Microinjection of L-glutamate into the nucleus tractus solitarii increases arterial pressure in conscious rats.

Microinjection of L-glutamate into the nucleus tractus solitarii (NTS) of anesthetized rats produces a fall in mean arterial pressure (MAP) similar to that observed during activation of baroreceptor afferents. In the present study we examined the effect of bilateral microinjections of L-glutamate through chronically implanted cannulae in the NTS of conscious freely moving rats. Group I (n = 6) was studied under conscious conditions and 24 h later the rats were anesthetized with urethane and the effects of L-glutamate re-examined. In conscious rats, L-glutamate (30 pmol to 5 nmol/100 nl) produced dose-dependent increases in MAP (+37 +/- 7 mmHg, 5 nmol), whereas under urethane anesthesia falls in MAP were observed (-11 +/- 3 mmHg, 5 nmol). Group II (n = 7) was studied under conscious conditions and 1 h later the rats were anesthetized with chloralose and the effects of L-glutamate re-examined. In this group of conscious rats L-glutamate (300 pmol to 5 nmol/100 nl) also produced dose-dependent increases in MAP (+37 +/- 5 mmHg, 5 nmol), whereas under chloralose anesthesia a dose-dependent depressor response was observed (-33 +/- 6 mmHg, 5 nmol). Saline microinjections into the NTS of conscious and anesthetized rats produced negligible effects. These data demonstrate that microinjection of L-glutamate into the NTS of rats produces a pressor response in conscious animals in contrast to depressor responses in animals anesthetized with chloralose or urethane.(ABSTRACT TRUNCATED AT 250 WORDS)     

Glycine, like glutamate, microinjected into the nucleus tractus solitarii of rat decreases arterial pressure and heart rate

Glycinergic mechanisms have been implicated in central cardiovascular regulation. However, the inhibitory amino acid's role in the nucleus tractus solitarii (NTS), the site of termination of cardiovascular afferents, has not been clarified. Thus, we sought to determine if the microinjection of glycine into the NTS alters arterial pressure and heart rate. Microinjections of glycine, like glutamate, confined to the NTS decreased arterial pressure and heart rate in a neurally mediated, dose-dependent manner. The glycine antagonist strychnine completely blocked these effects of glycine but did not itself alter arterial pressure or heart rate, or interfere with the baroreceptor reflex. The acute hypotensive, bradycardic response to glycine was followed by a period during which glycine essentially eliminated the cardiovascular responses to the microinjection into the NTS of glutamate, an amino acid reputed to be a transmitter in the baroreceptor reflex arc. These data suggest that glycine is involved in cardiovascular regulation by the NTS but do not support its being an integral transmitter in the baroreceptor reflex.     

Microinjection of orexin into the rat nucleus tractus solitarius causes increases in blood pressure

Orexin A (OX-A) and orexin B (OX-B), also known as hypocretin-1 and hypocretin-2, have been suggested to play a role cardiovascular control. The nucleus tractus solitarius (NTS), located in the dorsal medulla plays an essential role in neural control of the cardiovascular system. Orexin-immunoreactive axons have been demonstrated within this nucleus suggesting that NTS may be a site through which OX acts to influence cardiovascular control. We report here that microinjection of OX-A into the NTS of urethane anesthetized rats causes increases in blood pressure (10⁻⁹ M, mean AUC=607.1±65.65 mmHg s, n=5) and heart rate (10⁻⁹ M, mean AUC=16.15±3.3 beats, n=5) which returns to baseline within 90 s. We show that these effects are dose related and site specific. Microinjection of OX-B into NTS elicited similar increases in BP (mean AUC=680.8±128.5 mmHg s, n=4) to that of OX-A suggesting specific actions at the OX₂R receptor. These observations support the conclusion that orexins act as chemical messengers in the NTS likely influencing the excitability of cardiovascular neurons in this region and thus regulating global cardiovascular function.     

Injection of antisense oligos to nNOS into nucleus tractus solitarii increases blood pressure.

