Regulatory role of brain angiotensins in the control of physiological and behavioral responses

Enhanced angiotensin II stimulates renal disorders in transgenic Tsukuba hypertensive mice

Tsukuba hypertensive mice (THMs) are transgenic mice carrying human renin-angiotensin system (RAS) genes. The aim of this study is to evaluate whether renal disorders are present in THMs. Twenty-week-old THMs and C57BL/6 mice (C57s) were used for this study. Each group consisted of 8 mice. Systolic blood pressure, urinary volume, water intake and urinary albumin excretion were measured in each mouse. Each mouse was then euthanized, and the renal glomerulosclerosis index and glomerular size were measured. Systolic blood pressure of THMs was about 40 mmHg higher than that of C57s. Urinary volume, water intake and urinary albumin excretion were significantly higher in THMs than in C57s. The renal glomerulosclerosis index and glomerular size were also significantly higher in THMs than in C57s. These results suggested that an enhanced renin-angiotensin system, including its hypertensive effects, stimulates albuminuria, renal glomerulosclerosis and glomerular hypertrophy in THMs.

Angiotensin, thirst, and sodium appetite

Angiotensin (ANG) II is a powerful and phylogenetically widespread stimulus to thirst and sodium appetite. When it is injected directly into sensitive areas of the brain, it causes an immediate increase in water intake followed by a slower increase in NaCl intake. Drinking is vigorous, highly motivated, and rapidly completed. The amounts of water taken within 15 min or so of injection can exceed what the animal would spontaneously drink in the course of its normal activities over 24 h. The increase in NaCl intake is slower in onset, more persistent, and affected by experience. Increases in circulating ANG II have similar effects on drinking, although these may be partly obscured by accompanying rises in blood pressure. The circumventricular organs, median preoptic nucleus, and tissue surrounding the anteroventral third ventricle in the lamina terminalis (AV3V region) provide the neuroanatomic focus for thirst, sodium appetite, and cardiovascular control, making extensive connections with the hypothalamus, limbic system, and brain stem. The AV3V region is well provided with angiotensinergic nerve endings and angiotensin AT1 receptors, the receptor type responsible for acute responses to ANG II, and it responds vigorously to the dipsogenic action of ANG II. The nucleus tractus solitarius and other structures in the brain stem form part of a negative-feedback system for blood volume control, responding to baroreceptor and volume receptor information from the circulation and sending ascending noradrenergic and other projections to the AV3V region. The subfornical organ, organum vasculosum of the lamina terminalis and area postrema contain ANG II-sensitive receptors that allow circulating ANG II to interact with central nervous structures involved in hypovolemic thirst and sodium appetite and blood pressure control. Angiotensin peptides generated inside the blood-brain barrier may act as conventional neurotransmitters or, in view of the many instances of anatomic separation between sites of production and receptors, they may act as paracrine agents at a distance from their point of release. An attractive speculation is that some are responsible for long-term changes in neuronal organization, especially of sodium appetite. Anatomic mismatches between sites of production and receptors are less evident in limbic and brain stem structures responsible for body fluid homeostasis and blood pressure control. Limbic structures are rich in other neuroactive peptides, some of which have powerful effects on drinking, and they and many of the classical nonpeptide neurotransmitters may interact with ANG II to augment or inhibit drinking behavior. Because ANG II immunoreactivity and binding are so widely distributed in the central nervous system, brain ANG II is unlikely to have a role as circumscribed as that of circulating ANG II. Angiotensin peptides generated from brain precursors may also be involved in functions that have little immediate effect on body fluid homeostasis and blood pressure control, such as cell differentiation, regeneration and remodeling, or learning and memory. Analysis of the mechanisms of increased drinking caused by drugs and experimental procedures that activate the renal renin-angiotensin system, and clinical conditions in which renal renin secretion is increased, have provided evidence that endogenously released renal renin can generate enough circulating ANG II to stimulate drinking. But it is also certain that other mechanisms of thirst and sodium appetite still operate when the effects of circulating ANG II are blocked or absent, although it is not known whether this is also true for angiotensin peptides formed in the brain. Whether ANG II should be regarded primarily as a hormone released in hypovolemia helping to defend the blood volume, a neurotransmitter or paracrine agent with a privileged role in the neural pathways for thirst and sodium appetite of all kinds, a neural organizer especially in sodium appetit

Purification, characterization, and cloning of a cytosolic aspartyl aminopeptidase

An aminopeptidase with a preference for N-terminal aspartyl and glutamyl residues but distinct from glutamyl aminopeptidase (EC 3.4. 11.7) was purified to near homogeneity from rabbit brain cytosol. Its properties were similar to an enzyme described previously (Kelly, J. A., Neidle, E. L., and Neidle, A. (1983) J. Neurochem. 40, 1727-1734). Aspartyl aminopeptidase had barely detectable activity toward simple aminoacyl-naphthylamide substrates. Its activity was determined with the substrate Asp-Ala-Pro-naphthylamide in the presence of excess dipeptidyl-peptidase IV (EC 3.4.14.5). The native enzyme has a molecular mass of 440 kDa and migrates as a single band of 55 kDa after SDS-polyacrylamide gel electrophoresis. The sequences of three tryptic peptides were used to screen the GenBankTM data base of expressed sequence tags. Human and mouse clones described as "similar to a yeast vacuolar aminopeptidase" and containing full-length cDNAs were identified and sequenced. The human cDNA was expressed in Escherichia coli. The amino acid sequence has significant homology to yeast aminopeptidase I, placing it as the first identified mammalian member of the M18 family of metalloproteinases. Homologous sequences in Caenorhabditis elegans and in prokaryotes revealed three conserved histidines, three conserved glutamates and five conserved aspartates. Aspartyl aminopeptidase is found at relatively high levels in all mammalian tissues examined and is likely to play an important role in intracellular protein and peptide metabolism.

The brain is a possible target for an angiotensin-converting enzyme inhibitor in the treatment of chronic heart failure

BACKGROUND

Although many lines of evidence have shown beneficial effects of angiotensin-converting enzyme (ACE) inhibitors on patients with chronic heart failure (CHF) after myocardial infarction (MI), the target of ACE inhibitors still remains unclear. The objectives of the present study were to evaluate the dipsogenic response to centrally administered angiotensin and to examine the effect of central administration of an ACE inhibitor on cardiac remodeling in rats with CHF after large MI.

METHODS AND RESULTS

The drinking responses to intracerebroventricular (i.c.v.) injections of saline and angiotensin I (100 ng) were measured in 22 male Sprague-Dawley rats with or without CHF at 2-5 weeks after the ligation of the left coronary artery. The dipsogenic responses to i.c.v. angiotensin I were significantly larger in rats with CHF and large MI (infarct size > 30%) than in sham-operated rats. Pretreatment with losartan abolished the significant difference between the two groups. Left ventricular (LV) weights of 32 surviving rats with CHF were measured after the 3-week subcutaneous infusions of vehicle (s.c.-VEH) and captopril (1 mg x kg(-1) x h(-1), s.c.-CAP) or the 3-week i.c.v. infusions of vehicle (i.c.v.-VEH) and captopril (50 microg x kg(-1) x h(-1), i.c.v.-CAP). The LV weights normalized by body weights of s.c.-CAP rats were significantly smaller than those of s.c.-VEH rats (1.73 +/- 0.04 vs 2.08 +/- 0.09 g x kg(-1); P < .01); those of i.c.v.-CAP rats were also significantly smaller than those of i.c.v.-VEH rats (1.84 +/- 0.08 vs 2.1 +/- 0.10 g x kg(-1); P < .05).

CONCLUSIONS

These results suggest that the brain is a possible target for ACE inhibitors in the treatment of CHF after MI.

