Differential baroreflex control of sympathetic drive by angiotensin II in the nucleus tractus solitarii

Regional Differences in Sympathetic Nerve Activity Are Generated by Multiple Arterial Baroreflex Loops Arranged in Parallel

In this review, by evaluating the responses during freezing, rapid eye movement (REM) sleep, and treadmill exercise, we discuss how multiple baroreflex loops arranged in parallel act on different organs to modulate sympathetic nerve activity (SNA) in a region-specific and coordinated manner throughout the body. During freezing behaviors, arterial pressure (AP) remains unchanged, heart rate (HR) persistently decreases, renal SNA (RSNA) increases, and lumbar SNA (LSNA) remains unchanged. The baroreflex curve for RSNA shifts upward; that for LSNA remains unchanged; and that for HR shifts to the left. These region-specific changes in baroreflex curves are responsible for the region-specific changes in RSNA, LSNA, and HR during freezing. The decreased HR could allow the heart to conserve energy, which is offset by the increased RSNA caused by decreased vascular conductance, resulting in an unchanged AP. In contrast, the unchanged LSNA leaves the muscles in readiness for fight or flight. During REM sleep, AP increases, RSNA and HR decrease, while LSNA is elevated. The baroreflex curve for RSNA during REM sleep is vertically compressed in comparison with that during non-REM sleep. Cerebral blood flow is elevated while cardiac output is decreased during REM sleep. To address this situation, the brain activates the LSNA selectively, causing muscle vasoconstriction, which overcomes vasodilation of the kidneys as a result of the decreased RSNA and cardiac output. Accordingly, AP can be maintained during REM sleep. During treadmill exercise, AP, HR, and RSNA increase simultaneously. The baroreflex curve for RSNA shifts right-upward with the increased feedback gain, allowing maintenance of a stable AP with significant fluctuations in the vascular conductance of working muscles. Thus, the central nervous system may employ behavior-specific scenarios for modulating baroreflex loops for differential control of SNA, changing the SNA in a region-specific and coordinated manner, and then optimizing circulatory regulation corresponding to different behaviors.

Mobile Heart Rate Variability Biofeedback Improves Autonomic Activation and Subjective Sleep Quality of Healthy Adults – A Pilot Study

Objective

Restorative sleep is associated with increased autonomous parasympathetic nervous system activity that might be improved by heart rate variability-biofeedback (HRV-BF) training. Hence the aim of this study was to investigate the effect of a four-week mobile HRV-BF intervention on the sleep quality and HRV of healthy adults.

Methods

In a prospective study, 26 healthy participants (11 females; mean age: 26.04 ± 4.52 years; mean body mass index: 23.76 ± 3.91 kg/m²) performed mobile HRV-BF training with 0.1 Hz breathing over four weeks, while sleep quality, actigraphy and HRV were measured before and after the intervention.

Results

Mobile HRV-BF training with 0.1 Hz breathing improved the subjective sleep quality in healthy adults [t(24) = 4.9127, p ≤ 0.001, d = 0.99] as measured by the Pittsburgh Sleep Quality Index. In addition, mobile HRV-BF training with 0.1 Hz breathing was associated with an increase in the time and frequency domain parameters SDNN, Total Power and LF after four weeks of intervention. No effect was found on actigraphy metrics.

Conclusions

Mobile HRV-BF intervention with 0.1 Hz breathing increased the reported subjective sleep quality and may enhance the vagal activity in healthy young adults. HRV-BF training emerges as a promising tool for improving sleep quality and sleep-related symptom severity by means of normalizing an impaired autonomic imbalance during sleep.

Differential shifts in baroreflex control of renal and lumbar sympathetic nerve activity induced by freezing behaviour in rats

NEW FINDINGS

What is the central question of this study? Is the arterial baroreflex involved in causing patterned, region-specific changes in sympathetic nerve activity during freezing behaviour in conscious rats? What is the main finding and its importance? Freezing behaviour is accompanied by differential shifts in the baroreflex control of renal and lumbar sympathetic nerve activity and heart rate. It is noteworthy that baroreflex pathways may be discretely separated, allowing differential modification of baroreflex curves that may generate differential changes in sympathetic nerve activity during freezing behaviour.

ABSTRACT

The present study was designed to test whether the baroreflex stimulus-response curves for renal sympathetic nerve activity (RSNA), lumbar sympathetic nerve activity (LSNA) and heart rate (HR) were shifted in a regionally specific manner during freezing behaviour in conscious rats. Male Wistar rats were chronically instrumented with electrodes and arterial and venous catheters for measurement of RSNA, LSNA and electrocardiogram. After a 60-min control period, freezing behaviour in conscious rats was induced by exposure to loud white noise (90 dB) for 10 min. The baroreflex curves for RSNA, LSNA and HR were generated by changing systemic arterial pressure using rapid intravenous infusions of vasoactive drugs and then fitted to an inverse sigmoid function curve. During the freezing behaviour, the baroreflex curve for RSNA was expanded upward with a significant (P < 0.001) increase (by 153% compared with the control level) in the upper plateau (maximum capacity of RSNA drive), whereas the baroreflex curve for LSNA remained unchanged. Conversely, the baroreflex curve for HR was shifted leftward with a significant (P = 0.004) decrease (by 11 mmHg relative to the control level) in the midpoint pressure. Our results indicate that baroreflex curve shifts for RSNA, LSNA and HR occur in a regionally specific manner during freezing behaviour. This indicates that baroreflex pathways may be discretely separated, allowing differential modification of baroreflex curves that may generate differential changes in sympathetic nerve activity during freezing behaviour.

Increased angiotensin II formation in the brain modulates cardiovascular homeostasis and erythropoiesis

In spite of the fact that the modulatory effects of angiotensin II (Ang II) on the sympathetic nerve activity to targeted organs involved in blood pressure (BP) regulation is well acknowledged, the local production of this peptide in the brain and the consequences of enhanced central Ang II beyond the cardiovascular system are not yet well comprehended. In the present study, we generated and validated a new transgenic mouse line overexpressing the rat full-length angiotensinogen (Agt) protein specifically in the brain (Agt-Tg). Adult Agt-Tg mice presented overall increased gene expression of total Agt in the brain including brainstem and hypothalamus. In addition, the excess of Agt led to abundantly detectable brain Ang II levels as well as increased circulating copeptin levels. Agt-Tg displayed raised BP in acute recordings, while long-term telemetrically measured basal BP was indistinguishable from wild-types. Agt-Tg has altered peripheral renin–angiotensin system and vasomotor sympathetic tone homeostasis because renal gene expression analysis, plasma Ang II measurements and ganglionic blockade experiments revealed suppressed renin expression and reduced Ang II and higher neurogenic pressure response, respectively. Plasma and urine screens revealed apparently normal fluid and electrolyte handling in Agt-Tg. Interestingly, hematological analyses showed increased hematocrit in Agt-Tg caused by enhanced erythropoiesis, which was reverted by submitting the transgenic mice to a long-term peripheral sympathectomy protocol. Collectively, our findings suggest that Agt-Tg is a valuable tool to study not only brain Ang II formation and its modulatory effects on cardiovascular homeostasis but also its role in erythropoiesis control via autonomic modulation.

