Total norepinephrine spillover, muscle sympathetic nerve activity and heart-rate spectral analysis in a patient with dopamine beta-hydroxylase deficiency

Skin biopsy and microneurography disclose selective noradrenergic dysfunction due to dopamine-β-hydroxylase deficiency

Skin biopsy and microneurography are autonomic tests directly evaluating adrenergic and cholinergic sympathetic fibers to identify selective deficiency of a specific peripheral sympathetic subdivision. We describe a patient with tomacular neuropathy due to a deletion of the PMP22 gene who complained of chronic orthostatic hypotension due to a dopamine-β-hydroxylase deficiency confirmed by genetic analysis demonstrating two novel mutations in the DβH gene. To further characterize autonomic dysfunctions the proband underwent skin biopsy and microneurography. These tests disclosed a selective peripheral adrenergic dysfunction demonstrating the possibility to ascertain DβH deficiency. In conclusion, skin biopsy and microneurography may help to increase the diagnosis of this peculiar disorder particularly when routine autonomic nervous system tests show uncertain results.

Adrenaline: insights into its metabolic roles in hypoglycaemia and diabetes

Adrenaline is a hormone that has profound actions on the cardiovascular system and is also a mediator of the fight-or-flight response. Adrenaline is now increasingly recognized as an important metabolic hormone that helps mobilize energy stores in the form of glucose and free fatty acids in preparation for physical activity or for recovery from hypoglycaemia. Recovery from hypoglycaemia is termed counter-regulation and involves the suppression of endogenous insulin secretion, activation of glucagon secretion from pancreatic α-cells and activation of adrenaline secretion. Secretion of adrenaline is controlled by presympathetic neurons in the rostroventrolateral medulla, which are, in turn, under the control of central and/or peripheral glucose-sensing neurons. Adrenaline is particularly important for counter-regulation in individuals with type 1 (insulin-dependent) diabetes because these patients do not produce endogenous insulin and also lose their ability to secrete glucagon soon after diagnosis. Type 1 diabetic patients are therefore critically dependent on adrenaline for restoration of normoglycaemia and attenuation or loss of this response in the hypoglycaemia unawareness condition can have serious, sometimes fatal, consequences. Understanding the neural control of hypoglycaemia-induced adrenaline secretion is likely to identify new therapeutic targets for treating this potentially life-threatening condition.

A patient with PMP22-related hereditary neuropathy and DBH-gene-related dysautonomia

Recurrent focal neuropathy with liability to pressure palsies is a relatively frequent autosomal-dominant demyelinating neuropathy linked to peripheral myelin protein 22 (PMP22) gene deletions. The combination of PMP22 gene mutations with other genetic variants is known to cause a more severe phenotype than expected. We present the case of a patient with severe orthostatic hypotension since 12 years of age, who inherited a PMP22 gene deletion from his father. Genetic double trouble was suspected because of selective sympathetic autonomic disturbances. Through exome-sequencing analysis, we identified two novel mutations in the dopamine beta hydroxylase gene. Moreover, with interactome analysis, we excluded a further influence on the origin of the disease by variants in other genes. This case increases the number of unique patients presenting with dopamine-β-hydroxylase deficiency and of cases with genetically proven double trouble. Finding the right, complete diagnosis is crucial to obtain adequate medical care and appropriate genetic counseling.

