Solitarial premotor neuron projections to the rat esophagus and pharynx: implications for control of swallowing

The Pathway from Anatomy and Physiology to Diagnosis: A Developmental Perspective on Swallowing and Dysphagia

Dysphagia results from diverse and distinct etiologies. The pathway from anatomy and physiology to clinical diagnosis is complex and hierarchical. Our approach in this paper is to show the linkages from the underlying anatomy and physiology to the clinical presentation. In particular, the terms performance, function, behavior, and physiology are often used interchangeably, which we argue is an obstacle to clear discussion of mechanism of pathophysiology. We use examples from pediatric populations to highlight the importance of understanding anatomy and physiology to inform clinical practice. We first discuss the importance of understanding anatomy in the context of physiology and performance. We then use preterm infants and swallow-breathe coordination as examples to explicate the hierarchical nature of physiology and its impact on performance. We also highlight where the holes in our knowledge lie, with the ultimate endpoint of providing a framework that could enhance our ability to design interventions to help patients. Clarifying these terms, and the roles they play in the biology of dysphagia will help both the researchers studying the problems as well as the clinicians applying the results of those studies.

Effect of pharmacological inhibition of the pontine respiratory group on swallowing interneurons in the dorsal medulla oblongata

OBJECTIVES

To examine the role of neurons of the pontine respiratory group (PRG) overlapping with the Kölliker-Fuse nucleus in the regulation of swallowing, we compared the activity of swallowing motor activities and interneuron discharge in the dorsal swallowing group in the medulla before and after pharmacological inhibition of the PRG.

METHODS

In 23 in situ perfused brainstem preparation of rats, we recorded the activities of the vagus (VNA), hypoglossal (HNA), and phrenic nerves (PNA), and swallowing interneurons of the dorsal medulla during fictive swallowing elicited by electrical stimulation of the superior laryngeal nerve or oral water injection. Subsequently, respiratory- and swallow-related motor activities and single unit cell discharge were assessed before and after local microinjection of the GABA-receptor agonist muscimol into the area of PRG ipsilateral to the recording sites of swallowing interneurons.

RESULTS

After muscimol injection, the amplitude and duration of swallow-related VNA bursts decreased to 71.3 ± 2.84 and 68.1 ± 2.76 % during electrically induced swallowing and VNA interburst intervals during repetitive swallowing decreased. Similar effects were observed for swallowing-related HNA. The swallowing motor activity was similarly qualitatively altered during physiologically induced swallowing. All 23 neurons were changed in either discharge duration or frequency after PRG inhibition, however, the general discharge patterns in relation to the motor output remained unchanged.

CONCLUSION

Descending synaptic inputs from PRG provide control of the primary laryngeal sensory gate and synaptic activity of the PRG partially determine medullary cell and cranial motor nerve activities that govern the pharyngeal stage of swallowing.

Functional anatomy of the vagus system: How does the polyvagal theory comply?

Due to its pivotal role in autonomic networks and interoception, the vagus attracts continued interest from both basic scientists and therapists of various clinical disciplines. In particular, the widespread use of heart rate variability as an index of autonomic cardiac control and a proposed central role of the vagus in biopsychological concepts, e.g., the polyvagal theory, provide a good opportunity to recall basic features of vagal anatomy. In addition to the "classical" vagal brainstem nuclei, i.e., dorsal motor nucleus, nucleus ambiguus and nucleus tractus solitarii, the spinal trigeminal and paratrigeminal nuclei come into play as targets of vagal afferents. On the other hand, the nucleus of the solitary tract receives and integrates not only visceral but also somatic afferents.

Firing characteristics of swallowing interneurons in the dorsal medulla during physiologically induced swallowing in perfused brainstem preparation in rats

Oropharyngeal swallowing is centrally mediated by a swallowing central pattern generator (Sw-CPG) in the medulla oblongata. The activity of the Sw-CPG depends on the sensory inputs determined by physical and chemical bolus properties. Here we investigate the sensory-motor integration during swallowing arising from different sensory sources. To do so we electrically stimulated the superior laryngeal nerve and we triggered swallowing with oral injections of distilled water or capsaicin solution and extracellularly recorded from swallowing interneurons in arterially perfused brainstem preparations of rats. We recorded the activities of 40 neurons, while monitoring the motor activities of the phrenic, vagal and hypoglossal nerves. Eighteen neurons responded to electrical stimulation of the ipsilateral superior laryngeal nerve, and 6 neurons were excited by oral fluid injection, while 16 non-respiratory neurons did not receive afferent inputs to either electrical or physiological stimuli. The cellular activities displayed by swallowing interneurons during electrical and physiological stimulation of pharyngeal and laryngeal afferent input reveal complex adaptations of the timing of firing patterns and frequencies. The modulation of neuronal activity is likely to contribute to the coordination of efficient bolus transfer during the pharyngeal stage of swallowing.

Functional anatomy of the vagus system - Emphasis on the somato-visceral interface

Due to its pivotal role in autonomic networks, the vagus attracts continuous interest from both basic scientists and clinicians. In particular, recent advances in vagus nerve stimulation strategies and their application to pathological conditions beyond epilepsy provide a good opportunity to recall basic features of vagal peripheral and central anatomy. In addition to the "classical" vagal brainstem nuclei, i.e., dorsal motor nucleus, nucleus ambiguus and nucleus tractus solitarii, the spinal trigeminal and paratrigeminal nuclei come into play as targets of vagal afferents. On the other hand, the nucleus of the solitary tract receives and integrates not only visceral but also somatic afferents. Thus, the vagus system participates significantly in what may be defined as "somato-visceral interface".

The role of nucleus of the solitary tract glucagon-like peptide-1 and prolactin-releasing peptide neurons in stress: anatomy, physiology and cellular interactions

Neuroendocrine, behavioural and autonomic responses to stressful stimuli are orchestrated by complex neural circuits. The caudal nucleus of the solitary tract (cNTS) in the dorsomedial hindbrain is uniquely positioned to integrate signals of both interoceptive and psychogenic stress. Within the cNTS, glucagon‐like peptide‐1 (GLP‐1) and prolactin‐releasing peptide (PrRP) neurons play crucial roles in organising neural responses to a broad range of stressors. In this review we discuss the anatomical and functional overlap between PrRP and GLP‐1 neurons. We outline their co‐activation in response to stressful stimuli and their importance as mediators of behavioural and physiological stress responses. Finally, we review evidence that PrRP neurons are downstream of GLP‐1 neurons and outline unexplored areas of the research field. Based on the current state‐of‐knowledge, PrRP and GLP‐1 neurons may be compelling targets in the treatment of stress‐related disorders.

Influences of GABAergic Inhibition in the Dorsal Medulla on Contralateral Swallowing Neurons in Rats

OBJECTIVES

We aimed to examine the effect of unilateral inhibition of the medullary dorsal swallowing networks on the activities of swallowing-related cranial motor nerves and swallowing interneurons.

METHODS

In 25 juvenile rats, we recorded bilateral vagal nerve activity (VNA) as well as unilateral phrenic and hypoglossal activity (HNA) during fictive swallowing elicited by electrical stimulation of the superior laryngeal nerve during control and following microinjection of the GABA agonist muscimol into the caudal dorsal medulla oblongata in a perfused brainstem preparation. In 20 animals, swallowing interneurons contralateral to the muscimol injection side were simultaneously recorded extracellularly and their firing rates were analyzed during swallowing.

