Regional distribution of the two subtypes of angiotensin II receptor in rat brain using selective nonpeptide antagonists

Comparison of 3 AT1 Receptor Binding Assays: Filtration Assay, ScreenReady™ Target, and WGA Flashplate®

In this article, the study of 3 different angiotensin II type 1 (AT1) receptor binding assays in terms of reproducibility, robustness, and feasibility for high-throughput screening (HTS) is described. The following methods were used: a nonhomogeneous filtration assay in a 96-well format using CHO-AT1 cell membranes and 2 homogeneous assays, which include the commercially available ScreenReady™ Target for the AT1 receptor and the wheat germ agglutinin (WGA) Flashplate®, which was coated “in-house” with the CHO-AT1 cell membranes. Receptors were labeled with [125I]-Sar1-Ile8-angiotensin II, and radioligand binding was displaced using the antagonist losartan and the natural agonist angiotensin II. Reproducible Kd, Bmax, and Ki values and good total binding/nonspecific binding (TB/NSB) ratios were obtained with both the ScreenReady™ Targets and the filtration assay, whereas the WGA Flashplates® showed unacceptably high nonspecific binding and high variation when applied as a homogeneous assay. However, when applied as a heterogeneous assay (i.e., when a wash step at the end of the assay is included), the results were significantly better. Interestingly, ligand affinities were consistently lower in Flashplate®-based assays than in the filtration assay. This may be due to the immobilization of the receptors onto the solid surface of the plate, affecting their conformation. In terms of reproducibility, robustness, and feasibility for HTS, the authors conclude that the ScreenReady™ Target plates are most suitable for AT1 receptor binding screening.

Vasopressin and angiotensin receptors of the medial septal area in the control of mean arterial pressure induced by vasopressin

Introduction. Brain arginine 8-vasopressin (AVP), through the V1a- and V2-receptors, is essential for the maintenance of mean arterial pressure (MAP). Central AVP interacts with the components of the renin-angiotensin system, which participate in MAP regulation. This study aimed to determine the effects of V1a-, V2- and V1a/V2-AVP selective antagonists and AT1- and AT2-angiotensin II (Ang II) selective antagonists on the MAP induced by AVP injected into the medial septal area (MSA) of the brain.

Materials and methods. Male Holtzman rats with stainless steel cannulae implanted into the MSA were used in experiments. Direct MAP was recorded in conscious rats.

Results. AVP administration into the MSA caused a prompt and potent pressor response in a dose-dependent fashion. Pretreatment with the V1a- and V2-antagonists reduced, whereas prior injection of the V1a/V2-antagonist induced a decrease in the MAP that remained below the baseline. Both AT 1- and AT2-antagonists elicited a decrease, while simultaneous injections of two antagonists were more effective in decreasing the MAP induced by AVP.

Conclusion. These results indicate there is a synergism between the V1a- and V2-AVP and AT1and AT2-Ang II receptors in the MSA in the regulation of MAP.

Central angiotensin II receptors mediate hemodynamic response variability to stressors

We examined whether ANG II receptors in the central nervous system mediate hemodynamic responses to pharmacological (cocaine) and behavioral (cold water) stressors. After administration of cocaine (5 mg/kg iv), rats were classified as vascular responders (VR) if their pressor response was due entirely to an increase in systemic vascular resistance (SVR) despite a decrease in cardiac output (CO). Cocaine elicited a pressor response in mixed responders (MR) that was dependent on small increases in both SVR and CO. ANG II (30 ng/5 μl icv, 5 min before cocaine) augmented the decrease in CO in VR and prevented the increase in CO in MR. Administration of [Sar1,Thr8]ANG II (20 μg/5 μl icv; sarthran) before cocaine attenuated the decrease in CO and the large increase in SVR in VR so that they were no longer different from MR. Losartan (20 μg icv) or captopril (50 μg icv) preceding cocaine administration also attenuated the decrease in CO and the large increase in SVR seen in VR only. The role of angiotensin was not specific for cocaine, because ANG II (icv) pretreatment before startle with cold water (1 cm deep) enhanced the decrease in CO and the increase in SVR in both MR and VR, whereas losartan (icv) pretreatment before startle attenuated the decrease in CO and the increase in SVR in VR so that they were no longer different from MR. These data suggest that central ANG II receptors mediate the greater vascular and cardiac responsiveness in vascular responders to acute pharmacological and behavioral stressors.

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.

Human angiotensin II type-2 receptor inhibition of insulin-mediated ERK-2 activity via a G-protein coupled signaling pathway

While it has been shown that the angiotensin type-2 (AT(2)) receptor plays an important role in the development and differentiation of many tissues, the second messengers involved in its signaling pathways are just beginning to be understood. To further determine the signaling pathways for the AT(2) receptor, we have investigated whether human angiotensin type-2 receptor transfected into Chinese hamster ovary (CHO) cells can modulate insulin-induced extracellular signal-related protein kinase (ERK-2) phosphorylation via a G-protein coupled mechanism. Our results indicate that the human AT(2) receptor decreases insulin-induced ERK-2 phosphorylation through a G-protein mediated pathway since inhibition was attenuated by pertussis toxin (a G(i)/G(0) inhibitor). Our findings further indicate that the inhibitory response was insensitive to sodium orthovanadate (a PTPase inhibitor), but sensitive (attenuated) to okadaic acid, suggesting an important role for protein phosphatase 2A (PP2A). We have also shown that alanine substitution of the putative G-protein coupling DRY(141-143) motif of the second intracellular loop significantly decreases the human AT(2) receptor's ability to inhibit insulin-induced ERK-2 phosphorylation. Our results support the hypothesis that the AT(2) receptor inhibits insulin-induced ERK-2 activity via a G-protein coupled pathway involving the up-regulation of PP2A.

Role of AT2 receptor in the brain in regulation of blood pressure and water intake

The effects of intracerebroventricular (ICV) injection of angiotensin II (ANG II) on blood pressure and water intake were examined with the use of ANG II receptor-deficient mice. ICV injection of ANG II increased systolic blood pressure in a dose-dependent manner in wild-type (WT) mice and ANG type 2 AT(2) receptor null (knockout) (AT(2)KO) mice; however, this increase was significantly greater in AT(2)KO mice than in WT mice. The pressor response to a central injection of ANG II in WT mice was inhibited by ICV preinjection of the selective AT(1) receptor blocker valsartan but exaggerated by the AT(2) receptor blocker PD-123319. ICV injection of ANG II also increased water intake. It was partly but significantly suppressed both in AT(2)KO and AT(1)aKO mice. Water intake in AT(2)/AT(1)aKO mice did not respond to ICV injection of ANG II. Both valsartan and PD-123319 partly inhibited water intake in WT mice. These results indicate an antagonistic action between central AT(1)a and AT(2) receptors in the regulation of blood pressure, but they act synergistically in the regulation of water intake induced by ANG II.

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.

Acute changes in 3H-PAC and 125I-PYY binding in the nucleus tractus solitarii and hypothalamus after a hypertensive stimulus

Activation of alpha-2-adrenergic and neuropeptide Y (NPY) receptors in the nucleus tractus solitarii (NTS) induces hypotension and bradycardia. On the contrary, activation of angiotensin II (Ang II) receptors leads to hypertension. Acute changes in binding parameters of alpha-2-adrenergic, NPY and Ang II receptors were evaluated in the NTS and paraventricular hypothalamic nucleus (PVN) of rats after a hypertensive stimulus employing quantitative receptor autoradiography. Saturation experiments showed a decrease in the number (Bmax) of alpha-2-adrenergic binding sites in the NTS 6 hours after coarctation-induced hypertension. Furthermore, the affinity of NPY receptors was diminished as seen by the increase in the KD value of 125I-PYY. Tyrosine hydroxylase and NPY immunoreactivities were increased in the NTS and ventral medulla. Binding of 125I-Ang II was not changed in the NTS. Binding of all ligands analyzed was not altered in the PVN. The results suggest an acute down-regulation of alpha-2-adrenergic and NPY receptors involved with hypotension in response to hypertensive stimulus, which might be related to an increased availability of catecholamines and NPY in the NTS.

