Regional origins of 3-methoxy-4-hydroxyphenylglycol in plasma: effects of chronic sympathetic nervous activation and denervation, and acute reflex sympathetic stimulation

Measurement of Noradrenaline and Serotonin Metabolites With Internal Jugular Vein Sampling: An Indicator of Brain Monoamine Turnover in Depressive Illness and Panic Disorder

In research spanning three decades we have estimated brain monoamine turnover (approximately equating with synthesis rate) with sampling from the internal jugular veins and measurement of trans-cerebral plasma monoamine metabolite concentration gradients. Here we report indices of brain noradrenaline and serotonin turnover in patients with major depressive illness (MDD) and panic disorder (PD). Brain noradrenaline turnover was assessed from the combined flux into the internal jugular veins of the metabolites dihydroxyphenylglycol (DHPG) and 3-hydroxy-4-methoxyphenylglycol (MHPG), and brain serotonin turnover from the overflow of the primary metabolite, 5-hydroxyindole acetic acid (5HIAA). Comparison was made with matched healthy research participants. In both MD and PD the estimate of brain noradrenaline turnover provided by metabolite overflow was unremarkable. In contrast, in both patient groups the estimate of brain serotonin turnover provided by 5HIAA overflow was increased 3-4-fold (P < 0.01). This neurotransmitter abnormality was normalized in MDD and PD in clinical remission, during selective serotonin reuptake blocker (SSRI) dosing. We cannot be sure if the brain serotonergic abnormality we find in MDD and PD is causal or a correlate. Measurements in PD were not made during a panic attack. The increased estimated serotonin turnover here may possibly be a substrate for panic attacks; serotonergic raphe nuclei participate in anxiety responses in experimental animals. It is puzzling that the findings were identical in MDD and PD, although it may be pertinent that these psychiatric diagnoses are commonly comorbid. It is unlikely that activation of brain serotonergic neurons is driving the sympathetic nervous activation present, which contributes to cardiovascular risk, persistent sympathetic activation in MDD and episodic activation in PD during panic attacks. We have previously demonstrated that the mechanism of activation of human central sympathetic outflow in other contexts (hypertension, heart failure) is activation of noradrenergic brainstem neurons projecting to the hypothalamus and amygdala.

Sympathetic activity in obesity: a brief review of methods and supportive data

The increase in the prevalence of obesity and the concomitant rise in obesity-related illness have led to substantial pressure on health care systems throughout the world. While the combination of reduced exercise, increased sedentary time, poor diet, and genetic predisposition is undoubtedly pivotal in generating obesity and increasing disease risk, a large body of work indicates that the sympathetic nervous system (SNS) contributes to obesity-related disease development and progression. In obesity, sympathetic nervous activity is regionalized, with activity in some outflows being particularly sensitive to the obese state, whereas other outflows, or responses to stimuli, may be blunted, thereby making the assessment of sympathetic nervous activation in the clinical setting difficult. Isotope dilution methods and direct nerve recording techniques have been developed and utilized in clinical research, demonstrating that in obesity there is preferential activation of the muscle vasoconstrictor and renal sympathetic outflows. With weight loss, sympathetic activity is reduced. Importantly, sympathetic nervous activity is associated with end-organ dysfunction and changes in sympathetic activation that accompany weight loss are often reflected in an improvement of end-organ function. Whether targeting the SNS directly improves obesity-related illness remains unknown, but merits further attention.

Heart rate variability in menopausal hot flashes during sleep

OBJECTIVE

The aim of this study was to determine if heart rate variability changes during hot flashes recorded during sleep.

METHODS

This study was performed in a university medical center laboratory with 16 postmenopausal women demonstrating at least four hot flashes per night. Polysomnography, heart rate, and sternal skin conductance to indicate hot flashes were recorded in controlled, laboratory conditions.

