The contribution of brain angiotensin II to the baroreflex regulation of renal sympathetic nerve activity in conscious normotensive and hypertensive rats

Afferent Renal Denervation Attenuates Sympathetic Overactivation From the Paraventricular Nucleus in Spontaneously Hypertensive Rats

BACKGROUND

The effectiveness of renal denervation (RDN) in reducing blood pressure and systemic sympathetic activity in hypertensive patients has been established. However, the underlying central mechanism remains unknown. This study aimed to investigate the role of RDN in regulating cardiovascular function via the central renin-angiotensin system (RAS) pathway.

METHODS

Ten-week-old spontaneously hypertensive rats (SHR) were subjected to selective afferent renal denervation (ADN) using capsaicin solution. We hypothesized that ADN would effectively reduce blood pressure and rebalance the RAS component of the paraventricular nucleus (PVN) in SHR.

RESULTS

The experimental results show that the ADN group exhibited significantly lower blood pressure, reduced systemic sympathetic activity, decreased chronic neuronal activation marker C-FOS expression in the PVN, and improved arterial baroreflex function, compared with the Sham group. Furthermore, ACE and AT1 protein expression was reduced while ACE2 and MAS protein expression was increased in the PVN of SHR after ADN.

CONCLUSIONS

These findings suggest that RDN may exert these beneficial effects through modulating the central RAS pathway.

Genetic Deletion of β-Arrestin 2 From the Subfornical Organ and Other Periventricular Nuclei in the Brain Alters Fluid Homeostasis and Blood Pressure

BACKGROUND

ANG (angiotensin II) elicits dipsogenic and pressor responses via activation of the canonical Gαq (G-protein component of the AT1R [angiotensin type 1 receptor])-mediated AT1R in the subfornical organ. Recently, we demonstrated that ARRB2 (β-arrestin 2) global knockout mice exhibit a higher preference for salt and exacerbated pressor response to deoxycorticosterone acetate salt. However, whether ARRB2 within selective neuroanatomical nuclei alters physiological responses to ANG is unknown. Therefore, we hypothesized that ARRB2, specifically in the subfornical organ, counterbalances maladaptive dipsogenic and pressor responses to the canonical AT1R signaling.

METHODS

Male and female Arrb2FLOX mice received intracerebroventricular injection of either adeno-associated virus (AAV)-Cre-GFP (green fluorescent protein) to induce brain-specific deletion of ARRB2 (Arrb2ICV-Cre). Arrb2FLOX mice receiving ICV-AAV-GFP were used as control (Arrb2ICV-Control). Infection with ICV-AAV-Cre primarily targeted the subfornical organ with few off targets. Fluid intake was evaluated using the 2-bottle choice paradigm with 1 bottle containing water and 1 containing 0.15 mol/L NaCl.

RESULTS

Arrb2ICV-Cre mice exhibited a greater pressor response to acute ICV-ANG infusion. At baseline conditions, Arrb2ICV-Cre mice exhibited a significant increase in saline intake compared with controls, resulting in a saline preference. Furthermore, when mice were subjected to water-deprived or sodium-depleted conditions, which would naturally increase endogenous ANG levels, Arrb2ICV-Cre mice exhibited elevated saline intake.

CONCLUSIONS

Overall, these data indicate that ARRB2 in selective cardiovascular nuclei in the brain, including the subfornical organ, counterbalances canonical AT1R responses to both exogenous and endogenous ANG. Stimulation of the AT1R/ARRB axis in the brain may represent a novel strategy to treat hypertension.

Angiotensin II and the Cardiac Parasympathetic Nervous System in Hypertension

The renin-angiotensin-aldosterone system (RAAS) impacts cardiovascular homeostasis via direct actions on peripheral blood vessels and via modulation of the autonomic nervous system. To date, research has primarily focused on the actions of the RAAS on the sympathetic nervous system. Here, we review the critical role of the RAAS on parasympathetic nerve function during normal physiology and its role in cardiovascular disease, focusing on hypertension. Angiotensin (Ang) II receptors are present throughout the parasympathetic nerves and can modulate vagal activity via actions at the level of the nerve endings as well as via the circumventricular organs and as a neuromodulator acting within brain regions. There is tonic inhibition of cardiac vagal tone by endogenous Ang II. We review the actions of Ang II via peripheral nerve endings as well as via central actions on brain regions. We review the evidence that Ang II modulates arterial baroreflex function and examine the pathways via which Ang II can modulate baroreflex control of cardiac vagal drive. Although there is evidence that Ang II can modulate parasympathetic activity and has the potential to contribute to impaired baseline levels and impaired baroreflex control during hypertension, the exact central regions where Ang II acts need further investigation. The beneficial actions of angiotensin receptor blockers in hypertension may be mediated in part via actions on the parasympathetic nervous system. We highlight important unknown questions about the interaction between the RAAS and the parasympathetic nervous system and conclude that this remains an important area where future research is needed.

