Glucocorticoid receptor regulation: the effects of adrenalectomy, exogenous glucocorticoid, and stress on hepatic receptor number in male and female mice

Trypanosoma cruzi disrupts thymic homeostasis by altering intrathymic and systemic stress-related endocrine circuitries

We have previously shown that experimental infection caused by Trypanosoma cruzi is associated with changes in the hypothalamus-pituitary-adrenal axis. Increased glucocorticoid (GC) levels are believed to be protective against the effects of acute stress during infection but result in depletion of CD4⁺CD8⁺ thymocytes by apoptosis, driving to thymic atrophy. However, very few data are available concerning prolactin (PRL), another stress-related hormone, which seems to be decreased during T. cruzi infection. Considering the immunomodulatory role of PRL upon the effects caused by GC, we investigated if intrathymic cross-talk between GC and PRL receptors (GR and PRLR, respectively) might influence T. cruzi-induced thymic atrophy. Using an acute experimental model, we observed changes in GR/PRLR cross-activation related with the survival of CD4⁺CD8⁺ thymocytes during infection. These alterations were closely related with systemic changes, characterized by a stress hormone imbalance, with progressive GC augmentation simultaneously to PRL reduction. The intrathymic hormone circuitry exhibited an inverse modulation that seemed to counteract the GC-related systemic deleterious effects. During infection, adrenalectomy protected the thymus from the increase in apoptosis ratio without changing PRL levels, whereas an additional inhibition of circulating PRL accelerated the thymic atrophy and led to an increase in corticosterone systemic levels. These results demonstrate that the PRL impairment during infection is not caused by the increase of corticosterone levels, but the opposite seems to occur. Accordingly, metoclopramide (MET)-induced enhancement of PRL secretion protected thymic atrophy in acutely infected animals as well as the abnormal export of immature and potentially autoreactive CD4⁺CD8⁺ thymocytes to the periphery. In conclusion, our findings clearly show that Trypanosoma cruzi subverts mouse thymus homeostasis by altering intrathymic and systemic stress-related endocrine circuitries with major consequences upon the normal process of intrathymic T cell development.

It is currently estimated that 90 million people in America are exposed to T. cruzi infection, the causative agent of Chagas disease. Despite the mortality and morbidity, this infection is yet considered a neglected disease, due to the lack of effective, safe, and affordable pharmaceuticals for controlling it. T. cruzi leads to immunosuppression of the T cell compartment and to chronic cardiac inflammation, which seems to be associated with the disruption of thymic homeostasis. Thymus atrophy, characteristic of acute infection, is mainly associated with the loss of immature CD4⁺CD8⁺ thymocytes, which in turn is associated with increased corticosterone systemic levels, together with their premature export to the periphery as potential autorreactive cells. Although being deleterious to the thymus, GCs are protective during this infection, for avoiding an exacerbated pro-inflammatory response. Here we demonstrate that the increase of GCs in plasma is related to the impairment of PRL systemic levels. The intrathymic hormonal circuitry is also altered during infection and an imbalance of the cross-talk involving GR and PRL is related with CD4⁺CD8⁺ depletion. The partial restoration of PRL levels prevented thymus atrophy of infected mice, thus partially reverting the T. cruzi-induced subversion of the organ, ultimately reestablishing thymus homeostasis.

Synaptic regulation of the hypothalamic-pituitary-adrenal axis and its modulation by glucocorticoids and stress

Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis has been implicated in a range of affective and stress-related disorders. The regulatory systems that control HPA activity are subject to modulation by environmental influences, and stressful life events or circumstances can promote subsequent HPA dysregulation. The brain is a major regulator of the HPA axis, and stress-induced plasticity of the neural circuitry involved in HPA regulation might constitute an etiological link between stress and the development of HPA dysregulation. This review focuses on the synaptic regulation of neuroendocrine corticotropin-releasing hormone (CRH) neurons of the hypothalamic paraventricular nucleus, which are the cells through which the brain predominantly exerts its influence on the HPA axis. CRH neuronal activity is largely orchestrated by three neurotransmitters: GABA, glutamate, and norepinephrine. We discuss our current understanding of the neural circuitry through which these neurotransmitters regulate CRH cell activity, as well as the plastic changes in this circuitry induced by acute and chronic stress and the resultant changes in HPA function.