New developments in the management of neurogenic orthostatic hypotension

Orthostatic hypotension (OH) is defined as a sustained reduction of ≥ 20 mmHg systolic blood pressure or ≥ 10 mmHg diastolic blood pressure upon standing for ≤ 3 min. Orthostatic hypotension is commonly associated with hypertension, and its prevalence is highest in those with uncontrolled hypertension compared to those with controlled hypertension or normotensive community elderly subjects. Orthostatic hypotension can cause significant disability, with patients experiencing dizziness, lightheadedness or syncope, and other problems that potentially have a profound negative impact on activities of daily living that require standing or walking. Furthermore, OH increases the risk of falls and, importantly, is an independent risk factor of mortality. Despite its importance, there is a paucity of treatment options for this condition. Most of the advances in treatment options have relied on small studies of repurposed drugs done in patients with severe OH due to rare neurodegenerative conditions. Midodrine, an oral prodrug converted to the selective α1-adrenoceptor agonist desglymidodrine, was approved by the FDA for the treatment of OH in 1996. For almost two decades, no other pharmacotherapy was developed specifically for the treatment of OH until 2014, when droxidopa was approved by the FDA for the treatment of neurogenic OH associated with primary autonomic neuropathies including Parkinson disease, multiple system atrophy, and pure autonomic failure. These are neurodegenerative diseases ultimately characterized by failure of the autonomic nervous system to generate norepinephrine responses appropriate to postural challenge. Droxidopa is a synthetic amino acid that is converted to norepinephrine by dopa-decarboxylase, the same enzyme that converts levodopa into dopamine in the treatment of Parkinson disease. We will review this and other advances in the treatment of OH in an attempt to provide a practical guide to its management.

Pediatric ptosis as a sign of treatable autonomic dysfunction

PURPOSE

To report the ophthalmic findings in young patients with dopamine β-hydroxylase deficiency and to assess them in the context of other reports in an attempt to discern if ophthalmic criteria may assist in early detection of this debilitating, yet treatable, disorder.

DESIGN

Prospective, observational case series.

METHODS

An ophthalmic examination, including measuring intraocular and systemic blood pressures while supine, sitting, and standing, and eyelid function and pupillary function testing, was completed on 3 young patients with recently documented dopamine β-hydroxylase deficiency at a single institution.

RESULTS

Mean arterial blood pressures were 90.1 ± 18.5 mm Hg supine, 79.1 ± 25.7 mm Hg sitting, and 45.8 ± 11.6 mm Hg standing (P = .021). Mean intraocular pressures in these patients were 15.8 ± 1.0 mm Hg supine, 15.0 ± 3.6 mm Hg sitting, and 7.7 ± 2.3 mm Hg standing (P = .03). Mean palpebral fissure, levator function, and margin reflex distance were 8.2 ± 1.0 mm, 16.0 ± 0 mm, and 2.8 ± 0.6 mm, respectively. Measurable miosis was present in only 1 patient, and pupillary supersensitivity to 2.5% phenylephrine was not observed.

CONCLUSIONS

The ophthalmologic findings of the patients in this case series documented mild ptosis and striking orthostatic reductions in intraocular pressure and mean arterial blood pressure, as might be expected with a lack of intrinsic sympathetic function. Orthostatic intraocular pressure and mean arterial blood pressure may be a helpful early screening tool for autonomic dysfunction in children undergoing a ptosis evaluation.

Recurrent postural vasovagal syncope: sympathetic nervous system phenotypes

BACKGROUND

The pathophysiology of vasovagal syncope is poorly understood, and the treatment usually ineffective. Our clinical experience is that patients with vasovagal syncope fall into 2 groups, based on their supine systolic blood pressure, which is either normal (>100 mm Hg) or low (70-100 mm Hg). We investigated neural circulatory control in these 2 phenotypes.

METHODS AND RESULTS

Sympathetic nervous testing was at 3 levels: electric, measuring sympathetic nerve firing (microneurography); neurochemical, quantifying norepinephrine spillover to plasma; and cellular, with Western blot analysis of sympathetic nerve proteins. Testing was done during head-up tilt (HUT), simulating the gravitational stress of standing, in 18 healthy control subjects and 36 patients with vasovagal syncope, 15 with the low blood pressure phenotype and 21 with normal blood pressure. Microneurography and norepinephrine spillover increased significantly during HUT in healthy subjects. The microneurography response during HUT was normal in normal blood pressure and accentuated in low blood pressure phenotype (P=0.05). Norepinephrine spillover response was paradoxically subnormal during HUT in both patient groups (P=0.001), who thus exhibited disjunction between nerve firing and neurotransmitter release; this lowered norepinephrine availability, impairing the neural circulatory response. Subnormal norepinephrine spillover in low blood pressure phenotype was linked to low tyrosine hydroxylase (43.7% normal, P=0.001), rate-limiting in norepinephrine synthesis, and in normal blood pressure to increased levels of the norepinephrine transporter (135% normal, P=0.019), augmenting transmitter reuptake.

