Ontogeny of atrial natriuretic factor (ANF) binding in various areas of the rat brain

Expression patterns of atrial natriuretic peptide and its receptors within the cochlear spiral ganglion of the postnatal rat

The spiral ganglion, which is primarily composed of spiral ganglion neurons and satellite glial cells, transmits auditory information from sensory hair cells to the central nervous system. Atrial natriuretic peptide (ANP), acting through specific receptors, is a regulatory peptide required for a variety of cardiac, neuronal and glial functions. Although previous studies have provided direct evidence for the presence of ANP and its functional receptors (NPR-A and NPR-C) in the inner ear, their presence within the cochlear spiral ganglion and their regulatory roles during auditory neurotransmission and development is not known. Here we investigated the expression patterns and levels of ANP and its receptors within the cochlear spiral ganglion of the postnatal rat using immunofluorescence and immunoelectron microscopy techniques, reverse transcription-polymerase chain reaction and Western blot analysis. We have demonstrated that ANP and its receptors colocalize in both subtypes of spiral ganglion neurons and in perineuronal satellite glial cells. Furthermore, we have analyzed differential expression levels associated with both mRNA and protein of ANP and its receptors within the rat spiral ganglion during postnatal development. Collectively, our research provides direct evidence for the presence and synthesis of ANP and its receptors in both neuronal and non-neuronal cells within the cochlear spiral ganglion, suggesting possible roles for ANP in modulating neuronal and glial functions, as well as neuron-satellite glial cell communication, within the spiral ganglion during auditory neurotransmission and development.

Expression and localization of atrial natriuretic peptide and its receptors in rat spiral ganglion neurons

Spiral ganglion neurons (SGNs) are the primary auditory neurons in the inner ear, conveying auditory information between sensory hair cells and the central nervous system. Atrial natriuretic peptide (ANP), acting through specific receptors, is a regulatory peptide required for a variety of cardiac and neuronal functions. While the localization of ANP and its receptors (NPR-A and NPR-C) in the inner ear has been widely studied, there is only limited information regarding their localization in cochlear SGNs and their regulatory roles during primary auditory neurotransmission. Here we have investigated the presence of ANP and its receptors in the cochlear spiral ganglion of the postnatal rat using immunohistochemistry, reverse transcription-polymerase chain reaction (RT-PCR) and Western blot analysis. ANP and its receptors are expressed in the cochlear SGNs at both the mRNA and protein level, and co-localize in the cochlear SGNs as shown by immunofluorescence. Our research provides a direct evidence for the presence and synthesis of ANP as well as its receptors in the cochlear SGNs, suggesting a possible role for ANP in modulating the neuronal functions of SGNs via its receptors.

Natriuretic peptides in brain physiology

Natriuretic peptides (NPs) regulate salt and water homeostasis by inducing natriuresis and diuresis in the kidney. These actions in addition to those via the heart and vascular system play important roles in the regulation of blood pressure. In the central nervous system NPs play a significant role in neuronal development, synaptic transmission and neuroprotection. Currently, six different human NPs have been described: atrial natriuretic peptide (ANP), urodilatin (URO, renal natriuretic peptide), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) as well as guanylin and uroguanylin. ANP, URO and BNP activate the natriuretic peptide receptor A (NPR-A or guanylate cyclase A (GC-A)) while CNP activates natriuretic peptide receptor B (NPR-B or guanylate cyclase B (GC-B)). Guanylin and uroguanylin are known to activate guanylate cyclase C (GC-C). The receptors GC-A, GC-B, and GC-C are widely expressed in the human body. Currently, GC-B and CNP seems to have the highest expression in central nervous system compared to other NPs and their receptors. All known NPs generate intracellular cyclic GMP (cGMP) by activating their specific guanylate cyclase receptors. Subsequently, cGMP is able to activate protein kinase I or II (PKG I or II) and/or directly regulate transmembrane proteins such as ion channels, transporters and pumps. NPs also bind to the natriuretic peptide receptor C (also called clearance receptor NPR-C) which is a major pathway for the degradation of NPs and has no guanylate cyclase activity. In this review we will focus on new insights regarding the physiological effects of NPs in the brain, especially specific areas of their signaling pathways in neurons and glial cells.

