Glucocorticoid and mineralocorticoid receptors in rat neocortical and hippocampal brain cells in culture: characterization and regulatory studies

Glucocorticoid-Dependent Mechanisms of Brain Tolerance to Hypoxia

Adaptation of organisms to stressors is coordinated by the hypothalamic-pituitary-adrenal axis (HPA), which involves glucocorticoids (GCs) and glucocorticoid receptors (GRs). Although the effects of GCs are well characterized, their impact on brain adaptation to hypoxia/ischemia is still understudied. The brain is not only the most susceptible to hypoxic injury, but also vulnerable to GC-induced damage, which makes studying the mechanisms of brain hypoxic tolerance and resistance to stress-related elevation of GCs of great importance. Cross-talk between the molecular mechanisms activated in neuronal cells by hypoxia and GCs provides a platform for developing the most effective and safe means for prevention and treatment of hypoxia-induced brain damage, including hypoxic pre- and post-conditioning. Taking into account that hypoxia- and GC-induced reprogramming significantly affects the development of organisms during embryogenesis, studies of the effects of prenatal and neonatal hypoxia on health in later life are of particular interest. This mini review discusses the accumulated data on the dynamics of the HPA activation in injurious and non-injurious hypoxia, the role of the brain GRs in these processes, interaction of GCs and hypoxia-inducible factor HIF-1, as well as cross-talk between GC and hypoxic signaling. It also identifies underdeveloped areas and suggests directions for further prospective studies.

Androgen and glucocorticoid receptor direct distinct transcriptional programs by receptor-specific and shared DNA binding sites

The glucocorticoid (GR) and androgen (AR) receptors execute unique functions in vivo, yet have nearly identical DNA binding specificities. To identify mechanisms that facilitate functional diversification among these transcription factor paralogs, we studied them in an equivalent cellular context. Analysis of chromatin and sequence suggest that divergent binding, and corresponding gene regulation, are driven by different abilities of AR and GR to interact with relatively inaccessible chromatin. Divergent genomic binding patterns can also be the result of subtle differences in DNA binding preference between AR and GR. Furthermore, the sequence composition of large regions (>10 kb) surrounding selectively occupied binding sites differs significantly, indicating a role for the sequence environment in guiding AR and GR to distinct binding sites. The comparison of binding sites that are shared shows that the specificity paradox can also be resolved by differences in the events that occur downstream of receptor binding. Specifically, shared binding sites display receptor-specific enhancer activity, cofactor recruitment and changes in histone modifications. Genomic deletion of shared binding sites demonstrates their contribution to directing receptor-specific gene regulation. Together, these data suggest that differences in genomic occupancy as well as divergence in the events that occur downstream of receptor binding direct functional diversification among transcription factor paralogs.

Basal glucocorticoid receptor activation induces proliferation and inhibits neuronal differentiation of human induced pluripotent stem cell-derived neuronal precursor cells

Glucocorticoids (GC) have first been shown to originate from the adrenal glands where synthesis and release is controlled by the hypothalamic-pituitary-adrenal (HPA) axis. Recently, it was shown that GC and other steroid hormones are also synthesized in the central nervous system, so-called neurosteroids. GC bind to specific GC receptors (GR) which function as ligand-activated transcription factors. GR are expressed in nearly all cell types in the brain, and therefore GC have a strong impact on neuronal development. Most knowledge of the influence of GC on neurodevelopment has been obtained from animal research. Recent advances in stem cell technology made it possible to generate neuronal precursor cells (NPCs) and neurons from human induced pluripotent stem cells (hiPSCs). To explore the cellular mechanism of GC affecting human neuronal development, we quantified the proliferation and differentiation of hiPSCs-derived NPCs in the absence and presence of the selective high-affinity GR agonist dexamethasone and the selective GR antagonist mifepristone, respectively. Our results show that inhibition of GR significantly reduced proliferation of NPCs and promoted differentiation whereas GR activation suppressed neuronal differentiation. This implies that neuronal GC must be present in NPCs for proliferation. Consequently we identified the presence of 11-β-hydroxylase CYP11B1, which hydroxylates the respective steroid precursors to bioactive GC, in NPCs. We propose that hiPSC technology offers an ideal system to get more insight into the synthesising and regulatory pathways in steroidogenesis in human neurons and to differentiate between the mechanism by which adrenal GC and neuronal GC impact on neurodevelopment.

Stress and the HPA Axis: Balancing Homeostasis and Fertility

An organism’s reproductive fitness is sensitive to the environment, integrating cues of resource availability, ecological factors, and hazards within its habitat. Events that challenge the environment of an organism activate the central stress response system, which is primarily mediated by the hypothalamic–pituitary–adrenal (HPA) axis. The regulatory functions of the HPA axis govern the cardiovascular and metabolic system, immune functions, behavior, and reproduction. Activation of the HPA axis by various stressors primarily inhibits reproductive function and is able to alter fetal development, imparting a biological record of stress experienced in utero. Clinical studies and experimental data indicate that stress signaling can mediate these effects through direct actions in the brain, gonads, and embryonic tissues. This review focuses on the mechanisms by which stress activation of the HPA axis impacts fertility and fetal development.

Glucocorticoids, epigenetic control and stress resilience

Glucocorticoid hormones play a pivotal role in the response to stressful challenges. The surge in glucocorticoid hormone secretion after stress needs to be tightly controlled with characteristics like peak height, curvature and duration depending on the nature and severity of the challenge. This is important as chronic hyper- or hypo-responses are detrimental to health due to increasing the risk for developing a stress-related mental disorder. Proper glucocorticoid responses to stress are critical for adaptation. Therefore, the tight control of baseline and stress-evoked glucocorticoid secretion are important constituents of an organism's resilience. Here, we address a number of mechanisms that illustrate the multitude and complexity of measures safeguarding the control of glucocorticoid function. These mechanisms include the control of mineralocorticoid (MR) and glucocorticoid receptor (GR) occupancy and concentration, the dynamic control of free glucocorticoid hormone availability by corticosteroid-binding globulin (CBG), and the control exerted by glucocorticoids at the signaling, epigenetic and genomic level on gene transcriptional responses to stress. We review the beneficial effects of regular exercise on HPA axis and sleep physiology, and cognitive and anxiety-related behavior. Furthermore, we describe that, possibly through changes in the GABAergic system, exercise reduces the impact of stress on a signaling pathway specifically in the dentate gyrus that is strongly implicated in the behavioral response to that stressor. These observations underline the impact of life style on stress resilience. Finally, we address how single nucleotide polymorphisms (SNPs) affecting glucocorticoid action can compromise stress resilience, which becomes most apparent under conditions of childhood abuse.

