Rapid glucocorticoid effects on excitatory amino acid levels in the hippocampus: a microdialysis study in freely moving rats

Cortisol rapidly disrupts prepulse inhibition in healthy men

Stress is known to affect sensorimotor gating (measured with prepulse inhibition of startle, or PPI), possibly improving perception of threat signals at the expense of other input during states of arousal. Stress also induces a variety of autonomic nervous system and endocrine responses, such as an activation of the hypothalamic-pituitary-adrenal axis. The latter will result in the release of the stress hormone cortisol which is known to exert rapid and sustained action on several CNS processes. Since previous studies have not clarified whether and which stress response components may mediate effects on sensorimotor gating, this study asked whether a link may exist between cortisol and sensorimotor gating. We tested whether cortisol may affect PPI by assessing PPI before, during, and after non-stressful, covert 1mg IV cortisol infusions in 27 healthy men in a single-blind and placebo-controlled within-subject design. Cortisol induced a rapid reduction of PPI, with its maximum at 20 min after administration, and PPI returned to baseline after another 20 min. Startle magnitude in the absence of a prepulse was not affected. This rapid effect of the IV cortisol infusions is probably mediated by a non-genomic mechanism. We conclude that stress effects on sensorimotor gating may be mediated by glucocorticoids. The disruption of sensorimotor gating by the stress hormone cortisol may serve the processing of intense and potentially dangerous startling stimuli.

Corticosterone decreases the activity of rat glutamate transporter type 3 expressed in Xenopus oocytes

Glucocorticoids can increase the extracellular concentrations of glutamate, the major excitatory neurotransmitter. We investigated the effects of corticosterone on the activity of a glutamate transporter, excitatory amino acid carrier 1 (EAAC1; also called excitatory amino acid transporter type 3 [EAAT3]), and the roles of protein kinase C (PKC) and phosphatidylinositol 3-kinase (PI3K) in regulating these effects. Rat EAAC1 was expressed in Xenopus oocytes by injecting mRNA. L-Glutamate (30 μM)-induced membrane currents were measured using the two-electrode voltage clamp technique. Exposure of these oocytes to corticosterone (0.01-1 μM) for 72 h decreased EAAC1 activity in a dose-dependent fashion, and this inhibition was incubation time-dependent. Corticosterone (0.01 μM for 72 h) significantly decreased the V(max), but not the K(m), of EAAC1 for glutamate. Furthermore, pretreatment of oocytes with staurosporine, a PKC inhibitor, significantly decreased EAAC1 activity (1.00±0.06 to 0.70±0.05 μC; P<0.05). However, no statistical differences were observed between oocytes treated with staurosporine, corticosterone, or corticosterone plus staurosporine. Similar patterns of responses were achieved by chelerythrine or calphostin C, other PKC inhibitors. Phorbol-12-myristate-13-acetate (PMA), a PKC activator, inhibited corticosterone-induced reduction in EAAC1 activity. Pretreating oocytes with wortmannin or LY294002, PI3K inhibitors, also significantly reduced EAAC1 activity, but no difference was observed between oocytes treated with wortmannin, corticosterone, or wortmannin plus corticosterone. The above results suggest that corticosterone exposure reduces EAAC1 activity and this effect is PKC- and PI3K-dependent.

Cage mate separation in pair-housed male rats evokes an acute stress corticosterone response

Corticosterone (CORT) release from the adrenal glands in response to acutely stressful stimuli is well-characterized, however several non-experimental, environmental stressors can also engender a CORT response. The aim of this study was to investigate an acute activation of the HPA axis in pair-housed animals in response to separation. We observed a rapid significant increase in CORT in the animal remaining in the home cage following cage mate removal, that was not caused by cage opening and transient removal of cage mate. In addition, we examined this response in both control, non-stressed animals and in animals subjected to chronic variable stress (CVS) and found that although basal levels of CORT differed between control and CVS animals, there was no significant difference in the acute CORT levels between the control and CVS animals after separation, indicating that this environmental event is perceived as acutely stressful in both conditions. Furthermore, we examined the time course of CORT activation and found that CORT levels rapidly rise within minutes of separation peaking at 15 min and returning to baseline by 90 min. The results of this study demonstrate that separation can induce an acute stress response in the remaining cage mate measured by increased CORT and should be considered in molecular, behavioral, and electrophysiological studies.

The enhancement of stress-related memory by glucocorticoids depends on synapsin-Ia/Ib

The activation of glucocorticoid receptors (GR) by glucocorticoids increases stress-related memory through the activation of the MAPK signaling pathway and the downstream transcription factor Egr-1. Here, using converging in vitro and in vivo approaches, respectively, GR-expressing cell lines, culture of hippocampal neurons, and GR genetically modified mice (GR(NesCre)), we identified synapsin-Ia/Ib as one of the effectors of the glucocorticoid signaling cascade. Stress and glucocorticoid-induced activation of the GR modulate synapsin-Ia/Ib through two complementary mechanisms. First, glucocorticoids driving Egr-1 expression increase the expression of synapsin-Ia/Ib, and second, glucocorticoids driving MAPK activation increase its phosphorylation. Finally, we showed that blocking fucosylation of synapsin-Ia/Ib in the hippocampus inhibits its expression and prevents the glucocorticoid-mediated increase in stress-related memory. In conclusion, our data provide a complete molecular pathway (GR/Egr-1/MAPK/Syn-Ia/Ib) through which stress and glucocorticoids enhance the memory of stress-related events and highlight the function of synapsin-Ia/Ib as molecular effector of the behavioral effects of stress.

Interaction of childhood stress with hippocampus and prefrontal cortex volume reduction in major depression

Early emotional stress is associated with a life-long burden of risk for later depression and stressful life events contribute to the development of depressive episodes. In this study we investigated whether childhood stress is associated with structural brain alterations in patients with major depression (MD). Forty-three patients with MD and 44 age as well as gender matched healthy control subjects were investigated using high-resolution magnetic resonance imaging (MRI). Region of interest analysis of the hippocampus, whole brain voxel-based morphometry (VBM) and assessment of childhood stress was carried out. Significantly smaller hippocampal white matter and prefrontal gray matter volume was observed in patients with MD compared to healthy controls. In particular left hippocampal white matter was smaller in patients, who had emotional childhood neglect, compared to those without neglect. For male patients this effect was seen in the left and right hippocampus. Moreover, physical neglect during childhood affected prefrontal gray matter volume in healthy subjects. Both emotional neglect and brain structural abnormalities predicted cumulative illness duration and there was a significant interaction between emotional neglect and prefrontal volumes as well as hippocampal white matter on the illness course. Childhood neglect resulted in hippocampal white matter changes in patients with major depression, pronounced at the left side and in males. Most interestingly, childhood stress and brain structure volumes independently predicted cumulative illness course. Subjects with both, structural brain changes and childhood emotional neglect seem to be at a very high risk to develop a more severe illness course.

