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

Histamine H4 receptor antagonists as potent modulators of mammalian vestibular primary neuron excitability

BACKGROUND AND PURPOSE

Betahistine, the main histamine drug prescribed to treat vestibular disorders, is a histamine H(3) receptor antagonist. Here, we explored the potential for modulation of the most recently cloned histamine receptor (H(4) receptor) to influence vestibular system function, using a selective H(4) receptor antagonist JNJ 7777120 and the derivate compound JNJ 10191584.

EXPERIMENTAL APPROACH

RT-PCR was used to assess the presence of H(4) receptors in rat primary vestibular neurons. In vitro electrophysiological recordings and in vivo behavioural approaches using specific antagonists were employed to examine the effect of H(4) receptor modulation in the rat vestibular system.

KEY RESULTS

The transcripts of H(4) and H(3) receptors were present in rat vestibular ganglia. Application of betahistine inhibited the evoked action potential firing starting at micromolar range, accompanied by subsequent strong neuronal depolarization at higher concentrations. Conversely, reversible inhibitory effects elicited by JNJ 10191584 and JNJ 7777120 began in the nanomolar range, without inducing neuronal depolarization. This effect was reversed by application of the selective H(4) receptor agonist 4-methylhistamine. Thioperamide, a H(3) /H(4) receptor antagonist, exerted effects similar to those of H(3) and H(4) receptor antagonists, namely inhibition of firing at nanomolar range and membrane depolarization above 100 µM. H(4) receptor antagonists significantly alleviated the vestibular deficits induced in rats, while neither betahistine nor thioperamide had significant effects.

CONCLUSIONS AND IMPLICATIONS

H(4) receptor antagonists have a pronounced inhibitory effect on vestibular neuron activity. This result highlights the potential role of H(4) receptors as pharmacological targets for the treatment of vestibular disorders.

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.

Neuropharmacology of vestibular system disorders

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

The role of histaminergic H1 and H3 receptors in food intake: a mechanism for atypical antipsychotic-induced weight gain?

Atypical antipsychotics such as olanzapine and clozapine are effective at treating the multiple domains of schizophrenia, with a low risk of extra-pyramidal side-effects. However a major downfall to their use is metabolic side-effects particularly weight gain/obesity, which occurs by unknown mechanisms. The present paper explores the potential candidature of histaminergic neurotransmission in the mechanisms of atypical antipsychotic-induced weight gain, with a focus on the histaminergic H1 and H3 receptors. Olanzapine and clozapine have a high affinity for the H1 receptor, and meta-analyses show a strong correlation between risk of weight gain and H1 receptor affinity. In addition, olanzapine treatment decreases H1 receptor binding and mRNA expression in the rat hypothalamus. Furthermore, a complex role is emerging for the histamine H3 receptor in the control of hunger. The H3 receptor is a pre-synaptic autoreceptor that inhibits the synthesis and release of histamine, and a heteroreceptor that inhibits other neurotransmitters such as serotonin (5-HT), noradrenaline (NA) and acetylcholine (ACh), which are also implicated in the regulation of food intake. Thus, the H3 receptor is in a prime position to regulate food intake, both through its control of histamine and its influence on other feeding pathways. We proposed that a mechanism for atypical antipsychotic-induced weight gain may be partly through the H3 receptor, as a drug-induced decrease in H1 receptor activity may decrease histamine tone through the H3 autoreceptors, compounding the weight gain problem. In addition, atypical antipsychotics may affect food intake by influencing 5-HT, NA and ACh release via interactions with the H3 heteroreceptor.

Comparable efficacy and tolerability between twice daily and three times daily betahistine for Ménière's disease

CONCLUSION

Betahistine at oral doses of 16 mg tid and 24 mg bid provides similar efficacy and tolerability in the treatment of vertigo in patients with Ménière's disease.

OBJECTIVE

To compare the efficacy and tolerability of betahistine 16 mg tid and 24 mg bid in the treatment of vertigo in patients with Ménière's disease.

PATIENTS AND METHODS

This was a randomized, open-label study of 120 consecutive patients with well-established Ménière's disease treated with betahistine 16 mg tid or 24 mg bid for 24 weeks. Treatment efficacy, assessed by clinical outcome level in terms of severity, frequency and duration of vertigo spells, was evaluated at baseline and at weeks 4, 12 and 24. Between-group comparisons of outcome data (Wilcoxon, Mann-Whitney U test) and adverse events (chi-squared test) were made.

