Rapidly adapting pulmonary receptor afferents: II. Fine structure and synaptic organization of central terminal processes in the nucleus of the tractus solitarius

Descending Modulation of Laryngeal Vagal Sensory Processing in the Brainstem Orchestrated by the Submedius Thalamic Nucleus

The nodose and jugular vagal ganglia supply sensory innervation to the airways and lungs. Jugular vagal airway sensory neurons wire into a brainstem circuit with ascending projections into the submedius thalamic nucleus (SubM) and ventrolateral orbital cortex (VLO), regions known to regulate the endogenous analgesia system. Here we investigate whether the SubM-VLO circuit exerts descending regulation over airway vagal reflexes in male and female rats using a range of neuroanatomical tracing, reflex physiology, and chemogenetic techniques. Anterograde and retrograde neuroanatomical tracing confirmed the connectivity of the SubM and VLO. Laryngeal stimulation in anesthetized rats reduced respiration, a reflex that was potently inhibited by activation of SubM. Conversely, inhibition of SubM potentiated laryngeal reflex responses, while prior lesions of VLO abolished the effects of SubM stimulation. In conscious rats, selective chemogenetic activation of SubM neurons specifically projecting to VLO significantly inhibited respiratory responses evoked by inhalation of the nociceptor stimulant capsaicin. Jugular vagal inputs to SubM via the medullary paratrigeminal nucleus were confirmed using anterograde transsynaptic conditional herpes viral tracing. Respiratory responses evoked by microinjections of capsaicin into the paratrigeminal nucleus were significantly attenuated by SubM stimulation, whereas those evoked via the nucleus of the solitary tract were unaltered. These data suggest that jugular vagal sensory pathways input to a nociceptive thalamocortical circuit capable of regulating jugular sensory processing in the medulla. This circuit organization suggests an intersection between vagal sensory pathways and the endogenous analgesia system, potentially important for understanding vagal sensory processing in health and mechanisms of hypersensitivity in disease.SIGNIFICANCE STATEMENT Jugular vagal sensory pathways are increasingly recognized for their important role in defensive respiratory responses evoked from the airways. Jugular ganglia neurons wire into a central circuit that is notable for overlapping with somatosensory processing networks in the brain rather than the viscerosensory circuits in receipt of inputs from the nodose vagal ganglia. Here we demonstrate a novel and functionally relevant example of intersection between vagal and somatosensory processing in the brain. The findings of the study offer new insights into interactions between vagal and spinal sensory processing, including the medullary targets of the endogenous analgesia system, and offer new insights into the central processes involved in airway defense in health and disease.

A Genetically Defined Circuit for Arousal from Sleep during Hypercapnia

The precise neural circuitry that mediates arousal during sleep apnea is not known. We previously found that glutamatergic neurons in the external lateral parabrachial nucleus (PBel) play a critical role in arousal to elevated CO2 or hypoxia. Because many of the PBel neurons that respond to CO2 express calcitonin gene-related peptide (CGRP), we hypothesized that CGRP may provide a molecular identifier of the CO2 arousal circuit. Here, we report that selective chemogenetic and optogenetic activation of PBel^CGRP neurons caused wakefulness, whereas optogenetic inhibition of PBel^CGRP neurons prevented arousal to CO2, but not to an acoustic tone or shaking. Optogenetic inhibition of PBel^CGRP terminals identified a network of forebrain sites under the control of a PBel^CGRP switch that is necessary to arouse animals from hypercapnia. Our findings define a novel cellular target for interventions that may prevent sleep fragmentation and the attendant cardiovascular and cognitive consequences seen in obstructive sleep apnea. VIDEO ABSTRACT.

Vagal Afferent Innervation of the Airways in Health and Disease

Vagal sensory neurons constitute the major afferent supply to the airways and lungs. Subsets of afferents are defined by their embryological origin, molecular profile, neurochemistry, functionality, and anatomical organization, and collectively these nerves are essential for the regulation of respiratory physiology and pulmonary defense through local responses and centrally mediated neural pathways. Mechanical and chemical activation of airway afferents depends on a myriad of ionic and receptor-mediated signaling, much of which has yet to be fully explored. Alterations in the sensitivity and neurochemical phenotype of vagal afferent nerves and/or the neural pathways that they innervate occur in a wide variety of pulmonary diseases, and as such, understanding the mechanisms of vagal sensory function and dysfunction may reveal novel therapeutic targets. In this comprehensive review we discuss historical and state-of-the-art concepts in airway sensory neurobiology and explore mechanisms underlying how vagal sensory pathways become dysfunctional in pathological conditions.

An essential component to brainstem cough gating identified in anesthetized guinea pigs

Coughing protects and clears the airways and lungs of inhaled irritants, particulates, pathogens, and accumulated secretions. An initial urge to cough, and an almost binary output suggests gating mechanisms that encode and modulate this defensive reflex. Whether this "gate" has a physical location for the physiological barrier it poses to cough is unknown. Here we describe a critical component to cough gating, the central terminations of the cough receptors. A novel microinjection strategy defined coordinates for microinjection of glutamate receptor antagonists that nearly abolished cough evoked from the trachea and larynx in anesthetized guinea pigs while having no effect on basal respiratory rate and little or no effect on reflexes attributed to activating other afferent nerve subtypes. Comparable microinjections in adjacent brainstem locations (0.5-2 mm distal) were without effect on coughing. Subsequent transganglionic and dual tracing studies confirmed that the central terminations of the cough receptors and their primary relay neurons are found bilaterally within nucleus tractus solitarius (nTS), lateral to the commissural subnucleus and perhaps in the medial subnuclei. These synapses possess the physiological characteristics of a cough gate. Their localization should facilitate more mechanistic studies of the encoding and gating of cough.

