Level Of Nucleus Tractus Solitarii Research Articles

Aldosterone activates ERK phosphorylation in the nucleus tractus solitarius

Sodium intake effect of aldosterone has attracted much attention. In our recent study, aldosterone can play a nongenomic regulatory role on rapid sodium intake in the NTS (nucleus tractus solitarius) by activating G protein-coupled estrogen receptor (GPER), and it exhibited an obvious time-dependent and concentration-dependent regulation. However, the molecular mechanism how aldosterone regulated sodium intake rapidly, is unclear. To determine the molecular mechanism of rapid sodium intake regulation of aldosterone, rats with a stainless-steel cannula in the NTS were used (n = 6 each subgroup), and were injected different concentrations of aldosterone/G1 (GPER agonist)/G15 (GPER antagonist) at different time points, then detected ERK1/2 protein expression. The results showed that aldosterone/G1 increased the ERK1/2 protein phosphorylation, and presented a time-dependent and concentration-dependent similar to sodium intake; Meanwhile, G15 partially blocked this effect at least. Taken together, we postulate that ERK1/2 protein may influence nongenomic sodium intake regulated by aldosterone at nucleus tractus solitarius level.

Adenosine-A2A Receptor Signaling Plays a Crucial Role in Sudden Unexpected Death in Epilepsy.

Adenosinergic activities are suggested to participate in SUDEP pathophysiology; this study aimed to evaluate the adenosine hypothesis of SUDEP and specifically the role of adenosine A2A receptor (A2AR) in the development of a SUDEP mouse model with relevant clinical features. Using a combined paradigm of intrahippocampal and intraperitoneal administration of kainic acid (KA), we developed a boosted-KA model of SUDEP in genetically modified adenosine kinase (ADK) knockdown (Adk+/-) mice, which has reduced ADK in the brain. Seizure activity was monitored using video-EEG methods, and in vivo recording of local field potential (LFP) was used to evaluate neuronal activity within the nucleus tractus solitarius (NTS). Our boosted-KA model of SUDEP was characterized by a delayed, postictal sudden death in epileptic mice. We demonstrated a higher incidence of SUDEP in Adk+/- mice (34.8%) vs. WTs (8.0%), and the ADK inhibitor, 5-Iodotubercidin, further increased SUDEP in Adk+/- mice (46.7%). We revealed that the NTS level of ADK was significantly increased in epileptic WTs, but not in epileptic Adk+/- mutants, while the A2AR level in NTS was increased in epileptic (WT and Adk+/-) mice vs. non-epileptic controls. The A2AR antagonist, SCH58261, significantly reduced SUDEP events in Adk+/- mice. LFP data showed that SCH58261 partially restored KA injection-induced suppression of gamma oscillation in the NTS of epileptic WT mice, whereas SCH58261 increased theta and beta oscillations in Adk+/- mutants after KA injection, albeit with no change in gamma oscillations. These LFP findings suggest that SCH58261 and KA induced changes in local neuronal activities in the NTS of epileptic mice. We revealed a crucial role for NTS A2AR in SUDEP pathophysiology suggesting A2AR as a potential therapeutic target for SUDEP risk prevention.

Astrocytic modulation of glutamatergic synaptic transmission is reduced in NTS of rats submitted to short-term sustained hypoxia.

