Rapid effects of corticosterone on cardiovascular neurons in the rostral ventrolateral medulla of rats

The impact of sciatic nerve injury and social interactions testing on glucocorticoid receptor expression in catecholaminergic medullary cell populations

Paradoxically, while acute pain leads to transiently elevated corticosterone, chronic pain does not result in persistently elevated corticosterone. In the sciatic nerve chronic constriction injury (CCI) model of chronic pain, we have shown that the same nerve injury produces a range of behavioural outcomes, each associated with distinctive adaptations to the HPA-axis to achieve stable plasma corticosterone levels. We also demonstrated that CRF and GR expression in the paraventricular hypothalamus (PVH) was increased in rats that showed persistent changes to their social behaviours during Resident-Intruder testing ('Persistent Effect' rats) when compared to rats that showed no behavioural changes ('No Effect' rats). In this study, we investigated whether these changes were driven in part by altered sensitivity of the brainstem catecholaminergic pathways (known to regulate the PVH) to glucocorticoids. GR expression in adrenergic (C1,C2) and noradrenergic (A1,A2) cells was determined using immunohistochemistry in behaviourally tested CCI rats and in uninjured controls. We found no differences between Persistent Effect and No Effect rats in (1) the glucocorticoid sensitivity of these cells, or (2) the numbers of adrenergic and noradrenergic cells in each region. However, we discovered an overall reduction in GR expression in the non-catecholaminergic cells of these regions in both experimental groups when compared to uninjured controls, most likely attributable to the repeated Resident-Intruder testing. Taken together, these data suggest strongly that brainstem mechanisms are unlikely to play a key role in the rebalancing of the HPA-axis triggered by CCI, increasing the probability that these changes are driven by supra-hypothalamic regions.

Differential Effects of Focused Attention and Open Monitoring Meditation on Autonomic Cardiac Modulation and Cortisol Secretion

Mindfulness-based interventions (MBIs) have been used widely as a useful tool for the alleviation of various stress-related symptoms. However, the effects of MBIs on stress-related physiological activity have not yet been ascertained. MBIs primarily consist of focused-attention (FA) and open-monitoring (OM) meditation. Since differing effects of FA and OM meditation on brain activities and cognitive tasks have been mentioned, we hypothesized that FA and OM meditation have also differing effects on stress-related physiological activity. In this study, we examined the effects of FA and OM meditation on autonomic cardiac modulation and cortisol secretion. Forty-one healthy adults (aged 20-46 years) who were meditation novices experienced 30-min FA and OM meditation tasks by listening to instructions. During resting- and meditation-states, electrocardiogram transducers were attached to participants to measure the R-R interval, which were used to evaluate heart rate (HR) and perform heart rate variability (HRV) analyses. Saliva samples were obtained from participants pre- and post-meditation to measure salivary cortisol levels. Results showed that FA meditation induced a decrease in HR and an increase in the root mean square of successive differences (rMSDD). In contrast, OM meditation induced an increase in the standard deviation of the normal-to-normal interval (SDNN) to rMSSD ratio (SDNN/rMSSD) and a decrease in salivary cortisol levels. These results suggest that FA meditation elevates physiological relaxation, whereas OM meditation elevates physiological arousal and reduces stress.

Cortisol rapidly increases baroreflex sensitivity of heart rate control, but does not affect cardiac modulation of startle

Cortisol, the final product of human HPA axis activation, rapidly modulates the cortical processing of afferent signals originating from the cardiovascular system. While peripheral effects have been excluded, it remains unclear whether this effect is mediated by cortical or subcortical (e.g. brainstem) CNS mechanisms. Cardiac modulation of startle (CMS) has been proposed as a method to reflect cardio-afferent signals at subcortical (potentially brainstem-) level. Using a single blind, randomized controlled design, the cortisol group (n = 16 volunteers) received 1 mg cortisol intravenously, while the control group (n = 16) received a placebo substance. The CMS procedure involved the assessment of eye blink responses to acoustic startle stimuli elicited at six different latencies to ECG-recorded R-waves (R + 0, 100, 200, 300, 400 and 500 ms). CMS was assessed at four measurement points: baseline, -16 min, +0 min, and +16 min relative to substance application. Baroreflex sensitivity (BRS) of heart rate (HR) control was measured non-invasively based on spontaneous beat-to-beat HR and systolic blood pressure changes. In the cortisol group, salivary cortisol concentration increased after IV cortisol administration, indicating effective distribution of the substance throughout the body. Furthermore, BRS increased in the cortisol group after cortisol infusion. There was no effect of cortisol on the CMS effect, however. These results suggest that low doses of cortisol do not affect baro-afferent signals, but central or efferent components of the arterial baroreflex circuit presumably via rapid, non-genomic mechanisms.

Nongenomic glucocorticoid effects and their mechanisms of action in vertebrates

Glucocorticoids (GC) act on multiple organ systems to regulate a variety of physiological processes in vertebrates. Due to their immunosuppressive and anti-inflammatory actions, glucocorticoids are an attractive target for pharmaceutical development. Accordingly, they are one of the most widely prescribed classes of therapeutics. Through the classical mechanism of steroid action, glucocorticoids are thought to mainly affect gene transcription, both in a stimulatory and suppressive fashion, regulating de novo protein synthesis that subsequently leads to the physiological response. However, over the past three decades multiple lines of evidence demonstrate that glucocorticoids may work through rapid, nonclassical mechanisms that do not require alterations in gene transcription or translation. This review assimilates evidence across the vertebrate taxa on the diversity of nongenomic actions of glucocorticoids and the membrane-associated cellular mechanisms that may underlie rapid glucocorticoid responses to include potential binding sites characterized to date.

