Metabotropic glutamate receptors depress vagal and aortic baroreceptor signal transmission in the NTS

Real-time closed-loop brainstem stimulation modality for enhancing temporal blood pressure reduction

BACKGROUND

Traditional pharmacological interventions are well tolerated in the management of elevated blood pressure (BP) for individuals with resistant hypertension. Although neuromodulation has been investigated as an alternative solution, its open-loop (OL) modality cannot follow the patient's physiological state. In fact, neuromodulation for controlling highly fluctuating BP necessitates a closed-loop (CL) stimulation modality based on biomarkers to monitor the patient's continuously varying physiological state.

OBJECTIVE

By leveraging its intuitive linkage with BP responses in ongoing efforts aimed at developing a CL system to enhance temporal BP reduction effect, this study proposes a CL neuromodulation modality that controls nucleus tractus solitarius (NTS) activity to effectively reduce BP, thus reflecting continuously varying physiological states.

METHOD

While performing neurostimulation targeting the NTS in the rat model, the arterial BP response and neural activity of the NTS were simultaneously measured. To evaluate the temporal BP response effect of CL neurostimulation, OL (constant parameter; 20 Hz, 200 μA) and CL (Initial parameter; 11 Hz, 112 μA) stimulation protocols were performed with stimulation 180 s and rest 600 s, respectively, and examined NTS activity and BP response to the protocols.

RESULTS

In-vivo experiments for OL versus CL protocol for direct NTS stimulation in rats demonstrated an enhancement in temporal BP reduction via the CL modulation of NTS activity.

CONCLUSION

This study proposes a CL stimulation modality that enhances the effectiveness of BP control using a feedback control algorithm based on neural signals, thereby suggesting a new approach to antihypertensive neuromodulation.

Frequency-dependent depression of the NTS synapse affects the temporal response of the antihypertensive effect of auricular vagus nerve stimulation (aVNS)

OBJECTIVES

Auricular vagus nerve stimulation (aVNS) has recently emerged as a promising neuromodulation modality for blood pressure (BP) reduction due to its ease of use although its efficacy is still limited compared to direct baroreflex stimulation. Previous studies have also indicated that synaptic depression of nucleus tractus solitarius (NTS) in the baroreflex pathway depends on stimulus frequency. However, the nature of this frequency dependence phenomenon on antihypertensive effect has been unknown for aVNS. We aimed to investigate the antihypertensive effect of aVNS considering frequency-dependent depression characteristic in the NTS synapse. We explored NTS activation and BP reduction induced by aVNS and by direct secondary neuron stimulation (DS).

APPROACH

Both protocols were performed with recording of NTS activation and BP response with stimulation for each frequency parameter (2, 4, 20, 50, and 80 Hz).

MAIN RESULTS

The BP recovery time constant was significantly dependent on the frequency of DS and aVNS (DS - 2 Hz: 8.17 ± 4.98; 4 Hz: 9.73 ± 6.3; 20 Hz: 6.61 ± 3.28; 50 Hz: 4.93 ± 1.65; 80 Hz: 4.00 ± 1.43, p < 0.001, Kruskal-Wallis H-test / aVNS - 2 Hz: 4.02 ± 2.55; 4 Hz: 8.13 ± 4.05; 20 Hz: 6.40 ± 3.16; 50 Hz: 5.18 ± 2.37; 80 Hz: 3.13 ± 1.29, p < 0.05, Kruskal-Wallis H-test) despite no significant BP reduction at 2 Hz compared to sham groups (p > 0.05, Mann-Whitney U-test).

SIGNIFICANCE

Our observations suggest that the antihypertensive effect of aVNS is influenced by the characteristics of frequency-dependent synaptic depression in the NTS neuron in terms of the BP recovery time. These findings suggest that the antihypertensive effect of aVNS can be improved with further understanding of the neurological properties of the baroreflex associated with aVNS, which is critical to push this new modality for clinical interpretation.

Effect of transient receptor potential vanilloid-1 on cough hypersensitivity induced by particulate matter 2.5

AIMS

The mechanism of cough hypersensitivity induced by particulate matter 2.5 (PM2.5) remains elusive. The current study was designed to explore the effect of transient receptor potential vanilloid-1 (TRPV1) on cough hypersensitivity in airway and central nervous system.

MAIN METHODS

The PM2.5-induced chronic cough model of guinea pig was established by exposure to different doses of PM2.5 for three weeks. After exposure, the animals were microinjected with TRPV1 agonist capsaicine, antagonist capsazepine in the dorsal vagal complex respectively. Cough sensitivity was measured by determining the provocative concentration of citric acid inducing 5 or more coughs (C5). Airway inflammation was detected by hematoxylin eosin (HE) staining and Evans blue fluorescence, and substance P (SP) and TRPV1 expressions in airway were observed by immunohistochemical staining. TRPV1 expressions in the dorsal vagal complex were observed by immunofluorescence. Retrograde tracing by pseudorabies virus-Bartha (PRV-Bartha) was conducted to confirm the regulatory pathway between airway and central nervous system.

KEY FINDINGS

PM2.5 induced TRPV1 expressions in both of airway and dorsal vagal complex and airway neurogenic inflammation. Airway vascular permeability increased after being exposed to PM2.5. The expressions of SP in the airway and airway inflammation was increased after microinjecting TRPV1 agonist, and decreased after microinjecting TRPV1 antagonist. PRV infected neurons in medulla oblongata mainly located in the dorsal vagal complex.

SIGNIFICANCE

These findings show that TRPV1 in the dorsal vagal complex could promote airway neurogenic inflammation and cough reflex sensitivity through neural pathways of vagal complex-airways, which indicate the therapeutic potential of specific inhibition of TRPV1 for chronic cough induced by PM2.5.

