Role of the median preoptic nucleus in the development of chronic deoxycorticosterone acetate (DOCA)-salt hypertension.

We have previously reported that the subfornical organ (SFO) does not contribute to the chronic hypertensive response to DOCA-salt in rats, and yet the organum vasculosum of the lamina terminalis (OVLT) plays a significant role in the development of deoxycorticosterone acetate (DOCA)-salt hypertension. Since efferent fibers of the OVLT project to and through the median preoptic nucleus (MnPO), the present study was designed to test the hypothesis that the MnPO is necessary for DOCA-salt hypertension in the rat. Male Sprague-Dawley rats underwent SHAM (MnPOsham; n = 5) or electrolytic lesion of the MnPO (MnPOx; n = 7) followed by subsequent unilateral nephrectomy and telemetry instrumentation. After recovery and during the experimental protocol, rats consumed a 0.1% NaCl diet and 0.9% NaCl drinking solution. Mean arterial pressure (MAP) was recorded telemetrically 5 days before and 21 days after DOCA implantation (100 mg/rat; SQ). The chronic pressor response to DOCA was attenuated in MnPOx rats by Day 11 of treatment and continued such that MAP increased 25 ± 3 mmHg in MnPOsham rats by Day 21 of DOCA compared to 14 ± 3 mmHg in MnPOx rats. These results support the hypothesis that the MnPO is an important brain site of action and necessary for the full development of DOCA-salt hypertension in the rat.

Neurons of the median preoptic nucleus contribute to chronic angiotensin II-salt induced hypertension in the rat.

Experiments were designed to test the hypothesis that median preoptic (MnPO) neurons are necessary for the full hypertensive response to chronic angiotensin II (AngII) in rats consuming a high salt diet. The MnPO is implicated in many of the physiologic actions of AngII, primarily acting as a downstream nucleus to AngII binding at circumventricular organs such as the organum vasculosum of the lamina terminalis (OVLT). We have previously shown a prominent effect of lesion of the OVLT on the chronic hypertensive effects of AngII in rats consuming high salt. Additionally, we have shown that lesion of the MnPO attenuated the hypertensive response to chronic intravenous infusion of AngII in rats. However, whether MnPO neurons or fibers of passage contribute to this response is not clear. Male Sprague Dawley rats were randomly assigned to either sham (SHAM; n = 8) or ibotenic acid lesion of the MnPO (MnPOx; n = 6). In the MnPOx group, 200 nl of ibotenic acid in phosphate buffer saline (5 µg/µl) was injected into each of 3 predetermined coordinates targeted at the entire MnPO. After a week of recovery, rats were instrumented with radiotelemetric pressure transducers, provided 2.0% NaCl diet and distilled water ad libitum and given another week to recover. After 3 days of baseline measurements, osmotic minipumps were implanted subcutaneously in all rats for administration of AngII at a rate of 150 ng/kg/min. Blood pressure measurements were made for 14 days after minipump implantation. By day 7 of AngII treatment, blood pressure responses appeared to plateau in both groups while the hypertensive response was markedly attenuated in MnPOx rats (MnPOx, 122 ± 6 mmHg; SHAM, 143 ± 8 mmHg). These results support the hypothesis that neurons of the MnPO are involved in the central pathway mediating the chronic hypertensive effects of AngII in rats consuming a high salt diet.

Development of a Chemical Reproductive Aging Model in Female Rats.