Sensory nerves and airway irritability

The lung, like many other organs, is innervated by a variety of sensory nerves and by nerves of the parasympathetic and sympathetic nervous systems that regulate the function of cells within the respiratory tract. Activation of sensory nerves by both mechanical and chemical stimuli elicits a number of defensive reflexes, including cough, altered breathing pattern, and altered autonomic drive, which are important for normal lung homeostasis. However, diseases that afflict the lung are associated with altered reflexes, resulting in a variety of symptoms, including increased cough, dyspnea, airways obstruction, and bronchial hyperresponsiveness. This review summarizes the current knowledge concerning the physiological role of different sensory nerve subtypes that innervate the lung, the factors which lead to their activation, and pharmacological approaches that have been used to interrogate the function of these nerves. This information may potentially facilitate the identification of novel drug targets for the treatment of respiratory disorders such as cough, asthma, and chronic obstructive pulmonary disease.

Central regulation of the cough reflex: therapeutic implications

In many species including humans, antagonists of NMDA-type glutamate receptors such as dextromethorphan, when used at sufficient doses, have been found to be relatively safe and effective antitussives. Similarly, now in five different species (guinea pigs, rabbits, cats, dogs and pigs), neurokinin receptor antagonists have also proven to be safe and effective antitussive agents. Both of these classes of drugs act centrally to prevent cough. A brief review of what is known about the central encoding of cough is presented, as are the advantages of centrally acting antitussives. Also discussed are new insights into cough and NMDA receptor signaling that may lead to the development of more effective antitussive agents with limited side effects and broad application in treating cough associated with a variety of aetiologies.

Cough sensors. I. Physiological and pharmacological properties of the afferent nerves regulating cough

The afferent nerves regulating cough have been reasonably well defined. The selective effects of general anesthesia on C-fiber-dependent cough and the opposing effects of C-fiber subtypes in cough have led to some uncertainty about their regulation of this defensive reflex. But a role for C-fibers in cough seems almost certain, given the unique pharmacological properties of these unmyelinated vagal afferent nerves and the ability of many C-fiber-selective stimulants to evoke cough. The role of myelinated laryngeal, tracheal, and bronchial afferent nerve subtypes that can be activated by punctate mechanical stimuli, inhaled particulates, accumulated secretions, and acid has also been demonstrated. These "cough receptors" are distinct from the slowly and rapidly adapting intrapulmonary stretch receptors responding to lung inflation. Indeed, intrapulmonary rapidly and slowly adapting receptors and pulmonary C-fibers may play no role or a nonessential role in cough, or might even actively inhibit cough upon activation. A critical review of the studies of the afferent nerve subtypes most often implicated in cough is provided.

Cardiorespiratory effects of intertrigeminal area stimulation in vagotomized rats

It has been recently shown that the pontine intertrigeminal region (ITR) plays an important role in respiratory regulation, including vagally mediated apneic reflexes. Neurons of the ITR have connections with the nucleus tractus solitarius and projections to the ventrolateral medulla. However, the functional targets of these projections are not fully defined. Stimulation of ITR neurons produced respiratory effects, but cardiovascular responses have not been explored. We investigated impact of bilateral vagotomy on respiratory and cardiovascular responses to glutamate microinjections within the ITR in ketamine/xylazine anesthetized rats. Cardiorespiratory indices, including breath duration (TT), tidal volume (VT), mean cardiac intervals (RR), systolic blood pressure (SBP), pulse pressure (PP) and their coefficients of variation (CVTT, CVVT, CVSBP, CVPP, respectively) were analyzed in 30 s segments before and after injection of glutamate (10 mM, 30 L) into the ITR. This assessment was carried out both before and after bilateral vagotomy. Glutamate injection evoked apnea and increased CVTT, but these responses were not altered by bilateral vagotomy. In contrast, removing vagal pathways significantly increased volume variability (CVVT), making tidal volume more vulnerable to perturbation from the ITR. Vagotomy prolonged the increase of mean systolic blood pressure observed after glutamate injection and unmasked a delayed but sustained elevation of PP and CVPP after ITR stimulation. The present findings indicate a broad involvement of the ITR in autonomic regulation, including at least cardiovascular and respiratory effects.

Baroreflexes of the rat. IV. ADN-evoked responses at the NTS

In a long-term (7-21 days) neuromuscular blocked (NMB) rat preparation, using precise single-pulse aortic depressor nerve (ADN) stimulation and stable chronic evoked response (ER) recordings from the dorsal-medial solitary nucleus (dmNTS), two different response patterns were observed: continuous and discrete. For the continuous pattern, activity began approximately 3 ms after the stimulus and persisted for 45 ms; for the discrete pattern, two complexes were separated by a gap from approximately 17 to 25 ms. The early complex was probably transmitted via A-fibers: it had a low stimulus current threshold and an average conduction velocity (CV) of 0.58-5.5 m/s; the high threshold late (HTL) complex had a CV = 0.26-0.58 m/s. The average stimulus amplitude-ER magnitude transduction curves for the A and HTL complexes were sigmoidal. For individual rats, in the linear range, mean r2 = 0.96 +/- 0.03 for both complexes. The average stimulus amplitude vs. the systolic blood pressure change (delta sBP) transduction curve was also approximately linear; however, for individual rats, the relationship was not consistently reliable: mean r2 = 0.48 +/- 0.19. Approximately 90% of recording sites had respiratory, and 50% had cardiac synchronism. The NMB preparation is useful for studying central baroreflex mechanisms that operate on time scales of days or weeks, such as adaptation and other kinds of neural plasticity.

