Hypertension caused by angiotensin II infusion involves increased superoxide production in the central nervous system

Hypertension caused by angiotensin II (Ang II) infusion is associated with oxidative stress in the peripheral vasculature and kidney. The role of redox mechanisms in the central nervous system (CNS), a tissue known to be pivotal in Ang II-dependent hypertension, has not been investigated. We recently identified superoxide (O2*-) in the brain as a key signaling intermediate in the transient pressor response elicited by acute injection of Ang II directly into the CNS. Here we tested the hypothesis that hypertension caused by chronic systemic infusion of Ang II is mediated by a central neurogenic mechanism involving O2*-. Infusion of Ang II (600 ng x kg(-1) x min(-1)) over a 2-week period in mice caused a gradually developing hypertension that was correlated with marked elevations in O2*- production specifically in the subfornical organ (SFO), a brain region lying outside the blood-brain barrier and known to be a primary sensor for blood-borne Ang II. Adenoviral-mediated delivery of cytoplasmically targeted superoxide dismutase (SOD) selectively to this site prevented the hypertension and the increased O2*- production, whereas gene transfer of SOD targeted to the extracellular matrix had no effect. These data suggest that increased intracellular O2*- production in the SFO is critical in the development of Ang II-induced hypertension.

Angiotensin II immunoreactivity in the neural afferents and efferents of the subfornical organ of the rat

Angiotensin II (AII) immunoreactive cells and fibers were identified in the subfornical organ (SFO) of the rat. Cells were distributed in an annulus around the periphery of the SFO and were most visible in the Brattleboro rat treated with colchicine. Fibers were observed in a plexus, located centrally within the ring of cells, and knife-cuts suggested that they arise from parent cell bodies lying outside of the SFO. Studies combining immunohistochemistry with retrograde transport identified the perifornical zone of the lateral hypothalamus, the rostral zona incerta, and the nucleus reuniens of the thalamus as the source of AII-stained inputs to the SFO, and the region of the median preoptic nucleus as a recipient of AII-immunoreactive SFO efferents. It is suggested that these biochemically defined connections of the SFO participate in the central neural control of fluid balance.

Glucagon-like peptide I receptors in the subfornical organ and the area postrema are accessible to circulating glucagon-like peptide I

The intestinal incretin hormone glucagon-like peptide I (GLP-I) inhibits gastric motility and secretion in normal, but not in vagotomized subjects, pointing to a centrally mediated effect. Therefore, our aim was to study the availability of rat brain GLP-I receptors to peripherally injected 125I-labeled GLP-I. The specificity of the binding was tested by co-injection of excess amounts of unlabeled GLP-I. Using light microscopical autoradiography of rat brain sections, we found specific 125I-GLP-I binding exclusively in the subfornical organ and the area postrema. This binding was abolished when an excess amount of unlabeled GLP-I was co-injected with the labeled GLP-I. We conclude that cells in the subfornical organ and the area postrema could be responsive to blood-borne GLP-I. The observed binding of peripherally administered GLP-I to the subfornical organ and the area postrema, which both have close neuroanatomical connections with hypothalamic areas involved in water and appetite homeostasis, is consistent with the potential roles of circulating GLP-I in the central regulation of appetite and autonomic functions.

Vasopressin secretion: osmotic and hormonal regulation by the lamina terminalis

The lamina terminalis, located in the anterior wall of the third ventricle, is comprised of the subfornical organ, median preoptic nucleus (MnPO) and organum vasculosum of the lamina terminalis (OVLT). The subfornical organ and OVLT are two of the brain's circumventricular organs that lack the blood-brain barrier, and are therefore exposed to the ionic and hormonal environment of the systemic circulation. Previous investigations in sheep and rats show that this region of the brain has a crucial role in osmoregulatory vasopressin secretion and thirst. The effects of lesions of the lamina terminalis, studies of immediate-early gene expression and electrophysiological data show that all three regions of the lamina terminalis are involved in osmoregulation. There is considerable evidence that physiological osmoreceptors subserving vasopressin release are located in the dorsal cap region of the OVLT and possibly also around the periphery of the subfornical organ and in the MnPO. The circulating peptide hormones angiotensin II and relaxin also have access to peptide specific receptors (AT(1) and LGR7 receptors, respectively) in the subfornical organ and OVLT, and both angiotensin II and relaxin act on the subfornical organ to stimulate water drinking in the rat. Studies that combined neuroanatomical tracing and detection of c-fos expression in response to angiotensin II or relaxin suggest that both of these circulating peptides act on neurones within the dorsal cap of the OVLT and the periphery of the subfornical organ to stimulate vasopressin release.

