Lesions of the anteroventral third ventricle prevent salt-induced hypertension in the borderline hypertensive rat

Physiological Mechanisms of Dietary Salt Sensing in the Brain, Kidney, and Gastrointestinal Tract

Excess dietary salt (NaCl) intake is strongly correlated with cardiovascular disease and is a major contributing factor to the pathogenesis of hypertension. NaCl-sensitive hypertension is a multisystem disorder that involves renal dysfunction, vascular abnormalities, and neurogenically-mediated increases in peripheral resistance. Despite a major research focus on organ systems and these effector mechanisms causing NaCl-induced increases in arterial blood pressure, relatively less research has been directed at elucidating how NaCl is sensed by various tissues to elicit these downstream effects. The purpose of this review is to discuss how the brain, kidney, and gastrointestinal tract sense NaCl including key cell types, the role of NaCl versus osmolality, and the underlying molecular and electrochemical mechanisms.

Altered Neuronal Discharge in the Organum Vasculosum of the Lamina Terminalis Contributes to Dahl Salt-Sensitive Hypertension

BACKGROUND

Salt-sensitive hypertension in humans and experimental models is associated with higher plasma and cerebrospinal fluid sodium chloride (NaCl) concentrations. Changes in extracellular NaCl concentrations are sensed by specialized neurons in the organum vasculosum of the lamina terminalis (OVLT). Stimulation of OVLT neurons increases sympathetic nerve activity (SNA) and arterial blood pressure (ABP), whereas chronic activation produces hypertension. Therefore, the present study tested whether OVLT neuronal activity was elevated and contributed to SNA and ABP in salt-sensitive hypertension.

METHODS

Male Dahl salt-sensitive (Dahl S) and Dahl salt-resistant (Dahl R) rats were fed 0.1% or 4.0% NaCl diets for 3 to 4 weeks and used for single-unit recordings of OVLT neurons or simultaneous recording of multiple sympathetic nerves during pharmacological inhibition of the OVLT.

RESULTS

Plasma and cerebrospinal fluid Na+ and Cl- concentrations were higher in Dahl S rats fed 4% versus 0.1% or Dahl R rats fed either diet. In vivo single-unit recordings revealed a significantly higher discharge of NaCl-responsive OVLT neurons in Dahl S rats fed 4% versus 0.1% or Dahl R rats. Interestingly, intracarotid infusion of hypertonic NaCl evoked greater increases in OVLT neuronal discharge of Dahl S versus Dahl R rats regardless of NaCl diet. The activity of non-NaCl-responsive OVLT neurons was not different across strain or diets. Finally, inhibition of OVLT neurons by local injection of the gamma-aminobutyric acid agonist muscimol produced a greater decrease in renal SNA, splanchnic SNA, and ABP of Dahl S rats fed 4% versus 0.1% or Dahl R rats.

CONCLUSIONS

A high salt diet activates NaCl-responsive OVLT neurons to increase SNA and ABP in salt-sensitive hypertension.

Sympathetic Nervous System Contributions to Hypertension: Updates and Therapeutic Relevance

The sympathetic nervous system plays a pivotal role in the long-term regulation of arterial blood pressure through the ability of the central nervous system to integrate neurohumoral signals and differentially regulate sympathetic neural input to specific end organs. Part 1 of this review will discuss neural mechanisms of salt-sensitive hypertension, obesity-induced hypertension, and the ability of prior experiences to sensitize autonomic networks. Part 2 of this review focuses on new therapeutic advances to treat resistant hypertension including renal denervation and carotid baroactivation. Both advances lower arterial blood pressure by reducing sympathetic outflow. We discuss potential mechanisms and areas of future investigation to target the sympathetic nervous system.

Neurogenic and sympathoexcitatory actions of NaCl in hypertension

Excess dietary salt intake is a major contributing factor to the pathogenesis of salt-sensitive hypertension. Strong evidence suggests that salt-sensitive hypertension is attributed to renal dysfunction, vascular abnormalities, and activation of the sympathetic nervous system. Indeed, sympathetic nerve transections or interruption of neurotransmission in various brain centers lowers arterial blood pressure (ABP) in many salt-sensitive models. The purpose of this article is to discuss recent evidence that supports a role of plasma or cerebrospinal fluid hypernatremia as a key mediator of sympathoexcitation and elevated ABP. Both experimental and clinical studies using time-controlled sampling have documented that a diet high in salt increases plasma and cerebrospinal fluid sodium concentration. To the extent it has been tested, acute and chronic elevations in sodium concentration activates the sympathetic nervous system in animals and humans. A further understanding of how the central nervous system detects changes in plasma or cerebrospinal fluid sodium concentration may lead to new therapeutic treatment strategies in salt-sensitive hypertension.

