Involvement of non-neuronal brain cells in AVP-mediated regulation of water space at the cellular, organ, and whole-body level

A population pharmacokinetics model of balovaptan to support dose selection in adult and pediatric populations

Balovaptan is a brain-penetrating vasopressin receptor 1a antagonist previously investigated for the core symptoms of autism spectrum disorder (ASD). A population pharmacokinetic (PK) model of balovaptan was developed, initially to assist clinical dosing for adult and pediatric ASD studies and subsequently for new clinical indications including malignant cerebral edema (MCE) and post-traumatic stress disorder. The final model incorporates one-compartment disposition and describes time- and dose-dependent non-linear PK through empirical drug binding and a gut extraction component with turnover. An age effect on clearance observed in children was modeled by an asymptotic function that predicts adult-equivalent exposures at 40% of the adult dose for children aged 2-4 years, 70% for 5-9 years, and at the full adult dose for ≥ 10 years. The model was adapted for intravenous (IV) balovaptan dosing and combined with in vitro and ex vivo pharmacodynamic data to simulate brain receptor occupancy as a guide for dosing in a phase II trial of MCE prophylaxis after acute ischemic stroke. A sequence of three stepped-dose daily infusions of 50, 25 and 15 mg over 30 or 60 min was predicted to achieve a target occupancy of ≥ 80% in ≥ 95% of patients over a 3-day period. This model predicts both oral and IV balovaptan exposure across a wide age range and will be a valuable tool to analyze and predict its PK in new indications and target populations, including pediatric patients.

Astrocytic insulin receptor controls circadian behavior via dopamine signaling in a sexually dimorphic manner

Mammalian circadian clocks respond to feeding and light cues, adjusting internal rhythms with day/night cycles. Astrocytes serve as circadian timekeepers, driving daily physiological rhythms; however, it's unknown how they ensure precise cycle-to-cycle rhythmicity. This is critical for understanding why mistimed or erratic feeding, as in shift work, disrupts circadian physiology- a condition linked to type 2 diabetes and obesity. Here, we show that astrocytic insulin signaling sets the free-running period of locomotor activity in female mice and food entrainment in male mice. Additionally, ablating the insulin receptor in hypothalamic astrocytes alters cyclic energy homeostasis differently in male and female mice. Remarkably, the mutants exhibit altered dopamine metabolism, and the pharmacological modulation of dopaminergic signaling partially restores distinct circadian traits in both male and female mutant mice. Our findings highlight the role of astrocytic insulin-dopaminergic signaling in conveying time-of-feeding or lighting cues to the astrocyte clock, thus governing circadian behavior in a sex-specific manner.

Tolvaptan attenuated brain edema in experimental intracerebral hemorrhage

Arginine-vasopressin (AVP) is believed to be positively correlated with the brain edema formation, but the underlying mechanism is still unclear. In this study, we explored the role of the V2 receptors antagonist tolvaptan on brain edema following intracerebral hemorrhage (ICH) with a rat model. Animals were randomly given tolvaptan or vehicle through oral gavage at 12 h, 36 h, and 60 h after ICH surgery. Brain swelling (%), brain water content(BWC), neurological scores, Evans blue fluorescence and blood-brain barrier (BBB) tight junction proteins were measured to evaluate the effect of tolvaptan in ICH. We found that tolvaptan alleviated the brain swelling (%), decreased the BWC growth, and attenuated the neurological deficits after ICH (p < 0.05, vs vehicle). What's more, tolvaptan increased the expression of ZO-1 and occludin (p < 0.05, vs vehicle), which might be attributed to the down-regulated effects of tolvaptan on MMP-9. These results provided evidence supporting the use of tolvaptan for ICH-induced brain edema. Large animal experiments are required to further explore the efficacy and mechanism of tolvaptan in ICH treatment.

