Central angiotensin II receptors mediate hemodynamic response variability to stressors

We examined whether ANG II receptors in the central nervous system mediate hemodynamic responses to pharmacological (cocaine) and behavioral (cold water) stressors. After administration of cocaine (5 mg/kg iv), rats were classified as vascular responders (VR) if their pressor response was due entirely to an increase in systemic vascular resistance (SVR) despite a decrease in cardiac output (CO). Cocaine elicited a pressor response in mixed responders (MR) that was dependent on small increases in both SVR and CO. ANG II (30 ng/5 microl icv, 5 min before cocaine) augmented the decrease in CO in VR and prevented the increase in CO in MR. Administration of [Sar(1),Thr(8)]ANG II (20 microg/5 microl icv; sarthran) before cocaine attenuated the decrease in CO and the large increase in SVR in VR so that they were no longer different from MR. Losartan (20 microg icv) or captopril (50 microg icv) preceding cocaine administration also attenuated the decrease in CO and the large increase in SVR seen in VR only. The role of angiotensin was not specific for cocaine, because ANG II (icv) pretreatment before startle with cold water (1 cm deep) enhanced the decrease in CO and the increase in SVR in both MR and VR, whereas losartan (icv) pretreatment before startle attenuated the decrease in CO and the increase in SVR in VR so that they were no longer different from MR. These data suggest that central ANG II receptors mediate the greater vascular and cardiac responsiveness in vascular responders to acute pharmacological and behavioral stressors.     

Central and peripheral administration of amylin induces energy expenditure in anesthetized rats

Amylin is a peptide hormone that is co-released with insulin from pancreatic β-cells following a meal. Intracerebroventricular (icv) administration of amylin (1–100 pmol), or an amylin agonist, salmon calcitonin, elicited dose-dependent thermogenic, tachycardic, and hyperthermic responses in urethane-anesthetized rats. Intravenous (iv) administration of higher doses of amylin (100 pmol–20 nmol) also induced similar responses, although the amplitudes of these responses were significantly smaller than those elicited by icv administration, suggesting the primary action of amylin to be in the brain. However, the iv administration of amylin induced the responses slightly faster than the icv injection, the former responses occurring <4 min and the latter, at 8–10 min, after the administration. The iv but not the icv injection of amylin increased the respiratory exchange ratio transiently (<20 min), though the thermogenic response lasted for a longer period after both injections, indicating a shift from mixed fuel to predominantly carbohydrate utilization in the initial phase of thermogenesis induced by the iv injection of amylin. The differences in substrate utilization and latency of the responses suggest that the actions of amylin include partly different targets when administered centrally and peripherally. Moreover, pretreatment with a β-adrenergic blocker, propranolol (5 mg kg⁻¹, iv), blocked all responses elicited by either icv or iv administration of amylin, whereas ablation of the area postrema in the hindbrain did not influence the effects of icv-administered amylin. These results suggest the involvement of amylin in postprandial energy expenditure, mediated by peripheral β-adrenoceptors.     

Angiotensin II-induced release of oxytocin: interaction with norepinephrine and role in lactation

These studies examined the receptors involved in angiotensin II (Ang II) stimulated secretion of systemic oxytocin (OT) and the role of this peptide in release of OT during suckling. Plasma OT concentrations were measured following intracerebroventricular (icv) injection of vehicle, Ang II, or Ang II following pretreatment with a selective AT1 (Losartan) or AT2 (PD 123319) receptor antagonist. Furthermore, we measured Ang II-induced OT release during central alpha-adrenergic receptor blockade (phentolamine). Finally, plasma OT concentrations before and during suckling were evaluated following central administration of Ang II receptor antagonists. The increase in systemic OT following central Ang II was abolished by AT1 receptor blockade and inhibited by the AT2 receptor antagonist. Furthermore, pretreatment with phentolamine significantly diminished systemic OT release in response to icv Ang II. Finally, central Ang II receptor blockade did not alter the increase in circulating OT during suckling. These data demonstrate that Ang II evoked OT release is mediated through activation of both AT1 and AT2 receptors and suggest that a component of Ang II-induced OT stimulation is due to norepinephrine release. Furthermore, central angiotensin systems do not have a direct role in stimulating OT release during suckling.     