Modulation of motor functions involving the dopaminergic system by AT1 receptor antagonist, losartan

Growing evidence has indicated the existence of a brain renin angiotensin system and its possible interaction with other putative neurotransmitters and their receptors. In the present study, the effect of losartan, an AT1 receptor antagonist, was studied on the motor functions involving the dopaminergic system. Losartan (5-30 mg/kg) per se decreased locomotor activity without producing motor toxicity. It partially reversed the apomorphine-induced hyperlocomotion and stereotypy in mice, and potentiated neuroleptic-induced catalepsy in rats. On chronic administration (once daily for 21 days) losartan failed to block apomorphine-induced hyperlocomotion, but the inhibition of stereotypic response and potentiation of neuroleptic-induced catalepsy remained unaltered. These observations suggest that losartan inhibited the release of dopamine through AT1 receptor and also suggest the existence of a compensatory mechanism in certain brain region concerned with dopamine motor function.

Neuronal ion channel signalling pathways: modulation by angiotensin II

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

Vasopressin mediates the inhibitory effect of central angiotensin II on cerebrospinal fluid formation

Angiotensin II infused at low doses into the cerebral ventricles decreases cerebrospinal fluid (CSF) production. Since central angiotensin II also activates the sympathetic nervous system and promotes vasopressin release, the roles of these two factors in mediating the inhibitory effect of angiotensin II on CSF formation were studied. CSF production was measured in rats by the ventriculocisternal perfusion method. During central angiotensin II infusion (5 pg min(-1)), the following adrenoceptor antagonists were administered intravenously (i.v.): phentolamine (alpha1/alpha2, 2 mg/kg per h), prazosin (alpha1, 1 mg/kg per h), and propranolol (beta, 1 mg/kg per h). None of these agents affected the inhibitory effect of angiotensin II on CSF formation. In comparison, in animals administered i.v., the vasopressin V1 receptor antagonist, d(CH2)5Tyr(Me)Arg-vasopressin (10 microg/kg per h), the angiotensin II-induced decrease in CSF production was abolished. Our observations indicate, therefore, that vasopressin mediates the inhibitory effect of central angiotensin II on CSF formation.

Distribution of angiotensin type-1 receptor messenger RNA expression in the adult rat brain

Angiotensin II and angiotensin III in the brain exert their various effects by acting on two pharmacologically well-defined receptors, the type-1 (AT1) and the type-2 (AT2) receptors. Receptor binding autoradiography has revealed the dominant presence of AT1 in brain nuclei involved in cardiovascular, body fluid and neuroendocrine control. The cloning of the AT1 complementary DNA has revealed the existence of two receptor subtypes in rodents, AT1A and AT1B. Using specific riboprobes for in situ hybridization, we have previously shown that the AT1A messenger RNA is predominantly expressed in the rat forebrain; in contrast the AT1B subtype predominates in the anterior pituitary. Using a similar technical approach, the aim of the present study was to establish the precise anatomical localization of cells synthetising the AT1A receptor in the adult rat brain. High AT1A messenger RNA expression was found in the vascular organ of the lamina terminalis, the median preoptic nucleus, the subfornical organ, the hypothalamic periventricular nucleus, the parvocellular parts of the paraventricular nucleus, the nucleus of the solitary tract and the area postrema, in agreement with previous autoradiographic studies, describing a high density of AT1 binding sites in these nuclei. In addition, AT1A messenger RNA expression was detected in several brain areas, where no AT1 binding was reported previously. Thus, we identify strong expression of AT1A messenger RNA expression in scattered cells of the lateral parts of the preoptic region, the lateral hypothalamus and several brainstem nuclei. In none of these structures was the AT1B messenger RNA detectable at the microscopic level. In conclusion, it is suggested that angiotensins may exert their central effects on body fluid and cardiovascular homeostasis mainly via the AT1A receptor subtype.

Central renin injections: effects on drinking and expression of immediate early genes

This study investigated the drinking response and the expression of Fos- and Egr-1-immunoreactivity (Fos-ir; Egr-1-ir) in the brain induced by endogenous angiotensin generated by intracerebroventricular (i.c.v.) injection of renin. Renin induced Fos-ir in the subfornical organ (SFO), median preoptic (MnPO), supraoptic and paraventricular nuclei (SON and PVN), area postrema (AP), nuclei of the solitary tract (NTS) and lateral parabrachial nuclei (LPBN). Renin-induced Egr-1-ir exhibited a similar pattern of distribution as that observed for Fos-ir. The dose of i.c.v. renin that induced expression of immediate early gene (IEG) product immunoreactivity also produced vigorous drinking. When renin-injected rats were pretreated with captopril, an angiotensin converting enzyme inhibitor, drinking was blocked. With the same captopril pretreatment, both Fos- and Egr-1-ir in the SFO, MnPO, SON, PVN, AP and LPBN were also significantly reduced.

Autoradiographic identification of kidney angiotensin IV binding sites and angiotensin IV-induced renal cortical blood flow changes in rats

The present investigation initially determined that specific binding sites for the hexapeptide angiotensin IV (AngIV) are present in the rat kidney cortex and outer medulla but not in the inner medulla, using in vitro autoradiographic techniques. This binding site has been termed AT4, is distinct from the previously characterized AT1 and AT2 sites, and does not bind the specific AT1 receptor antagonist DuP753 or the AT2 receptor antagonist PD123177. Renal artery infusions of AngIV produced a dose-dependent increase in cortical blood flow without altering systemic blood pressure. In contrast, the infusion of angiotensin II (AngII) induced a dramatic decrease in cortical blood flow, accompanied by a significant elevation in systemic blood pressure. The infusion of [D-Val(1)]AngIV, an analog that does not bind at the AT4 receptor site, and the C-terminal truncated analogs AngIV (1-4) and AngIV (1-5) that possess lower affinity for this site, produced no change in cortical blood flow. The infusion of [Nle1]AngIV and [Lys1]AngIV, analogs that bind with high affinity at the AT4 receptor site, produced increases in cortical blood flow with no influence on blood pressure. Pretreatment with a specific AT4 receptor antagonist, Divalinal-AngIV, completely blocked AngIV-induced elevations in blood flow, but failed to influence AngII-induced decreases in blood flow, suggesting that these ligands are acting at different receptor sites. Pretreatment with the nitric oxide synthase inhibitor, NG-Monomethyl-L-Arginine, also blocked subsequent AngIV-induced increases in cortical blood flow. These data support the notion that AngIV exerts a unique influence upon renal hemodynamics via the AT4 receptor subtype, and suggest that AngIV-induced elevations in blood flow may be mediated by nitric oxide.

Receptor-mediated angiotensin II transcytosis by brain microvessel endothelial cells

Angiotensin II (Ang II) uptake and transport across monolayers of bovine brain microvessel endothelial cells (BMECs) was demonstrated. Ang II transport was linear up to 2 h, saturable with a K(m) of 1.7 nM, and tended to be polarized with the apical-to-basolateral transport being greater. [3H]Ang II transport was found to be inhibited by excess unlabeled Ang II, by the Ang II analog sarathrin, and by the endocytic inhibitor phenylarsine oxide. Ang II-(2-8) and-(3-8) were shown to significantly increase the transport of Ang II. These results demonstrate for the first time the receptor-mediated transcytosis of Ang II across brain microvessel endothelium.

