Angiotensin II type-2 (AT2) receptor-mediated inhibition of NMDA receptor signalling in neuronal cells

Conventional cardiovascular risk factors in Transient Global Amnesia: Systematic review and proposition of a novel hypothesis

Transient Global Amnesia (TGA) is an enigmatic amnestic syndrome. We conducted a systematic review to investigate the relationship between the conventional cardiovascular risk factors and TGA. MEDLINE, CENTRAL, EMBASE and PsycINFO were comprehensively searched and 23 controlled observational studies were retrieved. The prevalence of hypertension, diabetes mellitus, dyslipidemia and smoking was lower among patients with TGA compared to Transient Ischemic Attack. Regarding the comparison of TGA with healthy individuals, there was strong evidence suggesting a protective effect of diabetes mellitus on TGA and weaker evidence for a protective effect of smoking. Hypertension was associated with TGA only in more severe stages, while dyslipidemia was not related. In view of these findings, a novel pathophysiological hypothesis is proposed, in which the functional interactions of Angiotensin-II type-1 and N-methyl-D-aspartate receptors are of pivotal importance. The whole body of clinical evidence (nature of precipitating events, associations with migraine, gender-based association patterns) was integrated.

Angiotensin II AT2 receptors regulate NGF-mediated neurite outgrowth via the NO-cGMP pathway

We investigated whether Angiotensin II type 2 (AT2) receptor activation was involved in NGF-induced nerve regeneration. NGF-mediated neurite outgrowth in cultured dorsal root ganglia (DRG) cells was significantly inhibited by AT2 receptor antagonist (PD123,319) treatment. AT2 receptor knockdown also inhibited NGF-mediated neurite outgrowth. To determine the mechanisms, we analyzed the NO-cGMP pathway. The cGMP analog increased NGF-mediated nerve elongation, which inhibited by PD123,319. Furthermore, soluble guanylate cyclase expression was significantly less in NGF and PD123,319 treatment DRG than in NGF treatment alone. These results suggest that NGF-mediated neurite outgrowth is suppressed by AT2 receptor signaling via the NO-cGMP-PKG pathway.

Hypotensive and sympathoinhibitory responses to selective central AT2 receptor stimulation in spontaneously hypertensive rats

The type 2 angiotensin receptor (AT2R) has been suggested to counterbalance the type 1 angiotensin receptor (AT1R) in the central regulation of blood pressure and sympathetic tone. In the present study we investigated the blood pressure responses to stimulation of central AT2Rs by the selective agonist Compound 21 in conscious spontaneously hypertensive rats (SHRs) and normotensive Wistar Kyoto rats (WKY rats). We also assessed the impact on noradrenaline [norepinephrine (NE)] plasma levels, autonomic function, spontaneous baroreflex sensitivity, and the possible involvement of the nitric oxide (NO) pathway and the AT1Rs. Chronic intracerebroventricular Compound 21 infusion lowered blood pressure and NE plasma levels in both rat strains. The night-time hypotensive effect was greater in SHRs compared with WKY rats. Compound 21 improved spontaneous baroreflex sensitivity more in SHRs than in WKY rats. These effects were abolished by co-administration of the AT2R antagonist PD123319 or the NO synthase inhibitor Nω-nitro-L-arginine methyl ester hydrochloride (L-NAME). Central AT1R blockade did not enhance the hypotensive response to Compound 21. Chronic selective stimulation of central AT2Rs lowers blood pressure through sympathoinhibition, and improves spontaneous baroreflex sensitivity more in SHRs than in WKY rats. These responses appear to require a functioning central NO pathway, but are not modified by central AT1R blockade. Collectively, the data demonstrate specific beneficial effects of stimulation of central AT2Rs in hypertension associated with increased sympathetic tone, and suggest that central AT2Rs may represent a potential new therapeutic target for the treatment of neurogenic hypertension.

