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

Multiscale model of dynamic neuromodulation integrating neuropeptide-induced signaling pathway activity with membrane electrophysiology

We developed a multiscale model to bridge neuropeptide receptor-activated signaling pathway activity with membrane electrophysiology. Typically, the neuromodulation of biochemical signaling and biophysics have been investigated separately in modeling studies. We studied the effects of Angiotensin II (AngII) on neuronal excitability changes mediated by signaling dynamics and downstream phosphorylation of ion channels. Experiments have shown that AngII binding to the AngII receptor type-1 elicits baseline-dependent regulation of cytosolic Ca(2+) signaling. Our model simulations revealed a baseline Ca(2+)-dependent response to AngII receptor type-1 activation by AngII. Consistent with experimental observations, AngII evoked a rise in Ca(2+) when starting at a low baseline Ca(2+) level, and a decrease in Ca(2+) when starting at a higher baseline. Our analysis predicted that the kinetics of Ca(2+) transport into the endoplasmic reticulum play a critical role in shaping the Ca(2+) response. The Ca(2+) baseline also influenced the AngII-induced excitability changes such that lower Ca(2+) levels were associated with a larger firing rate increase. We examined the relative contributions of signaling kinases protein kinase C and Ca(2+)/Calmodulin-dependent protein kinase II to AngII-mediated excitability changes by simulating activity blockade individually and in combination. We found that protein kinase C selectively controlled firing rate adaptation whereas Ca(2+)/Calmodulin-dependent protein kinase II induced a delayed effect on the firing rate increase. We tested whether signaling kinetics were necessary for the dynamic effects of AngII on excitability by simulating three scenarios of AngII-mediated KDR channel phosphorylation: (1), an increased steady state; (2), a step-change increase; and (3), dynamic modulation. Our results revealed that the kinetics emerging from neuromodulatory activation of the signaling network were required to account for the dynamical changes in excitability. In summary, our integrated multiscale model provides, to our knowledge, a new approach for quantitative investigation of neuromodulatory effects on signaling and electrophysiology.

The central renin-angiotensin system and sympathetic nerve activity in chronic heart failure

CHF (chronic heart failure) is a multifactorial disease process that is characterized by overactivation of the RAAS (renin-angiotensin-aldosterone system) and the sympathetic nervous system. Both of these systems are chronically activated in CHF. The RAAS consists of an excitatory arm involving AngII (angiotensin II), ACE (angiotensin-converting enzyme) and the AT1R (AngII type 1 receptor). The RAAS also consists of a protective arm consisting of Ang-(1-7) [angiotensin-(1-7)], the AT2R (AngII type 2 receptor), ACE2 and the Mas receptor. Sympatho-excitation in CHF is driven, in large part, by an imbalance of these two arms, with an increase in the AngII/AT1R/ACE arm and a decrease in the AT2R/ACE2 arm. This imbalance is manifested in cardiovascular-control regions of the brain such as the rostral ventrolateral medulla and paraventricular nucleus in the hypothalamus. The present review focuses on the current literature that describes the components of these two arms of the RAAS and their imbalance in the CHF state. Moreover, the present review provides additional evidence for the relevance of ACE2 and Ang-(1-7) as key players in the regulation of central sympathetic outflow in CHF. Finally, we also examine the effects of exercise training as a therapeutic strategy and the molecular mechanisms at play in CHF, in part, because of the ability of exercise training to restore the balance of the RAAS axis and sympathetic outflow.

Angiotensin type 2 receptors in the intermediolateral cell column of the spinal cord: negative regulation of sympathetic nerve activity and blood pressure

BACKGROUND

Our previous study demonstrated that AT2R in brainstem nuclei participated in the regulation of sympathetic outflow and cardiovascular function. However, the functional significance of AT2R in the intermediolateral cell column (IML) of the thoracic spinal cord in normal rats remains elusive. We hypothesized that AT2R activation in the IML exerts a sympatho-inhibitory effect.