Role of endogenous opioids and histamine in morphine induced emesis

The role of opioid and histaminergic system in morphine induced emesis was investigated in dogs. Morphine (25 micrograms, icv) consistently evoked emesis with an average latency of 195 +/- 29 sec which was fully accounted for by an action on the chemoreceptor trigger zone (CTZ) as its ablation rendered animals refractory to vomiting. Intraventricular pretreatment with opioid antagonist naloxone, histamine H1 antagonist mepyramine and H2 antagonists metiamide and cimetidine afforded protection to icv morphine emesis. The CSF histamine concentration was significantly raised 5 min after icv morphine administration. The results suggest that both endogenous opioid and histamine are involved in morphine emesis. Naloxone in high doses (1600 micrograms, icv) elicited emesis which was not blocked by CTZ ablation confirming our earlier report.     

Role of thromboxane receptors in the dipsogenic response to central angiotensin II

Central angiotensin II (ANG II) regulates thirst. Because thromboxane A2-prostaglandin H2 (TP) receptors are expressed in the brain and mediate some of the effects of ANG II in the vasculature, we investigated the hypothesis that TP receptors mediate the drinking response to intracerebroventricular (icv) injections of ANG II. Pretreatment with the specific TP-receptor antagonist ifetroban (Ifet) decreased water intake with 50 ng/kg icv ANG II (ANG II + Veh, 7.2 +/- 0.7 ml vs. ANG II + Ifet, 2.8 +/- 0.8 ml; n = 5 rats; P < 0.001) but had no effect on water intake induced by hypertonic saline (NaCl + Veh, 8.4 +/- 1.1 ml vs. NaCl + Ifet, 8.9 +/- 1.8 ml; n = 5 rats; P = not significant). Administration of 0.6 microg/kg icv of the TP-receptor agonist U-46,619 did not induce drinking when given alone but did increase the dipsogenic response to a near-threshold dose of 15 ng/kg icv ANG II (ANG II + Veh, 1.1 +/- 0.7 vs. ANG II + U-46,619, 4.5 +/- 0.9 ml; n = 5 rats; P < 0.01). We conclude that central TP receptors contribute to the dipsogenic response to ANG II.     

Overlapping distributions of receptors for atrial natriuretic peptide and angiotensin II visualized by in vitro autoradiography: morphological basis of physiological antagonism

Atrial natriuretic peptides exert actions on many key organs involved in blood pressure and water and electrolyte balance. Many of these actions result in a physiological antagonism of angiotensin. To investigate the morphological basis of this interaction, we have mapped the distribution of receptors for atrial natriuretic peptide and angiotensin II in a number of target organs, using 125I-labelled rat atrial natriuretic peptide (99-126) and 125I-labelled [Sar1,Ile8]angiotensin II. In the kidney both atrial natriuretic peptide and angiotensin II receptors were observed overlying glomeruli, vasa recta bundles (high densities), and the outer cortex (moderate density). In the other tissues studied, atrial natriuretic peptide and angiotensin II receptors were codistributed in the adrenal zona glomerulosa, cerebral circumventricular organs including the subfornical organ, organum vasculosum of the lamina terminalis and area postrema, and the external plexiform layer of the olfactory bulb. The concurrent distribution of specific receptors for both peptides at these sites provides the basis for atrial natriuretic peptide to exert a functional antagonism of the actions of angiotensin II on blood pressure and water and electrolyte homeostasis at multiple sites.     

Action sites of adrenomedullin in the rat brain: functional mapping by Fos expression.

The effects of intracerebroventricular (icv) administration of adrenomedullin (AM) and proadrenomedullin NH2-terminal 20 peptide (PAMP) on the expression of Fos in the central nervous system (CNS) were examined in conscious rats, using immunohistochemistry. Fos-like immunoreactivity (LI) was detected in various brain areas of the rats, including the supraoptic nucleus, the paraventricular nucleus, the locus coeruleus, the area postrema and the nucleus of the tractus solitarius 90 min after icv administration of AM. Few cells with Fos-LI were found in the CNS 90 min after icv administration of saline. Fos-LI was also detected in the various hypothalamic areas after icv administration of PAMP. These results suggest that centrally administered AM and PAMP may cause physiological responses through the activation of a neural network in the hypothalamus and the brainstem.     

