Commissural Nucleus Tractus Solitarius Research Articles

Inhibitory modulation of chemoreflex bradycardia by stimulation of the nucleus raphe obscurus is mediated by 5-HT 3 receptors in the NTS of awake rats

Several studies demonstrated the involvement of 5-hydroxytryptamine (5-HT) and its different receptor subtypes in the modulation of neurotransmission of cardiovascular reflexes in the nucleus tractus solitarii (NTS). Moreover, anatomic evidence suggests that nucleus raphe obscurus (ROb) is a source of 5-HT-containing terminals within the NTS. In the present study we investigated the possible changes in the cardiovascular responses to peripheral chemoreceptor activation by potassium cyanide (KCN, i.v.) following ROb stimulation with l-glutamate (10 nmol/50 nL) and also whether 5-HT 3 receptors in the caudal commissural NTS are involved in this neuromodulation. The results showed that stimulation of the ROb with l-glutamate in awake rats ( n = 15) produced a significant reduction in the bradycardic response 30 s after the microinjection (− 182 ± 19 vs − 236 ± 10 bpm; Wilcoxon test) but no changes in the pressor response to peripheral chemoreceptor activation (43 ± 4 vs 51 ± 3 mmHg; two-way ANOVA) in relation to the control. Microinjection of 5--HT 3 receptors antagonist granisetron (500 pmol/50 nL), but not the vehicle, into the caudal commissural NTS bilaterally prevented the reduction of chemoreflex bradycardia in response to microinjection of l-glutamate into ROb. These data indicate that 5-HT-containing projections from ROb to the NTS play an inhibitory neuromodulatory role in the chemoreflex evoked bradycardia by releasing 5-HT and activating 5-HT 3 receptors in the caudal NTS.

Chemoreflex sympathoexcitation was not altered by the antagonism of glutamate receptors in the commissural nucleus tractus solitarii in the working heart–brainstem preparation of rats

The changes in thoracic sympathetic nerve activity, heart rate and frequency of phrenic nerve discharge in response to chemoreflex activation before and after bilateral microinjections of glutamate receptor antagonists into the comissural nucleus tractus solitarii (cNTS) were evaluated in the working heart-brainstem preparation of rats. Microinjections of kynurenic acid (KYN, 250 mM), (+/-)-alpha-methyl-4-carboxyphenylglycine (MCPG, 100 mM), or KYN plus MCPG into the cNTS were performed in three different groups. These microinjections into the cNTS did not affect the increase in the thoracic sympathetic nerve activity elicited by chemoreflex activation (KYN, 54 +/- 3 versus 51 +/- 2%, n = 11; MCPG, 48 +/- 5 versus 54 +/- 5%, n = 7; and KYN plus MCPG, 57 +/- 6 versus 55 +/- 3%, n = 5). The increase in the frequency of the phrenic nerve discharge in response to chemoreflex activation was also not affected by KYN (0.28 +/- 0.02 versus 0.30 +/- 0.04 Hz), MCPG (0.27 +/- 0.03 versus 0.27 +/- 0.04 Hz), or KYN plus MCPG (0.30 +/- 0.04 versus 0.20 +/- 0.03 Hz). The bradycardic response to chemoreflex activation was significantly reduced after microinjection of KYN at 2 (-220 +/- 16 versus -50 +/- 6 beats min(-1)) and 10 min (-220 +/- 16 versus -65 +/- 9 beats min(-1)) and after microinjection of KYN plus MCPG into the NTS it was abolished at 2 (-192 +/- 14 versus -2 +/- 1 beats min(-1)) and 10 min (-192 +/- 14 versus -4 +/- 2 beats min(-1)). These data support the hypothesis that the neurotransmission of the sympathoexcitatory and respiratory components of the chemoreflex in the cNTS involves neurotransmitters other than L-glutamate and also the concept that the parasympathetic component of this reflex is mediated by L-glutamate.

Chemoreflex sympatho‐excitation in the working heart‐brainstem preparation (WHBP) of rat was not affected by the antagonism of glutamate receptors in the commissural nucleus tractus solitarius (NTS).