Calcium channels mediate angiotensin II-induced drinking behaviour and c-fos expression in the brain

It is widely accepted that calcium ions are critically important in both short- and/or long-lasting responses of neurons to a stimulus. We have shown previously that NMDA receptors play a role in dipsogenic responses and c-fos expression induced by intracerebroventricular (i.c.v.) infusion of angiotensin II (Ang II). Since NMDA receptors are known to be linked to receptor-operated calcium channels, this study determined whether voltage dependent calcium channels are also involved in Ang II-induced behavioural (drinking) and endocrine responses as well as c-fos expression. The antidipsogenic actions of three L-type calcium channel antagonists, nifedipine, diltiazem and verapamil on Ang II-induced drinking behaviour were studied. These bind to the dihydropyridine, phenylalkylamine and benzothiazepine sites respectively. Rats (Lister-hooded) pre-treated i.c.v. with either 25 or 100 microg nifedipine, followed by 25 pmol Ang II, drank significantly less water than controls during the first 15 min after infusion. However, rats pre-treated with i.c.v. 100 microg diltiazem or verapamil showed no change in Ang II-induced drinking behaviour. The antidipsogenic actions of N- and P-type calcium channel antagonists omega-conotoxin GVIA and omega-conotoxin MVIIC were also evaluated. Rats pre-treated with 5 pmol or 20 pmol omega-conotoxin GVIA did show a slight but not significant suppression of water intake, particularly after the higher dose. Rats pre-treated with omega-conotoxin MVIIC drank almost the same amount of water as those pre-treated with saline. Nifedipine was found to suppress both Ang II-induced corticosterone release and c-fos expression in the following areas: organum vasculosum of the lamina terminalis (OVLT), median preoptic nucleus (MNPO), hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON). The results described in this paper provide evidence that calcium channels play important roles in the Ang II-induced behavioural and endocrine responses, and in the expression of the immediate-early gene c-fos. This suggests that an L-type calcium channel may participate both short- and longer-term neuronal actions of Ang II.

Role of angiotensin II receptor subtypes in mediating the sympathoexcitatory effects of exogenous and endogenous angiotensin peptides in the rostral ventrolateral medulla of the rabbit

The pressor region in the rostral part of the ventrolateral medulla (VLM) in the rabbit contains a high density of the AT1 subtype of angiotensin (Ang) II receptor. In this study in anaesthetized barodenervated rabbits, we determined the effect of microinjection into the rostral VLM of the AT1 receptor antagonist losartan and the AT2 receptor antagonist PD123319 on resting arterial pressure and renal sympathetic nerve activity, and on the cardiovascular responses normally evoked by exogenous Ang II or Ang III in this region. Losartan (1 nmol) abolished the pressor and sympathoexcitatory responses normally evoked by exogenous Ang II, but PD123319 (1 nmol) had little effect on these responses. Both losartan (0.1-10 nmol) and PD123319 (0.1-1 nmol) had little effect on the resting arterial pressure and renal sympathetic nerve activity, except for a transient sympathoexcitatory response at the higher doses. In confirmation of previous findings, however, microinjection of the non-selective Ang receptor antagonist [Sar1,Thr8]Ang II (80 pmol) significantly decreased resting arterial pressure and sympathetic nerve activity. These results suggest that the sympathoexcitatory effects evoked by exogenous Ang II and III in the rostral VLM are mediated by AT1 receptors, but that the tonic sympathoexcitation produced by endogenous Ang peptides in the rostral VLM of the rabbit are mediated by receptors other than AT1 or AT2 receptors.

Expression of angiotensin type-1 (AT1) and type-2 (AT2) receptor mRNAs in the adult rat brain: a functional neuroanatomical review

The discovery that all components of the renin-angiotensin system (RAS) are present in the central nervous system led investigators to postulate the existence of a local brain RAS. Supporting this, angiotensin immunoreactive neurons have been visualized in the brain. Two major pathways were described: a forebrain pathway which connects circumventricular organs to the median preoptic nucleus, paraventricular nucleus, and supraoptic nucleus, and a second pathway connecting the hypothalamus to the medulla oblongata. Blood-brain barrier deficient circumventricular organs are rich in angiotensin II receptors. By activating these receptors, circulating angiotensin II may act on central cardiovascular centers via angiotensinergic neurons, providing a link between peripheral and central angiotensin II systems. Among the effector peptides of the brain RAS, angiotensin II and angiotensin III have the same affinity for the two pharmacologically well-defined receptors: type 1 (AT1) and type 2 (AT2). When injected in the brain, these peptides increase blood pressure, water intake, and anterior and posterior pituitary hormone release and may modify memory and learning. The cloning of AT1 and AT2 receptor cDNAs has revealed that these receptors belong to the seven transmembrane domain receptor family. In rodents, two AT1 receptor subtypes, AT1A and AT1B, have been isolated. Using specific riboprobes for in situ hybridization histochemistry, recent studies mapped the distribution of AT1A, AT1B, and AT2 receptor mRNAs in the adult rat and found a predominant expression of AT1A and AT2 mRNA in the brain and of AT1B in the pituitary. Very limited overlap was found between the brain expression of AT1A and AT2 mRNAs. In several functional entities of the brain, such as the preoptic region, the hypothalamus, the olivocerebellary system, and the brainstem baroreflex arc, the colocalization of receptor mRNA, binding sites, and angiotensin immunoreactive nerve terminals suggests local synthesis and expression of angiotensin II receptors. In other areas, such as the bed nucleus of the stria terminalis, the median eminence, or certain parts of the nucleus of the solitary tract, angiotensin II receptors are likely of extrinsic origin. The neuronal expression of AT1A and AT2 receptors was demonstrated in the subfornical organ, the hypothalamus, and the lateral septum. By using double label in situ hybridization, AT1A receptor expression was localized in corticotropin releasing hormone but not in vasopressin containing neurons in the hypothalamus. The information is discussed together with functional data concerning the role of brain angiotensins, in an attempt to provide a better understanding of the physiological and functional roles of each receptor subtype.

Dose-dependent inhibitory effects of angiotensin II on visual responses of the rat superior colliculus: AT1 and AT2 receptor contributions

Angiotensin II (Ang II) has traditionally been regarded as a peripherally circulating and acting hormone involved in fluid homeostasis and blood pressure regulation. With the rather recent localization of Ang II receptors within the mammalian brain, renewed interest has emerged in the hope of elucidating the central impact and function of this hormone. One region that has been clearly demonstrated to express Ang II receptors is the superior colliculus (SC). This mesencephalic structure plays an important role in sensory visuomotor integration. Receptors for Ang II (of both the AT1 and AT2 subtypes) have been localized within the superficial layers of this structure, i.e. the areas that are visually responsive. In the hopes of characterizing the role of Ang II in the SC, we have attempted to physiologically activate these receptors in vivo and observe the effects of Ang II on visually evoked responses. In the attempt to identify the receptor subtype(s) responsible in mediating these effects, Ang II was injected concomitantly with selective receptor ligands. Experiments were performed on adult rats prepared in classical fashion for electrophysiological studies. Through microinjection of Ang II, and the simultaneous recording of visually evoked potentials to flash stimulation, we have observed that this peptide yields a strong suppressive effect on visual neuronal activity. By injecting Ang II at various concentrations (10(-3)-10(-10) M), we have further observed that the effects of this peptide express a dose-related dependency. Injection of Ang II in progressively more ventral layers yielded less pronounced effects, demonstrating physiologically the discrete localization of these receptors in the stratum griseum superficiale. Coinjection of Ang II with Losartan yielded a near complete blockade of Ang II suppressive effects, suggesting that AT1 receptors play a prominent role in mediating these responses. However, coinjection of Ang II with PD 123,319 yielded a slight, yet significant partial blockade. Coinjection of Ang II with both the AT1 and AT2 receptor antagonists yielded a complete blockade of the Ang II effect. Finally, some of the results suggest that the AT2 receptor ligand CGP 42,112 may possess agonist properties. Taken together, these findings suggest that the AT1 receptor is predominantly involved in mediating Ang II responses in the SC and there also appears to be some indication of AT2 receptor involvement. However, the underlying mechanisms (such as receptor interactions), the exact specificity of the ligands used, and the possibility of other receptor subtype implication have yet to be explored fully.