Acute hydrocortisone administration reduces cardiovagal baroreflex sensitivity and heart rate variability in young men

KEY POINTS

A surge in cortisol during acute physiological and pathophysiological stress may precipitate ventricular arrhythmia and myocardial infarction. Reduced cardiovagal baroreflex sensitivity and heart rate variability are observed during acute stress and are associated with an increased risk of acute cardiac events. In the present study, healthy young men received either a single iv bolus of saline (placebo) or hydrocortisone, 1 week apart, in accordance with a randomized, placebo-controlled, cross-over study design. Hydrocortisone acutely increased heart rate and blood pressure and reduced cardiovagal baroreflex sensitivity and heart rate variability in young men. These findings suggest that, by reducing cardiovagal baroreflex sensitivity and heart rate variability, acute surges in cortisol facilitate a pro-arrhythmic milieu and provide an important mechanistic link between stress and acute cardiac events ABSTRACT: Surges in cortisol concentration during acute stress may increase cardiovascular risk. To better understand the interactions between cortisol and the autonomic nervous system, we determined the acute effects of hydrocortisone administration on cardiovagal baroreflex sensitivity (BRS), heart rate variability (HRV) and cardiovascular reactivity. In a randomized, placebo-controlled, single-blinded cross-over study, 10 healthy males received either a single iv bolus of saline (placebo) or 200 mg of hydrocortisone, 1 week apart. Heart rate (HR), blood pressure (BP) and limb blood flow were monitored 3 h later, at rest and during the sequential infusion of sodium nitroprusside and phenylephrine (modified Oxford Technique), a cold pressor test and a mental arithmetic stress task. HRV was assessed using the square root of the mean of the sum of the squares of differences between successive R-R intervals (rMSSD). Hydrocortisone markedly increased serum cortisol 3 h following infusion and also compared to placebo. In addition, hydrocortisone elevated resting HR (+7 ± 4 beats min^-1 ; P < 0.001) and systolic BP (+5 ± 5 mmHg; P = 0.008); lowered cardiovagal BRS [geometric mean (95% confidence interval) 15.6 (11.1-22.1) ms/mmHg vs. 26.2 (17.4--39.5) ms/mmHg, P = 0.011] and HRV (rMSSD 59 ± 29 ms vs. 84 ± 38 ms, P = 0.004) and increased leg vasoconstrictor responses to cold pressor test (Δ leg vascular conductance -45 ± 20% vs. -23 ± 26%; P = 0.023). In young men, an acute cortisol surge is accompanied by increases in HR and BP, as well as reductions in cardiovagal BRS and HRV, potentially providing a pro-arrhythmic milieu that may precipitate ventricular arrhythmia or myocardial infarction and increase cardiovascular risk.

Increase in salivary oxytocin and decrease in salivary cortisol after listening to relaxing slow-tempo and exciting fast-tempo music

Relaxation and excitation are components of the effects of music listening. The tempo of music is often considered a critical factor when determining these effects: listening to slow-tempo and fast-tempo music elicits relaxation and excitation, respectively. However, the chemical bases that underlie these relaxation and excitation effects remain unclear. Since parasympathetic and sympathetic nerve activities are facilitated by oxytocin and glucocorticoid, respectively, we hypothesized that listening to relaxing slow-tempo and exciting fast-tempo music is accompanied by increases in the oxytocin and cortisol levels, respectively. We evaluated the change in the salivary oxytocin and cortisol levels of participants listening to slow-tempo and fast-tempo music sequences. We measured the heart rate (HR) and calculated the heart rate variability (HRV) to evaluate the strength of autonomic nerve activity. After listening to a music sequence, the participants rated their arousal and valence levels. We found that both the salivary oxytocin concentration and the high frequency component of the HRV (HF) increased and the HR decreased when a slow-tempo music sequence was presented. The salivary cortisol level decreased and the low frequency of the HRV (LF) to HF ratio (LF/HF) increased when a fast-tempo music sequence was presented. The ratio of the change in the oxytocin level was correlated with the change in HF, LF/HF and HR, whereas that in the cortisol level did not show any correlation with indices of autonomic nerve activity. There was no correlation between the change in oxytocin level and self-reported emotions, while the change in cortisol level correlated with the arousal level. These findings suggest that listening to slow-tempo and fast-tempo music is accompanied by an increase in the oxytocin level and a decrease in the cortisol level, respectively, and imply that such music listening-related changes in oxytocin and cortisol are involved in physiological relaxation and emotional excitation, respectively.

Chronic glucocorticoid exposure activates BK-NLRP1 signal involving in hippocampal neuron damage

BACKGROUND

Neuroinflammation mediated by NLRP1 (nucleotide-binding oligomerization domain (NOD)-like receptor protein 1) inflammasome plays an important role in many neurological diseases such as Parkinson's disease (PD) and Alzheimer's disease (AD). Our previous studies showed that chronic glucocorticoid (GC) exposure increased brain inflammation via NLRP1 inflammasome and induce neurodegeneration. However, little is known about the mechanism of chronic GC exposure on NLRP1 inflammasome activation in hippocampal neurons.

METHODS

Hippocampal neurons damage was assessed by LDH kit and Hoechst 33258 staining. The expression of microtubule-associated protein 2 (MAP2), inflammasome complex protein (NLRP1, ASC and caspase-1), inflammatory cytokines (IL-1β), and large-conductance Ca²⁺ and voltage-activated K⁺ channel (BK channels) protein was detected by Western blot. The inflammatory cytokines (IL-1β and IL-18) were examined by ELISA kit. The mRNA levels of NLRP1, IL-1β, and BK were detected by real-time PCR. BK channel currents were recorded by whole-cell patch-clamp technology. Measurement of [K⁺]_i was performed by ion-selective electrode (ISE) technology.