Synaptic regulation of the hypothalamic-pituitary-adrenal axis and its modulation by glucocorticoids and stress

Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis has been implicated in a range of affective and stress-related disorders. The regulatory systems that control HPA activity are subject to modulation by environmental influences, and stressful life events or circumstances can promote subsequent HPA dysregulation. The brain is a major regulator of the HPA axis, and stress-induced plasticity of the neural circuitry involved in HPA regulation might constitute an etiological link between stress and the development of HPA dysregulation. This review focuses on the synaptic regulation of neuroendocrine corticotropin-releasing hormone (CRH) neurons of the hypothalamic paraventricular nucleus, which are the cells through which the brain predominantly exerts its influence on the HPA axis. CRH neuronal activity is largely orchestrated by three neurotransmitters: GABA, glutamate, and norepinephrine. We discuss our current understanding of the neural circuitry through which these neurotransmitters regulate CRH cell activity, as well as the plastic changes in this circuitry induced by acute and chronic stress and the resultant changes in HPA function.

Estradiol and corticosterone independently impair spatial navigation in the Morris water maze in adult female rats

The independent effects of ovarian and adrenal hormones on spatial place learning were examined in male and female Long-Evans hooded rats. Experimental groups received bilateral ovariectomy (females only) and adrenalectomy (both sexes), followed by hormone administration according to a predetermined schedule. Spatial and reversal training in the Morris water maze were used to measure behavioural performance in locating a hidden platform. General proficiency and strategies use were assessed using search times and time spent in the periphery, respectively. The number of direct and circle swims to the platform was used to assess memory for the location of the hidden platform. Experiment 1 investigated the roles of estradiol and progesterone in spatial navigation in the absence of high levels of adrenal steroids. The female group that received estradiol alone showed longer search times, greater periphery swimming, and fewer direct and circle swims to the target than all other female groups. Experiment 2 investigated the role of corticosterone (CORT) in spatial navigation in the absence of ovarian hormones. Male and female rats that received acute matched doses of exogenous CORT were equally impaired during spatial training. During reversal training, the impairment in search time, periphery swimming, and direct and circle swims persisted in the female CORT group only. Analysis of serum CORT levels in the male and female experimental groups revealed no significant differences. These data suggest that estradiol and CORT can independently impair acquisition of spatial navigation skills and strategies use in adult female rats.

Effect of acute adrenalectomy on rat liver glucocorticoid receptor

In order to improve current clinical treatment of human hypocortisolism, it is necessary to understand molecular aspects of this pathophysiology. In this study liver tissues from male Wistar rats were used as an experimental model to study structural and functional properties of glucocorticoid receptor (GR) in the absence of glucocorticoid hormones (GC). Results show that acute adrenalectomy (ADX) significantly increases the number of GR binding sites and GR protein content. In addition, acute ADX stimulates increase in stability of the GR, decrease in stability of the glucocorticoid- receptor complex (G-R), and changes in accumulation of the G-R complex in nuclei and its cellular distribution. .

Expression and binding activity of the glucocorticoid receptor are upregulated in septic muscle

We examined the influence of sepsis, induced by cecal ligation and puncture in rats, on the protein and gene expression and hormone binding activity of the glucocorticoid receptor (GR) in skeletal muscle. Sepsis resulted in increased GR mRNA and protein levels and upregulated hormone binding activity in extensor digitorum longus and soleus muscles. Scatchard analysis suggested that the increased GR hormone binding activity reflected an increased number of hormone binding sites, whereas receptor affinity for glucocorticoids was unchanged. The GR antagonist RU-38486 blocked the sepsis-induced increase in GR expression and hormone binding activity, implicating a positive regulatory effect of glucocorticoids on GR expression and binding activity under the present experimental conditions. The results suggest that glucocorticoid-dependent metabolic changes in skeletal muscle during sepsis may reflect not only high circulating glucocorticoid levels but increased amounts and hormone binding activity of the GR as well.

