The histamine H3 receptor: from gene cloning to H3 receptor drugs

Palladium-catalyzed intramolecular C-H difluoroalkylation: synthesis of substituted 3,3-difluoro-2-oxindoles

The synthesis of 3,3-difluoro-2-oxindoles through a robust and efficient palladium-catalyzed CH difluoroalkylation is described. This process generates a broad range of difluorooxindoles from readily prepared starting materials. The use of BrettPhos as the ligand was crucial for high efficiency. Preliminary mechanistic studies suggest that oxidative addition is the rate-determining step for this process.

Pharmacological and methodological aspects of the separation-induced vocalization test in guinea pig pups; a systematic review and meta-analysis

The separation-induced vocalization test in guinea pig pups is one of many that has been used to screen for anxiolytic-like properties of drugs. The test is based on the cross-species phenomenon that infants emit distress calls when placed in social isolation. Here we report a systematic review and meta-analysis of pharmacological intervention in the separation-induced vocalization test in guinea pig pups. Electronic databases were searched for original research articles, yielding 32 studies that met inclusion criteria. We extracted data on pharmacological intervention, animal and methodological characteristics, and study quality indicators. Meta-analysis showed that the different drug classes in clinical use for the treatment of anxiety disorders, have comparable effects on vocalization behaviour, irrespective of their mechanism of action. Of the experimental drugs, nociception (NOP) receptor agonists proved very effective in this test. Analysis further indicated that the commonly used read-outs total number and total duration of vocalizations are equally valid. With regard to methodological characteristics, repeated testing of pups as well as selecting pups with moderate or high levels of vocalization were associated with larger treatment effects. Finally, reporting of study methodology, randomization and blinding was poor and Egger's test for small study effects showed that publication bias likely occurred. This review illustrates the value of systematic reviews and meta-analyses in improving translational value and methodological aspects of animal models. It further shows the urgent need to implement existing publication guidelines to maximize the output and impact of experimental animal studies.

Histamine H4 receptor ligands: future applications and state of art

Histamine is a chemical transmitter found practically in whole organism and exerts its effects through the interaction with H1 to H4 histaminergic receptors. Specifically, H4 receptors are found mainly in immune cells and blood-forming tissues, thus are involved in inflammatory and immune processes, as well as some actions in central nervous system. Therefore, H4 receptor ligands can have applications in the treatment of chronic inflammatory and immune diseases and may be novel therapeutic option in these conditions. Several H4 receptor ligands have been described from early 2000's until nowadays, being imidazole, indolecarboxamide, 2-aminopyrimidine, quinazoline, and quinoxaline scaffolds the most explored and discussed in this review. Moreover, several studies of molecular modeling using homology models of H4 receptor and QSAR data of the ligands are summarized. The increasing and promising therapeutic applications are leading these compounds to clinical trials, which probably will be part of the next generation of blockbuster drugs.

Insufficient intake of L-histidine reduces brain histamine and causes anxiety-like behaviors in male mice

L-histidine is one of the essential amino acids for humans, and it plays a critical role as a component of proteins. L-histidine is also important as a precursor of histamine. Brain histamine is synthesized from L-histidine in the presence of histidine decarboxylase, which is expressed in histamine neurons. In the present study, we aimed to elucidate the importance of dietary L-histidine as a precursor of brain histamine and the histaminergic nervous system. C57BL/6J male mice at 8 wk of age were assigned to 2 different diets for at least 2 wk: the control (Con) diet (5.08 g L-histidine/kg diet) or the low L-histidine diet (LHD) (1.28 g L-histidine/kg diet). We measured the histamine concentration in the brain areas of Con diet-fed mice (Con group) and LHD-fed mice (LHD group). The histamine concentration was significantly lower in the LHD group [Con group vs. LHD group: histamine in cortex (means ± SEs): 13.9 ± 1.25 vs. 9.36 ± 0.549 ng/g tissue; P = 0.002]. Our in vivo microdialysis assays revealed that histamine release stimulated by high K(+) from the hypothalamus in the LHD group was 60% of that in the Con group (P = 0.012). However, the concentrations of other monoamines and their metabolites were not changed by the LHD. The open-field tests showed that the LHD group spent a shorter amount of time in the central zone (87.6 ± 14.1 vs. 50.0 ± 6.03 s/10 min; P = 0.019), and the light/dark box tests demonstrated that the LHD group spent a shorter amount of time in the light box (198 ± 8.19 vs. 162 ± 14.1 s/10 min; P = 0.048), suggesting that the LHD induced anxiety-like behaviors. However, locomotor activity, memory functions, and social interaction did not differ between the 2 groups. The results of the present study demonstrated that insufficient intake of histidine reduced the brain histamine content, leading to anxiety-like behaviors in the mice.

Synthesis and characterization of new bivalent agents as melatonin- and histamine H3-ligands

Melatonin is an endogenous molecule involved in many pathophysiological processes. In addition to the control of circadian rhythms, its antioxidant and neuroprotective properties have been widely described. Thus far, different bivalent compounds composed by a melatonin molecule linked to another neuroprotective agent were synthesized and tested for their ability to block neurodegenerative processes in vitro and in vivo. To identify a novel class of potential neuroprotective compounds, we prepared a series of bivalent ligands, in which a prototypic melatonergic ligand is connected to an imidazole-based H₃ receptor antagonist through a flexible linker. Four imidazolyl-alkyloxy-anilinoethylamide derivatives, characterized by linkers of different length, were synthesized and their binding affinity for human MT₁, MT₂ and H₃ receptor subtypes was evaluated. Among the tested compounds, 14c and 14d, bearing a pentyl and a hexyl linker, respectively, were able to bind to all receptor subtypes at micromolar concentrations and represent the first bivalent melatonergic/histaminergic ligands reported so far. These preliminary results, based on binding affinity evaluation, pave the way for the future development of new dual-acting compounds targeting both melatonin and histamine receptors, which could represent promising therapeutic agents for the treatment of neurodegenerative pathologies.

