Re-examination of the glucocorticoid hypothesis of stress and aging

Contributions of glucocorticoid receptors in cortical astrocytes to memory recall

Dysfunctions in memory recall lead to pathological fear; a hallmark of trauma-related disorders, like posttraumatic stress disorder (PTSD). Both, heightened recall of an association between a cue and trauma, as well as impoverished recall that a previously trauma-related cue is no longer a threat, result in a debilitating fear toward the cue. Glucocorticoid-mediated action via the glucocorticoid receptor (GR) influences memory recall. This literature has primarily focused on GRs expressed in neurons or ignored cell-type specific contributions. To ask how GR action in nonneuronal cells influences memory recall, we combined auditory fear conditioning in mice and the knockout of GRs in astrocytes in the prefrontal cortex (PFC), a brain region implicated in memory recall. We found that knocking out GRs in astrocytes of the PFC disrupted memory recall. Specifically, we found that knocking out GRs in astrocytes in the PFC (AstroGRKO) after fear conditioning resulted in higher levels of freezing to the CS+ tone when compared with controls (AstroGRintact). While we did not find any differences in extinction of fear toward the CS+ between these groups, AstroGRKO female but not male mice showed impaired recall of extinction training. These results suggest that GRs in cortical astrocytes contribute to memory recall. These data demonstrate the need to examine GR action in cortical astrocytes to elucidate the basic neurobiology underlying memory recall and potential mechanisms that underlie female-specific biases in the incidence of PTSD.

Becoming Stressed: Does the Age Matter? Reviewing the Neurobiological and Socio-Affective Effects of Stress throughout the Lifespan

Social and affective relations occur at every stage of our lives. Impairments in the quality of this “social world” can be exceptionally detrimental and lead to psychopathology or pathological behavior, including schizophrenia, autism spectrum disorder, affective disorders, social phobia or violence, among other things. Exposure to highly stressful or traumatic events, depending on the stage of life in which stress exposure occurs, could severely affect limbic structures, including the amygdala, and lead to alterations in social and affective behaviors. This review summarizes recent findings from stress research and provides an overview of its age-dependent effects on the structure and function of the amygdala, which includes molecular and cellular changes, and how they can trigger deviant social and affective behaviors. It is important to highlight that discoveries in this field may represent a breakthrough both for medical science and for society, as they may help in the development of new therapeutic approaches and prevention strategies in neuropsychiatric disorders and pathological behaviors.

From Gut to Brain: Alteration in Inflammation Markers in the Brain of Dextran Sodium Sulfate-induced Colitis Model Mice

Objective

Neuropsychiatric manifestations like depression and cognitive dysfunction commonly occur in inflammatory bowel disease (IBD). In the context of the brain-gut axis model, colitis can lead to alteration of brain function in a bottom-up manner. Here, the changes in the response of the hypothalamic-pituitary-adrenal axis and inflammation-related markers in the brain in colitis were studied.

Methods

Dextran sodium sulfate (DSS) was used to generate a mouse model of colitis. Mice were treated with DSS for 3 or 7 days and sacrificed. We analyzed the gene expression of brain-derived neurotrophic factor (BDNF), cyclo-oxygenase 2 (COX-2), and glial fibrillary acidic protein (GFAP), and the expression of GFAP, in the hippocampus, hypothalamus, and amygdala. Additionally, the levels of C-reactive protein (CRP) and serum cortisol/corticosterone were measured.

Results

Alteration of inflammatory-related markers varied depending on the brain region and exposure time. In the hippocampus, COX-2 mRNA, GFAP mRNA, and GFAP expression were upregulated during exposure to DSS. However, in the hypothalamus, COX-2 mRNA was upregulated only 3 days after treatment. In the amygdala, BDNF and COX-2 mRNAs were downregulated. CRP and corticosterone expression increased with DSS treatment at day 7.

Conclusion

IBD could lead to neuroinflammation in a bottom-up manner, and this effect varied according to brain region. Stress-related hormones and serum inflammatory markers, such as CRP, were upregulated from the third day of DSS treatment. Therefore, early and active intervention is required to prevent psychological and behavioral changes caused by IBD, and region-specific studies can help understand the precise mechanisms by which IBD affects the brain.

Neighborhood social stressors, fine particulate matter air pollution, and cognitive function among older U.S. adults

A growing number of studies have found a link between outdoor air pollution and cognitive function among older adults. Psychosocial stress is considered an important factor determining differential susceptibility to environmental hazards and older adults living in stressful neighborhoods may be particularly vulnerable to the adverse health effects of exposure to hazards such as air pollution. The objective of this study is to determine if neighborhood social stress amplifies the association between fine particulate matter air pollution (PM2.5) and poor cognitive function in older, community-dwelling adults. We use data on 779 U.S. adults ages 55 and older from the 2001/2002 wave of the Americans' Changing Lives study. We determined annual average PM2.5 concentration in 2001 in the area of residence by linking respondents with EPA air monitoring data using census tract identifiers. Cognitive function was measured using the number of errors on the Short Portable Mental Status Questionnaire (SPMSQ). Exposure to neighborhood social stressors was measured using perceptions of disorder and decay and included subjective evaluations of neighborhood upkeep and the presence of deteriorating/abandoned buildings, trash, and empty lots. We used negative binomial regression to examine the interaction of neighborhood perceived stress and PM2.5 on the count of errors on the cognitive function assessment. We found that the association between PM2.5 and cognitive errors was stronger among older adults living in high stress neighborhoods. These findings support recent theoretical developments in environmental health and health disparities research emphasizing the synergistic effects of neighborhood social stressors and environmental hazards on residents' health. Those living in socioeconomically disadvantaged neighborhoods, where social stressors and environmental hazards are more common, may be particularly susceptible to adverse health effects of social and physical environmental exposures.

