Nongenomic mechanism of glucocorticoid inhibition of bradykinin-induced calcium influx in PC12 cells: possible involvement of protein kinase C

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.

The Anti-allergic Cromones: Past, Present, and Future

The anti-allergic cromones were originally synthesized in the 1960s by Fisons Plc, and the first drug to emerge from this program, disodium cromoglycate was subsequently marketed for the treatment of asthma and other allergic conditions. Whilst early studies demonstrated that the ability of the cromones to prevent allergic reactions was due to their 'mast cell stabilizing' properties, the exact pharmacological mechanism by which this occurred, remained a mystery. Here, we briefly review the history of these drugs, recount some aspects of their pharmacology, and discuss two new explanations for their unique actions. We further suggest how these findings could be used to predict further uses for the cromones.

Changes in the glucocorticoid receptor and Ca²⁺/calreticulin-dependent signalling pathway in the medial prefrontal cortex of rats with post-traumatic stress disorder

The glucocorticoid receptor (GR), calreticulin (CRT) and protein kinase C (PKC) have all been implicated in the Ca(2+)-dependent signalling pathway, which plays an important role in the plasticity of the central nervous system, learning and memory. The medial prefrontal cortex (mPFC) is known to be involved in mechanisms of learning and memory. In the present study, single prolonged stress (SPS) was used as an animal model of post-traumatic stress disorder (PTSD). The Morris water maze test was used to detect rats' ability for spatial memory and learning. A fluorescence spectrophotometer was used to measure the concentration of intracellular Ca(2+) in mPFC. Immunohistochemistry, immunofluorescence, western blot and reverse transcription polymerase chain reaction were used to explore changes in GR, CRT and PKC in mPFC of SPS rats. The concentration of Ca(2+) in mPFC was increased in the SPS rats. We found increased intensity of GR and CRT immunoreactivity and increased messenger RNA (mRNA) levels of GR, CRT and PKC in mPFC of the SPS groups, although the degree and time of increase was different among them. The protein levels of cytoplasmic GR, cytoplasmic CRT and cytoplasmic pPKC increased in mPFC of the SPS groups, whereas the protein level of nuclear GR decreased in comparison with the control group. As a conclusion, changed CRT and GR/PKC were involved in the mechanism of SPS-induced dysfunctional mPFC.

Glucocorticoid acts on a putative G protein-coupled receptor to rapidly regulate the activity of NMDA receptors in hippocampal neurons

Glucocorticoids (GCs) have been demonstrated to act through both genomic and nongenomic mechanisms. The present study demonstrated that corticosterone rapidly suppressed the activity of N-methyl-D-aspartate (NMDA) receptors in cultured hippocampal neurons. The effect was maintained with corticosterone conjugated to bovine serum albumin and blocked by inhibition of G protein activity with intracellular GDP-β-S application. Corticosterone increased GTP-bound G(s) protein and cyclic AMP (cAMP) production, activated phospholipase Cβ(3) (PLC-β(3)), and induced inositol-1,4,5-triphosphate (IP(3)) production. Blocking PLC and the downstream cascades with PLC inhibitor, IP(3) receptor antagonist, Ca(2+) chelator, and protein kinase C (PKC) inhibitors prevented the actions of corticosterone. Blocking adenylate cyclase (AC) and protein kinase A (PKA) caused a decrease in NMDA-evoked currents. Application of corticosterone partly reversed the inhibition of NMDA currents caused by blockage of AC and PKA. Intracerebroventricular administration of corticosterone significantly suppressed long-term potentiation (LTP) in the CA1 region of the hippocampus within 30 min in vivo, implicating the possibly physiological significance of rapid effects of GC on NMDA receptors. Taken together, our results indicate that GCs act on a putative G protein-coupled receptor to activate multiple signaling pathways in hippocampal neurons, and the rapid suppression of NMDA activity by GCs is dependent on PLC and downstream signaling.

Long-term continuous corticosterone treatment decreases VEGF receptor-2 expression in frontal cortex

OBJECTIVE

Stress and increased glucocorticoid levels are associated with many neuropsychiatric disorders including schizophrenia and depression. Recently, the role of vascular endothelial factor receptor-2 (VEGFR2/Flk1) signaling has been implicated in stress-mediated neuroplasticity. However, the mechanism of regulation of VEGF/Flk1 signaling under long-term continuous glucocorticoid exposure has not been elucidated.

MATERIAL AND METHODS

We examined the possible effects of long-term continuous glucocorticoid exposure on VEGF/Flk1 signaling in cultured cortical neurons in vitro, mouse frontal cortex in vivo, and in post mortem human prefrontal cortex of both control and schizophrenia subjects.

