Baroreceptor-mediated activation of sympathetic nerve activity to salivary glands

Salivary gland function is regulated by both the sympathetic and parasympathetic nervous systems. Previously we showed that the basal sympathetic outflow to the salivary glands (SNA(SG)) was higher in hypertensive compared to normotensive rats and that diabetes reduced SNA(SG) discharge at both strains. In the present study we sought to investigate how SNA(SG) might be modulated by acute changes in the arterial pressure and whether baroreceptors play a functional role upon this modulation. To this end, we measured blood pressure and SNA(SG) discharge in Wistar-Kyoto rats (WRY-intact) and in WRY submitted to sinoaortic denervation (WRY-SAD). We made the following three major observations: (i) in WRY-intact rats, baroreceptor loading in response to intravenous infusion of the phenylephrine evoked an increase in SNA(SG) spike frequency (81%, p<0.01) accompanying the increase mean arterial pressure ((sic)MAP: +77 +/- 14 mmHg); (ii) baroreceptor unloading with sodium nitroprusside infusion elicited a decrease in SNA(SG) spike frequency (17%, p<0.01) in parallel with the fall in arterial blood pressure ((sic)MAP: 30 3 mmHg) in WRY-intact rats; iii) in the WRY-SAD rats, phenylephrine-evoked rises in the arterial pressure ((sic)MAP: +56 +/- 6 mmHg) failed to produce significant changes in the SNA(SG) spike frequency. Taken together, these data show that SNA(SG) increases in parallel with pharmacological-induced pressor response in a baroreceptor dependent way in anaesthetised rats. Considering the key role of SNA(SG) in salivary secretion, this mechanism, which differs from the classic cardiac baroreflex feedback loop, strongly suggests that baroreceptor signalling plays a decisive role in the regulation of salivary gland function. (C) 2012 Elsevier Inc. All rights reserved.

A search for activation of C nociceptors by sympathetic fibers in complex regional pain syndrome

Although the term 'reflex sympathetic dystrophy' has been replaced by 'complex regional pain syndrome' (CRPS) type I, there remains a widespread presumption that the sympathetic nervous system is actively involved in mediating chronic neuropathic pain ["sympathetically maintained pain" (SMP)], even in the absence of detectable neuropathophysiology.

Activation of nicotinic receptor-induced postsynaptic responses to luteinizing hormone-releasing hormone in bullfrog sympathetic ganglia via a Na+-dependent mechanism

Nicotine at very low doses (5–30 nM) induced large amounts of luteinizing hormone-releasing hormone (LHRH) release, which was monitored as slow membrane depolarizations in the ganglionic neurons of bullfrog sympathetic ganglia. A nicotinic antagonist, d-tubocurarine chloride, completely and reversibly blocked the nicotine-induced LHRH release, but it did not block the nerve-firing-evoked LHRH release. Thus, nicotine activated nicotinic acetylcholine receptors and produced LHRH release via a mechanism that is different from the mechanism for evoked release. Moreover, this release was not caused by Ca2+ influx through either the nicotinic receptors or the voltage-gated Ca2+ channels because the release was increased moderately when the extracellular solution was changed into a Ca2+-free solution that also contained Mg2+ (4 mM) and Cd2+ (200 μM). The release did not depend on Ca2+ release from the intraterminal Ca2+ stores either because fura-2 fluorimetry showed extremely low Ca2+ elevation (≈30 nM) in response to nicotine (30 nM). Moreover, nicotine evoked LHRH release when [Ca2+] elevation in the terminals was prevented by loading the terminals with 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid and fura-2. Instead, the nicotine-induced release required extracellular Na+ because substitution of extracellular NaCl with N-methyl-d-glucamine chloride completely blocked the release. The Na+-dependent mechanism was not via Na+ influx through the voltage-gated Na+ channels because the release was not affected by tetrodotoxin (1–50 μM) plus Cd2+ (200 μM). Thus, nicotine at very low concentrations induced LHRH release via a Na+-dependent, Ca2+-independent mechanism.