Possible dual effect of endogenous ANP on water and sodium intake and role of AII

Water intake may or may not be associated with sodium appetite. There are excitatory and inhibitory mechanisms that control these behaviors whose mediators and interactions are unclear. We investigated the effects of specific antisera against angiotensin II (AB-AII) and atrial natriuretic peptide (AB-ANP) on the induction of the two behaviors in rats deprived of water overnight or normally hydrated and submitted to intracerebroventricular (icv) microinjection of AII. AB-ANP reduced water intake induced by overnight deprivation but not by icv microinjection of AII, while AB-AII reduced water intake in both situations. AB-ANP and AB-AII increased saline intake in deprived animals and decreased saline intake induced by icv microinjection of AII in normally hydrated animals. The effect of AII on water and sodium intake may depend, at least in part, on an interaction with the system of ANP neurons. This peptide, in turn, may have different actions on water and sodium intake as a function of extracellular fluid conditions and of AII levels.

Differential action of AGCF2 upon cell type-dependent expression of human angiotensinogen gene

To investigate the regulatory mechanisms of human angiotensinogen (ANG) gene expression in the brain, we analyzed the 1.3-kb promoter by transfection studies and gel shift assays. The region from -106 to +44 was sufficient for promoter activity in glioblastoma cells, and multiple nuclear factors including AGCF2 (human ANG core promoter binding factor 2) bound within this 150-bp region. The mutations within AGCF2-binding elements decreased the transcriptional activity in glioblastoma cells but rather increased it in hepatoma cells. These results indicate that AGCF2 has a differential function between these cells and contributes to the glia-dependent angiotensinogen promoter activity.

Angiotensin II interacts with nitric oxide-cyclic GMP pathway in the central control of drinking behaviour: mapping with c-fos and NADPH-diaphorase

Recognition of the role of nitric oxide in cell-to-cell communication has changed the concept of traditional neurotransmission. We have shown previously that N-methyl-D-aspartate receptors mediate dipsogenic responses and c-Fos expression induced by intracerebroventricular infusion of angiotensin II. Since these receptors are known to be linked to the nitric oxide-cyclic GMP pathway, the present study explores the contribution of this path to the behavioural and cellular effects of intracerebroventricular angiotensin II by using behavioural testing, NADPH-diaphorase histochemistry and immunocytochemical staining for the immediate-early gene, c-fos. N(G)-nitro-L-arginine methyl ester (125 and 250 microg, intracerebroventricular), an inhibitor of nitric oxide synthase, and Methylene Blue (100 microg), an inhibitor of guanylate cyclase activation, antagonized water intake induced by intracerebroventricular injection of 25 pmol angiotensin II. The effects of N(G)-nitro-L-arginine methyl ester were reversed by co-injection of L-arginine, the substrate for nitric oxide synthase. However, N(G)-nitro-L-arginine methyl ester did not alter the pattern of angiotensin II-induced c-fos expression in the organum vasculosum of the lamina terminalis, median preoptic nucleus, hypothalamic paraventricular nucleus and supraoptic nucleus. Double staining with NADPH-diaphorase histochemistry and c-Fos immunocytochemistry showed that neurons staining for both were localized to the anterior third ventricle. However, only 19-25% of the c-Fos-positive neurons expressed NADPH. There were also substantial numbers of neurons in which angiotensin II induced c-Fos that were NADPH-negative. Extensive co-distribution of NADPH-diaphorase-stained cells and those expressing c-fos in response to intracerebroventricular injection of angiotensin II, especially in the median preoptic nucleus, imply that nitric oxide might participate in the mechanism of angiotensin II-induced drinking behaviour. However, a low rate of co-localization of the two markers to individual cells suggests that angiotensin II stimulated the production of nitric oxide and c-Fos in different populations of neurons. Since our previous results showed that glutamate blockade, but not nitric oxide synthase inhibition, suppressed angiotensin II-induced c-Fos, the experiments reported here further suggest that nitric oxide release is not an essential requirement for the expression of c-fos elicited by angiotensin II. They also provide evidence that the dipsogenic and c-Fos responses to angiotensin II are dissociated at a cellular level.

Aminopeptidase A: distribution in rat brain nuclei and increased activity in spontaneously hypertensive rats

Aminopeptidase A is a membrane-bound zinc metalloprotease which cleaves angiotensin II into angiotensin III. Using a new specific aminopeptidase A inhibitor, EC33, we evaluated its enzymatic activity in several microdissected brain nuclei involved in the control of cardiovascular functions and in the pituitary. We compared this distribution with that of the angiotensin I-converting enzyme which converts angiotensin I to angiotensin II. Aminopeptidase A activity was heterogenously distributed with a 150-fold difference between the lowest and the highest levels. The pituitary and the circumventricular organs were the richest source of enzyme, followed by the median eminence, the arcuate nucleus, the area postrema, the choroid plexus and the supraotic and paraventricular nuclei. We did not find any close parallel between aminopeptidase A and angiotensin I-converting enzyme distributions. We examined both enzymatic activities in brain nuclei of spontaneously hypertensive rats. Aminopeptidase A activity was higher in the spontaneously hypertensive rats than in age-matched Wistar Kyoto control rats. The difference was up to 2.5-fold in several brain nuclei involved in the blood pressure regulation; in contrast, no differences in angiotensin I-converting enzyme activity were found in the same regions. The close correspondence between the distribution of aminopeptidase A activity and angiotensin receptors and nerve terminals in the brain associated with the observation that aminopeptidase A activity was overactivated in the spontaneously hypertensive rats suggest that this enzyme may contribute, at least in part, to the regulation of cardiovascular functions by its ability to convert angiotensin II to angiotensin III.

A longitudinal study of cerebrospinal fluid angiotensin-converting enzyme in neuroleptic-treated schizophrenia

1. The aim of the study was to replicate our earlier finding of elevated cerebrospinal fluid (CSF) angiotensin-converting enzyme (ACE) in neuroleptic-treated schizophrenia and to elucidate the correlations between CSF ACE, neuroleptic treatment, and psychotic symptoms in a longitudinal study. 2. Levels of ACE were measured in CSF and serum from 9 acutely psychotic schizophrenic patients at two separate points of time; within a few days of admission and at follow-up after 3-4 weeks. CSF ACE was also determined from 9 healthy controls. 3. The schizophrenic patients showed non-significantly higher levels of CSF ACE than the controls. Although a significant clinical improvement was observed and the neuroleptic medication was reduced during the follow-up period, there were no significant differences in serum or CSF ACE between the two observation points in the schizophrenia group.

High levels of human chymase expression in the pineal and pituitary glands

The brain renin-angiotensin system plays a role in both cardiovascular homeostasis and neurosecretory functions. Since the mechanisms of angiotensin (Ang) II formation in the human brain have not been clarified, the aims of the present study were to determine the presence of human chymase and angiotensin I-converting enzyme (ACE) in human and non-human brains. In the human brain, the total Ang II-forming activity was significantly higher in the pineal and pituitary glands than those in other regions. In other species (rat, bovine and porcine), the level of chymase as well as total Ang II-forming activities in pineal glands were significantly lower than those in human glands. High levels of chymase-like immunoreactivity (ir) were found in the arteriolar endothelial cells, adventitial mesenchymal cells and in parenchymal cells of the human pineal and pituitary glands while ACE-ir was mostly observed in the endothelial cells and occasionally found in parenchymal cells. Our study provides the first evidence that human chymase exists in the pineal and pituitary glands. The remarkable regional and species differences in mechanisms of Ang II formation suggest a specific role of chymase or ACE in the human brain.

Electrophysiological and immunocytochemical evidence for a cGMP-mediated inhibition of subfornical organ neurons by nitric oxide

The activation of neurons in the subfornical organ (SFO) by angiotensin II (AngII) is well established and is widely regarded as the basis for the AngII-induced increase in water intake. Application of the nitric oxide (NO) donor sodium nitroprusside (SNP) led to an inhibition of the spontaneous electrical activity in 96% of the neurons sensitive for SNP (n = 50). In addition, the firing rate in 60% of the neurons inhibited by SNP decreased in response to superfusion with the natural substrate of the NO synthase (NOS) L-arginine whereas 70% increased their frequency after application of the NOS blocker NG-monomethyl-L-arginine (L-NMMA; n = 10). The inhibitory effect of SNP could be mimicked by application of membrane-permeable 8-Br-cGMP. The presence of nNOS, the neuronal isoform of NOS, was demonstrated immunocytochemically and using the NADPH-diaphorase technique on SFO slices. Using a highly selective antibody against cGMP in formaldehyde-fixed tissue, the NO donors SNP, 3-morpholinosydnonimine (SIN-1), and S-nitroso-N-acetyl-DL-penicillamine (SNAP) caused a strong increase in cGMP formation when applied under the same conditions as used for the electrophysiological recordings. These electrophysiological results suggest an important role for NO in SFO-mediated responses and offer a plausible explanation for the in vivo-observed opposite effects of AngII and NO on water intake.

