Biomedical subjects

Henk Karst

Publications and source records attributed to Henk Karst.

11 recordsLinked to original sources

PubMed2007

Brief RU 38486 treatment normalizes the effects of chronic stress on calcium currents in rat hippocampal CA1 neurons.

Chronic stress alters many properties in rat brain, like serotonin responsiveness and dendritic morphology. In the present study, we examined (i) whether unpredictable stress during 21 days affects calcium (Ca) currents of CA1 pyramidal neurons recorded on day 22; and (ii) if so, whether this change is normalized by treatment with the glucocorticoid receptor-antagonist RU 38486 during days 18-21. At 3 weeks of unpredictable stress increased the amplitude of the peak and sustained calcium current components, determined in hippocampal slices prepared from animals under rest (ie, with low corticosterone levels). The increased Ca-current amplitude was associated with an enhanced cell capacitance; current density was not significantly affected by chronic stress. In slices from stressed rats that received RU 38486, no stress-induced enhancement of calcium current amplitude was seen, while RU 38486 by itself did not alter calcium currents in handled controls. We confirmed earlier observations that brief in vitro treatment with 100 nM corticosterone, thus substantially activating the low-affinity glucocorticoid receptors, increases Ca-current amplitude recorded 1-4 h later in slices from naïve rats. However, Ca-current amplitude was not affected by corticosterone applied to slices from handled controls and currents were even decreased by corticosterone given to slices from chronically stressed rats, suggesting that corticosterone effects depend on the history of the animal. In conclusion, the data indicate that chronic stress, RU 38486 treatment as well as acute rises in corticosterone level strongly modulate calcium influx into CA1 neurons. This could have consequences for the viability of these neurons.

Animals↗

PubMed2005

Mineralocorticoid receptors are indispensable for nongenomic modulation of hippocampal glutamate transmission by corticosterone.

The adrenal hormone corticosterone transcriptionally regulates responsive genes in the rodent hippocampus through nuclear mineralocorticoid and glucocorticoid receptors. Via this genomic pathway the hormone alters properties of hippocampal cells slowly and for a prolonged period. Here we report that corticosterone also rapidly and reversibly changes hippocampal signaling. Stress levels of the hormone enhance the frequency of miniature excitatory postsynaptic potentials in CA1 pyramidal neurons and reduce paired-pulse facilitation, pointing to a hormone-dependent enhancement of glutamate-release probability. The rapid effect by corticosterone is accomplished through a nongenomic pathway involving membrane-located receptors. Unexpectedly, the rapid effect critically depends on the classical mineralocorticoid receptor, as evidenced by the effectiveness of agonists, antagonists, and brain-specific inactivation of the mineralocorticoid but not the glucocorticoid receptor gene. Rapid actions by corticosterone would allow the brain to change its function within minutes after stress-induced elevations of corticosteroid levels, in addition to responding later through gene-mediated signaling pathways.

Adrenal Cortex Hormones↗

PubMed2005

Corticosterone slowly enhances miniature excitatory postsynaptic current amplitude in mice CA1 hippocampal cells.

Corticosteroid hormones are released in high amounts after stress and bind to intracellular receptors in the brain, which in activated form function as transcription factors. We here tested the effect of a high dose of corticosterone on AMPA-receptor-mediated transmission in the CA1 hippocampal area, which is enriched in corticosteroid receptors. To focus on slow gene-mediated effects of the hormone, excitatory postsynaptic currents were measured at least 1 h after a brief application of 100 nM corticosterone to slices from adrenally intact adult mice. The amplitude but not frequency of miniature postsynaptic excitatory currents was found to be significantly enhanced. These effects were mimicked by 100 nM RU 28362, a selective agonist for intracellular glucocorticoid receptors. Evoked AMPA responses at the single cell were significantly enhanced when measured 2-4 h after application of 100 nM corticosterone, but not at earlier moments nor with a longer delay. In summary, the present results show that activation of hippocampal glucocorticoid receptors induces a slow enhancement of AMPA-receptor-mediated responses, at the single-cell level.

Adaptation, Physiological↗

PubMed2005

Corticosterone shifts different forms of synaptic potentiation in opposite directions.