CONCLUSIONS

Patients with recurrent vasovagal syncope, when phenotyped into 2 clinical groups based on their supine blood pressure, show unique sympathetic nervous system abnormalities. It is predicted that future therapy targeting the specific mechanisms identified in the present report should translate into more effective treatment.

Neurocognitive function in dopamine-β-hydroxylase deficiency

Dopamine-β-hydroxylase (DβH) deficiency is a rare genetic syndrome characterized by the complete absence of norepinephrine in the peripheral and the central nervous system. DβH-deficient patients suffer from several physical symptoms, which can be treated successfully with L-threo-3,4-dihydroxyphenylserine, a synthetic precursor of norepinephrine. Informal clinical observations suggest that DβH-deficient patients do not have obvious cognitive impairments, even when they are not medicated, which is remarkable given the important role of norepinephrine in normal neurocognitive function. This study provided the first systematic investigation of neurocognitive function in human DβH deficiency. We tested 5 DβH-deficient patients and 10 matched healthy control participants on a comprehensive cognitive task battery, and examined their pupil dynamics, brain structure, and the P3 component of the electroencephalogram. All participants were tested twice; the patients were tested once ON and once OFF medication. Magnetic resonance imaging scans of the brain revealed that the patients had a smaller total brain volume than the control group, which is in line with the recent hypothesis that norepinephrine has a neurotrophic effect. In addition, the patients showed an abnormally small or absent task-evoked pupil dilation. However, we found no substantial differences in cognitive performance or P3 amplitude between the patients and the control participants, with the exception of a temporal-attention deficit in the patients OFF medication. The largely spared neurocognitive function in DβH-deficient patients suggests that other neuromodulators have taken over the function of norepinephrine in the brains of these patients.

Tyrosine hydroxylase immunoreactivity as indicator of sympathetic activity: simultaneous evaluation in different tissues of hypertensive rats

Vasomotor control by the sympathetic nervous system presents substantial heterogeneity within different tissues, providing appropriate homeostatic responses to maintain basal/stimulated cardiovascular function both at normal and pathological conditions. The availability of a reproducible technique for simultaneous measurement of sympathetic drive to different tissues is of great interest to uncover regional patterns of sympathetic nerve activity (SNA). We propose the association of tyrosine hydroxylase immunoreactivity (THir) with image analysis to quantify norepinephrine (NE) content within nerve terminals in arteries/arterioles as a good index for regional sympathetic outflow. THir was measured in fixed arterioles of kidney, heart, and skeletal muscle of Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) (123 ± 2 and 181 ± 4 mmHg, 300 ± 8 and 352 ± 8 beats/min, respectively). There was a differential THir distribution in both groups: higher THir was observed in the kidney and skeletal muscle (∼3-4-fold vs. heart arterioles) of WKY; in SHR, THir was increased in the kidney and heart (2.4- and 5.3-fold vs. WKY, respectively) with no change in the skeletal muscle arterioles. Observed THir changes were confirmed by either: 1) determination of NE content (high-performance liquid chromatography) in fresh tissues (SHR vs. WKY): +34% and +17% in kidney and heart, respectively, with no change in the skeletal muscle; 2) direct recording of renal (RSNA) and lumbar SNA (LSNA) in anesthetized rats, showing increased RSNA but unchanged LSNA in SHR vs. WKY. THir in skeletal muscle arterioles, NE content in femoral artery, and LSNA were simultaneously reduced by exercise training in the WKY group. Results indicate that THir is a valuable technique to simultaneously evaluate regional patterns of sympathetic activity.

