Histamine immunoreactivity changes in vestibular-lesioned and histaminergic-treated cats

High prevalence of sleep disorders in Ménière's disease: Interplay between vestibular function and sleep

BackgroundIn Ménière's disease (MD), impaired sleep worsens the quality of life and triggers episodes, perpetuating a vicious cycle. The intricate connection between vestibular function and sleep regulation is poorly understood.ObjectiveWe aimed to investigate this link by studying sleep in confirmed MD patients.MethodsIn a single-center observational study, 26 consecutive patients with MD were enrolled. Sleep characterization included thorough symptom inquiry, questionnaires (PSQI, ESS, PFS, STOP-Bang, and AAO-HNS), and respiratory polygraphy (RP) or polysomnography (PSG).ResultsDespite 42% of patients reporting overall sleep satisfaction, further inquiry revealed 76% experiencing sleep disturbances per PSQI. STOP-Bang indicated 56% with a moderate-to-severe risk of OSAS. OSAS defined by apnea hypopnea index (AHI) ≥ 5/h was present in 75%. OSAS severity was categorized as mild (5/h ≤ AHI < 15/h, 33%), moderate (15/h ≤ AHI < 30/h, 21%), and severe (AHI ≥ 30/h, 21%). In patients with AAO-HNS score >3, migraines and insomnia were more prevalent. Patients with moderate-to-severe OSAS had poorer hearing. All recently diagnosed patients with moderate-to-severe OSAS had undergone more than one medical treatment compared to others (32%).ConclusionsThe high prevalence of sleep disorders in MD patients underscores the need for thorough screening, even without spontaneous complaints. Instrumental sleep exploration via RP or PSG is essential, as OSAS treatment could aid vestibular function.

Advanced progress of vestibular compensation in vestibular neural networks

Vestibular compensation is the natural process of recovery that occurs with acute peripheral vestibular lesion. Here, we summarize the current understanding of the mechanisms underlying vestibular compensation, focusing on the role of the medial vestibular nucleus (MVN), the central hub of the vestibular system, and its associated neural networks. The disruption of neural activity balance between the bilateral MVNs underlies the vestibular symptoms after unilateral vestibular damage, and this balance disruption can be partially reversed by the mutual inhibitory projections between the bilateral MVNs, and their top-down regulation by other brain regions via different neurotransmitters. However, the detailed mechanism of how MVN is involved in vestibular compensation and regulated remains largely unknown. A deeper understanding of the vestibular neural network and the neurotransmitter systems involved in vestibular compensation holds promise for improving treatment outcomes and developing more effective interventions for vestibular disorders.

Histaminergic System and Vestibular Function in Normal and Pathological Conditions

Most neurotransmitter systems are represented in the central and peripheral vestibular system and are thereby involved both in normal vestibular signal processing and the pathophysiology of vestibular disorders. However, there is a special relationship between the vestibular system and the histaminergic system. The purpose of this review is to document how the histaminergic system interferes with normal and pathological vestibular function. In particular, we will discuss neurobiological mechanisms such as neuroinflammation that involve histamine to modulate and allow restoration of balance function in the situation of a vestibular insult. These adaptive mechanisms represent targets of histaminergic pharmacological compounds capable of restoring vestibular function in pathological situations. The clinical use of drugs targeting the histaminergic system in various vestibular disorders is critically discussed.

The Effects of Unilateral Labyrinthectomy on Monoamine Neurotransmitters in the Medial Vestibular Nucleus of Rats

BACKGROUND

This study aimed to investigate the effects of unilateral labyrinthectomy (UL) on monoamine neurotransmitters in the medial vestibular nucleus (MVN) of rats.

METHODS

Adult Sprague-Dawley rats were utilized for the vestibular impaired animal model through UL. The success of the model establishment and the recovery process were evaluated using vestibular behavioral tests, including spontaneous nystagmus, postural asymmetry, and balance beam test. Additionally, the expression levels of c-Fos protein in the MVN were assessed by immunofluorescence. Furthermore, changes in the expression levels of monoamine neurotransmitters, including 5-hydroxytryptamine (5-HT), norepinephrine (NE), dopamine (DA), and histamine in the MVN, were analyzed using high-performance liquid chromatography (HPLC) at different time points after UL (4 h, 8 h, 1 day, 2 days, 4 days, and 7 days).