Angiotensin II-mediated posttranslational modification of nNOS in the PVN of rats with CHF: role for PIN

An increased sympathetic drive is an adverse characteristic in chronic heart failure (CHF). The protein expression of neuronal nitric oxide synthase (nNOS)- and hence nitric oxide (NO)-mediated sympathoinhibition is reduced in the paraventricular nucleus (PVN) of rats with CHF. However, the molecular mechanism(s) of nNOS downregulation remain(s) unclear. The aim of the study was to reveal the underlying molecular mechanism for the downregulation of nNOS in the PVN of CHF rats. Sprague-Dawley rats with CHF (6-8 wk after coronary artery ligation) demonstrated decreased nNOS dimer/monomer ratio (42%), with a concomitant increase in the expression of PIN (a protein inhibitor of nNOS known to dissociate nNOS dimers into monomers) by 47% in the PVN. Similarly, PIN expression is increased in a neuronal cell line (NG108) treated with angiotensin II (ANG II). Furthermore, there is an increased accumulation of high-molecular-weight nNOS-ubiquitin (nNOS-Ub) conjugates in the PVN of CHF rats (29%). ANG II treatment in NG108 cells in the presence of a proteasome inhibitor, lactacystin, also leads to a 69% increase in accumulation of nNOS-Ub conjugates immunoprecipitated by an antiubiquitin antibody. There is an ANG II-driven, PIN-mediated decrease in the dimeric catalytically active nNOS in the PVN, due to ubiquitin-dependent proteolytic degradation in CHF. Our results show a novel intermediary mechanism that leads to decreased levels of active nNOS in the PVN, involved in subsequent reduction in sympathoinhibition during CHF, offering a new target for the treatment of CHF and other cardiovascular diseases.

Potentiation of NMDA receptor-dependent cell responses by extracellular high mobility group box 1 protein

Background

Extracellular high mobility group box 1 (HMGB1) protein can operate in a synergistic fashion with different signal molecules promoting an increase of cell Ca²⁺ influx. However, the mechanisms responsible for this effect of HMGB1 are still unknown.

Principal Findings

Here we demonstrate that, at concentrations of agonist per se ineffective, HMGB1 potentiates the activation of the ionotropic glutamate N-methyl-D-aspartate receptor (NMDAR) in isolated hippocampal nerve terminals and in a neuroblastoma cell line. This effect was abolished by the NMDA channel blocker MK-801. The HMGB1-facilitated NMDAR opening was followed by activation of the Ca²⁺-dependent enzymes calpain and nitric oxide synthase in neuroblastoma cells, resulting in an increased production of NO, a consequent enhanced cell motility, and onset of morphological differentiation. We have also identified NMDAR as the mediator of HMGB1-stimulated murine erythroleukemia cell differentiation, induced by hexamethylenebisacetamide. The potentiation of NMDAR activation involved a peptide of HMGB1 located in the B box at the amino acids 130–139. This HMGB1 fragment did not overlap with binding sites for other cell surface receptors of HMGB1, such as the advanced glycation end products or the Toll-like receptor 4. Moreover, in a competition assay, the HMGB1_(130–139) peptide displaced the NMDAR/HMGB1 interaction, suggesting that it comprised the molecular and functional site of HMGB1 regulating the NMDA receptor complex.

Conclusion

We propose that the multifunctional cytokine-like molecule HMGB1 released by activated, stressed, and damaged or necrotic cells can facilitate NMDAR-mediated cell responses, both in the central nervous system and in peripheral tissues, independently of other known cell surface receptors for HMGB1.

The levels of renin-angiotensin related components are modified in the hippocampus of rats submitted to pilocarpine model of epilepsy

We previously showed that patients with temporal lobe epilepsy (TLE) present an increased expression of angiotensin II (AngII) AT1 and AT2 receptors in the hippocampus, supporting the idea of an upregulation of renin-angiotensin system (RAS) in this disease. This study aimed to verify the relationship between the RAS and TLE during epileptogenesis. Levels of the peptides angiotensin I (AngI), angiotensin II (AngII) and angiotensin 1-7 (Ang 1-7), were detected by HPLC assay. Angiotensin AT1 and AT2 receptors, Mas mRNA receptors and angiotensin converting enzyme (ACE), tonin and neutral endopeptidase (NEP) mRNA were also quantified at the hippocampus of Wistar rats by real time PCR, during acute (n=10), silent (n=10) and chronic (n=10) phases of pilocarpine-induced epilepsy. We observed an increased peptide level of Ang1-7 into acute and silent phases, decreasing importantly (p≤0.05) in the chronic phase, suggesting that AngI may be converted into Ang 1-7 by NEP, which is present in high levels in these periods. Our results also showed increased peptide level of AngII in the chronic phase of this model. In contraposition, the ACE expression is reduced in all periods. These data suggest that angiotensinogen or AngI may be cleaved to AngII by tonin, which showed increased expression in all phases. We found changes in AT1, AT2 and Mas mRNA receptors levels suggesting that Ang1-7 could act at Mas receptor during the silent period. Herein, we demonstrated for the first time, changes in angiotensin-related peptides, their receptors as well as the releasing enzymes in the hippocampus of rats during pilocarpine-induced epilepsy.