Aminopeptidase A antiserum inhibits intracerebroventricular angiotensin II-induced dipsogenic and pressor responses

Angiotensin II increases drinking and blood pressure when administered intracerebroventricularly. Intracerebroventricular injections of antiserum with anticatalytic activity against aminopeptidase A, the principal enzyme that metabolizes angiotensin II to angiotensin III, reduced the drinking and blood pressure responses to 10 pmol angiotensin II by 73% and 59%, respectively. APA antiserum had no effect on responses to angiotensin III administered intracerebroventricularly. A Glu-thiol inhibitor of aminopeptidase A also reduced angiotensin II-induced drinking. These results suggest that metabolism of angiotensin II to angiotensin III is an obligatory activation step for the brain angiotensin system.

Differential regulation of angiotensinogen and natriuretic peptide mRNAs in rat brain by osmotic stimulation: focus on anterior hypothalamus and supraoptic nucleus

Central angiotensin II and natriuretic peptide systems have been shown to be involved in the central regulation of blood fluid homeostasis with alterations in central peptide and/or receptor levels observed following changes in osmotic status. The present study investigated the effects of sodium loading on mRNA encoding the angiotensin II precursor, angiotensinogen (AOGEN), and the natriuretic peptides, atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) in rat brain using quantitative in situ hybridization histochemistry of [35S]- and [33P]-labeled oligonucleotide probes. Following 7 and 14 days of 2% sodium chloride in drinking water a significant increase was detected in preproAOGEN (ppAOGEN) mRNA in presumed astrocytes in regions of the anterior hypothalamus, including the periventricular nucleus, the medial preoptic area and medial preoptic nucleus, while a decrease was observed in astrocytes in the supraoptic nucleus. Other forebrain regions examined including the subfornical organ, bed nucleus of the stria terminalis and the arcuate nucleus showed no significant alteration in the level of ppAOGEN mRNA. Sodium loading did not appreciably alter ppANP or ppCNP mRNA levels in neurons of the anteromedial preoptic or arcuate nuclei or hippocampus at the times studied. PpANP mRNA levels were also unaltered in Barrington's nucleus following sodium loading, while preprocorticotropin-releasing hormone mRNA was significantly decreased. These results indicate that AOGEN mRNA transcription/stability in vivo is modulated by alterations in osmotic balance, consistent with previous reports of a central role for AII in cardiovascular and body fluid homeostasis. In contrast, despite reports of modulation of hypothalamic ANP-immunoreactivity following changes in osmotic status, it would appear that osmotic stimulation over periods of 7-14 days does not markedly alter the transcription or stability of hypothalamic natriuretic peptide mRNAs in vivo.

Lateral hypothalamic stimulation inhibits dentate granule cell LTP: direct connections

We discovered that angiotensin II (Ang II) applied directly to the dentate gyrus inhibited LTP induction in medial perforant path-dentate granule cell synapses and that the inhibition can be blocked by losartan, an Ang II AT1 receptor specific antagonist. In the first part of this study we found that electrical stimulation of the lateral hypothalamus (LH) inhibits LTP in these synapses and the inhibition can be blocked by pretreating the animals with losartan, indicating that LH angiotensin-containing neurons project to the dentate gyrus. Results of the second part of the study demonstrate clearly that some angiotensin-containing LH neurons project directly to dentate granule cells. LH neurons were identified by retrograde tracers applied to the granule cell layer. Double-labeled neurons containing angiotensin and HRP were sparsely distributed and both fusiform and multipolar LH neurons appeared in a small cluster lateral and ventral to the fornix at the level of the paraventricular nucleus. Large numbers of angiotensin staining neurons were observed in the hypothalamus. Results support our hypothesis that some angiotensin containing LH neurons project directly to the dentate gyrus.

Angiotensin II-induced fluid phase endocytosis in human cerebromicrovascular endothelial cells is regulated by the inositol-phosphate signaling pathway

The involvement of the early signaling messengers, inositol tris-phosphate (IP3), intracellular calcium, [Ca2+]i, and protein kinase C (PKC), in angiotensin II (AII)-induced fluid phase endocytosis was investigated in human brain capillary and microvascular endothelial cells (HCEC). ALL (0.01-10 microM) stimulated the uptake of Lucifer yellow CH, an inert dye used as a marker for fluid phase endocytosis, in HCEC by 50-230%. AII also triggered a fast accumulation of IP3 and a rapid increase in [Ca2+]i in cells loaded with the Ca(2+)-responsive fluorescent dye fura-2. The prompt AII-induced [Ca2+]i spike was not affected by incubating HCEC in Ca(2+)-free medium containing 2 mM EGTA or by pretreating the cultures with the Ca2+ channel blockers, methoxyverapamil (D600; 50 microM), nickel (1 mM), or lanthanum (1 mM), suggesting that the activation of AII receptors on HCEC triggers the release of Ca2+ from intracellular stores. The AII-triggered increases in IP3, [Ca2+]i, and Lucifer yellow uptake were inhibited by the nonselective AII receptor antagonist, Sar1, Val5, Ala8-AII (SVA-AII), and by the phospholipase C (PLC) inhibitors, neomycin and U-73122. By contrast, the protein kinase C (PKC) inhibitors, staurosporine and calphostin C, failed to affect any of these AII-induced events. This study demonstrates that increased fluid phase endocytotosis induced by AII in human brain capillary endothelium, an event thought to be linked to the observed increases in blood-brain barrier permeability in acute hypertension, is likely dependent on PLC-mediated changes in [Ca2+]i and independent of PKC.

Elevated plasma vasopressin and normal cerebrospinal fluid angiotensin-converting enzyme in chronic pain disorder

The study was performed proceeding from the hypothesis that pain proneness in chronic pain disorder (CPD) is a result of alterations in central mechanisms regulating pain sensations. To elucidate the function of the central renin-angiotensin system, the levels of angiotensin-converting enzyme (ACE) and arginine vasopressin (AVP) in cerebrospinal fluid (CSF) and peripheral blood were measured in 15 CPD patients and 19 healthy controls. Plasma AVP levels (p = .01) as well as the serum osmolality (p = .01) were significantly higher in the CPD group. No significant differences in CSF ACE levels were found. AVP is a stress-related peptide, but central antinociceptive effects have also been reported. Elevated plasma AVP levels possibly may constitute a response to chronic stress.

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.

