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

Effects of aging and levodopa on the laryngeal adductor reflex in rats

Dopaminergic neurotransmission plays an essential role in sensorimotor function, and declines with age. Previously, we found the laryngeal adductor reflex (LAR) was increased in excitation by a dopamine receptor antagonist. If this airway-protective reflex is similarly affected by aging, it will interfere with volitional control in older adults. The current study tested whether the LAR was affected by aging, and whether such deficits were reversed by levodopa administration in aging rats. We recorded thyroarytenoid (TA) muscle activity at rest and during elicitation of LAR responses by stimulation of the internal branch of the superior laryngeal nerve (iSLN) in 6-, 18- and 30-month-old rats under alpha-chloralose anesthesia. Using paired stimuli at different inter-stimulus intervals (ISIs), LAR central conditioning, resting muscle activity, and reflex latency and amplitudes were quantified. Numbers of dopaminergic neurons in the substantia nigra pars compacta (SNpc) were measured using tyrosine hydroxylase staining. We found: (1) increased resting TA muscle activity and LAR amplitude occurred with fewer dopaminergic neurons in the SNpc in 18- and 30-month-old rats; (2) decreases in LAR latency and increases in amplitude correlated with reduced numbers of dopaminergic neurons in the SNpc; (3) test responses were greater at 1000ms ISI in 18-month-old rats compared with 6-month-old rats; and (4) levodopa administration further increased response latency but did not alter muscle activity, response amplitude, or central conditioning. In conclusion, increases in laryngeal muscle activity levels and reflex amplitudes accompanied age reductions in dopaminergic neurons but were not reversed with levodopa administration.

Combined laryngeal inflammation and trauma mediate long-lasting immunoreactivity response in the brainstem sensory nuclei in the rat

Somatosensory feedback from the larynx plays a critical role in regulation of normal upper airway functions, such as breathing, deglutition, and voice production, while altered laryngeal sensory feedback is known to elicit a variety of pathological reflex responses, including persistent coughing, dysphonia, and laryngospasm. Despite its clinical impact, the central mechanisms underlying the development of pathological laryngeal responses remain poorly understood. We examined the effects of persistent vocal fold (VF) inflammation and trauma, as frequent causes of long-lasting modulation of laryngeal sensory feedback, on brainstem immunoreactivity in the rat. Combined VF inflammation and trauma were induced by injection of lipopolysaccharide (LPS) solution and compared to VF trauma alone from injection of vehicle solution and to controls without any VF manipulations. Using a c-fos marker, we found significantly increased Fos-like immunoreactivity (FLI) in the bilateral intermediate/parvicellular reticular formation (IRF/PCRF) with a trend in the left solitary tract nucleus (NTS) only in animals with combined LPS-induced VF inflammation and trauma. Further, FLI in the right NTS was significantly correlated with the severity of LPS-induced VF changes. However, increased brainstem FLI response was not associated with FLI changes in the first-order neurons of the laryngeal afferents located in the nodose and jugular ganglia in either group. Our data indicate that complex VF alterations (i.e., inflammation/trauma vs. trauma alone) may cause prolonged excitability of the brainstem nuclei receiving a direct sensory input from the larynx, which, in turn, may lead to (mal)plastic changes within the laryngeal central sensory control.

Caudal nuclei of the rat nucleus of the solitary tract differentially innervate respiratory compartments within the ventrolateral medulla