Nodose ganglionectomy reduces angiotensin II receptor binding in the rat brainstem

Angiotensin II (Ang II) receptor binding sites in the dorsomedial medulla of intact and unilaterally nodose ganglionectomized rats were identified and characterized using 125I-sarcosine,isoleucine Ang II. This radioligand bound saturably and with high affinity to rat brain homogenates and to sections of rat brainstem. Specific (1 microM angiotensin II displaceable) binding of 125I-sarcosine,isoleucine Ang II was displaced by angiotensin analogues with a potency order similar to that described for angiotensin II receptors. Unilateral nodose ganglionectomy caused a reduction in Ang II receptor binding in the medial solitary tract nucleus, dorsal motor nucleus of the vagus, and area postrema ipsilateral to the lesioned ganglion. This observation suggests that Ang II receptors in the dorsomedial medulla may be located on axon terminals of vagal afferents and cell bodies of vagal efferents.     

大鼠脑室注射高渗氯化钠和蔗糖诱导饮水行为和脑内c—fos的表达

用ＳＤ种系清醒大鼠,观察脑室注射高渗物质引起的饮水及ｃ－ｆｏｓ在脑内的表达部位。实验结果表明,脑室内微量注射１．５ｍｏｌ／Ｌ、３ｍｏｌ／Ｌ ＮａＣｌ或３ｍｏｌ／Ｌ蔗糖均可诱导饮水反应,并在前脑的终板血管器官、正中视前核和下丘脑视上核与室旁核中见到Ｆｏｓ样免疫反应阳性细胞,同样在后脑的最后区、臂旁外侧核与孤束核中也能见到Ｆｏｓ样免疫反应阳性细胞,同样在后脑的最后区、臂旁外侧核与孤束核中也能见到Ｆｏｓ     

Role of angiotensin II and corticotropin-releasing hormone in hemodynamic responses to cocaine and stress

Cocaine produces characteristic behavioral and autonomic responses due to its unique pharmacological properties. Many of the autonomic responses resemble those to acute behavioral stress. Both cocaine and behavioral stress have been shown to evoke an increase in sympathetic nerve activity that is primarily responsible for the peripheral cardiovascular responses. We noted varying hemodynamic and sympathetic response patterns to cocaine administration and to acute behavioral stress in rats that correlate with the predisposition to develop both a sustained increase in arterial pressure and cardiomyopathies. Several lines of evidence suggest that the autonomic response patterns are dependent on the actions of central peptides including angiotensin II (Ang II) and corticotropin-releasing hormone (CRH). This is based on observations demonstrating that intracerebroventricular (icv) administration of receptor antagonists for Ang II or CRH attenuated the decrease in cardiac output (CO) and increase in vascular resistance noted in some animals after cocaine administration or startle. In contrast, icv Ang II enhances the cardiodepression associated with cocaine administration or startle. Based on this and other evidence, we propose that the autonomic response patterns to startle and to cocaine are closely related and dependent on central Ang II and CRH. Furthermore, we suggest that these central peptides may be responsible for varying predisposition to cardiovascular disease.     

α1-Adrenergic Receptor-Mediated Downregulation of Angiotensin II Receptors in Neuronal Cultures

Previous evidence has suggested that brain catecholamine levels are important in the regulation of central angiotensin II receptors. In the present study, the effects of norepinephrine and 3,4-dihydroxyphenylethylamine (dopamine) on angiotensin II receptor regulation in neuronal cultures from rat hypothalamus and brainstem have been examined. Both catecholamines elicit significant decreases in [125I]angiotensin II-specific binding to neuronal cultures prepared from normotensive rats, effects that are dose dependent and that are maximal within 4-8 h of preincubation. Saturation and Scatchard analyses revealed that the norepinephrine-induced decrease in the binding is due to a decrease in the number of angiotensin II receptors in neuronal cultures, with little effect on the receptor affinity. Norepinephrine has no significant actions on [125I]angiotensin II binding in cultures prepared from spontaneously hypertensive rats. The downregulation of angiotensin II receptors by norepinephrine or dopamine is blocked by alpha 1-adrenergic and not by other adrenergic antagonists, a result suggesting that this effect is initiated at the cell surface involving alpha 1-adrenergic receptors. This is further supported by our data indicating a parallel downregulation of specific alpha 1-adrenergic receptors elicited by norepinephrine. In summary, these results show that norepinephrine and dopamine are able to alter the regulation of neuronal angiotensin II receptors by acting at alpha 1-adrenergic receptors, which is a novel finding.     