Cerebral aqueduct block attenuates cardio-renal injuries in post-DOCA-NaCl-hypertensive Dahl R rats

The systemic and/or local effects of the hydrocephalic brain were investigated in DOCA-NaCl-hypertensive Dahl R rats induced by 250 mg kg(-1) DOCA in silicone and 1% saline water. After a 1-week recovery with 0.3% NaCl chow and tap water, one group had the aqueduct of Sylvius blocked with silicone and epoxy materials with a control sham group matching mean blood pressure (BP) and body weight. The 4-week-postsurgery BP on the 0.3% NaCl diet averaged 161±3.2 in the sham group and 146±2.3 mm Hg in the blocked group (P<0.0001). Both groups were then given an 8% NaCl diet and after 4 weeks, the sham group's BP was increased further with markedly increased mortality: 186 mm Hg vs. 154 mm Hg (P<0.0001); 12 sham rats died after 11 weeks, while all the blocked rats survived (P<0.0001). A transient change in plasma Na levels was observed in the blocked group after 48 h on the 8% NaCl diet. At 14 weeks, 0 sham rats survived, compared with 10 out of 16 blocked rats (P<0.0001). After 11 weeks on 8% NaCl, the average tail venous pressure in the sham group was significantly higher than that of the blocked rats (P<0.0001) indicating the end stage of renal and heart failure. The hearts and kidneys weighed significantly more in the sham vs. the blocked rats (P<0.0001 for both groups). These results indicate that the aqueduct block prevents post-DOCA hypertension and cardio-renal injuries, suggesting that centralized third ventricular brain signaling has a role in salt-genetic hypertension.

Excess dietary salt intake alters the excitability of central sympathetic networks

The ingestion of excess dietary salt (defined as NaCl) is strongly correlated with cardiovascular disease, morbidity, mortality, and is regarded as a major contributing factor to the pathogenesis of hypertension. Although several mechanisms contribute to the adverse consequences of dietary salt intake, accumulating evidence suggests that dietary salt loading produces neurogenically-mediated increases in total peripheral resistance to raise arterial blood pressure (ABP). Evidence from clinical studies and experimental models clearly establishes a hypertensive effect of dietary salt loading in a subset of individuals who are deemed "salt-sensitive". However, we will discuss and present evidence to develop a novel hypothesis to suggest that while chronic increases in dietary salt intake do not elevate mean ABP in "non-salt-sensitive" animals, dietary salt intake does enhance several sympathetic reflexes thereby predisposing these animals and/or individuals to the development of salt-sensitive hypertension. Additional evidence raises an intriguing hypothesis that these enhanced sympathetic reflexes are largely attributed to the ability of excess dietary salt intake to selectively enhance the excitability of sympathetic-regulatory neurons in the rostral ventrolateral medulla. Insight into the cellular mechanisms by which dietary salt intake alters the responsiveness of RVLM circuits will likely provide a foundation for developing new therapeutic approaches to treat salt-sensitive hypertension. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

A high-salt diet does not influence renal sympathetic nerve activity: a direct telemetric investigation

