The subfornical organ, a specialized sodium channel, and the sensing of sodium levels in the brain

Constitutive cell proliferation and neurogenesis in the organum vasculosum lamina terminalis and subfornical organ of adult rats

Neurogenesis continues throughout adulthood in the subventricular zone, hippocampal subgranular zone, and the hypothalamic median eminence (ME) and the adjacent medio-basal hypothalamus. The ME is one of the circumventricular organs (CVO), which are specialized brain areas characterized by an incomplete blood-brain barrier and, thus, are involved in mediating communication between the central nervous system and the periphery. Additional CVOs include the organum vasculosum laminae terminalis (OVLT) and the subfornical organs (SFO). Previous studies have demonstrated that the ME contains neural stem cells (NSCs) capable of generating new neurons and glia in the adult brain. However, it remains unclear whether the OVLT and SFO also contain proliferating cells, the identity of these cells, and their ability to differentiate into mature neurons. Here we show that glial and mural subtypes exhibit NSC characteristics, expressing the endogenous mitotic maker Ki67, and incorporating the exogenous mitotic marker BrdU in the OVLT and SFO of adult rats. Glial cells constitutively proliferating in the SFO comprise NG2 glia, while in the OVLT, both NG2 glia and tanycytes appear to constitute the NSC pool. Furthermore, pericytes, which are mural cells associated with capillaries, also contribute to the pool of cells constitutively proliferating in the OVLT and SFO of adult rats. In addition to these glial and mural cells, a fraction of NSCs containing proliferation markers Ki67 and BrdU also expresses the early postmitotic neuronal marker doublecortin, suggesting that these CVOs comprise newborn neurons. Notably, these neurons can differentiate and express the mature neuronal marker NeuN. These findings establish the sensory CVOs OVLT and SFO as additional neurogenic niches, where the generation of new neurons and glia persists in the adult brain.

Salt-induced phosphoproteomic changes in the subfornical organ in rats with chronic kidney disease

OBJECTIVES

Subfornical organ (SFO) is vital in chronic kidney disease (CKD) progression caused by high salt levels. The current study investigated the effects of high salt on phosphoproteomic changes in SFO in CKD rats.

METHODS

5/6 nephrectomized rats were fed a normal-salt diet (0.4%) (NC group) or a high-salt diet (4%) (HC group) for three weeks, while sham-operated rats were fed a normal-salt diet (0.4%) (NS group). For phosphoproteomic analysis of SFO in different groups, TiO₂ enrichment, isobaric tags for relative and absolute quantification (iTRAQ) labeling, and liquid chromatography-tandem mass spectrometry (LC-MS/MS) were used.

RESULTS

There were 6808 distinct phosphopeptides found, which corresponded to 2661 phosphoproteins. NC group had 168 upregulated and 250 downregulated phosphopeptides compared to NS group. Comparison to NC group, HC group had 154 upregulated and 124 downregulated phosphopeptides. Growth associated protein 43 (GAP43) and heat shock protein 27 (Hsp27) were significantly upregulated phosphoproteins and may protect against high-salt damage. Differential phosphoproteins with tight functional connection were synapse proteins and microtubule-associated proteins, implying that high-salt diet disrupted brain's structure and function. Furthermore, differential phosphoproteins in HC/NC comparison group were annotated to participate in GABAergic synapse signaling pathway and aldosterone synthesis and secretion, which attenuated inhibitory neurotransmitter effects and increased sympathetic nerve activity (SNA).

DISCUSSION

This large scale phosphoproteomic profiling of SFO sheds light on how salt aggravates CKD via the central nervous system.

Effects of a high-salt diet on MAP and expression levels of renal ENaCs and aquaporins in SHR

An increase in blood pressure by a high-salt (HS) diet may change the expression levels of renal epithelial sodium channels (ENaCs) and aquaporins (AQPs). Spontaneously hypertensive rats (SHRs) and Wistar Kyoto (WKY) rats were exposed to HS and regular-salt (RS) diets for 6 weeks. Mean arterial pressure (MAP) and plasma atrial natriuretic peptide (ANP), angiotensin II (Ang II), aldosterone, and arginine vasopressin (AVP) levels were determined. Expression of mRNA levels of ENaCs and AQPs were quantified by real-time PCR. The MAP was higher in SHRs on the HS diet. Plasma Ang II and aldosterone levels were low while plasma ANP level was high in both strains of rats. Renal expression of mRNA levels of α-, β-, and γ-ENaCs was lowered in SHRs on the HS diet. Meanwhile, renal AQP1, AQP2, and AQP7 mRNA expression levels were lowered in both strains of rats on the HS diet. Suppression of mRNA expression levels of ENaC and AQP subunits suggests that the high-salt-induced increase in the MAP of SHR may not be solely due to renal sodium and water retention.

The organum vasculosum of the lamina terminalis and subfornical organ: regulation of thirst

Thirst and water intake are regulated by the organum vasculosum of the lamina terminalis (OVLT) and subfornical organ (SFO), located around the anteroventral third ventricle, which plays a critical role in sensing dynamic changes in sodium and water balance in body fluids. Meanwhile, neural circuits involved in thirst regulation and intracellular mechanisms underlying the osmosensitive function of OVLT and SFO are reviewed. Having specific Nax channels in the glial cells and other channels (such as TRPV1 and TRPV4), the OVLT and SFO detect the increased Na+ concentration or hyperosmolality to orchestrate osmotic stimuli to the insular and cingulate cortex to evoke thirst. Meanwhile, the osmotic stimuli are relayed to the supraoptic nucleus (SON) and paraventricular nucleus of the hypothalamus (PVN) via direct neural projections or the median preoptic nucleus (MnPO) to promote the secretion of vasopressin which plays a vital role in the regulation of body fluid homeostasis. Importantly, the vital role of OVLT in sleep-arousal regulation is discussed, where vasopressin is proposed as the mediator in the regulation when OVLT senses osmotic stimuli.

The Triad Na+ Activated Na+ Channel (Nax)-Salt Inducible KINASE (SIK) and (Na+ + K+)-ATPase: Targeting the Villains to Treat Salt Resistant and Sensitive Hypertension

The Na⁺-activated Na⁺ channel (Nax) and salt-inducible kinase (SIK) are stimulated by increases in local Na⁺ concentration, affecting (Na⁺ + K⁺)-ATPase activity. To test the hypothesis that the triad Nax/SIK/(Na⁺ + K⁺)-ATPase contributes to kidney injury and salt-sensitive hypertension (HTN), uninephrectomized male Wistar rats (200 g; n = 20) were randomly divided into 4 groups based on a salt diet (normal salt diet; NSD-0.5% NaCl-or high-salt diet; HSD-4% NaCl) and subcutaneous administration of saline (0.9% NaCl) or deoxycorticosterone acetate (DOCA, 8 mg/kg), as follows: Control (CTRL), CTRL-Salt, DOCA, and DOCA-Salt, respectively. After 28 days, the following were measured: kidney function, blood pressure, (Na⁺ + K⁺)-ATPase and SIK1 kidney activities, and Nax and SIK1 renal expression levels. SIK isoforms in kidneys of CTRL rats were present in the glomerulus and tubular epithelia; they were not altered by HSD and/or HTN. CTRL-Salt rats remained normotensive but presented slight kidney function decay. HSD rats displayed augmentation of the Nax/SIK/(Na⁺ + K⁺)-ATPase pathway. HTN, kidney injury, and kidney function decay were present in all DOCA rats; these were aggravated by HSD. DOCA rats presented unaltered (Na⁺ + K⁺)-ATPase activity, diminished total SIK activity, and augmented SIK1 and Nax content in the kidney cortex. DOCA-Salt rats expressed SIK1 activity and downregulation in (Na⁺ + K⁺)-ATPase activity in the kidney cortex despite augmented Nax content. The data of this study indicate that the (Na⁺ + K⁺)-ATPase activity response to SIK is attenuated in rats under HSD, independent of HTN, as a mechanism contributing to kidney injury and salt-sensitive HTN.

