Corticosteroid receptors and the central nervous system

Dimerization of the Glucocorticoid Receptor and Its Importance in (Patho)physiology: A Primer

The glucocorticoid receptor (GR) is a very versatile protein that comes in several forms, interacts with many proteins and has multiple functions. Numerous therapies are based on GRs’ actions but the occurrence of side effects and reduced responses to glucocorticoids have motivated scientists to study GRs in great detail. The notion that GRs can perform functions as a monomeric protein, but also as a homodimer has raised questions about the underlying mechanisms, structural aspects of dimerization, influencing factors and biological functions. In this review paper, we are providing an overview of the current knowledge and insights about this important aspect of GR biology.

Rapid effects of corticosterone in the mouse dentate gyrus via a nongenomic pathway

Corticosterone activates two types of intracellular receptors in the rodent brain: the high affinity mineralocorticoid receptor (MR) and lower affinity glucocorticoid receptor (GR). These receptors act as transcriptional regulators and mediate slow changes in neuronal activity in a region-dependent manner. For example, in CA1 pyramidal cells, corticosterone slowly changes Ca(2+) currents and glutamate transmission but dentate granule cells appear to be resistant. Recent studies have shown that corticosteroids also exert rapid MR-dependent, nongenomic effects on hippocampal CA1 cells [e.g. increasing the frequency of miniature excitatory postsynaptic currents (mEPSCs)]. In the present study, we investigated whether dentate granule cells are also resistant to the rapid effects of corticosterone. We found that, comparable to the CA1 area, corticosterone quickly and reversibly increases mEPSC frequency but not amplitude of dentate cells. This effect did not require protein synthesis and displayed the pharmacological profile of an MR- rather than GR-dependent event. These data support the hypothesis that, unlike the slow gene-mediated effects of corticosterone, rapid hormonal actions are quite similar for CA1 and dentate cells.

Primary aldosteronism: an update

Primary aldosteronism (PA) is common with an estimated prevalence rate of 10% in subjects with essential hypertension and higher in those with resistant hypertension. As well as contributing to hypertension, aldosterone has detrimental effects on the heart, vasculature and kidneys as well as adverse metabolic effects leading to an excess of cardiovascular morbidity. Therefore, recognition and appropriate treatment of PA is of increasing importance. However, the diagnosis of PA and determination of subtype can be problematic. The purpose of this review is to provide an overview of the evidence supporting this increased prevalence of PA, explore the metabolic and cardiovascular consequences of aldosterone excess and discuss optimal diagnostic and therapeutic strategies of PA.

Investigations of HPA function and the enduring consequences of stressors in adolescence in animal models

Developmental differences in hypothalamic-pituitary-adrenal (HPA) axis responsiveness to stressors and ongoing development of glucocorticoid-sensitive brain regions in adolescence suggest that similar to the neonatal period of ontogeny, adolescence may also be a sensitive period for programming effects of stressors on the central nervous system. Although research on this period of life is scarce compared to early life and adulthood, the available research indicates that effects of stress exposure during adolescence differ from, and may be longer-lasting than, effects of the same stress exposure in adulthood. Research progress in animal models in this field is reviewed including HPA function and the enduring effects of stress exposures in adolescence on sensitivity to drugs of abuse, learning and memory, and emotional behaviour in adulthood. The effects of adolescent stress depend on a number of factors, including the age, gender, the duration of stress exposure, the type of stressor, and the time between stress exposure and testing.

Insights Into Glucocorticoid-Associated Hypertension

The association between excess glucocorticoids and hypertension has been much discussed but poorly understood. From both clinical observations and laboratory studies, it is clear that glucocorticoids exert their effects at many different sites responsible for blood pressure regulation. Isoforms of the enzyme 11ss-hydroxysteroid dehydrogenase (11ss-HSD), located in steroid-responsive tissues, metabolize endogenously produced glucocorticoids. These enzymes limit steroid access to mineralocorticoid and/or glucocorticoid receptors. In the kidney, synthetic and endogenous glucocorticoids are capable of enhancing transepithelial sodium transport in the presence of 11ss-HSD inhibition. Proximal tubule reabsorption of sodium can be indirectly augmented after chronic exposure to glucocorticoids. In this segment, steroids have a permissive effect, increasing the expression of both Na(+), K(+) adenosine triphosphatase along the basolateral membrane and Na(+)-H(+) exchanger along the apical membrane of epithelial cells. Although glucocorticoids themselves produce no increase in sodium reabsorption in this segment, angiotensin II-stimulated sodium transport is significantly greater in proximal tubular cells pretreated with glucocorticoids. The increased transport in distal renal segments is more direct and stems in part from glucocorticoid cross-over binding to mineralocorticoid receptors. In vascular tissue, synthetic and endogenous glucocorticoids, after inhibition of the dehydrogenase reaction, magnify the response to circulating vasoconstrictors. The effects of glucocorticoids in vascular tissue is indirect, upregulating the expression of receptors to many vasoconstrictors and downregulating the effects of potential vasodilators. Thus, glucocorticoids have the potential to alter both circulating volume and vascular resistance.

