Intracellular CBG-like molecules in the rat pituitary

Light and electron microscopic studies on the influence of stress on prolactin-immunoreactivity in rat anterior pituitary lobe

An increasing number of evidence suggests an important role of prolactin in the modulation of stress response. However, the mechanisms of its action on the HPA axis are not yet understood. Glucocorticoids, liberated from adrenal cortex due to hormonal signals from pituitary corticotrophs are known to play a key role in systemic stress response. Previously we found evidence that corticosteroid-binding globulin (CBG) is involved in rapid, membrane-mediated actions of adrenal steroids. Here we studied qualitatively immunostainings for prolactin and CBG in pituitaries of male rats that had been subjected to osmotic challenge. We also examined late pregnant, parturient and early lactating rats, assuming that parturition represents a strong physiological stress. We employed double immunofluorescencent staining of semithin sections and immunoelectron microscopy. In stressed males we found increased prolactin immunofluorescence associated with membranes while in controls this staining was predominantly cytoplasmatic. CBG immunofluorescence was found in almost all prolactin cells of stressed males while such double staining was only occasionally observed in controls. Similar observations were made in females: While parturient rats showed intense membrane associated double staining for both antigens, late pregnant and early lactating animals showed patterns similar to that of male controls. Immunoelectron microscopy revealed increased exocytosis of prolactin containing vesicles in lactating rats. CBG was localized on cell membranes and additionally within prolactin vesicles. Our observations suggest prolactin liberation from pituitary lactotrophs along with CBG upon systemic stress response. Membrane effects of glucocorticoids mediated by CBG may be linked to stimulus secretion of prolactin.

The stressed brain: regional and stress-related corticosterone and stress-regulated gene expression in the adult zebra finch (Taeniopygia guttata)

Glucocorticoids (CORT) are well-known as important regulators of behaviour and cognition at basal levels and under stress. However, the precise mechanisms governing CORT action and functional outcomes of this action in the brain remain unclear, particularly in model systems other than rodents. In the present study, we investigated the dynamics of CORT regulation in the zebra finch, an important model system for vocal learning, neuroplasticity and cognition. We tested the hypothesis that CORT is locally regulated in the zebra finch brain by quantifying regional and stress-related variation in total CORT across brain regions. In addition, we used an ex vivo slice culture system to test whether CORT regulates target gene expression uniquely in discrete regions of the brain. We documented a robust increase in brain CORT across regions after 30 minutes of restraint stress but, interestingly, baseline and stress-induced CORT levels varied between regions. In addition, CORT treatment of brain slice cultures differentially affected expression of three CORT target genes: it up-regulated expression of FKBP5 in most regions and SGK1 in the hypothalamus only, whereas GILZ was unaffected by CORT treatment across all brain regions investigated. The specific mechanisms producing regional variation in CORT and CORT-dependent downstream gene expression remain unknown, although these data provide additional support for the hypothesis that the songbird brain employs regulatory mechanisms that result in precise control over the influence of CORT on glucocorticoid-sensitive neural circuits.

Hypothalamic-Pituitary--Adrenal Axis-Feedback Control

The hypothalamo-pituitary-adrenal axis (HPA) is responsible for stimulation of adrenal corticosteroids in response to stress. Negative feedback control by corticosteroids limits pituitary secretion of corticotropin, ACTH, and hypothalamic secretion of corticotropin-releasing hormone, CRH, and vasopressin, AVP, resulting in regulation of both basal and stress-induced ACTH secretion. The negative feedback effect of corticosteroids occurs by action of corticosteroids at mineralocorticoid receptors (MR) and/or glucocorticoid receptors (GRs) located in multiple sites in the brain and in the pituitary. The mechanisms of negative feedback vary according to the receptor type and location within the brain-hypothalmo-pituitary axis. A very rapid nongenomic action has been demonstrated for GR action on CRH neurons in the hypothalamus, and somewhat slower nongenomic effects are observed in the pituitary or other brain sites mediated by GR and/or MR. Corticosteroids also have genomic actions, including repression of the pro-opiomelanocortin (POMC) gene in the pituitary and CRH and AVP genes in the hypothalamus. The rapid effect inhibits stimulated secretion, but requires a rapidly rising corticosteroid concentration. The more delayed inhibitory effect on stimulated secretion is dependent on the intensity of the stimulus and the magnitude of the corticosteroid feedback signal, but also the neuroanatomical pathways responsible for activating the HPA. The pathways for activation of some stressors may partially bypass hypothalamic feedback sites at the CRH neuron, whereas others may not involve forebrain sites; therefore, some physiological stressors may override or bypass negative feedback, and other psychological stressors may facilitate responses to subsequent stress.

