Modulation of brain aging correlates by long-term alterations of adrenal steroids and neurally-active peptides

A Mechanistic Basis for the Beneficial Effects of Caloric Restriction On Longevity and Disease: Consequences for the Interpretation of Rodent Toxicity Studies

Caloric restriction in rodents has been repeatedly shown to increase life span while reducing the severity and retarding the onset of both spontaneous and chemically induced neoplasms. These effects of caloric restriction are associated with a spectrum of biochemical and physiological changes that characterize the organism's adaptation to reduced caloric intake and provide the mechanistic basis for caloric restriction's effect on longevity. Here, we review evidence suggesting that the primary adaptation appears to be a rhythmic hypercorticism in the absence of elevated adrenocorticotropin (ACTH) levels. This characteristic hypercorticism evokes a spectrum of responses, including reduced body temperature and increased metabolic efficiency, decreased mitogenic response coupled with increased rates of apoptosis, reduced inflammatory response, reduced oxidative damage to proteins and DNA, reduced reproductive capacity, and altered drug-metabolizing enzyme expression. The net effect of these changes is to (1) decrease growth and metabolism in peripheral tissues to spare energy for central functions, and (2) increase the organism's capacity to withstand stress and chemical toxicity. Thus, caloric restriction research has uncovered an evolutionary mechanism that provides rodents with an adaptive advantage in conditions of fluctuating food supply. During periods of abundance, body growth and fecundity are favored over endurance and longevity. Conversely, during periods of famine, reproductive performance and growth are sacrificed to ensure survival of individuals to breed in better times. This phenomena can be observed in rodent populations that are used in toxicity testing. Improvements over the last 30 years in animal husbandry and nutrition, coupled with selective breeding for growth and fecundity, have resulted in several strains now exhibiting larger animals with reduced survival and increased incidence of background lesions. The mechanistic data from caloric restriction studies suggest that these large animals will also be more susceptible to chemically induced toxicity. This creates a problem in comparing tests performed on animals of different weights and comparing data generated today with the historical database. The rational use of caloric restriction to control body weight to within preset guidelines is a possible way of alleviating this problem.

Glucocorticoid-dependent hippocampal transcriptome in male rats: pathway-specific alterations with aging

Although glucocorticoids (GCs) are known to exert numerous effects in the hippocampus, their chronic regulatory functions remain poorly understood. Moreover, evidence is inconsistent regarding the long-standing hypothesis that chronic GC exposure promotes brain aging/Alzheimer disease. Here, we adrenalectomized male F344 rats at 15 months of age, maintained them for 3 months with implanted corticosterone (CORT) pellets producing low or intermediate (glucocorticoid receptor-activating) blood levels of CORT, and performed microarray/pathway analyses in hippocampal CA1. We defined the chronic GC-dependent transcriptome as 393 genes that exhibited differential expression between intermediate and low CORT groups. Short-term CORT (4 days) did not recapitulate this transcriptome. Functional processes/pathways overrepresented by chronic CORT-up-regulated genes included learning/plasticity, differentiation, glucose metabolism, and cholesterol biosynthesis, whereas processes overrepresented by CORT-down-regulated genes included inflammatory/immune/glial responses and extracellular structure. These profiles indicate that GCs chronically activate neuronal/metabolic processes while coordinately repressing a glial axis of reactivity/inflammation. We then compared the GC transcriptome with a previously defined hippocampal aging transcriptome, revealing a high proportion of common genes. Although CORT and aging moved expression of some common genes in the same direction, the majority were shifted in opposite directions by CORT and aging (eg, glial inflammatory genes down-regulated by CORT are up-regulated with aging). These results contradict the hypothesis that GCs simply promote brain aging and also suggest that the opposite direction shifts during aging reflect resistance to CORT regulation. Therefore, we propose a new model in which aging-related GC resistance develops in some target pathways, whereas GC overstimulation develops in others, together generating much of the brain aging phenotype.

Structural plasticity of the adult brain: how animal models help us understand brain changes in depression and systemic disorders related to depression

The brain interprets experiences and translates them into behavioral and physiological responses. Stressful events are those which are threatening or, at the very least, unexpected and surprising, and the physiological and behavioral responses are intended to promote adaptation via a process called “allostasis. ” Chemical mediators of allostasis include cortisol and adrenalin from the adrenal glands, other hormones, and neurotransmitters, the parasympathetic and sympathetic nervous systems, and cytokines and chemokines from the immune system. Two brain structures, the amygdala and hippocampus, play key roles in interpreting what is stressful and determining appropriate responses. The hippocampus, a key structure for memories of events and contexts, expresses receptors that enable it to respond to glucocorticoid hormones in the blood, it undergoes atrophy in a number of psychiatric disorders; it also responds to stressors with changes in excitability, decreased dendritic branching, and reduction in number of neurons in the dentate gyrus. The amygdala, which is important for “emotional memories, ” becomes hyperactive in posttraumatic stress disorder and depressive illness, in animal models of stress, there is evidence for growth and hypertrophy of nerve cells in the amygdala. Changes in the brain after acute and chronic stressors mirror the pattern seen in the metabolic, cardiovascular, and immune systems, that is, short-term adaptation (allostasis) followed by long-term damage (allostatic load), eg, atherosclerosis, fat deposition obesity, bone demineralization, and impaired immune function. Allostatic load of this kind is seen in major depressive illness and may also be expressed in other chronic anxiety and mood disorders.

Immunosensor with fluid control mechanism for salivary cortisol analysis

The purpose of this research is to demonstrate a new design for a cortisol immunosensor for the noninvasive and quantitative analysis of salivary cortisol. We propose a cortisol immunosensor with a fluid control mechanism which has both a vertical flow and a lateral flow. The detected current resulting from a competitive reaction between the sample cortisol and a glucose oxidase (GOD)-labeled cortisol conjugate was found to be inversely related to the concentration of cortisol in the sample solution. A calibration curve using the relative detected current showed a R(2)=0.98 and CV=14% for a range of standard cortisol solutions corresponding to the concentrations of native salivary cortisol (0.1-10 ng/ml). The measurement could be accomplished within 35 min and the cortisol immunosensor could be reused. These results show promise for realizing an on-site and easy-to-use biosensor for cortisol. Used for evaluation of human salivary cortisol levels, the cortisol immunosensor measurement corresponded closely with commercially available ELISA method (R(2)=0.92). Our results indicate the promise of the new cortisol immunosensor for noninvasive, point of care measurement of human salivary cortisol levels.

