Synthesis and structure-activity relationships of conformationally constrained histamine H(3) receptor agonists

Ligand-directed biased agonism at human histamine H3 receptor isoforms across Gαi/o- and β-arrestin2-mediated pathways

The histamine H3 receptor (H3R) is a neurotransmitter receptor that is primarily found in the brain, where it controls the release and synthesis of histamine, as well as the release of other neurotransmitters (e.g. dopamine, serotonin). Notably, 20 H3R isoforms are differentially expressed in the human brain as a consequence of alternative gene splicing. The hH3R-445, -415, -365 and -329 isoforms contain the prototypical GPCR (7TM) structure, yet exhibit deletions in the third intracellular loop, a structural domain that is pivotal for G protein-coupling, signaling and regulation. To date, the physiological relevance underlying the individual and combinatorial function of hH3R isoforms remains poorly understood. Nevertheless, given their significant implication in physiological processes (e.g. cognition, homeostasis) and neurological disorders (e.g. Alzheimer's and Parkinson's disease, schizophrenia), widespread targeting of hH3R isoforms by drugs may lead to on-target side effects in brain regions that are unaffected by disease. To this end, isoform- and/or pathway-selective targeting of hH3R isoforms by biased agonists could be of therapeutic relevance for the development of region- and disease-specific drugs. Hence, we have evaluated ligand biased signaling at the hH3R-445, -415, -365 and -329 isoforms across various Gαi/o-mediated (i.e. [35S]GTPγS accumulation, cAMP inhibition, pERK1/2 activation, pAKT T308/S473 activation) and non Gαi/o-mediated (i.e. β-arrestin2 recruitment) endpoints that are relevant to neurological diseases. Our findings indicate that H3R agonists display significantly altered patterns in their degree of ligand bias, in a pathway- and isoform-dependent manner, underlining the significance to investigate GPCRs with multiple isoforms to improve development of selective drugs. SUBJECT CATEGORY: Neuropharmacology.

Structural and Molecular Determinants for Isoform Bias at Human Histamine H3 Receptor Isoforms

The human histamine H₃ receptor (hH₃R) is predominantly expressed in the CNS, where it regulates the synthesis and release of histamine and other neurotransmitters. Due to its neuromodulatory role, the hH₃R has been associated with various CNS disorders, including Alzheimer's and Parkinson's disease. Markedly, the hH₃R gene undergoes extensive splicing, resulting in 20 isoforms, of which 7TM isoforms exhibit variations in the intracellular loop 3 (IL3) and/or C-terminal tail. Particularly, hH₃R isoforms that display variations in IL3 (e.g., hH₃R-365) are shown to differentially signal via Gα_i-dependent pathways upon binding of biased agonists (e.g., immepip, proxifan, imetit). Nevertheless, the mechanisms underlying biased agonism at hH₃R isoforms remain unknown. Using a structure-function relationship study with a broad range of H₃R agonists, we thereby explored determinants underlying isoform bias at hH₃R isoforms that exhibit variations in IL3 (i.e., hH₃R-445, -415, -365, and -329) in a Gα_i-dependent pathway (cAMP inhibition). Hence, we systematically characterized hH₃R isoforms on isoform bias by comparing various ligand properties (i.e., structural and molecular) to the degree of isoform bias. Importantly, our study provides novel insights into the structural and molecular basis of receptor isoform bias, highlighting the importance to study GPCRs with multiple isoforms to better tailor drugs.

4-(3-Aminoazetidin-1-yl)pyrimidin-2-amines as High-Affinity Non-imidazole Histamine H3 Receptor Agonists with in Vivo Central Nervous System Activity

