Content and in vitro release of endogenous amino acids in the area of the nucleus of the solitary tract of the rat

Co-localisation of markers for glycinergic and GABAergic neurones in rat nucleus of the solitary tract: implications for co-transmission

Immunoreactive structures visualised with antibodies to glycine were prominent in areas of the nucleus of the solitary tract (NTS) surrounding the tractus solitarius, but scarcer in medial and ventral areas of the nucleus. This contrasted with a higher density, more homogenous distribution of structures labelled for gamma-aminobutyric acid (GABA). Immunolabelling of adjacent semi-thin sections nonetheless indicated a close correspondence between cells and puncta labelled by glycine and GABA antisera in certain NTS areas. With post-embedding electron microscopic immunolabelling, synaptic terminals with high, presumed transmitter levels of glycine were discriminated from terminals containing low, metabolic levels by quantitative analysis of gold particle labelling densities. In a random sample of terminals, 28.5% qualified on this basis as glycinergic (compared to 44.4% GABAergic); these glycinergic terminals targeted mainly dendritic structures and contained pleomorphic vesicles and symmetrical synapses. Serial section analysis revealed few terminals (5.2%) immunoreactive for glycine alone, with 82% of glycinergic terminals also containing high levels of GABA immunoreactivity. No evidence for co-localisation of glycine and glutamate was found. Light, confocal and electron microscopic labelling with antibodies to proteins specific for glycine and GABA synthesis, release and uptake confirmed that glycinergic terminals also containing GABA are found predominantly in more lateral areas of NTS, despite glycine receptors and the 'glial' glycine transporter (GLYT1) being expressed throughout all areas of the nucleus. The data suggest that synaptic terminals in certain functionally distinct areas of NTS co-release both inhibitory amino acids, which may account for the previously reported differential inhibitory effects of glycine and GABA on NTS neurones.

Glutamatergic neurons say NO in the nucleus tractus solitarii

Both glutamate and nitric oxide (NO) may play an important role in cardiovascular reflex and respiratory signal transmission in the nucleus tractus solitarii (NTS). Pharmacological and physiological data have shown that glutamate and NO may be linked in mediating cardiovascular regulation by the NTS. Through tract tracing, multiple-label immunofluorescent staining, confocal microscopic, and electronic microscopic methods, we and other investigators have provided anatomical evidence that supports a role for glutamate and NO as well as an interaction between glutamate and NO in cardiovascular regulation in the NTS. This review article focuses on summarizing and discussing these anatomical findings. We utilized antibodies to markers of glutamatergic neurons and to neuronal NO synthase (nNOS), the enzyme that synthesizes NO in NTS neurons, to study the anatomical relationship between glutamate and NO in rats. Not only were glutamatergic markers and nNOS both found in similar subregions of the NTS and in vagal afferents, they were also frequently colocalized in the same neurons and fibers in the NTS. In addition, glutamatergic markers and nNOS were often present in fibers that were in close apposition to each other. Furthermore, N-methyl-d-aspartate (NMDA) type glutamate receptors and nNOS were often found on the same NTS neurons. Similarly, alpha-amino-3-hydroxy-5-methylisoxozole-proprionic acid (AMPA) type glutamate receptors also frequently colocalized with nNOS in NTS neurons. These findings support the suggestion that the interaction between glutamate and NO may be mediated both through NMDA and AMPA receptors. Finally, by applying tracer to the cut aortic depressor nerve (ADN) to identify nodose ganglion (NG) neurons that transmit cardiovascular signals to the NTS, we observed colocalization of vesicular glutamate transporters (VGluT) and nNOS in the ADN neurons. Thus, taken together, these neuroanatomical data support the hypothesis that glutamate and NO may interact with each other to regulate cardiovascular and likely other visceral functions through the NTS.

Nitric oxide is involved in taurine release in the mouse brain stem under normal and ischemic conditions

Nitric oxide (NO) has been shown to regulate neurotransmitter release in the brain; both inhibitory and excitatory effects have been seen. Taurine is essential for the development and survival of neural cells and protects them under cell-damaging conditions. In the brain stem, it regulates many vital functions such as cardiovascular control and arterial blood pressure. Now we studied the effects of the NO-generating compounds hydroxylamine (HA), S-nitroso-N-acetylpenicillamine (SNAP) and sodium nitroprusside (SNP) on the release of preloaded [(3)H]taurine under normal and ischemic conditions in slices prepared from the mouse brain stem from developing (7-day-old) to young adult (3-month-old) mice. In general, the effects of NO on the release were somewhat complex and difficult to explain, as expected from the multifunctional role of NO in the central nervous system. The basal initial release under normal conditions was enhanced by the NO donors 5 mM HA and 1.0 mM SNAP at both ages, but SNP was inhibitory in developing mice. The release was markedly enhanced by K(+) stimulation. The effects of HA, SNAP and SNP on the basal release were not antagonized by the NO synthase inhibitor N(G)-nitro-L-arginine (L-NNA, 1.0 mM), demonstrating that mechanisms other than NO synthesis are involved. Taurine release in developing mice in the presence of SNP was reduced by the inhibitor of soluble guanylate cyclase, 1H-(1,2,3)oxadiazolo(4,3-a)quinoxalin-1-one (ODQ), indicating the possible involvement of cGMP. In normoxia, N-methyl-D-aspartate (NMDA, 1.0 mM) enhanced the SNAP- and HA-evoked taurine release in developing mice and the HA-evoked release in adults. In ischemia, both K(+) stimulation and NMDA potentiated the NO-induced release, particularly in the immature mice, probably without the involvement of the NO synthase or cGMP. The substantial release of taurine in the developing brain stem evoked by NO donors together with NMDA might represent signs of important mechanisms against excitotoxicity which protect the brain stem under cell-damaging conditions.

Activation of nucleus tractus solitarius 5-HT2A but not other 5-HT2 receptor subtypes inhibits the sympathetic activity in rats

Our first aim was to elucidate the mechanisms underlying the hypotensive response elicited by 5-HT(2) receptor activation in the nucleus tractus solitarius (NTS). In pentobarbitone-anaesthetized rats, intra-NTS administration of 2,5-dimethoxy-4-iodoamphetamine (DOI), a wide spectrum 5-HT(2) receptor agonist, but not an antagonist of selective 5-HT(2B) and 5-HT(2C) receptors, produced a decrease in blood pressure and heart rate. The maximal cardiovascular changes obtained by DOI (0.5 pmol) could be almost completely abolished by prior intra-NTS microinjection (10 pmol) of MDL-100907, a selective 5-HT(2A) receptor antagonist, but not by 5-HT(2B) or 5-HT(2C) receptor antagonists. In addition, using extracellular recordings we found that the large majority of identified cardiovascular rostroventrolateral medulla (RVLM) neurons were almost totally inhibited by NTS 5-HT(2A) receptor stimulation. We then investigated whether intra-NTS administration of a subthreshold dose (0.05 pmol) of DOI, known to facilitate the cardiovagal component of the baroreflex, could also modulate the sympathoinhibitory component of this reflex. These experiments showed that neither the decrease in the activity of the cardiovascular RVLM neurons and lumbar sympathetic nerve activities produced by aortic occlusion (gain of the baroreflex), nor the hypotensive response elicited by aortic nerve stimulation, were potentiated by the microinjection of DOI under such conditions. These data show that activation of 5-HT(2A), but not 5-HT(2B) or 5-HT(2C), receptors, located on NTS neurons, elicits depressor and bradycardic responses, and that this 5-HT(2A)-mediated hypotension is produced via the inhibition of RVLM cardiovascular neurons. In addition, NTS 5-HT(2A) receptor activation facilitates the cardiac but not the sympathetic baroreflex response.

