Effects of angiotensin II on the spontaneous activity of rostral ventrolateral medullary cardiovascular neurons and blood pressure in spontaneously hypertensive rats

The interactive role of rostral ventrolateral medulla (RVL) cardiovascular neurons and brain angiotensin II (Ang II) in regulating the arterial blood pressure was examined by recording simultaneously the spontaneous activity of these spinal projecting neurons and the arterial blood pressure in the pentobarbital-anesthetized spontaneously hypertensive rat (SHR) and its normotensive control, the Wistar Kyoto rat (WKY). It was found that Ang II elicited dose-dependent excitatory responses in a subpopulation of RVL cardiovascular neurons, followed by a subsequent increase in blood pressure. These effects of Ang II were significantly greater in SHR than in WKY. The effects were attenuated or abolished by co-administration of Ang II antagonist, [Sar¹, Ile⁸]-Ang II. Pre-administration of [Sar¹, Ile⁸]-Ang II to RVL using bilateral microinjection attenuated the blood pressure effects of intracerebroventricularly administered Ang II by as much as 70%. These results indicated that spinal projecting RVL cardiovascular neurons are important in mediating the pressor action of Ang II. The enhanced sensitivity and responsiveness of RVL cardiovascular neurons to Ang II may be pertinent to the genesis of hypertension in adult SHR.     

Protein Kinase C Agonists Increase the Expression of Angiotensin II Receptors in Neuronal Cultures

Previous studies have shown that norepinephrine is important in the regulation of central angiotensin II receptors, an effect mediated by alpha 1-adrenergic receptors. Because alpha 1-adrenergic stimulation leads to inositol phospholipid hydrolysis and activation of protein kinase C, we have examined a possible role of this enzyme in the regulation of central angiotensin II (Ang II) receptors. In the present study, we have examined the effects of protein kinase C activators, phorbol esters, on the expression of Ang II receptors in neuronal cultures prepared from 1-day-old rat brains. The active phorbol ester phorbol-12-myristate-13-acetate (TPA) caused time- and concentration-dependent increases in the specific binding of [125I]Ang II to its receptors in neuronal cultures of normotensive and spontaneously hypertensive rat brains. The stimulatory effect of TPA on Ang II receptors was apparent within 15 min and reached a maximum between 1 and 2 h. Ang II specific binding had returned to control levels by 24 h. Various phorbol esters increased [125I]Ang II binding in accordance with their order of potency in stimulating protein kinase C activity. Saturation and Scatchard analysis revealed that the phorbol ester-induced increase in [125I]Ang II binding was due to an increase in the number of Ang II receptors. These observations indicate that activation of protein kinase C results in an increased expression of Ang II receptors in neuronal cultures from both normotensive and spontaneously hypertensive rat brains. The results suggest a possible role of phosphorylation in Ang II receptor expression in neuronal cultures.     

Angiotensin II modulates tyr-phosphorylation of IRS-4, an insulin receptor substrate, in rat liver membranes

Angiotensin II (Ang II), a major regulator of blood pressure, is also involved in the control of cellular proliferation and hypertrophy and might exhibit additional actions in vivo by modulating the signaling of other hormones. As hypertension and Insulin (Ins) resistance often coexist and are risk factors for cardiovascular diseases, Ang II and Insulin signaling cross-talk may have an important role in hypertension development. The effect of Ins on protein tyrosine phosphorylation was assayed in rat liver membrane preparations, a rich source of Ins receptors. Following stimulation, Ins (10⁻⁷ M) induced tyr-phosphorylation of different proteins. Insulin consistently induced tyr-phosphorylation of a 160 kDa protein (pp160) with maximum effect between 1 and 3 min. The pp160 protein was identified by anti-IRS-4 but not by anti-IRS-1 antibody. Pre-stimulation with Ang II (10⁻⁷ M) diminishes tyr-phosphorylation level of pp160/IRS-4 in a dose-dependent manner. Okadaic acid, the PP1A and PP2A Ser/Thr phosphatase inhibitor, increases pp160 phosphorylation induced by Ins and prevents the inhibitory effect of Ang II pre-stimulation. Genistein, a tyrosine kinase inhibitor, diminishes tyr-phosphorylation level of IRS-4. PI3K inhibitors Wortmanin and LY294002, both increase tyr-phosphorylation of IRS-4, either in the presence of Ins alone or combined with Ang II. These results suggest that Ins and Ang II modulate IRS-4 tyr-phosphorylation in a PI3K-dependent manner. In summary, we showed that Ins induces tyr-phosphorylation of IRS-4, an effect modulated by Ang II. Assays performed in the presence of different inhibitors points toward a PI3K involvement in this signaling pathway.     