RESULTS

For the frequency bin of 0 to 0.15 Hz, spectral power was greater during waking compared with non-rapid eye movement sleep and less during stages 3 and 4 compared with stages 1 and 2. Power was greater during hot flashes compared with subsequent periods for all hot flashes. Power was greater during hot flashes compared with preceding and subsequent periods for those recorded during stage 1 sleep. For waking hot flashes, power in this band was higher before hot flashes than during or after them.

CONCLUSIONS

These data are consistent with our theory of elevated sympathetic activation as a trigger for menopausal hot flashes and with previous work on heart rate variability during the stages of sleep.

The role of central noradrenergic dysregulation in anxiety disorders: evidence from clinical studies

The nature of the noradrenergic dysregulation in clinical anxiety disorders remains unclear. In panic disorder, the predominant view has been that central noradrenergic neuronal networks and/or the sympathetic nervous system was normal in patients at rest, but hyper-reactive to specific stimuli, for example carbon dioxide. These ideas have been extended to other anxiety disorders, which share with panic disorder characteristic subjective anxiety and physiological symptoms of excess sympathetic activity. For example, Generalized Anxiety Disorder is characterized by chronic free-floating anxiety, muscle tension, palpitation and insomnia. It has been proposed that there is chronic central hypersecretion of noradrenaline in Generalized Anxiety Disorder, with consequent hyporesponsiveness of central post-synaptic receptors. With regards to other disorders, it has been suggested that there is noradrenergic involvement or derangement, but a more specific hypothesis has not been enunciated. This paper reviews the evidence for noradrenergic dysfunction in anxiety disorders, derived from indirect measures of noradrenergic function in clinical populations.

Jugular venous overflow of noradrenaline from the brain: a neurochemical indicator of cerebrovascular sympathetic nerve activity in humans

A novel neurochemical method was applied for studying the activity of sympathetic nerves in the human cerebral vascular system. The aim was to investigate whether noradrenaline plasma kinetic measurements made with internal jugular venous sampling reflect cerebrovascular sympathetic activity. A database was assembled of fifty-six healthy subjects in whom total body noradrenaline spillover (indicative of whole body sympathetic nervous activity), brain noradrenaline spillover and brain lipophlic noradrenaline metabolite (3,4-dihydroxyphenolglycol (DHPG) and 3-methoxy-4-hydroxyphenylglycol (MHPG)) overflow rates were measured. These measurements were also made following ganglion blockade (trimethaphan, n = 6), central sympathetic inhibition (clonidine, n = 4) and neuronal noradrenaline uptake blockade (desipramine, n = 13) and in a group of patients (n = 9) with pure autonomic failure (PAF). The mean brain noradrenline spillover and brain noradrenaline metabolite overflow in healthy subjects were 12.5 +/- 1.8, and 186.4 +/- 25 ng min(-1), respectively, with unilateral jugular venous sampling for both. Total body noradrenaline spillover was 605.8 ng min(-1) +/- 34.4 ng min(-1). As expected, trimethaphan infusion lowered brain noradrenaline spillover (P = 0.03), but perhaps surprisingly increased jugular overflow of brain metabolites (P = 0.01). Suppression of sympathetic nervous outflow with clonidine lowered brain noradrenaline spillover (P = 0.004), without changing brain metabolite overflow (P = 0.3). Neuronal noradrenaline uptake block with desipramine lowered the transcranial plasma extraction of tritiated noradrenaline (P = 0.001). The PAF patients had 77% lower brain noradrenaline spillover than healthy recruits (P = 0.06), indicating that in them sympathetic nerve degeneration extended to the cerebral circulation, but metabolites overflow was similar to healthy subjects (P = 0.3). The invariable discordance between noradrenline spillover and noradrenaline metabolite overflow from the brain under these different circumstances indicates that the two measures arise from different sources, i.e. noradrenaline spillover originates from the cerebral vasculature outside the blood-brain barrier, and the noradrenaline metabolites originate primarily from brain noradrenergic neurons. We suggest that measurements of transcranial plasma noradrenaline spillover have utility as a method for assessing the sympathetic nerve activity of the cerebral vasculature.