Chronic AT1 receptor blockade normalizes NMDA-mediated changes in renal sympathetic nerve activity and NR1 expression within the PVN in rats with heart failure

Exercise training normalizes enhanced glutamatergic mechanisms within the paraventricular nucleus (PVN) concomitant with the normalization of increased plasma ANG II levels in rats with heart failure (HF). We tested whether ANG II type 1 (AT(1)) receptors are involved in the normalization of PVN glutamatergic mechanisms using chronic AT(1) receptor blockade with losartan (Los; 50 mg.kg(-1).day(-1) in drinking water for 3 wk). Left ventricular end-diastolic pressure was increased in both HF + vehicle (Veh) and HF + Los groups compared with sham-operated animals (Sham group), although it was significantly attenuated in the HF + Los group compared with the HF + Veh group. The effect of Los on cardiac function was similar to exercise training. At the highest dose of N-methyl-d-aspartate (NMDA; 200 pmol) injected into the PVN, the increase in renal sympathetic nerve activity was 93 +/- 13% in the HF + Veh group, which was significantly higher (P < 0.05) than the increase in the Sham + Veh (45 +/- 2%) and HF + Los (47 +/- 2%) groups. Relative NMDA receptor subunit NR(1) mRNA expression within the PVN was increased 120% in the HF + Veh group compared with the Sham + Veh group (P < 0.05) but was significantly attenuated in the HF + Los group compared with the HF + Veh group (P < 0.05). NR(1) protein expression increased 87% in the HF + Veh group compared with the Sham + Veh group but was significantly attenuated in the HF + Los group compared with the HF + Veh group (P < 0.05). Furthermore, in in vitro experiments using neuronal NG-108 cells, we found that ANG II treatment stimulated NR(1) protein expression and that Los significantly ameliorated the NR(1) expression induced by ANG II. These data are consistent with our hypothesis that chronic AT(1) receptor blockade normalizes glutamatergic mechanisms within the PVN in rats with HF.

Neurosteroids alter glutamate-induced changes in neurite morphology of NG108-15 cells

Activation of the NMDA receptor leads to increased intracellular Ca2+ levels ([Ca2+]i) which induces outgrowth of and morphologic changes in the neurites of the NG108-15 cell line. This effect can be blocked by antagonists for this glutamate receptor subtype (e.g. ifenprodil or AP5). We have previously shown that nanomolar concentrations of various neurosteroids modulate ifenprodil binding to the NMDA receptor. To investigate whether this interaction affects the functioning of the receptor, we studied the effect of 24 and 48 h of pregnenolone sulphate (PS) or pregnanolone sulphate (3alpha5betaS) on glutamate-stimulated NG108-15 cells. Unexpectedly, the neurosteroids themselves had an inhibitory effect on glutamate-induced changes in neurite patterns. This effect was comparable to that of ifenprodil or AP5. Moreover, the effect of combined treatment with 3alpha5betaS and ifenprodil on neurite morphology indicated a functional interaction between the substances. Interestingly, PS induced cell detachment over time, an effect that was further enhanced by ifenprodil. Cell detachment was also seen after 48 h of treatment with 3alpha5betaS; however, the effect was blocked by ifenprodil and weaker than that of PS. The interaction with the NR2B-selective antagonist ifenprodil indicates that this NMDA receptor subunit may be involved in neurosteroid-induced NG108-15 cell detachment.

