Lateral parabrachial nucleus and angiotensin II-induced hypertension

Female protection from slow-pressor effects of angiotensin II involves prevention of ROS production independent of NMDA receptor trafficking in hypothalamic neurons expressing angiotensin 1A receptors

Renin–angiotensin system overactivity, upregulation of postsynaptic NMDA receptor function, and increased reactive oxygen species (ROS) production in the hypothalamic paraventricular nucleus (PVN) are hallmarks of angiotensin II (AngII)-induced hypertension, which is far more common in young males than in young females. We hypothesize that the sex differences in hypertension are related to differential AngII-induced changes in postsynaptic trafficking of the essential NMDA receptor GluN1 subunit and ROS production in PVN cells expressing angiotensin Type 1a receptor (AT1aR). We tested this hypothesis using slow-pressor (14-day) infusion of AngII (600 ng/kg/min) in mice, which elicits hypertension in males but not in young females. Two-month-old male and female transgenic mice expressing enhanced green fluorescent protein (EGFP) in AT1aR-containing cells were used. In males, but not in females, AngII increased blood pressure and ROS production in AT1aR–EGFP PVN cells at baseline and following NMDA treatment. Electron microscopy showed that AngII increased cytoplasmic and total GluN1–silver-intensified immunogold (SIG) densities and induced a trend toward an increase in near plasmalemmal GluN1–SIG density in AT1aR–EGFP dendrites of males and females. Moreover, AngII decreased dendritic area and diameter in males, but increased dendritic area of small (<1 µm) dendrites and decreased diameter of large (>1 µm) dendrites in females. Fluorescence microscopy revealed that AT1aR and estrogen receptor β do not colocalize, suggesting that if estrogen is involved, its effect is indirect. These data suggest that the sexual dimorphism in AngII-induced hypertension is associated with sex differences in ROS production in AT1aR-containing PVN cells but not with postsynaptic NMDA receptor trafficking.

A role for the lateral parabrachial nucleus in cardiovascular function and fluid homeostasis

The lateral parabrachial nucleus (LPBN) is located in an anatomical position that enables it to perform a critical role in relaying signals related to the regulation of fluid and electrolyte intake and cardiovascular function from the brainstem to the forebrain. Early neuroanatomical studies have described the topographic organization of blood pressure sensitive neurons and functional studies have demonstrated a major role for the LPBN in regulating cardiovascular function, including blood pressure, in response to hemorrhages, and hypovolemia. In addition, inactivation of the LPBN induces overdrinking of water in response to a range of dipsogenic treatments primarily, but not exclusively, those associated with endogenous centrally acting angiotensin II. Moreover, treatments that typically cause water intake stimulate salt intake under some circumstances particularly when serotonin receptors in the LPBN are blocked. This review explores the expanding body of evidence that underlies the complex neural network within the LPBN influencing salt appetite, thirst and the regulation of blood pressure. Importantly understanding the interactions among neurons in the LPBN that affect fluid balance and cardiovascular control may be critical to unraveling the mechanisms responsible for hypertension.

Adenoviral inhibition of AT1a receptors in the paraventricular nucleus inhibits acute increases in mean arterial blood pressure in the rat