Chronic sodium depletion suppresses the area postrema pressor pathway

Sodium depletion in dogs is known to affect both the renin-angiotensin as well as the sympathetic nervous system. The effect of this dietary regime upon the area postrema pressor pathway, as evaluated by the cardiovascular responses to centrally acting angiotensin II, has not been determined previously. With this in mind, male mongrel dogs were maintained on either a normal or a sodium restricted diet supplemented with furosemide and dose-response curves for intravertebral and intravenous angiotensin II (range: 1-20 ng/kg/min) were obtained. Sodium depletion results in not only a blunted intravenous pressor response to angiotensin II but also the abolition of the centrally mediated pressor responses mediated by the area postrema. Because accumulating evidence indicates that in sodium depleted dogs sympathetic nerve activity is reduced while central noradrenergic inhibitory activity is increased the reduced effects of angiotensin II upon the central sympathetically mediated pressor response may in part be related to decreases in sympathetic nerve activity.     

Effects of morphine and opioid peptides on plasma levels of atrial natriuretic peptide

The effect of opiate ligand administration on plasma levels of atrial natriuretic peptide (ANP) was studied in awake, freely moving Sprague-Dawley rats. Prior to and following the intracerebroventricular (icv) or central venous (iv) injection of morphine (MS), leu-enkephalin (Leu-enk), dynorphin (Dyn) or beta-endorphin (B-endor), plasma samples were obtained for measurement of ANP concentrations by radioimmunoassay. MS was 10 times more potent when given icv than when given iv to increase plasma ANP levels. Icv injection of Leu-enk decreased plasma ANP concentrations. Dyn and B-endor administration (iv or icv) did not alter the plasma concentration of ANP. These effects of MS and Leu-enk on plasma concentrations of ANP appear to be mediated through actions on the central nervous system. MS, Leu-enk, B-endor, and Dyn given icv, produced elevations of plasma norepinephrine (NE) and epinephrine (Epi) concentrations. When MS was given icv, mean Epi and NE plasma levels increased 10-50 times the increases noted with B-endor, Leu-enk and Dyn. A role of catecholamines in mediating MS-stimulated ANP release is supported by the observation that ganglionic blockade with chlorisondamine significantly attenuated the increase of plasma ANP levels. MS, but not B-endor, Leu-enk and Dyn, acts within the brain to increase plasma levels of ANP. MS-induced elevations of plasma ANP levels may be dependent on an intact autonomic nervous system.     

Central angiotensin II stimulates arteriolar vasomotion in conscious hamsters

Summary The effects of intracerebroventricular (icv) injections of 10 ng angiotensin II (ANG II) on mean arteriolar diameter and spontaneous arteriolar vasomotion were studied in subcutaneous tissue of conscious, restrained hamsters, using the skin fold window chamber preparation. Angiotensin II caused a significant decrease in mean arteriolar diameter which was associated with a significant elevation in the amplitude of vasomotion. The frequency of vasomotion did not change significantly. The central ANG II-induced effects on arteriolar vasomotion were not significantly altered by continuous intravenous (iv) infusion of hexamethonium (1 mg · kg^–1 · min^–1). In contrast, iv bolus injection of the vascular vasopressin receptor antagonist d(CH₂)₅Tyr(Me)AVP (10 g · kg^–1) 5 min prior to icv injection of ANG II significantly attenuated the effects of the neuropeptide on mean arteriolar diameter and the amplitude of vasomotion. These data indicate that central ANG II stimulation enhances arteriolar vasomotion in peripheral subcutaneous tissue of conscious hamsters and that this effect may be mediated by release of vasopressin.     