RESULTS

Betahistine 16 mg tid or 24 mg bid showed a significant improvement in clinical outcome level from baseline to week 24 (p < 0.01). There was no significant difference between dosage groups regarding improvement in vertigo at any time point during the study. There was no significant difference between groups in the incidence of adverse events, which was low (maximum: headache, 16 mg tid, 16.7% of patients at week 4; 6.7% at week 24). The number of patients reporting adverse events diminished with time.

Stress axis plasticity during vestibular compensation in the adult cat

The postural, ocular motor, perceptive and neurovegetative syndromes resulting from unilateral vestibular neurectomy (UVN) symptoms could generate a stress and thereby activate the hypothalamo-pituitary-adrenal (HPA) axis. This study was aimed at determining whether UVN causes changes in the activity of the HPA axis, and if so, evaluating the time course of changes associated with UVN syndrome. At the cellular level, corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) immunoreactivity (Ir) were analyzed and quantified in the paraventricular nucleus (PVN) and the vestibular nuclei (VN) complex of cats killed early (1 and 7 days) or late (30 and 90 days) after UVN. Dopamine-beta-hydroxylase (DbetaH), the enzyme synthesizing noradrenaline was examined in the locus coeruleus (LC) in these same cats. At the behavioral level, the time course of recovery of the postural and locomotor functions was quantified at the same postoperative delays in another group of UVN cats. Results showed a significant bilateral increase in the number of both AVP-Ir and CRF-Ir neurons in the PVN and an increase of DbetaH-Ir neurons in the LC at 1, 7 and 30 days after UVN. This increased number of neurons was no longer observed at 90 days. Conversely, a significant bilateral decrease of CRF-Ir neurons was observed in the VN at these same postlesion times, with a similar return to control values at 90 days. Our behavioral observations showed strong posturo-locomotor functional deficits early after UVN (1 and 7 days), which had recovered partially at 30 days and completely by 90 days postlesion. We demonstrate a long-lasting activation of the HPA axis, which likely reflects a chronic stress, experienced by the animals, which corresponds to the time course of full vestibular compensation, and which is no longer present when the animals are completely free of posturo-locomotor symptoms at 90 days.

Improving the quality of life in patients with vestibular disorders: the role of medical treatments and physical rehabilitation

Vertigo, dizziness and imbalance are the main symptoms of vestibular disorders. They can lead to physical consequences, such as reduced postural control and falls, to psychologic/psychiatric consequences, such as anxiety-depression symptoms, panic and agoraphobia, and to cognitive defects, especially in the elderly. Consequently, the general health status and the quality of life (QoL) of vestibular patients can be significantly impaired. Several questionnaires have been developed in an attempt to quantify the degree of handicap and disability, the self-perceived health status and the quality of life in vestibular patients with dizziness and imbalance. Additionally, the main goal of the treatments of vestibular disorders should be to control symptoms, reduce functional disability and to improve patients' QoL. This article reviews the physical and psychological consequences of the vestibular disorders, their impact on the patients' QoL, and the treatment options, including drug prescriptions and vestibular rehabilitation protocols. A profile of the compound betahistine and its efficacy on QoL indices in the treatment of Menière's disease and other forms of peripheral vertigo is also presented.

Optimizing the pharmacological component of integrated balance therapy

Summary

Drug treatment is an important option for the treatment of peripheral vestibular diseases.

Aim

To identify the drug component associated with optimal integrated balance therapy (IBT) for Ménière's disease or other peripheral vestibular disorders.

Materials and Methods

Analysis of a series of patients with Ménière's disease patients or patients with other peripheral vestibular disorders that received IBT involving either no medication or betahistine, cinnarizine, clonazepam, flunarizine or Ginkgo biloba during 120 days.

Results

In Ménière's disease, significant differences were observed for all drug therapies (60 days) versus no medication; betahistine was significantly more effective than all other drugs at 60 and 120 days. For non-Ménière's disorders, significant differences were observed among betahistine, cinnarizine, clonazepam and flunarizine and no medication after 60 days; all drug therapies were significantly more effective than no medication after 120 days; betahistine, cinnarizine or clonazepam were equally effective and betahistine was more effective than flunarizine and EGb 761. All treatment options were well tolerated.

Conclusions

Drug therapies were more effective than no medication in the IBT for patients with Ménière's disease or other peripheral vestibular disorders. Betahistine was the most effective medication for patients with Ménière's disease and was as effective as cinnarizine and clonazepam for other peripheral vestibular disorders.

