Immunohistochemical mapping of angiotensin type 2 (AT2) receptors in rat brain

Intrinsic organization of the corpus callosum

The corpus callosum-the largest commissural fiber system connecting the two cerebral hemispheres-is considered essential for bilateral sensory integration and higher cognitive functions. Most studies exploring the corpus callosum have examined either the anatomical, physiological, and neurochemical organization of callosal projections or the functional and/or behavioral aspects of the callosal connections after complete/partial callosotomy or callosal lesion. There are no works that address the intrinsic organization of the corpus callosum. We review the existing information on the activities that take place in the commissure in three sections: I) the topographical and neurochemical organization of the intracallosal fibers, II) the role of glia in the corpus callosum, and III) the role of the intracallosal neurons.

Neuropeptides and Their Roles in the Cerebellum

Although more than 30 different types of neuropeptides have been identified in various cell types and circuits of the cerebellum, their unique functions in the cerebellum remain poorly understood. Given the nature of their diffuse distribution, peptidergic systems are generally assumed to exert a modulatory effect on the cerebellum via adaptively tuning neuronal excitability, synaptic transmission, and synaptic plasticity within cerebellar circuits. Moreover, cerebellar neuropeptides have also been revealed to be involved in the neurogenetic and developmental regulation of the developing cerebellum, including survival, migration, differentiation, and maturation of the Purkinje cells and granule cells in the cerebellar cortex. On the other hand, cerebellar neuropeptides hold a critical position in the pathophysiology and pathogenesis of many cerebellar-related motor and psychiatric disorders, such as cerebellar ataxias and autism. Over the past two decades, a growing body of evidence has indicated neuropeptides as potential therapeutic targets to ameliorate these diseases effectively. Therefore, this review focuses on eight cerebellar neuropeptides that have attracted more attention in recent years and have significant potential for clinical application associated with neurodegenerative and/or neuropsychiatric disorders, including brain-derived neurotrophic factor, corticotropin-releasing factor, angiotensin II, neuropeptide Y, orexin, thyrotropin-releasing hormone, oxytocin, and secretin, which may provide novel insights and a framework for our understanding of cerebellar-related disorders and have implications for novel treatments targeting neuropeptide systems.

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

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

Contribution of amygdala to the pressor response elicited by microinjection of angiotensin II into the bed nucleus of the stria terminalis

The bed nucleus of the stria terminalis (BST) is part of the limbic system located in the rostral forebrain. BST is involved in behavioral, neuroendocrine and autonomic functions, including cardiovascular regulation. The amygdala, plays an important role in mediating the behavioral and physiological responses associated with fear and anxiety, including cardiovascular responses. In a previous study, we showed that microinjection of AngII into the BST produced a pressor and two types of single-unit responses in the BST, short excitatory and long inhibitory. This study was performed to find possible involvement of amygdala in cardiovascular responses elicited by microinjection of AngII into the BST, using blockade of the central nucleus of amygdala (CeA) and single unit recording from the CeA, while injecting AngII into the BST in anesthetized rat. Blockade of CeA attenuated the pressor response to microinjection of AngII into the BST. Eighty-six AngII microinjections were given into the BST and 198 single unit responses were recorded from CeA simultaneously, from which 89 showed a short duration excitatory response and 109 showed no responses. In conclusion, microinjection of AngII into the BST produces a short excitatory single unit response in the CeA, resulting in contribution of amygdala to the resulted pressor response. Taken together, our study and previous studies suggest a plausible hypothesis that these two nuclei perform their cardiovascular functions in cooperation with each other.

Cerebroprotective effects of RAS inhibitors: Beyond their cardio-renal actions

Work on the brain renin-angiotensin system has been explored by various researchers and has led to elucidation of its basic physiologies and behavior, including its role in reabsorption and uptake of body fluid, blood pressure maintenance with angiotensin II being its prominent effector. Currently, this system has been implicated for its newly established effects, which are far beyond its cardio-renal effects accounting for maintenance of cerebral blood flow and cerebroprotection, seizure, in the etiology of Alzheimer's disease, Parkinson's disease, multiple sclerosis, and bipolar disorder. In this review, we have discussed the distribution of angiotensin receptor subtypes in the central nervous system (CNS) together with enzymatic pathways leading to active angiotensin ligands and its interaction with angiotensin receptor 2 (AT2) and Mas receptors. Secondly, the use of angiotensin analogues (angiotensin converting enzyme inhibitors and AT1 and/or AT2 receptor blockers) in the treatment and management of the CNS disorders mentioned above has been discussed.

Reporter mouse strain provides a novel look at angiotensin type-2 receptor distribution in the central nervous system

Angiotensin-II acts at its type-1 receptor (AT1R) in the brain to regulate body fluid homeostasis, sympathetic outflow and blood pressure. However, the role of the angiotensin type-2 receptor (AT2R) in the neural control of these processes has received far less attention, largely because of limited ability to effectively localize these receptors at a cellular level in the brain. The present studies combine the use of a bacterial artificial chromosome transgenic AT2R-enhanced green fluorescent protein (eGFP) reporter mouse with recent advances in in situ hybridization (ISH) to circumvent this obstacle. Dual immunohistochemistry (IHC)/ISH studies conducted in AT2R-eGFP reporter mice found that eGFP and AT2R mRNA were highly co-localized within the brain. Qualitative analysis of eGFP immunoreactivity in the brain then revealed localization to neurons within nuclei that regulate blood pressure, metabolism, and fluid balance (e.g., NTS and median preoptic nucleus [MnPO]), as well as limbic and cortical areas known to impact stress responding and mood. Subsequently, dual IHC/ISH studies uncovered the phenotype of specific populations of AT2R-eGFP cells. For example, within the NTS, AT2R-eGFP neurons primarily express glutamic acid decarboxylase-1 (80.3 ± 2.8 %), while a smaller subset express vesicular glutamate transporter-2 (18.2 ± 2.9 %) or AT1R (8.7 ± 1.0 %). No co-localization was observed with tyrosine hydroxylase in the NTS. Although AT2R-eGFP neurons were not observed within the paraventricular nucleus (PVN) of the hypothalamus, eGFP immunoreactivity is localized to efferents terminating in the PVN and within GABAergic neurons surrounding this nucleus. These studies demonstrate that central AT2R are positioned to regulate blood pressure, metabolism, and stress responses.