Identification of metabolic pathways of brain angiotensin II and III using specific aminopeptidase inhibitors: predominant role of angiotensin III in the control of vasopressin release

Angiotensin (Ang) II and Ang III are two peptide effectors of the brain renin-angiotensin system that participate in the control of blood pressure and increase water consumption and vasopressin release. In an attempt to delineate the respective roles of these peptides in the regulation of vasopressin secretion, their metabolic pathways and their effects on vasopressin release were identified in vivo. For this purpose, we used recently developed selective inhibitors of aminopeptidase A (APA) and aminopeptidase N (APN), two enzymes that are believed to be responsible for the N-terminal cleavage of Ang II and Ang III, respectively. Mice received [3H]Ang II intracerebroventricularly (i.c.v.) in the presence or absence of the APN inhibitor, EC33 (3-amino-4-thio-butyl sulfonate) of the APN inhibitor, EC27 (2-amino-pentan-1,5-dithiol). [3H]Ang II and [3H]Ang III levels were evaluated from hypothalamus homogenates by HPLC. EC33 increased the half-life of [3H]Ang II 2.6-fold and completely blocked the formation of [3H]Ang III, whereas EC27 increased the half-life of [3H]Ang III 2.3-fold. In addition, the effects of EC33 and EC27 on Ang-induced vasopressin release were studied in mice. Ang II was injected i.c.v. in the presence or absence of EC33, and plasma vasopressin levels were estimated by RIA. While vasopressin levels were increased 2-fold by Ang II (5 ng), EC33 inhibited Ang II-induced vasopressin release in a dose-dependent manner. In contrast, EC27 injected alone increased in a dose-dependent manner vasopressin levels. The EC27-induced vasopressin release was completely blocked by the coadministration of the Ang receptor antagonist (Sar1-Ala8) Ang II. These results demonstrate for the first time that (i) APA and APN are involved in vivo in the metabolism of brain Ang II and Ang III, respectively, and that (ii) the action of Ang II on vasopressin release depends upon the prior conversion of Ang II to Ang III. This shows that Ang III behaves as one of the main effector peptides of the brain renin-angiotensin system in the control of vasopressin release.

Angiotensin II receptor subtype antagonists can both stimulate and inhibit salt appetite in rats

In urethane-anaesthetised male Wistar rats iontophoretic application of the angiotensin II (Ang II) type 1 (AT-1) receptor specific nonpeptide antagonist losartan in the septo-preoptic continuum can produce neuronal excitation of short- and long-term duration which has been interpreted as removal of tonic Ang II-induced inhibition. Mineralocorticoid pretreatment, which increases neuronal sensitivity to Ang II to enhance salt appetite, also removes this losartan-induced long-term excitation. These results suggested steroid modulation of the AT-1 receptor and a complex involvement of Ang II in salt appetite. To investigate the role of the inhibitory action of central Ang II on salt appetite, we gave intracerebroventicular injections of a single, low dose (10 ng) of losartan in normal euhydrated rats and this produced, paradoxically, a progressive increase in salt intake (1.6 +/- 0.3 ml/day to 3.5 +/- 0.9 ml/day, n = 15, P < 0.05). Treatment of these salt enhanced rats with DOCA (0.5 mg/day, s.c., for 3 days) further increased the salt appetite, but then a second i.c.v. injection of the same dose of losartan significantly inhibited the enhanced salt appetite (4.7 +/- 0.7 to 1.3 +/- 0.4, n = 9, P < 0.05). These results provide evidence for a complex action of Ang II on the At-1 receptor mediating the generation of salt appetite that appears to involve either at least two functional subtypes of this AT-1 receptor, as already suggested by previous electrophysiological experiments, or one AT-1 receptor with several activation states.

Effects of intracerebroventricular injections of losartan or PD123319 on arterial pressure and heart rate of sodium replete and sodium deplete rats

Angiotensin II (Ang II) non-peptide antagonists were injected i.c.v. (6.25-200 nmol, n = 5-8 rats/group). In sodium replete rats, losartan (AT1 receptor antagonist) induced an increase in mean arterial pressure (MAP) and in heart rate (HR) by 3rd ventricular (3rdV) injection, and an weaker pressor response and bradycardia by 4th ventricular (4thV) injection. PD123319 (AT2 receptor antagonist) induced an increase in MAP and in HR by 3rdV injection, and an increase in MAP and no alteration in HR by 4thV injection. In sodium deplete (furosemide plus removal of ambient sodium for 24 h) rats, losartan induced an increase in MAP and no alteration in HR by 3rdV injection, and no alteration in MAP and bradycardia by 4thV injection. PD123319 induced an increase in MAP and in HR by 3rdV injection, and an increase in MAP and bradycardia by 4thV injection. Thus, there were no fall in MAP by central injections of Ang II antagonists. Intravenous injection of losartan, but not of PD123319, induced a fall in MAP in both sodium replete and sodium deplete animals. Therefore, losartan and PD123319 can have similar effects on MAP and HR when injected intracerebroventricularly, although some differences are also present. The bradycardia is consistent with an withdrawal of Ang II inhibitory action on baroreflex.

Angiotensin II, via an action at AT1 receptors, may modulate the behavioural effects of ET-1 in conscious rats

The injection of endothelin-1 (ET-1) (1 pmol/rat) into the dorsolateral periaqueductal gray (PAG) area of freely moving rats induced rotation along the long axis of the body (barrel-rolling). Preinjection (10 min before) of losartan (a selective AT1 receptor antagonist; 50 nmol) to the PAG area reduced the behavioural response to ET-1. In contrast, pretreatment with PD123319 (a selective AT2 receptor antagonist; 50 nmol) did not affect the ET-1-induced barrel-rolling. These results indicate that endogenous angiotensin II, via an action on AT1 receptors, contributes to the barrel-rolling of ET-1 within the brain.

Distribution of angiotensin II type-2 receptor (AT2) mRNA expression in the adult rat brain

Radioactively labeled cRNA probes were used for in situ hybridization histochemistry to establish a detailed map of the sites of expression of the recently cloned angiotensin II, type 2 (AT2) receptor mRNA in the adult rat brain. The distribution of the AT2 receptor mRNA was consistent with that of the AT2 binding sites, which were previously established by autoradiographic binding studies. Thus, high AT2 receptor mRNA expression was observed in the lateral septum, in several thalamic nuclei, in the subthalamic nucleus, in the locus coeruleus, and in the inferior olive. Due to the superior resolution and sensitivity of in situ hybridization, AT2 receptor expression was localized at the cellular level, and some additional brain nuclei expressing AT2 receptor mRNA have been identified. These include the red nucleus, the pedunculopontine tegmental nucleus, the bed nucleus of the supraoptic decussation, the paragenual nucleus, and numerous brainstem nuclei. Several brain nuclei, such as the motor hypoglossal nucleus and the cerebellar nuclei, where AT2 receptor binding had previously been identified in young animals only, showed a high expression of the AT2 receptor mRNA in the adult rat. No correlation was found between the expression of the AT2 and the type 1 (AT1) receptor mRNAs. A combination of the in situ hybridization and glial fibrillary acidic protein (GFAP) immunohistochemistry shows that the AT2 receptor in the lateral septum showed that the AT2 receptor was not detected in GFAP immunoreactive astroglial cells, therefore indicating that AT2 is neuronal rather than glial in this brain region.

Functional evidence for the ability of angiotensin AT1 receptor antagonists to cross the blood-brain barrier in rats

The angiotensin AT1 receptor antagonists, losartan (2-n-butyl-4-chloro-5-hydroxymethyl-1-[(2'(1H-tetrazol-5-yl)biphen yl-4- yl)methyl]imidazole potassium salt), EXP3174 (2-n-butyl-4-chloro-1-[(2'(1 H-tetrazol-5-yl)biphenyl-4-yl)methyl]imidazole-5-carboxylic acid), GR117289 (1-[[3-bromo-2-[2-(1 H-tetrazol-5-yl)phenyl]-5-benzofuranyl]methyl]-2-butyl-4-chloro-1 H-imidazole-5-carboxylic acid) and LR-B/081 (methyl-2-[[4-butyl-2-methyl-6-oxo-5-[[2'-(1 H-tetrazol-5-yl)[1,1'-biphenyl]-4-yl]methyl]-1(6H)-pyrimidinyl]met hyl]-3- thiophenecarboxylate), given by intraperitoneal (i.p.) injection 15 min before intracerebroventricular administration of angiotensin II, inhibited drinking with the following order of potency: EXP3174 > GR117289 > losartan > LR-B/081. When 20 mumol/kg of each antagonist was i.p. injected 15 min, 4, 12 or 24 h before angiotensin II, EXP3174 and GR117289 inhibited water intake at each observation time, losartan at 4, 12 and 24 h, LR-B/081 only at 4 and 12 h. After per os administration of the same dose 4 or 12 h before angiotensin II, losartan reduced drinking at 4, but not at 12 h; LR-B/081 did not inhibit drinking either at 4 or 12 h. The present results suggest that EXP3174 and GR117289 cross the barrier readily. The effect of i.p. losartan on central angiotensin mechanisms is not prompt, suggesting that it may require conversion to EXP3174. LR-B/081 apparently crosses the barrier less readily than the other antagonists following both i.p. and per os administration.