Brain and peripheral renin-angiotensin systems are important in blood pressure maintenance. Circulating ANG II stimulates brain RAS to contribute to the increase mean arterial pressure (MAP). This mechanism has not been fully clarified, so it was hypothesized that reducing angiotensin type 1a (AT1a) receptors (AT1aRs) in the paraventricular nucleus (PVN) would diminish intravenous ANG II-induced increases in MAP. Adenoviruses (Ad) encoding AT1a small hairpin RNA (shRNA) or Ad-LacZ (marker gene) were injected into the PVN [1 × 109 plaque-forming units/ml, bilateral (200 nl/site)] of male Sprague-Dawley rats instrumented with radiotelemetry transmitters for MAP and heart rate measurements and with venous catheters for drug administration. No differences in weight gain or basal MAP were observed. ANG II (30 ng·kg−1·min−1 iv, 15 μl/min for 60 min) was administered 3, 7, 10, and 14 days after PVN Ad injection to increase blood pressure. ANG II-induced elevations in MAP were significantly reduced in PVN Ad-AT1a shRNA rats compared with Ad-LacZ rats (32 ± 6 vs. 8 ± 9 mmHg at 7 days, 35 ± 6 vs. 10 ± 6 mmHg at 10 days, and 32 ± 2 vs. 1 ± 5 mmHg at 14 days; P < 0.05). These observations were confirmed by acute administration of losartan (20 nmol/l, 100 nl/site) in the PVN prior to short-term infusion of ANG II; the ANG II-pressor response was attenuated by 69%. In contrast, PVN Ad-AT1a shRNA treatment did not influence phenylephrine-induced increases in blood pressure (30 μg·kg−1·min−1 iv, 15 μl/min for 30 min). Importantly, PVN Ad-AT1a shRNA did not alter superior mesenteric arterial contractility to ANG II or norepinephrine; ACh-induced arterial relaxation was also unaltered. β-Galactosidase staining revealed PVN Ad transduction, and Western blot analyses revealed significant reductions of PVN AT1 protein. In conclusion, PVN-localized AT1Rs are critical for short-term circulating ANG II-mediated elevations of blood pressure. A sustained suppression of AT1aR expression by single administration of shRNA can interfere with short-term actions of ANG II.

Fos-Related Antigen Immunoreactivity After Acute and Chronic Angiotensin II–Induced Hypertension in the Rabbit Brain

Several brain regions are proposed as contributing to chronic sympatho-excitatory effects of elevated circulating angiotensin II. However, earlier c-Fos studies have been limited to acute angiotensin II exposure. This study aims to determine brain regions responding with chronic elevated angiotensin II. Rabbits were administered angiotensin II (50 ng/kg per minute) or saline for 3 hours, 3 days, or 14 days. Basal mean arterial pressure was 71±2 mm Hg and increased 23±2 mm Hg, 32±4 mm Hg, and 22±2 mm Hg for 3 hours, 3 days, and 14 days, respectively, with angiotensin II infusion. Neuronal activation was detected using Fos-related antigens, which recognizes all of the known members of the Fos family. Neurons located in the amygdala and area postrema were activated transiently after acute infusion of angiotensin II but were not responsive by days 3 or 14. Neurons located in the nucleus of the solitary tract, caudal ventrolateral medulla, and lateral parabrachial nucleus were activated for ≤3 days after infusion of angiotensin II but were not responsive by day 14, which is consistent with their role in response to baroreceptor pathways that reset with sustained hypertension. The vascular organ of the lamina terminalis and subfornical organ showed sustained but diminishing activation over the 14-day period. However, the downstream hypothalamic nuclei that receive inputs from these nuclei, the paraventricular, supraoptic, and arcuate nuclei, showed marked sustained activation. These findings suggest that there is desensitization of circumventricular organs but sensitization of neurons in hypothalamic regions to long-term angiotensin II infusion.

Lesion of the caudal ventrolateral medulla prevents the induction of hypertension by adenosine receptor blockade in rats

The continuous infusion for 7 days of the adenosine receptor antagonist 1,3-dipropyl-8-sulfophenylxanthine (DPSPX) causes a sustained hypertension in rats, with an enhancement of sympathetic neurotransmission and activation of the renin-angiotensin system. We studied the involvement of the caudal ventrolateral medulla in the establishment of this hypertensive model by evaluating the effect of local lesioning in blood pressure (BP). Male adult Wistar rats received stereotaxic injections of 0.3 mul of saline or quinolinic acid (QA; 180 mM) in the caudal ventrolateral medulla followed by abdominal implant of minipump for infusion of saline or DPSPX (90 microg(-1) kg(-1) h(-1)). BP was measured in conscious animals every 2 days for 12 days. The sustained increase of BP (22.1 mm Hg; P < 0.001) detected in rats infused with DPSPX was reverted (6.7 mm Hg; P > 0.05) from day six onwards in animals with lesion of the lateralmost part of caudal ventrolateral medulla (VLMlat). The present results suggest that the development of hypertension induced by adenosine receptor antagonist involves the participation of the VLMlat. They further add new data as to the functional complexity of this medullary area involved in a variety of functions such as cardiovascular, respiratory, motor and pain control.