The Effects of Clinically Relevant Hypertonic Saline and Conivaptan Administration on Ischemic Stroke

Cerebral edema after stroke is associated with poor neurological outcomes. Current therapies are limited to osmotic agents, such as hypertonic saline (HS), which reduce intracranial pressure. Although studies have demonstrated edema reductions following HS, tissue survival has not been thoroughly examined. Additionally, the efficacy of promising pharmacological agents has not been evaluated for synergy with osmotic agents. Conivaptan is an FDA-approved vasopressin receptor antagonist that may exert both osmotic and anti-inflammatory effects. In this study, rats were subjected to middle cerebral artery occlusion prior to treatment with 5 % HS bolus +5 % HS maintenance (HS), conivaptan alone (Con), conivaptan +5 % HS maintenance (Con + HS), or conivaptan +5 % HS bolus +5 % maintenance (Con + HSb). Treatments were initiated at six (Early) or 24 h (Late) following stroke and rats were euthanized at 48 h to evaluate infarct volume, brain edema, and microglia/macrophage activation. Infarct volume and brain edema in the Early HS, Early Con, and Late HS groups were significantly reduced compared with controls. Interestingly, only the Early Con group demonstrated reduced microglia/macrophage activation. These data suggest an anti-inflammatory mechanism for conivaptan and provide support for a multipronged approach combining osmotic agents with compounds that inhibit the neuroinflammatory response to stroke.

Hyponatremia in Neurotrauma: The Role of Vasopressin

Hyponatremia is frequent in patients suffering from traumatic brain injury, subarachnoid hemorrhage, or following intracranial procedures, with approximately 20% having a decreased serum sodium concentration to <125 mmol/L. The pathophysiology of hyponatremia in neurotrauma is not completely understood, but in large part is explained by the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). The abnormal water and/or sodium handling creates an osmotic gradient promoting the shift of water into brain cells, thereby worsening cerebral edema and precipitating neurological deterioration. Unless hyponatremia is corrected promptly and effectively, morbidity and mortality increases through seizures, elevations in intracranial pressure, and/or herniation. The excess mortality in patients with severe hyponatremia (<125 mmol/L) extends beyond the time frame of hospital admission, with a reported mortality of 20% in hospital and 45% within 6 months of follow-up. Current options for the management of hyponatremia include fluid restriction, hypertonic saline, mineralocorticoids, and osmotic diuretics. However, the recent development of vasopressin receptor antagonists provides a more physiological tool for the management of excess water retention and consequent hyponatremia, such as occurs in SIADH. This review summarizes the existing literature on the pathophysiology, clinical features, and management of hyponatremia in the setting of neurotrauma.

Ion Homeostasis in Rhythmogenesis: The Interplay Between Neurons and Astroglia

Proper function of all excitable cells depends on ion homeostasis. Nowhere is this more critical than in the brain where the extracellular concentration of some ions determines neurons' firing pattern and ability to encode information. Several neuronal functions depend on the ability of neurons to change their firing pattern to a rhythmic bursting pattern, whereas, in some circuits, rhythmic firing is, on the contrary, associated to pathologies like epilepsy or Parkinson's disease. In this review, we focus on the four main ions known to fluctuate during rhythmic firing: calcium, potassium, sodium, and chloride. We discuss the synergistic interactions between these elements to promote an oscillatory activity. We also review evidence supporting an important role for astrocytes in the homeostasis of each of these ions and describe mechanisms by which astrocytes may regulate neuronal firing by altering their extracellular concentrations. A particular emphasis is put on the mechanisms underlying rhythmogenesis in the circuit forming the central pattern generator (CPG) for mastication and other CPG systems. Finally, we discuss how an impairment in the ability of glial cells to maintain such homeostasis may result in pathologies like epilepsy and Parkinson's disease.

Distinct functional states of astrocytes during sleep and wakefulness: Is norepinephrine the master regulator?

Astrocytes are the chief supportive cells in the central nervous system, but work over the past 20 years have documented that astrocytes also contribute to complex neural processes, such as working memory. Recent discoveries of norepinephrine-mediated astrocytic Ca²⁺ responses have raised the possibility that astrocytic activity in the adult brain is driven by global responses to changes in behavioral state. Moreover, analysis of the interstitial space volume suggests that astrocytes may undergo changes in cell volume in response to activation of norepinephrine receptors. This review will focus on what is known about astrocytic functions within the nervous system, and how these functions interrelate with rapid changes in behavioral state mediated by norepinephrine signaling.

