Tensor tympani reflex pathways studied with retrograde horseradish peroxidase and transneuronal viral tracing techniques

An Integrative Model Accounting for the Symptom Cluster Triggered After an Acoustic Shock

Acoustic shocks and traumas sometimes result in a cluster of debilitating symptoms, including tinnitus, hyperacusis, ear fullness and tension, dizziness, and pain in and outside the ear. The mechanisms underlying this large variety of symptoms remain elusive. In this article, we elaborate on the hypothesis that the tensor tympani muscle (TTM), the trigeminal nerve (TGN), and the trigeminal cervical complex (TCC) play a central role in generating these symptoms. We argue that TTM overuse (due to the acoustic shock), TTM overload (due to muscle tension), and ultimately, TTM injury (due to hypoxia and "energy crisis") lead to inflammation, thereby activating the TGN, TCC, and cortex. The TCC is a crossroad structure integrating sensory inputs coming from the head-neck complex (including the middle ear) and projecting back to it. The multimodal integration of the TCC may then account for referred pain outside the ear when the middle ear is inflamed and activates the TGN. We believe that our model proposes a synthetic and explanatory framework to explain the phenomena occurring postacoustic shock and potentially also after other nonauditory causes. Indeed, due to the bidirectional properties of the TCC, musculoskeletal disorders in the region of the head-neck complex, including neck injury due to whiplash or temporomandibular disorders, may impact the middle ear, thereby leading to otic symptoms. This previously unavailable model type is experimentally testable and must be taken as a starting point for identifying the mechanisms responsible for this particular subtype of tinnitus and its associated symptoms.

Ultrastructure of spines and associated terminals on brainstem neurons controlling auditory input

Spines are unique cellular appendages that isolate synaptic input to neurons and play a role in synaptic plasticity. Using the electron microscope, we studied spines and their associated synaptic terminals on three groups of brainstem neurons: tensor tympani motoneurons, stapedius motoneurons, and medial olivocochlear neurons, all of which exert reflexive control of processes in the auditory periphery. These spines are generally simple in shape; they are infrequent and found on the somata as well as the dendrites. Spines do not differ in volume among the three groups of neurons. In all cases, the spines are associated with a synaptic terminal that engulfs the spine rather than abuts its head. The positions of the synapses are variable, and some are found at a distance from the spine, suggesting that the isolation of synaptic input is of diminished importance for these spines. Each group of neurons receives three common types of synaptic terminals. The type of terminal associated with spines of the motoneurons contains pleomorphic vesicles, whereas the type associated with spines of olivocochlear neurons contains large round vesicles. Thus, spine-associated terminals in the motoneurons appear to be associated with inhibitory processes but in olivocochlear neurons they are associated with excitatory processes.

Tensor tympani motoneurons receive mostly excitatory synaptic inputs

The tensor tympani is a middle ear muscle that contracts in two different situations: in response to sound or during voluntary movements. To gain insight into the inputs and neural regulation of the tensor tympani, we examined the ultrastructure of synaptic terminals on labeled tensor tympani motoneurons (TTMNs) using transmission electron microscopy. Our sample of six TTMNs received 79 synaptic terminals that formed 126 synpases. Two types of synapses are associated with round vesicles and form asymmetric junctions (excitatory morphology). One of these types has vesicles that are large and round (Lg Rnd) and the other has vesicles that are smaller and round (Sm Rnd) and also contains at least one dense core vesicle. A third synapse type has inhibitory morphology because it forms symmetric synapses with pleomorphic vesicles (Pleo). These synaptic terminals can be associated with TTMN spines. Two other types of synapse are found on TTMNs but they are uncommon. Synaptic terminals of all types form multiple synapses but those from a single terminal are always the same type. Terminals with Lg Rnd vesicles formed the most synpases per terminal (avg. 2.73). Together, the synaptic terminals with Lg Rnd and Sm Rnd vesicles account for 62% of the terminals on TTMNs, and they likely represent the pathways driving the contractions in response to sound or during voluntary movements. Having a high proportion of excitatory inputs, the TTMN innervation is like that of stapedius motoneurons but proportionately different from other types of motoneurons.