RESULTS

We found that long-term continuous exposure to corticosterone (CORT, a natural glucocorticoid) reduced Flk1 protein levels both in vitro and in vivo. CORT treatment resulted in alterations in signaling molecules downstream to Flk1 such as PTEN, Akt and mTOR. We demonstrated that CORT-induced changes in Flk1 levels are mediated through glucocorticoid receptor (GR) and calcium. A significant reduction in Flk1-GR interaction was observed following CORT exposure. Interestingly, VEGF levels were increased in cortex, but decreased in serum following CORT treatment. Moreover, significant reductions in Flk1 and GR protein levels were found in postmortem prefrontal cortex samples from schizophrenia subjects.

CONCLUSIONS

The alterations in VEGF/Flk1 signaling following long-term continuous CORT exposure represents a molecular mechanism of the neurobiological effects of chronic stress.

A novel strategy for development of glucocorticoids through non-genomic mechanism

Glucocorticoids (GCs) are routinely believed to take effect through genomic mechanisms, which are also largely responsible for GCs' side effects. Beneficial non-genomic effects of GCs have been reported as being independent of the genomic pathway. Here, we synthesized a new type of GCs, which took effect mainly via non-genomic mechanisms. Hydrocortisone was conjugated with glycine, lysine and phenylalanine to get a bigger molecular structure, which could hardly go through the cell membrane. Evaluation of the anti-inflammatory efficacy showed that hydrocortisone-conjugated glycine (HG) and lysine could inhibit neutrophil degranulation within 15 min. HG could inhibit IgE-mediated histamine release from mast cells via a non-genomic pathway, and rapidly alleviate allergic reaction. Luciferase reporter assay showed that HG would not activate the glucocorticoid response element within 30 min, which verified the rapid effects independent of the genomic pathway. The work proposes a novel insight into the development of novel GCs, and provides new tools for experimental study on non-genomic mechanisms.

Rapid stress-induced corticosterone rise in the hippocampus reverses serial memory retrieval pattern

We previously showed that an acute stress (electric footshocks) induced both a rapid plasma corticosterone rise and a reversal of serial memory retrieval pattern in a contextual serial discrimination (CSD) task. This study is aimed at determining (i) if the rapid stress effects on CSD performance are mediated by the hippocampus; (ii) if hippocampal corticosterone membrane receptor activation is involved in the rapid stress effects on CSD performance. In experiment 1, microdialysis in the dorsal hippocampus (dHPC) was used to measure the stress-induced corticosterone rise; in parallel, the effect of acute stress on CSD performance was evaluated. In addition, the functional involvement of corticosterone in the behavioral effects of stress was assessed by administering metyrapone, a corticosterone synthesis inhibitor, before stress. In experiment 2, the involvement of hippocampal corticosterone membrane receptors in the stress-induced reversal of CSD performance was studied by injecting corticosterone-bovine serum albumin (BSA) (a membrane-impermeable complex) in the dHPC in non stressed mice. Results showed that (i) the acute stress induced a rapid (15 min) and transitory (90 min) corticosterone rise into the hippocampus dHPC, and a reversal of serial memory retrieval pattern; (ii) both the endocrinal and memory stress-induced effects were blocked by metyrapone; (iii) corticosterone-BSA injection into the dHPC in non stressed mice mimicked the effects of stress on serial retrieval pattern. Overall, our study is first to show that (i) a rapid stress-induced corticosterone rise into the dHPC transitorily reverses serial memory retrieval pattern and (ii) hippocampal corticosterone membrane receptors activation is involved in the rapid effects of acute stress on serial memory retrieval.

Inhibition of ATP-induced Ca2+ influx by corticosterone in dorsal root ganglion neurons

In addition to the classic genomic effects, it is well known that glucocorticoids also have rapid, nongenomic effects on neurons. In the present study, the effect of corticosterone (CORT) on ATP-induced Ca(2+) mobilization in cultured dorsal root ganglion (DRG) neurons were detected with confocal laser scanning microscopy using fluo-4/AM as a calcium fluorescent indicator that could monitor real-time alterations of intracellular calcium concentration ([Ca(2+)]i). ATP, an algesic agent, caused [Ca(2+)]i increase in DRG neurons by activation of P2X receptor. Pretreatment with CORT (1 nM-1 microM for 5 min) inhibited ATP-induced [Ca(2+)]i increase in DRG neurons. The rapid inhibition of ATP-induced Ca(2+) response by CORT was concentration-dependent, reversible and could be blocked by glucocorticoid receptor antagonist RU38486 (10 microM). Furthermore, the inhibitory effect of CORT was abolished by protein kinase A inhibitor H89 (10 microM), but was not influenced by protein kinase C inhibitor Chelerythrine chloride (10 microM). On the other hand, membrane-impermeable bovine serum albumin-conjugated corticosterone had no effect on ATP-induced [Ca(2+)]i transients. These observations suggest that a nongenomic pathways may be involved in the effect of CORT on ATP-induced [Ca(2+)]i transients in cultured DRG neurons.