Identifying the cellular targets of drug action in the central nervous system following corticosteroid therapy

Corticosteroid (CS) therapy is used widely in the treatment of a range of pathologies, but can delay production of myelin, the insulating sheath around central nervous system nerve fibers. The cellular targets of CS action are not fully understood, that is, "direct" action on cells involved in myelin genesis [oligodendrocytes and their progenitors the oligodendrocyte precursor cells (OPCs)] versus "indirect" action on other neural cells. We evaluated the effects of the widely used CS dexamethasone (DEX) on purified OPCs and oligodendrocytes, employing complementary histological and transcriptional analyses. Histological assessments showed no DEX effects on OPC proliferation or oligodendrocyte genesis/maturation (key processes underpinning myelin genesis). Immunostaining and RT-PCR analyses show that both cell types express glucocorticoid receptor (GR; the target for DEX action), ruling out receptor expression as a causal factor in the lack of DEX-responsiveness. GRs function as ligand-activated transcription factors, so we simultaneously analyzed DEX-induced transcriptional responses using microarray analyses; these substantiated the histological findings, with limited gene expression changes in DEX-treated OPCs and oligodendrocytes. With identical treatment, microglial cells showed profound and global changes post-DEX addition; an unexpected finding was the identification of the transcription factor Olig1, a master regulator of myelination, as a DEX responsive gene in microglia. Our data indicate that CS-induced myelination delays are unlikely to be due to direct drug action on OPCs or oligodendrocytes, and may occur secondary to alterations in other neural cells, such as the immune component. To the best of our knowledge, this is the first comparative molecular and cellular analysis of CS effects in glial cells, to investigate the targets of this major class of anti-inflammatory drugs as a basis for myelination deficits.

The hypersensitive glucocorticoid response specifically regulates period 1 and expression of circadian genes

Glucocorticoids regulate gene expression by binding and activating the glucocorticoid receptor (GR). While ligand affinity determines the global sensitivity of the response, additional proteins act on the genome to tune sensitivity of some genes. However, the genomic extent and specificity of dose-specific glucocorticoid responses are unknown. We show that dose-specific glucocorticoid responses are extraordinarily specific at the genomic scale, able to distinctly express a single gene, the circadian rhythm gene for Period 1 (PER1), at concentrations consistent with the nighttime nadir of human cortisol. We mapped the PER1 response to a single GR binding site. The specific GR binding sequence did not impact sensitivity, and we instead attributed the response to a combination of additional transcription factors and chromatin accessibility acting in the same locus. The PER1 hypersensitive response element is conserved in the mouse, where we found similar upregulation of Per1 in pituitary cells. Targeted and transient overexpression of PER1 led to regulation of additional circadian rhythm genes hours later, suggesting that hypersensitive expression of PER1 impacts circadian gene expression. These findings show that hypersensitive GR binding occurs throughout the genome, drives targeted gene expression, and may be important to endocrine mediation of peripheral circadian rhythms.

Glucocorticoids and serotonin alter glucocorticoid receptor mRNA levels in fetal guinea-pig hippocampal neurons, in vitro

The hypothalamic-pituitary-adrenal (HPA) axis is susceptible to programming during fetal life. Such programming occurs, at least partially, at the level of the hippocampus. The hippocampus plays a central role in regulation of the HPA axis and release of endogenous glucocorticoids, via mediation of glucocorticoid negative feedback. Fetal exposure to synthetic glucocorticoids can permanently alter glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) levels within the hippocampus, and serotonin is thought to be involved in this process. In the present study, we hypothesised that dexamethasone, cortisol and serotonin exposure would modify GR mRNA expression within fetal guinea-pig hippocampal cultures. Cultures were derived from 40-day-old guinea-pig fetuses, and were exposed to 0, 1, 10 and 100 nM dexamethasone, cortisol or serotonin for 4 days. Expression of GR and MR mRNA was examined by in situ hybridisation followed by high-resolution silver emulsion autoradiography. Four-day exposure to dexamethasone (P < 0.05; 100 nM) or cortisol (P = 0.08; 100 nM) downregulated the expression of GR mRNA within neurons. There was no change in the expression of MR mRNA levels following cortisol treatment. Exposure to serotonin (100 nM) significantly increased GR mRNA levels in hippocampal neurons. We conclude that synthetic and endogenous glucocorticoids, as well as serotonin, can influence GR expression during hippocampal development and in this way may act to permanently programme HPA function.

Alterations in stress reactivity after long-term treatment with paroxetine in women with posttraumatic stress disorder

Posttraumatic stress disorder (PTSD) is typically accompanied by both acute and chronic alterations in the stress response. These alterations have mostly been described in individuals under baseline conditions, but studies have also used a challenge model to assess the role of the hypothalamic-pituitary-adrenal (HPA) axis in the stress response. The purpose of this article was to assess the effect of long-term treatment with the selective reuptake inhibitor (SSRI), paroxetine, on stress reactivity in patients with PTSD. We assessed diurnal salivary cortisol and urinary cortisol as well as cortisol, heart rate, and behavioral responses to a standardized cognitive stress challenge, in 13 female patients with chronic PTSD before and after 12 months of paroxetine treatment. Treatment resulted in a significant decrease in PTSD symptoms. Twenty-four-hour urinary cortisol was lower compared to base line after successful treatment. Treatment resulted in a decrease of salivary cortisol levels on all time points on a diurnal curve. Despite similar stress perception, cortisol response to the cognitive stress challenge resulted in a 26.5% relative decrease in stress-induced salivary cortisol with treatment. These results suggest that successful treatment with SSRI in chronic PTSD is associated with a trend for a decrease in baseline diurnal cortisol and with reduced cortisol reactivity to stress.

Effects of oxidative challenge and calcium on ATP-levels in neuronal cells

BACKGROUND

Neurocellular overload with hydrogen peroxide (H2O2) induces oxidative stress and may initiate a cascade of intracellular toxic events leading to energy failure, increased lipid peroxidation and subsequently cell death. Studies suggest that hippocampal neurons may be more vulnerable to oxidative stress than cortical cells pointing to a differential vulnerability of neuronal cells. Since disturbed ATP- and calcium (Ca2+)-metabolism may be involved in this process, we here evaluated the effects of H2O2-induced oxidative stress and the involvement of Ca2+-regulation on neuronal energy metabolism.

METHODS

Using primary cortical and hippocampal neurons as well as immortalized hippocampal HT22 cells, we determined ATP-levels and accompanying cell death after oxidative challenge with H2O2. Additionally, the combined effects of H2O2 and alterations in Ca2+-concentrations were pharmacologically characterized in more detail.

RESULTS

H2O2-incubation decreased ATP-levels in a dose- and time-dependent manner in all neuronal cell systems tested. Such effects were most pronounced in primary hippocampal neurons. In cortical cells, increased ATP-levels were notable under low H2O2-concentrations. A dose-dependent decrease in ATP-concentrations was observed after treatment with Ca2+, which was further enhanced by additional H2O2-challenge.

CONCLUSIONS

Our data underline that both, H2O2- and Ca2+-treatment, are able to disturb intracellular energy metabolism. Out of the different systems studied, the ATP-decrease is most pronounced in hippocampal primary neurons, suggesting that this mechanism contributes to the selective neuronal vulnerability to oxidative stress in these neurons.

Mineralocorticoid and glucocorticoid receptor expressions in astrocytes and microglia in the gerbil hippocampal CA1 region after ischemic insult

In the present study, we observed expression and changes of mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) in the gerbil hippocampal CA1 region, but not in the CA2/3 region, after 5 min of transient forebrain ischemia. In blood, corticosterone levels were increased biphasically at 30 min and 12 h after ischemia/reperfusion, and thereafter its levels were decreased. In the sham-operated group, MR and GR immunoreactivities were weakly detected in the CA1 region. By 3 days after ischemia, MR and GR were not significantly altered in the CA1 region: at 12 h after ischemia, GR was expressed in a few neurons in the CA1 region, whereas MR was not expressed in any neurons after ischemic insult. From 4 days after ischemia, MR and GR immunoreactivities were detected in astrocytes and microglia in the CA1 region, and at 7 days after ischemia, MR and GR immunoreactivities peaked in the hippocampal CA1 region. At this time, 55% of astrocytes and 30% of microglia showed MR immunoreactivity, and 20% of astrocytes and 40% of microglia showed GR immunoreactivity. Western blot analyses showed that the pattern of changes in MR and GR protein levels was similar to the immunohistochemical changes observed after transient forebrain ischemia. From 4 days after ischemia, MR and GR protein levels were increased time-dependently after ischemia. In conclusion, enhanced MR and GR expressions in astrocytes and microglia were detected in the hippocampal CA1 region 4-7 days after ischemia/reperfusion. At this time, GR immunoreactivity was abundant in microglia, whereas MR immunoreactivity was prominent in astrocytes. The specific distribution of corticosteroid receptors in the astrocytes and microglia may be associated with the differences of MR and GR functions against ischemic damage.

