The physiological and pathophysiological roles of neuronal histamine: an insight from human positron emission tomography studies

Resting-State EEG Reveals Abnormal Microstate Characteristics of Depression with Insomnia

Previous research revealed various aspects of resting-state EEG for depression and insomnia. However, the EEG characteristics of depressed subjects with insomnia are rarely studied, especially EEG microstates that capture the dynamic activities of the large-scale brain network. To fill these research gaps, the present study collected resting-state EEG data from 32 subclinical depression subjects with insomnia (SDI), 31 subclinical depression subjects without insomnia (SD), and 32 healthy controls (HCs). Four topographic maps were generated from clean EEG data after clustering and rearrangement. Temporal characteristics were obtained for statistical analysis, including cross-group variance analysis (ANOVA) and intra-group correlation analysis. In our study, the global clustering of all individuals in the EEG microstate analysis revealed the four previously discovered categories of microstates (A, B, C, and D). The occurrence of microstate B was lower in SDI than in SD and HC subjects. The correlation analysis showed that the total Pittsburgh Sleep Quality Index (PSQI) score negatively correlated with the occurrence of microstate C in SDI (r = - 0.415, p < 0.05). Conversely, there was a positive correlation between Self-rating Depression Scale (SDS) scores and the duration of microstate C in SD (r = 0.359, p < 0.05). These results indicate that microstates reflect altered large-scale brain network dynamics in subclinical populations. Abnormalities in the visual network corresponding to microstate B are an electrophysiological characteristic of subclinical individuals with symptoms of depressive insomnia. Further investigation is needed for microstate changes related to high arousal and emotional problems in people suffering from depression and insomnia.

Updating functional brain units: Insights far beyond Luria

This paper reviews Luria's model of the three functional units of the brain. To meet this objective, several issues were reviewed: the theory of functional systems and the contributions of phylogenesis and embryogenesis to the brain's functional organization. This review revealed several facts. In the first place, the relationship/integration of basic homeostatic needs with complex forms of behavior. Secondly, the multi-scale hierarchical and distributed organization of the brain and interactions between cells and systems. Thirdly, the phylogenetic role of exaptation, especially in basal ganglia and cerebellum expansion. Finally, the tripartite embryogenetic organization of the brain: rhinic, limbic/paralimbic, and supralimbic zones. Obviously, these principles of brain organization are in contradiction with attempts to establish separate functional brain units. The proposed new model is made up of two large integrated complexes: a primordial-limbic complex (Luria's Unit I) and a telencephalic-cortical complex (Luria's Units II and III). As a result, five functional units were delineated: Unit I. Primordial or preferential (brainstem), for life-support, behavioral modulation, and waking regulation; Unit II. Limbic and paralimbic systems, for emotions and hedonic evaluation (danger and relevance detection and contribution to reward/motivational processing) and the creation of cognitive maps (contextual memory, navigation, and generativity [imagination]); Unit III. Telencephalic-cortical, for sensorimotor and cognitive processing (gnosis, praxis, language, calculation, etc.), semantic and episodic (contextual) memory processing, and multimodal conscious agency; Unit IV. Basal ganglia systems, for behavior selection and reinforcement (reward-oriented behavior); Unit V. Cerebellar systems, for the prediction/anticipation (orthometric supervision) of the outcome of an action. The proposed brain units are nothing more than abstractions within the brain's simultaneous and distributed physiological processes. As function transcends anatomy, the model necessarily involves transition and overlap between structures. Beyond the classic approaches, this review includes information on recent systemic perspectives on functional brain organization. The limitations of this review are discussed.

Single-Molecule Conductance of Intramolecular Hydrogen Bonding in Histamine on Gold

We perform single-molecule conductance measurements and DFT calculations on histamine, a biogenic amine that contains a flexible aliphatic linker and several nitrogen moieties with a potential for hydrogen bonding. Our study determines that junctions containing the free-base form of histamine can bridge through a molecular structure containing an intramolecular hydrogen bond. Conductance of this structure is higher than that through the saturated aliphatic linker. Flicker noise analysis of junction conductance confirms that transport occurs through the hydrogen bond and establishes a benchmark for noise measurements in hydrogen-bonded junctions. Overall, our work provides insights into the formation and conduction of intramolecular hydrogen bonding in single-molecule conductance measurements and into the conformations of the neurotransmitter histamine on noble metal surfaces.

Dynamic Interplay between Social Brain Development and Nutrient Intake in Young Children

Myelination of the brain structures underlying social behavior in humans is a dynamic process that parallels the emergence of social-emotional development and social skills in early life. Of the many genetic and environmental factors regulating the myelination processes, nutrition is considered as a critical and modifiable early-life factor for establishing healthy social brain networks. However, the impact of nutrition on the longitudinal development of social brain myelination remains to be fully understood. This study examined the interplay between childhood nutrient intake and social brain development across the first 5 years of life. Myelin-sensitive neuroimaging and food-intake data were analyzed in 293 children, 0.5 to 5 years of age, and explored for dynamic patterns of nutrient-social brain myelin associations. We found three data-driven age windows with specific nutrient correlation patterns, 63 individual nutrient-myelin correlations, and six nutrient combinations with a statistically significant predictive value for social brain myelination. These results provide novel insights into the impact of specific nutrient intakes on early brain development, in particular social brain regions, and suggest a critical age-sensitive opportunity to impact these brain regions for potential longer-term improvements in socio-emotional development and related executive-function and critical-thinking skills.

Are antimuscarinic effects common in hydroxyzine overdose? A cohort analysis of antimuscarinic effects in hydroxyzine and diphenhydramine-poisoned patients

INTRODUCTION

Exposures to hydroxyzine, a first-generation H1 antihistamine, have increased rapidly over the last two decades. Many assumptions about hydroxyzine poisoning are based on other antihistamines, like diphenhydramine. However, the receptor affinities of hydroxazine suggest that there should be fewer antimuscarinic findings than diphenhydramine.

METHODS

This was a cohort study that compared hydroxyzine and diphenhydramine exposures reported to the National Poison Data System between January 1, 2000, and December 31, 2020, and the Toxicologic Investigators Consortium Core Registry between January 1, 2010, and December 31, 2020. The primary outcome was to assess for antimuscarinic findings in hydroxyzine-poisoned patients, using diphenhydramine-poisoned patients as a comparison group. The secondary outcomes were to assess for markers of overall toxicity. Inclusion criteria were single-substance exposures with known outcomes. Exclusion criteria for National Poison Data System exposures were chronic exposures, unintentional exposures, and patients younger than 12 years old. There were no exclusion criteria for exposures reported to the Toxicologic Investigators Consortium Core Registry.

