Distribution and modulation of histamine H(3) receptors in basal ganglia and frontal cortex of healthy controls and patients with Parkinson's disease

Network connectivity and local transcriptomic vulnerability underpin cortical atrophy progression in Parkinson's disease

Parkinson's disease pathology is hypothesized to spread through the brain via axonal connections between regions and is further modulated by local vulnerabilities within those regions. The resulting changes to brain morphology have previously been demonstrated in both prodromal and de novo Parkinson's disease patients. However, it remains unclear whether the pattern of atrophy progression in Parkinson's disease over time is similarly explained by network-based spreading and local vulnerability. We address this gap by mapping the trajectory of cortical atrophy rates in a large, multi-centre cohort of Parkinson's disease patients and relate this atrophy progression pattern to network architecture and gene expression profiles. Across 4-year follow-up visits, increased atrophy rates were observed in posterior, temporal, and superior frontal cortices. We demonstrated that this progression pattern was shaped by network connectivity. Regional atrophy rates were strongly related to atrophy rates across structurally and functionally connected regions. We also found that atrophy progression was associated with specific gene expression profiles. The genes whose spatial distribution in the brain was most related to atrophy rate were those enriched for mitochondrial and metabolic function. Taken together, our findings demonstrate that both global and local brain features influence vulnerability to neurodegeneration in Parkinson's disease.

Literature-Based Discovery Predicts Antihistamines Are a Promising Repurposed Adjuvant Therapy for Parkinson's Disease

Parkinson's disease (PD) is a movement disorder caused by a dopamine deficit in the brain. Current therapies primarily focus on dopamine modulators or replacements, such as levodopa. Although dopamine replacement can help alleviate PD symptoms, therapies targeting the underlying neurodegenerative process are limited. The study objective was to use artificial intelligence to rank the most promising repurposed drug candidates for PD. Natural language processing (NLP) techniques were used to extract text relationships from 33+ million biomedical journal articles from PubMed and map relationships between genes, proteins, drugs, diseases, etc., into a knowledge graph. Cross-domain text mining, hub network analysis, and unsupervised learning rank aggregation were performed in SemNet 2.0 to predict the most relevant drug candidates to levodopa and PD using relevance-based HeteSim scores. The top predicted adjuvant PD therapies included ebastine, an antihistamine for perennial allergic rhinitis; levocetirizine, another antihistamine; vancomycin, a powerful antibiotic; captopril, an angiotensin-converting enzyme (ACE) inhibitor; and neramexane, an N-methyl-D-aspartate (NMDA) receptor agonist. Cross-domain text mining predicted that antihistamines exhibit the capacity to synergistically alleviate Parkinsonian symptoms when used with dopamine modulators like levodopa or levodopa-carbidopa. The relationship patterns among the identified adjuvant candidates suggest that the likely therapeutic mechanism(s) of action of antihistamines for combatting the multi-factorial PD pathology include counteracting oxidative stress, amending the balance of neurotransmitters, and decreasing the proliferation of inflammatory mediators. Finally, cross-domain text mining interestingly predicted a strong relationship between PD and liver disease.

Ameliorating parkinsonian motor dysfunction by targeting histamine receptors in entopeduncular nucleus-thalamus circuitry

In Parkinson's disease (PD), reduced dopamine levels in the basal ganglia have been associated with altered neuronal firing and motor dysfunction. It remains unclear whether the altered firing rate or pattern of basal ganglia neurons leads to parkinsonism-associated motor dysfunction. In the present study, we show that increased histaminergic innervation of the entopeduncular nucleus (EPN) in the mouse model of PD leads to activation of EPN parvalbumin (PV) neurons projecting to the thalamic motor nucleus via hyperpolarization-activated cyclic nucleotide-gated (HCN) channels coupled to postsynaptic H2R. Simultaneously, this effect is negatively regulated by presynaptic H3R activation in subthalamic nucleus (STN) glutamatergic neurons projecting to the EPN. Notably, the activation of both types of receptors ameliorates parkinsonism-associated motor dysfunction. Pharmacological activation of H2R or genetic upregulation of HCN2 in EPNPV neurons, which reduce neuronal burst firing, ameliorates parkinsonism-associated motor dysfunction independent of changes in the neuronal firing rate. In addition, optogenetic inhibition of EPNPV neurons and pharmacological activation or genetic upregulation of H3R in EPN-projecting STNGlu neurons ameliorate parkinsonism-associated motor dysfunction by reducing the firing rate rather than altering the firing pattern of EPNPV neurons. Thus, although a reduced firing rate and more regular firing pattern of EPNPV neurons correlate with amelioration in parkinsonism-associated motor dysfunction, the firing pattern appears to be more critical in this context. These results also confirm that targeting H2R and its downstream HCN2 channel in EPNPV neurons and H3R in EPN-projecting STNGlu neurons may represent potential therapeutic strategies for the clinical treatment of parkinsonism-associated motor dysfunction.

Nanodelivery of Histamine H3/H4 Receptor Modulators BF-2649 and Clobenpropit with Antibodies to Amyloid Beta Peptide in Combination with Alpha Synuclein Reduces Brain Pathology in Parkinson's Disease

Parkinson's disease (PD) in military personnel engaged in combat operations is likely to develop in their later lives. In order to enhance the quality of lives of PD patients, exploration of novel therapy based on new research strategies is highly warranted. The hallmarks of PD include increased alpha synuclein (ASNC) and phosphorylated tau (p-tau) in the cerebrospinal fluid (CSF) leading to brain pathology. In addition, there are evidences showing increased histaminergic nerve fibers in substantia niagra pars compacta (SNpc), striatum (STr), and caudate putamen (CP) associated with upregulation of histamine H3 receptors and downregulation of H4 receptors in human brain. Previous studies from our group showed that modulation of potent histaminergic H3 receptor inverse agonist BF-2549 or clobenpropit (CLBPT) partial histamine H4 agonist with H3 receptor antagonist induces neuroprotection in PD brain pathology. Recent studies show that PD also enhances amyloid beta peptide (AβP) depositions in brain. Keeping these views in consideration in this review, nanowired delivery of monoclonal antibodies to AβP together with ASNC and H3/H4 modulator drugs on PD brain pathology is discussed based on our own observations. Our investigation shows that TiO₂ nanowired BF-2649 (1 mg/kg, i.p.) or CLBPT (1 mg/kg, i.p.) once daily for 1 week together with nanowired delivery of monoclonal antibodies (mAb) to AβP and ASNC induced superior neuroprotection in PD-induced brain pathology. These observations are the first to show the modulation of histaminergic receptors together with antibodies to AβP and ASNC induces superior neuroprotection in PD. These observations open new avenues for the development of novel drug therapies for clinical strategies in PD.