Interleukin-6 induces glutathione in hippocampal cells

The cytokine interleukin-6 (IL-6) increases the levels of the physiological antioxidant glutathione (GSH) in peripheral organ systems such as liver tissue. Only little evidence exists about the actions of this cytokine on GSH in neuronal cell systems despite its possible neuroprotective effects. Therefore, we here characterized the effects of IL-6 on GSH in clonal hippocampal HT22 cells and in rat neuronal primary hippocampal cells. Our results demonstrate significant increases of GSH under most conditions after treatment with IL-6 in a time range of 1 to 48 h (HT22 cells) and 1 to 72 h (primary rat neuronal hippocampal cells). Further studies with an IL-6 antibody strongly support the specificity of the effects. These results suggest that IL-6 plays a substantial role in the regulation of GSH in hippocampal cells.

Dye-Labeled Benzodiazepines: Development of Small Ligands for Receptor Binding Studies Using Fluorescence Correlation Spectroscopy

To investigate benzodiazepine receptor binding studies by fluorescence correlation spectroscopy (FCS), the four fluorophores fluorescein, tetramethylrhodamine, Oregon Green 488, and Alexa 532 were coupled to the benzodiazepine Ro 07-1986/602 (Ro). Binding assays to polyclonal antibodies to benzodiazepines and at the native benzodiazepine receptor on the membrane of rat hippocampal neurons were established to examine the dye-labeled ligands for their benzodiazepine character and their binding behavior. Both the fluorescein and the Oregon Green488 moiety led to a loss of the benzodiazepine receptor binding of the corresponding Ro derivatives. Antibody recognition and interactions to the receptor were observed for the tetramethylrhodamine derivative (K(D) = 96.0 +/- 9.5 nM) but with a high amount of nonspecific binding at the cell membrane of about 50%. In saturation experiments a K(D) value of 97.2 +/- 8.5 nM was found for the Alexa Fluor 532 derivative-antibody interaction. Investigation of the binding of this ligand to the benzodiazepine receptor in FCS cell measurements led to confirmation of high specific binding behavior with a K(D) value of 9.9 +/- 1.9 nM. A nonspecific binding of <10% was observed after coincubation with 1 microM of midazolam. The different properties of the labeled benzodiazepine derivatives and the requirements of the fluorophore in small dye-labeled ligands in FCS binding studies, at the membrane of living cells, are discussed.

Glucocorticoids modulate neurotransmitter-induced glycogen metabolism in cultured cortical astrocytes

Glucocorticoids (GC) are considered as key modulators of glycogen homeostasis in peripheral tissues, but their role in the central nervous system has only partially been characterized. Exposure of primary cultures of cortical astrocytes to dexamethasone (DEX), a synthetic glucocorticoid, results in the reduction of noradrenaline (NA)-induced glycogen synthesis in a concentration-dependent manner with a IC50 of 4.88 nm and a maximum inhibition of 51%. Such an effect is mediated via glucocorticoid receptors (GRs), since it is mimicked by the glucocorticoid analogue RU28362 (100 nm) and prevented by the GR antagonist RU38486 (1 micro m). DEX does not act through alteration of signal transduction mechanisms, as cAMP formation induced by noradrenergic stimulation was unchanged. Moreover, glycogen synthesis was inhibited to the same extent when DEX was applied either together or only after a brief NA application. Neither [3H]2-deoxyglucose uptake nor lactate release was altered by DEX in the presence of NA, demonstrating that inhibition of glycogen synthesis is not a consequence of reduced glucose utilization or availability. Interestingly, enhancement of glycogen synthase activity induced by NA was reduced in the presence of DEX (-27%). These results suggest that GC could have a significant influence on neuroenergetics as they could modulate activity-related changes in brain glycogen metabolism.

Effects of clozapine and its metabolites on the 5-HT2 receptor system in cortical and hippocampal cells in vitro

PURPOSE

The goal of the present study was to determine the effects of clozapine (Cloz) and its metabolites norclozapine (Norcloz) and clozapine-N-oxide (Cloz-N-oxide) on the 5-HT(2) receptor system on the levels of protein and gene expression in in vitro systems of primary cortical cells of the rat and human hippocampal SHS5Y5 neuroblastoma cells.

METHODS

Clinically relevant concentrations of Cloz (200/400 ng/ml) and its metabolites (200 ng/ml) were used for the examination of the effects of Cloz and its metabolites on serotoninergic 5-HT(2) receptor parameters (density, affinity and mRNA levels) as well as on glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA levels in primary cortical cells of the rat after treatment for 24 h under in vitro conditions. To compare the results to human cells, we also measured treatment-induced changes in 5-HT(2) and GAPDH mRNA levels in human hippocampal SHS5Y5 cells.

RESULTS

A significant decrease was found in primary cortical cells for 5-HT(2) receptor density (Cloz 200/Cloz 400/Norcloz 200 and Cloz-N-oxide 200 vs. control) and 5-HT(2A) receptor mRNA levels (Cloz 200 vs. control). 5-HT(2A) receptor mRNA levels were also significantly reduced (Norcloz 200 vs. control) in SHS5Y5 cells. GAPDH mRNA levels were not affected.

CONCLUSIONS

The results of the present study show that Cloz and Norcloz induce significant alterations on the 5-HT(2) receptor system in primary cortical cells of the rat and in human hippocampal cells.

Brain-derived neurotrophic factor rescues neuronal death induced by methamphetamine

BACKGROUND

Methamphetamine (MA) induces degeneration of various regions of the brain, resulting in neuropsychiatric damage. Although the underlying mechanisms of MA-induced neurotoxicity have been studied, there are few reports to date regarding the factor(s) that can effectively prevent MA-induced neurotoxicity. Because brain-derived neurotrophic factor (BDNF) has been known to prevent many kinds of neuronal cell death, we investigated whether BDNF inhibits MA-induced neuronal death.

METHODS

Using primary cortical neurons, we examined the effect of BDNF on MA-induced neuronal death. In addition, using pharmacologic and molecular biological tools, we elucidated which pathways are involved in this effect.

RESULTS

Brain-derived neurotrophic factor dose-dependently blocked MA-induced neuronal death, and this effect was inhibited by phosphatidylinositol-3-kinase inhibitors. In addition, overexpression of activated Akt protects neurons against MA. Furthermore, expression of kinase-defective Akt blocked the effect of BDNF on MA-induced neuronal death.

CONCLUSIONS

Brain-derived neurotrophic factor effectively blocks MA-induced neuronal death, and Akt activation is necessary and sufficient for this effect.