RESULTS

There were 17,265 hydroxyzine and 102,354 diphenhydramine exposures reported to the National Poison Data System and 134 hydroxyzine and 1,484 diphenhydramine exposures reported to the Toxicologic Investigators Consortium Core Registry that met inclusion criteria. In both datasets, hydroxyzine-poisoned patients had lower rates and relative risk of developing antimuscarinic findings or receiving physostigmine, with the exception of hyperthermia in the Toxicologic Investigators Consortium Core Registry dataset. Coma/central nervous system depression (major), respiratory depression, seizures, ventricular dysrhythmias, intubation, and benzodiazepine administration were less likely in hydroxyzine-poisoned patients, but central nervous system depression (mild) was more likely in exposures reported to the National Poison Data System. The mortality in hydroxyzine-poisoned patients was rare: 0.02% and 0.8% of exposures reported to the National Poison Data System and Toxicologic Investigators Consortium Core Registry, respectively.

DISCUSSION

The clinical manifestations of hydroxyzine exposures are consistent with the pharmacology of hydroxazine. The clinical effects were consistent across two United States national datasets. Clinicians should not generalize the illness script of diphenhydramine exposures to hydroxyzine exposures.

CONCLUSIONS

Hydroxyzine-poisoned patients were less likely to develop antimuscarinic findings than diphenhydramine-poisoned patients. Hydroxyzine-poisoned patients were more likely to have mild central nervous system depression than an antimuscarinic toxidrome.

Recent advances in dopamine D2 receptor ligands in the treatment of neuropsychiatric disorders

Dopamine is a biologically active amine synthesized in the central and peripheral nervous system. This biogenic monoamine acts by activating five types of dopamine receptors (D_1-5 Rs), which belong to the G protein-coupled receptor family. Antagonists and partial agonists of D₂ Rs are used to treat schizophrenia, Parkinson's disease, depression, and anxiety. The typical pharmacophore with high D₂ R affinity comprises four main areas, namely aromatic moiety, cyclic amine, central linker and aromatic/heteroaromatic lipophilic fragment. From the literature reviewed herein, we can conclude that 4-(2,3-dichlorophenyl), 4-(2-methoxyphenyl)-, 4-(benzo[b]thiophen-4-yl)-1-substituted piperazine, and 4-(6-fluorobenzo[d]isoxazol-3-yl)piperidine moieties are critical for high D₂ R affinity. Four to six atoms chains are optimal for D₂ R affinity with 4-butoxyl as the most pronounced one. The bicyclic aromatic/heteroaromatic systems are most frequently occurring as lipophilic appendages to retain high D₂ R affinity. In this review, we provide a thorough overview of the therapeutic potential of D₂ R modulators in the treatment of the aforementioned disorders. In addition, this review summarizes current knowledge about these diseases, with a focus on the dopaminergic pathway underlying these pathologies. Major attention is paid to the structure, function, and pharmacology of novel D₂ R ligands, which have been developed in the last decade (2010-2021), and belong to the 1,4-disubstituted aromatic cyclic amine group. Due to the abundance of data, allosteric D₂ R ligands and D₂ R modulators from patents are not discussed in this review.

Molecular Characterization of Estrogen Receptor Agonists during Sewage Treatment Processes Using Effect-Directed Analysis Combined with High-Resolution Full-Scan Screening

Endocrine-disrupting potential was evaluated during the sewage treatment process using in vitro bioassays. Aryl hydrocarbon receptor (AhR)-, androgen receptor (AR)-, glucocorticoid receptor (GR)-, and estrogen receptor (ER)-mediated activities were assessed over five steps of the treatment process. Bioassays of organic extracts showed that AhR, AR, and GR potencies tended to decrease through the sewage treatment process, whereas ER potencies did not significantly decrease. Bioassays on reverse-phase high-performance liquid chromatography fractions showed that F5 (log K_OW 2.5-3.0) had great ER potencies. Full-scan screening of these fractions detected two novel ER agonists, arenobufagin and loratadine, which are used pharmaceuticals. These compounds accounted for 3.3-25% of the total ER potencies and 4% of the ER potencies in the final effluent. The well-known ER agonists, estrone and 17β-estradiol, accounted for 60 and 17% of the ER potencies in F5 of the influent and primary treatment, respectively. Fourier transform ion cyclotron resonance mass spectrometry analysis showed that various molecules were generated during the treatment process, especially CHO and CHOS (C: carbon, H: hydrogen, O: oxygen, and S: sulfur). This study documented that widely used pharmaceuticals are introduced into the aquatic environments without being removed during the sewage treatment process.

Contribution of astrocytic histamine N-methyltransferase to histamine clearance and brain function in mice

Brain histamine acts as a neurotransmitter in the regulation of various brain activities. Previous studies have shown that histamine N-methyltransferase (HNMT), a histamine-metabolizing enzyme, controls brain histamine concentration and brain function. However, the relative contribution of astrocytic or neuronal HNMT to the regulation of the histaminergic system is still inconclusive. Here, we phenotyped astrocytes-specific HNMT knockout (cKO) mice to clarify the involvement of astrocytic HNMT in histamine clearance and brain function. First, we performed histological examinations using HNMT reporter mice and showed a wide distribution of HNMT in the brain and astrocytic HNMT expression. Then, we created cKO mice by Cre-loxP system and confirmed that HNMT expression in cKO primary astrocytes was robustly decreased. Although total HNMT level in the cortex was not substantially different between control and cKO brains, histamine concentration after histamine release was elevated in cKO cortex. In behavioral tests, impaired motor coordination and lower locomotor activity were observed in the cKO mice. However, anxiety-like behaviors, depression-like behaviors, and memory functions were not altered by astrocytic HNMT disruption. Although sleep analysis demonstrated that the quantity of wakefulness and sleep did not change, the increased power density of delta frequency during wakefulness indicated lower cortical activation in cKO mice. These results demonstrate that astrocytic HNMT contributes to histamine clearance after histamine release in the cortex and plays a role in the regulation of motor coordination, locomotor activity, and vigilance state.

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.

Integration of the Cortical Haemodynamic Response Measured by Functional Near-Infrared Spectroscopy and Amino Acid Analysis to Aid in the Diagnosis of Major Depressive Disorder

Background: Major depressive disorder (MDD) is a debilitating condition with a high disease burden and medical comorbidities. There are currently few to no validated biomarkers to guide the diagnosis and treatment of MDD. In the present study, we evaluated the differences between MDD patients and healthy controls (HCs) in terms of cortical haemodynamic responses during a verbal fluency test (VFT) using functional near-infrared spectroscopy (fNIRS) and serum amino acid profiles, and ascertained if these parameters were correlated with clinical characteristics. Methods: Twenty-five (25) patients with MDD and 25 age-, gender-, and ethnicity-matched HCs were recruited for the study. Real-time monitoring of the haemodynamic response during completion of a VFT was quantified using a 52-channel NIRS system. Serum samples were analysed and quantified by liquid chromatography-mass spectrometry for amino acid profiling. Receiver-operating characteristic (ROC) curves were used to classify potential candidate biomarkers. Results: The MDD patients had lower prefrontal and temporal activation during completion of the VFT than HCs. The MDD patients had lower mean concentrations of oxy-Hb in the left orbitofrontal cortex (OFC), and lower serum histidine levels. When the oxy-haemoglobin response was combined with the histidine concentration, the sensitivity and specificity of results improved significantly from 66.7% to 73.3% and from 65.0% to 90.0% respectively, as compared to results based only on the NIRS response. Conclusions: These findings demonstrate the use of combination biomarkers to aid in the diagnosis of MDD. This technique could be a useful approach to detect MDD with greater precision, but additional studies are required to validate the methodology.