Glucocorticoid rapidly enhances NMDA-evoked neurotoxicity by attenuating the NR2A-containing NMDA receptor-mediated ERK1/2 activation

Glucocorticoid (GC) has been shown to affect the neuronal survival/death through a genomic mechanism, but whether or not it does through a nongenomic mechanism is unknown. Using a previously identified GR-deficient primary hippocampal neuron culture, we show here that a 15-min coexposure of N-methyl-D-aspartate (NMDA) with corticosterone at a stress-induced level significantly enhances neuronal death compared to NMDA alone. This enhancing effect of GC can be mimicked by the BSA-conjugated corticosterone, which is plasma membrane impermeable and cannot be blocked by RU38486 spironolactone. Furthermore, using a calcium-imaging technique, we found that B could increase both the percentage of neurons showing a significant increment of intracellular free calcium ([Ca2+](i)) due to NMDA stimulation and the amplitude of [Ca2+](i) increment in the individual responsive cells. Interestingly, this boosting effect of GC on [Ca2+](i) increment could be blocked by the NMDA receptor subunit 2A (NR2A)-specific antagonist [(R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydro-quinoxalin-5-yl)-methyl]-phosphonic acid (NVP-AAM077) but not by the NMDA receptor subunit 2B (NR2B)-specific antagonist Ro25-6981. Moreover, we also found that GC can dramatically attenuate the NMDA-induced activation of ERK1/2 without affecting that of p38; and that the NMDA-induced ERK1/2 activation and its attenuation by GC both can be occluded by the NVP-AAM077 but not by Ro25-6981. Consistently, the enhancing effect of GC on NMDA neurotoxicity can also be blocked by NVP-AAM077 and the ERK1/2 inhibitor PD98059 but not by Ro25-6981 and p38 inhibitor SB203580. Indeed, the NMDA neurotoxicity itself can be blocked by Ro25-6981 or SB203580, whereas it is increased by NVP-AAM077 and PD98059. Therefore, it is probable that NMDA triggers a prodeath signaling through the NR2B-p38 MAPK pathway, and a prosurvival signaling through the NR2A-ERK1/2 MAPK pathway, whereas the latter was negatively regulated by rapid GC action. Taken together, the present data suggest a nongenomic action by GC that enhances NMDA neurotoxicity through facilitating [Ca2+](i) increment and attenuating the NR2A-ERK1/2-mediated neuroprotective signaling, implicating a novel pathway underlying the regulatory effect of GC on neuronal survival/death.

Proteomic profiling of glucocorticoid-exposed myogenic cells: Time series assessment of protein translocation and transcription of inactive mRNAs

BACKGROUND

Prednisone, one of the most highly prescribed drugs, has well characterized effects on gene transcription mediated by the glucocorticoid receptor. These effects are typically occurring on the scale of hours. Prednisone also has a number of non-transcriptional effects (occurring on minutes scale) on protein signaling, yet these are less well studied. We sought to expand the understanding of acute effects of prednisone action on cell signaling using a combination of SILAC strategy and subcellular fractionations from C2C12 myotubes.

RESULTS

De novo translation of proteins was inhibited in both SILAC labeled and unlabeled C2C12 myotubes. Unlabeled cells were exposed to prednisone while SILAC labeled cells remained untreated. After 0, 5, 15, and 30 minutes of prednisone exposure, labeled and unlabeled cells were mixed at 1:1 ratios and fractionated into cytosolic and nuclear fractions. A total of 534 proteins in the cytosol and 626 proteins in the nucleus were identified and quantitated, using 3 or more peptides per protein with peptide based probability < or = 0.001. We identified significant increases (1.7- to 3.1- fold) in cytoplasmic abundance of 11 ribosomal proteins within 5 minutes of exposure, all of which returned to baseline by 30 min. We hypothesized that these drug-induced acute changes in the subcellular localization of the cell's protein translational machinery could lead to altered translation of quiescent RNAs. To test this, de novo protein synthesis was assayed after 15 minutes of drug exposure. Quantitative fluorography identified 16 2D gel spots showing rapid changes in translation; five of these were identified by MS/MS (pyruvate kinase, annexin A6 isoform A and isoform B, nasopharyngeal epithelium specific protein 1, and isoform 2 of Replication factor C subunit 1), and all showed the 5' terminal oligopyrimidine motifs associated with mRNA sequestration to and from inactive mRNA pools.

