Neurotransmitters in the vestibular system.

Neuronal networks that are linked to the peripheral vestibular system contribute to gravitoinertial sensation, balance control, eye movement control, and autonomic function. Ascending connections to the limbic system and cerebral cortex are also important for motion perception and threat recognition, and play a role in comorbid balance and anxiety disorders. The vestibular system also shows remarkable plasticity, termed vestibular compensation. Activity in these networks is regulated by an interaction between: (1) intrinsic neurotransmitters of the inner ear, vestibular nerve, and vestibular nuclei; (2) neurotransmitters associated with thalamocortical and limbic pathways that receive projections originating in the vestibular nuclei; and (3) locus coeruleus and raphe (serotonergic and nonserotonergic) projections that influence the latter components. Because the ascending vestibular interoceptive and thalamocortical pathways include networks that influence a broad range of stress responses (endocrine and autonomic), memory consolidation, and cognitive functions, common transmitter substrates provide a basis for understanding features of acute and chronic vestibular disorders.

Posttraumatic dizziness and vertigo.

Traumatic brain injury is an increasingly common public health issue, with the mild variant most clinically relevant for this chapter. Common causes of mild traumatic brain injury (mTBI) include motor vehicle accidents, athletics, and military training/deployment. Despite a range of clinically available testing platforms, diagnosis of mTBI remains challenging. Symptoms are primarily neurosensory, and include dizziness, hearing problems, headaches, cognitive, and sleep disturbances. Dizziness is nearly universally present in all mTBI patients, and is the easiest symptom to objectify for diagnosis. Aside from a thorough history and physical exam, in the near future specialized vestibular function tests will be key to mTBI diagnosis. A battery of oculomotor (antisaccade, predictive saccade) and vestibular tasks (head impulse test) has been demonstrated to sensitively and specifically identify individuals with acute mTBI. Vestibular therapy and rehabilitation have shown improvements for mTBI patients in cognitive function, ability to return to activities of daily living, and ability to return to work. Dizziness, as a contributor to short- and long-term disability following mTBI, is ultimately crucial not only for diagnosis but also for treatment.

Selective anterograde tracing of the individual serotonergic and nonserotonergic components of the dorsal raphe nucleus projection to the vestibular nuclei.

It is well known that the dorsal raphe nucleus (DRN) sends serotonergic and nonserotonergic projections to target regions in the brain stem and forebrain, including the vestibular nuclei. Although retrograde tracing studies have reported consistently that there are differences in the relative innervation of different target regions by serotonergic and nonserotonergic DRN neurons, the relative termination patterns of these two projections have not been compared using anterograde tracing methods. The object of the present investigation was to trace anterogradely the individual serotonergic and nonserotonergic components of the projection from DRN to the vestibular nuclei in rats. To trace nonserotonergic DRN projections, animals were pretreated with the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), and then, after 7 days, the anterograde tracer biotinylated dextran amine (BDA) was iontophoretically injected into the DRN. In animals treated with 5,7-DHT, nonserotonergic BDA-labeled fibers were found to descend exclusively within the ventricular plexus and to terminate predominantly within the periventricular aspect of the vestibular nuclei. Serotonergic DRN projections were traced by injecting 5,7-DHT directly into DRN, and amino-cupric-silver staining was used to visualize the resulting pattern of terminal degeneration. Eighteen hours after microinjection of 5,7-DHT into the DRN, fine-caliber degenerating serotonergic terminals were found within the region of the medial vestibular nucleus (MVN) that borders the fourth ventricle, and a mixture of fine- and heavier-caliber degenerating serotonergic terminals was located further laterally within the vestibular nuclear complex. These findings indicate that fine-caliber projections from serotonergic and nonserotonergic DRN neurons primarily innervate the periventricular regions of MVN, whereas heavier-caliber projections from serotonergic DRN neurons innervate terminal fields located in more lateral regions of the vestibular nuclei. Thus, serotonergic and nonserotonergic DRN axons target distinct but partially overlapping terminal fields within the vestibular nuclear complex, raising the possibility that these two DRN projection systems are organized in a manner that permits regionally-specialized regulation of processing within the vestibular nuclei.