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.

Stress risk factors and stress-related pathology: neuroplasticity, epigenetics and endophenotypes

This paper highlights a symposium on stress risk factors and stress susceptibility, presented at the Neurobiology of Stress workshop in Boulder, CO, in June 2010. This symposium addressed factors linking stress plasticity and reactivity to stress pathology in animal models and in humans. Dr. J. Radley discussed studies demonstrating prefrontal cortical neuroplasticity and prefrontal control of hypothalamo-pituitary-adrenocortical axis function in rats, highlighting the emerging evidence of the critical role that this region plays in normal and pathological stress integration. Dr. M. Kabbaj summarized his studies of possible epigenetic mechanisms underlying behavioral differences in rat populations bred for differential stress reactivity. Dr. L. Jacobson described studies using a mouse model to explore the diverse actions of antidepressants in brain, suggesting mechanisms whereby antidepressants may be differentially effective in treating specific depression endophenotypes. Dr. R. Yehuda discussed the role of glucocorticoids in post-traumatic stress disorder (PTSD), indicating that low cortisol level may be a trait that predisposes the individual to development of the disorder. Furthermore, she presented evidence indicating that traumatic events can have transgenerational impact on cortisol reactivity and development of PTSD symptoms. Together, the symposium highlighted emerging themes regarding the role of brain reorganization, individual differences, and epigenetics in determining stress plasticity and pathology.

Diurnal salivary cortisol concentrations in Parkinson's disease: increased total secretion and morning cortisol concentrations

BACKGROUND

Parkinson's disease (PD) is a chronic neurodegenerative disorder. There is limited knowledge about the function of the hypothalamic-pituitary-adrenal axis in PD. The primary aim of this prospective study was to analyze diurnal salivary cortisol concentrations in patients with PD and correlate these with age, gender, body mass index (BMI), duration of PD, and pain. The secondary aim was to compare the results with a healthy reference group.

METHODS

Fifty-nine PD patients, 35 women and 24 men, aged 50-79 years, were recruited. The reference group comprised healthy individuals matched for age, gender, BMI, and time point for sampling. Salivary cortisol was collected at 8 am, 1 pm, and 8 pm, and 8 am the next day using cotton-based Salivette(®) tubes and analyzed using Spectria(®) Cortisol I(125). A visual analog scale was used for estimation of pain.

RESULTS

The median cortisol concentration was 16.0 (5.8-30.2) nmol/L at 8 am, 5.8 (3.0-16.4) at 1 pm, 2.8 (1.6-8.0) at 8 pm, and 14.0 (7.5-28.7) at 8 am the next day. Total secretion and rate of cortisol secretion during the day (8 am-8 pm) and the concentration of cortisol on the next morning were lower (12.5 nmol/L) in the reference group. No significant correlations with age, gender, BMI, duration of PD, Hoehn and Yahr score, Unified Parkinson's Disease Rating Scale III score, gait, pain, or cortisol concentrations were found.

CONCLUSION

The neurodegenerative changes in PD does not seem to interfere with the hypothalamic-pituitary-adrenal axis. Salivary cortisol concentrations in PD patients were increased in the morning compared with the reference group, and were not influenced by motor dysfunction, duration of disease, or coexistence of chronic or acute pain.

Antidepressant effects of the extract YZ-50 from Polygala tenuifolia in chronic mild stress treated rats and its possible mechanisms

YZ-50 is an active fraction obtained from the root of Polygala tenuifolia Willd. (Polygalaceae) extract and it has been reported previously to exert beneficial effects on mental health in depressed sufferers, however, its mechanism of action remains unresolved. This study utilized the chronic mild stress (CMS) model of depression in Sprague-Dawley rats to evaluate the effects of YZ-50 on depressive behaviors. Furthermore, we tested the hypothesis that the capacity of YZ-50 to reverse the harmful effects of CMS is relative to the hypothalamo-pituitary-adrenal (HPA) system and brain-derived neurotrophic factor (BDNF) in the hippocampus. Repeated administration of YZ-50 for 28 days at the doses of 140 and 280 mg/kg in CMS, YZ-50 reversed the CMS-induced changes in sucrose consumption, plasma corticosterone levels and open field activity. In addition, CMS significantly decreased hippocampal BDNF mRNA levels. However, YZ-50 counteracted a decrease in hippocampal BDNF mRNA caused by CMS. In conclusion, YZ-50 reversed the harmful effects of CMS on mood and behaviors in rats and it possesses an antidepressant property that is at least in part mediated by the neuroendocrine and neuropropective systems, and it is likely that the HPA system plays an important role in this process.

Beneficial effects of exercise and its molecular mechanisms on depression in rats

Exercise showed the beneficial effects on mental health in depressed sufferers, whereas, its underlying mechanisms remained unresolved. This study utilized the chronic unpredictable stress (CNS) animal model of depression to evaluate the effects of exercise on depressive behaviors and spatial performance in rats. Furthermore, we tested the hypothesis that the capacity of exercise to reverse the harmful effects of CNS was relative to the hypothalamo-pituitary-adrenal (HPA) system and brain-derived neurotrophic factor (BDNF) in the hippocampus. Animal groups were exposed to CNS for 4 weeks with and without access to voluntary wheel running. Stressed rats consumed significantly less of a 1% sucrose solution during CNS and exhibited a significant decrease in open field behavior. On the other hand, they showed impaired spatial performance in Morris water maze test 2 weeks after the end of CNS. Further, CNS significantly decreased hippocampal BDNF mRNA levels. However, voluntary exercise improved or even reversed these harmful behavioral effects in stressed rats. Furthermore, exercise counteracted a decrease in hippocampal BDNF mRNA caused by CNS. In addition, we also found that CMS alone increased circulating corticosterone (CORT) significantly and decreased hippocampal glucocorticoid receptor (GR) mRNA. At the same time, exercise alone increased CORT moderately and did not affect hippocampal GR mRNA levels. While, when both CNS and exercise were combined, exercise reduced the increase of CORT and the decrease of GR caused by CMS. The results demonstrated that: (1) exercise reversed the harmful effects of CNS on mood and spatial performance in rats and (2) the behavioral changes induced by exercise and/or CNS might be associated with hippocampal BDNF levels, and in addition, the HPA system might play different roles in the two different processes.