Natriuretic peptides and their receptors in the central nervous system

Natriuretic peptides (NPs), including atrial, brain and C-type NPs, are a family of structurally related but genetically distinct peptides. These peptides, along with their receptors (NPRs), are long known to be involved in the regulation of various physiological functions, such as diuresis, natriuresis, and blood flow. Recently, abundant evidence shows that NPs and NPRs are widely distributed in the central nervous system (CNS), suggesting possible roles of NPs in modulating physiological functions of the CNS. This review starts with a brief summary of relevant background information, such as molecular structures of NPs and NPRs and general intracellular mechanisms after activation of NPRs. We then provide a detailed description of the expression profiles of NPs and NPRs in the CNS and an in-depth discussion of how NPs are involved in neural development, neurotransmitter release, synaptic transmission and neuroprotection through activation of NPRs.

Embryonic expression and multifunctional actions of the natriuretic peptides and receptors in the developing nervous system

Atrial natriuretic peptide (ANP) binding sites have been detected in the embryonic brain, but the specific receptor subtypes and biological functions for ANP family ligands therein remain undefined. We now characterize the patterns of gene expression for the natriuretic peptides [ANP, brain natriuretic peptide (BNP), type-C natriuretic peptide (CNP)] and their receptors (NPR-A, NPR-B, NPR-C) at several early stages in the embryonic mouse nervous system by in situ hybridization, and begin to define the potential developmental actions using cell culture models of peripheral (PNS) and central nervous systems (CNS). In the CNS, gene transcripts for CNP were present at the onset of neurogenesis, embryonic day 10.5 (E10.5), primarily in the dorsal part of the ventricular zone (VZ) throughout the hindbrain and spinal cord. On E14.5, new CNP signals were observed in the ventrolateral spinal cord where motor neurons reside, and in bands of cells surrounding the spinal cord and hindbrain, localized to dura and/or cartilage primordia. ANP and BNP gene transcripts were not detected in embryonic brain, but were highly abundant in the heart. The CNP-specific receptor (NPR-B) gene was expressed in cells just outside the VZ, in regions where post-mitotic neurons are differentiating. Gene expression for NPR-C, which recognizes all natriuretic peptides, was present in the roof plate of the hindbrain and spinal cord and in bilateral stripes just dorsolateral to the floor plate at E12.5. In the PNS, NPR-B and NPR-C transcripts were highly expressed in dorsal root sensory (DRG) and cranial ganglia beginning at E10.5, with NPR-C signal also prominent in adjoining nerves, consistent with Schwann cell localization. In contrast, NPR-A gene expression was undetectable in neural tissues. To define ontogenetic functions, we employed embryonic DRG and hindbrain cell cultures. The natriuretic peptides potently stimulated DNA synthesis in neuron-depleted as well as neuron-containing Schwann cell cultures and differentially inhibited neurite outgrowth in DRG sensory neuron cultures. CNP also exhibited modest survival-promoting effects for sensory neurons. In marked contrast to PNS effects, the peptides inhibited proliferation of neural precursor cells of the E10.5 hindbrain. Moreover, CNP, alone and in combination with sonic hedgehog (Shh), induced the expression of the Shh target gene gli-1 in hindbrain cultures, suggesting that natriuretic peptides may also modify patterning events in the embryonic brain. These studies reveal widespread, but discrete patterns of natriuretic peptide and receptor gene expression in the early embryonic nervous system, and suggest that the peptides play region- and stage-specific roles during the development of the peripheral and central nervous systems.