Exacerbated effects of prorenin on hypothalamic magnocellular neuronal activity and vasopressin plasma levels during salt-sensitive hypertension

Accumulating evidence supports that the brain renin-angiotensin system (RAS), including prorenin (PR) and its receptor (PRR), two newly discovered RAS players, contribute to sympathoexcitation in salt-sensitive hypertension. Still, whether PR also contributed to elevated circulating levels of neurohormones such as vasopressin (VP) during salt-sensitive hypertension, and if so, what are the precise underlying mechanisms, remains to be determined. To address these questions, we obtained patch-clamp recordings from hypothalamic magnocellular neurosecretory neurons (MNNs) that synthesize the neurohormones oxytocin and VP in acute hypothalamic slices obtained from sham and deoxycorticosterone acetate (DOCA)-salt-treated hypertensive rats. We found that focal application of PR markedly increased membrane excitability and firing responses in MNNs of DOCA-salt, compared with sham rats. This effect included a shorter latency to spike initiation and increased numbers of spikes in response to depolarizing stimuli and was mediated by a more robust inhibition of A-type K⁺ channels in DOCA-salt compared with sham rats. On the other hand, the afterhyperpolarizing potential mediated by the activation of Ca²⁺-dependent K⁺ channel was not affected by PR. mRNA expression of PRR, VP, and the Kv4.3 K⁺ channel subunit in the supraoptic nucleus of DOCA-salt hypertensive rats was increased compared with sham rats. Finally, we report a significant decrease of plasma VP levels in neuron-selective PRR knockdown mice treated with DOCA-salt, compared with wild-type DOCA-salt-treated mice. Together, these results support that activation of PRR contributes to increased excitability and firing discharge of MNNs and increased plasma levels of VP in DOCA-salt hypertension.NEW & NOTEWORTHY Our studies support that prorenin (PR) and its receptor (PRR) within the hypothalamus contribute to elevated plasma vasopressin levels in deoxycorticosterone acetate-salt hypertension, in part because of an exacerbated effect of PR on magnocellular neurosecretory neuron excitability; Moreover, our study implicates A-type K+ channels as key underlying molecular targets mediating these effects. Thus, PR/PRR stands as a novel therapeutic target for the treatment of neurohumoral activation in salt-sensitive hypertension.

Molecular Basis of the Brain Renin Angiotensin System in Cardiovascular and Neurologic Disorders: Uncovering a Key Role for the Astroglial Angiotensin Type 1 Receptor AT1R

The central renin angiotensin system (RAS) is one of the most widely investigated cardiovascular systems in the brain. It is implicated in a myriad of cardiovascular diseases. However, studies from the last decade have identified its involvement in several neurologic abnormalities. Understanding the molecular functionality of the various RAS components can thus provide considerable insight into the phenotypic differences and mechanistic drivers of not just cardiovascular but also neurologic disorders. Since activation of one of its primary receptors, the angiotensin type 1 receptor (AT1R), results in an augmentation of oxidative stress and inflammatory cytokines, it becomes essential to investigate not just neuronal RAS but glial RAS as well. Glial cells are key homeostatic regulators in the brain and are critical players in the resolution of overt oxidative stress and neuroinflammation. Designing better and effective therapeutic strategies that target the brain RAS could well hinge on understanding the molecular basis of both neuronal and glial RAS. This review provides a comprehensive overview of the major studies that have investigated the mechanisms and regulation of the brain RAS, and it also provides insight into the potential role of glial AT1Rs in the pathophysiology of cardiovascular and neurologic disorders.

Dysregulation of the Renin-Angiotensin System and the Vasopressinergic System Interactions in Cardiovascular Disorders

Purpose of Review

In many instances, the renin-angiotensin system (RAS) and the vasopressinergic system (VPS) are jointly activated by the same stimuli and engaged in the regulation of the same processes.

Recent Findings

Angiotensin II (Ang II) and arginine vasopressin (AVP), which are the main active compounds of the RAS and the VPS, interact at several levels. Firstly, Ang II, acting on AT1 receptors (AT1R), plays a significant role in the release of AVP from vasopressinergic neurons and AVP, stimulating V1a receptors (V1aR), regulates the release of renin in the kidney. Secondly, Ang II and AVP, acting on AT1R and V1aR, respectively, exert vasoconstriction, increase cardiac contractility, stimulate the sympathoadrenal system, and elevate blood pressure. At the same time, they act antagonistically in the regulation of blood pressure by baroreflex. Thirdly, the cooperative action of Ang II acting on AT1R and AVP stimulating both V1aR and V2 receptors in the kidney is necessary for the appropriate regulation of renal blood flow and the efficient resorption of sodium and water. Furthermore, both peptides enhance the release of aldosterone and potentiate its action in the renal tubules.

Summary

In this review, we (1) point attention to the role of the cooperative action of Ang II and AVP for the regulation of blood pressure and the water-electrolyte balance under physiological conditions, (2) present the subcellular mechanisms underlying interactions of these two peptides, and (3) provide evidence that dysregulation of the cooperative action of Ang II and AVP significantly contributes to the development of disturbances in the regulation of blood pressure and the water-electrolyte balance in cardiovascular diseases.

Angiotensin 1-7 in the rostro-ventrolateral medulla increases blood pressure and splanchnic sympathetic nerve activity in anesthetized rats

Angiotensin 1-7 (ANG-(1-7)), a derivative of angiotensin I or II, is involved in the propagation of sympathetic output to the heart and vasculature, and the receptor for ANG-(1-7), the Mas receptor, is expressed on astrocytes in the rostral ventrolateral medulla (RVLM). We recorded blood pressure (BP) and splanchnic sympathetic nerve activity (SSNA) before and after focal injection of ANG-(1-7) into the RVLM of rats. Unilateral injection of ANG-(1-7) into the RVLM, acting through the Mas receptor, increased SSNA and BP, and glutamate receptor antagonists, CNQX and D-AP5, partially reduced the ANG-(1-7) effect. ATP is often co-released with glutamate, and blocking ATP with PPADS also reduced the pressor response to microinjection of ANG-(1-7) within the RVLM. The effects of ANG-(1-7) were blocked by the MAS receptor antagonist, A-779 (which had no consistent effect on blood pressure or sympathetic nerve activity when injected on its own). We conclude that astrocytes in the RVLM participate in central, angiotensin-dependent regulation of blood pressure and sympathetic nerve activity, and the Mas receptor, when activated by ANG-(1-7), elicits the release of the gliotransmitters, glutamate and ATP. These gliotransmitters then cause an increase in sympathetic nerve activity and blood pressure by interacting with AMPA/kainate and P2X receptors in the RVLM.

