Hippocampal aging and adrenocorticoids: quantitative correlations

What is the functional significance of chronic stress-induced CA3 dendritic retraction within the hippocampus?

Chronic stress produces consistent and reversible changes within the dendritic arbors of CA3 hippocampal neurons, characterized by decreased dendritic length and reduced branch number. This chronic stress-induced dendritic retraction has traditionally corresponded to hippocampus-dependent spatial memory deficits. However, anomalous findings have raised doubts as to whether a CA3 dendritic retraction is sufficient to compromise hippocampal function. The purpose of this review is to outline the mechanism underlying chronic stress-induced CA3 dendritic retraction and to explain why CA3 dendritic retraction has been thought to mediate spatial memory. The anomalous findings provide support for a modified hypothesis, in which chronic stress is proposed to induce CA3 dendritic retraction, which then disrupts hypothalamic-pituitary-adrenal axis activity, leading to dysregulated glucocorticoid release. The combination of hippocampal CA3 dendritic retraction and elevated glucocorticoid release contributes to impaired spatial memory. These findings are presented in the context of clinical conditions associated with elevated glucocorticoids.

Regulation and expression of heme oxygenase enzymes in aged-rat brain: age related depression in HO-1 and HO-2 expression and altered stress-response

The heme oxygenase isozymes, HO-1 and HO-2, oxidatively cleave the heme molecule to produce biliverdin and the gaseous messenger, CO. The cleavage results in the release of iron, a regulator of transferrin, ferritin, and nitric oxide (NO) synthase gene expression. Biliverdin reductase (BVR) then catalyzes the reduction of biliverdin, generating the potent intracellular antioxidant, bilirubin. We report an age-related decrease in HO-1 and HO-2 expression present in select brain regions including the hippocampus and the substantia nigra, that are involved in the high order cognitive processes of learning and memory. The age-related loss of monoxide-producing potential in select regions of the brain was not specific to the HO system but was also observed in neuronal NO-generating system. Furthermore, compared to 2-month old rats, the ability of aged brain tissue to respond to hypoxic/hyperthermia was compromised at both the protein and the transcription levels as judged by attenuated induction of HO-1 immunoreactive protein and its 1.8 Kb transcript. Neotrofin (AIT), a cognitive-enhancing and neuroprotective drug, caused a robust increase in HO-1 immunoreactive protein in select neuronal regions and increased the expression of HO-2 transcripts. The potential interplay between regulation of HO-2 gene expression and the serum levels of the adrenal steroids is discussed. We suggest the search for therapeutic agents that reverse the decline and aberrant stress response of HO enzymes may lead to effective treatment regimens for age-associated neuronal deficits.

Chronic stress accelerates learning and memory impairments and increases amyloid deposition in APPV717I-CT100 transgenic mice, an Alzheimer's disease model

Although chronic stress is known to be linked with memory and other neurological disorders, little is known about the relationship between chronic stress and the onset or development of Alzheimer's disease (AD). In this study, we investigated the effects of long-term stress on the onset and severity of cognitive deficits and pathological changes in APPV717I-CT100 mice overexpressing human APP-CT100 containing the London mutation (V717I) after exposure to immobilization stress. We found that chronic immobilization stress accelerated cognitive impairments, as accessed by the Passive avoidance and the Social Transfer of Food Preference (STFP) tests. Moreover, the numbers and densities of vascular and extracellular deposits containing amyloid beta peptide (Abeta) and carboxyl-terminal fragments of amyloid precursor protein (APP-CTFs), which are pathologic markers of AD, were significantly elevated in stressed animals, especially in the hippocampus. Moreover, stressed animals, also showed highly elevated levels of neurodegeneration and tau phosphorylation and increased intraneuronal Abeta and APP-CTFs immunoreactivities in the hippocampus and in the entorhinal and piriform cortex. This study provides the first evidence that chronic stress accelerates the onset and severity of cognitive deficits and that these are highly correlated with pathological changes, which thus indicates that chronic stress may be an important contributor to the onset and development of AD.

Polymorphisms in the gene encoding 11B-hydroxysteroid dehydrogenase type 1 (HSD11B1) and lifetime cognitive change

A rare polymorphism in the gene encoding 11B-hydroxysteroid dehydrogenase type 1 (HSD11B1: rs846911-C/A) has been associated with an increased risk of Alzheimer's disease. We tested the hypothesis that this and 2 other HSD11B1 polymorphisms (rs12086634-G/T and rs846910-A/G) were associated with lifetime cognitive change in humans. Subjects were 194 participants of the Scottish Mental Survey of 1932 who took the same well-validated mental test at age 11 and age 79. The subjects represented the highest and lowest quintiles with respect to cognitive decline between ages 11 and 79. Despite having non-significantly different IQs at age 11, by age 79 the groups had mean (S.D.) IQs of 80.3 (14.1) and 109.6 (9.1), respectively (p<.001). The polymorphism rs846911-C/A was absent from both groups. There were no significant differences in the frequency of polymorphisms of rs12086634-G/T (p=.91) and rs846910-A/G (p=.90) between the groups. We conclude that these variants in HSD11B1 are not significant contributors to the range of cognitive ageing examined here.

Mid-life stress and cognitive deficits during early aging in rats: individual differences and hippocampal correlates

We explored here the possibility that mid-life stress in rats could have deleterious effects on cognitive abilities during early aging, as well as the potential role of inter-individual differences on the development of such effects. Male Wistar rats were classified according to their reactivity to novelty (4 months old) as highly (HR) or low (LR) reactive and, at mid-life (12 months old), either submitted to chronic stress (28 days) or left undisturbed. At early aging (18 months old), their learning abilities were tested in the water maze, and a number of neuroendocrine (plasma corticosterone; hippocampal corticosteroid receptors) and neurobiological (hippocampal expression of neuronal cell adhesion molecules) parameters were evaluated. Impaired performance was observed in stressed HR rats, as compared to unstressed HR and stressed LR rats. Increased hippocampal mineralocorticoid receptors were found in stressed LR rats when compared with stressed HR and control LR groups. In addition, mid-life stress-induced an increased corticosterone response and a reduction in NCAM-180 isoform and L1 regardless of the behavioral trait of novelty reactivity. These findings highlight a role of stress experienced throughout life on cognitive impairment occurring during the early aging period, as well as the importance of taking into account individual differences to understand variability in such cognitive decline.

Mechanisms of late-onset cognitive decline after early-life stress

Progressive cognitive deficits that emerge with aging are a result of complex interactions of genetic and environmental factors. Whereas much has been learned about the genetic underpinnings of these disorders, the nature of "acquired" contributing factors, and the mechanisms by which they promote progressive learning and memory dysfunction, remain largely unknown. Here, we demonstrate that a period of early-life "psychological" stress causes late-onset, selective deterioration of both complex behavior and synaptic plasticity: two forms of memory involving the hippocampus, were severely but selectively impaired in middle-aged, but not young adult, rats exposed to fragmented maternal care during the early postnatal period. At the cellular level, disturbances to hippocampal long-term potentiation paralleled the behavioral changes and were accompanied by dendritic atrophy and mossy fiber expansion. These findings constitute the first evidence that a short period of stress early in life can lead to delayed, progressive impairments of synaptic and behavioral measures of hippocampal function, with potential implications to the basis of age-related cognitive disorders in humans.