Chronic corticosterone administration down-regulates metabotropic glutamate receptor 5 protein expression in the rat hippocampus

Several lines of evidence suggest a dysfunctional glutamate system in major depressive disorder (MDD). Recently, we reported reduced levels of metabotropic glutamate receptor subtype 5 (mGluR5) in postmortem brains in MDD, however the neurobiological mechanisms that induce these abnormalities are unclear. In the present study, we examined the effect of chronic corticosterone (CORT) administration on the expression of mGluR5 protein and mRNA in the rat frontal cortex and hippocampus. Rats were injected with CORT (40 mg/kg s.c.) or vehicled once daily for 21 days. The expression of mGluR5 protein and mRNA was assessed by Western blotting and quantitative real-time PCR (qPCR). In addition, mGluR1 protein was measured in the same animals. The results revealed that while there was a significant reduction (-27%, P=0.0006) in mGluR5 protein expression in the hippocampus from CORT treated rats, mRNA levels were unchanged. Also unchanged were mGluR5 mRNA and protein levels in the frontal cortex and mGluR1 protein levels in both brain regions. Our findings provide the first evidence that chronic CORT exposure regulates the expression of mGluR5 and are in line with previous postmortem and imaging studies showing reduced mGluR5 in MDD. Our findings suggest that elevated levels of glucocorticoids may contribute to impairments in glutamate neurotransmission in MDD.

Stress hormones and AMPA receptor trafficking in synaptic plasticity and memory

The acquisition and consolidation of memories of stressful events is modulated by glucocorticoids, a type of corticosteroid hormone that is released in high levels from the adrenal glands after exposure to a stressful event. These effects occur through activation of mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs). The molecular mechanisms that underlie the effects of glucocorticoids on synaptic transmission, synaptic plasticity, learning and memory have recently begun to be identified. Glucocorticoids regulate AMPA (α-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate) receptor trafficking--which is crucially involved in synaptic transmission and plasticity--both rapidly and persistently. Stress hormones may, through modulation of AMPA receptor function, promote the consolidation of behaviourally relevant information.

Stress, sex, and neural adaptation to a changing environment: mechanisms of neuronal remodeling

The adult brain is much more resilient and adaptable than previously believed, and adaptive structural plasticity involves growth and shrinkage of dendritic trees, turnover of synapses, and limited amounts of neurogenesis in the forebrain, especially the dentate gyrus of the hippocampal formation. Stress and sex hormones help to mediate adaptive structural plasticity, which has been extensively investigated in the hippocampus and to a lesser extent in the prefrontal cortex and amygdala, all brain regions that are involved in cognitive and emotional functions. Stress and sex hormones exert their effects on brain structural remodeling through both classical genomic as well as non-genomic mechanisms, and they do so in collaboration with neurotransmitters and other intra- and extracellular mediators. This review will illustrate the actions of estrogen on synapse formation in the hippocampus and the process of stress-induced remodeling of dendrites and synapses in the hippocampus, amygdala, and prefrontal cortex. The influence of early developmental epigenetic events, such as early life stress and brain sexual differentiation, is noted along with the interactions between sex hormones and the effects of stress on the brain. Because hormones influence brain structure and function and because hormone secretion is governed by the brain, applied molecular neuroscience techniques can begin to reveal the role of hormones in brain-related disorders and the treatment of these diseases. A better understanding of hormone-brain interactions should promote more flexible approaches to the treatment of psychiatric disorders, as well as their prevention through both behavioral and pharmaceutical interventions.

Nongenomic actions of adrenal steroids in the central nervous system

Mineralocorticoids and glucocorticoids are steroid hormones that are released by the adrenal cortex in response to stress and hydromineral imbalance. Historically, adrenocorticosteroid actions are attributed to effects on gene transcription. More recently, however, it has become clear that genome-independent pathways represent an important facet of adrenal steroid actions. These hormones exert nongenomic effects throughout the body, although a significant portion of their actions are specific to the central nervous system. These actions are mediated by a variety of signalling pathways, and lead to physiologically meaningful events in vitro and in vivo. We review the nongenomic effects of adrenal steroids in the central nervous system at the levels of behaviour, neural system activity, individual neurone activity and subcellular signalling activity. A clearer understanding of adrenal steroid activity in the central nervous system will lead to a better ability to treat human disease as well as reduce the side-effects of the steroid treatments already in use.

Rapid stress-induced corticosterone rise in the hippocampus reverses serial memory retrieval pattern

We previously showed that an acute stress (electric footshocks) induced both a rapid plasma corticosterone rise and a reversal of serial memory retrieval pattern in a contextual serial discrimination (CSD) task. This study is aimed at determining (i) if the rapid stress effects on CSD performance are mediated by the hippocampus; (ii) if hippocampal corticosterone membrane receptor activation is involved in the rapid stress effects on CSD performance. In experiment 1, microdialysis in the dorsal hippocampus (dHPC) was used to measure the stress-induced corticosterone rise; in parallel, the effect of acute stress on CSD performance was evaluated. In addition, the functional involvement of corticosterone in the behavioral effects of stress was assessed by administering metyrapone, a corticosterone synthesis inhibitor, before stress. In experiment 2, the involvement of hippocampal corticosterone membrane receptors in the stress-induced reversal of CSD performance was studied by injecting corticosterone-bovine serum albumin (BSA) (a membrane-impermeable complex) in the dHPC in non stressed mice. Results showed that (i) the acute stress induced a rapid (15 min) and transitory (90 min) corticosterone rise into the hippocampus dHPC, and a reversal of serial memory retrieval pattern; (ii) both the endocrinal and memory stress-induced effects were blocked by metyrapone; (iii) corticosterone-BSA injection into the dHPC in non stressed mice mimicked the effects of stress on serial retrieval pattern. Overall, our study is first to show that (i) a rapid stress-induced corticosterone rise into the dHPC transitorily reverses serial memory retrieval pattern and (ii) hippocampal corticosterone membrane receptors activation is involved in the rapid effects of acute stress on serial memory retrieval.

Acute stress increases depolarization-evoked glutamate release in the rat prefrontal/frontal cortex: the dampening action of antidepressants

BACKGROUND

Behavioral stress is recognized as a main risk factor for neuropsychiatric diseases. Converging evidence suggested that acute stress is associated with increase of excitatory transmission in certain forebrain areas. Aim of this work was to investigate the mechanism whereby acute stress increases glutamate release, and if therapeutic drugs prevent the effect of stress on glutamate release.