We investigated the cardiovascular effects of bilateral microinjection of antisense oligodeoxynucleotides (oligos) into the nucleus tractus solitarii (NTS) to neuronal nitric oxide synthase (nNOS) to suppress the expression of nNOS molecular biologically. In urethane-anesthetized, paralyzed Wistar-Kyoto rats, bilateral microinjection of nNOS antisense oligos (20 pmol in a 50nl volume) into the NTS produced a significant increase in mean arterial blood pressure at 30-60min after injection, compared with rats injected with nNOS sense or scrambled oligos. Immunohistochemical study demonstrated that nNOS immunoreactivity in the rat NTS was suppressed by nNOS antisense oligos. These results indicate that suppression of the nNOS gene using antisense in the NTS increases blood pressure.     

Chemical stimulation of the nucleus tractus solitarii decreases cerebral blood flow in anesthetized rats

In anesthetized (chloralose and urethane), paralyzed and artificially ventilated rats, the neurons in the nucleus tractus solitarius (NTS) were chemically stimulated by microinjections of L-glutamate and the cerebral blood flow (CBF) was determined using a combination of labeled microspheres (either 57Co, 113Sn and 46Sc or 141Ce, 85Sr and 46Sc). Unilateral chemical stimulation of the NTS (n = 14) decreased CBF significantly in most brain areas. The decrease in CBF was not due to the decrease in arterial blood pressure (ABP) because the CBF of the whole cerebral cortex during the chemical stimulation of the NTS was significantly smaller (P less than 0.05) than the CBF during controlled hemorrhagic hypotension (n = 10). In another group of rats (n = 6), moderate hypertension was induced by blood transfusion. Unilateral chemical stimulation of the NTS in these rats decreased ABP but it remained within normotensive range. A significant (P less than 0.05) decrease in CBF (from 62 +/- 28 (mean +/- S.D.) to 48 +/- 23 ml.min-1.(100 g)-1) and increase in cerebrovascular resistance (from 1.9 +/- 1.2 to 2.6 +/- 1.2 mm Hg per [ml.min-1.(100 g)-1]) was observed in the whole cerebral cortex of these rats. Chemical stimulation of the NTS did not affect the reactivity of the cerebral vessels to hypercapnea (n = 5). These results suggest that the cell bodies within the NTS may play a role in the control of cerebral circulation.     

Catecholamine-synthesizing neuronal projections to the nucleus tractus solitarii of the rat

The objective of the present study was to determine the location of the neurons that give rise to catecholamine-containing terminals in the nucleus tractus solitarii. This was done by injecting rhodamine-filled latex microspheres into the nucleus tractus solitarii of rats to retrogradely label neuronal cell bodies and by processing sections from the brains of these animals to determine if the labelled neurons were immunoreactive for the catecholamine-synthesizing enzymes, dopamine-beta-hydroxylase (DBH) and phenylethanolamine-N-methyl transferase (PNMT). Approximately 60% of the DBH-immunoreactive neurons that projected to the nucleus tractus solitarii belonged to the A1/C1 cell group, while an additional 20% belonged to the A5 cell group. Thus, these two ventrolateral rhombencephalic cell groups accounted for nearly 80% of the total number of rhodamine-bead-labelled DBH-immunoreactive neurons in this series of experiments. Only a small number of DBH-immunoreactive neurons of the A2/C2 cell group contained rhodamine-filled latex microspheres. Rarely, DBH-immunoreactive neurons in the locus coeruleus and the nucleus subcoeruleus were found to project to the nucleus tractus solitarii. The majority of the PNMT-immunoreactive neurons that projected to the nucleus tractus solitarii belonged to the C1 cell group. Only small numbers of PNMT-immunoreactive neurons of the C2 and C3 groups were found to contain rhodamine-filled latex microspheres. It is concluded that neurons in the ventrolateral medulla and pons, some of which presumably utilize norepinephrine and/or epinephrine as a transmitter, could regulate autonomic function via direct projections to the nucleus tractus solitarii.     

Cardiovascular effects of substance P and capsaicin microinjected into the nucleus tractus solitarii of the rat

This report deals with the effect of substance P (SP) and capsaicin on blood pressure and heart rate after administration into different sites of the nucleus tractus solitarii (NTS) of urethane-anesthetized rats. Microinjection of SP at 6 different coordinates throughout the NTS showed 3 sites where SP administration evoked changes in blood pressure and heart rate. The most sensitive sites where application of SP into the NTS evoked dose-dependent hypotension and bradycardia were at the level of the posterior tip of the area postrema (zero level) and at the level of the obex. Capsaicin evoked dose-dependent hypotension and bradycardia at the same sites. These results further support the possibility that SP may be a neurotransmitter or neuromodulator of baroreceptor afferents in the NTS.     

