Autoradiography of atrial natriuretic peptide (ANP) receptors in the rat brain.

Quantitative autoradiography was used to localize and characterize atrial natriuretic peptide (ANP) receptors in the rat brain and to study their regulation. Peptide receptors are selectively located to circumventricular organs outside the blood brain barrier, such as the subfornical organ, and to brain areas involved in fluid and cardiovascular regulation. Dehydration, either by water deprivation of normal rats, or chronic dehydration present in homozygous Brattleboro rats lacking vasopressin, results in large increases in ANP binding in receptor number in the subfornical organ. In the deoxycorticosterone acetate (DOCA)-salt hypertensive model, only salt treatment, but not DOCA alone or the combination of DOCA-salt, increased the ANP receptor number in the subfornical organ and the choroid plexus. Both young and adult genetically hypertensive rats have a greatly decreased ANP receptor number in the subfornical organ and the choroid plexus. Selective displacement with an inactive analog lacking the disulfide bond (ANP 111-126) suggests that genetically hypertensive rats may lack C (clearance) atrial natriuretic peptide receptors. Our results implicate brain atrial natriuretic peptide receptors in the central response to alterations in fluid regulation and blood pressure.     

Selective Chronic Sodium or Chloride Depletion Specifically Modulates Subfornical Organ Atrial Natriuretic Peptide Receptor Number in Young Rats

1. We studied the effects of selective chronic sodium depletion of chloride depletion on atrial natriuretic peptide receptor number in the subfornical organ and paraventricular nucleus of young rats.2. Sodium or chloride depletion decreased plasma levels of atrial natriuretic peptide, increased plasma renin activity, and induced extracellular fluid volume contraction. Chloride depletion induced more significant changes in extracellular fluid volume contraction than sodium depletion.3. In the subfornical organ, atrial natriuretic peptide receptor number significantly decreased (30%) after sodium depletion, while chloride depletion induced a smaller, not statistically significant decrease. Conversely, atrial natriuretic peptide receptors located in the paraventricular nucleus of young rats were not significantly affected by sodium or chloride depletion.4. Water deprivation reversed the decrease in atrial natriuretic peptide receptors produced by sodium depletion. Water-deprived sodium-depleted rats actually had higher numbers of atrial natriuretic peptide receptors in the subfornical organ than control rats. These changes were associated with severe extracellular fluid volume contraction and up regulation of brain vasopressin mRNA steady-state levels. Thus, the direction of change in the number of subfornical organ atrial natriuretic peptide receptors was dependent on the degree of extracellular fluid volume contraction.5. Our results suggest that atrial natriuretic peptide receptors located in the subfornical organ, and not in the paraventricular nucleus, are selectively regulated by sodium depletion and extracellular fluid volume contraction.     

Increased atrial natriuretic peptide (6-33) binding sites in the subfornical organ of water deprived and Brattleboro rats

Binding sites for rat atrial natriuretic peptide (6-33) (ANP) were quantitated in the subfornical organ of chronically dehydrated homozygous Brattleboro rats unable to synthesize vasopressin; heterozygous Brattleboro rats, their controls, Long Evans rats and Long Evans rats after 4 days of water deprivation. Brain sections were incubated in the presence of 125I-ANP and the results analyzed by autoradiography coupled to computerized microdensitometry and comparison to 125I-standards. Brattleboro rats and water deprived Long Evans rats presented a higher number of ANP binding sites than their normally hydrated controls. Our results suggest a role of ANP binding sites in the subfornical organ in the central regulation of fluid balance and vasopressin secretion.     

Expression of certain proteins in the subfornical organ and cerebrospinal fluid of spontaneously hypertensive rats