METHODOLOGY/FINDINGS

Rats were chronically treated with vehicle or drugs employed for therapy of mood/anxiety disorders (fluoxetine, desipramine, venlafaxine, agomelatine) and then subjected to unpredictable footshock stress. Acute stress induced marked increase in depolarization-evoked release of glutamate from synaptosomes of prefrontal/frontal cortex in superfusion, and the chronic drug treatments prevented the increase of glutamate release. Stress induced rapid increase in the circulating levels of corticosterone in all rats (both vehicle- and drug-treated), and glutamate release increase was blocked by previous administration of selective antagonist of glucocorticoid receptor (RU 486). On the molecular level, stress induced accumulation of presynaptic SNARE complexes in synaptic membranes (both in vehicle- and drug-treated rats). Patch-clamp recordings of pyramidal neurons in the prefrontal cortex revealed that stress increased glutamatergic transmission through both pre- and postsynaptic mechanisms, and that antidepressants may normalize it by reducing release probability.

CONCLUSIONS/SIGNIFICANCE

Acute footshock stress up-regulated depolarization-evoked release of glutamate from synaptosomes of prefrontal/frontal cortex. Stress-induced increase of glutamate release was dependent on stimulation of glucocorticoid receptor by corticosterone. Because all drugs employed did not block either elevation of corticosterone or accumulation of SNARE complexes, the dampening action of the drugs on glutamate release must be downstream of these processes. This novel effect of antidepressants on the response to stress, shown here for the first time, could be related to the therapeutic action of these drugs.

Imaging the immediate non-genomic effects of stress hormone on brain activity

The stress hormones, glucocorticoids, bind to intracellular receptor proteins and act as transcription factors affecting gene activity. These genomic effects occur over hours and even days producing long-term changes in synaptic plasticity and neural transmission. In addition to this classic genomic pathway, there is evidence that stress hormones can have immediate, non-genomic effects on brain function. Using non-invasive functional magnetic resonance imaging, awake, adrenalectomized rats were given intravenous doses of corticosterone mimicking blood levels of hormone achieved with modest and intense stress. The dose of corticosterone mimicking high stress caused a significant increase in functional activity in the hippocampus, forebrain cortex and lateral hypothalamus within minutes of administration. This finding shows that stress hormones can have non-genomic effects on brain activity potentially affecting the immediate cognitive and behavioral response to a highly emotional experience.

Effects of stress and stress hormones on amyloid-beta protein and plaque deposition

Growing evidence indicates that physical and psychosocial stressors, in part acting through the hypothalamic-pituitary-adrenal (HPA) axis, may accelerate the process of Alzheimer's disease (AD). In this review, we summarize recent research related to the effects of stress and stress hormones on the various disease process elements associated with AD. Specifically, we focus on the relationships among chronic stressors, HPA axis activity, amyloid-beta protein, and amyloid-beta plaque deposition in mouse models of AD. The potential mechanisms by which stress and stress-related components, especially corticotrophin-releasing factor and its receptors, influence the pathogenesis of AD are discussed.

Modulation of presynaptic glucocorticoid receptors on glutamate release from rat hippocampal nerve terminals

In this study, we have examined the role of corticosterone (CORT) in the regulation of neuronal glutamate release using nerve terminals (synaptosomes) isolated from the rat hippocampus. Adult male Sprague-Dawley rats received either a chronic systemic administration of CORT (daily 25 mg/kg in sesame oil, subcutaneously) or long-term bilateral adrenalectomy (ADX) (3-4 weeks), and then the release of 4-aminopyridine (4AP)-evoked endogenous glutamate and the levels of glucocorticoid receptor (GR) expression from hippocampal nerve terminals were studied. Chronic administration of CORT resulted in a significant increase of 4AP-evoked glutamate release from hippocampal nerve terminals, whereas ADX reduced 4AP-evoked glutamate release. In addition, chronic administration of CORT and ADX induced a significant reduction and increase in GR expression in hippocampal synaptosomes, respectively, as detected by Western blots. Furthermore, acute treatment of CORT or dexamethasone facilitated 4AP-evoked glutamate release from synaptosomes freshly isolated from naïve rat hippocampus and this effect can be significantly prevented by pretreatment of GR antagonist mifepristone, but not by mineralocorticoid receptor (MR) antagonist RU28318. Together, our results strongly support the presence of GRs on presynaptic nerve terminals in the rat hippocampus acting to facilitate the release of neuronal glutamate.

Epileptogenesis is increased in rats with neonatal isolation and early-life seizure and ameliorated by MK-801: a long-term MRI and histological study

Early-life stress has been shown to destabilize the homeostatic synaptic plasticity and compromise the developing brain to the later encountered insults. This study would determine the long-term epileptogenic effect of neonatal isolation (NI) on early-life seizure. There were five groups: normal rearing (NR) rats; NI rats; NR rats suffering from status epilepticus (SE) at P12 (NR-SE); NI-SE rats; NI-SE-MK801 rats. All adult rats were video monitored to detect behavioral seizures, examined with brain magnetic resonance imaging, and assessed for hippocampal NeuN-immunoreactive (NeuN-IR) cells. Behavioral seizures were detected in one of six NR-SE rats, all the NI-SE rats (eight of eight), and none in the NR, NI, or NI-SE-MK801 rats. High hippocampal T2 signal were only found in three of five NR-SE rats, five of six NI-SE rats, and one of five NI-SE-MK801 rats. There was a significant decrease in the number of hippocampal NeuN-IR cells in the NR-SE and NI-SE groups, compared with the NR group, and MK-801 treatment ameliorated the neuronal loss. Our results demonstrated that NI led to an increase in epileptogenesis in rat pups with early-life SE, and treatment with MK-801 could ameliorate brain injuries, indicating a critical role of N-methyl-d-aspartic acid receptor in the epileptogenic process.

Stress exacerbates neuropathic pain via glucocorticoid and NMDA receptor activation

There is growing recognition that psychological stress influences pain. Hormones that comprise the physiological response to stress (e.g., corticosterone; CORT) may interact with effectors of neuropathic pain. To test this hypothesis, mice received a spared nerve injury (SNI) after exposure to 60 min restraint stress. In stressed mice, allodynia was consistently increased. The mechanism(s) underlying the exacerbated pain response involves CORT acting via glucocorticoid receptors (GRs); RU486, a GR antagonist, prevented the stress-induced increase in allodynia whereas exogenous administration of CORT to non-stressed mice reproduced the allodynic response caused by stress. Since nerve injury-induced microglial activation has been implicated in the onset and propagation of neuropathic pain, we evaluated cellular and molecular indices of microglial activation in the context of stress. Activation of dorsal horn microglia was accelerated by stress; however, this effect was transient and was not associated with the onset or maintenance of a pro-inflammatory phenotype. Stress-enhanced allodynia was associated with increased dorsal horn extracellular signal-regulated kinase phosphorylation (pERK). ERK activation could indicate a stress-mediated increase in glutamatergic signaling, therefore mice were treated prior to SNI and stress with memantine, an N-methyl-D-aspartate receptor (NMDAR) antagonist. Memantine prevented stress-induced enhancement of allodynia after SNI. These data suggest that the hormonal responses elicited by stress exacerbate neuropathic pain through enhanced central sensitization. Moreover, drugs that inhibit glucocorticoids (GCs) and/or NMDAR signaling could ameliorate pain syndromes caused by stress.