Impact of chronic hypercortisolemia on affective processing

Cushing syndrome (CS) is the classic condition of cortisol dysregulation, and cortisol dysregulation is the prototypic finding in Major Depressive Disorder (MDD). We hypothesized that subjects with active CS would show dysfunction in frontal and limbic structures relevant to affective networks, and also manifest poorer facial affect identification accuracy, a finding reported in MDD. Twenty-one patients with confirmed CS (20 ACTH-dependent and 1 ACTH-independent) were compared to 21 healthy control subjects. Identification of affective facial expressions (Facial Emotion Perception Test) was conducted in a 3 Tesla GE fMRI scanner using BOLD fMRI signal. The impact of disease (illness duration, current hormone elevation and degree of disruption of circadian rhythm), performance, and comorbid conditions secondary to hypercortisolemia were evaluated. CS patients made more errors in categorizing facial expressions and had less activation in left anterior superior temporal gyrus, a region important in emotion processing. CS patients showed higher activation in frontal, medial, and subcortical regions relative to controls. Two regions of elevated activation in CS, left middle frontal and lateral posterior/pulvinar areas, were positively correlated with accuracy in emotion identification in the CS group, reflecting compensatory recruitment. In addition, within the CS group, greater activation in left dorsal anterior cingulate was related to greater severity of hormone dysregulation. In conclusion, cortisol dysregulation in CS patients is associated with problems in accuracy of affective discrimination and altered activation of brain structures relevant to emotion perception, processing and regulation, similar to the performance decrements and brain regions shown to be dysfunctional in MDD. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Chronic 1α,25-(OH)2vitamin D3 treatment reduces Ca2+-mediated hippocampal biomarkers of aging

Aging in the hippocampus of several species is characterized by alterations in multiple Ca(2+)-mediated processes, including an increase in L-type voltage-gated Ca(2+) channel (L-VGCC) current, an enhanced Ca(2+)-dependent slow afterhyperpolarization (AHP), impaired synaptic plasticity and elevated Ca(2+) transients. Previously, we found that 1alpha,25-dihydoxyvitamin D(3) (1,25VitD), a major Ca(2+) regulating hormone, down-regulates L-VGCC expression in cultured hippocampal neurons. Here, we tested whether in vivo treatment of aged F344 rats with 1,25VitD would reverse some of the Ca(2+) -mediated biomarkers of aging seen in hippocampal CA1 neurons. As previously reported, L-VGCC currents and the AHP were larger in aged than in young neurons. Treatment with 1,25VitD over 7 days decreased L-VGCC activity in aged rats, as well as the age-related increase in AHP amplitude and duration. In addition, reduced L-VGCC activity was correlated with reduced AHPs in the same animals. These data provide direct evidence that 1,25VitD can regulate multiple Ca(2+)-dependent processes in neurons, with particular impact on reducing age-related changes associated with Ca(2+) dysregulation. Thus, these results may have therapeutic implications and suggest that 1,25VitD, often taken to maintain bone health, may also retard some consequences of brain aging.

Molecular endocrinology and physiology of the aging central nervous system

Aging is associated with a progressive decline in physical and cognitive functions. The impact of age-dependent endocrine changes regulated by the central nervous system on the dynamics of neuronal behavior, neurodegeneration, cognition, biological rhythms, sexual behavior, and metabolism are reviewed. We also briefly review how functional deficits associated with increases in glucocorticoids and cytokines and declining production of sex steroids, GH, and IGF are likely exacerbated by age-dependent molecular misreading and alterations in components of signal transduction pathways and transcription factors.

Allostatic load as a predictor of functional decline. MacArthur studies of successful aging

Allostatic load has been proposed as a cumulative measure of dysregulation across multiple physiological systems, and has been postulated to impact health risks. In the allostatic load model, increased risk is hypothesized to result not only from large and clinically significant dysregulation in individual systems, but also from more modest dysregulation, if present in multiple systems. Our objective was to construct an allostatic load score by optimally combining several physiologic measurements, and to examine its association with future functional decline. We analyzed data from a 7-year longitudinal study of a community-based cohort, whose age at baseline was between 70 and 79 years. Canonical correlation analysis was used to study the association of 10 biological measurements representing allostatic load with declines in scores on five tests each of physical and cognitive function over two follow-up periods: 1998-1991 and 1991-1995. We used bootstrapping to evaluate the stability of the canonical correlation and canonical weights. The canonical correlation between allostatic load and the 20 decline scores was 0.43 (P =.03) and the [25th, 75th] percentile interval of its distribution over 200 bootstrapped subsamples of the cohort was [0.48, 0.53]. These findings were not substantially affected by adjusting for covariates and cardiovascular disease. We conclude that a summary measure of physiologic dysregulation, such as allostatic load, is an independent predictor of functional decline in elderly men and women.

The impact of corticosteroids on the developing animal

Infants are subjected to both endogenous and exogenous corticosteroids in the pre- and postnatal periods. Stress to the mother before birth, or to the child postpartum, can give rise to high, chronic endogenous corticosteroid levels caused by activation of the hypothalamic-pituitary-adrenal (HPA) axis. Physician-administered exogenous corticosteroids are also used in the management of a wide spectrum of pre- and postnatal conditions. The long-term effects of corticosteroids in developing humans are not well known. Studies in animals, however, indicate that both natural stress and exogenous corticosteroids can have long-lasting and deleterious effects on the body, brain, behavior, and hypothalamic-pituitary-adrenal axis of developing infants. These data suggest that exogenous corticosteroids should be administered with caution, after careful benefit/risk analyses, and that, as far as possible, the developing brain should be protected against the effects of pre- and postnatal stress.

Calcineurin links Ca2+ dysregulation with brain aging

Brain aging is associated with altered Ca(2+) regulation. However, many Ca(2+) signal transduction mechanisms have not been explored in the aged brain. Here, we report that cytosolic expression and activity of the Ca(2+)-dependent protein phosphatase calcineurin (CaN) increases in the hippocampus during aging. CaN changes were paralleled by increased activation, but not expression, of CaN-regulated protein phosphatase 1 and a reduction in the phosphorylation state of CaN substrates involved in cell survival (i.e., Bcl-2-associated death protein and cAMP response element-binding protein). The age-related increase in CaN activity was not attributable to the inability of CaN to translocate to the membrane and was reduced by blocking L-type Ca(2+) channels. Finally, increased CaN activity correlated with memory function as measured with the Morris water escape task. The results suggest that altered regulation of CaN is one of the processes that could link Ca(2+) dyshomeostasis to age-related changes in neural function and cognition.

