Physiological assesment of vestibular function and toxicity in humans and animals

An improved method for toxicological profiling of chemical substances

Toxicity profiling is an integral part of the drug discovery pipeline. The 3Rs principle-Replacement, Reduction, and Refinement, is considered a golden rule in determining the most appropriate approach for toxicity studies. The acute toxicity study with proper estimate of median lethal dose (LD50) is usually an initial procedure for the determination of most suitable test doses for preclinical toxicological and pharmacological profiling. Several methods, which have been devised to determine the LD50, are faced with the challenge of using a large number of animals and time constraints. Despite the inherent advantage of the newer OECD Test Guidelines, the increasing concerns among toxicologists, the regulatory authorities and the general public, on the need to adhere to 3Rs principle, necessitated the need for an improved approach. Such an approach should not only minimize the time and number of animals required, but also take into cognizance animal welfare, and give accurate, comparable, and reproducible results across laboratories. While taking advantage of the inherent merits of the existing methods, here is presented the mathematical basis and evaluation of an improved method for toxicity profiling of test substances and estimation of LD50. The method makes use of the generated Table of values for the selection of appropriate test doses. Our proposed method has capacities to optimize the time and number of animal use, ensure more reliable and reproducible results across laboratories, allow for easy selection of doses for subsequent toxicity profiling, and be adaptable to other biological screening beyond toxicity studies.

[Animal models of balance pathologies: New tools to study peripheral vestibulopathies]

The different types of peripheral vestibulopathies (PVs) or peripheral vestibular disorders (PVDs) are essentially diagnosed on the basis of their clinical expression. The heterogeneity of vestibular symptoms makes it difficult to stratify patients for therapeutic management. Animal models of PVs are a good mean to search for clinical evaluation criteria allowing to objectively analyze the kinetics of expression of the vertigo syndrome and to evaluate the benefits of therapeutic strategies, whether they are pharmacological or rehabilitative. The question of the predictability of these animal models is therefore crucial for the identification of behavioral and biological biomarkers that could then be used in the human clinic. In this review, we propose an overview of the different animal models of PVs, and discuss their relevance for the understanding of the underlying pathophysiological mechanisms and the development of new and more targeted therapeutic approaches.

Pimozide Increases a Delayed Rectifier K+ Conductance in Chicken Embryo Vestibular Hair Cells

Pimozide is a conventional antipsychotic drug largely used in the therapy for schizophrenia and Tourette's syndrome. Pimozide is assumed to inhibit synaptic transmission at the CNS by acting as a dopaminergic D₂ receptor antagonist. Moreover, pimozide has been shown to block voltage-gated Ca²⁺ and K⁺ channels in different cells. Despite its widespread clinical use, pimozide can cause several adverse effects, including extrapyramidal symptoms and cardiac arrhythmias. Dizziness and loss of balance are among the most common side effects of pimozide. By using the patch-clamp whole-cell technique, we investigated the effect of pimozide [3 μM] on K⁺ channels expressed by chicken embryo vestibular type-II hair cells. We found that pimozide slightly blocks a transient outward rectifying A-type K⁺ current but substantially increases a delayed outward rectifying K⁺ current. The net result was a significant hyperpolarization of type-II hair cells at rest and a strong reduction of their response to depolarizing stimuli. Our findings are consistent with an inhibitory effect of pimozide on the afferent synaptic transmission by type-II hair cells. Moreover, they provide an additional key to understanding the beneficial/collateral pharmacological effects of pimozide. The finding that pimozide can act as a K⁺ channel opener provides a new perspective for the use of this drug.

What Predictability for Animal Models of Peripheral Vestibular Disorders?

The different clinical entities grouped under the term peripheral vestibulopathies (PVs) or peripheral vestibular disorders (PVDs) are distinguished mainly based on their symptoms/clinical expression. Today, there are very few commonly accepted functional and biological biomarkers that can confirm or refute whether a vestibular disorder belongs to a precise classification. Consequently, there is currently a severe lack of reliable and commonly accepted clinical endpoints, either to precisely follow the course of the vertigo syndrome of vestibular origin or to assess the benefits of therapeutic approaches, whether they are pharmacological or re-educational. Animal models of PV are a good means to identify biomarkers that could subsequently be exploited in human clinical practice. The question of their predictability is therefore crucial. Ten years ago, we had already raised this question. We revisit this concept today in order to take into account the animal models of peripheral vestibular pathology that have emerged over the last decade, and the new technological approaches available for the behavioral assessment of vestibular syndrome in animals and its progression over time. The questions we address in this review are the following: are animal models of PV predictive of the different types and stages of vestibular pathologies, and if so, to what extent? Are the benefits of the pharmacological or reeducational therapeutic approaches achieved on these different models of PV (in particular the effects of attenuation of the acute vertigo, or acceleration of central compensation) predictive of those expected in the vertiginous patient, and if so, to what extent?