Clustering of Kir4.1 at specialized compartments of the lateral membrane in ependymal cells of rat brain

Brain ependymal cells, which form an epithelial layer covering the cerebral ventricles, have been shown to play a role in the regulation of cerebrospinal and interstitial fluids. The machinery underlying this, however, remains largely unknown. Here, we report the specific localization of an inwardly rectifying K(+) channel, Kir4.1, on the ependymal cell membrane suggesting involvement of the channel in this function. Immunohistochemical study with confocal microscopy identified Kir4.1 labeling on the lateral but not apical membrane of ependymal cells. Ultrastructural analysis revealed that Kir4.1-immunogold particles were specifically localized and clustered on adjacent membranes at puncta adherens type junctions, whereas an aquaporin water channel, AQP4, that was also detected on the lateral membrane only occurred at components other than adherens junctions. Therefore, in ependymal cells, Kir4.1 and AQP4 are partitioned into distinct membrane compartments that might respectively transport either K(+) or water. Kir4.1 was also expressed in a specialized form of ependymal cell, namely the tanycyte, being abundant in tanycyte processes wrapping neuropils and blood vessels. These specific localizations suggest that Kir4.1 mediates intercellular K(+) exchange between ependymal cells and also K(+)-buffering transport via tanycytes that can interconnect neurons and vessels/ventricles. We propose that ependymal cells and tanycytes differentially operate Kir4.1 and AQP4 actively to control the property of fluids at local areas in the brain.

Neurosurgical Hyponatremia

Hyponatremia is a frequent electrolyte imbalance in hospital inpatients. Acute onset hyponatremia is particularly common in patients who have undergone any type of brain insult, including traumatic brain injury, subarachnoid hemorrhage and brain tumors, and is a frequent complication of intracranial procedures. Acute hyponatremia is more clinically dangerous than chronic hyponatremia, as it creates an osmotic gradient between the brain and the plasma, which promotes the movement of water from the plasma into brain cells, causing cerebral edema and neurological compromise. Unless acute hyponatremia is corrected promptly and effectively, cerebral edema may manifest through impaired consciousness level, seizures, elevated intracranial pressure, and, potentially, death due to cerebral herniation. The pathophysiology of hyponatremia in neurotrauma is multifactorial, but most cases appear to be due to the syndrome of inappropriate antidiuretic hormone secretion (SIADH). Classical treatment of SIADH with fluid restriction is frequently ineffective, and in some circumstances, such as following subarachnoid hemorrhage, contraindicated. However, the recently developed vasopressin receptor antagonist class of drugs provides a very useful tool in the management of neurosurgical SIADH. In this review, we summarize the existing literature on the clinical features, causes, and management of hyponatremia in the neurosurgical patient.

Ionic transporter activity in astrocytes, microglia, and oligodendrocytes during brain ischemia

Glial cells constitute a large percentage of cells in the nervous system. During recent years, a large number of studies have critically attributed to glia a new role which no longer reflects the long-held view that glia constitute solely a silent and passive supportive scaffolding for brain cells. Indeed, it has been hypothesized that glia, partnering neurons, have a much more actively participating role in brain function. Alteration of intraglial ionic homeostasis in response to ischemic injury has a crucial role in inducing and maintaining glial responses in the ischemic brain. Therefore, glial transporters as potential candidates in stroke intervention are becoming promising targets to enhance an effective and additional therapy for brain ischemia. In this review, we will describe in detail the role played by ionic transporters in influencing astrocyte, microglia, and oligodendrocyte activity and the implications that these transporters have in the progression of ischemic lesion.

Role of vasopressin V(1a) and V2 receptors for the development of secondary brain damage after traumatic brain injury in mice