A substantial array of respiratory, cardiovascular, visceral and somatic afferents are relayed via the nucleus of the solitary tract (NTS) to the brainstem (and forebrain). Despite some degree of overlap within the NTS, specificity is maintained in central respiratory reflexes driven by second order afferent relay neurons in the NTS. While the topographic arrangement of respiratory-related afferents targeting the NTS has been extensively investigated, their higher order brainstem targets beyond the NTS has only rarely been defined with any precision. Nonetheless, the various brainstem circuits serving blood gas homeostasis and airway protective reflexes must clearly receive a differential innervation from the NTS in order to evoke stimulus appropriate behavioral responses. Accordingly, we have examined the question of which specific NTS nuclei project to particular compartments within the ventral respiratory column (VRC) of the ventrolateral medulla. Our analyses of NTS labeling after retrograde tracer injections in the VRC and the nearby neuronal groups controlling autonomic function indicate a significant distinction between projections to the Bötzinger complex and preBötzinger complex compared to the remainder of the VRC. Specifically, the caudomedial NTS, including caudal portions of the medial solitary nucleus and the commissural division of NTS project relatively densely to the region of the retrotrapezoid nucleus and rostral ventrolateral medullary nucleus as well as to the rostral ventral respiratory group while avoiding the intervening Bötzinger and preBötzinger complexes. Area postrema appears to demonstrate a pattern of projections similar to that of caudal medial and commissural NTS nuclei. Other, less pronounced differential projections of lateral NTS nuclei to the various VRC compartments are additionally noted.

Silent synapses in developing rat nucleus tractus solitarii have AMPA receptors

NMDA-only synapses, called silent synapses, are thought to be the initial step in synapse formation in several systems. However, the underlying mechanism and the role in circuit construction are still a matter of dispute. Using combined morphological and electrophysiological approaches, we searched for silent synapses at the level of the nucleus tractus solitarii (NTS), a brainstem structure that is a gateway for many visceral sensory afferent fibers. Silent synapses were detected at birth by using electrophysiological recordings and minimal stimulation protocols. However, anatomical experiments indicated that nearly all, if not all, NTS synapses had AMPA receptors. Based on EPSC fluctuation measurements and differential blockade by low-affinity competitive and noncompetitive glutamate antagonists, we then demonstrated that NTS silent synapses were better explained by glutamate spillover from neighboring fibers and/or slow dynamic of fusion pore opening. Glutamate spillover at immature NTS synapses may favor crosstalk between active synapses during development when glutamate transporters are weakly expressed and contribute to synaptic processing as well as autonomic circuit formation.

Ultrastructure and synaptology of the paratrigeminal nucleus in the rat: primary pharyngeal and laryngeal afferent projections

The paratrigeminal nucleus (PTN) receives primary afferent projections from the aerodigestive tract and orofacial regions and plays a role in the integration of visceral and somatic information. This study describes the fine structure of the rat PTN and the synaptology of primary afferent projections from the pharynx and larynx. Injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) or cholera toxin-HRP (CT-HRP) were made into the wall of the pharynx or larynx to label primary afferent axon terminals. Light microscopic observations demonstrated that afferent axons terminated bilaterally in overlapping fields in the PTN. Electron microscopic observations of the PTN revealed that there were three distinct classes of neurons, based on morphology and axosomatic contacts. The most abundant neurons, Type 1, were fusiform in shape and received very few or no axosomatic contacts. Type 2 neurons contained prominent Nissl substance (rough endoplasmic reticulum) and few axosomatic contacts, while Type 3 neurons had many axosomatic synapses. Terminals containing round, clear vesicles and forming asymmetric contacts (round asymmetric, RA) with dendrites were the predominant synaptic type in the PTN. Primary afferent terminals from the pharynx and larynx were of the RA type and formed synaptic contacts with small-diameter (<1 microm) dendrites. Visceral primary afferent inputs from the pharynx and larynx overlap with trigeminal somatic afferents in the PTN and have similar synaptic morphology. The results support the concept that the PTN provides an anatomical substrate for mediating viscerovisceral and somatovisceral reflexes via efferent connections with autonomic centers in the brainstem.