Norepinephrine activity, as measured by MHPG, is associated with menopausal hot flushes

OBJECTIVES

Baseline norepinephrine levels, as measured by a metabolite (plasma 3-methoxy-4-hydroxyphenolglycol, MHPG), have been reported to increase in women who experience hot flushes. However, norepinephrine is also discharged in a counter-regulatory attempt to increase brain glucose as normal daily variations occur. The purpose of this analysis is to examine the relationship between hot flush frequency and MHPG under conditions of experimental glucose manipulation.

METHODS

A repeated-measures experimental design study was conducted with ten postmenopausal women taking hormone therapy between the ages of 38 and 55 years. In a 30-h experimental protocol, participants received normal saline and 20% glucose intravenous infusions on sequential days and were monitored for hot flushes and blood glucose changes. MHPG levels were evaluated before and after each experimental condition as a biomarker of norepinephrine activity.

RESULTS

Although hot flush frequency was significantly different between infusion periods, mean MHPG levels were not statistically different (normal saline period, 3.1 ng/ml; glucose infusion, 3.2 ng/ml). No distinct patterns of MHPG change were found in this sample.

CONCLUSIONS

In this study, there was no consistent pattern of MHPG increase or decrease in the women experiencing hot flushes.

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.

Increased brain serotonin turnover in panic disorder patients in the absence of a panic attack: reduction by a selective serotonin reuptake inhibitor

Since the brain neurotransmitter changes characterising panic disorder remain uncertain, we quantified brain noradrenaline and serotonin turnover in patients with panic disorder, in the absence of a panic attack. Thirty-four untreated patients with panic disorder and 24 matched healthy volunteers were studied. A novel method utilising internal jugular venous sampling, with thermodilution measurement of jugular blood flow, was used to directly quantify brain monoamine turnover, by measuring the overflow of noradrenaline and serotonin metabolites from the brain. Radiographic depiction of brain venous sinuses allowed differential venous sampling from cortical and subcortical regions. The relation of brain serotonin turnover to serotonin transporter genotype and panic disorder severity were evaluated, and the influence of an SSRI drug, citalopram, on serotonin turnover investigated. Brain noradrenaline turnover in panic disorder patients was similar to that in healthy subjects. In contrast, brain serotonin turnover, estimated from jugular venous overflow of the metabolite, 5-hydroxyindole acetic acid, was increased approximately 4-fold in subcortical brain regions and in the cerebral cortex (P < 0.01). Serotonin turnover was highest in patients with the most severe disease, was unrelated to serotonin transporter genotype, and was reduced by citalopram (P < 0.01). Normal brain noradrenaline turnover in panic disorder patients argues against primary importance of the locus coeruleus in this condition. The marked increase in serotonin turnover, in the absence of a panic attack, possibly represents an important underlying neurotransmitter substrate for the disorder, although this point remains uncertain. Support for this interpretation comes from the direct relationship which existed between serotonin turnover and illness severity, and the finding that SSRI administration reduced serotonin turnover. Serotonin transporter genotyping suggested that increased whole brain serotonin turnover most likely derived not from impaired serotonin reuptake, but from increased firing in serotonergic midbrain raphe neurons projecting to both subcortical brain regions and the cerebral cortex.

Acute response to intracisternal bupivacaine in patients with refractory pain of the head and neck

