Regulation of Angiotensinogen in the Central Nervous System

Several interventions known to alter plasma renin substrate in rats such as nephrectomy (NX), adrenalectomy (ADX) and glucocorticoid treatment changed the angiotensinogen content in the cerebrospinal fluid (CSF) in the same direction. However, peripheral and central angiotensinogen could be dissociated from each other by ADX and NX in combination, as well as by chronic converting enzyme blockade.

The regulation of brain angiotensinogen was further investigated in stroke-prone spontaneously hypertensive rats (SHR-sp) in comparison with normotensive Wistar Kyoto (WKY) rats. The angiotensinogen levels of the anterior hypothalamus and of the septal area showed strain and age-related differences. Chronic converting enzyme blockade, which kept SHR-sp normotensive, stimulated angiotensinogen in the anterior hypothalamus of both SHR-sp and WKY rats, but suppressed plasma renin substrate. A specific radioimmunoassay (RIA) for renin substrate of rat     

Delivery of angiotensin II type 1 receptor antisense inhibits angiotensin action in neurons from hypertensive rat brain.

Increased brain angiotensin II (AII) type 1 receptor (AT1R) expression has been implicated in the hyperactive brain angiotensin system and the development and maintenance of hypertension in the genetically spontaneously hypertensive (SH) rat. Neuronal cells in primary culture from the cardioregulatory-relevant brain areas (hypothalamus/brainstem) mimic increased brain AT1R gene expression and AT1R function of the adult SH rat. They have been utilized in the present study to determine whether cellular actions of AII could be regulated by the transfer of AT1R antisense (AT1R-AS) with the use of a retroviral-mediated gene delivery system developed for the central nervous system cultures. AII stimulates norepinephrine (NE) uptake in neuronal cultures of both normotensive (Wistar Kyoto) and SH rat brains. This neuromodulatory action is mediated by the AT1R subtype, is significantly higher in SH neurons, and is associated with a parallel stimulation of mRNAs for c-fos and NE transporter. Infection of neuronal cultures with a retrovirus vector that contains AT1R-AS (LNSV-AT1R-AS) results in an inhibition of AT1R-mediated stimulation of both c-fos and NE transporter mRNA, as well as NE uptake in both strains of rats; however, the inhibition is more pronounced in SH neurons compared with Wistar Kyoto rat brain neurons. The higher sensitivity of the SH rat brain neurons is further supported by our observation that a certain dose of LNSV-AT1R-AS that fails to induce inhibition of cellular actions of AII in WKY neurons causes a significant inhibition of AII actions in SH neurons. These observations show that retrovirally mediated delivery of AT1R-AS could be used to selectively control the actions of AII in primary neuronal cultures from SH rat brain.     

Angiotensin-like immunoreactivity in the brain of the spontaneously hypertensive rat

Evidence for the brain renin-angiotensin system being involved in the hypertension of the spontaneously hypertensive rat (SHR) includes central administration of angiotensin II (AII) antagonists and converting enzyme inhibitors that lower blood pressure in SHR. Using the unlabeled antibody enzyme method, we have found a significant difference in the distribution of brain angiotensin in SHR and Wistar-Kyoto controls (WKY). Six rats of each group were perfused with buffered picric acid-paraformaldehyde, and their brains sectioned at 50 and 100 mu. The sections were reacted with a 1:1000 dilution of AII antiserum for 36 hours followed by goat antirabbit immunoglobulin G and rabbit peroxidase antiperoxidase. For controls, preabsorption with AII, arginine vasopressin or preimmune serum were evaluated. The results showed over twice as many cells and fibers staining for AII-like immunoreactivity in SHR. The AII immunoreactive cell bodies were localized, in the order of their relative preponderance, in supraoptic and paraventricular nuclei of the hypothalamus, hippocampus, and cortex. The most prominent demonstration of AII-like immunoreactivity was observed in fiber profiles containing densely stained varicosities, which were present in many neuroanatomical subdivisions of the brain and brain stem including anterior and middle hypothalamus, basal ganglia, thalamus, locus coeruleus, nucleus of the solitary tract, limbic structures, and reticular formation. The increased fiber staining in the SHR was particularly evident in the frontal hypothalamic region, medial preoptic, and stria terminalis. We conclude that the results support the hypothesis of brain AII involvement in hypertension.     