Despite the high diversity of histamine H3 receptor (H3R) antagonist/inverse agonist structures, partial or full H3R agonists have typically been imidazole derivatives. An in-house screening campaign intriguingly afforded the non-imidazole 4-(3-azetidin-1-yl)pyrimidin-2-amine 11b as a partial H3R agonist. Here, the design, synthesis, and structure-activity relationships of 11b analogues are described. This series yields several non-imidazole full agonists with potencies varying with the alkyl substitution pattern on the basic amine following the in vitro evaluation of H3R agonism using a cyclic adenosine monophosphate response element-luciferase reporter gene assay. The key compound VUF16839 (14d) combines nanomolar on-target activity (pKi = 8.5, pEC50 = 9.5) with weak activity on cytochrome P450 enzymes and good metabolic stability. The proposed H3R binding mode of 14d indicates key interactions similar to those attained by histamine. In vivo evaluation of 14d in a social recognition test in mice revealed an amnesic effect at 5 mg/kg intraperitoneally. The excellent in vitro and in vivo pharmacological profiles and the non-imidazole structure of 14d make it a promising tool compound in H3R research.

Activation of orexinergic and histaminergic pathway involved in therapeutic effect of histamine H4 receptor antagonist against cisplatin-induced anorexia in mice

We previously reported that hypothalamic tumor necrosis factor-alpha (TNF-α) mRNA expression via histamine H₄ receptors contributes to the development of cisplatin-induced anorexia; however, its precise mechanisms remain unclear. It has been reported that chemotherapeutic agents induce the suppression of orexin neuron activity, and the administration of orexin inhibits chemotherapeutic agent-induced gastric discomfort. Other studies demonstrated that the central administration of TNF-α impairs the orexinergic system, and that orexin excites the histaminergic system. We investigated the involvement of orexinergic and histaminergic systems in the therapeutic effect of an H₄ receptor antagonist against cisplatin-induced anorexia. Cisplatin decreased the expression of prepro-orexin mRNA, which encodes precursors of orexin, in the hypothalamus of mice. The period of expression decreased in parallel with the onset of anorexia, and treatment with an H₄ receptor antagonist (JNJ7777120, 10 mg/kg) inhibited the decrease in expression. The effect of the H₄ receptor antagonist on cisplatin-induced anorexia in mice was antagonized by an orexin OX₂ receptor antagonist (JNJ10397049, 5 mg/kg) rather than an orexin OX₁ receptor antagonist (SB408124, 30 mg/kg). Although an OX₂ receptor agonist (YNT-185, 20 mg/kg) or a histamine H₃ receptor inverse agonist (ciproxifan, 1 mg/kg) inhibited the cisplatin-induced anorexia, the inhibitory effect of the OX₂ receptor agonist was antagonized by an H₃ receptor silent antagonist (VUF5681, 5 mg/kg). The combination of JNJ7777120 (10 mg/kg) and ciproxifan (0.5 mg/kg) completely resolved the cisplatin-induced anorexia. These results suggest that activation of the orexinergic and histaminergic pathway is involved in the therapeutic effect of an H₄ receptor antagonist against cisplatin-induced anorexia.

Visualization of the activation of the histamine H3 receptor (H3R) using novel fluorescence resonance energy transfer biosensors and their potential application to the study of H3R pharmacology

Activation of the histamine-3 receptor (H3R) is involved in memory processes and cognitive action, while blocking H3R activation can slow the progression of neurological disorders, such as Alzheimer's disease, schizophrenia and narcolepsy. To date, however, no direct way to examine the activation of H3R has been utilized. Here, we describe a novel biosensor that can visualize the activation of H3R through an intramolecular fluorescence resonance energy transfer (FRET) signal. To achieve this, we constructed an intramolecular H3R FRET sensor with cyan fluorescent protein (CFP) attached at the C terminus and yellow fluorescent protein (YFP) inserted into the third intracellular loop. The sensor was found to internalize normally on agonist treatment. We measured FRET signals between the donor CFP and the acceptor YFP in living cells in real time, the results of which indicated that H3R agonist treatment (imetit or histamine) increases the FRET signal in a time- and concentration-dependent manner with Kon and Koff values consistent with published data and which maybe correlated with decreasing cAMP levels and the promotion of ERK1/2 phosphorylation. The FRET signal was inhibited by H3R antagonists, and the introduction of mutations at F419A, F423A, L426A and L427A, once again, the promotion of ERK1/2 phosphorylation, was diminished. Thus, we have built a H3R biosensor which can visualize the activation of receptor through real-time structure changes and which can obtain pharmacological kinetic data at the same time. The FRET signals may allow the sensor to become a useful tool for screening compounds and optimizing useful ligands.

