Transneuronal labeling in hamster brainstem following lingual injections with herpes simplex virus-1

Extraneural prion neuroinvasion without lymphoreticular system infection

While prion infection of the lymphoreticular system (LRS) is necessary for neuroinvasion in many prion diseases, in bovine spongiform encephalopathy and atypical cases of sheep scrapie there is evidence to challenge that LRS infection is required for neuroinvasion. Here we investigated the role of prion infection of LRS tissues in neuroinvasion following extraneural inoculation with the HY and DY strains of the transmissible mink encephalopathy (TME) agent. DY TME agent infectivity was not detected in spleen or lymph nodes following intraperitoneal inoculation and clinical disease was not observed following inoculation into the peritoneum or lymph nodes, or after oral ingestion. In contrast, inoculation of the HY TME agent by each of these peripheral routes resulted in replication in the spleen and lymph nodes and induced clinical disease. To clarify the role of the LRS in neuroinvasion, the HY and DY TME agents were also inoculated into the tongue because it is densely innervated and lesions on the tongue, which are common in ruminants, increase the susceptibility of hamsters to experimental prion disease. Following intratongue inoculation, the DY TME agent caused prion disease and was detected in both the tongue and brainstem nuclei that innervate the tongue, but the prion protein PrP(Sc) was not detected in the spleen or lymph nodes. These findings indicate that the DY TME agent can spread from the tongue to the brain along cranial nerves and neuroinvasion does not require agent replication in the LRS. These studies provide support for prion neuroinvasion from highly innervated peripheral tissues in the absence of LRS infection in natural prion diseases of livestock.

Activation of XII motoneurons and premotor neurons during various oropharyngeal behaviors

Neural control of tongue muscles plays a crucial role in a broad range of oropharyngeal behaviors. Tongue movements must be rapidly and accurately adjusted in response to the demands of multiple complex motor tasks including licking/mastication, swallowing, vocalization, breathing and protective reflexes such as coughing. Yet, central mechanisms responsible for motor and premotor control of hypoglossal (XII) activity during these behaviors are still largely unknown. The aim of this article is to review the functional organization of the XII motor nucleus with particular emphasis on breathing, coughing and swallowing. Anatomical localization of XII premotor neurons is also considered. We discuss results concerned with multifunctional activity of medullary and pontine populations of XII premotor neurons, representing a single network that can be reconfigured to produce different oromotor response patterns. In this context, we introduce new data on swallowing-related activity of XII (and trigeminal) motoneurons, and finally suggest a prominent role for the pontine Kölliker-Fuse nucleus in the control of inspiratory-related activity of XII motoneurons supplying tongue protrusor and retrusor muscles.

Synaptic remodeling in the nucleus ambiguus following vagal–hypoglossal nerve anastomosis in the cat

We reported recently the occurrence of a massive and selective elimination of synaptic boutons on motoneurons in the dorsal motor nucleus of the vagus (DMV) in the cat following vagal-hypoglossal nerve anastomosis (VHA) [J. Comp. Neurol. 458 (2003) 195]. This study was aimed to explore the synaptic reorganization in the other major nucleus associated with the vagus, namely, the nucleus ambiguus (NA) following the same treatment. In view of the tremendous difference in function, the NA and DMV are considered to be two ideal nuclei for explanatory studies seeking to elucidate how VHA could induce different plasticity of brainstem neurons influenced by the newly reestablished neural pathway. The present results showed that the vagal efferent neurons in the NA had responded to VHA in a different manner compared with those in the DMV. Firstly, the numbers of axon terminals containing round (R), round with dense-cored (R+D), pleomorphic (P) or flattened (F) synaptic vesicles contacting the NA motoneurons were markedly increased at 500-day postoperation, the longest reinnervation interval. The percent increases in the synapse frequency for R, R+D, P and F boutons were 8.6%, 274.4%, 238.3% and 400.0%, respectively. Secondly, the formation of astroglial ensheathment around the motoneurons in the DMV following VHA was not evident in the NA. Another striking difference was the extensive dendritic sprouting of the NA neurons as opposed to the dendritic retraction of the DMV neurons as shown by a significant increase in distal dendrites of NA motoneurons. The different modes of neural remodeling between NA and DMV may be attributed to the unique nature of the two nuclei to structures they normally supply and their different compatibility with the newly innervated target, viz. tongue skeletal musculature.