Women are born with an abundant but finite pool of ovarian follicles, which naturally and progressively decreased during their reproductive years until menstrual periods stop permanently (menopause). Perimenopause represents the transition from reproductive to non-reproductive life. It is usually characterized by neuroendocrine, metabolic and behavioral changes, which result from a follicular depletion and reduced number of ovarian follicles. During this period, around 45-50 years old, women are more likely to express mood disorders, anxiety, irritability and vasomotor symptoms. The current animal models of reproductive aging do not successfully replicate human perimenopause and the gradual changes that occur in this phase. While the traditional rat model of menopause involves ovariectomy or surgical menopause consisting of the rapid and definitive removal of the ovaries resulting in a complete loss of all ovarian hormones, natural or transitional menopause is achieved by the selective loss of ovarian follicles (perimenopause period). However, the natural aging rodent (around 18-24 months) model fails to reach very low estrogen concentrations and overlaps the processes of somatic and reproductive aging. The chronic exposure of young rodents to 4-vinylcyclohexene diepoxide (VCD) is a well-established experimental model for perimenopause and menopause studies. VCD induces loss of ovarian small follicles (primary and primordial) in mice and rats by accelerating the natural process of atresia (apoptosis). The VCD, ovary-intact or accelerated ovarian failure (AOF) model is the experimental model that most closely matches natural human progression to menopause mimicking both hormonal and behavioral changes typically manifested by women in perimenopause. Graphical abstract: The female reproductive system is regulated by a series of neuroendocrine events controlled by central and peripheral components. (A). The mechanisms involved in this control are extremely complex and have not yet been fully clarified. In female mammals whose ovulation (the most important event in a reproductive cycle) occurs spontaneously, reproductive success is achieved through the precise functional and temporal integration of the hypothalamus-pituitary-ovary (HPO) axis. (B). In women, loss of fertility appears to be primarily associated with exhaustion of ovarian follicles, and this process occurs progressively until complete follicular exhaustion marked by the final menstrual period (FMP). (C). While in female rodents, reproductive aging seems to begin as a neuroendocrine process, in which changes in hypothalamic/pituitary function appear independently of follicular atresia. The traditional rat model of menopause, ovariectomy or surgical menopause consists of the rapid and definitive removal of the ovaries resulting in a complete loss of all ovarian hormones. (D). The chronic exposure (15-30 days) to the chemical compound 4-vinylcyclehexene diepoxide (VCD) in young rodents accelerates gradual failure of ovarian function by progressive depletion of primordial and primary follicles, but retains residual ovarian tissue before brain alterations that occurs in women in perimenopause. Low doses of VCD cause the selective destruction of the small preantral follicles of the ovary without affecting other peripheral tissues.

DOCA-salt hypertension and the role of the OVLT-sympathetic-gut microbiome axis.

Hypertension is a multifaceted condition influenced by genetic and environmental factors and estimated to cause 9.4 million deaths globally every year. Recently, there has been growing interest in understanding the gut microbe-host interaction in the maintenance of health or disease states, but relatively few studies have shown an association between the gut microbiome and specific types of hypertension. The deoxycorticosterone acetate (DOCA)-salt model of hypertension in rats is known to have a neurogenic component linked to increased sympathetic nervous system activity. As such, our lab has recently shown the hypertensive response in DOCA treated rats requires an intact organum vasculosum of the lamina terminalis (OVLT), a central hypothalamic circumventricular organ. Currently, we hypothesize the OVLT mediates changes in the gut microbiome associated with concomitant hypertension. Herein, we report that the hypertensive effects of DOCA-salt treatment were significantly attenuated throughout the 24-hour day/night cycle in OLVT lesioned rats on days 1, 3, and 9-21 of DOCA treatment compared with sham rats. Increased blood pressure (BP) in DOCA-salt treated rats was accompanied by specific changes in regional gut microbial populations yet was mitigated and offset by lesion of the OVLT. Furthermore, bacterial populations in OVLT-lesioned rats with attenuated hypertension more closely resembled those in normal control rats. We conclude that DOCA-salt hypertension is associated with specific microbiome changes in the gut, and the attenuated hypertensive effects of DOCA-salt in OVLT-lesioned rats is mediated in part through counteracting changes in these bacterial populations.

Development of the Deoxycorticosterone Acetate (DOCA)-salt Hypertensive Rat Model.

