Regulation of atrial natriuretic peptide receptors in the rat brain

ENaC-expressing neurons in the sensory circumventricular organs become c-Fos activated following systemic sodium changes

The sensory circumventricular organs (CVOs) are specialized collections of neurons and glia that lie in the midline of the third and fourth ventricles of the brain, lack a blood-brain barrier, and function as chemosensors, sampling both the cerebrospinal fluid and plasma. These structures, which include the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), and area postrema (AP), are sensitive to changes in sodium concentration but the cellular mechanisms involved remain unknown. Epithelial sodium channel (ENaC)-expressing neurons of the CVOs may be involved in this process. Here we demonstrate with immunohistochemical and in situ hybridization methods that ENaC-expressing neurons are densely concentrated in the sensory CVOs. These neurons become c-Fos activated, a marker for neuronal activity, after various manipulations of peripheral levels of sodium including systemic injections with hypertonic saline, dietary sodium deprivation, and sodium repletion after prolonged sodium deprivation. The increases seen c-Fos activity in the CVOs were correlated with parallel increases in plasma sodium levels. Since ENaCs play a central role in sodium reabsorption in kidney and other epithelia, we present a hypothesis here suggesting that these channels may also serve a related function in the CVOs. ENaCs could be a significant factor in modulating CVO neuronal activity by controlling the magnitude of sodium permeability in neurons. Hence, some of the same circulating hormones controlling ENaC expression in kidney, such as angiotensin II and atrial natriuretic peptide, may coordinate ENaC expression in sensory CVO neurons and could potentially orchestrate sodium appetite, osmoregulation, and vasomotor sympathetic drive.

ANP-mediated cGMP signaling and phosphodiesterase inhibition in the rat cervical spinal cord

Natriuretic peptides (NP) and the corresponding receptors are present in the rodent spinal cord. We have studied the structures which respond to atrial natriuretic peptide, brain natriuretic peptide, or C-type natriuretic peptide with an increased synthesis of cGMP. NP-responsive cGMP-producing structures were observed in laminae I-III, and X, and in addition in ependymal cells, astrocytes and a subpopulation of dorsal root ganglion cells. As the cGMP concentration is controlled by the rate of synthesis and the rate of breakdown by phosphodiesterases, we studied NP-responsive structures in spinal cord slices incubated in the presence of different phosphodiesterase inhibitors. We studied EHNA and BAY 60-7550 as selective PDE2 inhibitors, sildenafil as a selective PDE5 inhibitors, dipyridamole as a mixed type PDE5 and PDE10 inhibitor, rolipram as a PDE4 inhibitor, and SCH 81566 as a selective PDE9 inhibitor. Double immunostainings showed that cGMP-IR colocalized partial with the vesicular acetylcholine transporter molecule in lamina X, with Substance P in a subpopulation of neuronal fibers situated dorsolateral, and with a subpopulation of CGRP-IR dorsal root ganglion neurons. Colocalization of cGMP-IR was absent with parvalbumin, synaptophysin, and the vesicular transporter molecules for GABA and glutamate. It is concluded that NPs in the spinal cord are probably involved in integrating intersegmental sensory processing in the spinal cord although the greater part of the NP-responsive cGMP-producing fibers could not be characterized. PDE2, 5, and 9 are involved in regulating NP-stimulated cGMP levels in the spinal cord. NPs may have a role in regulating cerebrospinal fluid homeostasis.

The role of carbon monoxide and nitric oxide in hyperosmolality-induced atrial natriuretic peptide release by hypothalamus in vitro

We evaluated the participation of the nitrergic and carbon monoxide (CO) systems in the atrial natriuretic peptide (ANP) release induced by osmotic stimulation of the rat anterior and medial basal hypothalamus (BH) fragments in vitro. The increase in the medium osmolality (NaCl, 340 mOsm/kg H2O) induced an elevated ANP release, which was associated with a decrease in nitric oxide synthase (NOS) activity (p<0.001), nitric oxide (NO) production and nitrate (p<0.001) release into the medium. The NO donors sodium nitroprusside (SNP, 300 microM), S-nitroso-N-acetylpenicillamine (SNAP, 300 microM) and 3-morpholinylsydnoneimine chloride (SIN-1, 300 microM) promoted a significant decrease in ANP release in response to hyperosmolality (p<0.001). ANP release observed in the present study did not result from injury to the BH caused by the increase in medium osmolality nor a toxic effect of the NO donors as demonstrated by the ANP release after incubation with KCl (56 mM). Furthermore, hyperosmolality or NO donors did not increase the LDH content in the medium. The hyperosmotic-induced ANP release and reduction of NOS activity were prevented by the heme oxygenase inhibitor, zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG). In conclusion, these results suggest that NO, the production of which is dependent on CO, modulates the osmolality-induced ANP release by BH fragments.