Effects of myocardial infarction and captopril therapy on anxiety-related behaviors in the rat

The angiotensin-converting enzyme inhibitor, captopril, is used in the treatment of heart failure after myocardial infarction. This study evaluated whether different behavioral parameters of anxiety are affected by captopril therapy after myocardial infarction in rats. Myocardial infarction was induced by ligation of the left coronary artery and captopril therapy was started after 3 weeks. After 2 weeks of captopril therapy, anxiety-related behaviors were successively measured in four different tests: open field, elevated plus maze, home cage emergence, and open field escape. Myocardial infarction and captopril therapy affected behavior in the home cage emergence test and open field escape test. On the basis of the data from the open field escape test, captopril therapy appeared to decrease anxiety in infarcted rats and increase anxiety in sham rats. Because myocardial infarction and captopril therapy did not affect anxiety-related behaviors in the open field and elevated plus maze tests, it is assumed that these interventions affect anxiety-related behaviors depending on the type of test. This was partially supported by correlation analysis, which suggested that the behavior of the rats in the different tests of anxiety may reflect different anxiety-related traits.

Effect of renin on brain arterioles and cerebral blood flow in rabbits

There is evidence of an intrinsic renin-angiotensin system in the brain. The goal of the study was to determine whether stimulation of endogenous angiotensin production by applying renin to the brain surface has an effect on pial arteriolar caliber and CBF. Pial vessel diameters were measured through a closed cranial window in anesthetized rabbits. Percent changes of blood flow in the cortical area under the cranial window were simultaneously measured by laser-Doppler flowmetry. Topical application of 0.01-0.1 U/ml renin induced maximum dilation of 18.9 +/- 4% (mean +/- SD) of pial arterioles within 2 min. Arteriolar calibers thereafter decreased slowly. Flow gradually increased to peak at 38 +/- 15% 50 min after renin application. Angiotensin I levels in jugular blood, as measured by radioimmunoassay, increased to a peak 40 min after topical renin application. Angiotensin II levels in jugular blood and both angiotensin I and II levels in blood samples from the femoral artery did not change. Diameter and flow changes were inhibited by intravenous pretreatment with the converting enzyme blocker captopril (10 mg/kg body wt i.v.). Captopril did not affect the vasodilation and flow increase in response to hypercapnia. Topically applied captopril (10(-5) M) blocked renin-induced arteriolar dilation. We conclude that renin increases pial arteriolar diameters and cortical blood flow in the rabbit brain. Stimulation of angiotensin production is likely to be a mediator of this response.

Brain angiotensin and circulatory control

1. Components of the renin-angiotensin system (RAS) are found in the brain; both outside and inside the blood-brain barrier. 2. Almost all of the classical actions of the brain RAS are attributable to angiotensin (Ang) II and mediated by AT1 receptors. 3. Circumventricular organs (CVO), which lack the blood-brain barrier, are rich in AngII receptors and monitor circulating AngII levels. In vivo binding studies suggest that the CVO are also accessible to cerebrospinal fluid-derived AngII. 4. The median preoptic nucleus, paraventricular hypothalamic nucleus, supraoptic nucleus, nucleus tractus solitarius and ventrolateral medulla are inside the blood-brain barrier and are sites of action of brain AngII. In these nuclei, AngII seems to act as an excitatory neurotransmitter or neuromodulator. 5. Actions of AngII in the brain, both inside and outside the blood-brain barrier, are implicated in the central regulation of blood pressure and sympathetic outflow, release of hypothalamic and pituitary hormones and renal sodium handling. 6. Alterations in the activity of brain AngII may be involved in the mechanisms of some types of hypertension.

Physiological Role of Brain Angiotensin III in Drinking Behavior in the Rat

We examined the physiologic role of endogenous brain angiotensin III (AIII), an active degradative product of angiotensin II, in drinking behavior. Adult, male spontaneously hypertensive (SH) and Wistar-Kyoto normotensive (WKY) rats that were instrumented with an intracerebroventricular (i.c.v.) cannula connected to an osmotic minipump for chronic infusion were used. 7-day i.c.v. infusion of the specific AIII antagonist, Ile(7)-AIII (10 or 100 pmol/min), resulted in no significant alteration in daily (24 h), diurnal (8:00 a.m.-8.00 p.m.) or nocturnal (8:00 p.m.-8:00 a.m.) basal water intake in both SH and WKY rats. Similar results were obtained with i.c.v. infusion of the aminopeptidase inhibitor, bestatin (150 or 300 pmol/min), given alone or simultaneously with Ile(7)-AIII (10 pmol/min). Rats that were water-deprived for the first 3 days of 7-day infusion of Ile(7)-AIII consumed significantly less water during the first 2 h after water became available. Furthermore, the accumulated water intake during the first 24 h was appreciably greater in SH than WKY rats. We interpret these results to suggest that the endogenous brain AIII may not be tonically involved in fluid homeostasis. Instead, it must be activated under conditions of dehydration, such as water deprivation, particularly in the SHRs, to initiate drinking behavior. Copyright 1996 S. Karger AG, Basel

Angiotensin II receptor content within the subfornical organ and organum vasculosum lamina terminalis increases after experimental subarachnoid haemorrhage in rats

Nests of cells within the central nervous system, namely the circumventricular organs (CVOs) which include the subfornical organ (SFO), organum vasculosum lamina terminalis (OVLT), area postrema (AP) and the median eminence (ME) are known to contain not only receptors for angiotensin II (ANG II) but also ANG II itself. Though the significance of this central ANG II network in the pathophysiology of certain conditions like hypertension is well established, there appears to be a lack of knowledge as to how this system might be involved after subarachnoid haemorrhage (SAH). In this study, we have investigated ANG II receptor content change at various circumventricular organs after experimental subarachnoid haemorrhage in rats using a transcervical transclival model. ANG II receptor content was detected by in vivo autoradiography using intracisternal ANG II Sar 1, Ile 8 labelled with iodine (I) 125 both at 30 minutes and 48 hours after the SAH. Serum angiotensin converting enzyme activity was also detected during the time course reflecting the involvement of the peripheral angiotensin system and showed an early rise and a fall after two days. Immunohistochemistry was utilized to show the ANG II-containing cells within the circumventricular organs. SFO and OVLT were found to have a statistically significant increase in ANG II receptor content persisting over two days after the SAH. These alterations in the receptor content of CVOs may indicate their possible role in delayed ischaemic deficits seen after SAH.

