Organization of the coeruleo-vestibular pathway in rats, rabbits, and monkeys

Commissural and monosynaptic inputs to medial vestibular nucleus GABAergic neurons in mice

Objective

MVN GABAergic neurons is involved in the rebalance of commissural system contributing to alleviating acute peripheral vestibular dysfunction syndrome. This study aims to depict monosynaptic inputs to MVN GABAergic neurons.

Methods

The modified rabies virus-based retrogradation method combined with the VGAT-IRES-Cre mice was used in this study. Moreover, the commissural connections with MVN GABAergic neurons were analyzed.

Results

We identified 60 nuclei projecting to MVN GABAergic neurons primarily distributed in the cerebellum and the medulla. The uvula-nodulus, gigantocellular reticular nucleus, prepositus nucleus, intermediate reticular nucleus, and three other nuclei sent dense inputs to MVN GABAergic neurons. The medial (fastigial) cerebellar nucleus, dorsal paragigantocellular nucleus, lateral paragigantocellular nucleus and 10 other nuclei sent moderate inputs to MVN GABAergic neurons. Sparse inputs to MVN GABAergic neurons originated from the nucleus of the solitary tract, lateral reticular nucleus, pedunculopontine tegmental nucleus and 37 other nuclei. The MVN GABAergic neurons were regulated by the contralateral MVN, lateral vestibular nucleus, superior vestibular nucleus, and inferior vestibular nucleus.

Conclusion

Our study contributes to further understanding of the vestibular dysfunction in terms of neural circuits and search for new strategies to facilitate vestibular compensation.

Noradrenergic current responses of neurons in rat oculomotor neural integrators

The prepositus hypoglossi nucleus (PHN) and the interstitial nucleus of Cajal (INC) are involved in the control of horizontal and vertical gaze, respectively. A previous study showed that PHN neurons exhibit depolarized or hyperpolarized responses to noradrenaline (NA). However, the adrenoceptor types that participate in NA-induced responses and the effects of NA on INC neurons have not yet been investigated. Furthermore, the relationship between NA-induced responses and neuron types defined by neurotransmitter phenotypes has not been determined. In this study, we investigated NA-induced current responses in PHN and INC neurons and the relationships between these responses and neuron types using whole cell recordings in wild-type and transgenic rat brainstem slices. Local application of NA to the cell soma induced slow inward (SI) and slow outward (SO) currents that were mainly mediated by α1 and α2 adrenoceptors, respectively. These current responses were observed in both PHN and INC neurons, although the proportion of INC neurons that responded to NA was low. Analyses of the distributions of the current responses revealed that in the PHN, all fluorescently identified inhibitory neurons exhibited SI currents, whereas glutamatergic and cholinergic neurons exhibited both SI and SO currents. In the INC, glutamatergic and inhibitory neurons preferentially exhibited SI and SO currents, respectively. When the PHN and INC neurons were characterized by their firing pattern, we found that the proportions of the currents depended on their firing pattern. These results suggest that various modes of noradrenergic modulation in horizontal and vertical neural integrators are dependent on neuron type.NEW & NOTEWORTHY Noradrenergic modulation of oculomotor neural integrators involved in gaze control has not been elucidated. Here, we report that noradrenaline (NA)-induced slow inward (SI) and outward (SO) currents are mediated mainly by α1 and α2 adrenoceptors in neurons that participate in horizontal and vertical gaze control. The NA-induced current responses differed depending on the neurotransmitter phenotype and firing pattern. These results suggest various modes of noradrenergic modulation in horizontal and vertical integrator neurons.

Post-stroke dizziness, depression and anxiety

OBJECTIVE

Vestibular and psychiatric disorders are very closely related. Previous research shows that the discomfort and dysfunction caused by dizziness in patients can affect psychological processes, leading to anxiety and depression, and the irritation of anxiety and depression can aggravate the discomfort of dizziness. But the causal relationship between dizziness in the recovery period of stroke and Post-stroke depression (PSD) / Post-stroke anxiety (PSA) is not clear. Identifying the causal relationship between them can enable us to conduct more targeted treatments.

METHODS

We review the epidemiology and relationship of dizziness, anxiety, and depression, along with the related neuroanatomical basis. We also review the pathophysiology of dizziness after stroke, vestibular function of patients experiencing dizziness, and the causes and mechanisms of PSD and PSA. We attempt to explore the possible relationship between post-stroke dizziness and PSD and PSA.

CONCLUSION

The treatment approach for post-stroke dizziness depends on its underlying cause. If the dizziness is a result of PSD and PSA, addressing these psychological factors may alleviate the dizziness. This can be achieved through targeted treatments for PSD and PSA, such as psychotherapy, antidepressants, or anxiolytics, which could indirectly improve dizziness symptoms. Conversely, if PSA and PSD are secondary to vestibular dysfunction caused by stroke, a thorough vestibular function assessment is crucial. Identifying the extent of vestibular impairment allows for tailored interventions. These could include vestibular rehabilitation therapy and medication aimed at vestibular restoration. By improving vestibular function, secondary symptoms like anxiety and depression may also be mitigated.

Static Subjective Visual Vertical (SVV) in Patients with Vestibular Migraine

BACKGROUND

 Vestibular migraine (VM) is one of the common causes of episodic dizziness, but it is underdiagnosed and poorly understood. Previous research suggests that otolith reflex pathway performance is often impaired in this patient group, leading to altered perception of roll plane stimuli. Clinically, this perception can be measured with subjective visual vertical (SVV) testing.

PURPOSE

 The aim of this study is to compare static SVV performance (absolute mean SVV tilt, variance) in a cohort of patients diagnosed with VM to results obtained from clinically derived normative data.

STUDY DESIGN

 Retrospective case review.

STUDY SAMPLE

 Ninety-four consecutive patients between 18 and 65 years of age diagnosed with VM were included in this comparison to clinically derived normative data.

DATA COLLECTION AND ANALYSIS

 Retrospective chart review was completed. Demographic data, symptom report, and vestibular laboratory results were documented. SVV performance was documented in terms of absolute mean SVV tilt and response variance.

RESULTS

 Abnormal mean SVV tilt was described in 54% (n = 51) of patients with VM. Including abnormal response variance increased those identified with abnormal presentation to 67% (n = 63). Laboratory findings were insignificant for semicircular canal function, but of those with abnormal ocular vestibular myogenic potential results (n = 30), 77% (n = 23) demonstrated both abnormal SVV and utriculo-ocular reflex performance. There were no associations noted for SVV performance and demographic or other self-report variables.