Changes in histaminergic system in neuropsychiatric disorders and the potential treatment consequences

In contrast to that of other monoamine neurotransmitters, the association of the histaminergic system with neuropsychiatric disorders is not well documented. In the last two decades, several clinical studies involved in the development of drugs targeting the histaminergic system have been reported. These include the H3R-antagonist/inverse agonist, pitolisant, used for the treatment of excessive sleepiness in narcolepsy, and the H1R antagonist, doxepin, used to alleviate symptoms of insomnia. The current review summarizes reports from animal models, including genetic and neuroimaging studies, as well as human brain samples and cerebrospinal fluid measurements from clinical trials, on the possible role of the histaminergic system in neuropsychiatric disorders. These studies will potentially pave the way for novel histamine-related therapeutic strategies.

Neuromyelitis optica, aquaporin-4 antibodies, and neuroendocrine disorders

Neuromyelitis optica (NMO) is an autoimmune disorder of the central nervous system that preferentially affects the optic nerve and the spinal cord. In around 80% of NMO patients, autoantibodies binding to aquaporin-4 (AQP4) are detected. AQP4-IgG unifies a spectrum of disorders (NMOSD) that include not only optic neuritis, longitudinally extensive transverse myelitis but also syndromes caused by lesion of the diencephalic region and the circumventricular organs (CVOs). The distinctive immunopathological characteristics of NMOSD lesions, occurring in regions where AQP4 is highly expressed, supports a central role for AQP4-IgG in disease pathogenesis. AQP4 expression is concentrated in CVOs and in the hypothalamus, mainly in the dorsal hypothalamic area, dorsomedial hypothalamic nucleus and suprachiasmatic nucleus. Several neuroendocrine disorders caused by inflammatory lesions involving the diencephalic region have been described in patients with NMOSD, including syndrome of inappropriate antidiuresis, sleep disorders, and other endocrinopathies caused by hypothalamic injury. Focus of this chapter is the involvement of hypothalamus and CVOs in AQP4 autoimmunity.

Cryo-EM structure of the human histamine H1 receptor/Gq complex

Histamine receptors play important roles in various pathophysiological conditions and are effective targets for anti-allergy treatment, however the mechanism of receptor activation remain elusive. Here, we present the cryo-electron microscopy (cryo-EM) structure of the human H₁R in complex with a G_q protein in an active conformation via a NanoBiT tethering strategy. The structure reveals that histamine activates receptor via interacting with the key residues of both transmembrane domain 3 (TM3) and TM6 to squash the binding pocket on the extracellular side and to open the cavity on the intracellular side for G_q engagement in a model of "squash to activate and expand to deactivate". The structure also reveals features for G_q coupling, including the interaction between intracellular loop 2 (ICL2) and the αN-β junction of G_q/11 protein. The detailed analysis of our structure will provide a framework for understanding G-protein coupling selectivity and clues for designing novel antihistamines.

Differential Regulation of Bilastine Affinity for Human Histamine H1 Receptors by Lys 179 and Lys 191 via Its Binding Enthalpy and Entropy

Bilastine, a zwitterionic second-generation antihistamine containing a carboxyl group, has higher selectivity for H₁ receptors than first-generation antihistamines. Ligand-receptor docking simulations have suggested that the electrostatic interaction between the carboxyl group of second-generation antihistamines and the amino group of Lys179^ECL2 and Lys191^5.39 of human H₁ receptors might contribute to increased affinity of these antihistamines to H₁ receptors. In this study, we evaluated the roles of Lys179^ECL2 and Lys191^5.39 in regulating the electrostatic and hydrophobic binding of bilastine to H₁ receptors by thermodynamic analyses. The binding enthalpy and entropy of bilastine were estimated from the van ’t Hoff equation using the dissociation constants. These constants were obtained from the displacement curves against the binding of [³H] mepyramine to membrane preparations of Chinese hamster ovary cells expressing wild-type human H₁ receptors and their Lys179^ECL2 or Lys191^5.39 mutants to alanine at various temperatures. We found that the binding of bilastine to wild-type H₁ receptors occurred by enthalpy-dependent binding forces and, more dominantly, entropy-dependent binding forces. The mutation of Lys179^ECL2 and Lys191^5.39 to alanine reduced the affinity of bilastine to H₁ receptors by reducing enthalpy- and entropy-dependent binding forces, respectively. These results suggest that Lys179^ECL2 and Lys191^5.39 differentially contribute to the increased binding affinity to bilastine via electrostatic and hydrophobic binding forces.

Organic Cation Transporters in Brain Histamine Clearance: Physiological and Psychiatric Implications

Histamine acts as a neurotransmitter in the central nervous system and is involved in numerous physiological functions. Recent studies have identified the causative role of decreased histaminergic systems in various neurological disorders. Thus, the brain histamine system has attracted attention as a therapeutic target to improve brain function. Neurotransmitter clearance is one of the most important processes for the regulation of neuronal activity and is an essential target for diverse drugs. Our previous study has shown the importance of histamine N-methyltransferase for the inactivation of brain histamine and the intracellular localization of this enzyme; the study indicated that the transport system for the movement of positively charged histamine from the extracellular to intracellular space is a prerequisite for histamine inactivation. Several studies on in vitro astrocytic histamine transport have indicated the contribution of organic cation transporter 3 (OCT3) and plasma membrane monoamine transporter (PMAT) in histamine uptake, although the importance of these transporters in in vivo histamine clearance remains unknown. Immunohistochemical analyses have revealed the expression of OCT3 and PMAT on neurons, emphasizing the importance of investigating neuronal histamine uptake. Further studies using knockout mice or fast-scan cyclic voltammetry will accelerate the research on histamine transporters. In this review article, we summarize histamine transport assays and describe the candidate transporters responsible for histamine transport in the brain.