METHODS AND RESULTS

Using Western-blot analysis, immunohistochemical staining and quantitative real-time PCR, both AT1R and AT2R expressions were detected in the spinal cord. The highest AT2R protein expression was found in the IML, while AT1R expression didn't display regional differences within the gray matter. Microinjection of Ang II into the IML dose-dependently elevated mean blood pressure (MAP, employing a transducer-tipped catheter) and renal sympathetic nerve activity (RSNA, using a pair of platinum-iridium recording electrodes), which were completely abolished by Losartan, and attenuated by TEMPOL and apocynin. Activation of AT2R in the IML with CGP42112 evoked hypotension (ΔMAP: -21 ± 4 mmHg) and sympatho-inhibition (RSNA: 73 ± 3% of baseline), which were completely abolished by PD123319 and l-NAME. Blockade of AT2R in the IML with PD123319 significantly increased MAP (11 ± 1 mmHg) and sympathetic nerve activity (RSNA: 133 ± 13% of baseline). Moreover, PD123319 significantly enhanced the Ang II induced pressor response. Furthermore, in isolated IML neurons, CGP42112 treatment augmented potassium current and decreased resting membrane potential by employing whole-cell patch clamp.

CONCLUSION

In the normal condition, AT2R in the IML tonically inhibits sympathetic activity through an NO/NOS dependent pathway and subsequent potassium channel activation.

Angiotensin II increased neuronal stem cell proliferation: role of AT2R

Angiotensin II (Ang II), known a potent vasoactive substance in the renin-angiotensin system in the brain, plays a critical role in systemic blood pressure control. However, increasing evidence indicated that the physiological role of Ang II go beyond its vasoactive effect. In the present study, we demonstrated that Ang II type-1 receptor (AT1R) and type-2 receptor (AT2R) were expressed in primary rat hippocampal neuronal stem cells (NSCs). Treatment of rat hippocampal NSCs with Ang II increased cell proliferation. Pretreatment of NSCs with specific AT2R, but not AT1R, antagonist significantly suppressed Ang II-induced cell proliferation. Furthermore, Ang II stimulated ERK and Akt phosphorylation in NSCs. Pretreatment of MEK inhibitor, but not PI3K inhibitor, inhibited Ang II-induced ERK phosphorylation as well as cell proliferation. In addition, stimulation of NSCs with Ang II decreased expression of K_V 1.2/K_V 3.1 channels and blocked K⁺ currents which lie downstream of ERK activation. Taken together, these findings underpin the role of AT2R as a novel target that regulates cell proliferation mediated by Ang II with implications for therapeutic intervention for regulation of neurogenesis.

A current view of brain renin-angiotensin system: Is the (pro)renin receptor the missing link?

The renin-angiotensin system (RAS) plays a central role in the brain to regulate blood pressure (BP). This role includes the modulation of sympathetic nerve activity (SNA) that regulates vascular tone; the regulation of secretion of neurohormones that have a critical role in electrolyte as well as fluid homeostasis; and by influencing behavioral processes to increase salt and water intake. Based on decades of research it is clear that angiotensin II (Ang II), the major bioactive product of the RAS, mediates these actions largely via its Ang II type 1 receptor (AT1R), located within hypothalamic and brainstem control centers. However, the mechanisms of brain RAS function have been questioned, due in large part to low expression levels of the rate limiting enzyme renin within the central nervous system. Tissue localized RAS has been observed in heart, kidney tubules and vascular cells. Studies have also given rise to the hypothesis for localized RAS function within the brain, so that Ang II can act in a paracrine manner to influence neuronal activity. The recently discovered (pro)renin receptor (PRR) may be key in this mechanism as it serves to sequester renin and prorenin for localized RAS activity. Thus, the PRR can potentially mitigate the low levels of renin expression in the brain to propagate Ang II action. In this review we examine the regulation, expression and functional properties of the various RAS components in the brain with particular focus on the different roles that PRR may have in BP regulation and hypertension.