Cardiovascular autonomic regulation, inflammation and pain in rheumatoid arthritis

Background

Rheumatoid arthritis (RA) is a chronic inflammatory condition characterised by reduced heart rate variability (HRV) of unknown cause. We tested the hypothesis that low HRV, indicative of cardiac autonomic cardiovascular dysfunction, was associated with systemic inflammation and pain. Given the high prevalence of hypertension (HTN) in RA, a condition itself associated with low HRV, we also assessed whether the presence of hypertension further reduced HRV in RA.

Methods

In RA-normotensive (n = 13), RA-HTN (n = 17), normotensive controls (NC; n = 17) and HTN (n = 16) controls, blood pressure and heart rate were recorded. Time and frequency domain measures of HRV along with serological markers of inflammation (high sensitivity C-reactive protein [hs-CRP], tumour necrosis factor-α [TNF-α] and interleukins [IL]) were determined. Reported pain was assessed using a visual analogue scale.

Results

Time (rMSSD, pNN50%) and frequency (high frequency power, low frequency power, total power) domain measures of HRV were lower in the RA, RA-HTN and HTN groups, compared to NC (p = 0.001). However, no significant differences in HRV were noted between the RA, RA-HTN and HTN groups. Inverse associations were found between time and frequency measures of HRV and inflammatory cytokines (IL-6 and IL-10), but were not independent after multivariable analysis. hs-CRP and pain were independently and inversely associated with time domain (rMMSD, pNN50%) parameters of HRV.

Conclusions

These findings suggest that lower HRV is associated with increased inflammation and independently associated with increased reported pain, but not compounded by the presence of HTN in patients with RA.

•

Rheumatoid arthritis (RA) is a chronic inflammatory condition accompanied by low heart rate variability (HRV).

•

Important autonomic-immune interactions are suggested, but have not been thoroughly examined in RA.

•

We show that low HRV in RA is associated with increased serum inflammatory cytokine levels and patient-reported pain.

•

In our patients with RA, reductions in HRV were not compounded by the presence of hypertension.

A-type K+ channels contribute to the prorenin increase of firing activity in hypothalamic vasopressin neurosecretory neurons

Recent studies have supported an important contribution of prorenin (PR) and its receptor (PRR) to the regulation of hypothalamic, sympathetic, and neurosecretory outflows to the cardiovascular system, including systemic release of vasopressin (VP), both under physiological and cardiovascular disease conditions. Still, the identification of precise cellular mechanisms and neuronal/molecular targets remain unknown. We have recently shown that PRR is expressed in VP neurons and that their activation increases neuronal activity. However, the underlying ionic channel mechanisms are undefined. Here, we performed patch-clamp electrophysiology from identified VP neurons in acute hypothalamic slices obtained from enhanced green fluorescent protein-VP transgenic rats. Voltage-clamp recordings showed that PR inhibited the magnitude of A-type K⁺ current (I_A; ~50% at -25 mV), a subthreshold voltage-dependent current that restrains VP firing activity. PR also increased the inactivation rate of I_A and shifted the steady-state voltage-dependent inactivation function toward more hyperpolarized membrane potential (~7 mV shift), thus resulting in less channel availability to be activated at any given membrane potential. PR also inhibited a sustained component of I_A ("window" current). PR-mediated changes in action potential waveform and increased firing activity were occluded when I_A was blocked by 4-aminopyridine. Finally, PR failed to increase superoxide production within the supraoptic nucleus/paraventricular nucleus, and PR excitatory effects persisted in slices treated with the SOD mimetic tempol. Taken together, these experiments indicated that PR excitatory effects on vasopressin neurons involve inhibition of I_A, due, in part, to increases in its voltage-dependent inactivation properties. Moreover, our results indicate that PR effects did not involve an increase in oxidative stress.NEW & NOTEWORTHY Here, we demonstrate that prorenin/the prorenin receptor is an important signaling unit for the regulation of vasopressin firing activity and, thus, systemic hormonal release. We identified A-type K⁺ channels as key molecular targets mediating prorenin stimulation of vasopressin neuronal activity, thus standing as a potential therapeutic target for neurohumoral activation in cardiovascular disease.

Baroreflex control of renal sympathetic nerve activity in early heart failure assessed by the sequence method

KEY POINTS

The integrity of the baroreflex control of sympathetic activity in heart failure (HF) remains under debate. We proposed the use of the sequence method to assess the baroreflex control of renal sympathetic nerve activity (RSNA). The sequence method assesses the spontaneous arterial pressure (AP) fluctuations and their related changes in heart rate (or other efferent responses), providing the sensitivity and the effectiveness of the baroreflex. Effectiveness refers to the fraction of spontaneous AP changes that elicits baroreflex-mediated variations in the efferent response. Using three different approaches, we showed that the baroreflex sensitivity between AP and RSNA is not altered in early HF rats. However, the sequence method provided evidence that the effectiveness of baroreflex in changing RSNA in response to AP changes is markedly decreased in HF. The results help us better understand the baroreflex control of the sympathetic nerve activity.

ABSTRACT

In heart failure (HF), the reflex control of the heart rate is known to be markedly impaired; however, the baroreceptor control of the sympathetic drive remains under debate. Applying the sequence method to a series of arterial pressure (AP) and renal sympathetic nerve activity (RSNA), we demonstrated a clear dysfunction in the baroreflex control of sympathetic activity in rats with early HF. We analysed the baroreflex control of the sympathetic drive using three different approaches: AP vs. RSNA curve, cross-spectral analysis and sequence method between AP and RSNA. The sequence method also provides the baroreflex effectiveness index (BEI), which represents the percentage of AP ramps that actually produce a reflex response. The methods were applied to control rats and rats with HF induced by myocardial infarction. None of the methods employed to assess the sympathetic baroreflex gain were able to detect any differences between the control and the HF group. However, rats with HF exhibited a lower BEI compared to the controls. Moreover, an optimum delay of 1 beat was observed, i.e. 1 beat is required for the RSNA to respond after AP changing, which corroborates with the findings related to the timing between these two variables. For delay 1, the BEI of the controls was 0.45 ± 0.03, whereas the BEI of rats with HF was 0.29 ± 0.09 (P < 0.05). These data demonstrate that while the gain of the baroreflex is not affected in early HF, its effectiveness is markedly decreased. The analysis of the spontaneous changes in AP and RSNA using the sequence method provides novel insights into arterial baroreceptor reflex function.

Increased sympathetic nerve activity and reduced cardiac baroreflex sensitivity in rheumatoid arthritis

KEY POINTS

Rheumatoid arthritis (RA) is a chronic inflammatory condition associated with an increased risk of cardiovascular mortality. Increased sympathetic nerve activity and reduced cardiac baroreflex sensitivity heighten cardiovascular risk, althogh whether such autonomic dysfunction is present in RA is not known. In the present study, we observed an increased sympathetic nerve activity and reduced cardiac baroreflex sensitivity in patients with RA compared to matched controls. Pain was positively correlated with sympathetic nerve activity and negatively correlated with cardiac baroreflex sensitivity. The pattern of autonomic dysfunction that we describe may help to explain the increased cardiovascular risk in RA, and raises the possibility that optimizing pain management may resolve autonomic dysfunction in RA.