A single-point mutation (Ala280Val) in the third intracellular loop alters the signalling properties of the human histamine H₃ receptor stably expressed in CHO-K1 cells

BACKGROUND AND PURPOSE

An alanine to valine exchange at amino acid position 280 (A280V) in the third intracellular loop of the human histamine H₃ receptor was first identified in a patient suffering from Shy-Drager syndrome and later reported as a risk factor for migraine. Here, we have compared the pharmacological and signalling properties of wild-type (hH₃ R(WT)) and A280V mutant (hH₃ R(A280V)) receptors stably expressed in CHO-K1 cells.

EXPERIMENTAL APPROACH

The hH₃ R(A280V) cDNA was created by overlapping extension PCR amplification. Receptor expression and affinity were assessed by radioligand (N-α-[methyl-³H]-histamine) binding to cell membranes, and receptor function by the inhibition of forskolin-induced cAMP accumulation and stimulation of ERK1/2 phosphorylation in intact cells, as well as stimulation of [³⁵S]-GTPγS binding to cell membranes.

KEY RESULTS

Both receptors were expressed at similar levels with no significant differences in their affinities for H₃ receptor ligands. Upon activation the hH₃ RWT was significantly more efficacious to inhibit forskolin-induced cAMP accumulation and to stimulate [³⁵S]-GTPγS binding, with no difference in pEC50 estimates. The hH₃ RWT was also more efficacious to stimulate ERK1/2 phosphorylation, but this difference was not significant. The inverse agonist ciproxifan was more efficacious at hH3 RWT to reduce [³⁵S]-GTPγS binding but, for both receptors, failed to enhance forskolin-induced cAMP accumulation.

CONCLUSIONS AND IMPLICATIONS

The A280V mutation reduces the signalling efficacy of the human H₃ receptor. This effect may be relevant to the pathophysiology of disorders associated with the mutation.

Highly selective palladium-catalyzed cross-coupling of secondary alkylzinc reagents with heteroaryl halides

The highly selective palladium-catalyzed Negishi coupling of secondary alkylzinc reagents with heteroaryl halides is described. The development of a series of biarylphosphine ligands has led to the identification of an improved catalyst for the coupling of electron-deficient heterocyclic substrates. Preparation and characterization of oxidative addition complex (L)(Ar)PdBr provided insight into the unique reactivity of catalysts based on CPhos-type ligands in facilitating challenging reductive elimination processes.

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.

(R)-alpha-methylhistamine suppresses inhibitory neurotransmission in hippocampal CA1 pyramidal neurons counteracting propofol-induced amnesia in rats

BACKGROUND

Propofol is a short-acting, intravenous general anesthetic that is widely used in clinical practice for short procedures; however, it causes depressed cognitive function for several hours thereafter. (R)-alpha-methylhistamine (RAMH), a selective histamine H3 receptor agonist, can enhance memory retention and attenuates memory impairment in rats. In this study, we investigated whether RAMH could rescue propofol-induced memory deficits and the underlying mechanisms partaking in this process.

METHODS

In the modified Morris water maze (MWM) test, rats were randomized into the following groups: control, propofol (25 mg/kg, i.p., 30 min before training), RAMH (10 mg/kg, i.p., 60 min before training), and propofol plus RAMH. All randomized rats were subjected to 2 days of training, and a probe test was conducted on day 3. Field excitatory postsynaptic potentials were recorded from CA1 neurons in rat hippocampal slices, and long-term potentiation (LTP) was induced by either theta-burst stimulation (TBS) or high-frequency tetanic stimulation (HFS). Spontaneous and miniature inhibitory (sIPSCs, mIPSCs) or excitatory (sEPSCs, mEPSCs) postsynaptic currents were recorded from CA1 pyramidal neurons by whole-cell patch clamp.

RESULTS

In the MWM task, propofol injection significantly impaired spatial memory retention. Pretreatment with RAMH reversed propofol-induced memory retention. In hippocampal CA1 slices, propofol perfusion markedly inhibited TBS- but not HFS-induced LTP. Co-perfusion of RAMH reversed the inhibitory effect of propofol on TBS-induced LTP reduction. Furthermore, in hippocampal CA1 pyramidal neurons, RAMH significantly suppressed the frequency but not the amplitude of sIPSCs and mIPSCs and had little effects on both the frequency and amplitude of sEPSCs and mEPSCs.

CONCLUSIONS

Our results suggest that RAMH, by inhibiting presynaptic GABAergic neurotransmission, suppresses inhibitory neurotransmission in hippocampal CA1 pyramidal neurons, which in turn reverses inhibition of CA1 LTP and the spatial memory deficits induced by propofol in rats.