Mechanism of Action of Atypical Antipsychotic Drugs in Mood Disorders

Atypical antipsychotic drugs were introduced in the early 1990s. Unlike typical antipsychotics, which are effective only against positive symptoms of schizophrenia, atypical antipsychotics are effective against negative and cognitive symptoms as well. Furthermore, they are effective not only in psychotic but also in affective disorders, on their own or as adjuncts to antidepressant drugs. This review presents the neural mechanisms of currently existing atypical antipsychotics and putative antipsychotics currently being investigated in preclinical and clinical studies and how these relate to their effectiveness in mood disorders such as depression, anxiety, and post-traumatic stress disorder (PTSD). Typical antipsychotics act almost exclusively on the dopamine system. Atypical drugs, however, modulate serotonin (5-HT), norepinephrine, and/or histamine neurotransmission as well. This multimodal mechanism of action putatively underlies the beneficial effect of atypical antipsychotics in mood and anxiety disorders. Interestingly, novel experimental drugs having dual antipsychotic and antidepressant therapeutic potential, such as histamine, adenosine, and trace amine-associated receptors (TAAR) ligand, are also characterized by a multimodal stimulatory effect on central 5-HT, norepinephrine, and/or histamine transmission. The multimodal stimulatory effect on central monoamine neurotransmission may be thus primarily responsible for the combined antidepressant and antipsychotic therapeutic potential of certain central nervous system (CNS) drugs.

Restless legs syndrome induced by fexofenadine/pseudoephedrine

Antihistamines are known risk factors for restless legs syndrome (RLS). However, reports on RLS associated with fexofenadine or its combinations are rare. Here, we report a 30-year-old woman with RLS that was induced by fexofenadine/pseudoephedrine. She had been taking fexofenadine/pseudoephedrine for three months and felt a strong urge to move her legs at night, which was relieved by movement. Her condition improved by taking pramipexole, which she discontinued subsequently because of dizziness. One month later, she quitted taking fexofenadine/pseudoephedrine, after which her symptoms disappeared a week later. This case study demonstrates that RLS can be induced by fexofenadine/pseudoephedrine and we should always consider the possibility of drug-induced RLS.

Histaminergic neurons in the tuberomammillary nucleus as a control centre for wakefulness

Histamine plays pleiotropic roles as a neurotransmitter in the physiology of brain function, this includes the maintenance of wakefulness, appetite regulation and memory retrieval. Since numerous studies have revealed an association between histaminergic dysfunction and diverse neuropsychiatric disorders, such as Alzheimer's disease and schizophrenia, a large number of compounds acting on the brain histamine system have been developed to treat neurological disorders. In 2016, pitolisant, which was developed as a histamine H₃ receptor inverse agonist by Schwartz and colleagues, was launched for the treatment of narcolepsy, emphasising the prominent role of brain histamine on wakefulness. Recent advances in neuroscientific techniques such as chemogenetic and optogenetic approaches have led to remarkable progress in the understanding of histaminergic neural circuits essential for the control of wakefulness. In this review article, we summarise the basic knowledge about the histaminergic nervous system and the mechanisms underlying sleep/wake regulation that are controlled by the brain histamine system. LINKED ARTICLES: This article is part of a themed issue on Neurochemistry in Japan. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.4/issuetoc.

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.

Roles of Lys191 and Lys179 in regulating thermodynamic binding forces of ligands to determine their binding affinity for human histamine H1 receptors

Docking simulations based on the crystal structure of human histamine H₁ receptors have predicted crucial roles of Lys191^5.39 and Lys179^ECL2, which exist at the entrance of the ligand-binding pocket, in increasing the H₁-receptor selectivity for carboxylated second-generation antihistamines via electrostatic interaction. In this study, we evaluated the roles of Lys191^5.39 and Lys179^ECL2 in regulating the thermodynamic binding forces of non-carboxylated and carboxylated antihistamines that determine their binding affinity for human H₁ receptors. The binding enthalpy and entropy of the 3 sets of non-carboxylated and corresponding carboxylated antihistamines (doxepin and olopatadine, desloratadine and loratadine, and terfenadine and fexofenadine, respectively) were estimated using the van't Hoff equation with the dissociation constants obtained from the displacement curves of the non-carboxylated and carboxylated antihistamines against the binding of [³H]mepyramine to the membrane preparations of Chinese hamster ovary cells expressing human H₁ receptors at various temperatures, ranging from 4 °C to 37 °C. We found that the affinity for carboxylated antihistamines was lower than that for the corresponding non-carboxylated compounds due to lower enthalpy-dependent electrostatic binding forces and/or entropy-dependent hydrophobic binding forces. Mutations of Lys191^5.39 and/or Lys179^ECL2 to alanine mostly increased the binding affinity for antihistamines due to a variety of changes in both enthalpy- and entropy-dependent binding forces. These results suggest that Lys191^5.39 and Lys179^ECL2 may not contribute to selectively increasing the binding affinity for carboxylated antihistamines via electrostatic interaction, but that they can negatively modulate the binding affinity for non-carboxylated and carboxylated antihistamines non-selectively by affecting their electrostatic as well as hydrophobic binding forces.

Anomalous intrinsic connectivity within and between visual and auditory networks in major depressive disorder

OBJECTIVE

Major depressive disorder (MDD) is a ubiquitous mental illness with heterogeneous symptoms, however, the pathophysiology mechanisms are still not fully understood. Clinical and preclinical studies suggested that depression could cause disturbances in sensory perception systems, disruptions in auditory and visual functions may serve as an essential clinical features underlying MDD.

METHODS

The current study investigated the abnormal intrinsic connectivity within and between visual and auditory networks in 95 MDD patients and 97 age-, gender-, education level-matched healthy controls (HCs) by using resting-state functional magnetic resonance imaging (fMRI). One auditory network (AN) and three visual components including visual component 1 (VC1), VC2, and VC3 were identified by using independent component analysis method based on the fMRI networks during the resting state with the largest spatial correlations, combining with brain regions and specific network templates.

RESULTS

We found that MDD could be characterized by the following disrupted network model relative to HCs: (i) reduced within-network connectivity in the AN, VC2, and VC3; (ii) reduced between-network connectivity between the AN and the VC3. Furthermore, aberrant functional connectivity (FC) within the visual network was linked to the clinical symptoms.

CONCLUSIONS

Overall, our results demonstrated that abnormalities of FC in perception systems including intrinsic visual and auditory networks may explain neurobiological mechanisms underlying MDD and could serve as a potential effective biomarker.

Cell-based, animal and H1 receptor binding studies relative to the sedative effects of ketotifen and norketotifen atropisomers

OBJECTIVES

Ketotifen (K) and its active metabolite norketotifen (N) exist as optically active atropisomers. They both have antihistaminic and anti-inflammatory properties but the S-atropisomer of N (SN) causes less sedation than K and RN in rodents. This study investigated whether this could be related to a lower concentration of SN in brain or a lower affinity of SN for rat brain H₁ receptors.

