Biomedical subjects

C Sumners

Publications and source records attributed to C Sumners.

At least 37 records · Page 2Linked to original sources

PubMed1999

Angiotensin II type 1 receptor-modulated signaling pathways in neurons.

Mammalian brain contains high densities of angiotensin II (Ang II) type 1 (AT1) receptors, localized mainly to specific nuclei within the hypothalamus and brainstem regions. Neuronal AT1 receptors within these areas mediate the stimulatory actions of central Ang II on blood pressure, water and sodium intake, and vasopressin secretion, effects that involve the modulation of brain noradrenergic pathways. This review focuses on the intracellular events that mediate the functional effects of Ang II in neurons, via AT1 receptors. The signaling pathways involved in short-term changes in neuronal activity, membrane ionic currents, norepinephrine (NE) release, and longer-term neuromodulatory actions of Ang II are discussed. It will be apparent from this discussion that the signaling pathways involved in these events are often distinct.

Angiotensin II↗

PubMed1999

Angiotensin II decreases neuronal delayed rectifier potassium current: role of calcium/calmodulin-dependent protein kinase II.

Angiotensin II (Ang II) acts at specific receptors located on neurons in the hypothalamus and brain stem to elicit alterations in blood pressure, fluid intake, and hormone secretion. These actions of Ang II are mediated via Ang II type 1 (AT1) receptors and involve modulation of membrane ionic currents and neuronal activity. In previous studies we utilized neurons cultured from the hypothalamus and brain stem of newborn rats to investigate the AT1 receptor-mediated effects of Ang II on neuronal K+ currents. Our data indicate that Ang II decreases neuronal delayed rectifier (Kv) current, and that this effect is partially due to activation of protein kinase C (PKC), specifically PKCalpha. However, the data also indicated that another Ca2+-dependent mechanism was also involved in addition to PKC. Because Ca2+/calmodulin-dependent protein kinase II (CaM KII) is a known modulator of K+ currents in neurons, we investigated the role of this enzyme in the AT1 receptor-mediated reduction of neuronal Kv current by Ang II. The reduction of neuronal Kv current by Ang II was attenuated by selective inhibition of either calmodulin or CaM KII and was mimicked by intracellular application of activated (autothiophosphorylated) CaM KIIalpha. Concurrent inhibition of CaM KII and PKC completely abolished the reduction of neuronal Kv by Ang II. Consistent with these findings is the demonstration that Ang II increases CaM KII activity in neuronal cultures, as evidenced by increased levels of autophosphorylated CaM KIIalpha subunit. Last, single-cell reverse transcriptase (RT)-PCR analysis revealed the presence of AT1 receptor-, CaM KIIalpha-, and PKCalpha subunit mRNAs in neurons that responded to Ang II with a decrease in Kv current. The present data indicate that the AT1 receptor-mediated reduction of neuronal Kv current by Ang II involves a Ca2+/calmodulin/CaM KII pathway, in addition to the previously documented involvement of PKC.

Angiotensin II↗

PubMed1999

Angiotensin II type 2 receptor-mediated apoptosis of cultured neurons from newborn rat brain.

Angiotensin II (Ang II) type 2 (AT2) receptors are highly expressed in neonate brain and may have a role in developmental processes such as apoptosis. Concurrent activation of c-Jun N-terminal kinase (JNK) and inhibition of Erk mitogen-activated protein kinase activities is important for apoptosis in many cells, and we previously demonstrated that stimulation of AT2 receptors causes decreased mitogen-activated protein kinase activity in neurons cultured from newborn rat hypothalamus and brain stem. Using such cultures we have employed terminal deoxynucleotidyl transferase-mediated deoxy-UTP nick end labeling and internucleosomal DNA fragmentation to assess the role of AT2 receptors in neuronal apoptosis. Ang II (100 nM; 4-72 h) alone produced no significant neuronal apoptosis, and AT2 receptor activation did not stimulate JNK activity. However, exposure of cultures to UV radiation (6 J/m2/sec for 4 sec) to stimulate JNK elicited neuronal apoptosis that was significantly enhanced by Ang II, an effect that was abolished by the AT2 receptor antagonist PD 123,319 (1 microM) or the serine/threonine phosphatase inhibitor okadaic acid (3 nM). Additionally, Ang II enhanced the UV radiation-induced decrease in the levels of the DNA repair enzyme poly-(ADP-ribose) polymerase. These data indicate that Ang II, via AT2 receptors and activation of a serine/threonine phosphatase, contributes to neuronal apoptosis.