Auditory brainstem circuits that mediate the middle ear muscle reflex

The middle ear muscle (MEM) reflex is one of two major descending systems to the auditory periphery. There are two middle ear muscles (MEMs): the stapedius and the tensor tympani. In man, the stapedius contracts in response to intense low frequency acoustic stimuli, exerting forces perpendicular to the stapes superstructure, increasing middle ear impedance and attenuating the intensity of sound energy reaching the inner ear (cochlea). The tensor tympani is believed to contract in response to self-generated noise (chewing, swallowing) and non-auditory stimuli. The MEM reflex pathways begin with sound presented to the ear. Transduction of sound occurs in the cochlea, resulting in an action potential that is transmitted along the auditory nerve to the cochlear nucleus in the brainstem (the first relay station for all ascending sound information originating in the ear). Unknown interneurons in the ventral cochlear nucleus project either directly or indirectly to MEM motoneurons located elsewhere in the brainstem. Motoneurons provide efferent innervation to the MEMs. Although the ascending and descending limbs of these reflex pathways have been well characterized, the identity of the reflex interneurons is not known, as are the source of modulatory inputs to these pathways. The aim of this article is to (a) provide an overview of MEM reflex anatomy and physiology, (b) present new data on MEM reflex anatomy and physiology from our laboratory and others, and (c) describe the clinical implications of our research.

A morphologic study of Fluorogold labeled tensor tympani motoneurons in mice

The tensor tympani is one of two middle ear muscles that regulates the transmission of sound through the middle ear. Contraction of the tensor tympani in response to both auditory and non-auditory stimulation is mediated by the tensor tympani motoneurons (TTMNs). There are interesting differences among species in the acoustic thresholds for contraction of the middle ear muscles, which may be a reflection of underlying anatomical differences such as the number of TTMNs. However anatomical data for mice are lacking, even though the mouse is becoming the most common animal model for auditory and neuroscience research. We investigated the number and morphology of TTMNs in mice using Fluorogold, a retrograde neuronal tracer. After injections of Fluorogold into the tensor tympani muscle, a column of labeled TTMNs was identified ventro-lateral to the ipsilateral trigeminal nucleus. The labeled TTMNs were classified according to their morphological characteristics into three subtypes: "octopus-like", "fusiform" and "stellate", suggesting underlying differences in function. All three subtypes formed sparsely branched and radiating dendrites, some longer than 600 microm. Dendrites were longest and most numerous in the dorso-medial direction. In 18 cases, the mean number of mouse TTMNs was 51; the largest numbers were 70, 74 and 90 (n=3 injections). The mean size of mouse TTMNs was 13.0 microm (minor axis) and 23.5 microm (major axis). Compared with studies of TTMNs in larger species (cats and rats), mouse TTMNs are both fewer in number and smaller in size.

Neurochemistry of identified motoneurons of the tensor tympani muscle in rat middle ear

The objective of the present study was to identify efferent and afferent transmitters of motoneurons of the tensor tympani muscle (MoTTM) to gain more insight into the neuronal regulation of the muscle. To identify MoTTM, we injected the fluorescent neuronal tracer Fluoro-Gold (FG) into the muscle after preparation of the middle ear in adult rats. Upon terminal uptake and retrograde neuronal transport, we observed FG in neurons located lateral and ventrolateral to the motor trigeminal nucleus ipsilateral to the injection site. Immunohistochemical studies of these motoneurons showed that apparently all contained choline acetyltransferase, demonstrating their motoneuronal character. Different portions of these cell bodies were immunoreactive to bombesin (33%), cholecystokinin (37%), endorphin (100%), leu-enkephalin (25%) or neuronal nitric oxide synthase (32%). MoTTM containing calcitonin gene-related peptide, tyrosine hydroxylase, substance P, neuropeptide Y or serotonin were not found. While calcitonin gene-related peptide was not detected in the region under study, nerve fibers immunoreactive to tyrosine hydroxylase, substance P, neuropeptide Y or serotonin were observed in close spatial relationship to MoTTM, suggesting that these neurons are under aminergic and neuropeptidergic influence. Our results demonstrating the neurochemistry of motoneuron input and output of the rat tensor tympany muscle may prove useful also for the general understanding of motoneuron function and regulation.