Sustained hypoxia (SH) activates chemoreceptors to produce cardiovascular and respiratory responses to bring the arterial partial pressure of O2 back to the physiological range. We evaluated the effect of SH (fraction of inspired O2 = 0.10, 24 h) on glutamatergic synaptic transmission and the interaction neuron-astrocyte in neurons of the nucleus tractus solitarii (NTS). Tractus solitarius (TS) fiber stimulation induced glutamatergic currents in neurons and astrocytes. SH increased α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate (AMPA/kainate) [-183 ± 122 pA (n = 10) vs. -353 ± 101 pA (n = 10)] and N-methyl-d-aspartate (NMDA) current amplitude [61 ± 10 pA (n = 7) vs. 102 ± 37 pA (n = 10)]. To investigate the effects of SH, we used fluoroacetate (FAC), an astrocytic inhibitor, which revealed an excitatory modulation on AMPA/kainate current and an inhibitory modulation of NMDA current in control rats. SH blunted the astrocytic modulation of AMPA [artificial cerebrospinal fluid (aCSF): -353 ± 101 pA vs. aCSF + FAC: -369 ± 76 pA (n = 10)] and NMDA currents [aCSF: 102 ± 37 pA vs. aCSF + FAC: 108 ± 32 pA (n = 10)]. SH increased AMPA current density [control: -6 ± 3.5 pA/pF (n = 6) vs. SH: -20 ± 12 pA/pF (n = 7)], suggesting changes in density, conductance, or affinity of AMPA receptors. SH produced no effect on astrocytic resting membrane potential, input resistance, and AMPA/kainate current. We conclude that SH decreased the neuron-astrocyte interaction at the NTS level, facilitating the glutamatergic transmission, which may contribute to the enhancement of cardiovascular and respiratory responses to baro- and chemoreflexes activation in SH rats. NEW & NOTEWORTHY Using an electrophysiological approach, we have shown that in nucleus tractus solitarii (NTS) from control rats, astrocytes modulate the AMPA and NMDA currents in NTS neurons, changing their excitability. Sustained hypoxia (SH) increased both glutamatergic currents in NTS neurons due to 1) a reduction in the astrocytic modulation and 2) an increase in the density of AMPA receptors. These new findings show the importance of neuron-astrocyte modulation in the excitatory synaptic transmission in NTS of control and SH rats.

Inhibitory control of the cough reflex by galanin receptors in the caudal nucleus tractus solitarii of the rabbit.

The caudal nucleus tractus solitarii (NTS) is the main central station of cough-related afferents and a strategic site for the modulation of the cough reflex. The similarities between the characteristics of central processing of nociceptive and cough-related inputs led us to hypothesize that galanin, a neuropeptide implicated in the control of pain, could also be involved in the regulation of the cough reflex at the level of the NTS, where galanin receptors have been found. We investigated the effects of galanin and galnon, a nonpeptide agonist at galanin receptors, on cough responses to mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. Drugs were microinjected (30-50 nl) into the caudal NTS of pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Galnon antitussive effects on cough responses to the mechanical stimulation of the airway mucosa via a custom-built device were also investigated. Bilateral microinjections of 1 mM galanin markedly decreased cough number, peak abdominal activity, and increased cough-related total cycle duration. Bilateral microinjections of 1 mM galnon induced mild depressant effects on cough, whereas bilateral microinjections of 10 mM galnon caused marked antitussive effects consistent with those produced by galanin. Galnon effects were confirmed by using the cough-inducing device. The results indicate that galanin receptors play a role in the inhibitory control of the cough reflex at the level of the caudal NTS and provide hints for the development of novel antitussive strategies.

Brainstem sites controlling the lower esophageal sphincter and crural diaphragm in the ferret: A neuroanatomical study

The lower esophageal sphincter (LES) and the crural diaphragm (CD) surrounding the esophagogastric junction are key components of the gastroesophageal reflex mechanism, which engages the vago-vagal brainstem circuitry. Although both components work in conjunction to prevent gastroesophageal reflux, little is known about the brain area(s) where this integration takes place. The aims of this study were to: (1) trace the brainstem circuitry associated with the CD and the LES, and (2) determine possible sites of convergence. Experiments were done in adult male ferrets. Under isoflurane anesthesia, recombinant strains of the transneuronal pseudorabies virus (PRV-151 or PRV-Bablu) or the monosynaptic retrograde tracer cholera toxin beta-subunit (CTb) were injected into either the CD or the LES. Following a survival period of 5–7 days, animals were euthanized, perfused and their brains removed for dual-labeling immunofluorescence processing. In animals injected with recombinants of PRV into the CD and the LES, distinct labeling was found in various brainstem nuclei including: area postrema, DMV, nucleus tractus solitarius (NTS), medial reticular formation (MRF) and nucleus ambiguous (NA). Double-labeled cells were only evident in the DMV, NTS and MRF. Injections of CTb into the CD or the LES resulted in retrograde labeling only in the DMV. These findings demonstrate the presence of a direct projection from the DMV to the CD. They further suggest that the neuronal connections responsible for CD or LES function are contained in circuitries that, though largely independent, may converge at the level of DMV, NTS and MRF.