Neuroendocrine control of body fluid metabolism

Mammals control the volume and osmolality of their body fluids from stimuli that arise from both the intracellular and extracellular fluid compartments. These stimuli are sensed by two kinds of receptors: osmoreceptor-Na+ receptors and volume or pressure receptors. This information is conveyed to specific areas of the central nervous system responsible for an integrated response, which depends on the integrity of the anteroventral region of the third ventricle, e.g., organum vasculosum of the lamina terminalis, median preoptic nucleus, and subfornical organ. The hypothalamo-neurohypophysial system plays a fundamental role in the maintenance of body fluid homeostasis by secreting vasopressin and oxytocin in response to osmotic and nonosmotic stimuli. Since the discovery of the atrial natriuretic peptide (ANP), a large number of publications have demonstrated that this peptide provides a potent defense mechanism against volume overload in mammals, including humans. ANP is mostly localized in the heart, but ANP and its receptor are also found in hypothalamic and brain stem areas involved in body fluid volume and blood pressure regulation. Blood volume expansion acts not only directly on the heart, by stretch of atrial myocytes to increase the release of ANP, but also on the brain ANPergic neurons through afferent inputs from baroreceptors. Angiotensin II also plays an important role in the regulation of body fluids, being a potent inducer of thirst and, in general, antagonizes the actions of ANP. This review emphasizes the role played by brain ANP and its interaction with neurohypophysial hormones in the control of body fluid homeostasis.

Central mechanisms of acute ANG II modulation of arterial baroreflex control of renal sympathetic nerve activity

Short-term intravenous infusion of angiotensin II (ANG II) into conscious rabbits reduces the range of renal sympathetic nerve activity (RSNA) by attenuating reflex disinhibition of RSNA. This action of ANG II to attenuate the arterial baroreflex range is exaggerated when ANG II is directed into the vertebral circulation, which suggests a mechanism involving the central nervous system. Because an intact area postrema (AP) is required for ANG II to attenuate arterial baroreflex-mediated bradycardia and is also required for maintenance of ANG II-dependent hypertension, we hypothesized that attenuation of maximum RSNA during infusion of ANG II involves the AP. In conscious AP-lesioned (APX) and AP-intact rabbits, we compared the effect of a 5-min intravenous infusion of ANG II (10 and 20 ng x kg(-1) x min(-1)) on the relationship between mean arterial blood pressure (MAP) and RSNA. Intravenous infusion of ANG II into AP-intact rabbits resulted in a dose-related attenuation of maximum RSNA observed at low MAP. In contrast, ANG II had no effect on maximum RSNA in APX rabbits. To further localize the central site of ANG II action, its effect on the arterial baroreflex was assessed after a midcollicular decerebration. Decerebration did not alter arterial baroreflex control of RSNA compared with the control state, but as in APX, ANG II did not attenuate the maximum RSNA observed at low MAP. The results of this study indicate that central actions of peripheral ANG II to attenuate reflex disinhibition of RSNA not only involve the AP, but may also involve a neural interaction rostral to the level of decerebration.

Peripheral and central interactions between the renin-angiotensin system and the renal sympathetic nerves in control of renal function

Increases in renal sympathetic nerve activity (RSNA) regulate the functions of the nephron, the vasculature, and the renin-containing juxtaglomerular granular cells. As increased activity of the renin-angiotensin system can also influence nephron and vascular function, it is important to understand the interactions between RSNA and the renin-angiotensin system in the control of renal function. These interactions can be intrarenal, that is, the direct (via specific innervation) and indirect (via angiotensin II) contributions of increased RSNA to the regulation of renal function. The effects of increased RSNA on renal function are attenuated when the activity of the renin-angiotensin system is suppressed or antagonized with angiotensin-converting enzyme inhibitors or angiotensin II-type AT1 receptor antagonists. The effects of intrarenal administration of angiotensin II are attenuated following renal denervation. These interactions can also be extrarenal, that is, in the central nervous system, wherein RSNA and its arterial baroreflex control are modulated by changes in activity of the renin-angiotensin system. In addition to the circumventricular organs, the permeable blood-brain barrier of which permits interactions with circulating angiotensin II, there are interactions at sites behind the blood-brain barrier that depend on the influence of local angiotensin II. The responses to central administration of angiotensin II type AT1 receptor antagonists, into the ventricular system or microinjected into the rostral ventrolateral medulla, are modulated by changes in activity of the renin-angiotensin system produced by physiological changes in dietary sodium intake. Similar modulation is observed in pathophysiological models wherein activity of both the renin-angiotensin and sympathetic nervous systems is increased (e.g., congestive heart failure). Thus, both renal and extrarenal sites of interaction between the renin-angiotensin system and RSNA are involved in influencing the neural control of renal function.