Proliferative actions of natriuretic peptides on neuroblastoma cells. Involvement of guanylyl cyclase and non-guanylyl cyclase pathways

To identify neural tumor cell lines that could be used as models to study growth-related natriuretic peptide actions, we determined the effects of these peptides on the proliferation of human and rodent neuroblastoma cell lines. Subnanomolar concentrations of atrial natriuretic peptide (ANP) and type C natriuretic peptide (CNP) stimulated proliferation in all four cell lines. These actions were associated with cGMP elevation and were blocked by a protein kinase G inhibitor. These data imply the involvement of guanylyl cyclase (GC)-coupled natriuretic receptors. However, higher concentrations of ANP and CNP, and low concentrations of des-[Gln(18),Ser(19),Gly(20),Leu(21),Gly(22)]-ANP(4-23)-NH(2) (desANP(4-23)) (analog for NPR-C receptor) exerted antiproliferative actions in three of the cell lines. These effects were insensitive to a protein kinase G inhibitor and to HS-142-1, suggesting that growth-inhibitory actions involved a non-GC receptor. They did not appear to involve cAMP, protein kinase A, protein kinase C, or calcium mobilization but were abolished when constitutive mitogen-activated protein kinase activity was inhibited. Radioligand binding experiments revealed the presence of a uniform class of binding sites in NG108 cells and multiple binding sites in Neuro2a cells. Northern and reverse transcriptase-polymerase chain reaction analyses revealed differential gene expression for NPR-A/B/C in NG108 and Neuro2a cells. The results indicate that natriuretic peptides stimulate neuroblastoma cell proliferation through type NPR-A/B (GC) receptors. Higher concentrations of ANP and CNP exerted a mitogen-activated protein kinase-dependent antiproliferative action mediated by a non-GC receptor that interacts with desANP(4-23) with relatively high affinity.

Cardiac atrial natriuretic peptide (ANP) in rat dams and fetuses developed in space (NIH-R1 and NIH-R2 experiments)

NIH-R1 and R2 missions, conducted by NASA, allowed us to study the effects of the microgravitational environment 1) on cardiac ANP in pregnant rats, spaceflown for 11 days and dissected after a 2-day readaptation to Earth's gravity, after natural delivery, and 2) on maturation of cardiac ANP system in rat fetuses developed for 11 days in space and dissected on the day of landing, 2 days before birth. Immunocytochemical and electron microscopy analyses showed a typical formation of ANP-containing granules in atrial myocytes, in both dams and fetuses. Using competitive RT-PCR and radioimmunoassays, we observed that, after 2 days of readaptation to Earth's gravity, cardiac ANP biosynthesis of rat dams flown in space was increased by about twice, when compared to Synchronous and Vivarium Control rats. More obviously, rat fetuses developed in space and dissected on the day of landing displayed an altered maturation of cardiac ANP, evidenced by an increased mRNA biosynthesis (by about 6 fold, p<0.05), whereas the cardiac ANP storage was slightly reduced (by about twice, p<0.05) in both Flight and Synchronous Control groups, in comparison with Vivarium Control rats. These last results suggest that ANP metabolism during development is impacted by the microgravitational environment, but also by the housing conditions designed for space flight.

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.

Ontogenic expression of natriuretic peptide mRNAs in postnatal rat brain: implications for development?

