Effects of atrial natriuretic peptide on ischemic brain edema in rats evaluated by proton magnetic resonance method

Role of uroguanylin's signaling pathway in the development of ischemic stroke

Stroke is one of the leading causes of mortality and disability worldwide. By affecting bradykinin function, activation of guanylate cyclase (GC)‐A has been shown to have a neuroprotective effect after ischaemic stroke, whereas the same has not been confirmed for GC‐B; therefore, we aimed to determine the possible role of GC‐C and its agonist, uroguanylin (UGN), in the development of stroke. In this study, middle cerebral artery occlusion (MCAO) was performed on wild‐type (WT), GC‐C KO and UGN KO mice. MR images were acquired before and 24 h after MCAO. On brain slices 48 h after MCAO, the Ca²⁺ response to UGN stimulation was recorded. Our results showed that the absence of GC‐C in GC‐C KO mice resulted in the development of smaller ischaemic lesions compared with WT littermates, which is an opposite effect compared with the effects of GC‐A agonists on brain lesions. WT and UGN KO animals showed a stronger Ca²⁺ response upon UGN stimulation in astrocytes of the peri‐ischaemic cerebral cortex compared with the same cortical region of the unaffected contralateral hemisphere. This stronger activation was not observed in GC‐C KO animals, which may be the reason for smaller lesion development in GC‐C KO mice. The reason why GC‐C might affect Ca²⁺ signalling in peri‐ischaemic astrocytes is that GC‐C is expressed in these cells after MCAO, whereas under normoxic conditions, it is expressed mainly in cortical neurons. Stronger activation of the Ca²⁺‐dependent signalling pathway could lead to the stronger activation of the Na⁺/H⁺ exchanger, tissue acidification and neuronal death.

Inhibition of the Epigenetic Regulator REST Ameliorates Ischemic Brain Injury

Cerebral ischemia is known to activate the repressor element-1 (RE1)-silencing transcription factor (REST) which silences neural genes via epigenetic remodeling and promotes neurodegeneration. We presently determined if REST inhibition derepresses target genes involved in synaptic plasticity and promotes functional outcome after experimental stroke. Following transient focal ischemia induced by middle cerebral artery occlusion (MCAO) in adult rats, REST expression was upregulated significantly from 12 h to 1 day of reperfusion compared to sham control. At 1 day of reperfusion, REST protein levels were increased and observed in the nuclei of neurons in the peri-infarct cortex. REST knockdown by intracerebral REST siRNA injection significantly reduced the post-ischemic expression of REST and increased the expression of several REST target genes, compared to control siRNA group. REST inhibition also decreased post-ischemic markers of apoptosis, reduced cortical infarct volume, and improved post-ischemic functional recovery on days 5 and 7 of reperfusion compared to the control siRNA group. REST knockdown resulted in a global increase in synaptic plasticity gene expression at 1 day of reperfusion compared to the control siRNA group and significantly increased several synaptic plasticity genes containing RE-1 sequences in their regulatory regions. These results demonstrate that direct inhibition of the epigenetic remodeler REST prevents secondary brain damage in the cortex and improves functional outcome potentially via de-repression of plasticity-related genes after stroke.

Molecular mechanisms underlying the neuroprotective role of atrial natriuretic peptide in experimental acute ischemic stroke

Along with its role in regulating blood pressure and fluid homeostasis, the natriuretic peptide system could be also part of an endogenous protective mechanism against brain damage. We aimed to assess the possibility that exogenous atrial natriuretic peptide (ANP) could protect against acute ischemic stroke, as well as the molecular mechanisms involved. Three groups of rats subjected to transient middle cerebral artery occlusion (tMCAO, intraluminal filament technique, 60 min) received intracerebroventricular vehicle, low-dose ANP (0.5 nmol) or high-dose ANP (2.5 nmol), at 30 min reperfusion. Neurofunctional condition, and brain infarct and edema volumes were measured at 24 h after tMCAO. Apoptotic cell death and expression of natriuretic peptide receptors (NPR-A and NPR-C), K⁺ channels (K_ATP, K_V and BK_Ca), and PI3K/Akt and MAPK/ERK1/2 signaling pathways were analyzed. Significant improvement in neurofunctional status, associated to reduction in infarct and edema volumes, was shown in the high-dose ANP group. As to the molecular mechanisms analyzed, high-dose ANP: 1) reduced caspase-3-mediated apoptosis; 2) did not modify the expression of NPR-A and NPR-C, which had been downregulated by the ischemic insult; 3) induced a significant reversion of ischemia-downregulated K_ATP channel expression; and 4) induced a significant reversion of ischemia-upregulated pERK2/ERK2 expression ratio. In conclusion, ANP exerts a significant protective role in terms of both improvement of neurofunctional status and reduction in infarct volume. Modulation of ANP on some molecular mechanisms involved in ischemia-induced apoptotic cell death (K_ATP channels and MAPK/ERK1/2 signaling pathway) could account, at least in part, for its beneficial effect. Therefore, ANP should be considered as a potential adjunctive neuroprotective agent improving stroke outcome after successful reperfusion interventions.