ABSTRACT

Rheumatoid arthritis (RA) is a chronic inflammatory condition associated with increased cardiovascular morbidity/mortality and an incompletely understood pathophysiology. In animal studies, central and blood borne inflammatory cytokines that can be elevated in RA evoke pathogenic increases in sympathetic activity and reductions in baroreflex sensitivity (BRS). We hypothesized that muscle sympathetic nerve activity (MSNA) was increased and BRS decreased in RA. MSNA, blood pressure and heart rate (HR) were recorded in age- and sex-matched RA-normotensive (n = 13), RA-hypertensive patients (RA-HTN; n = 17), normotensive (NC; n = 17) and hypertensive controls (HTN; n = 16). BRS was determined using the modified Oxford technique. Inflammation and pain were determined using serum high sensitivity C-reactive protein (hs-CRP) and a visual analogue scale (VAS), respectively. MSNA was elevated similarly in RA, RA-HTN and HTN patients (32 ± 9, 35 ± 14, 37 ± 8 bursts min^-1 ) compared to NC (22 ± 9 bursts min^-1 ; P = 0.004). Sympathetic BRS was similar between groups (P = 0.927), whereas cardiac BRS (cBRS) was reduced in RA, RA-HTN and HTN patients [5(3-8), 4 (2-7), 6 (4-9) ms mmHg^-1 ] compared to NC [11 (8-15) ms mmHg^-1 ; P = 0.002]. HR was independently associated with hs-CRP. Increased MSNA and reduced cBRS were associated with hs-CRP although confounded in multivariable analysis. VAS was independently associated with MSNA burst frequency, cBRS and HR. We provide the first evidence for heightened sympathetic outflow and reduced cBRS in RA that can be independent of hypertension. In RA patients, reported pain was positively correlated with MSNA and negatively correlated with cBRS. Future studies should assess whether therapies to ameliorate pain and inflammation in RA restores autonomic balance and reduces cardiovascular events.

Arterial baroreflex control of sympathetic nerve activity and heart rate in patients with type 2 diabetes

Despite greater blood pressure reactivity to acute cardiovascular stressors and a higher prevalence of hypertension in type 2 diabetes (T2D) patients, limited information is available regarding arterial baroreflex (ABR) control in T2D. We hypothesized that ABR control of muscle sympathetic nerve activity (MSNA) and heart rate (HR) are attenuated in T2D patients. Seventeen T2D patients (50 ± 2 yr; 31 ± 1 kg/m²), 9 weight-matched controls (WM-CON, 46 ± 2 yr; 32 ± 2 kg/m²) and 10 lean controls (Lean-CON, 49 ± 3 yr; 23 ± 1 kg/m²), underwent bolus infusions of sodium nitroprusside (100 μg) followed 60 s later by phenylephrine (150 μg) and weighted linear regression performed. No group differences in overall sympathetic baroreflex gain were observed (T2D: -2.5 ± 0.3 vs. WM-CON: -2.6 ± 0.2 vs. Lean-CON: -2.7 ± 0.4 arbitrary units·beat·mmHg^-1, P > 0.05) or in sympathetic baroreflex gain when derived separately during blood pressure (BP) falls (nitroprusside) and BP rises (phenylephrine). In contrast, overall cardiac baroreflex gain was reduced in T2D patients compared with Lean-CON (T2D: 8.2 ± 1.5 vs. Lean-CON: 15.6 ± 2.9 ms·mmHg^-1, P < 0.05) and also tended to be reduced in WM-CON (9.3 ± 1.9 ms·mmHg^-1) compared with Lean-CON (P = 0.059). Likewise, during BP rises, cardiac baroreflex gain was reduced in T2D patients and weight-matched controls compared with lean controls (P < 0.05), whereas no group differences were found during BP falls (P > 0.05). Sympathetic and cardiac ABR gains were comparable between normotensive and hypertensive T2D patients (P > 0.05). These findings suggest preserved ABR control of MSNA in T2D patients compared with both obese and lean age-matched counterparts, with a selective impairment in ABR HR control in T2D that may be related to obesity.

Lead toxicity promotes autonomic dysfunction with increased chemoreceptor sensitivity

Mortality and morbidity by toxic metals is an important issue of occupational health. Lead is an ubiquitous heavy metal in our environment despite having no physiological role in biological systems. Being an homeostatic controller is expected that the autonomic nervous system would show a degree of impairment in lead toxicity. In fact, sympathoexcitation associated to high blood pressure and tachypnea has been described together with baroreflex dysfunction. However, the mechanisms underlying the autonomic dysfunction and the interplay between baro- and chemoreflex are not yet fully clarified. The angiotensinogenic PVN-NTS axis (paraventricular nucleus of the hypothalamus - nucleus tractus solitarius axis) is a particularly important neuronal pathway that could be responsible for the autonomic dysfunction and the cardiorespiratory impairment in lead toxicity. Within the current work, we addressed in vivo, baro- and chemoreceptor reflex behaviour, before and after central angiotensin inhibition, in order to better understand the cardiorespiratory autonomic mechanisms underlying the toxic effects of long-term lead exposure. For that, arterial pressure, heart rate, respiratory rate, sympathetic and parasympathetic activity and baro- and chemoreceptor reflex profiles of anaesthetized young adult rats exposed to lead, from foetal period to adulthood, were evaluated. Results showed increased chemosensitivity together with baroreceptor reflex impairment, sympathetic over-excitation, hypertension and tachypnea. Chemosensitivity and sympathetic overexcitation were reversed towards normality values by NTS treatment with A-779, an angiotensin (1-7) antagonist. No parasympathetic changes were observed before and after A-799 treatment. In conclusion, angiotensin (1-7) at NTS level is involved in the autonomic dysfunction observed in lead toxicity. The increased sensitivity of chemoreceptor reflex expresses the clear impairment of autonomic outflow to the cardiovascular and respiratory systems induced by putative persistent, long duration, alert reaction evoked by the long term exposure to lead toxic effects. The present study brings new insights on the central mechanisms implicated in the autonomic dysfunction induced by lead exposure which are relevant for the development of additional therapeutic options to tackle lead toxicity symptoms.