Glial functions in the blood-brain communication at the circumventricular organs

The circumventricular organs (CVOs) are located around the brain ventricles, lack a blood-brain barrier (BBB) and sense blood-derived molecules. This review discusses recent advances in the importance of CVO functions, especially glial cells transferring periphery inflammation signals to the brain. The CVOs show size-limited vascular permeability, allowing the passage of molecules with molecular weight <10,000. This indicates that the lack of an endothelial cell barrier does not mean the free movement of blood-derived molecules into the CVO parenchyma. Astrocytes and tanycytes constitute a dense barrier at the distal CVO subdivision, preventing the free diffusion of blood-derived molecules into neighboring brain regions. Tanycytes in the CVOs mediate communication between cerebrospinal fluid and brain parenchyma via transcytosis. Microglia and macrophages of the CVOs are essential for transmitting peripheral information to other brain regions via toll-like receptor 2 (TLR2). Inhibition of TLR2 signaling or depletion of microglia and macrophages in the brain eliminates TLR2-dependent inflammatory responses. In contrast to TLR2, astrocytes and tanycytes in the CVOs of the brain are crucial for initiating lipopolysaccharide (LPS)-induced inflammatory responses via TLR4. Depletion of microglia and macrophages augments LPS-induced fever and chronic sickness responses. Microglia and macrophages in the CVOs are continuously activated, even under normal physiological conditions, as they exhibit activated morphology and express the M1/M2 marker proteins. Moreover, the microglial proliferation occurs in various regions, such as the hypothalamus, medulla oblongata, and telencephalon, with a marked increase in the CVOs, due to low-dose LPS administration, and after high-dose LPS administration, proliferation is seen in most brain regions, except for the cerebral cortex and hippocampus. A transient increase in the microglial population is beneficial during LPS-induced inflammation for attenuating sickness response. Transient receptor potential receptor vanilloid 1 expressed in astrocytes and tanycytes of the CVOs is responsible for thermoregulation upon exposure to a warm environment less than 37°C. Alternatively, Nax expressed in astrocytes and tanycytes of the CVOs is crucial for maintaining body fluid homeostasis. Thus, recent findings indicate that glial cells in the brain CVOs are essential for initiating neuroinflammatory responses and maintaining body fluid and thermal homeostasis.

Comparison of dipsogenic responses of adult rat offspring as a function of different perinatal programming models

The perinatal environment interacts with the genotype of the developing organism resulting in a unique phenotype through a developmental or perinatal programming phenomenon. However, it remains unclear how this phenomenon differentially affects particular targets expressing specific drinking responses depending on the perinatal conditions. The main goal of the present study was to compare the dipsogenic responses induced by different thirst models as a function of two perinatal manipulation models, defined by the maternal free access to hypertonic sodium solution and a partial aortic ligation (PAL-W/Na) or a sham-ligation (Sham-W/Na). The programmed adult offspring of both perinatal manipulated models responded similarly when was challenged by overnight water dehydration or after a sodium depletion showing a reduced water intake in comparison to the non-programmed animals. However, when animals were evaluated after a body sodium overload, only adult Sham-W/Na offspring showed drinking differences compared to PAL and control offspring. By analyzing the central neurobiological substrates involved, a significant increase in the number of Fos + cells was found after sodium depletion in the subfornical organ of both programmed groups and an increase in the number of Fos + cells in the dorsal raphe nucleus was only observed in adult depleted PAL-W/Na. Our results suggest that perinatal programming is a phenomenon that differentially affects particular targets which induce specific dipsogenic responses depending on matching between perinatal programming conditions and the osmotic challenge in the latter environment. Probably, each programmed-drinking phenotype has a particular set point to elicit specific repertoires of mechanisms to reestablish fluid balance.

Aprocitentan and the endothelin system in resistant hypertension

Endothelin has emerged as a target for therapeutic intervention in systemic hypertension. As a vasoconstrictor, co-mitogenic agent, linking pulse pressure and vascular remodeling, and mediator of aldosterone and catecholamine release, endothelin is a key player in hypertension and end-organ damage. In 10-20% of the hypertensive population, the high blood pressure is resistant to administration of antihypertensive drugs of different classes in combination. Because endothelin is not targeted by the current antihypertensive drugs this may suggest that this resistance is due, in part at least, to a dependence on endothelin. This hypothesis is supported by the observation that this form of hypertension is often salt-sensitive, and that the endothelin system is stimulated by salt. In addition, the endothelin system is activated in subjects at risk of developing resistant hypertension, such as African-Americans or patients with obesity or obstructive sleep apnea. Aprocitentan is a novel, potent, dual endothelin receptor antagonist (ERA) currently in phase 3 development for the treatment of difficult-to-treat hypertension. This article discusses the research which underpinned the discovery of this ERA and the choice of its first clinical indication for patients with forms of hypertension which cannot be well controlled with classical antihypertensive drugs.

Signal Transduction of Mineralocorticoid and Angiotensin II Receptors in the Central Control of Sodium Appetite: A Narrative Review

Sodium appetite is an innate behavior occurring in response to sodium depletion that induces homeostatic responses such as the secretion of the mineralocorticoid hormone aldosterone from the zona glomerulosa of the adrenal cortex and the stimulation of the peptide hormone angiotensin II (ANG II). The synergistic action of these hormones signals to the brain the sodium appetite that represents the increased palatability for salt intake. This narrative review summarizes the main data dealing with the role of mineralocorticoid and ANG II receptors in the central control of sodium appetite. Appropriate keywords and MeSH terms were identified and searched in PubMed. References to original articles and reviews were examined, selected, and discussed. Several brain areas control sodium appetite, including the nucleus of the solitary tract, which contains aldosterone-sensitive HSD2 neurons, and the organum vasculosum lamina terminalis (OVLT) that contains ANG II-sensitive neurons. Furthermore, sodium appetite is under the control of signaling proteins such as mitogen-activated protein kinase (MAPK) and inositol 1,4,5-thriphosphate (IP3). ANG II stimulates salt intake via MAPK, while combined ANG II and aldosterone action induce sodium intake via the IP3 signaling pathway. Finally, aldosterone and ANG II stimulate OVLT neurons and suppress oxytocin secretion inhibiting the neuronal activity of the paraventricular nucleus, thus disinhibiting the OVLT activity to aldosterone and ANG II stimulation.

Adipsic diabetes insipidus

Adipsic diabetes insipidus (ADI) is a rare but devastating disorder of water balance with significant associated morbidity and mortality. Most patients develop the disease as a result of hypothalamic destruction from a variety of underlying etiologies. Damage to osmolar-responsive neuroreceptors, primarily within the supraoptic and paraventricular nuclei, results in impaired production and release of arginine vasopressin (AVP). Important regulating circuits of thirst sense and drive are regionally colocalized with AVP centers and therefore are also injured. Patients with central diabetes insipidus with impaired thirst response, defined as ADI, suffer from wide swings of plasma osmolality resulting in repeated hospitalization, numerous associated comorbidities, and significant mortality. Treatment recommendations are based largely on expert advice from case series owing to the rarity of disease prevalence. Acute disease management focuses on fixed dosing of antidiuretic hormone analogues and calculated prescriptions of obligate daily water intake. Long-term care requires patient/family education, frequent reassessment of clinical and biochemical parameters, as well as screening and treatment of comorbidities.

Imiquimod-induced skin inflammation is relieved by knockdown of sodium channel Nax

Na_x is an atypical sodium channel that mediates inflammatory pathways in pathological conditions of the skin. In this study, we developed a skin inflammation model in the rabbit ear through application of imiquimod (IMQ). Knockdown of Na_x using RNAi attenuated IMQ-induced skin inflammation, including skin erythema, scaling and papule formation. Histologic analysis showed that thickening and insufficient differentiation of the epidermis found in psoriasis-like skin were normalized by administration of Na_x -RNAi. Excessive infiltration of inflammatory cells found in inflammatory lesions, such as mast cells, eosinophils, neutrophils, T cells and macrophages, was reduced by Na_x -RNAi. Expression of S100A9, which is a downstream gene of Na_x and a mediator of inflammation, was decreased by Na_x -RNAi. Our results demonstrated that knockdown of Na_x ameliorated IMQ-induced psoriasis-like skin inflammation in vivo. Thus, targeting of Na_x may represent a potential therapeutic option for the treatment of psoriasis.