Effects of intracerebroventricular leptin and orexin-A on the baroreflex control of renal sympathetic nerve activity in conscious rats fed a normal or high-fat diet

This study examined the effect of leptin and orexin-A on autonomic baroreflex control in conscious Wistar rats exposed to high-fat (45% fat) or normal (3.4%) diet for 4 weeks. Renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP) and heart rate (HR) were monitored during the generation of baroreflex gain curves and acute volume expansion (VEP). Intracerebroventricular (ICV) leptin (1 μg/min) increased RSNA in the normal diet group (0.31 ± 0.04 vs 0.23 ± 0.03 mV/s) and MAP in the high-fat diet group (115 ± 5 vs 105 ± 5 mm Hg, P < .05). Orexin-A (50 ng/min) increased RSNA, HR and MAP in the high-fat diet group (0.26 ± 0.03 vs 0.22 ± 0.02 mV/s, 454 ± 8 vs 417 ± 12 beats/min, 117 ± 1 vs 108 ± 1 mm Hg) and the normal diet group (0.18 ± 0.05 vs 0.17 ± 0.05 mV/s, 465 ± 10 vs 426 ± 6 beats/min, 116 ± 2 vs 104 ± 3 mm Hg). Baroreflex sensitivity for RSNA was increased during ICV leptin by 50% in the normal diet group, compared to 14% in the high-fat diet group (P < .05). Similarly, orexin-A increased baroreflex sensitivity by 56% and 50% in the high-fat and normal diet groups, respectively (all P < .05). During ICV saline, VEP decreased RSNA by 31 ± 5% (P < .05) after 10 minutes and the magnitude of this response was blunted during ICV infusion of leptin (17 ± 2%, P < .05) but not orexin-A in the normal diet group. RSNA response to VEP was not changed during ICV leptin or orexin-A in the high-fat diet group. These findings indicate possible central roles for leptin and orexin-A in modulating the baroreflexes under normal or increased fat intake in conscious rats and potential therapeutic approaches for obesity associated hypertension.

Comparison of high-fat style diet-induced dysregulation of baroreflex control of renal sympathetic nerve activity in intact and ovariectomized female rats: Renal sympathetic nerve activity in high-fat style diet fed intact and ovariectomized female rats

IMPACT STATEMENT

Over activation of renal sensory nerve in obesity blunts the normal regulation of renal sympathetic nerve activity. To date, there is no investigation that has been carried out on baroreflex regulation of renal sympathetic nerve activity in obese ovarian hormones deprived rat model, and the effect of renal denervation on the baroreflex regulation of renal sympathetic nerve activity. Thus, we investigated the role of renal innervation on baroreflex regulation of renal sympathetic nerve activity in obese intact and ovariectomized female rats. Our data demonstrated that in obese states, the impaired baroreflex control is indistinguishable between ovarian hormones deprived and non-deprived states. This study will be of substantial interest to researchers working on the impact of diet-induced hypertension in pre- and postmenopausal women. This study provides insight into health risks amongst obese women regardless of their ovarian hormonal status and may be integrated in preventive health strategies.

Acute Management of Hypertension Following Intracerebral Hemorrhage

Intracerebral hemorrhage (ICH) is responsible for approximately 15% of strokes annually in the United States, with nearly 1 in 3 of these patients dying without ever leaving the hospital. Because this disproportionate mortality risk has been stagnant for nearly 3 decades, a main area of research has been focused on the optimal strategies to reduce mortality and improve functional outcomes. The acute hypertensive response following ICH has been shown to facilitate ICH expansion and is a strong predictor of mortality. Rapidly reducing blood pressure was once thought to induce cerebral ischemia, though has been found to be safe in certain patient populations. Clinicians must work quickly to determine whether specific patient populations may benefit from acute lowering of systolic blood pressure (SBP) following ICH. This review provides nurses with a summary of the available literature on blood pressure control following ICH. It focuses on intravenous and oral antihypertensive medications available in the United States that may be utilized to acutely lower SBP, as well as medications outside of the antihypertensive class used during the acute setting that may reduce SBP.

Acute hypertensive response in patients with intracerebral hemorrhage pathophysiology and treatment

Acute hypertensive response is a common systemic response to occurrence of intracerebral hemorrhage which has gained unique prominence due to high prevalence and association with hematoma expansion and increased mortality. Presumably, the higher systemic blood pressure predisposes to continued intraparenchymal hemorrhage by transmission of higher pressure to the damaged small arteries and may interact with hemostatic and inflammatory pathways. Therefore, intensive reduction of systolic blood pressure has been evaluated in several clinical trials as a strategy to reduce hematoma expansion and subsequent death and disability. These trials have demonstrated either a small magnitude benefit (second intensive blood pressure reduction in acute cerebral hemorrhage trial and efficacy of nitric oxide in stroke trial) or no benefit (antihypertensive treatment of acute cerebral hemorrhage 2 trial) with intensive systolic blood pressure reduction compared with modest or standard blood pressure reduction. The differences may be explained by the variation in intensity of systolic blood pressure reduction between trials. A treatment threshold of systolic blood pressure of ≥180 mm with the target goal of systolic blood pressure reduction to values between 130 and 150 mm Hg within 6 h of symptom onset may be best supported by current evidence.

The role of brain angiotensin II (type 2) receptors and nitric oxide in the renal sympathoinhibitory response to acute volume expansion in conscious rats

OBJECTIVE

The study was performed to investigate the role of angiotensin II type 2 (AT2) receptors and nitric oxide in the renal sympathoinhibitory response to volume expansion (VEP).

METHOD

Conscious rats were subjected to volume expansion (VEP) [0.25% body weight/min saline for 10 min intravenously (i.v.)] following intracerebroventricular (i.c.v.) infusion of either saline or angiotensin II (Ang II), or a combination of Ang II with either losartan, PD123319, or N-nitro-L-arginine methyl ester (L-NAME).