RESULTS

Integrated VNA and HNA to the injection side decreased to 49.0 ± 16.6% and 32.3 ± 17.9%, respectively. However, the VNA on the uninjected side showed little change after muscimol injection. Following local inhibition, 11 out of 20 contralateral swallowing interneurons showed either increased or decreased of their respective firing discharge during evoked-swallowing, while no significant changes in activity were observed in the remaining nine neurons.

CONCLUSION

The neuronal networks underlying the swallowing pattern generation in the dorsal medulla mediate the ipsilateral motor outputs and modulate the contralateral activity of swallowing interneurons, suggesting that the bilateral coordination of the swallowing central pattern generator regulates the spatiotemporal organization of pharyngeal swallowing movements.

LEVEL OF EVIDENCE

NA Laryngoscope, 131:2187-2198, 2021.

Activity of swallowing-related neurons in the medulla in the perfused brainstem preparation in rats

OBJECTIVES/HYPOTHESIS

We aimed to investigate and validate the cellular activity patterns and the potential topographical organization of neurons of the medullary swallowing pattern generator (Sw-CPG). We used the perfused brainstem preparation as an innovative experimental model that allows for stable neuronal recording in the brainstem.

STUDY DESIGN

Animal model.

METHODS

Experiments were conducted in 14 juvenile Wistar rats. The activities of the phrenic, vagus, and hypoglossal nerves were recorded at baseline, and fictive swallowing was elicited by stimulation of the superior laryngeal nerve. Extracellular action potentials of 72 swallowing-related neurons were recorded in the Sw-CPG of the dorsal medulla oblongata.

RESULTS

Neurons could be classified into three types: sensory relay, and neurons that were excited or inhibited during fictive swallowing. Approximately one-third of the neurons likely received monosynaptic input from the laryngeal afferents. One-third of neurons recorded showed respiratory-related activity, most of which exhibited inspiratory modulation. The neurons were widely distributed in the nucleus tractus solitarius and reticular formation.

CONCLUSIONS

The perfused brainstem preparation of rat fully preserves the Sw-CPG. The recorded cellular activities and general topographical organization of swallowing neurons are in accordance with previous in vivo studies. Thus, the perfused brainstem preparation is an ideal experimental model to advance the understanding of neuronal mechanisms underlying swallowing.

LEVEL OF EVIDENCE

NA Laryngoscope, 129:E72-E79, 2019.

Clinical course and outcome in patients with severe dysphagia after lateral medullary syndrome

Background

The objective of this study was to investigate the clinical course and final outcome in patients afflicted with severe dysphagia following a diagnosis of lateral medullary syndrome (LMS).

Methods

The patients with severe dysphagia after LMS admitted to a rehabilitation unit were included and their respective clinical data were prospectively collected. The criteria of ‘severe dysphagia’ was defined as the condition that showed decreased pharyngeal constriction with no esophageal passage in a videofluoroscopic swallowing study (VFSS) and initially required enteral tube feeding. The data included VFSS findings, types of diet and postural modification, penetration-aspiration scale (PAS) and functional oral intake scale (FOIS).

Results

A total of 11 patients were included and VFSS was performed every 2 weeks after stroke onset. Esophageal passage began to show at an average 34.7 ± 18.3 days, and the patients were able to begin consuming a partial oral diet with postural modification. It was 52.2 ± 21.8 days till they were advanced to a full oral diet. PAS and FOIS were significantly improved over time.

Conclusions

Patients with severe dysphagia after LMS were able to tolerate a partial oral diet at about 5 weeks following onset, and they were advanced to a normal diet after 10 weeks. This clinical course might help in predicting the prognosis, as well as assist in making practical decisions regarding a rehabilitation program.

Critical Points and Traveling Wave in Locomotion: Experimental Evidence and Some Theoretical Considerations

The central pattern generator (CPG) architecture for rhythm generation remains partly elusive. We compare cat and frog locomotion results, where the component unrelated to pattern formation appears as a temporal grid, and traveling wave respectively. Frog spinal cord microstimulation with N-methyl-D-Aspartate (NMDA), a CPG activator, produced a limited set of force directions, sometimes tonic, but more often alternating between directions similar to the tonic forces. The tonic forces were topographically organized, and sites evoking rhythms with different force subsets were located close to the constituent tonic force regions. Thus CPGs consist of topographically organized modules. Modularity was also identified as a limited set of muscle synergies whose combinations reconstructed the EMGs. The cat CPG was investigated using proprioceptive inputs during fictive locomotion. Critical points identified both as abrupt transitions in the effect of phasic perturbations, and burst shape transitions, had biomechanical correlates in intact locomotion. During tonic proprioceptive perturbations, discrete shifts between these critical points explained the burst durations changes, and amplitude changes occurred at one of these points. Besides confirming CPG modularity, these results suggest a fixed temporal grid of anchoring points, to shift modules onsets and offsets. Frog locomotion, reconstructed with the NMDA synergies, showed a partially overlapping synergy activation sequence. Using the early synergy output evoked by NMDA at different spinal sites, revealed a rostrocaudal topographic organization, where each synergy is preferentially evoked from a few, albeit overlapping, cord regions. Comparing the locomotor synergy sequence with this topography suggests that a rostrocaudal traveling wave would activate the synergies in the proper sequence for locomotion. This output was reproduced in a two-layer model using this topography and a traveling wave. Together our results suggest two CPG components: modules, i.e., synergies; and temporal patterning, seen as a temporal grid in the cat, and a traveling wave in the frog. Animal and limb navigation have similarities. Research relating grid cells to the theta rhythm and on segmentation during navigation may relate to our temporal grid and traveling wave results. Winfree's mathematical work, combining critical phases and a traveling wave, also appears important. We conclude suggesting tracing, and imaging experiments to investigate our CPG model.

Biomechanical Quantification of Mendelsohn Maneuver and Effortful Swallowing on Pharyngoesophageal Function

Objective

To quantify the effects of 2 swallowing maneuvers used in dysphagia rehabilitation—the Mendelsohn maneuver and effortful swallowing—on pharyngoesophageal function with novel, objective pressure-flow analysis.

Study Design

Evaluation of intervention effects in a healthy control cohort.

Setting

A pharyngoesophageal motility research laboratory in a tertiary education facility.

Subjects

Twelve young healthy subjects (9 women, 28.6 ± 7.9 years) from the general public, without swallowing impairment, volunteered to participate in this study.

Methods

Surface electromyography from the floor-of-mouth musculature and high-resolution impedance manometry–based pressure flow analysis were used to assess floor-of-mouth activation and pharyngoesophageal motility, respectively. Subjects each performed 10 noneffortful control swallows, Mendelsohn maneuver swallows, and effortful swallows, with a 5-mL viscous bolus. Repeated measures analyses of variance was used to compare outcome measures across conditions.

Results

Effortful and Mendelsohn swallows generated greater floor-of-mouth contraction ( P = .001) and pharyngeal pressure ( P < .0001) when compared with control swallows. There were no changes at the level of the upper esophageal sphincter, except for a faster opening to maximal diameter during maneuver swallows ( P = .01). The proximal esophageal contractile integral was reduced during Mendelsohn swallows ( P = .001).

Conclusion

Effortful and Mendelsohn maneuver swallows significantly alter the pharyngoesophageal pressure profile. Faster opening of the upper esophageal sphincter may facilitate bolus transfer during maneuver swallows; however, reduced proximal esophageal contractility during Mendelsohn maneuver swallows may impair bolus flow and aggravate dysphagic symptoms.

