Morphological and functional properties of rat dentate granule cells after adrenalectomy

Ion-channel degeneracy and heterogeneities in the emergence of signature physiological characteristics of dentate gyrus granule cells

Complex systems are neither fully determined nor completely random. Biological complex systems, including single neurons, manifest intermediate regimes of randomness that recruit integration of specific combinations of functionally specialized subsystems. Such emergence of biological function provides the substrate for the expression of degeneracy, the ability of disparate combinations of subsystems to yield similar function. Here, we present evidence for the expression of degeneracy in morphologically realistic models of dentate gyrus granule cells (GCs) through functional integration of disparate ion-channel combinations. We performed a 45-parameter randomized search spanning 16 active and passive ion channels, each biophysically constrained by their gating kinetics and localization profiles, to search for valid GC models. Valid models were those that satisfied 17 sub- and suprathreshold cellular-scale electrophysiological measurements from rat GCs. A vast majority (>99%) of the 15,000 random models were not electrophysiologically valid, demonstrating that arbitrarily random ion-channel combinations would not yield GC functions. The 141 valid models (0.94% of 15,000) manifested heterogeneities in and cross-dependencies across local and propagating electrophysiological measurements, which matched with their respective biological counterparts. Importantly, these valid models were widespread throughout the parametric space and manifested weak cross-dependencies across different parameters. These observations together showed that GC physiology could neither be obtained by entirely random ion-channel combinations nor is there an entirely determined single parametric combination that satisfied all constraints. The complexity, the heterogeneities in measurement and parametric spaces, and degeneracy associated with GC physiology should be rigorously accounted for while assessing GCs and their robustness under physiological and pathological conditions.NEW & NOTEWORTHY A recent study from our laboratory had demonstrated pronounced heterogeneities in a set of 17 electrophysiological measurements obtained from a large population of rat hippocampal granule cells. Here, we demonstrate the manifestation of ion-channel degeneracy in a heterogeneous population of morphologically realistic conductance-based granule cell models that were validated against these measurements and their cross-dependencies. Our analyses show that single neurons are complex entities whose functions emerge through intricate interactions among several functionally specialized subsystems.

The role of suboptimal mitochondrial function in vulnerability to post-traumatic stress disorder

Post-traumatic stress disorder remains the most significant psychiatric condition associated with exposure to a traumatic event, though rates of traumatic event exposure far outstrip incidence of PTSD. Mitochondrial dysfunction and suboptimal mitochondrial function have been increasingly implicated in several psychopathologies, and recent genetic studies have similarly suggested a pathogenic role of mitochondria in PTSD. Mitochondria play a central role in several physiologic processes underlying PTSD symptomatology, including abnormal fear learning, brain network activation, synaptic plasticity, steroidogenesis, and inflammation. Here we outline several potential mechanisms by which inherited (genetic) or acquired (environmental) mitochondrial dysfunction or suboptimal mitochondrial function, may contribute to PTSD symptomatology and increase susceptibility to PTSD. The proposed pathogenic role of mitochondria in the pathophysiology of PTSD has important implications for prevention and therapy, as antidepressants commonly prescribed for patients with PTSD have been shown to inhibit mitochondrial function, while alternative therapies shown to improve mitochondrial function may prove more efficacious.

Memory performance is related to the cortisol awakening response in older people, but not to the diurnal cortisol slope

There are large individual differences in age-related cognitive decline. Hypothalamic-pituitary-adrenal axis (HPA-axis) functioning has been suggested as one of the mechanisms underlying these differences. This study aimed to investigate the relationships between the diurnal cortisol cycle, measured as the cortisol awakening response (CAR), and the diurnal cortisol slope (DCS) and the memory performance of healthy older people. To do so, we assessed the verbal, visual, and working memory performance of 64 participants (32 men) from 57 to 76 years old who also provided 14 saliva samples on two consecutive weekdays to determine their diurnal cortisol cycle. The CAR was linearly and negatively associated with verbal (significantly) and visual (marginally) memory domains, but not with working memory. Sex did not moderate these relationships. Furthermore, no associations were found between the DCS and any of the three memory domains assessed. Our results indicate that the two components of the diurnal cortisol cycle have different relationships with memory performance, with the CAR being more relevant than DCS in understanding the link from HPA-axis activity and regulation to different types of memory. These results suggest that the CAR is related to memory domains dependent on hippocampal functioning (i.e., declarative memory), but not to those that are more dependent on prefrontal cortex functioning (i.e., working memory).