Central and peripheral mechanisms of narcotic antitussives: codeine-sensitive and -resistant coughs

Narcotic antitussives such as codeine reveal the antitussive effect primarily via the mu-opioid receptor in the central nervous system (CNS). The kappa-opioid receptor also seems to contribute partly to the production of the antitussive effect of the drugs. There is controversy as to whether delta-receptors are involved in promoting an antitussive effect. Peripheral opioid receptors seem to have certain limited roles. Although narcotic antitussives are the most potent antitussives at present, certain types of coughs, such as chronic cough, are particularly difficult to suppress even with codeine. In guinea pigs, coughs elicited by mechanical stimulation of the bifurcation of the trachea were not able to be suppressed by codeine. In gupigs with sub-acute bronchitis caused by SO2 gas exposure, coughing is difficult to inhibit with centrally acting antitussives such as codeine. Some studies suggest that neurokinins are involved in the development of codeine-resistant coughs. However, evidence supporting this claim is still insufficient. It is very important to characterize opiate-resistant coughs in experimental animals, and to determine which experimentally induced coughs correspond to which types of cough in humans. In this review, we describe the mechanisms of antitussive effects of narcotic antitussives, addressing codeine-sensitive and -resistant coughs, and including our own results.

GABAergic mechanisms of the lateral parabrachial nucleus on sodium appetite

GABAergic activation in the lateral parabrachial nucleus (LPBN) induces sodium and water intake in satiated and normovolemic rats. In the present study we investigated the effects of GABAA receptor activation in the LPBN on 0.3M NaCl, water, 2% sucrose and food intake in rats submitted to sodium depletion (treatment with the diuretic furosemide subcutaneously+sodium deficient food for 24h), 24h food deprivation or 24 h water deprivation. Male Holtzman rats with bilateral stainless steel cannulas implanted into the LPBN were used. In sodium depleted rats, muscimol (GABAA receptor agonist, 0.5 nmol/0.2 microl), bilaterally injected into the LPBN, produced an inconsistent increase of water intake and two opposite effects on 0.3M NaCl intake: an early inhibition (4.3+/-2.7 versus saline: 14.4+/-1.0 ml/15 min) and a late facilitation (37.6+/-2.7 versus saline: 21.1+/-0.9 ml/180 min). The pretreatment of the LPBN with bicuculline (GABAA receptor antagonist, 1.6 nmol) abolished these effects of muscimol. Muscimol into the LPBN also reduced food deprivation-induced food intake in the first 30 min of test (1.7+/-0.6g versus saline: 4.1+/-0.6g), without changing water deprivation-induced water intake or 2% sucrose intake in sodium depleted rats. Therefore, although GABAA receptors in the LPBN are not tonically involved in the control of sodium depletion-induced sodium intake, GABAA receptor activation in the LPBN produces an early inhibition and a late facilitation of sodium depletion-induced sodium intake. GABAA activation in the LPBN also inhibits food intake, while it consistently increases only sodium intake and not water, food or sucrose intake.

Taurine release in mouse brain stem slices under cell-damaging conditions

Taurine has been thought to be essential for the development and survival of neural cells and to protect them under cell-damaging conditions. In the brain stem taurine regulates many vital functions, including cardiovascular control and arterial blood pressure. We have recently characterized the release of taurine in the adult and developing brain stem under normal conditions. Now we studied the properties of preloaded [3H]taurine release under various cell-damaging conditions (hypoxia, hypoglycemia, ischemia, the presence of metabolic poisons and free radicals) in slices prepared from the mouse brain stem from developing (7-day-old) and young adult (3-month-old) mice, using a superfusion system. Taurine release was greatly enhanced under these cell-damaging conditions, the only exception being the presence of free radicals in both age groups. The ischemia-induced release was characterized to consist of both Ca2+-dependent and -independent components. Moreover, the release was mediated by Na+-, Cl--dependent transporters operating outwards, particularly in the immature brain stem. Cl- channel antagonists reduced the release at both ages, indicating that a part of the release occurs through ion channels, and protein kinase C appeared to be involved. The release was also modulated by cyclic GMP second messenger systems, since inhibitors of soluble guanylyl cyclase and phosphodiesterases suppressed ischemic taurine release. The inhibition of phospholipases also reduced taurine release at both ages. This ischemia-induced taurine release could constitute an important mechanism against excitotoxicity, protecting the brain stem under cell-damaging conditions.

Vesicular glutamate transporters and neuronal nitric oxide synthase colocalize in aortic depressor afferent neurons

The aortic depressor nerve (ADN) primarily transmits baroreceptor signals from the aortic arch to the nucleus tractus solitarii. Cell bodies of neurons that send peripheral fibers to form the ADN are located in the nodose ganglion (NG). Studies have implicated glutamate and nitric oxide in transmission of baroreflex signals; therefore, we tested the hypothesis that ADN neurons contain either vesicular glutamate transporters (VGLUTs) or neuronal nitric oxide synthase (nNOS) or both. We applied a fluorescent tracer, tetramethyl rhodamine dextran (TRD), to rat ADN to identify ADN neurons and then performed immunofluorescent labeling for nNOS and VGLUTs 1, 2, and 3 in NG sections. We found that VGLUT2-immunoreactivity (IR) and VGLUT3-IR was present in a significantly higher proportion of TRD positive neurons than in TRD negative neurons. In contrast, the percentage of TRD positive neurons containing VGLUT1-IR or nNOS-IR did not differ from that of TRD negative neurons. We also observed that the percentage of TRD positive neurons containing both VGLUT2-IR and nNOS-IR and the percentage of TRD positive neurons containing both VGLUT3-IR and nNOS-IR were significantly higher than that of TRD negative neurons. On the other hand, colocalization of VGLUT1-IR and nNOS-IR in TRD positive neurons did not differ from that of TRD negative neurons. These results support our hypothesis and suggest prominent roles of VGLUT2-IR containing neurons and VGLUT3-IR containing neurons in transmitting cardiovascular signals via the ADN to the brain stem.

Characteristics of taurine release in slices from adult and developing mouse brain stem

Taurine has been thought to function as a regulator of neuronal activity, neuromodulator and osmoregulator. Moreover, it is essential for the development and survival of neural cells and protects them under cell-damaging conditions. Taurine is also involved in many vital functions regulated by the brain stem, including cardiovascular control and arterial blood pressure. The release of taurine has been studied both in vivo and in vitro in higher brain areas, whereas the mechanisms of release have not been systematically characterized in the brain stem. The properties of release of preloaded [(3)H]taurine were now characterized in slices prepared from the mouse brain stem from developing (7-day-old) and young adult (3-month-old) mice, using a superfusion system. In general, taurine release was found to be similar to that in other brain areas, consisting of both Ca(2+)-dependent and Ca(2+)-independent components. Moreover, the release was mediated by Na(+)-, Cl(-)-dependent transporters operating outwards, as both Na(+)-free and Cl(-) -free conditions greatly enhanced it. Cl(-) channel antagonists and a Cl(-) transport inhibitor reduced the release at both ages, indicating that a part of the release occurs through ion channels. Protein kinases appeared not to be involved in taurine release in the brain stem, since substances affecting the activity of protein kinase C or tyrosine kinase had no significant effects. The release was modulated by cAMP second messenger systems and phospholipases at both ages. Furthermore, the metabotropic glutamate receptor agonists likewise suppressed the K(+)-stimulated release at both ages. In the immature brain stem, the ionotropic glutamate receptor agonists N-methyl-D-aspartate (NMDA) and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) potentiated taurine release in a receptor-mediated manner. This could constitute an important mechanism against excitotoxicity, protecting the brain stem under cell-damaging conditions.

