Effect of hyperthermia and anoxia on glucocorticoid and mineralocorticoid receptor expression in neonatal rat hippocampus

Do Corticosteroid Receptor mRNA Levels Predict the Expression of Their Target Genes?

The glucocorticoid stress hormones affect brain function via high-affinity mineralocorticoid receptors (MRs) and lower-affinity glucocorticoid receptors (GRs). MR and GR not only differ in affinity for ligands, but also have distinct, sometimes opposite, actions on neuronal excitability and other cellular and higher-order parameters related to cerebral function. GR and MR messenger RNA (mRNA) levels are often used as a proxy for the responsiveness to glucocorticoids, assuming proportionality between mRNA and protein levels. This may be especially relevant for the MR, which because of its high affinity is already largely occupied at low basal (trough) hormone levels. Here we explore how GR and MR mRNA levels are associated with the expression of a shared target gene, glucocorticoid-induced leucine zipper (GILZ, coded by Tsc22d3) with basal and elevated levels of corticosterone in male mice, using in situ hybridization. Depending on the hippocampal subfield and the corticosterone levels, mRNA levels of MR rather than GR mostly correlated with GILZ mRNA in the hippocampus and hypothalamus at the bulk tissue level. At the individual cell level, these correlations were much weaker. Using publicly available single-cell RNA sequencing data, we again observed that MR and GR mRNA levels were only weakly correlated with target gene expression in glutamatergic and GABAergic neurons. We conclude that MR mRNA levels can be limiting for receptor action, but many other cell-specific and region-specific factors ultimately determine corticosteroid receptor action. Altogether, our results argue for caution while interpreting the consequences of changed receptor expression for the response to glucocorticoids.

New View on the Impact of the Low-Frequency Electromagnetic Field (50 Hz) on Stress Responses: Hormesis Effect

INTRODUCTION

Low-frequency electromagnetic field (50 Hz) (EMF) can modify crucial neuronal processes. Existing data indicate that exposure to EMF may represent a mild stressor and contribute to disturbances of the hypothalamic-pituitary-adrenal (HPA) axis. The important regulatory pathways controlling HPA axis activity include two types of corticosteroid receptors: mineralocorticoid receptors (MRs) and glucocorticoid receptors. They are particularly abundant in the hippocampus, a key locus of HPA axis feedback control. The research aimed at determining whether (1) EMF exhibits hormesis, it means bidirectional action depending on EMF intensity (1 or 7 mT) and (2) repeated EMF exposure changes stress response to subsequent stress factors.

METHODS

The exposure (7 days, 1 h/day) of adult rats to EMF (1 mT and 7 mT) was repeated 3 times. HPA axis hormones and their receptors were analysed after each following exposure. Moreover, the impact of EMF exposure on hormonal and behavioural responses to subsequent stress factor - open-field test was evaluated.

RESULTS

Our data suggest that exposure to EMF can establish a new "set-point" for HPA axis activity. The direction and dynamics of this process depend on the intensity of EMF and the number of exposures. EMF of 1 mT induced an adaptive stress response, but 7 mT EMF caused sensitization. Consequently, EMF changed the vulnerability of the organism to a subsequent stress factor. We have also shown the increase in MR mRNA abundance in the hippocampus of 1 mT EMF-exposed rats, which can represent the possible neuroprotective response and suggest therapeutic properties of EMFs.

Glucocorticoid receptors function in the pathophysiology of brain hypoxia

Glucocorticoid receptors are ligand-activated transcription factors, which play an important role in the brain, mainly in stress response regulation. There are two types of receptors for glucocorticosteroids: mineralocorticoid receptors (MR) with high-affinity for the ligands and glucocorticoid receptors (GR) with a tenfold lower affinity. Selective activation of the receptors during hypoxia may decide neuronal fate, especially in the hippocampus. Depending on the severity of hypoxia-induced damage, neurons undergo necrosis or apoptosis. In the penumbral region, where neurons die mainly through the process of apoptosis, selective GR activation increases excitotoxicity, interferes with apoptotic signalling pathways and causes energy deficit in the cells, all of which promote cell death. On the other hand, selective MR activation seems to be neuroprotective. It is suggested that the main role of MR in neuroprotection is to regulate the balance between anti- and proapoptotic proteins from bcl-2 family.