CONCLUSION

 Absolute mean SVV tilt and response variance are often abnormal in patients diagnosed with VM. These findings support theories suggesting atypical intralabyrinthine integration within the vestibular nuclei and cerebellar nodular pathways.

Assessment of the Clinical Use of Vestibular Evoked Myogenic Potentials and the Video Head Impulse Test in the Diagnosis of Early-Stage Parkinson's Disease

OBJECTIVES

To explore the usefulness of vestibular tests including "vestibular evoked myogenic potentials" (VEMPs) and the video head impulse test (vHIT) in the early diagnosis of "idiopathic Parkinson's disease" (PD).

MATERIALS AND METHODS

The study involved 80 participants including 40 patients (24 males, 16 females; age average 63.20 ± 7.94 years) with PD and 40 healthy individuals (18 males and 22 females; age average of 60.36 ± 7.68 years). The Modified Hoehn and Yahr (H&Y) scale was used to measure how Parkinson's symptoms progress and the level of disability. Patients with PD underwent cVEMPs, oVEMPs, and vHIT and the results were compared with those of 40 age-matched healthy control (HC) subjects. vHIT results and VEMP responses were registered in all patients and HCs.

RESULTS

One-sided absent cVEMP responses were found in 6 (15%) patients with PD and 8 (20%) patients had bilaterally absent responses. Five (12.5%) patients had 1-sided absent oVEMP responses and it was bilateral in 6 (15%). Patients with PD had significantly shorter cVEMP P1, N1 latency, lower cVEMP amplitudes, and oVEMP amplitudes than the HC group. The cVEMP and oVEMP amplitude asymmetry ratio was significantly higher in the PD group (P < .05). Evaluation of vHIT results and vestibular-ocular reflex (VOR) gain between the groups revealed that anterior canal and posterior canal VOR gains results were remarkably lower in the PD group than in the HCs (P < .05). There was no difference in right and left lateral canal VOR gains between the groups (P > .05).

CONCLUSION

The results of this study suggest that cVEMP and vHIT can be used to evaluate the vestibular system in patients with early-stage Parkinson's disease.

Vestibular-autonomic interactions: beyond orthostatic dizziness

PURPOSE OF REVIEW

This review aims to summarize the current literature describing vestibular-autonomic interactions and to describe their putative role in various disorders' clinical presentations, including orthostatic dizziness and motion sensitivity.

RECENT FINDINGS

The vestibular-autonomic reflexes have long been described as they relate to cardiovascular and respiratory function. Although orthostatic dizziness may be in part related to impaired vestibulo-sympathetic reflex (orthostatic hypotension), there are various conditions that may present similarly. A recent clinical classification aims to improve identification of individuals with hemodynamic orthostatic dizziness so that appropriate recommendations and management can be efficiently addressed. Researchers continue to improve understanding of the underlying vestibular-autonomic reflexes with recent studies noting the insular cortex as a cortical site for vestibular sensation and autonomic integration and modulation. Work has further expanded our understanding of the clinical presentation of abnormal vestibular-autonomic interactions that may occur in various conditions, such as aging, peripheral vestibular hypofunction, traumatic brain injury, and motion sensitivity.

SUMMARY

The vestibular-autonomic reflexes affect various sympathetic and parasympathetic functions. Understanding these relationships will provide improved identification of underlying etiology and drive improved patient management.

The Neuropsychology of Dizziness and Related Disorders

There is a reciprocal relationship between vestibular and neuropsychological disorders. People with vertigo and dizziness are at higher risk of various psychiatric disorders, particularly anxiety, depression, and panic disorder. On the other hand, people with mood disorders are at higher risk of experiencing vertigo and dizziness. Vestibular information plays a crucial role in cognitive processes, especially visuo-spatial abilities. Consequently, vestibular disorders (both peripheral and central) often result in visuo-spatial deficits. In addition, lesions of the cortical and subcortical components of the vestibular system result in disorders of higher vestibular function, such as hemispatial neglect, pusher syndrome, and topographagnosia.

Altered monoaminergic levels, spasticity, and balance disability following repetitive blast-induced traumatic brain injury in rats

Spasticity and balance disability are major complications following traumatic brain injury (TBI). Although monoaminergic inputs provide critical adaptive neuromodulations to the motor system, data are not available regarding the levels of monoamines in the brain regions related to motor functions following repetitive blast TBI (bTBI). The objective of this study was to determine if mild, repetitive bTBI results in spasticity/balance deficits and if these are correlated with altered levels of norepinephrine, dopamine, and serotonin in the brain regions related to the motor system. Repetitive bTBI was induced by a blast overpressure wave in male rats on days 1, 4, and 7. Following bTBI, physiological/behavioral tests were conducted and tissues in the central motor system (i.e., motor cortex, locus coeruleus, vestibular nuclei, and lumbar spinal cord) were collected for electrochemical detection of norepinephrine, dopamine, and serotonin by high-performance liquid chromatography. The results showed that norepinephrine was significantly increased in the locus coeruleus and decreased in the vestibular nuclei, while dopamine was significantly decreased in the vestibular nuclei. On the other hand, serotonin was significantly increased in the motor cortex and the lumbar spinal cord. Because these monoamines play important roles in regulating the excitability of neurons, these results suggest that mild, repetitive bTBI-induced dysregulation of monoaminergic inputs in the central motor system could contribute to spasticity and balance disability. This is the first study to report altered levels of multiple monoamines in the central motor system following acute mild, repetitive bTBI.

Age-related features in vestibular migraine onset: A multiparametric analysis

BACKGROUND

Clinical heterogeneity is a peculiarity of vestibular migraine, in contrast to other vestibular disorders that have a more stereotypical expression. Migraine presents a range of variability in symptoms depending on the age of the patient. Supposing that migraine headache and vestibular migraine share the same pathogenetic mechanisms, a multiparametric analysis was performed to verify the hypotheses of an age-related influence on the clinical features of vestibular migraine at the onset.

METHODS

In this retrospective study, we analysed the clinical records of 72 consecutive patients affected by vestibular migraine from June 2012 to November 2018: 64 females and eight males; mean age 38.2 ± 9.6. We considered only patients that reported onset of vestibular symptoms within 12 months preceding inclusion into the study.