Cardiovascular effects of microinjections of angiotensin II into the nucleus tractus solitarii

The role of the angiotensin II system within the nucleus tractus solitarii (NTS) in central cardiovascular control was investigated by local microinjections of angiotensin II and the angiotensin II receptor antagonist saralasin. Microinjections of 1 ng angiotensin II into the NTS resulted in a monophasic depressor response (-7.3 +/- 1.7 mm Hg), while higher doses were characterized by a biphasic response, with an initial decrease followed by a subsequent increase in blood pressure (10 ng: -4.7 +/- 1.3/+7.9 +/- 1.1 and 100 ng: -2.4 +/- 1.2/+7.5 +/- 1.2 mm Hg). Heart rate decreased significantly following microinjections of 1 and 10 ng angiotensin II (-27 +/- 5.0 and -15 +/- 5.9 bpm), while with 100 ng angiotensin II there was no significant effect on heart rate. Prior i.v. administration of atropine (1 mg/kg) abolished the bradycardia, but did not significantly affect the blood pressure response. Microinjections of saralasin into the NTS elicited a dose-dependent pressor response (10 ng: 6.0 +/- 1.5 mm Hg; 100 ng: 16.8 +/- 3.4 mm Hg) and tachycardia (10 ng: 5 +/- 3.2 bpm; 100 ng: 17 +/- 4.4 bpm). Our data support the hypothesis that angiotensin II acts on specific receptors within the NTS to modulate peripheral cardiovascular responses. The cardiovascular effects elicited by microinjections of the peptide exhibit complicated dose-response relationships. The effects on heart rate, but not on blood pressure, appear to be mediated by parasympathetic activation.     

Hypotensive effect of des-acyl ghrelin at nucleus tractus solitarii of rat

Ghrelin is a gut-brain peptide and its endocrine activities are mediated by GH secretagogue receptor (GHSR)-1a. Des-acyl ghrelin does not activate GHSR-1a and is devoid of endocrine activities. While the microinjection of ghrelin into rat nucleus tractus solitarii (NTS) elicited hypotensive effects, this was not the case upon injection into GHSR-expressing rostral ventrolateral medulla or caudal ventrolateral medulla. To make clear the reason of the discrepancy between receptor distribution and neuronal responses, we examined the cardiovascular response of rats microinjected with des-acyl ghrelin into NTS. Intra-NTS injection of des-acyl ghrelin significantly reduced mean arterial pressure and heart rate. The hypotensive and bradycardic activity evoked by des-acyl ghrelin was not significantly different from that of native ghrelin. These results suggest that des-acyl ghrelin contribute to the regulation of cardiovascular control and that a receptor other than GHSR-1a exists in NTS.     

Glycine elicits release of acetylcholine from the nucleus tractus solitarii in rat

Previous studies have suggested that cardiovascular responses elicited by injection of glycine into the nucleus tractus solitarii (NTS) depend upon interactions between glycinergic and cholinergic neuronal elements in NTS. Release of acetylcholine in response to glycine is one such interaction that has been shown in slices of hippocampus and striatum. In this study we sought to test the hypothesis that glycine causes release of acetylcholine from neurotransmitter stores in NTS. We compared release from NTS with that from adjacent hypoglossal nucleus and from caudate nucleus. Release of radiolabeled acetylcholine was determinedin vitro after incubating NTS with [³H]choline. Exposure of NTS and caudate nucleus, but not hypoglossal nucleus, to glycine caused release of acetylcholine in a calcium-dependent manner that varied with concentration of glycine in the incubation medium. The maximally effective concentration (1 mM) of glycine elicited 136% increases over basal levels. Glycine did not elicit release of [³H]acetylcholine from tissue when calcium ion had been removed from the bath. Acetylcholine also was not released if tissue was incubated with either strychnine (10 μM) or hemicholinium-3 (1 mM) prior to exposure to glycine (1 mM). Thus, glycine, acting at strychnine-sensitive receptors in NTS, elicits release of acetylcholine from a portion of locally synthetized neurotransmitter stores.     