Overview of brain microdialysis

The technique of microdialysis enables sampling and collecting of small-molecular-weight substances from the interstitial space. It is a widely used method in neuroscience and is one of the few techniques available that permits quantification of neurotransmitters, peptides, and hormones in the behaving animal. More recently, it has been used in tissue preparations for quantification of neurotransmitter release. This unit provides a brief review of the history of microdialysis and its general application in the neurosciences. The authors review the theoretical principles underlying the microdialysis process, methods available for estimating extracellular concentration from dialysis samples (i.e., relative recovery), the various factors that affect the estimate of in vivo relative recovery, and the importance of determining in vivo relative recovery to data interpretation. Several areas of special note, including impact of tissue trauma on the interpretation of microdialysis results, are discussed. Step-by-step instructions for the planning and execution of conventional and quantitative microdialysis experiments are provided.

Cellular and molecular mechanisms in the long-term action of antidepressants

The hypotheses on the pathophysiology of depression/mood disorders and on antidepressant mechanisms have greatly changed in recent years. The classical monoamine hypothesis was revealed to be simplistic, in that it could not explain the temporal delay in the therapeutic action of antidepressants. Converging lines of evidence have shown that adaptive changes in the several mechanisms of neuroplasticity are likely to be the cellular and molecular correlates of therapeutic effect. In this article, several mechanisms of neuroplasticity are analyzed in relation to the mechanism of antidepressants, ranging from changes in gene expression (including neurotrophic mechanisms), to synaptic transmission and plasticity, and neurogenesis. We propose that the current version of the hypothesis of antidepressant mechanism simply be called the "hypothesis of neuroplasticity". In the final section, we also briefly review the main current novel strategies in the pharmacology of depression and the new putative targets for antidepressants, with particular emphasis on nonmonoaminergic mechanisms.

Motivations and methods for analyzing pulsatile hormone secretion

Endocrine glands communicate with remote target cells via a mixture of continuous and intermittent signal exchange. Continuous signaling allows slowly varying control, whereas intermittency permits large rapid adjustments. The control systems that mediate such homeostatic corrections operate in a species-, gender-, age-, and context-selective fashion. Significant progress has been made in understanding mechanisms of adaptive interglandular signaling in vivo. Principal goals are to understand the physiological origins, significance, and mechanisms of pulsatile hormone secretion. Key analytical issues are: 1) to quantify the number, size, shape, and uniformity of pulses, nonpulsatile (basal) secretion, and elimination kinetics; 2) to evaluate regulation of the axis as a whole; and 3) to reconstruct dose-response interactions without disrupting hormone connections. This review will focus on the motivations driving and the methodologies used for such analyses.

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.

Linear and non-linear dose-response functions reveal a hormetic relationship between stress and learning

Over a century of behavioral research has shown that stress can enhance or impair learning and memory. In the present review, we have explored the complex effects of stress on cognition and propose that they are characterized by linear and non-linear dose-response functions, which together reveal a hormetic relationship between stress and learning. We suggest that stress initially enhances hippocampal function, resulting from amygdala-induced excitation of hippocampal synaptic plasticity, as well as the excitatory effects of several neuromodulators, including corticosteroids, norepinephrine, corticotropin-releasing hormone, acetylcholine and dopamine. We propose that this rapid activation of the amygdala-hippocampus brain memory system results in a linear dose-response relation between emotional strength and memory formation. More prolonged stress, however, leads to an inhibition of hippocampal function, which can be attributed to compensatory cellular responses that protect hippocampal neurons from excitotoxicity. This inhibition of hippocampal functioning in response to prolonged stress is potentially relevant to the well-described curvilinear dose-response relationship between arousal and memory. Our emphasis on the temporal features of stress-brain interactions addresses how stress can activate, as well as impair, hippocampal functioning to produce a hormetic relationship between stress and learning.

Rapid changes in hippocampal CA1 pyramidal cell function via pre- as well as postsynaptic membrane mineralocorticoid receptors

Corticosterone (100 nm) rapidly increases the frequency of miniature excitatory postsynaptic currents in mouse CA1 pyramidal neurons via membrane-located mineralocorticoid receptors (MRs). We now show that a presynaptic ERK1/2 signalling pathway mediates the nongenomic effect, as it was blocked by the MEK inhibitors U0126 (10 microm) and PD098059 (40 microm) and occluded in H-Ras(G12V)-mutant mice with constitutive activation of the ERK1/2 presynaptic pathway. Notably, the increase in mEPSC frequency was not mediated by retrograde signalling through endocannabinoids or nitric oxide, supporting presynaptic localization of the signalling pathway. Unexpectedly, corticosterone was also found to have a direct postsynaptic effect, rapidly decreasing the peak amplitude of I(A) currents. This effect takes place via postsynaptic membrane MRs coupled to a G protein-mediated pathway, as the effect of corticosterone on I(A) was effectively blocked by 0.5 mm GDP-beta-S administered via the recording pipette into the postsynaptic cell. Taken together, these results indicate that membrane MRs mediate rapid, nongenomic effects via pre- as well as postsynaptic pathways. Through these dual pathways, high corticosterone concentrations such as occur after stress could contribute to enhanced CA1 pyramidal excitability.

Chronic stress enhances methamphetamine-induced extracellular glutamate and excitotoxicity in the rat striatum

Striking parallels exist between the neurochemical and toxic effects of stress and methamphetamine. Despite these similarities, no studies have examined how stress may promote the toxic effects of methamphetamine (METH). The current study tested the hypothesis that chronic stress enhances METH toxicity by augmenting glutamate (GLU) release and excitotoxicity in response to METH administration. Adult male Sprague-Dawley rats were exposed to 10 days of unpredictable stress and then received either saline or METH (7.5 mg/kg, i.p., once every 2 h x four injections). Prior exposure to unpredictable stress acutely enhanced the striatal extracellular GLU concentrations in response to METH, and eventually caused proteolysis of the cytoskeleton protein spectrin. Administration of the corticosterone synthesis inhibitor, metyrapone (25 mg/kg, i.p., prior to each stressor), during unpredictable stress attenuated the enhanced striatal GLU release in response to METH, blocked spectrin proteolysis, and attenuated METH-associated toxicity measured by long-term depletions in the dopamine and serotonin tissue content as well as depletions in dopamine and serotonin transporter immunoreactivity of the striatum. In summary, prior exposure to unpredictable stress enhances METH-induced elevations of GLU in the striatum, resulting in long-term excitotoxic damage and an augmentation of damage to dopamine and serotonin terminals. These studies provide a neurochemical basis for how stress contributes to the deleterious effects of METH abuse.

