Phenotypic evidence of faulty neuronal norepinephrine reuptake in essential hypertension

Clinical neurocardiology: defining the value of neuroscience-based cardiovascular therapeutics - 2024 update

The intricate role of the autonomic nervous system (ANS) in regulating cardiac physiology has long been recognized. Aberrant function of the ANS is central to the pathophysiology of cardiovascular diseases. It stands to reason, therefore, that neuroscience-based cardiovascular therapeutics hold great promise in the treatment of cardiovascular diseases in humans. A decade after the inaugural edition, this White Paper reviews the current state of understanding of human cardiac neuroanatomy, neurophysiology and pathophysiology in specific disease conditions, autonomic testing, risk stratification, and neuromodulatory strategies to mitigate the progression of cardiovascular diseases.

Palmitoylation regulates norepinephrine transporter uptake, surface localization, and total expression with pathogenic implications in postural orthostatic tachycardia syndrome

Postural orthostatic tachycardia syndrome (POTS) is an adrenergic signaling disorder characterized by excessive plasma norepinephrine, postural tachycardia, and syncope. The norepinephrine transporter (NET) modulates adrenergic homeostasis via the reuptake of extracellular catecholamines and is implicated in the pathogenesis of adrenergic and neurological disorders. In this study, we reveal NET is palmitoylated in male Sprague-Dawley rats and Lilly Laboratory Cell Porcine Kidney (LLC-PK1) cells. S-palmitoylation, or the addition of a 16-carbon saturated fatty acid, is a reversible post-translational modification responsible for the regulation of numerous biological mechanisms. We found that LLC-PK1 NET is dynamically palmitoylated, and that inhibition with the palmitoyl acyltransferase (DHHC) inhibitor, 2-bromopalmitate (2BP) results in decreased NET palmitoylation within 90 min of treatment. This result was followed closely by a reduction in transport capacity, cell surface, and total cellular NET expression after 120 min of treatment. Increasing 2BP concentrations and treatment time revealed a nearly complete loss of total NET protein. Co-expression with individual DHHCs revealed a single DHHC enzyme, DHHC1, promoted wild-type (WT) hNET palmitoylation and elevated NET protein levels. The POTS-associated NET mutant, A457P, exhibits dramatically decreased transport capacity and cell surface levels which we have confirmed in the current study. In an attempt to recover A457P NET expression, we co-expressed the A457P variant with DHHC1 to drive expression as seen with the WT protein but instead saw an increase in NET N-terminal immuno-detectable forms and fragments. Elimination of a potential palmitoylation site at cysteine 44 in the N-terminal tail of hNET resulted in a low expression phenotype mimicking the A457P hNET variant. Further investigation of A457P NET palmitoylation and surface expression is necessary, but our preliminary novel findings reveal palmitoylation as a mechanism of NET regulation and suggest that dysregulation of this process may contribute to the pathogenesis of adrenergic disorders like POTS.

Sympathetic Pathophysiology in Hypertension Origins: The Path to Renal Denervation

The importance of the sympathetic nervous system in essential hypertension has been recognized in 2 eras. The first was in early decades of the 20th century, through to the 1960s. Here, the sympathetic nervous system was identified as a target for the treatment of hypertension, and an extensive range of antiadrenergic therapies were developed. Then, after a period of lapsed interest, in a second era from 1985 on, the development of precise measures of human sympathetic nerve firing and transmitter release allowed demonstration of the importance of neural mechanisms in the initiation and maintenance of the arterial blood pressure elevation in hypertension. This led to the development of a device treatment of hypertension, catheter-based renal denervation, which we will discuss.

Palmitoylation regulates norepinephrine transporter trafficking and expression and is potentially involved in the pathogenesis of postural orthostatic tachycardia syndrome

Postural orthostatic tachycardia syndrome (POTS) is an adrenergic signaling disorder characterized by excessive plasma norepinephrine, postural tachycardia, and syncope. The norepinephrine transporter (NET) modulates adrenergic homeostasis via reuptake of extracellular catecholamines and is implicated in the pathogenesis of adrenergic and neurological disorders. Previous research has outlined that NET activity and trafficking is modulated via reversible post-translational modifications like phosphorylation and ubiquitylation. S-palmitoylation, or the addition of a 16-carbon saturated fatty acid, is another post-translational modification responsible for numerous biological mechanisms. In this study, we reveal that NET is dynamically palmitoylated and inhibition of this modification with the palmitoyl acyltransferase (DHHC) inhibitor, 2-bromopalmitate (2BP), results in decreased NET palmitoylation within 90 min of treatment. This result was followed closely with a reduction in transport capacity, cell surface, and total cellular NET expression after 120 min of treatment. Increasing 2BP concentrations and treatment time revealed a nearly complete loss of total NET protein. Co-expression with individual DHHCs revealed a single DHHC enzyme, DHHC1, promoted WT hNET palmitoylation and elevated NET protein levels. The POTS associated NET mutant, A457P, exhibits dramatically decreased transport capacity and cell surface levels which we have confirmed in the current study. In an attempt to recover A457P NET expression we co-expressed the A457P variant with DHHC1 to drive expression as seen with the WT protein but instead saw an increase in NET N-terminal immuno-detectable fragments. Further investigation of A457P NET palmitoylation and surface expression is necessary, but our preliminary novel findings reveal palmitoylation as a mechanism of NET regulation and suggest that dysregulation of this process may contribute to the pathogenesis of POTS.

5-Aminolevulinic acid hydrochloride enhances bupivacaine-induced hypotension in spontaneously hypertensive rats

PURPOSE

Oral administration of 5-aminolevulinic acid hydrochloride (5-ALA-HCl) has been reported to enhance the hypotensive effects associated with anesthetics, especially in elderly hypertensive patients treated with antihypertensive agents. The present study aimed to clarify the effects of antihypertensive-agent- and anesthesia-induced hypotension by 5-ALA-HCl in spontaneously hypertensive rats (SHRs).

METHODS

We measured blood pressure (BP) of SHRs and normotensive Wistar Kyoto (WKY) rats treated with amlodipine or candesartan before and after administration of 5-ALA-HCl. We also investigated the change in BP following intravenous infusion of propofol and intrathecal injection of bupivacaine in relation to 5-ALA-HCl administration.

FINDINGS

Oral administration of 5-ALA-HCl significantly reduced BP in SHRs and WKY rats with amlodipine and candesartan. Infusion of propofol significantly reduced BP in SHRs treated with 5-ALA-HCl. Intrathecal injection of bupivacaine significantly declined SBP and DBP in both SHRs and WKY rats treated with 5-ALA-HCl. The bupivacaine-induced decline in SBP was significantly larger in SHRs compared with WKY rats.

CONCLUSION

These findings suggest that 5-ALA-HCl does not affect the antihypertensive agents-induced hypotensive effect, but enhances the bupivacaine-induced hypotensive effect, especially in SHRs, indicating that 5-ALA may contribute to anesthesia-induced hypotension via suppression of sympathetic nerve activity in patients with hypertension.

Sympathetic Neural Control in Humans with Anxiety-Related Disorders

Numerous conceptual models are used to describe the dynamic responsiveness of physiological systems to environmental pressures, originating with Claude Bernard's milieu intérieur and extending to more recent models such as allostasis. The impact of stress and anxiety upon these regulatory processes has both basic science and clinical relevance, extending from the pioneering work of Hans Selye who advanced the concept that stress can significantly impact physiological health and function. Of particular interest within the current article, anxiety is independently associated with cardiovascular risk, yet mechanisms underlying these associations remain equivocal. This link between anxiety and cardiovascular risk is relevant given the high prevalence of anxiety in the general population, as well as its early age of onset. Chronically anxious populations, such as those with anxiety disorders (i.e., generalized anxiety disorder, panic disorder, specific phobias, etc.) offer a human model that interrogates the deleterious effects that chronic stress and allostatic load can have on the nervous system and cardiovascular function. Further, while many of these disorders do not appear to exhibit baseline alterations in sympathetic neural activity, reactivity to mental stress offers insights into applicable, real-world scenarios in which heightened sympathetic reactivity may predispose those individuals to elevated cardiovascular risk. This article also assesses behavioral and lifestyle modifications that have been shown to concurrently improve anxiety symptoms, as well as sympathetic control. Lastly, future directions of research will be discussed, with a focus on better integration of psychological factors within physiological studies examining anxiety and neural cardiovascular health. © 2022 American Physiological Society. Compr Physiol 12:1-33, 2022.