The changes in thoracic sympathetic nerve activity (tSNA), heart rate (HR) and frequency of phrenic nerve discharge (PND) in response to chemoreflex activation before and after bilateral microinjections of glutamate receptor antagonists into the NTS were evaluated in the WHBP. Microinjections of kynurenic acid (KYN, 250 mM), MCPG (100 mM) or KYN plus MCPG into the NTS were performed in 3 different groups. Microinjections of KYN (54±3 vs 51±2 %, n=11), MCPG (48±5 vs 54±5 %, n=7) or KYN plus MCPG (57±6 vs 55±3 %, n=5) into the NTS did not affect the increase in the tSNA elicited by chemoreflex activation. The increase in the frequency of the PND in response to chemoreflex activation was also not affected by microinjections of glutamate receptor antagonists: KYN (0.68±0.03 vs 0.76±0.05 Hz), MCPG (0.60±0.03 vs 0.61±0.03 Hz) and KYN plus MCPG (0.62±0.03 vs 0.62±0.02 Hz). The bradycardic response to chemoreflex activation was significantly reduced at 2 (−220±16 vs −50±6 bpm) and 10 min (−220±16 vs −65±9 bpm) after microinjection of KYN and was abolished at 2 (−192±14 vs −2±1 bpm) and 10 min (−192±14 vs −4±2 bpm) after microinjections of KYN plus MCPG into the NTS. These data support the hypothesis that the neurotransmission of the sympatho-excitatory and respiratory components of the chemoreflex in the NTS involves neurotransmitters other than L-glutamate and also the concept that the parasympathetic component of this reflex is mediated by L-glutamate. Supported by CNPq and FAPESP.

Functional and morphological organization of the nucleus tractus solitarius in the fictive cough reflex of guinea pigs

Projection of the superior laryngeal nerve (SLN) afferent fibers into the nucleus tractus solitarius (NTS) was investigated using a fluorescent tracer in guinea pigs. High density of fluorescence was detected in the ipsilateral NTS extending from 0.5 mm caudal to 1.2 mm rostral to the obex. At coronal slices, the fluorescent granules, lines and patches were located in the interstitial, medial and dorsal regions of NTS. Fluorescence was also found in the dorsal region of contralateral commissural NTS. Microstimulation of the rostral NTS, which corresponded to the region showing the strong fluorescence, induced an increase in the inspiratory discharge of phrenic nerve that was immediately followed by a large burst discharge of the iliohypogastric nerve in decerebrate, paralyzed and artificially ventilated guinea pigs. This serial response of the two nerves was identical to that induced by electrical stimulation of the SLN. Intravenous injection of codeine suppressed both NTS and SLN-induced responses. The SLN-induced response was inhibited by microinjection of codeine into the ipsilateral NTS and abolished by lesion of the ipsilateral NTS. These results suggest that the NTS has an integrative function in production of cough reflex and is possible sites of action of central antitussive agents.

Hypoxic activation of arterial chemoreceptors inhibits sympathetic outflow to brown adipose tissue in rats

In urethane-chloralose anaesthetized, neuromuscularly blocked, artificially ventilated rats, we demonstrated that activation of carotid chemoreceptors inhibits the elevated levels of brown adipose tissue (BAT) sympathetic nerve activity (SNA) evoked by hypothermia, by microinjection of prostaglandin E2 into the medial preoptic area or by disinhibition of neurones in the raphe pallidus area (RPa). Peripheral chemoreceptor stimulation with systemic administration of NaCN (50 microg in 0.1 ml) or with hypoxic ventilation (8% O2-92% N2, 30 s) completely inhibited BAT SNA. Arterial chemoreceptor-evoked inhibition of BAT SNA was eliminated by prior bilateral transections of the carotid sinus nerves or by prior inhibition of neurones within the commissural nucleus tractus solitarii (commNTS) with glycine (40 nmol/80 nl) or with the GABAA receptor agonist muscimol (160 pmol/80 nl; 77 +/- 10% attenuation), or by prior blockade of ionotropic excitatory amino acid receptors in the commNTS with kynurenate (8 nmol/80 nl; 82 +/- 10% attenuation). Furthermore, activation of commNTS neurones following local microinjection of bicuculline (30 pmol/60 nl) completely inhibited the elevated level of BAT SNA resulting from disinhibition of neurones in the RPa. These results demonstrate that hypoxic stimulation of arterial chemoreceptor afferents leads to an inhibition of BAT SNA and BAT thermogenesis through an EAA-mediated activation of second-order, arterial chemoreceptor neurones in the commNTS. Peripheral chemoreceptor-evoked inhibition of BAT SNA could directly contribute to (or be permissive for) the hypoxia-evoked reductions in body temperature and oxygen consumption that serve as an adaptive response to decreased oxygen availability.