RESULTS

Compared to the sham control group, the UL group exhibited the most pronounced vestibular impairment symptoms at 4 h post-UL, which significantly decreased at 4 days and almost fully recovered by 7 days. Immunofluorescence results showed a notable upregulation of c-Fos expression in the MVN subsequent to the UL-4 h, serving as a reliable indicator of heightened neuronal activity. In comparison with the sham group, HPLC analysis showed that the levels of 5-HT and NE in the ipsilesional MVN of the UL group were significantly elevated within 4 days after UL, and peaked on 1 day and 2 days, respectively. DA showed an increasing trend at different time points up to 7 days post-UL, while histamine levels significantly increased only at 1 day post-UL.

CONCLUSIONS

UL-induced dynamic changes in monoamine neurotransmitters during the early compensation period in the rat MVN may be associated with the regulation of the central vestibular compensation mechanism by the MVN.

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.

Betahistine Treatment in a Cat Model of Vestibular Pathology: Pharmacokinetic and Pharmacodynamic Approaches

This study is a pharmacokinetic (PK) and pharmacodynamics (PD) approach using betahistine doses levels in unilateral vestibular neurectomized cats (UVN) comparable to those used in humans for treating patients with Menière's disease. The aim is to investigate for the first time oral betahistine administration (0.2 and 2 mg/kg/day) with plasma concentrations of betahistine and its major metabolite 2-pyridylacetic acid (2-PAA) (N = 9 cats), the time course of posture recovery (N = 13 cats), and the regulation of the enzyme synthesizing histamine (histidine decarboxylase: HDC) in the tuberomammillary nuclei (TMN) of UVN treated animals (N = the same 13 cats plus 4 negative control cats). In addition the effect of co-administration of the lower betahistine dose (0.2 mg/kg/day) and selegiline (1 mg/kg/day), an inhibitor of the monamine oxidase B (MAOBi) implicated in betahistine catabolism was investigated. The PK parameters were the peak concentration (C_max), the time when the maximum concentration is reached (T_max) for both betahistine and 2-PAA and the area under the curve (AUC). The PD approach consisted at quantifying the surface support area, which is a good estimation of posture recovery. The plasma concentration-time-profiles of betahistine and 2-PAA in cats were characterized by early C_max-values followed by a phase of rapid decrease of plasma concentrations and a final long lasting low level of plasma concentrations. Co administration of selegiline and betahistine increased values of C_max and AUC up to 146- and 180-fold, respectively. The lowest dose of betahistine (0.2 mg/kg) has no effects on postural function recovery but induced an acute symptomatic effect characterized by a fast balance improvement (4–6 days). The higher dose (2 mg/kg) and the co-administration treatment induced both this acute effect plus a significant acceleration of the recovery process. The histaminergic activity of the neurons in the TMN was significantly increased under treatment with the 2 mg/kg betahistine daily dose, but not with the lower dose alone or in combination with selegiline. The results show for the first time that faster balance recovery in UVN treated cats is accompanied with high plasma concentrations of betahistine and 2-PAA, and upregulation of HDC immunopositive neurons in the TMN. The higher betahistine dose gives results similar to those obtained with the lower dose when co-administrated with an inhibitor of betahistine metabolism, selegiline. From a clinical point of view, the study provides new perspectives for Menière's disease treatment, regarding the daily betahistine dose that should be necessary for fast and slow metabolizers.