Continuous intracisternal infusion of bupivacaine for the management of intractable pain of the head and neck is effective in controlling pain in this patient group. With the catheter tip being located at the height of the C1 vertebral body, autonomic regulatory information may also be influenced by the infusion of bupivacaine. By combining direct sampling of cerebrospinal fluid (CSF), via a percutaneously placed catheter in the cisterna magna, with a noradrenaline and adrenaline isotope dilution method for examining sympathetic and adrenal medullary activity, we were able to quantify the release of brain neurotransmitters and examine efferent sympathetic nervous outflow in patients following intracisternal administration of bupivacaine. Despite severe pain, sympathetic and adrenal medullary activities were well within normal range (4.2 +/- 0.6 and 0.7 +/- 0.2 nmol min(-1), respectively, mean +/-S.E.M.). Intracisternal bupivacaine administration caused an almost instantaneous elevation in mean arterial blood pressure, increasing by 17 +/- 7 mmHg after 10 min (P < 0.01). Heart rate increased in parallel (17 +/- 5 beats min(-1)), and these changes coincided with an increase in sympathetic nervous activity, peaking with an approximately 50% increase over resting level 10 min after injection (P < 0.01). CSF levels of GABA were reduced following bupivacaine (P < 0.05). CSF catecholamines and serotonin, and EEG, remained unaffected. These results show that acutely administered bupivacaine in the cisterna magna of chronic pain sufferers leads to an activation of the sympathetic nervous system. The results suggest that the haemodynamic consequences occur as a result of interference with the neuronal circuitry in the brainstem. Although these effects are transient, they warrant caution at the induction of intracisternal local anaesthesia.

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.

Cardiac Sympathetic Nerve Biology and Brain Monoamine Turnover in Panic Disorder

Panic disorder serves as a clinical model for testing whether mental stress can cause heart disease. Our own cardiologic management of panic disorder provides case material of recurrent emergency room attendances with angina and electrocardiogram ischemia, triggered arrhythmias (atrial fibrillation, ventricular fibrillation), and documented coronary artery spasm, in some cases with coronary spasm being complicated by coronary thrombosis. Application of radiotracer catecholamine kinetics and clinical microneurography methodology suggests there is a genetic predisposition to panic disorder that involves faulty neuronal norepinephrine uptake, possibly sensitizing the heart to symptom generation. During panic attacks there are large sympathetic bursts, recorded by clinical microneurography in the muscle sympathetic nerve neurogram, and large increases in cardiac norepinephrine spillover, accompanied by surges of adrenal medullary epinephrine secretion. In other conditions such as heart failure and presumably here also, a high level of sympathetic nervous activation can mediate increased cardiac risk. The sympathetic nerve cotransmitter, neuropeptide Y (NPY), is released from the cardiac sympathetics during panic attacks, an intriguing finding given that NPY can cause coronary artery spasm. There is ongoing, continuous release of epinephrine from the heart in panic sufferers, perhaps attributable to epinephrine loading of cardiac sympathetic nerves by uptake from plasma during panic attacks, or possibly to in situ synthesis of epinephrine through the action of intracardiac phenylethanolamine-N-methytransferase (PNMT) activated by repeated cortisol responses. We have used internal jugular venous sampling and measurement of overflowing lipophilic brain monoamine metabolites to quantify brain norepinephrine and serotonin turnover in untreated patients with panic disorder. We find normal norepinephrine turnover but a marked increase in brain serotonin turnover in patients with panic disorder, in the absence of a panic attack, which presumably represents an underlying neurotransmitter substrate for the condition.

In vivo assessment of catechol O-methyltransferase activity in rabbit skeletal muscle

With the use of microdialysis technique in the anesthetized rabbit, we examined the catechol O-methyltransferase (COMT) activity at the skeletal muscle interstitium. We implanted a dialysis probe into the adductor muscle, and monitored dialysate catecholamines and their metabolites with chromatogram-electrochemical detection. Administration of COMT inhibitor (entacapone) decreased dialysate 3-methoxy 4-hydroxyphenylglycol (MHPG) levels. Local administration of dihydroxyphenylglycol induced increases in dialysate MHPG levels. These increases in dialysate MHPG levels were suppressed by the addition of entacapone. The concentration of MHPG in the skeletal muscle dialysate corresponded to the COMT activity in the skeletal muscle. Furthermore, local administration of norepinephrine or epinephrine increased normetanephrine or metanephrine levels in dialysate but not MHPG levels. Skeletal muscle microdialysis with local administration of catecholamine offers a new method for in vivo assessment of regional COMT activity.

