Binding of atrial and brain natriuretic peptides in brains of hypertensive rats

Multiplicity of cerebrospinal fluid functions: New challenges in health and disease

This review integrates eight aspects of cerebrospinal fluid (CSF) circulatory dynamics: formation rate, pressure, flow, volume, turnover rate, composition, recycling and reabsorption. Novel ways to modulate CSF formation emanate from recent analyses of choroid plexus transcription factors (E2F5), ion transporters (NaHCO3 cotransport), transport enzymes (isoforms of carbonic anhydrase), aquaporin 1 regulation, and plasticity of receptors for fluid-regulating neuropeptides. A greater appreciation of CSF pressure (CSFP) is being generated by fresh insights on peptidergic regulatory servomechanisms, the role of dysfunctional ependyma and circumventricular organs in causing congenital hydrocephalus, and the clinical use of algorithms to delineate CSFP waveforms for diagnostic and prognostic utility. Increasing attention focuses on CSF flow: how it impacts cerebral metabolism and hemodynamics, neural stem cell progression in the subventricular zone, and catabolite/peptide clearance from the CNS. The pathophysiological significance of changes in CSF volume is assessed from the respective viewpoints of hemodynamics (choroid plexus blood flow and pulsatility), hydrodynamics (choroidal hypo- and hypersecretion) and neuroendocrine factors (i.e., coordinated regulation by atrial natriuretic peptide, arginine vasopressin and basic fibroblast growth factor). In aging, normal pressure hydrocephalus and Alzheimer's disease, the expanding CSF space reduces the CSF turnover rate, thus compromising the CSF sink action to clear harmful metabolites (e.g., amyloid) from the CNS. Dwindling CSF dynamics greatly harms the interstitial environment of neurons. Accordingly the altered CSF composition in neurodegenerative diseases and senescence, because of adverse effects on neural processes and cognition, needs more effective clinical management. CSF recycling between subarachnoid space, brain and ventricles promotes interstitial fluid (ISF) convection with both trophic and excretory benefits. Finally, CSF reabsorption via multiple pathways (olfactory and spinal arachnoidal bulk flow) is likely complemented by fluid clearance across capillary walls (aquaporin 4) and arachnoid villi when CSFP and fluid retention are markedly elevated. A model is presented that links CSF and ISF homeostasis to coordinated fluxes of water and solutes at both the blood-CSF and blood-brain transport interfaces.

OUTLINE

1 Overview2 CSF formation2.1 Transcription factors2.2 Ion transporters2.3 Enzymes that modulate transport2.4 Aquaporins or water channels2.5 Receptors for neuropeptides3 CSF pressure3.1 Servomechanism regulatory hypothesis3.2 Ontogeny of CSF pressure generation3.3 Congenital hydrocephalus and periventricular regions3.4 Brain response to elevated CSF pressure3.5 Advances in measuring CSF waveforms4 CSF flow4.1 CSF flow and brain metabolism4.2 Flow effects on fetal germinal matrix4.3 Decreasing CSF flow in aging CNS4.4 Refinement of non-invasive flow measurements5 CSF volume5.1 Hemodynamic factors5.2 Hydrodynamic factors5.3 Neuroendocrine factors6 CSF turnover rate6.1 Adverse effect of ventriculomegaly6.2 Attenuated CSF sink action7 CSF composition7.1 Kidney-like action of CP-CSF system7.2 Altered CSF biochemistry in aging and disease7.3 Importance of clearance transport7.4 Therapeutic manipulation of composition8 CSF recycling in relation to ISF dynamics8.1 CSF exchange with brain interstitium8.2 Components of ISF movement in brain8.3 Compromised ISF/CSF dynamics and amyloid retention9 CSF reabsorption9.1 Arachnoidal outflow resistance9.2 Arachnoid villi vs. olfactory drainage routes9.3 Fluid reabsorption along spinal nerves9.4 Reabsorption across capillary aquaporin channels10 Developing translationally effective models for restoring CSF balance11 Conclusion.

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.

Expression of neuropeptide processing enzymes and neurosecretory proteins in ependyma and choroid plexus epithelium