Angiotensin II evokes hypotension and renal sympathoinhibition from a highly restricted region in the nucleus tractus solitarii

Microinjections of low doses (in the femtomolar or low picomolar range) of angiotensin II (Ang II) into the nucleus tractus solitarii (NTS) evoke depressor responses. In this study we have mapped in the rat the precise location of the subregion within the NTS at which Ang II evokes significant sympathoinhibitory and depressor responses. Microinjections of 1 pmol of Ang II evoked large decreases (≥20% of baseline) in renal sympathetic nerve activity (RSNA), from a highly restricted region in the medial NTS, at or very close to the level 0.2 mm caudal to the obex. Microinjections of the same dose of Ang II into the commissural or lateral NTS at the same rostrocaudal level, or into the medial and lateral NTS at the level of the obex evoked significantly smaller sympathoinhibitory responses, while microinjections into more rostral or caudal levels of the NTS evoked significant sympathoinhibitory responses even less frequently. In most cases (71%), the sympathoinhibitory responses were accompanied by depressor responses, the magnitudes of which were also greater within the medial NTS at the level 0.2 mm caudal to obex, as compared to the surrounding subregions. The results demonstrate that the cardiovascular effects of Ang II in the NTS are highly site-specific. Taken together with previous studies, the results also indicate that the neurons in the NTS that mediate the Ang II-evoked sympathoinhibition are a discrete subgroup of the population of sympathoinhibitory neurons within the nucleus.

Hemodynamic and respiratory responses to microinjection of ATP into the intermediate and caudal NTS of awake rats

The nucleus tractus solitarii (NTS) is the site of integration of the peripheral chemoreceptor afferents in the brainstem. Previous studies from our laboratory have shown that microinjection of ATP into the intermediate NTS produced increases in arterial pressure and bradycardia. In the present study, we evaluated the hemodynamic and respiratory responses to microinjection of ATP into the intermediate and caudal commissural NTS. In the same group of rats the responses were compared with cardiorespiratory responses to chemoreflex activation (KCN, i.v.). The data show that microinjection of ATP into the intermediate NTS produced pressor and bradycardic responses similar to those observed in response to chemoreflex activation but apnoea instead of tachypnoea. Microinjection of ATP into caudal commissural NTS produced increase in arterial pressure and tachypnoea similar to the chemoreflex but a minor bradycardia. The data show that microinjection of ATP into different sub-regions of the NTS produces a diverse pattern of hemodynamic and respiratory responses and suggest the involvement of this purine in the neurotransmission of the cardiovascular reflex in the NTS.

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.

Defining projections from the caudal pressor area of the caudal ventrolateral medulla