Electrophysiological properties of paraventricular magnocellular neurons in rat brain slices: modulation of IA by angiotensin II

Whole-cell patch-clamp recordings obtained from magnocellular neurons of the hypothalamic paraventricular nucleus in brain slice preparations of adult Sprague-Dawley rats have been utilized to examine three outward potassium conductances and the ionic mechanisms through which angiotensin II exerts its neurotransmitter actions within this region. Lucifer Yellow fills showed that neurons from which we recorded had large ovoid cell bodies 11-17 microns wide and 22-35 microns long, as well as 1-3 minimally branched processes, anatomical features in accordance with those previously described for magnocellular neuroendocrine neurons. These neurons had an average resting membrane potential of -58.3 +/- 0.9 (mean +/- S.E.M.) mV, spike amplitude of 92.8 +/- 1.4 mV, and input resistance of 788.9 +/- 50.4 M omega. Most of these cells displayed irregular or continuous spontaneous activity with a mean frequency of 2.44 +/- 0.33 Hz. Voltage-clamp recordings revealed three outward potassium currents; (1) a delayed outward current (IK), (2) a Ca(2+)-dependent outward current (IK(Ca)) and (3) a transient outward current (IA). These currents were classified according to their voltage dependence, inactivation, Ca2+ dependence and pharmacology. The IK was activated by depolarization beyond -40 mV and its amplitude consistently increased with depolarizing steps. The membrane conductance underlying this current was 27.3 +/- 3.8 nS for depolarization to +50 mV. In medium containing 2 mM Ca2+, depolarization to above -20 mV evoked a slowly-activating IK(Ca) which showed minimal inactivation. This current was suppressed in Ca(2+)-free/Co2+ medium and its membrane conductance was also smaller (19.4 +/- 3.5 nS at +50 mV) than that of IK. The IA demonstrated both fast activation and inactivation and was evoked only if depolarizing pulse steps were preceded by conditioning hyperpolarization. The activation threshold was approximately -65 mV and IA amplitude increased in non-linear fashion as test voltage steps became more positive. The 90% maximum of IA conductance was 15.7 +/- 1.1 nS, and was observed at membrane potentials around -15 mV. The reversal potentials of these currents were in accordance with the K+ equilibrium potential. Tetra-ethylammonium reversibly inhibited both the peak and steady-state currents of the IK, while 4-aminopyridine suppressed the IA. Replacement of 2 mM Ca2+ with 2 mM Co2+ in our bath solution or addition of Co2+ into Ca(2+)-free medium reduced the magnitude of IA, revealing the existence of a Co(2+)-sensitive IA. Bath administration of 10(-7) M angiotensin was without significant effect on IK, but resulted in a statistically significant reduction in IA (-31.0 +/- 4.1%) in 12 of 14 paraventricular nucleus cells tested, effects which were not observed following pretreatment with the AT1 receptor antagonist losartan. We conclude that in paraventricular nucleus magnocellular cells, like other CNS neurons, at least three sets of potassium channels contribute to the outward current evoked by depolarization. Our data also demonstrate ionic mechanisms through which angiotensin may act at AT1 receptors to influence the excitability of hypothalamic neuroendocrine cells.

Effect of central acute administration of cadmium on drinking behavior

The effect of acute third ventricle cadmium administration on the drinking behavior of adult male rats under different situations was studied. Injections of cadmium chloride (0.07, 0.7,and 7.0 ng/rat) significantly attenuated water intake in dehydrated rats. Drinking behavior induced by acute intracerebroventricular injections of carbachol (2 micrograms/rat) or angiotensin II (5 ng/rat) was also inhibited by central cadmium injections. Cadmium-induced blockade in water intake in dehydrated animals was reverted by the previous administration of a 5-HT2 antagonist (RP62203) in different doses (5 and 10 micrograms/rat). The data clearly reveal that cadmium elicits very fast actions on the central nervous system. It is suggested that cadmium-induced attenuation of water intake may rely on at least three different mechanisms: impairment of cholinergic and angiotensinergic systems in the brain and stimulation of a central serotonergic drive acting on 5-HT2 receptors. The study of cadmium neurotoxicity by observation of drinking behavior, a behavioral parameter easy to be recorded and measured, is proposed.

Permanent inhibition of angiotensinogen synthesis by antisense RNA expression

The renin-angiotensin system plays a pivotal role in blood pressure regulation. Recent molecular biological findings led to the new concept that in addition to the classic endocrine system, local tissue systems may also play an important role in cardiovascular diseases such as hypertension. In particular, the brain renin-angiotensin system was shown to influence the central control of blood pressure and is thought to contribute to the hypertensive phenotype of genetically hypertensive rat models. To identify the physiological role of these local systems, we established an antisense strategy to downregulate the expression of the precursor hormone angiotensinogen (AOGEN) in cell culture, which can also be used to establish transgenic rat lines. Plasmids encoding an RNA sequence complementary to the rat AOGEN mRNA under control of different viral and tissue-specific promoters were constructed and transfected into an AOGEN-expressing cell line. A competitive reverse transcription-polymerase chain reaction method was established for the quantification of AOGEN mRNA. Depending on the level of antisense RNA, the expression of the AOGEN gene was reduced down to 22% of control levels. Furthermore, the secretion of AOGEN protein was totally abolished. These results clearly demonstrate that the antisense constructs used are functional in reducing the AOGEN gene expression in vivo and can be used for the production of transgenic rats.

Medullary neurons activated by angiotensin II in the conscious rabbit

Previous studies have shown that angiotensin II (Ang II) can activate cardiovascular neurons within the medulla oblongata via an action on specific receptors. The purpose of this study was to determine the distribution of neurons within the medulla activated by infusion of Ang II into the fourth ventricle of conscious rabbits, using the expression of Fos, the protein product of the immediate early gene c-fos as a marker of neuronal activation. Experiments were done in both intact and barodenervated animals. In comparison with a control group infused with Ringer's solution alone, in both intact and barodenervated animals, fourth ventricular infusion of Ang II (4 to 8 pmol/min) induced a significant increase in the number of Fos-positive neurons in the nucleus of the solitary tract and in the rostral, intermediate, and caudal parts of the ventrolateral medulla. Double-labeling for Fos and tyrosine hydroxylase immunoreactivity showed that 50% to 75% of Fos-positive cells in the rostral, intermediate, and caudal ventrolateral medulla and 30% to 40% of Fos-positive cells in the nucleus of the solitary tract were also positive for tyrosine hydroxylase in both intact and barodenervated animals. The distribution of Fos-positive neurons corresponded very closely to the location of Ang II receptor binding sites as previously determined in the rabbit. The results indicate that medullary neurons activated by Ang II are located in discrete regions within the nucleus of the solitary tract and ventrolateral medulla and include, in all of these regions, both catecholamine and noncatecholamine neurons.

Angiotensin III and IV activation of the brain AT1 receptor subtype in cardiovascular function

The present investigation determined that native angiotensins II and III (ANG II and III) were equipotent as pressor agents when ICV infused in alert rats, whereas native angiotensin IV (ANG IV) was less potent. An analogue of each of these angiotensins was prepared with a hydroxyethylamine (HEA) amide bond replacement at the N-terminus, yielding additional resistance to degradation. These three angiotensin analogues, HEA-ANG II, HEA-ANG III, and HEA-ANG IV, were equivalent with respect to maximum elevation in pressor responses when ICV infused; and each evidenced significantly extended durations of effect compared with their respective native angiotensin. Comparing analogues, HEA-ANG II had a significantly longer effect compared with HEA-ANG III, and HEA-ANG IV, whereas the latter were equivalent. Pretreatment with the AT1 receptor subtype antagonist, Losartan (DuP753), blocked subsequent pressor responses to each of these analogues, suggesting that these responses were mediated by the AT1 receptor subtype. Pretreatment with the specific AT4 receptor subtype antagonist, Divalinal (HED 1291), failed to influence pressor responses induced by the subsequent infusion of these analogues. These results suggest an important role for Ang III, and perhaps ANG IV, in brain angiotensin pressor responses mediated by the AT1 receptor subtype.

Role of peripheral angiotensin in salt appetite of the sodium-deplete rat

Lines of evidence indicate that the importance of peripherally derived angiotensin II as a stimulus for salt appetite in rats has been underestimated. First, a series of observations is consistent with the idea that peripherally derived angiotensin acts at circumventricular organs of the brain to stimulate salt appetite following sodium depletion. Second, recent experiments show that depletion-induced salt appetite is abolished by the i.v. infusion of converting-enzyme inhibitor (captopril) at a dose that totally prevents the formation of angiotensin II within the peripheral circulation. This same dose of converting-enzyme inhibitor does not penetrate the blood-brain barrier to affect the actions of centrally derived angiotensin. These findings suggest that angiotensin II of peripheral origin is critical for the expression of salt appetite following extracellular fluid depletions. Together, these two lines of evidence suggest that the role of circulating angiotensin II in the stimulation of salt appetite in the rat should be re-examined.