METHODS

Ketotifen and norketotifen atropisomers were quantified using a validated chiral HPLC assay. RBE4 and Caco-2 cell monolayers were used in uptake and permeability studies, respectively. Free and total brain-to-plasma (B/P) ratios were determined after injecting racemic K and N into rat tail veins. Affinity for rat brain H₁ receptors (K_I ) was determined using the [³ H]mepyramine binding assay.

KEY FINDINGS

Uptake and permeation studies indicate no stereoselective transport for K or N. B/P ratios reveal the brain concentration of N is lower than K with no stereoselective transport into brain. Finally, the [³ H]mepyramine binding assay shows SN has the lowest affinity for rat brain H₁ receptors.

CONCLUSION

The lower sedative effect of SN in rodents is probably due to a combination of a lower uptake of N than K into the brain and less affinity of SN for CNS H₁ receptors.

Synthesis of anti-allergic drugs

Histamine is formed by the decarboxylation of histidine catalyzed by enzymes. It is an endogenous biologically active substance involved in multiple complex physiological processes as an important chemical transmitter. Histamine receptors have four subtypes, H₁, H₂, H₃ and H₄, all of which are G protein coupling receptors (GPCRs) with different physiological functions. Histamine plays an important role in the pathophysiological mechanism of allergic diseases, and the antagonistic effect of histamine has become an important way to study anti-allergic drugs, wherein the anti-allergic drugs used in clinical practice are mainly H₁ receptor antagonists. Currently, there are many varieties of H₁ receptor antagonists in clinical applications, which can be divided into ethylenediamine antagonists, amino ether antagonists, propylamine antagonists, tricyclic antagonists, piperazine antagonists and piperidine antagonists depending on their chemical structures. This article mainly reviews the research progress of allergic reactions with histamine H₁ receptor antagonists and expounds the important aspects of the design and synthesis of various new compounds.

Histamine is formed by the decarboxylation of histidine catalyzed by enzymes. It is an endogenous biologically active substance involved in multiple complex physiological processes as an important chemical transmitter.

Brain histamine H1 receptor occupancy after oral administration of desloratadine and loratadine

Some histamine H1 receptor (H1R) antagonists induce adverse sedative reactions caused by blockade of histamine transmission in the brain. Desloratadine is a second-generation antihistamine for treatment of allergic disorders. Its binding to brain H1Rs, which is the basis of sedative property of antihistamines, has not been examined previously in the human brain by positron emission tomography (PET). We examined brain H1R binding potential ratio (BPR), H1R occupancy (H1RO), and subjective sleepiness after oral desloratadine administration in comparison to loratadine. Eight healthy male volunteers underwent PET imaging with [11C]-doxepin, a PET tracer for H1Rs, after a single oral administration of desloratadine (5 mg), loratadine (10 mg), or placebo in a double-blind crossover study. BPR and H1RO in the cerebral cortex were calculated, and plasma concentrations of loratadine and desloratadine were measured. Subjective sleepiness was quantified by the Line Analogue Rating Scale (LARS) and the Stanford Sleepiness Scale (SSS). BPR was significantly lower after loratadine administration than after placebo (0.504 ± 0.074 vs 0.584 ± 0.059 [mean ± SD], P < 0.05), but BPR after desloratadine administration was not significantly different from BPR after placebo (0.546 ± 0.084 vs 0.584 ± 0.059, P = 0.250). The plasma concentration of loratadine was negatively correlated with BPR in subjects receiving loratadine, but that of desloratadine was not correlated with BPR. Brain H1ROs after desloratadine and loratadine administration were 6.47 ± 10.5% and 13.8 ± 7.00%, respectively (P = 0.103). Subjective sleepiness did not significantly differ among subjects receiving the two antihistamines and placebo. At therapeutic doses, desloratadine did not bind significantly to brain H1Rs and did not induce any significant sedation.

Antipsychotics: Mechanisms underlying clinical response and side-effects and novel treatment approaches based on pathophysiology

Antipsychotic drugs are central to the treatment of schizophrenia and other psychotic disorders but are ineffective for some patients and associated with side-effects and nonadherence in others. We review the in vitro, pre-clinical, clinical and molecular imaging evidence on the mode of action of antipsychotics and their side-effects. This identifies the key role of striatal dopamine D2 receptor blockade for clinical response, but also for endocrine and motor side-effects, indicating a therapeutic window for D2 blockade. We consider how partial D2/3 receptor agonists fit within this framework, and the role of off-target effects of antipsychotics, particularly at serotonergic, histaminergic, cholinergic, and adrenergic receptors for efficacy and side-effects such as weight gain, sedation and dysphoria. We review the neurobiology of schizophrenia relevant to the mode of action of antipsychotics, and for the identification of new treatment targets. This shows elevated striatal dopamine synthesis and release capacity in dorsal regions of the striatum underlies the positive symptoms of psychosis and suggests reduced dopamine release in cortical regions contributes to cognitive and negative symptoms. Current drugs act downstream of the major dopamine abnormalities in schizophrenia, and potentially worsen cortical dopamine function. We consider new approaches including targeting dopamine synthesis and storage, autoreceptors, and trace amine receptors, and the cannabinoid, muscarinic, GABAergic and glutamatergic regulation of dopamine neurons, as well as post-synaptic modulation through phosphodiesterase inhibitors. Finally, we consider treatments for cognitive and negative symptoms such dopamine agonists, nicotinic agents and AMPA modulators before discussing immunological approaches which may be disease modifying. This article is part of the issue entitled 'Special Issue on Antipsychotics'.

Antihistamines for Allergic Rhinitis Treatment from the Viewpoint of Nonsedative Properties

Antihistamines targeting the histamine H₁ receptor play an important role in improving and maintaining the quality of life of patients with allergic rhinitis. For more effective and safer use of second-generation drugs, which are recommended by various guidelines, a classification based on their detailed characteristics is necessary. Antihistamines for first-line therapy should not have central depressant/sedative activities. Sedative properties (drowsiness and impaired performance) are associated with the inhibition of central histamine neurons. Brain H₁ receptor occupancy (H₁RO) is a useful index shown to be correlated with indices based on clinical findings. Antihistamines are classified into non-sedating (<20%), less-sedating (20–50%), and sedating (≥50%) groups based on H₁RO. Among the non-sedating group, fexofenadine and bilastine are classified into “non-brain-penetrating antihistamines” based on the H₁RO. These two drugs have many common chemical properties. However, bilastine has more potent binding affinity to the H₁ receptor, and its action tends to last longer. In well-controlled studies using objective indices, bilastine does not affect psychomotor or driving performance even at twice the usual dose (20 mg). Upon selecting antihistamines for allergic rhinitis, various situations should be taken into our consideration. This review summarizes that the non-brain-penetrating antihistamines should be chosen for the first-line therapy of mild allergic rhinitis.