Renin Angiotensin System in Cognitive Function and Dementia

Angiotensin II represents a key molecule in hypertension and cerebrovascular pathology. By promoting inflammation and oxidative stress, enhanced Ang II levels accelerate the onset and progression of cell senescence. Sustained activation of RAS promotes end-stage organ injury associated with aging and results in cognitive impairment and dementia. The discovery of the angiotensin-converting enzyme ACE2-angiotensin (1–7)-Mas receptor axis that exerts vasodilator, antiproliferative, and antifibrotic actions opposed to those of the ACE-Ang II-AT1 receptor axis has led to the hypothesis that a decrease in the expression or activity of angiotensin (1–7) renders the systems more susceptible to the pathological actions of Ang II. Given the successful demonstration of beneficial effects of increased expression of ACE2/formation of Ang1–7/Mas receptor binding and modulation of Mas expression in animal models in containing cerebrovascular pathology in hypertensive conditions and aging, one could reasonably hope for analogous effects regarding the prevention of cognitive decline by protecting against hypertension and cerebral microvascular damage. Upregulation of ACE2 and increased balance of Ang 1–7/Ang II, along with positive modulation of Ang II signaling through AT2 receptors and Ang 1–7 signaling through Mas receptors, may be an appropriate strategy for improving cognitive function and treating dementia.

Immunohistochemical detection of angiotensin II receptors in mouse cerebellum and adrenal gland using "in vivo cryotechnique"

Angiotensin II (AT) receptors, including AT receptor type 1 (AT1R) and type 2 (AT2R), are expressed in the rodent central nervous system, but their distributions and activation states are still unclear. In this study, we have performed immunohistochemical analyses of AT receptors in mouse cerebellum and adrenal gland using our "in vivo cryotechnique" (IVCT). We used antibodies against amino-terminal domains of AT receptors, which are considered to undergo conformational changes upon the binding of AT. Immunoreactivity of AT1R was detected in mouse cerebellum, and was highest in the outer tissue areas of molecular layers using IVCT. The AT1R immunostaining largely overlapped with glial fibrillary acidic protein (GFAP), a marker of Bergmann glia. Surprisingly, the AT1R immunoreactivity in the cerebellar cortex was remarkably reduced following 5 and 10 min of hypoxia or direct administration of an AT1R antagonist, losartan. By contrast, in the adrenal cortex, such AT1R immunoreactivity detected at the zona glomerulosa did not change even after 15 min of hypoxia. The correlation of localization with GFAP and also hypoxia-induced decrease of its immunoreactivity were similarly observed by immunostaining of AT2R in the cerebellar specimens. These findings demonstrated that IVCT is useful to reveal dynamically changing immunoreactivities usually affected by receptor-ligand binding as well as hypoxia, and also suggested that functional activities of AT receptors are time-dependently modulated under hypoxia in the central nervous system in comparison with the adrenal glands.

Immunohistochemical Localization of AT1a, AT1b, and AT2 Angiotensin II Receptor Subtypes in the Rat Adrenal, Pituitary, and Brain with a Perspective Commentary

Angiotensin II increases blood pressure and stimulates thirst and sodium appetite in the brain. It also stimulates secretion of aldosterone from the adrenal zona glomerulosa and epinephrine from the adrenal medulla. The rat has 3 subtypes of angiotensin II receptors: AT1a, AT1b, and AT2. mRNAs for all three subtypes occur in the adrenal and brain. To immunohistochemically differentiate these receptor subtypes, rabbits were immunized with C-terminal fragments of these subtypes to generate receptor subtype-specific antibodies. Immunofluorescence revealed AT1a and AT2 receptors in adrenal zona glomerulosa and medulla. AT1b immunofluorescence was present in the zona glomerulosa, but not the medulla. Ultrastructural immunogold labeling for the AT1a receptor in glomerulosa and medullary cells localized it to plasma membrane, endocytic vesicles, multivesicular bodies, and the nucleus. AT1b and AT2, but not AT1a, immunofluorescence was observed in the anterior pituitary. Stellate cells were AT1b positive while ovoid cells were AT2 positive. In the brain, neurons were AT1a, AT1b, and AT2 positive, but glia was only AT1b positive. Highest levels of AT1a, AT1b, and AT2 receptor immunofluorescence were in the subfornical organ, median eminence, area postrema, paraventricular nucleus, and solitary tract nucleus. These studies complement those employing different techniques to characterize Ang II receptors.

Purkinje cells express Angiotensin II AT(2) receptors at different developmental stages

Angiotensin II (Ang II) binds and activates two major receptors subtypes, namely AT(1) and AT(2). In the fetus, AT(2) receptors predominate in all tissues and decline shortly after birth, being restricted to a few organs including brain. Interpretation of the function of Ang II in the cerebellum requires a thorough understanding of the localization of Ang II receptors. The aim of the present paper is to evaluate the localization of Ang II AT(2) receptors in the Purkinje cell (PC) layer during development. By binding autoradiography, a clear complementary pattern of AT(1) and AT(2) binding labeled by [(125)I] Ang II was observed in young rats within the cerebellar cortex. This pattern was present at the stages P8 and P15, but not at P30 and P60, where AT(2) binding appears low and superimposed with AT(1) binding. We demonstrate that AT(2) antibodies recognized postmitotic Purkinje cells, labeling the somata of these cells at all the stages studied, from P8 to P60, suggesting that PCs express these receptors from early stages of development until adulthood. In P8 and P15 animals, we observed a clear correspondence between immunolabeling and the well-defined layer observed by binding autoradiography. Confocal analysis allowed us to discard the co-localization of AT(2) receptors with glial fibrillary acidic protein (GFAP), a glial marker. Double immunolabeling allowed us to demonstrate the co-localization of Ang II AT(2) receptors with zebrin II, a specific PC marker. Since PCs are the sole output signal from the cerebellar cortex and considering the role of cerebellum in movement control, the specific receptor localization suggests a potential role for Ang II AT(2) receptors in the cerebellar function.

Systemic candesartan reduces brain angiotensin II via downregulation of brain renin-angiotensin system

The renin-angiotensin system has an important function in the regulation of blood pressure as well as in pathophysiological processes in the central nervous system. We examined the effects of the angiotensin receptor blocker candesartan (10 mg kg(-1) per day, p.o.) on brain angiotensin II levels in angiotensin II-infused hypertensive rats. Angiotensin II or vehicle was infused subcutaneously for 14 days in Sprague-Dawley rats. Angiotensin II infusion resulted in increased blood pressure, an effect that was blocked by candesartan treatment. There was no effect of the angiotensin II infusion on Angiotensin II levels in the brain or on blood-brain barrier permeability. Brain tissue angiotensinogen and angiotensin converting enzyme mRNA levels were not changed by angiotensin II infusion but were decreased by candesartan treatment. At 2 weeks of treatment, CV11974, an active form of candesartan, was detectable in the plasma but was not detectable in brain tissue. These data suggest that treatment with candesartan decreases brain angiotensin II by decreasing brain angiotensinogen and angiotensin converting enzyme gene expression.