Receptor autoradiography as mean to explore the possible functional relevance of neuropeptides: focus on new agonists and antagonists to study natriuretic peptides, neuropeptide Y and calcitonin gene-related peptides

Over the past 20 years, receptor autoradiography has proven most useful to provide clues as to the role of various families of peptides expressed in the brain. Early on, we used this method to investigate the possible roles of various brain peptides. Natriuretic peptide (NP), neuropeptide Y (NPY) and calcitonin (CT) peptide families are widely distributed in the peripheral and central nervous system and induced multiple biological effects by activating plasma membrane receptor proteins. The NP family includes atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP). The NPY family is composed of at least three peptides NPY, peptide YY (PYY) and the pancreatic polypeptides (PPs). The CT family includes CT, calcitonin gene-related peptide (CGRP), amylin (AMY), adrenomedullin (AM) and two newly isolated peptides, intermedin and calcitonin receptor-stimulating peptide (CRSP). Using quantitative receptor autoradiography as well as selective agonists and antagonists for each peptide family, in vivo and in vitro assays revealed complex pharmacological responses and radioligand binding profile. The existence of heterogeneous populations of NP, NPY and CT/CGRP receptors has been confirmed by cloning. Three NP receptors have been cloned. One is a single-transmembrane clearance receptor (NPR-C) while the other two known as CG-A (or NPR-A) and CG-B (or NPR-B) are coupled to guanylate cyclase. Five NPY receptors have been cloned designated as Y(1), Y(2), Y(4), Y(5) and y(6). All NPY receptors belong to the seven-transmembrane G-protein coupled receptors family (GPCRs; subfamily type I). CGRP, AMY and AM receptors are complexes which include a GPCR (the CT receptor or CTR and calcitonin receptor-like receptor or CRLR) and a single-transmembrane domain protein known as receptor-activity-modifying-proteins (RAMPs) as well as an intracellular protein named receptor-component-protein (RCP). We review here tools that are currently available in order to target each NP, NPY and CT/CGRP receptor subtype and establish their respective pathophysiological relevance.

Immunocytochemistry of cGMP in the Cerebellum of the Immature, Adult, and Aged Rat: the Involvement of Nitric Oxide. A Micropharmacological Study

In this study we describe the localization of formaldehyde-fixed cGMP-immunoreactivity (cGMP-IR) in rat cerebellar tissue slices incubated in vitro. In the absence of phosphodiesterase inhibition, cGMP-immunofluorescence was of low intensity in tissue slices prepared from immature cerebella. Addition of isobutylmethylxanthine (IBMX) to the incubation medium resulted in the appearance of cGMP-IR in clusters of astrocytes in the internal granular layer. Addition of N-methyl-d-aspartate (NMDA), kainic acid, atrial natriuretic factor (ANF), or sodium nitroprusside (SNP) gave an intense cGMP-IR in Bergmann fibres, Bergmann cell bodies, and astrocytes in the internal granular layer. Astrocytes in the white matter showed cGMP-IR after incubation of the slice in the presence of ANF or nitroprusside, but not after NMDA or kainic acid. In addition, after SNP stimulation of cGMP production, cGMP-IR was found in fibres which were not positive for glial fibrillary acidic protein (GFAP). In the adult cerebellar slice, intense basal cGMP-immunostaining was observed in Bergmann fibres, Bergmann cell bodies, and astrocytes in the granular layer. No cGMP-IR was observed in Purkinje cells. Stimulation of the cGMP-content in the glial structures by NMDA, ANF, or SNP, was suggested by the immunocytochemical results. However, when measured biochemically, only the effect of SNP was statistically significant, and immunocytochemistry showed that SNP clearly stimulated cGMP synthesis in neuronal cell structures. In the cerebellum of the aged rat a reduced cGMP-IR was found compared to the adult, in the same structures which showed cGMP-IR in the adult. Basal cGMP-immunostaining was reduced in the presence of haemoglobin, methylene blue, by inhibiting nitric oxide synthesis with NG-monomethyl-l-arginine (NGMAr), or by depletion of external Ca2+. Also the stimulatory effect of NMDA and of ANF (partly) on the cGMP-IR was inhibited by these compounds. cGMP-IR after stimulation of guanylate cyclase by SNP was reduced by the concomitant presence of haemoglobin or methylene blue, but not by NGMAr, or by omission of Ca2+. Our results point to an important role for cGMP in the functioning of glial tissue in the cerebellum and also suggest a role for nitric oxide as an intercellular mediator in the functioning of glutamate and ANF in the cerebellum.