MLR-induced inhibition of barosensory cells in the NTS

A central motor command arising from the mesencephalic locomotor region (MLR) is widely believed to be one of the neural mechanisms that reset the baroreceptor reflex upward during exercise. The nucleus tractus solitarius (NTS), a dorsal medullary site that receives input from baroreceptors, may be the site where central command inhibits baroreceptor input during exercise. We, therefore, examined the effect of electrical stimulation of the MLR on the impulse activity of cells in the NTS in decerebrate paralyzed cats. Of 129 NTS cells tested for baroreceptor input by injection of phenylephrine (7-25 microg/kg iv) or inflation of a balloon in the carotid sinus, 58 were stimulated and 19 were inhibited. MLR stimulation (80-150 microA) inhibited the discharge of 48 of the 58 cells stimulated by baroreceptor input. MLR stimulation had no effect on the discharge of the remaining 10 cells, each of which displayed no spontaneous activity. In contrast to the 77 NTS cells responsive to baroreceptor input, there was no change in activity of 52 cells when arterial pressure was increased by phenylephrine injection or balloon inflation. MLR stimulation activated each of the 52 NTS cells. For 23 of the cells, the onset latency to MLR stimulation was clearly discernable, averaging 6.4 +/- 0.4 ms. Our findings provide electrophysiological evidence for the hypothesis that the MLR inhibits the baroreceptor reflex by activating NTS interneurons unresponsive to baroreceptor input. In turn, these interneurons may release an inhibitory neurotransmitter onto NTS cells receiving baroreceptor input.

Time course of GABA in the synaptic clefts of inhibitory synapses in the rostral nucleus of the solitary tract

Concentration and time course of neurotransmitter in the synaptic cleft determines the amplitude and the duration of the resulting postsynaptic current. However, technical limitations involved in monitoring the time course of neurotransmitter concentration in the extra-cellular space have prevented direct evaluation of factors that influence neurotransmitter level in the cleft. Tetanic stimulation results in saturation of postsynaptic GABA(A) receptors in the rostral nucleus of the solitary tract (rNST) and GABA diffusion defines the decay time course of the inhibitory potentials or currents (IPSP/Cs). By applying a GABA concentration-response curve to these data it is possible to calculate the GABA concentration transient in the clefts of rNST inhibitory synapses. The analysis indicates that tetanic stimulation produces a GABA concentration that exceeds the concentration of neurotransmitter required to activate all postsynaptic GABA(A) receptors, resulting in short-term modification of the IPSP/Cs decay time. Moreover, the results also demonstrate that the rate of diffusion of GABA from the synaptic cleft is defined by two exponentials. A mathematical model of this process has been developed that supports these conclusions.

GABAA receptor activation in the lateral parabrachial nucleus induces water and hypertonic NaCl intake

Inhibitory serotonergic and cholecystokinergic mechanisms in the lateral parabrachial nucleus and central GABAergic mechanisms are involved in the regulation of water and NaCl intake. In the present study we investigated if the GABA(A) receptors in the lateral parabrachial nucleus are involved in the control of water, NaCl and food intake in rats. Male Holtzman rats with stainless steel cannulas implanted bilaterally into the lateral parabrachial nucleus were used. Bilateral injections of muscimol (0.2 nmol/0.2 microl) into the lateral parabrachial nucleus strongly increased 0.3 M NaCl (20.3+/-7.2 vs. saline: 2.6+/-0.9 ml/180 min) without changing water intake induced by the treatment with the diuretic furosemide combined with low dose of the angiotensin converting enzyme inhibitor captopril s.c. In euhydrated and satiated rats, bilateral lateral parabrachial nucleus injections of muscimol (0.2 and 0.5 nmol/0.2 microl) induced 0.3 M NaCl intake (12.1+/-6.5 and 32.5+/-7.3 ml/180 min, respectively, vs. saline: 0.4+/-0.2 ml/180 min) and water intake (5.2+/-2.0 and 7.6+/-2.8 ml/180 min, respectively, vs. saline: 0.8+/-0.4 ml/180 min), but no food intake (2+/-0.4 g/240 min vs. saline: 1+/-0.3 g/240 min). Bilateral lateral parabrachial nucleus injections of the GABA(A) antagonist bicuculline (1.6 nmol/0.2 microl) abolished the effects of muscimol (0.5 nmol/0.2 microl) on 0.3 M NaCl and water intake. Muscimol (0.5 nmol/0.2 microl) into the lateral parabrachial nucleus also induced a slight ingestion of water (4.2+/-1.6 ml/240 min vs. saline: 1.1+/-0.3 ml/240 min) when only water was available, a long lasting (for at least 2 h) increase on mean arterial pressure (14+/-4 mm Hg, vs. saline: -1+/-1 mm Hg) and only a tendency to increase urinary volume and Na+ and K+ renal excretion. Therefore the activation of GABA(A) receptors in the lateral parabrachial nucleus induces strong NaCl intake, a small ingestion of water and pressor responses, without changes on food intake.

NMDA receptors in nucleus tractus solitarii are linked to soluble guanylate cyclase

We sought to test the hypothesis that cardiovascular responses to activation of ionotropic, but not metabotropic, glutamate receptors in the nucleus tractus solitarii (NTS) depend on soluble guanylate cyclase (sGC) and that inhibition of sGC would attenuate baroreflex responses to changes in arterial pressure. In adult male Sprague-Dawley rats anesthetized with chloralose, the ionotropic receptor agonists N-methyl-d-aspartate (NMDA) and dl-α-amino-3-hydroxy-5-methylisoxazole-propionic acid (AMPA) and the metabotropic receptor agonist trans-dl-amino-1,3-cyclopentane-dicarboxylic acid (ACPD) were microinjected into the NTS before and after microinjection of sGC inhibitors at the same site. Inhibition of sGC produced significant dose-dependent attenuation of cardiovascular responses to NMDA but did not alter responses produced by injection of AMPA or ACPD. Bilateral inhibition of sGC did not alter arterial pressure, nor did it attenuate baroreflex responses to pharmacologically induced changes in arterial pressure. This study links sGC with NMDA, but not AMPA or metabotropic, receptors in cardiovascular signal transduction through NTS.

Contraction-sensitive skeletal muscle afferents inhibit arterial baroreceptor signalling in the nucleus of the solitary tract: role of intrinsic GABA interneurons

Arterial baroreceptor and skeletal muscle receptor afferents relay sensory information to the nucleus of the solitary tract (NTS) during exercise. Previous studies have suggested that skeletal muscle afferent input inhibits baroreflex function; however, detailed information on the role of muscle afferents and GABAergic mechanisms in the NTS is limited. Furthermore, identification of specific afferent modalities that activate GABAergic neurons in the NTS remains unknown. In the present study, we examined the neuroanatomical and physiological interactions between spinal dorsal horn cells that transmit contraction-sensitive input from skeletal muscle and GABAergic interneurons in the NTS. Biotinylated dextran amine (BDA, 10%, 25-100 nL) microinjection into dorsal horn of the cervical spinal cord was combined with glutamate decarboxylase (GAD) immunohistochemistry to visualize the nature of the relationship of BDA-labeled fibers in the NTS with GAD immunoreactivity (GAD-ir). BDA-labeled axons and terminal processes were localized in the medial, commissural, dorsomedial and dorsolateral subdivisions of the caudal NTS. Moreover, BDA-labeled fibers were observed in close proximity to GAD-ir structures throughout these regions of the NTS. The physiological interaction between skeletal muscle receptor and arterial baroreceptor afferents was investigated using an arterially perfused, decerebrate rat preparation. Activation of skeletal muscle afferents by electrically evoked twitch contraction of the forelimb attenuated baroreflex responsiveness (BR, calculated as the ratio of changes in heart rate to systemic pressure) from -1.5+/-0.3 bpm.mm Hg(-1) to -0.1+/-0.1 bpm.mm Hg(-1) (control versus contraction, P<0.05, n=15). However, forelimb contraction failed to inhibit the reflex bradycardia evoked by activation of peripheral chemoreceptor afferents, indicating a reflex-specific action. Bilateral microinjection of bicuculline methiodide (BIC, 10 microM, 40-60 nL) into the caudal NTS restored baroreflex responsiveness during contraction (-1.6+/-0.2 versus -0.1+/-0.1 versus -1.5+/-0.2 bpm.mmHg(-1), control versus contraction versus contraction+BIC P<0.05, n=8). We conclude that activation of ascending spinal neurons from the cervical dorsal horn by contraction-sensitive skeletal muscle afferents selectively inhibits arterial baroreceptor signaling in the NTS via activation of a GABAergic mechanism.