Effects of anesthetics on hypoglossal nerve discharge and c-Fos expression in brainstem hypoglossal premotor neurons

This study examined the effects of anesthesia on the hypoglossal nerve and diaphragm activities and on c-Fos expression in brainstem hypoglossal premotor neurons (pmXII). Experiments were performed in 71 rats by using halothane inhalation, pentobarbital sodium, or mixtures of alpha-chloralose and urethane or ketamine and xylazine. First, various cardiorespiratory parameters were measured in the rats (n = 31) during both awake and anesthetized conditions. The volatile anesthetic halothane, but not the other anesthetics, was always associated with a strong phasic inspiratory activity in the hypoglossal nerve. Second, a double-immunohistochemical study was performed in awake and anesthetized rats (n = 40) to gauge the level of activity of pmXII neurons. Brainstem pmXII neurons were identified after microiontophoresis of the retrograde tracer Fluoro-Gold in the right hypoglossal motor nucleus. Patterns of c-Fos expression at different brainstem levels were compared in five groups of rats (i.e., awake or anesthetized with halothane, pentobarbital, chloralose-urethane, and ketamine-xylazine). Sections were processed for double detection of c-Fos protein and Fluoro-Gold by using the standard ABC method and a two-color peroxidase technique. Anesthesia with halothane induced the strongest c-Fos expression in a restricted pool of pmXII located in the pons at the level of the Kölliker-Fuse nucleus and the intertrigeminal region. The results demonstrated a major effect of halothane in inducing changes in hypoglossal activity and revealed a differential expression of c-Fos protein in pmXII neurons among groups of anesthetized rats. We suggest that halothane mediates changes in respiratory hypoglossal nerve discharge by altering activity of premotor neurons in the Kölliker-Fuse and intertrigeminal region.

Identification of lingual motor control circuits using two strains of pseudorabies virus

First-order interneurons that project to hypoglossal motoneurons are distributed within reticular formation subdivisions in the pons and medulla in areas thought to control licking, swallowing, chewing, and respiration. Movement of the tongue in each of these functions is achieved by the coordinated action of both intrinsic and extrinsic lingual muscles. Interneuron populations that project to these different lingual motoneuronal pools appear to be largely overlapping in the reticular formation. Because of the functional coupling between intrinsic and extrinsic muscles during most tongue movements, one might predict that individual pre-hypoglossal interneurons project to multiple motoneuronal pools. To test this hypothesis, one strain of pseudorabies virus was injected into the styloglossus muscle (an extrinsic lingual muscle) and a second strain of pseudorabies virus was injected into the intrinsic lingual muscles of the anterior tongue in the same preparation. Rats were perfused with fixative 84-96 h later, and dual-labeling immunohistochemistry was performed to reveal populations of single- and double-labeled brainstem neurons. Motoneurons innervating the different lingual muscles were spatially segregated within the hypoglossal motor nucleus, and no double-labeled motoneurons were observed. In contrast, pre-hypoglossal neurons projecting to each lingual motoneuron pool were highly overlapping in the reticular formation, and many were double-labeled. These observations suggest that coactivation of lingual muscles can be achieved, at least in part, through divergent projections of first-order interneurons to anatomically and functionally distinct pools of lingual motoneurons in the hypoglossal nucleus.

Rapid prion neuroinvasion following tongue infection

Food-borne transmission of prions can lead to infection of the gastrointestinal tract and neuroinvasion via the splanchnic and vagus nerves. Here we report that the transmission of transmissible mink encephalopathy (TME) is 100,000-fold more efficient by inoculation of prions into the tongues of hamsters than by oral ingestion. The incubation period following TME agent (hereinafter referred to as TME) inoculation into the lingual muscles was the shortest among the five nonneuronal routes of inoculation, including another intramuscular route. Deposition of the abnormal isoform of the prion protein, PrP(Sc), was first detected in the tongue and submandibular lymph node at 1 to 2 weeks following inoculation of the tongue with TME. PrP(Sc) deposits in the tongue were associated with individual axons, and the initial appearance of TME in the brain stem was found in the hypoglossal nucleus at 2 weeks postinfection. At later time points, PrP(Sc) was localized to brain cell groups that directly project to the hypoglossal nucleus, indicating the transneuronal spread of TME. TME PrP(Sc) entry into the brain stem preceded PrP(Sc) detection in the rostral cervical spinal cord. These results demonstrate that TME can replicate in both the tongue and regional lymph nodes but indicate that the faster route of brain invasion is via retrograde axonal transport within the hypoglossal nerve to the hypoglossal nucleus. Topical application of TME to a superficial wound on the surface of the tongue resulted in a higher incidence of disease and a shorter incubation period than with oral TME ingestion. Therefore, abrasions of the tongue in livestock and humans may predispose a host to oral prion infection of the tongue-associated cranial nerves. In a related study, PrP(Sc) was detected in tongues following the intracerebral inoculation of six hamster-adapted prion strains, which demonstrates that prions can also travel from the brain to the tongue in the anterograde direction along the tongue-associated cranial nerves. These findings suggest that food products containing ruminant or cervid tongue may be a potential source of prion infection for humans.

