Histamine in the treatment of vertigo

Principles of vestibular pharmacotherapy

Ideally, vestibular pharmacotherapy is intended, through specific and targeted molecular actions, to significantly alleviate vertigo symptoms, to protect or repair the vestibular sensory network under pathologic conditions, and to promote vestibular compensation, with the eventual aim of improving the patient's quality of life. In fact, in order to achieve this aim, considerable progress still needs to be made. The lack of information on the etiology of vestibular disorders and the pharmacologic targets to modulate, as well as the technical challenge of targeting a drug to its effective site are some of the main issues yet to be overcome. In this review, my intention is to provide an account of the therapeutic principles that have shaped current vestibular pharmacotherapy and to further explore crucial questions that must be taken into consideration in order to develop targeted and specific pharmacologic therapies for each type and stage of vestibular disorders.

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.

Changes in Histamine Receptors (H1, H2, and H3) Expression in Rat Medial Vestibular Nucleus and Flocculus after Unilateral Labyrinthectomy: Histamine Receptors in Vestibular Compensation

Vestibular compensation is the process of behavioral recovery following peripheral vestibular lesion. In clinics, the histaminergic medicine is the most widely prescribed for the treatment of vertigo and motion sickness, however, the molecular mechanisms by which histamine modulates vestibular function remain unclear. During recovery from the lesion, the modulation of histamine receptors in the medial vestibular nucleus (MVN) and the flocculus may play an important role. Here with the means of quantitative real-time PCR, western blotting and immunohistochemistry, we studied the expression of histamine receptors (H1, H2, and H3) in the bilateral MVN and the flocculus of rats on the 1st, 3rd, and 7th day following unilateral labyrinthectomy (UL). Our results have shown that on the ipsi-lesional flocculus the H1, H2 and H3 receptors mRNA and the protein increased significantly on the 1st and 3rd day, with compare of sham controls and as well the contralateral side of UL. However, on the 7th day after UL, this expression returned to basal levels. Furthermore, elevated mRNA and protein levels of H1, H2 and H3 receptors were observed in the ipsi-lesional MVN on the 1st day after UL compared with sham controls and as well the contralateral side of UL. However, this asymmetric expression was absent by the 3rd post-UL. Our findings suggest that the upregulation of histamine receptors in the MVN and the flocculus may contribute to rebalancing the spontaneous discharge in bilateral MVN neurons during vestibular compensation.

Efficacy and Tolerability of a Fixed Combination of Cinnarizine and Dimenhydrinate versus Betahistine in the Treatment of Otogenic Vertigo : A Double-Blind, Randomised Clinical Study

INTRODUCTION

Peripheral vestibular disorders frequently lead to the manifestation of symptoms of vertigo. The objective of this study was to compare the efficacy and tolerability of a fixed combination of cinnarizine 20mg and dimenhydrinate 40mg per tablet with betahistine (betahistine dimesylate) 12mg per tablet in the treatment of patients with otogenic vertigo.

PATIENTS AND METHODS

Sixty-one patients with vertigo due to peripheral vestibular disorders (otogenic vertigo) participated in this prospective, double-blind, comparative, single-centre study. Patients were randomly allocated to treatment with betahistine 12mg or the fixed combination of cinnarizine 20mg and dimenhydrinate 40mg, both treatments given three times daily for 4 weeks. Efficacy was determined by patients' assessments of vertigo symptoms after 1 and 4 weeks of treatment using a visual analogue scale to determine a 'mean vertigo score'.

RESULTS

Treatment with the fixed combination led to significantly greater improvements in mean vertigo scores compared with the reference therapy betahistine. This was evident as early as 1 week after the onset of treatment (p = 0.002). Over 4 weeks of therapy, the fixed combination decreased the intensity of vertigo symptoms about 2-fold compared with betahistine (p = 0.001). Furthermore, reductions in symptoms typically associated with vertigo were more pronounced (p = 0.009) in the fixed-combination group compared with the betahistine group after 4 weeks of treatment. No serious adverse events were reported in either treatment group. Tolerability of the fixed combination was judged as 'very good' by 97% (betahistine 90%) and as 'good' by 3% (betahistine 10%) of patients.