Differential expression of vesicular glutamate transporters by vagal afferent terminals in rat nucleus of the solitary tract: projections from the heart preferentially express vesicular glutamate transporter 1

The central projections and neurochemistry of vagal afferent neurones supplying the heart in the rat were investigated by injecting cholera toxin B-subunit into the pericardium. Transganglionically transported cholera toxin B-subunit was visualized in the medulla oblongata in axons and varicosities that were predominantly aggregated in the dorsomedial, dorsolateral, ventrolateral and commissural subnuclei of the caudal nucleus of the solitary tract. Unilateral vagal section in control rats prevented cholera toxin B-subunit labeling on the ipsilateral side of the nucleus of the solitary tract. Fluorescent and electron microscopic dual labeling showed colocalization of immunoreactivity for vesicular glutamate transporter 1, but only rarely vesicular glutamate transporters 2 or 3 with cholera toxin B-subunit in terminals in nucleus of the solitary tract, suggesting that cardiac vagal axons release glutamate as a neurotransmitter. In contrast, populations of vagal afferent fibers labeled by injection of cholera toxin B-subunit, tetra-methylrhodamine dextran or biotin dextran amine into the aortic nerve, stomach or nodose ganglion colocalized vesicular glutamate transporter 2 more frequently than vesicular glutamate transporter 1. The presence of other neurochemical markers of primary afferent neurones was examined in nucleus of the solitary tract axons and nodose ganglion cells labeled by pericardial cholera toxin B-subunit injections. Immunoreactivity for a 200-kDa neurofilament protein in many large, cholera toxin B-subunit-labeled nodose ganglion cells indicated that the cardiac afferent fibers labeled are mostly myelinated, whereas binding of Griffonia simplicifolia isolectin B4 to fewer small cholera toxin B-subunit-labeled ganglion cells suggested that tracer was also taken up by some non-myelinated axons. A few labeled nucleus of the solitary tract axons and ganglion cells were positive for substance P and calcitonin gene-related peptide, which are considered as peptide markers of nociceptive afferent neurones. These data suggest that the population of cardiac vagal afferents labeled by pericardial cholera toxin B-subunit injection is neurochemically varied, which may be related to a functional heterogeneity of baroreceptive, chemoreceptive and nociceptive afferent fibers. A high proportion of cardiac neurones appear to be glutamatergic, but differ from other vagal afferents in expressing vesicular glutamate transporter 1.

Vesicular glutamate transporters type 1 and 2 expression in axon terminals of the rat nucleus of the solitary tract

The nucleus of the solitary tract is the site of termination of primary afferent fibers running in the facial, glossopharyngeal and vagus nerves. The present study was performed to map the distribution of glutamatergic axons terminals in the rat nucleus of the solitary tract using immunodetection of vesicular glutamate transporter 1 and vesicular glutamate transporter 2. The two vesicular glutamate transporters were differentially distributed among nucleus of the solitary tract subdivisions. Vesicular glutamate transporter 1 immunoreactivity was mostly found in the lateral part of the nucleus (ventrolateral, interstitial and intermediate subdivisions) whereas vesicular glutamate transporter 2 labeling was distributed throughout the nucleus of the solitary tract. Electron microscope examination indicated that vesicular glutamate transporter immunoreactivity was localized in axon terminals filled with round synaptic vesicles. After injection of cholera toxin B subunit in sensory ganglia, anterograde labeling was found in vesicular glutamate transporter 1, as well as vesicular glutamate transporter 2-immunoreactive boutons. Double immunolabeling experiments allowed distinctions between terminals expressing either vesicular glutamate transporter 1 or vesicular glutamate transporter 2 or both vesicular glutamate transporter 1 and vesicular glutamate transporter 2 immunoreactivities. The latter population, expressing both transporters immunolabeling, completely disappeared after deafferentation induced by removal of sensory ganglia. This study indicates that vesicular glutamate transporter content identifies three different subpopulations of glutamatergic boutons in the nucleus of the solitary tract and provides definitive evidence that primary afferent neurons contribute glutamatergic terminals to the nucleus of the solitary tract.

Central nervous system control of the laryngeal muscles in humans

Laryngeal muscle control may vary for different functions such as: voice for speech communication, emotional expression during laughter and cry, breathing, swallowing, and cough. This review discusses the control of the human laryngeal muscles for some of these different functions. Sensori-motor aspects of laryngeal control have been studied by eliciting various laryngeal reflexes. The role of audition in learning and monitoring ongoing voice production for speech is well known; while the role of somatosensory feedback is less well understood. Reflexive control systems involving central pattern generators may contribute to swallowing, breathing and cough with greater cortical control during volitional tasks such as voice production for speech. Volitional control is much less well understood for each of these functions and likely involves the integration of cortical and subcortical circuits. The new frontier is the study of the central control of the laryngeal musculature for voice, swallowing and breathing and how volitional and reflexive control systems may interact in humans.