The objective of this study was to analyze the proteins in the cerebrospinal fluid of spontaneously hypertensive rats and to study their possible role in the relationship between hydrocephalus, arterial hypertension and variations in the subfornical organ. Brains and cerebrospinal fluid from control Wistar-Kyoto rats and spontaneously hypertensive rats sacrificed with chloral hydrate were used. Cerebrospinal fluid and extract of subfornical organ were processed by protein electrophoresis. Antisera against protein bands of 141, 117 and 48 kDa and Concanavalin A were used for immunohistochemical and western blot study of the subfornical organ, adjacent circumventricular structures and cerebrospinal fluid. Ventricular dilation in the spontaneously hypertensive rats and the presence of quite a lot of protein bands in the cerebrospinal fluid of the hypertensive rats, which were either not observed or scarcely present in the cerebrospinal fluid of the Wistar-Kyoto rats, were confirmed. The subfornical organ, third ventricle ependyma and choroideus plexus showed immunoreactive material for antibodies against 141kDa, 117 and 48 kDa proteins band (anti-B1, anti-B2 and anti-B3). The larger amount of the immunoreactive material was found in the subfornical organ of the spontaneously hypertensive rat. Our results and the alterations observed by other authors in the subfornical organ in hydrocephalic and hypertensive rats support the possibility that this circumventricular organ, some proteins of the cerebrospinal fluid and ventricular dilation could be connected with the physiopathology of this type of hypertension.     

Overlapping distributions of receptors for atrial natriuretic peptide and angiotensin II visualized by in vitro autoradiography: morphological basis of physiological antagonism

Atrial natriuretic peptides exert actions on many key organs involved in blood pressure and water and electrolyte balance. Many of these actions result in a physiological antagonism of angiotensin. To investigate the morphological basis of this interaction, we have mapped the distribution of receptors for atrial natriuretic peptide and angiotensin II in a number of target organs, using 125I-labelled rat atrial natriuretic peptide (99-126) and 125I-labelled [Sar1,Ile8]angiotensin II. In the kidney both atrial natriuretic peptide and angiotensin II receptors were observed overlying glomeruli, vasa recta bundles (high densities), and the outer cortex (moderate density). In the other tissues studied, atrial natriuretic peptide and angiotensin II receptors were codistributed in the adrenal zona glomerulosa, cerebral circumventricular organs including the subfornical organ, organum vasculosum of the lamina terminalis and area postrema, and the external plexiform layer of the olfactory bulb. The concurrent distribution of specific receptors for both peptides at these sites provides the basis for atrial natriuretic peptide to exert a functional antagonism of the actions of angiotensin II on blood pressure and water and electrolyte homeostasis at multiple sites.     

Aquaporin-1 in blood vessels of rat circumventricular organs

Although the water channel protein aquaporin-1 (AQP1) is widely observed outside the rat brain in continuous, but not fenestrated, vascular endothelia, it has not previously been observed in any endothelia within the normal rat brain and only to a limited extent in the human brain. In this immunohistochemical study of rat brain, AQP1 has also been found in microvessel endothelia, probably of the fenestrated type, in all circumventricular organs (except the subcommissural organ and the vascular organ of the lamina terminalis): in the median eminence, pineal, subfornical organ, area postrema and choroid plexus. The majority of microvessels in the median eminence, pineal and choroid plexus, known to be exclusively fenestrated, are shown to be AQP1-immunoreactive. In the subfornical organ and area postrema in which many, but not all, microvessels are fenestrated, not all microvessels are AQP1-immunoreactive. In the AQP1-immunoreactive microvessels, the AQP1 probably facilitates water movement between blood and interstitium as one component of the normal fluxes that occur in these specialised sensory and secretory areas. AQP1-immunoreactive endothelia have also been seen in a small population of blood vessels in the cerebral parenchyma outside the circumventricular organs, similar to other observations in human brain. The proposed development of AQP1 modulators to treat various brain pathologies in which AQP1 plays a deleterious role will necessitate further work to determine the effect of such modulators on the normal function of the circumventricular organs.     

Blood-brain barrier and atrial natriuretic factor

In brain, binding sites for atrial natriuretic factor (ANF) have been characterized in areas such as circumventricular organs that lack the tight capillary endothelial junctions of the blood-brain barrier and therefore are exposed to circulating peptides. Since atrial natriuretic factor acts directly on vascular endothelium and has been proposed to be actively involved in blood pressure regulation and fluid homeostasis, it is interesting to know whether ANF receptors exist on brain capillaries that constitute the blood-brain barrier and participate in the constant fluid exchange between blood and brain. The present paper reports recent evidence of the presence of ANF receptors located on the structure. It assesses the specific binding of 125I-labelled ANF on bovine brain microvessel preparations and its coupling with a guanylate cyclase system. The potential physiological role of ANF on brain microcirculation and blood-brain barrier functions is discussed.     