Binding of angiotensin and atrial natriuretic peptide in brain of hypertensive rats

Atrial natriuretic peptides, produced in the mammalian cardiac atrium, are released into the general circulation and may be actively involved in the control of blood pressure and in fluid homeostasis as antagonists of the peripheral angiotensin system. Certain cardiovascular effects of atrial natriuretic peptides may be centrally mediated, as binding sites for atrial natriuretic factor (8-33) (ANF) have been localized to the subfornical organ. This circumventricular structure lacks a blood-brain barrier and is therefore accessible to circulating peptides. It contains large numbers of angiotensin II (AII) binding sites, and has been suggested as the main central site of action for circulating AII in the regulation of blood pressure and fluid metabolism. Here we have studied binding sites for rat atrial natriuretic peptide(6-33) (rANP) and AII in the brains of spontaneously (genetic) hypertensive rats (SHR) and their normotensive controls, Wistar Kyoto (WKY) rats, by quantitative autoradiography. Binding sites for both peptides were highly localized in the subfornical organ. The number of rANP binding sites was decreased in the subfornical organ of both young (4 weeks old) and adult (14 weeks old) SHR compared with age-matched normotensive controls. Conversely, the number of AII binding sites was higher in both young and adult SHR compared with WKY rats. Our results suggest a central role for rANP and AII in genetic hypertension; they may act as mutual antagonists in brain areas involved in control of blood pressure and fluid regulation.

Angiotensin II may mediate excitatory neurotransmission from the subfornical organ to the hypothalamic supraoptic nucleus: an anatomical and electrophysiological study in the rat

In the rat, it has been proposed that angiotensin II (AII) neurons in the subfornical organ, a midline circumventricular structure, participate in the activation of hypothalamic neurosecretory neurons and promote a rise in plasma vasopressin and oxytocin. In this study, we observed AII-immunoreactive fibers coursing throughout the supraoptic nucleus as well as in other magnocellular cell groups of the hypothalamus. Moreover, following retrograde transport of Fast blue deposited within the supraoptic nucleus, cell counts in our best case revealed that 40% of AII-immunoreactive neurons in subfornical organ contained Fast blue, and 46% of the retrogradely labeled subfornical organ cells contained AII. In separate electrophysiological studies, post-stimulus histograms from 18 of 28 supraoptic neurons displayed a 30-55% reversible reduction in the excitation evoked by an electrical stimulus in the subfornical organ during local pressure applications of 100 microM to 1 mM saralasin. In 2 of 14 other cells, tubocurare (100 microM) produced only a 10% reduction in subfornical organ excitation. These observations indicate that AII may mediate an excitatory input to supraoptic neurons from the subfornical organ.

Repeated stress increases the density of angiotensin II binding sites in rat paraventricular nucleus and subfornical organ

We have studied the properties of angiotensin II binding sites in the paraventricular nucleus, subfornical organ and anterior pituitary lobe of rats subjected to repeated immobilization stress. This treatment produced significant increase in the density of angiotensin II binding sites in these two nuclei without any significant alteration in binding affinity. Repeated stress did not alter angiotensin II binding properties in the anterior pituitary lobe. Our results suggest that brain angiotensin binding sites may have a role in regulation of the stress response.

Quantitative determination of angiotensin II binding sites in rat brain and pituitary gland by autoradiography

Rat brain and pituitary angiotensin II (AII) binding sites were quantitated by incubation of tissue sections with 125I-[Sar1] AII, Ultrofilm radioautography, computerized densitometry, and comparison with 125I-standards at appropriate film exposure times. The highest number of AII binding sites was found in anterior pituitary and the circumventricular organs, organon subfornicalis and organon vasculosum laminae terminalis.