Knockdown of sodium channel Nax reduces dermatitis symptoms in rabbit skin

The skin plays a critical role in maintenance of water homeostasis. Dysfunction of the skin barrier causes not only delayed wound healing and hypertrophic scarring, but it also contributes to the development of various skin diseases. Dermatitis is a chronic inflammatory skin disorder that has several different subtypes. Skin of contact dermatitis and atopic dermatitis (AD) show epidermal barrier dysfunction. Na_x is a sodium channel that regulates inflammatory gene expression in response to perturbation of barrier function of the skin. We found that in vivo knockdown of Na_x using RNAi reduced hyperkeratosis and keratinocyte hyperproliferation in rabbit ear dermatitic skin. Increased infiltration of inflammatory cells (mast cells, eosinophils, T cells, and macrophages), a characteristic of dermatitis, was reduced by Na_x knockdown. Upregulation of PAR-2 and thymic stromal lymphopoietin (TSLP), which induce Th2-mediated allergic responses, was inhibited by Na_x knockdown. In addition, expression of COX-2, IL-1β, IL-8, and S100A9, which are downstream genes of Na_x and are involved in dermatitis pathogenesis, were also decreased by Na_x knockdown. Our data show that knockdown of Na_x relieved dermatitis symptoms in vivo and indicate that Na_x is a novel therapeutic target for dermatitis, which currently has limited therapeutic options.

Effects of Gut Bacteria Depletion and High-Na+ and Low-K+ Intake on Circulating Levels of Biogenic Amines

SCOPE

High-sodium and low-potassium (HNaLK) content in Western diets increases the risk of hypertension and cardiovascular disease (CVD). It is investigated if the dietary minerals interact with gut bacteria to modulate circulating levels of biogenic amines, which are implicated in various pathologies, including hypertension and CVD.

METHODS AND RESULTS

Using a metabolomic approach to target biogenic amines, the effects of gut bacteria depletion and HNaLK intake on circulating levels of biogenic amines in rats are examined. Forty-five metabolites whose plasma levels are significantly altered by gut bacteria depletion (p < 0.05) are found, indicating their regulation by gut bacteria. Many of them are not previously linked to gut bacteria; therefore, these data provide novel insights into physiological or pathological roles of gut bacteria. A number of plasma metabolites that are altered both by gut bacteria and HNaLK intake are also found, suggesting possible interactions of the diet and gut bacteria in the modulation of these metabolites. The diet effects are observed with significant changes in the gut bacterial taxa Porphyromonadaceae and Prevotellaceae (p < 0.05).

CONCLUSION

The dietary minerals may regulate abundances of certain gut bacteria to alter circulating levels of biogenic amines, which may be linked to host physiology or pathology.

Targeted Ablation of Distal Cerebrospinal Fluid-Contacting Nucleus Alleviates Renal Fibrosis in Chronic Kidney Disease

The potential function of distal cerebrospinal fluid-contacting nucleus (dCSF-CNs) in chronic kidney disease (CKD) development is poorly understood. We hypothesized that dCSF-CNs might affect the renin-angiotensin system (RAS) in kidney injury progression, with dCSF-CNs ablation potentially alleviating local RAS and renal fibrosis in rats after five-sixths nephrectomy (5/6Nx). Part of rats were randomly administered artificial cerebrospinal fluid (aCSF) intracerebroventricularly (icv), followed by 5/6Nx or sham operation; and other part of rats were administered Cholera toxin B subunit conjugated with saporin (CB-SAP) for dCSF-CNs lesion before 5/6Nx. The effect of CB-SAP on dCSF-CNs ablation was confirmed by double immunofluorescence staining. RAS component, NOX₂ and c-fos levels in the subfornical organ (SFO), hypothalamic paraventricular nucleus (PVN) and hippocampus, as well as tyrosine hydroxylase (TH) and c-fos positive cells in rostral ventrolateral medulla (RVLM) were assessed. Next, the levels of RAS components (angiotensinogen [AGT], angiotensin-converting enzyme [ACE], Ang II type 1 receptor [AT1R], angiotensin-converting enzyme 2 [ACE2], and Mas receptor), NADPH oxidases (NOX₂ and catalase), inflammatory cytokines (monocyte chemotactic protein 1 [MCP-1] and IL-6), and fibrotic factors (fibronectin and collagen I) were assessed. Less CB-labeled neurons were found in dCSF-CNs of CB-SAP-treated rats compared with 5/6Nx animals. Meanwhile, CB-SAP downregulated AGT, Ang II, AT1R, NOX₂, catalase, MCP-1, IL-6, fibronectin, and collagen I, and upregulated ACE2 and Mas receptor, compared with CKD rats. More TH and c-fos positive cells were found in RVLM of 5/6Nx rats but the number decreased after dCSF-CNs ablation. Targeted dCSF-CNs ablation could alleviate renal inflammation and fibrosis in chronic kidney injury by inhibiting cerebral and renal RAS/NADPH oxidase.

Understanding the Two Faces of Low-Salt Intake

Fierce debate has developed whether low-sodium intake, like high-sodium intake, could be associated with adverse outcome. The debate originates in earlier epidemiological studies associating high-sodium intake with high blood pressure and more recent studies demonstrating a higher cardiovascular event rate with both low- and high-sodium intake. This brings into question whether we entirely understand the consequences of high- and (very) low-sodium intake for the systemic hemodynamics, the kidney function, the vascular wall, the immune system, and the brain. Evolutionarily, sodium retention mechanisms in the context of low dietary sodium provided a survival advantage and are highly conserved, exemplified by the renin-angiotensin system. What is the potential for this sodium-retaining mechanism to cause harm? In this paper, we will consider current views on how a sodium load is handled, visiting aspects including the effect of sodium on the vessel wall, the sympathetic nervous system, the brain renin-angiotensin system, the skin as "third compartment" coupling to vascular endothelial growth factor C, and the kidneys. From these perspectives, several mechanisms can be envisioned whereby a low-sodium diet could potentially cause harm, including the renin-angiotensin system and the sympathetic nervous system. Altogether, the uncertainties preclude a unifying model or practical clinical guidance regarding the effects of a low-sodium diet for an individual. There is a very strong need for fundamental and translational studies to enhance the understanding of the potential adverse consequences of low-salt intake as an initial step to facilitate better clinical guidance.

Distinct neural mechanisms for the control of thirst and salt appetite in the subfornical organ

Body fluid conditions are continuously monitored in the brain to regulate thirst and salt-appetite sensations. Angiotensin II drives both thirst and salt appetite; however, the neural mechanisms underlying selective water- and/or salt-intake behaviors remain unknown. Using optogenetics, we show that thirst and salt appetite are driven by distinct groups of angiotensin II receptor type 1a-positive excitatory neurons in the subfornical organ. Neurons projecting to the organum vasculosum lamina terminalis control water intake, while those projecting to the ventral part of the bed nucleus of the stria terminalis control salt intake. Thirst-driving neurons are suppressed under sodium-depleted conditions through cholecystokinin-mediated activation of GABAergic neurons. In contrast, the salt appetite-driving neurons were suppressed under dehydrated conditions through activation of another population of GABAergic neurons by Na_x signals. These distinct mechanisms in the subfornical organ may underlie the selective intakes of water and/or salt and may contribute to body fluid homeostasis.

Sodium channel Nax is a regulator in epithelial sodium homeostasis

The mechanisms by which the epidermis responds to disturbances in barrier function and restores homeostasis are unknown. With a perturbation of the epidermal barrier, water is lost, resulting in an increase in extracellular sodium concentration. We demonstrate that the sodium channel Nax functions as a sodium sensor. With increased extracellular sodium, Nax up-regulates prostasin, which results in activation of the sodium channel ENaC, resulting in increased sodium flux and increased downstream mRNA synthesis of inflammatory mediators. Nax is present in multiple epithelial tissues, and up-regulation of its downstream genes is found in hypertrophic scars. In animal models, blocking Nax expression results in improvement in scarring and atopic dermatitis-like symptoms, both of which are pathological conditions characterized by perturbations in barrier function. These findings support an important role for Nax in maintaining epithelial homeostasis.