Adenosine protects against angiotensin II-induced apoptosis in rat cardiocyte cultures

Adenosine has been found to be cardioprotective during episodes of cardiac ischemia/reperfusion through activation of the A₁ and possibly A₃ receptors. Therefore, we have investigated whether activation of these receptors can protect also against apoptotic death induced by angiotensin II (Ang II) in neonatal rat cardiomyocyte cultures. Exposure to Ang II (10 nM) resulted in a 3-fold increase in programmed cell death (p < 0.05). Pretreatment with the A₁ adenosine receptor agonist 2-chloro-N⁶-cyclopentyladenosine (CCPA, 1 M), abolished the effects of Ang II on programmed cardiomyocyte death. Moreover, exposure of cells to the A₁ adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (CPX) before pretreatment with CCPA, prevented the protective effect of the latter. Pretreatment with the A₃ adenosine receptor agonist N⁶-(3-iodobenzyl) adenosine-5-N-methyluronamide (IB-MECA, 0.1 M), led to a partial decrease in apoptotic rate induced by Ang II. Exposure of myocytes to Ang II caused an immediate increase in the concentration of intracellular free Ca²⁺ that lasted 40–60 sec. Pre-treatment of cells with CCPA or IB-MECA did not block Ang II-induced Ca²⁺ elevation. In conclusion, activation of adenosine A₁ receptors can protect the cardiac cells from apoptosis induced by Ang II, while activation of the adenosine A₃ receptors confers partial cardioprotection.     

β2 Mediated inhibition of [3H]dopamine release from nucleus accumbens slices and monoamine levels in a rat model for attention-deficit hyperactivity disorder

The spontaneously hypertensive rat (SHR) has been proposed as an animal model for attentiondeficit disorder (ADHD). The behavioural problems have been suggested to be secondary to altered reinforcement mechanisms in which nucleus accumbens dopaminergic activity plays an important role. Interaction between the noradrenergic and dopaminergic system in the nucleus accumbens has been implicated in the locomotor hyperactivity and impaire discriminative performance of SHR. The present study therefore investigated whether there was any change in the ₂-adrenoceptor mediated inhibition of dopamine release from nucleus accumbens slices of SHR in comparison with their normotensive Wistar-Kyoto (WKY) controls. The electrically stimulated release of [³H]dopamine (DA) from nucleus accumbens slices was decreased to a similar extent by UK14,304, an ₂-adrenoceptor agonist, in SHR and WKY. Basal norepinephrine (NE) levels were increased in locus coeruleus (LC) and A₂ noradrenergic nuclei, but not in the A₁ nucleus of SHR, while basal serotonin (5-HT) levels were increased in all these pons-medulla nuclei. These results suggest that a primarily dysfunctional LC and A₂ nucleus does not have a secondary effect on dopaminergic transmission in the nucleus accumbens via ₂-adrenoceptor mediated inhibition of DA release. Basal monoamine levels in several brain areas of SHR were significantly different from that of WKY. DA, and 5-HT turnover were decreased in SHR versus WKY suggesting hypofunctional dopaminergic and serotonergic systems in some brain areas of SHR.     

α1-Adrenergic Receptors in Neuronal Cultures from Rat Brain: Increased Expression in the Spontaneously Hypertensive Rat

Neuronal cells in primary culture from 1-day-old brains of normotensive, Wistar-Kyoto strain (WKY) and spontaneously hypertensive (SH) rats have been utilized to study the expression of alpha 1-adrenergic receptors. Binding of a selective alpha 1 antagonist, [125I]2-[beta-(4-hydroxy-3-iodophenyl)-ethylaminomethyl]-tetralone ([125I]HEAT) to neuronal membranes prepared from primary brain cultures of WKY and SH rats was 75-80% specific, rapid, and time-dependent although the binding was 1.5-2 times higher in neuronal membranes from SH rat brain cultures. Kinetic analysis of the association and dissociation data demonstrated no significant differences between rat strains. Competition-inhibition experiments provided IC50 values for various antagonists and agonists in the following order: prazosin less than phentolamine less than yohimbine less than phenylephrine less than norepinephrine less than propranolol, suggesting that [125I]HEAT bound selectively to alpha 1-adrenergic receptors. Scatchard analysis of the binding data provided straight lines for both strains of rats, indicating the presence of a homogeneous population of binding sites. It also showed that the increase in the binding in neuronal cells from SH rat brains over those from normotensive WKY controls was a result of an increase in the number of alpha 1-adrenergic receptors. Incubation of neuronal cultures from both strains of rats with phenylephrine, an alpha 1-adrenergic agonist, caused a time- and dose-dependent decrease in the binding of [125I]HEAT. This decrease was due to a decrease in the number of alpha 1-adrenergic receptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Hypothalamic cardiovascular effects of angiotensin-(1-7) in spontaneously hypertensive rats