Binding of the Dual-Action Anti-Parkinsonian Drug AG-0029 to Dopamine D2 and Histamine H3 Receptors: A PET Study in Healthy Rats

Introduction: Parkinson’s disease (PD) is a neurodegenerative disorder characterized by motor dysfunction and a diverse range of nonmotor symptoms. Functional relationships between the dopaminergic and histaminergic systems suggest that dual-action pharmaceuticals like AG-0029 (D₂/D₃ agonist/H₃ antagonist) could ameliorate both the motor and cognitive symptoms of PD. The current study aimed to demonstrate the interaction of AG-0029 with its intended targets in the mammalian brain using positron emission tomography (PET). Methods: Healthy male Wistar rats were scanned with a small-animal PET camera, using either the dopamine D₂/D₃ receptor ligand [¹¹C]raclopride or the histamine H₃ receptor ligand [¹¹C]GSK-189254, before and after treatment with an intravenous, acute, single dose of AG-0029. Dynamic [¹¹C]raclopride PET data (60 min duration) were analyzed using the simplified reference tissue model 2 (SRTM2) with cerebellum as reference tissue and the nondisplaceable binding potential as the outcome parameter. Data from dynamic [¹¹C]GSK-189254 scans (60 min duration) with arterial blood sampling were analyzed using Logan graphical analysis with the volume of distribution (V_T) as the outcome parameter. Receptor occupancy was estimated using a Lassen plot. Results: Dopamine D_2/3 receptor occupancies in the striatum were 22.6 ± 18.0 and 84.0 ± 3.5% (mean ± SD) after administration of 0.1 and 1 mg/kg AG-0029, respectively. In several brain regions, the V_T values of [¹¹C]GSK-189254 were significantly reduced after pretreatment of rats with 1 or 10 mg/kg AG-0029. The H₃ receptor occupancies were 11.9 ± 8.5 and 40.3 ± 11.3% for the 1 and 10 mg/kg doses of AG-0029, respectively. Conclusions: Target engagement of AG-0029 as an agonist at dopamine D₂/D₃ receptors and an antagonist at histamine H₃ receptors could be demonstrated in the rat brain with [¹¹C]raclopride and [¹¹C]GSK-189254 PET, respectively. The measured occupancy values reflect the previously reported high (subnanomolar) affinity of AG-0029 to D₂/D₃ and moderate (submicromolar) affinity to H₃ receptors.

Extraction of Reduced Infrared Biomarker Signatures for the Stratification of Patients Affected by Parkinson’s Disease: An Untargeted Metabolomic Approach

An untargeted Fourier transform infrared (FTIR) metabolomic approach was employed to study metabolic changes and disarrangements, recorded as infrared signatures, in Parkinson’s disease (PD). Herein, the principal aim was to propose an efficient sequential classification strategy based on SELECT-LDA, which enabled optimal stratification of three main categories: PD patients from subjects with Alzheimer’s disease (AD) and healthy controls (HC). Moreover, sub-categories, such as PD at the early stage (PDI) from PD in the advanced stage (PDD), and PDD vs. AD, were stratified. Every classification step with selected wavenumbers achieved 90.11% to 100% correct assignment rates in classification and internal validation. Therefore, selected metabolic signatures from new patients could be used as input features for screening and diagnostic purposes.

Histamine and histamine receptors: Roles in major depressive disorder

Although the incidence of major depressive disorder (MDD) is high and its social impact is great, we still know very little about the pathophysiology of depression. The monoamine hypothesis of depression suggests that 5-HT, NE, and DA synergistically affect mood, which is the basis of current drug therapy for depression. However, histamine as a monoamine transmitter is rarely studied. Our review is the first time to illustrate the effect of histaminergic system on depression in order to find the way for the development of new antidepressant drugs. The brain neurotransmitter histamine is involved in MDD, and the brain histaminergic system operates through four receptors. Histamine and its receptors can also regulate the immune response to improve symptoms of depression. In addition, H3R can interact with other depression-related transmitters (including 5-HT, DA, GLU, and MCH); thus, histamine may participate in the occurrence of depression through other neural circuits. Notably, in rodent studies, several H3R and H1R antagonists were found to be safe and effective in alleviating depression-like behavior. To highlight the complex functions of histamine in depression, and reveals that histamine receptors can be used as new targets for antidepressant therapy.

Histamine H3 and H4 receptors modulate Parkinson's disease induced brain pathology. Neuroprotective effects of nanowired BF-2649 and clobenpropit with anti-histamine-antibody therapy

Military personnel deployed in combat operations are highly prone to develop Parkinson's disease (PD) in later lives. PD largely involves dopaminergic pathways with hallmarks of increased alpha synuclein (ASNC), and phosphorylated tau (p-tau) in the cerebrospinal fluid (CSF) precipitating brain pathology. However, increased histaminergic nerve fibers in substantia nigra pars Compacta (SNpc), striatum (STr) and caudate putamen (CP) associated with upregulation of Histamine H3 receptors and downregulation of H4 receptors in human cases of PD is observed in postmortem cases. These findings indicate that modulation of histamine H3 and H4 receptors and/or histaminergic transmission may induce neuroprotection in PD induced brain pathology. In this review effects of a potent histaminergic H3 receptor inverse agonist BF-2549 or clobenpropit (CLBPT) partial histamine H4 agonist with H3 receptor antagonist, in association with monoclonal anti-histamine antibodies (AHmAb) in PD brain pathology is discussed based on our own observations. Our investigation shows that chronic administration of conventional or TiO₂ nanowired BF 2649 (1mg/kg, i.p.) or CLBPT (1mg/kg, i.p.) once daily for 1 week together with nanowired delivery of HAmAb (25μL) significantly thwarted ASNC and p-tau levels in the SNpC and STr and reduced PD induced brain pathology. These observations are the first to show the involvement of histamine receptors in PD and opens new avenues for the development of novel drug strategies in clinical strategies for PD, not reported earlier.

In Vivo Receptor Visualization and Evaluation of Receptor Occupancy with Positron Emission Tomography

Positron emission tomography (PET) is useful for noninvasive in vivo visualization of disease-related receptors, for evaluation of receptor occupancy to determine an appropriate drug dosage, and for proof-of-concept of drug candidates in translational research. For these purposes, the specificity of the PET tracer for the target receptor is critical. Here, we review work in this area, focusing on the chemical structures of reported PET tracers, their K_i/K_d values, and the physical properties relevant to target receptor selectivity. Among these physical properties, such as cLogP, cLogD, molecular weight, topological polar surface area, number of hydrogen bond donors, and pK_a, we focus especially on LogD and LogP as important physical properties that can be easily compared across a range of studies. We discuss the success of PET tracers in evaluating receptor occupancy and consider likely future developments in the field.