Hypothalamic-pituitary-adrenal axis activity under resting conditions and cardiovascular risk factors in adolescents

BACKGROUND

Activation of the hypothalamic-pituitary-adrenal (HPA) axis has been associated with higher levels of cardiovascular (CVD) risk factors in adults. This study aimed to assess the relation between measures of HPA axis activity under resting conditions and CVD risk factors in a general population of adolescents at 17 years.

METHODS

A total of 1134 adolescents from the Western Australian Pregnancy Cohort (Raine) Study had phenotypic and socio-demographic data. The associations between HPA axis measures (plasma ACTH, total cortisol, calculated free cortisol, corticosteroid binding globulin (CBG), and salivary cortisol) and a range of cardiovascular risk factors were examined using multivariable linear regression models, with adjustment for gender, adiposity, birth weight, gestational age, and socio-behavioural factors.

RESULTS

Plasma total cortisol was positively associated with systolic blood pressure (SBP) (p=0.011), total cholesterol, HDL-cholesterol, and triglycerides (all p<0.001), and hs-CRP (p=0.047). Salivary cortisol was associated positively with HDL-C (p=0.033) and negatively with LDL-cholesterol (p=0.016); plasma calculated free cortisol was positively associated with triglycerides (p=0.006); plasma CBG was positively associated with total cholesterol and HDL-cholesterol (both p<0.001), LDL-cholesterol (p=0.022), and hs-CRP (p=0.001). After correction for multiple comparisons, significant associations remained for total cortisol with total cholesterol, HDL-C, and triglycerides; for calculated free cortisol with triglycerides; and for CBG with HDL-C, total cholesterol, and hs-CRP. Plasma ACTH was not associated with any cardiovascular risk factor. There was no association between BMI and any measure of HPA axis activity.

CONCLUSION

In an adolescent population, HPA axis measures under resting conditions are associated with a range of CVD risk factors. Clarification of the mechanisms underlying these associations in adolescence would be an important step in understanding the evolution of adult CVD.

Adrenal steroids in the brain: role of the intrinsic expression of corticosteroid-binding globulin (CBG) in the stress response

The complex interaction between hypothalamus, pituitary and adrenal glands is a key component of the neuroendocrine stress response. The major stress hormones--glucocorticoids--have both central and peripheral effects. Among the factors regulating their availability to target tissues are levels of corticosteroid-binding globulin, as the major transport protein for glucocorticoids in systemic circulation. Our recent findings demonstrated expression of corticosteroid-binding globulin in various brain regions and in different cell populations (neurons and glial cells). We showed at the cellular level the presence of corticosteroid-binding globulin in the human hypothalamus, where it was co-localized with the classical neurohypophyseal neurohormones--vasopressin and oxytocin. For the first time we demonstrated in mouse that the same gene encodes brain and liver corticosteroid-binding globulin. The full-length sequencing of hypothalamic corticosteroid-binding globulin revealed a full homology with liver corticosteroid-binding globulin cDNA. Thus, we confirmed that corticosteroid-binding globulin mRNA is produced locally within various cerebral regions and thus not transported from blood. However, the amounts of mRNA encoding corticosteroid-binding globulin are in liver about 200 times higher than in brain. The wide distribution of corticosteroid-binding globulin, distinct from the localization of glucocorticoid receptors, observed in our comparative study in rodents, led us to propose two possibilities: (1) corticosteroid-binding globulin is made in certain neurons to deliver glucocorticoids into the cell and within the cell in the absence of cytoplasmic glucocorticoid receptors or (2) is internalized into neurons specifically to deliver glucocorticoids to classical glucocorticoid receptors. Brain corticosteroid-binding globulin may be involved in the response to changing systemic glucocorticoid levels either additionally to known nuclear and membrane corticosteroid receptors or in glucocorticoid responsive brain regions devoid of these receptors. Clearly the multiple locations of corticosteroid-binding globulin within the central nervous system of humans and rodents imply multiple functional properties in normal and/or pathological conditions, which are yet to be determined. Most likely, the importance of brain corticosteroid-binding globulin exceeds the function of a mere steroid transporter.