Dietary sodium deprivation evokes activation of brain regional neurons and down-regulation of angiotensin II type 1 receptor and angiotensin-convertion enzyme mRNA expression

Previous studies have indicated that the renin-angiotensin-aldosterone system (RAAS) is implicated in the induction of sodium appetite in rats and that different dietary sodium intakes influence the mRNA expression of central and peripheral RAAS components. To determine whether dietary sodium deprivation activates regional brain neurons related to sodium appetite, and changes their gene expression of RAAS components of rats, the present study examined the c-Fos expression after chronic exposure to low sodium diet, and determined the relationship between plasma and brain angiotensin I (ANG I), angiotensin II (ANG II) and aldosterone (ALD) levels and the sodium ingestive behavior variations, as well as the effects of prolonged dietary sodium deprivation on ANG II type 1 (AT1) and ANG II type 2 (AT2) receptors and angiotensin-convertion enzyme (ACE) mRNA levels in the involved brain regions using the method of real-time polymerase chain reaction (PCR). Results showed that the Fos immunoreactivity (Fos-ir) expression in forebrain areas such as subfornical organ (SFO), paraventricular hypothalamic nuclei (PVN), supraoptic nucleus (SON) and organum vasculosum laminae terminalis (OVLT) all increased significantly and that the levels of ANG I, ANG II and ALD also increased in plasma and forebrain in rats fed with low sodium diet. In contrast, AT1, ACE mRNA in PVN, SON and OVLT decreased significantly in dietary sodium depleted rats, while AT2 mRNA expression did not change in the examined areas. These results suggest that many brain areas are activated by increased levels of plasma and/or brain ANG II and ALD, which underlies the elevated preference for hypertonic salt solution after prolonged exposure to low sodium diet, and that the regional AT1 and ACE mRNA are down-regulated after dietary sodium deprivation, which may be mediated by increased ANG II in plasma and/or brain tissue.

Angiotensin II type 2 receptors have a major somatodendritic distribution in vasopressin-containing neurons in the mouse hypothalamic paraventricular nucleus

The hypothalamic paraventricular nucleus (PVN) and angiotensin II (AngII) play critical roles in cardiovascular and neurohumoral regulation ascribed in part to vasopressin (VP) release. The AngII actions in the PVN are mediated largely through angiotensin II type 1 (AT1) receptors. However, there is indirect evidence that the functionally elusive central angiotensin II type 2 (AT2) receptors are also mediators of AngII signaling in the PVN. We used electron microscopic dual immunolabeling of antisera recognizing the AT2 receptor and VP to test the hypothesis that mouse PVN neurons expressing VP are among the cellular sites where this receptor has a subcellular distribution conducive to local activation. Immunoreactivity for the AT2 receptor was detected in somatodendritic profiles, of which approximately 60% of the somata and approximately 28% of the dendrites also contained VP. In comparison with somata and dendrites, axons, axon terminals, and glia less frequently contained the AT2 receptor. Somatic labeling for the AT2 receptor was often seen in the cytoplasm near the Golgi lamellae and other endomembrane structures implicated in receptor trafficking. AT2 receptor immunoreactivity in dendrites was commonly localized to cytoplasmic endomembranes, but was occasionally observed on extra- or peri-synaptic portions of the plasma membrane apposed by astrocytic processes or by unlabeled axon terminals. The labeled dendritic plasmalemmal segments containing AT2 receptors received asymmetric excitatory-type or more rarely symmetric inhibitory-type contacts from unlabeled axon terminals containing dense core vesicles, many of which are known to store neuropeptides. These results provide the first ultrastructural evidence that AT2 receptors in PVN neurons expressing VP and other neuromodulators are strategically positioned for surface activation by AngII and/or intracellular trafficking.

A peripherally administered, centrally acting angiotensin II AT2 antagonist selectively increases brain AT1 receptors and decreases brain tyrosine hydroxylase transcription, pituitary vasopressin and ACTH

The physiological actions of brain Angiotensin II AT(2) receptors and their relationship to Angiotensin II AT(1) receptors remain controversial. To further clarify their role, we determined to what extent systemic administration of an AT(2) receptor antagonist affected AT(2) receptor binding within the brain and the expression of AT(1) receptors. For this purpose, we subcutaneously administered the AT(2) receptor antagonist PD123319 (1 mg/kg/day) to adult male rats for two weeks via osmotic minipumps. We also studied the content of pituitary adrenocorticotropic hormone and vasopressin, representative of hypothalamic-pituitary-adrenal axis activation, and the tyrosine hydroxylase gene expression in the locus coeruleus as a measure of central norepinephrine function. We found significant decreases in AT(2) receptor binding in brain areas inside the blood brain barrier, the inferior olive and the locus coeruleus. AT(2) receptor blockade increased AT(1) receptor binding and mRNA expression not only in the subfornical organ and the median eminence, situated outside the blood brain barrier, but also in the hypothalamic paraventricular nucleus, located inside the blood brain barrier. These changes paralleled decreased expression of tyrosine hydroxylase mRNA in the locus coeruleus and decreased pituitary adrenocorticotropic and vasopressin content. Our results demonstrate that sustained peripheral administration of an AT(2) antagonist decreases binding to brain AT(2) receptors, indicating that this drug is a useful tool for the study of their central role. AT(2) receptor activity inhibition up-regulates AT(1) receptor expression in specific brain areas. Blockade of brain AT(2) receptors is compatible with enhanced hypothalamic-pituitary-adrenal axis and decreased central sympathetic system activity.

Functional roles of neuropeptides in cerebellar circuits

Whereas the cerebellum contains 22 different types of neuropeptides as presently known, their expression is generally weak and diffusely dispersed in cerebellar tissues, which often makes their functional significance doubtful. Nevertheless, our knowledge about certain neuropeptides has advanced to the extent that we can figure out their unique functional roles in cerebellar circuits. Throughout the cerebellum, CRF is contained in climbing fibers and its spontaneous release is required for the induction of cerebellar long-term depression (LTD), a cellular mechanism of motor learning. Corticotropin-releasing factor (CRF) is also expressed in the paraventricular nucleus-pituitary system and amygdala-lower brainstem system, both of which are involved in coping responses to stress. In view that motor learning requires stressful efforts for correcting errors in repeated trials, CRF in climbing fibers may imply that the olivocerebellar system is part of a large CRF-operated functional system that acts to cope with various stressors. Orexin, on the other hand, is contained in beaded fibers, which, originating from the hypothalamus, project to various brainstem nuclei and also to the cerebellum, exclusively the flocculus. Currently available evidence suggests that, in fight-or-flight situations, orexinergic neurons switch the state of cardiovascular control systems including the flocculus to secure blood supply to working muscles. Considerable knowledge has also been accumulated about angiotensin II, galanin, and cerebellin, but there is still a gap in defining their unique functional roles in cerebellar circuits.