Neurohumoral Integration of Cardiovascular Function by the Lamina Terminalis

The mechanisms involved in cardiovascular regulation, such as vascular tone, fluid volume and blood osmolarity, are quite often mediated by signals circulating in the periphery, such as angiotensin II and sodium concentration. Research has identified areas within the lamina terminalis (LT), specifically the sensory circumventricular organs (CVOs), the subfornical organ and the organum vasculosum of the lamina terminalis, as playing crucial roles detecting and integrating information derived from these circulating signals. The median preoptic nucleus (MnPO) is a third integrative structure within the LT that influences cardiovascular homeostasis, although to date, its role is not as clearly elucidated. More recent studies have demonstrated that the CVOs are not only essential in the detection of traditional cardiovascular signals but also signals primarily considered to be important in the regulation of metabolic, reproductive and inflammatory processes that have now also been implicated in cardiovascular regulation. In this review, we highlight the critical roles played by the LT in the detection and integration of circulating signals that provide critical feedback control information contributing to cardiovascular regulation.

New aspects in fenestrated capillary and tissue dynamics in the sensory circumventricular organs of adult brains

The blood-brain barrier (BBB) generally consists of endothelial tight junction barriers that prevent the free entry of blood-derived substances, thereby maintaining the extracellular environment of the brain. However, the circumventricular organs (CVOs), which are located along the midlines of the brain ventricles, lack these endothelial barriers and have fenestrated capillaries; therefore, they have a number of essential functions, including the transduction of information between the blood circulation and brain. Previous studies have demonstrated the extensive contribution of the CVOs to body fluid and thermal homeostasis, energy balance, the chemoreception of blood-derived substances, and neuroinflammation. In this review, recent advances have been discussed in fenestrated capillary characterization and dynamic tissue reconstruction accompanied by angiogenesis and neurogliogenesis in the sensory CVOs of adult brains. The sensory CVOs, including the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), and area postrema (AP), have size-selective and heterogeneous vascular permeabilities. Astrocyte-/tanycyte-like neural stem cells (NSCs) sense blood- and cerebrospinal fluid-derived information through the transient receptor potential vanilloid 1, a mechanical/osmotic receptor, Toll-like receptor 4, a lipopolysaccharide receptor, and Nax, a Na-sensing Na channel. They also express tight junction proteins and densely and tightly surround mature neurons to protect them from blood-derived neurotoxic substances, indicating that the NSCs of the CVOs perform BBB functions while maintaining the capacity to differentiate into new neurons and glial cells. In addition to neurogliogenesis, the density of fenestrated capillaries is regulated by angiogenesis, which is accompanied by the active proliferation and sprouting of endothelial cells. Vascular endothelial growth factor (VEGF) signaling may be involved in angiogenesis and neurogliogenesis, both of which affect vascular permeability. Thus, recent findings advocate novel concepts for the CVOs, which have the dynamic features of vascular and parenchymal tissues.

Channel properties of Nax expressed in neurons

Nax is a sodium-concentration ([Na+])-sensitive Na channel with a gating threshold of ~150 mM for extracellular [Na+] ([Na+]o) in vitro. We previously reported that Nax was preferentially expressed in the glial cells of sensory circumventricular organs including the subfornical organ, and was involved in [Na+] sensing for the control of salt-intake behavior. Although Nax was also suggested to be expressed in the neurons of some brain regions including the amygdala and cerebral cortex, the channel properties of Nax have not yet been adequately characterized in neurons. We herein verified that Nax was expressed in neurons in the lateral amygdala of mice using an antibody that was newly generated against mouse Nax. To investigate the channel properties of Nax expressed in neurons, we established an inducible cell line of Nax using the mouse neuroblastoma cell line, Neuro-2a, which is endogenously devoid of the expression of Nax. Functional analyses of this cell line revealed that the [Na+]-sensitivity of Nax in neuronal cells was similar to that expressed in glial cells. The cation selectivity sequence of the Nax channel in cations was revealed to be Na+ ≈ Li+ > Rb+ > Cs+ for the first time. Furthermore, we demonstrated that Nax bound to postsynaptic density protein 95 (PSD95) through its PSD95/Disc-large/ZO-1 (PDZ)-binding motif at the C-terminus in neurons. The interaction between Nax and PSD95 may be involved in promoting the surface expression of Nax channels because the depletion of endogenous PSD95 resulted in a decrease in Nax at the plasma membrane. These results indicated, for the first time, that Nax functions as a [Na+]-sensitive Na channel in neurons as well as in glial cells.

Sodium sensing in the brain

Sodium (Na) homeostasis is crucial for life, and the Na⁺ level ([Na⁺]) of body fluids is strictly maintained at a range of 135–145 mM. However, the existence of a [Na⁺] sensor in the brain has long been controversial until Na_x was identified as the molecular entity of the sensor. This review provides an overview of the [Na⁺]-sensing mechanism in the brain for the regulation of salt intake by summarizing a series of our studies on Na_x. Na_x is a Na channel expressed in the circumventricular organs (CVOs) in the brain. Among the CVOs, the subfornical organ (SFO) is the principal site for the control of salt intake behavior, where Na_x populates the cellular processes of astrocytes and ependymal cells enveloping neurons. A local expression of endothelin-3 in the SFO modulates the [Na⁺] sensitivity for Na_x activation, and thereby Na_x is likely to be activated in the physiological [Na⁺] range. Na_x stably interacts with Na⁺/K⁺-ATPase whereby Na⁺ influx via Na_x is coupled with activation of Na⁺/K⁺-ATPase associated with the consumption of ATP. The consequent activation of anaerobic glucose metabolism of Na_x-positive glial cells upregulates the cellular release of lactate, and this lactate functions as a gliotransmitter to activate GABAergic neurons in the SFO. The GABAergic neurons presumably regulate hypothetic neurons involved in the control of salt intake behavior. Recently, a patient with essential hypernatremia caused by autoimmunity to Na_x was found. In this case, the hypernatremia was considered to be induced by the complement-mediated cell death in the CVOs, where Na_x specifically populates.

Salt and gene expression: evidence for [Na+]i/[K+]i-mediated signaling pathways

Our review focuses on the recent data showing that gene transcription and translation are under the control of signaling pathways triggered by modulation of the intracellular sodium/potassium ratio ([Na+]i/[K+]i). Side-by-side with sensing of osmolality elevation by tonicity enhancer-binding protein (TonEBP, NFAT5), [Na+]i/[K+]i-mediated excitation-transcription coupling may contribute to the transcriptomic changes evoked by high salt consumption. This novel mechanism includes the sensing of heightened Na+ concentration in the plasma, interstitial, and cerebrospinal fluids via augmented Na+ influx in the endothelium, immune system cells, and the subfornical organ, respectively. In these cells, [Na+]i/[K+]i ratio elevation, triggered by augmented Na+ influx, is further potentiated by increased production of endogenous Na+,K+-ATPase inhibitors documented in salt-sensitive hypertension.

Extracellular Na(+) levels regulate formation and activity of the NaX/alpha1-Na(+)/K(+)-ATPase complex in neuronal cells

MnPO neurons play a critical role in hydromineral homeostasis regulation by acting as sensors of extracellular sodium concentration ([Na⁺]_out). The mechanism underlying Na⁺-sensing involves Na⁺-flow through the Na_X channel, directly regulated by the Na⁺/K⁺-ATPase α1-isoform which controls Na⁺-influx by modulating channel permeability. Together, these two partners form a complex involved in the regulation of intracellular sodium ([Na⁺]_in). Here we aim to determine whether environmental changes in Na⁺ could actively modulate the Na_X/Na⁺/K⁺-ATPase complex activity. We investigated the complex activity using patch-clamp recordings from rat MnPO neurons and Neuro2a cells. When the rats were fed with a high-salt-diet, or the [Na⁺] in the culture medium was increased, the activity of the complex was up-regulated. In contrast, drop in environmental [Na⁺] decreased the activity of the complex. Interestingly under hypernatremic condition, the colocalization rate and protein level of both partners were up-regulated. Under hyponatremic condition, only Na_X protein expression was increased and the level of Na_X/Na⁺/K⁺-ATPase remained unaltered. This unbalance between Na_X and Na⁺/K⁺-ATPase pump proportion would induce a bigger portion of Na⁺/K⁺-ATPase-control-free Na_X channel. Thus, we suggest that hypernatremic environment increases Na_X/Na⁺/K⁺-ATPase α1-isoform activity by increasing the number of both partners and their colocalization rate, whereas hyponatremic environment down-regulates complex activity via a decrease in the relative number of Na_X channels controlled by the pump.