Fine structural survey of the intermediate subnucleus of the nucleus tractus solitarii and its glossopharyngeal afferent terminals

The intermediate subnucleus of the nucleus tractus solitarii (imNTS) receives somatosensory inputs from the soft palate and pharynx, and projects onto the nucleus ambiguus, thus serving as a relay nucleus for swallowing. The ultrastructure and synaptology of the rat imNTS, and its glossopharyngeal afferent terminals, have been examined with cholera toxin-conjugated horseradish peroxidase (CT-HRP) as an anterograde tracer. The imNTS contained oval or ellipsoid-shaped, small to medium-sized neurons (18.2 x 11.4 microm) with little cytoplasm, few cell organelles and an irregularly shaped nucleus. The cytoplasm often contained one or two nucleolus-like stigmoid bodies. The average number of axosomatic terminals was 1.8 per profile. About 83% of them contained round vesicles and formed asymmetric synaptic contacts (Gray's type I), while about 17% contained pleomorphic vesicles and formed symmetric synaptic contacts (Gray's type II). The neuropil contained small or large axodendritic terminals, and about 92% of them were Gray's type I. When CT-HRP was injected into the nodose ganglion, many labeled terminals were found in the imNTS. All anterogradely labeled terminals contacted dendrites but not somata. The labeled terminals were usually large (2.69+/-0.09 mum) and exclusively of Gray's type I. They often contacted more than two dendrites, were covered with glial processes, and formed synaptic glomeruli. A small unlabeled terminal occasionally made an asymmetric synaptic contact with a large labeled terminal. The large glossopharyngeal afferent terminals and the neurons containing stigmoid bodies characterized the imNTS neurons that received pharyngeal afferents.

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.

Neuronal activation in the medulla oblongata during selective elicitation of the laryngeal adductor response

Swallow and cough are complex motor patterns elicited by rapid and intense electrical stimulation of the internal branch of the superior laryngeal nerve (ISLN). The laryngeal adductor response (LAR) includes only a laryngeal response, is elicited by single stimuli to the ISLN, and is thought to represent the brain stem pathway involved in laryngospasm. To identify which regions in the medulla are activated during elicitation of the LAR alone, single electrical stimuli were presented once every 2 s to the ISLN. Two groups of five cats each were studied; an experimental group with unilateral ISLN stimulation at 0.5 Hz and a surgical control group. Three additional cats were studied to evaluate whether other oral, pharyngeal, or respiratory muscles were activated during ISLN stimulation eliciting LAR. We quantified < or = 22 sections for each of 14 structures in the medulla to determine if regions had increased Fos-like immunoreactive neurons in the experimental group. Significant increases (P < 0.0033) occurred with unilateral ISLN stimulation in the interstitial subnucleus, the ventrolateral subnucleus, the commissural subnucleus of the nucleus tractus solitarius, the lateral tegmental field of the reticular formation, the area postrema, and the nucleus ambiguus. Neither the dorsal motor nucleus of the vagus, usually active for swallow, nor the nucleus retroambiguus, retrofacial nucleus, and the lateral reticular nucleus, usually active for cough, were active with elicitation of the laryngeal adductor response alone. The results demonstrate that the laryngeal adductor pathway is contained within the broader pathways for cough and swallow in the medulla.