RESULTS

Chronic dexamethasone (DEX) treatment significantly increased LDH release and neuronal apoptosis and decreased expression of MAP2. The mechanistic studies revealed that chronic DEX exposure significantly increased the expression of NLRP1, ASC, caspase-1, IL-1β, L-18, and BK protein and NLRP1, IL-1β and BK mRNA levels in hippocampal neurons. Further studies showed that DEX exposure results in the increase of BK channel currents, with the subsequent K⁺ efflux and a low concentration of intracellular K⁺, which involved in activation of NLRP1 inflammasome. Moreover, these effects of chronic DEX exposure could be blocked by specific BK channel inhibitor iberiotoxin (IbTx).

CONCLUSION

Our findings suggest that chronic GC exposure may increase neuroinflammation via activation of BK-NLRP1 signal pathway and promote hippocampal neurons damage, which may be involved in the development and progression of AD.

Neurochemistry of neurons in the ventrolateral medulla activated by hypotension: Are the same neurons activated by glucoprivation?

Previous studies have demonstrated that a range of stimuli activate neurons, including catecholaminergic neurons, in the ventrolateral medulla. Not all catecholaminergic neurons are activated and other neurochemical content is largely unknown hence whether stimulus specific populations exist is unclear. Here we determine the neurochemistry (using in situ hybridization) of catecholaminergic and noncatecholaminergic neurons which express c-Fos immunoreactivity throughout the rostrocaudal extent of the ventrolateral medulla, in Sprague Dawley rats treated with hydralazine or saline. Distinct neuronal populations containing PPCART, PPPACAP, and PPNPY mRNAs, which were largely catecholaminergic, were activated by hydralazine but not saline. Both catecholaminergic and noncatecholaminergic neurons containing preprotachykinin and prepro-enkephalin (PPE) mRNAs were also activated, with the noncatecholaminergic population located in the rostral C1 region. Few GlyT2 neurons were activated. A subset of these data was then used to compare the neuronal populations activated by 2-deoxyglucose evoked glucoprivation (Brain Structure and Function (2015) 220:117). Hydralazine activated more neurons than 2-deoxyglucose but similar numbers of catecholaminergic neurons. Commonly activated populations expressing PPNPY and PPE mRNAs were defined. These likely include PPNPY expressing catecholaminergic neurons projecting to vasopressinergic and corticotrophin releasing factor neurons in the paraventricular nucleus, which when activated result in elevated plasma vasopressin and corticosterone. Stimulus specific neurons included noncatecholaminergic neurons and a few PPE positive catecholaminergic neuron but neurochemical codes were largely unidentified. Reasons for the lack of identification of stimulus specific neurons, readily detectable using electrophysiology in anaesthetized preparations and for which neural circuits can be defined, are discussed.

Role of nitric oxide on purinergic signalling in the cochlea

In the inner ear, there is considerable evidence that extracellular adenosine 5'-triphosphate (ATP) plays an important role in auditory neurotransmission as a neurotransmitter or a neuromodulator, although the potential role of adenosine signalling in the modulation of auditory neurotransmission has also been reported. The activation of ligand-gated ionotropic P2X receptors and G protein-coupled metabotropic P2Y receptors has been reported to induce an increase of intracellular Ca(2+) concentration ([Ca(2+)](i)) in inner hair cells (IHCs), outer hair cells (OHCs), spiral ganglion neurons (SGNs), and supporting cells in the cochlea. ATP may participate in auditory neurotransmission by modulating [Ca(2+)](i) in the cochlear cells. Recent studies showed that extracellular ATP induced nitric oxide (NO) production in IHCs, OHCs, and SGNs, which affects the ATP-induced Ca(2+) response via the NO-cGMP-PKG pathway in those cells by a feedback mechanism. A cross-talk between NO and ATP may therefore exist in the auditory signal transduction. In the present article, I review the role of NO on the ATP-induced Ca(2+) signalling in IHCs and OHCs. I also consider the possible role of NO in the ATP-induced Ca(2+) signalling in SGNs and supporting cells.

Plasma corticosterone levels is elevated in rats submitted to chronic intermittent hypoxia

In the present study we investigated whether plasma corticosterone is altered in rats exposed to chronic intermittent hypoxia (CIH). Rats were submitted to a fraction of inspired oxygen of 6%, for 40 s, every 9 min, 8 h a day, for 35 days (CIH rats, n=17), while control rats were maintained under normoxic conditions (n=16). After CIH, the rats presented a significant increase in baseline mean arterial pressure (118+/-2 vs 106+/-3, mmHg) but not in baseline heart rate (381+/-17 vs 362+/-12 bpm) when compared to the control rats. Besides, a significant increase in plasma corticosterone was observed in CIH rats in comparison to the control rats (39+/-4 vs 20+/-2 microg/dl). Considering that corticosterone can affect both peripheral and central sympathetic mechanisms, the elevated plasma corticosterone may represent a new insight on the mechanisms underlying the hypertension observed after CIH.