Vesicular glutamate transporter 2 immunoreactivity in putative vagal mechanosensor terminals of mouse and rat esophagus: indication of a local effector function?

Intraganglionic laminar endings (IGLEs) represent the major vagal afferent structures throughout the gastrointestinal tract. Previous ultrastructural investigations have revealed synaptic contacts of IGLEs on myenteric neurons. Thus, in addtion to functioning probably as mechanosensors, IGLEs may also synaptically influence myenteric neurons. In search of clues for potential transmitters in IGLEs, we investigated, by combined neuronal tracing and immunocytochemistry in the esophagus, the correlation between IGLEs and vesicular glutamate transporter 2 (VGLUT2), which is considered a reliable marker for glutamatergic neurons. In rat esophagus, IGLEs were immunostained with calretinin. In the mouse, anterograde wheat germ agglutinin/horseradish peroxidase (WGA-HRP) tracing from nodose ganglion was used in order to label esophageal IGLEs. Confocal laser scanning microscopy demonstrated that VGLUT2 immunoreactivity was highly colocalized with synaptophysin and that both calretinin and tyramide amplified WGA-HRP in rat and mouse esophagus, respectively. No colocalization was found with calcitonin gene-related peptide, a marker for spinal primary afferents. Thus, VGLUT2 is found in vagal afferent endings in the esophagus, suggesting that glutamate is contained in, and probably released from, synaptic vesicles previously described in IGLEs. Functional evidence pending, this finding is in favor of a local effector function of IGLEs onto myenteric neurons.

Activity-dependent role of NMDA receptors in transmission of cardiac mechanoreceptor input to the NTS

Evidence suggests that transmission of barosensitive input from arterial baroreceptors and cardiac mechanoreceptors at nucleus tractus solitarius (NTS) neurons involves non-N-methyl-d-aspartate (NMDA) glutamate receptors, but there is a possibility that the contribution of NMDA receptors might increase during periods of increased afferent input, when enhanced neuronal depolarization could increase the activation of NMDA receptors by removal of a Mg(2+) block. Thus the effects of NMDA on cardiac mechanoreceptor-modulated NTS neuronal discharges were examined at different levels of arterial pressure used to change cardiac mechanoreceptor afferent input. To determine whether the response was specific to NMDA, (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) was also administered at different levels of neuronal discharge. In anesthetized dogs, neuronal activity was recorded from the NTS while NMDA or AMPA was picoejected at high versus low arterial stimulating pressures. NMDA, but not AMPA, produced a significantly greater discharge of mechanoreceptor-driven NTS neurons at higher versus lower levels of stimulating pressure. These data suggest that the role played by NMDA receptors is greater during periods of enhanced neuronal depolarization, which could be produced by increases in afferent barosensitive input.

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 nuclei of the medulla and suggests that this subtype may be involved in shaping synaptic transmission in these central nuclei.

Carbon monoxide and metabotropic glutamate receptors in rat nucleus tractus solitarii: participation in cardiovascular effect

Carbon monoxide (CO) has been identified as an endogenous biological messenger in the brain. Heme oxygenase catalyzes the metabolism of heme to biliverdin and CO. Recent studies have demonstrated that CO is involved in central cardiovascular regulation and modulates the baroreflex in the nucleus tractus solitarii of rats. The purpose of the present study was to investigate the possible interaction of CO and excitatory amino acids in the nucleus tractus solitarii. In anesthetized male Sprague-Dawley rats, unilateral intranucleus tractus solitarii microinjection of hematin, a heme molecule cleaved by heme oxygenase to yield CO, or excitatory amino acids L-glutamate produced depressor and bradycardiac effects. Similar cardiovascular effects were observed with several agonists for ionotropic glutamate receptors such as N-methyl-D-aspartate (NMDA), (+/-)-alpha-amino-3-hydroxyl-5-methylisoxazole-4-propanoic acid (AMPA), kainic acid and for metabotropic glutamate (mGlu) receptors, trans-(+/-)-1-amino-(1S,3R)-cyclopentanedicarboxylic acid (ACPD). Among these agonists, prior administration of the heme oxygenase inhibitor, zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG) (1 nmol), significantly attenuated the cardiovascular effects of hematin, L-glutamate and ACPD. Furthermore, the cardiovascular effects of ACPD were prevented by the selective mGlu receptors antagonist L-2-amino-3-phosphonoprionate (L-AP3). However, pretreatment with ZnDPBG failed to prevent the cardiovascular responses to microinjection of NMDA, AMPA and kainic acid. On the other hand, prior administration of the NMDA receptor antagonist, diazocilpine (MK-801), or (+/-)-2-amino-5-phosphonopentanoic acid (APV) attenuated the depressor and bradycardiac effect of hematin. These results demonstrated that mGlu receptors may couple to the activation of heme oxygenase via the liberation of CO to participate in central cardiovascular regulation. They also suggested that CO and excitatory amino acids may interact in the nucleus tractus solitarii of rats.

Antinociceptive effect of cardiopulmonary chemoreceptor and baroreceptor reflex activation in the rat

The effect on the nociceptive tail-flick (TF) reflex of cardiopulmonary chemoreceptor and arterial baroreceptor activation, producing Bezold-Jarisch like- and baro-reflex responses, respectively, was analysed in lightly halothane-anaesthetized rats. Intra-cardiac administration of phenylbiguanide (5-100 microg/kg, into the right atrium) or veratrine (30-150 microg/kg, into the left ventricle), which both elicited the characteristic Bezold-Jarisch-like cardiovascular reflex responses (hypotension and bradycardia), produced a dose-dependent increase in TF latency. A similar inhibitory influence on the TF reflex was noted upon baroreflex activation by acute administration of phenylephrine (15-50 microg/kg i.v.) or aortic depressor nerve stimulation (100-400 microA). As expected from the involvement of local excitatory amino acid receptors in both vagally mediated cardiovascular reflex responses and inhibition of the TF reflex, microinjections of kynurenic acid (3 nmol/0.1 microl), an N-methyl-D-aspartate (NMDA) and non-NMDA receptor antagonist, into the nucleus tractus solitarius, prevented the cardiovascular responses as well as the concomitant increase in TF latency produced by cardiopulmonary chemoreceptor and baroreceptor stimulations. The present data show that induction of the cardiopulmonary chemoreceptor and baroreceptor reflexes produces an antinociceptive effect which can be assessed using the TF test, and that glutamate ionotropic receptors within the nucleus tractus solitarius mediate this effect.

Glutamate and NO mediation of the pressor response to 5-HT3 receptor stimulation in the nucleus tractus solitarii

The possible participation of glutamate and NO/cGMP in the pressor response to 5-HT3 receptor activation in the nucleus tractus solitarii (NTS) was investigated using selective antagonists in urethane-anaesthetized rats. Intra-NTS administration of NMDA and non-NMDA receptor antagonists, but not metabotropic glutamate receptor antagonists, markedly reduced (70%) the increase in blood pressure caused by local application of the potent 5-HT3 receptor agonist, 1-(m-chlorophenyl)-biguanide. The 5-HT3 receptor-mediated pressor response was also significantly attenuated by the local blockade of nitric oxide synthase and soluble guanylyl cyclase. These data suggest that ionotropic glutamate receptors and the associated NO/cGMP transduction mechanism contribute downstream to the pressor effect elicited by 5-HT3 receptor stimulation in the NTS.