Serotonin-induced plasma extravasation in the murine inner ear: possible mechanism of migraine-associated inner ear dysfunction.

Sensitivity to sound and vertigo are often components of migraine. Recent studies suggest that plasma extravasation from intradural blood vessels may contribute to migraine pain. This study documented plasma extravasation in the mouse inner ear after intravenous administration of serotonin (5-HT). Horseradish peroxidase (HRP) was injected intravenously to trace protein extravasation in mice, followed 15 min later by intravenous 5-HT or saline. Forty-five minutes later, mice were euthanized. HRP extravasation was visualized immunohistochemically and quantified densitometrically. Baseline and evoked extravasation in stria vascularis and tectorial membrane were indistinguishable from skin, dura mater and tympanic membrane. Brain parenchyma, Scarpa's ganglion, basal spiral ganglion and modiolus, and the central vestibular nerve segment showed no significant 5-HT-induced extravasation. In contrast, 5-HT produced extravasation in the apical spiral ganglion, modiolus, and intralabyrinthine superior and inferior vestibular nerve. Thus, inner ear plasma extravasation is a potential mechanism for migraine-associated vertigo and sound sensitivity.

Anterograde tracing of projections from the dorsal raphe nucleus to the vestibular nuclei.

This study used the anterograde transport of biotinylated dextran amine (BDA) to identify the course and terminal distribution of projections from the dorsal raphe nucleus (DRN) to the vestibular nuclei in rats. After iontophoretic injection of BDA into the medial and lateral regions of DRN, anterogradely labeled fibers descend within the medial longitudinal fasciculus and the ventricular fiber plexus to terminate within two discrete regions of the vestibular nuclear complex. One terminal field was located primarily ipsilateral to the injection site and involved rostrodorsal aspects of the vestibular nuclei, including superior vestibular nucleus and rostral portions of the medial vestibular nucleus (MVN) and lateral vestibular nucleus (LVN). The other terminal field involved caudoventral aspects of both ipsilateral and contralateral MVN and LVN and was less heavily innervated. These findings confirm that the vestibular nuclei are targeted by a regionally-selective projection from the DRN. The segregation of DRN terminals into anatomically distinct fields indicates that the DRN-vestibular nucleus projections are organized to selectively modulate processing within specific functional domains of the vestibular nuclear complex. In particular, these terminal fields may be organized to modulate vestibular regions involved in eye movement-related velocity storage, coordination of vestibular and affective responses, and the bilateral coordination of horizontal eye movement reflexes.

Serotonergic and nonserotonergic neurons in the dorsal raphe nucleus send collateralized projections to both the vestibular nuclei and the central amygdaloid nucleus.

Using a combination of double retrograde tracing and serotonin immunofluorescence staining, we examined whether individual serotonergic and nonserotonergic neurons in the dorsal raphe nucleus are sources of collateralized axonal projections to vestibular nuclei and the central amygdaloid nucleus in the rat. Following unilateral injections of Diamidino Yellow into the vestibular nuclei and Fast Blue into the central amygdaloid nucleus, it was observed that approximately one-fourth of the dorsal raphe nucleus neurons projecting to the vestibular nuclei send axon collaterals to the central amygdaloid nucleus. Immunofluorescence staining for serotonin revealed that more than half of the dorsal raphe nucleus neurons from which these collateralized projections arise contain serotonin-like immunoreactivity. These findings indicate that a subpopulation of serotonergic and nonserotonergic dorsal raphe nucleus cells may act to co-modulate processing in the vestibular nuclei and the central amygdaloid nucleus, regions implicated in the generation of emotional and affective responses to real and perceived motion.

Regulation of mitochondrial uncoupling proteins in mouse inner ear ganglion cells in response to systemic kanamycin challenge.