Plasticity of the histamine H3 receptors after acute vestibular lesion in the adult cat

After unilateral vestibular neurectomy (UVN) many molecular and neurochemical mechanisms underlie the neurophysiological reorganizations occurring in the vestibular nuclei (VN) complex, as well as the behavioral recovery process. As a key regulator, the histaminergic system appears to be a likely candidate because drugs interfering with histamine (HA) neurotransmission facilitate behavioral recovery after vestibular lesion. This study aimed at analyzing the post-lesion changes of the histaminergic system by quantifying binding to histamine H₃ receptors (H₃R; mediating namely histamine autoinhibition) using a histamine H₃ receptor agonist ([³H]N-α-methylhistamine). Experiments were done in brain sections of control cats (N = 6) and cats submitted to UVN and killed 1 (N = 6) or 3 (N = 6) weeks after the lesion. UVN induced a bilateral decrease in binding density of the agonist [³H]N-α-methylhistamine to H₃R in the tuberomammillary nuclei (TMN) at 1 week post-lesion, with a predominant down-regulation in the ipsilateral TMN. The bilateral decrease remained at the 3 weeks survival time and became symmetric. Concerning brainstem structures, binding density in the VN, the prepositus hypoglossi, the subdivisions of the inferior olive decreased unilaterally on the ipsilateral side at 1 week and bilaterally 3 weeks after UVN. Similar changes were observed in the subdivisions of the solitary nucleus only 1 week after the lesion. These findings indicate vestibular lesion induces plasticity of the histamine H₃R, which could contribute to vestibular function recovery.

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.

Histaminergic ligands improve vestibular compensation in the cat: behavioural, neurochemical and molecular evidence

This study analysed the effects of betahistine and thioperamide, two histamine H(3) receptor antagonists, on the recovery process after unilateral vestibular neurectomy (UVN) in the cat. In UVN animals untreated or treated with betahistine or thioperamide, recovery was evaluated by recording the horizontal spontaneous nystagmus and the postural and locomotor performances. The neurochemical effects of these drugs were determined by examining their impact on the histaminergic system. We quantified the mRNA coding for histidine decarboxylase (enzyme synthesizing histamine) by in situ hybridisation in the tuberomammillary nuclei, while binding density to histamine H(3) receptors was assessed using a histamine H(3) receptor agonist ([(3)H]N-alpha-methylhistamine) and autoradiography methods in the tuberomammillary and the vestibular nuclei. Relative to the UVN-untreated group, cats treated with betahistine or thioperamide showed strongly accelerated behavioural recovery. UVN-induced 1) an up-regulation of histidine decarboxylase mRNA in the tuberomammillary nuclei, strongly accentuated under betahistine and thioperamide, 2) a reduction of the binding to histamine H(3) receptors in the vestibular and tuberomammillary nuclei, also strongly enhanced in both groups of treated cats. This study demonstrates that betahistine and thioperamide strongly improve the recovery of vestibular functions in UVN cats by interacting with the histaminergic system.

Histaminergic and glycinergic modulation of GABA release in the vestibular nuclei of normal and labyrinthectomised rats

Vestibular compensation (the behavioural recovery that follows unilateral vestibular de-afferentation), is facilitated by histamine, and is associated with increased central histamine release and alterations in histamine H(3) receptor expression in the vestibular nuclei. However, little is known of the effects of histamine on neurotransmission in the vestibular nuclei, and the mechanisms by which histamine may influence compensation are unclear. Here we examined the modulatory effects of histaminergic agents on the release of amino acid neurotransmitters in slices of the medial vestibular nucleus (MVN) prepared from normal and labyrinthectomised rats. The release of GABA, but not glutamate, glycine or aspartate, was robustly and reproducibly evoked by a high-K(+) stimulus applied to normal MVN slices. Histamine inhibited the evoked release of GABA, both through a direct action on presynaptic H(3) receptors (presumably located on GABAergic terminals), and through a novel, indirect pathway that involved the increased release of glycine by activation of postsynaptic H(1)/H(2) receptors (presumably on glycinergic neurons). After unilateral labyrinthectomy (UL), the direct H(3) receptor-mediated inhibition of GABA release was profoundly downregulated in both ipsi-lesional and contra-lesional MVNs. This effect appeared within 25 h post-UL and persisted for at least 3 weeks post-UL. In addition, at 25 h post-UL the indirect glycinergic pathway caused a marked suppression of GABA release in the contra-lesional but not ipsi-lesional MVN, which was overcome by strychnine. Stimulation of histamine H(3) receptors at 25 h post-UL restored contra-lesional GABA release to normal, suggesting that acutely after UL H(3) receptors may strongly modulate glycinergic and GABAergic neurotransmission in the MVN. These findings are the first to demonstrate the modulatory actions of the histaminergic system on neurotransmission in the vestibular nuclei, and the changes that occur during vestibular system plasticity. During vestibular compensation, histaminergic modulation of glycine and GABA release may contribute to the rebalancing of neural activity in the vestibular nuclei of the lesioned and intact sides.