Central AT1 and AT2 receptors mediate chronic intracerebroventricular angiotensin II-induced drinking in rats fed high sodium chloride diet from weaning

Intracerebroventricular (ICV) angiotensin (AIl) administration stimulates central AII receptors to induce water consumption in rats. The aim of this study was to determine the role of brain AT1 and AT2 receptors in mediating chronic ICV AII-induced drinking in rats raised on normal or high sodium chloride diets from weaning. Rats were weaned at 21 days of age and placed on normal or high sodium chloride diet for 10-12 weeks. At adulthood, the animals were instrumented with brain lateral ventricular cannulas and femoral arterial catheters. Low dose chronic central AII infusion (20 ng min(-1)) significantly (P < 0.05) increased water intake in both groups of rats when compared with their respective controls of 24 h artificial cerebrospinal fluid infusions. In a separate group of high sodium fed rats, coinfusion of AII with the AT1 receptor antagonist, losartan (0.25 microg min(-1)) or the AT2 receptor blocker, PD 123319 (0.50 microg min(-1)) blocked chronic ICV AII-induced drinking. Upon reinfusion of AII water intake increased above control. Following the cessation of AII infusions, water intake returned to values not significantly different from control (P > 0.05). In contrast, in the normal sodium fed rats losartan, but not PD 123319, blocked the AII-mediated water intake. The data demonstrate that in high sodium chloride fed rats AII stimulates both central AT1 and AT2 receptors to induce drinking, while in the normal sodium chloride fed rats the peptide activates the drinking response primarily by stimulation of central AT1 receptors.

A new angiotensinergic system in the CNS of the rat

The use of two different polyclonal, affinity-purified, monospecific antibodies to ANG II (called BODE and BODE 1) revealed dissimilar distribution of ANG II immunoreactivity within the rat central nervous system (CNS). The ANG II-like material detected using BODE was concentrated in the neurosecretory hypothalamic nuclei, in the inner layer of the median eminence and in the posterior lobe of the pituitary. In contrast, the BODE 1 antibody did not stain the hypothalamic-neurohypophysial angiotensinergic system, and the staining pattern was much more broadly distributed throughout the CNS. BODE 1 is the first antibody that can be used to verify the locations of endogenous angiotensin and their receptor sites in the CNS. The diverse distribution of the ANG II-like material detected by the two antibodies provides strong evidence for the existence of at least two different angiotensinergic systems in the CNS.

Upregulation of immunoreactive angiotensin II release and angiotensinogen mRNA expression by high-frequency preganglionic stimulation at the canine cardiac sympathetic ganglia

The possible involvement of the local angiotensin system in ganglionic functions was investigated in the canine cardiac sympathetic ganglia. Positive chronotropic responses to preganglionic stellate stimulation at high frequencies, after intravenous administration of pentolinium plus atropine, were inhibited by the nonpeptide angiotensin AT(1) receptor antagonist forasartan or the angiotensin I-converting enzyme inhibitor captopril, whereas the rate increases elicited by the postganglionic stellate stimulation and norepinephrine given intravenously failed to be inhibited by these antagonists. The levels of endogenous immunoreactive angiotensin II, as determined by radioimmunoassay in the incubation medium of the stellate and inferior cervical ganglia, were increased after the high-frequency preganglionic stimulation of the isolated ganglia. The increment of the peptide was also antagonized by the pretreatment with captopril but not by a chymase inhibitor, chymostatin. The expression of angiotensinogen mRNA was observed in the stellate ganglion, adrenal, liver, and lung but not in the ovary and spleen. The expression of the mRNA in the stellate and inferior cervical ganglia increased after high-frequency preganglionic stimulation of the in vivo dogs for a period of 1 hour. These results indicate that an intrinsic angiotensin I-converting enzyme-dependent angiotensin system exists in the cardiac sympathetic ganglia, which is activated by high-frequency preganglionic stimulation.

Renal effects of angiotensin II receptor subtype 1 and 2-selective ligands injected into the paraventricular nucleus of conscious rats

We determined the effects of losartan and CGP42112A (selective ligands of the AT1 and AT2 angiotensin receptors, respectively) and salarasin (a relatively nonselective angiotensin receptor antagonist) on urinary volume and urinary sodium and potassium excretion induced by administration of angiotensin II (ANG II) into the paraventricular nucleus (PVN) of conscious rats. Both the AT1 and AT2 ligands and salarasin administered in the presence of ANG II elicited a concentration-dependent inhibition of urine excretion, but losartan inhibited only 75% of this response. The IC50 for salarasin, CGP42112A, and losartan was 0.01, 0.05, and 6 nM, respectively. Previous treatment with saralasin, CGP42112A and losartan competitively antagonized the natriuretic responses to PVN administration of ANG II, and the IC50 values were 0.09, 0.48, and 10 nM, respectively. The maximum response to losartan was 65% of that obtained with saralasin. Pretreatment with saralasin, losartan, and CGP42112A injected into the PVN caused shifts to the right of the concentration-response curves, but the losartan concentrations were disproportionately greater compared with salarasin or CGP42112A. The IC50 values were 0.06, 0.5, and 7.0 for salarasin, CGP42112A, and losartan, respectively. These results suggest that both AT1 and AT2 receptor subtypes in the PVN are involved in ANG II-related urine, sodium, and potassium excretion, and that the inhibitory responses to AT2 blockade are predominant.

Non-peptide angiotensin type 1 receptor antagonists in the treatment of hypertension

Angiotensin II (Ang II) acts at the cellular level on two receptor subtypes: the AT1 receptor which can be blocked by losartan and its analogues (the 'sartan family'), and the AT2 receptor that does not react with the above antagonists but which can be blocked by different compounds, such as PD123319. AT1 receptor blockade has proven to be a highly effective means of interference with the renin-angiotensin system (RAS) and hence of reducing high blood pressure. As a result of the terminal blockade of the RAS cascade, circulating Ang II levels tend to rise two- to threefold. The free access of such enhanced levels to uninhibited AT2 receptors may be clinically relevant, as argued in the present review. The most extensive experimental and clinical experience with AT1 receptor blockade so far has been obtained with the pioneer drug losartan, although major contributions have also been made on candesartan cilexetil, irbesartan and valsartan. All of these four drugs have been instrumental in substantial clinical trials, serving as sources of information in the clinically oriented part of this review. AT1 receptor blocking drugs generally provide a relatively gradual decrease in blood pressure, which is comparable to that obtained with conventional anti-hypertensive drugs. Clinical trials reveal an astounding lack of drug-related adverse effects, scoring even better than placebo in terms of frequencies and sometimes patterns. The trough/peak ratio on single dosages seems to have been mastered, particularly with the second generation of AT1 receptor blockers, as is evident from 24 h ambulatory blood pressure monitoring. Combination with low-dose thiazide regimens is well established. Intermediate endpoints (micro-albuminuria and left ventricular hypertrophy) appear to be controllable. Morbid cardiovascular sequelae are currently under study in comparison with beta- and calcium channel blockade.