Sympathetic overactivity occurs before hypertension in the two-kidney, one-clip model

Our knowledge of mechanisms responsible for both the development and the maintenance of hypertension remains incomplete in the Goldblatt (two-kidney, one-clip; 2K1C) model. We tested the hypothesis that elevated sympathetic nerve activity (SNA) occurs before the onset of hypertension in 2K1C rats, considering the time course of the increase in SNA in relationship to the onset of the hypertension. We used a decorticated in situ working heart-brainstem preparation of three groups of male Wistar rats, namely sham-operated animals (SHAM, n = 7) and animals 3 weeks post-2K1C, of which some were hypertensive (2K1C-H, n = 6) and others normotensive (2K1C-N, n = 9), as determined in vivo a priori. Perfusion pressure was higher in both 2K1C groups (2K1C-H, 76 ± 1 mmHg; 2K1C-N, 74 ± 3 mmHg; versus SHAM, 60 ± 2 mmHg, P < 0.05). The SNA was significantly elevated in both 2K1C groups (2K1C-H, 47.7 ± 6.1 μV; 2K1C-N, 32.8 ± 2.8 μV; versus SHAM, 20.5 ± 2.5 μV, P < 0.05) owing to its increased respiratory modulation; the chemoreflex was augmented and baroreflex depressed. Precollicular transection reduced SNA in all groups (2K1C-H, -32.5 ± 7.5%; 2K1C-NH, -48 ± 6.9%; versus SHAM, -13.2 ± 1%, P < 0.05). Subsequent medullary spinal cord transection abolished SNA in both SHAM and 2K1C-N groups, but decreased it by only 57 ± 5.5% in 2K1C-H preparations. Thus, SNA is raised before the onset of hypertension, by the third week after renal artery clipping, and this originates, in part, from its enhanced respiratory modulation. Spinal circuits contribute to the elevation of SNA in the 2K1C model, but only after hypertension has developed.

Role of neurons and glia in the CNS actions of the renin-angiotensin system in cardiovascular control

Despite tremendous research efforts, hypertension remains an epidemic health concern, leading often to the development of cardiovascular disease. It is well established that in many instances, the brain plays an important role in the onset and progression of hypertension via activation of the sympathetic nervous system. Further, the activity of the renin-angiotensin system (RAS) and of glial cell-mediated proinflammatory processes have independently been linked to this neural control and are, as a consequence, both attractive targets for the development of antihypertensive therapeutics. Although it is clear that the predominant effector peptide of the RAS, ANG II, activates its type-1 receptor on neurons to mediate some of its hypertensive actions, additional nuances of this brain RAS control of blood pressure are constantly being uncovered. One of these complexities is that the RAS is now thought to impact cardiovascular control, in part, via facilitating a glial cell-dependent proinflammatory milieu within cardiovascular control centers. Another complexity is that the newly characterized antihypertensive limbs of the RAS are now recognized to, in many cases, antagonize the prohypertensive ANG II type 1 receptor (AT1R)-mediated effects. That being said, the mechanism by which the RAS, glia, and neurons interact to regulate blood pressure is an active area of ongoing research. Here, we review the current understanding of these interactions and present a hypothetical model of how these exchanges may ultimately regulate cardiovascular function.

Reporter mouse strain provides a novel look at angiotensin type-2 receptor distribution in the central nervous system

Angiotensin-II acts at its type-1 receptor (AT1R) in the brain to regulate body fluid homeostasis, sympathetic outflow and blood pressure. However, the role of the angiotensin type-2 receptor (AT2R) in the neural control of these processes has received far less attention, largely because of limited ability to effectively localize these receptors at a cellular level in the brain. The present studies combine the use of a bacterial artificial chromosome transgenic AT2R-enhanced green fluorescent protein (eGFP) reporter mouse with recent advances in in situ hybridization (ISH) to circumvent this obstacle. Dual immunohistochemistry (IHC)/ISH studies conducted in AT2R-eGFP reporter mice found that eGFP and AT2R mRNA were highly co-localized within the brain. Qualitative analysis of eGFP immunoreactivity in the brain then revealed localization to neurons within nuclei that regulate blood pressure, metabolism, and fluid balance (e.g., NTS and median preoptic nucleus [MnPO]), as well as limbic and cortical areas known to impact stress responding and mood. Subsequently, dual IHC/ISH studies uncovered the phenotype of specific populations of AT2R-eGFP cells. For example, within the NTS, AT2R-eGFP neurons primarily express glutamic acid decarboxylase-1 (80.3 ± 2.8 %), while a smaller subset express vesicular glutamate transporter-2 (18.2 ± 2.9 %) or AT1R (8.7 ± 1.0 %). No co-localization was observed with tyrosine hydroxylase in the NTS. Although AT2R-eGFP neurons were not observed within the paraventricular nucleus (PVN) of the hypothalamus, eGFP immunoreactivity is localized to efferents terminating in the PVN and within GABAergic neurons surrounding this nucleus. These studies demonstrate that central AT2R are positioned to regulate blood pressure, metabolism, and stress responses.

The effect of late gestation foetal hypoglycaemia on cardiovascular and endocrine function in sheep

An appropriate foetal cardiovascular (CV) response to reduced substrate supply (e.g. oxygen or other nutrients) is vital for growth and development, and may impact on CV control. The prevailing nutritional environment and associated CV changes may influence subsequent CV responses to challenges during late gestation, for example, umbilical cord occlusion (UCO). We investigated the effect of low-circulating glucose on foetal CV control mechanisms and response to UCO. Under general anaesthesia, late gestation foetal sheep (n = 7, 119 days gestational age (dGA), term ∼147 days) were implanted with vascular catheters, a bladder catheter, electrocardiogram electrodes and an umbilical cord occluder. Mean arterial pressure (MAP), heart rate (HR) and kidney function were monitored during maternal saline (MSAL, 125dGA) and insulin (MINS, 126dGA) infusion, and foetal CV responses were assessed during incremental doses of angiotensin II, a 90-s total UCO, and administration of phenylephrine to assess baroreflex function. During MINS infusion, the decrease in maternal and foetal blood glucose was associated with a small but significant decrease in foetal HR and reduced foetal baroreflex sensitivity (P < 0.05). The increase in foetal MAP during a 90-s UCO was greater during hypoglycaemia (P < 0.05). The MAP response to angiotensin II was not affected by hypoglycaemia. Decreased foetal HR and baroreflex sensitivity and increased CV responsiveness to UCO during hypoglycaemia indicates altered CV homoestatic mechanisms. The combination of altered nutrition and a CV challenge, such as UCO, during late gestation may have a cumulative effect on foetal CV function.

Heart rate variability biofeedback: how and why does it work?

In recent years there has been substantial support for heart rate variability biofeedback (HRVB) as a treatment for a variety of disorders and for performance enhancement (Gevirtz, 2013). Since conditions as widely varied as asthma and depression seem to respond to this form of cardiorespiratory feedback training, the issue of possible mechanisms becomes more salient. The most supported possible mechanism is the strengthening of homeostasis in the baroreceptor (Vaschillo et al., 2002; Lehrer et al., 2003). Recently, the effect on the vagal afferent pathway to the frontal cortical areas has been proposed. In this article, we review these and other possible mechanisms that might explain the positive effects of HRVB.