Characterization, distribution and plasticity of atrial natriuretic factor binding sites in brain

The [125I]iodotyrosyl derivative of atrial natriuretic factor [( 125I])ANF) apparently binds to a single class of high affinity sites in guinea pig brain membrane preparations. Ligand selectivity pattern reveals that the structural requirements of brain [125I]ANF binding sites are similar to those reported in most peripheral tissues. In vitro receptor autoradiographic studies demonstrate that the brain distribution of [125I]ANF binding sites is species dependent. In rat, high levels of binding are found in olfactory bulb, subfornical organ, area postrema, choroid plexus, and ependyma. In guinea pig, these regions are also enriched with [125I]ANF binding in addition to various thalamic nucleic, amygdala, hippocampus, and cerebellum. In monkey, high densities of sites are seen in the cerebellar cortex. This suggests that brain ANF receptor sites could mediate ANF effects related to the central integration of cardiovascular parameters, as well as other actions not associated with these systems. As in the periphery, it appears that brain [125I]ANF binding sites are associated with guanylate cyclase. Moreover, the density of [125I]ANF receptor binding sites is altered in certain brain regions in spontaneously hypertensive rats and in cardiomyopathic hamsters, demonstrating the plasticity of brain ANF receptors. Thus, ANF and ANF receptors are complementary facets of a new neurotransmitter-neuromodulator system present in mammalian brain.     

Atrial natriuretic peptide binding sites in the brain and pituitary gland of the toad, Bufo marinus: localisation and receptor characterisation

The distribution and nature of 125I-atrial natriuretic peptide binding sites have been examined in the brain and pituitary gland of the toad, Bufo marinus, using tissue section autoradiography, affinity cross-linking and electrophoresis, guanylyl cyclase assays and molecular analysis of natriuretic peptide receptor C (NPR-C) and NPR-GC mRNA expression. The highest density of 125I-atrial natriuretic peptide binding sites occurred in the dorsal pallium, the habenular region, the torus semicircularis, the choroid plexus, and the pituitary gland. Less dense binding was observed in the medial pallium, the thalamic region, the hypothalamus, the optic tectum, and the interpeduncular nucleus. The natriuretic peptide receptor-C specific ligand, C-ANF, displaced the binding in all brain regions; however, some residual binding was observed in the habenular region, the hypothalamus, the choroid plexus, and the pituitary gland. In isolated brain membranes, 1 microM rat atrial natriuretic peptide increased cyclic guanosine monophosphate levels to 90% above basal. Affinity cross-linking followed by reducing electrophoresis showed that 125I-atrial natriuretic peptide bound to proteins of 65 kDa and 135 kDa respectively. Furthermore, molecular analysis demonstrated that natriuretic peptide receptor-C and guanylyl cyclase messenger ribonucleic acid are expressed in the brain. In combination with the autoradiography, the data indicated that atrial natriuretic peptide acting via specific receptors could be important in natriuretic peptide regulation of the brain.     

Atriopeptin prevents angiotensin II-stimulated glucose utilization in the subfornical organ

Previous studies have shown that angiotensin II (ANG II) increases glucose utilization in the subfornical organ and stimulates drinking behavior. We investigated with the deoxyglucose method whether atriopeptin III, an atrial natriuretic peptide (ANP), would prevent this enhanced glucose metabolism and interfere with the drinking response in the presence of ANG II. Two rat models with high circulating levels of ANG II were studied: the homozygous Brattleboro and ANG II-infused Sprague-Dawley rats. ANP decreased the normally enhanced glucose utilization in the subfornical organ in the Brattleboro rat and inhibited ANG II-stimulated glucose metabolism in the subfornical organ of Sprague-Dawley rats. This effect was accompanied by decreased ANG II-stimulated water intake. These findings indicate that ANP may act at the level of subfornical organ to antagonize the dipsogenic action of ANG II.     