Lack of tissue glucocorticoid reactivation in 11beta -hydroxysteroid dehydrogenase type 1 knockout mice ameliorates age-related learning impairments

11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD-1) intracellularly regenerates active corticosterone from circulating inert 11-dehydrocorticosterone (11-DHC) in specific tissues. The hippocampus is a brain structure particularly vulnerable to glucocorticoid neurotoxicity with aging. In intact hippocampal cells in culture, 11beta-HSD-1 acts as a functional 11beta-reductase reactivating inert 11-DHC to corticosterone, thereby potentiating kainate neurotoxicity. We examined the functional significance of 11beta-HSD-1 in the central nervous system by using knockout mice. Aged wild-type mice developed elevated plasma corticosterone levels that correlated with learning deficits in the watermaze. In contrast, despite elevated plasma corticosterone levels throughout life, this glucocorticoid-associated learning deficit was ameliorated in aged 11beta-HSD-1 knockout mice, implicating lower intraneuronal corticosterone levels through lack of 11-DHC reactivation. Indeed, aged knockout mice showed significantly lower hippocampal tissue corticosterone levels than wild-type controls. These findings demonstrate that tissue corticosterone levels do not merely reflect plasma levels and appear to play a more important role in hippocampal functions than circulating blood levels. The data emphasize the crucial importance of local enzymes in determining intracellular glucocorticoid activity. Selective 11beta-HSD-1 inhibitors may protect against hippocampal function decline with age.

The neurobiology of stress: from serendipity to clinical relevance

The hormones and other physiological agents that mediate the effects of stress on the body have protective and adaptive effects in the short run and yet can accelerate pathophysiology when they are over-produced or mismanaged. Here we consider the protective and damaging effects of these mediators as they relate to the immune system and brain. 'Stress' is a principle focus, but this term is rather imprecise. Therefore, the article begins by noting two new terms, allostasis and allostatic load that are intended to supplement and clarify the meanings of 'stress' and 'homeostasis'. For the immune system, acute stress enhances immune function whereas chronic stress suppresses it. These effects can be beneficial for some types of immune responses and deleterious for others. A key mechanism involves the stress-hormone dependent translocation of immune cells in the blood to tissues and organs where an immune defense is needed. For the brain, acute stress enhances the memory of events that are potentially threatening to the organism. Chronic stress, on the other hand, causes adaptive plasticity in the brain, in which local neurotransmitters as well as systemic hormones interact to produce structural as well as functional changes, involving the suppression of ongoing neurogenesis in the dentate gyrus and remodelling of dendrites in the Ammon's horn. Under extreme conditions only does permanent damage ensue. Adrenal steroids tell only part of the story as far as how the brain adapts, or shows damage, and local tissue modulators - cytokines for the immune response and excitatory amino acid neurotransmitters for the hippocampus. Moreover, comparison of the effects of experimenter-applied stressors and psychosocial stressors show that what animals do to each other is often more potent than what experimenters do to them. And yet, even then, the brain is resilient and capable of adaptive plasticity. Stress-induced structural changes in brain regions such as the hippocampus have clinical ramifications for disorders such as depression, post-traumatic stress disorder and individual differences in the aging process.

Allostasis, allostatic load, and the aging nervous system: role of excitatory amino acids and excitotoxicity

The adaptive responses of the body to challenges, often known as "stressors", consists of active responses that maintain homeostasis. This process of adaptation is known as "allostasis", meaning "achieving stability through change". Many systems of the body show allostasis, including the autonomic nervous system and hypothalamo-pituitary-adrenal (HPA) axis and they help to re-establish or maintain homeostasis through adaptation. The brain also shows allostasis, involving the activation of nerve cell activity and the release of neurotransmitters. When the individual is challenged repeatedly or when the allostatic systems remain turned on when no longer needed, the mediators of allostasis can produce a wear and tear on the body that has been termed "allostatic load". Examples of allostatic load include the accumulation of abdominal fat, the loss of bone minerals and the atrophy of nerve cells in the hippocampus. Circulating stress hormones play a key role, and, in the hippocampus, excitatory amino acids and NMDA receptors are important mediators of neuronal atrophy. The aging brain seems to be more vulnerable to such effects, although there are considerable individual differences in vulnerability that can be developmentally determined. Yet, at the same time, excitatory amino acids and NMDA receptors mediate important types of plasticity in the hippocampus. Moreover, the brain retains considerable resilience in the face of stress, and estrogens appear to play a role in this resilience. This review discusses the current status of work on underlying mechanisms for these effects.

Opposite effects on NCAM expression in the rat frontal cortex induced by acute vs. chronic corticosterone treatments

The temporal pattern of exposure to glucocorticoids has been reported to be a critical factor in determining the outcome of glucocorticoid actions at the brain. In this work, the effects of different regimes of subcutaneous corticosterone administration (acute-single injection-vs. chronic-daily injection for 21 days) on the expression of the neural cell adhesion molecule (NCAM) were evaluated in different rat brain regions (CA1-CA4, dentate gyrus, frontal cortex, striatum, and hypothalamus). The treatments were selected according to previous studies in which we showed biphasic effects of corticosterone on memory formation, with acute corticosterone effects being facilitating and chronic effects being deleterious. In addition, the chronic treatment was shown by others to result in structural alterations at the hippocampus. NCAM was evaluated given its cell-cell recognition and adhesion properties, and the involvement on synaptic stabilisation subserving long-term memory formation. The results showed a biphasic modulation of NCAM levels at the frontal cortex, with acute corticosterone resulting in enhanced NCAM levels at 8 h and 24 h posttraining, and the chronic treatment decreasing its expression. None of the other brain areas examined showed significant changes in NCAM expression with corticosterone treatments, except for the hypothalamus that showed reduced NCAM levels after the chronic corticosterone regime. These results support the view that NCAM regulation at the frontal cortex might be a mechanism by which corticosterone treatments influence memory formation. They also highlight the hypothalamus as a brain area particularly sensitive to NCAM regulation by prolonged exposure to elevated glucocorticoids.

Corticosteroids, the Aging Brain and Cognition

The hippocampal region of the brain is a useful model system for understanding the plasticity and resilience of brain cells to stress hormone action and aging. Hippocampal neurons show both structural and functional plasticity, and individual differences in hippocampal function are shaped by early life experiences. For human brain aging, there are new non-invasive imaging tools to relate to the animal models, and these can help to assess the vulnerability of the aging hippocampus in relation to stress and Alzheimer's disease.