Benzodiazepine effect of 125I-iomazenil–benzodiazepine receptor binding and serum corticosterone level in a rat model

To test the change in free or unoccupied benzodiazepine receptor (BZR) density in response to diazepam, we investigated (125)I-iomazenil ((125)I-IMZ) binding and serum corticosterone levels in a rat model. Wistar male rats, which received psychological stress using a communication box for 5 days, were divided into two groups according to the amount of administered diazepam: no diazepam [D (0)] group and 10 mg/kg per day [D (10)] group of 12 rats each. The standardized uptake value (SUV) of (125)I-IMZ of the D (10) group were significantly lower (P < .05) than those of the D (0) group in the frontal, parietal and temporal cortices, globus pallidus, hippocampus, amygdala and hypothalamus. The serum corticosterone level ratio in the D (10) group was significantly lower than that in the D (0) group (P < .05). From the change in serum corticosterone levels, diazepam attenuated the psychological stress produced by the physical stress to animals in adjacent compartments. From the reduced binding of (125)I-IMZ, it is clear that diazepam competed with endogenous ligand for the free BZR sites, and the frontal, parietal and temporal cortices, globus pallidus, hippocampus, amygdala and hypothalamus are important areas in which (125)I-IMZ binding is strongly affected by administration of diazepam.

125I-iomazenil - benzodiazepine receptor binding and serum corticosterone level during psychological stress in a rat model

To test the hypothesis that benzodiazepine receptor density decreases in response to stress, we correlated (125)I-iomazenil ((125)I-IMZ) binding with serum corticosterone levels in a rat model. Wistar male rats were divided into four groups; control group (CON, 10 rats), no physical or psychological stress; and one-, three-, and five-day stress groups of 12 rats each (1-DAY, 3-DAY, and 5-DAY, respectively), receiving psychological stress for the given number of days. Psychological stress were given to rats with a communication box. The standardized uptake value (SUV) of (125)I-iomazenil of the 3-DAY and 5-DAY showed that (125)I-iomazenil-benzodiazepine receptor binding was significantly reduced in the cortices, accumbens nuclei, amygdala and caudate putamen (p<0.05). Serum corticosterone level ratio appeared to be slightly elevated in 3-DAY and 5-DAY, although this elevation was not significant. These data suggest that (125)I-IMZ is a useful radioligand to reflect received stress and its binding in the cortices, accumbens nuclei, amygdala and caudate putamen is strongly affected by psychological stress.

Chronic stress attenuates glucocorticoid negative feedback: involvement of the prefrontal cortex and hippocampus

Disruption of the glucocorticoid negative feedback system is observed in approximate one half of human depressives, and a similar condition is induced in animals by chronic stress. This disruption is thought to involve down-regulation of glucocorticoid receptors (GRs) in the feedback sites of the brain. However, the responsible site of the brain has not been well elucidated. Here we examined the effects of chronic stress induced by water immersion and restraint (2 h/day) for 4 weeks followed by recovery for 10 days on the GR levels in the prefrontal cortex (PFC), hippocampus, and hypothalamus of rats using a Western immunoblot technique. In the PFC, the cytosolic GR levels were decreased, but the nuclear GR levels were not changed. In the hippocampus, the levels of cytosolic and nuclear GRs were increased. However, there were no marked changes in the GR levels in the hypothalamus. The changes in the cytosolic GR levels were confirmed at the mRNA level by an in situ hybridization technique. We next examined the suppressive effects of dexamethasone (DEX) infusions into these regions on the circulating corticosterone levels. When DEX was infused into the PFC or hippocampus of the chronically stressed rats, the suppressive response to DEX was abolished, but the response was normal in the hypothalamus. In addition, when DEX was injected systemically to the chronically stressed rats, the suppressive response to DEX was significantly attenuated. These results suggest that the abnormal changes in GRs in the higher centers of the hypothalamo-pituitary-adrenal axis are involved in the chronic stress-induced attenuation of the feedback. Since dysfunction of the PFC or hippocampus is implicated in the pathogenesis of depression, the present findings would help to understand the mechanisms underlying the disrupted feedback system and its relation to brain dysfunction in depression.

In vitro regulation of corticotropin-releasing hormone

Studies involving regulation of corticotropin-releasing hormone (CRH) in vitro have been used to validate findings obtained in vivo and more importantly have been used as model systems to better understand signalling mechanisms responsible for the expression of the CRH gene and peptide. Many in vitro studies examining CRH have utilized hypothalamic tissue while a few have focused on the amygdala. Clonal cell lines have also been utilized as models of central nervous system CRH neurons. Stimuli that have been implicated in regulating hypothalamic CRH regulation in vitro include protein kinase A (PKA) and protein kinase C (PKC) activators, glucocorticoids, biogenic amines, cytokines and the gaseous neurotransmitters. Amygdalar CRH levels in vitro are affected by some of the same stimuli that regulate hypothalamic CRH; however there is evidence supporting differential regulation of CRH in these two brain regions by some of the same stimuli. Only a few studies in aggregate have investigated signal transduction mechanisms and these studies have focused on PKA- and glucocorticoid-mediated changes in CRH expression. Thus, much more investigative work in better understanding CRH regulation in vitro is needed.