RESULTS

Intracerebroventricular losartan, PD123319, or L-NAME did not change baseline mean arterial pressure, heart rate, or renal sympathetic nerve activity (RSNA). However, i.c.v. Ang II increased mean arterial pressure and decreased heart rate and RSNA baselines (113 ± 2 vs. 107 ± 2 mmHg, 365 ± 7 vs. 379 ± 5 beats/min, 1.03 ± 0.13 vs. 1.29 ± 0.15 μV.s, respectively, all P < 0.05). During i.c.v. saline infusion, VEP decreased RSNA by 27 ± 2% (P < 0.05) after 10 min and the magnitude of this response was unchanged during i.c.v. infusion of Ang II, losartan, or PD123319 but was decreased by L-NAME compared with that obtained with i.c.v. saline (14 ± 3 vs. 30 ± 5%, P < 0.05). i.c.v. Ang II in combination with losartan enhanced (41 ± 3 vs. 29 ± 5%) but with PD123319 decreased (15 ± 2 vs. 28 ± 4%, P < 0.05) the renal sympathoinhibition compared with Ang II alone. The renal sympathoinhibitory response was enhanced (43 ± 5 vs. 29 ± 1%, P < 0.05) by i.c.v. infusion of an AT2 agonist, CGP42112 the magnitude of which was unchanged when combined with L-NAME. The sympathoinhibitory response to VEP following Ang II plus L-NAME was similar to Ang II alone.

CONCLUSION

These findings suggest that activation of central AT2 receptors enhances the renal sympathoinhibitory response to VEP but this effect is not dependent on nitric oxide.

Impaired sodium-evoked paraventricular nucleus neuronal activation and blood pressure regulation in conscious Sprague-Dawley rats lacking central Gαi2 proteins

AIM

We determined the role of brain Gαi2 proteins in mediating the neural and humoral responses of conscious male Sprague-Dawley rats to acute peripheral sodium challenge.

METHODS

Rats pre-treated (24-h) intracerebroventricularly with a targeted oligodeoxynucleotide (ODN) (25 μg per 5 μL) to downregulate brain Gαi2 protein expression or a scrambled (SCR) control ODN were challenged with an acute sodium load (intravenous bolus 3 m NaCl; 0.14 mL per 100 g), and cardiovascular parameters were monitored for 120 min. In additional groups, hypothalamic paraventricular nucleus (PVN) Fos immunoreactivity was examined at baseline, 40, and 100 min post-sodium challenge.

RESULTS

In response to intravenous hypertonic saline (HS), no difference was observed in peak change in mean arterial pressure between groups. In SCR ODN pre-treated rats, arterial pressure returned to baseline by 100 min, while it remained elevated in Gαi2 ODN pre-treated rats (P < 0.05). No difference between groups was observed in sodium-evoked increases in Fos-positive magnocellular neurons or vasopressin release. V1a receptor antagonism failed to block the prolonged elevation of arterial pressure in Gαi2 ODN pre-treated rats. A significantly greater number of Fos-positive ventrolateral parvocellular, lateral parvocellular, and medial parvocellular neurons were observed in SCR vs. Gαi2 ODN pre-treated rats at 40 and 100 min post-HS challenge (P < 0.05). In SCR, but not Gαi2 ODN pre-treated rats, HS evoked suppression of plasma norepinephrine (P < 0.05).

CONCLUSION

This highlights Gαi2 protein signal transduction as a novel central mechanism acting to differentially influence PVN parvocellular neuronal activation, sympathetic outflow, and arterial pressure in response to acute HS, independently of actions on magnocellular neurons and vasopressin release.

Open-loop static and dynamic characteristics of the arterial baroreflex system in rabbits and rats

The arterial baroreflex system is the most important negative feedback system for stabilizing arterial pressure (AP). This system serves as a key link between the autonomic nervous system and the cardiovascular system, and is thus essential for understanding the pathophysiology of cardiovascular diseases and accompanying autonomic abnormalities. This article focuses on an open-loop systems analysis using a baroreceptor isolation preparation to identify the characteristics of two principal subsystems of the arterial baroreflex system, namely, the neural arc from pressure input to efferent sympathetic nerve activity (SNA) and the peripheral arc from SNA to AP. Studies on the static and dynamic characteristics of the two arcs under normal physiological conditions and also under various interventions including diseased conditions are to be reviewed. Quantitative understanding of the arterial baroreflex function under diseased conditions would help develop new treatment strategies such as electrical activation of the carotid sinus baroreflex for drug-resistant hypertension.

Early Training-Induced Reduction of Angiotensinogen in Autonomic Areas-The Main Effect of Exercise on Brain Renin-Angiotensin System in Hypertensive Rats

Background

Exercise training (T) blunts functional deficits and renin-angiotensin system (RAS) hyperactivity in hypertensive individuals. There is no information on T-induced temporal changes of brain RAS. We evaluate now the simultaneous effects of T on functional responses and time course changes in the expression/activity of brain RAS components in autonomic cardiovascular-controlling areas.

Methods and Results

Spontaneously hypertensive rats (SHR) and age-matched normotensive controls (WKY) were trained for 0, 1, 2, 4, 8 and 12 weeks. Sedentary (S) groups served as time-controls. After arterial pressure (AP) and heart rate (HR) recordings at rest, fresh and fixed brains were harvested for qPCR and immunofluorescence assays. SHR-S vs. WKY-S exhibited higher mean AP (MAP) and HR, increased pressure variability and sympathetic activity, elevated AT₁ receptor (AT₁) expression in nucleus tractus solitarii (NTS) and higher Mas receptor expression in the rostroventrolateral medulla (RVLM). In SHR, T promptly (T₂ on) reduced sympathetic variability to heart/vessels and largely decreased angiotensinogen expression in the paraventricular hypothalamic nucleus (PVN) and NTS, with a late RVLM reduction (T₄). AT₁ expression was only reduced at T₁₂ (PVN and NTS) with transient, not maintained Mas receptor changes in PVN and RVLM. These responses were accompanied by baseline MAP and HR reduction in the SHR-T (from T₄ on). In the SHR group, PVN angiotensinogen expression correlated positively with sympathetic activity, resting MAP and HR. In WKY-T, a precocious (T₂-T₁₂) RVLM AT₁ decrease preceded the appearance of resting bradycardia (from T₈ on).