Recent studies suggest that brain ependyma and choroid plexus produce neuropeptide processing enzymes. To facilitate the understanding of these cells and their ability to produce biologically active peptides, we developed cultures of defined cell type. Ependymal cells were characterized by morphological criteria, and choroid plexus epithelial cell lines were characterized by the presence of the mRNA for IGF-II and transthyretin, a thyroxine binding protein produced in liver and choroid plexus. The ependymal cells and the choroid plexus epithelial cell lines were then examined for the presence of mRNAs for various neuropeptide processing enzymes. Northern blot analysis revealed high levels of furin, carboxypeptidase E, and peptidyl glycine alpha-amidating monooxygenase mRNAs, with levels in ependymal cells comparable to those in brain or pituitary. Carboxypeptidase E activity was detected in medium from cultured ependymal cells; this activity was identified as carboxypeptidase E based on the acidic pH optimum and sensitivity to various inhibitors. The mRNAs for other neuropeptide processing enzymes, such as prohormone convertases 1 and 2, were not detected on Northern blots of RNA from ependyma or choroid plexus epithelium. Since ependyma and choroid plexus epithelium express a subset of processing enzymes, we suggest that these cells have the capacity to produce biologically active peptides. Initial screening by reverse transcriptase-polymerase chain reaction assays has demonstrated the presence of mRNA for the neurosecretory proteins chromogranin B and secretogranin II in both ependyma and choroid plexus epithelium.

ANP receptors in neurons and astrocytes from spontaneously hypertensive rat brain

In this study we compared the levels and responsiveness of atrial natriuretic peptide (ANP) receptors in neuronal and astrocyte glial cultures from spontaneously hypertensive (SH) and normotensive (Wistar-Kyoto: WKY) rat brain. Both neuronal and astrocyte glial cultures from the hypothalamus and brain stem of 1-day-old SH and WKY rats display specific high-affinity binding sites for 125I-labeled ANP. The presence of a large population of ANP-C receptors in each type of culture is indicated by the strong competition of 125I-ANP binding by the ring-deleted analogue of ANP [C-ANF-(4-23)]. In neuronal cultures from both strains, C-type natriuretic peptide (CNP-22) was the most effective natriuretic peptide in stimulating guanosine 3',5'-cyclic monophosphate (cGMP) levels, suggesting the presence of ANP-B receptors in these cells. By contrast, ANP was the most effective stimulator of cGMP levels in SH and WKY rat astrocyte glial cultures, suggesting the presence of ANP-A receptors. Here, we have determined that there is a decrease in the maximum binding capacity for 125I-ANP-specific binding in both SH rat neuronal and astrocyte glial cultures compared with their respective control cells. The stimulatory effects of CNP-22 on cGMP levels in SH rat neurons and of ANP on cGMP levels in SH rat astrocytes were significantly reduced compared with their respective WKY rat cultures. Our data suggest that the lower number of ANP receptors in SH rat neuronal and astrocyte glial cultures includes a reduction in the guanylate cyclase-coupled ANP receptors.

Selective localization of C atrial natriuretic peptide receptors in the rat brain

We studied the distribution of C atrial natriuretic peptide (C-ANP) receptors in the rat brain after incubation with 125I-rANP, and competition with the selective C-ANP receptor competitor C-ANP(4-23) (10(-6) M). C-ANP receptors were selectively localized to a few structures, the external plexiform layer of the olfactory bulb, the arachnoid mater and the choroid plexus, and their amount corresponds to 25%, 75% and 35%, respectively, of the ANP specific binding. These data suggest specialized functions for C-ANP receptors in brain.

Porcine brain natriuretic peptide-like immunoreactivity in rat tissues

The presence of immunoreactive porcine brain natriuretic peptide in rat tissues was studied with a specific radioimmunoassay for porcine brain natriuretic peptide-26. The cross-reactivity of the antiserum used was less than 0.001% with rat atrial natriuretic peptide, rat brain natriuretic peptide-32 and rat brain natriuretic peptide-45. Immunoreactive porcine brain natriuretic peptide was detectable in various tissues of the rat, and high concentrations of immunoreactive porcine brain natriuretic peptide were found in the brain and cardiac atrium, with the highest level in the hypothalamus (159 +/- 30 fmol/gram wet tissue, mean +/- SEM, n = 4). Reverse phase high performance liquid chromatography showed that the immunoreactive porcine brain natriuretic peptide of the whole brain and heart extracts eluted mainly at an identical position to synthetic porcine brain natriuretic peptide-26. These findings indicate that porcine brain natriuretic peptide-like substance, distinct from rat brain natriuretic peptide, is present in high concentrations in the rat brain and cardiac atrium.

A modified quantitative autoradiographic assay for atrial natriuretic peptide receptors in rat brain

New conditions for quantitative autoradiographic determination of natriuretic peptide (ANP) binding in rat brain were developed in order to inhibit hormone degradation during incubation without the use of proteinase inhibitors. This procedure includes 1-h preincubation of tissue slices in hypotonic buffer and incubation at 4 degrees C in the presence of 4% proteinase-free albumin. ANP binding to choroid plexus and subfornical organ under above conditions reached equilibrium after 90 min. ANP degradation at the end of the incubation did not exceed 2%. Binding data analysis revealed the previously undetected presence of two distinct ANP binding sites in the rat choroid plexus.