Astrocytes in sensory circumventricular organs of the rat brain express functional binding sites for endothelin

Sensory Circumventricular Organs, Neuroendocrine Control, and Metabolic Regulation

The central nervous system is critical in metabolic regulation, and accumulating evidence points to a distributed network of brain regions involved in energy homeostasis. This is accomplished, in part, by integrating peripheral and central metabolic information and subsequently modulating neuroendocrine outputs through the paraventricular and supraoptic nucleus of the hypothalamus. However, these hypothalamic nuclei are generally protected by a blood-brain-barrier limiting their ability to directly sense circulating metabolic signals—pointing to possible involvement of upstream brain nuclei. In this regard, sensory circumventricular organs (CVOs), brain sites traditionally recognized in thirst/fluid and cardiovascular regulation, are emerging as potential sites through which circulating metabolic substances influence neuroendocrine control. The sensory CVOs, including the subfornical organ, organum vasculosum of the lamina terminalis, and area postrema, are located outside the blood-brain-barrier, possess cellular machinery to sense the metabolic interior milieu, and establish complex neural networks to hypothalamic neuroendocrine nuclei. Here, evidence for a potential role of sensory CVO-hypothalamic neuroendocrine networks in energy homeostasis is presented.

Cellular Microbiology of Mycoplasma canis

Mycoplasma canis can infect many mammalian hosts but is best known as a commensal or opportunistic pathogen of dogs. The unexpected presence of M. canis in brains of dogs with idiopathic meningoencephalitis prompted new in vitro studies to help fill the void of basic knowledge about the organism's candidate virulence factors, the host responses that it elicits, and its potential roles in pathogenesis. Secretion of reactive oxygen species and sialidase varied quantitatively (P < 0.01) among strains of M. canis isolated from canine brain tissue or mucosal surfaces. All strains colonized the surface of canine MDCK epithelial and DH82 histiocyte cells and murine C8-D1A astrocytes. Transit through MDCK and DH82 cells was demonstrated by gentamicin protection assays and three-dimensional immunofluorescence imaging. Strains further varied (P < 0.01) in the extents to which they influenced the secretion of tumor necrosis factor alpha (TNF-α) and the neuroendocrine regulatory peptide endothelin-1 by DH82 cells. Inoculation with M. canis also decreased major histocompatibility complex class II (MHC-II) antigen expression by DH82 cells (P < 0.01), while secretion of gamma interferon (IFN-γ), interleukin-6 (IL-6), interleukin-10 (IL-10), and complement factor H was unaffected. The basis for differences in the responses elicited by these strains was not obvious in their genome sequences. No acute cytopathic effects on any homogeneous cell line, or consistent patterns of M. canis polyvalent antigen distribution in canine meningoencephalitis case brain tissues, were apparent. Thus, while it is not likely a primary neuropathogen, M. canis has the capacity to influence meningoencephalitis through complex interactions within the multicellular and neurochemical in vivo milieu.

Functional expression of P2 purinoceptors in a primary neuroglial cell culture of the rat arcuate nucleus

