Intrinsic gamma-aminobutyric acid neurons in the nucleus of the solitary tract and the rostral ventrolateral medulla of the rat: an immunocytochemical and biochemical study

Physiological and pathophysiological evaluation of baroreflex functionality with concurrent diffusion tensor imaging of its neural circuit in the rat

Background

By measuring the prevalence of neuronal traffic between two brain structures based on the notion that diffusion of water molecules along the axon in parallel bundles will create prominent anisotropy in the direction of the passage of action potentials, diffusion tensor imaging (DTI) may be taken as an effective tool for functional investigations. Demonstration of complementary results obtained from synchronized DTI of the baroreflex neural circuit and physiological or pathophysiological evaluation of baroreflex functionality should validate this notion.

Methods

We implemented concurrent changes in neuronal traffic within the neural circuit of the baroreflex-mediated sympathetic vasomotor tone in the brain stem and alterations of its experimental surrogate under physiological and pathophysiological conditions. We further evaluated the functional and clinical implications of results obtained from this experimental paradigm in conjunction with baroreflex induction and a mevinphos intoxication model of brain stem death.

Results

We found that robust connectivity existed between the nucleus tractus solitarii and rostral ventrolateral medulla, the afferent and efferent nuclei of the baroreflex-mediated sympathetic vasomotor. Intriguingly, this connectivity was either reversibly disrupted or irreversibly severed to reflect alterations in baroreflex responses to physiological or pathophysiological challenges.

Conclusions

The capability to observe simultaneous and complementary changes in neuronal traffic within the neural circuit of the baroreflex-mediated sympathetic vasomotor tone and alterations of its experimental surrogate that bears technical, scientific and clinical implications sustains the notion that coupled with relevant physiological phenotypes, DTI can be an effective investigative tool for functional evaluations of brain stem activities.

Baroreflex functionality in the eye of diffusion tensor imaging

By applying diffusion tensor imaging (DTI) as a physiological tool to evaluate changes in functional connectivity between key brainstem nuclei in the baroreflex neural circuits of mice and rats, recent work has revealed several hitherto unidentified phenomena regarding baroreflex functionality. (1) The presence of robust functional connectivity between nucleus tractus solitarii (NTS) and nucleus ambiguus (NA) or rostral ventrolateral medulla (RVLM) offers a holistic view on the moment-to-moment modus operandi of the cardiac vagal baroreflex or baroreflex-mediated sympathetic vasomotor tone. (2) Under pathophysiological conditions (e.g. neurogenic hypertension), the disruption of functional connectivity between key nuclei in the baroreflex circuits is reversible. However, fatality ensues on progression from pathophysiological to pathological conditions (e.g. hepatic encephalopathy) when the functional connectivity between NTS and NA or RVLM is irreversibly severed. (3) The absence of functional connectivity between the NTS and caudal ventrolateral medulla (CVLM) necessitates partial rewiring of the classical neural circuit that includes CVLM as an inhibitory intermediate between the NTS and RVLM. (4) Sustained functional connectivity between the NTS and NA is responsible for the vital period between brain death and the inevitable cardiac death. (5) Reduced functional connectivity between the NTS and RVLM or NA points to inherent anomalous baroreflex functionality in floxed and Cre-Lox mice. (6) Disrupted NTS-NA functional connectivity in Flk-1 (VEGFR2) deficient mice offers an explanation for the hypertensive side-effect of anti-vascular endothelial growth factor therapy (anti-VEGF) therapy. These newly identified baroreflex functionalities revealed by DTI bear clinical and therapeutic implications.

MRI/DTI of the Brain Stem Reveals Reversible and Irreversible Disruption of the Baroreflex Neural Circuits: Clinical Implications

Baroreflex is the physiological mechanism for the maintenance of blood pressure and heart rate. Impairment of baroreflex is not a disease per se. However, depending on severity, the eventuality of baroreflex dysfunction varies from inconvenience in daily existence to curtailment of mobility to death. Despite universal acceptance, neuronal traffic within the contemporary neural circuits during the execution of baroreflex has never been visualized. By enhancing signal detection and fine-tuning the scanning parameters, we have successfully implemented tractographic analysis of the medulla oblongata in mice that allowed for visualization of connectivity between key brain stem nuclei in the baroreflex circuits. When viewed in conjunction with radiotelemetric analysis of the baroreflex, we found that under pathophysiological conditions when the disrupted connectivity between key nuclei in the baroreflex circuits was reversible, the associated disease condition (e.g. neurogenic hypertension) was amenable to remedial measures. Nevertheless, fatality ensues under pathological conditions (e.g. hepatic encephalopathy) when the connectivity between key substrates in the baroreflex circuits was irreversibly severed. MRI/DTI also prompted partial re-wiring of the contemporary circuit for baroreflex-mediated sympathetic vasomotor tone, and unearthed an explanation for the time lapse between brain death and the inevitable asystole signifying cardiac death that follows.

Effects of anesthetics on the renal sympathetic response to anaphylactic hypotension in rats

The autonomic nervous system plays an important role in rat anaphylactic hypotension. It is well known that sympathetic nerve activity and cardiovascular function are affected by anesthetics. However, the effects of different types of anesthesia on the efferent renal sympathetic nerve activity (RSNA) during anaphylactic hypotension remain unknown. Therefore, we determined the renal sympathetic responses to anaphylactic hypotension in anesthetized and conscious rats and the roles of baroreceptors in these responses. Sprague-Dawley rats were randomly allocated to anesthetic groups that were given pentobarbital, urethane, or ketamine-xylazine and to a conscious group. The rats were sensitized using subcutaneously injected ovalbumin. The systemic arterial pressure (SAP), RSNA and heart rate (HR) were measured. The effects of sinoaortic baroreceptor denervation on RSNA during anaphylaxis were determined in pentobarbital-anesthetized and conscious rats. In all of the sensitized rats, the RSNA increased and SAP decreased after antigen injection. At the early phase within 35 min of the antigen injection, the antigen-induced sympathoexcitation in the conscious rats was significantly greater than that in the anesthetized rats. Anaphylactic hypotension was attenuated in the conscious rats compared to the anesthetized rats. The anesthetic-induced suppression of SAP and RSNA was greater in the order ketamine-xylazine >urethane = pentobarbital. Indeed, in the rats treated with ketamine-xylazine, RSNA did not increase until 40 min, and SAP remained at low levels after the antigen injection. The baroreceptor reflex, as evaluated by increases in RSNA and HR in response to the decrease in SAP induced by sodium nitroprusside (SNP), was suppressed in the anesthetized rats compared with the conscious rats. Consistent with this finding, baroreceptor denervation attenuated the excitatory responses of RSNA to anaphylaxis in the conscious rats but not in the pentobarbital-anesthetized rats. RSNA was increased markedly in conscious rats during anaphylactic hypotension. Anesthetics attenuated this antigen-induced renal sympathoexcitation through the suppression of baroreceptor function.

Brain sources of inhibitory input to the rat rostral ventrolateral medulla

The rostral ventrolateral medulla (RVLM) contains neurons critical for cardiovascular, respiratory, metabolic, and motor control. The activity of these neurons is controlled by inputs from multiple identified brain regions; however, the neurochemistry of these inputs is largely unknown. Gamma-aminobutyric acid (GABA) and enkephalin tonically inhibit neurons within the RVLM. The aim of this study was to identify all brain regions that provide GABAergic or enkephalinergic input to the rat RVLM. Neurons immunoreactive for cholera toxin B (CTB-ir), retrogradely transported from the RVLM, were assessed for expression of glutamic acid decarboxylase (GAD67) or preproenkephalin (PPE) mRNA using in situ hybridization. GAD67 mRNA was expressed in CTB-ir neurons in the following regions: the nucleus of the solitary tract (NTS, 6% of CTB-ir neurons), area postrema (AP, 8%), caudal ventrolateral medulla (17%), midline raphe (40%), ventrolateral periaqueductal gray (VLPAG, 15%), lateral hypothalamic area (LHA, 25%), central nucleus of the amygdala (CeA, 77%), sublenticular extended amygdala (SLEA, 86%), interstitial nucleus of the posterior limb of the anterior commissure (IPAC, 56%), bed nucleus of the stria terminals (BNST, 59%), and medial preoptic area (MPA, 53%). PPE mRNA was expressed in CTB-ir neurons in the following regions: the NTS (14% of CTB-ir neurons), midline raphe (26%), LHA (22%), zona incerta (ZI, 15%), CeA (5%), paraventricular nucleus (PVN, 13%), SLEA (66%), and MPA (26%). Thus, limited brain regions contribute GABAergic and/or enkephalinergic input to the RVLM. Multiple neurochemically distinct pathways originate from these brain regions projecting to the RVLM.