Stimulation of 5-HT2 receptors in the nucleus tractus solitarius enhances NMDA receptor-mediated reflex-evoked bradycardiac responses in the rat

The modulation by 5-HT2 receptors in the nucleus tractus solitarius of the reflex bradycardia evoked by stimulation of peripheral baroreceptors and cardiopulmonary chemoreceptors, and their possible functional interactions with local NMDA receptors, were investigated in pentobarbital- and urethane-anaesthetized rats, respectively. Microinjection of the 5-HT2 receptor agonist, 2,5-dimethoxy-4-iodoamphetamine (0.1-0.5 pmol), into the nucleus tractus solitarius elicited a dose-dependent hypotension and bradycardia. Bilateral microinjections at the same site of a subthreshold dose of 2,5-dimethoxy-4-iodoamphetamine (0.05 pmol) significantly enhanced the aforementioned reflex-evoked bradycardiac responses. In contrast, local bilateral microinjections of the NMDA receptor antagonist, 2-amino-5-phosphonopentanoic acid (500 and 1000 pmol), reduced, in a dose-dependent manner, both reflex-evoked responses. The facilitatory effect of 2,5-dimethoxy-4-iodoamphetamine upon these reflex-evoked bradycardiac responses was prevented by prior local microinjection of low doses of either the selective 5-HT2 receptor antagonist, ketanserin (10 pmol), or 2-amino-5-phosphonopentanoic acid (100 pmol), which, on their own, did not affect the reflex-associated bradycardia. These data suggest that 5-HT2 receptors within the nucleus tractus solitarius participate in a facilitatory modulation of the reflex control of heart rate, probably through functional interactions with local NMDA receptors.

Expression of metabotropic glutamate receptors in nodose ganglia and the nucleus of the solitary tract

The purpose of this study was to identify the complement of metabotropic glutamate receptors (mGluRs) expressed in nodose ganglia and the nucleus tractus solitarius (NTS). mRNA from these tissues was isolated and amplified with standard RT-PCR with primers specific for each mGluR subtype. The results of this analysis showed that the NTS expresses all eight mGluR subtypes, whereas nodose ganglia express only group III mGluRs: mGluR4, mGluR6, mGluR7, and mGluR8. Application of the group III-specific mGluR agonist L-(+)-2-amino-4-phosphonobutyric acid (100 microM) reversibly inhibited voltage-gated calcium currents isolated from DiI-labeled aortic baroreceptor neurons and unlabeled nodose neurons. The results of this study suggest that group III mGluRs are the primary mGluR subtype expressed in visceral afferent neurons and that these receptors may be involved in afferent central transmission.

Properties of NTS neurons receiving input from barosensitive receptors

Afferent input from barosensitive receptors, including carotid baroreceptors and cardiac mechanoreceptors, has been found to produce different types of discharge patterns in neurons in the nucleus tractus solitarius (NTS). The discharge patterns of the neurons may be dependent on many factors, including input from the different barosensitive receptor subtypes, the contribution of different ionotropic glutamate receptors [NMDA (N-methyl-D-aspartate) versus nonNMDA receptors] in transmission of the input, effects of different neuropeptide neurotransmitters/neuromodulators on afferent transmission, or the order of the neuron within the barosensitive reflex arc. It is not clear if the roles of the glutamate receptor subtypes are the same for neurons activated by the different barosensitive inputs. In addition, the amount of afferent input from the barosensitive receptors, due to increases or decreases in stimulating pressures, may result in altering the roles of the ionotropic glutamate receptor subtypes. While most evidence suggests that nonNMDA receptors play the greatest role in the transmission of afferent activity to second-orders NTS neurons, it is possible that increases in afferent input may lead to an enhanced role for NMDA receptors in the transmission of the barosensitive input, since increased depolarization of the NTS neurons may lead to removal of a Mg2+ block of the NMDA channel. Transmission of baroreceptor input at third- and higher-order neurons has been found to involve both nonNMDA and NMDA receptors, suggesting a possible functional role for the distribution of these receptor types. The roles of these different factors in the initiation of NTS neuronal discharge will be discussed.

Optical approaches to embryonic development of neural functions in the brainstem

The ontogenetic approach to physiological events is a useful strategy for understanding the functional organization/architecture of the vertebrate brainstem. However, conventional electrophysiological techniques are difficult or impossible to employ in the early embryonic central nervous system. Optical techniques using voltage-sensitive dyes have made it possible to monitor neural activities from multiple regions of living systems, and have proven to be a useful tool for analyzing the embryogenetic expression of brainstem neural function. This review describes recent progress in optical studies made on embryonic chick and rat brainstems. Several technical issues concerning optical recording from the embryonic brainstem preparations are discussed, and characteristics of the optical signals evoked by cranial nerve stimulation or occurring spontaneously are described. Special attention is paid to the chronological analyses of embryogenetic expression of brainstem function and to the spatial patterning of the functional organization/architecture of the brainstem nuclei. In addition, optical analyses of glutamate, GABA, and glycine receptor functions during embryogenesis are described in detail for the chick nucleus tractus solitarius. This review also discusses intrinsic optical signals associated with neuronal depolarization. Some emphases are also placed on the physiological properties of embryonic brainstem neurons, which may be of interest from the viewpoint of developmental neurobiology.

Substance P in the nucleus of the solitary tract augments bronchopulmonary C fiber reflex output

Bronchopulmonary C fibers defend the lungs against injury from inhaled agents by a central nervous system reflex consisting of apnea, cough, bronchoconstriction, hypotension, and bradycardia. Glutamate is the putative neurotransmitter at the first central synapses in the nucleus of the solitary tract (NTS), but substance P, also released in the NTS, may modulate the transmission. To test the hypothesis that substance P in the NTS augments bronchopulmonary C fiber input and hence reflex output, we stimulated the C fibers with left atrial capsaicin (LA CAP) injections and compared the changes in phrenic nerve discharge, tracheal pressure (TP), arterial blood pressure (ABP), and heart rate (HR) in guinea pigs before and after substance P injections (200 microM, 25 nl) in the NTS. Substance P significantly augmented LA CAP-evoked increases in expiratory time by 10-fold and increases in TP and decreases in ABP and HR by threefold, effects prevented by neurokinin-1 (NK1) receptor antagonism. Thus substance P acting at NTS NK1 receptors can exaggerate bronchopulmonary C fiber reflex output. Because substance P synthesis in vagal airway C fibers may be enhanced in pathological conditions such as allergic asthma, the findings may help explain some of the associated respiratory symptoms including cough and bronchoconstriction.

Bradycardic responses to microinjection of N-methyl-D-aspartate into the nucleus tractus solitarius are inhibited by local activation of 5-HT(3) receptors

Previous reports have described that glutamate ionotropic receptors in the nucleus tractus solitarius (NTS) are involved in the reflex control of heart rate, and that such a control can be inhibited by NTS-5-HT(3) receptor stimulation. In the present study, we examined in urethane anaesthetized rats the effects of intra-NTS microinjection of 1-(m-chlorophenyl)-biguanide (CPBG), a potent and selective 5-HT(3) receptor agonist, on the cardiovascular responses to local administration of glutamate ionotropic receptor agonists. Intra-NTS microinjection of CPBG reduced the atropine-sensitive bradycardia elicited by local microinjection of NMDA without affecting the cardiovascular responses to intra-NTS microinjections of AMPA or kainic acid. The reduction by CPBG of the NMDA-evoked cardiac response was blocked by prior intra-NTS microinjection of granisetron, a 5-HT(3) receptor antagonist, as well as bicuculline, a GABA(A) receptor antagonist. These results suggest that the stimulation of NTS 5-HT(3) receptors specifically reduces, via a GABA-dependent mechanism, the cardiac response to local NMDA administration.