Intra-arterial transplantation of human bone marrow mesenchymal stem cells (hBMMSCs) improves behavioral deficits and alters gene expression in rodent stroke model

Stroke is a multi-factorial polygenic disease and is a major cause of death and adult disability. Administration of bone marrow stem cells protects ischemic rat brain by facilitating recovery of neurological functions. But the molecular mechanism of stem cells action and their effect on gene expression is not well explored. In this study, we have transplanted 1 × 10⁶ human bone marrow mesenchymal stem cells (hBMMSCs) in middle cerebral artery occluded (MCAo) adult male Wistar rats through intracarotid artery route at 24 h after surgery. Motor behavioral tests (rotarod and open field) were performed to assess the changes in motor functions at day 0 and day1, 4, 8 and 14. The expression of studied genes at mRNA and protein level was quantified by using Q-PCR and western blotting, respectively. Further, we have assessed the methylation pattern of promoter of these genes by using methylation-specific PCR. Data were analyzed statistically and correlated. A significant improvement in behavioral deficits was observed in stem cells treated group after 14th day post stroke. Significantly (p < 0.05) increased mRNA and protein levels of brain derived neurotrophic factor and ANP genes in hBMMSCs treated group along with decrease in methylation level at their promoter was observed. On the other hand, significantly decreased mRNA and protein level of TSP1 and WNK1 in hBMMSCs treated group was observed. In conclusion, hBMMSCs administration significantly improves the behavioral deficits by improving motor and locomotor coordination. The promoter of TSP1 and WNK1 genes was found to be hyper-methylated in hBMMSCs group resulting in their decreased expression while the promoter of ANP and brain derived neurotrophic factor was found to be hypo-methylated. This study might shed a light on how hBMMSCs affect the gene expression by modulating methylation status.

Natriuretic peptides in the central nervous system: Novel targets for cognitive impairment

Natriuretic peptides (NPs) are traditionally known as cardiac hormones with diuretic, natriuretic and blood pressure lowering properties. Evidence indicates that NPs and their receptors are abundant in the central nervous system, suggesting their involvement in regulation of various brain functions. It has been shown that NPs are involved in the regulation of neurovascular and blood-brain barrier integrity, neuro-inflammation, neuroprotection, synaptic transmission and brain fluid homeostasis. In addition, NPs might contribute to the brain's inhibitory control over the hypothalamic-pituitary-adrenal axis. Studies have also shown that high systemic levels of NPs are associated with cognitive impairment independent of cardiovascular risk factors. In this review we discuss the potential roles of NPs in regulating structural and functional integrity of the brain. Based on the available neurobiological and clinical evidence, we propose that NPs might represent as potential novel diagnostic and therapeutic targets for cognitive impairment.

Phosphodiesterase inhibitors as therapeutics for traumatic brain injury

Developing therapeutics for traumatic brain injury remains a challenge for all stages of recovery. The pathological features of traumatic brain injury are diverse, and it remains an obstacle to be able to target the wide range of pathologies that vary between traumatic brain injured patients and that evolve during recovery. One promising therapeutic avenue is to target the second messengers cAMP and cGMP with phosphodiesterase inhibitors due to their broad effects within the nervous system. Phosphodiesterase inhibitors have the capability to target different injury mechanisms throughout the time course of recovery after brain injury. Inflammation and neuronal death are early targets of phosphodiesterase inhibitors, and synaptic dysfunction and circuitry remodeling are late potential targets of phosphodiesterase inhibitors. This review will discuss how signaling through cyclic nucleotides contributes to the pathology of traumatic brain injury in the acute and chronic stages of recovery. We will review our current knowledge of the successes and challenges of using phosphodiesterase inhibitors for the treatment of traumatic brain injury and conclude with important considerations in developing phosphodiesterase inhibitors as therapeutics for brain trauma.

Natriuretic hormones in brain function

Natriuretic hormones (NH) include three groups of compounds: the natriuretic peptides (ANP, BNP and CNP), the gastrointestinal peptides (guanylin and uroguanylin), and endogenous cardiac steroids. These substances induce the kidney to excrete sodium and therefore participate in the regulation of sodium and water homeostasis, blood volume, and blood pressure (BP). In addition to their peripheral functions, these hormones act as neurotransmitters or neuromodulators in the brain. In this review, the established information on the biosynthesis, release and function of NH is discussed, with particular focus on their role in brain function. The available literature on the expression patterns of each of the NH and their receptors in the brain is summarized, followed by the evidence for their roles in modulating brain function. Although numerous open questions exist regarding this issue, the available data support the notion that NH participate in the central regulation of BP, neuroprotection, satiety, and various psychiatric conditions, including anxiety, addiction, and depressive disorders. In addition, the interactions between the different NH in the periphery and the brain are discussed.

Enhanced post-ischemic angiogenesis in mice lacking RNF213; a susceptibility gene for moyamoya disease

Moyamoya disease (MMD) is a chronic occlusive cerebrovascular disease with unknown etiology that is characterized by the development of abnormal vascular networks at the base of the brain. Recent genome-wide studies identified RNF213 as an important MMD susceptibility gene. However, the exact mechanism by which the RNF213 abnormality leads to MMD remains unknown. Thus, we sought to clarify the role of RNF213 in angiogenesis under ischemic conditions using conventional RNF213 knockout mice. We assessed the infarction volume, cerebral edema, and vascular density in the ischemic brain after transient middle cerebral artery occlusion (tMCAO). To further evaluate systemic angiogenesis following chronic ischemia, we investigated blood flow recovery using laser speckle flowmetry, the severity of ambulatory impairments, and vascular density in the hind-limb after permanent femoral artery ligation. Results were compared between homozygous RNF213 knockout mice (RNF213 -/-) and wild-type littermates (Wt). No significant differences were observed in infarction volume or the formation of edema following tMCAO, or in vascular density 28 days after tMCAO between RNF213 -/- and Wt. Blood flow recovery was significantly improved in RNF213 -/- from 3 to 28 days after femoral artery ligation, and angiogenesis as shown by vascular density in the hind-limb was significantly enhanced in RNF213 -/- at 28 days. The amelioration of ambulatory impairments was also evident in RNF213 -/-. Angiogenesis was enhanced in mice lacking RNF213 after chronic hind-limb ischemia, which suggested the potential role of the RNF213 abnormality in the development of pathological vascular networks in chronic ischemia.