Chronic effects of intracerebral ventricular enterostatin in Osborne-Mendel rats fed a high-fat diet

Enterostatin, a pentapeptide found in the procolipase molecule of the pancreas, has been infused chronically into the lateral ventricle of Osborne-Mendel rats for 11 days. Treatment with enterostatin acutely lowered food intake more in the treated animals than in the vehicle-treated controls. Weight gain and serum insulin were significantly reduced by enterostatin. Corticosterone was significantly increased in rats receiving chronic infusions of enterostatin into the lateral ventricle and was also increased following an acute ICV injection of enterostatin. CRH antagonist did not block the acute feeding response to enterostatin in Sprague-Dawley rats. The elevation of corticosterone during chronic infusion of enterostatin reduced the level of hepatic glucocorticoid receptor as measured by Western blot. We conclude that body weight gain of OM rats is reduced by centrally administered enterostatin, and that the acute effects of enterostatin on food intake are not mediated through stimulation of CRH secretion.

Dehydroepiandrosterone regulation of the hepatic glucocorticoid receptor in the Zucker rat. The obesity research program

Dehydroepiandrosterone (DHEA) decreases the activity of hepatic tyrosine aminotransferase (TAT), a glucocorticoid-inducible enzyme, in the obese, hypercorticosteronemic Zucker rat. To investigate the mechanism of this antiglucocorticoid action, the effect of exogenous DHEA on hepatic glucocorticoid receptor (GC) number and affinity was quantitated. Food supplementation with DHEA (0.6% w/w) for 1 or 7 days had no effect on either receptor number or affinity in obese Zucker rats. After 28 days, however, DHEA treatment resulted in a nearly 40% decrease in cytosolic hepatic receptor content (Bmax; fmol/mg cytosolic protein) without any change in affinity (Kd) in both lean and obese rats. DHEA treatment for 28 days also resulted in an increased liver size and cytosolic protein content. When the hepatic GC receptor content was normalized based on the change in liver size and protein content, the apparent number of GC binding sites per liver was not affected by DHEA treatment. This observation suggests that DHEA's effect on GC receptor content may not be a specific action and that downregulation of the GC receptor is not the mechanism of DHEA action on GC induced TAT activity. This is supported by the effect of DHEA on obese rat TAT activity in the same experiment where the greatest inhibition occurred after only 1 day of treatment. From these experiments it is concluded that although long-term DHEA treatment may decrease the relative concentration of GC receptors in rat liver, this change is not the mechanism through which DHEA mediates its acute antiglucocorticoid action.

Liver and brain glucocorticoid receptor in rainbow trout, Oncorhynchus mykiss: down-regulation by dexamethasone

Total glucocorticoid binding sites were identified and quantitated in liver and brain of rainbow trout using an exchange method and [3H]dexamethasone as the ligand. Both tissues contained a predominantly cytosolic moiety that bound dexamethasone with high specificity. Binding was saturable, time dependent, and completely reversible. Scatchard analysis showed a linear relationship suggesting that receptors belong to a single class. Dexamethasone down-regulated both liver and brain receptors. Down-regulation was rapid (within hours) and dose dependent (ED50 = 1.5 mg/kg body weight). Dexamethasone-induced down-regulation was not a result of cytoplasm to nuclei translocation or due to increases in tissue concentrations of steroid. Dexamethasone administration resulted in a lowering of Bmax (82.3 +/- 2.5 to 20.6 +/- 10.5 fmol/mg protein) and an increase in Kd (15.6 +/- 0.2 to 44.3 +/- 5.0 nM) suggesting a conformational change in the receptor molecule as part of the mechanism. The brain and liver of the rainbow trout thus have glucocorticoid receptors similar to those described in the mammalian system. Further, these receptors are subjected to autologous regulation similar to their counterparts in other systems.

Stress-induced changes of glucocorticoid receptor in rat liver

The effect of corticosterone injection and of acute and repeated stress on rat liver cytosol glucocorticoid receptor was studied to ascertain whether corticosterone-induced glucocorticoid receptor (GR) regulation also takes place in intact animals as it does in adrenalectomized ones. Adult male rats were exposed to six different stressors (swimming, 10 mg/kg histamine i.p., 500 mU/kg vasopressin s.c., heat, immobilization and cold) acutely or three times daily for 18 days (repeated stress). Each of the stressors applied acutely provoked a pronounced increase of plasma corticosterone with subsequent induction of hepatic tyrosine aminotransferase activity. Depletion of cytosol receptor was however only noticed after swimming and histamine injection. On the other hand, sustained hypersecretion of corticosterone evoked by repeated stress significantly reduced the number of GR in rat liver cytosol without any change in Kd. It is concluded that in the presence of intact adrenal glands cytosol receptors are more resistant to corticosterone-induced depletion than in their absence. Further, repeated stress causes down-regulation of GR in the liver, most probably by sustained corticosterone secretion, yet the effect of other stress factors cannot be excluded.