CONCLUSION

We describe novel approaches of subcellular proteomic profiling and assessment of acute changes on a minute-based time scale. These data expand the current knowledge of acute, non-transcriptional activities of glucocorticoids, including changes in protein subcellular localization, altered translation of quiescent RNA pools, and PKC-mediated cytoskeleton remodeling.

Cromoglycate drugs suppress eicosanoid generation in U937 cells by promoting the release of Anx-A1

Using biochemical, epifluorescence and electron microscopic techniques in a U937 model system, we investigated the effect of anti-allergic drugs di-sodium cromoglycate and sodium nedocromil on the trafficking and release of the anti-inflammatory protein Annexin-A1 (Anx-A1) when this was triggered by glucocorticoid (GC) treatment. GCs alone produced a rapid (within 5 min) concentration-dependent activation of PKCα/β (Protein Kinase C; EC 2.7.11.13) and phosphorylation of Anx-A1 on Ser²⁷. Both phosphoproteins accumulated at the plasma membrane and Anx-A1 was subsequently externalised thereby inhibiting thromboxane (Tx) B₂ generation. When administered alone, cromoglycate or nedocromil had little effect on this pathway however, in the presence of a fixed sub-maximal concentration of GCs, increasing amounts of the cromoglycate-like drugs caused a striking concentration-dependent enhancement of Anx-A1 and PKCα/β phosphorylation, membrane recruitment and Anx-A1 release from cells resulting in greatly enhanced inhibition of TxB₂ generation. GCs also stimulated phosphatase accumulation at the plasma membrane of U937 cells. Both cromoglycate and nedocromil inhibited this enzymatic activity as well as that of a highly purified PP2A phosphatase preparation. We conclude that stimulation by the cromoglycate-like drugs of intracellular Anx-A1 trafficking and release (hence inhibition of eicosanoid release) is secondary to inhibition of a phosphatase PP2A (phosphoprotein phosphatase; EC 3.1.3.16), which probably forms part of a control loop to limit Anx-A1 release. These experiments provide a basis for a novel mechanism of action for the cromolyns, a group of drugs that have long puzzled investigators.

Protein kinase C signaling in the hypothalamic arcuate nucleus regulates sexual receptivity in female rats

Rapid membrane-mediated estradiol signaling regulating sexual receptivity requires the interaction of the estrogen receptor (ER)-alpha and the metabotropic glutamate receptor 1a (mGluR1a). A cell signaling antibody microarray revealed that estradiol activated 42 proteins in the arcuate nucleus of the hypothalamus (ARH). To begin an analysis of various signaling pathways, protein kinase A and protein kinase C (PKC)-theta, whose signaling pathways have been implicated in the estradiol regulation of sexual receptivity, were examined. In the ARH sample, the increase in phospho-protein kinase A could not be confirmed by Western blotting, in either cytosolic or membrane fractions. However, the increase in phosphorylated PKCtheta seen with the pathway array was verified by Western blotting. To study whether rapid estradiol activation of PKC regulates the ARH-medial preoptic nucleus pathway regulating lordosis, mu-opioid receptor (MOR) internalization and lordosis reflex were tested. Blocking PKC in ARH with 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]3-(1H-indol-3-yl) maleimide significantly attenuated estradiol-induced MOR internalization. Furthermore, disruption of PKC signaling within the ARH at the time of estradiol treatment significantly diminished the lordosis reflex. Moreover, blocking PKC prevented MOR internalization when the circuit was activated by the mGluR1a agonist, (RS)-3,5-dihydroxyphenylglycine. Activation of PKC with phorbol 12, 13-dibutyrate induced MOR internalization, indicating that PKC was a critical step for membrane ERalpha-initiated mGluR1a-mediated cell signaling and phorbol 12, 13-dibutyrate significantly facilitated the lordosis reflex. Together these findings indicate that rapid membrane ERalpha-mGluR1a interactions activate PKCtheta cell signaling, which regulates female sexual receptivity.