Catecholamines 101

This review of clinical catecholamine neurochemistry is based on the Streeten Memorial Lecture at the 19th annual meeting of the American Autonomic Society and lectures at a satellite of the 6th Congress of the International Society of Autonomic Neuroscience. Here I provide historical perspective, describe sources and meanings of plasma levels of catecholamines and their metabolites, present a model of a sympathetic noradrenergic neuron that conveys how particular aspects of sympathetic nervous function affect plasma levels of catecholamines and their metabolites, and apply the model to understand plasma neurochemical patterns associated with some drugs and disease states.

The 2009 Carl Ludwig Lecture: pathophysiology of the human sympathetic nervous system in cardiovascular diseases: the transition from mechanisms to medical management

Sympathetic nervous system responses typically are regionally differentiated, with activation in one outflow sometimes accompanying no change or sympathetic inhibition in another. Regional sympathetic activity is best studied in humans by recording from postganglionic sympathetic efferents (multiunit or single fiber recording) and by isotope dilution-derived measurement of organ-specific norepinephrine release to plasma (regional “norepinephrine spillover”). Evidence assembled in this review indicates that sympathetic nervous system abnormalities are crucial in the development of cardiovascular disorders, notably heart failure, essential hypertension, disorders of postural circulatory control causing syncope, and “psychogenic heart disease,” heart disease attributable to mental stress and psychiatric illness. These abnormalities involve persistent, adverse activation of sympathetic outflows to the heart and kidneys in heart failure and hypertension, episodic or ongoing cardiac sympathetic activation in psychogenic heart disease, and defective sympathetic circulatory reflexes in disorders of postural circulatory control. An important goal for clinical scientists is translation of knowledge of pathophysiology, such as this, into better treatment for patients. The achievement of this “mechanisms-to-management” transition is at differing stages of development with the different conditions. Clinical translation is mature in cardiac failure, knowledge of cardiac neural pathophysiology having led to introduction of β-adrenergic blockers, an effective therapy. With essential hypertension, perhaps we are on the cusp of effective translation, with recent successful testing of selective catheter-based renal sympathetic nerve ablation in patients with resistant hypertension, an intervention firmly based on demonstration of activation of the renal sympathetic outflow. With psychogenic heart disease and postural syncope syndromes, knowledge of the neural pathophysiology is emerging, but clinical translation remains for the future.

Polymorphism of the serotonin transporter gene modulates brain and physiological responses to acute stress in Japanese men

A short (S) variant, compared to a long (L) variant, of the promoter region of the serotonin transporter gene-linked polymorphic region (5HTTLPR) has been related to emotional hyper-reactivity. We tested whether the 5HTTLPR could modulate acute stress responses in the brain and, the cardiovascular and neuroendocrine systems. Ten Japanese male participants carrying double copies of the S alleles and 10 Japanese males carrying S and L alleles conducted a mental arithmetic task, and their regional cerebral blood flow by (15)O positron emission tomography and cardiovascular and neuroendocrine parameters were measured. During the acute stress task, the participants with the SS alleles showed stronger reactivity in blood pressure and secretion of epinephrine, compared to the participants with the SL and LL alleles. Furthermore, the SS carriers showed greater activation in stress-related brain regions such as the hypothalamus, cerebellum, midbrain, and pulvinar compared to the SL and LL carriers during the acute stress task. The present findings indicated that the S allele of the 5HTTLPR is associated with greater brain and physiological reactivity to acute stress in Japanese men.

