The histamine H3 receptor inverse agonist SAR-152954 reverses deficits in long-term potentiation associated with moderate prenatal alcohol exposure.

Prenatal alcohol exposure can have persistent effects on learning, memory, and synaptic plasticity. Previous work from our group demonstrated deficits in long-term potentiation (LTP) of excitatory synapses on dentate gyrus granule cells in adult offspring of rat dams that consumed moderate levels of alcohol during pregnancy. At present, there are no pharmacotherapeutic agents approved for these deficits. Prior work established that systemic administration of the histaminergic H3R inverse agonist ABT-239 reversed deficits in LTP observed following moderate PAE. The present study examines the effect of a second H3R inverse agonist, SAR-152954, on LTP deficits following moderate PAE. We demonstrate that systemic administration of 1 mg/kg of SAR-152954 reverses deficits in potentiation of field excitatory post-synaptic potentials (fEPSPs) in adult male rats exposed to moderate PAE. Time-frequency analyses of evoked responses revealed PAE-related reductions in power during the fEPSP, and increased power during later components of evoked responses which are associated with feedback circuitry that are typically not assessed with traditional amplitude-based measures. Both effects were reversed by SAR-152954. These findings provide further evidence that H3R inverse agonism is a potential therapeutic strategy to address deficits in synaptic plasticity associated with PAE.

Histamine 3 receptor inverse agonist Samelisant (SUVN-G3031): Pharmacological characterization of an investigational agent for the treatment of cognitive disorders.

BACKGROUND: Central histamine H3 receptors are a family of presynaptic auto and heteroreceptors. Blockade of the presynaptic H3 receptors activates the downstream pathway(s) involved in the processes of learning and memory, making it a potential therapeutic option for ameliorating cognitive dysfunction. Samelisant (SUVN-G3031) is a potent and selective inverse agonist at the H3 receptors. AIM: The aim of this research is to study the effects of Samelisant in diverse animal models of cognitive functions. METHODS: The effects of Samelisant on cognitive functions were studied using social recognition, object recognition and Morris water maze tasks. Neurochemical and electrophysiological effects of Samelisant were monitored using microdialysis and electroencephalography techniques. RESULTS: Samelisant showed procognitive effects in diverse animal models of cognition at doses ranging from 0.3 to 3 mg/kg, per os (p.o.) (social recognition and object recognition task). Samelisant significantly increased the brain acetylcholine levels in the cortex at doses of 10 and 20 mg/kg, p.o. In the Morris water maze task, combined administration of suboptimal doses of Samelisant and donepezil resulted in procognitive effects significantly larger than the either treatment. Similarly, Samelisant significantly potentiated the effects of donepezil on pharmacodynamic biomarkers of cognition i.e. acetylcholine levels in brain and neuronal theta oscillations. CONCLUSION: Samelisant may have potential utility in the treatment of cognitive deficits associated with hypocholinergic state.

Pitolisant for treating patients with narcolepsy.

Introduction: Narcolepsy is a rare sleep disorder characterized by excessive daytime sleepiness, cataplexy, disturbed nocturnal sleep, hypnagogic hallucinations, and sleep paralysis. Pitolisant is a first-in-class drug acting on histamine 3 receptors and indicated for the treatment of narcolepsy. This article aims to review pitolisant.Areas covered: In this paper the chemical properties, mechanism of action, pharmacokinetics, clinical efficacy and safety of pitolisant was introduced, and the development course of drugs for treating narcolepsy is also briefly described. We performed a systematic review of the literature using PubMed and the following keywords were used: 'pitolisant' and 'narcolepsy', 'cataplexy' and 'excessive daytime sleepiness' and 'histamine 3 receptor'.Expert opinion: Pitolisant is a histamine 3 receptor antagonist/inverse agonist. It can activate histamine release in the brain and enhances wakefulness. Clinical studies showed that pitolisant significantly decreased excessive daytime sleepiness and cataplexy rate versus placebo. Pitolisant was well tolerated, common adverse reactions were headache, insomnia, nausea, and anxiety.

In silico and in vitro studies of two non-imidazole multiple targeting agents at histamine H3 receptors and cholinesterase enzymes.