CONCLUSION

The fixed combination of cinnarizine and dimenhydrinate was shown to be an effective and very well tolerated treatment option for patients with otogenic vertigo. It proved to be statistically more efficient in reducing vertigo than the widely used betahistine. Therefore, the fixed combination of cinnarizine and dimenhydrinate may be considered a first-line treatment option for the treatment of otogenic vertigo.

Betahistine treatment improves the recovery of static symptoms in patients with unilateral vestibular loss

Vestibular loss induces a combination of postural, oculomotor, and perceptive symptoms that are compensated over time. The aim of this study was to analyze the influence of betahistine dihydrochloride on vestibular compensation. A randomized, double-blind, placebo-controlled study was performed in Menière's disease patients who underwent a curative unilateral vestibular neurotomy (UVN). The effects of betahistine treatment were investigated on a broad spectrum of vestibular-induced changes resulting from vestibular loss: body sway, head orientation, ocular cyclotorsion, spontaneous nystagmus, verticality perception, and self-evaluation of the postural stability. The time course of the recovery was compared in 16 patients who received either a placebo or betahistine (24 mg b.i.d.) from 3 days up to 3 months after UVN. Patients were examined before (day -1) and after UVN (days 7, 30, and 90). Results indicate that betahistine reduces the time to recovery by 1 month or more depending on the tested functions. Betahistine was effective as soon as 4 days after treatment administration, and the effect remained during the whole compensation period (up to 3 months). The observed clinical effects may be attributed to an action of betahistine in rebalancing the neuronal activity between contralateral vestibular nuclei.

A fixed combination of cinnarizine/dimenhydrinate for the treatment of patients with acute vertigo due to vestibular disorders : a randomized, reference-controlled clinical study

BACKGROUND AND OBJECTIVE

Vestibular dysfunction commonly leads to - often severe - vertigo symptoms. The objective of this study was to compare the antivertiginous efficacy and tolerability of a fixed combination of cinnarizine/dimenhydrinate with those of betahistine in patients with acute vertigo due to vestibular disorders.

METHODS

Sixty-six patients experiencing acute vertigo attacks participated in this prospective, double-blind, three-centre, comparative study. Patients who assessed at least one vertigo symptom as being of medium intensity (> or =2) on a 5-point visual analogue scale (VAS; from 0 = no symptoms to 4 = very severe symptoms) were randomly allocated to treatment with the fixed combination of cinnarizine 20 mg and dimenhydrinate 40 mg three times daily or betahistine 12 mg three times daily for 4 weeks. The primary efficacy endpoint was change in mean vertigo score, as determined by patients' assessments of 12 individual vertigo symptoms on the 5-point VAS after 4 weeks of treatment.

RESULTS

Treatment with the fixed combination led to significantly greater improvements in mean vertigo scores than the reference therapy betahistine after 4 weeks of therapy (p = 0.013). The differences were clinically relevant, based on the Mann-Whitney estimator. Furthermore, the incidence of vertigo-associated vegetative symptoms was significantly reduced after 1 (p = 0.004) and 4 weeks (p = 0.023) in the fixed-combination group relative to the betahistine group. Three patients, all of them in the betahistine group, reported adverse events, none of which was considered serious. Almost all patients (n = 62) rated the tolerabilities of both medications as very good or good.

CONCLUSION

The fixed combination of cinnarizine/dimenhydrinate was shown to be an effective and very well tolerated treatment option for patients with acute vertigo due to vestibular disorders. The combination proved to be significantly more efficient in reducing vertigo and associated vegetative symptoms than betahistine in such patients.

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.