The tuberomamillary nucleus in neuropsychiatric disorders

The tuberomamillary nucleus (TMN) is located within the posterior part of the hypothalamus. The histamine neurons in it synthesize histamine by means of the key enzyme histidine decarboxylase (HDC) and from the TMN, innervate a large number of brain areas, such as the cerebral cortex, hippocampus, amygdala as well as the thalamus, hypothalamus, and basal ganglia. Brain histamine is reduced to an inactivated form, tele-methylhistamine (t-MeHA), by histamine N-methyltransferase (HMT). In total, there are four types of histamine receptors (H_1-4Rs) in the brain, all of which are G-protein coupled. The histaminergic system controls several basal physiological functions, including the sleep-wake cycle, energy and endocrine homeostasis, sensory and motor functions, and cognitive functions such as attention, learning, and memory. Histaminergic dysfunction may contribute to clinical disorders such as Parkinson's disease, Alzheimer's disease, Huntington's disease, narcolepsy type 1, schizophrenia, Tourette syndrome, and autism spectrum disorder. In the current chapter, we focus on the role of the histaminergic system in these neurological/neuropsychiatric disorders. For each disorder, we first discuss human data, including genetic, postmortem brain, and cerebrospinal fluid studies. Then, we try to interpret the human changes by reviewing related animal studies and end by discussing, if present, recent progress in clinical studies on novel histamine-related therapeutic strategies.

Region-specific regulation of central histaminergic H3 receptor expression in a mouse model of cow's milk allergy

Central histaminergic H3 receptor (H3R) has been extensively investigated as a potential therapeutic target for various neurological and neurodegenerative disorders. Despite promising results in preclinical rodent models, clinical trials have not provided conclusive evidence for the benefit of H3R antagonists to alleviate cognitive and behavioral symptoms of these disorders. Inconsistent pharmacological efficacies may arise from aberrant changes in H3R over time during disease development. Because H3R is involved in feedback inhibition of histamine synthesis and secretion, the expression of the autoreceptor may also be reciprocally regulated by altered histamine levels in a pathological condition. Thus, we investigated H3R expression in a mouse model of cow's milk allergy, a condition associated with increased histamine levels. Mice were sensitized to bovine whey proteins (WP) over 5 weeks and H3R protein and transcript levels were examined in the brain. Substantially increased H3R immunoreactivity was observed in various brain regions of WP-sensitized mice compared to sham mice. Elevated H3R expression was also found in the thalamic/hypothalamic region. The expression of histaminergic H1, but not H2, receptor subtype was also increased in this and the midbrain regions. Unlike the brain, all three histaminergic receptors were increased in the small intestine. These results indicated that the central histaminergic receptors were altered in WP-sensitized mice in a subtype- and region-specific manner, which likely contributed to behavioral changes we observed in these mice. Our study also suggests that altered levels of H3R could be considered during a pharmacological intervention of a neurological disease.

Pitolisant and Other Histamine-3 Receptor Antagonists-An Update on Therapeutic Potentials and Clinical Prospects

Background: Besides its well-known role as a peripheral chemical mediator of immune, vascular, and cellular responses, histamine plays major roles in the central nervous system, particularly in the mediation of arousal and cognition-enhancement. These central effects are mediated by the histamine-3 auto receptors, the modulation of which is thought to be beneficial for the treatment of disorders that impair cognition or manifest with excessive daytime sleepiness. Methods: A database search of PubMed, Google Scholar, and clinicaltrials.gov was performed in June 2020. Full-text articles were screened and reviewed to provide an update on pitolisant and other histamine-3 receptor antagonists. Results: A new class of drugs—histamine-3 receptor antagonists—has emerged with the approval of pitolisant for the treatment of narcolepsy with or without cataplexy. At the recommended dose, pitolisant is well tolerated and effective. It has also been evaluated for potential therapeutic benefit in Parkinson disease, epilepsy, attention deficit hyperactivity disorder, Alzheimer’s disease, and dementia. Limited studies have shown pitolisant to lack abuse potential which will be a major advantage over existing drug options for narcolepsy. Several histamine-3 receptor antagonists are currently in development for a variety of clinical indications. Conclusions: Although limited clinical studies have been conducted on this new class of drugs, the reviewed literature showed promising results for future additions to the clinical indications of pitolisant, and the expansion of the list of approved drugs in this class for a variety of indications.

Genetic lack of histamine upregulates dopamine neurotransmission and alters rotational behavior but not levodopa-induced dyskinesia in a mouse model of Parkinson's disease

The brain histaminergic and dopaminergic systems closely interact, and some evidence also suggests significant involvement of histamine in Parkinson's disease (PD), where dopaminergic neurons degenerate. To further investigate histamine-dopamine interactions, particularly in the context of PD, a genetic lack of histamine and a mouse model of PD and levodopa-induced dyskinesia were here combined. Dopaminergic lesions were induced in histidine decarboxylase knockout and wildtype mice by 6-hydroxydopamine injections into the medial forebrain bundle. Post-lesion motor dysfunction was studied by measuring drug-induced rotational behavior and dyskinesia. Striatal tissue from both lesioned and naïve animals was used to investigate dopaminergic, serotonergic and histaminergic biomarkers. Histamine deficiency increased amphetamine-induced rotation but did not affect levodopa-induced dyskinesia. qPCR measurements revealed increased striatal expression of D1 and D2 receptor, DARPP-32, and H3 receptor mRNA, and synaptosomal release experiments in naïve mice indicated increased dopamine release. A lack of histamine thus causes pre- and postsynaptic upregulation of striatal dopaminergic neurotransmission which may be reflected in post-lesion motor behavior. Disturbances or manipulations of the histaminergic system may thus have significant consequences for dopaminergic neurotransmission and motor behavior in both healthy and disease conditions. The findings also represent new evidence for the complex interplay between dopamine and histamine within the nigrostriatal pathway.

Histamine-induced plasticity and gene expression in corticostriatal pathway under hyperammonemia

Aims

Histamine H3 receptor (H3R) antagonists/inverse agonists increase vigilance. We studied brain histaminergic pathways under hyperammonemia and the transcriptome of receptors and their signaling cascades to provide a rationale for wake‐promoting therapies.

Methods

We analyzed histamine‐induced long‐lasting depression of corticostriatal synaptic transmission (LLDhist). As the expression of dopamine 1 receptors (D1R) is upregulated in LGS‐KO striatum where D1R‐H3R dimers may exist, we investigated actions of H3R and D1R agonists and antagonists. We analyzed transcription of selected genes in cortex and dorsal striatum in a mouse model of inborn hyperammonemia (liver‐specific glutamine synthetase knockout: LGS‐KO) and compared it with human hepatic encephalopathy.

Results

LGS‐KO mice showed significant reduction of the direct depression (DD) but not the long‐lasting depression (LLD) by histamine. Neither pharmacological activation nor inhibition of D1R significantly affected DDhist and LLDhist in WT striatum, while in LGS‐KO mice D1R activation suppressed LLDhist. Histaminergic signaling was found unchanged at the transcriptional level except for the H2R. A study of cAMP‐regulated genes indicated a significant reduction in the molecular signature of wakefulness in the diseased cortex.

Conclusions

Our findings provide a rationale for the development of aminergic wake‐promoting therapeutics in hyperammonemic disorders.