Imbalance of angiotensin type 1 receptor and angiotensin II type 2 receptor in the rostral ventrolateral medulla: potential mechanism for sympathetic overactivity in heart failure

Upregulation of angiotensin II type 1 receptors (AT(1)R) in the rostral ventrolateral medulla (RVLM) contributes to the sympathoexcitation in the chronic heart failure (CHF). However, the role of angiotensin II type 2 receptor (AT(2)R) is not clear. In this study, we measured AT(1)R and AT(2)R protein expression in the RVLM and determined their effects on renal sympathetic nerve activity, blood pressure, and heart rate in anesthetized sham and CHF rats. We found that (1) although AT(1)R expression in the RVLM was upregulated, the AT(2)R was significantly downregulated (CHF: 0.06+/-0.02 versus sham: 0.15+/-0.02, P<0.05); (2) simultaneously stimulating RVLM AT(1)R and AT(2)R by angiotensin II evoked sympathoexcitation, hypertension, and tachycardia in both sham and CHF rats with greater responses in CHF; (3) stimulating RVLM AT1R with angiotensin II plus the specific AT(2)R antagonist PD123319 induced a larger sympathoexcitatory response than simultaneously stimulating AT(1)R and AT(2)R in sham rats, but not in CHF; (4) activating RVLM AT(2)R with CGP42112 induced a sympathoinhibition, hypotension, and bradycardia only in sham rats (renal sympathetic nerve activity: 36.4+/-5.1% of baseline versus 102+/-3.9% of baseline in artificial cerebrospinal fluid, P<0.05); (5) pretreatment with 5,8,11,14-eicosatetraynoic acid, a general inhibitor of arachidonic acid metabolism, into the RVLM attenuates the CGP42112-induced sympathoinhibition. These results suggest that AT(2)R in the RVLM exhibits an inhibitory effect on sympathetic outflow, which is, at least partially, mediated by an arachidonic acid metabolic pathway. These data implicate a downregulation in the AT(2)R as a contributory factor in the sympathoexcitation in CHF.

NADPH oxidase contributes to angiotensin II signaling in the nucleus tractus solitarius

Angiotensin II (AngII), acting through angiotensin type 1 (AT1) receptors, exerts powerful effects on central autonomic networks regulating cardiovascular homeostasis and fluid balance; however, the mechanisms of AngII signaling in functionally defined central autonomic neurons have not been fully elucidated. In vascular cells, reactive oxygen species (ROS) generated by the enzyme NADPH oxidase play a major role in AngII signaling. Thus, we sought to determine whether NADPH oxidase is present in central autonomic neurons and, if so, whether NADPH oxidase-derived ROS are involved in the effects of AngII on these neurons. The present studies focused on the intermediate dorsomedial nucleus of the solitary tract (dmNTS) because this region receives autonomic afferents via the vagus nerve and is an important site of AngII actions. Using double-label immunoelectron microscopy, we found that the essential NADPH oxidase subunit gp91phox is present in somatodendric and axonal profiles containing AT1 receptors. The gp91phox-labeled dendrites received inputs from large axon terminals resembling vagal afferents. In parallel experiments using patch clamp of dissociated NTS neurons anterogradely labeled via the vagus, we found that AngII potentiates the L-type Ca2+ currents, an effect mediated by AT1 receptors and abolished by the ROS scavenger Mn(III) tetrakis (4-benzoic acid) porphyrin chloride. The NADPH oxidase assembly inhibitor apocynin and the peptide inhibitor gp91phox docking sequence, but not its scrambled version, also blocked the potentiation. The results provide evidence that NADPH oxidase-derived ROS are involved in the effects of AngII on Ca2+ influx in NTS neurons receiving vagal afferents and support the notion that ROS are important signaling molecules in central autonomic networks.