In vivo monitoring of norepinephrine and its metabolites in skeletal muscle

Although skeletal muscle sympathetic nerve activity plays an important role in the regulation of vascular tone and glucose metabolism, relatively little is known about regional norepinephrine (NE) kinetics in the skeletal muscle. With use of the dialysis technique, we implanted dialysis probes in the adductor muscle of anesthetized rabbits and examined whether dialysate NE and its metabolites were influenced by local administration of pharmacological agents through the dialysis probes. Dialysate dihydroxyphenylglycol (DHPG) and 3-methoxy-4-hydroxyphenylglycol (MHPG) were measured as two major metabolites of NE. The skeletal muscle dialysate NE, DHPG and MHPG were 11.7+/-1.2, 38.1+/-3.2, and 266.1+/-28.7 pg/ml, respectively. Basal dialysate NE levels were suppressed by tetrodotoxin (Na(+) channel blocker, 10 microM) (5.1+/-0.6 pg/ml), and augmented by desipramine (NE uptake blocker, 100 microM) (25.8+/-3.2 pg/ml). Basal dialysate DHPG levels were suppressed by pargyline (monoamine oxidase blocker, 1mM) (24.3+/-4.6 pg/ml) and augmented by reserpine (vesicle NE transport blocker, 10 microM) (75.8+/-2.7 pg/ml). Basal dialysate MHPG levels were not affected by pargyline, reserpine, or desipramine. Addition of tyramine (sympathomimetic amine, 600 microM), KCl (100 mM), and ouabain (Na(+)-K(+) ATPase blocker, 100 microM) caused brisk increases in dialysate NE levels (200.9+/-14.2, 90.6+/-25.7, 285.3+/-46.8 pg/ml, respectively). Furthermore, increases in basal dialysate NE levels were correlated with locally administered desipramine (10, 100 microM). Thus, dialysate NE and its metabolite were affected by local administration of pharmacological agents that modified sympathetic nerve endings function in the skeletal muscle. Skeletal muscle microdialysis with local administration of a pharmacological agent provides information about NE release, uptake, vesicle uptake and degradation at skeletal muscle sympathetic nerve endings.

What we know about managing menopausal hot flashes: navigating without a compass

Hot flashes and night sweats are frequently experienced as the cardinal symptoms of menopause. However, their physiological basis has not been explained; nor have any potential risks been explored. Current knowledge and theoretical perspectives regarding hot flashes will be presented and contrasted with evidence for an emerging hypothesis of altered brain glucose availability as the hot flash trigger. Perspectives regarding hormone therapy and alternative therapies for treatment of hot flashes will be presented and directions for future research reviewed.

Monoamine metabolism and sympathetic nervous activation following subarachnoid haemorrhage: influence of gender and hydrocephalus

Subarachnoid haemorrhage is a serious condition, often accompanied by cerebral vasospasm and hydrocephalus, which may result in delayed cerebral ischaemia and neurological deterioration. While the mechanisms responsible remain unknown, activation of the sympathetic nervous system, leading to elevated levels of circulating catecholamines is, at least in part, implicated. In this study, we sought to examine the importance of sympathetic nervous activation and its relation to brain monoaminergic neurotransmission in 25 patients following subarachnoid haemorrhage by examining plasma and cerebrospinal fluid levels of the catecholamines noradrenaline, adrenaline and dopamine, and their metabolites. Total body sympathetic activity was concurrently assessed using isotope dilution methodology. In the early phase following subarachnoid haemorrhage patients exhibited markedly elevated rates of spillover of noradrenaline to plasma (9.11 +/- 1.12 vs. 3.39 +/- 0.26 nmol/min, p < 0.01), with rates being higher in those patients in whom hydrocephalus developed (11.15 +/- 1.40 vs. 7.90 +/- 1.41 nmol/min, p = 0.05). The degree of sympathetic nervous activation tended to be higher in females compared with males. Lower cerebral perfusion pressures were observed in those patients in whom cerebrospinal fluid concentrations of noradrenaline and dopamine metabolites were high. A marked sympathetic nervous activation, more pronounced in women and in those with hydrocephalus, occurs following subarachnoid haemorrhage. The diminished cerebral perfusion seen following subarachnoid bleeding may occur as a result of activation of central catecholaminergic neurones.