The importance of dietary salt in the development of hypertension has long been a source of controversy. Recent studies suggest a combination of high-salt and ANG II infusion may increase sympathetic drive; however, the effect of a change in dietary salt alone is unclear. Using telemetry, we recorded renal sympathetic nerve activity (RSNA), arterial pressure (MAP), and heart rate (HR) in seven New Zealand white rabbits before and during a 6-day period of increased salt intake (normal NaCl 0.5 g·kg−1·day−1, high NaCl 2.5 g·kg−1·day−1) and a second group of seven rabbits with normal salt intake throughout. The responses to stressful stimuli encountered in the laboratory were recorded and compared with rest in control and high-salt groups. Resting MAP, HR, and RSNA were not significantly altered with high salt intake [88 ± 5 vs. 91 ± 6 mmHg; 251 ± 8 vs. 244 ± 9 beats per minute (bpm); 9.7 ± and 1.2 vs. 10.8 ± 1.7 normalized units (nu)] despite significant reductions in plasma renin activity (1.88 ± 0.18 vs. 1.27 ± 0.15 nmol ANG I·l−1·h−1; P < 0.05) and ANG II (7.5 ± 1.2 vs. 4.3 ± 0.8 pmol/l). Increasing levels of stressful stimuli (resting in home cage, containment in box, handling, and nasopharyngeal activation) in animals on a normal salt diet caused graded increases in MAP (89 ± 2 mmHg, 95 ± 2 mmHg, 107 ± 4 mmHg, and 122 ± 5 mmHg, respectively) and RSNA (9.7 ± 0.9 nu; 11.8 ± 2.7 nu; 31.4 ± 3.7 nu; 100 nu) but not HR (245 ± 8 bpm; 234 ± 8 bpm; 262 ± 9 bpm; 36 ± 5 bpm). High dietary salt did not significantly alter the responses to stress. We conclude that a 6-day period of high salt intake does not alter the level of RSNA, with non-neural mechanisms primarily responsible for the observed renin-angiotensin system suppression.

Whole body norepinephrine kinetics in ANG II-salt hypertension in the rat

The purpose of this study was to investigate total body norepinephrine (NE) kinetics as an index of global sympathetic nervous system (SNS) outflow in a rat model of chronic ANG II-salt hypertension. Male Sprague-Dawley rats fed a 0.4% (normal salt, NS) or 2% (HS) NaCl diet were instrumented with arterial and venous catheters. After 5 days of recovery and a 3-day control period, ANG II (150 ng.kg(-1).min(-1)) was given subcutaneously by minipump for 14 days. Plasma NE levels and total body NE spillover and clearance were determined on control day 3 and ANG II infusion days 7 and 14 using radioisotope dilution principles. To perform this analysis, 3H-NE and NE were measured in arterial plasma after a 90-min infusion of tracer amounts of 3H-NE. Mean arterial pressure (MAP) was similar during the control period in NS and HS rats; however, MAP increased to a higher level in HS rats. During the control period, plasma NE tended to be lower in rats on HS, whereas NE clearance tended to be higher in HS rats. As a result NE spillover was similar in NS and HS rats during the control period. In NS rats, plasma NE, NE spillover, and NE clearance were unchanged by ANG II. In contrast, in rats on the HS diet, plasma NE and NE spillover increased during ANG II infusion, whereas NE clearance was unchanged. In conclusion, a HS diet alone or ANG II infusion in animals fed NS do not affect global sympathetic outflow. However, the additional hypertensive response to ANG II in animals fed HS is accompanied by SNS activation.

Effect of chronic social stress on nitric oxide synthesis and vascular function in rats with family history of hypertension

Genetic predisposition and psychosocial stress are known risk factors in the aetiology of hypertension. The aim of this study was to investigate the as yet unknown role of nitric oxide (NO) in mechanisms of social stress-induced hypertension in rats with a family history of hypertension. Male adult rats used in the study were offspring of normotensive (Wistar) dams and spontaneously hypertensive sires. The rats were exposed to 6-week crowding stress (5 rats/cage, 200 cm2/rat). Control rats were kept four per cage (480 cm2/rat). Blood pressure was determined non-invasively on the tail. Basal blood pressure of all rats was 131 +/- 2 mm Hg. Crowding stress increased significantly blood pressure (p < 0.02 vs. basal value). Crowding had no influence on NO synthase activity in the left ventricle, adrenal glands and kidney. However, crowding stress reduced significantly NO synthase activity in the aorta by 37% (p < 0.01 vs. control). Acetylcholine-induced relaxation and noradrenaline-induced vasoconstriction of the femoral artery were reduced in stressed rats by 58% (p < 0.001) and 41% (p < 0.003), respectively. On balance then, the results indicate that chronic social stress produced by crowding was associated with reduced vascular NO synthesis and altered vascular function in adult borderline hypertensive rats of normotensive mothers.

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.