Central pathways of pulmonary and lower airway vagal afferents

Lung sensory receptors with afferent fibers coursing in the vagus nerves are broadly divided into three groups: slowly (SAR) and rapidly (RAR) adapting stretch receptors and bronchopulmonary C fibers. Central terminations of each group are found in largely nonoverlapping regions of the caudal half of the nucleus of the solitary tract (NTS). Second order neurons in the pathways from these receptors innervate neurons located in respiratory-related regions of the medulla, pons, and spinal cord. The relative ease of selective activation of SARs, and to a lesser extent RARs, has allowed for more complete physiological and morphological characterization of the second and higher order neurons in these pathways than for C fibers. A subset of NTS neurons receiving afferent input from SARs (termed pump or P-cells) mediates the Breuer-Hering reflex and inhibits neurons receiving afferent input from RARs. P-cells and second order neurons in the RAR pathway also provide inputs to regions of the ventrolateral medulla involved in control of respiratory motor pattern, i.e., regions containing a predominance of bulbospinal premotor neurons, as well as regions containing respiratory rhythm-generating neurons. Axon collaterals from both P-cells and RAR interneurons, and likely from NTS interneurons in the C-fiber pathway, project to the parabrachial pontine region where they may contribute to plasticity in respiratory control and integration of respiratory control with other systems, including those that provide for voluntary control of breathing, sleep-wake behavior, and emotions.

Characterisation of c-Fos expression in the central nervous system of mice following right atrial injections of the 5-HT3 receptor agonist phenylbiguanide

Cardiopulmonary receptors relay signals to the central nervous system via vagal and spinal visceral afferents. To date there are no detailed topographical studies in mice indicating the distribution of central neurones activated following stimulation of cardiopulmonary afferents. In anaesthetised mice, we injected the 5-HT(3) receptor agonist phenylbiguanide (PBG), a drug that is known to stimulate cardiopulmonary afferent C-fibres, into the right atrium of the heart and mapped c-Fos expression within specific regions of the central nervous system. Intra-atrial injection of PBG produced a reflex cardiorespiratory response including a pronounced bradycardia and a respiratory depression. Using immunohistochemical detection of the protein product of the immediate-early gene c-fos, we mapped the brain regions affected by cardiopulmonary 5-HT(3) receptor stimulation. Within the nucleus of the solitary tract (nTS) of PBG-injected mice, we detected an increased number of c-Fos-positive nuclei in the dorsolateral and gelatinous parts at the level of the area postrema (-7.48 mm bregma) but not at more rostral or caudal levels (-7.76, -7.20, -6.84 and -6.36 mm bregma) relative to vehicle-injected control mice. In addition, c-Fos expression in the crescent part of the lateral parabrachial nucleus was decreased in PBG-injected mice whereas no significant differences were detected between PBG-injected and control mice in the number of c-Fos-positive nuclei in the dorsal part of the lateral parabrachial nucleus. PBG injections had no significant effects on the number of c-Fos-positive catecholaminergic neurones within the C1/A1, C2/A2, A5, A6 and A7 cell groups. Likewise, PBG injections had no significant effects on c-Fos expression in other central regions involved in cardiorespiratory control or cardiorespiratory reflexes (selected non-catecholaminergic nuclei in the medulla and midbrain periaqueductal gray, the paraventricular nucleus of the hypothalamus and the central nucleus of the amygdala). Identification of specific regions of the nTS complex involved in relaying signals from afferent cardiopulmonary C-fibres to the central nervous system will be useful for future studies aimed at understanding neural mechanisms underlying cardiopulmonary reflexes and physiological responses to cardiopulmonary disease.

Functional organization of the parabrachial complex and intertrigeminal region in the control of breathing

Although the medulla oblongata contains the epicenter for respiratory rhythm generation, many other parts of the neuraxis play significant substratal roles in breathing. Accumulating evidence suggests that the pons contains several groups of neurons that may belong to the central respiratory system. This article will review data from microstimulation mapping and tract-tracing studies of the parabrachial complex (PB) and intertrigeminal region (ITR). Chemical activation of neurons in these areas has distinct effects on ventilatory and airway muscle activity. Tract-tracing experiments from functionally identified sites reveal specific respiratory-related sensory inputs and outputs that are likely anatomical substrates for these effects. The data suggest that an important physiological role for the rostral pons may be reflexive respiratory responses to airway stimuli.

Direct projections from the cardiovascular nucleus tractus solitarii to pontine preganglionic parasympathetic neurons: a link to cerebrovascular regulation

Peripheral or central interruption of the baroreflex or the parasympathetic innervation of cerebral vessels leads to similar changes in regulation of cerebral blood flow. Therefore, we sought to test the hypothesis that the cardiovascular nucleus tractus solitarii, the site of termination of arterial baroreceptor nerves, projects to pontine preganglionic neurons whose stimulation elicits cerebral vasodilatation. The current study utilized both light and electron microscopic techniques to analyze anterograde tracing from the cardiovascular nucleus tractus solitarii to preganglionic parasympathetic neurons in the pons. We further used retrograde tracing from that same pontine region to the cardiovascular nucleus tractus solitarii and evaluated the confluence of tracing from the cardiovascular nucleus tractus solitarii to pontine preganglionic neurons labeled retrogradely from the pterygopalatine ganglia. The cardiovascular nucleus tractus solitarii projected to pontine preganglionic parasympathetic neurons, but more rostral and caudal regions of nucleus tractus solitarii did not. In contrast, all three regions of nucleus tractus solitarii projected to the nucleus ambiguus and dorsal motor nucleus of the vagus. Although not projecting to pontine preganglionic parasympathetic neurons, regions lateral, rostral, and caudal to cardiovascular nucleus tractus solitarii sent projections through the pons medial to the preganglionics. The study establishes the presence of a direct monosynaptic pathway from neurons in the cardiovascular nucleus tractus solitarii to pontine preganglionic parasympathetic neurons that project to the pterygopalatine ganglia, the source of nitroxidergic vasodilatory innervation of cerebral blood vessels. It provides evidence that activation of those preganglionic neurons can cause cerebral vasodilatation and increased cerebral blood flow. Finally, it demonstrates differential innervation of medullary and pontine preganglionic parasympathetic neurons by different regions of the nucleus tractus solitarii.