Immunohistochemical localization of atrial natriuretic factor and autoradiographic distribution of atrial natriuretic factor-binding sites in the brain of the cave salamander Hydromantes genei (Amphibia, Plethodontidae)

The distribution of atrial natriuretic factor (ANF)-like immunoreactivity in the central nervous system of the cave salamander Hydromantes genei (Amphibia, Plethodontidae) was investigated by using antisera raised against rat and human ANF(1-28). Concurrently, the location of ANF-binding sites was determined by autoradiography, using radioiodinated human ANF(1-28) as a tracer. In several regions of the brain, including the olfactory bulb, the preoptic area, the ventral thalamus, the tectum of the mesencephalon, and the choroid plexuses inside the ventricles, a good correlation was observed between the distribution of ANF-immunoreactive elements and the location of ANF-binding sites. Mismatching was found in the habenular nucleus, the commissura habenularis, the fasciculum retroflexus, and the interpeduncular nucleus, which contained high levels of binding sites but were devoid of ANF-immunoreactive structures. In contrast, a few other regions, such as the pineal gland and the subcommissural organ, showed a high concentration of ANF-like immunoreactivity but did not contain ANF-binding sites. This study provides the first localization of ANF-like immunoreactivity and ANF-binding sites in the brain of an urodele amphibian. The results show that the ANF peptidergic system in the cave salamander has an organization more simple than the organizations described for the brain of frog or other vertebrates. This feature is probably related to the expression of highly pedomorphic characters in plethodontids. The anatomical distribution of ANF-immunoreactive elements and ANF-binding sites suggests that ANF-related peptides may act as hypophysiotropic hormones as well as neurotransmitters and/or neuromodulators in the salamander brain.

Decreased expression of natriuretic peptide A receptors and decreased cGMP production in the choroid plexus of spontaneously hypertensive rats

Atrial natriuretic peptide receptor (ANP) subtypes and their signal transduction response were characterized in choroid plexus of spontaneously hypertensive (SHR) and normotensive (WKY) rats. We found two ANP receptor subtypes, guanylate cyclase coupled and uncoupled, in both rat strains. Binding of ANP was lower in SHR choroid plexus when compared to WKY. The lower ANP binding in SHR was the result of a decrease of binding to the guanylate cyclase-coupled receptor subtype A, a decrease that correlated well with the decreased ANP-induced cGMP formation in SHR. Forskolin stimulated cGMP production to the same extent in both strains. In WKY rats, ANP increased basal and forskolin-stimulated cAMP production; conversely, in SHR, ANP did not affect the basal level of cAMP and inhibited the forskolin-stimulated cAMP production. These results demonstrate differences in ANP receptor subtype expression, and ANP signal transduction in choroid plexus of hypertensive and normotensive rats, which is of possible significance to the central mechanisms of blood pressure control.

Biochemical and cytochemical analyses of BNP-stimulated guanylate cyclase in frog choroid plexus

This study shows that in the choroid plexus of Rana esculenta particulate guanylate cyclase (GC) is appreciably stimulated by porcine brain natriuretic peptide (BNP). Ultracytochemical tests for GC show that BNP notably increases the enzymatic reaction product along the apical surfaces of the epithelial cells. It can therefore be hypothesized that the apical zone of the epithelial cells possess receptors which have a particular affinity for BNP produced in the central nervous system and dumped into the cerebrospinal fluid. These results, together with those of a previous study [32], confirm that the choroid plexus is an organ which has receptors for the natriuretic peptides which are involved in the processes of osmoregulation and the control of cerebrospinal fluid production.

Influence of humoral control peptides on medullary vasomotor control neurons: microstimulation and double-labeling studies using SHR and WKY rats

To study the influence of humoral control peptides on medullary vasomotor control neurons, angiotensin II (AII), arginine vasopressin (AVP) and atrial natriuretic peptide (ANP) were microinjected into three vasomotor control areas, i.e., the area postrema (AP), the nucleus tractus solitarii (NTS) and the rostral ventrolateral medulla (RVLM), of spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY), and the evoked cardiovascular response was observed. Unlike the injection areas, the threshold dose of one peptide for the cardiovascular response was similar, but the threshold dose differed from peptide to peptide. The threshold dose was lower for AII (0.15-0.29 pmol), in-between for ANP (0.9-1.5 pmol) and higher for AVP (14-30 pmol). No significant difference in the threshold dose was observed between SHR and WKY, suggesting that hypertension in SHR may not be due to the abnormal sensitivity to the three peptides of the vasomotor control neurons in the AP, NTS, and RVLM. The structural basis of the results of the microstimulation experiment was supported by the double-labeling study. The NTS neurons were innervated by (1) the AII-immunoreactive (ir) neurons in both sides of the lateral hypothalamic area (LH), the RVLM and the caudal ventrolateral medulla, and (2) the ANP-ir neurons in both sides of the paraventricular nucleus (Pa) and the LH. The RVLM neurons were innervated by (1) the AII-ir neurons in both sides of the LH and ipsilateral side of the lateral parabrachial nucleus (Pbl) and (2) the ANP-ir neurons in the ipsilateral Pbl. There was no evidence that the AVP-ir neurons in the Pa and the supraoptic nucleus innervate the NTS and the RVLM neurons, or that the AII, ANP or AVP-ir neurons innervate the AP neurons. This study suggests that in common with SHR and WKY rats AII and ANP may influence both the NTS and RVLM not by the humoral pathway but by the neural pathway, and AVP may not influence the three vasomotor control areas by the neural pathway.