Cellular mechanisms of activity-dependent BDNF expression in primary sensory neurons

Brain-derived neurotrophic factor (BDNF) is abundantly expressed by both developing and adult rat visceral sensory neurons from the nodose ganglion (NG) in vivo and in vitro. We have previously shown that BDNF is released from neonatal NG neurons by activity and regulates dendritic development in their postsynaptic targets in the brainstem. The current study was carried out to examine the cellular and molecular mechanisms of activity-dependent BDNF expression in neonatal rat NG neurons, using our established in vitro model of neuronal activation by electrical field stimulation with patterns that mimic neuronal activity in vivo. We show that BDNF mRNA (transcript 4) increases over threefold in response to a 4-h tonic or bursting pattern delivered at the frequency of 6 Hz, which corresponds to the normal heart rate of a newborn rat. No significant increase in BDNF expression was observed following stimulation at 1 Hz. The latter effect suggests a frequency-dependent mechanism of regulated BDNF expression. In addition to BDNF transcript 4, which is known to be regulated by activity, transcript 1 also showed significant upregulation. The increases in BDNF mRNA were followed by BDNF protein upregulation of a similar magnitude after 24h of stimulation at 6 Hz. Electrical stimulation-evoked BDNF expression was inhibited by pretreating neurons with the blocker of voltage-gated sodium channels tetrodotoxin and by removing extracellular calcium. Moreover, our data show that repetitive stimulation-evoked BDNF expression requires calcium influx through N-, but not L-type, channels. Together, our study reveals novel mechanisms through which electrical activity stimulates de novo synthesis of BDNF in sensory neurons, and points to the role of N-type calcium channels in regulating BDNF expression in sensory neurons in response to repetitive stimulation.

Peptide and lipid modulation of glutamatergic afferent synaptic transmission in the solitary tract nucleus

The brainstem nucleus of the solitary tract (NTS) holds the first central neurons in major homeostatic reflex pathways. These homeostatic reflexes regulate and coordinate multiple organ systems from gastrointestinal to cardiopulmonary functions. The core of many of these pathways arise from cranial visceral afferent neurons that enter the brain as the solitary tract (ST) with more than two-thirds arising from the gastrointestinal system. About one quarter of ST afferents have myelinated axons but the majority are classed as unmyelinated C-fibers. All ST afferents release the fast neurotransmitter glutamate with remarkably similar, high-probability release characteristics. Second order NTS neurons receive surprisingly limited primary afferent information with one or two individual inputs converging on single second order NTS neurons. A- and C-fiber afferents never mix at NTS second order neurons. Many transmitters modify the basic glutamatergic excitatory postsynaptic current often by reducing glutamate release or interrupting terminal depolarization. Thus, a distinguishing feature of ST transmission is presynaptic expression of G-protein coupled receptors for peptides common to peripheral or forebrain (e.g., hypothalamus) neuron sources. Presynaptic receptors for angiotensin (AT1), vasopressin (V1a), oxytocin, opioid (MOR), ghrelin (GHSR1), and cholecystokinin differentially control glutamate release on particular subsets of neurons with most other ST afferents unaffected. Lastly, lipid-like signals are transduced by two key ST presynaptic receptors, the transient receptor potential vanilloid type 1 and the cannabinoid receptor that oppositely control glutamate release. Increasing evidence suggests that peripheral nervous signaling mechanisms are repurposed at central terminals to control excitation and are major sites of signal integration of peripheral and central inputs particularly from the hypothalamus.

Upregulation of brain-derived neurotrophic factor expression in nodose ganglia and the lower brainstem of hypertensive rats

Hypertension leads to structural and functional changes at baroreceptor synapses in the medial nucleus tractus solitarius (NTS), but the underlying molecular mechanisms remain unknown. Our previous studies show that brain-derived neurotrophic factor (BDNF) is abundantly expressed by rat nodose ganglion (NG) neurons, including baroreceptor afferents and their central terminals in the medial NTS. We hypothesized that hypertension leads to upregulation of BDNF expression in NG neurons. To test this hypothesis, we used two mechanistically distinct models of hypertension, the spontaneously hypertensive rat (SHR) and the deoxycorticosterone acetate (DOCA)-salt rat. Young adult SHRs, whose blood pressure was significantly elevated compared with age-matched Wistar-Kyoto (WKY) control rats, exhibited dramatic upregulation of BDNF mRNA and protein in the NG. BDNF transcripts from exon 4, known to be regulated by activity, and exon 9 (protein-coding region) showed the largest increases. Electrical stimulation of dispersed NG neurons with patterns that mimic baroreceptor activity during blood pressure elevations led to increases in BDNF mRNA that were also mediated through promoter 4. The increase in BDNF content of the NG in vivo was associated with a significant increase in the percentage of BDNF-immunoreactive NG neurons. Moreover, upregulation of BDNF in cell bodies of NG neurons was accompanied by a significant increase in BDNF in the NTS region, the primary central target of NG afferents. A dramatic increase in BDNF in the NG was also detected in DOCA-salt hypertensive rats. Together, our study identifies BDNF as a candidate molecular mediator of activity-dependent changes at baroafferent synapses during hypertension.

Metabotropic glutamate receptors as novel therapeutic targets on visceral sensory pathways

Metabotropic glutamate receptors (mGluR) have a diverse range of structures and molecular coupling mechanisms. There are eight mGluR subtypes divided into three major groups. Group I (mGluR1 and 5) is excitatory; groups II (mGluR2 and 3) and III (mGluR 4, 6, and 7) are inhibitory. All mGluR are found in the mammalian nervous system but some are absent from sensory neurons. The focus here is on mGluR in sensory pathways from the viscera, where they have been explored as therapeutic targets. Group I mGluR are activated by endogenous glutamate or constitutively active without agonist. Constitutive activity can be exploited by inverse agonists to reduce neuronal excitability without synaptic input. This is promising for reducing activation of nociceptive afferents and pain using mGluR5 negative allosteric modulators. Many inhibitory mGluR are also expressed in visceral afferents, many of which markedly reduce excitability. Their role in visceral pain remains to be determined, but they have shown promise in inhibition of the triggering of gastro-esophageal reflux, via an action on mechanosensory gastric afferents. The extent of reflux inhibition is limited, however, and may not reach a clinically useful level. On the other hand, negative modulation of mGluR5 has very potent actions on reflux inhibition, which has produced the most likely candidates so far as therapeutic drugs. These act probably outside the central nervous system, and may therefore provide a generous therapeutic window. There are many unanswered questions about mGluR along visceral afferent pathways, the answers to which may reveal many more therapeutic candidates.