Differential Regulation of Thermodynamic Binding Forces of Levocetirizine and (S)-Cetirizine by Lys191 in Human Histamine H₁ Receptors

Cetirizine is a zwitterionic second-generation antihistamine containing R- and S-enantiomers, levocetirizine, and (S)-cetirizine. Levocetirizine is known to have a higher affinity for the histamine H₁ receptors than (S)-cetirizine; ligand-receptor docking simulations have suggested the importance of the formation of a salt bridge (electrostatic interaction) between the carboxylic group of levocetirizine and the Lys191 residue at the fifth transmembrane domain of human histamine H₁ receptors. In this study, we evaluated the roles of Lys191 in the regulation of the thermodynamic binding forces of levocetirizine in comparison with (S)-cetirizine. The binding enthalpy and entropy of these compounds were estimated from the van 't Hoff equation, by using the dissociation constants obtained from their displacement curves against the binding of [³H]mepyramine to the membrane preparations of Chinese hamster ovary cells expressing wild-type human H₁ receptors and their Lys191 mutants to alanine at various temperatures. We found that the higher binding affinity of wild-type H₁ receptors for levocetirizine than (S)-cetirizine was achieved by stronger forces of entropy-dependent hydrophobic binding of levocetirizine. The mutation of Lys191 to alanine reduced the affinities for levocetirizine and (S)-cetirizine, through a reduction in the entropy-dependent hydrophobic binding forces of levocetirizine and the enthalpy-dependent electrostatic binding forces of (S)-cetirizine. These results suggested that Lys191 differentially regulates the binding enthalpy and entropy of these enantiomers, and that Lys191 negatively regulates the enthalpy-dependent electrostatic binding forces of levocetirizine, contrary to the predictions derived from the ligand-receptor docking simulations.

An mGlu4-Positive Allosteric Modulator Alleviates Parkinsonism in Primates

BACKGROUND

Levodopa remains the gold-standard treatment for PD. However, it becomes less effective as the disease progresses and produces debilitating side effects, such as motor fluctuations and l-dopa-induced dyskinesia. Modulation of metabotropic glutamate receptor 4 represents a promising antiparkinsonian approach in combination with l-dopa, but it has not been demonstrated in primates.

OBJECTIVE

We studied whether a novel positive allosteric modulator of the metabotropic glutamate receptor 4, PXT002331 (foliglurax), could reduce parkinsonism in primate models.

METHODS

We assessed the therapeutic potential of PXT002331 in three models of MPTP-induced parkinsonism in macaques. These models represent three different stages of disease evolution: early stage and advanced stage with and without l-dopa-induced dyskinesia.

RESULTS

As an adjunct to l-dopa, PXT002331 induced a robust and dose-dependent reversal of parkinsonian motor symptoms in macaques, including bradykinesia, tremor, posture, and mobility. Moreover, PXT002331 strongly decreased dyskinesia severity, thus having therapeutic efficacy on both parkinsonian motor impairment and l-dopa-induced dyskinesia. PXT002331 brain penetration was also assessed using PET imaging in macaques, and pharmacodynamic analyses support target engagement in the therapeutic effects of PXT002331.

CONCLUSIONS

This work provides a demonstration that a positive allosteric modulator of metabotropic glutamate receptor 4 can alleviate the motor symptoms of PD and the motor complications induced by l-dopa in primates. PXT002331 is the first compound of its class to enter phase IIa clinical trials. © 2018 International Parkinson and Movement Disorder Society.

Fexofenadine Suppresses Delayed-Type Hypersensitivity in the Murine Model of Palladium Allergy

Palladium is frequently used in dental materials, and sometimes causes metal allergy. It has been suggested that the immune response by palladium-specific T cells may be responsible for the pathogenesis of delayed-type hypersensitivity in study of palladium allergic model mice. In the clinical setting, glucocorticoids and antihistamine drugs are commonly used for treatment of contact dermatitis. However, the precise mechanism of immune suppression in palladium allergy remains unknown. We investigated inhibition of the immune response in palladium allergic mice by administration of prednisolone as a glucocorticoid and fexofenadine hydrochloride as an antihistamine. Compared with glucocorticoids, fexofenadine hydrochloride significantly suppressed the number of T cells by interfering with the development of antigen-presenting cells from the sensitization phase. Our results suggest that antihistamine has a beneficial effect on the treatment of palladium allergy compared to glucocorticoids.

The clinical pharmacology of non-sedating antihistamines

We previously reported on brain H₁ receptor occupancy measurements of antihistamines in human brain using [¹¹C]doxepin and positron emission tomography (PET). We proposed the use of brain H₁ receptor occupancy to classify antihistamines objectively into three categories of sedating, less-sedating, and non-sedating antihistamines according to their sedative effects. Non-sedating antihistamines are recommended for the treatment of allergies such as pollinosis and atopic dermatitis because of their low penetration into the central nervous system. Physicians and pharmacists are responsible for fully educating patients about the risks of sedating antihistamines from pharmacological points of view. If a sedating antihistamine must be prescribed, its sedative effects should be thoroughly considered before choosing the drug. Non-sedating antihistamines should be preferentially used whenever possible as most antihistamines are equally efficacious, while adverse effects of sedating antihistamines can be serious. This review summarizes the pharmacological properties of clinically useful non-sedating antihistamines from the perspective of histamine function in the CNS.

GABAA receptor occupancy by subtype selective GABAAα2,3 modulators: PET studies in humans

RATIONALE

Sedation, dependence, and abuse liability limit the use of non-selective γ-aminobutyric acid (GABA_A) receptor positive modulators for the treatment of anxiety. AZD7325 and AZD6280 are novel, subtype-selective GABA_Aα2,3 receptor positive modulators with limited sedative effects.

OBJECTIVES

The current study aimed to confirm target engagement at GABA_A receptors by AZD7325 and AZD6280 in humans and to determine the relationship between exposure, GABA_A receptor occupancy, and tolerability.

METHOD

Two PET studies, using high-resolution research tomography (HRRT) and the radioligand [¹¹C]flumazenil, were performed in 12 subjects at baseline and after administration of single oral doses of AZD7325 (0.2 to 30 mg) and AZD6280 (5 to 40 mg). PET images were analyzed using a simplified reference tissue model, and regional binding potentials (BP_ND) were obtained. The relationship between plasma concentration of AZD7325 or AZD6280 and GABA_A receptor occupancy was described by hyperbolic function, and K _i,plasma (plasma concentration required for 50% receptor occupancy) was estimated. Assessments of safety and tolerability included recording of adverse events, vital signs, electrocardiogram, and laboratory tests.

RESULTS

The [¹¹C]flumazenil binding was reduced in a dose-dependent, saturable manner by both agents. Maximum receptor occupancy could be reached for both compounds without causing sedation or cognitive impairment. The K _i,plasma estimates for AZD7325 and AZD6280 were 15 and 440 nmol/l, respectively.