Hair cortisol and cognitive performance in healthy older people

Worse cognitive performance in older people has been associated with hypothalamic-pituitary-adrenal axis dysregulation (in particular, higher cortisol levels). Analysis of hair cortisol concentrations (HCC) is a novel method to measure long-term cortisol exposure, and its relationship with cognition in healthy older people has not yet been studied. We investigated whether HCC (measured in hair scalp) and diurnal salivary cortisol levels (awakening, 30min after awakening, and evening, across two days) were related to cognitive performance (assessed with the Trail-making Test A and B, Digit Span Forward and Backward, word list-RAVLT and Stories subtest of the Rivermead) in 57 healthy older people (mean age=64.75 years, SD=4.17). Results showed that lower HCC were consistently related to worse working memory, learning, short-term verbal memory (RAVLT first trial and immediate recall) and long-term verbal memory. In contrast, higher mean levels and higher diurnal area under the curve of diurnal salivary cortisol were related to worse attention and short-term verbal memory (immediate story recall), respectively. Interestingly, a higher ratio of mean levels of diurnal salivary cortisol over HCC were related to worse performance on working memory and short-term verbal memory, suggesting that those individuals with lower long-term cortisol exposure might be more vulnerable to the negative effect of HPA-axis dysregulation on these cognitive processes. Our findings suggest that both low long-term cortisol exposure and a possible dysregulation of the diurnal rhythm of the HPA-axis may account, at least in part, for the inter-individual variability in cognitive performance in healthy older people.

Knockdown of the glucocorticoid receptor alters functional integration of newborn neurons in the adult hippocampus and impairs fear-motivated behavior

Glucocorticoids (GCs) secreted after stress reduce adult hippocampal neurogenesis, a process that has been implicated in cognitive aspects of psychopathology, amongst others. Yet, the exact role of the GC receptor (GR), a key mediator of GC action, in regulating adult neurogenesis is largely unknown. Here, we show that GR knockdown, selectively in newborn cells of the hippocampal neurogenic niche, accelerates their neuronal differentiation and migration. Strikingly, GR knockdown induced ectopic positioning of a subset of the new granule cells, altered their dendritic complexity and increased their number of mature dendritic spines and mossy fiber boutons. Consistent with the increase in synaptic contacts, cells with GR knockdown exhibit increased basal excitability parallel to impaired contextual freezing during fear conditioning. Together, our data demonstrate a key role for the GR in newborn hippocampal cells in mediating their synaptic connectivity and structural as well as functional integration into mature hippocampal circuits involved in fear memory consolidation.

The interplay between rapid and slow corticosteroid actions in brain

Stress causes the release of many transmitters and hormones, including corticosteroids. These molecules enter the brain and exert their effects through the mineralo- and glucocorticoid receptor. The former receptor plays an important role in neuronal stability. However, it also mediates rapid non-genomic corticosteroid effects that in synergy with other stress mediators activate limbic cells and promote behavioral choices allowing the organism to quickly respond to the imminent danger. Glucocorticoid receptors primarily mediate slow genomic effects, for instance in the hippocampus and prefrontal cortex, which are thought to contribute to contextual and higher cognitive aspects of behavioral performance several hours after stress. Rapid and slow effects interact and collectively contribute to successful behavioral adaptation. Long-term disturbances in the release pattern of corticosteroid hormones and in the responsiveness of their receptors give rise to structural and functional changes in neuronal properties which may contribute to the expression of psychopathology.

