Dendritic architecture of hypoglossal motoneurons projecting to extrinsic tongue musculature in the rat

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 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.

Elucidation of neuronal circuitry: protocol(s) combining intracellular labeling, neuroanatomical tracing and immunocytochemical methodologies

We describe a protocol combining either intracellular biotinamide staining or anterograde biotinylated dextran amine (BDA) tracing with retrograde horseradish peroxidase (HRP) labeling and immunocytochemistry in order to map physiologically identified neuronal pathways. Presynaptic neurons including their boutons are labeled by either intracellular injection of biotinamide or extracellular injection of BDA while postsynaptic neurons are labeled with HRP via retrograde transport. Tissues are first processed to detect HRP using a tetramethylbenzidine and sodium-tungstate method. Biotinamide or BDA staining is then visualized using an ABC-diaminobenzidine-Ni method and finally the tissue is immunocytochemically stained using choline acetyltransferase (ChAT) or parvalbumin antibodies and a peroxidase-anti-peroxidase method. After processing, biotinamide, BDA, HRP and immunocytochemical staining can readily be distinguished by differences in the size, color and texture of their reaction products. We have utilized this methodology to explore synaptic relationships between trigeminal primary afferent neurons and brainstem projection and motoneurons at both the light and electron microscopic levels. This multiple labeling methodology could be readily adapted to characterize the physiological, morphological and neurochemical properties of other neuronal pathways.

Axonal projections and synapses from the supratrigeminal region to hypoglossal motoneurons in the rat

Neural circuits from the supratrigeminal region (Vsup) to the hypoglossal motor nucleus were studied in rats using anterograde and retrograde neuroanatomical tracing methodologies. Iontophoretic injection of 10% biotinylated dextran amine (BDA) unilaterally into the Vsup anterogradely labeled axons and axon terminals bilaterally in the hypoglossal nucleus (XII) as well as other regions of the brainstem. In the ipsilateral XII, the highest density of BDA labeling was found in the dorsal compartment and the ventromedial subcompartment of the ventral compartment, where BDA labeling formed a dense, patchy distribution. Microinjection of 20% horseradish peroxidase (HRP) ipsilaterally or bilaterally into the tongue resulted in retrograde labeling of XII motoneurons confined to the dorsal and ventral compartments of the hypoglossal motor nucleus. Under light microscopical examination, BDA-labeled terminals were observed closely apposing the somata and primary dendrites of HRP-labeled hypoglossal motoneurons. Two hundred and sixty-five of these BDA-labeled terminals were examined at the ultrastructural level. One hundred and twelve BDA-labeled axon terminals were observed synapsing with either the somata (39%, 44/112) or the large or medium-size dendrites (61%, 68/112) of retrogradely labeled hypoglossal motoneurons. Axon terminals containing spherical vesicles (S-type) formed asymmetric synapses with HRP-labeled hypoglossal motoneuron dendrites. In contrast to this, F(F)-type axon terminals, containing flattened vesicles, formed symmetric synapses with both the somata and dendrites of HRP-labeled hypoglossal motoneurons with a preponderance of the contacts on their somata. Axon terminals containing pleomorphic vesicles (F(P)-type) were noted forming both symmetric and asymmetric synapses with HRP-labeled hypoglossal motoneuron somata and dendrites. The present study provides anatomical evidence of neuronal projections and synaptic connections from the supratrigeminal region to hypoglossal motoneurons. These data suggest that the supratrigeminal region, as one of the premotor neuronal pools of the hypoglossal nucleus, may coordinate and modulate the activity of tongue muscles during oral motor behaviors.