Histamine, histamine H3 receptor, and alcohol use disorder

Alcohol use disorder is associated with several mental, physical, and social problems. Its treatment is difficult and often requires a combination of pharmacological and behavioural therapy. The brain histaminergic system, one of the wake-active systems that controls whole-brain activity, operates through three neuronal GPCRs. The histamine H₃ receptor (Hrh3), which is expressed in many brain areas involved in alcohol drinking and alcohol reward, can be targeted with a number of drugs developed initially for cognitive disorders and/or disorders related to sleep, wakefulness, and alertness. In all rodent alcohol drinking models tested so far, H₃ receptor antagonists have reduced alcohol drinking and alcohol-induced place preference and cue-induced alcohol reinstatement. Several H₃ receptor antagonists tested and found to be safe for humans could be subjected to clinical tests to treat alcohol use disorder. Preference should be given to short-acting drugs to avoid the sleep problems associated with the wake-maintaining effects of the drugs. LINKED ARTICLES: This article is part of a themed section on New Uses for 21st Century. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.3/issuetoc.

Regularizing firing patterns of rat subthalamic neurons ameliorates parkinsonian motor deficits

The subthalamic nucleus (STN) is an effective therapeutic target for deep brain stimulation (DBS) for Parkinson's disease (PD), and histamine levels are elevated in the basal ganglia in PD patients. However, the effect of endogenous histaminergic modulation on STN neuronal activities and the neuronal mechanism underlying STN-DBS are unknown. Here, we report that STN neuronal firing patterns are more crucial than firing rates for motor control. Histamine excited STN neurons, but paradoxically ameliorated parkinsonian motor deficits, which we attributed to regularizing firing patterns of STN neurons via the hyperpolarization-activated cyclic nucleotide-gated channel 2 (HCN2) channel coupled to the H2 receptor. Intriguingly, DBS increased histamine release in the STN and regularized STN neuronal firing patterns under parkinsonian conditions. HCN2 contributed to the DBS-induced regularization of neuronal firing patterns, suppression of excessive β oscillations, and alleviation of motor deficits in PD. The results reveal an indispensable role for regularizing STN neuronal firing patterns in amelioration of parkinsonian motor dysfunction and a functional compensation for histamine in parkinsonian basal ganglia circuitry. The findings provide insights into mechanisms of STN-DBS as well as potential therapeutic targets and STN-DBS strategies for PD.

Repurposing drugs to treat l-DOPA-induced dyskinesia in Parkinson's disease

In this review, we discuss the opportunity for repurposing drugs for use in l-DOPA-induced dyskinesia (LID) in Parkinson's disease. LID is a particularly suitable indication for drug repurposing given its pharmacological diversity, translatability of animal-models, availability of Phase II proof-of-concept (PoC) methodologies and the indication-specific regulatory environment. A compound fit for repurposing is defined as one with appropriate human safety-data as well as animal safety, toxicology and pharmacokinetic data as found in an Investigational New Drug (IND) package for another indication. We first focus on how such repurposing candidates can be identified and then discuss development strategies that might progress such a candidate towards a Phase II clinical PoC. We discuss traditional means for identifying repurposing candidates and contrast these with newer approaches, especially focussing on the use of computational and artificial intelligence (AI) platforms. We discuss strategies that can be categorised broadly as: in vivo phenotypic screening in a hypothesis-free manner; in vivo phenotypic screening based on analogy to a related disorder; hypothesis-driven evaluation of candidates in vivo and in silico screening with a hypothesis-agnostic component to the selection. To highlight the power of AI approaches, we describe a case study using IBM Watson where a training set of compounds, with demonstrated ability to reduce LID, were employed to identify novel repurposing candidates. Using the approaches discussed, many diverse candidates for repurposing in LID, originally envisaged for other indications, will be described that have already been evaluated for efficacy in non-human primate models of LID and/or clinically. This article is part of the Special Issue entitled 'Drug Repurposing: old molecules, new ways to fast track drug discovery and development for CNS disorders'.

Development of [ Carbonyl-11C]AZ13198083, a Novel Histamine Type-3 Receptor Radioligand with Favorable Kinetics

The histamine subtype-3 receptor (H₃R) is implicated in a range of central nervous system disorders, and several radioligands have been developed for H₃R positron emission tomography imaging. However, a limitation of currently used PET radioligands for H₃R is the slow binding kinetics in high density brain regions. To address this, we herein report the development of three novel candidate H₃R radioligands, namely, [ carbonyl-¹¹C]AZ13153556 ([ carbonyl-¹¹C]4), [ carbonyl-¹¹C]AZD5213([ carbonyl-¹¹C]5), and [ carbonyl-¹¹C]AZ13198083 ([ carbonyl-¹¹C]6), and their subsequent preclinical evaluation in nonhuman primates (NHP). Radioligands [ carbonyl-¹¹C]4-6 were produced and isolated in high radioactivity (>1000 MBq), radiochemical purity (>99%), and moderate molar activity (19-28 GBq/μmol at time of injection) using a palladium-mediated ¹¹C-aminocarbonylation protocol. All three radioligands showed high brain permeability as well as a regional brain radioactivity distribution in accordance with H₃R expression (striatum > cortex > cerebellum). [ Carbonyl-¹¹C]6 displayed the most favorable in vivo kinetics and brain uptake, with an early peak in the striatal time-activity curve followed by a progressive washout from the brain. The specificity and on-target kinetics of [ carbonyl-¹¹C]6 were next investigated in pretreatment and displacement studies. After pretreatment or displacement with 5 (0.1 mg/kg), a uniformly low distribution of radioactivity across the NHP brain was observed. Collectively, this work demonstrates that [ carbonyl-¹¹C]6 is a promising candidate for H₃R imaging in human subjects.

Knockout of histamine receptor H3 alters adaptation to sudden darkness and monoamine levels in the zebrafish

AIM

Histamine receptor H3 (HRH3) has substantial neuropharmacological potential. Currently, knockout models of this receptor have been investigated only in mice. We characterized the expression of this receptor in the zebrafish and generated a zebrafish HRH3 knockout line. Using this model, we studied the role of HRH3 in important behaviours. We also analysed the effect of HRH3 knockout on monoaminergic systems, which has not been thoroughly studied in any animal model.

METHODS

Generation of a mutant zebrafish line using the CRISPR-Cas9 system. Analysis of locomotor and social behaviour. Expression of HRH3 was characterized using in situ hybridization. Analysis of monoamine networks using HPLC, immunohistochemistry and quantitative PCR.

RESULTS

We found that HRH3 knockout zebrafish larvae showed a shorter period of increased locomotion after a sudden onset of darkness, while the knockout larvae had a wild-type-like acute response to sudden darkness. Adult knockout fish showed decreased swimming velocity, although locomotor activity of knockout larvae was unaltered. Additionally, levels of dopamine and serotonin were significantly decreased in the knockout fish, while monoamine-related gene expression and immunohistochemistry patterns were unchanged.