High dose hydrocortisone immediately after trauma may alter the trajectory of PTSD: interplay between clinical and animal studies

High-dose corticosteroids have been reported to reduce symptoms of acute stress and post-traumatic stress in polytrauma patients and in animal studies. The underlying mechanism of action remains largely unclear. These issues were addressed in parallel in the clinical and preclinical studies below. In this preliminary study, 25 patients with acute stress symptoms were administered a single intravenous bolus of high-dose hydrocortisone (100-140 mg) or placebo within 6 h of a traumatic event in a prospective, randomized, double-blind, placebo-controlled pilot study. Early single high-dose hydrocortisone intervention attenuated the core symptoms of both the acute stress and of subsequent PTSD in patients. High-dose hydrocortisone treatment given in the first few hours after a traumatic experience was associated with significant favorable changes in the trajectory of exposure to trauma, as expressed by the reduced risk of the development of PTSD post-trauma. In parallel, a comparative study of morphological arborization in dentate gyrus and its modulating molecules was performed in stress-exposed animals treated with high-dose hydrocortisone. Steroid-treated stressed animals displayed significantly increased dendritic growth and spine density, with increased levels of brain-derived neurotrophic factor (BDNF) and obtunded postsynaptic density-95 (PSD-95) levels. The animal study provided insights into the potential mechanism of this intervention, as it identified relevant morphological and biochemical associations to the clinical observations. Thus, evidence from clinical and animal studies suggests that there is a "window of opportunity" in the early aftermath of trauma to help those who are vulnerable to the development of chronic PTSD.

A novel animal model of hippocampal cognitive deficits, slow neurodegeneration, and neuroregeneration

Long-term adrenalectomy (ADX) results in an extensive and specific loss of dentate gyrus granule cells in the hippocampus of adult rats. This loss of granule cells extends over a period of weeks to months and ultimately results in cognitive deficits revealed in a number of tasks that depend on intact hippocampal function. The gradual nature of ADX-induced cell death and the ensuing deficits in cognition resemble in some important respects a variety of pathological conditions in humans. Here, we characterize behavioural and cellular processes, including adult neurogenesis, in the rat ADX model. We also provide experimental evidence for a neurogenic treatment strategy by which the lost hippocampal cells may be replaced, with the goal of functional recovery in mind.

Severe early life stress hampers spatial learning and neurogenesis, but improves hippocampal synaptic plasticity and emotional learning under high-stress conditions in adulthood

Early life stress increases the risk for developing stress-related pathologies later in life. Recent studies in rats suggest that mild early life stress, rather than being overall unfavorable, may program the hippocampus such that it is optimally adapted to a stressful context later in life. Here, we tested whether this principle of "adaptive programming" also holds under severely adverse early life conditions, i.e., 24 h of maternal deprivation (MD), a model for maternal neglect. In young adult male rats subjected to MD on postnatal day 3, we observed reduced levels of adult hippocampal neurogenesis as measured by cell proliferation, cell survival, and neuronal differentiation. Also, mature dentate granule cells showed a change in their dendritic morphology that was most noticeable in the proximal part of the dendritic tree. Lasting structural changes due to MD were paralleled by impaired water maze acquisition but did not affect long-term potentiation in the dentate gyrus. Importantly, in the presence of high levels of the stress hormone corticosterone, even long-term potentiation in the dentate gyrus of MD animals was facilitated. In addition to this, contextual learning in a high-stress environment was enhanced in MD rats. These morphological, electrophysiological, and behavioral observations show that even a severely adverse early life environment does not evolve into overall impaired hippocampal functionality later in life. Rather, adversity early in life can prepare the organism to perform optimally under conditions associated with high corticosteroid levels in adulthood.

Corticosterone reduces dendritic complexity in developing hippocampal CA1 neurons

Although prolonged stress and corticosteroid exposure induce morphological changes in the hippocampal CA3 area, the adult CA1 area is quite resistant to such changes. Here we addressed the question whether elevated corticosteroid hormone levels change dendritic complexity in young, developing CA1 cells. In organotypic cultures (prepared from P5 rats) that were 14-21 days cultured in vitro, two doses of corticosterone (30 and 100 nM) were tested. Dendritic morphology of CA1 neurons was established by imaging neurons filled with the fluorescent dye Alexa. Application of 100 nM corticosterone for 20 minutes induced atrophy of the apical dendritic tree 1-4 hours later. Fractal analysis showed that total neuronal complexity was reduced twofold when compared with vehicle-treated neurons. Exposing organotypic slices to 30 nM corticosterone reduced apical length in a more delayed manner: only neurons examined more than 2 hours after exposure to corticosterone showed atrophy of the apical dendritic tree. Neither dose of corticosterone affected the length of basal dendrites or spine density. Corticosterone was ineffective in changing morphology of the apical dendrites when tested in the presence of the glucocorticoid receptor antagonist RU38486. These results suggest that high physiological levels of corticosterone, via activation of the glucocorticoid receptor, can, during the course of only a few hours, reduce the dendritic complexity of CA1 pyramidal neurons in young, developing hippocampal tissue. These findings suggest that it is relevant to maintain plasma corticosterone levels low during hippocampal development.