Angiotensin II↗

PubMed1998

Angiotensin II in central nervous system physiology.

In summary, the prevailing concept is that brain Ang II increases blood pressure by activating AT1 receptors, and that these have a neuromodulating effect to increase the activity of autonomic nervous system. Pathways for Ang II stimulating thirst and blood pressure, increased vasopressin release and sympathetic activation have been outlined. Brain RAS synthesis, while incompletely understood, is active in the absence of a peripheral RAS. Angiotensin elicits specific receptor mediated signals in neurons, particularly in the hypothalamus and brainstem. These actions are due to neuronal membrane ionic currents and the regulation of transcription factors. The areas to be explored further are characterization and functional roles of the other AT receptor subtypes, such as AT4, AT(1-7) and nuclear AT-R. Their interactions with other peptides and transmitters, and their signaling pathways need to be investigated. The story that began 100 years ago with renin is certainly not ended and will continue to unfold as further investigations with new techniques progress.

Angiotensin II↗

PubMed1998

Angiotensin receptors and norepinephrine neuromodulation: implications of functional coupling.

The objective of this review is to examine the role of neuronal angiotensin II (Ang II) receptors in vitro. Two types of G protein-coupled Ang II receptors have been identified in cardiovascularly relevant areas of the brain: the AT1 and the AT2. We have utilized neurons in culture to study the signaling mechanisms of AT1 and AT2 receptors. Neuronal AT1 receptors are involved in norepinephrine (NE) neuromodulation. NE neuromodulation can be either evoked or enhanced. Evoked NE neuromodulation involves AT1 receptor-mediated, losartan-dependent, rapid NE release, inhibition of K+ channels and stimulation of Ca2+ channels. AT1 receptor-mediated enhanced NE neuromodulation involves the Ras-Raf-MAP kinase cascade and ultimately leads to an increase in NE transporter, tyrosine hydroxylase and dopamine beta-hydroxylase mRNA transcription. Neuronal AT2 receptors signal via a Gi protein and are coupled to activation of PP2A and PLA2 and stimulation of K+ channels. Finally, putative cross-talk pathways between AT1 and AT2 receptors will be discussed.

Animals↗

PubMed1998

Angiotensin II type 2 receptor stimulation of neuronal delayed-rectifier potassium current involves phospholipase A2 and arachidonic acid.

Angiotensin II (Ang II) elicits an Ang II type 2 (AT2) receptor-mediated increase in delayed-rectifier K+ current (IK) in neurons cultured from newborn rat hypothalamus and brainstem. This effect involves a pertussis toxin (PTX)-sensitive Gi protein and is abolished by inhibition of serine and threonine phosphatase 2A (PP-2A). Here, we determined that Ang II stimulates [3H]arachidonic acid (AA) release from cultured neurons via AT2 receptors. This effect of Ang II was blocked by inhibition of phospholipase A2 (PLA2) and by PTX. Because AA and its metabolites are powerful modulators of neuronal K+ currents, we investigated the involvement of PLA2 and AA in the AT2 receptor-mediated stimulation of IK by Ang II. Single-cell reverse transcriptase (RT)-PCR analyses revealed the presence of PLA2 mRNA in neurons that responded to Ang II with an increase in IK. The stimulation of neuronal IK by Ang II was attenuated by selective inhibitors of PLA2 and was mimicked by application of AA to neurons. Inhibition of lipoxygenase (LO) enzymes significantly reduced both Ang II- and AA-stimulated IK, and the 12-LO metabolite of AA 12S-hydroxyeicosatetraenoic acid (12S-HETE) stimulated IK. These data indicate the involvement of a PLA2, AA, and LO metabolite intracellular pathway in the AT2 receptor-mediated stimulation of neuronal IK by Ang II. Furthermore, the demonstration that inhibition of PP-2A abolished the stimulatory effects of Ang II, AA, and 12S-HETE on neuronal IK but did not alter Ang II-stimulated [3H]-AA release suggests that PP-2A is a distal event in this pathway.