Functional anatomy of the hypoglossal innervated muscles of the rat tongue: a model for elongation and protrusion of the mammalian tongue

This anatomical investigation in the rat was designed to illustrate the detailed organization of the tongue's muscles and their innervation in order to elucidate the actions of the muscles of the higher mammalian tongue and thereby clarify the protrusor subdivision of the hypoglossal-tongue complex. The hypoglossal innervated, extrinsic styloglossus, hyoglossus, and genioglossus and the intrinsic transversus, verticalis and longitudinalis linguae muscles were observed by microdissection and analysis of serial transverse-sections of the tongue. Sihler's staining technique was applied to whole rat tongues to demonstrate the hypoglossal nerve branching patterns. Dissections of the tongue demonstrate the angles at which the extrinsic muscles act on the base of the tongue. The Sihler stained hypoglossal nerves demonstrate branches to the styloglossus and hyoglossus emanating from its lateral division while branches to the genioglossus muscle exit from its medial division. The largest portions of both XIIth nerve divisions can be seen to enter the body of the tongue to innervate the intrinsic muscles. Transverse sections of the tongue demonstrate the organization of the intrinsic muscle fibers of the tongue. Longitudinal muscle fibers run along the entire circumference of the tongue. Alternating sheets of transverse lingual and vertical lingual muscles can be observed to insert into the circumference of the tongue. Most importantly in clarifying tongue protrusion, we demonstrate the transversus muscle fibers enveloping the most superior and inferior portions of the longitudinalis muscles. Longitudinal muscle fascicles are completely encircled and thus are likely to be compressed by transverse muscle fascicles resulting in elongation of the tongue. We discuss our findings in relation to biomechanical studies, that describe the tongue as a muscular hydrostat and thereby define the "elongation-protrusion apparatus" of the mammalian tongue. In so doing, we clarify the functional organization of the hypoglossal-tongue complex.

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

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

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.

Contractile properties of the tongue's genioglossus muscle and motor units in the rat

The contractile characteristics of individual mammalian tongue muscles have rarely been investigated, in contrast to spinal cord-innervated and extraocular muscles. Therefore, whole muscle and motor unit contractile forces, plus muscle fiber types, were studied in the genioglossus, the major protrusor muscle, of the rat tongue. The muscle, exclusively composed of fast-contracting units, could be activated from rostroventral hypoglossal nucleus sites only. The following figures represent the means of the contractile measures. Whole muscle twitch tension was 7.02 g, contraction time was 14.22 ms, fusion frequency was 104 Hz, maximum tetanic tension was 37.22 g, and fatigue index was 0.72. Single motor unit twitch tension was 45. 9 mg, contraction time was 11.7 ms, fusion frequency was 94.8 Hz, maximum tetanic tension was 241.95 mg, and fatigue index was 0.68. The genioglossus muscle appeared qualitatively similar to the rat styloglossus muscle, one of the two major retractor muscles of the tongue. The delineation of motor unit contractile characteristics in tongue muscle is important in our understanding of the control of tongue movement.

Substance P in the hypoglossal nucleus of the rat

The distribution of substance P (SP)-containing synaptic terminals in the hypoglossal nucleus (XII) of adult rats was examined by retrograde peroxidase labelling and immunocytochemistry. From the location of peroxidase injections into the tongue and of labelled neurones in the ventral lamina of XII, motor neurones that supply intrinsic vertical, longitudinal and transverse fibres as well as the extrinsic muscle genioglossus appear to have been labelled. SP-containing terminals were found making contact, and sometimes dual synapses, with unlabelled neuronal dendrites but not with retrogradely labelled somata or dendrites. These findings suggest that SP terminals may contact dendrites of interneurones or of neurones supplying other extrinsic muscles located in the anterior part of the tongue. Dual SP-containing synapses between XII motor neurones may be the means by which tongue muscle fibres are recruited and their function synchronized.