Functional actions of corticosteroids in the hippocampus

Corticosteroid hormones are released in high amounts after stress. The hormones enter the brain compartment and bind to high affinity mineralocorticoid receptors--particularly enriched in limbic regions--as well as to lower affinity glucocorticoid receptors which are more ubiquitous. Shortly after the stressful event, corticosteroids (in concert with specific monoamines and neuropeptides) have the potential to increase cellular excitability in subfields of the hippocampus, like the CA1 area. These effects are rapid in onset and occur via a nongenomic pathway. At the same time, however, the hormones also start slower, gene-mediated processes. These cause attenuation of excitatory information flow through the CA1 hippocampal area. Induction of long-term potentiation at that time is impaired. This may help to normalize hippocampal activity some hours after the stressful event and preserve information encoded within the context of the event. These adaptational effects of the hormones may become maladaptive if the stressful event is associated with other challenges of the network (like ischemic insults) or when stress occurs repetitively, in an uncontrollable and unpredictable manner. In that case, i) normalization of activity seems to be less efficient (particularly when other limbic areas like the amygdala nuclei are activated during stress), ii) induction of long-term potentiation is hampered at all times and iii) serotonin responses are attenuated. This may contribute to the precipitation of clinical symptoms in stress-related disorders such as major depression. A better understanding of the corticosteroid actions could lead to a more rational treatment strategy of stress-related disorders.

Chronic stress: implications for neuronal morphology, function and neurogenesis

In normal life, organisms are repeatedly exposed to brief periods of stress, most of which can be controlled and adequately dealt with. The presently available data indicate that such brief periods of stress have little influence on the shape of neurons or adult neurogenesis, yet change the physiological function of cells in two time-domains. Shortly after stress excitability in limbic areas is rapidly enhanced, but also in brainstem neurons which produce catecholamines; collectively, during this phase the stress hormones promote focused attention, alertness, vigilance and the initial steps in encoding of information linked to the event. Later on, when the hormone concentrations are back to their pre-stress level, gene-mediated actions by corticosteroids reverse and normalize the enhanced excitability, an adaptive response meant to curtail defense reactions against stressors and to enable further storage of relevant information. When stress is experienced repetitively in an uncontrollable and unpredictable manner, a cascade of processes in brain is started which eventually leads to profound, region-specific alterations in dendrite and spine morphology, to suppression of adult neurogenesis and to inappropriate functional responses to a brief stress exposure including a sensitized activation phase and inadequate normalization of brain activity. Although various compounds can effectively prevent these cellular changes by chronic stress, the exact mechanism by which the effects are accomplished is poorly understood. One of the challenges for future research is to link the cellular changes seen in animal models for chronic stress to behavioral effects and to understand the risks they can impose on humans for the precipitation of stress-related disorders.

Role of corticosteroid hormones in the dentate gyrus

Dentate granule cells are enriched with receptors for the stress hormone corticosterone, i.e., the high-affinity mineralocorticoid receptor (MR), which is already extensively occupied with low levels of the hormone, and the glucocorticoid receptor (GR), which is particularly activated after stress. More than any other cell type in the brain studied so far, dentate granule cells require hormone levels to be within the physiological range. In the absence of corticosteroids, proliferation and apoptotic cell death are dramatically enhanced. Dendritic morphology and synaptic transmission are compromised. Conversely, prolonged exposure of animals to a high level of corticosterone suppresses neurogenesis and presumably makes dentate granule cells more vulnerable to delayed cell death. These corticosteroid effects on dentate cell and network function are translated into behavioral consequences, in health and disease.