Organic cation transporter 3 (Slc22a3) is implicated in salt-intake regulation

Organic cation transporters (OCTs) are carrier-type permeases known to participate in general detoxification functions in peripheral tissues. Previous in vitro studies have suggested that OCTs ensure Uptake2, a low-affinity, corticosteroid-sensitive catecholamine removal system, which was characterized initially in sympathetically innervated tissues. Although the presence of both Uptake(2)-like transport and most OCT subtypes has also been demonstrated in the brain, the physiological role of this family of transporters in CNS remained totally unknown. In the present work, we show that the OCT3 transporter is found throughout the brain and highly expressed in regions regulating fluid exchange, including circumventricular organs such as area postrema and subfornical organ (SFO), and in other structures implicated in the sensing of changes in blood osmolarity and regulation of salt and water ingestion. OCT3/Slc22a3-deficient mice show an increase in the level of ingestion of hypertonic saline under thirst and salt appetite conditions, as well as alterations of the neural response in the SFO after sodium deprivation, as monitored by Fos immunoreactivity. This work demonstrates that the presence of OCT3 is critical for the balanced neural and behavioral responses to environmentally induced variations in osmolarity and provides for the first time physiological evidence of the importance of OCTs for CNS function.

Localization of changes in immediate early genes in brain in relation to hydromineral balance: intravenous angiotensin II

Immediate early genes, detected by Fos- and Jun-like immunoreactivity (FLI, JLI), were induced in discrete regions of the rat brain by intravenous infusion of angiotensin II (Ang II) at dipsogenic doses. The regions included subfornical organ (SFO), organum vasculosum laminae terminalis (OVLT), median preoptic nucleus (MnPO), supraoptic nucleus (SON), and the magnocellular part of the paraventricular hypothalamus (PVH). These responses were sustained for up to 6 h of infusion. In SFO, FLI was induced preferentially in the posterior part, while JLI occurred in more central regions. Cerebroventricular (ICV) injection of the Ang II type 1 receptor (AT-1) antagonist, losartan potassium, completely prevented the FLI induced by Ang II in these brain regions. ICV injection of the Ang II type 2 receptor (AT-2) antagonist, PD 123319, did not reduce Ang II-induced FLI in SFO, OVLT and MnPO, but markedly attenuated the activation in SON and PVH. To determine whether SFO is the primary site for transduction of the circulating Ang II signal, electrolytic lesions were made in or rostral to the SFO. Rats with complete lesions showed a complete absence of Ang-induced FLI in SON and PVH. The data are discussed in terms of functional mapping of the brain regions activated by circulating Ang II and neural circuitry for water intake, including the possible role of AT-2 receptors in PVH and SON.

Chronic intermittent hypoxia increases blood pressure and expression of FosB/DeltaFosB in central autonomic regions

Chronic intermittent hypoxia (CIH) models repetitive bouts of arterial hypoxemia that occur in humans suffering from obstructive sleep apnea. CIH has been linked to persistent activation of arterial chemoreceptors and the renin-angiotensin system, which have been linked to chronic elevations of sympathetic nerve activity (SNA) and mean arterial pressure (MAP). Because Fos and FosB are transcription factors involved in activator protein (AP)-1 driven central nervous system neuronal adaptations, this study determined if CIH causes increased Fos or FosB staining in brain regions that regulate SNA and autonomic function. Male Sprague Dawley rats were instrumented with telemetry transmitters for continuous recording of MAP and heart rate (HR). Rats were exposed to continuous normoxia (CON) or to CIH for 8 h/day for 7 days. CIH increased MAP by 7-10 mmHg without persistently affecting HR. A separate group of rats was killed 1 day after 7 days of CIH for immunohistochemistry. CIH did not increase Fos staining in any brain region examined. Staining for FosB/ΔFosB was increased in the organum vasculosum of the lamina terminalis (CON: 9 ± 1; CIH: 34 ± 3 cells/section), subfornical organ (CON: 7 ± 2; CIH: 31 ± 3), median preoptic nucleus (CON 15 ± 1; CIH: 38 ± 3), nucleus of the solitary tract (CON: 9 ± 2; CIH: 28 ± 4), A5 (CON: 3 ± 1; CIH: 10 ± 1), and rostral ventrolateral medulla (CON: 5 ± 1; CIH: 17 ± 2). In the paraventricular nucleus, FosB/ΔFosB staining was located mainly in the dorsal and medial parvocellular subnuclei. CIH did not increase FosB/ΔFosB staining in caudal ventrolateral medulla or supraoptic nucleus. These data indicate that CIH induces an increase in FosB/ΔFosB in autonomic nuclei and suggest that AP-1 transcriptional regulation may contribute to stable adaptive changes that support chronically elevated SNA.