Angiotensin II disrupts inhibitory avoidance memory retrieval

The brain renin-angiotensin system (RAS) is involved in learning and memory, but the actual role of angiotensin II (A(II)) and its metabolites in this process has been difficult to comprehend. This has been so mainly due to procedural issues, especially the use of multi-trial learning paradigms and the utilization of pre-training intracerebroventricular infusion of RAS-acting compounds. Here, we specifically analyzed the action of A(II) in aversive memory retrieval using a hippocampal-dependent, one-trial, step-down inhibitory avoidance task (IA) in combination with stereotaxically localized intrahippocampal infusion of drugs. Rats bilaterally implanted with infusion cannulae aimed to the CA1 region of the dorsal hippocampus were trained in IA and tested for memory retention 24 h later. We found that when given into CA1 15 min before IA memory retention test, A(II), but not angiotensin IV or angiotensin(1-7) induced a dose-dependent and reversible amnesia without altering locomotor activity, exploratory behavior or anxiety state. The effect of A(II) was blocked in a dose-dependent manner by the A(II)-type 2 receptor (AT(2)) antagonist PD123319 but not by the A(II)-type 1 receptor (AT(1)) antagonist losartan. By themselves, neither PD123319 nor losartan had any effect on memory expression. Our data indicate that intra-CA1 A(II) hinders retrieval of avoidance memory through a process that involves activation of AT(2) receptors.

Angiotensinase activity is asymmetrically distributed in the amygdala, hippocampus and prefrontal cortex of the rat

There are important asymmetries in brain functions such as emotional processing and stress response in humans and animals. Knowledge of the bilateral distribution of brain neurotransmitters is important to appropriately understand its functions. Some peptides such as those included in the renin-angiotensin system (RAS) and cholecystokinin (CCK) are related to modulation of behavior and stress. However, although angiotensin AT1 and CCK type 2 receptors were found in adult rat brain, there are no studies of their bilateral distribution in stress-related areas. The function of angiotensin peptides is depending on the action of several aminopeptidases (AP) called angiotensinases, some of them being also involved in the metabolism of CCK. We have studied the bilateral distribution of soluble (SOL) and membrane-bound (MEM) alanyl- (AlaAP), cystinyl- (CysAP), glutamyl- (GluAP) and aspartyl- (AspAP) AP activities in stress-related areas such as amygdala, hippocampus and medial prefrontal cortex of adult male rats in resting conditions. These enzymes are involved in the metabolism of angiotensins (AlaAP, CysAP, GluAP, AspAP) and CCK (GluAP, AspAP). In the amygdala, all the activities studied showed a right predominance with a significant difference ranging from 30% for SOL CysAP to 125% for SOL GluAP. In the hippocampus, there was a left predominance for SOL AlaAP, SOL and MEM CysAP and MEM AspAP activities (100, 80, 300 and 100% higher, respectively). In contrast, GluAP predominated remarkably in the right hippocampus (eight-fold for SOL and three-fold for MEM). In the prefrontal cortex, SOL and MEM CysAP and SOL AspAP predominated in the left hemisphere (40, 100 and 40% higher, respectively). These results demonstrated a heterogeneous bilateral pattern of angiotensinase activities in motivation and stress-related areas. This may reflect an uneven asymmetrical distribution of their endogenous substrates depending on the brain location and consequently, it would be also a reflect of the asymmetries in the functions they are involved in.

Angiotensin II attenuates NMDA receptor-mediated neuronal cell death and prevents the associated reduction in Bcl-2 expression