Corticosteroids: sculptors of the hippocampal formation

The influence of corticosteroids on hippocampus-dependent learning and memory processes is now indisputable. On the other hand, closer scrutiny of early studies together with interpretations from newer studies would suggest that the proposition that corticosteroid-induced hippocampal cell death accounts fully for the associated cognitive deficits is only partially correct. Firstly, it is now clear that a specific sub-population of hippocampal neurons, the granule cells of the dentate gyrus, is more sensitive to changes in the corticosteroid environment; this fact raises the interesting question of what might be the unique properties of granule cells that render them more vulnerable to these hormones, since virtually all hippocampal cells express corticosteroid receptors. Secondly, from a critical analysis of the available data, the picture that emerges is that corticosteroids, by acting through two distinct receptors, influence not only cell birth and death, but probably also cell differentiation. Mineralocorticoid receptor (MR) occupation appears to be essential for the survival of existing and newly generated granule neurons. In contrast, while glucocorticoid receptors (GR) can induce loss of neurons in the absence of MR activation, it appears that their occupation usually results in less drastic effects involving only dendritic atrophy and loss of synaptic contacts. This revised scheme of corticosteroid actions on hippocampal structure should explain earlier observations that many of the cognition- impairing effects of corticosteroids are reversible.

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.

Cushing’s disease and melancholia

Striking similarities exist in the endocrinology of Cushing's disease and melancholic depression.Laboratory abnormalities, which have been found in both, include raised urinary,plasma and salivary cortisol, non-suppression of cortisol in the dexamethasone suppression test and adrenocorticotrophin (ACTH) hypersecretion. The hypercortisolism can be so severe in melancholic depression that it is difficult to distinguish from Cushing's disease and has been described as a "pseudo-Cushing's" state. Cerebrospinal fluid corticotrophin-releasing hormone (CRH) levels have been found to be lower in patients with Cushing's disease than in depressed subjects. Dynamic endocrine tests may help to distinguish between the two disorders.An exaggerated response to synacthen has been found in both but a reduced ACTH response to CRH occurs in depression, unlike those with Cushing's disease who show ACTH hyper-responsiveness. Other tests, which may help to distinguish between the two disorders,include the dexamethasone-CRH test, the naloxone test, the insulin-induced hypoglycemia test and the desmopressin stimulation test. Similarities in psychiatric symptoms have been recognised for many years. More recently, the physical complications of melancholic depression have been noted. These include osteoporosis, an increased risk of death from cardiovascular disease, hypertension, a redistribution of fat to intra abdominal sites and insulin resistance. Cushing's disease shares these physical complications and we propose that the common underlying factor is excessive plasma glucocorticoids. The increasing recognition of the physical complications and the increased morbidity and mortality in those who suffer from depression underscores the necessity for early detection and treatment of this illness and screening for undetected physical complications.

Regulation of corticotropin-releasing hormone in vitro

Studies examining regulation of corticotropin-releasing hormone (CRH) in vitro have been used to validate findings obtained in vivo and more importantly have been used as model systems to better understand signalling mechanisms responsible for the expression of the CRH gene and peptide. Most in vitro studies examining CRH have utilized hypothalamic tissue while a few have focused on the amygdala. Furthermore, clonal cell lines have also been utilized as models of central nervous system CRH neurons. Stimuli that have been implicated in regulating hypothalamic CRH in vitro include protein kinase A (PKA) and protein kinase C (PKC) activators, glucocorticoids, biogenic amines, cytokines and the gaseous neurotransmitters. CRH levels in the amygdala in vitro are affected by some of the same stimuli that regulate hypothalamic CRH; however there is evidence supporting differential regulation of CRH in these two brain regions by some of the same stimuli. Only a few studies in aggregate have investigated the signal transduction mechanisms responsible for CRH expression. These mechanistic studies have focused on PKA- and glucocorticoid-mediated changes in CRH expression. Clearly much more investigative work in better understanding CRH regulation in vitro is needed.

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.

Regional differences in brain monoamine oxidase subtypes in an animal model of geriatric depression: effects of olfactory bulbectomy in young versus aged rats

Geriatric depression is often associated dysregulation of the hypothalamus-pituitary-adrenal (HPA) axis, and with poor responsiveness to antidepressants that work through inhibition of monoamine reuptake; accordingly, it has been suggested that MAO inhibitors may represent a therapeutic alternative in this group. In the current study, we evaluated expression of MAO subtypes in brain regions of young and aged rats subjected to olfactory bulbectomy (OBX), a procedure that reproduces many of the biochemical and functional changes associated with human depression. Activities of both MAO A and B were elevated in aged rats as compared to young rats in most regions, but not in the midbrain, and the OBX lesion failed to produce any change in this pattern. These results stand in contrast to the differential effects of glucocorticoids, which reduce brain MAO in young animals but induce activity in aged rats. Our results support the view that the aged brain possesses biochemical characteristics that distinguish its monoamine biochemistry from that of young brain, and that these distinctions may work in conjunction with HPA axis dysregulation to influence the etiology and therapy of geriatric depression. The use of appropriate animal models for depression and for disruption of HPA axis function can allow for the testing of potential human biomarkers (such as platelet MAO) that may serve to predict treatment outcome.