Restoration of vestibular function: basic aspects and practical advances for rehabilitation

BACKGROUND

Normal balanced functioning of the human vestibular system is required to achieve an upright stance and locomotion, head and eye stabilization and internal spatial representation; any lesion in this system will disrupt these functions.

SCOPE

This review synthesizes previous work performed by the author and his research group in both animal models and vestibular defective patients over the last three decades. The author presents both an updated view on the basic mechanisms underlying the two main theories of vestibular compensation and his views on the principles that should guide management and rehabilitation of patients with vestibular loss.

FINDINGS

Static deficits, following the loss or disruption of vestibular functions, are fully compensated; this is explained by the vestibulo-centric theory that suggests different plastic changes occurring in the vestibular nuclei complexes. In contrast, dynamic deficits remain poorly compensated; the restoration of dynamic vestibular functions results from substitution processes and vicarious strategies. The practical advances in the rehabilitation of vestibular defective patients are as follows: (1) perform rehabilitation at an early stage; (2) favour active retraining; (3) do not use stereotyped rehabilitation programs but adapt exercises to the patients; (4) examine patients in standardized environments; (5) use both static and dynamic tests; and (6) avoid drugs with sedative effects (or limit them to the very acute stage only) and prescribe those accelerating the recovery process (e.g. betahistine dihydrochloride).

CONCLUSION

Recovery of vestibular function is greatest when early active retraining and adequate pharmacological treatments are used in combination.

Changes in the histaminergic system during vestibular compensation in the cat

To determine how the histaminergic system is implicated in vestibular compensation, we studied the changes in histidine decarboxylase (HDC; the enzyme synthesizing histamine) mRNA regulation in the tuberomammillary (TM) nuclei of cats killed 1 week, 3 weeks and 3 months after unilateral vestibular neurectomy (UVN). We also used one- and two-step bilateral vestibular neurectomized (BVN) cats to determine whether HDC mRNA regulation depended on the asymmetrical vestibular input received by the TM nuclei neurons. In addition, we analysed the HDC mRNA changes in the TM nuclei and the recovery of behavioural functions in UVN cats treated with thioperamide, a pure histaminergic drug. Finally, we quantified binding to histamine H3 receptors (H3Rs) in the medial vestibular nucleus (VN) by means of a histamine H3R agonist ([3H]N-alpha-methylhistamine) in order to further investigate the sites and mechanisms of action of histamine in this structure. This study shows that UVN increases HDC mRNA expression in the ipsilateral TM nucleus at 1 week. This increased expression persisted 3 weeks after UVN, and regained control values at 3 months. HDC mRNA expression was unchanged in the one-step BVN cats but showed mirror asymmetrical increases in the two-step BVN compared to the 1 week UVN cats. Three weeks' thioperamide treatment induced a bilateral HDC mRNA up-regulation in the UVN cats, which was higher than in the untreated UVN group. Binding to histamine H3Rs in the MVN showed a strong bilateral decrease after thioperamide treatment, while it was reduced ipsilaterally in the UVN cats. That such changes of the histaminergic system induced by vestibular lesion and treatment may play a functional role in vestibular compensation is strongly supported by the behavioural data. Indeed, spontaneous nystagmus, posture and locomotor balance were rapidly recovered in the UVN cats treated with thioperamide. These results demonstrate that changes in histamine levels are related to vestibular compensation.