While angiotensin II (Ang II) plays a major role in the regulation of blood pressure, fluid homeostasis and neuroendocrine function, recent studies have also implicated the peptide hormone in cell growth, differentiation and apoptosis. In support of this, we have previously demonstrated that Ang II attenuates N-methyl-D-aspartate (NMDA) receptor signaling [Molec. Brain Res. 48 (1997) 197]. To further examine the modulatory role of Ang II on NMDA receptor function, we investigated the effect of angiotensin receptor (AT) activation on NMDA-mediated cell death and the accompanying decrease in Bcl-2 expression. The viability of differentiated N1E-115 and NG108-15 neuronal cell lines was reduced following exposure to NMDA in a dose-dependent manner. MTT analysis (mitochondrial integrity) revealed a decrease in cell survival of 49.4+/-12.3% in NG108 cells and 79.9+/-6.8% in N1E cells following treatment with 10 mM NMDA for 20 h. Cytotoxicity in N1E cells was inhibited by the noncompetitive NMDA receptor antagonist, MK-801. Further, NMDA receptor-mediated cell death in NG108 cells was attenuated by treatment with Ang II. The Ang II effect was inhibited by both AT1 and AT2 receptor antagonists, losartan and PD123319, respectively, suggesting that both receptor subtypes may play a role in the survival effect of Ang II. Since it has been shown that activation of NMDA receptors alters the expression of Bcl-2 family proteins, Western blot analysis was performed in N1E cells to determine whether Ang II alters the NMDA-induced changes in Bcl-2 expression. A concentration-dependent decrease of intracellular Bcl-2 protein levels was observed following treatment with NMDA, and this reduction was inhibited by MK801. Addition of Ang II suppressed the NMDA receptor-mediated reduction in Bcl-2. The Ang II effect on NMDA-mediated changes in Bcl-2 levels was blocked by PD123319, but was not significantly changed by losartan, suggesting AT2 receptor specificity. Taken together, these results suggest that Ang II attenuates NMDA receptor-mediated neurotoxicity and that this effect may be due, in part, to an alteration in Bcl-2 expression.

Nitric oxide and angiotensin II: neuromodulation of thermoregulation during combined heat and hypohydration stress

We investigated the central role of nitric oxide and AngII on thermoregulation in rats (Rattus norvegicus, Sabra strain,) undergoing heat-stress in euhydration or hypohydration (water deprivation, -10% b.wgt). Experimental rats received AngII (100 pm), 7-nitroindazole-an antagonist of neuronal nitric oxide synthase (7NI-100 nm), or AngII+7NI in a 5-microl bolus intracerebroventricularly (i.c.v.) under light chloroform anesthesia; untreated control rats received saline or DMSO (5%). We used three experimental paradigms: (1) heat defense responses [salivation (STsh), vasodilatation (VTsh) temperature thresholds and heat-endurance] in conscious, heat-stressed (39 degrees C) rats; (2) Western immunoblotting to detect AngII AT(1) and AT(2) receptors and nNOS protein expression; (3) real-time PCR to measure gene transcripts. In the in vivo experiment, 7NI decreased thermoregulatory thresholds, namely, NO had a reciprocal effect that was more pronounced during hypohydration (e.g. euhydration: STsh: -0.7+/-0.01 degrees C, hypohydration: -0.9+/-0.18 degrees C, p<0.05). AngII decreased STsh by 0.9+/-0.18 degrees C (p<0.05) upon euhydration but increased it in hypohydration (+1.7+/-0.28 degrees C, p<0.05). A novel finding was the involvement of AT(2) receptors in thermoregulation, which was more pronounced upon hypohydration. The response to NO was mediated via AT(1) and AT(2) receptors signaling, as well as independently. A synthesis of the results from all experimental paradigms suggests (1) a dominant influence (decrease) of NO on AT(1) receptors, thereby changing AT(1)/AT(2) receptor ratio and their signaling pathway; primarily upon hypohydration; (2) an influence of AngII (increase) on receptor density, more pronounced during hypohydration, at both gene transcription and translation levels; and (3) an effect of AngII on nNOS protein levels, implying a mutual effect of AngII and NO.

Inhibitory effects of angiotensin on NMDA-induced cytotoxicity in primary neuronal cultures

Primary cultures from the hypothalamus/thalamus/septum/midbrain (HTSM) region of 1-day-old mice were used to investigate the effects of angiotensin on NMDA excitotoxicity. Cell viability was determined following exposure to 1-10 mM glutamate or 0.01-10 mM NMDA. Cells exposed to 1 mM glutamate or 1 mM NMDA for 24 h showed a significant increase in cell death as determined by propidium iodide staining. HTSM cultures treated with 0.1 mM NMDA revealed both DNA laddering and positive staining for TUNEL, suggesting apoptosis as the primary mechanism for the cell death. We also determined whether angiotensin II (Ang II) modulated NMDA-induced cell death in HTSM-cultured neurons. Cells pre-treated with 10 nM Ang II showed a decrease in NMDA-induced cytotoxicity, TUNEL staining and DNA laddering. NMDA-induced cell death was also reduced when cells were pre-treated with the AT1 receptor antagonist losartan. In this study, we have shown that NMDA and glutamate induce cell death through the NMDA receptor, and that Ang II, acting primarily through the AT2 receptor, can attenuate this response.