Roadbumps at the Crossroads of Integrating Behavioral and In Vitro Approaches for Neurotoxicity Assessment

With the appreciation that behavior represents the integration and complexity of the nervous system, neurobehavioral phenotyping and assessment has seen a renaissance over the last couple of decades, resulting in a robust database on rodent performance within various testing paradigms, possible associations with human disorders, and therapeutic interventions. The interchange of data across behavior and other test modalities and multiple model systems has advanced our understanding of fundamental biology and mechanisms associated with normal functions and alterations in the nervous system. While there is a demonstrated value and power of neurobehavioral assessments for examining alterations due to genetic manipulations, maternal factors, early development environment, the applied use of behavior to assess environmental neurotoxicity continues to come under question as to whether behavior represents a sensitive endpoint for assessment. Why is rodent behavior a sensitive tool to the neuroscientist and yet, not when used in pre-clinical or chemical neurotoxicity studies? Applying new paradigms and evidence on the biological basis of behavior to neurobehavioral testing requires expertise and refinement of how such experiments are conducted to minimize variability and maximize information. This review presents relevant issues of methods used to conduct such test, sources of variability, experimental design, data analysis, interpretation, and reporting. It presents beneficial and critical limitations as they translate to the in vivo environment and considers the need to integrate across disciplines for the best value. It proposes that a refinement of behavioral assessments and understanding of subtle pronounced differences will facilitate the integration of data obtained across multiple approaches and to address issues of translation.

Age-Related Changes in the Cochlea and Vestibule: Shared Patterns and Processes

Both age-related hearing loss (ARHL) and age-related loss in vestibular function (ARVL) are prevalent conditions with deleterious consequences on the health and quality of life. Age-related changes in the inner ear are key contributors to both conditions. The auditory and vestibular systems rely on a shared sensory organ - the inner ear - and, like other sensory organs, the inner ear is susceptible to the effects of aging. Despite involvement of the same sensory structure, ARHL and ARVL are often considered separately. Insight essential for the development of improved diagnostics and treatments for both ARHL and ARVL can be gained by careful examination of their shared and unique pathophysiology in the auditory and vestibular end organs of the inner ear. To this end, this review begins by comparing the prevalence patterns of ARHL and ARVL. Next, the normal and age-related changes in the structure and function of the auditory and vestibular end organs are compared. Then, the contributions of various molecular mechanisms, notably inflammaging, oxidative stress, and genetic factors, are evaluated as possible common culprits that interrelate pathophysiology in the cochlea and vestibular end organs as part of ARHL and ARVL. A careful comparison of these changes reveals that the patterns of pathophysiology show similarities but also differences both between the cochlea and vestibular end organs and among the vestibular end organs. Future progress will depend on the development and application of new research strategies and the integrated investigation of ARHL and ARVL using both clinical and animal models.

Effect of Oral Allylnitrile Administration on Cochlear Functioning in Mice Following Comparison of Different Anesthetics for Hearing Assessment

Background: Allylnitrile is a compound found in cruciferous vegetables and has the same lethality and toxic effects as the other nitriles. In 2013, a viable allylnitrile ototoxicity mouse model was established. The toxicity of allylnitrile was limited through inhibition of CYP2E1 with trans-1,2-dichloroethylene (TDCE). The allylnitrile intoxication model has been extensively tested in the 129S1 mouse strain for vestibular function, which showed significant HC loss in the vestibular organ accompanied by severe behavioral abnormalities. However, the effect of allylnitrile on auditory function remains to be evaluated. Commonly used anesthetics to conduct hearing measurements are isoflurane and ketamine/xylazine anesthesia but the effect of these anesthetics on hearing assessment is still unknown. In this study we will evaluate the otovestibular effects of oral allylnitrile administration in mice. In addition, we will compare the influence of isoflurane and ketamine/xylazine anesthesia on hearing thresholds.

Methods and Materials: Fourteen Coch+/– CBACa mice were randomly allocated into an allylnitrile (n = 8) and a control group (n = 6). Baseline measurements were done with isoflurane and 1 week later under ketamine/xylazine anesthesia. After baseline audiovestibular measurements, mice were co-administered with a single dose of allylnitrile and, to reduce systemic toxicity, three intraperitoneal injections of TDCE were given. Hearing loss was evaluated by recordings of auditory brainstem responses (ABR) and distortion product otoacoustic emissions (DPOAE). Specific behavioral test batteries for vestibular function were used to assess alterations in vestibular function.

Results: Hearing thresholds were significantly elevated when using isoflurane anesthesia compared to ketamine/xylazine anesthesia for all frequencies of the ABR and the mid-to-high frequencies in DPOAE. Allylnitrile-treated mice lacked detectable ABR thresholds at each frequency tested, while DPOAE thresholds were significantly elevated in the low-frequency region of the cochlea and completely lacking in the mid-to high frequency region. Vestibular function was not affected by allylnitrile administration.

Conclusion: Isoflurane anesthesia has a negative confounding effect on the measurement of hearing thresholds in mice. A single oral dose of allylnitrile induced hearing loss but did not significantly alter vestibular function in mice. This is the first study to show that administration of allylnitrile can cause a complete loss of hearing function in mice.