Aging, stress and the hippocampus

Functional loss often occurs in many body systems (e.g., endocrine, cognitive, motor) with the passage of years, but there is great individual variation in the degree of compromise shown. The current focus on brain aging will continue because demographic trends indicate that the average lifespan will show a continued increase. There is increasing emphasis on understanding how aging contributes to a decline in brain functions, cognition being a prime example. This is due in part to the fact that dementias and other losses in brain function that sometimes accompany aging cause an obvious decline in the quality of life and these deficits are of more concern as the number of elderly increase. Stress also is a ubiquitous aspect of life and there is now a greater interest in understanding the role of stress and the stress response in brain aging. The key role of the hippocampus and its related brain structures in cognition, as well as in the feedback control of the response to stress, have made this brain area a logical focus of investigation for those interested in the impact of stress on brain aging. Here, we describe how the hippocampus changes with age and we examine the idea that age-related changes in the secretion patterns of the hypothalamic-pituitary adrenal (HPA) axis can contribute to aging of this structure. We also examine the proposal that stress, perhaps due to compromised HPA axis function, can contribute to hippocampal aging through exposure to excessive levels of glucocorticoids. The aging hippocampus does not appear to suffer a generalized loss of cells or synapses, although atrophy of the structure may occur in humans. Thus, age-related cognitive impairments are likely related to other neurobiological alterations that could include changes in the signaling, information encoding, plasticity, electrophysiological or neurochemical properties of neurons or glia. Although excessive levels of glucocorticoids are able to interfere with cognition, as well as hippocampal neuronal integrity, and aging is sometimes accompanied by an increase in these steroids because of inadequate feedback control of the HPA axis, none of these are a foregone consequence of aging. The general preservation of cells and the plastic potential of the hippocampus provide a focus for the development of pharmacological, nutritive or lifestyle strategies to combat age-related declines in the hippocampus as well as other brain areas.

The stress system in the human brain in depression and neurodegeneration

Corticotropin-releasing hormone (CRH) plays a central role in the regulation of the hypothalamic-pituitary-adrenal (HPA)-axis, i.e., the final common pathway in the stress response. The action of CRH on ACTH release is strongly potentiated by vasopressin, that is co-produced in increasing amounts when the hypothalamic paraventricular neurons are chronically activated. Whereas vasopressin stimulates ACTH release in humans, oxytocin inhibits it. ACTH release results in the release of corticosteroids from the adrenal that, subsequently, through mineralocorticoid and glucocorticoid receptors, exert negative feedback on, among other things, the hippocampus, the pituitary and the hypothalamus. The most important glucocorticoid in humans is cortisol, present in higher levels in women than in men. During aging, the activation of the CRH neurons is modest compared to the extra activation observed in Alzheimer's disease (AD) and the even stronger increase in major depression. The HPA-axis is hyperactive in depression, due to genetic factors or due to aversive stimuli that may occur during early development or adult life. At least five interacting hypothalamic peptidergic systems are involved in the symptoms of major depression. Increased production of vasopressin in depression does not only occur in neurons that colocalize CRH, but also in neurons of the supraoptic nucleus (SON), which may lead to increased plasma levels of vasopressin, that have been related to an enhanced suicide risk. The increased activity of oxytocin neurons in the paraventricular nucleus (PVN) may be related to the eating disorders in depression. The suprachiasmatic nucleus (SCN), i.e., the biological clock of the brain, shows lower vasopressin production and a smaller circadian amplitude in depression, which may explain the sleeping problems in this disorder and may contribute to the strong CRH activation. The hypothalamo-pituitary thyroid (HPT)-axis is inhibited in depression. These hypothalamic peptidergic systems, i.e., the HPA-axis, the SCN, the SON and the HPT-axis, have many interactions with aminergic systems that are also implicated in depression. CRH neurons are strongly activated in depressed patients, and so is their HPA-axis, at all levels, but the individual variability is large. It is hypothesized that particularly a subgroup of CRH neurons that projects into the brain is activated in depression and induces the symptoms of this disorder. On the other hand, there is also a lot of evidence for a direct involvement of glucocorticoids in the etiology and symptoms of depression. Although there is a close association between cerebrospinal fluid (CSF) levels of CRH and alterations in the HPA-axis in depression, much of the CRH in CSF is likely to be derived from sources other than the PVN. Furthermore, a close interaction between the HPA-axis and the hypothalamic-pituitary-gonadal (HPG)-axis exists. Organizing effects during fetal life as well as activating effects of sex hormones on the HPA-axis have been reported. Such mechanisms may be a basis for the higher prevalence of mood disorders in women as compared to men. In addition, the stress system is affected by changing levels of sex hormones, as found, e.g., in the premenstrual period, ante- and postpartum, during the transition phase to the menopause and during the use of oral contraceptives. In depressed women, plasma levels of estrogen are usually lower and plasma levels of androgens are increased, while testosterone levels are decreased in depressed men. This is explained by the fact that both in depressed males and females the HPA-axis is increased in activity, parallel to a diminished HPG-axis, while the major source of androgens in women is the adrenal, whereas in men it is the testes. It is speculated, however, that in the etiology of depression the relative levels of sex hormones play a more important role than their absolute levels. Sex hormone replacement therapy indeed seems to improve mood in elderly people and AD patients. Studies of rats have shown that high levels of cumulative corticosteroid exposure and rather extreme chronic stress induce neuronal damage that selectively affects hippocampal structure. Studies performed under less extreme circumstances have so far provided conflicting data. The corticosteroid neurotoxicity hypothesis that evolved as a result of these initial observations is, however, not supported by clinical and experimental observations. In a few recent postmortem studies in patients treated with corticosteroids and patients who had been seriously and chronically depressed no indications for AD neuropathology, massive cell loss, or loss of plasticity could be found, while the incidence of apoptosis was extremely rare and only seen outside regions expected to be at risk for steroid overexposure. In addition, various recent experimental studies using good stereological methods failed to find massive cell loss in the hippocampus following exposure to stress or steroids, but rather showed adaptive and reversible changes in structural parameters after stress. Thus, the HPA-axis in AD is only moderately activated, possibly due to the initial (primary) hippocampal degeneration in this condition. There are no convincing arguments to presume a causal, primary role for cortisol in the pathogenesis of AD. Although cortisol and CRH may well be causally involved in the signs and symptoms of depression, there is so far no evidence for any major irreversible damage in the human hippocampus in this disorder.

Stress hormones and human memory function across the lifespan

In this paper, we summarize the data obtained in our laboratory showing the effects of glucocorticoids on human cognitive function in older adults, young adults and children. We first present data obtained in the aged human population which showed that long-term exposure to high endogenous levels of glucocorticoids is associated with both memory impairments and a 14% smaller volume of the hippocampus. We then report on studies showing that in older adults with moderate levels of glucocorticoids, memory performance can be acutely modulated by pharmacological manipulations of glucocorticoids. In young adults, we present data obtained in our laboratory showing that cognitive processing sustained by the frontal lobes is also sensitive to acute increases of glucocorticoids. We also summarize studies showing that just as in older adults, memory performance in young adults can be acutely modulated by pharmacological manipulations of glucocorticoids. We then present a study in which we showed a differential involvement of adrenergic and glucocorticoid hormones for short- and long-term memory of neutral and emotional information. In the last section of the paper, we present data obtained in a population of young children and teenagers from low and high socioeconomic status (SES), where we showed that children from low SES present significantly higher levels of basal cortisol when compared to children from high SES. We then present new data obtained in this population showing that children and teenagers from low and high SES do not process the plausibility of positive and negative attributes in the same way. Children from low SES tended to process positive and negative attributes on a more negative note than children from high SES, and this type of processing was significantly related to basal cortisol at age 10, 12 and 14. Altogether, the results of these studies show that both bottom-up (effects of glucocorticoids on cognitive function), and top-down (effects of cognitive processing on glucocorticoid secretion) effects exist in the human population.