CONCLUSIONS

Our results show that HRH3 knockout larvae adapt faster to sudden darkness, suggesting a role for this receptor in regulating responses to changes in the environment. The decreased levels of dopamine and serotonin provide the first direct evidence that knockout of HRH3 alters these systems.

Histamine Excites Striatal Dopamine D1 and D2 Receptor-Expressing Neurons via Postsynaptic H1 and H2 Receptors

The central histaminergic nervous system, originating from the tuberomammillary nucleus (TMN) of the hypothalamus, widely innervates almost the whole brain, including the basal ganglia. Intriguingly, the histaminergic system is altered in parkinsonian patients. Yet, little is known about the effect and mechanisms of histamine on different types of neurons in the basal ganglia circuitry. Here, by using anterograde tracing, immunostaining, patch clamp recording, and single-cell qPCR techniques, we investigate the histaminergic afferents in the striatum, the major input structure of the basal ganglia, as well as the effect of histamine on the striatal GABAergic medium spiny projection neurons (MSNs). We report a direct histaminergic projection from the hypothalamic TMN to the striatum in rats. Furthermore, histamine exerts a strong postsynaptic excitatory effect on both dopamine D1 and D2 receptor-expressing MSNs. The concentration-response curves and the EC₅₀ values for histamine on these two types of MSNs are similar. In addition, dopamine D1 and D2 receptor-expressing MSNs co-express histamine H1 and H2 receptor mRNAs. Both histamine H1 and H2 receptors are co-localized on dopamine D1 and D2 receptor-expressing MSNs and co-mediate the histamine-induced excitation on the two types of neurons. These results suggest that the histaminergic afferent inputs in the striatum may modulate both dopamine D1 and D2 receptor-expressing MSNs by activation of postsynaptic histamine H1 and H2 receptors and thus serve as an important extrastriatal modulator for biasing the direct and indirect pathways to actively regulate functions of the basal ganglia and participate in the pathogenesis and pathophysiology of basal ganglia diseases.

New Insights into the Potential Roles of 3-Iodothyronamine (T1AM) and Newly Developed Thyronamine-Like TAAR1 Agonists in Neuroprotection

3-Iodothyronamine (T1AM) is an endogenous high-affinity ligand of the trace amine-associated receptor 1 (TAAR1), detected in mammals in many organs, including the brain. Recent evidence indicates that pharmacological TAAR1 activation may offer a novel therapeutic option for the treatment of a wide range of neuropsychiatric and metabolic disorders. To assess potential neuroprotection by TAAR1 agonists, in the present work, we initially investigated whether T1AM and its corresponding 3-methylbiaryl-methane analog SG-2 can improve learning and memory when systemically administered to mice at submicromolar doses, and whether these effects are modified under conditions of MAO inhibition by clorgyline. Our results revealed that when i.p. injected to mice, both T1AM and SG-2 produced memory-enhancing and hyperalgesic effects, while increasing ERK1/2 phosphorylation and expression of transcription factor c-fos. Notably, both compounds appeared to rely on the action of ubiquitous enzymes MAO to produce the corresponding oxidative metabolites that were then able to activate the histaminergic system. Since autophagy is key for neuronal plasticity, in a second line of experiments we explored whether T1AM and synthetic TAAR1 agonists SG1 and SG2 were able to induce autophagy in human glioblastoma cell lines (U-87MG). After treatment of U-87MG cells with 1 μM T1AM, SG-1, SG-2 transmission electron microscopy (TEM) and immunofluorescence (IF) showed a significant time-dependent increase of autophagy vacuoles and microtubule-associated protein 1 light chain 3 (LC3). Consistently, Western blot analysis revealed a significant increase of the LC3II/LC3I ratio, with T1AM and SG-1 being the most effective agents. A decreased level of the p62 protein was also observed after treatment with T1AM and SG-1, which confirms the efficacy of these compounds as autophagy inducers in U-87MG cells. In the process to dissect which pathway induces ATG, the effects of these compounds were evaluated on the PI3K-AKT-mTOR pathway. We found that 1 μM T1AM, SG-1 and SG-2 decreased pAKT/AKT ratio at 0.5 and 4 h after treatment, suggesting that autophagy is induced by inhibiting mTOR phosphorylation by PI3K-AKT-mTOR pathway. In conclusion, our study shows that T1AM and thyronamine-like derivatives SG-1 and SG-2 might represent valuable tools to therapeutically intervene with neurological disorders.

Ligand autoradiographical quantification of histamine H3 receptor in human dementia with Lewy bodies

Dementia with Lewy bodies (DLB) is a serious age-dependent human neurodegenerative disease, with multiple debilitating symptoms, including dementia, psychosis and significant motor deficits, but with little or no effective treatments. This comparative ligand autoradiographical study has quantified histamine H₃ receptors (H₃R) in a series of major cortical and basal ganglia structures in human DLB and Alzheimer’s (AD) post-mortem cases using the highly selective radioligand, [³H] GSK189254.

In the main, the levels of H₃ receptor were largely preserved in DLB cases when compared with aged-matched controls. However, we provide new evidence showing variable levels in the globus pallidus, and, moreover, raised levels of Pallidum H₃ correlated with positive psychotic symptoms, in particular delusions and visual hallucinations, but not symptoms associated with depression. Furthermore, no correlation was detected for H₃ receptor levels to MMSE or IUPRS symptom severity.

This study suggests that H₃R antagonists have scope for treating the psychotic symptomologies in DLB and other human brain disorders.

Histamine and the striatum

The neuromodulator histamine is released throughout the brain during periods of wakefulness. Combined with an abundant expression of histamine receptors, this suggests potential widespread histaminergic control of neural circuit activity. However, the effect of histamine on many of these circuits is unknown. In this review we will discuss recent evidence for histaminergic modulation of the basal ganglia circuitry, and specifically its main input nucleus; the striatum. Furthermore, we will discuss recent findings of histaminergic dysfunction in several basal ganglia disorders, including in Parkinson's disease and most prominently, in Tourette's syndrome, which has led to a resurgence of interest in this neuromodulator. Combined, these recent observations not only suggest a central role for histamine in modulating basal ganglia activity and behaviour, but also as a possible target in treating basal ganglia disorders. This article is part of the Special Issue entitled 'Histamine Receptors'.

Thr105Ile (rs11558538) polymorphism in the histamine N-methyltransferase (HNMT) gene and risk for Parkinson disease: A PRISMA-compliant systematic review and meta-analysis

BACKGROUND/AIMS

Several neuropathological, biochemical, and pharmacological data suggested a possible role of histamine in the etiopathogenesis of Parkinson disease (PD). The single nucleotide polymorphism (SNP) rs11558538 in the histamine N-methyltransferase (HNMT) gene has been associated with the risk of developing PD by several studies but not by some others. We carried out a systematic review that included all the studies published on PD risk related to the rs11558538 SNP, and we conducted a meta-analysis following Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines.