Immethridine, histamine H3-receptor (H3R) agonist, alleviated experimental autoimmune encephalomyelitis via inhibiting the function of dendritic cells

Multiple sclerosis (MS) is an inflammatory disease that is characterized by immune-mediated demyelination and degeneration of the central nervous system (CNS). Experimental autoimmune encephalomyelitis (EAE) is the preferential experimental rodent model for MS. Previous study demonstrated histamine H3 receptor (H3R) was an important factor in pathophysiology of EAE and immethridine was the most selective agonist of H3R. However, whether immethridine has therapeutic effect on EAE and its mechanism remained to be defined. Here we constructed EAE mouse model by immunization of MOG_35-55 peptides with complete Freund’s adjuvant, immethridine was used to treat EAE and its therapeutic effect was evaluated. The results showed that the treatment of immethridine could alleviate EAE. The percentage of Th1 and Th17 in the spleen from the treated EAE mice decreased and the surface molecules such as CD40, CD86 or MHCII on dendritic cells (DCs) were also down-regulated. To understand the effect of immethridine on DCs, bone marrow-derived DCs were prepared and the immunological functions were analyzed. The data demonstrated that immethridine could change the expression profiles of cytokines in DCs and inhibit the expression of the co-stimulatory molecules such as CD40 and CD86. Furthermore, immethridine also inhibited the antigen-presenting function of DCs and T cell differentiation induced by DCs. Signaling pathway analysis demonstrated that the phosphorylation of NF-κB p65 but not ERK1/2 in DCs was inhibited after the treatment of immethridine. These data strongly suggested that immethridine could inhibit the function of DCs and indicated the therapeutic potential on EAE.

Discovery of nanomolar ligands with novel scaffolds for the histamine H4 receptor by virtual screening

The involvement of histamine H4 receptor (H4R) in immune cells chemotaxis and mediator release makes it an attractive target for the treatment of inflammation disorders. A decade of medicinal chemistry efforts has led to several promising ligands, although the chemical structures described so far possesses a singular limited diversity. We report here the discovery of novel structures, belonging to completely different scaffolds. The virtual screening was planed as a two-steps process. First, using a "scout screening" methodology, we have experimentally probed the H4R ligand binding site using a small size chemical library with very diverse structures, and identified a hit that further assist us in refining a raw 3D homology model. Second, the refined 3D model was used to conduct a widened virtual screening. This two-steps strategy proved to be very successful, both in terms of structural diversity and hit rate (23%). Moreover, the hits have high affinity for the H4R, with most potent ligands in the nanomolar range.

The Histamine H3 Receptor: Structure, Pharmacology, and Function

Among the four G protein-coupled receptors (H₁-H₄) identified as mediators of the biologic effects of histamine, the H₃ receptor (H₃R) is distinguished for its almost exclusive expression in the nervous system and the large variety of isoforms generated by alternative splicing of the corresponding mRNA. Additionally, it exhibits dual functionality as autoreceptor and heteroreceptor, and this enables H₃Rs to modulate the histaminergic and other neurotransmitter systems. The cloning of the H₃R cDNA in 1999 by Lovenberg et al. allowed for detailed studies of its molecular aspects. In this work, we review the characteristics of the H₃R, namely, its structure, constitutive activity, isoforms, signal transduction pathways, regional differences in expression and localization, selective agonists, antagonists and inverse agonists, dimerization with other neurotransmitter receptors, and the main presynaptic and postsynaptic effects resulting from its activation. The H₃R has attracted interest as a potential drug target for the treatment of several important neurologic and psychiatric disorders, such as Alzheimer and Parkinson diseases, Gilles de la Tourette syndrome, and addiction.

Synthesis, biological evaluation and molecular modeling studies of phthalazin-1(2H)-one derivatives as novel cholinesterase inhibitors

A series of donepezil analogues based on phthalazin-1(2H)-one scaffold was studied as hChEIs. The biological results revealed that the structural modifications proposed significantly affected ChE inhibitory potency as well as selectivity AChE/BuChE.