Neurons in the cochlear nuclei controlling the tensor tympani muscle in the rat: a study using pseudorabies virus

The middle ear muscle reflex has been implicated in modulation of auditory input and protection of the inner ear from acoustic trauma. However, the identification of neurons in the cochlear nuclei participating in this reflex has not been fully elucidated. In the present study, we injected the retrograde transynaptic tracer pseudorabies virus into single tensor tympani (TT) muscles, and identified transynaptically labeled cochlear nucleus neurons at multiple survival times. Motoneurons controlling TT were located ventral to the ipsilateral motor trigeminal nucleus and extended rostrally towards the medial aspect of the lateral lemniscus. Transynaptically labeled neurons were observed bilaterally in the dorsal and dorso-medial parts of ventral cochlear nuclei as early as 48 h after virus injection, and had morphological features of radiate multipolar cells. After >or=69 h, labeled cells of different types were observed in all cochlear nuclei. At those times, labeling was also detected bilaterally in the medial nucleus of the trapezoid body and periolivary cell groups in the superior olivary complex. Based on the temporal course of viral replication, our data strongly suggest the presence of a direct projection of neurons from the ventral cochlear nuclei bilaterally to the TT motoneuron pool in rats. The influence of neurons in the cochlear nuclei upon TT activity through direct and indirect pathways may account for multifunctional roles of this muscle in auditory functions.

Audio-vocal interaction in the pontine brainstem during self-initiated vocalization in the squirrel monkey

The adjustment of the voice by auditory input happens at several brain levels. The caudal pontine brainstem, though rarely investigated, is one candidate area for such audio-vocal integration. We recorded neuronal activity in this area in awake, behaving squirrel monkeys (Saimiri sciureus) during vocal communication, using telemetric single-unit recording techniques. We found audio-vocal neurons at locations not described before, namely in the periolivary region of the superior olivary complex and the adjacent pontine reticular formation. They showed various responses to external sounds (noise bursts) and activity increases (excitation) or decreases (inhibition) to self-produced vocalizations, starting prior to vocal onset and continuing through vocalizations. In most of them, the responses to noise bursts and self-produced vocalizations were similar, with the only difference that neuronal activity started prior to vocal onset. About one-third responded phasically to noise bursts, independent of whether they increased or decreased their activity to vocalization. The activity of most audio-vocal neurons correlated with basic acoustic features of the vocalization, such as call duration and/or syllable structure. Auditory neurons near audio-vocal neurons showed significantly more frequent phasic response patterns than those in areas without audio-vocal activity. Based on these findings, we propose that audio-vocal neurons showing similar activity to external acoustical stimuli and vocalization play a role in olivocochlear regulation. Specifically, audio-vocal neurons with a phasic response to external auditory stimuli are candidates for the mediation of basal audio-vocal reflexes such as the Lombard reflex. Thus, our findings suggest that complex audio-vocal integration mechanisms exist in the ventrolateral pontine brainstem.

Central auditory pathways mediating the rat middle ear muscle reflexes

The middle ear muscle (MEM) reflexes function to protect the inner ear from intense acoustic stimuli and to reduce acoustic masking. Sound presented to the same side or to the opposite side activates the MEM reflex on both sides. The ascending limbs of these pathways must be the auditory nerve fibers originating in the cochlea and terminating in the cochlear nucleus, the first relay station for all ascending auditory information. The descending limbs project from the motoneurons in the brainstem to the MEMs on both sides, causing their contraction. Although the ascending and descending pathways are well described, the cochlear nucleus interneurons that mediate these reflex pathways have not been identified. In order to localize the MEM reflex interneurons, we developed a physiologically based reflex assay in the rat that can be used to determine the integrity of the reflex pathways after experimental manipulations. This assay monitored the change in tone levels and distortion product otoacoustic emissions within the ear canal in one ear during the presentation of a reflex-eliciting sound stimulus in the contralateral ear. Preliminary findings using surgical transection and focal lesioning of the auditory brainstem to interrupt the MEM reflexes suggest that MEM reflex interneurons are located in the ventral cochlear nucleus.

Responses of medial olivocochlear neurons. Specifying the central pathways of the medial olivocochlear reflex

Medial olivocochlear (MOC) neurons project to outer hair cells (OHC), forming the efferent arm of a reflex that affects sound processing and offers protection from acoustic overstimulation. The central pathways that trigger the MOC reflex in response to sound are poorly understood. Insight into these pathways can be obtained by examining the responses of single MOC neurons recorded from anesthetized guinea pigs. Response latencies of MOC neurons are as short as 5 ms. This latency is consistent with the idea that type I, but not type II, auditory-nerve fibers provide the major inputs to the reflex interneurons in the cochlear nucleus. This short latency also implies that the cochlear-nucleus interneurons have rapidly conducting axons. In the cochlear nucleus, lesions of the posteroventral subdivision (PVCN), but not the anteroventral (AVCN) or dorsal (DCN) subdivisions, produce permanent disruption of the MOC reflex, based on a metric of adaptation of the distortion-product otoacoustic emission (DPOAE). This finding supports earlier anatomical results demonstrating that some PVCN neurons project to MOC neurons. Within the PVCN, there are two general types of units when classified according to poststimulus time histograms: onset units and chopper units. The MOC response is sustained and cannot be produced solely by inputs having an onset pattern. The MOC reflex interneurons are thus likely to be chopper units of PVCN. Also supporting this conclusion, chopper units and MOC neurons both have sharp frequency tuning. Thus, the most likely pathway for the sound-evoked MOC reflex begins with the responses of hair cells, proceeds with type I auditory-nerve fibers, PVCN chopper units, and MOC neurons, and ends with the MOC terminations on OHC.