Anxiolytic and antidepressant-like activities of the novel and potent non-imidazole histamine H₃ receptor antagonist ST-1283

Previous studies have suggested a potential link between histamine H₃ receptors (H₃R) signaling and anxiolytic-like and antidepressant-like effects. The aim of this study was to investigate the acute effects of ST-1283, a novel H₃R antagonist, on anxiety-related and depression-related behaviors in comparison with those of diazepam and fluoxetine. The effects of ST-1283 were evaluated using the elevated plus maze test, open field test, marbles burying test, tail suspension test, novelty suppressed feeding test, and forced swim test in male C57BL/6 mice. The results showed that, like diazepam, ST-1283 (7.5 mg/kg) significantly modified all the parameters observed in the elevated plus maze test. In addition, ST-1283 significantly increased the amount of time spent in the center of the arena without altering general motor activity in the open field test. In the same vein, ST-1283 reduced the number of buried marbles as well as time spent digging in the marbles burying test. The tail suspension test and forced swim test showed that ST-1283 was able to reduce immobility time, like the recognized antidepressant drug fluoxetine. In the novelty suppressed feeding test, treatment with ST-1283 decreased latency to feed with no effect on food intake in the home cage. Importantly, pretreatment with the H₃R agonist R-α-methylhistamine abrogated the anxiolytic and antidepressant effects of ST-1283. Taken together, the present series of studies demonstrates the novel effects of this newly synthesized H₃R antagonist in a number of preclinical models of psychiatric disorders and highlights the histaminergic system as a potential therapeutic target for the treatment of anxiety-related and depression-related disorders.

Mathematical analysis of the sodium sensitivity of the human histamine H3 receptor

Purpose

It was shown by several experimental studies that some G protein coupled receptors (GPCR) are sensitive to sodium ions. Furthermore, mutagenesis studies or the determination of crystal structures of the adenosine A_2A or δ-opioid receptor revealed an allosteric Na⁺ binding pocket near to the highly conserved Asp^2.50. Within a previous study, the influence of NaCl concentration onto the steady-state GTPase activity at the human histamine H₃ receptor (hH₃R) in presence of the endogenous histamine or the inverse agonist thioperamide was analyzed. The purpose of the present study was to examine and quantify the Na⁺-sensitivity of hH₃R on a molecular level.

Methods

To achieve this, we developed a set of equations, describing constitutive activity and the different ligand-receptor equilibria in absence or presence of sodium ions. Furthermore, in order to gain a better understanding of the ligand- and Na⁺-binding to hH₃R on molecular level, we performed molecular dynamic (MD) simulations.

Results

The analysis of the previously determined experimental steady-state GTPase data with the set of equations presented within this study, reveals that thioperamide binds into the orthosteric binding pocket of the hH₃R in absence or presence of a Na⁺ in its allosteric binding site. However, the data suggest that thioperamide binds preferentially into the hH₃R in absence of a sodium ion in its allosteric site. These experimental results were supported by MD simulations of thioperamide in the binding pocket of the inactive hH₃R. Furthermore, the MD simulations revealed two different binding modes for thioperamide in presence or absence of a Na⁺ in its allosteric site.

Conclusion

The mathematical model presented within this study describes the experimental data regarding the Na⁺-sensitivity of hH₃R in an excellent manner. Although the present study is focused onto the Na⁺-sensitivity of the hH₃R, the resulting equations, describing Na⁺- and ligand-binding to a GPCR, can be used for all other ion-sensitive GPCRs.

Procognitive properties of drugs with single and multitargeting H3 receptor antagonist activities

The histamine H3 receptor (H3 R) is an important modulator of numerous central control mechanisms. Novel lead optimizations for H3 R antagonists/inverse agonists involved studies of structure-activity relationships, cross-affinities, and pharmacokinetic properties of promising ligands. Blockade of inhibitory histamine H3 autoreceptors reinforces histaminergic transmission, while antagonism of H3 heteroreceptors accelerates the corticolimbic liberation of acetylcholine, norepinephrine, glutamate, dopamine, serotonin and gamma-aminobutyric acid (GABA). The H3 R positioned at numerous neurotransmission crossroads indicates therapeutic applications of small-molecule H3 R modulators in a number of psychiatric and neurodegenerative diseases with various clinical candidates available. Dual target drugs displaying H3 R antagonism/inverse agonism with inhibition of acetylcholine esterase (AChE), histamine N-methyltransferase (HMT), or serotonin transporter (SERT) are novel class of procognitive agents. Main chemical diversities, pharmacophores, and pharmacological profiles of procognitive agents acting as H3 R antagonists/inverse agonists and dual H3 R antagonists/inverse agonists with inhibiting activity on AChE, HMT, or SERT are highlighted here.

Histamine H3 receptors aggravate cerebral ischaemic injury by histamine-independent mechanisms

The role of the histamine H3 receptor (H3R) in cerebral ischaemia/reperfusion (I/R) injury remains unknown. Here we show that H3R expression is upregulated after I/R in two mouse models. H3R antagonists and H3R knockout attenuate I/R injury, which is reversed by an H3R-selective agonist. Interestingly, H1R and H2R antagonists, a histidine decarboxylase (HDC) inhibitor and HDC knockout all fail to compromise the protection by H3R blockade. H3R blockade inhibits mTOR phosphorylation and reinforces autophagy. The neuroprotection by H3R antagonism is reversed by 3-methyladenine and siRNA for Atg7, and is diminished in Atg5^−/− mouse embryonic fibroblasts. Furthermore, the peptide Tat-H3R_CT414-436, which blocks CLIC4 binding with H3Rs, or siRNA for CLIC4, further increases I/R-induced autophagy and protects against I/R injury. Therefore, H3R promotes I/R injury while its antagonism protects against ischaemic injury via histamine-independent mechanisms that involve suppressing H3R/CLIC4 binding-activated autophagy, suggesting that H3R inhibition is a therapeutic target for cerebral ischaemia.