Pharmacological treatment of vertigo

This review discusses the physiology and pharmacological treatment of vertigo and related disorders. Classes of medications useful in the treatment of vertigo include anticholinergics, antihistamines, benzodiazepines, calcium channel antagonists and dopamine receptor antagonists. These medications often have multiple actions. They may modify the intensity of symptoms (e.g. vestibular suppressants) or they may affect the underlying disease process (e.g. calcium channel antagonists in the case of vestibular migraine). Most of these agents, particularly those that are sedating, also have a potential to modulate the rate of compensation for vestibular damage. This consideration has become more relevant in recent years, as vestibular rehabilitation physical therapy is now often recommended in an attempt to promote compensation. Accordingly, therapy of vertigo is optimised when the prescriber has detailed knowledge of the pharmacology of medications being administered as well as the precise actions being sought. There are four broad causes of vertigo, for which specific regimens of drug therapy can be tailored. Otological vertigo includes disorders of the inner ear such as Ménière's disease, vestibular neuritis, benign paroxysmal positional vertigo (BPPV) and bilateral vestibular paresis. In both Ménière's disease and vestibular neuritis, vestibular suppressants such as anticholinergics and benzodiazepines are used. In Ménière's disease, salt restriction and diuretics are used in an attempt to prevent flare-ups. In vestibular neuritis, only brief use of vestibular suppressants is now recommended. Drug treatments are not presently recommended for BPPV and bilateral vestibular paresis, but physical therapy treatment can be very useful in both. Central vertigo includes entities such as vertigo associated with migraine and certain strokes. Prophylactic agents (L-channel calcium channel antagonists, tricyclic antidepressants, beta-blockers) are the mainstay of treatment for migraine-associated vertigo. In individuals with stroke or other structural lesions of the brainstem or cerebellum, an eclectic approach incorporating trials of vestibular suppressants and physical therapy is recommended. Psychogenic vertigo occurs in association with disorders such as panic disorder, anxiety disorder and agoraphobia. Benzodiazepines are the most useful agents here. Undetermined and ill-defined causes of vertigo make up a large remainder of diagnoses. An empirical approach to these patients incorporating trials of medications of general utility, such as benzodiazepines, as well as trials of medication withdrawal when appropriate, physical therapy and psychiatric consultation is suggested.

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.

The physiology of brain histamine

Histamine-releasing neurons are located exclusively in the TM of the hypothalamus, from where they project to practically all brain regions, with ventral areas (hypothalamus, basal forebrain, amygdala) receiving a particularly strong innervation. The intrinsic electrophysiological properties of TM neurons (slow spontaneous firing, broad action potentials, deep after hyperpolarisations, etc.) are extremely similar to other aminergic neurons. Their firing rate varies across the sleep-wake cycle, being highest during waking and lowest during rapid-eye movement sleep. In contrast to other aminergic neurons somatodendritic autoreceptors (H3) do not activate an inwardly rectifying potassium channel but instead control firing by inhibiting voltage-dependent calcium channels. Histamine release is enhanced under extreme conditions such as dehydration or hypoglycemia or by a variety of stressors. Histamine activates four types of receptors. H1 receptors are mainly postsynaptically located and are coupled positively to phospholipase C. High densities are found especially in the hypothalamus and other limbic regions. Activation of these receptors causes large depolarisations via blockade of a leak potassium conductance, activation of a non-specific cation channel or activation of a sodium-calcium exchanger. H2 receptors are also mainly postsynaptically located and are coupled positively to adenylyl cyclase. High densities are found in hippocampus, amygdala and basal ganglia. Activation of these receptors also leads to mainly excitatory effects through blockade of calcium-dependent potassium channels and modulation of the hyperpolarisation-activated cation channel. H3 receptors are exclusively presynaptically located and are negatively coupled to adenylyl cyclase. High densities are found in the basal ganglia. These receptors mediated presynaptic inhibition of histamine release and the release of other neurotransmitters, most likely via inhibition of presynaptic calcium channels. Finally, histamine modulates the glutamate NMDA receptor via an action at the polyamine binding site. The central histamine system is involved in many central nervous system functions: arousal; anxiety; activation of the sympathetic nervous system; the stress-related release of hormones from the pituitary and of central aminergic neurotransmitters; antinociception; water retention and suppression of eating. A role for the neuronal histamine system as a danger response system is proposed.

Betahistine increases vestibular blood flow

Betahistine is used for treatment of several vestibular disorders. Despite the accepted use of this histamine-like substance, its mechanism of action is not well understood. The purpose of this study was to assess the possibility that one of the activities of betahistine is increasing blood flow in the peripheral vestibular end organs. Using a novel surgical approach, we identified the posterior semicircular canal ampulla of guinea pigs and placed a laser Doppler probe in position to obtain blood flow measurements from the posterior semicircular canal ampulla. Blood pressure, heart rate, and vestibular blood flow were continuously recorded. Concentration-response curves were obtained for betahistine (2.5, 5, 7.5, and 10 mg/kg) and control-vehicle (0.15 mol/L NaCl) infusions. A separate group of subjects was pretreated with the competitive selective H3 agonist, thioperimide maleate, before betahistine treatment. Increases in vestibular blood flow and decreases in blood pressure were observed in response to betahistine infusions. Pretreatment with thioperamide maleate abolished these changes at low doses of betahistine and attenuated the responses at higher doses of betahistine. These results show that betahistine administration induces increases in vestibular blood flow. These findings support the potential use of betahistine for treatment of vestibular disorders, which may be caused by compromised circulation.