AT1 receptors mediate chronic central nervous system AII hypertension in rats fed high sodium chloride diet from weaning

CNS angiotensin II (AII) hypertension is induced by chronic, low dose intracerebroventricular (ICV) AII infusion only in rats raised on a relatively high sodium chloride diet (250 meq kg(-1)food) from weaning. This experimental model of hypertension is dependent upon renal sympathetic innervation and associated with neurogenic sodium retention. This study determined whether AT1 and/or AT2 receptor subtypes in the CNS mediate this neurogenic ICV AII hypertension. Rats were weaned at 21 days of age and fed a 1.5% sodium chloride diet for 10-12 weeks. At adulthood, animals were instrumented with CNS lateral ventricular cannulas, femoral arterial and vein catheters and housed in metabolic pens for chronic study. Low dose ICV AII infusion (20 ng min(-1) )increased mean arterial pressure by 12+/-2 mm Hg and decreased urinary sodium excretion for three consecutive days. Subsequent ICV AT1 blockade with losartan abolished both the pressor and antinatriuretic responses to low dose ICV AII. In contrast, ICV AT2 receptor blockade with PD 123319 did not affect either angiotensin induced pressor or antinatriuretic responses. Following cessation of ICV AII infusion, arterial pressure and sodium excretion returned to values not significantly different from control in both groups of rats. These data confirm that low dose ICV AII causes hypertension and sodium retention in rats raised from early age on moderately elevated sodium intakes. This AII mediated neurogenic hypertension and antinatriuresis is transduced by activation of CNS AT1 receptors and not by activation of central AT2 receptors.

Hypothalamic angiotensin receptor subtypes in normotensive and hypertensive rats

The binding of 125I-labeled [Sar1,Ile8]angiotensin II to the hypothalamic membranes of the normotensive Wistar-Kyoto rat (WKY) and the spontaneously hypertensive rat (SHR) was studied. Displacement experiments with four centrally active angiotensins, losartan, and PD-123319 confirm the known existence of angiotensin AT1 and AT2 receptors in the rat hypothalamus. The values of the inhibitory constants for angiotensin II and PD-123319 in the SHR were significantly lower than the corresponding values in the WKY, indicating the possible existence of high-affinity hypothalamic AT1 and AT2 receptors for the two ligands in the SHR. The angiotensin AT1 receptor was further separated into a 5'-guanylyl imidodiphosphate-sensitive and -nonsensitive subtype, indicating that one of the subtypes is G protein coupled. The SHR has significantly higher numbers of measurable AT1-receptor subtypes as well as AT2 receptor subtypes. The former data support the findings of other investigators showing that the hypothalamus of the SHR expressed more AT1A and AT1B mRNAs than that of the normotensive rat. Des-Asp1-angiotensin I, which is known to attenuate the central pressor action of angiotensin II and angiotensin III, acts on both the AT1 and AT2 receptors, although it has a higher affinity for the AT1 receptors. The overall increase in the number of AT1 and AT2 receptors in the SHR is in line with the contention that the brain of the hypertensive rat, compared with that of the WKY, has a hyperactive renin-angiotensin system.

Sciatic nerve transection evokes lasting up-regulation of angiotensin AT2 and AT1 receptor mRNA in adult rat dorsal root ganglia and sciatic nerves

The angiotensin AT2 receptor is involved in tissue repair and cellular stress responses in non-neuronal cells. We have previously observed that the AT2 receptor-induced neurite formation in PC12W cells is paralleled by a reduced neurofilament M expression as it occurs in nerve fiber regeneration. Here we show that transection and crush of sciatic nerve fibers of adult rats results in dramatic changes of AT2, AT1a and AT1b receptor mRNA in dorsal root ganglion neurons (DRGs) and in sciatic nerves 3, 14 and 28 days after axotomy and crush. The expression patterns were determined by reverse transcription polymerase chain reaction (RT-PCR) assay, and the specificity of amplification products was verified by Southern blot hybridization. Whereas axotomy evoked a transient increase of AT2 receptor mRNA by more than 1000% after 3 days in proximal and after 14 days in distal sciatic nerve stumps (510%), the maximum expression in DRGs was observed after 14 days (1100%). Sciatic nerve crush resulted in a time-dependent up-regulation of AT2 receptor mRNA in sciatic nerve segments coinciding with the successful regeneration of nerve fibers. In sciatic nerves, AT1a and AT1b receptor mRNA levels were increased within different time-courses and to different extents with a maximum expression of 570%. In contrast to AT1a receptor mRNAs, AT1b receptor mRNA levels were increased in DRGs by maximally 800%. These results suggest that AT2 and AT1 receptor-mediated pathways are involved in Schwann cell-mediated myelination and in neuroregenerative responses of DRGs.

Stimulation of ACTH and GH release by angiotensin II in normal men is mediated by the AT1 receptor subtype

This study was performed in order to determine whether the stimulatory effect of plasma angiotensin II (ANG II) on Adrenocorticotropic hormone (ACTH) and growth hormone (GH) secretion in humans is mediated by AT1 subtype receptors. For this purpose, the effects of the administration of the AT1 receptor antagonist, losartan (50 mg p.o.) or a placebo on the ACTH and GH responses to ANG II (i.v. infusion for 60 min of successively increasing doses (4, 8 and 16 ng/kg/min); each dose for 20 min) were evaluated in eight normal men. ANG II infusion induced significant increases in both serum ACTH and GH levels (mean peaks were 1.6- and four-times higher than baseline, respectively). The ACTH response to ANG II was completely abolished by pretreatment with losartan. Also, the ANG II-induced GH rise was reduced by administration of losartan, but the GH response was still significantly higher than the basal value (mean peak was twice as high as the baseline). These data provide evidence of AT1 receptor involvement in mediation of the ANG-II stimulating effect on ACTH and GH secretion.

Action of AT1 subtype angiotensin II receptors of the medial preoptic area on gonadotropins and prolactin release

This study determined the effect of the selective angiotensin II (A II) AT1 receptor subtype antagonist losartan in the medial preoptic area (MPOA) of ovariectomized rats, treated with estrogen or untreated, on the release of gonadotropins (LH and FSH) and prolactin (PRL). The MPOA is sensitive to the action of A II and contains cell bodies of neurons producing luteinizing hormone-releasing hormone and a large density of estradiol receptors. Plasma FSH was not altered in any situation. However, losartan blocked and estradiol facilitated the stimulating and inhibitory effects of A II microinjection into the MPOA on LH and PRL secretion, respectively. The results indicate that these effects are mediated by AT1 receptors in the MPOA and that estradiol may modulate them. On the other hand, losartan itself reduced LH secretion in ovariectomized rats, indicating that the increase in the secretion of this hormone, after removal of the negative feedback caused by estradiol, is due, at least in part, to the action of A II on AT1 receptors of the MPOA.

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

The N-methyl-D-aspartate (NMDA) receptor has been reported to be important in synaptic plasticity, neuronal development, normal brain function and neurologic disease. We have recently shown that PC12W cells, a subclone of rat pheochromocytoma PC12 cell line, release nitric oxide (NO), as measured by in vitro spin-trapping combined with electron paramagnetic resonance (EPR) spectroscopy, when challenged with NMDA [Norby, S.W., Weyhenmeyer, J.A. and Clarkson, R.B., Stimulation and inhibition of NO production in macrophages and neuronal cells as observed by spin trapping, Free Rad. Biol. Med., 22 (1997) 1-9]. In the present study, we provide immunochemical evidence for the expression of both the NMDAR1 and NMDAR2A/B receptor subunits in PC12W cells, that express only the angiotensin type-2 (AT2) receptor subtype, and in NG108-15 (NG108) cells, a murine neuroblastoma x glioma hybrid that expresses both the angiotensin type-1 (AT1) and AT2 receptor subtypes. We also show that treatment of PC12W cells with angiotensin (Ang II) decreases NMDA-induced NO release by 28.0 +/- 4.2%, and that this response can be attenuated by pre-treating the cells with the isoform-specific AT2 antagonist, PD 123319. Interestingly, there was no effect on cGMP accumulation in PC12W cells treated with NMDA. Similar experiments were carried out using NG108 cells since the binding properties and functional characteristics of their NMDA receptors have been previously described [Ohkuma, S., Katsura, M., Chen, D., Chen, S. and Kuriyama, K., Presence of N-methyl-D-aspartate (NMDA) receptors in neuroblastoma x glioma hybrid NG 108-15 cells-analysis using 45Ca2+ influx and [3H]MK-801 binding as functional measures, Mol. Brain Res. 22 (1994) 166-172]. Our results show that NG108 cells significantly increase cGMP levels when challenged with NMDA (21.2 +/- 5.0% over control levels), and that this response can be attenuated by the addition of angiotensin (57.1 +/- 6.2% of stimulated levels). The effect of angiotensin on NMDA-mediated changes in cGMP levels was blocked by the AT2 antagonist, PD 123319, but was not significantly changed by the addition of the AT1 antagonist, losartan. Further, Ang II action on NMDA signalling in NG108 cells was completely inhibited by the addition of both the AT1 and AT2 antagonists. Taken together, these results suggest that AngII inhibits NMDA-mediated NO and cGMP production through a mechanism involving the AT2 receptor subtype.