The association between the polymorphisms in a sodium channel gene SCN7A and essential hypertension: a case-control study in the Northern Han Chinese

Nax , an α-subunit of the sodium channel encoded by the SCN7A gene, has been deemed to be a sensor of the concentration of sodium in the brain and may be involved in salt intake behavior. We inferred that Nax /SCN7A may participate in the regulation of blood pressure and the pathogenesis of essential hypertension (EH). The present case-control study involving 615 hypertensives and 617 normotensives was performed to investigate the association between SCN7A polymorphisms and EH in the Northern Han Chinese population. The three common single nucleotide polymorphisms (SNPs) (rs3791251, rs6738031, rs7565062) in the exons of SCN7A were genotyped with the TaqMan assay. Significant association between SNP rs7565062 and EH was found under the addictive and dominant genetic models (P = 0.024, OR = 1.283, 95%CI [1.033-1.592]; P = 0.013, OR = 1.203, 95%CI [1.040-1.392]; respectively). The three SNPs were in close pair-wise linkage disequilibrium with each other and the haplotype analyses indicated that haplotype G-A-T was significantly associated with increased risk of EH (P = 0.023, OR = 1.290). In conclusion, our data showed that SNP rs7565062 of SCN7A was significantly associated with EH and the allele T of rs7565062 or the related haplotype G-A-T will be a genetic risk factor for EH in the Northern Han Chinese population.

Mineralocorticoid-induced sodium appetite and renal salt retention: evidence for common signaling and effector mechanisms

An increase in renal sodium chloride (salt) retention and an increase in sodium appetite are the body's responses to salt restriction or depletion in order to restore salt balance. Renal salt retention and increased sodium appetite can also be maladaptive and sustain the pathophysiology in conditions like salt-sensitive hypertension and chronic heart failure. Here we review the central role of the mineralocorticoid aldosterone in both the increase in renal salt reabsorption and sodium appetite. We discuss the working hypothesis that aldosterone activates similar signaling and effector mechanisms in the kidney and brain, including the mineralocorticoid receptor, the serum- and glucocorticoid-induced kinase SGK1, the ubiquitin ligase NEDD4-2, and the epithelial sodium channel ENaC. The latter also mediates the gustatory salt sensing in the tongue, which is required for the manifestation of increased salt intake. Effects of aldosterone on both the brain and kidney synergize with the effects of angiotensin II. Thus, mineralocorticoids appear to induce similar molecular pathways in the kidney, brain, and possibly tongue, which could provide opportunities for more effective therapeutic interventions. Inhibition of renal salt reabsorption is compensated by stimulation of salt appetite and vice versa; targeting both mechanisms should be more effective. Inhibiting the arousal to consume salty food may improve a patient's compliance to reducing salt intake. While a better understanding of the molecular mechanisms is needed and will provide new therapeutic options, current pharmacological interventions that target both salt retention and sodium appetite include mineralocorticoid receptor antagonists and potentially inhibitors of angiotensin II and ENaC.

Salt appetite is reduced by a single experience of drinking hypertonic saline in the adult rat

Salt appetite, the primordial instinct to favorably ingest salty substances, represents a vital evolutionary important drive to successfully maintain body fluid and electrolyte homeostasis. This innate instinct was shown here in Sprague-Dawley rats by increased ingestion of isotonic saline (IS) over water in fluid intake tests. However, this appetitive stimulus was fundamentally transformed into a powerfully aversive one by increasing the salt content of drinking fluid from IS to hypertonic saline (2% w/v NaCl, HS) in intake tests. Rats ingested HS similar to IS when given no choice in one-bottle tests and previous studies have indicated that this may modify salt appetite. We thus investigated if a single 24 h experience of ingesting IS or HS, dehydration (DH) or 4% high salt food (HSD) altered salt preference. Here we show that 24 h of ingesting IS and HS solutions, but not DH or HSD, robustly transformed salt appetite in rats when tested 7 days and 35 days later. Using two-bottle tests rats previously exposed to IS preferred neither IS or water, whereas rats exposed to HS showed aversion to IS. Responses to sweet solutions (1% sucrose) were not different in two-bottle tests with water, suggesting that salt was the primary aversive taste pathway recruited in this model. Inducing thirst by subcutaneous administration of angiotensin II did not overcome this salt aversion. We hypothesised that this behavior results from altered gene expression in brain structures important in thirst and salt appetite. Thus we also report here lasting changes in mRNAs for markers of neuronal activity, peptide hormones and neuronal plasticity in supraoptic and paraventricular nuclei of the hypothalamus following rehydration after both DH and HS. These results indicate that a single experience of drinking HS is a memorable one, with long-term changes in gene expression accompanying this aversion to salty solutions.

Efficient transduction of 11 poly-arginine peptide in an ischemic lesion of mouse brain

Direct intracellular delivery of intact proteins has been successfully achieved by tagging cell-penetrating peptide (CPP), which consists of short positively charged amino acids, such as 11 poly-arginine (11R); however, in vivo delivery of the proteins to the brain has remained challenging because it is unclear whether CPP would enable proteins to cross the blood-brain barrier (BBB). In this study, we conducted an in vivo kinetic study to investigate the efficiency of 11R-mediated peptide delivery in the normal and ischemic brain. The 11R was observed in the microvessels and neurons surrounding the microvessels throughout the brain 1 hour after systemic administration, but the signal of the peptide was faint after 2 hours. In a transient middle cerebral artery occlusion mouse model, 11R was markedly enhanced and remained detectable in the cells on the ipsilateral side for as long as 8 hours after administration compared with the contralateral side. These results suggest that 11R is capable of in vivo delivery to the brain by passing through the BBB. Furthermore, 11R-mediated protein transduction could be used for the delivery of therapeutic molecules in cerebral ischemia.

The Nax Channel

Na x, which is preferentially expressed in the glial cells of sensory circumventricular organs in the brain, is a sodium channel that is poorly homologous to voltage-gated sodium channels. We previously reported that Na x is a sodium concentration ([Na+])-sensitive, but not a voltage-sensitive channel that is critically involved in body-fluid homeostasis. Nax-knockout mice do not stop ingesting salt even when dehydrated and transiently develop hypernatremia. [Na+] in body fluids is strictly controlled at 135 to 145 mM in mammals. Although the set point must be within this range, Na x was shown to have a threshold value of ~150 mM for extracellular [Na+] ([Na+]o) for activation in vitro. Therefore, the [Na+]o dependency of Na x in vivo is presumably modified by an as yet unidentified mechanism. We recently demonstrated that the [Na+]o dependency of Na x in the subfornical organ was adjusted to the physiological range by endothelin-3. Pharmacological experiments revealed that endothelin receptor B signaling was involved in this modulation of Na x gating through protein kinase C and ERK1/2 activation. In addition, we identified a case of essential hypernatremia caused by autoimmunity to Na x. Occurrence of a ganglioneuroma composed of Schwann-like cells that robustly expressed Na x was likely to induce the autoimmune response in this patient. An intravenous injection of the immunoglobulin fraction of the patient’s serum, which contained anti-Na x antibodies, into mice reproduced the patient’s symptoms. This review provides an overview of the physiological functions of Na x by summarizing our recent studies.

Essential hypertension: an approach to its etiology and neurogenic pathophysiology

Essential hypertension, a rise in blood pressure of undetermined cause, includes 90% of all hypertensive cases and is a highly important public health challenge that remains, however, a major modifiable cause of morbidity and mortality. This review emphasizes that, from an evolutionary point of view, we are adapted to ingest and excrete <1 g of sodium (2.5 g of salt) per day and that essential hypertension develops when the kidneys become unable to excrete the amount of sodium ingested, unless blood pressure is increased. The renal-mean arterial pressure set-point model is briefly described to explain that a shift of the pressure natriuresis relationship toward abnormally high pressure levels is a pathophysiological characteristic of essential hypertension. Evidence indicating that this anomaly in the pressure natriuresis relationship arises from a sympathetic nervous system dysfunction is briefly formulated, and the most widely accepted pathophysiologic proposal to explain the development of this sympathetic dysfunction is described, with commentaries about novel action mechanisms of some drugs currently used in essential hypertension treatment.

