Characterization of Thyroid Hormones Antivertigo Effects in a Rat Model of Excitotoxically-Induced Vestibulopathy

Plastic Events of the Vestibular Nucleus: the Initiation of Central Vestibular Compensation

Vestibular compensation is a physiological response of the vestibular organs within the inner ear. This adaptation manifests during consistent exposure to acceleration or deceleration, with the vestibular organs incrementally adjusting to such changes. The molecular underpinnings of vestibular compensation remain to be fully elucidated, yet emerging studies implicate associations with neuroplasticity and signal transduction pathways. Throughout the compensation process, the vestibular sensory neurons maintain signal transmission to the central equilibrium system, facilitating adaptability through alterations in synaptic transmission and neuronal excitability. Notable molecular candidates implicated in this process include variations in ion channels and neurotransmitter profiles, as well as neuronal and synaptic plasticity, metabolic processes, and electrophysiological modifications. This study consolidates the current understanding of the molecular events in vestibular compensation, augments the existing research landscape, and evaluates contemporary therapeutic strategies. Furthermore, this review posits potential avenues for future research that could enhance our comprehension of vestibular compensation mechanisms.

[Research advances in the mechanism of vestibular compensation and treatment]

Unlike other sensory systems, since the vestibular system maintains the tension balance of the entire system in a"push-pull" mode, local dysfunction in the system will cause the balance of the entire system to collapse. Unilateral peripheral vestibular dysfunction will cause severe vestibular symptoms, but it can recover spontaneously within a few days to several weeks. This phenomenon is called "vestibular compensation"（VC）. Since the peripheral vestibular impact in most cases is irreversible, it is widely believed that the central mechanism plays a key role in the vestibular compensation process. Static symptom is fully compensated within a few weeks, which is in parallel with the restored balance in the resting discharge of the vestibular nucleus on both sides; the incomplete compensation of dynamic deficits takes longer and is achieved mainly through the mechanism of sensory substitution and behavioral substitution. Here we briefly reviewed the mechanism of vestibular compensation and treatment in order to provide an insight into further study and clinical treatment strategies.

Statistical Associations between Vestibular Pathologies and Hypothyroidism: A Retrospective Study

The association between vestibular pathologies and thyroid hormone disorders has been known for several decades. However, very little information is available on the types of vestibular symptoms that may be affected by altered thyroid hormone levels. The aim of this study was to provide patient data in order to identify statistical associations between vestibular pathologies and thyroid hormone disorders. A retrospective review of the records of 422 patients seen for physiotherapy treatment of vertigo was carried out. Statistical analysis of the data was performed using logistic regression, providing Chi2 and Odds Ratio statistics. Our results show that hypothyroidism statistically significantly increases the expression of certain symptoms, such as vestibular instability and gait disorders, in vestibular pathologies such as Menière's disease or central vertigo. By analyzing patient data, our study provides new evidence of dependence between altered thyroid status and the expression of vestibular pathologies.

Thyroid Axis and Vestibular Physiopathology: From Animal Model to Pathology

A recent work of our group has shown the significant effects of thyroxine treatment on the restoration of postural balance function in a rodent model of acute peripheral vestibulopathy. Based on these findings, we attempt to shed light in this review on the interaction between the hypothalamic-pituitary-thyroid axis and the vestibular system in normal and pathological situations. Pubmed database and relevant websites were searched from inception through to 4 February 2023. All studies relevant to each subsection of this review have been included. After describing the role of thyroid hormones in the development of the inner ear, we investigated the possible link between the thyroid axis and the vestibular system in normal and pathological conditions. The mechanisms and cellular sites of action of thyroid hormones on animal models of vestibulopathy are postulated and therapeutic options are proposed. In view of their pleiotropic action, thyroid hormones represent a target of choice to promote vestibular compensation at different levels. However, very few studies have investigated the relationship between thyroid hormones and the vestibular system. It seems then important to more extensively investigate the link between the endocrine system and the vestibule in order to better understand the vestibular physiopathology and to find new therapeutic leads.

Effect of Fluoxetine and Acacetin on Central Vestibular Compensation in an Animal Model of Unilateral Peripheral Vestibulopathy

Damage to the peripheral vestibular system is known to generate a syndrome characterized by postural, locomotor, oculomotor, perceptual and cognitive deficits. Current pharmacological therapeutic solutions for these pathologies lack specificity and efficacy. Recently, we demonstrated that apamin, a specific SK channel blocker, significantly reduced posturo-locomotor and oculomotor deficits in the cat and the rat. The aim of the present study was to test the antivertigo potential of compounds belonging to the SK antagonists family, such as Acacetin and Fluoxetine. Young rats were subjected to unilateral ototoxic lesions of the vestibular organ using transtympanic administration of arsanilic acid (TTA) to evoke unilateral vestibular loss (UVL). Vestibular syndrome was monitored using behavioural evaluation allowing appreciation of the evolution of static and dynamic posturo-locomotor deficits. A significant effect of the TTA insult was only found on the distance moved, the mean body velocity and the not moving time. From day 2 to week 2 after TTA, the distance moved and the mean body velocity were significantly decreased, while the not moving time was significantly increased. Acacetin does not evoke any significant change in the vestibular posturo-locomotor parameters' kinetics. Administration of Fluoxetine two weeks before TTA and over three weeks after TTA (preventive group) does not evoke any significant change in the vestibular posturo-locomotor parameters' kinetics. Administration of Fluoxetine from three weeks after TTA significantly delayed the functional recovery. This study demonstrates that Acacetin or Fluoxetine in TTA vestibulo-injured rats does not bring any significant benefit on the posture and locomotor balance deficits.