CONCLUSION

High GABA_A receptor occupancy by AZD7325 and AZD6280 could be reached without clear sedative effects.

Changes in Histidine Decarboxylase, Histamine N-Methyltransferase and Histamine Receptors in Neuropsychiatric Disorders

Compared to other monoamine neurotransmitters, information on the association between the histaminergic system and neuropsychiatric disorders is scarce, resulting in a lack of histamine-related treatment for these disorders. The current chapter tries to combine information obtained from genetic studies, neuroimaging, post-mortem human brain studies and cerebrospinal fluid measurements with data from recent clinical trials on histamine receptor agonists and antagonists, with a view to determining the possible role of the histaminergic system in neuropsychiatric disorders and to pave the way for novel histamine-related therapeutic strategies.

Asp73-dependent and -independent regulation of the affinity of ligands for human histamine H1 receptors by Na. Biochem Pharmacol 2016;128:46-54. [PMID: 28040476 DOI: 10.1016/j.bcp.2016.12.021]

The affinity of ligands for G-protein-coupled receptors (GPCRs) is allosterically regulated by Na⁺ via a highly conserved aspartate residue (Asp^2.50) in the second transmembrane domain of GPCRs. In the present study, we examined the Na⁺-mediated regulation of the affinity of ligands for G_q/11-protein-coupled human histamine H₁ receptors in Chinese hamster ovary cells. The affinities of 3 agonists and 20 antihistamines were evaluated by their displacement curves against the binding of [³H]-mepyramine to membrane preparations in the presence or absence of 100mM NaCl. The affinities of most drugs including histamine, an agonist, and d-chlorpheniramine, a first-generation antihistamine, were reduced by NaCl, with the extent of NaCl-mediated changes varying widely between drugs. In contrast, the affinities of some second-generation antihistamines such as fexofenadine were increased by NaCl. These changes were retained in intact cells. The mutation of Asp^2.50 (Asp73) to asparagine abrogated NaCl-induced reductions in affinities for histamine and d-chlorpheniramine, but not NaCl-induced increases in the affinity for fexofenadine. Quantitative structure-activity relationship (QSAR) analyses showed that these Na⁺-mediated changes were explained and predicted by a combination of the molecular energies and implicit solvation energies of the compounds. These results suggest that Na⁺ diversely regulates the affinity of ligands for H₁ receptors from the extracellular sites of receptors via Asp73-dependent and -independent mechanisms in a manner that depends on the physicochemical properties of ligands. These results may contribute to a deeper understanding of the fundamental mechanisms by which the affinity of ligands for their receptors is allosterically regulated by Na⁺.

Quantitative prediction of histamine H1 receptor occupancy by the sedative and non-sedative antagonists in the human central nervous system based on systemic exposure and preclinical data

Significant histamine H1 receptor occupation in the central nervous system (CNS) is associated with sedation. Here we examined the time profiles of the H1 receptor occupancy (RO) in the CNS using sedative (diphenhydramine and ketotifen) and non-sedative (bepotastine and olopatadine) antagonists at their therapeutic doses by integrating in vitro and animal data. A pharmacokinetic model was constructed to associate plasma concentrations and receptor binding in the brain. Dissociation and association rate constants with the H1 receptor and plasma and brain unbound fractions were determined in vitro. Passive and active clearances across the blood-brain barrier (BBB) were estimated based on physicochemical properties and microdialysis studies in mice and monkeys. The estimated RO values were comparable with the reported values determined at time to maximum concentration (T_max) of plasma by positron-emission tomography in humans. The simulation suggested that the predicted maximum ROs by bepotastine and olopatadine were greater than the reported values. Sensitivity analysis showed that active transport across BBB had a significant impact on the RO duration of the H1 antagonists examined. The present study demonstrated that modeling and simulation permits a reasonable RO estimation in the human CNS. Our findings will facilitate the development of CNS-acting drugs.

C-terminal of human histamine H1 receptors regulates their agonist-induced clathrin-mediated internalization and G-protein signaling

It has been suggested that the agonist-induced internalization of G-protein-coupled receptors from the cell surface into intracellular compartments regulates cellular responsiveness. We previously reported that G_q/11 -protein-coupled human histamine H₁ receptors internalized via clathrin-dependent mechanisms upon stimulation with histamine. However, the molecular determinants of H₁ receptors responsible for agonist-induced internalization remain unclear. In this study, we evaluated the roles of the intracellular C-terminal of human histamine H₁ receptors tagged with hemagglutinin (HA) at the N-terminal in histamine-induced internalization in Chinese hamster ovary cells. The histamine-induced internalization was evaluated by the receptor binding assay with [³ H]mepyramine and confocal immunofluorescence microscopy with an anti-HA antibody. We found that histamine-induced internalization was inhibited under hypertonic conditions or by pitstop, a clathrin terminal domain inhibitor, but not by filipin or nystatin, disruptors of the caveolar structure and function. The histamine-induced internalization was also inhibited by truncation of a single amino acid, Ser487, located at the end of the intracellular C-terminal of H₁ receptors, but not by its mutation to alanine. In contrast, the receptor-G-protein coupling, which was evaluated by histamine-induced accumulation of [³ H]inositol phosphates, was potentiated by truncation of Ser487, but was lost by its mutation to alanine. These results suggest that the intracellular C-terminal of human H₁ receptors, which only comprises 17 amino acids (Cys471-Ser487), plays crucial roles in both clathrin-dependent internalization of H₁ receptors and G-protein signaling, in which truncation of Ser487 and its mutation to alanine are revealed to result in biased signaling toward activation of G-proteins and clathrin-mediated internalization, respectively.

The involvement of central nervous system histamine receptors in psychological stress-induced exacerbation of allergic airway inflammation in mice

BACKGROUND

Psychological stress is one of the major risk factors for asthma exacerbation. Although histamine in the brain acts as an excitatory and inhibitory neurotransmitter associated with psychological stress, the contribution of brain histamine to psychological stress-induced exacerbation of asthma remains unclear. The objective of this study was to investigate the role of histamine receptors in the CNS on stress induced asthma aggravation.

METHODS

We monitored the numbers of inflammatory cells and interleukin (IL)-13 levels in bronchoalveolar lavage fluid, airway responsiveness to inhaled methacholine, mucus secretion in airway epithelial cells, and antigen-specific IgE contents in sera in a murine model of stress-induced asthma treated with epinastine (an H1R antagonist), thioperamide (an H3/4R antagonist), or solvent.

RESULTS

All indicators of stress-induced asthma exacerbation were significantly reduced in stressed mice treated with epinastine compared with those treated with solvent, whereas treatment with thioperamide did not reduce the numbers of inflammatory cells in the stressed mice.