Recently, multi-target directed ligands have been of research interest for multifactorial disorders such as Alzheimer's disease (AD). Since H3 receptors (H3 Rs) and cholinesterases are involved in pathophysiology of AD, identification of dual-acting compounds capable of improving cholinergic neurotransmission is of importance in AD pharmacotherapy. In the present study, H3 R antagonistic activity combined with anticholinesterase properties of two previously computationally identified lead compounds, that is, compound 3 (6-chloro-N-methyl-N-[3-(4-methylpiperazin-1-yl)propyl]-1H-indole-2-carboxamide) and compound 4 (7-chloro-N-[(1-methylpiperidin-3-yl)methyl]-1,2,3,4-tetrahydroisoquinoline-2-carboxamide), was tested. Moreover, molecular docking and binding free energy calculations were conducted for binding mode and affinity prediction of studied ligands toward cholinesterases. Biological evaluations revealed inhibitory activity of ligands in nanomolar (compound 3: H3 R EC50  = 0.73 nM; compound 4: H3 R EC50  = 31 nM) and micromolar values (compound 3: AChE IC50  = 9.09 µM, BuChE IC50  = 21.10 µM; compound 4: AChE IC50  = 8.40 µM, BuChE IC50  = 4.93 µM) for H3 R antagonism and cholinesterase inhibition, respectively. Binding free energies yielded good consistency with cholinesterase inhibitory profiles. The results of this study can be used for lead optimization where dual inhibitory activity on H3 R and cholinesterases is needed. Such ligands can exert their biological activity in a synergistic manner resulting in higher potency and efficacy.

Histamine H1 and H3 receptor activation increases the expression of Glucose Transporter 1 (GLUT-1) in rat cerebro-cortical astrocytes in primary culture.

Astrocytes take up glucose via the 45 kDa isoform of the Glucose Transporter 1 (GLUT-1), and in this work we have investigated whether histamine regulates GLUT-1 expression in rat cerebro-cortical astrocytes in primary culture. Cultured astrocytes expressed histamine H1 and H3 receptors (H1Rs and H3Rs) as evaluated by radioligand binding. Receptor functionality was confirmed by the increase in the intracellular concentration of Ca2+ (H1R) and the inhibition of forskolin-induced cAMP accumulation (H3R). Quantitative RT-PCR showed that histamine and selective H1R and H3R agonists (1 h incubation) significantly increased GLUT-1 mRNA to 153 ±â€¯7, 163 ±â€¯2 and 168 ±â€¯13% of control values, respectively. In immunoblot assays, incubation (3 h) with histamine or H1R and H3R agonists increased GLUT-1 protein levels to 224 ±â€¯12, 305 ±â€¯11 and 193 ±â€¯13% of control values, respectively, an action confirmed by inmunocytochemistry. The effects of H1R and H3R agonists were blocked by the selective antagonists mepyramine (H1R) and clobenpropit (H3R). The pharmacological inhibition of protein kinase C (PKC) prevented the increase in GLUT-1 protein induced by either H1R or H3R activation. Furthermore, histamine increased ERK-1/2 phosphorylation, and the effect of H1R and H3R activation on GLUT-1 protein levels was reduced or prevented, respectively, by MEK-1/2 inhibition. These results indicate that by activating H1Rs and H3Rs histamine regulates the expression of GLUT-1 by astrocytes. The effect appears to involve the phospholipase C (PLC) → diacylglycerol (DAG)/Ca2+→ PKC and PLC → DAG/Ca2+ → PKC → MAPK pathways.

Metabolomic study reveals the acute hypotensive effect of S-1-propenylcysteine accompanied by alteration of the plasma histidine level in spontaneously hypertensive rats.