Quantitative Assessment of Anti-Gravity Reflexes to Evaluate Vestibular Dysfunction in Rats

The tail-lift reflex and the air-righting reflex are anti-gravity reflexes in rats that depend on vestibular function. To obtain objective and quantitative measures of performance, we recorded these reflexes with slow-motion video in two experiments. In the first experiment, vestibular dysfunction was elicited by acute exposure to 0 (control), 400, 600, or 1000 mg/kg of 3,3'-iminodipropionitrile (IDPN), which causes dose-dependent hair cell degeneration. In the second, rats were exposed to sub-chronic IDPN in the drinking water for 0 (control), 4, or 8 weeks; this causes reversible or irreversible loss of vestibular function depending on exposure time. In the tail-lift test, we obtained the minimum angle defined during the lift and descent maneuver by the nose, the back of the neck, and the base of the tail. In the air-righting test, we obtained the time to right the head. We also obtained vestibular dysfunction ratings (VDRs) using a previously validated behavioral test battery. Each measure, VDR, tail-lift angle, and air-righting time demonstrated dose-dependent loss of vestibular function after acute IDPN and time-dependent loss of vestibular function after sub-chronic IDPN. All measures showed high correlations between each other, and maximal correlation coefficients were found between VDRs and tail-lift angles. In scanning electron microscopy evaluation of the vestibular sensory epithelia, the utricle and the saccule showed diverse pathological outcomes, suggesting that they have a different role in these reflexes. We conclude that these anti-gravity reflexes provide useful objective and quantitative measures of vestibular function in rats that are open to further development.

Calyx junction dismantlement and synaptic uncoupling precede hair cell extrusion in the vestibular sensory epithelium during sub-chronic 3,3'-iminodipropionitrile ototoxicity in the mouse

The cellular and molecular events that precede hair cell (HC) loss in the vestibular epithelium during chronic ototoxic exposure have not been widely studied. To select a study model, we compared the effects of sub-chronic exposure to different concentrations of 3,3'-iminodipropionitrile (IDPN) in the drinking water of two strains of mice and of both sexes. In subsequent experiments, male 129S1/SvImJ mice were exposed to 30 mM IDPN for 5 or 8 weeks; animals were euthanized at the end of the exposure or after a washout period of 13 weeks. In behavioral tests, IDPN mice showed progressive vestibular dysfunction followed by recovery during washout. In severely affected animals, light and electron microscopy observations of the vestibular epithelia revealed HC extrusion towards the endolymphatic cavity. Comparison of functional and ultrastructural data indicated that animals with fully reversible dysfunction did not have significant HC loss or stereociliary damage, but reversible dismantlement of the calyceal junctions that characterize the contact between type I HCs (HCI) and their calyx afferents. Immunofluorescent analysis revealed the loss of calyx junction proteins, Caspr1 and Tenascin-C, during exposure and their recovery during washout. Synaptic uncoupling was also recorded, with loss of pre-synaptic Ribeye and post-synaptic GluA2 puncta, and differential reversibility among the three different kinds of synaptic contacts existing in the epithelium. qRT-PCR analyses demonstrated that some of these changes are at least in part explained by gene expression modifications. We concluded that calyx junction dismantlement and synaptic uncoupling are early events in the mouse vestibular sensory epithelium during sub-chronic IDPN ototoxicity.

Combined exposure to carbon disulfide and low-frequency noise reversibly affects vestibular function

Chronic occupational exposure to carbon disulfide (CS2) has debilitating motor and sensory effects in humans, which can increase the risk of falls. Although no mention of vestibulotoxic effects is contained in the literature, epidemiological and experimental data suggest that CS2 could cause low-frequency hearing loss when associated with noise exposure. Low-frequency noise might also perturb the peripheral balance receptor through an as-yet unclear mechanism. Here, we studied how exposure to a low-frequency noise combined with 250-ppm CS2 affected balance in rats. Vestibular function was tested based on post-rotary nystagmus recorded by a video-oculography system. These measurements were completed by behavioral tests and analysis of the cerebellum to measure expression levels for gene expression associated with neurotoxicity. Assays were performed prior to and following a 4-week exposure, and again after a 4-week recovery period. Functional measurements were completed by histological analyses of the peripheral organs.Nystagmus was unaltered by exposure to noise alone, while CS2 alone caused a moderate 19% decrease of the saccade number. In contrast, coexposure to 250-ppm CS2 and low-frequency noise decreased both saccade number and duration by 33% and 34%, respectively. After four weeks, recovery was only partial but measures were not significantly different from pre-exposure values. Real-time quantitative polymerase chain reaction (RT-qPCR) analysis of cerebellar tissue revealed a slight but significant modification in expression levels for two genes linked to neurotoxicity in CS2-exposed animals. However, neither histopathological changes to the peripheral receptor nor behavioral differences were observed. Based on all these results, we propose that the effects of CS2 were due to reversible neurochemical disturbance of the efferent pathways managing post-rotatory nystagmus. Because the nervous structures involving the vestibular function appear particularly sensitive to CS2, post-rotary nystagmus could be used as an early, non-invasive measurement to diagnose CS2 intoxication as part of an occupational conservation program.