Are ATP and glutamate released from slowly adapting pulmonary stretch receptor afferents in the NTS?

Are ATP and glutamate released from slowly adapting pulmonary stretch receptor afferents in the NTS?

Adrenomedullin 2 microinjection into the nucleus tractus solitarius elevates arterial pressure and heart rate in rats

Adrenomedullin 2 (AM2), a novel member of the calcitonin gene-related peptide family, has emerged as a multifunctional peptide controlling endocrine and cardiovascular functions and physiological behaviors. Both central and peripheral administration of AM2 can have profound effects on systemic and/or pulmonary circulation in mammals. However, the target nuclei of AM2 and the role of central AM2 in cardiovascular regulation remain unknown. In the present study, we microinjected AM2 into the rat nucleus tractus solitarius (NTS), the central termination site of baroreceptor afferents. Consistent with previous reports showing the hypertensive effect of intracerebroventricular administration of AM2, the direct microinjection of the peptide into the NTS increased arterial pressure as well as heart rate in rats. Importantly, this effect of AM2 on cardiovascular regulation was significantly attenuated by an antagonist of receptor components for AM2 that were abundant in the rat NTS. Our results indicate that AM2 may play an important role in the regulation of the cardiovascular system at the NTS level.

Antagonism of glutamate receptors in the intermediate and caudal NTS of awake rats produced no changes in the hypertensive response to chemoreflex activation

The role of excitatory amino acid (EAA) receptors in the neurotransmission of the sympathoexcitatory component of the chemoreflex (pressor response) in the intermediate and caudal aspects of the commissural nucleus tractus solitarii (NTS) of awake rats was evaluated. Microinjection of kynurenic acid, a non-selective antagonist of EAA receptors, into the intermediate and caudal commissural NTS produced a large increase in the baseline mean arterial pressure (MAP), which may reduce the magnitude of the pressor response to chemoreflex activation. To avoid this problem sodium nitroprusside (SNP) was infused (i.v.) after microinjections of kynurenic acid (2 nmol/50 nl) into the NTS, in order to normalize the MAP and then the chemoreflex was activated and the magnitude of the pressor response evaluated. Microinjection of kynurenic acid into the intermediate (bilaterally) and caudal (midline) commissural NTS ( n=6) produced a significant increase in baseline MAP (103±5 vs. 137±6 mm Hg) normalized by SNP infusion (107±4 mm Hg) and under this experimental condition the pressor response to chemoreflex activation was not statistically different in relation to the control (37±7 vs. 44±6 mm Hg). Bilateral microinjections of kynurenic acid into the caudal NTS ( n=8) also produced a significant increase in baseline MAP (109±4 vs. 145±6 mm Hg) normalized by SNP infusion (109±6 mm Hg). After normalization of MAP, the pressor response to chemoreflex activation at 3 (34±6 mm Hg) and 10 min (37±6 mm Hg) was also not different in relation to the control (46±5 mm Hg). These data indicate that the antagonism of EAA receptors simultaneously in the intermediate (bilateral) and caudal (midline) commissural NTS or only in the caudal commissural NTS (bilateral) of awake rats had no effect on the hypertensive response to chemoreflex activation. We suggest that neurotransmitter other than l-glutamate may take part in the neurotransmission of the sympathoexcitatory component of the chemoreflex at the NTS level.

Effect of losartan microinjections into the NTS on the cardiovascular components of chemically evoked reflexes in a rabbit model of acute heart ischemia.