Our previous study has shown that a single dose of S-1-propenylcysteine (S1PC) exerted an antihypertensive effect in spontaneously hypertensive rats (SHR), while its mode of action remained to be further investigated. The aim of this study was to explore the potential mechanism of the antihypertensive effect of S1PC in SHR using a liquid chromatography-mass spectrometry (LC-MS)-based metabolomic approach. Blood samples were serially collected from SHR after a single oral administration of S1PC (6.5 mg/kg body weight). The metabolomics data acquired from the LC-MS analysis of plasma samples were processed using principal component analysis (PCA) and partial least squares discriminant analysis (PLS-DA). In addition, the SHR were treated with S1PC or histidine (10 mg/kg body weight) with and without intravenous preinjection of thioperamide, a histamine H3 receptor antagonist. The blood pressure of SHR was measured by the tail-cuff method at different times after administration. In the PLS-DA score plots, the clusters of the S1PC groups were clearly or partly separated from those of the control groups at 1.5 and 3 h after administration, indicating the metabolic profiles were substantially altered by the S1PC treatment at these time points. Comparative analysis based on variable importance in the projection (VIP) values obtained from PLS-DA led to the identification of 14 and 15 metabolites differing between the two groups at 1.5 and 3 h, respectively, which included various amino acids. Among the metabolites identified, the plasma histidine level in the S1PC group significantly increased at 1.5 and 3 h, and decreased to that in the control group at 6 h. Moreover, pretreatment with thioperamide inhibited the blood pressure lowering effect of S1PC as well as that of histidine. These results suggested that S1PC alters histidine metabolism and consequently exerts the antihypertensive effect via the central histamine H3 receptor.

Discovery and Development of N-[4-(1-Cyclobutylpiperidin-4-yloxy)phenyl]-2-(morpholin-4-yl)acetamide Dihydrochloride (SUVN-G3031): A Novel, Potent, Selective, and Orally Active Histamine H3 Receptor Inverse Agonist with Robust Wake-Promoting Activity.

A series of chemical optimizations guided by in vitro affinity at a histamine H3 receptor (H3R), physicochemical properties, and pharmacokinetics in rats resulted in identification of N-[4-(1-cyclobutyl-piperidin-4-yloxy)phenyl]-2-(morpholin-4-yl)acetamide dihydrochloride (17v, SUVN-G3031) as a clinical candidate. Compound 17v is a potent (hH3R Ki = 8.73 nM) inverse agonist at H3R with selectivity over other 70 targets, Compound 17v has adequate oral exposures and favorable elimination half-lives both in rats and dogs. It demonstrated high receptor occupancy and marked wake-promoting effects with decreased rapid-eye-movement sleep in orexin-B saporin lesioned rats supporting its potential therapeutic utility in treating human sleep disorders. It had no effect on the locomotor activity at doses several fold higher than its efficacious dose. It is devoid of hERG and phospholipidosis issues. Phase-1 evaluation for safety, tolerability, and pharmacokinetics, and long-term safety studies in animals have been successfully completed without any concern for further development.

Clobenpropit, a histamine H3 receptor antagonist/inverse agonist, inhibits [3H]-dopamine uptake by human neuroblastoma SH-SY5Y cells and rat brain synaptosomes.

BACKGROUND: Clobenpropit, a potent antagonist/inverse agonist at the histamine H3 receptor (H3R), reduced the cytotoxic action of 6-hydroxydopamine (6-OHDA) in neuroblastoma SH-SY5Y cells transfected with the human H3R. We therefore set out to study whether this effect involved a receptor-independent action on dopamine transport. METHODS: The uptake of [3H]-dopamine was assayed in SH-SY5Y cells and rat striatal or cerebro-cortical isolated nerve terminals (synaptosomes). Clobenpropit binding to the human norepinephrine (NET) and dopamine (DAT) transporters was analyzed by molecular modeling. RESULTS: In SH-SY5Y cells, [3H]-dopamine uptake was inhibited by desipramine (selective NET inhibitor), GBR-12909 (selective DAT inhibitor), and fluoxetine (selective inhibitor of the serotonin transporter, SERT) with IC50 values 37, 537, and 2800nM, respectively. The potency rank order indicates that [3H]-dopamine uptake is primarily performed by NET. Clobenpropit inhibited [3H]-dopamine uptake (maximum inhibition 82.7±2.8%, IC50 490nM), and the effect was reproduced by the H3R antagonist/inverse agonist iodophenpropit, but not by the agonists R-α-methylhistamine and immepip or the antagonists/inverse agonists ciproxifan and A-331440. Clobenpropit also inhibited [3H]-dopamine uptake by rat striatal and cerebro-cortical synaptosomes (-54.6±11.3% and -46.3±9.6%, respectively, at 10µM). Modeling of the human NET and DAT obtained by homology from the crystal of Drosophila melanogaster DAT showed that clobenpropit can bind to a site also recognized in both transporters by nisoxetine, a potent NET inhibitor. CONCLUSION: These data indicate a direct inhibitory effect of clobenpropit on catecholamine transport.