Reliability of monosynaptic sensory transmission in brain stem neurons in vitro

The timing of events within the nervous system is a critical feature of signal processing and integration. In neurotransmission, the synaptic latency, the time between stimulus delivery and appearance of the synaptic event, is generally thought to be directly related to the complexity of that pathway. In horizontal brain stem slices, we examined synaptic latency and its shock-to-shock variability (synaptic jitter) in medial nucleus tractus solitarius (NTS) neurons in response to solitary tract (ST) electrical activation. Using a visualized patch recording approach, we activated ST 1-3 mm from the recorded neuron with short trains (50-200 Hz) and measured synaptic currents under voltage clamp. Latencies ranged from 1.5 to 8.6 ms, and jitter values (SD of intraneuronal latency) ranged from 26 to 764 micros (n = 49). Surprisingly, frequency of synaptic failure was not correlated with either latency or jitter (P > 0.147; n = 49). Despite conventional expectations, no clear divisions in latency were found from the earliest arriving excitatory postsynaptic currents (EPSCs) to late pharmacologically polysynaptic responses. Shortest latency EPSCs (<3 ms) were mediated by non-N-methyl-D-aspartate (non-NMDA) glutamate receptors. Longer latency responses were a mix of excitatory and inhibitory currents including non-NMDA EPSCs and GABAa receptor-mediated currents (IPSC). All synaptic responses exhibited prominent frequency-dependent depression. In a subset of neurons, we labeled sensory boutons by the anterograde fluorescent tracer, DiA, from aortic nerve baroreceptors and then recorded from anatomically identified second-order neurons. In identified second-order NTS neurons, ST activation evoked EPSCs with short to moderate latency (1.9-4.8 ms) but uniformly minimal jitter (31 to 61 micros) that were mediated by non-NMDA receptors but had failure rates as high as 39%. These monosynaptic EPSCs in identified second-order neurons were significantly different in latency and jitter than GABAergic IPSCs (latency, 2.95 +/- 0.71 vs. 5.56 +/- 0.74 ms, mean +/- SE, P = 0.027; jitter, 42.3 +/- 6.5 vs. 416.3 +/- 94.4 micros, P = 0.013, n = 4, 6, respectively), but failure rates were similar (27.8 +/- 9.0 vs. 9.7 +/- 4.4%, P = 0.08, respectively). Such results suggest that jitter and not absolute latency or failure rate is the most reliable discriminator of mono- versus polysynaptic pathways. The results suggest that brain stem sensory pathways may differ in their principles of integration compared with cortical models and that this importantly impacts synaptic performance. The unique performance properties of the sensory-NTS pathway may reflect stronger axosomatic synaptic processing in brain stem compared with dendritically weighted models typical in cortical structures and thus may reflect very different strategies of spatio-temporal integration in this NTS region and for autonomic regulation.

Sensitivity to naloxone of the behavioral signs of morphine withdrawal and c-Fos expression in the rat CNS: a quantitative dose-response analysis

Several studies have used c-Fos expression to delineate the neural substrate underlying naloxone-precipitated morphine withdrawal (MW). However, because behavioral manifestations of MW depend on both the degree of dependence and the doses of naloxone (NAL), a comprehensive study would require examining c-Fos expression in relation with the degree of MW. Here, changes in behavior and in c-Fos-like immunoreactivity (FLI) were studied in the same rats after injection of three doses of NAL to precipitate various degrees of MW. Fifteen established signs of MW were examined for 1 hour after NAL injection, and FLI was quantified in 52 regions of the brain and in the lumbosacral spinal cord. Linear regression analyses were used to examine changes in numbers of signs and FLI neurons with the doses of NAL, and data were considered dose-related for a statistical level of significance of P < 0.05. In summary, autonomic signs of MW increased in a dose-related manner, whereas somatomotor signs did not. After MW, 33 central nervous system regions exhibited significant increases in FLI and were, thus, considered as important neural correlates of MW. Twenty of them displayed dose-related increases in c-Fos expression and correspond to regions related to autonomic functions. Low c-Fos expression was detected in some regions involved in motor control or in reward, suggesting either their minor role in MW or a limitation of the technique. This dose-response analysis suggests that the increase in the severity of autonomic manifestations of MW is associated with a gradual activation of major structures of the autonomic nervous system.

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.

Ultrastructural localization of P2X3 receptors in rat sensory neurons

We used isolated IgG antibodies selective for P2X3 receptors to study the ultrastructural distribution of these receptors in rat sensory neurons. In trigeminal ganglia, P2X3 receptor immunoreactivity occurred in small and large nerve cell bodies and their processes. Endoplasmic reticulum and Golgi apparatus were heavily stained; cytoplasmic matrix was faintly to moderately stained. In synaptic glomeruli in lamina II of cervical dorsal horn, P2X3 receptor-immunoreactive core terminals were postsynaptic to unlabelled vesicle-containing dendrites and axons. In the nucleus of the solitary tract, receptor-positive boutons synapsed on dendrites and cell bodies and had complex synaptic relationships with other axon terminals and vesiculated dendrites. These observations identify sites from which ATP could be released to influence sensory signalling within the central nervous system.