Differential regulation of parvocellular neuronal activity in the paraventricular nucleus of the hypothalamus following single vs. repeated episodes of water restriction-induced drinking

Acute activation of the hypothalamic-pituitary-adrenal (HPA) axis releases glucocorticoids to maintain homeostasis, whereas prolonged exposure to elevated glucocorticoids has deleterious effects. Due to the potential benefits of limiting stress-induced glucocorticoid secretion, the present study uses drinking in dehydrated rats as a model to delineate mechanisms mobilized to rapidly inhibit HPA activity during stress. Using Fos expression as an indicator of neuronal activation, the effect of a single or repeated episode of dehydration-induced drinking on the activity of magnocellular and parvocellular neurons in the paraventricular nucleus (PVN) of the hypothalamus was examined. Adult male rats underwent a single episode or repeated (six) episodes of water restriction and were sacrificed before or after drinking water in the AM. Plasma osmolality, vasopressin (AVP), adrenocorticotropic hormone (ACTH) and corticosterone were elevated by water restriction and reduced after drinking in both models. Fos expression was elevated in AVP-positive magnocellular PVN neurons and AVP- and corticotropin releasing hormone (CRH)-positive parvocellular PVN neurons after water restriction. Fos expression was reduced in magnocellular AVP neurons after both models of restriction-induced drinking. In contrast, Fos expression did not change in AVP and CRH parvocellular neurons after a single episode of restriction-induced drinking, but was reduced after repeated episodes of restriction-induced drinking. These data indicate that drinking-induced decreases in glucocorticoids in dehydrated rats involve multiple factors including reduction in magnocellular release of vasopressin and reduction in parvocellular neuronal activity. The differential inhibition of PVN parvocellular neurons after repeated rehydration may reflect a conditioned response to repeated stress reduction.

Beta2 and beta4 subunits of BK channels confer differential sensitivity to acute modulation by steroid hormones

Membrane-associated receptors for rapid, steroidal neuromodulation remain elusive. Estradiol has been reported to facilitate activation of voltage- and Ca(2+)-dependent BK potassium channels encoded by Slo, if associated with beta1 subunits. We show here that 1) multiple members of the beta family confer sensitivity to multiple steroids on BK channels, 2) that beta subunits differentiate between steroids, and 3) that different betas have distinct relative preferences for particular steroids. Expressed in HEK 293 cells, inside-out patches with channels composed of Slo-alpha alone showed no steroid sensitivity. Cells expressing alphabeta4 exhibited potent, rapid, reversible, and dose-dependent potentiation by corticosterone (CORT; a glucocorticoid), and were potentiated to a lesser degree by other sex and stress steroids. In contrast, alphabeta2 channels were potentiated more strongly by dehydroepiandrosterone (DHEA; an enigmatic, stress-related adrenal androgen), and to a lesser extent by CORT, estradiol, testosterone, and DHEA-S. Cholesterol had no effect on any BK channel compositions tested. Conductance-voltage plots of channels composed of alpha plus beta2 or beta4 subunits were shifted in the negative direction by steroids, indicating greater activation at negative voltages. Thus our results argue that the variety of Slo-beta subunit coexpression patterns occurring in vivo expands the repertoire of Slo channel gating in yet another dimension not fully appreciated, rendering BK gating responsive to dynamic fluctuations in a multiple of steroid hormones.

Inhibition of Cortisol Production With Metyrapone Prevents Mental Stress-Induced Endothelial Dysfunction and Baroreflex Impairment

OBJECTIVES

This study was designed to investigate the role of cortisol in stress-induced endothelial dysfunction and impaired baroreflex sensitivity (BRS) by blocking cortisol production with metyrapone before subjecting healthy volunteers to mental stress.

BACKGROUND

Mental stress raises cortisol levels and is associated with increased coronary heart disease (CHD) morbidity and mortality, especially from sudden cardiac death. It also causes endothelial dysfunction and impaired BRS.

METHODS

We measured brachial artery flow-mediated dilation (FMD), a measure of endothelial function, and BRS in 36 subjects without CHD risk factors who were then randomized in a double-blind fashion to oral metyrapone 750 mg x 2 or placebo. Five hours later we subjected subjects to mental stress and then remeasured endothelial function and BRS.

RESULTS

Prestress cortisol levels were significantly higher in the placebo group at 270.5 (30.9) nmol/l versus 89.1 (11.8) nmol/l (p = 0.01), and the increase with stress was higher at 57.9 (17.9) nmol/l versus 11.2 (2.2) nmol/l (p < 0.001). In the placebo group, compared to baseline, FMD and BRS fell significantly from 4.5% (0.7%) to 1.4% (1.1%) (p = 0.02) and 21.4 (2.3) ms/mmHg to 16.3 (1.5) ms/mmHg (p = 0.04), respectively. In the metyrapone group, FMD and BRS were unchanged from baseline: 4.3% (0.9%) versus 5.1% (0.8%) (p = 0.48) and 26.4 (2.9) ms/mmHg versus 24.9 (2.6) ms/mmHg (p = 0.62), respectively. Analysis of covariation showed a significant effect of metyrapone on change in both FMD (p = 0.009) and BRS (p = 0.024).

CONCLUSIONS

Stress-related endothelial dysfunction and BRS impairment can be prevented by blocking cortisol production with metyrapone, demonstrating a direct or facilitative role for cortisol in these phenomena and suggesting mechanisms by which stress contributes to CHD and sudden cardiac death.