Conclusions

Early and maintained reduction of angiotensinogen content in autonomic areas of the SHR is the most prominent effect of training on brain RAS. Down-regulation of PVN RAS expression is an essential factor to drive cardiovascular benefits in SHR-T, while resting bradycardia in WKY-T is correlated to RVLM AT₁ reduction.

Role of the renin-angiotensin system, renal sympathetic nerve system, and oxidative stress in chronic foot shock-induced hypertension in rats

OBJECTIVE

The renin-angiotensin system (RAS) and renal sympathetic nerve system (RSNS) are involved in the development of hypertension. The present study is designed to explore the possible roles of the RAS and the RSNS in foot shock-induced hypertension.

METHODS

Male Sprague-Dawley rats were divided into six groups: control, foot shock, RSNS denervation, denervation plus foot shock, Captopril (angiotensin I converting enzyme inhibitor, ACE inhibitor) plus foot shock, and Tempol (superoxide dismutase mimetic) plus foot shock. Rats received foot shock for 14 days. We measured the quantity of thiobarbituric acid reactive substances (TBARS), corticosterone, renin, and angiotensin II (Ang II) in plasma, the activities of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px), and renal noradrenaline content. RAS component mRNA and protein levels were quantified in the cerebral cortex and hypothalamus.

RESULTS

The two week foot shock treatment significantly increased systolic blood pressure, which was accompanied by an increase in angiotensinogen, renin, ACE1, and AT1a mRNA and protein expression in the cerebral cortex and hypothalamus, an increase of the plasma concentrations of renin, Ang II, corticosterone, and TBARS, as well as a decrease in plasma SOD and GSH-Px activities. Systolic blood pressure increase was suppressed by denervation of the RSNS or treatment with Captopril or Tempol. Interestingly, denervation or Tempol treatment both decreased main RAS components not only in the circulatory system, but also in the central nervous system. In addition, decreased antioxidant levels and increased TBARS and corticosterone levels were also partially restored by denervation or treatment with Tempol or Captopril.

CONCLUSIONS

RAS, RSNS and oxidative stress reciprocally potentiate to play important roles in the development of foot shock-induced hypertension.

Nitric oxide impacts on angiotensin AT2 receptor modulation of high-pressure baroreflex control of renal sympathetic nerve activity in anaesthetized rats

Aim

Nitric oxide (NO) interacts with the local brain renin-angiotensin system to modulate sympathetic outflow and cardiovascular homoeostasis. This study investigated whether NO influenced the ability of angiotensin AT2 receptor activation to modify the high-pressure baroreceptor regulation of renal sympathetic nerve activity (RSNA) and heart rate (HR).

Methods

Anaesthetized (chloralose/urethane) rats were prepared to allow generation of baroreflex gain curves for RSNA or HR following intracerebroventricular (I.C.V.) CGP42112 (AT2 receptor agonist), PD123319 (AT2 receptor antagonist) or losartan (AT1 receptor antagonist), and then in combination with L-NAME (NO synthase inhibitor).

Results

I.C.V. PD123319, CGP42112, and Losartan did not change baseline mean arterial pressure, HR or RSNA. Baroreflex sensitivities for RSNA and HR were increased following AT2 receptor activation with CGP42112 by 112 and 157%, respectively, but were reduced following PD123319 by 20% (all P < 0.05). L-NAME alone increased baroreflex sensitivity for both RSNA and HR, by 62 and 158%, respectively, but when co-infused with either CGP42112 or PD123319, the baroreflex sensitivity fell to values comparable to those obtained during I.C.V. saline infusion. The baroreflex sensitivities for RSNA and HR were increased by losartan by 92% and 192%, respectively, but in the presence of L-NAME were no different from those obtained during I.C.V. saline infusion.

Conclusion

There is an important facilitatory role for AT2 receptors in the high-pressure baroreflex regulation of RSNA and HR which is dependent on a functional NO/NOS system. Conversely, AT1 receptors have an inhibitory effect on the baroreflex, an action that relies on a tonic inhibition of NO.

Renal denervation restores the baroreflex control of renal sympathetic nerve activity and heart rate in Wistar-Kyoto rats with cisplatin-induced renal failure

AIM

There is evidence that in chronic renal failure, the sympathetic nervous system is activated. This study investigated the role of the renal innervation in suppressing high- and low-pressure baroreflex control of renal sympathetic nerve activity and heart rate in cisplatin-induced renal failure.

METHODS

Renal failure was induced using cisplatin (5 mg kg(-1) , i.p.) and the rats used 7 days later. Groups of rats were anaesthetized and prepared for measurement of renal sympathetic nerve activity and heart rate. Acute unilateral or bilateral renal denervation was performed, and renal sympathetic nerve activity and heart rate baroreflex gain curves were generated while the cardiopulmonary receptors were stimulated using an acute saline volume load.

RESULTS

Cisplatin administration reduced (P < 0.05) glomerular filtration rate by 27%, increased sodium fractional excretions fourfold, plasma creatinine and kidney index by 39 and 30% respectively, (all P < 0.05) compared with control rats. In the renal failure rats, baroreflex sensitivity for renal sympathetic nerve activity and heart rate was reduced (P < 0.05) by 29% and 27% (both P < 0.05) compared with control animals. Bilateral, but not unilateral, renal denervation restored baroreflex sensitivity to normal values. Volume expansion reduced (P < 0.05) renal sympathetic nerve activity by 34% in control rats, but remained unchanged in the renal failure rats. Unilateral and bilateral renal denervation progressively restored the volume expansion induced renal sympathoinhibition to control values.