Angiotensinogen levels in the brain and cerebrospinal fluid of the genetically hypertensive rat

The present experiments were designed to document changes in the regional distribution of angiotensinogen in the rat brain with the development of hypertension in spontaneously hypertensive rats (SHR) relative to age-matched normotensive Wistar-Kyoto rats (WKY). Levels of angiotensinogen were measured in discrete brain nuclei and cerebrospinal fluid from rats at 4, 7, and 16 weeks of age and in cerebrospinal fluid obtained by cisternal puncture at 7 and 16 weeks. Age-dependent changes in angiotensinogen were found, with levels higher in both strains at 4 weeks of age compared with 7 or 16 weeks. In contrast, plasma levels of angiotensinogen were essentially the inverse of the brain levels, low at 4 weeks and higher at 7 and 16 weeks. Levels in a number of regions adjacent to the rostral third ventricle from the 4-week-old SHR (prehypertensive phase) were significantly elevated relative to the WKY (p less than 0.05), while levels in the amygdala and posterior hypothalamus were significantly lower in the SHR (p less than 0.05). In 7-week-old rats (evolving phase), levels in nine brain regions were significantly elevated in the SHR relative to the WKY and included the nucleus tractus solitarii (p less than 0.01). Unlike the prehypertensive and evolving phases, in 16-week-old rats (maintenance phase) only two brain areas, the nucleus of the diagonal band and the lateral hypothalamus, had significantly elevated levels in the SHR (p less than 0.05). Cerebrospinal fluid levels of angiotensinogen did not correlate well with brain levels of angiotensinogen.(ABSTRACT TRUNCATED AT 250 WORDS)     

Central and peripheral mechanisms of the enhanced hypertension following long-term salt loading in spontaneously hypertensive rats

We evaluated whether or not increased sodium (Na) concentrations of cerebrospinal fluid (CSF) and stimulated activities of brain renin-angiotensin system (RAS) contribute to an enhanced hypertension by salt overload in spontaneously hypertensive rats (SHR). Long-term salt loading (1% NaCl solution as drinking fluid) accelerated the development of hypertension in SHR, but did not alter the blood pressure (BP) in normotensive Wistar-Kyoto rats (WKY). CSF Na concentration was elevated in uninephrectomized (Nx) group as compared to that in control SHR, while in WKY CSF Na was not influenced by the treatment. A fall in BP by intravenous AVP antagonist or hexamethonium was greater in salt-loaded SHR than in controls. This hypotensive response to the combined blockade of AVP and SNS correlated with CSF Na in SHR but not in WKY. Plasma concentration of AVP and epinephrine tended to increase in relation to the degree of salt loading in SHR but not in WKY. Pressor responses to intracerebroventricular (ICV) angiotensin II (AII) and NaCl were greater in SHR than in WKY, although these responses were not influenced by chronic salt load in either SHR or WKY. The enhanced hypertensive action of ICV NaCl in SHR was abolished by pretreatment with ICV AII antagonist. Chronic saline drinking enhanced the depressor effect of ICV captopril in SHR but not in WKY. These observations suggest that salt overload in SHR may cause an elevated CSF Na concentration and an enhanced activity of brain RAS, which may increase activity of SNS and release of AVP, resulting in an enhanced development of hypertension.     

High NaCl diet reduces hypothalamic norepinephrine turnover in hypertensive rats

The current study tested the hypothesis that high NaCl diets elevate blood pressure in NaCl-sensitive spontaneously hypertensive rats (SHR-S) by reducing noradrenergic input to depressor neurons in the anterior hypothalamus. SHR-S were studied at 7 weeks of age, and age-matched salt resistant SHR (SHR-R) and normotensive Wistar-Kyoto rats (WKY) were controls. Rats were fed either high (8%) NaCl or control (1% NaCl) diets for 2 weeks, following which norepinephrine turnover in hypothalamus (anterior, posterior, and ventral regions), brainstem (pons and medulla), and thoracic spinal cord was assessed using the dopamine beta-hydroxylase inhibitor 1-cyclohexyl-2-mercapto-imidazole (CHMI). Regional brain catecholamines were measured by high performance liquid chromatography with electrochemical detection following intraperitoneal injection of CHMI or vehicle. Disappearance of norepinephrine following CHMI was used as an index of noradrenergic neuronal activity. The 8% NaCl diet caused a significant elevation in blood pressure in SHR-S but not in SHR-R or WKY. Endogenous norepinephrine levels and turnover were lower in the anterior hypothalamus of SHR-S fed 8% NaCl than in those fed 1% NaCl but were not significantly different in other groups. Endogenous norepinephrine levels and turnover were greater in pons of 8% NaCl--fed SHR-S than in those fed 1% NaCl but were not significantly different in other groups. These observations support the hypothesis that reduced noradrenergic input to depressor neurons in the anterior hypothalamus and increased noradrenergic input to neurons in the pons are related to NaCl sensitivity in the SHR-S.     