Acute effects of glucocorticoids on ATP-induced Ca2+ mobilization and nitric oxide production in cochlear spiral ganglion neurons

Rapid, non-genomic effects of glucocorticoids on extracellular adenosine 5'-triphosphate (ATP)-induced intracellular Ca(2+) concentration ([Ca(2+)](i)) changes and nitric oxide (NO) production were investigated in type I spiral ganglion neurons (SGNs) of the guinea-pig cochlea using the Ca(2+)-sensitive dye fura-2 and the NO-sensitive dye 4,5-diaminofluorescein (DAF-2). Pretreatment of SGNs with 1 microM dexamethasone for 10 min, a synthetic glucocorticoid hormone, enhanced the ATP-induced [Ca(2+)](i) increase in SGNs. RU 38486, a competitive glucocorticoid receptor antagonist eliminated the effects of dexamethasone on the ATP-induced [Ca(2+)](i) increase in SGNs. These acute effects of dexamethasone were dependent on the presence of extracellular Ca(2+), thereby suggesting that dexamethasone may rapidly enhance the Ca(2+) influx through the activation of ionotropic P2X receptors which may interact with glucocorticoid-mediated membrane receptors. Extracellular ATP increased the intensity of DAF-2 fluorescence, indicating NO production in SGNs. The ATP-induced NO production was mainly due to the Ca(2+) influx through the activation of P2 receptors. S-nitroso-N-acetylpenicillamine, a NO donor, enhanced the ATP-induced [Ca(2+)](i) increase in SGNs while L-N(G)-nitroarginine methyl ester (L-NAME), a NO synthesis inhibitor, inhibited it. Dexamethasone enhanced the ATP-induced NO production in SGNs. The augmentation of dexamethasone on ATP-induced NO production was abolished in the presence of l-NAME. It is concluded that the ATP-induced [Ca(2+)](i) increase induces NO production which enhances a [Ca(2+)](i) increase in SGNs by a positive-feedback mechanism. Dexamethasone enhances the ATP-induced [Ca(2+)](i) increase in SGNs which results in the augmentation of NO production. The present study suggests that NO may play an important role in auditory signal transduction. Our results also indicate that glucocorticoids may rapidly affect auditory neurotransmission due to a novel non-genomic mechanism.

The role of CRF1 receptors for sympathetic nervous response to laparotomy in anesthetized rats

Corticotropin-releasing factor (CRF) is released in response to various types of stressors and mediates endocrine, autonomic, immune, and behavioral responses to stress through interaction with CRF1 and CRF2 receptors. To investigate the role of CRF1 receptors in physiological responses to surgical stress, we analyzed the effects of two different non-peptide selective CRF1 receptor antagonists (JTC-017 and CP-154,526) and a peptide non-selective CRF receptor antagonist (astressin) on laparotomy-induced sympathetic nervous responses in isoflurane-anesthetized rats. JTC-017, CP-154,526, and astressin similarly suppressed plasma ACTH elevation induced by laparotomy. JTC-017 and CP-154,526 significantly augmented plasma noradrenaline and adrenaline responses to laparotomy, while astressin showed no effect on these responses. Laparotomy-induced maximum increases in mean blood pressure and heart rate were augmented by JTC-017, but were not affected by astressin. The results suggested for the first time that there was a pathway to attenuate sympathetic nervous response to surgical stress through CRF1 receptors in the central nervous system.

Electrophysiological evidences for the contribution of NMDA receptors to the inhibition of clonidine on the RVLM presympathetic neurons

The main objective of this study is to test the hypothesis that N-methyl-D-aspartate (NMDA) receptors within the rostral ventrolateral medulla (RVLM) are involved in the inhibition of clonidine on the RVLM presympathetic neurons. Totally, 22 presympathetic neurons were recorded in anesthetized and paralyzed rats. The majority of these neurons (n=16 of 22) were significantly inhibited by iontophoretic (30 nA) clonidine, the other 6 neurons were insensitive to clonidine. In seven clonidine-sensitive neurons, iontophoretic clonidine (30 nA) antagonized the neuronal excitation of iontophoretic NMDA receptor agonist NMDA (20 nA). In remaining nine clonidine-sensitive neurons, iontophoretic NMDA receptor antagonist MK801 (60 nA) significantly attenuated the neuronal inhibition of iontophoretic (30 nA) clonidine. In conclusion, these results suggest that NMDA receptors contribute to the inhibition of clonidine on the RVLM presympathetic neurons.

Stress and immunological phagocytosis: possible nongenomic action of corticosterone

Some immunological responses triggered by stress can be mediated by corticosterone activity through cytosolic receptors regulating gene expression. There are, however some reports on the possibility of a nongenomic effect of this hormone to explain phenomena observed in a few minutes. We have found that macrophages from mice subjected to 10 min of cold stress (at -15 degrees C) showed a lower phagocytic capacity mediated by Fcgamma-receptors than cells from control animals. Treating mice with glucocorticoid antagonist RU 486 did not block the decrease in phagocytic capacity. This inhibitory effect on phagocytosis was also observed by experiments in vitro with corticosterone in the concentration found in serum after stress, and could not be prevented by RU 486, actinomicyn D or cycloheximide. These results indicate that corticosterone could affect phagocytosis by macrophages through a nongenomic mechanism, and may have physiological implications.

Blockade of N-methyl-d-aspartate receptors within the rostral ventrolateral medulla antagonizes clonidine-induced cardiovascular effects

There is wide agreement that the rostral ventrolateral medulla (RVLM) plays a crucial role in the regulation of blood pressure (BP), and that there may be a close correlation between the actions of centrally acting antihypertensive agents and N-methyl-D-aspartate (NMDA) receptor functional states. The present study was done to test the hypothesis that NMDA receptors within the RVLM were involved in the cardiovascular effects of centrally acting antihypertensive drug clonidine in anesthetized and paralyzed rats. Prior unilateral microinjection of NMDA receptor antagonist dizocilpine (MK801, 500 pmol) into the RVLM significantly attenuated (p<0.01, n=9) the reductions of BP (-24+/-6 to -8+/-4 mm Hg) and heart rate (-49+/-9 to -14+/-7 bpm) induced by unilaterally injected clonidine (5 nmol) into the RVLM. Prior bilateral microinjection of MK801 (500 pmol for each side) into the RVLM effectively (p<0.01, n=7) antagonized the hypotension (-25+/-5 to -8+/-2 mm Hg) and bradycardia (-43+/-7 to -11+/-4 bpm) of intravenously administered clonidine (10 microg kg(-1)). Importantly, iontophoretic application of MK801 (60 nA) significantly (p<0.01, n=9) prevented the inhibitory effect of intravenously (10 microg kg(-1)) injected clonidine on the discharge of presympathetic neurons in the RVLM (neuronal inhibition: -39+/-6 to -10+/-2%). In conclusion, the present study shows that the RVLM administrated MK801 effectively antagonizes clonidine-induced cardiovascular effects, and suggests that NMDA receptors within the RVLM contribute to clonidine actions.