Histamine H3 receptor dysregulation is a hallmark of pathological conditions in the central nervous system, and H3 receptor antagonism is neuroprotective. Here Chen et al. show that histamine-independent H3 receptor activation can enhance neuronal cell death during cerebral ischaemia by suppressing autophagy.

Discovery of (1R,6S)-5-[4-(1-cyclobutyl-piperidin-4-yloxy)-phenyl]-3,4-diaza-bicyclo[4.1.0]hept-4-en-2-one (R,S-4a): histamine H(3) receptor inverse agonist demonstrating potent cognitive enhancing and wake promoting activity

A series of fused cyclopropyl-4,5-dihydropyridazin-3-one (3,4-diaza-bicyclo[4.1.0]hept-4-en-2-one) phenoxypiperidine analogs was designed and synthesized, leading to the identification of (1R,6S)-5-[4-(1-cyclobutyl-piperidin-4-yloxy)-phenyl]-3,4-diaza-bicyclo[4.1.0]hept-4-en-2-one (R,S-4a) as a second-generation pyridazin-3-one H3R antagonist. Compound R,S-4a was a potent H3R functional antagonist in vivo in the rat dipsogenia model, demonstrated potent wake activity in the rat EEG/EMG model, and enhanced short-term memory in the rat social recognition memory model at doses as low as 0.03-0.3 mg/kg po.

Histamine in migraine and brain

BACKGROUND

Histamine has been studied in both health and disease since the initial description a century ago. With its vasodilative effect, it was suggested early on to be involved in the pathophysiology of migraine. Over the past 25 years, much has been learned about histamine as a neurotransmitter in the central nervous system. The role of this neurotransmitter system in migraine has not been previously reviewed.

OBJECTIVE

Discuss a potential role of the brain histaminergic system in migraine.

METHODS

Unstructured literature search with a no specific hypothesis-driven approach.

RESULTS

There is substantial evidence that systemically given histamine may elicit, maintain, and aggravate headache. The mechanisms for this are not known, and histamines do not penetrate the blood-brain barrier (BBB). However, circulating histamine may influence hypothalamic activity via the circumventricular organs that lack BBB. In the rat, prolonged activation of meningeal nociceptors induced by dural mast cell degranulation has been observed. Subcutaneous injections of N-alpha-methyl histamine, a catabolite of histamine with high affinity to the histamine H3 receptor, probably have some migraine preventive effect. A negative feedback on histamine release from mast cells in proximity to C-fiber endings has been a postulated mechanism. Most antihistamines have shown to be ineffective as acute medication for migraine. Two centrally acting potent H1 receptor antagonists (cinnarizine and cyproheptadine) have been reported to be efficacious in preventing migraine. However, the proof for this is limited, and their efficacy has been ascribed other actions than the antihistaminergic. In general, lack of specificity and side effects limit the potential use of centrally acting H1 and H2 antagonists. Brain histamine is synthesized by neurons that are restricted to the posterior basal hypothalamus, more specific to the tuberomamillary nucleus (TMN), and that project practically to the whole central nervous system. The posterior hypothalamus is a suspected locus in quo in several primary headaches. Recently, a positron emission tomography study performed in the prodromal phase of migraine attacks supported the idea of initial involvement of this area. In another recent study, the thalamic nuclei receiving trigeminal output was also shown to have direct connections with the ventral TMN. The central histaminergic system plays an important role in the complex sleep-wake cycle, promoting cortical excitability during wakening and attention, and it consolidates the wake state. The period of the day, in the evenings and during the night, when there is reduced susceptibility for migraine attacks corresponds with less central histaminergic firing. Activation of both the H3 and the H4 receptor promotes inhibitory actions on neurons. The H3 receptor causes autoinhibition of the histaminergic neurons themselves, and centrally acting H3 receptor agonist prodrugs have shown to both inhibit neurogenic inflammation in dura, to induce sleep, and to produce antinociception. There are no registered ongoing studies on H3 and H4 receptor ligands in migraine.

CONCLUSION

The role of the central histaminergic system in migraine is largely unexplored, but findings from preclinical research may be linked to several aspects of the disorder. The histaminergic system of the brain may play an important role, especially in the initial phase of an attack, and histamine H3 and H4 receptor ligands may potentially have migraine prophylactic properties. However, the basis for this is still circumstantial, and the evidence is lacking.