Changes in pericytic expression of NG2 and PDGFRB and vascular permeability in the sensory circumventricular organs of adult mouse by osmotic stimulation

The blood-brain barrier (BBB) is a barrier that prevents free access of blood-derived substances to the brain through the tight junctions and maintains a specialized brain environment. Circumventricular organs (CVOs) lack the typical BBB. The fenestrated vasculature of the sensory CVOs, including the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO) and area postrema (AP), allows parenchyma cells to sense a variety of blood-derived information, including osmotic ones. In the present study, we utilized immunohistochemistry to examine changes in the expression of NG2 and platelet-derived growth factor receptor beta (PDGFRB) in the OVLT, SFO and AP of adult mice during chronic osmotic stimulation. The expression of NG2 and PDGFRB was remarkably prominent in pericytes, although these angiogenesis-associated proteins are highly expressed at pericytes of developing immature vasculature. The chronic salt loading prominently increased the expression of NG2 in the OVLT and SFO and that of PDGFRB in the OVLT, SFO and AP. The vascular permeability of low-molecular-mass tracer fluorescein isothiocyanate was increased significantly by chronic salt loading in the OVLT and SFO but not AP. In conclusion, the present study demonstrates changes in pericyte expression of NG2 and PDGFRB and vascular permeability in the sensory CVOs by chronic osmotic stimulation, indicating active participation of the vascular system in osmotic homeostasis.

Regulation of central Na+ detection requires the cooperative action of the NaX channel and α1 Isoform of Na+/K+-ATPase in the Na+-sensor neuronal population

The median preoptic nucleus (MnPO) holds a strategic position in the hypothalamus. It is adjacent to the third ventricle; hence, it can directly access the ionic composition of the CSF. MnPO neurons play a critical role in hydromineral homeostasis regulation by acting as central sensors of extracellular Na(+) concentration ([Na(+)](ext)). The mechanism underlying Na(+) sensing involves the atypical Na(+) channel, Na(X). Here we sought to determine whether Na(+) influx in Na(+) sensors is actively regulated via interaction with other membrane proteins involved in cellular Na(+) homeostasis, such as Na(+)/K(+)-ATPase. The Na(+)/K(+)-ATPase role was investigated using patch-clamp recordings in rat MnPO dissociated neurons. Na(+) current evoked with hypernatriuric solution was diminished in the absence of ATP/GTP, indicating that Na(+)/K(+)-ATPase play a central role in [Na(+)](ext) detection. Specific blockers of α1 and α3 isoforms of Na(+)/K(+)-ATPase, ouabain or strophanthidin, inhibited this Na(+) current. However, strophanthidin, which selectively blocks the α1 isoform, was more effective in blocking Na(+) current, suggesting that the Na(+)/K(+)-ATPase-α1 isoform is specifically involved in [Na(+)](ext) detection. Although strophanthidin did not alter either the membrane resistance or the Na(+) reversal potential, the conductance and the permeability of the Na(X) channel decreased significantly. Our results suggest that Na(+)/K(+)-ATPase interacts with the Na(X) channel and regulates the high [Na(+)](ext)-evoked Na(+) current via influencing the Na(+) influx rate. This study describes a novel intracellular regulatory pathway of [Na(+)](ext) detection in MnPO neurons. The α1 isoform of Na(+)/K(+)-ATPase acts as a direct regulatory partner of the Na(X) channel and influences Na(+) influx via controlling the Na(+) permeability of the channel.

Endothelin-3 expression in the subfornical organ enhances the sensitivity of Na(x), the brain sodium-level sensor, to suppress salt intake

Salt homeostasis is essential to survival, but brain mechanisms for salt-intake control have not been fully elucidated. Here, we found that the sensitivity of Na(x) channels to [Na(+)](o) is dose-dependently enhanced by endothelin-3 (ET-3). Na(x) channels began to open when [Na(+)](o) exceeded ~150 mM without ET-3, but opened fully at a physiological [Na(+)](o) (135–145 mM) with 1 nM ET-3. Importantly, ET-3 was expressed in the subfornical organ (SFO) along with Nax, and the level was robustly increased by dehydration. Pharmacological experiments revealed that endothelin receptor B (ET(B)R) signaling is involved in this modulation of Na(x) gating through protein kinase C and ERK1/2 activation. ET(B)R agonists increased the firing rate of GABAergic neurons via lactate in the SFO, and an ET(B)R antagonist attenuated salt aversion during dehydration. These results indicate that ET-3 expression in the SFO is tightly coupled with body-fluid homeostasis through modulation of the [Na(+)](o) sensitivity of Na(x).

Neurogenesis in the circumventricular organs of adult mouse brains

The circumventricular organs (CVOs), including the organum vasculosum of the lamina terminalis (OVLT), subfornical organ (SFO), median eminence (ME), and area postrema (AP), allow parenchyma cells to sense a variety of blood-derived substances and/or secreted peptides into blood circulation. In the present study, we examined continuous neurogenesis in the CVOs of adult mice. The immunohistochemistry of neural progenitor cell (NPC) marker proteins revealed that Math1- and Mash1-positive cells were observed in the discrete regions of CVOs, including the capillary plexus in the OVLT, the internal zone of the ME, and the lateral zone in the AP. A few Mash1- and Math1-positive cells were seen throughout the SFO, and many Math1- but not Mash1-positive cells were observed at the arcuate nucleus. Math-positive cells were often seen to localize in close proximity to the vasculature. Bromodeoxyuridine (BrdU) immunohistochemistry revealed the incorporation of BrdU in a subpopulation of Mash1-, Math1-, HuC/D-, and microtubule-associated protein 2 (MAP2)-positive cells. Mash1- and Math1-positive cells expressed exclusively high level of plasminogen, whereas a subpopulation of HuC/D- and MAP2-positive neurons expressed low or undetectable level of plasminogen. Thus, the present study demonstrates that newborn cells express NPC marker proteins and plasminogen to localize closely at vascular matrix and moreover differentiate into neurons expressing mature neuron marker proteins, indicating that new neurons are possibly generated to integrate into new neural circuits.

VEGF-dependent continuous angiogenesis in the median eminence of adult mice

Brain vasculature forms the blood-brain barrier (BBB) that restricts the movement of molecules between the brain and blood, but the capillary of the median eminence (ME) lacks the BBB for secretion of adenohypophysial hormone-releasing peptides. In the present study, we aimed to elucidate whether continuous angiogenesis occurs in the ME of adult mice. By using a mitotic marker, bromodeoxyuridine (BrdU), we demonstrated that new endothelial cells were born continuously in the ME of adults. Prominent expression of NG2, platelet-derived growth factor receptor B (PDGFRB), and delta-like ligand 4 was observed at pericytes of adults, although the expression of these angiogenesis-associated proteins has been shown to be at low or trace levels in adult mature capillary. In addition, vascular endothelial growth factor (VEGF), a key regulator of angiogenesis, was expressed highly in the nervous parenchyma of the ME. Expression of VEGF receptor 2 (VEGFR2) was observed at endothelial cells in the external zone and at somatodendrites in the internal zone. Finally, a VEGFR- and PDGFR-associated tyrosine kinase inhibitor, SU11248, significantly decreased the number of BrdU-positive proliferating endothelial cells and parenchyma cells. In conclusion, the present study demonstrates VEGF-dependent continuous angiogenesis in the ME of adult mouse brains under normal conditions, which provides new insight into our understanding of neurosecretion in the ME.