3D MRI studies of neuroanatomic changes in unipolar major depression: the role of stress and medical comorbidity

Increasing evidence has accumulated for structural brain changes associated with unipolar recurrent major depression. Studies of neuroanatomic structure in early-onset recurrent depression have only recently found evidence for depression-associated structural change. Studies using high-resolution three-dimensional magnetic resonance imaging (MRI) are now available to examine smaller brain structures with precision. Brain changes associated with early-onset major depression have been reported in the hippocampus, amygdala, caudate nucleus, putamen, and frontal cortex, structures that are extensively interconnected. They comprise a neuroanatomic circuit that has been termed the limbic-cortical-striatal-pallidal-thalamic tract. Of these structures, volume loss in the hippocampus is the only consistently observed change to persist past the resolution of the depression. Possible mechanisms for tissue loss include neuronal loss through exposure to repeated episodes of hypercortisolemia; glial cell loss, resulting in increased vulnerability to glutamate neurotoxicity; stress-induced reduction in neurotrophic factors; and stress-induced reduction in neurogenesis. Many depressed patients, particularly those with late-onset depression, have comorbid physical illnesses producing a high rate of hyperintensities in deep white matter and subcortical gray matter and brain damage to key structures involved in the modulation of emotion. Combining MRI studies with functional studies has the potential to localize abnormalities in blood flow, metabolism, and neurotransmitter receptors and provide a better integrated model of depression.

Reorganization of the morphology of hippocampal neurites and synapses after stress-induced damage correlates with behavioral improvement

We recently demonstrated that stress-induced cognitive deficits in rats do not correlate with hippocampal neuronal loss. Working on the premise that subtle structural changes may however be involved, we here evaluated the effects of chronic stress on hippocampal dendrite morphology, the volume of the mossy fiber system, and number and morphology of synapses between mossy fibers and CA3 dendritic excrescences. To better understand the mechanisms by which stress exerts its structural effects, we also studied these parameters in rats given exogenous corticosterone. Further, to search for signs of structural reorganization following the termination of the stress and corticosterone treatments, we analysed groups of rats returned to treatment-free conditions. All animals were assessed for spatial learning and memory performance in the Morris water maze. Consistent with previous findings, dendritic atrophy was observed in the CA3 hippocampal region of chronically stressed and corticosterone-treated rats; in addition, we observed atrophy in granule and CA1 pyramidal cells following these treatments. Additionally, profound changes in the morphology of the mossy fiber terminals and significant loss of synapses were detected in both conditions. These alterations were partially reversible following rehabilitation from stress or corticosterone treatments. The fine structural changes, which resulted from prolonged hypercortisolism, were accompanied by impairments in spatial learning and memory; the latter were undetectable following rehabilitation. We conclude that there is an intimate relationship between corticosteroid levels, hippocampal neuritic structure and hippocampal-dependent learning and memory.

Biological effects of single and repeated swimming stress in male rats: beneficial effects of glucocorticoids

We have examined the biological effects of single (45 min at 22 degrees C) and repeated swimming stress (45 min at 22 degrees C for 7 d) using male Sprague-Dawley rats. Repeated swimming for a week resulted in a significant inhibition in total body weight (25%) as compared to control unstressed animals. There was significant increase in adrenal and kidney relative weight and decreases in relative thymus weight in repeated swimming-stressed animals as compared to control animals. Repeated swimming stress resulted in almost threefold increase in plasma corticosterone levels with concomitant dramatic decrease in total glucocorticoid receptor (GR) levels in liver, thymus, and heart as compared to control unstressed animals. Interestingly, single swimming stress resulted in a significant elevation in lipid peroxidation levels in the liver and heart. In contrast, there was no change in the lipid per oxidation levels in the liver and heart between chronic stressed and control unstressed animals. Finally, both single and repeated swimming-stress animals had almost 50% reduction in plasma triglyceride levels as compared to control unstressed animals. It is concluded that elevated plasma corticosterone levels by downregulating GR during repeated swimming stress exerts beneficial effects in rats by retarding the total body weight gain and lowering plasma triglyceride levels without affecting free-radicals-induced oxidative stress.

Developmental stress disrupts habituation but spares prepulse inhibition in young rats

Stress has long been recognized as a factor that contributes to the induction of schizophrenia and results in abnormal sensorimotor functioning and information processing. Patients with schizophrenia show disrupted habituation and prepulse inhibition of the acoustic startle response. This study examined the effects of maternal isolation in rats on the habituation of startle and PPI to assess the potential impact of developmental stress on schizophrenic symptomatology. Evaluation of performance in young adulthood (3-4 months) revealed a disruption of habituation in the isolated group; response amplitude increased over time. PPI was not altered. These results suggest that the disruption of habituation may involve acute effects of elevated stress hormones on neuronal functions. In contrast, disturbance of PPI may require an accrual of neuronal insult and damage to ultimately undermine neurologic function, possibly through impact on N-methyl-D-aspartate-mediated transmission. An analysis of effects at middle age is planned to address this possibility.

Expression of the glucocorticoid receptor in early and late passage C-6 glioma cells and in normal astrocytes derived from aged mouse cerebral hemispheres

The presence of the glucocorticoid receptor in early and late passage C-6 glioma cells 2B clone and in astrocytes derived from aged mouse cerebral hemispheres has been documented by immunoblotting and/or immunofluorescence labelling. All cell types studied express the glucocorticoid receptor of molecular weight 97 KDa. In addition, in astrocytes derived from aged mouse cerebral hemispheres a smaller molecular weight polypeptide reacting with anti-glucocorticoid receptor antibody was also demonstrated. No difference in the amount of the 97 KDa glucocorticoid receptor between early and late C-6 2B cells was observed, whereas the astrocytes from aged cerebral hemispheres contained considerably reduced amounts of the glucocorticoid receptor compared to C-6 2B cells. Late passage C-6 2B cells were immunofluorescence labelled with the anti-glucocorticoid antibody, the receptor being almost exclusively present in the cytoplasm, with particular concentration in the perinuclear region. The presence of glucocorticoid receptor of molecular weight 97 KDa in glial cells corroborates and expands the existing data based on radioligand binding and immunocytochemical studies. These cell populations can be exploited as a model system for the study of the effects of glucocorticoids on senescence and brain aging.