Brain corticotropin releasing factor in the spontaneously hypertensive rat

Corticotropin releasing factor and vasopressin were measured in major brain regions including the neurohypophysis in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY) during development of hypertension. The highest concentration of corticotropin releasing factor was found in the hypothalamus in both strains. Corticotropin releasing factor was decreased in most major brain regions of SHR. In the hypothalamus, corticotropin releasing factor was lower in 3- and 6-week-old SHR than in age-matched WKY (p less than 0.01), but was similar at 12 and 24 weeks of age. The content of corticotropin releasing factor did not differ in the neurohypophysis in 3-week-old rats but began to decrease at 6 weeks of age (p less than 0.01) and continued to decrease during the development of hypertension (p less than 0.01). Brain vasopressin concentration did not differ between SHR and WKY except in the hypothalamus. The level of hypothalamic vasopressin was consistently lower in SHR than in WKY (p less than 0.01). These peptides are thought to be associated with autonomic nervous regulation, and our results may further strengthen the possibility that the deficit of the peptides may be involved in the development of spontaneous hypertension.     

Cerebrovascular and brain microanatomy in spontaneously hypertensive rats with streptozotocin-induced diabetes

The influence of hypertension associated with diabetes on cerebrovascular and frontal cortex or hippocampus microanatomy was investigated in 20-week-old spontaneously hypertensive rats (SHR) in which diabetes was induced by treatment with streptozotocin (STZ) and in control or STZ-diabetic age-matched normotensive Wistar Kyoto (WKY) rats. At the beginning of experiment, systolic pressure values were similar in WKY rats either control, or exposed to STZ and remarkably higher in control or STZ-treated SHR. Systolic pressure values increased in the different animal groups examined along the course of experiment. Blood glucose levels were increased in either STZ-WKY rats or -SHR compared to WKY rats and SHR respectively. The main changes occurring in pial and intracerebral arteries of SHR and STZ-SHR were thickening of the arterial wall accompanied by luminal narrowing. In medium sized pial arteries of STZ-WKY rats luminal narrowing and a decreased thickness of arterial wall were noticeable. Intracerebral arteries of STZ-WKY diabetic rats showed a not homogeneous sensitivity of different sized branches. The volume of zones III and IV of frontal cortex was decreased in SHR and STZ-SHR compared to control WKY rats. The number of nerve cells in these cerebrocortical layers was decreased to a similar extent in SHR. STZ-WKY rats or STZ-SHR compared to control WKY rats. In dentate gyrus, followed by the CA1 subfield of hippocampus, decreased volume and number of neurons were found in SHR and STZ-SHR compared to control WKY rats. The occurrence of astrogliosis was observed in hypertensive, diabetic or hypertensive plus diabetic rats. The above findings indicate the occurrence of cerebrovascular and brain microanatomical changes in SHR and to a lesser extent in STZ-diabetic rats compared to control normotensive and normoglicemic WKY rats. Association of hypertension and diabetes caused more pronounced changes than in the single disease models. These results support the view that hypertension and diabetes affect the structure of cerebrovascular tree and of brain and that association of the two diseases results in an increased risk of target-organ damage, involving brain.     