A survey of oral cavity afferents to the rat nucleus tractus solitarii

Visualization of myelinated fiber arrangements, cytoarchitecture, and projection fields of afferent fibers in tandem revealed input target selectivity in identified subdivisions of the nucleus tractus solitarii (NTS). The central fibers of the chorda tympani (CT), greater superficial petrosal nerve (GSP), and glossopharyngeal nerve (IX), three nerves that innervate taste buds in the oral cavity, prominently occupy the gustatory-sensitive rostrocentral subdivision. In addition, CT and IX innervate and overlap in the rostrolateral subdivision, which is primarily targeted by the lingual branch of the trigeminal nerve (LV). In the rostrocentral subdivision, compared with the CT terminal field, GSP appeared more rostral and medial, and IX was more dorsal and caudal. Whereas IX and LV filled the rostrolateral subdivision diffusely, CT projected only to the dorsal and medial portions. The intermediate lateral subdivision received input from IX and LV but not CT or GSP. In the caudal NTS, the ventrolateral subdivision received notable innervation from CT, GSP, and LV, but not IX. No caudal subnuclei medial to the solitary tract contained labeled afferent fibers. The data indicate selectivity of fiber populations within each nerve for functionally distinct subdivisions of the NTS, highlighting the possibility of equally distinct functions for CT in the rostrolateral NTS, and CT and GSP in the caudal NTS. Further, this provides a useful anatomical template to study the role of oral cavity afferents in the taste-responsive subdivision of the NTS as well as in subdivisions that regulate ingestion and other oromotor behaviors.     

Increased anti-apoptotic conditions in the nucleus tractus solitarii of spontaneously hypertensive rat

Since the nucleus tractus solitarii (NTS) is a pivotal region for regulating the set-point of arterial pressure, we propose here its role in the development of neurogenic hypertension. Given the findings of recent studies suggesting that the NTS of spontaneously hypertensive rats (SHR) exhibits a specific inflammatory state characterized by leukocyte accumulation within the NTS microvasculature, we hypothesized that gene expression levels of apoptotic factors are altered in the NTS of SHR compared to normotensive Wistar-Kyoto rats (WKY). To test this hypothesis, we used RT(2) Profiler PCR arrays targeting apoptosis-related factors. We found that gene expression of the death receptor Fas (tumor necrosis factor receptor superfamily, member 6) and the cysteine-aspartic acid protease caspase 12 were down-regulated in the NTS of both adult hypertensive and young pre-hypertensive SHR compared to age-matched WKY. On the other hand, an anti-apoptotic factor, neuronal apoptosis inhibitory protein, was highly increased in the NTS of SHR. These results suggest that the NTS of SHR exhibits an anti-apoptotic condition. Furthermore, this profile appears not to be secondary to hypertension. Whether this differential gene expression in the NTS contributes to the hypertensive state of the SHR via alteration of neuronal circuitry regulating cardiovascular autonomic activity awaits elucidation.     

Perinatal development of inhibitory synapses in the nucleus tractus solitarii of the rat

The nucleus tractus solitarii (NTS) plays a key role in the central control of the autonomic nervous system. In adult rats, both GABA and glycine are used as inhibitory neurotransmitter in the NTS. Using a quantitative morphological approach, we have investigated the perinatal development of inhibitory synapses in the NTS. The density of both inhibitory axon terminals and synapses increased from embryonic day 20 until the end of the second postnatal week (postnatal day 14). Before birth, only GABAergic axon terminals developed and their number increased during the first postnatal week. Mixed GABA/glycine axon terminals appeared at birth and their number increased during the first postnatal week. This suggests the development of a mixed GABA/glycine inhibition in parallel to pure GABA inhibition. However, whereas GABAergic axon terminals were distributed throughout the NTS, mixed GABA/glycine axon terminals were strictly located in the lateral part of the NTS. Established at birth, this specific topography remained in the adult rat. From birth, GABA_A receptors, glycine receptors and gephyrin were clustered in inhibitory synapses throughout the NTS, revealing a neurotransmitter–receptor mismatch within the medial part of the NTS. Together these results suggest that NTS inhibitory networks develop and mature until postnatal day 14. Developmental changes in NTS synaptic inhibition may play an important role in shaping neural network activity during a time of maturation of autonomic functions. The first two postnatal weeks could represent a critical period where the impact of the environment influences the physiological phenotypes of adult rats.     