Corticosteroid-binding globulin contributes to the neuroendocrine phenotype of mice selected for extremes in stress reactivity

Increasing evidence indicates an important role of steroid-binding proteins in endocrine functions, including hypothalamic-pituitary-adrenal (HPA) axis activity and regulation, as they influence bioavailability, local delivery, and cellular signal transduction of steroid hormones. In the plasma, glucocorticoids (GCs) are mainly bound to the corticosteroid-binding globulin (CBG) and to a lesser extend to albumin. Plasma CBG levels are therefore involved in the adaptive stress response, as they determine the concentration of free, biologically active GCs. In this study, we investigated whether male mice with a genetic predisposition for high-reactivity (HR), intermediate-reactivity (IR), or low-reactivity (LR) stress-induced corticosterone (CORT) secretion present different levels of free CORT and CORT-binding proteins, basally and in response to stressors of different intensity. Our results suggest a fine control interaction between plasma CBG expression and stress-induced CORT release. Although plasma CBG levels, and therefore CBG binding capacity, were higher in HR animals, CORT secretion overloaded the CBG buffering function in response to stressors, resulting in clearly higher free CORT levels in HR compared with IR and LR mice (HR>IR>LR), resembling the pattern of total CORT increase in all three lines. Both stressors, restraint or forced swimming, did not evoke fast CBG release from the liver into the bloodstream and therefore CBG binding capacity was not altered in our three mouse lines. Thus, we confirm CBG functions in maintaining a dynamic equilibrium between CBG-bound and unbound CORT, but could not verify its role in delaying the rise of plasma free CORT immediately after stress exposure.

A role for corticosteroid-binding globulin variants in stress-related disorders

Primary stress-related diseases such as chronic fatigue syndrome, fibromyalgia or chronic widespread pain have been associated with altered activity of the hypothalamic-pituitary-adrenal axis due to measured relative hyper- or hypo-cortisolism in basal or experimentally stimulated states. A hereditary risk to development of these diseases has been proposed. Corticosteroid-binding globulin (CBG), a plasma transport vehicle for cortisol, may play a more active role in the hypothalamic-pituitary-adrenal axis. Chronically altered hypothalamic-pituitary-adrenal axis has been associated with common medical problems. Hypocortisolism has been observed in kindred studies of rare mutations of the SERPIN A6 (CBG) gene and more common SERPIN A6 polymorphisms associated with reduced CBG levels or CBG:cortisol-binding affinity. Over the last decade, studies of five different CBG gene mutations in humans, human genetic associations and transgenic mouse models have suggested that CBG may have hitherto unexpected roles in modulation of the stress response. Naturally occurring CBG variants may alter susceptibility to disorders associated with chronic stress and relative hypocortisolism. On the other hand, hypercortisolism has been linked with Cushing's disease and metabolic syndrome and CBG gene polymorphisms have been linked to obesity in animal models. In this article, we look at the evidence suggesting a role for CBG in stress-related disorders, focusing particularly on CBG gene polymorphisms and chronic pain/fatigue syndromes.

Intrinsic expression of transcortin in neural cells of the mouse brain: a histochemical and molecular study

Corticosteroid binding globulin (CBG, transcortin) has been shown to be expressed in the brain of rat and human species. In this study we examined the CBG brain expression and cDNA structure in mice, comparing wild-type (Cbg+/+) and Cbg knockout mice (Cbg-/-, obtained by genetic disruption of the SerpinA6 alias Cbg gene). We used double immunofluorescence labelling with specific neuronal and glial markers to analyze the cellular localization of CBG in various regions of the mouse brain. In wild-type (Cbg+/+) mice we found CBG immunoreactivity in neuronal perikarya of the magnocellular hypothalamic nuclei, amygdala, hippocampus, cerebral cortex, cerebellum and pituitary. A portion of glial cells (astrocytes, oligodendrocytes) contained CBG immunoreactivity, including some of the ependymal cells and choroid plexus cells. No CBG immunoreactivity was detected in Cbg-/- brain tissues. We showed by RT-PCR that the full-length Cbg mRNA is present in those regions, indicating an intrinsic expression of the steroid-binding globulin. Furthermore, we found by sequencing analysis that Cbg cDNA obtained from the mouse hypothalamus was homologous to Cbg cDNA obtained from the liver. Finally, we have evaluated the relative levels of CBG expression by quantitative PCR in various brain regions and in the liver. We found that brain levels of Cbg mRNA are low compared to the liver but significantly higher than in CBG-deficient mice. Although derived from the same gene than liver CBG, brain CBG protein may play a specific or complementary role that requires the production and analysis of brain-specific Cbg knockout models.