Temporal profile and mechanisms of the prompt sympathoexcitation following coronary ligation in Wistar rats

Our aim was to assess the timing and mechanisms of the sympathoexcitation that occurs immediately after coronary ligation. We recorded thoracic sympathetic (tSNA) and phrenic activities, heart rate (HR) and perfusion pressure in Wistar rats subjected to either ligation of the left anterior descending coronary artery (LAD) or Sham operated in the working heart-brainstem preparation. Thirty minutes after LAD ligation, tSNA had increased (basal: 2.5±0.2 µV, 30 min: 3.5±0.3 µV), being even higher at 60 min (5.2±0.5 µV, P<0.01); while no change was observed in Sham animals. HR increased significantly 45 min after LAD (P<0.01). Sixty minutes after LAD ligation, there was: (i) an augmented peripheral chemoreflex - greater sympathoexcitatory response (50, 45 and 27% of increase to 25, 50 and 75 µL injections of NaCN 0.03%, respectively, when compared to Sham, P<0.01); (ii) an elevated pressor response (32±1 versus 23±1 mmHg in Sham, P<0.01) and a reduced baroreflex sympathetic gain (1.3±0.1 versus Sham 2.0±0.1%.mmHg-1, P<0.01) to phenylephrine injection; (iii) an elevated cardiac sympathetic tone (ΔHR after atenolol: -108±8 versus -82±7 bpm in Sham, P<0.05). In contrast, no changes were observed in cardiac vagal tone and bradycardic response to both baroreflex and chemoreflex between LAD and Sham groups. The immediate sympathoexcitatory response in LAD rats was dependent on an excitatory spinal sympathetic cardiocardiac reflex, whereas at 3 h an angiotensin II type 1 receptor mechanism was essential since Losartan curbed the response by 34% relative to LAD rats administered saline (P<0.05). A spinal reflex appears key to the immediate sympathoexcitatory response after coronary ligation. Therefore, the sympathoexcitatory response seems to be maintained by an angiotensinergic mechanism and concomitant augmentation of sympathoexcitatory reflexes.

Putative roles of neuropeptides in vagal afferent signaling

The vagus nerve is a major pathway by which information is communicated between the brain and peripheral organs. Sensory neurons of the vagus are located in the nodose ganglia. These vagal afferent neurons innervate the heart, the lung and the gastrointestinal tract, and convey information about peripheral signals to the brain important in the control of cardiovascular tone, respiratory tone, and satiation, respectively. Glutamate is thought to be the primary neurotransmitter involved in conveying all of this information to the brain. It remains unclear how a single neurotransmitter can regulate such an extensive list of physiological functions from a wide range of visceral sites. Many neurotransmitters have been identified in vagal afferent neurons and have been suggested to modulate the physiological functions of glutamate. Specifically, the anorectic peptide transmitters, cocaine and amphetamine regulated transcript (CART) and the orexigenic peptide transmitters, melanin concentrating hormone (MCH) are differentially regulated in vagal afferent neurons and have opposing effects on food intake. Using these two peptides as a model, this review will discuss the potential role of peptide transmitters in providing a more precise and refined modulatory control of the broad physiological functions of glutamate, especially in relation to the control of feeding.

Glutamatergic activities in neonatal rat spinal cord heterogeneously regulate single-fiber splanchnic nerve discharge

Kynurenic acid (KYN) is a metabolite of tryptophan and is involved in various neurological disorders. Using whole-bundle nerve recording techniques, we previously observed that applications of KYN to block endogenous ionotropic glutamate receptor activities in neonatal rat spinal cords in vitro cause a reversible fluctuation of splanchnic sympathetic nerve discharge (SND). We hypothesized that the SND fluctuation was due to a heterogeneous single-fiber response. To detail individual fiber activities, we used the so-called 'oligofiber recordings'. Spontaneous single-fiber activities were recorded from the collagenase-dissociated splanchnic nerve fascicles. Applications of KYN increased, decreased or did not change firing rates. The heterogeneous responses in spontaneous spiking activities were confirmed by applications of APV or CNQX, suggesting an effect mediated by endogenous NMDA- or non-NMDA receptor activities. In addition to changes in firing rates, apparent drug-induced changes in firing patterns were also observed in some fiber activities. Using the oligofiber recording techniques, we confirmed a differential role of endogenous ionotropic glutamate receptor activities in regulating sympathetic outflows from the spinal cord of neonatal rats. Fine-tuning of ionotropic glutamate receptor activities in the spinal cord may serve as a simple way for heterogeneous regulation of various sympathetic-targeting tissues.

Neuroimmune communication in hypertension and obesity: a new therapeutic angle?

Hypertension is an epidemic health concern and a major risk factor for the development of cardiovascular disease. Although there are available treatment strategies for hypertension, numerous hypertensive patients do not have their clinical symptoms under control and it is imperative that new avenues to treat or prevent high blood pressure in these patients are developed. It is well established that increases in sympathetic nervous system (SNS) outflow and enhanced renin-angiotensin system (RAS) activity are common features of hypertension and various pathological conditions that predispose individuals to hypertension. More recently, hypertension has also become recognized as an immune condition and accumulating evidence suggests that interactions between the RAS, SNS and immune systems play a role in blood pressure regulation. This review summarizes what is known about the interconnections between the RAS, SNS and immune systems in the neural regulation of blood pressure. Based on the reviewed studies, a model for RAS/neuroimmune interactions during hypertension is proposed and the therapeutic potential of targeting RAS/neuroimmune interactions in hypertensive patients is discussed. Special emphasis is placed on the applicability of the proposed model to obesity-related hypertension.

Brain stem oxidative stress and its associated signaling in the regulation of sympathetic vasomotor tone

There is now compelling evidence from studies in humans and animals that overexcitation of the sympathetic nervous system plays an important role in the pathogenesis of cardiovascular diseases. An excellent example is neurogenic hypertension, in which central sympathetic overactivation is involved in the development, staging, and progression of the disease, and one of the underlying mechanisms involves oxidative stress in key brain stem sites that are engaged in the regulation of sympathetic vasomotor tone. Using the rostral ventrolateral medulla (RVLM) and nucleus tractus solitarii (NTS) as two illustrative brain stem neural substrates, this article provides an overview of the impact of reactive oxygen species and antioxidants on RVLM and NTS in the pathogenesis of neurogenic hypertension. This is followed by a discussion of the redox-sensitive signaling pathways, including several kinases, ion channels, and transcription factors that underpin the augmentation in sympathetic vasomotor tone. In addition, the emerging view that brain stem oxidative stress is also causally related to a reduction in sympathetic vasomotor tone and hypotension during brain stem death, methamphetamine intoxication, and temporal lobe status epilepticus will be presented, along with the causal contribution of the oxidant peroxynitrite formed by a reaction between nitric oxide synthase II (NOS II)-derived nitric oxide and superoxide. Also discussed as a reasonable future research direction is dissection of the cellular mechanisms and signaling cascades that may underlie the contributory role of nitric oxide generated by different NOS isoforms in the differential effects of oxidative stress in the RVLM or NTS on sympathetic vasomotor tone.