In the acute phase of myocardial infarction (MI) there are modifications of the autonomic outflow with an increase in the sympathetic tone and changes in cardiovascular reflexes. Activation of angiotensin AT1 receptors in the nucleus tractus solitarii (NTS) inhibits the baroreceptor and enhances the carotid chemoreflex. In this study, we investigated the role of NTS-AT1 receptors on the cardiovascular reflex responses evoked on stimulation of carotid chemoreflex and cardiac chemosensitive fibres in the acute phase of MI. We also test the hypothesis that changes in cardiovascular responses to activation of carotid chemo and cardiac chemosensitive reflexes are secondary to changes in haemodynamic conditions due to infarction or to the activation of nociceptors of the heart. Rabbits were anaesthetised, paralysed and artificially ventilated. Carotid chemoreceptors and cardiac chemosensitive fibres were stimulated with lobeline and ATP, respectively. Arterial blood pressure, electrocardiogram and heart rate were monitored. A craniotomy was made to expose the caudal portions of the medulla and a multibarreled glass microelectrode was inserted in order to allow the identification of NTS and the microinjection of losartan (an angiotensin AT1 receptor antagonist). The heart was exposed by a midline thoracotomy and cardiac ischemia was produced by ligating the left descending coronary artery. The carotid chemoreflex and cardiac chemosensitive reflexes were evoked before and following the coronary ligation. The effect of losartan injection into the NTS on these reflexes was also assessed. In control experiments reflexes were assessed before and after the administration of capsaicin and procainamide. Results show that the activation of carotid chemoreflex elicited a greater increase of blood pressure and bradycardia after MI and that this was partially reversed by losartan microinjection after MI. Also the stimulation of cardiac chemosensitive fibres evoked a larger decrease on blood pressure and heart rate after MI and these were also partially reversed by losartan. The same enhancement of cardiovascular carotid chemo and cardiac chemosensitive receptors was observed after administration of capsaicin on the ventricular surface but not after procainamide. In conclusion, this study strongly suggests that, at NTS level, angiotensin AT1 receptors are involved in the modifications of autonomic outflow observed in the acute phase of MI.

Antagonism of glutamatergic metabotropic receptors in the NTS of awake rats does not affect the gain of the baroreflex

There is evidence suggesting that metabotropic receptors may play a role in the neurotransmission of the baroreflex in the nucleus tractus solitarius (NTS) of rats. In a recent study from our laboratory, we verified that microinjection of a metabotropic receptor agonist, trans-1-amino-1,3-cyclopentanediocarboxylic acid, into the NTS of awake and anesthetized rats produced baroreflex-like responses (hypotension and bradycardia). In the present study, we evaluated the possible role of l-glutamate metabotropic receptors of the NTS in the neuromodulation of the parasympathetic component of baroreflex activation in awake rats. Bilateral microinjection (50 nl) of a metabotropic receptor antagonist (α-methyl-4-carboxyphenylglycine, MCPG, 100 mM) into the rostral commissural NTS produced no change in the gain of the baroreflex bradycardia. In addition, microinjection of MCPG into the NTS produced no changes in baseline mean arterial pressure (MAP) or heart rate (HR), indicating that metabotropic receptors play no tonic role in the neurotransmission of the baroreflex. The dose of MCPG used to block the metabotropic receptors was effective in reducing the bradycardic and hypotensive responses to microinjection (50 nl) of trans-1-amino-1,3-cyclopentanediocarboxylic acid (5 mM) into the NTS. The data show that metabotropic glutamate receptors play no major role in the neuromodulation of the parasympathetic component of the baroreflex at the NTS level.

Antagonism of adenosine A(1) receptors in the NTS does not affect the chemoreflex in awake rats.

The possible involvement of adenosine A(1) receptors in neurotransmission of the sympathoexcitatory component of the chemoreflex in the nucleus tractus solitarii (NTS) of awake rats was evaluated. Unilateral microinjection of increasing doses of adenosine (0.01, 0.06, 0.12, 1.25, 2.5, and 5.0 nmol/50 nl) into the lateral aspect of the commissural NTS produced a long-lasting increase in baseline mean arterial pressure (MAP) and no changes in baseline heart rate (HR). Microinjection of adenosine at 1.25 nmol/50 nl (ED(50)) into the NTS (n = 9) produced a significant increase in baseline MAP (119 +/- 3, 122 +/- 4, and 117 +/- 4 mmHg at 30 s, 1 min, and 2 min, respectively) compared with control (102 +/- 3 mmHg) but no significant changes after previous microinjection of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), an adenosine A(1) receptor antagonist (107 +/- 3, 107 +/- 3, and 106 +/- 3 mmHg at 30 s, 1 min, and 2 min, respectively) compared with control (102 +/- 3 mmHg). Microinjection of adenosine before and after DPCPX into the same site of the lateral commissural NTS produced no changes in baseline HR. In another group of rats (n = 8), microinjection of DPCPX (0.285 nmol/50 nl) into lateral and midline aspects of the commissural NTS produced no significant changes in pressor (+46 +/- 4 vs. +47 +/- 2 mmHg) or bradycardic responses (-216 +/- 9 vs. -226 +/- 12 beats/min) to chemoreflex activation with intravenous potassium cyanide compared with control responses. These data show that microinjection of adenosine into the NTS produced a small and long-lasting pressor response by activating A(1) receptors and that blockade of these receptors produced no changes in cardiovascular responses to chemoreflex activation. We conclude that adenosine A(1) receptors are not involved in processing of the chemoreflex afferents at the NTS level.