Pregnenolone sulfate enhances neurogenesis and PSA-NCAM in young and aged hippocampus

Age-dependent cognitive impairments have been correlated with functional and structural modifications in the hippocampal formation. In particular, the brain endogenous steroid pregnenolone-sulfate (Preg-S) is a cognitive enhancer whose hippocampal levels have been linked physiologically to cognitive performance in senescent animals. However, the mechanism of its actions remains unknown. Because neurogenesis is sensitive to hormonal influences, we examined the effect of Preg-S on neurogenesis, a novel form of plasticity, in young and old rats. We demonstrate that in vivo infusion of Preg-S stimulates neurogenesis and the expression of the polysialylated forms of NCAM, PSA-NCAM, in the dentate gyrus of 3- and 20-month-old rats. These influences on hippocampal plasticity are mediated by the modulation of the gamma-aminobutyric acid receptor complex A (GABA(A)) receptors present on hippocampal neuroblasts. In vitro, Preg-S stimulates the division of adult-derived spheres suggesting a direct influence on progenitors. These data provide evidence that neurosteroids represent one of the local secreted signals controlling hippocampal neurogenesis. Thus, therapies which stimulate neurosteroidogenesis could preserve hippocampal plasticity and prevent the appearance of age-related cognitive disturbances.

The Use of Dehydroepiandrosterone Therapy in Clinical Practice

Dehydroepiandrosterone (DHEA) therapy is controversial due to sensationalized reports of epidemiologic studies and the over-the-counter availability of DHEA. Human clinical trials have investigated the potential efficacy of DHEA therapy in multiple conditions with resultant inconsistencies in findings. DHEA is unique compared with other adrenal steroids because of the fluctuation in serum levels found from birth into advancing age. The lower endogenous levels of DHEA and DHEA sulfate found in advancing age have been correlated with a myriad of health conditions. Also, some studies suggest gender-specific actions of endogenous and exogenous DHEA. We reviewed only pharmacokinetic studies and human clinical trials investigating the efficacy of DHEA therapy that were placebo-controlled as these provided the most reliable scientific basis for the evaluation of DHEA therapy. Pharmacodynamic studies suggest that doses of 30-50mg of oral DHEA may produce physiologic androgen levels, especially in women. These studies report a dose-dependent effect and lack of accumulation of serum androgen levels. Pharmacologic studies also reveal a gender-specific response to DHEA therapy such that testosterone levels are increased in women but not in men. Clinical trials suggest that 50mg of oral DHEA, but not <30mg, can increase serum androgen levels to within the physiologic range for young adults with primary and secondary adrenal insufficiency, possibly improve sexual function, improve mood and self-esteem, and decrease fatigue/exhaustion. Whereas DHEA replacement therapy may be effective in treating patients with adrenal insufficiency, human clinical trials investigating its efficacy in conditions such as systemic lupus erythematosus, HIV, Alzheimer disease, advancing age, male sexual dysfunction, perimenopausal symptoms, depression, and cardiovascular disease have not provided consistent findings.

11beta-hydroxysteroid dehydrogenase type 1 as a modulator of glucocorticoid action: from metabolism to memory

Increases in plasma cortisol and glucocorticoid pharmacotherapy cause myriad adverse effects from obesity and diabetes to impairments in memory. The common metabolic syndrome phenotypically resembles the rare disorder Cushing's syndrome, but plasma cortisol levels are usually normal. 11beta-Hydroxysteroid dehydrogenase type 1 (11beta-HSD1) catalyses the regeneration of active glucocorticoids (cortisol and corticosterone) from inert 11-keto forms in specific tissues, notably liver, adipose and brain. Recent work shows that obese humans and rodents have increased 11beta-HSD1 activity selectively in adipose tissue. By locally amplifying glucocorticoid action, this increase in activity might explain the Cushing's syndrome/metabolic syndrome paradox. Indeed, mice deficient in 11beta-HSD1 resist both the metabolic syndrome that develops with dietary obesity and glucocorticoid-associated cognitive impairments that develop with ageing. The ongoing development of selective 11beta-HSD1 inhibitors affords the opportunity to explore a new approach to some major common disorders.

Changes of Calbindin D-28k Immunoreactivity in the Hippocampus after Adrenalectomy in the Seizure Sensitive Gerbil

Calbindin D-28k (CB), a calcium-binding protein, containing neurons in the hippocampus plays an important role in hippocampal excitability in epilepsy. In the present study, we investigated changes of CB immunoreactivity after adrenalectomy (ADX) in the hippocampus and dentate gyrus of the seizure sensitive gerbil, which is susceptible to seizure to identify roles of CB in epileptogenesis. The changes of the CB immunoreactivity after ADX were significant in the hippocampal CA1 region. By 24 h after ADX, CB-immunoreactive CA1 pyramidal cells and CB immunoreactivity increased. At this time, well-stained dendrites projected to the stratum radiatum. Thereafter, the CB immunoreactivity decreased time dependently by 96 h after ADX. In the dentate gyrus, the changes of CB-immunoreactive neurons were mainly observed in the granule cell layer. The number and immunoreactivity of CB-immunoreactive neurons was high at 24 h after ADX, thereafter, those decreased by 96 h after ADX. These results suggest that glucocorticoid has an important role in modulating the seizure activity and CB serves an inhibitory function, which regulates the seizure activity and output signals from the hippocampus.

Age- and sex-dependent development of adrenocortical hyperactivity in a transgenic mouse model of Alzheimer's disease

In this study, we investigated mice of the TgCRND8 line, an APP transgenic mouse model of Alzheimer's disease (AD), with respect to behavioral, endocrinological, and neuropathological parameters. Our results show that transgenic and wild-type mice did not differ in their general health status, exploratory and anxiety related behavior as well as in the activity of their sympathetic-adrenomedullary system. Significant differences, however, were found regarding body weight, amyloid plaque formation, and the activity of the hypothalamic-pituitary-adrenocortical (HPA) axis. Continuous monitoring of glucocorticoid (GC) concentrations over a period of 120 days, utilizing a noninvasive technique to measure corticosterone metabolites in fecal samples, revealed that transgenic animals showed adrenocortical hyperactivity, starting very early in males (from day 30) and later in females (around day 90). It is hypothesized that these changes in the activity of the HPA axis are linked to amyloid-beta associated pathological alterations in the hippocampus, causing degenerations in the negative feedback regulation of the HPA axis leading to hypersecretion of GC. Thus, the development of adrenocortical hyperactivity might be a key-element in the understanding of AD.

Impact of age and caloric restriction on neurogenesis in the dentate gyrus of C57BL/6 mice

Age-related changes in neurogenesis and its modulation by caloric restriction (CR) were studied in C57BL/6 mice. To this end, bromodeoxyuridine (BrdU) labeling was used to assess neuronal and glial precursor proliferation and survival in the granular cell layer (GCL) and the hilus of the dentate gyrus of 2-, 12-, 18-, and 24-month-old mice. For both regions, we found an age-dependent decrease in proliferation but not in survival of newborn cells. Interestingly, the reduction in proliferation occurred between 2 and 18 months of age with no additional decline between 18- and 24-month-old mice. Phenotyping of the newborn cells revealed a decrease in the neuron fraction in the GCL between 2 and 12 months of age but not thereafter. The majority of BrdU cells in the hilus colocalized with astrocytic but none with neuronal markers. CR from 3 to 11 months of age had no effect on neurogenesis in the GCL, but had a survival-promoting effect on newly generated glial cells in the hilus of the dentate gyrus. In conclusion, C57BL/6 mice reveal a substantial reduction in neurogenesis in the dentate gyrus until late adulthood with no further decline with aging. Long-term CR does not counteract this age-related decline in neurogenesis but promotes survival of hilar glial cells.