METHODS

We used several databases to perform the systematic review, the software Meta-DiSc 1.1.1 to perform the meta-analysis of the eligible studies, and the Q-statistic to test heterogeneity between studies.

RESULTS

The meta-analysis included 4 eligible case-control association studies for the HNMT rs11558538 SNP and the risk for PD (2108 patients, 2158 controls). The frequency of the minor allele positivity showed a statistically significant association with a decreased risk for PD, both in the total series and in Caucasians. Although homozygosity for the minor allele did not reach statistical significance, the test for trend indicates the occurrence of a gene-dose effect. Global diagnostic odds ratios (95% confidence intervals) for rs11558538T were 0.61 (0.46-0.81) for the total group, and 0.63 (0.45-0.88) for Caucasian patients.

CONCLUSION

The present meta-analysis confirms published evidence suggesting that the HNMT rs11558538 minor allele is related to a reduced risk of developing PD.

International Union of Basic and Clinical Pharmacology. XCVIII. Histamine Receptors

Histamine is a developmentally highly conserved autacoid found in most vertebrate tissues. Its physiological functions are mediated by four 7-transmembrane G protein-coupled receptors (H1R, H2R, H3R, H4R) that are all targets of pharmacological intervention. The receptors display molecular heterogeneity and constitutive activity. H1R antagonists are long known antiallergic and sedating drugs, whereas the H2R was identified in the 1970s and led to the development of H2R-antagonists that revolutionized stomach ulcer treatment. The crystal structure of ligand-bound H1R has rendered it possible to design new ligands with novel properties. The H3R is an autoreceptor and heteroreceptor providing negative feedback on histaminergic and inhibition on other neurons. A block of these actions promotes waking. The H4R occurs on immuncompetent cells and the development of anti-inflammatory drugs is anticipated.

Role of the Histamine H3 Receptor in the Central Nervous System

The G_i/o protein-coupled histamine H₃ receptor is distributed throughout the central nervous system including areas like cerebral cortex, hippocampus and striatum with the density being highest in the posterior hypothalamus, i.e. the area in which the histaminergic cell bodies are located. In contrast to the other histamine receptor subtypes (H₁, H₂ and H₄), the H₃ receptor is located presynaptically and shows a constitutive activity. In detail, H₃ receptors are involved in the inhibition of histamine release (presynaptic autoreceptor), impulse flow along the histaminergic neurones (somadendritic autoreceptor) and histamine synthesis. Moreover, they occur as inhibitory presynaptic heteroreceptors on serotoninergic, noradrenergic, dopaminergic, glutamatergic, GABAergic and perhaps cholinergic neurones. This review shows for four functions of the brain that the H₃ receptor represents a brake against the wake-promoting, anticonvulsant and anorectic effect of histamine (via postsynaptic H₁ receptors) and its procognitive activity (via postsynaptic H₁ and H₂ receptors). Indeed, H₁ agonists and H₃ inverse agonists elicit essentially the same effects, at least in rodents; these effects are opposite in direction to those elicited by brain-penetrating H₁ receptor antagonists in humans. Although the benefit for H₃ inverse agonists for the symptomatic treatment of dementias is inconclusive, several members of this group have shown a marked potential for the treatment of disorders associated with excessive daytime sleepiness. In March 2016, the European Commission granted a marketing authorisation for pitolisant (Wakix^R) (as the first representative of the H₃ inverse agonists) for the treatment of narcolepsy.

Histamine H3 receptor antagonist decreases cue-induced alcohol reinstatement in mice

We have earlier found that the histamine H3 receptor (H3R) antagonism diminishes motivational aspects of alcohol reinforcement in mice. Here we studied the role of H3Rs in cue-induced reinstatement of alcohol seeking in C57BL/6J mice using two different H3R antagonists. Systemic administration of H3R antagonists attenuated cue-induced alcohol seeking suggesting that H3R antagonists may reduce alcohol craving. To understand how alcohol affects dopamine and histamine release, a microdialysis study was performed on C57BL/6J mice and the levels of histamine, dopamine and dopamine metabolites were measured in the nucleus accumbens. Alcohol administration was combined with an H3R antagonist pretreatment to reveal whether modulation of H3R affects the effects of alcohol on neurotransmitter release. Alcohol significantly increased the release of dopamine in the nucleus accumbens but did not affect histamine release. Pretreatment with H3R antagonist ciproxifan did not modify the effect of alcohol on dopamine release. However, histamine release was markedly increased with ciproxifan. In conclusion, our findings demonstrate that H3R antagonism attenuates cue-induced reinstatement of alcohol seeking in mice. Alcohol alone does not affect histamine release in the nucleus accumbens but H3R antagonist instead increases histamine release significantly suggesting that the mechanism by which H3R antagonist inhibits alcohol seeking found in the present study and the decreased alcohol reinforcement, reward and consumption found earlier might include alterations in the histaminergic neurotransmission in the nucleus accumbens. These findings imply that selective antagonists of H3Rs could be a therapeutic strategy to prevent relapse and possibly diminish craving to alcohol use. This article is part of the Special Issue entitled 'Histamine Receptors'.

Interactions of the orexin/hypocretin neurones and the histaminergic system

Histaminergic and orexin/hypocretin systems are components in the brain wake-promoting system. Both are affected in the sleep disorder narcolepsy, but the role of histamine in narcolepsy is unclear. The histaminergic neurones are activated by the orexin/hypocretin system in rodents, and the development of the orexin/hypocretin neurones is bidirectionally regulated by the histaminergic system in zebrafish. This review summarizes the current knowledge of the interactions of these two systems in normal and pathological conditions in humans and different animal models.

The human histaminergic system in neuropsychiatric disorders

Histaminergic neurons are exclusively located in the hypothalamic tuberomamillary nucleus, from where they project to many brain areas. The histaminergic system is involved in basic physiological functions, such as the sleep-wake cycle, energy and endocrine homeostasis, sensory and motor functions, cognition, and attention, which are all severely affected in neuropsychiatric disorders. Here, we present recent postmortem findings on the alterations in this system in neuropsychiatric disorders, including Parkinson's disease (PD), Alzheimer's disease (AD), Huntington's disease (HD), depression, and narcolepsy. In addition, we highlight the need to validate animal models for these diseases and also for Tourette's syndrome (TS) in relation to alterations in the histaminergic system. Moreover, we discuss the potential for, and concerns over, the use of novel histamine 3 receptor (H3R) antagonists/inverse agonists as treatment for such disorders.