Functional genomics as an emerging strategy for the investigation of central mechanisms in experimental hypertension

Centrally mediated increases in sympathetic nerve activity and attenuated arterial baroreflexes contribute to the pathogenesis of hypertension. Despite the characterization of cellular and physiological mechanisms that regulate blood pressure and alterations that contribute to hypertension, the genetic and molecular basis of this pathophysiology remains poorly understood. Strategies to identify genes that contribute to central pathophysiologic mechanisms in hypertension include integrative biochemistry and physiology as well as functional genomics. This article summarizes recent progress in applying functional genomics to elucidate the genetic basis of altered central blood pressure regulatory mechanisms in hypertension. We describe approaches others and we have undertaken to investigate gene expression profiles in hypertensive models in order to identify genes that contribute to the pathogenesis of hypertension. Finally, we provide the readers a roadmap for negotiating the route from experimental findings of gene expression profiling to translating their therapeutic potential. The combination of gene expression profiling and the phenotypic characterization of in vitro and in vivo loss or gain of function experiments for candidate genes have the potential to identify genes involved in the pathogenesis of hypertension and may present novel targets for therapy.

PI3-kinase inhibitors abolish the enhanced chronotropic effects of angiotensin II in spontaneously hypertensive rat brain neurons

Angiotensin II (Ang II), acting at Ang II type 1 receptors (AT1Rs), increases the firing rate of neurons from Wistar-Kyoto (WKY) rat brain via protein kinase C (PKC)- and calcium-calmodulin kinase II (CaMKII)-dependent mechanisms. The objectives of this study were twofold; first, to compare the Ang-II-stimulated increase in firing of neurons from WKY and spontaneous hypertensive rats (SHR) and second, to elucidate the signaling mechanisms involved. Action potentials were measured in neurons cultured from SHR and WKY rat brains using the whole cell configuration of the patch-clamp technique in the current-clamp mode. Ang II (100 nM) caused three- and sixfold increases in neuronal firing rate in WKY rat and SHR neurons, respectively; effects that were abolished by the AT1R antagonist Losartan (1 microM). Co-administration of calphostin C (10 microM, a PKC inhibitor) and KN-93 (10 microM, a CaMKII inhibitor) completely blocked this Ang II action in WKY rat neurons, while they caused only a approximately 50% attenuation in SHR neurons. The residual increase in firing rate produced by Ang II in SHR neurons was blocked by inhibitors of phosphatidylinositol 3 kinase (PI3-kinase), either LY 294002 (10 microM) or wortmannin (100 nM). These observations suggest that a PI3-kinase signaling pathway may be responsible for the enhanced chronotropic effect produced by Ang II in SHR neurons.

Angiotensin II-induced ionic currents and signalling pathways in submandibular ganglion neurons

Angiotensin II (Ang II) is one of the most important vasoconstrictive hormones but is also known to act as a neuromodulator and a neurotransmitter in the central and peripheral nervous system. The submandibular ganglion (SMG) neuron is a parasympathetic ganglion which receives inputs from preganglionic cholinergic neurons, and innervates the submandibular salivary gland to control saliva secretion. In this study, the effects of Ang II on SMG neurons were investigated using the whole-cell patch clamp technique. Membrane currents evoked by a ramp pulse from +50 to -100 mV (-150 mV/500 ms) were compared in both the absence and presence of Ang II. In eight neurons tested, 1 microM Ang II increased inward currents by 42.0+/-8.2%. The reversal potentials of the Ang II-induced current were 0.2+/-0.6 mV. These increase of inward currents by Ang II were antagonized by losartan, a selective antagonist of AT(1) receptors. Intracellular dialysis with 0.1mM guanosin 5'-O-(2-thiodiphosphate) (GDP-beta-S), a G-proteins blocker, and anti-G(q/11) antibody attenuated Ang II-induced ionic current. In addition, pretreatment of neurons with 10 microM staurosporine (stauro), a protein kinase C (PKC) inhibitor, 0.5 microM PMA, a PKC activator, and 10 microM KN-93, a Ca2+/calmodulin-dependent protein kinase II (CaM K II) inhibitor, attenuated Ang II-induced ionic current in SMG neurons. The data presented here demonstrated that Ang II-induced ionic current via G(q/11)-proteins involving both PKC and CaM K II pathways in SMG neurons.