The neuronal role of angiotensin II in thirst, sodium appetite, cognition and memory

Within the past two decades, a great deal has been learnt about the renin-angiotensin system in the brain. The renin-angiotensin system is one of the best-studied enzyme-neuropeptide systems in the brain. The diversity of localization of this peptide throughout the brain has implied a variety of potential functions. Besides its classical role in the regulation of blood pressure and body-fluid homeostasis, it has more subtle functions involving complex mechanisms such as learning and memory. The profound effects on behaviour produced by angiotensin are of broad interest to neuroscientists. The mechanisms of action differ depending on whether angiotensin is locally synthesized and whether regulation is governed by neural or metabolic inputs impinging on the neurones. Its central action is mediated through peptidergic receptors present on neurones. The description of the receptor subtypes AT1 and AT2 for angiotensin II and the development of non-peptidic specific angiotensin receptor subtype antagonists have opened a new area in this field of research. The AT1 site, which preferentially binds to angiotensin II and angiotensin III, appears to mediate the classical angiotensin functions concerned with maintenance of blood pressure and body-fluid control. In addition, most of the behavioural effects described so far are linked with AT1, although so-called psychotropic effects are presumed to be mediated by receptor systems other than the known specific angiotensin receptors. In fact, evidence for the existence of such receptors with high-affinity binding has been reported. The central action of angiotensin II mediated by AT2 is as yet unclear. Most reports concerning this receptor subtype suggest a role in differentiation and development, since the number of binding sites is higher in fetal and young rats than in adults. Furthermore, the neuronal effect of angiotensin II in the inferior olivary nucleus which is blocked specifically by AT2 antagonists suggests an involvement in motor control. Over the next few years we should find answers to many of the questions currently unanswered about angiotensin function and, given the rapid progress in research on this neuropeptide, it may serve as a model for the action of peptides on neuronal function in general.

The angiotensin II AT2 receptor inhibits proliferation and promotes differentiation in PC12W cells

Angiotensin II (ANG II) has been implicated in cell growth and differentiation. We investigated the effect of AT2 receptor stimulation on proliferation and morphological differentiation in cells of neuronal origin by using the pheochromocytoma derived cell line, PC12W. ANG II (10(-8)-10(-6) M) inhibited fetal calf serum (FCS)-induced cell proliferation in a concentration dependent manner. In half of the experiments, the epidermal growth factor (EGF) exerted a mitogenic action which was concentration-dependently inhibited by ANG II. In the other half of the experiments, EGF had an antimitogenic effect which was further enhanced by ANG II (maximally at 10(-6) M). Treatment with nerve growth factor (NGF) induced an inhibition of [3H]thymidine incorporation, which was enhanced by ANG II, maximally 25% at the highest concentration. The effects of ANG II on [3H]thymidine incorporation were reflected by those on cell number and were prevented by the AT2 receptor antagonist, PD123177, but not influenced by the AT1 receptor antagonist, losartan. The ANG II-induced inhibition of cell proliferation was paralleled by morphological differentiation in response to daily treatment with ANG II. ANG II also enhanced low-dose NGF-induced neurite formation. Again, these effects of ANG II were abolished by the AT2 receptor antagonist, PD123177. Our data in PC12W cells show that the AT2 receptor not only inhibits growth factor-induced proliferation and enhances the NGF-mediated growth arrest but also induces morphological differentiation in cells of neuronal origin. These findings strongly support the hypothesis that the AT2 receptor promotes differentiation in neuronal cells.

Angiotensin II induces the expression of c-fos mRNA in the central nervous system of the rat

We have investigated the effects of intracerebroventricular injections of angiotensin II in conscious rats on the expression of c-fos messenger RNA (mRNA) in the caudate nucleus, hypothalamus, midbrain and brainstem using semi-quantitative polymerase chain reaction and Northern Blots. RNA analysis revealed the presence of c-fos transcripts in the midbrain and brainstem following icv injections of ANG II. ANG II (1, 10, 100 ng) induced a substantial increase in c-fos mRNA in the brainstem which was significant after 10 ng ANG II, and less after 100 ng. This effect was time-dependent being detectable within 15 minutes and maximal after 60 minutes. This ANG II-induced c-fos mRNA expression was totally inhibited by icv pretreatment with the ANG II-AT1 receptor antagonist, losartan. Our data show for the first time that stimulation of central periventricular AT1 receptors induces the expression of c-fos mRNA in the brain. Thus, ANG II, in addition to its short-term regulatory actions, can participate through transcription factors in neuroplastic processes.

Evidence for a differential modulation of the alpha-2 adrenoceptors by angiotensin II in the nucleus tractus solitarii of the spontaneously hypertensive and the Wistar-Kyoto normotensive rats

An interaction between angiotensin II (Ang II) receptors and alpha 2-adrenoceptors was evaluated in the nucleus tractus solitarii (NTS) of the normotensive Wistar-Kyoto rat (WKY) and of the spontaneously hypertensive rat (SHR) using quantitative receptor autoradiography and cardiovascular analysis. In the WKY rat, Ang II promoted a dose-dependent increase in the IC50 value of l-noradrenaline when competing for ([3H]p-aminoclonidine ([3H]PAC) binding sites, which reached a maximum of 400% with 10 nM of Ang II and was associated with a small decrease in the B0 value (20%). In the SHR Ang II (0.1 nM) had an opposite effect leading to a decrease in the IC50 value of about 57%, and no change was observed in the B0 value. Saturation analysis also showed that Ang II (0.1 nM) increased the KD value of [3H]PAC in the WKY strain but in contrast decreased the KD value of [3H]PAC in the SHR. The Bmax value was not significantly changed neither in the WKY rat nor in the SHR. The cardiovascular analysis showed that a threshold dose of Ang II (0.05 pmol) counteracted the vasodepressor effect produced by l-noradrenaline coinjected in the NTS of the WKY rat. No effect was observed in heart rate. In the SHR no counteraction of the l-noradrenaline-induced vasodepressor effect was found, and in contrast a slight increase of the vasodepressor effect associated with a significant increase in the bradycardiac response was observed. The results give evidence for an antagonistic Ang II/alpha 2 receptor interaction in the cardiovascular part of the NTS of the WKY rat as previously observed in the Sprague-Dawley rat. However, this interaction is altered in the SHR, so that in this strain the Ang II/alpha 2 receptor interaction enhances alpha 2 affinity and possibly alpha 2 receptor function. This opposite effect observed in the SHR may represent one compensatory mechanism to counteract the development of high blood pressure in the SHR.