Direct evidence for nitric oxide synthase in vagal afferents to the nucleus tractus solitarii

The anatomical relationship between vagal afferents and brain nitric oxide synthase containing terminals in the nucleus tractus solitarii was studied by means of anterograde tracing combined with immunocytochemistry and immuno-electron microscopy. Biotinylated dextran amine was injected into the nodose ganglion with a glass micropipette. Four to eight days following the injection, regions of the nucleus tractus solitarii containing biotinylated dextran amine-labelled vagal afferents and those containing nitric oxide synthase-immunopositive terminals were congruent. Many neurons exhibiting nitric oxide synthase immunoreactivity were found within the biotinylated dextran amine-containing terminal field. However dense labeling of terminals with biotinylated dextran amine precluded determination if the terminals were nitric oxide synthase-immunoreactive. Therefore, we combined degeneration of vagal afferents after removal of one nodose ganglion with nitric oxide synthase immuno-electron microscopy. Axon terminals that possessed characteristic vesicle clusters and were partially or completely engulfed by glial processes were identified as degenerating vagal afferents. Degenerating axon terminals comprised 38% of the total axon terminals in the nucleus tractus solitarii in a sample of sections; and of the degenerating axon terminals, 67% were nitric oxide synthase-immunoreactive. Nitric oxide synthase immunoreactivity was present in 41% of the non-degenerating axon terminals. Prominent staining of dendrites for nitric oxide synthase immunoreactivity indicated that much of the nitric oxide synthase in the nucleus tractus solitarii is not derived from peripheral afferents. Of the total number of dendritic profiles sampled, half were nitric oxide synthase-immunoreactive. Our data support the hypothesis that nitric oxide or nitric oxide donors may be present in primary vagal afferents that terminate in the nucleus tractus solitarii. While this study confirms that vagal afferents contain brain nitric oxide synthase, it demonstrates for the first time that the majority of nitric oxide synthase immunoreactivity in the nucleus tractus solitarii is found in intrinsic structures in the nucleus. In addition, our data show that second or higher order neurons in the nucleus tractus solitarii may be nitroxidergic and receive both nitroxidergic and non-nitroxidergic vagal input.

Rapidly deactivating AMPA receptors determine excitatory synaptic transmission to interneurons in the nucleus tractus solitarius from rat

Excitatory synaptic transmission was investigated in interneurons of the parvocellular nucleus tractus solitarius (pNTS) by performing patch-clamp experiments in thin slice preparations from rat brain stem. Stimulation of single afferent fibers evoked excitatory postsynaptic currents (EPSCs) mediated by glutamate receptors of the DL-alpha-amino-3-hydroxy-5-methylisoxazole-propionic acid (AMPA) and N-methyl-D-aspartate types. AMPA-receptor-mediated EPSCs displayed decay time constants of 3.5 +/- 1.2 (SD) ms (13 cells), which were slow compared with EPSC decay time constants in neurons of the cerebellum or hippocampus. Slow EPSC decay was not explained by dendritic filtering, because the passive membrane properties of pNTS interneurons provided favorable voltage-clamp conditions. Also, the slowness of EPSC decay did not result from slow deactivation of AMPA receptors (0.7 +/- 0.2 ms, 5 cells), which was investigated during rapid application of agonist to outside-out patches. Comparison of AMPA receptor kinetics with EPSC decay time constants suggested that the slow time course of EPSCs resulted from the prolonged presence of glutamate in the synaptic cleft.

Glutamate immunoreactivity of insular cortex afferents to the nucleus tractus solitarius in the rat: a quantitative electron microscopic study

Corticosolitary axons and their terminals were labeled by the anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase, after injections into the rat insular cortex. The ultrastructure of these cortical afferents was analysed in the medial and commissural subnuclei of the nucleus tractus solitarius. Cortical terminals had a mean area of 0.36 microns 2, and were among the smallest terminals in the nucleus. They made single, asymmetric synaptic contacts with thin dendritic stems or with spines. The average diameter of the dendrites postsynaptic to cortical axons was 0.59 microns, and significantly smaller (P < 0.01, Kolmogorov-Smirnov test) than the mean (0.87 microns) of the population of dendrites in the same region of the nucleus tractus solitarius. Cortical boutons contained closely packed round and clear synaptic vesicles of diameter ca. 28 nm, a few mitochondria, and no dense core vesicles. Postembedding immunogold analysis showed that the anterogradely labeled cortical axon terminals were immunoreactive to glutamate, but not to GABA. Cortical afferents had on average four times the glutamate immunoreactivity (assessed by gold particle density) than local dendrites or terminals making symmetric synaptic contacts. Similarly, most of the unlabeled axon terminals participating in asymmetric synaptic contacts were highly enriched in glutamate immunoreactivity, suggesting that glutamate may be a most prevalent transmitter in the nucleus tractus solitarius. Terminals immunoreactive to GABA always made symmetric synapses, mostly with dendritic shafts and perikarya. We concluded that insular cortex axons made single, asymmetric synaptic contacts with thin, probably distal dendrites in the nucleus tractus solitarius. Cortical terminals are immunoreactive to glutamate, and morphologically different from primary afferents and from terminals immunoreactive to GABA.

Synaptic interactions of substance P immunoreactive nerve terminals in the baro- and chemoreceptor reflexes of the cat

The neurochemical anatomy and synaptic interactions of morphologically identified chemoreceptor or baroreceptor afferents in the nucleus of the solitary tract (NTS) are poorly understood. A substantial body of physiological and light microscopic evidence suggests that substance P (SP) may be a neurotransmitter contained in first order sensory chemo- or baroreceptor afferents, however ultrastructural support of this hypothesis is lacking. In the present report we have traced the central projections of the carotid sinus nerve (CSN) in the cat by utilizing the transganglionic transport of horseradish peroxidase. Medullary tissues including the commissural NTS (cNTS) were processed for the histochemical visualization of transganglionically labeled CSN afferents and for the immunocytochemical detection of SP by dual labeling light and electron microscopic methods. At the light microscopic level, dense bilateral labeling with TMB was found in the tractus solitarius (TS) and cNTS, caudal to the obex. Rostral to the obex, significant ipsilateral TMB labeling was detected in the dorsal, dorso-lateral, and medial subnuclei of the NTS, as well as in the TS. Significant staining of SP immunoreactive processes was detected in most subnuclei of the NTS. The cNTS was examined by electron microscopy. Either HRP or SP were readily identified in single labeled unmyelinated axons, myelinated axons, and nerve terminals in the cNTS. SP immunoreactivity was also identified in unmyelinated axons, myelinated axons, and nerve terminals in the cNTS which were simultaneously identified as CSN primary afferents. These ultrastructural data support the hypothesis that SP immunoreactive first order neurons are involved in the origination of the chemo- and baroreceptor reflexes. Axo-axonic synapses were observed between CSN primary afferent terminals and: (a) unlabeled nerve terminals; (b) other CSN primary afferent terminals; and (c) terminals containing SP. Axo-axonic synapses were also observed between CSN primary afferents which contained SP, and other SP terminals. These observations may mediate the morphological bases for multiple forms of presynaptic inhibition in the cNTS, including those involved in cardiorespiratory integration. In conclusion, our results indicate that SP immunoreactive nerve terminals may be important in both the origination and the modulation of the chemo- and/or baroreceptor reflexes.