Effect of a high-salt diet on gamma-aminobutyric acid-mediated responses in the nucleus tractus solitarius of Sprague-Dawley rats

Previous study using an indirect gamma-aminobutyric acid (GABA) agonist indicated that high salt intake enhances sensitivity of nucleus tractus solitarius (NTS) projecting inhibitory input to rostral ventrolateral medulla sympathoexcitatory neurons. We further investigated the relationship between salt intake and the GABA system in NTS. Sprague-Dawley (S-D) rats consuming high dietary salt (8%) or low dietary salt (0.3%) for 3 weeks were used. Under chloralose-anesthesia, baseline arterial pressure (AP) and heart rate (HR) were similar in both groups. Bilateral injection into NTS of nipecotic acid, GABA(A) receptor agonist (muscimol), or GABA(B) receptor agonist (baclofen) elicited greater pressor responses in high-salt group. GABA(A) receptor antagonist, bicuculline and GABA(B) receptor antagonist, CGP-35348 elicited greater depressor responses. Phenylephrine or nitroprusside (i.v.) elicited similar respective increases or decreases in AP in both groups. Baroreflex sensitivity was similar. Thus, high-salt intake enhances both GABA(A) receptor- and GABA(B) receptor-mediated responses within NTS, thereby inhibiting elevation of AP.

High salt diet enhances cardiovascular responses from the nucleus tractus solitarius and ventrolateral medulla of Sprague-Dawley rats

High salt intake has been shown to augment the sensitivity of rostral ventrolateral medulla (RVLM) sympathoexcitatory neurons. We examined the effects of 4 weeks of high dietary salt (8%) on the sensitivity of nucleus tractus solitarius (NTS) and caudal ventrolateral medulla (CVLM) in controlling RVLM. In chloralose-anesthetized Sprague-Dawley rats, high salt intake did not elevate baseline arterial pressure or heart rate (HR). In high-salt group, NTS, CVLM, and RVLM responses to glutamate were greater. NTS responses to acetylcholine or serotonin, which is independent of baroreflex, also were greater. Phenylephrine or nitroprusside (i.v.) elicited similar changes in arterial pressure and heart rate, the baroreflex sensitivity also was similar in both groups of rats. These results suggest that high salt intake augments the sensitivity of NTS and CVLM sending inhibitory input to RVLM. This presumably may inhibit the RVLM, thereby inhibiting the elevation of arterial pressure.

Increased dietary sodium alters neural control of blood pressure during intravenous ANG II infusion

Increased dietary sodium enhances both excitatory and inhibitory blood pressure responses to stimulation of the central sympathetic nervous system (SNS) centers. In addition, long-term (hours to days) administration of ANG II increases blood pressure by activation of the SNS. These studies investigated the effects of increased dietary sodium on SNS control of blood pressure during 0- to 24-h infusion of ANG II in conscious, male rats consuming either tap water or isotonic saline (Iso) for 2 to 3 wk. The SNS component (evaluated by ganglionic blockade with trimetaphan) of both control blood pressure and the pressor response to intravenous ANG II was reduced in Iso animals. Furthermore, although the pressor response to intravenous ANG II infusion was similar between groups, the baroreflex-induced bradycardia during the initial 6 h of ANG II infusion was significantly greater, whereas the tachycardia accompanying longer infusion periods was significantly attenuated in Iso animals. These data suggest that in normal rats increased dietary sodium enhances sympathoinhibitory responses during intravenous ANG II.

The role of nitric oxide in the reversal of hemorrhagic shock by oxotremorine

In the present study, the effect of the nitric oxide synthase inhibitor, N(G)-nitro-L-arginine methylester (L-NAME), on the antishock actions of oxotremorine was investigated in rats subjected to hemorrhagic shock under urethane anesthesia. L-citrulline production in the AV3V region, as an indicator of nitric oxide (NO) synthesis, was assayed by high-performance liquid chromatography (HPLC) with fluorescent detection throughout the experiment. The rats were pretreated with either intravenous (i.v.) physiological saline or L-NAME (2.5 mg/kg) before bleeding. L-NAME potentiated the reversal of hypotension by oxotremorine (25 microg/kg, i.v.). However, oxotremorine either alone or in combination with L-NAME did not produce any significant change in 60-min survival rate at this low dose. Analysis of microdialysis samples collected from the AV3V region showed that L-citrulline concentration increased during bleeding and that this increase was abolished by L-NAME pretreatment. These results may suggest that nitric oxide production contributes to hypotension in rats bled to shock since nitric oxide levels in the AV3V region increased in response to bleeding and nitric oxide synthase (NOS) inhibition abolished this increase and potentiated the oxotremorine-induced reversal of hypotension.

