The role of the central nervous system in NaCl-sensitive hypertension in spontaneously hypertensive rats

Effect of oxidative stress in rostral ventrolateral medulla on sympathetic hyperactivity after traumatic brain injury

Sympathetic hyperactivity occurs in a subgroup of patients after traumatic brain injury (TBI). The rostral ventrolateral medulla (RVLM) is a key region for the activity of sympathetic nervous system. Oxidative stress in the RVLM is proved to be responsible for the increased level of sympathetic activity in animal models of hypertension and heart failure. In this study, we investigated whether oxidative stress in the RVLM contributed to the development of sympathetic hyperactivity after TBI in rats. Model of diffuse axonal injury was induced using Sprague-Dawley rats, and level of mean arterial pressure (MAP) and plasma Norepinephrine (NE) was measured to evaluate the sympathetic activity. For the assessment of oxidative stress, expression of reactive oxygen species (ROS), malondialdehyde (MDA), and superoxide dismutase (SOD) in the RVLM was determined. Microinjection of Tempol into the RVLM was performed to determine the effect of oxidative stress on sympathetic hyperactivity. According to the results, TBI led to elevated MAP and plasma NE in rats. It also induced a significantly increased level of ROS, MDA production and decreased level of SOD in the RVLM. The sympathetic activity, ROS, and MDA in the RVLM decreased significantly after microinjection of Tempol. Therefore, the present results suggested that oxidative stress in the RVLM was involved in the development of sympathetic hyperactivity following TBI.

Inhibition of Renin-Angiotensin System from Conception to Young Mature Life Induces Salt-Sensitive Hypertension via Angiotensin II-Induced Sympathetic Overactivity in Adult Male Rats

Previous studies indicate that perinatal compromise of taurine causes cardiovascular disorders in adults via the influence of taurine on renin-angiotensin system (RAS). This study tested whether perinatal inhibition of the RAS would itself alter the adult cardiovascular system in a similar way. Female Sprague-Dawley rats were fed normal rat chow and given water alone (Control) or water containing captopril (400 mg/l) from conception until weaning. Then, the male offspring drank water or water containing captopril until 5 weeks of age followed by normal rat chow and water alone until 7 weeks of age. Thereafter, they drank water alone (Control, Captopril) or 1% NaCl solution (Control+1%, Captopril+1%). At 9 weeks of age, all animals were implanted with femoral arterial and venous catheters. Forty-eight hours later, blood chemistry, glucose tolerance, and hemodynamic parameters were determined in freely moving conscious rats. Then, the same experiments were repeated 2 days after captopril treatment. Body weights, kidney and heart to body weight ratios, fasting and non-fasting blood sugar, glucose tolerance, and heart rates were not significantly different among groups. Further, plasma sodium, mean arterial pressure, and sympathetic activity significantly increased whereas baroreflex sensitivity decreased in Captopril+1% compared to other groups. These changes were normalized by acute captopril treatment and the arterial pressure differences also by acute ganglionic and central adrenergic blockade. The present study suggests that inhibition of the RAS in the early life induces RAS overactivity, leading to salt-sensitive hypertension via sympathetic nervous system overactivity and depressed baroreflex sensitivity in adult male rats.

Effect of High-Salt Diet on Age-Related High Blood Pressure and Hypothalamic Redox Signaling