CONCLUSION

These findings reveal a significant role of the renal sensory innervation in cisplatin-damaged kidneys which blunt the normal baroreflex control of renal sympathetic nerve activity.

Neuron-specific (pro)renin receptor knockout prevents the development of salt-sensitive hypertension

The (pro)renin receptor (PRR), which binds both renin and prorenin, is a newly discovered component of the renin-angiotensin system that is highly expressed in the central nervous system. The significance of brain PRRs in mediating local angiotensin II formation and regulating blood pressure remains unclear. The current study was performed to test the hypothesis that PRR-mediated, nonproteolytic activation of prorenin is the main source of angiotensin II in the brain. Thus, PRR knockout in the brain is expected to prevent angiotensin II formation and development of deoxycorticosterone acetate-salt-induced hypertension. A neuron-specific PRR (ATP6AP2) knockout mouse model was generated using the Cre-LoxP system. Physiological parameters were recorded by telemetry. PRR expression, detected by immunostaining and reverse transcription-polymerase chain reaction, was significantly decreased in the brains of knockout mice compared with wild-type mice. Intracerebroventricular infusion of mouse prorenin increased blood pressure and angiotensin II formation in wild-type mice. This hypertensive response was abolished in PRR-knockout mice in association with a reduction in angiotensin II levels. Deoxycorticosterone acetate-salt increased PRR expression and angiotensin II formation in the brains of wild-type mice, an effect that was attenuated in PRR-knockout mice. PRR knockout in neurons prevented the development of deoxycorticosterone acetate-salt-induced hypertension as well as activation of cardiac and vasomotor sympathetic tone. In conclusion, nonproteolytic activation of prorenin through binding to the PRR mediates angiotensin II formation in the brain. Neuron-specific PRR knockout prevents the development of deoxycorticosterone acetate-salt-induced hypertension, possibly through diminished angiotensin II formation.

Role of angiotensin AT2 receptors and nitric oxide in the cardiopulmonary baroreflex control of renal sympathetic nerve activity in rats

OBJECTIVES

This study investigated the hypothesis that angiotensin II (type 2) (AT2) receptor activation to modulate the renal sympatho-inhibition to saline volume expansion was dependent on nitric oxide production.

METHODS

Renal sympatho-inhibition to a saline volume expansion (VEP, 0.25% body weight/min i.v. for 30 min) was studied following intracerebroventricular (ICV) saline, CGP42112 (CGP, AT2 agonist), PD123319 (AT2 antagonist), and losartan (AT1 antagonist), and then in combination with N-nitro-L-arginine methyl ester (L-NAME) (nitric oxide synthase inhibitor).

RESULTS

ICV saline, PD123319, CGP, and losartan did not change baseline mean arterial pressure, heart rate, or renal sympathetic nerve activity (RSNA). VEP decreased RSNA in all groups by 58-62% (P<0.05). CGP enhanced the decrease in RSNA compared to saline (74 vs. 60%; P<0.05), whereas PD123319 was without effect (58 vs. 57%). L-NAME only increased baseline RSNA when co-administered with PD123319 (P<0.05). VEP-induced reduction in RSNA following L-NAME was less than during ICV saline (46 vs. 62%; P<0.05). In the group where PD123319 preceded L-NAME, the fall in RSNA was smaller than when PD123319 was infused alone (40 vs. 63%; P<0.05), but not if PD123319 followed L-NAME (52 vs. 44%). L-NAME did not change the magnitude of VEP-induced sympatho-inhibition following CGP (67 vs. 60%). Losartan enhanced the renal sympatho-inhibition to VEP (70 vs. 62%; P<0.05), the magnitude of which was unchanged when L-NAME was present (70 vs. 65%).

CONCLUSION

AT2 receptor activation enhances the VEP-induced reduction in RSNA. Although nitric oxide is important in allowing the normal renal sympatho-inhibitory response to VEP, this is not dependent on AT2 receptors.

Translational physiology and SND recordings in humans and rats: a glimpse of the recent past with an eye on the future

The sympathetic nervous system (SNS) plays an important role in cardiovascular function, and based on the critical mechanistic relationship between altered sympathetic neural mechanisms and cardiovascular disease, it is important that the autonomic research community identifies deficiencies in the translational exchange of information and strives for a more thorough understanding of the translational significance of findings from studies involving sympathetic nerve discharge (SND) regulation in human and animal subjects. The present review assesses the state of the literature regarding studies that have used direct recordings of SND during the past three decades in humans and rats, focusing on; 1) identifying the number of studies reporting SND recordings in humans and rats, 2) briefly describing the translational exchange of SND regulation information from these studies, 3) contrasting the number of studies completed in anesthetized and conscious rats, and 4) assessing the prevalence of long-term SND recording studies in conscious rats. The majority of SND recordings in rats have been completed using anesthetized preparations, although a substantial number of studies have been completed in conscious rats. However, few studies have completed long-term (>5 days) SND recordings in freely-behaving rats, and even fewer studies have used experimental preparations that combine long-term nerve recordings with the capacity for completing central neural microinjections, or have been completed in animal models of cardiovascular disease. The wide-spread implementation of long-term SND recordings in rodent models of cardiovascular disease would be expected to enhance the translational exchange of clinically-relevant information between animals and humans.