The objective of the present work was to study the cardiovascular actions of the intrahypothalamic injection of Ang-(1-7) and its effects on the pressor response to Ang II in spontaneously hypertensive (SH) rats and Wistar Kyoto (WKY) animals. In anaesthetized SH and WKY rats, a carotid artery was cannulated for mean arterial pressure (MAP) measurement and a stainless-steel needle was inserted into the anterior hypothalamus for drug administration. The cardiovascular effects of the intrahypothalamic administration of Ang-(1-7) were determined in SH and WKY rats. In SH rats, the effect of irbesartan and D-Ala-Ang-(1-7) on Ang-(1-7) cardiovascular effect was also evaluated. Ang II was administered in the hypothalamus of SH and WKY rats and changes in blood pressure and heart rate were measured followed by the administration of Ang II, Ang II+Ang-(1-7) or Ang II+D-Ala-Ang-(1-7). Ang-(1-7) did not the change basal MAP in WKY rats, but induced a pressor response in SH animals. Whilst the co-administration of D-Ala-Ang-(1-7) did not affect the response to Ang-(1-7), the previous administration of irbesartan prevented the effect of the peptide. The intrahypothalamic injection of Ang II induced a significantly greater pressor response in SH animals compared to normotensive rats. The co-administration of Ang-(1-7) with Ang II did not affect the pressor response to Ang II in the WKY group. In SH rats, whilst the co-administration of Ang-(1-7) with Ang II reduced the pressor response to Ang II, the concomitant application of D-Ala-Ang-(1-7) with Ang II increased the pressor response to the octapeptide after 5 and 10 min of intrahypothalamic administration. In conclusion, our result demonstrated that the biologically active peptide Ang-(1-7) did not participate in the hypothalamic blood pressure regulation of WKY animals. In SH rats, Ang-(1-7) exerted pleiotropic effects on blood pressure regulation. High dose of the heptapeptide produced a pressor response because of an unspecific action by activation of AT1 receptors. The concomitant administration of lower doses of Ang-(1-7) with Ang II reduced the pressor response to the octapeptide. Finally, the effect of AT(1-7) antagonist on Ang II pressor response suggested that hypothalamic formed Ang-(1-7) are implicated in the regulation of the cardiovascular effects of Ang II.     

Molecular comparison of α2-adrenergic receptors from rat adrenocortical carcinoma and human blood platelet

Summary We have previously described a simple two-step purification technique to isolate ₂-adrenergic receptors from the rat adrenocortical carcinoma (Jaiswal, R. K. and Sharma, R. K. (1985) Biochem. Biophys. Res. Commun. 130, 58–64). Utilizing this technique we have now achieved 77 000-fold purification to apparent homogeneity of ₂-adrenergic receptors from human platelets. We have compared the biochemical characteristics of these receptors with those from the rat, which were purified 40000-fold to homogeneity.The [¹²⁵I] receptor proteins from two sources showed: (a) a single radioactive band with a Mr of 64000 as evidenced by one- and two-dimensional sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDSPAGE); and (b) a single symmetrical peak with a pl of 4.2 by isoelectric focusing polyacrylamide gel electrophoresis. Both proteins showed typical ₂-adrenergic binding characteristics with specific binding activities of 13.85 nmol/mg and 14.17 nmol/mg protein. These values are close to the theoretical binding activity of 15.6 nmol/mg protein for 1 mol of the ligand binding 1 mol of the receptor protein. These results attest to the purity of the receptors, to its Mr of 64000, and to its acidic nature. However, the peptide maps of the radioiodinated ₂-adrenergic receptors from rat adrenocortical carcinoma and human blood platelets reveal some distinct differences which may relate to the differences in the pharmacological specificities between rodent and nonrodent ₂-adrenergic receptors.Abbreviations PAC p-aminoclonidine - PMSF Phenylmethyl-sulfonylfluoride - DTT Dithiothreitol - HPLC High Performance Liquid Chromatography     