RESULTS

Statistical analysis shows a significant increase in the diagnosis of probable vestibular migraine with increasing age and a decrease in vestibular migraine diagnosis (p = 0.034). The incidence of spontaneous dizziness increases with age (p = 0.012); by contrast, external spontaneous vertigo, and visually induced vertigo decrease after 40 years of age (p = 0.018), clinically characterising the onset of juvenile forms. Spontaneous vertigo, head motion-induced vertigo/dizziness, and positional vertigo did not show significant variations with age.

CONCLUSION

Our data show that the type of vestibular symptoms in vestibular migraine varies according to the age of onset.

Self-motion perception is sensitized in vestibular migraine: pathophysiologic and clinical implications

Vestibular migraine (VM) is the most common cause of spontaneous vertigo but remains poorly understood. We investigated the hypothesis that central vestibular pathways are sensitized in VM by measuring self-motion perceptual thresholds in patients and control subjects and by characterizing the vestibulo-ocular reflex (VOR) and vestibular and headache symptom severity. VM patients were abnormally sensitive to roll tilt, which co-modulates semicircular canal and otolith organ activity, but not to motions that activate the canals or otolith organs in isolation, implying sensitization of canal-otolith integration. When tilt thresholds were considered together with vestibular symptom severity or VOR dynamics, VM patients segregated into two clusters. Thresholds in one cluster correlated positively with symptoms and with the VOR time constant; thresholds in the second cluster were uniformly low and independent of symptoms and the time constant. The VM threshold abnormality showed a frequency-dependence that paralleled the brain stem velocity storage mechanism. These results support a pathogenic model where vestibular symptoms emanate from the vestibular nuclei, which are sensitized by migraine-related brainstem regions and simultaneously suppressed by inhibitory feedback from the cerebellar nodulus and uvula, the site of canal-otolith integration. This conceptual framework elucidates VM pathophysiology and could potentially facilitate its diagnosis and treatment.

Vestibular Evoked Myogenic Potentials Are Abnormal in Idiopathic REM Sleep Behavior Disorder

Objectives: To investigate brainstem function in idiopathic REM sleep Behavior Disorder (iRBD), a condition occurring as a result of a derangement of connections within brainstem structures, with a battery of Vestibular Evoked Myogenic Potentials (VEMPs), neurophysiological tools suited for the functional investigation of the brainstem. Neurophysiological data were correlated with clinical characteristics of patients.

Methods: Twenty patients with iRBD and 22 healthy controls underwent cervical (cVEMP), masseter (mVEMP) and ocular (oVEMP) VEMP recording. Patients were assessed clinically according to presence of motor as well as non-motor symptoms such as constipation, depression, and hyposmia. Also, they were screened for postural instability through the Berg Balance Scale (BBS). VEMPs were categorized as for increasing degrees of abnormalities, namely latency delay, amplitude reduction and absence; a VEMP score was built accordingly.

Results: Compared with controls, iRBD had higher rates of abnormalities both in the VEMP battery (iRBD 75%, Controls 23%; p < 0.01) as well as in each single VEMP (cVEMP: 45 vs. 5%; mVEMP: 65 vs. 13.6%; oVEMP: 50 vs. 5%; p < 0.01), which exhibited significantly lower amplitudes (cVEMP and oVEMP: p < 0.0001; mVEMP: p = 0.001) in iRBD. Within altered reflexes, absence was predominant in oVEMP (81%), amplitude reduction in mVEMP (50%) and cVEMP (70%). Severity of VEMP alterations was significantly higher in iRBD compared with controls (p < 0.05 for all VEMPs), as indicated by the larger VEMP scores in the former. The oVEMP score correlated inversely with poor performances on the BBS.

Conclusion: VEMPs unveil consistent and extensive brainstem abnormalities in iRBD patients. Further studies are warranted for testing the potential of VEMPs in the monitoring of the evolution of iRBD over time.

Characterization of postural control impairment in women with fibromyalgia

The main goal of this cross-sectional study was to detect whether women with fibromyalgia syndrome (FMS) have altered postural control and to study the sensory contribution to postural control. We also explored the possibility that self-induced anxiety and lower limb strength may be related to postural control. For this purpose, 129 women within an age range of 40 to 70 years were enrolled. Eighty of the enrolled women had FMS. Postural control variables, such as Ellipse, Root mean square (RMS) and Sample entropy (SampEn), in both directions (i.e. mediolateral and anteroposterior), were calculated under five different conditions. A force plate was used to register the center of pressure shifts. Furthermore, isometric lower limb strength was recorded with a portable dynamometer and normalized by lean body mass. The results showed that women with FMS have impaired postural control compared with healthy people, as they presented a significant increase in Ellipse and RMS values (p<0.05) and a significant decrease in SampEn in both directions (p<0.05). Postural control also worsens with the gradual alteration of sensory inputs in this population (p<0.05). Performing a stressor dual task only impacts Ellipse in women with FMS (p>0.05). There were no significant correlations between postural control and lower limb strength (p>0.05). Therefore, women with FMS have impaired postural control that is worse when sensory inputs are altered but is not correlated with their lower limb strength.

Postural threat influences vestibular-evoked muscular responses

Standing balance is significantly influenced by postural threat. While this effect has been well established, the underlying mechanisms of the effect are less understood. The involvement of the vestibular system is under current debate, and recent studies that investigated the effects of height-induced postural threat on vestibular-evoked responses provide conflicting results based on kinetic (Horslen BC, Dakin CJ, Inglis JT, Blouin JS, Carpenter MG. J Physiol 592: 3671-3685, 2014) and kinematic (Osler CJ, Tersteeg MC, Reynolds RF, Loram ID. Eur J Neurosci 38: 3239-3247, 2013) data. We examined the effect of threat of perturbation, a different form of postural threat, on coupling (cross-correlation, coherence, and gain) of the vestibulo-muscular relationship in 25 participants who maintained standing balance. In the "No-Threat" conditions, participants stood quietly on a stable surface. In the "Threat" condition, participants' balance was threatened with unpredictable mediolateral support surface tilts. Quiet standing immediately before the surface tilts was compared to an equivalent time from the No-Threat conditions. Surface EMG was recorded from bilateral trunk, hip, and leg muscles. Hip and leg muscles exhibited significant increases in peak cross-correlation amplitudes, coherence, and gain (1.23-2.66×) in the Threat condition compared with No-Threat conditions, and significant correlations were observed between threat-related changes in physiological arousal and medium-latency peak cross-correlation amplitude in medial gastrocnemius (r = 0.408) muscles. These findings show a clear threat effect on vestibular-evoked responses in muscles in the lower body, with less robust effects of threat on trunk muscles. Combined with previous work, the present results can provide insight into observed changes during balance control in threatening situations.