Parabrachial nucleus modulates cardiovascular responses to blood loss

The goal of this study was to determine the role of the pontine lateral parabrachial nucleus (LPBN) in the compensatory responses to blood loss. Conscious unrestrained rats with complete, partial, or sham bilateral ibotenic acid lesions of the LPBN were subjected to a hypotensive 16-ml/kg blood withdrawal via arterial catheter. Complete lesions (LPBNx) encompassed the entire LPBN and extended into the ventrolateral parabrachial region to encroach on the Kolliker-Fuse nucleus. Partial lesions were restricted to the body of the LPBN and spared the outer rim of the external lateral subnucleus of the LPBN. In all three groups, serum corticosterone concentration and plasma renin activity increased four- to fivefold after hemorrhage (P < 0.01), and immunocytochemistry demonstrated numerous Fos-positive neurons in the hypothalamic supraoptic nucleus. However, the cardiovascular responses to hypotensive blood loss differed for complete and partial lesions. Blood pressure failed to recover in LPBNx rats and was significantly lower in LPBNx (66 +/- 4 mmHg) than in rats with partial or sham lesions (98 +/- 4 and 85 +/- 5 mmHg, respectively) at 40 min posthemorrhage. In contrast, rats with partial lesions had a significant attenuation of the posthemorrhage bradycardia. This implies that a population of neurons within the body of the LPBN is essential for full expression of the bradycardia that accompanies hemorrhagic hypotension, whereas the ventrolateral parabrachial region is essential for normal restoration of arterial pressure after hypotensive hemorrhage.

Antisense inhibition of brain renin-angiotensin system decreased blood pressure in chronic 2-kidney, 1 clip hypertensive rats

The systemic renin-angiotensin system (RAS) plays an important role in blood pressure (BP) regulation during the development of 2-kidney, 1 clip (2K1C) hypertension. Its contributions decrease with time after constriction of the renal artery. During the chronic phase, the peripheral RAS returns to normal, but the hypertension is sustained for months. We hypothesized that in this phase the brain RAS contributes to the maintenance of high BP. To test the hypothesis, we studied the role of brain RAS by decreasing the synthesis of angiotensinogen (AGT) and the angiotensin II (Ang II) type 1a receptor (AT(1)R) with intracerebroventricular injections of antisense oligonucleotides (AS-ODNs). The response of systolic BP (SBP) to AS-ODNs to AGT mRNA was studied in 2K1C rats at 6 months after clipping, and the response to AS-ODNs to AT(1)R mRNA was studied at 10 months after clipping. Intracerebroventricular injection of AS-ODN-AGT (200 microgram/kg, n=5) significantly decreased SBP (-22+/-6 mm Hg, P<0.05) compared with the sense ODN (n=5) and saline (n=3) groups. Intracerebroventricular injection of AS-ODN-AGT reduced the elevated hypothalamic Ang II level. The hypothalamic Ang II content in sense ODN and saline groups was significantly (P<0.05) higher than in the nonclipped group. Compared with inverted ODN, intracerebroventricular injection of AS-ODN-AT(1)R (250 microgram/kg, n=6) significantly decreased SBP (-26+/-8 mm Hg, P<0.05) for 3 days after injection. This was a brain effect because intravenous AS-ODN-AT(1)R at a dose of 250 to 500 microgram/kg did not affect SBP. These results suggest that the brain RAS plays an important role in maintaining the elevated SBP in chronic 2K1C hypertension.