Anesthesia alters pulmonary vasoregulation by angiotensin II and captopril.

We investigated the effects of an intravenous (pentobarbital sodium) and inhalational (halothane) general anesthetic on the pulmonary vascular responses to angiotensin II and angiotensin-converting enzyme inhibition (CEI). Multipoint pulmonary vascular pressure-flow (P/Q) plots were generated in conscious pentobarbital- (30 mg/kg iv) and halothane-anesthetized (approximately 1.2% end-tidal) dogs in the intact (no drug) condition, during angiotensin II administration (60 ng.kg-1.min-1 iv), and during CEI (captopril 1 mg/kg plus 1 mg.kg-1.h-1 iv). In conscious dogs, angiotensin II increased (P less than 0.001) the pulmonary vascular pressure gradient [pulmonary arterial pressure--pulmonary arterial wedge pressure (PAP-PAWP)] over the empirically measured range of Q; i.e., angiotensin II caused pulmonary vasoconstriction. Pulmonary vasoconstriction (P less than 0.01) in response to angiotensin II was also observed during pentobarbital sodium anesthesia. In contrast, angiotensin II had no effect on the P/Q relationship during halothane anesthesia. In conscious dogs, CEI decreased (P less than 0.001) PAP-PAWP over the empirically measured range of Q; i.e., CEI caused pulmonary vasodilation. However, CEI caused pulmonary vasoconstriction (P less than 0.02) during pentobarbital sodium and had no effect on the P/Q relationship during halothane. Thus, compared with the conscious state, the pulmonary vasoconstrictor response to angiotensin II is unchanged or abolished, and the pulmonary vasodilator response to CEI is reversed to vasoconstriction or abolished during pentobarbital sodium and halothane anesthesia, respectively.     

Regulation of angiotensin II binding sites in the subfornical organ and other rat brain nuclei after water deprivation

1. Binding sites for angiotensin II have been localized in forebrain and brain-stem areas of water-deprived and control Sprague-Dawley rats, employing autoradiography with computerized microdensitometry. 2. Angiotensin II receptor sites were identified in the organum vasculosum of the lamina terminalis, subfornical organ, paraventricular nucleus, median preoptic nucleus, area postrema, nucleus of the solitary tract, and inferior olive. 3. After dehydration a significant increases in the concentration of angiotensin II receptors was detected only in the subfornical organ. Although there was an increased concentration of angiotensin II binding sites in the organum vasculosum of the lamina terminalis, the median preoptic nucleus, and the paraventricular nucleus after dehydration, these changes did not reach statistical significance. Other brain nuclei investigated did not show differences in angiotensin II binding sites in the dehydrated rats compared to controls. 4. These results indicate that angiotensin II receptors in the subfornical organ may play an important role in fluid homeostasis during dehydration.     

Central Tempol alters basal sympathetic nerve discharge and attenuates sympathetic excitation to central ANG II

In the present study, we established dose-response relationships between central administration of 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl (Tempol, a superoxide dismutase mimetic) and the level of renal sympathetic nerve discharge (SND) and tested the hypothesis that intracerebroventricular (icv) Tempol pretreatment would attenuate centrally mediated changes in SND produced by icv ANG II administration. Urethane-chloralose-anesthetized, baroreceptor-denervated, normotensive rats were used. We found that icv Tempol administration produced dose-dependent sympathoinhibitory, hypotensive, and bradycardic responses. Mean arterial pressure and SND values were significantly increased after icv ANG II (150 ng/kg) administration, and these responses were abrogated after icv pretreatment with Tempol (75 micromol/kg) or losartan. Brain superoxide levels tended to be higher in ANG II-treated rats compared with rats treated with Tempol and ANG II. Tempol pretreatment did not prevent increases in SND level that were produced by acute heat stress, which indicates specificity in the effect of Tempol in reducing sympathoexcitation. These results demonstrate that icv Tempol administration influences central sympathetic neural circuits in a dose-dependent manner and attenuates SND responses to central ANG II infusion.     