Angiotensin II↗

PubMed1998

Neuronal ion channel signalling pathways: modulation by angiotensin II.

The brain contains both angiotensin II (Ang II) type 1 (AT1) and Ang II type 2 (AT2) receptors. Neuronal AT1 receptors mediate the stimulatory actions of Ang II on blood pressure, water and salt intake, and secretion of vasopressin. In contrast, neuronal AT2 receptors have been implicated in the stimulation of apoptosis and as being antagonistic to AT1 receptors. The physiological actions of Ang II in the brain, whether mediated by AT1 or AT2 receptors, involve changes in neuronal activity that are initiated by changes in the activity of membrane ionic currents and channels. This review focusses on the intracellular signalling pathways that couple neuronal AT1 and AT2 receptors to changes in the activity of membrane K+ and Ca2+ currents and channels. As will become clear from our discussion, the signalling pathways that are modulated by neuronal AT1 and AT2 receptors are quite distinct.

Angiotensin II↗

PubMed1998

Regulation of alpha2A-adrenergic receptor expression by epinephrine in cultured astroglia from rat brain.

Epinephrine (Epi) mediates various physiological effects via alpha2A-adrenergic receptors (alpha2A-ARs). Studies in mice with a point mutation in the gene for alpha2A-AR have shown that these receptors are responsible for the centrally mediated depressor effects of alpha2-AR agonists. These studies underscore the importance of understanding the basic cellular mechanisms involved in the expression of alpha2A-ARs, of which little is known. We use astroglia cultured from the hypothalamus and brainstem of adult Sprague-Dawley rats as a model system in which to study factors that regulate alpha2A-AR expression. These cells contain alpha2-ARs, which are predominately of the alpha2A-AR subtype. Our studies have shown that Epi causes a dose- and time-dependent decrease in steady-state levels of alpha2A-AR mRNA and number of alpha2A-ARs, effects that are mediated via alpha1- and beta-adrenergic receptors (alpha1-ARs and beta-ARs). These effects of Epi on alpha2A-AR mRNA and alpha2A-AR number are mimicked by activation of protein kinase C or increases in cellular cyclic AMP, which are intracellular messengers activated by alpha1-ARs and beta-ARs, respectively. Taken together, these results indicate that expression of alpha2A-ARs is regulated in a heterologous manner by Epi, via alpha1-AR- and beta-AR-mediated intracellular pathways.

Animals↗

PubMed1998

Angiotensin II stimulates activation of Fos-regulating kinase and c-Jun NH2-terminal kinase in neuronal cultures from rat brain.

c-Fos/c-Jun dimers (activating protein-1 transcription factor) are involved in the modulatory actions of angiotensin II (Ang II) on brain norepinephrine neurons, effects mediated via Ang II type 1 (AT1) receptors. The transcriptional activities of c-Fos and c-Jun can be augmented by Fos-regulating kinase (FRK) and c-Jun NH2-terminal kinase (JNK), respectively. In this study, we investigated the effects of Ang II on FRK and JNK activities in neurons cultured from newborn rat hypothalamus and brain stem, which include a population of catecholaminergic cells containing AT1 receptors. Ang II caused time-dependent increases in the activation of FRK and JNK, effects completely inhibited by the AT1 receptor antagonist losartan but not by the Ang II type 2 (AT2) receptor blocker PD123,319. The stimulation of FRK activity by Ang II was abolished by the protein kinase C (PKC) inhibitor GF109203X or the calcium chelator BAPTA, but not by inhibition of calmodulin or calcium/calmodulin-dependent protein kinase II. However, the activation of JNK by Ang II was not dependent on PKC or another calcium-dependent mechanism. These data demonstrate that Ang II stimulates activation of FRK and JNK in neuronal cells, actions that may contribute to the neuromodulatory effects of this peptide.