Angiotensin II disrupts inhibitory avoidance memory retrieval

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

Physiology of Local Renin-Angiotensin Systems

Since the first identification of renin by Tigerstedt and Bergmann in 1898, the renin-angiotensin system (RAS) has been extensively studied. The current view of the system is characterized by an increased complexity, as evidenced by the discovery of new functional components and pathways of the RAS. In recent years, the pathophysiological implications of the system have been the main focus of attention, and inhibitors of the RAS such as angiotensin-converting enzyme (ACE) inhibitors and angiotensin (ANG) II receptor blockers have become important clinical tools in the treatment of cardiovascular and renal diseases such as hypertension, heart failure, and diabetic nephropathy. Nevertheless, the tissue RAS also plays an important role in mediating diverse physiological functions. These focus not only on the classical actions of ANG on the cardiovascular system, namely, the maintenance of cardiovascular homeostasis, but also on other functions. Recently, the research efforts studying these noncardiovascular effects of the RAS have intensified, and a large body of data are now available to support the existence of numerous organ-based RAS exerting diverse physiological effects. ANG II has direct effects at the cellular level and can influence, for example, cell growth and differentiation, but also may play a role as a mediator of apoptosis. These universal paracrine and autocrine actions may be important in many organ systems and can mediate important physiological stimuli. Transgenic overexpression and knock-out strategies of RAS genes in animals have also shown a central functional role of the RAS in prenatal development. Taken together, these findings may become increasingly important in the study of organ physiology but also for a fresh look at the implications of these findings for organ pathophysiology.

The CNS renin-angiotensin system

The renin-angiotensin system (RAS) is one of the best-studied enzyme-neuropeptide systems in the brain and can serve as a model for the action of peptides on neuronal function in general. It is now well established that the brain has its own intrinsic RAS with all its components present in the central nervous system. The RAS generates a family of bioactive angiotensin peptides with variable biological and neurobiological activities. These include angiotensin-(1-8) [Ang II], angiotensin-(3-8) [Ang IV], and angiotensin-(1-7) [Ang-(1-7)]. These neuroactive forms of angiotensin act through specific receptors. Only Ang II acts through two different high-specific receptors, termed AT1 and AT2. Neuronal AT1 receptors mediate the stimulatory actions of Ang II on blood pressure, water and salt intake, and the secretion of vasopressin. In contrast, neuronal AT2 receptors have been implicated in the stimulation of apoptosis and as being antagonistic to AT1 receptors. Among the many potential effects mediated by stimulation of AT2 are neuronal regeneration after injury and the inhibition of pathological growth. Ang-(1-7) mediates its antihypertensive effects by stimulating the synthesis and release of vasodilator prostaglandins and nitric oxide and by potentiating the hypotensive effects of bradykinin. New data concerning the roles of Ang IV and Ang-(1-7) in cognition also support the existence of complex site-specific interactions between multiple angiotensins and multiple receptors in the mediation of important central functions of the RAS. Thus, the RAS of the brain is involved not only in the regulation of blood pressure, but also in the modulation of multiple additional functions in the brain, including processes of sensory information, learning, and memory, and the regulation of emotional responses.

Angiotensin II blocks memory consolidation through an AT2 receptor-dependent mechanism

RATIONALE AND OBJECTIVES

Several studies suggest that the brain renin-angiotensin system is involved in memory consolidation. However, the participation of angiotensin II (AII) in this process is controversial. This is probably due to the fact that many of the studies carried out to elucidate this matter employed multitrial learning paradigms together with pretraining intracerebroventricular infusions, and therefore were unable to distinguish between consolidation and retrieval related events and lacked anatomical specificity. To circumvent this problem, we analyzed the role played in memory consolidation by AII using the hippocampal-dependent, one-trial, step-down inhibitory avoidance task (IA) in combination with stereotaxically localized intrahippocampal infusion of drugs.

METHODS AND RESULTS

Rats bilaterally implanted with infusion cannulae into the CA1 region of the dorsal hippocampus (CA1) were trained in IA and tested for memory retention 24 h later. We found that when infused into CA1 immediately or 30 min after training but not later, AII produced a dose-dependent amnesic effect without altering locomotor activity, exploratory behavior or anxiety state. The amnesic effect of AII was not mimicked by angiotensin IV (AIV) and was totally blocked by the AII-type 2 receptor (AT2) antagonist, PD123319, but not by the AII-type 1 receptor (AT1) antagonist, losartan. Importantly, when infused alone, neither PD123319 nor losartan produced any effect on memory retention.

CONCLUSIONS

Our data indicate that, when given into CA1, AII blocks memory formation through a mechanism involving activation of AT2 receptors; however, endogenous AII does not seem to participate in the consolidation of IA long-term memory.

The Effect of Central Injection of Angiotensin-Converting Enzyme Inhibitor and the Angiotensin Type 1 Receptor Antagonist on the Induction by Lipopolysaccharide of Fever and Brain Interleukin-1β Response in Rats

We recently reported an involvement of peripheral angiotensin II (ANG II) in the development of both the fever and the peripheral interleukin (IL)-1beta production induced in rats by a systemic injection of lipopolysaccharide (LPS). The present study was performed to investigate whether brain ANG II contributes to the fever and IL-1beta production in the rat brain induced by i.c.v. injection of LPS. LPS (0.2 and 2 microg i.c.v.) induced dose-related fevers and increases in the brain (hypothalamus, hippocampus, and cerebellum) concentrations of IL-1beta. These effects were significantly inhibited by i.c.v. administration of either an angiotensin-converting-enzyme (ACE) inhibitor or an angiotensin type 1 (AT(1)) receptor antagonist. By contrast, the ACE inhibitor had no effect on the IL-1beta (i.c.v.)-induced fever, whereas the AT(1) receptor antagonist enhanced (rather than reduced) it. The AT(1) receptor antagonist had no effect on the brain levels of prostaglandin E(2) in rats given an i.c.v. injection of IL-1beta. These results suggest that in rats, brain ANG II and AT(1) receptors are involved in the LPS-induced production of brain IL-1beta, thus contributing to the fever induced by the presence of LPS within the brain.