Non-esterified fatty acids increase arterial pressure via central sympathetic activation in humans

Previous studies have shown that acute increases in plasma NEFAs (non-esterified fatty acids) raise SVR (systemic vascular resistance) and BP (blood pressure). However, these studies have failed to distinguish between CNS (central nervous system) mechanisms that raise sympathetic activity and paracrine mechanisms that increase SVR directly, independent of CNS involvement. The aim of the present study was to directly determine whether the sympathetic nervous system contributes to the pressor response to NEFAs. On 2 days separated by at least 2 weeks, 17 lean healthy volunteers (ten male/seven female; age, 22±1 years; body mass index, 23±1 kg/m2; values are means±S.E.M.) received a 4-h intravenous infusion of 20% Intralipid® or placebo (in a single-blind randomized balanced order). MSNA (muscle sympathetic nerve activity), HR (heart rate), BP (oscillometric brachial measurement) and (cardiac output; acetylene rebreathing) were measured before and throughout infusion. The change in HR (+8.2±1.0 and +2.4±1.2 beats/min), systolic BP (+14.0±1.6 and +3.2±2.5 mmHg) and diastolic BP (+8.2±1.0 and −0.1±1.7 mmHg) were significantly greater after the 4-h infusion of Intralipid® compared with placebo (P<0.001). The change in BP with Intralipid® resulted from an increase in SVR (/mean arterial pressure; P<0.001) compared with baseline, without a change in . MSNA burst frequency increased during Intralipid® infusion compared with baseline (+4.9±1.3 bursts/min; P<0.05), and total MSNA (frequency×amplitude) was augmented 65% (P<0.001), with no change during placebo infusion. Lipid infusion increased insulin, aldosterone and F2-isoprostane, but not leptin, concentrations. On the basis of the concomitant increase in BP, MSNA and SVR, we conclude that central sympathetic activation contributes to the pressor response to NEFAs.

Relative efficiencies of plasma catechol levels and ratios for neonatal diagnosis of menkes disease

BACKGROUND

Menkes disease is an X-linked recessive neurodevelopmental disorder resulting from mutation in a copper-transporting ATPase gene. Menkes disease can be detected by relatively high concentrations of dopamine (DA) and its metabolites compared to norepinephrine (NE) and its metabolites, presumably because dopamine-beta-hydroxylase (DBH) requires copper as a co-factor. The relative diagnostic efficiencies of levels of catechol analytes, alone or in combination, in neonates at genetic risk of Menkes disease have been unknown.

METHODS

Plasma from 44 at-risk neonates less than 30 days old were assayed for DA, NE, and other catechols. Of the 44, 19 were diagnosed subsequently with Menkes disease, and 25 were unaffected.

RESULTS

Compared to unaffected at-risk infants, those with Menkes disease had high plasma DA (P < 10(-6)) and low NE (P < 10(-6)) levels. Considered alone, neither DA nor NE levels had perfect sensitivity, whereas the ratio of DA:NE was higher in all affected than in all unaffected subjects (P = 2 x 10(-8)). Analogously, levels of the DA metabolite, dihydroxyphenylacetic acid (DOPAC), and the NE metabolite, dihydroxyphenylglycol (DHPG), were imperfectly sensitive, whereas the DOPAC:DHPG ratio was higher in all affected than in all unaffected subjects (P = 2 x 10(-4)). Plasma dihydroxyphenylalanine (DOPA) and the ratio of epinephrine (EPI):NE levels were higher in affected than in unaffected neonates (P = 0.0015; P = 0.013).

CONCLUSIONS

Plasma DA:NE and DOPAC:DHPG ratios are remarkably sensitive and specific for diagnosing Menkes disease in at-risk newborns. Affected newborns also have elevated DOPA and EPI:NE ratios, which decreased DBH activity alone cannot explain.