Decrease in the expression of N-methyl-D-aspartate receptors in the nucleus tractus solitarii induces antinociception and increases blood pressure

N-methyl-D-aspartate receptors (NMDAR) have a role in cardiovascular control at the nucleus tractus solitarii (NTS), eliciting increases or decreases in blood pressure (BP), depending on the area injected with the agonists. In spite of the association between cardiovascular control and pain modulation, the effects of manipulating NMDAR in pain responses have never been evaluated. In this study, we decreased the expression of NMDAR in the NTS using gene transfer to target receptor subunits and evaluate long-term effects. Seven days after the injection of lentiviral vectors containing the NR1a subunit cDNA of NMDAR, in antisense orientation, into the intermediate NTS of Wistar rats, BP was measured, and the formalin test of nociception was performed. The antisense vector induced a decrease of NR1 expression in the NTS and elicited BP rises and hypoalgesia. Antisense vectors inhibited formalin-evoked c-Fos expression in the spinal cord, indicating decreased nociceptive activity of spinal neurons. Using a time-course approach, we verified that the onset of both the increases in BP and the hypoalgesia was at 4 days after vector injection into the NTS. The injection of NMDA into the NTS reversed the effects of antisense vectors in pain behavioral responses and spinal neuronal activation and decreased BP and heart rate. The present study shows that the NR1 subunit of the NMDAR at the NTS is critical in the regulation of tonic cardiovascular and nociceptive control and shows an involvement of the nucleus in the modulation of sustained pain.     

Serotonin- and substance P-containing projections to the nucleus tractus solitarii of the rat

The objective of the present study was to determine the location of the neurons that give rise to serotonin- and substance P-containing terminals in the nucleus tractus solitarii. This was done by injecting rhodamine-filled latex microspheres into the nucleus tractus solitarii of rats to retrogradely label neuronal cell bodies and by processing sections from the brains of these animals to determine whether the labelled neurons contained serotonin or substance P immunoreactivity. Serotonin-immunoreactive neurons that projected to the nucleus tractus solitarii were found in the nucleus raphe magnus, nucleus raphe obscurus, nucleus raphe pallidus, and in the ventral medulla, lateral to the pyramidal tract. Substance P-immunoreactive neurons that projected to the nucleus tractus solitarii were found in similar areas but were proportionately less numerous in the nucleus raphe magnus and proportionately more numerous in the nucleus raphe pallidus. It is concluded that neurons in the medullary raphe nuclei, some of which presumably utilize serotonin or substance P as a neurotransmitter, could regulate autonomic function via direct projections to the nucleus tractus solitarii.     

Intracellular study of nucleus parabrachialis and nucleus tractus solitarii interconnections

Responses of the nucleus parabrachialis (PBN) neurons to electrical stimulation of the nucleus tractus solitarii (NTS) were investigated by intracellular recording technique in anesthetized rats. Excitatory postsynaptic potentials (EPSPs) were evoked by ipsilateral NTS stimulation in 38 PBN neurons. They were considered monosynaptic because their latencies did not change with either variations in stimulus intensity or high-frequency repetitive stimulation. The latencies of EPSPs ranged from 1.2 to 6.9 ms. PBN neurons were also antidromically activated by NTS stimulation, giving a mean axonal conduction velocity of 4.6 m/s. Some of these neurons also responded with monosynaptic EPSPs to NTS stimulation. Direct stimulation of these neurons by depolarizing current pulses elicited repetitive firing with frequencies up to 350 Hz. The morphological analysis of 5 PBN neurons labeled with horseradish peroxidase (HRP) indicates that the soma were fusiform in shape, and the size varied from 163 to 783 microns 2. All neurons had 3-5 spiny primary dendrites which extended in a predominantly mediolateral direction. Axons arose from a proximal dendritic trunk, close to the soma. The results indicated that PBN is reciprocally connected with the NTS which elicits an excitatory effect on PBN neurons.     

Direct catecholaminergic projection from nucleus tractus solitarii to supraoptic nucleus

To determine whether the supraoptic nucleus (SON) receives a direct projection from catecholamine cells of the nucleus tractus solitarii (NTS), retrograde transport of rhodamine-tagged latex microspheres was combined with a procedure for the fluorescence histochemical visualization of catecholamines. SON tracer injections, made via transpharyngeal approach, retrogradely labelled cells at all levels of NTS, although the majority were located caudal to obex with an ipsilateral predominance. Approximately half of these cells were also identified as catecholaminergic; the relatively caudal level in the dorsomedial medulla of most of these cells suggests that they probably correspond to the A2 catecholamine cell group.     