Localization of AT(2) receptors in the nucleus of the solitary tract of spontaneously hypertensive and Wistar Kyoto rats using [125I] CGP42112: upregulation of a non-angiotensin II binding site following unilateral nodose ganglionectomy

We have examined the binding distribution of a selective AT(2) receptor ligand [125I] CGP42112 in the brain of adult Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). AT(2) receptor localization was also examined in the rat brainstem following unilateral nodose ganglionectomy. Specific [125I] CGP42112 binding was observed in discrete brain regions from both rat strains, including the nucleus of the solitary tract (NTS), and did not differ between WKY and SHR. [125I] CGP42112 binding in the NTS revealed an AT(2) receptor component that was displaceable by PD 123319 and Ang II (50-58%), as well as a non-angiotensin II receptor component (42-49%). Following unilateral nodose ganglionectomy, [125I] CGP42112 binding density on the denervated side of the NTS was increased approximately two-fold in both WKY and SHR. This increased [125I] CGP42112 binding density in the ipsilateral NTS was comprised of a greater non-angiotensin II component than that observed in the sham groups, since only approximately 30% was displaced by PD123319 and angiotensin II. Furthermore, [125I] CGP42112 also revealed high binding density on the denervated side in the dorsal motor nucleus and the nucleus ambiguus in both WKY and SHR. AT(2) receptor immunoreactivity was also visualised in the NTS of sham operated rats, but was not observed in the dorsal motor nucleus or the nucleus ambiguus, nor was it up-regulated following nodose ganglionectomy. These results demonstrate, for the first time, an AT(2) receptor binding site in the NTS, as well as a non-angiotensin II [125I] CGP42112 binding site. These studies also demonstrate that nodose ganglionectomy represents a useful model in which to study a non-angiotensin II [125I] CGP42112 binding site that is up-regulated following degeneration of afferent vagal nerves.

Effects of angiotensin II microinjected into medial amygdala on male sexual behavior in rats

Research was undertaken to study the role of central angiotensin in the modulation of male sexual behavior, testing the effect of angiotensin II (Ang II) injections into the medial amygdaloid nucleus (MeA). The sexual behavior of adult male Wistar rats was evaluated, 15 min after bilateral intra-amygdaloid microinjection (0.3 microl) of saline and 5 doses of Ang II: 10; 25; 50; 100, and 150 fmol. The effects of the Ang II receptor blockade were also studied. We tested the effect of coinjection of Ang II (50 fmol) with the AT1 antagonist, losartan (20 pmol) and the AT2 antagonist, CGP 42112 (1 pmol). Ang II inhibited sexual behavior and this inhibition was prevented by the coinjection of AT1 antagonist, losartan, or the AT2 antagonist, CGP 42112. Results show that Ang II has a powerful effect on male sexual behavior, which may be mediated by both AT1 and AT2 receptors.

Angiotensin receptor-like immunoreactivity in adult brain white matter astrocytes and oligodendrocytes

Most of the physiological effects of brain angiotensins are currently believed to be mediated by angiotensin receptors located principally on neurons. However, numerous studies in vitro have demonstrated the presence of functional angiotensin receptors on brain astrocytes, raising the possibility that glial cells may also participate in mediating the effects of the central renin-angiotensin system. Nevertheless, it is uncertain whether these cells in situ express angiotensin receptors, raising questions about the physiological significance of results observed in cell cultures. We have examined the distribution of angiotensin receptor-like immunoreactivity in glial cells in white matter tracts in the adult CNS, using a panel of antisera to the AT1 and AT2 angiotensin receptors. Antiserum preadsorption and/or Western blot demonstrated the specificity of the antisera in brain tissue. In immunohistochemical experiments, the AT1 antisera selectively labeled AT1-expressing neurons in the piriform cortex, whereas the AT2 antiserum stained cells in the trigeminal motor nucleus, these being nuclei known to express AT1 and AT2 receptors, respectively. Using double-label immunohistochemistry, we observed AT1- and AT2-immunoreactive astrocytes and oligodendrocytes in white matter tracts, which include the rat cerebellar white matter, periventricular white matter, and optic nerve, in addition to the bovine corpus callosum and human subcortical white matter. In contrast, astrocytes in the gray matter region of the cerebral cortex were not found to be angiotensin receptor-like immunoreactive. These results demonstrate the presence of AT1 and/or AT2 angiotensin receptor-like immunoreactivity in brain white matter macroglial cells in situ and support the idea that glial cells may play a more important role in the central renin-angiotensin system than previously thought.

Genetic deletion of angiotensin AT2 receptor leads to increased cell numbers in different brain structures of mice

Angiotensin II (Ang II) is a potent vasoactive peptide and displays growth factor-like properties. Different high-affinity Ang II receptor subtypes (AT1A, AT1B and AT2) have been cloned. They are expressed in various brain structures. Additionally, it has been assumed that Mas could interact directly or indirectly with the renin-angiotensin system. The AT1 receptor mediates pressor and mitogenic effects of Ang II, whereas physiological function and signaling mechanisms of the AT2 receptor remain poorly understood. Recent reports have shown that Ang II could mediate apoptosis through AT2 receptors. Since the AT1A, AT2 and Mas knockout mice provide new tools for uncovering potential actions of Ang II, the cell number in different brain structures of male adult wild-type mice and mice deficient for AT1A, AT2 or Mas was evaluated to get more insight into the role of Ang II in central nervous system development. In nearly all investigated brain structures (cortex, hippocampus, amygdala, thalamus), the cell number was significantly higher in AT2-deficient mice in comparison to wild-type mice. To the contrary, in AT1A-deficient mice the cell number was significantly less than in controls in the lateral geniculate and the medial amygdaloid nucleus. However, cell numbers were not changed in Mas-knockout mice compared to their wild-types. These results show the contrary effects of both angiotensin receptors on cell growth and represent the first demonstration of their action on neuronal cell development evidenced in the adult mouse brain.

Expression of the genes encoding the vasopressin-activated calcium-mobilizing receptor and the dual angiotensin II/vasopressin receptor in the rat central nervous system

The distributions of two newly discovered receptors, the vasopressin-activated calcium-mobilizing receptor (VACM-1) and the dual angiotensin II/vasopressin receptor (AII/AVP), in the central nervous system (CNS) of the rat were determined using reverse transcriptase-polymerase chain reaction and in situ hybridization. The sequence of the rat VACM-1 cDNA was determined and found very homologous to the rabbit and human sequences. Both VACM-1 and AII/AVP receptor genes were widely expressed in the brain, but differed according to the cell type studied. Glial cells were very faintly labelled. The epithelial cells of the choroid plexuses, the ependymal cells and the pia mater were all labelled. Both genes were most active in neurones throughout the CNS. VACM-1 and AII/AVP receptors were detected in neurones previously shown to possess V1a and V1b vasopressin receptors, and/or the AT1 and AT2 angiotensin II receptors in many brain areas. This was the case for the magnocellular neurones of the supraoptic and paraventricular nuclei of the hypothalamus. We suggest that the VACM-1 and AII/AVP receptors may account for the V2-like responses to vasopressin by these neurones which lack a genuine V2 vasopressin receptor.

