Role of central catecholaminergic pathways in the actions of endogenous ANG II on sympathetic reflexes

Brain angiotensin peptides regulate sympathetic tone and blood pressure

Brain angiotensin II (Ang II) induces tonic sympathoexcitatory effects through AT1 receptor stimulation of glutamatergic neurons and sympathoinhibitory effects via GABAergic neurons in the rostral ventrolateral medulla, the brainstem 'pressor area'. NADPH-derived superoxide production and reactive oxygen species signalling is critical in these actions, and AT2 receptors in the rostral ventrolateral medulla appear to mediate opposing effects on sympathetic outflow. In the hypothalamic paraventricular nucleus, Ang II has AT1 receptor-mediated sympathoexcitatory effects and enhances nitric oxide formation, which in turn inhibits the Ang II effects through a GABAergic mechanism. Ang II also decreases the tonic sympathoinhibitory effect of gamma amino butyric acid within the paraventricular nucleus. Angiotensin III and Angiotensin IV increase blood pressure via brain AT1 receptor stimulation. Angiotensin (1-7) influences cardiovascular function through a specific Mas-receptor. This review examines the evidence that brain angiotensin peptides, glutamate, gamma amino butyric acid and nitric oxide interact within the rostral ventrolateral medulla and paraventricular nucleus to control sympathetic tone and blood pressure.

Blockade of AT1 receptors in the rostral ventrolateral medulla increases sympathetic activity under hypoxic conditions

The role of ANG type 1 (AT1) receptors in the rostral ventrolateral medulla (RVLM) in the maintenance of sympathetic vasomotor tone in normotensive animals is unclear. In this study, we tested the hypothesis that AT1 receptors make a significant contribution to the tonic activity of presympathetic neurons in the RVLM of normotensive rats under conditions where the excitatory input to these neurons is enhanced, such as during systemic hypoxia. In urethane-anesthetized rats, microinjections of the AT1 receptor antagonist candesartan in the RVLM during moderate hypoxia unexpectedly resulted in substantial increases in arterial pressure and renal sympathetic nerve activity (RSNA), whereas under normoxic conditions the same dose resulted in no significant change in arterial pressure and RSNA. Under hypoxic conditions, and after microinjection of the GABA(A) receptor antagonist bicuculline in the RVLM, subsequent microinjection of candesartan in the RVLM resulted in a significant decrease in RSNA. In control experiments, bilateral microinjections in the RVLM of the compound [Sar1,Thr8]ANG II (sarthran), which decreases sympathetic vasomotor activity via a mechanism that is independent of AT1 receptors, significantly reduced arterial pressure and RSNA under both normoxic and hypoxic conditions. The results indicate that, at least under some conditions, endogenous ANG II has a tonic sympathoinhibitory effect in the RVLM, which is dependent on GABA receptors. We suggest that the net effect of endogenous ANG II in this region depends on the balance of both tonic excitatory and inhibitory actions on presympathetic neurons and that this balance is altered in different physiological or pathophysiological conditions.

Expression of angiotensin II type 1 (AT(1)) receptor in the rostral ventrolateral medulla in rats

In the present study, the changes of amino acids release in the spinal cord after the application of angiotensin II (ANG II) in the rostral ventrolateral medulla (RVLM) and the distribution of ANG receptors on neurons of the RVLM were investigated. A microdialysis experiment showed that microinjection of angiotensin II into the RVLM significantly (P < 0.01) increased the release of aspartate and glutamate in the intermediolateral column of the spinal cord. Immunofluorescence technique combined with confocal microscopy demonstrated that most of the glutamatergic and GABAergic neurons in the RVLM of both Wistar and spontaneously hypertensive rats (SHR) were double labeled with ANG type 1 (AT1) receptor. Immunocytochemical studies demonstrated that the mean optic density of AT1 receptor of the cell surface as well as the whole cell was higher (P < 0.05) in SHR than that in Wistar rats, indicating that the higher expression of AT1 receptors in the RVLM may contribute to the higher responsiveness of SHR to ANG II stimulation. Immunogold staining and electronmicroscopic study demonstrated that AT1 receptor in the RVLM was distributed on the rough endoplasmic reticulum, cell membrane, and nerve processes. The results suggest that effects evoked by ANG II in the RVLM are closely related to glutamatergic and GABAergic pathways. These results indirectly support the hypothesis that ANG II in the RVLM may activate vasomotor sympathetic glutamatergic neurons, leading to an increase in sympathetic nerve activity and arterial blood pressure.

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.