The influence of aging on the human sympathetic nervous system and brain norepinephrine turnover

Investigating aging effects on the sympathetic nervous system and ascertaining underlying central nervous system (CNS) mechanisms mediating sympathetic stimulation is clinically pertinent because of the possible interconnection of cardiovascular disease development with age-dependent sympathetic nervous changes. Because of previous evidence linking human CNS neuronal noradrenergic function and sympathetic activity, we investigated the influence of aging on brain norepinephrine turnover in 22 healthy men aged 20-30 yr and 16 healthy men aged 60-75 yr by measuring the internal jugular venous overflow of norepinephrine and its lipophilic metabolites. Sympathoneural and adrenal medullary function was also studied, using plasma catecholamine isotope dilution methodology and regional central venous sampling. In the older men there was increased norepinephrine turnover in suprabulbar subcortical brain regions, 317 +/- 50 ng/min compared with 107 +/- 18 ng/min in younger men. A differentiated sympathetic nervous activation was also present in older men. Overall, levels of both cardiac and hepatomesenteric norepinephrine spillover were directly correlated with subcortical norepinephrine turnover. These findings suggest that in sympathetic nervous activation accompanying aging, as has previously been demonstrated with the sympathetic nervous stimulation in human hypertension and heart failure, there is an underlying sympathoexcitatory influence of noradrenergic projections to suprabulbar subcortical regions.

Physiology of hot flashes

Hot flashes are the most common symptom of the climacteric, although prevalence estimates are lower in some rural and non-Western areas. The symptoms are characteristic of a heat-dissipation response and consist of sweating on the face, neck, and chest, as well as peripheral vasodilation. Although hot flashes clearly accompany the estrogen withdrawal at menopause, estrogen alone is not responsible since levels do not differ between symptomatic and asymptomatic women. Until recently it was thought that hot flashes were triggered by a sudden, downward resetting of the hypothalamic setpoint, since there was no evidence of increased core body temperature. Evidence obtained using a rapidly responding ingested telemetry pill indicates that the thermoneutral zone, within which sweating, peripheral vasodilation, and shivering do not occur, is virtually nonexistent in symptomatic women but normal (about 0.4 degrees C) in asymptomatic women. The results suggest that small temperature elevations preceding hot flashes acting within a reduced thermoneutral zone constitute the triggering mechanism. Central sympathetic activation is also elevated in symptomatic women which, in animal studies, reduces the thermoneutral zone. Clonidine reduces central sympathetic activation, widens the thermoneutral zone, and ameliorates hot flashes. Estrogen virtually eliminates hot flashes but its mechanism of action is not known.

Central nervous system norepinephrine metabolism in hypertension

Although the pivotal role played by the brain in the maintenance of optimal physiologic and psychologic health has long been recognized, methods for the direct examination of human central nervous system processes have only recently been developed. A growing body of evidence indicates that central nervous systemmonoaminergic cell groups, in particular those utilizing norepinephrine as their neurotransmitter, participate in the excitatory regulation of the sympathetic nervous system and the development of the hypertensive state.

Increased sympathetic nervous activity in patients with nontraumatic subarachnoid hemorrhage

BACKGROUND AND PURPOSE

Activation of the sympathetic nervous system, which leads to elevation of circulating catecholamines, is implicated in the genesis of cerebral vasospasm and cardiac aberrations after subarachnoid hemorrhage. To this juncture, sympathetic nervous testing has relied on indirect methods only.

METHODS

We used an isotope dilution technique to estimate the magnitude and time course of sympathoadrenal activation in 18 subarachnoid patients.