Effect of corticosterone on noradrenergic nuclei in the pons-medulla and [3H]NA release from terminals in hippocampal slices

The aim of the present study was to investigate possible membrane and genomic effects of corticosterone on the noradrenergic system of the rat brain. Corticosterone effects were studied in vivo by treating rats s.c. with 10 mg/kg corticosterone for 7 or 14 days. In the first two experiments corticosterone significantly decreased the noradrenaline (NA) and dopamine (DA) levels in the pons-medulla, an area which contains the A1-A7 noradrenergic cell groups, while the NA and DA levels in the dorsal hippocampus remained unchanged. In a third experiment where the locus coeruleus (LC) and the A1 and A2 nuclei (A1,A2) were analysed separately, NA levels were unchanged but total MHPG levels and the total MHPG/NA ratio were decreased in the A1,A2 area. Chronic corticosterone treatment (14 days) did not alter the alpha 2-adrenoceptor-mediated modulation of [3H]NA release from dorsal hippocampal slices. Neither the spontaneous outflow nor the electrically stimulated release of [3H]NA from dorsal hippocampal slices of untreated rats was affected by exposure of the slices to corticosterone (10(-7) M - 10(-4) M) in the superfusion buffer. Thus, chronic corticosterone treatment of rats altered the noradrenergic system of the pons-medulla, but did not change the alpha 2-adrenoceptor-mediated modulation of NA release in the dorsal hippocampus, a major terminal area of the LC neurons. Corticosterone also did not appear to have a direct membrane effect on the NA terminals in the dorsal hippocampus of the rat.

Epidermal dexamethasone receptors in dogs with confirmed hyperadrenocorticalism, hypothyroidism or undiagnosed hormonal alopecia

Low capacity, high affinity [3H] dexamethasone binding receptors were identified in cytosolic preparations of the skin (mean number 42.0 +/- 25.2 fmol mg-1 protein, apparent dissociation constant (1 nM +/- 0.23) of clinically normal dogs. No [3H] dexamethasone binding was observed in the skin of nine out of 10 dogs with confirmed spontaneous hyperadrenocorticism or in the skin of three out of six dogs with undiagnosed hormonal alopecia. A reduction was detected in the number of [3H] dexamethasone binding receptors in the skin of one dog with confirmed hypothyroidism. This study provides evidence for the susceptibility of canine glucocorticoid receptors to down regulation by imbalances of endogenous hormones, particularly increased glucocorticoid concentrations.

Mouse major histocompatibility complex and lung development: haplotype variation, H-2 immunolocalization, and progressive maturation

The association of the mouse major histocompatibility complex (H-2), lung maturation, and corticosteroid responsiveness has recently been demonstrated in congenic B10 (H-2b) and B10.A (H-2a) mice (Hu et al.: American Journal of Medical Genetics 35:126-131, 1990). We have investigated additional haplotypes [B10.BR (H-2k) and B10.D2 (H-2d)] to confirm that there is a strong association between H-2 haplotype variation and the degree of pulmonary maturation. Lungs of B10.D2, B10.BR, B10, and B10.A congenic mice achieve haplotypic specific maturation: B10.D2 lungs greater than B10 lungs = B10.BR lung greater than B10.A lungs. It appears that the expression of these developmental potentials is under corticosteroid regulation. Further, to test the hypothesis that H-2 antigens would be expressed earlier in embryonic lungs with the H-2b (B10) or H-2k (B10.BR) haplotype than with the H-2a (B10.A) haplotype, we investigated the spatiotemporal patterns of H-2 antigen localization in B10, B10.BR, and B10.A congenic mouse strains with and without corticosteroid treatment. The spatial patterns of H-2 antigen localization was similar in the B10, B10.BR, and B10.A mouse lungs; however, these patterns appeared earlier in both untreated and treated B10 and B10.BR mice as compared with untreated B10.A mice, suggesting an H-2 haplotype associated rate of pulmonary maturation. Following corticosteroid treatment, all congenic strains had a temporally comparable spatial distribution of H-2 antigens. Our results provide preliminary evidence suggesting that both a lung "developmental gene(s)" and a "glucocorticoid responsiveness gene(s)" are most likely outside the K-D subregions of the H-2 complex. A model of the H-2 regulation of lung maturation and corticosteroid responsiveness is discussed.

