Corticosteroid receptors in liver cytosol of the clawed toad, Xenopus laevis: daily and seasonal variations

Estradiol and reproduction in the South American toad Rhinella arenarum (Amphibian, Anura)

Rhinella arenarum is a South American toad with wide geographic distribution. Testes of this toad produce high amount of androgens during the non reproductive season and shift steroid synthesis from androgens to 5α-pregnanedione during the breeding. In addition, plasma estradiol (E₂) in males of this species shows seasonal variations but, since testes of R. arenarum do not express aromatase, the source of plasma E₂ remained unknown for several years. However, the Bidder's organ (BO), a structure located at one pole of each testis, is proposed to be the main source of E₂ in male's toads since it expresses several steroidogenic enzymes and is able to produce E₂ from endogenous substrates throughout the year. In addition, there were significant correlations between plasma E₂ and total activity of BO aromatase, and between plasma E₂ and the amount of hormone produced by the BO in vitro. In the toad, apoptosis induced by in vitro treatment with E₂ was mostly detected in spermatocytes during the breeding and in spermatids during the post-reproductive season, suggesting that this steroid has an important role in controlling spermatogenesis. However, in vitro treatment with E₂ had no effect on proliferation. This evidence suggests that the mechanism of action of E₂ on amphibian spermatogenesis is complex and more studies are necessary to fully understand the role of estrogens regulating the balance between cellular proliferation and apoptosis. In addition, in R. arenarum in vitro studies suggested that E₂ has no effect on CypP450c17 protein levels or enzymatic activity, while it reduces 3β-hydroxysteroid dehydrogenase/isomerase (3β-HSD/I) activity during the post reproductive season. As well, E₂ regulates FSHβ mRNA expression all over the year suggesting a down regulation process carried out by this steroid. The effect on LHβ mRNA is dual, since during the reproductive season estradiol increases the expression of LHβ mRNA while in the non-reproductive season it has no effect. In conclusion, the effect of E₂ on gonadotropins and testicular function is complex, not clearly understood and probably varies depending on the species. The aim of the current article is to review evidence on reproductive endocrinology and on the role of estradiol regulating reproduction in amphibians, with emphasis on the South American species Rhinella arenarum.

Effect of glucocorticoids on androgen biosynthesis in the testes of the toad Rhinella arenarum (Amphibia, Anura)

In rat Leydig cells, glucocorticoids (GCs) inhibit testosterone production through the interaction with the glucocorticoid receptor (GR). However, the sensitivity of those cells to GCs is regulated by the enzyme 11β-hydroxysteroid dehydrogenase Type 1 (11β-HSD1). In the testes of the toad Rhinella arenarum, the presence of an 11β-HSD similar to type 2 and a cytosolic GR has also been described. However, there is a lack of information regarding the effects of GCs on amphibian testicular steroidogenesis. In this study, the effects of corticosterone on androgen production, and the activity of two steroidogenic enzymes in toad testes were reported. Corticosterone inhibits androgen production via the GR because the GR antagonist RU486 prevents corticosterone-induced inhibition of testosterone. Corticosterone also reduced the activity of the cytochrome P450 17-hydroxylase, C17,20-lyase (Cyp450 _c17 ) without affecting the 3β-hydroxysteroid dehydrogenase/isomerase activity. This effect on Cyp450 _c17 was likewise inhibited by RU486. On the other hand, corticosterone had no effect on the amount of steroidogenic acute regulator protein. These results suggest that GCs inhibit steroidogenesis in toad testes by reducing of Cyp450 _c17 activity via a GR-mediated mechanism.

The effect of long-term corticosterone treatment on blood cell differentials and function in laboratory and wild-caught amphibian models

The effect of long-term stress on amphibian immunity is not well understood. We modeled a long-term endocrine stress scenario by elevating plasma corticosterone in two species of amphibians and examined effects on white blood cell differentials and innate immune activity. Plasma corticosterone was elevated in American bullfrogs (Lithobates catesbeianus) by surgically implanting corticosterone capsules and in African clawed frogs (Xenopus laevis) by immersion in corticosterone-treated water. To provide a context for our results within endogenous corticosterone fluctuations, diurnal plasma corticosterone cycles were determined. A daily low of corticosterone was observed in X. laevis at 12:00, while a significant pattern was not observed in L. catesbeianus. Elevated plasma corticosterone levels increased the ratio of peripheral neutrophils to lymphocytes, in both species, and decreased eosinophil concentrations in L. catesbeianus over a long-term period. Whole blood oxidative burst generally correlated with neutrophil concentrations, and thus was increased with corticosterone treatment, significantly in L. catesbeianus. In L. catesbeianus, an endogenous response of eosinophils and lymphocytes to implanted empty (sham) capsules was observed, but this effect was attenuated by corticosterone. Peripheral monocyte and basophil concentrations were not significantly altered by corticosterone treatment in either species. Our results show that long-term stress can alter amphibian immune parameters for extended periods and may play a role in susceptibility to disease.