Various effects of angiotensin II on amygdaloid neuronal activity in normotensive control and hypertensive transgenic [TGR(mREN-2)27] rats

The effects of iontophoretically ejected angiotensin II (Ang II) on the firing rate of neurons in the basolateral complex and the central and cortical amygdala were investigated in two strains of urethane anesthetized rats. In normotensive Sprague-Dawley rats, Ang II induced a significant increase in the discharge rate of responsive amygdaloid neurons. In contrast, in the hypertensive transgenic [TGR(mREN-2)27] rats with higher brain Ang II level, Ang II more often caused inhibitory effects on the amygdaloid firing rate in comparison with controls. The distribution of nonresponsive, excited, and inhibited neurons differed significantly in the two rat strains. Moreover, the responsiveness of amygdaloid neurons was significantly higher in transgenic rats in comparison with controls. Both the increase and the decrease in the firing rate caused by Ang II could be blocked either by angiotensin AT(1) or by AT(2) receptor-specific antagonists. In many cases, the Ang II-induced decrease in the firing rate was antagonized by bicuculline, a gamma-aminobutyric acid (GABA(A)) antagonist. The higher responsiveness of amygdaloid neurons in transgenic rats as well as the predominance of inhibitory effects, presumedly mediated by GABAergic interneurons, could change the output of the amygdala and its influence on thirst, kidney, and cardiovascular function or on processes of learning and anxiety.

Interaction between Mas and the angiotensin AT1 receptor in the amygdala

The Mas-protooncogene is a maternally imprinted gene encoding an orphan G protein-coupled receptor expressed mainly in limbic structures of the rodent CNS. Because Mas and the product of the Mas-related gene enhance the effects of angiotensins on cells expressing angiotensin receptors of the AT1 subtype, we first compared the distribution of cells expressing AT1 receptors in different limbic and thalamic brain structures in Mas-knockout mice and in wildtype mice by an immunohistochemical approach. No significant differences could be found between the two strains. The Mas-protooncogene seems to be implicated in the signal transduction of angiotensin receptors and is expressed in the amygdala. Therefore we then analyzed whether field potentials are altered by angiotensin II in brain slices of the basolateral amygdala. An opposite action of angiotensin II was obtained in mice lacking the Mas-protooncogene in comparison to wildtype mice. The use of different angiotensin receptor antagonists provides the first in vitro evidence for a functional interaction between the Mas-protooncogene and the AT1 receptor.

Angiotensin II type 1 receptor-modulated signaling pathways in neurons

Mammalian brain contains high densities of angiotensin II (Ang II) type 1 (AT1) receptors, localized mainly to specific nuclei within the hypothalamus and brainstem regions. Neuronal AT1 receptors within these areas mediate the stimulatory actions of central Ang II on blood pressure, water and sodium intake, and vasopressin secretion, effects that involve the modulation of brain noradrenergic pathways. This review focuses on the intracellular events that mediate the functional effects of Ang II in neurons, via AT1 receptors. The signaling pathways involved in short-term changes in neuronal activity, membrane ionic currents, norepinephrine (NE) release, and longer-term neuromodulatory actions of Ang II are discussed. It will be apparent from this discussion that the signaling pathways involved in these events are often distinct.

Angiotensin II in central nervous system physiology

In summary, the prevailing concept is that brain Ang II increases blood pressure by activating AT1 receptors, and that these have a neuromodulating effect to increase the activity of autonomic nervous system. Pathways for Ang II stimulating thirst and blood pressure, increased vasopressin release and sympathetic activation have been outlined. Brain RAS synthesis, while incompletely understood, is active in the absence of a peripheral RAS. Angiotensin elicits specific receptor mediated signals in neurons, particularly in the hypothalamus and brainstem. These actions are due to neuronal membrane ionic currents and the regulation of transcription factors. The areas to be explored further are characterization and functional roles of the other AT receptor subtypes, such as AT4, AT(1-7) and nuclear AT-R. Their interactions with other peptides and transmitters, and their signaling pathways need to be investigated. The story that began 100 years ago with renin is certainly not ended and will continue to unfold as further investigations with new techniques progress.

Mapping of angiotensin AT1 receptors in the rat limbic system

The AT1 receptor is one of the two receptor subtypes able to bind angiotensin II. In the present study, immunohistochemical examination of the distribution of the AT1 receptor in several limbic structures of female rats has been done, revealing new aspects of the distribution of AT1-positive cells. The presence of AT1 receptor expressing cells in the hippocampus and the amygdala is described, but their distribution in these regions has not been examined in a detailed way. We found some notable differences in the distribution of these cells: in female rats, we detected high amounts of labeled cells in the hippocampus, the entorhinal cortex and piriform cortex. In somewhat lower amounts, stained cells could be found in several nuclei of the amygdala (in the basomedial, basolateral, lateral, central and medial nucleus of the amygdala, in the amygdalopiriform transition area and in the amygdalohippocampal transition area as well as in the bed nucleus of the stria terminalis).

Visualization of specific angiotensin II binding sites in the rat limbic system

The present study examined the distribution of angiotensin-binding cells by using a fluorescence-coupled angiotensin II in fixed horizontal sections that contained several limbic structures. In normal female rats, dense staining was found in the CA3 and CA1 regions and the dentate gyrus of the hippocampus--in the subiculum as well as in the entorhinal cortex and piriform cortex. Moderate staining was found in the CA2 region, in the central and medial nuclei of the amygdala. Low-level staining was obtained in the basolateral and lateral nucleus of the amygdala as well as in the bed nucleus of the stria terminalis. The co-incubation of fluorescence-coupled angiotensin II together with angiotensin II in excess and with saralasin, respectively, suppressed the angiotensin staining in structures investigated.

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.

Analysis of angiotensin type 2 receptors in vasopressinergic neurons and pituitary in the rat

Previous functional studies indicated that an angiotensin type 2 (AT2) receptor subtype may participate in the regulation of vasopressin release by angiotensin II (AngII). In the present study, AT2 receptor-directed antiserum immunohistochemically detected AT2 receptors within the hypothalamic paraventricular (PVN) and the supraoptic nuclei (SON) of the rat brain, more specifically, in identified vasopressinergic neurons. Considering the lack of AT2 binding in the PVN and the SON using receptor autoradiography, we tested the hypothesis that these AT2 receptors are transported to the posterior pituitary. Western blot analysis detected AT2 immunoreactivity in the posterior pituitary. However, no AT2 binding was detected in posterior pituitary membranes, and no AT2 binding was detected with quantitative receptor autoradiography in the neurohypophysis. Thus, if AT2 receptors are transported from the magnocellular vasopressin neurons to the posterior pituitary, their role in AngII regulation of vasopressin release at the neurohypophyseal terminals remains to be clarified.