Whole-muscle and motor-unit contractile properties of the styloglossus muscle in rat

Investigations of whole muscle and motor-unit contractile properties have provided valuable information for our understanding of the spinal cord and extraocular motor systems. However, no previous investigation has examined these properties in an isolated tongue muscle. The purpose of this study was to determine the contractile properties and muscle fiber types of the rat styloglossus muscle. The styloglossus is one of three extrinsic tongue muscles and serves to retract the tongue within the oral cavity. Adult male Sprague-Dawley rats (n = 19) were used in these experiments. The contractile characteristics of the whole styloglossus muscle (n = 9) were measured in response to stimulation of the hypoglossal nerve branch to the muscle. The average twitch tension produced was 3.30 g with a mean twitch contraction time of 13.81 ms. The mean maximum tetanic tension was 19.66 g and occurred at or near the fusion frequency, which averaged 109 Hz. The styloglossus muscle was resistant to fatigue [fatigue index (F. I.) = 0.76]. In separate experiments (n = 7), the contractile characteristics of 37 single motor units were measured in response to extracellular stimulation of hypoglossal motoneurons. The twitch tension generated by styloglossus motor units averaged 35.7 mg, and the mean twitch contraction time was 12.46 ms. The mean fusion frequency was 92 Hz. Maximum tetanic tension averaged 177.8 mg. Styloglossus single motor units were resistant to fatigue (F. I. = 0.74). The sites of stimulation that yielded a contractile response in the styloglossus muscle were consistent with the location of the styloglossus motoneuron pool reported in earlier anatomy studies. Muscle fiber typing was determined in three animals based on the myofibrillar ATPase reaction at pH 9.8, 4.6, and 4.3. The styloglossus muscle was composed of approximately 99% type IIA fibers with a few scattered type I fibers present in the study sample. On the basis of the combined findings of the physiology and histochemistry experiments, the styloglossus muscle appeared to be a homogeneous muscle composed almost exclusively of fast, fatigue-resistant motor units. These properties of the styloglossus muscle and its motor units were compared with findings in other rat skeletal muscles.

Substance P-immunoreactive boutons closely appose inspiratory protruder hypoglossal motoneurons in the cat

In anesthetized cats, we recorded intracellularly from 26 hypoglossal motoneurons which were antidromically activated following electrical stimulation of either the medial or lateral branches of the hypoglossal nerve. Twenty-one of these neurons were protruder motoneurons 6 of which had inspiratory activity. Three of the protruder motoneurons with inspiratory activity were filled with Neurobiotin and found to be closely apposed to substance P-like immunoreactive nerve terminals.

Inhibition of styloglossus motoneurons during the palatally induced jaw-closing reflex

The inhibition of hypoglossal motoneurons innervating the styloglossus muscle during transient jaw closing, the so-called jaw-closing reflex, was studied in cats. The application of diffuse pressure stimulation to the posterior palatal surface produced the jaw-closing reflex and inhibitory postsynaptic potentials in the styloglossus motoneurons, indicating that mechanosensory inputs from the posterior palatal mucosa sent inhibitory synaptic inputs to styloglossus motoneurons. We also demonstrated that, during the palatally induced jaw-closing reflex, the tongue extended at jaw closure and was still extended forward in the initial part of the opening phase. In all of 22 styloglossus motoneurons studied, the depression of firing was elicited after the onset of jaw closure. In 14 of 22 styloglossus motoneurons, the depression of firing was elicited in the closing phase, and in the remaining cells it was elicited in the occlusal phase. By increasing the intracellular concentration of chloride ions, the inhibitory postsynaptic potential elicited in the styloglossus motoneuron converted to a depolarizing potential. It is concluded that the inhibition of styloglossus motoneurons may be involved in the maintenance of tongue protrusions during the palatally induced jaw-closing reflex, and that inhibitory postsynaptic potentials evoked in the styloglossus motoneurons are partly due to a chloride-dependent inhibitory postsynaptic potential.