Selective suppression of late laryngeal adductor responses by N-methyl-D-aspartate receptor blockade in the cat

Laryngeal adductor responses to afferent stimulation play a key role in airway protection. Although vital for protection during cough and swallow, these responses also must be centrally controlled to prevent airway obstruction by laryngospasm during prolonged stimulation. Our purpose was to determine the role of N-methyl-D-aspartate (NMDA) receptors in modulating early R1 responses (at 9 ms) and/or later more prolonged R2 responses (at 36 ms) during electrical stimulation of the laryngeal afferent fibers contained in the internal branch of the superior laryngeal nerve in the cat. The percent occurrence, amplitude, and conditioning of muscle responses to single superior laryngeal nerve (SLN) stimuli presented in pairs at interstimulus intervals of 250 ms were measured in three experiments: 1) animals that had ketamine as anesthetic premedication were compared with those who did not, when both were maintained under alpha-chloralose anesthesia. 2) The effects of administering ketamine in one group of animals were compared with increasing the depth of alpha-chloralose anesthesia without NMDA receptor blockade in another group of animals. 3) The effects of dextromethorphan (without anesthetic effects) were examined in another group of animals. In the first experiment, the occurrence of R2 responses were reduced from 95% in animals without ketamine premedication to 25% in animals with ketamine premedication (P = 0.015). No differences occurred in the occurrence, amplitude, latency, or conditioning effects on R1 responses between these groups. In the second experiment, the occurrence of R2 responses was reduced from 96 to 79% after an increase in the depth of anesthesia with alpha-chloralose in contrast with reductions in R2 occurrence from 98 to 19% following the administration of ketamine to induce NMDA receptor blockade along with increased anesthesia (P = 0.025). In the third experiment, R2 occurrence was reduced from 89 to 27% (P = 0.017) with administration of dextromethorphan while R1 response occurrence and amplitude did not change. In each of these experiments, NMDA receptor blockade did not have significant effects on cardiac or respiratory rates in any of the animals. The results demonstrate that NMDA receptors play an essential role in long latency R2 laryngeal responses to laryngeal afferent stimulation. On the other hand, early R1 laryngeal adductor responses are likely to involve non-NMDA receptor activation.

Neural control of tongue movement with respect to respiration and swallowing

The tongue must move with remarkable speed and precision between multiple orofacial motor behaviors that are executed virtually simultaneously. Our present understanding of these highly integrated relationships has been limited by their complexity. Recent research indicates that the tongue s contribution to complex orofacial movements is much greater than previously thought. The purpose of this paper is to review the neural control of tongue movement and relate it to complex orofacial behaviors. Particular attention will be given to the interaction of tongue movement with respiration and swallowing, because the morbidity and mortality associated with these relationships make this a primary focus of many current investigations. This review will begin with a discussion of peripheral tongue muscle and nerve physiology that will include new data on tongue contractile properties. Other relevant peripheral oral cavity and oropharyngeal neurophysiology will also be discussed. Much of the review will focus on brainstem control of tongue movement and modulation by neurons that control swallowing and respiration, because it is in the brainstem that orofacial motor behaviors sort themselves out from their common peripheral structures. There is abundant evidence indicating that the neural control of protrusive tongue movement by motoneurons in the ventral hypoglossal nucleus is modulated by respiratory neurons that control inspiratory drive. Yet, little is known of hypoglossal motoneuron modulation by neurons controlling swallowing or other complex movements. There is evidence, however, suggesting that functional segregation of respiration and swallowing within the brainstem is reflected in somatotopy within the hypoglossal nucleus. Also, subtle changes in the neural control of tongue movement may signal the transition between respiration and swallowing. The final section of this review will focus on the cortical integration of tongue movement with complex orofacial movements. This section will conclude with a discussion of the functional and clinical significance of cortical control with respect to recent advances in our understanding of the peripheral and brainstem physiology of tongue movement.

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.

Central nervous pathways and control of the airways

Neural control of airway muscles and secretions is predominantly by excitatory parasympathetic and non-adrenergic, non-cholinergic innervations (excitatory and/or inhibitory depending on the species). Functionally distinct afferents effecting airway reflexes terminate in different but overlapping parts of the nucleus tractus solitarius, where integration of simultaneously evoked reflex responses occurs. Parasympathetic preganglionic neurones are located in the dorsal vagal nucleus and nucleus ambiguus, which also contains upper airway motoneurones. These output neurones receive inputs from the central respiratory network which modify the effectiveness of reflex activity. This is particularly important since many afferents evoking airway reflexes concurrently modify respiratory drive. Thus, their effect on the outflow is twofold, a direct reflex effect and an indirect respiratory action and these may facilitate or antagonise one another. Although there is reflex control of individual motor outflows, in some defined situations, e.g. swallowing and coughing a stereotypical pattern of motor outflow is evoked. The neural mechanisms underlying these aspects of airway control are discussed.