Nervous kidney. Interaction between renal sympathetic nerves and the renin-angiotensin system in the control of renal function

Increases in renal sympathetic nerve activity regulate the functions of the nephron, the vasculature, and the renin-containing juxtaglomerular granular cells. Because increased activity of the renin-angiotensin system can also influence nephron and vascular function, it is important to understand the interactions between the renal sympathetic nerves and the renin-angiotensin system in the control of renal function. These interactions can be intrarenal, for example, the direct (by specific innervation) and indirect (by angiotensin II) contributions of increased renal sympathetic nerve activity to the regulation of renal function. The effects of increased renal sympathetic nerve activity on renal function are attenuated when the activity of the renin-angiotensin system is suppressed or antagonized with ACE inhibitors or angiotensin II-type AT(1)-receptor antagonists. The effects of intrarenal administration of angiotensin II are attenuated after renal denervation. These interactions can also be extrarenal, for example, in the central nervous system, wherein renal sympathetic nerve activity and its arterial baroreflex control are modulated by changes in activity of the renin-angiotensin system. In addition to the circumventricular organs, whose permeable blood-brain barrier permits interactions with circulating angiotensin II, there are interactions at sites behind the blood-brain barrier that depend on the influence of local angiotensin II. The responses to central administration of angiotensin II-type AT(1)-receptor antagonists into the ventricular system or microinjected into the rostral ventrolateral medulla are modulated by changes in activity of the renin-angiotensin system produced by physiological changes in dietary sodium intake. Similar modulation is observed in pathophysiological models wherein activity of both the renin-angiotensin and sympathetic nervous systems is increased (eg, congestive heart failure). Thus, both renal and extrarenal sites of interaction between the renin-angiotensin system and renal sympathetic nerve activity are involved in influencing the neural control of renal function.

Angiotensin II acutely attenuates range of arterial baroreflex control of renal sympathetic nerve activity

Acutely increasing peripheral angiotensin II (ANG II) reduces the maximum renal sympathetic nerve activity (RSNA) observed at low mean arterial blood pressures (MAPs). We postulated that this observation could be explained by the action of ANG II to acutely increase arterial blood pressure or increase circulating arginine vasopressin (AVP). Sustained increases in MAP and increases in circulating AVP have previously been shown to attenuate maximum RSNA at low MAP. In conscious rabbits pretreated with an AVP V1 receptor antagonist, we compared the effect of a 5-min intravenous infusion of ANG II (10 and 20 ng x kg(-1) x min(-1)) on the relationship between MAP and RSNA when the acute pressor action of ANG II was left unopposed with that when the acute pressor action of ANG II was opposed by a simultaneous infusion of sodium nitroprusside (SNP). Intravenous infusion of ANG II resulted in a dose-related attenuation of the maximum RSNA observed at low MAP. When the acute pressor action of ANG II was prevented by SNP, maximum RSNA at low MAP was attenuated, similar to that observed when ANG II acutely increased MAP. In contrast, intravertebral infusion of ANG II attenuated maximum RSNA at low MAP significantly more than when administered intravenously. The results of this study suggest that ANG II may act within the central nervous system to acutely attenuate the maximum RSNA observed at low MAP.

Parabrachial nucleus induces suppression of baroreflex bradycardia by the release of glutamate in the rostral ventrolateral medulla of the rat

The involvement of glutamatergic neurotransmission in the rostral ventrolateral medulla (RVLM) in the suppression of baroreflex bradycardia by the parabrachial nucleus (PBN) was investigated. Repeated electrical activation of the PBN increased the concentration of glutamate in the dialysate collected from the RVLM. The same stimulation also suppressed baroreflex bradycardia in response to transient hypertension evoked by phenylephrine (5 microg/kg, intravenously). Microinfusion of L-glutamate (10, 50 or 100 microM) via the microdialysis probe into the RVLM dose-dependently elicited a significant inhibition of baroreflex bradycardia that paralleled the concentration and time course of the PBN-elicited elevation in extracellular glutamate in the RVLM. The suppression of baroreflex bradycardia elicited by microinjection of L-glutamate (1 nmol) into the RVLM was appreciably reversed by coinjection of the NMDA receptor antagonist, dizocilpine (500 pmol), or the non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2, 3-dione (50 pmol). These results suggest that an increase in the extracellular concentration of glutamate and activation of both NMDA and non-NMDA receptors in the RVLM may mediate the suppression of baroreflex bradycardia by activation of the PBN.