Homologous desensitization of human histamine H₃ receptors expressed in CHO-K1 cells

Histamine H₃ receptors (H₃Rs) modulate the function of the nervous system at the pre- and post-synaptic levels. In this work we aimed to determine whether, as other G protein-coupled receptors (GPCRs), H₃Rs desensitize in response to agonist exposure. By using CHO-K1 cells stably transfected with the human H₃R (hH3R) we show that functional responses (inhibition of forskolin-induced cAMP accumulation in intact cells and stimulation of [(35)S]-GTPγS binding to cell membranes) were markedly reduced after agonist exposure. For cAMP accumulation assays the effect was significant at 60 min with a maximum at 90 min. Agonist exposure resulted in decreased binding sites for the radioligand [(3)H]-N-methyl-histamine ([(3)H]-NMHA) to intact cells and modified the sub-cellular distribution of H₃Rs, as detected by sucrose density gradients and [(3)H]-NMHA binding to cell membranes, suggesting receptor internalization. The reduction in the inhibition of forskolin-stimulated cAMP formation observed after agonist pre-incubation was prevented by incubation in hypertonic medium or in ice-cold medium. Agonist-induced loss in binding sites was also prevented by hypertonic medium or incubation at 4 °C, but not by filipin III, indicating clathrin-dependent endocytosis. Immunodetection showed that CHO-K1 cells express GPCR kinases (GRKs) 2/3, and both the GRK general inhibitor ZnCl₂ and a small interfering RNA against GRK-2 reduced receptor desensitization. Taken together these results indicate that hH₃Rs experience homologous desensitization upon prolonged exposure to agonists, and that this process involves the action of GRK-2 and internalization via clathrin-coated vesicles.

The prototypical histamine H3 receptor inverse agonist thioperamide improves multiple aspects of memory processing in an inhibitory avoidance task

Numerous studies have found that histamine plays a major role in memory and that the histamine H3 receptor (H3R) inverse agonist thioperamide improves cognitive performance in various animal models. However, little is known about the stages of memory that are specifically affected by thioperamide. The purpose of the present study was to investigate the effects of thioperamide on acquisition, consolidation and retrieval processes in a one-trial inhibitory avoidance task in female C57BL/6J mice. In addition, potential state-dependency effects were studied by injecting thioperamide before the training and the test sessions in order to induce similar physiological states during acquisition and retrieval. Our results indicate that post-training systemic administration of thioperamide facilitated consolidation. Moreover, the administration of thioperamide before the training session had no effect on latency to enter the black compartment during training but enhanced memory during the retention test. The administration of thioperamide before the retention test also increased performance, which indicates that this compound ameliorates memory retrieval. Finally, when animals received thioperamide before the training session and before the retention test, the cognitive enhancing effects of thioperamide were not significantly changed. Together, our results show that thioperamide improves cognitive performance in an inhibitory avoidance task through actions on different memory stages. Furthermore, inducing a similar physiological state with thioperamide during acquisition and retrieval do not significantly affect cognitive enhancement. Our results suggest that the blockade of H3R can be helpful for the treatment of neuropsychiatric conditions characterized by deficits affecting several stages of memory processing.

Histamine-HisCl1 receptor axis regulates wake-promoting signals in Drosophila melanogaster

Histamine and its two receptors, histamine-gated chloride channel subunit 1 (HisCl1) and ora transientless (Ort), are known to control photoreception and temperature sensing in Drosophila. However, histamine signaling in the context of neural circuitry for sleep-wake behaviors has not yet been examined in detail. Here, we obtained mutant flies with compromised or enhanced histamine signaling and tested their baseline sleep. Hypomorphic mutations in histidine decarboxylase (HDC), an enzyme catalyzing the conversion from histidine to histamine, caused an increase in sleep duration. Interestingly, hisCl1 mutants but not ort mutants showed long-sleep phenotypes similar to those in hdc mutants. Increased sleep duration in hisCl1 mutants was rescued by overexpressing hisCl1 in circadian pacemaker neurons expressing a neuropeptide pigment dispersing factor (PDF). Consistently, RNA interference (RNAi)-mediated depletion of hisCl1 in PDF neurons was sufficient to mimic hisCl1 mutant phenotypes, suggesting that PDF neurons are crucial for sleep regulation by the histamine-HisCl1 signaling. Finally, either hisCl1 mutation or genetic ablation of PDF neurons dampened wake-promoting effects of elevated histamine signaling via direct histamine administration. Taken together, these data clearly demonstrate that the histamine-HisCl1 receptor axis can activate and maintain the wake state in Drosophila and that wake-activating signals may travel via the PDF neurons.

Assessment of the abuse liability of ABT-288, a novel histamine H₃ receptor antagonist

RATIONALE

Histamine H3 receptor antagonists, such as ABT-288, have been shown to possess cognitive-enhancing and wakefulness-promoting effects. On the surface, this might suggest that H3 antagonists possess psychomotor stimulant-like effects and, as such, may have the potential for abuse.

OBJECTIVES

The aim of the present study was to further characterize whether ABT-288 possesses stimulant-like properties and whether its pharmacology gives rise to abuse liability.

METHODS

The locomotor-stimulant effects of ABT-288 were measured in mice and rats, and potential development of sensitization was addressed. Drug discrimination was used to assess amphetamine-like stimulus properties, and drug self-administration was used to evaluate reinforcing effects of ABT-288. The potential development of physical dependence was also studied.

RESULTS

ABT-288 lacked locomotor-stimulant effects in both rats and mice. Repeated administration of ABT-288 did not result in cross-sensitization to the stimulant effects of d-amphetamine in mice, suggesting that there is little overlap in circuitries upon which the two drugs interact for motor activity. ABT-288 did not produce amphetamine-like discriminative stimulus effects in drug discrimination studies nor was it self-administered by rats trained to self-administer cocaine. There were no signs of physical dependence upon termination of repeated administration of ABT-288 for 30 days.