Discharge patterns of hypoglossal motoneurons during fictive breathing, coughing, and swallowing

We performed a series of experiments to study the intracellular activity of 58 hypoglossal motoneurons (HMs) in decerebrate, paralyzed, and ventilated cats. Changes in membrane potentials (MP) and discharge activities were evaluated during fictive breathing (FB), swallowing (FS), and coughing (FC). FS and FC were elicited by electrical stimulation of the superior laryngeal nerves. FB, FS, and FC all exhibited characteristic discharge patterns of the phrenic, abdominal, pharyngeal branch of the vagus, and hypoglossal nerves. Thirty-nine HMs displayed respiratory modulation, and 19 were nonrespiratory modulated. Nine HMs did not exhibit MP changes during FB, FS, and FC. During FS, 49 HMs exhibited MP changes consisting of depolarization, hyperpolarization or hyperpolarization-depolarization. HMs involved in FS were either respiratory modulated (n = 38) or not (n = 11). Only 20 HMs displayed MP changes and/or discharge activity during FC. All but two HMs fired during the expiratory phase of FC or at the end of this reflex. All HMs involved in FC (n = 20) were also modulated during both FB and FS. Our results suggest that the XII nucleus is functionally divided into common and distinct subsets of HMs based on their spontaneous activities and responses observed during FS and FC. The changes in MP and discharge frequencies observed during the three behaviors also suggest that HMs are driven by specific premotor neurons during FS, whereas a common premotor pathway is involved during FB and FC.

CNS innervation of posterior cricoarytenoid muscles: a transneuronal labeling study

The CNS cell groups that project to neurons, which innervate the posterior cricoarytenoid muscles (PCA), were identified by the viral retrograde transneuronal labeling method. Pseudorabies virus (PRV) was injected into the PCA of C8 spinal rats and after 5 days survival, brain tissue sections were processed for immunohistochemical detection of PRV. Retrogradely labeled motor neurons innervating the PCA were seen in the nucleus ambiguus and in the area ventral to it. Neurons innervating the PCA motoneurons were found throughout the ventral aspect of the medulla oblongata, in the nucleus tractus solitarius, and in the pons. Labeling was present in the midbrain periaquaductal gray, in the lateral and paraventricular hypothalamic nuclei, in the amygdaloid complex, in the hippocampus, and within the piriform cortex. In summary, the motor neurons that control PCA activity are innervated predominantly by a network of neurons that lie along the neuraxis, in the regions known to be involved in regulation of respiratory output and autonomic functions.

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.

Jaw-tongue reflex: afferents, central pathways, and synaptic potentials in hypoglossal motoneurons in the cat

The tongue position is reflexively controlled by the jaw position (the jaw-tongue reflex). The purpose of this study was to clarify the mechanism of this reflex in terms of afferents, central pathways, and synaptic potentials in hypoglossal motoneurons in the cat. Intracellular recordings from hypoglossal motoneurons revealed that electrical stimulation of the temporalis muscle nerve evoked excitatory and inhibitory post-synaptic potentials in hypoglossal motoneurons. The threshold of temporalis muscle nerve stimulation for evoking the synaptic potentials was higher than 2.0 times the nerve threshold. The amplitude of the potentials increased with stimulus intensity up to 5.0 times the nerve threshold. Punctate light pressure applied to the temporalis muscle induced a tonic depolarizing potential in hypoglossal motoneurons on which action potentials as well as depolarizing synaptic activation noise were superimposed. On the other hand, electrical stimulation of the temporalis muscle during jaw-opening could slightly inhibit the electromyographic activities in the genioglossus and styloglossus muscles. Lesions including the Probst's tract at the level caudal to the trigeminal motor nucleus abolished both excitation and inhibition in hypoglossal motoneurons induced by tonic depression of the lower jaw, but exerted no effects on either the tonic stretch reflex or the trigemino-hypoglossal reflex. In contrast, lesions including the trigeminal spinal tract produced no changes in either excitation or inhibition of hypoglossal motoneurons induced by temporalis muscle afferents, whereas the excitation of hypoglossal motoneurons was abolished by the lesions. We conclude that the group II muscle spindle afferents from the temporalis muscle are primarily responsible for evoking the jaw-tongue reflex.