Heterosynaptic crosstalk: GABA-glutamate metabotropic receptors interactively control glutamate release in solitary tract nucleus

Synaptic terminals often contain metabotropic receptors that act as autoreceptors to control neurotransmitter release. Less appreciated is the heterosynaptic crossover of glutamate receptors to control GABA release and vice versa GABA receptors which control glutamate release. In the brainstem, activation of solitary tract (ST) afferents releases glutamate onto second-order neurons within the solitary tract nucleus (NTS). Multiple metabotropic receptors are expressed in NTS for glutamate (mGluRs) and for GABA (GABA(B)). The present report identifies mGluR regulation of glutamate release at second and higher order sensory neurons in NTS slices. We found strong inhibition of glutamate release to group II and III mGluR activation on mechanically isolated NTS neurons. However, the same mGluR-selective antagonists paradoxically decreased glutamate release (miniature, mEPSCs) at identified second-order NTS neurons. Unaltered amplitudes were consistent with selective presynaptic mGluR actions. GABA(B) blockade in slices resolved the paradoxical differences and revealed a group II/III mGluR negative feedback of mEPSC frequency similar to isolated neurons. Thus, the balance of glutamate control is tipped by mGluR receptors on GABA terminals resulting in predominating heterosynaptic GABA(B) inhibition of glutamate release. Regulation by mGluR or GABA(B) was not consistently evident in excitatory postsynaptic currents (EPSCs) in higher-order NTS neurons demonstrating metabotropic receptor distinctions in processing at different NTS pathway stages. These cellular localizations may figure importantly in understanding interventions such as brain-penetrant compounds or microinjections. We conclude that afferent glutamate release in NTS produces a coordinate presynaptic activation of co-localized mGluR and GABA(B) feedback on cranial afferent terminals to regulate glutamate release.

Distinct target cell-dependent forms of short-term plasticity of the central visceral afferent synapses of the rat

Background

The visceral afferents from various cervico-abdominal sensory receptors project to the dorsal vagal complex (DVC), which is composed of the nucleus of the solitary tract (NTS), the area postrema and the dorsal motor nucleus of the vagus nerve (DMX), via the vagus and glossopharyngeal nerves and then the solitary tract (TS) in the brainstem. While the excitatory transmission at the TS-NTS synapses shows strong frequency-dependent suppression in response to repeated stimulation of the afferents, the frequency dependence and short-term plasticity at the TS-DMX synapses, which also transmit monosynaptic information from the visceral afferents to the DVC neurons, remain largely unknown.

Results

Recording of the EPSCs activated by paired or repeated TS stimulation in the brainstem slices of rats revealed that, unlike NTS neurons whose paired-pulse ratio (PPR) is consistently below 0.6, the distribution of the PPR of DMX neurons shows bimodal peaks that are composed of type I (PPR, 0.6-1.5; 53% of 120 neurons recorded) and type II (PPR, < 0.6; 47%) neurons. Some of the type I DMX neurons showed paired-pulse potentiation. The distinction of these two types depended on the presynaptic release probability and the projection target of the postsynaptic cells; the distinction was not dependent on the location or soma size of the cell, intensity or site of the stimulation, the latency, standard deviation of latency or the quantal size. Repeated stimulation at 20 Hz resulted in gradual and potent decreases in EPSC amplitude in the NTS and type II DMX neurons, whereas type I DMX neurons displayed only slight decreases, which indicates that the DMX neurons of this type could be continuously activated by repeated firing of primary afferent fibers at a high (~10 Hz) frequency.

Conclusions

These two general types of short-term plasticity might contribute to the differential activation of distinct vago-vagal reflex circuits, depending on the firing frequency and type of visceral afferents.

A novel postsynaptic group II metabotropic glutamate receptor role in modulating baroreceptor signal transmission

The nucleus tractus solitarius (NTS) is essential for orchestrating baroreflex control of blood pressure. When a change in blood pressure occurs, the information is transmitted by baroreceptor afferent fibers to the central network by glutamate binding to ionotropic glutamate receptors on second-order baroreceptor neurons. Glutamate also activates presynaptic group II and III metabotropic glutamate receptors (mGluRs), depressing both glutamate and GABA release to modulate baroreceptor signal transmission. Here we present a novel role for postsynaptic group II mGluRs to further fine-tune baroreceptor signal transmission at the first central synapses. In a brainstem slice with ionotropic glutamate and GABA receptors blocked, whole-cell patch-clamp recordings of second-order baroreceptor neurons revealed that two group II mGluR agonists evoked concentration-dependent membrane hyperpolarizations. The hyperpolarization remained when a presynaptic contribution was prevented with Cd(2+), was blocked by a postsynaptic intervention of intracellular dialysis of the G-protein signaling inhibitor, was mimicked by endogenous release of glutamate by tractus solitarius stimulation, and was prevented by a group II mGluR antagonist. Postsynaptic localization of group II mGluRs was confirmed by fluorescent confocal immunohistochemistry and light microscopy. Group II mGluR induced-currents consisted of voltage-dependent outward and inward components, prevented by tetraethylammonium chloride and tetrodotoxin, respectively. In contrast to group II mGluR-induced hyperpolarization, there was no effect on intrinsic excitability as determined by action potential shape or firing in response to depolarizing current injections. The data suggest a novel mechanism for postsynaptic group II mGluRs to fine-tune baroreceptor signal transmission in the NTS.