Angiotensin II↗

PubMed1997

Angiotensin receptors and norepinephrine neuromodulation: implications of functional coupling.

Animals↗

PubMed1997

Modulation of angiotensin II type 2 receptor mRNA in rat hypothalamus and brainstem neuronal cultures by growth factors.

This study investigates the regulatory effects of growth factors upon angiotensin II type 2 (AT2) mRNA levels in neurons co-cultured from newborn rat hypothalamus and brainstem. Incubation of cultured neurons with nerve growth factor (NGF; 5-50 ng/ml) caused time-dependent changes in the steady-state levels of AT2 receptor mRNA. Short-term (0.5-1.0 h) incubations with NGF resulted in significant increases in AT2 receptor mRNA, whereas longer-term incubations (4-24 h) caused significant decreases. Activation of NGF receptors is known to stimulate phospholipase C-gamma and subsequently activate protein kinase C (PKC). Incubation of cultures with the PKC activator, phorbol-12-myristate-13-acetate (PMA; 100 nM), caused temporal changes in AT2 receptor mRNA levels similar to those observed with NGF. By contrast, insulin (0.1-10 microg/ml) elicited only significant decreases in AT2 receptor mRNA levels. The observed abilities of NGF and insulin to regulate the expression of AT2 receptor mRNA are consistent with the fact that the AT2 receptor gene promoter region contains several cis DNA regulatory elements that respond to growth factor-stimulated transcription factors. These novel observations which show that NGF and insulin can regulate AT2 receptor mRNA in neurons derived from neonatal rat CNS lend support to the idea that AT2 receptors have a role in development and differentiation.

Animals↗

PubMed1997

Regulation of alpha 2A-adrenergic receptor mRNA in rat astroglial cultures: role of cyclic AMP and protein kinase C.

In this study we investigated regulation of alpha 2A-adrenergic receptor (alpha 2A-AR) mRNA in rat astroglial cultures by increases in intracellular cyclic AMP levels and by protein kinase C (PKC) activation. Treatment of astroglial cultures with forskolin (FSK), an adenylyl cyclase activator, or with the membrane-permeable cyclic AMP analogues dibutyryl cyclic AMP or Sp-adenosine 3',5'-cyclic monophosphothioate triethylamine caused time- and concentration-dependent decreases in the levels of a approximately 4.0-kb alpha 2A-AR mRNA transcript. Levels of alpha 2A-AR mRNA were reduced to approximately 10% of control levels within 4 h of 1 microM FSK treatment. PKC agonists [phorbol 12-myristate 13-acetate (PMA), phorbol 12,13-dibutyrate, and mezerein] also decreased alpha 2A-AR mRNA levels in a time- and concentration-dependent fashion. A 90% decrease in alpha 2A-AR mRNA levels was observed with 50 nM PMA in 4 h. The decrease in alpha 2A-AR mRNA levels caused by FSK and PMA treatment appears to be the result of decreases in transcription of the alpha 2A-AR gene and is not due to decreases in alpha 2A-AR mRNA degradation rate. These observations suggest that alpha 2A-AR mRNA levels are regulated by cyclic AMP and PKC.

Animals↗

PubMed1997

Cytokine- and endotoxin-induced nitric oxide synthase in rat astroglial cultures: differential modulation by angiotensin II.

Recent studies have shown that the stimulatory effects of bacterial endotoxin [lipopolysaccharide (LPS)] on inducible nitric oxide (NO) synthase (iNOS) in astroglia are significantly reduced by the peptide angiotensin II (Ang II). In the present study we have compared the modulatory actions of Ang II on cytokine- and LPS-stimulated iNOS in astroglia cultured from adult rat brain. Incubation of astroglia with LPS (100 ng/ml; 24 h) and/or combinations of interleukin-1 beta (IL-1 beta; 10 ng/ml, 24 h), interferon-gamma (IFN-gamma; 100 U/ml, 24 h), or tumor necrosis factor-alpha (TNF-alpha; 100 ng/ml, 24 h) resulted in significant increases of iNOS mRNA, iNOS protein, and NO production, with the latter indicated by increased nitrite accumulation. The effects of LPS, IL-1 beta, and TNF-alpha were significantly decreased by coincubation with Ang II (100 ng/ ml, 24 h). In contrast, Ang II did not alter the stimulation of iNOS mRNA levels and NO production elicited by IFN gamma. Therefore, Ang II differentially modulates the stimulatory actions of LPS and cytokines on iNOS, and subsequently NO production, in astroglia. These data suggest that Ang II may have an important modulatory role in intracerebral immune responses that involve production of NO by astroglia.