Assessment of [125I]WYE-230949 as a novel histamine H3 receptor radiopharmaceutical

Histamine H3 receptor therapeutics have been proposed for several diseases such as schizophrenia, attention deficit hyperactivity disorder, Alzheimer's disease and obesity. We set out to evaluate the novel compound, [125I]WYE-230949, as a potential radionuclide imaging agent for the histamine H3 receptor in brain. [125I]WYE-230949 had a high in vitro affinity for the rat histamine H3 receptor (Kd of 6.9 nM). The regional distribution of [125I]WYE-230949 binding sites in rat brain, demonstrated by in vitro autoradiography, was consistent with the known distribution of the histamine H3 receptor. Rat brain uptake of intravenously injected [125I]WYE-230949 was low (0.11 %ID/g) and the ratio of specific: non-specific binding was less than 1.4, as determined by ex vivo autoradiography. In plasma, metabolism of [125I]WYE-230949 into a less lipophilic species occurred, such that less than 38% of the parent compound remained 30 minutes after injection. Brain uptake and metabolism of [125I]WYE-230949 were increased and specific binding was reduced in anaesthetised compared to conscious rats. [125I]WYE230949 is not a potential radiotracer for imaging rat histamine H3 receptors in vivo due to low brain uptake, in vivo metabolism of the parent compound and low specific binding.

Histamine is required for H₃ receptor-mediated alcohol reward inhibition, but not for alcohol consumption or stimulation

BACKGROUND AND PURPOSE

Conflicting data have been published on whether histamine is inhibitory to the rewarding effects of abused drugs. The purpose of this study was to clarify the role of neuronal histamine and, in particular, H₃ receptors in alcohol dependence-related behaviours, which represent the addictive effects of alcohol.

EXPERIMENTAL APPROACH

Alcohol-induced conditioned place preference (alcohol-CPP) was used to measure alcohol reward. Alcohol-induced locomotor stimulation, alcohol consumption and kinetics were also assessed. mRNA levels were quantified using radioactive in situ hybridization.

KEY RESULTS

Low doses of H₃ receptor antagonists, JNJ-10181457 and JNJ-39220675, inhibited alcohol reward in wild-type (WT) mice. However, these H₃ receptor antagonists did not inhibit alcohol reward in histidine decarboxylase knock-out (HDC KO) mice and a lack of histamine did not alter alcohol consumption. Thus H₃ receptor antagonists inhibited alcohol reward in a histamine-dependent manner. Furthermore, WT and HDC KO mice were similarly stimulated by alcohol. The expression levels of dopamine D₁ and D₂ receptors, STEP61 and DARPP-32 mRNA in striatal subregions were unaltered in HDC KO mice. No differences were seen in alcohol kinetics in HDC KO compared to WT control animals. In addition, JNJ-39220675 had no effect on alcohol kinetics in WT mice.

CONCLUSIONS AND IMPLICATIONS

These data suggest that histamine is required for the H₃ receptor-mediated inhibition of alcohol-CPP and support the hypothesis that the brain histaminergic system has an inhibitory role in alcohol reward. Increasing neuronal histamine release via H₃ receptor blockade could therefore be a novel way of treating alcohol dependence.

Histamine H₃ receptors, the complex interaction with dopamine and its implications for addiction

Histamine H₃ receptors are best known as presynaptic receptors inhibiting the release of histamine, as well as other neurotransmitters including acetylcholine and dopamine. However, in the dorsal and ventral striatum, the vast majority of H₃ receptors are actually located postsynaptically on medium sized spiny output neurons. These cells also contain large numbers of dopamine (D₁ and D₂) receptors and it has been shown that H₃ receptors form heterodimers with both D₁ and D₂ receptors. Thus, the anatomical localization of H₃ receptors suggests a complex interaction that could both enhance and inhibit dopaminergic neurotransmission. Dopamine, especially within the striatal complex, plays a crucial role in the development of addiction, both in the initial reinforcing effects of drugs of abuse, as well as in maintenance, relapse and reinstatement of drug taking behaviour. It is, therefore, conceivable that H₃ receptors can moderate the development and maintenance of drug addiction. In the present review, we appraise the current literature on the involvement of H₃ receptors in drug addiction and try to explain these data within a theoretical framework, as well as provide suggestions for further research.

The histamine autoreceptor is a short isoform of the H₃ receptor

BACKGROUND AND PURPOSE The histamine H(3) receptor was identified as the autoreceptor of brain histaminergic neurons. After its cloning, functional H(3) receptor isoforms generated by a deletion in the third intracellular loop were found in the brain. Here, we determined if this autoreceptor was the long or the short isoform. EXPERIMENTAL APPROACH We hypothesized that the deletion would affect H(3) receptor stereoselectivity. The effects of the enantiomers of two chiral ligands, N(α)-methyl-α-chloromethylhistamine (N(α) Me-αClMeHA) and sopromidine, were investigated on cAMP formation at the H(3(445)) and H(3(413)) receptor isoforms, common to all species. They were further compared with their effects at autoreceptors. They were also compared on [(35)S]GTPγ[S] binding to membranes of rat cerebral cortex, striatum and hypothalamus, the richest area in autoreceptors. KEY RESULTS The stereoselectivity of N(α) Me-αClMeHA enantiomers as agonists was similar at the H(3(413)) receptor isoform and autoreceptors, but lower at the long isoform. While (S) sopromidine did not discriminate between the isoforms, (R) sopromidine was an antagonist at the H(3(413)) receptor isoform and autoreceptors, but a full agonist at the long isoform. In rat brain, stereoselectivity of N(α) Me-αClMeHA was higher in the hypothalamus than in cerebral cortex or striatum, whereas the opposite pattern was found for sopromidine. CONCLUSIONS AND IMPLICATIONS The pharmacological profiles of H(3) receptor isoforms differed markedly, showing that the function of autoreceptors was fulfilled by a short isoform, such as the H(3(413)) receptor. Development of drugs selectively targeting autoreceptors might enhance their therapeutic efficacy and/or decrease incidence of side effects.

Nondopaminergic treatments for Parkinson’s disease

SUMMARY The main treatment strategy for Parkinson’s disease (PD) is focused on dopamine replacement. However, PD is no longer seen purely as a disease of the dopaminergic system, as the pathological processes involve neurodegeneration and altered neurotransmission of several nondopaminergic systems that are involved in both motor and nonmotor features of the disease. This article reviews current and experimental nondopaminergic pharmacological approaches to treatments for PD with a focus on motor symptoms, treatments of L-dopa-induced motor complications and treatments of nonmotor symptoms including mood disorders, cognition, psychosis and autonomic problems.

Alterations in the histaminergic system in the substantia nigra and striatum of Parkinson's patients: a postmortem study

Earlier studies showed neuronal histamine production in the hypothalamic tuberomamillary nucleus to be unchanged in Parkinson's disease (PD), whereas the histamine levels and innervation in the substantia nigra (SN) increased. In the present study we used quantitative polymerase chain reaction (qPCR) to assess the changes in the histaminergic system in the SN, caudate nucleus (CN), and putamen (PU) in 7 PD patients and 7 controls. The messenger RNA (mRNA) expression of the histamine receptor-3 (H(3)R), which was localized immunocytochemically in the large pigmented neurons, was significantly decreased in the SN in PD, while histamine receptor-4 (H(4)R)-mRNA expression showed a significant increase in caudate nucleus and PU. In addition, significantly increased mRNA levels of histamine methyltransferase (HMT), a key enzyme involved in histamine metabolism, were found in the SN and in the PU in PD. Moreover, in the SN, the histamine methyltransferase-mRNA showed a strong negative correlation with PD disease duration. Our observations imply the presence of local changes in the histaminergic system that may contribute to PD pathology, and may thus provide a rationale for possible novel therapeutic strategies.