Alterations of Central Vasopressinergic System in Transgenic Rats with Low Brain Angiotensinogen

P188 The consequence of a permanent alteration of the brain renin-angiotensin system (RAS) on central vasopressinergic system was studied at the level of the nucleus tractus solitarii (NTS) of transgenic rats with a deficit in brain angiotensinogen, TGR(ASrAOGEN). Vasopressin (AVP) (100 pmol/ 50nl) or desmopressin (DDAVP, V2 receptor agonist) (100pmol/ 50nl) were microinjected into the NTS of urethane-anesthetized TGR(ASrAOGEN) and Sprague-Dawley (SD) rats. Blood pressure and heart rate were measured via a femoral artery catheter and the baroreflex induced by phenylephrine (baroreflex bradycardia) evaluated. Hypothalamic levels of AVP mRNA were measured by ribonuclease protection assay (RPA). AVP microinjections induced a greater decrease of the MAP in TGR(ASrAOGEN) in comparison with SD rats (-19.9 ± 5.2 vs. -7.5 ± 0.7 mmHg, p<0.05). The significantly higher baroreflex sensitivity observed in TGR compared to that of SD rats was normalized after AVP microinjection (0.76 ± 0.1 vs. 0.3 ± 0.1 ms/ mmHg, before and after AVP microinjection in TGR rats, p<0.05). The microinjections of DDAVP in the NTS produced also a decrease of MAP, which was significantly higher in TGR(ASrAOGEN) in comparison with the control strain (-21.2 ± 0.9 vs. -10 ± 1.2 mmHg, p<0.05). Hypothalamic AVP mRNA levels were not significantly altered in TGR rats. These results demonstrate that a permanent deficit in the brain angiotensinogen synthesis can alter the functionality of central vasopressinergic system. However, the hypothalamic AVP synthesis is not altered. The increased sensitivity to AVP and DDAVP in TGR(ASrAOGEN) rats indicates a functional upregulation of AVP receptors in the NTS and further evidences the interaction between RAS and the vasopressinergic system within the brain. Furthermore, this study for the first time shows the existence of functional DDAVP-sensitive vasopressin receptors at the NTS level.

Role of endogenous nitric oxide in the nucleus tratus solitarii on baroreflex control of heart rate in spontaneously hypertensive rats.

Toinvestigate the modulatory effect of endogenous nitric oxide (NO) in the nucleus tractus solitarii (NTS) on the baroreceptor reflex control of heart rate in conscious spontaneously hypertensive (SHR) and normotensive (WKY) rats. Male age- and weight-matched SHR and WKY chronically instrumented with cannulas in the NTS, artery and vein were used. Basal pressure (AP), heart rate (HR) and reflex HR responses during loading/unloading of baroreceptors (phenylephrine/sodium nitroprusside, iv) were recorded during vehicle (3 nl/min) NG-monomethyl-L-arginine (L-NMMA) and L-arginine (L-Arg) infusions into the NTS. Constitutive NO synthase (NOS) activity was inferred by 3H-citrulline formation in the dorsal brain stem of other SHR and WKY groups. In SHR a small dose of L-NMMA (30 ng/kg/min) restricted to the NTS did not change AP and HR (185+/-4 mmHg, 373+/-12 beats/min, respectively), but decreased the HR range (57+/-7 beats/min, a 34% reduction, P< 0.05) without changing further the impaired gain of baroreceptor reflex control of HR. In the WKY group similar results (significant 32% reduction in HR range, gain unchanged) were only attained with a dose 10 times higher (L-NMMA(NTS) = 300 ng/kg/min), no effect being observed with the small dose (HR range = 163+/-12 beats/min). In SHR, L-Arg(NTS) (900 ng/kg/min) did not improve baroreflex control of HR, but restored the depression of HR range when given after L-NMMA(NTS). Basal NOS activity in the dorsal brain stem was reduced in SHR (P < 0.05) when compared to WKY group. NO modulates, at the NTS level, the baroreceptor reflex control of HR in both SHR and WKY not by altering the gain, but by increasing HR range during afferent stimulation. In SHR the depressed NO modulation is in accordance with the smaller NOS activity in the dorsal brain stem.