Depression and anxiety in Parkinson's disease: possible effect of genetic variation in the serotonin transporter

Recently, a functional polymorphism in the promoter region of the serotonin transporter gene has been linked to anxiety. In cell culture, the short allele of this polymorphism synthesizes less serotonin transporter, resulting in a reduction of the removal of serotonin from the synaptic cleft. This pilot study examines depression and anxiety in Parkinson's disease patients as a function of the variation in this polymorphism. Thirty-two patients were genotyped and then blindly administered the Hamilton Depression and Anxiety Scales. Clinical data on the neurologic features of the disease were also gathered. Patients with the short allele of the serotonin transporter promotor scored significantly higher on both the depression and anxiety measures. There were no differences between groups for any neurologic variable. Patients with the short allele were more likely to have scores for anxiety and depression that indicated "caseness." This study suggests that the short allele of the serotonin transporter gene may represent a significant risk factor for the development of anxiety and depression in Parkinson's disease patients.

Frailty and the older man

Frailty is a wasting syndrome of advanced age that leaves a person vulnerable to falls, functional decline, morbidity, and mortality. The cause of this syndrome is complex but likely has a biologic basis. Studies by the authors' research group have validated a phenotype of frailty [table: see text] and have established a gender difference in prevalence with women twice as likely to develop the syndrome as men. Using a biologic model that includes sarcopenia, neuroendocrine decline, and immune dysfunction as potential causes, several physiologic gender differences may explain these differing levels of frailty. First, higher baseline levels of muscle mass may protect men from reaching a threshold of weakness and muscle mass loss that may put them into a category of frailty. Specific neuroendocrine and hormonal factors that may make men less likely to develop frailty than women include testosterone and GH, which may provide advantages in muscle mass maintenance, and cortisol, which is likely less dysregulated in older men as compared to older women. There is also evidence of immune system dimorphism that is, in part, responsive to sex steroids, perhaps making men more vulnerable to sepsis and infection and women more vulnerable to chronic inflammatory conditions and muscle mass loss. The net effect of the hormonal dysregulation and immune system dysfunction is an accelerated loss of muscle mass. There is also evidence that lower levels of activity and lower caloric intake in women as compared to men may also influence the phenotype of frailty and make women more vulnerable then men to the syndrome.

Glucocorticoids, stress, and their adverse neurological effects: relevance to aging

Glucocorticoids, the adrenal steroids secreted during stress, while critical for successful adaptation to acute physical stressors, can have a variety of deleterious effects if secreted in excess. It has come to be recognized that glucocorticoid excess can have adverse effects in the nervous system, particularly the hippocampus. These effects include disruption of synaptic plasticity, atrophy of dendritic processes, compromising the ability of neurons to survive a variety of coincident insults and, at an extreme, overt neuron death. This review considers the current cellular and molecular bases underlying these adverse glucocorticoid actions, and their relevance to brain aging.

Ligand and subfield specificity of corticoid-induced neuronal loss in the rat hippocampal formation

Adult male rats were treated chronically with the selective type II corticosteroid receptor agonist dexamethasone, with dexamethasone plus aldosterone, a selective type I receptor agonist, and with a supraphysiological dose of corticosterone sufficient to occupy both type I and type II receptors; injection-free and oil (vehicle)-treated rats served as controls. Following one month of treatment, the animals were killed and their brains were processed for stereological assessment of volumes and total numbers of neurons in the hippocampal formation. Dexamethasone treatment resulted in significant reductions in the total number of dentate granule and the CA3 pyramidal cells and in the volumes of some layers of these subfields; however, this steroid did not influence any morphometric parameter in the CA1 subfield, and the number of hilar cells was also unaltered. In contrast to the results obtained with dexamethasone, the other two groups of corticoid injected animals did not reveal changes in total cell numbers in any of the subfields of the hippocampal formation, although in the corticosterone-treated group a reduction in the volumes of the hilus and of the stratum radiatum of the CA3 subfield was observed. The present data show that the exclusive activation of type II corticosteroid receptors results in subfield-specific neuronal loss in the hippocampal formation of rats. This type II receptor-mediated neuronal loss can, however, be abrogated by the simultaneous stimulation of type I corticosteroid receptors. Together, these findings extend and support previous studies which suggested that activation of type I corticosteroid receptors may promote neuronal survival and that neurodegeneration may be triggered by type II corticosteroid receptor stimulation. An important implication of this result is that elevated levels of the endogenous corticosteroid receptor ligands (e.g., during stress) is unlikely to cause severe structural damage to the hippocampal formation due to the contemporaneous occupation of type I receptors.

Effects of alprazolam on the free-choice ethanol consumption induced by isolation stress in aged rats

Late-onset drinking is a common problem in elderly people related to stress induced by social isolation. Experiments were performed in order to evaluate the effects of alprazolam, a benzodiazepine agonist anxiolytic, on the free-choice ethanol consumption in aged rats subjected to isolation stress. The animals we offered a two-bottle choice consumption (one of 0.2% saccharin and the other with 10% ethanol/0.2% saccharin) and then exposed to 4 days of isolation stress on an irregular, unpredictable schedule. Stress resulted in significant increase in ethanol consumption. Treatment with alprazolam (1 mg/Kg) partially reversed this adverse effect of stress.