Crosstalk between the sympathetic nervous system, inflammation and coagulation in gestational diabetes; a therapeutic approach in postmenopausal hypertension

The occurrence of gestational diabetes (GDM) during pregnancy is a powerful sign of a risk of later type 2 diabetes (T2D) and cardiovascular diseases (CVDs). The physiological basis for this disease progression is not yet fully understood, but increasing evidence exists on interplay of insulin resistance, subclinical inflammation, and more recently, on unbalance of the autonomic nervous system. Since the delay in development of T2D and CVD after GDM ranges from years to decades, better understanding of the pathophysiology of GDM could give us new tools for primary prevention. The present study was aimed at investigating the role of the sympathetic nervous system (SNS) in GDM and its associations with insulin and a variety of inflammatory cytokines and coagulation and fibrinolysis markers. This thesis covers two separate study lines. Firstly, we investigated 41 women with GDM and 22 healthy pregnant and 14 non-pregnant controls during the night in hospital. Blood samples were drawn at 24:00, 4:00 and 7:00 h to determine the concentrations of plasma glucose, insulin, noradrenaline (NA) and adrenomedullin, markers of subclinical inflammation, coagulation and fibrinolysis variables and platelet function. Overnight holter ECG recording was performed for analysis of heart rate variability (HRV). Secondly, we studied 87 overweight hypertensive women with natural menopause. They were randomised to use a central sympatholytic agent, moxonidine (0.3mg twice daily), the β-blocking agent atenolol (50 mg once daily+blacebo once daily) for 8 weeks. Inflammatory markers and adiponectin were analysed at the beginning and after 8 weeks. Activation of the SNS (increase in NA, decreased HRV) was seen in pregnant vs. non-pregnant women, but no difference existed between GDM and normal pregnancy. However, modulation (internal rhythm) of HRV was attenuated in GDM. Insulin and inflammatory cytokine levels were comparable in all pregnant women but nocturnal variation of concentrations of C-reactive protein, serum amyloid A and insulin were reduced in GDM. Levels of coagulation factor VIII were lower in GDM compared with normal pregnancy, whereas no other differences were seen in coagulation and fibrinolysis markers. No significant associations were seen between NA and the studied parameters. In the study of postmenopausal women, moxonidine treatment was associated with favourable changes in the inflammatory profile, seen as a decrease in TNFα concentrations (increase in atenolol group) and preservation of adiponectin levels (decrease in atenolol group). In conclusion, our results did not support our hypotheses of increased SNS activity in GDM or a marked association between NA and inflammatory and coagulation markers. Reduced biological variation of HRV, insulin and inflammatory cytokines suggests disturbance of autonomic and hormonal regulatory mechanisms in GDM. This is a novel finding. Further understanding of the regulatory mechanisms could allow earlier detection of risk women and the possibility of prevention. In addition, our results support consideration of the SNS as one of the therapeutic targets in the battle against metabolic diseases, including T2D and CVD.

Activation du système nerveux autonome lors du sommeil nocturne et lors du sommeil de récupération diurne chez les somnambules et les sujets contrôles

Les somnambules présentent des caractéristiques qui suggèrent un dysfonctionnement dans la régulation du sommeil lent profond (SLP). La fonction autonome est étroitement liée à la régulation des stades de sommeil et reflète l’intensité du SLP. Notre objectif est d’étudier la fonction autonome pendant le SLP des somnambules et des sujets témoins avant et après une privation de sommeil. Quatorze somnambules adultes (9 femmes et 5 hommes ; 28,1 ± 5,8 ans) et 14 sujets témoins appariés pour l’âge et le sexe (27,8 ± 6,0 ans) ont été évalués par vidéo - polysomnographie pour une nuit de base et pendant le sommeil de récupération après 25 heures de privation de sommeil. La fréquence cardiaque (FC) et les composantes spectrales de la variabilité de la FC ont été évaluées. Les composantes de basses (LF) et de hautes fréquences (HF) en valeur absolue et en unités normalisées (LFn et HFn) ainsi que le ratio LF/HF ont été analysés à partir de segments de 5 minutes d’électrocardiogramme sélectionnés lors du SLP des deux premiers cycles de sommeil. Au cours du premier cycle de sommeil, les somnambules, mais pas les sujets témoins, ont montré une diminution des LFn et du ratio LF/HF ainsi qu’une augmentation des HFn lors du sommeil de récupération par rapport au sommeil normal. Au cours du deuxième cycle, les somnambules ont montré une FC plus élevée en sommeil de récupération par rapport au sommeil de base et l’inverse a été trouvé chez les sujets témoins. Les somnambules ont montré une augmentation de l’activité parasympathique ainsi qu’une diminution de l’activité sympathique au cours du premier cycle du sommeil de récupération par rapport à la valeur initiale. Puisque cette fenêtre de temps est fortement associée à la survenue d'épisodes de somnambulisme chez les sujets prédisposés, cette hyperactivité parasympathique pourrait être impliquée dans la physiopathologie de somnambulisme.