Apposition of neuronal elements containing nitric oxide synthase and glutamate in the nucleus tractus solitarii of rat: a confocal microscopic analysis

The distribution of glutamate and neuronal nitric oxide synthase in the rat nucleus tractus solitarii was investigated by double fluorescent immunohistochemistry combined with confocal laser scanning microscopy. Cells and fibers that exhibited neuronal nitric oxide synthase immunoreactivity alone, glutamate immunoreactivity alone or both immunolabels were present in all subnuclei of the nucleus tractus solitarii, but staining intensities differed between the subnuclei. The percentages of double-labeled glutamate-immunoreactive cells also differed between the subnuclei. The central subnucleus contained the highest percentage of double-labeled glutamate-immunoreactive cells and the medial subnucleus contained the lowest. The percentages of double-labeled neuronal nitric oxide synthase-immunoreactive neurons likewise differed between the subnuclei. The central subnucleus contained the highest percentage of double-labeled neuronal nitric oxide synthase-immunoreactive neurons and the commissural subnucleus contained the lowest. Because of our interest in cardiovascular regulation, the anatomical relationship between glutamate-immunoreactive and neuronal nitric oxide synthase-immunoreactive fibers in the dorsolateral and commissural subnuclei was further examined at higher magnification. Close appositions were observed between neuronal nitric oxide synthase-immunoreactive and glutamate-immunoreactive fibers, between double-labeled and glutamate-immunoreactive fibers, and between neuronal nitric oxide synthase-immunoreactive and double-labeled fibers. We recognized that a single visual perspective might cause labeled fibers that pass in close proximity to appear to make contact. Therefore, we constructed three-dimensional images from serial optical sections obtained from the dorsolateral and commissural subnuclei by means of a confocal scanning microscope. Rotation of the three-dimensional images caused some fibers that had seemed to be in close apposition to other structures to separate from those structures. In contrast, some glutamate-immunoreactive and some neuronal nitric oxide synthase-immunoreactive fibers remained in close apposition regardless of the angle at which they were viewed. This study supports there being an anatomical link between glutamatergic and nitroxidergic systems in the nucleus tractus solitarii. Recognized physiological interactions between the two systems could occur through such a link.

Glycine receptor (gephyrin) immunoreactivity is present on cholinergic neurons in the dorsal vagal complex

We previously demonstrated that microinjection of exogenous glycine into the nucleus tractus solitarii of anesthetized rats elicits responses that are qualitatively like those elicited by microinjection of acetylcholine at the same site. The responses to glycine, like those to acetylcholine, are blocked by administration of a muscarinic receptor antagonist and prolonged by administration of an acetylcholinesterase inhibitor. Furthermore, glycine leads to release of acetylcholine from the nucleus tractus solitarii and surrounding dorsal vagal complex. An anatomical framework for interactions between glycinergic and cholinergic neurons was established by studies that identified glycine terminals and receptors in the dorsal vagal complex. The current study investigated the relationship between glycine receptors and neuronal elements that were immunoreactive for choline acetyltransferase in the dorsal vagal complex. Neurons that were immunoreactive for choline acetyltransferase were located in the dorsal motor nucleus of the vagus, hypoglossal nucleus and nucleus ambiguus, and stained cells were also present in medial, intermediate, and ventrolateral subnuclei of the nucleus tractus solitarii. We found that glycine receptors, immunolabeled with an antibody to gephyrin, were present on cholinergic dendrites in the nucleus tractus solitarii. Gephyrin immunoreactivity was also present on dendrites that did not stain for choline acetyltransferase. These data further support the contribution of cholinergic neurons in mediating cardiovascular responses to glycine in the nucleus tractus solitarii.

Modulation of the carotid baroreceptor reflex by substance P in the nucleus tractus solitarius

Previous studies have shown that administration of substance P (SP) into the nucleus tractus solitarius (NTS) can evoke a depressor response similar to that produced by activation of the arterial baroreceptors. In addition, some studies have suggested that SP increases the reflex responses to activation of baroreceptor input. The present study was performed to determine the effects of SP on the carotid sinus baroreceptor reflex at the level of the NTS by examining the effects of both exogenous SP microinjected into different rostrocaudal locations in the NTS and blockade of the effects of endogenous SP, through the microinjection of a substance P antagonist (SPa; [D-Pro, D-Trp]-substance P). Changes in pressure in an isolated carotid sinus in anesthetized dogs were used to evoke baroreflex changes in arterial blood pressure (BP) before and after microinjection of SP (0.5 microM) or SPa (10 microM) into barosensitive regions of the NTS. Microinjection of SP or its antagonist did not alter baseline, resting BP but did produce significant changes in baroreflex sensitivity. Microinjection of SP into different rostrocaudal regions of the NTS produced different responses, with rostral and caudal NTS microinjections producing significant increases in sensitivity. No effects on baroreflex sensitivity were obtained in response to SP microinjections into the intermediate NTS. Unlike SP, microinjection of the SPa significantly decreased baroreflex sensitivity at all rostrocaudal levels of the NTS. These data demonstrated that SP has the capability to modulate the carotid baroreflex at the level of the NTS and support a physiological role for endogenously released SP.

Role of glutamate receptors in transmission of vagal cardiac input to neurones in the nucleus tractus solitarii in dogs

1. Vagal afferent input from cardiac mechanoreceptors excites neurones in the nucleus tractus solitarii (NTS), but discharge patterns evoked by physiological activation of pressure-sensitive cardiac mechanoreceptors have not been studied in vivo. The role of glutamate receptor subtypes in transmission of afferent activity to the NTS neurones has not been determined. The present study therefore has two aims: first, to characterise the discharge patterns of neurones in the NTS that receive pressure-sensitive vagal cardiac receptor input and second, to determine the roles of ionotropic glutamate receptor subtypes in the transmission of this putative cardiac mechanoreceptor-related activity to NTS neurones. 2. Pulse-synchronous activity of neurones in the NTS evoked by vagal afferent input was recorded extracellularly in an anaesthetised dog model using multibarrel glass electrodes, which allowed picoejection of the glutamate receptor antagonists NBQX or AP5 to block either non-NMDA or NMDA receptors, respectively, during the neuronal recording. Pressure sensitivity of the recorded neurones was examined by monitoring their response to a small increase in arterial blood pressure. Selective pressure activation of carotid sinus baroreceptors in an isolated sinus or selective denervation of aortic baroreceptors were used to test for convergent excitation of the neurones by arterial baroreceptors. 3. Pulse-synchronous cardiac-related neuronal activity recorded from neurones in both the right and left NTS was eliminated following section of the left (n = 17) or right (n = 1) vagus nerves. No spontaneous, non-pulsatile activity was observed in these neurones before or after vagotomy. Activity transmitted via left vagal afferents was found to be sensitive to changes in arterial blood pressure. In these neurones, activity was blocked in 13 of 17 neurones by picoejection of NBQX, with the remainder requiring both NBQX and AP5. None of the cardiac-related neurones responded to activation of carotid baroreceptors or denervation of aortic baroreceptors, indicating no convergence of activity from carotid baroreceptors or aortic baroreceptors with pressure thresholds of approximately 130 mmHg or less. 4. The results suggest that vagal pressure-sensitive afferent input from cardiac mechanoreceptors is transmitted primarily by left vagal afferent fibres via non-NMDA receptors to neurones in both the ipsilateral and contralateral NTS. NMDA receptors were also found to have a role in the activation of a small subpopulation of neurones.