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.

Glucocorticoid-targeting of the adenylyl cyclase signaling pathway in the cerebellum of young vs. aged rats

Glucocorticoids exacerbate aging-induced cell death, but relatively little is known about other CNS effects in senescence. We examined noradrenergic/adenylyl cyclase signaling in the cerebellum, which is a brain region that is susceptible to deterioration of synaptic function in aging. Aged control rats had increased total cyclase catalytic activity, but showed deficits in basal adenylyl cyclase. Deficits resolved when G-proteins were stimulated with GTP, GTP and fluoride, or GTP and isoproterenol, despite reductions in beta-receptors. In young rats, long-term dexamethasone infusions evoked the same types of changes that had been seen in aging, including induction of cyclase catalytic activity and enhanced G-protein responsiveness. The same dexamethasone regimens given to aged rats failed to cause stimulation of these processes in the cerebellum, but did so in a peripheral tissue (kidney). These data indicate homology between the cellular events involved in noradrenergic signaling during aging and after glucocorticoid administration to young animals; the absence of glucocorticoid effects in the elderly cohort supports a convergent mechanism with aging. Given the high incidence of HPA axis dysregulation in the elderly, and particularly in elderly depression, effects of glucocorticoids on cell signaling may contribute to disrupted function and to altered drug reactivity.

Uniform abnormalities in the hippocampus of five chronic schizophrenic men compared with age-matched controls

In a post-mortem study five chronic schizophrenic men were matched with respect to age and sex to five control subjects without a known history of psychiatric illness, and were compared in pairs with regard to neurone number and pyramidal cell orientation in the left hippocampus. All five schizophrenics had significantly more disoriented pyramidal cells in the Cornu Ammonis subregions CA1-CA3 than their matched controls. They also had significantly fewer pyramidal cells in the observed areas of CA1 and CA3, but not in CA2 and CA4. There was no difference in the number of granular cells of the dentate gyrus. In pairwise comparisons the pattern was never reversed with respect to these two parameters, except in a single case where one schizophrenic proband had more cells in subregion CA2. In the absence of gliosis the findings were interpreted as sequelae of an early, presumably prenatal, injury.

Cortisol secretion and Alzheimer's disease progression

BACKGROUND

Mild hypercortisolemia is a frequent concomitant of Alzheimer's disease (AD). In an effort to ascertain the relationship between serum cortisol concentration (CORT) and disease progression, aging, and survival, we followed 9 persons with AD, ages from 56 to 84 years, from an original cohort of 19 enrollees with serial cognitive testing and CORT determinations.

METHODS

The cognitive instrument was a modification of the Alzheimer's Disease Assessment Scale-Cognitive (mADAS-COG). Serum cortisol determinations were performed at noon, and an Afternoon Cortisol Test (ACT) was used to obtain an estimate of average CORT.

RESULTS

Baseline 12:00 hours CORT but not ACT correlated significantly with the change in mADAS-COG (r = .90, p < .01). ACT levels increased as the mADAS-COG increased over time (p = .037), by 0.156 +/- 0.06 microgram/dL for each one-point increase (indicating greater impairment) in cognitive test score. ACT levels did not increase significantly simply with aging. For the entire cohort of 19 subjects, neither baseline ACT nor 12:00 hours CORT was significantly related to survival.

CONCLUSIONS

Hypercortisolemia in AD appears related to the clinical progression of the disease, but not to aging or length of survival.

McEwen-Induced Modulation of Endocrine History: A Partial Review

In endless facets of physiology, there are points of homeostatic balance, such that too much or too litttle of something can both be deleterious (i.e., an "inverse U" pattern). This is particularly true when considering glucocorticoids (GCs), the adrenals steroid secreted during stress. In the first part of this paper, I review a number of realms in which a paucity and an excess of GCs are both damaging. Some findings are classical (for example, concerning GC effects upon body weight), while some are quite recent and have considerable implications for both physiology and pathophysiology (for example, inverse U's of GC actions in the realm of immunity and neuronal survival). The second part of the review considers the far thornier issue of how such inverse U's of GC actions are generated on a cellular and molecular level. One solution that has evolved, primarily in the hippocampus within the nervous system, involves the presence of two different types of receptors for GCs within the same cells; so long as the two receptors have very different affinities and mediate opposing effects on some cellular endpoint, an inverse U will emerge. The second solution, found in a number of peripheral tissues, involves GCs having opposing effects on the amount of some signal being generated (e.g., an immune cytokine) and the sensitivity of target tissues to that signal; under conditions that appear to be physiologically relevant, inverse U's emerge from this pattern as well. The final section of this review considers the enormous role played by Bruce McEwen in the emergence of this literature. I suggest that while much of this obviously has to do with the facts that have come from his group, another substantial contribution is from his steadying and supportive personality, the veritable embodiment of homeostatic balance.

Aging in the hippocampus: interrelated actions of neurotrophins and glucocorticoids

Over the past two decades, evidence has been accumulating that diffusible molecules, such as growth factors and steroids hormones, play an important part in neural senescence, particularly in the hippocampus. There is also evidence that these molecules do not act as independent signals, but show interrelated regulation and cooperative control over the aging process. Here, we review some of the changes that occur in the hippocampus with age, and the influence of two classes of signaling substances: glucocorticoids and neurotrophins. We also examine the interactions between these substances and how this could influence the aging process.

