Selective reduction of glucocorticoid receptor immunoreactivity in the hippocampal formation and central amygdaloid nucleus of the aged rat

A unifying hypothesis of Alzheimer's disease. III. Risk factors

Normal ageing and Alzheimer's disease (AD) have many features in common and, in many respects, both conditions only differ by quantitative criteria. A variety of genetic, medical and environmental factors modulate the ageing-related processes leading the brain into the devastation of AD. In accordance with the concept that AD is a metabolic disease, these risk factors deteriorate the homeostasis of the Ca(2+)-energy-redox triangle and disrupt the cerebral reserve capacity under metabolic stress. The major genetic risk factors (APP and presenilin mutations, Down's syndrome, apolipoprotein E4) are associated with a compromise of the homeostatic triangle. The pathophysiological processes leading to this vulnerability remain elusive at present, while mitochondrial mutations can be plausibly integrated into the metabolic scenario. The metabolic leitmotif is particularly evident with medical risk factors which are associated with an impaired cerebral perfusion, such as cerebrovascular diseases including stroke, cardiovascular diseases, hypo- and hypertension. Traumatic brain injury represents another example due to the persistent metabolic stress following the acute event. Thyroid diseases have detrimental sequela for cerebral metabolism as well. Furthermore, major depression and presumably chronic stress endanger susceptible brain areas mediated by a host of hormonal imbalances, particularly the HPA-axis dysregulation. Sociocultural and lifestyle factors like education, physical activity, diet and smoking may also modulate the individual risk affecting both reserve capacity and vulnerability. The pathophysiological relevance of trace metals, including aluminum and iron, is highly controversial; at any rate, they may adversely affect cellular defences, antioxidant competence in particular. The relative contribution of these factors, however, is as individual as the pattern of the factors. In familial AD, the genetic factors clearly drive the sequence of events. A strong interaction of fat metabolism and apoE polymorphism is suggested by intercultural epidemiological findings. In cultures, less plagued by the 'blessings' of the 'cafeteria diet-sedentary' Western lifestyle, apoE4 appears to be not a risk factor for AD. This intriguing evidence suggests that, analogous to cardiovascular diseases, apoE4 requires a hyperlipidaemic lifestyle to manifest as AD risk factor. Overall, the etiology of AD is a key paradigm for a gene-environment interaction. Copyright 2000 John Wiley & Sons, Ltd.

Age-related changes in rat hippocampal theta rhythms: a difference between type 1 and type 2 theta

The age-related changes in two types of theta rhythms recorded from the hippocampus in young (4 months-old), mature (12-13 months-old) and aged (22-25 months-old) rats were investigated. The type 1 theta rhythm was measured from hippocampal EEG recorded from walking rats and the type 2 theta was measured from the EEG induced by reticular pontin oralis nucleus (PON) stimulation in urethane anesthetized rats. The peak frequency and the peak power were detected from power spectra calculated on each theta sample by fast Fourier transformation (FFT). No age-related alteration was observed on the peak frequency of type 1 theta rhythm. However, on type 2 theta rhythm, the peak frequency was decreased in the aged rats compared with the young and the mature rats. The type 2 theta rhythm is cholinergic, and therefore this result suggests that age-related deterioration can be clearly observed in the cholinergic system including the hippocampus in rats.

On the role of glucocorticoid receptors in brain plasticity

1. The mapping of glucocorticoid receptors (GR) in the rat central nervous system (CNS) has demonstrated their widespread presence in large numbers of nerve and glial cell populations also outside the classical stress regions. 2. The present paper summarizes the evidence that glucocorticoids via GR in the CNS can act as lifelong organizing signals from development to aging. The following examples are given. (a) In the prepubertal and adult offspring, prenatal corticosterone treatment can produce long-lasting changes in striatal dopaminergic communication. (b) In adulthood, the evidence suggests complex regulation by adrenocortical hormones of neurotrophic factors and their receptors in the hippocampal formation. (c) In aging, the strongly GR-immunoreactive pyramidal cell layer of the CA1 hippocampal area appears to be preferentially vulnerable to neurotoxic actions of glucocorticoids, especially in some rat strains. 3. Strong evidence suggests that each nerve cell in the CNS is supported by a trophic unit, consisting of other nerve cells and glial cells, blood vessels, and extracellular matrix molecules. Due to multiple actions on nerve and glial cell populations of the different trophic units, the glucocorticoids may exert either an overall trophic or a neurotoxic action. It seems likely that with increasing age, the endangering actions of glucocorticoids on nerve cells prevail over the neurotrophic ones, leading to reduced nerve cell survival in some trophic units.