The arcuate nucleus (ARC) plays an important role in the hypothalamic control of energy homeostasis. Expression of various purinoceptor subtypes in the rat ARC and physiological studies suggest a modulatory function of P2 receptors within the neuroglial ARC circuitry. A differentiated mixed neuronal and glial microculture was therefore established from postnatal rat ARC, revealing neuronal expression of ARC-specific transmitters involved in food intake regulation (neuropeptide Y (NPY), proopiomelanocortin (POMC), tyrosine hydroxylase (TH)). Some NPYergic neurons cosynthesized TH, while POMC and TH expression proved to be mutually exclusive. Stimulation with the general purinoceptor agonists 2-methylthioadenosine-5'triphosphate (2-MeSATP) and ATP but not the P2X1/P2X3 receptor subtype agonist α,β-methyleneadenosine-5'triphosphate (α,β-meATP) induced intracellular calcium signals in ARC neurons and astrocytes. Some 5-10% each of 2-MeSATP responsive neurons expressed POMC, NYP or TH. Supporting the calcium imaging data, radioligand binding studies to hypothalamic membranes showed high affinity for 2-MeSATP, ATP but not α,β-meATP to displace [α-(35)S]deoxyadenosine-5'thiotriphosphate ([(35)S]dATPαS) from P2 receptors. Repetitive superfusion with equimolar 2-MeSATP allowed categorization of ARC cells into groups with a high or low (LDD) degree of purinoceptor desensitization, the latter allowing further receptor characterization. Calcium imaging experiments performed at 37°C vs. room temperature showed further reduction of desensitization. Agonist-mediated intracellular calcium signals were suppressed in all LDD neurons but only 25% of astrocytes in the absence of extracellular calcium, suggestive of metabotropic P2Y receptor expression in the majority of ARC astrocytes. The highly P2Y1-selective receptor agonists MRS2365 and 2-methylthioadenosine-5'diphosphate (2-MeSADP) activated 75-85% of all 2-MeSATP-responsive ARC astrocytes. Taking into consideration the high potency to dose-dependently stimulate ARC cells of the LDD group, the high affinity for rat P2X(1-3) and low affinity for rat P2X4, P2X7 and P2Y receptor subtypes except P2Y1 and P2Y13, the agonist 2-MeSATP primarily acted upon P2X2 and P2Y1 purinoceptors to trigger intracellular calcium signaling in ARC neurons and astrocytes.

Glial and perivascular structures in the subfornical organ: distinguishing the shell and core

The subfornical organ (SFO) is a circumventricular organ with a chemosensitive function, and its vessels have no blood-brain barrier. Our study investigated the glial and vascular components in the SFO to determine whether their distributions indicate subdivisions, how to characterize the vessels and how to demarcate the SFO. To this end, we investigated glial markers (GFAP, glutamine synthetase, S100) and other markers, including vimentin and nestin (immature glia), laminin (basal lamina), β-dystroglycan (glio-vascular connections), and aquaporin 4 (glial water channels). We determined that the 'shell' of the SFO was marked by immunoreactivity for S100, GFAP and aquaporin 4. Nestin immunoreactivity was characteristic of the 'core'. Vimentin was almost evenly distributed. Glutamine synthetase immunoreactivity occurred in the shell but its expression was sparse. Vessels in the core were decorated with laminin but showed a discontinuous expression of aquaporin 4. Vimentin and GFAP staining was usually in separate glial elements, which may be related to their functional differences. Similar to other vessels in the brain, β-dystroglycan was detected along the shell vessels but laminin was not. The gradual disappearance of the laminin immunopositivity was attributed to the gradual disappearance of the perivascular space. Thus, our findings suggest that the shell and core glio-vascular structures are adapted to different sensory functions: osmoperception and the perception of circulating peptides, respectively.

ENaC γ-expressing astrocytes in the circumventricular organs, white matter, and ventral medullary surface: sites for Na+ regulation by glial cells

Using a double immunofluorescence procedure, we report the discovery of a novel group of fibrous astrocytes that co-express epithelial sodium channel (ENaC) γ-subunit protein along with glial acidic fibrillary protein (GFAP). These cells are concentrated along the borders of the sensory circumventricular organs (CVOs), embedded in the white matter (e.g., optic nerve/chiasm, anterior commissure, corpus callosum, pyramidal tract) and are components of the pia mater. In the CVOs, a compact collection of ENaC γ-immunoreactive glial fibers form the lamina terminalis immediately rostral to the organum vasculosum of the lamina terminalis (OVLT). Astrocyte processes can be traced into the median preoptic nucleus - a region implicated in regulation of sodium homeostasis. In the subfornical organ (SFO), ENaC γ-GFAP astrocytes lie in its lateral border, but not in the ventromedial core. In the area postrema (AP), a dense ENaC γ-GFAP glial fibers form the interface between the AP and nucleus tractus solitarius; this area is termed the subpostremal region. Antibodies against the ENaC α- or β-subunit proteins do not immunostain these regions. In contrast, the antibodies against the ENaC γ-subunit protein react weakly with neuronal cell bodies in the CVOs. Besides affecting glial-neural functions in the CVOs, the astrocytes found in the white matter may affect saltatory nerve conduction, serving as a sodium buffer. The ENaC γ-expressing astrocytes of the ventral medulla send processes into the raphe pallidus which intermingle with the serotoninergic (5-HT) neurons found in this region as well as with the other nearby 5-HT neurons distributed along ventral medullary surface.