Atrial natriuretic peptide in the central nervous system of the rat

1. Studies of the presence of atrial natriuretic peptide immunoreactivity and receptor binding sites in the central nervous system have revealed unusual sites of interest. 2. As a result, numerous studies have appeared that indicate that brain atrial natriuretic peptide is implicated in the regulation of blood pressure, fluid and sodium balance, cerebral blood flow, brain microcirculation, blood-brain barrier function, and cerebrospinal fluid production. 3. Alteration of the atrial natriuretic peptide system in the brain could have important implications in hypertensive disease and disorders of water balance in the central nervous system.     

Asymmetric localization of Notch2 on the microvillous surface in choroid plexus epithelial cells

Notch family molecules are transmembrane receptors that play various roles in contact-dependent cell–cell interactions in a wide range of organs. In the brain, Notch2, but not the other members of Notch, is expressed in the choroid plexus at an exceptionally high level. We immunohistochemically examined the cellular and subcellular localization of Notch2 protein in the choroid plexus using confocal and electron microscopy. Unexpectedly, Notch2 was asymmetrically localized on the microvillous surface of epithelial cells in the choroid plexus of both postnatal and adult rats. This localization pattern of Notch2 suggests its novel and unknown role independent of contact with adjacent cells in the choroid plexus. In organotypic cultures of the choroid plexus, the addition of anti-Notch2 antibody resulted in deformation of microvilli in epithelial cells, which suggests a role of Notch2 in the maintenance of the microvillous structure in choroid plexus epithelial cells.     

Regulation of angiotensin II binding sites in the subfornical organ and other rat brain nuclei after water deprivation

1. Binding sites for angiotensin II have been localized in forebrain and brain-stem areas of water-deprived and control Sprague-Dawley rats, employing autoradiography with computerized microdensitometry. 2. Angiotensin II receptor sites were identified in the organum vasculosum of the lamina terminalis, subfornical organ, paraventricular nucleus, median preoptic nucleus, area postrema, nucleus of the solitary tract, and inferior olive. 3. After dehydration a significant increases in the concentration of angiotensin II receptors was detected only in the subfornical organ. Although there was an increased concentration of angiotensin II binding sites in the organum vasculosum of the lamina terminalis, the median preoptic nucleus, and the paraventricular nucleus after dehydration, these changes did not reach statistical significance. Other brain nuclei investigated did not show differences in angiotensin II binding sites in the dehydrated rats compared to controls. 4. These results indicate that angiotensin II receptors in the subfornical organ may play an important role in fluid homeostasis during dehydration.     

The endocrine heart and natriuretic peptides: histochemistry, cell biology, and functional aspects of the renal urodilatin system

 This review focuses on some selected aspects of the endocrine heart and natriuretic peptides. The endocrine heart is composed of specific myoendocrine cells of the cardiac atria. The myoendocrine cells synthesize and secrete the natriuretic peptide hormones which exhibit natriuretic, diuretic, and vasorelaxant properties. Immunohistochemical analyses show that natriuretic peptides of the A-type and B-type are localized not only in the specific granules of these myoendocrine cells but also in many other organs including the brain, adrenal medulla, and kidney. Also, their receptors are detected in many organs showing the multiple functions of these regulatory peptides. Of the members of the natriuretic peptide family, ANP (ANP for atrial natriuretic peptide; also denominated cardiodilatin, CDD), brain natriuretic peptide (BNP), C-type natriuretic peptide (CNP), and the A-type, including its renal form, urodilatin, are emphasized in this review. Urodilatin is localized in the kidney, differentially processed, and secreted into the urine. The intrarenal synthesis and secretion is the basis for a paracrine system regulating water and sodium reabsorption at the level of the collecting duct. CDD/ANP-1-126, cleaved from a precursor of 126 amino acids in the heart to a 28-amino acid-containing circulating molecular form (CDD/ANP-99-126), and urodilatin (CDD/ANP-95-126) share similar biochemical features and biological functions, but urodilatin may be more involved in the regulation of body fluid volume and water–electrolyte excretion, while circulating CDD/ANP-99-126 is responsible for blood pressure regulation. The physiological and pharmacological properties of these peptides have great clinical impact, and as a consequence urodilatin is involved in drug development for the treatment of acute renal failure, cardiomyopathia, and acute asthma. Accepted: 8 July 1998     

Arginine vasopressin causes morphological changes suggestive of fluid transport in rat choroid plexus epithelium