Subnuclear distribution of afferents from the oral, pharyngeal and laryngeal regions in the nucleus tractus solitarii of the rat: a study using transganglionic transport of cholera toxin

The central distributions of afferents from the oral cavity, the pharynx, the larynx and the esophagus to the nucleus tractus solitarii (NTS) were examined by using transganglionic anterograde transport of the cholera toxin B subunit (CT-b). Injections of CT-b into the body of the tongue and the hard palate resulted in heavy labeling of the lateral subnucleus (l-NTS) of the NTS rostral to the area postrema. Injection into the root of the tongue resulted in heavy labeling of the l-NTS, the dorsal half of the medial (m-NTS), the intermediate (im-NTS) and the interstitial (is-NTS) subnuclei rostral to the area postrema. Injections into the soft palate and the pharynx resulted in a similar labeling pattern in the is-NTS, im-NTS and m-NTS to that in the case of the root of the tongue, but this labeling extended rostrocaudally. Heavy labeling of the medial aspect of the l-NTS was found in the case of the soft palate, but the labeling was sparse in the case of the pharynx. Moderate labeling was also found in the commissural subnucleus (co-NTS). Injection into the larynx resulted in labeling of the is-NTS throughout the NTS, and of the rostral half of im-NTS. Injection into the esophagus resulted in heavy labeling of the central subnucleus, and moderate labeling of the co-NTS and the caudal half of im-NTS. A few but consistent anterogradely labeled terminals were found to appose retrogradely labeled small neurons in the rostral tip of the dorsal motor nucleus of vagus in the cases of injections into the root of the tongue, the soft palate, the pharynx, and the larynx. These results have characterized the viscerotopic representation of afferent projections from the oral and the cervical visceral organs to the subnuclei of the NTS.

Ultrastructural synaptic organization of axon terminals in the ventral part of the cat oral pontine reticular nucleus

In an attempt to contribute to the current knowledge of the brainstem reticular formation synaptic organization, the ultrastructure and distribution of synaptic terminal profiles on neurons in the ventral part of the oral pontine reticular nucleus (vRPO), the rapid eye movement (REM) sleep-induction site, were studied quantitatively. Terminals with asymmetric contacts and rounded vesicles were classified according to vesicle density as type I or II (high or low density, respectively). The area, apposed perimeter length, and mitochondrial area of type I terminals, on average, were significantly smaller than those of type II terminals. Type III and IV terminals had symmetric contacts and oval and/or flattened vesicles; type III terminals formed synapses between them and on initial axons. Type V and VI terminals showed characteristics intermediate to those of asymmetric and symmetric synapses. Interestingly, some terminal features were related to both terminal area and postsynaptic dendritic diameter. The percentages of different synapses sampled on somata were as follows: asymmetric synapses (usually formed by type II terminals; mean +/- S.D.), 26.4% +/- 3%; symmetric synapses, 46.7% +/- 5.2%; and intermediate synapses, 26.9% +/- 6.1%. The percentages of different synapses sampled on dendrites were asymmetric synapses, 62.1% +/- 9%; symmetric synapses, 25.6% +/- 8.1%; and intermediate synapses, 12.3% +/- 1.7%. Comparison between large- and small-diameter dendrites revealed that the percentages of symmetric synapses and type II terminals decreased, whereas the percentages of type I terminals increased as postsynaptic dendritic diameters became smaller. Synaptic density was approximately four times lower on somata than on dendrites. The vRPO synaptic organization reflects some patterns that are similar to those found in other regions of the central nervous system as well as specific synaptic patterns that are probably related to its functions: the generation and maintenance of REM sleep and the control of eye movement or limb muscle tone.