Angiotensin II receptor subtypes and phosphoinositide hydrolysis in rat adrenal medulla

Angiotensin II (ANG) receptor subtypes were characterized by quantitative autoradiography after incubation with the ANG agonist [124I]Sar1-ANG in rat adrenal medulla. ANG receptors are highly localized in adrenal medulla. Specific binding was displaced by 4% and by 95% with the AT, receptor blocker losartan and the AT2 receptor competitor CGP 42112A, respectively. Analysis of competition curves indicated relative binding potencies for the AT2 population of CGP 42112A>PD 123319> PD 123177. ANG stimulated +-nositol phosphate formation in a dose-dependent manner in rat adrenal medulla. Losartan at concentrations of 10(-9) to 10(-5) M antagonized the effect of ANG, whereas PD 123177 or PD 123319 had no antagonistic action. However, at a higher concentration (10(-5) M) PD 123177 or PD 123319 potentiated the effect of ANG on InsP1-accumulation. In the presence of PD 123319 (10(-5) M) ANG dose-response curve was shifted to the left with no change in the maximal effect. This affect was blocked by the addition of losartan (10(-5) M). On the contrary, the addition of CGP 42112A (10(-6) M) inhibited ANG-induced increase in InsP1-accumulation. On the other hand, ANG and CGP 42112A reduced basal cyclic GMP formation, this effect was partially reverted by sodium orthovanadate, a phosphotyrosine phosphatase inhibitor. Our results further demonstrate the presence of two ANG receptor subtypes in adrenal medulla: ANG binding to AT, receptor stimulates inositol phospholipid metabolism, whereas ANG binding to AT2 receptors decreases both inositol phosphate production and cGMP formation.

Tissular expression and regulation of type 1 angiotensin II receptor subtypes by quantitative reverse transcriptase-polymerase chain reaction analysis

Recent studies have revealed that angiotensin II (Ang II) interacts with two pharmacologically different types of seven-transmembrane domain receptors, hence named Ang II type 1 and type 2 (AT1 and AT2) receptors. cDNAs for the AT1 receptor have been cloned, and the existence of two receptor subtypes, AT1A and AT1B, has been revealed in rat and mouse. This study presents a new approach for the specific quantification of AT1A and AT1B receptor mRNAs by reverse transcription and polymerase chain reaction amplification in the presence of an AT1 receptor mutant cRNA as internal standard. Absolute quantities of mRNA are then determined by extrapolation using the standard curve generated with the internal standard. Moreover, addition of this internal standard to each tube controls for both reverse transcription and polymerase chain reaction amplification in each sample. In male Wistar rats, the highest absolute AT1A receptor mRNA levels were found in liver and kidney and those for AT1B receptor mRNA in the pituitary. Expressed as a percentage of total AT1A+AT1B receptor mRNA content, AT1A receptor mRNA content was 100% in liver, 85% in lung, 73% in kidney, 65% in aorta, 48% in adrenals, and 15% in the hypophysis. Since this approach can determine absolute AT1A and AT1B receptor mRNA quantities in different organs, it allows the study of the regulation of their expression under different pathophysiological conditions. After sodium depletion, known to induce hyperactivity of the renin-angiotensin system, adrenal AT1A and AT1B receptor mRNA levels were increased by 60% and 110%, respectively. In contrast, in renovascular hypertension (two-kidney, one clip), also associated with elevated circulating plasma renin activity, adrenal AT1B receptor mRNA levels decreased by 50%, whereas there was no change in those of AT1A. Therefore, the differential distribution and regulation of these two receptor subtypes suggest that each of them might be involved in the mediation of different biological effects of Ang II.

Evidence for an antagonistic angiotensin II/alpha 2-adrenoceptor interaction in the nucleus tractus solitarii

Interactions between alpha 2-adrenoceptors and angiotensin II receptors were evaluated in the nucleus tractus solitarii of the rat by means of quantitative receptor autoradiography and cardiovascular analysis. In binding experiments using l-noradrenaline to compete for [3H]p-aminoclonidine binding sites, angiotensin II (1 nM) increased the IC50 value of l-noradrenaline by 50%. The angiotensin AT1 receptor antagonist, DUP753 (losartan), not only blocked this action but also decreased the IC50 value of l-noradrenaline. The modulatory effect of angiotensin II was also evaluated after addition of both DUP753 and PD123319, an angiotensin AT2 receptor antagonist, and counteraction of the reduction in the IC50 value of l-noradrenaline was observed. In saturation experiments angiotensin II increased the KD and Bmax values of [3H]p-aminoclonidine binding sites, compatible with possible uncoupling of the alpha 2-adrenoceptors. Cardiovascular analysis demonstrated that a threshold dose of angiotensin II (0.05 pmol) counteracted the vasodepressor effect produced by an ED50 dose of l-adrenaline, l-noradrenaline or clonidine coinjected in the nucleus tractus solitarii. DUP753 fully blocked this in vivo modulation of alpha 2-adrenoceptors by angiotensin II. These findings suggest the existence of an antagonistic angiotensin AT1/alpha 2-adrenoceptor interaction in the nucleus tractus solitarii. Therefore, it can be surmised that the activation of angiotensin II AT1 receptors may reduce the transduction of the alpha 2-adrenoceptors and thus the alpha 2-mediated vasodepressor responses.

Angiotensin II receptor antagonists: a new approach to blockade of the renin-angiotensin system

A-II exerts its activity on various target tissues by binding to its receptors. The discovery of local RASs and A-II receptors within various tissues has generated interest in the clinical usefulness of RAS inhibition by directly blocking the action of A-II at the receptor level. Different A-II receptor subtypes have been identified and subsequently termed AT1 and AT2. AT1-receptor subtypes are the predominant receptor subtypes existing in most organs and, by coupling to a transmembrane G protein, seem to be the main subtypes participating in the vasoactive responses of A-II. Saralasin, a peptide with specific A-II receptor-antagonistic activity, had limited practical long-term usefulness as a result of its short half-life, significant agonistic properties, and lack of oral bioavailability. The discovery of simple benzyl-substituted imidazoles, which possess weak but highly selective A-II receptor antagonistic properties, led to the development of losartan (DuP 753). Losartan is a potent, orally active, specific, competitive nonpeptide A-II receptor antagonist that appears to be an effective antihypertensive agent both in animal studies and in preliminary clinical trials. The therapeutic usefulness of losartan, however, is not limited to its antihypertensive effects. The potential benefits of A-II receptor antagonists include roles in postmyocardial infarction therapy, slowing A-II-induced cardiac hypertrophy, 154, 155 slowing the progression of heart failure, preventing postangioplasty restenosis, and in slowing the progression of renal disease. Furthermore, losartan, a selective A-II type 1 (AT1) receptor antagonist, has also been a valuable pharmacologic probe for studying the mechanism of A-II stimulation of its receptors. A-II receptor antagonism appears to be as effective as ACE inhibition in the treatment of hypertension and other pathologic processes that involve the RAS and may offer an alternative to those patients who cannot tolerate ACE inhibitors because of their side effects.

Selective changes in angiotensin II AT1 and AT2 receptor subtypes in the rat superior colliculus following eye enucleation

In the brain of young (two weeks old) rats, angiotensin II receptors (AT2 receptors) are found in brain nuclei which receive and integrate direct visual input from the retina, the suprachiasmatic nuclei (containing only AT1 receptors), the lateral geniculate nuclei (containing AT2 receptors) and the superior colliculus (which contains both receptor types with a majority of AT2). In adult rats, angiotensin II receptors are present in the suprachiasmatic nuclei and the superior colliculus but not in the lateral geniculate. Using quantitative autoradiography we found that, in adult rats, bilateral eye enucleation caused a significant decrease in AT2 receptor binding, but not in AT1 receptor binding, and only in the superior colliculus. Unilateral enucleation of 12-day-old pups led to a decrease in AT2 receptor binding from the contralateral superior colliculus, as early as day 2 post-enucleation. Conversely, there was a significant increase in binding to AT1 receptors in the ipsilateral superior colliculus after seven days. No changes were seen in the lateral geniculate or suprachiasmatic nuclei. Angiotensin II binding to subcellular fractions of tissue from the superior colliculus region of 19-day-old pups suggested that AT2 receptor sites were present on the plasma membrane of the postsynaptic cell body. Membrane binding studies also showed a significant decrease in AT2 receptor binding to the same subcellular fractions when 19-day-old pups, enucleated seven days earlier, were compared to sham-operated animals. Our results suggest that expression of AT1 and AT2 receptors in the superior colliculus may be regulated by retinal input.