Sympathetic nerve traffic and circulating norepinephrine levels in RGS2-deficient mice

Regulator of G protein signaling 2 (RGS2-/-) deficient mice feature an increased resting blood pressure and an excessive pressor response to stress. We measured renal sympathetic nerve activity (RSNA) directly to test the hypothesis that RSNA is increased in RGS2-/- mice, compared to RGS2+/+ mice. Seventeen mice (RGS2-/-, n=9; RGS2+/+, n=8) were anesthetized with isoflurane. We cannulated the left jugular vein for drug administration. Renal sympathetic nerve activity (RSNA) was recorded using bipolar electrodes. Arterial blood pressure (BP) from the femoral artery, ECG (needle electrodes), and RSNA were recorded (sample rate 10 kHz) simultaneously. RSNA was analysed off-line using a modified wavelet de-noising technique and the classical discriminator method. RSNA detected during phenylephrine bolus injections or after the animals death was subtracted from baseline values. Mean arterial blood pressure, norepinephrine plasma levels, the responsiveness to vasoactive drugs, and the sympathetic baroreflex gain were similar in anesthetized RGS2+/+ and RGS2-/- animals. RSNA was lower in RGS2-/- mice compared to wild-type controls (wavelet: spike rate in Hz: RGS2+/+ 25.5+/-5.1; RGS2-/- 17.4+/-4.0; discriminator method: RGS2+/+ 41.4+/-5.7, RGS2-/- 22.0+/-4.3, p<0.05). Thus, the expected result proved not to be the case. Our data suggest a mismatch between sympathetic nerve traffic and plasma norepinephrine concentrations. This observation may depend on altered coupling between electrical nerve activity and norepinephrine release and/or a changed norepinephrine uptake in RGS2-/- mice.

Microneurography as a tool in clinical neurophysiology to investigate peripheral neural traffic in humans

Microneurography is a method using metal microelectrodes to investigate directly identified neural traffic in myelinated as well as unmyelinated efferent and afferent nerves leading to and coming from muscle and skin in human peripheral nerves in situ. The present paper reviews how this technique has been used in clinical neurophysiology to elucidate the neural mechanisms of autonomic regulation, motor control and sensory functions in humans under physiological and pathological conditions. Microneurography is particularly important to investigate efferent and afferent neural traffic in unmyelinated C fibers. The recording of efferent discharges in postganglionic sympathetic C efferent fibers innervating muscle and skin (muscle sympathetic nerve activity; MSNA and skin sympathetic nerve activity; SSNA) provides direct information about neural control of autonomic effector organs including blood vessels and sweat glands. Sympathetic microneurography has become a potent tool to reveal neural functions and dysfunctions concerning blood pressure control and thermoregulation. This recording has been used not only in wake conditions but also in sleep to investigate changes in sympathetic neural traffic during sleep and sleep-related events such as sleep apnea. The same recording was also successfully carried out by astronauts during spaceflight. Recordings of afferent discharges from muscle mechanoreceptors have been used to understand the mechanisms of motor control. Muscle spindle afferent information is particularly important for the control of fine precise movements. It may also play important roles to predict behavior outcomes during learning of a motor task. Recordings of discharges in myelinated afferent fibers from skin mechanoreceptors have provided not only objective information about mechanoreceptive cutaneous sensation but also the roles of these signals in fine motor control. Unmyelinated mechanoreceptive afferent discharges from hairy skin seem to be important to convey cutaneous sensation to the central structures related to emotion. Recordings of afferent discharges in thin myelinated and unmyelinated fibers from nociceptors in muscle and skin have been used to provide information concerning pain. Recordings of afferent discharges of different types of cutaneous C-nociceptors identified by marking method have become an important tool to reveal the neural mechanisms of cutaneous sensations such as an itch. No direct microneurographic evidence has been so far proved regarding the effects of sympathoexcitation on sensitization of muscle and skin sensory receptors at least in healthy humans.