Brain edema is still one of the most deleterious sequels of traumatic brain injury (TBI), and its pathophysiology is not sufficiently understood. The goal of the current study was to investigate the role of arginine vasopressin (AVP), also known as antidiuretic hormone (ADH), an important regulator of tissue water homeostasis, for the formation of post-traumatic brain edema, intracranial pressure (ICP), brain damage, and functional deficits following brain trauma. C57/B16 mice (n=112) were subjected to controlled cortical impact (CCI; 8m/s, 1 mm). At 3 min after trauma, animals received 500 ng of the AVP V(1a)-receptor antogonist (deamino-Pen(1), O-Me-Tyr(2), Arg(8)]-Vasopressin) or 500 ng of the AVP V2-receptor antagonist (adamantaneacetyl(1), O-Et-D-Tyr(2),Val(4), Abu(6),Arg(8,9)]-Vasopressin) by intracerebroventricular injection. After trauma, cerebral water content (24 h), ICP (24 h), contusion volume (24 h and 7 days), and functional outcome (1-7 days) were assessed (n=8 per experimental group). Post-traumatic inhibition of AVP V(1A) receptors reduced ICP by 29% (p < 0.05), brain water content by 45% (p < 0.05), and secondary contusion expansion by 37% (p < 0.05), and it significantly improved motor function 6 and 7 days after trauma (p < 0.05). Inhibition of AVP V2 receptors had no significant effect. The current results demonstrate that vasopressin V(1A) receptors are involved in the pathogenesis of brain edema formation and the subsequent development of secondary brain damage after traumatic brain injury. Accordingly, our study suggests that vasopressin V(1A) receptors may represent a novel therapeutic target for the treatment of post-traumatic brain edema and secondary brain damage.

Changes and effects of plasma arginine vasopressin in traumatic brain injury

BACKGROUND

Traumatic brain injury (TBI) is a common disease accompanied by chronic morbidity and mortality. The pathological mechanism and effective pharmacological treatments of TBI remain undetermined. It is suggested that AVP is involved in TBI. It is thus interesting to investigate the changes and effects of plasma AVP in clinical trials.

METHODS

The serum concentrations of AVP, serum electrolytes, and serum osmolarity in a total of 23 TBI patients were dynamically monitored (on admission, Day 1, Day 3, and Day 5). Relationship between AVP and severity of brain injury and functional outcome were evaluated, respectively.

RESULTS

The mean AVP serum concentrations in the TBI group were significantly higher than those recorded in the control (CTRL) group on intensive care unit (ICU) admission and Day 1 (p<0.05). On Day 3 and Day 5, the differences between those groups were not significant (p>0.05). The negative correlations were found between sodium and AVP (r=-0.35; p<0.05) and between osmolarity and AVP (r=-0.42; p<0.05). In poor outcome group, the mean AVP serum concentrations were significantly higher than in good outcome group and CTRL group (p<0.05). A statistically significant correlation was also found between AVP on ICU admission and the initial Glasgow Coma Scale (r=0.47; p<0.05).

CONCLUSION

We suggest that AVP is involved in the pathophysiology process of secondary brain damage after TBI. It seems that AVP antagonist is a promising target for the treatment of TBI, while further studies should be carried out.

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.

The vasopressin system--from antidiuresis to psychopathology

Vasopressin is a neuropeptide with multiple functions. In addition to its predominantly antidiuretic action after peripheral secretion from the posterior pituitary, it seems to fulfill--together with its receptor subtype--all requirements for a neuropeptide system critically involved in higher brain functions, including cognitive abilities and emotionality. Following somatodendritic and axonal release in distinct brain areas, vasopressin acts as a neuromodulator and neurotransmitter in multiple and varying modes of interneuronal communication. Accordingly, changes in vasopressin expression and release patterns may have wide-spread consequences. As shown in mice, rats, voles, and humans, central vasopressin release along a continuum may be beneficial to the individual, serving to adjust physiology and behavior in stressful scenarios, possibly at the potential expense of increasing susceptibility to disease. Indeed, if over-expressed and over-released, it may contribute to hyper-anxiety and depression-like behaviors. A vasopressin deficit, in turn, may cause signs of both diabetes insipidus and total hypo-anxiety. The identification of genetic polymorphisms underlying these phenomena does not only explain individual variation in social memory and emotionality, but also help to characterize potential targets for therapeutic interventions. The capability of both responding to stressful stimuli and mediating genetic polymorphisms makes the vasopressin system a key mediator for converging (i.e., environmentally and genetically driven) behavioral regulation.