Pharmacological characterization of intracellular, membrane, and plasma binding sites for corticosterone in house sparrows

The diversity and specificity of glucocorticoid effects are dependent on cell-specific receptor mechanisms. Three known corticosteroid receptors mediate tissue effects of glucocorticoids in vertebrates: two intracellular receptors that act primarily as ligand-activated transcription factors, and a membrane-associated receptor. The intracellular receptor sub-types have been well characterized in mammals, however relatively little is known about them across non-mammalian vertebrates. The membrane-associated receptors are poorly characterized in most vertebrate taxa. To explore the basis for glucocorticoid action in birds, we pharmacologically characterized the three putative corticosteroid receptors in the brain, as well as a plasma corticosterone binding globulin, in the house sparrow (Passer domesticus). We found that house sparrow brain cytosol contained two distinguishable binding sites for corticosterone. A high affinity, mineralocorticoid-like receptor had subnanomolar affinity for corticosterone (K(d) approximately 0.2 nM). However, this 'MR-like' high-affinity receptor did not bind RU28318 or canrenoic acid, two compounds that bind mammalian MR with high affinity. A lower-affinity, glucocorticoid-like receptor in brain cytosol bound corticosterone with an average K(d)=5.61 nM. This GR-like receptor showed subnanomolar affinity for RU 486. MR- and GR-like receptors were found in equal numbers in whole brain assays (average B(max)=69 and 62 fmol/mg protein, respectively). House sparrow brain membranes contain a single binding site specific for glucocorticoids, with characteristics consistent with a steroid/receptor interaction. Corticosterone affinity for this putative membrane receptor was approximately 24 nM, with apparent B(max)=177 fmol/mg protein. House sparrow plasma contained a single binding site for [(3)H]corticosterone. Specific binding to plasma sites was inhibited by glucocorticoids, progesterone, and testosterone. Testosterone binding to this corticosteroid binding globulin is noteworthy as sex steroid-specific binding globulins have not been identified in birds. Taken together, these data extend our ability to evaluate the comparative actions of glucocorticoids, increase our understanding of mechanisms behind the tissue specificity of glucocorticoid action, and offer insight into the evolution of glucocorticoid action in vertebrates.

Cytosolic glucocorticoid receptor in the testis of Bufo arenarum: seasonal changes in its binding parameters

Glucocorticoids (GC) are the hormonal mediators of stress. In mammals, high levels of GC have negative effects on reproductive physiology. For instance, GC can inhibit testicular testosterone synthesis by acting via glucocorticoid receptors (GR), the extent of the inhibition being dependent on GC levels. However, the effect of GC on testicular function and even the presence of GR in amphibians are still unclear. The purpose of this work was to characterise testicular cytosolic GR in Bufo arenarum, determining the seasonal changes in its binding parameters as well as the intratesticular localisation. The binding assays were performed in testis cytosol with [3H]dexamethasone (DEX) and [3H]corticosterone (CORT). Binding kinetics of DEX and CORT fitted to a one-site model. Results were expressed as means +/- standard error. Apparent number of binding sites (Bapp) was similar for both steroids (Bapp DEX = 352.53 +/- 72.08 fmol/mg protein; Bapp CORT = 454.24 +/- 134.97 fmol/mg protein) suggesting that both hormones bind to the same site. Competition studies with different steroids showed that the order of displacement of [3H]DEX and [3H]CORT specific binding is: DEX approximately RU486 approximately deoxycorticosterone (DOC) > CORT > aldosterone > RU28362 > progesterone >>> 11-dehydroCORT. The affinity of GR for DEX (Kd = 11.2 +/- 1.5 nM) remained constant throughout the year while circulating CORT clearly increased during the reproductive season. Therefore, testis sensitivity to GC action would depend mainly on inactivating mechanisms (11beta-hydroxysteroid dehydrogenase type 2) and CORT plasma levels. Since total and free CORT are higher in the reproductive than in the non-reproductive period, the magnitude of GC actions could be higher during the breeding season. The intratesticular localisation of the GR was determined after separation of cells by a Percoll density gradient followed by binding assays in each fraction. DEX binds to two different fractions corresponding to Leydig and Sertoli cells. In conclusion, in the testis of B. arenarum GC could regulate the function of both cellular types particularly during breeding when CORT reaches the highest plasma concentration.