Enhanced SCN7A/Nax expression contributes to bone cancer pain by increasing excitability of neurons in dorsal root ganglion

Bone pain is one of the most common complications in cancer patients with bone metastases, and has the most significant impact on quality of life for patients. Patients with bone cancer pain may be difficult to treat due to the poor understanding of the mechanisms; therefore, the mechanisms of bone cancer pain required elucidation for developing new therapeutics. Recent studies show that SCN7A/Nax channel serves as a sodium-level sensor of the body fluid that controls the Na-intake behavior by changing the excitability of neurons. In the current study, the expression of SCN7A/Nax and the excitability of primary sensory neurons in bone cancer pain rats were examined. The analgesic effects of knockdown SCN7A/Nax channel using RNAi lentivirus intrathecal treatment were evaluated with a behavioral test. The results showed that implantation of sarcoma induced ongoing and movement-evoked pain behaviors, whereas SCN7A/Nax knockdown prevented the onset of these hyperalgesia. Immunohistochemistry showed that SCN7A/Nax was located in the medium- to large-sized neurons in dorsal root ganglions (DRGs). The proportion of SCN7A/Nax-positive cells was significantly increased in DRGs ipsilateral to sarcoma implantation. Immunostaining results were further confirmed by Western blot and real time-polymerase chain reaction (RT-PCR) analyses. Recording from primary sensory neurons in excised rat dorsal root ganglias, we found that most of SCN7A/Nax-positive neurons exhibited subthreshold oscillations, depolarized resting membrane potential and more negative threshold of action potential. These electrophysiological changes of neurons increased ectopic spike discharge which was thought to be an important generator of chronic pain, however, the hyperexcitability was completely reversed by SCN7A/Nax knockdown. These results demonstrate that enhanced expression of SCN7A/Nax channel within distinct subpopulation of DRG neurons contributes to bone cancer pain by increasing the excitability of these neurons. These findings may lead to novel strategies for the treatment of bone cancer pain.

Voltage-gated sodium channels: biophysics, pharmacology, and related channelopathies

Voltage-gated sodium channels (VGSC) are multi-molecular protein complexes expressed in both excitable and non-excitable cells. They are primarily formed by a pore-forming multi-spanning integral membrane glycoprotein (α-subunit) that can be associated with one or more regulatory β-subunits. The latter are single-span integral membrane proteins that modulate the sodium current (I_Na) and can also function as cell adhesion molecules. In vitro some of the cell-adhesive functions of the β-subunits may play important physiological roles independently of the α-subunits. Other endogenous regulatory proteins named “channel partners” or “channel interacting proteins” (ChiPs) like caveolin-3 and calmodulin/calmodulin kinase II (CaMKII) can also interact and modulate the expression and/or function of VGSC. In addition to their physiological roles in cell excitability and cell adhesion, VGSC are the site of action of toxins (like tetrodotoxin and saxitoxin), and pharmacologic agents (like antiarrhythmic drugs, local anesthetics, antiepileptic drugs, and newly developed analgesics). Mutations in genes that encode α- and/or β-subunits as well as the ChiPs can affect the structure and biophysical properties of VGSC, leading to the development of diseases termed sodium “channelopathies”.  This review will outline the structure, function, and biophysical properties of VGSC as well as their pharmacology and associated channelopathies and highlight some of the recent advances in this field.

Different vascular permeability between the sensory and secretory circumventricular organs of adult mouse brain

The blood-brain barrier (BBB) prevents free access of circulating molecules to the brain and maintains a specialized brain environment to protect the brain from blood-derived bioactive and toxic molecules; however, the circumventricular organs (CVOs) have fenestrated vasculature. The fenestrated vasculature in the sensory CVOs, including the organum vasculosum of lamina terminalis (OVLT), subfornical organ (SFO) and area postrema (AP), allows neurons and astrocytes to sense a variety of plasma molecules and convey their information into other brain regions and the vasculature in the secretory CVOs, including median eminence (ME) and neurohypophysis (NH), permits neuronal terminals to secrete many peptides into the blood stream. The present study showed that vascular permeability of low-molecular-mass tracers such as fluorescein isothiocyanate (FITC) and Evans Blue was higher in the secretory CVOs and kidney as compared with that in the sensory CVOs. On the other hand, vascular permeability of high-molecular-mass tracers such as FITC-labeled bovine serum albumin and Dextran 70,000 was lower in the CVOs as compared with that in the kidney. Prominent vascular permeability of low- and high-molecular-mass tracers was also observed in the arcuate nucleus. These data demonstrate that vascular permeability for low-molecular-mass molecules is higher in the secretory CVOs as compared with that in the sensory CVOs, possibly for large secretion of peptides to the blood stream. Moreover, vascular permeability for high-molecular-mass tracers in the CVOs is smaller than that of the kidney, indicating that the CVOs are not totally without a BBB.

High-sweat Na+ in cystic fibrosis and healthy individuals does not diminish thirst during exercise in the heat

Sweat Na(+) concentration ([Na(+)]) varies greatly among individuals and is particularly high in cystic fibrosis (CF). The purpose of this study was to determine whether excess sweat [Na(+)] differentially impacts thirst drive and other physiological responses during progressive dehydration via exercise in the heat. Healthy subjects with high-sweat [Na(+)] (SS) (91.0 ± 17.3 mmol/l), Controls with average sweat [Na(+)] (43.7 ± 9.9 mmol/l), and physically active CF patients with very high sweat [Na(+)] (132.6 ± 6.4 mmol/l) cycled in the heat without drinking until 3% dehydration. Serum osmolality increased less (P < 0.05) in CF (6.1 ± 4.3 mosmol/kgH(2)O) and SS (8.4 ± 3.0 mosmol/kgH(2)O) compared with Control (14.8 ± 3.5 mosmol/kgH(2)O). Relative change in plasma volume was greater (P < 0.05) in CF (-19.3 ± 4.5%) and SS (-18.8 ± 3.1%) compared with Control (-14.3 ± 2.3%). Thirst during exercise and changes in plasma levels of vasopressin, angiotensin II, and aldosterone relative to percent dehydration were not different among groups. However, ad libitum fluid replacement was 40% less, and serum NaCl concentration was lower for CF compared with SS and Control during recovery. Despite large variability in sweat electrolyte loss, thirst appears to be appropriately maintained during exercise in the heat as a linear function of dehydration, with relative contributions from hyperosmotic and hypovolemic stimuli dependent upon the magnitude of salt lost in sweat. CF exhibit lower ad libitum fluid restoration following dehydration, which may reflect physiological cues directed at preservation of salt balance over volume restoration.

Oestradiol potentiates hormone secretion and neuronal activation in response to hypertonic extracellular volume expansion in ovariectomised rats

Secretion of vasopressin (VP), oxytocin (OT) and atrial natriuretic peptide (ANP) is an essential mechanism for the maintenance of hydromineral homeostasis. Secretion of these hormones is modulated by several circulating factors, including oestradiol. However, it remains unclear how oestradiol exerts this modulation. In the present study we investigated the participation of oestradiol in the secretion of VP, OT and ANP and in activation of vasopressinergic and oxytocinergic neurones of the supraoptic (SON) and paraventricular (PVN) nuclei of the hypothalamus in response to extracellular volume expansion (EVE). For this purpose, ovariectomised (OVX) rats treated for 7 days with vehicle (corn oil, 0.1 ml/rat, OVX+O group) or oestradiol (oestradiol cypionate, 10 μg/kg, OVX+E group) were subjected to either isotonic (0.15 m NaCl, 2 ml/100 g b.w., i.v.) or hypertonic (0.30 m NaCl, 2 ml/100 g b.w., i.v.) EVE. Blood samples were collected for plasma VP, OT and ANP determination. Another group of rats was subjected to cerebral perfusion, and brain sections were processed for c-Fos-VP and c-Fos-OT double-labelling immunohistochemistry. In OVX+O rats, we observed that both isotonic and hypertonic EVE increased plasma OT and ANP concentrations, although no changes were observed in VP secretion. Oestradiol replacement did not alter hormonal secretion in response to isotonic EVE, but it increased VP secretion and potentiated plasma OT and ANP concentrations in response to hypertonic EVE. Immunohistochemical data showed that, in the OVX+O group, hypertonic EVE increased the number of c-Fos-OT and c-Fos-VP double-labelled neurones in the PVN and SON. Oestradiol replacement did not alter neuronal activation in response to isotonic EVE, but it potentiated vasopressinergic and oxytocinergic neuronal activation in the medial magnocellular PVN (PaMM) and SON. Taken together, these results suggest that oestradiol increases the responsiveness of vasopressinergic and oxytocinergic magnocellular neurones in the PVN and SON in response to osmotic stimulation.