Mitochondrial uncoupling proteins are a proton transporter family involved in regulation mitochondrial superoxide and ATP production. Uncoupling proteins are expressed by rat spiral ganglion and vestibular ganglion cells [Hear Res 196 (2004) 39]. This study tests the hypothesis that uncoupling protein expression is up-regulated in response to the reactive oxygen species challenge imposed by kanamycin and antioxidant (2,3-dihydroxybenzoate) treatment in mice. In control C57BL/6, CBA/J and BALB/c mice, mRNA for uncoupling protein 1, uncoupling protein 2, uncoupling protein 3, Slc25a27 (uncoupling protein 4) and Slc25a14 (uncoupling protein 5/BMCP1) was expressed in the spiral and vestibular ganglia. After kanamycin-treatment (700 mg/kg twice daily for 14 days s.c.), uncoupling protein 2 and uncoupling protein 3 mRNA expression increased significantly in spiral and vestibular ganglia and kidney, but was unaffected in cerebral cortex. Significant Slc25a27 (uncoupling protein 4) mRNA up-regulation was also observed in spiral and vestibular ganglia, but not in kidney or cerebral cortex. These effects were blocked by simultaneous administration of kanamycin and 2,3-dihydroxybenzoate (300 mg/kg twice daily for 14 days s.c.). Western immunoblotting and immunohistochemistry confirmed the uncoupling protein 2 and uncoupling protein 3 up-regulation in inner ear. Finally, 2,3-dihydroxybenzoate treatment alone produced an upregulation of uncoupling protein 1 mRNA in the spiral ganglion, vestibular ganglion and cerebral cortex, but not the kidney. Uncoupling protein 2 and uncoupling protein 3 upregulation in the kidney and inner ear ganglia likely reflects their general role as a feedback pathway to reduce mitochondrial superoxide generation. Slc25a27 (uncoupling protein 4) upregulation in the inner ear ganglia, by contrast, is likely to be a secondary response to kanamycin-induced hair cell death. We propose that increased uncoupling protein 2, uncoupling protein 3 and Slc25a27 expression has several neuroprotective effects via reduction in mitochondrial superoxide generation and local thermogenesis, including: (1) reducing mean ROS load to prevent apoptosis, (2) increasing signal-to-noise characteristics of intracellular ROS signaling pathways (e.g. lipoxygenases, growth factor and transcription factors), (3) heat-related alteration of enzyme kinetics and (4) promotion of cell depolarization (activation of heat-gated ion channels).

Zonal organization of the vestibulo-cerebellar pathways controlling the horizontal eye muscles using two recombinant strains of pseudorabies virus.

Many studies have documented the influence of the flocculus upon vestibulo-ocular reflex eye movements. Electrical stimulation of Purkinje cells in a central longitudinal zone evoked slow ipsilateral eye movements in the horizontal plane. Recently, the organization of neurons in the vestibulo-cerebellar pathways controlling single lateral rectus and medial rectus muscles was identified in rats using the transynaptic transport of pseudorabies virus. Overlapping distributions of neurons innervating single muscles were located predominantly in a central longitudinal zone of ventral paraflocculi/dorsal flocculi, and the rostral half of ventral flocculi. This study used two isogenic pseudorabies virus recombinants to determine whether individual cells in those brain regions have collateralized projections to motoneuron pools innervating the right lateral rectus and the left medial rectus muscles using different survival times and dual injection paradigms. The infected neurons were detected using dual-labeling immunofluorescence. Three populations of labeled neurons were observed: two populations replicated only one reporter while a third contained both viruses (i.e. dual-labeled). Most dual-labeled cells were located in a central longitudinal zone of the ventral paraflocculus, ipsilateral to the injection into the medial rectus, whereas very few were in the flocculus. This finding suggests that the flocculus and ventral paraflocculus may exert influence upon distinct vestibulo-cerebellar pathways. Most Purkinje cells in the ventral paraflocculus may influence the vestibulo-ocular reflex pathways through collateralization, whereas those in the flocculus may instead provide a monocular control of eye movements.

Zonal organization of the vestibulo-cerebellum in the control of horizontal extraocular muscles using pseudorabies virus: I. Flocculus/ventral paraflocculus.