The neuroanatomic binding pattern of [125I]atrial natriuretic factor in the Japanese quail brain

Application of quantitative autoradiography technique provided a discrete anatomical distribution pattern of the atrial natriuretic factor (ANF) in the Japanese quail, Coturnix coturnix japonica, brain. The highest binding levels of [125I]ANF were shown to occur in telencephalon areas, such as fasciculus diagonalis Brocae (232 fmol/mg protein), septum (194 fmol/mg protein) and olfactory bulb (153 fmol/mg protein), and in posterior sites, such as nucleus interpeduncularis (177 fmol/mg protein), while lower levels (> 51 < 87 fmol/mg protein) were found in the hypothalamic sites of the diencephalon. The similar ANF receptor density levels in some brain areas of the quail as well as both mammalian and non-mammalian species suggest that this peptide might be involved in osmoregulatory activities (at the brain level) and furthermore indicate a probable functional conservation of ANF in vertebrates.

Neuroendocrine regulatory mechanisms in the choroid plexus-cerebrospinal fluid system

The CSF is often regarded as merely a mechanical support for the brain, as well as an unspecific sink for waste products from the CNS. New methodology in receptor autoradiography, immunohistochemistry and molecular biology has revealed the presence of many different neuroendocrine substances or their corresponding receptors in the main CSF-forming structure, the choroid plexus. Both older research on the sympathetic nerves and recent studies of peptide neurotransmitters in the choroid plexus support a neurogenic regulation of choroid plexus CSF production and other transport functions. Among the endocrine substances present in blood and CSF, 5-HT, ANP, vasopressin and the IGFs have high receptor concentrations in the choroid plexus and have been shown to influence choroid plexus function. Finally, the choroid plexus produces the growth factor IGF-II and a number of transport proteins, most importantly transthyretin, that might regulate hormone transport from blood to brain. These studies suggest that the choroid plexus-CSF system could constitute an important pathway for neuroendocrine signalling in the brain, although clearcut evidence for such a role is still largely lacking.

Localization of binding sites for atrial natriuretic factor and angiotensin II in the central nervous system of the clawed toad Xenopus laevis

The distribution of binding sites for atrial natriuretic factor (ANF) and angiotensin II (A II) was investigated in the central nervous system (CNS) of the clawed toad Xenopus laevis by means of in vitro autoradiography using [125I]-rat ANF(99-126) or [125I] [Val5] A II and [125I]human A II as labeled ligands. The highest densities of specific ANF-binding were detected in the nucleus habenularis, thalamic regions, hypophyseal pars nervosa and nucleus interpeduncularis. Moderate ANF-binding was found in the bulbus olfactorius, pallium, septum, striatum, lateral forebrain bundle, nucleus infundibularis, hypophyseal pars distalis and tectum. The highest levels of specific A II binding sites were observed in the nucleus praeopticus, nucleus habenularis, hypophyseal pars nervosa and pars distalis, whereas the amygdala contained moderate A II binding. The existence of specific binding sites for ANF and A II in the CNS of Xenopus laevis suggests that both peptides act as neurotransmitters or neuromodulators in the amphibian CNS. The co-localization of dense binding sites for both peptides in the nucleus habenularis, hypophyseal pars nervosa and pars distalis supports the view that ANF and A II have opposite regulatory functions in these regions.

HS-142-1, a novel non-peptide ANP antagonist, blocks the cyclic GMP production elicited by natriuretic peptides in PC12 and NG108-15 cells

HS-142-1 is a novel non-peptide antagonist for atrial natriuretic peptide (ANP) receptor. The effect of HS-142-1 on the cyclic GMP production elicited by natriuretic peptides in neuronal cell lines, PC12 and NG108-15 was examined. Natriuretic peptides such as ANP, brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) enhanced cyclic GMP production in a dose-dependent manner. HS-142-1 inhibited cyclic GMP accumulation elicited by natriuretic peptides in a dose-dependent fashion in both cells. The results suggest that HS-142-1 will be an important tool for identification and understanding of the mechanisms by which natriuretic peptides act in nervous systems.