Organization of motoneurons in the dorsal hypoglossal nucleus that innervate the retrusor muscles of the tongue in the rat

This anatomical investigation was prompted by the incomplete knowledge of the myotopic organization of the dorsal subdivison of the hypoglossal nucleus. Intrinsic muscle motoneurons were not segregated and labeled previously with regard to the lateral division of the hypoglossal nerve. Also, motoneuron number and cell size, in relation to the individual retrusor tongue musculature, were rarely addressed previously. Retrograde labeling ofretrusor muscle motoneurons in the dorsal subdivision of the rat hypoglossal nucleus was done. Cholera toxin conjugate horseradish peroxidase (CTHRP) was injected into the retrusor tongue muscles with only the lateral division of the hypoglossal nerve intact. The dorsal subdivision of the hypoglossal nucleus contained approximately 800 motoneurons ranging in cell body size from 19 to 41 microm. When either the styloglossus, hyoglossus, superior longitudinal, or inferior longitudinal muscle was isolated and injected with CTHRP, a separate motoneuron pool for each muscle was seen. The extrinsic muscle motoneurons, styloglossus and hyoglossus, were found rostrolateral and caudolateral respectively. In contrast, the intrinsic superior and inferior longitudinal muscle motoneurons were found more central and medial in the nucleus. Extrinsic muscle motoneurons were larger (approximately 30 microm) than intrinsic muscle motoneurons (approximately 26 microm; P < .0001). Intrinsic muscle motoneurons account for a great majority of the motoneurons in the dorsal aspect of the hypoglossal nucleus and their axons have been shown to be contained in the lateral (retrusor) division of the hypoglossal nerve. This study revealed the myotopic organization of the retrusor subdivision of the rat hypoglossal nucleus.

The enkephalinergic innervation of the genioglossus musculature in the rat: implications for the respiratory control of the tongue

This study sought to determine if the enkephalinergic (ENK) innervation of the hypoglossal nucleus (nXII) in the rat was organized differentially for the control of the genioglossus musculature whose activity is essential in maintaining the patency of the upper airway. Immunocytochemical results revealed that the genioglossus motoneuron pool, comprising the ventrolateral subcompartment of the nXII, was consistently and heavily labeled throughout its rostrocaudal dimension. Labeling was characterized by dense focal clustering throughout the neuropil, and by the appearance of numerous perisomatic-like profiles. Similarly, the ventromedial subcompartment mainly rostrally, and the dorsal compartment caudally, whose motoneurons control the caudal intrinsic protrusor and rostral retrusor muscles, respectively, were also consistently labeled. While these results demonstrate that the genioglossus musculature is targeted by ENK inputs, they also suggest that other selected musculature of the tongue is controlled by ENK. It is argued that the innervation pattern identified in the present study is consistent with a functional role for ENK in the respiratory control of the tongue.

Autoradiographic localization of neurotransmitter binding sites in the hypoglossal and motor trigeminal nuclei of the rat

The hypoglossal and motor trigeminal nuclei contain somatic motoneurons innervating the tongue, jaw, and palate. These two cranial motor nuclei are myotopically organized and contain neurotransmitter binding sites for thyrotropin-releasing hormone, substance P, and serotonin. Quantitative autoradiography was used to localize thyrotropin-releasing hormone, substance P, and serotonin-1A and serotonin-1B binding sites in the hypoglossal and motor trigeminal nuclei and to relate the relative distributions of these binding sites to the myotopic organizations of the two nuclei. In the hypoglossal nucleus, high-to-moderate concentrations of all four binding sites were present in the dorsal and ventromedial subnuclei, whereas low concentrations were noted in the ventrolateral subnucleus. In the motor trigeminal nucleus, high concentrations of serotonin-1B, moderate densities of thyrotropin-releasing hormone, and low levels of substance P and serotonin-1A binding sites were present in both the ventromedial and dorsolateral subnuclei. These observations demonstrate that neurotransmitter binding sites in the hypoglossal and motor trigeminal nuclei are heterogeneously localized and that their distributions correspond to the previously described myotopic organizations of each nucleus.