Arginine vasopressin increases glutamate release and intracellular Ca2+ concentration in hippocampal and cortical astrocytes through two distinct receptors

Arginine vasopressin (AVP), released from the CNS, plays an important role in regulating several aspects of CNS functions including aggression, anxiety, and cognition. In this study, we report a novel finding that AVP induces glutamate release from astrocytes isolated from the cerebral cortex and hippocampus. We also investigated the types of AVP receptors involved in the AVP-induced increase in glutamate release from astrocytes isolated from the hippocampus and cortex of neonatal rats. We showed that the AVP (0.1-1000 nmol/L) induced increase in glutamate release and [Ca(2+)](i) is brought about by two distinct subtypes of V(1) receptors (V(1a) and V(1b)). Our results suggested that V(1b) receptors are predominantly expressed in astrocytes isolated from the hippocampus and V(1a) receptors are solely expressed in astrocytes isolated from the cerebral cortex of neonatal rats. The results of the western blot analyses confirmed these pharmacological data. In addition, the AVP-induced increase in glutamate did not contribute to an increase in [Ca(2+)](i), as blockade of metabotropic glutamate receptors did not alter the AVP-induced increase in [Ca(2+)](i). In addition, the administration of a phospholipase A(2) inhibitor failed to alter AVP-induced [Ca(2+)](i) increase suggesting the lack of involvement of this enzyme.

Presence of D1- and D2-like dopamine receptors in the rat, mouse and bovine multiciliated ependyma

The multiciliated ependyma forms an epithelial-like layer that could act as a selective barrier between the brain parenchyma and cerebrospinal fluid. In the present study, tyrosine hydroxylase-containing fibres have been detected in the basal pole of the ependymal cells of the lateral ventricles of rat, mouse and calf. The use of antibodies against at least two different peptide sequences of each D(2), D(3), D(4) and D(5) dopamine receptor subtype has allowed their detection in: (i) sections of mouse, rat and bovine lateral ventricles, by means of immunocytochemistry; and (ii) membrane protein extracts obtained from the ependymal layer of the bovine lateral ventricles, using immunoblotting. The immunocytochemical study has shown the presence of all these subtypes of dopamine receptors in the ependymal cells. Immunoblotting demonstrated similar immunoreactive bands for all receptor subtypes in both ependymal and corpus striatum membrane extracts.

PSA-NCAM in mammalian structural plasticity and neurogenesis

Polysialic acid (PSA) is a linear homopolymer of alpha2-8-N acetylneuraminic acid whose major carrier in vertebrates is the neural cell adhesion molecule (NCAM). PSA serves as a potent negative regulator of cell interactions via its unusual biophysical properties. PSA on NCAM is developmentally regulated thus playing a prominent role in different forms of neural plasticity spanning from embryonic to adult nervous system, including axonal growth, outgrowth and fasciculation, cell migration, synaptic plasticity, activity-induced plasticity, neuronal-glial plasticity, embryonic and adult neurogenesis. The cellular distribution, developmental changes and possible function(s) of PSA-NCAM in the central nervous system of mammals here are reviewed, along with recent findings and theories about the relationships between NCAM protein and PSA as well as the role of different polysialyltransferases. Particular attention is focused on postnatal/adult neurogenesis, an issue which has been deeply investigated in the last decade as an example of persisting structural plasticity with potential implications for brain repair strategies. Adult neurogenic sites, although harbouring all subsequent steps of cell differentiation, from stem cell division to cell replacement, do not faithfully recapitulate development. After birth, they undergo morphological and molecular modifications allowing structural plasticity to adapt to the non-permissive environment of the mature nervous tissue, that are paralled by changes in the expression of PSA-NCAM. The use of PSA-NCAM as a marker for exploring differences in structural plasticity and neurogenesis among mammalian species is also discussed.

Atrial natriuretic peptide: its putative role in modulating the choroid plexus-CSF system for intracranial pressure regulation

Evidence continues to build for the role of atrial natriuretic peptide (ANP) in reducing cerebrospinal fluid (CSF) formation rate, and thus, intracranial pressure. ANP binds to choroid plexus (CP) epithelial cells. This generates cGMP, which leads to altered ion transport and the slowing of CSF production. Binding sites for ANP in CP are plentiful and demonstrate plasticity in fluid imbalance disorders; however, specific ANP receptors in epithelial cells need confirmation. Using antibodies directed against NPR-A and NPR-B, we now demonstrate immunostaining not only in the choroidal epithelium (including cytoplasm), but also in the ependyma and some endothelial cells of cerebral microvessels in adult rats (Sprague-Dawley). The choroidal and ependymal cells stained almost universally, thus substantiating the initial autoradiographic binding studies with 125I-ANP. Because ANP titers in human CSF have previously been shown to increase proportionally to increments in ICP, we propose a compensatory ANP modulation of CP function to down-regulate ICP in hydrocephalus. Further evidence for this notion comes from the current finding of increased frequency of "dark" epithelial cells in CP of hydrocephalic (HTx) rats, which fits our earlier observation that the "dark" choroidal cells, associated with states of reduced CSF formation, are increased by elevated ANP in CSF. Altogether, ANP neuroendocrine-like regulation at CSF transport interfaces and blood-brain barrier impacts brain fluid homeostasis.