Characterisation of endothelin-1-induced intrastriatal lesions within the juvenile and adult rat brain using MRI and 31P MRS

Improved non-invasive magnetic resonance (MR) characterisation of in vivo models of focal ischaemic insults such as transient ischaemic attack (TIA) and perinatal arterial ischaemic stroke (AIS) may assist diagnosis, outcome prediction and treatment design. The classic middle cerebral artery occlusion (MCAO) model of ischaemic stroke is well documented in MR studies but generates extensive and complex lesions involving an acute inflammatory response and de-occlusion that immediately restores circulation. By contrast, intrastriatal microinjection of the potent vasoconstrictor, endothelin-1 (ET-1), induces a focal, reversible and low-flow ischaemia in the absence of a typical inflammatory response, which gradually restores blood flow over several hours and may be more relevant to TIA and AIS pathology. This study presents the first comprehensive longitudinal MR characterisation of the real-time anatomical [T1-weighted (T1-w)/T2-weighted (T2-w)], pathophysiological [apparent diffusion coefficient (ADC), cerebral blood volume, gadolinium contrast imaging of blood-brain barrier (BBB) integrity] and metabolic [phosphorus magnetic resonance spectroscopy (31P MRS)] evolution of a purely ischaemic ET-1-induced lesion within the juvenile and adult rat brain. ET-1-induced cytotoxic oedema was visualised on T2-w magnetic resonance imaging (MRI), inconsistent with the conventional notion that it cannot be detected using anatomical MRI. There was no immunohistochemical evidence of an acute inflammatory response or loss of BBB integrity, thus excluding a vasogenic oedema contribution to the pathology. Maximal T2-w intensity correlated with the lowest ADC value in both age groups, re-emphasising the purely ischaemic nature of the lesion and the absence of vasogenic oedema. Furthermore, extensive acute T1-w hypointensity was observed in the presence of cytotoxic oedema-induced T2-w changes, whereas other authors have shown that increased T1 values following MCAO reflect vasogenic oedema. Intriguingly, the lesion border exhibited hyperintensity on T2-w and ADC MRI at later time points, and the former may be a consequence of phagocytosis-induced fatty droplet deposition by macrophages detected immunohistochemically. In spite of a chronically reduced ADC, typically associated with ischaemia-induced energy failure, a 31P MRS-detectable reduction in the phosphocreatine (PCr) to gamma adenosine triphosphate (γATP) ratio was not observed at any time point in either age group, suggesting dissociation of tissue water diffusion and metabolic changes within the ET-1-induced lesion.

Interaction between bradykinin and natriuretic peptides via RGS protein activation in HEK-293 cells

In this study, the interaction of natriuretic peptides (NP) and bradykinin (BK) signaling pathways was identified by measuring membrane potential (V(m)) and intracellular Ca(2+) using the patch-clamp technique and flow cytometry in HEK-293 cells. BK and NP receptor mRNA was identified using RT-PCR. BK (100 nM) depolarized cells activating bradykinin receptor type 2 (B(2)R) and Ca(2+)-dependent Cl(-) channels inhibitable by 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB; 10 μM). The BK-induced Ca(2+) signal was blocked by the B(2)R inhibitor HOE 140. [Des-Arg(9)]-bradykinin, an activator of B(1)R, had no effect on intracellular Ca(2+). NP [atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), C-type natriuretic peptide (CNP), and urodilatin] depolarized HEK-293 cells inhibiting K(+) channels. ANP, urodilatin, BNP [binding to natriuretic peptide receptor (NPR)-A] and 8-bromo-(8-Br)-cGMP inhibited the BK-induced depolarization while CNP (binding to NPR-Bi) failed to do so. The inhibitory effect on BK-triggered depolarization could be reversed by blocking PKG using the specific inhibitor KT 5823. BK-stimulated depolarization as well as Ca(2+) signaling was completely blocked by the phospholipase C (PLC) inhibitor U-73122 (10 nM). The inositol 1,4,5-trisphosphate receptor blocker 2-aminoethoxydiphenyl borate (2-APB; 50 μM) completely inhibited the BK-induced Ca(2+) signaling. UTP, another activator of the PLC-mediated Ca(2+) signaling pathway, was blocked by U-73122 as well but not by 8-Br-cGMP, indicating an intermediate regulatory step for NP via PKG in BK signaling such as regulators of G-protein signaling (RGS) proteins. When RGS proteins were inhibited by CCG-63802 in the presence of BK and 8-Br-cGMP, cells started to depolarize again. In conclusion, as natural antagonists of the B(2)R signaling pathway, NP may also positively interact in pathological conditions caused by BK.

Atrial natriuretic peptide is eliminated from the brain by natriuretic peptide receptor-C-mediated brain-to-blood efflux transport at the blood-brain barrier

Cerebral atrial natriuretic peptide (ANP), which is generated in the brain, has functions in the regulation of brain water and electrolyte balance, blood pressure and local cerebral blood flow, as well as in neuroendocrine functions. However, cerebral ANP clearance is still poorly understood. The purpose of this study was to clarify the mechanism of blood-brain barrier (BBB) efflux transport of ANP in mouse. Western blot analysis showed expression of natriuretic peptide receptor (Npr)-A and Npr-C in mouse brain capillaries. The brain efflux index (BEI) method confirmed elimination of [(125)I]human ANP (hANP) from mouse brain across the BBB. Inhibition studies suggested the involvement of Npr-C in vivo. Furthermore, rapid internalization of [(125)I]hANP by TM-BBB4 cells (an in vitro BBB model) was significantly inhibited by Npr-C inhibitors and by two different Npr-C-targeted short interfering RNAs (siRNAs). Finally, treatment with 1α,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) significantly increased Npr-C expression in TM-BBB4 cells, as determined by liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based targeted absolute proteomics. Our results indicate that Npr-C mediates brain-to-blood efflux transport of ANP at the mouse BBB as a pathway of cerebral ANP clearance. It seems likely that levels of natriuretic peptides in the brain are modulated by 1,25(OH)(2)D(3) through upregulation of Npr-C expression at the BBB.