Afferent projections of the superior olivary complex

Based on current literature, the afferents of the superior olivary complex (SOC) are described including those from the cochlear nucleus, inferior colliculus, thalamus, and auditory cortex. Intrinsic SOC afferents and non-auditory afferents from the serotoninergic and noradrenergic systems are also described. New data are provided that show a differential distribution of serotoninergic afferents within the SOC: serotoninergic fibers were relatively sparse in the lateral and medial superior olives and the medial nucleus of the trapezoid body and were most numerous in periolivary regions. There are variations in the density of serotoninergic fibers within periolivary regions themselves. New data is also provided on auditory and non-auditory afferents to SOC neurons, which have known targets. These include: cochlear nucleus afferents to periolivary (lateral nucleus of the trapezoid body, LNTB) cells that project to the inferior colliculus; cortical afferents to periolivary (ventral nucleus of the trapezoid body, VNTB) cells that project to the cochlear nucleus; and serotoninergic and noradrenergic afferents to periolivary (LNTB and VNTB) cells that project to the cochlear nucleus. The relationships between other types of afferents and SOC neurons with known projections are also described as functional circuits. The circuits include those that are part of the ascending auditory system (to the inferior and superior colliculi, lateral lemniscus, and medial geniculate nucleus), the descending auditory system (to the cochlea and cochlear nucleus), and the middle ear reflex circuits.

Retinal projections in mice with inherited retinal degeneration: implications for circadian photoentrainment

The availability of naturally occurring and transgenic retinal mutants has made the mouse an attractive experimental model to address questions regarding photoentrainment of circadian rhythms. However, very little is known about the retinal cells and the retinal projections to the nuclei of the murine circadian timing system. Furthermore, the effect of inherited retinal degeneration on these projections is not understood. In this report, we have used pseudorabies virus as a neuroanatomical tract tracer in mice to address a series of questions: Which retinal cells mediate circadian responses to light? What is the nature of the retinohypothalamic projection? What is the impact of the inherited retinal disorder, retinal degenerate (rd/rd), on the structures of the photoentrainment pathway? Our results show that a class ofretinal ganglion cell, morphologically similar to the type III ganglion cells of the rat, appears to project to central circadian structures of the mouse. They are few in number and sparsely distributed throughout the retina. The low number and broad distribution of these specialized retinal ganglion cells may be an adaptive mechanism to integrate environmental irradiance without compromising the spatial resolution required for vision. In addition, viral infection of conelike and rodlike photoreceptors and amacrinelike cells suggest that these cells may mediate or contribute to circadian responses to light. Inherited retinal degeneration has no obvious effect on the anatomy of the retinal cells or their projections to the circadian axis. These anatomical findings are consistent with our previous findings showing that aged rd/rd mice are capable of regulating their circadian rhythms by light with unattenuated sensitivity.

Serotoninergic innervation of stapedial and tensor tympani motoneurons

Retrograde tracing and neurotransmitter immunohistochemistry were combined to determine whether serotonin neurons innervated stapedial and tensor tympani motoneurons. With high-power light microscopy, putative axo-somatic and axo-dendritic contacts were observed between serotonin-positive endings and both stapedial and tensor tympani motoneurons, indicating that serotonin neurons terminate on brainstem motoneurons innervating the middle-ear muscles. With this connection, the serotonin system may directly modulate middle-ear muscle activity.