Group III metabotropic glutamate receptors (mGluRs) modulate transmission of gustatory inputs in the brain stem

Glutamate is the principal neurotransmitter at the primary sensory afferent synapse in the medulla for the taste system. At this synapse, glutamate activates N-methyl-D-aspartate (NMDA) and non-NMDA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA] and kainate) ionotropic receptors to effect a response in the second-order neurons. The current experiment is the first to examine the role of metabotropic glutamate receptors (mGluRs) in the transmission of taste information. In an in vitro slice preparation of the primary vagal gustatory nucleus in goldfish, primary gustatory afferent fibers were stimulated electrically, whereas evoked dendritic field potentials were recorded in the sensory layers. Recordings were made before, during, and after bath application of mGluR agonists for various mGluR groups and subtypes. Whereas L-AP4, a group III agonist, reduced the field potential, group I and group II agonists had no effect. Furthermore, the selective mGluR4 agonist ACPT-III and mGluR8 agonist PPG were effective at reducing the field potential, whereas agonists selective for mGluR6 and 7 were not. MAP4, a group III mGluR antagonist, attenuated frequency-dependent depression, indicating that endogenous glutamate binds to presynaptic mGluRs under normal conditions. Furthermore, polymerase chain reaction showed that mRNA for mGluR4 and 8 is expressed in the vagal ganglia, a prerequisite if those receptors are expressed presynaptically in the vagal lobe. Collectively, these experiments indicate that mGluR4 and 8 are presynaptic at the primary gustatory afferent synapse and that their activation inhibits glutamatergic release.

Plasticity in glutamatergic NTS neurotransmission

Changes in the physiological state of an animal or human can result in alterations in the cardiovascular and respiratory system in order to maintain homeostasis. Accordingly, the cardiovascular and respiratory systems are not static but readily adapt under a variety of circumstances. The same can be said for the brainstem circuits that control these systems. The nucleus tractus solitarius (NTS) is the central integration site of baroreceptor and chemoreceptor sensory afferent fibers. This central nucleus, and in particular the synapse between the sensory afferent and second-order NTS cell, possesses a remarkable degree of plasticity in response to a variety of stimuli, both acute and chronic. This brief review is intended to describe the plasticity observed in the NTS as well as the locus and mechanisms as they are currently understood. The functional consequence of NTS plasticity is also discussed.

Brain-derived neurotrophic factor in arterial baroreceptor pathways: implications for activity-dependent plasticity at baroafferent synapses

Functional characteristics of the arterial baroreceptor reflex change throughout ontogenesis, including perinatal adjustments of the reflex gain and adult resetting during hypertension. However, the cellular mechanisms that underlie these functional changes are not completely understood. Here, we provide evidence that brain-derived neurotrophic factor (BDNF), a neurotrophin with a well-established role in activity-dependent neuronal plasticity, is abundantly expressed in vivo by a large subset of developing and adult rat baroreceptor afferents. Immunoreactivity to BDNF is present in the cell bodies of baroafferent neurons in the nodose ganglion, their central projections in the solitary tract, and terminal-like structures in the lower brainstem nucleus tractus solitarius. Using ELISA in situ combined with electrical field stimulation, we show that native BDNF is released from cultured newborn nodose ganglion neurons in response to patterns that mimic the in vivo activity of baroreceptor afferents. In particular, high-frequency bursting patterns of baroreceptor firing, which are known to evoke plastic changes at baroreceptor synapses, are significantly more effective at releasing BDNF than tonic patterns of the same average frequency. Together, our study indicates that BDNF expressed by first-order baroreceptor neurons is a likely mediator of both developmental and post-developmental modifications at first-order synapses in arterial baroreceptor pathways.

Expression of Group I metabotropic glutamate receptors on phenotypically different cells within the nucleus of the solitary tract in the rat

Group I metabotropic glutamate receptors (mGluRs) are G-coupled receptors that modulate synaptic activity. Previous studies have shown that Group I mGluRs are present in the nucleus of the solitary tract (NTS), in which many visceral afferents terminate. Microinjection of selective Group I mGluR agonists into the NTS results in a depressor response and decrease in sympathetic nerve activity. There is, however, little evidence detailing which phenotypes of neurons within the NTS express Group I mGluRs. In brainstem slices, we performed immunohistochemical localization of Group I mGluRs and either glutamic acid decarboxylase 67 kDa isoform (GAD67), neuronal nitric oxide synthase (nNOS) or tyrosine hydroxylase (TH). Fluoro-Gold (FG, 2%; 15 nl) was microinjected in the caudal ventrolateral medulla (CVLM) of the rat to retrogradely label NTS neurons that project to CVLM. Group I mGluRs were distributed throughout the rostral-caudal extent of the NTS and were found within most NTS subregions. The relative percentages of Group I mGluR expressing neurons colabeled with the different markers were FG (6.9+/-0.7) nNOS (5.6+/-0.9), TH (3.9+/-1.0), and GAD67 (3.1+/-1.4). The percentage of FG containing cells colabeled with Group I mGluR (13.6+/-2.0) was greater than the percent colabeled with GAD67 (3.1+/-0.5), nNOS (4.7+/-0.5), and TH (0.1+/-0.08). Cells triple labeled for FG, nNOS, and Group I mGluRs were identified in the NTS. Thus, these data provide an anatomical substrate by which Group I mGluRs could modulate activity of CVLM projecting neurons in the NTS.