NMDA induces NOS 1 translocation to the cell membrane in NGF-differentiated PC 12 cells

Glutamatergic-mediated nitric oxide (NO) production occurs via the N-methyl-D-aspartic acid (NMDA) postsynaptic density protein 95 (PSD95)-neuronal nitric oxide synthase (NOS1) ternary complex. To determine whether NOS1 is targeted to the membrane subsequent to NMDA receptor activation, we examined the effect of NMDA on NOS1 subcellular localization in nerve growth factor (NGF) differentiated PC12 cells. No effect on cell viability was observed using a range of NMDA concentrations from 500 to 1000 microM. Within 3 min of stimulation with 750 microM NMDA, increased cytoplasmic NOS1 immunostaining was observed with rapid membrane staining thereafter. This was inhibited by NMDAR inhibition with MK801. This observation was confirmed using subcellular fractionation and immunoblotting. Using 4, 5-diaminofluorescein diacetate (DAF2-DA) staining and a diazotization assay, concurrent NO production was observed. When PC 12 cells were co-treated with either NMDA and N(6)-nitro-L-arginine methyl ester hydrochloride (L-NAME) or (5R, 10S)-(+)-5-methyl-10, 11-dihydro-5H-dibenzo [a, d] cyclohepten-5, 10-imine hydrogen maleate (MK-801), nitric oxide (NO) generation was inhibited. Stimulation in a calcium-free medium did not increase NO levels. Although no evidence of cytotoxicity was observed utilizing either the MTT assay or measures of apoptosis within the maximal interval of NOS1 translocation, cell viability was reduced following 10 h of continuous NMDA exposure. While it has been shown that NMDA triggers NOS1 activation, these results indicate that NMDAR activation also mediates NOS1 targeting to the membrane. Our data validate that NGF-differentiated PC12 cells may be employed as a useful in vitro model to further study the regulation of NOS1 subsequent to NMDAR activation.

Effects of mutations in the highly conserved DRY motif on binding affinity, expression, and G-protein recruitment of the human angiotensin II type-2 receptor

The signaling pathways for the seven transmembrane G-protein coupled angiotensin II receptors (AT(1) and AT(2)) are just beginning to be understood. While these receptors play an important role in the development and differentiation of many tissues, including the cardiovascular and central nervous systems, information about amino acid motifs involved in angiotensin II-mediated signaling is only available for the AT(1) receptor subtype. In the present study, we mutated the conserved DRY(141-143) motif in the AT(2) receptor, which is thought to be involved in G-protein recruitment. Expression of wild type and mutant receptors in CHO-K1 cell plasma membranes was confirmed using radioligand binding analyses. Our findings indicate a significant change in the binding affinities (kD) and capacities (B(max)) of the mutant receptors relative to wild type. Alanine substitutions of D(141) and DRY(141-143) resulted in a significant decrease of binding affinity for both Sar(1)Ile(8)-angiotensin II (SarIle-Ang II) (mixed agonist/antagonist) and angiotensin II (agonist). The binding affinities following alanine substitutions of R(142) and Y(143) were not significantly different from wild type receptor. Interestingly, the R(142)-A and Y(143)-A mutants revealed a significant decrease in binding levels from wild type with SarIle-Ang II, but not angiotensin II. The effect of GTPgammaS on angiotensin II binding affinity between wild type and mutant receptors was similarly significant. The D(141)-A, Y(143)-A, and DRY(141-143)-AAA mutant receptors showed a marked decrease in GTPgammaS-induced angiotensin II affinity shift. The R(142)-A GTPgammaS binding affinity shift was not different from the wild type receptor. Our results support the hypothesis that the DRY motif plays a significant role in the binding affinity, structural stability and G-protein recruiting of the AT(2) receptor.