Pupil abnormalities in selected autonomic neuropathies

Examination of the pupil provides an opportunity to detect disturbances in the autonomic innervation of the eye. The pupil is frequently affected in patients with generalized autonomic neuropathies. This literature review confirms a high prevalence of sympathetic deficits and parasympathetic deficits in acute or subacute dysautonomia, diabetes, amyloidosis, pure autonomic failure, paraneoplastic syndromes, Sjögren syndrome, familial dysautonomia, and dopamine beta-hydroxylase deficiency. It confirms the relative scarcity of a pupil abnormality in patients with multiple system atrophy. There are difficulties in clinical diagnosis of pupil abnormalities and interpretation of pupil pharmacologic tests, particularly when combined sympathetic and parasympathetic deficits are present.

L-Dihydroxyphenylserine (L-DOPS): A Norepinephrine Prodrug

L-threo-3,4-dihydroxyphenylserine (L-DOPS, droxydopa) is a synthetic catecholamino acid. When taken orally, L-DOPS is converted to the sympathetic neurotransmitter, norepinephrine (NE), via decarboxylation catalyzed by L-aromatic-amino-acid decarboxylase (LAAAD). Plasma L-DOPS levels peak at about 3 h, followed by a monoexponential decline with a half-time of 2 to 3 h. Plasma levels of NE and of its main neuronal metabolite, dihydroxyphenylglycol (DHPG) peak approximately concurrently but at much lower concentrations. The relatively long half-time for disappearance of L-DOPS from plasma, compared to that of NE, explains their very different attained plasma concentrations. In patients with neurogenic orthostatic hypotension, L-DOPS increases blood pressure and ameliorates orthostatic intolerance. Inhibition of LAAAD, such as by treatment with carbidopa, which does not penetrate the blood-brain barrier, prevents the blood pressure effects of the drug, indicating that L-DOPS increases blood pressure by augmenting NE production outside the brain. Patients with pure autonomic failure (which usually entails loss of sympathetic noradrenergic nerves), and patients with multiple system atrophy (in which noradrenergic innervation remains intact) have similar plasma NE responses to L-DOPS. This suggests mainly non-neuronal production of NE from L-DOPS. L-DOPS is very effective in treatment of deficiency of dopamine-beta-hydroxylase (DBH), the enzyme required for conversion of dopamine to NE in sympathetic nerves. L-DOPS holds promise for treating other much more common conditions involving decreased DBH activity or NE deficiency, such as a variety of syndromes associated with neurogenic orthostatic hypotension.

Sympathetic nerve activity in hypotension and orthostatic intolerance

AIM

The present paper reviews how changes in sympathetic nerve activity are related to hypotensive episodes and orthostatic intolerance in humans.

RESULTS

It has been well documented that sympathetic neural traffic to skeletal muscles (muscle sympathetic nerve activity; MSNA) plays an essential role in maintaining blood pressure homeostasis mainly through baroreflex. The MSNA responded to gravitational loading from the head to the leg (+Gz) during passive head-up tilt (HUT). Patients who suffered from orthostatic hypotension with or without syncope were classified into at least two groups; low and high responders of MSNA to orthostatic loading. The typical examples belonging to the former group were patients of multiple system atrophy who had very low basal sympathetic outflow to muscle which responded extremely poorly to HUT. Patients of multiple system atrophy presented also postprandial hypotension in which muscle sympathetic response to oral glucose administration was absent. The latter group was represented by subjects who manifested vasovagal syncope with normal or even higher muscle sympathetic response to HUT, which was suddenly withdrawn concomitantly with bradycardia and hypotension. Similar withdrawal of sympathetic nerve traffic to muscle was encountered in a rare case of idiopathic non-orthostatic episodic hypotension which accompanied bradycardia. The MSNA was suppressed by short-term exposure to microgravity but was enhanced after long-term exposure to microgravity. Orthostatic intolerance after long-term exposure to microgravity was related to progressive reduction of muscle sympathetic response to orthostatic loading with impaired arterial baroreflex.

CONCLUSION

It is concluded that hypotensive episodes are closely related to poor or lack of muscle sympathetic outflow, but may depend on various neural mechanisms to induce it.