Nav1.7 is the predominant sodium channel in rodent olfactory sensory neurons

Background

Voltage-gated sodium channel Na_v1.7 is preferentially expressed in dorsal root ganglion (DRG) and sympathetic neurons within the peripheral nervous system. Homozygous or compound heterozygous loss-of-function mutations in SCN9A, the gene which encodes Na_v1.7, cause congenital insensitivity to pain (CIP) accompanied by anosmia. Global knock-out of Na_v1.7 in mice is neonatal lethal reportedly from starvation, suggesting anosmia. These findings led us to hypothesize that Na_v1.7 is the main sodium channel in the peripheral olfactory sensory neurons (OSN, also known as olfactory receptor neurons).

Methods

We used multiplex PCR-restriction enzyme polymorphism, in situ hybridization and immunohistochemistry to determine the identity of sodium channels in rodent OSNs.

Results

We show here that Na_v1.7 is the predominant sodium channel transcript, with low abundance of other sodium channel transcripts, in olfactory epithelium from rat and mouse. Our in situ hybridization data show that Na_v1.7 transcripts are present in rat OSNs. Immunostaining of Na_v1.7 and Na_v1.6 channels in rat shows a complementary accumulation pattern with Na_v1.7 in peripheral presynaptic OSN axons, and Na_v1.6 primarily in postsynaptic cells and their dendrites in the glomeruli of the olfactory bulb within the central nervous system.

Conclusions

Our data show that Na_v1.7 is the dominant sodium channel in rat and mouse OSN, and may explain anosmia in Na_v1.7 null mouse and patients with Na_v1.7-related CIP.

Neuronal sodium leak channel is responsible for the detection of sodium in the rat median preoptic nucleus

Sodium (Na(+)) ions are of primary importance for hydromineral and cardiovascular homeostasis, and the level of Na(+) in the body fluid compartments [plasma and cerebrospinal fluid (CSF)] is precisely monitored in the hypothalamus. Glial cells seem to play a critical role in the mechanism of Na(+) detection. However, the precise role of neurons in the detection of extracellular Na(+) concentration ([Na(+)](out)) remains unclear. Here we demonstrate that neurons of the median preoptic nucleus (MnPO), a structure in close contact with the CSF, are specific Na(+) sensors. Electrophysiological recordings were performed on dissociated rat MnPO neurons under isotonic [Na(+)] (100 mM NaCl) with local application of hypernatriuric (150, 180 mM NaCl) or hyponatriuric (50 mM NaCl) external solution. The hyper- and hyponatriuric conditions triggered an in- and an outward current, respectively. The reversal potential of the current matched the equilibrium potential of Na(+), indicating that a change in [Na(+)](out) modified the influx of Na(+) in the MnPO neurons. The conductance of the Na(+) current was not affected by either the membrane potential or the [Na(+)](out). Moreover, the channel was highly selective for lithium over guanidinium. Together, these data identified the channel as a Na(+) leak channel. A high correlation between the electrophysiological recordings and immunofluorescent labeling for the Na(X) channel in dissociated MnPO neurons strongly supports this channel as a candidate for the Na(+) leak channel responsible for the Na(+)-sensing ability of rat MnPO neurons. The absence of Na(X) labeling and of a specific current evoked by a change in [Na(+)](out) in mouse MnPO neurons suggests species specificity in the hypothalamus structures participating in central Na(+) detection.

Effects of central sodium on epithelial sodium channels in rat brain

We evaluated the effects of intracerebroventricular (icv) infusion of Na+-rich artificial cerebrospinal fluid (aCSF), with or without the mineralocorticoid receptor (MR) blocker spironolactone, on epithelial Na+ channel (ENaC) subunits and regulators, such as MR, serum/glucocorticoid-inducible kinase 1, neural precursor cells expressed developmentally downregulated 4-like gene, 11β-hydroxylase, and aldosterone synthase, in brain regions of Wistar rats. The effects of icv infusion of the amiloride analog benzamil on brain tissue and CSF Na+ concentration ([Na+]) were also assessed. In the choroid plexus and ependyma of the anteroventral third ventricle, ENaC subunits are present in apical and basal membranes. Na+-rich aCSF increased β-ENaC mRNA and immunoreactivity in the choroid plexus and increased α- and β-ENaC immunoreactivities in the ependyma. Na+-rich aCSF increased α- and β-ENaC-gold-labeled particles in the microvilli of the choroid plexus and in basolateral membranes of the ependyma. Spironolactone only prevented the increase in β-ENaC immunoreactivity in the choroid plexus and ependyma. In the supraoptic nucleus, paraventricular nucleus, and subfornical organ, Na+-rich aCSF did not affect mRNA expression levels of the studied genes. Benzamil significantly increased CSF [Na+] in the control, but not Na+-rich, aCSF group. In contrast, benzamil prevented the increase in hypothalamic tissue [Na+] by Na+-rich aCSF. These results suggest that CSF Na+ upregulates ENaC expression in the brain epithelia, but not in the neurons of hypothalamic nuclei. ENaC in the choroid plexus and ependyma appear to contribute to regulation of Na+ homeostasis in the brain.

Autoimmunity to the sodium-level sensor in the brain causes essential hypernatremia

Na(x) is the sodium-level sensor of body fluids in the brain involved in sodium homeostasis. Na(x)-knockout mice do not stop ingesting salt even when dehydrated. Here we report a case with clinical features of essential hypernatremia without demonstrable hypothalamic structural lesions, who was diagnosed as a paraneoplastic neurologic disorder. The patient had autoantibodies directed against Na(x), along with a ganglioneuroma composed of Schwann-like cells robustly expressing Na(x). The removal of the tumor did not reduce the autoantibody levels or relieve the symptoms. Intravenous injection of the immunoglobulin fraction of the patient's serum into mice induced abnormalities in water/salt intake and diuresis, which led to hypernatremia. In the brains of these mice, cell death was observed along with focal deposits of complement C3 and inflammatory infiltrates in circumventricular organs where Na(x) is specifically expressed. Our findings thus provide new insights into the pathogenesis of hypernatremia relevant to the sodium-level-sensing mechanism in humans.

Circulating signals as critical regulators of autonomic state--central roles for the subfornical organ

To maintain homeostasis autonomic control centers in the hypothalamus and medulla must respond appropriately to both external and internal stimuli. Although protected behind the blood-brain barrier, neurons in these autonomic control centers are known to be influenced by changing levels of important signaling molecules in the systemic circulation (e.g., osmolarity, glucose concentrations, and regulatory peptides). The subfornical organ belongs to a group of specialized central nervous system structures, the circumventricular organs, which are characterized by the lack of the normal blood-brain barrier, such that circulating lipophobic substances may act on neurons within this region and via well-documented efferent neural projections to hypothalamic autonomic control centers, influence autonomic function. This review focuses on the role of the subfornical organ in sensing peripheral signals and transmitting this information to autonomic control centers in the hypothalamus.

Identification and functional characterization of the promoter of the mouse sodium-activated sodium channel Na(x) gene (Scn7a)

Na(x) is a sodium channel, thought to be a descendant of the voltage-gated sodium channel family. Nevertheless, Na(x) is not activated by voltage but rather by augmentation of extracellular sodium over 150 mM. In the brain, it is localized to the circumventricular organs, important regions for salt and water homeostasis in mammals, where it operates as a sodium-level sensor of body fluid. Na(x) channel is expressed in lung, uterus, and heart, and it is also found in trigeminal and dorsal root ganglia and in nonmyelinating Schwann cells, where its physiological role remains unclarified. Here we identified the promoter and transcription start sites of Na(x) sodium channel in dorsal root ganglia neurons from mouse. We report a characterization of the basal TATA-less promoter and the sequence requirements for promoter activity in Neuro 2A neuroblastoma cells and in dorsal root ganglia neurons, where basal promoter activity seems to require NGFI-C and Ebox DNA elements. Finally, we provide evidence that a repression mechanism that inhibits Na(x) expression may be present in certain tissues. These findings provide the basis with which to understand tissue-specific regulation of Na(x) sodium channel gene (Scn7a) expression.