Stress and the aging hippocampus

The "glucocorticoid cascade hypothesis" of hippocampal aging has stimulated a great deal of research into the neuroendocrine aspects of aging and the role of glucocorticoids, in particular. Besides strengthening the methods for investigating the aging brain, this research has revealed that the interactions between glucocorticoids and hippocampal neurons are far more complicated than originally envisioned and involve the participation of neurotransmitter systems, particularly the excitatory amino acids, as well as calcium ions and neurotrophins. New information has provided insights into the role of early experience in determining individual differences in brain and body aging by setting the reactivity of the hypothalamopituitary-adrenal axis and the autonomic nervous system. As a result of this research and advances in neuroscience and the study of aging, we now have a far more sophisticated view of the interactions among genes, early development, and environmental influences, as well as a greater appreciation of events at the cellular and molecular levels which protect neurons, and a greater appreciation of pathways of neuronal damage and destruction. While documenting the ultimate vulnerability of the brain to stressful challenges and to the aging process, the net result of this research has highlighted the resilience of the brain and offered new hope for treatment strategies for promoting the health of the aging brain.

Plasticity of hippocampal corticosteroid receptors during aging in the rat

Aging is commonly associated with dysregulation of the hypothalamo-pituitary-adrenal axis and cognitive impairment. On the basis of suggestions that these disruptions ensue from changes in the hippocampal complement of corticosteroid (mineralocorticoid and glucocorticoid) receptors (MR and GR), we examined the availability of hippocampal MR and GR by measuring the in vivo uptake of 3H-aldosterone and 3H-dexamethasone (selective MR and GR agonists, respectively); MR and GR mRNA levels were also measured. We observed age-related declines in both the synthesis of MR and GR and the uptake of their respective ligands. Whereas MR mRNA levels and ligand uptake declined in parallel, GR binding declined more steeply than GR mRNA. This latter result, together with our finding that aged rats show impaired corticosteroid receptor mRNA and protein up-regulation after corticosteroid withdrawal, indicates decreased transcription of MR and GR genes and posttranslational modification of GR mRNA during aging. Given that corticosteroids can influence MR and GR synthesis and binding, and based on the finding that aged subjects show reduced basal secretion of corticosterone, we propose that this relative hypocorticalism may be responsible for the changes observed in MR and GR activity, which then leads to disturbances in neuroendocrine regulation and cognitive function in aged subjects.

Neurotrophic action of lipocortin 1 derived from astrocytes on cultured rat cortical neurons

The lipocortins are a family of structurally related proteins, namely an annexin family, that exerts a variety of cellular functions through Ca2+-dependent binding to phospholipase A2 [EC 3.1. 1.4], including a crucial role in the central nervous system (CNS) such as antipyrogenic, thermoregulatory and neuroprotective agents in vivo. To elucidate the paradigm of lipocortin 1 functions in the CNS, we have first demonstrated (1) the induction and subsequent extracellular secretion of LC1 by glucocorticoid in cultured rat astrocytes, and (2) neurotrophic activities (survival-promoting, neuritogenic and synaptogenic actions on rat cortical neurons) of recombinant LC1. Time-and dose-dependent experiments of a synthetic glucocorticoid, dexamethasone (DEX), on rat cortical astrocytes in culture revealed that the expression of the intracellular LC1 mRNA and protein were significantly augmented by DEX (1 microM). In addition, DEX evoked an extracellular secretion of LC1 without its cytotoxic effects. Furthermore, the recombinant LC1 appeared to promote not only the survival and neurite outgrowth but also the synaptogenesis of embryonal rat cortical neurons. These results suggest that LC1 induced and selectively released from astrocytes by either endogenously or exogenously introduced glucocorticoids may play a specific and essential role on development and regeneration of the central nervous system.

Do early-life events permanently alter behavioral and hormonal responses to stressors?

Early-life stimulation (e.g., brief handling) attenuates the behavioral and neuroendocrine responses to stressors encountered in adulthood, particularly with respect to activation of hypothalamic-pituitary-adrenal (HPA) activity. In contrast, if neonates were subjected to a more severe stressor, such as protracted separation from the dam or exposure to an endotoxin, then the adult response to a stressor was exaggerated. These early-life experiences program HPA functioning, including negative feedback derived from stimulation of hippocampal glucocorticoid receptors, and corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) coexpression in PVN neurons, to modify the response to subsequent stressor experiences. The persistent variations of HPA activity observed in handled/stimulated animals may stem from alterations in dam-pup interactions (e.g. increased arched-back feeding, licking, grooming). In addition genetic makeup is critical in determining stress reactivity. For instance, BALB/cByJ mice are more reactive to stressors than C57BL/6ByJ mice, exhibiting greater HPA hormonal alterations and behavioral disturbances. BALB/cByJ also fail to acquire a spatial learning response in a Morris water-maze paradigm, which has been shown to be correlated with hippocampal cell loss associated with aging. Early-life handling of BALB/cByJ mice prevented these performance deficits and attenuated the hypersecretion of ACTH and corticosterone elicited by stressors. The stressor reactivity may have been related to maternal and genetic factors. When BALB/cByJ mice were raised by a C57BL/6ByJ dam, the excessive stress-elicited HPA activity was reduced, as were the behavioral impairments. However, cross-fostering the more resilient C57BL/6ByJ mice to a BALB/cByJ dam failed to elicit the behavioral disturbances. It is suggested that genetic factors may influence dam-pup interactive styles and may thus proactively influence the response to subsequent stressors among vulnerable animals. In contrast, in relatively hardy animals the early-life manipulations may have less obvious effects.