Mechanism of the permissive action of dexamethasone on the glucagon-dependent activation of the phosphoenolpyruvate carboxykinase gene in cultured rat hepatocytes

Rat hepatocytes were cultured for 24 h in the presence or absence of 100 nM dexamethasone (DX). After a medium change, phosphoenolpyruvate carboxykinase (PCK) was induced by addition of glucagon at different concentrations, from physiological 0.1 nM to hyperphysiological 10 nM, again in the presence or absence of 100 nM dexamethasone. 1. With dexamethasone addition during the culture and induction phase (DX+/+), 10 nM glucagon increased PCK mRNA abundance (Northern blot analysis) and activity (in vitro translation) synchronously to the same extent with maxima after 2 h and PCK enzyme activity after a time lag with a maximum after 6 h. The total detectable PCK mRNA amount was apparently also translationally active. 10 microM N6,2'-O-dibutyryladenosine 3',5'-(cyclic)phosphate (Bt2cAMP) as the second messenger had essentially the same effect as 10 nM glucagon. 2. In the absence of dexamethasone during the preculture and the induction phase (DX-/-), 10 nM glucagon and 10 microM Bt2cAMP could enhance PCK mRNA only about half-maximally. Glucagon or dexamethasone added alone in physiological concentrations of 0.1 nM and 100 nM, respectively, were unable to increase PCK mRNA. However, treatment of the cells with dexamethasone also enabled 0.1 nM glucagon to enhance PCK mRNA to a maximum after 2 h, independent of the presence of dexamethasone during the induction period (DX+/+ and DX+/- cells). Thus, dexamethasone was a permissive agent in that it shifted the sensitivity of the cells towards glucagon into the physiological concentration range. 3. In the presence of dexamethasone during the culture and induction phase (DX+/+) 0.1 nM glucagon maximally enhanced the transcription of the PCK gene (nuclear run on) fourfold after 30 min; in the absence of dexamethasone during both phases (DX-/-) glucagon was without any effect. The overall transcriptional rate was not significantly different in cells with and without dexamethasone during the culture and induction phase (DX+/+ vs. DX-/-). Thus, dexamethasone acted permissively mainly on the transcription of the PCK gene. 4. With culture in the presence of dexamethasone over decreasing periods of time, 1 nM glucagon could induce submaximal PCK mRNA amounts already after 1-3 h steroid culture. This restitution by dexamethasone of the PCK mRNA inducibility by glucagon was inhibited by cycloheximide. This suggested that ongoing protein synthesis was required for the permissive action of dexamethasone on the expression of the PCK gene. The results allow the following conclusions.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Depletion of rat liver glucocorticoid receptor hormone-binding and its mRNA in sepsis

Glucocorticoid receptor (GR) hormone-binding activity, its physical characteristics, and GR mRNA levels were studied in the liver, brain and muscle of normal (saline-injected) and hypermetabolic septic rats 24 h after the subcutaneous injections of E. coli. The GR levels (hormone-binding activity) declined by about 40%, 56%, and 40% in septic liver, brain, and muscle cytosol, respectively. The mechanism of the decrease in the GR levels in sepsis was studied in liver. The GR levels remained low (45% of control hormone-binding) even after 48 h of E. coli administration. The decrease in the liver GR occurred in the 9S untransformed GR. The 9S GR from septic liver transformed to the 4S form in proportions comparable to the control liver GR. In addition, the 4S GR from control and septic liver was capable of binding to DNA-cellulose to a similar extent. The GR mRNA level in septic liver declined by about 30%. Thus, a decrease in GR hormone-binding activity in sepsis appears to be due to a decline in the steady-state GR mRNA level and not from a change in the qualitative properties of the GR protein.