Central neural responses to restraint stress are altered in rats with an early life history of repeated brief maternal separation

Repeated brief maternal separation (i.e. 15 min daily, MS15) of rat pups during the first one to two postnatal weeks enhances active maternal care received by the pups and attenuates their later behavioral and neuroendocrine responses to stress. In previous work, we found that MS15 also alters the developmental assembly and later structure of central neural circuits that control autonomic outflow to the viscera, suggesting that MS15 may alter central visceral circuit responses to stress. To examine this, juvenile rats with a developmental history of either MS15 or no separation (NS) received microinjection of retrograde neural tracer, FluoroGold (FG), into the hindbrain dorsal vagal complex (DVC). After 1 week, FG-injected rats and surgically intact littermates were exposed to either a 15-min restraint stress or an unrestrained control condition, and then perfused 1 h later. Brain tissue sections from surgically intact littermates were processed for Fos alone or in combination with phenotypic markers to examine stress-induced activation of neurons within the paraventricular nucleus of the hypothalamus (PVN), bed nucleus of the stria terminalis (BNST), and hindbrain DVC. Compared to NS controls, MS15 rats displayed less restraint-induced Fos activation within the dorsolateral BNST (dBNST), the caudal PVN, and noradrenergic neurons within the caudal DVC. To examine whether these differences corresponded with altered neural inputs to the DVC, sections from tracer-injected rats were double-labeled for FG and Fos to quantify retrogradely labeled neurons within hypothalamic and limbic forebrain regions of interest, and the proportion of these neurons activated after restraint. Only the dBNST displayed a significant effect of postnatal experience on restraint-induced Fos activation of DVC-projecting neurons. The distinct regional effects of MS15 on stress-induced recruitment of neurons within hypothalamic, limbic forebrain, and hindbrain regions has interesting implications for understanding how early life experience shapes the functional organization of stress-responsive circuits.

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.

GABAergic neurons in the ventrolateral subnucleus of the nucleus tractus solitarius are in contact with Kölliker-Fuse nucleus neurons projecting to the rostral ventral respiratory group and phrenic nucleus in the rat

After ipsilateral injections of biotinylated dextran amine (BDA) into the ventrolateral subnucleus of the nucleus tractus solitarius (vlNTS) and Fluoro-gold (FG) into the rostral ventral respiratory group (rVRG) region or into the phrenic nucleus (PhN) region in the rat, an overlapping distribution of BDA-labeled axon terminals and FG-labeled neurons was found in the Kölliker-Fuse (KF) nucleus ipsilateral to the injection sites. Using retrograde tracing combined with immunohistochemistry for glutamic acid decarboxylase isoform 67 (GAD67), we indicated that as many as 40% of the vlNTS neurons projecting to the KF were immunoreactive for GAD67. Using a combination of anterograde and retrograde tracing techniques, and immunohistochemistry for GAD67, we further demonstrated that the vlNTS axon terminals with GAD67 immunoreactivity established close contact to the rVRG- or PhN-projecting KF neurons. The present results suggest that GABAergic vlNTS fibers may exert inhibitory influences on the rVRG- as well as PhN-projecting KF neurons and these circuits may be involved in the respiratory reflexes such as the Hering-Breuer reflex.

Quantitative and immunohistochemical analysis of neuronal types in the mouse caudal nucleus tractus solitarius: focus on GABAergic neurons

gamma-Aminobutyric acid-ergic (GABAergic) neurons are major inhibitory interneurons that are widely distributed in the central nervous system. The caudal nucleus tractus solitarius (cNTS), which plays a key role in respiratory, cardiovascular, and gastrointestinal function, contains GABAergic neurons for regulation of neuronal firing. In the present study, GABAergic neuronal organization was analyzed in relation to the location of subnuclei in the mouse cNTS. According to the differential expression of glutamate decarboxylase 67 (GAD67), vesicular glutamate transporter 2 (VGLUT2), calbindin, and tyrosine hydroxylase (TH) mRNAs, the cNTS was divided into four subnuclei: the subpostrema, dorsomedial, commissural, and medial subnuclei. The numerical density and size of soma in the four subnuclei were then quantified by an unbiased dissector analysis. Calbindin-positive cells constituted subpopulations of small non-GABAergic neurons preferentially localized in the subpostrema subnucleus. TH-positive cells constituted large neurons preferentially localized in the medial subnucleus. GABAergic neurons constituted a subpopulation of small neurons, preferentially localized in the commissural and medial subnuclei, which represented > or =50% of small cells in these subnuclei. Thus, the GABAergic small neurons were located around TH-positive large cells in the ventrolateral portion of the cNTS. This finding, in combination with results of previous studies in the rat cNTS showing that large cells originate efferents from the cNTS, suggests that GABAergic small neurons in the commissural and medial subnuclei might regulate output from the cNTS.

Inhibitory modulation of chemoreflex bradycardia by stimulation of the nucleus raphe obscurus is mediated by 5-HT3 receptors in the NTS of awake rats

Several studies demonstrated the involvement of 5-hydroxytryptamine (5-HT) and its different receptor subtypes in the modulation of neurotransmission of cardiovascular reflexes in the nucleus tractus solitarii (NTS). Moreover, anatomic evidence suggests that nucleus raphe obscurus (ROb) is a source of 5-HT-containing terminals within the NTS. In the present study we investigated the possible changes in the cardiovascular responses to peripheral chemoreceptor activation by potassium cyanide (KCN, i.v.) following ROb stimulation with L-glutamate (10 nmol/50 nL) and also whether 5-HT3 receptors in the caudal commissural NTS are involved in this neuromodulation. The results showed that stimulation of the ROb with L-glutamate in awake rats (n=15) produced a significant reduction in the bradycardic response 30 s after the microinjection (-182+/-19 vs -236+/-10 bpm; Wilcoxon test) but no changes in the pressor response to peripheral chemoreceptor activation (43+/-4 vs 51+/-3 mmHg; two-way ANOVA) in relation to the control. Microinjection of 5--HT3 receptors antagonist granisetron (500 pmol/50 nL), but not the vehicle, into the caudal commissural NTS bilaterally prevented the reduction of chemoreflex bradycardia in response to microinjection of L-glutamate into ROb. These data indicate that 5-HT-containing projections from ROb to the NTS play an inhibitory neuromodulatory role in the chemoreflex evoked bradycardia by releasing 5-HT and activating 5-HT3 receptors in the caudal NTS.

Immunohistochemical localization of GAD67-expressing neurons and processes in the rat brainstem: subregional distribution in the nucleus tractus solitarius

The role of gamma-aminobutyric acid (GABA) in homeostatic control in the brainstem, in particular, in the nucleus tractus solitarius (NTS), is well established. However, to date, there is no detailed description of the distribution of GABAergic neurons within the NTS. The goal of the current study was to reexamine the efficacy of immunohistochemical localization of glutamic acid decarboxylase (GAD) protein, specifically the 67-kDa isoform (GAD67), as a marker for GABAergic neurons in the medulla and to provide a detailed map of GAD67-immunoreactive (-ir) cells within rat NTS by using a recently developed mouse monoclonal antibody. We describe a distribution of GAD67-ir cells in the medulla similar to that reported previously from in situ hybridization study. GAD67-ir cells were localized in regions known to contain high GABA content, including the ventrolateral medulla, raphe nuclei, and area postrema, but were absent from all motor nuclei, although dense terminal labeling was discerned in these regions. In the NTS, GAD67-ir was localized in all subregions. Semiquantitative analysis of the GAD67-ir distribution in the NTS revealed greater numbers of GAD67-ir cells medial to the solitary tract. Finally, dense GAD67 terminal labeling was found in the medial, central, intermediate, commissural, and subpostremal subregions, whereas sparse labeling was observed in the ventral subregion. Our findings support the use of immunohistochemistry for GAD67 as a marker for the localization of GABAergic cells and terminal processes in the rat brainstem. Furthermore, the reported heterogeneous distribution of GAD67-ir in the NTS suggests differential inhibitory modulation of sensory processing.

Effect of a high-salt diet on gamma-aminobutyric acid-mediated responses in the nucleus tractus solitarius of Sprague-Dawley rats

Previous study using an indirect gamma-aminobutyric acid (GABA) agonist indicated that high salt intake enhances sensitivity of nucleus tractus solitarius (NTS) projecting inhibitory input to rostral ventrolateral medulla sympathoexcitatory neurons. We further investigated the relationship between salt intake and the GABA system in NTS. Sprague-Dawley (S-D) rats consuming high dietary salt (8%) or low dietary salt (0.3%) for 3 weeks were used. Under chloralose-anesthesia, baseline arterial pressure (AP) and heart rate (HR) were similar in both groups. Bilateral injection into NTS of nipecotic acid, GABA(A) receptor agonist (muscimol), or GABA(B) receptor agonist (baclofen) elicited greater pressor responses in high-salt group. GABA(A) receptor antagonist, bicuculline and GABA(B) receptor antagonist, CGP-35348 elicited greater depressor responses. Phenylephrine or nitroprusside (i.v.) elicited similar respective increases or decreases in AP in both groups. Baroreflex sensitivity was similar. Thus, high-salt intake enhances both GABA(A) receptor- and GABA(B) receptor-mediated responses within NTS, thereby inhibiting elevation of AP.