Seasonal regulation of membrane and intracellular corticosteroid receptors in the house sparrow brain

A number of studies have demonstrated seasonal regulation of the adrenocortical response to stress, or of corticosteroid binding globulins, but very few studies have examined seasonal regulation of corticosteroid receptor levels. As a result, there have been few attempts to produce an integrated picture of seasonal plasticity of the stress response. We measured baseline and stress-induced corticosterone (CORT), corticosteroid binding globulin and neuronal cytosolic and membrane corticosteroid receptor levels in male and female, wild-caught house sparrows (Passer domesticus) during three different seasons over the annual cycle (nesting, molting and winter). We identified three neuronal corticosteroid receptors in the house sparrow brain: two intracellular receptors and one membrane-associated receptor. Little is known about corticosteroid receptors in neuronal membranes of avian and mammalian species, but we found that the levels of membrane corticosteroid receptors varied seasonally, being lowest during the nesting season. Cytosolic corticosteroid receptor numbers (both low and high affinity receptors) also varied seasonally. In contrast to the membrane bound receptors, however, the numbers of low and high affinity cytosolic receptors were lowest during winter. In addition, mean levels of total basal and stress-induced CORT in the plasma varied seasonally. Both basal and stress-induced levels of total CORT were significantly higher during nesting than during winter or molt. Finally, corticosteroid binding globulin levels in plasma were also seasonally regulated, in a pattern similar to total CORT, so that estimated free CORT levels did not vary between seasons. These data indicate that multiple components of the stress response are seasonally regulated in birds obtained from wild populations. Interactions between these regulated components provide a basis for seasonal differences in behavioural and physiological responses to stress.

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.

Intracellular pH and calcium in frog early distal tubule: effects of transport inhibitors

1. The K+ channels of the apical membrane of the diluting segment (early distal tubule, EDT) of the frog are involved in the regulation of transepithelial NaCl transport. These channels are sensitive to pHi and intracellular Ca2+ (Ca2+i). Inhibition of transport by furosemide (frusemide) results in a compensatory increase in K+ channel activity. The aims of the present study were to determine whether pHi or Ca2+i were altered by furosemide, and to identify the means by which such changes were brought about. 2. Experiments were performed using single, microperfused EDT segments. Measurements of pHi and Ca2+i were made using the intracellular fluorescent probes, 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF) and fura-2, respectively. 3. Furosemide increased pHi and Ca2+i. The intracellular alkalinization was the result of an alkaline shift in the set-point of the basolateral Na(+)-H+ exchanger. This response was dependent upon the increase in Ca2+i. 4. The increase in Ca2+i produced by furosemide was due to the release of Ca2+ from intracellular stores. Depletion of these stores, by 2,5-di-t-butylhydroquinone (TBQ) and caffeine, prevented the furosemide-induced changes in Ca2+ and pH. 5. Furosemide-induced activation of Na(+)-H+ exchange was prevented by the calmodulin antagonist, W-7. 6. Thus furosemide elicits a rise in Ca2+i which, via calmodulin, results in activation of Na(+)-H+ exchange. The resulting intracellular alkalinization would be expected to increase channel activity.

Transcriptional regulation of the Xenopus laevis B beta fibrinogen subunit gene by glucocorticoids and hepatocyte nuclear factor 1: analysis by transfection into primary liver cells

The blood-clotting protein fibrinogen is composed of three subunits, designated A alpha, B beta, and gamma, which are encoded by a family of related genes. As part of the acute-phase response, expression of the fibrinogen genes is coordinately regulated in the liver by glucocorticoids. To understand the factors underlying this hormonal response, we have examined control of transcription from fibrinogen gene fragments transfected into hepatocytes from the frog Xenopus laevis. This analysis is the first in any species to define transcriptional regulatory elements for the fibrinogen genes by transfection into primary liver cells, rather than liver-derived cell lines. A transfection vector was constructed containing the Xenopus B beta gene transcription start site and 1293 bp of the 5' flanking sequence linked to the firefly luciferase gene. When this construct was transfected into primary liver parenchymal cells, luciferase expression was induced approximately 14-fold by glucocorticoids, an increase similar to the transcriptional stimulation of the endogenous B beta subunit gene. DNA fragments with as little as 284 bases of upstream sequence retained full hormone responsiveness. This region contains a sequence resembling the canonical glucocorticoid response element (GRE) at bases -148 to -162. Deletions or specific point mutations eliminating this putative GRE led to complete loss of glucocorticoid inducibility. Physical association of the steroid hormone receptor with this functional GRE was demonstrated with a truncated form of the rat glucocorticoid receptor containing the DNA-binding domain. A second possible GRE at positions -526 to -540 was not hormone-responsive, in either the presence or the absence of the more proximal GRE. The regulatory region also has a sequence similar to the binding site for a liver-specific transcription factor, hepatocyte nuclear factor 1 (HNF-1), at positions -120 to -132. Specific point mutations in the HNF-1-binding site, in a construct containing a wild-type GRE, reduced promoter activity by a factor of 10, while stimulation by glucocorticoids was retained. Binding studies confirmed specific interaction between this site and the transcription factor HNF-1 alpha from mouse. Thus, we have identified a GRE sufficient to account for full glucocorticoid inducibility and an HNF-1 site close to the promoter that are major determinants of transcriptional control of the Xenopus fibrinogen B beta subunit gene in cells from normal liver tissue.