Corticosteroid hormones in the central stress response: quick-and-slow

Recent evidence shows that corticosteroid hormones exert rapid non-genomic effects on neurons in the hypothalamus and the hippocampal CA1 region. The latter depend on classical mineralocorticoid receptors which are accessible from the outside of the plasma membrane and display a 10-fold lower affinity for corticosterone than the nuclear version involved in neuroprotection. Consequently, this 'membrane' receptor could play an important role while corticosteroid levels are high, i.e. during the initial phase of the stress response. We propose that during this phase corticosterone promotes hippocampal excitability and amplifies the effect of other stress hormones. These permissive non-genomic effects may contribute to fast behavioral effects and encoding of stress-related information. The fast effects are complemented by slower glucocorticoid receptor-mediated effects which facilitate suppression of temporary raised excitability, recovery from the stressful experience and storage of information for future use.

The effects of non-genomic glucocorticoid mechanisms on bodily functions and the central neural system. A critical evaluation of findings

Mounting evidence suggests that--beyond the well-known genomic effects--glucocorticoids affect cell function via non-genomic mechanisms. Such mechanisms operate in many major systems and organs including the cardiovascular, immune, endocrine and nervous systems, smooth and skeletal muscles, liver, and fat cells. Non-genomic effects are exerted by direct actions on membrane lipids (affecting membrane fluidity), membrane proteins (e.g. ion channels and neurotransmitter receptors), and cytoplasmic proteins (e.g. MAPKs, phospholipases, protein kinases, etc.). These actions are mediated by the glucocorticoids per se or by the proteins dissociated from the liganded glucocorticoid receptor complex. The MR and GR also activate non-genomic mechanisms in certain cases. Some effects of glucocorticoids are shared by a variety of steroids, whereas others are more selective. Moreover, "ultra-selective" effects-mediated by certain glucocorticoids only-were also shown. Disparate findings suggest that non-genomic mechanisms also show "demand-specificity", i.e. require the coincidence of two or more processes. Some of the non-genomic mechanisms activated by glucocorticoids are therapeutically relevant; moreover, the "non-genomic specificity" of certain glucocorticoids raises the possibility of therapeutic applications. Despite the large body of evidence, however, the non-genomic mechanisms of glucocorticoids are still poorly understood. Criteria for differentiating genomic and non-genomic mechanisms are often loosely applied; interactions between various mechanisms are unknown, and non-genomic mechanism-specific pharmacological (potentially therapeutic) agents are lacking. Nevertheless, the discovery of non-genomic mechanisms is a major breakthrough in stress research, and further insights into these mechanisms may open novel approaches for the therapy of various diseases.

Enhancement of hippocampal mossy fiber activity in zinc deficiency and its influence on behavior

The extracellular concentration of glutamate in the hippocampus is increased by hippocampal perfusion with CaEDTA, a membrane-impermeable zinc chelator, suggesting that the activity of glutamatergic neurons in the hippocampus are influenced by the extracellular concentrations of zinc. In the present study, the relationship between the extracellular concentrations of zinc and mossy fiber activity in the hippocampus was examined in mice and rats fed a zinc-deficient diet for 4 weeks. Timm's stain, by which histochemically reactive zinc in the presynaptic vesicles is detected, was attenuated in the hippocampus in zinc deficiency. The extracellular signal of ZnAF-2, a membrane-impermeable zinc indicator, was also lower in the hippocampal CA3, suggesting that the basal extracellular concentrations of zinc are lower maintained in zinc deficiency. To check mossy fiber activity after 4-week zinc deprivation, the decrease in the signal of FM4-64, an indicator of presynaptic activity (exocytosis), at mossy fiber synapses was measured under the condition of spontaneous depolarization. The decrease was significantly facilitated by zinc deficiency, suggesting that the basal exocytosis at mossy fiber synapses is enhanced by zinc deficiency. On the other hand, the increase in anxiety-like behavior was observed in the open-field test after 4-week zinc deprivation. The present study demonstrates that the decrease in the basal extracellular concentrations of zinc may be linked to the enhancement of the basal mossy fiber activity in zinc deficiency. This decrease seems to be also involved in neuropsychological behavior in zinc deficiency.

Stress-induced changes in sleep and associated neuronal activity in rat hippocampus and amygdala

Stress increases vulnerability to anxiety and depression. We have investigated the effect of acute immobilization stress in amygdalohippocampal circuits by measuring the electroencephalogram (EEG) in male Wistar rats during rapid eye movement (REM) sleep. Electrodes were implanted stereotaxically in the hippocampus (CA1 and CA3 subregions of the hippocampus) and the amygdala (lateral nucleus). Prior to the stress, two baseline recordings were taken. Twenty-four hours later rats were exposed once to acute immobilization stress (AIS) session for 2 h. After the release and on subsequent days, electrophysiological changes that occurred due to stress during REM sleep were analyzed by comparing them with baseline measurements. Our results suggest that acute immobilization stress induced significant increase in REM sleep in the first 24 h after the exposure. In addition to changes in the sleep patterns, we have observed increased theta oscillations in CA1 area of the hippocampus with decreased coherence at theta range (4-8 Hz) between hippocampus and amygdala. These results suggest that single exposure to aversive experience such as immobilization stress can lead to dynamic changes in neuronal activities with altered sleep morphology. The results obtained in the present study are comparable to those seen in human patients suffering from panic, and anxiety due to posttraumatic stress disorder (PTSD).

Central effects of stress hormones in health and disease: Understanding the protective and damaging effects of stress and stress mediators

Stress begins in the brain and affects the brain, as well as the rest of the body. Acute stress responses promote adaptation and survival via responses of neural, cardiovascular, autonomic, immune and metabolic systems. Chronic stress can promote and exacerbate pathophysiology through the same systems that are dysregulated. The burden of chronic stress and accompanying changes in personal behaviors (smoking, eating too much, drinking, poor quality sleep; otherwise referred to as "lifestyle") is called allostatic overload. Brain regions such as hippocampus, prefrontal cortex and amygdala respond to acute and chronic stress and show changes in morphology and chemistry that are largely reversible if the chronic stress lasts for weeks. However, it is not clear whether prolonged stress for many months or years may have irreversible effects on the brain. The adaptive plasticity of chronic stress involves many mediators, including glucocorticoids, excitatory amino acids, endogenous factors such as brain neurotrophic factor (BDNF), polysialated neural cell adhesion molecule (PSA-NCAM) and tissue plasminogen activator (tPA). The role of this stress-induced remodeling of neural circuitry is discussed in relation to psychiatric illnesses, as well as chronic stress and the concept of top-down regulation of cognitive, autonomic and neuroendocrine function. This concept leads to a different way of regarding more holistic manipulations, such as physical activity and social support as an important complement to pharmaceutical therapy in treatment of the common phenomenon of being "stressed out". Policies of government and the private sector play an important role in this top-down view of minimizing the burden of chronic stress and related lifestyle (i.e. allostatic overload).