Synaptic organization of the interstitial subdivision of the nucleus tractus solitarii and of its laryngeal afferents in the rat

The nucleus tractus solitarii, the first central relay for gustatory and a variety of visceral afferents, is also an integrative center for numerous functions. Its interstitial subdivision is involved in swallowing and respiratory reflexes. The ultrastructural characteristics of this subdivision and of its laryngeal afferents were investigated in adult rat by a serial-section study and by application of wheat germ agglutinin-horseradish peroxidase conjugate to the peripheral afferent fibers. The interstitial subnucleus contained scattered small neuronal cell bodies with such ultrastructural features as a large nucleus with deep indentations and an organelle-poor cytoplasm. On the basis of their size and vesicular content, the axon terminals were classified into three categories. Group I and group II terminals were small or large, respectively, and contained mainly small, round, and clear synaptic vesicles. Group III terminals were also small but contained small, pleomorphic, and clear vesicles. Axodendritic synapses were the most numerous. They were either asymmetrical, comprised of group I and II terminals, or symmetrical, comprised of group III terminals. More than 50% were part of complex synaptic arrangements in the form of rosettes or glomeruli. Axosomatic contacts involved both group I and group III terminals and were always symmetrical. A high frequency of axoaxonic synapses was found. They were symmetrical, comprised of group III terminals on group I or II terminals. Different types of symmetrical synaptic contacts made by dendrites were also found. This study indicates also that the ipsilateral interstitial subdivision constitutes the preferential site of termination for superior laryngeal afferents. The labeled axon terminals belonged exclusively to groups I and II and were involved in both axodendritic and axoaxonic synapses. Some of the axodendritic synapses were part of rosettes or glomeruli. All these synaptic arrangements may be considered a morphological substrate for important processing of afferent information in the nucleus tractus solitarii. They may account for some of the integrative functions of the interstitial subnucleus such as physiological processes triggered from the superior laryngeal nerve.

Localization of glucose transporter GLUT 3 in brain: comparison of rodent and dog using species-specific carboxyl-terminal antisera

The carboxyl-terminal amino acid sequences of the canine and gerbil glucose transporter GLUT3 were determined and compared to the published rat sequence. Eleven of 16 amino acids comprising the carboxyl terminus of GLUT3 were found to be identical in rat and dog. However, the canine sequence "ATV" substitutes for the rat sequence "PGNA" at the end of the molecule. The gerbil sequence has 12 of 16 amino acids identical to the rat, including the PGNA terminus. Based on these sequences, four peptides were synthesized, and two polyclonal antisera (one to the canine sequence and one to the rat sequence) were raised to examine the distribution of GLUT3 in canine and rodent brain. Immunoblots of brain membrane preparations showed that both antisera identified peptide-inhibitable protein bands of molecular weight 45,000-50,000. Immunocytochemical studies demonstrated that binding sites for these antisera were abundantly distributed in neuropil in all brain regions. Areas rich in synapses and areas surrounding microvessels exhibited especially high reactivity. GLUT3 reactivity was similarly distributed in canine and rodent brain, except at the blood-brain barrier. GLUT3 was not detected in the blood-brain barrier in gerbil and rat but was present in many canine cerebral endothelial cells, particularly in cerebellum and brain stem. The carboxyl-terminal antisera employed in this study exhibited high degrees of species specificity, indicating that the three or four terminal amino acids of the immunizing peptides (ATV and PGNA) are important epitopes for binding the polyclonal antibodies. These antisera exhibited only minimal binding to brain tissue of non-target species, yet yielded similar staining patterns in neuropil of rodent and canine brain. This finding provides strong evidence that the observed staining patterns accurately reflect the distribution of GLUT3 in brain. In addition, the presence of vascular GLUT3 in dog brain suggests that the canine blood-brain barrier may be preferable to that of the rat as a model for studies of glucose transport relevant to human brain.

Synapses between slowly adapting lung stretch receptor afferents and inspiratory beta-neurons in the nucleus of the solitary tract of cats: a light and electron microscopic analysis

Previous neuroanatomic and physiologic studies indicated that afferent fibres from slowly adapting pulmonary stretch receptors (SAR) project to the nuclei of the solitary tract and terminate on inspiratory beta-neurons. In the present study we combined electrophysiologic and morphologic approaches to verify the presumed monosynaptic connections between SARs and beta-neurons. Single identified beta-neurons and single identified SAR afferent fibres were labelled intrasomally and intraaxonally, respectively, with horseradish peroxidase (HRP) in the same anesthetized cats. Under the light microscope, we analyzed the morphology of beta-neurons and their dendritic fields and of the terminal projection pattern of fibres from SARs and identified potential synaptic connections between boutons of SAR afferent fibres and the soma and dendrites of beta-neurons. The identified tissue was then processed further for electron microscopic analysis. On average, beta-neurons had 6 primary dendrites that bifurcated 3-8 times. The dendritic trees extended 1.5 mm both rostrocaudally in the ventrolateral nucleus of the solitary tract and medially into the intermediate subnucleus. Axons of beta-neurons curved toward the midline and no collateral branches were evident over its stained length (2.5-3.4 mm). Axodendritic synaptic contacts between SAR fibres and beta-neurons were identified electron microscopically in four of six tissue samples chosen by light microscopy. In addition, we located 2 axodendritic and 2 axosomatic synaptic contacts that were not observed under light microscopic screening. The boutons of SAR fibres contained clear, round vesicles and formed asymmetrical synapses with beta-neurons. Multiple synaptic connections were found between collaterals of a single SAR and single beta-neurons, indicating a dense terminal projection of single SAR afferent fibres onto beta-neurons. These morphologic data prove monosynaptic connections between electrophysiologically identified SAR afferent fibres and beta-neurons.