CONCLUSIONS

The sum of these preclinical data, the first of their kind applied to H3 antagonists, indicates that ABT-288 is unlikely to possess a high potential for abuse in the human population and suggests that H3 antagonists, as a class, are similar in this regard.

A proof-of-concept study of the effect of a novel H3-receptor antagonist in allergen-induced nasal congestion

BACKGROUND

H1-receptor inverse agonists are used effectively for treating several symptoms of allergic rhinitis, including nasal itching, rhinorrhea, and sneezing, although most agents are not very effective in treating nasal congestion.

OBJECTIVE

This study evaluated the relative efficacy of a novel selective H3-receptor antagonist, JNJ-39220675, in preventing nasal congestion induced by exposing participants with ragweed allergy to ragweed allergen in an environmental exposure chamber model.

METHODS

In this single-dose, patient-blind, double-dummy, placebo- and active-controlled, phase IIa cross-over study, 53 participants were randomized to JNJ-39220675 plus placebo, placebo plus pseudoephedrine, or only placebo. The primary efficacy assessment was change in nasal patency assessed by measuring the minimal cross-sectional area of the nasal cavity by using acoustic rhinometry. Secondary assessment included total nasal symptom scores (TNSSs) over the 8-hour environmental exposure chamber exposure period.

RESULTS

Smaller decreases in minimal cross-sectional area were observed after JNJ-39220675 (least square mean difference, -0.126; P = .06) and pseudoephedrine (least square mean difference, -0.195; P = .004) treatment compared with placebo. The means for the baseline-adjusted area under the curve of TNSSs were significantly smaller for JNJ-39220675 (P = .0003) and pseudoephedrine (P = .04) versus placebo. JNJ-39220675 was significantly effective in treating all 4 individual symptoms (P ≤ .05 for all scores) compared with placebo, whereas pseudoephedrine only showed a trend for improvement in individual symptom scores of the TNSS. Insomnia was the most frequent adverse event (17.3%) associated with JNJ-39220675 treatment.

CONCLUSION

Prophylactic treatment with the H3-antagonist JNJ-39220675 relieved allergen-induced nasal congestion by using standard nasal symptom scoring; however, in contrast to pseudoephedrine, it only showed a trend for increasing nasal patency by using objective measures.

Proxyfan acts as a neutral antagonist of histamine H3 receptors in the feeding-related hypothalamic ventromedial nucleus

BACKGROUND AND PURPOSE

Centrally acting histamine H(3) receptor ligands are proposed as potential treatments for obesity, although the value of inverse agonists at these receptors is still debated. Functional inhibition of H(3) autoreceptors activates neurones in a hypothalamic 'satiety' centre. The H(3) receptor antagonist, proxyfan was used as a tool to assess the action of histaminergic compounds in this model.

EXPERIMENTAL APPROACH

We compared the actions of histamine on feeding with those of an H(3) receptor agonist (imetit) and inverse agonist (thioperamide) in rats and mice. Sites of action were identified by immunohistochemistry and the hypothalamic ventromedial nucleus (VMN) was investigated using electrophysiological techniques.

KEY RESULTS

Central histamine or thioperamide decreased fast-induced feeding, whereas imetit increased feeding. Systemic thioperamide entered the brain to activate hypothalamic feeding centres and to reduce feeding without causing any adverse behaviours. Thioperamide activated neurones in the VMN through an action on histamine autoreceptors, whilst imetit had the opposite effect. Proxyfan administered alone did not affect either feeding or electrical activity. However, it blocked the actions of both thioperamide and imetit, acting as a neutral antagonist in this system.

CONCLUSIONS AND IMPLICATIONS

The H(3) receptor inverse agonist, thioperamide, potently reduced appetite without adverse behavioural effects. This action was blocked by proxyfan, acting as a neutral antagonist in this model and, therefore, this compound is useful in determining the selectivity of H(3) receptor-directed drugs. A major action of thioperamide is through presynaptic autoreceptors, inducing stimulation by endogenous histamine of postsynaptic H(1 ) receptors on anorectic hypothalamic neurones.

Mild and general conditions for negishi cross-coupling enabled by the use of palladacycle precatalysts

A wide range of biaryls were synthesized by palladium-catalyzed Negishi cross-couplings at ambient temperature or with low catalyst loading. This protocol features the use of a recently reported aminobiphenyl palladacycle precatalyst to generate the catalytically active XPhosPd(0) species. Significantly, a wide range of challenging heterocyclic and polyfluorinated aromatic substrates can be employed to give products in excellent yields.

Molecular and cellular analysis of human histamine receptor subtypes

The human histamine receptors hH(1)R and hH(2)R constitute important drug targets, and hH(3)R and hH(4)R have substantial potential in this area. Considering the species-specificity of pharmacology of H(x)R orthologs, it is important to analyze hH(x)Rs. Here, we summarize current knowledge of hH(x)Rs endogenously expressed in human cells and hH(x)Rs recombinantly expressed in mammalian and insect cells. We present the advantages and disadvantages of the various systems. We also discuss problems associated with the use of hH(x)R antibodies, an issue of general relevance for G-protein-coupled receptors (GPCRs). There is much greater overlap in activity of 'selective' ligands for other hH(x)Rs than the cognate receptor subtype than generally appreciated. Studies with native and recombinant systems support the concept of ligand-specific receptor conformations, encompassing agonists and antagonists. It is emerging that for characterization of hH(x)R ligands, one cannot rely on a single test system and a single parameter. Rather, multiple systems and parameters have to be studied. Although such studies are time-consuming and expensive, ultimately, they will increase drug safety and efficacy.