Sympathetic nerve activity and neurotransmitter release in humans: translation from pathophysiology into clinical practice

AIM

There has been a revolution in cardiovascular neuroscience in recent years with, in some cases, translation into clinical practice of the knowledge of pathophysiology gained through application of sympathetic nerve recording and catecholamine isotope dilution methodology. OBESITY-RELATED HYPERTENSION: An earlier hypothesis, based on findings in most models, was that weight gain in obesity is due in part to sympathetic nervous underactivity reducing thermogenesis. Microneurography and regional noradrenaline spillover measurements in human obesity have disproven this hypothesis, weakening the case for the use of beta3-adrenergic agonists to stimulate thermogenesis. Sympathetic nerve firing rates in post-ganglionic fibres directed to the skeletal muscle vasculature are increased, as is renal sympathetic tone, with a doubling of the spillover rate of noradrenaline from the kidneys. Given these findings, antiadrenergic antihypertensive drugs may be the preferred agents for obesity-related hypertension, but this has not been adequately tested.

ESSENTIAL HYPERTENSION

Whether stress causes high blood pressure, previously hotly debated, has been under recent review by an Australian Government body, the Specialist Medical Review Council. Despite medicolegal implications, the ruling was that stress is one proven cause of hypertension. The judgment was reached after consideration of the epidemiological evidence, but in particular the described neural pathophysiology of essential hypertension: (a) persistent sympathetic nervous stimulation is commonly present, (b) suprabulbar projections of noradrenergic brainstem neurones are activated and (c) adrenaline is released as a cotransmitter in sympathetic nerves. These were taken to be biological markers of stress.

CARDIAC FAILURE

At one time, the failing heart was thought to be sympathetically denervated. Longterm administration of inotropic adrenergic agonists, to provide the cardiac catecholamine stimulation thought to be lacking, increased mortality. Noradrenaline isotope dilution methodology subsequently demonstrated that the sympathetic outflow to the heart was preferentially activated, cardiac noradrenaline spillover being increased as much as 50-fold. The level of stimulation of the cardiac sympathetic nerves was the most powerful predictor of death. These observations provide the theoretical foundation for the very successful introduction of beta-adrenergic blockers for treatment of heart failure.

Muscle sympathetic nerve activity in blood pressure control against gravitational stress

Muscle sympathetic nerve activity (MSNA) can be directly recorded from human peripheral nerves in situ using microneurography. MSNA plays an essential role to control systemic blood pressure against gravitational stress. MSNA was enhanced by changing posture against terrestrial gravity from lying to sitting, and from sitting to standing. This activity was enhanced by head-up tilt depending on the gravitational input from the head to the leg (+Gz) in the human body. Orthostatic hypotension occurred when MSNA response to gravitational stress was impaired both in high and low responders of this sympathetic outflow. Syncope was preceded and/or associated by a withdrawal of MSNA. MSNA was suppressed by short-term exposure to microgravity but was enhanced after long-term exposure to microgravity. Orthostatic intolerance after exposure to prolonged microgravity was associated with a reduction of increased MSNA response to gravitational stress. Aging influenced gravity-related responses of MSNA.

Microneurographic research on sympathetic nerve responses to environmental stimuli in humans

The sympathetic nervous system plays an important role to maintain the homeostasis of vital functions in humans against environmental stimuli. Sympathetic nerve responses to environmental stimuli in humans have been assessed conventionally using rather indirect methods by analyzing the responses of effector organs or by measuring the changes in plasma norepinephrine level. Meanwhile, the microneurography technique has enabled us to approach the sympathetic nervous system in humans more directly. By applying this technique, it has become possible to investigate how the human sympathetic nervous system responds to different kinds of environmental stimuli. In this paper, the usefulness of microneurography as a research tool in environmental physiology is shown together with a review of microneurographic findings on sympathetic nerve responses to environmental stimuli in humans.