Benzodiazepine binding studies on living cells: application of small ligands for fluorescence correlation spectroscopy

We demonstrate the applicability of fluorescence correlation spectroscopy (FCS) for receptor binding studies using low molecular weight ligands on the membranes of living nerve cells. The binding of the benzodiazepine Ro 7-1986/602 (N-des-diethyl-fluorazepam), labeled with the fluorophore Alexa 532, to the benzodiazepine receptor was analyzed quantitatively at the membrane of single rat hippocampal neurons. The values obtained for the dissociation constant Kd = (9.9 +/- 1.9) nm and the rate constant for ligand-receptor dissociation kdisS = (1.28 +/- 0.08) x 10(-3) s(-1) show that there is a specific and high affinity interaction between the dye-labeled ligand (Ro-Alexa) and the receptor site. The binding was saturated at approx. 100 nM and displacement of 10 nM Ro-Alexa, with a 1,000-fold excess of midazolam, showed a non-specific binding of 7-10%. Additionally, two populations of the benzodiazepine receptor that differed in their lateral mobility were detected in the membrane of rat neurons. The diffusion coefficients for these two populations [D(bound1) = (1.32 +/- 0.26) microm2/s; D(bound2) = (2.63 +/- 0.63) x 10(-2) microm2/s] are related to binding sites, which shows a mono-exponential decay in a time-dependent dissociation of the ligand-receptor complex.

Glucocorticoid receptors in hippocampal neurons that do not engage proteasomes escape from hormone-dependent down-regulation but maintain transactivation activity

The glucocorticoid receptor (GR) protein is subjected to hormone-dependent down-regulation in most cells and tissues. This reduction in receptor levels that accompanies chronic hormone exposure serves to limit hormone responsiveness and operates at transcriptional, posttranscriptional, and posttranslational levels. The ability of glucocorticoid hormones to trigger GR down-regulation may be not universal, particularly in mature and developing neurons in which conflicting results regarding hormone control of GR protein have been reported. We find that endogenous GR is not down-regulated in the HT22 mouse hippocampal cell line and in primary hippocampal neurons derived from embryonic rats. Because GR has the capacity to be ubiquitylated in HT22 cells, receptor down-regulation must be limited by defects in either targeting of polyubiquitylated receptor to the proteasome or processing of the targeted receptor by the proteasome. Despite the lack of GR down-regulation in the HT22 cells, glucocorticoid-induced transcription from transiently transfected templates is attenuated upon prolonged hormone treatment. This termination of GR transactivation is not due to inefficient nuclear import or nuclear retention of the receptor. Furthermore, GR efficiently exports from HT22 cell nuclei in hormone-withdrawn cells, indicating that the receptor has access to both nuclear and cytoplasmic degradation pathways. Our results suggest that appropriate maturation of proteasomal degradative or targeting activities may be required, particularly in hippocampal neurons, for hormone-dependent down-regulation of GR.

Differential effects of glucocorticoids and gonadal steroids on glutathione levels in neuronal and glial cell systems

The aim of the present study was to investigate the short- and long-term effects of glucocorticoids [corticosterone (CORT), dexamethasone (DEX), 6-methylprednisolone (6-MP)] and gonadal steroids [17beta-estradiol (E(2)), progesterone (PROG), testosterone (TEST)] on the levels of the antioxidant glutathione (GSH) in different cell systems of the CNS (neuronal hippocampal HT22 cells, primary hippocampal and neocortical brain cells, and C(6) glioma cells). In HT22 cells, steroids exerted mainly long-term effects. Significant increases of GSH levels were detectable after a 24 hr treatment with 10(-7) M of DEX (122% +/- 5%), 6-MP (208% +/- 32%), E(2) (134% +/- 10%), and TEST (155% +/- 17%). A significant decrease occurred after incubation with PROG for 24 hr (79% +/- 9%). In primary hippocampal cultures, a 24 hr treatment with DEX (140% +/- 8%), E(2) (123% +/- 6%), and PROG (118% +/- 5%) led to significant increases of the GSH levels, whereas, in neocortical primary cultures, only an incubation with E(2) increased GSH (149% +/- 8%). In C(6) cells, hormone treatment led to both significant short-term (1 hr: CORT 114% +/- 5%, DEX 90% +/- 3%, E(2) 88% +/- 3%; 3 hr: DEX 115% +/- 5%, E(2) 122% +/- 6%, TEST 78% +/- 4%) and significant long-term (24 hr: CORT 74% +/- 4%, 6-MP 84% +/- 5%, E(2) 115% +/- 6%, PROG 91% +/- 4%, TEST 116% +/- 5%) effects. In summary, we were able to demonstrate differential effects of steroids on GSH levels in different cellular CNS models, showing an important influence of steroids and especially E(2) on antioxidative cellular functions in neuronal and glial cells.

Altered dendritic spine density in animal models of depression and in response to antidepressant treatment

Olfactory bulbectomy, neonatal clomipramine administration, and maternal deprivation have been employed as animal models of depression. Each model is unique with respect to the experimental manipulations required to produce "depressive" signs, expression and duration of these signs, and response to antidepressant treatments. Dendritic spines represent a possible anatomical substrate for the enduring changes seen with depression and we have previously shown that chronic antidepressant drug exposure alters the density of hippocampal dendritic spines in an enduring fashion. The purpose of the present study was to determine whether persistent alteration of hippocampal spine density is a common element in each of these different models of depression and whether such alterations could be reversed with chronic antidepressant treatment. The results show that olfactory bulbectomy reduced spine density in CA1, CA3, and dentate gyrus compared to sham-operated controls. Chronic treatment with amitriptyline, a tricyclic antidepressant, reversed the bulbectomy- induced reduction in dendritic spine density in CA1, CA3, and dentate gyrus, whereas treatment with mianserin, an atypical antidepressant, reversed this reduction only in dentate gyrus. On the other hand, neither neonatal clomipramine administration nor maternal deprivation affected hippocampal dendritic spine density. Repeated neonatal handling, however, as a control or as part of the maternal deprivation procedure, elevated spine density in dentate gyrus. These data suggest that long-lasting alterations in hippocampal dendritic spine density contribute to the neural mechanism underlying the olfactory bulbectomy model of depression, but not the neonatal clomipramine or maternal deprivation models.

Psychological stress increases hippocampal mineralocorticoid receptor levels: involvement of corticotropin-releasing hormone

We investigated whether acute stressors regulate functional properties of the hippocampal mineralocorticoid receptor (MR), which acts inhibitory on hypothalamic-pituitary-adrenocortical activity. Exposure of rats to forced swimming or novelty evoked a significant rise in density of MR immunoreactivity in all hippocampal subfields after 24 hr, whereas exposure to a cold environment was ineffective. Time course analysis revealed that the effect of forced swimming on MR peaked at 24 hr and returned to control levels between 24 and 48 hr. In pyramidal neurons of CA2 and CA3, marked rises were already observed after 8 hr. Radioligand binding assays showed that corticotropin-releasing hormone (CRH) injected intracerebroventricularly into adrenalectomized rats also produced a rise in hippocampal MR levels; an effect for which the presence of corticosterone, but not dexamethasone, at the time of injection was a prerequisite. Moreover, pretreatment with the CRH receptor antagonist (d-Phe(12),Nle(21,38),alpha-Me-Leu(37))-CRH(12-41) blocked the effect of forced swimming on hippocampal MR levels. To investigate whether the rise in MR levels had any functional consequences for HPA regulation, 24 hr after forced swimming, a challenge test with the MR antagonist RU 28318 was conducted. The forced swimming exposed rats showed an enhanced MR-mediated inhibition of HPA activity. This study identifies CRH as an important regulator of MR, a pathway with marked consequence for HPA axis regulation. We conclude that the interaction between CRH and MR presents a novel mechanism involved in the adaptation of the brain to psychologically stressful events.