Histamine (H3) receptors modulate the excitatory amino acid receptor response of the vestibular afferents

Although the effectiveness of histamine-related drugs in the treatment of peripheral and central vestibular disorders may be explained by their action on the vestibular nuclei, it has also been shown that antivertigo effects can take place at the peripheral level. In this work, we examined the actions of H3 histaminergic agonists and antagonists on the afferent neuron electrical discharge in the isolated inner ear of the axolotl. Our results indicate that H3 antagonists such as thioperamide, clobenpropit, and betahistine (BH) decreased the electrical discharge of afferent neurons by interfering with the postsynaptic response to excitatory amino acid agonists. These results lend further support to the idea that the antivertigo action of histamine-related drugs may be caused, at least in part, by a decrease in the sensory input from the vestibular endorgans. The present data show that the inhibitory action of the afferent neurons discharge previously described for BH is not restricted to this molecule but is also shared by other H3 antagonists.

Dose- and duration-dependent effects of betahistine dihydrochloride treatment on histamine turnover in the cat

Drugs interacting with the histaminergic system are currently used for vertigo treatment and it was shown in animal models that structural analogues of histamine like betahistine improved the recovery process after vestibular lesion. This study was aimed at determining the possible dose and duration effects of betahistine treatment on histamine turnover in normal adult cats, as judged by the level of messenger RNA for histidine decarboxylase (enzyme synthesizing histamine) in the tuberomammillary nuclei. Experiments were conducted on betahistine-treated cats receiving daily doses of 2, 5, 10, or 50 mg/kg during 1 week, 3 weeks, 2 months, or 3 months. The 1-week, 3-week, and 2- and 3-month treatments correspond to the acute, compensatory, and sustained compensatory stages of vestibular compensation, respectively. The lowest dose (2 mg/kg) given the longest time (3 months) was close to the dosage for vestibular defective patients. Data from the experimental groups were compared to control, untreated cats and to placebo-treated animals. The results clearly show that betahistine dihydrochloride administered orally in the normal cat interferes with histamine turnover by increasing the basal expression level of histidine decarboxylase mRNA of neurons located in the tuberomammillary nuclei of the posterior hypothalamus. The effects were both dose- and time-dependent. In conclusion, compensation of both static and dynamic deficits is subtended by long-term adaptive mechanisms that could be facilitated pharmacologically using betahistine dihydrochloride.

Co-localization of histamine and dopamine-beta-hydroxylase in sympathetic ganglion and release of histamine from cardiac sympathetic terminals of guinea-pig

1. The aim of this study was to investigate the co-localization of histamine and dopamine-beta-hydroxylase in the superior cervical ganglion of guinea-pig and release of histamine from cardiac sympathetic terminals in guinea-pig isolated atrium. 2. Histidine decarboxylase (a histamine-synthesizing enzyme) mRNA signals were detected in the neurones of superior cervical ganglion of guinea-pig by in situ hybridization. The results of double-labelled immunofluorescence further confirmed the co-localization of histamine and dopamine-beta-hydroxylase in the large principle neurons and small intensely fluorescent cells in the superior cervical ganglion. The immunoreactivities of both histamine and dopamine-beta-hydroxylase were significantly attenuated after 6-hydroxydopamine-induced lesion of sympathetic nerves. 3. The refractory electrical field stimulation caused the release of histamine from cardiac sympathetic terminals of guinea-pig isolated atria (112.14 +/- 40.34 ng x ml(-1)), which was significantly attenuated to 35 +/- 15.57 ng x ml(-1) by reserpine pretreatment. Following administering compound 48/80, a mast cell degranulator, electrical field stimulation induced a dramatic increase of endogenous histamine release from isolated atria (303.57 +/-72.93 ng x ml(-1)). When compound 48/80 was added to the reserpine-treated atria, the release of histamine induced by field stimulation was decreased to 207.14 +/- 76.39 ng x ml(-1). 4 These results provide novel evidence that histamine co-exists with noradrenaline in sympathetic nerves and might act as a neurotransmitter to modulate sympathetic neurotransmission.