According to the World Health Organization (WHO), nearly 1.13 billion people worldwide have hypertension, a major factor responsible for premature death globally. The inherent multifactorial nature of hypertension makes its study difficult since the chronic rise in blood pressure depends on the intricate connection between dietary, genetic and environmental factors. Therefore, the pathophysi-ology of hypertension is not completely understood. For these reasons, there is an ongoing search for animal models that better mimic changes resulting from this disease. Because of its complexity, the use of animal models aimed at elucidating the pathogenesis of hypertension and to evaluate new therapeutic possibilities is an important tool for understanding this disease since it enables consistent experimental strategies that are impractical in humans. Over time, many animal models have been developed for the study of chronic increases in blood pressure ranging from genetic models that include the spontaneously hypertensive rat (SHR) and genetic manipulations, such as the TGR (mRen2) rat, as well as neurogenic or endocrine models. One of the most commonly used hypertensive rat models today is that of hypertension induced by treatment with deoxycorticosterone acetate associated with high sodium intake, i.e., the DOCA-salt model. This model is known to have a neurogenic component linked to increased sympathetic nervous system activity, and as such the DOCA-salt model promotes cross-talk between endocrine and neural components that lead to increased blood pressure, and may impact the functioning of other organs.

Lesion of the OVLT markedly attenuates chronic DOCA-salt hypertension in rats.

Lesions of the anteroventral third ventricle (AV3V region) are known to prevent many forms of experimental hypertension, including mineralocorticoid [deoxycorticosterone acetate (DOCA)-salt] hypertension in the rat. However, AV3V lesions include the organum vasculosum of the lamina terminalis (OVLT), portions of the median preoptic nucleus, and efferent fibers from the subfornical organ (SFO), thereby limiting the ability to define the individual contribution of these structures to the prevention of experimental hypertension. Having previously reported that the SFO does not play a significant role in the development of DOCA-salt hypertension, the present study was designed to test the hypothesis that the OVLT is necessary for DOCA-salt hypertension in the rat. In uninephrectomized OVLT-lesioned (OVLTx; n = 6) and sham-operated ( n = 4) Sprague-Dawley rats consuming a 0.1% NaCl diet and 0.9% NaCl drinking solution, 24-h mean arterial pressure (MAP) was recorded telemetrically 5 days before and 21 days after DOCA implantation (100 mg sc per rat). No differences in control MAP were observed between groups. The chronic pressor response to DOCA was attenuated in OVLTx rats such that MAP increased to 133 ± 3 mmHg in sham-operated rats by day 21 of DOCA compared with 120 ± 4 mmHg (means ± SE) in OVLTx rats. These results support the hypothesis that the OVLT is an important brain site of action for the pathogenesis of DOCA-salt hypertension in the rat.

Role of afferent and efferent renal nerves in the development of AngII-salt hypertension in rats.

Hypertension is the leading modifiable risk factor for death worldwide, yet the causes remain unclear and treatment remains suboptimal. Catheter-based renal denervation (RDNX) is a promising new treatment for resistant hypertension, but the mechanisms underlying its antihypertensive effect remain unclear. We recently found that RDNX attenuates deoxycorticosterone acetate-salt hypertension and that this is dependent on ablation of afferent renal nerves and is associated with decreased renal inflammation. To determine if this is common to other models of salt-sensitive hypertension, rats underwent complete RDNX (n = 8), selective ablation of afferent renal nerves (n = 8), or sham denervation (n = 8). Mean arterial pressure (MAP) and heart rate were measure by telemetry and rats were housed in metabolic cages for measurement of sodium and water balance. Rats were then subjected to angiotensin II (AngII)-salt hypertension (10 ng/kg/min, intravenous + 4% NaCl diet) for 2 weeks. At the end of the study, renal T-cell infiltration was quantified by flow cytometry. AngII resulted in an increase in MAP of ~50 mmHg in all three groups with no between group differences, and a transient bradycardia that was blunted by selective ablation of afferent renal nerves. Sodium and water balance were unaffected by AngII-salt treatment and similar between groups. Lastly, AngII infusion was not associated with T-cell infiltration into the kidneys, and T-cell counts were unaffected by the denervation procedures. These results suggest that AngII-salt hypertension in the rat is not associated with renal inflammation and that neither afferent nor efferent renal nerves contribute to this model.