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.

Glial cells as sources and targets of natriuretic peptides

Natriuretic peptides and their receptors are widely expressed in mammalian CNS and increasing evidence implicates them in the regulation of neural development, synaptic transmission and processing of information, and neuroprotection. Although the peptides have been mainly localized in neuronal populations they are also produced in glial cells. Astroglia and microglia also express functional natriuretic peptide receptors that can regulate important physiological responses. In this article we review evidence on the localization of natriuretic peptides and their receptors in astroglial and microglial cells and summarize data supporting the participation of this signalling system in neuron-glia and glia-brain blood vessel communication relevant to CNS function.

Atrial natriuretic peptide inhibits osmotical glial cell swelling in the ischemic rat retina: Dependence on glutamatergic-purinergic signaling

Atrial natriuretic peptide (ANP) is a regulator of the water and electrolyte content in the brain which also mediates cell volume homeostasis. Here, we determined whether the expression of ANP in the retina of the rat undergoes changes during ischemia-reperfusion, and whether ANP affects the osmotic swelling of Müller glial cells in postischemic retinas under hypotonic conditions. Transient retinal ischemia was induced by elevation of the intraocular pressure above systolic blood pressure for 1h. At 1 and 3 days after reperfusion, there was an increased content of ANP protein in the retina, as determined by Western blotting. The increase of the retinal ANP content was markedly reduced when triamcinolone acetonide (10 mM in 2 microl vehicle) was intravitreally injected before ischemia. ANP inhibited the osmotic swelling of Müller cell somata in retinal slices. The effect of ANP was mediated by activation of NP receptors expressed by retinal neurons which evoked a release of glutamate. The stimulation of metabotropic glutamate receptors expressed by Müller cells evoked an autocrine purinergic signaling mechanism that resulted in the opening of K(+) and Cl(-) channels; the ion efflux counteracted the osmotic swelling of Müller cells. It is concluded that the expression of ANP is transiently upregulated in the postischemic retina of the rat. The increased expression of ANP may represent a part of the retinal response to transient ischemia and may inhibit cytotoxic glial cell swelling.

Differential effects of atrial natriuretic peptide on the brain water and sodium after experimental cortical contusion in the rat

Atrial natriuretic peptide (ANP) plays an important role in the regulation of water and sodium in the body via cyclic GMP (cGMP) pathway. Although ANP has been shown to be protective in cerebral ischemia or intracerebral hemorrhage, its role in traumatic brain injury (TBI) has yet to be elucidated. We herein assessed ANP effects on brain water and sodium in TBI. Controlled cortical impact (3 mm depth, 6 m/sec) was used to induce an experimental cortical contusion in rats. Continuous administration of ANP 0.2 (n = 6) or 0.7 microg/kg/24 h (n = 6), cGMP analogue (8-Bromo-cGMP) 0.1 (n = 5) or 0.3 mg/kg/24 h (n = 5), or vehicle (n = 6) was begun 15 minutes after injury, using a mini-osmotic pump implanted into the peritoneal cavity. At 24 hours after injury, ANP significantly exacerbated brain edema in the injured hemisphere in a dose-dependent manner while it reduced brain sodium concentrations in both hemispheres. These ANP effects could be mimicked by a cGMP analogue. In the second series (n = 20), BBB integrity was assessed by evaluating the extravasation of Evans blue dye. ANP or cGMP analogue significantly worsened BBB disruption in the injured hemisphere at 24 hours after injury. These findings suggest that ANP administration exacerbates brain edema after the experimental cortical contusion in rats, possibly because of an increase in the BBB permeability via cGMP pathway, whereas it reduces brain sodium levels.

Cortical spreading depression and gene regulation: relevance to migraine

Cortical spreading depression (CSD) may be the underlying mechanism of migraine aura. The role of CSD in initiating a migraine headache remains to be determined, but it might involve specific changes in gene expression in the brain. To examine these changes, four episodes of CSD at 5-minute intervals were induced in the mouse brain by application of 300mM KCl, and gene expression was examined 2 hours later using cDNA array and reverse transcriptase-polymerase chain reaction. Controls consisted of groups that received anesthesia only, attachment of recording electrodes only, and application of 0.9% NaCl. Of the over 1,180 genes examined in our experiments, those consistently regulated by CSD included vasoactive peptides; the vasodilator atrial natriuretic peptide was induced by CSD, while the vasoconstrictor neuropeptide Y was downregulated. Other genes specifically regulated by CSD were involved in oxidative stress responses (major prion protein, glutathione-S-transferase-5, and apolipoprotein E). L-type calcium channel mRNA was upregulated. In summary, CSD regulates genes that are intrinsic to its propagation, that identify accompanying vascular responses as a potential source of pain, and that protect against its potential pathological consequences. We believe these observations have strong relevance to the mechanisms of migraine and its outcomes.