Neurootological differential therapy for vertigo patients

Major neurootological complaints, which mostly need drug treatment, are: giddiness, dizziness, hearing loss and tinnitus. The neurootological differential diagnosis is the basis for planning the mostly supportive treatment of vertigo patients. In planning the therapy, we are utilizing a computerbased expert system Clamedex for establishing the neurootological diagnosis through history, ORL inspection, ENG, calorics, rotatory chair test, cranio-corpo-graphy (CCG), optokinetics, psychophysical audiometry, acoustic brainstem and late evoked potentials, visually evoked potentials etc. On this knowledge base we are designing an individually adapted case oriented drug therapy.Nausea and vomitus are the important subjective complications of dysequilibrium states. Therefore antivertiginous and antiemetic therapies have to be applied if necessary. Usually the duration is of limited time.Other drugs being chosen for a supportive pharmaco therapy according to the functional topodiagnostics of the lesions usually possess one or more of the following actions upon the equilibrium regulating network:increase of cerebral blood flow,enhanced penetration through the blood brain barrier,increase of neuronal metabolism andstabilization of specific neurotransmitters.The neurootological patients are treated and monitored by regular neurootometric follow up investigations.

The vestibular system as a model of sensorimotor transformations. A combined in vivo and in vitro approach to study the cellular mechanisms of gaze and posture stabilization in mammals

To understand the cellular mechanisms underlying behaviours in mammals, the respective contributions of the individual properties characterizing each neuron, as opposed to the properties emerging from the organization of these neurons in functional networks, have to be evaluated. This requires the use, in the same species, of various in vivo and in vitro experimental preparations. The present review is meant to illustrate how such a combined in vivo in vitro approach can be used to investigate the vestibular-related neuronal networks involved in gaze and posture stabilization, together with their plasticity, in the adult guinea-pig. Following first a general introduction on the vestibular system, the second section describes various in vivo experiments aimed at characterizing gaze and posture stabilization in that species. The third and fourth parts of the review deal with the combined in vivo-in vitro investigations undertaken to unravel the physiological and pharmacological properties of vestibulo-ocular and vestibulo-spinal networks, together with their functional implications. In particular, we have tried to use the central vestibular neurons as examples to illustrate how the preparation of isolated whole brain can be used to bridge the gap between the results obtained through in vitro, intracellular recordings on slices and those collected in vivo, in the behaving animal.

Physiopathology of H3-receptors and pharmacology of betahistine

This article reviews published research on the physiology of histamine H3-receptors. The function of this inhibitory autoreceptor, its localisation and influence on histaminergic neurones as well as histamine-controlled processes are presented together with the role of H3-receptors in the vestibular system. In addition to a summary of the properties of the main H3-agonists and -antagonists, the pharmacology of betahistine and its place in the treatment of vertigo are discussed.

Effects of histamine and betahistine on rat medial vestibular nucleus neurones: possible mechanism of action of anti-histaminergic drugs in vertigo and motion sickness

The tonic discharge of 71 medial vestibular nucleus (MVN) neurones was recorded in slices of the dorsal brainstem of young adult rats. Bath application of histamine caused a dose-related excitation in 59 of the 71 cells (83%), the remaining 12 (17%) being unresponsive. Dimaprit, a selective H2 agonist, also caused excitation in all 20 cells tested. The histamine-induced excitation and the response to dimaprit were antagonised by the selective H2 antagonist ranitidine, confirming that the H2 subtype of histamine receptor is involved in mediating the effects of histamine on these cells. Triprolidine, a selective H1 antagonist, also antagonised the excitation caused by histamine, at a concentration (0.3 microM) which left the H2 receptor-mediated response to dimaprit unchanged. Thus the excitatory effects of histamine on MVN cells in the rat involve two components mediated through H1 and H2 receptor-linked mechanisms, respectively. Betahistine, a weak H1 agonist and H3 antagonist, had little excitatory action when applied on its own, but significantly reduced the excitation caused by histamine when the two drugs were applied together. The effects of betahistine were consistent with a partial-agonist action at H1 receptors on MVN cells, reducing the excitatory responses to histamine presumably by occupying these receptor sites in competition with the exogenously applied neurotransmitter. This partial-agonist action of betahistine may be an important part of its mechanism of action in the symptomatic treatment of vertigo and motion sickness, since it is likely to occur not only in the MVN but also in many brain regions, including the thalamus and cortex, which express H1 receptors and which are innervated by the hypothalamic histaminergic system. Thus the effectiveness of betahistine and other anti-H1 drugs against motion sickness may be explained by their action in reducing the effects of the excess histamine release induced in such conditions in various brain areas, including the MVN.