Autonomic processing of the cardiovascular reflexes in the nucleus tractus solitarii.

The nucleus tractus solitarii (NTS) receives afferent projections from the arterial baroreceptors, carotid chemoreceptors and cardiopulmonary receptors and as a function of this information produces autonomic adjustments in order to maintain arterial blood pressure within a narrow range of variation. The activation of each of these cardiovascular afferents produces a specific autonomic response by the excitation of neuronal projections from the NTS to the ventrolateral areas of the medulla (nucleus ambiguous, caudal and rostal ventrolateral medulla). The neurotransmitters at the NTS level as well as the excitatory amino acid (EAA) receptors involved in the processing of the autonomic responses in the NTS, although extensively studied, remain to be completely elucidated. In the present review we discuss the role of the EAA L-glutamate and its different receptor subtypes in the processing of the cardiovascular reflexes in the NTS. The data presented in this review related to the neurotransmission in the NTS are based on experimental evidence obtained in our laboratory in unanesthetized rats. The two major conclusions of the present review are that a) the excitation of the cardiovagal component by cardiovascular reflex activation (chemo- and Bezold-Jarisch reflexes) or by L-glutamate microinjection into the NTS is mediated by N-methyl-D-aspartate (NMDA) receptors, and b) the sympatho-excitatory component of the chemoreflex and the pressor response to L-glutamate microinjected into the NTS are not affected by the NMDA receptors antagonist, suggesting that the sympatho-excitatory component of these responses is mediated by non-NMDA receptors.

Inhibition of stimulated cyclic AMP production by multiple neuropeptide Y receptors in the rat brainstem

Neuropeptide Y (NPY) has been shown to modulate blood pressure, heart rate and to inhibit the baroreceptor reflex at the level of nucleus tractus solitarius (NTS). The aim of this study was to examine effects of NPY and its related peptides on forskolin (1 μM)-stimulated cyclic AMP production in slices of the rat NTS. Each peptide was present at 0.3 μM. Pretreatment with NPY inhibited the stimulated increase in cyclic AMP levels in slices of rat NTS. Also [Pro 34]NPY, an analog, which activates Y1, Y3 (and Y5) receptors inhibited the stimulated increase in cyclic AMP levels. However, pretreatment with the Y1 receptor-selective antagonist BIBP3226 (3 μM) did not affect the [Pro 34]NPY-evoked inhibition of cyclic AMP levels. In addition, [Leu 31,Pro 34]NPY, an Y1 (and PP1/Y4 and Y5) receptor agonist did not inhibit the stimulated increase in cyclic AMP production. Also the Y2 receptor-selective agonist C2-NPY inhibited the stimulated elevation of cyclic AMP levels, while peptide YY, which does not recognize Y3 receptors did not significantly affect the stimulated cyclic AMP production. In conclusion, it seems that Y2 and Y3 receptors are coupled to inhibition of adenylate cyclase activity in the rat NTS.

Is L-Dopa a Neurotransmitter of the Primary Baroreceptor Afferents Terminating in the Nucleus Tractus Solitarii of Rats?