Plasma arginine-vasopressin following experimental stroke: effect of osmotherapy

Neurohumoral responses have been implicated in the pathogenesis of ischemia-evoked cerebral edema. In a well-characterized animal model of ischemic stroke, the present study was undertaken to 1) study the profile of plasma arginine-vasopressin (AVP), and 2) determine whether osmotherapy with mannitol and various concentrations of hypertonic saline (HS) solutions influence plasma AVP levels. Halothane-anesthetized adult male Wistar rats were subjected to 2 h of middle cerebral artery occlusion with the intraluminal filament technique. Plasma AVP levels (means +/- SD) were significantly elevated at 24 h (42 +/- 21 pg/ml), 48 h (50 +/- 28 pg/ml), and 72 h (110 +/- 47 pg/ml), and returned to baseline at 96 h (22 +/- 15 pg/ml) following middle cerebral artery occlusion compared with sham-operated controls (14 +/- 7 pg/ml). Plasma AVP levels at 72 h were significantly attenuated with 7.5% HS (37 +/- 8 pg/ml; 360 +/- 11 osmol/l) compared with 0.9% saline (73 +/- 6; 292 +/- 6 osmol/l), 3% HS (66 +/- 8 pg/ml; 303 +/- 12 osmol/l), or mannitol (74 +/- 9 pg/ml; 313 +/- 14 osmol/l) treatment. HS (7.5%) significantly attenuated water content in the ipsilateral and contralateral hemispheres compared with surgical shams, 0.9% saline, 3% HS, and mannitol treatments. Peak plasma AVP levels were not associated with direct histopathological injury to the anterior hypothalamus. Attenuation of brain water content with 7.5% HS treatment coincides with attenuated serum AVP levels, and we speculate that this may represent one additional mechanism by which osmotherapy attenuates edema associated with ischemic stroke.

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

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

Bumetanide inhibition of the blood-brain barrier Na-K-Cl cotransporter reduces edema formation in the rat middle cerebral artery occlusion model of stroke

Increased transport of Na+ across an intact blood-brain barrier (BBB) participates in edema formation during the early hours of cerebral ischemia. In previous studies, the authors showed that the BBB Na-K-Cl cotransporter is stimulated by factors present during ischemia, suggesting that the cotransporter may contribute to the increased brain Na+ uptake in edema. The present study was conducted to determine (1) whether the Na-K-Cl cotransporter is located in the luminal membrane of the BBB, and (2) whether inhibition of the BBB cotransporter reduces brain edema formation. Perfusion-fixed rat brains were examined for cotransporter distribution by immunoelectron microscopy. Cerebral edema was evaluated in rats subjected to permanent middle cerebral artery occlusion (MCAO) by magnetic resonance diffusion-weighted imaging and calculation of apparent diffusion coefficients (ADC). The immunoelectron microscopy studies revealed a predominant (80%) luminal membrane distribution of the cotransporter. Magnetic resonance imaging studies showed ADC ratios (ipsilateral MCAO/contralateral control) ranging from 0.577 to 0.637 in cortex and striatum, indicating substantial edema formation. Intravenous bumetanide (7.6-30.4 mg/kg) given immediately before occlusion attenuated the decrease in ADC ratios for both cortex and striatum (by 40-67%), indicating reduced edema formation. Bumetanide also reduced infarct size, determined by TTC staining. These findings suggest that a luminal BBB Na-K-Cl cotransporter contributes to edema formation during cerebral ischemia.