Expression of glutamate transporters and ionotropic glutamate receptors in GLAST knockout mice

In order to investigate the molecular mechanism underlying high seizure susceptibility of GLAST knockout mice, we carried out Western blotting for the expression of GLT-1, EAAC-1, and several kinds of glutamate receptors in the hippocampus and the cortex. Although no significant difference was observed between GLAST (+/+) and (-/-) mice in terms of expression of GLT-1 and EAAC-1 in the hippocampus, these proteins were over-expressed in the frontal cortex in GLAST (-/-) mice (GLT-1, about 210% increase; EAAC-1, about 180% increase). Expression of hippocampal Glu-R1 and Glu-R2 in GLAST (-/-) mice was remarkably increased (Glu-R1, about 140% increase; Glu-R2, about 160% increase), while Glu-R3 and NMDA receptors levels (NMDA-R1, 2A and 2B) were equal to those in control. Cortical levels of Glu-R1, -R2 and -R3 receptors in GLAST (-/-) mice were remarkably decreased (Glu-R1, about 60% decrease; Glu-R2, about 60% decrease; Glu-R3, about 70% decrease), while NMDA receptors were remarkably increased in comparison to those in GLAST (+/+) mice (N-R1, about 150% increase; N-R2A, about 150% increase; N-R2B, about 140% increase). These data suggest that the increased susceptibility to seizures in GLAST (-/-) mice might be derived from increased expression of Glu-R1 in the hippocampus coupled with decreased cortical expression of Glu-R2 and increased NMDA-R1 and -2A, -2B expression.

Nitric oxide and hydroxyl radicals initiate lipid peroxidation by NMDA receptor activation

In this experiment, we used direct electron paramagnetic resonance (EPR) spectra to measure lipid peroxidation by hydroxyl radical (.OH), nitric oxide (.NO) and lipid radical (.L). NMDA-receptor associated lipid peroxidation is thought to act through .OH in induction of neurotoxicity. The origin of .OH generation was found to arise mainly from peroxynitrite anion produced from O(2)(-) and .NO rather than from Fenton's reaction. This study verified that .OH generation from interactive reactions between .NO and O(2)(-) initiates NMDA-induced lipid peroxidation of PC12 cells.

Angiotensin protects cortical neurons from hypoxic-induced apoptosis via the angiotensin type 2 receptor

The effects of angiotensin on mouse cortical neuronal cultures exposed to chemical-induced hypoxia was investigated. Cultures exposed to 10 mM sodium azide for 5 min showed a 17% increase in apoptosis when assayed 24 h postinsult. The N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 blocked sodium azide-induced cell death suggesting that the NMDA receptor contributes to the mediated cell death. Pretreatment of cultured neurons with angiotensin decreased sodium azide-induced apoptosis by 94%. When the AT(1) receptor was blocked by its receptor antagonist, losartan, angiotensin activation of the AT(2) receptor completely inhibited sodium azide-induced apoptosis. Pretreatment of neurons with the AT(2) receptor antagonist PD123319 resulted in angiotensin reducing sodium azide-induced apoptosis by 48%. These results demonstrate that angiotensin can significantly attenuate sodium azide-induced apoptosis primarily through activation of the AT(2) receptor and suggests that angiotensin may have a protective role in neurons undergoing ischemic injury.

Nitric oxide and cyclic GMP are involved in angiotensin II AT(2) receptor effects on neurite outgrowth in NG108-15 cells

In their undifferentiated state, NG108-15 cells express only the angiotensin II (Ang II) type 2 receptor (AT(2)). We have previously shown that Ang II induced neurite outgrowth of NG108-15 cells, a process involving sustained activation of p42/p44(mapk) activity. We have also shown that Ang II stimulates nitric oxide (NO) production. The aim of the present study was to investigate the role of the NO/cyclic GMP (cGMP) cascade in the signal transduction of the AT(2) receptor-stimulated neurite outgrowth. Three-day treatment of cells with dbcGMP induced neurite outgrowth as did Ang II. Preincubation with an inhibitor of cGMP-dependent protein kinase, KT5823, resulted in the formation of short neurites, while in the presence of LY83583 or methylene blue, two inhibitors of guanylyl cyclase, cells resembled control cells with only one or two thin processes. Western blot analyses indicated that nNOS was present in NG108-15 cells. Immunoprecipitation with antiphosphotyrosine antibodies showed that Ang II induced NOS activity and increased cGMP production through a Gi-dependent pathway. However, neither L-NAME, KT5823, nor LY83583 affected the activation of p42/p44(mapk) induced by Ang II, indicating that the pathway NO/guanylyl cyclase/cGMP was not involved in Ang II-induced activation of MAPK. The present results suggest that the neurite outgrowth induced by Ang II results from at least parallel but complementary pathways, one involved in neurite elongation (through the cooperation of MAPK and PKG) and the other involved in sprouting (through cGMP).