The role of vagal neurocircuits in the regulation of nausea and vomiting

Nausea and vomiting are among the most frequently occurring symptoms observed by clinicians. While advances have been made in understanding both the physiological as well as the neurophysiological pathways involved in nausea and vomiting, the final common pathway(s) for emesis have yet to be defined. Regardless of the difficulties in elucidating the precise neurocircuitry involved in nausea and vomiting, it has been accepted for over a century that the locus for these neurocircuits encompasses several structures within the medullary reticular formation of the hindbrain and that the role of vagal neurocircuits in particular are of critical importance. The afferent vagus nerve is responsible for relaying a vast amount of sensory information from thoracic and abdominal organs to the central nervous system. Neurons within the nucleus of the tractus solitarius not only receive these peripheral sensory inputs but have direct or indirect connections with several other hindbrain, midbrain and forebrain structures responsible for the co-ordination of the multiple organ systems. The efferent vagus nerve relays the integrated and co-ordinated output response to several peripheral organs responsible for emesis. The important role of both sensory and motor vagus nerves, and the available nature of peripheral vagal afferent and efferent nerve terminals, provides extensive and readily accessible targets for the development of drugs to combat nausea and vomiting.

Differential activation of medullary vagal nuclei caused by stimulation of different esophageal mechanoreceptors

Esophageal mechanoreceptors, i.e. muscular slowly adapting tension receptors and mucosal rapidly adapting touch receptors, mediate different sets of reflexes. The aim of this study was to determine the medullary vagal nuclei involved in the reflex responses to activation of these receptors. Thirty-three cats were anesthetized with alpha-chloralose and the esophagus was stimulated by slow balloon or rapid air distension. The physiological effects of the stimuli (N=4) were identified by recording responses from the pharyngeal, laryngeal, and hyoid muscles, esophagus, and the lower esophageal sphincter (LES). The effects on the medullary vagal nuclei of the stimuli: slow distension (N=10), rapid distension (N=9), and in control animals (N=10) were identified using the immunohistochemical analysis of c-fos. The experimental groups were stimulated three times per minute for 3h. After the experiment, the brains were removed and processed for c-fos immunoreactivity or thioinin. We found that slow balloon distension activated the esophago-UES contractile reflex and esophago-LES relaxation response, and rapid air injection activated the belch and its component reflexes. Slow balloon distension activated the NTSce, NTSdl, NTSvl, DMNc, DMNr and NAr; and rapid air injection primarily activated AP, NTScd, NTSim, NTSis, NTSdm, NTSvl, NAc and NAr. We concluded that different sets of medullary vagal nuclei mediate different reflexes of the esophagus activated from different sets of mechanoreceptors. The NTScd is the primary NTS subnucleus mediating reflexes from the mucosal rapidly adapting touch receptors, and the NTSce is the primary NTS subnucleus mediating reflexes from the muscular slowly adapting tension receptors. The AP may be involved in mediation of belching.

Differential activation of pontomedullary nuclei by acid perfusion of different regions of the esophagus

The objective of this study was to determine the brain stem nuclei and physiological responses activated by esophageal acidification. The effects of perfusion of the cervical (ESOc), or thoracic (ESOt) esophagus with PBS or HCl on c-fos immunoreactivity of the brain stem or on physiological variables, and the effects of vagotomy were examined in anesthetized cats. We found that acidification of the ESOc increased the number of c-fos positive neurons in the area postrema (AP), vestibular nucleus (VN), parabrachial nucleus (PBN), nucleus ambiguus (NA), dorsal motor nucleus (DMN), and all subnuclei of the nucleus tractus solitarius (NTS), but one. Acidification of the ESOt activated neurons in the central (CE), caudal (CD), dorsomedial (DM), dorsolateral (DL), ventromedial (VM) subnuclei of NTS, and the DMN. Vagotomy blocked all c-fos responses to acid perfusion of the whole esophagus (ESOw). Perfusion of the ESOc or ESOt with PBS activated secondary peristalsis (2P), but had no effect on blood pressure, heart rate, or respiratory rate. Perfusion of the ESOc, but not ESOt, with HCl activated pharyngeal swallowing (PS), profuse salivation, or physiological correlates of emesis. Vagotomy blocked all physiological effects of ESOw perfusion. We conclude that acidification of the ESOc and ESOt activate different sets of pontomedullary nuclei and different physiological responses. The NTSce, NTScom, NTSdm, and DMN are associated with activation of 2P, the NTSim and NTSis, are associated with activation of PS, and the AP, VN, and PBN are associated with activation of emesis and perhaps nausea. All responses to esophageal fluid perfusion or acidification are mediated by the vagus nerves.

Brain stem control of the phases of swallowing

The phases of swallowing are controlled by central pattern-generating circuitry of the brain stem and peripheral reflexes. The oral, pharyngeal, and esophageal phases of swallowing are independent of each other. Although central pattern generators of the brain stem control the timing of these phases, the peripheral manifestation of these phases depends on sensory feedback through reflexes of the pharynx and esophagus. The dependence of the esophageal phase of swallowing on peripheral feedback explains its absence during failed swallows. Reflexes that initiate the pharyngeal phase of swallowing also inhibit the esophageal phase which ensures the appropriate timing of its occurrence to provide efficient bolus transport and which prevents the occurrence of multiple esophageal peristaltic events. These inhibitory reflexes are probably partly responsible for deglutitive inhibition. Three separate sets of brain stem nuclei mediate the oral, pharyngeal, and esophageal phases of swallowing. The trigeminal nucleus and reticular formation probably contain the oral phase pattern-generating neural circuitry. The nucleus tractus solitarius (NTS) probably contains the second-order sensory neurons as well as the pattern-generating circuitry of both the pharyngeal and esophageal phases of swallowing, whereas the nucleus ambiguus and dorsal motor nucleus contain the motor neurons of the pharyngeal and esophageal phases of swallowing. The ventromedial nucleus of the NTS may govern the coupling of the pharyngeal phase to the esophageal phase of swallowing.

Brainstem neuropathology in a mouse model of Niemann-Pick disease type C

Niemann-Pick disease type C (NPC) is a neurovisceral lipid storage disorder characterized by progressive and widespread neurodegeneration. Although some characteristic symptoms of NPC result from brainstem dysfunction, little information is available about which brainstem structures are affected. In this study, the brainstems of mutant BALB/c NPC1-/- mice with a retroposon insertion in the NPC1 gene were examined for neuropathological changes. In the midbrain, the integrated optic density (IOD) and cell count density of tyrosine-hydroxylase (TH) immunostained neurons were decreased in the substantia nigra. In the pons, TH immunoreactivity in the locus ceruleus (LC) neurons was decreased, while the IOD and the neuronal density of choline acetyltransferase (ChAT)-immunostained neurons in the pedunculopontine tegmental nucleus were preserved. The ChAT immunoreactivity of the hypoglossal nucleus (12N) neurons was not decreased, but Klüver-Barrera staining showed that neuronal density in the nucleus of the solitary tract (NTS) was decreased. Klüver-Barrera and neuronal nuclei (NeuN) staining showed a decrease in neuronal density in the ventral cochlear nucleus, but not in the dorsal cochlear nucleus. Gliosis was widely identified by GFAP staining in various brainstem structures, including the superior and inferior colliculi, the rostral interstitial nucleus of the medial longitudinal fasciculus, the oculomotor complex, the medial geniculate nucleus, the nucleus ambiguus, and the 12N. However, GFAP expression was not augmented in the LC, the cochlear nucleus, or the NTS. These neuropathological findings suggest a basis for the neurological syndromes observed in NPC, such as rigidity, oculomotor symptoms, cataplexy and sleep disturbance, dysphagia, and perceptive deafness.