Brain corticosteroid receptor balance in health and disease

In this review, we have described the function of MR and GR in hippocampal neurons. The balance in actions mediated by the two corticosteroid receptor types in these neurons appears critical for neuronal excitability, stress responsiveness, and behavioral adaptation. Dysregulation of this MR/GR balance brings neurons in a vulnerable state with consequences for regulation of the stress response and enhanced vulnerability to disease in genetically predisposed individuals. The following specific inferences can be made on the basis of the currently available facts. 1. Corticosterone binds with high affinity to MRs predominantly localized in limbic brain (hippocampus) and with a 10-fold lower affinity to GRs that are widely distributed in brain. MRs are close to saturated with low basal concentrations of corticosterone, while high corticosterone concentrations during stress occupy both MRs and GRs. 2. The neuronal effects of corticosterone, mediated by MRs and GRs, are long-lasting, site-specific, and conditional. The action depends on cellular context, which is in part determined by other signals that can activate their own transcription factors interacting with MR and GR. These interactions provide an impressive diversity and complexity to corticosteroid modulation of gene expression. 3. Conditions of predominant MR activation, i.e., at the circadian trough at rest, are associated with the maintenance of excitability so that steady excitatory inputs to the hippocampal CA1 area result in considerable excitatory hippocampal output. By contrast, additional GR activation, e.g., after acute stress, generally depresses the CA1 hippocampal output. A similar effect is seen after adrenalectomy, indicating a U-shaped dose-response dependency of these cellular responses after the exposure to corticosterone. 4. Corticosterone through GR blocks the stress-induced HPA activation in hypothalamic CRH neurons and modulates the activity of the excitatory and inhibitory neural inputs to these neurons. Limbic (e.g., hippocampal) MRs mediate the effect of corticosterone on the maintenance of basal HPA activity and are of relevance for the sensitivity or threshold of the central stress response system. How this control occurs is not known, but it probably involves a steady excitatory hippocampal output, which regulates a GABA-ergic inhibitory tone on PVN neurons. Colocalized hippocampal GRs mediate a counteracting (i.e., disinhibitory) influence. Through GRs in ascending aminergic pathways, corticosterone potentiates the effect of stressors and arousal on HPA activation. The functional interaction between these corticosteroid-responsive inputs at the level of the PVN is probably the key to understanding HPA dysregulation associated with stress-related brain disorders. 5. Fine-tuning of HPA regulation occurs through MR- and GR-mediated effects on the processing of information in higher brain structures. Under healthy conditions, hippocampal MRs are involved in processes underlying integration of sensory information, interpretation of environmental information, and execution of appropriate behavioral reactions. Activation of hippocampal GRs facilitates storage of information and promotes elimination of inadequate behavioral responses. These behavioral effects mediated by MR and GR are linked, but how they influence endocrine regulation is not well understood. 6. Dexamethasone preferentially targets the pituitary in the blockade of stress-induced HPA activation. The brain penetration of this synthetic glucocorticoid is hampered by the mdr1a P-glycoprotein in the blood-brain barrier. Administration of moderate amounts of dexamethasone partially depletes the brain of corticosterone, and this has destabilizing consequences for excitability and information processing. 7. The set points of HPA regulation and MR/GR balance are genetically programmed, but can be reset by early life experiences involving mother-infant interaction. 8. (ABSTRACT TRUNCATED)

Steroid hormones and their receptors in the brain

Steroid hormones regulate several important functions of the brain by altering the expression of particular genes through their receptors. First in this paper the localization of glucocorticoid receptor immunoreactivity and mRNA in the brain was examined. Second biphasic effects of glucocorticoid on the hippocampus was described and particular emphasis was given on the apoptosis. Third the significance of estrogen receptor in the sexually dimorphic areas was discussed. These results suggest that steroids modulate the gene expression along with the alteration of cell structures in a different manner in a tissue-specific pattern.

Modulation of hypothalamic-pituitary-adrenal function by transgenic expression of interleukin-6 in the CNS of mice

Interleukin-6 (IL-6) and IL-6 receptor mRNA and protein have been reported in different brain regions under normal and pathophysiological conditions. Although much is known about the hypothalamic-pituitary-adrenal (HPA) axis stimulation after acute administration, less is known about the chronic effects of IL-6 on the function of the HPA axis. In the present study, we examined the function of the HPA axis in transgenic mice in which constitutive expression of IL-6 under the control of the glial fibrillary acidic protein (GFAP) promoter was targeted to astrocytes in the CNS. GFAP-IL6 mice heterozygous or homozygous for the IL-6 transgene had normal basal plasma corticosterone levels but, after restraint stress, showed abnormally increased levels in a gene dose-dependent manner. The increased plasma corticosterone levels in the IL-6 transgenic mice were associated with increased adrenal corticosterone content and hyperplasia of both adrenal cortex and medulla. Notably, plasma adrenocorticotrophic hormone (ACTH) levels and pituitary ACTH content were either not changed or decreased in these mice, whereas plasma arginine vasopressin (AVP) was increased, supporting a role for AVP in response to acute immobilization stress. The reduced ACTH response together with the adrenal hyperplasia in the IL-6 transgenic mice suggests direct activation at the level of the adrenal gland that may be directly activated by AVP or sensitized to ACTH. A similar mechanism may play a role in the blunted ACTH response and elevated corticosterone levels under pathophysiological conditions observed in humans with high brain levels of IL-6.

Prevention of stress-induced morphological and cognitive consequences

Atrophy and dysfunction of the human hippocampus is a feature of aging in some individuals, and this dysfunction predicts later dementia. There is reason to believe that adrenal glucocorticoids may contribute to these changes, since the elevations of glucocorticoids in Cushing's syndrome and during normal aging are associated with atrophy of the entire hippocampal formation in humans and are linked to deficits in short-term verbal memory. We have developed a model of stress-induced atrophy of the hippocampus of rats at the cellular level, and we have been investigating underlying mechanisms in search of agents that will block the atrophy. Repeated restraint stress in rats for 3 weeks causes changes in the hippocampal formation that include suppression of 5-HT1A receptor binding and atrophy of dendrites of CA3 pyramidal neurons, as well as impairment of initial learning of a radial arm maze task. Because serotonin is released by stressors and may play a role in the actions of stress on nerve cells, we investigated the actions of agents that facilitate or inhibit serotonin reuptake. Tianeptine is known to enhance serotonin uptake, and we compared it with fluoxetine, an inhibitor of 5-HT reuptake, as well as with desipramine. Tianeptine treatment (10 mg/kg/day) prevented the stress-induced atrophy of dendrites of CA3 pycamidal neurons, whereas neither fluoxetine (10 mg/kg/day) nor desipramine (10 mg/kg/day) had any effect. Tianeptine treatment also prevented the stress-induced impairment of radial maze learning. Because corticosterone- and stress-induced atrophy of CA3 dendrites is also blocked by phenytoin, an inhibitor of excitatory amino acid release and actions, these results suggest that serotonin released by stress or corticosterone may interact pre- or post-synaptically with glutamate released by stress or corticosterone, and that the final common path may involve interactive effects between serotonin and glutamate receptors on the dendrites of CA3 neurons innervated by mossy fibers from the dentate gyrus. We discuss the implications of these findings for treating cognitive impairments and the risk for dementia in the elderly.