Autonomic regulation of kidney function

The kidneys play a central role in cardiovascular homeostasis by ensuring a balance between the fluid taken in and that lost and excreted during everyday activities. This ensures stability of extracellular fluid volume and maintenance of normal levels of blood pressure. Renal fluid handling is controlled via neural and humoral influences, with the former determining a rapid dynamic response to changing intake of sodium whereas the latter cause a slower longer-term modulation of sodium and water handling. Activity in the renal sympathetic nerves arises from an integration of information from the high and low pressure cardiovascular baroreceptors, the somatosensory and visceral systems as well as the higher cortical centers. Each sensory system provides varying input to the autonomic centers of the hypothalamic and medullary areas of the brain at a level appropriate to the activity being performed. In pathophysiological states, such as hypertension, heart failure and chronic renal disease, there may be an inappropriate sympathoexcitation causing sodium retention which exacerbates the disease process. The contribution of the renal sympathetic nerves to these cardiovascular diseases is beginning to be appreciated with the demonstration that renal denervation of resistant hypertensive patients results in a long-term normalization of blood pressure.

Inhibition of P-glycoprotein enhances transport of imipramine across the blood-brain barrier: microdialysis studies in conscious freely moving rats

BACKGROUND AND PURPOSE

Recent studies indicate that efflux of antidepressants by the multidrug resistance transporter P-glycoprotein (P-gp) at the blood-brain barrier (BBB) may contribute to treatment-resistant depression (TRD) by limiting intracerebral antidepressant concentrations. In addition, clinical experience shows that adjunctive treatment with the P-gp inhibitor verapamil may improve the clinical outcome in TRD. Therefore, the present study aimed to investigate the effect of P-gp inhibition on the transport of the tricyclic antidepressant imipramine and its active metabolite desipramine across the BBB.

EXPERIMENTAL APPROACH

Intracerebral microdialysis in rats was used to monitor brain levels of imipramine and desipramine following i.v. imipramine administration, with or without pretreatment with one of the P-gp inhibitors verapamil or cyclosporin A (CsA). Plasma drug levels were also determined at regular intervals.

KEY RESULTS

Pretreatment with either verapamil or CsA resulted in significant increases in imipramine concentrations in the microdialysis samples, without altering imipramine plasma pharmacokinetics. Furthermore, pretreatment with verapamil, but not CsA, led to a significant elevation in plasma and brain levels of desipramine.

CONCLUSIONS AND IMPLICATIONS

The present study demonstrated that P-gp inhibition enhanced the intracerebral concentration of imipramine , thus supporting the hypothesis that P-gp activity restricts brain levels of certain antidepressants, including imipramine. These findings may help to explain reports of a beneficial response to adjunctive therapy with verapamil in TRD.

Differential control of efferent sympathetic activity revisited

This article reviews 40 years of research (1970-2010) into the capability of the efferent sympathetic nervous system to display differential responsiveness. Discovered first were antagonistic changes of activity in sympathetic filaments innervating functionally different sections of the cardiovascular system in response to thermal stimulation. During the subsequent four decades of investigation, a multitude of differential sympathetic efferent response patterns were identified, ranging from opposing activity changes at the level of multi-fiber filaments innervating different organs to the level of single fibers controlling functionally different structures in the same organ. Differential sympathetic responsiveness was shown to be displayed in response to exogenous or artificial stimulation of afferent sensory fibers transmitting particular exogenous stimuli, especially those activating peripheral nociceptors. Moreover, sympathetic differentiation was found to be characteristic of autonomic responses to environmental changes by which homeostasis in the broadest sense would be challenged. Heat or cold loads or their experimental equivalents, altered composition of inspired air or changes in blood gas composition, imbalances of body fluid control, and exposure to agents challenging the immune system were shown to elicit differential efferent sympathetic response patterns which often displayed a high degree of specificity. In summary, autonomic adjustments to changes of biometeorological parameters may be considered as representative of the capability of the sympathetic nervous system to exert highly specific efferent control of organ functions by which bodily homeostasis is maintained.

A bedtime dose of ARB was better than a morning dose in improving baroreflex sensitivity and urinary albumin excretion--the J-TOP study

The hypothesis that the bedtime dosing of angiotensin receptor blocker (ARB) is superior to morning dose in improving baroreflex sensitivity (BRS) and urinary albumin/creatinine ratio (UACR) was tested in this study. Baroreflex sensitivity was measured at baseline and at 6th month (N = 109) and was found to increase in the bedtime-dose group (P = .004), but not in the morning-dose group. The correlations between the change in BRS and the change in UACR were insignificant in the morning-dose group (r = 0.17, P = .26), but were significant in the bedtime-dose group (r = -0.29, P = .04). In conclusion, the improvement of BRS could be one of the mechanisms by which bedtime dosing of ARB confers renal protection.

Nitric oxide and angiotensin II regulate cardiovascular homeostasis and the arterial baroreflex control of heart rate in conscious lambs

To investigate the potential role of angiotensin II (Ang II) type 1 receptors (AT1Rs) as well as endogenously produced nitric oxide (NO) in regulating cardiovascular homeostasis during ontogeny, experiments were carried out in conscious lambs aged approximately 1 week ( N = 9) and 6 weeks ( N = 11). The arterial baroreflex control of heart rate (HR) was assessed before and after intravenous (IV) infusion of the selective AT1R antagonist, ZD 7155, before and after IV administration of the L-arginine analogue, NG-nitro-L-arginine methyl ester (L-NAME). In both groups, after ZD 7155 alone, mean arterial pressure decreased then increased after L-NAME. At 1 but not 6 weeks, HR decreased after ZD 7155 as well as after L-NAME. At 1 but not 6 weeks, there was a decrease in the HR range after ZD 7155 and after ZD 7155 + L-NAME, as compared to control. There was also a decrease in minimum HR after ZD 7155 + L-NAME at 1 week. These data provide new evidence that, together, Ang II and NO regulate cardiovascular homeostasis as well as the arterial baroreflex of HR early in life which may help to explain the activation of these two systems early in life.