11Beta-hydroxysteroid dehydrogenase inhibition improves cognitive function in healthy elderly men and type 2 diabetics

In aging humans and rodents, inter-individual differences in cognitive function have been ascribed to variations in long-term glucocorticoid exposure. 11beta-Hydroxysteroid dehydrogenase type 1 (11beta-HSD1) regenerates the active glucocorticoid cortisol from circulating inert cortisone, thus amplifying intracellular glucocorticoid levels in some tissues. We show that 11beta-HSD1, but not 11beta-HSD2, mRNA is expressed in the human hippocampus, frontal cortex, and cerebellum. In two randomized, double-blind, placebo-controlled crossover studies, administration of the 11beta-HSD inhibitor carbenoxolone (100 mg three times per day) improved verbal fluency (P < 0.01) after 4 weeks in 10 healthy elderly men (aged 55-75 y) and improved verbal memory (P < 0.01) after 6 weeks in 12 patients with type 2 diabetes (52-70 y). Although carbenoxolone has been reported to enhance hepatic insulin sensitivity in short-term studies, there were no changes in glycemic control or serum lipid profile, nor was plasma cortisol altered. 11beta-HSD1 inhibition may be a new approach to prevent/ameliorate cognitive decline.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Environmentally induced long-term structural changes: cues for functional orientation and vulnerabilities

Environmental challenges profoundly modify phenotypes and disrupt inherent developmental programs both at functional and structural levels. As an example, we have studied the impact of these environmental influences on adult neurogenesis in the dentate gyrus. Neurogenesis results from an inherent program, participates to hippocampal network organization and, as a consequence, to the various functional abilities depending on this region, including memories. In preclinical studies of aging we have shown that phenotypes vulnerable to the development of spatial memory disorders are characterized by lower hippocampal neurogenesis. We have hypothesized that these interindividual variations in functional expression of neurogenesis in senescent subjects could be predicted early in life. Indeed, a behavioral response (novelty-induced locomotor reactivity) and a biological trait (hypothalamo-pituitary-adrenal axis activity), which are predictive of cognitive impairments later in life, are related to neurogenesis in young adult rats. This suggests that subjects starting off with an impaired neurogenesis, here rats that are high reactive to stress, are predisposed for the development of age-related cognitive disorders. We have further shown that these inter-individual differences result from early deleterious life events. Indeed, prenatal stress orients neurogenesis in pathological ways for the entire life, and precipitates age-related cognitive impairments. Altogether these data suggest first that hippocampal neurogenesis plays a pivotal role in environmentally-induced vulnerability to the development of pathological aging, and second that environmental challenges and life events orient structural developments, leading to different phenotypes.

11β-Hydroxysteroid Dehydrogenases in the Brain

Glucocorticoids affect a wide range of processes in the brain, altering neurotransmission, electrophysiological activity, metabolism, cell division, and death. These actions are mediated by corticosteroid receptors (glucocorticoid and mineralocorticoid) that modify transcriptional activity of target genes. The amount of steroid available to activate these receptors is not only dependent on the circulating levels but also on pre-receptor metabolism of glucocorticoids occurring intracellularly. This metabolism is carried out by the enzymes 11beta-hydroxysteroid dehydrogenases (11beta-HSDs). There are two distinct isozymes, the products of distantly related genes. 11beta-HSD type 2 inactivates glucocorticoids to its inert 11-keto derivative, while 11alpha-HSD type 1 elevates intracellular glucocorticoid levels by regenerating active glucocorticoids from circulating 11-dehydrocorticosterone or cortisone. This review highlights the important and very different roles the two enzymes play in the brain, outlining recent results obtained from studying mice with a targeted gene deletion in the 11beta-HSD1 or 11beta-HSD2 genes.

Spatial memory performances of aged rats in the water maze predict levels of hippocampal neurogenesis

Neurogenesis occurs within the adult dentate gyrus of the hippocampal formation and it has been proposed that the newly born neurons, recruited into the preexistent neuronal circuits, might be involved in hippocampal-dependent learning processes. Age-dependent spatial memory impairments have been related to an alteration in hippocampal plasticity. The aim of the current study was to examine whether cognitive functions in aged rats are quantitatively correlated with hippocampal neurogenesis. To this end, we took advantage of the existence of spontaneous individual differences observed in aged subjects in a hippocampal-dependent task, the water maze. We expected that the spatial memory capabilities of aged rats would be related to the levels of hippocampal neurogenesis. Old rats were trained in the water maze, and, 3 weeks after training, rats were injected with 5-bromo-2'-deoxyuridine (BrdUrd, 50 or 150 mg/kg) to label dividing cells. Cell proliferation was examined one day after the last BrdUrd injection, whereas cell survival and differentiation were determined 3 weeks later. It is shown that a quantitative relationship exists between learning and the number of newly generated neurons. Animals with preserved spatial memory, i.e., the aged-unimpaired rats, exhibited a higher level of cell proliferation and a higher number of new neurons in comparison with rats with spatial memory impairments, i.e., the aged-impaired rats. In conclusion, the extent of memory dysfunction in aged rats is quantitatively related to the hippocampal neurogenesis. These data reinforce the assumption that neurogenesis is involved in memory processes and aged-related cognitive alterations.

Central mechanisms of stress integration: hierarchical circuitry controlling hypothalamo-pituitary-adrenocortical responsiveness

Appropriate regulatory control of the hypothalamo-pituitary-adrenocortical stress axis is essential to health and survival. The following review documents the principle extrinsic and intrinsic mechanisms responsible for regulating stress-responsive CRH neurons of the hypothalamic paraventricular nucleus, which summate excitatory and inhibitory inputs into a net secretory signal at the pituitary gland. Regions that directly innervate these neurons are primed to relay sensory information, including visceral afferents, nociceptors and circumventricular organs, thereby promoting 'reactive' corticosteroid responses to emergent homeostatic challenges. Indirect inputs from the limbic-associated structures are capable of activating these same cells in the absence of frank physiological challenges; such 'anticipatory' signals regulate glucocorticoid release under conditions in which physical challenges may be predicted, either by innate programs or conditioned stimuli. Importantly, 'anticipatory' circuits are integrated with neural pathways subserving 'reactive' responses at multiple levels. The resultant hierarchical organization of stress-responsive neurocircuitries is capable of comparing information from multiple limbic sources with internally generated and peripherally sensed information, thereby tuning the relative activity of the adrenal cortex. Imbalances among these limbic pathways and homeostatic sensors are likely to underlie hypothalamo-pituitary-adrenocortical dysfunction associated with numerous disease processes.

HPA axis and memory

The hormones of the hypothalamus-pituitary-adrenal (HPA) axis influence memory in situations of acute and chronic stress. The present review tries to summarize the current state of knowledge by describing the enhancing as well as the impairing effects of stress or glucocorticoid (GC) treatment documented in animals and humans. GCs secreted during the acquisition of a stressful task facilitate consolidation. However, acute stress (or GC treatment) unrelated to the task impairs performance. The effects of acute stress are additionally modulated by gender, age and the emotional valence of the learning material. Chronic stress in rodents has mostly impairing effects on memory and hippocampal integrity. However, other regions of the brain, such as the prefrontal cortex, are also sensitive to stress. In humans, similar observations have been reported in several patient populations as well as in older subjects. The potential to reverse these effects using behavioural or pharmacological approaches needs to be explored.

Chronic glucocorticoid receptor activation impairs CREB transcriptional activity in clonal neurons

Excessive circulating levels of glucocorticoids are thought to be associated with cognitive impairment. We provide evidence that chronic activation of the glucocorticoid receptor (GR) in clonal neurons inhibits the transcriptional activity of the cyclic AMP response element-binding protein (CREB), which is believed to be involved in memory processes. To investigate the underlying mechanism we studied the phosphorylation of CREB and found altered phosphorylation kinetics in neurons chronically treated with glucocorticoids. Our results demonstrate a hitherto unrecognized crosstalk between the cyclic AMP and glucocorticoid pathway and may provide the molecular basis for the effects of long-term glucocorticoid exposure on cognitive function.

Adaptogenic activity of a novel withanolide-free aqueous fraction from the roots of Withania somnifera Dun. (Part II)

In the Indian traditional system of medicine Withania somnifera Dun. is widely regarded as the Indian Ginseng. A new withanolide-free hydrosoluble fraction was isolated from the roots of Withania somnifera Dun. and was evaluated for putative antistress activity against a battery of tests to delineate the activity of this fraction. The latter fraction exhibited significant antistress activity in a dose-related manner (Singh et al., 2001) and was further studied against chemical and physical induced stress in rats and mice. The extract of Withania somnifera root (a commercial preparation available locally) was also used to compare the results. A preliminary acute toxicity study in mice showed a good margin of safety with a high therapeutic index.