We previously defined a functional area in the caudal medulla oblongata that elicits an increase in arterial pressure when stimulated (Sun and Panneton [2002] Am. J. Physiol. 283:R768-R778). In the present study, anterograde and retrograde tracing techniques were used to investigate the projections of this caudal pressor area (CPA) to the medulla and pons. Injections of biotinylated dextran amine into the CPA resulted in numerous labeled fibers with varicosities in the ipsilateral subnucleus reticularis dorsalis, commissural subnucleus of the nucleus tractus solitarii, lateral medulla, medial facial nucleus, A5 area, lateral vestibular nucleus, and internal lateral subnucleus of the parabrachial complex. Sparser projections were found ipsilaterally in the pressor and depressor areas of the medulla and the spinal trigeminal nucleus and contralaterally in the CPA. Injections of the retrograde tracer Fluoro-Gold into these areas labeled neurons in the CPA as well as the nearby medullary dorsal horn and reticular formation. However, we conclude that the CPA projects preferentially to the subnucleus reticularis dorsalis, commissural nucleus tractus solitarii, lateral medulla, A5 area, and internal lateral parabrachial nucleus. Weaker projections were seen to the CVLM and RVLM and to the contralateral CPA. The projection to the facial nucleus arises from nearby reticular neurons, whereas projections to the vestibular nucleus arise from the lateral reticular nucleus. Labeled neurons in the CPA consisted mostly of small bipolar and some triangular neurons. The projection to the CVLM, or to A5 area, may provide for the increase in arterial pressure with CPA stimulation. However, most of the projections described herein are to nuclei implicated in the processing of noxious information. This implies a unique role for the CPA in somatoautonomic regulation.

Cardiovascular responses to microinjection of AMPA into the rostral commissural nucleus tractus solitarius of awake rats

Previous studies have shown that microinjection of NMDA or non-NMDA receptor agonists into the nucleus tractus solitarius of anesthetized rats produces bradycardia and hypotension. In the present study, we evaluated the autonomic components of the cardiovascular responses to the microinjection of α-amine-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) into the nucleus tractus solitarius of awake rats. AMPA (0.05 nmol 50 nl −1) was microinjected into the nucleus tractus solitarius before and at 2, 10, 30 and 180 min after intravenous injection of methylatropine (2 mg kg −1). The control response to the microinjection of AMPA into the lateral aspect of the commissural nucleus tractus solitarius consisted of bradycardia and hypotension, (−190±20 bpm and −49±7 mm Hg), which were effectively blocked at 2 min (−5±2 bpm and −4±5 mm Hg), 10 min (−3±2 bpm and −6±3 mm Hg) and 30 min (−9±4 bpm and −5±5 mm Hg) after methylatropine. Although the bradycardic and hypotensive responses remained significantly reduced 180 min after injection of methylatropine, the responses tended to normalize. The data show that microinjection of AMPA into the rostral commissural nucleus tractus solitarius of awake rats produced intense bradycardia essentially mediated by parasympathetic excitation, which was the cause of the concomitant hypotensive response.

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.

Expression of metabotropic glutamate receptor 8 in autonomic cell groups of the medulla oblongata of the rat

Metabotropic glutamate receptors (mGluRs) in the medulla oblongata have been suggested to have a functional role in the regulation of cardiovascular baroreflexes. The present study examines the localization of mGluR8 autonomic nuclei of the medulla of the rat. mGluR8 immunoreactivity was observed in the cell bodies and/or processes of the dorsolateral, interstitial, medial, intermediate, ventral, ventrolateral, subpostremal, commissural, parvicellular and gelatinosus subnuclei of the nucleus tractus solitarius (NTS). The intensity of mGluR8 staining was highest in the commissural and interstitial subnuclei at the level of the area postrema. Commissural NTS is involved in regulation of baro-, and chemo-reflexes whereas the interstitial nucleus mediates respiratory reflexes. In the area postrema, diffuse staining was observed in the cell bodies, dendrites and fibers of the dorsal and central regions. In vagal outflow nuclei, mGluR8 immunoreactivity was observed in: (1) the cell bodies and processes of the dorsal motor nucleus of the vagus (DMN) throughout the rostro-caudal extent; and (2) the cell bodies and fibers throughout the rostro-caudal extent of the dorsal and ventral division the nucleus ambiguus (NA). Staining in the ventrolateral medulla was restricted to regions ventral to the nucleus ambiguus and dorsal to the lateral reticulate nucleus. The present study is the first to provide a detailed mapping of mGluR8 within the autonomic nucleii of the medulla and suggests that this subtype may be involved in shaping synaptic transmission in these central nuclei.

Sensory afferent selective role of P2 receptors in the nucleus tractus solitarii for mediating the cardiac component of the peripheral chemoreceptor reflex in rats.