Increased Expression of Vasopressin V1aReceptors after Traumatic Brain Injury

Experimental evidence obtained in various animal models of brain injury indicates that vasopressin promotes the formation of cerebral edema. However, the molecular and cellular mechanisms underlying this vasopressin action are not fully understood. In the present study, we analyzed the temporal changes in expression of vasopressin V1a receptors after traumatic brain injury (TBI) in rats. In the intact brain, the V1a receptor was expressed in neurons located in all layers of the frontoparietal cortex. The V1a receptor-immunoreactive product was predominantly localized to neuronal nuclei and had both a diffused and punctate staining pattern. The V1a receptors were also expressed in astrocytes, especially in layer 1 of the frontoparietal cortex. In these cells, two distinctive patterns of immunopositive staining for V1a receptors were observed: a diffused cytosolic staining of cell bodies and processes and a clearly punctate staining pattern that was predominantly localized to the astrocytic cell bodies. The real-time reverse-transcriptase polymerase chain reaction analysis of changes in mRNA for the V1a receptor demonstrated that after TBI, there is an early (4 h post-TBI) increase in the number of transcripts in the ipsilateral frontoparietal cortex, when compared to the contralateral hemisphere or the sham-injured rats. This increase in the message was followed by the up-regulation of expression of the V1a receptors at the protein level. This was most evident in cortical astrocytes in the areas surrounding the lesion. The number of the V1a receptor-immunopositive astrocytes in the traumatized parenchyma gradually increased, starting at 8 h and peaking at 4-6 days after TBI. Furthermore, a redistribution of V1a receptors from the astrocytic cell bodies to the astrocytic processes was observed. In addition to astrocytes, an increased expression of V1a receptors was found in the endothelium of both blood microvessels and the large-diameter blood vessels in the frontoparietal cortex ipsilateral to injury. This increase in the V1a receptor expression was apparent between 2 and 4 days after TBI. As early as 1-2 h following the impact, there was also a striking increase in the number of the V1a receptor-immunopositive beaded axonal processes, with greatly enlarged varicosities, that were localized to various areas of the injured parenchyma. It is suggested that the increased expression of V1a receptors plays an important role in the vasopressin-mediated formation of edema in the injured brain.

Membrane potential stabilization in amphibian skeletal muscle fibres in hypertonic solutions

This study investigated membrane transport mechanisms influencing relative changes in cell volume (V) and resting membrane potential (E(m)) following osmotic challenge in amphibian skeletal muscle fibres. It demonstrated a stabilization of E(m) despite cell shrinkage, which was attributable to elevation of intracellular [Cl(-)] above electrochemical equilibrium through Na(+)-Cl(-) and Na(+)-K(+)-2Cl(-) cotransporter action following exposures to extracellular hypertonicity. Fibre volumes (V) determined by confocal microscope x z - scanning of cutaneous pectoris muscle fibres varied linearly with [1/extracellular osmolarity], showing insignificant volume corrections, in fibres studied in Cl(-)-free, normal and Na(+)-free Ringer solutions and in the presence of bumetanide, chlorothiazide and ouabain. The observed volume changes following increases in extracellular tonicity were compared with microelectrode measurements of steady-state resting potentials (E(m)). Fibres in isotonic Cl(-)-free, normal and Na(+)-free Ringer solutions showed similar E(m) values consistent with previously reported permeability ratios P(Na)/P(K)(0.03-0.05) and P(Cl)/P(K) ( approximately 2.0) and intracellular [Na(+)], [K(+)] and [Cl(-)]. Increased extracellular osmolarities produced hyperpolarizing shifts in E(m) in fibres studied in Cl(-)-free Ringer solution consistent with the Goldman-Hodgkin-Katz (GHK) equation. In contrast, fibres exposed to hypertonic Ringer solutions of normal ionic composition showed no such E(m) shifts, suggesting a Cl(-)-dependent stabilization of membrane potential. This stabilization of E(m) was abolished by withdrawing extracellular Na(+) or by the combined presence of the Na(+)-Cl(-) cotransporter (NCC) inhibitor chlorothiazide (10 microM) and the Na(+)-K(+)-2Cl(-) cotransporter (NKCC) inhibitor bumetanide (10 microM), or the Na(+)-K(+)-ATPase inhibitor ouabain (1 or 10 microM) during alterations in extracellular osmolarity. Application of such agents after such increases in tonicity only produced a hyperpolarization after a time delay, as expected for passive Cl(-) equilibration. These findings suggest a model that implicates the NCC and/or NKCC in fluxes that maintain [Cl(-)](i) above its electrochemical equilibrium. Such splinting of [Cl(-)](i) in combination with the high P(Cl)/P(K) of skeletal muscle stabilizes E(m) despite volume changes produced by extracellular hypertonicity, but at the expense of a cellular capacity for regulatory volume increases (RVIs). In situations where P(Cl)/P(K) is low, the same co-transporters would instead permit RVIs but at the expense of a capacity to stabilize E(m).