Tactical Approaches to Interconverting GPCR Agonists and Antagonists

There are many reported examples of small structural modifications to GPCR-targeted ligands leading to major changes in their functional activity, converting agonists into antagonists or vice versa. These shifts in functional activity are often accompanied by negligible changes in binding affinity. The current perspective focuses on outlining and analyzing various approaches that have been used to interconvert GPCR agonists, partial agonists, and antagonists in order to achieve the intended functional activity at a GPCR of therapeutic interest. An improved understanding of specific structural modifications that are likely to alter the functional activity of a GPCR ligand may be of use to researchers designing GPCR-targeted drugs and/or probe compounds, specifically in cases where a particular ligand exhibits good potency but not the preferred functional activity at the GPCR of choice.

Donepezil + propargylamine + 8-hydroxyquinoline hybrids as new multifunctional metal-chelators, ChE and MAO inhibitors for the potential treatment of Alzheimer's disease

The synthesis, biochemical evaluation, ADMET, toxicity and molecular modeling of novel multi-target-directed Donepezil + Propargylamine + 8-Hydroxyquinoline (DPH) hybrids 1-7 for the potential prevention and treatment of Alzheimer's disease is described. The most interesting derivative was racemic α-aminotrile4-(1-benzylpiperidin-4-yl)-2-(((8-hydroxyquinolin-5-yl)methyl)(prop-2-yn-1-yl)amino) butanenitrile (DPH6) [MAO A (IC50 = 6.2 ± 0.7 μM; MAO B (IC50 = 10.2 ± 0.9 μM); AChE (IC50 = 1.8 ± 0.1 μM); BuChE (IC50 = 1.6 ± 0.25 μM)], an irreversible MAO A/B inhibitor and mixed-type AChE inhibitor with metal-chelating properties. According to docking studies, both DPH6 enantiomers interact simultaneously with the catalytic and peripheral site of EeAChE through a linker of appropriate length, supporting the observed mixed-type AChE inhibition. Both enantiomers exhibited a relatively similar position of both hydroxyquinoline and benzyl moieties with the rest of the molecule easily accommodated in the relatively large cavity of MAO A. For MAO B, the quinoline system was hosted at the cavity entrance whereas for MAO A this system occupied the substrate cavity. In this disposition the quinoline moiety interacted directly with the FAD aromatic ring. Very similar binding affinity values were also observed for both enantiomers with ChE and MAO enzymes. DPH derivatives exhibited moderate to good ADMET properties and brain penetration capacity for CNS activity. DPH6 was less toxic than donepezil at high concentrations; while at low concentrations both displayed a similar cell viability profile. Finally, in a passive avoidance task, the antiamnesic effect of DPH6 was tested on mice with experimentally induced amnesia. DPH6 was capable to significantly decrease scopolamine-induced learning deficits in healthy adult mice.

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.

Luciferase reporter gene assay on human, murine and rat histamine H4 receptor orthologs: correlations and discrepancies between distal and proximal readouts

The investigation of the (patho)physiological role of the histamine H4 receptor (H4R) and its validation as a possible drug target in translational animal models are compromised by distinct species-dependent discrepancies regarding potencies and receptor subtype selectivities of the pharmacological tools. Such differences were extremely pronounced in case of proximal readouts, e. g. [(32)P]GTPase or [(35)S]GTPγS binding assays. To improve the predictability of in vitro investigations, the aim of this study was to establish a reporter gene assay for human, murine and rat H4Rs, using bioluminescence as a more distal readout. For this purpose a cAMP responsive element (CRE) controlled luciferase reporter gene assay was established in HEK293T cells, stably expressing the human (h), the mouse (m) or the rat (r) H4R. The potencies and efficacies of 23 selected ligands (agonists, inverse agonists and antagonists) were determined and compared with the results obtained from proximal readouts. The potencies of the examined ligands at the human H4R were consistent with reported data from [(32)P]GTPase or [(35)S]GTPγS binding assays, despite a tendency toward increased intrinsic efficacies of partial agonists. The differences in potencies of individual agonists at the three H4R orthologs were generally less pronounced compared to more proximal readouts. In conclusion, the established reporter gene assay is highly sensitive and reliable. Regarding discrepancies compared to data from functional assays such as [(32)P]GTPase and [(35)S]GTPγS binding, the readout may reflect multifactorial causes downstream from G-protein activation, e.g. activation/amplification of or cross-talk between different signaling pathways.