The pressor and renal sympathetic nerve responses to vascular and spinal V1 receptor activation after manipulation of dietary sodium intake

OBJECTIVE

Excessive dietary Na intake can enhance the autonomic control of blood pressure, but the physiological mechanisms are unclear. This study examined how low (0.03%) and high (3.0%) dietary Na intake, from weaning (4 weeks) to adulthood (11 weeks), altered the pressor and renal sympathoexcitatory responses to peripheral and spinal V1 receptor activation.

METHODS

Mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) were monitored in α-chloralose/urethane anaesthetized male Wistar rats.

RESULTS

Dose-dependent increases in MAP were observed in all groups to intravenous (i.v.) vasopressin [arginine vasopressin (AVP); 1-10 ng 0.2 ml] and phenylephrine (1-10 μg 0.2 ml), and in the high Na group, these responses were enhanced but to a greater extent for AVP than phenylephrine (P<0.001). A direct dose-dependent rise in RSNA to intrathecal (10 μl) AVP (1-100 μmol/l) and glutamate (10-100 mmol/l) was observed in the normal Na group. The RSNA responses were enhanced in the high Na group at lower doses of intrathecal AVP (1 μmol/l, P<0.01; 5 μmol/l, P<0.05) and all doses of glutamate (P<0.001) compared to the normal Na group. In the low Na group, the RSNA responses to intrathecal AVP were suppressed, but those to intrathecal glutamate were enhanced compared to normal Na (P<0.001) and similar to the high Na group.

CONCLUSION

These data demonstrated that high Na enhanced peripheral and spinal V1-mediated responses. Interestingly, low Na intake blunted the spinal V1-mediated RSNA responses, but sensitized those to spinal glutamate, which may be a compensatory mechanism to ensure adequate neural control of the kidney when dietary Na intake is reduced.

Pregnancy and the endocrine regulation of the baroreceptor reflex

The purpose of this review is to delineate the general features of endocrine regulation of the baroreceptor reflex, as well as specific contributions during pregnancy. In contrast to the programmed changes in baroreflex function that occur in situations initiated by central command (e.g., exercise or stress), the complex endocrine milieu often associated with physiological and pathophysiological states can influence the central baroreflex neuronal circuitry via multiple sites and mechanisms, thereby producing varied changes in baroreflex function. During pregnancy, baroreflex gain is markedly attenuated, and at least two hormonal mechanisms contribute, each at different brain sites: increased levels of the neurosteroid 3alpha-hydroxy-dihydroprogesterone (3alpha-OH-DHP), acting in the rostral ventrolateral medulla (RVLM), and reduced actions of insulin in the forebrain. 3alpha-OH-DHP appears to potentiate baroreflex-independent GABAergic inhibition of premotor neurons in the RVLM, which decreases the range of sympathetic nerve activity that can be elicited by changes in arterial pressure. In contrast, reductions in the levels or actions of insulin in the brain blunt baroreflex efferent responses to increments or decrements in arterial pressure. Although plasma levels of angiotensin II are increased in pregnancy, this is not responsible for the reduction in baroreflex gain, although it may contribute to the increased level of sympathetic nerve activity in this condition. How these different hormonal effects are integrated within the brain, as well as possible interactions with additional potential neuromodulators that influence baroreflex function during pregnancy and other physiological and pathophysiological states, remains to be clearly delineated.

Influence of dietary sodium on the blood pressure and renal sympathetic nerve activity responses to intracerebroventricular angiotensin II and angiotensin III in anaesthetized rats

The regulation of blood pressure and sympathetic outflow by the brain renin-angiotensin system in animals subjected to raised or lowered dietary Na(+) intake is unclear. This study compared the mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA) responses to intracerebroventricular (i.c.v.) infusion of angiotensin II (AngII) and III (AngIII) before and after peripheral V(1) receptor blockade (V(1)B) in alpha-chloralose-urethane-anaesthetized rats fed a low (0.03%, LNa(+)), normal (0.3%, NNa(+)) or high Na(+) diet (3.0%, HNa(+)) from 4 to 11 weeks of age. The rise in MAP 2 min post AngII i.c.v. was greater in HNa(+) (14 +/- 3 mmHg) versus LNa(+) (8 +/- 1 mmHg, P < 0.05) and after AngIII i.c.v. in HNa(+) (14 +/- 3 mmHg) versus NNa(+) (6 +/- 1 mmHg, P < 0.05) and LNa(+) (7 +/- 1 mmHg, P < 0.05). The MAP responses to AngII and AngIII i.c.v. were abolished after V(1)B in LNa(+), but were only attenuated in HNa(+). In NNa(+), V(1)B blunted the MAP responses to AngII and abolished those to AngIII. The MAP remained elevated 30 min after AngII in all groups, but returned to baseline levels 15 min after AngIII in NNa(+) and HNa(+) (P < 0.01). Twenty minutes after i.c.v. AngII, RSNA rose above baseline in HNa(+) (112 +/- 1%), a response not observed in the LNa(+) and NNa(+) groups. Twenty minutes post AngIII i.c.v., RSNA was elevated in both HNa (109 +/- 2%) and NNa(+) (109 +/- 2%). After V(1)B, RSNA rose only in the HNa(+) group 15 min post AngIII infusion (109 +/- 1%). Together, these findings: (1) suggest that HNa(+) intake augments the MAP and RSNA responses to i.c.v. AngII and AngIII; (2) highlight an important role for peripheral V(1) receptors during these responses; and (3) differentiate the effects of AngII and AngIII on blood pressure and RSNA.