The determination of alpha-adrenergic receptor concentration on rat pancreatic islet cells

The selective a2 adrenergic antagonist yohimbine has been shown to prevent the noradrenaline induced inhibition of insulin secretion from isolated rat islets of Langerhans, Binding studies utilizing [³H]yohimbine showed specific binding to dispersed rat islet cells with a K_d of 2.9 nM and receptor concentration of 645 fmols/mg protein. The use of chloroquine to inhibit receptor recycling did not affect binding of the ligand. Binding studies and secretion data are consistent with the suggestion that adrenergic receptors of the ₂ sub-type may play a dominant role in the regulation of insulin secretion.     

Autoradiographic Localization of [125I-Tyr0]Bradykinin Binding Sites in Brains of Wistar-Kyoto and Spontaneously Hypertensive Rats

1. The present study was undertaken to localize and characterize bradykinin (BK) binding sites in brains from Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR).2. Serial sections of brains were cut from adult WKY and SHR and specific [¹²⁵I-Tyr⁰]bradykinin ([¹²⁵I-Tyr⁰]BK) binding was determined using in vitro quantitative receptor autoradiographic techniques.3. Specific binding of [¹²⁵I Tyr⁰]BK was localized in the medulla oblongata to the regions of the nucleus of the solitary tract (NTS), area postrema (AP), dorsal motor nucleus of the vagus (X), and caudal subnucleus of the spinal trigeminal nucleus in both strains of rat. The specific binding (85–90% of total binding) was of high affinity and saturable with K _D values in the range of 100 pM and a B _max of 0.75 fmol per mg tissue equivalent in the NTS–X–AP complex of both the WKY and SHR. In competition studies, the rank order of potencies was similar in both strains with BK = Lys-BK > icatibant >>> DesArg⁹-BK. The B₂ receptor antagonist icatibant inhibited [¹²⁵I-Tyr⁰]BK binding with a K _i value of 0.63 ± 0.19 nM in WKY and 0.91 ± 0.73 nM in SHR, while K _i values for the B> ₁ receptor agonist DesArg⁹-BK were 1475 ± 1055 and 806 ± 362 nM in WKY and SHR, respectively.4. Our finding of specific high-affinity [¹²⁵I-Tyr⁰]BK B₂ binding sites in the NTS, AP, and the X of WKY and SHR is important because these brain areas are associated with central cardiovascular regulation. However, alterations in BK B₂ receptors in the medulla that could contribute to the hypertensive state in the SHR were not detected.     

Angiotensin and calcium signaling in the pituitary and hypothalamus

1) In the rat pituitary, angiotensin type 1B receptors (AT_1B) are located in lactotrophs and corticotrophs.2) Activation of AT_1B receptors are coupled to G_q/11 (Guanine protein coupled receptor, or GPCR); they increase phospholipase C (PLC) activity resulting in inositol 1,4,5 triphosphate (InsP₃) and diacylglycerol (DAG) formation. A biphasic increase in [Ca²⁺]_itriggered by InsP₃ and DAG ensues.3) As many GPCRs, AT_1B pituitary receptors rapidly desensitize.4) This was observed in the generation of InsP3, the mobilization of intracellular Ca²⁺, and in prolactin release. Both homologous and heterologous desensitization was evidenced.5) Desensitization of the angiotensin II type 1 (AT1) receptor in the pituitary shares similarities and differences with endogenously expressed or transfected AT1 receptors in different cell types.6) In the pituitary hyperplasia generated by chronic estrogen treatment there was desensitization or alteration in angiotensin II (Ang II) evoked intracellular Ca²⁺ increase, InsP3 generation, and prolactin release. This correlates with a downregulation of AT1 receptors.7) In particular, in hyperplastic cells Ang II failed to evoke a transient acute peak in [Ca²⁺]_i, which was replaced by a persistent plateau phase of [Ca²⁺]_i increase.8) Different calcium channels participate in Ang II induced [Ca²⁺]_i increase in control and hyperplastic cells. While spike phase in control cells is dependent on intracellular stores sensitive to thapsigargin, in hyperplastic cells plateau increase is dependent on extracellular calcium influx.9) Signal transduction of the AT1 pituitary receptor is greatly modified by hyperplasia, and it may be an important mechanism in the control of the hyperplastic process.10) In the hypothalamus and brain stem there is a predominant expression of AT_1A and AT₂ mRNA.11) 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.12) Calcium channels play important roles in the Ang II induced behavioral and endocrine responses.13) Ang II, in physiological concentrations, can activate AT1 receptors to stimulate both Ca²⁺ release from intracellular stores and Ca²⁺ influx from the extracellular space to increase [Ca²⁺]_i in polygonal and stellate astroglia of the hypothalamus and brain stem.14) In primary cell culture of neurons from newborn rat hypothalamus and brain stem, it has also been determined that Ang II elicits an AT1 receptor mediated inhibition of delayed rectifier K(+) current and a stimulation of Ca²⁺ current.15) In primary cell cultures derived from the subfornical organ or the organum vasculosum laminae terminalis of newborn rat pups, Ang II produced a pronounced desensitization of the [Ca²⁺]_i response.16) Hypothalamic and pituitary Ang II systems are involved in different functions, some of which are related. At both levels Ang II signals through [Ca²⁺]_i in a characteristic way.     

