Effects of thyroid hormones and aldosterone on mineralocorticoid binding sites in the toad bladder

Summary In the urinary bladder of the toadBufo marinus triiodothyronine selectively inhibits the late effect of aldosterone on Na⁺ transport. We have investigated whether T₃ might mediate its antimineralocorticoid action by controlling: i) the level of aldosterone binding sites in the soluble (cytosolic) pool isolated from tissues treated with T₃ (60nm) for up to 20 hr of incubation; ii) the kinetics of uptake of³H-aldosterone into cytoplasmic and nuclear fractions after 2 or 20 hr of exposure to T₃. The number and the affinity of Type I (high affinity, low capacity) and Type II (low affinity, high capacity) cytosolic binding sites (measured at 0°C) did not vary significantly after 18 hr of exposure to T₃, while aldosterone-dependent Na⁺ transport was significantly inhibited. In addition, T₃ did not modify the kinetics of uptake (90 min) of³H-aldosterone into cytoplasmic and nuclear fractions of toad bladder incubatedin vitro at 25°C. By contrast, aldosterone itself was able to down-regulate its cytosolic and nuclear binding sites after an 18-hr exposure to the steroid hormone (10 or 80nm). T₃ slightly (20%) but significantly potentiated the down regulation of nuclear binding sites. In conclusion, T₃ does not appear to have major effects on the regulation of the aldosterone receptor, which could explain in a simple manner its antimineralocorticoid action.     

Effect of adrenalectomy and aldosterone on the modulation of mineralocorticoid receptors in rat kidney

Adrenalectomized rat kidney is commonly used for the study of mineralocorticoid mechanism of action in mammals. In this model, aldosterone is known to bind to two classes of binding sites: type I (mineralocorticoid) and type II (glucocorticoid). The study of the aldosterone binding in normal rat kidney requires the elimination of endogenous hormones bound to each type of receptor. Thus, a suitable technique was developed using in situ perfusion of the kidneys. The efficacy of this method was of about 85 to 90% at the level of both cytoplasm and nucleus. Aldosterone binding capacity was checked in normal rat kidney after in situ perfusion and was found to be 300 to 500% lower than in adrenalectomized rat kidney, both in cytoplasm and nuclei. Computer analysis of aldosterone binding parameters in the cytoplasm (30,000 X g supernatant) of rat kidney suggested that adrenalectomy might induce an important rise in the number of mineralocorticoid receptors (congruent to 260%). An increase in the number of glucocorticoid receptors was also observed but appeared to be lower. Aldosterone, when perfused during 24 h in adrenalectomized rats, lowered the number of type I sites to the same level as observed in normal rat kidney. This effect was fully reversible after interruption of aldosterone perfusion. These results suggested an aldosterone-induced down regulation of mineralocorticoid receptors.     

Specific hydroxylations determine selective corticosteroid recognition by human glucocorticoid and mineralocorticoid receptors

The ligand binding domains of the human mineralocorticoid receptor (hMR) and glucocorticoid receptor (hGR) display a high sequence homology. Aldosterone and cortisol, the major mineralocorticoid and glucocorticoid hormones, are very closely related, leading to the cross-binding of these hormones to both receptors. The present study reports on the mechanism by which hMR and hGR are activated preferentially by their cognate hormones. We found that the ability of corticosteroids to stimulate the receptor's transactivation function is depending on the stability of the steroid-receptor complexes. In the light of a hMR structural model we propose that contacts through the corticosteroid C21 hydroxyl group are sufficient to stabilize hMR but not hGR and that additional contacts through the C11- and C17-hydroxyl groups are required for hGR.     

Corticosteroid hormones and the brain]

The anatomical and functional links between the hormone stress axis and the cortico-limbic brain regions which integrate emotion and motivation are well documented. It is important, considering the consequences of stress on the brain, to take into account the regulatory buffer capacities of the personality-cognitive processes. Another point of interest is evaluation of the long term effects of repeated life events on chronic environmental pressures which induce brain negative feedback defects and, subsequently, insidious cellular changes in regions such as the hippocampus that lead to memory or adaptive impairments. An example is provided by perinatal stress that induces, later in life, both hormonal and cognitive deleterious changes.     