Summary The experiments described herein use an in vitro preparation of choroid plexus to demonstrate that it is a vasopressin-responsive organ by morphologic criteria. Choroid plexus from rats was incubated for one hour in graded concentrations of arginine vasopressin (AVP). Within physiologic range of molar concentration, incubation in vasopressin induced a decrease in basal and lateral spaces in choroid plexus epithelial cells as well as an increase in number of dark cells. The number of cells with basal spaces decreased significantly from 82.7±9.2 in control tissue to 19±18 in tissue incubated in 10^-12 M AVP; similarly, the number with lateral cellular spaces decreased from 20±8.8 to 7.6±2.2 cells in 10^-10 M AVP. Dark cells increased in number from 3.8±2.6 in control conditions to 49±4 with 10^-9 M vasopressin. These data suggest important effects of arginine vasopressin in cerebrospinal fluid (CSF) on choroid plexus, compatible with enhanced fluid transport across choroid epithelial cells.     

The membrane-associated progesterone-binding protein 25-Dx is expressed in brain regions involved in water homeostasis and is up-regulated after traumatic brain injury

After traumatic brain injury, progesterone has important neuroprotective effects in the nervous system. There is better functional outcome and less oedema formation in pseudopregnant rat females (high levels of endogenous progesterone) than in males. In addition to intracellular progesterone receptors, membrane binding sites of the hormone such as 25-Dx may also be involved in neuroprotection. In the present study we investigated the distribution of the membrane-associated progesterone-binding protein 25-Dx in rat brain. Immunohistochemical analysis showed that 25-Dx is particularly abundant in the hypothalamic area, circumventricular organs, and ependymal cells of the lateral walls of the third and lateral ventricles. A strong signal was also detected in the meninges. Double immunofluorescence immunolabelling and confocal microscopy showed that 25-Dx is co-expressed with vasopressin in neurones of the paraventricular, supraoptic and retrochiasmatic nuclei. Levels of 25-Dx expression were higher in pseudopregnant females than in males. After traumatic brain injury, 25-Dx expression was up-regulated in neurones and induced in astrocytes, which play an important role in regulating water and ion homeostasis. The expression of 25-Dx in structures involved in CSF production (choroid plexus) and in osmoregulation (circumventricular organs, hypothalamus and meninges), and its up-regulation after brain damage, point to a novel and potentially important role of this progesterone-binding protein in the maintenance of water homeostasis after traumatic brain injury.     

Natriuretic peptides in fish physiology

Natriuretic peptides exist in the fishes as a family of structurally-related isohormones including atrial natriuretic peptide (ANP), C-type natriuretic peptide (CNP) and ventricular natriuretic peptide (VNP); to date, brain natriuretic peptide (or B-type natriuretic peptide, BNP) has not been definitively identified in the fishes. Based on nucleotide and amino acid sequence similarity, the natriuretic peptide family of isohormones may have evolved from a neuromodulatory, CNP-like brain peptide. The primary sites of synthesis for the circulating hormones are the heart and brain; additional extracardiac and extracranial sites, including the intestine, synthesize and release natriuretic peptides locally for paracrine regulation of various physiological functions. Membrane-bound, guanylyl cyclase-coupled natriuretic peptide receptors (A- and B-types) are generally implicated in mediating natriuretic peptide effects via the production of cyclic GMP as the intracellular messenger. C- and D-type natriuretic peptide receptors lacking the guanylyl cyclase domain may influence target cell function through G(i) protein-coupled inhibition of membrane adenylyl cyclase activity, and they likely also act as clearance receptors for circulating hormone. In the few systems examined using homologous or piscine reagents, differential receptor binding and tissue responsiveness to specific natriuretic peptide isohormones is demonstrated. Similar to their acute physiological effects in mammals, natriuretic peptides are vasorelaxant in all fishes examined. In contrast to mammals, where natriuretic peptides act through natriuresis and diuresis to bring about long-term reductions in blood volume and blood pressure, in fishes the primary action appears to be the extrusion of excess salt at the gills and rectal gland, and the limiting of drinking-coupled salt uptake by the alimentary system. In teleosts, both hypernatremia and hypervolemia are effective stimuli for cardiac secretion of natriuretic peptides; in the elasmobranchs, hypervolemia is the predominant physiological stimulus for secretion. Natriuretic peptides may be seawater-adapting hormones with appropriate target organs including the gills, rectal gland, kidney, and intestine, with each regulated via, predominantly, either A- or B-type (or C- or D-type?) natriuretic peptide receptors. Natriuretic peptides act both directly on ion-transporting cells of osmoregulatory tissues, and indirectly through increased vascular flow to osmoregulatory tissues, through inhibition of drinking, and through effects on other endocrine systems.     