Stress-induced changes in hippocampal function

Exposure of an organism to stress leads to activation of the sympatho-adrenomedullary system and the hypothalamo-pituitary-adrenal axis. Consequently, levels of noradrenaline, peptides like vasopressin and CRH, and corticosteroid hormones in the brain rise. These hormones affect brain function at those sites where receptors are enriched, like the hippocampus, lateral septum, amygdala nuclei, and prefrontal cortex. During the initial phase of the stress response, when hormone levels are high, these compounds mostly enhance excitability and promote long-term potentiation. Later on, when hormone levels have subsided but gene-mediated effects of corticosteroids start to appear, the excitability is normalized to the pre-stress level, in the CA1 hippocampal area, but possibly less so in the dentate gyrus and amygdala. A disturbed balance between these early and late phases of the stress response as well as a shift toward the relative contribution of the dentate/amygdala pathways may explain why the normal restorative capacity fails in vulnerable people experiencing a life-threatening situation, which could contribute to the development of PTSD.

Stress and memory: behavioral effects and neurobiological mechanisms

Stress is a potent modulator of learning and memory processes. Although there have been a few attempts in the literature to explain the diversity of effects (including facilitating, impairing, and lack of effects) described for the impact of stress on memory function according to single classification criterion, they have proved insufficient to explain the whole complexity of effects. Here, we review the literature in the field of stress and memory interactions according to five selected classifying factors (source of stress, stressor duration, stressor intensity, stressor timing with regard to memory phase, and learning type) in an attempt to develop an integrative model to understand how stress affects memory function. Summarizing on those conditions in which there was enough information, we conclude that high stress levels, whether intrinsic (triggered by the cognitive challenge) or extrinsic (induced by conditions completely unrelated to the cognitive task), tend to facilitate Pavlovian conditioning (in a linear-asymptotic manner), while being deleterious for spatial/explicit information processing (which with regard to intrinsic stress levels follows an inverted U-shape effect). Moreover, after reviewing the literature, we conclude that all selected factors are essential to develop an integrative model that defines the outcome of stress effects in memory processes. In parallel, we provide a brief review of the main neurobiological mechanisms proposed to account for the different effects of stress in memory function. Glucocorticoids were found as a common mediating mechanism for both the facilitating and impairing actions of stress in different memory processes and phases. Among the brain regions implicated, the hippocampus, amygdala, and prefrontal cortex were highlighted as critical for the mediation of stress effects.

Stress, depression, and neuroplasticity: a convergence of mechanisms

Increasing evidence demonstrates that neuroplasticity, a fundamental mechanism of neuronal adaptation, is disrupted in mood disorders and in animal models of stress. Here we provide an overview of the evidence that chronic stress, which can precipitate or exacerbate depression, disrupts neuroplasticity, while antidepressant treatment produces opposing effects and can enhance neuroplasticity. We discuss neuroplasticity at different levels: structural plasticity (such as plastic changes in spine and dendrite morphology as well as adult neurogenesis), functional synaptic plasticity, and the molecular and cellular mechanisms accompanying such changes. Together, these studies elucidate mechanisms that may contribute to the pathophysiology of depression. Greater appreciation of the convergence of mechanisms between stress, depression, and neuroplasticity is likely to lead to the identification of novel targets for more efficacious treatments.

Social isolation rearing-induced impairment of the hippocampal neurogenesis is associated with deficits in spatial memory and emotion-related behaviors in juvenile mice

Experiences during brain development may influence the pathogenesis of developmental disorders. Thus, social isolation (SI) rearing after weaning is a useful animal model for studying the pathological mechanisms of such psychiatric diseases. In this study, we examined the effect of SI on neurogenesis in the hippocampal dentate gyrus (DG) relating to memory and emotion-related behaviors. When newly divided cells were labeled with 5-bromo-2'-deoxyuridine (BrdU) before SI, the number of BrdU-positive cells and the rate of differentiation into neurons were significantly decreased after 4-week SI compared with those in group-housed mice. Repeated treatment of fluoxetine prevented the SI-induced impairment of survival of newly divided cells and ameliorated spatial memory impairment and part of aggression in SI mice. Furthermore, we investigated the changes in gene expression in the DG of SI mice by using DNA microarray and real-time PCR. We finally found that SI reduced the expression of development-related genes Nurr1 and Npas4. These findings suggest that communication in juvenile is important in the survival and differentiation of newly divided cells, which may be associated with memory and aggression, and raise the possibility that the reduced expression of Nurr1 and/or Npas4 may contribute to the impairment of neurogenesis and memory and aggression induced by SI.

Stress and memory: behavioral effects and neurobiological mechanisms

Stress is a potent modulator of learning and memory processes. Although there have been a few attempts in the literature to explain the diversity of effects (including facilitating, impairing, and lack of effects) described for the impact of stress on memory function according to single classification criterion, they have proved insufficient to explain the whole complexity of effects. Here, we review the literature in the field of stress and memory interactions according to five selected classifying factors (source of stress, stressor duration, stressor intensity, stressor timing with regard to memory phase, and learning type) in an attempt to develop an integrative model to understand how stress affects memory function. Summarizing on those conditions in which there was enough information, we conclude that high stress levels, whether intrinsic (triggered by the cognitive challenge) or extrinsic (induced by conditions completely unrelated to the cognitive task), tend to facilitate Pavlovian conditioning (in a linear-asymptotic manner), while being deleterious for spatial/explicit information processing (which with regard to intrinsic stress levels follows an inverted U-shape effect). Moreover, after reviewing the literature, we conclude that all selected factors are essential to develop an integrative model that defines the outcome of stress effects in memory processes. In parallel, we provide a brief review of the main neurobiological mechanisms proposed to account for the different effects of stress in memory function. Glucocorticoids were found as a common mediating mechanism for both the facilitating and impairing actions of stress in different memory processes and phases. Among the brain regions implicated, the hippocampus, amygdala, and prefrontal cortex were highlighted as critical for the mediation of stress effects.

The coming out of the brain mineralocorticoid receptor

Corticosteroids - secreted after stress - have profound effects on brain and behavior. These effects are mediated by mineralocorticoid and glucocorticoid receptors, which are abundantly expressed in limbic neurons. The role of mineralocorticoid receptors in higher brain functions has never been well understood. Here we argue that the recently discovered low-affinity membrane version of the mineralocorticoid receptor contributes to the initial phase of the stress reaction; this is complemented by the glucocorticoid receptor which terminates the stress response. This concept may explain why human carriers of a mineralocorticoid receptor gene variant display enhanced neuroendocrine and autonomic responsiveness to a psychological stressor.