A second enzyme protecting mineralocorticoid receptors from glucocorticoid occupancy

We have confirmed that A6 cells (derived from kidney of Xenopus laevis), which contain both mineralocorticoid and glucocorticoid receptors, do not normally possess 11 beta-hydroxysteroid dehydroxgenase (11 beta-HSD1 or 11 beta-HSD2) enzymatic activity and so are without apparent "protective" enzymes. A6 cells do not convert the glucocorticoid corticosterone to 11-dehydrocorticosterone but do, however, possess steroid 6 beta-hydroxylase that transforms corticosterone to 6 beta-hydroxycorticosterone. This hydroxylase is cytochrome P-450 3A (CYP3A). We have now determined the effects of 3 alpha,5 beta-tetrahydroprogesterone and chenodeoxycholic acid (both inhibitors of 11 beta-HSD1) and 11-dehydrocorticosterone and 11 beta-hydroxy-3 alpha,5 beta-tetrahydroprogesterone (inhibitors of 11 beta-HSD2) and carbenoxalone, which inhibits both 11 beta-HSD1 and 11 beta-HSD2, on the actions and metabolism of corticosterone and active Na+ transport [short-circuit current (Isc)] in A6 cells. All of these 11 beta-HSD inhibitory substances induced a significant increment in corticosterone-induced Isc, which was detectable within 2 h. However, none of these agents caused an increase in Isc when incubated by themselves with A6 cells. In all cases, the additional Isc was inhibited by the mineralocorticoid receptor (MR) antagonist, RU-28318, whereas the original Isc elicited by corticosterone alone was inhibited by the glucocorticoid receptor antagonist, RU-38486. In separate experiments, each agent was shown to significantly inhibit metabolism of corticosterone to 6 beta-hydroxycorticosterone in A6 cells, and a linear relationship existed between 6 beta-hydroxylase inhibition and the MR-mediated increase in Isc in the one inhibitor tested. Troleandomycin, a selective inhibitor of CYP3A, inhibited 6 beta-hydroxylase and also significantly enhanced corticosterone-induced Isc at 2 h. These experiments indicate that the enhanced MR-mediated Isc in A6 cells may be related to inhibition of 6 beta-hydroxylase activity in these cells and that this 6 beta-hydroxylase (CYP3A) may be protecting the expression of corticosterone-induced active Na+ transport in A6 cells by MR-mediated mechanism(s).

Effect of adrenalectomy and corticosterone substitution on glucose and glycogen metabolism in rat brain

In non-nervous tissues, glucocorticoids (GCs) counteract the effects of insulin and stimulate gluconeogenesis. The present study was designed to investigate whether or not adrenalectomy (ADX) and glucocorticoid substitution influence the pathway of both glucose and glycogen metabolism in cerebral parietotemporal cortex and hippocampus, and if so how. The activities of respective key enzymes, such as hexokinase (HK), phosphofructokinase (PFK), pyruvate kinase (PK), glucose-6-phosphatase (G6Pase) and phosphorylase a (PLa), and the concentrations of the intermediates, such as glucose (Glu), glucose-6-phosphate (G6P), fructose-6-phosphate (F6P), fructose-1,6-bisphosphate (F16PP), pyruvate (Pyr), lactate (Lac), glycogen (Glyc) and glucose-1-phosphate (G1P), were measured in the brains of 1-year-old male Wistar rats under controlled conditions 3 days after ADX or sham operation and in a pilot study after ADX and substitution with corticosterone (CST) suspended in sesame oil or after ADX and subcutaneous administration of the vehicle only. An increase in both glycolytic flux and glycogen breakdown and a decrease in gluconeogenesis in cerebral cortex but not in hippocampus were observed after ADX. After substitution with CST in adrenalectomized rats the effect of ADX on enzyme activities was reversed: significant differences from adrenalectomized rats that received vehicle only was shown for PK and G6Pase activities in both areas of the rat brain investigated.

GABAA receptor pharmacology

gamma-Aminobutyric acid (GABA)A receptors for the inhibitory neurotransmitter GABA are likely to be found on most, if not all, neurons in the brain and spinal cord. They appear to be the most complicated of the superfamily of ligand-gated ion channels in terms of the large number of receptor subtypes and also the variety of ligands that interact with specific sites on the receptors. There appear to be at least 11 distinct sites on GABAA receptors for these ligands.

Messenger RNAs encoding steroidogenic enzymes are expressed in rodent brain

Using the reverse transcription polymerase chain reaction, mRNAs encoding steroidogenic P450s as well as NADPH-cytochrome P450 reductase (P450 reductase), adrenodoxin and the transcription factor steroidogenic factor 1 (SF-1) were all detected in rodent brain, but their distribution between brain regions varied. Adrenodoxin and P450 reductase were detected in all regions, suggesting the presence of both mitochondrial and microsomal P450s throughout the brain. Messenger RNAs encoding P450scc (CYP11A1) and P45017 alpha (CYP17) were also detected in all brain regions, this being the first report of CYP17 in the brain. P450c21 (CYP21) was detected only in the brain stem. P45011 beta (CYP11B1) and P450aldo (CYP11B2) are expressed in rat brain, but not in mouse brain; CYP11B1 primarily in the cerebrum, whereas CYP11B2 was detected in all brain regions. In both species, highest levels of aromatase P450 (CYP19) mRNA were detected in the cerebrum. SF-1 expression was restricted to the cerebrum minus cortex. Thus, although SF-1 is required for high level expression of the steroidogenic enzymes in adrenals and gonads, other factors may influence the expression of these genes in the brain. If the mRNAs detected by RT-PCR are indeed translated into functional enzymes, these studies suggest that different brain regions have different capacities for local steroid hormone production and metabolism. This raises the technical challenge of locating the specific sites of synthesis as well as the function of such locally produced ligands.