Increase in urinary cortisol excretion and memory declines: MacArthur studies of successful aging

Cortisol production is increased during stress, and the actions of cortisol on receptors in the brain and other body organs are involved in allostasis, the process of adaptation to stress, as well as in allostatic load, the wear and tear associated with excessive exposure to cortisol. Using data from a community-based longitudinal study of older men and women, aged 70-79 yr, we tested the hypothesis that exposure to increasing levels of cortisol is associated with declines in memory performance. Associations between 12-h urinary free cortisol excretion and performance on tests of memory (delayed verbal recall and spatial recognition), abstraction, and spatial ability were examined. Among the women, greater cortisol excretion was associated with poorer baseline memory performance, independent of socio-demographic, health status, health behavior, and psychosocial characteristics. Moreover, women who exhibited increases in cortisol excretion over a 2.5-yr follow-up period were more likely to show declines in memory performance. By contrast, women who experienced declines in cortisol exhibited improvements in memory performance. No significant associations were found among the men. The results for the women suggest that decrements in memory performance associated with increases in cortisol may not represent irreversible effects, as declines in cortisol were associated with improvements in memory.

Stress effects on morphology and function of the hippocampus

The hippocampal formation, which contains high levels of adrenal steroid receptors, is vulnerable to insults such as stroke, seizures, and head trauma, and it is also sensitive and vulnerable to the effects of stress. We have discovered that the hippocampus of rodents and tree shrews shows atrophy of pyramidal neurons in the CA3 region. Psychosocial stress and restraint stress produce atrophy over approximately 3-4 weeks. Atrophy is blocked by inhibiting adrenal steroid formation and by blocking the actions of excitatory amino acids using Dilantin or NMDA receptor inhibitors. Glucocorticoid administration also blocks CA3 atrophy, but Dilantin administration blocks this as well, indicating that excitatory amino acid release mediates the atrophy, which likely involves disassembly of the dendritic cytoskeleton. Studies with in vivo microdialysis in several laboratories have shown that glutamate release in the hippocampus increases in stress and that stress-induced glutamate release is reduced by adrenalectomy. Recent electron microscopy of mossy fiber terminals on CA3 neurons has revealed a depletion of synaptic vesicles as a result of repeated stress. The mossy fiber terminals appear to be responsible for driving atrophy of CA3 neurons, which involves principally atrophy of the apical dendrites. These results are discussed in relation to data from MRI showing atrophy of the whole human hippocampus in Cushing's disease, recurrent depressive illness, PTSD, and normal aging as well as dementia.

Permanent upregulation of hippocampal mineralocorticoid receptors after neonatal administration of ACTH-(4-9) analog ORG 2766 in rats

The development of brain corticosteroid receptors may be permanently modified by perinatal hormone treatments, in particular by hormones of the hypothalamic-pituitary-adrenal axis. Changes in binding characteristics of corticosteroid receptors were investigated in rats treated subcutaneously with 1 microg/g body wt of the ACTH-(4-9) analog peptide ORG 2766 once daily at postnatal days 1, 3 and 5. [3H]Corticosterone (CORT) binding capacity (Bmax) and affinity (Kd) were determined at 1-, 2- and 3-weeks old and adult ages in the hippocampal cytosol by using saturation analysis. Mineralocorticoid type receptor (MR) and glucocorticoid receptor (GR) sites were measured separately with single-point analysis applying a selective glucocorticoid ligand RU 28362 saturating GR. An increase in [3H]CORT binding capacity was found during postnatal development which remained permanently high up to adult age. Separate analysis of MR and GR expression indicated that the increment in the number of corticoid receptor sites was due to an increase in number of MRs in both the young and adult rats. It was concluded that neonatal injections of ACTH-(4-9) peptide resulted in a permanent and selective upregulation of hippocampal MRs, which may underlie the previously observed increased vigilance and novelty-induced behavioral reactivity of the peptide-treated adult rats (Felszeghy, K., Sasvari, M. and Nyakas, C., Horm. Behav., 27 (1993) 380-396).

Gonadal and adrenal steroids regulate neurochemical and structural plasticity of the hippocampus via cellular mechanisms involving NMDA receptors

1. The hippocampus is an important brain structure for working and spatial memory in animals and humans, and it is also a vulnerable as well as plastic brain structure as far as sensitivity to epilepsy, ischemia, head trauma, stress, and aging. 2. The hippocampus is also a target brain area for the actions of hormones of the steroid/thyroid hormone family, which traditionally have been thought to work by regulating gene expression. "Genomic" actions of steroid hormones involve intracellular receptors, whereas "nongenomic" effects of steroids involve putative cell surface receptors. Although this distinction is valid, it does not go far enough in addressing the variety of mechanisms that steroid hormones use to produce their effects on cells. This is because cell surface receptors may signal changes in gene expression, while genomic actions sometimes affect neuronal excitability, often doing so quite rapidly. 3. Moreover, steroid hormones and neurotransmitters may operate together to produce effects, and sometimes these effects involve collaborations between groups of neurons. For example, a number of steroid actions in the hippocampus involve the coparticipation of excitatory amino acids. These interactions are evident for the regulation of synaptogenesis by estradiol in the CA1 pyramidal neurons of hippocampus and for the induction of dendritic atrophy of CA3 neurons by repeated stress as well as by glucocorticoid injections. In addition, neurogenesis in the adult and developing dentate gyrus is "contained" by adrenal steroids as well as by excitatory amino acids. In each of these three examples, NMDA receptors are involved. 4. These results not only point to a high degree of interdependency between certain neurotransmitters and the actions of steroid hormones, but also emphasize the degree to which structural plasticity is an important aspect of steroid hormone action in the adult as well as developing nervous system.

Aging and the hypothalamus: research perspectives

There are several hypothalamic theories of aging, none of which has been validated. An approach to validation is to search for consequences of anatomic ablations of hypothalamic regions that are functional hallmarks of aging, or consequences of ablation that postpone the appearance of hallmarks of aging or extend longevity. Ablation of the hypothalamic ventromedial nucleus (VMN) in the weanling rat is associated with subsequent increased body fat, glucose intolerance, hyperlipidemia, and decreased renal function. Each of these consequences is characteristic of aging in humans and in several animal models of aging. Ablation of the hypothalamic dorsomedial nucleus (DMN) in the weanling rat leads to a symmetrically smaller animal with normal glucose and lipid metabolism, decreased body fat for size, and reduced risk of decreased renal function and circulating IGF-I levels. These are findings consistent with calorie restriction models in rodents that significantly extend life span. This review compares outcomes of lesions in the VMN, DMN, and lateral hypothalamic area (LHA) for relevance to aging. To establish a relationship between these anatomic areas of the hypothalamus and aging, it is concluded that the VMN, DMN, and LHA lesions should be examined for impact on longevity and compared with data obtained from simultaneously studied intact ad-lib-fed and 40% calorie-restricted animals. Lesioned animals also should be rigorously studied for neurotransmitters (e.g., neuropeptide Y, beta-endorphin, serotonin, corticotropin-releasing factor, and galanin), and for behavioral changes consistent with aging, for accumulation of specific tissue lipofuscin and amyloid that are associated with normal aging and for other age-dependent findings, such as incidence of tumors and cataract.