Ischaemic brain oedema

Ischaemic brain oedema appears to involve two distinct processes, the relative contribution and time course of which depend on the duration and severity of ischaemia, and the presence of reperfusion. The first process involves an increase in tissue Na+ and water content accompanying increased pinocytosis and Na+, K+ ATPase activity across the endothelium. This is apparent during the early phase of infarction and before any structural damage is evident. This phenomenon is augmented by reperfusion. A second process results from a more indiscriminate and delayed BBB breakdown that is associated with infarction of both the parenchyma and the vasculature itself. Although, tissue Na+ level still seems to be the major osmotic force for oedema formation at this second stage, the extravasation of serum proteases is an additional potentially deleterious factor. The relative importance of protease action is not yet clear, however, degradation of the extracellular matrix conceivably leads to further BBB disruption and softening of the tissue, setting the stage for the most pronounced forms of brain swelling. A number of factors mediate or modulate ischaemic oedema formation, however, most current information comes from experimental models, and clinical data on this microcosmic level is lacking. Clinically significant brain oedema develops in a delayed fashion after large hemispheric strokes and is a cause of substantial mortality. Neurological signs appear to be at least as good as direct ICP measurement and neuroimaging in detecting and gauging the secondary damage produced by stroke oedema. The neuroimaging characteristics of the stroke, specifically the early involvement of greater than half of the MCA territory, are, however, highly predictive of the development of severe oedema over the subsequent hours and days. None of the available medical therapies provide substantial relief from the oedema and raised ICP, or at best, they are temporizing in most cases. Hemicraniectomy appears most promising as a method of avoiding death from brain compression, but the optimum timing and manner of patient selection are currently being investigated. All approaches to massive ischaemic brain swelling are clouded by the potential for survival with poor functional outcome. It is possible to manage blood pressure, serum osmolarity by way of selective fluid administration, and a number of other systemic factors that exaggerate brain oedema. Broad guidelines for treatment of stroke oedema can therefore be given at this time.

Effects of centrally administered arginine vasopressin and atrial natriuretic peptide on the development of brain edema in hyponatremic rats

OBJECTIVE

Centrally released arginine vasopressin (AVP) and atrial natriuretic peptide (ANP) have been shown to participate in brain volume regulation. The aim of the present study was to evaluate the effects of centrally administered AVP and ANP on the time course of development of brain edema in vivo in hyponatremic rats, using diffusion-weighted magnetic resonance imaging.

METHODS

We performed intracerebroventricular (ICV) administration of 120 microg AVP, 20 microg ANP, or physiological saline into the right lateral ventricle in 18 rats. Twenty-five minutes after the treatment, we induced systemic hyponatremia by the intraperitoneal administration of 140 mmol/L dextrose solution. Serial diffusion-weighted imaging scans were obtained up to 96 minutes after the start of the hyponatremia. Changes in the brain extra-to intracellular volume fraction ratio were estimated as changes in the apparent diffusion coefficient (ADC).

RESULTS

No change in the ADC was observed after the ICV injection of saline or AVP. The onset of hyponatremia induced a rapid and marked ADC reduction in both groups, indicating an increased intracellular space. However, the ADC decrease became significantly more pronounced in the ICV AVP group (83.3+/-4.7% of baseline level, mean +/- standard deviation) than in the saline group (93.7+/-3.3% of baseline, P < 0.001) after 78 minutes of hyponatremia. The ICV injection of ANP induced a prompt ADC increase to 111.5+/-10.0% (P < 0.05) of the baseline level, indicating a rapid reduction in the intracellular compartment. In the initial phase of hyponatremia, the ADC values in the ANP group were consistently higher than those in the saline group, decreasing finally to 86.9+/-9.6% after 96 minutes of hyponatremia.

CONCLUSION

Our findings demonstrate the opposite effects of AVP and ANP on the intracellular volume fraction of the brain during the development of cellular brain edema, with an immediate effect on ANP and a delayed effect on AVP. The results emphasize the direct effects of these hormones on the cellular volume regulatory mechanisms in the brain during the development of cerebral edema.

MRI study of transient cerebral ischemia in the gerbil: interest of T2 mapping

RATIONALE AND OBJECTIVES

The aim of this study was to evaluate the diagnostic use of MRI and, more precisely, the use of quantitative T2 imaging at 7 T for the early detection of neuronal cerebral alterations after transient ischemia in the gerbil.

METHODS

One hundred forty-seven Mongolian gerbils were separated into four groups for which a bicarotid artery occlusion lasted for 4, 6, 8, or 10 minutes, respectively. The animals were scanned before carotid artery occlusion and at 3, 6, 10, 24, and 48 hours and 5 days after the ischemic incident. MR images were acquired on a Bruker Avance DRX300 mini-imaging system.

RESULTS

Our results show that T2 mapping is able to localize brain damage induced by transient ischemia and to detect early perturbations in water content (as early as 6 hours after ischemia).

CONCLUSIONS

T2 measurements in the striata are correlated with the severity of the ischemic incident, since the changes observed on the T2 images are directly proportional to the duration of occlusion.

Immunohistochemical modifications of vasoactive neuropeptides and excitatory amino acids in the nervous tissue of the Mongolian gerbil after transient cerebral ischemia

Modifications in the tissue concentration of vasoactive peptides (Endothelin, Calcitonin Gene Related Peptide, Atrial Natriuretic Peptide) and excitatory amino acids (glutamate, aspartate) were found in the nervous tissue of Mongolian gerbils after transient cerebral ischemia which was induced by unilateral occlusion of the common carotid artery for 30 min 4 h. In fact, immunostaining for these peptides was more intense in the ischemic tissue: the greatest increases of tissue immunoreactivity were observed for Endothelin; smaller differences were found for Calcitonin Gene Related Peptide and Atrial Natriuretic Peptide. Immunostaining for Neuropeptide Y, another vasoactive neuropeptide, was virtually unchanged. Infarct areas, when present, contained numerous Endothelin-immunoreactive cell bodies. On the contrary, the same areas were completely void of glutamate- or aspartate-immunostained neurons, normally present in the correspondent regions of the control tissue. The present results suggest that severe cerebral ischemia is paralleled by an unbalance of local vasoactive factors. The predominance of vasoconstrictor action of Endothelin might play a major role in the irreversible damage, together with the excitotoxic effect of the extracellular accumulation of excitatory amino acids, probably due to a leakage from neuronal cell somata, as suggested by the disappearance of glutamate- or aspartate-immunostained neurons.