The brain renin-angiotensin system modulates angiotensin II-induced hypertension and cardiac hypertrophy

The potential involvement of the brain renin-angiotensin system in the hypertension induced by subpressor doses of angiotensin II was tested by the use of newly developed transgenic rats with permanent inhibition of brain angiotensinogen synthesis [TGR(ASrAOGEN)]. Basal systolic blood pressure monitored by telemetry was significantly lower in TGR(ASrAOGEN) than in Sprague-Dawley rats (parent strain) (122.5+/-1.5 versus 128.9+/-1.9 mm Hg, respectively; P<0.05). The increase in systolic blood pressure induced by 7 days of chronic angiotensin II infusion was significantly attenuated in TGR(ASrAOGEN) in comparison with control rats (29.8+/-4.2 versus 46. 3+/-2.5 mm Hg, respectively; P<0.005). Moreover, an increase in heart/body weight ratio was evident only in Sprague-Dawley (11.1%) but not in TGR(ASrAOGEN) rats (2.8%). In contrast, mRNA levels of atrial natriuretic peptide (ANP) and collagen III in the left ventricle measured by ribonuclease protection assay were similarly increased in both TGR(ASrAOGEN) (ANP, x2.5; collagen III, x1.8) and Sprague-Dawley rats (ANP, x2.4; collagen III, x2) as a consequence of angiotensin II infusion. Thus, the expression of these genes in the left ventricle seems to be directly stimulated by angiotensin II. However, the hypertensive and hypertrophic effects of subpressor angiotensin II are at least in part mediated by the brain renin-angiotensin system.

Glutamatergic projection to RVLM mediates suppression of reflex bradycardia by parabrachial nucleus

We investigated the role of glutamatergic projection from the parabrachial nucleus (PBN) complex to the rostral ventrolateral medulla (RVLM) in the PBN-induced suppression of reflex bradycardia in adult Sprague-Dawley rats that were maintained under pentobarbital anesthesia. Under stimulus conditions that did not appreciably alter the baseline systemic arterial pressure and heart rate, electrical (10-s train of 0.5-ms pulses, at 10-20 microA and 10-20 Hz) or chemical (L-glutamate, 1 nmol) stimulation of the ventrolateral regions and Köelliker-Fuse (KF) subnucleus of the PBN complex significantly suppressed the reflex bradycardia in response to transient hypertension evoked by phenylephrine (5 micrograms/kg iv). The PBN-induced suppression of reflex bradycardia was appreciably reversed by bilateral microinjection into the RVLM of the N-methyl-D-aspartate (NMDA)-receptor antagonist MK-801 (500 pmol) or the non-NMDA-receptor antagonist 6-cyano-7-nitroquinoxaline-2, 3-dione (50 pmol). Anatomically, most of the retrogradely labeled neurons in the ventrolateral regions and KF subnucleus of the ipsilateral PBN complex after microinjection of fast blue into the RVLM were also immunoreactive to anti-glutamate antiserum. These results suggest that a direct glutamatergic projection to the RVLM from topographically distinct regions of the PBN complex may participate in the suppression of reflex bradycardia via activation of both NMDA and non-NMDA receptors at the RVLM.

Splanchnic and vagal denervation attenuate central Fos but not AVP responses to intragastric salt in rats

We have recently reported that an acute intragastric hypertonic saline load increases plasma arginine vasopressin (PAVP) and Fos immunoreactivity in several central nuclei, including the supraoptic nucleus (SON), paraventricular nucleus (PVN), nucleus of the solitary tract (NTS), area postrema (AP), and lateral parabrachial nucleus (LPBN). We hypothesized that these responses are mediated by stimulation of peripheral osmoreceptors with splanchnic and/or vagal afferent projections. To test this hypothesis, we examined the effect of bilateral subdiaphragmatic vagotomy and bilateral splanchnic denervation on the PAVP and Fos immunoreactivity responses to intragastric hypertonic saline infusion in awake rats. Compared with responses in sham rats, Fos immunoreactivity responses were significantly reduced in vagotomized rats in the AP, SON, and PVN, whereas normal Fos levels were observed in the LPBN. However, vagotomized rats exhibited a normal increase in PAVP. Splanchnic-denervated rats also exhibited similar changes in PAVP in response to intragastric hypertonic saline compared with sham-denervated rats, and no differences were observed in Fos immunoreactivity in the LPBN, SON, and PVN compared with sham rats. However, splanchnic-denervated rats were observed to have significantly lower Fos staining in the NTS and AP compared with sham rats. The inability of splanchnic or vagal denervation alone to block the PAVP response to intragastric hypertonic saline suggests that either peripheral osmoreceptors project via both splanchnic and vagal afferents to mediate AVP release or that the observed response of PAVP is due to the activation of central osmoreceptors in the absence of measurable changes in plasma osmolality.