Sympathetic hyperactivity differentially affects skeletal muscle mass in developing heart failure: role of exercise training

Bacurau AV, Jardim MA, Ferreira JC, Bechara LR, Bueno CR Jr, Alba-Loureiro TC, Negrao CE, Casarini DE, Curi R, Ramires PR, Moriscot AS, Brum PC. Sympathetic hyperactivity differentially affects skeletal muscle mass in developing heart failure: role of exercise training. J Appl Physiol 106: 1631-1640, 2009. First published January 29, 2009; doi:10.1152/japplphysiol.91067.2008.-Sympathetic hyperactivity (SH) is a hallmark of heart failure (HF), and several lines of evidence suggest that SH contributes to HF-induced skeletal myopathy. However, little is known about the influence of SH on skeletal muscle morphology and metabolism in a setting of developing HF, taking into consideration muscles with different fiber compositions. The contribution of SH on exercise tolerance and skeletal muscle morphology and biochemistry was investigated in 3- and 7-mo-old mice lacking both alpha(2A)- and alpha(2C)-adrenergic receptor subtypes (alpha(2A)/alpha(2C)ARKO mice) that present SH with evidence of HF by 7 mo. To verify whether exercise training (ET) would prevent skeletal muscle myopathy in advanced-stage HF, alpha(2A)/alpha(2C)ARKO mice were exercised from 5 to 7 mo of age. At 3 mo, alpha(2A)/alpha(2C)ARKO mice showed no signs of HF and preserved exercise tolerance and muscular norepinephrine with no changes in soleus morphology. In contrast, plantaris muscle of alpha(2A)/alpha(2C)ARKO mice displayed hypertrophy and fiber type shift (IIA -> IIX) paralleled by capillary rarefaction, increased hexokinase activity, and oxidative stress. At 7 mo, alpha(2A)/alpha(2C)ARKO mice displayed exercise intolerance and increased muscular norepinephrine, muscular atrophy, capillary rarefaction, and increased oxidative stress. ET reestablished alpha(2A)/alpha(2C)ARKO mouse exercise tolerance to 7-mo-old wild-type levels and prevented muscular atrophy and capillary rarefaction associated with reduced oxidative stress. Collectively, these data provide direct evidence that SH is a major factor contributing to skeletal muscle morphological changes in a setting of developing HF. ET prevented skeletal muscle myopathy in alpha(2A)/alpha(2C)ARKO mice, which highlights its importance as a therapeutic tool for HF.

Ventrolateral medulla mechanisms involved in cardiorespiratory responses to central chemoreceptor activation in rats

A rise in arterial PCO(2) stimulates breathing and sympathetic activity to the heart and blood vessels. In the present study, we investigated the involvement of the retrotrapezoid nucleus (RTN) and glutamatergic mechanisms in the Botzinger/C1 region (Botz/C1) in these responses. Splanchnic sympathetic nerve discharge (sSND) and phrenic nerve discharge (PND) were recorded in urethane-anesthetized, sino-aortic-denervated, vagotomized, and artificially ventilated rats subjected to hypercapnia (end-expiratory CO(2) from 5% to 10%). Phrenic activity was absent at end-expiratory CO(2) of 4%, and strongly increased when end-expiratory CO(2) reached 10%. Hypercapnia also increased sSND by 103 +/- 7%. Bilateral injections of the GABA-A agonist muscimol (2 mM) into the RTN eliminated the PND and blunted the sSND activation (Delta = +56 +8%) elicited by hypercapnia. Injections of NMDA receptor antagonist AP-5 (100 mM), non-NMDA receptor antagonist 6,7-dinitro-quinoxaline-2,3-dione (DNQX; 100 mM) or metabotropic glutamate receptor antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG; 100 mM) bilaterally into the Botz/C1 reduced PND (Delta = +43 +/- 7%, +52 +/- 6% or +56 +/- 11%, respectively). MCPG also reduced sSND (Delta = +41 +/- 7%), whereas AP-5 and DNQX had no effect. In conclusion, the increase in sSND caused by hypercapnia depends on increased activity of the RTN and on metabotropic receptors in the Botz/C1, whereas PND depends on increased RTN activity and both ionotropic and metabotropic receptors in the Botz/C1.