We have assessed the functional role of type 2 purinergic (P2) receptors within the caudal aspect of the commissural nucleus tractus solitarii (NTS) in mediating the peripheral chemoreceptor reflex cardiorespiratory response in the arterially perfused in situ working heart-brainstem preparation of rats. Microinjection in NTS of either suramin (100 pmol) or pyrinoxalphosphate-6-azophenyl-2',4'-disulphonic acid tetrasodium salt (PPADS; 10 pmol) depressed the reflex bradycardia (by approximately 50 %), but not the tachypnoea, following peripheral chemoreceptor stimulation. In contrast, the reflex bradycardia produced by stimulation of pharyngo-oesophageal receptors was unaffected. Furthermore, microinjections in NTS of the P2X receptor agonist alpha,beta-methyleneadenosine 5'-triphosphate (10 pmol) evoked a bradycardia which was antagonized by suramin (100 pmol). This P2X agonist reversibly potentiated the peripheral chemoreceptor-evoked bradycardia. The effect of suramin was selective to purinergic receptors because the bradycardia evoked by microinjection of alpha,beta-methyleneadenosine 5'-triphosphate was blocked while the bradycardic responses to microinjections of NMDA or non-NMDA receptor agonists were not affected. From whole-cell recordings, some NTS neurones received convergent excitatory synaptic inputs from both peripheral chemoreceptors and receptors at the pharyngo-oesophageal junction. The excitatory postsynaptic response evoked by chemoreceptor stimulation was depressed by suramin, but convergent excitatory inputs from pharyngo-oesophageal receptors were unperturbed. Our findings support the hypothesis that caudal commissural NTS P2 purinergic receptors play a role in the neurotransmission of the parasympathetic (bradycardic) component of the chemoreceptor reflex. This effect is highly selective in that the chemoreceptor afferent-evoked tachypnoea, as well as other visceral receptor-mediated reflex bradycardia, remain unaffected.

On the immunohistochemical distribution of ionotropic P2X receptors in the nucleus tractus solitarius of the rat

ATP has been shown to excite neurones of the nucleus tractus solitarius (NTS) via the activation of P2X receptors. In the present study, the distribution of six P2X receptors (P2X 1–P2X 6) within the rat NTS was investigated by peroxidase immunohistochemistry. Immunopositive neurones for P2X receptor subtypes were detected in all divisions of the NTS, although the staining densities differed according to receptor subtype and sub-nuclei. P2X 1-immunopositive cells were distributed throughout the rostro-caudal extent of the NTS, while varicose fibres were mainly located along the postremal border. P2X 2 immunoreactivity was present in neurones and fibres located throughout the NTS. In the commissural NTS intense staining was observed medial of the solitary tract while in the sub-postremal NTS neurones were observed along the postremal border. A high density of P2X 3-positive neurones and fibres was observed in the sub-postremal NTS along the border of the area postrema and in the rostral NTS in the medial subdivision. In comparison to the staining observed with the other receptor antibodies, there was considerably reduced P2X 4 receptor immunoreactivity. P2X 4-positive neurones tended to be more sparsely distributed, and found mainly in the intermediate portion of the commissural NTS, and along the postremal border. In contrast, we observed dense staining for the P2X 5 receptor subtype in a majority of regions within the NTS. The most striking staining was observed in the intermediate subdivision at the level of the sub-postremal NTS and the medial portion of the rostral NTS. P2X 6 immunoreactive neurones were observed in the medial commissural NTS, along the postremal border and in the dorso-medial and medial subdivisions of the rostral NTS. Taken together, our findings confirm the presence of six P2X receptor subtypes within the NTS of the rat, consistent with a neurotransmitter role for ATP in the rat NTS. These results indicate the need for more extensive functional studies to elucidate the roles of the individual and heterodimeric assemblies of P2X receptor subtypes within the NTS.

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.

Role of the medulla oblongata in hypertension.