Stretch induced endothelin-1 secretion by adult rat astrocytes involves calcium influx via stretch-activated ion channels (SACs)

The expression of endothelins (ETs) and ET-receptors is often upregulated in brain pathology. ET-1, a potent vasoconstrictor, also inhibits the expression of astrocyte glutamate transporters and is mitogenic for astrocytes, glioma cells, neurons, and brain capillary endothelia. We have previously shown that mechanical stress stimulates ET-1 production by adult rat astrocytes. We now show in adult astrocytes that ET-1 production is driven by calcium influx through stretch-activated ion channels (SACs) and the ET-1 production correlates with cell proliferation. Mechanical stimulation using biaxial stretch (<20%) of a rubber substrate increased ET-1 secretion, and 4 μM GsMTx-4 (a specific inhibitor of SACs) inhibited secretion by 30%. GsMTx-4 did not alter basal ET-1 levels in the absence of stretch. Decreasing the calcium influx by lowering extracellular calcium also inhibited stretch-induced ET-1 secretion without effecting ET-1 secretion in unstretched controls. Furthermore, inhibiting SACs with the less specific inhibitor streptomycin also inhibited stretch-induced ET-1 secretion. The data can be explained with a simple model in which ET-1 secretion depends on an internal Ca(2+) threshold. This coupling of mechanical stress to the astrocyte endothelin system through SACs has treatment implications, since all pathology deforms the surrounding parenchyma.

Physiological response of bovine subcommissural organ to endothelin 1 and bradykinin

The circumventricular organs (CVOs) regulate certain vegetative functions. Receptors for bradykinin (BDK) and endothelin (ET) have been found in some CVOs. The subcommissural organ (SCO) is a CVO expressing BDK-B2 receptors and secreting Reissner's fiber (RF) glycoproteins into the cerebrospinal fluid. This investigation was designed to search for ET receptors in the bovine SCO and, if found, to study the functional properties of this ET receptor and the BDK-B2 receptor. Cryostat sections exposed to (125)I ET1 showed dense labeling of secretory SCO cells, whereas the adjacent ciliated ependyma was devoid of radiolabel. The binding of (125)I ET1 was abolished by antagonists of ETA and ETB receptors. The intracellular calcium concentration ([Ca(2+)](i)) was measured in individual SCO cells prior to and after exposure to ET1, BDK, or RF glycoproteins. ET1 (100 nM) or BDK (100 nM) caused an increase in [Ca(2+)](i) in 48% or 53% of the analyzed SCO-cells, respectively. RF glycoproteins had no effect on [Ca(2+)](i) in SCO cells. ET and BDK evoked two types of calcium responses: prolonged and short responses. Prolonged responses included those with a constant slow decline of [Ca(2+)](i), biphasic responses, and responses with a plateau phase at the peak level of [Ca(2+)](i). ET1-treated SCO explants contained a reduced amount of intracytoplasmic AFRU (antiserum to RF glycoproteins)-immunoreactive material compared with sham-treated control explants. Our data suggest that ET1 and BDK regulate [Ca(2+)](i) in bovine SCO cells, and that the changes in [Ca(2+)](i) influence the secretory activity of these cells.