Corticosteroid receptors in the avian kidney

The binding $\text{in vitro}$ of tritiated aldosterone to domestic duck ($\text{Anas platyrhynchos}$) kidney tissue has been investigated. Using tissue from animals on a normal diet, tritiated aldosterone was specifically bound to kidney cytosol with an apparent equilibrium dissociation constant of about 9 nM and number of binding sites in the 20 fmol/mg protein range. These values did not show statistically significant changes when the cytosol originated from animals with salt activated nasal glands. Kidney cytosols labeled with tritiated aldosterone sedimented with a single peak at 8S in a linear sucrose gradient (10–30%) and this peak was quenched by excess, radioinert aldosterone. Following incubation of labeled cytosols with crude nuclei, the cytosols became depleted of the label and aldosterone was translocated to the Tris-soluble and Tris-insoluble, 0,4 M KCl soluble nuclear fractions. Kidney cytosols metabolized aldosterone extensively to a compound presumed to be 3α,5β-tetrahydroaldosterone. However, only unchanged aldosterone became receptor-bound. It was concluded that the duck kidney possesses aldosterone receptors, though competition studies indicated that the specificity of these receptors might be different from those described in the mammalian kidney.     

Regulation of mineralocorticoid secretion by the superfused fetal monkey adrenal gland: lack of stimulation of aldosterone by ACTH

The rhesus monkey fetal adrenal secretion of mineralocorticoids was studied in vitro. Superfusion of fetal adrenal minces (n = 6) demonstrated that the fetal adrenal secretes aldosterone as well as desoxycorticosterone, 18 hydroxydesoxy corticosterone, and 18 hydroxycorticosterone. Addition of 250 ng/ml ACTH to the superfusion medium did not result in stimulation of aldosterone, but did increase these other mineralocorticoids. These data indicate that aldosterone production is not readily stimulated by ACTH in the fetal rhesus monkey, although other steroids in the mineralocorticoid pathway are.     

Neurobiological mechanisms involved in recognition of olfactory signature of the young in sheep

The following review focuses on neurobiological mechanisms responsible for the individual recognition of the olfactory signature of the young by the ewe at parturition. Steroids and vaginocervical stimulation are responsible for neurochemical and electrophysiological changes within the olfactory bulb that are part of the learning mechanisms of the individual lamb odour, thus allowing the establishment of a selective bond between the ewe and her lamb. There is an increase in the number of mitral cells, the principal cells of the olfactory bulb that respond to lamb odours, which is associated with increased release of glutamate and gamma-aminobutyric acid from the dendrodendritic synapses between the mitral and granule cells. The relation between the release of the two transmitters after birth suggests an increased efficacy of glutamate evoked gamma-aminobutyric acid release. Parturition is also accompanied by increased oxytocinergic, cholinergic and noradrenergic neurotransmitter release that are essential for selective recognition of lambs. These increases in transmitter release depend on maternal experience, so that greater amounts have been found in multiparous than primiparous ewes. Therefore maternal experience seems to induce a neural maturation process that facilitates effective transmitter release in the olfactory bulb.     

Elevated cardiac tissue level of aldosterone and mineralocorticoid receptor in diastolic heart failure: Beneficial effects of mineralocorticoid receptor blocker

Cardiac aldosterone levels have not been evaluated in diastolic heart failure (DHF), and its roles in this type of heart failure remain unclear. This study aimed to detect cardiac aldosterone by use of a liquid chromatographic-mass spectrometric method and to assess the effects of mineralocorticoid receptor blockade on hypertensive DHF. Dahl salt-sensitive rats fed 8% NaCl diet from 7 wk (hypertensive DHF model) were divided at 13 wk into three groups: those treated with subdepressor doses of eplerenone (12.5 or 40 mg x kg(-1) x day(-1)) and an untreated group. Dahl salt-sensitive rats fed 0.3% NaCl diet served as controls. Cardiac aldosterone was detected in the DHF rats but not in the control rats, with increased ventricular levels of mineralocorticoid receptor. Cardiac levels of 11-deoxycorticosterone, corticosterone, and 11-dehydrocorticosterone were not different between the control and DHF rats, but the tissue level of corticosterone that has an affinity to mineralocorticoid receptor was 1,000 times as high as that of aldosterone. Aldosterone synthase activity and CYP11B2 mRNA were undetectable in the ventricular tissue of the DHF rats. Administration of eplerenone attenuated ventricular hypertrophy, ventricular fibrosis, myocardial stiffening, and relaxation abnormality, leading to the prevention of overt DHF. In summary, the myocardial aldosterone level increased in the DHF rats. However, its value was extremely low compared with corticosterone, and no evidence for enhancement of intrinsic myocardial aldosterone production was found. The upregulation of mineralocorticoid receptor may play a central role in the pathogenesis of DHF, and blockade of mineralocorticoid receptor is likely an effective therapeutic regimen of DHF.     

Corticosteroid hormones in pathogenetic heterogenety of post-stress depression

The role of corticosteroid hormones in post-stress depression progress and their usage for correction of this psychopathology was studied in active and passive Wistar rats. It was found that only passive individuals had sensitivity to corticosteroids and, among them, only adrenal gland hormones lead to corrections of post-stress depression.     