Dietary salt intake alters cardiovascular responses evoked from the rostral ventrolateral medulla

The present experiments examined whether in rats consuming diets with either high NaCl content (8%) or low Na+ content (0.01%) for 2 wk excitatory inputs to the rostral ventrolateral medulla (RVLM) would be altered. In chloralose-anesthetized rats, injection of glutamate into the RVLM elicited a pressor response that, compared with rats fed a control diet, was 50% larger in rats fed a diet containing 8% NaCl and was 25% smaller in rats fed a diet containing 0.01% Na+. Pressor responses produced by electrical stimulation of sciatic nerve afferents, as well as by microinjections into the RVLM of L-dihydroxyphenylalanine or carbachol, were all potentiated by high dietary salt intake and reduced by low dietary salt intake. Dietary salt intake had no effect on pressor responses produced by intravenous injection of phenylephrine, indicating that salt-related alterations in cardiovascular responses produced by central activation could not be accounted for by changes in peripheral vascular reactivity. The decrease in arterial pressure produced by injection of glutamate into the nucleus of the solitary tract was also potentiated by the high salt diet, suggesting that the sensitivity of central baroreceptor reflex pathways may be altered by dietary NaCl. These results indicate that the amount of NaCl consumed in the diet can change the sensitivity of RVLM sympathoexcitatory neurons, and this change in sensitivity is not restricted to any particular class of cell surface receptors.

Role of the AV3V region in the pressor responses induced by amygdala stimulation

The role of the anteroventral third ventricle (AV3V) region in the pressor responses to carbachol injected into the lateral cerebral ventricle (i.c.v.), the electrical stimulation of and carbachol-induced stimulation of, the central nucleus of the amygdala were investigated in conscious, unrestrained Sprague-Dawley rats. I.c.v. and intra-amygdalar carbachol caused a significant rise in blood pressure of 22.9 +/- 2.8 and 16.8 +/- 2.2 mmHg, respectively. Electrical stimulation (1 ms, 80 Hz, 50-300 microA, for 30 s) of the central nucleus of amygdala also produced intensity-dependent pressor effects. Electrolytic lesion of the AV3V region abolished the pressor responses induced by carbachol and by electrical amygdala stimulation. The heart rate changes were also significantly inhibited in the AV3V-lesioned rats. These results indicate that the integrity of the AV3V region is essential for the central cholinergic cardiovascular changes induced by central amygdaloid nucleus stimulation.

Brain "ouabain" and angiotensin II in salt-sensitive hypertension in spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) received from 5 to 9 weeks of age a high or regular sodium diet and concomitant intracerebroventricular infusions via minipumps of the following compounds: antibody Fab fragments (200 micrograms/d), which bind ouabain and related steroids with high affinity; the angiotensin II (Ang II) type 1 receptor blocker losartan (1 mg/kg per day); a combination of Fab fragments and losartan; and as control, gamma-globulins (200 micrograms/d). The same doses of Fab fragments and losartan were also given intravenously. At 9 weeks of age, compared with SHR on regular sodium, SHR on high sodium that were treated with gamma-globulins had higher resting blood pressure and showed significantly enhanced excitatory responses of blood pressure, renal sympathetic nerve activity, and heart rate to air stress and inhibitory responses to the central alpha 2-agonist guanabenz. Central Fab fragments and losartan alone or combined prevented all these effects of high sodium. Intravenous Fab fragments or losartan was ineffective. Compared with control SHR on high sodium, SHR on high sodium that were treated with Fab fragments had significantly increased sympathoexcitatory and pressor responses to central Ang II injection, consistent with a decrease in brain Ang II receptor occupancy. These data indicate that both increased brain "ouabain" and Ang II contribute to salt-sensitive hypertension in SHR. Brain Ang II receptor stimulation appears to be downstream of "ouabain" in the pathways mediating sympathoexcitatory and pressor effects of high sodium.