Differential effects of chronic partial sleep deprivation and stress on serotonin-1A and muscarinic acetylcholine receptor sensitivity

Disrupted sleep and stress are often linked to each other, and considered as predisposing factors for psychopathologies such as depression. The depressed brain is associated with reduced serotonergic and enhanced cholinergic neurotransmission. In an earlier study, we showed that chronic sleep restriction by forced locomotion caused a gradual decrease in postsynaptic serotonin-1A receptor sensitivity, whilst chronic forced activity alone, with sufficient sleep time, did not affect receptor sensitivity. The first aim of the present study was to examine whether the sleep loss-induced change in receptor sensitivity is mediated by adrenal stress hormones. The results show that the serotonin-1A receptor desensitization is independent of adrenal hormones as it still occurs in adrenalectomized rats. The second aim of the study was to establish the effects of sleep restriction on cholinergic muscarinic receptor sensitivity. While sleep restriction affected muscarinic receptor sensitivity only slightly, forced activity significantly hypersensitized the muscarinic receptors. This hypersensitization is because of the stressful nature of the forced activity protocol as it did not occur in adrenalectomized rats. Taken together, these data confirm that sleep restriction may desensitize the serotonin-1A receptor system. This is not a generalized effect as sleep restriction did not affect the sensitivity of the muscarinic cholinergic receptor system, but the latter was hypersensitized by stress. Thus, chronic stress and sleep loss may, partly via different pathways, change the brain into a direction as it is seen in mood disorders.

Effect of chronic stress and mifepristone treatment on voltage-dependent Ca2+ currents in rat hippocampal dentate gyrus

Chronic unpredictable stress affects many properties in rat brain. In the dentate gyrus, among other things, increased mRNA expression of the Ca2+ channel alpha1C subunit has been found after 21 days of unpredictable stress in combination with acute corticosterone application (100 nM). In the present study, we examined: (i) whether these changes in expression are accompanied by altered Ca2+ currents in rat dentate granule cells recorded on day 22 and (ii) whether treatment with the glucocorticoid receptor antagonist mifepristone during the last 4 days of the stress protocol normalises the putative stress-induced effects. Three weeks of unpredictable stress did not affect Ca2+ current amplitude in dentate granule cells under basal conditions (i.e. after incubation with vehicle solution). However, the sustained Ca2+ current component (which largely depends on the alpha1C subunit) was significantly increased in amplitude after chronic stress when slices had been treated with corticosterone 1-4 h before recording. These findings suggest that dentate granule cells are exposed to an increased calcium load after exposure to an acute stressor when they have a history of chronic stress, potentially leading to increased vulnerability of the cells. The present results are in line with the molecular data on Ca2+ channel alpha1C subunit expression. A significant three-way interaction between chronic stress, corticosterone application and mifepristone treatment was found, indicating that the combined effect of stress and corticosterone depends on mifepristone cotreatment. Interestingly, current density (defined as total current divided by capacitance) did not differ between the groups. This indicates that the observed changes in Ca2+ current amplitude could be attributable to changes in cell size.

Temporal and region-dependent changes in muscarinic M4 receptors in the hippocampus and entorhinal cortex of adrenalectomized rats

Long-term adrenalectomy induces a dramatic loss of cells in the dentate gyrus and CA1-CA4 fields of the hippocampus resulting in an impairment of cognitive functions such as spatial learning, memory and exploratory behaviour. Muscarinic M1 and M4 receptor levels in the hippocampus and entorhinal cortex of adult male Wistar rats were examined 3, 14, 30, 90, and 150 days after adrenalectomy. Receptor levels in the entorhinal cortex and the hippocampus were determined by quantitative autoradiography using 125I-M1-toxin-1 and 125I-M4-toxin-1, M1 and M4 subtype selective antagonists, respectively. Moreover, the level of hippocampal M1 and M4 muscarinic receptors were evaluated 1 month after adrenalectomy by immunoblot analysis. Adrenalectomy induced apoptotic processes were examined by analysing apoptotic markers using Western blot analysis. No significant changes were observed in the level of muscarinic M1 receptors in the entorhinal cortex, the dentate gyrus and in the different CA fields of the hippocampus of adrenalectomized (ADX) rats. However, M4 receptors showed a significant decrease in the entorhinal cortex (at 3 days), dentate gyrus and CA4 (at 14 days), CA3 (at 30 days), and CA2 and CA1 (at 90 days) after adrenalectomy. Moreover, a decrease in the level of M4 receptors was detected in ADX rats 1 month after adrenalectomy as compared with sham groups using M4 specific antibody. Apoptotic markers such as PARP and p53 were significantly increased whereas Bcl-2 marker was decreased in ADX rat brain homogenates compared to controls. Our results show that M1 and M4 receptors are differentially affected by adrenalectomy and indicate that these subtypes have different functions in the hippocampus. Our data on time and region-dependent decreases in hippocampal M4 receptors indicate that the M4 receptor subtype is influenced by adrenal hormones and suggest that the M4 receptor might be linked to memory function in the hippocampus.