Contribution of excitatory amino acid receptors of the retrotrapezoid nucleus to the sympathetic chemoreflex in rats

In the present study, we evaluated the role of glutamatergic mechanisms in the retrotrapezoid nucleus (RTN) in changes of splanchnic sympathetic nerve discharge (sSND) and phrenic nerve discharge (PND) elicited by central and peripheral chemoreceptor activation. Mean arterial pressure (MAP), sSND and PND were recorded in urethane-anaesthetized, vagotomized, sino-aortic denervated and artificially ventilated male Wistar rats. Hypercapnia (10% CO(2)) increased MAP by 32 +/- 4 mmHg, sSND by 104 +/- 4% and PND amplitude by 101 +/- 5%. Responses to hypercapnia were reduced after bilateral injection of the NMDA receptor antagonist D,L-2-amino-5-phosphonovalerate (AP-5; 100mm in 50 nl) in the RTN (MAP increased by 16 +/- 3 mmHg, sSNDby 82 +/- 3% and PND amplitudeby 63 +/- 7%). Bilateral injection of the non-NMDA receptor antagonist 6,7-dinitro-quinoxaline-2,3-dione(DNQX; 100 mm in 50 nl) and the metabotropic receptor antagonist (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG; 100mm in 50 nl) in the RTN did not affect sympathoexcitatory responses induced by hypercapnia. Injection of DNQX reduced hypercapnia-induced phrenic activation, whereas MCPG did not. In animals with intact carotid chemoreceptors, bilateral injections of AP-5 and DNQX in the RTN reduced increases in MAP, sSND and PND amplitude produced by intravenous injection of NaCN (50 mu g kg(-1)). Injection of MCPG in the RTN did not change responses produced by NaCN. These data indicate that RTN ionotropic glutamatergic receptors are involved in the sympathetic and respiratory responses produced by central and peripheral chemoreceptor activation.

Switching control of sympathetic activity from forebrain to hindbrain in chronic dehydration

We investigated the mechanisms responsible for increased blood pressure and sympathetic nerve activity (SNA) caused by 2-3 days dehydration (DH) both in vivo and in situ preparations. In euhydrated (EH) rats, systemic application of the AT(1) receptor antagonist Losartan and subsequent pre-collicular transection (to remove the hypothalamus) significantly reduced thoracic (t) SNA. In contrast, in DH rats, Losartan, followed by pre-collicular and pontine transections, failed to reduce tSNA, whereas transection at the medulla-spinal cord junction massively reduced tSNA. In DH but not EH rats, selective inhibition of the commissural nucleus tractus solitarii (cNTS) significantly reduced tSNA. Comparable data were obtained in both in situ and in vivo (anaesthetized/conscious) rats and suggest that following chronic dehydration, the control of tSNA transfers from supra-brainstem structures (e. g. hypothalamus) to the medulla oblongata, particularly the cNTS. As microarray analysis revealed up-regulation of AP1 transcription factor JunD in the dehydrated cNTS, we tested the hypothesis that AP1 transcription factor activity is responsible for dehydration-induced functional plasticity. When AP1 activity was blocked in the cNTS using a viral vector expressing a dominant negative FosB, cNTS inactivation was ineffective. However, tSNA was decreased after pre-collicular transection, a response similar to that seen in EHrats. Thus, the dehydration-induced switch in control of tSNA from hypothalamus to cNTS seems to be mediated via activation of AP1 transcription factors in the cNTS. If AP1 activity is blocked in the cNTS during dehydration, sympathetic activity control reverts back to forebrain regions. This unique reciprocating neural structure-switching plasticity between brain centres emphasizes the multiple mechanisms available for the adaptive response to dehydration.

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.

Reduced preprandial dipping accounts for rapid elevation of blood pressure and renal sympathetic nerve activity in rabbits fed a high-fat diet

Consumption of a high-fat diet (HFD) by rabbits results in increased blood pressure (BP), heart rate (HR), and renal sympathetic nerve activity (RSNA) within 1 wk. Here, we determined how early this activation occurred and whether it was related to changes in cardiovascular and neural 24-h rhythms. Rabbits were meal-fed a HFD for 3 wks, then a normal-fat diet (NFD) for 1 wk. BP, HR, and RSNA were measured daily in the home cage via implanted telemeters. Baseline BP, HR, and RSNA over 24 h were 71 ± 1 mm Hg, 205 ± 4 beats/min and 7 ± 1 normalized units (nu). The 24-h pattern was entrained to the feeding cycle and values increased from preprandial minimum to postprandial maximum by 4 ± 1 mm Hg, 51 ± 6 beats/min, and 1.6 ± .6 nu each day. Feeding of a HFD markedly diminished the preprandial dip after 2 d (79–125% of control; p < 0.05) and this reduction lasted for 3 wks of HFD. Twenty-four-hour BP, HR, and RSNA concurrently increased by 2%, 18%, and 22%, respectively. Loss of preprandial dipping accounted for all of the BP increase and 50% of the RSNA increase over 3 wks and the 24-h rhythm became entrained to the light-dark cycle. Resumption of a NFD did not alter the BP preprandial dip. Thus, elevated BP induced by a HFD and mediated by increased sympathetic nerve activity results from a reduction in preprandial dipping, from the first day. Increased calories, glucose, insulin, and leptin may account for early changes, whereas long-term loss of dipping may be related to increased sensitivity of sympathetic pathways.