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

Oromotor behavior results from the complex interaction between jaw, facial, and lingual muscles. The experiments in this and subsequent papers identify the sources of multisynaptic input to the trigeminal, facial, and hypoglossal motor nuclei. In the current experiments, pseudorabies virus (PRV-Ba) was injected into the jaw-opening (anterior digastric and mylohyoid) and jaw-closing muscles (masseter, medial pterygoid, and temporalis) in bilaterally sympathectomized rats. Injection volumes ranged from 2 to 21 microl with average titers of 2.8 x 10(8) pfu/ml and maximum survival times of 96 h. The labeling patterns and distributions were consistent between each of the individual muscles and muscle groups. A predictable myotopic labeling pattern was produced in the trigeminal motor nucleus (Mo 5). Transneuronally labeled neurons occurred in regions known to project directly to Mo 5 motoneurons including the principal trigeminal sensory and supratrigeminal areas, Kölliker-Fuse region, nucleus subcoeruleus, and the parvicellular reticular formation. Maximum survival times revealed polysynaptic connections from the periaqueductal gray, laterodorsal and pedunculopontine tegmental areas, and the substantia nigra in the midbrain, ventromedial pontine reticular regions including the gigantocellular region and pars alpha and ventralis in the pons and medulla, and the nucleus of the solitary tract, paratrigeminal region, and paramedian field in the medulla. Thus, the results define the structure of the multisynaptic brainstem neural circuits controlling mandibular movement in the rat.

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.

Transneuronal labeling of a nociceptive pathway, the spino-(trigemino-)parabrachio-amygdaloid, in the rat

Transneuronal tracing of a nociceptive pathway, the spino-(trigemino)-parabrachio-amygdaloid pathway, was performed using an alpha-herpes virus, the Bartha strain of pseudorabies virus (PRV). Microinjection of PRV into the central nucleus of the amygdala (Ce) resulted in progressive retrograde and transneuronal infection of a multisynaptic circuit involving neurons in the brainstem and spinal cord as detected immunocytochemically. At short survival (26 hr), retrogradely labeled neurons were concentrated in the external lateral nucleus of the parabrachial complex (elPB) but were absent from both the trigeminal nucleus caudalis (TNC) and the spinal cord. At longer survivals (52 hr), labeled cells were present in lamina I of both the TNC and spinal dorsal horn. Retrograde labeling from the Ce with Fluoro-gold demonstrated that elPB neurons have long dendrites extending laterally into the terminal field of spinal and trigeminal afferents, where transneuronal passage of PRV to these afferents could occur. Even longer survivals (76 hr) resulted in a columnar pattern of cell labeling in the TNC and spinal dorsal horn that extended from lamina I into lamina II. At this longest survival, primary sensory neurons became infected. Bilateral excitotoxic lesions of the elPB blocked almost all viral passage from the Ce to superficial laminae of the TNC and spinal dorsal horn. These results demonstrate that nociceptive input to the amygdala is relayed from neurons in lamina I through the elPB. We propose that this modular arrangement of lamina I and II neurons may provide the basis for spinal processing of peripheral input to the amygdala.

Interneurons presynaptic to rat tail-flick motoneurons as mapped by transneuronal transport of pseudorabies virus: few have long ascending collaterals

The method of transneuronal retrograde transport of the Bartha strain of the swine alpha-herpes virus, pseudorabies virus, was used to identify putative interneurons presynaptic to motoneurons that supply a tail-flick muscle in the rat. We also investigated whether these interneurons also contribute to ascending somatosensory pathways. Two to five days after injection of pseudorabies virus into the left abductor caudae dorsalis muscle, and cholera toxin B into the right somatosensory thalamus and midbrain, rats were perfused and spinal cord sections processed immunohistochemically in a two-step procedure to stain cholera toxin B-immunoreactive cells black and pseudorabies virus-immunoreactive cells brown. At short (two-day) survivals, the first spinal neurons to be pseudorabies virus-immunoreactive were in the ipsilateral abductor caudae dorsalis motoneuron pool (S3-S4) and intermediolateral cell column (T12-L2), with a few (0 to five/section) bilaterally in the intermediate zone and around the central canal (all lumbosacral levels). With longer (three- to four-day) survival, more cells were noted (20-50/section) bilaterally (ipsilateral preponderance) in the dorsal and ventral horns of the lumbosacral cord. Many were in lamina I (marginal layer), while few were in lamina II (substantia gelatinosa). At four- and five-day survivals, the numbers of cells increased (20 to 100/section) bilaterally and now included lamina II. The fact that unilateral rhizotomy at L4-Co1 failed to change the distribution of spinal pseudorabies virus labeling suggests that the labeling was due to retrograde transport via the ventral root. In support, bilateral removal of the lumbar sympathetic ganglia, which receive their preganglionic innervation through the ventral root, reduced pseudorabies virus immunoreactivity throughout the thoracic and rostral lumbar spinal cord. These data indicate that there are (i) direct projections from intermediate and dorsal horn cells to abductor caudae dorsalis motoneurons, and (ii) disynaptic connections from dorsal horn (possibly including lamina II) cells to more ventral last-order interneurons. We also suggest that some lamina II cells are presynaptic to lamina I cells that project directly to abductor caudae dorsalis motoneurons. We observed cholera toxin B-immunoreactive cells (five to 20/section) in the expected locations (contralateral lamina I, deep dorsal horn and intermediate zone; lateral spinal nucleus bilaterally). Double-labeled (i.e. pseudorabies virus- and cholera toxin B-immunoreactive) neurons were only occasionally seen in the lateral spinal nucleus and were absent in the spinal gray matter, indicating that segmental interneurons do not collateralize in long ascending sensory pathways to the midbrain and somatosensory thalamus.