Pivotal role of the sympathetic nerves of the human heart in mental stress responses and triggered cardiovascular catastrophes

Mental stress can trigger cardiac catastrophes, explicitly evident during national disasters such as earthquakes. Activation of the cardiac sympathetic outflow and inhibition of the cardiac vagus are important mediating mechanisms. This manuscript describes efforts by the Human Neurotransmitters Research Laboratory of the Baker Institute in Melbourne to develop investigative methods to study the sympathetic nerves of the human heart, and to apply these in mental stress research. With laboratory mental stress, activation of the adrenal medulla was found to occur, accompanied by a regionalized sympathetic nervous response directed to the heart, but sparing the sympathetic outflow to the skeletal muscle vasculature. Patients with panic disorder are at increased cardiovascular risk. They exhibit high-level sympathetic activation during a panic attack, sometimes accompanied by coronary artery spasm. Patients with sudden ventricular arrhythmias causing collapse in the community were found to have as the predisposing substrate high baseline cardiac sympathetic activity, from previously unrecognized mild heart failure; it was surprising at the time that we did not find critical coronary artery stenosis as the substrate. In some the arrhythmia event had a behavioural trigger. In Takotsubo cardiomyopathy ("Broken Heart Syndrome") the myocardial stunning appears to represent a catecholamine cardiomyopathy, from astronomically high plasma adrenaline concentrations, rather than be caused by activation of the cardiac sympathetic nerves. Some diseases (essential hypertension, heart failure, panic disorder) have forms of sympathetic neural enhancement which contribute to cardiovascular risk: reuptake of noradrenaline by sympathetic nerves after release is faulty and single sympathetic fibres fire in multiple salvos within a single cardiac cycle. Paradoxically, obesity-hypertension does not share in this sympathetic neural augmentation, which is present only in normal-weight hypertensive patients, providing the possible basis for an observed "Obesity Paradox" (longer survival in obesity-hypertension than in normal weight hypertension). Community-wide specific prevention of cardiovascular triggering is not currently possible, due to there being no available simple screening tests which could be applied to the community at-large for the commonest substrates, silent coronary artery disease and mild heart failure. Standard medical preventive measures for coronary atherosclerosis will of course be helpful. Targeted prevention of triggering can be done in those with a detected predisposing substrate, such as genetic Long QT Syndrome, and in survivors of a serious triggered event, who need detailed, appropriate testing.

Assessment of Cardiovascular Risk Factors and Their Interactions in the Risk of Coronary Heart Disease in Patients with Type 2 Diabetes with Different Weight Levels, 2013-2018

PURPOSE

In addition to hyperglycemia and hyperlipidemia, obesity and hypertension are important cardiovascular risk factors for coronary heart disease (CHD) in patients with type 2 diabetes mellitus (T2DM). This study aims to explore the interaction of these risk factors.

PATIENTS AND METHODS

Data of hospitalized patients with T2DM from 2013 to 2018 were collected. A multivariate logistic regression model was established. Patients with normal weight and blood pressure were recruited as controls. The interaction on the risk of CHD was evaluated by relative excess risk due to interaction (RERI).

RESULTS

Among the 30,693 patients with T2DM, 7202 (23.5%) had CHD. In the low weight group, the prevalence of CHD in patients with hypertension was nearly four times higher than that in patients without hypertension (42.7% vs 11.3%, P < 0.01). The OR value of hypertension alone on CHD increased from 1.29 (95% CI 1.06-1.56) in the body mass index (BMI) 30.0-34.9 kg/m² group to 1.35 (95% CI 1.11-1.62) in the BMI ≤ 18.5 kg/m² group. Additive interaction was observed between hypertension and BMI in CHD risk, especially in the low weight group (RERI:2.2, 95% CI 0.9-3.5). BMI and CHD risk showed a smile curve relationship. The attributive proportion in the low weight group was higher than that in the severe obesity group, that is, 0.52 (95% CI 0.35-0.69) vs 0.18 (95% CI -0.59 to 0.95).

CONCLUSION

Maintaining normal weight and avoiding low weight are particularly important for patients with co-occurring diabetes and hypertension to prevent the risk of CHD.

Impact of sex and age on metabolism, sympathetic activity, and hypertension

In this brief review, we summarize the current knowledge on the complex interplay between metabolism, sympathetic activity and hypertension with a focus on sex differences and changes with age in humans. Evidence suggests that in premenopausal women, sex hormones, particularly estrogen exerts a profound cardioprotective effect which may be associated with favorable metabolic profiles, as well as lower sympathetic activity and blood pressure at rest and any given physiological and environmental stimuli compared with men of a similar age. Along this line, premenopausal women seem to be generally protected from obesity-induced metabolic and cardiovascular complications. However, postmenopausal estrogen deprivation during midlife and older age has a detrimental impact on metabolism, may lead to adipose tissue redistribution from the subcutaneous to abdominal area, and augments sympathetic activity. All these changes could contribute significantly to the higher prevalence of hypertension and greater cardiometabolic risk in older women than older men. It is proposed that obesity-related hypertension has a neurogenic component which is characterized by sympathetic overactivity, but the impact of sex and age remains largely unknown. Understanding sex and age-specific differences in obesity and sympathetic neural control of blood pressure is important in the prevention and/or risk reduction of cardiometabolic disorders for both men and women.

Neurocardiac regulation: from cardiac mechanisms to novel therapeutic approaches

Cardiac sympathetic overactivity is a well-established contributor to the progression of neurogenic hypertension and heart failure, yet the underlying pathophysiology remains unclear. Recent studies have highlighted the importance of acutely regulated cyclic nucleotides and their effectors in the control of intracellular calcium and exocytosis. Emerging evidence now suggests that a significant component of sympathetic overactivity and enhanced transmission may arise from impaired cyclic nucleotide signalling, resulting from compromised phosphodiesterase activity, as well as alterations in receptor-coupled G-protein activation. In this review, we address some of the key cellular and molecular pathways that contribute to sympathetic overactivity in hypertension and discuss their potential for therapeutic targeting.

Impact of body mass index on cardiac adrenergic derangement in heart failure patients: a 123I-mIBG imaging study

PURPOSE

To assess the impact of body mass index (BMI) on cardiac adrenergic derangement, measured by iodine-123 meta-iodobenzylguanidine (¹²³I-mIBG) imaging in heart failure (HF) patients. Overweight and obesity represent relevant health issues, and augmented sympathetic tone has been described in patients with increased BMI. An extensive literature supports that HF-dependent cardiac denervation, measured through mIBG parameters, is an independent predictor of cardiovascular outcomes and mortality. However, the influence of BMI on cardiac mIBG uptake has not been largely investigated.

METHODS

We prospectively enrolled patients with systolic HF, collecting demographic, clinical, echocardiographic data, and mIBG imaging parameters. In order to detect the factors associated with mIBG parameters, a model building strategy, based on the Multivariable Fractional Polynomial algorithm, has been employed.

RESULTS

We studied 249 patients with systolic HF, mean age of 66.4 ± 10.6 years, and mean left ventricular ejection fraction (LVEF) of 30.7% ± 6.4, undergoing cardiac ¹²³I-mIBG imaging to assess HF severity and prognosis. Seventy-eight patients (31.3%) presented a BMI ≥ 30 kg/m² and obese patients showed a significant reduction in early heart to mediastinum (H/M) ratio (1.66 ± 0.19 vs. 1.75 ± 0.26; p = 0.008) and a trend to reduction in washout rate (33.6 ± 18.3 vs. 38.1 ± 20.1; p = 0.092) compared with patients with BMI < 30 kg/m². Multiple regression analysis revealed that BMI, age, and LVEF were significantly correlated with early and late H/M ratios.

CONCLUSIONS

Results of the present study indicate that BMI, together with LVEF and age, is independently correlated with cardiac mIBG uptake in HF patients.

Pre-synaptic sympathetic calcium channels, cyclic nucleotide-coupled phosphodiesterases and cardiac excitability

In sympathetic neurons innervating the heart, action potentials activate voltage-gated Ca²⁺ channels and evoke Ca²⁺ entry into presynaptic terminals triggering neurotransmitter release. Binding of transmitters to specific receptors stimulates signal transduction pathways that cause changes in cardiac function. The mechanisms contributing to presynaptic Ca²⁺ dynamics involve regulation of endogenous Ca²⁺ buffers, in particular the endoplasmic reticulum, mitochondria and cyclic nucleotide targeted pathways. The purpose of this review is to summarize and highlight recent findings about Ca²⁺ homeostasis in cardiac sympathetic neurons and how modulation of second messengers can drive neurotransmission and affect myocyte excitability in cardiovascular disease. Moreover, we discuss the underlying mechanism of abnormal intracellular Ca²⁺ homeostasis and signaling in these neurons, and speculate on the role of phosphodiesterases as a therapeutic target to restore normal autonomic transmission in disease states of overactivity.