Differential sensitivity of laryngeal and pharyngeal motoneurons to iontophoretic application of serotonin

Serotonergic neurons decrease their activity during sleep, especially rapid eye movement sleep, thereby reducing their facilitatory effect on upper airway motoneurons. The magnitude of teh sleep-related loss of tone varies among upper airway muscles (e.g., pharyngeal dilator motoneurons are more suppressed than laryngeal motoneurons). We hypothesized that these differences may be related to the sensitivity of different groups of upper airway motoneurons to serotonin. Experiments were done on decerebrate, vagotomized, paralysed and artificially-ventilated cats. Hypoglossal and laryngeal motoneurons were recorded extracellularly using five-barrel pipettes filled with: serotonin, glutamate and methysergide (serotonergic antagonist) for iontophoresis, and NaCl for recording and current balancing. All but two of the 65 hypoglossal motoneurons (45 inspiratory, 10 expiratory, 10 tonic) and 27 out of 32 laryngeal motoneurons (14 inspiratory, 18 expiratory) were excited by serotonin, and the excitation was abolished by methysergide. To compare the magnitude of the excitatory effect among distinct motoneuronal groups, we applied small ejection currents in a standardized manner (+15 nA for 3 min; 10 mM serotonin in 150 NaCl) onto spontaneously active motoneurons (13 inspiratory hypoglossal, 11 inspiratory laryngeal and 11 expiratory laryngeal). Serotonin increased the number of spikes per respiratory burst of inspiratory hypoglossal motoneurons from 19 +/- 4.0 (S.E.M.) to 35 +/- 4.8, of inspiratory laryngeal motoneurons from 44 +/- 8.3 to 55 +/- 8.8, and of expiratory laryngeal motoneurons from 23 +/- 4.8 to 33 +/- 6.2. The relative increases in activity (to 220% +/- 24, 147% +/- 23 and 148% +/- 9 of control, respectively) were significantly higher in hypoglossal than in laryngeal motoneurons. In addition, the excitatory effect developed significantly faster in hypoglossal than in laryngeal motoneurons. Methysergide reduced the spontaneous activity of about half the hypoglossal and laryngeal motoneurons to 66% +/- 5 of control. Thus, the sensitivity to the excitatory effects of serotonin varies among different pools of upper airway motoneurons. These differences correlate with the pattern of airway muscle hypotonia seen during sleep.

Identification of rat brainstem multisynaptic connections to the oral motor nuclei using pseudorabies virus. III. Lingual muscle motor systems

The present experiments complete our investigations of higher order afferent control of the orofacial muscles by examining the premotor systems controlling the lingual musculature. Pseudorabies virus (PRV) was injected into the extrinsic (protruders: genioglossus and geniohyoid; retractors: hyoglossus and styloglossus) and intrinsic tongue muscles in bilaterally sympathectomized rats. Injection volumes ranged from 1 to 12 microl with average titers of 4 x 10(8) pfu/ml and maximum survival times of 90 h. Consistent labeling patterns and distributions occurred across each of the individual muscles and between extrinsic and intrinsic muscle groups, as well as in comparison to the results from the previous masticatory and facial muscle experiments. Virus injections produced a predictable myotopic labeling pattern in the hypoglossal nucleus (Mo 12). Transneuronally labeled neurons occurred in regions known to project directly to Mo 12 motoneurons including the nucleus subcoeruleus, trigeminal sensory areas, parvicellular reticular formation, and the dorsal medullary reticular fields. Maximum survival times revealed more distant connections from medial and lateral reticular zones including the periaqueductal gray, dorsal raphe, laterodorsal and pedunculopontine tegmental areas, and substantia nigra in the midbrain, the gigantocellular region, pontine nucleus caudalis and ventralis, and lateral paragigantocellular region in the pons, and the nucleus of the solitary tract, paratrigeminal region, and paramedian field in the medulla. Thus, injections of PRV into the orofacial muscles revealed a complex, but remarkably uniform network of multisynaptic connections in the brainstem that control and coordinate the activity of the masticatory, facial, and lingual muscles.