Overexpression of copper/zinc superoxide dismutase in the median preoptic nucleus improves cardiac function after myocardial infarction in the rat.

Previous reports indicate that overexpression of copper/zinc superoxide dismutase (CuZnSOD), an intracellular superoxide (O2 (•-) ) scavenging enzyme, in the brain subfornical organ improves cardiac function in a mouse model of heart failure (HF). A downstream hypothalamic site, the MnPO, may act as a relay centre for O2 (•-) to serve as a mediator in the pathophysiology of HF. To test the hypothesis that elevated O2 (•-) in the MnPO contributes to the pathophysiology of HF and decreased cardiac function, we injected adenovirus encoding CuZnSOD (AdCuZnSOD, n=7) or control empty adenovirus vector (AdEmpty, n=7) into the MnPO of normal rats. Subsequently, rats were subjected to coronary artery ligation to create a myocardial infarct (MI) of the left ventricle. Cardiac function was monitored via echocardiography. Upon completion, rat brains were examined for CuZnSOD expression in MnPO via immunofluorescence and histopathological analyses of cardiac infarct size were conducted. Baseline (EF) ejection fractions (%) of AdCuZnSOD and AdEmpty rats were 73 ± 1 and 71 ± 1, respectively. Two weeks after MI, EF was significantly decreased in both groups of rats (AdCuZnSOD: 51 ± 3, AdEmpty: 46 ± 1). In contrast, by 4 weeks post MI, EF had improved to 64 ± 2 in AdCuZnSOD rats, yet was only 52 ± 1 in AdEmpty rats, and this was accompanied by lower plasma noradrenaline levels in AdCuZnSOD rats (0.49 ± 0.19 ng/mL) compared to AdEmpty rats (1.20 ± 0.32 ng/mL). In conclusion, despite decreases in EF early after MI, overexpression of CuZnSOD in the MnPO was related to an improvement in left ventricular function and concomitant decreased plasma noradrenaline levels 4 weeks post MI.

Angiotensin II-Induced Hypertension Is Attenuated by Overexpressing Copper/Zinc Superoxide Dismutase in the Brain Organum Vasculosum of the Lamina Terminalis.

Angiotensin II (AngII) can access the brain via circumventricular organs (CVOs), including the subfornical organ (SFO) and organum vasculosum of the lamina terminalis (OVLT), to modulate blood pressure. Previous studies have demonstrated a role for both the SFO and OVLT in the hypertensive response to chronic AngII, yet it is unclear which intracellular signaling pathways are involved in this response. Overexpression of copper/zinc superoxide dismutase (CuZnSOD) in the SFO has been shown to attenuate the chronic hypertensive effects of AngII. Presently, we tested the hypothesis that elevated levels of superoxide (O2 (∙-)) in the OVLT contribute to the hypertensive effects of AngII. To facilitate overexpression of superoxide dismutase, adenoviral vectors encoding human CuZnSOD or control adenovirus (AdEmpty) were injected directly into the OVLT of rats. Following 3 days of control saline infusion, rats were intravenously infused with AngII (10 ng/kg/min) for ten days. Blood pressure increased 33 ± 8 mmHg in AdEmpty rats (n = 6), while rats overexpressing CuZnSOD (n = 8) in the OVLT demonstrated a blood pressure increase of only 18 ± 5 mmHg after 10 days of AngII infusion. These results support the hypothesis that overproduction of O2 (∙-) in the OVLT plays an important role in the development of chronic AngII-dependent hypertension.

Over-expression of copper/zinc superoxide dismutase in the median preoptic nucleus attenuates chronic angiotensin II-induced hypertension in the rat.