Brain stem melanocortinergic modulation of meal size and identification of hypothalamic POMC projections

Metabolic, cognitive, and environmental factors processed in the forebrain modulate food intake by changing the potency of direct controls of meal ingestion in the brain stem. Here, we behaviorally and anatomically test the role of the hypothalamic proopiomelanocortin (POMC) system in mediating some of these descending, indirect controls. Melanotan II (MTII), a stable melanocortin 4 receptor (MC4R) and melanocortin 3 receptor (MC3R) agonist injected into the fourth ventricle near the dorsal vagal complex, potently inhibited 14-h food intake by decreasing meal size but not meal frequency; SHU9119, an antagonist, increased food intake by selectively increasing meal size. Furthermore, MTII injected into the fourth ventricle increased and SHU9119 tended to decrease heart rate and body temperature measured telemetrically in freely moving rats. Numerous alpha-melanocyte-stimulating hormone-immunoreactive axons were in close anatomical apposition to nucleus tractus solitarius neurons showing c-Fos in response to gastric distension, expressing neurochemical phenotypes implicated in ingestive control, and projecting to brown adipose tissue. In retrograde tracing experiments, a small percentage of arcuate nucleus POMC neurons was found to project to the dorsal vagal complex. Thus melanocortin signaling in the brain stem is sufficient to alter food intake via changing the potency of satiety signals and to alter sympathetic outflow. Although the anatomical findings support the involvement of hypothalamomedullary POMC projections in mediating part of the descending, indirect signal, they do not rule out involvement of POMC neurons in the nucleus tractus solitarius in mediating part of the direct signal.

GABA, in some cases together with glycine, is used as the inhibitory transmitter by pump cells in the Hering-Breuer reflex pathway of the rat

The Hering-Breuer reflex is one of the fundamental respiratory reflexes and is mediated by second-order relay neurons of the slowly adapting lung stretch receptors. These neurons, which are called pump cells, are located in the nucleus tractus solitarii and include a population of inhibitory neurons. We aimed to determine which transmitter, GABA or glycine, the inhibitory pump cells use. In addition, we examined whether or not second-order relay neurons of the rapidly-adapting lung stretch receptors (RAR-cells), whose excitatory or inhibitory nature is not known, use these inhibitory neurotransmitters. In Nembutal-anesthetized, neuromuscularly blocked and artificially ventilated rats, we labeled pump cells (n=33) and RAR-cells (n=26) with Neurobiotin and processed the tissues for detection of mRNA encoding either glutamic acid decarboxylase isoform 67 (GAD67) or glycine transporter 2 (GLYT2) using in situ hybridization. The pump cells were located in the interstitial nucleus and its vicinity and the RAR-cells in the commissural subnucleus. The majority (64%) of the pump cells examined for GAD67 mRNA and many (26%) of the pump cells examined for GLYT2 mRNA expressed respective mRNAs. Of the eight pump cells in which both mRNAs were double-detected, three expressed both mRNAs and one expressed GAD67 mRNA but not GLYT2 mRNA, the other four expressing neither mRNAs. On the other hand, RAR-cells expressed neither GAD67 mRNA nor GLYT2 mRNA. The results suggest that the inhibitory pump cells are basically GABAergic and some of them may corelease GABA and glycine, and that RAR-cells are neither GABAergic nor glycinergic. These findings expand our understanding of the networks of lung receptor-mediated reflexes including the Hering-Breuer reflex.

Functional organization of brain pathways subserving the baroreceptor reflex: studies in conscious animals using immediate early gene expression

1. This paper reviews studies carried out in our laboratory in which we have used the c-fos functional mapping method, in combination with other methods, to determine the functional organization of central baroreceptor pathways as they operate in the conscious rabbit. 2. First, we showed that periods of induced hypertension or hypotension each result in a specific and reproducible pattern of activation of neurons in the brainstem and forebrain. In particular, hypotension (but not hypertension) results in the activation of catecholamine neurons in the medulla and pons and vasopressin-synthesizing neurons in the hypothalamus. 3. The activation of medullary cell groups in response to induced hypertension or hypotension in the conscious rabbit is almost entirely dependent on inputs from arterial baroreceptors, while the activation of hypothalamic vasopressin-synthesising neurons in response to hypotension is largely dependent on baroreceptors, although an increase in circulating angiotensin also appears to contribute. 4. Discrete groups of neurons in the rostral ventrolateral medulla (RVLM) and A5 area in the pons are the major groups of spinally projecting neurons activated by baroreceptor unloading. In contrast, spinally projecting neurons in the paraventricular nucleus in the hypothalamus appear to be largely unaffected by baroreceptor signals. 5. Direct afferent inputs to RVLM neurons in response to increases or decreases in arterial pressure originate primarily from other medullary nuclei, particularly neurons located in the caudal and intermediate levels of the ventrolateral medulla (CVLM and IVLM), as well as in the nucleus tractus solitarius (NTS). 6. There is also a direct projection from barosensory neurons in the NTS to the CVLM/IVLM region, which is activated by baroreceptor inputs. 7. Collectively, the results of our studies in conscious animals indicate that baroreceptor signals reach all levels of the brain. With regard to the baroreceptor reflex control of sympathetic activity, our studies are consistent with previous studies in anesthetized animals, but in addition reveal other previously unrecognized pathways that also contribute to this reflex regulation.

Neurokinin-1 receptors in the rat nucleus tractus solitarius: pre- and postsynaptic modulation of glutamate and GABA release

Neurokinins such as substance P and neurokinin A have long been thought to act as neurotransmitters or modulators in the nucleus tractus solitarius. However, the role and location of the receptors for these peptides have remained unclear. We examined the consequences of activation of the neurokinin-1 (NK1) receptor subtype in the rat nucleus tractus solitarius using whole-cell patch clamp recordings in brain slices. Application of delta-Ala-Phe-Phe-Pro-MeLeu-D-Pro[spiro-gamma-lactam]-Leu-Trp-NH2 (a specific NK1 agonist) or neurokinin A resulted in depolarization, evident as a slow inward current, mediated by direct postsynaptic NK1 receptor activation. The effect was conserved in the presence of tetrodotoxin, and protein kinase C-dependent since it was blocked by 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl)maleimide, a specific protein kinase C inhibitor. In addition, an increase in the frequency and amplitude of spontaneous excitatory postsynaptic currents was observed, reflecting increased glutamate release induced by NK1 receptor activation. This effect was abolished by tetrodotoxin, suggesting that it resulted from increased firing in afferent neurons, subsequent to somatodendritic excitation via NK1 receptors. Furthermore, spontaneous inhibitory postsynaptic currents were increased in frequency and amplitude showing that GABA release was promoted by NK1 receptor activation. However, amplitude of miniature inhibitory postsynaptic currents was unaltered by NK1 receptor activation, but the increase in frequency persisted. These findings suggest that NK1 receptors are located on presynaptic terminals as well as at somatodendritic sites of GABAergic neurons. The increase in GABA release was also shown to be protein kinase C-dependent. The data presented here show NK1 receptors in the rat nucleus tractus solitarius are present both excitatory and inhibitory neurons. Activation of these receptors can result in increases in release of both GABA and glutamate, suggesting a crucial modulatory role for NK1 receptors in the rat nucleus tractus solitarius.

Distribution of glycine transporter 2 mRNA-containing neurons in relation to glutamic acid decarboxylase mRNA-containing neurons in rat medulla

We studied the distribution of medullary glycinergic neurons in relation to GABAergic neurons, by using in situ hybridization method for mRNA encoding either glycine transporter 2 (GLYT2) or glutamic acid decarboxylase isoform 67 (GAD67). GLYT2 mRNA-positive (GLYT2+) neurons were distributed widely and clustered in (1). the respiration-related area of the ventrolateral medulla called the Bötzinger complex, (2). the nucleus retroambiguus caudal to the obex or the caudal ventral respiratory group, (3). the spinal trigeminal nucleus, (4). a small area immediately dorsal to the inferior olivary nucleus, and (5). the border zone between the hypoglossal nucleus and the surrounding reticular formation. It was characteristic that in the dorsomedial medulla, GLYT2+ neurons were distributed only sparsely in contrast to dense GAD67+ neurons. Only few GLYT2+ neurons were distributed in the medial and interstitial subnuclei of the nucleus tractus solitarii. In particular virtually no GLYT2+ neurons were found in the area postrema. Furthermore, in the reticular formation and the spinal trigeminal nucleus, GAG67+ neurons tended to be distributed in the area where GLYT2+ neurons were sparse, and vice versa. These results provide useful information for the effort of determining neurotransmitters involved in the medullary neurons.