Blood flow and assessment of capillaries in the aging rat posterior canal crista

Vascular change has been proposed as an etiological factor in inner ear aging and in several inner ear disorders. Moreover, some successful medical management of the episodic vertigo and tinnitus associated with Ménière's disease has been directed toward pharmacologically increasing blood flow, changing vascular permeability or ion homeostasis. While there are many studies of cochlear capillary morphology and blood flow, there are very few examining these variables in the vestibular system and none with respect to aging. The purpose of this study was to examine the rat posterior canal ampullary crista for age-related changes in blood flow and capillary morphology. By combining stereological techniques with microsphere injection, we have determined that in the rat posterior canal crista there is a statistically significant age-related decrease in blood flow (75%), mean capillary diameter (31%), and volume fraction of capillary lumen (31%). There is also an overall 18% decrease in the volume of the ampullary crista, a 72% decrease in blood flow/unit volume and a 36% increase in capillary length/unit volume. There were no significant changes in the capillary surface area/unit volume, the absolute capillary length, or the absolute capillary surface area. These data suggest impaired blood flow and degenerative loss of the ampullary crista may be relate to impaired end organ function.

Medial vestibular nucleus in the guinea-pig: histaminergic receptors. I. An in vitro study

Antihistaminergic drugs are currently used for the symptomatic treatment of vestibular-related syndromes such as vertigo and motion sickness. We therefore investigated whether histamine could modulate the firing of medial vestibular nuclei neurons (MVNn). Recently, we have demonstrated that different cell types are present among MVNn in guinea-pig brainstem slices. Bath-application of histamine at 10(-4) or 10(-5) M induced a small membrane depolarization accompanied by a slight decrease in membrane resistance and a reversible increase in spontaneous firing in all MVN cell types. These effects were presumably postsynaptic as they persisted in a low-calcium/high-magnesium solution. Using a variety of agonists and antagonists of histamine receptors (H1, H2 and H3), we conclude that these effects are mediated by H2 receptors. The companion paper is concerned with an in vivo study of the histaminergic modulation of the vestibular function (Yabe et al., in press).

Medial vestibular nucleus in the guinea-pig: histaminergic receptors. II. An in vivo study

In a companion paper (Serafin et al. 1992) we have demonstrated in vitro that histamine depolarizes three previously described medial vestibular nucleus neuron (MVNn) types (Serafin et al. 1991a, b). It has also been shown that this effect was exclusively mediated through postsynaptic H2 receptors. All the same, the eventual contribution of presynaptic H3 receptors to the physiological response of the MVNn to histamine remained an open question since, during the slicing procedure, any histaminergic axons projecting to the vestibular nuclei would have been interrupted. This rendered our study of H3-mediated effects of histamine difficult. Hence, in the present in vivo study our aim was three-fold: (1) to investigate the presence of H3 receptors at the vestibular nuclei level; (2) to evaluate the functional importance of MVNn H2 receptors; and (3) to explore whether H3 ligands, when injected intraperitoneally (i.p.), could modulate dynamic vestibular functions. In order to address the first two questions, we investigated postural changes induced by perfusion of the guinea-pig's vestibular nuclear complex with specific ligands of the H2 and H3 receptors. Our data extend the conclusions of our in vitro study and suggest that lateral vestibular nuclei neurons and the MVNn are endowed with both H2 and H3 receptors. Our results indicate furthermore that histamine can modulate, quite effectively, static vestibular reflexes. Finally, the present study demonstrates that i.p. injection of thioperamide, an H3 antagonist, induces a significant decrease in the horizontal vestibular-ocular reflex gain and, by contrast to most of the clinically used antihistaminics, has no detrimental effect on the alertness level. Our results may thus lead to clinical testing and use of H3 antagonists as antivertigo or anti motion-sickness drugs.