Repeated antenatal glucocorticoid treatment decreases hypothalamic corticotropin releasing hormone mRNA but not corticosteroid receptor mRNA expression in the fetal guinea-pig brain

Approximately 10% of pregnant women are treated with synthetic glucocorticoids in late gestation, to promote fetal lung maturation. The effectiveness of this treatment has led to the use of repeated dose regimens, with little knowledge of the impact on neuroendocrine development. Animal studies have recently shown that repeated fetal glucocorticoid exposure can lead to permanent changes in hypothalamic-pituitary-adrenal (HPA) function in offspring. In this study, we hypothesized that such treatment modifies corticotropin releasing hormone (CRH), glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) systems in the developing limbic system and hypothalamus. Pregnant guinea-pigs were treated with dexamethasone, betamethasone or vehicle on days 40,41,50,51,60 and 61 of gestation (birth = 68 days). On day 62, guinea-pigs were killed and the fetuses rapidly removed. Glucocorticoid treatment resulted in a dose-dependent reduction in plasma cortisol concentrations in both male and female fetuses. There was also a significant reduction in CRH mRNA expression in the hypothalamic paraventricular nucleus. In contrast, exposure to glucocorticoid increased MR mRNA expression in the hippocampus (CA1/2 and CA3) and dentate gyrus of female fetuses. There was a small but significant increase in GR mRNA expression in limbic structures in male fetuses following treatment with 1 mg/kg dexamethasone. However, there was no significant effect of glucocorticoid exposure on hippocampal GR mRNA expression in female fetuses, or hypothalamic GR mRNA in either males or females. In conclusion, repeated maternal glucocorticoid treatment inhibits fetal HPA function. The fact that CRH mRNA levels were reduced indicates that synthetic glucocorticoids enter the fetal brain. By contrast, fetal glucocorticoid exposure does not downregulate GR mRNA, and increases MR mRNA expression. The latter likely reflects removal of circulating endogenous ligand (cortisol). These alterations may form the basis for permanently modified HPA activity in later life.

Glucocorticoids and serotonin alter glucocorticoid receptor (GR) but not mineralocorticoid receptor (MR) mRNA levels in fetal mouse hippocampal neurons, in vitro

Studies utilizing rats and guinea pigs have demonstrated that the hypothalamo-pituitary-adrenal (HPA) axis can be programmed by glucocorticoids during fetal life. Such programming is believed to occur, at least partially, at the level of hippocampal glucocorticoid receptors (GR) and mineralocorticoid receptors (MR). Studies have also demonstrated that serotonin up regulates GR levels within the developing hippocampus. However, the cell type in which these changes take place has not been determined. We hypothesized that dexamethasone, corticosterone and serotonin exposure modify GR and MR mRNA levels in fetal mouse hippocampal cultures, and that these effects are confined to neurons. Cultures were derived from CD1 mouse fetuses on day 18 of gestation (n=8 dams). Fetal hippocampi were dissected, then mechanically and chemically dispersed. Cultures were exposed to dexamethasone, corticosterone or serotonin (1-100 nM) for 4 days. Levels of GR and MR mRNA were examined by in situ hybridization and high-resolution silver emulsion autoradiography. Four days exposure to dexamethasone or corticosterone (10 or 100 nM) decreased levels of GR mRNA within neurons. There was no significant change in MR mRNA in either experiment. Exposure to serotonin (100 nM) significantly increased expression of GR mRNA in hippocampal neurons. MR mRNA levels were unaffected by serotonin treatment. Dexamethasone, corticosterone or serotonin exposure did not alter expression of GR mRNA within glial cells. We conclude that synthetic and endogenous glucocorticoids, as well as serotonin, can influence neuronal levels of GR mRNA during hippocampal development. However, whether these effects are permanent remains to be determined.

Expression and glucocorticoid regulation of macrophage migration inhibitory factor (MIF) in hippocampal and neocortical rat brain cells in culture

The biosynthesis of macrophage-migration inhibitory factor (MIF) and its regulation by the glucocorticoid dexamethasone was examined in cultured hippocampal and neocortical embryonic rat cells. Using immunohistochemical methods, MIF was found to be localized in neuronal as well as in non-neuronal cells. During the whole 12 day culture period, levels of MIF transcripts were detectable in both hippocampal and neocortical cells with an apparent increase in extracellular MIF protein at the later time points examined. Treatment with even very low concentrations (10(-11) M) of dexamethasone did not alter MIF mRNA levels but resulted in a rapid release of intracellular MIF protein within 1 and 4 h and a subsequent replenishment after 24 h. These data suggest that glucocorticoids do not affect the transcriptional activity of the MIF gene but induce the secretion of the protein, which suggests a close functional relationship of both mediators in the CNS.

The effects of serotonin on glucocorticoid receptor binding in rat raphe nuclei and hippocampal cells in culture

The raphe-hippocampal serotonin (5-HT) system is involved in the regulation of the hypothalamus-pituitary-adrenal axis. The purpose of this study was to determine and compare the roles of 5-HT in the regulation of glucocorticoid receptor (GR) binding in the raphe nuclei and in the hippocampus. The effects of 5-HT, 5-HT agonists, and the 5-HT reuptake inhibitor citalopram on GR binding sites were studied in primary cultures of the fetal raphe nuclei and the hippocampus. Exposure of hippocampal cells to 5-HT, (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI; a 5-HT2 agonist), or citalopram resulted in an increase in number of GR binding sites. The effect of DOI was blocked by ketanserin (a 5-HT2 antagonist). Specific and saturable GR binding was found in raphe cells. Exposure of raphe cells to 5-HT, (+/-)-8 hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; a 5-HT1A agonist), or citalopram induced a significant decrease in number of GR binding sites. The effect of 8-OH-DPAT was reversed by WAY 100135 [N-tert-butyl-3-[1-[1-(2-methoxy)phenyl]piperazinyl]-1-phenylpropiona mide; a 5-HT1A antagonist]. These results show that the regulation of GRs during fetal life is structure-dependent and involves different 5-HT receptor subtypes. Moreover, the regulation of hippocampal GRs by citalopram suggests an action of antidepressants independent of their effects on monoamines.