A corticosteroid receptor in neuronal membranes

Steroids may rapidly alter neuronal function and behavior through poorly characterized, direct actions on neuronal membranes. The membrane-bound receptors mediating these behavioral responses have not been identified. [3H]Corticosterone labels a population of specific, high-affinity recognition sites (dissociation constant = 0.51 nanomolar) in synaptic membranes from an amphibian brain. These binding sites were localized by receptor autoradiography in the neuropil, outside the regions of perikarya. The affinities of corticoids for this [3H]corticosterone binding site were linearly related to their potencies in rapidly suppressing male reproductive behavior. Thus, it appears that brain membranes contain a corticosteroid receptor that could participate in the regulation of behavior.

Metamorphic rate as a function of the light/dark cycle in Rana pipiens larvae

1. The rate of development of Rana pipiens tadpoles in spontaneous and thyroxine (T4)-induced metamorphosis was studied on light/dark (LD) cycles in which the photophase was held constant while the scotophase was progressively extended or vice versa. 2. Metamorphic rate fluctuated in both types of experiments as the LD cycle lengthened. However, the pattern of resonance differed with the length of the photophase. For example, with an 8 hr light phase, development rate slowed and then increased as the cycle was extended from 24 to to 36 hr, whereas with a 12 hr photophase the reverse took place. 3. The findings are compatible with the occurrence of a rhythm of light sensitivity in photoperiodic time measurement in this amphibian. 4. From the viewpoint of hormonal mechanisms, it is suggested that photoperiodic effects on metamorphic rate result from different patterns of melatonin secretion under the various LD cycles, since melatonin can modify the action of T4 in metamorphosis.

Seasonal variation in hepatic binding of estrogen in the turtle, Chrysemys picta

The present study describes the seasonal changes of the estrogen receptor (ER) system in the liver of the turtle, Chrysemys picta. [3H]Estradiol ([3H]E2) binding capacities and affinities of liver cytosols and nuclear extracts were measured with established procedures and analyzed by Scatchard plots. Our data revealed significant seasonal variations in both receptor content and the ER's affinity for [3H]E2 in the liver of the turtle. Nuclear ER content remained at a fairly stable level of 70 fmol/g tissue throughout the year, but exhibited two sharp increases in the months of May and October, attaining values of approximately 150 fmol/g tissue. These rises in nuclear ER content coincided temporally with the two peaks of vitellogenin accumulation and estrogen surge in the annual cycle previously reported for the female turtle. Cytosolic ER level in the liver exhibited a much more complex pattern. It fluctuated with a much wider range of 80 to 250 fmol/g tissue. Right after oviposition in July, it declined to its lowest value of 80 +/- 9 fmol/g tissue but gradually increased to 160 +/- 15 fmol/g tissue in September and remained at this medium range for the remaining of the fall season but rose sharply again to the highest value of the year in December, reaching a value of 270 +/- 7 fmol/g tissue. In the following winter months, cytosolic ER level declined until March before it rebounded to a second peak value of 242 +/- 6 fmol/g tissue in May.(ABSTRACT TRUNCATED AT 250 WORDS)

Corticosteroid receptors in liver cytosol of the clawed toad, Xenopus laevis: influence of thyroid and ovarian hormones

The glucocorticoid receptor capacity Ro and the dissociation constant Kd were determined in the liver of Xenopus laevis by Scatchard analysis. In 5-year-old female toads Ro was about three times higher than that in males (153.9, 54.3 fmol/mg protein) and Kd was similar in both sexes (4.0, 4.1 nM). Some of the animals used had abnormal enlarged thyroid glands, atrophic ovaries, or both defects in connection with different levels of Ro, but not of Kd, compared to those of normal animals. Females with ovarian atrophy showed significantly lower Ro values, in the same range as in normal males, and a high liver weight. In male and female toads with enlarged thyroid glands and in animals with both defects a significantly higher Ro occurred compared to that of the corresponding group without this abnormality. To study the influence of thyroid hormones on glucocorticoid receptors, young toads (2-3 years old) received injections of 4-phenyl-2-thiouracil, T3, or T4 on 7 consecutive days. Ro and Kd were determined on the following day. Doses of 50 and 500 ng T3 and of 500 and 5000 ng T4 per gram of body weight and day resulted in an increase of Ro up to 250% of the controls. Injections of T3 were more efficient in males than in females. The effect of thyroxine was about the same in both sexes. These observations suggest that thyroid and ovarian hormones exert an influence on glucocorticoid receptor capacity and may belong to the factors which regulate glucocorticoid receptors.