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.

Immunohistochemical localization of type-II (AT2) angiotensin receptors with a polyclonal antibody against a peptide from the C-terminal tail

A polyclonal antibody has been prepared against a synthetic peptide derived from the C-terminal tail of the cloned rat AT2 angiotensin receptor, corresponding to amino acid residue 341-351. The antibody was of high titer and displayed monospecific activity toward the synthetic peptide in the ELISA assay. Western blot analysis indicated that the antiserum recognised only a single protein band with a mean apparent molecular mass of 75.4 kDa in the rat adrenals. Immunohistochemical studies with affinity purified antibody localised immunoreactive AT2 angiotensin receptor in medulla cells of the adrenals. Immunoreactivity was also observed in pyramidal tract, but no specific immunoreactivity can be detected in regions of rat brain that are known to express AT2 angiotensin receptors, including inferior olive, locus coeruleus and cerebellum.

Histochemistry in rat brain and spinal cord with an antibody directed at the cholecystokininA receptor

Various neurobiological evidence indicates that A-subtype cholecystokinin (CCKA) receptors are widely distributed through the mammalian neuroaxis despite the sparse localization found by receptor autoradiography. To address this paradox, immunohistochemistry has been performed in rat brain and spinal cord using an antibody directed at a portion of the amino terminal sequence of the CCKA receptor. Immunoreactivity, visualised using diaminobenzidine, was widely and topographically distributed being most concentrated in medulla and spinal cord. Many forebrain areas contained specifically labelled neurones, notably the nucleus accumbens, septum, stria terminalis, habenula, substantia nigra, ventral tegmental area and lateral geniculate nucleus. In medulla, heavily labelled perikarya were found in parabrachial and trigeminal nuclei, while in spinal cord immunoreactivity was localized in dorsal horn. Localization of immunoreactivity was consistent with the reported distribution of CCKA receptor-mediated mechanisms. Our observations represent the first attempt to describe the localization of the CCKA receptor in brain using immunohistochemistry and support its wide functional involvement in the central nervous system.

Cloning and expression of angiotensin II type 2 (AT2) receptors from murine neuroblastoma N1E-115 cells: evidence for AT2 receptor heterogeneity

Homology-based PCR was used to isolate angiotensin II type 2 (AT2) receptor cDNA from murine neuroblastoma N1E-115 cells. Despite subtle differences in the nucleotide sequence (the N1E-115 clone coded for Phe133 as TTC and Gln326 as CAG; base substitutions are in bold-italics), the AT2 receptor protein was identical to other reported murine AT2 clones. When transfected into COS-1 cells, the expressed AT2 receptor displayed high affinity for AngII and for AT2-selective compounds, GTP gamma S-insensitive agonist binding and enhanced agonist binding by dithiothreitol. Previously, we have demonstrated that N1E-115 cells possess two distinct subpopulations of AT2 receptors, defined as peak I and peak III receptors, that can be separated by heparin-sepharose chromatography. The two subpopulations differ pharmacologically, biochemically and immunologically. The binding properties of the cloned AT2 receptor closely resembled that of peak III receptors. Moreover, antisera raised against peak I AT2 receptors failed to immunoreact to either peak III receptors or cloned AT2 receptors expressed in COS-1 cells. Collectively, these data suggest that the cloned AT2 receptor is identical to peak III receptors from N1E-115 cells and that a novel AT2 receptor (peak I) remains to be cloned.

Sodium intake, angiotensin II receptor blockade, and baroreflex function in conscious rats

The hypothesis that endogenous angiotensin II (Ang II) chronically supports baroreflex control of lumbar sympathetic nerve activity (LSNA) and heart rate (HR) via AT1 but not AT2 receptors was tested in conscious, normotensive rats. Rats were fed either a sodium deficient diet (LS) to increase circulating Ang II or a high-sodium diet (HS) for 2 to 3 weeks. One to two days after surgery to implant catheters and nerve electrodes, baroreflex curves were produced before and 40 minutes after intravenous administration of the AT1 antagonist losartan (10 mg/kg) or the AT2 antagonist PD123319 (500 micrograms/kg + 50 micrograms.kg-1.min-1). Mean arterial pressure (MAP) after losartan was maintained at basal levels with methoxamine. Forty minutes after losartan in LS rats, LSNA (46 +/- 5 to 22 +/- 1% max) and HR (414 +/- 7 to 387 +/- 8 bpm) were decreased (P < .05). Losartan decreased reflex control of LSNA more in LS than in HS rats (P < .05), as indicated by reductions in maximum LSNA (98 +/- 2 to 78 +/- 3% max) and minimum LSNA (42 +/- 5 to 21 +/- 5% max). Losartan also shifted reflex control of LSNA to a lower pressure in both groups, but the effect was larger in LS rats (-21 +/- 3 [LS] versus -9 +/- 2 [HS] mm Hg at basal LSNA; P < .05). Maximum gain was unaltered in either group. Similarly, losartan reduced maximum HR (534 +/- 6 to 495 +/- 9 bpm) and shifted the HR curve leftward (114 +/- 5 to 105 +/- 4 mm Hg) in LS but not in HS rats. In general, no changes were observed in MAP or baroreflex control of LSNA and HR after PD123319 in LS rats. These results suggest that in conscious, normotensive LS rats, endogenous Ang II supports LSNA and HR over a wide MAP range via AT1 but not AT2 receptors.

Transient upregulation of the AT2 receptor mRNA level after global ischemia in the rat brain

The present study examined changes in angiotensin receptors (AT1 and AT2) and angiotensinogen mRNA level after global ischemia in the rat brain. The AT2 mRNA level increased by three-fold in both the cortex and hippocampus, which are known to be sensitive to ischemic injury, 3 h after ischemia. The increase thus appeared only during the early reperfusion period. In the striatum, amygdala and cerebellum, the level increased moderately 3 h and/or 24 h after ischemia; there was no change in the hypothalamus. On the other hand, the AT1A and AT1B receptor mRNA levels were not altered in the cortex or hippocampus during the early reperfusion period, even 3 h and 24 h after ischemia. There was no significant alteration in angiotensinogen mRNA level 3 h or 24 h after ischemia. These results suggest that the transient upregulation of AT2 receptor mRNA occurs in the cortex and hippocampus after injury and these changes may be in some way related to the molecular events which lead to delayed neuronal cell death.