Long-term treatment with antidepressants increases glucocorticoid receptor binding and gene expression in cultured rat hippocampal neurones

Since the glucocorticoid receptor (GR) and/or mineralocorticoid receptor (MR) in the hippocampus have been implicated in cortisol feedback of the hypothalamus-pituitary-adrenal (HPA) axis, abnormalities in those receptors might underlie the hyperactivity of the HPA axis described in patients with major depression. Animal studies have shown that long-term in-vivo treatment with antidepressants up-regulates hippocampal GR and/or MR, but it is not clear whether this up-regulation is evoked through a direct action of antidepressants on these receptors. We therefore examined the direct effects of long-term antidepressant treatment on GR binding and the levels of GR messenger RNA (mRNA) in primary cultures of rat hippocampal neurones. The time course of the effects of the tricyclic antidepressants desipramine and amitriptyline on GR binding, as assessed by [3H]dexamethasone binding using RU 28362, a specific agonist for GR, showed a biphasic mode of stimulation: desipramine significantly increased the GR binding with 2-day exposure by 36% over that in controls and by 99% and 60% with 10- and 14-day exposures, respectively. Amitriptyline also led to a significant increase in GR binding, with peaks at 2 (by 60%) and 14 days of exposure (by 60%). The effects of 14-day treatment with desipramine required at least the first 4-day exposure, and the first 10-day exposure was required for the full effect. Northern blot analysis demonstrated that the GR mRNA level was significantly increased by 14-day treatment with desipramine (+142% over control), amitriptyline (+108%), mianserin (+124%), paroxetine (+42%) and sulpiride (+92%), but not with haloperidol. Immunocytochemistry for GR revealed that 2- or 14-day treatment with desipramine significantly increased the number of GR-positive cells with dominant immunoreactivity in the nuclei of granule cell-like neurones or in perikarya of pyramidal cell- and granule cell-like neurones. These findings suggest that tricyclic antidepressants directly increase hippocampal GR by short-term (2-day) and long-term (14-day) exposure, and that the increase by long-term exposure is evoked commonly with different classes of antidepressants through transcriptional up-regulation of GR expression.

Brain-derived neurotrophic factor prevents neuronal cell death induced by corticosterone

Corticosterone (CORT), one of the glucocorticoids, causes neuronal damage in the hippocampus, but the mechanism(s) of action underlying its effects remains unknown. Brain-derived neurotrophic factor (BDNF) is a neurotrophic factor that belongs to the neurotrophin family, affects the survival and/or differentiation of various types of neurons in vitro, and is able to antagonize neuronal death induced by various brain insults or neurotoxins in vivo. In this study, the effects of CORT on BDNF protein contents and mRNA expression were investigated in relation to neuronal survival/death of cultured rat hippocampal neurons, because the colocalization of BDNF with its receptor, TrkB, suggests that BDNF may exert its putative protective and trophic effects through an autocrine mechanism in the hippocampus. Administration of CORT accelerated the neuronal death that proceeds after serum deprivation, and simultaneously reduced the levels of BDNF mRNA and intracellular BDNF content. Exogenously added BDNF actually attenuated CORT-induced neuronal death, but not in the presence of K252a, an inhibitor of the tyrosine kinase activity of Trk family receptors. These observations suggest that CORT induces damage to hippocampal neurons, at least partly, via reducing their BDNF synthesis.

Amphetamines induce apoptosis and regulation of bcl-x splice variants in neocortical neurons

Amphetamineanalogs have emerged as popular recreational drugs of abuse. The number of reports of these substances producing severe acute toxicity and death is increasing. In 'Ecstasy' -associated deaths, focal necrosis in the liver and individual myocytic necrosis has been reported. Furthermore, serotonergic and dopaminergic neuronal cell damage has been observed in experimental amphetamine intoxication in laboratory animals. Here we demonstrate that subchronic exposure to D-amphetamine, methamphetamine, methylenedioxyamphetamine, and methylenedioxymethamphetamine ('Ecstasy') results in significant neurotoxicity in rat neocortical neurons in vitro. This neuronal cell death is accompanied by endonucleosomal DNA cleavage and differential expression of anti- and proapoptotic bcl-xL/S splice variants. In addition, we observed pronounced induction of cell stress-associated transcription factor c-jun and translation initiation inhibitor p97 after amphetamine treatment. These data support that the neurotoxic effects of different amphetamines are extended to rat neocortical neurons and that apoptotic pathways are involved in amphetamine-induced neurotoxicity.

Bidirectional effects of the neuroactive steroid tetrahydrodeoxycorticosterone on GABA-activated Cl- currents in cultured rat hypothalamic neurons

1. The non-genomic effects of tetrahydrodeoxycorticosterone (THDOC; 5-alpha-pregnane-3-alpha, 21-diol-20-one) were studied in cultured hypothalamic neurons of the rat. 2. The effects of THDOC (10 nM - 1 microM) on responses to different concentrations of exogenously applied GABA and on spontaneous inhibitory postsynaptic currents (IPSCs) were measured with whole-cell voltage clamp recordings. 3. Application of GABA induced inward currents with dose-dependently increasing amplitudes (up to 3.9 nA at a holding potential of -20 mV). High doses of THDOC (100 nM-1 microM) induced small inward currents on its own (14+/-3 and 24+/-3 pA, respectively). 4. Simultaneous application of 10 microM GABA with 100 nM or 1 microM THDOC increased current amplitudes by 125 and 128%, respectively. At 10 nM THDOC exerted no consistent effects on GABA currents. 5. Responses to 1 microM of GABA were modulated in a bidirectional manner by different doses of THDOC: 10 nM THDOC reduced the amplitude of GABA responses to 80% (P=0.018, n=15), whereas 100 nM and 1 microM THDOC enhanced the GABA response to 115 and 180% (P=0.0007, n = 15), respectively. 6. The time constant of decay of spontaneous inhibitory postsynaptic currents (IPSCs) was reversibly increased from 91+/-10 to 314+/-34 ms (n=3) by the application of THDOC (1 microM). The amplitudes of the IPSCs were not affected by THDOC. 7. These data indicate that THDOC modulates GABA responses of hypothalamic neurons in a bidirectional manner, resulting in a complex tuning of neuronal excitability in the hypothalamus.

Estrogen hormones reduce lipid peroxidation in cells and tissues of the central nervous system

Effects of estrogen hormones on lipid peroxidation (LPO) were examined in rat brain homogenates (RBHs), hippocampal HT 22 cells, rat primary neocortical cultures, and human brain homogenates (HBHs). Dose-response curves indicated half-maximal effective concentrations (EC50) of 5.5 and 5.6 mM for iron-induced LPO in RBHs and HT 22 homogenates. Incubation of living rat primary neocortical cultures with iron resulted in an EC50 of 0.5 mM, whereas culture homogenates showed an EC50 of 1.2 mM. Estrogen hormones reduced LPO in all systems: In RBHs, estrone inhibited iron-induced LPO to 74.1 +/- 5.8% of control levels (17beta-estradiol: 71.3 +/- 0.1%) at a concentration of 10 microM. In hippocampal HT 22 cell homogenates, levels of LPO were reduced to 74.8 +/- 5.5% by estrone and to 47.8 +/- 6.2% by 17beta-estradiol. In living neocortical cultures, 17beta-estradiol decreased iron-induced LPO to 79.2 +/- 4.8% and increased the survival of cultured neuronal cells. Of the other steroid compounds tested (corticosterone, progesterone, testosterone), only progesterone decreased LPO in HT 22 cell homogenates. In HBHs, LPO was dose-dependently increased by iron concentrations from 2.7 to 6.0 mM. Incubation with estrogens resulted in a dose-dependent inhibition of LPO to 53.8 +/- 8.6% with 10 microM 17beta-estradiol, whereas estrone failed to affect iron-induced LPO to a significant extent. Nonestrogenic steroids, including hydrocortisol, did not show significant effects on LPO in HBHs.