ANF-induced modulation of ADH-release in the rabbit and Pekin duck

The atrial natriuretic factor (ANF) as an osmoregulatory hormone causes a reduction of extracellular fluid volume primarily through stimulation of renal and extrarenal water and sodium elimination. Consequently, ANF counteracts the renin-angio-tensin II-aldosterone (RAAS) and the antidiuretic hormone (ADH) systems at their target organ level. The possible direct interaction of ANF with the hypothalamo-neurohypophyseal ADH system was investigated in conscious ducks and rabbits during conditions of eu- and dehydration. In euhydrated animals, the plasma concentration of ADH remained unchanged during the systemic infusion of species-specific ANF, whereas in dehydrated rabbits but not ducks, the plasma concentration of ADH was significantly decreased. These differences in ADH modulation were supported by the localization of binding sites for radiolabeled ANF at the sites of ADH release, the median eminence (ME) and neurohypophysis (NH) of the rabbit but not duck brain, using receptor-autoradiography. For both species, circumventricular organs lacking a functional blood-brain barrier (BBB) such as the subfornical organ (SFO), the organum vasculosum of the laminae terminalis (OVLT), the pineal and the choroid plexus (ChP), as well as the ependymal lining of the third ventricle (VIII) were labeled specifically. Within the BBB, binding sites for ANF could not be detected in the ADH-synthesizing paraventricular (PVN) and supraoptic nuclei (SON) of either species, however, sites were observed in the anterior median nucleus of the hypothalamus (AM) of the duck brain. In the AM as well as the PVN and ME, the existence of a brain-intrinsic ANF system could be demonstrated for the Pekin duck using immunocytochemistry.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic peptide in the locus coeruleus and its possible role in the regulation of arterial blood pressure, fluid and electrolyte homeostasis

Atrial natriuretic factor (ANP) is present in neuronal cells of the locus coeruleus and its vicinity in the pontine tegmentum and moderate amount of ANP is detectable in this area by radioimmunoassay. The ANP (both peripheral and brain-born) is known as a neuropeptide which may influence the body salt and water homeostasis and blood pressure by targeting both central and peripheral regulatory mechanisms. Whether this pontine ANP cell group is involved in any of these regulatory mechanisms, the effect of various types of hypertension and experimental alterations in the salt and water balance on ANP levels was measured by radioimmunoassay in the locus coeruleus of rats. Adrenalectomy, as well as aldosterone and dexamethasone treatments failed to alter ANP levels in the locus coeruleus. Reduced ANP levels were measured in spontaneously hypertensive (both young and adult) rats, and in diabetes insipidus (Brattleboro) rats with vasopressin replacement. In contrast to these situations, elevated ANP levels were found in rats with DOCA-salt or 1-kidney-1-clip hypertension. These data suggest a link between ANP levels in the locus coeruleus and fluid volume homeostasis. Whether this link is causal and connected with the major activity of locus coeruleus neurons (noradrenergic influence on brain regulatory activities) needs further informations.

Atrial natriuretic Peptide secretion from fetal rat diencephalon in culture

Abstract The presence of a distinct brain pool of the atrial natriuretic peptides (ANP) has been established. To determine the molecular forms and regulation of secretion of ANP, we studied fetal rat diencephalic neurons and glia in primary culture. ANP immunoreactivity determined by radioimmunoassay was found only in the neuron predominant cultures. The neurons contained mainly ANP (103-126) and less ANP (102-126), but secreted only ANP (103-126) into the medium after potassium and glutamate-dependent depolarization. Little, if any, ANP (99-126), the predominant form which circulates in plasma and originates from the heart, was secreted. The ability of potassium and glutamate to cause a mean 50% increase of ANP secretion above baseline was abolished after deleting calcium chloride from the medium. In contrast, hypo- or hyperosmolarity or increased sodium content in the incubation medium did not influence ANP secretion. These studies indicate that regulative secretion of ANP occurs from primary cultures of predominantly diencephalic neurons, probably accounting for the high concentrations of these peptides in this area of the brain. The forms of ANP contained within the cells and secreted after depolarization are different from ANP secreted from neonatal rat atrial myocytes. In contrast to myocytes, varying sodium or osmolarity did not cause ANP secretion. We postulate that influences on ANP production/secretion in the brain may be distinct from the heart.