Modulation of activity in swallowing motor cortex following esophageal acidification: a functional magnetic resonance imaging study

Esophageal acid exposure induces sensory and motility changes in the upper gastrointestinal tract; however, the mechanisms involved and the effects on activity in the brain regions that control swallowing are unknown. The aim of this study was to examine functional changes in the cortical swallowing network as a result of esophageal acidification using functional magnetic resonance imaging (fMRI). Seven healthy volunteers (3 female, age range=20-30 years) were randomized to receive either a 0.1 M hydrochloric acid or (control) saline infusion for 30 min into the distal esophagus. Postinfusion, subjects underwent four 8 min blocks of fMRI over 1 h. These alternated between 1 min swallowing water boluses and 1 min rest. Three-dimensional cluster analysis for group brain activation during swallowing was performed together with repeated-measures ANOVA for differences between acid and saline. After acid infusion, swallowing-induced activation was seen predominantly in postcentral gyrus (p<0.004). ANOVA comparison of acid with saline showed a significant relative reduction in activation during swallowing of the precentral gyrus (M1) BA 4 (p<0.008) in response to acid infusion. No areas of increased cortical activation were identified with acid vs. saline during swallowing. Esophageal acidification inhibits motor and association cortical areas during a swallowing task, probably via changes in vagal afferent or nociceptive input from the esophagus. This mechanism may play a protective role, facilitating acid clearance by reduced descending central motor inhibition of enteric/spinal reflexes, or by preventing further ingestion of injurious agents.

Pathophysiological changes of the gastrointestinal tract in ischemic stroke

OBJECTIVE

Dysphagia is common after stroke and represents a marker of poor prognosis. After ischemic stroke, dysphagia represents only one part of the clinical spectrum of changes in the gastrointestinal (GI) tract and includes GI hemorrhage, delayed GI emptying, and colorectal dysfunction. State-of-the-art imaging techniques have started to revolutionize to study the cortical and brainstem control of these GI symptoms. It has become increasingly obvious that GI alterations after stroke are complex and its recovery following stroke is even more so.

METHODS

In this review, an electronic database research was performed in MEDLINE, EMBASE, and the COCHRANE database using the terms stroke, dysphagia, GI motility, or cortical reorganization; an extensive manual searching was additionally conducted.

RESULTS

Cerebral ischemia may lead to an interruption of the axis between central nervous system and GI system. This altered interrelation between the central nervous system and the GI system may cause, among other things, mainly dysphagia, GI dysmotility, and GI hemorrhage. The consecutive clinical symptoms can often be directly attributed to specific cerebral ischemic lesions involving the brain stem as well as certain cortical and subcortical structures. However, in some cases the pathophysiological mechanisms leading to GI symptoms are incompletely understood. Recent improvement of imaging techniques, especially in functional imaging, has lead to new insights of the central control of the GI tract, suggesting that its cortical and medullar organization is multifocal, and bilateral with handness-independent hemispheric dominance.

CONCLUSIONS

Following stroke, patients may have swallowing impairment and other changes of the GI tract that could affect nutritional and hydration status and that lead to aspiration pneumonia. Impaired nutritional status is associated with reduced functional improvement, increased complication rates, and prolonged hospital stays.

Innervation of the mammalian esophagus

Understanding the innervation of the esophagus is a prerequisite for successful treatment of a variety of disorders, e.g., dysphagia, achalasia, gastroesophageal reflux disease (GERD) and non-cardiac chest pain. Although, at first glance, functions of the esophagus are relatively simple, their neuronal control is considerably complex. Vagal motor neurons of the nucleus ambiguus and preganglionic neurons of the dorsal motor nucleus innervate striated and smooth muscle, respectively. Myenteric neurons represent the interface between the dorsal motor nucleus and smooth muscle but they are also involved in striated muscle innervation. Intraganglionic laminar endings (IGLEs) represent mechanosensory vagal afferent terminals. They also establish intricate connections with enteric neurons. Afferent information is implemented by the swallowing central pattern generator in the brainstem, which generates and coordinates deglutitive activity in both striated and smooth esophageal muscle and orchestrates esophageal sphincters as well as gastric adaptive relaxation. Disturbed excitation/inhibition balance in the lower esophageal sphincter results in motility disorders, e.g., achalasia and GERD. Loss of mechanosensory afferents disrupts adaptation of deglutitive motor programs to bolus variables, eventually leading to megaesophagus. Both spinal and vagal afferents appear to contribute to painful sensations, e.g., non-cardiac chest pain. Extrinsic and intrinsic neurons may be involved in intramural reflexes using acetylcholine, nitric oxide, substance P, CGRP and glutamate as main transmitters. In addition, other molecules, e.g., ATP, GABA and probably also inflammatory cytokines, may modulate these neuronal functions.

Characterization of neurons of the nucleus tractus solitarius pars centralis

Esophageal sensory afferent inputs terminate principally in the central subnucleus of the tractus solitarius (cNTS). Neurons of the cNTS comprise two major neurochemical subpopulations. One contains neurons that are nitric oxide synthase (NOS) immunoreactive (-IR) while the other comprises neurons that are tyrosine hydroxylase (TH)-IR. We have shown recently that TH-IR neurons are involved in esophageal-distention induced gastric relaxation. We used whole cell patch clamp techniques in rat brainstem slices combined with immunohistochemical and morphological reconstructions to characterize cNTS neurons. Postrecording reconstruction of cNTS neurons revealed two morphological neuronal subtypes; one group of cells (41 out of 131 neurons, i.e., 31%) had a multipolar soma, while the other group (87 out of 131 neurons, i.e., 66%) had a bipolar soma. Of the 43 cells in which we conducted a neurochemical examination, 15 displayed TH-IR (9 with bipolar morphology, 6 with multipolar morphology) while the remaining 28 neurons did not display TH-IR (18 with bipolar morphology, 10 with multipolar morphology). Even though the range of electrophysiological properties varied significantly, morphological or neurochemical distinctions did not reveal characteristics peculiar to the subgroups. Spontaneous excitatory postsynaptic currents (sEPSC) recorded in cNTS neurons had a frequency of 1.5 +/- 0.15 events s(-1) and an amplitude of 27 +/- 1.2 pA (Vh = -50 mV) and were abolished by pretreatment with 30 muM AP-5 and 10 muM CNQX, indicating the involvement of both NMDA and non-NMDA receptors. Some cNTS neurons also received a GABAergic input that was abolished by perfusion with 30-50 muM bicuculline. In conclusion, our data show that despite the heterogeneity of morphological and neurochemical membrane properties, the electrophysiological characteristics of cNTS neurons are not a distinguishing feature.