Increased Abundance of Lactobacillales in the Colon of Beta-Adrenergic Receptor Knock Out Mouse Is Associated With Increased Gut Bacterial Production of Short Chain Fatty Acids and Reduced IL17 Expression in Circulating CD4+ Immune Cells

Emerging evidence suggests an associative link between gut dysbiosis, the autonomic nervous system (ANS) and the immune system in pathophysiology of neurogenic hypertension (HTN). However, the close interplay between these three systems presents us with difficulties in deciphering the cause-effect relationship in disease. The present study utilized beta 1 and 2 adrenergic receptor knock out (AdrB1^tm1BkkAdrB2^tm1Bkk/J KO) mice to isolate the effects of reduced overall sympathetic drive on gut microbiota and systemic immune system. We observed the following: (i) Diminished beta adrenergic signaling mainly reflects in shifts in the Firmicutes phyla, with a significant increase in abundance of largely beneficial Bacilli Lactobacillales in the KO mice; (ii) This was associated with increased colonic production of beneficial short chain fatty acids (SCFAs) butyrate, acetate and propionate, confirming functional microbiota shifts in the KO mice; (iii) Dampened systemic immune responses in the KO mice reflected in reduction on circulating CD4⁺.IL17⁺ T cells and increase in young neutrophils, both previously associated with shifts in the gut microbiota. Taken together, these observations demonstrate that reduced expression of beta adrenergic receptors may lead to beneficial shifts in the gut microbiota and dampened systemic immune responses. Considering the role of both in hypertension, this suggests that dietary intervention may be a viable option for manipulation of blood pressure via correcting gut dysbiosis.

Polymorphism of SLC6A2 gene does not influence outcome of myocardial 123I-mIBG scintigraphy in patients with chronic heart failure

AIM

The NET, encoded by SLC6A2, is responsible for presynaptic NE-reuptake. 123I-mIBG is clinically used to evaluate cardiac sympathetic function. However, it is unknown if polymorphism of SLC6A2 influences cardiac sympathetic activity as assessed with 123I-mIBG. Therefore we studied the influence of SLC6A2 SNPs on myocardial 123I-mIBG parameters in CHF.

MATERIALS AND METHODS

Forty-nine adults with stable CHF (age 66.5 ± 8.1 years, LVEF 22.3 ± 6.4) were enrolled. Fifteen minutes (early) and 4 hours (late) after administration of 123I-mIBG planar images were acquired. The H/M ratio was calculated from the manually drawn ROI over the left ventricle and a fixed mediastinal ROI. Fourteen exons of the SLC6A2 gene were analyzed from whole blood samples.

RESULTS

We found 6 different SLC6A2 SNPs, although none were functional. LVEF was the only independent predictor for early (adjusted R 2 = 0.063, p = 0.045) and late H/M ratio (adjusted R 2 = 0.116, p = 0.010). NT-proBNP was the only independent predictor for 123I-mIBG WO (adjusted R 2 = 0.074, p = 0.032). SLC6A2 SNPs were not associated with any myocardial 123I-mIBG-derived parameter.

CONCLUSION

In this specific CHF population polymorphism of SLC6A2 gene was not associated with any 123I-mIBG derived parameters.

Clozapine-Induced Cardiovascular Side Effects and Autonomic Dysfunction: A Systematic Review

Background: Clozapine is the antipsychotic of choice for treatment-resistant schizophrenia and has minimal risk for extrapyramidal symptoms. Therapeutic benefits, however, are accompanied by a myriad of cardiometabolic side-effects. The specific reasons for clozapine's high propensity to cause adverse cardiometabolic events remain unknown, but it is believed that autonomic dysfunction may play a role in many of these.

Objective: This systematic review summarizes the literature on autonomic dysfunction and related cardiovascular side effects associated with clozapine treatment.

Method: A search of the EMBASE, MEDLINE, and EBM Cochrane databases was conducted using the search terms antipsychotic agents, antipsychotic drug^*, antipsychotic^*, schizophrenia, schizophren^*, psychos^*, psychotic^*, mental ill^*, mental disorder^*, neuroleptic^*, cardiovascular^*, cardiovascular diseases, clozapine^*, clozaril^*, autonomic^*, sympathetic^*, catecholamine^*, norepinephrine, noradrenaline, epinephrine, adrenaline.

Results: The search yielded 37 studies that were reviewed, of which only 16 studies have used interventions to manage cardiovascular side effects. Side effects reported in the studies include myocarditis, orthostatic hypotension and tachycardia. These were attributed to sympathetic hyperactivity, decreased vagal contribution, blockade of cholinergic and adrenergic receptors, reduced heart rate variability and elevated catecholamines with clozapine use. Autonomic neuropathy was identified by monitoring blood pressure and heart rate changes in response to stimuli and by spectral analysis of heart rate variability. Metoprolol, lorazepam, atenolol, propranolol, amlodipine, vasopressin and norepinephrine infusion were used to treat tachycardia and fluctuations in blood pressure, yet results were limited to case reports.

Conclusion: The results indicate there is a lack of clinical studies investigating autonomic dysfunction and a limited use of interventions to manage cardiovascular side effects associated with clozapine. As there is often no alternative treatment for refractory schizophrenia, the current review highlights the need for better designed studies, use of autonomic tests for prevention of cardiovascular disease and development of novel interventions for clozapine-induced side effects.

Cardiac sympatho-vagal balance and ventricular arrhythmia

A hallmark of cardiovascular disease is cardiac autonomic dysregulation. The phenotype of impaired parasympathetic responsiveness and sympathetic hyperactivity in experimental animal models is also well documented in large scale human studies in the setting of heart failure and myocardial infarction, and is predictive of morbidity and mortality. Despite advances in emergency revascularisation strategies for myocardial infarction, device therapy for heart failure and secondary prevention pharmacotherapies, mortality from malignant ventricular arrhythmia remains high. Patients at highest risk or those with haemodynamically significant ventricular arrhythmia can be treated with catheter ablation and implantable cardioverter defibrillators, but the morbidity and reduction in quality of life due to the burden of ventricular arrhythmia and shock therapy persists. Therefore, future therapies must aim to target the underlying pathophysiology that contributes to the generation of ventricular arrhythmia. This review explores recent advances in mechanistic research in both limbs of the autonomic nervous system and potential avenues for translation into clinical therapy. In addition, we also discuss the relationship of these findings in the context of the reported efficacy of current neuromodulatory strategies in the management of ventricular arrhythmia.

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We review advances in mechanistic research in the cardiac autonomic nervous system.

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This is discussed in relation to neuromodulatory therapy for ventricular arrhythmia.

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Neuromodulation therapies can influence both neurotransmitters and co-transmitters.

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This may therefore improve on conventional medical treatment.

Cyclic nucleotide regulation of cardiac sympatho-vagal responsiveness

Cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) are now recognized as important intracellular signalling molecules that modulate cardiac sympatho-vagal balance in the progression of heart disease. Recent studies have identified that a significant component of autonomic dysfunction associated with several cardiovascular pathologies resides at the end organ, and is coupled to impairment of cyclic nucleotide targeted pathways linked to abnormal intracellular calcium handling and cardiac neurotransmission. Emerging evidence also suggests that cyclic nucleotide coupled phosphodiesterases (PDEs) play a key role limiting the hydrolysis of cAMP and cGMP in disease, and as a consequence this influences the action of the nucleotide on its downstream biological target. In this review, we illustrate the action of nitric oxide-CAPON signalling and brain natriuretic peptide on cGMP and cAMP regulation of cardiac sympatho-vagal transmission in hypertension and ischaemic heart disease. Moreover, we address how PDE2A is now emerging as a major target that affects the efficacy of soluble/particulate guanylate cyclase coupling to cGMP in cardiac dysautonomia.