Transduction of a functional domain of the AT1 receptor in neurons by HIV-Tat PTD

Despite advances in transgenic and gene transfer technologies, in vivo structure-function studies of the angiotensin II type I receptor (AT1R) have revealed limited information on the diverse actions of angiotensin II. Our objective in the present study was to determine if protein transduction technology with the use of the HIV-Tat protein transduction domain could fill this gap. Recombinant HIV-Tat protein transduction domain fused to EGFP and to the third intracellular loop of the AT1R was expressed. Incubation of hypothalamus and brainstem neurons with this peptide indicated an efficient transport of the protein to most of the cells. This transduction was accompanied by an increase in neuronal firing rate, an effect similar to that observed with angiotensin II stimulation of the neuronal AT1R. The characteristics of the chronotropic effects of recombinant third intracellular loop and its synthetic counterpart were similar and comparable to the effects of angiotensin II on these neurons. In addition, in the presence of the protein kinase C inhibitor calphostin C, the peptide failed to increase firing rate. These observations demonstrated that transduction of neurons with the third intracellular loop of the AT1R produces chronotropic effects similar to those induced by angiotensin II. The data suggests that protein transduction technology could be useful for in vivo AT1R domain transduction.

Angiotensin II-induced inhibition of calcium currents via G(q/11)-protein involving protein kinase C in hamster submandibular ganglion neurons

Angiotensin II (AngII) is one of the most important vasoconstrictive hormones but is also known to act as a neuromodulator and a neurotransmitter in the central and peripheral nervous systems. In a previous study, we have shown that AngII, mediated by AT(1) receptors, inhibits voltage-dependent calcium channel (VDCC) currents (I(Ca)) via G-proteins in submandibular ganglion (SMG) neurons. In this study, we further characterized the signal transduction of AngII-induced inhibition of I(Ca). Application of 1 microM AngII inhibited I(Ca) by 32.1+/-2.7% (mean+/-S.E.M., n=9). Intracellular dialysis of anti-G(q/11) antibodies attenuated these inhibition (8.8+/-1.3%, n=6). In addition, treatment of protein kinase C (PKC) activator and inhibitor also attenuated these inhibition (8.0+/-0.9 and 9.8+/-0.9%, n=6 and 9, respectively). We therefore conclude that AngII inhibits VDCC via G(q/11)-proteins involving in SMG neurons. In addition, such PKC-dependent pathways mediated mainly L-type VDCC inhibition.

Decay in prepulse facilitation of calcium channel currents by Gi/o-protein attenuation in hamster submandibular ganglion neurons, but not Gq/11

The calcium ion influx through voltage-dependent calcium channels (VDCCs) has a vital role in the control of neurotransmitter release and membrane excitability. Prepulse facilitation is a phenomenon in which a strong depolarizing pulse induces a form of the VDCCs that exhibits an increased opening probability in response to a given test potential; this persists for several seconds after repolarization. It has been reported that prepulse facilitation occurs via dissociation of the guanosine triphosphate (GTP)-binding proteins (G-proteins) from the VDCCs and that recovery from facilitation involves rebinding of the G-proteins. The heterotrimeric G-proteins act as switches that regulate information processing circuits connecting cell surface G-protein-coupled-receptors to a variety of effectors. In this study, we have studied the characterization of G-protein subtypes in prepulse facilitation of VDCCs currents (Ica) in hamster submandibular ganglion (SMG) neurons, using whole-cell patch clamp recordings. Under control conditions, with GTP (0.1 mM) in the recording pipette, the rate of prepulse facilitation was 19.0 +/- 1.9% (n = 13). Intracellular dialysis with GDP-beta-S (0.1 mM), G-protein blocker, and pretreatment of neurons with N-ethylmaleimide (NEM) (100 microM for 2 min), Gi/o blocker, attenuated the rate of prepulse facilitation. Intracellular dialysis of anti-Gq/11-antibody did not alter it. These results suggest that prepulse facilitation of VDCCs is due to Gi/o-types of G-protein, but not to the Gq/11-type, in SMG neurons.