CONCLUSIONS

These results suggest that H1R, but not H3/4R, may be involved in stress-induced asthma exacerbations in the central nervous system.

Methods for preclinical assessment of antipruritic agents and itch mechanisms independent of mast-cell histamine

Itch is a sensation that provokes a desire to scratch. Mast-cell histamine was thought to be a key itch mediator. However, histamine and mast-cell degranulation were reported not to elicit scratching in animals. It was difficult to investigate the pathophysiology of itching and to evaluate the antipruritic efficacy of chemical agents in the early 1990 s. We showed that hind-paw scratching and biting were elicited by stimulation with pruritogenic agents in mice. Those results demonstrated for the first time that cutaneous itching could be evaluated behaviorally in animals. We established various animal models of pathological itch of the skin (dry skin, mosquito allergy, surfactant-induced pruritus, and herpes zoster) and mucus membranes (pollen allergy). Mast-cell histamine did not play a key role in itching in any animal model examined except for the pollen allergy model. Histamine is not an exclusive itch mediator of mast cells; tryptase and leukotriene B4 released from mast cells also act as itch mediators. Epidermal keratinocytes release several itch mediators, such as leukotriene B4, sphingosylphosphorylcholine, thromboxane A2, nociceptin, nitric oxide, and histamine, which may play important roles in pathological itching. Appropriate animal models of pathological itching are needed for pharmacological evaluation of the antipruritic efficacy of chemical agents.

Gas phase equilibrium structure of histamine

The first gas electron diffraction (GED) experiment for histamine was carried out. The equilibrium structure of histamine in the gas phase was determined on the basis of the data obtained. The refinement was also supported by the rotational constants obtained in previous studies [B. Vogelsanger, et al., J. Am. Chem. Soc., 1991, 113, 7864-7869; P. Godfrey, et al., J. Am. Chem. Soc., 1998, 120, 10724-10732] and quantum chemical calculations. The proposed mechanism of tautomerization by simultaneous intermolecular transfer of hydrogens in a histamine dimer helps to explain the distribution of tautomers in different experiments. The estimations of the conformational interconversion times provided the explanation for the absence of some conformers in the rotational spectroscopy experiments.

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.

Development of TASP0410457 (TASP457), a novel dihydroquinolinone derivative as a PET radioligand for central histamine H3 receptors

Background

Histamine H₃ receptor (H₃R) is a potential therapeutic target of sleep- and cognition-related disorders. The purpose of the present study is to develop a novel positron emission tomography (PET) ligand for H₃Rs from dihydroquinolinone derivatives, which we previously found to have high affinity with these receptors.

Methods

Six compounds were selected from a dihydroquinolinone compound library based on structural capability for ¹¹C labeling and binding affinity for H₃Rs. Their in vivo kinetics in the rat brain were examined in a comparative manner by liquid chromatography and tandem mass spectrometry (LC-MS/MS). Chemicals with appropriate kinetic properties were then labeled with ¹¹C and evaluated in rats and monkeys using PET.

Results

Of the six compounds, TASP0410457 (also diminutively called TASP457) and TASP0434988 exhibited fast kinetics and relatively high brain uptakes in ex vivo LC-MS/MS and were selected as candidate PET imaging agents. PET data in rat brains were mostly consistent with LC-MS/MS findings, and rat and monkey PET scans demonstrated that [¹¹C]TASP0410457 was superior to [¹¹C]TASP0434988 for high-contrast H₃R PET imaging. In the monkey brain PET, distribution volume for [¹¹C]TASP0410457 could be quantified, and receptor occupancy by a nonradioactive compound was measurable using this radioligand. The specific binding of [¹¹C]TASP0410457 to H₃Rs was confirmed by autoradiography using rat and monkey brain sections.

Conclusions

We developed [¹¹C]TASP0410457 as a radioligand enabling a robust quantification of H₃Rs in all brain regions and demonstrated the utility of ex vivo LC-MS/MS and in vivo PET assays for selecting appropriate imaging tracers. [¹¹C]TASP0410457 will help to examine the implication of H₃Rs in neuropsychiatric disorders and to characterize emerging therapeutic agents targeting H₃Rs.

Electronic supplementary material

The online version of this article (doi:10.1186/s13550-016-0170-2) contains supplementary material, which is available to authorized users.

Histamine Clearance Through Polyspecific Transporters in the Brain

Histamine plays an important role as a neurotransmitter in diverse brain functions, and clearance of histamine is essential to avoid excessive histaminergic neuronal activity. Histamine N-methyltransferase, which is an enzyme in the central nervous system that metabolizes histamine, is localized to the cytosol. This suggests that a histamine transport process is essential to inactivate histamine. Previous reports have shown the importance of astrocytes for histamine transport, although neuronal histamine transport could not be ruled out. High-affinity and selective histamine transporters have not yet been discovered, although it has been reported that the following three polyspecific transporters transport histamine: organic cation transporter (OCT) 2, OCT3, and plasma membrane monoamine transporter (PMAT). The K _m values of human OCT2, OCT3, and PMAT are 0.54, 0.64, and 4.4 mM, respectively. The three transporters are expressed in the brain, and their regional distribution is different. Recent studies revealed the contribution of OCT3 and PMAT to histamine transport by primary human astrocytes. Several investigations using mice supported the importance of OCT3 for histamine clearance in the brain. However, further studies are required to elucidate the detailed mechanism of histamine transport in the brain.

Bilastine: a new antihistamine with an optimal benefit-to-risk ratio for safety during driving

INTRODUCTION

Rational selection of a second-generation H1-antihistamine requires efficacy and safety considerations, particularly regarding central nervous system (CNS) effects (cognitive and psychomotor function), potential for driving impairment, minimal sedative effects and a lack of interactions. This review evaluates the key safety features of the non-sedating antihistamine, bilastine, during driving and in preventing road traffic accidents.

AREAS COVERED

Among the second-generation H1-antihistamines, sedative effects which can affect cognitive and psychomotor performance, and possibly driving ability, may not be similar. Bilastine is absorbed rapidly, undergoes no hepatic metabolism or cytochrome P450 interaction (minimal drug-drug interaction potential), and is a substrate for P-glycoprotein (limiting CNS entry). Positron emission tomography showed that, compared with other second-generation H1-antihistamines, bilastine has the lowest cerebral histamine H1-receptor occupancy. Bilastine 20 mg once daily (therapeutic dose) is non-sedating, does not enhance the effects of alcohol or CNS sedatives, does not impair driving performance and has at least similar efficacy as other second-generation H1-antihistamines in the treatment of allergic rhinoconjunctivitis and urticaria.

EXPERT OPINION

Current evidence shows that bilastine has an optimal benefit-to-risk ratio, meeting all conditions for contributing to safety in drivers who need antihistamines, and hence for being considered as an antihistamine of choice for drivers.