Angiotensin II receptor blockade: an innovative approach to cardiovascular pharmacotherapy

Through the multiple actions of angiotensin II (AII), the renin-angiotensin system (RAS) participates in cardiovascular homeostasis. Angiotensin II acts by binding to specific membrane-bound receptors, which are coupled to one of several signal transduction pathways. These AII receptors exhibit heterogeneity, represented by AT1 and AT2 receptor subtypes. The AT1 receptor mediates the major cardiovascular action of the RAS. This receptor has been cloned from multiple species, disclosing features consistent with a transmembrane, G-protein-linked receptor. Further AII receptor heterogeneity is evident by the cloning of isotypes of the AT1 receptor. Blocking the interaction of AII with its receptor is the most direct site to inhibit the actions of the RAS. Many AII receptor antagonists, including peptide analogs of AII and antibodies directed against AII, possess unfavorable properties that have limited their clinical utility. The discovery and further development of imidazole compounds with AII antagonist properties and favorable characteristics, however, has promise for clinical utility. The leader in this field is a selective AT1 receptor antagonist losartan (previously known as DuP 753 or MK-954). Losartan was demonstrated to be an effective antagonist of many AII-induced actions and an effective antihypertensive agent in many animal models of hypertension (HTN). Losartan also demonstrated secondary benefits in preventing stroke, treating congestive heart failure (CHF), and delaying the progression of renal disease in animal models. Clinical studies confirm the AII antagonist action of losartan and suggest that losartan will be effective in the treatment of essential HTN. AII antagonism is likely to provide useful treatment in essential HTN and CHF, conditions in which the RAS is known to play a major role. The utility of AII antagonism may extend beyond that of HTN and CHF, as suggested by the potential usefulness of angiotensin-converting enzyme (ACE) inhibition in the treatment or prevention of many other diseases. The key advantage AII antagonists provide over ACE inhibitors is that they may avoid unwanted side effects, related to bradykinin potentiation with the latter drugs. The AII antagonists will help determine the role of the RAS in physiologic regulation and in the pathophysiology of various disease states.

Differential effect of dithiothreitol and DuP 753 on angiotensin II receptor subtypes in bovine cerebellar cortex

Angiotensin II (AII) receptor subtypes were investigated in bovine cerebellar cortical membranes (BCCM) using agonists and antagonists. Differences were observed whether the tissue for the characterization was used fresh or frozen. In fresh BCCM, AII and angiotensin III (AIII) (not selective), p -aminophenylalanine6 -AII ([pNH2Phe6]AII) (AT2 selective), and DuP 753 (AT1 selective) interacted with two sites, at high and low affinity. In the presence of 10 mM dithiothreitol (DTT), the low-affinity site for AIII and [pNH2Phe6]AII and the high-affinity site for DuP 753 were no longer detectable. The same effect was obtained with 1 microM DuP 753, with only the low-affinity component of [pNH2Phe6]AII being resistant to this blocker. In frozen BCCM, DuP 753, PD 123319 (AT2 selective), and sarcosine1, isoleucine8-AII ([Sar1Ile8] AII) yielded monophasic curves. These data confirm the presence of multiple receptors (AT1 and AT2) for AII on BCCM and suggest the possibility that within the AT2 receptors, one subtype, DTT sensitive, may exist. Alternatively, a heterogeneity in the AT1 class, with respect to the sensitivity to DuP753, may be considered.

The neurogenic origin of hypertension in SHR may be mediated by angiotensin II through a receptor different from AT1 and AT2

In spontaneously hypertensive rats (SHR) 15-18 weeks old, the intracerebroventricular (icv) administration of peptide angiotensin II (Ang) antagonists results in a short (30-60 min) decrease (15-25 mmHg) of blood pressure (BP). Both EXP-3174, a known metabolite of Losartan and AT1 selective, and PD-123177, a receptor AT2 specific compound, do not affect SHR BP following icv administration. The receptor AT1 selective non-peptide Ang antagonists, Losartan and L-158809, induce long-lived (days) significant BP reductions (< or = 40 mmHg) in SHR, but only 18 h after icv injection. The slow development of BP reduction and its persistence might be due to the formation of an active metabolite, different from EXP-3174, a Losartan metabolite. In older SHR (25-28 weeks), the hypotensive effect of Losartan and L-158809 is not significant. These results suggest that in the CNS of the young SHR, an active Renin-Ang-System is implicated in the establishment of the hypertensive state, and that the receptor for this function is different from AT1 and AT2, since it has a selectivity profile different from AT1 and AT2 receptor types.

The renin-angiotensin system in the brain: an update 1993

The renin-angiotensin system is considered to be one of the most important hormonal systems in the regulation of blood pressure and body fluid homeostasis. Ever since this system has been demonstrated to be present also in the brain, vast efforts have been made in investigating its central impact and function. The last few years, and especially the development of non-peptidic angiotensin II receptor subtype specific antagonists and the subsequent pharmacological characterization of these subtypes, brought this field of research a large step forward. This progress also might have opened up new avenues of developing highly specific anti-hypertensive drugs and thereby new ways of treating hypertension. This paper intends to provide a summary of the knowledge about the brain renin-angiotensin system accumulated during recent years; an update 1993.

Binding of [3H]angiotensin II and [3H]DuP 753 (Losartan) to rat liver homogenates reveals multiple sites. Relationship to AT1a- and AT1b-type angiotensin receptors and novel nonangiotensin binding sites

The binding characteristics of radiolabeled angiotensin II and the nonpeptidergic angiotensin AT1 receptor antagonist, DuP 753 (Losartan), were studied in rat liver homogenates. Competition experiments with human angiotensin I, II, and III and with the angiotensin antagonists, CGP 42114A, saralasin, DuP 753, and PD123177, confirmed that the [3H]angiotensin II binding was to an AT1-type receptor. Computer analysis of the competition studies using the human angiotensins demonstrated that the data could be best fitted to a model that considers interaction at two sites. Angiotensin II, angiotensin III, and an angiotensin II analogue, [Sar1]angiotensin II, were calculated to have approximately one hundredfold selectivity at each of the two binding sites, but angiotensin I and the antagonists did not show a difference in affinity between the two sites. The addition of 120 mM NaCl and the nonhydrolyzable analogue of GTP, GppNHp (100 microM) to the buffer resulted in a reduction in [3H]angiotensin II binding at both sites. Thus, we suggest that the two sites may represent distinct angiotensin AT1-type receptors, possibly AT1a and AT1b subtypes. The addition of dithiothreitol (DTT) reduced [3H]angiotensin II binding, confirming the binding to AT1-type receptors. Binding studies using the selective AT1 angiotensin II receptor antagonist, [3H]DuP 753, were also performed on the rat liver homogenates. Saturation studies using both angiotensin II and DuP 753 to define nonspecific binding showed that [3H]DuP 753 bound to at least two types of site, one smaller population of receptors that was sensitive to both angiotensin II and DuP 753 and a second site that was sensitive to DuP 753 only.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II responsiveness of rat paraventricular and subfornical organ neurons in vitro

The responsiveness of neurons in the hypothalamic paraventricular nucleus to angiotensin II was investigated using extracellular single unit recording techniques in rat brain slices. Bath application of angiotensin II at a concentration of 3 x 10(-7) M for 2-5 min resulted in excitatory responses in 50.4% of 141 paraventricular cells tested. The mean increase in firing rate was 2.12 +/- 0.20 (mean +/- S.E.M.) spikes/s, which represents a mean increase in activity of 149.8 +/- 16.5%. Angiotensin II-sensitive neurons usually displayed irregular, phasic, or very slow spontaneous activity, with the majority of these neurons located in the magnocellular region. Under physiological blockade of synaptic transmission with low Ca2+/high Mg2+ medium, neuronal responses to this peptide remained in 12 (92.3%) of 13 cells tested. Application of three successive doses of angiotensin II ranging from 3 x 10(-9)-3 x 10(-7) M showed that neuronal responses were dose-dependent with an estimated threshold of 10(-8) M. In comparison with angiotensin III, angiotensin II not only stimulated more paraventricular cells, but usually induced larger excitatory responses. Angiotensin II subtype 1 receptor antagonist losartan completely blocked angiotensin II responsiveness in each of 14 paraventricular cells tested whereas PD 123319, an angiotensin II subtype 2 receptor antagonist, exhibited a partial inhibitory effect in about one half of another 13 cells. In addition, single unit in vitro subfornical organ recordings demonstrate that angiotensin II evokes greater excitatory responses than in the paraventricular nucleus and that these effects are abolished by losartan application. These results support the hypothesis that within both the paraventricular nucleus and subfornical organ angiotensin II is a bioactive peptide which modulates neuronal activity and thus may exert significant control over neuroendocrine and autonomic functions.