Identification and characterization of ZEL-H16 as a novel agonist of the histamine H3 receptor

The histamine H3 receptor (H3R) has been recognized as a promising target for the treatment of various central and peripheral nervous system diseases. In this study, a non-imidazole compound, ZEL-H16, was identified as a novel histamine H3 receptor agonist. ZEL-H16 was found to bind to human H3R with a Ki value of approximately 2.07 nM and 4.36 nM to rat H3R. Further characterization indicated that ZEL-H16 behaved as a partial agonist on the inhibition of forskolin-stimulated cAMP accumulation (the efficacy was 60% of that of histamine) and activation of ERK1/2 signaling (the efficacy was 50% of that of histamine) at H3 receptors, but acted as a full agonist just like histamin in the guinea-pig ileum contraction assay. These effects were blocked by pertussis toxin and H3 receptor specific antagonist thioperamide. ZEL-H16 showed no agonist or antagonist activities at the cloned human histamine H1, H2, and H4 receptors and other biogenic amine GPCRs in the CRE-driven reporter assay. Furthermore, our present data demonstrated that treatment of ZEL-H16 resulted in intensive H3 receptor internalization and delayed recycling to the cell surface as compared to that of control with treatment of histamine. Thus, ZEL-H16 is a novel and potent nonimidazole agonist of H3R, which might serve as a pharmacological tool for future investigations or as possible therapeutic agent of H3R.

Synthesis, biological evaluation, and molecular modeling of donepezil and N-[(5-(benzyloxy)-1-methyl-1H-indol-2-yl)methyl]-N-methylprop-2-yn-1-amine hybrids as new multipotent cholinesterase/monoamine oxidase inhibitors for the treatment of Alzheimer's disease

A new family of multitarget molecules able to interact with acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE), as well as with monoamino oxidase (MAO) A and B, has been synthesized. Novel compounds (3-9) have been designed using a conjunctive approach that combines the benzylpiperidine moiety of the AChE inhibitor donepezil (1) and the indolyl propargylamino moiety of the MAO inhibitor N-[(5-benzyloxy-1-methyl-1H-indol-2-yl)methyl]-N-methylprop-2-yn-1-amine (2), connected through an oligomethylene linker. The most promising hybrid (5) is a potent inhibitor of both MAO-A (IC50=5.2±1.1 nM) and MAO-B (IC50=43±8.0 nM) and is a moderately potent inhibitor of AChE (IC50=0.35±0.01 μM) and BuChE (IC50=0.46±0.06 μM). Moreover, molecular modeling and kinetic studies support the dual binding site to AChE, which explains the inhibitory effect exerted on Aβ aggregation. Overall, the results suggest that the new compounds are promising multitarget drug candidates with potential impact for Alzheimer's disease therapy.

Histamine and H3 receptor in alcohol-related behaviors

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

Antagonist affinity measurements at the Gi-coupled human histamine H3 receptor expressed in CHO cells

BACKGROUND

The H3 histamine receptor is a Gi-coupled GPCR that has been proven to exist in different agonist-induced states, including that defined by the protean agonist proxyfan. Several GPCRs are now known to exist in different states. For some of these, antagonist affinity measurement remain constant regardless of the state of the receptor, for others e.g. the beta-adrenoceptors, the antagonist affinity measurements vary considerably depending on which agonist-dependent state is being identified. The purpose of this study was to examine the antagonist affinity measurements at the Gi-coupling human H3 receptor, paying particular attention to measurements made in the presence of full agonists, partial agonists and the proxyfan protean agonist-induced state of the receptor.