Bi-directional, chemically specified neural connections between the subfornical organ and the midbrain raphe system

Reports of a serotonin-immunoreactive fiber plexus in the subfornical organ (SFO) and evidence that this input is relevant to the control of thirst and blood pressure by the SFO prompted an investigation of neural connections between the SFO and the serotonergic neurons in the raphe system. Serotonin-immunoreactive fibers were found to enter the SFO by a number of different routes and to form a plexus in the central part, which contains a bed of fenestrated capillaries. Injections of the anterogradely transported lectin, phaseolis vulgaris leucoagglutinin (PHA-L), into the dorsal raphe nucleus produced a pattern of labeled fibers in the SFO comparable to that revealed with the anti-serotonin staining. Injections into the SFO of the retrogradely transported dye, Fast Blue, labeled cell bodies in the dorsal and median raphe nuclei of the midbrain, and at least 60% of these cells also demonstrated serotonin immunoreactivity. Injections of PHA-L into the SFO identified a return projection to this part of the raphe system, where an angiotensin-immunoreactive plexus of fibers and varicosities is found. Injections of True Blue into various parts of the midbrain raphe nuclei reliably labeled neurons in the SFO, and some 56% of them could also be stained with anti-angiotensin. It is suggested that this circuitry is involved in the control of fluid balance by the SFO and that the serotonergic-raphe projection to the SFO may participate in the relay of visceral sensory information, perhaps related to blood pressure, from the nucleus of the solitary tract and the lateral parabrachial nucleus.

Elevated blood pressure in transgenic mice with brain-specific expression of human angiotensinogen driven by the glial fibrillary acidic protein promoter

In addition to the circulatory renin (REN)-angiotensin system (RAS), a tissue RAS having an important role in cardiovascular function also exists in the central nervous system. In the brain, angiotensinogen (AGT) is expressed in astrocytes and in some neurons important to cardiovascular control, but its functional role remains undefined. We generated a transgenic mouse encoding the human AGT (hAGT) gene under the control of the human glial fibrillary acidic protein (GFAP) promoter to experimentally dissect the role of brain versus systemically derived AGT. This promoter targets expression of transgene products to astrocytes, the most abundant cell type expressing AGT in brain. All transgenic lines exhibited hAGT mRNA expression in brain, with variable expression in other tissues. In one line examined in detail, transgene expression was high in brain and low in tissues outside the central nervous system, and the level of plasma hAGT was not elevated over baseline. In the brain, hAGT protein was mainly localized in astrocytes, but was present in neurons in the subfornical organ. Intracerebroventricular (ICV) injection of human REN (hREN) in conscious unrestrained mice elicited a pressor response, which was abolished by ICV preinjection of losartan. Double-transgenic mice expressing the hREN gene and the GFAP-hAGT transgene exhibited a 15-mm Hg increase in blood pressure and an increased preference for salt. Blood pressure in the hREN/GFAP-hAGT mice was lowered after ICV, but not intravenous losartan. These studies suggest that AGT synthesis in the brain has an important role in the regulation of blood pressure and electrolyte balance.

Localization of calcium receptor mRNA in the adult rat central nervous system by in situ hybridization

The capacity to sense changes in the concentrations of extracellular ions is an important function in several cell types. For example, hormone secretion by parathyroid cells and thyroid C-cells is primarily regulated by the level of extracellular ionized calcium (Ca2+). The G-protein-coupled receptor that mediates the parathyroid cell response to Ca2+ has been cloned and we have used in situ hybridization to map calcium receptor (CaR) mRNA expression in the adult rat brain. Cells expressing CaR mRNA were present in many areas of the brain suggesting that a variety of cell types express the CaR. Particularly high numbers of CaR expressing cells were found in regions associated with the regulation of fluid and mineral homeostasis, most notably the subfornical organ. These data suggest that the capacity to detect changes in extracellular Ca2+ concentrations may have important functional consequences in several neural systems.