The brain senses circulating levels of angiotensin II (AngII) via circumventricular organs, such as the subfornical organ (SFO), and is thought to adjust sympathetic nervous system output accordingly via this neuro-hormonal communication. However, the cellular signaling mechanisms involved in these communications remain to be fully understood. Previous lesion studies of either the SFO, or the downstream median preoptic nucleus (MnPO) have shown a diminution of the hypertensive effects of chronic AngII, without providing a clear explanation as to the intracellular signaling pathway(s) involved. Additional studies have reported that over-expressing copper/zinc superoxide dismutase (CuZnSOD), an intracellular superoxide (O2·-) scavenging enzyme, in the SFO attenuates chronic AngII-induced hypertension. Herein, we tested the hypothesis that overproduction of O2·- in the MnPO is an underlying mechanism in the long-term hypertensive effects of chronic AngII. Adenoviral vectors encoding human CuZnSOD (AdCuZnSOD) or control vector (AdEmpty) were injected directly into the MnPO of rats implanted with aortic telemetric transmitters for recording of arterial pressure. After a 3 day control period of saline infusion, rats were intravenously infused with AngII (10 ng/kg/min) for ten days. Rats over-expressing CuZnSOD (n = 7) in the MnPO had a blood pressure increase of only 6 ± 2 mmHg after ten days of AngII infusion while blood pressure increased 21 ± 4 mmHg in AdEmpty-infected rats (n = 9). These results support the hypothesis that production of O2·- in the MnPO contributes to the development of chronic AngII-dependent 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.

OVLT lesion decreases basal arterial pressure and the chronic hypertensive response to AngII in rats on a high-salt diet.

We have reported that lesion of the organum vasculosum of the lamina terminalis (OVLT) has no effect on basal levels of mean arterial pressure (MAP) but abolishes the hypertensive effects of angiotensin II (AngII) in rats consuming a normal-salt diet. These results suggest that the OVLT does not contribute to regulation of MAP under conditions of normal salt intake, but it is an important brain site for the hypertensive actions of AngII. The OVLT has been proposed as a major sodium sensor in the brain and the hypertensive effects of AngII are exacerbated by high-salt intake. Therefore, the objective of this study was to investigate the role of the OVLT during AngII-induced hypertension in rats fed a high-salt diet. Male Sprague-Dawley rats underwent sham (Sham; n = 9) or OVLT lesion (OVLTx; n = 8) surgery and were placed on a high-salt (2% NaCl) diet. MAP was measured by radio telemetry during three control days, 10 days of AngII infusion (10 ng/kg/min, i.v.), and a 3-day recovery period. MAP was significantly lower in OVLTx (97 ± 2 mmHg) compared to Sham (106 ± 1 mmHg) rats during the control period (P < 0.05). Moreover, the chronic pressor response to AngII was markedly attenuated in OVLTx rats. MAP increased 58 ± 3 mmHg in Sham rats by Day 10 of AngII compared to a 40 ± 7 mmHg increase in OVLTx rats (P < 0.05). We conclude that (1) the OVLT regulates the basal levels of MAP in rats consuming a high-salt and (2) the OVLT is an important brain site of action for the pathogenesis of AngII-salt hypertension in the rat. Supported by HL076312.

The role of the subfornical organ in angiotensin II-salt hypertension in the rat.

Hypertension caused by chronic infusion of angiotensin II (Ang II) in experimental animals is dependent, in part, on increased activity of the sympathetic nervous system. This chronic sympathoexcitatory response is amplified by a high-salt diet, suggesting an interaction of circulating Ang II and dietary salt on sympathetic regulatory pathways in the brain. The present study tested the hypothesis that the subfornical organ (SFO), a forebrain circumventricular organ known to be activated by circulating Ang II, is crucial to the pathogenesis of hypertension induced by chronic Ang II administration in rats on a high-salt diet (Ang II-salt model). Rats were randomly selected to undergo either subfornical organ lesion (SFOx) or sham surgery (Sham) and then placed on a high-salt (2% NaCl) diet. One week later, rats were instrumented for radiotelemetric measurement of mean arterial pressure (MAP) and heart rate (HR) and placed in metabolic cages to measure sodium and water balance. Baseline MAP was slightly (but not statistically) lower in SFOx compared with Sham rats during the 5 day control period. During the subsequent 10 days of Ang II administration, MAP was statistically lower in SFOx rats. However, when MAP responses to Ang II were analysed by comparing the change from the 5 day baseline period, only on the fifth day of Ang II was MAP significantly different between groups. There were no differences between groups for water or sodium balance throughout the protocol. We conclude that, although the SFO is required for the complete expression of Ang II-salt hypertension in the rat, other brain sites are also involved.