RESULTS

CHO cells stably expressing the human histamine H3 receptor and a CRE-SPAP reporter were used. Measurements of CRE-gene transcription and 3H-cAMP accumulation were made. A range of ligands of different agonist efficacies were determined, including some partial agonists e.g. VUF 5681. Unlike other Gi-coupled receptors, no Gs-coupled state of the receptor was detected with these ligands. Antagonist affinity measurements were constant, whether the measurements were made in the presence of a full agonist, a partial agonist or the protean agonist proxyfan.

CONCLUSION

In contrast to all three subtypes of the beta-adrenoceptors, but in keeping with the traditional pharmacological dogma, antagonist affinity measurements remained constant at the human H3 receptor, including the medium-efficacy proxyfan-induced state of the receptor and the VUF5681-induced state of the receptor.

Molecular aspects of the histamine H3 receptor

The cloning of the histamine H(3) receptor (H(3)R) cDNA in 1999 by Lovenberg et al. [10] allowed detailed studies of its molecular aspects and indicated that the H(3)R can activate several signal transduction pathways including G(i/o)-dependent inhibition of adenylyl cyclase, activation of phospholipase A(2), Akt and the mitogen activated kinase as well as the inhibition of the Na(+)/H(+) exchanger and inhibition of K(+)-induced Ca(2+) mobilization. Moreover, cloning of the H(3)R has led to the discovery several H(3)R isoforms generated through alternative splicing of the H(3)R mRNA. The H(3)R has gained the interest of many pharmaceutical companies as a potential drug target for the treatment of various important disorders like obesity, myocardial ischemia, migraine, inflammatory diseases and several CNS disorders like Alzheimer's disease, attention-deficit hyperactivity disorder and schizophrenia. In this paper, we review various molecular aspects of the hH(3)R including its signal transduction, dimerization and the occurrence of different H(3)R isoforms.

Constitutive activity of H3 autoreceptors modulates histamine synthesis in rat brain through the cAMP/PKA pathway

We previously described that agonist-activated histamine H3 autoreceptors inhibit the stimulation of histamine synthesis mediated by calcium/calmodulin- and cAMP-dependent protein kinases (CaMKII and PKA respectively) in histaminergic nerve endings. In the absence of an agonist H3 receptors show partial constitutive activity, so we hypothesized that suppression of constitutive activity by an inverse agonist could stimulate these transduction pathways. We show here that the H3 inverse agonist thioperamide increases histamine synthesis in rat brain cortical slices independently from the amounts of extracellular histamine. Thioperamide effects were mimicked by the inverse agonists clobenpropit and A-331440, but not by the neutral antagonist VUF-5681. In contrast, coincubation with VUF-5681 suppressed thioperamide effects. The effects of thioperamide were completely blocked by the PKA inhibitor peptide myristoyl-PKI14-22, a peptide that did not block depolarization stimulation of histamine synthesis. In addition, thioperamide effects required depolarization and were impaired by blockade of N-type calcium channels (mediating depolarization), but not by CaMKII inhibition. These results indicate that constitutive activity of H3 receptors in rat brain cortex inhibits the adenylate cyclase/PKA pathway, and perhaps also the opening of N-type voltage sensitive calcium channels, but apparently not CaMKII.

A Chemical Switch for the Modulation of the Functional Activity of Higher Homologues of Histamine on the Human Histamine H3 Receptor: Effect of Various Substitutions at the Primary Amino Function

In an effort to establish the structural requirements for agonism, neutral antagonism, and inverse agonism at the human histamine H(3) receptor (H(3)R) we have prepared a series of higher homologues of histamine in which the terminal nitrogen of the side chain has been either mono- or disubstituted with several aliphatic, alicyclic, and aromatic moieties or incorporated in cyclic systems. The novel ligands have been pharmacologically investigated in vitro for their affinities on the human H(3)R and H(4)R subtypes by radioligand displacement experiments and for their intrinsic H(3)R activities via a CRE-mediated beta-galactosidase reporter gene assay. Subtle changes of the substitution pattern at the side chain nitrogen alter enormously the pharmacological activity of the ligands, resulting in a series of compounds with a wide spectrum of pharmacological activities. Among the several neutral H(3)R antagonists identified within this series, compounds 2b and 2h display an H(3)R affinity in the low nanomolar concentration range (pK(i) values of 8.1 and 8.4, respectively). A very potent and selective H(3)R agonist (1l, pEC(50) = 8.9, alpha = 0.94) and a very potent, though not highly selective, H(3)R inverse agonist (2k, pIC(50) = 8.9, alpha = -0.97) have been identified as well.