Lateral parabrachial nucleus modulates baroreflex regulation of sympathetic nerve activity

Previous studies have demonstrated that the lateral parabrachial nucleus (LPBN) is an important site for descending modulation of baroreflex control of heart rate. In the present study it was hypothesized that the LPBN neurons may also modulate baroreflex control of arterial pressure and sympathetic nerve activity. In urethan-anesthetized rats, electrical or chemical activation of the LPBN produced a significant reduction in the magnitude of the baroreflex inhibition of mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) elicited by aortic depressor nerve stimulation. Chemical inactivation of the LPBN resulted in a small increase in baroreflex control of MAP, but baroreflex control of RSNA was not affected. The results suggest that LPBN neurons have little tonic influence over baroreflex control of MAP and RSNA in the anesthetized rat. When the LPBN is activated, however, LPBN neurons may function to reduce the capacity of the baroreflex to regulate sympathetically mediated increases in arterial pressure.

Long-term sympatho-excitatory effect of angiotensin II: a mechanism of spontaneous and renovascular hypertension

1. The peptide hormone angiotensin II (AngII) is acknowledged to be an important factor in the pathophysiology of hypertension. This is particularly the case in hypertension caused by luminal narrowing of one renal artery, (i.e. renovascular hypertension). The primary mechanism by which AngII raises blood pressure, however, is disputed. Strong arguments can be made supporting either vascular contraction, effects on renal excretion of sodium and water, or trophic actions on cardiovascular structures as the key element. In this paper I review evidence that AngII influences blood pressure by modulating autonomic nervous system activity. Modulation can occur at both the peripheral and central aspects of the autonomic system, but I focus on brain pathways involved in determining sympathetic nervous system activity. 2. Experimental and clinical investigations are cited to support the hypothesis that sympathetically mediated pressor effects are increased by both circulating and brain-derived AngII in hypertension. Recent work points specifically to sympathetic pre-motor neurons in the rostral ventrolateral medulla (RVLM) as a critical site of action of brain AngII in normotensive and hypertensive animals. 3. This same set of neurons appears to be an important relay in the sympatho-excitatory response to circulating AngII initiated at circumventricular organs, particularly the area postrema. AngII has important effects on the baroreflex. These do not mediate the sympatho-excitation elicited by circulating AngII, but rather mask its expression. 4. Substantial data support the hypothesis that increased blood concentrations of AngII in renovascular hypertension elevate blood pressure by causing neurogenic vasoconstriction mediated through the area postrema and RVLM.

Preoptic recess lesions reduce right atrial pressure responses to volume expansion

Ablation of the periventricular tissue surrounding the anteroventral portion of the third cerebral ventricle (AV3V-X) abolishes natriuresis and diuresis during volume expansion. Although deficits in several efferent mechanisms have been identified, effects of AV3V-X on afferent input to the volume reflex have not been investigated. Therefore, these experiments measured right atrial pressure (RAP) in conscious AV3V-X and control-operated (CONT) rats before, during, and following acute (60 s) or continuous (60 min) infusion of isotonic saline, or vascular volume expansion with whole blood (15 min). Changes in RAP were significantly smaller in AV3V-X rats than CONT animals during acute isotonic saline infusion and during whole-blood expansion. Treatment with hexamethonium abolished the difference in RAP during acute volume infusion. However, there was no significant difference in RAP between groups during continuous isotonic saline expansion. These data suggest that AV3V-X reduces afferent input from cardiopulmonary stretch receptors during acute volume expansion, which may contribute to diminished natriuresis and diuresis observed in these animals.