Shift to an involvement of phosphatidylinositol 3-kinase in angiotensin II actions on nucleus tractus solitarii neurons of the spontaneously hypertensive rat

RATIONALE

Central angiotensin (Ang) II inhibits baroreflex and plays an important role in the pathogenesis of hypertension. However, the underlying molecular mechanisms are still not fully understood.

OBJECTIVE

Our objective in the present study was to characterize the signal transduction mechanism of phosphatidylinositol 3-kinase (PI3K) involvement in Ang II-induced stimulation of central neuronal activity in cultured neurons and Ang II-induced inhibition of baroreflex in spontaneously hypertensive rats (SHR) versus WKY rats.

METHODS AND RESULTS

Application of Ang II to neurons produced a 42% greater increase in neuronal firing in cells from the SHR than the WKY rat. Although the Ang II-mediated increase in firing rate was abolished entirely by the protein kinase (PK)C inhibitor GF109230 in the WKY, blockade of both PKC and PI3K activity was necessary in the SHR. This was associated with an increased ability of Ang II to stimulate NADPH oxidase-reactive oxygen species (ROS)-mediated signaling involving phosphorylation of the p47phox subunit of the NADPH oxidase and was dependent on the activation of PI3K in the SHR. Inhibition of PI3K resulted in the reduction of levels of p47phox phosphorylation, NADPH oxidase activity, ROS levels, and ultimately neuronal activity in cells from the SHR but not the WKY rat. In addition, in working heart-brainstem preparations, inhibition of PKC activity in the nucleus of the solitary tract in situ abolished the Ang II-mediated depression of cardiac and sympathetic baroreceptor reflex gain in the WKY. In contrast, PKC inhibition in the nucleus of the solitary tract of SHR only partially reduced the effect of Ang II on the baroreceptor reflex gain.

CONCLUSIONS

These observations demonstrate that PI3K in the cardiovascular brainstem regions of the SHR may be selectively involved in Ang II-mediated signaling that includes a reduction in baroreceptor reflex function, presumably via a NADPH-ROS mediated pathway.

Long-range correlation of renal sympathetic nerve activity in both conscious and anesthetized rats

In this study we employed both detrended fluctuation analysis (DFA) and multiscale entropy (MSE) measurements to compare the long-range temporal correlation (LRTC) of multifibre renal sympathetic nerve activity (RSNA) between conscious and anesthetized Wistar rats. It was found that both methods showed the obvious LRTC properties in conscious state. Moreover, the scaling exponent of the RSNA in conscious rats was significantly higher than that in anesthetized rats. The results of MSE analysis showed that the entropy values, derived from the conscious group, increased on small time scales and then stabilized to a relatively constant value whereas the entropy measure, derived from anesthetized animals, almost monotonically decreased. This suggests that the fractal properties of underlying dynamics of the system have been reduced by anesthesia. The results demonstrate that apparently random fluctuations in multifibre RSNA are dictated by a complex deterministic process that imparts "long-term" memory to the dynamic system. However, this memory is significantly weakened by anesthesia.

Dietary salt loading exacerbates the increase in sympathetic nerve activity caused by intravenous insulin infusion in rats

Obesity and type 2 diabetes mellitus frequently produce chronic elevations in blood insulin levels. Importantly, hyperinsulinemia stimulates increases in sympathetic nerve activity that may predispose to hypertension, atherosclerosis, and end-organ damage. Because depletion of dietary salt (NaCl) increases angiotensin II levels, which has been shown to enhance sympathetic responses to excitatory stimuli such as thermal stimulation and bicuculline in the hypothalamus, we predicted that insulin-induced elevations in lumbar sympathetic activity would be augmented by low NaCl and suppressed by high dietary NaCl. Adult male Sprague-Dawley rats were randomized into groups receiving low (0.0 mEq/d, n = 10), normal (2.0 mEq/d, n = 10), and high (5.7 mEq/d, n = 10) NaCl for a period of 8 days. After this, the animals were anesthetized for measurement of heart rate, mean arterial pressure, and lumbar sympathetic nerve activity during 110 minutes of intravenous insulin infusion (15 mU/kg per minute) with euglycemic clamp. Insulin administration caused modest blood pressure decreases accompanied by heart rate increases that were similar across the 3 dietary groups. Unexpectedly, sympathetic increases to insulin were lowest in the low-NaCl group (100%-135% +/- 24%), moderate in the normal-NaCl group (100%-170% +/- 23%), and greatest in the high-NaCl group (100%-252% +/- 39%). Dietary NaCl level did not affect baseline blood glucose or insulin sensitivity as assessed by euglycemic clamp. These findings indicate that dietary salt loading exacerbates the lumbar sympathoexcitatory response to intravenous insulin infusion in rats.

Neural control of renal function

The renal nerves are the communication link between the central nervous system and the kidney. In response to multiple peripheral and central inputs, efferent renal sympathetic nerve activity is altered so as to convey information to the major structural and functional components of the kidney, the vessels, glomeruli, and tubules, each of which is innervated. At the level of each of these individual components, information transfer occurs via interaction of the neurotransmitter released at the sympathetic nerve terminal-neuroeffector junction with specific postjunctional receptors coupled to defined intracellular signaling and effector systems. In response to normal physiological stimuli, changes in efferent renal sympathetic nerve activity contribute importantly to homeostatic regulation of renal blood flow, glomerular filtration rate, renal tubular epithelial cell solute and water transport, and hormonal release. Afferent input from sensory receptors located in the kidney participates in this reflex control system via renorenal reflexes that enable total renal function to be self-regulated and balanced between the two kidneys. In pathophysiological conditions, abnormal regulation of efferent renal sympathetic nerve activity contributes significantly to the associated abnormalities of renal function which, in turn, are of importance in the pathogenesis of the disease.