Novel indanone derivatives as MAO B/H3R dual-targeting ligands for treatment of Parkinson's disease.

The design of multi-targeting ligands was developed in the last decades as an innovative therapeutic concept for Parkinson's disease (PD) and other neurodegenerative disorders. As the monoamine oxidase B (MAO B) and the histamine H3 receptor (H3R) are promising targets for dopaminergic regulation, we synthetized dual-targeting ligands (DTLs) as non-dopaminergic receptor approach for the treatment of PD. Three series of compounds were developed by attaching the H3R pharmacophore to indanone-related MAO B motifs, leading to development of MAO B/H3R DTLs. Among synthesized indanone DTLs, compounds bearing the 2-benzylidene-1-indanone core structure showed MAO B preferring inhibition capabilities along with nanomolar hH3R affinity. Substitution of C5 and C6 position of the 2-benzylidene-1-indanones with lipophilic substituents revealed three promising candidates exhibiting inhibitory potencies for MAO B with IC50 values ranging from 1931 nM to 276 nM and high affinities at hH3R (Ki < 50 nM). Compound 3f ((E)-5-((4-bromobenzyl)oxy)-2-(4-(3-(piperidin-1-yl)propoxy)benzylidene)-2,3-dihydro-1H-inden-1-one, MAO B IC50 = 276 nM, hH3R Ki = 6.5 nM) showed highest preference for MAO B over MAO A (SI > 36). Interestingly, IC50 determinations after preincubation of enzyme and DTLs revealed also nanomolar MAO B potency for 3e (MAO B IC50 = 232 nM), a structural isomer of 3f, and 3d (MAO B IC50 = 541 nM), suggesting time-dependent inhibition modes. Reversibility of inhibition for all three compounds were confirmed by dilution studies in excess of substrate. Thus, indanone-substituted derivatives are promising lead structures for the design of MAO B/hH3R DTLs as novel therapeutic approach of PD therapy.

Progress in the development of histamine H3 receptor antagonists/inverse agonists: a patent review (2013-2017).

INTRODUCTION: Since years, ligands blocking histamine H3 receptor (H3R) activity (antagonists/inverse agonists) are interesting targets in the search for new cures for CNS disorders. Intensive works done by academic and pharmaceutical company researchers have led to many potent and selective H3R antagonists/inverse agonists. Some of them have reached to clinical trials. AREAS COVERED: Patent applications from January 2013 to September 2017 and the most important topics connected with H3R field are analysed. Espacenet, Patentscope, Pubmed, GoogleScholar or Cochrane Library online databases were principially used to collect all the materials. EXPERT OPINION: The research interest in histamine H3R field is still high although the number of patent applications has decreased during the past 4 years (around 20 publications). Complexity of histamine H3R biology e.g. many isoforms, constitutive activity, heteromerization with other receptors (dopamine D2, D1, adenosine A2A) and pharmacology make not easy realization and evaluation of therapeutic potential of anti-H3R ligands. First results from clinical trials have verified potential utility of histamine H3R antagonist/inverse agonists in some diseases. However, more studies are necessary for better understanding of an involvement of the histaminergic system in CNS-related disorders and helping more ligands approach to clinical trials and the market. Lists of abbreviations: hAChEI - human acetylcholinesterase inhibitor; hBuChEI - human butyrylcholinesterase inhibitor; hMAO - human monoamine oxidase; MAO - monoamine oxidase.

Prenatal Alcohol Exposure Increases Histamine H3 Receptor-Mediated Inhibition of Glutamatergic Neurotransmission in Rat Dentate Gyrus.