Functional organisation of central cardiovascular pathways: studies using c-fos gene expression

Until about 10 years ago, knowledge of the functional organisation of the central pathways that subserve cardiovascular responses to homeostatic challenges and other stressors was based almost entirely on studies in anaesthetised animals. More recently, however, many studies have used the method of the expression of immediate early genes, particularly the c-fos gene, to identify populations of central neurons that are activated by such challenges in conscious animals. In this review we first consider the advantages and limitations of this method. Then, we discuss how the application of the method of immediate early gene expression, when used alone or in combination with other methods, has contributed to our understanding of the central mechanisms that regulate the autonomic and neuroendocrine response to various cardiovascular challenges (e.g., hypotension, hypoxia, hypovolemia, and other stressors) as they operate in the conscious state. In general, the results of studies of central cardiovascular pathways using immediate early gene expression are consistent with previous studies in anaesthetised animals, but in addition have revealed other previously unrecognised pathways that also contribute to cardiovascular regulation. Finally, we briefly consider recent evidence indicating that immediate early gene expression can modify the functional properties of central cardiovascular neurons, and the possible significance of this in producing long-term changes in the regulation of the cardiovascular system both in normal and pathological conditions.

Effects of GABA(A) and glycine receptor agonists on the medullary inspiratory neuronal activity during spontaneous augmented breaths in anesthetized rats

To clarify whether GABAergic or glycinergic transmission alters the activity of inspiratory neurons during spontaneous augmented breaths, we recorded the single unit activity from inspiratory neurons in the dorsal and ventral respiratory groups in the medulla of pentobarbital anesthetized rats and applied GABA(A) and glycine receptor agonists by iontophoresis using multibarrel microelectrodes. The spontaneous augmented breath was divided into two different phases; the first phase (phase I) resembled a normal inspiration but the second phase (phase II) indicated a marked increase in diaphragm electromyogram activity. During application of either muscimol or glycine, the discharge of inspiratory neurons during the phase I of spontaneous augmented breaths was suppressed, but the augmenting discharge of the phase II did not change significantly in any cell type of the neurons (I-augmenting, I-decrementing and I-other). These results suggested that the excitatory inputs to inspiratory neurons during the phase II of augmented breaths may not be significantly influenced by the activation of either GABA(A) receptors or glycine receptors.

Somatic nociception activates NK1 receptors in the nucleus tractus solitarii to attenuate the baroreceptor cardiac reflex

There is limited information regarding the integration of visceral and somatic afferents within the nucleus of the solitary tract (NTS). We studied the interaction of nociceptive and baroreceptive inputs in this nucleus in an in situ arterially perfused, un-anaesthetized decerebrate preparation of rat. At the systemic level, the gain of the cardiac component of the baroreceptor reflex was attenuated significantly by noxious mechanical stimulation of a forepaw. This baroreceptor reflex depression was mimicked by NTS microinjection of substance P and antagonized by microinjection of either bicuculline (a GABAA receptor antagonist) or a neurokinin type 1 (NK1) receptor antagonist (CP-99994). The substance P effect was also blocked by a bilateral microinjection of bicuculline, at a dose that was without effect on basal baroreceptor reflex gain. Baroreceptive NTS neurons were defined by their excitatory response following increases in pressure within the ipsilateral carotid sinus. In 27 of 34 neurons the number of evoked spikes from baroreceptor stimulation was reduced significantly by concomitant electrical stimulation of the brachial nerve (P < 0.01). Furthermore, the attenuation of baroreceptor inputs to NTS neurons by brachial nerve stimulation was prevented by pressure-ejection of bicuculline from a multi-barrelled microelectrode (n = 8). In a separate population of 17 of 45 cells tested, brachial nerve stimulation evoked an excitatory response that was antagonized by blockade of NK1 receptors. We conclude that nociceptive afferents activate NK1 receptors, which in turn excite GABAergic interneurons impinging on cells mediating the cardiac component of the baroreceptor reflex.

GABAergic neurotransmission at the nucleus tractus solitarii in the suppression of reflex bradycardia by parabrachial nucleus

We investigated the role of GABAergic neurotransmission at the nucleus tractus solitarii (NTS) in the suppression of cardiac baroreceptor reflex (BRR) response induced by parabrachial nucleus (PBN) complex in adult Sprague-Dawley rats maintained under pentobarbital anesthesia. Based on in vivo microdialysis coupled with high-performance liquid chromatography-fluorescence detection for gamma-aminobutyric acid (GABA), we found that electrical stimulation of the ventrolateral regions and Koelliker-Fuse (KF) subnucleus of PBN complex resulted in a site-specific increase in GABA concentration in the dialysate collected from the NTS. The temporal increase in extracellular GABA concentration in the NTS coincided with the time course of PBN-induced cardiac BRR inhibition. In addition, the PBN-induced cardiac BRR suppression was reversed by microinjection bilaterally into the NTS of a GABA(A) receptor antagonist, bicuculline methiodide (5 pmol), or a GABA(B) receptor antagonist, 2-OH saclofen (500 pmol). Blockade of neuronal activity in the ventrolateral regions and KF subnucleus of PBN complex with lidocaine (5%) elicited an enhancement of the same reflex response. The time course of this facilitatory effect of lidocaine correlated positively with the temporal decrease in extracellular GABA concentration in the NTS. Anatomically, Fast Blue-labeled neurons were identified in the same subnuclei of the PBN complex after microinjection of the retrograde transport tracer into the NTS. Some of these Fast Blue-labeled neurons were also immunoreactive to glutamic acid decarboxylase. These results suggest that a direct GABAergic descending projection from the KF subnucleus and surrounding areas of the PBN complex to the NTS may inhibit cardiac BRR response by activating GABA(A) and GABA(B) receptors at the NTS.

Brain stem control of swallowing: neuronal network and cellular mechanisms

Swallowing movements are produced by a central pattern generator located in the medulla oblongata. It has been established on the basis of microelectrode recordings that the swallowing network includes two main groups of neurons. One group is located within the dorsal medulla and contains the generator neurons involved in triggering, shaping, and timing the sequential or rhythmic swallowing pattern. Interestingly, these generator neurons are situated within a primary sensory relay, that is, the nucleus tractus solitarii. The second group is located in the ventrolateral medulla and contains switching neurons, which distribute the swallowing drive to the various pools of motoneurons involved in swallowing. This review focuses on the brain stem mechanisms underlying the generation of sequential and rhythmic swallowing movements. It analyzes the neuronal circuitry, the cellular properties of neurons, and the neurotransmitters possibly involved, as well as the peripheral and central inputs which shape the output of the network appropriately so that the swallowing movements correspond to the bolus to be swallowed. The mechanisms possibly involved in pattern generation and the possible flexibility of the swallowing central pattern generator are discussed.

Glutamatergic and GABAergic inputs to the RVL mediate cardiovascular adjustments to noxious stimulation

Stimulation of cutaneous and muscle afferents induces several cardiovascular adjustments such as hypertension, tachycardia, and muscle vasodilation. Although previous studies have demonstrated that the rostral ventrolateral medulla (RVL) mediates sympathoexcitation and pressor responses to sciatic nerve stimulation (SNS), whether it also mediates blood flow adjustments remains unclear. Therefore, in the present study, we examined the role of the RVL in the vasodilation induced by SNS and the possible neurotransmitters involved. In Urethane-anesthetized, paralyzed, and artificially ventilated rats, SNS (square pulses, 1 ms, 20 Hz, 800--1200 microA, 10 s) produced increases in blood pressure, heart rate, blood flow, and vascular conductance of the stimulated limb. Unilateral microinjection of kainic acid (2 nmol/100 nl) into the RVL contralateral to the stimulated limb abolished cardiovascular adjustments to SNS. Unilateral microinjections of kynurenic acid (2 nmol/100 nl) selectively abolished the pressor response to SNS, whereas bicuculline (400 pmol/100 nl) abolished the increases in blood flow without changing the pressor response. These results suggest that glutamatergic synapses within the RVL mediate pressor responses, whereas GABAergic synapses may mediate the vasodilation to SNS.

Rostral ventrolateral medulla suppresses reflex bradycardia by the release of gamma-aminobutyric acid in nucleus tractus solitarii of the rat

We investigated the role of gamma-aminobutyric acid (GABA) in the nucleus tractus solitarii (NTS), the principal recipient of baroreceptor afferent fibers in the medulla oblongata, in the suppression of cardiac baroreceptor reflex (BRR) response by the rostral ventrolateral medulla (RVLM). Direct microinfusion via reverse microdialysis of L-glutamate (50 microM) into the RVLM promoted an inhibition of the BRR response, alongside an increase in the concentration of GABA in the dialysate collected from the ipsilateral NTS. Such an increase in GABA concentration in the NTS to RVLM activation was site-specific, as microinfusion of L-glutamate into areas outside the confines of RVLM resulted in no discernible change in GABA concentration in the dialysate of the NTS and minimal effect on the cardiac BRR response. The RVLM-induced BRR suppression of cardiac BRR response to microinjection into the bilateral RVLM of L-glutamate (1 nmol) was antagonized by administration into the bilateral NTS of the GABA(A) receptor antagonist, bicuculline methiodide (1 or 5 pmol), or the GABA(B) receptor antagonist, 2-hydroxy-saclofen (100 or 500 pmol). These results suggest that GABA released in the NTS may participate in cardiac BRR suppression induced by glutamatergic activation of the RVLM, via an action on both GABA(A) and GABA(B) receptor subtypes.