Glycine-immunoreactive synaptic terminals in the nucleus tractus solitarii of the cat: ultrastructure and relationship to GABA-immunoreactive terminals

Postembedding immunogold labeling methods applied to ultrathin and semithin sections of cat dorsomedial medulla showed that neuronal perikarya, dendrites, myelinated and nonmyelinated axons, and axon terminals in the nucleus tractus solitarii contain glycine immunoreactivity. Light microscopic observations on semithin sections revealed that these immunoreactive structures were unevenly distributed throughout the entire nucleus. At the electron microscopic level, synaptic terminals with high levels of glycine-immunoreactivity, assumed to represent those releasing glycine as a neurotransmitter, were discriminated from terminals containing low, probably metabolic levels of glycine-immunoreactivity, by a quantitative analysis method. This compared the immunolabeling of randomly sampled terminals with a reference level of labeling derived from sampling the perikarya of dorsal vagal neurones. The vast majority of these "glycinergic" terminals contained pleomorphic vesicles, formed symmetrical synaptic active zones, and targeted dendrites. They appeared to be more numerous in areas of the nucleus tractus solitarii adjoining the tractus solitarius, but rather scarce caudally, medially, ventrally, and in the dorsal motor vagal nucleus. In a random analysis of the entire nucleus tractus solitarii, 26.2% of sampled terminals were found to qualify as glycine-immunoreactive. In contrast, boutons immunoreactive for gamma-aminobutyric acid (GABA) were more evenly distributed throughout the dorsal vagal complex and accounted for 33.7% of the synaptic terminals sampled. A comparison of serial ultrathin sections suggested three subpopulations of synaptic terminals: one containing high levels of both GABA- and glycine-immunoreactivities (21% of all terminals sampled), one containing only GABA-immunoreactivity (12.7%), and relatively few terminals (5.2%) that were immunoreactive for glycine alone. These results were confirmed by dual labeling of sections using gold particles of different sizes. This study reports the first analysis of the ultrastructure of glycinergic nerve terminals in the cat dorsal vagal complex, and the pattern of coexistence of glycine and GABA observed provides an anatomical explanation for our previously reported inhibitory effects of glycine and GABA on neurones with cardiovascular and respiratory functions in the nucleus tractus solitarii.

Cardiorespiratory responses to intracerebroventricular injection of neuropeptide Y in anaesthetised dogs

Cardiovascular and respiratory effects of intracerebroventricular (icv) administration of neuropeptide Y (NPY) and separate, preferential agonists for NPY Y1 and Y2 receptors were observed in anaesthetised dogs. Central injections of NPY resulted in significant cardiac slowing and decreases in arterial pressure. These cardiovascular effects were blocked by central injection of the NPY Y1- preferring antagonist 1229U91. Central injection of NPY did not have a significant effect on ventilation, but the NPY Y1 antagonist 1229U91 administered alone caused a significant increase in ventilation. The NPY Y1-receptor agonist [Leu31Pro34] NPY significantly decreased ventilation while the NPY Y2 receptor agonist N-acetyl [Leu28Leu31] NPY 24--36 significantly increased it. A similar inverse relationship was seen with respect to blood pressure, with the NPY Y1-receptor agonist [Leu31Pro34] NPY significantly decreasing blood pressure, while the NPY Y2 receptor agonist N-acetyl [Leu28Leu31] NPY 24-36 significantly increased it. These findings suggest a role for NPY Y1 receptors in pathways mediating decreases in ventilation and blood pressure, and for NPY Y2 receptors in those mediating increased ventilation and blood pressure.

Inspiration-promoting vagal reflex under NMDA receptor blockade in anaesthetized rabbits

1. This study describes a novel vagal respiratory reflex in anaesthetized rabbits. In contrast to the well-known inspiratory (I) off-switching by vagal afferent excitation, this vagal reflex initiates and maintains the central I activity of phrenic nerve discharges in rabbits pre-treated with antagonists of N-methyl-D-aspartate-type excitatory amino acid receptors (NMDA-Rs). 2. Under NMDA-R blockade with either dizocilpine (0.025-0.3 mg kg-1), D-2-amino-5-phosphonopentanoic acid (AP5, 0.5-1 mg, i.c.v.) or ketamine (10 mg kg-1), vagal stimulation at low frequencies (5-40 Hz) during the I phase prevented or markedly delayed the spontaneous I termination. In contrast, stimulation of the same vagal afferent at the same intensity but at a higher frequency (100-160 Hz) during the I phase immediately terminated the I phase. 3. In non-vagotomized rabbits, maintaining the tidal volume at end-expiratory levels during the I phase prevented spontaneous I termination and maintained apneusis after NMDA-R blockade with dizocilpine. 4. Brief stimulation of vagal afferents at low frequency (5-40 Hz) during the expiratory (E) phase constantly initiated phrenic I discharge after NMDA-R block. 5. We conclude that low-frequency discharge of vagal pulmonary stretch receptor afferents, as when lung volume is near functional residual capacity, promotes central I activity under NMDA-R blockade.

A subpopulation of neurons in the rat rostral nucleus of the solitary tract that project to the parabrachial nucleus express glutamate-like immunoreactivity

In rodents, gustatory information is transmitted from second order neurons in the rostral nucleus of the solitary tract (rNST) to the parabrachial nucleus (PBN) in the pons. The chemical nature of this projection is unknown. Therefore, the goal of the current study was to determine if rNST neurons that project to the PBN express glutamate-like immunoreactivity. Projection neurons were retrogradely labeled following stereotaxic injection of rhodamine-filled latex microspheres into the right PBN of seven rats while glutamate-immunoreactive (GLU-IR) structures were visualized in the same tissue using an immunoperoxidase procedure. The number of single- and double-labeled neurons located in the right (ipsilateral) and left rNST, in each of the nuclear subdivisions as well as their position along the rostral-caudal axis of the rNST was determined. GLU-IR cell bodies were located throughout the rNST. Although the rostral central subdivision contained the highest percentage (33.8%) of GLU-IR perikarya, immunolabeled neurons were most concentrated (number/area of subdivision) within the medial subnucleus. The rostral third of the rNST contained the fewest (20. 5%) and lowest density of GLU-IR cell bodies. The highest percentage of rNST neurons retrogradely labeled from the PBN were located ipsilateral (85.4%) to the pontine injection site, in the middle third of the nucleus (44.2%) and within the rostral central subdivision (52.4%). Overall, 18% of the labeled rNST projection neurons were GLU-IR. The distribution of double-labeled neurons mirrored that of the projection neurons with the largest number located in the ipsilateral rNST (84.5%), middle third of the nucleus (40.5%) and rostral central subdivision (64.7%). These results indicate that glutamate may be a main component of the ascending pathway from the rNST to the PBN. In addition, since GLU-IR neurons were located throughout the rNST and most were not retrogradely-labeled, the current results suggest that glutamate may be an important neurotrans-mitter within the medulla.

AMPA receptor activation of area postrema neurons

This study reports on the effects of activation of ionotropic glutamate receptors on area postrema neuron cytosolic calcium concentration ([Ca2+]i). In 140 of 242 area postrema neurons isolated from postnatal rats, application of 100 microM L-glutamate (L-Glu) resulted in a significant increase in [Ca2+]i. The remaining neurons were unaffected. The effects of L-Glu on area postrema [Ca2+]i were dose dependent, with a threshold of response near 1.0 microM and maximal response near 100 microM. To determine if the response of L-Glu in area postrema neurons was due to activation of ionotropic glutamate receptors, the effects of the broad-spectrum ionotropic glutamate receptor antagonist kynurinic acid (Kyn) was determined. Application of 1.0 mM Kyn resulted in a 62.6 +/- 4% inhibition of the L-Glu-evoked response. Application of the selective N-methyl-D-aspartic acid (NMDA) antagonist 2-amino-5-phosphonopentanoic acid had no effect on the response of area postrema neurons to 100 microM L-Glu. In contrast, application of the selective DL-alpha-amino-3-hydroxy-5-methylisoxazole-propionic acid (AMPA)/kainate receptor antagonist 6,7-dinitroquinoxaline (DNQX) effectively blocked the 100 microM L-Glu response. Application of (+/-)-AMPA mimicked the effects observed with L-Glu and was selectively blocked by DNQX. These results suggest that L-Glu activation of area postrema neurons involves activation of AMPA receptors but not NMDA receptors.