The neurobiology of glia in the context of water and ion homeostasis

Astrocytes are highly complex cells that respond to a variety of external stimulations. One of the chief functions of astrocytes is to optimize the interstitial space for synaptic transmission by tight control of water and ionic homeostasis. Several lines of work have, over the past decade, expanded the role of astrocytes and it is now clear that astrocytes are active participants in the tri-partite synapse and modulate synaptic activity in hippocampus, cortex, and hypothalamus. Thus, the emerging concept of astrocytes includes both supportive functions as well as active modulation of neuronal output. Glutamate plays a central role in astrocytic-neuronal interactions. This excitatory amino acid is cleared from the neuronal synapses by astrocytes via glutamate transporters, and is converted into glutamine, which is released and in turn taken up by neurons. Furthermore, metabotropic glutamate receptor activation on astrocytes triggers via increases in cytosolic Ca(2+) a variety of responses. For example, calcium-dependent glutamate release from the astrocytes modulates the activity of both excitatory and inhibitory synapses. In vivo studies have identified the astrocytic end-foot processes enveloping the vessel walls as the center for astrocytic Ca(2+) signaling and it is possible that Ca(2+) signaling events in the cellular component of the blood-brain barrier are instrumental in modulation of local blood flow as well as substrate transport. The hormonal regulation of water and ionic homeostasis is achieved by the opposing effects of vasopressin and atrial natriuretic peptide on astroglial water and chloride uptake. In conjuncture, the brain appears to have a distinct astrocytic perivascular system, involving several potassium channels as well as aquaporin 4, a membrane water channel, which has been localized to astrocytic endfeet and mediate water fluxes within the brain. The multitask functions of astrocytes are essential for higher brain function. One of the major challenges for future studies is to link receptor-mediated signaling events in astrocytes to their roles in metabolism, ion, and water homeostasis.

Branched-chain Ketoacyl Dehydrogenase Deficiency: Maple Syrup Disease

Classic maple syrup disease can be managed to allow a benign neonatal course, normal growth, and low hospitalization rates. The majority of affected infants that are prospectively managed have good neurodevelopmental outcome; however, acute metabolic intoxication and neurologic deterioration can develop rapidly at any age. Each episode is associated with a risk for cerebral edema, cerebrovascular compromise, and brain herniation. High plasma leucine and, possibly, alpha-ketoisocaproate are the principal neurotoxins in maple syrup disease. Plasma levels rise rapidly in association with net protein catabolism provoked by common infections and injuries. Transient periods of maple syrup disease encephalopathy appear fully reversible, leaving no clinically detectable neurologic sequelae. In contrast, prolonged amino acid imbalance, particularly if occurring during the critical period of brain development, leads to neuronal hypoplasia, a paucity of synapses, and undermyelination. Stagnated maturation and inadequate nutritional maintenance of brain structure have lifelong neurologic and behavioral consequences. Core elements of effective long-term therapy include screening and identification of asymptomatic newborns, frequent plasma amino acid monitoring, careful attention to branched-chain amino acid nurtriture, prevention of cerebral essential amino acid deficiencies, adequate provision of essential omega-3 class fatty acids and micronutrients deficient in commercial formulas, methods for home monitoring of metabolic control, and a commitment to lifelong therapy. Recognizing the risk for acute leucine intoxication depends on anticipating effects of common childhood infection and physiologic stresses on whole body protein turnover. Successful management of metabolic decompensation is based on the use of home sick-day regimens, rapid availability of branched-chain amino acid-free hyperalimentation solutions for hospitalized children, prevention of hyponatremia in patients with leucinosis, and frequent adjustments of intravenous therapies guided by plasma amino acid levels and indices of metabolic and clinical response.