Novel insights into mechanisms of glucocorticoid action and the development of new glucocorticoid receptor ligands

Glucocorticoids (GCs) are potent anti-inflammatory and immunosuppressant agents. Unfortunately, they also produce serious side effects that limit their usage. This discrepancy is the driving force for the intensive search for novel GC receptor ligands with a better benefit-risk ratio as compared to conventional GCs. A better understanding of the molecular mode of GC action might result in the identification of novel drug targets. Genomic GC effects are mediated by transrepression or transactivation, the latter being largely responsible for GC side effects. We here discuss novel GC receptor ligands, such as selective glucocorticoid receptor agonists (SEGRAs), which might optimize genomic GC effects as they preferentially induce transrepression with little or no transactivating activity. In addition to genomic GC effects, GCs also produce rapid genomic-independent activities, termed nongenomic, and we here review the possible implications of a recently reported mechanism underlying nongenomic GC-induced immunosuppression in T cells. It was shown that the synthetic GC dexamethasone targets membrane-bound GC receptors leading to impaired T cell receptor signaling. As a consequence, membrane-linked GC receptors might be a potential candidate target for GC therapy. The ultimate goal is to convert these molecular insights into new GC receptor modulators with an improved therapeutic index.

Glucocorticoids inhibit degranulation of mast cells in allergic asthma via nongenomic mechanism

BACKGROUND

Glucocorticoids (GCs) are the most potent anti-inflammatory agents available for allergic diseases including asthma, which are routinely believed to need several hours to take effect through regulating gene expression. Our previous report had shown that GCs could inhibit allergic asthma within 10 min, which the classical mechanism could not explain.

OBJECTIVE

To confirm the existence and verify the sites of GCs' rapid action, we investigated nongenomic effects of GCs on degranulation of mast cells in allergic asthma.

METHODS

The GCs' rapid action on airway mast cells deregulations was evaluated in the allergic asthma model of guinea pigs by the computer-assisted morphometry. Using whole-cell patch clamp and fluorometric assay, we examined GCs' nongenomic effect on IgE-mediated exocytosis and histamine release of rat basophilic leukaemia-2H3 mast cells. Employing the flash photolysis technique, we studied the role of Ca(2+) signal in the GCs' nongenomic effect.

RESULTS

Inhaled GCs significantly inhibited airway mast cells degranulation in the allergic asthma model of guinea pigs within 10 min. In vitro, GCs could rapidly inhibit IgE-mediated exocytosis and histamine release of mast cells, and neither GC nuclear receptor antagonist nor protein synthesis inhibitor could block the rapid action. We further demonstrated that GCs' nongenomic effect was not through direct action on secretory machinery, but was mediated by a reduction in the [Ca(2+)](i) elevation.

CONCLUSIONS

The study suggested for the first time that nongenomic pathway was involved in GCs' rapid inhibition on allergic asthma, and raised the possibility of new therapeutic strategies for allergic diseases including asthma.

Membrane estrogen receptor-alpha interactions with metabotropic glutamate receptor 1a modulate female sexual receptivity in rats

In rats, female sexual behavior is regulated by a well defined limbic-hypothalamic circuit that integrates sensory and hormonal information. Estradiol activation of this circuit results in mu-opioid receptor (MOR) internalization in the medial preoptic nucleus, an important step for full expression of sexual receptivity. Estradiol acts through both membrane and intracellular receptors to influence neuronal activity and behavior, yet the mechanism(s) and physiological significance of estradiol-mediated membrane responses in vivo have remained elusive. Recent in vitro evidence found that stimulation of membrane-associated estrogen receptor-alpha (ER alpha) led to activation of metabotropic glutamate receptor 1a (mGluR1a). Furthermore, mGluR1a signaling was responsible for the observed downstream effects of estradiol. Here we present data that show that ER alpha and mGluR1a directly interact to mediate a rapid estradiol-induced activation of MOR in the medial preoptic nucleus, leading to female sexual receptivity. In addition, blockade of mGluR1a in the arcuate nucleus of the hypothalamus resulted in a significant attenuation of estradiol-induced MOR internalization, leading to diminished female sexual behavior. These results link membrane-initiated estradiol actions to neural events modulating behavior, demonstrating the physiological importance of ER alpha-to-mGluR1a signaling.