Robust dynamic balance of AP-1 transcription factors in a neuronal gene regulatory network

BACKGROUND

The octapeptide Angiotensin II is a key hormone that acts via its receptor AT1R in the brainstem to modulate the blood pressure control circuits and thus plays a central role in the cardiac and respiratory homeostasis. This modulation occurs via activation of a complex network of signaling proteins and transcription factors, leading to changes in levels of key genes and proteins. AT1R initiated activity in the nucleus tractus solitarius (NTS), which regulates blood pressure, has been the subject of extensive molecular analysis. But the adaptive network interactions in the NTS response to AT1R, plausibly related to the development of hypertension, are not understood.

RESULTS

We developed and analyzed a mathematical model of AT1R-activated signaling kinases and a downstream gene regulatory network, with structural basis in our transcriptomic data analysis and literature. To our knowledge, our report presents the first computational model of this key regulatory network. Our simulations and analysis reveal a dynamic balance among distinct dimers of the AP-1 family of transcription factors. We investigated the robustness of this behavior to simultaneous perturbations in the network parameters using a novel multivariate approach that integrates global sensitivity analysis with decision-tree methods. Our analysis implicates a subset of Fos and Jun dependent mechanisms, with dynamic sensitivities shifting from Fos-regulating kinase (FRK)-mediated processes to those downstream of c-Jun N-terminal kinase (JNK). Decision-tree analysis indicated that while there may be a large combinatorial functional space feasible for neuronal states and parameters, the network behavior is constrained to a small set of AP-1 response profiles. Many of the paths through the combinatorial parameter space lead to a dynamic balance of AP-1 dimer forms, yielding a robust AP-1 response counteracting the biological variability.

CONCLUSIONS

Based on the simulation and analysis results, we demonstrate that a dynamic balance among distinct dimers of the AP-1 family of transcription factors underlies the robust activation of neuronal gene expression in the NTS response to AT1R activation. Such a differential sensitivity to limited set of mechanisms is likely to underlie the stable homeostatic physiological response.

Differential control of renal and lumbar sympathetic nerve activity during freezing behavior in conscious rats

The present study was designed to document changes in sympathetic nerve activity and cardiovascular function when conscious rats were challenged with a noise stressor to induce freezing behavior. The potential contribution of the arterial baroreceptors in regulating sympathetic nerve activity and cardiovascular adjustments during the freezing behavior was then examined. Wistar male rats were assigned to sham-operated (SO) and sinoaortic-denervated (SAD) groups and instrumented chronically with electrodes for measurements of renal (RSNA) and lumbar (LSNA) sympathetic nerve activity, electroencephalogram, electromyogram, and electrocardiogram and catheters for measurements of systemic arterial and central venous pressure. Both SO and SAD rats were exposed to 90 dB of white noise for 10 min, causing freezing behavior in both groups. In SO rats, freezing behavior was associated with an immediate and significant (P < 0.05) increase in RSNA, no changes in LSNA or mean arterial pressure, and a significant (P < 0.05) decrease in heart rate. SAD attenuated the magnitude of the immediate increase in RSNA and had no influence on the response in LSNA during freezing behavior compared with SO rats. Moreover, in SAD rats, mean arterial pressure increased significantly (P < 0.05) while heart rate did not change during the freezing behavior. These data indicate that freezing behavior evokes regionally different changes in sympathetic outflows, which may be involved in generating the patterned responses of cardiovascular function to stressful or threatening sensory stimulation. Moreover, it is suggested that the arterial baroreceptors are involved in generating the differential changes in RSNA and LSNA and thus the patterned changes in cardiovascular functions observed during freezing behavior in conscious rats.

Topographical specificity of regulation of respiratory and renal sympathetic activity by the midbrain dorsolateral periaqueductal gray

The midbrain periaqueductal gray (PAG) mediates the physiological responses to a wide range of stressors. It consists of four longitudinal columns that have different anatomical connections and functional properties. Previous anatomical and behavioral studies have led to the hypothesis that the dorsolateral PAG, but not the adjacent lateral and dorsomedial subregions, is a key center that integrates the behavioral response to acute psychological threatening stimuli. In this study, we tested whether, consistent with this hypothesis, activation of neurons in the dorsolateral PAG evokes a pattern of cardiovascular and respiratory responses that is distinct from that evoked from surrounding regions. Arterial pressure, heart rate, renal sympathetic nerve activity (RSNA), and phrenic nerve activity (PNA) were recorded simultaneously in urethane-anesthetized rats. Microinjections of very small amounts of d,l-homocysteic acid (750 pmol in 15 nl) were made in sites throughout the dorsomedial, dorsolateral, and lateral PAG subregions. Increases in RSNA of similar magnitude accompanied by small to moderate increases in arterial pressure and heart rate were evoked from all three PAG subregions. In contrast, large increases in both PNA burst rate (respiratory rate) and overall respiratory activity were evoked only from a highly circumscribed region that corresponded closely to the dorsolateral PAG subregion at an intermediate to caudal level. Within this region, the evoked increases in RSNA and respiratory activity were highly correlated (r = 0.914, P < 0.001), suggesting the possibility that a common population of "command neurons" within the dorsolateral PAG may generate both sympathetic and respiratory responses from this region.

Region-specific changes in sympathetic nerve activity in angiotensin II-salt hypertension in the rat

It is now well accepted that many forms of experimental hypertension and human essential hypertension are caused by increased activity of the sympathetic nervous system. However, the role of region-specific changes in sympathetic nerve activity (SNA) in the pathogenesis of hypertension has been difficult to determine because methods for chronic measurement of SNA in conscious animals have not been available. We have recently combined indirect, and continuous and chronic direct, assessment of region-specific SNA to characterize hypertension produced by administration of angiotensin II (Ang II) to rats consuming a high-salt diet (Ang II-salt hypertension). Angiotensin II increases whole-body noradrenaline (NA) spillover and depressor responses to ganglionic blockade in rats consuming a high-salt diet, but not in rats on a normal-salt diet. Despite this evidence for increased 'whole-body SNA' in Ang II-salt hypertensive rats, renal SNA is decreased in this model and renal denervation does not attenuate the steady-state level of arterial pressure. In addition, neither lumbar SNA, which largely targets skeletal muscle, nor hindlimb NA spillover is changed from control levels in Ang II-salt hypertensive rats. However, surgical denervation of the splanchnic vascular bed attenuates/abolishes the increase in arterial pressure and total peripheral resistance, as well as the decrease in vascular capacitance, observed in Ang II-salt hypertensive rats. We hypothesize that the 'sympathetic signature' of Ang II-salt hypertension is characterized by increased splanchnic SNA, no change in skeletal muscle SNA and decreased renal SNA, and this sympathetic signature creates unique haemodynamic changes capable of producing sustained hypertension.

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.