A functional parameter to study heterogeneity of glial cells in rat brain slices: cyclic guanosine monophosphate production in atrial natriuretic factor (ANF)-responsive cells

Stimulation of guanylate cyclase in vitro by atrial natriuretic factor (ANF) or sodium nitroprusside was studied in rat brain tissue slices biochemically as well as by means of cyclic guanosine monophosphate (cGMP) immunocytochemistry. The ANF-responsive, cGMP-producing cells were studied in the olfactory bulb, the septal area, the hippocampus, the medial amygdala, and the medial preoptic area. These cells, having the ANF-stimulated particulate guanylate cyclase, were characterized as astroglial cells on the basis of their glial fibrillary acidic protein (GFAP) immunostaining, although not all astroglial cells in these areas could be identified as cGMP-immunoreactive cells. Sodium nitroprusside-stimulated soluble guanylate cyclase activity was demonstrated in neuronal cell bodies and varicose fibers and was associated with blood vessel walls. Upon maturation, a significant decrease in cGMP production was found after stimulation by 100 nM ANF-(103-126) in the olfactory bulb, the medial amygdala, and the hippocampus, but not in the septal area; no change was found in these areas in cGMP content after stimulation of cGMP production by 10 microM sodium nitroprusside. Via cGMP immunocytochemistry, no qualitative differences were seen in the ANF-responsive, cGMP-producing cells upon maturation.

Central and peripheral effects of the peptide ANF on renal function and blood pressure in hypertensive rats

1. The present study assesses renal and blood pressure effects of systemically and intracerebroventricularly (I.C.V.) administered atrial natriuretic factor (ANF) in anaesthetized New Zealand genetically hypertensive (NZGH) rats and their normotensive substrain (NZN). 2. Plasma ANF concentration was significantly raised in NZGH compared with NZN animals. Intravenous ANF administration increased circulating ANF concentration to similar levels in NZGH and NZN rats. Plasma aldosterone concentrations were initially similar in normotensive and hypertensive animals and were reduced markedly by I.V. ANF administration in both groups. 3. Peripheral administration of ANF produced a significant and sustained hypotension during the period of the hormone administration in NZGH animals, while arterial mean blood pressure (MBP) was not altered significantly in NZN rats. Central infusion of ANF produced no change in MBP in either NZGH or NZN animals. There was no detectable change in heart rate during central or peripheral administration of the hormone in either group. 4. By comparison with animals maintained on hormone-free infusate, urine flow changed little over the 80 min period of I.V. ANF infusion in NZGH rats but was markedly increased in NZN animals. Urinary sodium excretion was elevated during ANF infusion in both NZGH and NZN animals. Central infusion of ANF produced a diuresis in NZN rats but a slight reduction in urine flow in NZGH animals, while sodium excretion was not affected in either group. 5. It appears from the present study that central and peripheral ANF effects on renal function and blood pressure differ in hypertensive and normotensive animals. These differences may in part reflect adjustments to long-term elevation in plasma ANF in hypertension and in part are possibly associated with disturbances in related endocrine mechanisms.

cGMP-Producing, Atrial Natriuretic Factor-Responding Cells in the Rat Brain

Using an in vitro incubation method, we stimulated cGMP production in rat brain slices by rat ANF-(103 - 126). The localization of the cells responding to this ANF stimulation with an increase in cGMP production was studied by cGMP immunocytochemistry. ANF-responding cells were found in specific loci throughout the central nervous system of the rat. Regions containing the highest number of these cells were: the olfactory bulb, the lateral septum, the bed nucleus of the accessory olfactory tract, the mediobasal amygdala, the central grey area, the medial vestibular nucleus, and the nucleus of the solitary tract. Scattered ANF-responding, cGMP-immunoreactive cells were found in the hippocampus, the cingulate cortex, the ventral pallidum, the medial preoptic area, and the endopeduncular nucleus. ANF-responding cells in these areas had the same morphology, that is, multipolar with numerous processes. The nature of these ANF-responding cells was studied by sequential staining with an antiserum against glial fibrillary acidic protein (GFAP). In the hippocampus it was demonstrated that all ANF-responding cells are astroglial cells. However, not all astroglial cells in this area showed a cGMP response, demonstrating a regional heterogeneity. ANF-responding cells, having the appearance of neuronal cell bodies, could be found in the subfornical organ, and the hypothalamic paraventricular nucleus. Fibres producing cGMP immunoreactivity in response to ANF were found in the median preoptic nucleus, the medial preoptic area, and the dorsal hypothalamus.