Altered responsiveness of the kidney to activation of the renal nerves in fat-fed rabbits

We tested whether mild adiposity alters responsiveness of the kidney to activation of the renal sympathetic nerves. After rabbits were fed a high-fat or control diet for 9 wk, responses to reflex activation of renal sympathetic nerve activity (RSNA) with hypoxia and electrical stimulation of the renal nerves (RNS) were examined under pentobarbital anesthesia. Fat pad mass and body weight were, respectively, 74% and 6% greater in fat-fed rabbits than controls. RNS produced frequency-dependent reductions in renal blood flow, cortical and medullary perfusion, glomerular filtration rate, urine flow, and sodium excretion and increased renal plasma renin activity (PRA) overflow. Responses of sodium excretion and medullary perfusion were significantly enhanced by fat feeding. For example, 1 Hz RNS reduced sodium excretion by 79 ± 4% in fat-fed rabbits and 46 ± 13% in controls. RNS (2 Hz) reduced medullary perfusion by 38 ± 11% in fat-fed rabbits and 9 ± 4% in controls. Hypoxia doubled RSNA, increased renal PRA overflow and medullary perfusion, and reduced urine flow and sodium excretion, without significantly altering mean arterial pressure (MAP) or cortical perfusion. These effects were indistinguishable in fat-fed and control rabbits. Neither MAP nor PRA were significantly greater in conscious fat-fed than control rabbits. These observations suggest that mild excess adiposity can augment the antinatriuretic response to renal nerve activation by RNS, possibly through altered neural control of medullary perfusion. Thus, sodium retention in obesity might be driven not only by increased RSNA, but also by increased responsiveness of the kidney to RSNA.

Commissural nucleus of the solitary tract is important for cardiovascular responses to caudal pressor area activation

The nucleus of the solitary tract (NTS) is the site of the first synapse of cardiovascular afferent fibers in the central nervous system. Important mechanisms for cardiovascular regulation are also present in the caudal pressor area (CPA) localized at the caudal end of the ventrolateral medulla. In the present study we sought to investigate the role of the commissural subnucleus of the NTS (commNTS) on pressor and tachycardic responses induced by L-glutamate injected into the CPA. Male Holtzman rats (n=8 rats/group) anesthetized with urethane (1.2 g/kg of body weight, iv) received injections of the GABAA receptor agonist muscimol into the commNTS. Unilateral injection of L-glutamate (10 nmol/ 100 nL) into the CPA increased mean arterial pressure (MAP, 31 4 mm Hg, vs. saline: 3 +/- 2 mm Hg) and heart rate (HR, 44 8 bpm, vs. saline: 10 7 bpm). inhibition of commNTS neurons with muscimol (120 pmol/60 nL) abolished the increase in MAP (9 4 mm Hg) and HR (17 7 bpm) produced by L-glutamate into the CPA. The present results suggest that the pressor and tachycardic responses to CPA activation are dependent on commNTS mechanisms.

Sympathetic mediation of salivation induced by intracerebroventricular pilocarpine in rats

Central cholinergic activation by pilocarpine induces salivation dependent on the integrity of forebrain areas. The present work investigates the autonomic mediation of this salivation. Pilocarpine (500 nmol/rat) was injected into the lateral ventricle (LV) of tribromoethanol-anesthetized adult male rats. Preweighed cotton balls were inserted into the oral cavity and weighed again 7 min later. ol-adrenoceptor antagonists (3-50 mu mol/kg) prazosin (alpha(1)), yohimbine (alpha(2)) or propranolol (beta) injected intraperitoneally (i.p.) produced, 80%, 20% and 0% inhibition respectively of the LV pilocarpine-induced salivation. Intracerebroventricular injections (160 nmol) of the antagonists did not alter the effects of pilocarpine injected into the LV. Bilateral section of chorda tympani nerve or bilateral sympathetic cervical ganglionectomy produced 0% and 40% inhibition of pilocarpine-induced salivation, respectively. Ganglionectomy did not alter salivation induced by i.p, injection of pilocarpine (4 mu mol/kg). The results indicate that there is a large sympathetic contribution to the salivation induced by central cholinergic activation. (C) 1999 Elsevier B.V. B.V. All rights reserved.