Endomorphin-2 is released from newborn rat primary sensory neurons in a frequency- and calcium-dependent manner

Recent evidence indicates that endomorphins, endogenous mu-opioid receptor (MOR) agonists, modulate synaptic transmission in both somatic and visceral sensory pathways. Here we show that endomorphin-2 (END-2) is expressed in newborn rat dorsal root ganglion (DRG) and nodose-petrosal ganglion complex (NPG) neurons, and rarely co-localizes with brain-derived neurotrophic factor (BDNF). In order to examine activity-dependent release of END-2 from neurons, we established a model using dispersed cultures of DRG and NPG cells activated by patterned electrical field stimulation. To detect release of END-2, we developed a novel rapid capture enzyme-linked immunosorbent assay (ELISA), in which END-2 capture antibody was added to neuronal cultures shortly before their electrical stimulation. The conventional assay was effective at reliably detecting END-2 only when the cells were stimulated in the presence of CTAP, a MOR-selective antagonist. This suggests that the strength of the novel assay is related primarily to rapid capture of released END-2 before it binds to endogenous MORs. Using the rapid capture ELISA, we found that stimulation protocols known to induce plastic changes at sensory synapses were highly effective at releasing END-2. Removal of extracellular calcium or blocking voltage-activated calcium channels significantly reduced the release. Together, our data provide the first evidence that END-2 is expressed by newborn DRG neurons of all sizes found in this age group, and can be released from these, as well as from NPG neurons, in an activity-dependent manner. These results point to END-2 as a likely mediator of activity-dependent plasticity in sensory pathways.

Calcium-fluxing glutamate receptors associated with primary gustatory afferent terminals in goldfish (Carassius auratus)

Presynaptic ionotropic glutamate receptors modulate transmission at primary afferent synapses in several glutamatergic systems. To test whether primary gustatory afferent fibers express Ca(2+)-permeable AMPA/kainate receptors, we utilized kainate-stimulated uptake of Co(2+) along with immunocytochemistry for the Ca(2+)-binding proteins (CaBPs) calbindin and calretinin to investigate the primary gustatory afferents in goldfish (Carassius auratus). In goldfish, the primary gustatory nucleus (equivalent to the gustatory portion of the nucleus of the solitary tract) includes the vagal lobe, which is a large, laminated structure protruding dorsally from the medulla. Kainate-stimulated uptake of Co(2+) (a measure of Ca(2+)-fluxing glutamate receptors) shows punctate staining distributed in the distinct laminar pattern matching the layers of termination of the primary gustatory afferent fibers. In addition, CaBP immunocytochemistry, which correlates highly with expression of Ca(2+)-permeable AMPA/kainate receptors, shows a laminar pattern of distribution similar to that found with kainate-stimulated cobalt uptake. Nearly all neurons of the vagal gustatory ganglion show Co(2+) uptake and are immunopositive for CaBPs. Transection of the vagus nerve proximal to the ganglion results in loss of such punctate Co(2+) uptake and of punctate CaBP staining as soon as 4 days postlesion. These results are consonant with the presence of Ca(2+)-fluxing glutamate receptors on the presynaptic terminals of primary gustatory terminals, providing an avenue for modulation of primary gustatory input.

Peripheral versus central modulation of gastric vagal pathways by metabotropic glutamate receptor 5

Metabotropic glutamate receptors (mGluR) are classified into group I, II, and III mGluR. Group I (mGluR1, mGluR5) are excitatory, whereas group II and III are inhibitory. mGluR5 antagonism potently reduces triggering of transient lower esophageal sphincter relaxations and gastroesophageal reflux. Transient lower esophageal sphincter relaxations are mediated via a vagal pathway and initiated by distension of the proximal stomach. Here, we determined the site of action of mGluR5 in gastric vagal pathways by investigating peripheral responses of ferret gastroesophageal vagal afferents to graded mechanical stimuli in vitro and central responses of nucleus tractus solitarius (NTS) neurons with gastric input in vivo in the presence or absence of the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP). mGluR5 were also identified immunohistochemically in the nodose ganglia and NTS after extrinsic vagal inputs had been traced from the proximal stomach. Gastroesophageal vagal afferents were classified as mucosal, tension, or tension-mucosal (TM) receptors. MPEP (1-10 microM) inhibited responses to circumferential tension of tension and TM receptors. Responses to mucosal stroking of mucosal and TM receptors were unaffected. MPEP (0.001-10 nmol icv) had no major effect on the majority of NTS neurons excited by gastric distension or on NTS neurons inhibited by distension. mGluR5 labeling was abundant in gastric vagal afferent neurons and sparse in fibers within NTS vagal subnuclei. We conclude that mGluR5 play a prominent role at gastroesophageal vagal afferent endings but a minor role in central gastric vagal pathways. Peripheral mGluR5 may prove a suitable target for reducing mechanosensory input from the periphery, for therapeutic benefit.

Disinhibition of the cardiac limb of the arterial baroreflex in rat: a role for metabotropic glutamate receptors in the nucleus tractus solitarii

The nucleus tractus solitarii (NTS) is the first site of integration for primary baroreceptor afferents, which release glutamate to excite second-order neurones through ionotropic receptors. In vitro studies indicate that glutamate may also activate metabotropic receptors (mGluRs) to modulate the excitability of NTS neurones at pre- and postsynaptic loci. We examined the functional role of metabotropic glutamate receptors (mGluRs) in modulating the baroreceptor reflex in the rat NTS. Using the working heart-brainstem preparation, the baroreflex was activated using brief pressor stimuli and the consequent cardiac (heart rate change) and non-cardiac sympathetic (T8-10 chain) baroreflex gains were obtained. Microinjections of glutamate antagonists were made bilaterally into the NTS at the site of termination of baroreceptor afferents. NTS microinjection of kynurenate (ionotropic antagonist) inhibited both the cardiac and sympathetic baroreflex gains (16 +/- 5% and 59 +/- 11% of control, respectively). The non-selective mGluR antagonist MCPG produced a dose-dependent inhibition of the cardiac gain (30 +/- 3% of control) but not the sympathetic gain. Selective inhibitions of the cardiac gain were also seen with LY341495 and EGLU suggesting the response was mediated by group II mGluRs. This effect on cardiac gain involves attenuation of the parasympathetic baroreflex as it persists in the presence of atenolol. Prior NTS microinjection of bicuculline (GABA(A) antagonist) prevented the mGluR-mediated attenuation of the cardiac gain. These results are consistent with the reported presynaptic inhibition of GABAergic transmission by group II mGluRs in the NTS and constitute a plausible mechanism allowing selective feed-forward disinhibition to increase the gain of the cardiac limb of the baroreflex without changing the sympathoinhibitory component.