Lesion of the Subfornical Organ attenuates Neuronal Activation of the Paraventricular Nucleus in response to Angiotensin II in normal rats.

INTRODUCTION/HYPOTHESIS: The subfornical organ, one of the central circumventricular organs, has been shown to mediate many of the effects of circulating angiotensin II (AngII). Where these signals are processed downstream is not fully understood. The SFO does indeed project to prominent cardiovascular regulatory centers such as the paraventricular nucleus (PVN), of whose neurons are activated by central AngII. We reasoned that AngII sensed at the SFO would cause neuronal activation at downstream hypothalamic areas such as the median preoptic nucleus and paraventricular nucleus, and as such would be diminished in animals with lesions of the SFO. MATERIALS AND METHODS: To test this hypothesis, groups of rats underwent either SFO lesion (SFOx) or sham operation. Five days later rats were instrumented with radiotelemetry transducers for monitoring of mean arterial pressure (MAP) and venous catheters for infusions. MAP and heart rate were measured continuously. After a 4 day control period, infusion of AngII (0.575 µg/kg/min) was begun for a period of 2 hours. Rats were then sacrificed and brains were processed for neuronal Fos expression. RESULTS: AngII produced Fos expression in the SFO, MnPO and PVN of sham rats. Fos expression was greatly attenuated in the PVN of SFOx rats. CONCLUSION: These results support our hypothesis, suggesting that AngII sensitive neurons of the SFO can mediate neuronal activation in the PVN.

Role of the organum vasculosum of the lamina terminalis for the chronic cardiovascular effects produced by endogenous and exogenous ANG II in conscious rats.

Endogenous and exogenous circulating ANG II acts at one of the central circumventricular organs (CVOs), the subfornical organ (SFO), to modulate chronic blood pressure regulation. However, at the forebrain, another important CVO is the organum vasculosum of the lamina terminalis (OVLT). In the present study, we tested the hypothesis that the OVLT mediates the hypertension or the hypotension produced by chronic infusion of ANG II or losartan (AT1 antagonist), respectively. Six days after sham or OVLT electrolytic lesion, male Sprague-Dawley rats (280-320 g, n = 6 per group) were instrumented with intravenous catheters and radiotelemetric blood pressure transducers. Following another week of recovery, rats were given 3 days of saline control infusion (7 ml/day) and were then infused with ANG II (10 ng·kg(-1)·min(-1)) or losartan (10 mg·kg(-1)·day(-1)) for 10 days, followed by 3 recovery days. Twenty-four hour average measurements of mean arterial pressure (MAP) and heart rate (HR) were made during this protocol. Hydromineral balance (HB) responses were measured during the experimental protocol. By day 9 of ANG II treatment, MAP had increased 16 ± 4 mmHg in sham rats but only 4 ± 1 mmHg in OVLT lesioned rats without changes in HR or HB. However, the hypotension produced by 10 days of losartan infusion was not modified in OVLT lesioned rats. These results suggest that the OVLT might play an important role during elevation of plasma ANG II, facilitating increases of blood pressure but is not involved with baseline effects of endogenous ANG II.

Simultaneous administration of Ang(1-7) or A-779 does not affect the chronic hypertensive effects of angiotensin II in normal rats.