Projections of the dorsomedial nucleus of the intercollicular complex (DM) in relation to respiratory-vocal nuclei in the brainstem of pigeon (Columba livia) and zebra finch (Taeniopygia guttata)

Injections of neuronal tracers were made into the dorsomedial nucleus of the intercollicular complex (DM) of pigeons and zebra finches in order to investigate the projections of this nucleus which has long been implicated in respiratory-vocal control. Despite the fact that pigeons are nonsongbirds and zebra finches are songbirds, the projections were very similar in both species. Most descended throughout the brainstem, taking ventral and dorsal trajectories, which merged in the medulla. Those descending ventrally terminated upon the ventrolateral parabrachial nucleus (PBvl), the nucleus infraolivaris superior, a nucleus of the rostral ventrolateral medulla (RVL), and the nucleus retroambigualis (RAm). Those taking a dorsal trajectory via the occipitomesencephalic tract terminated in the tracheosyringeal part of the hypoglossal nucleus (XIIts), the suprahypoglossal region, and nucleus retroambigualis. There were also substantial projections throughout an arc extending between XIIts and RVL rostrally, and XIIts and RAm caudally. Neurons throughout this arc, which include inspiratory premotor neurons at levels straddling the obex and expiratory premotor neurons more caudally (in RAm), were retrogradely labeled from spinal injections. The DM projections were predominantly ipsilateral, but there were distinct contralateral projections to all the homologous nuclei in both species. All but the projections to PBvl and XIIts were reciprocal. In summary, the projections of DM suggest that it is able to influence all the key motor and premotor nuclei involved in patterned respiratory-vocal activity.

Compartmental organization of styloglossus and hyoglossus motoneurons in the hypoglossal nucleus of the rat

Surgical techniques were used to isolate the extrinsic bellies of the styloglossus and hyoglossus muscles from the body of the tongue for cholera toxin HRP injection. An average of 53 styloglossus and 121 hyoglossus motoneurons in the dorsal subdivision of the hypoglossal nucleus were demonstrated using tetramethyl benzidine histochemistry. Styloglossus motoneurons were restricted to the rostral 25% of the nucleus while hyoglossus motoneurons occupied other regions of the dorsal nucleus.

Axotomy induces a different modulation of both low-affinity nerve growth factor receptor and choline acetyltransferase between adult rat spinal and brainstem motoneurons

Adult rat spinal and brainstem motoneurons re-express low-affinity nerve growth factor receptor (p75) after their axotomy. We have previously reported and quantified the time course of this reexpression in spinal motoneurons following several types of injuries of the sciatic nerve. Other studies reported the reexpression of p75 in axotomized brainstem motoneurons. Results of these previous studies differed regarding the type of the most effective triggering injury for p75 reexpression, the relative duration of this reexpression and the decrease of choline acetyltransferase (ChAT) immunoreactivity (-IR) following a permanent axotomy of spinal or brainstem motoneurons. These differences suggest that these two populations of motoneurons respond to axotomy with a different modulation of p75 and ChAT expression. The aim of the present study was to determine whether differential modulation exists. We have analyzed and quantified the presence of p75- and ChAT-IR motoneurons in the hypoglossal nucleus following the same types of injury and the same time course we previously used for sciatic motoneurons. The results show that a nerve crush is the most effective triggering injury for p75 and that it induces similar temporal patterns of p75 and ChAT expression for sciatic and hypoglossal motoneurons. In contrast, a cut injury of the sciatic and hypoglossal nerves resulted in distinct temporal courses of both p75 and ChAT expression between these two populations of motoneurons. In fact, a permanent axotomy of the hypoglossal motoneurons induced i) a much longer maintenance phase for p75 than in sciatic motoneurons and ii) a progressive loss of ChAT-IR with a successive return to normal values in contrast to the modest decrease in the sciatic motoneurons. This evidence indicates that spinal and brainstem motoneurons respond to a permanent axotomy with a different modulation of p75 and ChAT expression. Altogether, the present data and the reported evidence of a differential post-axotomy cell death support the hypothesis that these two populations of motoneurons undergo different dynamic changes after axotomy.