Differential activation of medullary vagal nuclei during different phases of swallowing in the cat

The aim of this study was to identify the medullary vagal nuclei involved in the different phases of swallowing activated physiologically in a species with an esophagus similar to human. In decerebrate cats, the pharyngeal (0.5-1.0 ml water in pharynx (N=6)) or esophageal (1-3 ml air in esophagus (N=5)) phases of swallowing were stimulated separately once per minute for 3 h, and we compared the resulting c-fos immunoreactivity within neuronal cell nuclei of the dorsal motor nucleus (DMN), nucleus tractus solitarius (NTS) and nucleus ambiguus (NA) with a sham control group (N=5). We found that the pharyngeal phase was associated with an elevated number of c-fos positive neurons in the intermediate (NTSim), interstitial (NTSis), ventromedial (NTSvm) subnuclei of the NTS, caudal DMN, and dorsal NA; and the esophageal phase was associated with an elevated number of c-fos positive neurons in the central (NTSce), ventral, dorsolateral, ventrolateral subnuclei of the NTS, rostral DMN, and ventral NA. We concluded that the pharyngeal and esophageal phases of swallowing are associated with different sets of NTS subnuculei; and the DMN and NA may contain functionally different populations of motor neurons situated rostrocaudally and dorsoventrally associated with the different phases of swallowing. The central pattern generator (CPG) for swallowing probably receives significant peripheral feedback, and the NTSvm may participate in the transition of the pharyngeal to the esophageal phase of swallowing.

Role of intrinsic nitrergic neurones on vagally mediated striated muscle contractions in the hamster oesophagus

Oesophageal peristalsis is controlled by vagal motor neurones, and intrinsic neurones have been identified in the striated muscle oesophagus. However, the effect(s) of intrinsic neurones on vagally mediated contractions of oesophageal striated muscles has not been defined. The present study was designed to investigate the role of intrinsic neurones on vagally evoked contractions of oesophageal striated muscles, using hamster oesophageal strips maintained in an organ bath. Stimulation (30 micros, 20 V) of the vagus nerve trunk produced twitch contractions. Piperine inhibited vagally evoked contractions, while capsaicin and NG-nitro-L-arginine methyl ester (L-NAME) abolished the inhibitory effect of piperine. The effect of L-NAME was reversed by subsequent addition of L-arginine, but not by D-arginine. L-NAME did not have any effect on the vagally mediated contractions and presumed 3H-ACh release. NONOate, a nitric oxide donor, and dibutyryl cyclic GMP inhibited twitch contractions. Inhibition of vagally evoked contractions by piperine and NONOate was fully reversed by ODQ, an inhibitor of guanylate cyclase. Immunohistochemical staining showed immunoreactivity for nitric oxide synthase (NOS) in nerve cell bodies and fibres in the myenteric plexus and the presence of choline acetyltransferase and NOS in the motor endplates. Only a few NOS-immunoreactive portions in the myenteric plexus showed vanilloid receptor 1 (VR1) immunoreactivity. Our results suggest that there is a local neural reflex that involves capsaicin-sensitive neurones, nitrergic myenteric neurones and vagal motor neurones.

Intracellular activity of superior laryngeal nerve motoneurons during fictive swallowing in decerebrate rats

We examined the swallowing-related intracellular activity of motoneurons of the superior laryngeal nerve (SLN) in decerebrate, paralyzed and artificially-ventilated rats, to elucidate the neuronal mechanism of the pharyngo-esophageal and laryngo-esophageal coordination during swallowing. The majority of the SLN motoneurons exhibited respiratory rhythm (n=16; 13 inspiratory, one expiratory and two non-respiratory neurons). During fictive swallowing evoked by electrical stimulation of the SLN, all these motoneurons showed a hyperpolarization-depolarization sequence in their membrane potentials. The hyperpolarization, which was shown to consist of inhibitory postsynaptic potentials, started at the onset of the hypoglossal swallowing burst, lasted during the burst, and was followed by a depolarization at the end of the burst. This hyperpolarization-depolarization pattern implies that the SLN motoneurons may be involved in the 'inhibitory chain' within the swallowing pattern generator, which may be cardinal in the sequential activation of different populations of motoneurons innervating the swallowing-related muscles.

Swallowing-related activities of respiratory and non-respiratory neurons in the nucleus of solitary tract in the rat

Swallowing-related activity was examined in respiratory (n = 60) and non-respiratory (n = 82) neurons that were located in and around the nucleus of the solitary tract (NTS) in decerebrated, neuromuscularly blocked and artificially ventilated rats. Neurons that were orthodromically activated by electrical stimulation of the superior laryngeal nerve (SLN) were identified, and fictive swallowing was evoked by SLN stimulation. The pharyngeal phase of swallowing was monitored by hypoglossal nerve activity. Two types of non-respiratory neurons with swallowing-related bursts were identified: 'early' swallowing neurons (n = 24) fired during periods of hypoglossal bursts, and 'late' swallowing neurons (n = 8) fired after the end of hypoglossal bursts. The remaining non-respiratory neurons were either suppressed (n = 21) or showed no change in activity (n = 29) during swallowing. On the other hand, respiratory neurons with SLN inputs included 56 inspiratory and four expiratory neurons. Inspiratory neurons were classified into two major types: a group of neurons discharged simultaneously with hypoglossal bursts (type 1 neurons, n = 19), while others were silent during bursts but were active during inter-hypoglossal bursts when swallowing was provoked repetitively (type 2 neurons, n = 34). Three of the expiratory neurons fired during hypoglossal bursts. Many of the swallowing-related non-respiratory neurons and the majority of the inspiratory neurons received presumed monosynaptic inputs from the SLN. Details of the distribution and firing patterns of these NTS neurons, which have been revealed for the first time in a fictive swallowing preparation in the rat, suggest their participation in the initiation, pattern formation and mutual inhibition between swallowing and respiration.

Abnormal esophageal motility in children with congenital central hypoventilation syndrome

BACKGROUND & AIMS

Congenital central hypoventilation syndrome, an unexplained disorder of the central control of breathing that may reflect widespread dysfunction of brainstem structures, is regarded as a form of neuro cristopathy. Because swallowing-induced peristalsis is centrally controlled and depends on neural crest-derived esophageal innervation, we looked for esophageal dysmotility in patients with congenital central hypoventilation syndrome.

METHODS

Seven patients without dysphagia or any other upper gastrointestinal tract symptoms were studied prospectively (5 girls and 2 boys; median age, 14 years; range, 11-18 years). They were compared with 7 age- and sex-matched controls. Esophageal manometry was performed using a low-compliance infusion system and the station pull-through technique. At least 10 wet swallows were analyzed in each subject.

RESULTS

Pressure wave propagation was abnormal in all 7 patients (median percentage of swallows propagated, 18%, and range, 0-66; controls, 90% and 80-100; P < 0.001). Lower esophageal sphincter relaxation was abnormal in 5 patients (patients, 73% and 53-100; controls, 95% and 90-100; P = 0.01). In 2 patients, lower esophageal sphincter pressure was above the 95th percentile of control values.

CONCLUSIONS

These abnormalities are strong evidence of lower esophageal dysfunction in congenital central hypoventilation syndrome. We speculate that the underlying mechanism may be dysfunction of the central structures that control swallowing.

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.