Neonatal Hypoxia Ischaemia: Mechanisms, Models, and Therapeutic Challenges

Neonatal hypoxia-ischaemia (HI) is the most common cause of death and disability in human neonates, and is often associated with persistent motor, sensory, and cognitive impairment. Improved intensive care technology has increased survival without preventing neurological disorder, increasing morbidity throughout the adult population. Early preventative or neuroprotective interventions have the potential to rescue brain development in neonates, yet only one therapeutic intervention is currently licensed for use in developed countries. Recent investigations of the transient cortical layer known as subplate, especially regarding subplate's secretory role, opens up a novel set of potential molecular modulators of neonatal HI injury. This review examines the biological mechanisms of human neonatal HI, discusses evidence for the relevance of subplate-secreted molecules to this condition, and evaluates available animal models. Neuroserpin, a neuronally released neuroprotective factor, is discussed as a case study for developing new potential pharmacological interventions for use post-ischaemic injury.

Glucocorticoids as an emerging pharmacologic agent for cardiopulmonary resuscitation

Although cardiac arrest (CA) constitutes a major health problem with dismal prognosis, no specific drug therapy has been shown to improve survival to hospital discharge. CA causes adrenal insufficiency which is associated with poor outcome and increased mortality. Adrenal insufficiency may manifest as an inability to increase cortisol secretion during and after cardiopulmonary resuscitation (CPR). Several studies suggest that glucocorticoids during and after CPR seem to confer benefits with respect to return of spontaneous circulation (ROSC) rates and long term survival. They have beneficial hemodynamic effects that may favor their use during CPR and in the early post-resuscitation period. Moreover, they have anti-inflammatory and anti-apoptotic properties that improve organ function by reducing ischemia/reperfusion (I/R) injury. However, glucocorticoid supplementation has shown conflicting results with regard to survival to hospital discharge and neurological outcome. The purpose of this article is to review the pathophysiology of hypothalamic-pituitary-adrenal (HPA) axis during CPR. Furthermore, this article reviews the effects of glucocorticoids use during CRP and the post-resuscitation phase.

Cortisol stimulates proliferation and apoptosis in the late gestation fetal heart: differential effects of mineralocorticoid and glucocorticoid receptors

We have previously found that modest chronic increases in maternal cortisol result in an enlarged fetal heart. To explore the mechanisms of this effect, we used intrapericardial infusions of a mineralocorticoid receptor (MR) antagonist (canrenoate) or of a glucocorticoid receptor (GR) antagonist (mifepristone) in the fetus during maternal infusion of cortisol (1 mg·kg⁻¹·day⁻¹). We have shown that the MR antagonist blocked the increase in fetal heart weight and in wall thickness resulting from maternal cortisol infusion. In the current study we extended those studies and found that cortisol increased Ki67 staining in both ventricles, indicating cell proliferation, but also increased active caspase-3 staining in cells of the conduction pathway in the septum and subendocardial layers of the left ventricle, suggesting increased apoptosis in Purkinje fibers. The MR antagonist blocked the increase in cell proliferation, whereas the GR antagonist blocked the increased apoptosis in Purkinje fibers. We also found evidence of activation of caspase-3 in c-kit-positive cells, suggesting apoptosis in stem cell populations in the ventricle. These studies suggest a potentially important role of corticosteroids in the terminal remodeling of the late gestation fetal heart and suggest a mechanism for the cardiac enlargement with excess corticosteroid exposure.

Androgen receptor overexpression is neuroprotective in experimental stroke

Male sex is a known risk factor in human stroke. However, the role of the cognate receptor for androgens-the androgen receptor (AR)-in stroke outcome remains unclear. Here, we found that AR mRNA is downregulated in the peri-infarct tissue of gonadally intact male mice subjected to middle cerebral artery occlusion (MCAO) and 6 h reperfusion. We then used genetically engineered mice overexpressing AR in brain (AR-Tg) to compare outcomes from MCAO in intact or castrated males and to evaluate the neuroprotective role of dihydrotestosterone (DHT) replacement in AR-Tg castrates. A further evaluation of AR overexpression in ischemic paradigms was performed using rat PC12 cells transfected with human AR and treated with oxidative and apoptotic stressors. We then studied the role of DHT in cultures overexpressing AR. Our results show (1) ischemia alters the expression of AR by decreasing AR mRNA levels, (2) AR overexpression is protective in vivo against MCAO in intact and castrated AR-Tg mice and in vitro against oxidative and apoptotic stressors in AR-PC12 cells, and (3) DHT does not enhance the protection triggered by AR overexpression in AR-Tg castrated mice nor in AR-PC12 cells.