Calcium entering the cell via different routes, e.g.,N-methyl-D-aspartate (NMDA) receptors or voltage-dependent calcium channels (VDCCs), plays a pivotal role in hippocampal synaptic potentiation. Since corticosteroid hormones have been reported to enhance calcium influx through VDCCs, one may predict that these hormones facilitate hippocampal synaptic efficacy. Surprisingly, though, stress and corticosteroids have so far been found to reduce synaptic potentiation. Here, we addressed this apparent paradox and examined synaptic potentiation in the CA1 area of hippocampal slices from mice with low basal corticosterone levels 1--4 h after a brief in vitro administration of corticosterone. Nifedipine and APV were used to isolate NMDA receptor-mediated and VDCC-mediated long-term potentiations (LTPs), respectively. We report that corticosterone facilitates synaptic potentiation that depends on activation of VDCCs while impairing synaptic plasticity that is mediated by NMDA receptor activation. The glucocorticoid-receptor (GR) antagonist RU 38486 blocked both the effects of corticosterone. These results indicate that the net effect of corticosteroid hormones on synaptic plasticity is determined by the balance between different types of potentiation, a balance that may be region specific and depends on the experimental conditions. We speculate that these opposite effects on synaptic efficacy are involved in the bidirectional modulation of cognitive performance by corticosteroid hormones.

Animals↗

PubMed2005

GABAergic transmission in the rat paraventricular nucleus of the hypothalamus is suppressed by corticosterone and stress.

Parvocellular neurons in the hypothalamic paraventricular nucleus receive hormonal inputs mediated by corticosterone as well as neuronal inputs, prominent among which is a GABAergic inhibitory projection. In the present study we examined the functional properties of this GABAergic innervation when corticosteroid levels fluctuate. Frequency, amplitude and kinetic properties of miniature inhibitory postsynaptic potentials (mIPSCs), mediated by gamma amino butyric acid (GABA) were studied with whole cell recording in parvocellular neurons. Injection of a high dose of corticosterone in vivo suppressed the frequency but did not change the amplitude and kinetic properties of mIPSCs recorded 1-5 h later in vitro. Similar effects were observed after restraint stress. The corticosteroid actions do not require involvement of extrahypothalamic brain regions, because in vitro administration of 100 nM corticosterone (20 min) directly to a hypothalamic slice also suppressed the frequency of mIPSCs recorded several hours later. Corticosterone administration to hypothalamic slices from restraint rats did not result in stronger reduction of mIPSC frequency than either treatment alone, pointing to a common underlying mechanism. Paired pulse response inhibition was reduced by corticosterone, suggesting that the hormone decreases the release probability of GABA-containing vesicles. Unlike neurosteroids, corticosterone induced no rapid effects on mIPSC properties. These results indicate that increases in glucocorticoid level due to stress can slowly but persistently inhibit the GABAergic tone on parvocellular hypothalamic neurons via a hitherto unknown local mechanism independent of limbic projections.

2-Amino-5-phosphonovalerate↗

PubMed2004

Effects of chronic stress on structure and cell function in rat hippocampus and hypothalamus.

It has become increasingly clear that the increase in corticosteroid levels, e.g. after a brief stressor induce molecular and cellular changes in brain, including the hippocampal formation. These effects eventually result in behavioral adaptation. Prolonged exposure to stress, though, may lead to mal-adaptation and even be a risk factor for diseases like major depression in genetically predisposed individuals. We conducted a series of experiments where changes in brain function were examined after 3 weeks of unpredictable stress. After unpredictable stress, inhibitory input to neurons involved in the hypothalamus-pituitary-adrenal (HPA) axis regulation was suppressed, which may dysregulate the axis and lead to overexposure of the brain to glucocorticoids. Furthermore, glutamate transmission in the dentate gyrus (DG) was enhanced, possibly through transcriptional regulation of receptor subunits. Combined with enhanced calcium channel expression this could increase vulnerability to cell death. Neurogenesis and apoptosis in the dentate were diminished. Synaptic plasticity was suppressed both in the dentate and CA1 area. Collectively, these effects may give rise to deficits in memory formation. Finally, we observed reduced responses to serotonin in the CA1 area, which could contribute to the onset of symptoms of depression in predisposed individuals. All of these endpoints provide potential targets for novel treatment strategies of stress-related brain disorders.

Animals↗

PubMed2004

Chronic unpredictable stress alters gene expression in rat single dentate granule cells.

The rat adrenal hormone corticosterone binds to low and high affinity receptors, discretely localized in brain, including the dentate gyrus. Differential activation of the two receptor types under physiological conditions alters gene expression and functional characteristics of hippocampal neurones. In the present study we examined gene expression of electrophysiologically identified dentate granule cells, 1 day after 21 days of unpredictable stress or control treatment, both under basal corticosteroid conditions and after brief in vitro exposure to a high dose of corticosterone. From each cell, RNA was collected, linearly amplified and hybridized with cDNA clones for three calcium channel subunits, four NMDA, two AMPA and 11 GABA receptor subunits. We found that gene expression was not extensively changed after chronic stress when neurones were studied under basal corticosteroid conditions. However, combined with acute exposure to a high corticosterone dose, relative expression of eight transcripts was significantly altered, including a consistent up-regulation of GABAa receptor alpha 1 and down-regulation of calcium channel alpha 1A and alpha 1G subunit expression. If translated to the protein level, this may result in more transient GABAa receptor mediated responses and less transient calcium influx. This could contribute to enhanced excitability and vulnerability of granule cells after chronic stress.