Impact of the renin-angiotensin system on cerebral perfusion following subarachnoid haemorrhage in the rat

1. This study investigated the effects of blocking the AT1 angiotensin receptors with irbesartan, either peripherally or centrally, on systemic blood pressure, intracranial pressure and cerebral perfusion pressure following experimental subarachnoid haemorrhage (SAH) in urethane-anaesthetized rats. Sympathetic nervous activation was determined by measuring plasma noradrenaline levels. 2. In untreated animals, SAH induced a sustained increased in intracranial pressure from 2.1 +/- 0.3 to 16 +/- 2 mmHg (3 h, P < 0.001). Cerebral perfusion pressure was reduced by 20 % (P < 0.001), this reduction being maintained for 3 h. Sympathetic activation was evident in the high level of plasma noradrenaline measured 3 h post-SAH (751 +/- 104 vs. 405 +/- 33 pg ml(-1), P < 0.05). 3. Acute peripheral pretreatment with irbesartan (3 mg kg(-1), I.V.) prevented the rise in plasma noradrenaline and further aggravated the decrease in cerebral perfusion pressure by producing transient systemic hypotension (blood pressure was 85 +/- 6 mmHg at 2 h post-SAH vs. 100 +/- 3 mmHg, P < 0.01). 4. Intracisternal pretreatment with irbesartan (0.035 mg) did not prevent the rise in plasma noradrenaline post-SAH but enhanced the rise in intracranial pressure by 75 % compared with untreated animals. 5. This study demonstrates that peripheral endogenous angiotensin II interacts with the sympathetic nervous system in order to maintain an adequate cerebral perfusion following SAH. Endogenous angiotensin II in the brain seems to exert a protective effect by counteracting the elevation in intracranial pressure that occurs following experimental SAH.

AT1 receptor block does not affect arterial baroreflex during pregnancy in rabbits

The role of ANG II in the arterial baroreflex control of renal sympathetic nerve activity (RSNA) in eight term-pregnant (P) and eight nonpregnant (NP) conscious rabbits was assessed using sequential intracerebroventricular and intravenous infusions of losartan, an AT1 receptor antagonist. The blood pressure (BP)-RSNA relationship was generated by sequential inflations of aortic and vena caval perivascular occluders. Pregnant rabbits exhibited a lower maximal RSNA reflex gain (-44%) that was primarily due to a reduction in the maximal sympathetic response to hypotension (P, 248 +/- 20% vs. NP, 357 +/- 41% of rest RSNA, P < 0.05). Intracerebroventricular losartan decreased resting BP in P (by 9 +/- 3 mmHg, P < 0.05) but not NP rabbits, and had no effect on the RSNA baroreflex in either group. Subsequent intravenous losartan decreased resting BP in NP and further decreased BP in P rabbits, but had no significant effect on the maximal RSNA reflex gain. ANG II may have an enhanced role in the tonic support of BP in pregnancy, but does not mediate the gestational depression in the arterial baroreflex control of RSNA in rabbits.

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.

Increased central AT(1)-receptor activation, not systemic vasopressin, sustains hypertension in ANP knockout mice

We tested the hypothesis that hypertension in atrial natriuretic peptide (ANP) knockout mice is caused in part by disinhibition of angiotensin II-mediated vasopressin release. Inactin-anesthetized F(2) homozygous ANP gene-disrupted mice (-/-) and wild-type (+/+) littermates were surgically prepared for carotid arterial blood pressure measurement (ABP) and background intravenous injection of physiological saline or vasopressin V(1)-receptor antagonist (Manning compound, 10 ng/g body wt) and subsequent intracerebroventricular (left lateral ventricle) injection of saline (5 microl) or ANP (0.5 microg) or angiotensin II AT(1)-receptor antagonist losartan (10 microg). Only (-/-) showed significant decrease in ABP after intracerebroventricular ANP or losartan. Both showed significant hypotension after intravenous V(1) antagonist, but there was no difference between their responses. We conclude that 1) vasopressin contributes equally to ABP maintenance in ANP-disrupted mice and wild-type controls; 2) permanently elevated ABP in ANP knockouts is associated with increased central nervous angiotensin II AT(1)-receptor activation; 3) disinhibition of central nervous angiotensin II AT(1) receptors in ANP-deficient animals does not lead to a significant increase in the importance of vasopressin as a mechanism for blood pressure maintenance.

Effects of central infusion of ANG II and losartan on the cardiac baroreflex in rabbits

The effect of chronic activation or inhibition of central ANG II receptors on cardiac baroreflex function in conscious normotensive rabbits was examined. Animals received a fourth ventricular (4V) infusion of ANG II (30 and 100 ng/h), losartan (3 and 30 microg/h), or Ringer solution (2 microl/h) for 2 wk. After 1 and 2 wk, ANG II (100 ng/h) decreased cardiac baroreflex gain by 20 and 37%, respectively (P = 0.015), whereas losartan (30 microg/h) increased baroreflex gain by 24 and 58%, respectively (P = 0.02). Within 1 wk of the end of the infusions, cardiac baroreflex gain had returned to control. Ringer solution or the lower doses of ANG II or losartan did not modify the cardiac baroreflex function. Blood pressure and heart rate were not altered by any treatment, nor was their variability affected. These data demonstrate a novel long-term modulation of cardiac baroreflexes by endogenous ANG II that is independent of blood pressure level.