Complementation analysis of pseudorabies virus gE and gI mutants in retinal ganglion cell neurotropism

Pseudorabies virus glycoproteins gE and gI are required to infect some, but not all, regions of the rodent central nervous system after peripheral injection. After infection of the retina, pseudorabies virus mutants lacking either gE or gI can subsequently infect neural centers involved in the control of circadian function but cannot infect visual circuits mediating visual perception or the reflex movement of the eyes. In this study, we used genetic complementation to test the hypothesis that gE and gI are required for entry into the specific retinal ganglion cells that project to visual centers. These data strongly suggest that gE and gI must function after the viruses enter primary neurons in the retina.

Specificity of pseudorabies virus as a retrograde marker of sympathetic preganglionic neurons: implications for transneuronal labeling studies

The purpose of the present study was to examine the specificity of the Bartha strain of pseudorabies virus (PRV) as a CNS retrograde marker. This information is critical in assessing whether this virus has potential value as a specific transneuronal marker. The model system chosen for analysis was the intermediolateral cell column (IML)--the principal site of origin of sympathetic preganglionic neurons (SPNs). Two experiments were performed. The first experiment established the usefulness of this model system and the second examined the properties of PRV as a retrograde cell body marker. In the first experiment, injections of two different conventional retrograde cell body markers (cholera toxin-beta subunit (CTb) and Fluoro-Gold) were made in two ipsilateral sympathetic structures (viz., stellate ganglion and adrenal gland) in the same rat. This experiment established that (1) heterogenous SPNs originate in the same cell clusters that form the IML at the T4-T8 levels and 2) SPNs innervate specific sympathetic targets with almost none providing a dual innervation of the stellate ganglion and adrenal gland. This mosaic arrangement of target-specific SPNs makes the IML an excellent CNS site for this type of study. The second experiment followed the same paradigm: PRV was injected into the stellate ganglion and CTb into the adrenal gland (and vice versa). These experiments established that PRV infections of one functional class of SPNs did not produce infections in nearby, functionally unrelated SPNs and did not cause a reduction in the SPN cell population, except under conditions of severe gliosis. These two properties increase the probability that Bartha PRV may be used as a specific retrograde transneuronal marker of central autonomic pathways.

Specific pseudorabies virus infection of the rat visual system requires both gI and gp63 glycoproteins

Transneuronal transport of pseudorabies virus (PRV) from the retina to visual centers that mediate visual discrimination and reflexes requires specific genes in the unique short region of the PRV genome. In contrast, these same viral genes are not required to infect retinorecipient areas of the brain involved in circadian rhythm regulation. In this report, we demonstrate that viral mutants carrying defined deletions of the genes encoding glycoprotein gI or gp63, or both, result in the same dramatic transport defect. Efficient export of either gI or gp63 from the endoplasmic reticulum to the Golgi apparatus in a fibroblast cell line requires the presence of both proteins. We also show that gI and gp63 physically interact, as demonstrated by pulse-chase and sucrose gradient sedimentation experiments. Complex formation is rapid compared with homodimerization of PRV glycoprotein gII. We suggest that gI and gp63 function in concert to affect neurotropism in the rat visual circuitry and that a heterodimer is likely to be the unit of function.

Cholera toxin B-HRP and wheat germ agglutinin-HRP tracing of tensor tympani muscle motoneurons and processes in rabbits