Animals↗

PubMed2003

Gene expression patterns in rat dentate granule cells: comparison between fresh and fixed tissue.

RNA from brain tissue (in particular human brain) can often only be extracted from fixed material. As brain tissue is very heterogeneous with regard to cell type, it is important to obtain RNA from small samples of identified cells. The aim of this study was (a) to generate expression profiles from small yet homogeneous samples of fixed brain cells in rat and (b) to verify the reliability of these profiles by comparing them with expression profiles obtained from single fresh neurons of the same cell type. Samples (n=12) of 50 rat dentate granule cells were isolated, using Laser Microdissection and Pressure Catapulting, from paraformaldehyde fixed, paraffin embedded tissue or from frozen, ethanol fixed tissue. In addition, RNA was extracted under visual control from individual dentate granule cells (n=12) in hippocampal slices, after electrophysiological recording with patch clamp electrodes. Our data show that RNA was successfully extracted from ethanol fixed sections yielding expression profiles highly comparable to those from non-fixed, single granule cells. RNA extraction from paraformaldehyde fixed, paraffin embedded tissue was less reliable. The present approach validates expression profiling from small amounts of fixed neurons as a powerful tool to investigate molecular processes if fresh tissue is not available.

Animals↗

PubMed2003

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.

Adrenalectomy↗

PubMed2003

Effect of chronic stress on synaptic currents in rat hippocampal dentate gyrus neurons.

We investigated the effect of chronic stress on synaptic responses of rat dentate granule cells to perforant path stimulation. Rats were subjected for 3 wk to unpredictable stressors twice daily or to control handling. One day after the last stressor, hippocampal slices were prepared and synaptic responses were determined with whole-cell recording. At that time, adrenal weight was found to be increased and thymus weight as well as gain in body weight were decreased in the stressed versus control animals, indicative of corticosterone hypersecretion during the stress period. In slices from rats with basal corticosteroid levels (at the circadian trough, under rest), no effect of prior stress exposure was observed on synaptic responses. However, synaptic responses of dentate granule cells from chronically stressed and control rats were differently affected by in vitro activation of glucocorticoid receptors, i.e., 1-4 h after administration of 100 nM corticosterone for 20 min. Thus the maximal response to synaptic activation of dentate cells at holding potential of -70 mV [when N-methyl-D-aspartate (NMDA) receptors are blocked by magnesium] was significantly enhanced after corticosterone administration in chronically stressed but not in control animals. In accordance, the amplitude of alpha-amino-3-hydroxy-5-methylisolazole-4-propionic acid (AMPA) but not of NMDA receptor-mediated currents was increased by corticosterone in stressed rats, over the entire voltage range. Corticosterone treatment also decreased the time to peak of AMPA currents, but this effect did not depend on prior stress exposure. The data indicate that following chronic stress exposure synaptic excitation of dentate granule cells may be enhanced when corticosterone levels rise. This enhanced synaptic flow could contribute to enhanced excitation of projection areas of the dentate gyrus, most notably the CA3 hippocampal region.

Adrenal Glands↗

PubMed2002

Glucocorticoids alter calcium conductances and calcium channel subunit expression in basolateral amygdala neurons.

Glucocorticoid hormones, which are released in high amounts after stress, enter the brain where they bind to intracellular receptors that are abundant in limbic areas, in particular the hippocampus and amygdala nuclei. Behavioural studies indicate that glucocorticoids modulate learning and memory processes via receptors in the hippocampus and amygdala. So far, the effects of glucocorticoids on amygdala neurons have not been investigated at the cellular and molecular level. We report here that in vitro application of glucocorticoids for 20 min increases 1-4 h later the amplitude of sustained, high-voltage-activated calcium currents in principal neurons of the basolateral amygdala. In contrast, the transient, low-voltage-activated currents were decreased. We examined whether these functional changes in calcium conductance were accompanied by transcriptional regulation of calcium channel subunits. Analysis of the RNA - collected after recording and then linearly amplified - revealed that glucocorticoid-mediated increases in sustained calcium currents are associated with a parallel shift in the relative expression of the alpha1 subunit constituting the pore of the sustained, high-voltage-activated (L-type) calcium channel. These data indicate that glucocorticoids, probably by selectively targeting genes encoding calcium channel subunits, largely alter the calcium influx into basolateral amygdala neurons. These actions could modify amygdala network function and thus contribute to the behavioural effects exerted by the stress hormones via the basolateral amygdala.

Amygdala↗