Angiotensin II↗

PubMed1997

Chronic ethanol increases N-methyl-D-aspartate-stimulated nitric oxide formation but not receptor density in cultured cortical neurons.

The effects of prolonged ethanol exposure on excitatory amino acid receptor stimulated nitric oxide (NO) formation were examined in primary rat cortical neuronal cultures. Chronic ethanol (4 days, 100 mM) potentiated N-methyl-D-aspartate (NMDA)-stimulated NO formation as determined by measuring the conversion of [3H]arginine to [3H]citrulline. In contrast, chronic ethanol had no effect on NO formation stimulated by kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxalonepropionic acid, or the calcium ionophore ionomycin. Potassium chloride-stimulated NO formation was also enhanced by chronic ethanol treatment, but this effect was not seen in the presence of the ionotropic glutamate receptor antagonists MK-801 and 6-cyano-7-nitroquinoxaline-2,3-dione. Immunoblot analysis of expression of NR1, NR2A, and NR2B receptor subunits showed no difference between control and chronic ethanol-treated cultures. In support of this apparent lack of change in receptor density, there was no difference in the specific binding of 125I-MK-801 between control and chronic ethanol-treated groups. These results demonstrate that prolonged ethanol exposure selectively enhanced NMDA receptor-stimulated NO formation, which may play an important role in alcohol dependence, withdrawal, and alcohol-associated brain damage. These results also suggest that chronic ethanol-induced increases in NMDA receptor function may not be due to a simple increase in the number of NMDA receptors or change in NMDA receptor subunit composition but may instead reflect more complicated and subtle changes.

Animals↗

PubMed1997

Modulation of K+ and Ca2+ currents in cultured neurons by an angiotensin II type 1a receptor peptide.

Angiotensin II (ANG II) inhibits delayed rectifier K+ current (IK) and stimulates total Ca2+ current (ICa) in neurons cocultured from newborn rat hypothalamus and brain stem, effects mediated via ANG II type 1 (AT1) receptors. Here, we identify potential G protein activator regions of the AT1 receptor responsible for initiating the intracellular changes that lead to alterations in these currents. Intracellular application into cultured neurons of a peptide corresponding to the third cytoplasmic loop of the AT1 receptor (AT1a/i3) mimicked the actions of ANG II on IK and ICa, whereas application of a peptide corresponding to the second cytoplasmic loop (AT1a/i2) did not alter these currents. This modulation of IK and ICa by AT1a/i3 involves intracellular messengers (G alpha q, protein kinase C, and intracellular Ca2+) that are identical to those involved in the modulation of IK and ICa following ANG II activation of AT1 receptors. These data provide functional evidence for a role of the third cytoplasmic loop of the AT1 receptor in G protein coupling and subsequent modulation of ion channel effectors.

Amino Acid Sequence↗

PubMed1997

Mechanisms underlying the chronotropic effect of angiotensin II on cultured neurons from rat hypothalamus and brain stem.