Corticosteroid receptors, 11 beta-hydroxysteroid dehydrogenase, and the heart

Mineralocorticoid and glucocorticoid hormones are known as corticosteroid hormones and are synthesized mainly in the adrenal cortex; however, more recently the enzymes involved in their synthesis have been found in a variety of cells and tissues, including the heart. The effects of these hormones are mediated via both cytoplasmic mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs), which act as ligand-inducible transcription factors. In addition, rapid, nongenomically mediated effects of these steroids can occur that may be via novel corticosteroid receptors. The lipophilic nature of these hormones allows them to pass freely through the cell membrane, although the intracellular concentration of mineralocorticoids and glucocorticoids is dependent on several cellular factors. The main regulators of intracellular glucocorticoid levels are 11 beta-hydroxysteroid dehydrogenase (11 beta HSD) isoforms. 11 beta HSD1 acts predominantly as a reductase in vivo, facilitating glucocorticoid action by converting circulating receptor-inactive 11-ketoglucocorticoids to active glucocorticoids. In contrast, 11 beta HSD 2 acts exclusively as an 11 beta-dehydrogenase and decreases intracellular glucocorticoids by converting them to their receptor-inactive 11-ketometabolites. Furthermore, P-glycoproteins, by actively pumping steroids out of cells, can selectively decrease steroids and local steroid synthesis can increase steroid concentrations. Receptor concentration, receptor modification, and receptor-protein interactions can also significantly impact on the corticosteroid response. This review details the receptors and possible mechanisms involved in both mediating and modulating corticosteroid responses. In addition, direct effects of corticosteroids on the heart are described including a discussion of the corticosteroid receptors and the mechanisms involved in mediating their effects.

Nongenomic steroid action: controversies, questions, and answers

Steroids may exert their action in living cells by several ways: 1). the well-known genomic pathway, involving hormone binding to cytosolic (classic) receptors and subsequent modulation of gene expression followed by protein synthesis. 2). Alternatively, pathways are operating that do not act on the genome, therefore indicating nongenomic action. Although it is comparatively easy to confirm the nongenomic nature of a particular phenomenon observed, e.g., by using inhibitors of transcription or translation, considerable controversy exists about the identity of receptors that mediate these responses. Many different approaches have been employed to answer this question, including pharmacology, knock-out animals, and numerous biochemical studies. Evidence is presented for and against both the participation of classic receptors, or proteins closely related to them, as well as for the involvement of yet poorly understood, novel membrane steroid receptors. In addition, clinical implications for a wide array of nongenomic steroid actions are outlined.

Glucocorticoids reduce responses to AMPA receptor activation and blockade in nucleus tractus solitarius

We tested the hypothesis that glucocorticoids attenuate changes in arterial pressure and renal sympathetic nerve activity (RSNA) in response to activation and blockade of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors within the nucleus of the solitary tract (NTS). Experiments were performed in Inactin-anesthetized male Sprague-Dawley rats treated for 7 +/- 1 days with a subcutaneous corticosterone (Cort) pellet or in control rats. Baseline mean arterial pressure (MAP) was significantly higher in Cort-treated rats (109 +/- 2 mmHg, n = 39) than in control rats (101 +/- 1 mmHg, n = 48, P < 0.05). In control rats, microinjection of AMPA (0.03, 0.1, and 0.3 pmol/100 nl) into the NTS significantly decreased MAP at all doses and decreased RSNA at 0.1 and 0.3 pmol/100 nl. Responses to AMPA in Cort-treated rats were attenuated at all doses of AMPA (P < 0.05). Responses to the AMPA-kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) were also significantly reduced in Cort-treated rats relative to control rats. Blockade of glucocorticoid type II receptors with mifepristone significantly enhanced responses to CNQX in both control and Cort rats. We conclude that glucocorticoids attenuate MAP and RSNA responses to activation and blockade of AMPA receptors in the NTS.

Involvement of I(1)-imidazoline receptors in baroreceptor reflex in the caudal ventrolateral medulla of rats

There is ample evidence to show the existence of center I(1)-imidazoline receptors that are involved in the regulation of cardiovascular activities. The purpose of this study was to examine the possible role of I(1)-imidazoline receptors and alpha(2)-adrenoceptors within the caudal ventrolateral medulla (CVLM) in mediating the baroreceptor reflex in anesthetized rats. Unilateral microinjection of idazoxan (2 nmol in 50 nl), a mixed antagonist of I(1)-imidazoline receptors and alpha(2)-adrenoceptors, into the CVLM significantly (P<0.01) decreased blood pressure (BP), heart rate (HR), and the firing rate of presympathetic neurons in the rostral ventrolateral medulla (RVLM) by 21+/-6 mmHg, 25+/-5 beats per min and 3.5+/-0.9 spikes/s, respectively. Moreover, unilateral injection of idazoxan into the CVLM significantly (P<0.01) reduced the inhibitory responses of the ipsilateral RVLM presympathetic neurons evoked by stimulation of aortic nerve and elevation of BP, and partially inhibited the neuronal cardiac cycle-related rhythm. Depressor responses evoked by aortic nerve stimulation were significantly (P<0.01) attenuated 10 and 20 min after bilateral microinjection of idazoxan (2 nmol in 50 nl for each side) into the CVLM (-20+/-4 and -30+/-4 vs. -40+/-1 mmHg). However, injection of yohimbine (500 pmol in 50 nl), a selective alpha(2)-adrenoceptor antagonist, into the CVLM did not affect the resting cardiovascular activities and baroreceptor reflex. It is concluded that the CVLM I(1)-imidazoline receptors are involved in maintenance of tonic cardiovascular activities and transmission of the baroreceptor reflex.