Aim: The aim of this study was to investigate the effect of a high salt (HS) diet on age-related changes in blood pressure (BP) and the possible role played by regulatory central mechanisms. Methods: Young (5 months) and old (27 months) male Fischer 344 × Brown Norway (F344/BN) rats were fed standard chow or 8% HS diet for 12 days. BP and heart rate (HR) were measured by telemetry. Results: Mean arterial BP (MAP) was significantly elevated in old rats during the day and night when compared with young animals. The HS diet further elevated MAP in both age groups, and the increase was more pronounced in the old animals, while HR was not altered by age or HS diet. In addition, cardiovascular responses to restraint stress were diminished in the old when compared with the young and were unchanged with HS diet in either age group. Both age and the HS diet elevated the adrenomedullary mRNA levels of tyrosine hydroxylase, an indicator for sympathoexcitation. HS diet enhanced intracerebroventricular angiotensin II (AngII)-induced BP and HR elevations in both age groups. AngII type 1 receptor mRNA increased significantly in the hypothalamus with age and HS diet. Furthermore, hypothalamic p22phox mRNA and gp91phox protein, subunits of NADPH oxidase, as well as NADPH oxidase activity increased with the HS diet in the old animals, whereas antioxidant enzymes that decreased with age yet remained unaltered with the HS diet. Conclusion: Our findings indicate that sensitivity of BP to HS diet increases with age, and that central AngII-induced pressor responses are diminished in old rats compared with the young both under control conditions and during HS diet treatment. These changes are paralleled by increases in the expression and NADPH oxidase activity in the hypothalamus, possibly leading to central oxidative stress-mediated sympathoexcitation and high BP.

Abnormal autonomic nervous system function in rural Thai men: A potential contributor to their high risk of sudden unexplained nocturnal death syndrome

BACKGROUND

Rural compared to urban Thai populations have a higher incidence of sudden unexplained nocturnal death syndrome (SUNDS). This study tests the hypothesis that compared to young urban Thai men, the young rural northeast Thai men display autonomic system dysfunction that may contribute to their relatively high risk to develop SUNDS.

METHODS

Forty-seven healthy second and third year students from Khon Kaen University (20-22years old) were divided into central, urban northeastern, and rural northeastern groups, based on the locality in which they had grown up and in which their parents had lived prior to their birth.

RESULTS

Body weight, body height, serum sodium, serum potassium, fasting blood sugar, glucose tolerance, resting mean arterial pressure, resting heart rate, ulnar nerve conduction velocity, and sympathetic and parasympathetic nervous system activity were not significantly different among the three groups. In contrast, compared to urban northeasterners and central Thais, rural northeasterners displayed low sympathetic and high parasympathetic responses to cold stress and oral saline load; however, baroreflex sensitivity and the autonomic nervous system responses to upright tilt were not significantly different among the three groups. In addition, respiratory rates at rest and in response to upright tilt, cold stress, and oral saline load were not significantly different among the three groups.

CONCLUSIONS

These data indicate that compared to central or urban, individuals from rural origin display decreased sympathetic and increased parasympathetic responses to stresses. These altered responses could predispose the individuals to inappropriate autonomic control during the stresses, including those resulting in SUNDS.

Inhibitory effects of alpha-lipoic acid on oxidative stress in the rostral ventrolateral medulla in rats with salt-induced hypertension

Oxidative stress in the rostral ventrolateral medulla (RVLM) plays an important role in the pathophysiology of hypertension. Alpha‑lipoic acid (ALA) is widely recognized for its potent superoxide inhibitory properties, and it can safely penetrate deep into the brain. The aim of this study was to explore whether ALA supplementation attenuates hypertensive responses and cardiac hypertrophy by decreasing the NAD(P)H oxidase (NOX)-derived overproduction of reactive oxygen species (ROS) in the mitochondria in the RVLM, and thus attenuating the development of salt‑induced hypertension. For this purpose, male Wistar rats were randomly divided into 2 groups and either fed a high-salt diet or not. After 8 weeks, the rats were either administered ALA or an equal volume of the vehicle for 8 weeks. The rats fed a high‑salt diet exhibited higher mean arterial pressure (MAP) and higher plasma noradrenaline (NE) levels, as well as cardiac hypertrophy, as evidence by the increased whole heart weight/body weight (WHW/BW) ratio, WHW/tibia length (TL) ratio and left‑ventricular weight (LVW)/TL ratio. Compared with the rats in the NS group, the rats in the HS group only exhibited increased levels of superoxide, NOX2, NOX4 and mitochondrial malondialdehyde (MDA), but also decreased levels of copper/zinc (Cu/Zn)-superoxide dismutase (SOD), mitochondrial SOD and glutathione (GSH) in the RVLM. The supplementation of ALA decreased MAP, plasma NE levels and the levels of cardiac hypertrophy indicators. It also decreased the levels of superoxide, NOX2, NOX4 and mitochondrial MDA, and increased the levels of Cu/Zn‑SOD, mitochondrial SOD and GSH in the RVLM compared with the rats fed a high-salt diet and not treated with ALA. On the whole, our findings indicate that long‑term ALA supplementation attenuates hypertensive responses and cardiac hypertrophy by decreasing the expression of NAD(P)H subunits (NOX2 and NOX4), increasing the levels of mitochondrial bioenergetic enzymes, and enhancing the intracellular antioxidant capacity in the RVLM during the development of hypertension.