Reversal of hemorrhagic shock in rats by oxotremorine: the role of muscarinic and nicotinic receptors, and AV3V region

In an experimental model of hemorrhagic shock resulting in the death of almost all rats within 20-30 min, centrally active cholinomimetic drugs are reported to induce a prompt, sustained and dose-dependent improvement in blood pressure and survival rate claimed to be due to nicotinic, but not muscarinic actions. In the present study, cholinergic receptor agonist, oxotremorine (50 micrograms/kg, i.v.) increased mean arterial pressure (from 22 +/- 1 to 123 +/- 3 mm Hg) and 60 min-survival rate (from 0% to 92%) in rats bled to hypovolemic shock. Atropine (2 mg/kg, i.v.) pretreatment inhibited the pressor effect of oxotremorine significantly, but did not modify its effect on survival rate. On the other hand, pretreatment with mecamylamine (50 micrograms, i.c.v.) almost abolished the reduction in mortality rate, but inhibited the pressor effect of oxotremorine, partially. These results indicate that oxotremorine-induced pressor response and decrease in mortality in rats with severe hemorrhagic shock are primarily mediated via central muscarinic and nicotinic receptors, respectively. AV3V region was previously reported to be involved in pressor and natriuretic effects of i.c.v. carbachol in normotensive rats. In the present study, the electrolytic lesions of AV3V region significantly inhibited oxotremorine-induced increases in both blood pressure and survival rate in rats subjected to hemorrhagic shock. These findings indicate that AV3V region plays a major role in cholinergic cardiovascular control in hypotensive animals as well as normotensives.

Colchicine injected into the anterior forebrain in rats decreases blood pressure without changing the responses to haemorrhage

In a previous publication rats were shown to develop a marked positive sodium balance 6 days after injection of 1 microgram colchicine into their anterior forebrains. This was thought to be the normal hydromineral balance response to correct an hypothesised colchicine-induced decrease in blood volume (and/or pressure). To test this hypothesis, fluid and sodium intakes and excretion were measured before and then for 6 days following injection of colchicine (1 microgram in 250 nl) into the anterior forebrain, around the diagonal band of Broca, in male Wistar rats. At the end of the 6 days the animals were anaesthetised with urethane and blood pressure measured continuously before during and after a 3.5 ml haemorrhage. Blood pressure was significantly reduced (86.4 +/- 3.9 n = 11 vs. 67.7 +/- 3.4 mm Hg n = 12, p < 0.01) in the colchicine-treated rats compared to the controls and failed to recover following the haemorrhage. Measured blood parameters were similar in both colchicine-treated and nontreated groups, including plasma levels of vasopressin both before and following the haemorrhage. These results suggest that the colchicine injections may have compromised a central component of the sympathetic nervous system, thereby leading to the significant decrease in blood pressure without compensatory vasopressin release, and the lack of recovery of the decrease in blood pressure following a haemorrhage.

Increased dietary salt sensitizes vasomotor neurons of the rostral ventrolateral medulla

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

Strategies for investigation of CNS mechanisms of phenotypic variation in blood pressure and salt appetite in genetic hypertensive rats

Several characteristics of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY), make these homozygous strains particularly well suited for investigating the interactions of salt appetite, blood pressure control, and their neuroendocrine substrates. Appropriate genetic and developmental investigations of sources of variation in salt appetite, blood pressure, and their putative neuroendocrine substrates in these homozygous strains can provide valuable insights into fundamental mechanisms of disease, as well as factors controlling homeostatic behavioral and physiological processes. However, inappropriate use of these strains can produce misleading, although seductively plausible, conclusions regarding mechanisms. Selective inbreeding for hypertension has concentrated in SHR the "high pressure" allele for several genes that influence blood pressure, whereas breeding for normal blood pressure has left WKY with the "normal pressure" allele for all or most of these genes. In principle, inbred hypertensive strains could provide information about specific genetic alterations that mediate the hypertensive phenotype. The benefits of work with these strains are discussed, but several false assumptions and logical pitfalls are described that might cause misleading or erroneous interpretations of results from work with such strains. These problems illustrate the importance of the research strategy in elucidating the particular information that can be provided by these inbred animal models of hypertension. Two strategic approaches for studying hypertension and other genetically determined or influenced characteristics in inbred animal models such as SHR are discussed: cosegregation analysis for identifying or rejecting genetic linkage, and brain graft techniques for identifying brain specific genetic influences on cardiovascular or behavioral phenotypes. Examples of each approach are described.