Expression of corticosteroid binding globulin in the rat central nervous system

Immunoreactivity for corticosteroid binding globulin was observed in the hypothalamus of intact male rats in the magnocellular nuclei and in single neurons in the periventricular nucleus and the lateral hypothalamus. The suprachiasmatic and the arcuate nuclei contained parvocellular neurons with specific immunoreactivity. Extensive networks of immunopositive fibers were observed in the lateral hypothalamus, the preoptic region, the bed nucleus of the stria terminalis and along the third ventricle. Immunostained axons often exhibited varicosities. The internal and the external layer of the median eminence showed numerous bundles of immunostained axons. Herring bodies in the posterior pituitary lobe contained specific immunoreactivity while pituicytes remained unstained. A portion of the Purkinje cells in the cerebellum and mossy fibers in the cerebellar granular layer stained for corticosteroid binding globulin. Some of the pyramidal cells in the hippocampus were corticosteroid binding globulin positive. Immunostained fibers occurred in the mesencephalon in the periaqueductal grey and in the medulla oblongata. A small fraction of the ependymal cells was also stained. In the spinal cord we observed specific immunoreactivity in a portion of the neurons in the dorsal horn. With polymerase chain reaction we confirmed the presence of the respective transcripts in the different brain regions. The multiple locations of corticosteroid binding globulin throughout the central nervous system suggest multiple functional properties, including neuroendocrine and neurohumoral functions.

Distinct specificity for corticosteroid binding sites in amphibian cytosol, neuronal membranes, and plasma

To address mechanisms of corticosteroid action, one needs tools for distinguishing between the major classes of corticosteroid binding sites: neuronal membrane-associated receptors, intracellular ligand-activated transcription factors, and corticosteroid binding globulins (CBG) in plasma. We characterized the binding parameters for three classes of binding sites in an amphibian, Ambystoma tigrinum, and found that each class had a distinct pharmacological specificity. Equilibrium saturation and kinetic experiments indicated that [3H]corticosterone binds to neuronal membranes with high affinity (Kd approximately 0.37 nM). Aldosterone and two synthetic ligands for mammalian intracellular receptors, dexamethasone and RU486, displayed low affinity for brain membrane sites. In cytosol prepared from brain and liver, [3H]corticosterone bound to a single class of receptors with high affinity (Kd approximately 0.75 and 4.69 nM, respectively) and the rank order potencies for steroid inhibition of [3H]corticosterone binding was RU486 > dexamethasone approximately = corticosterone > aldosterone. In kidney and skin cytosol, [3H]corticosterone binding was best fit with a model having a high-affinity and a lower-affinity site; these sites are not consistent with the pharmacology of mammalian Type I (MR) and Type II (GR) receptors. [3H]Corticosterone also bound to presumed CBG in plasma with high affinity (Kd approximately 2.7 nM), but dexamethasone and androgens bound to plasma CBG with equivalently high affinity. These data demonstrate that pharmacological specificity can be a useful tool for distinguishing corticosteroid binding to different classes of binding sites. These data also indicate that there may be marked species differences in the specificity of corticosteroid binding sites.