Cholinergic signal transduction in the mouse sphenopalatine ganglion

The sphenopalatine ganglia (SPG) receive their preganglionic innervation from the ventro-lateral reticular formation and nuclei of the caudal pons, and are involved in parasympathetic control of cranial glandular and vascular components including the blood supply to specific brain areas. In 53% of all SPG neurons, a particular member (MOL2.3) of the odorant receptor superfamily is co-expressed with green fluorescent protein (GFP) in MOL2.3 transgenic mouse pups. Choline acetyltransferase and vesicular acetylcholine transporter (VAChT) could be demonstrated in 90% of the GFP-positive, and 60% of the GFP-negative cells, these cells thus representing cholinergic neurons. Some 50% of all SPG neurons were nitrergic at a high rate of VAChT co-expression, the majority of them being GFP-positive. Most SPG neurons received cholinergic innervation as demonstrated by perineuronal VAChT immunoreactive nerve terminals. To characterize cholinergic signal transduction in SPG neurons, calcium imaging experiments were performed in a SPG primary culture system containing GFP-positive and -negative neurons. Ganglionic neurons could repeatedly be activated by cholinergic stimulation in a dose-dependent manner, with calcium entering all cells from the extracellular compartment. Stimulation with specific agonists supported prevalence of nicotinic cholinergic receptors (nAChRs). Inhibition of cholinergically induced intracellular calcium signalling by various omega-conotoxins indicated functional expression of alpha 3 beta 4 and alpha 7 nAChR subtypes in murine SPG cells, which could be supported by RT-PCR analysis of the neonatal mouse SPG. With regard to secondary cholinergic activation, L- but not N-subtype voltage-gated calcium channels might represent a prime target. Nicotinic signal transduction did not prove to be different in GFP-positive as compared to-negative murine SPG neurons.

Differential uptake of molecules from the circulation and CSF reveals regional and cellular specialisation in CNS detection of homeostatic signals

The uptake of hydroxystilbamidine (OHSt, FluoroGold equivalent) and wheat germ agglutinin (WGA), into the hypothalamus, two hours after injections into either the circulation or the cerebrospinal fluid, were compared in adult rats. Following intravenous injection, OHSt was found in astrocytes of the median eminence and medial part of the arcuate nucleus whereas WGA intensely labelled the blood vessels and ependymal cells throughout the hypothalamus. In complete contrast, intracerebroventricular (icv) injection into the lateral ventricle resulted in OHSt uptake by ependymocytes and astrocytes in the area adjacent to the third ventricle, with virtually no uptake in regions taking up this dye following systematic injections, i.e., the median eminence and medial arcuate. Following icv injection WGA labelling was intense in all parts of the ependymal layer of the third ventricle, including the alpha- and beta-tanycytes. Injections into the cisterna magna gave a different pattern of uptake with OHSt being found only in astrocytes in the ventral part of the hypothalamus lateral to the arcuate nucleus whilst WGA uptake was virtually absent. This highlights the regional and cellular specialisation for uptake of molecules from the circulation and CSF. The median eminence and medial arcuate take up molecules from the circulation, with different cell types taking up different molecules. As the CSF flows through the ventricular system, different cells lining the ventricular and subarachnoid spaces take up molecules differentially. Molecules in the CSF appear to be excluded from the median eminence and medial arcuate region.