Effects of long-term air jet noise and dietary sodium chloride in borderline hypertensive rats

The hypothesis that simultaneous exposure to a high (8%) sodium chloride diet and behavioral stress (air jet noise) would act synergistically to increase blood pressure was investigated in male borderline hypertensive rats. Rats were fed either a 1% or an 8% sodium chloride diet beginning at 6 weeks of age. Rats in the Air Noise condition were restrained and exposed to random blasts of air jet noise for 2 h/d, 5 d/wk, from 7 to 17 weeks of age. Controls either were placed in identical restrainers and test chambers but not exposed to air jet noise (Restrained Control) or were left undisturbed (Maturation Control). Biweekly indirect blood pressure measurements showed that by 17 weeks of age, the high-sodium chloride diet and air jet noise exposure produced additive increases in blood pressure. Direct blood pressure measurements at 18 weeks of age confirmed the higher systolic pressures in borderline hypertensive rats exposed to both an 8% sodium chloride diet and air jet noise. After ganglionic blockade, the blood pressure of rats in the Air Noise group remained higher than that of Restrained and Maturation Controls, suggesting that the increased blood pressure of air jet noise-exposed rats was not maintained by increased autonomic activity. Blood pressure after maximal vasodilation by hydralazine was increased in rats exposed to both an 8% sodium chloride diet and air jet noise compared with other groups. Baroreceptor reflex sensitivity (tested by graded doses of angiotensin II) did not differ among groups.(ABSTRACT TRUNCATED AT 250 WORDS)

The borderline hypertensive rat (BHR) as a model for environmentally-induced hypertension: a review and update

In recent years, the burgeoning disciplines of health psychology and behavioral medicine have renewed interest in the important role that environmental factors can play in the disease process. Nowhere is this concern more well-founded than in the area of cardiovascular disorders, particularly hypertension. Epidemiologists and clinicians have long suspected that stressful life events can be a sufficient trigger for the expression of hypertension in some individuals. To understand better the ways in which these variables interact in the disease process, researchers have tried, with limited success, to produce experimental hypertension in animals by exposing them to stressful environmental paradigms. Additionally, recent investigations using the borderline hypertensive rat (BHR) have demonstrated the important role genetic factors can play in mediating both the behavioral and cardiovascular responses to environmental stressors. The current paper will review these attempts and discuss recent data from experiments using a relatively new animal model that appears to be especially appropriate for the study of environmental-genetic factors in the elaboration of essential hypertension. We will also discuss potential mechanisms by which environmental stress influences arterial pressure and suggest avenues for further inquiry into the stress-disease relationship.

Aqueduct block markedly reduces mortality and hypertension in post-deoxycorticosterone acetate Dahl salt-resistant rats

When Dahl salt-resistant (DR) rats are given mild post-deoxycorticosterone acetate (DOCA) hypertension, they will have, within 8 weeks, a 53% mortality on a high NaCl diet, without a rise of blood pressure. Forty-two DR rats were given DOCA in silicone (250 mg/kg) and 1% NaCl to drink. After 4 weeks, the DOCA and 1% saline were removed and replaced with a low NaCl diet and tap water. One week later, they were divided into two groups perfectly matched for blood pressure (154 mm Hg). One group had the aqueduct of Sylvius blocked with silicone and epoxy materials; the other group had a sham block. After 4 more recovery weeks on a low NaCl diet, blood pressure averaged 171 mm Hg in sham rats and 147 mm Hg in truly blocked rats (p less than 0.0001). Thus, the aqueduct block prevented most of the post-DOCA hypertension and permitted a strong post-DOCA recovery from the acute DOCA hypertension. The rats with the sham block had an actual rise in blood pressure during the post-DOCA recovery period. The vicious cycle leading to permanent post-DOCA NaCl hypertension was broken by the aqueduct block. Then both groups began an 8% high NaCl diet, and after 4 weeks, blood pressure averaged 184 mm Hg in sham and 155 mm Hg in truly blocked rats (p less than 0.0001). After 12 weeks on 8% NaCl, all sham rats had died (28 of 28), whereas only one of 14 truly blocked rats had died (93% reduction in mortality, p less than 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Role of anteroventral third ventricle and vasopressin in renal response to stress in borderline hypertensive rats