Laryngeal and respiratory protective reflexes

Swallowing is a complex motor behavior that relies on an interneuronal network of premotor neurons (PMNs) to organize the sequential activity of motor neurons that are active during the buccopharyngeal and esophageal phases. Swallowing PMNs are highly interconnected to multiple areas of the brain stem and the central nervous system and provide a potential anatomic substrate integration of swallowing activity with airway protective reflexes. Because these neurons have synaptic contact with both afferent inputs and motor neurons and exhibit a true central activity, they appear to constitute the swallowing central pattern generator. We studied the viscerotopic organization of the nucleus of the solitary tract (NTS), the nucleus ambiguus (NA), the dorsal motor nucleus (DMN), and the hypoglossal nucleus (XII) using cholera toxin horseradish peroxidase (CT-HRP), a sensitive antegrade and retrograde tracer that effectively labels afferent terminal fields within the NTS as well as swallowing motor neurons and their dendritic fields within the NA, DMN, and XII. We used CT-HRP to provide a comprehensive description of the dendritic architecture of NA motor neurons innervating swallowing muscles. We also conducted studies using pseudorabies virus (PRV), a swine alpha-herpesvirus, to map central neural circuits after injection in the peripheral or central nervous systems. One attenuated vaccine strain, Bartha PRV, has preferential affinity for sites of afferent synaptic contact on the cell body and dendrites and a reactive gliosis that effectively isolates the infected neurons and provides a barrier to the nonspecific spread to adjacent neurons. The findings provide a basis for the central integration of swallowing and respiratory protective reflexes.

Neural circuits in swallowing and abdominal vagal afferent-mediated lower esophageal sphincter relaxation

The purpose of this review is to identify the medullary subnuclei that house neural circuits for lower esophageal sphincter (LES) relaxation. LES relaxation may occur as a component of primary peristalsis elicited by superior laryngeal nerve (SLN) afferent stimulation, secondary peristalsis elicited by esophageal distention or as a component of belch reflex, and transient LES relaxation elicited by gastric vagal afferent stimulation. In mice, SLN stimulation at 10 Hz elicited complete swallowing reflex, including pharyngeal and esophageal peristalsis, and LES relaxation. SLN stimulation at 5 Hz elicited pharyngeal contractions and isolated LES relaxation, which is not accompanied by esophageal peristalsis. Electric stimulation of afferents in the ventral branch of the subdiaphragmatic vagus (vSDV) at 10 Hz also elicited isolated LES relaxation. Using these defined stimuli, c-fos expression was examined in the entire craniocaudal extent of the medullary nuclei. SLN stimulation at 10 Hz induced c-fos expression in neurons in: (1) interstitial (SolI), intermediate (SolIM), central (SolCe), occasional medial (SolM), and dorsomedial (SolDM) solitary subnuclei; (2) motor neurons in the nucleus ambiguus, including its semicompact (NAsc), loose (NAl), and compact (NAc) formations; and (3) dorsal motor nucleus of vagus, including its rostral (DMVr) and caudal (DMVc) parts. The activated neurons represent neurons involved with afferent SLN-mediated reflexes, including swallowing. SLN stimulation at 5 Hz evoked c-fos expression in neurons in SolI, SolIM, SolM, and SolDM but not in SolCe; and motor neurons in NAsc, NAl, and DMVc but not in NAc or DMVr. Stimulation of vSDV induced c-fos expression in neurons in SolM and SolDM and in motoneurons in DMVc. When considered with published reports in other animal species, these data support the speculation that (1) swallow-evoked primary peristalsis involves the following neural circuits: SolI/SolIM --> NAsc/NAl for pharyngeal and SolCe --> NAc for esophageal (striated muscle) peristalsis, SolM/SolDM --> preganglionic neurons in DMVc and DMVr and nitrergic and cholinergic neurons in myenteric plexus for esophageal (smooth muscle) peristalsis, and SolM/SolDM --> preganglionic neurons in DMVc --> postganglionic nitrergic neurons in the myenteric plexus for LES relaxation; and (2) abdominal vagus-stimulated isolated LES relaxation may involve neurons in SolM and SolDM --> preganglionic motor neurons in DMVc --> postganglionic nitrergic neurons in the myenteric plexus.

Rhombencephalic pathways and neurotransmitters controlling deglutition

Information from neuronal pathway tracing and pharmacologic microstimulation studies, in conjunction with electrophysiological data, has begun to coalesce into a coherent, if still incomplete, picture of the brain stem circuitry responsible for generating the motor patterns underlying deglutition and esophageal peristalsis in the rat. The intermediate, interstitial, and ventral subnuclei of the solitarius complex appear to play a pivotal part, as evidenced by their viscerosensory inputs and extensive projections to the parvicellular intermediate reticular formation of the medulla, to the brain stem deglutitive motor neuron pools, and to general visceral efferent preganglionic neurons controlling the upper alimentary tract striated and smooth musculature, respectively. The dense projections of the solitarial central subnucleus form a separate subcircuit controlling esophageal, and also some aspects of gastric, motility. Although not extensive, direct connections between the latter subnucleus and interneurons coordinating the buccopharyngeal stage of swallowing appear to exist. In both subcircuits, fast information transfer uses excitatory amino acidergic transmission by means of several glutamate-receptor subtypes. Release from tonic GABAergic inhibition exerted by local solitarial interneurons may provide a mechanism for triggering deglutitive premotoneuronal activity. Local or reticular cholinergic neurons are implicated in pharyngoesophageal coupling and the generation of propulsive esophagomotor output. The solitary interneurons under investigation engage in complex local dendritic and axonal projections within the solitarius complex. Further analysis of these local circuits and their transmitters should yield essential clues regarding the mechanisms underlying deglutitive motor pattern generation.

Characterization of the central nervous system innervation of the rat spleen using viral transneuronal tracing

Splenic immune function is modulated by sympathetic innervation, which in turn is controlled by inputs from supraspinal regions. In the present study, the characterization of central circuits involved in the control of splenic function was accomplished by injecting pseudorabies virus (PRV), a retrograde transynaptic tracer, into the spleen and conducting a temporal analysis of the progression of the infection from 60 hours to 110 hours postinoculation. In addition, central noradrenergic cell groups involved in splenic innervation were characterized by dual immunohistochemical detection of dopamine-beta-hydroxylase and PRV. Infection in the CNS first appeared in the spinal cord. Splenic sympathetic preganglionic neurons, identified in rats injected with Fluoro-Gold i.p. prior to PRV inoculation of the spleen, were located in T(3)-T(12) bilaterally; numerous infected interneurons were also found in the thoracic spinal cord (T(1)-T(13)). Infected neurons in the brain were first observed in the A5 region, ventromedial medulla, rostral ventrolateral medulla, paraventricular hypothalamic nucleus, Barrington's nucleus, and caudal raphe. At intermediate survival times, the number of infected cells increased in previously infected areas, and infected neurons also appeared in lateral hypothalamus, A7 region, locus coeruleus, subcoeruleus region, nucleus of the solitary tract, and C3 cell group. At longer postinoculation intervals, infected neurons were found in additional hypothalamic areas, Edinger-Westphal nucleus, periaqueductal gray, pedunculopontine tegmental nucleus, caudal ventrolateral medulla, and area postrema. These results demonstrate that the sympathetic outflow to the spleen is controlled by a complex multisynaptic pathway that involves several brainstem and forebrain nuclei.

Dysphagia in a patient with lateral medullary syndrome: insight into the central control of swallowing

BACKGROUND & AIMS

Central control of swallowing is regulated by a central pattern generator (CPG) positioned dorsally in the solitary tract nucleus and neighboring medullary reticular formation. The CPG serially activates the cranial nerve motor neurons, including the nucleus ambiguus and vagal dorsal motor nucleus, which then innervate the muscles of deglutition. This case provides insight into the central control of swallowing.