Wheat germ agglutinin conjugated to TRITC: a novel approach for labeling primary projection neurons of peripheral afferent nerves

Wheat germ agglutinin conjugated to tetramethylrhodamine isothiocyanate-dextran (WGA-TRITC) was studied as a novel tracer of primary projection neurons of pharyngeal (PhN) and superior laryngeal (SLN) branches of the vagus nerve. The SLN and PhN were dissected from rat cervical tissues and the proximal end of the nerves were bathed in tracer for 60-90 min. The animals were sacrificed 42-72 h later. The tissue was fixed, sliced, mounted on slides and viewed under epifluorescence. The clarity of the fluorescent label in projection neurons was confounded in some regions of the brainstem by autofluorescence. A computer image analysis method was developed to quantify fluorescence intensity for definitive identification of labeled neurons. Brainstem neurons labeled by afferent projections of the SLN and PhN were localized to the nucleus tractus solitarius. Efferents were identified in the nucleus ambiguus. WGA-TRITC labeled cells were observed in the ipsilateral brainstem at intensities significantly different from the fluorescence observed in controls (P<0.01). The distribution and density of labeling is in agreement with results of previous investigations, suggesting that WGA-TRITC is a useful alternative for tracing SLN and PhN projections to brainstem nuclei.

Direct parabrachial nuclear projections to the pharyngeal motoneurons in the rat: an anterograde and retrograde double-labeling study

The parabrachial nucleus consists of several subnuclei which contains autonomic, gustatory, visceral sensory, nociceptive, and respiratory related neurons. We have investigated the direct projections from the rat parabrachial region, including the K olliker-Fuse nucleus, to the pharyngeal motoneurons with an anterograde and retrograde double-tracing technique. The cholera toxin subunit-B was injected into the lower pharynx or the esophagus after injection of biotinylated dextran amine into the ventrolateral parabrachial nuclear region, including the external medial, the external lateral, and the crescent area of the central lateral parabrachial nuclei and into the Kölliker-Fuse nucleus. The anterogradely dextran amine-labeled fibers from these nuclei projected to the semicompact, loose and external formations besides the compact formation of the nucleus ambiguus. Many anterogradely labeled fibers and terminals were found to contact retrogradely cholera toxin-labeled pharyngeal neuronal soma and dendrites in the semicompact formation of the nucleus ambiguus. The medial half of the parabrachial nucleus, including the medial and the medial part of the central lateral parabrachial nuclei, sent a few fibers to the reticular formation just dorsal to the esophageal motoneurons but no fibers to either the pharyngeal or to the esophageal motoneurons. These results suggested that the visceral sensory, gustatory, nociceptive or respiratory related neurons in the parabrachial nucleus project directly to the pharyngeal motoneurons, but there are no parabrachial projections to the esophageal motoneurons in the nucleus ambiguus.

Crossing inputs of the superior laryngeal nerve afferents to medullary swallowing-related neurons in the cat

To understand the neural mechanism for generation of synchronous activity on both sides during swallowing, we examined the convergence of inputs from the bilateral superior laryngeal nerves (SLNs) in the urethane-anesthetized cat medulla and we also examined the changes in swallowing outputs after a longitudinal brain-stem split in decerebrate cats. Twenty-six (31%) of 84 swallowing-related neurons (SRNs) that were oligosynaptically activated by ipsilateral SLN stimulation and recorded mostly in the reticular formation received contralateral inputs, which were confirmed by orthodromic spike responses (n = 16) or were detected as subliminal facilitatory or inhibitory inputs (n = 10) using conditioning-test stimuli. The rate of convergence of inputs from bilateral SLNs in these SRNs was significantly higher than that (4%) in the SRNs that were regarded as sensory-relay neurons in the nucleus tractus solitarius (NTS). The SRNs receiving signals from the contralateral SLN were located diffusely from the NTS and the adjacent reticular formation to the nucleus ambiguus (NA) and the reticular formation dorso-medial to the NA. A midsagittal split from 3 mm caudal to 6 mm rostral to the obex could change symmetrical swallowing to unilateral swallowing. Thus the crossing projections to the contralateral SRNs appear to contribute to symmetrical swallowing.