RESULTS

Compared with 2 different control groups, the patients with subarachnoid hemorrhage exhibited an approximately 3-fold increase in total-body norepinephrine spillover into plasma within 48 hours after insult (3.2+/-0.3 and 4.2+/-0.7 versus 10.2+/-1.4 nmol/L; P<0.05 versus both). This sympathetic activation persisted throughout the 7- to 10-day examination period and was normalized at the 6-month follow-up visit.

CONCLUSIONS

The present study has established that massive sympathetic nervous activation occurs in patients after subarachnoid hemorrhage. This overactivation may relate to the well-known cardiac complications described in subarachnoid hemorrhage.

Internal jugular venous spillover of noradrenaline and metabolites and their association with sympathetic nervous activity

It is recognized that the brain plays a pivotal role in the maintenance of blood pressure and the control of myocardial function. By combining direct sampling of internal jugular venous blood with a noradrenaline isotope dilution method, for examining neuronal transmitter release, and microneurographic nerve recording, we were able to quantify the release of central nervous system noradrenaline and its metabolites and investigate their association with efferent sympathetic nervous outflow in healthy subjects and patients with pure autonomic failure. To further investigate the relationship between brain noradrenaline, sympathetic nervous activity and blood pressure regulation we examined brain catecholamine turnover, based on the internal jugular venous overflow of noradrenaline and its principal central nervous system metabolites, in response to a variety of pharmacological challenges. A substantial increase was seen in brain noradrenaline turnover following trimethaphan, presumably resulting from a compensatory response in sympathoexcitatory forebrain noradrenergic neurones in the face of interruption of sympathetic neural traffic and reduction in arterial blood pressure. In contrast, reduction in central nervous system noradrenaline turnover accompanied the blood pressure fall produced by intravenous clonidine administration, thus representing the blood pressure lowering action of the drug. Following vasodilatation elicited by intravenous adrenaline infusion, brain noradrenaline turnover increased in parallel with elevation in muscle sympathetic nervous activity. While it is difficult to assess the source of the noradrenaline and metabolites determined in our studies, available evidence implicates noradrenergic cell groups of the posterolateral hypothalamus, amygdala, the A5 region and the locus coeruleus as being involved in the regulation of sympathetic outflow and autonomic cardiovascular control.

Cerebral noradrenaline spillover and its relation to muscle sympathetic nervous activity in healthy human subjects

Studies using internal jugular vein blood sampling in human subjects have demonstrated the release of noradrenaline from the brain and have provided a link between central nervous system noradrenergic neuronal activity and renal, cardiac and total body sympathetic activity. The aim of this study was to further categorise the dependence of regional sympathetic nervous function on central nervous system noradrenergic neuronal processes by combining measures of internal jugular venous noradrenaline spillover, as an indicator of brain noradrenaline release, and cerebral blood flow scans with measures of the overall integrated neuronal firing rate for the body as a whole, the spillover of noradrenaline into the coronary sinus and with measurements of resting muscle sympathetic nerve activity. Positive veno-arterial plasma noradrenaline gradients were found across the brain, with the plasma concentration being 17 +/- 3% (p < 0.01) greater in the internal jugular vein. Linear regression analysis revealed a significant relationship between the degree of muscle sympathetic nerve activity and the spillover of noradrenaline from subcortical brain regions (y = 0.1 x + 16.0; r = 0.81, p < 0.02). The rate of spillover of noradrenaline for the body as a whole also bore a significant association with the rate of subcortical noradrenaline spillover (y = 0.01x + 2.33; r = 0.71, p < 0.05). Cortical noradrenaline spillover was not related to any of the sympathetic nervous system parameters measured in this study. The demonstration of a direct relationship between the rate of peroneal nerve firing and the spillover of noradrenaline from subcortical brain regions provides further support for the concept of central nervous system noradrenergic cell groups behaving in a sympathoexcitatory role.