Transmitter-like Dihydroxyphenylalanine (DOPA) is released from rat striata and blood pressure regulation centers in the lower brain stem. Exogenous DOPA itself elicits in vitro presynaptic and in vivo postsynaptic responses. All are stereoselective, and most are antagonized by DOPA methyl ester, a competitive DOPA antagonist. DOPA does not displace selective binding of α 2 , β , D 1 , and D 2 ligands in brain membrane preparations. It is possible to cooperate for effectiveness in Parkinson's disease. Meanwhile, DOPA induces DOPA ester-sensitive neuronal glutamate release from slices, even under inhibition of aromatic L-amino acid decarboxylase (AADC). The release by baroreceptor stimulation with phenylephrine has been abolished by bilateral sinoaortic denervation without modification of hypertension. DOPA microinjected into depressor sites of the medial nucleus tractus solitarii (NTS) identified by prior glutamate leads to dose-dependent decreases in blood pressure and heart rate (HR) in untreated rats and in those treated with central AADC inhibitor. The discussion also includes interactions between DOPAergic and GABAergic systems. GABA is an inhibitory neuromodulator for baroreflex at the level of NTS. GABA functions tonically via GABA A receptors to elicit increases in blood pressure and HR and to inhibit decreases in those by electrical aortic nerve stimulation. The third topic under discussion in this chapter is altered functions of the DOPA system in the NTS of adult spontaneously hypertensive rats (SHR). Basal DOPA release is lower in SHR than age-matched Wistar-Kyoto (WKY) rats. This release has been reduced by tetrodotoxin (TTX) to the same absolute levels in the two strains.

Medullary visceral reflex circuits: local afferents to nucleus tractus solitarii synthesize catecholamines and project to thoracic spinal cord.

Visceral feedback circuits in lower brainstem were elucidated with retrograde tracers by mapping neurons that issue local projections to the general visceral afferent division of the nucleus tractus solitarii (NTS) and dorsomotor vagal nucleus (DMX) in adult male rats. In study 1, spinal and intramedullary afferents to the visceral-sensorimotor complex (NTS-X) were traced to contiguous populations of cell bodies arranged in cylindrical segmental organization. NTS-X afferents derive from curvilinear arrays of neurons that parallel the efferent radiations of the solitariotegmental tract. Newly discovered afferents arise from circumscribed cell groups in the dorsal reticular formation and periventricular zone. Another source was traced to a paraambigual cell column in the apex of the rostral ventrolateral reticular nucleus (n.RVL). In study 2, catecholaminergic afferents were initially defined with combined retrograde transport-immunocytochemical methods. Deposits of retrograde tracers into NTS-X transported to neurons containing tyrosine hydroxylase (TH) in the A1, C1, and C3 areas or phenylethanolamine N-methyltransferase (PNMT) in the C1 area of the n.RVL and C3 area. In study 3, it was revealed that NTS-X afferents arise, in part, as collaterals of thoracic reticulospinal neurons. Deposits of the retrograde fluorescent tracer Fluorogold into the upper thoracic cord and rhodamine-labeled microbeads into NTS-X transported to the same neurons within a subambigual locus in n.RVL and parts of nucleus raphe magnus. In study 4, dual retrograde tracer-immunocytochemical analysis demonstrated that catecholamines are synthesized by a subset of neurons in the n.RVL that issue collaterals to the NTS-X and thoracic cord. Double retrogradely labeled TH- or PNMT-immunoreactive cell bodies were restricted to the C1 area within a 450-microns column bordered rostrally by the facial nucleus and ventrally by the medullary subpial surface. We conclude that visceral reflex arcs are reciprocally organized. Targets of NTS projection are also sources of local NTS-X afferent innervation. Catecholaminergic and other local afferents from reticular formation, periventricular, and spinal gray may, via collaterals, simultaneously modulate visceral reflex excitability at the level of NTS and the outflow of autonomic and respiratory motoneurons.

Activation of N-methyl-d-aspartate Receptors Induces Endogenous Rhythmic Bursting Activities in Nucleus Tractus Solitarii Neurons: An Intracellular Study on Adult Rat Brainstem Slices.