Distribution of [3H]quinuclidinyl benzilate, [3H]nicotine, and [125I]alpha-bungarotoxin binding sites in the nucleus tractus solitarii of the cat

The distribution of muscarinic and nicotinic cholinergic binding sites in the cat nucleus tractus solitarii was studied by the technique of in vitro autoradiography. Using the antagonist [3H]quinuclidinyl benzilate, muscarinic binding sites were differentially located in subdivisions of the nucleus tractus solitarii. The majority of muscarinic binding sites were located predominantly in the caudal half of the nucleus, reaching their greatest amounts at the mid levels of the nucleus tractus solitarii. The medial, dorsolateral, intermediate, and interstitial subdivisions contained the highest densities of quinuclidinyl benzilate binding sites. Nicotinic cholinergic binding sites, using [3H]nicotine and [125I]alpha-bungarotoxin, had unique patterns of distribution. With [3H]nicotine the majority of binding sites were located in rostral levels of the nucleus with very few binding sites present in the caudal half. In contrast, [125I]alpha-bungarotoxin binding sites were present mainly in subdivisions located in the caudal half of the nucleus, i.e., commissural, ventrolateral, dorsolateral, medial, and intermediate subdivisions, and dropped off precipitously at more rostral levels. The differential distribution of [3H]nicotine and [125I]alpha-bungarotoxin suggests the two ligands may be labeling different types of nicotinic binding sites in the nucleus tractus solitarii. The unique distribution of muscarinic and nicotinic cholinergic binding sites in the various subdivisions of the nucleus solitarii suggests that muscarinic and nicotine mechanisms may play an active role in the regulation of the diverse autonomic functions at the level of the nucleus tractus solitarii.

Cytochrome oxidase activity in the nucleus of the tractus solitarius of the cat

We studied the cytochrome oxidase (CO) activity in the nucleus of the tractus solitarius (NTS) of normal cats and in animals subjected to unilateral removal of vagal and glossopharyngeal afferents. In normal cats CO activity was higher in the ventrolateral, dorsolateral, interstitial and ventral NTS subnuclei. The dorsal, medial, commissural and gelatinosus subdivisions showed lower levels of CO activity. The peripheral deafferentation up to 47 days did not reduce the CO activity, suggesting an important role for the central inputs in determining the neural activity of the NTS.

Identification of neurons receiving input from pulmonary rapidly adapting receptors in the cat

1. Extracellular and intracellular recordings were made in the caudal subdivisions of the nucleus tractus solitarii (NTS) to locate and characterize neurons excited by afferents from pulmonary rapidly adapting receptors (RARs) in Nembutal-anaesthetized cats. 2. Neurons identified as second-order cells activated by RARs (RAR-cells) were activated by electrical stimulation of myelinated afferents in the cervical portion of the vagus nerve(s) and by at least two of the following 'physiological' stimuli: (a) collapse of the lungs to atmospheric pressure; (b) hyperinflation of the lungs by either increasing tidal volume or maintained lung inflations; and (c) brief inhalation of ammonia vapour. 3. Of the ninety-nine RAR-cells identified and studied extracellularly, eighty-four were localized within the commissural nucleus of the NTS. Seventy-four cells responded monosynaptically to electrical stimulation of both ipsi- and contralateral vagal stimulation. The remaining ten RAR-cells located in the commissural nucleus and the fifteen located in the caudal portion of the medial subnucleus of the NTS rostral to the obex, responded to the ipsilateral vagus only. 4. Under control ventilatory conditions (bilateral pneumothorax, positive end-expiratory pressure of approx. 2 cmH2O), forty-eight of the ninety-nine RAR-cells showed spontaneous ventilator-related activity, occurring primarily during ventilator-induced deflations (thirty of forty-eight). 'Reversal' of this ventilator-related modulation, from firing predominantly during lung deflations to lung inflations, and vice versa, could be induced in eleven of the RAR-cells by changing the lung volume or by the inhalation of ammonia vapour. 5. Modulation of firing in synchrony with the central respiratory rhythm was observed in the activity of fourteen of the ninety-nine RAR-cells. 6. Intracellular recordings were made from twenty-two NTS neurons caudal to the obex that received monosynaptic excitatory postsynaptic potentials (EPSPs) from both ipsi- and contralateral vagus nerves. Two were positively identified as RAR-cells. 7. It is concluded that the commissural nucleus of the NTS is the major location of RAR-cells, and that the response characteristics of these neurons largely, but not entirely, correspond to the known properties of RARs.