Angiotensin II-induced inhibition of calcium currents in hamster submandibular ganglion neurons

Angiotensin II (Ang II) is one of the most important vasoconstrictive hormones but is also known to act as a neuromodulator and a neurotransmitter in the central and peripheral nervous systems. In a previous study, we have shown that Ang II, via AT1 receptors, induced depolarization by inhibition of M-type K(+) channels and SK channels in submandibular ganglion (SMG) neurons. In this study, we investigated the effects of Ang II on calcium channel current (I(Ca)) in acutely dissociated SMG neurons by the patch-clamp technique using the whole-cell configuration. Ang II inhibited total I(Ca) by 32.1+/-2.7%. The half-maximum inhibitory concentration (IC(50)) of Ang II for inhibiting I(Ca) was 0.8 microM. In the presence of 1 microM losartan, which is a selective antagonist of AT1 receptors, the effect of Ang II was attenuated (7.6+/-1.5%). Application of a strong depolarizing voltage prepulse did not affect the Ang II-induced inhibition of I(Ca) (32.8+/-2.8%). Intracellular dialysis of GDP-beta-S attenuated the inhibition of I(Ca) (6.8+/-2.1%). The mean percentage inhibitions of L-, N- and P/Q-type VDCCs by Ang II were 29.1+/-1.7, 16.3+/-6.0 and 1.2+/-0.8%, respectively, of the total I(Ca).

ANG II-mediated inhibition of neuronal delayed rectifier K+ current: role of protein kinase C-alpha

It was previously determined that ANG II and phorbol esters inhibit Kv current in neurons cultured from newborn rat hypothalamus and brain stem in a protein kinase C (PKC)- and Ca2+-dependent manner. Here, we have further defined this signaling pathway by investigating the roles of "physiological" activators of PKC and different PKC isozymes. The cell-permeable PKC activators, diacylglycerol (DAG) analogs 1,2-dioctanoyl-sn-glycerol (1 micromol/l, n = 7) and 1-oleoyl-2-acetyl-sn-glycerol (1 micromol/l, n = 6), mimicked the effect of ANG II and inhibited Kv current. These effects were abolished by the PKC inhibitor chelerythrine (1 micromol/l, n = 5) or by chelation of internal Ca2+ (n = 8). PKC antisense (AS) oligodeoxynucleotides (2 micromol/l) against Ca2+-dependent PKC isoforms were applied to the neurons to manipulate the endogenous levels of PKC. PKC-alpha-AS (n = 4) treatment abolished the inhibitory effects of ANG II and 1-oleoyl-2-acetyl-sn-glycerol on Kv current, whereas PKC-beta-AS (n = 4) and PKC-gamma-AS (n = 4) did not. These results suggest that the angiotensin type 1 receptor-mediated effects of ANG II on neuronal Kv current involve activation of PKC-alpha.

Angiotensin II and calcium channels

Sixty years after its initial discovery, the octapeptide hormone angiotensin II (AngII) has proved to play numerous physiological roles that reach far beyond its initial description as a hypertensive factor. In spite of the host of target tissues that have been identified, only two major receptor subtypes, AT1 and AT2, are currently fully identified. The specificity of the effects of AngII relies upon numerous and complex intracellular signaling pathways that often mobilize calcium ions from intracellular stores or from the extracellular medium. Various types of calcium channels (store- or voltage-operated channels) endowed with distinct functional properties play a crucial role in these processes. The activity of these channels can be modulated by AngII in a positive and/or negative fashion, depending on the cell type under observation. This chapter reviews the main characteristics of AngII receptor subtypes and of the various calcium channels as well as the involvement of the multiple signal transduction mechanisms triggered by the hormone in the cell-specific modulation of the activity of these channels.