NEW & NOTEWORTHY

This is the first study to show increases in vestibular-evoked responses of the lower body muscles under conditions of increased threat of postural perturbation. While robust findings were observed in hip and leg muscles, less consistent results were found in muscles of the trunk. The present findings provide further support in the ongoing debate for arguments that vestibular-evoked balance responses are influenced by fear and anxiety and explain previous threat-related changes in balance.

The Vestibulo-Collic Reflex is Abnormal in Migraine

Interictal evoked central nervous system responses are characterized in migraineurs by a deficit of habituation, at both cortical and subcortical levels. The click-evoked vestibulo-collic reflex (VCR) allows the assessment of otolith function and an oligosynaptic pathway linking receptors in the saccular macula to motoneurons of neck muscles. Three blocks of 75 averaged responses to monaural 95-dB normal hearing level 3-Hz clicks were recorded over the contracted ipsilateral sternocleidomastoid muscle in 25 migraineurs between attacks and 20 healthy subjects, without vestibular symptoms. Amplitudes, raw and corrected for baseline electromyography, were significantly smaller in migraine patients. Whereas in healthy volunteers the VCR habituated during stimulus repetition (-4.96% ± 14.3), potentiation was found in migraineurs (4.34% ± 15.3; P = 0.04). The combination with a reduced mean amplitude does not favour vestibular hyperexcitability as an explanation for the habituation deficit in migraine, but rather an abnormal processing of repeated stimuli in the reflex circuit.

Vestibular migraine: diagnosis challenges and need for targeted treatment

ABSTRACT Approximately 1% of the general population suffers from vestibular migraine. Despite the recently published diagnostic criteria, it is still underdiagnosed condition. The exact neural mechanisms of vestibular migraine are still unclear, but the variability of symptoms and clinical findings both during and between attacks suggests an important interaction between trigeminal and vestibular systems. Vestibular migraine often begins several years after typical migraine and has a variable clinical presentation. In vestibular migraine patients, the neurological and neurotological examination is mostly normal and the diagnosis will be based in the patient clinical history. Treatment trials that specialize on vestibular migraine are scarce and therapeutic recommendations are based on migraine guidelines. Controlled studies on the efficacy of pharmacologic interventions in the treatment of vestibular migraine should be performed.

Vestibular migraine

During the last decades a new vestibular syndrome has emerged that is now termed vestibular migraine (VM). The main body of evidence for VM is provided by epidemiologic data demonstrating a strong association between migraine and vestibular symptoms. Today, VM is recognized as one of the most common causes of episodic vertigo. The clinical presentation of VM is heterogeneous in terms of vestibular symptoms, duration of episodes, and association with migrainous accompaniments. Similar to migraine, there is no clinical or laboratory confirmation for VM and the diagnosis relies on the history and the exclusion of other disorders. Recently, diagnostic criteria for VM have been elaborated jointly by the International Headache Society and the Bárány Society. Clinical examination of patients with acute VM has clarified that the vast majority of patients with VM suffer from central vestibular dysfunction. Findings in the interval may yield mild signs of damage to both the central vestibular and ocular motor system and to the inner ear. These interictal clinical signs are not specific to VM but can be also observed in migraineurs without a history of vestibular symptoms. How migraine affects the vestibular system is still a matter of speculation. In the absence of high-quality therapeutic trials, treatment is targeted at the underlying migraine.

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.

Visuo-vestibular contributions to anxiety and fear

The interactive roles of the visual and vestibular systems allow for postural control within boundaries of perceived safety. In specific circumstances, visual vestibular and postural interactions act as a cue that trigger fear, similarly to what occurs in motion sickness. Unusual patterns of visuo-vestibular interaction that emerge without warning can elicit fear, which can then become associated to a certain stimuli or situation, creating a CS-US association, (i.e., phobia), or can emerge without warning but also without becoming associated to a particular concomitant event (i.e., panic). Depending on the individual sensitivity to visuo-vestibular unusual patterns and its impact in postural control, individuals will be more or less vulnerable to develop these disorders. As such, the mechanism we here propose is also sufficient to explain the lack of certain fears albeit exposure. Following this rationale, a new subcategory of anxiety disorders, named visuo-vestibular fears can be considered. This model brings important implications for developmental and evolutionary psychological science, and invites to place visuo-vestibular fears in a particular subtype or specification within the DSM-5 diagnostic criteria.

Spatial cognition, body representation and affective processes: the role of vestibular information beyond ocular reflexes and control of posture

A growing number of studies in humans demonstrate the involvement of vestibular information in tasks that are seemingly remote from well-known functions such as space constancy or postural control. In this review article we point out three emerging streams of research highlighting the importance of vestibular input: (1) Spatial Cognition: Modulation of vestibular signals can induce specific changes in spatial cognitive tasks like mental imagery and the processing of numbers. This has been shown in studies manipulating body orientation (changing the input from the otoliths), body rotation (changing the input from the semicircular canals), in clinical findings with vestibular patients, and in studies carried out in microgravity. There is also an effect in the reverse direction; top-down processes can affect perception of vestibular stimuli. (2) Body Representation: Numerous studies demonstrate that vestibular stimulation changes the representation of body parts, and sensitivity to tactile input or pain. Thus, the vestibular system plays an integral role in multisensory coordination of body representation. (3) Affective Processes and Disorders: Studies in psychiatric patients and patients with a vestibular disorder report a high comorbidity of vestibular dysfunctions and psychiatric symptoms. Recent studies investigated the beneficial effect of vestibular stimulation on psychiatric disorders, and how vestibular input can change mood and affect. These three emerging streams of research in vestibular science are-at least in part-associated with different neuronal core mechanisms. Spatial transformations draw on parietal areas, body representation is associated with somatosensory areas, and affective processes involve insular and cingulate cortices, all of which receive vestibular input. Even though a wide range of different vestibular cortical projection areas has been ascertained, their functionality still is scarcely understood.