Distribution of Non-AT1, Non-AT2 Binding of 125I-Sarcosine1, Isoleucine8 Angiotensin II in Neurolysin Knockout Mouse Brains

The recent identification of a novel binding site for angiotensin (Ang) II as the peptidase neurolysin (E.C. 3.4.24.16) has implications for the renin-angiotensin system (RAS). This report describes the distribution of specific binding of ¹²⁵I-Sarcosine¹, Isoleucine⁸ Ang II (¹²⁵I-SI Ang II) in neurolysin knockout mouse brains compared to wild-type mouse brains using quantitative receptor autoradiography. In the presence of p-chloromercuribenzoic acid (PCMB), which unmasks the novel binding site, widespread distribution of specific (3 µM Ang II displaceable) ¹²⁵I-SI Ang II binding in 32 mouse brain regions was observed. Highest levels of binding >700 fmol/g initial wet weight were seen in hypothalamic, thalamic and septal regions, while the lowest level of binding <300 fmol/g initial wet weight was in the mediolateral medulla. ¹²⁵I-SI Ang II binding was substantially higher by an average of 85% in wild-type mouse brains compared to neurolysin knockout brains, suggesting the presence of an additional non-AT₁, non-AT₂, non-neurolysin Ang II binding site in the mouse brain. Binding of ¹²⁵I-SI Ang II to neurolysin in the presence of PCMB was highest in hypothalamic and ventral cortical brain regions, but broadly distributed across all regions surveyed. Non-AT₁, non-AT₂, non-neurolysin binding was also highest in the hypothalamus but had a different distribution than neurolysin. There was a significant reduction in AT₂ receptor binding in the neurolysin knockout brain and a trend towards decreased AT₁ receptor binding. In the neurolysin knockout brains, the size of the lateral ventricles was increased by 56% and the size of the mid forebrain (−2.72 to +1.48 relative to Bregma) was increased by 12%. These results confirm the identity of neurolysin as a novel Ang II binding site, suggesting that neurolysin may play a significant role in opposing the pathophysiological actions of the brain RAS and influencing brain morphology.     

Molecular cloning,sequencing and expression of an α2-adrenergic receptor complementary DNA from rat brain

We have isolated a cDNA clone from rat brain using a human platelet ₂-adrenergic receptor genomic clone as a probe. Comparison of the deduced amino acid sequence (450 residues) corresponding to the rat brain cDNA with that of the human platelet and human kidney ₂-adrenergic receptors showed 84% and 44% sequence similarity, respectively. The major sequence difference between the rat brain and human platelet proteins, was a stretch of 48 amino acids within the third cytosolic loop in which the similarity was only 42%. Analysis of the 48 amino acid-region indicated that the two receptors significantly differ in terms of their primary amino acid sequence and the predicted secondary and tertiary structural features. There was no sequence similarity between the human platelet and rat brain clone over the 177 bases of 3-noncoding sequence and a less than 50% similarity over a stretch of 210 nucleotides in the 5-untranslated region. Southern-blot analysis with a human platelet ₂-adrenergic receptor probe revealed the existence of a single 5.2 kb restriction fragment (KpnI/SacI) in both human and rat genomic DNA; the rat brain ₂-receptor probe, however, hybridized to a single 1.9 kb band in rat DNA. Northern-blot analysis of rat brain poly(A⁺) RNA with the rat brain cDNA probe under stringent hybridization conditions revealed a single 4.5 kb mRNA; none was detected by the human platelet receptor probe. The rat brain 4.5 kb mRNA was not detected in any (other than brain) tested rat tissues utilizing either rat brain or human platelet DNA probes. The rat brain cDNA was expressed in a mammalian cell line (COS-2A) and found to bind the ₂-adrenergic antagonist [³H]yohimbine; based on the binding-affinity for prazosin, the presently cloned receptor was pharmacologically closer to the _2A subclass. We conclude that the rat brain cDNA encodes a new ₂-adrenergic receptor subtype that may be brain-specific.Abbreviations G protein guanine nucleotide-binding proteins - cA2-47 ₂-adrenergic receptor cDNA from rat brain - SSC (1X SSC contains 0.15 M NaCl, 15 mM Na₃citrate, pH 7.0)     