Endothelin-1 as a central mediator of LPS-induced fever in rats

Fever induced by E. coli lipopolysaccharide (LPS) in rats is substantially reduced by blockade of central endothelin ET(B) receptors. This study explores the role of endothelin-1 as a central mediator of fever in rats, by investigating the effect of a pyrogenic dose of LPS on the levels of big endothelin-1 and endothelin-1 in the cerebrospinal fluid (CSF) and endothelin-1 in the plasma. We further assessed whether the increase in body temperature caused by central injection of endothelin-1 constitutes solely a hyperthermia or a true integrated febrile response. LPS (5 mug kg(-1), i.v.) induced fever which peaked at 1.16 +/- 0.24 degrees C within 2 h and remained stable up to 5 h. CSF levels of immunoreactive (ir) big endothelin-1 decreased to undetectable levels at 3 h after LPS, returning only partially at 5 h post-injection. CSF ir-endothelin-1 levels were undetectable in saline-treated animals, but reached 21.9 +/- 5.2 fmol ml(-1) at 3 h after LPS treatment. Plasma ir-endothelin-1 levels were unchanged after saline or LPS. Central injection of endothelin-1 (1 pmol, i.c.v.) caused long-lasting increases in body temperature (0.81 +/- 0.17 degrees C, 3 h), but simultaneously decreased tail skin temperature (-1.10 +/- 0.26 degrees C), indicating cutaneous vasoconstriction. Moreover, endothelin-1 induced fever (1.0 +/- 0.3 degrees C, 3 h) when injected into the preoptic area of the anterior hypothalamus (100 fmol), but not i.v. (1 or 10 pmol). These data suggest that endothelin-1 is produced in the brain and acts centrally as a mediator of LPS-induced fever.

Interleukin-1 mediates endothelin-1-induced fever and prostaglandin production in the preoptic area of rats

The intracerebroventricular injection of endothelin-1 (ET-1) induces fever and increases PG levels in the cerebrospinal fluid of rats. Likewise, the injection of IL-1 into the preoptic area (POA) of the rat hypothalamus causes both fever and increased PG production. In this study, we conducted in vivo and in vitro experiments in the rat to investigate 1) the hypothalamic region involved in ET-1-induced fever and PG biosynthesis and 2) whether hypothalamic IL-1 plays a role as a mediator of the above ET-1 activities. One hundred femtomoles of ET-1 increased body temperature when injected in the POA of conscious Wistar rats; this effect was significantly counteracted by the coinjection of 600 pmol IL-1 receptor antagonist (IL-1ra). In experiments on rat hypothalamic explants, 100 nM ET-1 caused a significant increase in PGE2 production and release from the whole hypothalamus and from the isolated POA, but not from the retrochiasmatic region, in 1-h incubations. Six nanomoles of IL-1ra or 10 nM of a cell-permeable interleukin-1 converting enzyme inhibitor completely counteracted the effect of ET-1 on PGE2 release from the POA. One hundred nanomoles ET-1 also caused a significant increase in IL-1beta immunoreactivity released into the bath solution of hypothalamic explants after 1 h of incubation, although during such time ET-1 failed to modify the gene expression of IL-1beta and other pyrogenic cytokines within the hypothalamus. In conclusion, our results show that ET-1 increases IL-1 production in the POA, and this effect appears to be correlated to ET-1-induced fever in vivo, as well as to PG production in vitro.

Mechanosensation and endothelin in astrocytes--hypothetical roles in CNS pathophysiology

Endothelin (ET) is a potent autocrine mitogen produced by reactive and neoplastic astrocytes. ET has been implicated in the induction of astrocyte proliferation and other transformations engendered by brain pathology, and in promoting the malignant behavior of astrocytomas. Reactive astrocytes containing ET are found in the periphery/penumbra of a wide array of CNS pathologies. Virtually all brain pathology deforms the surrounding parenchyma, either by direct mass effect or edema. Mechanical stress is a well established stimulus for ET production and release by other cell types, but has not been well studied in the brain. However, numerous studies have illustrated that astrocytes can sense mechanical stress and translate it into chemical messages. Furthermore, the ubiquitous reticular meshwork formed by interconnected astrocytes provides an ideal morphology for sensing and responding to mechanical disturbances. We have recently demonstrated stretch-induced ET production by astrocytes in vitro. Inspired by this finding, the purpose of this article is to review the literature on (1) astrocyte mechanosensation, and (2) the endothelin system in astrocytes, and to consider the hypothesis that mechanical induction of the ET system may influence astrocyte functioning in CNS pathophysiology. We conclude by discussing evidence supporting future investigations to determine whether specific inhibition of stretch-activated ion channels may represent a novel strategy for treating or preventing CNS disturbances, as well as the relevance to astrocyte-derived tumors.