Double-blind, randomized, multicenter study comparing the effect of betahistine and flunarizine on the dizziness handicap in patients with recurrent vestibular vertigo

OBJECTIVE

The aim of this double-blind, randomized, multicenter study was to compare the efficacy of betahistine dihydrochloride (BH) and flunarizine (FL) using the Dizziness Handicap Inventory (DHI), a validated self-assessment questionnaire that has not previously been used in a clinical trial to evaluate antivertigo drugs.

MATERIAL AND METHODS

Patients with recurrent vertigo of peripheral vestibular origin and who were severely handicapped by vertigo were randomized to an 8-week course of treatment with oral BH 48 mg daily or oral FL 10 mg daily. The efficacy endpoints were the total DHI score and the physical, functional and emotional subscores.

RESULTS

Fifty-two patients completed the study. After 8 weeks of treatment the mean total DHI score and the physical subscore were significantly lower in the BH group compared to the FL group (7.5 and 3.6 points, respectively). The mean total DHI score as well as the three subscores decreased significantly after 4 and 8 weeks in both treatment groups.

CONCLUSION

This study showed that at 8 weeks BH is significantly more effective than FL in terms of improving the total DHI score and the physical subscore. It was also established that the DHI is a useful and reliable method for evaluating the efficacy of antivertigo drugs.

Betahistine dihydrochloride interaction with the histaminergic system in the cat: neurochemical and molecular mechanisms

Drugs interfering with the histaminergic system facilitate behavioral recovery after vestibular lesion, likely by increasing histamine turnover and release. The effects of betahistine (structural analogue of histamine) on the histaminergic system were tested by quantifying messenger RNA for histidine decarboxylase (enzyme synthesizing histamine) by in situ hybridization and binding to histamine H(3) receptors (mediating, namely, histamine autoinhibition) using a histamine H(3) receptor agonist ([(3)H]N-alpha-methylhistamine) and radioautography methods. Experiments were done in brain sections of control cats (N=6) and cats treated with betahistine for 1 (N=6) or 3 (N=6) weeks. Betahistine treatment induced symmetrical changes with up-regulation of histidine decarboxylase mRNA in the tuberomammillary nucleus and reduction of [(3)H]N-alpha-methylhistamine labeling in both the tuberomammillary nucleus, the vestibular nuclei complex and nuclei of the inferior olive. These findings suggest that betahistine upregulates histamine turnover and release, very likely by blocking presynaptic histamine H(3) receptors, and induces histamine H(3) receptor downregulation. This action on the histaminergic system could explain the effectiveness of betahistine in the treatment of vertigo and vestibular disease.

Histamine and betahistine in the treatment of vertigo: elucidation of mechanisms of action

The aim of this review is to provide clinicians with a picture of the mechanisms by which: histamine and histaminergic agonists act on the vestibular system both peripherally and centrally; and histaminergic agonists and antagonists interfere with the recovery process after peripheral vestibular lesion. We have focused on betahistine, a structural analogue of histamine with weak histamine H(1) receptor agonist and more potent H(3) receptor antagonist properties, to review the currently available data on the role of the histaminergic system in the recovery process after peripheral vestibular deficits and the effects of histamine analogues in the clinical treatment of vertigo. This review provides new insights into the basic mechanisms by which betahistine improves vestibular compensation in animal models of unilateral vestibular dysfunction, and elucidates particularly the mechanisms of action of this substance at the level of the CNS. It is proposed that betahistine may reduce peripherally the asymmetric functioning of the sensory vestibular organs in addition to increasing vestibulocochlear blood flow by antagonising local H(3) heteroreceptors. Betahistine acts centrally by enhancing histamine synthesis within tuberomammillary nuclei of the posterior hypothalamus and histamine release within vestibular nuclei through antagonism of H(3) autoreceptors. This mechanism, together with less specific effects of betahistine on alertness regulation through cerebral H(1) receptors, should promote and facilitate central vestibular compensation. Elucidation of the mechanisms of action of betahistine is of particular interest for the treatment of vestibular and cochlear disorders and vertigo.