A brainstem slice preparation and intracellular recording techniques were used to examine the effects of N-methyl-d-aspartate (NMDA) application on neurons within the swallowing area of the nucleus tractus solitarii (NTS). According to their cellular properties, NTS neurons were classified into type I and type II neurons. The most striking difference was the occurrence of delayed excitation in type I but not in type II neurons, when they were depolarized from membrane potentials more negative than -60 mV. Bath application of NMDA (30 - 60 microM) elicited depolarization and triggered stable repetitive firing in all the NTS neurons but one. During the NMDA-induced depolarization, hyperpolarization below -60 mV elicited, in some type I neurons, a rhythmic bursting pattern. The duration of the bursts (300 - 1000 ms) and their frequency (0.5 - 2 Hz) depended on the membrane potential. With hyperpolarizations below -75 mV, rhythmic bursting was converted into rhythmic single discharges, a pattern elicited directly in the other type I neurons. In all cases, rhythmic patterns were superimposed on cyclic depolarizations of the membrane potential characterized by an initial ramp-shaped phase. In type II neurons, rhythmic bursting discharges, superimposed on rhythmic oscillations of the membrane potential, were also obtained upon hyperpolarization during the NMDA-induced depolarization. In all type I neurons tested, NMDA-induced cyclic ramp-shaped depolarizations continued after addition of tetrodotoxin to the medium. Rhythmic bursting was not elicited by bath application of kainate (10 - 20 microM). Application of d-2-amino-5-phosphonovalerate (50 microM) blocked NMDA-induced depolarizations without modifying those elicited by kainate, which were selectively depressed by 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM). Moreover, removal of Mg2+ from the medium suppressed NMDA-induced cyclic depolarizations. Results demonstrate that both NMDA and non-NMDA receptors are present in NTS neurons and that selective activation of NMDA receptors induced rhythmic bursting and/or rhythmic single discharges. Rhythmic patterns were not driven by synaptic mechanisms but originated from endogenous properties of NTS neurons activated by NMDA. Thus, NTS neurons can be considered as conditional pacemakers. According to the location of the neurons, the conditional properties shown in these in vitro experiments might be involved in vivo in the generation of rhythmic motor activities set up at the NTS level, such as swallowing.

Altered Cardiovascular Responses to Glutamate and Acetylcholine Microinjected into the Nucleus Tractus Solitarii of the SHR

L-glutamate (GLU) or acetylcholine (ACh) injected into the nucleus tractus solitarii (NTS) of rat elicits a decrease in arterial pressure (AP) and heart rate (HR) similar to that seen with activation of the baroreflex. Both compounds may play a role in the mediation of the baroreflex at the level of NTS. The baroreflex may be disturbed in the spontaneously hypertensive rat (SHR). Thus we sought to determine if the dose-related responses to microinjection (50 nl) of the two putative transmitters into NTS may be altered in SHR vs Wistar Kyoto (WKY) controls. Both GLU and ACh produced significant dose-related decreases in AP and HR in SHR and WKY. The dose-related changes in AP elicited by GLU were shifted significantly to the right in SHR compared to WKY. Responses of AP were only different after intermediate doses of ACh. The dose-related HR responses to GLU were the same in SHR and WKY. However, the HR response to ACh was significantly reduced in the SHR. These data suggest that cardiovascular regulation in the NTS through GLU and ACh mechanisms may be disturbed in the SHR.

Rhythmic bursting patterns induced in neurons of the rat nucleus tractus solitarii, in vitro, in response to N-methyl- d-aspartate

The activity of nucleus tractus solitarii (NTS) neurons was recorded extracellularly on rat brainstem slices. Depending on the neuron, bath application of N-methyl- d-aspartate (NMDA; 30–120 μM) elicited either a pattern of rhythmic bursting or repetitive firing. The rhythmic bursting pattern was characterized by trains of action potentials occurring at a rate of 0.36-2 Hz. With most of the neurons, the mean burst duration and the mean discharge frequency ranged between 200 and 800 ms and between 20 and 40 Hz, respectively. Both the repetitive and the rhythmic bursting patterns were reversibly blocked when dl-2-amino-5-phosphonovalerate (80 μM) was applied. Application of quisqualate (30–60 μM) or glutamate (300–1200 μM) on NTS neurons induced only repetitive firing even in neurons exhibiting a rhythmic bursting pattern under NMDA. The present findings show that rhythmic bursting patterns can be generated within the isolated NTS under activation of NMDA receptors. The rhythmic bursting resulted probably from local NTS mechanisms (synaptic interactions or neuronal intrinsic properties) which might be involved in physiological rhythmic activities organized at the NTS level, such as swallowing.