Effects of fetal ovine adrenalectomy on sympathetic and baroreflex responses at birth

Studies were performed to test the hypothesis that the absence of adrenal glucocorticoids late in gestation alters sympathetic and baroreflex responses before and immediately after birth. Fetal sheep at 130-131 days gestation (term 145 days) were subjected to bilateral adrenalectomy before the normal prepartum increase in plasma cortisol levels. One group of fetuses (n = 5) received physiological cortisol replacement with a continuous infusion of hydrocortisone (2 mg x day(-1) x kg(-1) for 10 days), whereas the other group received 0.9% NaCl vehicle (n = 5). All animals underwent a second surgery 48 h before the study for placement of a renal nerve recording electrode. Heart rate (HR), mean arterial blood pressure (MABP), renal sympathetic nerve activity (RSNA), and baroreflex control of HR and RSNA were studied before and after cesarean section delivery. At the time of study (140-141 days gestation), fetal plasma cortisol concentration was undetectable in adrenalectomized (ADX) fetuses and 58 +/- 9 ng/ml in animals receiving cortisol replacement (ADX + F). Fetal and newborn MABP was significantly greater in ADX + F relative to ADX animals. One hour after delivery, MABP increased 13 +/- 3 mmHg and RSNA increased 91 +/- 12% above fetal values in ADX + F (both P < 0.05) but remained unchanged in ADX lambs. The midpoint pressures of the fetal HR and RSNA baroreflex function curves were significantly greater in ADX + F (54 +/- 3 and 56 +/- 3 mmHg for HR and RSNA curves, respectively) than ADX fetuses (45 +/- 2 and 46 +/- 3 mmHg). After delivery, the baroreflex curves reset toward higher pressure in ADX + F but not ADX lambs. These results suggest that adrenal glucocorticoids contribute to cardiovascular regulation in the late-gestation fetus and newborn by modulating arterial baroreflex function and sympathetic activity.

Opposing actions of adrenal androgens and glucocorticoids on alternative splicing of Slo potassium channels in bovine chromaffin cells

Pituitary ablation (hypophysectomy) in rats was previously reported to cause a precipitous change in the relative abundance of two alternative splice variants of the "BK"- or "Maxi K"-encoding Slo gene in adrenal chromaffin cells. Inclusion of the optional "STREX" exon (STRess axis-regulated EXon) in a C-terminal splice site was reduced, in preference to the variant lacking an insert at this site. Adrenocorticotropic hormone (ACTH) injections prevented the drop in STREX inclusion, implicating stress-axis function, as opposed to other pituitary functions. Because ACTH promotes synthesis and release of glucocorticoids (corticosterone or cortisol, depending on species), we hypothesized that glucocorticoids applied directly would promote STREX inclusion. Contrary to predictions, we report that direct application of glucocorticoids to bovine cells in vitro decreased STREX inclusion. This effect was blocked by the glucocorticoid receptor antagonist RU38486. As with glucocorticoids, synthesis and release of the adrenal androgen dehydroepiandrosterone (DHEA) increases in response to stress or elevated ACTH levels in some species. We report that direct application of DHEA increased expression of the STREX variant in cultured bovine cells. Two other androgens, androstenedione and testosterone, had similar effects. We hypothesize that Slo splicing in adrenal chromaffin cells in vivo is differentially regulated by the integrative, concentration- and time-dependent actions of glucocorticoids and androgens, with potentially important ramifications for stress-evoked catecholamine secretion.

Rapid changes in monoamine levels following administration of corticotropin-releasing factor or corticosterone are localized in the dorsomedial hypothalamus

Monoaminergic systems are important modulators of the neuroendocrine, autonomic, and behavioral responses to stress-related stimuli. The male roughskin newt (Taricha granulosa) was used as a model system to investigate the effects of corticotropin-releasing factor (CRF) or corticosterone administration on tissue concentrations of norepinephrine, epinephrine, dopamine, 3,4-dihydroxyphenylacetic acid, serotonin, and 5-hydroxyindoleacetic acid (5-HIAA) in microdissected brain areas. Intracerebroventricular infusion of 25 or 50 ng of CRF increased locomotor activity and site-specifically increased dopamine concentrations within the dorsomedial hypothalamus 30 min after treatment when compared to vehicle-treated controls. In further studies, male newts were treated as follows: (1) no injection, no handling, (2) saline injection, or (3) 10 microg corticosterone and then placed in a novel environment. Monoamine and monoamine metabolite concentrations were similar in the unhandled and saline-injected controls 20 min after treatment. In contrast, corticosterone-injected newts had elevated concentrations of dopamine, serotonin, and 5-HIAA in the dorsomedial hypothalamus (a region that contains dopamine- and serotonin-accumulating neuronal cell bodies in representatives of all vertebrate classes) but not in several other regions studied. These site-specific neurochemical effects parallel neurochemical changes observed in the dorsomedial hypothalamic nucleus of mammals following exposure to a variety of physical and psychological stress-related stimuli. Therefore, these changes may reflect highly conserved, site-specific neurochemical responses to stress and stress-related neurochemicals in vertebrates. Given the important role of the dorsomedial hypothalamus in neuroendocrine, autonomic, and behavioral responses to stress, and a proposed role for this region in fast-feedback effects of glucocorticoids on the hypothalamo-pituitary-adrenal axis, these stress-related monoaminergic changes are likely to have important physiological or behavioral consequences.