Modulation of arterial baroreflexes by antisense oligodeoxynucleotides to NMDAR1 receptors in the nucleus tractus solitarius

An antisense oligodeoxynucleotide which specifically blocked the production of the glutamate receptor subtype NMDAR1 was administered to the nucleus tractus solitarius (NTS) in order to examine the role of this receptor in baroreflex control of heart rate. Baseline blood pressure and heart rate were unchanged by NTS treatment with the antisense oligodeoxynucleotide to the NMDAR1 receptor subunit. However, the reflex bradycardia evoked in response to the hypertension induced by bolus administration of phenylephrine was significantly attenuated following bilateral NTS antisense oligodeoxynucleotide treatment. Administration of the corresponding mismatched antisense oligodeoxynucleotide did not significantly alter the reflex bradycardia. These data indicate that NMDAR1 receptors are involved in neurotransmission in the baroreflex arc at the level of the NTS. The specificity of antisense oligodeoxynucleotides may prove to be a useful technique to analyze the role of receptor subtypes mediating neurotransmission in central pathways.

Type I and II metabotropic glutamate receptors regulate the outflow of [3H]D-aspartate and [14C]gamma-aminobutyric acid in rat solitary nucleus

Metabotropic glutamate (mGlu) receptors modulating amino acid outflow were examined in a model system in order to further characterize the pharmacological nature of the mGlu receptors involved in viscerosensory processing in the nucleus tractus solitarii. The actions of a number of subtype-selective mGlu receptor agonists and antagonists were monitored on the K+-evoked outflow of [3H]D-aspartate and [14C]gamma-aminobutyric acid (GABA) from superfused slices of rat nucleus tractus solitarii. (+/-)1S,3R-1-Amino-cyclopentane-1,3-dicarboxylate (10-300 microM), produced a concentration-dependent increase in outflow, which was attenuated by a number of phenylglycine antagonists. (2S,3S,4S)-alpha-(Carboxycyclopropyl)-glycine (30-300 microM) had mixed effects on outflow. The type I-selective agonist (RS)-3,5-dihydroxyphenylglycine (300 microM) also increased outflow and these effects were reversed by the type I antagonist (RS)-1-aminoindan-1,5-dicarboxylate (100 microM). Activation of type II mGlu receptors with (2R,4R)-aminopyrrolidine-2,4-dicarboxylate (300 microM), however, decreased outflow, and this effect was antagonized by the type II antagonist LY307452 (200 microM). Interestingly, LY307452 (200 microM) alone, enhanced outflow of [3H]D-aspartate, but not [14C]GABA. Type III mGlu receptors may not be involved in outflow of [3H]D-aspartate and [14C]GABA in the nucleus tractus solitarii, as L-2-amino-4-phosphonobutyrate (30-300 microM) had no effect under the present experimental conditions. These in vitro studies provide new evidence for roles for Type I and II mGlu receptors in viscerosensory processing in nucleus tractus solitarii.

Mediation by N-methyl- d-aspartate and non- N-methyl- d-aspartate receptors in the expression of fos protein at the nucleus tractus solitarii in response to baroreceptor activation in the rat

We investigated the role of glutamatergic synapses in the expression of Fos protein at the nucleus tractus solitarii following baroreceptor activation in rats anaesthetized with pentobarbital sodium. Microinjection of L-glutamate (1 nmol) bilaterally into the nucleus tractus solitarii elicited significant hypotension and bradycardia. There was a concurrent increase, as determined immunohistochemically, in the expression of Fos protein at the commissural, medial and dorsomedial subnuclei of the caudal nucleus tractus solitarii. These effects were blunted when L-glutamate was co-administered with either the selective N-methyl-D-aspartate or non-N-methyl-D-aspartate glutamate receptor antagonist, dizocilpine maleate (200 pmol) or 6-cyano-7-nitroquinoxaline-2,3-dione (8 pmol), into the caudal nucleus tractus solitarii. Repeated and scheduled transient hypertension evoked by phenylephrine (2.5, 5.0 or 10.0 microg/kg, i.v.) also appreciably increased the number of Fos-immunoreactive neurons at the commissural, medial and dorsomedial subnuclei of the caudal nucleus tractus solitarii. The expression of Fos protein in this fashion was reduced, simultaneous with a discernible depression in baroreceptor reflex response, when baroreceptor activation was coupled with microinjection bilaterally of dizocilpine maleate (200 pmol) or 6-cyano-7-nitroquinoxaline-2,3-dione (8 pmol) into the nucleus tractus solitarii. Regression analysis showed that the depressive action on the baroreceptor reflex response by both glutamate receptor antagonists correlated positively to the reduction in Fos-immunoreactivity in the nucleus tractus solitarii after baroreceptor activation. Double immunohistochemical staining revealed that nucleus tractus solitarii neurons that showed Fos immunoreactivity were generally also immunoreactive to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor subunit 1. On the other hand, Fos immunoreactivity was usually absent from neurons in the nucleus tractus solitarii that were immunoreactive to N-methyl-D-aspartate receptor subunit 1. These results suggest that glutamatergic neurotransmission plays an active role, via comparable contributions from both N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors, in the expression of Fos protein at the caudal nucleus tractus solitarii in response to baroreceptor activation.

Immunohistochemical detection of glutamate in rat vagal sensory neurons

Vagal primary afferent neurons have their cell bodies located in the nodose (inferior) and jugular (superior) vagal ganglia and send terminals into the nucleus tractus solitarii (NTS) which lies in the dorsomedial medulla. The presence of glutamate (Glu)-containing neurons in the rat nodose ganglion was investigated using immunohistochemistry. Glu-immunoreactivity on nodose sections was found in neuronal perikarya and nerve fibers, but not in non-neuronal elements such as Schwann cells and satellite cells. Both immunoreactive and non-immunoreactive ganglion cells were observed. The immunoreactive ganglion cells amounted to about 60% of the nodose population. No specific intraganglionic localization was observed for the non-immunoreactive cells. Immunoreactive perikarya were slightly smaller than the non-immunoreactive ones, but no relationship was found between size and staining intensities of immunoreactive neurons. The present data indicate that immunodetectable Glu is present in a large population of vagal afferent neurons. They therefore add to a growing body of evidence suggesting that Glu may be the main neurotransmitter released by vagal afferent terminals within the nucleus tractus solitarii.

Demonstration of glutamate immunoreactivity in vagal sensory afferents in the nucleus tractus solitarius of the rat

To investigate whether glutamate is a neurotransmitter in vagus nerve sensory afferents terminating in the nucleus tractus solitarius, these terminals were identified by the anterograde transport and their glutamate content examined using the post-embedding immunogold technique. After injection of horseradish peroxidase into the nodose ganglion anterogradely labelled axonal boutons were visualized throughout the nucleus of the solitary tract (nTS), the dorsal motonucleus of the vagus nerve (DVN), predominantly ipsilateral to the injection, and to a lesser extent in the area postrema. Electron microscopic analysis of 47 anterogradely labelled boutons in the nTS following post-embedding immunocytochemistry for glutamate revealed that 43 of these boutons (> 91%) contained a level of glutamate immunoreactivity significantly greater (P < 0.001%) than that observed in the surrounding tissue. The observed enrichment of glutamate immunoreactivity in boutons identified as vagus nerve sensory afferents indicate that glutamate may be a transmitter in these neurones.