilization of neurotrophins; e) that GCs have a variety of electrophysiological effects which can damage neurons. The relevance of each of those mechanisms to GC-induced atrophy, neurotoxicity and neuroendangerment is considered, as are the likely interactions among them.

Synergy between chronic corticosterone and sodium azide treatments in producing a spatial learning deficit and inhibiting cytochrome oxidase activity

Previously, we developed a rat model of persistent mitochondrial dysfunction based upon the chronic partial inhibition of the mitochondrial enzyme cytochrome oxidase (EC 1.9.3.1). Continuous systemic infusion of sodium azide at approximately 1 mg/kg per hr inhibited cytochrome oxidase activity and produced a spatial learning deficit. In other laboratories, glucocorticoids have been reported to exacerbate neuronal damage from various acute metabolic insults. Therefore, we tested the hypothesis that corticosterone, the primary glucocorticoid in the rat, would potentiate the sodium azide-induced learning deficit. To this end, we first identified nonimpairing doses of sodium azide (approximately 0.75 mg/kg per hr) and corticosterone (100-mg pellet, 3-week sustained-release). We now report that chronic co-administration of these individually nonimpairing treatments produced a severe learning deficit. Moreover, the low dose of corticosterone, which did not elevate serum corticosterone, acted synergistically with sodium azide to inhibit cytochrome oxidase activity. The latter result represents a previously unidentified effect of glucocorticoids that provides a candidate mechanism for glucocorticoid potentiation of neurotoxicity induced by metabolic insult. These results may have the clinical implication of expanding the definition of hypercortisolism in patient populations with compromised oxidative metabolism. Furthermore, they suggest that glucocorticoid treatment may contribute to pathology in disease or trauma conditions that involve metabolic insult.