Histamine H3 receptor antagonists in relation to epilepsy and neurodegeneration: a systemic consideration of recent progress and perspectives

The central histaminergic actions are mediated by H(1) , H(2) , H(3) and H(4) receptors. The histamine H(3) receptor regulates the release of histamine and a number of other neurotransmitters and thereby plays a role in cognitive and homeostatic processes. Elevated histamine levels suppress seizure activities and appear to confer neuroprotection. The H(3) receptors have a number of enigmatic features like constitutive activity, interspecies variation, distinct ligand binding affinities and differential distribution of prototypic splice variants in the CNS. Furthermore, this Gi/Go-protein-coupled receptor modulates several intracellular signalling pathways whose involvement in epilepsy and neurotoxicity are yet to be ascertained and hence represent an attractive target in the search for new anti-epileptogenic drugs. So far, H(3) receptor antagonists/inverse agonists have garnered a great deal of interest in view of their promising therapeutic properties in various CNS disorders including epilepsy and related neurotoxicity. However, a number of experiments have yielded opposing effects. This article reviews recent works that have provided evidence for diverse mechanisms of antiepileptic and neuroprotective effects that were observed in various experimental models both in vitro and in vivo. The likely reasons for the apparent disparities arising from the literature are also discussed with the aim of establishing a more reliable basis for the future use of H(3) receptor antagonists, thus improving their utility in epilepsy and associated neurotoxicity.

The histamine autoreceptor is a short isoform of the H₃ receptor

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

Histamine H3 receptor antagonists/inverse agonists on cognitive and motor processes: relevance to Alzheimer's disease, ADHD, schizophrenia, and drug abuse

Histamine H3 receptor (H3R) antagonists/inverse agonists possess potential to treat diverse disease states of the central nervous system (CNS). Cognitive dysfunction and motor impairments are the hallmark of multifarious neurodegenerative and/or psychiatric disorders. This review presents the various neurobiological/neurochemical evidences available so far following H3R antagonists in the pathophysiology of Alzheimer's disease (AD), attention-deficit hyperactivity disorder (ADHD), schizophrenia, and drug abuse each of which is accompanied by deficits of some aspects of cognitive and/or motor functions. Whether the H3R inverse agonism modulates the neurochemical basis underlying the disease condition or affects only the cognitive/motor component of the disease process is discussed with the aim to provide a rationale for their use in diverse disease states that are interlinked and are accompanied by some common motor, cognitive and attentional deficits.

Effects of L-histidine and histamine H3 receptor modulators on ethanol-induced sedation in mice

Recent studies suggest that the brain histaminergic system and especially the H3 receptors are involved in the regulation of alcohol consumption and alcohol-induced behaviors. Part of this effect might be due to a modulation of ethanol-induced sedation by central histamine. The aim of the present study was to investigate the effects of several histaminergic drugs on ethanol-induced sedation using the loss of righting reflex experimental protocol in female Swiss mice. A pretreatment with L-histidine, the histamine precursor, significantly reduced ethanol-induced sedation, suggesting that brain histamine protects against the sedative effects of ethanol. In a second set of experiments, several H3 receptor agonists (immepip or imetit) and inverse agonists/antagonists (thioperamide, A331440, or BF2.649) were tested. Surprisingly, both H3 receptor agonists and antagonists potentiated the sedative effects of ethanol. This paradoxical effect might be due to the subtle regulatory actions related to the H3 heteroreceptor function.

Total synthesis and biological evaluation of phidianidines A and B uncovers unique pharmacological profiles at CNS targets

The synthesis of phidianidines A and B, the first 1,2,4-oxadiazole-containing alkaloid, from the marine opisthobranch mollusk Phidiana militaris is reported. The synthesis proceeds in six steps from known indole acetic acids in 39.9% (phidianidine A) and 21% (phidianidine B) overall yields from commercially available materials. Biological characterization found that phidianidines A and B are selective inhibitors of DAT (versus SERT and NET) and a selective, potent ligand and partial agonist of the μ opioid receptor (versus δ- and κ-opioid receptors). Moreover, neither phidianidines A and B are cytotoxic, and thus represent an attractive starting point for chemical optimization; therefore, we piloted a number of chemistries and prepared a diverse series of unnatural analogs.

Natriuretic peptide-induced catecholamine release from cardiac sympathetic neurons: inhibition by histamine H3 and H4 receptor activation