Magnetic resonance imaging during cerebral hypoxia-ischemia: T2 increases in 2-week-old but not 4-week-old rats

We investigated whether the changes detectable with magnetic resonance imaging techniques during and after an episode of cerebral hypoxia-ischemia differ in immature and older brain. Diffusion weighted (DW) and T2-weighted (T2W) images were repeatedly acquired before, during, and after an episode of cerebral hypoxia-ischemia (unilateral carotid artery occlusion plus hypoxia) in 2- and 4-wk-old rats lightly anesthetized with isoflurane. Areas of increased brightness were detected in DW images from both 2- and 4-wk-old rats by 10-20 min after the start of hypoxia. These hyperintense areas increased during hypoxia, comprising 60.8+/-4.9% and 30.5+/-2.7% of the brain image at the level of the thalamus in 2-wk-old and 4-wk-old animals, respectively (p < 0.003). Hyperintense areas (e.g. 27.0+/-8.3%) also appeared in T2W images during hypoxia-ischemia in 2-wk-old animals, but these did not occur in 4-wk-old animals (p < 0.02). This observation was reflected in T2, which increased during hypoxia-ischemia in the 2-wk-old but not the 4-wk-old group. By 60 min after the termination of hypoxia-ischemia in either age group, areas of hyperintensity resolved and then reappeared 24 h later on both DW and T2W images. Thus, irrespective of age, magnetic resonance imaging changes during transient hypoxia-ischemia generally recover with a delayed or secondary increase in DW and T2W hyperintensity hours later. Immature brain differs from older brain primarily with respect to some combination of hypoxic/ischemic cellular or biochemical changes, that are detectable as increases in T2 within 2-wk-old but not 4-wk-old animals.

Effect of benzodiazepines and neurosteroids on ammonia-induced swelling in cultured astrocytes

Astroglial swelling occurs in acute hyperammonemic states, including acute hepatic encephalopathy. In these conditions, the peripheral-type benzodiazepine receptor (PBR), a receptor associated with neurosteroidogenesis, is up-regulated. This study examined the potential involvement of PBRs and neurosteroids in ammonia-induced astrocyte swelling in culture. At low micromolar concentrations, the PBR antagonist PK 11195, atrial natriuretic peptide, and protoporhyrin IX, which are known to interact with the PBR, attenuated (16-100%) the effects of ammonia, whereas the PBR agonists Ro5-4864, diazepam binding inhibitor (DBI51-70), and octadecaneuropeptide exacerbated (10-15%) the effects of ammonia. At micromolar concentrations, diazepam, which interacts with both the PBR and the central-type benzodiazepine receptor (CBR), increased swelling by 11%, whereas flumazenil, a CBR antagonist, had no effect. However, at 100 nM diazepam and flumazenil abrogated ammonia-induced swelling. The neurosteroids dehydroepiandrosterone sulfate, tetrahydroprogesterone, pregnenolone sulfate, and tetrahydrodeoxycorticosterone (THDOC), products of PBR stimulation, at micromolar concentrations significantly enhanced (70%) ammonia-induced swelling. However, at nanomolar concentrations, these neurosteroids, with exception of THDOC, blocked ammonia-induced swelling. We conclude that neurosteroids and agents that interact with the PBR influence ammonia-induced swelling. These agents may represent novel therapies for acute hyperammonemic syndromes and other conditions associated with brain edema and astrocyte swelling.

The effects of a butanediol treatment on acute focal cerebral ischemia assessed by quantitative diffusion and T2 MR imaging

Increased water T2 values indicates the presence of vasogenic edema. Decreased apparent diffusion coefficient (ADC) maps reveal ischemic areas displaying cytotoxic edema. ADC and T2 abnormalities spread through the middle cerebral artery (MCA) territory up to 24 h after middle cerebral artery occlusion (MCAO). Also, it was found that ADC and T2 contours closely match at 3.5 and 24 h. Since butanediol reduces vasogenic edema and improves energy status in various models of ischemia, we used these two techniques to investigate putative improvements in cytotoxic and vasogenic edema after permanent MCAO performed on rats. Rats were given no treatment (n = 8), or a treatment with 25 mmol/kg intraperitoneal (i.p.) butanediol (n = 5), 30 min before and 2.5 h after MCAO. Quantitative ADC and T2 maps of brain water were obtained, from which the volumes presenting abnormalities were calculated at various time points up to 24 h. Effects of butanediol on the ADC and T2 values in these areas were determined. Butanediol reduced neither the ADC volume nor the initial ADC decline. However, it reduced T2 volumes by 32% at 3.5 h and 15% at 24 h (p < 0.05), and reduced T2 increase in the striatum at 3.5 h post-MCAO. Therefore, our results show for the first time that a pharmacological agent such as butanediol can delay the development of vasogenic edema but does not limit the development of vasogenic edema but does not limit the development of cytotoxic edema. ADC imaging detects areas of severe metabolic disturbance but not moderately ischemic peripheral areas where butanediol is presumed to be more efficacious.