The central natriuretic peptide system is composed of at least three structurally homologous and uniquely distributed peptides and receptors which are thought to be involved in the central regulation of cardiovascular and autonomic function and more recently been shown to affect cellular growth and proliferation, processes pertinent to mammalian development. As such, following our initial mapping of preproatrial natriuretic peptide (ppANP) mRNA in adult brain [M.C. Ryan, A.L. Gundlach, Anatomical localization of preproatrial natriuretic peptide mRNA in the rat brain by in situ hybridization histochemistry: in olfactory regions, J. Comp. Neurol., 356 (1995) 168-182], it was of interest to determine the ontogenic expression of natriuretic peptide mRNAs in the developing rat brain. Using in situ hybridization histochemistry of specific [35S]- or [33P]-labeled oligonucleotides, ppANP and preproC-type natriuretic peptide (ppCNP) mRNAs were detected in the developing rat brain from postnatal day 4 to day 60 (adult). PpANP mRNA was observed in many hindbrain, but only some forebrain, regions at postnatal day 4. Regional differences in the temporal expression of ppANP mRNA were apparent with ppANP mRNA detected in the medial preoptic area, mammillary nuclei and medial habenular nucleus at postnatal day 4 and in other areas including the arcuate and dorsomedial hypothalamic nuclei and in olfactory and limbic regions at postnatal day 10. A number of regions also exhibited transient expression of ppANP mRNA such as the bed nucleus of the stria terminalis and the medial cerebellar nucleus. In contrast, ppCNP mRNA was detected at relatively high levels in several regions on postnatal day 4 including olfactory nuclei, the hippocampus and particularly the pontine nucleus. The level of expression appeared to increase markedly in most regions including forebrain olfactory and hippocampal areas and in brainstem regions including the pontine nucleus, the parvocellular and lateral reticular and spinal trigeminal nuclei by postnatal days 10 and 13, but decreased from this peak to equivalent to adult levels by postnatal day 28. The differential and transient expression of the natriuretic peptides during postnatal development, together with previous reports of the ontogenic regulation of natriuretic peptide receptor expression and binding patterns, further suggests their involvement in developmental processes in the rat CNS and provides information relevant to the likely functional development of natriuretic peptide-utilizing pathways.

Region-specific developmental patterns of atrial natriuretic factor- and nitric oxide-activated guanylyl cyclases in the postnatal frontal rat brain

In the rat central nervous system, cyclic GMP can be produced by two isoforms of guanylyl cyclase: a cytosolic isoform, which is activated by nitric oxide, and a membrane-bound isoform, activated by atrial natriuretic factor. We studied the development of guanylyl cyclase activity upon maturation of the rat forebrain from postnatal days 4 to 24, using a combined immunocytochemical and biochemical approach. Atrial natriuretic factor-activated particulate guanylyl cyclase activity was found to decrease in the frontal cortex, in the lateral septum and in the piriform cortex upon maturation. A transient expression of atrial natriuretic factor-sensitive guanylyl cyclase activity was observed at postnatal day 8 in the caudate putamen complex, whereas an increase was observed in the lateral olfactory tract from postnatal days 8 to 24. Biochemical and immunocytochemical studies using the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester, or the inhibitor of soluble guanylyl cyclase 1H-[1,2,4]oxadiazolo[4,3-a]quinaloxin-1-one, indicated high levels of endogenous nitric oxide release at postnatal days 4 and 8. This activity decreased strongly in all brain areas examined. From postnatal day 8 onwards, atrial natriuretic factor-responsive cyclic GMP-immunoreactive cells could be characterized as astrocytes, with the exception of those in the the lateral olfactory tract, where the myelinated fibers became cyclic GMP producing. Furthermore, our results on activation of both guanylyl cyclases at postnatal day 8 leads to the suggestion that both isoforms might be found in the same cells. This study shows that there are pronounced differences between various frontal brain areas in the development of the responsiveness of both the particulate and soluble isoforms of guanylyl cyclase, and lends further support to the hypothesis that natriuretic peptides have a role in neuronal growth and plasticity of the rat brain.