Cardiovascular effects produced by bradykinin microinjection into the nucleus tractus solitarii of anesthetized rats

In this study, we characterized the cardiovascular effects produced by microinjection of doses in the femtomole range of bradykinin (BK) into the nucleus tractus solitarii of male Wistar rats (230–280 g, n = 120) anesthetized with urethane (1.2 g/kg, i.p.). Microinjections of BK (1, 10, 100 fmol, and 1 and 10 pmol, in 50 nl) or vehicle (NaCl, 0.9%) were made by using a triple-barreled glass micropipette into the medial nTS (0.4 mm anterior, 0.3 mm lateral to the obex and 0.3 mm deep from the dorsal surface). Microinjection of BK produced a shallow dose-dependent decrease in mean arterial pressure and heart rate reaching −18 ± 6 mmHg and −21 ± 5 beats/min, with the dose of 10 pmol. The peripheral mechanism of these effects, tested in animals treated with methylatropine (2 mg/kg, i.V.), or propranolol (2 mg/kg, i.v.) or prazosin (30 μg/kg, i.v.), was shown to be mainly dependent on an increase in vagal efferent activity for bradycardia and a decrease in sympathetic activity for hypotension. In order to investigate the receptor subtype involved in these effects, BK was microinjected into the nTS before and after the injection of the B₁ receptor antagonist, Des-Arg⁹-Leu⁸-BK (DALBK) (11.5 pmol) or before and after the B₂ receptor antagonist, HOE-140 (7.7 pmol). The cardiovascular effects of BK were significantly attenuated by the microinjection of HOE-140 and DALBK into the nTS. The effect of BK microinjected into the nTS on the baroreflex modulation was also investigated. While BK produced a significant facilitation of the baroreflex, HOE-140 and DALBK produced a significant attenuation of the baroreceptor control of heart rate. Taken together, the data presented in this study indicate the nTS as a site, in the central nervous system, for the modulatory effect of BK on the central cardiovascular control.     

Projections from the nucleus tractus solitarii to the spinal cord

Projections from the nucleus tractus solitarii (NTS) to the spinal cord were demonstrated in the male Sprague-Dawley rat. In retrograde transport studies, a horseradish peroxidase conjugate or a fluorescent dye, Fluoro Gold, were injected into midcervical or upper thoracic spinal segments. Most solitariospinal neurons were multipolar or bipolar and located between the obex and spinomedullary junction. Solitariospinal neurons were concentrated in proximity to the ventral border of the solitary tract and extended dorsally into the intermediate division and ventrolaterally into the intermediate reticular zone (IRt) of the lateral tegmental field. This subgroup predominantly projects to midcervical spinal segments. A subset of small neurons was retrogradely labeled from cervical or thoracic spinal segments in the medial commissural nucleus and contiguous with a periventricular group surrounding the central canal. In anterograde transport studies, iontophoretic deposits of Phaseolus vulgaris leucoagglutinin were centered stereotaxically on sites in NTS identified by retrograde transport data. The lectin was incorporated by neurons of the solitary complex and transported bilaterally by axons that emerged from the nucleus and entered the reticular formation. The solitario-reticular (transtegmental) pathway irradiated diagonally across the IRt and extended caudally into the cervical lateral funiculus and spinal gray. A small periventricular-spinal pathway also descended longitudinally to the neuraxis. Solitariospinal neurons project to superficial lamina of the dorsal horn, laminae VII and X and ventral horn. The projections are predominantly contralateral to phrenic and intercostal motor nuclei and ipsilateral to the intermediolateral cell column. The solitariospinal projection represents the shortest route in the central nervous system, other than the local intraspinal reflex, through which first order visceral afferents signal cardiorespiratory and alimentary motor nuclei. © 1993 Wiley-Liss, Inc.     

Baroreflex actions of substance P microinjected into the nucleus tractus solitarii in rat: a consequence of local distortion

Biologically active substance P (SP) (1000 ng in 0.1 μl saline) microinjected into the nucleus tractus solitarii (NTS) of 25 rats did not affect arterial pressure, heart rate, or the baroreceptor reflex. However, microinjection of saline alone in volumes greater than 0.3 μl consistently elicited hypotension and bradycardia followed occasionally by transient hypertension. These data suggest that previously reported cardiovascular effects of SP microinjected into the NTS resulted from local distortion.