The neural basis of salt sensitivity in the rat: altered hypothalamic function

Dietary NaCl supplementation in NaCl-sensitive spontaneously hypertensive rats (SHR-S) elevates blood pressure, increases peripheral sympathetic nervous system activity and depresses endogenous noradrenaline stores and noradrenaline release in the anterior hypothalamus. NaCl-resistant spontaneously hypertensive rats (SHR-R) and normotensive Wistar Kyoto (WKY) rats are resistant to the NaCl-induced alterations in blood pressure and central and peripheral noradrenergic activity, suggesting that the alterations observed in the SHR-S during NaCl loading are genetically mediated. The anterior hypothalamus is a major cardiovascular regulatory region, and depressor responses elicited by pharmacologic (alpha 2 adrenoceptor) stimulation of this area are exaggerated in SHR-S fed a high NaCl diet compared with SHR-S fed a basal diet and compared with SHR-R and WKY fed a high or basal NaCl diet. Membrane-binding techniques confirm that alpha 2 adrenoceptors in the anterior hypothalamic area are increased in number in SHR-S fed a high NaCl diet, presumably reflecting upregulation in response to reduced local noradrenaline release. These findings are consistent with the hypothesis that decreased noradrenergic activity of sympathoinhibitory neurons in the anterior hypothalamic area may mediate the exacerbation in hypertension that occurs in SHR-S during dietary NaCl supplementation.     

Hypertension-linked decrease in the expression of brain gamma-adducin

Gene profiling data coupled with adducin polymorphism studies led us to hypothesize that decreased expression of this cytosolic protein in the brain could be a key event in the central control of hypertension. Thus, our objectives in the present study were to (1) determine which adducin subunit gene demonstrates altered expression in the hypothalamus and brainstem (two cardioregulatory-relevant brain areas) in two genetic strains of hypertensive rats and (2) analyze the role of adducins in neurotransmission at the cellular level. All three adducin subunits (alpha, beta, and gamma) were present in the hypothalamus and brainstem of Wistar Kyoto (WKY) and spontaneously hypertensive (SH) rats. However, only the gamma-adducin subunit expression was 40% to 60% lower in the SH rat compared with WKY rat. A similar decrease in gamma-adducin expression was observed in the hypothalamus and brainstem of the renin transgenic rat compared with its normotensive control. Losartan treatment of the SH rat failed to normalize gamma-adducin gene expression. A hypertension-linked decrease of gamma-adducin was confirmed by demonstrating a decrease in gamma-adducin expression in hypothalamic/brainstem neuronal cultures from prehypertensive SH rats. Neuronal firing rate was evaluated to analyze the role of this protein in neurotransmission. Perfusion of a gamma-adducin-specific antibody caused a 2-fold increase in the neuronal firing rate, an effect similar to that observed with angiotensin II. Finally, we observed that preincubation of neuronal cultures for 8 hours with 100 nmol/L angiotensin II caused a 60% decrease in endogenous gamma-adducin and was associated with a 2-fold increase in basal firing rate. These observations support our hypothesis that a decrease in gamma-adducin expression in cardioregulatory-relevant brain areas is linked to hypertension possibly by regulating the release of neurotransmitters.     

NaCl does not affect hypothalamic noradrenergic input in deoxycorticosterone acetate/NaCl and Dahl salt-sensitive rats

Previous studies from our laboratories demonstrated that dietary NaCl supplementation in NaCl-sensitive spontaneously hypertensive rats elevates blood pressure, increases peripheral sympathetic nervous system activity, and depresses endogenous norepinephrine stores and turnover in the anterior hypothalamus. These findings suggest that reduced noradrenergic input to sympathoinhibitory neurons in anterior hypothalamus contributes to NaCl-sensitive hypertension in spontaneously hypertensive rats. The current study tested the hypothesis that dietary NaCl supplementation depresses endogenous norepinephrine stores and turnover in anterior hypothalamus of two other NaCl-sensitive models of hypertension, the Dahl salt-sensitive rat and the deoxycorticosterone acetate/NaCl hypertensive rat, thus increasing blood pressure by reducing noradrenergic input to the anterior hypothalamus. Dahl salt-sensitive rats were fed a high (8%) NaCl diet, and deoxycorticosterone acetate/NaCl rats rats drank 1% NaCl solution ad libitum for 2 or 4 weeks. Age-matched Dahl salt-sensitive rats fed a basal 1% NaCl diet and uninephrectomized Sprague-Dawley rats drinking tap water were controls. Regional brain catecholamines were determined by high-performance liquid chromatography with electrochemical detection. Norepinephrine turnover in hypothalamus (anterior, posterior, and ventral regions) and brain stem (pons and medulla) was assessed using the dopamine beta-hydroxylase inhibitor 1-cyclohexyl-2-mercapto-imidazole. High NaCl treatment caused significant elevations in blood pressure in Dahl salt-sensitive and deoxycorticosterone acetate/NaCl rats, but endogenous norepinephrine levels and turnover rates were not significantly different in anterior hypothalamus or any other brain region studied between the NaCl-supplemented and control groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of angiotensins injected into various brain areas on luteinizing hormone release in female rats