The brain stem position, organization and number of motoneurons innervating the rabbit tensor tympani muscle (TTM) were determined by retrograde axonal transport of cholera toxin B/horseradish peroxidase conjugate (CTB-HRP) and wheat germ agglutinin HRP conjugate (WGA-HRP) tracers. The synaptic input to the TTM motoneurons was examined with WGA-HRP. The results show the motoneurons of the TTM to be localized in a cluster ventro-lateral to the outer margin of the ipsilateral trigeminal motor nucleus (VMN) and dorso-lateral to the superior olive. The number of labeled cells was greater in the combined CTB-HRP/WGA-HRP injected cases. The TTM motoneurons were triangular and elongated in shape and smaller than those of the VMN. An extensive network of dendritic branches was present ventro-laterally in the vicinity of the superior olive. Similar, but less extensive collections of dendritic processes were observed to course dorso-medially, rostrally and caudally. Axons were observed to project first dorsally or laterally, towards the trigeminal motor root, then after a sharp turn coursed ventrally within the trigeminal motor root (VMR). Transneuronal transport of the WGA-HRP was not accomplished in any preparation, suggesting among other things, system or species differences in the effectiveness of the WGA-HRP conjugate as a transynaptic tracer. It is concluded that the TTM acoustic reflex in rabbits and other mammals, its threshold, prolonged contraction capacity, and its influence on middle ear sound transmission may be related to its demonstrated extensive synaptic field in the reflex chain, particularly in the area of the superior olive, while its many other physiological functions may be made possible by the number, location, and multi-dimensional orientation of its motoneurons and dendrites.

beta-Galactosidase expressing recombinant pseudorabies virus for light and electron microscopic study of transneuronally labeled CNS neurons

A beta-galactosidase expression pseudorabies virus (Bartha strain) was constructed, injected into the adrenal gland of rats, and subsequently shown to transneuronally label the CNS autonomic neurons that project to the sympathoadrenal preganglionic neurons. Virally infected neurons were visualized with a one-step histochemical reaction using the Bluo-Gal substrate (halogenated indolyl-beta-D-galactoside) for the localization of beta-galactosidase activity. In some infected neurons, a Golgi-like staining of the primary and sometimes secondary dendrites could be obtained. For electron microscopic studies, the Bluo-Gal substrate produces an electron-dense reaction product that is easily identified at both low and high magnification. This virus may be useful for the study of the cell architecture and synaptic organization of transneuronally labeled neurons of functionally defined neural circuits. These results also demonstrate that it is possible to deliver foreign genes into specific chains of neurons in the mammalian CNS by means of the retrograde transneuronal vial labeling method.

Two alpha-herpesvirus strains are transported differentially in the rodent visual system

Uptake and transneuronal passage of wild-type and attenuated strains of a swine alpha-herpesvirus (pseudorabies [PRV]) were examined in rat visual projections. Both strains of virus infected subpopulations of retinal ganglion cells and passed transneuronally to infect retino-recipient neurons in the forebrain. However, the location of infected forebrain neurons varied with the strain of virus. Intravitreal injection of wild-type virus produced two temporally separated waves of infection that eventually reached all known retino-recipient regions of the central neuraxis. By contrast, the attenuated strain of PRV selectively infected a functionally distinct subset of retinal ganglion cells with restricted central projections. The data indicate that projection-specific groups of ganglion cells are differentially susceptible to the two strains of virus and suggest that this sensitivity may be receptor mediated.

The descending auditory pathway and acousticomotor systems: connections with the inferior colliculus

In this review the following major points are emphasized. First, the descending auditory system includes 3 separate, but parallel pathways connecting the AC, MGB and IC. Each pathway makes a strong set of connections with a distinctive area from each of 3 auditory centers. The three sets of connections are mutually exclusive, such that the pathways describe 3 separate corticocolliculo-geniculate systems. Thus, multiple feedback loops between the AC and the IC are formed which create a great capacity for parallel processing of auditory information. Second, the IC projects to the SOC and, in particular, to the source of olivocochlear efferent neurons. The connections of the IC with the AC rostrally, and with the olivocochlear neurons caudally, imply a descending trisynaptic pathway from the cortex to the cochlea whose travel time could better that of the ascending pathway and thus provide an efficient feedback mechanism. It is probable that the IC influences cochlear signal processing. The reciprocal connectivity between any two of either the IC, SOC or the CN, again, affords to the auditory system remarkable parallel processing capabilities. Finally, the descending auditory, and 'extra-auditory' connections of the IC bestow a functional separateness to the 3 nuclei of the IC, a view that is best illustrated by description of the ICX as an acousticomotor nucleus, having connections with the SC, cerebellum and somatosensory and vocalization systems. More sophisticated questions about the descending auditory system will incorporate these present observations and test functional implications to which they allude.