Long-term changes in mineralocorticoid and glucocorticoid receptor occupancy following exposure to an acute stressor

Stressors produce rapid activation of the hypothalamic-pituitary-adrenal axis, which typically resolves within 60-90 min following termination of the stressor. In addition, some stressors such as inescapable tailshock (IS) also produce elevated basal levels of corticosterone (CORT), and reduced serum levels of corticosteroid binding globulin (CBG). The elevated basal levels of CORT produced by IS are only observed at the trough of the circadian rhythm of CORT secretion, and are sustained for 2-3 days following stressor termination. The goal of the following experiments was to determine the extent to which the elevated basal levels of CORT observed following IS exposure produced greater corticosteroid receptor occupancy in the brain and pituitary. To do so, rats (n=8-10 per group) received either sham or bilateral adrenalectomy (with CORT replacement in their drinking water; 25 microg/ml) and were given 3 days to recover. Rats were then exposed to 100 ISs (1.6 mA, 5 s each) administered on a 60 s variable intertrial interval, or remained in their home cages. As seen previously, IS produced an increase in basal CORT (5 microg/dl) and a decrease in CBG (30% decrease). Rats were sacrificed 24 h following IS for trunk blood samples and brain dissections. IS exposure had very little effect on corticosteroid receptor protein expression as determined by mineralocorticoid receptor (MR) and glucocorticoid receptor (GR) binding levels in ADX rats. In addition, no changes in whole cell GR levels (as detected by Western blot) were observed in sham rats exposed to IS. On the other hand, IS exposure led to greater occupancy of MR (ranging from 25%-50%) in hippocampus, hypothalamus, pituitary, and posterior cortex. IS also produced greater occupancy of GR (approximately 20%) in hypothalamus and posterior cortex. These long-term changes in corticosteroid receptor activation, evident 24 h after IS exposure, may be responsible for some of the long-term neural, behavioral and immune changes observed following this acute stress procedure.

Regulation of corticosteroid-binding globulin synthesis by 1alpha,25-dihyroxy-vitamin D3 (calcitriol), 9-cis-retinoic acid and triiodothyronine in cultured rat fetal hepatocytes

Evidence regarding the nature of the regulatory factors which directly act upon liver cells and extra-hepatic tissues to alter CBG synthesis is scarce. The present study used cultured rat fetal hepatocytes to investigate the involvement and possible interplay in this process of several members of the nuclear receptors superfamily: vitamin D (VDR), retinoic acids (RAR/RXR) and thyroid hormones (TR). Treatment of cells with 1alpha,25-(OH)2D3 (1,25-D) elicited a dose-dependent inhibition of basal CBG concentration in culture medium. Maximum inhibition to about 15% of control level was achieved with 0.1-1.0 nM, with an IC50 of 3.8 x 10(-12) M and with no significant change in binding affinity. Differential activation of RAR and RXR with either 9-cis-retinoic acid (9-cis-RA) or the RAR-selective synthetic retinoid TTNPB revealed that high doses of both drugs diminished CBG expression, though the former proved about 10-times more potent than the latter in this regard. Amplification by triiodothyronine (T3) of CBG synthesis failed to block the inhibitory effects of either 1,25-D or retinoids, as revealed by both binding capacity and mRNA measurements. Relative to CBG, 1,25-D similarly depressed the synthesis of alpha-fetoprotein (AFP), while on the contrary, retinoids and T3 were shown to cause opposite effects, as 9-cis-RA and TTNPB elevated and T3 decreased AFP expression. The present findings identify for the first time ligands of VDR and RAR/RXR as powerful negative regulators of both basal and T3-stimulated CBG biosynthesis in fetal hepatocytes and suggest lack of a functional interplay between TR and VR or RAR/RXR in these processes.

Synthesis, tissue distribution in rats and PET studies in baboon brain of no-carrier-added [18F]RU 52461: in vivo evaluation as a brain glucocorticoid receptor radioligand

11,17 beta-Dihydroxy-6-methyl-17 alpha-(3-[18F]fluoro-prop-1- ynyl)androsta-1,4,6-trien-3-one [( 18F]RU 52461), an 18F-analog of RU 28362, was synthesized by bromide displacement with [18F]fluoride in 12-30% overall radiochemical yield (decay-corrected) within 140 min from end of bombardment (EOB). The specific activity was 900-1500 mCi/mumol (33.3-55.5 GBq/mumol) at the end of synthesis (EOS). Biodistribution studies indicated high adrenal and pituitary retention, and uniformly low uptake of [18F]RU 52461 in all other brain regions of the rat. Except for the pituitary, no specific receptor-mediated uptake of [18F]RU 52461 could be demonstrated using saturating doses of unlabeled RU 52461 in rat brain. While no change was observed throughout the brain areas in adrenalectomized rats and in animals coinjected with dexamethasone, when compared to controls. PET studies revealed extremely low levels of radioactivity in baboon brain. Therefore, [18F]RU 52461 does not appear to cross the blood-brain barrier, suggesting that this radiopharmaceutical is not suitable to visualize the brain glucocorticoid binding sites by PET.