Differential Internalization Rates and Postendocytic Sorting of the Norepinephrine and Dopamine Transporters Are Controlled by Structural Elements in the N Termini

The norepinephrine transporter (NET) mediates reuptake of synaptically released norepinephrine in central and peripheral noradrenergic neurons. The molecular processes governing availability of NET in the plasma membrane are poorly understood. Here we use the fluorescent cocaine analogue JHC 1-64, as well as several other approaches, to investigate the trafficking itinerary of NET in live noradrenergic neurons. Confocal imaging revealed extensive constitutive internalization of JHC 1-64-labeled NET in the neuronal somata, proximal extensions and presynaptic boutons. Phorbol 12-myristate 13-acetate increased intracellular accumulation of JHC 1-64-labeled NET and caused a parallel reduction in uptake capacity. Internalized NET strongly colocalized with the "long loop" recycling marker Rab11, whereas less overlap was seen with the "short loop" recycling marker Rab4 and the late endosomal marker Rab7. Moreover, mitigating Rab11 function by overexpression of dominant negative Rab11 impaired NET function. Sorting of NET to the Rab11 recycling compartment was further supported by confocal imaging and reversible biotinylation experiments in transfected differentiated CATH.a cells. In contrast to NET, the dopamine transporter displayed markedly less constitutive internalization and limited sorting to the Rab11 recycling compartment in the differentiated CATH.a cells. Exchange of domains between the two homologous transporters revealed that this difference was determined by non-conserved structural elements in the intracellular N terminus. We conclude that NET displays a distinct trafficking itinerary characterized by continuous shuffling between the plasma membrane and the Rab11 recycling compartment and that the functional integrity of the Rab11 compartment is critical for maintaining proper presynaptic NET function.

Respiratory modulated sympathetic activity: a putative mechanism for developing vascular resistance?

KEY POINTS

Sympathetic activity exhibits respiratory modulation that is amplified in hypertensive rats. Respiratory modulated sympathetic activity produces greater changes in vascular resistance than tonic stimulation of the same stimulus magnitude in normotensive but not hypertensive rats. Mathematical modelling demonstrates that respiratory modulated sympathetic activity may fail to produce greater vascular resistance changes in hypertensive rats because the system is saturated as a consequence of a dysfunctional noradrenaline reuptake mechanism. Respiratory modulated sympathetic activity is an efficient mechanism to raise vascular resistance promptly, corroborating its involvement in the ontogenesis of hypertension.

ABSTRACT

Sympathetic nerve activity (SNA) exhibits respiratory modulation. This component of SNA is important - being recruited under cardiorespiratory reflex conditions and elevated in the spontaneously hypertensive (SH) rat - and yet the exact influence of this modulation on vascular tone is not understood, even in normotensive conditions. We constructed a mathematical model of the sympathetic innervation of an arteriole, and used it to test the hypothesis that respiratory modulation of SNA preferentially increases vasoconstriction compared to a frequency-matched tonic pattern. Simulations supported the hypothesis, where respiratory modulated increases in vasoconstriction were mediated by a noradrenergic mechanism. These predictions were tested in vivo in adult Wistar rats. Stimulation of the sympathetic chain (L3) with respiratory modulated bursting patterns, revealed that bursting increases vascular resistance (VR) more than tonic stimulation (57.8 ± 3.3% vs. 44.8 ± 4.2%; P < 0.001; n = 8). The onset of the VR response was also quicker for bursting stimulation (rise time constant = 1.98 ± 0.09 s vs. 2.35 ± 0.20 s; P < 0.01). In adult SH rats (n = 8), the VR response to bursting (44.6 ± 3.9%) was not different to tonic (37.4 ± 3.5%; P = 0.57). Using both mathematical modelling and in vivo techniques, we have shown that VR depends critically on respiratory modulation and revealed that this pattern dependency in Wistar rats is due to a noradrenergic mechanism. This respiratory component may therefore contribute to the ontogenesis of hypertension in the pre-hypertensive SH rat - raising VR and driving vascular remodelling. Why adult SH rats do not exhibit a pattern-dependent response is not known, but further modelling revealed that this may be due to dysfunctional noradrenaline reuptake.

Acute effects of arterial baroreflex on sympathetic nerve activity and plasma norepinephrine concentration

Arterial pressure (AP) elevates as a logarithmic function of exogenously administered dose of norepinephrine (NE). In contrast, AP is nearly linearly correlated with efferent sympathetic nerve activity (SNA) during acute baroreflex intervention. The present study aimed at quantifying the relationship between SNA and plasma NE concentration during acute baroreflex intervention. Carotid sinus regions were isolated from systemic circulation in five Wistar Kyoto rats, and carotid sinus pressure was changed among 60, 100, 120, 140, and 180 mm Hg every 2 min. Arterial blood (0.2 ml) was obtained at each pressure level for plasma NE measurement. Maximum AP and minimum AP were 153.34 ± 6.28 and 67.31 ± 4.92 mm Hg, respectively, in response to pressure perturbation. Plasma NE correlated linearly with SNA for individual animal data (slope: 0.957 ± 0.090 pg · ml(-1) · %(-1), intercept: 46.57 ± 7.22 pg/ml, r(2): ranged from 0.923 to 0.992) and also for group averaged data (NE = 0.956 × SNA + 47.97, r(2 )= 0.982). Blockade of neuronal NE uptake by intravenous desipramine (1 mg/kg) administration increased the slope (2.966 ± 0.686 pg · ml(-1) · %(-1), P < 0.05) and the intercept (168.73 ± 28.53 pg/ml, P < 0.01) of the plasma NE-SNA relationship. These results indicate that the relationship between SNA and plasma NE concentration was nearly linear within the normal physiological range of acute baroreflex control of AP. While plasma NE concentration can reflect changes in SNA, it may also overestimate the sympathetic outflow from the central nervous system when neuronal NE uptake is impaired systemically.

Low-dose B-type natriuretic peptide raises cardiac sympathetic nerve activity in sheep

The reported effects of atrial natriuretic peptide (ANP) on sympathetic nerve activity (SNA) are variable, dependent on concomitant hemodynamic actions, and likely to be regionally differentiated. There are few reports of the effect of B-type natriuretic peptide (BNP) on SNA and none have measured cardiac SNA (CSNA) by direct microneurography. We measured the effects of low-dose ANP and BNP (2.4 pmol·kg−1·min−1 infused for 120 min) on CSNA and hemodynamics in conscious sheep ( n = 8). While there was a trend for mean arterial pressure and cardiac output to fall with both ANP and BNP, changes were not significant compared with vehicle control. However, BNP did significantly reduce systolic arterial (97 ± 4.2 vs. 107 ± 6.8 mmHg during control; P = 0.043) and pulse pressures (0.047) and increase heart rate (110 ± 6.7 vs. 96 ± 7.3 beats/min; P = 0.044). Trends for these hemodynamic parameters to change with ANP did not achieve statistical significance. ANP also had no significant effect on any CSNA parameters measured. In contrast, BNP induced a rise in both CSNA burst frequency (∼20 bursts/min higher than control, P = 0.011) and burst area (∼40% higher than control, P = 0.013). BNP-induced rises in burst incidence (bursts/100 beats), and burst area per 100 beats, however, were not significant. In conclusion, BNP infused at low doses that only had subtle effects on hemodynamics increased CSNA burst frequency and burst are per minute. This increase in CSNA may in large part be secondary to an increase in heart rate as CSNA burst incidence and burst area per 100 beats were not significantly increased. This study provides no evidence for inhibition of CSNA by natriuretic peptides.

Peripheral cardiac sympathetic hyperactivity in cardiovascular disease: role of neuropeptides

High levels of sympathetic drive in several cardiovascular diseases including postmyocardial infarction, chronic congestive heart failure and hypertension are reinforced through dysregulation of afferent input and central integration of autonomic balance. However, recent evidence suggests that a significant component of sympathetic hyperactivity may also reside peripherally at the level of the postganglionic neuron. This has been studied in depth using the spontaneously hypertensive rat, an animal model of genetic essential hypertension, where larger neuronal calcium transients, increased release and impaired reuptake of norepinephrine in neurons of the stellate ganglia lead to a significant tachycardia even before hypertension has developed. The release of additional sympathetic cotransmitters during high levels of sympathetic drive can also have deleterious consequences for peripheral cardiac parasympathetic neurotransmission even in the presence of β-adrenergic blockade. Stimulation of the cardiac vagus reduces heart rate, lowers myocardial oxygen demand, improves coronary blood flow, and independently raises ventricular fibrillation threshold. Recent data demonstrates a direct action of the sympathetic cotransmitters neuropeptide Y (NPY) and galanin on the ability of the vagus to release acetylcholine and control heart rate. Moreover, there is as a strong correlation between plasma NPY levels and coronary microvascular function in patients with ST-elevation myocardial infarction being treated with primary percutaneous coronary intervention. Antagonists of the NPY receptors Y1 and Y2 may be therapeutically beneficial both acutely during myocardial infarction and also during chronic heart failure and hypertension. Such medications would be expected to act synergistically with β-blockers and implantable vagus nerve stimulators to improve patient outcome.