METHODS

A 65-year-old man with a right superior lateral medullary syndrome presented with a constellation of symptoms, including dysphagia. The swallow was characterized using videofluoroscopy and esophageal motility and the results were compared with magnetic resonance imaging (MRI) findings.

RESULTS

Videofluoroscopy showed intact lingual propulsion and volitional movements of the larynx. Distal pharyngeal peristalsis was absent, and the bolus did not pass the upper esophageal sphincter. Manometry showed proximal pharyngeal contraction and normal peristaltic activity in the lower esophagus (smooth muscle), but motor activity of the upper esophageal sphincter and proximal esophagus (striated muscle) was absent. MRI showed a lesion of the dorsal medulla.

CONCLUSIONS

These findings are compatible with a specific lesion of the connections from a programming CPG in the solitary tract nucleus to nucleus ambiguus neurons, which supply the distal pharynx, upper esophageal sphincter, and proximal esophagus. There is functional preservation of the CPG control center in the solitary tract nucleus and of the vagal dorsal motor nucleus neurons innervating the smooth muscle esophagus.

Brain stem control of swallowing: neuronal network and cellular mechanisms

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

Swallowing reflex and brain stem neurons activated by superior laryngeal nerve stimulation in the mouse

The purpose of the present study was to identify vagal subnuclei that participate in reflex swallowing in response to electrical stimulation of the left superior laryngeal nerve (SLN). SLN stimulation at 10 Hz evoked primary peristalsis, including oropharyngeal and esophageal peristalsis, and LES relaxation. It also induced c-fos expression in interneurons in the interstitial (SolI), intermediate (SolIM), central (SolCe), dorsomedial (SolDM) and commissural (SolC) solitary subnuclei. Neurons in parvicellular reticular nucleus (PCRt) and area postrema (AP) and motoneurons in the semicompact (NAsc), loose (NAl), and compact (NAc) formations of the nucleus ambiguus and both rostral (DMVr) and caudal (DMVc) parts of the dorsal motor nucleus of vagus were also activated. The activated neurons represent all neurons concerned with afferent SLN-mediated reflexes, including the swallowing-related neurons. SLN stimulation at 5 Hz elicited oropharyngeal and LES but not esophageal responses and evoked c-fos expression in neurons in SolI, SolIM, SolDM, PCRt, AP, NAsc, NAl, and DMVc but not in SolCe, NAc, or DMVr. These data are consistent with the role of SolI, SolIM, SolDM, NAsc, NAl, and DMVc circuit in oropharyngeal peristalsis and LES relaxation and SolCe, NAc, DMVc, and DMVr in esophageal peristalsis and LES responses.

Lower esophageal sphincter relaxation and activation of medullary neurons by subdiaphragmatic vagal stimulation in the mouse

BACKGROUND & AIMS

Isolated lower esophageal sphincter (LES) relaxation associated with belching and vomiting and the transient LES relaxation associated with gastroesophageal reflux are gastric afferent-mediated vagovagal reflexes. We aimed to identify the brain stem vagal subnuclei involved in these reflexes.

METHODS

In anesthetized mice, LES pressures were recorded using a manometric technique and response to electrical stimulation of the ventral trunk of subdiaphragmatic vagus was investigated. Anatomy of the vagal subnuclei was defined, and activated subnuclei with ventral subdiaphragmatic vagus stimulation were detected by c-fos immunohistochemical staining.

RESULTS

Ventral subdiaphragmatic vagal stimulation elicited frequency-dependent LES relaxation without evoking esophageal contractions and induced c-fos expression in interneurons in medial, dorsomedial, and commissural subnuclei along with outer shell of area postrema and motoneurons in the caudal dorsal motor nucleus of vagus. Brain stem subnuclei including interstitial, intermediate, and central subnuclei, and nucleus ambiguous, which have been reported to be involved in the response to swallowing, were not activated.

CONCLUSIONS

Stimulation of the ventral subdiaphragmatic vagus causes isolated LES relaxation and activates neurons in select vagal subnuclei that may represent the brain stem circuit involved in the abdominal vagal-afferent-evoked isolated LES relaxation. These observations suggest that different brain stem circuits are involved in swallow-induced and gastric afferent-mediated isolated LES relaxations.

Central nervous system nitric oxide induces oropharyngeal swallowing and esophageal peristalsis in the cat

BACKGROUND & AIMS

The functional role of brainstem nitric oxide (NO) in swallowing and esophageal peristalsis remains unknown. We examined the effects of blockade of central nervous system (CNS) NO synthase (NOS) on swallowing and on primary and secondary peristalsis.

METHODS

(1) The effect of intravenous (IV) NOS inhibitor N(G)-nitro-L-arginine (L-NNA) on swallowing and swallowing-induced peristalsis was examined. (2) An NOS inhibitor (N(G)-monomethyl-L-arginine [L-NMMA]) was administered into the fourth ventricle intracerebroventricularly (ICV), and its effects on swallowing and primary and secondary peristalsis were examined.

RESULTS

(1) IV L-NNA significantly reduced the number of oropharyngeal swallows and the induction of primary peristalsis in the smooth muscle portion of the esophageal body; the change was not significant within the striated muscle portion. (2) L-NMMA given ICV significantly reduced the number of oropharyngeal swallows and the incidence of primary peristalsis in both smooth and striated muscle, but the reduction in amplitude was significant only for the smooth muscle contraction. There was a significant reduction in both the amplitude and incidence of secondary peristalsis, only in the smooth muscle portion.

CONCLUSIONS

CNS NO is an important neurotransmitter in the induction of oropharyngeal swallowing and esophageal peristalsis. The neural substrates mediating striated and smooth muscle peristalsis may be both anatomically and neurochemically distinct.

Retinoic acid exposure on gestational days 11 to 13 impairs swallowing in rat offspring

We have previously reported that exposure to 10 mg/kg of all-trans-retinoic acid (RA) daily on the 11th, 12th, and 13th days of rat gestation is lethal to all fetuses so exposed, due to an inability to suckle [R.R. Holson et al., Neurotoxicol Teratol 19 (1997) 347-353]. Because this lethal RA effect could be due to any of a variety of causes, from olfactory problems in locating the nipple to a motor problem in sucking or swallowing, we performed the following experiment. Albino dams were exposed to 10-mg/kg RA or vehicle daily over gestational days (GDs) 11 to 13. On the afternoon of GD 21 all pups were delivered by c-section. Tongue cannulae were inserted into the oral cavity of these offspring, and used to infuse a solution of condensed milk directly into the mouth. During and after each of four infusions, the behavioral response to the infusion (typically rolling and curling) was recorded. Controls responded well to this procedure, typically swallowing all milk so infused. In contrast, almost no RA-exposed neonates were able to swallow milk infused into the oral cavity. In such cases the milk simply dribbled out of the mouth, while the stomach was found to be empty at autopsy. However, the RA-treated animals did seem aware that milk was entering their mouths, because they showed a normal behavioral response to milk infusion. We conclude that GD 11-13 retinoid lethality is due to motor not sensory problems in the control of swallowing.