Experimental muscle pain produces central modulation of proprioceptive signals arising from jaw muscle spindles

The aim of the present study was to investigate the effects of intramuscular injection with hypertonic saline, a well-established experimental model for muscle pain, on central processing of proprioceptive input from jaw muscle spindle afferents. Fifty-seven cells were recorded from the medial edge of the subnucleus interpolaris (Vi) and the adjacent parvicellular reticular formation from 11 adult cats. These cells were characterized as central units receiving jaw muscle spindle input based on their responses to electrical stimulation of the masseter nerve, muscle palpation and jaw stretch. Forty-five cells, which were successfully tested with 5% hypertonic saline, were categorized as either dynamic-static (DS) (n=25) or static (S) (n=20) neurons based on their responses to different speeds and amplitudes of jaw movement. Seventy-six percent of the cells tested with an ipsilateral injection of hypertonic saline showed a significant modulation of mean firing rates (MFRs) during opening and/or holding phases. The most remarkable saline-induced change was a significant reduction of MFR during the hold phase in S units (100%, 18/18 modulated). Sixty-nine percent of the DS units (11/16 modulated) also showed significant changes in MFRs limited to the hold phase. However, in the DS neurons, the MFRs increased in seven units and decreased in four units. Finally, five DS neurons showed significant changes of MFRs during both opening and holding phases. Injections of isotonic saline into the ipsilateral masseter muscle had little effect, but hypertonic saline injections made into the contralateral masseter muscle produced similar results to ipsilateral injections with hypertonic saline. These results unequivocally demonstrate that intramuscular injection with an algesic substance, sufficient to produce muscle pain, produces significant changes in the proprioceptive properties of the jaw movement-related neurons. Potential mechanisms involved in saline-induced changes in the proprioceptive signals and functional implications of the changes are discussed.

Fos-like immunoreactivity in the brain stem following oral quinine stimulation in decerebrate rats

The present study compared the distribution of Fos-like immunoreactivity (FLI) following intraoral stimulation with quinine monohydrochloride (QHCl) in awake intact rats to the pattern obtained in chronic supracollicular decerebrate (CD) rats. Because the behavioral rejection response to QHCl is evident in the CD rat, it was hypothesized that the pattern of FLI in the lower brain stem should be similar in both groups. Overall, the distribution of FLI in the brain stem was quite similar in both intact and CD groups, and QHCl stimulation increased FLI in the rostral (gustatory) nucleus of the solitary tract, the parabrachial nucleus (PBN), and the lateral reticular formation (RF) compared with an unstimulated control group. The CD group differed from the intact group, however, with a trend toward less FLI in the RF and a shift in the pattern of label away from the external subdivision of the PBN. CD rats also had increased FLI in the caudal nucleus of the solitary tract, with or without intraoral infusions. The distribution of QHCl-induced FLI in the brain stem of intact rats thus indicates both local sensorimotor processing as well as the influence of forebrain structures.

Herpes simplex virus as a transneuronal tracer

Determining the connections of neural systems is critical for determining how they function. In this review, we focus on the use of HSV-1 and HSV-2 as transneuronal tracers. Using HSV to examine neural circuits is technically simple. HSV is injected into the area of interest, and after several days, the animals are perfused and processed for immunohistochemistry with antibodies to HSV proteins. Variables which influence HSV infection include species of host, age of host, titre of virus, strain of virus and phenotype of infected cell. The choice of strain of HSV is critically important. Several strains of HSV-1 and HSV-2 have been utilized for purposes of transneuronal tract-tracing. HSV has been used successfully to study neuronal circuitry in a variety of different neuroanatomical systems including the somatosensory, olfactory, visual, motor, autonomic and limbic systems.

Distribution of fos-like immunoreactivity in the medullary reticular formation of the rat after gustatory elicited ingestion and rejection behaviors

The distribution of neurons in the medullary reticular formation (RF) activated by the ingestion of sucrose or rejection of quinine was examined using standard immunohistochemical techniques to detect the expression of the Fos protein product of the immediate-early gene c-fos. Double-labeling techniques were used to gain further insight into the possible functional significance of RF neurons exhibiting Fos-like immunoreactivity (FLI). Compared with sucrose and unstimulated controls, quinine elicited significantly more FLI neurons in three specific RF subdivisions: parvocellular reticular nucleus (PCRt), intermediate reticular nucleus (IRt), and dorsal medullary reticular nucleus (MdD). Moreover, the number of FLI neurons in the RF of quinine-stimulated animals was significantly correlated with the degree of oromotor activity. Thus, the distinct distribution of FLI neurons throughout the RF after quinine may reflect the activation of a specific oral rejection circuit. The double-labeling results indicated a high degree of segregation between FLI neurons and premotor projection neurons to the hypoglossal nucleus (mXII) retrogradely labeled with Fluorogold. Thus, although there were a significant number of double-labeled neurons in the RF, the major concentration of premotor projection neurons to mXII in IRt were medial to the preponderance of FLI neurons in the PCRt. In contrast, there was substantial overlap between FLI neurons in the RF and labeled fibers after injections of the anterograde tracer, biotinylated dextran into the rostral (gustatory) portion of the nucleus of the solitary tract. These results support a medial (premotor)/lateral (sensory) functional topography of the medullary RF.