The role of angiotensin II in ingestive behaviour: a brief review of angiotensin II, thirst and Na appetite

From the outset, the study of angiotensin II (Ang II) in body fluid homeostasis has been both complicated and intriguing. Since the publication of an early report of the dipsogenic action of this peptide, the pursuit of the role of Ang II in thirst and Na appetite has continued for the last 25 years. This pursuit captured the attention of all workers interested in the behavioural/physiological regulation of body fluid balance, with major contributions being made by James T. Fitzsimons and his colleagues. In spite of its powerful dipsogenic actions, delineation of its precise role in physiological thirst has been elusive and difficult to demonstrate. The influence of Ang II on Na intake took longer to show convincingly. However, in contrast to thirst, the role of Ang II in physiological Na appetite has been demonstrated clearly. The technological advances made during the recent years have greatly increased our ability to delineate the neurobiological context of Ang II-mediated responses. Thus, the future is promising in regard to illuminating the subtleties of the role of Ang II in body fluid balance.

Effects of non-peptide angiotensin II-receptor antagonists on pentylenetetrazol kindling in mice

The effects of non-peptide AT1- and AT2-receptor antagonists DuP 753 (losartan) and PD 123319 on the intensity of pentylenetetrazol (PTZ)-kindled seizures in mice were studied. PTZ was injected intraperitoneally at a subconvulsive dose of 40 mg/kg at 48 h until the appearance of clonic seizures. DuP 753 administered intracerebroventricularly (i.c.v.) tended to decrease seizure intensity. Successive administration of ineffective doses of DuP 753 (losartan) and AT2 (angiotensin II) significantly decreased seizure intensity. PD 123319 (i.c.v.) decreased seizure intensity. Combination of ineffective doses of PD 123319 and AT2 also significantly decreased seizure intensity. The results suggest the role of AT2 receptor and its subtypes in PTZ-kindled seizures as well as an action of DuP 753 and PD 123319 similar to the action of AT2, an AT2-receptor agonist.

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.

Immunological analysis of angiotensin AT2 receptors in peripheral tissues of neonatal and adult rats

The peptide hormone angiotensin II (Ang II) is an important regulator of cardiovascular and body fluid homeostasis. Circulating Ang II mediates its physiological actions by activating Ang II receptors in both the periphery and the brain. Previous studies have demonstrated that Ang II receptor expression is high in neonates and decreases to adult levels as the animal matures. A greater proportion of this decline is due to reduced expression of the Type 2 (AT2) receptor subtype. In order to further investigate the expression of this receptor subtype, AT2-directed antisera were utilized to determine the expression of AT2 receptors in both neonatal and adult rat peripheral tissues by immunoblot analysis. The pattern of AT2 receptor immunoreactivity was largely consistent with previous studies employing autoradiographic and radioligand binding assays in peripheral tissues. However, AT2 receptor immunoreactivity was not seen in the adrenal, despite earlier reports of AT2 receptor expression in this peripheral tissue. These immunohistochemical studies also suggested that AT2 receptors undergo tissue-specific post-translational processing during development. Collectively, these results identify immunoreactive AT2 receptor populations in neonatal and adult rat peripheral tissues and further strengthen the hypothesis of AT2 receptor heterogeneity.

Localization of AT2 angiotensin II receptor gene expression in rat brain by in situ hybridization histochemistry

To localize the gene expression of AT2 angiotensin II receptors in rat brain we performed in situ hybridization histochemistry using 35S-labeled antisense riboprobes. The AT2 receptor mRNA expression pattern was compared in consecutive brain sections, from 2 week old rats, with the receptor expression by means of [125I]Sar1-ANG II binding and displacement with AT2 selective ligands followed by autoradiography. Expression of AT2 receptor mRNA was found in several thalamic nuclei (ventral posterolateral, mediodorsal, central medial, paracentral, and paraventricular), the medial geniculate nuclei, the nucleus of the optic tract, the subthalamic nucleus, the interposed nucleus of the cerebellum, and in the inferior olive. In these areas the AT2 receptor gene expression corresponds well with [125I]Sar1-ANG II binding. In addition, AT2 receptor mRNA expression was found in the red nucleus where no [125I]Sar1-ANG II binding was present. No significant hybridization of the AT2 receptor antisense probe was found in septal nuclei, the locus coeruleus, the dorsolateral geniculate nucleus, or the cerebellar cortex, areas rich in [125I]Sar1-ANG II binding. Our results indicate that some brain regions may be involved in AT2 receptor formation, transporting the receptor protein to other brain areas. However, in most structures, both the formation and expression of receptors occur, suggesting the existence of local AT2 receptor circuits, or that of AT2 autoreceptors. Other structures express only the receptor protein, indicating that these AT2 receptors are produced elsewhere. Our present data are the basis for further studies on the clarification of AT2 receptor pathways in the brain.

Spatial heterogeneity of tumour blood flow modification induced by angiotensin II: relationship to receptor distribution

Angiotensin II (ATII) has potential for improving delivery of blood-borne anti-cancer agents to tumours by increasing tumour blood flow. However, ATII-induced hypertension is not always accompanied by an increase in tumour blood flow due to significant constriction of the tumour vasculature. Such unpredictability in tumour response to ATII limits the clinical usefulness of this approach. In this study, the potential of assessing numbers of binding sites for ATII as a predictor of tumour blood flow response to intravenous administration of ATII was investigated. The distribution of ATII receptors in the rat P22 carcinosarcoma was related to tumour blood flow distribution and blood flow response to ATII using an autoradiographic approach. ATII (0.2 microgram x kg-1 x min-1) increased mean arterial blood pressure of anaesthetized BD9 rats from 92.2 +/- 1.4 mmHG to 145.6+- 1.3 mmHg. Despite this increase in perfusion pressure, overall tumour blood flow to viable regions decreased by 20%, indicating significant constriction of tumour blood vessels. Autoradiographic localisation of tumour blood flow showed that the decrease in flow was confined to the tumour periphery, with no change at the tumour centre. This pattern was consistent with 10% more binding sites for ATII at the tumour periphery than at the tumour centre. Maximum number of binding sites (BLmax) for the P22 tumour was 0.38 +/- 0.09 fmol x mg-1, which is approximately a factor of 10 lower than published values for various normal tissues. The dissociation constant Kd was l.16 +/- 0.18 nM. These results encourage the development of techniques for analysis of receptor binding characteristics for predicting the response of individual tumours to blood flow manipulation using vasoactive agents.