Angiotensin II (AII; 1.0 microliter of 10(-10) M) injected bilaterally into the anterior hypothalamus-preoptic area of the brain produced a 100% increase in mean whole blood LH concentrations in ovariectomized rats treated with estradiol and progesterone. Injections of equal volumes and concentrations of the peptide into the third cerebral ventricle or into the posterior hypothalamus did not affect blood LH values. The effects of AII administration into the rostral hypothalamus were dose-related and specific. Equimolar injections of angiotensin I or angiotensin III into this region did not significantly affect blood LH levels. Taken together, these data demonstrate that the anterior hypothalamus-preoptic area, compared to the third ventricle or more caudal brain sites, is particularly sensitive to the specific stimulatory actions of AII on LH release.     

Changes of retinal neurons and glial fibrillary acid protein immunoreactive astrocytes in spontaneously hypertensive rats

OBJECTIVE: The influence of arterial hypertension on retinal neurons and glial fibrillary acid protein (GFAP) immunoreactive astrocytes was investigated in spontaneously hypertensive rats (SHRs). METHODS: The retinas of 4- and 6-month-old SHRs and age-matched Wistar-Kyoto rats (WKY) were investigated. A group of SHRs, treated from 4 to 6 months with the hypotensive drug hydralazine, was also examined. Microanatomical and immunohistochemical techniques associated with image analysis and the terminal deoxyribonucleotidyl transferase (TdT)-mediated biotin-16-dUTP nick-end labelling (TUNEL) technique for apoptosis or necrosis were used, as well as astrocyte molecular biology (Western blot) techniques. RESULTS: In 4-month-old SHR and WKY rats, retinal morphology and the number of retinal neurons and of GFAP-immunoreactive astrocytes were similar, with the exception of the occurrence of 1% of TUNEL-positive ganglionic neurons in SHRs. In 6-month-old SHRs a decrease of retinal volume and of the number of ganglionic neurons and photoreceptors was observed, compared with age-matched normotensive WKY rats or younger SHR and WKY rats. Two per cent of ganglionic neurons and 5% of photoreceptors were also TUNEL positive. In 6-month-old SHRs, hypertrophic perivascular GFAP-immunoreactive astrocytes were found, whereas their number was unchanged compared to younger cohorts or WKY rats. An increased expression of GFAP was also noticeable in SHRs by Western blot analysis. Hypotensive treatment with hydralazine partly countered retinal changes occurring in SHRs. CONCLUSIONS: The occurrence of neuronal and astroglial changes when a stable hypertension was developed, and their sensitivity to antihypertensive treatment, suggest that they may represent a hypertension-related phenomenon.     

Tissue renin-angiotensin systems in renal hypertension.

Angiotensinogen messenger RNA (mRNA) levels were measured in the brain (hypothalamus, lower brain stem, cerebellum), liver, kidneys, and adrenal glands of rats made hypertensive by ligation of the aorta between the renal arteries. We also measured renin mRNA in the kidneys of these renal hypertensive rats. The early phase of hypertension (day 6) was associated with significant increases in plasma renin activity and levels of circulating angiotensin II. The circulating renin-angiotensin system was not activated in the later phase of hypertension (day 24). Angiotensinogen mRNA levels were elevated in the lower brain stem of hypertensive rats at both stages of hypertension. In contrast, angiotensinogen mRNA levels in the hypothalamus were increased only at day 6 after aortic ligation. Decreased levels of angiotensinogen mRNA were observed in the cerebellum in both the early and later phases of the hypertension. Angiotensinogen mRNA levels in the adrenal gland below the ligature fell in the early phases but rose in the later phases of hypertension. Renin mRNA levels of the ischemic kidney remained elevated at both the early and later phases, whereas in both ischemic and nonischemic kidneys, levels of angiotensinogen mRNA remained below sham values throughout the period of study. These results indicate differential expression of renin-angiotensin system mRNAs in tissues of renal hypertensive rats. The differential changes in the expression of angiotensinogen mRNA over the course of development and maintenance of renal hypertension suggest that factors in addition to angiotensin II are important in modulating the expression of renin-angiotensin system genes.     