HYPOTHESIS: The following studies were designed to test the hypothesis that simultaneous administration of either Ang(1-7) or its antagonist A-779 would affect the chronic hypertensive effects of angiotensin II (Ang II). INTRODUCTION: Despite the well-described actions of Ang(1-7) and its role possessing opposite actions to Ang II, there have been few studies examining the role of Ang(1-7) in a chronic setting. It is well established that Ang(1-7) plays a protective role in preventing deleterious effects of Ang II in the heart, but little is known of its role in modulating the chronic hypertensive effects of Ang II. MATERIALS AND METHODS: Rats were instrumented with venous catheters and telemetric pressure transducers. Arterial pressure responses were measured in rats treated with Ang II (10 ng/kg/ min) (n=9) and compared with those treated with Ang II and Ang(1-7) (24 microg/kg/h) (n=8), or the Ang(1-7) antagonist A-779 (24 microg/kg/h) (n=7) for 8 days. RESULTS: Mean arterial pressure rose significantly and similarly in all three groups of rats, such that by day 8 of Ang II infusion, pressures had risen 25-30 mmHg in all rats. CONCLUSIONS: These results do not support the hypothesis that the chronic hypertensive effects of Ang II in normal rats are altered by co-administration of either Ang(1-7) or A-779.

Role of the median preoptic nucleus in the chronic hypotensive effect of losartan in sodium-replete normal rats.

1. We have shown previously that the chronic hypotensive effect of the angiotensin II AT1 receptor antagonist losartan is mediated, in part, by the subfornical organ (SFO). However, the neural pathway(s) mediating this central effect of losartan downstream from the SFO has not been completely elucidated. 2. The present study was designed to test the hypothesis that the median preoptic nucleus (MnPO) is a crucial part of the neural pathway necessary for the chronic hypotensive effect of losartan. To test this hypothesis, male Sprague-Dawley rats were subjected to either Sham or electrolytic lesion of the MnPO (MnPOx). Rats were instrumented with radiotelemetric transducers and aortic flow probes for the continuous measurement of mean arterial pressure (MAP) and heart rate and cardiac output (CO), respectively. Total peripheral resistance (TPR) was calculated as MAP/CO. After 3 days of baseline measurements, rats were infused intraperitoneally with losartan (10 mg/kg per day) via an osmotic minipump at a rate of 5 microL/min. 3. The data revealed that, by Day 9 of losartan treatment, MAP had decreased 34 +/- 2 mmHg in MnPOx rats (n = 9), whereas the MAP of Sham-lesioned rats (n = 8) had only decreased 24 +/- 3 mmHg. These findings were accompanied by a greater decrease in TPR in MnPOx compared with Sham rats (-0.464 vs-0.237 mmHg/mL per min, respectively), whereas CO remained unchanged throughout the study protocol. 4. These results do not support the hypothesis that an intact MnPO is necessary to mediate the full chronic hypotensive effect of losartan in normal rats. Instead, they appear to suggest that the MnPO may play an important role in buffering the profound hypotension induced by losartan.

The cardiovascular response of normal rats to dual lesion of the subfornical organ and area postrema at rest and to chronic losartan.

The subfornical organ (SFO) and the area postrema (AP), two of the sensory circumventricular organs (CVO), are known to play a role in the chronic central control of blood pressure. In previous studies in which these regions were independently lesioned, the chronic hypotensive effects of the AT(1) receptor blocker losartan (10 mg/kg/day) were attenuated by ~15 mm Hg. In the present study, we sought to investigate the effect of concurrent lesion of both the SFO and the AP on the cardiovascular effects of chronic losartan infusion in order to test the hypothesis that a greater attenuation of the hypotensive effects of losartan would be observed in rats with dual lesions. To do so, arterial pressure and heart rate responses to 10-day infusion of losartan were compared in sham rats and those with dual lesions of the AP and SFO. Two important findings resulted from this study. First, dual lesion rats exhibited a sustained and significant decrease in resting blood pressure (83+/-1 mm Hg vs. 104+/-1 mm Hg, respectively) and heart rate (356+/-3 bpm vs. 398+/-6 bpm, respectively) compared to sham animals. Secondly, rats with concurrent lesion of both the AP and the SFO demonstrated a significantly attenuated response to losartan compared to sham animals but showed no greater attenuation of losartan's chronic hypotensive effects than animals with lesion of either the SFO or the AP (approximately 15 mm Hg). Although these results do not support the stated hypothesis, they do suggest redundancy and compensatory roles of the AP and SFO in basal cardiovascular control.