Potentiation of mouse vagal afferent mechanosensitivity by ionotropic and metabotropic glutamate receptors

Glutamate acts at central synapses via ionotropic (iGluR--NMDA, AMPA and kainate) and metabotropic glutamate receptors (mGluRs). Group I mGluRs are excitatory whilst group II and III are inhibitory. Inhibitory mGluRs also modulate peripherally the mechanosensitivity of gastro-oesophageal vagal afferents. Here we determined the potential of excitatory GluRs to play an opposing role in modulating vagal afferent mechanosensitivity, and investigated expression of receptor subunit mRNA within the nodose ganglion. The responses of mouse gastro-oesophageal vagal afferents to graded mechanical stimuli were investigated before and during application of selective GluR ligands to their peripheral endings. Two types of vagal afferents were tested: tension receptors, which respond to circumferential tension, and mucosal receptors, which respond only to mucosal stroking. The selective iGluR agonists NMDA and AMPA concentration-dependently potentiated afferent responses. Their corresponding antagonists AP-5 and NBQX alone attenuated mechanosensory responses as did the non-selective antagonist kynurenate. The kainate selective agonist SYM-2081 had minor effects on mechanosensitivity, and the antagonist UBP 302 was ineffective. The mGluR5 antagonist MTEP concentration-dependently inhibited mechanosensitivity. Efficacy of agonists and antagonists differed on mucosal and tension receptors. We conclude that excitatory modulation of afferent mechanosensitivity occurs mainly via NMDA, AMPA and mGlu5 receptors, and the role of each differs according to afferent subtypes. PCR data indicated that all NMDA, kainate and AMPA receptor subunits plus mGluR5 are expressed, and are therefore candidates for the neuromodulation we observed.

Group I Metabotropic Glutamate Receptors on Second-Order Baroreceptor Neurons Are Tonically Activated and Induce a Na+–Ca2+Exchange Current

The nucleus tractus solitarius (NTS) is essential for coordinating baroreflex control of blood pressure. The baroreceptor sensory fibers make glutamatergic synapses onto second-order NTS neurons. Glutamate spillover activates Group II and III presynaptic metabotropic glutamate receptors (mGluRs) on the baroreceptor central terminals to inhibit synaptic transmission, but the role of postsynaptic mGluRs is less understood. We used whole cell patch-clamping in anatomically identified second-order baroreceptor neurons in a brain stem slice to test whether Group I, II, and III mGluRs had postsynaptic effects at this first central synapse in the baroreceptor afferent pathway. The Group I agonist DHPG induced a depolarization and spiking that was mimicked by endogenous glutamate. Group I mGluR blockade prevented the depolarization and slightly hyperpolarized the neurons, suggesting a small tonic Group I mGluR activation. The DHPG-induced inward current consisted of voltage-dependent and -independent components; the former was blocked by TEA and the latter was blocked by replacing extracellular NaCl with LiCl or Tris-HCl. The DHPG current was potentiated in a Ca2+-free external solution and was diminished by intracellular dialysis with BAPTA and by perfusion with Na+–Ca2+exchanger blockers, KB-R7943 or 3′,4′-dichlorobenzamil. Intracellular dialysis with GDPβS or heparin and perfusion with the PLC inhibitor U-73122 or the Ca2+-calmodulin inhibitor W-7 significantly decreased the DHPG current. The data suggest that Group I mGluRs on baroreceptor neurons are functional; are activated by endogenous glutamate; and activate a Na+–Ca2+exchanger through G-protein, PLC, IP3, and Ca2+-calmodulin mechanisms to excite the cell, thus providing postsynaptic mechanisms to enhance or prolong baroreceptor signal transmission.

Brain stem excitatory and inhibitory signaling pathways regulating bronchoconstrictive responses

This review summarizes recent work on two basic processes of central nervous system (CNS) control of cholinergic outflow to the airways: 1) transmission of bronchoconstrictive signals from the airways to the airway-related vagal preganglionic neurons (AVPNs) and 2) regulation of AVPN responses to excitatory inputs by central GABAergic inhibitory pathways. In addition, the autocrine-paracrine modulation of AVPNs is briefly discussed. CNS influences on the tracheobronchopulmonary system are transmitted via AVPNs, whose discharge depends on the balance between excitatory and inhibitory impulses that they receive. Alterations in this equilibrium may lead to dramatic functional changes. Recent findings indicate that excitatory signals arising from bronchopulmonary afferents and/or the peripheral chemosensory system activate second-order neurons within the nucleus of the solitary tract (NTS), via a glutamate-AMPA signaling pathway. These neurons, using the same neurotransmitter-receptor unit, transmit information to the AVPNs, which in turn convey the central command to airway effector organs: smooth muscle, submucosal secretory glands, and the vasculature, through intramural ganglionic neurons. The strength and duration of reflex-induced bronchoconstriction is modulated by GABAergic-inhibitory inputs and autocrine-paracrine controlling mechanisms. Downregulation of GABAergic inhibitory influences may result in a shift from inhibitory to excitatory drive that may lead to increased excitability of AVPNs, heightened airway responsiveness, and sustained narrowing of the airways. Hence a better understanding of these normal and altered central neural circuits and mechanisms could potentially improve the design of therapeutic interventions and the treatment of airway obstructive diseases.