Brain pathways controlling arterial pressure are distributed throughout the neuraxis and are organized in topographically selective networks. In this brief review, we will focus on the medulla oblongata. The nucleus tractus solitarius (NTS) is the primary site of cardiorespiratory reflex integration. It is well accepted that lesions or other perturbations in the NTS can result in elevations of arterial pressure (AP), with many of the associated features so commonly found in humans. However, recent studies have shown 2 distinct subpopulations of neurons within the NTS that can influence AP in opposite ways. Commissural NTS neurons located on the midline may contribute to maintenance of hypertension in spontaneously hypertensive rats (SHR), because small lesions in this area result in a very significant reduction in AP. Also involved in this blood pressure regulation network are 2 distinct regions of the ventrolateral medulla: caudal (CVLM) and rostral (RVLM). Neurons in CVLM are thought to receive baroreceptor input and to relay rostrally to control the activity of the RVLM. Projections from CVLM to RVLM are inhibitory, and a lack of their activity may contribute to development of hypertension. The RVLM is critical to the tonic and reflexive regulation of AP. In different experimental models of hypertension, RVLM neurons receive significantly more excitatory inputs. This results in enhanced sympathetic neuronal activity, which is essential for the development and maintenance of the hypertension.

Neurons of nucleus of the solitary tract synchronize the EEG and elevate cerebral blood flow via a novel medullary area

In anesthetized spinalized rat, electrical stimulation of the nucleus tractus solitarius (NTS) synchronizes the EEG by increasing the power of 4–6-Hz waves (>100%; P<0.01), and elevates cerebral blood flow (rCBF) by 18±5% ( P<0.05). The coordinated response appears within seconds, is global, reversible, graded, evoked from the commissural sub-nucleus, and replicated by l-glutamate. The responses are markedly reduced by bilateral lesions or muscimol microinjections restricted to a region of ventral medullary reticular formation, the medullary cerebral vasodilator area (MCVA), a region from which stimulation elicits identical responses and mediates the comparable responses to hypoxic/ischemic excitation of sympathoexcitatory neurons of rostral ventrolateral medulla (RVLM). We conclude that: (a) excitation of intrinsic neurons of commissural NTS synchronizes the EEG and coordinately elevates rCBF; (b) the responses are mediated by excitation of neurons in MCVA; (c) the MCVA may be a common final pathway mediating cerebrovascular and EEG responses from multiple areas of CNS; and (d) the NTS–MCVA pathway may be a part of the anatomical substrate for behaviors, including slow-wave sleep and seizure suppression evoked by stimulation of visceral afferents terminating in NTS.

Microinjection of a 5-HT 3 receptor agonist into the NTS of awake rats inhibits the bradycardic response to activation of the von Bezold-Jarisch reflex

In the present study we investigated the effects of bilateral microinjection into the lateral commissural nucleus tractus solitarius (NTS) of 2-methyl-5-HT, a 5-HT 3 receptor agonist, on the bradycardic response of the von Bezold-Jarisch reflex of awake rats. We evaluated mainly the bradycardic response because in previous studies we documented that the hypotensive response of the von-Bezold-Jarisch reflex in awake rats is secondary to the intense bradycardic response. The Bezold-Jarisch reflex was activated by intravenous injection of serotonin (8 μg/kg) in awake rats before and 1, 3, 10, 20 and 60 min after bilateral microinjection of 2-methyl-5-HT (5 nmol/50 nl, n = 8) into the NTS. Microinjections of 2-methyl-5-HT into the NTS produced a significant increase in basal mean arterial pressure [(MAP), 97 ± 4 vs. 114 ± 4 mmHg), no changes in basal heart rate and a significant reduction in bradycardic (−78 ± 19; −94 ± 24 and −107 ± 21 bpm) and hypotensive (−16 ± 4; −10 ± 5 and −17 ± 4 mmHg) responses to activation of the von Bezold-Jarisch reflex at 3, 10 and 20 min, respectively, when compared with the control value (−231 ± 13 bpm and −43 ± 4 mmHg). The data of the present study suggest that serotonin acting on 5-HT 3 receptors in the NTS may play an important inhibitory neuromodulatory role in the bradycardic response to activation of the von Bezold-Jarisch reflex.