Corticosterone regulation of type I and type II adrenal steroid receptors in brain, pituitary, and immune tissue

Type I and Type II adrenal steroid receptor levels were compared in the brain, pituitary and immune system of adrenalectomized rats in the presence or absence of several replacement doses of corticosterone. Six days of adrenalectomy produced an up-regulation of Type II adrenal steroid receptors in the brain and spleen. The lowest replacement dose of corticosterone (equivalent to resting levels of this hormone) blocked this Type II receptor up-regulation, while higher replacement doses of corticosterone were associated with widespread Type I and Type II adrenal steroid receptor down-regulation. However, the dose of corticosterone required for receptor down-regulation varied between tissues. Specifically, hippocampal receptors were most sensitive to corticosterone, whereas pituitary receptors were the least sensitive. All tissues examined, except the pituitary, exhibited a down-regulation of Type II receptors with a high corticosterone replacement dose which approximated acute stress levels of this hormone. In summary, physiologically relevant concentrations of corticosterone were capable of down-regulating Type I and Type II adrenal steroid receptors in multiple brain areas and peripheral immune tissues, including peripheral blood mononuclear cells. In contrast, adrenal steroid receptor levels in the pituitary were relatively insensitive to regulation by corticosterone.

Corticosteroids and the brain

Mineralocorticoid (MR) and glucocorticoid receptors (GR) are expressed in the central nervous system. Radioligand binding studies, autoradiography, immunocytochemistry and in situ hybridization have shown that MR and GR are found in abundance in neurons of the limbic system (hippocampus), a structure involved in mood, affect and subtle control of the hypothalamic-pituitary-adrenal (HPA) axis. In the hippocampus MR binds corticosterone (CORT) as well as aldosterone (ALDO) with high affinity. MR seems mainly occupied by CORT in the face of its 2-3 order higher circulating concentration. GR binds CORT with a 6-10-fold lower affinity. MR and GR gene expression, as well as the native receptor proteins, seem to be controlled in a coordinative manner. When GR is down-regulated by excess homologous steroid, MR appears to be increased. Down regulation of MR reduces GR as well. MR and GR display a differential ontogenetic pattern. Ontogeny, particularly that of GR, can be permanently influenced when animals are exposed during the first post-natal week of maternal deprivation, handling, CORT or ACTH1-24 injections. These MR and GR changes persist into senescence and have been proposed to result in altered CORT responsiveness, stress regulation, behavioural adaptation and brain aging.

Estrophilic 3 alpha,3 beta,17 beta,20 alpha-hydroxysteroid dehydrogenase from rabbit liver--I. Isolation and purification

A procedure for isolation of a highly-purified estrophilic hydroxysteroid dehydrogenase (EHSD) from rabbit liver, including ammonium sulphate fractionation, gel filtration, ion-exchange and affinity chromatography on estradiol-Sepharose, has been developed. The enzyme possesses NADP-dependent 3 alpha,3 beta,17 beta,20 alpha-HSD activities with a wide spectrum of androgenic, progestagenic, and estrogenic substrates. EHSD is a monomeric protein whose molecular mass determined by different methods is 35,000-39,000. The protein exhibits microheterogeneity due to the differences in molecular surface charge. The catalytic and hormone-binding properties and molecular sizes of the two protein fractions obtained by chromatography on DEAE-Toyopearl are close or identical. The enzymatic activity of EHSD is minor as compared to other HSDs from rabbit liver. However, the low values of Km, the high affinity for steroid ligands, and high tissue levels of EHSD suggest the protein to play a role in the biodynamics of sex hormones.