The ACTH(4-9) analog ORG 2766 and recovery after brain damage in animal models--a review

Treatment with adrenocorticotrophic hormone (ACTH), as well as with ACTH fragments and analogues, can influence behaviour of animals and humans. Furthermore it facilitates recovery of damaged peripheral nervous tissue. The question whether ACTH/MSH peptides affect recovery processes after injury to the central nervous system as well is addressed in the present review. The effects of administration of the ACTH(4-9) analog ORG 2766 after brain lesions has been studied frequently. However, the interpretation of the available data is confused by the variability of the results. Several factors can be identified which influence the efficacy of the peptide: (i) not all behavioural tests are equally suitable to reveal a peptide effect on behavioural recovery; (ii) the affected brain area; (iii) whether cell bodies or terminals are affected; (iv) the post-operative housing conditions; and (v) the onset and duration of peptide administration. Two possible explanations of peptide efficacy on functional recovery are considered: first, the peptide may accelerate spontaneously occurring recovery processes and second, the peptide may induce compensatory mechanisms underlying functional recovery without recuperation of the damaged neurons. These compensatory mechanisms seem to rely mainly on enhanced non-selective attention by activation of limbic structures. It is as yet unknown to which receptor system ORG 2766 binds; the analog lacks affinity for the known melanocortin (MC) receptors in brain, yet ORG 2766 is able to modulate the activity of endogenous opioids and the NMDA-receptor. A modulating influence of the peptide on NMDA-receptor activity might indirectly account for both enhanced attention--with ensuing behavioural recovery--and the acceleration of spontaneous recovery.

Effects of stress on neurotrophic factor expression in the rat brain

Changes in neurotrophic factor expression in the brain are part of the stress response. Decreased BDNF may contribute to hippocampal damage that occurs during chronic stress or aging. Stress-induced increases in NT-3 may be important for neural plasticity and adaptation or sensitization to repeated stress. Stress-induced changes in neurotrophic factors may be particularly relevant to the cognitive changes that occur in recurrent depression, aging, and posttraumatic stress disorder.

Roles of steroid hormones and their receptors in structural organization in the nervous system

Due to their chemical properties, steroid hormones cross the blood-brain barrier where they have profound effects on neuronal development and reorganization both in invertebrates and vertebrates, including humans mediated through their receptors. Steroids play a crucial role in the organizational actions of cellular differentiation representing sexual dimorphism and apoptosis, and in the activational effects of phenotypic changes in association with structural plasticity. Their sites of action are primarily the genes themselves but some are coupled with membrane-bound receptor/ion channels. The effects of steroid hormones on gene transcription are not direct, and other cellular components interfere with their receptors through cross-talk and convergence of the signaling pathways in neurons. These genomic and non-genomic actions account for the divergent effects of steroid hormones on brain function as well as on their structure. This review looks again at and updates the tremendous advances made in recent decades on the study of the role of steroid (gonadal and adrenal) hormones and their receptors on developmental processes and plastic changes in the nervous system.

Stress-induced atrophy of apical dendrites of hippocampal CA3c neurons: involvement of glucocorticoid secretion and excitatory amino acid receptors

Repeated restraint stress of rats for 21 days causes atrophy of apical dendrites of hippocampal CA3c pyramidal neurons. This effect is mimicked by daily corticosterone treatment for 21 days and is prevented y the anti-epileptic drug, phenytoin, known to interfere with excitatory amino acid release and action. The present study was designed to investigate the involvement of endogenous corticosterone secretion and excitatory amino acid receptors in the stress-induced hippocampal dendritic atrophy. Treatment of chronically stressed rats with the steroid synthesis blocker cyanoketone prevented stress-induced dendritic atrophy. Cyanoketone-treated animals showed an impaired corticosterone secretion in response to the stressor, while basal levels were maintained. Besides the involvement of endogenous corticosterone secretion, N-methyl-D-aspartate receptors also play a role, since the competitive receptor antagonist, CGP 43487, blocked stress-induced dendritic atrophy. In contrast, NBQX, a competitive inhibitor of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors, was ineffective at a dose that blocks ischemic damage. These results indicate that the reversible atrophy induced by 21 days of daily restraint stress requires corticosterone secretion and that excitatory mechanisms involving N-methyl-D-aspartate receptors play a major role in driving the atrophy.

Chronic corticosterone treatment-induced ultrastructural changes at rat neuromuscular junction

BACKGROUND

Chronic exposure to glucocorticoids affects both the structure and function of vertebrate skeletal muscles. As little is known about the effects of such steroids on the neuromuscular junctions (NMJs) of different muscle fiber types, the influence of chronic corticosterone (CORT) administration on the ultrastructure of NMJs of soleus (SOL) and extensor digitorum longus (EDL) was studied.

METHODS

Ten Fischer 344 male rats, the same animals used previously, were either injected daily with 5-10 mg CORT or received vehicle as control animals for 3 months and were sacrificed at 5 months of age. Muscles were bathed in situ in 4% phosphate buffered glutaraldehyde for ten minutes, then removed and conventional electron microscopic procedures were followed. Qualitative and quantitative observations of nerve terminal ultrastructures were statistically treated with multivariate analysis of variance to determine differences between control and CORT-treated animals.

RESULTS

Fast-twitch EDL muscles were more affected by CORT-treatment than slow-twitch SOL muscles. Morphometric analysis of NMJ's in CORT-treated rats revealed significant decrease in fiber diameter, nerve terminal area and synaptic vesicle density, but a significant increase in synaptic cleft (P < 0.05). The NMJ's underwent partial denervation and reinnervation processes as demonstrated by large areas of presynaptic nerve terminal occupied by microtubules and electron dense granular material.

CONCLUSIONS

Chronic CORT-treatments induced degenerative changes which were more pronounced in fast-twitch EDL muscles than slow-twitch SOL muscles, suggesting that pattern or amount of activity affect the CORT-treatment outcome. These steroid-induced stress changes are similar to those observed in aging and disuse studies of NMJ. Thus, glucocorticoid hormones may play an etiological role in the homeostasis of the NMJ in response to various stimuli.

Desensitization of the neuronal insulin receptor: a new approach in the etiopathogenesis of late-onset sporadic dementia of the Alzheimer type (SDAT)?