Membrane properties of area postrema neurons

Intrinsic membrane properties, voltage-dependent sodium and voltage-dependent potassium currents of area postrema neurons in culture have been characterized with respect to their voltage dependence, time dependence and sensitivity to specific blocking agents. The area postrema is a hindbrain circumventricular organ which is known to have an important role in the central regulation of cardiovascular function. This study is the first to describe the biophysical properties of ion channels present in rat area postrema neurons. Recordings in current-clamp mode revealed a mean resting membrane potential of -55.0 +/- 1.6 (n = 24) mV and an input resistance of 213.6 +/- 23 M omega. For the 24 neurons tested, the evoked action potential had a mean threshold of 38.8 +/- 2 mV and a mean amplitude of 107.3 +/- 15 mV. Our results show that the area postrema possesses only one principle sodium current which is completely abolished by 5 microM tetrodotoxin (TTX) (n = 28). This current activated near -50 mV and reached peak amplitude at -30 mV. The area postrema does not possess a TTX insensitive sodium current. The area postrema has at least two types of potassium currents. All area postrema neurons studied with tetraethylamonium (TEA) (n = 40) showed the presence of a slowly activating outward current which was present at voltages greater than -40 mV and was blocked by 10 mM TEA. In addition, 75% of the neurons studied (n = 30/40) also showed a rapidly inactivating, 4-AP sensitive IA type current which activated near -30 mV. Angiotensin II attenuated both the peak and the steady-state potassium currents, suggesting that angiotensin II may modulate area postrema activity by inhibiting voltage-gated potassium channels.

Effects of lateral parabrachial nucleus lesions in chronic renal hypertensive rats

Neuroanatomic studies describing forebrain projections to the lateral parabrachial nucleus suggest a central integrative role in cardiovascular regulation. We performed this study to examine the role of this pontine nucleus in the maintenance of one-kidney, figure-8 renal-wrap hypertension. Bilateral ibotenic acid ablation of the lateral parabrachial nucleus was performed 4 weeks after induction of hypertension or sham operation. In hypertensive rats, ablation produced a significant reduction in mean arterial pressure from 160 +/- 4 to 118 +/- 2 mm Hg and a transient but significant increase in heart rate from 381 +/- 5 to 408 +/- 8 beats per minute on the first day after ablation; arterial pressure returned to preablation values by day 5 after ablation. In sham-operated, normotensive animals, arterial pressure was not altered by ablation, and a transient but significant increase in heart rate from 384 +/- 8 to 419 +/- 7 beats per minute was again observed. Before ablation, trimethaphan administration produced a significantly greater drop in arterial pressure in hypertensive (delta-72.8 +/- 4.6 mm Hg) versus normotensive (delta-55.7 +/- 4.1 mm Hg) animals. This effect was eliminated on day 1 after ablation yet returned on day 4 after ablation. In blood samples obtained before ablation and on days 1 and 4 after ablation, circulating plasma catecholamine concentrations in both groups remained unchanged. These observations suggest that, because of possible alternate neural compensatory mechanisms, lateral parabrachial nucleus ablation produces a significant yet transient reversal of renal-wrap hypertension. Thus, the lateral parabrachial nucleus may contribute to the increased sympathetic nervous system function associated with this model.

Increased dietary salt sensitizes vasomotor neurons of the rostral ventrolateral medulla

Excess dietary sodium is a major contributing factor to the incidence and severity of hypertension. However, the precise mechanism or mechanisms by which salt contributes to the severity of hypertension are unknown. The region of the rostral ventrolateral medulla (RVLM) is a principal brain stem locus critical for the regulation of arterial blood pressure by the sympathetic nervous system. The purpose of this study was to determine if excess dietary sodium chloride might alter the function or responsiveness of neurons in the RVLM. Male Sprague-Dawley rats were given either tap water or 0.9% sodium chloride solution to drink for 10 to 14 days. Excess sodium chloride did not affect baseline blood pressure. However, when neurons of the RVLM were stimulated by microinjections of L-glutamate, evoked increases in arterial pressure were potentiated in rats given sodium chloride. Augmented pressor responses could not be accounted for by increased vascular reactivity because both groups responded similarly to intravenously administered phenylephrine and norepinephrine. Additionally, electrical stimulation of descending spinal sympathoexcitatory axons produced identical pressor responses in both groups, indicating that altered synaptic transmission at central or peripheral neuroeffector junctions distal to the RVLM could not explain enhanced pressor responses produced by direct stimulation of RVLM cell somata. Finally, impaired arterial baroreceptor reflexes could not account for augmented RVLM pressor responses, as depressor and bradycardic responses produced by electrical stimulation of aortic baroreceptor afferents were not reduced in rats given excess dietary sodium chloride.(ABSTRACT TRUNCATED AT 250 WORDS)