Distribution of, and a putative role for, the cell-surface neutral metallo-endopeptidases during mammalian craniofacial development

Endopeptidase-24.11 (neutral endopeptidase, neprilysin, ‘enkephalinase’, EC 3.4.24.11) and endopeptidase-24.18 (endopeptidase-2, meprin, EC 3.4.24.18) are cell-surface zinc-dependent metallo-endopeptidases able to cleave a variety of bioactive peptides including growth factors. We report the first study of the cellular and tissue distribution of both enzymes and of the mRNA for NEP during embryonic development in the rat. Endopeptidase-24.11 protein was first detected at E10 in the lining of the gut and, at E12, the enzyme was present on the notochord, medial and lateral nasal processes, otocyst, mesonephros, heart and neuroepithelium. In contrast, at this time endopeptidase-24.18 was present only on the apical surface of the neuroepithelial cells. By E14 and E16, NEP was also detected in a wide range of craniofacial structures, notably the palatal mesenchyme, the choroid plexus, tongue and perichondrium. The distribution of endopeptidase-24.18 at these stages was restricted to the inner ear, the nasal conchae, and ependymal layer of the brain ventricles and the choroid plexus. Although endopeptidase-24.11 had been detectable in the craniofacial vasculature at E12 and E14, this was no longer apparent at E16. Significantly, the distribution of endopeptidase-24.11 mRNA closely matched the immunolocalization of the protein at all stages investigated. In order to explore the functional role of these enzymes, inhibition studies were carried out using two selective inhibitors of endopeptidase-24.11, phosphoramidon and thiorphan. E9.5 and E10.5 embryos exposed to either inhibitor displayed a characteristic, asymmetric abnormality consisting of a spherical swelling, possibly associated with a haematoma, predominantly on the left side of the prosencephalon, and the severity of this defect appeared to be a dose-dependent phenomenon. This study suggests that these enzymes play previously unrecognized roles during mammalian embryonic development.

Localization and characteristics of atrial natriuretic peptide receptors in prenatal and postnatal rat brain

We studied the expression of atrial natriuretic peptide (ANP) receptor subtypes during development in the rat forebrain, using quantitative autoradiography. Highest ANP binding was observed in the cortical neuroepithelium at embryonic day 17. Lower ANP binding was found in cingulate and frontal cortices at postnatal day 10, but none was detectable at 8 weeks of age. In the neuroepithelium of the embryonic rat, binding was displaced with a potency of rat ANP-(1-28) (rANP) > porcine type-C natriuretic peptide (pCNP-22) = rat ANP fragment C-ANP-(4-23) (rC-ANP-(4-23)) = rat brain natriuretic peptide (rBNP-32), different from that of any of the well-characterized (ANPA, ANPB, and ANPC) natriuretic peptide receptors. The present results support the hypothesis of a role for ANP during brain maturation and indicate that the ANP receptors highly expressed in the embryonic neuroepithelium may belong to a new ANP receptor subtype not yet characterized.

Pre- and post-natal ontogeny of neutral endopeptidase 24-11 ('enkephalinase') studied by in vitro autoradiography in the rat

Neutral endopeptidase (NEP, enkephalinase, CALLA) which is present in various neural and non-neural tissues, is able to cleave a variety of regulatory peptides. The distribution of NEP has been studied during rat pre- and post-natal development by autoradiography after in vitro binding of the tritiated inhibitor [3H]HACBO-Gly to whole-body and organ sections. In the central nervous system (CNS), where the presence of NEP has been related to the termination of the action of enkephalins, the external layer of the olfactory bulbs is the only structure prominently labeled before birth. Other CNS structures rich in NEP in the adult, such as the nigrostriatal tract, are progressively labeled after birth. Outside the CNS, the progressive appearance of NEP in the kidney, the lungs and the salivary glands suggests its concomitant involvement in adult physiological functions, including fluid balance control, possibly by cleaving the atrial natriuretic peptide (ANP) and other peptides. On the other hand, transient or enhanced expression of NEP is observed during the development of several organs such as the sensory organs, the heart and the major blood vessels, the intestine, the bones and the genital tubercle. In addition to the still incompletely known physiological functions of the enzyme, the developmental pattern of its expression in several tissues strongly suggests a modulatory role for NEP in the ontogeny of a large number of organs.