Specificity of rabies virus as a transneuronal tracer of motor networks: transfer from hypoglossal motoneurons to connected second-order and higher order central nervous system cell groups

The specificity of transneuronal transfer of rabies virus [challenge virus standard (CVS) strain] was evaluated in a well-characterized neuronal network, i.e., retrograde infection of hypoglossal motoneurons and transneuronal transfer to connected (second-order) brainstem neurons. The distribution of the virus in the central nervous system was studied immunohistochemically at sequential intervals after unilateral inoculation into the hypoglossal nerve. The extent of transneuronal transfer of rabies virus was strictly time dependent and was distinguished in five stages. At 1 day postinoculation, labelling involved only hypoglossal motoneurons (stage 1). Retrograde transneuronal transfer occurred from 2.0-2.5 days postinoculation (stage 2). In stages 2-4, different groups of second-order neurons were labelled sequentially, depending on the strength of their input to the hypoglossal nucleus. In stages 4 and 5, labelling extended to several cortical and subcortical cell groups, which can be regarded as higher order because they are known to control tongue movements and/or to provide input to hypoglossal-projecting cell groups. The pattern of transneuronal transfer of rabies virus resembles that of alpha-herpesviruses with regard to the nonsynchronous labelling of different groups of second-order neurons and the transfer to higher order neurons. In striking contrast to alpha-herpesviruses, the transneuronal transfer of rabies is not accompanied by neuronal degeneration. Moreover, local spread of rabies from infected neurons and axons to adjoining glial cells, neurons, or fibers of passage does not occur. The results show that rabies virus is a very efficient transneuronal tracer. Results also provide a new insight into the organization of cortical and subcortical higher order neurons that mediate descending control of tongue movements indirectly via hypoglossal-projecting neurons.

Subcompartmental organization of the ventral (protrusor) compartment in the hypoglossal nucleus of the rat

The extent and myotopic organization of the ventral (protrusor) compartment of the hypoglossal nucleus (nXII) in the rat is controversial. Of particular concern is the location of motoneurons that innervate the intrinsic (verticalis, transversus) as compared to extrinsic (genioglossus) tongue protrusor muscles. These issues were investigated with retrograde transport, lesion/degeneration/immunocytochemical, and classic Golgi staining techniques. Results from these experiments demonstrate the following: (1) the ventral compartment extends the entire rostrocaudal length of nXII and is organized into three longitudinally oriented subcompartments, one medial and one lateral within the boundaries of nXII, and one outside the confines of nXII, defined as the lateral accessory subcompartment; 2) the medial and lateral subcompartments contain motoneurons that innervate the intrinsic (verticalis, transversus) and extrinsic (genioglossus) tongue protrusor muscles, respectively, while the lateral accessory subcompartment innervates the geniohyoid muscle; (3) ventral subcompartments are unequal in size and vary along the rostrocaudal dimension of nXII. The medial subcompartment is largest caudally and smallest rostrally, while the converse is true for the lateral subcompartment. By contrast, the lateral accessory subcompartment is present only along the caudal one-half of nXII; (4) medial and lateral subcompartments are further organized into smaller subgroups. Medial and centromedial subgroups are discernible within the medial subcompartment, lateral and centrolateral subgroups within the lateral subcompartment. Both medial and lateral subgroups extend throughout the rostrocaudal length of nXII, whereas the centromedial and centrolateral subgroups are present only along the middle two-thirds of nXII where they form a central motoneuron band; (5) there is an inverse myotopic organization within the medial and lateral subcompartments such that proximal and distal portions of intrinsic and extrinsic protrusor muscles receive innervation from rostral and caudal motoneurons, respectively; and (6) there is a correlation between motoneuron morphology (size, shape and dendritic field domains), subcompartment localization, and myotopic specificity. Motoneurons in the medial subcompartment are small (mean = 23.08 microns), round to globular, with dendrites oriented medially, dorsomedially, dorsolaterally, and caudally, whereas lateral subcompartment motoneurons are large (mean = 29.49 microns), round to triangular, with dendrites directed mainly mediolaterally and dorsally. These data are relevant to understanding the functional organization of nXII and the motor control of the tongue. Results are further discussed relative to the convergence of multifunctional afferent systems in the ventromedial subcompartment of nXII.