Alterations in the balance of protein kinase and phosphatase activities and age-related impairments of synaptic transmission and long-term potentiation

Aging is associated with an impaired ability to maintain long-term potentiation (LTP), but the underlying cause of the impairment remains unclear. To gain a better understanding of the cellular and molecular mechanisms responsible for this impairment, the synaptic transmission and plasticity were studied in the CA1 region of hippocampal slices from adult (6-8 months) and poor-memory (PM)-aged (23-24 months) rats. The one-way inhibitory avoidance learning task was used as the behavioral paradigm to screen PM-aged rats. With intracellular recordings, CA1 neurons of PM-aged rats exhibited a more hyperpolarized resting membrane potential, reduced input resistance, and increased amplitude of afterhyperpolarization and spike threshold, compared with those in adult rats. Although a reduction in the size of excitatory synaptic response was observed in PM-aged rats, no obvious differences were found between adult and PM-aged rats in the pharmacological properties of excitatory synaptic response, paired-pulse facilitation, or frequency-dependent facilitation, which was tested with trains of 10 pulses at 1, 5, and 10 Hz. Slices from the PM-aged rats displayed significantly reduced early-phase long-term potentiation (E-LTP) and late-phase LTP (L-LTP), and the entire frequency-response curve of LTP and LTD is modified to favor LTD induction. The susceptibility of time-dependent reversal of LTP by low-frequency afferent stimulation was also facilitated in PM-aged rats. Bath application of the protein phosphatase inhibitor, calyculin A, enhanced synaptic response in slices from PM-aged, but not adult, rats. In contrast, application of the cAMP-dependent protein kinase inhibitors, Rp-8-CPT-cAMPS and KT5720, induced a decrease in synaptic transmission only in slices from the adult rats. Furthermore, the selective beta-adrenergic receptor agonist, isoproterenol, and pertussis toxin-sensitive G-protein inhibitor, N-ethylmaleimide, effectively restored the deficit in E-LTP and L-LTP of PM-aged rats. These results demonstrate that age-related impairments of synaptic transmission and LTP may result from alterations in the balance of protein kinase/phosphatase activities.

Cortisol, DHEAS and aging: resistance to cortisol suppression in frail institutionalized elderly

Convincing evidences has linked the hypothalamus-pituitary-adrenal (HPA) axis to aging patterns. F excess is implicated in the development of frailty characteristics whereas DHEAS is positively correlated to successful aging. We compared serum F and DHEAS levels of independent community-living (successful group, 19 M and 28 F, 69 to 87 yr) with those of institutionalized elderly (frail group, 20 M and 30 F, 65 to 95 yr). Serum F was determined at 1) baseline (08:00 h, 16:00 h and 23:00 h), 2) after 2 overnight dexamethasone (DEX) suppression tests (DST, using 0.25 and 1.0 mg doses), and 3) 60 min after ACTH stimulation (250 microg i.v. bolus); serum DHEAS was determined at 08:00 h. Basal serum F at 08:00 h, 16:00 h and 23:00 h and serum DHEAS levels were similar in both groups; however F: DHEAS ratio at 08:00 h was higher in the frail, compared to the successful group (mean +/- SD: 0.55 +/- 0.53 and 0.35 +/- 0.41, respectively; p = 0.04). In response to DST, F suppression was less effective in frail elderly after either 0.25 or 1.0 mg doses (9.0 +/- 6.0 and 2.0 +/- 0.9 microg/dl), as compared to the successful group (5.8 +/- 4.4 and 1.5 +/- 0.5 microg/dl) (p = 0.01). In addition, a significant correlation was observed between post-DEX F levels (both doses) and parameters of cognitive and physical frailty. Normal and similar F levels were observed after ACTH stimulation in both groups. Our data suggest a deficient feedback regulation of the HPA axis in frail institutionalized elderly, as demonstrated by a higher set point for F suppression. This augmented HPA tonus enforces the hypothesis that even milder F excess may be related to characteristics of frailty in the elderly.

Early disruption of the mother-infant relationship: effects on brain plasticity and implications for psychopathology

Early environmental manipulations can impact on the developing nervous system, contributing to shape individual differences in physiological and behavioral responses to environmental challenges. In particular, it has been shown that disruptions in the mother-infant relationship result in neuroendocrine, neurochemical and behavioural changes in the adult organism, although the basic mechanisms underlying such changes have not been completely elucidated. Recent data suggest that neurotrophins might be among the mediators capable of transducing the effects of external manipulations on brain development. Nerve growth factor and brain-derived neurotrophic factor are known to play a major role during brain development, while in the adult animal they are mainly responsible for the maintenance of neuronal function and structural integrity. Changes in the levels of neurotrophic factors during critical developmental stages might result in long-term changes in neuronal plasticity and lead to increased vulnerability to aging and to psychopathology.

Increased astrocyte reactivity in the hippocampus of murine models of type 1 diabetes: the nonobese diabetic (NOD) and streptozotocin-treated mice

Diabetes can be associated with cerebral dysfunction in humans and animal models of the disease. Moreover, brain anomalies and alterations of the neuroendocrine system are present in type 1 diabetes (T1D) animals, such as the spontaneous nonobese diabetic (NOD) mouse model and/or the pharmacological streptozotocin (STZ)-induced model. Because of the prevalent role of astrocytes in cerebral glucose metabolism and their intimate connection with neurones, we investigated hippocampal astrocyte alterations in prediabetic and diabetic NOD mice and STZ-treated diabetic mice. The number and cell area related to the glial fibrillary acidic protein (GFAP)-immunoreactive astrocytes were quantified in the stratum radiatum region of the hippocampus by computerized image analysis in prediabetic (2, 4 and 8 weeks of age) and diabetic (16-week-old) NOD female mice, age and sex-matched lymphocyte-deficient NODscid and C57BL/6 control mice and, finally, STZ-induced diabetic and vehicle-treated nondiabetic 16-week-old C57BL/6 female mice. Astrocyte number was higher early in life in prediabetic NOD and NODscid mice than in controls, when transient hyperinsulinemia and low glycemia were found in these strains. The number and cell area of GFAP(+) cells further increased after the onset of diabetes in NOD mice. Similarly, in STZ-treated diabetic mice, the number of GFAP(+) cells and cell area were higher than in vehicle-treated mice. In conclusion, astrocyte changes present in genetic and pharmacological models of T1D appear to reflect an adaptive process to alterations of glucose homeostasis.

Molecular inflammation hypothesis of aging based on the anti-aging mechanism of calorie restriction

Accumulating evidence strongly suggests that oxidative stress underlies aging processes. Research provides consistent evidence that calorie restriction (CR) reduces age-related oxidative stress and has anti-inflammatory properties. However, information is lacking on the molecular mechanism that would better define the interrelation of reactive oxygen species and nitrogen species and the pro-inflammatory states of the aging process. In this review, the biochemical and molecular bases of the inflammatory process in the aging process are analyzed to delineate the molecular inflammation hypothesis of aging. The key players involved in the proposed hypothesis are the age-related upregulation of NF-kappa B, IL-1 beta, IL-6, TNFalpha, cyclooxygenase-2, and inducible NO synthase, all of which are attenuated by CR. Furthermore, age-related NF kappa B activation is associated with phosphorylation by I kappa B kinase/NIK and MAPKs, while CR blocked these activation processes. The modulation of these factors provides molecular insights of the anti-inflammatory action of CR in relation to the aging process. Based on available finding and our recent supporting evidence, we prefer to use "molecular inflammation" to emphasize the importance of the molecular reaction mechanisms and their aberrance, predisposing to fully expressed chronic inflammatory phenomena. It was further proposed that CR's major force of the regulation of redox-sensitive inflammation may well be its life-prolonging action.