BACKGROUND: We have reported that prenatal alcohol exposure (PAE)-induced deficits in dentate gyrus, long-term potentiation (LTP), and memory are ameliorated by the histamine H3 receptor inverse agonist ABT-239. Curiously, ABT-239 did not enhance LTP or memory in control offspring. Here, we initiated an investigation of how PAE alters histaminergic neurotransmission in the dentate gyrus and other brain regions employing combined radiohistochemical and electrophysiological approaches in vitro to examine histamine H3 receptor number and function. METHODS: Long-Evans rat dams voluntarily consumed either a 0% or 5% ethanol solution 4 hours each day throughout gestation. This pattern of drinking, which produces a mean peak maternal serum ethanol concentration of 60.8 ± 5.8 mg/dl, did not affect maternal weight gain, litter size, or offspring birthweight. RESULTS: Radiohistochemical studies in adult offspring revealed that specific [3 H]-A349821 binding to histamine H3 receptors was not different in PAE rats compared to controls. However, H3 receptor-mediated Gi /Go protein-effector coupling, as measured by methimepip-stimulated [35 S]-GTPγS binding, was significantly increased in cerebral cortex, cerebellum, and dentate gyrus of PAE rats compared to control. A LIGAND analysis of detailed methimepip concentration-response curves in dentate gyrus indicated that PAE significantly elevates receptor-effector coupling by a lower affinity H3 receptor population without significantly altering the affinities of H3 receptor subpopulations. In agreement with the [35 S]-GTPγS studies, a similar range of methimepip concentrations also inhibited electrically evoked field excitatory postsynaptic potential responses and increased paired-pulse ratio, a measure of decreased glutamate release, to a significantly greater extent in dentate gyrus slices from PAE rats than in controls. CONCLUSIONS: These results suggest that a PAE-induced elevation in H3 receptor-mediated inhibition of glutamate release from perforant path terminals as 1 mechanism contributing the LTP deficits previously observed in the dentate gyrus of PAE rats, as well as providing a mechanistic basis for the efficacy of H3 receptor inverse agonists for ameliorating these deficits.

Time-dependent effects of olanzapine treatment on the expression of histidine decarboxylase, H1 and H3 receptor in the rat brain: The roles in olanzapine-induced obesity.

Antipsychotic treatment, particularly olanzapine and clozapine, induces severe obesity. The Histamine H1 receptor is considered to be an important contributor to olanzapine-induced obesity, however how olanzapine modulates the histaminergic system is not sufficiently understood. This study examined the effect of olanzapine on key molecules of the histaminergic system, including histidine decarboxylase (HDC), H1 receptor (H1R) and H3 receptor (H3R), in the brain at different stages of olanzapine-induced obesity. During short-term treatment (8-day), olanzapine increased hypothalamic HDC mRNA expression and H1R binding in the arcuate nucleus (Arc) and ventromedial hypothalamus (VMH), without changing H3R binding density. HDC mRNA and Arc H1R binding were positively correlated with increased food intake, feeding efficiency and weight gain. When the treatment was extended to 16 and 36 days, H1R binding was increased not only in the hypothalamic Arc and VMH but also in the brainstem dorsal vagal complex (DVC). The H1R bindings in the Arc, VMH and DVC were positively correlated with weight gain induced by olanzapine treatment. However, the expression of HDC and H3R mRNA was not increased. These results suggest that olanzapine time-dependently modulates histamine neurotransmission, which suggested the different neuronal mechanisms underlying different stages of weight gain development. Treatment targeting the H1R may be effective for both short- and long-term olanzapine-induced weight gain.

The European Medicines Agency review of pitolisant for treatment of narcolepsy: summary of the scientific assessment by the Committee for Medicinal Products for Human Use.

On 31 March 2016, the European Commission issued a decision for a marketing authorisation valid throughout the European Union (EU) for pitolisant (Wakix) for the treatment of narcolepsy with or without cataplexy in adults. Pitolisant is an antagonist/inverse agonist of the human histamine H3 receptor. The dose should be selected using an up-titration scheme depending on individual patient response and tolerance and should not exceed 36 mg/day. The main evidence of efficacy of pitolisant was based on two Phase III clinical trials. The improvement on excessive daytime sleepiness was shown against placebo in the Harmony I study (-3.33 points; 95% confidence interval (CI) [-5.83; -0.83]; p = 0.024) and in Harmony CTP (-3.41 points; 95% CI [-4.95; -1.87]; p < 0.0001). The daily cataplexy rate in Harmony I improved against placebo with a rate ratio (rR) of 0.38 whilst in the Harmony CTP the ratio of improvement on weekly cataplexy rate against placebo was 0.512. The most commonly reported adverse reactions were headache, insomnia and nausea. This article summarizes the scientific review leading to approval of pitolisant in the EU. The assessment report and product information are available on the European Medicines Agency website (http://www.ema.europa.eu).