Hormonal and neurotransmitter roles for angiotensin in the regulation of central autonomic function

In this review we present the case for both hormonal and neurotransmitter actions of angiotensin II (ANG) in the control of neuronal excitability in a simple neural pathway involved in central autonomic regulation. We will present both single-cell and whole-animal data highlighting hormonal roles for ANG in controlling the excitability of subfornical organ (SFO) neurons. More controversially we will also present the case for a neurotransmitter role for ANG in SFO neurons in controlling the excitability of identified neurons in the paraventricular nucleus (PVN) of the hypothalamus. In this review we highlight the similarities between the actions of ANG on these two populations of neurons in an attempt to emphasize that whether we call such actions "hormonal" or "neurotransmitter" is largely semantic. In fact such definitions only refer to the method of delivery of the chemical messenger, in this case ANG, to its cellular site of action, in this case the AT1 receptor. We also described in this review some novel concepts that may underlie synthesis, metabolic processing, and co-transmitter actions of ANG in this pathway. We hope that such suggestions may lead ultimately to the development of broader guiding principles to enhance our understanding of the multiplicity of physiological uses for single chemical messengers.

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.

G-proteins are involved in 5-HT receptor-mediated modulation of N- and P/Q- but not T-type Ca2+ channels

5-HT produces voltage-independent inhibition of the N-, P/Q-, and T-type Ca2+ currents in sensory neurons of Xenopus larvae by acting on 5-HT1A and 5-HT1D receptors. We have explored the underlying mechanisms further and found that the inhibition of high voltage-activated (HVA) currents by 5-HT is mediated by a pertussis toxin-sensitive G-protein that activates a diffusible second messenger. Although modulation of T-type currents is membrane-delimited, it was not affected by GDP-beta-S (2 mM), GTP-gamma-S (200 microM), 5'-guanylyl-imidodiphosphate tetralithium (200 microM), aluminum fluoride (AlF4-, 100 microM), or pertussis toxin, suggesting that a GTP-insensitive pathway was involved. To investigate the modulation of the T currents further, we synthesized peptides that were derived from conserved cytoplasmic regions of the rat 5-HT1A and 5-HT1D receptors. Although two peptides derived from the third cytoplasmic loop inhibited the HVA currents by activating G-proteins and occluded the modulation of HVA currents by 5-HT, two peptides from the second cytoplasmic loop and the C tail had no effect. None of the four receptor-derived peptides had any effect on the T-type currents. We conclude that 5-HT modulates T-type channels by a membrane-delimited pathway that does not involve G-proteins and is mediated by a functional domain of the receptor that is distinct from that which couples to G-proteins.

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.

Structure of two fragments of the third cytoplasmic loop of the rat angiotensin II AT1A receptor. Implications with respect to receptor activation and G-protein selection and coupling

The structural bases that render the third intracellular loop (i3) of the rat angiotensin II AT1A receptor one of the cytoplasmic domains responsible for G-protein coupling are still unknown. The three-dimensional structures of two overlapping peptides mapping the entire i3 loop and shown to differently interact with purified G-proteins have been obtained by simulated annealing calculations, using NMR-derived constraints collected in 70% water/30% trifluoroethanol solution. While the NH2-terminal half, Ni3, residues 213-231, adopts a stable amphipathic alpha-helix, extending over almost the entire peptide, a more flexible conformation is found for the COOH-terminal half, Ci3, residues 227-242. For this peptide, a cis-trans isomerization around the Lys6-Pro7 peptide bond generates two exchanging isomers adopting similar conformations, with an alpha-helix spanning from Asn9 to Ile15 and a poorly defined NH2 terminus. A quite distinct structural organization is found for the sequence EIQKN, common to Ni3 and Ci3. The data do suggest that the extension and orientation of the amphipathic alpha-helix, present in the proximal part of i3, may be modulated by the distal part of the loop itself through the Pro233 residue. A molecular model where this possibility is considered as a mechanism for G-protein selection and coupling is presented.

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 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.

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 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.