Atrial natriuretic peptide (ANP) attenuates brain oedema accompanying experimental subarachnoid haemorrhage

The effects of centrally administered atrial natriuretic peptide (ANP) on the brain water and electrolyte contents were investigated in a rodent subarachnoid haemorrhage (SAH) model. SAH caused statistically significant increases in the brain sodium and water contents, while the potassium content did not change significantly, indicating that the brain oedema could be classified as having a primarily vasogenic component. Two micrograms or 5 micrograms of rat ANP administered into the lateral ventricle at the time of SAH induction statistically significantly decreased the water and sodium accumulation measured 90 minutes following SAH. The same treatment did not inhibit development of brain oedema measured 3 hours following SAH. However, when 5 micrograms of ANP was administered intraventricularly at the time of SAH induction and also 90 minutes later, the brain oedema 3 hours following SAH was again reduced statistically significantly. These effects of ANP were found not to be mediated by primary changes in serum osmolality and electrolyte concentrations. The present results confirm that centrally administered ANP may act directly on the central nervous system to inhibit brain water and sodium accumulation in SAH-induced brain oedema. The potentials of influencing the central neuro-endocrine system as a novel way of the treatment of brain oedema are discussed.

Insight into the regulation by second messenger molecules of the permeability of the blood-brain barrier

Recent advances in our knowledge of the blood-brain barrier have in part been made by studying the properties and function of cerebral endothelial cells in vitro. After an era of working with a fraction, enriched in cerebral microvessels by centrifugation, the next generation of in vitro blood-brain barrier model systems was introduced, when the conditions for routinely culturing the endothelial cells were established. This review summarizes the results obtained mainly from this in vitro approach. Different elements of the intracellular signaling messenger systems have been detected in the course of our studies in the cerebral endothelial cells. It has been shown that the synthesizing enzymes of and substrate proteins for the second messenger molecules are present in the cerebral endothelial cells, and their activity and/or amount can change in pathological circumstances, i.e., during the formation of brain oedema. Pharmacological treatments interfering with the second messenger systems proved to be effective in the prevention of brain oedema formation.

Oedema reduction by levemopamil in focal cerebral ischaemia of spontaneously hypertensive rats studied by magnetic resonance imaging

The effect of treatment with the Ca2+ channel blocker and 5-HT2 receptor antagonist levemopamil (recommended INN for (S)-emopamil) on the extent of ischaemic brain oedema was studied by magnetic resonance imaging in vivo. Focal cerebral ischaemia was induced in spontaneously hypertensive rats by permanent middle cerebral artery occlusion. The treatment consisted of slow intravenous injections of an aqueous solution of levemopamil given immediately after middle cerebral artery occlusion and again 2 h and 4 h later. One group of animals (n = 17) received 3 x 2 mg/kg of levemopamil (total dose: 6 mg/kg) and another group (n = 13) received 3 x 4 mg/kg (total dose: 12 mg/kg). Saline was administered to the controls (n = 16) at corresponding times. High-resolution T2-weighted spin echo images were obtained 24 h after middle cerebral artery occlusion from two transversal brain planes (4.5 mm and 6.5 mm dorsal to the interaural line). Dose-dependent reductions of brain oedema were achieved in both brain planes. The lower dose of levemopamil reduced the extent of oedema significantly (P < 0.05) by 20 +/- 3.7% in the upper and by 21 +/- 3.8% in the lower brain plane as compared to the controls (means +/- S.E.M.). The higher dose diminished the extent of oedema in the same planes by 30 +/- 3.5% and 31 +/- 4.0%, respectively. Dose-dependent reductions of infarct size, as determined by vital tissue staining using 2,3,5-triphenyltetrazolium chloride (TTC), were observed in the levemopamil-treated groups. Body temperature was not affected by levemopamil, suggesting direct cerebroprotection by this drug.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of atrial natriuretic peptide on brain edema, intracranial pressure and cerebral energy metabolism in rat congenital hydrocephalus

Atrial natriuretic peptide (ANP) regulates fluid and electrolyte homeostasis in the central nervous system. In this study, we evaluated the effects of ANP on brain edema, intracranial pressure (ICP) and cerebral energy metabolism in congenital hydrocephalus in rats. Brain edema, indicated by the longitudinal relaxation time (T1), was evaluated by 1H-magnetic resonance imaging (MRI). The ICP was monitored with a miniature pressure-transducer with telemetric system. Cerebral energy metabolism, indicated by PCr/Pi ratio, was measured by 31P-magnetic resonance spectroscopy (MRS). The rats were given 10 microliters of ANP in the left cerebral ventricle. Three different concentrations of ANP were given; 0.2 (group I), 2.0 (group II) and 20.0 (group III) micrograms/10 microliters, respectively. 10 microliters of saline was injected into the ventricle of the control group rats. There were no significant changes of ICP, T1 value and PCr/Pi ratio among the control group, group I and group II. In group III, in contrast, ICP decreased significantly at 20 minutes after ANP administration and stayed at this ICP level for 60 minutes. The T1 value decreased and PCr/Pi ratio increased 30 minutes after ANP administration. This study revealed that intraventricularly administered ANP could decrease ICP, reduce brain edema and improve the cerebral energy metabolism in rats with congenital hydrocephalus.