Cimetidine disrupts the renewal of testicular cells and the steroidogenesis in a hermaphrodite fish.

The importance of histamine in the physiology of the testis in mammals and reptiles has been recently shown. Histamine receptors (Hrs) are well conserved in fish and are functional in several fish species. We report here for the first time that histamine and the mRNA of Hrh1, Hrh2 and Hrh3 are all present in the gonad of the hermaphrodite teleost fish gilthead seabream. Moreover, cimetidine, which acts in vitro as an agonist of Hrh1 and Hrh2 on this species, was intraperitoneally injected in one and two years old gilthead seabream males. After three and five days of cimetidine injection, we found that this compound differently modified the gonadal hrs transcript levels and affects the testicular cell renewal and the gene expression of steroidogenesis-related molecules as well as the serum steroid levels. Our data point to cimetidine as a reproductive disruptor and elucidate a role for histamine in the gonad of this hermaphrodite fish species through Hr signalling.

Histaminergic Pharmacology Modulates the Analgesic and Antiedematogenic Effects of Spinally Injected Morphine.

BACKGROUND: Histamine receptors are known to participate in spinal cord nociceptive transmission, and previous studies have suggested that histaminergic receptors are involved in the analgesic effects of morphine. Herein, we investigated the effect of intrathecal injection of histaminergic agonists and antagonists in a model of acute articular inflammation and their interaction with morphine. METHODS: After carrageenan injection in the right knee joint, articular incapacitation was measured hourly, for up to 6 hours, by the paw elevation time during 1-minute periods of stimulated walking. Inflammatory edema was also assessed hourly by determining an increase in articular diameter. Spinal treatments were administered 20 minutes before knee-joint carrageenan injection and were compared with the saline-treated control group. RESULTS: Intrathecally injected histamine increased incapacitation and articular edema, whereas the H1R antagonist, cetirizine, decreased both parameters. The H3R agonist, immepip, decreased both incapacitation and edema, but the H3R antagonist, thioperamide, increased both incapacitation and edema. Morphine inhibited both incapacitation and edema. Furthermore, combining a subeffective dose of morphine with cetirizine or immepip potentiated the analgesic and antiedematogenic effect. CONCLUSIONS: Histamine seems to act at the spinal level via H1 and H3 receptors to modulate acute arthritis in rats. An H1R antagonist and H3R agonist were found to potentiate the analgesic and antiedematogenic effects of morphine, suggesting that histaminergic and opioid spinal systems may be explored for means of improving analgesia, as well as peripheral anti-inflammatory effects.

Ciproxifan, an H3 receptor antagonist, improves short-term recognition memory impaired by isoflurane anesthesia.

BACKGROUND: Exposure to volatile anesthetics has been reported to cause temporary or sustained impairments in learning and memory in pre-clinical studies. The selective antagonists of the histamine H3 receptors (H3R) are considered to be a promising group of novel therapeutic agents for the treatment of cognitive disorders. The aim of this study was to evaluate the effect of H3R antagonist ciproxifan on isoflurane-induced deficits in an object recognition task. METHODS: Adult C57BL/6 J mice were exposed to isoflurane (1.3 %) or vehicle gas for 2 h. The object recognition tests were carried at 24 h or 7 days after exposure to anesthesia to exploit the tendency of mice to prefer exploring novel objects in an environment when a familiar object is also present. During the training phase, two identical objects were placed in two defined sites of the chamber. During the test phase, performed 1 or 24 h after the training phase, one of the objects was replaced by a new object with a different shape. The time spent exploring each object was recorded. RESULTS: A robust deficit in object recognition memory occurred 1 day after exposure to isoflurane anesthesia. Isoflurane-treated mice spent significantly less time exploring a novel object at 1 h but not at 24 h after the training phase. The deficit in short-term memory was reversed by the administration of ciproxifan 30 min before behavioral training. CONCLUSION: Isoflurane exposure induces reversible deficits in object recognition memory. Ciproxifan appears to be a potential therapeutic agent for improving post-anesthesia cognitive memory performance.