Central action of adrenal steroids during stress and adaptation

Corticosteroids interact with receptors in the central nervous system. These receptors display heterogeneity and can be distinguished as corticosterone- and aldosterone-binding mineralocorticoid receptors and dexamethasone-binding glucocorticoid receptors. Ligand specificity of mineralocorticoid receptors for either corticosterone or aldosterone seems to be determined by co-localized transcortin and the enzyme, 11 beta-hydroxysteroid dehydrogenase. Aldosterone-selective mineralocorticoid receptors appear to be present in the circumventricular organs and the AV3V region of the hypothalamus and mediate behavior that is driven by salt appetite. Highest concentrations of mineralocorticoid receptors are found in neurons of the hippocampus. These limbic mineralocorticoid receptor sites mediate tonic influences of corticosterone on brain processes. Glucocorticoid receptors bind corticosterone with a tenfold lower affinity than do mineralocorticoid receptors, and are widely distributed in neuronal and glial cells of the brain. Glucocorticoid receptors are involved in the termination of the stress response (negative feedback). Studies involving measurement of glucocorticoid receptor mRNA and binding sites have revealed that glucocorticoid receptors are subject to autoregulation. After ADX, glucocorticoid receptor concentration increases, but is reduced after chronic stress, chronic administration of glucocorticoids, and at senescence. A diminished glucocorticoid receptor concentration may compromise the negative feedback action exerted by glucocorticoids after stress. After ADX, mineralocorticoid receptor binding is acutely up-regulated and reaches its maximum between 7 and 24 hours post-ADX. Mineralocorticoid receptor mRNA level shows a transient increase following ADX. Long-term ADX has no effect on the mineralocorticoid receptor concentration, but, interestingly, chronic dexamethasone treatment results in an up-regulation of mineralocorticoid receptors. Mineralocorticoid receptor level is decreased at senescence, but this age-related decrement can be reversed by chronic treatment with the ACTH4-9 analog, ORG 2766. Functionally, mineralocorticoid receptors and glucocorticoid receptors are involved in different aspects of the organization of the stress response, and in conjunction they control the stress responsiveness of the animal.

Feedback action and tonic influence of corticosteroids on brain function: a concept arising from the heterogeneity of brain receptor systems

Two types of corticosteroid receptors can be distinguished in rat brain. The type 1 receptor resembles the kidney mineralocorticoid receptor and has two functional expressions in brain, i.e. type 1 corticosterone (CORT) preferring sites (CR) and mineralocorticoid receptors (MR). The type 2 receptor is similar to the liver glucocorticoid receptor (GR). CORT binds to both CR and GR. The localization, binding specificity, and capacity of the receptor systems have served as criteria to evaluate steroid dependent events in brain biochemistry and behaviour. The GR is widely distributed in neurons and glial cells, with the highest density in frontal brain regions. The GR becomes occupied concomitant with rising plasma CORT levels after stress and as part of the circadian rhythm. The GR mediates the feedback action of CORT on stress-activated brain processes. The CR has its predominant localization in neurons of the septo-hippocampal complex and has a ten-fold higher affinity for CORT than that of the GR. The CR is, at all times of intact adrenocortical secretion, 90% or more occupied by endogenous hormone. The CR mediates a tonic influence exerted with stringent specificity by CORT on hippocampus-associated functions, e.g. cognition, mood, and affect. CORT, via the CR, thus contributes to hippocampus function in interpretation of sensory information, leading to appropriate neuroendocrine and behavioural responses, which are themselves subsequently subject to feedback action via the GR. The MR mediates the mineralocorticoid effect on salt and water balance and its behavioural corollary of salt appetite. The anatomical localization of the MR system is as yet ill-defined, although functional studies suggest circumventricular organs as mineralocorticoid target sites. The CR and the MR have in common the high affinity for mineralocorticoids, but the CR is defined by its exclusive responsiveness to CORT as its agonist. The CR and MR probably represent the same chemical receptor modality (type 1), which is expressed differentially depending on the presence of extravascular corticosteroid binding globulin (CBG) in the vicinity of the receptor. GR capacity is subject to autoregulation. Chronic stress, senescence, and chronic CORT administration reduce GR number, with, as a consequence, a less efficient feedback signal. The CR number seems not to be under the control of corticosteroids, probably since the receptor sites are extensively occupied by endogenous hormones. The CR number displays a circadian rhythm and is reduced during senescence.(ABSTRACT TRUNCATED AT 400 WORDS)