Regional changes in cardiac and stellate ganglion norepinephrine transporter in DOCA-salt hypertension

Uptake of norepinephrine via the neuronal norepinephrine transporter is reduced in the heart during deoxycorticosterone (DOCA)-salt hypertension. We hypothesized that this was due to reduced norepinephrine transporter mRNA and/or protein expression in the stellate ganglia and heart. After 4 weeks of DOCA-salt treatment there was no change in norepinephrine transporter mRNA in either the right or the left stellate ganglia from hypertensive rats (n=5-7, p>0.05). Norepinephrine transporter immunoreactivity in the left stellate ganglion was significantly increased (n=4, p<0.05) while the right stellate ganglion was unchanged (n=4, p>0.05). Whole heart norepinephrine content was significantly reduced in DOCA rats consistent with reduced uptake function; however, when norepinephrine was assessed by chamber, a significant decrease was noted only in the right atrium and right ventricle (n=6, p<0.05). Cardiac norepinephrine transport binding by chamber revealed that it was only reduced in the left atrium (n=5-7, p>0.05). Therefore, 1) contrary to our hypothesis reduced reuptake in the hypertensive heart is not exclusively due to an overall reduction in norepinephrine transporter mRNA or protein in the stellate ganglion or heart, and 2) norepinephrine transporter regulation occurs regionally in the heart and stellate ganglion in the hypertensive rat heart.

Norepinephrine transporter variant A457P knock-in mice display key features of human postural orthostatic tachycardia syndrome

Postural orthostatic tachycardia syndrome (POTS) is a common autonomic disorder of largely unknown etiology that presents with sustained tachycardia on standing, syncope and elevated norepinephrine spillover. Some individuals with POTS experience anxiety, depression and cognitive dysfunction. Previously, we identified a mutation, A457P, in the norepinephrine (NE; also known as noradrenaline) transporter (NET; encoded by SLC6A2) in POTS patients. NET is expressed at presynaptic sites in NE neurons and plays a crucial role in regulating NE signaling and homeostasis through NE reuptake into noradrenergic nerve terminals. Our in vitro studies demonstrate that A457P reduces both NET surface trafficking and NE transport and exerts a dominant-negative impact on wild-type NET proteins. Here we report the generation and characterization of NET A457P mice, demonstrating the ability of A457P to drive the POTS phenotype and behaviors that are consistent with reported comorbidities. Mice carrying one A457P allele (NET(+/P)) exhibited reduced brain and sympathetic NE transport levels compared with wild-type (NET(+/+)) mice, whereas transport activity in mice carrying two A457P alleles (NET(P/P)) was nearly abolished. NET(+/P) and NET(P/P) mice exhibited elevations in plasma and urine NE levels, reduced 3,4-dihydroxyphenylglycol (DHPG), and reduced DHPG:NE ratios, consistent with a decrease in sympathetic nerve terminal NE reuptake. Radiotelemetry in unanesthetized mice revealed tachycardia in NET(+/P) mice without a change in blood pressure or baroreceptor sensitivity, consistent with studies of human NET A457P carriers. NET(+/P) mice also demonstrated behavioral changes consistent with CNS NET dysfunction. Our findings support that NET dysfunction is sufficient to produce a POTS phenotype and introduces the first genetic model suitable for more detailed mechanistic studies of the disorder and its comorbidities.

Investigation of the correlation between norepinephrine transporter gene polymorphisms and essential hypertension

Essential hypertension (EH) is an etiological risk factor that poses a serious threat to human health. The aim of this study therefore was to investigate the correlation between the two SNPs (rs1805067 and rs2397771) of the norepinephrine (NE) transporter gene in the sympathetic nervous system and essential hypertension (EH). A total of 96 hypertensive patients (EH group) and 96 normal subjects (control group) were selected by epidemiological investigation and the sequences at rs1805067 and rs2397771 in the two groups were investigated using polymerase chain reaction (PCR) and Sanger sequencing. In the EH group, systolic and diastolic blood pressure, body mass index (BMI), levels of creatinine, low-density lipoprotein cholesterol (LDL-C), triglycerides (TG), and the percentage of drinkers were found to be higher compared with those of the control group (P<0.05). The results of the analysis of the allele frequencies of rs1805067 (A/G) revealed that there was no difference between the two groups (P>0.05). The analysis results of the allele frequencies of rs2397771 (C/G) in the EH group revealed that G accounted for 48.96% and C for 51.04% of the patients, while in the control group, these values were 64.58 and 35.42%, respectively. These results indicated a significant difference between the two groups (P<0.05). In the population studied, the onset of EH may be correlated with BMI, TG, LDL-C and high-density lipoprotein cholesterol (HDL-C). EH may be related to the rs2397771 (C/G) C allele polymorphism of the NE transporter gene but not to rs1805067 (A/G).

Stress-dependent hypertension and the role of T lymphocytes

Hypertension is a significant global health burden that is associated with an increased risk of stroke, atherosclerosis and other cardiovascular diseases. Several risk factors, including high dietary salt, obesity, genetics and race, as well as behavioural and psychological factors, contribute to development of this complex disease. Various hypertensive stimuli enhance sympathetic drive and promote autonomic dysfunction leading to elevated blood pressure. As our understanding of the pathogenesis and end-organ damage associated with hypertension increases, mounting evidence also highlights the role of inflammation in this process and, in particular, the role of the adaptive immune system and T cells. This review discusses recent findings regarding the role of the central nervous system, T lymphocytes and the impact of cardiovascular risk factors, such as psychological stress, in hypertension.

Overview of endocrine systems in primary hypertension

Multiple hormonal factors play a major role in the functional and structural abnormalities of hypertension (HT). At present, the kidneys and, in particular, renal Na(+) retention are thought to constitute a primary and sustaining mechanism in the development of HT. However, the precise renal and hormonal mechanisms leading to increased Na(+) reabsorption and HT remain unknown. Because the vast majority of HT is primary, this article focuses on the major endocrine systems, the RAS, aldosterone, and the SNS, that play a prominent role in the pathogenesis of HT.

Regulation of monoamine transporters: Role of transporter phosphorylation

Presynaptic biogenic amine transporters mediate reuptake of released amines from the synapse, thus regulating serotonin, dopamine and norepinephrine neurotransmission. Medications utilized in the treatment of depression, attention deficit-hyperactivity disorder and other psychiatric disorders possess high affinity for amine transporters. In addition, amine transporters are targets for psychostimulants. Altered expression of biogenic amine transporters has long been implicated in several psychiatric and degenerative disorders. Therefore, appropriate regulation and maintenance of biogenic amine transporter activity is critical for the maintenance of normal amine homoeostasis. Accumulating evidence suggests that cellular protein kinases and phosphatases regulate amine transporter expression, activity, trafficking and degradation. Amine transporters are phosphoproteins that undergo dynamic control under the influence of various kinase and phosphatase activities. This review presents a brief overview of the role of amine transporter phosphorylation in the regulation of amine transport in the normal and diseased brain. Understanding the molecular mechanisms by which phosphorylation events affect amine transporter activity is essential for understanding the contribution of transporter phosphorylation to the regulation of monoamine neurotransmission and for identifying potential new targets for the treatment of various brain diseases.