Maternal deprivation stress exacerbates cognitive deficits in immature rats with recurrent seizures

PURPOSE

Maternal deprivation is stressful for the neonate. The aim of this study was to investigate the short- and long-term effects of maternal separation on recurrent seizures in the developing brain.

METHODS

Rats were divided into four groups according to whether the rat pups were treated with maternal deprivation from postnatal day 2 (P2) to P9 or neonatal seizures induced by intraperitoneal (i.p.) injection of pentylenetetrazol (PTZ) from P10 to P14. Rats in the control group received saline i.p. injection from P10 to P14; rats in the isolation group underwent daily separation from their dams from P2 to P9; rats in the PTZ-treated group were subjected to PTZ-induced recurrent seizures from P10 to P14; rats in the isolation plus PTZ-treated group were subjected to maternal deprivation from P2 to P7 followed by serial seizures from P10 to P14. In addition, subsets of rats at P15 were killed and the brains assessed for acute neuronal degeneration. Visual-spatial memory test using the Morris water maze task was performed at P80. After testing, the hippocampus was evaluated for histologic lesions and cyclic adenosine monophosphate (cAMP)-responsive element-binding protein phosphorylation at serine-133 (pCREBSer-133), an important transcription factor underlying learning and memory.

RESULTS

All rats given PTZ developed recurrent seizures. After PTZ administration, rats with a history of maternal deprivation had more intense impairment than did rats with maternal deprivation and neonatal seizures than those without deprivation. Neuronal degeneration was most prominent in the rats exposed to maternal deprivation plus recurrent seizures. Rats receiving maternal deprivation or PTZ-induced recurrent seizures exhibited only spatial deficits, but no morphologic changes in the hippocampus. However, rats with maternal deprivation plus PTZ-induced recurrent seizures exhibited worse visual-spatial learning compared with rats with either isolation or PTZ-induced recurrent seizures alone. The levels of pCREBSer-133 may play a role in the decrease in the hippocampus from the rats subjected to maternal deprivation and/or PTZ-induced recurrent seizures, as compared with rats exposed to vehicle-control saline. These results indicate that repeated maternal deprivation can exacerbate long-term cognitive deficits resulting from neonatal seizures. In addition, impaired phosphorylation of CREBSer-133.

CONCLUSIONS

Repeated maternal deprivation stress has synergistic effects with recurrent seizures in inducing neurologic damage in the developing brain.

Timing of neurodegeneration and beta-amyloid (Abeta) peptide deposition in the brain of aging kokanee salmon

Brains of kokanee salmon (Oncorhynchus nerka kennerlyi) in one of four reproductive stages (sexually immature, maturing, sexually mature, and spawning) were stained with cresyl violet and silver stain to visualize neurodegeneration. These reproductive stages correlate with increasing somatic aging of kokanee salmon, which die after spawning. Twenty-four regions of each brain were examined. Brains of sexually immature fish exhibited low levels of neurodegeneration, whereas neurodegeneration was more marked in maturing fish and greatest in spawning fish. Neurodegeneration was present in specific regions of the telencephalon, diencephalon, mesencephalon, and rhombencephalon. Pyknotic neurons were observed in all regions previously reported to be immunopositive for A beta. Regions that did not exhibit neurodegeneration during aging included the magnocellular vestibular nucleus, the nucleus lateralis tuberis of the hypothalamus, and Purkinje cells of the cerebellum, all of which also lack A beta; perhaps these regions are neuroprotected. In 14 of 16 brain areas for which data were available on both the increase in A beta deposition and pyknosis, neurodegeneration preceded or appeared more or less simultaneously with A beta production, whereas in only two regions did A beta deposition precede neurodegeneration. This information supports the hypothesis that A beta deposition is a downstream product of neurodegeneration in most brain regions. Other conclusions are that the degree of neurodegeneration varies among brain regions, neurodegeneration begins in maturing fish and peaks in spawning fish, the timing of neurodegeneration varies among brain regions, and some regions do not exhibit accelerated neurodegeneration during aging.

Individual vulnerability to substance abuse and affective disorders: role of early environmental influences

One of the most important questions raised by modern psychiatry and experimental psychopathology is the origin of mental diseases. More concisely, clinical and experimental neurosciences are increasingly concerned with the factors that render one individual more vulnerable than another to a given pathological outcome. Animal models are now available to understand the sources of individual differences for specific phenotypes prone to behavioral disadaptations. Over the last 10 years we have explored the consequences of environmental perinatal manipulations in the rat. We have shown that prenatal stress is at the origin of a wide range of physiological and behavioral aberrances such as alterations in the activity of the hormonal stress axis, increased vulnerability to drug of abuse, emotional liability, cognitive impairments and predisposition to pathological aging. Taken together, these abnormalities define a bio-behavioral syndrome. Furthermore, the cognitive disabilities observed in prenatally-stressed rats were recently related to an alteration of neurogenesis in the dentate gyrus, thus confirming the impact of early life events on brain morphology. A second model (handling model) has also been developed in which pups are briefly separated from their mothers during early postnatal life. In contrast with prenatally-stressed animals, handled rats exhibited a reduced emotion response when confronted with novel situations and were protected against age-induced impairments of both the hormonal stress axis and cognitive functions. Taken together, the results of these investigations show that the bio-behavioral phenotype that characterizes each individual is strongly linked to the nature and timing of perinatal experience. Furthermore, data collected in prenatally-stressed animals indicate that this model could be used profitably to understand the etiology and pathophysiology of affective disorders.

Effects of estrogens on choline-acetyltransferase immunoreactivity and GAP-43 mRNA in the forebrain of young and aging male rats

1. Previous work demonstrated that estradiol (E2) treatment prevented the abnormal response to stress and the reduction of glucocorticoid receptors (GR) in hippocampus from aging male rats. The mechanisms originating these effects were unknown. 2. In the present work, we investigated the E2 effects on the cholinergic, growth-associated protein (GAP-43) expressing neurons of the medial septum (MS) and vertical limb of diagonal band of Broca (VDB). These areas project to the hippocampus, and may be involved in the mentioned E2 effects in aging animals. Therefore, the response to E2 of choline-acetyltransferase (ChAT) in neurons and cell processes and GAP-43 mRNA as a marker of neurite outgrowth was studied in young and old male rats. 3. Young (3-4 months) and old (18-20 months) male Sprague-Dawley rats remained untreated or were implanted s.c. with a 14 mg pellet of E2 benzoate during 6 weeks. We used immoucytochemistry to determine ChAT and isotopic in situ hybridization to analyze GAP-43 mRNA expression. 4. Aging males showed a reduction in the number and length of ChAT-immunoreactive cell processes, but not in the number of positive neurons in MS and VDB. E2 reverted both parameters in old rats to levels of young animals. Regarding basal levels of GAP-43 mRNA, they were similar in old and young animals, but E2 treatment up-regulated GAP-43 mRNA expression in MS and VDB of old animals only. 5. Our data suggest that prolonged E2 treatment may affect hippocampal function of aging male rats by regulating in part the plasticity of cholinergic, GAP-43 expressing neurones of the basal forebrain. Without discarding a direct E2 effect on the limbic tissue, effects on the cholinergic system may have a pronounced impact on the neuroendocrine and stress responses of the aging hippocampus.

Impact of the GH-cortisol ratio on the age-dependent changes in body composition

Aging is associated with a decrease in GH levels and this is paralleled by changes in body composition, i.e., increased visceral fat, and decreased lean body mass and bone mineral density. Similar changes in body composition are seen in the state of hypercortisolism. Increasing age has been shown to be associated with elevated evening cortisol levels in men. An increased exposure of several tissues to glucocorticoids with aging, i.e., visceral fat cells, in combination with the reduction of the lipolytic effects of declining GH levels, may contribute to the age-dependent increase of visceral fat accumulation. We hypothesize that the age-dependent changes in body fat are the result of an age-dependent decrease of the GH/cortisol ratio at the level of the adipocyte. This is caused by the decline in GH concentrations and the increase in cortisol levels and/or metabolism at the adipocyte.