Much literature has studied the relationship between the organization of neurons in the flocculus/ventral paraflocculus and vestibulo-ocular reflex pathways. Although activation of a flocculus central zone produces ipsilateral horizontal eye movement, anatomical tracing evidence in rats suggests that there may not be a simple one-to-one correspondence between flocculus/ventral paraflocculus zones and control of single extraocular muscles or coplanar pairs of antagonistic extraocular muscles. This study used the retrograde transynaptic transport of pseudorabies virus to identify the topographical organization of Purkinje cells in the flocculus/ventral paraflocculus that control the lateral rectus (LR) and medial rectus (MR) muscles in rats. A survival time of 80 h and 84 h was necessary to observe consistent transynaptically labeled cells in the flocculus/ventral paraflocculus following injections of pseudorabies virus into the MR and LR, respectively. The organization of Purkinje cells in the dorsal flocculus and ventral paraflocculus abided by the traditional boundaries, whereas the labeling pattern in the ventral flocculus showed a more complex, interdigitated arrangement. In agreement with prior studies, transynaptically labeled neurons were also observed in specific vestibular nuclear regions within the medial and superior vestibular nuclei and dorsal Y group. The distribution of labeled neurons in ipsilateral and contralateral vestibular nuclei was associated with features of ipsilateral and contralateral retrograde labeling of Purkinje cells in flocculus/ventral paraflocculus. Importantly, this study provides the first evidence of vestibulo-cerebellar zones controlling individual extraocular muscles and also overlapping distribution of neurons in flocculo-vestibular zones that influence the LR and MR motoneuron pools. This suggests that some of these neurons may be responsible for controlling both muscles.

Organization of projections from the raphe nuclei to the vestibular nuclei in rats.

Previous anatomic and electrophysiological evidence suggests that serotonin modulates processing in the vestibular nuclei. This study examined the organization of projections from serotonergic raphe nuclei to the vestibular nuclei in rats. The distribution of serotonergic axons in the vestibular nuclei was visualized immunohistochemically in rat brain slices using antisera directed against the serotonin transporter. The density of serotonin transporter-immunopositive fibers is greatest in the superior vestibular nucleus and the medial vestibular nucleus, especially along the border of the fourth ventricle; it declines in more lateral and caudal regions of the vestibular nuclear complex. After unilateral iontophoretic injections of Fluoro-Gold into the vestibular nuclei, retrogradely labeled neurons were found in the dorsal raphe nucleus (including the dorsomedial, ventromedial and lateral subdivisions) and nucleus raphe obscurus, and to a minor extent in nucleus raphe pallidus and nucleus raphe magnus. The combination of retrograde tracing with serotonin immunohistofluorescence in additional experiments revealed that the vestibular nuclei receive both serotonergic and non-serotonergic projections from raphe nuclei. Tracer injections in densely innervated regions (especially the medial and superior vestibular nuclei) were associated with the largest numbers of Fluoro-Gold-labeled cells. Differences were observed in the termination patterns of projections from the individual raphe nuclei. Thus, the dorsal raphe nucleus sends projections that terminate predominantly in the rostral and medial aspects of the vestibular nuclear complex, while nucleus raphe obscurus projects relatively uniformly throughout the vestibular nuclei. Based on the topographical organization of raphe input to the vestibular nuclei, it appears that dense projections from raphe nuclei are colocalized with terminal fields of flocculo-nodular lobe and uvula Purkinje cells. It is hypothesized that raphe-vestibular connections are organized to selectively modulate processing in regions of the vestibular nuclear complex that receive input from specific cerebellar zones. This represents a potential mechanism whereby motor activity and behavioral arousal could influence the activity of cerebellovestibular circuits.

Acrylamide axonopathy revisited.

Distal swelling and eventual degeneration of axons in the CNS and PNS have been considered to be the characteristic neuropathological features of acrylamide (ACR) neuropathy. These axonopathic changes have been the basis for classifying ACR neuropathy as a central-peripheral distal axonopathy and, accordingly, research over the past 30 years has focused on the primacy of axon damage and on deciphering underlying mechanisms. However, based on accumulating evidence, we have hypothesized that nerve terminals, and not axons, are the primary site of ACR action and that compromise of corresponding function is responsible for the autonomic, sensory, and motor defects that accompany ACR intoxication (NeuroToxicology 23 (2002) 43). In this paper, we provide a review of data from a recently completed comprehensive, longitudinal silver stain study of brain and spinal cord from rats intoxicated with ACR at two different daily dosing rates, i.e., 50 mg/kg/day, ip or 21 mg/kg/day, po. Results show that, regardless of dose-rate, ACR intoxication was associated with early, progressive nerve terminal degeneration in all CNS regions and with Purkinje cell injury in cerebellum. At the lower dose-rate, initial nerve terminal argyrophilia was followed by abundant retrograde axon degeneration in white matter tracts of spinal cord, brain stem, and cerebellum. The results support and extend our nerve terminal hypothesis and suggest that Purkinje cell damage also plays a role in ACR neurotoxicity. Substantial evidence now indicates that axon degeneration is a secondary effect and is, therefore, not pathophysiologically significant. These findings have important implications for future mechanistic research, classification schemes, and assessment of neurotoxicity risk.