Ultrastructural organization of the interstitial subnucleus of the nucleus of the tractus solitarius in the cat: identification of vagal afferents

This electron microscopic study, based on serial section analysis, describes the synaptic organization of the interstitial subnucleus of the nucleus of the solitary tract and identifies the terminals of the vagal primary afferents utilizing degeneration and HRP transport. The interstitial subnucleus contains sparsely scattered cell bodies, numerous dendrites and axon terminals, and bundles of unmyelinated and myelinated axons. The cell bodies which are small in diameter have an organelle poor cytoplasm and a large invaginated nucleus. Axon terminals can be classified into two main types according to their vesicular shape. The first type contains clear, round vesicles and can be further subdivided into two subgroups on the basis of their morphology and the size of their vesicles. In the first subgroup the terminals are small, contain a few mitochondria and their vesicles are densely packed with an homogeneous size. In the second subgroup the terminals which vary from small to large, contain many mitochondria and contain round vesicles which are heterogeneous in size. The second main terminal type consists of axon terminals containing pleomorphic vesicles which are associated with asymmetrical or symmetrical synaptic contacts on dendrites. Axo-axonic contacts are present in the interstitial subnucleus. In general, the presynaptic axon terminals contain pleomorphic vesicles and the postsynaptic elements contain round vesicles of varying size. In some dendrites, identified by the presence of ribosomes, groups of round and/or pleomorphic vesicles are found associated with synaptic contacts. These dendrites are presynaptic to conventional dendrites and postsynaptic to axon terminals. After removal of the nodose ganglion, degenerative alterations are seen only at the caudal and middle levels of the interstitial subnucleus. Degeneration occurs in a few myelinated axons and in axon terminals which usually contain a mixture of small and larger round, clear vesicles. After HRP injection into the vagus nerve, the HRP reaction product is visible in axon terminals filled with clear, round vesicles which are heterogeneous in size. The labelled axon terminals establish single or multiple synaptic contacts. This study demonstrates that terminals of vagal primary afferents consist principally of terminals of the second subgroup. The morphology of these terminals are compared to primary afferents in the brainstem and spinal cord.

[The nucleus tractus solitarius: neuroanatomic, neurochemical and functional aspects]

The nucleus tractus solitarii (NTS) has long been considered as the first central relay for gustatory and visceral afferent informations only. However, data obtained during the past ten years, with neuroanatomical, biochemical and electrophysiological techniques, clearly demonstrate that the NTS is a structure with a high degree of complexity, which plays, at the medullary level, a key role in several integrative processes. The NTS, located in the dorsomedial medulla, is a structure of small size containing a limited number of neurons scattered in a more or less dense fibrillar plexus. The distribution and the organization of both the cells and the fibrillar network are not homogeneous within the nucleus and the NTS has been divided cytoarchitectonically into various subnuclei, which are partly correlated with the areas of projection of peripheral afferent endings. At the ultrastructural level, the NTS shows several complex synaptic arrangements in form of glomeruli. These arrangements provide morphological substrates for complex mechanisms of intercellular communication within the NTS. The NTS is not only the site of vagal and glossopharyngeal afferent projections, it receives also endings from facial and trigeminal nerves as well as from some renal afferents. Gustatory and somatic afferents from the oropharyngeal region project with a crude somatotopy within the rostral part of the NTS and visceral afferents from cardiovascular, digestive, respiratory and renal systems terminate viscero-topically within its caudal part. Moreover the NTS is extensively connected with several central structures. It projects directly to multiple brain regions by means of short connections to bulbo-ponto-mesencephalic structures (parabrachial nucleus, motor nuclei of several cranial nerves, ventro-lateral reticular formation, raphe nuclei...) and long connections to the spinal cord and diencephalic and telencephalic structures, in particular the hypothalamus and some limbic structures. The NTS is also the recipient of several central afferent inputs. It is worth to note that most of the structures that receive a direct projection from the NTS project back to the nucleus. Direct projections from the cerebral cortex to the NTS have also been identified. These extensive connections indicate that the NTS is a key structure for autonomic and neuroendocrine functions as well as for integration of somatic and autonomic responses in certain behaviors. The NTS contains a great diversity of neuroactive substances. Indeed, most of the substances identified within the central nervous system have also been detected in the NTS and may act, at this level, as classical transmitters and/or neuromodulators.(ABSTRACT TRUNCATED AT 400 WORDS)

Rapidly adapting pulmonary receptor afferents: I. Arborization in the nucleus of the tractus solitarius

The organization of axon collaterals, preterminal processes, and presumptive synaptic boutons of single physiologically identified rapidly adapting receptor (RAR) pulmonary afferent fibers was examined following the intraaxonal application of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP). The RAR axons were injected 200-300 microns lateral to the nucleus of the tractus solitarius (nTS) at a number of different rostrocaudal levels in seven individual experiments. The trajectories of the stained axons were reconstructed from individual 50-microns-thick serial sections. The rostrocaudal extent, as well as the distribution of the trajectory of each RAR afferent, was reconstructed from every section by using a camera lucida attachment. In this first of two papers, we describe the pattern of organization of bouton terminals of RAR afferents related to cytoarchitectonically distinct subnuclei of the nTS. In the companion paper, morphological details of the fine structure of these synaptic boutons and axonal branches are described in different subnuclei in order to illustrate morphological differences in these functionally distinct regions. A number of significant findings have resulted from this light microscopic study. The central process of a single RAR afferent fiber arborized in the medulla oblongata over a considerable distance in the rostrocaudal plane (2.5 mm rostral to 1.4 mm caudal to the obex). A single RAR afferent fiber terminated in numerous bouton terminals (range 500-1,050), and these terminals arose from over 400 segments of branches of the parent injected axon. A small number of en passant bouton terminals were found. There appeared to be a remarkable degree of consistency in the subnuclei of the nTS where these terminals arborized. The dorsal and dorsolateral subnuclei of the nTS received 144-647 bouton terminals. The second-largest concentration of bouton terminals of RAR afferents was found in the intermediate (nI) subnucleus of the nTS. No labeled bouton terminal was found in the ventral and ventrolateral subnuclei of the nTS. This finding is in sharp contrast to the terminations of SAR afferents which terminated predominantly in the ventral and ventrolateral nuclei of the nTS, the interstitial nucleus of the nTS, and the nI. The parent RAR axon could be traced as far rostrally as 2.5 mm, even though the region of terminal arborization could not be followed beyond 0.8 mm. The destination of this rostrally projecting RAR afferent could not be determined in this study.(ABSTRACT TRUNCATED AT 400 WORDS)