Subfornical organ mediates pressor effect of angiotensin: Influence of nitric oxide synthase inhibitors, AT1 and AT2 angiotensin antagonist's receptors

AT1 receptors in the subfornical organ modulate arterial pressure and the baroreflex in two-kidney, one-clip hypertensive rats

The subfornical organ (SFO), a forebrain circumventricular organ that lies outside the blood-brain barrier, has been implicated in arterial pressure and baroreflex responses to angiotensin II (ANG II). We tested whether pharmacological inhibition or selective silencing of SFO ANG II type 1 receptors (AT₁R) of two-kidney, one-clip rats with elevated plasma ANG II decreases resting arterial pressure and renal sympathetic nerve activity (RSNA) and/or modulates arterial baroreflex responses of heart rate (HR) and RSNA. Male Sprague-Dawley rats underwent renal artery clipping [2-kidney, 1-clip (2K,1C)] or sham clipping (sham). After 6 wk, conscious rats instrumented with vascular catheters, renal nerve electrodes, and a cannula directed to the SFO were studied. In another set of experiments, rats were instrumented with hemodynamic and nerve radio transmitters and injected with scrambled RNA or silencing RNA targeted against AT₁R. Mean arterial pressure (MAP) was significantly higher in 2K,1C rats. Acute SFO injection with the AT₁R inhibitor losartan did not change MAP in sham or 2K,1C rats. Baroreflex curves of HR and RSNA were shifted rightward in 2K,1C rats. Losartan exerted no effect. SFO AT₁R knockdown did not influence MAP in sham rats but decreased MAP in 2K,1C rats, despite no change in plasma ANG II or resting RSNA. AT₁R knockdown prevented the reduction in maximum gain and slope of baroreflex responses of HR and RSNA; the reduced RSNA response to baroreceptor unloading was partially restored in 2K,1C rats. These findings show that AT₁R activation within the SFO contributes to hypertension and baroreflex dysfunction in 2K,1C rats and highlight the temporal requirement for reversal of these effects.

Tumor necrosis factor-α potentiates the effects of angiotensin II on subfornical organ neurons

Inflammation is thought to play a fundamental role in the pathophysiology of hypertension and heart failure, although the mechanisms for this remain unclear. Proinflammatory cytokines, such as tumor necrosis factor-α (TNF-α), influence the subfornical organ (SFO) to modulate sympathetic activity and blood pressure. The pressor effects of TNF-α in the SFO are partially mediated by angiotensin II (ANG II) receptor type 1 (AT1R), and TNF-α is known to potentiate ANG II-induced hypertension. However, the cellular mechanism of the interaction between TNF-α and ANG II/AT1R signaling remains unknown. In the present study, we performed Ca2+ imaging on dissociated SFO neurons in vitro from male Sprague-Dawley rats to determine whether TNF-α modulates ANG II-induced increases in intracellular Ca2+ in SFO neurons. We first established that a proportion of SFO neurons respond to ANG II, an effect that required AT1R signaling and extracellular Ca2+. We then tested the hypothesis that TNF-α may modulate the effects of ANG II on SFO neurons by examining the effects of TNF-α treatment on the ANG II-induced rise in intracellular Ca2+. We discovered that TNF-α potentiated the ANG II-induced rise in intracellular Ca2+, an effect that was dependent on the duration of TNF-α treatment. Finally, we determined that this potentiation of ANG II-induced Ca2+ activity relied on tetrodotoxin-sensitive voltage-gated Na+ (vgNa+) channels. These data suggest that the potentiation of ANG II/AT1R activity by TNF-α in SFO neurons results from the previously demonstrated ability of this cytokine to modulate the activation threshold of vgNa+ currents.

COX-1-derived PGE2 and PGE2 type 1 receptors are vital for angiotensin II-induced formation of reactive oxygen species and Ca(2+) influx in the subfornical organ

Regulation of blood pressure by angiotensin II (ANG II) is a process that involves the reactive oxygen species (ROS) and calcium. We have shown that ANG-II type 1 receptor (AT1R) and prostaglandin E2 (PGE2) type 1 receptors (EP1R) are required in the subfornical organ (SFO) for ROS-mediated hypertension induced by slow-pressor ANG-II infusion. However, the signaling pathway associated with this process remains unclear. We sought to determine mechanisms underlying the ANG II-induced ROS and calcium influx in mouse SFO cells. Ultrastructural studies showed that cyclooxygenase 1 (COX-1) codistributes with AT1R in the SFO, indicating spatial proximity. Functional studies using SFO cells revealed that ANG II potentiated PGE2 release, an effect dependent on AT1R, phospholipase A2 (PLA2) and COX-1. Furthermore, both ANG II and PGE2 increased ROS formation. While the increase in ROS initiated by ANG II, but not PGE2, required the activation of the AT1R/PLA2/COX-1 pathway, both ANG II and PGE2 were dependent on EP1R and Nox2 as downstream effectors. Finally, ANG II potentiated voltage-gated L-type Ca(2+) currents in SFO neurons via the same signaling pathway required for PGE2 production. Blockade of EP1R and Nox2-derived ROS inhibited ANG II and PGE2-mediated Ca(2+) currents. We propose a mechanism whereby ANG II increases COX-1-derived PGE2 through the AT1R/PLA2 pathway, which promotes ROS production by EP1R/Nox2 signaling in the SFO. ANG II-induced ROS are coupled with Ca(2+) influx in SFO neurons, which may influence SFO-mediated sympathoexcitation. Our findings provide the first evidence of a spatial and functional framework that underlies ANG-II signaling in the SFO and reveal novel targets for antihypertensive therapies.