Functional reconstitution of the angiotensin II type 2 receptor and G(i) activation

On the basis of the patterns of conserved amino acid sequence, the angiotensin II type 2 (AT(2)) receptor belongs to the family of serpentine receptors, which relay signals from extracellular stimuli to heterotrimeric G proteins. However, the AT(2) receptor signal transduction mechanisms are poorly understood. We have measured AT(2)-triggered activation of purified heterotrimeric proteins in urea-extracted membranes from cultured COS-7 cells expressing the recombinant receptor. This procedure removes contaminating GTP-binding proteins without inactivating the serpentine receptor. Binding studies using [(125)I] angiotensin (Ang) II revealed a single binding site with a K(d)=0.45 and a capacity of 627 fmol/mg protein in the extracted membranes. The AT(2) receptor caused a rapid activation of alpha(i) and alpha(o) but not of alpha(q) and alpha(s), as measured by radioactive guanosine 5'-3-O-(thio)triphosphate (GTPgammaS) binding. Activation required the presence of activated receptors, betagamma, and alpha subunits. As a first step aimed at developing an in vitro assay to examine AT(2) receptor pharmacology, we tested a battery of Ang II-related ligands for their ability to promote AT(1) or AT(2) receptor-catalyzed G(i) activation. Two proteolytic fragments of Ang II, Ang III and Ang1-7, also promoted activation of alpha(i) through the AT(2) receptor. Furthermore, we found that [Sar(1),Ala(8)]Ang II is an antagonist for both AT(1) and AT(2) receptors and that CPG42112 behaves as a partial agonist for the AT(2) receptor. In combination with previous observations, these results show that the AT(2) receptor is fully capable of activating G(i) and provides a new tool for exploring AT(2) receptor pharmacology and interactions with G-protein trimers.

Angiotensin II and nitric oxide: a question of balance

The vasoconstrictor peptide angiotensin II (Ang II) and the endogenous vasodilator nitric oxide (NO) have many antagonistic effects, as well as influencing each other's production and functioning. In the short-term, Ang II stimulates NO release, thus modulating the vasoconstrictor actions of the peptide. In the long-term, Ang II influences the expression of all three NO synthase (NOS) isoforms, while NO downregulates the Ang II Type I (AT1) receptor, contributing to the protective role of NO in the vasculature. Within the cardiovascular system, Ang II and NO also have antagonistic effects on vascular remodeling and apoptosis. In the kidney, the distribution of the NOS isoforms coincides with the sites of the components of the renin-angiotensin system. NO influences renin secretion from the kidney, and NO-Ang II interactions are important in the control of glomerular and tubular function. In the adrenal gland, NO has been shown to affect Ang II-induced aldosterone synthesis, while in the brain NO appears to influence Ang II-induced drinking behavior, although conflicting data have been reported. In this review, we focus on the diverse ways in which Ang II and NO interact, and on the importance of maintaining a balance between these two important mediators.

Nitric oxide, a new second messenger involved in the action of angiotensin II on neuronal differentiation of NG108-15 cells

Nitric Oxide (NO) is a gas that diffuses freely through membranes of target cells to activate cGMP formation. NO is synthesised from arginine, by a family of Nitric Oxide Synthase (NOS). In the brain, NO influences synaptic plasticity, apoptosis and development. It has been recently shown that angiotensin II (Ang II) could mediate NO production by its two types of receptors, AT1 and AT2. Since we have shown that Ang II, via the AT2 receptor could induce neurite outgrowth and morphological differentiation of NG108-15 cells, the aim of the study was to investigate if NO could be one of the second messengers involved in the Ang II effect. Using the Griess colorimetric assay, we found that Ang II, by its AT2 receptor, induced nitrite formation from NO. This effect was abolished by the N-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor. We also found that treatment of the cells with S-nitroso-N-acetylpenicillamine (SNAP), an exogenous source of NO, induced the same morphological differentiation. These results demonstrate that the morphological differentiation induced by the AT2 receptor is partly due to an increase in NO production.