Increased level of atrial natriuretic peptide messenger RNA in the hypothalamus and brainstem of spontaneously hypertensive rats.

OBJECTIVE: The aim of this study was to investigate atrial natriuretic peptide (ANP) gene expression in the central nervous system (CNS) during hypertension. METHODS: We measured and compared immunoreactive atrial natriuretic peptide (irANP) and ANP messenger RNA (mRNA) in the hypothalamus and brainstem of 17-week-old spontaneously hypertensive rats (SHR) with those of age-matched Wistar-Kyoto (WKY) rats using ribonuclease (RNase) protection assay for ANP mRNA and a specific radioimmunoassay for irANP. RESULTS: RNase protection assay revealed that the concentrations of ANP mRNA in the hypothalamus and brainstem of SHR were higher than those of WKY rats. IrANP concentrations in the hypothalamus and brainstem of SHR were determined by a specific radioimmunoassay and found to be higher than those of WKY rats. Elevated mRNA levels in the hypothalamus and brainstem of SHR indicated that increased level of irANP in the CNS resulted from increased synthesis of ANP. CONCLUSION: We propose that increased synthesis of brain ANP in SHR may reflect a compensatory mechanism induced by hypertension.     

Effect of lesions of the anteroventral third ventricle (AV3V) on the development of hypertension in spontaneously hypertensive rats

Lesions of the anteroventral third ventricle (AV3V), an angiotensin and osmosensitive region of the anterior hypothalamus, prevent or abort hypertension in a number of rat models. To determine if AV3V lesions alter hypertension in spontaneously hypertensive rats (SHR), lesions and control sham lesions were made in young SHR at 28 days of age. AV3V lesions had no effect on the development of hypertension in SHR. However, lesioned rats demonstrated significantly reduced pressor responses to intracerebroventricular injections of angiotensin II (AII) and hypertonic NaCl, and drinking produced by centrally administered AII. The depressor effect of central AII receptor blockade was also significantly attenuated in lesioned SHR. These effects appeared to be of central origin since the lesion did not affect the pressor action of intravenous AII or norepinephrine (NE). It is concluded that unlike other models of experimental hypertension (steroid-salt, one-and two-kidney renal, neurogenic) the development of hypertension in SHR does not depend upon the integrity of the AV3V region.     

Brain nitric oxide synthase activity in spontaneously hypertensive rats during the development of hypertension

OBJECTIVES: Blockade of neuronal nitric oxide synthase (nNOS) in the brain induced an increase in mean arterial pressure of spontaneously hypertensive rats (SHR). We hypothesize that increased nitric oxide (NO) synthesis in the brain compensates for hypertension. Therefore, we measured NOS activity in different brain regions in SHR at prehypertensive, onset and established hypertension, and compared with age-matched Wistar-Kyoto (WKY) rats. METHOD: NOS activity was measured by the ability of tissue homogenate to convert [3H]l-arginine to [3H]l-citrulline in a Ca2+- and NADPH-dependent manner. RESULTS: NOS activity was impaired in the cerebral cortex and brainstem of prehypertensive SHR. At established hypertension, SHR showed an augmentation in NOS activity in hypothalamus and brainstem. Chronic treatment of SHR with the angiotensin-1 converting enzyme (ACE)-inhibitor, enalapril, and the AT(1) receptor antagonist, losartan, normalized NOS activity in the hypothalamus but not in the brainstem. Treatment with a peripheral vasodilator, hydralazine, did not affect NOS activity. CONCLUSION: Attenuated NOS activity in the cortex and brainstem of prehypertensive SHR may play a role in the pathogenesis of hypertension. The upregulated NOS activity in the hypothalamus and brainstem of SHR possibly serves to compensate for hypertension. Hypothalamic, but not brainstem, NO is involved in antihypertensive effects of ACE inhibition and AT(1) receptor blockade. Since a blood pressure decrease per se had no effect on NOS activity, it appears that central sympathetic activity influenced by endogenous angiotensin II, rather than blood pressure, represents the stimulus for the increased NOS activity in the hypothalamus of SHR.     