Strain differences in corticosteroid receptor efficiencies and regulation in Brown Norway and Fischer 344 rats

During the dark phase of the diurnal cycle, and during recovery from restraint stress, Brown Norway (BN) rats secrete less corticosterone than Fischer 344 (F344) rats. These strains also display different levels of corticosteroid receptors in the hippocampus, and of plasma transcortin. Because corticosteroid receptors, plasma transcortin and corticosterone secretion are mutually regulated, we examined brain and pituitary mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) expression and some of the parameters modulated by these receptors (i.e. body and thymus weight, fluid intake, plasma transcortin) in BN and F344 rat strains, by comparing the effects of either hormone deprivation by long-term (21 days) adrenalectomy (ADX), or chronic elevation of corticosterone given in drinking fluid to ADX rats. In BN rats, body weight gain and fluid intake were insensitive to corticosterone deprivation, suggesting that MR-related mechanisms are constitutively active in this strain. Body weight (b.w.) gain, plasma transcortin and thymus weight were reduced to a greater extent by chronic corticosterone in BN rats than in F344 rats, possibly as a consequence of higher free, active fraction of plasma corticosterone due to lower plasma transcortin concentrations and/or a greater efficiency of GR-related mechanisms in BN rats. F344 rats displayed twofold higher brain and pituitary MR levels than BN rats, whereas tissue-and strain-specific regulations were observed for GR levels. The differences in MR levels observed between BN and F344 strains cannot completely explain the differences in corticosterone actions, suggesting that strain differences in response to ADX or corticosterone treatment result from variable receptor efficiencies.

Antidepressants restore hypothalamic-pituitary-adrenal feedback function in aged, cognitively-impaired rats

Aged, cognitively-impaired rats (and humans) show hypothalamic-pituitary-adrenal (HPA) hyperactivity that correlates with hippocampal damage. The resultant increase in plasma glucocorticoid exposure is thought to contribute to impaired hippocampal function and to potentiate hippocampal neuron death. In young, adult rats antidepressant drugs increase corticosteroid receptor expression in brain regions known to regulate the HPA axis, leading to increased negative-feedback control and decreased HPA activity. In the present study we examined basal levels of plasma adrenocorticotropin hormone (ACTH) and corticosterone in aged, cognitively-impaired (AI), aged, cognitively-unimpaired (AU) and young, adult (Yg) rats. Plasma ACTH and corticosterone levels were significantly elevated in the AI rats, but only in samples obtained during the diurnal peak. Five weeks of treatment with desipramine (15 mg/kg) significantly reduced evening levels of both ACTH and corticosterone in all groups, and eliminated the group differences. We then examined delayed, glucocorticoid negative feedback in these animals. Among vehicle-treated animals, a bolus injection of corticosterone (10 mg/kg), administered 3 hours prior to testing, completely inhibited the plasma ACTH response to restraint in AU and Yg, but not AI animals. In contrast, plasma ACTH responses to restraint were completely inhibited in AI rats following chronic treatment with desipramine. These findings indicate that the antidepressant, desipramine, decreases HPA activity and increases glucocorticoid negative-feedback sensitivity in AI rats, suggesting that antidepressant drugs may form a useful therapeutic approach to HPA dysfunction in elderly human populations.

Corticosterone is involved in foot shock-induced inactivity in rats

Inescapable shock (IS) exposure induces behavioral inactivity, related to behavioral alterations in subsequent tests (i.e., escape failure, and inactivity during shuttle box task). Metyrapone (150 mg/kg, IP), a corticosterone (CS) synthesis inhibitor, administered 3 h prior to IS reduced inactivity during this aversive experience. Forty-eight hours later, when these rats were submitted to a shuttle box task, a reduction in both escape failure and inactivity was observed. These effects were reversed by CS (20 mg/kg, SC) and dose dependent of the synthetic glucocorticoid dexamethasone, both administered 1 h before IS. When metyrapone was administered 3 h before the shuttle box task to IS-exposed animals, escape failures and inactivity were markedly reduced. This effect was subsequently reversed by CS. The dynamics of changes in serum CS concentrations after both IS and shuttle box task paralleled behavioral changes. Animals injected with metyrapone before IS, which displayed active behavior, showed serum CS levels stable at their basal levels after shock, and their secretion pattern was quite attenuated after the shuttle box task, whereas vehicle-, CS alone-, and metyrapone + CS-injected animals showed higher serum CS concentrations post-IS, which slowly decreased to their corresponding basal levels. CS secretion after the shuttle box task was similar for the three groups: it had the same magnitude as after IS, though the decrease was faster. In all groups, animals displayed passive behavior. These results indicate that glucocorticoids are involved in the onset and expression of passive behaviors induced by uncontrollable stressors. Therefore, it is possible to suggest a functional relationship between CS released by exposure to inescapable stressor and the behavioral strategies adopted by rats under this stressful condition.

Distinction between binding of [3H]triamcinolone acetonide to a ligand binding domain on the glucocorticoid receptor complex in cytosol fractions of brain and liver from the rat with intact adrenals

Binding of [3H]triamcinolone acetonide (TA) was examined to evaluate possible differences in biochemical and pharmacological profiles of a ligand binding domain on the naive glucocorticoid (GC) receptor complex between cytosol fractions of brain and liver obtained from the rat with intact adrenals. Unlabeled TA was 3-times less potent in displacing [3H]TA binding at equilibrium in the presence of Na2MoO4 at 2 degrees C in brain than in liver, while dexamethasone was more than 15-times more potent in displacing brain binding than liver binding. Both progesterone and aldosterone caused more than 20-times more potent displacement of [3H]TA binding in brain than in liver. However, zinc ions were effective in similarly inhibiting binding at equilibrium via lowering the affinity without affecting the density. In contrast to these experiments done at 2 degrees C, the addition of Na2MoO4 potentiated binding in brain and liver in a transient manner at 30 degrees C. Prolongation of incubation period from 30 min to 5 h induced a rightward shift of concentration-response curves of Na2MoO4 for [3H]TA binding at 30 degrees C in brain and liver in an irreversible fashion. The potentiation by Na2MoO4 was prevented by the addition of leupeptin but not 5 other protease inhibitors in brain cytosol fractions. Ion exchange chromatography revealed that elution profiles were entirely different from each other for [3H]TA binding between brain and liver cytosol fractions. These results suggest that GC receptors may respond to a variety of signals mediated by different steroid hormones through broader pharmacological spectrum of the ligand binding domain in brain than in liver.

Neurotrophin expression modulated by glucocorticoids and oestrogen in immortalized hippocampal neurons

We have used reverse transcription followed by polymerase chain reaction amplification to investigate changes in expression of nerve growth factor (NGF) mRNA in immortalized hippocampal neurons after treatment with the glucocorticoids dexamethasone and corticosterone, the glucocorticoid antagonist RU38486, and the gonadal steroids progesterone and 17-beta oestradiol. We found that NGF mRNA levels rise after application of either dexamethasone or corticosterone, and that this rise is prevented by the antagonist. Thus, neurotrophin expression is modulated by the physiological glucocorticoid and is mediated by type II glucocorticoid receptors. Progesterone has no effect, while 17-beta oestradiol suppresses NGF mRNA in a postnatally-derived cell line but does not change levels in an embryonic line. An increase in neurotrophin expression is therefore not a general response to steroid hormone application, and may be a specific defence against the presence of metabolically endangering glucocorticoids.