Noradrenergic modulation of neuronal responses to n-methyl-d-aspartate in the vestibular nuclei: an electrophysiological and immunohistochemical study

Excitatory responses evoked by N-methyl-d-aspartate (NMDA) in the vestibular nuclei (VN) of the rat were studied in vivo during microiontophoretic application of noradrenaline (NA) and/or its agonists and antagonists. Ejection of NA-modified excitatory responses mediated by NMDA receptors (NMDAR) in all neurons tested; the effect was enhancement in 59% of cases and depression in the remaining 41%. Enhancements prevailed in all VN with the exception of the lateral vestibular nucleus, where both effects were recorded in an equal number of cases. The enhancing action of NA on NMDAR-mediated responses was mimicked by the noradrenergic beta-receptor agonist isoproterenol, the beta1 specific agonist denopamine and the alpha2 agonist clonidine. These effects were blocked respectively by the generic beta-receptor antagonist timolol, the beta1 antagonist atenolol and the alpha2 antagonist yohimbine. In contrast, application of the alpha1 receptor agonist cirazoline and the specific alpha1 antagonist prazosin respectively mimicked and partially antagonized the depression of NMDAR-mediated excitations induced by NA. Double-labeling immunohistochemical techniques demonstrated broad colocalization of NMDAR (specifically NR1 and NR2 subunits) with noradrenergic receptors (alpha1, alpha2 and beta1) in many VN neurons; only minor differences were found between nuclei. These results indicate that NA can produce generalized modulation of NMDAR-mediated excitatory neurotransmission in VN, which may in turn modify synaptic plasticity within the nuclei.

Identification of neural networks that contribute to motion sickness through principal components analysis of fos labeling induced by galvanic vestibular stimulation

Motion sickness is a complex condition that includes both overt signs (e.g., vomiting) and more covert symptoms (e.g., anxiety and foreboding). The neural pathways that mediate these signs and symptoms are yet to identified. This study mapped the distribution of c-fos protein (Fos)-like immunoreactivity elicited during a galvanic vestibular stimulation paradigm that is known to induce motion sickness in felines. A principal components analysis was used to identify networks of neurons activated during this stimulus paradigm from functional correlations between Fos labeling in different nuclei. This analysis identified five principal components (neural networks) that accounted for greater than 95% of the variance in Fos labeling. Two of the components were correlated with the severity of motion sickness symptoms, and likely participated in generating the overt signs of the condition. One of these networks included neurons in locus coeruleus, medial, inferior and lateral vestibular nuclei, lateral nucleus tractus solitarius, medial parabrachial nucleus and periaqueductal gray. The second included neurons in the superior vestibular nucleus, precerebellar nuclei, periaqueductal gray, and parabrachial nuclei, with weaker associations of raphe nuclei. Three additional components (networks) were also identified that were not correlated with the severity of motion sickness symptoms. These networks likely mediated the covert aspects of motion sickness, such as affective components. The identification of five statistically independent component networks associated with the development of motion sickness provides an opportunity to consider, in network activation dimensions, the complex progression of signs and symptoms that are precipitated in provocative environments. Similar methodology can be used to parse the neural networks that mediate other complex responses to environmental stimuli.

Vestibular insights into cognition and psychiatry

The vestibular system has traditionally been thought of as a balance apparatus; however, accumulating research suggests an association between vestibular function and psychiatric and cognitive symptoms, even when balance is measurably unaffected. There are several brain regions that are implicated in both vestibular pathways and psychiatric disorders. The present review examines the anatomical associations between the vestibular system and various psychiatric disorders. Despite the lack of direct evidence for vestibular pathology in the key psychiatric disorders selected for this review, there is a substantial body of literature implicating the vestibular system in each of the selected psychiatric disorders. The second part of this review provides complimentary evidence showing the link between vestibular dysfunction and vestibular stimulation upon cognitive and psychiatric symptoms. In summary, emerging research suggests the vestibular system can be considered a potential window for exploring brain function beyond that of maintenance of balance, and into areas of cognitive, affective and psychiatric symptomology. Given the paucity of biological and diagnostic markers in psychiatry, novel avenues to explore brain function in psychiatric disorders are of particular interest and warrant further exploration.

Postural threat differentially affects the feedforward and feedback components of the vestibular-evoked balance response

Circumstances may render the consequence of falling quite severe, thus maximising the motivation to control postural sway. This commonly occurs when exposed to height and may result from the interaction of many factors, including fear, arousal, sensory information and perception. Here, we examined human vestibular-evoked balance responses during exposure to a highly threatening postural context. Nine subjects stood with eyes closed on a narrow walkway elevated 3.85 m above ground level. This evoked an altered psycho-physiological state, demonstrated by a twofold increase in skin conductance. Balance responses were then evoked by galvanic vestibular stimulation. The sway response, which comprised a whole-body lean in the direction of the edge of the walkway, was significantly and substantially attenuated after ~800 ms. This demonstrates that a strong reason to modify the balance control strategy was created and subjects were highly motivated to minimise sway. Despite this, the initial response remained unchanged. This suggests little effect on the feedforward settings of the nervous system responsible for coupling pure vestibular input to functional motor output. The much stronger, later effect can be attributed to an integration of balance-relevant sensory feedback once the body was in motion. These results demonstrate that the feedforward and feedback components of a vestibular-evoked balance response are differently affected by postural threat. Although a fear of falling has previously been linked with instability and even falling itself, our findings suggest that this relationship is not attributable to changes in the feedforward vestibular control of balance.

Altered brain metabolism in vestibular migraine: comparison of interictal and ictal findings

BACKGROUND

Vestibular symptoms/signs frequently coexist with migraine, but the mechanisms of migraine-related vestibular dysfunction remain to be elucidated. This study aimed to determine altered brain metabolism in vestibular migraine.

METHODS

Two patients with vestibular migraine underwent (18)F-fluorodeoxy glucose (FDG) positron-emission tomography (PET) during and between attacks of vestibular migraine in addition to detailed neurotological evaluation. We analyzed the regional brain metabolism of the patients in comparison with that of age-matched healthy controls in each patient. We also compared ictal with interictal FDG PET using a subtraction method.

RESULTS

During the attacks, both patients showed an activation of the bilateral cerebellum and frontal cortices, and deactivation of the bilateral posterior parietal and occipitotemporal areas. One patient also showed hypermetabolism in the dorsal pons and midbrain, right posterior insula, and right temporal cortex while the other patient had an additional activation of the left temporal cortex. Compared with interictal images, ictal PET showed increased metabolism in the bilateral cerebellum, frontal cortices, temporal cortex, posterior insula, and thalami.

CONCLUSION

During the attacks of vestibular migraine, the increased metabolism in the temporo-parieto-insular areas and bilateral thalami indicates activation of the vestibulo-thalamo-cortical pathway, and decreased metabolism in the occipital cortex may represent reciprocal inhibition between the visual and vestibular systems.