Chronic corticosterone injections induce a decrease of ATP levels and sustained activation of AMP-activated protein kinase in hippocampal tissues of male mice

Chronic corticosterone injections induce hippocampus tissue damage and depression-like behavior in rodent animals, the cause of which is not known. Nevertheless, increasing evidence shows that adenylate kinase (AK) and AMP-activated protein kinase (AMPK) play a very important role in intracellular energy metabolism and are especially critical for neurons which are known to have very small energy reserves and narrow margin of safety between the energy that can be generated and the energy required for maximum activity. Abnormalities of AK or AMPK system have detrimental effects on neurons or brain function especially at times of increased activity. In this study, we investigated the effects of chronic corticosterone exposure on energy metabolism, as well as AK and AMPK in hippocampal tissues in male C57BL/6N mice. Our results show that chronic corticosterone injection induced depression-like behavior in male mice, significantly decreased the energy levels and caused a sustained increase of AMP:ATP ratio in hippocampal tissues. Interestingly, chronic corticosterone injections did not produce obvious effects on AK1 protein and mRNA levels, but caused a sustained activation of AMPK. The results indicate that sustained AMPK activation might be a mechanism by which chronic corticosterone treatment causes depression-like behavior in male mice.

Exposure to novelty and forced swimming evoke stressor-dependent changes in extracellular GABA in the rat hippocampus

In the hippocampus, a brain structure critically important in the stress response, GABA controls neuronal activity not only via synaptic inhibition, but also via tonic inhibition through stimulation of extrasynaptic GABA receptors. The extracellular level of GABA may represent a major determinant for tonic inhibition and, therefore, it is surprising that its responsiveness to stress has hardly been investigated. To clarify whether hippocampal extracellular GABA levels change in response to acute stress, we conducted an in vivo microdialysis study in rats. We found that dialysate GABA levels respond to various neuropharmacological manipulations such as reuptake inhibition, elevated concentrations of K(+), tetrodotoxin and baclofen, indicating that a large proportion of hippocampal extracellular GABA depends on neuronal release and that GABA re-uptake plays a role in determining the extracellular levels of this neurotransmitter. Next, rats were exposed to a novel cage or to forced swimming in 25 degrees C water. Interestingly, these two stressors resulted in opposite effects. Novelty caused a fast increase in GABA (120% of baseline), whereas forced swimming resulted in a profound decrease (70% of baseline). To discriminate between the psychological and physical aspects (i.e. the effects on body temperature) of forced swimming, another group of animals was forced to swim at 35 degrees C. This stressor, like novelty, caused an increase in hippocampal GABA, suggesting a stimulatory effect of psychological stress. The effects of novelty could not be blocked by the corticotropin-releasing factor receptor antagonist D-Phe-CRF(12-41). These results are the first to demonstrate stressor-dependent changes in hippocampal extracellular GABA; an observation which may be of particular significance for GABAergic tonic inhibition of hippocampal neurons.

Metabolic regulatory network alterations in response to acute cold stress and ginsenoside intervention

Acute stress may trigger systemic biochemical and physiological changes in living organisms, leading to a rapid loss of homeostasis, which can be gradually reinstated by self-regulatory mechanisms and/or drug intervention strategy. However, such a sophisticated metabolic regulatory process has so far been poorly understood, especially from a holistic view. Urinary metabolite profiling of Sprague-Dawley rats exposed to cold temperature (-10 degrees C) for 2 h using GC/MS in conjunction with modern multivariate statistical techniques revealed drastic biochemical changes as evidenced by fluctuations of urinary metabolites and demonstrated the protective effect of total ginsenosides (TGs) in ginseng extracts on stressed rats. The metabonomics approach enables us to visualize significant alterations in metabolite expression patterns as a result of stress-induced metabolic responses and post-stress compensation, and drug intervention. Several major metabolic pathways including catecholamines, glucocorticoids, the tricarboxylic acid (TCA) cycle, tryptophan (nicotinate), and gut microbiota metabolites were identified to be involved in metabolic regulation and compensation required to restore homeostasis.

The principle of homeostasis in the hypothalamus-pituitary-adrenal system: new insight from positive feedback

Feedback control, both negative and positive, is a fundamental feature of biological systems. Some of these systems strive to achieve a state of equilibrium or “homeostasis”. The major endocrine systems are regulated by negative feedback, a process believed to maintain hormonal levels within a relatively narrow range. Positive feedback is often thought to have a destabilizing effect. Here, we present a “principle of homeostasis,” which makes use of both positive and negative feedback loops. To test the hypothesis that this homeostatic concept is valid for the regulation of cortisol, we assessed experimental data in humans with different conditions (gender, obesity, endocrine disorders, medication) and analyzed these data by a novel computational approach. We showed that all obtained data sets were in agreement with the presented concept of homeostasis in the hypothalamus-pituitary-adrenal axis. According to this concept, a homeostatic system can stabilize itself with the help of a positive feedback loop. The brain mineralocorticoid and glucocorticoid receptors—with their known characteristics—fulfill the key functions in the homeostatic concept: binding cortisol with high and low affinities, acting in opposing manners, and mediating feedback effects on cortisol. This study supports the interaction between positive and negative feedback loops in the hypothalamus-pituitary-adrenal system and in this way sheds new light on the function of dual receptor regulation. Current knowledge suggests that this principle of homeostasis could also apply to other biological systems.

A rapid inhibition of NMDA receptor current by corticosterone in cultured hippocampal neurons

The stress level of corticosterone (CORT) may enhance the vulnerability of neurons to insult by increasing N-methyl-D-aspartate (NMDA) receptor activity. In this study, we present data showing that CORT could exert an inhibitory effect on NMDA currents in cultured neonatal hippocampal neurons. Extracellular application of 0.1,1,10 and 100 microM CORT significantly reduced the inward current evoked by co-application of NMDA (100 microM). Extracellular application of a membrane-impermeable CORT-BSA (10 microM) maintained the CORT effect. RU38486 (10 microM) failed to block the CORT (1 microM) inhibitory effect. Additionally, intracellular application of CORT (10 microM) showing the lack of effectiveness indicated that a non-genomic mechanism mediated the CORT suppression on NMDA receptors in hippocampal neurons. Furthermore, to elevate the activity of protein kinase A by intracellular 8-Br-cAMP maintained the suppressive effect of CORT on NMDA current. Intracellular blockade of protein kinase A by Rp-cAMP (10 microM) or staurosporine (50 nM) reduced NMDA currents and abolished CORT depression of NMDA currents. These data indicated that NMDA current itself is dependent on protein kinase A (PKA) activity and CORT depression of the current could be PKA-dependent at the same time. The rapid inhibitory effect of the stress level CORT on NMDA current might suggest a protective mechanism for neurons exposed to a transient increase in glucocorticoids.