Glucocorticoid and mineralocorticoid receptors differentially modulate cultured dopaminergic neurons of rat ventral mesencephalon

Activation of kappa-opioid receptors by U69,593 in a concentration of 1 microM maximally inhibited dopamine release. Withdrawal from chronic (4 days) treatment with U69,593 (1 microM) induced a long-lasting (at least 7 days) increase (30-50%) in [3H]dopamine release induced by 25 mM K+ from cultured rat mesencephalic neurons without causing functional kappa-opioid receptor desensitization. Incubation of these cultured neurons with the glucocorticoid agonist, dexamethasone (3 nM), had no effect on basal or 25 mM K(+)-induced dopamine release, whereas the steroid completely blocked the development of opiate-induced neuronal supersensitivity to depolarization. This blockade was found to be concentration-dependent, with an EC50 of about 0.3 nM and was not associated with changes in corticotropin-releasing factor (CRF) receptor, dopamine D2 autoreceptor or kappa-opioid receptor functioning. Therefore, opioid and glucocorticoid receptors appear to be co-localized in dopaminergic neurons of rat ventral mesencephalon and interact in a functionally antagonistic manner. Interestingly, the mineralocorticoid agonist, aldosterone (5 nM), itself increased K(+)-stimulated [3H]dopamine release by about 25%, consistent with the putative role of mineralocorticoid receptors in maintaining neuronal excitability. Moreover, whereas 1 nM corticosterone appeared to mimic the dopamine release-facilitating effect of aldosterone, 30 nM corticosterone displayed a modulatory effect similar to that of dexamethasone. Thus, whereas mineralocorticoid receptors that are already activated at low plasma corticosterone levels may uphold neuronal sensitivity to depolarizing stimuli, activation of glucocorticoid receptors by higher plasma corticosterone levels may prevent the development of neuronal supersensitivity, e.g. following chronic activation of kappa-opioid receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Neonatal handling alters serotonin (5-HT) turnover and 5-HT2 receptor binding in selected brain regions: relationship to the handling effect on glucocorticoid receptor expression

Neonatal handling permanently alters hypothalamic-pituitary-adrenal responses to stress. This effect is, in part, mediated by a handling-induced increase in forebrain glucocorticoid receptor gene expression. The effect of postnatal handling on glucocorticoid receptor expression appears to be mediated by an increase in serotonin (5-HT) activity, acting via a 5-HT2 receptor with a high affinity for 5-HT (i.e. the 5-HT2H receptor). In the present study we examined the nature of the effects of handling on the relevant 5-HT systems. We found that: (1) handling increases 5-HT turnover in regions of the neonatal rat brain where glucocorticoid receptor expression is altered (i.e. the hippocampus and frontal cortex), but not in regions where glucocorticoid receptor expression in unaffected (e.g. hypothalamus and amygdala); (2) handling has no long-term effects on hippocampal or frontal cortex 5-HT turnover, and is actually associated with a decrease in 5-HT concentrations; and (3) handling does not alter 5-HT2 receptor density in the hippocampus or frontal cortex in neonates (although there are surprising effects on 5-HT2 receptor density in the frontal cortex of adult animals). Taken together these data provide further evidence for the importance of 5-HT in mediating the effects of handling on the development of glucocorticoid receptor expression, but suggest that the role of 5-HT is unique to early development; differences in glucocorticoid receptor expression in adult handled and non-handled animals are not associated with long-term differences in either 5-HT levels or 5-HT2 receptors.

Hippocampal glucocorticoid receptor expression in the tree shrew: regulation by psychosocial conflict

1. This study was conducted to determine whether chronic psychosocial conflict alters the expression of glucocorticoid receptor (GR) mRNA in the hippocampus of male tree shrews (Tupaia belangeri). 2. To generate probes for the in situ hybridization, the tree shrew GR gene was partly cloned. There was a 90% homology between the deduced amino acid sequence of the cloned tree shrew GR and that of the corresponding human GR sequence. 35S-Labeled riboprobes which had been transcribed from the tree shrew GR clone hybridized to pyramidal neurons in all subregions of the tree shrew hippocampal formation and to granule neurons in the dentate gyrus. 3. After in situ hybridization, the expression of GR mRNA was semiquantitatively determined by counting silver grains over single neurons of the hippocampal formation of psychosocially stressed tree shrews and control animals. After 12 days of social conflict, the number of silver grains in the CA1 and CA3 pyramidal neurons of stressed animals was significantly lower than in controls. No statistically significant differences in mRNA expression were observed in the pyramidal neurons of the subiculum and in the granule neurons of the dentate gyrus. 4. The present results suggest that psychosocial stress leads to a site-specific down-regulation of hippocampal GR via modification of mRNA expression.

Methamphetamine-induced decrease in neural glucocorticoid receptors: relationship to monoamine levels

Methamphetamine (MA) is a potent psychostimulant drug which is neurotoxic to dopamine (DA) and serotonin (5-HT) neurons. It has been previously reported that acute MA administration to adrenalectomized rats produced large dose-related decreases in hippocampal and striatal glucocorticoid receptors (GR). The present study was designed to determine if MA could decrease neural and peripheral GR when administered to adrenal-intact rats using a neurotoxic dosing regimen which produces depletions of brain DA and 5-HT levels. MA (0, 6.25, 12.5 and 25 mg/kg) was administered to adrenal-intact rats every 2 h for a total of 4 doses. Rats were adrenalectomized (ADX) 6 days later and subsequently sacrificed 24 h later. GR and mineralocorticoid receptors (MR) were measured using radioligand binding assays. Tissue levels of 5-HT and DA were measured in order to confirm the neurotoxic effects of MA and also to relate corticosteroid receptor levels to monoamine concentrations. MA produced dose-related decreases in GR levels in the hippocampus, striatum, frontal cortex and hypothalamus. Hippocampal MR were not affected by MA. 5-HT was also decreased in all of these same 4 brain regions, whereas DA was significantly decreased only in the striatum. MA did not decrease GR in cerebellum and similarly had no effect on DA and 5-HT in this region. MA also did not decrease GR or 5-HT levels in the spleen. These results demonstrate that MA produces a decrease in GR in a variety of brain areas, which is related primarily to 5-HT depletions.(ABSTRACT TRUNCATED AT 250 WORDS)

Aldosterone modulates glucocorticoid receptor binding in hippocampal cell cultures via the mineralocorticoid receptor

The regulation of corticosteroid receptor expression in the rat brain by adrenal steroids remains controversial. The results of in vivo studies [Brinton and McEwen, 1988; Luttge et al., 1989] suggest that activation of type I receptors can modulate both mineralocorticoid (MR; type I) and glucocorticoid (GR; type II) receptor binding in selected brain regions. The present study utilized primary hippocampal cell cultures from rats sacrificed at E19-20 days of gestation to examine the effects of RU 28362, corticosterone (CORT) and aldosterone (ALDO) on GR binding ([3H]dexamethasone +/- RU 28362). Four days of exposure to 10 nM RU 28362, a highly selective GR agonist, resulted in a robust (approximately 70%) decrease in GR binding. Similar exposure to 10 nM of either CORT or ALDO also produced a significant (50-55%) decrease in GR binding capacity. Scatchard analyses confirmed that the diminished GR binding capacity in response to ALDO was due to a decrease in total number of binding sites (Bmax for Control = 112 +/- 25 fmol/mg vs. ALDO = 48 +/- 12 fmol/mg) with no significant change in the affinity constant. The calculated EC50 from the ALDO concentration response curve was 3.5 nM. Competition studies demonstrated that such low nM concentrations of ALDO were unable to displace specific [3H]dexamethasone +/- RU 28362 binding. Spironolactone, a highly specific MR antagonist, inhibited the ALDO-induced down-regulation of GR binding. These findings support the hypothesis that MR activation can modulate GR binding in hippocampal cells.