Acrylamide neuropathy. II. Spatiotemporal characteristics of nerve cell damage in brainstem and spinal cord.

Previous studies of acrylamide (ACR) neuropathy in rat PNS [Toxicol. Appl. Pharmacol. 151 (1998) 211] and cerebellum [NeuroToxicology 23 (2002) 397] have suggested that axon degeneration was not a primary effect and was, therefore, of unclear neurotoxicological significance. To continue morphological examination of ACR neurotoxicity in CNS, a cupric silver stain method was used to define spatiotemporal characteristics of nerve cell body, dendrite, axon and terminal degeneration in brainstem and spinal cord. Rats were exposed to ACR at a dose-rate of either 50 mg/kg per day (i.p.) or 21 mg/kg per day (p.o.), and at selected times brains and spinal cord were removed and processed for silver staining. Results show that intoxication at the higher ACR dose-rate produced a nearly pure terminalopathy in brainstem and spinal cord regions, i.e. widespread nerve terminal degeneration and swelling were present in the absence of significant argyrophilic changes in neuronal cell bodies, dendrites or axons. Exposure to the lower ACR dose-rate caused initial nerve terminal argyrophilia in selected brainstem and spinal cord regions. As intoxication continued, axon degeneration developed in white matter of these CNS areas. At both dose-rates, argyrophilic changes in brainstem nerve terminals developed prior to the onset of significant gait abnormalities. In contrast, during exposure to the lower ACR dose-rate the appearance of axon degeneration in either brainstem or spinal cord was relatively delayed with respect to changes in gait. Thus, regardless of dose-rate, ACR intoxication produced early, progressive nerve terminal degeneration. Axon degeneration occurred primarily during exposure to the lower ACR dose-rate and developed after the appearance of terminal degeneration and neurotoxicity. Spatiotemporal analysis suggested that degeneration began at the nerve terminal and then moved as a function of time in a somal direction along the corresponding axon. These data suggest that nerve terminals are a primary site of ACR action and that expression of axonopathy is restricted to subchronic dosing-rates.

Acrylamide neuropathy. III. Spatiotemporal characteristics of nerve cell damage in forebrain.

Previous studies of acrylamide (ACR) neuropathy in rat PNS [Toxicol. Appl. Pharmacol. (1998) 151:211-221] and in spinal cord, brainstem and cerebellum [NeuroToxicology (2002a) 23:397-414; NeuroToxicology (2002b) 23:415-429] have suggested that axon degeneration was not a primary effect and was, therefore, of unclear neurotoxicological significance. To conclude our studies of neurodegeneration in rat CNS during ACR neurotoxicity, a cupric silver stain method was used to define spatiotemporal characteristics of nerve cell body, dendrite, axon and terminal argyrophilia in forebrain regions and nuclei. Rats were exposed to ACR at a dose-rate of either 50 mg/kg per day (i.p.) or 21 mg/kg per day (p.o.) and at selected times brains were removed and processed for silver staining. Results show that intoxication at either ACR dose-rate produced a terminalopathy, i.e. nerve terminal degeneration and swelling were present in the absence of significant argyrophilic changes in neuronal cell bodies, dendrites or axons. Exposure to the higher ACR dose-rate caused early onset (day 5), widespread nerve terminal degeneration in most of the major forebrain areas, e.g. cerebral cortex, thalamus, hypothalamus and basal ganglia. At the lower dose-rate, nerve terminal degeneration in the forebrain developed early (day 7) but exhibited a relatively limited spatial distribution, i.e. anteroventral thalamic nucleus and the pars reticulata of the substantia nigra. Several hippocampal regions were affected at a later time point (day 28), i.e. CA1 field and subicular complex. At both dose-rates, argyrophilic changes in forebrain nerve terminals developed prior to the onset of significant gait abnormalities. Thus, in forebrain, ACR intoxication produced a pure terminalopathy that developed prior to the onset of significant neurological changes and progressed as a function of exposure. Neither dose-rate used in this study was associated with axon degeneration in any forebrain region. Our findings indicate that nerve terminals were selectively affected in forebrain areas and, therefore, might be primary sites of ACR action.