Significance of brain renin for pathogenesis of hypertension: effect of antihypertensive drugs on active and inactive renins in the brain of spontaneously hypertensive rat

Relationship between blood pressure and brain renin was studied in four groups of spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY); as controls (n = 5), administered captopril (n = 5), trichlormethiazide (n = 5) and atenolol (n = 5). 1) Inactive renin in the hypothalamus of captopril-administered SHR was significantly lower than that of control SHR and captopril-administered WKY. On the other hand, active renin in the hypothalamus, thalamus and striatum of captopril-administered SHR was significantly lower than that of control SHR and captopril-administered WKY. 2) Inactive renin in the hypothalamus of trichlormethiazide administered SHR was significantly lower than that of control SHR and trichlormethiazide-administered WKY. On the other hand, active renin in the hypothalamus, thalamus and midbrain of trichlormethiazide-administered SHR was significantly lower than that of control SHR and trichlormethiazide-administered WKY. 3) Inactive renin in the hypothalamus of atenolol-administered SHR was significantly lower than that of control SHR and atenolol-administered WKY. On the other hand, active renin in the hypothalamus, thalamus and midbrain of atenolol-administered SHR was significantly lower than that of control SHR and atenolol-administered WKY. These results suggest that the production and/or activation of renin in the hypothalamus, thalamus, midbrain and striatum play an important role in the initiation and/or development of hypertension of SHR by the local generation of angiotensin II.     

Glutamatergic inputs in the hypothalamic paraventricular nucleus maintain sympathetic vasomotor tone in hypertension

The paraventricular nucleus (PVN) of the hypothalamus is critical to the regulation of sympathetic output. The PVN hyperactivity is known to cause increased sympathetic nerve activity in spontaneously hypertensive rats (SHRs). The purpose of this study was to determine whether glutamatergic input to the PVN contributes to heightened sympathetic outflow in hypertension. Lumbar sympathetic nerve activity, mean arterial blood pressure, and heart rate were recorded from anesthetized SHRs and Wistar-Kyoto (WKY) rats. Bilateral microinjection of an N-methyl-D-aspartate receptor antagonist, 2-amino-5-phosphonopentanoic acid, or a non-N-methyl-D-aspartate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, into the PVN dose-dependently decreased lumbar sympathetic nerve activity, mean arterial blood pressure, and heart rate in SHRs but not in WKY rats. Bilateral microinjection of kynurenic acid into the PVN also significantly decreased lumbar sympathetic nerve activity, mean arterial blood pressure, and heart rate in SHRs but not in WKY rats. Furthermore, microinjection of gabazine, a specific GABA(A) receptor antagonist, into the PVN increased lumbar sympathetic nerve activity, mean arterial blood pressure, and heart rate in both SHRs and WKY rats. Notably, this response was significantly attenuated in SHRs compared with that in WKY rats. In addition, kynurenic acid abolished the sympathoexcitatory and pressor responses to microinjection of gabazine into the PVN in both SHRs and WKY rats. Thus, this study provides new functional evidence that resting sympathetic vasomotor tone is maintained by tonic glutamatergic input in the PVN in SHRs. Removal of GABAergic inhibition results in augmented glutamatergic input in the PVN, which probably constitutes an important source of excitatory drive to the brain stem vasomotor neurons in hypertension.     

Angiotensin II in the Brain and Brainstem of the Doca Salt Hypertensive Rat

We previously reported that immunoreactive angiotensin II (AII) containing nerve fibers and cell bodies were increased in the brain and brainstem of the spontaneously hypertensive (SH) rat compared with its normotensive control, the Wistar Kyoto (WKY) rat. Since earlier studies from other laboratories described the distribution of AII in normotensive Sprague Dawley rat brain, it was the intent of this investigation to examine the localization of AII in only DOCA-salt hypertensive rat. We unilaterally nephrectomized Sprague Dawley rats and administered multiple subcutaneous injections of 30 mg/kg body weight of DOCA with saline substituted for drinking water to significantly increase blood pressure. Using the peroxidase anti-peroxidase method for immunocytochemical localization of a tissue antigen, we characterized AII distribution and density in the brain and brainstem of DOCA-salt hypertensive rat. Positively stained cell bodies and fiber profiles were found in discrete anatomical subdivisions, including the limbic system, hypothalamus, basal ganglia, circumventricular organs, reticular formation and ependymal tissue immediately adjacent to the ventricles. These findings support the hypothesis that AII is widely distributed indiscrete regions of the DOCA-salt hypertensive ratbrain and brainstem, and that the distribution of A11 is consistent with a potential functional significance in the regulation of cardiovascular activity and neuroendocrine function.