Roles of the subfornical organ and area postrema in arterial pressure increases induced by 48-h water deprivation in normal rats

In rats, water deprivation (WD) increases arterial blood pressure (BP) in part due to actions of elevated osmolality in the brain to increase vasopressin levels and sympathetic activity. However, the osmoreceptors that mediate this response have not been identified. To test the hypothesis that osmoregulatory circumventricular organs are involved, BP and heart rate (HR) were continuously recorded telemetrically during 48 h of WD in normal rats with lesions (x) or sham lesions (sham) of the subfornical organ (SFO) or area postrema (AP). Although WD increased BP in SFOx and SFOsham rats, no significant difference in the hypertensive response was observed between groups. HR decreased transiently but similarly in SFOx and SFOsham rats during the first 24 h of WD. When water was reintroduced, BP and HR decreased rapidly and similarly in both groups. BP (during lights off) and HR were both lower in APx rats before WD compared to APsham. WD increased BP less in APx rats, and the transient bradycardia was eliminated. Upon reintroduction of drinking water, smaller falls in both BP and HR were observed in APx rats compared to APsham rats. WD increased plasma osmolality and vasopressin levels similarly in APx and APsham rats, and acute blockade of systemic V1 vasopressin receptors elicited similar depressor responses, suggesting that the attenuated BP response is not due to smaller increases in vasopressin or osmolality. In conclusion, the AP, but not the SFO, is required for the maximal hypertensive effect induced by WD in rats.

Taurine supplementation in spontaneously hypertensive rats: Advantages and limitations for human applications

Taurine (2-aminoethanesulfonic acid) is a β-amino acid found in many tissues particularly brain, myocardium, and kidney. It plays several physiological roles including cardiac contraction, antioxidation, and blunting of hypertension. Though several lines of evidence indicate that dietary taurine can reduce hypertension in humans and in animal models, evidence that taurine supplementation reduces hypertension in humans has not been conclusive. One reason for the inconclusive nature of past studies may be that taurine having both positive and negative effects on cardiovascular system depending on when it is assessed, some effects may occur early, while others only appear later. Further, other consideration may play a role, e.g., taurine supplementation improves hypertension in spontaneously hypertensive rats on a low salt diet but fails to attenuate hypertension on a high salt diet. In humans, some epidemiologic studies indicate that people with high taurine and low salt diets display lower arterial pressure than those with low taurine and high salt diets. Differences in techniques for measuring arterial pressure, duration of treatment, and animal models likely affect the response in different studies. This review considers both the positive and negative effects of taurine on blood pressure in animal models and their applications for human interventions.

Perinatal taurine depletion increases susceptibility to adult sugar-induced hypertension in rats

This study tests the hypothesis that perinatal taurine depletion produces autonomic nervous system dysregulation and increases arterial pressure in young male rats maintained on a high sugar diet. Sprague-Dawley dams were either taurine depleted (beta-alanine 3% in water) or left untreated from conception to weaning. Their male offspring were fed normal rat chow with or without 5% glucose. At 7-8 weeks of age, the male offspring were either tested in a conscious, unrestrained state or after anesthetia. Body weight was slightly lower in the taurine-depleted rats although their heart or kidneys to body weight ratios were similar. Plasma potassium, blood urea nitrogen, plasma creatinine, hematocrit, fasting blood glucose concentrations and glucose tolerance were all similar. In the taurine-depleted, high glucose group, mean arterial pressure and sympathetic nervous system activity were increased while baroreflex function was impaired. These findings suggest that in this model perinatal taurine depletion causes autonomic nervous system dysfunction that may contribute to dietary high sugar-induced hypertension.