Microinjection of a 5-HT3 receptor agonist into the NTS of awake rats inhibits the bradycardic response to activation of the von Bezold-Jarisch reflex

In the present study we investigated the effects of bilateral microinjection into the lateral commissural nucleus tractus solitarius (NTS) of 2-methyl-5-HT, a 5-HT3 receptor agonist, on the bradycardic response of the von Bezold-Jarisch reflex of awake rats. We evaluated mainly the bradycardic response because in previous studies we documented that the hypotensive response of the von-Bezold-Jarisch reflex in awake rats is secondary to the intense bradycardic response. The Bezold-Jarisch reflex was activated by intravenous injection of serotonin (8 microg/kg) in awake rats before and 1, 3, 10, 20 and 60 min after bilateral microinjection of 2-methyl-5-HT (5 nmol/50 nl, n = 8) into the NTS. Microinjections of 2-methyl-5-HT into the NTS produced a significant increase in basal mean arterial pressure [(MAP), 97 +/- 4 vs. 114 +/- 4 mmHg), no changes in basal heart rate and a significant reduction in bradycardic (-78 +/- 19; -94 +/- 24 and -107 +/- 21 bpm) and hypotensive (-16 +/- 4; -10 +/- 5 and -17 +/- 4 mmHg) responses to activation of the von Bezold-Jarisch reflex at 3, 10 and 20 min, respectively, when compared with the control value (-231 +/- 13 bpm and -43 +/- 4 mmHg). The data of the present study suggest that serotonin acting on 5-HT3 receptors in the NTS may play an important inhibitory neuromodulatory role in the bradycardic response to activation of the von Bezold-Jarisch reflex.

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

In the present study we evaluated the effects of bilateral microinjection of muscimol (a GABA(A) receptor agonist) and baclofen (a GABA(B) receptor agonist) into the lateral commissural nucleus tractus solitarii (NTS) of awake rats on the gain of the baroreflex (BG) activated by a short duration (10-15 s) infusion of phenylephrine (Phe, 2.5 microg/0.05 ml, i.v.). Microinjection of muscimol (50 pmol/50 nl, n=8) into the NTS produced a significant increase in baseline mean arterial pressure ((MAP) 122+/-6 vs. 101+/-2 mmHg), no changes in baseline heart rate (HR) and a reduction in BG (-1.59+/-0. 1 vs. -0.69+/-0.1 beats/mmHg). Microinjection of baclofen (6.25 pmol/50 nl, n=6) into the NTS also produced a significant increase in baseline MAP (138+/-5 vs. 103+/-2 mmHg), no changes in baseline HR and a reduction in BG (-1.54+/-0.3 vs. -0.53+/-0.2 beats/mmHg). Considering that the reduction in BG could be secondary to the increase in MAP in response to microinjection of muscimol (n=6) or baclofen (n=7) into the NTS, in these two groups of rats we brought the MAP back to baseline by infusion of sodium nitroprusside (NP, 3.0 microg/0.05 ml, i.v.). Under these conditions, we verified that the BG remained significantly reduced after muscimol (-1.49+/-0.2 vs. -0.35+/-0.2 beats/mmHg) and after baclofen (-1.72+/-0.2 vs. -0.33+/-0.2 beats/mmHg) when compared to control. Reflex tachycardia was observed during the normalization of MAP by NP infusion and, in order to prevent the autonomic imbalance from affecting BG, we used another group of rats treated with atenolol (5 mg/kg, i.v.), a beta1 receptor antagonist. In rats previously treated with atenolol and submitted to NP infusion, we verified that BG remained reduced after microinjection of muscimol or baclofen into the NTS. The data show that activation of GABA(A) and GABA(B) receptors, independently of the changes in the baseline MAP or HR, inhibited the neurons of the NTS involved in the parasympathetic component of the baroreflex.

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.

Brainstem viscerotopic organization of afferents and efferents involved in the control of swallowing

Cholera toxin horseradish peroxidase (CT-HRP), a sensitive antegrade and retrograde tracer, is effective at labeling swallowing motoneurons and their dendritic fields within the nucleus ambiguus (NA), nucleus of the solitary tract (NTS), dorsal motor nucleus of the vagus nerve, and hypoglossal nucleus. Using this tracer to label motoneurons within the NTS demonstrates that palatal, pharyngeal, and laryngeal afferents overlap considerably within the interstitial and intermediate subnuclei. These afferents have a pattern of distribution within the NTS similar to the labeling observed after application of the same tracer to the superior laryngeal nerve. Esophageal afferents, however, terminate entirely within the central (NTScen) subnucleus and do not overlap their distribution with palatal, pharyngeal, or laryngeal afferents. Within the nodose ganglion (NG), sensory neurons projecting to the soft palate and pharynx are located superiorly, and those projecting to the esophagus and stomach are located inferiorly, an organization that indicates rostrocaudal positioning along the alimentary tract. Sensory neurons within the NG and NTS contain, among others, the major excitatory and inhibitory amino acid neurotransmitters glutamate (Glu) and gamma-aminobutyric-acid (GABA). Both Glu and GABA help to coordinate esophageal peristalsis. Using pseudorabies virus as a transsynaptic tracer demonstrates the role of GABA and Glu as mediators of synaptic transmission within the swallowing central pattern generator, a fact further supported by the presence of specific receptors for each neurotransmitter within the NTScen. Anatomic studies using CT-HRP have been effective in revealing the total extent of extranuclear dendritic projections and the organization of dendrites within the confines of a nucleus; further studies have produced the following data. Motoneurons innervating the soft palate, pharynx, larynx, and cervical esophagus have extensive dendrites that extend into the adjacent reticular formation with a distinct pattern for each muscle group. Motoneurons of the musculature active during the buccopharyngeal phase of swallowing (soft palate, pharynx, cricothyroid, and cervical esophagus) have extensive dendritic arborizations that terminate within the adjacent reticular formation of the NA. Swallowing premotor neurons located in the reticular formation surrounding the NA are active during the buccopharyngeal phase of swallowing. These data provide an anatomic basis for interaction of swallowing motoneurons with premotor neurons located in this area. Motoneurons innervating all levels of the esophagus are confined to the compact formation (NAc), whereas those motoneurons projecting to the pharynx and cricothyroid muscle are located in the semicompact formation (NAsc). The intrinsic laryngeal muscles were represented within the loose formation (NAI) and the heart within the external formation. In contrast, the dendrites of motoneurons projecting to the thoracic and subdiaphragmatic esophagus are confined to the NAc. Both the NAsc and NAc have extensive longitudinal bundling of dendrites within the confines of the nucleus, resulting in the formation of a rostrocaudal dendritic plexus where dendrites crisscross between bundles. Intranuclear bundling of dendrites is evident in the soft palate, pharynx, and esophagus and is lacking only for the cricothyroid muscle. Moreover, ventrolateral- and dorsomedial-oriented dendritic bundles are present within the NAsc. In contrast to the longitudinal dendritic bundles, the ventrolateral- and dorsomedial-oriented dendritic bundles exit the NAsc and penetrate the adjacent reticular formation. The extensive bundling of motoneuronal dendrites within the NA supports the hypothesis that these structures serve as networks for the generation of complex motor activities, such as swallowing.

Activation of GABAA but not GABAB receptors in the NTSblocked bradycardia of chemoreflex in awake rats

In the present study we analyzed effects of bilateral microinjections of muscimol (a GABAA agonist) and baclofen (a GABAB agonist) into the nucleus tractus solitarius (NTS) on bradycardic and pressor responses to chemoreflex activation (potassium cyanide, 40 micrograms/rat iv) in awake rats. Bilateral microinjections of muscimol (25 and 50 pmol/50 nl) into the NTS increased baseline mean arterial pressure (MAP): 119 +/- 8 vs. 107 +/- 2 mmHg (n = 6) and 121 +/- 8 vs. 103 +/- 3 mmHg (n = 6), respectively. Muscimol at 25 pmol/50 nl reduced the bradycardic response to chemoreflex activation 5 min after microinjection; with 50 pmol/50 nl the bradycardic response to chemoreflex activation was reduced 5, 15, 30, and 60 min after microinjection. Neither muscimol dose produced an effect on the pressor response of the chemoreflex. Effects of muscimol (50 pmol/50 nl) on basal MAP and on the bradycardic response of the chemoreflex were prevented by prior microinjection of bicuculline (a GABAA antagonist, 40 pmol/50 nl) into the NTS. Bilateral microinjections of baclofen (12.5 and 25 pmol/50 nl) into the NTS produced an increase in baseline MAP [137 +/- 9 vs. 108 +/- 4 (n = 7) and 145 +/- 5 vs. 105 +/- 2 mmHg (n = 7), respectively], no changes in basal heart rate, and no effects on the bradycardic response; 25 pmol/50 nl only attenuated the pressor response to chemoreflex activation. The data show that activation of GABAA receptors in the NTS produces a significant reduction in the bradycardic response, whereas activation of GABAB receptors produces a significant reduction in the pressor response of the chemoreflex. We conclude that 1) GABAA but not GABAB plays an inhibitory role in neurons of the lateral commissural NTS involved in the parasympathetic component of the chemoreflex and 2) attenuation of the pressor response of the chemoreflex by activation of GABAB receptors may be due to inhibition of sympathoexcitatory neurons in the NTS or may be secondary to the large increase in baseline MAP produced by baclofen.