Chronic treatment with the mineralocorticoid hormone aldosterone results in increased anxiety-like behavior

Aldosterone is the last component of the renin-angiotensin-aldosterone system inducing its peripheral effects via mineralocorticoid receptors (MR). Brain MR bind preferentially glucocorticoids. So far, the role of MR in behavioral functions has been investigated almost exclusively in relation to glucocorticoids. Recently, aldosterone itself has been linked to affective disorders. The aim of this study was to test the hypothesis that chronic elevation of circulating levels of aldosterone leads to increased anxiety. We have investigated the effects of chronic aldosterone treatment on (1) anxiety-like behavior, and (2) basal and stress-induced levels of selected hormones. Forty male Wistar rats were subcutaneously implanted with osmotic minipumps and treated with aldosterone (2 µg/100 g/day) or vehicle for two weeks. Aldosterone concentrations in plasma showed a mild (approximately four-fold) increase at the end of two-week aldosterone treatment. This mild hyperaldosteronism resulted in a significant enhancement of anxiety as demonstrated by alterations in all indicators of anxiety-like behavior measured in the open field and elevated plus-maze tests, without significant changes in measures of general locomotor activity. Aldosterone treatment affected not only the spatiotemporal measures of anxiety, but also the ethological parameters related to exploration and risk assessment. Chronic treatment with aldosterone was associated with increased water intake and decreased plasma renin activity, but failed to modify basal or stress-induced activity of the hypothalamic-pituitary-adrenocortical axis. The results provide evidence on anxiogenic action of prolonged increase in circulating aldosterone concentrations. Thus, aldosterone may represent an important target for future antidepressant and anxiolytic drug development.     

A critical region in the mineralocorticoid receptor for aldosterone binding and activation by cortisol: evidence for a common mechanism governing ligand binding specificity in steroid hormone receptors

The amino acids that confer aldosterone binding specificity to the mineralocorticoid receptor (MR) remain to be determined. We had previously analyzed a panel of chimeras created between the MR and the glucocorticoid receptor and determined that amino acids 804-874 of the MR ligand binding domain are critical for aldosterone binding. In the present study a further series of chimeras was created within this region. The chimeras were analyzed by a transactivation assay and [(3)H]aldosterone binding, and the critical region was narrowed down to amino acids 820-844. Site-directed mutagenesis was used to create single and multiple amino acid substitutions in this region. These studies identified 12 of the 16 amino acids that differ in the MR and the glucocorticoid receptor in this region as being critical to conferring aldosterone responsivity. The amino acids that differ in the region 820-844 lie on the surface of the molecule and, therefore, it appears that MR ligand binding selectivity is conferred by residues that do not form part of the ligand binding pocket. Other studies have found that the corresponding regions of the androgen and glucocorticoid receptors are critical for the binding of natural and synthetic ligands, suggesting a common mechanism governing ligand binding specificity. The new chimeras also displayed, as previously reported, a dissociation between cortisol binding and transactivation and, intriguingly, only those that bound aldosterone with high affinity were activated by cortisol, suggesting a common mechanism that underlies specificity of aldosterone binding and the ability of cortisol to activate the MR.     

Evolutionary conservation of tissue-specific lymphocyte-endothelial cell recognition mechanisms involved in lymphocyte homing

Tissue-specific interactions with specialized high endothelial venules (HEV) direct the homing of lymphocytes from the blood into peripheral lymph nodes, mucosal lymphoid organs, and tissue sites of chronic inflammation. These interactions have been demonstrated in all mammalian species examined and thus appear highly conserved. To assess the degree of evolutionary divergence in lymphocyte-HEV recognition mechanisms, we have studied the ability of lymphocytes to interact with HEV across species barriers. By using an in vitro assay of lymphocyte binding to HEV in frozen sections of lymphoid tissues, we confirm that mouse, guinea pig, and human lymphocytes bind to xenogeneic as well as homologous HEV. In addition, we show that mouse and human lymphoid cell lines that bind selectively to either peripheral lymph node or mucosal vessels (Peyer's patches, appendix) in homologous lymphoid tissues exhibit the same organ specificity in binding to xenogeneic HEV. Furthermore, monoclonal antibodies that recognize lymphocyte "homing receptors" and block homologous lymphocyte binding to peripheral lymph node or to mucosal HEV, also inhibit lymphocyte interactions with xenogeneic HEV in a tissue-specific fashion. Similarly, anti-HEV antibodies against organ-specific mouse high endothelial cell "addressins" involved in lymphocyte homing to peripheral lymph node or mucosal lymphoid organs, not only block the adhesion of mouse lymphocytes but also of human lymphocytes to target mouse HEV. The results illustrate a remarkable degree of functional conservation of elements mediating these cell-cell recognition events involved in organ-specific lymphocyte homing.