Dendritic stability in a model of adult-onset IGF-I deficiency

OBJECTIVE

A significant decrease in plasma levels of insulin-like growth factor-I (IGF-I) is one of the most robust hallmarks of aging and may contribute to functional changes associated with senescence. This study examined the role of IGF-I in the maintenance of adult dendritic morphology.

DESIGN

We utilized a model of the aging-related decrease in plasma IGF-I to examine whether such a decrease, in itself, leads to dendritic changes in the cerebral cortex. The dw/dw rat, originally of the Lewis strain, suffers from a spontaneous mutation in which growth hormone (GH) production is severely decreased. Since GH is responsible for the production of circulating IGF-I by the liver, these animals are deficient in plasma IGF-I. Homozygous dw/dw rats were administered porcine GH to sustain IGF-I levels during development and then GH injections were stopped as adults in order to examine the effects of adult-onset GH and IGF-I deficiency. Animals sacrificed after two or eight weeks of GH and IGF-I deficiency were compared to age-matched dw/dw animals that received GH both developmentally and throughout adulthood (GH/IGF-I replete). The dendritic arbors of pyramidal neurons in cingulate cortex were labeled by intracellular injection and reconstructed in three dimensions.

RESULTS

Comparing GH/IGF-I replete and deficient dw/dw rats, we found no differences in the apical or basal arbors of either layer two or layer five pyramidal neurons.

CONCLUSIONS

These findings indicate that a decrease in plasma levels of IGF-I is not sufficient in itself to produce dendritic changes like those seen in aging animals.

Stress and the brain: from adaptation to disease

In response to stress, the brain activates several neuropeptide-secreting systems. This eventually leads to the release of adrenal corticosteroid hormones, which subsequently feed back on the brain and bind to two types of nuclear receptor that act as transcriptional regulators. By targeting many genes, corticosteroids function in a binary fashion, and serve as a master switch in the control of neuronal and network responses that underlie behavioural adaptation. In genetically predisposed individuals, an imbalance in this binary control mechanism can introduce a bias towards stress-related brain disease after adverse experiences. New candidate susceptibility genes that serve as markers for the prediction of vulnerable phenotypes are now being identified.

Prominent decline of newborn cell proliferation, differentiation, and apoptosis in the aging dentate gyrus, in absence of an age-related hypothalamus-pituitary-adrenal axis activation

Neurogenesis and apoptosis in the hippocampal dentate gyrus (DG) occur during development and adulthood. However, little is known about how these two processes relate to each other during aging. In this study, we examined apoptosis, proliferation, migration, and survival of newborn cells in the young (2 weeks), young-adult (6 weeks), middle-aged (12 months), and old (24 months) rat DG. We also measured dentate volume and cell numbers, along with basal corticosterone and stress response parameters. We show that new cell proliferation and apoptosis slow down profoundly over this time period. Moreover, migration and differentiation into a neuronal or glial phenotype was strongly reduced from 6 weeks of age onwards; it was hardly present in middle-aged and old rats as confirmed by confocal analysis. Surprisingly, we found no correlation between cell birth and corticosterone levels or stress response parameters in any age group.