Top-down approach to vestibular compensation: translational lessons from vestibular rehabilitation

This review examines vestibular compensation and vestibular rehabilitation from a unified translational research perspective. Laboratory studies illustrate neurobiological principles of vestibular compensation at the molecular, cellular and systems levels in animal models that inform vestibular rehabilitation practice. However, basic research has been hampered by an emphasis on 'naturalistic' recovery, with time after insult and drug interventions as primary dependent variables. The vestibular rehabilitation literature, on the other hand, provides information on how the degree of compensation can be shaped by specific activity regimens. The milestones of the early spontaneous static compensation mark the re-establishment of static gaze stability, which provides a common coordinate frame for the brain to interpret residual vestibular information in the context of visual, somatosensory and visceral signals that convey gravitoinertial information. Stabilization of the head orientation and the eye orientation (suppression of spontaneous nystagmus) appear to be necessary by not sufficient conditions for successful rehabilitation, and define a baseline for initiating retraining. The lessons from vestibular rehabilitation in animal models offer the possibility of shaping the recovery trajectory to identify molecular and genetic factors that can improve vestibular compensation.

Reconsidering the role of neuronal intrinsic properties and neuromodulation in vestibular homeostasis

The sensorimotor transformations performed by central vestibular neurons constantly adapt as the animal faces conflicting sensory information or sustains injuries. To ensure the homeostasis of vestibular-related functions, neural changes could in part rely on the regulation of 2° VN intrinsic properties. Here we review evidence that demonstrates modulation and plasticity of central vestibular neurons’ intrinsic properties. We first present the partition of Rodents’ vestibular neurons into distinct subtypes, namely type A and type B. Then, we focus on the respective properties of each type, their putative roles in vestibular functions, fast control by neuromodulators and persistent modifications following a lesion. The intrinsic properties of central vestibular neurons can be swiftly modulated by a wealth of neuromodulators to adapt rapidly to temporary changes of ecophysiological surroundings. To illustrate how intrinsic excitability can be rapidly modified in physiological conditions and therefore be therapeutic targets, we present the modulation of vestibular reflexes in relation to the variations of the neuromodulatory inputs during the sleep/wake cycle. On the other hand, intrinsic properties can also be slowly, yet permanently, modified in response to major perturbations, e.g., after unilateral labyrinthectomy (UL). We revisit the experimental evidence, which demonstrates that drastic alterations of the central vestibular neurons’ intrinsic properties occur following UL, with a slow time course, more on par with the compensation of dynamic deficits than static ones. Data are interpreted in the framework of distributed processes that progress from global, large-scale coping mechanisms (e.g., changes in behavioral strategies) to local, small-scale ones (e.g., changes in intrinsic properties). Within this framework, the compensation of dynamic deficits improves over time as deeper modifications are engraved within the finer parts of the vestibular-related networks. Finally, we offer perspectives and working hypotheses to pave the way for future research aimed at understanding the modulation and plasticity of central vestibular neurons’ intrinsic properties.

Neurologic bases for comorbidity of balance disorders, anxiety disorders and migraine: neurotherapeutic implications

The comorbidity among balance disorders, anxiety disorders and migraine has been studied extensively from clinical and basic research perspectives. From a neurological perspective, the comorbid symptoms are viewed as the product of sensorimotor, interoceptive and cognitive adaptations that are produced by afferent interoceptive information processing, a vestibulo-parabrachial nucleus network, a cerebral cortical network (including the insula, orbitofrontal cortex, prefrontal cortex and anterior cingulate cortex), a raphe nuclear-vestibular network, a coeruleo-vestibular network and a raphe-locus coeruleus loop. As these pathways overlap extensively with pathways implicated in the generation, perception and regulation of emotions and affective states, the comorbid disorders and effective treatment modalities can be viewed within the contexts of neurological and psychopharmacological sites of action of current therapies.

Neuropharmacology of vestibular system disorders

This work reviews the neuropharmacology of the vestibular system, with an emphasis on the mechanism of action of drugs used in the treatment of vestibular disorders. Otolaryngologists are confronted with a rapidly changing field in which advances in the knowledge of ionic channel function and synaptic transmission mechanisms have led to the development of new scientific models for the understanding of vestibular dysfunction and its management. In particular, there have been recent advances in our knowledge of the fundamental mechanisms of vestibular system function and drug mechanisms of action. In this work, drugs acting on vestibular system have been grouped into two main categories according to their primary mechanisms of action: those with effects on neurotransmitters and neuromodulator receptors and those that act on voltage-gated ion channels. Particular attention is given in this review to drugs that may provide additional insight into the pathophysiology of vestibular diseases. A critical review of the pharmacology and highlights of the major advances are discussed in each case.

Dizziness, migrainous vertigo and psychiatric disorders

OBJECTIVES

This study sought to establish the prevalence of vestibular disorders, migraine and definite migrainous vertigo in patients with psychiatric disorders who were referred for treatment of dizziness, without a lifetime history of vertigo.

STUDY DESIGN

Retrospective study.

SETTING

Out-patients in a university hospital.

MATERIALS AND METHODS

Fifty-two dizzy patients with panic disorders and agoraphobia, 30 with panic disorders without agoraphobia, and 20 with depressive disorders underwent otoneurological screening with bithermal caloric stimulation. The prevalence of migraine and migrainous vertigo was assessed. The level of dizziness was evaluated using the Dizziness Handicap Inventory.

RESULTS

Dizzy patients with panic disorders and agoraphobia had a significantly p = 0.05 regarding the prevalence of peripheral vestibular abnormalities in the group of subjects with PD and agoraphobia and in those with depressive disorders. Migraine was equally represented in the three groups, but panic disorder patients had a higher prevalence of migrainous vertigo definite migrainous vertigo. Almost all patients with a peripheral vestibular disorder had a final diagnosis of definite migrainous vertigo according to Neuhauser criteria. These patients had higher Dizziness Handicap Inventory scores. The Dizziness Handicap Inventory total score was higher in the subgroup of patients with panic disorders with agoraphobia also presenting unilateral reduced caloric responses or definite migrainous vertigo, compared with the subgroup of remaining subjects with panic disorders with agoraphobia (p < 0.001).

CONCLUSIONS

Our data support the hypothesis that, in patients with panic disorders (and especially those with additional agoraphobia), dizziness may be linked to malfunction of the vestibular system. However, the data are not inconsistent with the hypothesis that migrainous vertigo is the most common pathophysiological mechanism for vestibular disorders.