Glucocorticoids modulate baroreflex control of renal sympathetic nerve activity

Experiments were performed to determine the effects of glucocorticoids on arterial baroreceptor reflex control of renal sympathetic nerve activity (RSNA). Intravenous infusions of phenylephrine and nitroprusside were used to produce graded changes in arterial pressure (AP) in Inactin-anesthetized male Sprague-Dawley rats. Baroreflex control of RSNA was determined during a baseline period and 2 and 3 h after administration of the glucocorticoid type II receptor antagonist Mifepristone (30 mg/kg sc) or vehicle (oil). Corticosterone (cort) treatment (100 mg cort pellet sc for 2-3 wk) increased baseline AP from 115 +/- 2 to 128 +/- 1 mmHg. Cort treatment also decreased the gain coefficient and increased the midpoint of the baroreflex curve. Treatment of cort rats with Mifepristone decreased AP within 2 h and increased the gain coefficient and decreased the midpoint of the baroreflex function curve back toward values measured in control rats. Mifepristone altered the baroreflex function curve even when AP was maintained at baseline levels. Therefore, these data demonstrate for the first time that glucocorticoids can modulate baroreflex control of RSNA by a mechanism that is, in part, independent of changes in AP.

Nongenomic membrane actions of glucocorticoids in vertebrates

For decades, it was widely assumed that glucocorticoids (GCs) work solely through changes in gene expression to exert their physiological actions, a process that normally takes several hours to occur. However, recent evidence indicates that GCs might also act at the membrane through specific receptors to exert multiple rapid effects on various tissues and cells. GCs modulate hormone secretion, neuronal excitability, behavior, cell morphology, carbohydrate metabolism and other processes within seconds or minutes. These early actions occur independent of the genome and are transduced by the same biochemical effector pathways responsible for mediating rapid responses to neurotransmitters. The biological significance of most rapid GC effects are not well understood, but many might be related to the important functions that this hormone plays in modulating stress responses.

Corticosteroid actions from neuronal membrane to behavior: Neurophysiological mechanisms underlying rapid behavioral effects of corticosterone

Investigation of the rapid suppression of male courtship clasping behavior by corticosterone in roughskin newts (Taricha granulosa) has led to the identification of a specific neuronal membrane receptor for this stress steroid. This paper describes studies of the neurophysiological effects of the rapid, membrane receptor mediated action of corticosterone on neurons that are involved in the control of clasping. In freely behaving newts, medullary neurons, including reticulospinal neurons, process clasp-triggering sensory signals and participate in control of clasping movements. Corticosterone injection causes these brainstem neurons to show selective depression of clasping-related sensorimotor function. These corticosterone effects appear in 3-10 min and are closely associated with the simultaneous depression of clasping. In addition to these functionally specific effects, corticosterone simultaneously causes widespread, primarily depressive effects on neuronal activity and excitability in the medulla and elsewhere in the brain. Thus, the membrane actions of corticosterone lead to diverse neural effects, including changes in membrane excitability as well as specific, network-level actions that are apparent only during behavior. These rapid corticosterone effects strongly interact with actions of the neuropeptides vasotocin and corticotropin-releasing factor, such that the form and magnitude of the steroid's effects depend on the prevailing neuroendocrine state of the brain.Key words: glucocorticoid, membrane receptor, non-genomic, amphibian, reproduction.

Effects of cortisol on brain alpha2-adrenoceptors: potential role in stress

It has been proposed that behavioural changes induced by chronic psychosocial stress in male tree shrews might be related to alterations in the central nervous alpha2-adrenoceptor system. In the noradrenergic centres of the brain, alpha2-adrenoceptors function as autoreceptors regulating noradrenaline release. Chronic stress downregulates these receptors in several brain regions. Since during stress, the activity of the hypothalamus-pituitary-adrenal axis is increased leading to high concentrations of plasma glucocorticoids, we investigated whether the effects of chronic stress can be mimicked by cortisol treatments. Two experiments were performed: a short-term treatment (males were injected i.v. with 1.5 mg cortisol and brains were dissected 2 h later) and a long-term treatment (animals received the hormone in their drinking water for 5 days; daily uptake 3-7 mg). The short-term treatment (injection), similar to the stress effects, downregulated alpha2-adrenoceptors in several brain regions. In contrast, the long-term oral treatment induced regional receptor upregulation. These data show: (i) that glucocorticoids regulate alpha2-adrenoceptors in the brain; (ii) that the duration and/or the route of cortisol application determines the results: and (iii) that chronic stress effects are not only due to the long-term glucocorticoid exposure, but also to other elements of the stress response.

Pleiotropic signaling pathways in rapid, nongenomic action of glucocorticoid

The traditional genomic theory of steroid action does not fully explain the rapid effects of hormonal steroids, and it is thought that the nongenomic actions mediated by a putative membrane receptor may provide a plausible explanation. Although there is a rich body of evidence to substantiate the rapid, nongenomic effects of steroid hormones, the signal transduction pathways involved have proved to be complex and pleiotropic. Based on previous studies on the rapid, nongenomic actions of glucocorticoid (GC) and the G-protein-protein kinase pathways involved, including our own studies on PC12, SK-N-SH, BT-325 cells, and synaptosomes, in this review we will discuss the issue of multiple signal transduction pathways involved in the rapid, nongenomic effects of GC.