Function and plasticity of brain corticosteroid receptor systems: action of neuropeptides

Two types of corticosteroid receptors may be distinguished in rat brain. Type 1 resembles the kidney mineralocorticoid receptor and Type 2 is similar to the liver glucocorticoid receptor (GR). Type 1 receptor system displays two functional expressions, i.e. Type 1 corticosterone (CORT)-preferring sites (CR) and Type 1 mineralocorticoid receptors (MR). MR occurs in circumventricular organs and mediates behaviors such as salt appetite. CR has its principal localization in neurons of the hippocampus, and mediates tonic influences of CORT on hippocampus-associated functions. CR responds with stringent specificity to CORT. Differentiation between CR and MR is due to a different accessibility of the receptor by CORT and ALDO, which seems dependent on the presence of extravascular corticosteroid binding globulin (CBG). GR has a wide distribution in brain, occurs in neurons and glial cells and has its principal localization in such regions as the paraventricular nucleus and the n. tractus solitarii (site of CRF synthesis and of blood pressure regulation, respectively). GR mediates the feedback action of CORT on stress-activated brain processes. GR is subject to autoregulation by CORT. Chronic stress, senescence, and chronic CORT administration reduce receptor number, while GR capacity is increased after adrenalectomy. Reduced GR receptor number results in a less-efficient feedback action. The CORT signal via CR probably can only be altered via changes in CR density evoked rather by neural factors than by autoregulation. CR density is reduced at senescence, but is increased to receptor number of young control animals after chronic treatment with a behaviorally potent ACTH-(4-9) analogue, Org 2766. CR plasticity is prominent for the hippocampus, which is a structure involved in cognition, emotional state and subtle regulation of pituitary-adrenal function.

Anatomical resolution of two types of corticosterone receptor sites in rat brain with in vitro autoradiography and computerized image analysis

The rat brain contains two receptor systems for corticosterone (CORT): the glucocorticoid (GR) and corticosterone or mineralocorticoid-like (CR) receptor sites. We have studied the localization of these receptors by in vitro autoradiography and by in vitro cytosol binding assays in microdissected brain areas. In vitro autoradiography revealed that CR receptor sites are almost entirely restricted to the septal-hippocampal complex, whereas the presence of GR extends throughout the brain. Highest levels of GR are present in the lateral septum, hippocampal, cortical and thalamic regions and the paraventricular nucleus. In vitro determination of binding of 3H-labelled steroids to CR and GR in cytosol of "punched out" brain tissue revealed a similar neuroanatomical distribution as observed with the autoradiographic analysis. In addition, it was found that CORT binds to CR (KD approximately 0.5 nM) with 5-10-fold higher affinity than to GR (KD approximately 2.5-5 nM).

Relative binding affinity of steroids for the corticosterone receptor system in rat hippocampus

In cytosol of the hippocampus corticosterone displays highest affinity for the sites that remain available for binding in the presence of excess RU 26988, which is shown to be a "pure" glucocorticoid. A rather high affinity (greater than or equal to 25%) was found for 11 beta-hydroxyprogesterone, 21-hydroxyprogesterone, 5 alpha-corticosterone, 19-nor-deoxycorticosterone, 11-deoxycorticosterone and cortisol. A moderate affinity (greater than 5% and less than 25%) was displayed by about 14 steroids among which progesterone, aldosterone, 9 alpha-fluorocortisol and dexamethasone. Corticosterone also shows highest affinity to plasma transcortin and thymus cytosol in the presence of RU 26988. However, the rank-order in affinity by the competing steroids was distinctly different from that observed in the hippocampus; cf. aldosterone and dexamethasone displaced [3H]corticosterone from sites unoccupied by RU 26988 in the hippocampus but not from transcortin or sites in thymus cytosol. In thymus cytosol some potent glucocorticoids have higher affinity for the [3H]dexamethasone labeled sites than dexamethasone. The binding of [3H]dexamethasone in thymus cytosol is completely abolished in the presence of a 100-fold excess of RU 26988. We conclude that our data support the evidence for RU 26988 as a selective ligand for glucocorticoid receptors. RU 26988 leaves binding sites available with highest affinity for corticosterone in hippocampus cytosol that are distinct from transcortin-like sites as found in thymus cytosol or from plasma transcortin.