Insulin reveals Akt signaling as a novel regulator of norepinephrine transporter trafficking and norepinephrine homeostasis

Noradrenergic signaling in the CNS plays an essential role in circuits involving attention, mood, memory, and stress as well as providing pivotal support for autonomic function in the peripheral nervous system. The high-affinity norepinephrine (NE) transporter (NET) is the primary mechanism by which noradrenergic synaptic transmission is terminated. Data indicate that NET function is regulated by insulin, a hormone critical for the regulation of metabolism. Given the high comorbidity of metabolic disorders such as diabetes and obesity with mental disorders such as depression and schizophrenia, we sought to determine how insulin signaling regulates NET function and thus noradrenergic homeostasis. Here, we show that acute insulin treatment, through the downstream kinase protein kinase B (Akt), significantly decreases NET surface expression in mouse hippocampal slices and superior cervical ganglion neuron boutons (sites of synaptic NE release). In vivo manipulation of insulin/Akt signaling, with streptozotocin, a drug that induces a type 1-like diabetic state in mice, also results in aberrant NET function and NE homeostasis. Notably, we also demonstrate that Akt inhibition or stimulation, independent of insulin, is capable of altering NET surface availability. These data suggest that aberrant states of Akt signaling such as in diabetes and obesity have the potential to alter NET function and noradrenergic tone in the brain. Furthermore, they provide one potential molecular mechanism by which Akt, a candidate gene for mood disorders such as schizophrenia and depression, can impact brain monoamine homeostasis.

Rab11 supports amphetamine-stimulated norepinephrine transporter trafficking

The norepinephrine transporter (NET) is a presynaptic plasma membrane protein that mediates reuptake of synaptically released norepinephrine. NET is also a major target for medications used for the treatment of depression, attention deficit/hyperactivity disorder, narcolepsy, and obesity. NET is regulated by numerous mechanisms, including catalytic activation and membrane trafficking. Amphetamine (AMPH), a psychostimulant and NET substrate, has also been shown to induce NET trafficking. However, neither the molecular basis nor the nature of the relevant membrane compartments of AMPH-modulated NET trafficking has been defined. Indeed, direct visualization of drug-modulated NET trafficking in neurons has yet to be demonstrated. In this study, we used a recently developed NET antibody and the presence of large presynaptic boutons in sympathetic neurons to examine basal and AMPH-modulated NET trafficking. Specifically, we establish a role for Rab11 in AMPH-induced NET trafficking. First, we found that, in cortical slices, AMPH induces a reduction in surface NET. Next, we observed AMPH-induced accumulation and colocalization of NET with Rab11a and Rab4 in presynaptic boutons of cultured neurons. Using tagged proteins, we demonstrated that NET and a truncated Rab11 effector (FIP2DeltaC2) do not redistribute in synchrony, whereas NET and wild-type Rab11a do. Analysis of various Rab11a/b mutants further demonstrates that Rab11 regulates NET trafficking. Expression of the truncated Rab11a effector (FIP2DeltaC2) attenuates endogenous Rab11 function and prevented AMPH-induced NET internalization as does GDP-locked Rab4 S22N. Our data demonstrate that AMPH leads to an increase of NET in endosomes of single boutons and varicosities in a Rab11-dependent manner.

Genetics of anxiety and trauma-related disorders

Anxiety disorders are the most common psychiatric illnesses in the United States with approximately 30% of the population experiencing anxiety-related symptoms in their lifetime [Kessler RC, Berglund P, Demler O, Jin R, Merikangas KR, Walters EE (2005) Lifetime prevalence and age-of-onset distributions of Diagnostic and Statistical Manual of Mental Disorders-IV (DSM-IV) disorders in the National Comorbidity Survey Replication. Arch Gen Psychiatry 62:593-60]. Notably, a variety of studies have demonstrated that 30-40% of the variance contributing to these disorders is heritable. In the present review, we discuss the latest findings regarding the genetic and environmental influences on the development and symptomatology of anxiety disorders. Specific emphasis is placed on posttraumatic stress disorder (PTSD) due to its uniqueness as an anxiety disorder; its diagnosis is dependent on a precipitating traumatic event and its development appears to be mediated by both genetic and environmental contributions. The co-morbidity of anxiety disorders and the potential re-classification of anxiety disorders as part of DSM-V are reviewed given the potential impact on the interpretation and design of genetic investigations. Lastly, several keys to future genetic studies are highlighted. Thorough analyses of the gene by environment (GxE) interactions that govern one's vulnerability to anxiety disorder(s), the effectiveness of individual treatment strategies, and the severity of symptoms may lead to more effective prophylactic (e.g. social support) and treatment strategies.

Cardiovascular abnormalities in patients with major depressive disorder: autonomic mechanisms and implications for treatment

This article provides a detailed review of the association of major depression with coronary heart disease (CHD), examines the biological variables underpinning the linkage and discusses the clinical implications for treatment. When considering the co-morbidity between major depressive disorder (MDD) and CHD it is important to differentiate between (i) the prevalence and impact of MDD in those with existing CHD and (ii) MDD as a risk factor for the development of CHD. Whether the same biological mechanisms are at play in these two instances remains unknown. Depression is common in patients with CHD. Importantly, depression in these patients increases mortality. There is also consistent evidence that MDD is a risk factor for the development of CHD. The relative risk of developing CHD is proportional to the severity of depression and is independent of smoking, obesity, hypercholesterolaemia, diabetes mellitus and hypertension. There is a clear need to identify the underlying neurochemical mechanisms responsible for MDD and their linkage to the heart and vascular system. Of particular interest are activation of stress pathways, including both the sympathetic nervous system and hypothalamic-pituitary-adrenal axis, and inflammatory-mediated atherogenesis. Elevated sympathetic activity, reduced heart rate variability and increased plasma cortisol levels have been documented in patients with MDD. In addition to direct effects on the heart and vasculature, activation of stress pathways may also be associated with increased release of inflammatory cytokines such as interleukin-6 and tumour necrosis factor-alpha. Elevated levels of C-reactive protein are commonly observed in patients with MDD. The majority of investigations examining treatment of depression following myocardial infarction have focused on safety and efficacy; there is little evidence to indicate that treating depression in these patients improves survival. Given that strategies for preventive therapy remain incompletely formulated, future research should focus on generating a better understanding of the neurobiology of MDD and heart disease as a basis for rational and effective therapy.

Subcellular localization of the antidepressant-sensitive norepinephrine transporter

BACKGROUND

Reuptake of synaptic norepinephrine (NE) via the antidepressant-sensitive NE transporter (NET) supports efficient noradrenergic signaling and presynaptic NE homeostasis. Limited, and somewhat contradictory, information currently describes the axonal transport and localization of NET in neurons.

RESULTS

We elucidate NET localization in brain and superior cervical ganglion (SCG) neurons, aided by a new NET monoclonal antibody, subcellular immunoisolation techniques and quantitative immunofluorescence approaches. We present evidence that axonal NET extensively colocalizes with syntaxin 1A, and to a limited degree with SCAMP2 and synaptophysin. Intracellular NET in SCG axons and boutons also quantitatively segregates from the vesicular monoamine transporter 2 (VMAT2), findings corroborated by organelle isolation studies. At the surface of SCG boutons, NET resides in both lipid raft and non-lipid raft subdomains and colocalizes with syntaxin 1A.

CONCLUSION

Our findings support the hypothesis that SCG NET is segregated prior to transport from the cell body from proteins comprising large dense core vesicles. Once localized to presynaptic boutons, NET does not recycle via VMAT2-positive, small dense core vesicles. Finally, once NET reaches presynaptic plasma membranes, the transporter localizes to syntaxin 1A-rich plasma membrane domains, with a portion found in cholera toxin-demarcated lipid rafts. Our findings indicate that activity-dependent insertion of NET into the SCG plasma membrane derives from vesicles distinct from those that deliver NE. Moreover, NET is localized in presynaptic membranes in a manner that can take advantage of regulatory processes targeting lipid raft subdomains.

Human sympathetic nerve biology: parallel influences of stress and epigenetics in essential hypertension and panic disorder

Patients with panic disorder provide a clinical model of stress. On a "good day," free from a panic attack, they show persistent stress-related changes in sympathetic nerve biology, including abnormal sympathetic nerve single-fiber firing ("salvos" of multiple firing within a cardiac cycle) and release of epinephrine as a cotransmitter. The coreleased epinephrine perhaps originates from in situ synthesis by phenylethanolamine N-methyltransferase (PNMT). In searching for biological evidence that essential hypertension is caused by mental stress--a disputed proposition--we note parallels with panic disorder, which provides an explicit clinical model of stress: (1) There is clinical comorbidity; panic disorder prevalence is increased threefold in essential hypertension. (2) For both, epinephrine cotransmission is present in sympathetic nerves. (3) In panic disorder and essential hypertension, but not in health, single-fiber sympathetic nerve firing salvos occur. (4) Tissue nerve growth factor is increased in both conditions (nerve growth factor is a stress reactant). (5) There is induction of PNMT in sympathetic nerves. Essential hypertension exhibits a further manifestation of mental stress: there is activation of noradrenergic brain stem neurons projecting to the hypothalamus and amygdala. These pathophysiological findings strongly support the view that chronic mental stress is important in the pathogenesis of essential hypertension. A hypothesis now under test is whether in both disorders, under prevailing conditions of ongoing stress, PNMT induced in sympathetic nerves acts as a DNA methylase, causing the norepinephrine transporter (NET) gene silencing that is present in both conditions. PNMT can have an intranuclear distribution, binding to DNA. We have demonstrated that the reduced neuronal noradrenaline reuptake present in both disorders does have an epigenetic mechanism, with demonstrable reduction in the abundance of the transporter protein, the NET gene silencing being associated with DNA binding by the methylation-related inhibitory transcription factor MeCP2.