Betahistine, vestibular function and compensation: in vitro studies of vestibular function and plasticity

Histamine has an excitatory action on rat medial vestibular nucleus neurones in vitro, an effect that is mediated by histamine H1 and H2 receptors. Betahistine, which is a weak agonist at the H1 receptor and a moderate antagonist at the presynaptic H3 autoreceptor, weakly excites medial vestibular nucleus cells but antagonizes their responses to histamine. Experiments were carried out on rat medial vestibular nucleus cells in vitro using slices prepared from animals that had undergone unilateral labyrinthectomy (UL). There was a significant increase in the intrinsic excitability of medial vestibular nucleus cells in the rostral region of the ipsi-lesional nucleus within 4 h post-UL, which was sustained for the following week. These changes in intrinsic excitability of the medial vestibular nucleus neurones were abolished in animals that were not exposed to the secretion of stress hormones that normally occurs following UL. Histamine is also released in response to the stress associated with vestibular dysfunction. It is possible that the beneficial effects of betahistine on vestibular compensation are related to an interaction between histaminergic receptors activated by the parallel release of histamine and the activation of glucocorticoid receptors through the activation of the stress axis. Further study of the interactions between histamine receptors and the activation of the stress axis may be useful in understanding the effects of betahistine on vestibular plasticity.

Evaluation of the vestibular aqueduct in vestibulocochlear disorders by magnetic resonance imaging

The visibility of the vestibular aqueduct (VA) was examined using magnetic resonance imaging (MRI) in 95 patients: 15 patients with Meniere's disease, 4 with vestibular Meniere's disease, 4 with cochlear Meniere's disease and 72 patients with other vestibular and/or cochlear disorders. In order to visualize the VA, the T2-weighted image (T2WI) and the proton-density weighted image (PDWI) were obtained in the sagittal plane with a head coil. The visibility of the VA was classified into 4 grades, i.e. grade 0 (not visible), grade 1 (partially visible with PDWI), grade 2 (partially visible with T2WI) and grade 3 (clearly visible with T2WI). The visibility of the VA was significantly lower bilaterally in Meniere's disease, vestibular Meniere's disease, cochlear Meniere's disease and idiopathic bilateral sensorineural hearing loss (IBSNHL) than in the other diseases. The differences among Meniere's disease, vestibular Meniere's disease and cochlear Meniere's disease were not significant. The significance of decreased visibility in IBSNHL is unknown so far. The VA studies using MRI strongly suggest that the pathogenesis of Meniere's disease, vestibular Meniere's disease, cochlear Meniere's disease and IBSNHL is, at least in part, related to the findings of the VA.

Vestibular compensation in the cat: the role of the histaminergic system

Histamine is thought to be involved in the recovery of vestibular function as histaminergic medications are effective in vestibular-related syndromes. We conducted studies in the cat to assess the effects of betahistine (a histamine-like substance) on the behavioural recovery process after unilateral vestibular neurectomy (UVN). We also investigated histamine immunoreactivity changes in the vestibular and tuberomammillary nuclei of betahistine-treated lesioned cats compared with untreated and unlesioned cats. Betahistine strongly accelerated the behavioural recovery process after UVN, with a time benefit of approximately 2 weeks for both static posture (support surface) and dynamic equilibrium function (locomotor balance) compared with untreated animals. A bilateral decrease in histamine immunoreactivity was seen in both acute and compensated UVN cats; this effect was strongly accentuated with betahistine treatment. In conclusion, the results indicate that vestibular lesion reduces histamine staining due to an increase in histamine release in the vestibular and tuberomammillary nuclei that promote vestibular recovery. Betahistine dihydrochloride should contribute to this process by acting on both the presynaptic histamine H3 and postsynaptic histamine H1 receptors.