Role of AT1 receptors in the resetting of the baroreflex control of heart rate by angiotensin II in the rabbit

Angiotensin II (Ang II) resets the baroreflex control of heart rate to a higher blood pressure. This action is apparently mediated via Ang II receptors in the area postrema, but it is not known if these are of the AT1 or AT2 subtype. In the present study the effects of losartan, a selective AT1 receptor antagonist, and PD 123319, a selective AT2 antagonist, on the cardiac baroreflex response to Ang II were investigated in conscious rabbits with chronically implanted arterial and venous catheters. Baroreflex curves were generated with intravenous infusions of phenylephrine and nitroprusside (2.6-25 micrograms/kg per min) and analyzed using a four-parameter logistic model to yield their upper and lower plateaus, arterial pressure at the midpoint of the heart rate range (BP50), and slope coefficient. From these four parameters, the gain and range of the baroreflex were calculated. Background intravenous infusion of Ang II at 10 ng/kg per min increased mean arterial pressure by 17 mmHg but did not change heart rate. Ang II shifted the baroreflex curve to the right as indicated by an increase in BP50 from 70.9 +/- 2.0 to 89.3 +/- 2.7 mmHg (P < 0.05), but did not change baroreflex gain significantly. Ang II did not alter the upper plateau of the baroreflex, but decreased the lower plateau from 119.4 +/- 10.3 to 73.6 +/- 11.5 beats per minute (bpm) (P < 0.05), extending the heart rate range by 52.5 bpm. Pretreatment with losartan completely abolished the pressor and cardiac baroreflex responses to Ang II. In contrast, PD 123319 had no effect on these responses. Administration of losartan alone to block endogenous Ang II shifted the baroreflex curve to the left as indicated by a decrease in BP50 from 71.2 +/- 2.7 to 64.7 +/- 2.5 mmHg (P < 0.05). These results demonstrate that the resetting of the baroreflex control of heart rate by Ang II is mediated by AT1 receptors, and that basal levels of endogenous Ang II exert a tonic action on the cardiac baroreflex to increase the setpoint around which the baroreflex regulates heart rate.

Functional roles of brain AT1 and AT2 receptors in the central angiotensin II pressor response in conscious young spontaneously hypertensive rats

Areas of adult rat brain that mediate the cardiovascular effects of central angiotensin II (ANG II) predominantly express AT1 ANG II receptors. In contrast, AT2 receptor expression in young rats is transiently increased, reaching a maximum during the first few weeks of life. This study was designed to determine the roles of brain AT1 and AT2 receptors in mediating the central pressor effects of ANG II in young (4-week-old) conscious spontaneously hypertensive rats (SHR). Mean arterial pressure responses to intracerebroventricular (i.c.v.) ANG II (100 ng in 5 microliters) were determined 10 minutes after i.c.v. injection of either the AT1 receptor antagonist Losartan (1.0, 2.5, 5.0, and 10.0 micrograms), the AT2 receptor ligand PD 123319 (3.5 x [10(-6), 10(-4), 10(-2), 10(0)] micrograms), or both. In control rats, i.c.v. Losartan prevented the pressor response to i.c.v. ANG II in a dose-dependent manner (P < 0.05), while i.c.v. PD 123319 alone was without effect. In other animals, pressor responses caused by i.c.v. ANG II-induced vasopressin secretion (VP-component) and sympathetic nervous system activation (SNS-component) were studied individually, with similar result; Losartan prevented the SNS-component, but reduced the VP-component by only 45%, indicating that both pressor components involve AT1 receptor activation. However, doses of Losartan were more effective when combined with 3.5 micrograms of PD 123319 than when given alone (P < 0.05); nearly eliminating the VP-component. These results suggest that i.c.v. ANG-II-induced pressor effects may involve activation of multiple receptor subtypes.

Cellular localization of angiotensin type 1 receptor and angiotensinogen mRNAs in the subfornical organ of the rat brain

The cellular localization of angiotensin type 1 receptor (AT 1) and angiotensinogen mRNA expression in the subfornical organ (SFO) of the rat brain has been studied by means of non-radioactive in situ hybridization combined with immunocytochemistry for glial fibrillary acidic protein (GFAP) and Neutral red staining. The AT 1 receptor mRNA expression is shown to be within putative nerve cells without any association with the glial fibrillary acidic protein (GFAP)-immunoreactive (IR) cells. In contrast the angiotensinogen cRNA expression is associated predominantly with GFAP-IR cells. The results demonstrate that a neuronal AT 1 receptor mediates the actions of circulating angiotensin II on the SFO and that the angiotensinogen mRNA is predominantly expressed in the SFO astroglial cells.

Functional evidence that the angiotensin antagonist losartan crosses the blood-brain barrier in the rat

Losartan is a novel nonpeptidergic antagonist of angiotensin (ANG) II subtype 1 (AT1) receptors, which effectively lowers blood pressure in high-renin hypertensive rat and blocks the pressor response to systemic ANG II. It is well known that high densities of ANG II receptors exist in the hypothalamic paraventricular nucleus (PVN). In addition, activation of putative angiotensinergic afferents to the PVN originating in subfornical organ (SFO) elevates blood pressure and facilitates the activity of PVN neurons. We report here that systemic administration of losartan (3 mg/kg) significantly attenuates the pressor response to electrical stimulation of SFO. The excitatory responses of PVN neurons to SFO stimulation or local pressure microinjection of ANG II were also significantly inhibited in 58.8% and 88.9% of PVN cells, respectively, by intravenous administration of losartan. These pharmacological effects were rapid and reversible, and were accompanied by little change of basal arterial blood pressure or spontaneous neuronal activity. These observations suggest that systemic losartan crosses the blood-brain barrier (BBB) and acts at AT1 receptors within the PVN.

Characterization of biochemical responses of angiotensin II (AT2) binding sites in the rat pheochromocytoma PC12W cells

Rat pheochromocytoma PC12W cell membranes have previously been shown to exclusively contain the AT2 receptor subtype. The present study extended these binding data and explored the functional expression of these binding sites. Our binding competition studies show a potency series of Ang II = Ang III greater than saralasin greater than Ang I = PD123177 much greater than Ang II(1-7) much much greater than losartan. PD123177 (1 microM) completely eliminated [125I]Ang II binding to PC12W cells. Competitive displacement of [125I]Ang II with Ang II shows a dissociation equilibrium constant (Kd) of 1.79 nM and a binding site maximum (Bmax) of 3.97 fmol/mg protein. Investigating several Ang II signal transduction pathways on these cells, we found that Ang II (10(-8) to 10(-6) M) does not affect basal cAMP, cGMP, arachidonic acid release, prostacyclin release, intracellular Ca2+ mobilization or thymidine incorporation in the PC12W cells. Nerve growth factor, cAMP, 5-fluorouridine deoxyriboside modulation of the number of AT2 receptor sites in PC12W cells failed to unmask any Ang II effects on basal cAMP, cGMP and intracellular Ca2+ mobilization. In conclusion, the present study confirms the exclusive presence of AT2 binding sites in the PC12W cells. However, these binding sites are not functionally coupled to common signal transduction pathways.