Acrylamide neuropathy. I. Spatiotemporal characteristics of nerve cell damage in rat cerebellum.

Based on evidence from morphometric studies of PNS, we suggested that acrylamide (ACR)-induced distal axon degeneration was a secondary effect related to duration of exposure [Toxicol. Appl. Pharmacol. 151 (1998) 211]. To test this hypothesis in CNS, the cupric-silver stain method of de Olmos was used to define spatiotemporal characteristics of nerve somal, dendritic, axonal and terminal degeneration in rat cerebellum. Rats were exposed to ACR at either 50 mg/kg per day (i.p.) or 21 mg/kg per day (p.o.) and at selected times (i.p. = 5, 8 and 11 days; p.o. = 7, 14, 21, 28 and 38 days) brains were removed and processed for silver staining. Results demonstrate that intoxication at the higher ACR dose-rate produced early (day 5) and progressive degeneration of Purkinje cell dendrites in cerebellar cortex. Nerve terminal degeneration occurred concurrently with somatodendritic argyrophilia in cerebellar and brainstem nuclei that receive afferent input from Purkinje neurons. Relatively delayed (day 8), abundant axon degeneration was present in cerebellar white matter but not in cortical layers or in tracts carrying afferent fibers (cerebellar peduncles) from other brain nuclei. Axon argyrophilia coincided with the appearance of perikaryal degeneration, which was selective for Purkinje cells since silver impregnation of other cerebellar neurons was not evident in the different cortical layers or cerebellar nuclei. Intoxication at the lower ACR dose-rate produced simultaneous (day 14) dendrite, axon and nerve terminal argyrophilia and no somatic Purkinje cell degeneration. The spatiotemporal pattern of dendrite, axon and nerve terminal loss induced by both ACR dose-rates is consistent with Purkinje cell injury. Injured neurons are likely to be incapable of maintaining distal processes and, therefore, axon degeneration in the cerebellum is a component of a "dying-back" process of neuronal injury. Because cerebellar coordination of somatomotor activity is mediated solely through efferent projections of the Purkinje cell, injury to this neuron might contribute significantly to gait abnormalities that characterize ACR neurotoxicity.

Acrylamide neuropathy. II. Spatiotemporal characteristics of nerve cell damage in brainstem and spinal cord.

Previous studies of acrylamide (ACR) neuropathy in rat PNS [Toxicol. Appl. Pharmacol. 151 (1998) 211] and cerebellum [Neurotoxicology, 2002a] have suggested that axon degeneration was not a primary effect and was, therefore, of unclear neurotoxicological significance. To continue morphological examination of ACR neurotoxicity in CNS, a cupric silver stain method was used to define spatiotemporal characteristics of nerve cell body, dendrite, axon and terminal degeneration in brainstem and spinal cord. Rats were exposed to ACR at a dose-rate of either 50 mg/kg per day (i.p.) or 21 mg/kg per day (p.o.), and at selected times brains and spinal cord were removed and processed for silver staining. Results show that intoxication at the higher ACR dose-rate produced a nearly pure terminalopathy in brainstem and spinal cord regions, ie. widespread nerve terminal degeneration and swelling were present in the absence of significant argyrophilic changes in neuronal cell bodies, dendrites or axons. Exposure to the lower ACR dose-rate caused initial nerve terminal argyrophilia in selected brainstem and spinal cord regions. As intoxication continued, axon degeneration developed in white matter of these CNS areas. At both dose-rates, argyrophilic changes in brainstem nerve terminals developed prior to the onset of significant gait abnormalities. In contrast, during exposure to the lower ACR dose-rate the appearance of axon degeneration in either brainstem or spinal cord was relatively delayed with respect to changes in gait. Thus, regardless of dose-rate, ACR intoxication produced early, progressive nerve terminal degeneration. Axon degeneration occurred primarily during exposure to the lower ACR dose-rate and developed after the appearance of terminal degeneration and neurotoxicity. Spatiotemporal analysis suggested that degeneration began at the nerve terminal and then moved as a function of time in a somal direction along the corresponding axon. These data suggest that nerve terminals are a primary site of ACR action and that expression of axonopathy is restricted to subchronic dosing-rates.