Genetic variation within adrenergic pathways determines in vivo effects of presynaptic stimulation in humans

BACKGROUND

Catecholamines govern stress blood pressure responses. Catecholaminergic responses may be partially genetic and contribute to the complex heritability of hypertension.

METHODS AND RESULTS

To evaluate catecholaminergic responses without systemic counterregulation, we infused graded concentrations of tyramine, an indirect presynaptic norepinephrine releaser, into dorsal hand veins of 49 normotensive men and women of 5 ethnicities. Vascular responses were coupled to common (minor allele frequency >10%) single-nucleotide polymorphisms at adrenergic target loci within presynaptic pathways. Significance was set at P<0.003 after Bonferroni correction. Generalized analysis of molecular variance (GAMOVA) was performed to determine whether genetic admixture contributed to results. Venoconstriction progressed to 47% with increasing concentrations of tyramine (0.129 to 25.8 mmol/L; P<0.001). Family history of hypertension (P<0.001) and female sex (P=0.02) predicted blunted tyramine responses. Two genetic loci significantly predicted vascular response: chromogranin B, which encodes a protein that catalyzes catecholamine vesicle formation (CHGB, exon 4, Glu348Glu; P=0.002), and cytochrome b-561 (CYB561, intron 1, C719G; P<0.001), an electron shuttle for catecholamine synthesis. Stepwise regression suggested important effects for the CHGB locus, with polymorphisms for the vacuolar-ATPase beta-subunit (ATP6V1B1, exon 1, Ile30Thr) and flavin-containing monooxygenase-3 (FMO3, exon 3, Lys158Glu, P=0.002). GAMOVA did not show a significant relationship between overall genetic profile and hand-vein constriction (P=0.29), which indicates that population stratification did not contribute to this phenotype.

CONCLUSIONS

Locally infused tyramine produced dose-dependent pressor responses, predicted by family history of hypertension, sex, and genetic variants at loci, particularly CHGB, that encode the biosynthesis, storage, and metabolism of catecholamines. Such variants may influence the complex heritability of adrenergic responses and perhaps hypertension.

Sympathetic activity in major depressive disorder: identifying those at increased cardiac risk?

BACKGROUND

Evidence exists linking major depressive disorder (MDD) with clinical cardiovascular events. The importance of the sympathetic nervous system in the generation of cardiac risk in other contexts is established.

OBJECTIVE

To examine the importance of the sympathetic nervous system in the generation of cardiac risk in patients with major depressive disorder (MDD).

METHODS

Studies were performed in 39 patients meeting the Diagnostic and Statistical Manual of Mental Disorders IV (DSM-IV) criteria for MDD and in 76 healthy subjects. Treatment for patients consisted of selective serotonin reuptake inhibition (SSRI) for 12 weeks. Whole body and cardiac sympathetic activity were examined using noradrenaline isotope dilution methodology and sympathetic nerve recording techniques. Measurement of the extraction of infused tritiated noradrenaline by the heart, and estimation of cardiac dihydroxyphenylglycol production provided direct quantification of neuronal noradrenaline reuptake.

RESULTS

Sympathetic activity, particularly in the heart and for the whole body, in patients with MDD followed a bimodal distribution. Elevated values were observed in patients with co-morbid panic disorder (P = 0.006). Consistent with a defect in noradrenaline reuptake, the cardiac extraction of tritiated noradrenaline (0.80 +/- 0.01 versus 0.56 +/- 0.04%, P < 0.001) and cardiac dihydroxyphenylglycol overflow (109 +/- 8 versus 73 +/- 11, P = 0.01) were reduced in patients with MDD. SSRI therapy abolished the excessive sympathetic activation, with whole body noradrenaline spillover falling from 518 +/- 83 to 290 +/- 41 ng/min (P = 0.008).

CONCLUSIONS

We have identified a subset of patients with MDD in whom sympathetic nervous activity is extraordinarily high, including in the sympathetic outflow to the heart. Treatment with an SSRI may reduce sympathetic activity in a manner likely to reduce cardiac risk.

Molecular cloning and functional expression of the murine noradrenaline transporter

The cDNA of the murine noradrenaline transporter (mNAT) was cloned from the RNA of the placenta of a C57BL/6 mouse. The cloned mNAT differs from a previously published sequence in two amino acids within the C-terminal region. A cDNA obtained from an inbred mouse strain showed a further amino acid exchange (Ile(505)Val) within the fifth intracellular loop. The pharmacological properties of both, the wild-type mNAT and the variant (mNAT-I(505)V), were studied in human embryonic kidney HEK293 cells transfected with the corresponding cDNA. The kinetic constants for transport (K (m), V (max)) of [(3)H]noradrenaline ([(3)H]-NA) and binding (K (D), B (max)) of the selective NAT inhibitor [(3)H]nisoxetine were not different between the two isoforms; the mean kinetic constants amounted to about 4 microM and 120pmol/mg protein for K (m) and V (max) and 6nM and 18pmol/mg protein for K (D) and B (max), respectively. [(3)H]-NA transport by both isoforms showed the typical properties of an NAT because it was dependent on sodium and chloride and inhibited with almost identical K (i) values by various NAT substrates and inhibitors. The only significant pharmacological difference identified between the two mNAT isoforms was an about threefold higher affinity for cocaine of the very rare mNAT-I(505)V variant.

Determination of endogenous norepinephrine levels in different chambers of the rat heart by capillary electrophoresis coupled with amperometric detection

Capillary electrophoresis with end-column amperometric detection (CE-EC) was used to determine the regional distribution of norepinephrine (NE) in the hearts of sympathetically innervated (control) and chemically sympathectomized rats. Key features of the method are (i) the sample preparation and clean-up step that involved the application of off-line solid phase extraction (SPE) with a 95% NE recovery and (ii) the use of a diamond microelectrode for detection. NE was quantified in the left and right ventricle, the ventricular septum, and the left and right atrium. The NE concentration in the atria was three to five times higher than in the ventricles and ventricular septum of control rats. Basal NE levels in the left and right ventricle and the ventricular septum were reduced to below the detection limit (0.034 microg/g tissue) in tissues treated with the neurotoxin, 6-hydroxydopamine (6-OHDA), while only a moderate reduction was observed in the left and right atrium. Importantly, the diamond microelectrode provided low and stable background current and low peak-to-peak noise Collapse

Key Words

• sympathetic nervous system
• heart
• norepinephrine
• capillary electrophoresis with electrochemical detection

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MESH Headings

• Adrenergic Agents/pharmacology
• Animals
• Electric Stimulation
• Electrochemistry/methods
• Electrophoresis, Capillary/methods
• Heart Atria/drug effects
• Heart Atria/metabolism
• Heart Atria/radiation effects
• Heart Septum/drug effects
• Heart Septum/metabolism
• Heart Septum/radiation effects
• Heart Ventricles/drug effects
• Heart Ventricles/metabolism
• Heart Ventricles/radiation effects
• In Vitro Techniques
• Ion-Selective Electrodes
• Male
• Myocardium/cytology
• Myocardium/metabolism
• Norepinephrine/metabolism
• Oxidopamine/pharmacology
• Rats
• Rats, Sprague-Dawley

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Grants

• P01 HL070687 NHLBI NIH HHS
• R01 HL084258 NHLBI NIH HHS
• HL084258 NHLBI NIH HHS
• HL70687 NHLBI NIH HHS

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Affiliation(s)

Martin Novotny Department of Chemistry and the Neuroscience Program, Michigan State University, East Lansing, MI, 48824
Veronika Quaiserova-Mocko Department of Chemistry and the Neuroscience Program, Michigan State University, East Lansing, MI, 48824
Erica A. Wehrwein Department of Physiology and the Neuroscience Program, Michigan State University, East Lansing, MI, 48824
David L. Kreulen Department of Physiology and the Neuroscience Program, Michigan State University, East Lansing, MI, 48824
Greg M. Swain Department of Chemistry and the Neuroscience Program, Michigan State University, East Lansing, MI, 48824