Effects of antidepressant drugs on inositol phospholipid hydrolysis in rat cerebral cortical slices

The effects of several different types of antidepressant drugs on phosphoinositide hydrolysis by slices of rat cerebral cortex was investigated by prelabeling inositol phospholipids with [³H]inositol and then measuring the formation of [³H]inositol phosphates (a total fraction consisting of the mono-and poly-phosphates was collected) in the presence of 10 mM LiCl. All of the drugs tested (amitriptyline, trimipramine, mianserin, desipramine, tranylcypromine, and citalopram) inhibited NE-stimulated [³H]inositol phosphate formation. This inhibition appeared to be due to antagonism of ₁-receptors. In addition to inhibiting the effects of NE, the tricyclic antidepressants themselves were able to stimulate [³H]inositol phosphate formation. This stimulation occurred at drug concentrations higher than that needed to inhibit stimulation by NE. Stimulatory effects of the antidepressants themselves were not blocked by the ₁-antagonist, prazosin. An examination of the types of inositol phosphates formed revealed that formation of inositol monophosphate was stimulated, but that inostiol biphosphate production was decreased by tricyclic antidepressants compared to control.     

Age-Related Development of γ-Aminobutyric Acid (GABA)BReceptor Functions in Various Brain Regions of Spontaneously Hypertensive Rats

The age-related development of GABA_Breceptors and their coupling to adenylate cyclase were studied in the brains of spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats. Compared with WKY rats, the specific [³H]GABA binding to GABA_Breceptors showed a significant decrease not only in the posterior hypothalamus, midbrain, hippocampus and striatum of eleven-week-old SHR, which maintain a hypertensive state, but also in the posterior hypothalamus of four-week-old normotensive SHR. Similarly, the GABA_Breceptor agonists (baclofen and DN-2327)-induced suppression of adenylate cyclase activity showed a decrease in the posterior hypothalamus of four-week-old SHR as well as in the posterior hypothalamus and striatum of eleven-week-old SHR. These results suggest that the functions of the GABA_Breceptor in the brain of SHR may be decreased independently from the occurrence of blood pressure elevation and that such changes may even be involved in the pathogenesis of SHR.     

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.     

Regulation of N-Methyl-d-aspartate receptor-mediated calcium transport and norepinephrine release in rat hippocampus synaptosomes by polyamines

The role of polyamines (PA) synthesis in NMDA receptor-mediated⁴⁵Ca²⁺ fluxes and norepinephrine release was studied in rat hippocampal synaptosomes. NMDA (50M) caused a sharp (>2-fold) transient increase in PA synthesis regulating enzyme, ornithine decarboxylase (ODC) activity with concomitant elevation in PA levels in the order putrescine>spermidine>spermine. ODC inhibitor, -difluoromethylornithine (DFMO), and NMDA antagonist, 2-amino-5-phosphonovaleric acid (D-AP5), both blocked increases in ODC activity and PA levels. Activation of NMDA receptors induced a sharp (3 to 4-fold) and quick (15 seconds) increase in⁴⁵Ca²⁺ uptake by synaptosomes within 15 seconds of exposure at 37°C. The efflux of⁴⁵Ca²⁺ and³H-norepinephrine (NE) release at 22°C from pre-loaded synaptosomes was also significantly (2 to 4-fold) enhanced by NMDA within 15 seconds. These NMDA receptor-mediated effects on calcium fluxes and NE release were blocked by NMDA receptor-antagonists (DAP-5 and MK-801) and PA synthesis inhibitor, DFMO and the DFMO inhibition nullified by exogenous putrescine. These observations establish that ODC/PA cascade play an important role in transduction of excitatory amino acid mediated signals at NMDA receptors.Special issue dedicated to Dr. Sidney Ochs.