Persistence of circadian variation in arterial blood pressure in beta1/beta2-adrenergic receptor-deficient mice

The beta-adrenergic pathway has been considered one important effector of circadian variation in arterial pressure. Experiments were performed in beta1/beta2-adrenergic receptor-deficient mice (beta1/beta2ADR-/-) to assess whether this pathway is required for circadian variation in mean arterial pressure (MAP) and to determine the impact of its loss on the response to changes in dietary salt. Twenty-four-hour recordings of MAP, heart rate (HR), and locomotor activity were made in conscious 16- to 17-wk-old mice [wild-type, (WT), n = 7; beta1/beta2ADR-/-, n = 10] by telemetry. Both WT and beta1/beta2ADR-/- mice demonstrated robust circadian variation in MAP and HR, although 24-h mean MAP was 10% lower (102.02 +/- 1.81 vs. 92.11 +/- 2.62 mmHg) in beta1/beta2ADR-/- than WT, HR was 16% lower and day-night differences reduced. Both WT and beta1/beta2ADR-/- mice adapted to changed salt intake without changed MAP. However, the beta1/beta2ADR-/- mice demonstrated a striking reduction in locomotor activity in light and dark phases of the day. In WT mice, MAP was markedly affected by locomotor activity, resulting in bimodal distributions in both light and dark. When MAP was analyzed using only intervals without locomotor activity, bimodality and circadian differences were reduced, and there was no significant difference between the two genotypes. The results indicate that there is no direct effect or role for the beta-adrenergic system in circadian variation of arterial pressure in mice, aside from the indirect consequences of altered locomotor activity. Our results also confirm that locomotor activity contributes strongly to circadian variation in blood pressure in mice.

Upregulation of GAD65 mRNA in the medulla of the rat model of metabolic syndrome

Metabolic syndrome is characterized by obesity, elevated blood pressure (BP), insulin resistance, and hypercholesterolemia. Recently an animal model of this disorder has been proposed in rats selectively bred based on their performance on a treadmill-running task. Accordingly, low capacity runner (LCR) rats exhibited all of the diagnostic criteria for metabolic syndrome, including elevated BP, as compared to their high capacity runner (HCR) counterparts [U. Wisløff, S.M. Najjar, O. Ellingsen, P.M. Haram, S. Swoap, Q. Al-Share, M. Fernstrom, K. Rezaei, S.J. Lee, L.G. Koch, S.L. Britton, Cardiovascular risk factors emerge after artificial selection for low aerobic capacity, Science 307 (2005) 418-420]. Previous studies have highlighted the importance of GABAergic neurotransmission in the medullary cardiovascular-regulatory areas in the central control of BP. Thus, we hypothesized a dysregulation in GABAergic transmission in the medullary cardiovascular-regulatory nuclei of LCR rats. To begin testing this hypothesis we carried out experiments examining expression of the GABA synthetic enzymes, GAD65 and GAD67, mRNAs in the two rat strains via radioactive in situ hybridization. Our results showed GAD65 and GAD67 mRNAs were widely expressed throughout the brainstem; quantification revealed increased GAD65 mRNA expression in LCR animals in the caudal nucleus tractus solitarius (NTS) and rostral ventrolateral medulla (VLM) as compared to HCR rats. Conversely, no differences in the expression of GAD67 were detected in these regions. These data are consistent with the notion of altered GABAergic neurotransmission in the NTS and VLM in metabolic syndrome, and point to the importance of these regions in cardiovascular regulation.