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Collaborators Collapse

The norepinephrine transporter in physiology and disease

The norepinephrine transporter (NET) terminates noradrenergic signalling by rapid re-uptake of neuronally released norepinephrine (NE) into presynaptic terminals. NET exerts a fine regulated control over NE-mediated behavioural and physiological effects including mood, depression, feeding behaviour, cognition, regulation of blood pressure and heart rate. NET is a target of several drugs which are therapeutically used in the treatment or diagnosis of disorders among which depression, attention-deficit hyperactivity disorder and feeding disturbances are the most common. Individual genetic variations in the gene encoding the human NET (hNET), located at chromosome 16q12.2, may contribute to the pathogenesis of those diseases. An increasing number of studies concerning the identification of single nucleotide polymorphisms in the hNET gene and their potential association with disease as well as the functional investigation of naturally occurring or induced amino acid variations in hNET have contributed to a better understanding of NET function, regulation and genetic contribution to disorders. This review will reflect the current knowledge in the field of NET from its initial discovery until now.

The neuronal noradrenaline transporter, anxiety and cardiovascular disease

Panic disorder can serve as a clinical model for testing whether mental stress can cause heart disease. Potential neural mechanisms of cardiac risk are the sympathetic activation during panic attacks, continuing release of adrenaline as a co-transmitter in the cardiac sympathetic nerves, and impairment of noradrenaline neuronal reuptake, augmenting sympathetic neural respnses. The phenotype of impaired neuronal reuptake of noradrenaline: an epigenetic mechanism? We suspect that this phenotype, in sensitizing people to heart symptom development, is a cause of panic disorder, and by magnifying the sympathetic neural signal in the heart, underlies increased cardiac risk. No loss of function mutations of the coding region of the norepinephrine transporter (NET) are evident, but we do detect hypermethylation of CpG islands in the NET gene promoter region. Chromatin immunoprecipitation methodology demonstrates binding of the inhibitory transcription factor, MeCP2, to promoter region DNA in panic disorder patients. Cardiovascular illnesses co-morbid with panic disorder. Panic disorder commonly coexists with essential hypertension and the postural tachycardia syndrome. In both of these cardiovascular disorders the impaired neuronal noradrenaline reuptake phenotype is also present and, as with panic disorder, is associated with NET gene promoter region DNA hypermethylation. An epigenetic 'co-morbidity' perhaps underlies the clinical concordance. Brain neurotransmitters. Using internal jugular venous sampling, in the absence of a panic attack we find normal norepinephrine turnover, but based on measurements of the overflow of the serotonin metabolite, 5HIAA, a marked increase (six to sevenfold) in brain serotonin turnover in patients with panic disorder. This appears to represent the underlying neurotransmitter substrate for the disorder. Whether this brain serotonergic activation is a prime mover, or consequential on other primary causes of panic disorder, including cardiac sensitization by faulty neuronal noradrenaline reuptake leading to cardiac symptoms and the enhanced vigilance which accompanies them, is unclear at present.

Neurotensin-Induced Myocardial Noradrenergic Effects in Spontaneously Hypertensive Rats

Although increases in myocardial synaptic norepinephrine concentrations contribute toward the progression to heart failure in hypertension, the stimuli for norepinephrine release are unclear. In this study we explored whether neurotensin, a neuropeptide found in heart tissue, could modify myocardial norepinephrine release in spontaneously hypertensive rats (SHR). Prior to the development of cardiac decompensation, baseline coronary effluent norepinephrine concentrations were higher in isolated heart preparations of spontaneously hypertensive rats than in Wistar Kyoto (WKY) control rat hearts. Neurotensin increased coronary effluent norepinephrine concentrations and induced positive inotropic responses, effects that were enhanced in spontaneously hypertensive rats compared with Wistar Kyoto rats. Although the neurotensin receptor antagonist, SR 48692, did not modify either baseline coronary effluent norepinephrine concentrations or left ventricular systolic function in spontaneously hypertensive rats, it dose dependently abolished neurotensin-induced cardiac norepinephrine release and contractile responses. Neurotensin-mediated inotropic responses were also abolished by co-administration of the beta-adrenoreceptor blockers, propranolol and atenolol. Inotropic responses to exogenous norepinephrine were similar in SHR and WKY rats. In summary, in the hypertensive heart there is an increased sensitivity to neurotensin's actions on myocardial norepinephrine release and subsequent contractile changes. Therefore, neurotensin receptor blockade may represent a novel therapeutic target in preventing the progression to heart failure in hypertension.

Psychophysiological mechanisms in panic disorder: a correlative analysis of noradrenaline spillover, neuronal noradrenaline reuptake, power spectral analysis of heart rate variability, and psychological variables

BACKGROUND

The risk of adverse clinical cardiac events is increased in patients with panic disorder (PD). We evaluated possible mechanistic links between PD and heart disease. We estimated cardiac vagal activity from heart rate variability (HRV) measurements and quantified sympathetic nervous system (SNS) activity using plasma noradrenaline tracer kinetics methodology.

METHODS

Thirty-nine people with PD and 39 age- and gender-matched healthy volunteers were studied. In 19 participants with PD, both HRV and plasma noradrenaline kinetics were tested; in 20 with PD and 20 healthy volunteers, HRV measurements only were made, whereas in 19 healthy volunteers, noradrenaline kinetics only was tested. All panic disorder participants completed psychological measures of anxiety sensitivity and state and trait anxiety; healthy volunteers in whom HRV was measured also provided psychological measures.

RESULTS

Sympathetic nervous tone in the heart, based on rates of cardiac noradrenaline spillover, was normal in PD. Noradrenaline and adrenaline plasma clearance and plasma tritiated noradrenaline and adrenaline extraction in transit through the heart, all dependent on the noradrenaline transporter (NET), were reduced in PD. Psychometric testing linked inhibition of anger to this deficit in NET functioning. Anxiety sensitivity was specifically associated with impaired cardiac NET. High- and low-frequency heart rate spectral power was unrelated to all plasma noradrenaline kinetics measurements.

CONCLUSION

Defective neuronal reuptake of noradrenaline, by augmenting the sympathetic neural signal in the heart, might have a dual effect, sensitizing the heart such as to lead to symptom development (and thus perhaps causing panic disorder) and, second, potentially contributing to adverse cardiac events in established PD.

Catecholamine metabolism: a contemporary view with implications for physiology and medicine

This article provides an update about catecholamine metabolism, with emphasis on correcting common misconceptions relevant to catecholamine systems in health and disease. Importantly, most metabolism of catecholamines takes place within the same cells where the amines are synthesized. This mainly occurs secondary to leakage of catecholamines from vesicular stores into the cytoplasm. These stores exist in a highly dynamic equilibrium, with passive outward leakage counterbalanced by inward active transport controlled by vesicular monoamine transporters. In catecholaminergic neurons, the presence of monoamine oxidase leads to formation of reactive catecholaldehydes. Production of these toxic aldehydes depends on the dynamics of vesicular-axoplasmic monoamine exchange and enzyme-catalyzed conversion to nontoxic acids or alcohols. In sympathetic nerves, the aldehyde produced from norepinephrine is converted to 3,4-dihydroxyphenylglycol, not 3,4-dihydroxymandelic acid. Subsequent extraneuronal O-methylation consequently leads to production of 3-methoxy-4-hydroxyphenylglycol, not vanillylmandelic acid. Vanillylmandelic acid is instead formed in the liver by oxidation of 3-methoxy-4-hydroxyphenylglycol catalyzed by alcohol and aldehyde dehydrogenases. Compared to intraneuronal deamination, extraneuronal O-methylation of norepinephrine and epinephrine to metanephrines represent minor pathways of metabolism. The single largest source of metanephrines is the adrenal medulla. Similarly, pheochromocytoma tumor cells produce large amounts of metanephrines from catecholamines leaking from stores. Thus, these metabolites are particularly useful for detecting pheochromocytomas. The large contribution of intraneuronal deamination to catecholamine turnover, and dependence of this on the vesicular-axoplasmic monoamine exchange process, helps explain how synthesis, release, metabolism, turnover, and stores of catecholamines are regulated in a coordinated fashion during stress and in disease states.