Cranial afferent glutamate heterosynaptically modulates GABA release onto second-order neurons via distinctly segregated metabotropic glutamate receptors

The balance between excitation and inhibition dictates central integration. Glutamatergic and GABAergic neurotransmission dominate this process. Cranial primary afferents enter the brainstem to release glutamate (Glu) onto second-order neurons within the caudal nucleus tractus solitarius (NTS) to initiate autonomic reflexes. The simplest pathways for these reflexes contain as few as two central neurons, but display robust frequency-dependent behavior. Within NTS, multiple metabotropic Glu receptors (mGluRs) are present, but their roles are poorly understood. Using synaptically discriminated second-order NTS neurons in brainstem slices and mechanically dissociated NTS neurons with intact boutons, we show that Glu differentially controls GABA release via distinct presynaptic mGluRs. In second-order NTS neurons recorded in slices, activation of primary afferents at frequencies as low as 10 shocks per second released sufficient Glu to alter rates of spontaneous IPSCs (sIPSCs). In both approaches, group I mGluRs increased GABA release in some neurons, but, on different neurons, group II and group III mGluRs decreased the sIPSC rate. mGluR actions were remarkably rapid, with onset and reversal beginning within 100 msec. In all cases, mGluR actions were exclusively presynaptic, and mGluRs did not alter postsynaptic properties in second-order neurons in either slices or isolated neurons. Tests with capsaicin and alphabeta-methylene ATP suggest that myelinated and unmyelinated afferent pathways engage both mGluR-GABA mechanisms. Afferent Glu spillover provides heterosynaptic cross talk with GABAergic inhibition in NTS. This process may critically shape the dynamic character and use dependence for cranial afferent transmission at the first stage of autonomic reflexes.

Glutamate suppresses GABA release via presynaptic metabotropic glutamate receptors at baroreceptor neurones in rats

The nucleus tractus solitarii (NTS) is essential for coordinating arterial baroreflex control of blood pressure. The primary baroreceptor afferent fibres make their first excitatory synaptic contact at second-order NTS neurones with glutamate as the major neurotransmitter. Glutamate regulates its own release by activating presynaptic metabotropic glutamate autoreceptors (mGluRs) on the baroreceptor central terminals to suppress its further release in frequency-dependent manner. Gamma-aminobutyric acid (GABA) interneurones provide the major inhibitory synaptic input. It is the integration of excitatory and inhibitory inputs that shapes the NTS output of baroreceptor signals. We hypothesized that glutamate released from the primary central afferent terminals can spill over to presynaptic mGluRs on GABA interneurones to suppress GABA release at the second-order baroreceptor neurones. We assessed GABA transmission in second-order baroreceptor neurones identified by attached aortic depressor nerve (ADN) boutons. The medial NTS was stimulated to evoke GABA inhibitory postsynaptic currents (eIPSCs). Glutamate spillover, generated by brief 2 s, 25 Hz trains of stimuli applied to the tractus solitarius (TS), induced a small (10%) but significant reduction in the eIPSC amplitudes. The depression was enhanced to a 25% decrease by increasing glutamate in the cleft with a glutamate-uptake inhibitor (M-trans-pyrrolidine-2,4-dicarboxylic acid, 1 mum), blocked by a Group II mGluR antagonist (LY341495, 200 nm) and mimicked by a Group II agonist ((2S,3S,4S)-CCG/(2S,1'S,2'S)-2-carboxycyclopropyl; L-CCG-I). A presynaptic mGluR locus was established by the mGluR agonist-mediated increase in the paired-pulse ratio of two consecutive eIPSCs in conjunction with the decrease in the first eIPSC, and a decrease in the frequency (39-46% reduction at EC(50) concentration), but not amplitude, of spontaneous and miniature GABA IPSCs. The data indicate that endogenous glutamate activation of Group II presynaptic mGluRs can decrease GABA release at the first central synapses, suggesting a heterosynaptic role for the Group II mGluRs in shaping baroreceptor signal transmission.

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.

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.

Postexercise hypotension in conscious SHR is attenuated by blockade of substance P receptors in NTS

In hypertensive subjects, a single bout of dynamic exercise results in an immediate lowering of blood pressure back toward normal. This postexercise hypotension (PEH) also occurs in the spontaneously hypertensive rat (SHR). In both humans and SHRs, PEH features a decrease in sympathetic nerve discharge, suggesting the involvement of central nervous system pathways. Given that substance P is released in the nucleus tractus solitarius (NTS) by activation of baroreceptor and skeletal muscle afferent fibers during muscle contraction, we hypothesized that substance P acting at neurokinin-1 (NK-1) receptors in the NTS might contribute to PEH. We tested the hypothesis by determining, in conscious SHRs, whether NTS microinjections of the NK-1 receptor antagonist SR-140333 before exercise attenuated PEH. The antagonist, in a dose (60 pmol) that blocked substance P- and spared D,L-homocysteic acid-induced depressor responses, significantly attenuated the PEH by 37%, whereas it had no effect on blood pressure during exercise. Vehicle microinjection had no effect. The antagonist also had no effect on heart rate responses during both exercise and the PEH period. The data suggest that a substance P (NK-1) receptor mechanism in the NTS contributes to PEH.