Estrogen decreases hypothalamic angiotensin II AT1 receptor binding and mRNA in the female rat

Estrogen has been shown to modulate angiotensin II (AngII)-regulated behaviors, such as thirst, and may do so by influencing the central renin-angiotensin system (RAS). While numerous studies have attempted to correlate changes in AngII receptors or other components of the RAS with estrogen treatment, the low abundance of these genes has made comparisons difficult. Generally, such experiments have relied on traditional approaches to analyze gene expression that often restrict the experimenter to studying only a few mRNA species, whereas a behavior as complex as thirst may be influenced by changes in multiple genes. The present experiments utilized quantitative receptor autoradiography and mRNA expression profiling to identify and compare AngII receptors and their mRNA levels as well as other components of the RAS in female rat pituitary and hypothalamic-thalamic-septal (HTS) tissue samples. This relatively new approach to the study of gene expression permits the simultaneous comparison of multiple genes from a single tissue sample. These studies revealed that ovariectomized (OVX) female rats treated with estradiol benzoate (EB) had a 30%-40% reduction in the levels of AT(1) receptor mRNA in pituitary and HTS samples as compared to OVX, control animals. In the pituitary, the mRNA levels for angiotensinogen (AGT) were increased by 45% following estrogen administration. In addition, a reduction in [125I]-AngII binding to AT(1) receptors in the pituitary and the subfornical organ was measured following estrogen treatment. These results suggest that estrogen may modulate the pituitary and central RAS through a coordinate regulation of the angiotensin receptors and the levels of newly synthesized AngII.

Peripheral amylin activates circumventricular organs expressing calcitonin receptor a/b subtypes and receptor-activity modifying proteins in the rat

The pancreatic hormone amylin (AMY) and the AMY-receptor-agonist salmon-calcitonin (sCT) reduce short-term food-intake after binding to the area postrema (AP), a circumventricular organ (CVO) lacking blood-brain-barrier characteristics. AMY has also been proposed to induce drinking via another CVO, the subfornical organ (SFO). In cellular systems, AMY-binding is generated by interaction of calcitonin-receptor a/b (CT((a))/CT((b))) with receptor-activity modifying proteins (RAMPs). By using in situ hybridization, the codistribution of CT((a))/CT((b)) with RAMP1-3 and c-fos was mapped in CVOs of rats. AMY and sCT induced c-fos within the SFO which contained CT((a)) and/or CT((b)) and RAMP1/2 mRNA. AMY and sCT also activated AP neurons, which express the CT((a)), but not the CT((b)), receptor and RAMP2/3 mRNA. These data emphasize the important role of these structures as primary targets for circulating AMY.

Increased specific binding of angiotensin II in the organum vasculosum of the laminae terminalis area of the spontaneously hypertensive rat brain

Three localized sites, the organum vasculosum, lamina terminalis (OVLT) subfornical organ (SFO) and cortex in the brains of spontaneously hypertensive (SH) rats and normotensive Wistar-Kyoto (WKY) rats were micro-dissected out. The membranes were assayed for 125I-labelled angiotensin II specific binding. The only significant difference was a 129% greater specific binding in the OVLT in SH compared to WKY rats. Specific binding of 125I-labelled angiotensin II in the SFO and the cerebral cortex was not significantly different between the two strains. SH rats are known to have a greater sensitivity to centrally administered angiotensin II, which may contribute to the hypertension. This data suggests that a difference in central angiotensin II receptors is at least partially responsible for the increased sensitivity.

Selected contribution: role of IL-6 in LPS-induced nuclear STAT3 translocation in sensory circumventricular organs during fever in rats

Interleukin-6 (IL-6) is regarded as an endogenous mediator of lipopolysaccharide (LPS)-induced fever. IL-6 is thought to act on the brain at sites that lack a blood-brain barrier, the circumventricular organs (CVOs). Cells that are activated by IL-6 respond with nuclear translocation of the signal transducer and activator of transcription 3 molecule (STAT3) and can be detected by immunohistochemistry. We investigated whether the LPS-induced release of IL-6 into the systemic circulation was accompanied by a nuclear STAT3 translocation within the sensory CVOs. Treatment with LPS (100 microg/kg) led to a slight (1 h) and then a strong increase (2-8 h) in plasma IL-6 levels, which started to decline at the end of the febrile response. Administration of both pyrogens LPS and IL-6 (45 microg/kg) induced a febrile response with IL-6, causing a rather moderate fever compared with the LPS-induced fever. Nuclear STAT3 translocation in response to LPS was observed within the vascular organ of the lamina terminalis (OVLT) and the subfornical organ (SFO) 2 h after LPS treatment. To investigate whether this effect was mediated by IL-6, the cytokine itself was systemically applied and indeed an identical pattern of nuclear STAT3 translocation was observed. However, nuclear STAT3 translocation already occurred 1 h after IL-6 application and proved to be less effective compared with LPS treatment when analyzing OVLT and SFO cell numbers that showed nuclear STAT3 immunoreactivity after the respective pyrogen treatment. Our observations represent the first molecular evidence for an IL-6-induced STAT3-mediated genomic activation of OVLT and SFO cells and support the proposed role of these brain areas as sensory structures for humoral signals created by the activated immune system and resulting in the generation of fever.