Angiotensin receptor subtypes in the brain

Development of specific angiotensin II receptor ligands has recently provided evidence for the existence of two angiotensin II receptor subtypes, termed AT1 and AT2, which differ in their signal transduction mechanisms and in the effects they mediate. In brain, both receptor subtypes are present. Most of the known central actions of angiotensin II, for example the regulation of blood pressure and of electrolyte and water balance, seem to be mediated by the AT1 receptor, while the role of the AT2 receptor is still an enigma. This review by Thomas Unger and colleagues summarizes the current knowledge and latest hypotheses in this rapidly developing field.

The distribution of angiotensin II AT1 receptor subtype mRNA in the rat brain

The present study demonstrates the existence and regional distribution of angiotensin II AT1 receptor subtype mRNA expression in the rat brain by the use of in situ hybridization and RNase protection assay. Substantial expression levels in the brain have only been detected in certain distinct areas, such as the subfornical organ, the parvocellular part of the paraventricular hypothalamic nucleus, and the median preoptic nucleus. The results give further evidence for the involvement of the angiotensin II AT1 receptor subtype in the classical functions of central angiotensin II, like blood pressure control, body fluid homeostasis and in corticotropin-releasing factor (CRF) secretion.

Characterization and distribution of angiotensin II binding sites in fetal and neonatal astrocytes from different rat brain regions

Although angiotensin II (Ang II) binding sites have been extensively investigated in brain, revealing the presence of both AT1 and AT2 subtypes in various areas, the question as to which cells express AT1 and AT2 sites is still open. We report here that primary cultures of astrocytes obtained from various brain regions of fetal (F17) and one-day-old rats express Ang II binding sites belonging only to the AT1 subtype. The binding sites have the same binding profile in all regions tested; however, much less binding was observed in membranes of astrocytes derived from cortical than from subcortical regions and almost none were found in neonatal cortex. In addition, the dispersion method used at the onset of culture affects the number of binding sites present at the end of the culture period.

Modulation of net outward current in cultured neurons by angiotensin II: involvement of AT1 and AT2 receptors

In this study we have used whole-cell, voltage-clamp procedures to determine the effects of angiotensin II (AII) on net outward current (I(no)) in neurons co-cultured from the hypothalamus and brainstem of 1-day-old rats. Ino is the sum of all inward and outward membrane currents (minus Na+, which is blocked by tetrodotoxin) which occur during the repolarization phase of the action potential. We have determined that AII elicits two separate effects on I(no) in cultured neurons. AII caused a reversible and concentration (0.1 nM-10 microM)-dependent increase in I(no). This effect is inhibited by the AT2 receptor-selective antagonists, PD123177 and PD123319 (both 100 nM), but not by the AT1-selective receptor blocker, DuP753 (Losartan; 100 nM), and so it is mediated by AT2 receptors. In a smaller number of neurons AII induced a reversible and concentration (0.01 nM-10 microM)-dependent decrease in I(no) that was blocked by Losartan (100 nM) but not by PD123177 (100 nM). Thus the decrease in I(no) is mediated by AT1 receptors. Additionally, some neurons displayed both AT1- and AT2 receptor-mediated effects on I(no). Our results demonstrate two distinct actions of AII on membrane ionic currents in cultured neurons, effects that are mediated by different AII receptor subtypes.

Effects of angiotensin analogues and angiotensin receptor antagonists on paraventricular neurones

In a previous study we observed that most neurones in the paraventricular nucleus are excited by angiotensin-(1-7). In comparison with angiotensin III this excitatory action was significantly delayed. The aim of the present microiontophoretic study of angiotensin II-sensitive rat paraventricular neurones was to compare the effect of the angiotensin-analogues angiotensin-(1-7), angiotensin-(2-7), angiotensin II and angiotensin III on the spontaneous activity of these neurones and to test angiotensin receptor subtype 1 antagonists (CGP 46027 or DuP 753) and subtype 2 selective antagonists (CGP 42112A and PD 123177) in order to acquire more evidence of the receptor subtype present. As previously observed angiotensin II, angiotensin III and angiotensin-(1-7) excited most neurones. The effect of angiotensin-(1-7) was usually weaker than that of angiotensin II, and in contrast to angiotensin III the latencies were not significantly different. Angiotensin-(1-7) seemed to be active by itself, because its effect was antagonised by angiotensin receptor antagonists. Angiotensin-(2-7) was mostly inactive, although a few cells were excited. Whereas the excitatory effects of angiotensin-(1-7), angiotensin II and angiotensin III could always be inhibited with both angiotensin receptor subtype antagonists 1 and 2, that produced by angiotensin-(2-7) was only weakly antagonised, if at all. Subtype 1 selective antagonists were effective at lower concentrations than selective subtype 2 antagonists.

Induction of the angiotensin AT2 receptor subtype expression by differentiation of the neuroblastoma x glioma hybrid, NG-108-15

In vitro differentiation of the mouse neuroblastoma-rat glioma hybrid cell line, NG-108-15, with dimethyl sulphoxide (1.5%) and low serum (0.5%), produced a marked increase in the number of angiotensin II receptors, from a level at the limit of sensitivity using labelled angiotensin II with a high specific activity ([125I]angiotensin II), in undifferentiated cells, to a Bmax of 1077 (1070-1268) fmol/mg in 5-day-differentiated cells. The affinity (Kd) of radiolabelled angiotensin II for the receptors in differentiated cells was 8.1 (7.5-10) nM. The recently available selective non-peptide antagonists, DuP 753 and PD 123177 and the peptide analogues of angiotensin II, CGP 42112A and p-aminophenylalanine6 angiotensin II, were used to characterize the angiotensin II receptors by competing for 125I-[Sar1-Ile8]angiotensin II binding to membranes prepared from undifferentiated and differentiated cells. The predominant angiotensin II receptor subtype expressed by undifferentiated cells was AT1 and after differentiation AT2. This change in receptor expression was evident 2 days after initiation of differentiation, was maximal at 4-5 days and was stable for at least 8 days. Administration of angiotensin II induced intracellular Ca2+ mobilization in both undifferentiated and differentiated cells. This was antagonised by the selective AT1 antagonist, DuP 753, indicating an action at the AT1 receptor subtype in both undifferentiated and differentiated cells. The selective AT2 antagonist, PD 123177 was without effect on the angiotensin II induced increase in intracellular Ca2+. This effect of DuP 753 on Ca2+ was specific for angiotensin II since the drug had no effect on bradykinin induced increases in intracellular Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II AT1 receptors in rat superior cervical ganglia: characterization and stimulation of phosphoinositide hydrolysis

Angiotensin II receptor number was higher in superior cervical ganglia of 2-week-old when compared to 8-week-old rats. In both young and adult rats, specific binding of [125I][Sar1]angiotensin II was displaced competitively by the AT1-receptor antagonist DuP 753 but not by the AT2-receptor competitor PD 123177. In ganglia from adult rats, DuP 753 competed with an IC50 of 113 nM. The stable guanine nucleotide GTP gamma S inhibited binding of [125I][Sar1]angiotensin II in young and adult rats by approximately 50% with IC50 values of 105 and 120 nM, respectively, suggesting that the angiotensin receptor is G-protein linked. Angiotensin II at a dose of 1 microM stimulated inositol phosphate formation 58% over control values in superior cervical ganglia from 8-week-old rats. This effect was totally blocked by 10 microM DuP 753 but not by 10 microM PD 123177. Our findings demonstrate that rat superior cervical ganglia contain AT1-type angiotensin receptors that are probably G-protein linked, and their stimulation results in increased inositol phospholipid metabolism.