CNS cell groups regulating the sympathetic outflow to adrenal gland as revealed by transneuronal cell body labeling with pseudorabies virus

The CNS cell groups that innervate the sympathoadrenal preganglionic neurons of rats were identified by a transneuronal viral cell body labeling technique combined with neurotransmitter immunohistochemistry. Pseudorabies virus was injected into the adrenal gland. This resulted in retrograde viral infections of the ipsilateral sympathetic preganglionic neurons (T4-T13) and caused retrograde transneuronal cell body infections in 5 areas of the brain: the caudal raphe nuclei, ventromedial medulla, rostral ventrolateral medulla, A5 cell group, and paraventricular hypothalamic nucleus (PVH). In the spinal cord, the segmental distribution of virally infected neurons was the same as the retrograde cell body labeling observed following Fluoro-gold injections in the adrenal gland except there was almost a 300% increase in the number of cells labeled and a shift in cell group distribution. These results imply there are local interneurons that regulate the sympathoadrenal preganglionic neurons. In the medulla oblongata, serotonin (5-HT)-, substance P (SP)-, thyrotropin-releasing hormone-, Met-enkephalin-, and somatostatin-immunoreactive neurons of the raphe pallidus and raphe obscurus nuclei and the ventromedial medulla were infected. In the ventromedial and rostral ventrolateral medulla, immunoreactive phenylethanolamine-N-methyltransferase, SP, neuropeptide Y, somatostatin, and enkephalin neurons were infected. The A5 noradrenergic cells were labeled, as were some somatostatin-immunoreactive neurons in this area. In the were infected. The A5 noradrenergic cells were labeled, as were some somatostatin-immunoreactive neurons in this area. In the hypothalamus, tyrosine hydroxylase- and SP-immunoreactive neurons of the dorsal parvocellular PVH were infected. Only a few immunoreactive vasopressin, oxytocin, Met-enkephalin, neurotensin, and somatostatin PVH neurons were labeled.

A general pattern of CNS innervation of the sympathetic outflow demonstrated by transneuronal pseudorabies viral infections

Pseudorabies virus (PRV) injections of various sympathetic ganglia and the adrenal gland were made in rats. These produced immunohistochemically detectable retrograde viral infections of ipsilateral sympathetic preganglionic neurons (SPNs) and transneuronal infections of the specific sets of second order neurons in the spinal cord and brain that innervate the infected SPNs. Five cell groups in the brain appear to regulate the entire sympathetic outflow: the paraventricular hypothalamic nucleus (PVH), A5 noradrenergic cell group, caudal raphe region, rostral ventrolateral medulla, and ventromedial medulla. In addition, local interneurons in laminae VII and X of the spinal cord are also involved. Other CNS areas also became transneuronally labeled after infections of certain sympathetic ganglia, most notably the superior cervical and stellate ganglia. These areas include the central gray matter and lateral hypothalamic area. The zona incerta was uniquely labeled after stellate ganglion infections. The cell body labeling was specific. This specificity was demonstrated in the PVH where the neurons of the parvocellular PVH that form the descending sympathetic pathway were labeled in a topographic fashion. Finally, we demonstrate that the retrograde transneuronal viral cell body labeling method can be used simultaneously with either neuropeptide transmitter or transmitter synthetic enzyme immunohistochemistry.

Neuronal organization of the stapedius reflex pathways in the rat: a retrograde HRP and viral transneuronal tracing study

The location of stapedius motoneurons in the rat was determined with horseradish peroxidase (HRP) retrograde tracing techniques. After injection of free HRP or wheat germ agglutinin-horseradish peroxidase (WGA-HRP) in the stapedius muscle on one side, labeled neurons were seen ipsilaterally in a region ventromedial to the rostral half of the facial motor nucleus (VII), extending rostrally to the caudal part of the superior olivary complex (SOC). These labeled neurons, located outside the SOC and facial motor nuclei themselves, constitute the pool of stapedius motoneurons, in agreement with previous descriptions for other species. In order to identify the origin of some inputs to the stapedius motoneurons, injections of herpes virus suis were performed in the stapedius muscle. After replication in the motoneurons, the viruses are transported transneuronally to some premotor neurons, as previously reported in other systems. The presence of the virus was detected by immunofluorescence in neurons corresponding to the stapedius motoneurons labeled with HRP or WGA-HRP. In addition, infected neurons were seen bilaterally at the level of the SOC, in the mediotrapezoid region, where no labeled cells were observed following HRP or WGA-HRP injections in the stapedius muscle. These neurons were considered as infected transneuronally and therefore providing inputs to the pool of stapedius motoneurons. No virus could be detected in cochlear nucleus neurons. These data are consistent with previous observations in the rabbit based on lesion experiments, suggesting that neurons at the level of the SOC are involved in the reflex arc of middle ear muscles.