[Mechanisms of hypertension in the central nervous system]

This article reviews studies by the author on central mechanisms of hypertension. Spontaneously hypertensive rats (SHR) have been developed as a rat model of genetic hypertension, and central acetylcholine has been implicated in hypertension in SHR. The rostral ventrolateral medulla (RVL), a major source of efferent sympathetic activity, has cholinergic pressor systems. The release of acetylcholine is enhanced in the RVL of SHR, leading to hypertension. The alteration of the RVL cholinergic system in SHR results from enhanced angiotensin systems in the anterior hypothalamic area (AHA). Angiotensin II-sensitive neurons are present in the AHA and they are tonically activated by endogenous angiotensins. The basal activity of AHA angiotensin II-sensitive neurons is enhanced in SHR, mainly due to enhanced sensitivity of AHA neurons to angiotensin II. The AHA angiotensin system is also responsible for hypertension induced by emotional stress and central Na(+) increases. These findings suggest that the AHA angiotensin system may play a critical role in the development of hypertension.

Sensitivity of pressor responses to central hypertonic saline is greatly enhanced even in pre-hypertensive spontaneously hypertensive rats

It has been suggested that intracerebroventricular injection of hypertonic saline mimics the effects of a high salt diet in spontaneously hypertensive rats (SHR), a genetic model of hypertension. Intracerebroventricular injection of hypertonic saline produces an increase in blood pressure and the pressor response to hypertonic saline is enhanced in adult hypertensive SHR. In this study, we examined whether the intracerebroventricular hypertonic saline-induced pressor response is enhanced even in pre-hypertensive SHR. The basal mean blood pressure was almost the same in 4-week-old SHR and age-matched Wistar Kyoto rats (WKY), whereas it was greater in 15-16-week-old SHR than in age-matched WKY. Intracerebroventricular injection of hypertonic saline (10 microl of 230 mM NaCl) produced an increase in blood pressure in both 4-week-old and 15-16-week-old SHR, whereas it did not affect blood pressure in both age-matched WKY. Intracerebroventricular injection of hypertonic saline (10 microl of 260 mM NaCl) produced an increase in blood pressure in all rats but the pressor response was greater in both 4-week-old and 15-16-week-old SHR than in respective age-matched WKY. Intracerebroventricular injection of Phe-Met-Arg-Phe amide (FMRF), an FMRF-inducible sodium channel activator, produced an increase in blood pressure in all rats but the pressor response was greater in SHR than in WKY at both ages. These findings indicate that the sensitivities of pressor responses to intracerebroventricular hypertonic saline and FMRF are enhanced not only in hypertensive but also in pre-hypertensive SHR.

Angiotensin II sensitivity of anterior hypothalamic area neurons is enhanced in both spontaneously hypertensive rats and Dahl salt-sensitive rats

We have previously demonstrated that some neurons in the anterior hypothalamic area (AHA) are tonically activated by endogenous angiotensins. Furthermore, we have demonstrated that intracerebroventricular injection of hypertonic saline increases the firing rate of AHA angiotensin II-sensitive neurons via angiotensins and that the central sodium-induced activation of AHA neurons is enhanced in spontaneously hypertensive rats (SHR) and Dahl salt-sensitive (Dahl S) rats. In this study, we examined whether sensitivities of AHA angiotensin II-sensitive neurons to angiotensin II are enhanced in SHR and Dahl S rats as compared with their respective controls. Male 15- to 16-week-old SHR and age-matched Wistar Kyoto rats (WKY), and male 15- to 16-week-old Dahl S rats and Dahl R rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. In SHR, pressure application of angiotensin II (3 x 10(-9) to 3 x 10(-8) M) onto AHA angiotensin II-sensitive neurons increased their firing rate in a concentration-dependent manner. In WKY, only the highest concentration of angiotensin II increased the firing rate, while the lower concentrations of angiotensin II did not affect it. In Dahl S rats, pressure application of angiotensin II (10(-8) and 3 x 10(-8) M) onto AHA neurons increased their firing rate, while angiotensin II (3 x 10(-9) M) did not affect it. In Dahl R rats, the highest concentration of angiotensin II increased the firing rate, while the lower concentrations of angiotensin II did not affect it. These findings indicate that the sensitivity of AHA neurons to angiotenisn II is enhanced in SHR and Dahl S rats as compared with their respective controls.