The blood-brain barrier in vitro: the second decade

Ever since the discovery of Paul Ehrlich (1885 Das Sauerstoff-bedürfnis des Organismus: Hirschwald, Berlin) about the restricted material exchange, existing between the blood and the brain, the ultimate goal of subsequent studies has been mainly directed towards the elucidation of relative importance of different cellular compartments in the peculiar penetration barrier consisting the structural basis of the blood-brain barrier (BBB). It is now generally agreed that, in most vertebrates, the endothelial cells of the central nervous system (CNS) are responsible for the unique penetration barrier, which restricts the free passage of nutrients, hormones, immunologically relevant molecules and drugs to the brain. After an era of studying with endogenous or exogenous tracers the unique permeability properties of cerebral endothelial cells in vivo, the next generation, i.e. the in vitro blood-brain barrier model system was introduced in 1973. Recent advances in our knowledge of the BBB have in part been made by studying the properties and function of cerebral endothelial cells (CEC) with this in vitro approach. This review summarizes the results obtained on isolated brain microvessels in the second decade of its advent.

Effects of arginine vasopressin and atriopeptin on glial cell volume measured as 3-MG space

This study evaluates the hypothesis that arginine vasopressin (AVP) and atriopeptin, peptide hormones synthesized and released within the brain, are regulators of brain cell volume using cultured astroglial cells derived from newborn rats. Cell water content, regarded as volume, was measured in defined, serum-free medium as the 3-O-methylglucose (3-MG) space. Initial experiments established conditions such that glucose, which competes with 3-MG for the glucose carrier, would not interfere with the measurement of the 3-MG space. AVP increased the 3-MG space of glial cells by an average of 25% between 30 and 120 min of exposure, whereas atriopeptin decreased it by 32%. The 3-MG space remained close to normal after coadministration of both peptides. The AVP-dependent increase in 3-MG space was blocked both by the V1 antagonist d(CH2)5Tyr(Me)AVP (Manning compound) and by the cotransport inhibitor, bumetanide. Results are consistent with a role for AVP and atriopeptin in the homeostasis of atroglial cell volume.

Volume regulation of the brain tissue--a survey

Though the brain bulk has been considered to be constant in several pressure homeostasis studies, the central nervous tissue may be responsible for the accommodation of extracerebral masses exceeding the volume regulation capacity of the cerebral blood and cerebrospinal fluid. Volume buffering of the nervous tissue may even be functioning in parallel, in conjunction with the "fluid" compartments. Of the existing volume regulatory models, the following are discussed: osmotic feedback (buffering) preventing major fluid shifts in osmotic or pressure disequilibrium at the blood brain barrier (BBB), and the 4-compartment model, which under steady-state conditions can be regarded as an analogue of systemic tissue volume regulation, i.e. secretion of fluid at the BBB, bulk flow of interstitial space fluid (ISF) in the brain and absorption via the cerebrospinal fluid (CSF). The most recent data are presented, confirming that accommodation of space occupation by the nervous tissue is achieved via shrinkage of the extracerebral fluid (ECF), while the cell volume remains relatively constant. These findings confirm Hakim's classical hypothesis, based on biomechanical considerations, that the brain behaves like a sponge. The data presented in this survey point to a more general hypothesis: the brain interstitial space can vary in volume according to physiological and pathological stress, within certain bounds this being a reversible process which does not affect brain function. The potential role of the central neuro-endocrine system in brain volume regulation is discussed. Vasopressin (AVP) and atriopeptin (ANP) probably, function within the brain via a paracrine mechanism, as physiological regulators of brain cell and ISF volume.(ABSTRACT TRUNCATED AT 250 WORDS)

Diffusion- and T2-weighted imaging: evaluation of oedema reduction in focal cerebral ischaemia by the calcium and serotonin antagonist levemopamil

Magnetic resonance imaging (MRI) was used to evaluate beneficial drug effects in a rat model of focal cerebral ischaemia. Extent of cerebral oedema was measured on T2-weighted images 24 hr after permanent middle cerebral artery occlusion (MCAO) in spontaneously hypertensive rats. Areas of increased signal intensity strongly correlate with histochemically determined areas of ischaemia in corresponding brain planes (r = 0.84; p < .001). In a separate cohort of animals, spatial progression of oedema formation was studied at 3, 24, 48, and 72 hr after MCAO showing a maximum extent at 48 hr. Early events in cerebral ischaemia were monitored using diffusion-weighted imaging. Effects of levemopamil [formerly (S)-emopamil], a calcium and serotonin antagonist, and the reference compound isradipine were quantified on high resolution T2-weighted spin-echo images 24 hr after MCAO. Combined pre- and posttreatment with isradipine showed a 21% inhibition of oedema progression. Application of a single dose (10 mg/kg) of levemopamil either 30 min before or 2 hr after MCAO revealed a diminution of oedematous areas by 19% and 25%, respectively. Levemopamil reduces the extent of ischaemic brain oedema in an established stroke model.

Atrial natriuretic peptide augments the blood-brain transfer of water but not leucine and glucose

Recent evidence predicts an effect of atrial natriuretic peptide (ANP) on the blood-brain transfer of water. To test this prediction, we measured the blood-brain transfer of water, L-leucine, and D-glucose in 9 brain regions of male rats after intravenous injection of 10 pmol ANP. The peptide elicited an increase of the permeability-surface area (PaS) product of labeled water by 28-108% while the PaS products of leucine and glucose remained unchanged. Cerebral blood flow increased 15-48% while cardiac output and plasma volume in brain did not alter, indicating no change of capillary surface area (CSA). Regionally, the CSA varied from 63 cm2/g (striatum) to 97 cm2/g (colliculi) and the fraction of capillaries contributing to the total vascular volume varied from 29% (olfactory bulb/lobe) to 62% (striatum). The blood-brain barrier (BBB) permeability to water (5.7 micron/s) was an order of magnitude higher than to glucose (0.4 micron/s) or to leucine (0.3 micron/s).