Glucocorticoid receptor mRNA in Alzheimer's diseased hippocampus

Hypothalamic-pituitary-adrenal (HPA) axis dysfunction is a common finding in Alzheimer's dementia. Since there is a loss of hippocampal corticosteroid receptors in animal models of aging, and since hippocampal cell loss occurs in Alzheimer's disease (AD), it has been suggested that a loss of hippocampal glucocorticoid receptors (GR) may underlie some aspects of HPA axis dysfunction in patients with AD. Levels of corticosteroid receptor protein are not reliably determined in postmortem human brain due to rapid lability. In contrast, levels of mRNA coding for GR are stable in postmortem tissue. We report here initial observations from in situ hybridization experiments which indicate that regional levels of glucocorticoid receptor mRNA in hippocampus, as determined by film autoradiography, are significantly higher in AD hippocampus than in controls. While neuronal levels of GR mRNA in AD, revealed by emulsion autoradiography, were equal in control and AD tissue. Taken together these results suggest that adrenal dysfunction in AD may relate to defects in receptor function rather than corticosteroid receptor loss in the hippocampus.

Evidence for a role of neurosteroids in modulation of diurnal changes and acute stress-induced corticosterone secretion in rats

The neurosteroid allopregnanolone has been shown to be a potent ligand of gamma-aminobutyric acid (GABA)-A receptors and enhances its receptor-mediated inhibitory events. Since central GABA plays a major inhibitory role, via GABA-A receptors, in hypothalamic-pituitary-adrenal (HPA) function in rats, the present study has evaluated the effect of passive immunoneutralization of allopregnanolone on diurnal changes in corticosterone secretion and acute stress-induced corticosterone secretion in rats. In the first protocol, four groups of male rats (prepubertal, fertile, castrated adult and aged) and three groups of female rats (prepubertal, fertile at different phases of the estrous cycle and aged) were studied. Rats were injected intracerebroventricularly (i.c.v.) with 10 microliters anti-allopregnanolone serum or 10 microliters normal rabbit serum (control) 24 h before exposure to an acute cold swimming stress, and sacrificed either before stress or after 5 min stress. In the second protocol, fertile male or female rats at diestrus II were injected i.c.v. with anti-allopregnanolone serum or normal rabbit serum and sacrificed on the following day at 10.00 or 18.00. Truncal blood samples were collected for measuring plasma corticosterone. Our results showed that there was no significant difference in basal plasma corticosterone levels between antiserum-treated and control rats of both sexes. However, in male rats, central injection of antiserum to allopregnanolone significantly potentiated plasma corticosterone response to stress in prepubertal and adult fertile rats as well as in castrated rats. Likewise, in female rats, the stress response of plasma corticosterone was enhanced by passive immunoneutralization of allopregnanolone in prepubertal and fertile rats throughout the estrous cycle.(ABSTRACT TRUNCATED AT 250 WORDS)

Glial and neuronal glucocorticoid receptor immunoreactive cell populations in developing, adult, and aging brain

A detailed mapping of glucocorticoid receptor (GR) immunoreactivity (IR) in rat CNS was performed employing a mouse monoclonal antibody against rat liver GR. Subjective comparisons were made between the present results and the available data in the literature. A semiquantitation of GR immunostaining was found necessary and was obtained by microdensitometric and morphometric techniques, which enabled the distinction of neuronal and glial cell populations containing GR IR in various CNS regions. GR IR in the CNS was mainly found in the nuclear compartment. The GR was present in neuronal populations with classical neurotransmitters, especially monoamines and glutamate and with various neuropeptides. The degree of colocalization varied according to the function of the brain area. Functional implications were made in relation to stress sensitivity, mood and nociception/antinociception. The global control of networks by glucocorticoids may allow an optimal integration of different types of circuits. The GR is found already in the fetal rat and the development of GR mRNA and receptor protein was followed during the pre- and postnatal periods. The GR appears to be a major factor in brain maturation and in modulation of stress responses. In aged Brown Norway rat brain GR IR but not mineralocorticoid receptor (MR) IR is reduced in the hippocampal nerve cells. The intensity of GR IR but not the number of nerve cells is altered, indicating a reduced activation of the GR in aging in this rat strain. Overall GR participates in neuronal plasticity from fetal and postnatal life to adult life and aging.