Identification of brainstem interneurons projecting to the trigeminal motor nucleus and adjacent structures in the rabbit

Neurons of several nuclei within the medial pontomedullar reticular formation are active during mastication, but their relationship with other elements of the pattern generating circuits have never been clearly defined. In this paper, we have studied the connection of this area with the trigeminal motor nucleus and with pools of last-order interneurons of the lateral brainstem. Retrograde tracing techniques were used in combination with immunohistochemistry to define populations of glutamatergic and GABAergic neurons. Injections of tracer into the Vth motor nucleus marked neurons in several trigeminal nuclei including the ipsilateral mesencephalic nucleus, the contralateral Vth motor nucleus, the dorsal cap of the main sensory nucleus and the rostral divisions of the spinal nucleus bilaterally. Many last-order interneurons formed a bilateral lateral band running caudally from Regio h (the zone surrounding the Vth motor nucleus), through the parvocellular reticular formation and Vth spinal caudal nucleus. Injections of tracer into Regio h, an area rich in last-order interneurons, marked, in addition to the areas listed above, a large number of neurons in the medial reticular formation bilaterally. The major difference between injection sites was that most neurons projecting to the Vth motor nucleus were located laterally, whereas most of those projecting to Regio h were found medially. Both populations contained glutamatergic and GABAergic neurons intermingled. Our results indicate that neurons of the medial reticular formation that are active during mastication influence Vth motoneurons output via relays in Regio h and other adjacent nuclei.

Connections of Barrington's nucleus to the sympathetic nervous system in rats

Barrington's nucleus (BN) has been considered a pontine center related exclusively to the control of pelvic parasympathetic activity. The present study demonstrates an anatomical linkage between BN and autonomic outflow to visceral targets innervated exclusively by the sympathetic division of the autonomic nervous system. Temporal analysis of infection after injection of pseudorabies virus (PRV), a retrograde transynaptic tracer, into two sympathetically innervated organs, the spleen and the kidney, revealed the presence of infected neurons in BN at early post-inoculation survival intervals. Immunohistochemical localization of PRV after spleen injections showed that a small subpopulation of BN neurons became labeled in a time frame coincident with the appearance of infected neurons in other brain regions known to project to sympathetic preganglionic neurons (SPNs) in the thoracic spinal cord; a larger number of infected neurons appeared in BN at intermediate intervals after PRV injections into the spleen or kidney. Coinjection of the retrograde tracer Fluoro-Gold i.p. and PRV into the spleen demonstrated that parasympathetic preganglionic neurons in the caudal medulla or lumbo-sacral spinal cord were not infected, indicating that infected BN neurons were not infected via a parasympathetic route. Thus, BN neurons become infected after PRV injections into the spleen or kidney either directly through BN projections to SPNs, or secondarily via BN projections to infected pre-preganglionic neurons. These results demonstrate an anatomical linkage, either direct or indirect, between BN and sympathetic activity. Because BN receives numerous inputs from diverse brain regions, the relation of BN with both branches of the autonomic nervous system suggests that this nucleus might play a role in the integration of supraspinal inputs relevant to the central coordination of sympathetic and parasympathetic activity.

Central integration of swallow and airway-protective reflexes

The relationship between the timing of respiration and swallowing has been proven not to be random. Using pseudorabies virus (PRV) as a transsynaptic neural tracer, a basis for the central integration of swallowing and airway-protective reflexes can be located in the neural circuits projecting to swallowing-related muscles. The premotor neurons (PMNs) that constitute the swallowing central pattern generators, interneuronal networks able to initiate repetitive rhythmic muscle activity independent of sensory feedback, connect with multiple areas of the brainstem and other areas of the central nervous system. Those PMNs that project to muscles used in swallowing have been localized within the nucleus of the solitary tract (NTS) and its adjacent reticular formation, and they are synaptically linked both to peripheral afferents and to cortical swallowing areas. Bartha PRV, an attenuated vaccine strain of swine alpha-herpesvirus with a long postinjection survival rate and the ability to produce controlled infections that spread in a hierarchical manner within synaptically linked neurons, can specifically label neurons projecting to PMNs of a given circuit. Thus, it has been used to isolate two neuroanatomically distinct subnetworks of PMNs involved in the buccopharyngeal and esophageal phases of swallowing. Use of PRV as a neural tracer shows that during the buccopharyngeal phase of swallowing, vagal afferents from the pharynx and larynx and from the superior laryngeal nerve terminate in the intermediate and interstitial subnuclei of the NTS. Motoneurons projecting to the pharynx and larynx are located in the semicompact and loose formations of the nucleus ambiguus (NA). Neural tracing with PRV also shows that esophageal PMNs have direct synaptic contact with esophageal motoneurons in the compact formation of the NA. Moreover, esophageal PMNs are localized exclusively to the central subnucleus of the NTS, a site that also is the sole point of termination of esophageal vagal afferents. Using PRV, one can identify third-order (neurons projecting to PMNs) esophageal neurons in sites where pharyngeal PMNs have been noted. Injection of PRV into the esophagus and subsequent detection using immunofluorescence found a subpopulation of neurons in the intermediate and interstitial subnuclei of the NTS. This subpopulation projects to pharyngeal motoneurons and buccopharyngeal PMNs, and it is synaptically linked to esophageal PMNs. The synaptic link between buccopharyngeal and esophageal PMNs provides a potential anatomic substrate within the NTS for the central integration of esophageal peristalsis with the pharyngeal phase of swallowing and airway-protective reflexes. Human studies and animal models investigating esophagoglottal closure and pharyngo-upper esophageal sphincter (pharyngo-UES) contractile reflexes have located the neural pathways that mediate airway-protective reflexes. Similar studies and models using two PRV strains injected simultaneously into different swallowing and respiration-related muscle groups may identify synaptic connectivity between laryngeal, esophageal, and pharyngeal PMNs and, thus, may help to demonstrate the central integration of swallowing and airway-protective reflexes.

Central control of lower esophageal sphincter relaxation

The lower esophageal sphincter is innervated by both parasympathetic (vagus) and sympathetic (primarily splanchnic) nerves; however, the vagal pathways are the ones that are essential for reflex relaxation of the lower esophageal sphincter (LES), such as that which occurs during transient LES relaxations. Vagal afferent sensory endings from the distal esophagus and LES terminate in the hindbrain nucleus tractus solitarius. The preganglionic motor innervation of the LES arises from the dorsal motor nucleus of the vagus. Together these nuclei comprise the dorsal vagal complex within which there is a neural network coordinating reflex control of the sphincter. Vagal efferent preganglionic neurons to the gastrointestinal tract are organized viscerotopically in the dorsal motor nucleus of the vagus. Stimulation of the dorsal motor nucleus of the vagus caudal to the opening of the fourth ventricle results in relaxations, whereas stimulation in the rostral portion of the nucleus evokes contractions of the LES. Few details are known about the neural circuitry that links sensory information from the stomach and esophagus within the nucleus tractus solitarius to these separate populations of neurons within the dorsal motor nucleus of the vagus. The motor vagal preganglionic output is primarily cholinergic, which ultimately stimulates excitatory or inhibitory motor neurons that control the smooth muscle tone. Excitatory neurons evoke muscarinic receptor-mediated muscle contraction. Inhibitory neurons evoke nitric oxide or vasoactive intestinal polypeptide-mediated relaxation of the lower esophageal sphincter. However, other neurotransmitters are found in vagal preganglionic neurons, including norepinephrine/dopamine and nitric oxide. A subpopulation of nitric oxide synthase-containing vagal preganglionic neurons innervate the upper gastrointestinal tract and mediate relaxation. The neurotransmitters and circuitry controlling lower esophageal sphincter pressure are important to characterize, because part of the dorsal vagal complex is outside of the blood-brain barrier and is a potential target for pharmacologic intervention in the treatment of such disorders as gastroesophageal reflux disease.