Glutamatergic projection to RVLM mediates suppression of reflex bradycardia by parabrachial nucleus

We investigated the role of glutamatergic projection from the parabrachial nucleus (PBN) complex to the rostral ventrolateral medulla (RVLM) in the PBN-induced suppression of reflex bradycardia in adult Sprague-Dawley rats that were maintained under pentobarbital anesthesia. Under stimulus conditions that did not appreciably alter the baseline systemic arterial pressure and heart rate, electrical (10-s train of 0.5-ms pulses, at 10-20 microA and 10-20 Hz) or chemical (L-glutamate, 1 nmol) stimulation of the ventrolateral regions and Köelliker-Fuse (KF) subnucleus of the PBN complex significantly suppressed the reflex bradycardia in response to transient hypertension evoked by phenylephrine (5 micrograms/kg iv). The PBN-induced suppression of reflex bradycardia was appreciably reversed by bilateral microinjection into the RVLM of the N-methyl-D-aspartate (NMDA)-receptor antagonist MK-801 (500 pmol) or the non-NMDA-receptor antagonist 6-cyano-7-nitroquinoxaline-2, 3-dione (50 pmol). Anatomically, most of the retrogradely labeled neurons in the ventrolateral regions and KF subnucleus of the ipsilateral PBN complex after microinjection of fast blue into the RVLM were also immunoreactive to anti-glutamate antiserum. These results suggest that a direct glutamatergic projection to the RVLM from topographically distinct regions of the PBN complex may participate in the suppression of reflex bradycardia via activation of both NMDA and non-NMDA receptors at the RVLM.

Ultrastructure of glutamate and GABA immunoreactive axon terminals of the rat nucleus tractus solitarius, with a note on infralimbic cortex afferents

The principal fast neurotransmitters in the CNS are glutamate and GABA. Our aim was to provide a baseline account on the ultrastructure of the axon terminals immunoreactive to glutamate or GABA present in the nucleus tractus solitarius (NTS) of the rat. In addition, we wanted to complete our study of cortico-solitary afferents at the electron microscopic level, by analyzing the inputs from the infralimbic cortex. Using post-embedding immunogold, we found that nearly 61% of the axon terminals were glutamatergic, and 36% were GABAergic in the rat visceral NTS. In general, axons making asymmetric synaptic contacts were enriched in glutamate, compared to axons involved in symmetric synapses. In contrast, the vast majority of the GABAergic axon terminals made symmetric synaptic contacts. We could discern five types of glutamatergic and two types of GABAergic axon terminals that differed in their fine structure. Afferents from the infralimbic cortex were small, with clear synaptic vesicles and no dense core vesicles; they made asymmetric contacts with fine dendrites, and were glutamatergic. We conclude that most axon terminals in the NTS use glutamate or GABA as fast transmitters, in addition to being a heterogeneous population of morphological types.

Ventrolateral medullary control of cardiovascular activity during muscle contraction

An overview of the role of ventrolateral medulla (VLM) in regulation of cardiovascular activity is presented. A summary of VLM anatomy and its functional relation to other areas in the central nervous system is described. Over the past few years, various studies have investigated the VLM and its involvement in cardiovascular regulation during static muscle contraction, a type of static exercise as seen, for example, during knee extension or hand-grip exercise. Understanding the neural mechanisms that are responsible for regulation of cardiovascular activity during static muscle contraction is of particular interest since it helps understand circulatory adjustments in response to an increase in physical activity. This review surveys the role of several receptors and neurotransmitters in the VLM that are associated with changes in mean arterial pressure and heart rate during static muscle contraction in anesthetized animals. Possible mechanisms in the VLM that modulate cardiovascular changes during static muscle contraction are summarized and discussed. Localized administration of an excitatory amino-acid antagonist into the rostral portion of the VLM (RVLM) attenuates increases in blood pressure and heart rate during static muscle contraction, whereas its administration into the caudal part of the VLM (CVLM) augments these responses. Opioid or 5-HT1A receptor stimulation in the RVLM, but not in the CVLM, attenuates cardiovascular responses to muscle contraction. Furthermore, intravenous, intracerebroventricular or intracisternal injection of an alpha 2-adrenoceptor agonist or a cholinesterase inhibitor attenuates increases in blood pressure and heart rate during static muscle contraction. Finally, the possible involvement of endogenous neurotransmitters in the RVLM and the CVLM associated with cardiovascular responses during static muscle contraction is discussed. An overview of the role of the VLM in the overall cardiovascular control network in the brain is presented and critically reviewed.

Differential effects of anesthetics on sympathetic nerve activity and arterial baroreceptor reflex in chronically instrumented rats

The effects of pentobarbital sodium, chloralose, and urethane on sympathetic nerve activity and arterial baroreceptor reflex were examined using rats chronically instrumented for recordings of blood pressure (BP), electrocardiogram and renal sympathetic nerve activity (RSNA). Pentobarbital sodium (30 mg/kg, i.v.) produced a decrease in BP with a transient decrease in heart rate (HR) and no change in RSNA. Chloralose (50 mg/kg, i.v.) also caused a decrease in BP and no change in HR and RSNA until a later increase in HR and RSNA, while urethane (800 mg/kg, i.v.) increased BP, HR, and RSNA. Baroreceptor reflex function was assessed by constructing a logistic function curve compiled from data obtained by intravenous infusion in increasing doses of phenylephrine and sodium nitroprusside. Both pentobarbital sodium and chloralose administration decreased the gain of baroreceptor reflex control of both HR and RSNA. Urethane also decreased the gain of baroreceptor reflex control of HR but elicited no change in that of RSNA. These results suggest that different intravenously administered anesthetics affect the peripheral sympathetic outflows in qualitatively and quantitatively different manners.

GABA- and glycine-mediated inhibitory postsynaptic potentials in neonatal rat rostral ventrolateral medulla neurons in vitro

Whole-cell patch recordings were made from rostral ventrolateral medulla neurons of two in vitro preparations: (i) brainstem spinal cords of two- to five-day-old rats, and (ii) coronal brainstem slices of eight- to 12-day-old rats, and the inhibitory synaptic activities in these neurons have been studied. In brainstem spinal cord preparations, Lucifer Yellow was diffused into the recording neurons at the end of experiments. Medullary neurons were characterized as: (i) spinally projecting by the appearance of an antidromic spike following electrical stimulation of the spinal tract between T2 and T3 segments, and (ii) adrenergic by the detection of phenylethanolamine-N-methyltransferase immunoreactivity in Lucifer Yellow-filled neurons. Of the 13 spinally projecting and phenylethanolamine-N-methyltransferase-positive medullary neurons, focal stimulation elicited in the presence of glutamate receptor antagonists an inhibitory postsynaptic potential in nine neurons. Inhibitory synaptic potentials were reversibly eliminated by the GABA(A) receptor antagonist bicuculline (10-20 microM) in six of nine neurons, by the glycine receptor antagonist strychnine (0.1-1 microM) in two and by a combination of bicuculline and strychnine in one neuron. In brainstem slice preparations, focal stimulation elicited three types of synaptic potential: (i) an excitatory postsynaptic potential, (ii) an inhibitory postsynaptic potential and (iii) a biphasic synaptic potential consisting of an excitatory synaptic potential followed by an inhibitory synaptic potential. Inhibitory synaptic potentials had a reversal potential between -70 and -80 mV, reversed their polarity in a low (6.7 mM) Cl- Krebs' solution, and suppressed or blocked by either bicuculline or strychnine or both. Elimination of inhibitory synaptic potentials unmasked in some cells an excitatory synaptic potential or enhanced the excitatory synaptic potential component in medullary neurons with a biphasic response, indicating a marked convergence of excitatory and inhibitory inputs onto a single neuron. A population of medullary neurons appeared to be pacemaker neurons whereby they discharged spontaneously. When discharges were suppressed by membrane hyperpolarization, focal stimulation elicited inhibitory synaptic potentials in 8/23 neurons tested. Our results suggest that inhibitory synaptic potentials in medullary neurons are mediated by either GABA and/or glycine which open primarily Cl- channels. The prevalence of inhibitory synaptic potentials in medullary neurons indicates an essential role of inhibitory transmission in controlling the input and output ratio of these neurons.