Interactions between methamphetamine and environmental stress: role of oxidative stress, glutamate and mitochondrial dysfunction

AIMS

Methamphetamine is an amphetamine derivative that is abused increasingly world-wide at an alarming rate over the last decade. Pre-clinical and human studies have shown that methamphetamine is neurotoxic to brain dopamine and serotonin. Other lines of study indicate that stress enhances the vulnerability to drug abuse. The purpose of this review is to shed light on the biochemical similarities between methamphetamine and stress in an effort to highlight the possibility that prior exposure to stress may interact with methamphetamine to exacerbate neurotoxicity.

METHODS

A review of the literature on methamphetamine and stress was conducted that focused on the common neurotoxic and biochemical consequences of methamphetamine administration and stress exposure.

RESULTS

Experimental findings of a large number of studies suggest that there are parallels between stress and methamphetamine with regard to their ability to increase glutamate release, produce a metabolic compromise and cause oxidative damage.

CONCLUSION

A combination of methamphetamine administration and stress can act synergistically and/or additively to cause or augment toxicity in brain regions such as striatum and hippocampus.

LTP after stress: up or down?

When an organism is exposed to a stressful situation, corticosteroid levels in the brain rise. This rise has consequences for behavioral performance, including memory formation. Over the past decades, it has become clear that a rise in corticosteroid level is also accompanied by a reduction in hippocampal long-term potentiation (LTP). Recent studies, however, indicate that stress does not lead to a universal suppression of LTP. Many factors, including the type of stress, the phase of the stress response, the area of investigation, type of LTP, and the life history of the organism determine in which direction LTP will be changed.

Plasma cortisol and progression of dementia in subjects with Alzheimer-type dementia

OBJECTIVE

Studies of subjects with dementia of the Alzheimer type have reported correlations between increases in activity of the hypothalamic-pituitary-adrenal (HPA) axis and hippocampal degeneration. In this study, the authors sought to determine whether increases in plasma cortisol, a marker of HPA activity, were associated with clinical and cognitive measures of the rate of disease progression in subjects with Alzheimer-type dementia.

METHOD

Thirty-three subjects with very mild and mild Alzheimer-type dementia and 21 subjects without dementia were assessed annually for up to 4 years with the Clinical Dementia Rating scale and a battery of neuropsychological tests. Plasma was obtained at 8 a.m. on a single day and assayed for cortisol. Rates of change over time in the clinical and cognitive measures were derived from growth curve models.

RESULTS

In the subjects with dementia, but not in those without dementia, higher plasma cortisol levels were associated with more rapidly increasing symptoms of dementia and more rapidly decreasing performance on neuropsychological tests associated with temporal lobe function. No associations were observed between plasma cortisol levels and clinical and cognitive assessments obtained at the single assessment closest in time to the plasma collection.

CONCLUSIONS

Higher HPA activity, as reflected by increased plasma cortisol levels, is associated with more rapid disease progression in subjects with Alzheimer-type dementia.

The temporal dynamics model of emotional memory processing: a synthesis on the neurobiological basis of stress-induced amnesia, flashbulb and traumatic memories, and the Yerkes-Dodson law

We have reviewed research on the effects of stress on LTP in the hippocampus, amygdala and prefrontal cortex (PFC) and present new findings which provide insight into how the attention and memory-related functions of these structures are influenced by strong emotionality. We have incorporated the stress-LTP findings into our "temporal dynamics" model, which provides a framework for understanding the neurobiological basis of flashbulb and traumatic memories, as well as stress-induced amnesia. An important feature of the model is the idea that endogenous mechanisms of plasticity in the hippocampus and amygdala are rapidly activated for a relatively short period of time by a strong emotional learning experience. Following this activational period, both structures undergo a state in which the induction of new plasticity is suppressed, which facilitates the memory consolidation process. We further propose that with the onset of strong emotionality, the hippocampus rapidly shifts from a "configural/cognitive map" mode to a "flashbulb memory" mode, which underlies the long-lasting, but fragmented, nature of traumatic memories. Finally, we have speculated on the significance of stress-LTP interactions in the context of the Yerkes-Dodson Law, a well-cited, but misunderstood, century-old principle which states that the relationship between arousal and behavioral performance can be linear or curvilinear, depending on the difficulty of the task.

Plasma cortisol and progression of dementia in subjects with Alzheimer-type dementia

OBJECTIVE

Studies of subjects with dementia of the Alzheimer type have reported correlations between increases in activity of the hypothalamic-pituitary-adrenal (HPA) axis and hippocampal degeneration. In this study, the authors sought to determine whether increases in plasma cortisol, a marker of HPA activity, were associated with clinical and cognitive measures of the rate of disease progression in subjects with Alzheimer-type dementia.

METHOD

Thirty-three subjects with very mild and mild Alzheimer-type dementia and 21 subjects without dementia were assessed annually for up to 4 years with the Clinical Dementia Rating scale and a battery of neuropsychological tests. Plasma was obtained at 8 a.m. on a single day and assayed for cortisol. Rates of change over time in the clinical and cognitive measures were derived from growth curve models.

RESULTS

In the subjects with dementia, but not in those without dementia, higher plasma cortisol levels were associated with more rapidly increasing symptoms of dementia and more rapidly decreasing performance on neuropsychological tests associated with temporal lobe function. No associations were observed between plasma cortisol levels and clinical and cognitive assessments obtained at the single assessment closest in time to the plasma collection.

CONCLUSIONS

Higher HPA activity, as reflected by increased plasma cortisol levels, is associated with more rapid disease progression in subjects with Alzheimer-type dementia.

Permissive influence of stress in the expression of a U-shaped relationship between serum corticosterone levels and spatial memory errors in rats

The relationship between glucocorticoids (GCs) and memory is complex, in that memory impairments can occur in response to manipulations that either increase or decrease GC levels. We investigated this issue by assessing the relationship between serum corticosterone (the primary rodent GC) and memory in rats trained in the radial arm water maze, a hippocampus-dependent spatial memory task. Each day, rats learned a new location of the hidden escape platform and then 30 min later their memory of the location of the platform was tested. Under control conditions, well-trained rats had excellent spatial memory and moderately elevated corticosterone levels (approximately 26 microg/dl versus a baseline of approximately 2 microg/dl). Their memory was impaired when corticosterone levels were either reduced by metyrapone (a corticosterone synthesis inhibitor) or increased by acute stress (predator exposure), forming an overall U-shaped relationship between corticosterone levels and memory. We then addressed whether there was a causal relationship between elevated corticosterone levels and impaired memory. If elevated corticosterone levels were a sufficient condition to impair memory, then exogenously administered corticosterone, alone, should have impaired performance. However, we found that spatial memory was not impaired in corticosterone-injected rats that were not exposed to the cat. This work demonstrates that an intermediate level of corticosterone correlated with optimal memory, and either a decrease or an increase in corticosterone levels, in conjunction with strong emotionality, impaired spatial memory. These findings indicate that fear-provoking conditions, which are known to engage the amygdala, interact with stress levels of corticosterone to influence hippocampal functioning.

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.