Betahistine: a retrospective synopsis of safety data

Betahistine is a structural analogue of histamine that is prescribed for the treatment of vestibular disorders such as Ménière's disease and the symptomatic treatment of vertigo. It is estimated from sales information that >130 million patients have been exposed to the drug since its registration in 1968. In this review we analyse the safety profile of betahistine based on data obtained during >35 years of worldwide postmarketing surveillance. Until 31 December 2005, 554 adverse drug reaction (ADR) reports with 994 individual signs and symptoms were received by the marketing authorisation holder from worldwide sources and were reviewed and evaluated. Signs and symptoms of cutaneous hypersensitivity reactions during betahistine therapy were the most frequently reported complaints. They consisted of usually mild and self-limiting rash, pruritus and urticaria, and all symptoms were reversible after drug discontinuation. Betahistine was reported to be involved in one anaphylactoid reaction and one case of Stevens-Johnson syndrome. Anaphylactic reactions with fatal outcome were not reported. The reports that describe gastrointestinal complaints mostly concern nausea and vomiting or unspecific abdominal pain. These were typically non-serious complaints. Hepatobiliary involvement was reported 25 times, including increases in alkaline phosphatase, gamma-glutamyltransferase, and alanine and aspartate aminotransferase levels. None of the patients concerned developed severe liver failure or died. ADRs related to the nervous system predominantly reveal heterogeneous events that are not suggestive of a specific adverse reaction profile for betahistine. A clinical intolerance to betahistine that gave rise to asthma or bronchospasm was only reported in eight ADRs. A total of three cases of neoplasm have been reported. One case concerned a male patient of unknown age who experienced weight loss, insomnia, impatience and irritability soon after the start of betahistine therapy. An undiagnosed phaeochromocytoma was suspected. The remaining two cases were assessed as being unrelated to betahistine by the reporter. Finally, four deaths have been reported during the course of postmarketing surveillance for betahistine. The reporter assessed the causal relationship to betahistine in two as unrelated, in one as unlikely and the other as unassessable. In summary, clinical and postmarketing studies have revealed a good safety profile of betahistine that was confirmed by the safety surveillance data presented.

Thioperamide delays vestibular compensation in goldfish

Unilateral lesion of the vestibular system induces posturo-locomotor deficits that are compensated for with time. Drug therapy is currently used to improve the recovery process and to facilitate vestibular compensation. We investigated the effects of thioperamide on functional recovery after unilateral labyrinthectomy in Carassius auratus. Approximately 24h after surgery, the animals were injected intraperitoneally with thioperamide (15 mg/kg) and saline (1.5 ml/kg). The injections were repeated daily for a total of 15 consecutive days. The substances were administered in a volume of 1.5 ml/kg body weight. Another group, which served as a non-lesion control, did not receive unilateral labyrinthectomy or system injections. Animals treated with saline presented a compensatory decrease in body tilt on the 7th day, while the animals treated with thioperamide presented a decrease in body tilt from the 13th day, suggesting a delay in the functional recovery process. These results suggest that an increase in cerebral histamine levels impairs vestibular compensation in goldfish.

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.

Antidepressants and dizziness

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

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.

Applicability of bioanalysis of multiple analytes in drug discovery and development: review of select case studies including assay development considerations

The development of sound bioanalytical method(s) is of paramount importance during the process of drug discovery and development culminating in a marketing approval. Although the bioanalytical procedure(s) originally developed during the discovery stage may not necessarily be fit to support the drug development scenario, they may be suitably modified and validated, as deemed necessary. Several reviews have appeared over the years describing analytical approaches including various techniques, detection systems, automation tools that are available for an effective separation, enhanced selectivity and sensitivity for quantitation of many analytes. The intention of this review is to cover various key areas where analytical method development becomes necessary during different stages of drug discovery research and development process. The key areas covered in this article with relevant case studies include: (a) simultaneous assay for parent compound and metabolites that are purported to display pharmacological activity; (b) bioanalytical procedures for determination of multiple drugs in combating a disease; (c) analytical measurement of chirality aspects in the pharmacokinetics, metabolism and biotransformation investigations; (d) drug monitoring for therapeutic benefits and/or occupational hazard; (e) analysis of drugs from complex and/or less frequently used matrices; (f) analytical determination during in vitro experiments (metabolism and permeability related) and in situ intestinal perfusion experiments; (g) determination of a major metabolite as a surrogate for the parent molecule; (h) analytical approaches for universal determination of CYP450 probe substrates and metabolites; (i) analytical applicability to prodrug evaluations-simultaneous determination of prodrug, parent and metabolites; (j) quantitative determination of parent compound and/or phase II metabolite(s) via direct or indirect approaches; (k) applicability in analysis of multiple compounds in select disease areas and/or in clinically important drug-drug interaction studies. A tabular representation of select examples of analysis is provided covering areas of separation conditions, validation aspects and applicable conclusion. A limited discussion is provided on relevant aspects of the need for developing bioanalytical procedures for speedy drug discovery and development. Additionally, some key elements such as internal standard selection, likely issues of mass detection, matrix effect, chiral aspects etc. are provided for consideration during method development.