Small temperature variations alter edaravone-induced neuroprotection of cortical cultures exposed to prolonged hypoxic episodes

BACKGROUND

Edaravone, a free radical scavenger, has been shown to be neuroprotective in vivo and in vitro. However, the impact of small temperature variations on its neuroprotective actions remains unknown.

METHODS

We examined the degree of neuroprotection conferred by various concentrations of edaravone on cortical cultures exposed to prolonged hypoxia (24 h) under three conditions: mild hypothermia (32 degrees C), normothermia (37 degrees C), and mild hyperthermia (39 degrees C). The survival of cortical neurones from E16 Wistar rats (SR) was evaluated using photomicrographs taken before and after exposure to hypoxia.

RESULTS

The mean survival of neurones exposed to hypoxia at normothermia was 14.7 (sem 1.8)%. The addition of 50 microM edaravone significantly improved the mean survival to 40.5 (4.7)%. This improvement was noted at higher doses of edaravone (5 microM < or =) but not at lower doses (< or =500 nM). With mild hypothermia and prolonged hypoxia without edaravone, neuroprotection was significantly improved with a mean survival of 63.0 (5.2)%. This neuroprotective effect was not enhanced with the addition of edaravone, even at the highest dose. Hypoxia-induced neurotoxicity was aggravated by mild hyperthermia as reflected by a mean survival of 9.1 (2.1)%. However, higher concentrations of edaravone inhibited the deleterious effect of mild hyperthermia, thereby demonstrating a significant neuroprotective effect. The survival of neurones subjected to both hyperthermia and edaravone was the same as that of neurones exposed to normothermia and edaravone.

CONCLUSIONS

Temperature is a potential factor in determining whether edaravone confers a neuroprotective effect when applied during prolonged hypoxic insults.

Mineralocorticoid and glucocorticoid receptors in hippocampus: their impact on neurons survival and behavioral impairment after neonatal brain injury

Glucocorticoids (GC) exert multiple effects within the central nervous system via mineralocorticoid receptors (MR) and glucocorticoid receptors (GR) activation. MR expression is associated with a neuroprotective phenotype, whereas GR activation is implicated in the induction of an endangered neural phenotype and the opposite actions are most evident in hippocampus, where these receptors are predominantly present. Hippocampus has an overall inhibitory influence on the activity of the hypothalamic-pituitary-adrenal (HPA) axis and it has been suggested that efficient learning and adequate stress response depend on the appropriate functioning of the axis brought by coordinated activation of MR and GR in this region. There is a growing body of evidence that perinatal asphyxia causes irreversible damage to the brain leading to neurons loss in regions vulnerable to oxygen shortage especially in hippocampus. In the present review, some aspects of recently acquired insight in the role of GC receptors in promoting neuronal death and survival after hippocampal injury are discussed. Since the unbalance of MR and GR in hippocampus creates a condition of disturbed neuroendocrine regulation their potential impact on behavioral impairment will also be reviewed.

Injury-induced mineralocorticoid receptor expression involves differential promoter usage: a novel role for the rat MRbeta variant

Neuronal injury results in increased mineralocorticoid receptor (MR) expression and is associated with increased neuronal survival, suggesting that enhancing MR signalling may have therapeutic implications. MR has a complex gene structure with at least three untranslated exons (alpha, beta, gamma) each with unique promoters and a common coding region. We examined whether distinct cellular stressors differentially regulate exon-specific MR transcripts. MRbeta transcript was specifically upregulated in rat primary cortical cultures undergoing hypothermic oxygen-glucose deprivation (OGD/H) through activation of its own promoter. This effect was mediated in part by ERK signalling as blockade with PD98059 inhibited OGD/H-induced MRbeta promoter activity. A specific increase in MRbeta transcript expression was also found in vivo in hypothermic anoxic neonatal rat hippocampus. These results demonstrate a novel key role for the MRbeta transcript in response to injury and suggest that some of the known neuroprotective effects of hypothermia may be mediated through increased MR expression.