Atrial natriuretic peptide and angiotensin II binding sites in cerebral capillaries of spontaneously hypertensive rats

1. We carried out investigations on specific atrial natriuretic peptide (ANP) and angiotensin II (ANG) binding sites in capillaries isolated from the cerebral cortex of spontaneously hypertensive rats (SHR), an animal model of human essential hypertension, and also from Wistar Kyoto rats (WKY). 2. In an equilibrium binding study done in the presence of increasing concentrations of the radiolabeled ligands, the binding of 125I-rat alpha-ANP (1-28) [ANF-(99-126)] (125I-rANP) and 125I-ANG (5-L-isoleucine) (125I-ANG) to the cerebral capillaries was single and of a high affinity. 3. The maximum binding capacity (Bmax) and dissociation constant (Kd) in the 125I-rANP binding of 20-week-old, hypertensive SHR was significantly lower than in age-matched, normotensive WKY. Conversely, a significant increase in the Bmax of 125I-ANG binding of adult SHR was observed, with a significant decrease in the Kd. 4. There was no differences in the Bmax of 125I-rANP and 125I-ANG binding between 4-week-old, prehypertensive SHR and age-matched WKY. However, there was a significant decrease in the Kd of 125I-rANP binding of SHR. 5. As a dramatic change in the binding kinetics of 125I-rANP and 125I-ANG was noted in the cerebral capillaries of adult sustained-hypertensive SHR, the possibility that ANP and ANG play a role in the etiology of dysfunction of the blood-brain barrier complicated with hypertension, by interacting with specific receptors, would have to be considered.

Localization of atrial natriuretic factor (ANF) binding sites in the central nervous system of the frog

The distribution of atrial natriuretic factor (ANF) binding sites was investigated in the central nervous system of the frog Rana ridibunda using the technique of in vitro receptor autoradiography by means of [125I]-labeled ANF-28. The anatomic distribution of ANF recognition sites was determined on Kodak ARX films apposed onto tissue sections, and their distribution was examined in greater detail by analysis of autoradiograms generated by using emulsion-coated sections. The highest levels of ANF binding sites were found in the olfactory bulb, the dorsal pallium, the septum, the habenular nucleus, the dorsal infundibular nucleus, the interpeduncular nucleus, and in the tectum. Moderate levels of ANF binding sites were observed in the thalamus and throughout the mesencephalon, whereas low levels were detected in the lateral and medial pallium, the medial forebrain bundle, and the nucleus rotondus. In the pituitary gland, the neural and distal lobes were densely loaded with ANF binding sites, whereas no autoradiographic labeling was observed in the pars intermedia. In general, there was a good correlation between the location of ANF receptors and the distribution of ANF-containing neurons, as previously determined by immunocytochemistry. Together these results support the view that ANF may act as a neurotransmitter or a neuromodulator in various regions of the frog brain.

Receptor sites for atrial natriuretic factors in brain and associated structures: an overview

1. Recent data have clearly shown the existence of specific receptor binding sites for atrial natriuretic factors (ANF) or polypeptides in mammalian brain tissues. 2. Ligand selectivity pattern and coupling to cGMP production suggest that brain ANF sites are similar to high-affinity/low-capacity sites found in various peripheral tissues (kidney, adrenal gland, blood vessels). These brain ANF sites possibly are of the B-ANP subtype. 3. High densities of ANF binding sites are found especially in areas of the central nervous system associated with the control of various cardiovascular parameters (such as the subfornical organ and area postrema). However, high densities of sites are also present in other regions such as the hippocampus, cerebellum, and thalamus in the brain of certain mammalian species, suggesting that brain ANF could act as a neuromodulator of noncardiovascular functions. 4. The density of brain ANF binding sites is modified in certain animal models of cardiovascular disorders and during postnatal ontogeny, demonstrating the plasticity of these sites in the central nervous system (CNS). 5. Specific ANF binding sites are also found in various other CNS-associated tissues such as the eye, pituitary gland, and adrenal medulla. In these tissues ANF appears to act as a modulator of fluid production and hormone release. 6. Thus, ANF-like peptides and ANF receptor sites are present in brain and various peripheral tissues, demonstrating the existence of a family of brain/heart peptides.

Atrial natriuretic peptide

This report has reviewed some of the cardiovascular aspects of ANP. The emergence of the heart as an endocrine organ requires that numerous questions be asked with regard to the importance of ANP to anesthesia and surgery. It is clear that the interaction of the hormone with other vasoactive compounds, including anesthetic agents, requires further elucidation. The accumulation of more information regarding the regulation of ANP and its cardiovascular setting will define its role in hemodynamic homeostasis in the acute clinical setting. Questions of specific interest to the anesthesiologist that require elucidation are: (1) Does the presence of abnormal ANP levels, associated with specific disease states, affect perioperative cardiovascular function? (2) Do cardiac surgery and CPB affect ANP-adrenergic interaction? (3) What is the relationship among blood volume, blood pressure, cation metabolism, and the ANP-renin-angiotensin system in perioperative patients? (4) What is the role of ANP as a therapeutic modality in surgical patients?