Glucocorticoid signaling: a nongenomic mechanism for T-cell immunosuppression

Glucocorticoids were long believed to exert their effects through transcriptional regulation of glucocorticoid-receptor target genes. However, there is accumulating evidence for nongenomic glucocorticoid-receptor-dependent modulation of signal transduction pathways. Here, we review rapid glucocorticoid activities and focus on a novel mechanism that underlies nongenomic glucocorticoid-induced immunosuppression in T cells. The findings demonstrate a physical and functional interaction between the glucocorticoid receptor and the T-cell receptor (TCR) complex. In its unligated state, the glucocorticoid receptor has an important role in TCR signaling but, after glucocorticoid-receptor-ligand binding (caused by short-term treatment with the synthetic glucocorticoid dexamethasone), the TCR complex is disrupted, leading to impaired TCR signaling. These data reveal a dichotomal functional role for glucocorticoid receptors: one in the cytosol as part of the TCR complex and the other as a nuclear regulator of gene transcription. Drugs that selectively target membrane-bound glucocorticoid receptors might represent a novel immunosuppressive approach.

Vasopressin-induced translocation and proteolysis of protein kinase Cα in an amphibian brain: Modulation by corticosterone

In urodele amphibians, the hypothalamic neuropeptide arginine vasotocin and the adrenal steroid corticosterone interact to regulate reproductive behavior by actions in the brain. The present study investigated signal transduction pathways underlying acute effects of vasotocin and corticosterone, presumably mediated via "non-genomic" steroid action, in an amphibian brain. We used Western blot to examine the effects of corticosterone and the vasotocin receptor agonist arginine vasopressin, alone and in combination, on the subcellular localization and proteolytic processing of protein kinase C-alpha (PKCalpha) in tiger salamander brain tissue. Treatment of whole brain minces with vasopressin or vasotocin led to increases in PKCalpha in membrane fractions and concurrent decreases in PKCalpha in cytosolic fractions. Vasopressin or vasotocin treatment also induced the appearance in membrane and cytosolic fractions of a PKCalpha-immunoreactive band that corresponds to PKMalpha, the proteolytically generated, free catalytic subunit of PKCalpha. Treatment with corticosterone alone had no consistent effect on either PKCalpha or PKMalpha in either fraction. However, pretreatment with corticosterone reliably blocked vasopressin-induced increases in cytosolic PKMalpha. These data provide new information about the cellular mechanisms of action of vasopressin and corticosterone in the vertebrate brain and suggest a cellular mechanism by which the two hormones interact to regulate neuronal physiology and behavior.

Minireview: rapid glucocorticoid signaling via membrane-associated receptors

Glucocorticoids are secreted into the systemic circulation from the adrenal cortex and initiate a broad range of actions throughout the organism that regulate the function of multiple organ systems, including the liver, muscle, the immune system, the pancreas, fat tissue, and the brain. Delayed glucocorticoid effects are mediated by classical steroid mechanisms involving transcriptional regulation. Relatively rapid effects of glucocorticoids also occur that are incompatible with genomic regulation and invoke a noncanonical mode of steroid action. Studies conducted in several labs and on different species suggest that the rapid effects of glucocorticoids are mediated by the activation of one or more membrane-associated receptors. Here, we provide a brief review focused on multiple lines of evidence suggesting that rapid glucocorticoid actions are triggered by, or at least dependent on, membrane-associated G protein-coupled receptors and activation of downstream signaling cascades. We also discuss the possibility that membrane-initiated actions of glucocorticoids may provide an additional mechanism for the regulation of gene transcription.

The adrenal gland of newt Triturus carnifex (Amphibia, Urodela) following in vivo betamethasone administration

The response of the adrenal gland of Triturus carnifex to betamethasone administration was studied; the effects were evaluated by examination of the ultrastructural morphological features of the tissues as well as the serum levels of aldosterone, corticosterone, norepinephrine and epinephrine. In March and June, betamethasone significantly decreased the serum levels of aldosterone and corticosterone and the lipid droplet content in the steroidogenic cells. Moreover, betamethasone influenced the chromaffin tissue, enhancing in March (when the chromaffin cells produce norepinephrine and epinephrine in almost equal quantities) epinephrine serum levels and the numeric ratio between norepinephrine and epinephrine granules in the chromaffin cells. In June, (when the chromaffin cells contain almost exclusively norepinephrine granules) betamethasone administration raised norepinephrine serum levels, whereas a decrease in the numeric ratio between norepinephrine and epinephrine granules in the chromaffin cells was found. Finally, betamethasone administration did not evoke in June any increase in the mean number of epinephrine granules in the chromaffin cells and/or in epinephrine serum levels, as would be expected if phenyletanolamine-N-methyl transferase (PNMT) enzyme, converting norepinephrine into epinephrine, were activated by corticosteroids. The results of this study showed that betamethasone decreased aldosterone and corticosterone serum levels and enhanced catecholamine serum concentrations. Moreover, the present results suggest that a stimulatory role of glucocorticoids on PNMT enzyme may be ruled out.