Noradrenergic regulation of GABAergic inhibition of main olfactory bulb mitral cells varies as a function of concentration and receptor subtype

The main olfactory bulb (MOB) receives a rich noradrenergic innervation from the pontine nucleus locus coeruleus (LC). Previous studies indicate that norepinephrine (NE) modulates the strength of GABAergic inhibition in MOB. However, the nature of this modulation and the NE receptors involved remain controversial. The goal of this study was to investigate the role of NE receptor subtypes in modulating the GABAergic inhibition of mitral cells using patch-clamp electrophysiology in rat MOB slices. NE concentration dependently and bi-directionally modulated GABA(A) receptor-mediated spontaneous and miniature inhibitory postsynaptic currents (sIPSCs/mIPSCs) recorded in mitral cells. Low doses of NE suppressed sIPSCs and mIPSCs because of activation of alpha2 receptors. Intermediate concentrations of NE increased sIPSCs and mIPSCs primarily because of activation of alpha1 receptors. In contrast, activation of beta receptors increased sIPSCs but not mIPSCs. These results indicate that NE release regulates the strength of GABAergic inhibition of mitral cells depending on the NE receptor subtype activated. Functionally, the differing affinity of noradrenergic receptor subtypes seems to allow for dynamic modulation of GABAergic inhibition in MOB as function of the extracellular NE concentration, which in turn, is regulated by behavioral state.

Selective anterograde tracing of nonserotonergic projections from dorsal raphe nucleus to the basal forebrain and extended amygdala

The dorsal raphe nucleus (DRN) contains both serotonergic and nonserotonergic projection neurons. Retrograde tracing studies have demonstrated that components of the basal forebrain and extended amygdala are targeted heavily by input from nonserotonergic DRN neurons. The object of this investigation was to examine the terminal distribution of nonserotonergic DRN projections in the basal forebrain and extended amygdala, using a technique that allows selective anterograde tracing of nonserotonergic DRN projections. To trace nonserotonergic DRN projections, animals were pretreated with nomifensine, desipramine and the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), 7 days prior to placing an iontophoretic injection of biotinylated dextran amine (BDA) into the DRN. In animals treated with 5,7-DHT, numerous nonserotonergic BDA-labeled fibers ascended to the basal forebrain in the medial forebrain bundle system. Some of these labeled fibers crossed through the lateral hypothalamus, bed nucleus of the stria terminalis, and substantial innominata. These fibers entered the amygdala through the ansa peduncularis and ramified within the central and basolateral amygdaloid nuclei. Other fibers entered the diagonal band of Broca and formed a dense plexus of labeled fibers in the dorsal half of the intermediate portion of the lateral septal nucleus and the septohippocampal nucleus. These findings demonstrate that the basal forebrain and extended amygdala receive a dense projection from nonserotonergic DRN neurons. Given that the basal forebrain plays a critical role in processes such as motivation, affect, and behavioral control, these findings support the hypothesis that nonserotonergic DRN projections may exert substantial modulatory control over emotional and motivational functions.

Noradrenaline modulates neuronal responses to GABA in vestibular nuclei

The effects of noradrenaline (NA) on the inhibitory responses to GABA were studied in vivo in neurons of the vestibular nuclei of the rat using extracellular recordings of single unit electrical activity and a microiontophoretic technique of drug application in loco. NA application influenced GABA-evoked inhibitions in 82% of tested neurons, depressing them in 42% and enhancing them in 40% of cases. The more frequent action of NA on GABA responses was depressive in lateral and superior vestibular nuclei (50% of neurons) and enhancing in the remaining nuclei (56% of neurons). The most intense effect of NA application was the enhancement of GABA responses induced in a population of lateral vestibular nucleus neurons, characterized by a background firing rate significantly higher than that of other units. The alpha(2) noradrenergic receptor agonist clonidine mimicked the enhancing action of NA on GABA responses; this action was blocked by application of the specific alpha(2) antagonist yohimbine. The beta adrenergic agonist isoproterenol induced either depressive or enhancing effects on GABA responses; the former more than the latter were totally or partially blocked by application of the beta antagonist timolol. It is concluded that NA enhances GABA responses by acting on noradrenergic alpha(2) and to a lesser extent beta receptors, whereas depressive action involves beta receptors only. These results confirm the hypothesis that the noradrenergic system participates in the regulation of the vestibulospinal and the vestibulo-ocular reflexes and suggest that conspicuous changes of NA content in brain due to aging or stress could lead to a deterioration in the mechanisms of normal vestibular function.

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.

Desensitization of 5-HT(1A) autoreceptors induced by neonatal DSP-4 treatment

To examine the effect of noradrenergic lesion on the reactivity of central 5-HT(1A) receptors, DSP-4 (50 mg/kg) was administered neonatally 30 min after zimelidine (10 mg/kg) administration. 5-HT(1A) autoreceptors are involved in the regulation of serotonin (5-HT) synthesis. In HPLC assay R-(+)-8-OH-DPAT (0.03 mg/kg) significantly decreased 5-HT synthesis rate in striatum, hypothalamus and frontal cortex of control, whilst nonsignificantly in DSP-4-lesioned adult rats (10-12 weeks old). To determine which type of receptor, pre- or postsynaptically located, is involved in the attenuated response to 5-HT(1A) receptors' agonist, behavioral tests were conducted. R-(+)-8-OH-DPAT (0.015 mg/kg) caused hyperphagia of control rats, but did not change feeding of DSP-4 treated rats. R-(+)-8-OH-DPAT (0.1 mg/kg) induced hypothermia and "5-HT(1A) syndrome" in both control and DSP-4-lesioned animals. The nature of this phenomenon is attributable to the presynaptic adaptive mechanism and suggests the desensitization of 5-HT(1A) autoreceptors of rats with neonatal lesion of the central noradrenergic system.

Maturation of otolith-related brainstem neurons in the detection of vertical linear acceleration in rats

To investigate the critical maturation time of otolith-related neurons in processing vertical orientations, rats (postnatal day 4 to adults) were studied for functional activation of c-fos expression in brainstem neurons by immuno-/hybridization histochemistry. Conscious rats were subjected to sinusoidal linear acceleration along the vertical plane. Labyrinthectomized and/or stationary controls showed only sporadically scattered Fos-labeled neurons in the vestibular nuclei, confirming an otolithic origin of c-fos expression. Functionally activated Fos expression in neurons of the medial and spinal vestibular nuclei and group x were identifiable by P7 and those in group y by P9. A small number of Fos-labeled neurons characterized by small soma size were found in the ventral part of lateral vestibular nucleus by P9. Other vestibular-related areas such as prepostitus hypoglossal nucleus, gigantocellular reticular nucleus and locus coeruleus of normal experimental rats showed functionally activated c-fos expression at P7. Neurons in dorsal medial cell column and beta subnucleus of the inferior olive only showed functionally activated c-fos expression by the second postnatal week. These findings revealed a unique critical maturation time for each of the vestibular-related brainstem areas in the recognition of gravity-related vertical head orientations. By mapping the three-dimensional distribution of Fos-immunoreactive neurons, we found an even distribution of otolith-related neurons within the spinal vestibular nucleus in groups x and y but a clustered distribution in the middle-lateral-ventral part of the medial vestibular nucleus. Taken together, our findings reveal the developmental profile of neuronal subpopulations within the vertical otolith system, thereby providing an anatomical basis for postnatal coding of gravity-related vertical head movements.