Glucocorticoids and 11 beta-hydroxysteroid dehydrogenase type 2 gene expression in the aging kidney

BACKGROUND

Aging is associated with increased concentrations of circulating glucocorticoids, a situation expected to induce a glucocorticoid-mediated mineralocorticoid effect, resulting in sodium retention and hypertension unless counteracting mechanisms are operative. Conversion of glucocorticoids to inert 11 beta-keto compounds by the enzyme 11 beta-hydroxysteroid dehydrogenase type 2 (11 beta-HSD2) is one of these mechanisms. We hypothesized therefore that 11 beta-HSD2 gene expression and/or activity increase with age in male WAG/Rij rats, a strain without increased blood pressure with age or senescence-related obesity or kidney disease.

MATERIALS AND METHODS

Corticosterone (B) concentrations in plasma and urinary excretion of corticosterone and dehydrocorticosterone (A) tetrahydro metabolites, THB + 5 alpha-THB + THA, were assessed by gas chromatography-mass spectrometry (GC-MS) in 10-month-old-rats (n = 6) and in 30-month-old rats (n = 6). Renal 11 beta-HSD2 messenger ribonucleic acid (mRNA) abundance was measured by real-time quantitative TaqMan polymerase chain reaction and microarray assays.

RESULTS

Thirty-month-old rats had significantly higher corticosterone concentrations in plasma and increased urinary excretion of corticosterone and dehydrocorticosterone tetrahydro metabolites. Conversion of B to A in kidney microsomes from 30-month-old rats was moderately but not significantly increased compared with 10-month-old rats. The urinary ratios of (THB + 5 alpha-THB)/THA and free B/A and renal 11 beta-HSD2 mRNA abundance were equal in 10- and 30-month-old rats.

CONCLUSIONS

There is no evidence for an enhanced gene expression or activity of renal 11 beta-HSD2 in these aging rats, suggesting either that endogenous 11 beta-HSD2 is able to cope with the increased corticosterone concentrations characteristic of the aging process or that alternative mechanisms contribute to the maintenance of a normal sodium excretion in these animals.

Inhibition of COX-2 reduces the age-dependent increase of hippocampal inflammatory markers, corticosterone secretion, and behavioral impairments in the rat

Brain aging as well as brain degenerative processes with accompanying cognitive impairments are generally associated with hyperactivity of the hypothalamus-pituitary-adrenal axis, the end product of which, the glucocorticoid hormone, has been warranted the role of cell damage primum movens ("cascade hypothesis"). However, chronic inflammatory activity occurs in the hippocampus of aged rats as well as in the brain of Alzheimer's disease patients. The concomitant increase in the secretion of the glucocorticoid hormone, the endogenous anti-inflammatory and pro-inflammatory markers, has prompted us to investigate the two phenomena in the aging rat, and to work out its meaning. This study shows that: (I) interleukin-1beta (IL-1beta), tumor necrosis factor alpha (TNFalpha), and prostaglandin E(2) (PGE(2)) increase with age in the rats hippocampus, and (II) chronic oral treatment with celecoxib, a selective cycloxygenase-2 (COX-2) inhibitor, is able to contrast the age-dependent increase in hippocampal levels of pro-inflammatory markers and circulating anti-inflammatory corticosterone, provided that it is started at an early stage of aging. Under these conditions, age-related impairments in cognitive ability may be ameliorated. Taken together, these results indicate that there is a natural tendency to offset the age-dependent increase in brain inflammatory processes via the homeostatic increase of the circulating glucocorticoid hormone.

Evidence for involvement of glucocorticoid response in the hippocampal changes in aged molarless SAMP8 mice

The involvement of glucocorticoid response in the hippocampal changes in aged SAMP8 mice after removal of their upper molar teeth (molarless condition) was examined using biochemical, morphological and behavioral techniques. Molarless mice showed plasma corticosterone levels to be significantly greater than those in molar-intact control mice. Pretreatment with metyrapone, which suppresses the stress-induced rise in plasma corticosterone levels, prevented the molarless condition-induced increase in plasma corticosterone levels, reduction in CA1 pyramidal neuron numbers, and impairment of spatial learning. The results suggest a link between the molarless condition and the glucocorticoid response, which may be involved in spatial learning deficits and hippocampal neuronal death in aged SAMP8 mice.

The modulatory effects of corticosteroids on cognition: studies in young human populations

In the present article, we report on two studies performed in young human populations which tested the cognitive impact of glucocorticoids (GC) in situations of decreased or increased ratio of mineralocorticoid (MR) and glucocorticoid (GR) receptor occupation. In the first study, we used a hormone replacement protocol in which we pharmacologically decreased cortisol levels by administration of metyrapone and then restored baseline cortisol levels by a subsequent hydrocortisone replacement treatment. Memory function was tested after each pharmacological manipulation. We observed that metyrapone treatment significantly impaired delayed recall, while hydrocortisone replacement restored performance at placebo level. In the second study, we took advantage of the circadian variation of circulating levels in cortisol and tested the impact of a bolus injection of 35 mg of hydrocortisone in the late afternoon, at a time of very low cortisol concentrations. In a previous study with young normal controls, we injected a similar dose of hydrocortisone in the morning, at the time of the circadian peak, and reported detrimental effects of GC on cognitive function. Here, when we injected a similar dose of hydrocortisone in the afternoon, at the time of the circadian trough, we observed positive effects of GC on memory function. The results of these two studies provide evidence that GC are necessary for learning and memory in human populations.

Persistent cognitive impairment following surgical treatment of Cushing's syndrome

Chronic exposure to elevated glucocorticoid (GC) levels in Cushing's syndrome (CS) is associated with deficits in cognitive function. It has already been shown that CS patients scored significantly lower than controls on several aspects of cognitive function (J. Int. Neuropsychol. Soc. 6 (2000) 20). In the present study, 13 subjects who presented with CS were investigated one year after surgical treatment to determine the extent to which the effects of hypercortisolism on cognitive function are reversible. Subjects were evaluated with a battery of tasks, similar to the original battery of a year earlier and including tests of attention, visuospatial processing, memory, reasoning and verbal fluency. Except for one task of visual organization, the results showed little change in performance, suggesting that prolonged exposure to high levels of GC can cause long-lasting deleterious effects on cognitive function. The data suggest that correction of hypercortisolism is not necessarily correlated with short-term improvement in cognitive function.

Stress, memory, and the hippocampus: can't live with it, can't live without it

Since the 1968s discovery of receptors for stress hormones (corticosteroids) in the rodent hippocampus, a tremendous amount of data has been gathered on the specific and somewhat isolated role of the hippocampus in stress reactivity. The hippocampal sensitivity to stress has also been extended in order to explain the negative impact of stress and related stress hormones on animal and human cognitive function. As a consequence, a majority of studies now uses the stress-hippocampus link as a working hypothesis in setting up experimental protocols. However, in the last decade, new data were gathered showing that stress impacts on many cortical and subcortical brain structures other than the hippocampus. The goal of this paper is to summarize the four major arguments previously used in order to confirm the stress-hippocampus link, and to describe new data showing the implication of other brain regions for each of these previously used arguments. The conclusion of this analysis will be that scientists should gain from extending the impact of stress hormones to other brain regions, since hormonal functions on the brain are best explained by their modulatory role on various brain structures, rather than by their unique impact on one particular brain region.