New high affinity H3 receptor agonists without a basic side chain

In this study, we replaced the basic amine function of the known histamine H(3) receptor agonists imbutamine or immepip with non-basic alcohol or hydrocarbon moieties. All compounds in this study show a moderate to high affinity for the cloned human H(3) receptor and, unexpectedly, almost all of them act as potent agonists. Moreover, in the alcohol series, we consistently observed an increased selectivity for the human H(3) receptor over the human H(4) receptor, but none of the compounds in this series possess increased affinity and functional activity compared to their alkylamine congeners. In this new series of compounds VUF5657, 5-(1H-imidazol-4-yl)-pentan-1-ol, is the most potent histamine H(3) receptor agonist (pK(i) = 8.0 and pEC(50) = 8.1) with a 320-fold selectivity at the human H(3) receptor over the human H(4) receptor.

N-substituted piperidinyl alkyl imidazoles: discovery of methimepip as a potent and selective histamine H3 receptor agonist

In this study, we continue our efforts toward the development of potent and highly selective histamine H(3) receptor agonists. We introduced various alkyl or aryl alkyl groups on the piperidine nitrogen of the known H(3)/H(4) agonist immepip and its analogues (1-3a). We observed that N-methyl-substituted immepip (methimepip) exhibits high affinity and agonist activity at the human histamine H(3) receptor (pK(i) = 9.0 and pEC(50) = 9.5) with a 2000-fold selectivity at the human H(3) receptor over the human H(4) receptor and more than a 10000-fold selectivity over the human histamine H(1) and H(2) receptors. Methimepip was also very effective as an H(3) receptor agonist at the guinea pig ileum (pD(2) = 8.26). Moreover, in vivo microdialysis (in rat brain) showed that methimepip reduces the basal level of brain histamine to about 25% after a 5 mg/kg intraperitoneal administration.

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

Since the cloning of the histamine H(3) receptor cDNA in 1999 by Lovenberg and co-workers, this histamine receptor has gained the interest of many pharmaceutical companies as a potential drug target for the treatment of various important disorders, including obesity, attention-deficit hyperactivity disorder, Alzheimer's disease, schizophrenia, as well as for myocardial ischaemia, migraine and inflammatory diseases. Here, we discuss relevant information on this target protein and describe the development of various H(3) receptor agonists and antagonists, and their effects in preclinical animal models.

Synthesis and structure–activity relationships of indole and benzimidazole piperazines as histamine H4 receptor antagonists

We describe the structure-activity relationships for a series of ligands structurally related to the recently identified (5-chloro-1H-indol-2-yl)-(4-methyl-piperazin-1-yl)-methanone (1) as histamine H(4) receptor (H(4)R) antagonists. Furthermore, we identified related benzimidazoles as novel lead compounds for the H(4)R. The ligands have been evaluated by radioligand displacement studies and functional assays for their interaction with both the human histamine H(3) and H(4) receptors and exhibit pK(i) values up to 7.5 at the human H(4)R.

Identification of 4-(1H-Imidazol-4(5)-ylmethyl)pyridine (Immethridine) as a Novel, Potent, and Highly Selective Histamine H3Receptor Agonist

In this study, the piperidine ring of immepip and its analogues was replaced by a rigid heterocyclic pyridine ring. Many compounds in the series exhibit high affinity and agonist activity at the human histamine H(3) receptor. Particularly, the 4-pyridinyl analogue of immepip (1c, immethridine) is identified as a novel potent and highly selective histamine H(3) receptor agonist (pK(i) = 9.07, pEC(50) = 9.74) with a 300-fold selectivity over the closely related H(4) receptor.