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.

Antidepressants and dizziness

Despite several evidences supporting a close relationship between dizziness and anxiety symptoms, the role of antidepressants in the treatment of such conditions remains poorly explored. The high prevalence of dizziness during serotonin reuptake inhibitors discontinuation syndrome and the few reports suggesting efficacy of antidepressants in treating some vestibular disorders justify a careful investigation. Neurophysiologic studies suggest possible focus of investigation on mechanisms of drug action. Psychophysiologic studies also suggest a possible role of antidepressant drugs in improving balance control and cognitive functioning. Controlled studies involving antidepressants with selective action in different neurotransmitters systems are necessary to elucidate the complex pathophysiologic mechanisms involving emotional and balance control. For future researches, special attention must be paid to the methodology of balance evaluation and the interaction between posture control and cognitive functioning.

Sexual dimorphism in the vomeronasal system of the rabbit

Studies have shown that the vomeronasal system (VNS), an olfactory neural network that participates in the control of reproductive physiology and behavior, is sexually dimorphic in the rat. These works have also shown two main characteristics of brain sexual dimorphism: (a) dimorphism appears in neural networks related to reproduction and (b) it can present two morphological patterns: one in which males present greater morphological measures than females (male > female) and another in which the opposite is true (female > male). The present work extends the hypothesis to the rabbit, as a representative species of Lagomorpha. In addition, the locus coeruleus (LC), which is known to send rich noradrenergic projections to VNS structures, was also studied. Sex differences were found in: (a) the number of mitral, and dark and light granule cells (female > male) of the accessory olfactory bulb (AOB); (b) the medial amygdala (Me) and its dorsal (Med) and ventral (Mev) subdivisions, males showing greater values than females in volume and number of neurons, while in the posteromedial cortical amygdala (PMCo or C(3)), females show greater density of neurons than males and (c) the posteromedial division of the bed nucleus of the stria terminalis (BSTMP) in which males have more neurons than females. No sex differences were seen in the bed nucleus of the accessory olfactory tract (BAOT) and the LC. These results evidence that, as it was observed in rodents, sex differences are also seen in the VNS of Lagomorpha and that these sex differences present the two morphological patterns seen in Rodentia. Differences between orders are discussed with respect to the species-specific physiological and behavioral peculiarities.

Nucleus prepositus

The cytoarchitecture and the histochemistry of nucleus prepositus hypoglossi and its afferent and efferent connections to oculomotor structures are described. The functional significance of the afferent connections of the nucleus is discussed in terms of current knowledge of the firing behavior of prepositus neurons in alert animals. The efferent connections of the nucleus and the results of lesion experiments suggest that it plays a role in a variety of functions related to the control of gaze.

Medial vestibular connections with the hypocretin (orexin) system

The mammalian medial vestibular nucleus (MVe) receives input from all vestibular endorgans and provides extensive projections to the central nervous system. Recent studies have demonstrated projections from the MVe to the circadian rhythm system. In addition, there are known projections from the MVe to regions considered to be involved in sleep and arousal. In this study, afferent and efferent subcortical connectivity of the medial vestibular nucleus of the golden hamster (Mesocricetus auratus) was evaluated using cholera toxin subunit-B (retrograde), Phaseolus vulgaris leucoagglutinin (anterograde), and pseudorabies virus (transneuronal retrograde) tract-tracing techniques. The results demonstrate MVe connections with regions mediating visuomotor and postural control, as previously observed in other mammals. The data also identify extensive projections from the MVe to regions mediating arousal and sleep-related functions, most of which receive immunohistochemically identified projections from the lateral hypothalamic hypocretin (orexin) neurons. These include the locus coeruleus, dorsal and pedunculopontine tegmental nuclei, dorsal raphe, and lateral preoptic area. The MVe itself receives a projection from hypocretin cells. CTB tracing demonstrated reciprocal connections between the MVe and most brain areas receiving MVe efferents. Virus tracing confirmed and extended the MVe afferent connections identified with CTB and additionally demonstrated transneuronal connectivity with the suprachiasmatic nucleus and the medial habenular nucleus. These anatomical data indicate that the vestibular system has access to a broad array of neural functions not typically associated with visuomotor, balance, or equilibrium, and that the MVe is likely to receive information from many of the same regions to which it projects.

Balance dysfunction in childhood anxiety: findings and theoretical approach

A recent special issue of the Journal of Anxiety Disorders, reviewed the experimental and clinical findings related to comorbidity of balance disorders and anxiety [J. Anxiety Disord. 15 (2001) 1.]. The studies mentioned in that issue were based mostly on adult subjects but prevalence of balance disorders in childhood anxiety is yet to be established. We have tested a small sample of children diagnosed for general or separation anxiety disorder and a control group of normal children. Extensive neurological examination revealed no clinically relevant vestibular impairment. Nevertheless, detailed questionnaires and balance tests confirmed an excessive sensitivity of anxiety disordered children to balance-challenging situations. Moreover, balance-challenging tasks triggered more balance mistakes and slower performance in anxiety versus control children. These findings support the notion of subclinical balance disorder in childhood anxiety. Results are discussed in terms of the two-stage theory of learning, which predicts that anxiety disorder may be an offshoot of lasting balance dysfunction.

[Anatomy and physiology of the vestibular system: review of the literature]

The vestibular system is a complex system involving not only posterior labyrinth but also central structures such as cerebellum, striatum, thalamus, frontal and prefrontal cortex to assure balance, movements and walking. Information reaching the vestibular complex are not purely vestibular but also from visual, somatosensory and cerebellar origins. The equilibrium is also a complex physiological function needing concordance of vestibular, visual and somatosensory information or either central compensation after an injury but also an integrity of the central nervous system.