Endocrine modulation of the neurotoxicity of gp120: implications for AIDS-related dementia complex

HIV infection often involves the development of AIDS-related dementia complex, a variety of neurologic, neuropsychologic, and neuropathologic impairments. A possible contributor to AIDS-related dementia complex is the HIV envelope glycoprotein gp120, which damages neurons via a complex glutamate receptor- and calcium-dependent cascade. We demonstrate an endocrine modulation of the deleterious effects of gp120 in primary hippocampal and cortical cultures. Specifically, we observe that gp120-induced calcium mobilization and neurotoxicity are exacerbated by glucocorticoids, the adrenal steroids secreted during stress. Importantly, this deleterious synergy can occur between gp120 and synthetic glucocorticoids (such as prednisone or dexamethasone) that are used clinically in high concentrations to treat severe cases of the Pneumocystis carinii pneumonia typical of HIV infection. Conversely, we also observe that estradiol protects neurons from the deleterious actions of gp120, reducing toxicity and calcium mobilization.

Long-term glucocorticoid treatments decrease local cerebral blood flow in the rat hippocampus, in association with histological damage

The present study examined the influence of a long-term treatment with glucocorticoid on local cerebral blood flow of the hippocampus in rats, estimated with the hydrogen clearance method. Either a cholesterol (100 mg, as a control) or corticosterone (100 mg) bead was implanted subcutaneously in rats for a period of three months, beginning at 12 weeks of age. The effects of the treatments on the local circulation of the hippocampus were evaluated three to four months after the termination of the treatments. Hippocampal cerebral blood flow in corticosterone-treated rats was significantly lower (P<0.05) than that in control rats, and fluctuated over a day in lower amplitude than the controls. Severe histological damage was observed in the CA1 and CA3 cell fields of the hippocampus in corticosterone-treated rats. These neuropathological changes were characterized by soma shrinkage and condensation, or nuclear pyknosis, as reported previously. We concluded that a long-term glucocorticoid exposure resulted in an impairment of the hippocampal functions, accompanied by neuronal damage similar to that found in aged hippocampus. The present results support the hypothesis that glucocorticoids accelerate age-related changes in the brain.

Controversies surrounding glucocorticoid-mediated cell death in the hippocampus

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

Serotonin transporter expression in rat brain regions and blood platelets: aging and glucocorticoid effects

Hyperactivity of the hypothalamus-pituitary-adrenal axis is more common in elderly depression than in younger cohorts and glucocorticoids are known to influence serotonergic systems. The current study explores the interaction of glucocorticoids with aging on serotonin transporter expression and function. Continuous infusions of dexamethasone (26 days) reduced transporter expression in the aged brain but the ability of imipramine to inhibit synaptosomal [3H]serotonin uptake was unimpaired. These effects were unique to aged animals, as prior work with young adults found no effects of dexamethasone on transporter expression. In contrast to the effects in the brain, there were no differences in platelet transporter expression between young and old rats nor did dexamethasone treatment affect the values in the aged group: thus, the platelet may not reliably model these aspects of CNS function. The results suggest that there are basic biologic differences in the effects of glucocorticoids in aged vs. young brain that could contribute to lowered effectiveness to antidepressants in elderly depression; if transport capacity is already reduced by the effects of increased glucocorticoids, further inhibition of transport by antidepressants would have proportionally less impact on synaptic serotonin concentrations.

Nerve growth factor, central nervous system apoptosis, and adrenocortical activity in aged Fischer-344/brown Norway F1 hybrid rats

During aging there is a progressive loss of neuronal function in the basal forebrain that results in cognitive impairment and cholinergic deficits. While altered neurotrophin (NT)-mediated signal transduction may account for some age-associated deficits, there are differences in the extent of NT responsiveness among different laboratory rat strains. Here we measured nerve growth factor (NGF) protein levels and fragmented DNA in the CNS, and basal and NGF-stimulated activity levels of the hypothalamus-pituitary-adrenocortical axis (HPAA) in 3-, 18-, and 30-month-old Fischer-344/Brown Norway rats. Our results show that while there is no age-associated differences in NGF protein levels, in aged Fischer-344/Brown Norway rats, there are increases in levels of immunoreactive fragmented DNA in the CNS and in adrenocortical responses to the peripheral administration of NGF. These data contribute to the characterization of the Fischer-344/Brown Norway F1 hybrid rat and provide baseline values useful for future studies on aged CNS.

Do glucocorticoids contribute to the abnormalities in serotonin transporter expression and function seen in depression? An animal model

Adrenocorticosteroids and serotonergic neurons exert reciprocal regulatory actions, and both are abnormal in depression. We evaluated whether glucocorticoids influence the serotonin transporter in rat platelets and brain by infusing dexamethasone for 26 days, sufficient for replacement of the entire platelet population. Effectiveness was verified by measurement of plasma dexamethasone levels, adrenal atrophy, and growth inhibition. At the end of the infusion, we examined [3H]paroxetine binding to platelet, hippocampal, and cerebrocortical membranes, and [3H]serotonin uptake into platelets and synaptosomes. Dexamethasone slightly reduced platelet [3H]paroxetine binding (12%) and had no effect on binding in brain. Platelet [3H]serotonin uptake was unaffected, but synaptosomal uptake was significantly reduced. In neither platelets nor synaptosomes did dexamethasone alter imipramine's ability to inhibit uptake. Thus, elevated glucocorticoids are not responsible for reduced platelet serotonin transporter expression in depression, nor for resistance to imipramine's effect in platelets in elderly depression; however, reduced synaptosomal [3H]serotonin uptake indicates that glucocorticoids can affect transport efficiency, even when the number of transporter molecules is unaltered.