Homeostatic maintenance in excitability of tree shrew hippocampal CA3 pyramidal neurons after chronic stress

The experience of chronic stress induces a reversible regression of hippocampal CA3 apical neuron dendrites. Although such postsynaptic membrane reduction will obviously diminish the possibility of synaptic input, the consequences for the functional membrane properties of these cells are not well understood. We tested the hypothesis that chronic stress affects the input-output characteristics and excitability of CA3 pyramidal cells. Somatic whole-cell current-clamp recording with parallel intracellular biocytin labeling was performed on CA3 neurons from in vitro hippocampal slices from male tree shrews, which were collected after 28 days of psychosocial stress exposure and compared to recordings obtained from control animals. Post hoc morphometric analysis of biocytin-labeled CA3 cells revealed branch regression, by fewer dendritic crossings and length, limited to a distance of approximately 280-340 microm from the soma only. The results from whole-cell recording indicate that chronic stress surprisingly reduced the apparent membrane time constant and input resistance 20-25%, accompanied by increased amplitude of the hyperpolarization-induced voltage "sag." All active membrane properties, including depolarization-induced action potential kinetics, complex spiking patterns, and afterhyperpolarization voltages, were indistinguishable from control recordings. Although linear association analysis confirmed that differences in geometry, such as apical length or branch number, were correlated to functional variability in properties of the AP current and voltage threshold, these changes were too marginal to be reflected in the group differences. However, the individual adrenal hormone status was associated significantly with the selective changes in subthreshold excitability. Taken together, the data provide evidence that despite long-term stress induces morphological changes, upregulates cortisol release and shifts the intrinsic membrane properties, the efficacy of somatic excitability of CA3 pyramidal neurons is largely preserved.

Gene expression changes in single dentate granule neurons after adrenalectomy of rats

Removal of corticosterone by adrenalectomy induces apoptosis 3 days later, in some, but not all, rat dentate granule cells. We hypothesized that individual dentate cells trigger specific gene expression profiles that partly determine their apoptosis susceptibility. RNA was collected from physiologically characterized granule cells at 2 or 3 days after adrenalectomy or sham operation, and linearly amplified. The amplified RNA was hybridized to cDNA clones of: (1) candidate genes earlier identified after adrenalectomy in whole hippocampi with SAGE; and (2) genes encoding growth factors and their receptors. We observed that based on the entire expression profile, cells relatively resistant to apoptosis 3 days after adrenalectomy clustered together with one-third of cells 2 days after adrenalectomy. Within the group of ADX cells, a limited number of transcript ratios were found to correlate-positively or negatively-with a known risk factor for apoptosis, calcium influx. The overall analysis of physiological properties and multiple gene expression in single cells can narrow down the number of critical genes involved in apoptosis identified with large scale gene screening methods and allows a first impression of their role as being a potential risk factor or neuroprotective.

Calcium currents in rat dentate granule cells are altered after adrenalectomy

Adrenal corticosteroid hormones modulate voltage-gated calcium currents in rat CA1 hippocampal neurons. In the present whole-cell recording study we examined whether calcium currents in dentate granule cells are also under control of corticosteroids. In a first series of experiments, in which the calcium chelator BAPTA was added to the recording solution, the amplitude of calcium currents induced by a voltage step to -10 mV was found to be enhanced shortly (1 or 2 days) after adrenalectomy compared to sham operation. No enhancement was seen when adrenalectomized animals received a low dose of corticosterone in the drinking water. By contrast, 3 or 7 days after adrenalectomy calcium current amplitude was decreased. Starting 3 days after adrenalectomy, some of the granule cells underwent apoptosis. This caused a bias in the recorded cell population towards relatively apoptosis-resistant cells, suggesting that restricted calcium influx may be a key feature of cells withstanding the apoptotic route. In accordance, cells from a small percentage ( approximately 20%) of animals that resisted apoptosis after adrenalectomy also displayed small calcium currents. In a second series without BAPTA, thus focusing on the endogenous calcium-buffering capacity, we found that the time constant for the decay of the calcium current was decreased after adrenalectomy, probably due to enhanced calcium-dependent inactivation of the current. The data indicate that cellular calcium current characteristics of dentate granule cells are altered after adrenalectomy and that the alterations may in part determine the vulnerability to undergo apoptosis.