Do Histamine receptor 3 antagonists have a place in the therapy for schizophrenia?

In spite of almost 60 years of experience with the pharmacological treatment of schizophrenia, there is still a large unmet medical need for better control of especially the negative and cognitive symptoms of schizophrenia. One potential new avenue is the selective blockade of histamine H3 receptors (H3R). Based on a large basis of preclinical data, H3R antagonists or inverse agonists have been suggested to improve cognition in a variety of neurological and psychiatric indications. The aim of the present paper is to review the potential usefulness of H3R antagonists for the treatment of schizophrenia. Although, so far no H3R antagonist has been marketed and many phase II and III studies are still underway, the available data seem to indicate that H3R antagonists may not be primarily effective against the positive symptoms (i.e. the psychotic symptoms such as hallucinations and delusions) but may hold a promise as add-on therapy for selectively improving cognitive and perhaps negative symptoms.

Synthesis and functional characterization of imbutamine analogs as histamine H3 and H4 receptor ligands.

Imbutamine (4-(1H-imidazol-4-yl)butanamine) is a potent histamine H3 (H3R) and H4 receptor (H4R) agonist (EC50 values: 3 and 66 nM, respectively). Aiming at improved selectivity for the H4R, the imidazole ring in imbutamine was methyl-substituted or replaced by various differently substituted heterocycles (1,2,3-triazoles, 1,2,4-triazoles, pyridines, pyrimidines) as potential bioisosteres. Investigations in [(35)S]GTPγS binding assays using membranes of Sf9 insect cells expressing the respective human histamine receptor subtype revealed only very weak activity of most of the synthesized hetarylalkylamines at both receptors. By contrast, the introduction of substituents at the 4-imidazolyl ring was most effective regarding H4R selectivity. This holds for methyl substitution in position 2 and, especially, in position 5. 5-Methylimbutamine (H4R: EC50 = 59 nM, α = 0.8) was equipotent with imbutamine at the hH4R, but revealed about 16-fold selectivity for the hH4R compared to the hH3R (EC50 980 nM, α = 0.36), whereas imbutamine preferred the hH3R. The functional activities were in agreement with radioligand binding data. The results support the hypothesis that, by analogy with histamine, methyl substitution in histamine homologs offers a way to shift the selectivity in favor of the H4R.

The expression and function of histamine H3 receptors in pancreatic beta cells.

BACKGROUND AND PURPOSE: Histamine and its receptors in the CNS play important roles in energy homeostasis. Here, we have investigated the expression and role of histamine receptors in pancreatic beta cells, which secrete insulin. EXPERIMENTAL APPROACH: The expression of histamine receptors in pancreatic beta cells was examined by RT-PCR, Western blotting and immunostaining. Insulin secretion assay, ATP measurement and calcium imaging studies were performed to determine the function and signalling pathway of histamine H3 receptors in glucose-induced insulin secretion (GIIS) from MIN6 cells, a mouse pancreatic beta cell line. The function and signalling pathway of H3 receptors in MIN6 cell proliferation were examined using pharmacological assay and Western blotting. KEY RESULTS: Histamine H3 receptors were expressed in pancreatic beta cells. A selective H3 receptor agonist, imetit, and a selective inverse H3 receptor agonist, JNJ-5207852, had inhibitory and facilitatory effects, respectively, on GIIS in MIN6 cells. Neither imetit nor JNJ-5207852 altered intracellular ATP concentration, or intracellular calcium concentration stimulated by glucose and KCl, indicating that GIIS signalling was affected by H3 receptor signalling downstream of the increase in intracellular calcium concentration. Moreover, imetit attenuated bromodeoxyuridine incorporation in MIN6 cells. The phosphorylation of cAMP response element-binding protein (CREB), which facilitated beta cell proliferation, was inhibited, though not significantly, by imetit, indicating that activated H3 receptors inhibited MIN6 cell proliferation, possibly by decreasing CREB phosphorylation. CONCLUSIONS AND IMPLICATIONS: Histamine H3 receptors were expressed in mouse beta cells and could play a role in insulin secretion and, possibly, beta cell proliferation.