The renin-angiotensin system and the central nervous system.

One of several factors affecting the secretion of renin by the kidneys is the sympathetic nervous system. The sympathetic input is excitatory and is mediated by beta-adrenergic receptors, which are probably located on the membranes of the juxtaglomerular cells. Stimulation of sympathetic areas in the medulla, midbrain and hypothalamus raises blood pressure and increases renin secretion, whereas stimulation of other parts of the hypothalamus decreases blood pressure and renin output. The centrally active alpha-adrenergic agonist clonidine decreases renin secretion, lowers blood pressure, inhibits ACTH and vasopressin secretion, and increases growth hormone secretion in dogs. The effects on ACTH and growth hormone are abolished by administration of phenoxybenzamine into the third ventricle, whereas the effect on blood pressure is abolished by administration of phenoxybenzamine in the fourth ventricle without any effect on the ACTH and growth hormone responses. Fourth ventricular phenoxybenzamine decreases but does not abolish the inhibitory effect of clonidine on renin secretion. Circulating angiotensin II acts on the brain via the area postrema to raise blood pressure and via the subfornical organ to increase water intake. Its effect on vasopressin secretion is debated. The brain contains a renin-like enzyme, converting enzyme, renin substrate, and angiotensin. There is debate about the nature and physiological significance of the angiotensin II-generating enzyme in the brain, and about the nature of the angiotensin I and angiotensin II that have been reported to be present in the central nervous system. However, injection of angiotensin II into the cerebral ventricles produces drinking, increased secretion of vasopressin and ACTH, and increased blood pressure. The same responses are produced by intraventricular renin. Angiotensin II also facilitates sympathetic discharge in the periphery, and the possibility that it exerts a similar action on the adrenergic neurons in the brain merits investigation.     

Blood pressure responsiveness during the development of hypertension in the conscious spontaneously hypertensive rat

Blood pressure responsiveness to iv noradrenaline and angiotensin II was studied in conscious, freely moving, age-matched spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats from 4 to 16 weeks of age. At 4 and 6 weeks the SHR showed small, but nonsignificant increases in responsiveness compared with WKY to both noradrenaline and angiotensin II. At 8 weeks they exhibited similar responses to the WKY. Subsequently, at 12 and 16 weeks decreased responsiveness to noradrenaline (nonsignificant) and angiotensin II (p less than 0.05 at 12 and 16 weeks) was observed in SHR versus WKY. At 16 weeks of age, hexamethonium caused potentiation of the blood pressure response to noradrenaline and angiotensin II, but to the same degree in the two strains. Captopril at this age did not elicit potentiation to noradrenaline or angiotensin II in either strain. These results indicate that there is no rise in blood pressure responsiveness to circulating pressor agents, parallel to the development of hypertension in SHR. Increased receptor occupancy or more active attenuating reflexes in SHR versus WKY appear not to be involved in the absence of hyperresponsiveness in intact conscious SHR at 16 weeks of age.     

Quantitative distribution of angiotensin II binding sites in rat brain by autoradiography

Angiotensin II binding sites were localized and quantified in individual brain nuclei from single rats by incubation of tissue sections with 1 nM 125I-[Sar1]-angiotensin II, [3H]-Ultrofilm autoradiography, computerized microdensitometry and comparison with 125I-standards. High angiotensin II binding was present in the circumventricular organs (organon vasculosum laminae terminalis, organon subfornicalis and area postrema), in selected hypothalamic nuclei (nuclei suprachiasmatis, periventricularis and paraventricularis) and in the nucleus tractus olfactorii lateralis, the nucleus preopticus medianus, the dorsal motor nucleus of the vagus and the nucleus tractus solitarii. High affinity (KA from 0.3 to 1.5 X 10(9) M-1) angiotensin II binding sites were demonstrated in the organon subfornicalis, the nucleus tractus solitarii and the area postrema after incubation of consecutive sections from single rat brains with 125I-[Sar1]-angiotensin II in concentrations from 100 pM to 5 nM. These results demonstrate and characterize brain binding sites for angiotensin II of variable high affinity binding both inside and outside the blood-brain barrier.