The borderline hypertensive rat is the first filial offspring of the spontaneously hypertensive rat and the Wistar-Kyoto rat. In response to acute environmental stress (air jet), the borderline hypertensive rat exhibits a diuretic response, whereas the parental strains exhibit an antidiuretic response (spontaneously hypertensive rat) or no change in urine flow rate (Wistar-Kyoto rat). This study sought to investigate the role of the periventricular tissue surrounding the anteroventral third ventricle and vasopressin release in the diuretic response of the borderline hypertensive rat to acute environmental stress. Sixteen-week-old borderline hypertensive rats who had consumed a 1% NaCl diet for 10-12 weeks were given either electrolytic lesions of the anteroventral portion of the third ventricle or sham lesions. When exposed to acute environmental stress 4 weeks later, the increase in volume of dilute urine seen in the sham-lesion rats was not observed in the lesion rats. Plasma vasopressin concentration was decreased by acute environmental stress in the sham-lesion rats (15.2 +/- 4.0 to 10.9 +/- 1.7 pg/ml, p less than 0.05) but was unchanged in the lesion rats (12.3 +/- 2.0 to 13.4 +/- 4.0 pg/ml). In a separate group of intact borderline hypertensive rats, a constant intravenous infusion of vasopressin prevented the diuretic response to acute environmental stress. The results suggest that acute environmental stress produces a diuresis in the borderline hypertensive rats via a decrease in plasma vasopressin concentration that is dependent on the integrity of the periventricular tissue of the anteroventral portion of the third ventricle.

The effects of exercise training on salt-induced hypertension in the borderline hypertensive rat

The borderline hypertensive rat (BHR) develops severe hypertension when chronically subjected to either a high salt diet or behavioral stress. Previous research has shown that daily exercise can attenuate the development of stress-induced hypertension in the BHR. The current study sought to determine whether exercise might also exert a similar protective effect on salt-induced hypertension. Two groups of BHR were placed on a high salt diet for 20 weeks; one of the groups also engaged in daily swim training for the entire 20 week period. A third group of BHR served as maturation controls. At the end of the experimental period, direct measurement of heart rate in conscious subjects revealed a significant resting bradycardia in the trained group on a high salt diet but no significant group differences with respect to blood pressure level. Cardiovascular responses to an episode of acute stress revealed that BHR are capable of elevating pressure in response to a novel stressor after swim training. These observations suggest that exercise may be an environmental intervention capable of increasing cardiovascular responses to acute stressors.

Renal manifestations of NaCl sensitivity in borderline hypertensive rats

Compared with the normotensive Wistar-Kyoto rat, the spontaneously hypertensive rat exhibits exaggerated alterations in renal sympathetic nerve activity and excretory function during volume expansion (exaggerated natriuresis) and environmental stress (antinatriuresis). The borderline hypertensive rat is the first filial offspring of the spontaneously hypertensive rat and the Wistar-Kyoto rat and develops hypertension with increased dietary NaCl intake. The present investigation sought to determine whether the dietary NaCl intake-induced transition from the normotensive state of the Wistar-Kyoto parent to the hypertensive state of the spontaneously hypertensive parent in the borderline hypertensive rat was accompanied by a similar transition of the renal sympathetic nerve activity and excretory responses to volume expansion and environmental stress. Borderline hypertensive rats fed a 1% NaCl diet remained normotensive and exhibited renal sympathetic nerve activity and excretory responses to volume expansion and environmental stress that were similar to those of their Wistar-Kyoto parent. Borderline hypertensive rats fed an 8% NaCl diet developed hypertension and exhibited responses that were similar to those of their spontaneously hypertensive parent. Thus, the dietary NaCl intake-induced transition from the normotensive state of the Wistar-Kyoto parent to the hypertensive state of the spontaneously hypertensive parent in the borderline hypertensive rat was accompanied by a similar transition of the renal sympathetic nerve activity and excretory responses to volume expansion and environmental stress. The results suggest that increased dietary NaCl intake is able to induce or unmask the capabilities for these responses, which are genetically conveyed to the borderline hypertensive rat by the spontaneously hypertensive rat parent in latent forms.