Role of the median preoptic nucleus in chronic angiotensin II-induced hypertension.

Several lines of evidence implicate the median preoptic nucleus (MnPO) as a downstream site of activation following binding of angiotensin II (ANG II) at the subfornical organ and organum vasculosum of the lamina terminalis. We have shown previously that electrolytic lesion of the MnPO attenuated the increased blood pressure response to chronic intravenous infusion of ANG II. However, whether MnPO neurons or fibers that pass through this region contribute to this response is not clear. In the present study, to distinguish the relative importance of MnPO neurons from fibers of passage in the hypertensive response to chronic ANG II administration, male Sprague Dawley rats were randomly assigned to either sham (iSHAM) or ibotenic acid lesion of the MnPO (iMnPOx). In the iMnPOx group, 200 nl of ibotenic acid in phosphate buffer saline (5 microg/microl) was injected into each of 3 predetermined coordinates targeted at the entire MnPO. After a week of recovery, rats were instrumented with venous catheters, and radiotelemetric transducers for the intravenous administration of ANG II and the measurement of mean arterial pressure (MAP) and heart rate, respectively. Rats were given another week to recover. iSHAM and iMnPOx animals were then infused with saline (7 ml 0.9% NaCl/day) for 3 days as a control period, followed by 10 consecutive days of intravenous ANG II infusion (10 ng kg(-1) min(-1)), and finally a recovery period similar to control. Throughout the protocol, a 0.4% NaCl diet and distilled water were provided ad libitum. By day 8 of ANG II infusion, MAP had increased 54+/-2 mm Hg in iSHAM rats (n=8). The hypertensive response to ANG II was significantly attenuated in the iMnPOx rats (n=9), in which MAP had only increased 29+/-3 mm Hg. These results support the hypothesis that neurons of the MnPO are involved in the central neural pathway mediating the chronic hypertensive effects of ANG II.

Changing dietary sodium alters the chronic cardiovascular effects of losartan in rats.

INTRODUCTION: We have previously demonstrated a profound hypotensive response to the angiotensin II type 1 (AT1)-receptor antagonist losartan in rats consuming a normal salt diet that is not seen in salt-loaded rats, presumably due to a suppression of the renin-angiotensin system (RAS) by high sodium levels. The purpose of the present study was to examine the cardiovascular effects of changing dietary sodium intake during chronic treatment with losartan. We hypothesised that during blockade of AT1-receptors by chronic losartan infusion, when renin levels would be elevated regardless of dietary sodium, changing diets from high to normal or normal to high salt would have no effect on mean arterial pressure (MAP). MATERIALS AND METHODS: To test this hypothesis, groups of rats instrumented with radiotelemetry transducers for MAP monitoring and venous catheters for infusion were initially placed on either a 0.4% salt content diet, referred to as Losartan Normal diet - High salt diet (LosN-HI, n = 7), or a 4.0% salt content diet, referred to as Losartan High salt diet - Normal diet (LosHI-N, n = 9). After a thee-day control period, infusion of losartan was begun in all rats (10 mg/kg/day in 7 ml/day isotonic saline i.v.). After 10 days, diets were switched between groups and data were collected for another 10 days, after which losartan infusion was terminated for a 10-day recovery period. RESULTS: At the start of losartan infusion MAP was observed to be similar between LosN-HI rats (101+/-2 mmHg) and LosHI-N rats (101+/-2 mmHg). By day seven of the first 10 day protocol, MAP in LosN-HI rats had fallen to 71+/-4 mmHg while decreasing to 90+/-2 mmHg in LosHI-N rats. Five days after switching diets, MAP in LosN-HI rats had risen back to 85+/-3 mmHg, while MAP in LosHI-N rats had fallen to 75+/-2 mmHg. CONCLUSIONS: These results do not support our hypothesis, suggesting that changing dietary sodium can alter the chronic hypotensive response to losartan regardless of the initial state of the RAS.