17β-Estradiol alters the response of subfornical organ neurons that project to supraoptic nucleus to plasma angiotensin II and hypernatremia

This study was done in urethane anesthetized, ovariectomized (OVX) female rats that were either implanted or not implanted with silastic capsules containing17β-estradiol (E2) to investigate the effect of systemic changes in E2 on the discharge rate of subfornical organ (SFO) neurons that projected to supraoptic nucleus (SON) and responded to changes in plasma levels of angiotensin II (ANG II) or hypernatremia. Extracellular single unit recordings were made from 146 histologically verified single units in SFO. Intra-carotid infusions of ANG II excited ~57% of these neurons, whereas ~23% were excited by hypertonic NaCl. Basal discharge rate of neurons excited by ANG II or hypertonic NaCl was significantly lower in OVX+E2 rats compared to OVX only animals. The response of SFO neurons antidromically activated by SON stimulation to intra-carotid injections of ANG II or hypertonic NaCl was greater in the OVX only compared to the OVX+E2 rats. Intra-carotid injections of E2 in either group attenuated not only the basal discharge of these neurons, but also their response to ANG II or hypertonic NaCl. In all cases this inhibitory effect of E2 was blocked by an intra-carotid injection of the E2 receptor antagonist ICI-182780, although ICI-182780 did not alter the neuron's response to ANG II or hypertonic NaCl. Additionally, ICI-182780 in the OVX+E2 animals significantly raised the basal discharge of SFO neurons and their response to ANG II or hypertonic NaCl. These data indicate that E2 alters the response of SFO neurons to ANG II or NaCl that project to SON, and suggest that E2 functions in the female to regulate neurohypophyseal function in response to circulating ANG II and plasma hypernatremia.

Role of angiotensin II type 1 receptors in the subfornical organ in the pressor responses to central sodium in rats

Central infusion of Na(+)-rich artificial cerebro-spinal fluid (aCSF) activates the brain renin-angiotensin system and causes sympatho-excitatory and pressor responses. We evaluated the role of the subfornical organ (SFO) and angiotensin II type 1 (AT1) receptors in the SFO in mediating the central Na(+)-induced pressor response. In conscious Wistar rats, intra SFO infusions of Na(+)-rich aCSF containing 0.45 and 0.6M Na(+) at 10 nl/min or injection of angiotensin II (Ang II) at 80 ng increased blood pressure (BP) by 15-22 mmHg, whereas mannitol with the same osmolarity as the Na(+)-rich aCSF had no effects. Intra SFO infusion of the AT1 receptor blocker candesartan abolished the pressor response induced by intra SFO administration of Na(+)-rich aCSF or Ang II. Intra cerebro-ventricular (icv) infusion of Na(+)-rich aCSF (0.3M Na(+)) at 3.8 μl/min for 10 min increased BP by 15-20 mmHg. Electrolytic lesion of the SFO attenuated these BP increases by 50-70%. Intra SFO infusion of candesartan also prevented 50% of these pressor responses. These data suggest that SFO neurons are indeed sensitive to Na(+), the SFO is a major - but not only - site in the brain to sense an increase in CSF [Na(+)], and activation of AT1 receptors in the SFO mediates the SFO component of the Na(+)-induced pressor response.

Stanniocalcin-1 in the subfornical organ inhibits the dipsogenic response to angiotensin II

Recently, receptors for the calcium-regulating glycoprotein hormone stanniocalcin-1 (STC-1) have been found within subfornical organ (SFO), a central structure involved in the regulation of electrolyte and body fluid homeostasis. However, whether SFO neurons produce STC-1 and how STC-1 may function in fluid homeostasis are not known. Two series of experiments were done in Sprague-Dawley rats to investigate whether STC-1 is expressed within SFO and whether it exerts an effect on water intake. In the first series, experiments were done to determine whether STC-1 was expressed within cells in SFO using immunohistochemistry, and whether protein and gene expression for STC-1 existed in SFO using Western blot and quantitative RT-PCR, respectively. Cells containing STC-1 immunoreactivity were found throughout the rostrocaudal extent of SFO. STC-1 protein expression within SFO was confirmed with Western blot, and SFO was also found to express STC-1 mRNA. In the second series, microinjections (200 nl) of STC-1, ANG II, a combination of the two or the vehicle were made into SFO in conscious, unrestrained rats. Water intake was measured at 0700 for a 1-h period after each injection in animals. Microinjections of STC-1 (17.6 or 176 nM) alone had no effect on water intake compared with controls. However, STC-1 not only attenuated the drinking responses to ANG II for about 30 min, but also decreased the total water intake over the 1-h period. These data suggest that STC-1 within the SFO may act in a paracrine/autocrine manner to modulate the neuronal responses to blood-borne ANG II. These findings also provide the first direct evidence of a physiological role for STC-1 in central regulation of body fluid homeostasis.