Immunohistochemical localization of GABAA receptors in comparison with GABA-immunoreactive structures in the nucleus tractus solitarii of the rat

The localization of GABAA receptors was studied by immunohistochemistry in the nucleus tractus solitarii of the rat using a monoclonal antibody (bd17) against the beta-subunit. The pattern of distribution was compared with that of GABA-immunoreactive axons and nerve terminals. Positive staining for GABAA receptors was confined to regions near the surface of neuronal somata and their processes. The highest density of positive staining for GABAA receptors was seen in the central part of the rostral nucleus tractus solitarii where GABA-positive terminals were also rather dense. At both intermediate and caudal levels of the nucleus tractus solitarii, a moderate density of positive staining for GABAA receptors was located in the ventrolateral part, including the ventrolateral subnucleus. In these regions, the density of GABA-positive terminals was low. In the medial nucleus tractus solitarii, including the medial subnucleus, very little or no positive staining for GABAA receptors was detected, although many GABA-positive terminals were observed. The results suggest that the central part of the rostral nucleus tractus solitarii is controlled by the GABAergic system via GABAA receptors, but in the medial subnucleus of the nucleus tractus solitarii the GABA neurons appear to act via receptors that are not detectable by the antibody used.

Organization and transmitter specificity of medullary neurons activated by sustained hypertension: implications for understanding baroreceptor reflex circuitry

In situ expression of c-fos observed in response to phenylephrine (PE)-induced hypertension provided a basis for characterizing the organization and neurotransmitter specificity of neurons at nodal points of medullary baroreflex circuitry. Sustained hypertension induced by a moderate dose of PE provoked patterns of c-fos mRNA and protein expression that conformed in the nucleus of the solitary tract (NTS) to the termination patterns of primary baroreceptor afferents and in the caudal ventrolateral medulla (CVLM) to a physiologically defined depressor region. A majority of barosensitive CVLM neurons concurrently displayed markers for the GABAergic phenotype; few were glycinergic. Phenylephrine-sensitive GABAergic neurons that were retrogradely labeled after tracer deposits in pressor sites of the rostral ventrolateral medulla (RVLM) occupied a zone extending approximately 1.4 mm rostrally from the level of the calamus scriptorius, intermingled partly with catecholaminergic neurons of the A1 and C1 cell groups. By contrast, barosensitive neurons of the NTS were found to be phenotypically complex, with very few projecting directly to the RVLM. Extensive colocalization of PE-induced Fos-IR and markers for the nitric oxide phenotype were seen in a circumscribed, rostral, portion of the baroreceptor afferent zone of the NTS, whereas only a small proportion of PE-sensitive neurons in the NTS were found to be GABAergic. PE treatment parameters have been identified that provide a basis for defining and characterizing populations of neurons at the first station in the central processing of primary baroreceptor input and at a key inhibitory relay in the CVLM.

Evidence of gamma-aminobutyric acidergic control over the catecholaminergic projection from the medulla oblongata to the central nucleus of the amygdala

It is known that the nucleus of the solitary tract (NTS) and the ventrolateral medulla (VLM) project to the central nucleus of the amygdala (Ce), conveying visceral information. Conversely, the Ce sends projections to the NTS and the VLM. To understand better the role of catecholamine and gamma-aminobutyric acid (GABA) in these reciprocal connections, experiments were performed by combining lectin-conjugated horseradish peroxidase (WGA-HRP) anterograde and retrograde transport with preembedding immunocytochemistry to detect tyrosine hydroxylase (TH), and postembedding immunocytochemistry to detect GABA. The light microscopic study suggested that the majority of neurons in the NTS and the VLM projecting to the Ce were TH immunoreactive (TH-IR). Most of them were located at the level of the obex. Under the electron microscope, the GABAergic and non-GABAergic terminals were found to form synaptic contacts with the TH-(IR) or Ce-projecting or TH-IR/Ce-projecting double-labelled neurons of the NTS and VLM. The GABAergic terminals mostly formed symmetrical synaptic contacts with the postsynaptic structure in which perikarya (14-19%), dendrites (79-84%), and spines (2%) were observed. Approximately 94% of the axon terminals in the NTS and 90% of those in the VLM arising from the Ce were GABAergic and appeared not to form synaptic contacts with the TH-IR or Ce-projecting neurons in these regions. The present results demonstrated that the catecholaminergic neurons of the NTS and VLM projecting to the Ce receive an extensive GABAergic innervation and that the amygdala projection to the medulla is mostly GABAergic.

Microinjection of a serotonin3 receptor agonist into the NTS of unanesthetized rats inhibits the bradycardia evoked by activation of the baro- and chemoreflexes

In the present study we investigated the effects of microinjection into the commissural nucleus tractus solitarius (NTS) of unanesthetized rats of 2-methylserotonin (2-methyl-5-HT), a 5-HT3 receptor agonist, on the cardiac component of the baro- and chemoreflexes. The study was performed in conscious freely moving rats in order to avoid the possible effects of anesthetics on the cardiovascular responses to microinjection of neuroactive substances into the NTS. The baroreflex (phenylephrine, 0.5-2.0 micrograms/kg, i.v.) and the chemoreflex (potassium cyanide, 40 micrograms/rat, i.v) were activated in different groups of rats before and after bilateral microinjection of 2-methyl-5-HT into the NTS. Microinjections of 2-methyl-5-HT (5 nmol/50 nl) into the NTS produced a significant increase in basal mean arterial pressure (101 +/- 3 versus 125 +/- 8 mmHg), no changes in basal HR and a significant reduction in the reflex bradycardia triggered by baroreflex activation at 3 (-28 +/- 7 bpm), 10 (-35 +/- 4 bpm) and 20 min (-34 +/- 5 bpm) in comparison with the control value (-68 +/- 9 bpm). A similar reduction in the bradycardic response to chemoreflex activation was observed at 3 (-94 +/- 35 bpm), 10 (-98 +/- 38 bpm) and 20 min (-110 +/- 29 bpm) after 2-methyl-5-HT in comparison with the control value (-178 +/- 19 bpm). The effect of 2-methyl-5-HT on the basal mean arterial pressure and on the bradycardia evoked by stimulation of the baro- and chemoreflexes was blocked by pretreatment with granisetron bilaterally microinjected (500 pmol/50 nl) into the NTS. The data show that the stimulation of 5-HT3 receptors in the NTS of unanesthetized rats elicits a significant increase in basal mean arterial pressure and decreases the bradycardic response to baro- or chemoreflex activation.

Adrenergic responses in silent and putative inhibitory pacemaker-like neurons of the rat rostral ventrolateral medulla in vitro

Noradrenaline and adrenergic agonists were tested on pacemaker-like and silent neurons of the rat rostral ventrolateral medulla using intracellular recording in coronal brainstem slices as well as in punches containing only the rostral ventrolateral medullary region. Noradrenaline (1-100 microM) depolarized or increased the frequency of discharge of all cells tested in a dose-dependent manner. The noradrenaline-induced depolarization was associated with an apparent increase in cell input resistance at low concentrations and a decrease or no significant change at higher concentrations. Moreover, it was voltage dependent and its amplitude decreased with membrane potential hyperpolarization. Noradrenaline caused a dose-related increase in the frequency and amplitude of spontaneous inhibitory postsynaptic potentials. The alpha 1-adrenoceptor antagonist prazosin (0.5 microM) abolished the noradrenaline depolarizing response as well as-the noradrenaline-evoked increase in synaptic activity and unmasked an underlying noradrenaline dose-dependent hyperpolarizing response associated with a decrease in cell input resistance and sensitive to the alpha 2-adrenoceptor/antagonist yohimbine (0.5 microM). The alpha 1-adrenoceptor agonist phenylephrine (10 microM) mimicked the noradrenaline depolarizing response associated with an increase in membrane resistance as well as the noradrenaline-induced increase in synaptic activity. The alpha 2-adrenoceptor agonists UK-14,304 (1-3 microM) and clonidine (10-30 microM) produced only a small hyperpolarizing response, whereas the beta-adrenoceptor agonist isoproterenol (10-30 microM) had no effect. Baseline spontaneous postsynaptic potentials were abolished by strychnine (1 microM), bicuculline (30 microM) or both. However, only the strychnine-sensitive postsynaptic potentials had their frequency increased by noradrenaline or phenylephrine and they usually occurred with a regular pattern. Tetrodotoxin (1 microM) eliminated 80-95% of baseline spontaneous postsynaptic potentials and prevented the increase in synaptic activity evoked by noradrenaline and phenylephrine. Similar results were obtained in rostral ventrolateral medulla neurons impaled in both coronal slices and punches of the rostral ventrolateral medulla. It is concluded that noradrenaline could play an important inhibitory role in the rostral ventrolateral medulla via at least two mechanisms: an alpha 2-adrenoceptor-mediated hyperpolarization and an enhancement of inhibitory synaptic transmission through activation of alpha 1-adrenoceptors located on the somatic membrane of glycinergic interneurons. Some of these interneurons exhibit a regular discharge similar to the pacemaker-like neurons and might, at least in part, constitute a central inhibitory link in the baroreceptor-vasomotor reflex pathway.