Brain natriuretic peptides: differential localization of a new family of neuropeptides

Brain natriuretic peptide (BNP) is a recently discovered neuropeptide, isolated from the porcine brain, that is highly homologous to atriopeptin (AP), the atrial natriuretic peptide. We used a set of highly selective antisera against the two peptides to map their differential distribution immunohistochemically in the rat central nervous system. BNP immunoreactivity has a distinct distribution, involving many central autonomic and endocrine control structures that contain little if any AP immunoreactivity. AP and BNP belong to a family of neuropeptides that may be important in central cardiovascular control.

Central nervous system mediated vasodepressor action of atrial natriuretic factor

Administration of 20, 4 or 2.5 micrograms/kg of atriopeptin III (AT III) into the fourth ventricle of the brain of spontaneously hypertensive rats produced a 13, 14 and 7 mm Hg decrease in MAP respectively, while 1 microgram/kg had no effect on MAP and was significantly different from 20 or 4 micrograms/kg (p less than 0.025). In contrast, injection of AT III 20 micrograms/kg into the lateral ventricle did not produce a change in MAP. To examine an interaction of AT III with the opioidergic system, the opiate antagonist, naloxone HCl, 10 micrograms, was given by ICV injection 10 minutes prior to AT III, and significantly prevented the depressor response to AT III (p less than 0.025 compared with AT III alone). Injection of specific anti-sera to beta-endorphin failed to prevent the AT III-induced depressor response. Our results demonstrate that AT III can act within the central nervous system to decrease the MAP of rats, most likely at a locus in proximity to the fourth ventricle of the brain. Further, an interaction with the central opioidergic nervous system underlies the central effects of AT III.

Brain neuropeptides: actions on central cardiovascular control mechanisms

The many peptides we have not considered (e.g. gastrin, motilin, FMRFamide, carnosine, litorin, dermorphin, casomorphin, eledoisin, prolactin, growth hormone, neuromedin U, proctolin, etc.) were omitted due to lack of information as far as any putative central cardiovascular effects are concerned. However, even for some of these peptide pariahs intriguing snippets of information are available now (e.g. ref. 85), although as we write, the list of possible candidates for investigation grows longer. On an optimistic note, it is becoming clear that many brain neuropeptides may have important effects on cardiovascular regulation. It seems feasible that 'chemically coded' pathways in the brain might be the neuroanatomical correlate of a 'viscerotopic' organization of cardiovascular control mechanisms, whereby the activity of the heart and flows through vascular beds are individually controlled, but in an integrated fashion, utilizing particular combinations of neurotransmitters and neuropeptides within the brain. Such possibilities can only be investigated, properly, by measurement of changes in cardiac output and regional haemodynamics in response to appropriate interventions, in conscious, unrestrained animals.

Effect of chronic administration of the converting enzyme inhibitor enalapril (MK 421) on brain atrial natriuretic peptide receptors in Wistar-Kyoto and spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) showed lower brain ANP binding density when compared with normotensive Wistar-Kyoto (WKY) rats. In the WKY, angiotensin converting enzyme inhibitor, enalapril (25 mg/kg, p.o. for 14 days), decreased the number of ANP binding sites selectively in the subfornical organ and area postrema. Conversely, enalapril increased ANP binding density in the SHR, but only in the area postrema. Enalapril has central effects on ANP binding sites, specific to the circumventricular organs.

Atrial natriuretic factor in specific brain areas of spontaneously hypertensive rats

Atrial natriuretic peptides (atrial natriuretic factor, ANF) are present in a great number of brain areas inside and outside of the blood-brain barrier. The pattern of distribution implies the involvement of ANF in different physiological functions, such as blood pressure regulation, electrolyte and fluid homeostasis, and modulation of the neuroendocrine system. To further investigate a possible involvement of central ANF in spontaneous hypertension, we measured levels of ANF in 18 selected, microdissected brain areas of prehypertensive (4-week-old) and hypertensive (12-week-old) spontaneously hypertensive rats (SHR) and their normotensive control, Wistar-Kyoto rats (WKY), by radio-immunoassay. ANF was significantly decreased in seven brain areas in SHR at both ages investigated; the most pronounced decreases were found in the subfornical organ, in the perifornical and periventricular hypothalamic nuclei, and in the medial preoptic nucleus. In addition, in young SHR ANF was significantly decreased in the organum vasculosum laminae terminalis and increased in the median eminence. After the development of hypertension, a significant decrease of ANF could be detected in four more brain areas (bed nucleus of the stria terminalis, paraventricular and arcuate nuclei, dorsal raphe nucleus) of SHR, as compared with normotensive controls, and the increase in the median eminence was no longer detectable. These results suggest a role for ANF in genetic hypertension and the specific importance of certain brain regions.