Allostasis, amygdala, and anticipatory angst

Regions of the amygdala are involved in anticipation of negative events. Chronic anticipation of negative events leads to what we call allostatic load, or arousal pathology. Two hormones appear to be involved in arousal pathology; corticotropin-releasing hormone in the brain and glucocorticoids. We suggest that increases in corticotropin-releasing hormone, by stress or glucocorticoids, in the amygdala may have functional consequences for allostatic load. Whereas, corticotropin-releasing hormone in the parvocellular region of the paraventricular nucleus of the hypothalamus is decreased by glucocorticoids thereby under negative feedback and homeostatic control, the central nucleus of the amygdala is to some extent under positive feedback and is increased by glucocorticoids, and perhaps under allostatic control. The human and animal literature suggest that a variety of psychopathologies (e.g., melancholia) may be tied to neurohormonal signals activating regions of the amygdala.

Basic fibroblast growth factor and steroid receptors in the aging hippocampus of the brown Norway rat: immunocytochemical analysis in combination with stereology

The effect of aging on the hippocampal formation of the male Brown Norway rat was studied by immunohistochemistry and measurements of the immunoreactive hippocampal cells using stereological techniques. The total estimated number of glucocorticoid receptor (GR) immunoreactive neurons of the CA1-CA2 area did not differ in the 3- and the 36-month-old rat. However, the intensity of the GR immunoreactivity was decreased in the aged animals. A gradual decrease of the immunoreactivity for the mineralocorticoid receptor was also observed in the CA1-CA2 area. In the stratum oriens and the stratum radiatum of the CA1-CA2 area the immunoreactivity for basic fibroblast growth factor (bFGF) present in the glia was found to be reduced [20,000 +/- 2100 (n = 6)] in the 36-month-old rat vs the 3-month-old rat [28,500 +/- 4500 (n = 4) (*P = 0.05)]. However, there was no difference in the number of glial fibrillary acidic protein immunoreactive cells of this area in these two age groups. The present findings give evidence that in the Brown Norway rat there is no loss of the neuronal population containing glucocorticoid receptors of the CA1-CA2 area during aging but suggest that aging is characterized by deficits of glially derived growth factors, such as bFGF.

Postmortem stability of mRNA for glucocorticoid and mineralocorticoid receptor in rodent brain

The relative postmortem stability of the mRNA's for glucocorticoid (GR) and mineralocorticoid (MR) receptor in rodent brain was determined using semi-quantitative in situ hybridization (ISH). Rats were killed by CO2 asphyxiation and their brains removed immediately (0 h) or following 12 h or 24 h delays. Specific hybridization of GR and MR anti-sense [35S]RNA-probe to tissue mRNA encoding these receptors was detected using film and emulsion autoradiography. The most intense labeling for GR mRNA was in the dentate gyrus followed by the CA1 hippocampal region. Lower, but still detectable signal, was apparent over CA3-CA4 pyramidal cell regions. MR mRNA was detected throughout the CA1-4 pyramidal cell fields of the hippocampus and the granular cells of the dentate gyrus. Film images demonstrated that even in the 24 h postmortem delay group intense specific signal was present in sections hybridized with both anti-sense GR and MR probes, although there was some diminution in signal intensity in cortical areas at this later postmortem delay. These initial experiments with rat brain demonstrate that the mRNA's for both GR and MR, as detected with ISH, are stable for up to 24 h following death.

Stress, glucocorticoids, and aging

Attention has long been focused on the relationship between stress and aging, both under the guise of stress as an accelerator of normal aging and of aging as a time of impaired ability to cope with stress. This review examines the considerable amount of evidence in support of these views. We address these ideas with respect to glucocorticoids, the adrenal steroid hormones secreted during stress. In particular, we concentrate on three model systems: 1) programmed senescence in marsupial mice and semelparous fish as mediated by glucocorticoid excess; 2) glucocorticoid hypersecretion in rats and its role in damaging the aging brain; and 3) potential human and primate adrenocortical dysfunction during aging. We discuss physical and cognitive consequences of adrenocortical dysfunction in these systems, and how they may relate to human aging.