Endothelin ETB receptor signaling in the median eminence and subfornical organ of the rat brain

We investigated the effect of endothelins (ETs) on receptor-mediated phosphoinositides (PI) turnover in whole subfornical organ (SFO) and median eminence (ME). Consistent with the presence of a high density of binding sites in the SFO and the ME of the rat brain, our results show an increase in PI hydrolysis induced by ETs in each structure, in a dose-dependent manner and with similar ED50 values. In addition, IRL 1620, a selective ETB receptor agonist, increased the inositol monophosphate (InsP1) accumulation in the SFO and the ME in a similar degree as ETs. With the use of selective agonists and antagonists of both endothelin receptor subtypes, we characterized the receptor subtype involved in ET-induced phosphoinositide metabolism. The addition of two selective ETA receptor antagonists, BQ 123 or BQ 610, did not alter the ETs-induced increase in the PI metabolism. While, IRL 1620- and ET3-induced InsP1 accumulation was completely blocked by BQ 788, a selective ETB receptor antagonist, in both brain structures evaluated. Our results demonstrate that in the SFO and the ME of the rat brain, stimulation of phosphoinositide turnover constitutes one of the signaling pathways of ETs, and this action is mediated through ETB receptor activation. These results support the concept that endothelin could play a role in the regulation of brain functions.

Sensory circumventricular organs: central roles in integrated autonomic regulation

Circumventricular organs (CVO) play a critical role as transducers of information between the blood, neurons and the cerebral spinal fluid (CSF). They permit both the release and sensing of hormones without disrupting the blood-brain barrier (BBB) and as a consequence of such abilities the CVOs are now well established to have essential regulatory actions in diverse physiological functions. The sensory CVOs are essential signal transducers located at the blood-brain interface regulating autonomic function. They have a proven role in the control of cardiovascular function and body fluid regulation, and have significant involvement in central immune response, feeding behavior and reproduction, the extent of which is still to be determined. This review will attempt to summarize the research on these topics to date. The complexities associated with sensory CVO exploration are intense, but should continue to result in valuable contributions to our understanding of brain function.

Plasma endothelin-1 and big endothelin-1 levels in superior and inferior vena cava during protracted antiorthostatic hypokinetic/hypodynamia in rats

The plasma levels of endothelin-1 and big endothelin-1 were evaluated in blood of rats in the superior and inferior vena cava, in normal posture (synchronous controls), and after 12 days head-down suspension and 1 day recovery in normal posture. In synchronous controls, the mean plasma concentration of endothelin-1 in inferior vena cava or superior vena cava was almost the same (5.89+/-0.63 pmol/l and 5.67+/-0.64 pmol/l, respectively), whereas the mean plasma concentration of big endothelin-1 was higher (p<0.05) in superior vena cava compared to inferior vena cava (5.49+/-0.75 pmol/l and 1.39+/-0.15 pmol/l, respectively). In samples from superior vena cava of head-down suspended rats big endothelin-1 levels were significantly lower (p<0.05) up to day 9 of suspension, compared to non-suspended synchronous controls, whereas endothelin-1 values were higher (p<0.05). Big endothelin-1 concentration was higher (p<0.05) in inferior vena cava compared to non-suspended synchronous controls. The behaviour of endothelin-1 was more complex, endothelin-1 levels were lower (p<0.05) on day 1 of head-down suspension and higher (p<0.05) in samples taken on days 9 and 12. After 1 day recovery endothelin-1 and big endothelin-1 concentrations returned to normal in both superior vena cava and inferior vena cava. These data indicate that the endothelial system involvement for the two venous beds is different and suggest that local rather than systemic evaluation could better explain endothelial involvement and the contribution of different anatomic sites to the biosynthesis, conversion and clearance of the various involved molecules.