Adjacent Expression of Renin and Angiotensinogen in the Rostral Ventrolateral Medulla Using a Dual-Reporter Transgenic Model

All components of the renin-angiotensin system are localized in the brain. However, because renin is present in very low concentrations, the mechanism by which angiotensin II is formed in the brain remains unclear. We previously reported the development of 2 transgenic mouse models using sensitive reporters, enhanced green fluorescent protein (eGFP) and beta-galactosidase (beta-Gal), to examine the cellular localization of renin and angiotensinogen in the mouse brain. To determine whether renin and angiotensinogen are coexpressed or present in neighboring cells in the rostral ventrolateral medulla (RVLM) and other cardiovascular control regions of the brain, we produced and examined double-transgenic mice, which express eGFP driven by the renin promoter (REN-1c/eGFP) and beta-gal driven by the human angiotensinogen promoter (hAGT/beta-gal). Using these reporter transgenes as sensitive markers for renin and angiotensinogen expression, we conclude that both proteins are coexpressed in the parabrachial nucleus and central nucleus of the amygdala and are in adjacent cells in the RVLM, reticular formation, bed nucleus of the stria terminalis, subfornical organ, and CA1-3 region. These data suggests that, in these areas, both renin and angiotensinogen are in close proximity providing the potential for the local formation of angiotensin I either intracellularly, when there is colocalization, or in the interstitium, when they are in juxtaposed cells.

Subfornical organ neurons projecting to paraventricular nucleus: whole-cell properties

The subfornical organ (SFO) has been repeatedly identified as a CNS site that plays a critical role in sensing multiple physiological variables of the "milieu interieur" and, through efferent projections to other CNS sites, initiating physiological responses to change. Many recent in vitro patch-clamp studies have examined the cellular mechanisms underlying the sensory abilities of these specialized CNS neurons. The primary limitation of these studies, however, has been the inability to identify homogeneous groups of SFO neurons for such investigation. We report here the development of techniques to permit patch clamp recording from dissociated SFO neurons identified according to their in vivo projection site. SFO neurons were labeled by injection of fluorescently labeled, retrogradely transported microspheres into the hypothalamic paraventricular nucleus (PVN) 3 days prior to cell dissociation. Patch-clamp recordings from these SFO-PVN neurons revealed both sodium currents, potassium currents, action potentials, input resistance and membrane potential which were all similar to SFO cells prepared from animals with no prior tracer injection. Labeled SFO-->PVN cells were also found to be osmosensitive and responsive to angiotensin II, suggesting specific functional roles for this anatomically defined group of SFO neurons. Intriguingly, our post hoc analysis also demonstrated that all labeled neurons demonstrated a unique electrophysiological profile dominated by a large transient potassium conductance such that the transient/sustained potassium current ratio, or degree of inactivation was, on average, greater than 4.0. Utilization of these tracing techniques to permit the in vitro recording from cells with known in vivo connections will permit study of intrinsic mechanisms that underlie physiological responses of anatomically defined populations of neurons.

Microglial Acid Sensing Regulates Carbon Dioxide-Evoked Fear

BACKGROUND

Carbon dioxide (CO2) inhalation, a biological challenge and pathologic marker in panic disorder, evokes intense fear and panic attacks in susceptible individuals. The molecular identity and anatomic location of CO2-sensing systems that translate CO2-evoked fear remain unclear. We investigated contributions of microglial acid sensor T cell death-associated gene-8 (TDAG8) and microglial proinflammatory responses in CO2-evoked behavioral and physiological responses.