Expression of spinal NMDA receptor and PKCgamma after chronic morphine is regulated by spinal glucocorticoid receptor

Spinal NMDA receptor (NMDAR), protein kinase C (PKC), and glucocorticoid receptor (GR) have all been implicated in the mechanisms of morphine tolerance; however, how these cellular elements interact after chronic morphine exposure remains unclear. Here we show that the expression of spinal NMDAR and PKCgamma after chronic morphine is regulated by spinal GR through a cAMP response element-binding protein (CREB)-dependent pathway. Chronic morphine (10 microg, i.t.; twice daily for 6 d) induced a time-dependent upregulation of GR, the NR1 subunit of NMDAR, and PKCgamma within the rat's spinal cord dorsal horn. This NR1 and PKCgamma upregulation was significantly diminished by intrathecal coadministration of morphine with the GR antagonist RU38486 or a GR antisense oligodeoxynucleotide. Intrathecal coadministration of morphine with an adenylyl cyclase inhibitor (2',5'-dideoxyadenosine) or a protein kinase A inhibitor (H89) also significantly attenuated morphine-induced NR1 and PKCgamma expression, whereas intrathecal treatment with an adenylyl cyclase activator (forskolin) alone mimicked morphine-induced expression of GR, NR1, and PKCgamma. Moreover, the expression of phosphorylated CREB was upregulated within the spinal cord dorsal horn after chronic morphine, and a CREB antisense oligodeoxynucleotide coadministered intrathecally with morphine prevented the upregulation of GR, NR1, and PKCgamma. These results indicate that spinal GR through the cAMP-CREB pathway played a significant role in NMDAR and PKCgamma expression after chronic morphine exposure. The data suggest that genomic interaction among spinal GR, NMDAR, and PKCgamma may be an important mechanism that contributes to the development of morphine tolerance.

Glucocorticoids stimulate the activity of large-conductance Ca2+ -activated K+ channels in pituitary GH3 and AtT-20 cells via a non-genomic mechanism

The effects of glucocorticoids on ion currents were investigated in pituitary GH3 and AtT-20 cells. In whole-cell configuration, dexamethasone, a synthetic glucocorticoid, reversibly increased the density of Ca2+ -activated K+ current (IK(Ca)) with an EC50 value of 21 +/- 5 microM. Dexamethasone-induced increase in IK(Ca) density was suppressed by paxilline (1 microM), yet not by glibenclamide (10 microM), pandinotoxin-Kalpha (1 microM) or mifepristone (10 microM). Paxilline is a blocker of large-conductance Ca2+ -activated K+ (BKCa) channels, while glibenclamide and pandinotoxin-Kalpha are blockers of ATP-sensitive and A-type K+ channels, respectively. Mifepristone can block cytosolic glucocorticoid receptors. In inside-out configuration, the application of dexamethasone (30 microM) into the intracellular surface caused no change in single-channel conductance; however, it did increase BKCa -channel activity. Its effect was associated with a negative shift of the activation curve. However, no Ca2+ -sensitiviy of these channels was altered by dexamethasone. Dexamethasone-stimulated channel activity involves an increase in mean open time and a decrease in mean closed time. Under current-clamp configuration, dexamethasone decreased the firing frequency of action potentials. In pituitary AtT-20 cells, dexamethasone (30 microM) also increased BKCa -channel activity. Dexamethasone-mediated stimulation of IK(Ca) presented here that is likely pharmacological, seems to be not linked to a genomic mechanism. The non-genomic, channel-stimulating properties of dexamethasone may partly contribute to the underlying mechanisms by which glucocorticoids affect neuroendocrine function.

Central glucocorticoid receptors modulate the expression and function of spinal NMDA receptors after peripheral nerve injury