Synaptic transmission in nucleus tractus solitarius is depressed by Group II and III but not Group I presynaptic metabotropic glutamate receptors in rats

Presynaptic metabotropic glutamate receptors (mGluRs) serve as autoreceptors throughout the CNS to inhibit glutamate release and depress glutamatergic transmission. Both presynaptic and postsynaptic mGluRs have been implicated in shaping autonomic signal transmission in the nucleus tractus solitarius (NTS). We sought to test the hypothesis that activation of presynaptic mGluRs depresses neurotransmission between primary autonomic afferent fibres and second-order NTS neurones. In second-order NTS neurones, excitatory postsynaptic currents (EPSCs) synaptically evoked by stimulation of primary sensory afferent fibres in the tractus solitarius (ts) and currents postsynaptically evoked by alpha-amino-3-hydroxy-4-isoxazoleproprionic acid (AMPA) were studied in the presence and absence of mGluR agonists and antagonists. Real-time quantitative RT-PCR (reverse transcription-polymerase chain reaction) was used to determine whether the genes for the mGluR subtypes were expressed in the cell bodies of the primary autonomic afferent fibres. Agonist activation of Group II and III but not Group I mGluRs reduced the peak amplitude of synaptically (ts) evoked EPSCs in a concentration-dependent manner while having no effect on postsynaptically (AMPA) evoked currents recorded in the same neurones. At the highest concentrations, the Group II agonist, (2S,3S,4S)-CCG/(2S,1'S,2'S)-2-carboxycyclopropyl (L-CCG-I), decreased the amplitude of the ts-evoked EPSCs by 39 % with an EC50 of 21 microM, and the Group III agonist, L(+)-2-amino-4-phosphonobutyric acid (L-AP4), decreased the evoked EPSCs by 71 % with an EC50 of 1 mM. mRNA for all eight mGluR subtypes was detected in the autonomic afferent fibre cell bodies in the nodose and jugular ganglia. Group II and III antagonists ((2S,3S,4S)-2-methyl-2-(carboxycyclopropyl)glycine (MCCG) and (RS)-alpha-methylserine-O-phosphate (MSOP)), at concentrations that blocked the respective agonist-induced synaptic depression, attenuated the frequency-dependent synaptic depression associated with increasing frequencies of ts stimulation by 13-34 % and 13-19 %, respectively (P < 0.05, for each). We conclude that Group II and III mGluRs (synthesized in the cell bodies of the primary autonomic afferent fibres and transported to the central terminals in the NTS) contribute to the depression of autonomic signal transmission by attenuating presynaptic release of glutamate.

Behavioral analysis of anorexia produced by hindbrain injections of AMPA receptor antagonist NBQX in rats

The caudal brain stem integrates short-term feedback signals from the oral cavity and the food-handling abdominal viscera, as well as long-term homeostatic, cognitive, and emotional signals from the forebrain, to control ingestive behavior. Glutamate, acting on various receptor subtypes, plays a prominent role in this integrative process. Fourth ventricular injection of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor blocker 1,2,3,4-tetrahydro-6-nitro-2,3-dioxobenzo[f]quinoxaline-7-sulfonamide (NBQX, 0.5-5 nmol/3 microl) dose dependently suppressed intake of 15% sucrose in food-deprived and non-food-deprived rats compared with saline injection. Two consecutive paired NBQX injections (5 nmol) into the fourth ventricle did not produce conditioned taste aversion to saccharin, but LiCl did. Intraburst lick rate and lick efficiency were not affected, but burst size and number and initial lick rate were significantly decreased by NBQX. Local injection of NBQX (2 nmol) into and near the nucleus tractus solitarius also suppressed sucrose intake. These results suggest a general role for non-N-methyl-D-aspartate receptors in the transmission of positive (feedforward) signals, but do not identify the exact processing step involved, such as taste input, sensory-motor processing, or descending facilitation. More localized injections and response measures will be necessary.

Frequency dependence of endocytosis in aortic baroreceptor neurons and role of group III mGluRs

Synaptic transmission between baroreceptor afferents and the nucleus tractus solitarius (NTS) is known to exhibit frequency-dependent depression. Reductions in neurotransmitter release and alterations in mechanisms regulating synaptic transmission are hypothesized to be involved in the activity-dependent depression observed in baroreceptor afferent neurons. The present study utilized cultured aortic baroreceptor neurons and the fluorescent dyes FM1-43 and FM2-10 to characterize the process of endocytosis or vesicle retrieval and its dependence on 1) frequency of neuronal activation, 2) metabotropic glutamate receptor (mGluR) activation, and 3) calcium concentrations inside and outside the cell. Endocytosis per spike, measured in fluorescence units after a 10-s stimulus applied at frequencies of 0.5 (53 +/- 4), 1.0 (23 +/- 1), and 10.0 Hz (2.7 +/- 0.2), was significantly depressed at higher frequencies. Blockade of group III mGluRs with (RS)-cyclopropyl-4-phosphonophenylglycine (CPPG) facilitated endocytosis at all frequencies, suggesting that this receptor subtype may be involved in the inhibition of endocytosis. Manipulating the extracellular and intracellular calcium concentrations subsequent to exocytosis had no effect on endocytosis. These results suggest that frequency-dependent depression of endocytosis observed in vitro could contribute to the frequency-dependent depression of baroreceptor afferent neurotransmission and that group III mGluRs inhibit endocytosis.

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.

Frequency limits on aortic baroreceptor input to nucleus tractus solitarii

The frequency of baroreceptor volleys to the central nervous system can influence the fidelity of baroreceptor signal transmission and thus may affect baroreflex regulation of blood pressure. We examined 1) the extent to which frequency-dependent depression of aortic baroreceptor signals was initiated at the first central synapse between primary baroreceptor fibers and second-order nucleus tractus solitarii (NTS) neurons; 2) whether the pattern of baroreceptor input influenced the depression; and 3) the potential relevance to baroreflex sympathoinhibition. In urethan-anesthetized rats, NTS action potential responses of neurons classified as second or higher order and averaged lumbar sympathetic nerve activity responses were simultaneously measured during 100 aortic depressor nerve stimuli delivered in constant or phasic patterns (0.8-48 Hz). Frequency-dependent depression was initiated at second-order neurons, with NTS responses decreasing to a 72% response rate at 48 Hz; the depression was greater at higher-order neurons; responses decreased to a 30% response rate. The depression was slightly but significantly greater with phasic inputs. Curve fitting suggested that synaptic depression may limit baroreflex sympathoinhibition. Thus frequency limits on baroreceptor inputs at NTS synapses may affect baroreflex function.