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.

Effects of betahistine on the spatiotemporal response properties of vestibulospinal neurons to labyrinthine volleys

Betahistine, a drug used in the treatment of vestibular disorders, speeds-up the recovery from hemilabyrinthectomy in experimental animals, likely through the activation of histamine receptors. In order to better understand the mechanism of action of this drug we investigated, in adult, urethane anesthetized rats, whether betahistine modifies the spatial (directional) and temporal response properties of vestibular nuclear neurons to the labyrinthine input, as well as the convergence of different labyrinthine signals on single units. Extracellular single-unit activity was recorded from the caudal, spinal-projecting region of the vestibular nuclei during tilt of the animal, before and after i.p. injection of betahistine. The two orthogonal directions of maximal and minimal response to tilt, as well as the corresponding gains were determined for each neuron. Betahistine reduced the maximal response gain of units showing larger basal values of this parameter and increased it in neurons with smaller basal values, while the minimal response gain was on the average raised. These changes led to a significant decrease in the spatial specificity of the neurons, suggesting that betahistine affects the process of spatiotemporal convergence on vestibular units, likely through a rearrangement of the various inputs. This could be related to the effect of the drug on vestibular compensation.

Assessment of pharmacology and potential anti-obesity properties of H3receptor antagonists/inverse agonists

Histamine is a key neurotransmitter that alters central nervous system functions in both behavioural and homeostatic contexts through its actions on the histamine (H) subreceptors H(1), H(2) and H(3) G-protein-coupled receptors. H(3)receptors have a diverse central nervous system distribution where they function as both homo- and hetero-receptors to modulate the synthesis and/or release of several neurotransmitters. H(3) receptors are constitutively active, which implies that antagonists of H(3) receptors may also function as inverse agonists to alter the basal state of the receptor and uncouple constitutive receptor-G-protein interactions. Reference H(3) antagonists such as thioperamide and ciproxifan, administered either centrally or systemically, have been shown to cause changes in food consumption and/or body weight in proof-of-concept studies. More recently, several non-imidazole-based H(3) antagonists/inverse agonists have also been described with efficacy in at least one animal model of human obesity. Considerable preclinical effort remains necessary before such compounds achieve therapeutic success or failure. Moreover, ongoing research in a number of laboratories has shed new insights into the effects of H(3) ligands in the control of feeding, appetite and body weight, which offer different results and conclusions. The goal of this review is to appraise these findings and forecast whether any H(3) antagonists/inverse agonists will provide clinical utility to treat human obesity.

The effect of betahistine, a histamine H1 receptor agonist/H3 antagonist, on olanzapine-induced weight gain in first-episode schizophrenia patients

Histamine antagonism has been implicated in antipsychotic drug-induced weight gain. Betahistine, a histamine enhancer with H1 agonistic/H3 antagonistic properties (48 mg t.i.d.), was coadministered with olanzapine (10 mg/day) in three first-episode schizophrenia patients for 6 weeks. Body weight was measured at baseline and weekly thereafter. Clinical rating scales were completed at baseline and at week 6. All participants gained weight (mean weight gain 3.1+/-0.9 kg) and a similar pattern of weight gain was observed: an increase during the first 2 weeks and no additional weight gain (two patients) or minor weight loss (one patient) from weeks 3 to 6. None gained 7% of baseline weight, which is the cut-off for clinically significant weight gain. Betahistine was safe and well tolerated and did not interfere with the antipsychotic effect of olanzapine. Our findings justify a placebo-controlled evaluation of the putative weight-attenuating effect of betahistine in olanzapine-induced weight gain.