Central glucocorticoid receptors (GRs) and NMDA receptors (NMDARs) have been shown to play a significant role in the mechanisms of neuropathic pain after peripheral nerve injury; however, how central GRs and NMDARs interact in this process remains unknown. Here we show that the expression and function of spinal NMDARs after peripheral nerve injury were modulated by central GRs. Chronic constriction nerve injury (CCI) in rats induced a time-dependent upregulation of NR1 and NR2 subunits of the NMDAR within the spinal cord dorsal horn ipsilateral to CCI. The upregulation of NMDARs was significantly diminished by intrathecal administration (twice daily for postoperative days 1-6) of either the GR antagonist RU38486 or an antisense oligonucleotide against GRs. Moreover, this CCI-induced expression of NMDARs was significantly attenuated in rats receiving intrathecal treatment with an interleukin-6 (IL-6) antiserum and in mice with protein kinase Cgamma (PKCgamma) knock-out. Because IL-6 and PKCgamma mediated the upregulation of central GRs after CCI as demonstrated previously, the results suggest that IL-6 and PKCgamma served as cellular mediators contributing to the GR-mediated expression of NMDARs after CCI. Functionally, nociceptive behaviors induced by NMDAR activation and CCI were reversed by a single intrathecal administration of the GR antagonist RU38486. Conversely, a single intrathecal injection with the noncompetitive NMDAR antagonist MK-801 reversed neuropathic pain behaviors exacerbated by the GR agonist dexamethasone in CCI rats. These data suggest that interactions between central GRs and NMDARs through genomic and nongenomic regulation may be an important mechanism critical to neuropathic pain behaviors in rats.

Effect of protein kinase C activation on intracellular Ca2+ signaling and integrity of intestinal epithelial cells

Protein kinase C (PKC) activation and increases in cytosolic Ca(2+) cause intestinal injury. Since PKC activation can alter Ca(2+) homeostasis and increase Ca(2+) levels, we examined the effects of PKC activation on intestinal cellular integrity and the role of Ca(2+) signaling in this response. The epithelial cell line, IEC-18 was incubated with the PKC activator phorbol myristate acetate (PMA; 0.1-1.0 microM). In some experiments, cells were incubated in Ca(2+)-free medium. PMA treatment produced a concentration-dependent increase in cell injury and PKC activity. This response was attenuated by addition of the pan-specific PKC inhibitor, GF 109203X. Furthermore, cell viability was maintained in cells preincubated with PKC isoform-specific inhibitors to PKCalpha, PKCdelta and PKCepsilon. Cell injury was also reduced if cells were incubated in Ca(2+)-free medium or in the presence of the Ca(2+) channel antagonist, verapamil or the intracellular chelator BAPTA-AM. PMA, but not the inactive phorbol ester, 4alphaPMA, induced a dose-dependent increase in cellular Ca(2+) that was characterized by a rapid, transient spike followed by a tonic plateau phase which approximated control levels. These responses were eliminated by the addition of BAPTA-AM. Furthermore the increase in the Ca(2+) spike was reduced or eliminated by co-incubation with the PKCdelta antagonist, rottlerin. Inhibition of PKCalpha or PKCepsilon was less effective or ineffective in this regard. These data suggest that PKC activation via PMA challenge affects the integrity of rat intestinal epithelial cells. PKCdelta, but not PKCepsilon or PKCalpha activation appears to mediate this effect via an increase in cellular Ca(2+).

Expression of central glucocorticoid receptors after peripheral nerve injury contributes to neuropathic pain behaviors in rats

Peripheral glucocorticoid receptors (GRs) play a significant role in the anti-inflammatory effects of glucocorticoids; however, the role of central GRs in nociceptive behaviors after peripheral nerve injury (neuropathic pain behaviors) remains unknown. Here we show that the development of neuropathic pain behaviors (thermal hyperalgesia and mechanical allodynia) induced by chronic constriction nerve injury (CCI) in rats was attenuated by either the GR antagonist RU38486 (4 = 2 > 1 = 0.5 microg) or a GR antisense oligonucleotide administered intrathecally twice daily for postoperative days 1-6. The development of thermal hyperalgesia and mechanical allodynia after CCI also was prevented in adrenalectomized rats, whereas the GR agonist dexamethasone (100 microg/kg) given subcutaneously twice daily for postoperative day 1-6 restored CCI-induced neuropathic pain behaviors in the adrenalectomized rats. Mechanistically, CCI induced a time-dependent and region-specific expression of neuronal GRs primarily within the spinal cord dorsal horn ipsilateral to nerve injury, which showed a time course parallel to that of the development of neuropathic pain behaviors. Moreover, the expression of neuronal GR after CCI was mediated in part through an elevated spinal level of interleukin-6 (IL-6) and protein kinase Cgamma (PKCgamma), because intrathecal treatment with an IL-6 antiserum, a PKC inhibitor (cheryrithrine), or PKCgamma knock-out substantially reduced the expression of neuronal GRs as well as neuropathic pain behaviors after CCI. These findings indicate a central role of neuronal GRs in the mechanisms of neuropathic pain behaviors in rats and suggest a potential role for GR antagonists in clinical management of neuropathic pain.

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.