METHODS

CO2-evoked freezing, autonomic, and respiratory responses were assessed in TDAG8-deficient ((-/-)) and wild-type ((+/+)) mice. Involvement of TDAG8-dependent microglial activation and proinflammatory cytokine interleukin (IL)-1β with CO2-evoked responses was investigated using microglial blocker, minocycline, and IL-1β antagonist IL-1RA. CO2-chemosensitive firing responses using single-cell patch clamping were measured in TDAG8(-/-) and TDAG8(+/+) mice to gain functional insights.

RESULTS

TDAG8 expression was localized in microglia enriched within the sensory circumventricular organs. TDAG8(-/-) mice displayed attenuated CO2-evoked freezing and sympathetic responses. TDAG8 deficiency was associated with reduced microglial activation and proinflammatory cytokine IL-1β within the subfornical organ. Central infusion of microglial activation blocker minocycline and IL-1β antagonist IL-1RA attenuated CO2-evoked freezing. Finally, CO2-evoked neuronal firing in patch-clamped subfornical organ neurons was dependent on acid sensor TDAG8 and IL-1β.

CONCLUSIONS

Our data identify TDAG8-dependent microglial acid sensing as a unique chemosensor for detecting and translating hypercapnia to fear-associated behavioral and physiological responses, providing a novel mechanism for homeostatic threat detection of relevance to psychiatric conditions such as panic disorder.

Angiotensin type 1 receptors in the subfornical organ mediate the drinking and hypothalamic-pituitary-adrenal response to systemic isoproterenol

Circulating angiotensin II (ANGII) elicits water intake and activates the hypothalamic-pituitary-adrenal (HPA) axis by stimulating angiotensin type 1 receptors (AT1Rs) within circumventricular organs. The subfornical organ (SFO) and the organum vasculosum of the lamina terminalis (OVLT) are circumventricular organs that express AT1Rs that bind blood-borne ANGII and stimulate integrative and effector regions of the brain. The goal of these studies was to determine the contribution of AT1Rs within the SFO and OVLT to the water intake and HPA response to increased circulating ANGII. Antisense oligonucleotides directed against the AT1R [AT1R antisense (AT1R AS)] were administered into the OVLT or SFO. Quantitative receptor autoradiography confirmed that AT1R AS decreased ANGII binding in the SFO and OVLT compared with the scrambled sequence control but did not affect AT1R binding in other nuclei. Subsequently, water intake, ACTH, and corticosterone (CORT) were assessed after administration of isoproterenol, a beta-adrenergic agonist that decreases blood pressure and elevates circulating ANGII. Delivery of AT1R AS into the SFO attenuated water intake, ACTH, and CORT after isoproterenol, whereas similar treatment in the OVLT had no effect. To determine the specificity of this blunted drinking and HPA response, the same parameters were measured after treatment with hypertonic saline, a stimulus that induces drinking independently of ANGII. Delivery of AT1R AS into the SFO or OVLT had no effect on water intake, ACTH, or CORT after hypertonic saline. The results imply that AT1R within the SFO mediate drinking and HPA responses to stimuli that increase circulating ANGII.

Autoradiographic localization of angiotensin receptors in the sheep brain

Binding of [125I]-(Sar1,Ile8)angiotensin II (AII) to frozen sections of sheep brain was determined by in vitro autoradiography. Greatest AII-binding occurred in the organum vasculosum of the lamina terminalis, subfornical organ, median preoptic and periventricular nuclei situated in the anterior third ventricle wall. Other binding sites included the hypothalamic supraoptic and paraventricular nuclei and the medullary nucleus tractus solitarius. These regions may be central receptor sites for AII involvement in fluid and electrolyte balance and blood pressure regulation.

Anatomical evidence that neural circuits related to the subfornical organ contain angiotensin II

Bidirectional connections between the subfornical organ and the hypothalamus are reviewed, and emphasis is placed on recent evidence for the presence of angiotensin II in some of these pathways. Additionally, evidence is presented suggesting that this peptide may serve as a neurotransmitter or neuroendocrine factor in the efferent projections of cell groups receiving neural inputs from the subfornical organ. It appears that angiotensin II may serve as one of the chemical messengers at each link in multi-synaptic pathways related to the subfornical organ.