Blockade of NK-1 receptors in the lateral commissural nucleus tractus solitarii of awake rats had no effect on the cardiovascular responses to chemoreflex activation.

The neurotransmission of the chemoreflex in the nucleus tractus solitarii (NTS), particularly of the sympatho-excitatory component, is not completely understood. There is evidence that substance P may play a role in the neurotransmission of the chemoreflex in the NTS. Microinjection of substance P (50 pmol/50 nl, N = 12, and 5 nmol/50 nl, N = 8) into the commissural NTS of unanesthetized rats produced a significant increase in mean arterial pressure (101 +/- 1 vs 108 +/- 2 and 107 +/- 3 vs 115 +/- 4 mmHg, respectively) and no significant changes in heart rate (328 +/- 11 vs 347 +/- 15 and 332 +/- 7 vs 349 +/- 13 bpm, respectively) 2 min after microinjection. Previous treatment with WIN, an NK-1 receptor antagonist (2.5 nmol/50 nl), microinjected into the NTS of a specific group of rats, blocked the pressor (11 +/- 5 vs 1 +/- 2 mmHg) and tachycardic (31 +/- 6 vs 4 +/- 3 bpm) responses to substance P (50 pmol/50 nl, N = 5) observed 10 min after microinjection. Bilateral microinjection of WIN into the lateral commissural NTS (N = 8) had no significant effect on the pressor (50 +/- 4 vs 42 +/- 6 mmHg) or bradycardic (-230 +/- 16 vs -220 +/- 36 bpm) responses to chemoreflex activation with potassium cyanide (iv). These data indicate that the activation of NK-1 receptors by substance P in the NTS produces an increase in baseline mean arterial pressure and heart rate. However, the data obtained with WIN suggest that substance P and NK-1 receptors do not play a major role in the neurotransmission of the chemoreflex in the lateral commissural NTS.

Activation of GABA receptors in the NTS of awake rats reduces the gain of baroreflex bradycardia

In the present study we evaluated the effects of bilateral microinjection of muscimol (a GABA A receptor agonist) and baclofen (a GABA B receptor agonist) into the lateral commissural nucleus tractus solitarii (NTS) of awake rats on the gain of the baroreflex (BG) activated by a short duration (10–15 s) infusion of phenylephrine (Phe, 2.5 μg/0.05 ml, i.v.). Microinjection of muscimol (50 pmol/50 nl, n=8) into the NTS produced a significant increase in baseline mean arterial pressure ((MAP) 122±6 vs. 101±2 mmHg), no changes in baseline heart rate (HR) and a reduction in BG (−1.59±0.1 vs. −0.69±0.1 beats/mmHg). Microinjection of baclofen (6.25 pmol/50 nl, n=6) into the NTS also produced a significant increase in baseline MAP (138±5 vs. 103±2 mmHg), no changes in baseline HR and a reduction in BG (−1.54±0.3 vs. −0.53±0.2 beats/mmHg). Considering that the reduction in BG could be secondary to the increase in MAP in response to microinjection of muscimol ( n=6) or baclofen ( n=7) into the NTS, in these two groups of rats we brought the MAP back to baseline by infusion of sodium nitroprusside (NP, 3.0 μg/0.05 ml, i.v.). Under these conditions, we verified that the BG remained significantly reduced after muscimol (−1.49±0.2 vs. −0.35±0.2 beats/mmHg) and after baclofen (−1.72±0.2 vs. −0.33±0.2 beats/mmHg) when compared to control. Reflex tachycardia was observed during the normalization of MAP by NP infusion and, in order to prevent the autonomic imbalance from affecting BG, we used another group of rats treated with atenolol (5 mg/kg, i.v.), a β 1 receptor antagonist. In rats previously treated with atenolol and submitted to NP infusion, we verified that BG remained reduced after microinjection of muscimol or baclofen into the NTS. The data show that activation of GABA A and GABA B receptors, independently of the changes in the baseline MAP or HR, inhibited the neurons of the NTS involved in the parasympathetic component of the baroreflex.