Evidence that interleukin-1beta and reactive oxygen species production play a pivotal role in stress-induced impairment of LTP in the rat dentate gyrus

Long-term potentiation (LTP) in both area CA1 and the dentate gyrus is attenuated by stress and the evidence is consistent with the view that this is a consequence of increased activation of glucocorticoid receptors, in the hippocampus, following the stress-induced increase in circulating corticosterone. It has been shown that expression of the pro-inflammatory cytokine, interleukin-1beta (IL-1beta), is increased in hippocampus in response to stress; this finding together with the observation that IL-1beta exerts an inhibitory effect on LTP, suggests that IL-1beta may play a key role in mediating this inhibitory effect of stress on LTP. In this study, we explore this possibility and report that stress is also associated with increased reactive oxygen species production. The evidence presented supports the view that this is secondary to the stress-induced increase in IL-1beta concentration, as IL-1beta increased activity of superoxide dismutase and increased reactive oxygen species accumulation in hippocampus in vitro. We report that the inhibitory effect of stress on LTP is mimicked by H2O2, which increases reactive oxygen species accumulation, and by IL-1beta, the effect of which is overcome by the antioxidant, phenylarsine oxide. The hypothesis that the stress-induced increase in reactive oxygen species production may underlie the suppression of LTP is further supported by the finding that the effect of stress is abrogated by dietary manipulation with antioxidant vitamins E and C.

Action of glucocorticoids on survival of nerve cells: promoting neurodegeneration or neuroprotection?

Extensive studies during the past decades provided compelling evidence that glucocorticoids (GCs) have the potential to affect the development, survival and death of neurones. These observations, however, reflect paradoxical features of GCs, as they may be critically involved in both neurodegenerative and neuroprotective processes. Hence, we first address different aspects of the complex role of GCs in neurodegeneration and neuroprotection, such as concentration dependent actions of GCs on neuronal viability, anatomical diversity of GC-mediated mechanisms in the brain and species and strain differences in GC-induced neurodegeneration. Second, the modulatory action of GCs during development and ageing of the central nervous system, as well as the contribution of altered GC balance to the pathogenesis of neurodegenerative disorders is considered. In addition, we survey recent data as to the possible mechanisms underlying the neurodegenerative and neuroprotective actions of GCs. As such, two major aspects will be discerned: (i) GC-dependent offensive events, such as GC-induced inhibition of glucose uptake, increased extracellular glutamate concentration and concomitant elevation of intracellular Ca(2+), decrease in GABAergic signalling and regulation of local GC concentrations by 11 beta-hydroxysteroid dehydrogenases; and (ii) GC-related cellular defence mechanisms, such as decrease in after-hyperpolarization, increased synthesis and release of neurotrophic factors and lipocortin-1, feedback regulation of Ca(2+) currents and induction of antioxidant enzymes. The particular relevance of these mechanisms to the neurodegenerative and neuroprotective effects of GCs in the brain is discussed.

Glucocorticoid hypersecretion following intracerebroventricular injection of ethylcholine mustard aziridinium ion in rats

To investigate whether cholinergic hypofunctions in the brain influence hypothalamic-pituitary-adrenal activity, we examined the effects of cholinergic neurotoxin ethylcholine mustard aziridinium ion on basal and stress-induced levels of corticosterone in rats. Blood sampling from rats following intracerebroventricular injection of saline (5 microl, as a control) or this neurotoxin (5 nmol/5 microl) was performed over a day in one series, and was taken before, during and after an immobilization stress exposure in another series. Plasma levels of corticosterone and adrenocorticotropin were determined by the radioimmunoassay. The basal levels of plasma corticosterone and adrenocorticotropin over a day were significantly higher in the neurotoxin-treated rats, compared with the control rats (corticosterone, P<0.001; adrenocorticotropin, P<0.05). Further, relative adrenal gland weight of the neurotoxin-treated rats was significantly greater than that of the control rats (P<0.05). However, responses in plasma corticosterone level caused by the immobilization stress in the neurotoxin-treated rats were not different from those in the control rats. The present study demonstrated that damage to the cholinergic neurons in the brain increased hypothalamic-pituitary-adrenal activity over a day, probably due to freedom from inhibitory influences of the hippocampal cholinergic system, but that this damage did not influence stress-induced changes in plasma glucocorticoid level.

Adrenalectomy causes loss of zinc ions in zinc-enriched (ZEN) terminals and decreases seizure-induced neuronal death

Chelatable zinc ions from synaptic vesicles have been suggested to be involved in neuronal death caused by stroke, epilepsy and head trauma. Elevated glucocorticoid concentration exacerbates such neuron loss, while low levels protect. We have tested the notion that the neuroprotective effect of prior glucocorticoid reduction is mediated by a reduction of zinc ions contained in zinc-enriched (ZEN) synaptic vesicles. The level of vesicular zinc ions was evaluated by toluene sulfonamide quinoline (TSQ) fluorometry and zinc autometallography (ZnS(AMG)) 10 and 30 days, respectively, after adrenalectomy. The hippocampus showed significant vesicular zinc ion depletion following adrenalectomy. After the kainate injection, adrenalectomized rats showed proconvulsive seizure behavior, i.e. shortened latency to seizure onset time and increased seizure score. Additionally they showed decreased hippocampal CA3 neuronal death as compared to control animals. The present data suggest that zinc ions released from damaged ZEN terminals are involved in seizure-induced neuronal death.

Hypothalamo-pituitary-adrenocortical dysregulation in aging F344/Brown-Norway F1 hybrid rats

Hypothalamo-pituitary-adrenocortical (HPA) axis aging was studied in young (3 mo), middle aged (15 mo) and aged (30 mo) F344/Brown Norway hybrid rats. This strain was selected to obviate HPA-relevant pathologies found in other aging models. Aged, unstressed rats showed enhanced central HPA drive, marked by elevated ACTH release and decreased pituitary proopiomelanocortin and corticotropin-releasing factor receptor 1 (CRH-R1) mRNAs. Acute corticosterone responses to spatial novelty were exacerbated in aged rats; however, responses to restraint or hypoxia were not affected. Chronic stress exposure also differentially increased HPA drive in aged animals, marked by elevated paraventricular nucleus CRH peptide levels and pituitary proopiomelanocortin mRNA. Plasma ACTH and pituitary POMC and CRH-R1 mRNA expression in middle-aged rats were intermediate those of young and aged animals. Middle-aged animals responded to chronic stress with disproportionate increases in CRH mRNA levels, and increased corticosterone secretion following hypoxia but not novelty. The results suggest a gradual increase in HPA tone across the aging process, culminating in marked hyperresponsivity to both acute and chronic stress in senescence.

Hippocampal apoptosis in major depression is a minor event and absent from subareas at risk for glucocorticoid overexposure

Glucocorticoid (GC) overexposure in animals has been implicated in hippocampal dysfunctioning and neuronal loss. In major depression, hypercortisolemia, hypothalamic-pituitary-adrenocortical-axis alterations, and reduced hippocampal volumes are commonly observed; hence, hippocampal neurodegeneration is also expected. To study possible GC-related pathology, we investigated hippocampal tissue of 15 major-depressed patients, 16 matched controls, and 9 steroid-treated patients, using in situ-end-labeling for DNA fragmentation and apoptosis, and heat-shock protein 70 and nuclear transcription factor kappaB immunocytochemistry for damage-related responses. No obvious massive cell loss was observed in any group. In 11 of 15 depressed patients, rare, but convincing apoptosis was found in entorhinal cortex, subiculum, dentate gyrus, CA1, and CA4. Also in three steroid-treated patients, apoptosis was found. Except for several steroid-treated patients, heat-shock protein 70 staining was generally absent, nor was nuclear transcription factor-kappaB activation found. The detection in 11 of 15 depressed patients, in three steroid-treated, and in one control patient, demonstrates for the first time that apoptosis is involved in steroid-related changes in the human hippocampus. However, in absence of major pyramidal loss, its rare occurrence, that notably was absent from areas at risk for GC damage such as CA3, indicates that apoptosis probably only contributes to a minor extent to the volume changes in depression.