Translation of salt retention to central activation of the sympathetic nervous system in hypertension

1. Increased dietary salt increases blood pressure in many hypertensive individuals, producing salt-sensitive hypertension (SSH). The cause is unknown, but a major component appears to be activation of the sympathetic nervous system. The purpose of this short review is to present one hypothesis to explain how increased dietary salt increases sympathetic activity in SSH. 2. It is proposed that increased salt intake causes salt retention and raises plasma sodium chloride (NaCl) concentrations, which activate sodium/osmoreceptors to trigger sympathoexcitation. Moreover, we suggest that small and often undetectable increases in osmolality can drive significant sympathoexcitation, because the gain of the relationship between osmolality and increased sympathetic activity is enhanced. Multiple factors may contribute to this facilitation, including inappropriately elevated levels of angiotensin II or aldosterone, changes in gene expression or synaptic plasticity and increased sodium concentrations in cerebrospinal fluid. 3. Future studies are required to delineate the brain sites and mechanisms of action and interaction of osmolality and these amplification factors to elicit sustained sympathoexcitation in SSH.

Enhanced central hypertonic saline-induced activation of angiotensin II-sensitive neurons in the anterior hypothalamic area of spontaneously hypertensive and Dahl S rats

High dietary salt intake activates the brain renin-angiotensin system in spontaneously hypertensive rats (SHR) and Dahl S rats, resulting in sympathetic hyperactivity and hypertension. Increases of sodium concentration in cerebrospinal fluid (CSF) and/or enhanced responses to CSF sodium are considered to be involved in the high dietary salt-induced activation of central nervous system pathways in those rats. Previously we have demonstrated that intracerebroventricular injection of hypertonic saline increases the neural activity of angiotensin II-sensitive neurons trans-synaptically via endogenous angiotensins in the anterior hypothalamic area (AHA) of rats. In the present study, we examined whether the AHA angiotensin II-sensitive neuron response to hypertonic saline would differ in SHR and Dahl S rats from those of their controls. Male 15- to 16-week-old SHR and age-matched Wistar Kyoto rats (WKY), Dahl S rats and Dahl R rats and Wistar rats were anesthetized and artificially ventilated. Extracellular potentials were recorded from single neurons in the AHA. Intracerebroventricular injection of hypertonic saline increased the firing rate of AHA angiotensin II-sensitive neurons. The threshold sodium concentration for the central sodium-induced increase of neural firing was lower in SHR than those of WKY, Dahl S rats, Dahl R rats and Wistar rats. The increase in neural firing induced by hypertonic saline (250 mM) was greater in SHR than those of other four kinds of rats. Similarly, the threshold sodium concentration was lower in Dahl S rats than those of WKY, Dahl R rats and Wistar rats and the increase in neural firing induced by hypertonic saline (250 mM) was greater in Dahl S rats than those of WKY, Dahl R rats and Wistar rats. In SHR, intracerebroventricular injection of the amiloride-sensitive sodium channel blocker benzamil abolished the hypertonic saline (250 mM)-induced increase in neural firing, but the sodium channel blocker itself did not affect the basal firing of these neurons. These findings indicate that central sodium-induced activation of AHA angiotensin II-sensitive neurons is enhanced in SHR and Dahl S rats.

Prenatal malnutrition-induced hypertension in young rats is prevented by neonatal capsaicin treatment

Prenatal malnutrition-induced fetal growth retardation in the rat results in elevated arterial blood pressure at adulthood. To test the contribution of cardiovascular sensory C fibers in the hypertensive state, arterial blood pressure was measured in prenatally undernourished rats treated at birth with capsaicin. The effects of the neonatal capsaicin treatment on heart rate and respiratory frequency were also evaluated. Maternal malnutrition resulted in body and brain weights deficits in the offspring that were not modified by neonatal capsaicin treatment. Capsaicin treatment did not change the cardiovascular parameters in normal rats, but prevented the elevation of arterial blood pressure and heart rate in malnourished animals. These results indicate that elevation of arterial blood pressure in prenatally malnourished rats depends on the activity of some sensory unmyelinated C fibers.