Glial angiotensinogen regulates brain angiotensin II receptors in transgenic rats TGR(ASrAOGEN)

TGR(ASrAOGEN)680, a newly developed transgenic rat line with specific downregulation of astroglial synthesis of angiotensinogen, exhibits decreased brain angiotensinogen content associated with a mild diabetes insipidus and lower blood pressure. Autoradiographic experiments were performed on TGR(ASrAOGEN) (TG) and Sprague-Dawley (SD) control rats to quantify AT(1) and AT(2) receptor-binding sites in different brain nuclei and circumventricular organs. Dose-response curves for drinking response to intracerebroventricular injections of ANG II were compared between SD and TG rats. In most of the regions inside the blood-brain barrier [paraventricular nucleus (PVN), piriform cortex, lateral olfactory tract (LOT), and lateral preoptic area (LPO)], AT(1) receptor binding (sensitive to CV-11974) was significantly higher in TG compared with SD. In contrast, in the circumventricular organs investigated [subfornical organ (SFO) and area postrema], AT(1) receptor binding was significantly lower in TG. AT(2) receptors (binding sensitive to PD-123319) were detected at similar levels in the inferior olive (IO) of both strains. Angiotensin-binding sites sensitive to both CV-11974 and PD-123319 were detected in the LPO of SD rats and specifically upregulated in LOT, IO, and most notably PVN and SFO of TG. The dose-response curve for water intake after intracerebroventricular injections showed a higher sensitivity to ANG II of TG (EC(50) = 3.1 ng) compared with SD (EC(50) = 11.2 ng), strongly suggesting that the upregulation of AT(1) receptors inside the blood-brain barrier of TG rats is functional. Finally, we showed that downregulation of angiotensinogen synthesized by astroglial cells differentially regulates angiotensin receptor subtypes inside the brain and in circumventricular organs.     

Vascular receptors for angiotensin, vasopressin, and atrial natriuretic peptide in experimental hypertension

Angiotensin II (ANG II) and vasopressin (AVP) are two powerful vasoconstrictors, and atrial natriuretic peptide (ANP) is a potent vasorelaxant. The changes in the density or affinity of binding sites for these agents that may alter target organ responsiveness in hypertension are reviewed. ANG II binding in mesenteric arteries was unaltered in one-kidney, one-clip (1-K, 1-C) and in 2-K, 1-C hypertensive rats, while in deoxycorticosterone acetate (DOCA)-salt hypertensive rats ANG II binding to blood vessels was significantly increased. A role of mineralocorticoids to increase the number of vascular ANG II sites in some hypertensive models is suggested. In spontaneously hypertensive rats (SHR) ANG II receptors were increased in young rats in the prehypertensive stage with respect to Wistar-Kyoto (WKY) control rats, but normal in older rats. AVP binding in the vasculature of hypertensive rats was uniformly decreased in inverse correlation to plasma AVP levels, but vascular responsiveness to AVP was exaggerated. Inositol trisphosphate production by blood vessels of SHR in response to AVP showed that increased AVP receptor-coupled phospholipase C activity may mediate in part the exaggerated pressor response in spite of reduced or normal density of receptors for vasoconstrictor peptides. Vascular ANP sites in 2-K, 1-C, 1-K,1-C, and DOCA-salt hypertensive rats varied inversely with plasma concentrations of ANP. Normal densities of ANP receptors in saralasin-sensitive 2-K, 1-C hypertensive rats correlated with ANP sensitivity, while saralasin-insensitive 2-K, 1-C hypertensive rats, which did not respond to ANP, had significantly decreased density of ANP vascular receptors.(ABSTRACT TRUNCATED AT 250 WORDS)