We reported previously that natriuretic peptides, including brain natriuretic peptide (BNP), promote norepinephrine release from cardiac sympathetic nerves and dopamine release from differentiated pheochromocytoma PC12 cells. These proexocytotic effects are mediated by an increase in intracellular calcium secondary to cAMP/protein kinase A (PKA) activation caused by a protein kinase G (PKG)-mediated inhibition of phosphodiesterase type 3 (PDE3). The purpose of the present study was to search for novel means to prevent the proadrenergic effects of natriuretic peptides. For this, we focused our attention on neuronal inhibitory Gα(i/o)-coupled histamine H(3) and H(4) receptors. Our findings show that activation of neuronal H(3) and H(4) receptors inhibits the release of catecholamines elicited by BNP in cardiac synaptosomes and differentiated PC12 cells. This effect results from a decrease in intracellular Ca(2+) due to reduced intracellular cAMP/PKA activity, caused by H(3) and H(4) receptor-mediated PKG inhibition and consequent PDE3-induced increase in cAMP metabolism. Indeed, selective H(3) and H(4) receptor agonists each synergized with a PKG inhibitor and a PDE3 activator in attenuating BNP-induced norepinephrine release from cardiac sympathetic nerve endings. This indicates that PKG inhibition and PDE3 stimulation are pivotal for the H(3) and H(4) receptor-mediated attenuation of BNP-induced catecholamine release. Cardiac sympathetic overstimulation is characteristic of advanced heart failure, which was recently found not to be improved by the administration of recombinant BNP (nesiritide), despite the predicated beneficial effects of natriuretic peptides. Because excessive catecholamine release is likely to offset the desirable effects of natriuretic peptides, our findings suggest novel means to alleviate their adverse effects and improve their therapeutic potential.

Identification of a histaminergic circuit in the caudal hypothalamus: an evidence for functional heterogeneity of histaminergic neurons

It is well established that histaminergic neurons in the posterior hypothalamus make connections with whole brain areas and regulate several functions. Recent evidence indicates that histaminergic neurons are heterogeneous cell group and organized into distinct circuits. However, functional circuits of histaminergic neurons have not been fully mapped so far. To address this issue, we have investigated antihistamine-sensitive neuronal activation in the hypothalamus to determine the hypothalamic region primarily innervated by histaminergic neurons. Here we review our recent findings showing the existence of the heterogeneous subpopulations of histaminergic neurons in the TMN that innervated distinct regions to regulate particular functions. We have identified the caudal part of the arcuate nucleus of hypothalamus (cARC) as a target region of histaminergic neurons in food-restricted rats by assessing suppression of c-Fos expression by pretreatment with antihistamines. Histaminergic neurons in the tuberomammillary nucleus (TMN) are morphologically subdivided into five groups (E1-E5). Among the subdivisions, the E3 group was found to be activated corresponding to the activation of cARC neurons. Our findings suggest that this subpopulation selectively innervate cARC neurons. Accumulating reports have also described c-Fos expression in other TMN subpopulations. Various stress challenge induced c-Fos expression primarily in E4 and E5 subpopulations. Motivation- and drug-induced arousal elicited in common activation of ventrolateral part of the TMN containing E1 and E2 subdivisions, which receive projections from wake-active orexin neurons and sleep-active GABA neurons. These lines of evidence support the hypothesis that there are heterogeneous subpopulations in the TMN that innervated distinct regions to regulate particular functions.

Improvement of the Prediction Power of the CoMFA and CoMSIA Models on Histamine H3 Antagonists by Different Variable Selection Methods

The aim of this study is to enhance the predictivity power of CoMFA and CoMSIA models by means of different variable selection algorithms. The genetic algorithm (GA), successive projection algorithm (SPA), stepwise multiple linear regression (SW-MLR), and the enhanced replacement method (ERM) were used and tested as variable selection algorithms. Then, the selected variables were used to generate a simple and predictive model by the multilinear regression algorithm. A set of 74 histamine H₃ antagonists were split into 40 compounds as a training set, and 17 compounds as a test set, by the Kennard-Stone algorithm. Before splitting the data, 17 compounds were randomly selected from the pool of the whole data set as an evaluation set without any supervision, pretreatment, or visual inspection. Among applied variable selection algorithms, ERM had noticeable improvement on the statistical parameters. The r² values of training, test, and evaluation sets for the ERM-MLR model using CoMFA fields were 0.9560, 0.8630, and 0.8460 and using the CoMSIA fields were 0.9800, 0.8521, and 0.9080, respectively. In this study, the principles of organization for economic cooperation and development (OECD) for regulatory acceptability of QSARs are considered.

Role of histamine and its receptor subtypes in stimulation of conjunctival goblet cell secretion

PURPOSE

The purpose of this study was to determine the effect of histamine and its receptors on goblet cell secretion.

METHODS

Cultured rat and human goblet cells were grown in RPMI 1640. Goblet cell secretion of high molecular weight glycoconjugate was measured by an enzyme-linked lectin assay. Cultured rat goblet cells were homogenized and either RNA was isolated for RT-PCR or proteins were isolated for Western blot analysis for presence of histamine receptors subtypes H₁ through H₄. The localization of these receptors was determined in rat and human goblet cells by immunofluorescence microscopy.

RESULTS

Histamine stimulated goblet cell secretion in a concentration- and time-dependent manner. All four histamine receptors were present in cultured rat and human goblet cells. Use of agonists specific to individual histamine receptor subtypes indicated that the rank order of agonist stimulation was H₁ = H₃ > H₄ > H₂. Using antagonists specific to individual histamine receptor subtypes determined that H₂ and H₃, but not the H₁ and H₄, antagonists, inhibited histamine-stimulated conjunctival goblet cell secretion.

CONCLUSIONS

Rat and human conjunctival goblet cells are a direct target of histamine, which induces secretion. All four histamine receptors are present in rat and human conjunctiva and are active in rat conjunctival goblet cells. These findings suggest that all four histamine receptor subtypes are important for conjunctival goblet cell secretion. Blockage of histamine receptor subtypes could prevent the excess mucus production associated with ocular allergy.