The histamine H3 receptor: from discovery to clinical trials with pitolisant

The third histamine receptor was discovered in 1983 by a traditional pharmacological approach, consisting of assessing the inhibitory effect of histamine on its own release from depolarized rat brain slices. The same in vitro test was used to design, in 1987, the first highly selective and potent H3-autoreceptor ligands, the antagonist thioperamide and the agonist (R)alphamethylhistamine which enhances and inhibits, respectively, the activity of histaminergic neurons in brain. The use of these research tools was instrumental in establishing the main functions of cerebral histaminergic neurons, namely their role in maintenance of wakefulness, attention, learning and other cognitive processes. In 1990, the cloning of the gene of the H3-receptor, a member of the superfamily of heptahelical receptors coupled to G proteins, paved the way to the demonstration of the high constitutive activity of the receptor, including its native form, and its participation in the tonic control of histamine release; it also facilitated the development of H3-receptor inverse agonist programs in many drug companies. Pitolisant (BF2.649, 1-{3-[3-(4-chlorophenyl)propoxy]propyl}piperidine, hydrochloride) is the first inverse agonist to be introduced in the clinics. Its wake-promotion activity was evidenced in excessive diurnal sleepiness of patients with narcolepsy, Parkinson's disease or Obstructive Sleep Apnea/Hypopnea, in which this activity is characterized by a mean decrease of the Epworth Sleepiness Scale by about five units. The procognitive activity of this novel class of drugs may also find therapeutic applications in dementias, schizophrenia or attention deficit hyperactivity disorder.

JNJ-39220675, a novel selective histamine H3 receptor antagonist, reduces the abuse-related effects of alcohol in rats

RATIONALE

A few recent studies suggest that brain histamine levels and signaling via H(3) receptors play an important role in modulation of alcohol stimulation and reward in rodents.

OBJECTIVE

The present study characterized the effects of a novel, selective, and brain penetrant H(3) receptor antagonist (JNJ-39220675) on the reinforcing effects of alcohol in rats.

METHODS

The effect of JNJ-39220675 on alcohol intake and alcohol relapse-like behavior was evaluated in selectively bred alcohol-preferring (P) rats using the standard two-bottle choice method. The compound was also tested on operant alcohol self administration in non-dependent rats and on alcohol-induced ataxia using the rotarod apparatus. In addition, alcohol-induced dopamine release in the nucleus accumbens was tested in freely moving rats.

RESULTS

Subcutaneous administration of the selective H(3) receptor antagonist dose-dependently reduced both alcohol intake and preference in alcohol-preferring rats. JNJ-39220675 also reduced alcohol preference in the same strain of rats following a 3-day alcohol deprivation. The compound significantly and dose-dependently reduced alcohol self-administration without changing saccharin self-administration in alcohol non-dependent rats. Furthermore, the compound did not change the ataxic effects of alcohol, alcohol elimination rate, nor alcohol-induced dopamine release in nucleus accumbens.

CONCLUSIONS

These results indicate that blockade of H(3) receptor should be considered as a new attractive mechanism for the treatment of alcoholism.

Effects of simvastatin and 6-hydroxydopamine on histaminergic H1 receptor binding density in rat brains

Statins have been widely used for the treatment of a variety of medical conditions including psychoneurological disorders beyond their original use in lowering cholesterol. Histamine receptors play an important role in the regulation of neural activity, however, it is unknown whether statins act on histamine receptors, particularly for their neural regulatory effects. This study examined the effects of simvastatin and 6-hydroxydopamine (6-OHDA) lesions on histamine H1 receptors using [(3)H] pyrilamine binding autoradiography. Compared to the saline group, simvastatin (1 mg/kg/day) significantly decreased H1 receptor bindings in the primary motor cortex (M1), ventromedial hypothalamic nucleus (VMH), caudate putamen (CPu), accumbens core (AcbC) and prefrontal cortex (PfC) (all p<0.05); however 10 mg/kg/day simvastatin increased H1 receptor density only in the medial amygdaloid nucleus (Mep) (p<0.05), but had no significant effect in other regions examined. The 6-OHDA lesion did not alter H1 receptor binding density in most brain areas, except a trend decrease in the hippocampus (p=0.07) and a trend increase in the cingulate cortex (p=0.06). These results suggested that simvastatin has different effects on the H1 receptors in different rat brain regions depending on the doses. Therefore, simvastatin can modulate histaminergic neurotransmission in the brain, and support the role of H1 receptors in psychoneurological disorders.

Histamine and H3 receptor in alcohol-related behaviors

Data from rat models for alcohol preference and histidine decarboxylase knockout (HDC KO) mice suggest that brain histamine regulates alcohol-related behaviors. Histamine levels are higher in alcohol-preferring than in alcohol-nonpreferring rat brains, and expression of histamine H(3) receptor (H(3)R) is different in key areas for addictive behavior. H(3)R inverse agonists decrease alcohol responding in one alcohol-preferring rat line. Conditioned place preference induced by alcohol is stronger in HDC KO mice than in control mice. The HDC KO mice display a weaker stimulatory response to acute alcohol than the wild-type (WT) mice. In male inbred C57BL/6 mice the H(3)R antagonist ciproxifan inhibits ethanol-evoked stimulation of locomotor activity. Ciproxifan also potentiates the ethanol reward, but does not alone result in the development of place preference. At least in one rat model developed to study alcohol sensitivity high histamine levels are characteristic of the alcohol-insensitive rat line, and lowering brain histamine with a HDC inhibitor increases alcohol sensitivity in the tilting plane test. However, the motor skills of HDC KO mice do not seem to differ from those of the WT mice. Current evidence suggests that the histaminergic system is involved in the regulation of place preference behavior triggered by alcohol, possibly through an interaction with the mesolimbic dopamine system. Histamine may also interact with dopamine in the regulation of the cortico-striato-pallido-thalamo-cortical motor pathway and cerebellar mechanisms, which may be important in different motor behaviors beyond alcohol-induced motor disturbances. H(3)R ligands may have significant effects on alcohol addiction.

An elderly female patient with tardive oromandibular dystonia after prolonged use of the histamine analog betahistine

Tardive oromandibular dystonia (OMD) is iatrogenic in origin and is characterised by orofacial and lingual stereotypes more frequently than the idiopathic form of OMD Tardive OMD is often associated with anti-dopaminergic treatment involving drugs such as anti-psychotics, anti-emetics, and anti-vertigo agents, although the syndrome can also be triggered by anti-epileptic or anti-depressant drugs that do not have anti-dopaminergic properties. We report an elderly female patient with OMD after prolonged, self-administered treatment with betahistine dihydrochloride, a histamine analogue.