Distribution of GABA and glycine in the lamb nucleus of the solitary tract

The inhibitory amino acids gamma-aminobutyric acid (GABA) and glycine are involved in several nucleus of the solitary tract (NST)-mediated functions. The distribution of these amino acids in the NST of the lamb, a species frequently used in investigations of NST-mediated behaviors, has not been described. Therefore, this study was designed to investigate the distribution of GABA and glycine in the lamb NST using immunohistochemistry. Both GABA and glycine immunoreactive cells and puncta were unevenly distributed in the lamb NST. The highest density of GABA immunoreactive cells was found in the intermediate zone of the NST, medial to the solitary tract (ST). High to moderate levels of puncta labeling were observed throughout the NST, particularly around the ST in intermediate and caudal zones. Moderate to low levels of glycine immunoreactivity were observed, with most glycine immunoreactive cells and puncta found in the caudal two-thirds of the NST in the medial, ventrolateral and dorsomedial NST. Only a few glycine immunoreactive cells and puncta were found in the rostral zone of the NST. The widespread distribution of GABA and glycine immunoreactivity in intermediate and caudal zones of the NST suggests that these inhibitory amino acids play an important role in modulating NST-mediated functions like swallowing, respiration and cardiovascular regulation in the lamb. The much higher density of GABA immunoreactivity compared to glycine immunoreactivity in the rostral zone of the NST suggests that GABA, but not glycine, is an important neurotransmitter in the processing of taste information by the lamb NST.

Organization of excitatory and inhibitory local networks in the caudal nucleus of tractus solitarius of rats revealed in in vitro slice preparation

Morphological and physiological properties of neurons in the caudal nucleus of tractus solitarius (NTS) of rats were studied in vitro by whole-cell recording and intracellular staining with biocytin. Synaptic responses following the solitary tract stimulation were also investigated to elucidate anatomical substrates of the underlying local circuits. Biocytin-filled NTS cells were divided into three groups according to the pattern of their axonal arborization: (1) local circuit neurons whose axon collaterals were extensively distributed within the NTS with the main axons leaving the NTS; (2) presumed interneurons whose axon collaterals seemed to be restricted within the NTS; and (3) projection neurons whose axons had few, if any, collaterals. Both local circuit neurons and presumed interneurons had small cell bodies (< 150 microns2 in somal area) and exhibited tonic regular spiking at depolarized membrane potentials. Polysynaptic excitatory background activity was increased and lasted for 300-1000 msec in these neurons following solitary tract stimulation. The projection neurons had medium to large cell bodies (> 150 microns2 in somal area). Inhibitory postsynaptic responses produced by an increased CI-conductance were recorded in these projection neurons. These findings suggest that excitatory local networks are organized by an assembly of the local circuit neurons in the caudal NTS, and that the interneurons are arranged to connect the excitatory local network with medium to large projection neurons via inhibitory synapses. Visceral afferent information is probably processed in the highly organized excitatory and inhibitory local networks within the caudal NTS and conveyed to other brain regions.

Localization of GABAA alpha 1 mRNA subunit in the brainstem nuclei controlling esophageal peristalsis

The nucleus of the solitary tract, the site of esophageal premotor neurons (PMN), is tonically inhibited by GABAergic neurons via the GABAA receptor. We investigated the expression of GABAA alpha 1 subunit mRNA within esophageal PMNs of the NTS utilizing transynaptic tracing with pseudorabies virus and nonisotopic in-situ hybridization. Double-labeling studies revealed that the majority of PRV-immunoreactive cells also expressed GABAA alpha 1 mRNA. The expression of GABAA subunits supports a role for GABA in the brainstem circuit controlling esophageal peristalsis.

GABAergic neurons in rat nuclei of solitary tracts receive inhibitory-type synapses from amygdaloid efferents lacking detectable GABA-immunoreactivity

Gamma-aminobutyric acid (GABA) is a prominent inhibitory transmitter in both the central nucleus of the amygdala (Ce) and the medial nuclei of the solitary tracts (mNTS). These regions are reciprocally connected by anatomical pathways mediating the coordinated visceral responses to emotional stress. To further determine whether GABA is present in the amygdaloid efferents or their targets in the mNTS, we combined peroxidase labeling of Phaseolus vulgaris leucoagglutinin (PHA-L) or biotinylated dextran amine (BDA) anterogradely transported from the Ce with immunogold-silver detection of antibodies against GABA in the rat mNTS. By light microscopy, peroxidase labeling for either PHA-L or BDA was seen in varicose processes, whereas immunogold-silver labeling for GABA was detected in perikarya and processes throughout the rostrocaudal mNTS. The intermediate mNTS at the level of the area postrema, a region receiving mainly cardiorespiratory and gastric visceral afferents, were examined by electron microscopy. In this region, anterograde labeling was observed exclusively in unmyelinated axons and axon terminals. These terminals lacked detectable GABA-immunoreactivity, but formed symmetric synapses that are associated with inhibition. The targets of the anterogradely labeled terminals were medium-sized dendrites both with and without GABA-labeling. These dendrites often also received convergent input from terminals that were intensely GABA-immunoreactive. We conclude that visceral activation accompanying emotional response to stress is likely to involve inhibition of GABAergic neurons in the mNTS by non-GABA-containing amygdaloid efferents. Furthermore, our results indicate that the inhibition of these GABAergic neurons may be further augmented by release of GABA from other converging terminals in the mNTS.

GABAergic synapses upon neurons expressing substance P receptors in the nucleus of the solitary tract: an immunocytochemical electron microscope study in the rat

Morphological substrates for interactions between gamma-aminobutyric acid (GABA) and substance P upon neurons expressing substance P receptor (SPR) in the nucleus of the solitary tract (NST) were investigated by immunocytochemical electron microscopy. In the NST of the rat, many GABA-like immunoreactive axon terminals were in symmetric synaptic contacts with dendritic profiles; they were observed on nearly a half of the SPR-like immunoreactive dendritic profiles in the medial part of the caudal half of the NST.

GABA-induced responses in electrophysiologically characterized neurons within the rat rostro-ventrolateral medulla in vitro

Rostro-ventrolateral medulla (RVL) neurons were recorded using conventional intracellular recording techniques in brain slices maintained in vitro at 32 degrees C and classified into 3 major groups. The first group included neurons having endogenous pacemaker-like (PL) activity with regular firing frequency (mean 8 Hz) and a linear current-voltage relationship (I-V). The second group of neurons were slowly and irregularly firing (IF) or quiescent, presenting membrane potential oscillations and their I-V usually displayed an inward rectification. These neurons had a relatively longer action potential duration. The third group included silent neurons (S) with no apparent membrane oscillations and they differed from the first two groups by having relatively shorter action potential duration and amplitude and lower cell input resistance. When recorded with KCl-filled electrodes, the majority of silent neurons displayed a time-dependent inward rectification. With KAc-filled electrodes, irregular slow hyperpolarizing and depolarizing spontaneous potentials could be recorded primarily on PL and IF neurons, respectively. Moreover, fast spontaneous inhibitory postsynaptic potentials (PSPs) were detected in about 15% of PL and S neurons. They generally exhibited a regular pattern and were depolarizing when KCl-filled electrodes were used for recording. The amplitude of these inhibitory PSPS was reversibly reduced by the GABA A antagonists bicuculline, SR 95531 and picrotoxin. With KAc-filled electrodes, pressure-applied GABA (20 mM) evoked complex responses. In PL neurons, it consisted of a fast hyperpolarization followed by a slower depolarization that were both sensitive to SR 95531 and picrotoxin. The response was terminated by a long-lasting hyperpolarization that was reduced, but not abolished, by the GABA B antagonist CGP 35348. In IF and S neurons, GABA application usually produced a fast followed by a slow monophasic hyperpolarization and depolarization, respectively. The fast component of these responses was sensitive to the GABA A antagonists. Pressure application of isoguvacine (10 mM) always induced monophasic responses in all types of neurons recorded. Baclofen (1-30 mu M) reduced the firing frequency and hyperpolarized PL and IF neurons, an effect that was antagonized by CGP 35348 (50-100 mu M); however, it had little effect on silent neurons. It is concluded that RVL neurons have heterogeneous electrophysiological characteristics. Their predominant synaptic input and GABA responsiveness might be additional criteria to identify the excitatory and inhibitory elements in the RVL circuitry. All neuronal types seem to have functional GABA A and GABA B receptors; however, only a subpopulation is under tonic inhibitory control in vitro, probably from local GABAergic pacemaker interneurons. Our results further emphasize the role of GABA as an important neurotransmitter in the RVL network.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.