Role of differential changes in sympathetic nerve activity in the preparatory adjustments of cardiovascular functions during freezing behaviour in rats

Freezing behaviour is associated with a distinct pattern of changes in cardiovascular function, which has been considered as a preparatory reflex for 'fight or flight' behaviour. However, the detailed mechanisms underlying preparatory cardiovascular adjustments and their physiological implications have received less attention. We studied responses in renal and lumbar sympathetic nerve activity and cardiovascular function during freezing behaviour in conscious rats, which was induced by exposure to loud white noise. Freezing behaviour was associated with regionally specific alterations in sympathetic nerve activity, in that renal sympathetic nerve activity increased while lumbar sympathetic nerve activity did not change. Moreover, freezing behaviour was associated with differential shifts in baroreflex control of sympathetic outflows, which could help to explain the selective responses in renal and lumbar sympathetic nerve activity during freezing behaviour. These differential changes in sympathetic outflows would result in a visceral vasoconstriction without having any impact on the skeletal muscle vasculature. These cardiovascular adjustments during freezing behaviour may help to explain the immediate and massive increase in muscular blood flow that occurs at the onset of fight or flight behaviour. It is hypothesized that central command originating from the defence area could somehow modulate separate baroreflex pathways, causing differential changes in sympathetic nerve activity to generate the preparatory cardiovascular adjustments during the freezing behaviour.

Changes in hemodynamic and neurohumoral control cause cardiac damage in one-kidney, one-clip hypertensive mice

Sympathovagal balance and baroreflex control of heart rate (HR) were evaluated during the development (1 and 4 wk) of one-kidney, one-clip (1K1C) hypertension in conscious mice. The development of cardiac hypertrophy and fibrosis was also examined. Overall variability of systolic arterial pressure (AP) and HR in the time domain and baroreflex sensitivity were calculated from basal recordings. Methyl atropine and propranolol allowed the evaluation of the sympathovagal balance to the heart and the intrinsic HR. Staining of renal ANG II in the kidney and plasma renin activity (PRA) were also evaluated. One and four weeks after clipping, the mice were hypertensive and tachycardic, and they exhibited elevated sympathetic and reduced vagal tone. The intrinsic HR was elevated only 1 wk after clipping. Systolic AP variability was elevated, while HR variability and baroreflex sensitivity were reduced 1 and 4 wk after clipping. Renal ANG II staining and PRA were elevated only 1 wk after clipping. Concentric cardiac hypertrophy was observed at 1 and 4 wk, while cardiac fibrosis was observed only at 4 wk after clipping. In conclusion, these data further support previous findings in the literature and provide new features of neurohumoral changes during the development of 1K1C hypertension in mice. In addition, the 1K1C hypertensive model in mice can be an important tool for studies evaluating the role of specific genes relating to dependent and nondependent ANG II hypertension in transgenic mice.

Signalling across the blood brain barrier by angiotensin II: novel implications for neurogenic hypertension

Angiotensin II (AngII) is a major culprit in essential hypertension. Based on a genetic rodent model of hypertension, we review here evidence that AngII may signal across the blood brain barrier to affect neuronal circuits within the nucleus tractus solitarii (NTS) of the brainstem, a pivotal region regulating both the baroreceptor reflex and set point control of arterial pressure. We have termed this form of signalling as vascular-neuronal signalling. We describe that the depressant action of AngII in NTS on the baroreceptor reflex is mediated via activation of endothelial nitric oxide synthase (eNOS) releasing NO that promotes release of the inhibitory transmitter-GABA. This could shunt the incoming excitatory baroreceptor afferent traffic impinging on NTS neurones. Chronic studies recording arterial pressure in conscious unrestrained rats using radio-telemetry have revealed that eNOS in NTS plays an endogenous physiological role in the homeostatic regulation of the gain of the cardiac baroreceptor reflex. However, in the spontaneously hypertensive rat, eNOS mRNA was higher (compared to normotensive rats), and its chronic blockade in NTS restored the abnormally depressed cardiac baroreceptor reflex to levels akin to normotensive rats, improved heart rate variability and lowered arterial pressure. Hence, it seems that excessive eNOS activity in NTS of the SHR contributes to the pathological state of this animal model's cardiovascular autonomic nervous system. We speculate on why eNOS activity may be up regulated in the NTS of the SHR and propose that it is a consequence of high cerebral vascular resistance and inadequate blood perfusion of the brainstem.

Dynamic transcriptomic response to acute hypertension in the nucleus tractus solitarius

Baroreceptor afferents project to the cardiovascular region of the nucleus tractus solitarius (cvNTS), and their cvNTS target neurons may play a role in governing the sensitivity and operating range of the arterial baroreceptor reflex (baroreflexes). Recent studies have shown differential gene and protein expression in the cvNTS in response to changed arterial pressure. However, the extent of these responses is unknown. Therefore, we collected differential global gene expression data in a time series following acute hypertension in awake, freely moving rats. To acquire statistically significant results and place them in functional context, we overcame several quality control requirements and developed novel analytical approaches. The physiologically new findings from the study are that acute hypertension causes very extensive, time-varying gene regulatory changes, many involving neuronal function-specific genes and systems of genes. We use standard genomic analysis methods to manage the large data sets and to develop results such as heat maps to examine patterns and clusters in the gene regulation. We used the Gene Ontology categories to provide functional context. To place our findings in the context of the relevant literature, we developed two graphical representations of the networks implicated, linking receptors and channels to signaling pathways. The results point to the multivariate complexity of the response and implicate a group of receptors as candidates for mediating nucleus tractus solitarius baroreflex function in hypertension by identifying concurrent upregulation of receptor genes. We were able to make transcription factor binding predictions and record dysregulation of heart rate correlated with the transcriptional response.

Brain renin angiotensin in disease

A brain renin angiotensin system (RAS) and its role in cardiovascular control and fluid homeostasis was at first controversial. This was because a circulating kidney-derived renin angiotensin system was so similar and well established. But, the pursuit of brain RAS has proven to be correct. In the course of accepting brain RAS, high standards of proof attracted state of the art techniques in all the new developments of biolo1gy. Consequently, brain RAS is a robust concept that has enlightened neuroscience as well as cardiovascular physiology and is a model neuropeptide system. Molecular biology confirmed the components of brain RAS and their location in the brain. Transgenic mice and rats bearing renin and extra copies of angiotensinogen genes revealed the importance of brain RAS. Cre-lox delivery in vectors has enabled pinpoint gene deletion of brain RAS in discrete brain nuclei. The new concept of brain RAS includes ACE-2, Ang1–7, and prorenin and Mas receptors. Angiotensin II (ANG II) generated in the brain by brain renin has many neural effects. It activates behavioral effects by selective activation of ANG II receptor subtypes in different locations. It regulates sympathetic activity and baroreflexes and contributes to neurogenic hypertension. New findings implicate brain RAS in a much wider range of neural effects. We review brain RAS involvement in Alzheimer’s disease, stroke memory, and learning alcoholism stress depression. There is growing evidence to consider developing treatment strategies for a variety of neurological disease states based on brain RAS.