The chronotropic effect of angiotensin II (Ang II) was studied in cultured neurons from rat hypothalamus and brain stem with the use of the patch-clamp technique. Ang II (100 nM) increased the neuronal spontaneous firing rate from 0.8 +/- 0.3 (SE) Hz in control to 1.3 +/- 0.4 Hz (n = 7, P < 0.05). The amplitude of threshold stimulation was decreased by Ang II (100 nM) from 82 +/- 4 pA to 62 +/- 5 pA (n = 4, P < 0.05). These actions of Ang II were reversed by the angiotensin type 1 (AT1) receptor antagonist losartan (1 microM). In the presence of tetrodotoxin, Ang II (100 nM) significantly increased the frequency and the amplitude of the Cd2+-sensitive subthreshold activity of the cultured neurons. Ang II also stimulated the subthreshold early afterdepolarizations (EADs) to become fully developed action potentials. Similar to the action of Ang II, the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA, 100 nM) increased the firing rate from 0.76 +/- 0.3 Hz to 2.3 +/- 0.5 Hz (n = 6, P < 0.05) and increased the neuronal subthreshold activity. After neurons were intracellularly dialyzed with PKC inhibitory peptide (PKCIP, 5 microM), PMA alone, Ang II alone, or PMA plus Ang II no longer increased the action potential firing initiated from the resting membrane potential level. However, superfusion of PMA plus Ang II or Ang II alone increased the number of EADs that reached threshold and produced action potentials even in the presence of PKCIP (5 microM, n = 4). The actions of Ang II could also be mimicked by depolarizing pulse and K+ channel blockers (tetraethylammonium chloride or 4-aminopyridine). These results indicate that Ang II by activation of AT1 receptors increases neuronal excitability and firing frequency, and that this may involve both PKC dependent and -independent mechanisms.

4-Aminopyridine↗

PubMed1997

A-type K+ current in neurons cultured from neonatal rat hypothalamus and brain stem: modulation by angiotensin II.

The regulation of A-type K+ current (I(A)) and the single channel underlying I(A) in neonatal rat hypothalamus/brain stem cultured neurons were studied with the use of the patch-clamp technique. I(A) had a threshold of activation between -30 and -25 mV (n = 14). Steady-state inactivation of I(A) occurred between -80 and -70 mV and had a membrane voltage at which I(A) was half-maximum of -52.2 mV (n = 14). The mean values for the activation and inactivation (decay) time constants during a voltage step to +20 mV were 2.1 +/- 0.3 (SE) ms (n = 8) and 13.6 +/- 1.9 ms (n = 8), respectively. Single-channel recordings from outside-out patches revealed A-type K+ channels with voltage-dependent activation, 4-aminopyridine (4-AP) sensitivity, and inactivation kinetics similar to those of I(A). The single-channel conductance obtained from cell-attached patches was 15.8 +/- 1.3 pS (n = 4) in a physiological K+ gradient and 41.2 +/- 3.7 pS (n = 5) in symmetrical 140 mM K+. Angiotensin II (Ang II, 100 nM) reduced peak I(A) by approximately 20% during a voltage step to +20 mV (n = 8). Similarly, Ang II (100 nM) markedly reduced single A-type K+ channel activity by decreasing open probability (n = 4). The actions of Ang II on I(A) and single A-type K+ channels were reversible either by addition of the selective angiotensin type 1 (AT1) receptor antagonist losartan (1 microM) or on washout of the peptide. Thus the activation of AT1 receptors inhibits a tetraethylammonium-chloride-resistant, 4-AP-sensitive I(A) and single A-type K+ channels, and this may underlie some of the actions of Ang II on electrical activity of the brain.

4-Aminopyridine↗

PubMed1997

Functional interactions between neuronal AT1 and AT2 receptors.

Angiotensin II (Ang II), via the activation of the AT1 and AT2 receptors regulates electrophysiological responses of catecholaminergic neurons. This study was designed to determine if functional interactions between AT1 and AT2 receptors exist in a single neuron. Ang II caused two unique electrophysiological responses characteristic of receptor crosstalk. First, Ang II elicited an AT1 receptor-mediated decrease in I(K) followed by an AT2 receptor-mediated increase in I(K). Second, Ang II elicited an AT2 receptor-mediated increase in I(K) followed by an AT1 receptor-mediated decrease in I(K). AT1 and AT2 receptors were co-localized on the catecholaminergic neurons. These observations suggest, for the first time, the existence of a crosstalk between Ang II receptor subtypes that may be significant in the physiological activity of catecholaminergic neurons.

Angiotensin II↗