Vestibular abnormalities in congenital disorders.

This paper reviews the histopathologic features of vestibular abnormalities in congenital disorders affecting the inner ear, based upon a comprehensive literature survey and a review of cases in our temporal bone collection. The review proceeds in three systematic steps. First, we surveyed associated diseases with the major phenotypic features of congenital abnormalities of the inner ear (including the internal auditory canal and otic capsule). Second, the vestibular anomalies are examined specifically. Finally, the anomalies are discussed from a developmental perspective. Among vestibular anomalies, a hypoplastic endolymphatic duct and sac are observed most frequently. Anomalies of the semicircular canals are also often observed. From embryological and clinical viewpoints, many of these resemble the structural features from fetal stages and appear to be associated with vestibular dysfunction. It is expected that progress in genetic analysis and accumulation of temporal bone specimens with vestibular abnormalities in congenital diseases will provide crucial information not only for pathology of those diseases, but also for genetic factors that are responsible for the specific vestibular abnormalities.

Role of gene regulation during vestibular compensation: an integrative approach.

Identification of the role of gene regulation in vestibular compensation is one aspect of a larger issue: the identification of molecular bases for plasticity in multiple vestibulo-ocular, vestibulo-spinal, vestibulo-collic, and vestibulo-autonomic responses. To achieve this goal, it is incumbent on investigators to examine molecular events within the contexts of the single neuron, the location of the neuron in pathways, and the timing of the molecular events relative to behavioral compensation. Hence, the goal of identifying molecular bases for a particular compensatory response (e.g., the disappearance of spontaneous nystagmus in the light or the disappearance of static head tilt) requires careful attention to the time course of physiologic compensation and the location of the effects within central pathways that have the potential to affect the responses. The effects of impeding these site-specific and time-specific changes can then be tested to determine their role in the compensatory process. A consideration of the recent literature on molecular events related to the resolution of spontaneous nystagmus in the light indicates that a meaningful approach to these issues requires a broadening of our conceptual approach. Specifically, one must consider the roles of transcriptional, translational, and posttranslational events on the turnover of critical signaling substrates for vestibular compensation.

Background and history of the interface between anxiety and vertigo.

The comorbidity of vertigo and anxiety has been an integral component of the medical literature since antiquity. In the works of Plato, the same terms were used in the context of vertigo, inebriation, height vertigo, disorientation, and mental confusion. In classical medicine, vertigo had the ambiguous status of being both a disease per se and a symptom of other diseases such as hypochondriacal melancholy. Further, two etiologies were described for vertigo: an origin in the head (brain) and an origin in the hypochondria (abdominal viscera). In the course of the development of a detailed neurologic taxonomy of vertigo in the latter half of the nineteenth century, a debate ensued whether agoraphobia was a form of vertigo or a distinct psychiatric condition. Elucidation of this forgotten debate, within its historical context, provides insights into the recent rediscovery of the balance-anxiety interface.

Neurological bases for balance-anxiety links.

This review paper examines neurologic bases of links between balance control and anxiety based upon neural circuits that are shared by pathways that mediate autonomic control, vestibulo-autonomic interactions, and anxiety. The core of this circuitry is a parabrachial nucleus network, consisting of the parabrachial nucleus and its reciprocal relationships with the extended central amygdaloid nucleus, infralimbic cortex, and hypothalamus. Specifically, the parabrachial nucleus is a site of convergence of vestibular information processing and somatic and visceral sensory information processing in pathways that appear to be involved in avoidance conditioning, anxiety, and conditioned fear. Monoaminergic influences on these pathways are potential modulators of both effects of vigilance and anxiety on balance control and the development of anxiety and panic. This neurologic schema provides a unifying framework for investigating the neurologic bases for comorbidity of balance disorders and anxiety.