Even in its incipient stage, late-onset sporadic dementia of the Alzheimer type (SDAT) is characterized by an abnormal reduction in brain glucose consumption and energy formation. Gathering evidence indicates that cerebral glucose metabolism is controlled by brain insulin/insulin receptors. This led us to hypothesize that the abnormal reduction in glucose utilization found in Alzheimer brains is preceded by a desensitization of cerebral insulin receptors which might be due to enhanced levels of stress factors such as cortisol and catecholamines. The hypothesis is supported by clinical findings of an abnormal response to the oral glucose tolerance test in AD patients. Furthermore, experimental desensitization of the cerebral insulin receptor resulted in both cognitive deficits and metabolic abnormalities in cerebral oxidative glucose metabolism resembling those described in incipient late-onset SDAT. Glucose is the major source of energy in the CNS, and any impairment in cerebral glucose oxidation can be expected to result in deficits in both acetylcholine synthesis and ATP formation, which might contribute to altered APP processing and enhanced susceptibility to neurotoxicity.

Corticosteroids and hippocampal plasticity

The unexpected discovery in 1968 that the hippocampus takes up and retains adrenal steroids has leds to equally unanticipated findings regarding the actions of hormones on the brain and the ways in which the brain is capable of changing in response to the hormonal milieu. First were indications that adrenal steroids adversely affect pyramidal neurons of the hippocampus and cause damage during aging and as a result of severe and prolonged stress. Atrophy of dendrites, particularly in the CA3 region, appears to be an early index of these effects. Second was evidence that the dentate gyrus undergoes atrophy and granule neuron death after adrenalectomy; perhaps as a result of this neuronal death, neurogenesis is stimulated in dentate gyrus of adult rats. Third are recent indications that excitability of hippocampal neurons, including the ability to generate long-term potentiation (LTP), is regulated biphasically by adrenal steroids. One important goal of current research is to understand the role of type I and type II receptors for adrenal steroids in hormonally-induced hippocampal plasticity. Type I receptors appear to play a role in containing programmed-cell death and the rate of neurogenesis; they also regulate key neurochemical features of dentate gyrus and Ammon's horn; and they facilitate LTP. Present information indicates that type II receptors inhibit LTP and may play a role in the degenerative changes in Ammon's horn.

Reactivity to novelty during youth as a predictive factor of cognitive impairment in the elderly: a longitudinal study in rats

A life-span study of certain behavioral traits was conducted in rats. Animals were repeatedly tested in a circular corridor for reactivity to novelty and in a recognition memory task for cognitive abilities. These measures revealed important inter-individual differences in young as well as in old subjects. Some of these differences appear with aging (memory deficits) and others disappear (high reactivity to novelty). Moreover, a relationship between high reactivity to novelty in youth and deficits in memory recognition in elderly was found. Rats that are high-responders to novelty had age-related memory impairments whereas the low-responder rats did not. While the biological mechanism linking these two behavioral traits remains to be demonstrated, this study shows that age-related impairments can be predicted by factors detectable early in life.

Zinc deficiency and corticosteroids in the pathogenesis of alcoholic brain dysfunction--a review

Chronic alcoholism is associated with hypercortisolemia and low serum zinc (Zn). Hypercortisolemia could be responsible for alcoholic cerebral atrophy and is also associated with enhanced NMDA neurotoxicity. It is hypothesized that low brain Zn, noted in chronic alcoholics, enhances NMDA excitotoxicity and ethanol withdrawal seizure susceptibility. Also, Zn deficiency can produce neuronal damage through increased free radical formation. Clinically, Zn replacement therapy may be a rational approach to the treatment of alcohol withdrawal seizures and alcohol-related brain dysfunction.

Acute and long-term effects of adrenocorticotropin and dexamethasone on the auditory brainstem response in multiple sclerosis patients

Auditory brain-stem responses (ABRs) were compared in two groups of multiple sclerosis (MS) patients receiving standard treatment with adrenocorticotropin (ACTH) and with dexamethasone (DEX). ABRs were recorded prior to treatment, on the 1st and 8th day of therapy, and 21 days after the hormonal treatment had been discontinued. ABRs in MS patients were within the normal range of variability. Latencies of ABR components increased with increasing rate of presentation, and with decreasing intensity of the click stimuli. Changes in ABRs displayed a consistent pattern in patients treated with ACTH, but showed less coherence after DEX. In ACTH treated patients' latencies of the late ABR waves V and Vn were prolonged after clicks of high intensity, and reduced following clicks of low intensity resulting in a decreased slope of the latency-intensity function of these ABR waves. This pattern became most prominent in the recordings after the treatment had been discontinued, and could reflect an improved transmission across both afferent excitatory and recurrent inhibitory synapses in the auditory pathways. The findings indicate that--besides a common anti-inflammatory action--therapies with ACTH and DEX differ with regard to their influence on central nervous functioning.

Effect of acute and short-term administration of cholinomimetic drugs on corticosterone secretion in the rat

Centrally acting cholinomimetic drugs have been proposed for the therapy of cognitive disorders in aged subjects. Among the possible adverse side effects of this class of compounds, of great relevance is the stimulatory action on the adrenocortical axis, in view of the toxicity of glucocorticoids for hippocampal neurons and the immune system. The aim of the present study was to evaluate in conscious male rats the effect of acute and short-term administration of three novel cholinomimetic drugs on the release of corticosterone. The potent agonist of muscarinic receptors RU 35963 strikingly increased corticosterone levels after acute but not after short-term (6 days) administration. Similar results were obtained after administration of the reversible inhibitor of cholinesterase, eptastigmine. In contrast to RU 35963 and eptastigmine, acute administration of a choline precursor, L-alpha-glycerylphosphorylcholine, only slightly affected plasma corticosterone concentrations after both acute and short-term administration. It is concluded that activation of adrenocortical function by cholinomimetic drugs is a short-lasting event which does not represent an important side effect of these compounds when given on a long-term basis.

Cognitive deficits induced in young rats by long-term corticosterone administration

Corticosterone slow-release pellets, implanted for 9 weeks in young Fischer 344 rats, resulted in continuous high plasma levels of the hormone which are comparable to those of rats under mild stress. One week following termination of the drug treatment, the rats were tested in an eight-arm radial maze. During the initial acquisition stages, corticosterone-treated rats exhibited cognitive impairments in contrast to placebo-treated rats. The deficits were observed in all three parameters which were monitored, the total number of errors, the number of correct entries out of the first eight, and the total time needed to complete the test. This study is the first to report specific behavioral decrements related to the previously observed morphological hippocampal changes induced by long-term corticosterone administration.